EFFECT

O F A G G R E S S I V E M E D I A ON T H E F A T I G U E L I F E O F P L A S T I C S

S. B. R a t n e r a n d K. S. S e r g e e v a Mekhanika

Polimerov,

V o l . 4, No. 3, p p . 4 8 9 - 4 9 3 ,

1968

UDC 678:620.169 The dependence of fatigue life on concentration has been obtained for PVC and polystyrene exposed to the action of acetone solutions. Asthe mechanical stress increases, the role of the aggressivity of the medium diminishes, and its corrosion activity a tends to a minimum (a = 1). When a polymer is exposed to the action of a binary liquid, both of whose components are aggressive, a transition region is observed on the log 1 / ' r z l o g C curve, tn this region the fracture rate increases sharply. In t h e p r e s e n c e o f m u l t i p l e d e f o r m a t i o n i n l i q u i d media the liquid may play a dual role: as an external c o o l i n g a g e n t i t p r o l o n g s [1], b u t a s a n a g g r e s s i v e m e d i u m i t s h o r t e n s t h e l i f e of p l a s t i c s . In t h e p r e s e n t s t u d y we h a v e d i s r e g a r d e d t h e c o o l i n g e f f e c t a n d c o n f i n e d o u r a t t e n t i o n t o t h e q u e s t i o n of t h e c o n c e n t r a t i o n of t h e a g g r e s s i v e m e d i u m . Experimental, In these experiments we employed binary liquids with different concentrations of the aggressive component. We tested the effect of aqueous solutions of acetone on PVC and polystyrene, acetic and formic acids on capron, and sulfuric acid on polyformaldehyde. Testing was carried out on a UKI-IOM machine: 1) on preswotten specimens without a supply of liquid, 2) on nonswollen specimens with the liquid supplied in drops. In both cases the tests were conducted on cantilever-loaded speci mens rotating at a frequency w = 3000 rpm. The temperature and humidity of the air remained constant throughout the experiment: T -= 20° C :~ 1" relative humidity 55-57%. In the second ease the tests were conducted in accordance with the method previously described in [1,3]. Drops of liquid were supplied to the rotating specimen at the rate of 60 m//hr. The changes in the thermophysical properties of the liquid medium caused by varying the concentration of aggressive agent did little to increase the scatter of the data; even supplying the liquid in a jet rather than dropwise had almost no effect on the endurance of the specimen. The fatigue process is accompanied by swelling and dissolving of the specimen. F a t i g u e e q u a t i o n . In o r d e r t o e s t a b l i s h t h e r e l a t i o n between the fatigue life ~ and the concentration C for a d r o p w i s e s u p p l y of l i q u i d w e c o n s t r u c t e d W S h l e r t y p e ( r - l o g ~- c u r v e s a t a s e r i e s of g i v e n c o n c e n t r a t i o n s o f t h e a g g r e s s i v e l i q u i d ( F i g . l a ) . E a c h p o i n t on t h e g r a p h c o r r e s p o n d s to a s i n g l e s p e c i m e n t e s t e d . F r o m t h e s a m e d a t a we c o n s t r u c t e d t h e d e p e n d e n c e o f t h e f r a c t u r e r a t e 1 / r on c o n c e n t r a t i o n C a t v a r i o u s s t r e s s e s ( F i g . l b ) . It i s c l e a r f r o m t h e f i g u r e t h a t ~=A-C=roa -c ,

(1)

w h e r e "r0 i s t h e i n i t i a l f a t i g u e l i f e of t h e m a t e r i a l b e f o r e b e i n g e x p o s e d to t h e a g g r e s s i v e l i q u i d , i . e . , a t C=0. T h e c o n s t a n t a r e p r e s e n t s t h e c o r r o s i v e a c t i v i t y of the medium, indicating the factor by which the fatigue l i f e i s r e d u c e d a t 100% c o n c e n t r a t i o n of t h e a g g r e s s i v e component. E q u a t i o n (1) d i f f e r s f r o m t h e e x p r e s s i o n const

