Recent progress in multiferroic antenna shows high promise for communication in rf-denied environment for transmitting signals in lossy electrically conductive media. The antenna utilizes the ultra-low (ULF/VLF) frequency band (1 – 30 kHz) while also maintaining a compact design [1]. In such multiferroic antennas, containing magnetostictive materials that are mechanically actuated, becoming a near-field magnetic field transmitter. This transduction mechanism is based on magnetization changes inside the magnetostrictive material, due to the application of time-variable stresses generated by an electrically-driven piezoelectric material (which is elastically coupled with the magnetostrictor). Magnetoelectric coupling between the piezoelectric driving element and magnetostrictive material allows for modulation of a large magnetic signal form the multiferroic antenna [2]. Of particular importance is the material with a high piezomagnetic coefficient coupled with a large change in the internal flux density. Iron-gallium alloys, Fe1-xGax (Galfenol) with 0.12 &lt; x &lt; 0.30, are promising magnetostrictive, transducer materials that combine highly desirable tensile and compressive mechanical excitation with superior magnetic properties [3]. Doping studies of Galfenol show a maximum in the magnetostrictive response when the Ga composition is x = 0 16 – 0.19; while there is an additional peak at higher gallium compositions, this range maintains a large internal flux response [3,4]. This work investigates the mechanically-driven magnetic response of several Galfenol compositions as a function of driving frequency. A solid and laminated rod of similar size are also compared to determine the effect of eddy current losses on the flux response of the rod. The laminated rod is composed of 12 μm, epoxy bonded Galfenol sheets with x = 0.186. Magnetostrictive transducer elements under test were placed in a nonmagnetic frame with an applied, axial static prestress and permanent magnets placed above the Galfenol rods. When placed in an optimal magnetic bias field and compressively actuated through driving with a Pb[Zr,Ti]O3 (PZT) stack the magnetostrictive Galfenol changes magnetic states depending upon the applied stress. The bias magnetic field was optimized by determining the distance where the permanent magnets would allow for the highest Galfenol flux density response. A fluxgate magnetometer at 1 m and search coil surrounding the Galfenol rods determined the radiated magnetic field and changing internal flux, respectively; both showed similar frequency responses. Figure 1 shows the near-field (1 m) response of 3 samples (2 solid rods and 1 laminated) revealing an improved response at the lower Ga composition. Interestingly, a larger signal was measured from the laminated sample at all frequencies despite being a higher composition (18.4 at. % Ga); it also showed little variation with increasing frequency over the measured range. The laminated sample showed better operational properties compared with the solid samples that decreased with increasing frequency. Mechanically driven laminated Galfenol samples showed an improved dynamic response when compared with solid rods. Additional optimizations to the device configuration are possible as variations in production quality, heat treatments, and microstructural properties such as grain size and domain boundaries can affect the magnetostrictive and magnetic response of the material. Further investigations are needed to verify if any additional improvements are possible through annealing.

**References:**

[1] Joseph D. Schneider, John P. Domann, and Gregory P. Carman, Journal of Applied Physics 126, 224104 (2019) [2] Junran Xu, Chung Ming Leung, and Dwight Viehland, Sensors, 19(4), 853 (2019) [3] G. Petculescu, K. B. Hathaway, and A. E. Clark, Journal of Applied Physics 97, 10M315 (2005) [4] A. Mahadevan, P. G. Evans, and M. J. Dapinoa, Appl. Phys. Lett. 96, 012502 (2010)

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/edccf778d2f230d28c475ab79dc55641.jpg)
Figure 1: Frequency response of the fluxgate magnetometer 1 meter from the actuated Galfenol rods (solid: 17 and 18 at. % Ga; laminated: 18.4 at. % Ga), showing a nearly flat frequency response for the laminated rod from 300Hz to 2kHz. Both solid rods decrease with increasing frequency. The 18% solid rod has been annealed under tension.

