Influence of particle size and filling factor of Galfenol flakes on sensing performance of magnetostrictive composite transducers

Ultrasonic guided wave (GW) techniques have been used widely for structural health monitoring applications in various engineering structures such as rods, tubes, pipes and plates. Magneto-strictive sensors/transducers provide an attractive alternative to piezoelectric transducers for generating and detecting GWs because there are some advantages such as cost-effectiveness, flexibility, durability, no depolarization effect, and non-contact sensing. Results from our previous work validated that a magnetostrictive patch transducer (MPT) composed of a highly textured Fe-Ga alloy (Galfenol) sheet with a <;100> preferred orientation was capable of directional sensing of incoming guided Lamb waves (GLWs) in thin aluminum plates. The sensing feature of the proposed MPT resulted from the unique anisotropic magnetostriction nature of the textured Galfenol patch. The Galfenol patch exhibited high magnetostriction values of ~270 ppm along the <;100> orientation, approaching to around 80% of the single crystal magnetostriction performance at the same composition. The use of the magnetostrictive component in the Galfenol-based MPT was recently extended to a polymer-bonded composite with Galfenol flakes for substituting the bulk Galfenol patch. The metal-polymer composite offers useful band width at high frequencies due to high electrical resistivity and a non-conducting binder. The prior work demonstrated the effect of particle shape on sensing performance and directional sensitivity of three different MPTs composed of spherical nickel, granular and flake-type Galfenol powders, respectively. The use of Galfenol flakes in the composite-based MPT showed the best sensing response among those three kinds of powders. It was because the demagnetizing field caused by shape anisotropy was possibly minimized in the flake-type particles along the in-plane direction. In addition to the small demagnetizing field effect, the packing density of composites can be enhanced if th- particles are well aligned. Thus, in order to make the composite compact, the processing parameters such as particle size and epoxy content should be optimized for maximizing sensing performance.