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T H E l a r g e v a r i a t i o n of fatigue c r a c k g r o w t h r a t e s of high s t r e n g t h a l u m i n u m a l l o y s can be a t t r i b u t e d to t h r e e m a i n f a c t o r s : v a r i a t i o n s in a l l o y c h e m i s t r y , m e t a l l u r g i c a l p r o c e s s i n g v a r i a b l e s , and e n v i r o n m e n t . T h e d o m i n a n t influence of the e n v i r o n m e n t , and m o r e s p e c i f i c a l l y of w a t e r , h a s b e e n w e l l d o c u m e n t e d d u r i n g the l a s t ten y e a r s , l-s The A I - Z n - M g (7075 type) a l l o y s a r e s p e c i f i c a l l y s u s c e p t i b l e to c o r r o s i o n fatigue in high h u m i d i t y a i r o r in aqueous e n v i r o n m e n t s . Since the e n v i r o n m e n t a l e f f e c t i s due to the r e a c t i o n b e t w e e n the c o r r o s i v e m e d i a and the f r e s h l y e x p o s e d m e t a l s u r f a c e s at the c r a c k t i p , the t i m e a v a i l a b l e for the r e a c t i o n should be the m o s t i m p o r t a n t t e s t v a r i a b l e . T h e t o t a l c r a c k tip e x p o s u r e t i m e p e r c y c l e can be v a r i e d by changing the t e s t f r e q u e n c y . Within e a c h c y c l e , h o w e v e r , i t i s i m p o r t a n t to d i s t i n g u i s h b e t w e e n the opening t i m e , the t i m e at p e a k l o a d , and the c l o s i n g t i m e . The r e a s o n for t h i s d i s t i n c t i o n is b a s e d on the f a c t that it h a s been c l e a r l y e s t a b l i s h e d that fatigue c r a c k e x t e n s i o n in an i n e r t e n v i r o n m e n t t a k e s p l a c e only d u r i n g the opening p a r t of the load c y c l e . 6 With the new h y d r a u l i c m a c h i n e s , the opening o r r i s e t i m e p e r c y c l e can be v a r i e d by u s i n g d i f f e r e n t s t r e s s wave s h a p e s ( s i n u s o i d a l , s q u a r e , p o s i t i v e - s a w t o o t h , n e g a t i v e - s a w t o o t h ) at 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 . The s t r e s s c o r r o s i o n c r a c k advance taking p l a c e d u r i n g the t i m e At at p e a k l o a d can be m i n i m i z e d by k e e p i n g the m a x i m u m s t r e s s i n t e n s i t y l e v e l below the s t r e s s c o r r o s i o n t h r e s h o l d KISCC. Although s o m e of the high s t r e n g t h a l u m i n u m a l l o y s a r e known to be highly s u s c e p t i b l e to s t r e s s c o r r o s i o n c r a c k i n g , KISCC in the l o n g i t u d i n a l and long t r a n s v e r s e d i r e c tions a r e above the r a n g e of K m a x i m u m u s e d for fatigue c r a c k g r o w t h t e s t i n g . A s a c o n s e q u e n c e , the r e s u l t s of t h i s study r e p r e s e n t the s y n e r g i s t i c i n t e r a c t i o n b e t w e e n c o r r o s i o n and fatigue c r a c k growth below KISCC. Some r e s u l t s on the e f f e c t s of the c y c l i c s t r e s s wave f o r m on the c o r r o s i o n fatigue c r a c k p r o p a g a t i o n r a t e below KISCC have been r e R. J. SELINE