Three dimensional experimental characterization of a NiMnGa alloy

Abstract

Magnetic shape memory alloys (MSMAs) are materials that can display up to 10% recoverable strain in response to the application of a magnetic field or compressive mechanical stress. The recoverable strain depends on the magnitude of the stress and magnetic field that is applied to the material. Due to their large strains as well as fast response, MSMAs are suitable for actuation, power harvesting, and sensing applications. Broadening the range of applications for MSMAs requires an understanding of their magneto-mechanical behavior beyond 2D loading cases that have been studied to date. The response of MSMAs is primarily driven by the reorientation of martensite variants. During the reorientation process a change in material’s magnetization occurs. Using a pick-up coil (placed around, or on the side, of the specimen) one may convert this change in magnetization into an electric potential/voltage, making the material act as a power harvester. The magnitude of the output voltage depends on the number of turns of the pick-up coil, the amplitude of the reorientation strain, the magnitude and direction of the biased magnetic field, and the frequency at which the reorientation occurs. This paper presents experimental results for the material behavior under 3D loading conditions, including power harvesting data generated under such loading conditions. The 3D experimental data includes the material’s response to two compressive mechanical stresses, applied in perpendicular directions, simultaneously with the application of a magnetic field applied in the remaining orthogonal direction. The power harvesting data includes magnetic fields in multiple directions and different orientations of the pick-up coil. Results indicate that the presence of a bias magnetic field along the specimen length (i.e. the direction of application of the compressive stress) in addition to that applied normal to the specimen length, leads to an increase in the electric potential output.