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Effect of resin impregnated core veneer on shear strength of Douglas-fir plywood Chow, Sue-Zone 1966

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EFFECT OF RESIN IMPREGNATED CORE VENEER ON SHEAR STRENGTH OF DOUGLAS-FIR PLYWOOD by SUE-ZONE CHOW B.Sc, National Taiwan University, Taiwan, 1959 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF FORESTRY in the Department of FORESTRY We accept this thesis a's conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA A p r i l , 1966 In p r e s e n t i n g t h i s t h e s i s in p a r t i a l f u l f i l m e n t of the requirements fo r an advanced degree at the U n i v e r s i t y of B r i t i s h Columbia, I agree that the L i b r a r y s h a l l make i t f r e e l y a v a i l a b l e fo r reference and study . I f u r t h e r agree that permiss ion fo r ex -t e n s i v e copying of t h i s t h e s i s f o r s c h o l a r l y purposes may be granted by the Head of my Department or by h i s r e p r e s e n t a t i v e s . It i s understood that copying or p u b l i c a t i o n of t h i s t h e s i s f o r f i n a n -c i a l gain s h a l l not be a l lowed wi thout my w r i t t e n p e r m i s s i o n . Department of The U n i v e r s i t y of B r i t i s h Columbia Vancouver 8, Canada Date A^le^ /o? , / f 6 <Z (i) ABSTRACT The influence of lathe checks on shear strength of Douglas f i r plywood was investigated by means of impregnating lathe checks of rotary-cut veneer to various depths using a phenol-formaldehyde r e s i n . Comparative t e n s i l e shear strength tests were conducted on a Table Model Instron machine and photographs taken at various stages of load a p p l i c a t i o n to i l l u s t r a t e the varied manner of f a i l u r e . Strength of rotary-cut veneer plywood was about 60 to 70% that of sawn veneer plywood, but a f t e r the lathe checks of core veneer were impregnated by r e s i n there was no s i g n i f i c a n t difference between them. The shear strength (Y) was found to be highly correlated with penetration depth of adhesive into lathe checks (X). The l i n e a r r e l a t i o n s h i p between these factors was: Y a 228.22 + 1.28052X (SE £ . 21.82; r = 0.893). Per cent wood f a i l u r e estimated by conventional methods f a i l e d to rel a t e to shear strength. Rather,, the per cent wood f a i l u r e occurring within 10$ of the i n i t i a t i o n of an annual increment was found to be a better indicator of shear strength. Use of photography helped to explain more c l e a r l y stress d i s t r i b u t i o n and wood f a i l u r e i n the specimens. It was found that the ultimate strength was reached i n conventional plywood when the lathe checks were just opening. ( i i ) Core-impregnated plywood was used i n a test to compare the t e n s i l e shear resistance when ti g h t - s i d e and loose-side of veneer was next to the glue l i n e . Neither strength nor wood f a i l u r e were s i g n i f i c a n t l y d i f f e r e n t between the two. Tensile shear strength f o r plywood made of impregnated core veneer and untreated face veneer was two to three times as high as that of conventional plywood. The per cent wood f a i l u r e i n core veneer and shear strength varied inversely. Results obtained i n t h i s study indicated that i t i s f e a s i b l e to develop a plywood which has shear strength as high as 500 p s i while remaining economical to manufacture. ( i i i ) ACKNOWLEDGEMENT The author wishes to express his gratitude to Dr. R.W. Wellwood of the Faculty of Forestry at the University of B r i t i s h Columbia, under whose d i r e c t i o n t h i s thesis was accomplished. Appreciation i s also due to Dr. J.W. Wilson f o r his advice and review of t h i s thesis, to Dr. A. Kozak f o r his help i n s t a t i s t i c a l analysis, and to Mrs. Chow f o r assistance i n preparation of the manuscript. Thanks are also due to the author*s parents, and to Professor T.T. Wang of the Department of Forestry at National Taiwan University, Taipei,' Taiwan, f o r encouraging the author to study i n t h i s country. P a c i f i c Veneer and Plywood D i v i s i o n of Canadian Forest Products Limited supplied the log and P a c i f i c Resins Limited provided the glues used i n the experiment. Their generosity i s g r a t e f u l l y acknowledged. The author i s also appreciative of f i n a n c i a l assistance obtained from the Faculty of Forestry, University of B r i t i s h Columbia, Vancouver, Canada, and the National Research Council of Canada. (iv) TABLE OF CONTENTS Page TITLE PAGE ABSTRACT i ACKNOWLEDGEMENT i i i TABLE OF CONTENTS i v LIST OF TABLES v i i LIST OF FIGURES x INTRODUCTION 1 REVIEW OF LITERATURE 3 A. Lathe Checks 3 i . Formation 3 i i . Influence on shear strength of plywood k B. Wood Fa i l u r e 6 i . Importance 6 i i . Uncertainty of rel a t i o n s h i p between wood f a i l u r e and shear strength 6 i i i ; 1 Relationship between wood f a i l u r e and lathe checks 8 C. Relationship of Resin-Penetration to Shear Strength of Plywood. 9 i . 1 Permeability of Douglas-fir wood 9 i i . Resin properties 10 i i i . Resin impregnation and shear strength 12 (v) Page MATERIALS AND METHODS 13 A. Experimental Design 13 B. Materials 16 i . Veneer collection 16 a. Log 16 b. Peeling and sawing 16 c. Selection and grouping of veneers 17 (1) . Group A 1? (2) . Group B 17 (3) . Group C 18 i i . Staining 19 i i i . Drying 19 iv. Impregnation 20 C. Plywood Panel Construction 23 D. Preparation and Test of Specimens 25 i . Sample size 25 i i . Trimming of samples and measurements 25 i i i . Measurements of lathe checks, adhesive penetration depth and the angle of lathe checks 26 iv. Testing 27 v. Estimation of wood failure and position of failure 28 E. Photographic Technique 28 (vi) Page RESULTS 29 A. Shear Strength" 2 9 i . Group A 3 0 i i . Group B 3 1 i i i . Group C 3 1 B. Per Cent Wood Fai l u r e 3 2 i . Group A 3 2 i i . Group B 3 3 i i i . Group C 3 3 C. Photographic Evidence 3 3 DISCUSSION 3 3 A. Influence of Lathe Checks on Shear Strength 3 4 B. Influence of Lathe Checks on Wood Fa i l u r e 4 l C. Influence of Rotary-cutting on Shear Strength and Wood Fa i l u r e i n Tight-side and Loose-side of Veneer 42 D. Influence of Resin-Impregnation on Shear Strength and Per Cent F a i l u r e of Core Veneer 4 3 CONCLUSIONS 46 LITERATURE CITED 5 0 ( v i i ) Table IIIA. LIST OF TABLES Table I. Summation of average shear strengths and per cent wood f a i l u r e s , Table II. Analysis of variance: Average shear strengths i n Groups A,B and C (dry t e s t ) . Analysis of variance: Average shear strengths i n Treatments 5. 8 and 9 of Groups A, B and C, respectively (dry test)„ Duncan 1s multiple range t e s t : Average shear strengths i n Treatments 5» 8 and 9 of Groups A, B and C, respectively (dry test). Table IVA. Analysis of variance: Average shear strengths i n Treatments 5» 8 a*id 9 of Group A, B and C, respectively (boil test). Table IIIB. Page 56 57 58 58 59 Table IVB. Duncan's multiple range t e s t : Average shear strengths i n Treatments 5» 8 and 9 of Groups A, B and C, respectively (boil t e s t ) , 59 Table V. Summation of averaged data of Group A. 60 Table VIA. Analysis of variance: Average penetration depths of adhesive into lathe checks i n Treatments 1 to kf Group A. 61 Table VIB. Duncan's multiple range t e s t : Average penetration depths of adhesive into lathe checks i n Treatments 1 to k, Group A. 6l Table VILA. Table VIIB. Analysis of variance: Average shear strengths i n Treatments 1 to 5» Group A (dry test). 62 Duncan's multiple range t e s t : Average shear strengths i n Treatments 1 to 5. Group A (dry test). 62 ( v i i i ) Page Table VIIIA. Table VIIIB. Table Table Table Table IX. X. XIA. XIB, Table XIIA. Table XIIB. A n a l y s i s of v a r i a n c e : Average shear s t r e n g t h s i n Treatments 1 to 5. Group A ( b o i l t e s t ) . 63 Duncan 1s m u l t i p l e range t e s t : Average shear s t r e n g t h s i n Treatments 1 to 5» Group A ( b o i l t e s t ) . 63 A n a l y s i s of v a r i a n c e : Average shear s t r e n g t h s i n Treatments 6 to 8, Group B (dry t e s t ) . 64 A n a l y s i s of v a r i a n c e : Average shear s t r e n g t h s i n Treatments 6 to 8. Group B ( b o i l t e s t ) . 64 A n a l y s i s of v a r i a n c e : Average shear s t r e n g t h s i n Treatments 9 to 12, Group C (dry t e s t ) . 65 Duncan's m u l t i p l e range t e s t : Average shear s t r e n g t h s i n Treatments 9 to 12, Group C (dry t e s t ) . 65 A n a l y s i s o f v a r i a n c e : Average shear s t r e n g t h s i n Treatments 9 to 12, Group G ( b o i l t e s t ) . 66 Duncan's m u l t i p l e range t e s t : Average shear s t r e n g t h s i n Treatments 9 to 12, Group C ( b o i l t e s t ) . 66 Table X I I I . Table XIV. A n a l y s i s of v a r i a n c e : Average per cent wood f a i l u r e by c o n v e n t i o n a l methods i n Treatments 1 to 5t Group A, 67 A n a l y s i s o f v a r i a n c e : Average p o s i t i o n of wood f a i l u r e i n an annual increment from i t s i n i t i a t i o n i n Treatments 2 to 5» Group A. 67 Table XVA. A n a l y s i s of v a r i a n c e : Average per cent wood f a i l u r e o c c u r r i n g i n e a r l y -wood i n Treatments 2 to 5» Group A . 68 Table XVB. Duncan's m u l t i p l e range t e s t : Average per cent wood f a i l u r e o c c u r r i n g i n earlywood In Treatments 2 to 5» Group A, 68 (ix) Page Table XVI. Analysis of variance: Average per cent wood failures by conventional method in Treatments 6 to 8 , Group B (These are a l l of Group B). 6 9 Table XVIIA. Table XVIIB. Analysis of variance: Average per cent wood failure.' by conventional methods in Treatments 9 to 1 2 , Group C. 7 0 Duncan's multiple range test: Average per cent wood failure?, by conventional methods in Treatments 9 to 1 2 , Group C. 7 0 (x) LIST OF FIGURES Page Fig.*5 1 . C r i t i c a l zones of stress i n veneer cutting without a nosebar . 7 1 Fig.* 2.* Lathe check orien t a t i o n i n plywood shear test specimens. 7 2 Fig. ; 3 » Position of sample veneer i n the l o g . 7 3 FlgJ 4 ; ' Camera setup and te s t i n g apparatus. 7 ^ Fig; 1 5 » Relationship between shear strength of plywood and penetration depth of adhesive into lathe checks. 7 5 Fig. ; l 6 . S t r e s s - s t r a i n curve f o r plywood made of sawn veneer,1 with accompanying photographs at the positions noted - Treatment 5 . 7 6 Fig. ! 7 « S t r e s s - s t r a i n curve f o r plywood made of rotary-cut veneer,1 with accompanying photographs at the positions noted (Sample 1 ) - Treatment 1 . 8 1 Fig.' 8 . ; f S t r e s s - s t r a i n curve f o r plywood made of rotary-cut veneer, with accompanying photographs at the positions noted (Sample 2 ) - Treatment 1 „ 8 8 Fig.' 9 . Comparison of s t r e s s - s t r a i n curves f o r plywoods made of sawn veneer (S), rotary-cut veneer with lathe checks f u l l y impregnated by r e s i n (L),' and rotary-cut veneer (R). 9 3 Fig.1" 1 0 . S t r e s s - s t r a i n curve f o r plywood made of rotary-cut veneer,1 with lathe checks f u l l y Impregnated by resin,' with accompanying photographs at the positions . noted (Sample 1 ) - Treatment 4 . 9 4 (xi) Page Fig. 1 11.' S t r e s s - s t r a i n curve f o r plywood made of rotary-cut veneer with lathe checks f u l l y impregnated by re s i n , with accompanying photographs at the positions noted (Sample 2) - Treatment 4. 98 F i g . 12. S t r e s s - s t r a i n curve f o r plywood made of rotary-cut veneer with lathe checks f u l l y impregnated by resin,- with accompanying photographs at the positions noted (Sample 3) - Treatment 4. 103 Fig;* 13'.' Test specimens a f t e r f a i l u r e . 10? INTRODUCTION Many studies have been done during recent years to determine the factors influencing the t e n s i l e shear strength of plywoods. Of these factors, lathe checks are always regarded to be of the greatest importance. However, several fundamental questions concerning the influence of lathe checks have not been answered c l e a r l y . For example, how deeply does the glue migrate into lathe checks i n proportion to t h e i r t o t a l depth? W i l l varying depths of glue migration influence the shear strength of plywood? How does the c r i t i c a l area of a shear specimen having lathe checks behave under the applied load, In comparison with that of a check-free ( i . e . sawn veneer) specimen? If these questions could be s a t i s f a c t o r i l y answered, i t i s believed that the new knowledge could lead to better techniques that would improve strength properties of plywoods. The value of wood f a i l u r e as a c r i t e r i o n of plywood qual i t y has been a subject on which many c o n f l i c t i n g opinions have been expressed. This may be due to lack of understanding of the mechanism of wood f a i l u r e i n r e l a t i o n to shear strength. It has long been recognized that veneer properties are affected by the peeling process. The loose- and t i g h t - s i d e of a veneer are expected to be d i f f e r e n t i n strength properties. The t i g h t - s i d e might have been compressed beyond the proportional l i m i t by the nose bar, and the t e n s i l e strength p a r a l l e l to the 2 g r a i n of the l o o s e - s i d e might be reduced by l a t h e checks. A t e s t of these v a r i a b l e s on plywood specimens has not been done. In view of the above, the f i r s t purpose of t h i s study was to i n v e s t i g a t e how mechanical behaviour of plywood i n t e n s i l e shear, p e r p e n d i c u l a r to the g r a i n of the core plies,, would be i n f l u e n c e d (a) by presence of l a t h e checks and (b) by d i f f e r e n t depths of adhesive p e n e t r a t i o n i n t o l a t h e checks. Shear s t r e n g t h s were determined w i t h plywood i n which the f a c e p l i e s were not t r e a t e d . The shear t e s t i n g was done on an I n s t r o n machine which d i r e c t l y r e c o r d s the s t r e s s - s t r a i n curve on a c h a r t . C l o s e -up photographs of the samples, i n d i f f e r e n t stages of l o a d a p p l i c a t i o n , were a l s o taken and used f o r a n a l y s e s of s t r e s s and wood f a i l u r e a t known p o s i t i o n s on the s t r e s s - s t r a i n curve. The second purpose was to determine the c o n t r i b u t i o n of the t i g h t - and l o o s e - s i d e of veneer i n the f a c e p l i e s t o the t e n s i l e shear s t r e n g t h of plywood made w i t h resin-impregnated core veneer, w i t h f a i l u r e f o r c e d by specimen d e s i g n to occur i n f a c e p l y . Another o b j e c t i v e was to examine the i n f l u e n c e of the resin-impregnated core veneer on the shear s t r e n g t h of plywood i n which the f a c e p l i e s were not t r e a t e d , w i t h f a i l u r e f o r c e d to occur i n c o r e . I t i s hoped t h a t the r e s u l t s of t h i s experiment w i l l l e a d to the development of a plywood which has improved shear s t r e n g t h and s t i l l w i l l remain economical to manufacture. 