An acoustic emission investigation of microscopic ductile fracture
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-[~-ELATIONSHIPS b e t w e e n f r a c t u r e t o u g h n e s s t e s t p a r a m e t e r s and the change in m i c r o s c o p i c c r a c k growth m e c h a n i s m s m u s t b e known in o r d e r to allow d e v e l o p m e n t of a l l o y s with i n c r e a s e d r e s i s t a n c e to c r a c k p r o p agatton. The g e n e r a l d e s c r i p t i o n of slow c r a c k growth can b e c h a r a c t e r i z e d through e x "post facto f r a c t u r e surface investigations via electron transmission or scanning fractographic techniques. Alternatively, stand a r d m e t a l l o g r a p h i c e x a m i n a t i o n p e r p e n d i c u l a r to the f r a c t u r e plane m a y be m a d e . However, t h e s e t e c h n i q u e s do not give unambiguous d e t a i l about the c h r o n o l o g y o r c h a r a c t e r i s t i c s of the i n d i v i d u a l c r a c k i n g e v e n t s . In situ m e a s u r e m e n t s a r e r e q u i r e d in o r d e r to know the e x t e r n a l c o n d i t i o n s at the e x a c t i n s t a n t of the c r a c k event, a s well a s the c h a r a c t e r i s t i c s (i.e., s i z e , f r e quency) of the i n t e r n a l event. The a c o u s t i c e m i s s i o n technique, which h a s the p o t e n t i a l of d e t e c t i n g i n d i v i d ual c r a c k e v e n t s , ~'2 coupled with g r o s s c r a c k - g r o w t h m e a s u r e m e n t t e c h n i q u e s , such as c r a c k - o p e n i n g d i s placement or electrical resistance measurement, pres e n t s the p o s s i b i l i t y of studying c h a n g e s in individual c r a c k j u m p c h a r a c t e r i s t i c s . The u t i l i z a t i o n of t h e s e t e c h n i q u e s along with the ones p r e v i o u s l y m e n t i o n e d p o t e n t i a l l y r e p r e s e n t t h e a b i l i t y to a c h i e v e c o m p l e t e c h a r a c t e r i z a t i o n of the v a r i o u s p h e n o m e n a affecting slow c r a c k growth p r o p a g a t i o n . The f i r s t s t e p in d e v e l o p i n g t h i s c a p a b i l i t y is to d e termine the relationship between a single acoustic e m i s s i o n e v e n t ( s t r e s s - w a v e a m p l i t u d e ) and the a r e a t r a v e r s e d b y the a d v a n c i n g c r a c k in the c o r r e s p o n d i n g instant. Several previous publications describe anal y t i c a l and s e m i - e m p i r i c a l e x p e r i m e n t a l r e l a t i o n s h i p s b e t w e e n c r a c k growth a r e a swept out b y an a d v a n c i n g c r a c k and the r e s u l t a n t SWE SWE.1-8 An e x a c t e x p e r i mental
r e l a t i o n s h i p b e t w e e n s t r e s s - w a v e a m p l i t u d e , load d r o p d u r i n g c r a c k e x t e n s i o n , and i n c r e m e n t a l c r a c k a r e a swept out h a s r e c e n t l y b e e n r e p o r t e d b y C-erberich and D e s a l f l Thus, this f i r s t s t e p h a s e s s e n t i a l l y b e e n c o m pleted. It was found that a s i n g l e a c o u s t i c e m i s s i o n event can b e c o r r e l a t e d to the a r e a t r a v e r s e d b y an a d v a n c
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