Microscale stored energy as a fatigue indicator for NiTi shape memory alloys via synchrotron X-ray diffraction

Although fatigue is closely related to microstructural changes, current fatigue criteria for shape memory alloys (SMAs) fail to account for this information due to the lack of research on quantifying microstructural defects associated with fatigue. In this study, we introduce local stored energy as a quantifiable parameter that reflects microstructural evolution and demonstrate its effectiveness as a reliable fatigue indicator. Ex-situ synchrotron X-ray diffraction tests were conducted on a series of NiTi specimens subjected to cyclic loading and stopped at different fatigue stages. The results revealed inhomogeneous microstructures along the gauge section, characterized by residual R-phase accumulation, defect density, and residual stress in active zones. These microstructural changes, resulting from localized deformation, were quantified by local stored energy at the microscale via X-ray peak analysis. Consistent with these inhomogeneous microstructures, the distribution of local stored energy was uneven, with maximum values in active zones where fatigue cracks preferentially occur. As fatigue progressed, local stored energy in these zones increased, eventually stabilizing at a steady state. This steady state exhibited a negative correlation with fatigue lifetimes, where higher loading frequencies resulted in increased stored energy and shorter lifetimes. These findings validate local stored energy as a crucial fatigue indicator, paving the way for development of a physically-grounded fatigue criterion based on this quantity.