| A series of new magnetorheological materials have been derived from the research on the settlement problem of magnetorheological fluid(MRFs).Among them,the magnetorheological grease(MRGs)material based on grease not only exhibits an excellent effect on settlement stability,but also inherits the excellent fluidity of MRFs,which provides a simple and reliable solution for optimizing the performance of devices such as vibration reduction,sealing,polishing,etc.However,the experimental research of MRGs in shear and squeeze working mode is still insufficient,and the theoretical model describing its shear yield stress needs to be further improved.In view of the development status of MRGs,the main contribution of this dissertation is listed as follows:1.On the basis of magnetic dipole theory,the simplified treatment that the distance from the center of the particle to any point around it is far greater than its radius was not adopted,which not only improves the calculation accuracy of magnetic field distribution around the magnetized particle,and strengthens the force between particles,but also reveals the mechanism of the evolution of particles to particle chains.This will explain the experimental phenomenon of MRGs from a micro perspective,and lay a theoretical foundation for the establishment of the shear model.2.MRGs samples were prepared for experimental testing.MCR-51 rheometer was used to carry out steady-state shear test,and the influence of external magnetic flux density and shear strain on shear stress and viscosity of MRGs was analyzed.The viscoelastic properties of MRGs were studied in the oscillation shear mode of MCR-51 rheometer,and the relationship between storage modulus and damping factor with external magnetic flux density and oscillation frequency was given.The squeeze test was carried out with the help of MCR-302 rheometer,and the changes of squeeze strain,storage modulus and loss modulus of MRGs under the action of external magnetic flux density and normal force were obtained.3.With the help of Ansoft Maxwell electromagnetic analysis software,combined with the distribution characteristics of the magnetic particles in MRGs under the action of external magnetic field,the magnetization of a single particle was first simulated to verify the magnetic dipole theory.Secondly,the influence of the arrangement of double particles on inter-particle force was discussed,and the optimal arrangement was obtained.Next,the relationship between the magnetic shear resistance of the single chain and its inclination angle was studied,and the action range of magnetic shear resistance was obtained.Then,the influence of particle embedding on the stability of a single chain was analyzed.Finally,it is concluded that the double chains arranged tightly along the direction of magnetic field do not affect each other.4.Establish the single chain shear model,and verify its reliability with the help of simulation results.In view of the concentration of inclination angle distribution of the single chain and the life characteristics of the chain structure,it is assumed that the inclination angle of the single chain conforms to normal distribution and Weibull distribution.Two types of micro-macro crossscale theoretical models were developed respectively,comparing the response characteristics of the two to the particle magnetic susceptibility,it is found that the latter has more advantages.The applicability of normal distribution and Weibull distribution was discussed on the basis of experiments,and it is found that the theoretical model based on Weibull distribution is more consistent and more suitable for describing the constitutive relationship of MRGs. |
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