Sources of acoustic emission generated during the plastic deformation of 7075 aluminum alloy
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AND
F. P. HIGGINS
The p l a s t i c d e f o r m a t i o n of c r y s t a l l i n e solids will in g e n e r a l r e s u l t in the g e n e r a t i o n of a c o u s t i c e m i s s i o n . Identifying the a c t u a l s o u r c e s of the acoustic e m i s s i o n is difficult s i n c e the data may be c o m p r i s e d of e m i s s i o n s f r o m s e v e r a l s o u r c e s which may be o p e r a t i n g e i t h e r independently or c o o p e r a t i v e l y . By u s i n g a s e r i e s of t e s t i n g p r o c e d u r e s and s i m u l t a n e o u s l y m e a s u r i n g the d i s l o c a t i o n d a m p i n g while testing, it has been p o s s i b l e to identify the m a j o r s o u r c e s of a c o u s t i c e m i s s i o n in 7075-T6 and 7075-T651 a l u m i n u m a l l o y s . F o r the 7075-T651 alloy e s s e n t i a l l y all of the e m i s s i o n is f r o m the f r a c t u r e of s m a l l i n t e r m e t a l l i c p r e c i p i t a t e s d i s p e r s e d within the m a t r i x . F o r the alloy in the T - 6 t e m p e r t h e r e is additional a c o u s t i c e m i s s i o n o c c u r r i n g at the o n s e t of p l a s t i c d e f o r m a t i o n due to b r e a k away and s u b s e q u e n t motion of d i s l o c a t i o n line s e g m e n t s f r o m t h e i r p i n n i n g points.
T H E p l a s t i c d e f o r m a t i o n of c r y s t a l l i n e s o l i d s , in g e n e r a l , will r e s u l t in the g e n e r a t i o n of a c o u s t i c e m i s s i o n , In such c a s e s , it is p o s s i b l e to define a c o u s t i c e m i s s i o n as the low l e v e l sound or noise given off s p o n t a n e o u s l y when t h e r e is a r e l a x a t i o n of s t r e s s within the test m a t e r i a l . If the s o u r c e of the e m i s s i o n can be identified f r o m m e a s u r a b l e c h a r a c t e r i s t i c s , i.e. amplitude of e m i s s i o n , r a t e of e m i s s i o n s , e n e r g y , f r e q u e n c y components, and so forth, then a c o u s t i c e m i s s i o n m e a s u r e m e n t s may p r o v i d e a v a l u able tool for i n v e s t i g a t i n g b a s i c d e f o r m a t i o n m e c h a n i s m s . The i d e n t i f i c a t i o n of s o u r c e s is a difficult p r o b l e m which has had l i m i t e d s u c c e s s . The s o u r c e s of m e c h a n i s m s r e s p o n s i b l e for the g e n e r a t i o n of a c o u s t i c e m i s s i o n d u r i n g p l a s t i c d e f o r m a t i o n have b e e n a t t r i b u t e d by different i n v e s t i g a t o r s to the following: L u d e r ' s band f o r m a t i o n , 1 a l t e r n a t e slip o n t r a n s i e n t p l a n e s , 2 slip line f o r m a t i o n , 3 m i c r o c r a c k f o r ruction, 3 d i s l o c a t i o n b r e a k a w a y , 7"~ the mobile d i s l o c a tion density, ~~ slip a d v a n c e s 12 and r a p i d s o u r c e o p e r a t i o n , i.e. d i s l o c a t i o n m u l t i p l i c a t i o n , x3-15 It is of i n t e r e s t to note that, in a l l of the i n v e s t i g a t i o n s r e p o r t e d above, the s o u r c e
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