Microstructural effects on high cycle fatigue

Mechanical components subjected to cyclic loadings below the ultimate strength of the material often fail due to a phenomenon known as fatigue. Fatigue failures occur in three stages; crack initiation, crack propagation, and final catastrophic failure. For high cycle fatigue the crack initiation component of the total life usually dominates. Crack initiation in high cycle fatigue originates at the microstructural level often occurring at initial material imperfections or at plastic slip bands that are developed in a few poorly oriented grains. In addition there are applications such as micro-electro-mechanical systems (MEMS) and rolling contacts where the stressed volume of the material is confined to a small number of grains. For these reasons in order to properly investigate the origins of high cycle fatigue the effects of the material microstructure must be considered. This work focuses on modeling procedures that include micro-level material features to predict fatigue life and fatigue life scatter in MEMS devices and rolling contacts. Simulated microstructures are used in conjunction with continuum damage mechanics to predict the fatigue life of a component. Two material modeling frameworks are used. The first approach uses a discontinuous representation of the material. The second approach uses a continuum finite element model which incorporates cohesive zone elements. Material disorder in the form of topological variation in the material microstructure, inhomogeneous material properties, and initial internal flaws are included in the model to study their effects on fatigue life scatter. The fatigue lives predicted by the model are found to be in good agreement with the experimental results available in the literature