Size effects in molecular dynamic simulations of fracture in bcc iron crystals

Abstract

Three-dimensional (3D) simulations via molecular dynamics (MD) show that the brittle or ductile behavior of the atomistic samples with the edge crack (001)[110] (crack plane/crack front) depend on size of the self-similar atomistic crystals. Since the basic continuum predictions concerning cracks do not consider the random thermal atomic motion, we are restricted in this study to MD simulations with initial temperature of 0 K. For all samples tested, the crack initiation is brittle. However, the subsequent crack growth can be inhibited by twin formation on oblique planes {112}, crack branching along {011} planes and new dislocation emissions on {123} slip planes and the final fracture can also be then ductile, which depends predominantly on the thickness of the atomistic sample. The representative quantity, the atomistic fracture toughness initially increases with increasing sample thickness and later saturates near Griffith level for plane strain state along the crack front. The tested loading rates are equivalent to a cross head speed of 0.833×10-4 m/s used in one our previous experiment. These new MD results comply with the stress analysis performed by the anisotropic linear fracture mechanics (LFM) and comply with some experimental observations.