Subcritical crack growth under sustained load
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u n d e r s u s t a i n e d loading has b e e n a t t r i b u t e d to e n v i r o n mentally activated s t r e s s corrosion, 4 internal hydrogen, s and c r e e p . 8 Thus, it is not c l e a r whether s u b c r i t i c a l c r a c k growth u n d e r s u s t a i n e d load is a m a n i f e s t a t i o n of s o m e s e c o n d a r y factor, such as s t r e s s c o r r o s i o n o r hydrogen e m b r i t t l e m e n t , or whether it is the n o r m a l r e s p o n s e of m a t e r i a l in the v i c i n i t y of a s h a r p c r a c k to the p r e s e n c e of a s u s t a i n e d load. A m o r e c o m p l e t e u n d e r s t a n d i n g of the m e c h a n i s m of c r a c k growth is i m p o r t a n t ff s e r i o u s p r o b l e m s with s u b c r i t i c a l c r a c k i n g in s e r v i c e a r e to be avoided.
PROCEDURES T i - 4 AI-3 M o - I V alloy m a t e r i a l f r o m a 2.16 c m thic plate hot r o l l e d in the a l p h a - b e t a field was evaluated d u r i n g this i n v e s t i g a t i o n . Blanks for 6.4 m m (1/4 in.) thick c o m p a c t t e n s i o n s p e c i m e n s w e r e m a c h i n e d f r o m this plate, f r o m a location about midway b e t w e e n the s u r f a c e and c e n t e r of the plate, and heat t r e a t e d in v a c u u m . The heat t r e a t m e n t c o n s i s t e d of a n n e a l i n g fox I h at 900~ f u r n a c e cooling to 650~ holding for 4 h at 650~ and f u r n a c e cooling to r o o m t e m p e r a t u r e . The a n n e a l e d m a t e r i a l showed a s l i g h t l y elongated a l p h a - b e t a s t r u c t u r e (see Fig. 8) and c o n t a i n e d 10 pprt hydrogen. The t e n s i l e yield s t r e n g t h , as m e a s u r e d in the t r a n s v e r s e d i r e c t i o n a f t e r v a c u u m a n n e a l i n g , was 806 MN/m 2 (117 ksi). Compact t e n s i o n s p e c i m e n s ( T I o r i e n t a t i o n ) w e r e m a c h i n e d f r o m the a n n e a l e d b l a n k s and p r e c r a c k e d in fatigue. Specimen design and fatigu c r a c k i n g p r o c e d u r e s c o n f o r m e d to r e c o m m e n d e d p r a c tice 7 except that the hole s p a c i n g was i n c r e a s e d to I W to f a c i l i t a t e o b s e r v a t i o n of c r a c k m o v e m e n t . R i s i n g load t e s t s gave a K c v a l u e in the T L o r i e n t a t i o n of 79. M N / m s/2 (72.5 k s i - i n . t / a ) c a l c u l a t e d f r o m m a x i m u m load. The c o m p a c t t e n s i o n s p e c i m e n s were dead weight loaded u s i n g a 20:1 l e v e r a r m . The load was applied to i n c r e a s e s t r e s s i n t e n s i t y in steps of a p p r o x i m a t e l y 5 MN/m s/2 at 1 m i n i n t e r v a l s until the d e s i r e d s t r e s s i n t e n s i t y was r e a c h e d . The t i m e u n d e r load was m e a VOLUME 5, NOVEMBER 1974-235
s u r e d s t a r t i n g 1/2 m i n a f t e r a d d i t i o n of the final l o a d i n c r e m e n t . If f a i l u r e did not o c c u r within about 8 d a y s , the s p e c i m e n was r e m o v e d f r o m t e s t and f r a c t u r e d in t e n s i o n t
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