3 REVIEW OF LITERATURE , A. Lathe Checks I". Formation The influence of peeling technique on veneer quality-has received considerable study i n recent years ( 12 , 23» 3 0 f 3*+t 35 , 39, k0r^5. 46.). F l e i s c h e r (23) obtained excellent close-up photographs of veneer cutting and showed that an increase i n nose-bar pressure was e f f e c t i v e i n reducing both the severity of lathe checks and roughness of the veneer. Excessive compression i n the case of some softwoods, however, resulted i n s h e l l i n g or s l i v e r i n g , due to a separation between earlywood and latewood. Variations i n frequency and depth of . checks, i n veneer thickness and i n surface q u a l i t y i n peeled veneers were observed by McMillin (46). He found that, as nose-bar pressure was increased stepwise so that compression of the veneer increased from zero to 15% of veneer thickness, the depth of lathe checks decreased, while t h e i r frequency (of checks) increased. The mechanism of veneer formation at the c e l l u l a r l e v e l was studied comprehensively by Leney (39, *K>). When his-study i s combined with the explanation postulated by Koch ( 3 5 ) . i t i s seen that stresses i n the zone adjacent to the cutting edge of the knife can be c l a s s i f i e d simply into tension, shear and compression. These c r i t i c a l zones of stress concentration are presented diagrammat1cally i n F i g . 1. Because of stresses, three types of rupture may occur, singly or i n combination, as veneer i s formed. Leney (39, 40) i d e n t i f i e d these types of rupture as tension checks, shear checks, and compression tearing. "Tension check i s due to tension stresses r e s u l t i n g from combination of cant i l e v e r beam action, compression just behind the cutting edge, and shear stresses caused by the cuttin g edge r e s i s t i n g the movement of the wood. Shear checks from behind the cutting edge when high compressive \ stresses combined with r e s u l t i n g f r i c t i o n r e s i s t the movement of the chip up the kn i f e . Compression tearing was at t r i b u t e d to a d u l l knife r e s i s t i n g the movement of the wood past the cutting edge." (39) . About the same time C o l l i n s ( 1 2 ) postulated that lathe checks i n Douglas-fir veneer were produced by an unstable "snap action" i n an area stressed under tension normal to the break. At f i r s t , the stress would be nearly normal to the cut, but would change rapidly to a d i r e c t i o n nearly p a r a l l e l to the cut, i i . Influence on shear strength of plywood. Tensile strength perpendicular to the grain i n rotary-cut veneer has been studied by Kivimaa (3*0 „ He found that increase of nose-bar pressure increased the t e n s i l e strength of veneer but reduced the depth of lathe checks. Curry ( 1 5 ) and Yavorsky and Cunningham ( 7 1 ) postulated that a large component of shear existed i n the plane of the core veneer of three-ply specimens. Existence of lathe checks i n the core veneer exerted a pronounced e f f e c t on the amount of shear s t r a i n i n t h i s region. A si m i l a r opinion was reported by Norris, Warren and McKinnon ( 4 8 ) . They added that lathe checks reduced the e f f e c t i v e area of each ply by reducing the cross-section area. 5 o f wood s u b j e c t e d t o s t r e s s a n d, f u r t h e r m o r e , l a t h e c h e c k s i n t r o d u c e a zone o f s t r e s s c o n c e n t r a t i o n t h a t s h o u l d f u r t h e r d e c r e a s e t h e s h e a r s t r e n g t h . P r a c t i c a l i n v e s t i g a t i o n s were a l s o done by B e t h e l a n d H u f f m a n (5), F e i h l (22), and P a l k a (5*0-. B e t h e l a n d H u f f m a n f o u n d t h a t , i n p l y w o o d o f p o p l a r c o r e and gum f a c e , t h e o r i e n t -a t i o n o f l a t h e c h e c k s w i t h r e s p e c t t o t h e sawn c u t s i n t h e t e s t s p e c i m e n c o u l d c a u s e s i g n i f i c a n t d i f f e r e n c e s i n t h e r o l l i n g s h e a r s t r e n g t h . S t r e n g t h was l o w e r f o r s p e c i m e n s p u l l e d open t h a n f o r t h o s e p u l l e d c l o s e d . A s i m i l a r s t u d y was done by F e i h l (22) who f o u n d t h a t t h e i n f l u e n c e o f l a t h e c h e c k s on t h e s t r e n g t h o f y e l l o w b i r c h p l y w o o d was o f a v e r y c o n t r a d i c t o r y n a t u r e . T h i s was e x p l a i n e d a s b e i n g c a u s e d m a i n l y by m a j o r d i f f e r e n c e s i n t h e d i s t r i b u t i o n o f s t r e s s e s i n t h e s p e c i m e n s s u b m i t t e d t o d i f f e r e n t t e s t s . The b e n d i n g s h e a r t e s t was f o u n d t o be b e t t e r t h a n t h e t e n s i o n s h e a r t e s t . He c o n c l u d e d t h a t s p e c i m e n s w i t h l a t h e c h e c k s p u l l e d c l o s e d have h i g h e r s t r e n g t h t h a n t h o s e w i t h t h e c h e c k s p u l l e d open. ( F i g . 2). P a l k a (5^), i n v e s t i g a t e d f a c t o r s i n f l u e n c i n g s e v e r a l s t r e n g t h p r o p e r t i e s o f D o u g l a s - f i r p l y w o o d . He c o n c l u d e d t h a t t h e most i m p o r t a n t s i n g l e f a c t o r was v e n e e r t y p e . The d o m i n a n t r o l e o f t h i s f a c t o r i s a t t r i b u t e d t o t h e e f f e c t of l a t h e c h e c k s a n d s u r f a c e q u a l i t y . The a v e r a g e r o l l i n g s h e a r s t r e n g t h o f sawn v e n e e r was a b o u t 1.5 t i m e s t h a t o f r o t a r y - c u t v e n e e r b l o c k s . However, he s u g g e s t e d t h a t t h e r e d u c t i o n i n r o l l i n g s h e a r r e s i s t a n c e a t t r i b u t a b l e t o t h e m e c h a n i c a l e f f e c t o f l a t h e c h e c k s was com-p a r a t i v e l y s m a l l a n d / o r p a r t l y c o u n t e r e d by t h e a d h e s i v e p e n e t r a t i n g i n t o them. The l a t t e r was more pronounced f o r blocks prepared under high g l u i n g pressure (350 p s i ) . B. Wood F a i l u r e i . Importance Tests of glue bonds are made i n an attempt to p r e d i c t the behaviour to be expected of the plywood i n s e r v i c e . I n most instances the c r i t e r i o n f o r acceptance or r e j e c t i o n i s that the t e s t specimen must have a glue bond stren g t h at l e a s t equal to that of the wood i n the specimen. This l i n e of reasoning l e d to the use of percentage wood f a i l u r e as the basis f o r e v a l u a t i n g bond stren g t h (72). I n Canada per cent x-jood f a i l u r e i s accepted as a standard f o r est i m a t i n g glue-bond q u a l i t y (10) f o r t e s t i n g e x t e r i o r - t y p e D o u g l a s - f i r plywood bonded w i t h waterproof phenol-formaldehyde r e s i n . According to Canadian s p e c i f i c a t i o n s ( 1 0 ) : " I f the average wood f a i l u r e of t e n t e s t specimens from a t e s t piece i s l e s s than 60 per cent, or i f more than one t e s t specimen i s l e s s than 30 V e r cent, t h a t t e s t piece f a i l s . I f more than one t e s t piece from a t e s t panel f a i l s , then that panel f a i l s . " Importance of per cent wood f a i l u r e , t h e r e f o r e , can be seen. i i . U n c ertainty of r e l a t i o n s h i p between wood f a i l u r e and shear s t r e n g t h The importance of wood f a i l u r e was not emphasized i n e a r l i e r research i n England or the U.S.A. (50, 6 6 ) . Shear strength of plywood was based upon breaking loads obtained from mechanical t e s t s of glue j o i n t s . Truax ( 6 5 ) , in. d i s c u s s i n g 7 r e s u l t s of h i s experiments, s t a t e d that under good g l u i n g c o n d i t i o n s the j o i n t s t r e n g t h was not s e r i o u s l y i n f l u e n c e d by the percentage of wood f a i l u r e developed by the t e s t . T h i s was l a t e r confirmed by Northcott ( 4 9 ) , who used D o u g l a s - f i r veneers bonded w i t h phenol-formaldehyde r e s i n . By 1 9 3 8 s e r i o u s c o n s i d e r a t i o n was being given t o the use of per cent wood f a i l u r e as a c r i t e r i o n of bond q u a l i t y f o r plywood s p e c i f i c a t i o n purposes, i f not f o r research purpose ( 5 0 ) . The amount of wood f a i l u r e was expected to be r e l a t e d to the s e r v i c e l i f e of plywood ( 5 7 ) • A review of various data and r e p o r t s from d i f f e r e n t sources, together w i t h h i s experimental r e s u l t s , l e d Northcott ( 4 9 , 5 0 ) to p o s t u l a t e that there was a poorer c o r r e l a t i o n between breaking load and s e r v i c e l i f e than between per cent wood f a i l u r e and s e r v i c e l i f e . He f u r t h e r showed that s t r e n g t h values from mechanical t e s t s gave more r e l i a b l e estimates of bond st r e n g t h than r e s u l t s obtained by the per cent wood f a i l u r e method. Shen ( 5 9 ) found that per cent wood f a i l u r e i n spruce plywood was not as accurate an i n d i c a t o r f o r est i m a t i n g the g l u e -l i n e q u a l i t y as was the breaking l o a d . Palka ( 5 ^ ) . using D o u g l a s - f i r veneers, p o s t u l a t e d t h a t an increase of shear stre n g t h was a s s o c i a t e d w i t h higher wood f a i l u r e . The simple c o r r e l a t i o n c o e f f i c i e n t between shear s t r e s s and wood f a i l u r e percentage was 0 . 5 ^ f o r sawn veneer panels and O . 7 6 f o r r o t a r y - c u t veneer panels. 8 In a recent report, Northcott and co-workers ((5.2,; 53) concluded that, within cer t a i n l i m i t s , the per cent wood f a i l u r e was a p o t e n t i a l l y good estimate of plywood d u r a b i l i t y . They suggested that i t was reasonable to accept per cent wood f a i l u r e as a worthwhile predictor of bond d u r a b i l i t y with hot-pressed, phenolic-glue-bonded Douglas-fir plywood. The per cent wood f a i l u r e was expected to be more sensitive to i n i t i a l bond degradation than was the breaking load. From the res u l t s and opinions above, i t may be con-cluded that the rel a t i o n s h i p of per cent wood f a i l u r e to breaking load i n tension shear test has not been adequately defined. Certain variations i n wood, glue, i n gluing, or i n test and measuring method, may cause the amount of wood f a i l u r e of a glue joint to vary considerably (59). The density of wood (3), wood species (51)» whether heartwood or sapwood (59)» latewood percentage, veneer thickness, gluing pressure^, moisture content • of veneer (54), angle of grain i n r e l a t i o n to the glue- l i n e (4),• thickness of glue- l i n e (11) and kind of glue used (19K were factors found influencing the per cent wood f a i l u r e of shear specimens. i i i . Relationship between wood f a i l u r e and lathe checks It had been observed (48) that the number of lathe checks within one l i n e a l inch of veneer (from i t s cross-section) was independent of veneer thickness and that t h e i r depth, expressed as a proportion of the veneer thickness, increased with t h i s thickness. Consequently, plywood panels made from t h i c k veneers had a lower shear strength than panels of the same thickness made from t h i n veneers (15). Considering t h i s e f f e c t , along with Palka's finding (5^) that reduction i n wood f a i l u r e accompanied increasing veneer thickness, i t i s possible that increase of lathe check; depth would cause reduction In per cent wood f a i l u r e . Prom r e s u l t s obtained by Bethel and Huffman (5), orientation of lathe checks with respect to the saw cut i n f l u -enced the per cent wood f a i l u r e ? a s well as shear strength (p.5). Regardless of whether specimens were tested a f t e r b o i l i n g or i n dry condition, the average wood f a i l u r e of specimens f o r which lathe checks were to be pulled open was lower than that of speci-mens f o r which lathe checks were to be pulled closed. C. Relationship of Resin-penetration to Shear Strength of Plywood i . Permeability of Douglas-fir wood Erickson and co-workers (21) i l l u s t r a t e d that per-meability of sapwood i n the longitudinal and r a d i a l anatomical d i r e c t i o n was greater than that of heartwood. The superior permeability of latewood over earlywood was