Sensor Behavior of Magneto-Rheological Elastomers

Magneto-Rheological Elastomers (MREs) are composite materials of an elastomer matrix with a magnetic, micron-sized, powder filler. These materials have gained notoriety because they change stiffness substantially when exposed to a magnetic field giving them the capability of acting as a variable spring for numerous applications. Magnetic field induced strain has also been measured in these materials making them feasible as future actuator materials. However, the inverse effect involving a mechanically induced change in the magnetic properties of these materials has yet to be studied in great detail. This paper presents the results of an experimental study of this sensor behavior. Sheets of 5 mm thick MRE are synthesized from silicone rubber (RTV6186) and carbonyl iron power with a diameter of 9 micrometers. The application of a magnetic field during the silicone curing process allows for the creation of samples with particles aligned along the length, width, and thickness of the sample as well as unaligned samples. These samples are then strained up to 100% of their test length while exposed to various constant bias fields. The change in the internal magnetic properties of the sample as it is strained induces a voltage in a pickup coil that surrounds the sample. This voltage is found to closely track the applied strain-rate making these materials promising for large strain, non-contact strain-rate sensors. In this paper this effect is described in detail experimentally and a theoretical mechanism is proposed to describe this sensing ability. The experimental results for testing MRE samples of 4 alignments in 3 bias fields and at 3 frequencies are presented for cyclic input and the sensitivity, linearity, and repeatability are discussed. Additionally, the results of tests with random inputs are also shown.