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Factors influencing the strength properties of Douglas fir plywood normal to glueline Palka, Laszlo Cezar 1964

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FACTORS INFLUENCING THE STRENGTH PROPERTIES OF DOUGLAS FIR PLYWOOD NORMAL TO GLUELINE by LASZLO CEZAR PALKA B.S.F. (Sopron Division) University of Briti s h Columbia 196l 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 as conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA September, 1964 In p r e s e n t i n g t h i s t h e s i s i n p a r t i a l f u l f i l m e n t of the requirements f o 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 f o r r e f e r e n c e and study, I f u r t h e r agree that per-m i s s i o n f o r extensive 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 . I t i s understood t h a t - c o p y i n g 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 allowed without my w r i t t e n permission* Department of F o r e s t r y .  The U n i v e r s i t y of B r i t i s h Columbia, Vancouver 8, Canada Date September, 196*». ABSTRACT The study was designed to evaluate the r e l a t i v e importance of c e r t a i n f a c t o r s i n f l u e n c i n g the str e n g t h p r o p e r t i e s of cold-pressed Douglas f i r plywoods normal to g l u e l i n e . I n a d d i t i o n , estimates o f s t r e n g t h values were a l s o sought. Rotary-cut veneers were obtained from plywood m i l l s ; sawn veneers were prepared from lumber. A 2 x 3 x 3 f a c t o r i a l design was followed u s i n g veneer thicknesses o f l / l O , l/7, and 1/5 i n c h , and g l u i n g pressures of 50, 200, and 350 p s i . A c o l d - s e t t i n g modified p o l y v i n y l adhesive (Duro-Lok 50) was used i n a l l 18 plywood bl o c k s f a b r i c a t e d . From each of these, 8 t e n s i o n , k compression and 3 glue shear specimens were prepared. T h e i r dimensions were -|- x 1 x inches, 1 x 1 x ^ inches and 1 x 3ii x 3/5 i n c h , r e s p e c t i v e l y . Plywoods of sawn veneers were o n l y h a l f as strong as s o l i d wood i n both compression and t e n s i o n . S o l i d wood exceeded the compressive str e n g t h of r o t a r y - c u t veneer b l o c k s by two, and t e n s i l e v alues by seven times. S t i f f n e s s of sawn veneers was twice that of r o t a r y cut ones. The r a t i o of moduli of e l a s t i c i t y i n compression to those i n t e n s i o n was found to a p p r o x i -mate seven and s i x f o r the two veneer types, r e s p e c t i v e l y . The d i f f e r e n c e between s o l i d wood and sawn-veneer b l o c k s t r e n g t h might be a t t r i b u t e d mainly to the i n f l u e n c e of a suspected a c i d h y d r o l y s i s a t the g l u e l i n e s or p o s s i b l y to specimen geometry. The much lower strength values of r o t a r y - c u t veneers must have r e s u l t e d from the presence of l a t h e checks, and the lower q u a l i t y o f veneer surfaces. The f u n c t i o n a l dependence of a l l s t r e n g t h p r o p e r t i e s upon some independent f a c t o r s , and the r a n k i n g o f the l a t t e r , was e s t a b l i s h e d and evaluated by m u l t i p l e r e g r e s s i o n analyses. The combination of the 16, 17 or 18 most important veneer and plywood v a r i a b l e s accounted f o r p r a c t i c a l l y a l l the v a r i a t i o n , e s p e c i a l l y f o r r o t a r y - c u t veneers. I n a d d i t i o n , the complete dependence of some plywood v a r i a b l e s on independent veneer character-i s t i c s and g l u i n g techniques were shown by r e g r e s s i o n equations. I t should be noted t h a t the -three e x p e r i m e n t a l l y c o n t r o l l e d f a c t o r s , veneer type, veneer t h i c k n e s s and g l u i n g pressure, were not always a l l i n c l u d e d i n the s i x most s i g n i f i c a n t ones. The rank of v a r i a b l e s was found t o d i f f e r f o r each of the v a r i o u s strength p r o p e r t i e s observed. Analyses of va r i a n c e were performed f o r both observed and adjusted values w i t h i n each veneer type, both p r o v i d i n g almost i d e n t i c a l r e s u l t s . The high s i g n i f i c a n c e of veneer t h i c k n e s s has been shown f o r a l l s t r e n g t h p r o p e r t i e s , b a r r i n g shear. This was explained by i t s strong c o r r e l a t i o n w i t h a number of independent v a r i a b l e s , such as glue content and s p e c i f i c g r a v i t y . G l u i n g pressure exerted a h i g h l y s i g n i f i c a n t i n f l u e n c e on a l l s t r e n g t h p r o p e r t i e s of r o t a r y - c u t veneer b l o c k s , and i n compressive s t r e s s and s t r a i n of sawn-veneer plywood c o n s t r u c t i o n . I t s i n f l u e n c e was a t t r i b u t e d t o the s t r o n g c o r r e l a t i o n s i n d i c a t e d between i t and other v a r i a b l e s , f o r example, f u l l compression and p l a s t i c deformation. F i n a l l y , the e x p l o r a t o r y nature of the experiment was emphasized. ACKMOWLEDGEMENT Deepest g r a t i t u d e i s expressed to a l l persons who generously-provided m a t e r i a l o r in f o r m a t i o n . E s p e c i a l l y , the p r o f e s s i o n a l and understanding guidance o f Dr. R.V/. Wellwood, A c t i n g Dean, F a c u l t y o f F o r e s t r y , d u r i n g my graduate t r a i n i n g and t h e s i s p r e p a r a t i o n , i s g r a t e f u l l y acknowledged. The a s s i s t a n c e given "by Dr. A. Eozak i n the s t a t i s t i c a l a n a l y s i s , and the v a l u a b l e suggestions, cooperation and i n t e r e s t shown by Messrs. C.K.A. S t i e d a , P.L. Northcott and Dr. W.V. Hancock, Research O f f i c e r s , Forest Products Research Branch of the Department o f F o r e s t r y , i s t h a n k f u l l y noted. The a s s i s t a n c e f r e e l y given by the f o l l o w i n g companies and i n s t i t u t i o n s i s acknowledged g r a t e f u l l y : Canadian Forest Products L i m i t e d , Evans Products Company L i m i t e d , N a t i o n a l Starch and Chemical Company (Canada) L i m i t e d , T e s t i n g Laboratory, F a c u l t y of A p p l i e d Science, and Vancouver Laboratory, Forest Products Research Branch of the Canada Department o f F o r e s t r y . i i i . TABLE OP CONTENTS Page ABSTRACT i TABLE OF CONTENTS .. .. i i i LIST OF TABLES v LIST OF FIGURES v i i ACKNOWLEDGEMENT v i i i INTRODUCTION 1 1. Uses o f plywood .. 1 2. Obj e c t i v e and scope 2 3. L i t e r a t u r e survey 3 (a) Wood and veneer k (b) Adhesives and g l u i n g 7 (c) T e s t i n g methods 10 EXPERIMENT , . • . ^ 1. T e c h n i c a l assumptions 1^ 2. Experimental design 16 3. S t a t i s t i c a l assumptions 18 k. P r e p a r a t i o n of m a t e r i a l 19 (a) Veneers 19 (b) Adhesive 2 2 (c) Plywood 23 (d) Test specimens 25 5. T e s t i n g procedures 2 ^ (a) Compression 2 7 (b) Tension 2 7 (c) Shear .. .. 2 8 i v . Page 6. R e s u l t s 28 (a) Computations 29 (b) M u l t i p l e r e g r e s s i o n equations . . . . 30 (c) Plywood s t r e n g t h values 31 (d) Analyses o f vari a n c e 31 DISCUSSION .. 32 1. V a l i d i t y o f assumptions 32 2. Influ e n c e of c o n t r o l l e d f a c t o r s 3k (a) Veneer type 35 (b) Veneer t h i c k n e s s k3 (c) G l u i n g pressure kk 3. Influ e n c e of concomitant v a r i a b l e s k6 (a) I n compression k6 ( i ) Sawn-veneer b l o c k s k6 ( i i ) Rotary-cut veneer b l o c k s . . . . k$ (b) I n t e n s i o n 51 ( i ) Sawn-veneer b l o c k s 51 ( i i ) Rotary-cut veneer blo c k s . . . . 55 (c) I n shear 58 ( i ) Sawn-veneer b l o c k s .. .. 58 ( i i ) Rotary-cut veneer b l o c k s . . . . 60 CONCLUSIONS .. .. .. ' 6 2 BIBLIOGRAPHY 6 6 LIST OP TABLES Page TABLE I . Average str e n g t h values o f small c l e a r specimens of coast-type Douglas f i r wood i n a i r - d r y c o n d i t i o n 75 TABLE I I . Summary of veneer v a r i a b l e s per plywood bloc k s 76 TABLE I I I . Manufacturer's s p e c i f i c a t i o n s f o r the modified p o l y v i n y l adhesive, Duro-Lok 50 77 TABLE IV. Summary of plywood v a r i a b l e s per plywood blocks 78 TABLE V. Changes i n g l u i n g pressure w i t h time 79 TABLE VI. Compression r a t e o f plywood blo c k s .. .. .. 80 TABLE V I I . Changes i n height o f plywood b l o c k s d u r i n g and a f t e r p r e s s i n g 81 TABLE V I I I . Comparison of veneer and plywood s p e c i f i c g r a v i t i e s 82 TABLE IX. F u n c t i o n a l dependence of plywood s t r e n g t h p r o p e r t i e s on some s e l e c t e d independent v a r i a b l e s f o r the sawn-veneer c o n s t r u c t i o n s .. 83 TABLE X. Rank and c o n t r i b u t i o n of some s e l e c t e d inde-pendent v a r i a b l e s to the v a r i o u s plywood st r e n g t h p r o p e r t i e s o f sawn-veneer blo c k s .. .. 8^ TABLE X I . F u n c t i o n a l dependence of plywood s t r e n g t h p r o p e r t i e s on some s e l e c t e d independent v a r i a b l e s f o r the r o t a r y - c u t veneer c o n s t r u c t i o n s 85 TABLE X I I . Rank and c o n t r i b u t i o n of some s e l e c t e d inde-pendent v a r i a b l e s to the v a r i o u s s t r e n g t h p r o p e r t i e s of r o t a r y - c u t veneer b l o c k s .. .. 86 TABLE X I I I . F u n c t i o n a l dependence of some plywood v a r i a b l e s on independent f a c t o r s f o r sawn- and r o t a r y - c u t veneer blo c k s 87 TABLE XIV. Rank and c o n t r i b u t i o n of independent f a c t o r s to some plywood v a r i a b l e s , as i n d i c a t e d by t h e i r c o e f f i c i e n t s of determination 88 vx. Page TABLE XV. F u n c t i o n a l dependence of plywood s t r e n g t h on some s e l e c t e d independent v a r i a b l e s , e x c l u d i n g the e x p e r i m e n t a l l y c o n t r o l l e d ones 89 TABLE XVI. Rank and c o n t r i b u t i o n of some s i g n i f i c a n t independent v a r i a b l e s , e x c l u d i n g the e x p e r i -m e n t a l l y c o n t r o l l e d ones, to v a r i o u s plywood s t r e n g t h p r o p e r t i e s 90 TABLE XVII. Simple c o r r e l a t i o n of veneer th i c k n e s s and some concomitant v a r i a b l e s 91 TABLE XVTII. Simple c o r r e l a t i o n o f g l u i n g pressure and some concomitant v a r i a b l e s .. .. .. .. .. 92 TABLE XIX. Summary of observed plywood s t r e n g t h values f o r sawn and r o t a r y - c u t veneer blo c k s 93 TABLE XX. Summary of adjusted plywood s t r e n g t h values f o r sawn and r o t a r y - c u t veneer blo c k s 9k TABLE XXI. R a t i o s comparing v a r i o u s plywood s t r e n g t h p r o p e r t i e s w i t h i n each veneer type 95 TABLE X X I I . R a t i o s comparing v a r i o u s plywood s t r e n g t h p r o p e r t i e s o f sawn- and r o t a r y - c u t veneer bloc k s 96 TABLE X X I I I . R a t i o s of plywood s t r e n g t h p r o p e r t i e s comparing adjusted and observed values 97 TABLE XXIV. Summary of analyses o f va r i a n c e f o r observed and adjusted plywood s t r e n g t h values 98 TABLE XXV. Rank of h i g h l y s i g n i f i c a n t c o n t r o l l e d f a c t o r s on the v a r i o u s plywood str e n g t h p r o p e r t i e s .. .. 99 v i i . LIST OF FIGURES Page Fig u r e 1» C u t t i n g p l a n of plywood b l o c k s .... ». 100 Fig u r e Z. Baldwin U n i v e r s a l T e s t i n g Machine i n the m a t e r i a l s t e s t i n g l a b o r a t o r y of the F a c u l t y of A p p l i e d Science •» ». .. 101 Figure 5.. Compression specimen i n the microformer extensometer .» ». «» .» .» 102 Figure 4. Table Model I n s t r o n T e s t i n g Instrument i n the wood technology l a b o r a t o r y of the F a c u l t y of F o r e s t r y .» .. »• •» .» 103> Fig u r e 5. Tension specimen i n the g r i p s of the t a b l e model I n s t r o n t e s t i n g instrument .. 104 Fig u r e 6. Infl u e n c e of c o n t r o l l e d f a c t o r s on modulus of e l a s t i c i t y of sawn—veneer b l o c k s i n compression .. ... ... 105 Figure 7* Influence o f c o n t r o l l e d f a c t o r s on modulus of e l a s t i c i t y of sawn—veneer b l o c k s i n t e n s i o n .. . • .» .. .. 106; Fig u r e 8* Influence o f c o n t r o l l e d f a c t o r s on modulus of e l a s t i c i t y of r o t a r y - c u t veneer b l o c k s i n compression .. .» .» 107 Figure 9. Influence o f c o n t r o l l e d f a c t o r s on modulus of e l a s t i c i t y of r o t a r y - c u t veneer b l o c k s i n t e n sion <,» ». .. .. .. .. .» 108 - 1 -INTRODUCTION Douglas f i r (Pseudotsuga t a x i f o l i a B r i t t . ) c o n s t i t u t e s the major source o f p e e l e r b o l t s f o r the softwood plywood i n d u s t r y . Since i t i s indigenous to the western part of t h i s c o n t i n e n t , North American c o n d i t i o n s , methods and references w i l l be emphasized. 1. Uses of plywood The s t r u c t u r a l p o t e n t i a l i t i e s o f plywood have long been recognized. The Douglas F i r Plywood A s s o c i a t i o n ( s i n c e 1938) was among the f i r s t propo-nents of plywood c o n s t r u c t i o n . Beginning i n 19^1, the Army-Navy-Civil (A.N.C.) Committee of the Ammunitions Board of the United States o f America, i n cooperation w i t h the Forest Products Laboratory a t Madison, Wisconsin, have pu b l i s h e d plywood s t r e n g t h values and design recommendations. These were summarized i n the A.N.C. B u l l e t i n No. 18 (l95l) andthe Wood Handbook (1955)- The B r i t i s h Columbia Plywood Manufacturers A s s o c i a t i o n has published t e c h n i c a l data on Douglas f i r plywood si n c e 1950• Markwardt and Freas ( r e v i s e d 1956) published approximate design methods. On the b a s i s of Russian, German and Hungarian l i t e r a t u r e , H i l v e r t (1956) summarized design methods and problems. I n the United S t a t e s , the Timber Design and Co n s t r u c t i o n Handbook (1959) described design s p e c i f i c a t i o n and p r a c t i c e . The Timber Co n s t r u c t i o n Manual (l96l) d i d the same f o r Canada. Establishment o f the Plywood F a b r i c a t o r S e r v i c e , an a f f i l i a t e o f the Douglas F i r Plywood A s s o c i a t i o n , now known as the American Plywood A s s o c i a t i o n , marked the beginning of a new age of plywood s t r u c t u r a l usage, according to - 2 -Schniewind (1962a). North American and European attitudes and approaches to plywood production and uses are reflected in the comprehensive books written by Perkins (1962) and Kollmann (1963). The world-wide significance of plywood and other wood-based panels has been illustrated by the recent International Conference held under the auspices of the Food and Agriculture Organization of the United Nations (Fleischer, 1963). In the American Marietta Economic Survey (1960) i t was predicted that Douglas f i r plywood production w i l l rise to 13.4 b i l l i o n square feet in the United States, on a 3/8-inch basis, by 1970. Blomquist (1962) reported a total Douglas f i r plywood production of 8.5 b i l l i o n square feet, on the above basis, for 1962 . Plywood manufactured in British Columbia in 1961 was worth almost 83 million dollars (Dominion Bureau of Statistics, 1962), approximately 61 per cent of the total value of Canadian plywood pro-duction. 2. Objective and scope Stress analysis of solid wood and plywood has been hindered since, i n order to use modern photoelastic methods or electrical strain gauges effectively, the moduli of elasticity must be known in advance for any direction in the material (Walker, 1961). The strength and elastic properties of plywood paral-l e l to glueline have been investigated theoretically by March (1944) and Hearman (1948), and summarized by Hoff in Dietz (1949) and Meredith (1953). Curry (1954) and Liska (revised 1955) measured these properties experimentally. Preston (1950) and Curry (1957) showed the influence of adhesive to be negligible on the strength properties of conventional plywood. Moduli - 3 -of e l a s t i c i t y i n t e n s i o n and compression are l a c k i n g f o r plywood s t r e s s e d normal to g l u e l i n e and have been assumed to be i d e n t i c a l . However, wood i n both r a d i a l and t a n g e n t i a l d i r e c t i o n s (Walker, 196l), and glue (Marian and Stumbo, 1962), e x h i b i t higher r e s i s t a n c e to compression than t o t e n s i o n . Subsequently, one would expect plywood to behave s i m i l a r l y normal to g l u e l i n e . S t i e d a (1962) and laworsky, Cunningham and Hindley (1955) proposed r o l l i n g shear t o be a d i a g o n a l t e n s i o n or " t e a r " f a i l u r e . Obviously, moduli of e l a s t i c i t y and s t r e n g t h values perpendicular t o the g l u e l i n e i n f l u e n c e r o l l i n g shear c o n s i d e r a b l y . Thus, i f the f a c t o r s determining these s t r e n g t h p r o p e r t i e s were known, r e s i s t a n c e t o r o l l i n g shear might be improved by changes i n manufacturing techniques. The standard plywood (glue) shear t e s t s , i f performed j o i n t l y w i t h moduli of e l a s t i c i t y determinations, would a l l o w a conventional e v a l u a t i o n o f bond q u a l i t y . One could a l s o r e l a t e data obtained from t h i s experiment to those of others, u s i n g plywood shear t e s t ( r o l l i n g shear) values f o r comparison. The above c o n s i d e r a t i o n s , coupled w i t h the challenge of e x p l o r i n g an apparently neglected f i e l d , prompted the w r i t e r t o s e l e c t the present t o p i c . I t was hoped t h a t the i n f o r m a t i o n obtained would be u s e f u l to research workers concerned w i t h s t r e s s a n a l y s i s and/or p o s s i b l e improvements of plywood. 3. L i t e r a t u r e survey Plywood c o n s i s t s of veneers and glue, j o i n e d through boundary l a y e r s . The weakest of these determines bond strength,' when plywood i s subjected t o s t r e s s e s normal t o g l u e l i n e (Bikerman, i960). Specimen h i s t o r y , geometry - 4 -and t e s t i n g methods a l s o i n f l u e n c e the apparent strength p r o p e r t i e s of m a t e r i a l s (Marin, 1962). Thus, these f a c t o r s should be reviewed c o n c i s e l y . (a) Wood and veneer Zahner (1963) s i n g l e d out s o i l moisture content as the most s i g n i f i c a n t s i n g l e f a c t o r i n f l u e n c i n g growth r a t e and anatomical f e a t u r e s of wood. Larson (1962) proposed wood c h a r a c t e r i s t i c s to be a product of h e r e d i t y and environ-N s t r e n g t h i n ment. Lee (1961) c o r r e l a t e d / t e n s i o n p a r a l l e l t o the g r a i n w i t h the c r y s t a l l i n i t y of c e l l u l o s e , as d i d I f j u (1963), w h i l e r e s i s t a n c e t o compression was b e l i e v e d t o depend on the l i g n i n content of c e l l w a l l s . Schniewind (1959) showed t h e transverse a n i s o t r o p y of wood to be a f u n c t i o n of gross anatomic s t r u c t u r e . Kubler (1957) showed how i n t e r n a l growth s t r e s s e s increase the r e s i s t a n c e o f - t r e e s t o e x t e r n a l s t r e s s e s . He concurred w i t h Haraszty (1956), who emphasized t h a t the bole s t r u c t u r e r e s u l t e d from adap t a t i o n t o environmental s t r e s s e s and m e t a b d l i s t i c f u n c t i o n s . E h e o l o g i c a l l y , wood might be c l a s s i f i e d as a l i n e a r v i s c o - e l a s t i c s o l i d (Pentoney and Davidson, 1962). As a high"polymer, i t d i d not' e x h i b i t p r o p o r t i o n a l s t r e s s - s t r a i n r e l a t i o n s . Walker (I96l), however, showed t h a t i t may be considered as an e l a s t i c m a t e r i a l , w i t h i n c e r t a i n l i m i t s . The s t r e n g t h p r o p e r t i e s of, and f a c t o r s r e l a t i n g t o , v a r i a t i o n i n s p e c i f i c g r a v i t y of young rapid-growth Douglas f i r were s t u d i e d by L i t t l e f o r d (1961) and McKimmy (1959), r e s p e c t i v e l y . The Forest Products Lab o r a t o r i e s of Canada (1956) and of the United States (-1955) published average s t r e n g t h values f o r s m a l l c l e a r specimens of coast-type Douglas f i r i n the a i r - d r y c o n d i t i o n as l i s t e d i n Table I . Allowable s t r e s s e s f o r v a r i o u s grades - 5 -of plywoods, p a r a l l e l to g l u e l i n e , were summarized i n the v a r i o u s design handbooks mentioned above. Schniewind (1962b), d i s c u s s i n g s o l i d wood, reported t h a t moduli of e l a s t i c i t y were grea t e r i n the r a d i a l than i n the t a n g e n t i a l d i r e c t i o n f o r every species i n v e s t i g a t e d ; f u r t h e r , the d i f f e r e n c e s between the above moduli were not s i g n i f i c a n t f o r t e n s i l e and compressive t e s t s . Walker (1961), however, reported t h a t modulus of e l a s t i c i t y c a l c u l a t e d from bending (tension) was s i g n i f i c a n t l y d i f f e r e n t from t h a t obtained by compression t e s t s ; u n l i k e values shown i n Table I . Moduli of e l a s t i c i t y t r a n s v e r s e t o g r a i n and the f r i c t i o n a l p r o p e r t i e s of wood were found t o be h i g h l y important i n r o t a r y veneer c u t t i n g (McKenzie, 1962). This conforms w i t h M c M i l l i n (1958), who showed compression and t e n s i o n perpendicular to g r a i n , and r o l l i n g shear, t o be the most important mechanical p r o p e r t i e s of wood th a t i n f l u e n c e veneer q u a l i t y . F e i h l , Colbeck and Godin (1963), a f t e r studying a l a r g e number of f a c t o r s , concluded t h a t poor q u a l i t y veneer r e s u l t e d from badly adjusted l a t h e s , i n most cases. Mote (1963) found t h a t , i n veneer l a t h e s , the s t r e s s d i s t r i b u t i o n i n the chips was independent of depth of cut, c u t t i n g d i r e c t i o n or chi p type. On the other hand, Hoadley (1962) reported that wood d e n s i t y , wood temperature at time of c u t t i n g , nominal veneer t h i c k n e s s , and degree of nosebar compression, determine the development and f i n a l p a t t e r n of the dynamic f o r c e d i s t r i b u t i o n i n the wood. The causes and c o n t r o l of common pe e l i n g d e f e c t s i n veneer were summarized by F e i h l and Godin (1962). C o l l i n s (I960) suggested l a t h e checks to be a r e s u l t o f a "snap a c t i o n " at the c u t t i n g edge. Leney (i960) showed the presence of t e n s i o n , - 6 -compression and shear "checking" as a corollary of the basic severance action at the knife edge. The cutting force was estimated to range from 5 to 7 pounds per inch of cutting edge. Wangaard and Saraos (1959), examining cutting variables, found a 30 per cent reduction, i n tensile strength (of lauan) veneers, due to cold cutting. Nosebar pressures were evaluated i n terms of the mechanical properties of wood by McMillin (1958). Fleischer (1949) gave the f i r s t comprehensive experimental evaluation of rotary cutting i n terms of veneer quality. On the basis of the above studies, one might reconstruct the actual variables that resulted i n a given quality of green veneer. The effect of drying, as evaluated by bond quality, remains contro-versial. This might be attributed to the complexity of factors that determine bond quality. Milligan and Davies (1963) showed that jet-air dryers, working at high temperatures, can be used without reducing veneer quality. They dried 1/8-inch thick Douglas f i r heartwood veneers i n 0.95 minute at 550°F. They noted, however, that veneer temperatures did not exceed 280°F. at 5 per cent moisture content. Northcott, Hancock and Colbeck (1962) found that heat treatment of wood tended to reduce wettability, but the caustic of the glue, when applied, acted to restore i t . The importance of th i s can be appreciated i n the light of Gray's (1962)' calcula-tions, which predicted that adequate wetting of wood surfaces i s more important than adequate adhesion. Barlai (1961), after discussing the chemical changes that would take place i n veneer or lumber, proposed that, by controlling the intensity and duration of heat treatment, wood properties could be altered to a desired degree. The theory of plywood casehardening or surface inactivation was examined by Northcott, Colbeck, Hancock and Shen - 7 -(1959), who a l s o proposed sanding as an e f f e c t i v e remedy. Northcott and Colbeck (1959) showed t h a t veneer s t r e n g t h i s reduced by over-drying veneer a t or above 450°F. This confirmed s i m i l a r conclusions reached by Northcott (1957), Bryant and Stensrud (1954), and others. (b) Adhesives and g l u i n g The g e n e r a l p r i n c i p l e s of wood g l u i n g have been summarised by v a r i o u s authors, such as P e r r y (1942), De Bruyne and Hovwink (1951), Brown, Panshin and F o r s a i t h (1952), Bakai and Salamon (1953) and o t h e r s . "-Die'Ez- (1949), Knight (1952), the Wood Handbook (1955), the Manufacturing Chemist's A s s o c i a -t i o n (1957), Bergin (1959), and others, have provided b a s i c i n f o r m a t i o n on the p r o p e r t i e s and uses of wood g l u e s . Up-to-date i n f o r m a t i o n may be obtained from annual reviews by Blomquist (1962 and i960) and Hemming (1963). The l a t t e r heralded the appearance of modified p o l y v i n y l glues as the g r e a t e s t glue news of the year. The room-temperature-setting p o l y v i n y l adhesives became popular f o l l o w i n g World War I I , although t h e i r creep property remained a s e r i o u s drawback. McCormack (1954) l i s t e d high s e t t i n g speed and r e l a t i v e immunity t o i n f l u e n c e s of g l u i n g temperature, pressure and humidity, as the most important c h a r a c t e r i s t i c s of p o l y v i n y l emulsions. Duro-Lok 50, the modified p o l y v i n y l emulsion used i n t h i s experiment, was reported t o be water r e s i s t a n t and thermo-setting ( N a t i o n a l Starch and Chemicals Co. (Canada) L t d . , 1963). The adhesive f i l m forms p a r t l y by water being absorbed i n t o the wood and p a r t l y by evaporation. A c a t a l y s e d chemical r e a c t i o n , w i t h i t s temperature dependent r a t e , develops the heat- and water-r e s i s t a n t bond. The f o l l o w i n g p h y s i c a l and chemical p r o p e r t i e s have been - 8 -published by the manufacturer: Type Thermo-setting emulsion P r o p e r t i e s .. .. Weight .; .. .. 