explained by G r i f f i n (26) as due to the greater c a p i l l a r y forces i n the lumina of latewood and the greater degree of a s p i r a t i o n i n p i t s of earlywood than of latewood. Krahmer and CSte ( 3 8 ) described three natural pro-cesses which modify the condition of bordered p i t - p a i r s . One was p i t aspiration, the second was p i t occultation with extractives, 10 and the third process was pit incrustation. Pit membranes of Douglas-fir studied were not heavily incrusted like those of western red-cedar, but pit aspiration was commonly found in the heartwood of both species. Recently, Koran (37) showed that the presence of extraneous materials in the capillary structure was an important factor prohibiting penetration. The influence of origin of the material was reported by Miller (47). He found that heartwood of Douglas-fir from eastern Oregon was generally much less per-meable than that from the western half of the State. Craig (14) reported that trees from high elevations had lowest permeability while trees from medium elevations had the highest variation in permeability. The styrene impregnation technique applied in this study had been developed by Erickson and Balatinecz (20). They found that in radial impregnation the liquid moved mainly in the ray tracheids. Ray parenchyma cells were mostly imper-meable. In tangential impregnation, the penetration was com-paratively slow. Movement was through tracheid bordered pits and into and across ray tracheids to some extent. The higher permeability of sapwood over heartwood in Douglas-fir was also demonstrated. i i . Resin properties Impregnating veneer with a synthetic resin to stabi-lize the dimensions, and increase the strength of plywood made from i t , can be effectively accomplished only when resin-forming 11 constituents penetrate the c e l l wall structure and become bonded to the active groups i n the wood upon formation of the r e s i n (62). Hence, use of a high molecular weight or prepolymerized r e s i n , or non-polar resin-forming constituents, i s not desirable because deposition within the coarse c a p i l l a r y structure of wood renders i t l e s s e f f i c i e n t . A comparison of commercial water soluble phenol-formaldehyde resinoids f o r wood impregnations has been made by Burr and Stamm (8). By using an a l k a l i n e c a t a l y s t , the degree of prepolymerization of the impregnant was s u f f i c i e n t l y lowered so as to be soluble i n water i n a l l proportions. This system was superior to the raw mix, since the r e s i n was l e s s v o l a t i l e and very small amounts were l o s t during drying p r i o r to curing of r e s i n ( 6 l ) . Another advantage of the phenol-formaldehyde system was that i t swelled wood about 25% beyond the swelling i n water, thus further opening up the structure. A f t e r cure of the r e s i n , the retained volume of wood was close to the normal water-swollen volume of that wood. The ap p l i c a t i o n of urea-formaldehyde r e s i n was found l e s s successful (61), mainly because the lower polymer was only about 20% soluble i n water and the resin-forming s o l i d tended to pr e c i p i t a t e i n the coarse void structure of the wood as i t was dried, rather than continuing to diff u s e into the f i b e r walls. Also, the cured urea r e s i n was f a r l e s s water r e s i s t a n t than the phenol-formaldehyde r e s i n . 1 2 G a p - f i l l i n g properties of several wood adhesives was compared by Goto, Kawamura and Sakuno ( 2 5 ) . They found that phenol-formaldehyde glue was the best, while r e s o r c i n o l and pol y v i n y l acetate were next. i i i . Resin impregnation and shear strength An i n t e r e s t i n g experiment examining the influence of adhesive on the strength of plywood has been done by Curry ( 1 6 ) . Plywoods were made from veneers ranging i n thickness from 1 / 1 0 inch to 1 / 5 0 inch, using phenol-formaldehyde r e s i n and two other types of adhesive, and tested. It was found that when the veneer thickness was not less than 1 / 1 6 inch, the contribution of adhesive to the t o t a l strength of plywood was l i t t l e . But f o r plywoods made from thinner veneers, the influence of adhesive became more s i g n i f i c a n t . Neglect of adhesive influence could lead to considerable error when cal c u l a t i n g strength values because of the structure of the wood being modified by the presence of adhesives. Though the penetration of adhesive had i r r e g u l a r and i n d e f i n i t e borders, ray tissue was deeply pene-trated. A s i m i l a r finding that the shear strength of. wood was the deciding f a c t o r f o r shear strength of glue joint specimens was shown i n a report by Marian and Stumbo ( 4 3 ) . Since wood i s t y p i c a l l y nonhomogeneous, whereas r e s i n i s s t r i c t l y homogeneous, impregnating r e s i n into i t increased uniformity and thereby strength ( 6 l ) . Strength properties of wood which had been r e s i n impregnated, then compressed, were obtained by Stamm and 13 Seborg (63). and Dadswell and co-workers (17). The r e s u l t s showed that phenol-formaldehyde r e s i n was e f f e c t i v e i n increasing the compressive and shear strengths, whereas toughness was decreased. In Impreg (wood impregnated with r e s i n but not com-pressed), treatment with synthetic resins was found to improve only the compressive strength properties p a r a l l e l and perpendicular to the grain, and hardness, while ultimate tension p a r a l l e l to grain, toughness and Izod impact strength were decreased. However, because of technical progress, and improved chemicals, greater toughness of treated wood has been obtained ( 2 7 ) . Solechnik and co-workers (60), on the other hand, found that bending resistance of wood impregnated with condensation products of/phenol-formaldehyde r e s i n was increased by ll - 2 5 # . MATERIAL AND METHODS A. Experimental Design In t h i s experiment, shear strength and per cent wood f a i l u r e were evaluated i n 12 treatments divided into three groups. Each treatment had 12 specimens randomly taken from k duplicate panels. The treatments were as follows: 14 GROUP A - Evaluation of shear strength f o r specimens having d i f f e r e n t depths of adhesive penetration into lathe checks i n core veneers due to varying periods of adhesive impregnation.* Treatment 1 - Conventional plywood made of rotary-cut veneers bonded by AMRES 2211 phenol-formal-dehyde glue only. Treatment 2 - Plywood made of rotary-cut veneer with 5-minute r e s i n (No. 4880) impregnation of core veneer. Tireatment 3 - Plywood made of rotary-cut veneer with 10-minute r e s i n impregnation of core veneer. Treatment 4 - Plywood made of rotary-cut veneer with 30-minute r e s i n impregnation of core veneers. Treatment 5 - Plywood made of sawn veneer without r e s i n impregnation, using AMRES 2211 glue only. GROUP B - Evaluation of shear strength f o r specimens having resin-impregnated core veneer with f a i l u r e forced by specimen design to occur i n face p l y. Treatment 6 - Plywood made of rotary-cut veneer with ti g h t - s i d e of face p l y being adjacent to f u l l y resin-impregnated core veneer. * Hereafter, the word "re s i n " w i l l r e f e r to r e s i n No. 4880. The term "glue" w i l l r e f e r to i n d u s t r i a l use AMRES 2211. And "adhesive" w i l l r e f e r to either glue or r e s i n according to the text. 15 Treatment 7 - Plywood made of rotary-cut veneer with loose-side of face ply being adjacent to f u l l y resin-impregnated core veneer. Treatment 8 - Plywood made of sawn veneer with f u l l y r e s i n -impregnated core veneer. GROUP C - Evaluation of shear strength f o r specimens having resin-impregnated core veneer with f a i l u r e forced to occur i n core. Treatment 9 - Plywood made of sawn veneer with core impreg-nated by methanol-resin mix, flattened and o dried at 140 P. and bonded by glue. Treatment 10- Plywood made of rotary-cut veneer with core impregnated by methanol-resin mix, and bonded d i r e c t l y into plywood. Treatment 11- Plywood made of rotary-cut veneer with core impregnated by methanol-resin mix, flattened and dried at l40°F. and bonded by glue. Treatment 12- plywood made of rotary-cut veneer with core impregnated by water-resin mix, flattened and , o dried at 140 F. and bonded by glue. Analysis of variance and Duncan's multiple range -the. test (41) were performed upon^data obtained. B. Materials i . Veneer c o l l e c t i o n a. Log A Douglas-fir (Pseudotsuga menziesii (Mirb.) Francd) log, 10.5 feet i n length and about 23 inches i n diameter, was chosen from the log-pond of the P a c i f i c Veneer and Plywood Division, Canadian Forest Products Ltd., New Westminster.v. Its o r i g i n was Vancouver Island but the exact location was not known. A 6-inch long section was cut from one end of the log and l a t e r sawn into the tangential plane. The remaining 10-foot log was peeled on a commercial lathe. B. Peeling and sawing Before peeling, every tenth growth r i n g from the pith was c l e a r l y marked by India ink at each end of the log (Fig. 3). One quadrant of the 80- to 100-year part of the log was sprayed with blue paint to serve as a reference mark f o r subsequent sampling. Sawn and rotary-cut veneers were taken from t h i s zone. Thus, the l o c a t i o n and hopefully the properties of rotary-cut and sawn veneers were closely matched. Peeling took place two hours and 15 minutes a f t e r the honing of the knife. According to the data available, the setting of the machine was as follows: Peeling speed 170 fpm. Knife angle 90 deg. Horizontal gap 0.125 i n . 17 The log was chucked at the pith. This enabled the longitudinal axis of veneer to be parallel to the grain direction. The resulting veneer was 1/7 inch in thickness. Facilities of the Vancouver Forest Products Laboratory of the Canada Department of Forestry were used to saw the short log tangentially into sheets of 1/7-inch thickness and 4-by 6-inch surface dimension. C. Selection and grouping of veneers (1) . Group A Four 36-by 30-inch rotary-cut veneers taken from the end of the bolt adjacent to the log used for sawn veneer were sawn into strips of 6-by 24-inches along the grain (total of 6 strips)• Four strips were randomly chosen to serve as core veneers. Each strip was sawn into four 6-by 6-inch veneers which were randomly assigned to four treatments. Hence, each treatment was applied to four veneers marked alphabetically. The other strips were also sawn into 6-by 6-inch veneers to be used later as face veneers. Twelve 4-by 6-inch sawn veneers were randomly chosen. Four of them were used as core and eight as face plies. This assignment allowed each treatment to have 12 veneers to be made into 4 panels. (2), Group B Five consecutive parallel strips of 6-by 24-inch rotary-cut veneer were cut into 6-by 6-inch samples yielding a total of 20. Sixteen of these were randomly chosen and 18 separated into two equal groups of 8 each. Both groups served as face p l i e s , one to test the strength of tig h t - s i d e of veneer, the other f o r test of loose-side. A l l 4-by-6-inch sawn veneers were chosen to be face veneers f o r test of shear strength of face ply. These three classes of material provided comparison with r e s u l t s of other treatments of rotary-out veneers. Eight 6-by 6-inch and four 4- by 6-lnch samples of 1/10-inch thick sapwood veneers were taken from another tree to serve as cores of plywoods. In a preliminary test of thi s study these veneers had proved to be eas i l y penetrated by the impregnating r e s i n . This assignment allowed each treatment to have 12 veneers to be made into 4 panels, a l l of which would force f a i l u r e upon stressing to occur i n the face p l i e s . (3). Group C Twelve 6-by 6-inch rotary-cut sapwood veneers from the long log were randomly selected from a p i l e of veneers which had been cut into consecutive p a r a l l e l s t r i p s . They were separated into three equal groups of four veneers, and l a t e r used as core veneers f o r resin-impregnation. Twenty-four rotary-cut veneers of the same dimensions as the core veneers were obtained from matched heartwood of the same log, and used as face p l i e s . The above mentioned veneers were applied to Treatments 10, 11 and 12. Samples f o r Treatment 9 were made of sawn veneers. Eight sawn sapwood veneers were taken and marked to indicate t h e i r p o s i t i o n with respect to the cambium. They were then impregnated with r e s i n . Pour of them were selected f o r use 19 as core veneers. The reason f o r t h i s arrangement i s described in sections i i i and i v . Eight non-impregnated sawn veneers of heartwood were used as face p l i e s . Again, t h i s assignment allowed each treatment to have 12 veneers to be made Into k panels. i i . Staining In order to render lathe checks easily observable, the peeled core veneers of Group A were submerged in about 0.5% by weight of Calcozine Rhodamin BPX solution (red) f o r 12 hours, then a i r - d r i e d f o r two weeks. Under microscopic observation, i t was found that the dye penetrated to the t i p of the lathe checks, but did not dispere into the wood surrounding the lathe checks. A pre-liminary test of t h i s study showed that the presence of the dye had no ef f e c t on r e s i n impregnation and gluing. i i i . Dry ing A f t e r more than two weeks air - d r y i n g i n the working room of the laboratory, the moisture content of veneer averaged 8 . 