Experimental results for the behavior of MSMAs subjected to loads seen in power harvesting applications and complex loads

Abstract

The microstructure of magnetic shape memory alloys (MSMAs) is comprised of tetragonal martensite variants, each with their preferred internal magnetization orientation. In the presence of an external magnetic field, the martensite variants tend to reorient so that the preferred internal magnetization aligns with the external magnetic field. Thus MSMAs exhibit the shape memory effect when there is a magnetic field in the vicinity of a material point. Furthermore, the tetragonal nature of the martensite variants allows for a compressive stress to cause variant reorientation. This paper focuses on the experimental evaluation of MSMAs in power harvesting as well as on the response of the material under complex loading conditions. The experimental data reported here provides a basis for the evaluation of MSMAs suitability for applications other than the traditional actuation under a constant magnetic field. For power harvesting applications, consider an MSMA element subject to a large enough magnetic field so that all the variants begin in a field preferred state. Keeping the magnetic field constant and adding a variable compressive stress in a direction normal to that of the magnetic field, some or all of the martensitic variants may rotate into a stress preferred state. As the variants reorient, the internal magnetization vectors rotate, and the specimen's magnetization changes. The change in magnetization induces a current in a pick-up coil, resulting in an output voltage at its terminals according to Faraday's law of inductance. For other applications, the loads that an MSMA element is subject to may be different. Investigation into other potential loadings of an MSMA will give a better overall understanding of the magneto-mechanical behavior of MSMAs and perhaps highlight potential applications of these materials. Thus complex loads on MSMAs should be investigated experimentally and eventually modeled mathematically. For example, this work will study variable field and variable stress loading, which might occur if an actuator is being designed to displace a variable load over a controlled distance.