 Dynamic response of Fe-Ga for multiferroic magnetic field transmitters

Magnetostrictive materials are distinguished by an inherent coupling between their mechanical and magnetic processes which is manifested as a direct (actuator) and an inverse (sensor) effect. Iron-gallium alloys (Galfenol, Fe100-xGax with 12&lt;x&lt;30) are a particularly promising magnetostrictive, transducer material that combines highly desirable tensile and compressive mechanical attributes with superior magnetic properties. These characteristics make it possible to design robust structural sensors and energy harvesters. However, the behavior of the mechanical and magnetic phenomena is essentially non-linear, making it difficult to develop analytical models to find an optimal operational condition. Because the magnetostriction strongly depends on both the pre-stress and magnetic field, the characterization of the material is usually conducted under controlled conditions for normalizing the parameters with respect to each other. In this study, large-grained, highly textured, polycrystalline Galfenol samples with 15, 17, 18, and 19 at. % Ga were evaluated. As shown in Figure 1(a), standard magnetic hysteresis loops were measured at various applied compressive stresses. Changes in the flux density, B, as a function of applied stress were determined as the intercept values at specific applied magnetic fields, shown by the red dotted line for the 50 Oe bias magnetic field condition. Figure 1(b) shows an example of the flux density as a function of the mechanical pre-stress at constant magnetic fields of 30, 50, 80, 100, 120, and 150 Oe for 18% Galfenol sample (colored dot symbols). To estimate the response of the magnetic flux density as a function of applied stress, a hyperbolic tangent function was used with 4-nonlinear parameters (a, b, c and d) which are functions of magnetic field, as defined below: B(T,H)=a(H)×tanh[b(H)×(T+c(H))]+d(H) (1) The calculated magnetization based on the fitted parameters is plotted in Figure 1 (b) with solid lines, which fits the experimental data well. The continuous magneto-mechanical sensitivity, defined as the slope of the magnetization versus stress data, was calculated from the fitted equation (1) and the magneto-mechanical sensitivity results are depicted in Figure 2 for various bias magnetic fields. The same data process procedure was applied for the experimental characterization data of 15, 17 and 19% Ga specimens and Figure 3 shows the maximum value of the magneto-mechanical sensitivity as a function of applied bias field for the various compositions of Galfenol. As can be seen in Figure 3, 17% Galfenol shows higher magneto-mechanical sensitivity than 18% Galfenol, which usually shows higher magnetostriction values. Based on the results in Figure 3, 17% Galfenol will give better performance for sensor and energy harvester applications as compared to 18% alloys, which are used for actuation.

**References:**



**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/ee2402680b3e12b17a954ad38c86b980.jpg)
Figure 1. (a) Flux-density (B) versus external applied magnetic field (H) at various pre-stress values (T). (b) Magnetic induction (B) versus external compressive stress (T) at various bias fields (H) and curve-fit results based on hyperbolic tangent function.

![](https://assets.underline.io/uploads/markdown_image/1/image/741346be7a511b6fbdbbcf08a654cc25.jpg)
Figure 2. (a) Magneto-mechanical sensitivity at various bias magnetic fields (30, 50, 80, 100, 120 and 150 Oe). (b) Magneto-mechanical sensitivity at various bias magnetic fields for various composition.

 A Calculation of Magneto-Mechanical Sensitivity for Various Compositions of Galfenol

Composite materials, in the form of laminated and polymer matrix composites, are being increasingly used in manned and unmanned vessels and aircraft structures mainly due to their high strength and high stiffness to weight ratios. Composite structures are susceptible to degradation due to prolonged use, exposure to severe service environments, fatigue, sand abrasion as well as operator abuse, and neglect. This degradation increases the risk of operation and the cost of maintenance. Over the last two decades, progress has been made in structural state awareness using various sensing techniques such as acoustic emission, eddy current, and ultrasonic pitch hatch to diagnose damage and predict remaining life of the composite. Despite this progress, reliable detection and quantification of composite damage, particularly in the early stage, has remained a major challenge. Structural damage may be considered as a process that compromises the integrity of a structure. Delamination and cracking, two of the most common safety concerns of airframe structures, propagate rather slowly in the early stage but grow at a much faster rate in the later stage. Since early stage damage is not detectable using current technologies, it may be possible that by the time this damage is detected, the remaining life of a structure or its capacity has been severely degraded. The objective of this study is to show the feasibility of early stage damage detection utilizing embedding magnetic particles (Terfenol-D powder, a magnetostrictive material) into the composite structure, which is affected by the compressive stress surrounding them. Magnetostrictive materials exhibit dimensional changes in response to external magnetic fields. The inverse is also true; magnetization changes due to applied external stresses. When a composite material is cured, the particles are brought into compression due to shrinking of the resin, which provides a localized magnetic response. In this study, Terfenol-D (Tb0.3Dy0.7Fe1.92) was embedded between layers of a composite structure. A laminated panel, 8 inches by 8 inches, was fabricated from thirteen 0.01 inch thick unidirectional plies of a pre-impregnated AS4/3501-6 carbon/epoxy material system. The panel was pressed in an aluminum mold and cured in autoclave at 175 oC under vacuum pressure for about 3.5 hours. The Rectangular panels were cut to a nominal size of 8 inch by 1-inch tensile specimens. A pair of tapered fiber glass tabs was attached at both ends of the rectangular specimens in accordance with the ASTM standard D-3039/D3479. Figure 1(a) shows the test specimen and a disk cut out of the specimen for built-in anisotropy measurement using a VSM (Vibrating Sample Magnetometer). The specimen was then subjected to a quasi-static load up to 70% of the UTS (ultimate tensile strength/ultimate tensile stress to failure). Figure 1(b) shows the magnetization measurement within the VSM system and shows the reference measurement angle for the built-in anisotropy. Figure 2 shows the magnetization results for the different measurement angles, showing the built-in anisotropy before (2a) and after (2b) the load cycle. Following unidirectional loading, the sample clearly shows a change in built-in anisotropy. The slope of the hysteresis loop is now dependent upon angle, demonstrating a possible way to detect early stage damage using magnetostrictive particles embedded in a composite structure. Additionally, it appears to also provide information on the major load direction.

