The thermal and electrical properties of shape memory alloys (SMA) are known to be different in their austenitic and martensitic phases. This paper addresses the determination of the phase-dependent heat capacity, thermal conductivity and electrical resistivity of a SMA wire. While the heat capacity measurements are relatively straightforward using a differential scanning calorimeter, the determination of the thermal conductivity and the electrical resistivity are more difficult in view of the possible non-uniformity in the material state of a SMA wire during a phase transformation. Experimental procedures are developed and used to determine these properties in either phase and, along with a previously developed finite-element code accounting for the non-uniform material states, the austenite and martensite properties are determined from the experimental data. For the SMA wire tested, the thermal conductivities of the austenite and martensite phases are determined to be 2.8×10-2 J (mm S K)-1 and 1.4×10-2 J (mm S K)-1 respectively, a difference of 100%. The electrical resistivities of the austenite and martensite phases are determined to be 8.371×10-4 Ω mm and 9.603×10-4 Ω mm respectively, a difference of about 14.7%.

In this paper, we address the issue of modeling the temperature distribution in a shape memory alloy (SMA) wire with variable thermal and electrical properties. This is done in the context of a one-dimensional (1D) boundary value problem where an initially martensitic SMA wire is electrically heated under zero-stress conditions. The model accounts for an evolution in the thermal conductivity, electrical resistivity and heat capacity during the phase transformation. The evolution in the 1D temperature field is found by implementing a Galerkin-based finite-element method. This is used in combination with a recursive iteration scheme to accurately determine the change in the material properties during a time step. The numerical approach is validated by comparing it with a known analytical solution with variable thermal properties. A parametric study on the SMA phase transformation indicates that, based on the considered values for the material properties, the heat capacity is the least important factor that needs to be accounted for, whereas the electrical resistivity is the most important. It is also demonstrated that the time required to complete a martensite to austenite transformation for a SMA wire subjected to an adiabatic boundary condition is lower if the model accounts for property variations. In fact, when the cyclic response of a SMA wire actuator subjected to an adiabatic boundary condition is the issue at hand, a model that does not account for property variations will predict a lower frequency of actuation than a model that does account for the property variations, as dealt with in this paper.

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