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Relaxation processes in magnetorheological fluids (the kinetics of establishment of shear stresses under shear at a constant rate) are studied when a magnetic field is turned on. The compositions of MRF with a dispersed phase were studied: 30 vol. % of carbonyl iron, 40 vol. % of carbonyl iron, as well as 30 vol. % of carbonyl iron + 10 vol. % of ferrimagnetic filler Co0.65Zn0.35Fe2O4. The experiments were performed on a NAAKE RV 12 viscometer with a magnetic field inductor. The range of shear rates in MRF is 0.01–32 s-1, the induction of the applied magnetic field is 62.5–625 mT. It has been established that the time to reach equilibrium flow (stable values of shear stresses) decreases with an increase in the shear rate in the fluid, i.e., an increase in the shear rate leads to an increase in the rate of structural relaxation in the MRF in a magnetic field. For MRF only with carbonyl iron at a shear rate of up to 1 s-1, the relaxation time decreases with a decrease in the magnetic field. At high shear rates, it decreases with an increase in the magnetic field. A decrease in the concentration of carbonyl iron particles leads to an increase in the relaxation time at low shear rates and field values. However, at a field induction of 375 mT and higher and shear rates of 8 s-1 or more, the time to achieve a stable flow at a lower concentration of particles of the dispersed phase becomes shorter than at a higher concentration. MRF containing an oxide filler is characterized by a slower establishment of dynamic equilibrium compared to MRF with only carbonyl iron. This is due to the presence of anisodiametric oxide filler particles, which create additional mechanical interference to structure formation in a magnetic field. The coefficient of thermal conductivity λ of these MRF compositions was determined by the unrestricted planar layer method in the range of magnetic field strengths 0–300 kA/m and temperature range 20 °С – 40 °С. It has been established that λ increases with increasing magnetic field strength according to a dependence close to linear and decreases with increasing temperature. At a magnetic field strength of 300 kA/m, the thermal conductivity coefficient is 1.5–2.2 times higher compared to the value in the absence of a field. As the temperature increases, λ decreases by almost a factor of two in the studied range. Thus, the relaxation time of the MRF when the magnetic field is turned on depends on the composition of the fluid and the relationship between the hydrodynamic forces of the fluid flow and the magnetic forces of interaction between the particles of the dispersed phase. The results obtained will make it possible to take into account the effect of relaxation when creating generalized control algorithms for magnetorheological materials for their use in power engineering, precision instrumentation, transport engineering, aircraft engineering and construction.

magnetorheological fluid;relaxation time;shear stress