EFFECT OF
OF
POLYMER A. A.
STRAIN
RATE
MATERIALS
N. Tynnyi, I. Soshko,
Yu. and
ON IN
THE LIQUID
A. Kolevatov, N. G. Kalinin
STRENGTH MEDIA UDC 620.171.3 : 6 7 8 . 7 4 4 / . 7 4 3
T h e e f f e c t of s t r a i n r a t e on the s t r e n g t h of p o l y ( m e t h y l m e t h a c r y l a t e ) and t e f l o n - 4 in liquid m e d i a w a s s t u d i e d . It w a s shown t h a t t h e i n f l u e n c e of liquid m e d i a on the s t r e n g t h of p o l y mer materials decreases with increasing strain rate.
In e s t i m a t i n g t h e i n f l u e n c e of w o r k i n g m e d i a on the s t r e n g t h of m a t e r i a l s in g e n e r a l and p o l y m e r s in p a r t i c u l a r one s h o u l d t a k e into a c c o u n t t h e p o s s i b l e i n f l u e n c e of s u c h f a c t o r s as s t r a i n r a t e , w h i c h to a d e f i n i t e e x t e n t d e t e r m i n e s t h e r a t e of p r o p a g a t i o n of f r a c t u r e c r a c k s and t h e p r o p e r t i e s of t h e m e d i a , t h e m o s t i m p o r t a n t o f w h i c h in t h i s c o n t e x t a r e t h e v i s c o s i t y and the r a t e of s u r f a c e m i g r a t i o n . T h e full m a n i f e s t a t i o n of t h e i n f l u e n c e of a w o r k i n g m e d i u m on m a t e r i a l s t r e n g t h d e p e n d s on t h e p r e s e n c e of a s u f f i c i e n t q u a n t i t y of the m e d i u m in t h e t i p of a c r a c k o r a g r o w i n g d e f e c t , w h i c h , in t u r n , d e p e n d s on the r a t i o of t h e r a t e of c r a c k o r d e f e c t p r o p a g a t i o n to t h e r a t e of s u r f a c e m i g r a t i o n of t h e m e d i u m (the k i n e t i c f a c t o r ) . It m a y be p o s t u l a t e d that t h e e f f e c t of a m e d i u m c a n b e a p p r o x i m a t e l y e s t i m a t e d f r o m d a t a on t h e s t r e n g t h of m a t e r i a l s at d i f f e r e n t s t r a i n r a t e s . T h e a i m of t h i s i n v e s t i g a t i o n w a s to s t u d y the c o m b i n e d e f f e c t of w o r k i n g m e d i a and s t r a i n r a t e on t h e s t r e n g t h of p o l y ( m e t h y l m e t h a c r y l a t e ) (PMM) and p o l y t e t r a f l u o r o e t h y l e n e ( t e f l o n - 4 ) . T h e e x p e r i m e n t a l w o r k w a s c a r r i e d out on f l a t s p e c i m e n s 3 m m t h i c k . T h e d i m e n s i o n s of the s p e c i m e n s and c h a r a c t e r i s t i c s of the a p p a r a t u s c o n f o r m e d to GOST s p e c i f i c a t i o n s (11262-65 and 4648-63). T h e t e s t s w e r e c a r r i e d out at s t r a i n r a t e s of 4 9 104-1 9 10 -2 s e c -1 on a u n i v e r s a l m a c h i n e [4] w i t h an e l e c t r o n i c a l l y c o n t r o l l e d r a t e of s h a c k l e t r a v e r s e . T h e l o a d s and s t r a i n s w e r e r e c o r d e d b y an e l e c t r o t e n s o m e t r i c m e t