The role of mechanical properties in low-stress fatigue crack propagation
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R.W. HERTZBERG
An e x p e r i m e n t a l i n v e s t i g a t i o n was u n d e r t a k e n to study the r e l a t i o n s h i p between m e c h a n i c a l p r o p e r t i e s and low s t r e s s fatigue c r a c k p r o p a g a t i o n . A t t e n t i o n was focused on the " f a t i g u e " o r " r e v e r s e d p l a s t i c z o n e " at the c r a c k tip, s i n c e it was felt that m a t e r i a l p r o p e r t i e s i n this r e gion were of p r i m e i m p o r t a n c e in the c r a c k propagation p r o c e s s . An effort was made to s i m ulate this r e g i o n through fully r e v e r s e d s t r a i n - c y c l i n g t e s t s on t e n s i l e s p e c i m e n s . M e c h a n i c a l p r o p e r t i e s o b t a i n e d from a n u m b e r of m a t e r i a l s b e f o r e a n d a f t e r s t r a i n cycling w e r e c o r r e lated with c r a c k p r o p a g a t i o n data f r o m the s a m e m a t e r i a l s . E v i d e n c e indicated that while monotonic t e n s i l e p r o p e r t i e s a r e i n a d e q u a t e for c o r r e l a t i o n p u r p o s e s , the cyclic s t r a i n - h a r d ening coefficient, the cyclic yield s t r e n g t h , and the e l a s t i c m o d u l u s a p p e a r to be i m p o r t a n t p a r a m e t e r s . T h i s was felt to be a n i n d i c a t i o n of the i m p o r t a n c e of s t r a i n cycling in the r e v e r s e d p l a s t i c zone in influencing the r a t e - g o v e r n i n g m e c h a n i s m s in fatigue c r a c k growth.
A p r o b l e m of s p e c i a l s i g n i f i c a n c e to the m a t e r i a l s e n g i n e e r is the s e l e c t i o n of m e t a l s and a l l o y s to r e s i s t fatigue c r a c k p r o p a g a t i o n . When adequate data is lacking o r a new a l l o y m u s t be c o n s i d e r e d , few g u i d e l i n e s a r e a v a i l a b l e to aid in making a sound choice a m o n g candidate m a t e r i a l s . This is p a r t i c u l a r l y t r u e when the a p p l i c a t i o n i n v o l v e s low s t r e s s fatigue. T h e o r e t i c a l c o n s i d e r a t i o n s a r e also of little help in s e p a r a t i n g o r r a n k i n g a l l o y s on the b a s i s of fatigue r e s i s t a n c e , s i n c e the m a t e r i a l p a r a m e t e r s c o n t r o l l i n g c r a c k propagation a r e not well u n d e r s t o o d . T h i s lack of a g r e e m e n t and u n d e r s t a n d i n g of the f u n d a m e n t a l s of fatigue c r a c k p r o p a g a t i o n has led to a p r o l i f e r a t i o n of t h e o r e t i c a l laws a n d equations for c r a c k growth. ~-x~ Of t h e s e , only T o m k i n s ' r e s u l t s a p p e a r u s e f u l f o r p r e dicting a b r o a d r a n g e of data, and here only i n the r a n g e of g r o s s p l a s t i c c y c l i n g . This is not to say that t h e s e t h e o r e t i c a l a p p r o a c h e s to fatigue a r e without v a l u e , for indeed, a n u m b e r of p e r t i n e n t c o n c l u s i o n s can be drawn from t h e s e s t u d i e s and c o r r e s p o n d i n g c r a c k growth rate o b s e
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