Thermal degradation of the tensile strength of unidirectional boron/aluminum composites

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T H E upper u s e - t e m p e r a t u r e for a m e t a l m a t r i x c o m p o s i t e is often l i m i t e d not by the a p p r o a c h to the s o f t e n ing point of the m a t r i x , but r a t h e r by p r e m a t u r e d e g r a dation of the c o m p o s i t e s t r e n g t h by f a c t o r s such as t h e r m a l l y induced s t r e s s e s and c h e m i c a l r e a c t i o n b e tween the c o m p o n e n t s . T h i s d e g r a d a t i o n m a y r e s u l t f r o m weakening or f a i l u r e of the m a t r i x , the f i b e r , a n d / o r the f i b e r m a t r i x i n t e r f a c e . All t h r e e f a i l u r e modes have b e e n s e p a r a t e l y given as the c a u s e of s t r e n g t h d e g r a d a t i o n of B-A1 c o m p o s i t e s . 1'2'3 T y p i c a l c o m p o s i t e c o m p o n e n t s d i s p l a y a wide d i s p a r i t y in t h e r m a l expansion that leads to the g e n e r a tion of l a r g e i n t e r f a c i a l s h e a r or debonding s t r e s s e s . In B-A1, these s t r e s s e s can exceed the yield s t r e s s of the m a t r i x a l l o y at m o d e r a t e t e m p e r a t u r e s . N e v e r t h e l e s s , in a m e t a l m a t r i x composite, the r e s u l t i n g p l a s t i c d e f o r m a t i o n of the m a t r i x u s u a l l y c o n t r i b u t e s little to the c o m p o s i t e d e g r a d a t i o n . E x c e p t i o n s may o c c u r , however, w h e r e the c o m p o s i t e is r e p e a t e d l y cycled to t e m p e r a t u r e and c u m u l a t i v e t h e r m a l fatigue d a m a g e to the m a t r i x can r e s u l t . Shahinian1 a s s o c i a t e s fatigue induced m a t r i x s w e l l i n g with weakening of B(SiC)-A1 c o m p o s i t e s . The s e r i o u s n e s s of this effect depends on the d e g r e e of c o n s t r a i n t the m a t r i x e x p e r i e n c e s ; e . g . , the v o l u m e f r a c t i o n of f i b e r , the layup, and so forth. I n t e r f a c e f a i l u r e will p r e v e n t the d e s i r e d t r a n s f e r of load b e t w e e n the f i b e r s when the c o m p o s i t e is s t r e s s e d c a u s i n g e a r l y f a i l u r e of w e a k e r f i b e r s . F i b e r m a t r i x i n t e r f a c e f a i l u r e m a y r e s u l t f r o m the t h e r m a l s t r e s s e s d i r e c t l y or f r o m t h e s e s t r e s s e s in c o m b i n a tion with i n c r e a s e d c h e m i c a l r e a c t i o n at the i n t e r f a c e . This e x p l a n a t i o n has b e e n s u g g e s t e d by St/ahrke 2 for d e g r a d a t i o n of B-A1 c o m p o s i t e s a f t e r s t a t i c heating.

H. H. GRIMES and R. A. LAD are Head, Physics of Solids Section, and Chief, Materials Science Branch, respectively, NASA-LewisResearch Center, Cleveland,OH 44135. J. E. MAISEL is with the Department of Electrical Engineering,Cleveland State University, Cleveland, OH44115. Manuscript submitted May 23, t977.

METALLURGICALTRANSACTIONSA

While the t h e r m a l s t r e s s e s alone a r e u s u a l l y i n s u f ficient to c a u s e f i b e r f a i l u r e in a c o m p o s

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