Niall Murphy

Equi-Biaxial Fatigue of Elastomers: the Effect of Oil Swelling on Fatigue Life

Abstract The effect of oil swelling on the fatigue life of ethylene propylene diene monomer rubber (EPDM) has been studied under conditions of equi-biaxial cycling using dynamic bubble inflation. Specimens were subjected to varying degrees of swelling in reference mineral oils and fatigued at constant engineering stress amplitudes. The reference oils used for swelling the EPDM had known aniline points, allowing the rubber-oil compatibility to be determined. The inflation fluid for fatigue testing was selected with a solubility parameter that would produce a desired level of incompatibility with the test specimens, thereby limiting the amount of additional swelling during cycling. Wohler (S-N) plots were generated for dry and swollen specimens and the changes in complex elastic modulus E* and dynamic stored energy were analyzed. Specimen fractures were analyzed using scanning electron microscopy. The fractures in the swollen samples show that the failure surfaces flowed more readily over each other than di...

Creating a Uniform Magnetic Field for the Equi-Biaxial Physical Testing of Magnetorheological Elastomers; Electromagnet Design, Development and Testing.

This paper investigates a method to provide the magnetic field requirements for physical testing of magnetorheological elastomers (MREs) subjected to equi-biaxial loading using the bubble inflation method.For accurate physical testing of MREs, detailed knowledge of the properties of the applied magnetic field is required. To obtain reliable data it is essential to determine the strength, uniformity and directionality of flux density. A Halbach cylinder array can produce a magnetic field of approximately uniform flux density in one direction for a reference plane perpendicular to the direction. However, it is limited by the fixed field strength. To overcome this significant limitation, an electromagnetic array based on the geometry of a Halbach cylinder is proposed. This electromagnetic array will be capable of generating a uniform magnetic field, for the reference plane and in the perpendicular direction, that is capable of having the flux density varied to offer a range of field strengths for tests on different elastomer samples.FEA simulations of uniform electromagnetic arrays have been modelled. Ultimately, a model is offered that simulates the behaviour of an electromagnetic array and the capability to generate a uniform magnetic field with different flux densities and directionality over the required volume. The advantages and disadvantages of an electromagnetic array over a fixed strength Halbach cylinder were investigated and a detailed comparison of both was carried out. Preliminary tests have been conducted on prototype electromagnets and the measured magnetic fields have been found to be in agreement with the FEA model. In addition to the magnetic field experiments, tests have been carried out on a compressed air cooling system to allow continuous operation of the electromagnets for the duration of a fatigue test without test samples becoming overheated and chemical degradation occurring. These tests are also necessary to establish conditions where there is minimal drop in field strength due to the increased resistance associated with temperature increases during prolonged dynamic testing. In conclusion a design for an electromagnetic array for the equi-biaxial testing of MREs is presented along with proposals for further testing to fully develop the array and establish standard dynamic test procedures for the material. magnetic iron particles in the MRE attempting to align in the direction of the applied magnetic field (Bica, 2009). This alignment is caused by the interactions of magnetic dipoles (G.V. Stepanov, 2007). The force on a magnetic dipole of moment (m) in a magnetic field (B) is given in equation 1.1 (I.S. Grant, 1990).

Generating a Variable Uniform Magnetic Field Suitable for Fatigue Testing Magnetorheological Elastomers Using the Bubble Inflation Method.

This paper investigates the magnetic field generated by an electromagnetic array and whether it is suitable for the physical testing of magnetorheological elastomers (MREs) using bubble inflation. This will be achieved by comparing the magnetic field generated by the array with the simulated field calculated by an FEA model. The array will be evaluated with detailed measurements of the magnetic flux strength and direction over the entire sample volume. The magnetic flux versus the time the array is powered in order to ensure it is capable of providing a reliable magnetic field for the duration of a biaxial fatigue test of an MRE. tween particles result in varying changes in mechanical properties. It has been reported by (Varga et al., 2006) and (Boczkowska and Awietjan, 2009) that when the magnetic field is applied parallel to the particle chains it produces a greater MR effect than the same flux density applied perpendicular to the particle chains. 2 MODELLING THE MAGNETIC FIELD 2.1 Uniform magnetic fields A Halbach cylinder is an array of permanent magnets arranged in a cylinder which produces a uniform magnetic field, in one plane and over acceptable limits. A model with four electromagnets has been proposed based on the geometry of an open access Halbach cylinder used by (Hills et al., 2005). The proposed model is shown Figure 1 The FEA model calculates the simulated field and typical results are shown in Figure 2 for the field operating between the poles of the central coils. Figure 2 depicts that the simulated field has both a relatively uniform flux density of over 400mT and is of uniform direction. Further details of the modelling process were presented in previous work. (Gorman et al., 2011). 3 MANUFACTURING A PROTOTYPE OF THE ELECTROMAGNETIC ARRAY