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Title: Manufacture, characterisation and modelling of magneto-rheological elastomers
Author: Schubert, Gerlind
ISNI:       0000 0004 5360 5943
Awarding Body: University of Glasgow
Current Institution: University of Glasgow
Date of Award: 2014
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This investigation is concerned with the large-strain characterisation of Magneto-Rheological Elastomers (MREs), with the main focus on experimental characterisation. Quasi-static uniaxial compression, uniaxial tension, pure shear and equi-biaxial tension experiments have been performed both in the absence and in the presence of magnetic fields. The experimental data generated during this investigation constitutes an extensive data set characterising MREs under various deformation modes. This is the first time that such a consistent data set has been produced, and such data are essential to develop accurate constitutive models characterising MREs under general deformations. Isotropic and anisotropic MREs composed of silicone rubber and up to 40 vol% carbonyl iron powder were manufactured using a reliable and repeatable process. Specimens of different shapes were made in specially designed moulds and anisotropic samples were produced by placing the moulds inside a strong magnetic field during the curing process. Understanding the magnetic permeability of MREs is a prerequisite for both the development of constitutive models and for quantifying the magnetic flux applied to specimens during testing. Accordingly, the magnetic permeability of MREs has been characterised using a novel, simple and low-cost method. Large-strain experiments were conducted using test rigs specially designed for use in universal test machines, while incorporating permanent magnets. The magnetic flux was applied in the loading direction and samples were aligned with their direction of particle alignment both parallel and perpendicular to the loading direction. Where possible, strains were measured using a digital image correlation system. MREs were found to be very sensitive to the stress-softening Mullins effect as such a novel testing strategy was used: MRE samples were repeatedly used in several cyclic tests in a test series and tests conducted in the absence of magnetic fields were repeated twice in the test series in order to verify the stress-strain results and importantly, to eliminate the influence of the Mullins effect when interpreting the final stress-strain results. Cyclic fatigue tensile tests were conducted to determine stability strain limits of MREs, and specimens were not tested beyond these limits in the experiments. The mechanical response of MREs was found to be strongly nonlinear when tested up to large strains. Anisotropic MREs with particle alignment in the loading direction are the stiffest specimens, followed by anisotropic MREs with their particle alignment perpendicular to the loading direction, while isotropic MREs are the softest type of MRE. Moduli were found to increase with increasing iron content and MREs preconditioned to larger levels of strain were measured to be softer than the same type of MRE tested to lower strain levels. The largest MR effects were found in the small-strain region for all MREs. Anisotropic MREs containing 30% volume iron fraction with particle chains aligned parallel to both the loading direction, and the direction of the magnetic field, were found to exhibit the greatest MR effect of all specimens tested. MR effects generally decrease rapidly in the mid-strain region, but increase again at larger strain (> 15%). MR effects can be enhanced by preconditioning the specimens to larger levels of strain. The largest relative MR effects were found in uniaxial and equi-biaxial tension tests. Data fitting to pre-established hyperelastic constitutive models were conducted to evaluate their ability to characterise MREs. The parameters of the Ogden model describing isotropic MREs under general deformations in the absence of a magnetic field were successfully de- termined when experimental data obtained from tests up to the same strain level were combined in multi-deformation mode data fitting. The Ogden-Roxburgh model was found to describe the stress-softening well. Data fitting of several transversely isotropic constitutive models to experimental data of anisotropic MREs was not successful and none of the models employed in this investigation could accurately represent the data.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: TA Engineering (General). Civil engineering (General) ; TJ Mechanical engineering and machinery ; TK Electrical engineering. Electronics Nuclear engineering ; TS Manufactures