Bending behavior of iron-gallium (Galfenol) alloys for sensor applications

Galfenol alloys (Fe100-x, Ga-x) have been shown to combine significant magnetostriction (similar to 400 ppm) with strong mechanical properties (tensile strengths similar to 500 MPa), making them well suited for use in robust actuators and sensors as an active structural material. This project investigates the magnetomechanical bending behavior of Galfenol to facilitate the design concepts for using Galfenol in a variety of novel sensor applications. To this end, a series of experiments are conducted on the magnetic response of cantilevered beams to dynamic bending loads. The samples studied include polycrystalline Fe81.6Ga18.4 and Fe80.5Ga19.5 (1/8" diameter x 2" long) and single crystal Fe84Ga16 and Fe79Ga21 (1/16" diameter x 1" long). Mechanical excitation was applied to the tip of each rod, with tests performed with sinusoidal and broadband random inputs. Measuring the magnetic response of the samples were a giant magnetoresistive (GMR) sensor located behind the beam and a pickup coil wound directly on each rod. A combination of permanent magnets and solenoid provided dc fields to magnetically bias the samples. Results of initial testing show that simisoidal bending produces measurable output in which the GMR sensor agrees well with the pickup coil, and that the output increases when subjected to increased magnetic bias. Random input tests confirm that the various system resonances can be detected from the frequency spectra. Other results examine the effects of composition, crystal structure, and z-axis position of the GMR sensor. The system is modeled by incorporating classical continuum mechanics, the constitutive magnetostriction equations, and nonlinear magnetization terms, the results of which are compared with the experiments.