| Magnetorheological（MR）grease is a smart material which consists of magnetic particles suspended in grease medium.Due to its many advantage（e.g.the excellent anti-settlement ability,good sealing and lubrication,long-term storage stability,high magnetic-responsive capacity）,MR grease possesses the extraordinary potential applications in the requirement of shock absorption,energy dissipation,stepless transmission of torque,environment of long-time laying aside,reduction of seal element,lean design of seal structure.Since the MR grease developed relatively late,the magnetorheological mechanism research for the shear rheology and tensile rheology is still in the preliminary stage.The mechanism between coupling of various factors and shear rheological behavior in MR grease have not been studied clearly.Research on the constitutive-relationship characterization of tension rheological mechanics and microstructure-evolution mechanism for MR grease have not yet begun.The lag of studies for shear rheology and tension rheology largely restrict the application and development of MR grease.Based on the above-mentioned insufficient research of shear rheology and tension rheology for MR grease,main contents of the current paper are launched as follows:（1）Study on mechanism between various factors and shear rheology behavior during the shear thinning and shear yield.The L9 orthogonal array program of Taguchi method was applied to this paper.The magnetic-particles fraction,silicone-oil viscosity（SOV）and magnetic-particle size were used as input parameters.The shear-viscosity-attenuation degree and shear yield stress,which were used as the responsive parameters,were applied to quantitively characterize the shear thinning behavior and shear yield behavior,respectively.The analysis of variance（ANOVA）for the input parameters were utilized to conduct the significance assessment,discrepancy-direction analysis,and percent-contribution comparison of magnetic-particles fraction,SOV and magnetic-particle size towards shear thinning behavior and shear yield behavior.According to the significance assessment and discrepancy-direction analysis,it was confirmed that the magnetic field-induced shear thinning and shear yield behavior had positive correlation with the magnetic-particles fraction and SOV.But the magnetic-particle size hardly affected the magnetic responsive shear thinning and shear yield behavior.Based upon the comparison of percent contribution（PC）,it was found that the PC of magnetic-particles fraction and SOV towards shear thinning were delimited as three domains by the critical magnetic fields.The PC of magnetic-particles fraction towards the shear thinning experienced the trends from the rapid drop,slow increase,and rapid increase.However,the PC of SOV towards the shear thinning underwent the trends from the rapid increment,nearly constant,and rapid drop.Unlike the shear thinning,as the magnetic field enhanced,the PC of magnetic-particles fraction towards the shear yield behavior kept growing but the PC of SOV towards the shear yield behavior maintained decrease.These differentiation of shear rheology behavior are closely related to the evolution of magnetic structures and grease structures at the different shearing state.（2）Research on the constitutive relation for tensile mechanical rheology in the MR grease.The current work developed a macroscopic tensile mechanics model by using the combination of control equation between fluids’radial pressure gradient and tangential stress gradient,magnetic field-induced Bi-Viscous model and magnetic field attenuation.The tensile mechanics model not only verified the correlation between tension force and shear yield stress,but also exactly described the complex nonlinear mechanical behavior at the phase of tension failure in MR grease.Besides,the test system for the magnetorheological tension force was built.The tension rheological mechanisms of MR grease at conditions of different contact interface,different initial sample thickness and different tension velocity were analyzed by combining the experimental results of the tension with the theoretical model.It was found that the MR tension yield force under the traction of upper cylinder made by the low-carbon steel was much higher than the MR tension yield force under the traction of upper cylinder made by the aluminum.Furthermore,the larger initial thickness of MR grease would produce the smaller MR tension yield force and the higher tensile velocity induced the MR grease to engender the bigger MR tension yield force.（3）Study about the normal magnetic mechanism in the microcosmic magnetic-structure system of MR grease.The enhanced normal magnetic-force model was constructed through using the magnetic diploe theory that considered the additional magnetic-field effect among magnetic particles.The enhanced normal magnetic mechanics model was not only applied to investigate the coupling mechanism between magnetic-particle normal magnetic force and microstructure parameters including the magnetic-particle size,the ratio of adjacent magnetic-particles centerline spacing and magnetic-particle radius,and magnetic-particles number of the magnetic chain,but also used to comparatively analyze normal magnetic force of each magnetic particle in the chain.Meanwhile,it was affirmed which magnetic particle emerged the weakest normal magnetic force in this magnetic chain.（4）Investigation on the constitutive-relationship exploration between microstructure parameters and macroscopic tension rheological mechanics.In the current work,these factors of the top and bottom contact interfaces,magnetic-chains fracture position,multiple dipole effects,viscous force of grease and magnetization of magnetic structures were introduced into the enhanced-dipole normal magnetic mechanics model to build the micro-macro tension yield mechanical model for MR grease.Afterwards,the micro-macro tension failure mechanical model at the process of nonlinear tension was established via the combinations of the exponential attenuation relation between magnetic field and thickness of MR grease,the microstructures evolution and the micro-macro tension yield mechanical model.Comparing with the test results,it was affirmed that the micro-macro tension failure mechanical model could efficiently depict the nonlinear tension rheological mechanics behavior for MR grease in the tensile failure stage.Shearing-rheology and tensile-rheology mechanism study for MR grease in the current work will provide favorable theoretical support and practical reference value in many aspects such as the regulatory relationship between material parameters of MR grease and its magnetorheological performances,improvement of its rheological properties,promotion of its application field,design and development of MR device. |