h o d u s i n g an E P P - 0 9 p o t e n t i o m e t e r . T h e f o l l o w i n g w o r k i n g m e d i a w e r e u s e d in t h e e x p e r i m e n t s : w a t e r , k e r o s e n e , e t h y l a l c o h o l , v a s e l i n e o i l , o l e i c a c i d , a c e t o n e , and h e p t a n e . G r a p h s p l o t t e d in s e m i l o g a r i t h m i c c o o r d i n a t e s in F i g . l a and b show the v a r i a t i o n in t h e b r e a k i n g s t r e s s cr and e l o n g a t i o n at f r a c t u r e e in r e l a t i o n to s t r a i n r a t e . V a l u e s of e and e a r e e x p r e s s e d as r a t i o s w i t h r e s p e c t to t h e c o r r e s p o n d i n g v a l u e s r e c o r d e d at a s t r a i n r a t e of 1 9 10 -3 s e c -1 (i.e., w i t h the s h a c k l e s m o v i n g at a r a t e of 5 m m / m i n ) . T e s t r e s u l t s show t h a t the e f f e c t of w o r k i n g m e d i a on ~ and ~ of PMM s p e c i m e n s v a r i e s . T h u s , cr r e c o r d e d at a s t r a i n r a t e of 5 m n ~ m i n in a c e t o n e and w a t e r a r e , r e s p e c t i v e l y , 56 and 18% l o w e r t h a n in a i r ( F i g . l a ) . I n c r e a s i n g t h e s t r a i n r a t e r e d u c e s t h e e f f e c t of a c e t o n e and a l c o h o l on e; t h e c o r r e s p o n d i n g r e d u c t i o n in the c a s e of w a t e r is m u c h s m a l l e r . A t a s t r a i n r a t e of 45 m m / m i n , t h e e f f e c t of a l l t h e m e d i a s t u d i e d b e c o m e s v e r y s m a l l . T h e s e d a t a i n d i c a t e t h a t i n c r e a s i n g t h e s t r a i n r a t e l e a d s to an i n c r e a s e in t h e c r a c k p r o p a g a t i o n r a t e , so t h a t liquid m e d i u m m o l e c u l e s do not r e a c h t h e t i p of t h e c r a c k w h i c h s u b s e q u e n t ly, due to the a b s e n c e of a w o r k i n g m e d i u m in t h i s r e g i o n , g r o w in t h e s a m e w a y a s in a i r o r in v a c u u m . E l o n g a t i o n of PMM (Fig. l b ) is a l s o r e d u c e d u n d e r t h e i n f l u e n c e of liquid w o r k i n g m e d i a ; o l e i c a c i d is an e x c e p t i o n in t h a t it i n c r e a s e s the e of P M M .
I n s t i t u t e of P h y s i c s and M e c h a n i c s , A c a d e m y of S c i e n c e s of the U k r a i n i a n SSR, L ' v o v . T r a n s l a t e d f r o m F i z i k o - K h i m i c h e s k a y a M e k h a n i k a M a t e r i a l o v , Vol. 5, No. 6, pp. 676-679, N o v e m b e r - D e c e m b e r , 1969. O r i g i n a l a r t i c l e s u b m i t t e d F e b r u a r y 19, 1969.
9 1972 Consultants Bureau, a division of Plenum Publishing Corporation, 227 West 17th Street, New York, N. Y. 10011. All rights reserved. This article cannot be reproduced for any purpose whatsoever without permission of the publisher. A copy of this article is available from the publisher for $15.00.
