Mechanical Fatigue Characterization of NiTiHf SMAs for a Unified Descriptor to Actuation Fatigue

Experimental studies have been carried out to characterize, model and predict actuation or thermomechanical fatigue response and lifetime of nickel-titanium-hafnium high-temperature shape memory alloys to assess their functional performance and longevity for optimal actuator design. For this, the shape memory effect is exploited wherein the sample is cycled between an upper and lower cycle temperature at constant load (isobaric) to failure. In this paper, this process is carried out to develop an empirical model to fit established actuation fatigue lifetime and work-output data with mechanical fatigue lifetime data of NiTiHf SMAs. This is done in order to bridge the gap between slow and expensive actuation fatigue tests in contrast to much faster mechanical fatigue tests due to differences in loading rates. Mechanical and actuation fatigue testing of NiTiHf High temperature SMAs showcased correlation when compared within the work output-life curves. The discovery would allow for potential prediction or at the least a screening method of actuation fatigue lifetimes and work outputs exhibited by the material. Although, at its current stage, the idea still relies on conducting at least one actuation fatigue test to create a baseline reference due to there being an offset between the mechanical and actuation curves. However, it would still result in the decrease of total actuation tests and saved time, especially for tests in the high cycle fatigue regime.