Effect of material rate sensitivity on failure modes in the Charpy V-notch test

For the Charpy V-notch test the influence of strain rate on competing failure mechanisms is analyzed numerically. The nucleation and growth of micro-voids is represented in terms of an elastic-viscoplastic constitutive model, which describes the mechanism of ductile fracture by void coalescence. Failure by cleavage is assumed to occur if the maximum principal tensile stress exceeds a certain critical value. Attention is focused on the temperature regime where the transition in fracture mode between cleavage and ductile rupture takes place. In the analyses the temperature is taken as constant and the effect of inertia is neglected, so that time dependence enters only through the material strain rate sensitivity. The material model is found to reproduce the experimentally observed change in failure mode from predominantly ductile fracture at low strain rates, to cleavage fracture at high strain rates. The numerical results show that in the transition regime, the porosity in the notch tip region plays a role in the fracture process even when failure occurs by cleavage. Once the transition of failure mode from cleavage to ductile rupture has occurred, the energy absorbed at low rates is greater than that absorbed at high rates.