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J Wood Sci (1998) 44:343-347 The Japan Wood Research Society 1998

H e n g X u T e t s u y a N a k a o C h i a k i T a n a k aM a s a h i r o Y o s h i n o b u H i r o y u k i K a ~ a ya m aE f f e c t s o f f i b e r l e n g t h a n d o r i e n t a t i o n o n e l a s t i c i t y o ff i b e r - r e i n f o r c e d plywood

Received: May 21, 1997 / Accepted: February 26, 1998A b s t r a c t Short carb on fibers, a reinforc ed material in woodveneer composites, were used to investigate the effectsof fiber length and orientation of fibers on the elasticity ofplywood. The technical feasibility, elasticity, and strength ofthe reinforced plywood with short carbon fiber were evalu-ated. In a short fiber reinforcement system, the fiber lengthdoes not directly influence the reinforcement in Cox'stheory when the fiber length exceeded a certain length.When the lengt h of short car bon fiber is beyond 3 mm, thehigh reinforced result was obtained in the experiment.However, if fiber length was too long, the reinforced resultwas less owing to the bridge be tween fibers and the increaseof holes. The optimum fiber length must be considered.The orientation of fibers has a strong influence on the rein-forcement. Unidirectional, perpendicular, and random ori-entation displayed different influence on the elasticity.Experimental results were discussed with Cox's method.Reinforced plywood with short carbon fibers in randomorientation has a higher shear modulus and bendingstrength than the controls, in addition to other mechanicalproperties.K e y w o r d s Short fiber Rein forc ement Fiber length Fiber orientation Plywood

have focused on fiber-reinforced wood composites. 1 5 Mostof these pursuits have involved nonmetallic synthetic fiber-woven-cloth reinforcement, such as glass fiber, carboncloth, and fiber-reinforced plastic (FRP). The fiber rein-forcement is beneficial for increasing the mechanical prop-erties of plywood, decreasing its variations, and improvingits durability. Up to now, fewer investigations have beendone on nonmetallic synthetic short fibers (discontinuousfilament) in wood composite reinforcemen t. 4

The elasticity and strength of a material depend on thedetail of its minute structure. The factors influencing elastic-ity and strength, such as fiber volume fraction, fiber orienta-tion, and compound structure, have been investigated infiber -reinforced wo od composi tes. 2'3'5Because of the substi-tuted fiber cloth with short fibers in the reinforcement sys-tem, the other factors, such as fiber length and orientationof short fibers, must be considered. 6 In this study, the contri-butions of fiber length and orientation of fibers to elasticityin a short carbon fiber reinforcement system were investi-gated using Cox's theory and were demonstrated experi-mentally. From a practical point of view, technicalfeasibility, elasticity, and strength of reinforced plywoodwith short carbon fibers were evaluated.

IntroductionThe design of wood composite with high strength and dura-bility is always an importa nt issue in construction. Adva ncesin fiber-reinforced plastics motivate d us to evaluate the fea-sibility of manufacturing reinforced wood composites withhigh-p erform ance synthetic fibers. Numer ous investigations

H. Xu T. Nakao ([]) C. Tanaka M. Yoshinobu. H. KatayamaFaculty of Science and Engineering, Shimane University, Matsue690-0823, JapanTel. +81-852-326564;Fax +81-852-326598e-mail: [email protected]

ExperimentalShort carbon fiber reinforcement systemTo investigate the contribution of fiber length and orienta-tion to elasticity, specimens of wood fiber reinforcementsystems with short carbon fibers were prepared in a labora-tory. The wood fiber reinforcement systems were com-pounded with three plies of parallel veneers and twofiber-resin lamina-inserted veneers, shown in Fig. 1. Birch(Becula spp.) sliced veneer, 0.Smm thick, was chosen as awood element. The short fiber was chopped carbon fiber(TO RAY CA, T008; Toray, Tokyo, Japan), which has beenwidely applied for reinforcing the matrix of resin, rubber,paper, and cement. It was made from high-performance


