Stable Crack Growth in Shape Memory Alloy Actuators
Shape Memory Alloys (SMAs) can recover large, apparently permanent, strains when subjected to particular thermomechanical inputs. The key physical mechanism that drives this shape recovery is a reversible diffussionless solid to solid phase transformation from austenite to martensite and vice-versa under applied load or temperature variations. SMA actuators take advantage of this property to provide a significant amount of actuation with an extremely small envelope volume.
In this work, the effect of thermo-mechanically-induced global phase transformation on crack growth in an SMA actuator is investigated by means of the finite element method. The prototype center-crack problem is analyzed during thermal cycles under isobaric loading conditions. The temperature variation is sufficient to induce global phase transformation. The virtual crack closure technique is employed to measure the crack tip energy release rate during the entire actuation cycle. Results show that during actuation the energy release rate can increase drastically, an order of magnitude for specific material systems. This in turn implies that crack growth may be triggered as a result of phase transformation, resulting eventually in the actuator's ultimate failure. Stable crack growth is initially observed, i.e., the resistance to fracture increases with growing crack size, however, the observed toughness enhancement associated with crack advance cannot prevent ultimate failure for more than a few thermal cycles after initiation of crack propagation.