C~

(2)

u s e d to d e s c r i b e [4] t h e s t a t i c t e s t d a t a f o r r u b b e r s i n a corrosive medium. Although our data are also app r o x i m a t e l y d e s c r i b e d b y a n e q u a t i o n of t y p e (2), w e consider the latter less useful, since it excludes the p o s s i b i l i t y of e x t r a p o l a t i o n to c o r r o s i o n l e s s f r a c t u r e . In f a c t , a s C - - 0 e x p r e s s i o n (2) g i v e s ~- - - ~ , w h e r e a s i t i s k n o w n t h a t i n t h e a b s e n c e of c o r r o s i v e a c t i o n t h e m a t e r i a l f a i l s i n a f i n i t e t i m e , T h i s a l s o a p p l i e s to t h e more general formula ~ C - ~ o -/~ e U/Rr ,

(3)

w h i c h a l s o r e q u i r e s t h e r e p l a c e m e n t of t h e c o n c e n t r a t i o n factor C -a by a-C,if the general formula is to be

applied to the fracture of a material in an inertliquid. It is clear from Fig. 113 that, as the stress increases, the slope of the straight lines becomes gradually flatter. The corresponding dependence of a on is shown in the table. It follows that for bo*~h materials a ~ 1 as ~ ~ ~, in complete agreement with the physical significance of a. The decrease in the corrosive activity of the medium with increase in stress may be associated with two factors. Firstly, as previously described, in the presence of a dropwise supply of liquid the greater the stress, the shorter the time during which the medium can act and, consequently, the lower the effective value of the corrosion activity a. However, it is clear from Fig. Ib that this effect does not play a part, at least at small values of a. The fact that the upper straight lines in Fig. Ib are horizontal indicates that the coneentration is no longer of importance, which is equivalent to saying that the duration of action of the medium does not affect the results. Accordingly, we will concentrate on the second physical factor. :In fact, an increase in the load reduces the fraction of bonds

C o n s t a n t s of E q . (1) f o r t h e F r a c t u r e of P l a s t i c s i n a S o l u t i o n of A c e t o n e i n W a t e r at Various Stress Amplitudes leg % Material

e, kgf/cm 2 Experimental

! O0

PVC

Potystyrene

6,0

Extrapointed

6.5

130 250 350 495 570

6.0 4.8 3.8 2.9 2.3

5,9 4~9 3.9 3.0 2.7

130 150

5.8

5A

210

284

5,2

4.1

3~3

4.8

3,9

3.2

31.6 ~3.5 4,6 1.8 1.0 i,0

53.7

20:9 3.7 i.0

389

b

a

t

F i g . 1. E f f e c t of a c e t o n e c o n c e n t r a t i o n on the fatigue s t r e n g t h a and f r a c t u r e r a t e of PVC (1) and p o l y s t y r e n e (2): a) fatigne c u r v e s for the d r o p w i s e s u p p l y of a g g r e s s i v e liquid in v a r i o u s c o n c e n t r a t i o n s ; b) f r a c t u r e r a t e as a function of a c e t o n e c o n c e n t r a t i o n . b r o k e n b y c o r r o s i o n , a s e s t a b l i s h e d f o r r u b b e r s in [2]; t h e r e f o r e , in the l i m i t , at i n f i n i t e l y l a r g e l o a d s , the c o r r o s i o n a c t i v i t y of the m e d i u m in any s y s t e m i s no

C L

'i

I Q

~4

~3

t

3

~0

30

tOO

Fig. 2. Effect of concentration of aggressive l i q u i d on the e n d u r a n c e (N) of p l a s t i c s : a ) p o l y f o r m a l d e h y d e s w o l l e n f o r 22 h r in a s u l f u r i c a c i d s o l u t i o n , (T = 398 k g f / c m 2. b) c a p r o n ; 1) d r o p w i s e s u p p l y of a c e t i c a c i d s o l u t i o n s , cr = = 260 kgf/cm2; 2) s w o l l e n in f o r m i c a c i d s o l u t i o n s f o r 120 h r , (~ = 130 kgf/cm2; 3) s w o l l e n in s o l u t i o n s of a c e t i c a c i d f o r 120 hr, a = 1 3 0 k g f / em 2. 390

