The effect of brittle anodic coatings on the stiffness of aluminum filaments
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T H E p u r p o s e of t h i s study w a s to d e t e r m i n e t h e e f f e c t of b r i t t l e continuous and c r a z e d anodic c o a t i n g s on the e l a s t i c m o d u l u s of a l u m i n u m f i l a m e n t s . When a coating is continuous, the c o m p o s i t e s t i f f n e s s i s g iv e n by t h e r u l e of m i x t u r e s to a c l o s e a p p r o x i m a t i o n . If t h e c o a t ing b e c o m e s c r a z e d , s t r e s s m u s t be t r a n s f e r r e d f r o m the f i l a m e n t to the c o a t in g at each d i s c o n t i n u i t y . I n c r e a s i n g the d en s i ty of t r a n s v e r s e c r a c k s d e c r e a s e s t he e f f i c i e n c y of s t r e s s t r a n s f e r and the s t i f f n e s s of t he c o m p o s i t e is s t e a d i l y r e d u c e d . Anodic oxide c o a t i n g s on a l u m i n u m can be c r a z e d r e a d i l y and u n i f o r m l y by s t r a i n i n g the c o m p o s i t e into the p l a s t i c r a n g e . Oxide c o a ti n g s e g m e n t s a s s h o r t a s t h e i r t h i c k n e s s can b e f o r m e d by t h i s m e t h o d . T h e t r a n s f e r of s t r e s s to a~short coating t i l e i s a difficult p r o b l e m to a n a l y z e b e c a u s e of the s t r e s s c o n c e n t r a t i o n s at t h e ends of t h e t i l e . F o r e x a m p l e , one would e x p e c t that an a n a l y s i s b a s e d on D o w ' s a p p r o a c h ~ would be difficult to apply in light of T y s o n and D a v i e s 2 r e s u l t s . T h e s e r e s u l t s s h o w e d that the Dow m o d e l of a d i s c o n t i n u o u s f i l a m e n t in a p l a s t i c m a t r i x w a s in a g r e e m e n t with the p h o t o e l a s t i c r e s u l t s e x c e p t at t h e ends of t h e f i b e r . An a l t e r n a t e a p p r o a c h i s to t r y a m o d i f i c a t i o n of the s h e a r - l a g m o d e l such a s u s e d by R o s e n . 3 Although t h i s t e c h n i q u e is quite i n e x a c t , it h as the a d v a n t a g e that the s h e a r s t r e s s e s a r e c o n c e n t r a t e d in a n a r r o w r e g i o n a d j a c e n t to the d i s c o n t i n u o u s r e i n forcement. In the p r e s e n t study, e x p e r i m e n t a l m e a s u r e m e n t s of t he s t i f f n e s s of f i l a m e n t s with continuous and d i s c o n tinuous c o a t i n g s a r e c o m p a r e d to p r e d i c t i o n s b a s e d on D o w ' s a p p r o a c h and a l s o on t h e a p p r o x i m a t e s h e a r lag m o d e l d e s c r i b e d in Appendix A. E X P E R I M E N T AND R E S U L T S Anodic c o a t i n g s u s e d in t h i s study w e r e f o r m e d in 20 wt pct HzSO4 at 10~ and at a constant c u r r e n t d e n sity of 0.04 a m p p e r c m 2. T h e coating t h i c k n e s s w a s c o n t r o l l e d by a n o d i z i n g t i m e . A s shown in T a b l e I, a r e a f r a c t i o n s of anodic oxide up to about 0.25 w e r e p r o d u c e d . IZongitudinal d e f e c t s a r e f o r m e d in t h e c o a t J. S. THORNTON is Group Leader, Materials, Horizons Incorporated, Cleveland, Ohio. A.
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