THE EFFECT OF HIGH-FREQUENCY

WORKING LOADS

MEDIA

ON

THE

FATIGUE

STRENGTH

OF

STEEL

UNDER

N. L. Pozen Fiziko-Khimicheskaya UDC

Mekhanika

Materialov,

Vol. 5, No. 4, pp. 506-507,

1969

620.178.382.4:620.194.23

The effect of working media on the fatigue strength of m e t a l s in the load frequency range 10-20 kttz/sec has not been extensively studied. While, a few y e a r s ago, it was believed [1] that in view of the short test duration at such high f r e q u e n c i e s the influence of c o r r o s i v e media may be neglected, it i s now known that this effect may be quite c o n s i d e r a b l e . According to some w o r k e r s [2], s p e c i m e n s tested u n d e r s y m m e t r i c a l l o a d - c y c l e conditions in different working m e d i a have different fatigue l i m i t s . No a n a l y s i s of the physicochemical phenomena taking place during these tests was c a r r i e d out. The a i m of this i n v e s t i g a t i o n was to find an explanation for the r e s u l t s of tests on St. 45 steel s p e c i m e n s fatigued i n two c o r r o s i v e m e d i a at loading f r e q u e n c i e s of 18.2-19.1 k c / s e c . The explanation was sought i n the f r a m e w o r k of the a d s o r p t i o n - e l e c t r o c h e m i c a l theory of c o r r o s i o n fatigue [3] taking into account p h y s i c o c h e m i c a l phenomena that occur in liquid m e d i a u n d e r the influence of u l t r a s o n i c v i b r a t i o n s . The t e s t s were c a r r i e d out on a high-frequency r e s o n a n c e fatigue t e s t i n g machine US-20 [4]; polished s p e c i m e n s with a g a u g e - p o r t i o n d i a m e t e r of 7.5 m m were subjected to s y m m e t r i c a l and a s y m m e t r i c a l t e n s i o n - c o m p r e s s i o n loading cycles. In these c i r c u m s t a n c e s , a large quantity of heat i s generated at the v i b r a t i o n node due to h y s t e r e s i s l o s s e s . To m a i n t a i n the s p e c i m e n s at n o r m a l t e m p e r a t u r e we i n i t i a l l y used w a t e r cooling; the test duration ranged f r o m 3 m i n to 15 hr. The cooling m e d i u m s e r v e d also as the working m e d i u m . After tests, the s p e c i m e n surface showed evidence of c o r r o s i o n (Fig. 1) which could be seen even with the naked eye. The e n t i r e s p e c i m e n surface was u n i f o r m l y corroded. To elucidate the effect of the c o r r o s i v e m e d i u m on the fatigue strength, a r e p l i c a t e s e r i e s of t e s t s was c a r r i e d out on s p e c i m e n s (of the s a m e batch of St. 45 steel) made i n the same way but cooled with t r a n s f o r m e r oil during the fatigue t e s t s . No evidence of c o r r o s i o n was found on the surface of s p e c i m e n s fatigued in t r a n s f o r m e r oil. Fatigue curves obtained for a m = 36.1 k g / m m 2 a r e reproduced in Fig. 2. The curve r e l a t i n g to s p e c i m e n s tested in t r a n s f o r m e r oil is situated above that obtained for w a t e r - c o o l e d s p e c i m e n s and is l e s s steeply inclined. In view of such a marked difference in fatigue s t r e n g t h levels it b e c a m e n e c e s s a r y to r e j e c t the a s s e r t i o n that the influence of c o r r o s i v e media i n these c i r c u m s t a n c e s is negligible and to analyze this influence f r o m the standpoint of the a d s o r p t i o n - e l e c t r o c h e m i c a l theory of c o r r o s i o n fatigue, taking into account phenomena o c c u r r i n g in liquid media u n d e r the influence of u l t r a s o n i c s .

