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Ferromagnetic smart materials, including rare-earth giant magnetostrictive materials and ferromagnetic shape memory alloys, can exhibit sensation and activation simultaneously. It means it can not only detect the ambient variation but also show the corresponding response. Therefore, it has attracted more and more attentions because of its promising applications in aeronautics and astronautics, MEMS, information storage, medicine and so on. There materials are always subjected to magnetic fields besides stresses. It is particular important to characterize and measure their properties of magnetism and mechanics under magnetomechanical loading. Ferromagnetic smart materials present complicated nonlinear responses under the application of stress, magnetic field and magnetomechanical field. For example, coercive field, saturation magnetization, permeability, and magnetostriction will change obviously under different stresses. Elastic modulus, strain derivative and damping capacity are sharply dependent on magnetic fields. However, until now it is still lack to study on the magnetomechanical coupled behaviors of ferromagnetic smart materials due to its complexity. Another critical reason is that it needs to develop the experimental technique, method and instruments to measure the magnetomechanical properties of ferromagnetic smart materials. It is very important to measure and characterize their physical and mechanical performances, particular the nonlinear, coupled magnetic and mechanical behaviors at strong mechanical and magnetic loading. The research is very useful for the design of smart materials and structure devices. 4 u y @ {
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In this dissertation, the experimental and theoretical studies of the magnetomechanical behavior of ferromagnetic smart materials have been carried out. A multiaxial and multifunctional automatic loading and measuring apparatus for carrying out magnetomechanical experiments has been constructed by ourselves. It can provide strong magnetic field and mechanical loading and allow the automatic measurement and control with high accuracy. Employing this apparatus, the systematical experiments have been carried out to investigate the magnetoelastic behavior of the giant magnetostrictive materials in the multiaxial intense magnetic field-large stress space, such as the magnetostriction, magnetic hysteresis, stress-strain behavior, stress demagnetization effect, damping property and so on. The phenomenon of the giant forced volume magnetostriction, the “drop” of the magnetostriction, the anisotropy of the Young’s modulus, the path-dependent effect, congruency and wiping-out properties has been reported. In theoretical research, a phenomenological anisotropic domain rotation model has been proposed, which can quantitatively predict the magnetostriction and magnetic hysteresis of giant magnetostrictive materials under the application of the magnetomechanical loading. Based on the thermodynamic framework, the Preisach function with magnetic hysteretic characters is introduced to describe the domain switch process under the application of the magnetomechanical loading, which can satisfy the congruency and wiping-out properties at the same time. Then, a domain switch pseudoelasticity phenomenological model has been proposed to predict the domain switch pseudoelasticity. At last, a model on the basis of transformation kinetics is developed, in which the magnetic-field-induced stress is introduced and the equivalence principal is employed for the mechanical and magnetoelastic deformation. It can be used to describe the magnetic field induced strain for the ferromagnetic shape memory alloys. In this dissertation, we have systematically study the nonlinear hysteretic behavior under a multiaxial magnetomechanical space as well as the physical mechanism. In these works, some creative achievements are obtained. /-{O\7-D
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1. A multiaxial and multifunctional automatic loading and measuring apparatus for carrying out magnetomechanical experiments has been constructed by ourselves. It can provide strong magnetic field and large mechanical load, which is very difficult to realize because the limit of space. It combines a hydraulically driven mechanical loading system with a dipole electromagnet, which is capable of producing mechanical forces up to 2000kg and magnetic fields up to 1910kA/m with a pole gap of 100mm. And the mechanical force can be applied in any angle with the magnetic field because the mechanical loading device can be rotated in the pole gap. Thus, the multiaxial stress-magnetic field can be realized. To eliminate the effect of the high magnetic force on the electromagnet gap and avoid the damage of the pole and the sample, a system with a servo-motor driven by the feedback encoding is designed to keep the pole gap constant. To combine the electromagnetic device and the hydraulically driven system perfectly, the insulation and shield technique is adopted. The software developed allows the automatic measurement of magnetomechanical parameters under on-line computer control of both the magnetic field and the mechanical force, so that the magnetostriction, magnetic hysteresis, stress demagnetization effect, domain switch pseudoelasticity and magnetic induced fracture can be carried out under the application of magnetomechnical coupled field. nUud?F^_
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2. The systematical experiments have been carried out to investigate the magnetoelastic behavior of the giant magnetostrictive materials in the multiaxial intense magnetic field-large stress space, such as the magnetostriction, magnetic hysteresis, stress-strain behavior, stress demagnetization effect, damping property and so on. The phenomenon of the giant forced volume magnetostriction, the “drop” of the magnetostriction, the anisotropy of the Young’s modulus, the path-dependent effect, congruency and wiping-out properties has been firstly studied and reported for the [110] oriented polycrystalline Tb0.3Dy0.7Fe1.95 alloys in the multiaxial magnetomechanical space. And a domain switch pseudoelasticity concept has been proposed based on the experimental results. r, .95@
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3. A phenomenological anisotropic domain rotation model has been proposed. On the basis of the experimental results, it is assumed that the energy distribution parameter is a linear function of the pre-stress. Then the anhysteretic behavior of giant magnetostrictive materials under the application of the magnetomechanical loading has been predicted quantitatively. Further more, we assume that the energy distribution parameter is the function of the magnetization state. Thus a dissipation parameter is introduced to describe the magnetic hysteresis and magnetostriction. A good agreement between the model and the experimental results in low and high stress regions is obtained. It has the clear physical meaning and simple express. '5xuT _
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4. On the other hand, based on the thermodynamic framework, a domain switch pseudoelasticity phenomenological model has been proposed. A Gibbs free energy is obtained by the analysis and simplification of the domain switch process. Then, a Preisach function with magnetic hysteretic characters is introduced to describe the domain switch process under the application of the magnetomechanical loading, which can satisfy the congruency and wiping-out properties at the same time. Based on the experimental process, a Gauss distribution is adopted to represent the Preisach function. At last, materials parameters obtained easily under zero magnetic field are used to describe the domain switch pseudoelastic deformation behavior under different magnetic fields. A good agreement between the model and the experimental results is obtained. oa& US_
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5. A model on the basis of transformation kinetics is developed to describe the martensite variants preferred reorientation for the ferromagnetic shape memory alloys. An inverse exponential expression is given to describe the field dependence of magnetization for martensitic variants. The magnetic-field-induced stress is introduced into transformation kinetics and the equivalence principal is employed for the mechanical and magnetoelastic deformation. The experimental results including the magnetization, stress-strain and magnetic-field-induced strain are all predicted well. <S8I"8
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Key words: Giant magnetostrictive material; Ferromagnetic shape memory alloy; Magnetomechanical; Constitutive relation；
