Strain gradient effects on steady state crack growth in rate-sensitive materials

Steady state crack propagation produce substantial plastic strain gradients near the tip, which are accompanied by a high density of geometrically necessary dislocations and additional local strain hardening. Here, the objective is to study these gradient effects on Mode I toughness of a homogeneous rate-sensitive metal, using a higher order plasticity theory. Throughout, emphasis is on the toughness rate-sensitivity, as a recent numerical study of a conventional material (no gradient effects) has indicated a significant influence of both strain rate hardening and crack tip velocity. Moreover, a characteristic velocity, at which the toughness becomes independent of the rate-sensitivity, has been observed. It is the aim to bring forward a similar characteristic velocity for the current strain gradient visco-plastic model, as-well as to signify its use in future visco-plastic material modeling.

► Fracture toughness rate-sensitivity is in focus and a newly found characteristic velocity is pursued.
► The hardening owing to strain gradients lowers the fracture toughness for all crack velocities.
► Strain gradient hardening suppresses stress triaxiality in the region near the fracture zone.
► Toughness dependence on velocity allows for the existence of the characteristic velocity.
► Novel approach to extract rate-independent toughness based on the characteristic velocity is suggested.