3 April 2013 Sensing of retained martensite during thermal cycling of shape memory alloy wires via electrical resistance
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Shape memory alloys (SMAs) remain one of the most commercially viable active materials, thanks to a high specific work and the wide availability of high quality material. Still, significant challenges remain in predicting the degradation of SMA actuators during thermal cycling. One challenges in both the motivation and verification of degradation models is the measurement of retained martensite fraction during cycling. Direct measurement via diffraction is difficult to perform in situ, impossible for thin wires, (< 0.5mm) and prohibitively difficult for lengthy studies. As an alternative, the temperature coefficient of electrical resistivity (TCR) is used as an indicator of martensite phase fraction during thermal cycling of SMA wires. We investigate this technique with an example cycling experiment, using the TCR to successfully measure a 20% increase in retained martensite fraction over 80000 thermal cycles. As SMA wire temperature is difficult to measure directly during resistive heating, we also introduce a method to infer temperature to within 5 °C by integrating the lumped heat equation.
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Christopher B. Churchill, "Sensing of retained martensite during thermal cycling of shape memory alloy wires via electrical resistance", Proc. SPIE 8689, Behavior and Mechanics of Multifunctional Materials and Composites 2013, 868912 (3 April 2013); doi: 10.1117/12.2012178; https://doi.org/10.1117/12.2012178


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