An analysis of the fatigue behavior of Al-Al 3 Ni composites
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M. FARAG
AND
MOHAMED
H. ABD
EL
LATIF
C o m m e r c i a l p u r i t y A1-AlsNi e u t e c t i c c o m p o s i t e s have b e e n p r e p a r e d by d i r e c t i o n a l s o l i d i f i c a t i o n at growth r a t e s r a n g i n g f r o m 9.63 • 10 -s to 1.0 m m / s . T h e c o m p o s i t e s w e r e t e s t e d u n d e r r o t a t i n g bending fatigue c o n d i t i o n s and the r e s u l t s w e r e a n a l y z e d using a m o d e l taking into c o n s i d e r a t i o n the d i f f e r e n c e in P o i s s o n ' s r a t i o of the p h a s e s , i n t e r f i b e r s p a c i n g and p r e m a t u r e f r a c t u r e of f i b e r s . The c a l c u l a t e d fatigue l i v e s w e r e shown to be in good a g r e e m e n t with the e x p e r i m e n t a l r e s u l t s at a t l s t r e s s l e v e l s . Both the e x p e r i m e n t a l r e s u l t s and c a l c u l a t i o n s show that the c o m p o s i t e fatigue l i f e i s not s e n s i t i v e to l a r g e c h a n g e s in the growth r a t e . The fatigue l i m i t was a l s o found to be i n s e n s i t i v e to the m o d e of fatigue loading and notches. At high s t r e s s l e v e l s , h o w e v e r , s m o o t h s p e c i m e n s t e s t e d under r e v e r s e d bending e x h i b i t e d l o n g e r l i v e s than notched s p e c i m e n s t e s t e d u n d e r cyclic tension-tension conditions.
P R E V I O U S w o r k on the fatigue b e h a v i o r of u n i d i r e c t i o n a l l y s o l i d i f i e d A1-A13Ni e u t e c t i c c o m p o s i t e s 1'2 h a s shown that t h e r e is a change in the m o d e of f r a c t u r e a s between high and low s t r e s s e s . At high s t r e s s a m p l i t u d e s the fatigue r e s i s t a n c e is c o n t r o l l e d by A13Ni f i b e r c r a c k i n g in p l a n e s of m a x i m u m s h e a r s t r e s s . At low s t r e s s a m p l i t u d e s w h e r e the s t r e s s c o n c e n t r a t i o n at the c r a c k tip is i n s u f f i c i e n t to c a u s e f i b e r f r a c t u r e , the fatigue r e s i s t a n c e of the c o m p o s i t e i s c o n t r o l l e d by the fatigue r e s i s t a n c e of the m a t r i x and the s t r e n g t h of the AlsNi f i b e r - A 1 m a t r i x i n t e r f a c i a l bond. In the l a t t e r c a s e e x c e s s i v e f i b e r b u c k l i n g was a l s o o b s e r v e d and was thought to c a u s e f a i l u r e b e c a u s e of l o c a l f i b e r m i s a l i g n m e n t . 2 T h e d i f f e r e n c e s in the m o d e of f r a c t u r e was r e f l e c t e d in the f r a c t u r e s u r f a c e which c o n s i s t e d p r i m a r i l y of e l o n g a t e d d i m p l e s at high s t r e s s a m p l i t u d e s . As the s t r e s s l e v e l d e c r e a s e d s h e a r w a i l s d i s p e r s e d a m o n g the e l o n g a t e d d i m p l e s i n c r e a s e d in n u m b e r and s i z e and at low s t r e s s l e v e l s the f r a c t u r e s u r f a c e c o n s i s t e d of s t e e p m a c r o s c o p i c f r a c t u r e w a l l s due to the fact that the c r a c k p r o p a
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