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34 4c a r b o n f i b er f il a m e n t a n d s e t w i t h e p o x y r e s in b e f o r e h a n d .T o e x p l o r e t h e e f f e c t o f fi b e r l e n g t h , 3 , 6 , a n d 1 4 m m l o n gc h o p p e d c a r b o n f i b e rs u n d e r u n i d i r e c t i o n a l o r i e n t a t i o no f f i b e r s w e r e u s e d a s a f i b e r e l e m e n t . P h e n o l - r e s o r c i n o l -f o r m a l d e h y d e r e s i n ( P R F r e si n : D - 3 3 ; O s h i k a R e s i n , T o -k y o , J a p a n ) w a s u s e d a s an a d h e s i v e b e t w e e n e l e m e n t s . T h er e s i n s p r e a d r a t e p e r a s i n g l e g l u e l i n e w a s 3 5 0 g / m 2 f o r t h ew o o d - t o - f i b e r g lu e l i n e i n f i b e r - w o o d c o m p o s i t e s . F i b e r v o l -u m e w a s a b o u t 5 0 % i n t h e f i b e r - r e s in la m i n a . T o o b t a i nY o u n g ' s m o d u l u s o f w o o d v e n e e r , a co n t r o l sp e c i m e n w a sp r o d u c e d i n w h i c h t h e r e s i n s p r e a d r a t e p e r a s in g l e g l u e l i n ew a s 2 5 0 g / m 2 f o r t h e w o o d - t o - w o o d g l u e l in e w i t h o u t f i b err e i n f o r c e m e n t . A f t e r t h e c o m p o s i t e s w e r e a s s e m b l e d , t h e yw e r e p r e s s e d t o g e t h e r f o r a t l e a s t 4 8 h a t r o o m t e m p e r a t u r ea n d a p r e s s u r e o f 9 8 k P a . I n t h e ca s e o f 6 m m l o n g c h o p p e dc a r b o n f ib e rs , sp e c i m e n s w e r e f a b r i c a t e d w i t h t h r e e o r i e n t a -t i o n s o f t h e f i b e rs : u n i d i r e c t i o n a l ( a l l f i b e r s i n t h e w o o dg r a i n d i r e c t i o n ) , p e r p e n d i c u l a r ( h a l f o f t h e f i b e r s i n e a c hd i r e c t io n ) , a n d r a n d o m o r i e n t a ti o n s . T h e o r i e n t a t i o n s o ff i b e r s a r e s h o w n i n F i g . 1. O t h e r f a b r i c a t i o n c o n d i t i o n sw e r e t h e s a m e a s s ta t e d a b o v e . T h e s p e c i m e n s a r e l is t e d i nT a b l e 1 .

V e n e e r - - - t - . . . . . . . . . . . . . . . . . . . . . 7 " "- ~ o a ~ , ~ z = z = z z z z z _ _ e e e e _ _ ~ z z z z z q f F ib er-resin am ina

Unidirectional U n i) Perp endic ularPerp) RandomFig. 1 . Compos i t ion of wood f ibe r r e inforcement sys t em ( top) andorienta t ion of shor t f ibers in the f iber-res in lamina (b ot tom )

T h e e l a st i ci t y o f t h e r e i n f o r c e m e n t s y s t e m w a s d e t e r -m i n e d b y t h e v i b r a t i o n m e t h o d . A s a m p l e ( 15 1 5 0 m m )w a s f i x ed a t o n e e n d a n d e x c i t e d b y i m p a c t i n g t h e o t h e r ,f r e e e n d . T h e r e s o n a n c e s i g n a l w a s d e t e c t e d b y a m i c r o -p h o n e a n d t h e r e s o n a n c e f re q u e n c ie s o b t a i n e d t h o u g h a nF F T a n a l y z e r . Y o u n g ' s m o d u l u s ( E ) o f t h e w o o d f i b er r e i n -f o r c e m e n t s y s t e m w a s o b t a i n e d b y t h e f o l lo w i n g e q u a t i o n .

4 ~ 2 1 4 p A f 2E - a21 ( 1 )w h e r e l i s t h e s a m p l e l e n g t h , p i s t h e d e n s i t y , A i s th e c r o s sa r e a ; I i s t h e m o m e n t o f i n e r t ia , f i s t h e r e s o n a n c e f r e -q u e n c y , a n d a n = 1 .8 7 5 w h e n f i s t h e f i rs t m o d e .

A c c o r d i n g t o t h e e x p e r i m e n t a l v a lu e o f t h e w o o d f i b err e i n f o r c e m e n t s y s t em , Y o u n g ' s m o d u l u s o f f i b e r - r e s i nl a m i n a ( E FR ) c a n b e c o n v e r s e l y c a l c u l a t e d b y t h e f o l l o w i n gm i x t u r e l a w in b e n d i n g .

( 2 )w h e r e E a n d ! = Y o u n g ' s m o d u l u s a n d th e m o m e n t o fi n e r ti a , r e s p e c ti v e l y , o f th e w o o d f i b e r r e i n f o r c e m e n t s y s-t e m , o b t a i n e d f r o m E q . ( 1) ; E w is Y o u n g ' s m o d u l u s o f t h ev e n e e r e l e m e n t c o m p u t e d f r o m t h e c o n t r o l sp e c im e n ; a n dI w a n d I FR a r e t h e m o m e n t s o f in e r t i a o f e ac h v e n e e r a n df i b e r - r e s i n l a m i n a e l e m e n t , r e s p e c t i v e ly .

S h o r t c a r b o n f i b e r - r e i n f o r c d p l y w o o dT o e x p l o r e t h e f e a s i b i li t y o f u s i n g s h o r t c a r b o n f i b er r e in -f o r c e m e n t i n w o o d c o m p o s i t e s , s h o r t c a r b o n f i b er -r e i n f o r c e d p l y w o o d s w e r e f a b r i c a t e d a n d t h e i r e l as t ic i t y a n ds t r e n g t h d e t e r m i n e d . T h e e x p e r i m e n t a l p l y w o o d s a r e l i st e di n T a b l e 2 .

