Secondary damage in corrosion-fatigue of aluminum
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Table I. Plate Flexure Fatigue Test Results 2024-T4 Aluminum
Sample No.
Freq., cpm
Max. Fiber Stress, psi
No. of Cycles to Failure
Durationof Test, hrs
R A Y M O N D D. D A N I E L S A N D Fatigue-Tested Under AtmosphericConditions
W I L L I A M M. L O R K O V I C
T H E q u a n t i t a t i v e s t u d y of s e c o n d a r y d a m a g e p r o d u c e d i n c o r r o s i o n - f a t i g u e , i.e., s e c o n d a r y c r a c k i n g a n d l o c a l i z e d c o r r o s i o n p r o d u c e d in a d d i t i o n to t h e p r i n c i p a l f r a c t u r e , c a n b e of s i g n i f i c a n t h e l p in d e l i n e a t i n g t h e c h a r a c t e r i s t i c f e a t u r e s of c o r r o s i o n - f a t i g u e f o r v a r i o u s t e s t c o n d i t i o n s . T h e v a l u e of i n f o r m a t i o n o n t h e n u m b e r of s e c o n d a r y f r a c t u r e s i n i t i a t e d a t e a c h s t r e s s l e v e l a n d t e s t i n g s p e e d , a n d t h e r e l a t i v e c o n t r i b u t i o n of c o r r o s i o n p r o c e s s e s to t h e m o r p h o l o g y of t h e f r a c t u r e s and other damage produced, can be illustrated by res u l t s o b t a i n e d in c o r r o s i o n - f a t i g u e t e s t s o n a n a l u m i n u m a l l o y in a b r i n e e n v i r o n m e n t . The alloy studied was 2024-T4 aluminum in the form of 0.25 i n . p l a t e . S a m p l e s w e r e t e s t e d in f u l l y - r e v e r s e d bending in a Krause-type, plate-flexure fatigue machine. For corrosion-fatigue tests the samples were c o v e r e d t o p a n d b o t t o m w i t h a l a y e r of h e a v y w o o l f e l t s a t u r a t e d w i t h a 3 p c t s o l u t i o n of s o d i u m - c h l o r i d e i n distilled water. 1 T h e w e d g e - s h a p e d f a t i g u e s a m p l e s w e r e t e s t e d to f a i l u r e a t t h r e e s t r e s s l e v e l s in " d r y " f a t i g u e . T h e s t r e s s l e v e l s w e r e c h o s e n to g i v e f a t i g u e l i v e s of a p p r o x i m a t e l y 5 x 104, 5 • 105, a n d 5 x l 0 B c y c l e s . T h r e e c o r r e s p o n d i n g l y l o w e r s t r e s s l e v e l s w e r e u s e d in c o r r o s i o n - f a t i g u e t e s t s to a c h i e v e a p p r o x i m a t e l y t h e s a m e n u m b e r s of c y c l e s to f a i l u r e . T h e t e s t s w e r e r u n a t two d i f f e r e n t c y c l i c f r e q u e n c i e s , 1200 a n d 1900 c y c l e s p e r min (cpm). Data on the stress levels, cyclic frequenc i e s , c y c l e s to f a i l u r e , a n d d u r a t i o n of t e s t s a r e s h o w n in T a b l e I, a n d t h e S - N d a t a a r e p l o t t e d in F i g . 1. T h e r e w e r e two s a m p l e s r u n f o r e a c h t e s t c o n d i t i o n . A f t e r t e s t i n g to f a i l u r e , e a c h f r a c t u r e d f a t i g u e a n d corrosion-fatigue sample was cut longitudinally the l e n g t h of t h e s e c t i o n of u n i f o r m b e n d i n g m o m e n t a n d m o u n t e d f o r m e t a l l o g r a p h i c e x a m i n a t i o n of t h e l o n g i t u d i n a l c r o s
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