Temperature changes in specimens in microplasticity tests
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A t e n s i l e s p e c i m e n and a gage a s s e m b l y were d e s i g n e d to m e a s u r e p l a s t i c s t r a i n at room t e m p e r a t u r e . The a s s e m b l y m o u n t s t h r e e c a p a c i t a n c e s t r a i n gages 120 deg a p a r t , and is held by r i b s with a 45 deg included angle so as to define a 2 in. (5.08 cm) gage length. T h i s novel cons t r u c t i o n e l i m i n a t e s s p u r i o u s effects due to the f i l l e t s and s h o u l d e r s of the s p e c i m e n . The s t r a i n s e n s i t i v i t y is + 1 x 10 -7. The cooling and h e a t i n g of a s p e c i m e n , a c c o m p a n y i n g the loading and u n loading, r e s p e c t i v e l y , in the e l a s t i c r a n g e , a r e shown to affect p l a s t i c s t r a i n r e a d i n g s in a m a n n e r not c o n s i d e r e d by other i n v e s t i g a t o r s of m i c r o p l a s t i c i t y . The t h e r m a l c o n t r i b u t i o n s to app a r e n t s t r a i n in s p e c i m e n s of n o r m a l i z e d 4340 s t e e l and a n n e a l e d I n v a r were d e t e r m i n e d and were found to be s i g n i f i c a n t at s t r a i n s l e s s than 10-5. S t r a i n m e a s u r e m e n t t e c h n i q u e s n e c e s s a r y for the e x c l u s i o n of the t h e r m a l d i m e n s i o n a l changes of the s p e c i m e n from the m e a s u r e m e n t of very small plastic strains are discussed. INVESTIGATIONS of m i c r o p l a s t i c i t y involve v e r y s m a l l s t r a i n s of the o r d e r of 10 -7 to 10 -4 in m a t e r i a l s that a r e s t r e s s e d , g e n e r a l l y much below t h e i r y i e l d s t r e n g t h s . The d i f f e r e n t s t r a i n m e a s u r i n g t e c h n i q u e s e m p l o y e d and the m i c r o p l a s t i c b e h a v i o r of s e l e c t e d m a t e r i a l s extended to v e r y s m a l l s t r a i n s a r e r e v i e w e d in a r e c e n t publication; 1 this publication r e p o r t s that m e a s u r e m e n t s of s m a l l s t r a i n s have b e e n made with c a p a c i t a n c e gages,2- 9 l i n e a r d i f f e r e n t i a l t r a n s f o r m e r s , ~~ T u c k e r m a n optical s t r a i n gages, 1s-x6 or e l e c t r i c a l r e s i s t a n c e s t r a i n gages. ~7-2z In p r e v i o u s i n v e s t i g a t i o n s a- 12 u s i n g c a p a c i t a n c e gages or l i n e a r d i f f e r e n t i a l t r a n s f o r m e r s , the gages were m o u n t e d on the s h o u l d e r s of the t e n s i l e s p e c i m e n or on the s p e c i m e n h o l d e r s . T h u s , t h e s e m e a s u r e m e n t s could not provide a t r u e m e a s u r e m e n t of the s t r a i n over the gage length alone, b e c a u s e the r e p o r t e d v a l u e s n e c e s s a r i l y i n c l u d e d the s t r a i n in the f i l l e t s and in those p o r t i o n s of the r e d u c e d s e c t i o n outside the gage length. In the i n v e s t i g a t i o n s e m p l o y i n g T u c k e r m a n optical s t r a i n gages ~3-'6 and those e m p l o y i n g e l e c t r i c a l r e s i s t a n c e s t
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