Relation Between Strain Localization Front Movement and Fatigue Lifetime in NiTi Shape Memory Alloys

Movement of transformation fronts is known to lead to transformation-induced slip and consequently structural and functional fatigue in NiTi shape memory alloys (SMAs). Thus, intuitively, larger transformation front movements should lead to larger transformation-induced slip accumulation and poorer fatigue lifetimes. In this work, we investigate the relation between the volume swept by the transformation fronts in tensile test specimens and the fatigue lifetime in NiTi SMAs. As part of this investigation, we present two standardized test specimens — one for tension and another for bending — that are designed to enable multi-modal characterization of microstructure and deformation in SMAs. Specifically, these test specimen designs are compatible with electron backscatter diffraction (EBSD), scanning electron microscopy (SEM), micro-scale computed tomography (μCT), high-energy X-ray diffraction, and digital image correlation measurement methods. We envision an easier sharing of fatigue testing data across the industry with broader adaption of these standardized test specimens.