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Fig. 1. a) The breaking s t r e s s and b) the elongation at f r a c ture plotted against strain rate of PMM specimens tested in uniaxial tension in the following working media: 1) air; 2) oleic acid; 3) vaseline oii; 4) water; 5) kerosene; 6) ethyl alcohol; 7) acetone. Since elongatio n at f r a c t u r e in the media investigated is decreased, it could be postulated that these media have an embrittling effect on PMM. At the s a m e time, the s t r e s s - s t r a i n d i a g r a m r e p r o d u c e d in Fig. 2 indicates an opposite effect: the relation ~(e) obtained for specimens tested in working media deviates from the r e c t i l i n e a r long before the breaking s t r e s s is reached. This, in the generally accepted sense, points to the appearance of plastic or, in the case of p o l y m e r s , f o r c e d - e l a s t i c strains; this fact may be taken as a definite proof of the plasticizing effect of the media studied. 3
The apparent embrittling effect of working media on I~MM (as indicated by s t r e s s - s t r a i n diagrams) are not c o n t r a d i c t o r y and can be explained in the following way. The working media studied differ in t h e i r wetting behaviour on PMM, i.e., in the degree of their s u r f a c e activity with r e s p e c t to this m a t e r i a l . Oleic acid-type media a r e typical s u r f a c e active substances, while acetone is a strong solvent which produces swelling even in the absence of mechanical s t r e s s . Liquid medium molecules Fig. 2. Tensile s t r e s s adsorbed on the specimen surface reduce the s u r f a c e free energy and s t r a i n d i a g r a m s of PMM facilitate the growth of defects and m i c r o c r a c k s that are always present in tested in 1) air, 2) k e r o real m a t e r i a l s , especially on their surface. Applied s t r e s s e s promote sene, and 3) acetone at a these phenomena and the m a x i m u m effect of working media is concentrated s t r a i n rate of 2.5 ram/rain. in defects with the m a x i m u m s t r e s s concentration (i.e., a l r e a d y growing at the f a s t e s t rate) and, in turn, facilitate their propagation. This is a manifestation of the selective nature of the action of working media, which is also proved by the fact that fewer c r a c k s are produced on the s u r f a c e of specimens s t r e s s e d under the influence of working media than in the c a s e of specimens s t r e s s e d in air. Numerous c r a c k s f o r m e d on the s u r f a c e of specimens tested in air r e sult in the leveling of s t r e s s concentration and grow at the s a m e r a t e almost to the instant of f r a c t u r e . In the p r e s e n c e of a liquid working medium one o r s e v e r a l c r a c k s grow preferentially to f o r m a c r a c k along which the f r a c t u r e , as a rule, takes place. This leads to a reduction in the elongation; at the s a m e time, however, the nature of c r a c k propagation is substantially changed: s e v e r a l small c r a c k s radiating f r o m the tip region leading to the appearance of f o r c e d - e l a s t i c s t r a i n s and to changes in the nature of the r e l a tion ~(e). The substantial reduction in the cr of PMM in acetone and alcohol may be attributed not only to the influence of adsorption phenomena but also, and to a l a r g e r extent, to sorption p r o c e s s e s which (especially in the c r a c k - t i p regions) a r e accompanied by the swelling of s u r f a c e layers leading to a reduction in the loadc a r r y i n g c r o s s section. I n c r e a s i n g the time of interaction of the p o l y m e r with a medium (in the p r e s e n c e of mechanical s t r e s s e s ) gradually leads to the onset of sorption phenomena, i.e., to the medium penetrating into the polym e r and facilitating the growth of internal defects, plastification, flow, and orientation of the m a t e r i a l . These phenomena are time and s t r e s s dependent and their effect is fully manifested when these two factors exert their influence for a sufficiently long period of time. Tests on teflon-4 at various s t r a i n rates (Fig. 3) showed that the nature of the relation o- =f (v) is qualitatively s i m i l a r to that obtained for PMM. However, the effect of liquid media, in this case, is less
567
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I$ I ~0 v mm/t~in
~-
~0
'
200
-~0 e.V,
Fig. 3. The dependence of the braking s t r e s s of teflon-4 on a) and b) elongation at f r a c t u r e in 1) air, 2) acetone, and 3) heptane. pronounced; changes in the mechanical p r o p e r t i e s reflected in the reduction in cr and ~ o c c u r only at low s t r a i n rates. Analysis of the results obtained leads to the following conclusions about the effect of the strain rate v and the liquid working media on (r and e.. 1) increasing v leads to an i n c r e a s e in cr for specimens tested in liquid media to a level approaching that r e c o r d e d in air; 2) t h e effect of liquid media on ~ and e is m o s t pronounced at low v. LITERATURE 1. 2. 3. 4.
568
CITED
B . D . S u m m e t al., DAN SSSR, 136, No. 6 (1961); 137, No. 6 (1961). A . I . Soshko, V. F. Koval, and A. N. Tynnyi, FKhMM [Soviet Materials Science], No. 6 (1967). G . M . Bartenev and I. V. Razumovskaya, DAN SSSR, 150, No. 4 (1963). S . I . Mikitishin and A. N. Tynnyi, ZL, No. 7 (1968).