3 7 $ . Veneers of a l l treatments were l i g h t l y sanded by hand to secure smooth surfaces which were then cleaned by a strong vacuum cleaner to remove f i n e wood p a r t i c l e s that otherwise would hinder adhesion. Veneers were p i l e d , supported by sticks between every tow veneers, and about 50 pounds of f l a t iron plates were placed on top of the p i l e to f l a t t e n them. 20 They were further dried in a small oven at l40°F for 6 hours. This temperature was used to minimize the degrad-ation of wood strength which otherwise might result upon exposure to higher temperature. The consequent moisture content of veneer averaged 5«50% which was believed to be suitable for the resin impregnation process (6l) . A high moisture content might result in the moisture in the wood diffusing out of the veneer as the resin-forming material diffused in. As Stamm stated (6l), this tends to dilute the resin. The volume of the resin increased so that eventually some had to be discarded. The core veneers of Treatments 2, 3 and 4 in Group A, and of all treatments of Group B and Group C, were stored in a plastic bag and later dried with the core veneers soon after the latter were impregnated, iv. Impregnation Two phenol-formaldehyde resins, No. 4000 and No. 4880, obtained from Pacific Resins Ltd., New Westminster, B.C. were tested in a preliminary study to determine their feasibility for the purpose intended. Resin specifications were as follows: Resin No. 4880 Resin No. 4000 Non-volatile fractions 63.3% 37.5% Viscosity (Centistokes) 600 15 Specific gravity 1.202 1.135 21 Resin No. 4000 was found to be more easily impreg-nated into the wood but, perhaps due to its short chain length, the resin did not hold the lathe checks strongly. Upon testing, the tension shear strength of treated plywood was found to be not significantly different from that of conventional, untreated plywood. Use of this resin was therefore abandoned. Work was then concentrated on the use of resin No. 4880. Since the solids content of this resin was 63.3$, dilution was necessary. However, like most short chain phenol-formaldehyde resins (6l), resin No. 4880 was not readily soluble in water at pH less than about 9. The resin was diluted by distilled water to 40$ solids content. Three per cent of sodium hydroxide by weight was added to retain pE^ O.5^  of Victoria blue dye by weight was added so that the cured resin could later be distinguished by stereomicroscope when within the lathe checks (lathe checks were indicated by red colour). The pH of the mixture, as indicated by a Beckman glass electrode pH meter, was 9»lt which proved adequate for the purpose of the experiment. The core veneers of Treatments 2, 3 and 4 of Gromp A, all treatments of Group B, and Treatment 12 of Group C, were immersed into the resin one sheet at a time. Each veneer, suitably weighted, was placed vertically in the container. The container was then placed in a pressure cylinder and 90 psi air pressure was applied. Treatments 2,3 and 4 of Group A were kept in the cylinder for 5» 10 and 30 minutes, respectively. 22 This operation was expected to r e s u l t i n varying depths of r e s i n penetration into lathe checks. Next, veneers were removed from the cylinder at the appropriate times and dri e d . Veneers of Group B and Treatment 12 of Group C remained i n the cylinder and were impregnated f o r 20 hours. The per cent r e s i n content of the veneers was determined on the oiren-dry basis by comparison with three extra veneers which served as controls and which were checked v i s u a l l y by stereoscope at 20 X magni-f i c a t i o n . Resin contents a f t e r 20 hours impregnation were 29.0% to 31«5% of oven-dry weight of wood. These per cent r e s i n contents were considered s u f f i c i e n t to saturate the c e l l wall ( 6 l ) . Microscopic observation supported t h i s . In order to speed the time of impregnation and at the same time obtain a reasonable amount of r e s i n i n the wood, a better solvent was sought which could d i l u t e the re s i n , allow i t s uniform d i s t r i b u t i o n i n wood substance, and dry f a s t . Methanol was found to be such a solvent. For colouring the re s i n , 0.5% by weight of V i c t o r i a blue dye was again added. The f i n a l s o l i d s content of the mixture was again made to 40%. Core veneers of Treatments 9» 10 and 11 were treated. Three hours was found to be long enough f o r impregnating sapwood veneers from the outer part of the stem. Ease of r e s i n impreg-nation was observed to decrease with an increase i n the distance of wood substance from the cambium. # (Weight of impregnated veneer a f t e r d r y i n g s oven-dry height of veneer)/ oven-dry weight of veneer X 100 23 In order to completely impregnate a l l the veneers, time of impregnation was prolonged to 5 hours. This enabled the veneer to have a r e s i n content of 20-22$ of i t s oven-dry weight. Again, t h i s was confirmed by microscopic observation. Together with core veneers of the other treatments and a l l face veneers, the impregnated veneers, with the exception of Treatment 10, were placed i n the oven and dried at l40°F. f o r j6 hours. This temperature and time f o r drying was chosen f i r s t l y because the b o i l i n g point of methanol i s o ° 148 F. (64.5 C.). If the drying temperature were higher than l48°F. the b o i l i n g of the solvent would expel the r e s i n from the c e l l walls. Secondly, f i n a l setting of the r e s i n was expected to be done i n a hot press at a temperature of 300°F. Thirdly, 36" hours drying at t h i s temperature would not degrade the strength of Douglas-fir wood ( 5 8 ) . Two p a r a l l e l s t i c k s were placed between the veneers as they were stacked. Several heavy i r o n plates were placed on the top to f l a t t e n the veneers. A f t e r drying, the moisture content of both impreg-nated and non-impregnated veneers averaged 2.02$. C. Plywood Panel Construction AMRES 2211 phenol-formaldehyde glue was used i n the major part of the experiment f o r bonding panels. The adhesive was mixed and applied according to the manufacturerJs s p e c i f i -cation. Except f o r core veneers of Treatment 10, which were not oven-dried, a l l the oven-dried impregnated veneers were l i g h t l y sanded a second time to provide a smooth surface, and cleaned by a strong vacuum cleaner. 2 4 The glue was uniformly spread on a l l veneers, with the exception of core veneers of Treatment 10, to ensure a spread of 50 pounds per thousand square feet per double glue l i n e . Veneers of alternating grain directions were c a r e f u l l y arranged, so that face p l i e s were exactly perpendicular to t h e i r respective centre p l y . Use of methanol as solvent f o r r e s i n might d i r e c t l y enable assembling impregnated veneers into plywood without the drying process p r i o r to the a p p l i c a t i o n of heat and pressure. An attempt was made to determine the f e a s i b i l i t y of t h i s hypothe-s i s . Impregnated veneers of Treatment 10 were exposed to room temperature f o r 30 minutes before a p p l i c a t i o n of the AMRES 2211 glue. At the same time,the face veneers were spread with the glue on t h e i r loose-side and allowed to stand f o r 10 minutes. Results of t h i s preliminary experiment proved that veneers could be strongly glued using a pressing time the same as that of conventional plywood, which i s normally 5 minutes. Treatment 6 was designed to test shear strength p a r a l l e l to grain i n plywood where the t i g h t - s i d e of face veneer was arranged to be i n contact with the core veneer. The assembled veneers were pressed at 150 p s i and 300°P. Temperature at the glue-line was measured by a portable potentiometer and time recorded when constant temperature was reached. It was found that : m i n u t e s was s u f f i c i e n t f o r plywood of core-dried veneer, and f i v e and one-half minutes for cores of Treatment 10, where the sudden b o i l i n g of methanol i n the centre ply was c l e a r l y v i s i b l e . 25 Immediately upon leaving the hot press the panels were stacked i n a thermostatically controlled oven (212°P.) f o r one hour, then placed i n another oven at l40°F. f o r 24 hours. The purpose of t h i s "hot stack" was to cool the plywood gradually and to allow complete curing of g l u e - l i n e s . The panels were then kept i n the laboratory f o r two weeks before sampling. D. Preparation and Test of Specimens i . Sample size Since tests of tension shear were conducted on a Table Model Instron Tester, i t was necessary to consider the maximum loading capacity of the machine (50 Kg) i n determining sample s i z e . In a preliminary study, d i f f e r e n t sizes of samples were tested to f i n d a suitable one. A width of about 0.2 inch was chosen, with the length between two notches- depending on treatments. For Groups A and B, length was about one inch, and f o r Group C about 0.5 inch. If specimens of Group C were not trimmed to 0 .5 inch i n length (between two notches), f a i l u r e i n the core f o r samples of Treatments 9 and 12 was impossible to obtain because of the resin-strengthened core veneer. i i . Trimming of samples and measurements Four panels of each treatment were cut into sample of sizes described above. The depth of notch was two-thirds of the core veneer thickness with the arrangement of lathe checks to be pulled open (5, 22), as shown i n F i g . 2. This i s 26 believed to be the most sensitive method to test r o l l i n g shear of plywood. Twenty-four samples were randomly chosen. Half were used f o r test when specimens were dry and the other half f o r test a f t e r b o i l i n g . The procedure for preparation of the l a t t e r was according to the CSA standard 0121-1961 (10). "Shear specimens . . . s h a l l be boiled i n water f o r 4 hours, then dried f o r 20 hours at a temperature of 145 ± 5°F. They s h a l l be boiled again f o r a period of 4 hours and tested while wet." i i i . Measurements of lathe checks, adhesive penetration depth and angle of lathe checks The end sections of each sample were l i g h t l y sanded using a b e l t sander. This enabled the stained part of wood to be observed and provided higher accuracy i n measurement of dimensions. The l i n e a r dimensions of samples were taken to an accuracy of 0.001 inch using a micrometer c a l i p e r . A stereomicroscope (5 X magnification) was used to measure veneer thicknesses, depth of lathe checks, depth of glue penetration (Treatment 1) and r e s i n penetration (Treatments 2, 3 and 4) and to estimate per cent wood f a i l u r e . The depth of lathe checks and of the penetrations were measured perpen-d i c u l a r l y from the deepest point of lathe check and that of the penetrations to the glue-line (48). As mentioned above, lathe checks had been stained by Calcozine Rhodamine BPX (red), r e s i n was coloured by V i c t o r i a blue and the colour of glue was black (Fig. 13). Therefore, depth of checks and that of the penetrations were ea s i l y i d e n t i f i e d . 27 In Group A, depth of each lathe check between the notches was "read" to give average depth. Veneer thickness was obtained by averaging three readings of core veneer thickness i n each sample. In order to rel a t e the depth to veneer thickness, the average depth of lathe check was divided by vener thickness to give a per cent depth of lathe check. The depth of adhesive penetration was divided by the depth of lathe check to give per cent depth of adhesive penetration (Table V). I n c l i n a t i o n of lathe check was obtained by measuring with a transparent protractor the angle of the three longest lathe checks to the g l u e - l i n e . The recorded values were averaged to represent the lathe check i n c l i n a t i o n of the sample and are shown i n Table V. i v . Testing A l l t e sts were performed with the same Table Model Instron Testing instrument. For accuracy, the test pieces were c a r e f u l l y aligned p a r a l l e l to the axis of loading. The grips holding the sample were uniformly tightened by means of a torgue-wrench set f o r 60 foot-pounds f o r dry samples and 35 foot-pounds f o r wet samples, which prevented the crushing of specimens before and during t e s t i n g . Load was applied with a continuous motion of the moveable crosshead at a rate of 0.02 inch per minute. Load was recorded automatically throughout the test on a chart. The ultimate load, load to unit s t r a i n within the proportional l i m i t , and ultimate s t r a i n are ava i l a b l e from t h i s record ( F i g . 4). 