This paper investigates the impact of temperature on the rheological properties of magnetorheological (MR) grease containing carbonyl iron suspended in lithium-based grease Lithium-based MR grease with 70% weight fraction of carbonyl iron is firstly prepared by mechanical mixing. The apparent viscosity and shear stress as a function of shear rate under different temperatures and magnetic field strengths are measured and discussed. It is found that the influence of temperature on apparent viscosity reduces with the increase of magnetic field strength. The dynamic properties of MR grease are obtained under oscillatory shear test. The influences of strain amplitude, driving frequency and magnetic field on the dynamic properties of MR grease at different temperature are discussed. The results demonstrate that the enhancement of temperature leads to the increase of storage modulus and the reduction of the loss factor. Microstructural variation of grease matrix at different temperature is proposed as an explanation of the rheological changes of MR grease.

Magnetorheological (MR) grease, which is a type of magneto-sensitive smart material, exhibits complex viscoelastic behavior under different magnetic fields. Storage and loss moduli from oscillatory test are commonly used to characterize such field-dependent viscoelastic behavior. However, they are not able to represent the higher harmonic nonlinearity appeared in oscillatory responses under large amplitude oscillatory (LAOS) shear. In this paper, an investigation is conducted on the characterization of the filed-dependent nonlinearity of MR grease under LAOS by utilizing of Fourier transform (FT)-Chebyshev analysis. To capture the higher harmonic nonlinearity for the modeling and analysis, a comprehensive test program has been conducted. Firstly, MR grease with 70% weight percentage of carbonyl iron particles (MRG-70) is prepared and the stress response of MRG-70 under the oscillatory shear from small to large strain amplitudes with different magnetic field are measured. Then the higher harmonics in stress response signal are used to detect the intercycle nonlinearity of MRG-70 and compare with the traditional analysis method of dynamic modulus. Further, the elastic and viscous measures from FT-Chebyshev analysis are obtained to capture the inter-/intracycle nonlinear behaviors, that is, strain stiffening/softening, shear thickening/thinning, of MRG-70 under LAOS. It is found that, in the presence of magnetic fields, the onset of the intercycle nonlinearities of MRG-70 is originated from shear thickening of MR grease. With the further increase of the shear strain, MRG-70 exhibits the nonlinearities with different combinations of strain stiffening/softening and shear thickening/thinning.

In this paper, a new type of heavy-duty magnetorheological fluid (MRF) mount is proposed and experimentally investigated. The MRF mount with both annular and radial channels are based on two operating modes: flow mode and squeeze mode. The combined paths can lengthen the MRF valve and hence maximize the MR effect in flow mode. A mechanical model is established to institutively predict the maximum damping force that the device can achieve. Magnetic field distribution of the MRF mount is then evaluated based on finite element analysis. Both squeeze and flow regions can obtain magnetic field intensities up to 0.742 T and 0.7 T under 1 A current, respectively. Finally, the quasi-static and dynamic tests of MRF mount under different loading conditions (frequency, amplitude, current) are performed and the results showed that the proposed MRF mount can provide large damping force up to 18.843 kN and wide tunable range (719% increase on damping force at 0.1 Hz with 2 mm amplitude).

Of all the smart materials that can vary with a change in external excitation, magnetorheological gel (MRG) is one of the most pre-eminent composites, having controllable and reversible responses according to the magnitude of the external magnetic field. Temperature has been identified as another important driver that can alter the dynamic properties of a MRG, and so far this has not been studied systematically. The temperature-dependent dynamic properties of a MRG under different magnetic field strengths were investigated by three kinds of experiments—strain amplitude, frequency and magnetic field sweep tests. The experimental results demonstrate that the storage and loss moduli of MRGs display a temperature-induced stiffening effect with a magnetic field but a temperature-induced softening effect in the absence of a magnetic field. The storage modulus improves with magnetic field strength, whereas the loss modulus first shows rapid growth and then a gradual reduction with increasing magnetic field strength. This temperature dependence of dynamic properties is also interpreted through different mechanisms related to the transformation of the MRG microstructure. Furthermore, a modified magnetic dipole model which could predict the relationship between storage modulus and magnetic field strength is combined with the classical Arrhenius equation expressing the effect of temperature on viscosity to describe the temperature dependence of the storage modulus of a MRG under different magnetic field strengths. This paper may provide some useful guidance for designing a magnetorheological device.

This paper investigates the quasi-static rheological properties of lithium-based magnetorheological (MR) grease under large deformation. Three types of lithium-based MR grease comprising different mass ratios of carbonyl iron (CI) particles and lithium-based grease were prepared. The dependence of the magneto-induced stress–strain curves for MR grease on CI particles content, shear rate, and shear deformation under quasi-static monotonic shear conditions were tested and discussed. The results demonstrate that the shear rate dependence of the maximum yield stress is significantly weakened by the magnetic field, and this weakening is further enhanced as the CI particles content of MR grease increases. In addition, to evaluate and characterize the behavior of the cyclic shear–stress curves of MR grease under quasi-static condition, cyclic shear tests under different controlled conditions, i.e., CI particles content, shear rate, shear strain amplitude, and magnetic field strength, were conduct and analyzed. The magneto-induced shear stress of MR grease with higher CI particles content shows a sharp decrease during the transition from loading to unloading. Moreover, the experiment results also show that the damping characteristics of MR grease are highly correlated with CI particles content, shear strain, and magnetic field strength.

Conventional magnetorheological (MR) dampers exhibit the drawbacks of excessively large and non-adjustable peak force/acceleration during the initial impact in the context of high-impact loads, limiting their applicability in the field of impact resistance. This paper introduces an innovative MR damper (MRD), which, coupled with multiple parallel relief orifices, is capable of effectively mitigating the initial impact peak and enhancing the plateau effect during the mid and late stages of impact. Firstly, the configuration and principle of the novel MRD with multiple orifices (MRDWO) are illustrated and its nonlinear model, based on Bingham-Plastic model and accounting for minor loss, is established. Secondly, the structural design and fabrication of MRDWO were conducted. Simultaneously, for comparative purposes, a traditional MRD with an identical structure to MRDWO, except for the absence of relief orifices, was also fabricated. Then, experimental investigations were conducted separately on MRDWO and MRD without relief orifices (MRDWOO) under various impact velocities and control currents to study their damping forces, accelerations, and displacement responses. Finally, the experimental results of MRDWO were utilized to validate the reliability of the aforementioned nonlinear model. Additionally, a comparative analysis was conducted between MRDWO and MRDWOO under various operating conditions in terms of impact forces, accelerations, and displacements. The results indicate that the novel MRDWO significantly reduces the impact force/acceleration peak during the initial impact and enhances the plateau effect of force/acceleration in mid to later stages, thereby reducing the required displacement for absorbing. This is crucial for mitigating the initial impact damage and improving the damping efficiency of the MR damper.