Characterization and Modeling of Thermo-Mechanical Fatigue in Equiatomic NiTi Actuators

Due to their high actuation energy density and ability to recover large deformation, Shape Memory Alloys (SMAs) have many promising engineering applications. However, applications are sometimes limited to non-structural or non-critical components due to the lack of fatigue characterization standards and general understanding of the actuation fatigue process. The purpose of this study was to characterize the actuation fatigue response of equiatomic Nickel Titanium (NiTi) with various heat treatments and loading paths and utilize the characterization data collected to calibrate a previously developed constitutive damage model for multiple loading paths. Dogbone actuators were processed from heat treated NiTi sheets; heat treatments ranged from 350°C to 400°C for one to three hours. The actuators were subjected to actuation fatigue via mechanical loading, resistive heating, and convective cooling. Two mechanical loading schemes were utilized: a constant load initially set at 200MPa and a linear or spring load ranging from 150MPa (fully martensite) to 250MPa (fully austenite), thus maintaining a similar work per cycle for each loading scheme. Linear loading schemes were introduced in order to better simulate actuation in an aerospace application, such as the morphing of semi-rigid surfaces. Specimens were thermally cycled to full actuation with a feedback (displacement and temperature) control scheme developed in LabVIEW. The observed fatigue responses varied widely as a result of different heat treatments and, to a lesser extent, loading schemes. For actuators with higher temperature heat treatments, the main observed failure mechanism was strain localization (necking). Data resulting from these experiments were used to calibrate a previously developed fatigue damage model, which is formulated such that the damage accumulation rate is general in terms of its dependence on current stress and actuation strain states. This allows the model to be fit to data from specimens subjected to variable loading paths described herein. Agreement between experiments and simulations is discussed.