28 v. Estimation of wood f a i l u r e and pos i t i o n of f a i l u r e The per cent wood f a i l u r e of broken specimens was evaluated by two methods: (1) the conventional way, which i s defined as being an estimate of the amount of wood f i b r e adhering to the surfaces of a fractured glue j o i n t , expressed as a percentage of the t o t a l j o i n t area, and (2) i n measuring the position within an annual increment, i n r e l a t i o n to i t s i n i t i a t i o n , at which f a i l u r e occurred, recorded as a per cent of the width. Amount of wood f a i l u r e i n t h i s p o s i t i o n was also estimated and expressed as a percentage of the t o t a l j o i n t area. The estimation was done under a stereomicroscope at a magnification of 20 X. E. Photographic Technique In order to take photographs of the development of f a i l u r e i n plywood during loading, a SAS Asahi Pentax camera was placed i n front of the loading side of the Instron machine. An extensible bellows unit was attached to the camera to magnify the object, with the lens focused on the c r i t i c a l shear area of the sample (Pig. 4). For the purpose of observing the whole c r i t i c a l area, the distance between the two notches was shortened to about 0.4 inch. When the sample was placed i n the machine and before a p p l i c a t i o n of load, the f i r s t photograph was taken for reference. As soon as the machine started, the ef f e c t of tension stress and s t r a i n on the sample was observed through the viewer. When a s l i g h t change was observed, the pen of the recording chart and the movement of loading arms were stopped. This avoided the vi b r a t i o n of the machine influencing the camera. The photograph was then taken and a number was marked 29 on the stress-strain curve as a reference point of the photo-graph being taken (Fig. 5 to 1 2 ) . A sequence of photographs was taken until the sample failed. A preliminary study was done to observe the effect of stopping the machine during test, since such action permitted stress relaxation and a possible alteration in the ultimate point of failure. The study showed that the stress-strain curves and shear strengths obtained in tests during which the machine was stopped for about 5 seconds were similar to results obtained in an uninterrupted test, hence the effect of these interruptions was ignored in this study. RESULTS A. Shear Strength The shear strengths of various groups and treatments were found to be significantly different (Tables I, II). Since the results of Group C gave data that varied greatly between treatments, a direct comparison of the strength of groups is not considered reasonable. Instead, the strength of specimens made of lathe -check-free sawn veneer from each group (i.e., Treatment 5 from Group A, Treatment 8 from Group B and Treatment 9 from Group C) were compared (Tables III, IV). The analyses of variance showed that the average strength of Treatment 5. Group A was significantly lower than that of Treatments 8 and 9 from the two other groups, whereas that of Treatment 9, Group C was not significantly different from Treatment 8, Group B, in the dry condition. 30 The variations between treatments of each group are as follows: i . Group A The differences of depth of r e s i n penetration into lathe checks i n Treatments 1 - 4 and differences of shear strength i n a l l treatments were found to be highly s i g n i f i c a n t (Tables VI, VII and V I I I ) . As shown i n Table V, average depths of adhesive pene-t r a t i o n into lathe checks were 16, 47, 88 and 97$ f o r Treatments 1, 2, 3 and 4 respectively. Average shear strengths were 252, 280, 343, 356 and 343 p s i f o r Treatments 1, 2, 3, k and 5 respectively. Regardless of the fact that the lathe checks of Treatment 1 were penetrated by glue only and those of Treatments 2, 3 and 4 were penetrated by r e s i n , a mathematical r e l a t i o n s h i p between penetration depth and shear strength of specimens was computed. This was done because i t i s believed that since either the glue or the r e s i n can hold lathe checks strongly during load, t h e i r relationship to the strength of specimen i s the same. This r e l a t i o n s h i p was found to be highly correlated as shown by the l i n e a r equation f o r the dry condition ( F i g . 5 ) . Y = 228.22 + 1.28052 X (SE E = 21.82; r a 0.893) where Y = shear strength and X = penetration depth as per cent of depth of lathe check. 31 The average s t r e n g t h v a l u e s o b t a i n e d from t e s t s of wet specimens f o l l o w e d the same p a t t e r n as those of dry specimens; they were 133, 175, 236, 226 and 229 p s i , r e s p e c t i v e l y , f o r Treatments 1, 2, 3, 4 and 5 (Table V I I I ) . i i . Group B As p r e v i o u s l y s t a t e d , t h i s group was designed to t e s t the shear s t r e n g t h p a r a l l e l t o the g r a i n i n u n t r e a t e d f a c e p l i e s and f o r e v a l u a t i n g the p o s s i b i l i t y of d e v e l o p i n g a s t r o n g resin-impregnated core plywood, The r e s u l t s i n d i c a t e d t h a t the average shear s t r e n g t h s of Treatments 6, 7 and 8 were 557, 506 and 544 p s i i n dry t e s t , and 370, 347 and 382 p s i i n the b o i l t e s t , r e s p e c t i v e l y . Analyses of v a r i a n c e showed t h a t there were no s i g n i f i c a n t d i f f e r e n c e s between the treatments i n e i t h e r t e s t ( Table IX and X ). For dry t e s t s , the shear s t r e n g t h s of t h i s group were s i g n i f i c a n t l y h i g h e r than those of any treatment of Group A. Average s t r e n g t h s of Group B a r e about two to three times as h i g h as those f o r Treatments 1 and 2, and 1.5 times as h i g h as Treatments 3, 4 and 5. i i i . Group C T h i s Group d i f f e r e d from Group A and Group B i n t h a t two d i f f e r e n t s o l v e n t s were used to impregnate r e s i n i n t o the core veneer i n order to o b t a i n a s t r o n g resin-wood complex. A l s o , the samples were designed to f a i l i n the core veneer. 32 The average shear strengths of the specimens of Treatments 9, 10, 11 and 12 were 589,156,376 and 530 p s i , respectively, i n dry t e s t . The analyses of variance showed that there were highly s i g n i f i c a n t differences between t r e a t -ments (Table XI). Treatment 9 was s i g n i f i c a n t l y higher than Treatments 10 and 11 at the 1% l e v e l , and Treatment 12 at the 5% l e v e l . Average wet strengths of Treatments 9, 10, 11 and 12 were 291>125,187 and 267 p s i , resp e c t i v e l y . Treatments 9 and 12 were not s i g n i f i c a n t l y d i f f e r e n t but were higher than Treatments 10 and 11 at the 1% l e v e l . Treatment 11 was higher than Treatment 10 at the 5% l e v e l (Table X I I ) . B. Per Cent Wood F a i l u r e i . Group A Average per cent wood f a i l u r e s estimated by the con-ventional method d i d not show s i g n i f i c a n t differences (Table X I I I ) . They were 94, 96, 89, 94 and 96%, respectively, f o r Treatments 1 to 5* Positions of f a i l u r e within an annual increment were found to be 7» 5. 6" and 7% f o r Treatments 2 to 5 respectively. Their differences were not s i g n i f i c a n t (Table XIV). The average per cent wood f a i l u r e s occurring i n these positions were 63, 75. 80 and 95% f o r Treatments 2 to 5» respectively. The analysis of variance showed that Treatment 5 (sawn veneer plywood) was s i g n i f i c a n t l y higher i n per cent wood f a i l u r e at the 1% l e v e l than Treatment 2, and Treatment 3 at the 5% l e v e l , but not s i g n i f i c a n t l y d i f f e r e n t from Treatment 4 (Table XV). 33 i i . Group B The p o s i t i o n of f a i l u r e within an annual increment was found to be inconsistent from sample to sample. Average per cent wood f a i l u r e s estimated by the conventional method were 84, 79 and 80$ f o r Treatments 6, 7 and 8, respectively. Their differences were not significant: (Table XVI). i i i . Group C The per cent wood f a i l u r e i n t h i s group was found to be s i g n i f i c a n t l y d i f f e r e n t between treatments. The order of magnitude of the data were 97» 95. 76 and 62$ f o r Treatments 10, 11, 12 and 9, respectively (Table XVII). C. Photographic Evidence Photographs of plywood samples made of sawn veneers, rotary-cut veneers with lathe checks f u l l y penetrated by r e s i n , andTiwAmpregnated rotary-cut veneers were s a t i s f a c t o r y . The setting of camera and t e s t i n g apparatus are shown i n F i g . 4. The s t r e s s - s t r a i n curves and the points at which the.photographs were taken are shown i n F i g . 6, 7, 8, 10, 11 and 12. The photographs of a l l test specimens a f t e r f a i l u r e are shown i n F i g . 13. DISCUSSION The study of Group A was designed to compare the shear strength of rotary-cut veneer specimens having lathe checks i n core veneer p a r t i a l l y or f u l l y penetrated by adhesive, 34 with the specimens made of sawn veneer. Consequently, t e n s i l e shear strengths of Group A specimens depend mainly on the shear resistance perpendicular to the grai n of wood. Group B was designed to evaluate shear resistance of specimens with the core veneer f u l l y impregnated by r e s i n and where f a i l u r e was forced to occur i n a face ply. Therefore, the strengths of specimens i n Group B depends on shear resistance p a r a l l e l to the g r a i n of wood. On the other hand, Group C was designed f o r evaluation of shear strength of specimens having r e s i n im-pregnated core veneer, with f a i l u r e forced to occur i n the core, so that strengths of this, group depend*!solely on the com-bination of r e s i n and wood. Hence, the order of magnitude of shear strength as shown by Table III i s considered reasonable. From t h i s , i t is clear that the hypothesis of t h i s thesis i s sound. That i s , plywood made of r e s i n impregnated core veneer and untreated face veneers can be a product having shear strengths about 1.5 times as high as that of sawn-veneer plywood, and two to three times as high as that of conventional plywood made of rotary-cut veneer. Further discussion of variations within groups i s included below. A. Influence of Lathe checks on Shear Strength The assumption which leads to the t h e o r e t i c a l analysis of f a i l u r e i n lap jo i n t s was given by Volkersen i n 1938 (18). According to him, the d i s t r i b u t i o n of shear stresses i n the 35 adhesive l a y e r a r i s e s s o l e l y from d i f f e r e n t i a l s t r a i n i n g i n the lap j o i n t . This hypothesis was modified by Goland and Reissner i n 1 9 ^ (24). They stated that tearing stresses, which were ignored by Volkersen, should be taken into account. Baud (72) employed an i s o t r o p i c model of plywood test specimen which traced the stress d i s t r i b u t i o n by photoelastic means. Thereby, he confirmed that tearing or t e n s i l e stresses e x i s t normal to the plane of the glue j o i n t i n the v i c i n i t y of the notch. In t h i s region, shear components occur i n the plane of the glue j o i n t . However, the main portion of the test section i s subjected only to t e n s i l e and compressive stresses. Consequently, the plane of the glue j o i n t i s free of shear stress. Similar r e s u l t s were reported by Ishihara and co-workers (33), Further a p p l i c a t i o n of a s t r a i n - i n d i c a t i n g b r i t t l e lacquer to wood and wood-glue combinations were investigated by Yavorsky and Cunningham (71)• They found that a large component of shear existed i n the plane of the core veneer of three-ply specimens.1 The presence of stress concentrations at the notches was demonstrated by the formation of i n i t i a l cracks in these areas at low loads, whereas the central region was free of cracks u n t i l considerable load had been applied. Any imperfection i n veneers w i l l therefore cause a reduction of t e n s i l e shear resistance i n plywood. This can be seen from the photograph of sawn veneer plywood i n t h i s experiment.' 3 6 In Pig.' 6 , photograph 2, the f i r s t crack occurred i n the upper notch.' The s t r e s s - s t r a i n curve (Fig. 6 ) indicated that the sample was stressed just over the proportional l i m i t at that point. Instead of the crack opening, a defect between the notches was pulled open by continuous loading (photograph 3 ) . Soon after,' the ultimate strength was reached and the sample f a i l e d suddenly i n the zone located within 10% from the i n i t i a t i o n of an annual increment. Later, the defect noted above was observed under stereomicroscope, on the end surface, and i d e n t i f i e d as ray ti s s u e . It i s known that ray i s the weakest tissue of wood when load i s applied i n the! d i r e c t i o n perpendicular to grain i n the r a d i a l face ( 6 8 ) . The defect might be due to the separation between rays and prosenchyma i n drying, where latewood prosenchyma of high pote n t i a l shrinkage i s i n contact with rays of r e l a t i v e l y small r a d i a l shrinkage potential ( 2 8 . 5 6 ) . The occurrence of wood f a i l u r e i n earlywood was found i n shear ( 4 3 ) , transverse compression ( 7 ) and bending ( 3 6 ) t e s t s . But t h i s i s the f i r s t report of shear f a i l u r e occurring within an annual increment i n the zone located within 1 0 % of i t s i n i t i a t i o n . This was reported as that region having maximum l i g n i n content i n an annual increment of Douglas^fir ( 7 0 ) . Results f o r the plywood made from^rotary-cut veneer (Fig. 7 and F i g . 8 ) were completely d i f f e r e n t from those of sawn veneer plywood (Fig. 6 ) . The fin e black declined l i n e s i n the core veneers of these figures are the red lathe checks i n samples 7 to 9 of F i g . 1 3 . ' Photographs 2 of F i g . 7 and F i g . 