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Influence of some characteristics of coniferous wood tissues on short-term creep El-Osta, Mohamed Lotfy Mahmoud 1971

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INFLUENCE OF SOME CHARACTERISTICS OF CONIFEROUS WOOD TISSUES ON SHORT-TERM CREEP by M. LOTFY MAHMOUD EL-OSTA B.S c . ( A g r i c u l t u r e ) A l e x a n d r i a U n i v e r s i t y , 1963 M.Sc. ( F o r e s t r y ) A l e x a n d r i a U n i v e r s i t y , 1966 A THESIS SUBMITTED I N P A R T I A L FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY i n t h e D e p a r t m e n t o f F o r e s t r y We a c c e p t t h i s t h e s i s a s c o n f o r m i n g t o t h e r e q u i r e d s t a n d a r d THE UNIVERSITY OF B R I T I S H COLUMBIA A u g u s t , 1971 In presenting th i s thes is in pa r t i a l fu l f i lment o f the requirements for an advanced degree at the Un ivers i ty of B r i t i s h Columbia, I agree that the L ibrary shal l make i t f ree ly ava i l ab le for reference and study. I fur ther agree that permission for extensive copying of th i s thes is for scho lar ly purposes may be granted by the Head of my Department or by his representat ives. It is understood that copying or pub l i ca t ion of th i s thesis fo r f inanc ia l gain sha l l not be allowed without my wr i t ten permission. Department of The Univers i ty of B r i t i s h Columbia Vancouver 8, Canada S u p e r v i s o r : P r o f e s s o r R.W. W e l l w o o d i i ABSTRACT The h y p o t h e s i s i s e x a m i n e d t h a t s h o r t - t e r m c r e e p r e s p o n s e o f e a r l y w o o d a nd l a t e w o o d t i s s u e s o f some c o n i f e r o u s s p e c i e s , s t r e s s e d i n t e n s i o n p a r a l l e l t o t h e g r a i n , i s a f u n c t i o n o f m i c r o f i b r i l a n g l e o f t h e S 2 l a y e r o f a n d r e l a t i v e d e g r e e o f c r y s t a l l i n i t y i n t h e t r a c h e i d c e l l w a l l , a l o n g w i t h s p e c i f i c g r a v i t y o f t h a t wood t i s s u e a n d i t s e x t r a c t i v e s c o n t e n t . A new t e c h n i q u e was d e v e l o p e d t o m e a s u r e t h e t o t a l c r e e p t h a t o c c u r r e d o v e r a 6 0 - m i n u t e p e r i o d o f t i m e f o r s m a l l s p e c i m e n s ( n o m i n a l l y 0.010 i n , t h i c k ) o f D o u g l a s - f i r ( P s e u d o t s u g a m e n z i e s i i ( M i r b . ) F r a n c o ) ( n o r m a l a n d c o m p r e s s i o n wood), S i t k a s p r u c e ( P i c e a s i t c h e n s i s (Bong.) C a r r . ) a n d w e s t e r n h e m l o c k ( T s u g a h e t e r o p h y l l a ( R a f . ) S a r g . ) , t a k e n f r o m e a r l y w o o d a nd l a t e w o o d z o n e s o f t h e same i n c r e m e n t . T o t a l c r e e p was d e t e r m i n e d a t two i n i t i a l d e f o r m a t i o n l e v e l s , 3,000 m i c r o i n . p e r i n . ( s t r a i n l e v e l No. 1) a n d 6,000 m i c r o i n . p e r i n . ( s t r a i n l e v e l No. 2 ) . M i c r o f i b r i l a n g l e was m e a s u r e d b y a m o d i f i e d m e r c u r y i m p r e g n a t i o n m e t h o d , w h i l e c e l l w a l l c r y s t a l l i n i t y was d e t e r m i n e d on s m a l l , u n o r i e n t e d p e l l e t s b y t h e X - r a y d i f f r a c t i o n t e c h n i q u e . A i r - d r y s p e c i f i c g r a v i t y ( o v e n - d r y w e i g h t ) a n d a l c o h o l - b e n z e n e p l u s h o t w a t e r e x t r a c t i v e s w e r e a l s o d e t e r m i n e d b y c o n v e n t i o n a l m e t h o d s . / M u l t i p l e r e g r e s s i o n a n a l y s e s w e r e c a r r i e d o u t a n d p r e d i c t i o n e q u a t i o n s , b a s e d on t h e e x p e r i m e n t a l r e s u l t s , h a v e b e e n c o n s t r u c t e d . I t i s shown t h a t t h e v a r i a b i l i t y i n t o t a l c r e e p r e s p o n s e c a n b e s t be e x p l a i n e d by u s i n g t h e p r e d i c t i o n e q u a t i o n w h i c h c o n t a i n s m i c r o f i b r i l a n g l e o f t h e S 2 l a y e r , s p e c i f i c g r a v i t y a n d e x t r a c t i v e s c o n t e n t . The 2 m u l t i p l e c o e f f i c i e n t s o f d e t e r m i n a t i o n (R ) u s i n g t h i s s u b s e t o f v a r i a b l e s a r e 0.7680 a n d 0.8550 f o r i n i t i a l s t r a i n Numbers 1 a n d 2, r e s p e c t i v e l y . C e l l w a l l c r y s t a l l i n i t y was e l i m i n a t e d f r o m t h e p r e d i c t i o n e q u a t i o n s a s t h e l e a s t i m p o r t a n t v a r i a b l e due t o i t s h i g h i n v e r s e c o r r e l a t i o n w i t h t h e m i c r o f i b r i l a n g l e o f t h e S2 l a y e r ( r = 0 . 9 2 8 4 ) . Two p o s s i b l e r e a s o n s a r e s u g g e s t e d t o e x p l a i n t h i s c o r r e l a t i o n . F i r s t , i n t h e c a s e o f a s m a l l a n g l e , t h e s c a t t e r a r o u n d t h e mean m i c r o f i b r i l a n g l e i s s m a l l e r a n d t h e m i c r o f i b r i l s p r o b a b l y , l i e a l m o s t p a r a l l e l t o e a c h o t h e r . A s a r e s u l t , t h e r e l a t i v e d e g r e e o f amorphous m a t e r i a l r e q u i r e d t o f i l l t h e m i c r o - s p a c e s b e t w e e n m i c r o -f i b r i l s w o u l d be s m a l l e r . C o n s i d e r i n g t h e c a s e o f a l a r g e m i c r o f i b r i l a n g l e , t h e m i c r o f i b r i l s a r e p r o b a b l y n o t p a r a l l e l t o e a c h o t h e r ? c o n s e q u e n t l y , r e l a t i v e l y l a r g e m i c r o - s p a c e s w o u l d be o c c u p i e d by t h e a m o r p h o u s m a t e r i a l . A s e c o n d p o s s i b l e r e a s o n f o r t h i s r e l a t i o n s h i p may be t h a t c e l l u l o s e c h a i n m o l e c u l e s , i n t h e c a s e o f a s m a l l m i c r o f i b r i l a n g l e , w i l l h a v e a b e t t e r c h a n c e f o r i n c r e a s e d i v f r e q u e n c y o f c r o s s l i n k s ( b o n d i n g b e t w e e n n e i g h b o u r i n g c h a i n s ) a l o n g t h e i r u n i t l e n g t h . C o n s e q u e n t l y , a t e n d e n c y o f i m p r o v e d g e o m e t r i c o r d e r s h o u l d be o b s e r v e d w i t h b e t t e r c h a i n c o h e r e n c e i n t h e r e s u l t i n g c e l l u l o s e as c o m p a r e d t o s i t u a t i o n s a s s o c i a t e d w i t h t r a c h e i d s c h a r a c t e r i z e d b y l a r g e r m i c r o f i b r i l a n g l e . I t m u s t be i n d i c a t e d t h a t r e a s o n s f o r t h i s h i g h d e g r e e o f c o r r e l a t i o n , a s n o t e d a b o v e , r e m a i n c o n j e c t u r a l . Among t h e s t r u c t u r a l f e a t u r e s s t u d i e d , m i c r o f i b r i l a n g l e was shown t o c o n t r o l c r e e p r e s p o n s e t o t h e g r e a t e s t e x t e n t A s i t i n c r e a s e s , t o t a l c r e e p i n c r e a s e s , t h e r e a s o n b e i n g t h a t w i t h a s m a l l a n g l e , m i c r o f i b r i l s a r e i n a p o s i t i o n t o b e a r m o s t o f t h e a p p l i e d l o a d a n d t h e r e f o r e t h e i r r e l a t i v e movement t o w a r d s a s m a l l e r a n g l e w o u l d be l e s s . T h i s r e s u l t s i n a s m a l l p l a s t i c d e f o r m a t i o n . I n t h e c a s e o f a l a r g e a n g l e , t h e r e i s a p o s s i b i l i t y t h a t t h e m i c r o f i b r i l s h a v e a l a r g e t e n d e n c y t o move t o a s m a l l e r a n g l e c a u s i n g a l a r g e c r e e p r e s p o n s e . Wood s a m p l e s o f l o w s p e c i f i c g r a v i t y c r e e p more t h a n t h o s e w i t h h i g h s p e c i f i c g r a v i t y . T h i s b e h a v i o r i s e x p l a i n e d b y t h e h i g h e r r e l a t i v e p e r c e n t o f t h e S2 l a y e r i n t h e l a t t e r . E x t r a c t i v e s a r e shown t o c o n t r i b u t e s i g n i f i c a n t l y t o t h e v a r i a t i o n i n t o t a l c r e e p . They p r o b a b l y a c t a s p l a s t i c i z e r s c a u s i n g a r e d u c t i o n i n t h e p r i m a r y and s e c o n d a r y b o n d i n g b e t w e e n m i c r o f i b r i l s . T h i s w o u l d f a c i l i -V t a t e t h e movement o f t h e s t i f f i n e x t e n s i b l e m i c r o f i b r i l s t o accommodate t h e c r e e p - i n d u c i n g s t r e s s e s . R e s u l t s o b t a i n e d i n t h i s s t u d y w e r e c o m p a t i b l e w i t h t h e p r o p o s e d h y p o t h e s i s . v i TABLE OF CONTENTS Page TITLE PAGE i ABSTRACT i i TABLE OF CONTENTS v i LIST OF TABLES i x LIST OF FIGURES X ACKNOWLEDGEMENT x i i i INTRODUCTION . . . . . 1 LITERATURE REVIEW 4 I. Minute Structure of Coniferous Woods 4 I I . Physical Nature of C e l l u l o s i c C e l l Wall Structure 6 A. C e l l Wall Organization 6 1. Primary Wall (P) 6 2. Secondary Wall 6 B. Supermolecular Arrangement of Cellulose Chains 10 1. Fringed M i c e l l a r Model 11 2. Fringed F i b r i l l a r Model . . . . 12 3. Continuous Model 13 4. Folded Chain Model 13 C. C e l l Wall C r y s t a l l i n i t y 15 1. Concept 15 2. Relation to Mechanical Properties 16 v i i Page 3. Va r i a t i o n i n Wood and Tracheids 22 D. M i c r o f i b r i l Angle 24 1. D e f i n i t i o n 24 2. Relation to Mechanical Properties 26 3. V a r i a t i o n i n Wood and Tracheids 34 E. Extractives Content 35 F. S p e c i f i c Gravity . . . . . . . . . . . 37 I I I . Creep Phenomenon . 40 A. E f f e c t of Wood Species 42 B. Relation to Moisture Content and Temperature 44 MATERIALS AND METHODS 45 I. Wood Samples 45 I I . Preparation of Test Specimens 46 I I I . Micro-creep Test 49 A. Testing Machine 49 B. Loading System 50 C. Selection of a Constant I n i t i a l S t r a i n Level 51 IV. M i c r o - s p e c i f i c Gravity . . . 52 V. Extractives Content 54 VI. M i c r o f i b r i l Angle Determination 55 A. X-ray D i f f r a c t i o n Method 55 B. Op t i c a l Method . . . 57 v i i i Page VII. Determination of C r y s t a l l i n i t y by X-ray Technique . . . . . 59 A. Specimen Preparation 59 B. X-ray D i f f r a c t i o n 60 C. Selection of an Equation for C r y s t a l l i n i t y Determination 62 RESULTS AND DISCUSSION 65 I. S t a t i s t i c a l Analyses and Interpretation of Results 65 II . Relationship of M i c r o f i b r i l Angle to Tota l Creep 7 1 I I I . Relationship of C e l l Wall C r y s t a l l i n i t y to Total Creep . 7 6 IV. Relationship of S p e c i f i c Gravity to Total Creep 8 3 V. Relationship of Extractives Content to Total Creep 8 6 VI. Significance of Results 91 RECOMMENDATIONS FOR FURTHER RESEARCH 94 CONCLUSION 96 LITERATURE CITED 99 i x LIST OF TABLES Page TABLE 1. TABLE 2. TABLE 3. SUMMARY OF MICROFIBRIL ANGLE DETERMINATIONS RELATIVE DEGREE OF CRYSTALLINITY (CRYSTALLINITY INDEX) 114 115 SPECIFIC GRAVITY, EXTRACTIVES CONTENT AND TOTAL CREEP FOR THE SAMPLES TESTED AT 3,000 AND 6,000 MICROIN. PER IN. INITIAL STRAIN 116 TABLE 4. MULTIPLE COEFFICIENTS OF DETERMIN-ATION (R ) AND STANDARD ERRORS OF ESTIMATE (SEE) BASED ON 34 WOOD SPECIMENS TESTED AT 3,000 MICROIN. PER IN. INITIAL STRAIN 118 TABLE 5. MULTIPLE«COEFFICIENTS OF DETERMIN-ATION (R ) AND STANDARD ERRORS OF ESTIMATE (SEE) BASED ON 34 WOOD SPECIMENS TESTED AT 6,000 MICROIN. PER IN. INITIAL STRAIN 120 TABLE 6. COVARIANCE ANALYSIS FOR TESTING THE DIFFERENCE IN TOTAL CREEP BETWEEN EARLYWOOD (EQUATION [5]) AND LATEWOOD (EQUATION [6]) AT 3,000 MICROIN. PER IN. INITIAL STRAIN 122 TABLE 7. COVARIANCE ANALYSIS FOR TESTING THE DIFFERENCE IN TOTAL CREEP BETWEEN EARLYWOOD (EQUATION [7]) AND LATEWOOD (EQUATION [8]) AT 6,000 MICROIN. PER IN. INITIAL STRAIN 123 TABLE 8. COVARIANCE ANALYSIS FOR TESTING THE DIFFERENCE IN TOTAL CREEP AT 3,000 MICROIN. PER IN. INITIAL STRAIN (EQUATION [3]) AND AT 6,000 MICROIN. PER IN. INITIAL STRAIN (EQUATION [4]) 124 TABLE 9. CORRELATION COEFFICIENTS FOR THE VARIABLES WHICH WERE DETERMINED ON (A) SPECIMENS TESTED AT 3,000 MICROIN. PER IN. AND (B) SPECIMENS TESTED AT 6,000 MICROIN. PER IN. INITIAL STRAIN 125 X LIST OF FIGURES Page FIG. 1. DIAGRAMMATIC REPRESENTATION OF CELL WALL ORGANIZATION OF A TYPICAL FIBER OR TRACHEID SHOWING THE TEXTURE OF THE DIFFERENT CELL WALL LAYERS, AFTER WARDROP (147) 127 FIG. 2. DIFFERENT CONCEPTS OF CELL WALL ORGAN-IZATION OF A TYPICAL FIBER OR TRACHEID, SHOWING FIBRILLAR AND/OR MICROFIBRILLAR DIRECTIONS. (A) FROM WARDROP AND BLAND (149); (B) HARADA ET AL. (34)• (C) FROM ; FORGACS (26). SEE TEXT FOR LEGEND . . . 127 FIG. 3. THE CLASSICAL MODEL OF MEYER AND MISCH-(86) FOR THE UNIT-CELL OF CRYSTAL LATTICE OF NATIVE CELLULOSE (CELLULOSE 1 ) . . . . 128 FIG. 4. FRINGED MICELLAR MODEL, AFTER HEARLE (36) 129 FIG. 5. FRINGED FIBRILLAR MODEL, AFTER HEARLE (36) 129 FIG. 6. DIAGRAMMATIC REPRESENTATION OF CELLULOSIC MICROFIBRILS ACCORDING TO CONCEPTS OF (A) HESS ET AL. (43) AND (B) RANBY (122) 130 FIG. 7. FOLDED CHAIN MODEL, AFTER MANLEY (79). THE MICROFIBRIL CONSISTS OF A TIGHTLY WOUND HELIX (A) FROM A RIBBON (B) ABOUT 8A THICK IN WHICH A SINGLE CELLULOSE CHAIN IS FOLDED REPEATEDLY 131 FIG. 8. A MODEL OF THE COTTON FIBER SHOWING THE SPIRAL STRUCTURE AND REVERSALS IN THE GROWTH LAYERj (A) PRIMARY'WALL; (B) GROWTH LAYERS; (C) REVERSALS, AFTER ORR ET AL. (103) 131 FIG. 9. DEFORMATION OF AN ELASTIC-PLASTIC BODY AS A FUNCTION OF TIME. LOADING DURING TIME T 0 TO T l f FOLLOWED BY UNLOADING TIME Tn TO T 9, AFTER KOLLMANN AND COTE (70) .7 132 x i Page FIG. 10. FOUR-ELEMENT SPRING AND DASHPOT MODEL, AFTER PENTONEY (108). SEE TEXT FOR LEGEND 132 FIG. 11. IDEALIZED LONG-TIME CREEP, AFTER PENTONEY (108) 133 FIG. 12. SCHEMATIC REPRESENTATION FOR SAMPLING PROCEDURE 134 FIG. 13. ARBOR PRESS WITH ADJUSTABLE CUTTING DIE USED FOR CREEP TEST SPECIMENS PREPARATION 135 FIG. 14. CREEP PARALLEL TO THE GRAIN SET-UP (A) TABLE MODEL INSTRON TESTING MACHINE (B) LOADING SYSTEM 136 FIG. 15. STRAIN-TIME RELATIONSHIP FOR DOUGLAS-FIR EARLYWOOD (NORMAL WOOD)AT (A) 3,000 MICROIN. PER IN. (SPECIMEN NO. 5) AND (B) 6,000 MICROIN. PER IN. (SPECIMEN NO. 4) INITIAL STRAIN I 3 7 FIG. 16. STRAIN-TIME RELATIONSHIP FOR SITKA SPRUCE LATEWOOD AT (A) 3,000 MICROIN. PER IN. (SPECIMEN NO. 4) AND (B) 6,000 MICROIN.PER IN. (SPECIMEN NO.l) INITIAL STRAIN 1 3 8 FIG. 17. CAHN ELECTRO-BALANCE USED FOR SPECIMEN WEIGHT DETERMINATION 139 FIG. 18. X-RAY INTENSITY AROUND THE (002) ARC FOR SITKA SPRUCE LATEWOOD USING TEXTURE GONIOMETER MACHINE 140 FIG. 19. RELATIONSHIP BETWEEN ANGLE "T" DERIVED FROM X-RAY AND MEAN MICROFIBRIL ANGLE MEASURED BY MERCURY IMPREGNATION (MA) 141 FIG. 20. X-RAY DIFFRACTION PATTERN OF DOUGLAS-FIR LATEWOOD (COMPRESSION WOOD) . . . . 142 FIG. 21. RELATIONSHIP BETWEEN TOTAL CREEP (Y) AND MICROFIBRIL ANGLE (X.. ) AT 3,000 MICROIN. PER IN. INITIAL STRAIN, SEE FIG. 19 FOR LEGEND 143 x i i Page FIG. 22. RELATIONSHIP BETWEEN TOTAL CREEP (Y) AND MICROFIBRIL ANGLE (X.. ) AT 6,000 MICROIN. PER IN. INITIAL STRAIN. SEE FIG. 19 FOR LEGEND 143 FIG. 23. RELATIONSHIP BETWEEN TOTAL CREEP (Y) AND RELATIVE DEGREE OF CRYSTALLINITY (X 2) AT 3,000 MICROIN. PER. IN. INITIAL STRAIN. SEE FIG. 19 FOR LEGEND . . . . 144 FIG. 24. RELATIONSHIP BETWEEN TOTAL CREEP (Y) AND RELATIVE DEGREE OF CRYSTALLINITY (X ?) AT 6,000 MICROIN. PER IN. INITIAL STRAIN. SEE FIG. 19 FOR LEGEND . . . . i 4 4 FIG. 25. RELATIONSHIP BETWEEN MICROFIBRIL ANGLE (X,) AND RELATIVE DEGREE OF CRYSTALLINITY (X^) . SEE FIG. 19 FOR LEGEND 145 FIG. 26. RELATIONSHIP BETWEEN TOTAL CREEP (Y) AND SPECIFIC GRAVITY (X 3) AT 3,000 MICROIN. PER IN. INITIAL STRAIN. SEE FIG. 19 FOR LEGEND 146 FIG. 27. RELATIONSHIP BETWEEN TOTAL CREEP (Y) AND SPECIFIC GRAVITY (X3) AT 6,000 MICROIN. PER IN. INITIAL STRAIN. SEE FIG. 19 FOR LEGEND 146 FIG. 28. RELATIONSHIP BETWEEN TOTAL CREEP (Y) AND EXTRACTIVES CONTENT (X 4) AT 3,000 MICROIN. PER IN. INITIAL STRAIN. SEE FIG. 19 FOR LEGEND 147 FIG. 29. RELATIONSHIP BETWEEN TOTAL CREEP (Y) AND EXTRACTIVES CONTENT (X 4) AT 6,000 MICROIN. PER IN. INITIAL STRAIN. SEE FIG. 19 FOR LEGEND 147 x i i i ACKNOWLEDGEMENT The w r i t e r i s i n d e b t e d t o h i s s u p e r v i s o r , D r . R. W. W e l l w o o d , P r o f e s s o r , F a c u l t y o f F o r e s t r y , U n i v e r s i t y o f B r i t i s h C o l u m b i a , f o r h i s c o n s c i e n t i o u s a n d s k i l l f u l g u i d a n c e a s w e l l a s h i s c o n s t r u c t i v e c r i t i c i s m d u r i n g t h e w h o l e o f a 3 j y e a r a c a d e m i c p r o g r a m . A p p r e c i a t i o n i s due t o t h e w r i t e r ' s a d v i s o r y c o m m i t t e e c o n s i s t i n g o f D r . C. A. B r o c k l e y , Mr. R. G. B u t t e r s , D r . N. C. F r a n z , D r . R. M. K e l l o g g , D r . A. K o z a k , D r . L. P a s z n e r a n d D r . R. W. W e l l w o o d f o r t h e i r v a l u a b l e s u g g e s t i o n s a n d r e v i e w o f t h i s t h e s i s . S p e c i a l t h a n k s a r e due t o D r . A. K o z a k , A s s o c i a t e P r o f e s s o r , f o r h i s a d v i c e o n s t a t i s t i c a l a n a l y s e s a n d com-p u t e r p r o g r a m m i n g ; D r . R. M. K e l l o g g , C a n a d a D e p a r t m e n t o f t h e E n v i r o n m e n t , W e s t e r n F o r e s t P r o d u c t s L a b o r a t o r y , V a n c o u v e r , f o r m a k i n g f a c i l i t i e s a n d e q u i p m e n t a t t h e F o r e s t P r o d u c t s L a b o r a t o r y a v a i l a b l e a n d f o r h i s h e l p a n d a d v i c e d u r i n g t h e p l a n n i n g a n d e x p e r i m e n t a l p h a s e s o f t h e t h e s i s . I n a d d i t i o n , a p p r e c i a t i o n i s e x t e n d e d t o Mr. R. G. B u t t e r s , A s s i s t a n t P r o f e s s o r , D e p a r t m e n t o f M e t a l l u r g y , f o r h i s v a l u a b l e s u g g e s t i o n s a n d f o r m a k i n g e q u i p m e n t i n h i s D e p a r t m e n t a v a i l a b l e . A p p r e c i a t i o n i s a l s o e x t e n d e d t o D r . E. P. Swan, A s s o c i a t e P r o f e s s o r ( p a r t - t i m e ^ f o r r e a d i n g t h e t h e s i s a n d x i v f o r h i s v a l u a b l e comments. The w r i t e r s i n c e r e l y t h a n k s D r . E. P. M e a g h e r , D e p a r t m e n t o f G e o l o g y , f o r p e r m i s s i o n t o u s e t h e X - r a y m a c h i n e ; M i s s L. C o w d e l l a n d M r s . K. H e j j a s f o r a s s i s t a n c e i n w r i t i n g c o m p u t e r p r o g r a m s ; M r s . M. Lambden f o r d r a f t i n g w o r k ; Mr. C. K. C h a n , g r a d u a t e s t u d e n t , f o r a s s i s t a n c e i n p r e p a r i n g m i c r o t o m e s e c t i o n s a n d Mr. G. Bohnenkamp f o r h i s t e c h n i c a l h e l p . The w r i t e r i s a l s o g r a t e f u l t o t h e N a t i o n a l R e s e a r c h C o u n c i l o f C a n a d a , t h e C o u n c i l o f t h e F o r e s t I n d u s t r i e s o f B r i t i s h C o l u m b i a a n d t h e U n i v e r s i t y o f B r i t i s h C o l u m b i a f o r f i n a n c i a l a s s i s t a n c e . F i n a l l y t h e w r i t e r w i s h e s t o t h a n k h i s p a r e n t s f o r t h e i r c o n t i n u i n g e n c o u r a g e m e n t . INTRODUCTION Wood has been used f o r many years as a s t r u c t u r a l m a t e r i a l i n b u i l d i n g s , bridges and innumerable other engineered f a b r i c a t i o n s . I t s mechanical behavior i s found to be i n f l u e n c e d by numerous f a c t o r s such as moisture content, temperature, species of wood and d i r e c t i o n of a p p l i e d s t r e s s or s t r a i n . In a d d i t i o n , the f a c t o r of time has been given e s p e c i a l c o n s i d e r a t i o n w i t h respect t o s t r e s s - s t r a i n r e l a t i o n s h i p s . Although s e v e r a l a p p l i c a t i o n s of wood are based on i t s e l a s t i c p r o p e r t i e s , r h e o l o g i c a l s t u d i e s r e v e a l t h a t wood e x h i b i t s both e l a s t i c and p l a s t i c p r o p e r t i e s . F r e q u e n t l y , the p l a s t i c p r o p e r t i e s dominate i n i t s behavior. Time-dependent behavior manifests i t s e l f i n s e v e r a l ways, namely creep, s t r e s s r e l a x a t i o n and damping c a p a c i t y . There i s evidence from l i n e a r v i s c o e l a s t i c s t u d i e s , conducted at low s t r e s s l e v e l s / t h a t these three r h e o l o g i c a l phenomena are r e l a t e d m e c h a n i s t i c a l l y . Creep, one of the important r h e o l o g i c a l p r o p e r t i e s e x h i b i t e d by wood, can be de f i n e d as a c o n t i n u i n g deformation due t o a constant load imposed over time. Since wood r e -q u i r e s s e v e r a l m o d i f i c a t i o n s f o r conversion t o a usable form, i t undergoes a p p l i e d s t r e s s e s i n manufacturing 2 processes such as machining, drying and pressing. In addition, i t can be subjected, i n use, to complex loading conditions. Whatever the source of stress, a resultant s t r a i n occurs and increases with elapsed time. The rheological response of wood under constant stress has been studied almost ex c l u s i v e l y using macro-samples. Very l i t t l e i s known about the microscopic and submicroscopic c h a r a c t e r i s t i c s contributing to var i a t i o n s i n creep response. Wood i s an extremely complex material. B a s i c a l l y i t s e l a s t i c properties, under the same environmental con-d i t i o n s , are determined by factors inherent i n i t s structure. These may be summarized as follows: 1. The amount of c e l l wall substance present i n a given volume of wood ( s p e c i f i c g r a v i t y ) ; 2. The proportionate chemical composition of the primary components of the c e l l wall ( c e l l u l o s e , hemicelluloses and lignin) ; 3. The supermolecular arrangement and o r i e n t a t i o n of wall material i n the d i f f e r e n t t i s s u e s ; 4. The kind, s i z e , proportions and arrangement of the c e l l s making up the woody t i s s u e s . I t i s anticipated that these factors would also influence the rheological properties of wood. But v i r t u a l l y nothing i s known about the role which the abovenoted variables play i n c o n t r o l l i n g creep response of wood. A thorough knowledge of the rel a t i o n s h i p s between 3 c r e e p r e s p o n s e a nd m i c r o s c o p i c a n d s u b m i c r o s c o p i c c h a r a c t e r -i s t i c s o f wood i s i m p o r t a n t . A t t h e m i c r o - l e v e l , s u c h i n f o r m a t i o n i s o f f u n d a m e n t a l s c i e n t i f i c i n t e r e s t b e c a u s e i t a l l o w s some i n f e r e n c e a s t o t h e m e c h a n i s m o f c r e e p . I n a d d i t i o n , i n f o r m a t i o n o b t a i n e d f r o m w e l l d e s i g n e d e x p e r i -m e n t s c o u l d be c o m b i n e d w i t h k n o w l e d g e o n o t h e r v a r i a b l e s , w h i c h m i g h t be r e s o l v e d a t t h e m a c r o - l e v e l , t o p r o v i d e g u i d e n c e t o t h e u s e r o f s t r u c t u r a l t i m b e r a s w e l l a s t o t h e wood s c i e n t i s t i n s e l e c t i n g m a t e r i a l o f t h e r e q u i r e d s t r e n g t h f o r a s p e c i f i c p u r p o s e . The o b j e c t i v e o f t h i s t h e s i s i s t o e x a m i n e t h e h y p o t h e s i s t h a t s h o r t - t e r m c r e e p o f e a r l y w o o d a n d l a t e w o o d t i s s u e s o f some c o n i f e r o u s s p e c i e s , s t r e s s e d i n t e n s i o n p a r a l l e l t o t h e g r a i n , i s a f u n c t i o n o f m i c r o f i b r i l a n g l e o f t h e S2 l a y e r o f t r a c h e i d w a l l , a n d r e l a t i v e d e g r e e o f c r y s t a l l i n i t y i n t h e c e l l w a l l , a l o n g w i t h s p e c i f i c g r a v i t y o f t h a t wood t i s s u e a n d i t s e x t r a c t i v e s c o n t e n t . Wood t i s s u e s t a k e n f r o m D o u g l a s - f i r ( P s e u d o t s u g a m e n z i e s i i ( M i r b . ) F r a n c o ) ( n o r m a l a n d c o m p r e s s i o n w o o d ) , S i t k a s p r u c e ( P i c e a s i t c h e n s i s (Bong.) C a r r . ) , a n d w e s t e r n h e m l o c k ( T s u g a h e t e r o p h y l l a ( R a f . ) S a r g . ) w e r e u s e d i n t h e p r e s e n t s t u d y . I t i s h o p e d t h a t k n o w l e d g e d e r i v e d f r o m a s t u d y o f t h i s n a t u r e w i l l p r o v i d e t h e means t o b e t t e r u n d e r s t a n d t h e r h e o l o g i c a l p r o p e r t i e s o f wood. \ LITERATURE REVIEW A l i t e r a t u r e review on creep response as a function of s t r u c t u r a l features of the woody c e l l w all, s p e c i f i c gravity and extractives content can be divided l o g i c a l l y into d i s t i n c t areas, which are described below. I. Minute Structure of Coniferous Woods Coniferous woods con s i s t of two major anatomical elements, the l o n g i t u d i n a l tracheids and the r a d i a l l y extended ray parenchyma c e l l s (wood rays) (105). The former con-s t i t u t e s up to 90 per cent of the wood volume and over 95 per cent of the wood weight (93, 105, 110, 124). In addition, d i f f e r e n t kinds of minor c e l l types may also be present, depending on species. The walls of tracheids consist of two ontogenetically d i s t i n c t structures, the primary wall and secondary w a l l . The primary wall, the outermost layer, i s formed at the time of c e l l d i v i s i o n i n the cambial zone of a tree. I t surrounds the protoplast during the surface growth phase of the plant c e l l under d i f f e r e n t i a t i o n (147) . After the surface growth has ceased, a secondary c e l l wall i s formed (Fig. 1). Anatomical elements i n wood are bonded together by an i n t e r - c e l l u l a r layer c a l l e d the middle lamella (ML ). 