MACHINE FOR INVESTIGATING C R E E P AND LONG-TERM STRENGTH OF POLYMER MATERIALS IN LIQUID MEDIA A T E L E V A T E D T E M P E R A T U R E S Yu. V. Kolevatov, A. I. Soshko, N. G. Kalinin, T. Yu. Stefyuk, and A. N. Tyanyi F i z i k o - K h i m i c h e s k a y a Mekhanika M a t e r i a l o v , Vol. 6, No. 1, pp. 8 2 - 8 4 , 1970 UDC 620.17(0678):539.376/434+539.4.010

Existing t es t i n g equipment is not suitable for investigating the c r e e p and l o n g - t e r m strength of p o l y m e r m a t e r i a l s u n d er a combined influence of liquid m e d i a and low and e l e v a t e d t e m p e r a t u r e s . A m a c h i n e of our design m a k e s it p o s s i b l e to c a r r y out t e s t s on p o l y m e r m a t e r i a l s in v ar i o u s a g g r e s s i v e me di a in the t e m p e r a t u r e i n t e r v a l 100-600 ~ K. A distinguishing f e a t u r e of the machine is a r e l a t i v e l y high (up to 1.0%) a c c u r a c y of m e a s u r i n g and r e c o r d i n g the elongation of s p e c i m e n s which m ay range f r o m 1.0 to 150070. The c r e e p and l o n g - t e r m s t r e n g t h c h a r a c t e r i s t i c s of p o l y m e r m a t e r i a l s a r e usually d e t e r m i n e d under a constant t e n s i l e load. The machine is so designed that it can be a s s e m b l e d in a fume cupboard or on a stand in an open l ab o r at o r y . To p r e v e n t c o r r o s i o n , m o s t of the machine p a r t s a r e made of s t a i n l e s s s t e e l .

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Fig. 1 Schematic of the apparatus f o r c r e e p and l o n g - t e r m s t r e n g t h t e s t s .

F i g u r e 1 shows a s c h e m a t i c of the m a c h i n e which works on the following p r i n c i p l e s . A disk 3 for mounting shackles 4 with t e s t p i e c e s 5 is attached to t h r e e b a r s 2 whose other ends a r e welded to a c o o l e r (condenser) 1. The c o n d e n s e r constitutes a welded c y l i n d e r with tubes 6 through which tie rods 7 pass. The tie rods a r e connected to cable 8 through which the f o r c e produced by weights 9 is t r a n s m i t t e d to the t e s t p i e c e s . The cable p a s s e s o v e r guide pulleys 10 and through a d e v i c e for m e a s u r i n g the s t r a i n of the t e s t p i e c e s . This device is mounted on columns 11 and is c o m p r i s e d of d r u m 12 with a h e li c a l groove for the cable and v a r i a b l e r e s i s t o r 13 b ased on the r h e o c h o r d of a PSR-03 automatic r e c o r d e r . The d r u m has t h r e e steps, whose d i a m e t e r s a r e s e l e c t e d to suit a given range of the elongation of t e s t p i e c e s . The cage with mounted s p e c i m e n s is placed in c o n t a i n e r 14 with a working m ed i u m . The co n t ai n er is fixed to heat ex ch an g er 15 which is c o m p r i s e d of a h e a t - i n s u l a t e d v e s s e l with a liquid heat t r a n s f e r m e d i u m 16, s p i r a l tube 17, and e l e c t r i c h e a t e r 18. The t e m p e r a t u r e inside t h e heat ex ch an g er is m e a s u r e d with t h e r m o c o u p l e 19 and a contact t h e r m o m e t e r 20. In t e s t s at high t e m p e r a t u r e s the e l e c t r i c h e a t e r and a t h e r m o s t a t a r e used. In this c a s e wa t e r or a high-boiling oil a r e used as the heat t r a n s f e r m ed i u m . C r e e p and l o n g - t e r m strength t e s t s at s u b z e r o t e m p e r a t u r e s a r e c a r r i e d out with the aid of a s p e c i a l apparatus (see Fig. 1). The a p p a r a t u s f o r maintaining low t e m p e r a t u r e s in the working c h a m b e r c o m p r i s e s a Dewar f l a s k 21 with liquid n i t r o g e n in which a tube 22 with an e l e c t r i c h e a t e r 23 is i m m e r s e d . The e x t e r n a l end of tube 22 is fitted with a magnetic valve 24 and an outlet to which a v a c u u m - t i g h t pipe 25 leading to the s p i r a l tube in the heat exchanger is connected. The Dewar flask is fitted with a contact m a n o m e t e r 26 and a safety valve 27. 86

