This is the fourth paper in our series, identifying unusual phenomena and providing recommendations for the thermo- mechanical characterization of shape memory alloy (SMA) wire. This paper (Part 4) investigates thermo-mechanical coupling effects focusing on the superelastic responses of SMA wire. Experiments on the same two NiTi alloys will show how this coupling, as well as the strain localization phenomenon explored in Part 3, combine to form unusual sensitivities to loading rate, thermal conditions, and specimen geometry. It is well known that the response of SMAs is loading-rate dependent. It is somewhat less appreciated that the response is also dependent on the nature of the ambient medium, even at the same loading rate. While conventional materials exhibit viscoelastic (or viscoplastic) rate dependence in certain loading rate regimes, we will show that SMAs are different. Rate dependence in SMAs should be understood in the broader subject of thermo-mechanical coupling, which originates from latent heats of transformation and the dependence of transformation stress on temperature. Five experiments on SM wire are presented to demonstrate typical ambient media (water versus air) and loading rate sensitivities in the superelastic response. Then, five experiments on SE wire in room temperature air across five decades in loading rate show how latent heat interactions between the SMA material and its surroundings lead to loading-rate effects and complex phase front kinetics. As will be shown, this loading rate sensitivity is exacerbated by the localized nature of transformation and the nonuniform, time-dependent temperature fields, causing hyper-sensitive dependence at loading-rates one might normally consider quasi-static for conventional materials.
Citation: Experimental Techniques
Pub Type: Journals
shape memory alloy, Nitinol wire, introduction, thermo-mechanical behavior