Optimization of the range of elastic behavior of unidirectional composites by prestraining
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R. J. ZAEHRING,
AND
H. P. CHESKIS
The effect of t e n s i l e p r e s t r a i n on the stage I t e n s i l e yield s t r e s s has b e e n studied both a n a l y t i c a l l y and e x p e r i m e n t a l l y for c o m p o s i t e s whose s t r e s s - s t r a i n c u r v e s obey the r u l e of m i x t u r e s . The m a t h e m a t i c a l a n a l y s i s p r o v i d e s a m e a n s for c a l c u l a t i n g the o p t i m u m a m o u n t of p r e s t r a i n , the r e s i d u a l s t r e s s e s (in the d i r e c t i o n of the f i b e r s ) in the m a t r i x and f i b e r m a t e r i a l s after unloading f r o m the p r e s t r a i n i n g , and the stage I yield s t r e s s in the c o m p o s i t e after the p r e s t r a i n t r e a t m e n t . It is shown that the i m p r o v e m e n t in stage I yield s t r e s s by p r e s t r a i n i n g is due to the d e v e l o p m e n t of n e g a t i v e r e s i d u a l s t r e s s e s in the m a t r i x . The stage I yield s t r e s s in c o m p o s i t e s with n e g l i g i b l e r e s i d u a l s t r e s s e s in the a s - f a b r i c a t e d condition can u s u a l l y be i m p r o v e d by a f a c t o r of two by p r e s t r a i n i n g ; the a m o u n t of i m p r o v e m e n t is even g r e a t e r if the a s - f a b r i c a t e d c o m p o s i t e s have the u s u a l s t a t e of r e s i d u a l s t r e s s , i.e., t e n s i o n i n the m a t r i x . E x p e r i m e n tal s t u d i e s on 2024 a l u m i n u m - t u n g s t e n c o m p o s i t e s ( f i l a m e n t - w o u n d ; h o t - p r e s s e d ) having t u n g s t e n f i b e r volume f r a c t i o n s between 0.08 and 0.40 v e r i f i e d the m a t h e m a t i c a l a n a l y s i s . The stage I yield s t r e s s e s m e a s u r e d in these c o m p o s i t e s after a p r e s t r a i n of 4.2 • 10-a w e r e in good a g r e e m e n t with p r e d i c t e d v a l u e s . I m p r o v e m e n t s of up to a factor of six w e r e found in the stage I yield s t r e s s as a r e s u l t of p r e s t r a i n ing. F I B E R r e i n f o r c e d c o m p o s i t e s c o n t a i n i n g l e s s than 0.50 v o l u m e f r a c t i o n of f i b e r s , Vf, u s u a l l y have stage II ( m a t r i x - p l a s t i c , f i b e r - e l a s t i c ) moduli which a r e s i g n i f i c a n t l y lower than the stage I ( m a t r i x and f i b e r both elastic) moduli. Thus, composite structural design is encumbered with a double modulus consideration. In addition, prediction of the onset of stage II, i.e., stage I yield point (al), is difficult because residual stresses which are developed in the composite because of differences in thermal and flow properties of the matrix and fibersI-8 can shift the stage I yield point.7-9 One solution to the problem is to increase Vf to a high enough level where the difference between the stage I and II moduli can be neglected in design considerations. The p r e s e n t c o s t s of r e i n f o r c i n g f i b e r s , h o w e v e r , make this solution r a t h e r c o s t l y . A n o t h e
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