Study On Rheological Characteristics Of Lithium-based Magnetorheological Grease And Its Basic Application In Vibration Isolation Mount

Magnetorheological(MR)grease are a class of smart materials whose rheological properties(such as viscosity and storage modulus)can be changed reversibly under the control of an external magnetic field.Owing to the use of viscoelastic grease matrix,MR grease presents a state similar to that of soft matter in the absence of magnetic field,which can effectively overcome the inevitable settlement and leakage problems of traditional MR fluid.When the magnetic field is applied,the ferromagnetic particles inside the MR grease can easily break through the constraints of the grease matrix and form the chain structures along the direction of the magnetic field.Along with the magnetic field changed,the internal microstructure of MR grease will also undergo corresponding evolution,showing different rheological properties in the macroscopic view.Due to excellent settlement stability and controllable performance,MR grease have huge application advantages over traditional MR fluid in vibration control fields such as MR mount,MR damper and MR absorber.At present,the research on MR grease is in its infancy,most of which focus on material preparation and process optimization,and lack of systematic characterization of rheological properties,so it is difficult to provide theoretical basis for the design of devices based on MR grease.For device application,the use of MR grease is almost in a blank state,which is not conducive to its popularization and application.In view of this,this paper conducts the study on the magnetic viscosity characteristics,quasi-static shear characteristics,and nonlinearities of MR grease through model establishment and experimental characterization.On this basis,in view of the performance degradation caused by the sedimentation of the MR fluid in the traditional MR mount and the limitation of the damping force that can be provided,MR grease is proposed to use as the medium material,a new type of compound MR mount is designed based on the squeeze valve mixed mode damper and the rubber main spring in parallel.The basic performance of the new vibration MR mount such as output force and controllability are studied through finite element numerical simulation,experimental testing and parameterized modeling methods.The specific research content and main conclusions are as follows:Firstly,in order to meet the requirements of no settlement and rapid response of MR materials in engineering application,the degree of entanglement of different grease soap fibers and the physical properties of Carbonyl iron powders(CIPs)were compared,lithium-based grease with a consistency grade of 0 and CN grade CIPs are selected as the matrix and ferromagnetic particles respectively,and the general preparation method of lithium based MR grease was proposed.Then,experimental methods such as scanning electron microscope were used to reveal the anti-settling mechanism of the grease matrix soap fiber in the MR grease from a microscopic point of view.Finally,the basic properties of lithium-based MR grease in shear and squeeze modes were studied,and the evolution of the internal CIPs chain and t soap fiber structures in different working modes were investigated.The results show that due to the three-dimensional network structure of soap fibers in the grease matrix,MR grease can maintain good sedimentation stability,and the lithium-based MR grease has a strong shear thinning effect,which helps to reduce the influence of the matrix on the chain formation of carbonyl iron particles during work.Secondly,owing to the current research on the rheological properties of MR greases are mostly concentrated under dynamic shear conditions,while the research under quasi-static conditions is almost blank.The mechanical behavior of MR grease under quasi-static monotonic shear is firstly studied,and the reasons for the macroscopic magneto-mechanical behavior were analyzed from the perspective of the formation and evolution of the CIPs structure.The mechanical approximation model between the magneto-mechanical performance parameters and the working conditions was discussed,and the change rule of the maximum yield stress was discussed.Then,the quasi-static cyclic shear tests under different conditions were carried out,the influence of CIPs contents,static shear rate and strain amplitude on the magnetic damping performance of lithium-based MR grease are discussed in detail.The results show that the macro-mechanical behavior of lithium-based MR grease can be described by a conventional viscoelastic-plastic Bingham fluid model,and the influence of carbonyl iron content on the magnetic damping performance can be further enhanced by the magnetic field,and the shear rate has a great influence on the magnetic damping performance.The influence of CIPs content on the magnetic damping properties can be further enhanced by the magnetic field,while the influence from shear rate will continue to weaken with the increase of the magnetic field.Thirdly,in view of the shortcomings that the widely used storage and loss modulus cannot fully characterize the nonlinear viscoelastic properties of MR grease under large amplitude oscillation shear(LAOS).The rheology analysis methods such as Fourier transform(FT)-rheology,strain amplitude sweep and Lissajous curve are firstly used to conduct the qualitative analysis on the nonlinear behavior of MR grease.On this basis,the FT-Chebyshev analysis is introduced to quantitatively characterize the nonlinear viscoelasticity of MR grease.The nonlinear viscoelasticity is regarded as a combination of elasticity and viscosity,and the influence of the applied magnetic field,strain amplitude,and oscillation frequency on the nonlinear behavior is investigated.The results show that the nonlinear viscoelasticity of MR grease is mainly caused by the shear thickening in the initial stage,which is accompanied by the energy dissipation caused by the yield of the CIPs structure.As the strain amplitude increased,it exhibits a combination of different nonlinear elasticity(strain hardening/strain softening)and nonlinear viscosity(shear thickening/shear thinning)behavior.Fourthly,due to the temperature dependence of the three-dimensional network structure inside the grease matrix,MR grease presents different temperature characteristics from traditional MR fluid.The rheometer was firstly used to analyze the steady-state shear test of lithium-based MR grease under different temperatures,and the influence of the magnetic field on the flow behavior was investigated.Bingham plastic model was used to describe the flow behavior of MR grease,and the yield stress under different temperatures and magnetic fields was obtained.Based on the classic magnetic dipole theory,the magnetic-temperature-yield stress model is established and verified.Then,in order to evaluate the dynamic properties of MR grease at different temperatures,the experimental study of lithium-based magnetorheological grease under oscillatory shear was carried out,and the changes of dynamic parameters with magnetic field,strain amplitude and frequency at different temperatures were analyzed.Based on the Maxwell model and Arrhenius formula,the magnetic-temperaturestorage/loss modulus model of MR grease was constructed and its effectiveness was verified.The results show that the increase in temperature helps to improve MR effect of MR grease,and higher magnetic field conditions can effectively reduce the influence of grease matrix on formation of the CIPs chains.In addition,the proposed magnetic-temperature-yield and magnetic-temperature-storage/loss modulus models can well characterize the temperature dependence of the MR grease under steady and oscillatory shear,respective.Finally,based on the research on the key rheology of MR greases,MR grease is proposed to use as the medium material,a new type of compound MR mount is designed based on the squeeze valve mixed mode damper and the rubber main spring in parallel.The magnetic circuit of the new mount is designed based on the electromagnetic theory and verified by using the finite element numerical simulation method.Then,the dynamic performance of the new mount under different working conditions is studied by using the electro-hydraulic servo universal material testing machine,focusing on the analysis of the controllable damping and stiffness characteristics that have an important impact on the vibration isolation performance.Considering the nonlinear relationship between the restoring force and the displacement amplitude,current and other parameters,the BWBN model is introduced to parameterize the hysteretic characteristic curve of the new mount.The results show that the new mount has excellent and reliable stability after a long time,it can output up to 17.8k N damping force and shows good stiffness and damping controllable characteristics.In addition,the BWBN model can well describe the dynamic nonlinear hysteretic characteristics of the new mount.This part about the design,experimental research and parameterized modeling of the new MR grease mount realizes the engineering application of lithium-based MR grease,which has certain significance for solving the problem of vibration isolation of power plants.