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our u n d e r s t a n d i n g of fatigue c r a c k p r o p a gation it is e s s e n t i a l to i n c r e a s e our knowledge of the r o l e played by d i s l o c a t i o n s in this p h e n o m e n o n . T h i s p a p e r r e p o r t s o b s e r v a t i o n s we have made on the v a r i a tion of d i s l o c a t i o n c o n f i g u r a t i o n s in the v i c i n i t y of fatigue c r a c k tips. Our o b s e r v a t i o n s w e r e made u n d e r the t r a n s m i s s i o n e l e c t r o n m i c r o s c o p e on s l i c e s cut f r o m m a t e r i a l in the r e g i o n s i m m e d i a t e l y ahead of fatigue c r a c k tips in copper s i n g l e c r y s t a l s . T h i s work follows the work of K l e s n i i and c o w o r k e r s x'2 on d i s location s u b s t r u c t u r e s developed a r o u n d the c r a c k tip in fatigued c o p p e r . E X P E R I M E N T A L PROCEDURE

Single c r y s t a l s of 99.999 pct p u r e OFHC copper w e r e grown by a modified B r i d g e m a n technique. T h e s e c r y s t a l s w e r e s p a r k cut into b a r s of d i m e n s i o n s 3 by 6 by 30 m m . The s p e c i m e n s w e r e notched with an Elox s p a r k cutting m a c h i n e u s i n g a 0.0031 in. (0.079 mm) w i r e saw. (The notch b e c a m e the fatigue c r a c k n u c l e a t i o n site and its e x i s t e n c e m i n i m i z e d the fatigue c r a c k n u c l e a t i o n time.) The s p e c i m e n o r i e n t a t i o n s w e r e d e t e r m i n e d f r o m Laue b a c k - r e f l e c t i o n X - r a y s . F i g . 1 shows the o r i e n t a t i o n s of our s p e c i m e n s . The s p e c i m e n s w e r e a n n e a l e d for 2 h in a v a c u u m at 800~ w e r e f u r n a c e cooled, and w e r e e l e c t r o p o l i s h e d to e l i m i n a t e s u r f a c e flaws. P u l l - p u l l fatigue t e s t s w e r e c a r r i e d out in a hydraulic a p p a r a t u s c o n s t r u c t e d in our l a b o r a t o r y . 3 The s p e c i m e n s w e r e c y c l i c a l l y s t r e s s e d for 9.5 x 10 -5 to 1.1 x 106 c y c l e s at a f r e q u e n c y of 40 Hz. The cyclic s t r e s s had an a m p l i t u d e of 73 M N / m 2 in t e n s i o n (35 M n / m 2 r e s o l v e d s h e a r s t r e s s on the m o s t favored slip plane). T h e fatigue c r a c k s w e r e b e t w e e n 0.6 and 0.8 m m in length when the s p e c i m e n s w e r e r e m o v e d f r o m the fatigue m a c h i n e . The s p e c i m e n s next w e r e s e c t i o n e d with the s p a r k m a c h i n e . A cut, p a r a l l e I to the plane of the fatigue c r a c k , was made on e i t h e r side of the fatigue c r a c k , F i g . 2. The cuts w e r e made 6 m m a p a r t . The c e n t e r A. H. PURCELL and J. WEERTMANare Graduate Research Assistant, Materials Science Department, and Walter P. Murphy Professor of Materials Science, respectively, Technological Institute, Northwestern University, Evanston, I11.60201. This paper is based upon a thesis submitted by A. H. PURCELL in partial fulfillment of the requirements of the degree of Master of Science a