Performance of Isotropic Magnetorheological Rubber Materials

Sammanfattning: Magnetorheological (MR) rubber materials are controllablecomposites that consist of magnetically polarisable particlesin a rubber matrix. They belong to a group of so-called "smartmaterials" that have seen an increased interest recently. Thesematerials have rheological properties that can be changedcontinuously, rapidly and reversibly by an applied magneticfield. With suitable control algorithms and solid stateelectronics, they can respond to changes in their environment.Although few applications of these materials have yet beenreported in the literature, the possibilities for materialswith controllable stiffness are numerous. Examples of potentialapplications are tuned vibration absorbers, andstiffness-tuneable mounts and suspensions.The purpose of this work is to increase our knowledgerelating to magnetorheological materials for dampingapplications, using construction rubber as the matrix. Thematerials should exhibit large responses to an applied magneticfield and have good mechanical properties. In order to simplifytheir manufacture, the use of a magnetic field duringproduction should, if possible, be avoided.MR rubber materials were made from nitrile and naturalrubber, and irregularly shaped iron particles severalmicrometers in size. The particles were not aligned by amagnetic field prior to the vulcanisation; hence, the materialscan be considered to be isotropic. These materials show a largeMR effect, e.g. an increase in the shear modulus when amagnetic field is applied, although the particles are notaligned within the material. This is explained by the lowcritical particle volume concentration (CPVC) of suchparticles. Similar behaviour can be obtained with materialscontaining carbonyl iron, if the particles are aggregated andthereby behave like large irregular particles. The ironparticle concentration has to be very close to the CPVC inorder to obtain a large MR effect.The absolute MR effect in isotropic MR rubber materials withlarge irregular iron particles is independent of the matrixmaterial, and the relative MR effect can thus be increased bythe addition of plasticisers. Other ways of increasing the MReffect are to increase the strength of the magnetic field,although the materials saturate magnetically at high fieldstrengths, or to use small strain amplitudes. The strong strainamplitude dependence of the MR effect suggests that isotropicMR rubber materials are most suitable for low amplitudeapplications, such as sound and vibration insulation.Measurements at frequencies within the audible frequency rangeshow that this is a promising application for isotropic MRrubber materials.