Effect of cyclic stresses and magnetic fields on stress-annealed Galfenol alloys

Abstract

Research has demonstrated that a built-in uniaxial stress can be achieved in Galfenol materials such that with no externally applied compressive stress, the material appears to be under compression of up to 48 MPa. This built-in stress creates the opportunity for Galfenol to be used under both tensile and compressive loads with full magnetostrictive capability. In order for this effect to be useful in real-world applications, limitations of the stress-annealing must be identified. Typical applications of magnetostrictive materials result in cyclic stress loads and cyclic magnetic fields being applied to the material along with other loading conditions such as elevated temperatures and shock loads. This research investigated the effect of cyclic stress loading and cyclic magnetic fields on the behavior of stress-annealed Galfenol 18.4 (Fe_81.6Ga_18.4) polycrystal samples with approximately 40 MPa of induced stress in the samples. Testing included cyclic stresses up to 55 MPa for as much as 106 cycles at low frequencies (<10 Hz) and cyclic magnetic fields of amplitudes from 4 kA/m up to 20 kA/m. Because of sample failure issues in the cyclic stress tests, the full 106 cycles were only applied at loads up to 28 MPa. Results of all testing show little or no change in the stress-annealed state of Galfenol 18.4 (Fe_81.6Ga_18.4) polycrystal samples. Future testing will increase levels of cyclic stress tests and combine stress and magnetic cyclic loads.