This paper presents dynamic viscoelastic properties of magnetorheological (MR) grease under variation of magnetic fields and magnetic particle fractions. The tests to discern the fielddependent properties are undertaken using both rotational and oscillatory shear rheometers. As a first step, the MR grease is developed by dispersing the carbonyl iron (CI) particles into grease medium with a mechanical stirrer. Experimental data are obtained by changing the magnetic field from 0 to 0.7 T at room temperature of 25 °C. It is found that a strong Payne effect limits the linear viscoelastic region of MR grease at strains above 0.1%. The results exhibit a high dynamic yield stress which is equivalent to Bingham plastic rheological model, and show relatively good MR effect at high shear rate of 2000 s −1 . In addition, high dispersion of the magnetic particles and good thermal properties are proven. The results presented in this work directly indicate that MR grease is a smart material candidate that could be widely applicable to various fields including vibration control.

In this study, a new magnetorheological (MR) grease was made featuring plate-like carbonyl iron (CI) particles, and its magnetic field-dependent rheological properties were experimentally characterized. The plate-like CI particles were prepared through high-energy ball milling of spherical CI particles. Then, three different ratios of the CI particles in the MR grease, varying from 30 to 70 wt% were mixed by dispersing the plate-like CI particles into the grease medium with a mechanical stirrer. The magnetic field-dependent rheological properties of the plate-like CI particle-based MR grease were then investigated using a rheometer by changing the magnetic field intensity from 0 to 0.7 T at room temperature. The measurement was undertaken at two different modes, namely, a continuous shear mode and oscillation mode. It was shown that both the apparent viscosity and storage modulus of the MR grease were heavily dependent on the magnetic field intensity as well as the CI particle fraction. In addition, the differences in the yield stress and the MR effect between the proposed MR grease featuring the plate-like CI particles and the existing MR grease with the spherical CI particles were investigated and discussed in detail.

In this study, three types of available processing aids; namely naphthenic oil (PTO), light mineral oil (MO) and epoxidised palm oil (EPO), were used as a dispersing agent to investigate the effects of physicochemical and viscoelastic properties of magnetorheological elastomers (MREs). EPO was selected as the natural-based processing aids, and its morphological, magnetic properties, mechanical and rheological properties of EPO based MREs was compared with the petroleum-based dispersing aids such as light MO and PTO at a fixed amount of 10 phr. The results indicated that EPO was the best alternative of dispersing agent for the MRE rather than the light MO and PTO. In addition, the morphological, magnetic and mechanical properties of the MRE suggested that the EPO could promote compatibility and provide a better lubricant property between the rubber molecular and magnetic particles. It has been observed that the field-dependent storage modulus and MR effect of MRE samples were improved with the addition of dispersing aids, where the MR effect of MRE with the EPO has showed the highest performance of 12%.

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This paper investigates the field-dependent rheological properties of magnetorheological (MR) fluid used to fill in MR dampers after long-term cyclic operation. For testing purposes, a meandering MR valve was customized to create a double-ended MR damper in which MR fluid flowed inside the valve due to the magnetic flux density. The test was conducted for 170,000 cycles using a fatigue dynamic testing machine which has 20 mm of stroke length and 0.4 Hz of frequency. Firstly, the damping force was investigated as the number of operating cycles increased. Secondly, the change in viscosity of the MR fluid was identified as in-use thickening (IUT). Finally, the morphological observation of MR particles was undertaken before and after the long-term operation. From these tests, it was demonstrated that the damping force increased as the number of operating cycles increases, both when the damper is turn on (on-state) and off (off-state). It is also observed that the particle size and shape changed due to the long operation, showing irregular particles.

During the last two decades, magnetorheological (MR) materials have attracted a significant amount of attention because of their tremendous potential for engineering applications. This review focuses on the role of various additives in enhancing the magnetic field-dependent rheological characteristics of solid and thixotropic matrice-based MR materials (hereafter referred to as MR solids). Typically, MR solids consist of solid or semi-solid matrices filled with magnetizable particles. However, additives need to be used to improve their performance such as the MR effect. This parameter is typically determined by the field-dependent dynamic modulus. Three different groups of additives would be introduced in the review namely plasticizers, carbon-and chromiumbased additives. Compared to particles in the common matrix without a softener, those in matrices with additives which act as matrix softeners will be aligned easier during curing. In fact, the interfaces bonding between matrixparticles would be improved that subsequently enhanced the magnetically induced viscoelastic properties of MRE. In this review, the influences of several additives on the MR effect of various MR solids including MR elastomers, MR greases, and MR gels, which are recognized as efficient smart materials for practical applications in various engineering fields, are surveyed and discussed. Figure 11. a) Storage modulus for different MR plastomers weight fractions and b) storage modulus for different particle sizes of MR plastomers containing embedded graphite. Reproduced with permission. [91]

The field-dependent viscoelastic and transient behaviours of plate-like-carbonyl-iron-particle-based magnetorheological greases are experimentally investigated in this study. The plate-like carbonyl-iron particles are made from spherical carbonyl-iron particles through a milling process using a rotary ball mill. Several samples of magnetorheological greases consisting of different weight percentages of plate-like carbonyl-iron particles are prepared. The surface microstructures and distributions of the plate-like carbonyl-iron particles in the grease are observed using various types of microscopic investigations. Subsequently, the viscoelastic and transient (dynamic) rheological properties of the plate-like carbonyl-iron particles are investigated using a commercial rheometer. The spherical carbonyl-iron particles are transformed into the flattened plate-like carbonyl-iron particles with a larger surface diameter. It is shown that the viscoelastic properties such as storage modulus and response time of the plate-like carbonyl-iron particles significantly depend on the shape and particle weight fraction. The findings reveal that the shape of the carbonyl-iron particles has a significant effect on the field-dependent behaviours. Thus, the results presented in this work provide very useful scientific contributions for devising appropriate applications utilizing the plate-like-carbonyl-iron-particle-based magnetorheological greases.

Engineering rubber composites have been widely used as main components in many fields including vehicle engineering and biomedical applications. However, when a rubber composite surface area is exposed to heat or sunlight and over a long-term accelerated exposure and lifecycle of test, the rubber becomes hard, thus influencing the mechanical and rheological behavior of the materials. Therefore, in this study, the deterioration of rheological characteristics particularly the phase shift angle (δ) of silicone rubber (SR) based magnetorheological elastomer (MRE) is investigated under the effect of thermal aging. SR-MRE with 60 wt% of CIPs is fabricated and subjected to a continuous temperature of 100 °C for 72 h. The characterization of SR-MRE before and after thermal aging related to hardness, micrograph, and rheological properties are characterized using low vacuum scanning electron microscopy (LV-SEM) and a rheometer, respectively. The results demonstrated that the morphological analysis has a rough surface and more voids occurred after the thermal aging. The hardness and the weight of the SR-MRE before and after thermal aging were slightly different. Nonetheless, the thermo-rheological results showed that the stress–strain behavior have changed the phase-shift angle (δ) of SR-MRE particularly at a high strain. Moreover, the complex mechanism of SR-MRE before and after thermal aging can be observed through the changes of the ‘in-rubber structure’ under rheological properties. Finally, the relationship between the phase-shift angle (δ) and the in-rubber structure due to thermal aging are discussed thoroughly which led to a better understanding of the thermo-rheological behavior of SR-MRE.