5 Wood c e l l w a l l s c o n s i s t o f t h r e e groups o f s t r u c t u r a l sub-stances which Wardrop (147) has c l a s s i f i e d as framework, m a t r i x and e n c r u s t i n g substances. The framework m a t e r i a l of the c e l l w a l l i s c e l l u l o s e , which i s aggregated i n the form of m i c r o f i b r i l s as developed d u r i n g w a l l f o r m a t i o n . These m i c r o f i b r i l s were p r e d i c t e d by F r e y - W y s s l i n g (28) as e a r l y as 1937 and confirmed by o t h e r s as s t r u c t u r a l u n i t s o f c e l l u l o s e . H e m i c e l l u l o s e s and o t h e r carbohydrate m a t e r i a l s , e x c l u d i n g c e l l u l o s e , are i n c o r p o r a t e d i n t o the c e l l w a l l as the m a t r i x forming substance, and are c o n s i d e r e d t o be amorphous m a t e r i a l s . L i g n i n i s the main e n c r u s t i n g substance which begins t o form w i t h the m a t r i x a f t e r some degree o f c e l l - w a l l f o r m a t i o n has taken p l a c e (147) . I t s c o n c l u s i v e c h e m i c a l a s s o c i a t i o n w i t h the framework ( c e l l u l o s e ) i s s t i l l w i d e l y debated. Thus, the c e l l w a l l can be c o n s i d e r e d as a three-phase m a t e r i a l . Ray parenchyma c e l l s are d i f f e r e n t i n t h e i r s t r u c t u r e from t r a c h e i d s and may a f f e c t .creep response o f wood due to the d i s l o c a t i o n which o c c u r s a t the c o n t a c t area between ray c e l l s and t r a c h e i d s . However, t r a c h e i d s are o n l y con-s i d e r e d i n t h i s l i t e r a t u r e review s i n c e they c o n s t i t u t e the main elements i n c o n i f e r o u s wood s t r u c t u r e . 6 I I . Physical Nature of C e l l u l o s i c C e l l Wall Structure A. C e l l Wall Organization C e l l wall organization studies are concerned 1) with the pattern of m i c r o f i b r i l o r i e n t a t i o n with respect to c e l l axes and 2) with the layering within the secondary w a l l . 1. Primary Wall (P) The primary wall consists of two kinds of m i c r o f i b r i l o r i e n t a t i o n . On i t s inner surface, the m i c r o f i b r i l s are oriented approximately i n a transverse d i r e c t i o n to the c e l l axis, but the or i e n t a t i o n d i f f e r s appreciably from t h i s on the outer surface (52, 145) (Figures 1 and 2). No lamellation has been observed by Wardrop (147) i n the primary wall layer which i s considered to be thinner (0.1 to 0.2 \i (138)) r e l a t i v e to the thickness of other layers i n the c e l l w a l l . Differences i n thickness and proportioning of primary wall layer among wood species, and between trees, t h e i r growth zones, and tissues are also expected. 2. Secondary Wall This wall i s characterized by two features: micro-scopic layering and submicroscopic lamellation (147)(Figures 1 and 2). Its layering feature was observed by Bailey and Kerr (4), who interpreted the o p t i c a l heterogeneity of the c e l l wall as being due to a d i f f e r i n g m i c r o f i b r i l l a r 7 arrangement i n the various layers. These layers are regarded by them as a thin outer layer (SI), a broad middle layer (S2) and a t h i n inner layer (S3) which may be absent i n some genera such as Picea (147). These layers also d i f f e r i n thickness. Timell (138) has reported values of 0.1 to 0.3y, 1.0 to 5.0y, and O.ly for the thickness of SI, S2 and S3, respectively. *The d i f f e r e n t m i c r o f i b r i l orientations i n tracheid wall layers have been confirmed on the basis of p o l a r i z a t i o n microscopy (154), l i g h t microscopy (156) , X-ray d i f f r a c t i o n analysis (113) and electron microscopy (83, 147). M i c r o f i b r i l s i n the SI layer are oriented at a r e l a t i v e l y large angle (close to 45°) with the l o n g i t u d i n a l c e l l axis (146). Lamellation of the SI layer was also indicated by the aid of electron microscopy. I t consists of a few lamellae of a l t e r n a t i n g S and Z h e l i c a l o r i e n t a t i o n ; the number of these lamellae i s d i f f e r e n t from one species to the other (23, 27, 52, 82, 146). However, further studies have shown that there may or may not be intermediate micro-f i b r i l o r i e n t a t i o n (S12) between SI and S2 layers (34, 147) (Fig. 2B). The middle layer of the secondary wall (S2) accounts for as much as 80 to 95 per cent of the tracheid wall volume, while the primary c e l l wall and SI layer account for only 5 to 15 per cent of the c e l l wall volume (59). Therefore, the S2 layer has been the one most thoroughly studied. In 8 t h i s l a y e r , t h e m i c r o f i b r i l s a r e a r r a n g e d a t a s t e e p ( s m a l l ) a n g l e t o t h e l o n g i t u d i n a l c e l l a x i s , u s u a l l y n o t i n e x c e s s o f a b o u t 30° (76), d e p e n d i n g t o some d e g r e e o n t h e s p e c i e s . F u r t h e r m o r e , t h e s e m i c r o f i b r i l s a r e a r r a n g e d i n n u m e r o u s c o n c e n t r i c l a m e l l a e m a k i n g a l m o s t t h e same a n g l e w i t h t h e l o n g i t u d i n a l c e l l a x i s (147) and e x h i b i t i n g , t h e r e -f o r e , a h i g h d e g r e e o f p a r a l l e l i s m (52) ( F i g u r e s 1 a n d 2). The m i c r o f i b r i l s i n t h e i n n e r l a y e r o f t h e s e c o n d a r y w a l l , S3, a r e a r r a n g e d a t q u i t e a l a r g e , b u t v a r i a b l e a n g l e t o t h e l o n g i t u d i n a l c e l l a x i s (19/ 34). I t i s r e -p o r t e d t h a t t h i s a n g l e r a n g e d f r o m 70 t o 90° (152). L a m e l l a t i o n o f t h e S3 l a y e r , w i t h a l t e r n a t i n g S o r Z h e l i c a l m i c r o f i b r i l l a r o r i e n t a t i o n , was a l s o o b s e r v e d (147). An i n t e r m e d i a t e m i c r o f i b r i l a r r a n g e m e n t (S23) h a s b e e n f o u n d b e t w e e n S2 a n d S3 l a y e r s b y H a r a d a e t a l . (34) ( F i g . 2B). B e s i d e s t h e a f o r e m e n t i o n e d l a y e r s , some s p e c i e s h a v e a n i n n e r m o s t l a y e r c a l l e d t h e w a r t y l a y e r (T) (75). I t may c o n s i s t o f t h e r e m a i n s o f c y t o p l a s m i c c o n t e n t s o f t h e c e l l w h i c h w e r e d e p o s i t e d on t h e i n n e r w a l l a t t h e c o n c l u s i o n o f w a l l d e v e l o p m e n t . H o w e v e r , e x i s t e n c e o f t h i s l a y e r i s s t i l l a c o n t r o v e r s i a l i s s u e i n t h e l i t e r a t u r e . F o r g a c s (26), f o r e x a m p l e , u s e d t h e w a r t y l a y e r t o r e f e r t o t h e l a y e r t h a t was c o n s i d e r e d b y t h e o t h e r s a s S3, w h i l e H a r a d a e t a l . (34) u s e d i t t o r e p r e s e n t a l a y e r a d d i t i o n a l t o SI, S2 and S3. I t s h o u l d a l s o be n o t e d t h a t l o c a l d e v i a t i o n s o c c u r i n t h e a f o r e m e n t i o n e d m i c r o f i b r i l o r g a n i z a t i o n due t o p i t c a v i t i e s i n t h e c e l l w a l l (34). 9 I n t h e c a s e o f c o m p r e s s i o n wood t r a c h e i d s (wood f o r m e d o n t h e l o w e r s i d e o f l e a n i n g s t e m s o f c o n i f e r s ) , t h e s e c o n d a r y c e l l w a l l l a y e r s a r e r e d u c e d t o t h e SI a n d a m o d i f i e d S2 l a y e r (16). T h i s m o d i f i e d S2 l a y e r h a s m i c r o -f i b r i l s o r i e n t e d a t an a n g l e o f a b o u t 45° t o t h e l o n g i t u d i n a l c e l l a x i s (16), w h i c h i s l a r g e r t h a n t h a t f o u n d i n t h e S2 l a y e r o f n o r m a l wood. I t i s a l s o c h a r a c t e r i z e d b y t h e p r e s e n c e o f an e x t r a l a y e r o f l i g n i n b e t w e e n SI a n d S2 (138). F u r t h e r m o r e , C o t e a nd Day (16) h a v e r e p o r t e d t h a t t h e SI l a y e r o f c o m p r e s s i o n wood t r a c h e i d s i s t h i c k e r t h a n t h a t i n n o r m a l wood. A n o t h e r u l t r a s t r u c t u r a l f e a t u r e o f c o m p r e s s i o n wood was o b s e r v e d b y W a r d r o p a n d D a v i e s (153), who i n d i c a t e d t h e p r e s e n c e o f t h e s o - c a l l e d ' h e l i c a l c h e c k s ' w h i c h f o l l o w t h e m i c r o f i b r i l o r i e n t a t i o n i n t h e S2 l a y e r . C o t e a n d Day (16) a g r e e d w i t h W a r d r o p a n d D a v i e s o n t h e r e a l i t y o f t h e s e c h e c k s w h i c h p r e s u m a b l y o r i g i n a t e d u r i n g t h e f o r m a t i o n o f t h e S2 l a y e r a n d w h i c h a r e n o t c a u s e d b y d r y i n g o f t h e c e l l w a l l . The c h e m i c a l c o m p o s i t i o n o f c o m p r e s s i o n wood i s a l s o d i f f e r e n t f r o m t h a t o f n o r m a l wood. I n g e n e r a l , t h e c e l l u l o s e c o n t e n t i s l o w e r , a s i s t h e amount o f galactoglu-» comannan, w h e r e a s g a l a c t a n c o n t e n t i s g r e a t e r a n d l i g n i n c o n t e n t i s c o n s i d e r a b l y h i g h e r t h a n i n n o r m a l wood (138). 10 B. Supermolecular Arrangement of Cellulose Chains Based on l i g h t microscopy studies, i t was found that tracheid walls consist l a r g e l y of a great number of filaments ( f i b r i l s ) wound h e l i c a l l y with respect to the tracheid axis. However, a f t e r the advent of electron microscopy, i t has been revealed that these f i b r i l s are aggregations of f i n e r filamentous units c a l l e d m i c r o f i b r i l s . Some research workers r e f e r to these m i c r o f i b r i l s as the basic supermolecular s t r u c t u r a l units i n the plant c e l l wall (16, 118), while others have indicated that the m i c r o f i b r i l s i n turn are composed of f i n e r or smaller elementary f i b r i l s o o (35 A by 35 A) (3, 30, 79, 91). The main component of m i c r o f i b r i l s and/or elementary f i b r i l s i s c e l l u l o s e , generally considered as being composed of 8-D-glucopyranose units linked together by a 1, 4 - g l y c o s i d i c bond (41). The actual state of hemicelluloses i n the l i v i n g tree i s l a r g e l y un-known. Examples taken from other carbohydrate containing plants and l i v i n g organisms suggest the p o s s i b i l i t y that the a b i l i t y to c r y s t a l l i z e or to form m i c r o f i b r i l s are c h a r a c t e r i s t i c s expandable to a l l native polysaccharides*. C e l l u l o s e i s p a r t l y c r y s t a l l i n e and gives an X-ray d i f f r a c t i o n diagram by which the dimensions of the c r y s t a l l o -graphic unit c e l l can be determined. Based on t h i s evidence, Meyer and Misch (86) were able to e s t a b l i s h the c r y s t a l l o -graphic dimensions for a monoclinic unit c e l l of C e l l u l o s e I. 11 T h e r e b y , e a c h u n i t c e l l c o n t a i n s f o u r g l u c o s e r e s i d u e s ( F i g . 3). The g l u c o s e r e s i d u e s a r e j o i n e d t o g e t h e r b y t h r e e k i n d s o f f o r c e s : a l o n g t h e a - a x i s b y f a i r l y s t r o n g h y d r o g e n b o n d s ; a l o n g t h e c - a x i s b y much w e a k e r v a n d e r W a a l ' s f o r c e s ; a n d a l o n g t h e b - a x i s , i . e . , a l o n g t h e c e l l u l o s e c h a i n , b y t h e 3-1/ 4 - g l u c o s i d i c b o n d s (70). C o n s e q u e n t l y , t h e c e l l u l o s e l a t t i c e i s b o t h a c h a i n l a t t i c e a n d a l a y e r l a t t i c e . A c o n v e n i e n t way o f e x p r e s s i n g t h e l e n g t h o f t h e c h a i n m o l e c u l e s i n c e l l u l o s i c f i b e r s i s b y u s i n g t h e t e r m ' d e g r e e o f p o l y m e r i z a t i o n ( D P ) ' , w h i c h r e p r e s e n t s t h e number o f g l u c o s e r e s i d u e s i n t h e c h a i n m o l e c u l e . The s u p e r m o l e c u l a r a r r a n g e m e n t o f t h e p o l y m e r c h a i n s w i t h i n t h e w a l l s t r u c t u r e h a s b e e n t h e s u b j e c t o f many m o d e l s . T h e r e i s now a v a s t l i t e r a t u r e on t h e s u b j e c t w i t h some d i v e r g e n c e o f o p i n i o n among e x p e r t s i n t h e f i e l d . The m o s t w i d e l y u s e d m o d e l s a r e d e s c r i b e d b e l o w ( F i g u r e s 4 t o 7). 1. F r i n g e d M i c e l l a r M o d e l The f r i n g e d m i c e l l a r m o d e l was p r o p o s e d b y H e r r m a n n e t a l . (42) e a r l y i n 1930 f o r c e l l w a l l s , a n d f o r . r e g e n e r a t e d c e l l u l o s e s b y many o t h e r s s u c h a s Hermans (41) and M a r k (80). B a s e d o n X - r a y d i f f r a c t i o n s t u d i e s , H e n g s t e n b e r g a nd M a r k (40) s u g g e s t e d t h a t c r y s t a l l i n e r e g i o n s i n n a t u r a l c e l l u l o s e o o f i b e r s h a d d i m e n s i o n s o f 50 A b y 500 A. S i n c e t h e m o l e c u l a r o l e n g t h was much l o n g e r t h a n 500 A, i t was p r o p o s e d , a c c o r d -i n g t o t h e f r i n g e d m i c e l l a r m o d e l , t h a t t h e m o l e c u l e s p a s s 12 a l t e r n a t e l y through many c r y s t a l l i n e and n o n - c r y s t a l l i n e r e g i o n s ( F i g . 4 ) . In a d d i t i o n , i t was assumed t h a t the molecules are e s s e n t i a l l y f u l l y extended c h a i n s (36) . Many mechanical and p h y s i c a l p r o p e r t i e s o f polymer b e h a v i o r were e x p l a i n e d on the b a s i s o f t h i s model i n s p i t e o f some di v e r g e n c e s o f view which appeared p a r t i c u l a r l y i n the e a r l y days and a l s o r e c e n t l y (134). 2. F r i n g e d F i b r i l l a r Model E l e c t r o n microscopy s t u d i e s have brought evidence f o r the e x i s t e n c e o f f i n e f i b r i l s w i t h widths of the o r d e r o of 100 A. Since i t i s i m p o s s i b l e t o f i t f i b r i l s o f t h i s s i z e i n s i d e f i b r i l s o f the f r i n g e d m i c e l l e s t r u c t u r e , H e a r l e (35, 37) proposed the f r i n g e d f i b r i l l a r model. A c c o r d i n g t o t h i s model, the f i b r i l s are assumed t o be l o n g , i m p e r f e c t c r y s t a l s winding t h e i r way through the f i b e r s t r u c t u r e and sep a r a t e d by n o n - c r y s t a l l i n e r e g i o n s ( F i g . 5 ) . The l o n g -c h a i n molecules are a l s o assumed t o pass a l t e r n a t e l y through c r y s t a l l i n e and n o n - c r y s t a l l i n e r e g i o n s , g i v i n g r i s e t o a continuous network s t r u c t u r e . The main d i f f e r e n c e between the two models, f r i n g e d f i b r i l and f r i n g e d m i c e l l e , i s t h a t i n the l a t t e r the n o n - c r y s t a l l i n e (amorphous) and c r y s t a l l i n e r e g i o n s a l t e r n a t e along the f i b r i l l a r a x i s , whereas i n the former the n o n - c r y s t a l l i n e r e g i o n s are l o c a t e d a l o n g the p e r i p h e r y o f the f i b r i l . J e n t z e n (60) has found t h a t the c a l c u l a t e d Young's modulus o f d e l i g n i f i e d l o n g l e a f pine (Pinus p a l u s t r i s M i l l . ) 13 tracheids agreed with the established t h e o r e t i c a l value for p e r f e c t l y oriented c r y s t a l l i n e c e l l u l o s e . Therefore, he suggested that the fringed f i b r i l l a r model i s a better representation of tracheid structure than the fringed micellar structure. This model also received the support of Tripp et aJL. (140) , and -Rebenfeld (123) for the structure of cotton c e l l u l o s e . But i t has been c r i t i c i z e d by Michie e_t a l . (89), due to the i n a b i l i t y of the model to explain the observed mechanical properties of regenerated c e l l u l o s e . 3. Continuous Model o This model was suggested by Hess e_t al_. (43) , Ranby (122), and Stuart (133). According to t h e i r model, the c r y s t a l l i n e l a t t i c e i n c e l l u l o s e f i b e r s i s continuous through-out the structure, although i t includes regions of lower order which occur at i n t e r v a l s along the length, rather than the width of the m i c r o f i b r i l . Two such proposals are shown in Fig.6. 4. Folded Chain Model In the abovementioned supermolecular organization of c e l l u l o s e chains, i t has been assumed that the molecules are e s s e n t i a l l y f u l l y extended chains. Recently there has been increasing evidence from u l t r a s t r u c t u r a l studies that t h i s organization i s an o v e r - s i m p l i f i c a t i o n . Consequently, i t has been suggested that the molecules i n single polymer c r y s t a l s are folded back and f o r t h i n layers. Since 1960, 14 a t l e a s t f i v e f o l d e d c o n f i g u r a t i o n s h a v e b e e n p r o p o s e d f o r n a t i v e c e l l u l o s e (3, 7, 21, 79, 139). A c c o r d i n g t o M a n l e y ' s m o d e l (79) , t h e m o l e c u l e s f o r m f l a t r i b b o n s by f o l d i n g b a c k a n d f o r t h , a n d t h e m o l e c u l a r c h a i n a x i s i s i n c l i n e d a t some p r e f e r r e d a n g l e t o t h e r i b b o n a x i s ( F i g . 7b). I f t h e r i b b o n i s wound as a h e l i x , t h e m o l e c u l a r c h a i n a x i s becomes p a r a l l e l t o t h e f i b r i l a x i s ( F i g . 7a). T h i s m o d e l was r e c e n t l y s u p p o r t e d b y S u l l i v a n (134) f o r c e l l u l o s e s f r o m s i x p i n e s and two p o r e d woods. Asunmaa (3) c r i t i c i z e d M a n l e y ' s m o d e l b a s e d on t h e c o n c e p t t h a t t h i s m o d e l p r o d u c e s a l o w e r a p p a r e n t d e n s i t y f o r t h e c e l l u l o s e i n a m i c r o f i b r i l t h a n t h a t m e a s u r e d f o r c r y s t a l l i n e c e l l u l o s e . M a r k (81) h a s a l s o c r i t i c i z e d t h e f o l d e d c h a i n m o d e l b e c a u s e o f i t s i n a b i l i t y t o e x p l a i n t h e l o w e x t e n s i b i l i t y , v e r y h i g h s t r e n g t h , a n d l a c k o f g l a s s t r a n s i t i o n " * " b e h a v i o r t y p i c a l o f s y n t h e t i c p o l y m e r s . I t m u s t be i n d i c a t e d t h a t m i c r o f i b r i l s c omposed o f f r i n g e d m i c e l l e s , f r i n g e d f i b r i l s , c o n t i n u o u s o r f o l d e d c h a i n c o n f i g u r a t i o n , w o u l d a l l h a v e d i f f e r e n t s t r e n g t h s and e l a s t i c p r o p e r t i e s f r o m one a n o t h e r (36, 81). T h e r e f o r e , b y s t u d y i n g t h e s t r e n g t h and e l a s t i c p r o p e r t i e s o f wood i n a n y f o r m o r d i r e c t i o n w o u l d , i n f a c t , o n l y r e f l e c t i t s u l t r a s t r u c t u r e . "*"Glass t r a n s i t i o n i s d e s c r i b e d a s t h e n a r r o w r e g i o n on t h e t e m p e r a t u r e s c a l e w h e r e t h e t h e r m a l e x p a n s i o n c o e f f i c -i e n t u n d e r g o e s a d i s c o n t i n u i t y and b e l o w w h i c h c o n f i g u r a t i o n a l r e - a r r a n g e m e n t o f p o l y m e r c h a i n b a c k b o n e s , i f t h e y o c c u r a t a l l , a r e e x t r e m e l y s l o w (1). C. C e l l Wall C r y s t a l l i n i t y 1. Concept Extensive studies over a period of years have led to the acceptance of the hypothesis that c e l l u l o s e f i b e r and wood tracheids consist of both ordered or c r y s t a l l i n e and disordered or amorphous regions. However, a proper d e f i n i t i o n of c r y s t a l l i n i t y has not been established. Those who deal with c r y s t a l l i n i t y from physical points of view define the ' c r y s t a l l i n e region' as the portion which i s i n a state of perfect, three dimensional order and which gives r i s e to s e l e c t i v e X-ray d i f f r a c t i o n patterns (41, 143). On the other hand, those who approach the fine structure of f i b e r s by chemical means consider the c r y s t a l l i n e region as the portion having extreme resistance to chemical treatments, as compared with the accessible amorphous region (41). I t should be pointed out that there i s no sharply defined borderline which can be drawn between the two states of a c e l l u l o s i c f i b e r , amorphous and c r y s t a l l i n e regions. Rather, the d i s t i n c t i o n i s made approximate (41). Furthermore, none of the physical or chemical methods a c t u a l l y measures l a t e r a l order or c r y s t a l l i n i t y d i s t r i b u t i o n of the sample, but they do rank various c e l l u l o s e materials according to t h e i r degree of c r y s t a l l i n i t y and they are thus useful for comparative purposes. Mark (81) has recently questioned the r e a l i t y of c r y s t a l l i n i t y measurements based on the idea that the presence 16 of voids on the surface of c r y s t a l l i t e s contributes also to X-ray scattering. Consequently he did not place much f a i t h i n r e l a t i n g mechanical behavior of f i b e r s to c r y s t a l l i n i t y . However, one cannot ignore t h i s parameter because i t i s one of the most important molecular c h a r a c t e r i s t i c s of c e l l u l o s i c f i b e r structure. 2. Relation to Mechanical Properties A large volume of work has been done to r e l a t e mechan-i c a l properties of natural t e x t i l e f i b e r s to t h e i r c r y s t a l l i n i t y . E a r l y i n 19 45 Conrad and Scroggie (15) found that elongation of regenerated c e l l u l o s e yarns increased with a c c e s s i b i l i t y , 2 which i s rel a t e d inversely to c r y s t a l l i n i t y number. Tripp et a l . (141) and Ward (143) also indicated that high strength and low e x t e n s i b i l i t y were associated with a high percentage of c r y s t a l l i n i t y . In an attempt to r e l a t e c r y s t a l l i n i t y of cotton f i b e r s to t e n s i l e strength properties, Krassig and Kitchen (72) used an approach s i m i l a r to that used for regenerated c e l l u l o s e . They assumed numbers of l i n k s between fringed 2 +. n • •*. u Z l ~ I m c r y s t a l l i n i t y number = -where: 1^ = average i n t e n s i t y of 101 interference, and I M = average i n t e n s i t y of minimum between 101 and the 101 interference. 17 m i c e l l e s . Such a study was c r i t i c i z e d by Mark (81) who stated that t h i s approach i s unsound from the standpoint of t h e o r e t i c a l mechanics. Unfortunately, he d i d not give reasons for his disagreement on t h e i r approach. The studies of amine d e c r y s t a l l i z a t i o n treatment c a r r i e d out by Orr e_t al_. (102) , Parker (106) , Segal e_t a l . (127), and Ward (143) indicate that the ultimate elongation increases but the strength may increase or decrease s l i g h t l y due to d e c r y s t a l l i z a t i o n . Parker also found that t h i s treatment caused abrupt decreases i n e l a s t i c modulus and zero-span t e n s i l e strength of paper handsheets. However, the f i r s t creep (creep of specimens which have not been previously subjected to external loading) increased sharply, while an increase i n r e c o v e r a b i l i t y of t e s t specimens was observed. Mark (80) indicated that e l a s t i c i t y and tenacity are f u n c t i o n a l l y r e l a t e d to the amount of c r y s t a l l i n e material, while swelling, drying and chemical reactions are to be associated with the amorphous parts. From the t h e o r e t i c a l point of view, the c r y s t a l l i t e forms the firm r e i n f o r c i n g parts of the structure, whereas the amorphous regions are the actual points of weakness (80, 100). The change of c r y s t a l l i n i t y when c e l l u l o s i c f i b e r s are stretched was also a subject for i n v e s t i g a t i o n . This was based on the hypothesis that c e l l u l o s i c f i b e r s undergo a process during stretching s i m i l a r to that experienced with natural rubber samples. Ce l l u l o s e suffers a p a r t i a l 18 c o n v e r s i o n o f t h e amorphous i n t o t h e c r y s t a l l i z e d s t a t e . The o n l y d i f f e r e n c e i s t h a t t h i s c h a n g e i s n o t a s r e v e r s i b l e a s w i t h r u b b e r a n d v e r y much l e s s p r o n o u n c e d (80), a s was o b s e r v e d b y S i s s o n (130). B e r k l e y a n d K e r r (8) i n v e s t i g a t e d t h e s t r u c t u r e o f u n d r i e d , f r e s h c o t t o n f i b e r w i t h X - r a y s , u s i n g p h o t o g r a p h i c m e t h o d s . X - r a y p a t t e r n s t a k e n f o r t h e s e f i b e r s showed l i t t l e o r no e v i d e n c e o f c r y s t a l l i n i t y , w h e r e a s u pon d r y i n g t h e u s u a l d i f f r a c t i o n d i a g r a m o f c e l l u l o s e a p p e a r e d . I f , b e f o r e d r y i n g , t h e c o t t o n f i b e r s h a d a l s o b e e n s t r e t c h e d i n t h e we t s t a t e , a f a i n t c e l l u l o s e d i f f r a c t i o n d i a g r a m a p p e a r e d . The d i f f r a c t i o n d i a g r a m r e m a i n e d p r e s e n t e v e n a f t e r a d r i e d b u n d l e o f c o t t o n was r e w e t t e d . The a u t h o r s i n t e r p r e t e d t h e s e f i n d i n g s t o i n d i c a t e t h a t u n d r i e d c o t t o n f i b e r , o f t h e a g e s w h i c h t h e y s t u d i e d , o c c u r s o n l y w i t h a n a m o r p h o u s s t r u c t u r e . T h e i r m o d e l o f u n d r i e d c o t t o n f i b e r c o n t a i n e d s e t s o f l o n g - c h a i n c e l l u l o s e m o l e c u l e s w h i c h , u p o n d r y i n g , u n d e r g o i n t e r n a l r o t a t i o n , b r i n g i n g a d j a c e n t h y d r o x y l g r o u p s i n t o c l o s e a n d p a r t i a l l y u n s e p a r a b l e p r o x i m i t y . The c h a i n s t h e n u n i t e a t i n t e r v a l s t o f o r m a p e r m a n e n t c r y s t a l l i n e s t r u c t u r e , l e a v i n g u n o r d e r e d m a t e r i a l b e t w e e n t h e c r y s t a l l i n e d o m a i n s . P r e s t o n (116) d i d n o t c o n s i d e r B e r k l e y a n d K e r r ' s r e s u l t s a s f i n a l , s i n c e t h e a b s e n c e o f a n X - r a y d i a g r a m d o e s n o t n e c e s s a r i l y i m p l y t h e a b s e n c e o f c r y s t a l l i n e r e g i o n s . T h e s e r e g i o n s may be s m a l l i n s i z e a n d , i n a d d i t i o n , t h e 19 c e l l u l o s e diagram of the fresh material may be masked by the water halos. But i t s t i l l remained to be explained why, i n the rewetted f i b e r , the water does not mask the c e l l u l o s e pattern. Quantitative X-ray d i f f r a c t i o n methods have been used by Ingersoll (57) to study the l a t e r a l ordering and the o r i e n t a t i o n of the c e l l u l o s e chains around the long chain axes i n various commercial and experimental rayons. His r e s u l t s showed that breaking length increased with l a t e r a l disordering of the c e l l u l o s e chains when the other factor was constant. He supports the concept that l o c a l chain disordering i n any l i n e a r polymer favors higher e x t e n s i b i l -i t y . In addition, he stated that i t i s important to make correctio n for the difference i n o r i e n t a t i o n , i f samples d i f f e r i n g widely i n t h i s factor are to be compared with regard to t h e i r l a t e r a l order. This l a s t concept i s also supported by Ward (143), who noted the d i f f i c u l t y i n obtaining a seri e s of f i b e r s which d i f f e r only i n t h e i r percentage of c r y s t a l l i n e material. The c r y s t a l l i n i t y of c e l l u l o s e i n the undried cotton f i b e r , fresh from the unopened but mature b o l l , was studied by Heyn (45) using X-ray diffractometer methods. I t was compared with the c r y s t a l l i n i t y of portions of f i b e r s from the same samples, e i t h e r dried d i r e c t l y or from which water had been removed by various i n d i r e c t ways. He found that stretching of the f i b e r i n water before drying resulted i n s l i g h t l y h i g h e r d i f f r a c t i o n p e a k s i n t h e d r i e d s a m p l e s a s c o m p a r e d t o u n s t r e t c h e d s a m p l e s , r e g a r d l e s s o f w h a t t h e s u b s e q u e n t p r e p a r a t i o n h a d b e e n . I t was a l s o e s t i m a t e d by N i c k e r s o n (100) t h a t a n o r m a l c o a g u l a t e d v i s c o s e f i l a m e n t h a d a b o u t 40 p e r c e n t c r y s t a l l i n e a n d 60 p e r c e n t amorphous m a t e r i a l , w h e r e a s s t r e t c h e d f i l a m e n t s o f t h e same m a t e r i a l c o n t a i n e d a b o u t 70 p e r c e n t c r y s t a l l i n e a n d 30 p e r c e n t a m o r p h o u s m a t e r i a l . T h i s i n d i c a t e s t h e e f f e c t o f s t r e t c h i n g o n c r y s t a l l i n i t y ; h o w e v e r , P a t i l e t a _ l . (107) i n d i c a t e d t h a t s t r e t c h i n g o f s w o l l e n f i b e r s " h a s n e x t t o no e f f e c t o n c r y s t a l l i n i t y o r c r y s t a l l i t e s i z e . " U n t i l a f e w y e a r s a g o , t h e q u e s t i o n o f c r y s t a l l i n i t y a s a f a c t o r i n f l u e n c i n g t h e m e c h a n i c a l b e h a v i o r o f wood was i g n o r e d . T h i s was p r o b a b l y due t o t h e d i f f i c u l t i e s i n -v o l v e d i n d e v e l o p i n g a t e c h n i q u e s u i t a b l e f o r d e t e r m i n i n g c r y s t a l l i n i t y i n wood and a l s o t h e v a r i a b i l i t y i n h e r e n t i n wood s t r u c t u r e . K o u r i s e t a _ l . (71) e x a m i n e d t h e e f f e c t o f v a r i o u s modes o f d r y i n g u p o n c r y s t a l l i n i t y o f s o f t w o o d s , u s i n g X - r a y d i f f r a c t i o n . T h e y d i d n o t o b s e r v e a n y c h a n g e i n c r y s t a l l i n i t y i n d e x when p r e v i o u s l y u n d r i e d , p u r e c e l l u l o s e f i b e r s w e r e d r i e d f r o m w a t e r u n d e r a w i d e r a n g e o f c o n d i t i o n s . On t h e o t h e r h a n d , t h e y o b t a i n e d l o w e r v a l u e s when t h e f i b e r s w e r e d r i e d f r o m b e n z e n e . C r y s t a l l i n i t y o f e a r l y w o o d a n d l a t e w o o d o f a l o n g -l e a f p i n e h o l o c e l l u l o s e p u l p was d e t e r m i n e d b y J e n t z e n (6 0) a s f i b e r s w e r e d r i e d u n d e r v a r i o u s a x i a l t e n s i l e l o a d s . 21 Although he observed an i n c r e a s e i n Young's modulus, t e n s i l e s t r e n g t h , work to r u p t u r e , and c r y s t a l l i t e o r i e n t a t i o n , he found a decrease i n u l t i m a t e e l o n g a t i o n o f i n d i v i d u a l f i b e r s , and no s i g n i f i c a n t change i n c r y s t a l l i n i t y . A s i m i l a r study c a r r i e d out by H i l l (47) on the same s p e c i e s supported Jentzen's f i n d i n g . I t should be p o i n t e d out t h a t these r e s u l t s are not i n agreement w i t h the observed change i n c r y s t a l l i n i t y of c o t t o n and r e g e n e r a t e d c e l l u l o s e f i b e r s f o l l o w i n g s t r e t c h i n g (8, 45, 57). The f i r s t study r e l a t i n g c e l l w a l l c r y s t a l l i n i t y d i r e c t l y t o mechanical b e h a v i o r o f wood was conducted by Murphey (92). A c c o r d i n g t o him, the c r y s t a l l i n i t y o f samples taken from y e l l o w b i r c h ( B e t u l a a l l e g h a n i e n s i s B r i t t o n ) and sugar maple (Acer saccharum Marsh.) i n c r e a s e d at a d e c r e a s i n g r a t e as a p p l i e d l o a d i n c r e a s e d . Although the c r y s t a l l i n i t y was c o n s t a n t over a 24 hour p e r i o d , i t d i d not r e t u r n t o the o r i g i n a l l e v e l a f t e r removal o f the a p p l i e d l o a d . Thus, upon a p p l i c a t i o n of t e n s i l e s t r e s s , some i n c r e a s e i n m o l e c u l a r l a t t i c e p e r f e c t i o n appears t o take p l a c e c a u s i n g an i n c r e a s e i n c r y s t a l l i n i t y . P a r t o f t h i s observed i n c r e a s e i s r e t a i n e d a f t e r removal o f the a p p l i e d s t r e s s . Murphey compared h i s r e s u l t s w i t h those of p u b l i s h e d r h e o l o g i c a l s t u d i e s o f s t r a i n v ersus the time e l a p s e d a f t e r l o a d i n g o r u n l o a d i n g . He concluded t h a t e l a s t i c s t r a i n as measured by X-ray d i f f r a c t i o n remained unchanged over a 24 hour p e r i o d , w h e r e a s a c o n t i n u e d c h a n g e i n s t r a i n o c c u r r e d d u r i n g t h e same p e r i o d . T h i s i n d i c a t e d t o h i m t h a t c o n t i n u e d d e f o r m a t i o n u n d e r l o a d , a f t e r i n i t i a l s t r a i n , i s v i s c o u s i n n a t u r e . F u r t h e r m o r e , t h e o b s e r v e d i m m e d i a t e r e c o v e r y u p o n r e m o v a l o f t h e l o a d was a n e l a s t i c r e s p o n s e . M u r p h e y i n t e r p r e t e d t h e r e s i d u a l d e f o r m a t i o n t o be due t o t h e b o n d s f o r m e d t h r o u g h t e n s i l e l o a d i n g w h i c h t h u s r e s t r i c t t h e r e t u r n o f t h e m o l e c u l e s t o t h e i r n o n - s t r e s s e d c o n d i t i o n . R e c e n t l y , Z i e g l e r (161) s u p p o r t e d M u r p h e y ' s r e s u l t s u s i n g D o u g l a s - f i r ( P s e u d o t s u g a m e n z i e s i i ( M i r b . ) F r a n c o ) and n o t e d t h a t t e n s i l e l o a d s r e s u l t e d i n i n c r e a s i n g c r y s t a l l i n i t y a n d d e f o r m a t i o n . The a c t u a l e x t e n t o f i n c r e a s e d c r y s t a l l i n i t y was n o t p u b l i s h e d . The o n l y c r i t i c i s m . t h e w r i t e r h a s o f M u r p h e y ' s w o r k i s t h e u n c e r t a i n t y o f c r y s t a l l i n i t y d e t e r m i n a t i o n w i t h t h e t y p e o f s p e c i m e n he u s e d . He u t i l i z e d s a m p l e s c u t o n t h e m i c r o t o m e a t 45° f r o m t h e r a d i a l d i r e c t i o n . T h e s e s a m p l e s h a v e , o f c o u r s e , p r e f e r r e d o r i e n t a t i o n , w h e r e a s a r a n d o m l y o r i e n t e d s p e c i m e n i s recommended f o r X - r a y d i f f r a c t i o n w o r k . T h e r e f o r e , h i s v a l u e s a r e n o t c o n s i d e r e d t o be p r e c i s e . 3. V a r i a t i o n i n Wood a n d T r a c h e i d s A f e w s t u d i e s h a v e b e e n c a r r i e d o u t by v a r i o u s w o r k e r s on v a r i a t i o n o f c r y s t a l l i n i t y i n wood and t r a c h e i d s . P r e s t o n e t a l . (12 0) c o m p a r e d t h e c r y s t a l l i n i t y o f C r o s s a n d B e v a n c e l l u l o s e t a k e n f r o m r a d i a t a p i n e ( P i n u s r a d i a t a D. Don) 23 o f d i f f e r e n t a g e s . T h e i r r e s u l t s r e v e a l e d t h a t c r y s t a l l i n i t y d e c r e a s e d f r o m p i t h t o b a r k . L e e (73) s t u d i e d t h e same r e l a t i o n s h i p , u s i n g two p u l p s p r e p a r e d f r o m w e s t e r n h e m l o c k ( T s u g a h e t e r o p h y l l a ( R a f . ) S a r g . ) b y p e r a c e t i c a c i d a n d c h l o r i t e m e t h o d , b u t f o u n d t h a t c r y s t a l l i n i t y i n c r e a s e d s i g n i f i c a n t l y f r o m p i t h t o a b o u t 15 i n c r e m e n t s f r o m p i t h , a f t e r w h i c h i t became a l m o s t c o n s t a n t . He f u r t h e r i n d i c a t e d t h a t c r y s t a l l i n i t y o f d e l i g n i f i e d t r a c h e i d s k e l e t o n s f r o m l a t e w o o d was s i g n i f i c a n t l y h i g h e r t h a n t h a t f r o m e a r l y w o o d . S i m i l a r r e s u l t s w e r e a l s o o b t a i n e d b y T a n i g u c h i (136), who f o u n d t h a t t h e t o t a l c r y s t a l l i n i t y c o n t e n t o f A k a m a t s u ( P i n u s d e n s i f l o r a S i e b . e t Zucc.) , a s d e t e r m i n e d b y a n a c i d h y d r o l y s i s m e t h o d , i n c r e a s e d s l o w l y w i t h age i n t h e e a r l y s t a g e o f g r o w t h . An X - r a y d i f f r a c t i o n s t u d y was c o n d u c t e d b y H o l z e r a n d L e w i s (53) o n D o u g l a s - f i r e a r l y w o o d a n d l a t e w o o d t r a c h e i d s k e l e t o n s . The l a t t e r e x h i b i t e d a n u n u s u a l l y h i g h d e g r e e o f p r e f e r r e d o r i e n t a t i o n among t h e c r y s t a l l i t e s , w h e r e a s t h e f i b e r s o f t h e f o r m e r w e r e f o u n d t o be much more a m o r p h o u s . U n f o r t u n a t e l y , t h e s e w o r k e r s d i d n o t g i v e n u m e r i c a l v a l u e s . L i n d g r e n (77) h a s a l s o p o i n t e d o u t t h a t S w e d i s h s p r u c e ( P i c e a e x c e l s a L i n k . ) l a t e w o o d g a v e a s h a r p e r d i a g r a m , w h i c h i n d i c a t e d a h i g h e r d e g r e e o f c r y s t a l l i n e o r d e r t h a n t h e c o r r e s p o n d i n g e a r l y w o o d . I n a d d i t i o n , d e l i g n i f i e d l o n g l e a f p i n e l a t e w o o d t r a c h e i d s e x h i b i t g r e a t e r c r y s t a l l i n i t y t h a n e a r l y w o o d t r a c h e i d s (60). 24 The m i c r o s c o p i c a n d s u b m i c r o s c o p i c s t r u c t u r e o f t e n s i o n wood h a s b e e n i n v e s t i g a t e d t h o r o u g h l y b y W a r d r o p (148), W a r d r o p and D a d s w e l l (150, 151), u s i n g s e v e r a l A u s t r a l i a n s p e c i e s . I t i s r e p o r t e d t h a t d e g r e e o f c r y s t a l l i n -i t y i n t e n s i o n wood i s h i g h e r t h a n i n n o r m a l wood (150, 151). T h e s e w o r k e r s a t t r i b u t e d t h e d i f f e r e n c e t o t h e e x i s t e n c e o f t h e i n n e r t h i c k g e l a t i n o u s l a y e r w h i c h c o n t a i n s a h i g h l y c r y s t a l l i n e c e l l u l o s e , p r o v i d e d t h a t t h e p a r a c r y s t a l l i n e ( a m o r p h o u s ) p h a s e i s s i m i l a r i n b o t h n o r m a l a n d t e n s i o n wood. L e e (73) f o u n d s i m i l a r r e s u l t s f o r h i s b l a c k C o t t o n w o o d ( P o p u l u s t r i c h o c a r p a T o r r , a n d G r a y ) p u l p s . I n c o n t r a s t , c r y s t a l l i n i t y o f D o u g l a s f i r c o m p r e s s i o n wood was c o n s i d e r a b l y l o w e r t h a n t h a t o f n o r m a l wood (73). D. M i c r o f i b r i l A n g l e 1. D e f i n i t i o n T h e r e i s g e n e r a l a g r e e m e n t among r e s e a r c h w o r k e r s t h a t m i c r o f i b r i l s i n c e l l w a l l l a y e r s a r e o r i e n t e d a t a n a n g l e t o t h e c e l l a x i s . S i n c e t h e S2 l a y e r c o n s t i t u t e s t h e b u l k o f t h e c e l l w a l l , t h e c o m p o s i t e m i c r o f i b r i l a n g l e i s u s u a l l y r e f e r r e d t o a s t h a t w h i c h t h e m i c r o f i b r i l s i n t h e S2 l a y e r make w i t h t h e c e l l a x i s . S e v e r a l t e r m s a r e u s e d t o d e s c r i b e t h i s k i n d o f o r i e n t a t i o n ; f o r e x a m p l e , W a r d r o p a n d P r e s t o n (155) u s e d t h e t e r m m i c e l l a r a n g l e , w h i c h h a s b e e n c r i t i c i z e d b y M a r k (81) b e c a u s e t h e o r i e n t a t i o n f o r m s a h e l i x , not a s p i r a l and the term micelle implies an 'archaic' concept. X-ray angle i s also used by Weiss et a_l. (157) to describe the o r i e n t a t i o n of the m i c r o f i b r i l s about the c e l l axis. This angle i s determined by X-ray technique, which i s considered by Mark (81) not to be a very accurate method for measurement of m i c r o f i b r i l angle, probably because the X-ray d i f f r a c t i o n diagram has to be interpreted before the angle can be determined. The term convolution angle i s u t i l i z e d by Betrabet and Iyengar (9) as an alterna-t i v e for m i c r o f i b r i l o r i e n t a t i o n . Recently, Mark (81) applied the standard i n d u s t r i a l term 1 filament winding angle 1 to describe the h e l i c a l organization of the micro-f i b r i l s with respect to the tracheid axis. Even Mark's terra has been misinterpreted to mean that the f i b e r wall i s b u i l t up from the inside out. M i c r o f i b r i l angle i s the most common term for describing m i c r o f i b r i l organization. I t i s used by Cave (13), Meylan (87), Meylan and Probine (88), Page (104) and many others. It has also been shown by Hearle (37), Preston (116) and Preston and Astbury (119), that the average c r y s t a l l i t e angle of i n c l i n a t i o n i n a c e l l wall layer of valonia, ramie, bamboo, cotton, s i s a l and c o n i f e r tracheids, corresponds to the mean m i c r o f i b r i l l a r o r i e n t a t i o n i n the same layer. Consequently these two expressions are used interchangeably i n the l i t e r a t u r e . 2. Relation to Mechanical Properties M i c r o f i b r i l angle of the S2 layer has been shown to be one of the most important supermolecular factors i n evaluating the strength properties of wood and other c e l l u l o s i c f i b e r s . M i c r o f i b r i l l a r arrangement of the S2 layer has been correlated with elongation, modulus of e l a s t i c i t y , tenacity and r e s i l i e n c e of cotton f i b e r s . Tripp et a l . (141), for example, observed the r e l a t i v e l y high elongation of cotton f i b e r concurrent with the a p p l i c a t i o n of a x i a l stress which they a t t r i b u t e d to the s p i r a l arrange-ment of the f i b r i l s . An increase i n Young's modulus and r e s i l i e n c e , i . e . , the a b i l i t y of a cotton f i b e r to absorb work without s u f f e r i n g permanent set, were also reported by them. H e l i c a l structure was also correlated s i g n i f i c a n t l y to the Pressley strength index (lb per mg) (44), shear strength (25) and to Young's modulus as well as tenacity (the number of unit lengths of the t e x t i l e material which can be supported before rupture) of a cotton bundle (123, 157). S i m i l a r l y , an inverse r e l a t i o n s h i p between convolution angle of 67 cotton v a r i e t i e s and t h e i r respective strength properties has been also obtained by Betrabet and Iyengar (9). Change of m i c r o f i b r i l l a r organization by stretching under t e n s i l e load was also studied. In 1946, Berkley and Kerr (8) found that undried cotton f i b e r s exhibited con-siderable p l a s t i c i t y and, when stretched, they increased 27 i n length and decreased approximately 25 per cent i n diameter. At the same time, the c e l l u l o s e s p i r a l i n the secondary wall approached the p a r a l l e l p o s i t i o n . Recently, Orr et a l . (103) recorded the untwisting of the h e l i c a l structure as load was applied to cotton f i b e r s . Much of t h i s untwisting was r e v e r s i b l e . They indicated that f r i c t i o n between growth layers and between f i b r i l s was a possible cause of permanent set, low i n t r i n s i c strength of high-angle cottons and weakness near the reversals (see F i g . 8). A p o s i t i v e r e -l a t i o n s h i p between f i b r i l angle of cotton f i b e r s and permanent set has also been found by Rebenfeld (123). Mechanical properties of s i s a l f i b e r s have been investigated i n r e l a t i o n to t h e i r h e l i x angle. Spark e_t a l . (131) developed a micro-extensometer for the automatic recording of load-extension curves of single s i s a l f i b e r s . Using t h i s instrument he found that f i b e r s with f l a t t e r s p i r a l s had a higher e x t e n s i b i l i t y , lower Young's modulus and lower breaking strength than had f i b e r s with steeper h e l i x . Preston (117) has also indicated s i m i l a r r e s u l t s with s i s a l f i b e r s . In addition, he reported that during the stretching of wet f i b e r s , the h e l i x angle changed i n such a way that the h e l i x became steeper just as would a s p i r a l spring. He observed 2 0 per cent extension to break for a f i b r i l l a r angle of about 50°. A great deal of t h i s extension was recoverable i n water. Therefore, he a n t i c i -28 pated a s l i p between m i c r o f i b r i l s to accommodate t h i s large amount of e x t e n s i b i l i t y . Further, Preston (117) has shown that the recovery was possible due to release of s t r a i n both i n the m i c r o f i b r i l s and i n the inc r u s t i n g substance between m i c r o f i b r i l s . Assuming good stress communication between the framework (cellulose) and surrounding material ( l i g n i n ) , v a l i d i t y of such an assumption i s supported by the work of Chow (13a) . I t i s reported that the highest strength and Young's modulus, and the lowest breaking extension of native c e l l u l o s e f i b e r s such as f l a x , hemp and ramie, and regenerated f i b e r s such as viscose rayon and acetate, coincide with the close s t approach to a x i a l o r i e n t a t i o n of c r y s t a l l i t e s (20, 41, 57, 84, 85, 129). I n t r i n s i c t e n a c i t i e s (tenacities of zero span length) of ramie, f l a x , and cotton f i b e r s were shown by DeLuca (20) to be related to the 50 per cent i n t e n s i t y angle of (002) arc. This r e l a t i o n s h i p was independent of t h e i r actual DP within the 1,800 to 5,000 DP range investigated. I n g e r s o l l (57) has indicated a substantial l i n e a r tenacity increase with increasing o r i e n t a t i o n (small angle) whereas elongation decreased h y p e r b o l i c a l l y . This increase i n tenacity with o r i e n t a t i o n has been explained q u a n t i t a t i v e l y on the basis that highly oriented materials have more c r y s t a l l i t e s p a r a l l e l to the f i b e r axis and hence are i n a 29 p o s i t i o n t o s u p p o r t l o a d m o s t e f f e c t i v e l y . I n a s i m i l a r m a n n e r , M e r e d i t h (84, 85) e x p l a i n e d t h e r e a s o n f o r t h e l o w e r t e n s i l e s t r e n g t h o b t a i n e d f r o m t h e p o o r l y o r i e n t e d c e l l u l o s e f i b e r s . I n t h e s e , o n l y a f r a c t i o n o f t h e c h a i n m o l e c u l e s , t h o s e w h i c h l i e p a r a l l e l t o t h e f i b e r a x i s , a r e i n a p o s i t i o n t o b e a r t e n s i o n s t r e s s e s w h e r e b y t h e y a r e f i r s t t o r u p t u r e , a n d t h e r u p t u r e o f t h e o t h e r s f o l l o w s d i r e c t l y . I n t h e c a s e o f s o l i d wood s p e c i m e n s , wood t i s s u e s a n d d e l i g n i f i e d t r a c h e i d s k e l e t o n s t e s t e d i n t e n s i o n , a n a l o g o u s h i g h c o r r e l a t i o n o f m i c r o f i b r i l a n g l e t o s t r e n g t h p r o p e r t i e s i s e v i d e n t . I n 1939, G a r l a n d (32) f o u n d t h a t ' s t r e n g t h -d e n s i t y ' a n d ' s t i f f n e s s - d e n s i t y ' o f s h o r t l e a f p i n e ( P i n u s  e c h i n a t a M i l l . ) a n d l o b l o l l y p i n e ( P i n u s t a e d a L.) l a t e w o o d s w e r e i n v e r s e l y c o r r e l a t e d t o t h e s i n e o f t h e m i c r o f i b r i l a n g l e . The b r e a k i n g l o a d i n t e n s i o n f o r r a d i a t a p i n e a n d D o u g l a s - f i r f i b e r s , t a k e n f r o m s u c c e s s i v e g r o w t h i n c r e m e n t s , was s t a t e d t o i n c r e a s e w i t h i n c r e a s i n g o r i e n t a t i o n (144). W a r d r o p (144) h a s a l s o i n d i c a t e d t h a t t h e l o n g e r t r a c h e i d s , w i t h more c e l l u l o s e c h a i n s f a v o r a b l y o r i e n t e d p a r a l l e l t o t h e c e l l a x i s , c a n t a k e a h i g h e r a p p l i e d l o a d i n t e n s i o n . I n a s t u d y b y F u j i s a k i (31), a n i n d i r e c t r e l a t i o n s h i p was f o u n d b e t w e e n i n c l i n a t i o n o f ' m i c e l l e s * i n t h e c e l l w a l l a n d m o d u l u s o f e l a s t i c i t y . T h i s r e l a t i o n s h i p stemmed f r o m t h e o b s e r v e d i n c r e a s e i n m o d u l u s o f e l a s t i c i t y f r o m p i t h t o b a r k o f s u g i ( C r y p t o m e r i a j a p o n i c a D. Don.) a n d t h e d e c r e a s e d m i c e l l a r a n g l e i n t h e same c a r d i n a l b o l e d i r e c t i o n . 30 Using the same species, Suzuki (135) obtained a close c o r r e l a t i o n between the ultimate s t r a i n and the m i c e l l a r angle. I f j u and Kennedy (55), u t i l i z i n g Douglas-fir, and Wellwood (158) using Douglas-fir and western hemlock, con-cluded that micro-tensile strength i s related to f i b r i l angle. When earlywood r e s u l t s were analyzed separately from latewood, the t e n s i l e strength of the l a t t e r only was shown to be highly correlated to f i b r i l angle (158). Cowdrey and Preston (17, 18) have indicated that Young's modulus of Sitka spruce (Picea s i t c h e n s i s (Bong.) Carr.) increased r a p i d l y across the annual increments from p i t h to bark. Tracheid length also increased and the h e l i c a l angle decreased along the same radius. Kellogg and I f j u (62) found that the r a t i o of l o n g i t u d i n a l to transverse thermal con-d u c t i v i t y of 20 hardwood and softwood species was the only f a c t o r influencing s p e c i f i c strength and s p e c i f i c s t i f f n e s s . Since a l l c e l l wall layers and types would contribute to t h e i r thermal conductivity r a t i o s , the authors suggested that t h i s r a t i o might be an evaluator to what they termed " i n t e g r a l f i b r i l angle 1. M i c r o f i b r i l ' angle has been related to the rh e o l o g i c a l properties of wood. Senft (128) attempted to examine the e f f e c t of creep-inducing loads, i n compression, on u l t r a -s t r u c t u r a l anatomy of Sitka spruce but was unable to measure the expected s h i f t i n m i c r o f i b r i l angle. He at t r i b u t e d t h i s , 31 h o w e v e r , t o t h e r e l a x a t i o n w h i c h p r e s u m a b l y h a d o c c u r r e d d u r i n g m i c r o f i b r i l a n g l e m e a s u r e m e n t , a n d a l s o t o t h e o b s e r v e d h i g h v a r i a b i l i t y o f t h e m i c r o f i b r i l a n g l e i n t h e s e c o n d a r y c e l l w a l l l a y e r s o f h i s s a m p l e s . H i l l (47) a n d J e n t z e n (60) h a v e shown, b y X - r a y a n a l y s i s , t h a t c r y s t a l l i t e o r i e n t a t i o n s o f l o n g l e a f p i n e h o l o c e l l u l o s e t r a c h e i d s k e l e t o n s f r o m e a r l y w o o d a n d l a t e w o o d t i s s u e s became s t e e p e r a s t h e f i b e r s w e r e d r i e d u n d e r v a r i o u s a x i a l t e n s i l e l o a d s . The t h e o r e t i c a l a n a l y s i s o f e x t e n s i o n l e d J e n t z e n t o c o n c l u d e t h a t t h e amount o f e x t e n s i o n s h o u l d be r e l a t i v e l y i n d e p e n d e n t o f t h e d r y i n g l o a d , b u t r e l a t e d t o t h e i n i t i a l f i b r i l a n g l e . H i s e x p e r i m e n t a l r e s u l t s s u p p o r t e d t h e s e t h e o r e t i c a l r e a s o n i n g s . F r e y - W y s s l i n g (29) h a s s p e c u l a t e d t h a t r h e o l o g i c a l p r o p e r t i e s a r e n o t d e p e n d e n t o n t h e f i b r i l a n g l e , b u t r a t h e r o n t h e r e l a t i v e l y p o o r c o h e s i o n b e t w e e n n e i g h b o u r i n g m i c r o -f i b r i l s . He p o s t u l a t e d t h a t t h e m i c r o f i b r i l s a r e ' c e m e n t e d ' t o g e t h e r i n s u c h a way t h a t t h e y c a n n o t e l o n g a t e i n d i v i d u a l l y u n l e s s t h e i r r e c i p r o c a l c o h e s i o n i s b r o k e n . T h e r e f o r e , he s u g g e s t e d t h a t t h e m a g n i t u d e o f e x t e n s i b i l i t y a n d d e g r e e o f e l a s t i c i t y ( e l a s t i c p o r t i o n o f t h e w o r k d o n e a s a p e r c e n t a g e o f t h e t o t a l w o r k ) m u s t d e p e n d on t h e f o r c e s w h i c h h o l d t h e m i c r o f i b r i l s t o g e t h e r i n t h e c e l l w a l l . H o w e v e r , t h e a b o v e r e p o r t e d w o r k by H i l l (47) a n d J e n t z e n (60) o n i n d i v i d u a l t r a c h e i d s d o e s n o t s u p p o r t F r e y - W y s s l i n g 1 s c o n c e p t b u t r a t h e r e m p h a s i z e s t h e m i c r o f i b r i l a n g l e a s a n i m p o r t a n t f a c t o r r e s p o n s i b l e f o r c r e e p . 