8 37 showed no cracks i n the face veneers i n v i c i n i t y of the notches, but the lathe checks had opened s l i g h t l y . When t h i s occurred, the f i n a l rupture stress was reached. Photograph 3 showed that the r e a l wood f a i l u r e started at that point. A l l t h i s evidence indicates that lathe checks d e f i n i t e l y reduce shear strength of plywood. A comparison between the s t r e s s - s t r a i n curves of plywood from sawn and rotary-cut veneers (Pig. 9)» showed that the former f a i l e d abruptly soon a f t e r maximum stress was reached whereas the l a t t e r had a long elongation i n the specimen. This might be caused by the opening of lathe checks which divided the c r i t i c a l area and resulted i n the formation of several flows of stress i n t h i s area ( 1 ) . The extent to which lathe checks influence f a i l u r e was demonstrated by the re s u l t s of Group A. Analyses of variance showed that differences i n shear strength between treatments of Group A were highly s i g n i f i c a n t (Tables V, VII and VIII). Treatment 1 (conventional plywood) gave s i g n i f i c a n t l y lower r e s u l t s than did any other treatment; whereas differences between Treatments 3 , 4 and 5 were not s i g n i f i c a n t , they were s i g n i f i c a n t l y higher than Treatment 2 (Table VII). This could be explained as being due to the varying penetration depths of adhesive into lathe checks i n the core veneers. It was observed that the penetration depth of glue into lathe checks of conventional plywood (Treatment 1) was 16% (Tables V and VI) of the average depth of lathe checks. This indicated 38 that the load area was decreased by about 55$* so that other 45$ of the area was subjected to f u l l load. The penetration of adhesive into lathe checks of Douglas-fir heartwood was d i f f i c u l t . Even when using a short chain resin, and impregnating under 90 psi a i r pressure f o r 5 minutes, as i n Treatment 2, the penetration depth of r e s i n was less than half the lathe check depth. This i s explained in the following way: (i) Nature of veneer. The curved nature of lathe checks, and the shear type of lathe checks (Pig. 1) which were concealed deep i n the veneer and therefore non-accessible to adhesive, should be considered. ( i i ) E f f e c t of pH of wood. The pH of mixed commercial phenol-formaldehyde i s about 12, but that of Douglas-fir heartwood i s only 3 to 4.5. It was found (13) that the pH of both f i l m and l i q u i d phenolic r e s i n decreased with increasing assembly time. High veneer moisture contents accentuated the decrease. According to stamm (6l),the decrease of pH causes reduced s o l u b i l i t y and glue transfer. It i s understandable that, since pH of Douglas-fir i s so low, when the glue i s spread i t s pH w i l l rapidly decrease. This i s provided that the rough surfaces at the entrances of lathe checks may absorb some amount of glue (6) and the excessive depth of lathe checks can a l t e r the proportion between the amount of glue and the surface area of lathe checks. Consequently, the buffering * , % depth of lathe c h e c k s ( 1 Q 0 _ £ d e p t h o f £ d h e s l v e penetration 39 p o t e n t i a l of wood i s s u f f i c i e n t l y great to overpower the pH. of glue (44). The v i s c o s i t y of glue greatly increases and no further penetration occurs. ( i i i ) Influence of extractives. The e f f e c t of extractives on glue bonds has been explained by Huffman (32), who stated: "The extractives may act as deterrents to adequate penetration of the f l u i d adhesive, they may retard the d i s s i p a t i o n of water or other solvents from the glue l i n e , and t h e i r chemical composition may act as a b a r r i e r to proper wetting or to the formation of molecular bonds", " k i l n drying resulted i n concentrating the extractives i n the outer layers". Hancock (29) found that i n the case of Douglas-fir veneer treated at high temperature, f a t t y acids concentrated at the surface, reducing the w e t t a b i l i t y of veneer. This chemical b a r r i e r w i l l no doubt slow the penetration rate and depth of glue. These influences, and the e f f e c t of high temperature of the hot press, accelerated the l o s s of water and polymerization of glue. The p o s s i b i l i t y of glue further penetrating into lathe checks i s thus greatly reduced. As mentioned above, there i s a highly s i g n i f i c a n t l i n e a r r e l a t i o n s h i p between the depth of adhesive penetration into the lathe checks (X) and the shear strength (Y), such that: Y * 228.22 + 1.28052 X. The depth of r e s i n penetration and shear strength i n Treatment 2 were s i g n i f i c a n t l y higher than those of Treatment 1. Si m i l a r l y f o r Treatments 3 and 4, which had s i m i l a r strengths, the depth was about two times higher than Treatment 2, and f i v e to s i x times higher than that of Treatment 1. It i s int e r e s t i n g to 1+0 note that shear strength of Treatments 3 and 4 were not s i g n i f i c a n t l y d i f f e r e n t (Tables VII and VIII); but the penetration depth of the l a t t e r was s i g n i f i c a n t l y higher than that of the former (Table VI). This could be interpreted as follows: When the penetration depth was beyond 80$ the shear strength of plywood made of rotary-cut veneer remained unchanged and no d i f f e r e n t from that of plywood made of sawn veneers (Tables VII and VIII). The r e l a t i o n s h i p between penetration depth and shear strength can also be observed from the photographic r e s u l t s . Figure 10 shows that the lathe checks were completely held by r e s i n , so that f a i l u r e occurred i n the face ply. From photograph 3 of the same Figure i t may be implied that the face f a i l u r e may have been due to the r e s i n strengthening the center ply, by the s l i g h t angle between the load axis and the sample causing misalignment of sample (2), or by defect i n the face ply." Figure 11 demonstrated that when a crack occurred i n the v i c i n i t y of the notches (photograph 3)t the specimen was already over the proportional l i m i t . When the shear lathe check* was s l i g h t l y opened, the maximum stress was reached. The sample then f a i l e d suddenly i n the c r i t i c a l area through the earlywood and the broken shear lathe check. Figure 12 shows the response of sample to the load when the lathe checks and other defects were completely held by r e s i n . A small crack was shown near to the upper notch i n * Shown by red color i n NO. 5 sample of F i g . 13, i t s p o s i t i o n i n photograph A, NO. 5 Is" indicated by an arrow, while i n photograph B, the red color i s v i s i b l e . 41 the face ply (Fig. 12, photograph 2) at which stage the specimen was stressed over the proportional l i m i t . Complete f a i l u r e occurred at the i n i t i a t i o n of one annual increment. The r e s i n was so strong that the course of breaking had passed or deviated around the t i p of glue. As a r e s u l t the s t r e s s - s t r a i n curves of plywood made from sawn veneers, and rotary-cut veneers with lathe checks f u l l y impregnated by r e s i n , were s i m i l a r . B. Influence of Lathe Checks on Wood Fa i l u r e Differences i n average per cent wood f a i l u r e in various treatments of Group A as estimated by the conventional method were not s i g n i f i c a n t (Table XIII). However, the character and po s i t i o n of the f a i l u r e i n Treatment 1 (conventional plywood made of rotary-cut veneer) varied greatly from that of Treatments 3, 4 and 5 (plywood made of veneer with lathe checks completely impregnated by r e s i n and plywood made of sawn veneer). In Treatment 1, the wood broke between the t i p s of the lathe checks i n core veneer. The wood gradually r o l l e d open a f t e r the ultimate strength was reached. But i n Treatments 3, 4 and 5* f a i l u r e occurred suddenly i n the beginning of earlywood. From t h i s i t can be seen that the per cent wood f a i l u r e of plywood made of rotary-cut veneer was not intimately related to the ultimate strength. Instead, the per cent wood f a i l u r e occurring within one or more annual increments i n the zone located within 10% from i t s i n i t i a t i o n gave a better i n d i c a t i o n of ultimate strength. This p o s i t i o n i n the annual r i n g was found to have the lowest t e n s i l e strength (69) and compression strength (31) i n Douglas^fir. Analysis of variance showed that average per 42 cent wood f a i l u r e of sawn veneer plywood (Treatment 5) i n t h i s region was s i g n i f i c a n t l y higher than i n Treatment 2 and Treatment 3, but not s i g n i f i c a n t l y d i f f e r e n t from Treatment 4 (Table XV). This r e s u l t c l e a r l y indicated that wood with no lathe checks and wood with lathe checks highly impregnated by r e s i n resulted i n higher per cent wood f a i l u r e . C. Influence of Rotary-cutting on Shear Strength and Wood Fail u r e i n Tight-side and Loose-side of Veneer A s t a t i s t i c a l analysis of r e s u l t s showed that there were no s i g n i f i c a n t differences either i n shear strength or wood f a i l u r e among a l l treatments of Group B. This f i n d i n g indicated that at the normal i n d u s t r i a l lathe settings, veneer i s not stressed beyond the proportional l i m i t . However, the pos i t i o n of wood f a i l u r e within the annual r i n g was no longer in the beginning of earlywood, which was varied from specimen to specimen. The shear strength of Group B samples averaged over 500 p s i i n dry te s t (Table 1). This i s much lower than comparable data from various sources (9,67), According to Stamm and Seborg (63), and Stieda (64), shear values are highly dependent upon the method of t e s t i n g . Data, therefore, are not comparable unless the method used and size of specimens are i d e n t i c a l . Results of t h i s experiment indicate a new f i n d i n g : that even i f the specimens do not f a i l i n core veneer, tension shear strength of plywood made from r e s i n impregnated core veneer and non-impregnated face veneers, w i l l be two to three 43 times as high as that of "conventional plywood made of comparable rotary-cut veneers. D. Influence of Resin Impregnation on Shear Strength and Per Cent Fa i l u r e of Core Veneer Two things have now become cle a r . F i r s t , the area of wood within an annual ri n g l e a s t able to r e s i s t stresses i s in the zone located within 10% from i t s i n i t i a t i o n . Second, the shear strength of latewood of Douglas-fir i s greater than, or at l e a s t equal to, that of phenol-formaldehyde g l u e - l i n e . Therefore, as shown by t h i s experiment, there i s no doubt that the a p p l i c a t i o n of r e s i n to strengthen earlywood, as i n Treatments 9 and 12, w i l l Increase shear strength of the wood. Average strength In dry test of Treatments 9 and 12 were 588 and 530 p s i , respectively. These figures were not s i g n i f i c a n t l y d i f f e r e n t from the average strengths of Group B, but were s i g n i f i c a n t l y higher than r e s u l t s f o r Treatments 10 and 11, and Treatment 5 (plywood made of sawn veneer without resin-impregnated core veneer). The per cent core veneer f a i l u r e , however, was higher i n Treatments 10 and 11 than i n 9 and 12 (Table XVII). A question a r i s e s i n that, since r e s i n should remedy the lathe defect, why were there s i g n i f i c a n t differences i n shear strengths between Treatments 9, 10, 11 and 12? An explanation i s given below: ( i ) . The highest strength i n Treatment 9 was due, f i r s t l y , to, the methanol-mixed r e s i n being uniformly d i s t r i b u t e d 44 in the wood tissue and, secondly, to the sawn veneer having minimum defects. Therefore, f a i l u r e i n the core veneer was only 62$ by standard wood f a i l u r e estimation procedures, ( i i ) . Samples of Treatment 12 were made from rotary-cut veneers, and hence contained many lathe checks. Through microscopic observation, i t was found that lathe checks were firmly held, but the d i s t r i b u t i o n of r e s i n in the wood was not as even as i n the samples of Treatment 9. These, factors were believed to account f o r the strength values of Treatment 12 being lower than those of Treatment 9. ( i i i ) . It i s inte r e s t i n g to note that the specimens of Treatment 10 have the lowest shear strengths but highest wood f a i l u r e values. This experiment indicated that d i r e c t assembly was possible. Because the b o i l i n g point of solvent was too low (l45°F.) i n comparison with the hot press temperature (300°F), pressing time was shortened to that used in making conventional plywood. Even so, rapid b o i l i n g of methanol expelled r e s i n from the wood and also pushed the lathe checks open. This resulted i n core-impregnated plywood being weaker than untreated plywood (Treatment 1). The c y c l i c b o i l i n g process ( 10, 55 ) could also have caused weakening of the wood. After samples were exposed to the f i r s t cycle of b o i l i n g and drying at l45°F.for 20 hours, most of the lathe checks in Treatment 1 were opened wide. Continuous b o i l i n g and drying subjected the samples to the stresses of shrinking and swelling, consequently the lathe checks were enlarged, r e s u l t i n g i n further decrease of loaded 45 area. In Table 1 i t i s shown that the shear strength of boiled samples of Treatment 1 was s i m i l a r to that of Treatment 10. Here, lathe checks had been already opened by methanol when the plywood was made. ( i v ) . The strength of samples from Treatment 11 i n dry test was equal to, or higher than, that of Treatments 3, 4 and 5 of Group A, but lower than that of Treatments 9 and 12. Prom observation of the manner and per cent wood f a i l u r e , t h i s was believed due to several lathe checks i n specimens not being strongly held when the plywood was made. It i s also possible that the setting of the oven-temperature was too high (l40° F.) for drying the methanol-resin mix. If t h i s i s so, the f a s t evaporation or b o i l i n g of methanol from wood would cause the larger lathe checks to open. Another reason might be that the v i s c o s i t y of methanol-diluted r e s i n was rather low, and did not hold the larger defects. The order of magnitude of shear strength in the b o l l test i n treatments of every group was s i m i l a r to that of the dry t e s t . Unexpectedly, the values of Treatments 9 and 12, which were highest i n dry te s t , were much lower than the average of Group B, and Treatments 3, 4 and 5 of Group A. These res u l t s are of doubtful value. The r e l a t i v e l y degraded strength was thought due to the c r i t i c a l area i n the samples of Group C being trimmed to one-half that of Group B. During b o i l i n g , the dimension of impregnated core veneer was almost constant, but 46 the untreated face veneers tended to swell. This mutual action between core and face p l i e s weakened the wood and glue t r a n s i t i o n zone (43). Consequently, i n tension shear specimens which Were alternately dried and wetted, the smaller the c r i t i c a l area the higher the stress concentration i n t h i s area w i l l be . This i s proved when examining the tested specimens of treatment 9. which showed that f a i l u r e occurred i n the wood and glue t r a n s i t i o n s . It i s highly possible that methanol can be used as a solvent f o r r e s i n impregnation of sawn veneer. For rotary-cut veneer, i f a method of fast drying the solvent at low temperature can be developed, a strong product can be obtained by d i r e c t assembly. This offers considerable commercial p o s s i b i l i t i e s . Otherwise impregnation as i n Group B would be required. CONCLUSIONS 1. The presence of lathe checks caused the tension shear strength of plywood made of rotary-cut veneers to be lower than that made of sawn veneers. Shear strength of conventional plywood with an average of 16% depth of glue penetration into lathe checks was only about 70% that of plywood made of sawn veneers i n dry, and about 60% i n wet, condition. 