Giant magnetostrictive composites for smart transducer and actuator applications

Abstract

Magnetostrictive materials are the most widely used magnetomechanically coupled smart materials. Terfenol-D (Tb₀.₃Dy₀.₇Fe₁.₉₂), a rare earths-iron alloy, is the best-known magnetostrictive material. While possessing giant magnetostrictive strain (~ 1000 ppm) and expeditious response (~ 1 μs) at room temperature and low fields (< 200 kA/m), [112]-textured monolithic Terfenol-D suffers from four intrinsic problems. The first is the eddy current-induced heating and bandwidth limitation to a few kilohertz; the second is the brittleness-imposed challenges to machining and shape novelty; the third is the lack of composition and property varieties; and the fourth is the high material cost. These problems have urged the necessity of developing polymer-bonded magnetostrictive composites in the recent decade. Laminated composites fabricated with Terfenol-D thin sheets have improved operating frequencies of about 20 kHz at the expense of high production cost and less shape flexibility. Particulate and particulate-chain composites based on irregularly shaped, randomly oriented Terfenol-D particles (10 300 μm size) are relatively practicable, but their strain values fall short of their monolithic material by at least 30 %. The recently reported short-fiber composites with needle-shaped, [112]-oriented Terfenol-D short fibers (~ 3 mm long and 0.8 mm wide) exhibit the largest strain outputs, reaching about 80 % of the monolithic Terfenol-D. In this study, we aimed to evolve the polymer-bonded magnetostrictive composites into a promising new type of crystallographically oriented continuous- (or long-) fiber composite, showing even larger magnetostrictive strains compared to the monolithic Terfenol-D, besides preserving the desired advantages of the existing magnetostrictive composites. Applications of the continuous-fiber composites in smart transducers and actuators were also realized. [112]-oriented Terfenol-D continuous fibers of 45 mm long and (1 mm × 1 mm) square cross section were prepared with the highly magnetostrictive [112] crystallographic axis of monolithic Terfenol-D oriented along their long axes. Bar-shaped epoxy-bonded Terfenol-D continuous-fiber composites with a preferred [112] crystallographic orientation were fabricated with length 45 mm and square cross section (12 mm × 12 mm) using six Terfenol-D volume fractions ranging from 0.2 to 0.7. Epoxy-bonded Terfenol-D short-fiber and particulate-chain composites of the same dimensions were also produced for comparison. The quasistatic magnetic and magnetostrictive properties of the composites were measured as functions of applied magnetic field, Terfenol-D volume fraction, and post-curing temperature with zero external stress loading and at room temperature. The proposed continuous-fiber composites, after being post-cured at 80 °C, exhibited extremely large saturation strains in excess of 1600 ppm at 400 kA/m. These saturation strains not only were the largest reported values in polymer-bonded Terfenol-D composites, but also exceeded the unloaded monolithic Terfenol-D value (= 1115 ppm at 400 kA/m) by 43 %. A physical model, based on the composite mechanics and filled with the requirements for temperature-dependent stress equilibrium within the composites and stress-dependent saturation strain of Terfenol-D, was presented to obtain an insight into the observed tremendous saturation strains in the continuous-fiber composites. It was found that the higher saturation strain compared to the monolithic Terfenol-D and their short-fiber and particulate-chain composites is mainly due to the residual compressive stresses developed in the continuous fibers during epoxy cure, a higher fiber aspect ratio for greater stress transfer from the fibers to the matrix, and the texturing of the fibers along the highly magnetostrictive [112] crystallographic axis, respectively. The dynamic magnetic and magnetostrictive properties of the continuous-fiber composites were evaluated as functions of frequency, magnetic bias field, and Terfenol-D volume fraction. The observed frequency dependent data indicated an insignificant eddy-current effect in the composites for operating frequencies up to 500 kHz. The bias field dependent data provided an improved understanding of the magnetization and magnetostriction processes in the composites. The volume fraction dependent data suggested an optimal device performance and cost by using composites with Terfenol-D volume fractions not less than 0.5. The overall property improvements in the continuous-fiber composites were expected to broaden the practical use of the magnetostrictive composites. A guide to designing and optimizing the composites for device applications was generated. Two distinct smart devices were developed to demonstrate the application potential of the continuous-fiber composites. These included a tunable vibration absorber for active absorption of vibrations in vibrating structures and a 64 kHz sandwich transducer for sonic and ultrasonic driving. The structure, operational principle, design modeling/simulation, fabrication, and characterization of the smart devices were included. A number of publications (comprising three published journal papers, three submitted journal papers, and two presented conference papers) were produced during the course of this study, elucidating the originality and practical applications of the present work.