Atomistic modeling of failure in iron

Abstract

This thesis is based on three research papers, all concerning molecular dynamics (MD) simulations of failure properties in pure iron. The first two papers examine iron crystals with cracks deformed in Mode I tensile testing, with different geometrical constraints. In Paper I we have reviewed fracture mechanics in light of atomistic simulations, and shown a possible way to link atomistic and multiscale simulations of cracks to the crack initiation toughness of the material. Stress-intensity factors and effective surface energies were calculated from atomistic penny-shaped cracks and multiscale edge crack simulations. The influence of T-stress/constraint level was examined.

Paper II is devoted to the study of penny-shaped cracks, comparing this geometry with the more commonly studied through-thickness cracks. It was found that the fracture mechanisms in specific crystallographic orientations were similar, but that the penny-shaped crack was able to change shape during loading in order to favor dislocation emission over unstable fracture.

The last paper, Paper III, is a study of size and strain rate effects in compression of nanopillars, where three crystallographic orientations were simulated. A size-strengthening effect was found for pillars compressed along (100) and (110) directions, and a lower strain rate was shown to result in lower maximal stress before deformation began.