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INTRODUCTION

EFFICIENT design of shape memory alloy (SMA)based devices requires accurate understanding of the material’s response to conditions that are normally encountered in applications. The signature features of the SMA response—pseudoelasticity, shape memory, and reorientation—have provided the designer with a lot of latitude in designing sensors and actuators. However, in an effort to connect materials characterization to the reality of actual applications, interruption of loading/unloading is a condition that needs to be anticipated and response of the material to such a condition needs to be understood. Recent work in our group and elsewhere has uncovered that an interruption of a pseudoelastic loading with a strain arrest (i.e., holding the strain at a constant value) results in creep-like stress relaxation, whereas interruption of pseudoelastic unloading with a strain arrest results in stress stiffening (this is unlike creep where the stress always relaxes during a strain arrest).

V.R. RUSSALIAN and A. BHATTACHARYYA are with the Smart Materials and MEMS Laboratory, Department of Systems Engineering, University of Arkansas at Little Rock, South University Ave, 2801, Little Rock, AR 72204. Contact e-mail: [email protected] Manuscript submitted November 30, 2015. Article published online August 7, 2017 METALLURGICAL AND MATERIALS TRANSACTIONS A

The creep-like behavior apparently is an outcome of thermomechanical coupling but it is really not so. This is underscored by the fact that such a behavior is present during reorientation loading as well. In that context, the rest of this paragraph is quoted almost verbatim from the first author’s dissertation.[1] The creep-like behavior was initially noticed during neutron diffraction studies on phase transformation-related behavior of single crystal 2 mm diameter CuAlNi wires.[2] This phenomenon was also observed by Bourke and Vaidyanathan[3,4] during their neutron diffraction studies of thick wires of polycrystal NiTi and NiTi TiC compounds having dimensions of 10 mm diameter and 24 mm length when it was required to interrupt the loading for 20 minutes to carry out neutron scattering measurements. Creep was initially observed in the SMA during this interruption. An apparently similar phenomenon was reported by Helm and Haupt,[5] and Pieczyska et al.[6] In the latter case, it was suggested that heating or cooling of the material during loading and unloading is the reason for this phenomenon. Lexcellent and Rejzner[7] tried to model this phenomenon based on the thermal kinetics of the material and thermomechanical coupling. Since the above experiments were conducted for pseudoelastic loading of the SMA materials where thermomechanical coupling is present, pseudocreep was assumed to have been caused by thermomechanical coupling. Studies conducted in our laboratory on NiTi polycrystal SMA have shown that the

VOLUME 48A, OCTOBER 2017—4511

Fig. 1—Pseudoelastic stress–strain response of NiTi wire at strain rate of 8.5E4 pct/s with strain arrests during loading and u