Geoffrey P. McKnight

Dynamic magnetomechanical properties of [112]-oriented Terfenol-D/epoxy 1–3 magnetostrictive particulate composites

The dynamic magnetomechanical properties of a Terfenol-D/epoxy 1–3 magnetostrictive particulate composite fabricated using 49% volume fraction of needle-shaped, [112]-oriented Terfenol-D particles were investigated as a function of magnetic bias field (HBias). A nonoriented composite with 51% volume fraction of irregular-shaped, ball-milled Terfenol-D particles was also prepared and characterized for comparison. It was found that the composites exhibit a similar qualitative trend in dynamic strain coefficient (d33) and dynamic susceptibility (χ33) with the oriented type possessing much higher values than the nonoriented type for all HBias. Data for the ratio d33/χ33 indicated that these increments in d33 and χ33 in the oriented composite are mainly attributed to [112] particulate crystallographic orientation.

Large magnetostriction in Terfenol-D particulate composites with preferred [112] orientation

Terfenol-D particulate composites have been fabricated with and without a preferred crystal orientation of the particles. A 25% volume fraction polymer matrix composite was fabricated in a magnetic field using geometric anisotropy to orient needle shaped particles with long axis [112] orientation along the length of the composite. Results demonstrate that the magnetostriction of a [112] oriented particle composite saturates near 1600 ppm. This is a significant increase when compared to composites without preferential orientation (1200 ppm). The oriented particle composite exhibits the largest reported magnetostriction for a particulate composite material. The magnetization-strain measurements indicate that the strain in the oriented composite is proportional to the (lambda) 112 saturation magnetostriction while the non-oriented composite is proportional to the polycrystalline saturation magnetostriction, (lambda) pc. In addition, the fields necessary for equivalent magnetostriction in the oriented particle composite are reduced when compared to the non-oriented composite, though both require higher fields than commercially available monolithic Terfenol-D.

Recent advances in magnetostrictive particulate composite technology

Recently, there have been significant advances in using magnetostrictive particles in a polymer matrix; finding uses in many applications, both as an active transducer and a passive damper. Termed magnetostrictive particulate composites (MPC), the material provides capabilities identical or superior to the monolithic material. Fortis Technologies has been pursuing improvements in the application and fabrication of this innovative material. The MPC technology provides a passive, broadband, large temperature range, high stiffness, dampling material to be used where current technologies fall short. Damping applications of this technology include sporting goods, power/hand tools, space launch and satellite design, noise abatement and vibration isolation. Energy absorption of the composites has been measured and is approaching that of the monolithic material. The material can also be actively controlled by a magnetic field, producing a transducer that can be used for sonar applications. The advantage of this technology over those currently in use is the large power density at relatively low frequencies and the ease of fabrication, allowing less expensive and more effective conformal arrays. Effective strain output and piezomagnetic coefficients have been measured, as have its dynamic properties. The results show significant improvement of the strain output and piezomagnetic coefficients, approaching the monolithic material.

Thin-layer magnetostrictive composite films for turbomachinery fan blade damping

Recently, there have been significant advances in using magnetostrictive particles in a polymer matrix; finding uses in many applications, both as an active transducer and a passive dumper. Termed magnetostrictive particulate composites (MPC), the material provides capabilities identical or superior to the monolithic material. Fortis Technologies has been pursuing improvements in the applications and fabrication of this innovative material. Specifically, this MPC technology provides a passive, broadband, large temperature range, high stiffness, damping material to be used where current technologies fall short. A novel manufacturing technique based on magnetic fields has been developed to distribute magnetostrictive particulates in a polymer resin and apply it in thin-layer on surfaces for vibration damping in environments typical of turbomachinery fan blades. These magnetostrictive particulates provide damping through domain wall switching, a non-conservative action which provides a high loss factor, and, in turn, significant vibration mitigation. The magnetostrictive damping composites can be easily fabricated into thin films, provide stiffness and strength while also incorporating damping capabilities which exceed in performance and temperature range viscoelastic materials, the current state of the art for applied blade damping. Analytical studies, a finite element analysis and experimental study of the new material in a typical turbomachinery blade loading condition has been conducted and has demonstrated the benefits of this technology.

