Crack-Parallel Stress Effects on Fracture at the Atomic Scale

Abstract Up to a few years ago, the fracture mechanics has focused on the role of singular stress field at the crack front or on a crack with scalar cohesive stress imagined existing near the front, while the influence of non-singular crack-parallel stresses has been ignored. However, recent studies show that different levels of such stresses can significantly alter fracture behaviors in many materials, often doubling the apparent fracture energy or reducing it nearly to zero. These findings challenge conventional linear elastic fracture mechanics (LEFM) and highlight the need to investigate the effect of non-singular stress states. In this study, we employ molecular dynamics models to examine crack-parallel stress effects at the atomistic level. We identify two distinct mechanisms of the crack-parallel effect on the atomic scale that explain the observed non-monotonic work-to-fracture response under increasing crack-parallel compression. Under moderate parallel compression, the displacements of surface atoms required by the creation of surface energy and the atomic-level densification increase the energy density and therefore enhances the material fracture energy. At higher levels of compression, generation of local defects destabilizes the fracture process zone, thus reducing the material fracture energy. By probing these mechanisms at the nanoscale, our study provides a computational foundation for fracture models that connect to the newly observed macroscale behaviors and informs the design of crack-tolerant quasi-brittle materials.