Padalia, Hrishikesh Rajeshkumar (2023-08). Mechanical Fatigue Characterization of NiTiHf SMA's for a Unified Descriptor to Actuation Fatigue. Master's Thesis.
Thesis
Due to its potential candidacy for use as solid-state actuators, experimental studies to characterize, model, and predict the actuation or thermomechanical fatigue response of NiTiHf high temperature SMA's (HTSMA) have been carried out to assess their functional performance and longevity for optimal actuator design. A drawback of actuation fatigue testing is the high cost and time span the tests require because of slow heating and cooling rates. This is especially applicable to high cycle actuation fatigue, which can take on the order of days to weeks to complete. On the other hand, mechanical or pseudoelastic fatigue is much quicker to carry out due to faster loading rates, with high cycle fatigue tests taking just hours. This work aims to experimentally characterize the mechanical fatigue lifetime and functional response of three Ni-rich NiTiHf HTSMA's (Ni50.3T i29.7Hf20, Ni50.5T i33.5Hf16, and Ni50.6T i30.4Hf19) under uniform testing parameters, and further develop an empirical model to fit established actuation lifetime and work-output data with mechanical fatigue lifetime data. Using this approach, similarity between the two fatigues types along different thermomechanical paths and loading rates, providing a unifying descriptor for tensile thermomechanical fatigue can be demonstrated. This approach showed that a similarity in the slopes of the two fatigue lifetime trends is observed across all the materials tested when using a mechanical work equivalent similar to the actuation work output model based on the Smith-Watson-Topper (SWT) parameter, with actuation data having a consistently higher work output, leading to an offset between both fatigue curves. Additionally, intrinsic properties of the materials such as ductility, is characterized by looking into the mechanical functional trends such as plastic strain accumulation, providing for preliminary material screening methods before extensive actuation fatigue analyses.
