TIPS AND TRICKS FOR CHARACTERIZING SHAPE MEMORY ALLOY WIRE: PART 3-LOCALIZATION AND PROPAGATION PHENOMENA

Abstract

This is the third paper in our series to identify unusual phenomena and to provide recommendations for the thermo-mechanical characterization of shape memory alloy (SMA) wire. Part 1 provided some basic background of the martensitic transformations responsible for the shape memory effect and superelasticity 1 . The characterization of two typical NiTi SMA alloys began with differential scanning calorimetry (DSC) thermograms to measure their respective transformation temperatures, specific heats, and latent heats of transformation. It included an experiment for each alloy showing both shape memory and superelasticity, but in different temperature regimes. Part 2 reviewed the various methods for obtaining a fundamental set of isothermal mechanical responses and provided data on the same two SMA wire alloys over their relevant temperature windows 2 . In the process, it showed stress-induced transformations, which lead to an introduction of strain localization and propagation of phase transformation fronts. In this paper(Part 3), we narrow ourfocus on certain unusual phenomena occurring during superelastic tension responses, namely localization of strain and temperature fields. These are often overlooked by the novice, yet they play an important role in the kinetics of stress-induced phase transformation, and in turn, exacerbate the material response’s sensitivities to loading rate and ambient media (which will be the subject of the next paper in this series). Here, we introduce special experimental techniques for (otherwise) difficult to measure features in the underlying material response. We focus on the superelastic response of one of the two NiTi alloys that was used in our previous articles, that is, the alloy with an Austenite finish temperature (Af) below room temperature (designated as superelastic wire). Figure 1 shows four isothermal superelastic experiments, magnified from the set of fundamental mechanical responses of Fig. 8 in 2 . Each was done on a virgin specimen under elongation control at a slow rate, ˙L =± 1 × 10 −4 s −1 , in a temperature-controlled air chamber at different Editor’s Note: This ET feature series is intended as an introduction to this exciting area of experimental mechanics. It aims to increase awareness of active materials and to promote their consistent characterization by disseminating best practices from leading researchers in the field. Each article in the series will address the characterization of one commercially significant active material. Series editors: Nilesh D. Mankame and Paul W. Alexander.