Material behaviourLarge-strain behaviour of Magneto-Rheological Elastomers tested under uniaxial compression and tension, and pure shear deformations
Introduction
Magneto-Rheological Elastomers (MREs) are smart materials whose properties can be altered reversibly and almost instantaneously by the application of external magnetic fields. This behaviour is caused by the interaction of micron-sized magnetisable particles dispersed in an elastomeric material. The magneto-rheological effect was first explored by Rabinow [1], working on Magneto-Rheological Fluids (MRFs). In MREs, the magnetic particles are locked in position by the solid rubber matrix. Anisotropic materials can be prepared by exposing the fluid MRE mixture to a magnetic field while curing, this forces the magnetised particles to align in chains, resulting in strong mechanical and magnetic anisotropy [2]. The first preliminary tests on MREs were performed by Rigbi and Jilken [3] and the dynamic small strain behaviour of MREs has since become a well-explored property [4], [5], [6], i.e.], using different types of matrix materials and magnetisable particles. Also, the influence of several factors on the final properties of MREs, such as the strength of the magnetic field used during manufacture of anisotropic MREs, was investigated by Chen et al. [7]. The magnetostriction [8] and the magnetic properties of MREs [9] have also been studied.
In order to develop constitutive models characterising the complex behaviour of MREs, extensive experimental data derived from uniaxial and multi-axial deformation modes on the same type of material are required [10], [11], [12]. The large strain behaviour of MREs has been studied mainly under compression and simple shear [13], [14], [15], [16], [17] while, to the best of the author's knowledge, the behaviour of MREs under pure shear or multi-axial deformations has yet to be investigated. So far, the variety of materials used in previous large-strain experiments makes it difficult to compare results from different investigations.
In this research work, uniaxial compression tests up to 50% strain, uniaxial tension tests up to a maximum of 100% strain and pure shear experiments up to a maximum of 70% strain were conducted to characterise both the mechanical behaviour and the MR effect of the manufactured MREs. Magnetic field strengths up to 450 mT were applied parallel to the loading direction. For anisotropic MREs, the particle alignment direction was oriented both parallel and perpendicular to the loading direction. The MR effect, defined here as the increase in tangent moduli due to the application of a magnetic field, is studied versus large strain. Together with earlier work in which MREs were studied under equi-biaxial tension up to 10% strain [18], the combined experimental data represent a comprehensive dataset essential for the development of accurate constitutive models for MREs.
Section snippets
Materials
Silicone rubber MM 240TV mixed with 30w% silicone oil ACC 34, both purchased from ACC Silicones, were used to create the elastomeric matrix material. Carbonyl iron particles (CIP), provided by BASF, were used as the magnetisable particles. The average particle size ranged from 3.7 to 4.7 μm (CIP type SQ). Samples of neat rubber material together with both isotropic and anisotropic MREs, each with volume particle concentrations of 10%, 20% and 30%, were prepared. All the components were mixed
Test setup and procedure
Large-strain experiments on both isotropic and anisotropic MREs with 0, 10, 20, and 30 vol% iron content have been conducted. The experiments were carried out using a Zwick Z250 uniaxial test machine equipped with a 250 kN load-cell for compression tests, whereas a 1 kN load-cell was used for the other tests. Bespoke test rigs were designed for each of the experiments, enabling the use of strong permanent magnets (Neodymium N52, measuring 50 × 50 × 25 mm), to induce magnetic fields during the
Method of analysis
The test machine recorded four-cycle load-displacement data. The third loading cycle of those data was extracted and shifted to zero displacement. To do so, the remnant deformation was determined manually by examining the change of slope of the load-displacement data. Engineering stresses were calculated with the reference area (original dimensions) determined using three measurements taken on each sample. In the case of compression samples, engineering strain values were calculated using the
DIC measurements
The Digital Image Correlation (DIC) system Limess was used to measure the strains in uniaxial tension and pure shear tests. Tests samples were sprayed with a white random speckle pattern. Fig. 4a shows a pure shear sample prepared for strain measurement using DIC. Grid lines were also drawn on the sample to enable calculation of the strains by measuring pixel positions. A series of images was recorded during the cyclic tests. The DIC software VIC- 3D performed correlation analysis by comparing
Conclusions
The behaviour of silicone-rubber based MREs with carbonyl iron as the magnetic particles were studied under large strain under compression, tension and pure shear deformations. Different iron particle contents, in both isotropic and anisotropic MREs, were tested. Magnetic flux densities applied in the loading direction were created with permanent magnets. For anisotropic MREs, samples were tested both parallel and perpendicular to the particle alignment direction. In tension tests, the samples
Acknowledgement
The project was funded by the Glasgow Research Partnership in Engineering (GRPE). The DIC system was loaned by the Engineering and Physical Sciences Research Council (EPSRC).
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2022, Polymer TestingCitation Excerpt :It has been shown that the stiffness and damping properties of MREs in shear mode increased with increasing frequency and magnetic flux density [6]. The influence of the particle concentration on the MS effect was studied by Schubert et al. [9] concluding that this effect increases with increasing iron particle concentration. As it has been explained, MREs can be divided into isotropic and anisotropic, depending on the particle arrangement.