On the fracture toughness and stable crack growth in shape memory alloy actuators in the presence of transformation-induced plasticity

The effect of transformation-induced plasticity (TRIP) on the fracture response of polycrystalline shape memory alloys is analyzed in the prototype infinite center-cracked plate subjected to thermal cycling under constant mechanical loading in plain strain. Finite element calculations are carried out to determine the mechanical fields and the crack-tip energy release rate using the virtual crack closure technique. Similar to phase transformation, TRIP is found to affect both the driving force for crack growth and the crack growth kinetics by promoting crack advance when occurring in a fan in front of the crack tip and providing a shielding effect when occurring behind that fan. Accumulation of TRIP strains over the cycles results in higher energy release rates from one cycle to another and may result in crack growth if the crack-tip energy release rate reaches a material specific critical value after a sufficient number of cycles. During crack advance, the shielding effect of the TRIP strains left in the wake of the growing crack dominates and therefore TRIP is found to both promote the initiation of crack growth and extend the stable crack growth regime.