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 (Fe81.6Ga18.4) 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 (Fe81.6Ga18.4) polycrystal samples. Future testing will increase levels of cyclic stress tests and combine stress and magnetic cyclic loads.
This investigation focused on dynamic characterization of a laminated polycrystalline sample of Fe81.6Ga18.4 alloy grown by the FSZM process. Previous studies using static characterization methods have shown unique properties of the alloy for use in transducers and active structures. Static characterization values were verified and material properties were contrasted with the "giant" magnetostrictive material Terfenol-D. Common test methods were used for dynamic characterization to calculate Young's modulus, coupling coefficient, magnetostrictive coefficient, and permeability. In addition, mechanical Q and potential efficiency were calculated. Comparison with static testing of single crystal samples showed that modulus, permeability, and magnetostrictive coefficient were very close to static values, with any differences being attributed to test fixture effects and material differences. Coupling coefficient appeared to be quite low, but no correction was applied for the test fixture losses and magnetic circuit effects. Comparisons with Terfenol-D show that with significantly less magnetic field, iron-gallium alloys can be used in high Q systems to achieve large acceleration and force output. The unique structural and magnetic properties of gallium-iron alloys enable applications that would be difficult or impossible with Terfenol-D.
The Zone Melt Crystal Growth Method (FSZM) has been used to produce polycrystalline Galfenol specimens, Fe81.6Ga18.4, with preferred {100} orientation. This crystal growth technique has advantages over conventional Bridgman methods in that zone rates used were at least an order of magnitude greater; 350 mm/hr versus 2-4 mm/hr. This material had measured magnetostrictions ranging from 168 ppm to 220 ppm compared to 290 ppm for a single crystal with a similar composition. It was discovered that upon machining a large increase in magnetostriction occurred, ~15%. Using Orientation Imaging Microscopy (OIM) techniques it was shown that the magnetostriction increase is due to the removal of off-axis grains located on the circumference of the FSZM samples. The room temperature mechanical properties were measured to be 72.4 GPa-86.3 GPa modulus of elasticity, 348 MPa-370 MPa ultimate strength, and elongation values of 0.81% - 1.2% depending upon zoning conditions.
It is well known that laminating TERFENOL-D drivers for high frequency operation reduces eddy current losses. However, there are questions regarding the effectiveness of the lamination if there is an electrical short between two adjacent laminae. Because of the high electrical resistivity of the epoxy used for lamination, these electrical shorts are most likely caused by metal particles or other impurities penetrating the epoxy layer, referred to here as 'point shorts'. The effects of electrical point shorts between adjacent laminae in TERFENOL-D drivers have been investigated. TERFENOL-D drivers with specific configurations of point shorts were fabricated and tested. The drivers were tested in two simple configurations, the first with its resonant frequency below the eddy current critical frequency for the lamina thickness, 7 kHz, and the second with its resonance above the critical frequency, 19 kHz. A comparison of solid drivers with completely shorted drivers show that the effect of laminating the TERFENOL-D is not nullified by the presence of an electrical short. In fact, comparing a completely shorted driver with a perfectly laminated driver indicates that there is very little, if any, performance degradation due to the presence of electrical shorts.
A large variety of applications exist which take advantage of the high dynamic strain and high energy coupling factor exhibited by 'giant' magnetostrictive materials. One such material is the rare earth-iron compound Terfenol-D (Tb0.3Dy0.7Fe1.95), which is being increasingly used in industrial, biomedical and defense applications. In order to fully realize the performance of this material, it is necessary to accurately model magnetostrictive transducer behavior from DC through the low ultrasonic regimes where the material is used. This paper has been motivated by the need for physically-based performance models of magnetostrictive materials as used in transducers. To this end, a recent magnetoelastic model for magnetostrictive transducers is employed to characterize the magnetization and strain behavior of Terfenol-D at frequencies from 1 Hz to 30 kHz. Model simulations were compared to experimental measurements at various drive levels and frequencies of operation, both under magnetically unbiased and biased conditions. The model provides an accurate characterization of the magnetization and strain for all operating conditions studied. However, further progress in strain simulations may be achieved through improvements in minor loop closure techniques and additional parameter identification.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.