T h e f i b e r - r e i n f o r c e d p l y w o o d s ( 3 0 0 3 0 0 r a m ) w e r et h r e e - p l y a n d f iv e - p ly p a n e l s , f a b r i c a t e d i n a l a b o r a t o r y .Y e l l o w m e r a t i ( S h o r e a s p p . ) r o t a r y v e n e e r s , 3 m m t h ic k ,w e r e u s e d . T h e P R F r e s in w a s a p p l i e d t o e a c h g l u e l in e a tt h e s a m e s p r e a d r a t e a s i n t h e a b o v e e x p e r i m e n t s . C h o p p e d

Table 1 . Desc r ip t ion of spec imens of woo d f ibe r r e inforcement sys t emTyp e of Fibe r - r e s in l amina Com pos i t espec imen Length ~ Or ienta t ionb Thickness c Thickness Av erag e(mm) (mm) (mm) SOW - - - 1.550 0.767FW3 3 Un i 0.224 1.998 1.010FW 6 6 Un i 0.267 2.084 0.972FWI4 14 Uni 0.284 2.118 0.946FWp 6 Pe rp 0.220 1.990 1.009FWR 6 Ran do m 0.306 2.162 0.953The measurem ent of t he compo s i t e th ickness w as ave raged for three point s on the spec imen.Va lues for t he th i ckness and spec if ic gravi ty (SG) of the com pos i t e we re obta ined f rom 10specimens.Length of shor t c a rbon f ibe r.bOrienta t ion of f ibers shown in Fig. 1.CThickness of the f iber-res in lamina es t im ated from the thickness of the com posi te


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T a b l e 2 . D e s c r i p t i o n o f s p e c i m e n s o f t e s t e d p l y w o o dT y p e s o f T h i c k n e s s A v e r a g e C o n s t r u c t i o nspecimen~ (mm) SG T h k . V b N o . o f V : N o . o f F - R dPl 9.23 0.50 3.2 3 -RP 1 9.82 0.55 3.2 3 2P2 15.54 0.47 3.2 5 -RP 2 15.79 0.56 3.2 5 4M e a s u r e m e n t o f t h e t h i c k n e s s o f p l y w o o d i s t h e s a m e a s i n T a b l e 1 .V a l u e s a n a v e r a g e o f 1 0 s p e c i m e n s ( 3 00 5 0 m m ) ."P~ , t h ree -p l y c o n t ro l p l y w o o d ; R P~ , t h ree -p l y f i b e r - re i n fo rced p l y -w o o d ; P2 , f i v e -p l y co n t ro l p l y w o o d ; RP2 , t h ree -p l y f i b e r - re i n fo rcedp l y w o o d .~ T h k . V , t h i c k n e s s o f v e n e e r ( r a m ) .* N u m b e r o f v e n e e r l a m i n a .N u m b e r o f f i b e r - r e si n la m i n a ( F - R )

c a r b o n f i b e r s , 6 m m l o n g , w e r e r a n d o m l y s p r e a d b e t w e e nv e n e e r s a t a r a t e 2 2 0 g / m 2. A s a r e f e r e n c e , c o n t r o l p l y w o o d sw i t h o u t f i b e r r e i n f o r c e m e n t w e r e p r o d u c e d . T h e f i b e r -r e i n f o r c e d p l y w o o d s w e r e f a b r i c a t e d a t 9 8 k P a a n d 1 4 5 Cf o r 6 m i n ( t h r e e - p l y p l y w o o d ) a n d 9 m i n ( f i ve - p l y p l y w o o d ) ;t h e c o n tr o l s w e r e a t t h e s a m e h o t p r e s s u r e a n d t e m p e r a t u r ef o r 5 a n d 8 m i n , r e s p e c t i v e l y .

U s i n g t h e f l ex u r al v i b r a t io n m e t h o d ( T G H m e t h o d ) , 7-mt h e d y n a m i c Y o u n g ' s m o d u l u s f o r b e n d i n g a n d p a n e l s h e a rm o d u l u s w e r e m e a s u r e d w i t h t h e b e a m . T h e p l a t e s h e a rm o d u l u s w a s o b t a i n e d u s i n g t o r s i o n a l v i b r a t i o n o f af r e e s q u a re p l a t e ( 30 0 3 0 0 m m ) / U s i n g t h e t h re e - p o i n ts t a t ic b e n d i n g t e s t , w e d e t e r m i n e d t h e b e n d i n g s t r e n g t h i nt h e d i r e c t i o n p a r a l l e l t o t h e f a c e g r a i n . B e a m s u s e d f o r t h ed y n a m i c a n d s t a t i c f l e x u r a l t e s t s w e r e 5 0 3 0 0 m m . T h es t a ti c b e n d i n g s p a n w a s 2 5 0 m m . T h e a v e r a g e o f 10 r e p l i -c a t e s c o m p r i s e d t h e v a l u e s f o r e l a s t i c it y a n d s t r e n g t h .

