Elastic-Plastic Fracture Mechanics Study of Tribology Problems.

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The elastic-plastic behavior of the subsurface layer under cyclic rolling-sliding contact loading is analyzed by the combination method (BFM+FEM) of the body force method(BFM) and the finite element method(FEM). Comparison between the amount of forward flow obtained numerically and the experimental measurements on a medium carbon steel shows that the material under the cyclic contact loading obeys the intermediate role of the isotropic hardening rule(IHR) and the kinematic hardening rule(KHR). The residual stresses and incremental cyclic plastic strains are also analyzed to investigate the easiness of crack initiation for driver and follower surface.In order to study the growth mechanism of surface crack ; pits and spall, on lubricated rolling-sliding contact surface, comouting programs for the analysis of cracks with curved and crossed planes were developed. In the numerical analysis, the Non-Hertzian loadings caused by surface roughness were considered. The effects of the contact of crack surfaces and friction between crack surfaces under compressive loading were also investigated.Considering the results of the crack initiation and growth analysis, it is concluded that the crack propagation condition is more crucial than the crack initiation condition as the cause of a pit formation under lubricated rolling-sliding contact loading.The importance of the mode II type crack growth in the early stage of crack propagation was pointed out and a new experimental method to measure the threshold stress intensity factor range for shear mode fatigue crack growth DELTAKtau_<th> was proposed.