1. Introduction
Magnetorheological (MR) grease is a new type of magneto-induced smart material composed of soft magnetic particles and grease matrix, which exhibits a transition state between traditional MR fluid (liquid state) [
1,
2,
3] and MR elastomer (solid-state) [
4,
5,
6] in the absence of a magnetic field. Compared with the severe problems of sedimentation and leakage faced by liquid MR fluid [
2,
7,
8], the MR grease has advantages of high stability and low leakage [
9]. Moreover, under the stimulation of external magnetic field, the magnetic particles in MR grease can break through the restraint of the grease matrix and form an ordered chain or cluster structure, which is expected to possess a higher magneto-induced effect than MR elastomer [
9,
10]. In a word, the excellent features of MR grease could upgrade the performances of the MR device, i.e., MR damper [
11,
12], MR clutch [
13], MR isolator, and MR actuator [
14,
15], compared with the use of MR fluid.
For MR grease, most researches focus on the rheological properties, such as viscosity, yield stress, storage modulus, loss modulus, and loss factor under different magnetic field strength and temperature. Rankin et al. investigated the yield stress of MR grease with 10 vol% of CI particle suspensions. They found that the yield stress of MR grease increased sub-quadratically with the external magnetic flux density [
16]. Sahin et al. tested the rheological properties of MR grease under steady shear test. The experimental results showed that the yield stress of MR grease is higher than that of traditional MR fluid, but the off-state viscosity is higher [
17]. Gordaninejad et al. studied the temperature characteristics of MR grease under various magnetic field strengths; the results demonstrated that the temperature dependence of MR grease is more pronounced than MR fluid [
18]. Choi et al. prepared MR grease by dispersing soft magnetic particles in a grease medium and examined its MR characteristics. The results proved that MR grease exhibit a strong solid-like structure with the applied magnetic field [
19]. Mohamad et al. studied the magnetic-induced properties of MR grease with varies weight percentages of the magnetic particles. The results demonstrated that the viscosity and storage modulus rapidly increase with the external magnetic field, and the maximum yield stress and relative MR effect are as high as 52.7 kPa and 952.38% respectively [
20]. Wang et al. tested the field-dependent characteristics of MR grease under different temperatures. They found that the temperature dependence of rheological properties of MR grease decrease with the increase of the external magnetic field [
10]. Most studies on the characteristics of MR grease are limited to dynamic shear test. The rheological properties demonstrated by MR grease under quasi-static shear test have been rarely investigated.
As known, in the practical application of MR devices, the external load applied on MR devices can be divided into three types: static load, quasi-static load, and dynamic load, among which dynamic load is the most common. However, in the case of vibration reduction of bridges and buildings, MR dampers often work under periodic quasi-static loads [
21,
22,
23,
24]. At this time, MR materials inside the damper are subjected to large deformation and quasi-static shearing. The field-induced properties of MR materials under quasi-static shear will determine the performance of MR damper used in bridges and buildings. Therefore, it is especially necessary to carry out investigations on quasi-static rheological properties of MR grease under large deformation to promote engineering application.
In this paper, the rheological properties of MR grease with different weight percentages of carbonyl iron (CI) particles under quasi-static shear mode and large shear deformation were investigated. Firstly, MR grease with 30%, 50%, 70% mass fraction of CI particles was prepared, respectively. The filed-induced stress of MR grease under the quasi-static monotonic shear condition with different shear rate and CI particles content were tested and discussed. Finally, the dependence of the stress–strain curves and damping performance for MR grease on shear rate, magnetic field strength, CI particles content under quasi-static cyclic shear condition were calculated and analyzed.
4. Conclusions
In this paper, MR grease with various weight percentages of CI particles was prepared by employing lithium-based grease as a continuous phase. The dependences of the magneto-induced rheological properties for MR grease on CI particles content, shear rate, and lager deformation under quasi-static monotonic/cyclic shear condition were tested and discussed. In the quasi-static monotonic shear test, it was found that, the maximum yield stress has a significant correlation with CI particles content, i.e., under the condition of shear rate of 0.05/s, the increase rate of the maximum yield stress of MRG-70 was as high as 6367% in the magnetic field strength range from 0 kA/m to 740 kA/m, while the maximum yield stress increase rate of MRG-30 was 1455%. In addition, the shear rate dependence of maximum yield stress was greatly weakened by the magnetic field, and this weakening was further enhanced as the CI particles content of MR grease increased. In the quasi-static cyclic shear test for MR grease with different CI particles content, the experiment showed that, in the absence of magnetic field, the cyclic shear stress–strain curves of three type MR grease all displayed an ellipse-like shape. However, with the increase of magnetic field strength, the shape of cyclic curves of MRG-30 gradually developed into rectangle-like, and MRG-50 and MRG-70 gradually changed to parallelogram-like shape. Moreover, shear stress of MRG-50 and MRG-70 changes sharply during the transition from loading to unloading, but this phenomenon is not evident for MRG-30. In the quasi-static cyclic shear test with different shear rate, the results showed that, at the different external magnetic field strength, the cyclic shear stress–strain curves of MRG-70 under different shear rate showed a different changing regularity at the beginning of the unloading stages, and the rate-dependent damping performance of MRG-70 decrease with the increase of the external magnetic field strength. In the quasi-static cyclic shear test with different shear strain, the results indicated that the shear strain-dependent damping performance of MR grease varied drastically with or without an applied magnetic field, which should be considered for the structural dimension design of MR dampers. It is finally remarked that a model investigation to characterize the properties of MR grease under quasi-static condition will be undertaken as the main work of the next phase. Magnetic field strength, frequency, strain amplitude, and temperature will be considered respectively.