Analysis of fatigue damage in shape memory alloys by nanoindentation

Microstructural damage induced by fatigue mechanical loads in nickel-titanium (NiTi) based shape memory alloys is analyzed by instrumented indentation. To this aim, two indentation daparameters have been defined, which are directly obtained from the force-penetration depth curve and describe the phase reversibility of the material under loading-unloading indentation cycles. These parameters have been used to analyze the microstructural damage mechanisms occurring in dog-bone specimens, which have been fatigued in the low cycle fatigue regime, and in single edge crack specimens during crack propagation. Systematic studies have been carried to correlate the nanoindentation response with the functional fatigue damage of the alloy, i.e. to study the relation between the damage mechanisms at the micro- and macro-scales. Finally, it has been shown that both structural and functional fatigue damage occurring at the macro-scale in NiTi alloys are strongly related to phase irreversibility occurring at the micro-scale, i.e. during loading-unloading indentation cycles. As a consequence, nanoindentation would be used as a non destructive method to estimate the fatigue damage in NiTi alloys.