The impact of boundary conditions on the response of NiMnGa samples in actuation and power harvesting applications

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 magnetomechanical response of the material makes MSMAs suitable for applications such as actuation, sensing, and power harvesting. While the magnetomechanical response of the material has been extensively investigated to date, there is no report in the literature on the effect of the boundary conditions (BCs) on its response. The response of MSMAs is primarily driven by the reorientation of internal martensite variants, in conjunction with rotation of magnetization vectors, and domain wall motion. During the reorientation process a change in material's magnetization occurs. For sensing and power harvesting applications, a pick-up coil may be used to convert this change in magnetization into an electric potential/voltage. To date, it has been confirmed experimentally that, according to Faraday's law of induction, 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. However, to our knowledge, no study has been carried out to investigate the effect of the BCs on the voltage output. This paper examines the effect of the BCs on the material's magnetomechanical response, as well as on the corresponding voltage output. Three BCs are considered in the performed experiments: i) simply supported, ii) clamped, and iii) mixed (i.e. one end clamped and one end guided). The difference observed in the magnetomechanical response of the material, between the tested BCs, is attributed to the local effects caused by the grips (particularly the clamped and mixed conditions) and by the rotation of the specimen within the grips (in the simply supported condition). The latter is facilitated by the difference between the cross section of the specimen and the cross section of the cavity receiving the sample and by the larger effective length of the specimen in this case.