Modeling of mixed-mode fatigue crack propagation

Abstract

Analytical and experimental approaches to determine mixed-mode fatigue crack growth threshold and growth rates are not well established and remain an active research topic. This dissertation compared the existing methods and developed some alternatives to address the problem with less assumptions and broader applicability. The derived models are based on a characteristic plane methodology and extend the stress/strain- based approach to fracture mechanics-based approach. Both shear-dominated failure and tension-dominated failure can be analyzed. The orientation of the characteristic plane changes according to the mode mixity, the ratio of shear fatigue limit over tensile fatigue limit, and the crack/notch tip radius for near threshold crack. It also depends on the grain orientation for microstructually small crack.

The effect of microstructure on the propagation of small fatigue cracks under rolling contact fatigue loading is examined in this dissertation. The local stress history is calculated using a macro-level 3-D elasto-plastic finite element model. A sub-modelling technique is used to achieve both computational efficiency and accuracy. The macro-level finite element model can accurately represent the contact stress of complex mechanical components and can consider the effect of loading non-proportionality. Then the equivalent stress amplitude at the critical location, which is calculated using a previously developed multiaxial fatigue limit criterion, is applied to a micro-level 2-D finite element model with center or edge crack. The fatigue model can automatically adapt for tensile/shear failure mechanisms according to material properties and loading conditions. Elasticity anisotropy, and randomness in both grain size and grain orientation are considered in the micro-level model. The geometric patterns of the grains in the polycrystalline wheel steel are generated using a 2D voronoi tessellation. The effects of applied load, crack size, grain orientation and grain disorientation on the mixed mode equivalent stress intensity factor are investigated using the developed models.