On the application of linear elastic fracture mechanics to hard steels
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R E C E N T L Y a few p a p e r s c o n c e r n i n g toughness t e s t ing of h a r d s t e e l s ~-3 call for attention about how to a s s e s s toughness in these b r i t t l e m a t e r i a l s . The i n h e r e n t p r o p e r t y dependence of o r i e n t a t i o n n o r m a l l y shown by high speed s t e e l s , due to the heavy f o r g i n g could not be accounted for by the c o n v e n t i o n a l t e s t p r o c e d u r e x'z i . e . i n a c c o r d a n c e with ASTM E-399. 4 It was f u r t h e r c o n s i d e r e d that the t e s t r e s u l t s did not comply with a c t u a l i n - o p e r a t i o n e x p e r i e n c e s n e i t h e r did the r a n k ing of the m a t e r i a l s yield the s a m e r e s u l t as a s t a n d a r d t e c h n o l o g i c a l t e s t such as the bend t e s t . However in the p r e s e n t i n v e s t i g a t i o n a bend t e s t p r o c e d u r e is applied followed by identification of the f r a c t u r e i n i t i a t i o n point u s i n g a s c a n n i n g e l e c t r o n m i c r o s c o p e . Consequently an a s s e s s m e n t is obtained of the c r i t i c a l flaw size as well as the c r i t i c a l s t r e s s int e n s i t y f a c t o r i . e . the f r a c t u r e toughness of the m a t r i x , if an a p p r o x i m a t i o n of the flaw by a s u i t a b l e g e o m e t r y is p e r f o r m e d , both of which d e t e r m i n e the load s u s t a i n ing capacity of the m a t e r i a l . MATERIALS AND TESTING PROCEDURE The test p r o g r a m included o r i g i n a l l y two high speed s t e e l s (M2, M3-2PM) and t h r e e tool s t e e l s (AISI 01, A2, D2) and in the case of the M2 s t e e l s p e c i m e n s were taken out of f o r g i n g s of t h r e e different s i z e s ; s q u a r e 140, 62 and 10 ram. In each case but the l a s t p r o p e r ties both in the r o l l i n g and in the t r a n s v e r s e d i r e c t i o n were tested. The t e s t i n g c o m p r i s e d a m i n i m u m of e l e v e n s p e c i m e n s for each v a r i a n t . U n f o r t u n a t e l y well defined f r a c t u r e o r i g i n s could only be detected in the M 3 - 2 P M s t e e l s u p p l e m e n t e d by a few c a s e s in the AISI 01 s t e e l and in the following it is t h e r e f o r e c o n c e n t r a t e d on these m a t e r i a l s . The c h e m i c a l c o m p o s i t i o n s a r e given in T a b l e I and the heat t r e a t m e n t s in T a b l e II. The t e s t i n g p r o c e d u r e c o n s i s t e d of four point b e n d i n g with an outer span of 56 m m and an i n n e r span of 24 m m which was cons i d e r e d s u i t a b l e for s p e c i m e n s with the d i m e n s i o n s 6 x 6 x 60 mm. A prn polishing was carried out on each specimen to reduce the possibility of surface crack initiation.
FRACTOGRAPHY The fractured specimens were examined in a binocular microscope and the point of fracture initiation was normally clearly seen even at low magnifications. A more careful examination was eventually performed in a scanning electron microscope and a selection o
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