32 B a s e d o n s u c h s t r o n g e x p e r i m e n t a l r e l a t i o n s h i p s b e t w e e n mean m i c r o f i b r i l a n g l e a n d e l a s t i c i t y , a s w e l l a s s t r e n g t h , t h e o r e t i c a l m o d e l s h a v e b e e n s u g g e s t e d t o e x p l a i n t h e m e c h a n i s m o f d e f o r m a t i o n i n p l a n t f i b e r s (17, 18, 38). H e a r l e (38), who h a s p r o p o s e d a f r i n g e d f i b r i l s t r u c t u r e f o r n a t i v e a nd r e g e n e r a t e d c e l l u l o s i c f i b e r s , h a s shown t h a t t h e d e f o r m a t i o n o f p l a n t f i b e r s c a n be e x p l a i n e d a s a c o m b i n a t i o n o f t w o p r o c e s s e s : (a) s t r e t c h i n g o f t h e f i b r i l s w i t h o u t a v o l u m e c h a n g e a n d (b) s t r e t c h i n g o f t h e f i b r i l s a s a s p r i n g a t c o n s t a n t f i b r i l l e n g t h , f o l l o w e d b y a r e -d u c t i o n i n v o l u m e . A c c o r d i n g t o H e a r l e * s m o d e l e a c h f i b e r i s a s o l i d c y l i n d e r a n d t h e r a d i i o f m i c r o f i b r i l s a r e t a k e n o t o b e o f t h e o r d e r o f 100 A. T h i s m o d e l h a s b e e n c r i t i c i z e d b y C o w d r e y a n d P r e s t o n (17, 18) a n d M a r k (81), t h e l a t t e r h a v i n g s t a t e d t h a t i f l a r g e r m e c h a n i c a l l y e f f e c t i v e r a d i i w e r e a s s u m e d a c c o r d i n g t o t h e f r i n g e d f i b r i l l a r s t r u c t u r e , s p r i n g - l i k e d e f o r m a t i o n s s h o u l d n o t be e x c l u d e d . C o w d r e y a n d P r e s t o n (17, 18) h a v e s u g g e s t e d two a p p r o a c h e s t o t h e d e f o r m a t i o n m e c h a n i s m a s an a l t e r n a t i v e t o t h e m o d e l o f H e a r l e . The f i r s t s t a t e s t h a t t h e m i c r o -f i b r i l s e x t e n d l i k e h e l i c a l s p r i n g s b y b e n d i n g , t w i s t i n g a n d s l i p p i n g o v e r e a c h o t h e r a n d n o t b y t h e s t r e t c h i n g o f i n d i v i d u a l f i b r i l s w h i c h m i g h t be e i t h e r i n d i v i d u a l m i c r o -f i b r i l s o r a g g r e g a t e s o f m i c r o f i b r i l s . T h e y h a d t o assume t h a t t h e m i c r o f i b r i l a g g r e g a t i o n i s 5 \i i n r a d i u s a n d t h a t t h e m i c r o f i b r i l l a r s u b s t a n c e i s i s o t r o p i c i n o r d e r t o b r i n g e x p e r i m e n t a n d t h e o r y i n t o m u t u a l p r o x i m i t y . The s e c o n d a p p r o a c h i s b a s e d on t h e s t r e t c h i n g o f m i c r o f i b r i l s i n H e a r l e ' s a n a l y s i s a n d c o n s i d e r s t h a t t h e c e l l w a l l i s a s i n g l e p h a s e s y s t e m i n w h i c h t h e w a l l s u b s t a n c e i s r e g a r d e d a s an a n i s o t r o p i c homogeneous medium. A c c o r d i n g t o t h i s a n a l y s i s , t h e c o r r e l a t i o n b e t w e e n t h e i n i t i a l com-p l i a n c e a n d t h e h e l i c a l a n g l e t a k e s t h e f o r m o f a q u a d r a t i c 2 e q u a t i o n i n s i n 8. M a r k (81) c o n s i d e r s t h e i r f i r s t a p p r o a c h n o t t o be a n u n r e a l i s t i c c o n c e p t ; a n d J e n t z e n ' s r e s u l t s n o t e d a b o v e f i t a s p r i n g m o d e l o f i n e x t e n s i b l e m a t e r i a l a n d o f f i x e d t o t a l l e n g t h . S i m i l a r w o r k h a s b e e n c o n d u c t e d b y B a l a s h o v e_t a l . ( 6 ) . U s i n g X - r a y d i a g r a m s , t h e y d e t e r m i n e d t h e c h a n g e i n t h e s p i r a l a n g l e a s a f i b e r b u n d l e o f s i s a l was s t r e t c h e d . The e x t e n s i o n o f t h e f i b e r s and t h e c h a n g e i n s p i r a l a n g l e w e r e r e l a t e d t o e a c h o t h e r e x p e r i m e n t a l l y i n t h e l i n e a r m anner t h a n w o u l d be p r e d i c t e d f o r a n e x t e n d e d i d e a l s p r i n g m o d e l . The a u t h o r s w e r e a l s o a b l e t o c a l c u l a t e t h e s l i p b e t w e e n m i c r o f i b r i l s a n d t h e c h a n g e i n d i s t a n c e b e t w e e n a d j a c e n t m i c r o f i b r i l s , i n t h e same c y l i n d r i c a l s h e e t and b e t w e e n c y l i n d r i c a l s h e e t s , a s a f i b e r u n d e r w e n t a maximum s t r a i n o f 3 t o 5 p e r c e n t . T h e r e f o r e , t h e y c o n c l u d e d t h a t r h e o l o g i c a l p r o p e r t i e s o f n a t u r a l f i b e r s a r e g r e a t l y d e p e n d e n t o n t h e m i c r o f i b r i l l a r o r g a n i z a t i o n o f t h e c e l l w a l l . 3. Va r i a t i o n i n Wood and Tracheids There has been increasing evidence i n the l i t e r a t u r e that m i c r o f i b r i l angle exhibits a high degree of v a r i a b i l i t y between species, within the same species and from p i t h to bark i n the same tree (48, 51, 111). I t also d i f f e r s within annual increments, with a steeper angle i n latewood than i n earlywood (5, 14, 50, 111, 159). In addition, compression wood i s characterized by a f l a t t e r angle than that of normal wood. At the micro l e v e l , not only i s there v a r i a t i o n of m i c r o f i b r i l angle from layer to layer i n the c e l l w a l l , but also within a given layer there may be a diffe r e n c e within the r a d i a l and tangential wall (81) . Maby (78) , Preston (115) and Preston and Wardrop (121) have shown that m i c r o f i b r i l angle i n the secondary wall i s related to c e l l breadth. H i l l e r (49) found a s i m i l a r r e l a t i o n s h i p for latewood of slash pine (Pinus e l l i o t t i i Engelm.) and of l o b l o l l y pine. It was shown some time ago by Preston (115, 118) and Wardrop and Preston (155) that the m i c r o f i b r i l angle (8) i n the S2 layer and the S i layer i s rela t e d to c e l l length (L) by the following equation: L = a + b cot 9 where: a, b = constants. Hence, t h i s equation agrees with the general observation that the m i c r o f i b r i l angle i n the longer latewood tracheids approaches the a x i a l d i r e c t i o n more c l o s e l y than i n shorter earlywood tracheids. E. Extractives Content In addition to i t s major components, c e l l u l o s e , hemicelluloses and l i g n i n , wood also contains small but i n some cases quite appreciable quantities of extraneous com-ponents. Many of these substances are extractable with neutral solvents, and are referred to as extractives (12). Wood extractives include a wide v a r i e t y of organic compounds (mainly polyphenols) of low and high molecular weight. D i s t r i b u t i o n of extractives i n wood i s not well established. They may be i n f i l t r a t e d completely into the amorphous region of the c e l l walls or they may be deposited mainly i n the parenchymatous c e l l lumen (105). On a macro-l e v e l , large differences are reported between species, with age from p i t h , r e l a t i v e maturity of trees, height i n stem, between heartwood and sapwood, and earlywood and latewood (132). Squire (132) has shown that the poly-phenolic compounds of Douglas-fir are confined to wood ray t i s s u e . It was shown by Tarkow and Krueger (137) that approximately 75 per cent of the t o t a l water soluble extrac-t i v e s of redwood (Sequoia sempervirens (D. Don ) endl.) are located i n the c e l l walls. The recent work by Morgen and Orsler (90) has also indicated that appreciable amounts 36 of extractives may be located i n the c e l l w a ll. Although the extractives constitute only a few per cent of the oven-dry weight of wood, t h e i r influence on mechanical properties may be considerable. In t h i s respect, knowledge of extractives l o c a t i o n i n c e l l s should help i n understanding t h e i r influence on mechanical properties of wood. Brown et a3_. (11) speculated that the e f f e c t of e x t r a c t i v e s ' removal from wood would be greatest on crushing strength p a r a l l e l to the grain, l e a s t on shock resistance. They a t t r i b u t e d the difference i n behavior to the fact that the former depends on the cumulative resistance of wood and ' i n f i l t r a t i o n ' present i n i t , whereas the l a t t e r i s contingent for the most part on the cohesive and adhesive forces acting i n the c e l l wall substance only, forces which are not altered s i g n i f i c a n t l y by removal of e x t r a c t i v e s . Recently, Arganbright (2) studied the influence of extractives on bending strength of redwood. He found that the green and a i r - d r y modulus of rupture (MOR) was unrelated to the amount of extractives, whereas modulus of e l a s t i c i t y (MOE) decreased with increasing amount of e x t r a c t i v e s . The author was not able to o f f e r a precise explanation f o r the observed r e l a t i o n s h i p between extractives and MOE. Further, on the r o l e of wood extractives on r h e o l o g i c a l properties of wood, Narayanamurti (94) and co-authors (95, 96) have indicated that removal of extractives caused a 37 decrease i n the r i g i d i t y modulus and an increase i n the e l a s t i c modulus of some hard- and softwood species, one of which was Picea morinda Link. The authors have also found that extractives affected rheological properties of wood at high temperature suggesting that these substances have a p l a s t i c i z i n g e f f e c t . Erickson and Sauer (24), following a s i m i l a r approach, have recently conducted a f l e x u r a l creep experiment under two drying conditions. According to t h e i r r e s u l t s , r e l a t i v e creep, defined as creep d e f l e c t i o n divided by the d e f l e c t i o n at one minute, was p o s i t i v e l y correlated with extractives content of redwood boards. The c o r r e l a t i o n values (r) were given as 0.77 and 0.76 for drying conditions of 106 °F and 150°F, resp e c t i v e l y . Elimination of one specimen, which developed a collapse streak during the t e s t at 150°F, improved the c o r r e l a t i o n between r e l a t i v e creep and extrac-t i v e s content (r = 0.95). F. S p e c i f i c Gravity S p e c i f i c gravity i s usually defined as a macroscopic measure of the amount of c e l l wall substance contained i n a uni t volume. It varies between and within species; r a d i a l l y and a x i a l l y i n the same tree; between earlywood and l a t e -wood; and across the growth increment (22). These va r i a t i o n s are due to differences i n the size of the c e l l s , the thickness of c e l l wall, and the r e l a t i v e volume of the S2 layer (24, 60, 64, 105). 38 Strength properties of wood are affected by i t s s p e c i f i c gravity. Based on extensive tests c a r r i e d out on many kinds of wood over a wide range of s p e c i f i c g r a v i t i e s , Newlin and Wilson (99) several decades ago established that the mathematical r e l a t i o n s h i p e x i s t i n g between s p e c i f i c g r a v i t y and various strength properties can be expressed as a parabolic equation of the nth degree, as follows: S = a G n where: S = any one of the strength properties, a = constant, n = constant varying from 1.00 to 2.25 depending on the p a r t i c u l a r strength property, and G = s p e c i f i c gravity Several authors, among them Homoky (54), I f j u and Kennedy (55), Kellogg and I f j u (61), Suzuki (135), Wellwood (158), and Wellwood et a l . (159) have found strong, p o s i t i v e r e l a t i o n s h i p s between s p e c i f i c gravity and the various strength properties of wood. Brown et a l . (11) have stated that t h i s r e l a t i o n s h i p requires no explanation since the load that a wood specimen w i l l bear i s determined to a great extent by the amount of c e l l - w a l l substance i t contains per u n i t of volume. However, two anomalies have been reported by Hearmon (39) and Mark (81) which indicate 39 the lack of correspondence between the e l a s t i c constants ( r i g i d i t y and Young's moduli) and de n s i t i e s of many species. L i t t l e has been reported concerning the e f f e c t of s p e c i f i c gravity on creep behavior of wood. In an attempt to account for species differences i n creep, Kellogg (61) found a negligable c o r r e l a t i o n between the species r e s i d u a l (an expression of the differences i n amount of creep between the creep curve of each species and the average creep curve for a l l 9 species studied) and s p e c i f i c gravity. In contrast to Kellogg's r e s u l t s , King (65) has concluded that s p e c i f i c gravity and maximum t e n s i l e s t r a i n can account for a high percentage of the differences between species with regard to the creep response i n and below the region of the threshold of set. He found that species having low s p e c i f i c gravity and high e x t e n s i b i l i t y creep more than those with a high s p e c i f i c gravity but low e x t e n s i b i l i t y . Erickson and Sauer (24), i n the work referred to above, reported a s i g n i f i c a n t inverse r e l a t i o n s h i p between r e l a t i v e creep and s p e c i f i c gravity at low drying temperature (106°F). The same r e l a t i o n s h i p was marked by the combined e f f e c t of i n i t i a l moisture and extractives content at high drying temperature (150°F). The authors had not expected any r e l a t i o n s h i p at the low temperature l e v e l since they had standardized the s p e c i f i c gravity e f f e c t by expressing creep d e f l e c t i o n r e l a t i v e to the i n i t i a l d e f l e c t i o n . They att r i b u t e d the obtained r e l a t i o n s h i p to a higher r a t i o of S2 layer to t o t a l c e l l wall substance as s p e c i f i c gravity increased, and not to the weight contribution of extractives to s p e c i f i c g ravity. Using plywood panels, made up from one softwood and six hardwood species, Higgins (46) found that the re s i d u a l deformation i n compression perpendicular to the grain varied from one species to another depending on the density of wood rather than the gross anatomical structure. I I I . Creep Phenomenon Wood i s commonly considered to behave e l a s t i c a l l y , i . e . , i t obeys Hooke's law, at stresses below the e l a s t i c l i m i t . However, i t has been indicated that wood also possesses properties dependent on time which are not f u l l y explainable by employing the Hookean e l a s t i c theory alone (47, 60, 61, 65, 68, 92, 108, 109, 128). I f a load i s applied to a wood-f i b e r or specimen at zero-time, there i s an instantaneous e l a s t i c deformation (OA, F i g . 9) followed by a continuing deformation (AB) over a period of time (T^-T^). This con-t i n u i n g deformation at constant stress i s c a l l e d creep. If the stress i s removed at time T^, an instantaneous e l a s t i c recovery (BC^) takes place, then a retarded p a r t i a l creep recovery (C^ to D) occurs at time T£. The part (DE) of the graph represents the permanent deformation remaining at the end of the loading-unloading c y c l e . 41 Many investigators have used rh e o l o g i c a l models i n an attempt to obtain a method by which the reaction of wood to an applied stress, over a long period of time, may be described phenomenologically. These models are composed of springs which deform under stress i n a Hookean manner, and dashpots which represent the viscous flow of a Newtonian l i q u i d . They are characterized by l i n e a r v i s c o e l a s t i c behavior because the springs are assumed to be Hookean and dashpots have flow rates which are Newtonian. A number of models, using various combinations of springs and dashpots, have been reported (1). The most common and more s a t i s f a c t o r y model used for wood and some high polymer s o l i d s , i s the four-element model shown i n F i g . 10 (7 0, 108). This model combines a Maxwell body (a spring and dashpot i n series) with a Kelvin body (a spring and dashpot i n p a r a l l e l ) . I f the model of F i g . 10 i s loaded over a period of time there i s an instantaneous e l a s t i c deformation represented by the spring constant K^. This i s a t t r i b u t e d to an e l a s t i c extension of the m i c r o f i b r i l s (38) or to t h e i r bending, twisting and s l i p p i n g (17, 18). The movement of the Kelvin body, represented by the spring constant ^ and damping c o e f f i c i e n t R^i r e s u l t s i n a retarded e l a s t i c response, i . e . , primary creep (Fig.11). Hearle (38) has indicated that t h i s primary creep i s caused by the v i s c o e l a s t i c compression of the non-crystalline material, and also by the increase i n length of the non-c r y s t a l l i n e matrix. I t i s also due to the adjustment between m i c r o f i b r i l s i n the non-crystalline portion of the c e l l w a l l. A l f r e y (1) claimed that the observed primary creep i s related to the uncurling of molecular chains. The R^  term (Fig. 10) represents a flow mechanism which i s usually associated with the l a t e r stages of the secondary creep, and the t e r t i a r y creep. This element i s said to be responsible for the permanent deformation. Upon removal of the load, i t i s assumed that there i s an instantaneous recovery of the spring. The ^ spring and R2 dashpot return slowly almost to t h e i r o r i g i n a l p o s i t i o n . F i n a l l y , the R^  dashpot may or may not change i t s p o s i t i o n , depending on the treatment given to the specimen. Even t h i s widely accepted four-element model has been c r i t i c i z e d by Pentoney (108) because of i t s i n a b i l i t y to describe the time-dependent behavior over a wide time scale. He then stated that a d i s t r i b u t i o n of retardation times might be proposed. This d i s t r i b u t i o n i s assumed to be composed of di s c r e t e retardation times through the addition of any number of Kelvin bodies i n series to the four element model. A. E f f e c t of Wood Species L i t t l e has been reported as to differences i n creep response among species. Creep behavior of 11 species (9 hardwoods and two softwoods, one of which was Douglas-fir) was compared by King (65) on an i n i t i a l s t r a i n basis and also on a stress l e v e l basis. He found substantial differences between species with respect to the magnitude of creep response, and he attr i b u t e d a large percentage of t h i s d i f f e r e n c e , i n and below the threshold of set, to s p e c i f i c g ravity and maximum s t r a i n . S i m i l a r l y , Kellogg (61) compared the creep response of 9 species, three of which were the same as King's, at the same i n i t i a l s t r a i n . He attempted to account for the v a r i a t i o n between species by p l o t t i n g the average r e s i d u a l from the general curve for each species against parameters such as s p e c i f i c gravity, slope of grain, ultimate s t r a i n , modulus of e l a s t i c i t y , and the r a t i o of modulus of e l a s t i c i t y to s p e c i f i c gravity, which he assumed to represent a measure of f i b r i l l a r o r i e n t a t i o n or c r y s t a l l i n i t y . The author concluded that the best parameter, which accounted for only 34.6 per cent of the t o t a l v a r i a t i o n between i n d i v i d u a l species, was the r a t i o of modulus of e l a s t i c i t y to s p e c i f i c gravity. Using i n d i v i d u a l longleaf pine h o l o c e l l u l o s e tracheid skeletons from a single growth increment, Jentzen (60) has indicated that those of earlywood underwent much greater elongations than those of latewood under the same applied drying load. He related these elongations to the reduction i n the f i b e r diameter and to the i n i t i a l f i b r i l o r i e n t a t i o n . Contrary to the aforementioned v a r i a b i l i t y between species with regard to creep response, Kingston (6 6) and Kingston and Clarke (67) reported that mountain ash 44 ( E u c a l y p t u s r e g n a n s F.V.M.) and h o op p i n e ( A r a u c a r i a c u n n i n g -h a m i i Sweet) beams d i d n o t , i n g e n e r a l , show w i d e l y d i f f e r e n t c r e e p b e h a v i o r i n b e n d i n g . U n f o r t u n a t e l y , t h e b a s i s o n w h i c h t h e s e s p e c i e s b e h a v e d s i m i l a r l y was n o t g i v e n by t h e a u t h o r s . B. R e l a t i o n t o M o i s t u r e C o n t e n t a n d T e m p e r a t u r e C r e e p r e s p o n s e d e p e n d s , t o a g r e a t e x t e n t , o n t h e e n v i r o n m e n t a l c o n d i t i o n s u n d e r w h i c h t h e t e s t s a r e c a r r i e d o u t . T h e r e b y m o i s t u r e c o n t e n t a n d t e m p e r a t u r e a r e c o n s i d e r e d t o be e x t r e m e l y i m p o r t a n t f a c t o r s . T h e i r e f f e c t was e l i m i n -a t e d i n t h i s s t u d y b y c a r r y i n g o u t t h e c r e e p t e s t s u n d e r c o n s t a n t t e m p e r a t u r e (73 ± 3.5°F) a n d r e l a t i v e h u m i d i t y (50 ± 2 p e r c e n t ) . The r e a d e r i s r e f e r r e d t o t h e r e c e n t e x c e l l e n t r e p o r t b y S c h n i e w i n d (125) c o n c e r n i n g t h e e f f e c t o f t e m p e r a t u r e and m o i s t u r e c o n t e n t on t h e r h e o l o g i c a l p r o p e r t i e s o f wood. MATERIALS AND METHODS I. Wood Samples The experimental material was c a r e f u l l y chosen from single trees of three western Canadian coniferous species, namely Douglas-fir normal and compression wood, Sitka spruce, and western hemlock, growing on a good s i t e (site index of 120) at the University of B r i t i s h Columbia Research Forest, Haney, B.C. The most important c h a r a c t e r i s t i c s taken into consideration i n the se l e c t i o n of t h i s material were s t r a i g h t -ness of stem, f a i r l y large diameter,and lack of leaning of the tree except for the one including compression wood. Based on these c h a r a c t e r i s t i c s , one tree was chosen from each of the abovenoted species i n addition to the tree which showed compression wood on one side. The diameters at breast height were 26, 22, 28 and 14 i n . outside bark for Douglas-fir normal and compression wood, Sitk a spruce and western hemlock, resp e c t i v e l y . As determined l a t e r by laboratory t e s t s , wood from these four trees furnished a wide range of m i c r o f i b r i l angle, s p e c i f i c gravity, c r y s t a l l i n -i t y per cent and extractives content. Af t e r f e l l i n g the trees one disc was chosen from each at breast height. In order to prevent drying below the f i b e r saturation point, discs were wrapped i n polyethylene sheets and transported to the Faculty of Forestry, where 46 t h e y w e r e s t o r e d i n a c o l d s t o r a g e r oom a t 35 ± 1°F. T h r e e a d j a c e n t t a n g e n t i a l b l o c k s w i t h s t r a i g h t -g r a i n e d wood w e r e c u t f r o m e a c h d i s c ( F i g . 1 2 ) . E a c h o f t h e s e t a n g e n t i a l b l o c k s i n c l u d e d a t l e a s t one a n n u a l i n c r e m e n t w h i c h e x h i b i t e d a minimum o f c u r v a t u r e . The d i m e n s i o n s o f e a c h b l o c k w e r e a n o m i n a l 2.5 i n . l o n g i t u d i n a l l y a n d 1.5 i n . r a d i a l l y w h e r e a s w i d t h ( t a n g e n t i a l l y ) was c o n t r o l l e d b y c u r v a t u r e o f t h e i n c r e m e n t s . The s e l e c t e d b l o c k s w e r e a s p i r a t e d u n d e r v a c u u m i n a v a c u u m / p r e s s u r e c y l i n d e r u n t i l w a t e r l o g g e d , p r i o r t o s e c t i o n i n g on a s l i d i n g m i c r o t o m e . I I . P r e p a r a t i o n o f T e s t S p e c i m e n s P r i m a r y e f f o r t s w e r e made t o c a r r y o u t a l l m e a s u r e -m e n t s f o r t h e s e l e c t e d f e a t u r e s , n a m e l y m i c r o f i b r i l a n g l e , r e l a t i v e d e g r e e o f c r y s t a l l i n i t y / e x t r a c t i v e s c o n t e n t , a n d s p e c i f i c g r a v i t y , o n t h e same s p e c i m e n s as u s e d f o r t h e c r e e p t e s t . B u t due t o p h y s i c a l l i m i t a t i o n s , s u c h a s amount o f m a t e r i a l r e q u i r e d f o r X - r a y d i f f r a c t i o n a n d t h e n e c e s s i t y o f a s p l i t s u r f a c e a l o n g t h e g r a i n f o r m i c r o f i b r i l a n g l e m e a s u r e m e n t , an a p p r o p r i a t e s a m p l i n g m e t h o d f o r s e c u r i n g m a t c h e d s p e c i m e n s was d e v i s e d and i s d e s c r i b e d b e l o w . P r e l i m i n a r y e x p e r i m e n t s h a v e shown t h a t m i c r o f i b r i l a n g l e a n d c r y s t a l l i n i t y d i d n o t d i f f e r s i g n i f i c a n t l y among t h r e e t a n g e n t i a l l y a d j a c e n t b l o c k s f o r a n y one d i s c . T h e r e -f o r e , t h i s number o f b l o c k s was c h o s e n t o r e p r e s e n t e a c h species. One block was used to provide material for the 3 3,000 microm. per xn. xnxtxal straxn l e v e l , the second for the 6,000 microin. per i n . i n i t i a l s t r a i n l e v e l , while the t h i r d block was used for obtaining the required amount of material for c r y s t a l l i n i t y determination. P r i o r to sectioning t a n g e n t i a l l y on a s l i d i n g microtome, each block was glued to a piece of plywood to ensure the required section q u a l i t y and to avoid stresses o r i g i n a t i n g from the microtome gri p s . It has been shown by Kennedy and Chan (63) that s l i c i n g angle, i . e . , the angle between grain d i r e c t i o n and the t r a v e l d i r e c t i o n of the microtome knife i s c r i t i c a l i f development of many s l i p planes i s to be avoided. Consequently, a s l i c i n g angle of 10° with the grain d i r e c t i o n p a r a l l e l to the knife edge was used. The increments previous to the selected ones (Numbers 71, 187, 60, and 80 for Douglas-fir normal wood, Sitka spruce, western hemlock and Douglas-fir compression wood, respectively) i n each block were sectioned and discarded. At the same time adjustments were made to ensure p a r a l l e l knife and wood block alignment. Once the tangential surface at the chosen p o s i t i o n i n e i t h e r earlywood or latewood region had been s a t i s f a c t o r i l y prepared, two contiguous microsections were One microin. = 10 6 i n . 