Fatigue hardening in annealed and deformed aluminum and copper
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W H E N s u b j e c t e d to an a l t e r n a t i n g s t r e s s , an a n n e a l e d m e t a l h a r d e n s . This c y c l i c - h a r d e n i n g b e h a v i o r can be m e a s u r e d by o b s e r v i n g the change in the width of the s t r e s s s t r a i n h y s t e r e s i s loop as a function of the n u m b e r of c y c l e s of constant s t r e s s a m p l i t u d e . C y c l i c h a r d e n i n g h as been s t u d ie d as a function of s e v e r a l p a r a m e t e r s including the t e s t t e m p e r a t u r e 1-3 and the s t r e s s (or s t r a i n ) a m p l i t u d e . 4-8 A l o w e r t e s t t e m p e r a t u r e r e s u l t s in a h i g h e r h a r d e n i n g r a t e while the amount of c y c l i c s t r a i n h a r d e n i n g of a n n e a l e d m e t a l s g e n e r a l l y i n c r e a s e s with i n c r e a s i n g a m p l i t u d e . 7 The o b j e c t i v e of the p r e s e n t p a p e r is to study c y c l i c h a r d e n i n g as a function of the mode and the amount of a p r e d e f o r m a tion. EXPERIMENTAL
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Most of the experiments were done on polycrystalline 2024 aluminum and a few on polycrystalline OFHC copper. Specimens were machined from bars to a gage length L o of 0.75 in. with a diam of 0.25 in., and annealed for 1 hr at 400~ All deformations were carried out at room temperature. An I n s t r o n t e s t i n g m a c h i n e was u s e d f o r the c y c l i c d e f o r m a t i o n s in t e n s i o n - c o m p r e s s i o n . P r e c i s e a l i g n m e n t b e t w e e n the u p p e r and l o w e r g r i p s was o b t a i n e d with a s p e c i a l g r i p f i x t u r e in which the l o w e r g r i p is e n c l o s e d in a s t e e l c r u c i b l e f i l l e d with Wood's a l l o y . By m e l t i n g the a l l o y , the l o w e r g r i p can m o v e f r e e l y to b e c o m e a l i g n e d with the u p p e r g r i p . No d e f o r m a t i o n of the s p e c i m e n was o b s e r v e d d u r i n g s o l i d i fication. The s p e c i m e n s w e r e s u b j e c t e d to the t e n s i o n - c o m p r e s s i o n c y c l e s at a r a t e of 5 c p m with a c o n s t a n t m a x i m u m s t r e s s a m p l i t u d e of 925 kg p e r sq c m f or 2024 a l u m i n u m and 600 kg p e r sq c m f o r c o p p e r . The e x p e c t e d fatigue l i v e s f o r t h e s e s t r e s s l e v e l s would be about 5 • 108 c y c l e s f o r 2024 a l u m i n u m 9 and about 3 • 107 c y c l e s f o r c o p p e r . 1~ The s t r e s s a m p l i t u d e s w e r e i m p o s e d f r o m the v e r y f i r s t c y c l e , and w e r e so s e l e c t e d to r e t a i n m e a s u r a b l e p l a s t i c s t r a i n in the f i r s t
A. VAN DEN BEUKEL is Research Associate, Delft Institute of Technology, The Netherlands. M. S. MISRA and H. I. DAWSONare Research Assistant and Associate Professor, respectively, Division of Metallurgical Engineering, University of Washington, Seattle, Washington. Manuscript submitted February 9, 1970. METALLURGICALTRANSACTIONS
few c y c l e s . The h y s t e
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