Magnetorheological Elastomers – Material Properties and Actuation Capabilities

Abstract

Magnetorheological (MR) elastomers as composite materials of magnetizable particles in an elastomer matrix are the soft solid relatives of the better known MR fluids [1]. They preferably consist of silicone elastomer filled with dispersed iron particles [2]. MR elastomers exhibit interesting viscoelastic properties, which strongly depend on the elastomer stiffness and the particle concentration and are controllable by a magnetic field. With rising iron particle concentration, the basic stiffness in terms of the elastic storage modulus increases, as expected. In a strong magnetic field, the storage modulus can be enhanced by a factor of about 10 (see Figure 1). A corresponding increase is observed also for the viscous loss modulus (see Figure 1). These effects are particularly pronounced for MR elastomers with soft silicone matrices, which exhibit low molecular crosslinking densities.In common MR elastomers, the iron particles are randomly distributed in the surrounding matrix material due to the mixing of the particles with the liquid silicone precursors in the preparation process. However, it is also possible to prepare MR elastomers with anisotropic particle distribution [3]. In order to achieve this modification, a magnetic field is applied already in the preparation procedure before and during the chemical crosslinking of the silicone elastomer in a thermal process, where the particles are arranged to chains along the magnetic field, in analogy to MR fluids. The magneto-mechanical properties of anisotropic MR are significantly different from those of their isotropic counterparts. A strong increase of the storage modulus of the anisotropic composites is observed already with lower iron particles concentrations.In other investigations, the magnetic properties of MR elastomer cubes with isotropic and anisotropic particle arrangements were studied and compared. Similar to MR fluids, even with high concentrations of iron particles, the magnetic susceptibility of MR elastomers is extremely low in comparison with pure solid iron, which is due to the dispersed distribution of the iron particles in the composite. The measured susceptibility of the anisotropic MR elastomer with a magnetic field perpendicular to the iron particle chains is comparable to the corresponding values of the isotropic samples. However, it could be shown that the susceptibility parallel to the particle chains in the anisotropic MR elastomer is significantly higher due to the particle contacts in this direction.Numerous interesting applications become possible with MR elastomers. Generally known are applications in vibration reduction. Here, the focus is on tunable vibration absorbers due to the dominant elastic behavior of elastomers [4] in contrast to the more viscous characteristics of MR fluids. However, promising exploitation potential of MR elastomers arises also for actuation devices, where an MR elastomer body deforms in a magnetic field. A speciality of these MR elastomer-based actuators is the possibility to realize different types of actuation depending on the orientation of the magnetic field.A basic type of actuation is caused by the linear deformation of an MR elastomer body in a corresponding magnetic field. Due to magnetic attraction forces on the soft-magnetic iron particles by a specially shaped yoke part in the magnetic circuit, the MR elastomer body such as a disk is deformed into a preferred direction. This type of actuator can be used for human-machine interfaces, where haptic feedback is required, when the user touches the interface. Another interesting application of linear MR elastomer actuators are pumping units. In these units, the periodic linear deformation of the actuator sucks the medium to be transported through a non-return valve into a chamber and pushes it out through another non-return valve.Moreover, with MR elastomers also another type of actuators with radial motion may be realized. In a magnetic field with radial orientation between an inner yoke and an annular outer yoke, a ring-shaped body of MR elastomer between the two yokes leaving an annular gap to the inner yoke can expand radially and close the gap (see Figure 2). This unusual kind of radial actuation offers a high potential for various applications. Especially interesting are valves with a ring-shaped opening, where the degree of closing is controlled by the magnetic field strength, i.e. the current in the coil of the electromagnet, and the corresponding radial deformation of the MR elastomer ring. With this mechanism, proportional valves can be realized.Another possible application of radial MR elastomer actuators are locking devices. Here, MR elastomer rings bridge the annular gaps between an inner rotatable shaft and an outer housing, Thereby blocking the rotation, when the magnetic field is activated. It could be demonstrated that locking torques of about 5 Nm can be achieved in a device with two MR elastomer rings. The locking effect is stronger than with an MR fluid in a device with a corresponding geometrical configuration. In this symposium contribution, material properties of MR elastomers and various actuator designs for different applications are discussed. **