11.0 l b Imp. g a l . S o l i d s content .. 48.0$ V i s c o s i t y .. .. 3000 cp Thinner .. .. .. H20, l e s s than 5% Freeze-thaw s t a b i l i t y F a i r pH 5.0 Storage c o n d i t i o n s . 45-65° (Optimum) Storage s t a b i l i t y .. 3 months (at 70°F) C a t a l y s t .. .. 42-2300 ( a c i d ) , use 5$ by weight (green g l u e l i n e ) 42-2301, use 10% by weight ( c o l o u r l e s s g l u e l i n e ) Working l i f e . .. 24 hr at 72°F, 2\ hr at 100°F. According t o the Manufacturing Chemist's A s s o c i a t i o n (1957) the p o l y v i n y l formal or b u t y r a l r e s i n s e x h i b i t the a b i l i t y t o be c r o s s - l i n k e d or i n s o l u b i l i z e d , thereby a c q u i r i n g thermo-setting p r o p e r t i e s . Thus, Duro-Lok 50 should belong to t h i s f a m i l y of adhesives. This would a l l o w e s t i m a t i o n o f t h e i r s t r e n g t h p r o p e r t i e s from data published by the above a s s o c i a t i o n f o r these p o l y v i n y l r e s i n s . Percentage e l o n g a t i o n and t e n s i l e s t r e n g t h f o r u n p l a s t i c i z e d p o l y v i n y l formal and b u t y r a l were given as approximately 3 per cent a t 11,000 and 4 per cent a t 10,000 p s i , r e s p e c t i v e l y . Modulus of e l a s t i c i t y i n t e n s i o n was reported t o v a r y from 500,000 t o 700,000 p s i a t room temperature. The boundary l a y e r s of the glued j o i n t determine i t s q u a l i t y . I n t e r -a c t i o n between wood and glue must be conceived s p a t i a l l y and through a water monolayer, according t o a proposal by Marian and Stumbo (I962). The - 9 -i n f l u e n c e o f various chemical and p h y s i c a l p r o p e r t i e s on bond q u a l i t y was summarized by the same authors as w e l l as by Bikerman (i960), who emphasized the r o l e of glue, as d i d N o r r i s (1958). Marian (1955) and Brown, Panshin and F o r s a i t h (1952) suggested q u a l i t a t i v e dependence between the various f a c t o r s . N o r t h c o t t , Hancock and Colbeck (I962) examined water r e l a t i o n s i n p h e n o l i c bonds, and Keylwerth (1962) studi e d s w e l l i n g of compressed wood. The e f f e c t of wood moisture content on g l u i n g was explored by Bergin (1959) and on shear s t r e n g t h by Sanborn (1945) and Lewis, Heebink and Cottingham (1945), who found 8 t o 12 per cent to be an optimum l e v e l of wood moisture content. Keylwerth and Hdfer (1962) showed t h a t plywood s t r e n g t h normal t o g l u e l i n e increased i n short time t e s t s (0.86 minute) as compared t o long time t e s t s (25 minutes) f o r p o l y v i n y l acetate g l u e s . Resorcinol-phenol-formaldehyde showed a reverse t r e n d . Driehuysen and Wellwood (i960) studied the i n f l u e n c e of temperature and r e l a t i v e humidity on open assembly time i n the manufacture of laminates. Freeman (1959) examined the r e l a t i o n s h i p between the p h y s i c a l and chemical p r o p e r t i e s of wood and adhesion, w h i l e Grantham and Atherton (1959) evaluated the o v e r a l l e f f e c t of pre-heating Douglas f i r b l o c k s . Curry (1957) concluded t h a t compression of veneers i s confined t o t h i n l a y e r s a t the g l u e l i n e . This i s i n agreement w i t h Preston (1950), who observed greater compression of plywood w i t h i n c r e a s i n g number of g l u e l i n e s . P o l e t i k a (1950) found t h a t t h i c k n e s s of laminates does not i n f l u e n c e s t r e n g t h , provided veneer thicknesses are not used. N o r r i s , Warren and McKinnon (1948) reported i n c r e a s i n g shear-through-thickness s t r e n g t h corresponding t o decreasing veneer t h i c k n e s s e s . C o c k r e l l and Bruce (1946) - 10 -found that r o l l i n g shear strength decreased with increasing glueline thicknesses. Murphey (1963) reported plastic deformations to be a result of permanent changes i n cry s t a l l i n i t y of cellulose. Wood deformations were shown by Perkit.ny and Helinska (1961) to be governed by a temperature-moisture content interaction, with a significant contribution from the release of growth stresses. Currier (I960) saw an opportunity for substantial savings of veneers through controlled reduction of pressure during hot pressing of Douglas f i r plywood. Baumann and Marian (I96l) studied gluing pressures as a function of the physical properties of wood. Carruthers (1959) investigated heat penetration i n hot pressing and found that compression of plywoods increased with increase i n pressing time, temperature, and moisture content of veneer. His findings agreed with those of Currier (i960), Sisterhenm (1958), and McDonald (1951), who was the f i r s t to propose the use of pressure (compression) control devices. Klein (1959) summarized a l l the advantages and disadvantages of both cold-and hot-pressing techniques. (c) Testing methods The problems of surface texture measurement were discussed by Stumbo (1963). Staining techniques for wood technologists were summarized by Wilson (1963). Currier (1962) gave detailed description of his methods of measuring and/or calculating plywood variables. A survey of methods for assessing veneer quality was undertaken by Newall (i960). The latter described methods used by Wangaard and Saraos (1959) for measuring--veneer thickness, smoothness and tightness; Suziko (1958) for estimating roughness; Hahn (1957) i n classifying wood surfaces; Higgins (1956) for determining veneer quality; Kivimaa (1956) i n veneer quality determination by tension - 11 -tests; Kaumann, Gottstein and Lantican (1956) in their comparison of numerical and subjective veneer quality evaluation; and the "droop" method of estimating veneer tightness. Standard methods for determining the strength parallel to glueline and the durability of plywood have been specified by the Canadian Standards Association (1961) and by the American Society for Testing Materials (1961). However, tests concerning plywood strength properties normal to glueline have, not been standardized. Marian and Stumbo (1962) proposed a tension test normal to glueline as the most sensitive method of evaluating bond quality. Keylwerth and HOfer (1962) used 3 by 4 by 16 cm. hyperbolically-necked specimens in tension perpendicular to gluelines, to investigate relaxation of adhesives. They found that the ultimate stress was i n f l u -enced by plastic deformations of the glue joints. Marra (1962) showed the influence of specimen geometry to be striking, on cross-lapped wooden blocks. He also noted that rheological factors controlled a large portion of the total strength. This is in close agreement with Biker-man's (1960) findings that bond strength is determined by specimen geometry and by the mechanical properties of adhesive, adherend or the boundary layer. Northcott (1958) evaluated percentage wood failure as a measure of bond quality. Rice (1957) showed that glueline shear stress at failure in compres-sion was over twice as great as in tension; further, that percentage wood failure was conspicuously lower in tension than in compression. Yaworsky, Cunningham and Hindley (1955), investigating standard shear specimens, concluded that the test results might be more influenced by stress d i s t r i -bution peculiar to the specimen than by the variables under investi-gation. Northcott (1954) presented similar arguments. He also - 12 -emphasized the importance of reproducibility of test results, acceptable unit of measurement, and ease of preparation of specimens. The problems of evaluating glues and glued products were also discussed by Blomquist (1954)• Wakefield (1947) studied the tension normal to glueline plywood test and found that ultimate strength was much lower than could be expected from solid wood having a similar density. He proposed wood permeability and grain direction as the most influential factors i n these tests. Osherovich (1955), who examined tension perpendicular to grain, found that with increasing r a d i i of curvature for necked-down specimens the stress increased as a result of reduced stress concentration. This i s supported by the earlier observations of Durelli (1942) and Frocht (1942). The former showed the presence of stress concentration i n necked-down tension specimens, whereas the latter found a uniform stress distribution i n circular shafts. Thus, one could expect uniform stress distribution by not using necked-down specimens. According to James (1962), moduli of e l a s t i c i t y of wood should not be influenced by the rate of deflection, but the stress to proportional limit should ichange., especially for plastic materials. This had been borne out experimentally by Liska (1955). It is also supported by the springwood failure theory proposed by Bodig (1963) for radially-loaded small Douglas f i r specimens. Rate of loading did not affect tension perpendicular to grain significantly within the range of 30 to 1000 kg/min., Osherovich (1955) reported. However, loading rates of 1 to 10 kg/min. resulted i n a 6 per cent strength reduction over higher rates. For compression perpendicular to grain, Stern (1944) found that twice or four times the standard speed did not cause any appreciable difference i n the stress values or moduli of e l a s t i c i t y . It - 13 -should be noted t h a t t h e importance of u n i f o r m i t y of t e s t i n g temperature and r e l a t i v e humidity, thus e q u i l i b r i u m moisture content of specimens, were emphasized by the standards adopted w i t h i n Canada and the United S t a t e s . - 14 -EXPERIMENT A l i s t i n g of assumptions concerning the v a r i a b l e s precedes an o u t l i n e of c o n s i d e r a t i o n s u n d e r l y i n g the s t a t i s t i c a l design and a n a l y s i s of the experiment. F o l l o w i n g t h i s , p r e p a r a t i o n of the specimens and v a r i o u s t e s t i n g methods are described. Experimental data are presented and analysed. 1. T e c h n i c a l assumptions C e r t a i n assumptions are proposed t o ensure a u n i f i e d approach t o the o b j e c t i v e s of the t h e s i s . T h e i r v a l i d i t y i s t o be evaluated i n the l i g h t o f the experimental evidence that w i l l be obtained. These t e c h n i c a l c o n s i d e r a t i o n s are: (1) A l l Douglas f i r veneers manufactured i n the Vancouver area c o n s t i t u t e a s i n g l e p o p u l a t i o n . (2) Plywood bond q u a l i t y w i l l be hig h and uniform i n the experiment. (3) Strength values obtained from t e n s i o n and compression t e s t s may be compared w i t h each other. (4) Magnitude and v a r i a t i o n o f st r e n g t h p r o p e r t i e s (S^) obtained may be adequately accounted f o r by veneer ( X j ) , plywood ( Y k ) , and t e s t i n g (Z-|_) v a r i a b l e s such t h a t : S^ = f ( X j , Y k, Zj). (5) Plywood, a composite m a t e r i a l , f a i l s i n i t s weakest l a y e r , i . e . veneer, when subjected to s t r e s s e s normal to g l u e l i n e . The f o l l o w i n g a l t e r n a t i v e s may be considered: (a) The b a s i c p r o p e r t i e s of wood (veneer) are not a l t e r e d by . , the manufacturing techniques,,so t h a t Sj_ = f ( X j ) , - 15 -(b) Manufacturing processes change the b a s i c p r o p e r t i e s of veneer, . so th a t S i = f ( Y k ) and Y k = f ( X j ) , (c) B e t t e r approximation of strength p r o p e r t i e s i s obtained from . , a combination of the most s i g n i f i c a n t veneer and plywood v a r i a b l e s , expressed as Sj_ = f (X^, Y^). (6) The p o s s i b l e random v a r i a t i o n s introduced by the t e s t i n g methods ca n c e l each other (on the average), thus t h e i r i n f l u e n c e may be n e g l i g i b l e . (7) Consequently, the measurement and/or c a l c u l a t i o n o f the f o l l o w i n g "independent" v a r i a b l e s , explained i n d e t a i l i n the t e x t , may prove adequate to account f o r the v a r i a t i o n i n plywood s t r e n g t h p r o p e r t i e s , (a) Veneer v a r i a b l e s : X I X2 X3 X4 X5 X6 X7 X8 X9 X10 X l l X12 X13 X14 X15 X16 X17 type thi c k n e s s growth r i n g s per i n c h summerwood per cent s p e c i f i c g r a v i t y moisture content l a t h e check depth 1 l a t h e checks per i n c h l a t h e check angle r a d i a l angle of growth r i n g l o n g i t u d i n a l angle of growth r i n g roughness t i g h t n e s s of cut ( X 2 ) 2 : ( t h i c k n e s s ) 2 (summerwood per c e n t ) 2 ( l a t h e check d e p t h ) 2 , ( s p e c i f i c g r a v i t y ) ? ( X 4 ) 2 ( X 7 ) 2 (15) 2 (b) Plywood v a r i a b l e s : Y l : g l u i n g pressure Y2: l o a d recovery Y 3 : number o f p l i e s Y4: height of b l o c k a t E.M.C. Y5: f u l l compression Y6: permanent compression Y7: weight l o s s i n press Y8: veneer d e n s i f i c a t i o n Y9: glue content ( s o l i d s ) Y10: i n c r e a s e i n s p e c i f i c - g r a v i t y Y l l : s p e c i f i c g r a v i t y - 16 -Y12: e q u i l i b r i u m moisture content (E.M.C.) Y13: days to E.M.C. Y14: ( Y l ) 2 : ( g l u i n g p r e s s u r e ) 2 Y15: ( Y 6 ) 2 : (permanent compression) 2 Y16: ( Y 8 ) 2 : (veneer d e n s i f i c a t i o n ) ? Y17: ( Y 1 0 ) 2 : ( i n c r e a s e i n s p e c i f i c , g r a v i t y ) 2 Y18: ( Y l l ) 2 : ( s p e c i f i c g r a v i t y ) 2 (8) These three independent v a r i a b l e s may be the most important: X I : veneer type X2: veneer thi c k n e s s Y l : g l u i n g pressure (9) The plywood s t r e n g t h p r o p e r t i e s , i . e . dependent v a r i a b l e s , considered are : S I : modulus of e l a s t i c i t y i n compression S2: u n i t s t r e s s i n compression S3: u n i t s t r a i n i n compression S4: modulus of e l a s t i c i t y i n t e n s i o n S5: u n i t s t r e s s i n t e n s i o n S6: u n i t s t r a i n i n t e n s i o n S7: wood f a i l u r e i n t e n s i o n S8: u n i t s t r e s s i n shear S9: wood f a i l u r e i n shear This n o t a t i o n of v a r i a b l e s s h a l l be adhered to i n d i s c u s s i o n s , t a b l e s and equations t h a t f o l l o w . 2. Experimental design I t has been assumed th a t wood and g l u e c h a r a c t e r i s t i c s , and manu-f a c t u r i n g techniques, determine the strength p r o p e r t i e s of plywood. An a n a l y s i s i n v o l v i n g many of these f a c t o r s could account f o r most of the v a r i a t i o n i n s t r e n g t h values. The p h y s i c a l c o n t r o l of a l l v a r i a b l e s would be i m p r a c t i c a b l e , i f not i m p o s s i b l e . T h e i r i n f l u e n c e , however, can be brought under s t a t i s t i c a l c o n t r o l and evaluated. Only the t h r e e f a c t o r s assumed t o be the most s i g n i f i c a n t are c o n t r o l l e d experimentally a t l e v e l s l a i d down by a f a c t o r i a l design. The - 17 -appropriate l e v e l s have been s e l e c t e d on the b a s i s t h a t three points may adequately d e f i n e the curvature of response s u r f a c e s . The 2 by 3 by 3 f a c t o r i a l design chosen i s o u t l i n e d below: Facto r A l l Factor B b l b 2 Factor C °1 c 2 c 3 type of veneer sawn r o t a r y cut veneer thicknesses 1/10 i n . 1/7 i n . 1/5 i n . g l u i n g pressure 50 p s i 200 p s i 350 p s i The treatment combinations have been assigned according to the f o l l o w i n g p a t t e r n , where numbers designate the experimental u n i t s , i . e . , the plywood b l o c k s : A a l a 2 B b l b 2 b 3 b l b 2 b 3 G l 1 4 7 10 13 16 C C 2 2 5 8 11 14 17 G 3 3 6 9 12 15 18 To rank the independent v a r i a b l e s according t o t h e i r i n f l u e n c e on .strength p r o p e r t i e s , and to a l l o w e v a l u a t i o n of assumptions about them, a-m u l t i p l e r e g r e s s i o n a n a l y s i s , w i t h automatic r e d u c t i o n , i s proposed. The s t a t i s t i c a l c o n t r o l c o n s i s t s of the r e c a l c u l a t i o n of plywood s t r e n g t h p r o p e r t i e s from r e g r e s s i o n equations t h a t exclude f a c t o r s A, B and C. This would e l i m i n a t e the e f f e c t of a l l other v a r i a b l e s . However, i t may s u f f i c e t o a d j u s t f o r the most s i g n i f i c a n t ones on l y . Then, the i n f l u e n c e - 18 -o f f a c t o r s A, B and C can be r e a l i s t i c a l l y evaluated by an a n a l y s i s of v a r i a n c e of the adjusted s t r e n g t h values. These adjusted values may a l s o be used t o construct the response surfaces to determine the optimum and/or worst combinations of the three c o n t r o l l e d independent v a r i a b l e s . I t should be noted t h a t the l a r g e number of v a r i a b l e s n e c e s s i t a t e s the use o f an e l e c t r o n i c computer. 3. S t a t i s t i c a l assumptions To achieve a r e l i a b l e and meaningful i n t e r p r e t a t i o n of the r e s u l t s , a c l o s e observance of the b a s i c assumptions of the s t a t i s t i c a l methods i s necessary. These assumptions are o u t l i n e d below. The r e g r e s s i o n equations are based on the assumptions t h a t the independent v a r i a b l e s are not i n f l u e n c e d by treatments and are measured without e r r o r . The dependent v a r i a b l e s are supposed t o be randomly and normally d i s t r i b u t e d , w i t h a common va r i a n c e . For c a l c u l a t i n g the adjusted means, that i s , u s i n g a covariance a n a l y s i s , the independence and n o r m a l i t y of r e s i d u a l s and the l i n e a r i t y of r e g r e s s i o n have t o be assumed. I t should be emphasized that a r e g r e s s i o n equation expresses a s t a t i s t i c a l law, i t holds t r u e on the average, but i t i s not an absolute mathematical t r u t h . The m u l t i p l e l i n e a r r e g r e s s i o n equations conform to the f o l l o w i n g model, according t o S t e e l and T o r r i e (I960)-: Y ± ~- A * B j ( X ^ ) -* Ej. where i = 1, 2, ..., n and j = 1, 2, p The c a l c u l a t i o n of adjusted means assumes the homogeneity of r e g r e s s i o n - 19 -c o e f f i c i e n t s , and i s i n d i c a t e d below, as g i v e n by S t e e l and T o r r i e (i960): A± = Y± __ B. ( X ± j _ X.j) The analyses of v a r i a n c e are based on the assumed a d d i t i v i t y of treatment and environmental f a c t o r s , and the independence, randomness and normal d i s t r i b u t i o n - w i t h a common variance about zero mean - o f the dependent v a r i a b l e s . The assumption o f n o r m a l i t y i s not r e q u i r e d f o r e s t i m a t i n g the components of va r i a n c e , but randomization i s necessary. When the independent v a r i a b l e s are f i x e d , t h a t i s , not i n f l u e n c e d by treatments, the e r r o r v a r i a n c e i s the appropriate term f o r t e s t i n g hypotheses about any source of v a r i a t i o n . For e v a l u a t i n g the adjusted means, the e r r o r degrees of freedom must be reduced by the number of independent v a r i a b l e s used i n the c a l c u l a t i o n of adjusted values. 