34 50 I0.8

0/mm06 V / / =50/ram

i I / / ~ - - ~ /3---20/mmo . 4 1 / / - - l O , m ma0 . 2 ~

0 .0 0 I 2 3F i b e r l e n g th ( r a m )

F i g . 2 . R e l a t i o n s b e t w e e n f i b e r l e n g t h a n d r e d u c e d f a c t o r u n d e r d i f f er -e n t c o n d i t i o n s

T a b l e 3 . S u m m a r y o f Y o u n g ' s m o d u l u s o f w o o d - f ib e r r e i n f o r c e m e n tsy s t em w i t h v a r i o u s f i b e r l en g t h sT y p e o f s p e c i m e n ~ E b (G P a) EFR (G P a)W 13.2 (0 .75) -FW3 20.3 (1.70) 88.1FW~, 21.5 (2.1 5) 8 7.2FW,4 19.8 (1.83) 68.6~ Ex p re ssed i n Tab l e 1 .b E x p e r i m e n t a l v a l u e s o f w o o d - f i b e r r e i n f o r c e m e n t s y s t e m a v e r a g e df o r 1 0 s p e c i m e n s . S t a n d a r d d e v i a t i o n s o f e x p e r i m e n t a l v a l u e s a r e i np a r e n t h e s e s .~ V a l ue s o f f ib e r - r e s i n l a m i n a c o m p u t e d f r o m E q . ( 2 )

Results and discussionE f f e c t o f f i b e r l e n g t h o n e l a s ti c i tyA c c o r d i n g t o C o x ' s t h e o r y , i f a s t r a i g h t f ib e r o f l e n g th l ise m b e d d e d i n a s o l i d m a t r i x ( r e s i n ) i n t h e f i b e r - r e s i n r e i n -f o r c e m e n t s y s t e m , t h e t r a n s f e r o f l o a d f r o m f i b e r to f i b e rc o u l d r e d u c e t h e m o d u l u s o f t h e r e i n fo r c e m e n t . C o n s i d e r -i n g t h e m e a n s o f t r a n s f e r o f l o a d f r o m f i b e r t o f i b e r , t h ered u ce d f ac to r F~ i s t h e r a t i o 6F ~ = 1 t a n h f i l / 2 (3 )

wh ere 1 i s f i b e r l en g th , an dIf i = I' 27~Gra ( 4 )i / .~ t G A F l n f 1 /~ F

w h e r e (7 m i s t h e s h e a r m o d u l u s o f t h e m a t r ix ; E F a n d A F a r eY o u n g ' s m o d u l u s a n d t h e a r e a o f s e c t i o n o f t h e f i b er , r e -s p e c t iv e l y ; a n d V F d e n o t e s t h e f i b e r v o l u m e f r a c t i o n i n a

f i b e r - r e s i n l a m i n a . I f i l l ~ 2 i s l a r g e , t h e r e d u c e d f a c t o r F ~a p p r o a c h e s 1 . I f i l l ~ 2 i s smal l , i t b eco mes c lo se t o 0 . I n t h es h o r t c a r b o n f i b e r a n d P R F r e s i n - r e i n f o r c e m e n t s y s t e m , fi i sb e l i e v e d t o b e i n t h e r a n g e o f 1 0 / r a m t o 1 0 0 / m m , i f i t iss u p p o s e d i n E q . ( 4 ) t h a t E F = 2 3 5 G P a , G m = 1 - 4 G P a , A F =3.52~Z btm2, an d VF = 5 0 % . T h e c h an g es o f F, co r r esp o n d in gwi th 1 i n v a r i o u s /3 v a lu e s a r e sh o w n in F ig . 2 . I f t h e f i b e rl en g th e x ce ed e d a ce r t a i n l en g th , t h a t t h e f ac to r F~ i s i co u ldb e b e l i e v e d . I n o t h e r w o r d s , t h e r e d u c t i o n c o u l d b e p r o b -a b l y n e g li g i b le i n t h e t h e o r y w h e n t h e f i b e r is l o n g e n o u g h .

D e m o n s t r a t i n g t h e e f f e c t o f f i be r l en g t h o n Y o u n g ' sm o d u l u s , t h e e x p e r i m e n t a l v a l u e s a r e g i v e n i n T a b l e 3 . Ah i g h e r Y o u n g ' s m o d u l u s i s f o u n d i n t h e u n i d i r e c t i o n a l o r i -en t a t i o n (FW3 , F W6 , an d F W~4 ), r eg a rd l es s o f f i b e r l en g th .T h e r e w e r e n o t s i g n i f i c a n t d i f f e r e n c e s a m o n g s p e c i m e n sr e i n f o r c e d u s i n g 3 - a n d 6 - r a m l o n g c h o p p e d c a r b o n f i b e r s .H o w e v e r , Y o u n g ' s m o d u l u s o f th e s p e c i m e n s r e i n f o rc e d b y1 4 - m m l o n g c h o p p e d c a r b o n f i b e r s w a s a l i t t l e l o w e r t h a nt h a t o f t h e o t h e r s . I t is s o m e w h a t d i f f e r e n t f r o m t h e a b o v et h e o r e t i c a n a l y si s . T h e r e a s o n s c o u l d l i e i n t h e f a c t t h a t t h el o n g e r f i b e r s a r e e a s i ly b r i d g e d a n d t h e h o l e s i n c r e a s e i n t h el a m i n a b e c a u s e t h e f i b e r - r e s i n l a m i n a o f F W 14 i s t h i c k e rt h a n t h e o t h e r s u n d e r t h e s a m e c o n d i t i o n s .


