The asymmetric evolution of grain-scale stresses in notched specimens under cyclic loads

Engineering components are often exposed to various cyclic loading conditions. The development of microscale stresses under such conditions depends on the material texture, local microstructure, the degree of crystals elastic and plastic anisotropy, as well as the presence of geometrical irregularities. In this study, the three-dimensional synchrotron X-ray diffraction (3D-XRD) technique is used and coupled with a crystal plasticity finite element (CPFE) model to deconvolute the contribution of such parameters on the magnitude of localized stresses. Special attention is paid to the notch geometry, where two double-edge-notched alpha-zirconium specimens with different geometries but the same texture were deformed in-situ while grain-scale stresses were measured both in the vicinity of the notches and in the far-field zones. Deep and shallow notches were cut to the dimensions comparable with the specimens' average grain sizes. For each specimen, the center of mass, orientation, elastic strain, stress, and relative volume of more than 6,000 grains were measured and studied. Post-deformation analysis with Electron Backscatter Diffraction (EBSD) was conducted to further validate the observed trends. It is shown that the distribution of grain-scale stresses is asymmetric within each specimen. This asymmetry develops from the early stages of the applied cyclic load and remains intact with further loading. A new method is introduced for quantifying the effects of grain neighbourhood on load sharing, where it is shown that such effects relax the stresses acting on "hard" grains, altering grain-scale stress concentration factors.