Study On Mechanical Property Of Electrorheological Fluids And Its Microstructure Simulation

As a typical intelligent material,the mechanical properties of the electrorheological(ER)fluids change drastically when an external electric field is applied,and this change is fast and reversible.The particular property makes ER fluids have a great application potential in electro-mechanical coupling.Therefore,the mechanism of the change in the mechanical properties of the ER fluids has always been focus and difficulty of the study of ER fluids.Under the applied electric field,the formation of chains structure between the electrodes makes the shear stress and apparent viscosity increase significantly.The relationship between the microstructure and the mechanical property in the working stat of the ER fluids is the key to explain the mechanism of ER fluids.Due to the limit of experimental conditions,the microstructure of ER particles is difficult to be observed directly,so the simulation becomes an important method to study the mechanism of ER fluid.The correctness of the model can be verified by the experimental phenomena,and the result of the simulation can be used to explain the phenomena in the experiment,so it is a common and effective method to study the mechanism of ER fluids.In this paper,the mechanical properties of ER fluids under compression and shear were studied through experiment and simulation,and the mechanism of different influence factors was explored.The details are as follows:1.The normal stress of ER fluid in compression mode was studied through experiment and simulation.The prepared ER fluid showed high normal stress under compression,and the influence of compressive speed on the normal stress of the ER fluid was found.The smaller the compressive speed is,the higher the normal stress of the ER fluid is.This is caused by the separation of the ER particles from the base liquid under electric field.The slower compressive speed corresponds to that the particles squeezed out will be less and the particle chains structure keeps more intact to support a higher load.Then,based on the dipole model,the compression model of the ER fluids was proposed.The simulation results were compared with the experimental results,and the rationality of the compression model was verified.The influences of the electric field strength,compressive strain and shear rate on the normal stress of ER fluid were studied using this model.When the shear rate is small,the influence of shear action on the normal stress of the ER fluid is quite small.With the increase of the shear rate,the normal stress of the ER fluid decreases gradually.Besides,the normal stress of the ER fluid under shear is oscillating.Through the calculation of the microstructure,it was found that the chains structure of ER particles under the large shear rate experiences destroy and reformation is the reason for the oscillation of normal stress.2.The influence of the dielectric loss on the mechanical properties of ER fluids was studied through experiment and simulation.The dielectric loss property of the modified particles by doping strontium ion was measured.The dielectric loss spectra of the modified particles and the shear rheological curves of the ER fluids were tested.With the increase of the strontium ion doped proportion,the relaxation frequency of the ER particles is reduced,the dielectric loss increases and the ER efficiency of the ER fluids decreases.When the relaxation frequency of the particles is less than 100 Hz,the ER fluids loses ER effect within a certain shear rate range.The influence of relaxation time on the mechanical properties of ER fluids was simulated.When the relaxation time exceeds 0.01 s,the critical shear rate decreases and the effective working range of the ER fluids decreases,which is consistent with the conclusion in the experiment.The mechanism of the influence of the dielectric loss on the ER fluids is proposed.When the relaxation frequency is too small,the direction of the dipole moment and the direction of the particle chains structure are different,so the interaction force along the direction of chains structure between the particles decreases,and even the attraction force may become repulsive force,then the strength of the particle chains structure is weakened.3.The change of the mechanical property of ER fluids under shear was studied through simulation,and the influence of shear rate on the shear stress of the ER fluids was studied together with experiment.The influences of volume fraction,electric field strength and shear rate on the mechanical property were studied using the model based on dipole polarization theory.Then,a two-dimensional simulation was conducted and the microstructure was calculated to analyze the influence of shear rate on shear stress.The influence of shear rate on the shear stress of ER fluid was found to be three different situations through experiment,and the simulation result was verified.The influence of shear rate on the mechanical property under different situations was explained by the simulation results of microstructure evolution.Under the situation of small shear rate,the particle chains structure tilt to the shear direction and the shear stress increases with the increase of shear rate.Under the situation of moderate shear rate,the particle chains structure keeps dynamic equilibrium because of destruction and recombination,and the shear stress oscillates with time.Under the situation of high shear rate,the ER fluid reaches shear-yield state.The chain structure is destroyed completely,so the effect of the electric field on the shear stress is weak.And the shear stress of the ER fluids is determined by the viscous force of the liquid,showing the nature of Bingham fluid.