Crack-tip thermal and mechanical hysteresis in Shape Memory Alloys under fatigue loading

Crack tip stress-induced phase transformation mechanisms in nickel–titanium alloys (NiTi), subjected to fatigue mechanical loads, have been analyzed by full field measurement techniques. In particular, Infrared thermography (IR) and Digital Image Correlation (DIC), have been applied to analyze the cyclic temperature and displacement evolutions in the crack tip region of a commercial pseudoelastic alloy, together with the associated thermal and mechanical hysteresis, by using Single Edge Crack (SEC) specimens. IR investigations revealed a global temperature variation of the specimen due to crack formation and propagation mechanisms, which is similar to common engineering metals, i.e. surface temperature rises quickly in an initial phase, then it reaches an almost constant value, and finally it increases rapidly as a consequence of the fatigue crack growth. In addition, cyclic thermal variation in the crack tip region and related hysteresis (temperature vs load) has been measured, which can been directly related to the thermal effects of the reversible stress-induced phase transformations. Further- more, a proper experimental setup has been made, based on a reflection microscope, for direct measurements of the crack tip displacement field by the DIC technique. Furthermore, a fitting procedure has been developed to calculate the mode I Stress Intensity Factor (SIF), starting from the displacement field, and the related mechanical hysteresis (SIF vs load).