Air fatigue of notched 1018 steel in the endurance limit range
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H. HARTT,
JOSEPH
E. FLUET,
AND
THOMAS
E. HENKE
R e v e r s e bend, c o n s t a n t d e f l e c t i o n f a t i g u e t e s t s w e r e p e r f o r m e d in a m b i e n t t e m p e r a t u r e a i r on notched 1018 s t e e l s p e c i m e n s . E n d u r a n c e l i m i t for t h e s e e x h i b i t e d s c a t t e r in e x c e s s of 25 pct, and t h i s was c o r r e l a t e d with c o n c e n t r a t i o n of m a n g a n e s e sulfide i n c l u s i o n s at the notch r o o t v i c i n i t y . F o r s p e c i m e n s d e s i g n a t e d as " h i g h i n c l u s i o n " the e n d u r a n c e l i m i t was 95 MN//m 2 (14 ksi); and f o r " l o w i n c l u s i o n " ones it w a s 130 M N / m 2 (19 k s i ) . By m o n i t o r i n g c r a c k length a s a function of n u m b e r of c y c l e s it w a s o b s e r v e d that u n b r o k e n s p e c i m e n s t e s t e d a b o v e a p p r o x i m a t e l y 85 MN//m 2 (12.5 k s i ) c o n t a i n e d n o n p r o p a g a t i n g c r a c k s . A l s o , high i n c l u s i o n s p e c i m e n s f a t i g u e d at s t r e s s e s a b o v e t h e i r e n d u r a n c e l i m i t , but l e s s than about 140 MN//m 2 (20 k s i ) e x h i b i t e d fatigue c r a c k h e s i t a t i o n ; that i s , c r a c k s i n i t i a t e d and g r e w to m i c r o s c o p i c s i z e but then a r r e s t e d for a finite n u m b e r of s u b s e quent c y c l e s . The e n d u r a n c e l i m i t f o r high i n c l u s i o n s p e c i m e n s was then a s t r e s s a b o v e which a r r e s t e d c r a c k s r e p r o p a g a t e and b e l o w which they do not. R e p r o p a g a t i o n of h e s i tant fatigue c r a c k s i s p r o j e c t e d to r e s u l t f r o m i n c l u s i o n r e l a t e d c y c l i c s t r e s s d a m a g e n e a r the c r a c k tip, which was p o s s i b l y s t i m u l a t e d by a t m o s p h e r i c i n t e r a c t i o n s .
R A T I O N A L I Z A T I O N of fatigue in notched m e m b e r s i s an i m p o r t a n t p r a c t i c a l p r o b l e m , and it i s f o r t h i s r e a s o n that e m p h a s i s h a s b e e n p l a c e d upon it h i s t o r i c a l l y (for e x a m p l e , s e e R e f s . 1,2). Of p a r t i c u l a r i n t e r e s t have been c r i t e r i a w h e r e b y fatigue life in t h e p r e s e n c e of s t r e s s c o n c e n t r a t i o n s can be p r e d i c t e d f r o m s m o o t h s p e c i m e n d a t a . 3-8 The r e s e a r c h a p p r o a c h f o r s u c h e f f o r t s has g e n e r a l l y m o n i t o r e d c y c l e s - t o f a i l u r e Nf a s a function of v a r i o u s t e s t a n d / o r m a t e r i a l v a r i a b l e s . M o r e r e c e n t and c r i t i c a l e v a l u a t i o n s c o n s i d e r , h o w e v e r , that if fatigue p r o c e s s e s a r e to b e u n d e r s t o o d m e c h a n i s t i c a l l y then Nf m u s t be r a t i o n a l i z e d in t e r m s of i t s component p a r t s , w h i c h inc l u d e : 1) c r a c k i n i t i a t i o n 2) Stage I c r a c k g r o w t h and 3) Stage II c r a c k growth. F o r fa
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