Inhomogeneous thermal changes in copper during plastic elongation
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L. MOSS
AND
ROBERT
B. POND,
SR.
The radiant energy emitted from electrolytic tough pitch copper during plastic tensile elongation was recorded with a photoconductive detection system sensitive to wavelengths from 2 to 30 pro. Radiation measurements were made while Cu samples were deformed at strain rates from 0.56 to 172 s -I. Abrupt changes in emission were correlated with the intermittent plastic action of Lfiders bands. It is emphasized that the measurements were of the differences in radiation emitted by nearby points on a sample. The differences in the emitted radiation were due to the inhomogeneous nature of the deformation. A new calibration technique is described that accounts for the optical effects of changes in surface topography and the thermal radiating characteristics of a metal that arise during plastic deformation. This calibration technique was used to associate temperature differences with the radiation measurements and the localized nature of the plasticity. Temperature differences were observed at low average strains that were large enough to imply crack nucleation. Changes in the inhomogeneous temperature-strain data occur at the critical strains that have been reported for many metals. This correlation is very clear at the critical strains of 1.5, 7.5 and 16.4 pct. Whether or not there are real changes at the other critical strains is more speculative. T H E R E have been n u m e r o u s i n v e s t i g a t i o n s of t h e r m a l changes d u r i n g the p l a s t i c d e f o r m a t i o n of m e t a l s , but these have a l m o s t i n v a r i a b l y d e a l t with the a v e r a g e t e m p e r a t u r e change. The a c c o m p a n y i n g a n a l y s e s have, t h e r e f o r e , b e e n p r i m a r i l y d e s i g n e d to r e l a t e the m e c h a n i c a l work done with the r e s u l t a n t h e a t i n g and to make an e n e r g y b a l a n c e . However, t h e r e is often as much o r m o r e i n t e r e s t in the l o c a l i z e d t h e r m a l changes that develop as t h e r e is in a v e r a g e t e m p e r a t u r e changes. The i n t e r e s t in the l o c a l i z e d changes is p r a c t i c a l l y , as well as a c a d e m i c a l l y , m o t i v a t e d s i n c e p h y s i c a l p r o p e r t i e s d u r i n g d e f o r m a t i o n depend on the t h e r m a l state, r a t h e r than on the a v e r a g e t e m p e r a t u r e , of the d e f o r m ing m a t e r i a l . Tl~ere have been a l i m i t e d n u m b e r of m e a s u r e m e n t s of t e m p e r a t u r e changes in p l a s t i c z o n e s . Most of these o b s e r v a t i o n s have b e e n made on s a m p l e s s u b j e c t e d to e x t e n s i v e s h e a r d e f o r m a t i o n s i m i l a r to those found in the m a c h i n i n g p r o c e s s . In t h e s e s i t u a t i o n s , the s t r a i n o c c u r s within the s a m p l e , and t h e r e a r e often o b s t r u c t i o n s that p r e v e n t t h e r m a l m e a s u r e m e n t s as d e f o r m a tion o c c u r s . In spite of these d i f f i c u l t
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