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High s t r e n g t h - l o w a l l o y (HSLA) s t e e l s a r e a r e l a t i v e l y new g r o u p of a l l o y s s i m i l a r to hot r o l l e d low c a r b o n s t e e l (HRLC) but having h i g h e r s t r e n g t h s a s a r e s u l t of c o m p o s i t i o n and p r o c e s s i n g v a r i a t i o n s . B e c a u s e t h e s e s t e e l s a r e of p o t e n t i a l use in a v a r i e t y of s t r u c t u r a l a p p l i c a t i o n s involving c y c l i c l o a d i n g a knowledge of t h e i r fatigue b e h a v i o r is i m p o r t a n t . F a t i g u e e x p e r i m e n t s w e r e p e r f o r m e d on s e v e r a l 80 k s i y i e l d s t r e n g t h HSLA s t e e l s and on c o n v e n t i o n a l HRLC s t e e l f o r c o m p a r i s o n . The HSLA s t e e l s w e r e a l l found to e x h i b i t s i m i l a r fatigue r e s i s t a n c e , and w e r e s u p e r i o r to HRLC s t e e l a t l o n g e r l i v e s . The e f f e c t s on f a t i g u e b e h a v i o r of two t y p e s of p l a s t i c p r e s t r a l n w e r e d e t e r m i n e d . W h i l e p r e s t r a i n s c a u s e d l a r g e i n c r e a s e s in m o n o t o n i c s t r e n g t h p r o p e r t i e s , such i m p r o v e m e n t s w e r e l a r g e l y l o s t in fatigue due to c y c l i c softening. T e n s i l e p r e s t r a i n s a r e m o r e d e t r i m e n t a l to f a t i g u e r e s i s t a n c e than c o m p r e s s i v e p r e s t r a i n s . F i n a l l y , it w a s found that HSLA s t e e l h a s a h i g h e r f a tigue notch s e n s i t i v i t y than HRLC s t e e l , h o w e v e r i t s notch f a t i g u e r e s i s t a n c e is s t i l l s u p e r i o r to t h a t of HRLC s t e e l .

T H E high s t r e n g t h - l o w a l l o y (HSLA) s t e e l s d e v e l o p e d in r e c e n t y e a r s have m a n y p o t e n t i a l a p p l i c a t i o n s . In p a r t i c u l a r , t h e s e m a t e r i a l s could r e p l a c e hot r o l l e d - l o w c a r b o n (HRLC) s t e e l w h e r e a d d i t i o n a l s t r e n g t h is r e q u i r e d o r w h e r e gage r e d u c t i o n s r e s u l t i n g in weight savings are desirable. A wide v a r i e t y of HSLA a l l o y s a r e now c o m m e r c i a l l y a v a i l a b l e , with y i e l d s t r e n g t h l e v e l s r a n g i n g up to 80 k s i (551 MPa); a l m o s t a t h r e e fold i n c r e a s e o v e r the 30 k s i (206 MPa) a v e r a g e y i e l d s t r e n g t h of c o n v e n t i o n a l hot r o l l e d s t e e l . 1 The i n c r e a s e d s t r e n g t h s a r e a c h i e v e d b y v a r i a t i o n s in c o m p o s i t i o n and p r o c e s s i n g , and a l l o y s having s i m i l a r s t r e n g t h l e v e l s a r e p r o d u c e d in d i f f e r e n t w a y s b y the v a r i o u s s t e e l c o m p a n i e s . F r o m the s t a n d point of c o m p o s i t i o n t h e r e a r e t h r e e p r i n c i p a l a l l o y i n g additions: vanadium, columbium, or titanium.2- 5 Other a d d i t i o n s a r e m a d e d e p e n d i n g on the p r o c e s s i n g c a p a b i l i t i e