Influence of inclusion content on fatigue crack
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F a t i g u e c r a c k growth r a t e s w e r e m e a s u r e d at r o o m t e m p e r a t u r e in d r y a i r for t h r e e 7075-T6 a l u m i n u m a l l o y s with d i f f e r e n t i n c l u s i o n content. Volume f r a c t i o n s of i n c l u s i o n s w e r e d e t e r m i n e d for e a c h a l l o y by the point count m e t h o d with two d i f f e r e n t a u t o m a t e d s y s t e m s . P l o t s of the fatigue c r a c k growth r a t e (da/dN) vs the s t r e s s - i n t e n s i t y - f a c t o r r a n g e (zXK) show a w e l l defined change of slope at the t r a n s i t i o n between plane s t r a i n and plane s t r e s s f r a c t u r e . T h i s t r a n s i t i o n is a s s o c i a t e d with a m a r k e d i n c r e a s e in the amount of f r a c t u r e by void growth a r o u n d i n c l u s i o n s . The volume f r a c t i o n and m e a n s p a c i n g of voids within the c y c l i c p l a s t i c zone have been d e t e r m i n e d a s a function of h K by q u a n t i t a tive f r a c t o g r a p h y . F r a c t u r e by v o i d s i s i m p o r t a n t when the m e a n s p a c i n g of such v o i d s i s a p p r o x i m a t e l y equal to the width of the c y c l i c p l a s t i c zone in the plane of the c r a c k . It i s concluded that the i n c l u s i o n content i n c r e a s e s the f a t i g u e - c r a c k growth r a t e s only within the plane s t r e s s r a n g e , that i s f o r v a l u e s of the s t r e s s - i n t e n s i t y - f a c t o r r a n g e AK > 20 k p s i i~-n--:. 1u high s t r e n g t h a l u m i n u m a l l o y s contain l a r g e (1 to 10 u) c o n s t i t u e n t P a r t i c l e s which a r e n o n e q u i l i brium primary phases formed during solidification. The volume f r a c t i o n of t h e s e c o n s t i t u e n t p a r t i c l e s d e pends upon the p r o c e s s i n g h i s t o r y of the ingot and the l e v e l of i r o n and s i l i c o n i m p u r i t i e s in the a l l o y . The e x a c t influence, if a n y , of t h e s e l a r g e p a r t i c l e s on the r a t e of fatigue c r a c k p r o p a g a t i o n in a l u m i n u m a l l o y s is s t i l l open to question. In t h i s w o r k , the w o r d s , " c o n s t i t u e n t p a r t i c l e s " , " l a r g e p a r t i c l e s " , and " i n c l u s i o n s " will be a s s u m e d to be s y n o n y m s . On one hand, F o r s y t h 1 s t a t e d that the i n c l u s i o n s c a u s e the f o r m a t i o n of s m a l l c r a c k s in the p l a s t i c zone a h e a d of the m a i n c r a c k with which t h e s e c r a c k s u l t i m a t e l y link up, thus a c c e l e r a t i n g the growth r a t e . M c E v i l y , 2 on the o t h e r hand, has s u g g e s t e d that the i n c l u s i o n s a c t a s c r a c k a r r e s t o r s by r e d u c i n g the c r a c k tip s h a r p n e s s and l e a d i n g to c r a c k b r a n c h i n g thus slowing down the c r a c k g r o w t h r a t e s . P e l l o u x 3 has c o n f i r m e d F o r s y t h ' s s t a t e m e n t by c o m p a r i n g the m a c r o s c o p i c and
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