l o n g e r s i g n i f i c a n t : a = 1. As f o r T0, the f a c t t h a t i t d e c r e a s e s with i n c r e a s e in s t r e s s (see t a b l e ) i s p e r f e c t l y natural. T h r e s h o l d e o n e e n t r a t i o m L e t us c o n s i d e r the e f f e c t of a b i n a r y liquid, both of whose c o m p o n e n t s a r e a g g r e s s i v e with r e s p e c t to the p o l y m e r , b u t in one of which the p o l y m e r s w e l l s , while in the o t h e r i t is d i s s o l v e d . The d a t a o b t a i n e d for s o l u t i o n s of a c e t i c and f o r m i c a c i d s a c t i n g on c a p r o n a r e p r e s e n t e d in F i g . 2b. E a c h p o i n t on the c u r v e s in F i g . 2 (and F i g . 3b) c o r r e s p o n d s to the l o g a r i t h m i c m e a n of the r e s u l t s of t e s t s on t h r e e s p e c i m e n s . It i s c l e a r f r o m F i g . 2b t h a t e a c h c u r v e is f o r m e d of t h r e e p a r t s . On the f i r s t s e c t i o n , a t low c o n c e n t r a t i o n s of a c e t i c a c i d , when i t s p r e s e n c e d o e s n o t a f f e c t the s w e l l i n g , the p o l y m e r s i m p l y s w e l l s in the w a t e r , and t h e r e is p r a c t i c a l l y no change in f r a c t u r e r a t e . On the t h i r d s e c t i o n the c h i e f p a r t i s p l a y e d b y s o l u t i o n of the c a p r o n in the acid, a s a r e s u l t of the s h a r p i n c r e a s e in c o n c e n t r a t i o n . On the m i d d l e s e c t i o n the f r a c t u r e r a t e i n c r e a s e s s h a r p l y , a s the a c i d b e g i n s to d i s s o l v e the c a p r o n . T h i s b e h a v i o r s u g g e s t s the e x i s t e n c e of a t h r e s h o l d c o n c e n t r a t i o n , at which a s h a r p fall in strength takes place. In fatigue t e s t s on c a p r o n with the s i m u l t a n e o u s d r o p w i s e s u p p l y of a c e t i c a c i d s o l u t i o n s (second method), i . e . , when the s o l v e n t p e n e t r a t e s both b y d i f f u s i o n and a l o n g f a t i g u e c r a c k s , the t h r e s h o l d c o n c e n t r a t i o n l i e s in the r e g i o n 3 - 6 % a c e t i c a c i d ( c u r v e 1 in Fig. 2b). If the c a p r o n is m e r e l y a l l o w e d to s w e l l in a c e t i c a c i d s o l u t i o n s and is then t e s t e d without the d r o p w i s e s u p p l y of liquid ( f i r s t method), i . e . , when only diffus i o n is involved, the a c t i o n of the a g g r e s s i v e m e d i u m i s l e s s i n t e n s e and i t s t h r e s h o l d c o n c e n t r a t i o n is h i g h e r :

