Fast atomic crack kinking and branching in WS₂

Abstract

Dynamic nanocrack propagation in 1T- and 2H-WS2 strips is studied by molecular dynamics, and the T-stress and circumferential stress in linear elastic fracture mechanics are considered. As the crack propagates, the crack-tip speed (v) experiences a rapid acceleration, and then oscillates at ~55% (1T) and ~65% (2H) of the Rayleigh-wave speed followed by crack kinking/branching. The critical energy release rates of crack instability are estimated to be ~1.5 J/m2 (1T) and ~4.0 J/m2 (2H). The crack path in 1T-WS2 exhibits higher sensitivity of strain rates for atomic asymmetry around the crack tip. Examination of the dynamic crack-tip field shows that the T-stress obtained by the over-deterministic method always fluctuates in negative, and the theoretical circumferential stress curve does not accurately capture the v-dependent atomic stress distribution. Consequently, both T-stress and circumferential stress are limited in predicting the crack kinking/branching directions, which can be attributed to the discrete crystal lattice and local anisotropy of WS2, where a preferred crack path along the zigzag surface is observed. The fracture properties of WS2 might provide useful physics for its applications in nano-devices.