A comparison of microcrack closure load development for stage I and II cracking events for Al 7075-T651
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I ~ E C E N T L Y it has b e e n p o s s i b l e to make d i r e c t m e a s u r e m e n t s of c r a c k c l o s u r e s t r e s s e s developed by p r o p a g a t i n g n o n c r y s t a l l o g r a p h i c s u r f a c e m i c r o c r a c k s in A1 2219-T851.~-3 L a r g e c l o s u r e s t r e s s e s have b e e n m e a s u r e d for c r a c k s of g r a i n size and l e s s i n length. F o r A1 2219 the n o n c r y s t a l l o g r a p h i c c r a c k s i n i t i a t e at s m a l l i n t e r m e t a l l i c p a r t i c l e s at the s p e c i m e n s u r f a c e and then p r o p a g a t e into the m a t r i x . C l o s u r e s t r e s s e s developed a r e l a r g e s t when the humidity is low or when the c r a c k path a s s e e n f r o m the s u r f a c e is p a r t i c u l a r l y i r r e g u l a r . F o r c e r t a i n s p e c i a l c i r c u m s t a n c e s it is p o s s i b l e to e s t i m a t e the c l o s u r e s t r e s s f r o m m e a s u r e m e n t s made to d e s c r i b e the s u r f a c e c r a c k path i r r e g u l a r i t y . 2 F o r A1 2219, h u m i d i t y affects the c r a c k path and the c r a c k path i r r e g u l a r i t y , and such a n a l y s i s aids in s e p a r a t i n g the d i r e c t effect of h u m i d i t y on c l o s u r e s t r e s s f r o m the i n d i r e c t path s e n s i t i v e c o n t r i b u t i o n to c l o s u r e . A l t e r n a t e l y , it is found that the r e s i d u a l m i c r o c r a c k opening at z e r o load, 5 (0), can be u s e d to e s t i m a t e c r a c k c l o s u r e s t r e s s , ecc, by the r e l a t i o n s h i p a c c / Cfmax = c 5 (O)/L where L is the c r a c k length, and c is a c o n s t a n t . This has b e e n found to be v a l i d even when c r a c k path a n a l y s i s cannot be u t i l i z e d . 2 L i m i t e d o b s e r v a t i o n s , however, for s h o r t m i c r o c r a c k s in A1 2219 s u g g e s t that an a d d i t i o n a l c o r r e c t i o n to the above e x p r e s s i o n is r e q u i r e d for s i t u a t i o n s w h e r e the o b s e r v e d c r a c k c o m p l i a n c e is not p r o p o r t i o n a l to c r a c k length. 3 The p r i m e p u r p o s e of the work r e p o r t e d h e r e is to extend the m i c r o c r a c k c l o s u r e study to an a d d i t i o n a l alloy, A1 7075-T651. I n i t i a l s u r f a c e c r a c k i n g modes for this m a t e r i a l include both c r y s t a l l o g r a p h i c (Stage I) and n o n c r y s t a l l o g r a p h i c (Stage II), which allows one to c o m p a r e c l o s u r e s t r e s s e s for the two m o d e s for m i c r o c r a c k s on the s a m e s p e c i m e n . To be p r e c i s e , t h r e e modes of c r a c k i n i t i a t i o n / p r o p a g a t i o n a r e d i s -
W. L. MORRIS is a Member of Techmcal Staff, Rockwell International Science Center, Thousand Oaks, CA 91360. Manuscript submitted October 8, 1976. METALLURGICALTRANSACTIONSA
t i n g u i s h e d in the p r e s e n t work: a) c r a c k s that develop in a p e r s i s t e n t slip band a r e t e r m e d Stag
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