LeAnn Faidley

A coupled structural-magnetic strain and stress model for magnetostrictive transducers

This paper addresses the modeling of strains and forces generated by magnetostrictive transducers in response to applied magnetic fields. The magnetostrictive effect is modeled by considering both the rotation of magnetic moments in response to the field and the elastic vibrations in the transducer. The former is modeled with the Jiles-Atherton model of ferromagnetic hysteresis in combination with a quartic magnetostriction law. The latter is modeled through force balancing which yields a PDE system with magnetostrictive inputs and boundary conditions given by the specific transducer design. The solution to this system provides both rod displacements and forces. The calculated forces are used to quantify the magnetomechanical effect in the transducer core, i.e., the stress-induced magnetization changes. This is done by considering a “law of approach” to the anhysteretic magnetization. The resulting model provides a representation of the bidirectional coupling between the magnetic and elastic states. It is demonstrated that the model accurately characterizes the magnetic hysteresis in the material, as well as the strains and forces output by the transducer under conditions typical of engineering applications.

Coupled magnetic field and viscoelasticity of ferrogel

Composed of a soft polymer matrix and magnetic filler particles, ferrogel is a smart material responsive to magnetic fields. Due to the viscoelasticity of the matrix, the behaviors of ferrogel are usually rate-dependent. Very few models with coupled magnetic field and viscoelasticity exist in the literature, and even fewer are capable of reliable predictions. Based on the principles of nonequilibrium thermodynamics, a field theory is developed to describe the magneto-viscoelastic property of ferrogel. The theory provides a guideline for experimental characterizations and structural designs of ferrogel-based devices. A specific material model is then selected and the theory is implemented in a finite-element code. As numerical examples, the responses of a ferrogel in uniform and non-uniform magnetic fields are analyzed. The dynamic response of a ferrogel to cyclic magnetic fields is also studied, and the prediction agrees with our experimental results. In the reversible limit, our theory recovers existing models for elastic ferrogel, and is capable of capturing some instability phenomena.

Terfenol-D elastomagnetic properties under varied operating conditions using hysteresis loop analysis

This paper presents an experimental study of the effects of varied magnetic bias, AC magnetic field amplitude and frequency on the characteristics of hysteresis loops produced in a magnetostrictive transducer. The study uses a magnetostrictive transducer designed at Iowa State University that utilizes an 11.5 cm (4.54 in) long by 1.27 cm (0.5 in) diameter cylindrical Terfenol-D rod. This transducer allows controlled variation of the following operating conditions: mechanical prestress, magnitude and frequency of AC magnetic field, and magnetic bias. By performing extensive experimental tests, material property trends can be developed for use in the optimization of transducer design parameters for different applications. For the results presented, the magnetic bias, the AC magnetic field amplitude, and the frequency of excitation were independently varied while temperature, mass load and prestress were kept constant. The minor hysteresis loops of the strain versus applied magnetic field, flux density versus applied magnetic field, and magnetization versus applied magnetic field are presented and compared. Material property trends identified from the minor loops are presented for the axial strain coefficient, permeability, susceptibility, and energy losses.

Axial strain of ferrogels under cyclic magnetic fields

Ferrogels are low stiffness polymer materials with embedded magnetic powder filler, giving them the capability to strain in a magnetic field. The large strains, high energy densities and fast responses that have been reported make these materials attractive for actuator applications; however, a full understanding of the dynamic behavior is lacking. This paper presents an experimental study of the cyclic behavior of these materials under various frequencies in both the upright and inverted configurations. A 1D phenomenological magneto-viscoelastic model is developed and used to capture this behavior. The trends in stiffness, viscosity and density are described and then used to predict the amplitude of the strain at different frequencies.

Modulus Increase with Magnetic Field in Ferromagnetic Shape Memory Ni–Mn–Ga

Ferromagnetic shape memory Ni–Mn–Ga has been shown to exhibit deformations of up to 9.5% when driven with quasistatic fields. This article is focused on the use of Ni–Mn–Ga as the active element in a dynamic transducer consisting of a solenoid and a low-reluctance, closed magnetic path. Despite the absence of a restoring force in this configuration, we have shown in prior studies recoverable compressible strains of 1/4 4100 m, which are attributable to internal bias stresses built in the material during manufacture. In this study, we experimentally establish the presence of a modulus defect in Ni50Mn28.7Ga21.3, whereby the elastic modulus increases as much as 255% upon increasing the applied magnetic field from zero to 380 kA/m DC. Experimental measurements are conducted under both mechanical and magnetic excitation, and analyzed in combination with vibratory models for the system. While in our experiments the attractive magnetic forces between the transducer poles may contribute to the total modulus increase, the presence of a modulus change associated with the Ni–Mn–Ga element is substantiated. Dynamic Ni–Mn–Ga transducers offer an attractive mechanism for electrical tuning of the modulus, with potential application in active vibration absorption problems.

