The determination of the relationship between strain and microhardness by means of visioplasticity
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I H E d e t e r m i n a t i o n of s t r a i n v a l u e s b y m e a n s of m i c r o h a r d n e s s m e a s u r e m e n t s is a technique which has b e e n applied by a n u m b e r of a u t h o r s , for example, Felix and Geiger, 1 Wilshaw 2 and Smith and Knott. 3 The technique is, of c o u r s e , only a p p l i c a b l e to m a t e r i a l s that s t r a i n h a r d e n . Its m a i n advantage is that it can be used to m e a s u r e p l a s t i c s t r a i n s e i t h e r on the s u r face or within the body of a s p e c i m e n o r s t r u c t u r a l component. Before it can be applied, it is n e c e s s a r y to e s t a b l i s h a c a l i b r a t i o n between m i c r o h a r d n e s s and s t r a i n . P r e v i o u s l y , c a l i b r a t i o n has b e e n a c c o m p l i s h e d b y s t r a i n i n g c o m p r e s s i o n o r t e n s i l e s p e c i m e n s and s e c t i o n i n g t h e s e p r i o r to h a r d n e s s m e a s u r e m e n t s . The p r i n c i p a l p r o b l e m s with p r e v i o u s c a l i b r a t i o n t e c h n i q u e s a r e that, f i r s t , s o m e p a r t i c u l a r , n o r m a l l y a u n i f o r m , s t r a i n d i s t r i b u t i o n m u s t be a s s u m e d in the c a l i b r a t i o n s p e c i m e n . Second, the m i c r o h a r d n e s s m u s t be a s s u m e d to be a function of s o m e s t r a i n component, such as the m a x i m u m t e n s i l e s t r a i n , as is effectively a s s u m e d by Wilshaw 2 or, a s s e e m s m o r e likely to be t r u e , to be a function of the effective s t r a i n . The e x p e r i m e n t d e s c r i b e d below m a k e s use of the v i s i o p l a s t i c i t y technique to d e t e r m i n e the s t r a i n d i s t r i b u t i o n in the c a l i b r a t i o n s p e c i m e n . This p r o v i d e s an a c c u r a t e c a l i b r a t i o n method v a l i d for c o m p l e x s t r a i n d i s t r i b u t i o n s . In addition, it is shown that m i c r o h a r d n e s s is a s i n g l e - v a l u e d function of effective s t r a i n . The r e l a t i o n s h i p b e t w e e n m i c r o h a r d n e s s and flow s t r e s s is i n v e s t i g a t e d and m i c r o s t r u c t u r e s of the d e f o r m e d m a t e r i a l a r e shown. 1. E X P E R I M E N T A L DETAILS An a x i s y m m e t r i c e x t r u s i o n p r o c e s s was s e l e c t e d for the d e t e r m i n a t i o n of the r e l a t i o n s h i p b e t w e e n m i c r o h a r d n e s s and s t r a i n . The r e a s o n s for this choice a r e that, f i r s t , the state of s t r e s s and s t r a i n v a r y f r o m point to point in the d e f o r m a t i o n r e g i o n 4,s and, t h e r e fore, the effect of a c o m p l e x state of s t r e s s and s t r a i n on the r e l a t i o n s h i p b e t w e e n m i c r o h a r d n e s s and s t r a i n J. N. ROBINSON and A. H. SHABAIKare Postgraduate Research Engineer and Associate Professor, respectively, Materials Department, School of Engineeringand Applied Science, University of California, Los Angeles,Calif. 90024. Man
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