Fig. 1. Specimen surface a) before and b) after a fatigue test (x 100). 402

Z

>L'I-~

~80 GO dO

N

Fig. 2. Fatigue curves of St. 45 steel specimens subjected to asymmetrical loading cycles at a frequency 18.1-19.1 kHz/sec in o) transformer oil and O) water.

Fig. 3. Schematic of the method for m e a s u r ing the potential i n the v i c i n i t y of a fatigued specimen.

Fig. 4. E l e c t r i c a l potential around a fatigued s p e c i m e n plotted against the v i b r a t i o n amplitude.

It is known [5] that e l e c t r i c a l c h a r g e s a r e produced i n liquid media through which u l t r a s o n i c waves are propagated and that a double e l e c t r i c a l l a y e r is f o r m e d at the m e t a l - d i e l e c t r i c contact i n t e r f a c e . When this contact is b r o k e n (at the expense of e x t e r n a l l y applied energy), e l e c t r i c a l d i s c h a r g e s were produced. This may happen as a r e s u l t of f r i c t i o n between, f o r instance, a fatigue s p e c i m e n and some liquid m e d i u m . In the case of s p e c i m e n s tested i n t r a n s f o r m e r oil, the p a s s a g e of e l e c t r i c c u r r e n t s produced by such charges is v i r t u a l l y i m p o s s i b l e . When steel s p e c i m e n s were tested i n water, which is a r e l a t i v e l y good conductor, the p r e s e n c e of an e l e c t r i c c u r r e n t produced as a r e s u l t of f r i c t i o n between the s p e c i m e n and liquid m e d i u m was detected. We m e a s u r e d the e l e c t r i c p o t e n t i a l s between the m e d i u m and the s p e c i m e n (in r e l a t i o n to the s p e c i m e n v i b r a t i o n amplitude) by a method shown s c h e m a t i c a l l y i n F i g . 3. A metal ring !, insulated f r o m s p e c i m e n 2 (to p r e v e n t shorting) i s connected to a C1-4 o s c i l l o g r a p h which has an input impedance of 0.5 1V[~ and which can be used to m e a s u r e both constant and a l t e r n a t i n g p o t e n t i a l s . The v i b r a t i o n amplitude was m e a s u r e d with m i c r o s c o p e 3. Since the e l e c t r i c a l r e s i s t a n c e of the water between the r i n g s is much l e s s than the input impedance of the oscillograph, such a connecting method has v i r t u a l l y no effect on the m e a s u r e d potential v a l u e s . The r e s u l t s of these m e a s u r e m e n t s a r e r e p r o d u c e d i n Fig. 4. It will be seen that the e l e c t r i c field potential i n c r e a s e s with the i n c r e a s i n g amplitude of s p e c i m e n v i b r a t i o n s which c o r r e s p o n d s to an i n c r e a s e in the l i n e a r velocity of the s p e c i m e n d i s p l a c e m e n t . It i s evident that e l e c t r i c a l charges p a s s i n g through w a t e r lead to its d i s s o c i a t i o n into hydrogen and oxygen; this, in turn, p r o m o t e s a d s o r p t i o n - e l e c t r o c h e m i c a l acts of c o r r o s i o n fatigue and, possibly, hydrogen charging of steel which i s r e f l e c t e d i n the nature of the f r a c t u r e of s p e c i m e n s (brittle f r a c t u r e without neck formation).

REFERENCES 1. 2. 3. 4. 5.

S. Tanaka, Rep. Inst. High Sp. Mech. Japan, 13, no. 129, 1961-1962o M. Kikukawa, K. Ohji, and K. Ogura, ASME, no. 4, 1965. G . V . Karpenko, C o r r o s i o n Fatigue of Metals [in Russian], Izd. Kamenyar, 1964. N. L~ Pozen, collection: Machines and I n s t r u m e n t s for T e s t i n g Metals [in Russian], Izdo Metallurgiya, 1968. B. V. D e r y a g i n and N. A. Krotova, Adgeziya, Izd. AN SSSR, !949.

8 J a n u a r y 1968 Institute for P r o b l e m s of Strength AS UkrSSR, Kiev

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