Crack tip displacements of microstructurally small cracks in 316L steel and their dependence on crystallographic orientations of grains

Abstract

ABSTRACTThe paper presents an analysis of the effect of the grain orientations on a short Stage I surface crack in a 316L stainless steel. The analysis is based on a plane‐strain finite element crystal plasticity model. The model consists of 212 randomly shaped, sized and oriented grains that is loaded monotonically in uniaxial tension to a maximum load of 1.12Rp0.2 (280 MPa). The influence of random grain structure on a crack is assessed by calculating the crack tip opening (CTOD) and sliding displacements (CTSD) for single crystal and polycrystal models, considering also different crystallographic orientations. In the single crystal case the CTOD and CTSD may differ by more than one order of magnitude. Near the crack tip slip is activated on all the slip planes whereby only two are active in the rest of the model. The maximum CTOD is directly related to the largest Schmid factors. For the more complex polycrystal cases it is shown that certain crystallographic orientations result in a cluster of soft grains around the crack‐containing grain. In these cases the crack tip can become a part of the localized strain, resulting in a large CTOD value. This effect, resulting from the overall grain orientations and sizes, can have a greater impact on the CTOD than the local grain orientation. On the other hand, when a localized soft response is formed away from the crack, the localized strain does not affect the crack tip directly, resulting in a small CTOD value. The resulting difference in CTOD can be up to a factor of 4, depending upon the crystallographic set. Grains as far as 6xCracklength significantly influence the crack tip parameters. It was also found that among grains with favourable orientation the CTOD increased with the size of such a grain. Finally, a significant change in CTOD and CTSD was observed when extending the crack into the second grain and placing it in the primary or the conjugate slip plane.