2. In plywood made of sawn veneer, cracking occurred i n the v i c i n i t y of one notch, indicating that the specimen was stressed beyond the proportional l i m i t . Ultimate strength 47 was reached when sudden f a i l u r e occurred i n earlywood of an annual r i n g . 3. During loading, cracking did not occur i n the v i c i n i t y of the notch, but opening of lathe checks i n the c r i t i c a l areas was observed i n conventional plywood made of rotary-cut veneer. Ultimate strength was reached when lathe checks were just opening. 4. When a l l lathe checks were completely impregnated with r e s i n , shear strength and manner of f a i l u r e of plywood were not d i f f e r e n t from that of plywood made of sawn veneer. Shear strength increased about 40% as a r e s u l t of t h i s treatment. 5. Shear strength was highly influenced by depth of adhesive penetration into lathe checks. Relationship between these two factors i s l i n e a r . When core veneer was f u l l y impregnated by r e s i n , shear strength of sawn-veneer plywood was about 1 .5 times as high as that of untreated plywood. 6. There was no s i g n i f i c a n t difference i n the per cent wood f a i l u r e as estimated by conventional method, f o r plywood made from rotary-cut and from sawn veneer. Use of photography i l l u s t r a t e s that r e l a t i n g per cent wood f a i l u r e to shear strength i s more meaningful i n plywood made of sawn than that of rotary-cut veneers. 48 7. The weakest plane to r e s i s t shear stress i n sawn veneer was found to be i n the zone within 10% of i n i t i a t i o n of an annual increment. Application of per cent wood f a i l u r e occurring i n t h i s area, f o r evaluatingi the strength of plywood, i s considered a more sensitive means than use of conventional methods of estimation; i . e . , an estimate of the amount of wood f i b e r s adhering to surfaces of a fractured glue j o i n t regardless of position of f a i l u r e i n an annual increment. 8. It was found highly f e a s i b l e to employ medium chain length phenol-formaldehyde r e s i n , with methanol as solvent, to strengthen the earlywood portion of veneers. Advantages of th i s process are: (i) a product with high strength properties r e s u l t s , and ( i i ) veneers can be d i r e c t l y assembled into plywood with the same hot press period as that of conventional plywood. However, i t must be noted that the influence of lathe checks l i m i t s the app l i c a t i o n of t h i s technique to plywood of rotary-cut veneer. If a method of fast drying of methanol from impregnated wood at low temperature could be obtained, a strong product could be produced by d i r e c t assembly. 9. Shear strength p a r a l l e l to grain of the ti g h t - s i d e and loose-side of rotary-cut veneer, obtained by tes t of standard tension shear specimens, showed no s i g n i f i c a n t difference. 49 This r e s u l t indicated that plywood made of r e s i n impregnated core veneer, and untreated face veneers, can be a product having shear strengths two to three times higher than that of conventional plywood. 50 LITERATURE CITED 1. Bach, L. 1966. Personal communication. Dept. of Forestry, For. Prod. Lab. Vancouver, Canada. 2. Bensend, D.W. and R.L. Preston. 19^6. Some causes of variability in the results of plywood shear tests. U.S. Forest Service, For. Prod. Lab. Rept. No. Rl6l5, 10 pp. 3. Bergln, E.G. 1953. The.significance of wood failure in glued joints. Dept. of Forestry, For. Prod. Lab., Canada. Reprinted from Canadian Woodworker. March, 1953» 2 pp. 4. . 1953. The gluing characteristics of various eastern Canadian wood species. Dept. of Forestry, For. Prod. Lab., Canada. Reprinted from Canadian Woodworker, December, 1953, 3 PP. 5. Bethel, J.S. and J.B. Huffman. 1952. Influence of lathe check orientation on plywood shear test results. School of Forestry, North Car. State Co l l . Tech. Rept. No. 1, 9 PP. 6. Blomquist, R.F. i960. Proceedings of the symposium on adhesives for the wood industry. U.S. Forest Service, For. Prod. Lab. Rept. No. 2183, 83 pp. 7. Bodig, J. 1965. The effect of anatomy on the i n i t i a l stress-strain relationship in transverse compression. Forest Prod. J. 15:197-202. 8. Burr, H.K. and A.J. Stamm. 195^. Comparison of commercial water - soluble phenol-formaldehyde resinoids for wood impregnation. U.S. Forest Service, For. Prod. Lab. Rept. No. 1384, 12 pp. 9. Canadian Forest Products Laboratory. 1956. Strength and related properties of woods grown in Canada. Technical Note. No. 3, 7 PP./\ 10. Canadian Standards Association. 196l. Douglas f i r plywood. CSA Standard 0121-1961, 15 PP. 11. Cockrell, R.A. and H.D. Bruce. 1946. Effect of thickness of glue line on strength and durability of glued wood joints. U.S. Forest Service, For. Prod. Lab. Rep. No. Rl6l6, 2 6 pp. 12. Collins, E.H. i960. Lathe check formation in Douglas f i r veneer. Forest Prods. J. 10:139-140. 13. Commonwealth Scientific and Industrial Research Organization. 1964. Plywood and gluing: Gluing Phenolic bonds. Division of Forest Products, Melbourne, Australia. Annual report 1963-1964, p. 44. 51 14. Craig, D.W. 1963. The permeability of Douglas f i r heartwood from various geographic sources i n the State of Washington. MP Thesis, University of Washington, 90 pp. 15. Curry, W.T. 195^. The strength properties of plywood. II. E f f e c t of the geometry of construction. D.S.I.R., H.M.S.O., London. Research B u l l . No. 33, 28 pp. 16. . 1957. The strength properties of plywood. II I . The influence of the adhesive. D.S.I.R., H.M.S.O., London. Research B u l l . No. 39, 27 pp. 17. Dadswell, H.E., Fi t z g e r a l d , J.S. and N. Tamblyn. 1952. Investigations of phenolic resins f o r making improved wood, 1942-44. II. The influence of r e s i n composition on the properties and structure of the improved wood. Aust. J. Appl. S c i . 3(D : 7 1 - 8 7 . 18. ' Eley, D.D. I96I. Adhesion. Oxford University Press, Oxford. 290 pp. 19. Erickner, H.W. 1955. The gluing c h a r a c t e r i s t i c s of 15 species of wood with cold-setting urea-resin glue. U.S. Forest Service, For. Prod. Lab. Rept. No. 1342, 2 pp. 20. Erickson, H.D. and J.J. Baltinecz. 1964. Liquid flow paths into wood using polymerization technique- Douglas f i r and styrene. Forest Prod. J. 14:293-299-21. , Schmitz, H. and R.A. Goertner, 1938. D i r e c t i o n a l permeability of seasoned woods to water and some factors which a f f e c t i t . J . Agric. Res. 56:711-746. 22. F e i h l , 0. 1958. Determination of the influence of lathe checks on strength of yellow b i r c h plywood t e s t specimens by means of two shear test methods. Dept. Forestry, For. Prod. Lab. Can. Pro;}. 0-144-5. Progress report No. 1 (unpublished). 23. Fleischer , H.O. 1949. Experiments i n rotary veneer cutting. Proc. Forest Prod. Res. Soc. 3:137-155-24. Goland, M. and E. Reissner. 1944. The stresses i n cement j o i n t s . J. Appl. Mech., Trans. Ameri. Soc. Mech. Engr. 66:17. 25. Goto, T., Kawamura, M. and T. Sakuno. 1963. Studies on the g a p - f i l l i n g property of wood adhesives. B u l l . Wood Res. Inst., Kyoto Univ. Japan, 31:59-74. 26. G r i f f i n , G.J. 1919- Bordered p i t s i n Douglas f i r : A study of the pos i t i o n of the torus i n mountain and low-land specimens i n r e l a t i o n ' t o creosote penetration. J. For. 17:813-822. 52 27. Gurvitch, J.E. 1957. The past developments and future prospects for compreg. Forest Prod. J. 7(9):16A-17A. 28. Hale, J.D. 1957* The anatomical basis of dimensional changes in moisture content. Forest Prod. J. 7:140-144. 29. Hancock, W.V. 1964. 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Stress-optical investigations on laminated bending test beam models. Holz Roh Werkstoff. 13:209-211. 37. Koran, Z. 1964. Air permeability and creosote retention of Douglas fir. Forest Prod. J. 14:159-166. 38. Krahmer, R.L. and W.A. Cote, Jr. I963. Changes in the coniferous wood cells associated with heartwood formation. Tappi, 46:42-49. 39. Leney, L. i 9 6 0 . Mechanism of veneer formation at the cellular level. Univ. of Missouri. Res. Bull. No. 744. 111pp. 40. _. i 9 6 0 . A photographic study of veneer formation. Forest Prod. J. 10:133-139. 41. Li, J.C.R. 1964. Statistical Inference I. Edwards Brothers, Inc. Ann Arbor, Michigan, 658 pp. 53 42. Lutz, J . 1964. How growth rate a f f e c t s properties of so f t -wood veneer. Forest Prod. J . 14:97-102. 43. Marian, J.E. and D.A. Stumbo. 1962. Adhesion i n wood. Part I. Physical factors. Holzforsch. 16:134-148. 44. Marra, A.A. 196l. Proceedings of the conference on theory of wood adhesion. Edited by A.A. Marra. Dept. of Wood S c i . and Technology. The Univ. of Michigan. P.l:3 :13. 45. McKenzie, W.M. 1962. The rela t i o n s h i p between the cutting properties of wood, and i t s physical and mechanical properties. Forest Prod. J. 12:287-294. 46. McMillin, G.W. 1958. The r e l a t i o n of mechanical properties of wood and nosebar pressure i n the production of veneer. Forest Prod. J. 8:23-32. 47. M i l l e r , D.J. I 9 6 I . Permeability of Douglas f i r i n Oregon. Forest Prod. J. 11:14-16. 48. Norris, C.B., Warren, F. and P.P. McKinnon. 1961. The ef f e c t of veneer thickness and grain d i r e c t i o n on the shear strength of plywood. U.S. Forest Service, For. Prod. Lab. Rept. No. 1801, 30 pp. 49. Northcott, P.L. 1952. The development of the g l u e l i n e -cleavage t e s t . Forest Prod. J. 5:216-224. 50. '\ 1955. Bond strength as indicated by wood f a i l u r e or mechanical t e s t . Forest Prod. J . 5:118-123. 51. " 1958. Wood f a i l u r e - within species and between species. Forest Prod. J. 8:180-181. 52. , and H.G.M. Colbeck. i 9 6 0 . Prediction of plywood bond d u r a b i l i t y . Forest Prod. J . 10:403-408. 53. , Hancock, W.V. and H.G.M. Colbeck. I 9 6 3 . Evidence of need f o r wood f a i l u r e standard. Adhesion. STP. No. 360 of the ASTM. 13 pp. 54. Palka, L.C. 1964. Factors influencing the strength properties of Douglas f i r plywood normal to glue l i n e . MF thesis. Faculty of Forestry, Univ. of B.C., 73 pp. 55. Pearson, W.J. 1956. Preliminary report on a proposed method of estimating service l i f e of exterior grade plywood. Forest Prod. J . 6:221-224. 54 56. Pentoney, R.E. 1953. Mechanisms a f f e c t i n g tangential vs. r a d i a l shrinkage. Proc. Forest Prod. Res. Soc. 2(2):27-32. 57. Perkins, N.S. 1950. Predicting exterior plywood perfor-mances. Prod. Forest Prod. Res. Soc. 4:352-363. 58. Salamon, M. 1963. Quality and strength properties of Douglas f i r dried at high temperature. Forest Prod. J. 13:339-344. 59. Shen, K.C. 1958. The e f f e c t s of dryer temperature, sapwood and heartwood, and time elapsing between drying and gluing on the gluing properties of Engelmann spruce veneer. MF th e s i s . Faculty of Forestry, Univ. of B.C., 36 pp. 60. Solechnik, N., Yaisolecn, J. and A.I. Novoselskays (Novoselskaja). 1959. Impregnation of wood with condensation products of phenol and formaldehyde. Naucnye Doklady Vyssej Skoly Lesoin Zenernoe Delo. No. 1:245-250. From abstr. i n Chem. Abstr. 5 4 : 9 2 8 4 (i960). 61. Stamm, A.J. 1964. Wood and Cellulose Science. The Ronald Press Co., New York, 549 pp. 62. , and R.M. Seborg. 1955. Forest Products Laboratory resin-treated wood (impreg). U.S. Forest Service, For. Prod. Lab. Rept. No. 1 3 8 0 , 8 pp. 63. . 1951. Forest Products Laboratory resin-treated, laminated, compressed wood (compreg). U.S. Forest Service, Forest Prod. Lab. Rept. No. 1381, 12 pp. 6 4 . Stieda, C.K.A. 1965. Personal communication. Canada Dept. Forestry, For. Prod. Lab. Vancouver. 65. Truax, T.R. 1929. The gluing of wood. U.S. Dept. Agric. B u l l . 1500, 78 pp. 66. , Browne, F.L. and D. Brouse. 1929. Significance of mechanical wood-joint tests f o r the selection of wood working glues. U.S. Forest Service, Forest Prod. Lab. Reprinted from Ind. Eng. Chem. Jan. 1929. 67. U.S. Department of Agriculture. 1955. Wood Handbook, Agric. handbook No. 72. P, 70. 68. Wardrop, A.B. and F.W. Addo-Ashong. 