I /iJ

*a F i g . 3. E f f e c t of a c e t o n e c o n c e n t r a t i o n on the s w e l l i n g (a) and e n d u r a n c e (b) of PVC: 1) s w e l l i n g for 23 hr; 2) 6 hr; 3) with d r o p w i s e s u p p l y of a c e t o n e s o l u t i o n s , a = 150 kgf/cm2; 4) the s a m e , cr = 250 kgf/cm2; 5) f o r s p e c i m e n s s w o l l e n for 48 h r , without a s u p p l y of liquid, (r = 150 k g f / c m 2. in the r e g i o n of 40% a c e t i c a c i d (curve 3 in F i g . 2b). H o w e v e r , a s the r a t e of d i f f u s i o n of the m e d i u m into the p o l y m e r i n c r e a s e s , f o r e x a m p l e , if c a p r o n is c a u s e d to s w e l l in f o r m i c a c i d ( r a t h e r than a c e t i c ) , when the s a m e s w o l l e n s p e c i m e n s a r e t e s t e d u n d e r the s a m e c o n d i t i o n s the t h r e s h o l d c o n c e n t r a t i o n is r e a c h e d e a r l i e r - - i n the r e g i o n of 5 - 1 0 % f o r m i c a c i d ( c u r v e 2 in Fig. 2b). A threshold concentration may also exist for other r e a s o n s , ff a p o l y m e r i s a l l o w e d to s w e l l in a b i n a r y l i q u i d in which one c o m p o n e n t i s a g g r e s s i v e with r e s p e c t to the p o l y m e r while the o t h e r i s not, i t i s M s o p o s s i b l e to o b s e r v e a r e g i o n of t h r e s h o l d c o n c e n t r a t i o n when the s w o l l e n s p e c i m e n s a r e s u b j e c t e d to f a t i g u e t e s t s in a c c o r d a n c e with the f i r s t m e t h o d , without a d r o p w i s e s u p p l y of liquid (curve 5 in F i g . 3b). If, h o w e v e r , the t e s t s a r e conducted with a d r o p w i s e s u p p l y of liquid to u n s w o l l e n m a t e r i a l , a t h r e s h o l d c o n c e n t r a tion is not o b s e r v e d ( c u r v e s 3 and 4 in F i g . 3b). It i s c l e a r f r o m Fig. 3a that for t h e s e m a t e r i a l s the e x i s t e n c e of a t h r e s h o l d c o n c e n t r a t i o n is a s s o c i a t e d with the r a t e at which t h e y s w e l l in a g g r e s s i v e m e d i a (each p o i n t on the c u r v e s in F i g . 3a c o r r e s p o n d s to the a r i t h m e t i c m e a n of the r e s u l t s of t e s t i n g t h r e e s p e c i m e n s ) . The t h r e s h o l d c o n c e n t r a t i o n i s o b s e r v e d in the s a m e r e g i o n a s the s h a r p i n c r e a s e in the d e g r e e of s w e l l i n g . It i s c l e a r f r o m the s a m e F i g . 3a that c h a n g ing the s w e l l i n g t i m e s h i f t s the r e g i o n in which the s h a r p i n c r e a s e in the d e g r e e of s w e l l i n g o c c u r s f u r t h e r to the r i g h t . T h e r e f o r e t h e r e i s no t h r e s h o l d c o n c e n t r a t i o n f o r s p e c i m e n s s u b j e c t e d to m u l t i p l e d e f o r m a t i o n with a d r o p w i s e s u p p l y of liquid, when the s w e l l i n g time is sharply reduced. E x p o s u r e of a p o l y m e r to a c h e m i c a l l y a g g r e s s i v e m e d i u m , when i n s t e a d of s w e l l i n g the s p e c i m e n m a y be o x i d i z e d and m a y even c r a c k , is a l s o a c c o m p a n i e d

b y a d e c r e a s e in fatigue l i f e d e p e n d i n g on the c o n c e n t r a t i o n of the a g g r e s s i v e agent, as o b s e r v e d in c o n n e c tion with the fatigue of p o l y f o r m a l d e h y d e in a s u l f u r i c a c i d s o l u t i o n (see Fig. 2a). CONCLUSIONS I. The concentration dependence of the fati~de life T = T0a -C has been obtained for PVC and polystyrene deformed in the presence of acetone solutions. As the mechanical stress increases, the part played by the corrosive action of the medium decreases and the corrosion a c t i v i t y t e n d s to a m i n i m u m (a ~ 1). 2. The f a t i g u e l i f e - c o n c e n t r a t i o n c u r v e s a r e c h a r a c t e r i z e d by a t r a n s i t i o n r e g i o n . In t h i s r e g i o n t h e r e is a s h a r p i n c r e a s e in f r a c t u r e r a t e at c e r t a i n c o n c e n t r a t i o n s of the a g g r e s s i v e liquid in w a t e r . T h i s d a n g e r o u s c o n c e n t r a t i o n is c a l l e d the t h r e s h o l d c o n c e n t r a t i o n ; i t f a l l s a s the a g g r e s s i v i t y of the m e d i u m i n c r e a s e s and when the p e n e t r a t i o n of the l a t t e r is i n t e n s i f i e d by a m e c h a n i c a l field. REFERENCES

1. S. B. R a t h e r and N. L B a r a s h , Mekh. p o l i m . [ P o l y m e r M e c h a n i c s ] , 1, 124, 1965. 2. S. B. R a t n e r and V. I. Korobov, Mekh. p o l i m . [ P o l y m e r M e c h a n i c s ] , 3, 93, 1965. 3. Yu. F. I
24 A p r i l 1967

Scientific-Research Institute of Plastics, Moscow

391