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34 6A s m e n t i o n e d a b o v e , t h e o r e t i c a l l y t h e d i r e c t e f f e c t o f

f ib e r l en g t h o n r e i n f o r c e m e n t c a n b e n e g l e c t e d w h e n t h ec a r b o n f i b e r s a r e l o n g e r t h a n 3 m m . H o w e v e r , w h e n t h ef i b e r i s t o o l o n g , o t h e r n e g a t i v e e f f ec t s c o u l d a p p e a r d u r i n ga r e a l a p p l i c a t i o n .E f f e c t o f o r i e n t a t i o n o f s h o r t f i b e rs o n e l a s ti c i tyI n a p l a n a r m a t o f f ib e rs , t h e e f f e c t o f o r i e n t a t i o n o f f ib e r so n t h e e l a s ti c i ty c a n b e a n a l y z e d u s i n g C o x ' s m e t h o d , tlW h e n f i b e r s a r r a n g e a t a n g l e 0 t o t h e a x i s d i r e c t i o n , t h el o a d - s t r a i n r e l a t i o n i s d e f i n e d a s

r l = K f ( g l c O s 2 0 -~- e 2 s i n 2 0 + ~ c o s O s i n O ) c o s 2 O f ( o ) d O0~z

P 2 = K f ( e l c s 2 0 + e2 s in2 0 + 4)cosOs inO)s in2Of(o )dO0

Q = K f ( e l c o s 2 0 + e2 s i n20 + q~ co s0s in0 )0cos 0 s i n Of(o)dO ( 5 )

w h e r e P I , P 2, e~, e 2 a r e t e n s i l e l o a d s ( P ) a n d s t r a i n s ( e ) i n t w oa x i s d i r e c t i o n s a t r i g h t a n g l e s , r e s p e c t i v e l y ; Q a n d q ) a r es h e a r l o a d s a n d s t r a i n s , r e s p e c t i v e l y ; K = a c o n s t a n t( i.e ., t h e p r o d u c t o f f ib e r Y o u n g ' s m o d u l u s a n d t h e t o t a ln u m b e r o f f ib e r s o f u n i t l e n g t h p e r u n i t o f a r e a ) ; f( 0) i s t h e" d i s t r i b u t i o n f u n c t i o n " e x p r e s s i n g t h e m a n n e r i n w h i c hf i b er s a r e g r o u p e d a c c o r d i n g t o t h e a n g l e 0 . I f f i b er s a r eu n i d i r e c t i o n a l l y o r i e n t e d a t o n e a x i s d i r e c t i o n i n w h i c h 0 =0 a n d fc0> = 1 , t h e Y o u n g ' s m o d u l u s o f t h e p l a n a r m a t o ff i b e r s i s K a t t h i s a x i s d i r e c t i o n f r o m E q . ( 5 ) . W h e n f i b e r sa r e h a l f a r r a n g e d a t t w o a x i s d i r e c t i o n s , r e s p e c t i v e l y , 0 = 0a n d r e/ 2, a n d f(0~ i s 1 /2 . T h e Y o u n g ' s m o d u l u s i s K / 2 a t e a c ha x is d i r e c t io n . W h e n t h e f i b er s a r e c o m p l e t e l y r a n d o m l yo r i e n te d , Y o u n g ' s m o d u l u s i s K / 3 , s u b s t i t u t i n g f(o) = 1/ re in toE q . ( 5 ) .

T h e e x p e r i m e n t a l v a l u es o f Y o u n g ' s m o d u l u s u n d e r v a r i-o u s o r i e n t a t i o n s o f f i b er s a r e l is t ed i n T a b l e 4 . A m o n g t h ef i b e r - r e s i n l a m i n a v a l u e s , d i f f e r e n c e s e x i s t a m o n g t h r e ec a s e s, i. e. , u n i d i r e c t io n a l , p e r p e n d i c u l a r , a n d r a n d o m o r i e n -t a t i o n ( F W 6 , F W p , a n d F W R ) . B a s e d o n t h e a d d i t i o n a l e f -f e c t s o f f i b e r le n g t h a n d o r i e n t a t i o n o f fi b er s , in t h e o r yY o u n g ' s m o d u l u s o f th e f i b e r - r e s i n l a m i n a ( ER R) w i t h s h o r tc a r b o n f i b e rs i n v a r i o u s o r i e n t a t i o n s c a n b e c a l c u l a t e d u s i n gt h e f o l l o w i n g r u l e o f m i x t u r e e q u a t i o n . 6E F R = E F V F F I C d -~ - ( 1 - - V F ) E R ( 6 )w h e r e E F a n d E R a r e Y o u n g ' s m o d u l i f o r c a r b o n f i b e r a n dm a t r i x ( r e s i n ) , r e s p e c t i v e l y ; V v d e n o t e s t h e f i b e r v o l u m ef r a c t io n ; F I a n d C a a r e t h e r e d u c e d f a c t o r s d r a w n b y l e n g t ha n d o r i e n t a t i o n o f s h o r t f ib e rs .

