Study On Normal Force Of Magnetorheological Fluid And Agnetorheological Damper

As a kind of smart materials, magnetorheological (MR) fluids are composed of magnetizable particles, carrier fluid and additives, which behave like Newtonian fluids in the absence of a magnetic field. After a magnetic field is applied, MR fluids begin to solidify and the particles suspended in the carrier fluids arrange themselves to form chains, columns and even intricate networks. Once magnetic field is removed, MR fluids transform back to Newtonian fluids again. Because of the fast, reversible and repeatable change, MR fluids have attracted many attentions in lots of fields. During the past decades, tremendous researches have been focused on the shear properties in the direction perpendicular to that of magnetic field, such as viscosity, shear yield stress and shear modulus.Under magnetic field, MR fluids generate normal force along the direction of magnetic field, and researches on normal forces have both important academic meaning and engineering value. On the one hand, the normal force changes the flowing gap of MR fluid in the MR device, which will affect the performance of the device, especially the precise device; on the other hand, as the normal stress of MR fluid is much greater than the shear stress, it can lead a smaller MR device when the same output force is needed. The MR device based on normal force of MR fluid can have a more and greater space for application. Besides, normal force of MR fluid is also an important elasticity property for evaluating the MR material, understanding the MR material and the structure deformation of the MR fluid.So far, researches on normal forces of MR fluids is still in the early stages, and understanding of the normal force of MR fluids is still far to be completed. The work in this dissertation mainly focuses on the normal forces of MR fluids and MR dampers in different modes. More details are as follows.1. Normal force of MR fluids in shear modeNormal forces of MR fluids in shear mode (including stationary, steady shear and oscillatory shear) were systematically investigated by using a commercial plate-plate magneto-rheometer, the relation among the normal force and testing time, magnetic field, temperature, shear rate, frequency and strain amplitude was acquired, and the results were verified by a dynamic simulation. It is found that with increasing of magnetic field, normal forces of MR fluids increase and then gradually saturate, which can be fitted according to a magnetization mode. At a constant magnetic field, the normal force at stationary keeps a steady value while the normal force at steady shear oscillates periodically, showing that the rotating plate with a very small tilt angle can lead a very large measurement error which has a very important effect on precise measurement. The normal forces at steady shear are not always greater than the normal forces at stationary, and comparisons between them depend on magnetic field, volume fraction of samples and shear rate, and both of them increase as the temperature increases. With the aid of dynamic signal analyzer, the normal forces at oscillatory shear were firstly tested. The relation between the normal force at oscillatory shear and testing time can be divided into two classes:in the linear elastic region the normal force keeps a steady value and in the nonlinear elastic region it oscillates periodically. Three regions can be obtained from the strain amplitude dependent normal force, but the normal force doesn’t change too much with increasing of the frequency, showing that measuring the normal forces supplies a new method to study the dynamic property of MR fluids. Dynamic simulation was introduced to calculate the normal stress of MR fluids in shear mode and the microstructure revolution of MR fluids, and the simulation results agreed with the test results qualitatively. This research will give us a comprehensive understanding of normal forces of MR fluids and lay the root for the application of MR fluids.2. Normal forces of MR fluids in squeeze modeUnder nonuniform magnetic field, normal forces of MR fluids in constant area squeeze mode were investigated by a commercial plate-plate rheometer, and the relation among normal forces and the external magnetic field, particle concentration, viscosity of carrier fluid, squeeze velocity and initial gap distance has been obtained. It is found that the normal forces of MR fluids increase with decreasing of the compressive gap and two regions (elastic deformation and plastic flow) are found through the normal force-gap distance curves. A power law relation can capture the relation between the normal force and the gap distance in the plastic flow region. Based on the continuum media theory, the theoretical model was developed to calculate the normal force under the nonuniform magnetic field. Besides, a range is obtained for the index of the power law relation between the normal force and the gap distance by considering the sealing effect and squeeze strengthening effect, which agrees well with the experimental results. Furthermore, to weaken the sealing effect, normal forces of MR fluids under constant volume squeeze and uniform magnetic field were tested by a home-made device. Different from the constant area mode, the squeeze velocity has an opposite effect on the normal force in constant volume squeeze, which reveals the microstructure revolution of MR fluids in squeeze mode. This research sets the stage and promotes the development of commercialization for the MR damper in squeeze mode.3. MR dampers in squeeze and valve modeOn the basis of squeeze mode of MR fluids, a MR damper in squeeze mode has been designed and manufactured. Electromagnetic finite element analysis was used to determine the magnetic field strength and the theoretical model was built to calculate the damping force. The dynamic behaviors of MR dampers were tested by using a MTS load frame. The results show that the damping forces of the MR damper are controlled, a small damper can generate large output damping forces and it can be used in vibration isolation for large equipment.In order to apply the MR damper to the high speed railway vehicle, a novel twin-tube and by-pass MR damper in valve mode has been designed. The damper has a long stroke and can guarantee the symmetry of output damping force, whose maximum is about10kN. The unique unidirectional flow path is beneficial to anti-sedimentation of MR damper. The response time of the damper is short and can meet the requirements of real-time control. Moreover, by using a platform of double mass spring system and control system, the damping effect of the MR damper was studied, showing that MR damper was effective for vibration reduction. These researches lay a first stone for applying the MR semi-active control technology to the high speed railway vehicle, and they help the localization development of vibration control system for the high speed railway vehicle.