**References:**



**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/ec7579761fac7b3092e06a41e62c5e2b.jpg)
Figure 1. (a) Tensile test specimen and a disk cut for magnetic characterization out of the Terfenol-D powder embedded composite panel. (b) Schematic of the magnetization measurement angle with respect to the tensile specimen loading direction.

![](https://assets.underline.io/uploads/markdown_image/1/image/f66edcf048cee46500e581eb85ca91bf.jpg)
Figure 2. Magnetic hysteresis loop measurement of the Terfenol-D embedded composite panel measured (a) before loading, and (b) after loading to 70% of UTS (Ultimate Tensile Strength).

 Measurement of Built-in Magnetic Anisotropy after Load Cycle for Polymer Composite Embedded Magnetostrictive Particles

Hybrid magneto-elasto-optical systems have the potential to translate surface strain into a color change for structural health monitoring (SHM), mediated by the magnetic coupling between two different functional materials, specifically magnetostrictive and magnetochromatic materials. This hybrid system has the potential to be applied as a paint to large structures, therefore small particle sizes are needed to allow the particles to stay suspended in solution. In this work, polymer-matrix composites with magnetostrictive Fe-Al alloy particles are evaluated to determine the magneto-elastic coupling properties in composite patches attached to a surface prior to understanding the hybrid magneto-elasto-optical interaction. Fe-Ga alloy (Galfenol) powders are usually considered for use in magnetorheological (MR) fluids, MR elastomers for tactile sensors, and SHM systems as composite patches to detect ultrasonic guided waves [1-5]. Figure 1 shows changes of magnetic properties as functions of Alfenol particle fraction and shape, associated with the magneto-elastic coupling effect under compressive stress along the out-of-plane direction. A tension apparatus was also designed to control applied stress precisely to the composite patches attached on dog-bone specimens in a solenoid coil. Flake-shaped particles are more sensitive to increases in applied stress than spherical particles. In addition to the static state, Alfenol MPCs with flake-type powders exhibited better sensing performance than Galfenol ones at frequencies of 120 kHz using ultrasonic guided wave techniques.

![](https://assets.underline.io/uploads/markdown_image/1/image/cecc9dc45f21565956e9a9cefcde1ca7.jpg)

Figure 1. Magnetic properties of composites and particle images; (a)-(b) magnetization and initial permeability as functions of particle shape and weight fraction, where average particle size in diameter was 80 μm and magnetic field was applied out of the plane of the composite patch during curing, (c) permeability changes as a function of compressive stress using a clamping shaft collar to squeeze disc patch on a vertical VSM sample holder, and (d) SEM images of flake and spherical type Alfenol powders with a size of 106-150 μm in diameter, where flake powders were prepared from gas-atomized powders using a wet ball milling process

Static and dynamic magneto-elastic sensing properties of Fe-Al alloy powder-epoxy composite patches

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Presentations

Dynamic response of Fe-Ga for multiferroic magnetic field transmitters

A Calculation of Magneto-Mechanical Sensitivity for Various Compositions of Galfenol

Measurement of Built-in Magnetic Anisotropy after Load Cycle for Polymer Composite Embedded Magnetostrictive Particles

Static and dynamic magneto-elastic sensing properties of Fe-Al alloy powder-epoxy composite patches

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