Fatigue crack initiation and early propagation in Al 2219-T851

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W H I L E it is not u n c o m m o n to s i m p l y c l a s s i f y the fatigue life of a t e s t s p e c i m e n into a c r a c k i n i t i a t i o n p h a s e and a c r a c k p r o p a g a t i o n phase, post c r a c k i n i t i a t i o n p r o c e s s e s can, r e c o g n i z a b l y , be m o r e c o m plex. In t e s t r e g i m e s o t h e r than that in which s p e c i m e n l i f e t i m e is e n t i r e l y i n i t i a t i o n d e t e r m i n e d , e.g., n e a r the fatigue e n d u r a n c e l i m i t , 1 t h e r e is the o p p o r tunity for c o o p e r a t i v e p h e n o m e n a such as m u l t i p l e m i c r o c r a c k c o a l e s c e n c e to s i g n i f i c a n t l y affect s p e c i m e n fatigue life. Such t r a n s i t i o n p e r i o d s b e t w e e n i n i t i a t i o n and the d e v e l o p m e n t of a t e r m i n a l c r a c k have, as d i s c u s s e d by F r e u d e n t h a l , 2 r e c e i v e d l i t t l e a t t e n t i o n . The r e s u l t s of Bowles and Schijve 3 s u g g e s t that c e r t a i n a l u m i n u m a l l o y s can have p a r t i c u l a r app l i c a b i l i t y in the study of i n i t i a t i o n and post i n i t i a t i o n phenomena. As in the case for other a l u m i n u m alloys, 3-5 i n i t i a t i o n i n A1 2219-T851 p r e d o m i n a n t l y o c c u r s at s u r f a c e s i t e s at b r i t t l e i n c l u s i o n s . The s u b s e q u e n t m i c r o c r a c k p r o p a g a t i o n is p r i n c i p a l l y n o n c r y s t a l l o g r a p h i c (Stage II): F a t i g u e above the e n d u r a n c e l i m i t p r o d u c e s m u l t i ple, s h o r t (100 ~ m ) data a c q u i s i t i o n .

PROCEDURES

Two heats of 2219-T851 aluminum (6.3Cu-0.3MnBal AI) having 75 and 200 ~m average transverse W. L. MORRIS is a Member of Technical Staff and O. BUCK is Group Leader, Science Center, Rockwell International, Thousand Oaks, CA 91360. H. L. MARCUS, formerly with the Science Center, Rockwell International, is now with the University of Texas at Austin, as a Professor in the MechanicalEngineering Department. Manuscript submitted June 9, 1975. METALLURGICALTRANSACTIONS A

RESULTS F o r both heats of A1 2219-T851, the v o l u m e d e n s i t y of i n t e r m e t a l l i c p a r t i c l e s l a r g e r t h a n 1 ~ m contained in t e s t s p e c i m e n s was i n e x c e s s of 108/cm 3. T h e s e tend to be elongated in the r o l l i n g d i r e c t i o n and a f t e r f i n a l s u r f a c e p r e p a r a t i o n m a n y of the i n c l u s i o n s s e c tioned by the s p e c i m e n s u r f a c e w e r e found to contain i n t e r n a l c r a c k s n o r m a l to the r o l l i n g d i r e c t i o n (Fig. VOLUME 7A, AUGUST 1976-1161

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