B e c a u s e t h e u s e d f i b e r w a s l o n g e n o u g h ( 6 m m ) , t h ea d d i t i o n a l e f f e c t o f f i b e r l e n g t h c a n b e n e g l e c t e d b a s e d o nt h e a b o v e a n a l y s is . I t c o u l d b e b e l i e v e d t h a t t h e r e d u c e df a c t o r F I = 1 i n E q . ( 6 ) b a s e d o n t h e a b o v e d i s cu s s i o n. I nr e g a r d t o t h e a d d i t i o n a l e f f e c t o f o r ie n t a t i o n , i f t h e r e d u c e d

T a b l e 4 . S u m m a r y o f Y o u n g ' s m o d u l u s o f w o o d - f i b e r r e i n f o r c e m e n ts y s t e m u n d e r v a r i o u s d i s t ri b u t i o n o f f i b e r sType o f E b(GPa) EFa (GPa)specimen" E x p . d E s t . e E s t . fFW 6 21 .5 (2 .15) 87 .2 122 .5 8 7 .2FW p 16 .3 (3 .28) 47 .6 63 .7 46 .1FW R 15 .0 (3 .75) 30 .0 43 .8 32 .1~Expressed in Table 1.UExperimental values o f wood-fiber reinforcement system averagedfor 10 specimens. Standard deviations of experimental values are inparentheses.CValues of fiber-resin lam ina.aExperimental values computed from Eq. (2).eEstimated values computed from Eq. (6) based on E F = 235 GP a andE a = 18.9GPa.fEstimated values computed from Eq. (6) based o n E F - 160GPa andE a - 14GP a

f a c t o r C d = 1 i s a s s u m e d f o r u n i d i r e c t i o n a l o r i e n t a t i o n , 6 i tc a n b e s e e n t h a t C a = 1 /2 f o r p e r p e n d i c u l a r o r i e n t a t i o n a n d1 /3 f o r r a n d o m o r i e n t a t i o n f r o m t h e a b o v e t h e r e s u l t s o n t h ee f f e c t o f f i b e r o r i e n t a t i o n o n e l a s ti c i t y . T h e c a l c u l a t e dr e s u l ts a r e c o n t a i n e d i n T a b l e 4 . I f E v = 2 3 5 G P a a n dE a = 1 8 . 9 G P a a r e a s s u m e d o n t h e b a s i s o f t h e s t a n d a r dc a r b o n f i b e r a n d r e s i n , ~2 t h e o r e t i c a l v a l u e s a r e l a r g e r t h a ne x p e r i m e n t a l o n e s b e c a u s e t h e r e w e r e s o m e d e f ec t s in t h ef i b e r - r e s i n l a m i n a i n p r a c t i c e ( e .g ., " b r i d g e s " b e t w e e nf i b e r s , h o l e s i n t h e r e s i n , o r o t h e r r e a s o n s ) . I f E v = 1 6 0 G p aa n d E R = 1 4 G P a , e s t i m a t e d f r o m t h e e x p e r i m e n t a l v a l u e o fF W 6 , a r e s u b s t i t u t e d , t h e c a l c u l a t e d v a l u e s a r e c l o s e t o t h ee x p e r i m e n t a l o n e s .

T h e d i f f e r e n t o r i e n t a t i o n s o f fi b e rs i n f l u e n c e d t h e e l a s ti c -i ty in t h e f i b e r r e i n f o r c e m e n t s y s t e m . T h e r e i n f o r c e d r e s u l tsf o r t h e e l a s t ic i ty a r e d i f f e r e n t w h e n f i b e r s a r e d i s t r i b u t e d a tu n i d i r e c t io n a l , p e r p e n d i c u l a r , a n d r a n d o m o r i e n t a t i o n s i n ac o m p o s i t e r e i n f o r c e m e n t .