4 . P r e p a r a t i o n of m a t e r i a l I d e a l l y , the plywood b l o c k s prepared i n t h i s experiment should d i f f e r o n l y i n three of t h e i r a t t r i b u t e s , namely, veneer type, veneer t h i c k n e s s , and g l u i n g pressure. To approach t h i s , one would need i d e n t i c a l and d e f e c t -f r e e sheets of veneers, processed by the same techniques, apparatus and people. Although the inh e r e n t v a r i a b i l i t y of wood, glue, and processing could not be e l i m i n a t e d , an attempt has been made to minimize i t s i n f l u e n c e by both experimental and s t a t i s t i c a l methods. (a) Veneers The r o t a r y - c u t veneer sheets were obtained from three d i f f e r e n t plywood m i l l s i n the Vancouver area, s i n c e a t t h a t time none of them - 20 -manufactured a l l three t h i c k n e s s e s . The samples were picked from veneers l e a v i n g the d r y e r s . Thus, they should represent the veneer p o p u l a t i o n r e s u l t i n g from standard i n d u s t r i a l p r a c t i c e s . To secure c o n t r o l specimens, two 1-in. by 6-in. f l a t - s a w n Douglas f i r boards were sawn i n t o sheets and planed t o the r e q u i r e d thicknesses i n the U n i v e r s i t y carpenter shop. The veneer sheets were then cut i n t o 4 8 - i n . by 5-in. s t r i p s along the g r a i n , u s i n g a t a b l e saw, and designated by c a p i t a l l e t t e r s . These, i n t u r n , were d i v i d e d i n t o 5 - i n . by 5-in. s e c t i o n s and i d e n t i f i e d by numbers from 1 t o 9. A l l s e c t i o n s showing v i s i b l e d e f e c t s were excluded from the subsequent phases o f the experiment. The 13 veneer v a r i a b l e s were then measured on each s e c t i o n and recorded f o r every plywood block t o be assembled. To render the l a t h e checks c l e a r l y v i s i b l e , the cross s e c t i o n o f each veneer piece had been p r e v i o u s l y s t a i n e d by I n d i a i n k , and sanded. To d i s t i n g u i s h springwood from summerwood, a s o l u t i o n c o n s i s t i n g o f equal p a r t s o f methyl blue and malachite green i n an a l c o h o l s o l v e n t was a p p l i e d t o the sanded cross s e c t i o n s . To ensure accurate measurements, a d i s s e c t i n g microscope - w i t h a c a l i b r a t e d eye-piece - was set up. This allowed readings accurate t o 1/10,000 i n . at a m a g n i f i c a t i o n o f 20, without touching (compressing) the specimens, a problem encountered w i t h mechanical gauges. Three s c a l e r a t i o s of deepest l a t h e checks t o the veneer thickness a t t h e same p o i n t s , were averaged and recorded as l a t h e check depth i n per cent of t h i c k n e s s . The mean of the above three t h i c k n e s s readings was converted t o inches t o give the recorded veneer t h i c k n e s s v a l u e s . S i m i l a r l y , the average of t h r e e s c a l e r a t i o s of summerwood over t o t a l growth r i n g width was c a l c u l a t e d as summerwood percentage. Growth r i n g - 21 -w i d t h was used as a d i v i s o r of one i n c h to ob t a i n the number of annual r i n g s per i n c h f o r the veneer s e c t i o n s . I n c l i n a t i o n of l a t h e checks to the veneer face was measured w i t h a transparent p r o t r a c t o r . The recorded values represent the mean of t h r e e readings taken t o the c l o s e s t 5°. An i n c h s c a l e scratched on the s t r a i g h t edge of the p r o t r a c t o r f a c i l i t a t e d the counting of l a t h e checks over the c e n t r a l two-inch p o r t i o n o f veneers, from which the average number of l a t h e checks was c a l c u l a t e d . O r i e n t a t i o n of growth r i n g s w i t h reference t o the veneer face was measured on the cross s e c t i o n ( r a d i a l angle) and along the g r a i n ( l o n g i t u d i n a l a n g l e ) . The l a t t e r was intended to serve as a measure of "short g r a i n " - a s e r i o u s d e f e c t i n plywoods - which, however, was not e n t i r e l y e l i m i n a t e d from t h i s experiment. Again, the average of t h r e e readings taken t o the c l o s e s t 5° was c a l c u l a t e d f o r every s e c t i o n . Most weighings were performed on a t o r s i o n balance, reading t o 0.5 gm (estimated t o 0.1 gm). I n d i v i d u a l veneer s e c t i o n s were weighed on a semi-micro balance w i t h a s e n s i t i v i t y o f 0.01 gm. A l l nine s e c t i o n s of a s t r i p were weighed before glue spreading and t h e i r weights averaged (Ws). One of these was d r i e d a t 100 i . 3°C f o r 24 hours t o o b t a i n the oven-dry weight (Wo). The average moisture content (M) of the " s t r i p " was determined as: Ww Wo x 100 = M {%). Wo The oven-dried pieces were dipped i n p a r a f f i n , and the weight of d i s t i l l e d water d i s p l a c e d (Wwo) determined by the standard water immersion method. The average s p e c i f i c g r a v i t y of the v a r i o u s veneer s t r i p s was c a l c u l a t e d - 22 -from: Wo = G Wwo Roughness and t i g h t n e s s of r o t a r y cut veneers were a l s o evaluated, a f t e r a v i s u a l ( s u b j e c t i v e ) i n s p e c t i o n under i n c i d e n t l i g h t . For purposes of s t a t i s t i c a l a n a l y s i s , these c l a s s e s had been gi v e n an a r b i t r a r y numerical value, as f o l l o w s : Loose s i d e : (a) rough - 7 (b) medium - 4 (c) smooth - 1 Tight s i d e : (a) t i g h t - 2 (b) medium - 3 (c) loose - 4 Measured or c a l c u l a t e d veneer v a r i a b l e s a r e presented as block averages i n Table I I . Sawn veneers should be stronger than r o t a r y - c u t samples, due.to t h e i r l a r g e r g r a i n angle, and b e t t e r ( e x c e l l e n t ) surface c o n d i t i o n , i n a d d i t i o n to the l a c k of l a t h e checks. A comparison o f the other v a r i a b l e s i n d i c a t e s a t r e n d i n the opposite d i r e c t i o n , t h a t would tend t o decrease the p o s s i b l e d i f f e r e n c e i n s t r e n g t h p r o p e r t i e s . (b) Adhesive * To produce plywood w i t h t h i c k n e s s e s i n excess of 4 inches, hot p r e s s i n g glues and techniques had t o be abandoned. Consequently, Duro-Lok 50, w i t h c a t a l y s t 42-2300 was s e l e c t e d . This allowed the completion of c o l d - p r e s s i n g of a l l 18 plywood blocks i n l e s s than three days. The adhesive was mixed and a p p l i e d i n accordance w i t h the manufacturers' i n s t r u c t i o n s , o u t l i n e d i n Table I I I . A small rubber r o l l e r glue spreader was used t o ensure a uniform - 23 -glue spread of 50 lb/M f t per double glue l i n e , and t o remain w i t h i n the al l o w a b l e open assembly time. Dummy veneer sec t i o n s were used to adjust the spreader t o t r a n s f e r 3.9 gm of.adhesive per double glue l i n e . Every second s e c t i o n was spread on both s i d e s . The a l t e r n a t i n g g r a i n d i r e c t i o n was c a r e f u l l y maintained i n a l l b l o c k s . To keep the edges of the plywood blo c k s p r o p e r l y a l i g n e d , a t l e a s t i n two d i r e c t i o n s , a s m a l l L-shaped frame of boards was n a i l e d together. This method f a c i l i t a t e d the handling of assemblies as w e l l . The dry weight of adhesive could not be checked a t the time of spreading, but was measured l a t e r and i s l i s t e d i n Table IV, as per cent of the t o t a l weight of the block. (c) Plywood The plywood assembly was pressed by a f i x e d compression head but, i n an attempt t o ensure a uniform pressure d i s t r i b u t i o n , i t was placed on a u n i v e r s a l p l a t e . The height of the block, as determined by the movement of the head w i t h reference t o a 5-inch high " z e r o - l e v e l " , was measured by a d i a l gauge reading t o 0.001 i n c h . Load readings were taken t o the c l o s e s t 5 pounds. I t was found necessary to ad j u s t the l o a d at 5-minute i n t e r v a l s i n the f i r s t 20 minutes of pressing time, t o m a i n t a i n (approach) the nominal g l u i n g pressure. The loads were noted before every adjustment. Load recovery was c a l c u l a t e d by s u b t r a c t i n g the a c t u a l l o a d before the f i r s t adjustment from that before the second. A l l values were expressed as a percentage of the nominal l o a d , and are presented i n Table IV. I t can be seen from the Table t h a t the average g l u i n g pressure a s y m p t o t i c a l l y approached the nominal l e v e l during the c y c l e . C l e a r l y , the water induced d i f f e r e n t s w e l l i n g and creep behavior i n the two veneer types. - 24 -Most l i k e l y , l a t h e checks allowed a r e l e a s e of s w e l l i n g pressures i n the r o t a r y cut veneers. A l s o , they could have been more p l a s t i c i z e d by the water than t h e i r sawn counterpart, s i n c e moisture movement i s f a c i l i t a t e d by the l a t h e checks. The i n i t i a l height ( t h i c k n e s s ) of plywood blocks (Table VI) was determined a f t e r a p p l y i n g a 50-pound l o a d to them t o f l a t t e n the cupped veneers. F o l l o w i n g t h i s , d i a l gauge readings were taken at f u l l pressure, before every adjustment, a t the end of the p r e s s i n g c y c l e , and a f t e r r e l e a s i n g pressure t o the i n i t i a l l e v e l of 50 l b . Height measurements were continued d a i l y f o r seven days a f t e r p r e s s i n g . The blocks were marked at f o u r points and readings were taken by a S t a r r e t t height gauge ( c a l i p e r ) to 0.001 i n c h . The averages of four readings taken at the above points are expressed i n per cent of i n i t i a l height, f o r ease of comparison, as shown i n Table V I . I t i s a l s o i n d i c a t e d t h a t both f u l l and permanent compressions increased p r o p o r t i o n a t e l y w i t h g l u i n g pressure i n most i n s t a n c e s , and that t h i n veneers were g e n e r a l l y compressed more than t h i c k ones. The average of 200 p s i g l u i n g pressure, as i n d i c a t e d i n Table IV, r e s u l t e d i n a permanent compression set of 1.99 and 4.29 per cent f o r the sawn and r o t a r y - c u t veneers, r e s p e c t i v e l y . I n Table V I I i t i s i l l u s t r a t e d t h a t a f t e r the pronounced i n i t i a l s w e l l i n g ( f i r s t day) a l l blocks reached t h e i r e q u i l i b r i u m moisture content (EMC) height w i t h i n 4 t o 5 days while being stored i n the wood technology l a b o r a t o r y . Veneers were kept i n the same room f o r a month p r i o r to g l u i n g . The same co n c l u s i o n had been reached by c o n s i d e r i n g changes i n weight of b l o c k s , as observed but not shown here. Specimens were cut from the plywood blocks to determine t h e i r moisture - 25 -content and s p e c i f i c g r a v i t y . The method, apparatus and formulae a p p l i e d were the same as f o r the veneers. Glue content was c a l c u l a t e d as the d i f f e r e n c e i n weight of the assembly p r i o r to g l u i n g and a t e q u i l i b r i u m moisture content of b l o c k s , and t a b u l a t e d as a per cent of the l a t t e r weight (Table V I I I ) . The plywood s p e c i f i c g r a v i t y was, t h e r e f o r e , expressed i n terms of veneer s p e c i f i c g r a v i t y to i n d i c a t e the gross increase i n d e n s i t y . By s u b t r a c t i n g glue content, converted t o percentage of veneer weight, from the gross i n c r e a s e i n d e n s i t y , the d e n s i f i c a t i o n of veneers was obtained. I n s p e c t i o n of Table V I I I r e v e a l s t h a t plywood blocks of r o t a r y - c u t veneers had a higher s p e c i f i c g r a v i t y and g l u e content than those of sawn veneers, and t h a t the l a t t e r e x h i b i t e d a higher degree of d e n s i f i c a t i o n . (d) Test specimens Since the t e s t i n g of plywood modulus of e l a s t i c i t y perpendicular to g l u e l i n e has not been standardized, specimen shape and s i z e had t o be s e l e c t e d a r b i t r a r i l y . The main concern was t o o b t a i n specimens that ensured a uniform s t r e s s d i s t r i b u t i o n , and of a s i z e t h a t allowed the use of the Table Model I n s t r o n T e s t i n g Instrument a v a i l a b l e i n the wood technology l a b o r a t o r y of the F a c u l t y of F o r e s t r y . A constant cross ; s e c t i o n allowed the f i r s t , a s m a l l s i z e could f u l f i l the second r e s t r i c t i o n . In a d d i t i o n , the plywood had to be of adequate thi c k n e s s (height) t o f a c i l i t a t e accurate s t r a i n measurements. I n the p r e l i m i n a r y experiment, a 1 - i n . by 1 - i n . by 4 - i n . r e c t a n g u l a r specimen had been t e n t a t i v e l y s e l e c t e d f o r both t e n s i o n and compression. T h i s seemed t o be j u s t i f i e d by data obtained from twelve 2 - i n . by 2 - i n . by 8 - i n . Douglas f i r plywood specimens hot-pressed w i t h phenol-formaldehyde - 2 6 -r e s i n (unpublished; S t i e d a , 1962). However, p r e l i m i n a r y specimens prepared by u s i n g c o l d - s e t t i n g urea r e s i n and cut t o the proposed dimensions, d i d not reach t h e i r p r o p o r t i o n a l l i m i t w i t h i n t h e capacity of the sm a l l t e s t i n g machine. By reducing the cross s e c t i o n t o \ i n . by 1 i n . , the t e n s i o n specimens could be t e s t e d but not the compression ones. The use of a l a r g e r t e s t i n g machine became necessary to avoid a f u r t h e r decrease i n cross s e c t i o n . To reduce the slenderness r a t i o of "the compression specimens, the o r i g i n a l 1 - i n . by 1- i n . cross s e c t i o n was r e t a i n e d . For checking bond q u a l i t y , three standard plywood shear specimens of three l a y e r s were cut out from a s e c t i o n of the plywood b l o c k . They were chosen so as t o c o i n c i d e with the f a i l u r e l i n e s i n the te n s i o n and/or compression specimens. Another s e c t i o n of the bl o c k was assigned f o r the determination of plywood moisture content and s p e c i f i c g r a v i t y . Results of the p r e l i m i n a r y experiment d i d not i n d i c a t e a s i g n i f i c a n t d i f f e r e n c e between the moduli of e l a s t i c i t y i n t e n s i o n and compression. They showed, however, t h a t the v a r i a t i o n of s t r e n g t h p r o p e r t i e s i n t e n s i o n i s g r e a t e r than i n compression. I t was found t h a t the mean s t r e n g t h value of a plywood block might be kept w i t h i n the 95 per cent confidence i n t e r v a l by t e s t i n g 3 compression and 6 t e n s i o n specimens. This l e d t o the c u t t i n g plan depicted i n Figure 1. The same p a t t e r n was used f o r a l l 18 plywood b l o c k s . 5. T e s t i n g procedures The specimens reached and maintained a f a i r l y uniform e q u i l i b r i u m moisture content while stored a t the t e s t i n g machines, as borne out by - 27 -moisture content determinations at the time of t e s t (Table I V ) . To minimize t e s t i n g time, a s t r a i n increment of 0.005 i n . / i n . / m i n had been s e l e c t e d f o r a l l t e s t s . This r e s u l t e d i n a head movement of 0.025 in./min f o r compression and 0.01 in./min f o r t e n s i o n t e s t i n g . Since d i f f e r e n t t e s t i n g machines were used f o r the v a r i o u s t e s t s , each set-up r e q u i r e s separate d e s c r i p t i o n . (a) Compression Compression t e s t s were performed on a h y d r a u l i c Baldwin U n i v e r s a l T e s t i n g Machine equipped w i t h an automatic X-Y recorder, as shown i n Figure 2. Only the lowest range of the machine, t h a t i s , 6000 pounds, was u t i l i z e d . Deformation was measured and t r a n s f e r r e d t o the recorder by a microformer extensometer. Only the c e n t r a l two inches- of the specimens were used i n measuring deformation, t o a v o i d p o s s i b l e excess compression i n the surface l a y e r s . The center l i n e of the specimens was marked by p e n c i l and the screws h o l d i n g the f l o a t i n g r i n g s were p o s i t i o n e d on them. A s p e c i a l frame was used i n s e t t i n g up the specimens t h a t ensured a span of two inches between the f l o a t i n g r i n g s . This arrangement i s i l l u s t r a t e d i n Figure 3. The r e c o r d e r was adjusted so as t o g i v e an e a s i l y d e f i n a b l e propor-t i o n a l ( e l a s t i c ) l i m i t . The u n i t s of the r e s p e c t i v e axes on the graph represented 0.02 i n c h of deformation, and 250 pounds of l o a d . (b) Tension The screw-gear type Table Model I n s t r o n T e s t i n g Instrument, complete w i t h a r e c o r d i n g u n i t , was used f o r t e n s i o n t e s t i n g . The machine i s shown i n F i gure 4. For most of'the specimens the maximum range, which i s 50 kilograms or 110 pounds, was needed. A u n i v e r s a l j o i n t - a standard f e a t u r e - 28 -on the machine - was thought to ensure a uniform s t r e s s d i s t r i b u t i o n . To prevent s l i p p a g e , s e r r a t e d t e n s i o n jaws were used. These were u n i f o r m l y t i g h t e n e d by means of a torque-wrench s et f o r 65 f t . l b . The deformation of the specimen over i t s c e n t r a l two-inch s e c t i o n was a u t o m a t i c a l l y recorded by the movement of the cross-bar t o which the upper p a i r of jaws was attached. An advantageous operating c h a r a c t e r i s t i c o f the machine allowed the jaws t o r e t u r n t o the set two-inch span a f t e r the completion of each t e s t . Figure 5 d e p i c t s a t e n s i o n specimen i n the t e s t i n g machine. A chart speed of 2 in./min was found t o g i v e a load-time curve of s u f f i c i e n t s e n s i t i v i t y . (c) Shear A Standard Shear T e s t i n g Machine was used t o perform the t e s t as s e t up i n the Plywood S e c t i o n of the Vancouver Laboratory, Forest Products Research Branch. The standard 1-inch by 3-inch shear specimens were t e s t e d i n the a i r - d r y c o n d i t i o n . They were not subjected to any soaking or b o i l i n g t e s t because, even without these treatments, the specimens should i n d i c a t e poor q u a l i t y bonds, i f present. C r i t i c a l shear area was one square i n c h ; hence u l t i m a t e l o a d was recorded d i r e c t l y i n p s i . The percentage of wood f a i l u r e was estimated o n l y a f t e r the e x p e r i -menter "standardized" h i s judgment by the use of s p e c i a l sets provided by the Forest Products Research Branch f o r t h i s s p e c i f i c purpose. 6. Re s u l t s Methods of c a l c u l a t i o n are o u t l i n e d , and r e s u l t s summarized i n the f o l l o w i n g s e c t i o n s . - 29 -(a) Computations Computations were performed on an IBM 1620 e l e c t r o n i c computer, u t i l i z i n g the l i b r a r y programs a v a i l a b l e f o r standard s t a t i s t i c a l techniques. Since t h e st r e n g t h p r o p e r t i e s of plywood blocks made of sawn and of r o t a r y -cut veneers appeared g r e a t l y d i f f e r e n t from one another, they were analysed s e p a r a t e l y . Thus, r e g r e s s i o n equations had t o be c a l c u l a t e d f o r both veneer types. A l s o , i n s t e a d of the 2 by 3 by 3 f a c t o r i a l a n a l y s i s planned, two separate 3 by 3 analyses of variance were r e q u i r e d t o be used. A s e l f - c o n t a i n e d F o r t r a n I I program was used f o r o b t a i n i n g the c o r r e l a t i o n and r e g r e s s i o n a n a l y s i s , w i t h s e l e c t i o n and automatic r e d u c t i o n , as programed by Dr. C. Froese i n 1962. This program was l i m i t e d t o a maximum of 20 v a r i a b l e s a t one time. The means, covariances, standard d e v i a t i o n s and simple c o r r e l a t i o n c o e f f i c i e n t s were p r i n t e d f o r each set o f data. For each r e g r e s s i o n a n a l y s i s , the r e g r e s s i o n c o e f f i c i e n t s ( B j ) , constant term f o r r e g r e s s i o n (A), r e s i d u a l v a r i a n c e , and c o e f f i c i e n t of determination (R ) were a l s o p r i n t e d . The v a r i a b l e c o n t r i b u t i n g t h e l e a s t t o the c o e f f i c i e n t of det e r m i n a t i o n was omitted, and the analyses repeated u n t i l a l l independent v a r i a b l e s had been e l i m i n a t e d . This f e a t u r e allowed the determination o f the most s i g n i f i c a n t f a c t o r or f a c t o r s i n an expected 99 per cent of the cases. Using the r e s u l t s of the above analyses, a simple F o r t r a n I I program was w r i t t e n by the experimenter to c a l c u l a t e the adjusted means. The c o r r e c t i o n was l i m i t e d t o the 11 most important independent v a r i a b l e s only, f o r blocks of both sawn and r o t a r y - c u t veneers. To evaluate the r o l e of veneer thickness and g l u i n g pressure on the plywood s t r e n g t h p r o p e r t i e s , a two-factor a n a l y s i s of variance w i t h r e p l i c a t e s was s e l e c t e d . This s e l f -- 30 -contained Fortran' I I program had been w r i t t e n by Dr. A. Kozak i h v i 9 6 2 . I t was designed t o compute the means and summarize sources of v a r i a t i o n degrees of freedom, sums o f squares, variances, and F - r a t i o s i n an a n a l y s i s of var i a n c e t a b l e . The analyses were performed f o r both observed and adjusted plywood s t r e n g t h v a l u e s . (b) M u l t i p l e r e g r e s s i o n equations F i r s t l y , the m u l t i p l e r e g r e s s i o n analyses were to e s t a b l i s h and evaluate the proposed f u n c t i o n a l dependence of plywood s t r e n g t h on veneer or plywood v a r i a b l e s and t h e i r combination. I t was found t h a t four t o seven s i g n i f i c a n t independent v a r i a b l e s could account f o r most of the v a r i a t i o n i n plywood s t r e n g t h p r o p e r t i e s . I n some cases, e i t h e r veneer thi c k n e s s or g l u i n g pressure and/or both were found unimportant f a c t o r s . Tables IX and 1 X I l i s t these r e g r e s s i o n equations f o r sawn and r o t a r y - c u t veneer b l o c k s , r e s p e c t i v e l y . The rank and c o n t r i b u t i o n of the most important v a r i a b l e s are summarized i n Tables X and X I I . Secondly, the dependence of some plywood v a r i a b l e s on independent f a c t o r s was determined by the use of the m u l t i p l e r e g r e s s i o n technique. These f a c t o r s had a l s o been ranked i n accordance w i t h t h e i r c o n t r i b u t i o n t o the variance of plywood str e n g t h p r o p e r t i e s . The above i n f o r m a t i o n i s summarized i n Tables X I I I and XIV. I n a d d i t i o n , r e g r e s s i o n equations were needed to c a l c u l a t e the adjusted plywood str e n g t h values. This time, the three e x p e r i m e n t a l l y c o n t r o l l e d f a c t o r s , namely, veneer type, veneer thickness and g l u i n g pressure, were excluded from the independent v a r i a b l e s . T h e i r rank and c o n t r i b u t i o n had a l s o been e v a l u a t e d . Although the means were adjusted f o r the most important 11 v a r i a b l e s , o n l y the f i r s t f o u r were summarized i n Tables XV -31-and XVI. L a s t l y , 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 veneer thickness or g l u i n g pressure and c e r t a i n concomitant v a r i a b l e s were l i s t e d i n Tables XVII and X V I I I r e s p e c t i v e l y . (c) Plywood s t r e n g t h values Block averages were c a l c u l a t e d t o provide an estimate of plywood str e n g t h p r o p e r t i e s normal t o g l u e l i n e , and t o assess the i n f l u e n c e of e x p e r i m e n t a l l y c o n t r o l l e d f a c t o r s . A summary of observed str e n g t h values i s g i v e n i n Table XIX. To o b t a i n a b e t t e r estimate of the r o l e of these c o n t r o l l e d f a c t o r s , the adjusted s t r e n g t h values were c a l c u l a t e d , c o n s i d e r a b l y reducing the e f f e c t of concomitant v a r i a b l e s . The adjusted means were c o l l e c t e d i n Table XX. F i n a l l y , t o f a c i l i t a t e comparisons between observed and adjusted means, sawn and r o t a r y cut b l o c k s , and the d i f f e r e n t s t r e n g t h p r o p e r t i e s , v a r i o u s strength r a t i o s were computed and are summarized i n Tables XXI, XXII and X X I I I . Figures 6 t o 9 a l l o w a g r a p h i c a l comparison of s t r e n g t h values, as response surfaces. (d) Analyses of v a r i a n c e The analyses of v a r i a n c e were performed on both observed and adjusted values, to t e s t the s i g n i f i c a n c e of veneer t h i c k n e s s , g l u i n g pressure and t h e i r i n t e r a c t i o n . R e s u l t s are summarized i n Table XXIV. To focus a t t e n t i o n on the h i g h l y s i g n i f i c a n t f a c t o r s only, a separate l i s t i n g was made i n Table XXV. - 32 -DISCUSSION A v a l i d i n t e r p r e t a t i o n of r e s u l t s requires a c l e a r understanding of the r e s t r i c t i o n s a s s o c i a t e d w i t h the t e c h n i c a l and s t a t i s t i c a l assumptions t h a t c o n s t i t u t e the b a s i s of the experiment. The v a r i o u s f a c t o r s may then be evaluated i n compression, t e n s i o n and shear, w i t h s p e c i a l emphasis on the e x p e r i m e n t a l l y c o n t r o l l e d v a r i a b l e s . F i n a l l y , the p o s s i b i l i t i e s of improving f u t u r e t e s t s should be considered. 