48 taken from each block. One was used for obtaining s t r i p s f o r creep experiments while the other was kept for micro-f i b r i l angle measurement. Special care was taken to obtain nearly the same section thickness from both earlywood and latewood zones. The thickness ranged from 0.009-0.011 i n . as determined by use of a TMI Model 549 micrometer. Punching out micro-creep t e s t specimens was c a r r i e d out using the s p e c i a l l y machined cu t t i n g die fixed to a h a l f -ton arbor press (Fig. 13). I t has been indicated by I f j u et a l . (56) that rectangular specimens have c e r t a i n advantages as f a r as stress behavior i s concerned. Accordingly, rectangular specimens with 0.098 i n . width and 2.5 i n . length were used i n t h i s study. Care was taken to ensure a cut p a r a l l e l to the grain, e i t h e r by tearing a s t r i p near the edge of the section, or by noting the d i r e c t i o n of ink flow along the grain. The t e s t specimens (four to six) obtained were then transferred to the c o n t r o l l e d temperature and humidity (CTH) room maintained at 73 ± 3.5°F and 50 ± 2 per cent r e l a t i v e humidity, where they were placed between layers of absorbent paper to prevent c u r l i n g . At the end of a storage period of eight days no change i n specimen weight could be observed to an accuracy of ±0.0005 g. The specimens were then checked for dimensions with the micrometer. 49 I I I . M i c r o - c r e e p T e s t A. T e s t i n g M a c h i n e A t a b l e m o d e l I n s t r o n t e s t i n g m a c h i n e (TM-M-L) was m o d i f i e d f o r c a r r y i n g o u t c r e e p t e s t s i n t e n s i o n p a r a l l e l t o t h e g r a i n . A s t r a i n g a uge e x t e n s o m e t e r w i t h 0.5 i n . g a uge l e n g t h and 10 p e r c e n t s t r a i n l i m i t was c o n n e c t e d t o a f i v e - p i n a d a p t e r f e e d i n g d i r e c t l y t o t h e l o a d c e l l a m p l i f i e r . A r e c o r d o f t h e s t r a i n b e h a v i o r was a u t o m a t i c a l l y r e c o r d e d by t h e I n s t r o n s t r i p c h a r t r e c o r d e r . T h i s i n s t r u m e n t i s on t h e l e f t h a n d s i d e o f t h e t e n s i l e t e s t i n g m a c h i n e ( F i g . 1 4 A ) . C a l i b r a t i o n o f t h e c h a r t was c a r r i e d o u t u s i n g an e x t e n s o m e t e r c a l i b r a t o r . W i t h t h e e x t e n s o m e t e r i n p l a c e , t h e c a l i b r a t o r was e x t e n d e d a known amount and c o r r e s p o n d i n g c h a r t m o t i o n was a d j u s t e d t o t h e d e s i r e d m a g n i f i c a t i o n . I t was a l s o p o s s i b l e t o i n c r e a s e t h e s e n s i t i v i t y b y u s i n g a l o w e r r a n g e , n a m e l y Range One on t h e I n s t r o n p a n e l , w h i c h p r o v i d e d a s e n s i t i v i t y o f ±5 m i c r o i n . p e r i n . To a c h i e v e t h i s l e v e l o f s e n s i t i v i t y , i t was n e c e s s a r y t o u n b a l a n c e t h e s t r a i n g a uge by a known amount o f s t r a i n . The z e r o s t r a i n p o i n t was s u p p r e s s e d o f f t h e s c a l e o f t h e c h a r t by e i t h e r 2,600 o r 5,600 m i c r o i n . p e r i n . The p r o c e d u r e f o l l o w e d was t h a t i n d i c a t e d i n t h e O p e r a t i n g I n s t r u c t i o n s m a n u a l o f t h e t e s t i n g m a c h i n e . 50 The chart was also c a l i b r a t e d e l e c t r o n i c a l l y using the coarse balance. One notch clockwise of the balance was equal to 1.05 per cent s t r a i n on the 10X range. Therefore, i t was convenient to check the c a l i b r a t i o n before and a f t e r each t e s t by simply turning the coarse balance one notch clockwise. B. Loading System Test specimens were glued at both ends between two aluminum sheets, using Eastman 910 adhesive. This was done to f a c i l i t a t e loading the specimens. The assembly was c a r e f u l l y aligned i n the upper gr i p of the Instron t e s t i n g machine. A hooked end wire was put through a hole i n the bottom of a bucket, used for loading the assembly, the bucket r e s t i n g on two supports on the lower crosshead of the machine. A 100-gram weight was hung at the lower end of the wire to straighten out the specimen and also to f a c i l i t a t e mounting the s t r a i n gauge extensometer on i t . The gauge was made almost weightless by hanging i t from the fi x e d upper crosshead of the Instron. Some lead shot, depending on the t e n s i l e strength of each species, was then put into the bucket without loading the specimen. A f t e r about 15 minutes, during which the zero s t r a i n point was suppressed to eith e r 2,600 or 5,600 microin. per i n . , the lower crosshead was automatically moved down. More lead shot was immediately poured into the bucket, so that the i n i t i a l required s t r a i n value of either 3,000 or 6,000 microin. per i n , was reached i n about 30 seconds. This system provided an accurate and rapid means of loading the assembly. The loading system i s shown i n Fi g . 14B. It should be indicated that the number of r e p l i c a t i o n s from earlywood and latewood used i n the s t a t i s t i c a l analyses i s not the same for each group of samples due to the following (a) unavoidable curvature of some of the annual increments; (b) specimen rupture during conduct of creep te s t at the higher s t r a i n l e v e l and (c) the i n a b i l i t y to apply the required i n i t i a l s t r a i n l e v e l to some tes t specimens. Measurements of t o t a l creep were taken over a 60-minute period of time at a chart speed of 0.2 i n . per minute. The Instron t e s t i n g machine was c a r e f u l l y checked for d r i f t every fourth t e s t . Two examples of tes t records are shown i n Figures 15 and 16. C. Selection of a Constant I n i t i a l S t r a i n Level Exploratory tests had shown that loading four to f i v e t e s t specimens, taken from the same section of wood ( r e p l i -cations) , to a s p e c i f i c percentage of the ultimate t e n s i l e strength, resulted i n a high v a r i a b i l i t y i n t o t a l creep response. However, loading s i m i l a r specimens to a constant i n i t i a l s t r a i n indicated a high degree of s i m i l a r i t y among them i n t o t a l creep values. This suggested that, i n a case where an explanation for the differences i n t o t a l creep i s required, one should f i x the e l a s t i c portion ( i n i t i a l strain) of the creep behavior. Previous work done by Kellogg (61) had also indicated a s i m i l a r trend. He proposed that s t r a i n s measured at the gross l e v e l remain comparable at the molecular l e v e l . There-fore, he considered t h i s single factor as the best i n d i c a t i o n of an equivalent stress condition at the molecular l e v e l . 3a Two i n i t i a l s t r a i n l e v e l s , namely 3,000 and 6,000 microin. per i n . , were chosen to be applied for a l l specimens used. Selection of these s t r a i n l e v e l s was based on the work done by King (65) who had found that the threshold of set occurred at almost 4,000 microin. per i n . i n i t i a l s t r a i n for a l l specimens of softwoods and hardwoods tested. He also indicated, by h i s graphs, that the v a r i a b i l i t y among species below the threshold of set was l e s s than that above i t . Therefore, s e l e c t i o n of the aforementioned s t r a i n l e v e l s was useful i n obtaining the creep behavior below and above one of the c r i t i c a l regions, i . e . , threshold of set. IV. M i c r o - s p e c i f i c Gravity Aft e r carrying out creep t e s t s , specimen thickness, width and length was measured to the nearest 0.001 i n . These specimens were used for micro-specific gravity determinations. 3 aThe equivalent stress l e v e l s are f i l e d i n Room 3 88, Faculty of Forestry. 53 The specimens were extracted i n sequence with ethyl ether, et h y l alcohol, and hot (about 80°C) d i s t i l l e d water. Eight hours were allowed for ether and alcohol extractions while hot water extractions were c a r r i e d out for four hours, changing water every hour. This extraction process was necessary i n order to base s p e c i f i c gravity c a l c u l a t i o n s on extr a c t i v e - f r e e c e l l wall substance. After extraction, specimens were put i n weighing dishes. Each tissu e group (earlywood or latewood) was placed i n one dish. They were then oven-dried at 100 ± 2°C for three hours. Upon completion of drying, specimens were transferred to a desiccator then immediately to a ple x i g l a s s glove box where they were cooled for 30 minutes inside the desiccator. Weights were determined using a Cahn E l e c t r o -balance contained i n the plexiglass glove box (Fig. 17). To maintain desiccated atmosphere for oven-dry specimens, f r e s h l y reconditioned s i l i c a - g e l drying medium was used. The box was t i g h t l y sealed providing the accurate moisture free atmosphere. Weighing was c a r r i e d out using the 20 mg range on the balance, which gave a s e n s i t i v i t y of 0.001 mg. Cali b r a t i o n s and zeroing of the balance were conducted a f t e r every f i f t h measurement. S p e c i f i c gravity based on volume at tes t and ext r a c t i v e - f r e e oven-dry weight was obtained by d i v i d i n g 3 weight i n mg by volume i n mm . This method of determining s p e c i f i c g r a v i t y h a s b e e n u s e d b y o t h e r w o r k e r s ( 5 4 , 5 6 ) , a n d h a s shown h i g h a c c u r a c y a n d e f f i c i e n c y . V. E x t r a c t i v e s C o n t e n t E x t r a c t i v e s c o n t e n t was d e t e r m i n e d o n t h e s p e c i f i c g r a v i t y s a m p l e s i n t h e f o l l o w i n g m anner. The s p e c i m e n s w e r e o v e n - d r i e d (100 ± 2°C) f o r t h r e e h o u r s p r i o r t o e x t r a c t i o n a n d w e i g h e d a s p r e v i o u s l y d e s c r i b e d o n t h e Cahn E l e c t r o -b a l a n c e . F o l l o w i n g e x t r a c t i o n t h e o v e n - d r y w e i g h t was o b t a i n e d a g a i n . E x t r a c t i v e s c o n t e n t was c a l c u l a t e d b a s e d o n e x t r a c t i v e - f r e e o v e n - d r y w e i g h t a s f o l l o w s : E x t r a c t i v e s c o n t e n t = ^ x 100 e f w h e r e : W Q d = o v e n - d r y w e i g h t , a n d = e x t r a c t i v e - f r e e o v e r - d r y w e i g h t . S i n c e m o s t o f t h e v o l a t i l e e x t r a c t i v e s h a v e b o i l i n g p o i n t s o f o v e r 100°C ( 1 2 ) , v e r y l i t t l e l o s s w o u l d be e x p e c t e d i n t h e s e c o n s t i t u e n t s d u r i n g o v e n - d r y i n g . 55 VI. M i c r o f i b r i l Angle Determination It i s well known that the middle layer (S2) of the c e l l wall constitutes as much as 80 to 95 per cent of the tracheid wall volume. Therefore, m i c r o f i b r i l angle i s defined, i n t h i s work, as the mean h e l i c a l angle between the d i r e c t i o n of the m i c r o f i b r i l s i n the S2 layer and the c e l l axis (88). During the course of the exploratory work several methods for m i c r o f i b r i l angle determination were t r i e d including the Bailey and Vestal (5) c l a s s i c a l iodine s t a i n i n g method. D i f f i c u l t i e s were experienced i n obtaining consistent development of iodine c r y s t a l s p a r a l l e l to the m i c r o f i b r i l s of the S2 layer. Consequently, t h i s technique was rejected as an appropriate method for determining m i c r o f i b r i l angle i n the species used. Two alternate techniques were chosen as indicated below. A. X-ray D i f f r a c t i o n Method The X-ray d i f f r a c t i o n technique based on the t h e o r e t i c a l model developed by Cave (13) was applied by Meylan (87) f o r measurement of m i c r o f i b r i l angle i n radiata pine. The technique involves exposing a 1.5 mm thick specimen to X-rays for two hours to obtain a photographic diagram of the (002) arc. Meylan measured the width of t h i s arc by means of the angular separation of the two tangents at the points of i n f l e c t i o n of the i n t e n s i t y curve obtained from a photometer recording. He drew tangents to the sides of the diagram and defined the width of the arc (2T) as the distance between the points of i n t e r s e c t i o n of these tangents with the zero i n t e n s i t y axis. This technique i s r e l a t i v e l y time consuming and tedious due to the numerous necessary steps involved i n obtaining the i n t e n s i t y curve of the (002) arc. A new technique was therefore developed which gives the X-ray d i f f r a c t i o n pattern d i r e c t l y i n 20 minutes with no need of the photometer scanning. A Texture Goniometer machine avail a b l e i n the Department of Metallurgy was used for t h i s purpose. The angle (2G) was set at 21° and the four to s i x s t r i p s of wood taken from the tangential section used for c r y s t a l l i n i t y determination were stacked together l o n g i t u d i n a l l y and placed perpendicular to the X-ray beam i n the middle of a sp e c i a l d i s c . This d i s c was set to rotate from the zero degree po s i t i o n to 360°, giving two peaks for the (002) plane, one at 90° and the other at 270°. The angle 'T' was defined as ha l f the angular distance between the points of i n t e r s e c t i o n of these tangents with the zero 4 i n t e n s i t y axis (Fig. 18). It should be noted that these 1T' values are affected by the mean m i c r o f i b r i l angle and the d i s t r i b u t i o n of micro-f i b r i l s i n the S2 layer of the c e l l wall (13). Because the The remaining diagrams are f i l e d i n Room 3 88, Faculty of Forestry. 57 "T' value does not give the average m i c r o f i b r i l angle d i r e c t l y but has to be ca l i b r a t e d with one of the microscopic methods for measuring the m i c r o f i b r i l angle, the method described below was used. B. Optical Method Since m i c r o f i b r i l s i n the S2 layer are arranged h e l i c a l l y with the lo n g i t u d i n a l axis of the tracheids, the angle on the opposite walls between the m i c r o f i b r i l s and the tracheid axis i s i n the opposite d i r e c t i o n . Thus the b i r e -fringences of the two opposite walls tend to compensate one another. Under t h i s condition, the tracheid functions approximately as an u n i a x i a l c r y s t a l with the maximum r e -f l e c t i v e index along i t s axis and independent of the micro-f i b r i l angle (104). In the case of the thick-walled f i b e r , the o p t i c a l theory of the system i s complex (114). Therefore, a single wall rather than a double wall i s required to determine the m i c r o f i b r i l angle. E a r l y i n 1934, Preston (112) overcame t h i s d i f f i c u l t y by proposing the method of longitud-i n a l sectioning of macerated f i b e r s . In doing so the opposite walls of the f i b e r s were removed and l i g h t was passed only through the remaining wall. A s i m i l a r approach was used recently by Page (104) , where mercury was introduced into the lumens ( r a d i a l surfaces) of wood samples or d e l i g n i f i e d tracheid skeletons. When an impregnated sample i s examined i n a p o l a r i z i n g microscope 58 u s i n g i n c i d e n t ( e p i - ) i l l u m i n a t i o n , t h e l i g h t p a s s e s t h r o u g h t h e t r a c h e i d w a l l , s u f f e r s r e f l e c t i o n a t t h e m e r c u r y s u r f a c e a n d e x i t s t h r o u g h t h e same w a l l . U n d e r e p i -i l l u m i n a t i o n w i t h t h e p o l a r i z e r a n d a n a l y z e r ( p o l a r s ) c r o s s e d , a t r a c h e i d w i t h a m e r c u r y - f i l l e d l u m e n h a s a n e x t i n c t i o n p o s i t i o n when t h e f i b r i l s i n t h e S2 l a y e r a r e p a r a l l e l t o t h e p l a n e o f one o f t h e p o l a r s . The m i c r o f i b r i l a n g l e i s m e a s u r e d a s t h e a n g l e b e t w e e n t h i s p l a n e a n d t h e a x i s o f t h e t r a c h e i d i n t h e e x t i n c t i o n p o s i t i o n . The o n l y p r e r e q u i s i t e i n t h e c a s e o f wood s a m p l e s i s t h a t t h e s u r f a c e t o be e x a m i n e d h a s b e e n s p l i t a l o n g t h e m i d d l e l a m e l l a . C o n s e q u e n t l y , P a ge u s e d m i l d d e l i g n i f i c a t i o n p r i o r t o wood s p l i t t i n g . P a g e ' s m e t h o d w i t h some m o d i f i c a t i o n s was u s e d i n t h e p r e s e n t s t u d y . The s a m p l e s o f w h i c h t h e t a n g e n t i a l s u r f a c e s w e r e m a t c h e d w i t h t h a t u s e d f o r c r e e p t e s t s , w e r e s u b j e c t e d t o a m i l d d e l i g n i f i c a t i o n ( s o l u t i o n c o n t a i n s e q u a l p a r t s o f a c e t i c a c i d a n d h y d r o g e n p e r o x i d e ) . T h i s f a c i l i t a t e d s p l i t t i n g t h e s e c t i o n s u r f a c e m a n u a l l y a l o n g t h e m i d d l e l a m e l l a , a s r e q u i r e d f o r o b t a i n i n g s i n g l e w a l l s . The s a m p l e s w e r e t h e n d e h y d r a t e d u s i n g a c e t o n e , f o l l o w i n g w h i c h t h e y w e r e p u t i n t o a s m a l l p r e s s u r e c y l i n d e r . M e r c u r y was i n t r o d u c e d a n d a p r e s s u r e o f 1,000 p s i was a p p l i e d t h r o u g h a p i s t o n i n t h e c y l i n d e r f o r a f e w m i n u t e s . The p r e s s u r e was t h e n r e l e a s e d a n d t h e s p e c i m e n s w e r e e x a m i n e d i n a p o l a r i z i n g m i c r o s c o p e u s i n g i n c i d e n t (epi-) 59 i l l u m i n a t i o n to determine m i c r o f i b r i l angle. T h i r t y 5 measurements were taken on each matched section. VII. Determination of C r y s t a l l i n i t y by X-ray Technique A. Specimen Preparation A section matched with that used for creep tests was avail a b l e from the t h i r d block, as noted above, for c r y s t a l l i n -i t y determination. The preliminary experiments during t h i s i n v e s t i g a t i o n showed that thickness as well as tracheid o r i e n t a t i o n are extremely important for c e l l - w a l l c r y s t a l l i n i t y determination. Since two d i f f e r e n t kinds of t i s s u e s , namely earlywood and latewood, are to be used as experimental materials and t h e i r s p e c i f i c g r a v i t i e s d i f f e r , i t was necessary to use the same mass per unit area from both earlywood and latewood. This was done by grinding the material to be used i n a Wiley m i l l and c o l l e c t i n g the p a r t i c l e s which passed through a 20 mesh screen but were retained on the 40 mesh screen. By using wood meal, the same r e l a t i v e thickness was maintained and f i b e r o r i e n t a t i o n was also minimized. Grinding the material to t h i s s i z e would not a f f e c t the d i f f r a c t i o n pattern (97). During the preliminary stages of t h i s work i t was found that 400 mg and 500 mg of material produced i d e n t i c a l diffractograms. Therefore, a standard sample of 400 mg i n a 1/2 x 1 1/2 i n . rectangular specimen holder was used. 5 The o r i g i n a l experimental data are f i l e d i n Room 388, Faculty of Forestry. 60 The ground wood was shaped i n t o a t h i n r e c t a n g u l a r p e l l e t (1/2 x 1 1/2 in. ) by compression a t about 1,000 p s i i n a s p e c i a l l y designed d i e . As a p r i o r s t e p , a drop o f d i l u t e g l u e s o l u t i o n (10 ml Duco cement p l u s 100 ml amyl acetate) was p l a c e d on 4 00 mg of ground wood a f t e r i t had been l i g h t l y compacted i n the d i e , then the f u l l s p e c i f i e d p r e s s u r e was a p p l i e d i n one to two minutes. I t was i n d i c a t e d by Nelson and S c h u l t z (98) t h a t the amount o f cement used would not a f f e c t the d i f f r a c t o g r a m . B. X-ray D i f f r a c t i o n A P h i l i p s X-ray D i f f r a c t o m e t e r was used f o r c r y s t a l l i n -i t y d e t e r m i n a t i o n . X-rays were generated from a wa t e r - c o o l e d copper t a r g e t passed through a 0.5° di v e r g e n c e s l i t and then to a n i c k e l f i l t e r i n o r d e r to e l i m i n a t e the K 3 r a d i a t i o n o f the X-ray tube. T e s t were made a t 40 Kv and 15 ma c u r r e n t . The goniometer c o n s i s t e d o f a p r o p o r t i o n a l counter w i t h a p r e a m p l i f i e r c i r c u i t mounted on a moveable arm, which was moved by a c o n s t a n t speed motor and a system o f gears. T h i s motor was operated at one degree (28) per minute. The sample s u r f a c e , a t the c e n t e r o f the c i r c l e surrounded by the goniometer/ moved a t h a l f the speed o f the goniometer. D i f f r a c t e d X-rays e n t e r i n g the p r o p o r t i o n a l counter through the 0.1° r e c e i v i n g s l i t and 0.5° s c a t t e r s l i t were amp-l i f i e d by. the p r e a m p l i f i e r c i r c u i t and f e d to a r e c o r d i n g p a n e l . The time constant, which controls the use and fluctuations of the rate meter, was maintained at eight seconds. A 2 2 scale factor of ei t h e r 2 x 10 or 4 x 10 , depending on the i n t e n s i t y of the peaks, was used. Chart speed was maintained at 0.39 i n . per minute. Test f o r a i r scatter indicated that correction for such e f f e c t was of a minor order and need not be made. Samples were mounted on a s p e c i a l l y designed aluminum holder, then placed appropriately i n the middle of the c i r c l e surrounded by the goniometer. I t was necessary to make sure that the specimen was at the same l e v e l as the holder. To achieve that, an aluminium sheet having the same thickness as that of the specimen was glued to the top of the holder. Previous work has shown that the d i f f r a c t i o n pattern developed over the range of 6 to 30° (28) angle i s s u f f i c i e n t for c r y s t a l l i n i t y determination (73, 92). Three peaks were obtained over t h i s range, namely (002) and (101 + 101 ) (Fig. 20). The same range was employed i n t h i s work and two diffractograms were taken for each specimen except for Sitka spruce latewood where one measurement was taken due to some experimental d i f f i c u l t i e s which did not allow taking the second measurement. The rest of diffractograms are f i l e d i n Room 388, Faculty of Forestry. 62 C. Selection of an Equation for C r y s t a l l i n i t y Determination The per cent of c r y s t a l l i n e material i n a p a r t i a l l y c r y s t a l l i n e polymer such as wood can be determined t h e o r e t i c a l l y with high p r e c i s i o n by comparing the i n t e n s i t y of X-rays d i f f r a c t e d by the c r y s t a l l i n e portion with that i n t e n s i t y d i f f r a c t e d by the non-crystalline portion i n the same samples (142). However, many d i f f i c u l t i e s have been faced by other workers. Assigning values to the c r y s t a l l i n e , amorphous, and background scatte r i n g , as well as the un-symmetrical peaks, have been among the major d i f f i c u l t i e s (33, 41). For the purpose of his work, Murphey (92)used the t o t a l integrated i n t e n s i t i e s minus the background (the pattern developed when the specimen was excluded from the apparatus). This included the X-ray d i f f r a c t e d by c r y s t a l l i t e s , the coherent amorphous sc a t t e r , and the incoherent and thermal a g i t a t i o n s c a t t e r . The disadvantage of t h i s method i s that one i s not able to separate the part d i f f r a c t e d by the c r y s t a l l i n e region of the c e l l wall from that of the amorphous region. Turley (142) has stated the advantage of using the r e l a t i v e per cent c r y s t a l l i n i t y value because of the d i f f i -c u l t i e s noted above. He drew the background l i n e between two points which were chosen so that a l l d i f f r a c t i o n patterns fo r the material had minima at these points. Then he s k e t c h e d t h e n o n - c r y s t a l l i n e p e a k i n an a r b i t r a r y manner b a s e d o n a s e t o f minimum p o i n t s . He c a l c u l a t e d t h e r e l a t i v e p e r c e n t c r y s t a l l i n i t y a s t h e r a t i o o f t h e a r e a u n d e r t h e c r y s t a l l i n e p e a k s a n d a b o v e t h e n o n - c r y s t a l l i n e p e a k t o t h e t o t a l a r e a a b o v e t h e b a c k g r o u n d l i n e , m u l t i p l i e d b y 100. T h i s m e t h o d i s u s e f u l i n r a n k i n g a s e r i e s o f s a m p l e s o f t h e same p o l y m e r a c c o r d i n g t o t h e i r c r y s t a l l i n i t y . H o w e v e r , i t i s n o t a n a c c u r a t e m e t h o d due t o t h e d i f f i c u l t i e s i n s e p a r a t i n g c r y s t a l l i n e f r o m n o n - c r y s t a l l i n e p e a k s . c r y s t a l l i n i t y p r o p o s e d b y S e g a l e_t a l _ . (126) was c h o s e n t o d e t e r m i n e t h e C r y s t a l l i n i t y I n d e x ( C r I ) , w h i c h e x p r e s s e s t h e r e l a t i v e d e g r e e o f c r y s t a l l i n i t y . The e q u a t i o n u s e d was a s f o l l o w s : An e m p i r i c a l m e t h o d f o r e s t i m a t i n g t h e d e g r e e o f C r I - I am x 100 w h e r e : I 002 t h e maximum i n t e n s i t y ( i n a r b i t r a r y u n i t s ) o f t h e (002) l a t t i c e d i f f r a c -t i o n , a n d I am t h e i n t e n s i t y o f d i f f r a c t i o n i n t h e same u n i t s a t 29 = 18°. T h i s e q u a t i o n was c o n s i d e r e d t h e b e s t a n d was u s e d f o r t h e p u r p o s e o f t h e p r e s e n t w o r k s i n c e t h e two p a r a m e t e r s i n t h e a b o v e e q u a t i o n a r e d e t e r m i n e d a t two f i x e d p o i n t s . The b a c k g r o u n d was d e s i g n a t e d a s t h e l i n e d r a w n b e t w e e n two p o i n t s w h i c h w e r e c h o s e n s o t h a t a l l d i f f r a c t i o n p a t t e r n s h a d m i n i m a h e r e ( s e e F i g . 