Crack velocity jumps engendered by a transformational process zone

Transformation toughness induced by surface tension of the crack-tip process zone interface: A field-theoretical approach

We study a crystal with a motionless crack exhibiting the transformational process zone at its tip within the field-theoretical approach. The latter enables us to describe the transformation toughness phenomenon and relate it to the solid's location on its phase diagram. We demonstrate that the zone extends backward beyond the crack tip due to the zone boundary surface tension. This setback engenders the crack-tip shielding, thus forming the transformation toughness. We obtain a quadrature expression for the effective fracture toughness using two independent approaches-(i) with the help of the elastic Green function and, alternatively, (ii) using the weight functions-and calculate it numerically applying the results of our simulations. Based on these findings, we derive an accurate analytical approximation that describes the transformation toughness. We further express it in terms of the experimentally accessible parameters of the phase diagram: the hysteresis width, the phase transition line slope, and the transformation strain.

Morphological transformation of the process zone at the tip of a propagating crack. I. Simulation

Stress concentration at a crack tip engenders a process zone, a small domain containing a phase, different from that in the bulk of the solid. We demonstrate that this zone at the tip of a propagating crack exhibits a morphological transformation with an increase of the crack velocity. The concave zone shape with an invagination in its back that is characteristic of a slow crack transforms into a droplet-shaped convex zone upon exceeding a critical velocity value, v_{G}. In this latter case, a metastable wake follows the propagating zone. We obtained this result by computer simulation of a crack propagating in a solid exhibiting a first-order phase transformation.

Crack-tip process zone as a bifurcation problem

Stress concentration at a crack tip generates a solid structural transformation in its vicinity, the process zone. We argue that its formation represents a local phase transition described by a multicomponent order parameter. We derive a system of equations describing the dynamics of the order parameter driven by an inhomogeneous, time-dependent stress field in the solid and show that it exhibits a bifurcation. The latter corresponds to the emergence of a process zone characterized by the distribution of the order parameter localized in the vicinity of the crack tip. The emergence temperature T_{*} considerably differs from the temperature of the bulk phase transformation T_{c}. We demonstrate that T_{*} exhibits a universal behavior T_{*}-T_{c}∼K_{I}^{4/3}, in terms of the stress intensity factor K_{I}, and that the zone universally vanishes upon achieving a critical velocity. These facts together give rise to a universal dynamic phase diagram.