1. V a l i d i t y of assumptions The t e c h n i c a l assumptions have been evaluated by s t a t i s t i c a l analyses of d a t a . Consequently, the l i m i t a t i o n s of the s t a t i s t i c a l methods should be f i r s t d i s c u s s e d . The veneer v a r i a b l e s comply t o the f i r s t assumption of r e g r e s s i o n a n a l y s i s , but most of the plywood v a r i a b l e s were i n f l u e n c e d by the treatments assigned t o each bl o c k . As a r e s u l t , r e g r e s s i o n equations i n c l u d i n g the l a t t e r v a r i a b l e s should be l e s s r e l i a b l e than those based on veneer v a r i a b l e s alone. The i n c l u s i o n of s i g n i f i c a n t i n t e r a c t i o n s i n r e g r e s s i o n equations i s a recommended s t a t i s t i c a l procedure. Some of the plywood v a r i a b l e s may be considered as an " i n t e r a c t i o n " of many independent f a c t o r s as i n d i c a t e d i n Tables X I I I and XXV. However, the c a l c u l a t i o n of adjusted means from r e g r e s s i o n equations based on plywood v a r i a b l e s i s bound to remove a part of the treatment e f f e c t s , thus reducing the s e n s i t i v i t y of subsequent analyses of v a r i a n c e . This a p p l i e s p a r t i c u l a r l y to adjusted s t r e n g t h values of r o t a r y - c u t veneer b l o c k s , where the i n f l u e n c e of plywood v a r i a b l e s i s more pronounced. - 33 -The d i s t r i b u t i o n o f dependent v a r i a b l e s was found t o be normal or near normal, w i t h the exception of wood f a i l u r e percentages. Consequently, no adjusted means were c a l c u l a t e d f o r wood f a i l u r e s . Observed values, however, were analysed to o b t a i n some i n f o r m a t i o n concerning the i n f l u e n c e of v a r i o u s veneer and plywood v a r i a b l e s on them. Since r e g r e s s i o n equations h o l d true on the average only, t h e i r f i t t o extreme values was expected to be poor. This was borne out by the u n r e a l i s t i c a l l y low adjusted values f o r blocks 13 and e s p e c i a l l y 16 (see Table XX). Presumably, a b e t t e r r e g r e s s i o n equation could have been f i t t e d t o the s t r e n g t h p r o p e r t i e s of r o t a r y - c u t veneer blocks by excluding the above two experimental u n i t s . This might p a r t l y account f o r the f a c t t h a t the adjusted values of sawn veneer blocks were much more uniform than the r o t a r y - c u t veneer b l o c k s . The assumptions of the analyses of v a r i a n c e were c l o s e l y approximated. The dependent v a r i a b l e s had a normal frequency d i s t r i b u t i o n and the treatments and measurements were randomized. Thus the q u a l i t a t i v e r e s u l t s o f analyses of variance should be r e l i a b l e . The t e c h n i c a l assumptions appeared to be j u s t i f i e d , w i t h two notable exceptions. F i r s t l y , as might have been expected, the sawn and r o t a r y - c u t veneer blocks formed two d i s t i n c t populations. This n e c e s s i t a t e d separate analyses f o r each. As a r e s u l t , i n s t e a d of the planned 18 hidden r e p l i c a t e s of the f a c t o r i a l design, the analyses were based on 9 o n l y . Since the f a c t o r i a l experiment was performed only once, the r e s u l t s are s t r i c t l y e x p l o r a t o r y i n nature. Secondly, g l u i n g pressure was not found to be an important f a c t o r i n many cases, e.g., compression modulus of e l a s t i c i t y and t e n s i o n s t r a i n f o r sawn veneer b l o c k s . I n s p e c t i o n of Tables X and X I I - 34 -supports t h i s statement. Further, blocks 13 and e s p e c i a l l y 16 had a low bond q u a l i t y , presumably a t t r i b u t a b l e t o the inadequacy o f the 50 p s i g l u i n g pressure used t o f l a t t e n the s l i g h t l y cupped veneers. T h e i r i n f l u e n c e on a n a l y s i s was discussed above. F i n a l l y , the e x i s t e n c e of f u n c t i o n a l dependencies S.= = f ( X . ) , S.j_ = f ( \ ) , S i = f ( X j , Y k) and Y k = f (Xj) have been adequately demonstrated. These accounted f o r approximately 65 to 94 per cent of the v a r i a t i o n i n s t r e n g t h of sawn-veneer b l o c k s , and 75 to 98 per cent i n r o t a r y - c u t veneer b l o c k s . Tables X and X I I l i s t these values as c o e f f i c i e n t s of determination ( R 2 ) . I t should be noted, however, t h a t about 50 per cent of the v a r i a t i o n i s unexplained f o r t e n s i o n s t r e s s , s t r a i n and wood f a i l u r e , a l s o f o r shear s t r e s s of the sawn-veneer b l o c k s . 2. I n f l u e n c e of c o n t r o l l e d f a c t o r s Since performance of sawn and r o t a r y - c u t veneer blo c k s was h i g h l y s i g n i f i c a n t l y d i f f e r e n t , t h e i n f l u e n c e of v a r i o u s f a c t o r s must be evaluated s e p a r a t e l y . A combined m u l t i p l e r e g r e s s i o n or a n a l y s i s of v a r i a n c e would have g i v e n u n r e a l i s t i c r e s u l t s , being based on the n o n e x i s t i n g "average" s t r e n g t h of blocks combining both veneer types. Veneer type alone was r e s p o n s i b l e f o r a c o n s i d e r a b l y l a r g e r v a r i a t i o n i n plywood s t r e n g t h p r o p e r t i e s than a l l the other c o n t r o l l e d and concomitant v a r i a b l e s combined. An attempt was made to account f o r i t s dominant r o l e . The i n f l u e n c e of veneer t h i c k n e s s and g l u i n g pressure was evaluated i n l i g h t of t h e i r a s s o c i a t i o n w i t h other v a r i a b l e s . Only f a c t o r s e x h i b i t i n g a simple c o r r e l a t i o n c o e f f i c i e n t (R) of a t l e a s t 0.40 were considered. - 35 -The response surfaces d e p i c t e d i n Figures 6 to 9 i l l u s t r a t e the dominant r o l e of veneer ty p e s . Although there i s a c o n s i d e r a b l e d i f f e r e n c e i n magnitude of s t i f f n e s s i n compression and t e n s i o n , or observed and adjusted values, the p a t t e r n of response t o the c o n t r o l l e d f a c t o r s remains b a s i c a l l y the same f o r each veneer type i n a l l cases. An attempt w i l l be made t o account f o r the i n f l u e n c e of veneer type by u s i n g a simple mechanical model. The r e l i a b i l i t y of t h i s model i s evaluated by comparing the v a r i o u s s t r e n g t h r a t i o s w i t h i n and between observed and adjusted values, o r of sawn and r o t a r y - c u t veneer b l o c k s . F i n a l l y , the p o s s i b l e r o l e of glue i s considered b r i e f l y along w i t h the r e l i a b i l i t y of the magnitude of observed values. An i n s p e c t i o n of the l i s t of independent veneer v a r i a b l e s measured, r e v e a l s the f a c t t h a t the o n l y p h y s i c a l d i f f e r e n c e s between sawn and r o t a r y -cut veneers are those r e l a t e d t o l a t h e checks and t o the q u a l i t y of s u r f a c e s . Consequently, these f a c t o r s must be r e s p o n s i b l e f o r most of the d i f f e r e n c e s i n s t r e n g t h p r o p e r t i e s . The c o r r e l a t i o n s of these and other independent v a r i a b l e s obtained f o r the S i r f (Xj) type r e g r e s s i o n equation i n t e n s i o n , are given below. (a) Veneer type F a c t o r s : R v a l u e s : ( l ) Depth of l a t h e checks. Veneer thi c k n e s s -0.52 Lathe check angle 0.51 Tightness 0.41 - 36 -(2) Number of la t h e checks per i n c h . Veneer thickness -0.75 Lathe check angle 0.52 S p e c i f i c g r a v i t y 0.46 (3) Lathe check angle Veneer thickness -0.75 Lathe checks per i n c h 0.52 Tightness 0.46 S p e c i f i c g r a v i t y -0.43 (4) Roughness Lathe check angle -0.37 (5) Tightness Lathe check angle 0.46 Lathe check depth 0.41 These v a r i a b l e s appear to be h i g h l y c o r r e l a t e d w i t h each other, but comparatively independent of other p h y s i c a l p r o p e r t i e s , except f o r veneer s p e c i f i c g r a v i t y . I t may be deducted from t h i s , t h a t veneer q u a l i t y i s determined mainly by the p e e l i n g process, b a r r i n g p o s s i b l e degradation i n subsequent d r y i n g . The extremely hi g h negative c o r r e l a t i o n observed between veneer thickness and l a t h e check v a r i a b l e s i s the l o g i c a l r e s u l t o f f o r c e s a c t i n g on wood chips of va r i o u s thicknesses i n the l a t h e , and i n the subsequent f l a t t e n i n g of curved veneers. This alone could account f o r t h e dominant r o l e of l a t h e checks i n determining plywood strength p r o p e r t i e s , s i n c e the r o l e of p l y thickness alone i s proven beyond doubt. The r o l e of roughness and t i g h t n e s s i n determining strength i s l i m i t e d . Loose veneers f a c i l i t a t e the pe n e t r a t i o n of glue, r e s u l t i n g - 37 -i n b e t t e r mechanical adhesion, and somewhat improved shear r e s i s t a n c e . I n t e n s i o n , however, they might be q u i t e d e t r i m e n t a l , since they represent a damaged (weakened) wood su r f a c e . The loose or crushed surface f i b e r s cannot o f f e r any s u b s t a n t i a l r e s i s t a n c e t o t e n s i l e f o r c e s . Even i n compression, these surfaces tend to increase s t r a i n , thus reducing s t i f f n e s s v a l u e s . Most of the r e d u c t i o n i n strength must be a t t r i b u t e d t o l a t h e checks. They may be considered as s l o t s i n the veneer, although some o f them might be f i l l e d and re-bonded by the r e s i n . The fewer, the steeper, the shallower they are, the l e s s they reduce s t r e n g t h . They might not a f f e c t u l t i m a t e s t r e s s i n compression, but by i n c r e a s i n g the i n i t i a l deformation, they decrease modulus of e l a s t i c i t y v a l u e s . A f t e r being compressed t i g h t , veneers may assume the s t r e n g t h of s o l i d wood. In t e n s i o n , the opening of the checks i s bound t o reduce both s t r e n g t h and s t i f f n e s s . Even i n i t i a l l y , t h e l o a d must be c a r r i e d o n l y by the s o l i d s e c t i o n s of veneers. T h e i r r e s i s t a n c e i s f u r t h e r reduced by the f a c t t h a t l a t h e checks o b v i o u s l y f a c i l i t a t e crack propagation a t lower loads than s o l i d wood l a y e r s . Thus t e n s i l e s t r e n g t h p r o p e r t i e s must be reduced c o n s i d e r a b l y more than those i n compression. Shear s t r e n g t h , on the other hand, may be higher as a r e s u l t of l a t h e checks f i l l e d w i t h g l u e . Depending on specimen o r i e n t a t i o n , l a t h e checks e i t h e r tend t o c l o s e or open i n shear. The former would o b v i o u s l y r e s u l t i n higher observed stresses than the l a t t e r . I n the present e x p e r i -ment, however, no a t t e n t i o n has been p a i d t o specimen o r i e n t a t i o n . This might account f o r the l a r g e r range of shear values f o r the r o t a r y - c u t veneers i n comparison w i t h t h e sawn ones. The v a l i d i t y of the proposed simple mechanical model ( " s l o t t e d sheet") i n e x p l a i n i n g the r o l e of l a t h e checks and surface q u a l i t y i s demonstrated - 33 -by the magnitude of observed s t r e n g t h values of sawn and r o t a r y - c u t veneer b l o c k s . T o - f a c i l i t a t e a comparison, the appropriate r a t i o s are summarized i n Table X X I I . I t i s shown t h a t both veneer types e x h i b i t the same s t r e s s i n compression on the average, but r o t a r y - c u t veneers deform twice as much as the sawn ones. T h e i r s t i f f n e s s i s , a c c o r d i n g l y , reduced by a f a c t o r o f two. The d e t r i m e n t a l i n f l u e n c e of weakened surface f i b e r s i s more pronounced i n t e n s i o n . ' Consequently, sawn veneers deform 2.2 times and c a r r y loads 3.6 times as much as t h e i r r o t a r y - c u t counterparts. The former f a i l g r a d u a l l y , whereas the l a t t e r f a i l by a sudden snap. Due t o the l a r g e r deformation induced by higher loads the s t i f f n e s s of sawn veneers exceeds t h a t of r o t a r y -cut ones by a f a c t o r of 1.8 only. The estimated wood f a i l u r e percentages i n both t e n s i o n and shear are almost i d e n t i c a l f o r t h e two veneer types. One would expect the adjusted s t r a i n values t o be i d e n t i c a l f o r both veneer types. This happens t o hold f o r compression t e s t s only. The extremely l a r g e d i f f e r e n c e i n t e n s i o n might be a t t r i b u t e d to the i n f l u e n c e of v a r i a b l e s not accounted f o r . These amount t o approximately 80 per cent f o r sawn and 40 per cent f o r r o t a r y - c u t veneers. Consequently, the r a t i o s of adjusted t e n s i o n values might be i n considerable e r r o r . The compression s t r e n g t h and s t i f f n e s s values t h a t a r e both c o r r e c t e d f o r about 60 per cent of the observed variance might be more comparable. They i n d i c a t e a t e n f o l d d i f f e r e n c e between the two veneer types. I n a d d i t i o n t o the i n f l u e n c e of l a t h e checks, t e n t a t i v e l y , most of t h i s should be assigned t o the d i f f e r e n c e i n d r y i n g schedules and the a s s o c i a t e d chemical degradation ( p y r o l y s i s ) and/or surface i n a c t i v a t i o n . Rotary-cut veneers are supposedly d r i e d at about 400°F. Lathe checks increase the exposed veneer surface manyfold. The sawn veneers, on the other hand, are s t r i p s cut out from 2- by 6-inch lumber, u n l i k e l y t o have been weakened by normal k i l n d r y i n g . Consequently, - 39 -sawn veneer blocks should be c o n s i d e r a b l y stronger than r o t a r y - c u t ones, e s p e c i a l l y i n t e n s i o n , when using adjusted values as a b a s i s of comparison. The r a t i o s o f approximately 1 to 90 and 1 t o 53 f o r t e n s i o n s t r e s s and s t r a i n , r e s p e c t i v e l y , must be considered erroneous. Comparison of the appropriate strength r a t i o s of adjusted and observed values i s f a c i l i t a t e d by Table X X I I I . I t i s i n t e r e s t i n g t o note t h a t , while the average r a t i o f o r sawn veneer blocks i s q u i t e high, t h a t f o r r o t a r y - c u t veneers approaches u n i t y . Thus i t i s demonstrated t h a t f o r the l a t t e r group, the i n f l u e n c e s of various f a c t o r s tend to c a n c e l each other. T r y i n g t o a d j u s t f o r them does not a l t e r the " s t a t u s quo" of s t r e n g t h p r o p e r t i e s . The v a r i o u s strength r a t i o s c a l c u l a t e d s e p a r a t e l y w i t h i n each veneer type should be i d e n t i c a l , because the mean of the nine blocks i n a l l s t r e n g t h p r o p e r t i e s should be i n f l u e n c e d by the v a r i o u s f a c t o r s t o approximately the same degree. The r a t i o s of adjusted s t r e n g t h values of sawn veneer blocks are expected t o be i n e r r o r , s i n c e they had been modified t o v a r i o u s degrees of accuracy. The adjusted values of r o t a r y - c u t veneer blocks should be comparable s i n c e almost a l l the s t r e n g t h p r o p e r t i e s are adjusted by about 60 per cent of the v a r i a n c e . The above assumptions are more or l e s s borne out by the r a t i o s of mean values, as l i s t e d i n Table XXI. The agreement between the r a t i o s of observed values of the two veneer types i s q u i t e good. The degree of correspondence among the r a t i o s of observed and adjusted s t r e n g t h values of the r o t a r y - c u t veneer blo c k s i s s a t i s f a c t o r y . The i r r e g u l a r p a t t e r n of adjusted values of sawn-veneer blocks i s not s u r p r i s i n g . The compression t o t e n s i o n s t r e s s r a t i o f o r Douglas f i r i s 2.05 from Canadian and 2.56 from U.S. data, as g i v e n i n Table I . The observed r a t i o f o r sawn-veneer blocks i s 2.54- The good agreement suggests the use of - 40 -r a t i o s obtained f o r the sawn-veneer bloc k s as a c o n t r o l f o r t h a t of r o t a r y -cut veneers. I t should be noted, however, t h a t the veneer s t r e n g t h values observed are o n l y h a l f of t h a t published f o r s o l i d wood. This c a l l s t o a t t e n t i o n the p o s s i b l e r o l e of d r y i n g techniques, glue, chemistry of adhesion, and specimen geometry. These are t o be discussed l a t e r . The compression to t e n s i o n s t r e s s r a t i o f o r r o t a r y - c u t veneer blocks was 8.59. Almost a l l of t h i s f o u r f o l d i n c r e a s e must be a t t r i b u t e d to a corresponding r e d u c t i o n i n t e n s i l e s t r e s s , since the two compression values have a r a t i o of 1.06. There i s , however, no way of e s t i m a t i n g from the experimental data how much of the r e d u c t i o n i s assignable t o l o o s e f i b e r s , l a t h e checks, chemical degradation or other causes. The s t r a i n r a t i o a l s o e x h i b i t s a f o u r f o l d i n c r e a s e . This corresponds t o o n l y a twofold i n c r e a s e i n s t r a i n o f r o t a r y - c u t veneers, since the two compression values have a r a t i o of 0.52. The moduli of e l a s t i c i t y are f a i r l y c l o s e because of the p r o p o r t i o n a l increase i n both s t r e s s and s t r a i n . They are 7-05 and 6.21 f o r sawn and r o t a r y - c u t veneer b l o c k s , r e s p e c t i v e l y . The s t r e n g t h r a t i o s comparing modulus of e l a s t i c i t y , u n i t s t r e s s , and u n i t s t r a i n t o shear s t r e s s a r e 913 and 681, 1.74 and 2.45, 0.68 and 0.29, r e s p e c t i v e l y , f o r the two veneer types. S i m i l a r r a t i o s might be used t o p r e d i c t plywood s t r e n g t h p r o p e r t i e s from any, a r b i t r a r i l y chosen, s i n g l e measured s t r e n g t h v a l u e . Although the e v a l u a t i o n of glue i s not one of the o b j e c t i v e s of the experiment, i t cannot be completely ignored. A . d e t a i l e d d i s c u s s i o n of i t i s not warranted e i t h e r s i n c e the bonds were s a t i s f a c t o r y , i . e . , not d i r e c t l y c r i t i c a l i n determining s t r e n g t h . Blocks 13 and 16 were the exceptions t o t h e r u l e . This may be a t t r i b u t e d t o the inadequacy of - 41 -the low g l u i n g pressure used t o f l a t t e n - t h e t h i c k veneers, t o wet the surfaces. Only f a c t o r s d e t r i m e n t a l t o veneer strength should be considered, s i n c e wood f a i l u r e was h i g h even at low t e n s i o n s t r e s s e s . F i r s t l y , the s o l i d i f i c a -t i o n of p o l y v i n y l r e s i n s i s accompanied by l o s s of water r e s u l t i n g i n wood expansion and glue c o n t r a c t i o n , f o l l o w e d by r e s t r i c t e d shrinkage of p l i e s . The r e s u l t i n g s t r e s s e s may damage the veneers. Rate of s o l i d i f i c a t i o n , as shown by the importance of weight l o s s i n press i n t h i s experiment, a l s o s t r o n g l y i n f l u e n c e s s t r e n g t h p r o p e r t i e s . T h i s , i n t u r n , i s p a r t l y deter-mined by the r e l a t i v e humidity of a i r , ambient temperature, and veneer moisture content at the time of pressing and c u r i n g . This might be respon-s i b l e f o r the important r o l e of veneer moisture content i n the experiment. F i n a l l y , s t r o n g l y a l k a l i n e o r a c i d i c glues may weaken wood surface by h y d r o l y s i s . Thus, t h e . a c i d c a t a l y s t used may be r e s p o n s i b l e f o r the r e l a t i v e l y high wood f a i l u r e observed even a t low t e n s i o n s t r e s s e s . Weakening o f surface f i b e r s i s a l s o augmented by p y r o l y s i s r e s u l t i n g from high tempera-t u r e or excessive d r y i n g . A c o r o l l a r y of the l a t t e r i s the formation of c h e m i c a l l y i n a c t i v a t e d s u r f a c e s , covered by a molecular l a y e r of f a t t y a c i d s . This prevents w e t t i n g and r e s u l t s i n p o s s i b l e l o c a l i s e d , poor bond q u a l i t y . The i n f l u e n c e of the l a t t e r two f a c t o r s i s n e g l i g i b l e , i f any, f o r the sawn-veneers. Since the "exposed" surface o f r o t a r y - c u t veneers exceeds t h a t of sawn ones many times, due t o the presence of l a t h e checks, the f o u r f o l d decrease i n t e n s i l e s t r e s s might be a t t r i b u t e d to the e f f e c t of these chemical f a c t o r s . The t e n t a t i v e nature of t h i s proposal should be emphasized, s i n c e there was no attempt made t o measure or evaluate e x p e r i m e n t a l l y any of the p o s s i b l e chemical f a c t o r s . As mentioned above, t e n s i o n t e s t s are much more - 42 -s e n s i t i v e to the c o n d i t i o n o f veneer surfaces than compression or even shear. Indeed, t e n s i o n s t r e s s normal t o g l u e l i n e appears t o be the most s e n s i t i v e s i n g l e measure of bond q u a l i t y . I t has been shown th a t p o l y v i n y l acetate makes stronger bonds w i t h b i r c h wood than does c a s e i n . The l a t t e r , i n t u r n , i s stronger than urea glues. Stronger bonds ar e made w i t h gaboon usi n g phenolic r e s i n s than u s i n g animal glues or urea r e s i n s . The p r e l i m i n a r y blocks prepared by us i n g c o l d -s e t t i n g urea e x h i b i t e d moduli of e l a s t i c i t y ranges of 30,000 t o 40,000 p s i , i n both t e n s i o n and compression normal t o g l u e l i n e . A small s e r i e s of bearing t e s t s performed by the w r i t e r f o r the Plywood Manufacturer's A s s o c i a t i o n o f B r i t i s h Columbia i n 1964, u s i n g i n d u s t r i a l l y produced e x t e r i o r grade (hot-press phenolr-formaldehyde r e s i n ) Douglas f i r plywood, i n d i c a t e d values of 20,000 to 40,000 p s i range. These