Exoelectron emission from torsional fatigue damage in hardened steels
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e a r l y m a n i f e s t a t i o n of the a c c u m u l a t i o n of f a t i g u e d a m a g e i s the d e v e l o p m e n t of s u r f a c e d e f o r m a t i o n at c e r t a i n l o c a t i o n s which e v e n t u a l l y b e c o m e s i t e s f o r n u c l e a t i o n of fatigue c r a c k s . H o w e v e r , long b e f o r e the a p p e a r a n c e of a c r a c k in the m e t a l i t s e l f , the s u r f a c e d e f o r m a t i o n p r o d u c e s m i c r o c r a c k s in the n a t u r a l l a y e r of b r i t t l e s u r f a c e oxide, r e v e a l i n g m i c r o s c o p i c r e g i o n s of b a r e m e t a l . U n d e r u l t r a v i o l e t i l l u m i n a t i o n , p h o t o e l e c t r o n s f r o m the u n d e r l y i n g m e t a l a r e a b l e to e s c a p e m o r e e a s i l y f r o m the f r e s h l y r e v e a l e d m e t a l s u r f a c e in the m i c r o c r a c k than f r o m the s u r r o u n d i n g oxide c o a t e d s u r f a c e . Thus the d e v e l o p m e n t of c r a c k s in the oxide, and hence the a s s o c i a t e d s u r f a c e d e f o r m a t i o n of the m e t a l , inc r e a s e s the p h o t o e l e c t r o n c u r r e n t . It i s this i n c r e ment which is known a s e x o e l e c t r o n e m i s s i o n . P r e v i o u s l y it has been shown, by s c a n n i n g the s u r f a c e of a fatigue s p e c i m e n with a s m a l l spot of u l t r a v i o l e t r a d i a t i o n , that l o c a l i z e d r e g i o n s of quite int e n s e e x o e l e c t r o n e m i s s i o n d e v e l o p e d v e r y e a r l y (
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