20). RESULTS AND DISCUSSION Experimental values for the three coniferous woods, Douglas-fir (normal and compression wood), Sitka spruce, and western hemlock tested at 3,000 microin. per i n . i n i t i a l s t r a i n ( i n i t i a l s t r a i n No. 1) and at 6,000 microin. per i n . i n i t i a l s t r a i n ( i n i t i a l s t r a i n No. 2) are summarized i n Tables 1, 2 and 3. Four independent v a r i a b l e s , m i c r o f i b r i l angle (X^), (Table 1, Column 2) r e l a t i v e degree of c r y s t a l l i n -i t y (X2) (Table 2) s p e c i f i c gravity (X^), extractives content (X^), and the dependent v a r i a b l e , t o t a l creep over a period of 60 minutes (Y) (Table 3) are used i n the s t a t i s t i c a l analyses. I. S t a t i s t i c a l Analyses and Interpretation of Results Multiple regression analysis was employed to f i n d a mathematical function that could be used to describe the r e l a t i o n s h i p which was assumed to e x i s t between t o t a l creep and the independent variables studied. The following l i n e a r function Y = b '+ b,X, + b 0X 0 + b-,X0 + b.X. 0 1 1 2 2 3 3 4 4 was found to be the best one to represent the anticipated r e l a t i o n s h i p for both l e v e l s of i n i t i a l s t r a i n used. 66 Accordingly, t h i s function was used for regression analyses to choose the most important variables which should be included i n the equation. The general r e l a t i o n s h i p between t o t a l creep i n a l l species and the independent variables was found to be as follows: Y x = -111.9230 + 13.6294X1 + 00.9093X2 + 144.0210X3 - 11.1135X, tl] SEE = 34.0773 O it ic IT = 0.7681 N =34 and, Y 2 = -1685.2100 + 31.5733X1 + 20.1994X2 + 451.2920X3 + 29.6591X, [2] SEE = 64.4748 IT = 0.8647 N = 34 for i n i t i a l s t r a i n s Numbers 1 and 2, res p e c t i v e l y . Computation of a l l possible combination was c a r r i e d out to s e l e c t the most s i g n i f i c a n t subset of the independent 7 Standard error of estimate. ** S i g n i f i c a n t at the one per cent l e v e l . variables which contribute s i g n i f i c a n t l y i n accounting for the v a r i a t i o n i n t o t a l creep. Results of t h i s procedure are summarized i n Tables 4 and 5. The following two regression equations were chosen to best represent the r e l a t i o n s h i p between t o t a l creep and the independent variables for i n i t i a l s t r a i n l e v e l s Numbers 1 and 2, res p e c t i v e l y : Y± = -50.6658 + 13.2377X-L + 143.2880X3 - 10.7829X4 . . [3] SEE = 33.5114 R 2 = 0.7680** N = 34 and, Y 2 = -297.2390 + 23.0248X1 + 407.8860X3 + 32.7107X4 [4] SEE = 65.6095 R 2 = 0.8550** N =34 These two regression equations were selected for three reasons. F i r s t l y , each of the variables ( m i c r o f i b r i l angle (X-^ ) , s p e c i f i c gravity (X^) and extractives content (X 4) i n d i v i d u a l l y contributed s i g n i f i c a n t l y to the v a r i a t i o n in t o t a l creep (Y). In other words, the p a r t i a l c o e f f i c i e n t s of determination are s i g n i f i c a n t , as indicated i n Tables 4 and 5. Secondly, the combination of X^, X^ and X 4 gives 2 the highest c o e f f i c i e n t of determination (R ) (Tables 4 and 5, Column 2). T h i r d l y , the standard error of estimate i n 68 t o t a l creep using these three variables together i s the lowest among the other combinations given i n Tables 4 and 5, Column 3. From the l i n e a r multiple regression analyses pre-sented above, two conclusions can be drawn up to t h i s point. F i r s t l y , f o r a l l coniferous wood tissues used, including both earlywood and latewood, the t o t a l creep response can be best explained by the mathematical functions [3] and [4] for i n i t i a l s t r a i n s Numbers 1 and 2, res p e c t i v e l y . Secondly, 76.80 and 85.50 per cent of the v a r i a b i l i t y i n t o t a l creep response i s accounted for by the v a r i a t i o n i n m i c r o f i b r i l angle, s p e c i f i c gravity and extractives content, under both s t r a i n conditions Numbers 1 and 2, r e s p e c t i v e l y . The f i r s t conclusion raises the question of whether separate pred i c t i o n equations should be used f o r earlywood and latewood or whether these two groups should be represented by a single equation. Therefore, covariance analyses were c a r r i e d out using the variables , X^ and X^. Equations [5] and [6] given with Table 6 represent earlywood and latewood regression model at 3,000 microin. per i n . s t r a i n l e v e l , r e s p e c t i v e l y . Examination of the r e s u l t s given i n Table 6 indicates that the differences for t e s t i n g slopes (creep response) and l e v e l s (magnitude of creep response) of Equations [5] and [6] are not s i g n i f i c a n t at the 5 per cent l e v e l . Consequently, t o t a l creep response of earlywood and latewood specimens tested under a 3,000 microin. per 69 i n . i n i t i a l s t r a i n can be explained by the same pre d i c t i o n regression Equation [3]. On the other hand, creep response of earlywood (Table 7, Equation [7]) and latewood (Table 7, Eq-uation [8]) specimens tested under a 6,000 microin. per i n . could be represented by two separate functions. Since the objective of t h i s i n v e s t i g a t i o n was to explain creep response i n r e l a t i o n to the abovenoted independent v a r i a b l e s , i t becomes evident that creep response, as a function of m i c r o f i b r i l angle, s p e c i f i c gravity, and extractives content under the higher s t r a i n l e v e l (No. 2), could be represented approximately by a single function (Equation [4]). This f a c i l i t a t e d establishment of the phenomenological model for creep response as a function of the s t r u c t u r a l features and other properties of the tracheid walls, as w i l l be discussed below. Examination of creep r e s u l t s given i n Table 3, reveals that t o t a l creep increases as the i n i t i a l applied s t r a i n (constant stress at the micro-level (61)) increases. It i s confirmed i n Table 8, where differences for t e s t i n g l e v e l s of Equations [3] and [4] are shown to be s i g n i f i c a n t l y d i f f e r e n t at the 0.5 per cent l e v e l . This implies that the higher i n i t i a l s t r a i n l e v e l (6,000 microin. per in.) would seem to bring into play the e f f e c t of the s t r u c t u r a l features, e s p e c i a l l y m i c r o f i b r i l s , f o r c i n g them to react to the applied s t r a i n i n a pronounced manner, i n addition, creep behavior at the lower s t r a i n l e v e l i s shown 70 i n T a b l e 8 t o be d i f f e r e n t f r o m t h a t a t t h e h i g h e r s t r a i n l e v e l . I n o t h e r w o r d s , s l o p e s o f t h e two E q u a t i o n s [3] and [4] a r e s i g n i f i c a n t l y d i f f e r e n t a t t h e 0.5 p e r c e n t l e v e l . The i m p l i c a t i o n i s t h a t t h e 6,000 m i c r o i n . p e r i n . a p p l i e d t o t h e t e s t s p e c i m e n s e x c e e d e d t h e w e l l e s t a b l i s h e d s t r a i n l e v e l ( a b o u t 4,000 m i c r o i n . p e r i n . (65)) a t w h i c h t h r e s h o l d o f s e t c o u l d h a v e a c t u a l l y b e g u n . A f t e r t h e d e v e l o p m e n t o f s e t , c r e e p i s r e p o r t e d t o i n c r e a s e 'more t h a n l i n e a r l y * w i t h i n c r e a s i n g s t r e s s l e v e l (10, 47, 65, 66). T h i s s u g g e s t s t h a t a b o v e t h e t h r e s h o l d o f s e t some f o r m o f y i e l d i n g b e g i n s (65, 6 9 ) . Y i e l d i n g i s p r o b a b l y due t o s t r u c t u r a l c h a n g e s s u c h a s f o r m a t i o n o f s e c o n d a r y b o n d s i n t h e amorphous r e g i o n o f t h e f i b e r (47, 9 2 ) . T h e s e new b o n d s w o u l d t e n d t o ' l o c k ' t h e m o l e c u l a r c h a i n s e g m e n t s i n t h e d e f o r m e d r e g i o n r e s u l t i n g i n a p e r m a n e n t d e f o r m a t i o n a f t e r r e m o v a l o f t h e a p p l i e d s t r e s s . B a s e d on t h e a b o v e r e s u l t s , i t c a n be s e e n t h a t t h e a n t i c i p a t e d c o r r e l a t i o n o f m i c r o f i b r i l a n g l e o f t h e S2 l a y e r , c e l l w a l l c r y s t a l l i n i t y , s p e c i f i c g r a v i t y a n d e x t r a c t i v e s c o n t e n t w i t h t o t a l c r e e p r e s p o n s e h a s b e e n e s t a b l i s h e d . The r o l e w h i c h t h e s e v a r i a b l e s p l a y i n c r e e p r e s p o n s e i s p r e -s e n t e d b e l o w . 71 II . Relationship of M i c r o f i b r i l Angle to Total Creep Before in v e s t i g a t i n g the m i c r o f i b r i l angle - t o t a l creep r e l a t i o n s h i p , i t i s important to discuss the r e s u l t s given i n Table 1. The Table includes 1T 1 values obtained by X-ray d i f f r a c t i o n technique, m i c r o f i b r i l angle obtained by mercury impregnation method and 0 angle obtained from Meylan's equation (87). I t was of i n t e r e s t to c a l i b r a t e the angle 1T 1 with the mercury angle. Regression analysis was ca r r i e d out using a l l eight samples from the species used i n t h i s experiment. The c o r r e l a t i o n c o e f f i c i e n t (r) was 0.7990 which i s s i g n i f i c a n t at the 2.5 per cent l e v e l . The regression equation i s given with F i g . 19. Examination of the r e s u l t s given i n Table 1 reveals that the available data are not s u f f i c i e n t to e s t a b l i s h a c a l i b r a t i o n curve for the species used; i n addition, the c o r r e l a t i o n c o e f f i c i e n t i s not good enough i n comparison with that obtained by other workers (Meylan (87)) . A further t r i a l was conducted to c a l i b r a t e the 'T* angle with Meylan's equation for radiata pine. From the re s u l t s given i n Table 1, i t i s clear that there i s a difference between the mercury angle and that obtained from Meylan's equation. A large difference i s noted i n the case of earlywoods of Douglas-fir compression and normal wood, Sitka spruce and western hemlock, and also for Douglas-fir latewood (normal wood). On the other hand, i t i s small i n thelatewoods of Douglas-fir compression wood, S i t k a s p r u c e a n d w e s t e r n h e m l o c k . F o r t h i s r e a s o n M e y l a n ' s e q u a t i o n w a s , i n g e n e r a l , c o n s i d e r e d n o t a c c u r a t e e n o u g h t o be u s e d f o r c a l i b r a t i n g t h e a n g l e "T' o b t a i n e d b y X - r a y t e c h n i q u e f o r s p e c i e s o t h e r t h a n r a d i a t a p i n e . A c c o r d i n g l y , t h e r e s u l t s s u g g e s t t h a t a c a l i b r a t i o n c u r v e i s r e q u i r e d f o r e a c h s p e c i e s . T h i s i s a s e r i o u s d i s a d v a n t a g e w h i c h d e t r a c t s f r o m t h e p r a c t i c a l a p p l i c a t i o n o f X - r a y t e c h n i q u e . S i n c e t h e m e r c u r y a n g l e i s o b t a i n e d b y a d i r e c t m e t h o d , i t i s c o n s i d e r e d t o be a more r e p r e s e n t a t i v e a n d a c c u r a t e v a l u e t h a n t h e one o b t a i n e d f r o m t h e M e y l a n ' s e q u a t i o n . C o n s e q u e n t l y , m i c r o f i b r i l a n g l e s u s e d i n t h e a n a l y s i s o f r e s u l t s w e r e t h o s e o b t a i n e d b y t h e m e r c u r y i m p r e g n a t i o n m e t h o d . I t s h o u l d be i n d i c a t e d t h a t t h e a v e r a g e s shown i n T a b l e 1, Column 2, a r e b a s e d u p o n 33 t o 73 o b s e r v a -g t i o n s . T h e y a r e c o n s i d e r e d t o r e p r e s e n t t h e mean m i c r o -f i b r i l a n g l e f o r e a c h o f t h e i n d i v i d u a l s t r i p s u s e d f o r c r e e p t e s t s . T h i s was n e c e s s a r y b e c a u s e i t was e x t r e m e l y d i f f i c u l t t o a s s i g n a p r e c i s e v a l u e f o r e a c h s t r i p i n d i v i d u a l l y . M i c r o f i b r i l s a r e c o m p o s e d o f a g g r e g a t i o n s o f c e l l u l o s e c h a i n m o l e c u l e s w h i c h r e p r e s e n t t h e m a i n s t r u c t u r a l c o m p o n e n t o f t h e f r a m e w o r k o f t r a c h e i d w a l l s . I t s h o u l d be n o t e d t h a t o n l y c e l l u l o s e i s c o n s i d e r e d h e r e i n t o c o n t r i b u t e t o c r e e p r e -s p o n s e o f wood t i s s u e s i n t h i s s t u d y , e v e n t h o u g h o t h e r c o m p o n e n t s s u c h a s l i g n i n a n d h e m i c e l l u l o s e s , a n d a l s o t h e f o r c e s ( p r i m a r y a n d s e c o n d a r y b o n d s ) w h i c h h o l d t h e m i c r o f i b r i l s g The o r i g i n a l e x p e r i m e n t a l d a t a a r e f i l e d i n Room 38 8, F a c u l t y o f F o r e s t r y . 73 together, may a f f e c t creep response (29, 117); but t h i s remains to be proven experimentally. Indications from r e l a t e d experiments on molecular stress relaxation by Chow (13a) show that a l l these major wood components ( c e l l u l o s e , l i g n i n and hemicelluloses)contribute to stress d i s t r i b u t i o n i n wood. However, they contribute at d i f f e r e n t l e v e l s and t h e i r response to stress i s highly time-dependent. I t has been shown that m i c r o f i b r i l s control the mechanical behavior of wood and other natural c e l l u l o s i c f i b e r s to a great extent by acting as elements for supporting the applied stress and the r e s u l t i n g s t r a i n (9, 18, 47, 60, 103, 117, 128). I f the m i c r o f i b r i l s were arranged p a r a l l e l to the lo n g i t u d i n a l axes of the f i b e r s or tracheids, they would be i n a po s i t i o n to r e s i s t deformation o r i g i n a t i n g from t e n s i l e loads v/ith maximum e f f i c i e n c y . Unfortunately, these s t r u c t u a l elements are oriented at an angle to the long i t u d i n a l axis of the tracheid i n the thick S2 layer. This angle d i f f e r s between species as well as across the annual increment from earlywood to latewood (48, 50, 51, 111). It can be seen from the experimental values presented i n Table 1 that Sitka spruce i s characterized by the lowest m i c r o f i b r i l angle, (9.22°) whereas the earlywood of Douglas-fir compression wood i s characterized by the highest angle (28.22°) among the wood tissues used. In general, earlywood has a larger m i c r o f i b r i l angle than latewood. This v a r i a b i l i t y i s d i r e c t l y related to tracheid 74 length, where, within one annual increment, earlywood has always shorter tracheids then latewood (115, 118, 155). Accordingly, species or wood tissues (earlywood or latewood) are expected to react d i f f e r e n t l y to the applied constant stress, depending l a r g e l y on the m i c r o f i b r i l angle. According to the s t a t i s t i c a l analyses, m i c r o f i b r i l angle i s the most important s t r u c t u r a l feature a f f e c t i n g creep response, whether as a single variable or combined with s p e c i f i c gravity and extractives content i n the multiple regression Equations [3] and [4]. Based on the 34 specimens from earlywood and latewood tissues of the abovenoted species, the rela t i o n s h i p s between m i c r o f i b r i l angle and t o t a l creep response under s t r a i n l e v e l No. 1 (Fig. 21) and s t r a i n l e v e l No. 2 (Fig. 22) are established herein. I t can be seen from the simple regression equation given i n F i g . 21 that 6 6.90 per cent of the v a r i a t i o n i n creep response i s a t t r i b u t a b l e to m i c r o f i b r i l angle at 3,000 microin. per i n . s t r a i n l e v e l . At the higher i n i t i a l s t r a i n l e v e l , 6,000 microin. per i n . , 68.19 per cent of the v a r i a t i o n i n creep response i s at t r i b u t a b l e to m i c r o f i b r i l angle (Fig. 22). This suggests that m i c r o f i b r i l angle must play an important and consistent part i n creep response. In general, t o t a l creep increases as m i c r o f i b r i l angle increases. A specimen having a large m i c r o f i b r i l angle p r i o r to ap p l i c a t i o n of a constant t e n s i l e s t r a i n could l o g i c a l l y be expected to react to a greater degree than a specimen having a r e l a t i v e l y s m a l l m i c r o f i b r i l a n g l e . T h i s r e a c t i o n w o u l d r e s u l t i n a l a r g e r d e f o r m a t i o n i n t h e c a s e o f t h e f o r m e r . D u r i n g t h e c o u r s e o f t h i s w o r k m i c r o f i b r i l a n g l e was n o t m e a s u r e d a f t e r a p p l y i n g s t r a i n due t o some e x p e r i -m e n t a l l i m i t a t i o n s . H o w e v e r , c l o s e e x a m i n a t i o n o f t h e 9 g r a p h s p r e s e n t e d i n F i g u r e s . 1 5 a n d 16, f o r e x a m p l e , r e v e a l s t h a t t h e r a t e o f c r e e p a t b o t h s t r a i n l e v e l s u s e d i s l a r g e r i n t h e c a s e o f D o u g l a s - f i r e a r l y w o o d ( a n a n g l e o f 21.63°) t h a n i n t h e c a s e o f s p r u c e l a t e w o o d ( a n a n g l e o f 9.22). T h i s l a r g e r r a t e o f c h a n g e c o u l d be c o n s i d e r e d a r e s u l t o f t h e e x p e c t e d l a r g e movement o f m i c r o f i b r i l s t o o r i e n t t h e m s e l v e s w i t h t h e a p p l i e d c o n s t a n t s t r a i n . T h i s a r g u m e n t i s s u p p o r t e d b y t h e w o r k o f J e n t z e n (60) o n l o n g l e a f p i n e e a r l y w o o d a n d l a t e w o o d p u l p s . J e n t z e n h a s s t a t e d t h a t e a r l y w o o d t r a c h e i d s k e l e t o n s u n d e r w e n t a g r e a t e r c h a n g e i n t h e i r ' c r y s t a l l i t e o r i e n t a t i o n ' t h a n d i d t h e d e l i g n i f i e d l a t e w o o d t r a c h e i d s k e l e t o n s u n d e r t h e same l o a d i n t e n s i o n . S i m i l a r r e s u l t s w e r e a l s o r e p o r t e d b y H i l l (47) f o r t h e l a t e w o o d p u l p o f t h e same s p e c i e s . W o r k i n g on s i s a l f i b e r s , B a l a s h o v e t a l . (6) w e r e a b l e t o r e c o r d a d e c r e a s e i n m i c r o f i b r i l a n g l e u n d e r t e n s i l e s t r a i n s t h a t i n c r e a s e d f r o m 5 t o 2 0 p e r c e n t . The r e s t o f c r e e p d i a g r a m s a r e f i l e d i n Room 388, F a c u l t y o f F o r e s t r y . 76 A c c e p t i n g t h e f a c t t h a t m i c r o f i b r i l a n g l e becomes s m a l l e r u n d e r a c o n s t a n t s t r a i n f o r a r e l a t i v e l y l o n g p e r i o d o f t i m e (60 m i n u t e s ) i t f o l l o w s t h a t t h e c h a n g e t o w a r d s a s m a l l e r a n g l e c a n be e x p e c t e d t o be l a r g e f o r a s p e c i m e n h a v i n g a l a r g e i n i t i a l a n g l e . T h i s w i l l i n t u r n l e a d t o h i g h e r c r e e p d e f o r m a t i o n due t o t h e l a r g e e x p e c t e d movement a s s o c i a t e d w i t h a d j u s t m e n t o f t h e s t i f f i n e x t e n s i b l e m i c r o -f i b r i l s , t o accommodate t h e a p p l i e d s t r a i n w i t h o u t f a i l u r e . The p o s s i b i l i t y a l s o e x i s t s t h a t t h e m i c r o f i b r i l s m i g h t s l i p p a s t e a c h o t h e r i f t h e a p p l i e d s t r a i n i s l a r g e e n o u g h , a s i n t h e c a s e o f i n i t i a l s t r a i n o f 6,000 m i c r o i n . p e r i n . , g i v i n g r i s e t o t h e h i g h o b s e r v e d c r e e p (6, 117). T h e s e r e s u l t s i n d i c a t e t h a t t h e m i c r o f i b r i l a n g l e o f t h e S2 l a y e r o f t h e t r a c h e i d w a l l i s one o f t h e f u n d a m e n t a l c h a r a c t e r i s t i c s o f t h e c e l l w a l l a n d e x e r t s a p r o f o u n d e f f e c t o n t h e t i m e - d e p e n d e n t b e h a v i o r o f wood. T h e r e f o r e , m i c r o -f i b r i l a n g l e s h o u l d be c o n s i d e r e d a s one i m p o r t a n t b a s i c m e a s u r e m e n t f o r s e l e c t i n g m a t e r i a l o f t h e r e q u i r e d s t r e n g t h f o r s p e c i f i c p u r p o s e s . I I I . R e l a t i o n s h i p o f C e l l W a l l C r y s t a l l i n i t y t o T o t a l C r e e p M i c r o f i b r i l a n g l e i s n o t t h e o n l y i m p o r t a n t s t r u c t u r a l f e a t u r e a f f e c t i n g t h e c r e e p r e s p o n s e o f wood. A c c o r d i n g t o t h e g e n e r a l l y a c c e p t e d f r i n g e d f i b r i l l a r m o d e l f o r t h e s u p e r -m o l e c u l a r a r r a n g e m e n t o f t h e c e l l u l o s e c h a i n s , t h e m i c r o -f i b r i l s a r e c o n s i d e r e d t o be l a r g e , i m p e r f e c t c r y s t a l s , s e p a r a t e d a l o n g t h e i r p e r i p h e r i e s b y n o n - c r y s t a l l i n e r e g i o n s ( 3 5 , 3 7 ) . C o n s e q u e n t l y , t h e r e l a t i v e d e g r e e o f c e l l w a l l c r y s t a l l i n i t y s h o u l d l o g i c a l l y a f f e c t c r e e p b e h a v i o r o f wood. A c c o r d i n g t o t h e r e s u l t s o f t h i s i n v e s t i g a t i o n , t o t a l c r e e p r e s p o n s e d e c r e a s e s a s t h e r e l a t i v e d e g r e e o f c r y s t a l l i n i t y i n t r a c h e i d w a l l s i n c r e a s e s ( F i g u r e s 23 a n d 2 4 ) . As a s i n g l e s t r u c t u r a l f e a t u r e , c e l l w a l l c r y s t a l l i n i t y was f o u n d t o c o n t r i b u t e up t o 64.27 and 54.11 p e r c e n t o f t h e t o t a l v a r i a b i l i t y i n c r e e p r e s p o n s e u n d e r i n i t i a l s t r a i n c o n d i t i o n s Numbers 1 a n d 2, r e s p e c t i v e l y . E x a m i n a t i o n o f T a b l e 2 i n d i c a t e s t h a t w i t h i n one a n n u a l i n c r e m e n t o f t h e s p e c i e s u s e d , e x c e p t w e s t e r n h e m l o c k , l a t e w o o d t i s s u e h a s h i g h e r r e l a t i v e d e g r e e o f c r y s t a l l i n i t y t h a n e a r l y w o o d . The a v e r a g e s o f t h i s c h a r a c t e r i s t i c a r e shown b e l o w . RELATIVE DEGREE OF C R Y S T A L L I N I T Y (PER CENT) E a r l y w o o d L a t e w o o d D o u g l a s - f i r N o r m a l wood C o m p r e s s i o n wood S i t k a s p r u c e W e s t e r n h e m l o c k 59.59 58.16 61.75 61.14 63.80 60.84 65.02 58.50 These r e s u l t s confirm the general trend observed by Holzer and Lewis (53) on Douglas-fir, and also by Lindgren (77) on Swedish spruce, that within an annual increment, earlywood i s lower i n c r y s t a l l i n i t y than latewood. The v a r i a b i l i t y between earlywood and latewood with regard to the abovenoted c h a r a c t e r i s t i c could possibly be due to two causes: 1. I t seems that the presence of l i g n i n reduces the degree of c e l l u l o s e l a t t i c e p erfection. This i s confirmed by two indica t i o n s i n the l i t e r a t u r e : (a) Within one annual increment, except for western hemlock, earlywood i s two to three per cent higher i n l i g n i n content than latewood (160) . (b) Removal of l i g n i n from slash pine samples increased the proportion of c r y s t a l l i n e material (98a). 