Acoustic Emission from metals during unloading and its relation to the bauschinger effect
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Acoustic Emission from Metals During Unloading and Its Relation to the Bauschinger Effect N. G. S A N K A R , J . R. F R E D E R I C K , AND D. K. F E L B E C K
unload emission from several annealed metals correl a t e s w i t h t h e m a g n i t u d e of t h e B a u s c h i n g e r e f f e c t in these metals. Hence, it appears that a plausible exp l a n a t i o n f o r t h e p r e s e n c e of u n l o a d e m i s s i o n i s t h a t i t i s t h e r e s u l t of t h e a b r u p t s p r i n g - b a c k of p i l e d - u p dislocations when the load on a specimen is removed. The acoustic emission tests were carried out using flat tensile specimens that were loaded in a specially d e s i g n e d low n o i s e - l e v e l t e n s i l e t e s t i n g m a c h i n e . T h e emission was detected by a piezoelectric transducer, t h e o u t p u t of w h i c h w a s f e d i n t o a l o w - n o i s e a m p l i f i e r . T h e a m o u n t of a m b i e n t n o i s e r e c o r d e d b y t h e s y s t e m w a s r e d u c e d by t h e u s e of a b a n d - p a s s f i l t e r w h i c h e l i m i n a t e d a l l f r e q u e n c i e s e x c e p t t h o s e in a b a n d r a n g i n g f r o m 6.0 to 1 2 . 0 kHz. T h e m e a s u r e d p e a k v a l u e of t h e b a c k g r o u n d n o i s e l e v e l w i t h i n t h i s b a n d w i d t h w a s 4 ~v. A l l s i g n a l s a b o v e t h i s l e v e l w e r e c o u n t e d . T h e l i m i t of s t r a i n s e n s i t i v i t y of t h e s y s t e m w a s c a l c u l a t e d to b e of t h e o r d e r of 10 -ix o n t h e b a s i s of t h e s i z e of s p e c i m e n u s e d a n d t h e c h a r a c t e r i s t i c s of t h e P Z T - 5 p i e z o e l e c t r i c t r a n s d u c e r e m ployed to detect the emission. T h e a m o u n t of e m i s s i o n w a s d e t e r m i n e d b y c o u n t i n g t h e b u r s t s of n o i s e on a n e l e c t r o n i c c o u n t e r a n d p l o t t i n g t h i s i n f o r m a t i o n on a n X - Y r e c o r d e r a s a f u n c t i o n of the a p p l i e d l o a d . T h e n e t p l a s t i c s t r a i n p r o d u c e d in t h e s p e c i m e n w a s m e a s u r e d by a s t r a i n g a g e a f t e r e a c h l o a d - u n l o a d c y c l e , A d e t a i l e d d e s c r i p t i o n of t h e e q u i p m e n t i s g i v e n in R e f . 6. T h e d e p e n d e n c e of t h e u n l o a d e m i s s i o n on t h e p l a s t i c strain is shown in Fig. 1for the materials that were s t u d i e d in t h e i n v e s t i g a t i o n . P l a s t i c s t r a i n i s s h o w n a s t h e i n d e p e n d e n t v a r i a b l e r a t h e r t h a n s t r e s s b e c a u s e of t h e c l o s e r e l a t i o n s h i p of t h e p h e n o m e n o n t o d i s l o c a t i o n motion.
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i F t h e r e i s a n a b r u p t r e l a x a t i o n of e i t h e r l o c a l i z e d o r long range stresses within a solid material, strain w a v e s a r e p r o d u c e d in t h e m a t e r i a l t h a t c a n b e d e t e c t e d b y m e a n s of a s e n s i t i v e p i e z o e l e c t r i c t r a n s d u
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