Micromechanical modeling of short crack nucleation and growth in high cycle fatigue of martensitic microstructures

Abstract

High cycle fatigue (HCF) is a frequently limiting failure mechanism of machine elements and modern high strength steels. Present day design rules rely on semi-empirical methods, guidelines and utilization of macroscopic analysis means in origin, such as fracture mechanics. The resulting challenge is that short crack regime, critical for HCF in terms of lifetime of components and products, is somewhat poorly handled. This is an outcome of the fact that the present means and methodologies do not explicitly account for effects arising from material microstructure, an oversight micromechanics aims to rectify.Micromechanical modeling operating on fatigue at the scale of material microstructure necessitates the introduction of suitable means to describe the mechanisms of cyclic plastic deformation and microstructural morphologies, considered critical for HCF especially at the early stages of micro-crack nucleation and damage evolution towards and within the short crack regime. In current work, a crystal plasticity based approach with combined hardening is utilized to capture the respective deformation response utilizing full field modeling. The modeling is carried out for both simplified prior austenite grain like microstructures as well as complex imaging based martensitic quenched and tempered steel microstructural models. A fully coupled damage modeling scheme is introduced to track damage nucleation and evolution at the scale of the studied microstructures. Crack closure is included within the approach to track behavior of microstructure scale defects under, e.g., fully reversed loading, more realistically. Model calibration is addressed and application cases involving damage and crack growth both under monotonic and cyclic loading are presented.The results demonstrate how the coupling of damage to crystal plasticity modeling can be utilized to identify and track the evolution of microstructure scale damage mechanisms in complex martensitic microstructures. Interactions between strain localization and damage accumulation are presented as well as transition from micro-cracking to short crack growth. The results show that the proposed approach can interpret the intricate dependencies and relations between complex microstructures, their (cyclic) deformation mechanisms and evolution of damage, the outcomes regarding crack formation and behavior are found to be in line with similar experimental studies.The proposed framework for modeling damage in polycrystalline microstructures is quite general in its capabilities. By solely introducing a suitable crystal plasticity based deformation model and a damage model describing nucleation and softening can plastic slip and damage interactions be studied in complex microstructures, and in principle, on any system where similar constitutive models are utilizable. The exploitation of the resulting micromechanical modeling and simulation capabilities lies both in simulation driven design of fatigue resistant components and high strength steels.