2. Earlywood has larger m i c r o f i b r i l angle than l a t e -wood (Table 1), within the same annual increment. In the present study, m i c r o f i b r i l angle i s shown to be strongly related (inversely) to the r e l a t i v e degree of c r y s t a l l i n i t y (Fig. 25). This r e l a t i o n -ship w i l l be discussed below. With regard to compression wood, the above r e s u l t s reveal that the r e l a t i v e degree of c r y s t a l l i n i t y i s consider-79 ably lower than that of Douglas-fir normal wood. The d i f f e r -ence i s more pronounced i n the case of latewood ti s s u e s . Numerical values of t h i s c h a r a c t e r i s t i c are higher than those obtained by Lee (73) . In the present study, averages are 61.7 and 59.5 per cent for Douglas-fir normal and compression wood, whereas those obtained by Lee were 54.3 and 4 6.4 per cent. This i s probably due to the fact that Lee used samples from d i f f e r e n t annual increments. In addition, he determined c r y s t a l l i n i t y on wood pulp prepared by the peracetic acid method. Recently, s i m i l a r differences were also reported between normal and compression wood of l o b l o l l y pine by Parham (105a). The higher l i g n i n content (138), larger m i c r o f i b r i l angle, and possibly the absence of the S3 layer of the compression wood tracheids appear to be responsible for i t s lower r e l a t i v e degree of c r y s t a l l i n i t y . The r o l e which c r y s t a l l i n i t y plays i n influencing creep response i s possible since the c r y s t a l l i t e s form the r i g i d r e i n f o r c i n g part of the tracheid wall structure. If t h i s well ordered portion i s predominant, wood exhibits a high degree of resistance to the stress which i s applied for a long period of time, thereby minimizing the develop-ment of excessive creep. A c o n t r o v e r s i a l issue has been reported i n the l i t e r a t u r e concerning the e f f e c t of applied load on c r y s t a l l i n i t y of the c e l l wall. Murphey (92) has reported 80 that c r y s t a l l i n i t y of yellow b i r c h and sugar maple increased as the t e n s i l e load was increased. This change was constant over a 24-hour period, but the c r y s t a l l i n i t y did not return to the o r i g i n a l l e v e l a f t e r a recovery period. On the other hand, H i l l (47) and Jentzen (60) , using longleaf pine h o l o c e l l u l o s e tracheid skeletons from earlywood and latewood t i s s u e s , did not observe any s i g n i f i c a n t change i n c r y s t a l l i n -i t y due to t e n s i l e load imposed during drying. In s p i t e of t h i s controversy, i t i s reasonable to believe that, under stretching conditions, there may be some molecular movement i n the amorphous portion of the c e l l wall which would r e s u l t i n improving the degree of s t r u c t u r a l perfection. This would happen through increase i n the degree of c r y s t a l l i n i t y . Whether the degree of c r y s t a l l i n i t y changes or not requires further i n v e s t i g a t i o n , but i t i s at l e a s t clear from the r e s u l t s of t h i s experiment that the i n i t i a l degree of c r y s t a l l i n i t y i s an important factor i n p r e d i c t i n g t o t a l creep response. Examination of the multiple regression.analyses (Tables 4 and 5) reveals that, a f t e r adjusting for the l i n e a r r e l a t i o n s h i p between t o t a l creep and each of the independent var i a b l e s , m i c r o f i b r i l angle, s p e c i f i c gravity, and extractives content, the r e l a t i v e degree of c r y s t a l l i n i t y i s no longer a s i g n i f i c a n t v a r i a b l e . This behavior was observed f o r both i n i t i a l s t r a i n l e v e l s used and, accordingly, the r e l a t i v e amount of c r y s t a l l i n i t y was the f i r s t independent 81 variable to be eliminated from the regression equations. The reason for t h i s may be found i n the fact that c r y s t a l l i n -i t y i s highly correlated with m i c r o f i b r i l angle (r = 0.9310 for the lower s t r a i n l e v e l and 0.9238 for the higher s t r a i n level) (Table 9). Therefore, i f m i c r o f i b r i l angle i s included i n the regression equation i t also takes care of the c r y s t a l l i n i t y e f f e c t . The r e l a t i o n s h i p between micro-f i b r i l angle and c r y s t a l l i n i t y i s shown i n F i g . 25. I t can be seen from t h i s figure that the small m i c r o f i b r i l angle i s associated with a high r e l a t i v e degree of c r y s t a l l i n i t y . Based on eight measurements only, the degree of c o r r e l a t i o n was found for the combined data to be very high (r = 0.9284) , as shown i n F i g . 25. C r y s t a l l i n i t y of c e l l u l o s e f i b e r s has been under i n v e s t i g a t i o n for a r e l a t i v e l y long period of time. The most d i f f i c u l t problem has been and s t i l l i s the i n a b i l i t y of s c i e n t i s t s to separate the e f f e c t of c r y s t a l l i n i t y on strength properties of natural and regenerated c e l l u l o s e f i b e r s from that of c r y s t a l l i t e s ^ o r i e n t a t i o n (57, 143). With a heterogenous material such as wood, the problem i s greater. The r e l a t i o n s h i p between c r y s t a l l i n i t y and micro-f i b r i l angle was reported by Lindgren (77) for Swedish spruce. A p o s i t i v e r e l a t i o n s h i p between the c r y s t a l l i n i t y of the paper fi b e r s and f i b e r length was reported by Ohta et a l . (101). Since the m i c r o f i b r i l angle decreases as the f i b e r length increases (115, 118, 155), the m i c r o f i b r i l 82 a n g l e s h o u l d be i n v e r s e l y r e l a t e d t o t h e c e l l w a l l c r y s t a l l i n -i t y . To e x p l a i n t h i s r e l a t i o n s h i p one c o u l d c o n s i d e r t h e t h e o r e t i c a l a n d e x p e r i m e n t a l w o r k b y Cave ( 1 3 ) , who h a s shown t h a t m i c r o f i b r i l d i r e c t i o n s c a t t e r i n c r e a s e s w i t h i n c r e a s i n g mean m i c r o f i b r i l a n g l e a b o u t t h e l o n g i t u d i n a l t r a c h e i d a x i s . T h i s , i n f a c t , w o u l d mean t h a t i n t h e c a s e o f a s m a l l ( s t e e p ) a n g l e , t h e s c a t t e r a r o u n d t h e mean m i c r o f i b r i l a n g l e i s s m a l l e r a n d t h e m i c r o f i b r i l s i n t h e S2 l a y e r p r o b a b l y l i e a l m o s t p a r a l l e l t o e a c h o t h e r . As a r e s u l t , t h e r e l a t i v e d e g r e e o f amorphous m a t e r i a l , ( a morphous c e l l u l o s e , h e m i c e l l u l o s e s a n d l i g n i n ) r e q u i r e d t o f i l l t h e m i c r o - s p a c e s b e t w e e n t h e m i c r o f i b r i l s w o u l d be s m a l l e r . C o n s i d e r i n g t h e c a s e o f a l a r g e a n g l e , t h e m i c r o f i b r i l s a r e p r o b a b l y n o t p a r a l l e l t o e a c h o t h e r ; c o n s e q u e n t l y , r e l a t i v e l y l a r g e m i c r o - s p a c e s b e t w e e n t h e m i c r o f i b r i l s w o u l d be o c c u p i e d by t h e amorph o u s m a t e r i a l . T h i s w o u l d t h e n r e s u l t i n a r e l a t i v e l y l o w e r c r y s t a l l i n i t y o f t h e c e l l w a l l as shown b y F i g . 2 5 , and T a b l e s 1 a n d 2. A s e c o n d p o s s i b l e r e a s o n f o r t h i s r e l a t i o n s h i p may be t h a t c e l l u l o s e c h a i n m o l e c u l e s , i n t h e c a s e o f a s m a l l m i c r o f i b r i l a n g l e , w i l l h a v e a b e t t e r c h a n c e f o r i n c r e a s e d f r e q u e n c y o f c r o s s l i n k s ( b o n d i n g b e t w e e n n e i g h -b o u r i n g c h a i n s ) a l o n g t h e i r u n i t l e n g t h . C o n s e q u e n t l y , a t e n d e n c y o f i m p r o v e d g e o m e t r i c o r d e r s h o u l d be o b s e r v e d w i t h b e t t e r c h a i n c o h e r e n c e i n t h e r e s u l t i n g c e l l u l o s e a s com-83 p a r e d t o s i t u a t i o n s a s s o c i a t e d w i t h t r a c h e i d s c h a r a c t e r i z e d b y l a r g e r m i c r o f i b r i l a n g l e . I t s h o u l d b e i n d i c a t e d t h a t t h e a b o v e n o t e d r e a s o n s a r e a t t e m p t s t o e x p l a i n t h e r e l a t i o n s h i p , h o w e v e r , t h e e x a c t n a t u r e o f i t s t i l l r e q u i r e s f u r t h e r i n v e s t i g a t i o n . I t s h o u l d be r e - e m p h a s i z e d t h a t , w i t h r e g a r d t o c r e e p r e s p o n s e o f wood i n t e n s i o n p a r a l l e l t o t h e g r a i n , m i c r o -f i b r i l a n g l e s h o u l d l o g i c a l l y be u s e d i n t h e m u l t i p l e r e -g r e s s i o n m o d e l ( E q u a t i o n s [3] a n d [4]). I n d o i n g s o , t h e e f f e c t o f c r y s t a l l i n i t y c o n t e n t i s i n c l u d e d p r i m a r i l y due t o i t s h i g h c o r r e l a t i o n w i t h m i c r o f i b r i l a n g l e . On t h e o t h e r h a n d , b e c a u s e o f t h i s same c o r r e l a t i o n , r e l a t i v e d e g r e e o f c r y s t a l l i n i t y c o u l d be u s e d i n t h e a b o v e n o t e d m o d e l s i n s t e a d o f m i c r o f i b r i l a n g l e b u t w i t h a b o u t 10 p e r c e n t r e d u c t i o n i n t h e R v a l u e s ( T a b l e s 4 a n d 5). R e a s o n s f o r t h i s h i g h d e g r e e o f c o r r e l a t i o n , a s n o t e d a b o v e , r e m a i n c o n j e c t u r a l . I V . R e l a t i o n s h i p o f S p e c i f i c G r a v i t y t o T o t a l C r e e p S p e c i f i c g r a v i t y , i . e . , t h e amount o f c e l l w a l l s u b s t a n c e c o n t a i n e d i n a u n i t v o l u m e , d i d n o t c o n t r i b u t e 2 s i g n i f i c a n t l y t o c r e e p r e s p o n s e a s a s i n g l e v a r i a b l e ( r = 0.0848 a n d 0.0148 f o r t h e l o w e r ( F i g . 26) a n d h i g h e r ( F i g . 27) i n i t i a l s t r a i n s , r e s p e c t i v e l y ) . On t h e o t h e r h a n d when s p e c i f i c g r a v i t y was c o m b i n e d w i t h m i c r o f i b r i l a n g l e a n d e x t r a c t i v e s c o n t e n t , i t c o n t r i b u t e d s i g n i f i c a n t l y t o c r e e p b e h a v i o r . I n o t h e r w o r d s , s p e c i f i c g r a v i t y became 84 an important variable a f t e r adjusting for the l i n e a r r e l a t i o n -ship between creep and each of m i c r o f i b r i l angle and extrac-t i v e s content. This i s due to the f a c t that s p e c i f i c gravity cannot act alone, but must be combined with the other v a r i a -bles . Examination of the r e s u l t s given i n Table 3, Columns 2 and 5, reveals that s p e c i f i c gravity values of Douglas-fir earlywood (normal wood) samples used for the lower i n i t i a l s t r a i n l e v e l are d i f f e r e n t from those used for the higher i n i t i a l s t r a i n l e v e l . This i s probably due to the morphological v a r i a t i o n such as c e l l wall thickness and c e l l diameter between the two groups of samples. In addition, i t i s supported by the unpublished r e s u l t s of Parker (106a) who found substantial differences i n s p e c i f i c gravity along the circumference of the same annual increment. The i n t e r p r e t a t i o n given to the e f f e c t of s p e c i f i c g r a v i t y i s the usual one. Besides representing the amount of c e l l wall material per unit volume, woods with high s p e c i f i c gravity also contain a high r a t i o of S2 layer to t o t a l tracheid wall thickness (24, 60, 64). The m i c r o f i b r i l s i n t h i s layer make a smaller angle with the l o n g i t u d i n a l tracheid axis than do the other c e l l wall layers. Consequently, a large portion of the c e l l wall would have a small micro-f i b r i l angle, which i n turn l i m i t s the deformation to a large extent. I t i s confirmed by the r e s u l t s given i n Table 9 that s p e c i f i c gravity i s inversely correlated with 85 the m i c r o f i b r i l angle. Therefore, t o t a l creep response decreases as s p e c i f i c gravity increases. The r e s u l t i s i n agreement with the work done by Erickson and Sauer (24) and also confirms King's findings (65). The l a t t e r has concluded that species of low density and high e x t e n s i b i l i t y e x h i b i t greater creep than species with a high s p e c i f i c gravity but low e x t e n s i b i l i t y , i n and below the region of the threshold of set. This reported inverse r e l a t i o n s h i p between exten-s i b i l i t y and s p e c i f i c gravity could also be explained by u t i l i z i n g an approach s i m i l a r to that used to explain the r e l a t i o n s h i p between creep and s p e c i f i c g r a vity. Another point of i n t e r e s t i s that the r e l a t i o n s h i p between the t o t a l creep response and s p e c i f i c gravity i s more pronounced under the 3,000 than under the 6,000 microin. per i n . s t r a i n l e v e l . Keeping i n mind King's work, i n which he stated that the creep - i n i t i a l s t r a i n r e l a t i o n s h i p was l i n e a r up to the threshold of set and c u r v i l i n e a r there-a f t e r , i t i s possible that below the threshold of set, s p e c i f i c gravity plays a r e l a t i v e l y important part. Once set occurs, other v a r i a b l e s , e s p e c i a l l y m i c r o f i b r i l angle, i n t e r a c t together and overshadow the importance of s p e c i f i c gravity i n r e l a t i o n to creep response. This a c t u a l l y confirms Kellogg 1s r e s u l t s (61), wherein l i t t l e or no c o r r e l a t i o n was found between the species r e s i d u a l and s p e c i f i c g ravity above the threshold of set. 86 It i s shown i n Table 9A that s p e c i f i c gravity i s p o s i t i v e l y correlated with the c e l l wall c r y s t a l l i n i t y . The r e l a t i o n s h i p does not require further explanation, since the c r y s t a l l i n e c e l l u l o s e i s supposed to have higher s p e c i f i c gravity than the amorphous one. This i s due to the fact that the amount of c e l l wall material per unit volume of the former i s greater than that of the l a t t e r . However, the same r e l a t i o n -ship did not reach s i g n i f i c a n t l e v e l for the samples tested at the higher s t r a i n l e v e l (Table 9B). This probably i s due to the o r i g i n a l differences i n s p e c i f i c gravity of groups of Douglas-fir earlywood (normal wood) samples tested at each s t r a i n l e v e l (Table 3). Nevertheless, i t i s u n l i k e l y that the above r e l a t i o n s h i p i s applicable at the macro-level. V. Relationship of Extractives Content to Total Creep Extractives content for each group of samples i s given i n Table 3. The averages are 6.14, 7.05, 4.16 and 4.89 per cent for Douglas-fir normal and compression wood, Sitka spruce and western hemlock, re s p e c t i v e l y . These averages are obtained by simple c a l c u l a t i o n of the values given i n Table 3 for each growth increment. Examining the published data of the extractives content for the same species revealed that the average extractives content d i f f e r s from one authority to the other, depending on the method used for determination and also on the basis for c a l c u l a t i o n (58, 74). However, the above values are not unreasonable i n comparison with those reported by Lewis ( 7 4 ) . ^ "^5.92, 4.90 and 5.30 per cent for Douglas-fir, black spruce and western hemlock, resp e c t i v e l y . 87 D e t e r m i n a t i o n o f t h e e x t r a c t i v e s c o n t e n t was d o n e on s a m p l e s a f t e r t h e y w e r e u s e d f o r c r e e p t e s t s ; t h e amount o f e x t r a c t i v e s was t h e n c o r r e l a t e d w i t h t h e t o t a l c r e e p r e s p o n s e . The i d e a l e x p e r i m e n t a l a p p r o a c h t o t h e c r e e p r e s p o n s e - e x t r a c t i v e s c o n t e n t r e l a t i o n s h i p w o u l d be t o r u n c r e e p t e s t s o n c o n t r o l s p e c i m e n s w i t h e x t r a c t i v e s r e m o v e d . B u t i t was r e a l i z e d t h a t , d u r i n g e x t r a c t i o n , s o m e c h a n g e s m i g h t h a v e t a k e n p l a c e i n t h e s a m p l e s due t o t h e e f f e c t o f d r y i n g and a l s o due t o s w e l l i n g c h a n g e s t h a t o c c u r d u r i n g e x t r a c t i o n . T h e r e f o r e , a c o m p a r i s o n b e t w e e n c o n t r o l a n d e x t r a c t e d s a m p l e s i s n o t v a l i d and w o u l d be m i s l e a d i n g , u n d e r t h i s c o n d i t i o n . The i m p o r t a n c e o f e x t r a c t i v e s e f f e c t on c r e e p r e s p o n s e h a s n o t b e e n c l e a r l y d e t e r m i n e d b y t h e f e w s t u d i e s d o n e i n t h e l a s t d e c a d e on s p e c i e s e n t i r e l y d i f f e r e n t f r o m t h e o n e s u s e d i n t h i s i n v e s t i g a t i o n . The r e a s o n l i e s i n t h e f a c t t h a t t h e e x a c t l o c a t i o n o f e x t r a c t i v e s i n wood t i s s u e s i s n o t w e l l d e f i n e d . I f e x t r a c t i v e s a r e l o c a t e d o n t h e l u m e n s u r f a c e s o f t r a c h e i d s a n d o f wood r a y s , t h e m a j o r r e p o s i t o r y i n wood s t r u c t u r e , t h e y w o u l d n o t b e e x p e c t e d t o i n f l u e n c e c r e e p r e s p o n s e and o t h e r m e c h a n i c a l p r o p e r t i e s ; i f t h e y a r e l o c a t e d w i t h i n t h e c e l l w a l l s t r u c t u r e , i . e . , i n t h e a m o r p h o u s r e g i o n , t h e y w o u l d p r o b a b l y a f f e c t c r e e p a n d o t h e r m e c h a n i c a l p r o p e r t i e s . A f e w s t u d i e s h a v e shown t h a t a p p r e c i a b l e amount o f e x t r a c t i v e s i s l o c a t e d i n t h e c e l l w a l l s t r u c t u r e (90, 105, 137). T a r k o w a n d K r u e g e r (137), f o r e x a m p l e , h a v e 88 found that approximately 75 per cent of the t o t a l water soluble extractives of redwood i s located within the c e l l wall structure, probably i n the amorphous region. But i t must be r e a l i z e d that redwood contains a large amount of water soluble extractives (over 20 per cent of the oven-dry weight); whereas the t o t a l amount of extractives, i n any one of the species used i n the present i n v e s t i g a t i o n , did not even reach 10 per cent of the ext r a c t i v e - f r e e oven-dry weight. The r e s u l t of t h i s author's work indicated that extractives content i s an important variable i n governing creep response whether as a single parameter (Figures 28 and 29) or combined with others. I t i s even more important than s p e c i f i c gravity (Figures 26, 27, 28 and 29). As the extractives content increases, more creep response i s expected. This r e l a t i o n s h i p between creep response and extractives content i s not only possible under high tempera-ture (24, 94, 96) but also under ambient temperature as used i n t h i s work (73 ± 3.5°F). 2 I t i s noted i n t h i s study that the r value for t o t a l creep-extractives content r e l a t i o n s h i p i s 16.98 per cent at the 3,000 microin. per i n . s t r a i n l e v e l (Fig. 28) and 47.88 per cent at the 6,000 microin. per i n . s t r a i n l e v e l (Fig. 29). This i s due to the fa c t that the samples used for the l a t t e r contain larger percentages of extractives than those used for the former, except for western hemlock earlywood (Table 3). Differences i n the extractive content 89 between the two cases are given i n percentage below. Earlywood Latewood Douglas-fir Normal wood Compression wood Sitka spruce Western hemlock 19.8 10.7 15.3 14.3 11 28.0 3.5 11.1 21.3 Accordingly, i f the amount of extractives increases, as pointed out above, higher c o r r e l a t i o n would be expected with t o t a l creep. Extractives are not expected to a f f e c t creep re-sponse of wood unless they are p a r t l y located i n the c e l l wall structure. Since i t was found i n t h i s study that creep response depends p a r t l y on extractives content, i t i s reasonable to believe that part of these extractives was located within the c e l l wall structure, e s p e c i a l l y i n the amorphous region. This i n d i r e c t evidence i s , i n general, i n agreement with the r e s u l t s obtained by Tarkow and Krueger (139) and also by Morgen and Orsler (90) on very d i f f e r e n t species, as indicated e a r l i e r . However, a more de t a i l e d study of the absolute creep values of extracted and unextracted wood sections might be i n d i c a t i v e of the r e a l action by which extractives exert t h e i r e f f e c t s . ''"''"This means that the average extractives content of Douglas-fir earlywood samples used for the higher s t r a i n l e v e l i s 19.8 per cent higher than that of the samples used for the lower s t r a i n l e v e l . 90 The r o l e w h i c h e x t r a c t i v e s p l a y i n c o n t r o l l i n g c r e e p r e s p o n s e o f wood i n t e n s i o n p a r a l l e l t o t h e g r a i n i s p r o b a b l y due t o t h e i r i n d u c e d p l a s t i c i z a t i o n e f f e c t . S i n c e c r y s t a l l i n e r e g i o n s o f t h e c e l l w a l l a r e b e l i e v e d t o be i m p e r m e a b l e t o t h e s e c o mpounds, i t i s n o t u n r e a s o n a b l e t o p r o p o s e t h a t p a r t o f them i s l o c a t e d i n t h e a m o r p h o u s r e g i o n s , b e t w e e n m i c r o f i b r i l s / a s i n d i c a t e d e a r l i e r . I t f o l l o w s t h a t t h e i r p r e s e n c e i n t h e s e p a r t i c u l a r m i c r o - s p a c e s r e d u c e s t h e b o n d i n g ( p r i m a r y and s e c o n d a r y ) b e t w e e n s t r u c t u r a l e l e m e n t s a n d h e n c e m i n i m i z e s t h e i r a b i l i t y t o r e s i s t a p p l i e d s t r e s s e s . A s a r e s u l t , t h e movement o f m i c r o f i b r i l s u n d e r t h e a p p l i e d s t r e s s i s f a c i l i t a t e d b y t h e e x i s t e n c e o f e x t r a c t i v e s . Some i n t e r e s t i n g c o r r e l a t i o n s w e r e f o u n d i n t h i s s t u d y b e t w e e n e x t r a c t i v e s a n d t h e o t h e r i n d e p e n d e n t v a r i a b l e s u s e d . E x t r a c t i v e s c o n t e n t was shown i n T a b l e 9 t o be r e l a t e d p o s i t i v e l y t o t h e m i c r o f i b r i l a n g l e . T h i s means t h a t a s t h e l a t t e r becomes l a r g e r t h e r e i s t h e p o s s i b i l i t y f o r more o f t h e f o r m e r t o e x i s t i n t h e a m o r p h o u s r e g i o n s . I t i s c o n f i r m e d b y t h e i n v e r s e c o r r e l a t i o n f o u n d b e t w e e n t h e r e l a t i v e d e g r e e o f c r y s t a l l i n i t y a n d e x t r a c t i v e s c o n t e n t . E x t r a c t i v e s c o n t e n t was a l s o i n v e r s e l y r e l a t e d t o s p e c i f i c g r a v i t y i n t h e c a s e o f t h e s a m p l e s t e s t e d a t t h e l o w e r s t r a i n l e v e l ( T a b l e 9 A ) . S i n c e i t i s p r o p o s e d t h a t e x t r a c t i v e s r e d u c e b o n d i n g b e t w e e n m i c r o f i b r i l s , t h e r e f o r e , t h e i r e x i s t e n c e w o u l d r e s u l t i n l e s s wood s u b s t a n c e p e r u n i t v o l u m e . The same two i n d e p e n d e n t v a r i a b l e s w e r e n o t s i g n i f i c a n t l y c o r r e l a t e d f o r s a m p l e s t e s t e d a t t h e h i g h e r s t r a i n l e v e l 91 ( T a b l e 9 B ) . The p r o b a b l e r e a s o n f o r t h i s a n o m a l y i s t h a t D o u g l a s - f i r e a r l y w o o d s a m p l e s ( n o r m a l wood) t e s t e d a t t h e 6,000 m i c r o i n . p e r i n . s t r a i n l e v e l h a v e h i g h e r s p e c i f i c g r a v i t i e s t h a n t h o s e t e s t e d a t t h e 3,000 m i c r o i n . p e r i n . s t r a i n l e v e l . I n s u m m a r i z i n g t h e a b o v e r e s u l t s , i t i s e v i d e n t t h a t s t r u c t u r a l f e a t u r e s , s u c h a s m i c r o f i b r i l a n g l e a n d s p e c i f i c g r a v i t y , i n a d d i t i o n t o e x t r a c t i v e s c o n t e n t , p l a y an i m p o r t a n t r o l e i n g o v e r n i n g t h e t o t a l c r e e p r e s p o n s e o f wood ( E q u a t i o n s [3] a nd [ 4 ] ) . T h i s r e s p o n s e i s e n h a n c e d i n e a r l y w o o d m a i n l y by t h e l a r g e m i c r o f i b r i l a n g l e ; i n a d d i t i o n , l o w s p e c i f i c g r a v i t y ( F i g u r e s 26 and 27) a n d h i g h e r e x t r a c t i v e s c o n t e n t ( F i g u r e s 29 a n d 2 9 ) , h e l p i n m a k i n g c r e e p r e s p o n s e more p r o n o u n c e d . V I . S i g n i f i c a n c e o f R e s u l t s The r e s u l t s o f t h i s s t u d y a p p e a r t o p r o v i d e new a n d w o r t h w h i l e i n f o r m a t i o n c o n c e r n i n g c r e e p r e s p o n s e as a f u n c t i o n o f some s t r u c t u r a l f e a t u r e s o f wood. The i n f o r m a t i o n o b t a i n e d i s o f f u n d a m e n t a l s c i e n t i f i c i n t e r e s t b e c a u s e o f t h e r e l a t i o n s h i p s r e v e a l e d b e t w e e n c r e e p r e s p o n s e i n t e n s i o n p a r a l l e l t o t h e g r a i n a n d e a c h o f m i c r o f i b r i l a n g l e o f t h e S2 l a y e r , r e l a t i v e d e g r e e o f c r y s t a l l i n i t y o f c e l l w a l l c e l l u l o s e , s p e c i f i c g r a v i t y a n d e x t r a c t i v e s c o n t e n t . T h i s i n c r e a s e s k n o w l e d g e a b o u t t h e r h e o l o g i c a l b e h a v i o r o f wood. S u c h k n o w l e d g e may a l s o g i v e g u i d a n c e t o t r e e b r e e d e r s a n d f o r e s t m a n a g e r s who, t h r o u g h t h e i r c o m b i n e d e f f o r t s , a i m a t f a s t e r t r e e g r o w t h a n d d e c r e a s e d r o t a t i o n s . The v a l u e o f s u c h e f f o r t s c a n be m e a s u r e d o n l y t h r o u g h m a i n t e n a n c e o r i m p r o v e m e n t o f i m p o r t a n t wood c h a r a c t e r i s t i c s . H o w e v e r , o f p a r t i c u l a r i m p o r t a n c e was t h e d e t e r m i n a t i o n o f t h e m o s t i m p o r t a n t s t r u c t u r a l f e a t u r e c o n t r o l l i n g t h e m a g n i t u d e o f c r e e p r e s p o n s e , n a m e l y m i c r o f i b r i l a n g l e o f t h e S2 l a y e r . T h i s s u p p o r t s t h e e a r l i e r e v i d e n c e t h a t m i c r o f i b r i l a n g l e s h o u l d be c o n s i d e r e d as one o f t h e i m p o r t a n t b a s i c m e a s u r e -m e n t s w h i c h i n f l u e n c e p h y s i c a l a n d m e c h a n i c a l p r o p e r t i e s o f wood. The r e l a t i o n s h i p s r e v e a l e d i n t h i s s t u d y , b e t w e e n t o t a l c r e e p and t h e a b o v e n o t e d c h a r a c t e r i s t i c s , c o u l d a l s o h e l p i n f o r m u l a t i n g an a p p r o p r i a t e r h e o l o g i c a l m o d e l f o r a c o m p l e t e d e s c r i p t i o n o f t h e c r e e