1963. The anatomy and fi n e structure of wood i n r e l a t i o n to i t s mechanical f a i l u r e . C.S.I.R.O., Department of Forest Prod. Reprinted from "Tewksbury Symposium on Fractures", 1963. 55 69. Wilson, J.W. and G. If j u . 1965. Wood c h a r a c t e r i s t i c s VII: Intra-increment r e l a t i o n s h i p of Douglas f i r wood density, t e n s i l e strength and s t i f f n e s s . P.P.R.I.C. Woodl. Res. Index No. 170, p. 19. 70. Wu, Y.T. 1964. Intra-increment l i g n i n content of f i v e western Canadian coniferous woods. MP thesis, Faculty of Forestry, Univ. of B.C., 48 pp. 71. Yavorsky, J.M. and J.H. Cunningham. 1955• S t r a i n d i s t r i b u t i o n i n maple glue block shear specimen as indicated by a b r i t t l e lacquer. Forest Prod. J . 5:80-84. 72. , and N.G. Hundley. 1955. Survey of factors a f f e c t i n g strength tests of glue j o i n t s . Forest Prod. J. 5:306-311. 56 Table I. Summation of average shear strengths and per cent wood f a i l u r e s roup Treatment Number of Shear strength Conventional Wood f a i l u r e * Remarks specimens dry p s i test S.D. b o i l psi test S.D. % A 1 12 252 20 133 14 94 * Dry tes t only A 2 12 280 17 175 14 96 A 3 12 343 23 236 24 89 S.D. Standard A 4 12 356 21 226 18 94 deviation A 5 12 343 34 229 26 96 ON B 6 12 557 82 370 34 84 B 7 12 506 78 347 47 79 B 8 12 544 83 382 45 80 C 9 12 589 65 291 3 8 62 C 10 12 156 46 125 49 97 c 11 12 376 65 187 33 95 c 12 12 530 56 267 31 76 57 Table I I . Analysis of variance: Average shear  strengths i n Groups A» B and C (dry t e s t ) . Source Groups Treatments Error Total Degrees Sum of Mean of freedom squares square 132 143 1108500 55^250 183.74 ** 1458400 162050 398170 2965070 3017 53.72 ** ** S i g n i f i c a n t at the 1% l e v e l 58 Table III A. Analysis of variance: Average shear  strengths i n Treatments 5t 8 and 9 of Groups A.' B and C,  respectively (dry t e s t ) . Source Degrees of freedom Sum of Mean P squares square Treatments 2 411157 205578 50.34 ** Error 33 134744 4083 Total 35 545901 ** S i g n i f i c a n t at the 1% l e v e l Table III B. Duncan's multiple range t e s t : Average shear strengths i n Treatments 5. 8 and 9 of Groups A, B and C, respectively (dry t e s t ) . Groups A B C Treatments 8. 9. Means (psi) 3^3 544 589 Any two means not underscored by the same l i n e are s i g n i f i c a n t l y d i f f e r e n t . Any two means underscored by the same l i n e are not s i g n i f i c a n t l y d i f f e r e n t . 59 Table IV A. Analysis of variance: Average shear strengths  i n Treatments 5» 8 and 9 of Groups A. B and C, respectively  ( b o i l test). : Source Degrees of freedom Sum of squares Mean square P Treatments 2 141668 70834 7.32 Error 33 319458 9681 Total 35 461126 ** S i g n i f i c a n t at the 1% l e v e l Table IV B.' Duncan's multiple range t e s t : Average shear  strengths i n Treatments 5; 8 and 9 of Groups A, B and C,  respectively ( b o i l t e s t ) . Groups A c B Treatments 5. 9. 8. Means (psi) 229 291 382 Any two means not underscored by the same l i n e are s i g n i f i c a n t l y d i f f e r e n t . Any two means underscored by the same l i n e are not s i g n i f i c a n t l y d i f f e r e n t . Table V. Summation of averaged data of Group A Treatment Lathe Checks Penetration Shear strength Wood f a i l u r e depths-Number Angle Depth (%) S.D. Dry test B o i l test A 2 B3 C^ D^ (per in.) (deg.) (%) (psi) (psi) (%) (%) (%) (%) 1 14 56 66 16* 9 252 133 94 2 13 54 65 47** 8 280 175 96 63 7 1.9 3 15 55 64 88** 7 343 236 89 75 5 1.5 4 14 55 66 97** 3 356 226 94 80 6 2.4 5 343 229 96 95 7 3.0 1. Penetration depth of adhesive into lathe checks.' * by AMRES glue only ** by r e s i n No. 4880 only S.D. Standard deviation 2. Conventional wood f a i l u r e 3. Wood f a i l u r e occurring i n C (see page 28) 4. P o s i t i o n of wood f a i l u r e i n an annual increment from i t s i n i t i a t i o n 5. ' Standard deviation of C 61 Table VI A.- Analysis of variance: Average penetration depths  of adhesive into lathe checks i n Treatments 1 to 4, Group A. Source Degrees Sum of Mean F of freedom squares square Treatments 3 50078.0 16693.0 301.7 ** Error 44 2434.3 55.3 Total 47 52512.O #* S i g n i f i c a n t at the 1% l e v e l Table VI B. Duncan's multiple range t e s t : Average penetration  depths of adhesive into lathe checks i n Treatments 1 to 4,  Group A. Treatments 1.' 2. 3 . 4.* Means (%) 16 47.1 88 97 Any two means not underscored by the same l i n e are s i g n i f i c a n t l y d i f f e r e n t . Any two means underscored by the same l i n e are not s i g n i f i c a n t l y d i f f e r e n t . 62 Table VII A. Analysis of variance; Average shear strengths  i n Treatments 1 to 5» Group A (dry t e s t ) . Source Treatments Error Total Degrees Sum of of freedom squares 4 55 59 1015^0 31136 132670 Mean square 25384 566 4 4 . 8 4 * * ** S i g n i f i c a n t at the 1% l e v e l Table VII B. Duncan's multiple range t e s t : Average shear  strengths in Treatments 1 to 5»' Group A (dry t e s t ) . Treatments Means (psi) 1. 252 2 . 2 8 0 5. 343 3. 343 4. 356 Any two means not underscored by the same l i n e are s i g n i f i c a n t l y d i f f e r e n t . Any two means underscored by the same l i n e are not s i g n i f i c a n t l y d i f f e r e n t . 63 Table VIII A. Analysis of variance: Average shear strengths  i n Treatments 1 to 5» Group A ( b o l l t e s t ) . Source Treatments Error Total Degrees of freedom 4 55 59 Sum of squares 95204 21831 117035 Mean square 23801.0 396.9 F 59.96 ** ** S i g n i f i c a n t at the 1% l e v e l Table VIII B. Dunoan's multiple range t e s t : Average shear  strengths i n Treatments 1 to 5. Group A ( b o i l t e s t ) . Treatments 1, 2. 4, 5. 3. Means (psi) 133 175 226 229 236 Any two means not underscored by the same l i n e are s i g n i f i c a n t l y d i f f e r e n t . Any two means underscored by the same l i n e are not s i g n i f i c a n t l y d i f f e r e n t . 64 Table IX. Analysis of variance: Average shear strengths  i n Treatments 6 to 8 t Group B (dry t e s t ) . Source Treatments Error Total Degrees of freedom 2 33 35 Sum of squares 16459 216070 232529 Mean square 8229.7 6547.6 1.26 NS Table X. Analysis of variance: Average shear strengths  i n Treatments 6 to 8. Group B ( b o l l t e s t ) . Source Treatments Error Total Degrees of freedom 2 33 35 Sum of squares 7475 59288 66763 Mean square 3737.6 1796.9 2.08 NS NS Non-significant 65 Table XI A. Analysis of variance: Average shear strengths  i n Treatments 9 to 12, Group G (dry t e s t ) . Source Treatments Error Total Degrees of freedom 3 44 47 Sum of squares 1340400 150970 1491400 Mean square 446810 3431 130.22 ** ** S i g n i f i c a n t at the l e v e l Table XI B. Duncan's multiple range t e s t : Average shear  strengths i n Treatments 9 to 12. Group C (dry t e s t ) . Treatments 10.' 11. 12. Means (psi) 156 376 530 589 Any two means not underscored by the same l i n e are s i g n i f i c a n t l y d i f f e r e n t . Any two means underscored by the same l i n e are not s i g n i f i c a n t l y d i f f e r e n t . 66 Table XII A. Analysis of variance: Average shear strengths  i n Treatments 9 to 12, Group C ( b o i l t e s t ) . Source Treatments Error Total Degrees of freedom 3 44 47 Sum of squares 208210 64981 273190 Mean square 69403 1477 46.99 ** ** S i g n i f i c a n t at the 1% l e v e l Table XII B. Duncan*s multiple range t e s t : Average shear  strengths i n Treatments 9 to 12. Group G ( b o i l t e s t ) . Treatments 10. 11. 12. 9. Means (psi) 125 I87 267 291 Any two means not underscored by the same l i n e are s i g n i f i c a n t l y d i f f e r e n t . Any two means underscored by the same l i n e are not s i g n i f i c a n t l y d i f f e r e n t . 6? Table XIII. Analysis of variance: Average per cent wood- f a i l u r e by conventional methods In Treatments 1 to 5 . Group A. Source Degrees Sum of Mean P of freedom squares square Treatments 4 386.1 96.53 1 .62 NS Error 55 3279.8 59.63 Total 59 3665.9 Table XIV. Analysis of variance: Average po s i t i o n of wood  f a i l u r e i n an annual increment from i t s i n i t i a t i o n i n  Treatments 2 to 5, Group A. Source Degrees Sum of Mean P of freedom squares square Treatments 3 37 .06 12.35^ 2.42 NS Error 44 224.75 5.108 Total 47 261.81 NS Non-significant 68 Table XV A. A n a l y s i s of va r i a n c e ; Average per cent wood  failures: o c c u r r i n g i n earlywood* i n Treatments 2 to 5»  Group A, Source Treatments E r r o r T o t a l Degrees of freedom 3 44 47 Sum of squares 6254.1 18147.0 24401.0 Mean square 2084.7 412.4 5.05 ** ** S i g n i f i c a n t at the 1% l e v e l Table XV B. Duncan's m u l t i p l e range t e s t : Average per  cent wood f a i l u r e n o c c u r r i n g i n earlywood-* i n Treatments  2 to 5. Group A. Treatments 2. 3. 4. 5. Means ($) 63 75 80 95 f W i t h i n 10$ of an annual increment from i t s i n i t i a t i o n . ' Any two means not underscored by the same l i n e are s i g n i f i c a n t l y d i f f e r e n t . Any two means underscored by the same l i n e are not s i g n i f i c a n t l y d i f f e r e n t . 69 Table X V I A n a l y s i s of variance: Average per cent wood  f a l l u r e r by conventional methods i n Treatments 6 to 8?  Group B (These are a l l of Group B) Source Degrees Sum of Mean F of freedom squares square Treatments 2 145.5 72.75 0.48 NS Error 33 4999.5 151.50 Total 35 51^5.0 NS Non-significant 70 Table XVII A. Analysis of variance: Average per cent wood  failure?;; by conventional methods i n Treatments 9 to 12. j  Group C. Source Treatments Error Total Degrees of freedom 3 44 47 Sum of squares 10114.0 8250.1 18364.0 Mean square 3371.2 187.5 F 17.98 ** ** S i g n i f i c a n t at the 1% l e v e l Table XVII B. Duncan 1s multiple range t e s t : Average per  cent wood f a i l u r e s by conventional methods i n Treatments 9  to 12. Group C; Treatments 9. 12, 11.' 10.' Means {%) 62 76 95 97 Any two means not underscored by the same l i n e are s i g n i f i c a n t l y d i f f e r e n t . Any two means underscored by the same l i n e are not s i g n i f i c a n t l y d i f f e r e n t . 71 Zone 8. Tension Zone 2. Shear Zone 3. Compression tearing (From Koch,R 1964. Wood machining processes. The Ronald Press Co , New York. p. 4 4 5 ) fig- I- Cr i t ical zones of stress in veneer cutting without a nosebor. 72 Specimen cut to pull open. 6 Specimen cut to pull closed / / / / / / / / / / / / B Fig. 2 - Lathe check orientation in plywood shear test specimens. 73 0 < ^ e e 30 40 50. 60 For oN face veneers o n d core veneers ' 2 . 2 " 0f Group A-ocf Only for the core v eneers of Qroup C . F\g.3- Potion of s ample veneer* * e , 0 9' 7k F i g . k. Camera setup and t e s t i n g apparatus 75 Correlation coefticient 0 .893 Standard error of estimate 21 82 psi Y = 228 22 + 128052 X Resin No. 4880 A A M R E S 2211 glue 0 10 20 30 40 50 60 70 80 90 100 X Penetration depth of adhesive into lathe checks (%) Fig. 5 Relationship between shear strength of plywood and penetration depth of adhesive into lathe checks. 400 76 3 350 300 250 • 200 150 00-50 0.05 010 0-15 020 Strain (in./irU Fig-6. Sfr@ss-strain cury® for plywood mad© of sawn v$netr,with eeeomponying pM®^mpfo® at positions psofdd —Treatment 5 . 77 P i g . 6 - Photograph 1 F i g . 6 - Photograph 2 . F i g . 6 - Photograph 3 8 0 F i g . 6 - Photograph 4 81 400 350 300 250 0 0.05 0 10 0.15 020 Strain (in./in.) Fig. 7. Stress-strain curve for plywood made of rotary-cut veneer,with accompanying photographs at the positions noted (Sample I) --Treatment 1. Fig. 7 - Photograph 1 83 F i g . 7 - Photograph 2 84 P i g . 7 - Photograph 3 P i g . 7 - Photograph 4 8 6 P i g . 7 - Photograph 5 87 F i g . 7 - Photograph 6 88 400 350 Rg. 8 . Stress-strain curve for plywood made of rotary-cut veneer, with accompanying photographs at the positions noted (Sampl® 2), —Treatment 1 . 8 9 F i g . 8 - Photograph 1 90 F i g . 8 - Photograph 2 91 Pig. 8 - Photograph 3 92 F i g . 8 - Photograph 4 93 4 0 0 4 0 0.05 0 0.05 0 0 0 5 010 0.15 Strain (in/in) Fig. 9. Comparison of stress-strain curves for plywoods made of sawn veneer (S), rotary-cut veneer with lathe checks fully impregnated by resin (L), and rotary-cut veneer(R). 94 4 0 0 • 350-0 0.05 0.10 0.15 020 Strain (in./in.) Fig-10. Stress-strain curve for plywood made of ro tary -cy t veneer with lath® checks fully 8fnpr@gfHQf@d by resin, with @ee@ra ponying photographs of the positions inoted (Sample I). --Treatment 4. 9 5 Pig. 10 - Photograph 1 96 F i g . 10 - Photograph 2 9 7 P i g . 10 - Photograph 3 400 0 0.05 0.10 0.15 Strain (in./in.) Fig. II. Stress-strain curve for plywood made of rotary-cut veneer with lathe checks fully impregnated by resin, with accompanying photographs at the positions noted(Sample-2) —Treatment 4. 99 Pig. 1 1 - Photograph 1 100 F i g . 11 - Photograph 2 101 F i g . 11 - Photograph 3 102 F i g . 11 - Photograph 4 103 0 0.05 0.10 0.15 0.20 Strain (in./in.) Fig. 12. Stress-strain curve for plywood mode of rotary-cut veneer with lathe checks fully impregnated by resin, with accompanying photographs o f fht positions noted (Sample 3) t —Treatment 4 104 F i g . 12 - Photograph 1 P i g . 12 - P h o t o g r a p h 2 Pig. 12 - Photograph 3 107 No. 1, 2 and 3* Plywood made of sawn veneer (No. 2 i s the sample f o r F i g . 6) No. 4, 5 and 6. Plywood made of rotary-cut veneer with lathe checks impregnated by r e s i n (No. 5 and No. 6 are the samples f o r F i g . 11 and 12) No. 7t 8 and 9. Plywood made of untreated rotary-cut veneer (No. 7 and No. 9 are the samples f o r F i g . 7 and 8) A 8 F i g . 13. Test specimens a f t e r f a i l u r e 

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