F l e x u r a l a n d s h e a r p r o p e r t i e s o f f i b e r - r e i n f o r c e d p l y w o o dB a s e d o n t h e a b o v e a n a l y s is , 6 m m l o n g f ib e r w a s s u i t a b lef o r r ei n f o r c e m e n t , a n d r a n d o m o r i e n t a t io n o f fi b er s c a u s e ds o m e i m p r o v e m e n t i n Y o u n g ' s m o d u l u s f o r t h e s h o r t fi b err e i n f o r c e m e n t s y s t e m . T h e s h e a r r i g i d it y o f f i b e r - r e i n f o r c e dp l y w o o d i n s e r t e d i n t h e f i b e r c l o t h in 7 t/ 4 o r i e n t a t i o n w a sc l e a r ly s u p e r i o r t o t h a t in s e r t e d i n p e r p e n d i c u l a r o r i e n t a -t i o n , a s d e m o n s t r a t e d i n a p r e v i o u s s t u d y . 3 S i m i l a r l y , re /4o r i e n t a t i o n a n d r a n d o m o r i e n t a t i o n o f s h o r t f ib e r s c a n d is -p l a y h i g h e r re i n f o r c e m e n t t h a n o t h e r o r i e n ta t i o n s f o r s h e a rr i g id i t y o n t h e b a s is o f C o x ' s t h e o r y , 1~ e v e n t h o u g h t h ed e t a i l s s h o u l d b e i n v e s t i g a t e d f u r t h e r . M o r e o v e r , r a n d o mo r i e n t a t i o n o f f i b e rs i s s i m p l e a n d a p p l i c a b l e i n p r a c t i c e . T or e i n f o r c e n o t o n l y f l e x u ra l p r o p e r t y b u t a l s o s h e a r r i g i d it y ,6 m m l o n g c a r b o n f ib e r i n r a n d o m o r i e n t a ti o n w a s c h o s e n a sr e i n f o r c e m e n t i n f i b e r- r e in f o r c e d p l y w o o d .

F i g u r e 3 s h o w s t h e s h e a r r i g i d it y o f t h e f i b e r - r e i n f o r c e dp l y w o o d . C o m p a r e d w i t h t he c o n t r o ls , t h e i n c re m e n t s o f th ep a n e l a n d p l a t e s h e a r m o d u l i o f p l y w o o d w e r e r e m a r k a b l e


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Fig. 4. B e n d i n g p r o p e r t i e s o f f i b e r- r e i n f o r c e d plywoods. Each t e s tc o m p r i s e d 1 5 p i e c e s fr o m t h r e e e x p e r i m e n t a l p a n e l s . Open bar s , c o n -t r o l p l y w o o d ; h a t c h e d b a r s ' , f ib e r - r e in f or c e d

f o r b o t h k i n d s o f t h e r e i n f o r c e d p l y w o o d ( t h r e e - p l y a n df i v e - p l y ) . T h e v a l u e s w e r e m u c h h i g h e r t h a n t h o s e o f t h ec o n t r o l s . I t i s c o n c l u d e d t h a t s h o r t c a r b o n f i b e r r e i n f o r c e -m e n t i n r a n d o m o r i e n t a t i o n b e n e f i c ia l l y e n h a n c e s p a n e l a n dp l a t e s h e a r p r o p e r t ie s . B e c a u s e o f t h e a p p l i c a t i o n o f p l y-w o o d i n c o n s t r u c t i o n ( e .g . , w a l l s h e a t h i n g a n d p a l l e t s ) t h ep a n e l a n d p l a t e s h e a r m o d u l i a r e i m p o r t a n t f a c t o r s t o h e l pr e s is t a n g u l a r o r s h a p e d e f o r m a t i o n s i n s i d e s a n d s u r f ac e s .T h e s e r e s u lt s a r e s im i l a r to t h o s e i n t h e r e i n f o r c e m e n t w i t hc a r b o n f i b e r c l o t h i n ~ / 4 o r i e n t a t i o n . 3

F i g u r e 4 s h o w s t h e b e n d i n g p r o p e r t i e s o f t h e f i b e r -r e i n f o r c e d p l y w o o d . C o m p a r e d w i t h t h e c o n t r o l s , Y o u n g ' sm o d u l u s a n d t h e s t r e n g t h i n c r e a s e d t o d i f f e r e n t e x t e n t s .B e c a u s e f i b e r s w e r e l a i d b e t w e e n v e n e e r s i n t h e r e i n f o r c e dp l y w o o d , t h e m o m e n t o f i n e r ti a a b o u t t h e n e u t r a l a x is in t h et h i c k n e s s o f t h e f ib e r - r e s i n l a m i n a w a s l o w e r . Y o u n g ' sm o d u l u s o f t h e r e i n f o r ce d p l y w o o d f o r b en d i n g i n c r e a s edo n l y a li tt le c o m p a r e d w i t h t h e v a l u e s o f c o r r e s p o n d i n gc o n t r o l s . I t i s k n o w n t h a t t h e r e i s a p r o p o r t i o n a l i n c l i n a t i o nb e t w e e n Y o u n g ' s m o d u l u s a n d b e n d i n g s t re n g t h . B y i n s er t -

34 7i n g s h o r t f ib e r r e i n f o r c e m e n t b e t w e e n v e n e e r s i n p l y w o o dt h e r e in f o r c e d e f f e c t o n b e n d i n g s t r e n g th i s m o r e m a r k e dt h a n t h e e f f e c t o n Y o u n g ' s m o d u l u s . T h i s m i g h t b e a t t r i b -u t e d t o a " h y b r i d e f f e c t , ''6 w h i c h w i l l b e i n v e s t i g a t e d i nf u r t h e r s t u d i e s .