p b e h a v i o r o f wood, i . e . , p h e n o m e n o l o g i c a l r h e o l o g y . The k n o w l e d g e o b t a i n e d f r o m t h i s s t u d y c o u l d a l s o be c o m b i n e d w i t h o t h e r p a r a m e t e r s , w h i c h may be r e v e a l e d a t t h e m i c r o - l e v e l ( f o r e x a m p l e , l i g n i n c o n t e n t , h e m i -c e l l u l o s e s c o n t e n t a n d p r i m a r y a n d s e c o n d a r y b o n d s w h i c h h o l d t h e m i c r o f i b r i l s t o g e t h e r ) , a n d a t t h e m a c r o - l e v e l s u c h a s g r a i n d e v i a t i o n , l a t e w o o d p e r c e n t a n d wood d e f e c t s , f o r a f u l l u n d e r s t a n d i n g o f c r e e p b e h a v i o r o f wood i n t e n s i o n p a r a l l e l t o t h e g r a i n . F o r p r a c t i c a l p u r p o s e s , s t r u c t u r a l members f r o m s a m p l e s w i t h s m a l l m i c r o f i b r i l a n g l e , h i g h d e g r e e o f c r y s t a l l i n i t y , h i g h s p e c i f i c g r a v i t y a n d l o w e x t r a c t i v e s c o n t e n t w o u l d show a l o w t e n d e n c y f o r c r e e p u n d e r p e r m a n e n t l o a d . A c c o r d i n g l y , t h e i r w o r k i n g s t r e s s e s c o u l d p e r h a p s be h i g h e r due t o t h e l o w e r a n t i c i p a t e d r e d u c t i o n i n t h e i r b a s i c s t r e s s e s . RECOMMENDATIONS FOR FURTHER RESEARCH The i n f o r m a t i o n d e r i v e d f r o m t h i s s t u d y w o u l d be u s e f u l i n d e t e r m i n i n g t h e m o s t i m p o r t a n t m i c r o s c o p i c , s u b -m i c r o s c o p i c a n d o t h e r f e a t u r e s o f wood w h i c h c o n t r o l c r e e p r e s p o n s e i n t e n s i o n p a r a l l e l t o t h e g r a i n . M i c r o f i b r i l a n g l e was f o u n d t o be t h e m o s t i m p o r t a n t and m o s t l o g i c a l s t r u c t u r a l f e a t u r e g o v e r n i n g t h e m a g n i t u d e o f c r e e p r e s p o n s e . I t i s s u g g e s t e d t h a t , w i t h an a p p r o p r i a t e e x p e r i m e n t a l t e c h n i q u e , f u r t h e r r e s e a r c h c o u l d be c o n d u c t e d t o d e t e r m i n e t h e c h a n g e i n t h e m i c r o f i b r i l a n g l e a s s o c i a t e d w i t h t h e d e f o r m a t i o n w h i c h r e s u l t s f r o m a s t r e s s a p p l i e d o v e r a l o n g p e r i o d o f t i m e . A p a r t o f t h i s d e f o r m a t i o n i s s a i d t o be n o n - r e c o v e r a b l e , w h i c h c o u l d be e x p l a i n e d b y t h e e x p e c t e d p e r m a n e n t d e c r e a s e i n t h e m i c r o f i b r i l a n g l e . T h i s , i f p r o v e n , w o u l d i n c r e a s e f u n d a m e n t a l k n o w l e d g e c o n c e r n i n g t h e r e l a t i o n s h i p b e t w e e n s u b m i c r o s c o p i c p r o p e r t i e s a n d t i m e - d e p e n d e n t b e h a v i o r o f wood, t h e l a t t e r o f w h i c h i s r e l a t i v e l y new. I t h a s b e e n o b s e r v e d i n t h i s s t u d y t h a t c e l l w a l l c r y s t a l l i n i t y i s h i g h l y c o r r e l a t e d w i t h t h e m i c r o f i b r i l a n g l e . I t i s recommended t h a t a s t u d y be i n i t i a t e d t o r e - e x a m i n e t h i s r e l a t i o n s h i p t o d e t e r m i n e w h e t h e r i t i s v a l i d f o r o t h e r c o n i f e r o u s woods a n d f o r h a r d w o o d s . T h i s w o u l d i n c r e a s e f u n d a m e n t a l k n o w l e d g e w i t h r e g a r d t o c o r r e l a t i o n s e x i s t i n g among wood p h y s i c a l p r o p e r t i e s . A n o t h e r a s p e c t w h i c h r e q u i r e s i n t e n s i v e i n v e s t i g a t i o n i s t h e d e v e l o p m e n t o f an a p p r o p r i a t e p h e n o m e n o l o g i c a l r h e o l o g i c a l m o d e l , w h i c h c o u l d c h a r a c t e r i z e wood by a c o m p l e t e d e s c r i p t i o n o f i t s m e c h a n i c a l b e h a v i o r u n d e r l o n g t e r m l o a d i n g . S i n c e l i g n i n a n d h e m i c e l l u l o s e s a r e e x p e c t e d t o i n f l u e n c e t h e r h e o l o g i c a l p r o p e r t i e s o f wood, i t i s recommended t h a t a s t u d y be c o n d u c t e d t o d e t e r m i n e t h e r o l e w h i c h t h e s e c o m p o n e n t s p l a y i n g o v e r n i n g c r e e p r e s p o n s e . CONCLUSION From the re s u l t s of t h i s study on creep response as a function of some s t r u c t u r a l features, s p e c i f i c gravity and extractives content of Douglas-fir normal and compression wood,and normal wood of Sitka spruce and western hemlock, the following conclusions are drawn: I. Under constant temperature and r e l a t i v e humidity (73 ± 3.5°F and 50 ± 2 per cent) conditions, t o t a l creep response of Douglas-fir, normal and compression wood, Sitka spruce and western hemlock, i s co n t r o l l e d by the m i c r o f i b r i l angle of the S2 layer, and the s p e c i f i c gravity and extractives content of the wood. These variables contribute up to 76.8 and 8 5.5 per cent of the t o t a l v a r i a b i l i t y i n creep response i n the case of 3,000 and 6,000 microin. per i n . i n i t i a l s t r a i n l e v e l , respectively. I I . M i c r o f i b r i l angle i s the most important and most l o g i c a l s t r u c t u r a l feature of the tracheid walls governing creep response. The t o t a l creep response increased as the m i c r o f i b r i l angle increased. It i s proposed that the creep-inducing stresses cause re o r i e n t a t i o n of the m i c r o f i b r i l s by forc i n g them to al i g n them-selves more nearly p a r a l l e l to the long i t u d i n a l tracheid axis. As a r e s u l t , the m i c r o f i b r i l angle i n the S2 layer of the tracheid wall becomes steeper 97 (smaller). This change i n the angle appears to be larger for a specimen having a larger angle p r i o r to the application of a constant s t r a i n . I I I . C e l l wall c r y s t a l l i n i t y , expressed as • j r e l a t i v e degree of c r y s t a l l i n i t y , i . e . , c r y s t a l l i n i t y i n -dex, a f f e c t s t o t a l creep response. A r e l a t i v e l y high percentage of c r y s t a l l i n i t y increases the r i g i d i t y of the c e l l wall which thereby r e s i s t s excessive creep. IV. C e l l wall c r y s t a l l i n i t y i s inversely correlated with the m i c r o f i b r i l angle of the S2 layer. V. S p e c i f i c gravity a f f e c t s creep response only when i t i s combined with m i c r o f i b r i l angle and extractives content. I t i s suggested that i f the s p e c i f i c gravity of a wood ti s s u e i s high, the r a t i o of S2 layer to t o t a l tracheid wall thickness i s also high, hence there i s a large volume of the c e l l wall i n which the m i c r o f i b r i l s are at a small angle to the long tracheid axis. This would decrease the t o t a l creep response. VI. Extractives content plays a r e l a t i v e l y important role i n c o n t r o l l i n g creep behavior of wood under a constant temperature of 73 ± 3.5°F and 50 ± 2 per cent r e l a t i v e humidity. The e x t r a c t i v e s -t o t a l creep r e l a t i o n s h i p suggests that a part of these extractives i s i n the amorphous region of the c e l l wall structure. 98 V I I . E x t r a c t i v e s i n t h e c e l l w a l l p r o b a b l y a c t a s p l a s t i c i z e r s c a u s i n g a r e d u c t i o n i n t h e p r i m a r y a n d s e c o n d a r y b o n d i n g b e t w e e n m i c r o f i b r i l s . T h i s w o u l d f a c i l i t a t e t h e movement o f t h e s t i f f i n e x t e n s i b l e m i c r o f i b r i l s t o accommodate t h e c r e e p - i n d u c i n g s t r e s s e s . V I I I . T o t a l c r e e p r e s p o n s e i s a f f e c t e d b y t h e i n i t i a l a p p l i e d s t r a i n , i n c r e a s i n g a s t h e i n i t i a l s t r a i n i n c r e a s e s . I X . R e s u l t s o b t a i n e d t h r o u g h o u t t h e t h e s i s a r e com-p a t i b l e w i t h t h e h y p o t h e s i s t h a t s h o r t - t e r m c r e e p r e s p o n s e o f e a r l y w o o d a n d l a t e w o o d t i s s u e s o f some c o n i f e r o u s s p e c i e s , s t r e s s e d i n t e n s i o n p a r a l l e l t o t h e g r a i n , i s a f u n c t i o n o f m i c r o f i b r i l a n g l e o f t h e S2 l a y e r o f t r a c h e i d w a l l a n d r e l a t i v e d e g r e e o f c r y s t a l l i n i t y i n t h e c e l l w a l l , a l o n g w i t h s p e c i f i c g r a v i t y o f t h a t wood t i s s u e a n d i t s e x t r a c t i v e s c o n t e n t . LITERATURE CITED A l f r e y , T., J r . 1948. M e c h a n i c a l b e h a v i o r o f h i g h p o l y m e r s . I n t e r s c i e n c e P u b l i s h e r s , I n c . , New Y o r k . 570 pp. A r g a n b r i g h t , D.C. 1971. 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Tech., Pennsylvania State Univ. 58pp. 114 TABLE 1 SUMMARY OF M I C R O F I B R I L ANGLE DETERMINATIONS S p e c i e s M e r c u r y A n g l e *T* A n g l e 8 A n g l e ( F r o m ( D e g r e e ) ( D e g r e e ) M e y l a n ' s E q u a t i o n (87)) ( D e g r e e ) D o u g l a s - f i r N o r m a l wood E a r l y w o o d 21.63(4.98) a 20.70 13.44 21.38 19.69 A v e r a g e 20.59 L a t e w o o d 12.95(2.48) 23.51 15.16 22.73 23.96 A v e r a g e 23.40 C o m p r e s s i o n Wood E a r l y w o o d 28.22(4.03) 38.70 25.35 39.94 41.51 A v e r a g e 40.05 L a t e w o o d 17.37(2.74) 26.55 16.93 27.11 25.20 A v e r a g e 26.29 S i t k a s p r u c e E a r l y w o o d 14.30(2.74) 15.64 10.65 15.98 16.43 A v e r a g e 16.02 L a t e w o o d 9.22(1.80) 15.53 10.28 15.08 15. 64 A v e r a g e 15.42 W e s t e r n h e m l o c k E a r l y w o o d 20.74(4.00) 22.28 14.96 24.19 22. 73 A v e r a g e 23.07 L a t e w o o d 21.26(2.01) 36.00 21.93 33.08 34.31 A v e r a g e 34.46 V a l u e s i n p a r e n t h e s e s a r e t h e s t a n d a r d d e v i a t i o n s o f t h e a v e r a g e s r e p o r t e d , a n d a r e b a s e d u p o n 33 t o 73 m e a s u r e m e n t s . 115 TABLE 2 RELATIVE DEGREE OF CRYSTALLINITY (CRYSTALLINITY INDEX) Species Relative Degree of C r y s t a l l i n i t y ( C r y s t a l l i n i t y Index) Per cent Douglas-fir Normal Wood Earlywood 1 60.99 2 58.18 Average 59.59 Latewood 1 64.22 2 63.38 Average 63.80 Compression Wood Earlywood 1 57.33 2 58.99 Average 58.16 Latewood 1 59.66 2 62.01 Average 60.84 Sitka spruce Earlywood 1 61.54 2 61. 95 Average 61.75 Latewood 1 65.02 Western hemlock Earlywood 1 60.73 2 61.54 Average 61.14 Latewood 1 60.82 2 56.18 Average 58.50 TABLE 3 SPECIFIC GRAVITY, EXTRACTIVES CONTENT AND TOTAL CREEP FOR THE SAMPLES TESTED AT 3,000 AND 6,000 MICROIN. PER IN. INITIAL STRAIN Samples Tested at 3,000 Microin. per Samples Tested at 6,000 Microin. per in. I n i t i a l Strain in. I n i t i a l S train Species S p e c i f i c a Extractives Total S p e c i f i c Extractives Total Gravity Content Based Creep Gravity Content Based Creep on Extractive- (Microin. on E x t r a c t i v e - (Microin. free Oven-dry per in.) free Oven-dry per in.) Weight Weight (Per cent) (Per cent) Douglas-fir Normal wood Earlywood 1 0.281 4.81 324 0.376 4.34 674 2 0.273 6.57 207 0.342 4.38 607 3 0.287 5.99 197 0. 329 9.38 . 724 4 0.284 5.46 180 0.332 9.59 610 5 0.282 5.20 234 0.297 7.3 8 596 6 0.295 6.28 214 0.300 7.68 616 Average 0.284 5 . 7 2 2 2 6 . 0 0.335 7.13 637.8 Latewood 1 0.773 3.72 200 0.700 5.17 485 Compression Wood Earlywood 1 0.303 7.05 370 0.278 8.87 714 2 0.290 7.57. 285 0.308 7.79 787 3 0.271 7.38 255 0.304 7.86 720 4 0.311 6.33 254 0.292 7.00 767 5 0.354 6.88 274 Average 0.306 7.04 287.6 0. 296 7. 88 747.0 Latewood 1 0.647 6.36 230 0.738 6.22 636 2 0.677 6.83 190 0.653 6.99 590 3 0.691 6.00 220 0.700 6.69 610 Average 0.672 6.40 213.3 0.697 6.63 612.0 TABLE 3 (continued) Samples Tested at 3,000 Microin. per Samples Tested at 6,000 Microin. per i n . I n i t i a l S t r a i n i n . I n i t i a l S t r a i n Species S p e c i f i c a Extractives Total S p e c i f i c Extractives Total Gravity Content Based Creep Gravity Content Based Creep on Extractive- (Microin. on E x t r a c t i v e - (Microin. free Oven-dry per in.) free Oven-dry per in.) Weiqht (Per cent) Weight (Per cent) Sitk a spruce Earlywood 1 0.381 3.98 140 0.314 6.21 314 2 0.312 4.37 145 0.300 6.03 327 3 0.382 4.92 130 0.358 4.47 307 4 0.391 4.30 164 0.341 3.44 293 5 0.362 3.15 160 0.368 4.09 323 6 0.402 3.92 110 Average 0.372 4.11 141.5 0.336 4.85 312.8 Latewood 1 0.604 4.01 84 0.557 4.73 270 2 0.575 4. 01 110 0.493 3.45 250 3 0.552 3.01 107 0.488 4.52 320 4 0.570 3.53 167 0.511 4.08 234 5 0.545 3.53 104 0.526 3.57 290 Average 0.569 3.62 114.4 0.515 4.07 272.8 Western hemlock Earlywood 1 0.262 5.16 217 0.262 5.83 427 2 0.248 8.60 182 0.259 5.26 394 3 0.246 5.87 177 0.261 4.52 454 4 0. 234 5.76 167 0.250 6.16 407 Average 0.248 6.35 185.8 0.258 5.44 420.5 Latewood 1 0.432 3.00 298 0.458 4.38 494 2 0.413 3.02 254 0.481 3.44 393 3 0.450 4.35 220 0.467 6.05 440 4 0.496 3.69 230 0.478 4.75 480 5 0.488 4.7 4 527 6 0.511 3.47 510 Average 0.448 3.52 250.5 0.481 4.47 474.0 aBased on oven-dry weight and volume at test. 118 TABLE 4 MULTIPLE C O E F F I C I E N T S OF DETERMINATION ( R 2 ) AND STANDARD ERRORS OF ESTIMATE (SEE) BASED ON 34 WOOD SPECIMENS TESTED AT 3,000 MICROIN. PER I N . I N I T I A L STRAIN C o r r e l a t i o n R 2 SEE L e v e l o f S i g n i f i c a n c e T o t a l c r e e p v s . m i c r o -f i b r i l a n g l e a n d r e l a t i v e d e g r e e o f c r y s t a l i n i t y 0 .6812 38 .6467 0.5 p e r c e n t T o t a l c r e e p v s . m i c r o -f i b r i l a n g l e a n d s p e c i f i c g r a v i t y 0 .7344 35 .2744 0.5 p e r * c e n t T o t a l c r e e p v s . m i c r o -f i b r i l a n g l e a n d e x t r a c t i v e s c o n t e n t 0 .7007 37 .4409 0.5 p e r c e n t T o t a l c r e e p v s . r e l a t i v e d e g r e e o f c r y s t a l l i n i t y a n d s p e c i f i c g r a v i t y 0 .6683 39 .4198 0.5 p e r c e n t T o t a l c r e e p v s . r e l a t i v e d e g r e e o f c r y s t a l l i n i t y a n d e x t r a c t i v e s c o n t e n t 0 .6434 40 .8708 0.5 p e r c e n t T o t a l c r e e p v s . s p e c i f i c g r a v i t y a n d e x t r a c t i v e s c o n t e n t 0 .1898 61 .6040 5.0 p e r c e n t T o t a l c r e e p v s . m i c r o -f i b r i l a n g l e , r e l a t i v e d e g r e e o f c r y s t a l l i n i t y a n d s p e c i f i c g r a v i t y 0 .7413 35 .3869 0.5 p e r c e n t T o t a l c r e e p v s . m i c r o -f i b r i l a n g l e , s p e c i f i c g r a v i t y a n d e x t r a c t i v e s c o n t e n t 0 .7680 33 .5114 0.5 p e r * c e n t T o t a l c r e e p v s . m i c r o -f i b r i l a n g l e , r e l a t i v e d e g r e e o f c r y s t a l l i n i t y a n d e x t r a c t i v e s c o n t e n t 0 .7014 38 .0191 0.5 p e r c e n t 119 TABLE 4 ( c o n t i n u e d ) C o r r e l a t i o n R 2 SEE L e v e l o f S i g n i f i c a n c e T o t a l c r e e p v s . r e l a t i v e d e g r e e o f c r y s t a l l i n i t y , s p e c i f i c g r a v i t y a n d e x t r a c t i v e s c o n t e n t 0.6714 39.8842 0.5 p e r c e n t T o t a l c r e e p v s . m i c r o -f i b r i l a n g l e , r e l a t i v e d e g r e e o f c r y s t a l -l i n i t y , s p e c i f i c g r a v i t y a n d e x t r a c t i v e s c o n t e n t 0.7681 34.0773 0.5 p e r c e n t * P a r t i a l c o r r e l a t i o n c o e f f i c i e n t s a r e s i g n i f i c a n t . 120 TABLE 5 9 MULTIPLE COEFFICIENTS OF DETERMINATION (R ) AND STANDARD ERRORS OF ESTIMATE (SEE) BASED ON 34 WOOD SPECIMENS TESTED AT 6,000 MICROIN. PER IN. INITIAL STRAIN Correlation R 2 SEE Level of Significance Total creep vs. micro-f i b r i l angle and r e l a t i v e degree of c r y s t a l l i n i t y 0 .6869 94 .8457 0 .5 per cent Total creep vs. micro-f i b r i l angle and s p e c i f i c gravity 0 .7698 81 .3324 0 .5 per * • cent To t a l creep vs. micro-f i b r i l angle and extractives content 0 .7610 82 .8748 0 .5 per * cent Total creep vs. r e l a t i v e degree of c r y s t a l l i n i t y and s p e c i f i c gravity 0 .5563 112 .9190 0 .5 per cent To t a l creep vs. r e l a t i v e degree of c r y s t a l l i n i t y and extractives content 0 .7153 90 .4467 0 .5 per * cent Total creep vs. s p e c i f i c gravity and extractives content 0 .4850 121 . 6500 0 .5 per cent Total creep vs. micro-f i b r i l angle, r e l a t i v e degree of c r y s t a l l i n i t y and s p e c i f i c gravity 0 .7996 77 .1387 0 .5 per * cent Total creep vs. micro-f i b r i l angle, s p e c i f i c g ravity and extractives content 0 .8550 65 . 6095 0 .5 per * cent Total creep vs. micro-f i b r i l angle, r e l a t i v e degree of c r y s t a l l i n i t y and extractives content 0 .7610 84 .2425 0 .5 per cent 121 TABLE 5 ( c o n t i n u e d ) C o r r e l a t i o n R 2 SEE L e v e l o f S i g n i f i c a n c e T o t a l c r e e p v s . r e l a t i v e d e g r e e o f c r y s t a l l i n i t y , s p e c i f i c g r a v i t y a n d e x t r a c t i v e s c o n t e n t 0.7541 85.4505 * 0.5 p e r c e n t T o t a l c r e e p v s . m i c r o -f i b r i l a n g l e , r e l a t i v e d e g r e e o f c r y s t a l l i n i t y , s p e c i f i c g r a v i t y a n d e x t r a c t i v e s c o n t e n t 0.8647 64.4748 0.5 p e r c e n t * P a r t i a l c o r r e l a t i o n c o e f f i c i e n t s a r e s i g n i f i c a n t . 122 TABLE 6 COVARIANCE ANALYSIS FOR TESTING THE DIFFERENCE IN TOTAL CREEP BETWEEN EARLYWOOD (EQUATION [ 5 ] ) a AND LATEWOOD (EQUATION [ 6 ] ) b AT 3,000 MICROIN. PER IN . INITIAL STRAIN Group DF SS MS F c Level of Significance Earlywood 17 24931.4 Latewood 9 7884.3 Total 26 32815.7 1262.1 Difference for t e s t i n g slopes 3 859.9 286.6 0.227 N.S. + Sums 29 33675.6 1161.2 Difference for t e s t i n g l e v e l s 1 15.2 15.2 0.013 N.S. Combined regression 30 33690.8 a Y = 1.6255 + 13.7204X1 + 11.0443X3 - 14.6503X4 . . .[5] b Y =.-66.4666 + 13.3015X-L + 189.6270X3 - 13.4366X4 . . .[6] +Not s i g n i f i c a n t at the 5 per cent l e v e l . 123 TABLE 7 COVARIANCE ANALYSIS FOR TESTING THE DIFFERENCE IN TOTAL CREEP BETWEEN EARLYWOOD (EQUATION [ 7 ] ) a AND LATEWOOD (EQUATION [ 8 ] ) b AT 6,000 MICROIN. PER IN. INITIAL STRAIN Group DF ss M.S F Level of Significance Earlywood 15 33106.3 Latewood 11 18684.8 Total 26 51791.1 1992.0 Difference for t e s t i n g slopes 3 44158.1 14719.4 7.389 0.5 per cent Sums 29 95949.2 3308.6 Difference for te s t i n g l e v e l s 1 33217.8 33217.8 10.040 0.5 per cent Combined regression 30 129167 a Y =-783.620 + 32.294X1 + 1660.890X3 + 19.769X4 , . .[7] b Y =-354.748 + 18.544X1 + 756.033X3 + 16.424X4 . . .[8] 124 TABLE 8 COVARIANCE ANALYSIS FOR TESTING THE DIFFERENCE IN TOTAL CREEP AT 3,000 MICROIN. PER IN. INITIAL STRAIN (EQUATION [ 3 ] ) a AND AT 6,000 MICROIN. PER IN. INITIAL STRAIN (EQUATION [ 4 ] ) b Group DF SS MS F Level of S i g n i f i -cance 3,000 microin. per i n . i n i t i a l s t r a i n 30 33697. 7 6,000 microin. per i n . i n i t i a l s t r a i n 30 129208. 3 Tota l 60 162906. 0 2715. 1 Difference for t e s t i n g slopes 3 169149. 0 56382. 9 20.766 0.5 per cent Sums 63 332055. 0 5270. 7 Difference for t e s t i n g l e v e l s Combined regression 1 64 1252030. 1584085. 0 0 1252030 .0 237.545 0.5 per cent Y = -50.6658 + 13.2377X1 + 143.2880X3 - 10.7829X4 . . .[3] Y = -297.2390 + 23.0248X1 + 407.8860X3 + 32.7107X4 . . .[4] TABLE 9 CORRELATION COEFFICIENTS FOR THE VARIABLES WHICH WERE DETERMINED ON (A) SPECIMENS TESTED AT 3,000 MICROIN. PER I N . AND (B) SPECIMENS TESTED AT 6,000 MICROIN. PER I N . I N I T I A L STRAIN A V a r i a b l e s M i c r o f i b r i l a n g l e (X^) R e l a t i v e d e g r e e o f c r y s t a l l i n i t y ( X 2 ) S p e c i f i c G r a v i t y (x 3) E x t r a c t i v e s C o n t e n t ( X 4 ) T o t a l C r e e p (Y) M i c r o f i b r i l a n g l e (X^) 1 -0.9310* -0.6048* 0.6662* 0.8179* R e l a t i v e d e g r e e o f c r y s t a l l i n -i t y (x 2) 1 0.5320* -0.4856* -0.8017* S p e c i f i c g r a v i t y (x 3) 1 -0.39071" -0.2911 N . S .+ E x t r a c t i v e s c o n t e n t ( X 4 ) 1 0.4121+ T o t a l c r e e p (Y) 1 TABLE 9 (continued) B Variables M i c r o f i b r i l angle (X^) Relative degree of c r y s t a l l i n i t y (X^) S p e c i f i c Gravity (x 3) Extractives Content (X 4) Tot a l Creep (Y) M i c r o f i b r i l angle (X^) 1 -0.9238* -0.4650* 0.5546* 0.8258* Relative degree of c r y s t a l l i n -i t y (X 0) 1 0.3233 N. S . -0.42791" -0.7356* S p e c i f i c gravity (x 3) 1 -0.2841 N.S. -0.1215 N.S. Extractives con-tent (x 4) 1 0.6920* Total creep (Y) 1 S i g n i f i c a n t at the one per cent l e v e l . S i g n i f i c a n t at the f i v e per cent l e v e l . Not s i g n i f i c a n t at the f i v e per cent l e v e l . to Outer surface Innsr surfoca \ / N Primary wall (Several Laminae (CA.30-150 (Several Laminae intermediate between Laminae) intermediate bet.veen those of the S2 and those of the SI and S 3 layers) S2 layers) FIG-1-DIAGRAMMATIC REPRESENTATION OF CELL WALL ORGANIZATION OF A TYPICAL FIBER OR TRACHEID SHOWING THE TEXTURE OF THE DIFFERENT CELL WALL LAYERS,AFTER WARDROP (147)-Secondary wall FIG-2- DIFFERENT CONCEPTS OF CELL WALL ORGANIZATION OF A TYPICAL FIBER OR TRACHEID,SHOWING FIBRIL-LAR AND/OR MICROFIBRILLAR DIRECTIONS-(A) FROM WARDROP AND BLAND (149), (B) HARADA ET AU (34); (C) FROM FORGACS (26)- SEE TEXT FOR LEGEND-128 FIG- 3- T H E C L A S S I C A L MODEL OF MEYER AND MISCH (86) FOR T H E UNIT C E L L OF C R Y S T A L L A T T I C E OF NATIVE C E L L U L O S E ( C E L L U L O S E ! ) • F I G - 4 - F R I N G E D M I C E L L A R M O D E L , AFTER H E A R L E (36)-FIG-5- FRINGED F I B R I L L A R M O D E L , A F T E R H E A R L E (36)-130 FIG- 6- DIAGRAMMATIC REPRESENTATION OF CELLULOSIC MICROFIBRILS ACCORDING TO CONCEPTS OF (A) HESS ET AU (43) AND (B) RXNBY (122)-131 F IG -7 - FOLDED CHAIN M O D E L , AFTER M A N L E Y (79)-THE MICROFIBRIL CONSISTS OF A TIGHTLY WOUND HEL IX (A) FROM A RIBBON (B) ABOUT 8 A THICK IN WHICH A SINGLE C E L L U L O S E CHAIN IS FOLDED R E P E A T E D L Y -F IG -8 - A MODEL OF THE COTTON F IBER SHOWING THE S P I R A L STRUCTURE AND R E V E R S A L S IN T H E GROWTH LAYER- - (A) P R I M A R Y W A L L ; (B) GROWTH L A Y E R S - , (C) R E V E R S A L S , A F T E R ORR ET AU (103)-132 FIG-9- DEFORMATION OF AN E L A S T I C - P L A S T I C BODY AS A FUNCTION OF TIME- LOADING DURING TIME T 0 TO T| .FOLLOWED BY UNLOADING TIME T , TO I2 , AFTER KOLLMANN AND C0TE ' (70) -K FIG- 10- F O U R - E L E M E N T SPRING AND DASHPOT MODEL, A F T E R PENTONEY (108) -SEE T E X T FOR LEGEND-133 FIG- II- IDEALIZED L O N G - T I M E C R E E P , A F T E R PENTONEY (108)-134 Fig- 12- SCHEMATIC REPRESENTATION FOR SAMPLING PROCEDURE-FIG- 13- ARBOR PRESS WITH ADJUSTABLE CUTTING DIE USED FOR CREEP TEST SPECIMEN PREPARATION-FIG- 14- C R E E P P A R A L L E L TO THE GRAIN S E T - U P -(A) T A B L E MODEL INSTRON TESTING MACHINE AND (B) LOADING S Y S T E M -3,400 (A) or 3.200 UJ CL z o 5 3,000 < cr _ J I 1 | | __ 10 15 20 25 30 35 T I M E , M I N U T E S 40 45 50 55 60 6.800 6,600 - 6,400 or LU CL cr 6,200 o 2 Z* jr 6,000 10 15 20 25 30 35 T I M E , M I N U T E S 40 45 50 55 60 FIG-15- STRAIN-TIME RELATIONSHIP FOR DOUGLAS-FIR EARLYWOOD (NORMAL WOOD) AT (A) 3,000 MICROIN-PER IN- (SPECIMEN NO-5) AND (B) 6,000 MICROIN PER IN-(SPECIMEN NO-4) INITIAL STRAIN-FIG-16- STRAIN-TIME RELATIONSHIP FOR SITKA SPRUCE LATEWOOD AT (A) 3,000 MICROIN- PER IN- (SPECIMEN NO-4) AND (8) 6,000 MICROIN- PER IN- (SPECIMEN NO-I) INITIAL STRAIN-FIG-17- CAHN ELECTRO-BALANCE USED FOR SPECIMEN WEIGHT DETERMINATION-5 FIG-I8- X - R A Y INTENSITY AROUND THE (002) ARC FOR SITKA S P R U C E LATEWOOD USING T E X T U R E GONIOMETER MACHINE-o i 1 4 1 cn. UJ LU Ce CO UJ Q ° LU I CO 2: CC c r> co ce LU •I-0.0 4-MA=4-2946 + 0 5 5 8 6 T S E E = 3-8974 0 -6385 N = 8 2 D . 0 X z o A + X X X Douglas-fir earlywood (normal wood) Douglas-fir latewood (normal wood) Douglas-fir earlywood (compression wood) Douglas-f ir latewood (compression wood ) Sitka spruce earlywood Sitka spruce latewood Western hemlock earlywood Western hemlock latewood 2 5 . 0 fiNGLE "T 2 0 . 0 DEGREES 3 5 . 0 4 0 . 0 4 5 . 0 F.IG- 19- RELATIONSHIP BETWEEN ANGLE "T n DERIVED FROM X - R A Y AND MEAN MICROFIBRIL ANGLE MEASURED BY MERCURY IMPREGNATION ; MA )• (002) F IG -20 - X - R A Y DIFFRACTION PATTERN OF D O U G L A S - F I R LATEWOOD (COMPRESSION WOOD)-M it* to RELATIONSHIP BETWEEN TOTAL CREEP (Y) AND MICRO F I B R I L ANGLE (X,) AT 3,000 MICROIN. PER I N . I N I T I A L STRAIN. SEE F I G . 19 FOR LEGEND. RELATIONSHIP BETWEEN TOTAL CREEP (Y) AND MICRO F I B R I L ANGLE (X,) AT 6,000 MICROIN. PER I N . I N I T I A L STRAIN. SEE F I G . 19 FOR LEGEND. 143 RELATIONSHIP BETWEEN TOTAL CREEP (Y) AND RELATIVE DEGREE OF CRYSTALLINITY (X„) AT 3,000 MICROIN. PER, IN. INITIAL STRAIN. SEE FIG. 19 FOR LEGEND. RELATIONSHIP BETWEEN TOTAL CREEP (Y) AND RELATIVE DEGREE OF CRYSTALLINITY (X„) AT 6,000 MICROIN. PER IN. INITIAL STRAIN. SEE FIG. 19 FOR LEGEND. X 145 FIG-25- RELATIONSHIP BETWEEN MICROFIBRIL ANGLE (X,) ANO RELATIVE DEGREE OF CRYSTALLINITY ( X 2 ) SEE FIG- 19 FOR LEGEND-RELATIONSHIP BETWEEN TOTAL CREEP (Y) AND SPECIFIC GRAVITY (X_J AT 3,000 MICROIN. PER IN. INITIAL STRAIN. SEE FIG. 19 FOR LEGEND. RELATIONSHIP BETWEEN TOTAL CREEP (Y) AND SPECIFIC GRAVITY (X ) AT 6,000 MICROIN. PER IN. INITIAL STRAIN. SEE FIG. 19 FOR LEGEND. 146 Y = 252-264-128069 X 3 SEE =64 446 r2 =0 0848 N=34 4' Y = 547290-142-924 X; SEE = 165 606 r 2 =00148 0 2 0.3 0.4 MICRO-SPECIFIC GRAVITY - 1 — 3 9 RELATIONSHIP BETWEEN TOTAL CREEP (Y) AND EXTRAC TIVES CONTENT (X.) AT 3,000 MICROIN. PER IN. INITIAL STRAIN. SEE FIG. 19 FOR LEGEND. RELATIONSHIP BETWEEN TOTAL CREEP (Y) AND EXTRAC TIVES CONTENT ( X j AT 6,000 MICROIN. PER IN. INITIAL STRAIN. SEE FIG. 19 FOR LEGEND. 

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