Analysis of magnetorheological grease normal force characteristics in static and dynamic shear modes

In this paper, the normal stress behavior of magnetorheological grease (MRG) in a quiescent state and its dynamic shear modes were systematically studied. Three types of magnetorheological greases were prepared, MRG-30, MRG-50, and MRG-70 with 30%, 50%, and 70% carbonyl iron(CI) powder mass, respectively. Furthermore, static and dynamic shear tests were carried out using a rheometer, the effects of time, magnetic field strength, shear rate and temperature on normal stress were studied and analyzed under both static and dynamic conditions. In the static mode, the research results show that the normal force is related to the magnetic field strength and the content of magnetic particles in the MR grease, but has nothing to do with historical time. the static normal stress generates the evident fluctuation with changing the temperature when magnetic field strengths of 96 kA m⁻¹ and 194 kA m⁻¹. The fluctuation is very small when the magnetic field strengths are 0kA m⁻¹, 391 kA m⁻¹ and 740kA m⁻¹. Similar to the static normal stress, the dynamic normal stress at the degaussing is greater than the normal stress during magnetic field loading. The dynamic normal stress are almost invariant with prolonging the time below 100 s⁻¹ shear rate under the medium magnetic field strength of 194 kA m⁻¹ whereas the stress is increased at the shear rate of 100 s⁻¹. Besides, the dynamic normal stress is basically independent from the time above the shear rate 0f 1 s-1 at the high magnetic field of 740 kA m⁻¹. Under other conditions, the normal force basically presents a stable value. At the absence of the magnetic field, there are quietly weak effects of temperature on the static and dynamic normal stress. Finally, when a magnetic field strength higher than 96 kA m⁻¹ is applied to the material, the value of dynamic normal stress increases with temperature. (paper)