Shear pipe effects and dynamic shear-strain localization in martensitic steels

A three-dimensional, multiple-slip, dislocation-density-based crystalline formulation, specialized finite-element formulations, and Voronoi tessellations adapted to martensitic orientations were used to investigate dislocation-density activities and shear-strain localization in high-strength martensitic steels under quasi-static and dynamic loading conditions. The formulation is based on accounting for variant morphologies and orientations, retained austenite, and initial dislocations densities that are uniquely inherent to martensitic microstructures. The effects of variant distributions and arrangements, loading directions, and microcracks on the evolution of shear-strain localization are investigated. The analysis indicates that shear-strain localization occurs due to slip-system compatibilities relative to the loading direction and the long direction of laths, which result in shear-strain accumulation. At specific triple junctions, rotation misalignments due to lattice and slip incompatibilities occur, and these incompatibilities are further exacerbated by the presence of defects, such as microcracks.