Neelesh N. Sarawate

Experimental characterization of the sensor effect in ferromagnetic shape memory Ni–Mn–Ga

The characterization of a commercial Ni–Mn–Ga alloy for use as a deformation sensor is addressed. The experimental determination of flux density as a function of strain loading and unloading at various fixed magnetic fields gives the bias field needed for maximum recoverable flux density change. This bias field is shown to mark the transition from irreversible (quasiplastic) to reversible (pseudoelastic) stress-strain behavior. A reversible flux density change of 145mT is observed over a range of 5.8% strain and 4.4MPa stress at a bias field of 368kA∕m. The alloy investigated shows potential as a high-compliance, high-displacement deformation sensor.

A continuum thermodynamics model for the sensing effect in ferromagnetic shape memory Ni–Mn–Ga

A magnetomechanical model based on continuum thermodynamics is presented which describes the sensing effect in single-crystal ferromagnetic shape memory Ni–Mn–Ga. The model quantifies the stress and magnetization dependence on strain at different values of bias fields under isothermal conditions. A magnetic Gibbs energy is considered as the thermodynamic potential with Zeeman, magnetostatic, and anisotropy energy contributions. Constitutive equations for stress and magnetization are obtained in the isothermal case after restricting the process through the Clausius-Duhem inequality for the second law of thermodynamics. Mechanical dissipation and the microstructure of Ni–Mn–Ga are incorporated in the continuum model through the internal state variables volume fraction, domain fraction, and magnetization rotation angle. Closed-form solutions describing the evolution of the internal state variables are developed. The model requires only seven parameters identified from simple experiments: stress-strain curve ...

A dynamic actuation model for magnetostrictive materials

We quantify the dependence of strain on dynamic magnetic field in magnetostrictive transducers. Dynamic eddy current losses are modeled as a one-dimensional (1D) magnetic diffusion problem in cylindrical coordinates. The constitutive magnetostrictive response to an average diffused magnetic field is quantified with the Jiles–Atherton model. The transducer is represented as a lumped-parameter, single-degree-of-freedom resonator with force input dictated by the magnetostriction. This equivalent force is expressed as a summation of Fourier series terms. The total dynamic strain output is obtained by superposition of strain solutions due to each harmonic of the force input.

Frequency Dependent Strain-Field Hysteresis Model for Ferromagnetic Shape Memory Ni–Mn–Ga

We quantify the relationship between magnetic fields and strains in dynamic Ni-Mn-Ga actuators. As a result of magnetic field diffusion and structural actuator dynamics, the strain-field relationship changes significantly relative to the quasistatic response as the magnetic field frequency increases. We model the magnitude and phase of the magnetic field inside a Ni-Mn-Ga sample as a 1-D magnetic diffusion problem with applied dynamic fields known on the surface of the sample, from which we calculate an averaged or effective field. We use a continuum thermodynamics constitutive model to quantify the hysteretic response of the martensite volume fraction due to this effective magnetic field. We postulate that the evolution of volume fractions with effective field exhibits a zero-order response. To quantify the dynamic strain output, we represent the actuator as a lumped-parameter, single-degree-of-freedom resonator with force input dictated by the twin-variant volume fraction. This results in a second-order, linear ordinary differential equation whose periodic force input is expressed as a summation of Fourier series terms. The total dynamic strain output is obtained by superposition of strain solutions due to each harmonic force input. The model accurately describes experimental measurements at frequencies up to 250 Hz.

Stiffness Tuning with Bias Magnetic Fields in Ferromagnetic Shape Memory Ni-Mn-Ga:

This article presents the dynamic characterization of mechanical stiffness changes under varied bias magnetic fields in single-crystal ferromagnetic shape memory Ni-Mn-Ga. The material is first converted to a single variant through the application and subsequent removal of a bias magnetic field. Mechanical base excitation is then used to measure the acceleration transmissibility across the sample, from where the resonance frequency is directly identified. The tests are repeated for various longitudinal and transverse bias magnetic fields ranging from 0 to 575 kA/m. A SDOF model for the Ni-Mn-Ga sample is used to calculate the mechanical stiffness and damping from the transmissibility measurements. An abrupt resonance frequency increase of 21% and a stiffness increase of 51% are obtained with increasing longitudinal fields. A gradual resonance frequency change of -35% and a stiffness change of -61% are obtained with increasing transverse fields. A constitutive model is implemented which describes the dependence of material stiffness on transverse bias magnetic fields. The damping exhibited by the system is low in all cases (~0.03). The measured dynamic behaviors make Ni-Mn-Ga well suited for vibration absorbers with electrically tunable stiffness.

