Micromechanical simulation for texture induced uncertainty in fatigue damage incubation using crystal plasticity model

Abstract

Crystallographic texture of wrought aluminum alloys has significant influence on the fatigue damage incubation and small-crack growth in the high cycle fatigue (HCF) regime. HCF lives have demonstrated fairly large scatters due to the random textural feature and its stochastic interaction with fatigue damage evolution. The fatigue life of an individual specimen is deterministic, which manifests the specific microstructure in the specimens and its interference with fatigue. In this paper, micromechanical simulations were conducted to quantify the grain orientation and grain boundary mismatch effects to fatigue damage incubation in a wrought 7075-T651 Al alloy. Fatigue damage incubated at fractured intermetallic particles near or at the surface [Xue et al. 2007]. Many intermetallic particles fractured after a few initial loading cycles; however, only one or a few fractured particles induced a crack into the matrix. One of these microcracks eventually grew and became a dominate crack and caused the final failure. The size of the particles, the orientation of the grain in which the particle resides, the particle spatial location to the grain boundary, the mismatch of the grain boundary, and the interaction of all these factors comprise the primary reason for the fatigue damage incubation. But no systematic investigation exists to provide general guideline for fatigue design and structural prognosis applications.In this research, a crystal plasticity constitutive model is implemented to simulate the microplasticity at the fractured intermetallic particles in single crystal and bicrystals, in which the crystallographic orientation mimics the typical rolling texture. The spatial location of the particle with respect to the grain and grain boundary is designed to represent those observed on the fractographs fatigued in the high cycle fatigue regime. The non-uniform loading distribution due to the heterogeneity of textured alloy is investigated by simulating on a representative unit cell with grain size and orientation taken from the realistic stereomicrograph of the sample. This approach is an initial high-order mechanistic multistage fatigue modeling approach, which can be implemented to estimate the uncertainty of the fatigue life prediction and fatigue life distribution using Monte Carlo simulation. Eventually, the reliability of fatigue life predictions will be accessed and the model evaluated as a mathematically rigorous tool for the structural integrity prognosis of aerospace structures that use wrought aluminum alloys.