Characteristics of a Magnetostrictive Composite Stress Sensor

A magnetostrictive composite material (MCM) of a giant magnetostrictive alloy, Terfenol-D (Tb0.3Dy0.7Fe2), and epoxy resin is fabricated to demonstrate an electrically isolated mechanical stress sensing mechanism that is based on the Villari effect (inverse magnetostriction). The Villari effect is the change in the magnetic property of a material when it is placed under an external mechanical stress. To monitor the Villari effect in the MCM, we detect the change in magnetic susceptibility by measuring the change in inductance when the material is under mechanical stress. The magnetic susceptibility changes due to the rotation and stretching of magnetic domains when the MCM is under stress. The magnetic susceptibility measurement setup consists of a tightly wound toroidal coil on a laminated silicon steel core with an air gap and an inductance meter with µH precision. Since the air gap in the silicon steel core is partially filled with the MCM, the percentage change in magnetic susceptibility of the MCM can be estimated with inductance measurements. This sensing technique can be useful for non-destructive evaluation and health monitoring of various structures. / To characterize the MCMs under stress, they are cured on the surfaces of aluminum substrates in the shape of a rectangular prism at various angles with respect to the stress axis. The field of the measurement coil traverses perpendicularly to the long axis of the MCM, in plane with the surface of the aluminum substrate. The MCMs are subjected to mechanical stress in the form of tension and compression ranging from 68 MPa to -68 MPa. It was observed that MCMs molded either parallel or perpendicular to the stress axis showed a nearly linear response to the applied mechanical stress with only a slight nonlinearity near zero stress and varied in the slope of the response. The MCMs molded at 30, 45, and 60 degrees with respect to the stress axis all showed strong nonlinear responses with varying behavior depending on the angle between the stress and magnetic field measurement axes. / In order to verify the experimentally observed behavior, theoretical derivation of magnetic susceptibility using free energy is performed. First, the mechanical energy is minimized with respect to each of the mechanical strains in order to obtain the equilibrium strains. Once equilibrium strains are found, they are substituted into the total energy equation in order to find the direction of magnetization for the estimate of the magnetic susceptibility