Magnetization dependence on dynamic strain in ferromagnetic shape memory Ni-Mn-Ga

The characterization of commercial Ni–Mn–Ga for use as a dynamic deformation sensor is addressed. The flux density is experimentally determined as a function of cyclic strain loading at frequencies from 0.2to160Hz. With increasing frequency, the stress versus strain response remains almost unchanged whereas the flux density versus strain response shows increasing hysteresis. This behavior indicates that twin-variant reorientation occurs in concert with the mechanical loading, whereas the rotation of magnetization vectors occurs with a delay as the loading frequency increases. The increasing magnetization hysteresis must be considered when utilizing the material in dynamic sensing applications.

Magnetic-field-induced stress and magnetization in mechanically blocked Ni-Mn-Ga

A single-crystal Ni–Mn–Ga sample (AdaptaMat, Ltd.) is first compressed from its longest shape to a given bias strain and subsequently subjected to a slowly alternating magnetic field while being prevented from deforming. The tests are repeated for several bias strains. The available blocking stress, or maximum field-induced stress relative to the bias stress, is critical for quantifying the work capacity of a material. The largest available blocking stress for this material is 1.47 MPa at a bias strain of 3% and field amplitude of 640u2002kA/m. The work capacity calculated as the area under the available blocking stress versus bias strain curve is 72.4u2002kJ/m3. An existing continuum thermodynamics model for Ni–Mn–Ga sensors is augmented by incorporating the magnetoelastic energy as a source of stress generation when the material is mechanically blocked. The strain and magnetization are described by fixing the variant volume fraction.

Magnetomechanical characterization and unified actuator/sensor modeling of ferromagnetic shape memory alloy Ni-Mn-Ga

A unified thermodynamic model is presented which describes the bulk magnetomechanical behavior of singlecrystal ferromagnetic shape memory Ni-Mn-Ga. The model is based on the continuum thermodynamics approach, where the constitutive equations are obtained by restricting the thermodynamic process through the Clausius-Duhem inequality. The total thermodynamic potential consists of magnetic and mechanical energy contributions. The magnetic energy consists of Zeeman, magnetostatic, and anisotropy energy contributions. The microstructure of Ni-Mn-Ga is included in the continuum thermodynamic framework through the internal state variables domain fraction, magnetization rotation angle, and variant volume fraction. The model quantifies the following behaviors: (i) stress and magnetization dependence on strain (sensing effect), and (ii) strain and magnetization dependence on field (actuation effect).

Electrical stiffness tuning in ferromagnetic shape memory Ni-Mn-Ga

This paper is focused on the dynamic characterization of field-induced mechanical stiffness changes under varied bias magnetic fields in commercial-quality, single-crystal ferromagnetic shape memory Ni-Mn-Ga. Prior to the dynamic measurements, a specified variant configuration is created in a prismatic Ni-Mn-Ga sample through the application and subsequent removal of collinear or transverse bias magnetic fields. Base excitation is used to measure the acceleration transmissibility across the sample, from where the resonance frequency is directly identified. These measurements are repeated for various collinear and transverse bias magnetic fields ranging from 0 to 575 kA/m, which are applied by a solenoid and an electromagnet, respectively. A 1-DOF model for the Ni-Mn-Ga sample is used to calculate the mechanical stiffness from resonance frequency measurements. A resonance frequency increase of 21% and a stiffness increase of 52% are observed in the collinear field tests. In the transverse field tests, a resonance frequency decrease of -36% is observed along with a stiffness decrease of -61%. The damping exhibited by this material is low in all cases (≈ 0.03). The measured dynamic behaviors make Ni-Mn-Ga well suited for vibration absorbers with electrically-tunable stiffness.

Dynamic strain-field hysteresis model for ferromagnetic shape memory Ni-Mn-Ga

Due to magnetic field diffusion and structural dynamics, the relationship between magnetic field and strain in Ni-Mn-Ga changes significantly as the frequency of applied field is increased. In order to describe this behavior, which is critical for actuator applications, we present a strain model for Ni-Mn-Ga driven with dynamic magnetic fields. The magnitude and phase of the magnetic field inside the sample are modeled as a 1-D magnetic diffusion problem, from where an averaged or effective field is calculated. A continuum thermodynamics constitutive model is used to quantify the hysteretic response of the martensite volume fraction due to this effective magnetic field. The evolution of volume fractions with effective field is proposed to behave as a zero order system. To quantify the dynamic strain output, the actuator is represented as a lumped-parameter 1-DOF resonator with force input dictated by the twin-variant volume fraction. This results in a second order, linear ODE whose periodic force input is expressed as a summation of Fourier series terms. The total dynamic strain output is obtained by superposition of strain solutions due to each harmonic force input. The model accurately describes experimental measurements at frequencies of up to 250 Hz.