Development of a two-scale damage model for incorporating the fatigue crack nucleation from surface inclusions

Abstract

In high cycle fatigue, damage and plasticity occur at the microscale, a scale smaller than the representative volume element (mesoscale). Therefore, the two-scale damage model is used for fatigue life assessment. In this model, a spherical inclusion is assumed to be fully embedded in the matrix. However, experimental observations show that the microplasticity and microcracks nucleate more easily in grains on the free surface rather than in the bulk. Hence, the aim of this work is to extend the two-scale damage model to take into account the nucleation of fatigue cracks on free surfaces. In order to do this, a surface scale transition law is derived to link the micro and mesoscales, considering a hemispherical damaged inclusion at the surface of an elasto-plastic matrix. Furthermore, a numerical scheme is proposed by developing an in-house Python code in conjunction with Abaqus software to compute the damage. The material parameters required in model are identified using monotonic and fatigue test results of smooth specimens. As a validation, for the butt-welded specimens subjected to fatigue loading, the obtained results of the proposed surface law are compared with the results of the pre-existing non-surface laws and the experimental fatigue data. It is confirmed that the use of surface localization law leads to a noticeable reduction in the number of cycles to crack initiation of about 25%.