values are based on a l i m i t e d number of specimens, thus are f a r from c o n c l u s i v e . The observed experimental values f o r r o t a r y - c u t veneer blo c k s glued w i t h Duro-Lok 50 range from 11,000 to 27,000 p s i and 85,000 t o 154,000 p s i i n t e n s i o n and compression, r e s p e c t i v e l y . Thus i t may be t e n t a t i v e l y proposed that t h i s glue produces plywoods s l i g h t l y i n f e r i o r i n s t i f f n e s s t o urea-bonded panels i n t e n s i o n , and c o n s i d e r a b l y s u p e r i o r to them i n compression normal t o g l u e l i n e . As long as th e i n f l u e n c e of p l y number, s i z e of c r i t i c a l ' s e c t i o n and specimen geometry are not known, these experimental values may serve only to evaluate the i n f l u e n c e of the v a r i o u s f a c t o r s . F u r t h e r , the experiment was planned t o be a p i l o t study o n l y i n a much neglected f i e l d . Thus, the f a c t o r i a l d e sign was not r e p l i c a t e d . The conclusions drawn are t e n t a t i v e i n nature. The gene r a l value of the s p e c i f i c mean st r e n g t h values observed i s open to question and needs f u r t h e r experimental evidence. - 43 -(b) Veneer thickness I n s p e c t i o n of Tables XXIV and XXV r e v e a l s t h a t a l l plywood str e n g t h p r o p e r t i e s normal t o g l u e l i n e of both veneer types were h i g h l y s i g n i f i c a n t l y a f f e c t e d by the t h i c k n e s s of p l i e s . The i n t e r a c t i o n s of the two c o n t r o l l e d f a c t o r s of veneer t h i c k n e s s and g l u i n g pressure were a l s o h i g h l y s i g n i f i c a n t , w i t h the exception of t e n s i o n s t r a i n and wood f a i l u r e . I t should be noted t h a t the analyses of variance performed t o evaluate the i n f l u e n c e of the above two c o n t r o l l e d f a c t o r s , u s i n g both observed and adjusted plywood str e n g t h v a l u e s , provided almost i d e n t i c a l r e s u l t s . Under the experimental c o n d i t i o n s , blocks of 1/7-inch t h i c k veneers y i e l d e d the highest s t r e n g t h values on the average, c l o s e l y f o l l o w e d by 1/10-inch, and c o n s i d e r a b l y t r a i l e d by 1/5-inch t h i c k sheets. This can be e a s i l y v e r i f i e d f o r s t i f f n e s s by a simple i n s p e c t i o n of Figures 6 t o 9• I t has a l r e a d y been shown on pages 35 and 36 t h a t p l y t h i c k n e s s i s h i g h l y c o r r e l a t e d w i t h l a t h e check depth, angle, and number per i n c h . This alone could e x p l a i n the r o l e of veneer t h i c k n e s s i n determining strength p r o p e r t i e s of r o t a r y - c u t veneer b l o c k s . I n a d d i t i o n , p l y t h i c k n e s s i s c l o s e l y a s s o c i a t e d w i t h a host of concomitant v a r i a b l e s , as shown i n Table XVII, which s i g n i f i c a n t l y i n f l u e n c e the various plywood s t r e n g t h p r o p e r t i e s . G e n e r a l l y , the same f a c t o r s were c o r r e l a t e d w i t h t h i c k n e s s i n both veneer types, although t h e i r rank was d i f f e r e n t . Rotary-cut veneers were a s s o c i a t e d w i t h more v a r i a b l e s . The high negative c o r r e l a t i o n c o e f f i c i e n t s observed support common-sense assumptions t h a t w i t h i n c r e a s i n g veneer t h i c k n e s s the p r o p o r t i o n of glue and the number of p l i e s to reach a g i v e n block height i s s m a l l e r . Permanent ( p l a s t i c ) deformation i s reduced a l s o , s i n c e the h i g h l y p l a s t i c i z e d - 44 -boundary l a y e r s c o n s t i t u t e a smaller percentage of the t o t a l t hickness i n blocks of t h i c k veneers. The cores of these p l i e s might r e t a i n t h e i r a i r -d r y s t i f f n e s s i n the press, r e s u l t i n g i n a decreased f u l l compression a l s o . Apparently, the moisture content of t h i n veneers was higher than that o f t h i c k ones, consequently t h e i r l o a d recovery i n press was lower t o o . R a d i a l g r a i n angle was p o s i t i v e l y c o r r e l a t e d w i t h p l y t h i c k n e s s and appeared t o be of consequence i n t e n s i o n and r o l l i n g shear. By chance, higher s p e c i f i c g r a v i t i e s were a s s o c i a t e d w i t h i n c r e a s i n g veneer t h i c k n e s s e s . I n summary, veneer thickness might e i t h e r be considered as an i n t e r a c t i o n term of a l l these v a r i a b l e s , or as t h e best s i n g l e measure of glue content, number of p l i e s , plywood s p e c i f i c g r a v i t y and p l a s t i c deformation. (c) G l u i n g pressure Both the compressive and t e n s i l e s t r e n g t h p r o p e r t i e s of r o t a r y - c u t veneer, and the former o n l y of sawn-veneer b l o c k s , were h i g h l y s i g n i f i c a n t l y i n f l u e n c e d by g l u i n g pressure, as shown i n Tables XXIV and XXV. R o l l i n g shear s t r e n g t h appeared t o be h i g h l y s i g n i f i c a n t l y a f f e c t e d by g l u i n g pressure alone. Optimum r e s u l t s were obtained by u s i n g 200 p s i g l u i n g pressure. The next i n rank was 350 p s i . For the t h i c k veneers used i n t h i s experiment, presumably due to t h e i r cupping, 50 p s i pressure was found t o be inadequate. The a n a l y s i s of v a r i a n c e , w i t h few exceptions, i n d i c a t e d t h a t g l u i n g pressure c o n t r i b u t e d l e s s t o the v a r i a t i o n i n s t r e n g t h than d i d veneer t h i c k n e s s . Again, F i g u r e s 6 to 9 i l l u s t r a t e these statements g r a p h i c a l l y f o r the moduli o f e l a s t i c i t y . I t i s shown i n Table X V I I I t h a t g l u i n g pressure was a s s o c i a t e d w i t h fewer concomitant v a r i a b l e s than veneer t h i c k n e s s . A l s o , the simple - 45 -c o r r e l a t i o n c o e f f i c i e n t s are lower. Further, the v a r i a b l e s d i f f e r more markedly i n kind and order between t e s t and veneer types than was observed f o r p l y t h i c k n e s s . I t might be a t t r i b u t e d t o t h e f a c t t h a t these f a c t o r s are mostly r h e o l o g i c a l i n nature, thus are dependent on a l a r g e number of veneer and/or plywood v a r i a b l e s i n t u r n . Another p o s s i b i l i t y i s t h a t these f a c t o r s c a n c e l each other's i n f l u e n c e , making g l u i n g pressure comparatively unimportant. Larger pressure i t s e l f causes an increased f u l l and permanent compression, r e s u l t i n g i n a higher degree of veneer d e n s i f i c a t i o n . I t a l s o f o r c e s more l i q u i d i n t o the veneers which f a c i l i t a t e s p l a s t i c deforma-t i o n f a r t h e r , thus c o n t r i b u t i n g t o a f a s t e r decay of pressure ( l o a d ) . C o r r e l a t i o n of g l u i n g pressure w i t h summerwood percentage and r a d i a l g r a i n angle ( f o r shear) might be considered spurious. The d i f f e r e n t p a t t e r n of weight l o s s i n d i c a t e d f o r the two veneer types, must be a s s o c i a t e d w i t h the higher degree of glue squeeze-out observed on sawn-veneer b l o c k s . The l a t h e checks would a l l o w more glue t o be r e t a i n e d i n t h e p l i e s and a t the same time ameliorate the movement of water, e s p e c i a l l y under high g l u i n g pressures, which i s the by-product of curing p o l y v i n y l r e s i n emulsions. Sawn-veneer b l o c k s , on the other hand, increased t h e i r weight l o s s under lower pressures.In t h i s way, l e s s l i q u i d was forced i n t o the s o l i d p l i e s t h a t hinder moisture movement, compared to the -rate of evapora-t i o n from the f r e e surfaces of squeezed out (excess) g l u e . This phenomenon was a l s o r e s p o n s i b l e f o r the higher glue content of r o t a r y - c u t veneer b l o c k s , i n s p i t e of u s i n g the same nominal glue spread f o r both veneer types. I n co n c l u s i o n , g l u i n g pressure might be considered as the best s i n g l e measure - 46 -of f u l l compression, p l a s t i c deformation, and veneer d e n s i f i c a t i o n , or as an i n t e r a c t i o n term of the f a c t o r s discussed above. 3. I n f l u e n c e of concomitant v a r i a b l e s The use of veneer v a r i a b l e s alone accounted f o r the l e a s t , but a q u i t e s u b s t a n t i a l part of the v a r i a t i o n i n plywood s t r e n g t h p r o p e r t i e s . Plywood v a r i a b l e s i n general explained more. A combination of the most important f a c t o r s might account f o r almost a l l of the v a r i a n c e , p a r t i c u l a r l y f o r r o t a r y - c u t veneer b l o c k s . (See Table X I I ) The p r o p o r t i o n of the v a r i a n c e accounted f o r by the sawn-veneer blocks was somewhat s m a l l e r . (See Table X) Appropriate r e g r e s s i o n equations, c o n s i s t i n g of four t o seven terms, were ta b u l a t e d w i t h t h e i r m u l t i p l e c o e f f i c i e n t of determination. (See Tables IX and XI.) I n the f o l l o w i n g d i s c u s s i o n , the above fo u r t a b l e s w i l l be r e f e r r e d t o e x c l u s i v e l y . As an a d d i t i o n a l r e s t r i c t i o n , o n l y the Sj_ = f (X^, Y k) type r e g r e s s i o n equations w i l l be considered. The simple c o r r e l a t i o n c o e f f i c i e n t s are not summarized i n t a b l e s , but w i l l be given i n the t e x t , when mentioned. The two veneer types have to be discussed s e p a r a t e l y w i t h i n each t e s t group, because the v a r i a b l e s and/or t h e i r rank d i f f e r s markedly. Modulus of e l a s t i c i t y i s defined as the r a t i o of u n i t s t r e s s to u n i t s t r a i n at or below the p r o p o r t i o n a l l i m i t . Consequently, v a r i a b l e s i n f l u e n c i n g s t r e s s and s t r a i n w i l l be considered before d i s c u s s i n g s t i f f n e s s i n some d e t a i l . (a) I n compression ( i ) Sawn-veneer blocks The most important 17 v a r i a b l e s s e l e c t e d accounted f o r 91.43 per cent of the v a r i a t i o n i n u n i t s t r e s s (Table X ) . Load recovery, as measured i n - 47 -the press, was r e s p o n s i b l e f o r 20.22 per cent of the v a r i a n c e . This i m p l i e s t h a t the r h e o l o g i c a l (flow) c h a r a c t e r i s t i c s of plywood p l a y an important r o l e i n determining i t s s t r e n g t h , even below the p r o p o r t i o n a l l i m i t . The second most important v a r i a b l e , compression at f u l l l o a d , u n d e r l i n e s t h e previous statement. These two f a c t o r s explained 36.28 per cent of the t o t a l v a r i a t i o n i n s t r e n g t h . They were f o l l o w e d by l e s s important v a r i a b l e s , such as growth r a t e ( r i n g s per i n c h ) , p e r m e a b i l i t y (number of days necessary t o reach an e q u i l i b r i u m moisture content), p l a s t i c deformation (permanent compression), and glue content of the plywood. Combined, they were r e s p o n s i b l e f o r 85.59 per cent of the v a r i a n c e . Most of the v a r i a t i o n i n u n i t s t r a i n , 94.28 per cent, was accounted f o r by t h e 17 independent v a r i a b l e s used. I t i s suggested t h a t s t r a i n v alues are much more s e n s i t i v e than s t r e s s t o r h e o l o g i c a l p r o p e r t i e s , s i n c e l o a d recovery alone was r e s p o n s i b l e f o r 47.89 per cent of the v a r i a n c e . An a d d i t i o n a l 29.75 per cent was gained by i n c l u d i n g veneer moisture content a t time of g l u i n g , veneer t h i c k n e s s (squared), weight l o s s i n pressing, and p e r m e a b i l i t y . The preponderance of these chemico-rheological f a c t o r s on s t r a i n i m p l i e s t h a t the veneers must have been c o n s i d e r a b l y m o d i f i e d ( p l a s t i c i z e d ) d u r i n g the p r e p a r a t i o n of plywood b l o c k s . The 17 most important v a r i a b l e s accounted f o r 94-30 per cent of the v a r i a t i o n i n the modulus of e l a s t i c i t y v alues. Of t h i s , 25.44 per cent may be a t t r i b u t e d t o veneer moisture content. The simplest assumption i s t h a t veneer moisture content increased p l a s t i c i t y , whereas a l a r g e r number of r i n g s per i n c h reduced i t . More l i k e l y , the i n f l u e n c e of the former was mainly due to i t s e f f e c t on r a t e of glue s e t t i n g , since d r i e r veneers take up water f a s t e r than wet ones. In c r e a s i n g veneer t h i c k n e s s a f f e c t e d - 48 -s t i f f n e s s i n the same manner as growth r a t e . Both in c r e a s e d the t o t a l amount of s t i f f e r latewood i n the p l i e s . The above three f a c t o r s e x p l a i n e d 65.70 per cent of the v a r i a n c e . Adding weight l o s s to the l i s t r a i s e d the percentage by almost 10 per cent. I t appears from the experiment that the l a r g e r the moisture r e t e n t i v e c a p a c i t y of the b l o c k s , the s t i f f e r they are. The exp l a n a t i o n i s proposed t h a t weight l o s s may be considered as a measure o f latewood p r o p o r t i o n , since moisture movement i s l e s s hindered i n earlywood than i n latewood of c o n i f e r s . The next independent v a r i a b l e i n importance, magnitude of f u l l com-pr e s s i o n , i s an obvious measure of plywood s t i f f n e s s . I t adds o n l y 1.75 per cent t o the variance, however. F i n a l l y , i t i s i n d i c a t e d t h a t t h e o r i e n t a t i o n o f latewood zones i s a l s o important i n determining modulus of e l a s t i c i t y . This r a i s e s the p r o p o r t i o n of the variance accounted f o r o n l y s l i g h t l y , t o 77.13 per cent. C u r i o u s l y , g l u i n g pressure was not among the s i x most important f a c t o r s a f f e c t i n g plywood s t i f f n e s s . The simple c o r r e l a t i o n c o e f f i c i e n t s between s t i f f n e s s , s t r e s s and s t r a i n are 0.29, -0.68, and 0.47, r e s p e c t i v e l y . Thus, in c r e a s e i n modulus of e l a s t i c i t y i s a s s o c i a t e d w i t h i n c r e a s i n g u n i t s t r e s s i n one t h i r d , and decreasing u n i t s t r a i n i n two-thirds of the cases. I n a d d i t i o n , an i n c r e a s i n g s t r a i n i s r e l a t e d t o i n c r e a s i n g s t r e s s i n about h a l f o f the specimens. These f i n d i n g s emphasize the r o l e of s t r a i n i n determining plywood s t r e n g t h p r o p e r t i e s normal t o g l u e l i n e . Deformation appeared to take place i n the s o f t , earlywood zones as expected. Thus, the experiment provided evidence f o r the c r i t i c a l r o l e of earlywood i n connection w i t h t h e s t r a i n f a i l u r e theory of plywoods (James, 1962). The c o r r e l a t i o n of s t r e n g t h p r o p e r t i e s and c o n t r o l l e d f a c t o r s as - 49 -i n d i c a t e d by R values i s t a b u l a t e d below: Modulus of e l a s t i c i t y U n i t s t r e s s U n i t s t r a i n Veneer t h i c k n e s s -0.22 0.07 0.33 Gluing pressure -0.16 0.34 0.32 P o s s i b l y , due to t h e i r l a r g e r i n f l u e n c e on s t r a i n , an i n c r e a s e i n the l e v e l o f c o n t r o l l e d f a c t o r s tends t o lower the modulus o f e l a s t i c i t y of sawn-veneer b l o c k s . The 17 v a r i a b l e s s e l e c t e d accounted f o r 95*43 per cent of the v a r i a t i o n i n u n i t s t r e s s (Table X I I ) . Of t h i s , 25-50 per cent was a t t r i b u t a b l e t o growth r a t e , i l l u s t r a t i n g t h a t above a l l t h e t o t a l amount of latewood i n the veneers i n f l u e n c e d s t r e n g t h . Plywood s p e c i f i c g r a v i t y ranked second i n importance. These two v a r i a b l e s e x p l a i n e d 36.45 per cent of the v a r i a n c e . Moisture content of veneers a t the time of g l u i n g ranked behind them, fo l l o w e d by l o n g i t u d i n a l g r a i n angle, plywood moisture content at the time of t e s t i n g , and th i c k n e s s of p l i e s . A t o t a l of 76.24 per cent of the v a r i a t i o n i n s t r e n g t h may be a t t r i b u t e d t o the independent f a c t o r s l i s t e d above. The pronounced e f f e c t of veneer v a r i a b l e s on plywood s t r e n g t h should be noted here. For u n i t s t r a i n , 93.24 per cent of the v a r i a n c e was explained by the 17 independent v a r i a b l e s . Plywood s p e c i f i c g r a v i t y (squared) alone was r e s p o n s i b l e f o r 77.56 per cent. Apparently, the s t r a i n r e s i s t a n c e of r o t a r y - c u t veneers when incorpo r a t e d i n t o plywood i s almost s o l e l y d e t e r -mined by t h e i r d e n s i t y . I t i s of i n t e r e s t t h a t s p e c i f i c g r a v i t y assumed ( i i ) Rotary-cut veneer blocks - 50 -i t s " t r a d i t i o n a l " importance f o r r o t a r y - c u t veneers, whereas i t s i n f l u e n c e on sawn ones was n e g l i g i b l e . I t s r o l e might be due t o the r e s t r i c t i v e i n f l u e n c e of latewood on l a t h e checks, or on compression damage to veneer surface ( C o l l i n s , 1960). Density and plywood moisture content were re s p o n s i b l e f o r 85.69 per cent of the v a r i a n c e . Thus, s t r a i n could be p r e d i c t e d from these two f a c t o r s . The i n c l u s i o n of glue content, l o a d recovery, growth r a t e and p e r m e a b i l i t y added o n l y 3 per cent t o the v a r i a n c e . The 17 most important v a r i a b l e s s e l e c t e d were r e s p o n s i b l e f o r 94.41 per cent of the t o t a l v a r i a t i o n i n the modulus of e l a s t i c i t y v a l u e s . Of t h i s , 38.03 per cent may be a t t r i b u t e d t o the glue content of plywoods. I t appears t h a t glue p e n e t r a t i o n i n l a t h e checks r e i n f o r c e d veneer surfaces, i . e . , r e s u l t e d i n a p o s s i b l y t h i c k e r boundary l a y e r . The s i g n i f i c a n c e of r a d i a l g r a i n o r i e n t a t i o n was i n d i c a t e d by the f a c t t h a t i t ranked second. L o g i c a l l y , i t i s f o l l o w e d by the (squared) latewood percentage due t o t h e i r c l o s e a s s o c i a t i o n . Plywood s t i f f n e s s i s bound to i n c r e a s e w i t h a l a r g e r p o r t i o n of s t r o n g latewood present, e s p e c i a l l y i f the growth r i n g s are o r i e n t e d n e a r l y perpendicular t o the g l u e l i n e s . C o n t r i b u t i o n of the above v a r i a b l e s amounted to 60.50 per cent of the v a r i a n c e . The s i g n i f i c a n c e of l o n g i t u d i n a l g r a i n o r i e n t a t i o n and roughness i s a l s o i n d i c a t e d . Their i n c l u s i o n b r i n g s the p o r t i o n of the v a r i a t i o n accounted f o r to 61.67 per cent. Veneer t h i c k n e s s and g l u i n g pressure seem to i n f l u e n c e plywood modulus of e l a s t i c i t y o n l y i n d i r e c t l y . F or the r o t a r y - c u t veneer blocks i n compression, the simple c o r r e l a - t i o n c o e f f i c i e n t s between s t i f f n e s s , s t r e s s , and s t r a i n were 0.45, -0.59, and 0.38. Thus, in c r e a s e i n s t r e s s r e s u l t e d i n higher s t i f f n e s s values i n almost h a l f , and a decrease i n s t r a i n i n more than h a l f of the specimens. - 51 -In addition, increasing strains were associated with higher stresses i n about one third of the cases. Again, the strain failure theory of wood seems to be i n evidence. The correlation of strength properties and controlled factors i s summarized below: Modulus of Unit Unit e l a s t i c i t y stress strain Veneer thickness -0.68 0.03 0.73 Gluing pressure 0.47 0.83 0.13 These coefficients indicate that increase i n veneer thickness causes a decrease i n modulus of e l a s t i c i t y because i t is associated with a large increase i n strain and a practically unchanged average stress. On the other hand, a larger gluing pressure increased strength considerably more than deformation. Consequently, the stiffness ratio rises with higher levels of the lat t e r factor. It should also be noted that gluing pressure seems to be a good measure of unit stress, and veneer thickness of unit strain, respectively. (b) In tension (i) Sawn-veneer blocks For wood failure percentage the 16 selected independent variables accounted fo r 48.93 per cent of the variance (Table X). Gluing pressure (squared) alone explained 25.34 per cent. Its importance must be inter-preted through i t s correlation with other factors. The conclusion may be drawn, however, that rheological phenomena play a dominant role i n deter-mining the ultimate strength of plywood. This i s underlined by the fact -52-t h a t the next most important v a r i a b l e s , namely, weight l o s s i n press, plywood s p e c i f i c g r a v i t y (squared), and number of days needed t o reach an e q u i l i b r i u m moisture content, are a l l i n f l u e n c e d by the manufacturing process. Together they were r e s p o n s i b l e f o r 44.66 per cent of the t o t a l v a r i a t i o n . The i n c l u s i o n of both squared and observed values of g l u i n g pressure among the s i x most important f a c t o r s , suggested the existence of an optimum pressure l e v e l . The low value of the c o e f f i c i e n t of determination suggests t h a t the a n a l y s i s f a i l e d t o i n c l u d e a number of important f a c t o r s i n f l u e n c i n g wood f a i l u r e . For i n s t a n c e , nothing i s known here of the d i s t r i b u t i o n of microscopic f a i l u r e o r s l i p planes and/or the p o s s i b l e s t r e s s concentrations r e s u l t i n g from the s m a l l bending moments i n t h e g r i p s , or the r o l e of r e s t r i c t e d s w e l l i n g i n press. Only 40.57 per cent of the t o t a l v a r i a t i o n i n u n i t s t r e s s was explained by the 16 v a r i a b l e s chosen. Apparently, the magnitude of permanent ( p l a s t i c ) deformation i s the most important s i n g l e f a c t o r , although i t i s r e s p o n s i b l e f o r o n l y 6.6l per cent of the v a r i a n c e . Combined w i t h the r e l a t e d g l u i n g pressure and veneer d e n s i f i c a t i o n , the three v a r i a b l e s account f o r 28.40 per cent. Plywood moisture content a t time of t e s t i n g and l o a d recovery add another 7 per cent. With the i n c l u s i o n of summerwood percnetage, 36.01 per cent of the variance can be accounted f o r . The importance and inadequacy of r h e o l o g i c a l c h a r a c t e r i s -t i c s i n e x p l a i n i n g s t r e n g t h properties- may be c a l l e d t o a t t e n t i o n . The 16 v a r i a b l e s s e l e c t e d were r e s p o n s i b l e f o r o n l y 41.73 per cent of the v a r i a t i o n i n u n i t s t r a i n . P l a s t i c deformation (squared) explained 8.46 per-cent of t h e v a r i a n c e . Combined w i t h g l u i n g pressure (squared), they accounted f o r 26.44 per cent. Next i n rank were p e r m e a b i l i t y and - 53 -the r e l a t e d weight l o s s i n press ( c u r i n g r a t e ) , plywood moisture content, and summerwood percentage. Their a d d i t i o n a l c o n t r i b u t i o n