The wall-slip effect is observed in areas with magnetorheological fluids (MRFs). A slip layer is formed, which reduces the friction between the solid particles and working surface that causes relative movement of the particles. This leads to errors in the measurement of rheological parameters and an inaccurate braking torque model. Thus, here, a rheometer with a sandpaper on the rotor is used to change the working surface roughness to analyze the wall-slip effect of the MRFs. Based on the experimental results, the influence patterns of wall-slip effect on fluid viscosity and yield stress are obtained. Furthermore, a MRF model is established that considers wall-slip effect, which is different from the conventional models. The model is employed to establish a magnetorheological (MR) braking torque model. To verify the braking torque model, a prototype was manufactured, and its mechanical properties were tested. When compared with a smooth rotor, the braking torque of MR brakes with rectangular grooves is increased. This confirms the existence of the wall-slip effect and shows that the wall-slip effect of MRF can be effectively suppressed by incorporating grooves on the rotor surface.

In order to study the influences of sealing gap on the seal of magnetorheological fluids (MRF), the quasi-static tensile tests along the direction of magnetic field have been carried out by using a commercial plate-plate rheometer. Based on the experimental results, the relationships among the height of sealing gap, magnetic field intensity, particle concentration and the sealing properties of MRF are discussed. The analysis results show that the sealing performances of MRF will deteriorate with the increase of sealing gap, especially when the MRF is exposed to a strong magnetic field. This is probably due to the fact that the material properties of MRF become more similar to the solid with the rise of magnetic field, which makes the sealing layer more brittle and prone to leakage. The tensile process in low magnetic field can be divided into the forming of seal region, stable seal, leak and balanced-pressure four stages, in which the stable seal stage shows that the MRF has certain self-healing ability of seal. Furthermore, both the initial compressive strength and the sealing stability of MRF can be significantly improved by increasing the volume fraction of MRF. Finally, a theoretical model for calculating the burst pressure of MRF seal is proposed according to the theory of magnetic dipole. The description of the relationship between various influencing factors and the sealing characteristics of MRF by this model is consistent with the experimental results, which indicates that the model has favorable instructing value in the practical engineering application.

Magnetorheological (MR) fluid damper is a high quality semi-active vibration damping device based on magnetorheological effect. However, one of the key factors restricting its development and application is the sedimentation of magnetic particles in MR fluids at present, which affects the working stability and service life of the damper, while there is a lack of effective method for monitoring sedimentation state of MR fluid inside the device in real time. A new structure of MR fluid damper for improving and monitoring the sedimentation stability of MR fluid is developed in this study based on the sedimentation principle and work characteristics of MR damper. Firstly, the theoretical analysis of the new MR fluid damper is carried out, then the prototype is designed and processed, the appropriate MR fluid is selected, finally the performance of the prototype is tested. The results of experiment show that the force of the new damper can be adjusted in both directions, and the output force can meet the requirements of working conditions. The resistance of MR fluid decreases and the conductivity of MR fluid increases in the middle of the damping channel, and the partial voltage at both ends of the non-inductive resistance increases gradually, which verifies the feasibility of the monitoring device for sedimentation. The voltage at both ends of the non-inductive resistance decreases slightly after placement for 24 weeks, so it can be judged that the sedimentation of MR fluid is weak in the new MR damper, which verifies the effectiveness of the new structure for improving sedimentation stability.

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