Usefulness of precracked charpy specimens for fracture toughness screening tests of titanium alloys
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HALL, AND C . M . PIERCE
Although the KIc v a l u e of a m a t e r i a l i s a v e r y useful m e a s u r e of f r a c t u r e t o u g h n e s s , i t s v a l i d e x p e r i m e n t a l d e t e r m i n a t i o n can be a complex and e x p e n s i v e p r o c e d u r e , not at p r e s e n t suited to r o u t i n e alloy s c r e e n i n g o r q u a l i t y c o n t r o l p u r p o s e s . To e x p l o r e the f e a s i b i l i t y of e s t i m a t i n g KIc in t i t a n i u m a l l o y s u s i n g t e c h n i q u e s that a r e m o r e convenient to p e r f o r m , i m pact and s l o w - b e n d t e s t s w e r e m a d e on e i t h e r V - n o t c h e d or f a t i g u e - p r e c r a c k e d Charpy s p e c i m e n s , and the r e s u l t i n g e n e r g y v a l u e s w e r e c o m p a r e d with the c o r r e s p o n d i n g a p p r o x i m a t e Klc v a l u e s . Of the v a r i o u s t e s t s studied, r e s u l t s f r o m five t i t a n i u m - b a s e d a l l o y s , two s t e e l s , and two a l u m i n u m a l l o y s showed that p r e c r a c k e d s p e c i m e n s b r o k e n i n s l o w - b e n d hold the m o s t p r o m i s e of giving e n e r g y v a l u e s that can be r e l a t e d to KIc. The b e s t c o r r e lation came f r o m s p e c i m e n s having m o s t l y flat f r a c t u r e s , but in the h i g h e r - t o u g h n e s s c a s e s , w h e r e s h e a r lips of an a p p r e c i a b l e size w e r e f o r m e d , j u s t a s r e a s o n a b l e a r e l a t i o n ship b e t w e e n e n e r g y and KIc was o b s e r v e d when f l a t - f r a c t u r e e n e r g i e s w e r e u s e d .
JLHE v a l u e of KIc, the c r i t i c a l plane s t r a i n s t r e s s i n t e n s i t y f a c t o r , is a n i m p o r t a n t m e a s u r e of t o u g h n e s s . It is a m a t e r i a l c o n s t a n t , it can be used in d e s i g n to c a l c u l a t e a c r i t i c a l c r a c k s i z e for a given a p p l i e d s t r e s s , and it can be d e t e r m i n e d e x p e r i m e n t a l l y u s i n g s u i t a b l y d e s i g n e d l a b o r a t o r y s p e c i m e n s and t e s t s . U n f o r t u n a t e l y , the b a s i c e x p e r i m e n t a l p r o c e d u r e for the m e a s u r e m e n t of v a l i d KIc v a l u e s , while quite w e l l e s t a b l i s h e d , r e m a i n s somewhat complex, r e q u i r i n g c a r e f u l a t t e n t i o n to s p e c i m e n design, f a b r i c a t i o n , and t e s t i n g , and c a l l i n g for exacting t e c h n i q u e s f o r a n a l y s i s of the r e s u l t s . 1-4 T h o s e t i m e - c o n s u m i n g and expensive r e q u i r e m e n t s d e t r a c t f r o m the p r a c t i c a l v a l u e of the t e s t , and t h e r e a r e m a n y c a s e s w h e r e it would be des i r a b l e t o ' h a v e a s i m p l e r method for e v a l u a t i n g f r a c t u r e t o u g h n e s s , even if only a n e s t i m a t e of KIc w e r e obtained. In the p a s t , a Charpy i m p a c t t e s t was c o m m o n l y u s e d to m e a s u r e the t o u g h n e s s of a m
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