Local yielding attending fatigue crack growth

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C Y C L I C c r a c k g r o w t h - - s t a g e II f a t i g u e ~ i s r e l a t i v e l y i n s e n s i t i v e to yield s t r e n g t h l e v e l and m i c r o s t r u c t u r e . 1 It follows f r o m this that h i g h - s t r e n g t h alloys e m p l o y e d at s t r e s s l e v e l s c o m m e n s u r a t e with t h e i r yield s t r e n g t h s a r e e s p e c i a l l y v u l n e r a b l e to fatigue c r a c k growth. It also i m p l i e s that growth r a t e m e a s u r e m e n t s have so far offered l i t t l e i n s i g h t as to the m e a n s and p r o s p e c t s of r e t a r d i n g c r a c k growth by m e t a l l u r g i c a l c h a n g e s . F i n a l l y , it r a i s e s q u e s t i o n s about the m e c h a n i s m of cyclic c r a c k growth (see P e l l o u x 2 for a c o m p r e h e n s i v e r e v i e w ) . The m e c h a n i s m s that have b e e n p r o p o s e d i n v a r i a b l y r e l a t e growth to the s t r a i n s and d i s p l a c e m e n t s within the p l a s t i c a l l y d e f o r m e d r e g i o n at the c r a c k tip which should be s e n s i t i v e to the y i e l d s t r e n g t h of the m a t e r i a l . The p r o b l e m of studying the c r a c k - t i p d e f o r m a t i o n is compounded by the s m a l l size of the d e f o r m e d r e gion and the r e l a t i v e i n a c c e s s i b i l i t y of the c r a c k tip. R i c e ' s 3 ~ a l e u l a t i o n s indicate that s t r e s s cycling c a u s e s a fourfold r e d u c t i o n in the p l a s t i c zone size of the c r a c k (independent of any effects of s t r e s s r e v e r s a l on the flow p r o p e r t i e s ) . A c c o r d i n g l y , for growth r a t e s in the neighborhood of ~0.1 # m / c y c l e the d e f o r m e d r e gion only extends N10 ~ m ahead of the c r a c k tip, wit i the l a r g e p e r - c y c l e s t r a i n s confined to an even s m a l l e r r e g i o n c o m p a r a b l e to the COD, -1 ~ m in s i z e . 4 V a r i o u s a p p r o a c h e s have been u s e d to study the zone. K l e s n i l and Lucas ~ have e x a m i n e d thinned s e c t i o n s d e r i v e d f r o m the zone ahead of the c r a c k on the e l e c t r o n m i c r o s c o p e ; Bowles and B r o e k 6 have studied thinned s e c t i o n s of the s t r a i n e d m a t e r i a l behind the c r a c k tip. Yokobori and c o w o r k e r s 7 have conducted X - r a y m i c r o b e a m s t u d i e s on the fatigue c r a c k zone. Liu and Iino 8 have m e a s u r e d c r a c k tip s t r a i n s by way of the Moir~ technique. Bowles 9 has r e c e n t l y m e a s u r e d C O D - v a l u e s of fatigue c r a c k s by i n f i l t r a t i n g the c r a c k cavity with a p l a s t i c . F i n a l l y , Hahn and c o w o r k e r s 1'~~have applied the F e - 3 S i etching technique to r e v e a l the fatigue c r a c k p l a s t i c zone in the i n t e r i o r of the m e t a l . G. T. HAHN, R. G. HOAGLAND,and A. R. ROSENFIELD are Division Chief, Research Metallurgist,and Associate DivisionChief, respectively, Deformation and Fracture Research Division, Metal Sc

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