A thermodynamic driving force approach for analyzing functional degradation of shape memory alloy components
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2018, 2018 Taylor & Francis Group, LLC. With growing use of shape memory alloy (SMA) components in different engineering applications, the issue of material performance over its designed life is of great concern to researchers lately. In order to analyze SMA components performance under coupled thermomechanical effects, theories considering both mechanical and thermal effects in a single framework must be employed rather than modifying classical empirical fatigue theories (like SN or N) developed for capturing pure mechanical loading effects. The core idea is to use driving force versus number of cycles rather than classical SN or N curves to quantify thermomechanically coupled functional degradation and life under cyclic thermomechanical loads. To this end, a two-species Gibbs potential is employed to develop thermodynamic driving force for the phase transformation and extent of phase transformation relationships by separating the thermoelastic and dissipative part of the SMA component responses under tension and torsion loading cases. Such an approach can be used to analyze shakedown effects for cyclic superelastic responses and capture coupled thermomechanical responses and functional degradation for SMA. To demonstrate the application of this approach, experiments on SMA extension springs are performed using a custom designed thermomechanical test rig capable of simulating shape memory effect during thermal cycling of SMA springs held under constant deformation. For every thermomechanical cycle, load and thermocouple (temperature) are continually recorded as a function of time using LabVIEW software. The sensor data over the specimen lifetime is used to construct driving force amplitude v/s no. of cycles variation that can be used as a potential substitute to classical fatigue theories for analyzing functional degradation of SMA components. It is shown that different combinations of mechanical and thermal loads with approximately the same calculated driving force lead to approximately similar lives. In addition, cyclic stressstrain results on SMA wire capturing the superelastic responses are used to analyze shakedown effects in SMA components.
