The effect of heat treatment on the fracture toughness and subcritical crack growth characteristics of a 350-grade marag
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The r e s u l t s of the t e s t s were plotted as the i n i t i a l applied s t r e s s i n t e n s i t y l e v e l K i i v s t i m e to f a i l u r e , and the plane s t r a i n * t h r e s h o l d s t r e s s i n t e n s i t y level *The specimensweresufficientlythick that planestramconditionsexisted.2 Kiscc , below which c r a c k growth did not o c c u r , was determined. Fatigue precracked single-edge-notched s p e c i m e n s 7.5 in. long, 1.5 in. wide, and 0.48 in. thick were u s e d to d e t e r m i n e the c r a c k v e l o c i t y c h a r a c t e r i s t i c s . T h e c r a c k length was m e a s u r e d o p t i c a l l y , u s ing l o w - p o w e r m a g n i f i c a t i o n , at s e l e c t e d time i n t e r v a l s . F r o m these o b s e r v a t i o n s , a c u r v e of c r a c k length vs t i m e was c o n s t r u c t e d . The slope of the c u r v e , which r e p r e s e n t s the i n s t a n t a n e o u s c r a c k growth r a t e , was d e t e r m i n e d g r a p h i c a l l y for v a r i o u s c r a c k length v a l u e s ; f r o m t h i s , the r e l a t i o n b e t w e e n s t r e s s i n t e n sity and c r a c k v e l o c i t y was d e t e r m i n e d . To d e t e r m i n e the amount of a u s t e n i t e , the X - r a y t e c h n i q u e of L i n d g r e n was u s e d . s T h i s technique c o m p a r e s the i n t e g r a t e d i n t e n s i t i e s of the (220) a u s t e n i t e line and the (200) m a r t e n s i t e line and is c l a i m e d to give an a c c u r a c y of 1 pct.
E X P E R I M E N T A L PROCEDURE All s p e c i m e n s were m a c h i n e d in the l o n g i t u d i n a l dir e c t i o n u n l e s s o t h e r w i s e noted. To d e t e r m i n e t e n s i l e properties, 0.25-in.-diam tension specimens were loaded in an I n s t r o n m a c h i n e at a s t r a i n r a t e of 0.005 in. p e r in. per m i n . F a t i g u e p r e c r a c k e d , s t a n d a r d size C h a r p y s p e c i m e n s were loaded in t h r e e - p o i n t b e n d i n g to d e t e r m i n e the plane s t r a i n f r a c t u r e toughn e s s Klc ; the data were a n a l y z e d using the r e c o m m e n d e d ASTM p r o c e d u r e . 2 S i m i l a r s p e c i m e n s were u s e d to e s t a b l i s h the subc r i t i c a l c r a c k growth c h a r a c t e r i s t i c s . T h e s e were deadweight loaded in c a n t i l e v e r b e n d i n g u s i n g the technique d e s c r i b e d by B r o w n / The e n v i r o n m e n t was e i t h e r l a b o r a t o r y a i r or 3.5 pct aqueous s o d i u m chlor i d e solution, with the l a t t e r c o n t i n u o u s l y d r i p p e d into the s p e c i m e n notch. S t r e s s i n t e n s i t y was c a l c u l a t e d f r o m the r e l a t i o n s h i p given by Brown and S r a w l e y 4 by a s s u m i n g that c a n t i l e v e r b e n d i n g c o r r e s p o n d e d to t h r e e - p o i n t loading with a s p a n - t o - w i d t h r a t i o of eight. C. S. CARTER is Senior Engineer, CommercialAirplane Group, The Boeing Company, Renton, Wash. Manuscript submitted Augus
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