When heater 23 is on, the pressure of nitrogen gas in the Dewar flask increases and forces the liquid nitrogen up the tube 22 through the magnetic valve to the spiral tube of the heat exchanger, thereby cooling the container with test pieces. When the temperature drops to the required level, the flow of liquid nitrogen is cut off by the magnetic valve which is controlled by a PSR-03 potentiometer with a thermocouple or by an electric circuit in conjunction with the contact thermometer. In the case of excessive pressure build-up in the Dewar flask the surplus nitrogen is let out to the atmosphere through the safety valve. The electric heater is controlled by an electronic circuit incorporating the contact manometer. For tests at very low temperatures liquid nitrogen can be introduced directly in the container with the test pieces. VKI

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~________-~TC Fig. 2. E l e c t r i c c i r c u i t of the t e m p e r a t u r e controlling system. The c i r c u i t of the e l e c t r o n i c t e m p e r a t u r e c o n t r o l l e r is shown in Fig. 2. The n o r m a l l y c l o s e d m a n o m e t e r contact CM is in the c i r c u i t of a r e l a y R 3 through which power is supplied to the n i t r o g e n h e a t e r EH!. The voltage on the h e a t e r is c o n t r o l l e d by a t r a n s f o r m e r T r l w i t h t a p s in the p r i m a r y winding. The e l e c t r o m a g n e t i c v al v e EMV is a c ti v at ed by de voltage f r o m a t r a n s f o r m e r T r l and r e c t i f i e r (D1-D 4, C1). The e l e c t r o m a g n e t i c valve c o m p r i s e s two c oil s L 1 and L2; the input voltage is e o m m u t a t e d by r e l a y R 4 and p o t e n t i o m e t e r PSR-03 with a t h e r m o c o u p l e TC. C u r r e n t in the c i r c u i t of EMV is c o n t r o l l e d by a c u r r e n t r e l a y CR~ and r e l a y R 2. The operation of the e l e c t r o m a g n e t i c valve is indicated by a w a r n i n g light L1. High t e m p e r a t u r e s a r e m e a s u r e d with a contact t h e r m o m e t e r CT, w h i c h is in the c i r c u i t of a 6PES c o n t r o l valve; the anode c i r c u i t includes an i n t e r m e d i a t e r e l a y R 1 which switches on the magnetic s t a r t e r MS 1. The r e q u i r e d voltage on the e l e c t r i c h e a t e r is s e t with the aid of an a u t o t r a n s f o r m e r A u T r l . The e l e c t r o n i c c i r c u i t f o r m e a s u r i n g and r e c o r d i n g the elongation of t e s t p i e c e s constitutes a s t a n d a r d s y s t e m with an IPS-020 s t a b i l i z e r , two r e s i s t o r s , and a r h e o e h o r d , which s e r v e s as a d i s p l a c e m e n t gauge. The r e s i s t a n c e of r h e o c h o r d 13 (see Fig. 1) is changed when the drum 12 is r o t a t e d by cable 8 as a r e s u l t of the d e f o r m a t i o n of a t e s t piece. The signal f r o m the r h e o e h o r d is t r a n s m i t t e d to a PSR-O3 or E P P - 0 9 s e l f - r e c o r d i n g p o t e n t i o m e t e r . The choice of the p o t e n t i o m e t e r depends on the r e q u i r e d speed of r e c o r d i n g the s t r a i n of t e s t p i e c e s . The r e l i a b i l i t y of the a b o v e - d e s c r i b e d equipment has been proved by operational e x p e r i e n c e .

llMarch

1969

Institute of Physics and Mechanics, AS UkrSSR, L'vov

87