Physics-based modelling of cyclic deformation and microstructure-sensitive fatigue crack propagation from shallow scribes

Abstract

Face-centered cubic (FCC) metals with low to medium stacking fault energy (SFE) develop similar mesoscale substructures under cyclic loading. The formation of these substructures is controlled by dislocation interactions and loading conditions. For instance, cross slip facilitates cell formation and Hirth locks define the labyrinth structure. In the case of aluminium (high SFE metal), cross slip is easily activated and a cell structure is often observed. However, it is not always recognised that aluminium can also form PSBs at low temperatures. This highlights that the underlying mechanism controlling the cyclic response in aluminium is not different from other FCC metals. This work proposes the role of mesoscale substructure as a material-invariant among FCC metals to predict the cyclic response of aluminium. The effect of number of cycles on modelling dislocation substructures is explored, which is found to trigger a change in dislocation structures in aluminium at 298K. A crystal plasticity framework based on mesoscale substructures is developed to study the cyclic response of aluminium under different crystal orientations, strain amplitudes, number of cycles, and temperatures. Finally, this work implemented the crystal plasticity model to study the microstructure-sensitive crack propagation from shallow scribes in pure aluminium. The gradient of fatigue indicator parameters (FIPs) is estimated as crack extends inside a grain with explicit microstructure simulations, which followed the same decaying trend predicted by experiments. Thereby, an engineering solution is proposed to couple microstructural and geometric gradients at the crack tip independently. The model predicted the transgranular fatigue life with independently coupled gradients that agree well with experiments.