Bending of Iron-Gallium (Galfenol) Alloys for Sensor Applications
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Bending of Iron-Gallium (Galfenol) Alloys for Sensor Applications Patrick R. Downey and Alison B. Flatau Aerospace Engineering, University of Maryland, 3181 Martin Hall, College Park, MD, 20742 ABSTRACT This project investigates the magnetomechanical sensing behavior of iron-gallium alloys in response to applied bending loads to identify the relevant design criteria for novel magnetostrictive sensor applications. A series of experiments are conducted on the magnetic induction response of cantilevered beams to dynamic bending loads. Analytic models of the system are formulated from both the constitutive magnetostriction equations and a free energy derivation. Both the experimental and analytical results show a change of as much as 0.3 T of induction can be measured in the samples in response to relatively small applied forces, with the output magnetic signal appearing at twice the frequency of beam vibration. INTRODUCTION Galfenol alloys (Fe100-x Gax 1≤x≤25 at. %) have been shown to combine significant magnetostriction (~400 ppm) with strong mechanical properties (tensile strengths ~500 MPa), [1-3] making them well suited for use in robust actuators and sensors as an active structural material. Recent studies have focused on dynamic testing [4,5], rolling operations [6], and maximizing tensile performance through stress annealing [7]. Of particular interest is the development of Galfenol nanowires created with electrochemical deposition [8-10], where the inspiration to mimic the hair-like cilia cells critical in biological sensing is aided by Galfenol’s unique combination of significant Villari effect magnetostriction and mechanical robustness. This work details testing of the sensing performance of Galfenol alloys when subjected to transverse bending loads, as this is still a novel operating regime for magnetostrictive materials outside of the conventional axial compression. Research has shown the general feasibility of using this material as a Villari effect sensor [11-13], and this current study advances these concepts with a physical description of the nature of the magnetic response, improved analytic models of the magnetomechanical coupling, and some of the first experimental sensing results from a sample of rolled Galfenol. EXPERIMENTAL DETAILS An investigation of the output magnetic response of Galfenol to applied bending loads was performed initially on cantilevered beams of single crystal Fe84Ga16 (1.58 mm diameter by 32.74 mm long) extracted from [100] ingots grown using the modified Bridgman technique. Mechanical excitation was applied to the tip of the sample manually and via a dynamic shaker, and the corresponding change in magnetic induction was measured with both a pickup coil of AWG34 magnet wire wound directly on the rod and a giant magnetoresistive (GMR) sensor located at the base. Large cylindrical permanent magnets surrounded the Galfenol to provide dc bias fields up to approximately 120 Oe. A series of tests were run to verify the magnetostrictive response of the material and the
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