Branch crack behaviour in bearing steel under rolling contact fatigue

摘要

In rolling contact fatigue, cracks grow by tensile (Mode I) or shear modes (Mode II and III) under the material surface due to the complicated tri-axial stress state. Flaking type failure, such as bearings and rails, is essentially due to the crack growth behaviour of these cracks. In order to evaluate the fatigue strength under rolling contact loading, it is important to understand the characteristics of such cracks under complicated stress state.The aim of this thesis was to study mixed mode behaviour of fatigue cracks, under cyclic shear stress and static compressive stress. Both experimental and numerical methods are applied. Experimental work consisted of the fatigue testing of bearing steel (SAE52100) under compressive mean stress. The test results show that resistance to mode I crack propagation increases with increasing compressive mean stress. In addition, non-propagating cracks, which are hardly observed at stress ratio R of -1 in high strength steel, were observed at R = -5 and -10. In the numerical investigations, stress intensity factor of complicated branch cracks in a plate (2D) under combined cyclic shear stress and static compressive stress was determined by finite element method. This study reveal the interaction behavior between the mode I and II cracks. The experimental and simulation results showed that there is a competitive behaviour between the Mode I and Mode II crack growth. The role of the compressive mean stress is to suppress the Mode I crack propagation. The magnitude of the compressive mean stress required to suppress Mode I crack growth depends on the Mode II crack length and applied cyclic shear stress. It was also observed that mode I crack lengths, if long enough, may reduce the KII of the Mode II crack even by 63%. The observation of non-propagating cracks for high compressive mean stress is one of novel finding of this study. However, the physical reason for this is not yet know. This is left future work. Furthermore, the numerical investigations can be extended to study more detail the influence of crack surface friction on crack growth mechanics.