In-Situ Studies During Cyclic Loading Of NiTi-Based Shape Memory Alloy Actuators

One of the biggest challenges in the field of SMA remains the functional degradation of material during thermomechanical cycling and related fatigue damage. In this work we focus on superelastic NiTi wires with special microstructures prepared by special thermomechanical treatment where we can observe microstructural evolutions taking massively place during first few cycles leading to stabilisation of the stress-strain curve. We believe that this part of cycling, when the microstructure undergoes intense changes such as accommodation of residual stresses, residual martensite and emersion of plasticity, plays the key role in the fatigue life of NiTi based materials, just number of cycles can vary significantly. The combination of these three phenomena results in a complex microstructure in which the cracks are nucleated. The experimental methods include in situ synchrotron X‑ray diffraction, electric resistance measurements applied during cyclic thermomechanical loading combined with mechanics modelling.