A kinetic model for ductile-brittle fracture mode transition behavior

Abstract A kinetic model for ductile-brittle fracture mode transition has been developed. In the ductile-brittle transition temperature range, brittle and ductile fracture are characterized in terms of thermally activated growth processes of tensile model I and shear mode II cracks, respectively. The thermally activated bond breaking in tension and shear are assumed to proceed under the condition of local stress relaxation due to dislocation nucleation at the crack tip. The disclocation nucleation rate is considered to be proportional to the strain rate. The tensile and shear fracture probabilities are defined by the product of the jump probability from the stretching to the breaking state of bonds at the crack tip, the active volume, and the density of microcracks or microvoids. The ductile-brittle transition temperature can be determined provided both the tensile and shear fracture probabilities are the same. The ductile-brittle transition temperature is given as a function of the activation energies for bond breaking in tension and shear, and dislocation nucleation, as well as the microstructural and mechanical parameters. This kinetic model provides a rationale for an experimentally determined linear relationship of the ductile-brittle transition temperature measured by small punch tests to that by Charpy V-notch tests on temper embrittled steels.