amounted to 10.38 per cent. Repeatedly, the r o l e of r h e o l o g i c a l f a c t o r s should be noted. Most of the v a r i a t i o n i n modulus of e l a s t i c i t y , 76.58 per cent, was accounted f o r by the 16 independent v a r i a b l e s chosen. For the r e g r e s s i o n a n a l y s i s i n t e n s i o n , the main f a c t o r s i n f l u e n c i n g s t i f f n e s s were s e l e c t e d . These were not n e c e s s a r i l y the most important v a r i a b l e s f o r the other s t r e n g t h p r o p e r t i e s . This might e x p l a i n the low c o e f f i c i e n t s of c o r r e l a t i o n encountered p r e v i o u s l y . S p e c i f i c g r a v i t y (squared) alone was r e s p o n s i b l e f o r 29.26 per cent of the v a r i a t i o n . S u r p r i s i n g l y , s t i f f n e s s was the o n l y s t r e n g t h property of sawn-veneer bloc k s dominated by plywood d e n s i t y . I t s i n f l u e n c e , however, was contrary t o the expected p a t t e r n . I n s p e c t i o n of data r e v e a l s t h a t 1/10-inch t h i c k veneer blocks possessed the lowest d e n s i t y and the highest modulus of e l a s t i c i t y values. The e f f e c t of other f a c t o r s , e.g., glue content, was such t h a t i t overrode the r e l a t i v e l y s m all i n f l u e n c e of d e n s i t y . This r e s u l t e d i n the m u l t i p l e r e g r e s s i o n a n a l y s i s p r e d i c t i n g i n c r e a s i n g s t i f f n e s s f o r decreasing s p e c i f i c g r a v i t y . Obviously, t h i s i s a spurious r e l a t i o n s h i p . An i n c r e ase i n the number of experimental u n i t s might have e l i m i n a t e d i t . The v a r i a b l e second i n rank, weight l o s s i n press, adds 10.86 per cent to the p o r t i o n of v a r i a n c e accounted f o r . I t may be considered as a measure of the r a t e of glue s o l i d i f i c a t i o n . S u r p r i s i n g l y , an increase i n c u r i n g r a t e reduces s t i f f n e s s . By adding lo a d recovery t o the r e g r e s -s i o n equation, 60.77 per cent of the v a r i a n c e may be explained. The l e s s a plywood b l o c k creeps under pressure the s t i f f e r i t i s . This f a c t o r - 54 -i n d i c a t e s the importance of r h e o l o g i c a l p r o p e r t i e s even under t h e p r o p o r t i o n a l l i m i t , where t h e i r r o l e i s assumed t o be n e g l i g i b l e i n short-time t e s t s . Slower growth r a t e reduces s t i f f n e s s , as i n d i c a t e d by the r e g r e s s i o n equation. T h i s accounts f o r another 5.66 per cent of the v a r i a t i o n . Permanent compression (squared) r a i s e s t h e percentage t o 72.68. I t seems l o g i c a l t h a t the amount of p l a s t i c deformation i s i n v e r s e l y p r o p o r t i o n a l t o the s t i f f n e s s of plywood. F i n a l l y , permanent compression brings the v a r i a t i o n accounted f o r t o 74.55 per cent. From the presence of the l a s t two f a c t o r s , the exis t e n c e of an optimum degree of p l a s t i c deformation may be deduced. Modulus of e l a s t i c i t y does not seem t o be d i r e c t l y i n f l u e n c e d by e i t h e r veneer t h i c k n e s s or g l u i n g pressure. For the sawn-veneer bloc k s i n t e n s i o n , the simple c o r r e l a t i o n  c o e f f i c i e n t s between modulus of e l a s t i c i t y , u n i t s t r e s s , and u n i t s t r a i n are 0.19, -0*11, and 0.95. Thus, an in c r e a s e i n s t i f f n e s s i s a s s o c i a t e d w i t h an increase of s t r e s s i n 19, and a decrease of s t r a i n i n 11 per cent of the cases. On the other hand, hig h s t r a i n values seem to be an a t t r i b u t e of strong specimens. The c o r r e l a t i o n o f wood f a i l u r e percentage to the above stre n g t h p r o p e r t i e s , i n t h e i r previous order, i s measured by the f o l l o w i n g R-values: 0.19, 0.29, and 0.26. Thus, t e n s i o n wood f a i l u r e i s an inadequate measure of s t i f f n e s s , s t r e s s , or s t r a i n . The c o r r e l a t i o n of s t r e n g t h p r o p e r t i e s and c o n t r o l l e d f a c t o r s i s summarized below: Modulus of U n i t U n i t Wood e l a s t i c i t y s t r e s s s t r a i n f a i l u r e Veneer t h i c k n e s s -0.46 0.16 0.29 -0.35 Gl u i n g pressure -0.13 0.10 0.15 0.18 - 55 -Inspection of these values c l a r i f i e s why the analyses of variance indicate veneer thickness as highly significant for a l l strength properties, compared to the non-significance of gluing pressure. Again, due to their larger influence on strain, the highest level of both factors tends to reduce the stiffness. It may be noted that thickness of plies i s apparently a better indicator of plywood stiffness than i s wood failure. Also, measurement of thickness i s non-destructive, whereas that of wood failure i s destructive. ( i i ) Rotary-cut veneer blocks The 16 independent variables selected account for 93.85 per cent of the variation i n wood failure percentages (Table XII). The most important single factor, veneer thickness (squared), explained 10.22 per cent of the variance. It appears that increasing veneer thickness results i n reducing wood failure percentages. Inclusion of veneer densification i n the regression equation raises the portion of variance accounted for to 47.44 per cent. The above variables are followed by roughness, plywood specific gravity (squared), permeability, and summerwood percentage. Their combined contribution to total variation amounts to 80.45 per cent. Wood failure appears to be influenced mainly by veneer variables, as expected. Most of the variation i n unit stress, 95.18 per cent, i s accounted for by the 16 variables selected. Apparently, veneer thickness (squared) i s the best single measure of plywood strength, since i t explains 55.83 per cent of the variance. It clearly indicates that veneer strength must be the limiting factor i n determining the magnitude of stress normal to glueline. The combination of veneer thickness with weight loss i n press i s responsible for 82.35 per cent of the variance. Inclusion of the factors of permeability, gluing pressure, plywood specific gravity (squared), - 56 -and p r o p o r t i o n of latewood, increased the p o r t i o n of variance accounted f o r t o 90.89 per cent. Most of the v a r i a t i o n i n u n i t s t r a i n , 81.12 per cent, may be accounted f o r by the 16 independent v a r i a b l e s . Veneer t h i c k n e s s (squared) alone explained 53.68 per cent of t h e v a r i a n c e . Coupled w i t h a measure of c u r i n g r a t e , they are r e s p o n s i b l e f o r 67.55 per cent. This percentage i s i n c r e a s e d to 74.77 per cent by the a d d i t i o n of g l u i n g pressure. The. i n c l u s i o n of plywood s p e c i f i c g r a v i t y (squared), veneer t i g h t n e s s , and percentage of latewood i n the r e g r e s s i o n equation r a i s e d the p o r t i o n o f the va r i a n c e accounted f o r t o 76.44 per cent. The 16 independent v a r i a b l e s accounted f o r 98.00 per cent, i . e . , almost a l l , of the v a r i a t i o n i n modulus o f e l a s t i c i t y . Again, veneer t h i c k n e s s appeared t o be the most important s i n g l e f a c t o r , being r e s p o n s i b l e f o r 55.87 per cent of the v a r i a n c e . This seems t o imply t h a t s t i f f n e s s i s mainly determined by veneer v a r i a b l e s . Combined w i t h plywood moisture content, they e x p l a i n 74.05 per cent of the v a r i a t i o n . As expected, i n c r e a s i n g moisture contents reduce modulus of e l a s t i c i t y by i n c r e a s i n g the p l a s t i c i t y of veneers. Plywood from t h i n veneers, through t h e i r higher glue and s l i g h t l y lower moisture contents, e x h i b i t more s t i f f n e s s than blocks of t h i c k p l i e s . I n c l u s i o n of plywood s p e c i f i c g r a v i t y explained only an a d d i t i o n a l 1.50 per cent of the v a r i a t i o n . I t s negative e f f e c t must be due t o the f a c t t h a t specimens of the lowest d e n s i t y e x h i b i t the highest s t r e n g t h and s t i f f n e s s . This r e s u l t s from the combined i n f l u e n c e of a multi t u d e of f a c t o r s , whose net e f f e c t o v e r r i d e s t h a t of s p e c i f i c g r a v i t y alone. The v a r i a b l e next i n rank, g l u i n g pressure, c o n t r i b u t e d 16.41 per cent t o the v a r i a n c e . An increase i n pressure seemed to r e s u l t - 57 -i n higher s t i f f n e s s , as would be expected. Since higher pressures are accompanied by l a r g e r f u l l compression, an increase i n the l a t t e r must r e s u l t i n higher modulus of e l a s t i c i t y a l s o . L a s t l y , i t i s i n d i c a t e d by the m u l t i p l e r e g r e s s i o n equation t h a t a decrease i n summerwood percentage reduced s t i f f n e s s . The combined e f f e c t of the above independent v a r i a b l e s accounted f o r 93.99 per cent of the v a r i a n c e . Repeatedly f o r t e n s i o n specimens, both c o n t r o l l e d f a c t o r s are i n c l u d e d among the f o u r most s i g n i -f i c a n t v a r i a b l e s . For the r o t a r y - c u t veneer blocks i n t e n s i o n , the simple c o r r e l a t i o n  c o e f f i c i e n t s are 0.93, 0.76, and 0.91. These f i g u r e s suggest t h a t an i n c r e a s e i n s t i f f n e s s i s almost always a s s o c i a t e d w i t h higher s t r e n g t h , and l e s s r e g u l a r l y , w i t h l a r g e r deformation. Furthermore, hi g h s t r a i n values seem t o be an a t t r i b u t e of strong specimens. The a s s o c i a t i o n of wood f a i l u r e percentage w i t h the above s t r e n g t h p r o p e r t i e s , i n t h e i r previous order, i s estimated by the f o l l o w i n g c o r r e l a t i o n c o e f f i c i e n t s : 0.56, 0.53, and 0.54. This suggests t h a t an increase i n s t r e n g t h p r o p e r t i e s induces a correspondingly higher wood f a i l u r e i n more than h a l f o f the specimens. Thus, wood f a i l u r e percentage might be considered as a rough i n d i c a t o r of plywood s t r e n g t h p r o p e r t i e s , f o r the r o t a r y - c u t veneer b l o c k s . The c o r r e l a t i o n of s t r e n g t h p r o p e r t i e s and c o n t r o l l e d f a c t o r s are as f o l l o w s : Modulus of U n i t U n i t Wood e l a s t i c i t y s t r e s s s t r a i n f a i l u r e Veneer t h i c k n e s s -0.75 -0.74 -0.73 -0.32 G l u i n g pressure 0.29 0.36 0.41 0.60 - 58 -These values i l l u s t r a t e t h e dominant r o l e of veneer t h i c k n e s s . Thick-ness i n d i c a t e s a t r e n d i n three-quarters of the cases observed, thus might be accepted as the simp l e s t and a reasonably c o n s i s t e n t plywood s t r e n g t h i n d i c a -t o r , f o r t h e experimental data a t l e a s t . The l e s s e r , but s t i l l h i g h l y s i g n i f i c a n t , a s s o c i a t i o n of g l u i n g pressure and s t r e n g t h p r o p e r t i e s i s shown. I t i s odd t h a t an i n c r e a s e i n thickness•tends t o reduce wood f a i l u r e , whereas an i n c r e a s e i n pressure tends to augment i t . This might be a spurious r e l a t i o n s h i p , or the r e s u l t of s t r e s s r e v e r s a l on veneer behavior, o r a phenomenon a s s o c i a t e d w i t h glue p e n e t r a t i o n and i t s e f f e c t s . (c) I n shear ( i ) Sawn-veneer blocks For wood f a i l u r e percentage, 68.93 per cent of the v a r i a n c e was explained by the 18 f a c t o r s chosen (Table X). Summerwood percentage (squared) alone was r e s p o n s i b l e f o r 33.95 per cent of the v a r i a t i o n . I n c r e a s i n g l a t e -wood proport i o n s seemed t o reduce wood f a i l u r e as expected. Coupled w i t h veneer t h i c k n e s s , i t accounted f o r 40.81 per cent. A d d i t i o n of g l u i n g pressure and f u l l compression r a i s e d t h i s to 48.17 per cent. The next two v a r i a b l e s , namely, veneer moisture content and l o a d recovery c o n t r i b u t e d o n l y 0.44 per cent to the v a r i a n c e . The 18 independent v a r i a b l e s accounted f o r o n l y 56.67 per cent of the t o t a l v a r i a t i o n i n shear s t r e s s . L o n g i t u d i n a l g r a i n o r i e n t a t i o n appeared to be the most important s i n g l e f a c t o r , e x p l a i n i n g 11.99 per cent -of the v a r i a n c e . A steeper angle increased the number of strong latewood zones t o be sheared, r e s u l t i n g i n higher s t r e s s or f a i l u r e . A decrease i n t h i c k n e s s , perhaps through i t s r e l a t i o n w i t h glue content and d e n s i t y , i n c r e a s e d shear r e s i s t a n c e . These two v a r i a b l e s are r e s p o n s i b l e f o r 23.38 per cent of the v a r i a n c e . Veneer s p e c i f i c g r a v i t y , as expected, i s important, b r i n g i n g the p o r t i o n of v a r i a n c e accounted f o r t o 33.95 per cent. Decreasing plywood moisture content increased r e s i s t a n c e t o shear s t r e s s e s , and added another 6.75 per cent to the v a r i a n c e . The r o l e of r a d i a l g r a i n angle was s i m i l a r t o t h a t of l o n g i t u d i n a l g r a i n o r i e n t a t i o n , i t s c o n t r i b u -t i o n amounting t o 8.13 per cent. F u l l compression, perhaps through i t s a s s o c i a t i o n w i t h i n c r e a s e i n d e n s i t y , i n f l u e n c e d shear s t r e s s p o s i t i v e l y . These f a c t o r s accounted f o r 49.37 per cent of the v a r i a n c e . G l u i n g pressure was not among the s i x most important v a r i a b l e s . 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 i s 0.54. Thus, an increase i n s t r e s s i s a s s o c i a t e d w i t h higher wood f a i l u r e values i n s l i g h t l y more than h a l f of the cases. Therefore, wood f a i l u r e may be considered o n l y as a v e r y rough e s t i m a t i o n of shear s t r e s s . Degree of c o r r e l a t i o n between strength p r o p e r t i e s and c o n t r o l l e d f a c t o r s i s i n d i c a t e d below: Unit Wood s t r e s s f a i l u r e Veneer thi c k n e s s -0.30 -0.43 Glu i n g pressure 0.18 0.35 I t seems c o n t r a d i c t o r y t h a t i n c r e a s e i n veneer thickness reduces wood f a i l u r e , while i n c r e a s i n g the p r o p o r t i o n of wood i n t h e b l o c k . This may be a t t r i b u t e d to the net e f f e c t o f f a c t o r s a s s o c i a t e d w i t h veneer t h i c k n e s s . - 60 -( i i ) Rotary-cut veneer blocks Most of the v a r i a t i o n i n wood f a i l u r e percentage, 94-01 per cent (Table X I I ) , could be accounted f o r by the 18 independent v a r i a b l e s . Veneer moisture content alone explained 27.90 per cent and, coupled w i t h glue content, they were r e s p o n s i b l e f o r 39.66 per cent of the variance. Higher' values of both increased the magnitude of the dependent v a r i a b l e s . The other v a r i a b l e s are veneer d e n s i f i c a t i o n (squared), permanent compression (squared), l a t h e check depth and veneer roughness, i n order of importance. Together, they accounted f o r 78.70 per cent of the v a r i a n c e . The 18 independent v a r i a b l e s were r e s p o n s i b l e f o r 87.61 per cent of the v a r i a t i o n i n shear s t r e s s . Of t h i s , 45.39 per cent was explained by veneer moisture content alone. As a general r u l e , i n c r e a s i n g moisture content reduces s t r e n g t h . I t i s known f o r shear s t r e s s , however, that i t reaches an optimum when glued i n the moisture content range of 8 t o 12 per cent ; according t o Lewis and co-workers (1945)• The experimental range was o n l y 7.3 to 9.3 per cent. Thus, an i n c r e a s e i n moisture should improve shear s t r e n g t h , t o conform with the expected p a t t e r n . Larger g l u i n g pressures, probably by f a c i l i t a t i n g glue p e n e t r a t i o n , r e s u l t i n higher s t r e s s v a l u e s . T h e i r c o n t r i b u t i o n t o the v a r i a n c e i s a f u r t h e r 13.18 per cent. I n c l u s i o n of the v a r i a b l e next i n rank, i . e . , glue content, increased the p o r t i o n of t h e v a r i a b l e accounted f o r t o 64.19 per cent. Apparently, the more glue a plywood contained, the stronger i t was i n shear The t o t a l c o n t r i b u t i o n of r a d i a l g r a i n o r i e n t a t i o n and roughness of veneers was o n l y 4-07 per cent. From the r e g r e s s i o n equation i t may be deduced t h a t an i n c r e a s e i n latewood surfaces bonded and rougher veneers, improve shear s t r e n g t h . F i n a l l y , l a t h e check depth was shown to be a d e t r i m e n t a l - 61 -f a c t o r . The above v a r i a b l e s accounted f o r 71.51 per cent of the va r i a n c e . Veneer thickness was not one of them. 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 i s 0.76. I n approximately three-quarters of the specimens, higher wood f a i l u r e s c o i n c i d e d w i t h higher s t r e n g t h . This seems t o j u s t i f y the use of percentage wood f a i l u r e as a simple and f a i r l y c o n s i s t e n t measure of glue bond q u a l i t y , f o r r o t a r y - c u t veneers. C o r r e l a t i o n o f s t r e n g t h p r o p e r t i e s and c o n t r o l l e d f a c t o r s i s t a b u l a t e d below: U n i t s t r e s s Wood f a i l u r e Veneer t h i c k n e s s -0.36 -0.41 Gluing pressure (squared) 0.67 0.51 The i n d i c a t e d r e d u c t i o n i n wood f a i l u r e , accompanied by i n c r e a s i n g veneer t h i c k n e s s e s , i s c o n t r a r y to the expected t r e n d . I t may be a t t r i b u t e d t o the net e f f e c t of f a c t o r s a s s o c i a t e d w i t h veneer t h i c k n e s s . - 62 -CONCLUSIONS (1) The most important s i n g l e f a c t o r i n f l u e n c i n g a l l plywood s t r e n g t h p r o p e r t i e s normal to g l u e l i n e , and standard plywood glue shear t e s t , was veneer type. The dominant r o l e of t h i s f a c t o r i s a t t r i b u t e d t o the e f f e c t of l a t h e checks and surface q u a l i t y . Both appear t o be determined mainly by the techniques of veneer p r e p a r a t i o n which a l t e r the mechanical and chemical ( s u r f a c e r e a c t i v i t y ) p r o p e r t i e s of wood. A l s o , both are h i g h l y c o r r e l a t e d w i t h veneer t h i c k n e s s . (2) The analyses o f variance performed to evaluate the i n f l u e n c e of veneer t h i c k n e s s and g l u i n g pressure, u s i n g both observed and adjusted s t r e n g t h v a l u e s , provided almost i d e n t i c a l r e s u l t s . Apparently, the i n f l u e n c e of c o n t r o l l e d f a c t o r s was independently superimposed upon the e f f e c t of concomitant v a r i a b l e s . Removal of the l a t t e r a l t e r e d (increased) the absolute value of s t r e n g t h p r o p e r t i e s , but ha r d l y changed the p a t t e r n of response. (3) Veneer t h i c k n e s s a f f e c t e d a l l plywood s t r e n g t h p r o p e r t i e s normal t o g l u e l i n e of both veneer types h i g h l y s i g n i f i c a n t l y . The importance o f veneer t h i c k n e s s may be a t t r i b u t e d to i t s h i g h c o r r e l a t i o n w i t h a number of independent v a r i a b l e s . These i n c l u d e glue content, number of p l i e s , plywood s p e c i f i c g r a v i t y , and p l a s t i c deformation. (4) Both the compressive and t e n s i l e s t r e n g t h p r o p e r t i e s of r o t a r y - c u t veneer, and the former only of sawn-veneer b l o c k s , were h i g h l y s i g n i f i c a n t l y i n f l u e n c e d by g l u i n g pressure. This f a c t o r a f f e c t e d r o l l i n g shear str e n g t h s i m i l a r l y . The r o l e of g l u i n g pressure may r e s u l t from i t s c l o s e a s s o c i a t i o n w i t h other independent v a r i a b l e s , such as compression a t f u l l l o a d , p l a s t i c deformation, and veneer d e n s i f i c a t i o n . (5) The i n t e r a c t i o n of veneer thi c k n e s s and g l u i n g pressure was a l s o h i g h l y s i g n i f i c a n t f o r a l l plywood s t r e n g t h p r o p e r t i e s w i t h the exception of t e n s i o n s t r a i n and wood f a i l u r e . This may be a t t r i b u t e d l a r g e l y to the p e c u l i a r s t r e n g t h p a t t e r n observed on blocks of 1/5-inch t h i c k veneers, p a r t i c u l a r l y the r o t a r y - c u t ones. (6) I n a d d i t i o n t o the th r e e c o n t r o l l e d f a c t o r s , i . e . , veneer type, veneer t h i c k n e s s , and g l u i n g pressure, the f o l l o w i n g independent v a r i a b l e s appeared t o i n f l u e n c e v a r i a t i o n i n plywood stre n g t h p r o p e r t i e s most. Sawn-veneer b l o c k s : Veneer moisture content Summerwood percentage Growth r a t e L o n g i t u d i n a l g r a i n angle Veneer d e n s i f i c a t i o n P e r m e a b i l i t y Rate of cure (weight l o s s ) F u l l compression Load recovery i n press P l a s t i c deformation Plywood s p e c i f i c g r a v i t y Rotary-cut veneer b l o c k s ; Veneer moisture content Summerwood percentage Growth r a t e R a d i a l g r a i n angle Tightness of cut Veneer d e n s i f i c a t i o n P e r m e a b i l i t y Rate of cure (weight l o s s ) Glue content Plywood moisture content Plywood s p e c i f i c g r a v i t y The use of veneer v a r i a b l e s alone accounted f o r t h e l e a s t , but a q u i t e s u b s t a n t i a l part of the v a r i a t i o n i n plywood s t r e n g t h p r o p e r t i e s . Plywood - 6 4 -v a r i a b l e s u s u a l l y explained more. A combination of the most important f a c t o r s accounted f o r almost a l l of the v a r i a n c e , p a r t i c u l a r l y f o r r o t a r y -cut veneer b l o c k s . (7) The average compressive strengths of sawn and r o t a r y - c u t veneer blo c k s were p r a c t i c a l l y the same. They amounted to approximately h a l f t h a t f o r s o l i d Douglas f i r wood. Apparently, compression str e n g t h normal t o g l u e l i n e was h a r d l y i n f l u e n c e d by the l a t h e checks. The d i f f e r e n c e between plywood and s o l i d wood may then be a t t r i b u t e d t o d i f f e r e n t specimen geometries, moisture contents, suspected a c i d h y d r o l y s i s at the g l u e l i n e s , and/or other causes. (8) The average t e n s i l e s t r e n g t h of sawn-veneer b l o c k s was h a l f that f o r s o l i d wood, and exceeded t h a t of r o t a r y - c u t veneer blocks 3 .5 times. The l a r g e d i f f e r e n c e s i n t e n s i l e s t r e n g t h values of sawn and r o t a r y -cut veneer blocks must r e s u l t from weakened (mechanically and chemically) surface f i b e r s , since bond q u a l i t y was acceptable as i n d i c a t e d by the h i g h wood f a i l u r e percentages. Thus, t e n s i o n normal to g l u e l i n e seems to be the best measure of bond q u a l i t y . (9) The average r o l l i n g shear s t r e s s of sawn-veneer exceeded t h a t of r o t a r y - c u t veneer blocks by about 1 .5 . I t i s suggested t h a t the r e d u c t i o n i n r o l l i n g shear r e s i s t a n c e a t t r i b u t a b l e t o the mechanical e f f e c t of l a t h e checks was comparatively s m a l l and/or p a r t l y countered by the adhesive penetrating i n t o them. The l a t t e r was more pronounced f o r blocks prepared under hig h g l u i n g pressure. I t was a l s o found that the standard plywood glue shear t e s t may be accepted as a rough i n d i c a t o r of plywood str e n g t h p r o p e r t i e s (normal to g l u e l i n e ) f o r r o t a r y - c u t , but not f o r sawn veneers. - 65 -(10) The moduli of e l a s t i c i t y of sawn-veneer exceeded those of r o t a r y - c u t veneer blocks by approximately two times. Plywoods of r o t a r y - c u t veneers deformed h a l f as much i n compression and twice as much i n t e n s i o n as those of sawn veneers, before reaching t h e i r r e s p e c t i v e p r o p o r t i o n a l l i m i t s . (11) Under the experimental c o n d i t i o n s , 1/7 i n c h , c l o s e l y f o l lowed by 1/10 i n c h , and 200 p s i appeared t o be the optimum l e v e l s of veneer t h i c k n e s s and g l u i n g pressure, r e s p e c t i v e l y . Consequently, the strongest (normal t o g l u e l i n e ) Douglas f i r plywood panels, glued w i t h the room-temperature s e t t i n g Duro-Lok 50 would be obtained by u s i n g veneer thicknesses of 1/7 or 1/10 i n c h a t 200 p s i g l u i n g pressure. R o l l i n g shear s t r e n g t h could be improved by employing higher g l u i n g pressures. The need f o r p r e c i s e l y c o n t r o l l e d manufacturing processes, t h a t would r e s u l t i n improved plywood s t r e n g t h values, i s i m p l i e d by the dominant r o l e of the techniques of prep a r a t i o n i n t h i s experiment, as i n d i c a t e d by t h e i r best s i n g l e measure: veneer type. - 66 -. BIBLIOGRAPHY American Marietta Company, 1960. National economic trends of significance to forest products industries. Special report. The Lumberman 87 (7): 117-176. American Society for Testing Materials, 1961. Testing veneer, plywood and other glued construction. 