Dynamic Magnetomechanical Properties of Terfenol-D/Epoxy 1-3 Particulate Composites

This paper describes the effect of particulate crystallographic orientation on the dynamic magnetomechanical properties of Terfenol-D/epoxy 1–3 magnetostrictive particulate composites. Two different types of composites with approximately 50% Terfenol-D volume fraction were fabricated for comparison with [112]-textured monolithic Terfenol-D. In the first type, needle-shaped, [112]-oriented particles cut from the monolithic Terfenol-D were used and in the second type, irregular-shaped, randomly oriented particles ball-milled from the monolithic material were employed. Elastic moduli (E33 H and E33 B ), dynamic strain coefficient (d33 ), and magnetomechanical coupling coefficient (k33 ) were investigated as a function of bias field. Both composites demonstrate similar property trends with the negative-ΔE, d33 , and k33 values maximizing near 30 kA/m. The maximum values achieved in the oriented type are up to 67% larger than the non-oriented type and approaches 65% of the monolithic Terfenol-D. The property improvement in the oriented type is shown to be attributed to [112] preferential particulate orientation.Copyright

Mechanical Deformation of Field-Coupled Materials

This article describes remarkable similarities in the nonlinear mechanical response of different active/smart materials despite fundamental differences in the underlying mechanisms associated with each material. Active/smart materials (i.e., piezoelectric (PZT-5H), magnetostrictive (Terfenol-D), and shape memory alloys (NiTi)) exhibit strong non-linear mechanical behavior produced by changing non-mechanical internal states such as polarization, magnetization, and phase/twin configuration. In active/smart materials the initial deformation proceeds linearly followed by a jump in strain associated with the transformation of an internal non-mechanical state. After the transformation, the mechanical response returns to linear elastic. Upon unloading, a residual strain is observed which can be recovered with the application of a corresponding external field (i.e., electric, magnetic, or thermal). Due to coupling between applied fields and non-mechanical internal states, mechanical deformation is also a function of applied external fields. At a critical applied field, the residual strain is eliminated, providing repeatable cyclic characteristics that can be used in passive damping applications. Even though different intrinsic processes (i.e., polarization, magnetization, and phase/twin variant composition) govern the deformation of each material, their macroscopic behavior is explained using a unified volume fraction concept. That is, the deformation of piezoelectric material is described in terms of the volume fraction of ferroelectric domains with polarization parallel or orthogonal to the applied load; the deformation of magnetostrictive materials is described in terms of the volume fraction of magnetic domains with magnetization parallel or orthogonal to the applied load; and the deformation of shape memory material is described in terms of the volume fraction of twin variants that are oriented favorably to the applied load. Although the qualitative behavior of each material is similar, the average magnitude of stress required to induce non-linearity varies from ~10 MPa for Terfenol-D to ~65 MPa for PZT-5H to ~300 MPa for NiTi shape memory alloy. It is hypothesized that a composite material made of these materials connected in series would exhibit passive damping over a wide range of applied stress.Copyright

Energy absorption in axial and shear loading of particulate magnetostrictive composites

Energy absorption properties of polymer matrix Terfenol-D particulate composites have been experimentally measured. In this work two volume fractions of Terfenol-D were investigated and both exhibited peak energy absorption of up to 25 percent per cycle. The tests include mechanical loading in both axial and shear combined with applied axial magnetic fields. The results show that the energy absorbed in a cycle of loading is a strong function of stress amplitude. The peak energy absorption for the zero magnetic field case in both axial and shear loading occurs near zero amplitude and decreases with increasing stress amplitude. The maximum energy absorption near zero stress amplitude has been observed previously in monolithic Terfenol-D and is a result of the low magnetic anisotorpy of Terfenol-D. Combined magnetic-mechanical loading demonstrated the influence of magnetic field on energy absorption properties. The energy absorption is decreased as the static magnetic field is increased if the cyclic stress amplitude is held constant. If however, we hold a constant magnetic field and vary the cyclic stress amplitude is held constant. If however, we hold a constant magnetic field and vary the cyclic mechanical loading amplitude, it has been observed that the peak energy absorption curve is shifted to higher stress values. This suggests stress tunable dampers are possible.