Dynamics Compensation and Rapid Resonance Identification in Ultrasonic-Vibration-Assisted Microforming System Using Magnetostrictive Actuator

In this paper, a mechatronic system is developed to compensate for the hardware dynamics effect, and to achieve rapid resonance identification for an ultrasonic-vibration-assisted microforming system. Microforming has recently attracted great interests due to the need for miniaturized manufacturing systems in emerging applications. It has been demonstrated that significant benefits, such as the reduction of input energy and the prolongation of tool life, can be gained by introducing ultrasonic vibration into the microforming process, particularly when the vibration is maintained at the resonant frequency of the vibrating workpiece. However, the fundamental mechanism of ultrasonic vibration effect on the microforming process has not yet been understood; the electrical actuators currently used to generate the ultrasonic vibration are bulky and not suitable for miniaturization of the microforming system, and control of the ultrasonic vibration is primitive and far from being optimal. To tackle these challenges, a microforming platform based on a magnetostrictive actuator has been developed. The main contributions of this paper are two-fold: first, the use of a novel iterative learning control technique along with a vibration oscillation regulation circuit to compensate for the effect of the magnetostrictive actuator dynamics on the ultrasonic vibration generation, and thereby, maintain the same vibration amplitude across a large excitation frequency range; and secondly, the use of the Fibonacci search algorithm to achieve rapid online identification of the resonant frequency. Experimental results obtained on the developed magnetostrictive-actuator-based microforming system are presented and discussed to demonstrate the efficacy of the proposed approach.

Acoustic Softening and Hardening of Aluminum in High-Frequency Vibration-Assisted Micro/Meso Forming

A hybrid micro/meso forming assisted by high-frequency vibration was experimentally investigated by upsetting aluminum. Experiments with various vibration amplitudes and durations were conducted. The high-frequency vibration resulted in both acoustic softening and hardening behavior. Results showed that the overall forming stress reduced by 30% when a transverse vibration of 9.3 kHz was applied, but the stress recovered once the vibration stopped. On the other hand, a hardening behavior was observed during the vibration and resulted in a permanent hardening of the material even after the vibration had stopped. The effects of acoustic softening and hardening were coupled during the vibration-assisted upsetting. It was found that larger vibration amplitude led to a more significant acoustic softening and hardening. The findings of this study provided a basis to understand the underlying mechanisms of vibration-assisted forming.

Magnetostriction and Field Stiffening of Magneto-Active Elastomers

Filled with certain amount of magnetic particles, an elastomer can be made magneto active for numerous applications. When a magneto-active elastomer (MAE) is subject to a homogeneous magnetic field, both magnetostriction and field-stiffening effect can be observed. Inspired by experimental observations and microstructure simulations in the literature, this paper presents a simplified phenomenological model for MAEs by considering a uniaxial deformation state. The model hypothesizes the field-stiffening effect to be a direct consequence of the inverse magnetostriction, i.e., the strain-dependent magnetization, in the context of finite deformation. By taking the elastic energy to be independent of magnetic field and the magnetization energy to be strain dependent, the model can capture both magnetostriction and field stiffening of an MAE. The functional form of the strain-dependent magnetization energy is determined by the underlying microstructure. MAEs with different microstructures exhibit different magnetostriction and field-stiffening behaviors. To predict the behavior of a specific MAE, one only needs to measure the effective permeability of an MAE as a function of the axial strain. The mathematical simplicity of the model could enable simulation and optimization of MAE-based devices under complex loading conditions.

Microstructure-based modeling of magneto-rheological elastomers

Filled with iron particles, polymers can be made responsive to magnetic fields. Specifically, the elastomers that change stiffness in response to a magnetic field are usually called magneto-rheological elastomers (MREs). Anisotropic MREs, in which the particles are aligned during curing and form chain-like structures, exhibit a more significant magneto-rheological (MR) effect, i.e. the field-induced stiffening. In this paper, we first develop a constitutive model for the nonlinear behavior of deformable solids under magnetic field. Based on the filler-substrate microstructure of MREs, we further implement the theory into a finite element method. The magneto-mechanical response of a representative unit cell of MRE is studied using the finite element method. The MR effect in both the shear modulus and the tensile modulus of an MRE is studied. In addition, we consider the viscoelasticity of the polymer matrix and study its effect on the properties of an MRE. Using the viscoelastic model for MRE, we also investigate the frequency dependence of the MR effect.

Micro Pin Extrusion of Metallic Materials Assisted by Ultrasonic Vibration

Micro extrusion is an economically competitive process to fabricate micro metallic parts. However, fabrication of extremely small geometric features leads to challenges in tool wear due to localized high stress and friction increase at the interface. This study focuses on micro pin extrusion of aluminum with assistance of ultrasonic vibration. Experiments were conducted with and without ultrasound using magnetostrictive actuator. Load-displacement curves from the experiments showed a load reduction when ultrasonic vibration was applied. Experiments of ultrasonic micro pin extrusion with two configurations were performed. The load reduction behaviors at off-resonance and in-resonance conditions were compared. The reduction can be explained by stress superposition of ultrasonic vibration.Copyright