A study of subsurface crack initiation produced by rolling contact fatigue
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INTRODUCTION
O N E o f the important factors that determines the n u m b e r o f contact cycles to failure in a rolling element is the amount of continuing plastic deformation. In simple push-pull fatigue, the n u m b e r o f cycles to failure is directly related to the plastic strain amplitude, given by the classic Coffin-Manson relation. Under repeated, welllubricated rolling contact, cyclic stresses produced by the contact pressure result in continuing cyclic plastic deformation when the contact pressure exceeds the shakedown pressure,tl'2j The shakedown pressure is determined by the cyclic strength, the constitutive relations, and the shape o f the contact a r e a .t~'2'3~ In the case o f perfectly plastic material behavior, the cycles o f continuing plasticity are not fully reversed. This leads to the accumulation o f unidirectional plasticity, ratchetting, or incremental collapse,I4] which may be more damaging than fully reversed plasticity. In kinematic hardening materials (such as bearing steels), cyclic plasticity gradually tends to promote fully reversed plastic strain cycles. ]SJ In either case, the continuing plasticity in the stressed region b e l o w the surface is expected to cause subsurface c r a c k initiation. Nonmetallic inclusions in SAE 52100 bearing steel have been studied by Becker.E6] His work revealed that the presence o f inclusions and a gradual breakdown o f the surrounding martensite into ferrite and carbide near the inclusions aid crack initiation and growth. L a t e r work on high-strength bearing steels showed that prior to crack initiation, several microstructural changes, such as the formation o f white and dark etching bands a f t e r several millions o f cycles, occurred. ]7.8] The orientations o f the white etching bands and cracks with respect to the rolling direction was recently shown to be related to the inclinations o f plastic strain concentrations calculated f o r idealized kinematic hardening material behavior, t9] There ARUN M. K U M A R , Postdoctoral Fellow, is with the Department of Mechanical and Materials Engineering, Washington State University, Pullman, WA 99164-2920. G E O R G E • T. HAHN and C A R O L A. RUBIN, Professors, are with the Center for Materials Tribology, Vanderbilt University, Nashville, T N 37235. Manuscript submitted April 20, 1992.
METALLURGICAL TRANSACTIONS A
is also evidence that the white etching bands o f f e r planes of weakness f o r c r a c k growth. I1°] The subsurface microstructural changes in bearing steels and the attending c r a c k initiation observed a f t e r a large number o f cycles are difficult and time consuming to reproduce in a laboratory. Further, it is difficult to characterize the microstructural changes produced by rolling contact except by detailed transmission electron microscopy. For these reasons, rolling contact fatigue studies o f a m o d e l material that fails more quickly, such as the high-strength 7075-T6 aluminum alloy, were undertaken to provide insight into the process o f d a m a g e accumulation
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