Temperature dependent fracture properties of shape memory alloys: Novel findings and a comprehensive model

Abstract

Temperature dependent fracture properties of NiTi-based Shape Memory Alloys (SMAs), within the pseudoelastic regime, were analyzed. In particular, the efective Stress Intensity Factor (SIF) was estimated, at diferent values of the testing temperature, by a ftting of the William's expansion series, based on Digital Image Correlation (DIC) measurements. It was found that temperature plays an important role on SIF and on critical fast fracture conditions. As a consequence, Linear Elastic Fracture Mechanics (LEFM) approaches are not suitable to predict fracture properties of SMAs, as they do not consider the efects of temperature. On the contrary, good agreements between DIC results and the predictions of an ad-hoc analytical model were observed. In fact, the model takes into account the whole thermo mechanical loading condition, including both mechanical load and temperature. Results revealed that crack tip stress-induced transformations do not represent a toughening efect and this is a completely novel result within the SMA community. Furthremore, it was demonstrated that the analytical model can be actually used to defne a temperature independent fracture toughness parameter. Therefore, a new approach is proposed, based on the analytical model, where both mechanical load and temperature are considered as loading parameters in SIF computation.