This study will present a proof-of-concept non-contact strain sensor, utilizing a prototype magnetostrictive (Fe-Ga alloy, Galfenol) strip on a steel plate; coupled mechanical-magnetostrictive equations will be used to evaluate sensor performance prediction. In contrast with typical piezo-ceramic materials, Galfenol is ductile and has an excellent ability to withstand mechanical shock. Galfenol also changes its permeability in response to applied stress. This stress-based permeability change is not time dependent and can measure static loads. The variation of magnetic induction with stress depends strongly on bias magnetic field. Typically, the performance of Galfenol is measured in a compressional load region because it has higher response there. However, in this study, we are aiming to develop a sensor for tensile stress measurement. To achieve a compression load in the sensing element, a Galfenol strip is aligned perpendicular to a tension bar, so that tension in the bar creates compression in the strip, via the Poisson effect.
The experimental setup in this study consists of a polycrystalline Galfenol strip bonded in the horizontal direction of a steel dog-bone shaped tension specimen. Two permanent magnets are attached at both ends of the Galfenol strip to provide a magnetic bias field through the strip. The magnetic flux through the Galfenol strip is measured with a non-contact Hall sensor during the tensile load test. The design reported here aims at low frequency applications, such as static and dynamic tension monitoring.