Conclus ionO n t h e b a s is o f t h e a b o v e t h e o r e t i c a l p r e s e n t a t i o n a n d e x -p e r i m e n t a l d e m o n s t r a t i o n , t h e e f f e c t s o f le n g t h a n d o r i e n t a -t i o n o f s h o r t f i b e r s o n t h e e l a s t i c p r o p e r t i e s o f s h o r t f i b e rr e i n f o r c e m e n t w e r e s t u d ie d . F r o m a p ra c t ic a l p o in t o f v i e w ,t h e t e c h n i c a l f e a s ib i l it y a n d r e i n f o r c e d f u n c t i o n s o f t h e re i n -f o r c e d p l y w o o d w i t h s h o r t f ib e r s w e r e e v a l u a t e d . T h e f o l -l o w i n g c o n c l u s i o n s c a n b e d r a w n .

T h e o r e t i c a l ly , i n t h e s h o r t f ib e r r e i n f o r c e m e n t s y s t e m t h ef i b e r l e n g t h d o e s n o t d i r e c t l y i n f l u e n c e t h e r e i n f o r c e m e n tw h e n t h e c a r b o n f i b e r l e n g t h i s l o n g e n o u g h . H o w e v e r , i ft h e f i b e r l e n g t h i s t o o l o n g , t h e r e i n f o r c e d r e s u l t i s n o t a sg o o d o w i n g t o t h e b r i d g e b e t w e e n f i b er s a n d th e i n c r e a s e o fh o l e s i n t h e g l u e l a y e r .

O r i e n t a t i o n s o f fi b er s in t h e w o o d - f i b e r c o m p o s i t e h a s am a r k e d i m p a c t o n t h e r e i n f o r c e m e n t . T h a t is , th e u n i d i re c -t i o n a l, p e r p e n d i c u la r , a n d r a n d o m o r i e n t a t i o n o f f i b er s i nf i b e r r e i n f o r c e m e n t e x h i b i t e d d i f f e r e n t e f f e c t s o n e l a s t i c i t y .F i b e r - r e i n f o r c e d p l y w o o d , w h i c h i s i n t e r n a l l y r e i n f o r c e dw i t h s h o r t c a r b o n f i be r s in r a n d o m o r i e n t a t i o n , s h o w e d ab e t t e r r e i n f o r c i n g i n f l u e n c e o n p a n e l s h e a r , p l a t e s h e a rm o d u l i, a n d b e n d i n g s t r e n g t h t h a n o t h e r m e c h a n i c a lp r o p e r t i e s .

R e f e r e n c e s

l . Spaun FD (1981) R e i n f o r c e m e n t o f w o o d w i t h f i b e rg l a s s . F o r P r o dJ 31(4):91-962. Smulski SJ, Ifju G (1987) Flexural b e h a v i o r o f g l a s s f i b e r r e i n -f o r c e d h a r d b o a r d . W o o d F i b e r S c i 19:313-3273 . X u H , Tana ka C , Nakao T , N is ino Y , Katayam a H (1996) F lexura ia n d s h e a r p r o p e r t i e s o f f i b e r - r e i n f o r c e d p l y w o o d . M o k u z a iGakkaishi 42:376-3824 . Tom im ura Y , Suzuk i I (1987) M a n u f a c t u r e o f m e d i u m d e n s i tyf i b e r b o a r d c o n t a i n i n g c a r b o n f i b e rs i n t h e c o r e l a y e r ( i n Japanese) .Mok uza i G akka ish i 33 :645-6495. Y anaga wa Y (1994) M e c h a n i c a l p r o p e r t i e s o f w o o d a n d g l a ss f i b e rc o m p o s i t e s ( i n Japanese) . Mok uza i G akka ish i 40 :1341-13476 . Fukuda H (1994) Fukugou za i ryou k iso kougaku ( in Japanese) .G e n d a i Koug aku Sya , Tokyo , pp 205-2637 . T im oshen ko SP (1921) On t h e c o r r e c t i o n f o r s h e a r o f t h e d i f f e r e n -t i a l e q u a t i o n f o r t r a n s v e r s e v i b r a t i o n s o f p r i s m a t i c b a r s . P h i l M agS e r 41:744-7648. Go ens E (1931) D e t e r m i n a t i o n o f Y o u n g ' s m o d u l u s f r om flexuralv i b r a t i o n s . A n n P h y s i k S e r 11:649-6789 . Hearm on RFS (1958) T h e i n f l u e n c e o f s h e a r a n d r o t a t o r y i n e r t i ao n t h e f r e e fl e x u r a l v i b r a t i o n o f w o o d e n b e a m s . B r J A p p l P h y s9:381-38810 . Nakao T , Okano T (1987) Eva lua t ion o f m o d u l u s o f r i g i d i t y b yd y n a m i c p l a t e s h e a r t e s t i n g . W o o d F i b e r S c i 19:332-338l i . C o x H L ( 1 95 2 ) T h e e l a s t i c i t y a n d s t r e n g t h o f p a p e r a n d o t h e rf ib r ou s m at e r ia l s . Br J App l Phys 3 (March) :72-7912 . Maki H , S im am ura S (1973) Fukugou Za i ryou G i tyu tus G i jyu tsuSyuuse i ( in Japanese) . Sangyou G i juu tsu Cen te r , Tokyo , pp 215 ,24 9

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