Stephen Jerrams

A rheological model of the dynamic behavior of magnetorheological elastomers

A rheological model is described that was developed to simulate the dynamic behavior of magnetorheological elastomers (MREs). The viscoelasticity of the polymer composite, magnetic field-induced properties and interfacial slippage between the matrix and particles were modeled by analogy with a standard linear solid model, a stiffness variable spring, and a spring-Coulomb friction slider, respectively. The loading history and rate dependent constitutive relationships for MREs were derived from the rheological model. The hysteresis loop from shear strain-shear stress plots, which determines the shear modulus and loss factor, were obtained from substituting cyclic loading into these constitutive relationships. The dynamic behavior of MREs was simulated by changing parameters in the rheological model to influence MREs’ performance. The simulation results verified the effectiveness of the model.

Eliminating electromechanical instability in dielectric elastomers by employing pre-stretch

Electromechanical instability (EMI) is one of most common failure modes for dielectric elastomers (DEs). It has been reported that pre-stretching a DE sample can suppress EMI due to strain stiffening taking place for larger strains and a higher elastic modulus are achieved at high stretch ratios when a voltage is applied to the material. In this work, the influence of equi-biaxial stretch on DE secant modulus was studied using VHB 4910 and silicone rubber (SR) composites containing barium titanate (BaTiO3, BT) particles and also dopamine coated BT (DP-BT) particles. The investigation of equi-biaxial deformation and EMI failure for VHB 4910 was undertaken by introducing a voltage-stretch function. The results showed that EMI was suppressed by equi-biaxial pre-stretch for all the DEs fabricated and tested. The stiffening properties of the DE materials were also studied with respect to the secant modulus. Furthermore, a voltage-induced strain of above 200% was achieved for the polyacrylate film by applying a pre-stretch ratio of 2.0 without EMI occurring. However, a maximum voltage-induced strain in the polyacrylate film of 78% was obtained by the SR/20 wt% DP-BT composite for a lower applied pre-stretch ratio of 1.6 and again EMI was eliminated.

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).

Removing Barriers to Conducting Research in Ireland’s Institutes of Technology

Institutes of Technology (IoT) in Ireland didn’t engage in significant levels of research until comparatively recently. With Ireland’s move towards a knowledge society and to position Irish Industry to take full advantage of economic recovery when it arrives, it is imperative that the research resources in the IoT sector are utilised more effectively. Most national programmatic research schemes support new research initiatives including greater 4th level activity and IoTs have or are developing research initiatives to respond to these schemes. Over the past 5 years significant increases have occurred in the numbers of graduates (at both masters and doctorate levels) from the IoT sector. Such graduates have garnered considerable research experience making their skills very attractive for employment in industry, the professions and wider society.

The Tromped: A Solution for Flight-Related Deep Vein Thrombosis?

Flight-related deep vein thrombosis (DVT) is well recognized. Reduced venous return occurs during immobility. This alteration in venous hemodynamics may contribute to DVT development. A prototype design of an in-flight exercise device to stimulate ambulatory bloodflow while seated has been developed, consisting of a foot pedal attached to a base by a hinge mechanism. Four devices of differing resistance were evaluated. Calf muscle pump function was assessed by air plethysmography in 10 healthy volunteers. Ejection volume fraction and RVF were determined in the standing position (control values) and were compared with those achieved by depression of the 4 devices while seated. Similar EVF and RVF values were achieved by the control and 2 of the devices. Plantar flexion against a predetermined resistance can effectively activate the calf muscle pump while seated and may reduce the incidence of flight-related DVT.

Equi-Biaxial Fatigue Behaviour of Magnetorheological Elastomers in Magnetic Fields

The equi-biaxial fatigue behaviour of silicone-based magnetorheological elastomers in external magnetic fields was studied. Wöhler curves relating fatigue life to stress amplitude and dynamic stored energy for magnetorheological elastomers with a range of magnetic particle contents were derived. It was found that the fatigue life of magnetorheological elastomers in magnetic fields was higher than that without magnetic fields. Under constant stress amplitude conditions, the presence of magnetic fields resulted in longer times for the samples to undergo large deformations and thus complex modulus (E*) decreased at a slower rate during the fatigue process, especially for low stress amplitudes. Magnetorheological elastomer samples tested in the presence of magnetic fields reached limiting values of E* at failure ranging from 1.28 to 1.44 MPa. The application of magnetic fields was found to have negligible influence on the damping loss factor of magnetorheological elastomers containing various volume fractions of carbonyl iron particles.

The Determination of Multi-Axial Fatigue in Magnetorheological Elastomers Using Bubble Inflation

Fatigue life is one of the most important physical characteristic that is investigated by materials engineers and scientists. The high dynamic loading experienced by machine parts necessitates understanding fatigue properties in critical components. Despite this requirement, elastomer fatigue criteria are imperfectly understood and even less is known about fatigue resistance in the emerging range of smart elastomers. In this paper, initial research into the equi-biaxial fatigue behaviour of magnetorheological elastomers (MREs) is described. Physical testing was carried out using a bubble inflation testing system. Silicone rubber based test samples were fatigued at different stress amplitudes ranging between 0.75MPa and 1.4MPa using engineering stress as the control mode. S-N (Wöhler) curves showing the plots of stress amplitude (σa) versus cycles to failure (N) are presented. Stress-strain behaviour throughout fatigue process is also described. For a fatigue test at a stress amplitude of 0.75MPa and no pre-stressing, it was found that stress softening occurred for the entire duration of the test, but was particularly pronounced in the first 100 cycles of testing.

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