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The e f f e c t of veneer t h i c k n e s s and g r a i n d i r e c t i o n on the shear s t r e n g t h of plywood. U.S.D.A., Forest Prod. Lab. Rept. No. 1801, Madison, W i s e , pp. 40. Osherovich, L . J . , 1955. T e n s i l e t e s t s on timber perpendicular t o g r a i n . Trans. No. 3128, C.S.I.R.O., A u s t r a l i a pp. 3. Pentoney, R.E. and R.W. Davidson, 1962. Rheology and the study of wood. Forest Prod. J . 12 (4): 243-248. P e r k i n s , N.S., 1962. Plywood p r o p e r t i e s , design and c o n s t r u c t i o n . Douglas F i r Plywood Assoc., Tacoma, Wash. pp. 132. P e r k i t n y , T. and L. H e l i n s k a , 1961. Uber den E i n f l u s s g l o i c h z e i t i g e r Temperatur - und Feuchtigkcitsanderung auf d i e Verformungen des Holzes. Holz a l s o Roh - und Werkstoff 19 (7): 259-269. - 72 -Perry, T.D., 1942. Modern plywood. Pitman P u b l . Corp,, New York pp. 366. Plywood Manufacturers A s s o c i a t i o n of B r i t i s h Columbia, 1958. 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Zahner, R., 1963- I n t e r n a l moisture s t r e s s and wood for m a t i o n i n c o n i f e r s . Forest Prod. J . 13 (6): 240-247-laworsky, J.M., J.H. Cunningham and N.Y. Hindley, 1955- Survey of f a c t o r s a f f e c t i n g s t r e n g t h t e s t s of glue j o i n t s . F orest Prod. J . 5 (5): 306-311. - 74 -TABLES AND FIGURES - 75 -TABLE I . AVERAGE STRENGTH VALUES OF SMALL CLEAR SPECIMENS OF COAST-TYPE DOUGLAS FIRWOOD IN AIR-DRY CONDITION Canada U.S. S p e c i f i c g r a v i t y ( b a s i c ) : 0.49 0.48 Volumetric shrinkage ($) 5-3 Stre s s e s p a r a l l e l to g r a i n : S t a t i c bending (tension) S t r e s s at p r o p o r t i o n a l l i m i t ( p s i ) .. 7,700 7,800 Modulus o f e l a s t i c i t y (lOOO p s i ) .. .. 1,980 1,950 Compression S t r e s s a t p r o p o r t i o n a l l i m i t ( p s i ) .. 4,830 5>850 Modulus of e l a s t i c i t y (1000 p s i ) .. .. 1,950 1,950 Stre s s e s p e r p e n d i c u l a r to g r a i n : Tension: maximum s t r e s s R a d i a l plane ( p s i ) 420 340 Tangential plane ( p s i ) 470 Compression S t r e s s a t p r o p o r t i o n a l l i m i t ( p s i ) .. 860 870 Cleavage: R a d i a l plane ( l b / i n . ) 1,430 Tangential plane ( l b / i n . ) 1,370 Rotary-cut veneers Sawn veneers Type fc. Block No. < < Qto' K-J h-'- 'H- J f—' I—1 I—' I—4 I—J 1—' 0>3 (D 00 -O .. ON vj\ -P" AO N> O (D vo CD ON vj\ -P" v j ro I—• |—> t—< I—1 l—• I—• I—• I—• I—• h - ' I V > r v > W H J l - ' l - ' h - ' l - ' l - J ^-s. H-- p - v o v o v o - p - - p - - p - 0 0 0 O N I - J O t - ' - p - - p - . p - O O I - ' ! x ! H - O I-1 M 1 1 t—1 i—' M -1 V) ro ro 1—' I-1 M 1—1 VO -p- P" -p- o O o v M  t—• -p- •P- •P- o O 1—' 00 ro VjJ VO VJ\ N> CO o OV ON l- 1 1 -P-•P" -0 Ov co -P" ro M (—1 I-* h-1 ON ON VJ\ O 1—' VO • r o CO CQ row ^ o ^ a ^ o ^ - J v J J ^ 3 v J ^ l - ' l - l ^ - , r o M v j J H ' i - ' H - ' M v j r o v J X H F -P CO s; co U ^ J r ^ f - p U V J U U V j J v J v J V j J W v j J V j j r O V j J v j J o fi S O N ^ N H N ^ V O S I O v J V j v O - O v j O V j J V J V O M r O l x ! O H p- Pi 3 v o - 0 0 - ^ J C O C O U \ ^ a O N I - , H O N I ^ 3 H ^ O \ 0 \ 0 \ V O W 4 I 0*3 CO H i-a p U \ U \ v j i p P P u > P P ^ - 4 r 4 r v j \ ^ p - - p - ^ - P " P ^ - p - X Pro V O ^ ) M C 0 - < | V O V J \ O V J I V J \ C N O o r o i - J 0 \ v j \ 0 \ - P " - p - - P : 0 \ < ! o O N M - O - p - ^ l v O v j r O v J O O O v v o C O r O v j i - p - O H ' O N H - H -O s & o o C O C O O O C O V O C O C O C O C O C O C O C O C O V O C O C O C O C O - x l C O j x i £S F -. . . . . . . . . . ON<rt-C0 - v 3 C O O O C T v O O O O \ v o r O O O P O \ O U U i U \ 0 \ P U P <DC+ co VJ o V J \ co o co -<1 CO v_0 v_n CO co co -P-CO o a> o -p- OO co o o U i U P w ON Ov Ov ON CO O VjJ VJV o O U -P" VJ Ov U% ON Cfv CO VO IV) vo o VJ -v] M C\ P v>> IV3 o X CO o ro .o 13" ro a FT vo O NO VO V J V b o b oo v j O H 00 VJO U\ V O b 00 H-1 -p- o t—1 VjJ VO 00 o b ON ro vo vj\ ro ON vo VJO -vj t-i VjJ P ON u o h-1 t-1 VjJ VjO ON b 1-J O Pi £» P 3 o hi tr" H -o ro F - P S H -p, O • p V o J v / v v J v J v J V j J v J v J v J - p - *3j . . . . . . . . . . JxJIlJO C O I - ' ^ C O O V - O I - ' V J V O O i—> i—> i—> i—' i—• i—• i—1 i—• i—' i—» i—• roC ro co at} cn ts* I r o r o r o r o r o v j r o r o r o r o i-3 £ - O O O N C o v j J o r o o v n r o r o r o r o r o r o r o r o r o r o i - ' rootj vjj co CQ c+ I - 91 ~ - 77 -TABLE I I I . MANTJFACTURER'S SPECIFICATIONS FOR THE  MODIFIED POLYVINYL ADHESIVE, DURO-LOK 50 A p p l i c a t i o n Brush, r o l l e r o r glue-spreader; g e n e r a l l y any method except spray. One-side a p p l i c a t i o n normally acceptable. Coverage Smooth s u r f a c e s : 25-35 l b M f t . 2 per s i n g l e glue l i n e . Screen-backs, foams or absorbent m a t e r i a l s : 40-55 ID. Temperature Over 65°F at glue l i n e s , 75-85°F p r e f e r r e d . Open assembly 5 min. Closed assembly 30 min. Press time 30 min. Machining time 1 hr a f t e r press c y c l e . Strength b u i l d - u p Wood to wood: over 400 p s i immediately from press, over 500 p s i one hour a f t e r press. Clean up 80-90°F water, before glue i s f u l l y dry. Hot water w i l l s o f t e n f i l m s u f f i c i e n t l y to scrape o f f d u r i n g next 24 hours. Cure time • .. B o i l i n g water r e s i s t a n c e i s developed very r a p i d l y and meets s p e c i f i c a t i o n s i n 24 hours a t ?0°F. Cold water r e s i s t a n c e i s developed approximately t o 50$ of f i n a l s t r e n g t h i n 24 hours a t 70°F, but i s s t i l l i n c r e a s -i n g a f t e r 30 days. Complete cure i s achieved i n approximately 3 minutes at 300°F. Rotary-cut veneers <J M M CRJ 03 S3 CD 0\ Uv ^ U N M O CD Sawn veneers N o o o s 3 0 N N A - P ~ N j r o Type w >2! M O O !N> VJO O V A o o N A N A o o o o NvJ NA NA o o ro O o NA o ro NJ o V A ro o o VjJ V A V A ro o o vo V A V A O O ro o o V A o CO CD H' • CO 00 -o 00 ro o\ S J •p- CO W M vo O N S3 Co u si u> ,vo ro o r o - f r - f ^ o r o - P O O N r o c o ro O S 3 o vo oo CD ro o tr> as t-< o ro £» OS CD pi V0 VJ ^ r ^ ^ r r o r o r j o o v j j c o . M SI ro ro M u u u .p- . .p- .p- ^ S 3 1 tr1 o t?d M • H- M CD o CO • .p-.p--p-.p-.p-.p--p-.p--p-V A V A V A S3 ON V A S3 O u u -P -p--p-ro vo V A S3 co O oo ro M oo ro ro ; P " NO • p - - p - - p - V A - p - - p - - p - - p -VjO V0 -p- -p-S ] N O N O S3 N O 00 O N V A V O O Vn M NO oo ro -p- -p-O N O N O N SI S3 -p" -P" V A N O O 00 V n 0 0 - p - - P " C A V A - p - S 3 . p - - p - V j r 0 M M V 0 r 0 N O O N O N M O N I - J - p - 0 0 0 3 0 N N O O O O N S 3 0 S 3 M M r o M -p-si - P " Vn vo sj vo N O -p-vo O N vo V A oo co H -p- vo V A V A V A M r o s i | r ^ H N W U \ N C A V J > - p - H H H to M VA FO M K! IN) VA M 03 VA O N O M S3 ON N O V A M NO tN3 NO S3 V0 V0 VA ON N O ^ N A S 3 - p " N o v ^ N j r o r o N o r o v A N A O N t - ' s i o o c x ) 0 ro M V A M M M M r o r o r o o r v > v o M - p - . p - O N V j j - p - r o v J J V A t - - 3 V j J O \ S 3 V J M C D V j O D O O N O C D . p - N O O \ r O O V A O O O S 3 C X > C 0 N O C D S 3 O N 0 0 N O S 3 r o O 3 O 3 S ] 0 0 O 0 0 0 C D N O C D C 0 r o o r o v j V A C o v o v o s i v o v A ro' N O S3 oo vo ro -p- O N V A oo f f f 4r f m -p-ON -p-NO N A N A -P" -P" -P" V A V A O N V A NO 1 * ? » —-- c+-.Hj O H CD V CO CD CO B O c+ 0 ^M CD .^O H-—'CO Ot) CO & ct-ct- fO o CO 1=1 M o ro t H 0\ U C\ N -p-IN) O\CQS3VAS3O\-p-O0O\ONHrf pj 0 • M CO v j M C X > r o v j o o O D r v ) N o - - o c » r o N o c x > - p - - p - s i o c ^ M O — ' H - H -O H, I o o -p- vo v>> V J -P~ - P - V A V A o\ o u ro ro ro -p- vo -p- V A V A £j <^ P_ Q -o.p-.p--p-s3VJOVAOoroNOM-p-s3toNoroovoNOM ^ro £ P CD c+ H 013 H - 3 (-'(-' M 'MM I—1 I—* I—• M M M M M H 3 O N O N A O O N M S J O N O O S l M O O O O r o O N O N j J r O M i - d y-to H M CQ CD V A M - P " 0 0 N A -p-ro ro -P "N*> o V A N A S3 M co u O N N A -p-ro ^^••^ jo c+* CD CO «<! O CD • H W ^ O N N A O N V ^ V A ^ N A N A - p - N A N A 4 ^ V J \ N A N A N A N A - p - - P " l - d PCD - p - M S 3 r 0 N j 0 N O N A O C 0 M N j 0 S 1 0 N M O O O N 0 N O M <0 v O M M 4 ^ r O N j O N j O N a ) M S 3 - p - N j O O - p - S 3 0 0 N S 3 V j J H-H-C+- H> o o I o 1=) 65 s o bd o o - ez -TABLE V. CHANGES IN GLUING PRESSURE WITH TIME Block No. Nominal load ( l b ) 0 Min. 5 Min. A c t u a l 10 Min. load (fo) a t 15 Min. 20 Min. 30 Min. Load Recov. (*) Veneer M.C. (<fo 1 1250 100 70.63 97.22 100.80 104.00 112.00 26.59 8.4 2 5000 100 83.20 94.00 98.40 98.80 100.80 10.80 7-3 03 3 8750 100 80.00 87.31 90.7^ 91.88 88.91 7.31 8.4 u o 4 1250 100 73.20 93.20 100.00 104.80 113.20 20.00 8.6 n a> »-> 5 5000 100 82.40 95-20 98.80 100.00 104.80 12.80 8.5 P i 6 8750 100 83.^2 92.57 94.40 95.5^ 92.80 9.15 8.5 CO 7 1250 .100 65.20 84.80 96.00 100.00 104.00 19.60 9.3 8 5000 100 88.20 93.20 97.20 98.20 97.20 5.00 8.9 9 8750 100 88.11 95.5^ 97.83 98.97 97.37 7.^3 8.0 Avge. 79.37 92.56 97.13 99-13 101.23 13.20 8.4 10 1250 100 76.OO 84.00 86.00 92.00 82.00 8.00 8.8 CQ 11 5000 100 77.60 85.00 90.80 92.80 88.00 7.40 8.2 u 12 8750 100 76.80 83.20 91.20 91.^3 85.03 6.40 8.9 S t> 13 1250 100 68.80 82.00 90.00 96.80 94.00 13.20 8.6 -P 14 5000 100 75.00 86.00 92.00 83.00 88.00 11.00 8.8 tary-c 15 8750 100 73. I** 80.45 86.86 88.46 81.37 7.31 9.0 tary-c 16 1250 100 77.60 84.40 90.00 95-20 86.00 6.80 8.6 o 17 5000 100 7^.80 87.20 93.20 94.40 89-00 12.40 8.8 18 8750 100 81.37 89.Ik 92.80 94.40 90.60 7-77 8.8 Avge. 75.68 84.60 90.31 93.16 87-11 8.90 8.7 Rotary-cut veneers Sawn veneers Type CD ro ,_ t d W I — > 1 — ' I — » l — ' I — ' l — ' I — • H - ' H - ' H feil-' p ( J ) \ I O N V J i P U I O H O ! l l V O C O - O O N v j i - f r V j O r o h - 1 O O OO . Ct} • O CD <D !V —v H CO U N U N U M V J M U N V J O N ) » ( D H v A O y > u \ O y i v n O y v v n O v/v V J \ O V A V J I o V J I r. w $ O O O O O O O O O O O O O O O O O O S2.C0 H-~$rl CD -p- -p- -p- -p- -p-- O -<1 O N C O V J \ O N -<] r o .p- r o CD O ^ P v o is- H -p- •P- VJ\ -P- VJl •P- V J N VJl VJl -P- -p- •P" •p- -P- -P- > CD H* H- H-o \ O v Vn M C O O O O O N - o O N VO C O 00 PS C R } d -V J O VO V J I -P" O v v o M VO r o r o t o • 3* H* -O - 0 - ^ 3 VjO -P- VJl V J J ON c o VJl t-1 v o VJ\ VA v—d- al I—• 1—" 1—« I—• I—• i—'. I — • I—' I—' O O O O O O O O O O O O O O O O O O h - ' h - ' l - ' f - ' M I - ' M h - ' l - ' O O O O O O O O O O O O O O O O O O 3 H-d -H-p 1 td VO -p-VjO vo v o v o v o v o v o v o v o v o Vji vji VjO -P - V J I r o V J I V J I vo vo VO VO vo vo ON -0 CO CO ON -N3 v o O vo vo V J O V O r o o 0 3 V A v*) -o I—' I—1 VjO O v -O O vo vo p -vi m V J O r o r o v o r o v o r o o o O N - o c o C o u i U l U l VJl \ o v o VO v o VO VJl vjv CK vj-i VO v o v o -P" -P" V A vo vo vo vo vo v_0 ON V A -0 V A v o VO v o VO 00 v o v o 00 O V J O V J O VO O o o V J O o o 0 3 -P- -o VA H1 V J O vji Ov O -O C O 00 VO -O O vo vo vji vji VO C D VO vo VO -O. -P- -P" VO VO VO VO - v l Vj P VJl V O 00 VO VO VO vo 00 00 VO -O r o o o v o o o o vo V J O V O O V J O VO oo r o V J I 00 V J O VO vo vo vo M -P" - J 00 oo v o v o v o v o VO -P" -n3 00 -p- c o VJl VJl V J O -P" VO ON M o -v3 co - > 3 r o c o V J O CO -p- CO o r o v j i VJI -p-O r o V O V J O o \ r o r o O W P H d-CD CD K , £• p CD W CD $ Ct-rl CD P M CD d -P CD co 4 CD P d -Pi P CO o O w Ul M § O g 1 o o bd tr" O O VJl CO P P ON VJl ON V J O •P- -P-•p- C O V J O VO CO -p- VjO r o V J I V J I r o vo VJ\ r—1 -P- VJl CO VJl - a -p-O N V J O r o 00 o r o V J O VA O -P" P -p- - > 3 -p- -P- V J O r o (-> V J O r o VO ON ON M r\ d -vo O O Ov - 0 o - o M t—1 r o -P- - a -p- \J p Hj ON V J O V J V OO o o M -p- V J O VJl VA VJV r o p< Ul] O O & •cf C D H r o ON VJl -P- r-> M r-> O r o M O V J \ r o H ro C O vo r-> - > 3 ON VO VJl VO r o vo - > 3 V J O V J O VJl p co V J O V J O r o r o VO OO V A V J V ON -0 o o o o O 3 C D o 3 3 d -H i 1 ro r o O i — 1 O I—* o O O O O C O h- 1 -p- o o O vo ON o - 0 -p- VJl M C O V J O V J O O V J O o v j V J O o ^ 3 ON VO V J J vo 00 vo -P- C O V J O -p- V J O C D - 08 -TABLE V I I . CHANGES IN HEIGHT OP PLYWOOD BLOCKS DURING AND AFTER PRESSING EH CQ U Q) CD c CD > c8 CO CQ U Q) Q) a CD > o I tf n3 +> M Block No. No. o f p l i e s 0 Height d i f f e r e n c e i n per cent o f EMC height, a t days: 1/48 1 2 3 4 5 6 7 Avge. veneer t h i c k n , ( i n . ) 1 kd 1.61 .37 .56 .48 .37 .18 .06 .00 .00 .1149 2 k8 2.43 •31 1.16 .84 .40 .12 .00 .00 .00 .1011 3 k8 5.80 -.24 .62 .54 .37 .32 .22 .11 .00 .1011 32 .78 -8.20 .35 .20 .18 .02 .00 .02 .04 .1460 5 32 1.97 .22 .40 .33 .05 .07 .07 .00 .05 a465 6 32 2.31 1.22 .44 .28 .22 .09 .00 .00 .00 .1456 7 2k .97 .03 .43 .25 .15 .13 .00 .11 .09 .2106 8 2k 1.21 -.11 .37 .15 .13 .11 .00 .05 .09 .2081 9 2k 1.60 .16 .60 .20 .12 .08 .00 .08 .10 .2121 Avge. 2.08 -.69 .55 .36 .22 .05 .04 .04 .04 .1651 10 48 4.86 .37 .39 .33 .26 .08 .02 .02 .00 .1051 11 43 6.10 2.36 .65 .49 .31 .17 .00 .05 .12 .1092 12 48 6.54 -.95 .51 .44 .42 .24 .11 .00 .05 .1074 13 32 3.03 -1.72 • 55 .43 .17 .10 .00 .00 .00 .1438 14 32 5-36 -.14 .86 .18 .13 .09 .00 .07 .09 .1426 15 32 5.89 -.35 .88 .74 .21 .21 .00 .00 .09 .1437 16 24 2.95 -2.08 .32 .19 .15 .15 .00 • 09 .13 .1984 17 24 1.16 -3.32 .22 .02 .05 .00 .07 .15 .07 .1989 18 24 4.77 -.44 .42 .11 .13 .00 .09 .18 .07 .1968 Avge. 4.45 -.70 .53 .33 .20 .12 .03 .06 .07 .1495 Rotary-cut veneers Sawn veneers Type tc- If bd ' l — ' l — ' r — ' l — ' I — • ! — ' » — ' » — > < feSr-1 ( H C 0 S l O \ V n - P U N H O l>) V O C O S I C A m - f - U N H OO CD CO • O VA -p-NO ON VA i— 1 -a ON NJ\ ro vo -p- ro VA VA VA VA ' -p* ON O VA O OO V J O VO O VO VA VA -P" VA VA M VO SI ON M (-• S3 -p" VO 03 VA O -P" VA VA O O S3 O VA CO vo NO VA S3 ro oo ro O ro M O VO VA r - 1 O ON VA VO VA VA o h-1 NO r - 1 r—1 O O -0 ro r—1 O 00 NO o o o ON S3 VA NA VA VO VA o ro ON VA VA O O M VO -0 vo -P" VA -P" -P" ON ro o\ VA ON VO ON vo C O V A O O O V O V O O V A •p- -p-NQ NO ON S3 S3 NO o VA -P- VA -p* ro ON .p- NO ON VO 00 ON -p- -p- -p- -p-00 03 S3 S3 -P" ON VA O vo o ct-CD CD O ct-r—1 o o P" CQ id ro o H" H> o 1 < ct--P" VA VA VA -P" NO VO I—1 00 S3 ON I—1 S3 -P" S3 -p- -p-NO VA NO vo VA -P- -P-O VA VA ro vo o -P- -P- -P- VA O N C O r O r - ' O N V A O N ^ r O O N N o C o r o v A - p - O -P- -p- -P- -P_; -Pj t-> ON t — 1 VA ro vo r - ' ro o vo ON I-1 o ON U\ ON 00 VA ro vo S3 ON S3 o VA o -p-\-> o S3 r—1 vo o o ro o o S3 o ON o oo o ro o ro o -p-ON NO ro vo ro ON co > a ct-ro » ro ro H P co t-1 ^ ro o H-Hj & 3 ro £u p, <i H-ct--P-vo ON VO oo ro ON vo ro oo O oo -p-ro ro NO o S3 ON 03 03 ro S3 NO VA 00 S3 ON -p" V SI CO b ON ON ON M <! CD CQ H . C>H> CO S^vO CD P H ct-H- P o ro 3 2 -p-VOVOVO -p--P"VAVA S3 VO O 00 vo vo ON VA S3 ro vo ro O VA ro o ON ON ON VO S3 ON NO O ro \-> vo ro ro-p-vo-p-VAVA-p--P- S3 O O r—1 NO oo -p- r—1 o ro co co vo oo ON o o v—'ro ro NO VA VA M O ON H" S3 -P" 00 VA -P" ON O O S3 ro ro -p- vo O O oo ro o NO I—1 VO ro VA VA oo vo ON VA 0*JF- H H 0 a ^CD o CO 4 -^H-W CD ct- CD 93 Vj O CO " 28 ~ Shear Tension Compr. CQ CQ CQ VO .00 S3 CQ CO CQ CQ CQ CQ ONVA p U N H II II II II II II II II II r—1 3—1 p N> S3 O 00 00 VO » • • S3 ON S3 S3 I—" ON I I + S3 VA O V O p • • VA ON S3 • ON ON ro p co ON ro i— 1 vo P ro S3 ro vo oo NO VO O M vn. O • S3 vo -P" ro • M • • O Ovo vn vo ro ON VA S3 VA . ON 00 VO + I o o V O V A O O ON vo -p- vo vo ss o ro Vn I I O r—1 • vo 00 S3 P co ro p H • -p- VA vo Shear Tension Compr. Shear Tension Compr. CQ CQ CQ NO OO S3 II II II vo ro P ON p vo NO vn • . • ON H O N ro NO vo + + + o o o . . . O vo vo ro ON co -P- P vo ro p ON CQ CQ CQ ONVA P CQ CQ CQ vo ro t-> CQ CQ CQ NO OO S3 CQ CQ CQ CQ CQ CQ ONVA p vo ro H' II II II II II II II II II II II II II II II r—1 r—1 H 1 r—1 VO P H 0 \ 0 P O vo M ONVO -P-VA M S3 ro vo -P-NQ S3 00 NO VA vo ro VA ON M ro J—' S3 • ro t—1 M • • • S3 • • VO 00 VO OO • ON . . vo VOH P • vo P • • S3 • vo ro M vo oo P ON ro VA vo ro oo vo ON Vr\ S3 VO oo ON ro M VO VA M U H p 1 + + i i + 1 1 1 + + 1 + + 1 ro o O VA 1—1 VA ro o vo y-> ro vo VA M M -p- • • • O VO • • VO ON vo ro VA i — 1 ONVA vo NO • S3 ON p • ON OO • O S3 p • ON oo VA ro ro 1—1 o o CO • o • • VA O VA vo S3 O « r 1 ON CO • O CO vo J—1 . VO S3 ON VO NO S3 vo -P- o P ON O vo ro VA t-1 X X K| ro ro h-1 X X X -p- -p-vo ON vo vo £ |3 « ro h<$ t—2 H<J r-^ P H* ro ro ro vo X X X X X X ro ro ro ro ro vo X i o + + - o p".c VO VO S3 NO ro O ro + .+ + ro o o S3 • » S3 ON ro • o ro vo VA S3 ON oo ro I 1 T. M H-1 P • O O ON • ON V O O N V A S3 S3 P oo P + ro I V A V O • • . vo vo ON VA VA VO 00 CO S3 -p-NO I + + M VO O o • • S3 oo vo • H VA -p-si 00 ON NO NO + + + O M NO • VA NO -p- « M ON ro o NO I—1 • S3 ro ro i + i o vo o . O • ro S3 o NO • VA VO OO S3 ro S3 o i i i M O O • • * -P-S3 O O H VA VO -P-S3 ro P o + o t p VO VO VO oo co oo PCX) H ro p X X H4 I + + M ro VA . . V j i O S3 • V A V A V A ro vo M 00 NO 00 K HTj S3 M ro + + L O ro p • • S3 • 00 VO o NO o -p" • • ro ON • ON oo vo S3 O ro H1 ro oo -p- ON ro ON oo NO P NO i—' VA O SI NO • SI O - O O P ON O VA 00 S3 vo + k + ' N! O H O • V A • O ro O V A -P oo o • P H S3 H -P-K j K K S3. ON VA ^ « | Hd S3 NO S3 + + O M + o NO VO OO M M S3 S3 -p- M ro o ro I I I M ro ON O M S3 S3 • • • -p-ro -p- ro NO H ONVO + t- 1 M P V O P M vo . . NO -P- O S3 V A V A P 0 3 NO + I I M ro ro . . . ro vo si si M vo -O 00 vo M ro o i i i p ro ro VA o • • ro sj oo • vo vo o vo vo vo o ro P M -POM . VO vo ro VA vo VA vn O -p- ro I i + O r-> O . ON • O S3 O ro vo p O • ro S3 ON H I VO + O vo ro ro NO S3 oo ro VA ON H o P i + i -p-VA 3—1 ON • VO • 00 NO S3 NA NO NO S3 ON 00 -p- O CO & Jj 'VA ro• h-^ r-^ H CO H •00 VO' M H £ vo vo ro vo M ro vo^--ro i i i -p- ro ro • ro S3 S3 • • oo VA ro oo -p- oo ON H ro i i i VA ro o VA oo • • • P VA PVO ro S3 S3 V A 00 S 3 + I I O0 VA M -p- ro VA ON • 00 • -p-vo P O N ro NO oo vo i o I I S3 ro oo • • . o NO ro NO ON NO o NO NO -P-NO O - N S3 o O -p-NO O S3 ON W H VO VO S3 vo I I -I O ON S3 . ro ss t—' • NO H U H P ON VO r—1 ON £ £ £ 03 ONVA 1-3 K j I—' 1—' i—' P VO VA X r p p VOVOS3 S] CO S3 VO VO S3 VO VO S3O0O3 u P P V0 h-'h-'VO VA M ON ,_, Go NO p ONONP S3 VA S3 txi S3 P vo S3VAVO ptoro po Pvoro p p r o o ro si W ON VO ON OO O V A V A V A M rONOrOON VOPON OOOH N OCOH H N H O W P to H S3 ON ro H vn O NO VO P M O VOHO VOOVO PONO N VO O VO IV) S3 - CQ -TABLE X. RAM AND CONTRIBUTION OF SOME SELECTED INDEPENDENT VARIABLES TO THE VARIOUS STRENGTH PROPERTIES OF SAWN-VENEER BLOCKS COMPRESSION SI S2 S3 Var. R 2 Var. R 2 Var. R 2 X6 .2544 X3 .0997 X6 .1771 X3 .4206 X l l .1094 X2 .4309 X l 4 .6570 X2 .1133 X3 .4976 X l l .6669 X6 .1222 X4 .4989 XIO .6747 XIO .1222 . X5 .5003 A l l .8228 .5841 .6045 Y5 .0581 Y2 .2021 Y2 .4789 Y13 .2131 Y 5 .3626 Y7 .6594 Y12 .3240 Y13 • 5073 Y13 .6987 Y l l .3311 Y6 .7027 Y5 .6988 Y3 .7299 Y9 .8277 Y15 .7034 Y7 .8213 Y8 .8280 Y l l .7409 A l l .9233 .8713 .9148 X6 .2544 Y2 .2022 Y2 .4789 X3 .4206 Y5 .3628 X6 .4944 X l 4 .6570 X3 .4507 X14 .5603 Y? • 7532 Y13 .5360 Y7 .7452 Y5 .7707 Y6 .7232 Y13 .7714 X l l .7713 Y9 .8559 X l l .7750 A l l .9430 .9143 .9428 TENSION S4 S5 S6 Var. R 2 Var. R 2 Var. R2 Var. S i " f < Xj) X2 .2088 X3 .0884 X2 .0814 X2 X3 .3103 X4 .0925 X3 .1118 X3 X4 .3171 X2 .1102 X6 .1272 X4 XIO .3206 X l l .1385 X4 .1312 XIO X l l .3210 X6 .1423 XIO .1412 X l l .5308 .2581 .2582 S i = f ( Y k ) Y7 . 1 5 9 7 Y6 .0661 Y6 .0708 Y7 Y2 .4463 Y l .1764 Y14 .2829 Y2 Y 5 .4732 Y8 .2840 Y13 .3285 Y 5 Y 1 5 .5030 Y2 .3499 Y7 .342? Y 1 5 Y6 . 7 1 5 8 Y 1 3 . 3 5 1 7 Y4 .3728 Y6 Y 1 2 .7360 Y l l .3541 Y 1 2 .3739 Y 1 2 .7511 .3680 . 3 8 3 7 S i Y18 .2926 Y6 . .0661 Y15 .0846 Y14 Y7 .4012 Y l .1764 Y14 .2644 Y7 Y2 .6077 Y8 .2840 Y13 .3194 Y18 X3 . 6 5 7 3 Y12 .2859 Y7 .3317 Y13 Y15 .7268 Y2 .3571 Y12 .3325 Y15 Y6 .7455 X4 .3601 X4 .3682 Y l . 7 6 5 8 .4057 .4173 SHEAR S7 S8 S9 R 2 Var. R 2 Var. R 2 .2088 X l l .1199 XI5 .3395 .3103 X14 .2508 Xl4 .4137 .3171 X3 . 3 6 3 6 X6 .4238 .3205 X 5 .3761 X l l .4353 .3210 XIO .4386 XIO .4404 .5308 .5136 .8089 .1597 Y l .0307 Y4 .2546 .4463 Y16 .2829 Y l . 3 3 3 1 .4732 Y2 .3318 Y18 .3448 . 5 0 3 0 Y7 .3414 Y7 .3727 .7158 Y9 .3416 Y13 .3734 .7360 Y13 .3421 Y5 .3794 .7511 . 3 4 3 0 .4657 .2534 X l l - .1199 XI5 .3395 .2727 X2 .2338 X2 .4081 .3440 X 5 • 3395 Y l .4633 .4249 Y13 .4070 Y5 .4817 .4398 XIO .4883 X6 .4847 .4466 Y5 .4937 Y2 .4861 .4893 • 5667 .6893 Note: Var. = V a r i a b l e TABLE X I . FUNCTIONAL DEPENDENCE OF PLYWOOD STRENGTH PROPERTIES ON SOME SELECTED INDEPENDENT VARIABLES FOR THE ROTARY-CUT VENEER CONSTRUCTIONS 75661 -348343 -1123.0 -4.3102 55.709 +1625.2 27.359 37.603 248.69 125.57 83527 1821.5 -648.65 64.587 40.346 .142.16 64.587 293.48 113.48 59-873 128.41 185.83 214.67 72.879 -41.396 +0.2988 +1.318 -0.9737 +1.8541 127.48 -432.17 129.44 -298.19 -4647.9 +16.845 -26.864 +0.0117 +0.0181 +0.2392 +0.0117 -73.832 +7.8445 58791 -1703.9 -875.79 -472.49 -131.45 -1.1214 -98.170 -951.63 -3173.0 -1402.3 +20.515 +10.494 (XIO) (X2) (X3) (X14) (X14) (Xl4) (X14) (X6 (X6 +13036 +2.6573 -3.7015 +0.2955 +2.2884 +3.7155 +1.4463 +32.498 +11.542 +3799.9 -897.54 +450.91 +0.9826 -3.7355 +13.551 +0.9826 +29.631 +11.969 +4173-5 -2.5791 -12.389 +0.1217 +0.3644 +1.0706 +0.5502 -1.2359 -1.0858 ( X 6 ) ~X4) X3) (X8 (X8 fX4 (X8 (Y5 (Y5. (Y6) (T5) (Y5) (Y9) Si = f (xd) - 1 3 6 9 . 2 '(xio) +1499.6 (X5) -21.889 ( X 6 ) - 0 . 3 7 6 9 (xio) - 1 . 1 5 4 (XIO) +2.9881 (X8) - 1 . 