The aerospace, automotive, and energy industries have seen the potential benefits of using shape
memory alloys (SMAs) as solid state actuators. Thus far, however, these actuators are generally
limited to non-critical components or over-designed due to a lack of understanding regarding how
SMAs undergo thermomechanical or actuation fatigue and the inability to accurately predict failure
in an actuator during use. The purpose of this study was to characterize the actuation fatigue response
of Nickel-Titanium-Hafnium (NiTiHf) axial actuators and, in turn, use this characterization to predict
failure and monitor damage in dogbone actuators undergoing various thermomechanical loading
paths. Calibration data was collected from constant load, full cycle tests ranging from 200-600MPa.
Subsequently, actuator lifetimes were predicted for four additional loading paths. These loading paths
consisted of linearly varying load with full transformation (300-500MPa) and step loads which
transition from zero stress to 300-400MPa at various martensitic volume fractions. Thermal cycling
was achieved via resistive heating and convective cooling and was controlled via a state machine
developed in LabVIEW. A previously developed fatigue damage model, which is formulated such that
the damage accumulation rate is general in terms of its dependence on current and local stress and
actuation strain states, was utilized. This form allows the model to be utilized for specimens
undergoing complex loading paths. Agreement between experiments and simulations is discussed.