Holographic detection of fatigue-induced surface deformation and crack growth in a high-strength aluminum alloy
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W. L. HAWORTH, V. K. SINGH, AND R. K. MUELLER Changes in optical c o r r e l a t i o n intensity (I c) a r e o b s e r v e d during fatigue cycling of 2024-T3 aluminum a l l o y . The I c m e a s u r e m e n t s a r e made by t r a n s m i t t i n g light s c a t t e r e d f r o m the s p e c i m e n s u r f a c e through a holographic f i l t e r containing information about the s u r f a c e topography at an e a r l i e r t i m e . Topographic changes such as slip band development, m i c r o c r a c k i n g , and c r a c k propagation a r e o b s e r v e d and r e c o r d e d in s itu during fatigue c y cling of individual s p e c i m e n s and cause c o r r e s p o n d i n g changes in c o r r e l a t i o n intensity. A t h r e e - s t a g e curve log (Ic) v s number of fatigue c y c l e s is o b s e r v e d for both urmotched and notched s p e c i m e n s . The o v e r a l l shape of the curve is not affected by the applied s t r e s s levels in constant amplitude t e s t s . The in s i t u m e t a l l o g r a p h i c o b s e r v a t i o n s confirm that region A of the c o r r e l a t i o n intensity curve c o r r e s p o n d s to p r o g r e s s i v e roughening of the s p e c i m e n surface caused by slip during the e a r l y p a r t of the fatigue life, together with a r a p i d i n c r e a s e in the number of m i c r o c r a c k s of the o r d e r of a few m i c r o m e t e r s in length. Few m e t a l l o g r a p h i c changes a r e o b s e r v e d during region B of the curve, where the c o r r e lation intensity r e m a i n s r e l a t i v e l y constant. The a c c e l e r a t i n g l o s s of c o r r e l a t i o n intensity in region C of the curve a r i s e s f r o m the e l a s t i c and p l a s t i c d i s p l a c e m e n t s which occur a s a c r a c k or c r a c k s grow beyond about 10 ~m in length. The m e t a l l o g r a p h i c o b s e r v a t i o n s a l s o show that for both notched and unnotched s p e c i m e n s , the c o r r e l a t i o n intensity r e a d ings in region C a r e sensitive to f a c t o r s such as c r a c k branching, c r a c k - t i p p l a s t i c i t y , and changes in c r a c k growth d i r e c t i o n as well as to the o v e r a l l i n c r e a s e in c r a c k length. The total l o s s of c o r r e l a t i o n intensity f r o m the beginning of fatigue cycling to the d e v e l opment of a c r a c k about 800 p m in length can be m o r e than eight o r d e r s of magnitude at the p r e s e n t s e n s i t i v i t y of our e x p e r i m e n t s . The optical c o r r e l a t i o n technique is an e x t r e m e l y sensitive method of detecting r e m o t e l y , in a i r , fatigue damage, and the p r o p a g a tion of fatigue c r a c k s f r o m ten to s e v e r a l hundred m i c r o m e t e r s in length. The c o r r e l a tion intensity curve provides an indication of developing fatigue damage and impending fatigue failure in individual s p e c i m e n s , and d e t e c t s the onset of c r a c k propagation with no p r i o r knowledge of the p r e s e n c e or p
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