5 4 2 5 (X12) (X6) -1.2860 (XIO) (X6) -1.247 (X7). S i = f ( Y k ) (Y8) +12315 (Y9) (Y4) -45.360 (Y4) -23.537 ( X l l ) (X3) (Y2) ( X 4 ) ( X 4 ) (X7) (X7) -3.3799 +7.5505 +29.678 -3.3799 -409.23 +4.7985 S i = f ( +18019 +84.075 -32.699 +0.0132 +0.0459 +2.1327 +6.0329 -1.3764 +0.4738 +3797.4 +84.045 +10.343 -148.19 -628.45 -2742.3 -24.460 +10.505 +6.8661 -3282.5 +115.88 +13.749 -O.6727 +16.101 -9.8713 -O.6727 +16.113 +0.0002 -2999.2 +26.561 +1405.5 +0.8355 +9.2430 +0.1556 -169.73 +0.0007 +0.3094 -88431 +18.221 +1762.1 -73.398 -175.59 -372.49 -282.54 +0.0215 -119.77 -7284.1 +124.52 -51.617 +O.0769 -3.6930 +0.9841 +0.0769 +21.176 +1.1946 -5008.5 +1521.1 +59.101 -75.454 -62.666 +15.429 -17.917 +17.226 +15.259 +1538.3 +2617.5 -132.07 -87.619 +2.2783 +3.4416 -87.619 +1.3783 +0.3259 +17.940 +20.727 -22.721 -3.6628 -8.7388 +12.272 -12.773 +11.348 +7.1466 TABLE X I I . RANK AND CONTRIBUTION OF SOME SELECTED INDEPENDENT VARIABLES TO THE VARIOUS STRENGTH PROPERTIES OF ROTARY-CUT VENEER BLOCKS COMPRESSION TENSION SHEAR SI S2 S3 S4 S5 S6 S7 S8 S9 Var. R 2 Var. R 2 Var. R 2 Var. R 2 Var. R 2 Var. R 2 Var. R 2 Var. R2 Var. R 2 S i = f (Xj) -X2 .4620 X3 .2550 X15 .6224 X14 .5587 X l 4 .5538 X14 .5369 X13 .1761 X6 • 3613 X6 .2790 X6 • 5565 X5 .3887 X2 .7063 X16 .6350 Xl6 • 5721 X8 • 5528 X16 .3711 X2 .3903 X2 .3422 XIO .6008 X6 .6767 X3 .8781 XIO .7034 X8 .5769 X16 .5529 X4 • 5748 X12 .4371 X7 .4948 X15 .6009 X l l .7225 X6 .8877 X8 .7043 XIO .6215 X4 .6468 X12 .5848 XIO .4416 X16 .5092 X l l .6139 X4 .7406 X12 .8947 X4 .8017 X4 .7604 X3 .6859 X8 .6011 X15 .4707 X12 .5893 A l l .8827 .9017 .9288 .9111 .8708 .7583 .8195 .7321 .8283 S i = * ( Xk) Y15 .5893 Y17 .1694 Y9 .5194 Y18 . 4 5 0 5 Y6 .5657 Y7 .5281 Y18 . 4 5 0 5 Y5 .4075 Y14 . 2 5 6 9 Y9 .6283 Y3 .3960 Y2 . 7 6 4 3 Y12 .6783 Y7 . 7 5 7 4 Y6 . 6 1 2 4 Y12 .6783 Y15 . 5 6 3 1 Y9 . 4 8 2 6 Y12 .6378 Y12 .3983 Y4 .8023 Y l .9048 Y12 .7626 Y l .6125 Y l .9048 Y l l .6569 Y16 .5330 Y13 .6664 Y9 .3996 Y8 .8030 Y15 .9140 Y13 .7633 Y8 .6796 Y15 .9140 Y13 .6611 Y13 • 5369 Y8 .7137 Y13 .4106 Y13 .8987 Y5 .9223 Y l .7688 Y13 .6809 Y5 .9223 Y4 .6611 Y15 .5398 Y5 .7210 Y4 .8927 Y7 .9023 Y13 .9254 Y5 .7837 Y12 .6809 Y13 .9254 Y12 .6739 Y2 .7750 A l l • 9316 .9456 .9271 .9781 .9501 . 7922 .9781 .7817 • 7750 S i = f (X Y9 .3803 X3 .2550 Y18 .7756 X14 .5587 X14 • 5583 X14 .5368 X l 4 .1022 X6 .4539 X6 .2790 XIO .4911 Y18 .3645 Y12 .8569 Y12 .7405 Y7 .8235 Y7 .6755. Y8 .4744 Y l l .5851 Y9 .3966 XI5 .6050 X6 .6155 Y9 .8618 Y18 .7555 Y13 .8563 Y l .7477 X13 .5904 Y9 .6419 Y16 .5988 Y12 .6080 X l l .7471 Y2 .8848 Y l .9196 Y l .8986 Y18 .7487 Y18 .6206 XIO .6531 Y15 .6054 X l l .6108 Y12 .7583 X3 .8878 Y5 .9266 Y18 .9005 XI3 .7552 Y13 .7951 X12 .6826 X7 .7010 X12 .6167 X2 .7624 Y13 .8963 X4 .9399 X4 .9089 X4 .7644 X4 .8045 X7 .7151 X12 .7870 A l l .9441 .9543 .9324 .9800 .9518 .8112 .9385 .8761 .9401 Note: Var. = V a r i a b l e s TABLE X I I I . FUNCTIONAL DEPENDENCE OF SOME PLYWOOD VARIABLES ON INDEPENDENT FACTORS FOR SAWN AND ROTARY-CUT VENEER BLOCKS SAWN VENEER BLOCKS R 2 Y2 = -18.55 +0.3235 (X3) +2.864 (X4) +0.2284 (X6) -1.936 ( x i o ) +3.216 ( X l l ) -0.0095 ( Y l ) -10.68 (Y4) .9941 Y5 = .115.3 -0.8299 (X4) -0.8794 (X6) +5.375 ( X l l ) -0.0426 ( Y l ) -15.33 (Y4) -1.079 (Y9) +0.00008 (Yl4).9811 Y6 = 8.706 +0.3600 (X3) +55-95 (X5) -0.1181 (X6) +0.7422 (XIO) +0.4679 ( x i i ) -10.11 (Y4) -0.000006 (Yl4).9944 Y8 = -4.293 +0.0889 (X3) -0.2944 (X6) +0.3297 ( X l l ) -0.9223--(Xll) +1.690 (Y4) +0.00001 (Y14) .9999 Y l l = -0.1226 +1.083 (X5) +0.0013 (X6) ^ -0.0022 ( x i i ) +0.00006(xii) +0.0277 ( Y l ) -0.5974 (Y4) -0.0000004 (Yl4).9996 Y12 = -1.014 -0.0048 (X3) +5-575 (X5) +0.5584 (X6) +0.2274 ( X l l ) +0.0431 (Y3) +0.0911 (Y9) +0.0000006 (Yl4).95l4 Y7 = 3.356 -1.657 (X5) -0.0302 (XIO)-0.1398 ( X l l ) -1.375 ( X l l ) +0.7299 (Y4) +78.63 (Y9) .9897 ROTARY--CUT VENEER BLOCKS Y2 = -0.5947 +0.3752 (X4) -26.48 (X5) +0.9566 (X6) +0.4079 (XIO) -2.835 ( x i i ) -0.1255 (X12)-0.00004 (Yl4).9205 Y5 = -22.77 +0.0700 (X3) +9.442 (X5) +0.0394 (X7) -0.1332 (XIO) +0.0108 (X12) +3.344 ( Y l ) +0.6913 (Y4) .9994 Y6 = 17.81 -0.0394 (X4) -1.303 (X6) -0.3159 ( x i o ) +6.976 ( X l l ) +4.249 (X14) -53.04 (Y4) +0.00002 (Y14).9977 Y8 = -9.066 +0.5883 (X6) +0.2882 (xl0)-0.4779 ( x i i ) +0.0150 (X12) -1.653 (X14) +2.039 ( Y l ) +110.5 (Y4) .9999 Y l l = 0.2308 -0.0004 (X3) +0.9852 (X5) +0.0061 (X6) -0.0020 (X7) -0.0004 (X9) +0.00006 ( Y l ) -0.0082 (Y4) .9876 Y12 = 7.017 +25.60 (X2) -0.0157 (X3) -0.1686 (X4) -4.591 (X5) +0.0045 (X12) +2.287 ( Y l ) -1.225 (Y4) .8546 Y13 = 9.424 -0.0181 (X3) +1.410 (X6) -0.2130 (X9) -0.0323 (XIO) -0.0028 (X14) -83.55 ( Y l ) +0.0319 (Y14).9999 TABLE XIV. RAM AMD CONTRIBUTION OF INDEPENDENT FACTORS TO SOME PLYWOOD VARIABLES AS INDICATED BY THEIR COEFFICIENTS OF DETERMINATION Var. R 2 Var. R 2 Var. R 2 Var. R 2 Var. R 2 Var. R 2 Var. R 2 Sawn Veneers Load F u l l Permanent Veneer Plywood Plywood Weight recovery compression compression d e n s i f i c a t i o n d e n s i t y E.M.C. l o s s Y2 Y5 Y6 - Y8 Y l l Y12 Y7 XIO .4328 X4 .2993 Y14 .3695 Y14 .6650 X5 • 9575 Y9 .3164 X l l .2755 X4 .9575 X l l .7854 X3 .6048 XIO .6683 Yl4 .9802 Y14 .5158 XIO .2978 Y l .9702 X4 .8672 Y4 .6803 X3 .9228 Y l .9810 X3 .5190 Y4 .3113 Y4 .9742 Y l .9197 XIO .7464 Y4 .9453 X l l .9811 X5 .8408 Y9 .3730 X3 • 9778 Y14 .9559 X l l .8051 X l l .9872 Y4 .9840 X l l .8453 X14 .9518 X6 .9809 X6 .9559 X6 .8402 X6 .9998 X6 .9878 Y3 .8909 X5 .9397 X l l .9941 Z9 .9811 X5 .9944 Y9 1.0000 X14 .9996 X6 .9514 X3 .9900 Xl4 1.0000 X3 1.0000 X14 1.0000 Y9 1.0000 Y4 1.0000 Y3 1.0000 Rotary-Cut Veneers Load F u l l Permanent Veneer Plywood Plywood Days to recovery compression compression d e n s i f i c a t i o n d e n s i t y E.M.C. E.M.C. Y2 Y5 Y6 Y8 Y l l Y12 Y13 X l l .2127 Y l .5983 X14 .5040 Y l .5463 X5 .9287 Y4 .4363 X3 .2070 Y14 .5009 X12 .6622 Y14 • 7785 X14 .9210 Y l .9639 X12 .5547 X14 .2906 XIO .7841 X5 .6849 XIO .9056 X12 .9733 X7 .9841 X2 .6258 X9 .7064 X5 .7858 XIO .7054 X4 .9066 XIO . 9860 X3 .9889 X3 .6319 X6 .7650 X4 .9151 X7 .7167 X6 .9516 X6 .9860 X9 .9870 X4 .6322 XIO .8285 X12 .9188 X3 .8823 Y4 .9834 Y4 .9981 X6 .9872 Y l .8497 Y14 .8300 X6 .9205 Y4 .9994 X l l . .9977 X l l .9999 Y4 .9876 X5 .8546 Y l .9999 Yk 1.0000 X6 1.0000 Y9 1.0000 X3 1.0000 X2 1.0000 X6 1.0000 Y4 1.0000 Note: Var. = V a r i a b l e Sh. Tension Ccmpr. Ul CQ W M CQ CQ CQ OO O N V n P U N H II II II II II II II M H U - f - -N] N P NO O CO • NO H O V A M • 00 I—1 00 ON • • S 3 ON • • V O V O V A -P" M 00 ON V n OO O V n vo V n Sh. Tension Compr. CQ CQ CQ CQ CO CQ CQ oo O N Vn -P" vo ro M II II II II II II II ro vo i—1 ss 1 VO Vn M ro vo Vn O N VO M VO vo 1—1 O N • H vo vo • • • ro • • O Nvo ro vn ro O -p" 3—1 00 o -p-vo -P- O r-> + + + + 1 1 + ro o VO VO r—1 O N • • • • • O N • M O 3—1 O S 3 O N ro -p- 00 -P" N O -p- ro O V O S 3 o ro ro vn N ro vo oo ro vo i 1 1 i + 1 1 o VO H o O r—' r—1 • • • • • • O oo o o o •p-vo O r—' vo vn oo Vn O N 00 ro ro N O o ro o I - 1 vo o O O vn X O N S 3 -p-vo X X X V O V O O N X X X X vo vo vo vo X X X vo vo vo I r—1 • O N v n O Co X i - 1 o + + + r-> O O v n • • • vo vo N O ro O O N N O -p-vo O N O X X X 00 S 3 S 3 I + v n O N O N • • r-> V n S 3 S 3 N O N O N O X X ON -p-N O r o N O X 00 o p I o ct-< CD £S CD + O I + I H S 3 O I P M O oo oo ro vo ro s i O N vn ro ro O N X X X X r - 1 -P* O N r—1 O O N t o • o • -P-NO S 3 O 00 N O S 3 vn ro N O X KJ X O N ro O N + 1 + + 1 + + H + + i + t + .+ ro N O -P- H vo ro -p- W H O t-1 M ro ro • • . ro S 3 O cy* • S 3 « • • ro N O • ro oo ro • • oo ro • 3-1 H ro • S 3 N O ro O N M h-1 S 3 00 O ro -p-si VO H O N S 3 M -p-si vn o \ p p o N O vo ro ro VO vn VO O N O N O N S 3 ro ro vo M S 3 X r—1 ro X X X H CO CD X X X 00 O N 3—* X X X K! M O N M ro X H< X f—1 ^ - ^ r—1 + + 1 1 + + 1 + 1 1 1 1 1 + ro H M V O O N V O 3—1 O N V O M 00 v o ro v n • V n t-J • 3— j V0 O N • V O O 00 • -p-ro vo • • O N • • 00 O O • • • S 3 • N O V O S 3 v n . ro S 3 -P- O vo -p- 00 V O S 3 N O O O 00 O N -p- N O P V O -P- O ro ro O V O v n N O oo V n V n O N V n v n v n co ro V n $ 0 $ v_^ ro ro ro • - ro O N H< >-< O N O N 3—' Hd !-< IV) O N V n Vn Vn O N O N O D ON O N N H H P U\ ONVn 00 S 3 S 3 S 3 OONON £d vo roovoo M ro N O -p- vn vo oo H s i vo ro ONVosi-p-Noroo vn 00 S 3 ro vo vn vo N O ro O O N VO SI -p-- 68 -TABLE XVI. RANK AND CONTRIBUTION OF SOME SIGNIFICANT INDEPENDENT VARIABLES. EXCLUDING THE EXPERIMENTALLY CONTROLLED ONES, TO VARIOUS PLYWOOD STRENGTH PROPERTIES S i = f ( X i , Y k) COMPRESSION TENSION SHEAR SI S2 S3 S4 S5 S6 S8 Var. R 2 Var. R 2 Var. R 2 Var. R 2 Var. R 2 Var. R 2 Var. R 2 Sawn veneers X6 .2543 Y2 .2022 Y2 .4790 Y l l .2987 X3 .0884 Y6 .0708 X l l .1199 X3 .4206 Y5 .3627 X6 .4944 Y2 .3781 Y6 .1349 X3 .1091 X3 .1869 Y5 .5761 X3 .4507 X3 .5172 X3 .4842 XIO .1625 X6 .1208 Y6 .2364 X l l .5904 Y6 .6227 X l l • 5193 X l l .4846 X6 .1670 X4 .1232 XIO .2369 Y2 .6007 X l l .6339 Y12 .5414 Y6 .4931 X4 .1671 Y8 .2357 Y2 .3463 Y12 .6078 Y8 .6960 Y5 .5420 XIO .4964 Y5 .1683 Y2 .2406 Y5 .3578 A l l .7420 . .8392 .8846 .6030 .3372 .3415 .3807 Rotary-cut Veneers X6 .3443 Y5 .3582 X8 .4346 Y12 .2839 X8 .3370 X8 .3850 Y5 .4075 X8 .4630 X3 .5260 X6 • 5833 X8 .5114 Y12 .5286 X l l .4453 X6 .5612 Y12 .5233 X4 .6408 Y5 .6899 X7 .5598 X7 .5802 Y5 .5612 XIO .5825 XIO .6633 X6 .6675 X3 .8013 X3 .6782 X4 .6736 X? .5758 XI2 .5845 X3 .6881 Y12 .7233 Y12 .8262 Y5 .7405 Y5 .7161 X4 .6403 X8 .7037 X4 .7368 XIO .7299 X4 .8293 X l l .7461 X l l .7222 Y12 .6541 X7 .7071 A l l .8273 .8190 .8912 .8687 .7973 .6954 .7394 Note: Var. = V a r i a b l e - 91 -TABLE XVII. SIMPLE CORRELATION OF VENEER THICKNESS AND SOME CONCOMITANT VARIABLES Concomitant v a r i a b l e s R Glue content ( s o l i d s ) -0.98' Number of p l i e s • -0.96 Permanent compression -0.56 Plywood moisture content -0.55 F u l l compression i n press -0.50 Plywood s p e c i f i c g r a v i t y 0.45 Number o f p l i e s -O.96 Glue content ( s o l i d s ) -0.93 Plywood s p e c i f i c g r a v i t y (squared) 0.80 Permanent compression (squared) -0.74 Plywood s p e c i f i c g r a v i t y O.56 Veneer moisture content -0.46 Plywood s p e c i f i c g r a v i t y (squared) 0.91 F u l l compression i n press 0.55 Plywood moisture content -0.53 Load recovery i n press -0.50 Permanent compression (squared) -0.48 R a d i a l g r a i n angle 0.44 Plywood s p e c i f i c g r a v i t y (squared) 0.83 Number o f l a t h e checks per i n c h -0.73 Permanent compression -0.70 F u l l compression i n press -O.69 Veneer d e n s i f i c a t i o n 0.58 Glue content ( s o l i d s ) -0.98 Height o f bl o c k s 0.79 Plywood s p e c i f i c g r a v i t y 0.54 Glue content ( s o l i d s ) -0.93 Plywood s p e c i f i c g r a v i t y 0.79 Permanent compression (squared) -0.72 Veneer d e n s i f i c a t i o n (squared) 0.57 R a d i a l g r a i n angle 0.42 - 92 -TABLE X V I I I . SIMPLE CORRELATION OF GLUING PRESSURE AND SOME CONCOMITANT VARIABLES Test Block co o •H CO co +5 h o O ct3 -p o o3 CO o •H -P CQ 3 C o CD I f? o j -P O « c6 CO -p o a? h CD H CO -P o Concomitant v a r i a b l e s R Load recovery i n press Permanent compression Veneer moisture content Weight l o s s i n press ( r a t e of cure) -O.Sk 0.62 -0.42 -0.41 Veneer d e n s i f i c a t i o n Permanent compression (squared) Veneer moisture content Growth r a t e 0.74 0.59 0.49 -0.40 Veneer d e n s i f i c a t i o n • Load recovery i n press Permanent compression Permanent compression (squared) Weight l o s s i n press ( r a t e o f cure) 0.86 -0.84 0.62 0.53 -0.41 F u l l compression i n press Veneer d e n s i f i c a t i o n Permanent compression (squared) Permanent compression Weight l o s s i n press ( r a t e of cure) 0.74 0.73 0.54 0.46 0.42 Weight l o s s i n press ( r a t e of cure) Load recovery i n press F u l l compression i n press 0.86 -0.84 0.41 F u l l compression i n press Veneer d e n s i f i c a t i o n Permanent compression (squared) Summerwood percentage (squared) R a d i a l g r a i n angle Weight l o s s i n press ( r a t e of cure) 0.86 0.64 0.59 0.46 -0.44 0.41 53 O c+ CO r-3 CO t—1 CD ro & o P< ro >d ro P ro P3 4 & H ro co ~^Ud co O vn>d co O O o co VnCD O O O •d CO H CO H -o Vn -P" Vn -p- vo co S3 C0OC0-ro ro ro vo ONVn -O •p- ON • Vn H3 ro Rotary-cut veneers Shear Tension Compr. CO CO CQ CQ CQ CQ C O C Q C Q CQ VO OO -vj O W n P U N H H -H ro ro -p- M VO VO NO NO ON IN) ONOVJV vo ro vo -p- ro H vn oo -p-ro vo P U \ H N O Vn N O O Vn H to vn VO - O O 00 O N -P" S3 H U H 9 CO ro Sawn veneers Shear Tension. Compr. CO CQ V O oo * ^ V d CO H ro vo ro vn si ro vo vo S3 vn ro vo -p-oo ro CO CQ CQ CQ CQ CQ CQ S3 ON Vn -P" VO IN) H vnco H o O O >d co P H V J N O S3 vn ro ON O -P" • S3 VO H VO oo co ro ro N O O ON * N O vo H ro 00 S3 O N O oo ro N O • ro i >d vnco H O O O >d co H ro ro ro si ro ro vo vo ro -p- ro vo -p- O O O N oo N O ro vo H O ON S3 IN) vo NO ro CQ H c+ CO IN) VO CQ c+ 4 ro el-s' O o c*-ro 4 H co cl-H-O CQ P o O 4 CO H3 4 &• ro CO O CO bf n P ro CD co CO •U s—>. CO H-H' V y O V ' vn O ro o o vo vn o g H3 p O H3 CO Wj < CD O >d {o C*5 CD CO p p-00 VO O vo to O0 H VO H S3 O ON S3 H ro oo S3 oo ON ON ro VO S3 ro vn O IN) . Vn 00 00 CO vn vo oo H oo ro N O N O O Vn vo ro ro vo O Oo O -P-vo • ro ro vo H vo oo vo ONVO H vo ON VO VO H PONO ro -p- ON Vn Vn ro N O -P" vo vo H ro o -p-O 00 H ro O vn O Vn vn H vo N O N O CO H H O ON . -P" vn H VO NO P M tO ON S3 SI • ON S3 ro ro VO Vn O S3 NO -P" ON • vo H -P- ro S3 ON ON ON O H H -P- tO O N -p-vo N O ro O N . ON ro -p- ro ro oo H -p- 00 S3 NO Vn ON to1 H o o cr1 CD 4 CD vn o ro O O vo vn O ON H VO VO vn ro -p-si -p-O 00 O N ONVO -p-•p- O ON vn ro « ON ON ro ON H vn vn Vn H ro Vn O VO S3 ro -p-vo • ro ro vo H O Vn S3 00 O 00 S3 • ON vn o ON H S3 O co N O ro H P f O H O 00 -p- -p-00 H CO S3 VO Vn S 3 H 00 S3 O Vn vn H ro VO H N P ON H -P" • Vn ro VO H S3 NO -P" 00 vn vo Vn ro o o ro H vn vn H NOH oovoroH O H O S30 OOS3S3VO NO NO W . . NO H 00 ro N O vo S3 S3 O N ro vo • N i p O H OO S3 Vn • oo H vo ro oo vo ro H vo O vo ro vo vn o S3 vn S3 -P-ro S3 -p- -P- H si ro -p- O N NO VO VO H oo si £ -p- oo V O 8s ro N O vo ro H vn H ro oo vo vn N O S3 H OO . N O H -P" to S N O o H ro vo ro o g N O - £6 -TABLE XX. SUMMARY OF ADJUSTED PLYWOOD STRENGTH VALUES FOR SAWN AND ROTARY-CUT VENEER BLOCKS Thickness (inch) F a c t o r s _. / . \ Pressure (,psx; Strength c h a r a c t e r i s t i c s Type CQ U CD <u CD > ctJ CO Type CQ u CD CD S CD ! > o o5 • P S i & si § S2 ° S3 S4 S5 S6 S7 o •H CQ CD EH <D CO C4 o •iH CQ CD EH u a CD CO 1 / 10 1 / 7 50 200 350 50 200 350 Block number 50 1 / 5 200 350 U n i t s l 2 3 4 5 6 7- 8 9 Mean 1000 p s i 1418 1361 1341 1498 1498 1391 1346 1280 1410 1393 p s i 3069 3413 3263 3450 3397 3483 3241 3342 . 3318 3331 10 "5 112 327 265 217 210 324 272 407 231 263 1000 p s i 320 318 313 317 319 310 308 304 316 314 p s i 4493 4451 4398 4548 4527 4584 4464 4441 4582 4498 10 -5 12582 12486 12368 12776 12706 13089 12666 12670 12918 12696 S8 p s i 789 810 829 824 823 829 779 702 807 799 S9 1o S i U n i t s 10 11 12 13 14 15 16 17 18 Mean SI 1000 p s i 122 210 175 93 180 134 13 84 115 125 S2 p s i 140 343 685 -7 350 352 196 403 588 339 S3 10 -5 40 29 357 -25 96 227 789 482 501 277 S4 1000 p s i 24 28 22 20 32 35 -15 8 15 19 S5 p s i 63 92 72 38 89 117 -55 7 28 50 S6 10 "5 267 354 360 177 291 379 -15 150 179 238 S7 1o S8 p s i 154 84 292 111 169 210 -76 49 263 140 S9 1° TABLE XXI. RATIOS COMPARING VARIOUS PLYWOOD STRENGTH PROPERTIES WITHIN EACH VENEER TYPE Thickness (inch) Pressure ( p s i ) R a t i o 51 / S4 52 / S5 53 / S6 SI / S8 ffl S2 / S8 % S5 / S8 ^ S7 / S9 > CD CQ o CD - P CQ 3 R a t i o SI / S4 0 3 S2 / S5 S3 / S6 51 / S8 52 / S8 S5 / S8 S7 / S9 R a t i o 51 / S4 52 / S5 53 / S6 co SI / S8 § S2 / S8 d S$ / S8 R a t i o SI / S4 5 S2 / S5 ^ S3 / S6 51 / S8 52 / S8 S5 / S8 Block CQ 03 CD CO Pi CD i> Block •p 0 CQ 1 h h> CD H CD CS Pi - P CD P-i Block CQ Pi U g CD cd CD CO pi CD > Block 0 CQ 1 u >s 0) U CD a PI - P CD .2 > 1 / 10 1 / 7 1 / 5 50 200 350 50 200 350 50 200 350 1 2 3 4 5 6 7 8 9 Mean 7.09 6.35 6.13 8.32 8.08 7.98 6.57 5.93 6.92 7.05 1.77 3.54 3.33 2.47 2.50 2.37 2.52 3.22 , 1.94 2.54 .26 .61 .54 .30 .31 .30 .41 .54 .28 .37 983 889 740 1076 1063 855 831 831 929 913 1.20 1.90 1.50 1.89 1.78 1.91 1.63 2.28 1.68 1.74 .68 .53 .45 .76 .71 .81 .65 .71 • 87 .68 1.01 .92 .90 .98 .96 .99 .80 1.20 .80 .95 10 11 12 13 14 15 16 17 18 Mean 5.08 7.30 7.50 5.25 5-85 4.21 10.13 7.87 7.35 6.21 5-33 6.58 10.24 5.67 6.38 4.55 43.14 19.67 19.22 8.59 1.06 .90 1.38 1.10 1.12 1.07 4.36 2.54 2.58 1.59 621 1167 525 671 744 576 2330 773 486 681 1.63 3.09 2.21 1.53 2.16 1.97 8.63 3.75 2.47 2.45 .30 .47 .22 .27 .34 .43 .20 .19 .13 .29 .62 1.00 1.01 1.09 .99 1.01 1.54 I.25 .91 1.00 1 2 3 4 5 6 7 8 9 Mean 4.43 4.28 4.28 4.72 4.67 4.49 4.37 4.21 4.46 4.44 .68 .77 .74 .76 .75 .76 .73 .75 .72 .74 .009 .026 .021 .017 .017 .024 .024 .032 .018 .021 1797 1680 1618 1818 1809 1678 1728 1823 1747 1743 3.89 4.21 3.94 4.19 4.13 4.20 4.16 4.76 4.11 4.17 5.69 5-50 5.31 5.52 5.50 5-53 5.73 6.33 5.68 5.63 10 11 12 13 14 15 16 17 18 Mean 5.08 7.50 7-95 4.65 5.62. 3.83 - 10.50 7.67 6.58 2.22 3.73 9.52 - 3.93 3.01 - - - 6.78 .15 .08 .99 - .33 .60 - 3.21 2.80 1.16 792 2500 599 838 IO65 638 - 1714 437 893 .91 4.08 2.35 - 2.07 1.68 - 8.22 2.24 2.42 .41 1.10 .25 .34 .53 .56 .72 .15 .11 .36 TABLE X X I I . RATIOS COMPARING VARIOUS PLYWOOD STRENGTH PROPERTIES OP SAWN AND ROTARY-CUT VENEER BLOCKS Thickness ( i n c h ) Pressure ( p s i ) Ra t i o SI / SI S2 / S2 S3 / S3 S4 / S4 S5 / S5 S6 / S6 S? / S7 S8 / S8 S9 / S9 R a t i o SI / SI S2 / S2 S3 / S3 S4 / S4 S5 / S5 S6 / S6 S8 / S8 Block 0) m > CD U 3 <D iH CQ efl & i> O Block CD CQ -P <D CQ 3 ^ d «5 1 / 10 c 1 / 7 1 / 5 50 200 350 50 200 350 50 200 350 l / l O 2/11 3/12 4/13 5/14 6/15 7/16 8/17 9/18 Mean 2.29 1.36 1.37 2.93 1.80 2.05 3.83 1.71 2.16 2.01 1.07 1.09 .66 2.25 1.15 1.34 1.16 .97 .77 1.06 .46 .87 .48 .76 • 57 .62 .32 .56 .36 .52 1.70 1.56 1.67 1.85 1.45 1.08 - 2.27 2.29 1.77 3.21 < 2.03 2.02 5.17 2.95 2.58 - 5.90 7.59 3.59 1.88 1.29 1.22 2.81 2.06 2.36 - 2.66 3.28 2.19 2.00 .96 .90 1.04 1.17 1.11 2.60 1.14 .89 1.13 1.45 1.78 .97 1.83 1.40 1.38 - 1.59 1.13 1.50 1.22 1.04 1.01 1.16 1.20 1.14 5.00 1.19 1.00 1.19 l / l O 2 / l l 3/12 4/13 5/14 6/15 7/16 8/17 9/18 Mean 11.62 6.48 7.66 8.27 10.38 _ 15.24 12.26 11.14 21.92 9.95 4.76 - 9.71 9.89 - 8.29 5.64 9.82 2.80 11.28 .74 - 2.19 1.43 - .84 .46 .95 13.33 11.36 14.23 15.85 9.67 8.86 - - 11.28 16.52 71.32 43.38 61.08 119.68 50.86 39.18 - 163.64 89.96 47.12 35.27 34.36 72.18 43.66 34.54 - 84.46 - 72.17 53.34 5.12 9.64 2.84 7.42 4.87 3.95 - 14.33 3.06 5.71 TABLE X X I I I . RATIOS OF PLYWOOD STRENGTH PROPERTIES COMPARING ADJUSTED AND OBSERVED VALUES Thickness ( i n c h ) Pressure ( p s i ) Ra t i o SI / SI S2 / S2 S3 / S3 S4 / S4 S5 / S5 S6 / S6 S8 / S8 R a t i o SI / SI S2 / S2 S3 / S3 S4 / S4 S5 / S5 S6 / S6 S8 / S8 Block CQ 5 CD Block 0 co 1 U CD 5 > 1 / 10 1/7 1 / 5 50 200 350 50 200 350 50 200 350 l / l 2/2 3/3 4/4 5/5 6/6 7/7 8/8 9/9 Mean 6.35 6.51 7.^ 9 5.73 5.65 6.38 7.5^ 8.80 6.40 6.60 11.24 7.65 8.99 7.50 7.68 7.14 9.26 8.38 8.32 8.29 .92 1.42 1.31 1.23 1.24 1.45 1.31 1.48 1.28 1.32 9.78 9.66 10.72 10.10 9.78 11.36 11.32 12.41 9.94 10.50 29.18 35-32 40.35 24.45 25-58 22.25 32.12 35.81 22.35 28.47 26.77 32.86 33.24 21.65 23.22 17.36 25.13 24.79 19.96 23.91 3.48 3.48 3.43 3.39 3.32 3.25 3.62 4.01 3.41 3.46 10/10 l l / l l 12/12 13/13 14/14 15/15 16/16 17/17 18/18 Mean 1.25 1.36 1.33 1.04 1.37 1.26 _ .99 1.13 1.19 .56 .84 1.24 - • 91 .97 .65 .975 1.13 .90 .15 .11 • 85 - .32 .67 1.22 .99 .98 .72 1.25 1.33 1.26 1.18 1.42 1.37 - .74 1.08 1.12 1.31 1.48 1.33 1.06 1.48 1.46 - .33 1.04 1.14 1.07 1.20 1.18 .84 1.10 1.19 - • 78 .91 .98 .98 .64 1.17 .83 .95 1.14 - .44 1.25 .91 NO SI Shear Tension Compression CO co CO S3 CO ON CO Vn p CQ VO CQ ro CQ 53 O ct-CD S3 CQ 3 O et-co H-Hj H-O S» O P . ^ P> H-M 4 (D H M H3 ct- CD ' CO CO co o 8 6 o 4 CO ct- CD CD O 3 OCQ ct-4 CD CO CO h i 3" ct- CD H CD CO O 4 CD ~-O 3 ri- CD CO CO cf o p-£D CO ct- CD H-CD CQ O 4 co pj-P p- y o H co c+ CD CO H' o 3 CQ ct-H* 3 I—I *x) *-3 ct- CD H-CD CQ O 4 co [V o CD ct- CD CO H- CO O CO ct-4 CD CO CQ M 1-3 3 4 pr c+ CD ' CD4 s e ct- CD H-O 3 CD O CO o ro vo o O ro vo vo s i H ro co ro i—• ON P P (—• co ct-CD o ct-H-O 3 ro vo CD co co ro O CQ ct-3 CQ ct-4 CD CO CO cf p. p P CO H hrj H •T) 1-3 H *tj t3 3 4 3* 3 4 3- 3 4 3-ct- CD H- ct- CD H- ct- CD P-CD CO O CD CO o CO CO o 4 CO 5" 4 co B* 4 co P p 3 P3 £ 3 PJ p B O 4 CO o 4 CD O 4 CD ct- CO CO ct- CD CD ct- CD CO H- CQ H- CD H' CO O o o 3 3 3 vj P ro h-> ro P r—1 P co vn ro ro O vn -p- ro vo vo s i vn S3 -p- vo oo vo ro VO vn P S3 VO NO VO -P- M oo o vo oo oo vo Vn vo -P" 00 vn VO vo ro o ro vn O vo ro vo oo vo oo VO S3 ON p oo -p- O vn OO ro ro vn co «8 H* Hj H-o P5 3 ct-53 53 CQ CQ -p- M VO 53 SI 53 • • • CQ CQ CQ ro VO O ON I * % P P V O O ON NO 53 53 • • CQ CQ ro vn (—' P ON 53 is! • • CQ CO ro ro oo oo o s i ON CQ r—1 S3 Vn vn VO CD P P * * * * * * r—1 I—1 VO vn NO O $ $ >!= S3 VO NO vn s j NO NO r—1 VO OO ON S3 VO OO ON O OO S3 S3 ro NO O S3 oo o ro ON ro M ON O P NO NO ON VO P P Oo vn vn S3 ro NO NO 3—1 NO CQ S3 00 S3 NO S3 NO OD S3 00 OO O P P VO O 53 * • * CQ * * * * * * I t t % t * * * * * % * * 3" CQ H-<g H-H" O 3 ct-O O ro • • • ON NO r—1 00 00 NO 53 is! 53 • • • CO CQ CQ 3-" M VO P P v n ro t—1 I—1 p I—1 Vn vo vo vn vnivoM ro VO VO O Vn M VO 03 O NONOP VO O M co ro S3 * 53 O P v n P V O H P ON ro - -CQ * 53 * * • ¥ CO P M ro vovnro vnvovo 3—1 NO ON O O VO O O O N H H O Vn $ * $ U; * * 1 $ * ro P H U i • • • O H - N l NO vo ro VO ON 3—' P v n ro ON VO P M M vn ro h-J oo ON ON ON vn ON S3 vn ro OO O S3 OO M vn P S 3 O OO O S3 VO H H ro S3 o CO VO O S3 ro p vn vo vo Vn S3 P S 3 ON 3-J O v n VO S3 P CO vo ro ON CQ tit ttt ttt at ttt - 86 -TABLE XXV. RANK OP HIGHLY SIGNIFICANT CONTROLLED FACTORS ON THE VARIOUS PLYWOOD STRENGTH PROPERTIES Strength property-Modulus Stress S t r a i n Rank o f c o n t r o l l e d f a c t o r s on Observed values Sawn veneers Thickness Thickness Pressure Pressure Thickness Rotary veneers Thickness Pressure Pressure Thickness Thickness Pressure Adjusted values Sawn veneers Rotary veneers Thickness Pressure Thickness Pressure Pressure Thickness Thickness Pressure Pressure Thickness Thickness Pressure Modulus Stress S t r a i n Wood f a i l u r e Thickness Thickness Thickness Thickness Thickness Pressure Thickness Pressure Thickness Pressure Pressure Thickness Thickness Thickness Thickness Thickness Pressure Thickness Pressure Thickness Pressure Pressure - Pressure Stress - Thickness Thickness Pressure Wood f a i l u r e - Thickness 5" Figure 1. C u t t i n g plan of plywood b l o c k s . - 101 -Figure 2. Baldwin U n i v e r s a l T e s t i n g Machine i n the m a t e r i a l s t e s t i n g l a b o r a t o r y of the F a c u l t y of A p p l i e d Science. - 102 -Figure 3. Compression specimen i n the microformer extensometer. - 103 -Figure k. Table Model I n s t r o n T e s t i n g Instrument i n the wood technology l a b o r a t o r y of the F a c u l t y of F o r e s t r y . Figure 5« Tension specimen i n the g r i p s of the t a b l e model I n s t r o n t e s t i n g instrument. - 105 -Gluing pressure (psi) Figure <S. Influence of controlled factors on modulus of elasticity of sawn- veneer blocks in compression. - 106 -Gtluin g pre s s ure (psi) Figure 7. Influence of controlled factors on modulus of elasticity of sawn-veneer blocks in tension. - 10? -Gluing pressure (psi) Figure 3. Influence of controlled factors on modulus of elasticity of rotary - cuf veneer blocks in compression. » 108 = i i 50 ZOO *>50 G/uing pressure. (ps>Q Figure 9. Influence of controlled factors on modulus of elasticity of rotary - cut veneer blocks in tension. 

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