| Magnetorheological elastomer(MRE)is composed of ferromagnetic particles and the polymer matrix.It is a kind of magneto-sensitive intelligent composite whose properties(such as modulus,damping,deformation,and impedance)can be controlled rapidly,continuously and reversibly by an external magnetic field.Moreover,the performance of the MRE has the advantages of stability and reproducibility since the matrix of the MRE is a solid polymer material.Therefore,MRE has been studied extensively and applied deeply in practical applications,especially in the field of vibration control.At present,the related researches on the mechanical properties and model of MRE mainly focus on the magnetic interaction of internal particles.The viscoelastic characteristics,as one of the most characteristic properties of a polymer matrix,are hardly taken into account in the theoretical modeling,so it is often difficult to explain the performance of MRE with viscoelastic characteristics,such as time-dependent and temperature-dependent properties.This will not only hinder the development of a comprehensive theoretical model but also limit the practical application of MRE as a unique smart polymer composite.In order to further develop the mechanical model and expand the engineering application of MRE,the MRE was regarded as a kind of particle-reinforced polymer composite material in this dissertation for a series of experimental studies.The influences of both the particles' magnetic properties and the matrix's mechanical properties on the magneto-mechanical properties of the MRE were taken into account,and the viscoelastic model of the polymer matrix was introduced into the theoretical analysis of the MRE.The details are as follows.1.The magneto-mechanical properties of the MRE based on hard magnetic particles were studied.To realize the engineering needs of negative stiffness,hard magnetic MRE was prepared based on hard magnetic particles(NdFeB),and the modulus curve of hard magnetic MRE under cyclic magnetic field was studied.The results showed that the modulus of hard magnetic MRE could be increased and decreased by loading the external magnetic field in different directions.In other words,the hard magnetic MRE has both negative stiffness and positive stiffness.It was also found that the change of MRE modulus was closely related to the magnetic properties of the internal hard magnetic particles.The symmetry of the modulus curve was determined by the relation between the test magnetic field and the magnetizing magnetic field.When the magnitude of the test magnetic field was equal to or larger than the magnetizing field,the modulus curve was symmetrical.When the magnitude of the test magnetic field was smaller than the magnetizing field,the modulus curve exhibited different degrees of asymmetry.Finally,according to the classical magnetic dipole hypothesis and the unique magnetic properties of hard magnetic materials,the various mechanical behaviors of hard magnetic MRE under the external magnetic field of different loading modes were explained.The method of applying a magnetic field as the first order reversal curve(FORC)was an effective means to solve the problem of reproducibility in practical applications.2.The time-dependent magneto-mechanical properties of MRE under different step magnetic fields were studied.The anisotropic MRE based on carbonyl iron powder and PDMS was prepared,and the shear storage modulus and loss modulus of MRE were measured under different step magnetic fields.The results showed that the storage modulus and loss modulus of MRE showed a significant"viscoelastic”characteristic over time.This viscoelastic characteristic was attributed to the rearrangement of the magnetic particle structure by the external magnetic field.A magnetic dipole-viscoelastic comprehensive mechanical model of MRE was proposed based on the classical magnetic dipole hypothesis and the analysis of viscoelastic properties of the polymer matrix.The characteristics of the particle motion inside the matrix were derived by the analysis of the interaction between the particles and the matrix.In the loading stage of the magnetic field,the characteristic time of the particle motion decreased with the increasing of the step magnetic field.While in the removal stage of the magnetic field,the characteristic time remained unchanged with the step magnetic field.Finally,the expressions of macroscopic mechanical properties(magnetic-induced storage modulus and loss modulus)related to the particle motion were obtained as a function of time,which were in good agreement with the experimental data.3.The temperature-dependent magneto-mechanical properties of MRE under different magnetic fields were studied.The shear storage modulus of MRE under different temperatures and magnetic fields was tested,and the change of MRE's magneto-modulus at different temperatures was analyzed.It was found that both the initial modulus and the magnetic-induced modulus of the MRE decreased with the increasing of temperature.As the temperature increased,the thermal motion of the molecular chain increased greatly.This resulted in the relative sliding between the molecular chains,and the initial modulus was decreased.The decrease of the magnetic-induced modulus was due to the rotation of the particle chain inside the matrix.With the increase of temperature and magnetic field,the rotation angle of the particle chain would increase,resulting in a decreasing of magnetic-induced modulus.Based on the analysis of the particle chain's rotation and the interaction between the particle chain and the matrix,the expression of the magnetic-induced modulus was obtained as the function of the initial modulus and the magnetic field.In order to demonstrate the existence of the particle chain rotation inside the matrix and the effect of temperature on particle rotation,a two-stage shear stress relaxation experiment was designed.It further proved the existence of the particle chain's rotation under the external magnetic field,and the rotation angle increased with the temperature. |
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