| Magnetorheological plastomer (MRP) is a new kind of soft magneto-sensitive polymeric material, which usually composes of micro-sized magneto-sensitive particles dispersing in magnetically insensitive polymer matrix. MRP presents a soft plasticine-like state, which is an intermediate state between traditional fluid-like magnetorheological fluid (MRF) and conventional solid-like magnetorheological elastomer (MRE). In contrast to the easily sedimentary MRF and the relatively low-magnetorheological-effect MRE, MRP attractively possesses long-time stability and a higher magnetorheological effect. Under usual conditions, the internal magnetizable magneto-sensitive particles in an MRP cannot move by themselves due to the constraint of the matrix. However, when an external magnetic field is applied on the MRP, the internal particles will overcome the constraint of the matrix and can rearrange into some ordered microstructures. With the formation of the microstructures proceeding, the macroscopic physical or mechanical properties of the MRP will vary synchronously, which makes the MRP have much potential in magnetic field-controllable dampers, absorbers, isolators, actuators, sensors, etc.Today, a typical soft polyurethane-based MRP composited with carbonyl iron powder has been reported. The non-linear dynamic magneto-viscoelastic properties, the static time-dependent creep and recovery behaviors, the dynamic tensile and squeeze flowing behaviors, and even the impedance spectroscopy properties of this MRP have been investigated. It is shown that the MRP is a high-performance magnetorheological material. However, it is still necessary to point out that many practical damping or actuating apparatuses work in the case of large deformation and quasi-static shear conditions. For these apparatuses, the magneto-mechanical behaviors of the useable materials have great effect on the performance of the apparatuses, and the materials with larger magneto-deforming capacity and higher magneto-damping performance are highly desirable. Therefore, for the high-performance MRP, it is imperative to study its magneto-deforming effect and magneto-damping performance under cyclically and quasi-statically shearing conditions. More importantly, it is vital to construct the analytical model of the microstructure of MRP and then to study how the particle-aggregated microstructure varies with the changing of the external applied magnetic field. Furthermore, it is necessary and significant to reveal the relationships between the MRP’s macroscopic mechanical properties and the MRP’s microscopic particle-aggregated structures. Given this, this thesis will firstly study the quasi-static magneto-mechanical behaviors of MRP, and then focus on the revealing of the relationship between the macroscopic mechanical properties and the microscopic particle-aggregated structures of MRP.Firstly, we observed the magnetic field-induced deformation of MRP and tested the evolution of magnetic field-induced stress of MRP under static constraint. The magneto-mechanical properties of MRP under cyclically and quasi-statically shearing condition were studied, and the effects of the magneto-sensitive particle content, the shear rate and the shear amplitude were investigated in the absence/presence of external magnetic field. In addition, the magneto-normal stress that rose in the shearing process was specially studied. The above researches show that the MRP possesses a large magneto-deforming effect and a high magneto-damping performance. In the absence of external magnetic field, MRP behaves as a material of low yield strength, high viscosity, high plastic malleability and high rate dependence. The mechanical behaviors of MRP can be analytically described by conventional Bingham-fluid model. In the presence of external magnetic field, it is shown that MRP is highly magneto-sensitive and the mechanical behaviors of MRP can be described by magneto-enhanced Bingham-fluid model. Under static constraint, the magnetic field-induced stress in MRP can be changed in a wide range by adjusting the external magnetic strength. However, it can be found that the magneto-stress will unexpectedly and abruptly experience a slight decrease due to the instability of the particle-aggregated microstructure of MRP. The magnetic strength and shear rate affect a lot on the mechanically shearing performance of MRP, and the damping performance of MRP can be largely enhanced by the external magnetic field. Moreover, the magneto-normal stress will periodically change a lot within the cyclically and linearly shearing process in the presence of external magnetic field.Secondly, we built an analytical model for the particle-aggregated microstructure of MRP, and studied the formation and evolution of the microstructure by using particle-level dynamics simulation when the MRP was subjected into a steady uniform magnetic field or an unsteady magnetic field. In the study, an easy-to-use, modified magnetic dipole model was proposed to calculate the interparticle magnetic force. Furthermore, the relationship between the macroscopic mechanical properties and the microscopic particle-aggregated structures was discussed. It is shown that the particle-aggregated microstructure of MRP will firstly experience a randomly dispersing state, then unsteady short chain-like structured state, followed by long chain-like structured state, and finally steady column-like structured state. Along with the changing of the microstructure, the magnetic potential energy of MRP will sharply decrease and gradually approach to a minimum value, while the magnetic field-induced stress in MRP will sharply increase and get approach to a maximum value. This approaching of magnetic potential energy matches well with the energy loss principle. Under an in-plane rotational magnetic field, the magneto-particles in MRP will aggregate into separate layered microstructures paralleling to the rotating plane of the field. Under a spatially changing magnetic field, it is found that the microstructure of MRP can be controlled by adjusting the field and various particle-aggregated microstructures can be obtained. In the meantime, the magnetic potential energy and magnetic field-induced stress of MRP will vary dramatically, and the larger the variation, the more different the microstructures. In short, the variation of the macroscopic magnetic potential energy and the magnetic field-induced stress of MRP directly depend on the formation and evolution of the microscopic magnetic field-induced particle-aggregated structures in MRP.Thirdly, to expand the discussion on magneto-controllable particle-aggregated microstructures, we built the analytical model of a ferrofluid-based bi-dispersing suspension of para-/dia-/non-magnetic and superparamagnetic particles. Then the formation and evolution of the magnetic field-induced microstructure, as well as the mechanical performance based on the microstructure, of the suspension under a steady, uniform magnetic field was studied. For this kind of suspension, it is shown that the particles in the suspension will aggregate into two-dimensional net-like microstructure or three-dimensional embedded chain-like or column-like microstructure. In the case of the volume fraction of total particles in the suspension being larger than12.5%, the magnetic field-induced axial stress along with the direction of the external magnetic field, or called axial tress, can be enhanced with the ratio of diamagnetic particle getting larger and larger to that of superparamagnetic particle. Besides, the magnetic field-induced axial stress perpendicular to the direction of the field, or called transverse stress, can be also largely enhanced. The enhancement of transverse stress results from the particles’ transverse expansion induced by superparamagnetic particle chains paralleling to diamagnetic particle chains. The enhancement of the axial stress results from the extension of the chains formed by particles with same magnetic property and the attraction between the two kinds of particle chains with diferrent magnetic properties. In addition, the magntic field-induced microstructure of the suspension of paramagnetic and non-magnetic particles was also studied. It can be found that the microstructure of this suspension differs much from that of mono-dispersing suspension and bi-dispersing suspension with diamagnetic and paramagnetic particles. The addition of non-magnetic particle to the hybrid suspension weakens the static magneto-mechanical performance and in the same time affects the dynamics magneto-mechanical performance a lot.In a brief summary, this research built the analytical model of microscopic particle-level structure of MRP and revealed that the microscopic particle-aggregated structure is the basis of the macroscopic magneto-mechanical behavior for MRP. The result of this research can be helpful to further understand the microstructure-based mechanism of the macroscopic physical properties of MRP. |
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