Articles

Prediction of Compressive Creep Bhaviour in Flexible Polyurethane Foam Over Long Time Scales and at Elevated Temperatures

Conor Briody et al. — Tue, 04 Sep 2012 04:00:19 PDT

Compressive creep gradually affects the structural performance of flexible polymeric foam material over extended time periods. When designing components, it is often difficult to account for long-term creep, as accurate creep data over long time periods or at high temperatures is often unavailable. This is mainly due to the lengthy testing times and/or inadequate high temperature testing facilities. This issue can be resolved by conducting a range of short-term creep tests and applying accurate prediction methods to the results. Short-term creep testing was conducted on viscoelastic polyurethane foam, a material commonly used in seating and bedding systems. Tests were conducted over a range of temperatures, providing the necessary results to allow for the generation of predictions of long-term creep behaviour using time-temperature superposition. Additional predictions were generated, using the William Landel Ferry time-temperature empirical relations, for material performance at temperatures above and below the reference temperature range. Further tests validated the results generated from these theoretical predictions.

The Implementation of a Visco-hyperelastic Numerical Material Model for Simulating the Behaviour of Polymer Foam Materials

Conor Briody et al. — Tue, 08 May 2012 06:02:17 PDT

Polyurethane foam has been in use for some time in wheelchair seating systems as it offers good pressure relieving capabilities in most cases. However, little characterisation work has gone into seating foam materials by comparison with conventional elastomeric materials. Accurate material models could allow better prediction of foam in-service behaviour, which could potentially improve seating design practises. The objective of this work was to develop an approach for the validation of hyperelastic and viscoelastic material model parameters used to simulate polyurethane foam behaviour. Material parameters were identified from relevant test procedures and implemented in a Finite Element simulation of an ISO foam indentation procedure. Physical test results were compared to results predicted using the identified material parameters. Simulations suggest a good overall agreement between test and model results.

A Rheological Model of the Dynamic Behaviour of Magnetorheological Elastomers

Lin Chen et al. — Wed, 01 Jun 2011 08:56:47 PDT

A rheological model is described that was developed to simulate the dynamic behaviour of magnetorheological elastomers (MREs). The viscoelasticity of the polymer composite, magnetic field induced properties and interfacial slippage between the matrix and particles were modelled 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 behaviours of MREs were simulated by changing parameters in the rheological model to influence MREs’ performances. The simulation results verified the effectiveness of the model.

Microstructures and Viscoelastic Properties of Anisotropic Magnetorheological Elastomers

Lin Chen — Tue, 17 May 2011 09:11:30 PDT

The microstructures and viscoelastic properties of anisotropic magnetorheological elastomers were investigated in this paper. The measurement results show that their mechanical properties are greatly dependent on the magnetic flux density applied in preparation. A finite-column model was proposed to describe the relations between the microstructures and the viscoelastic properties. The simulation results agree well with the experimental results.

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

Stephen Jerrams et al. — Mon, 09 May 2011 02:09:55 PDT

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. Wöhler (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 did those of the dryer specimens.

Effect of Carbon Black on the Mechanical Performances of Magnetorheological Elastomers

Lin Chen — Mon, 09 May 2011 02:09:54 PDT

Several magnetorheological elastomer (MRE) samples, with different weight percentages of carbon black, were fabricated under a constant magnetic field. Their microstructures were observed by using an environmental scanning electron microscope (SEM), and their mechanical performance including magnetorheological (MR) effect, damping ratio and tensile strength were measured with a dynamic mechanical analyzer (DMA) system and an electronic tensile machine. The experimental results demonstrate that carbon black plays a significant role in improving the mechanical performance of MR elastomers. Besides the merits of high MR effect and good tensile strength, the damping ratio of such materials is much reduced. This is expected to solve a big problem in the application of MR elastomers in practical devices, such as in adaptive tuned vibration absorbers.

Damping of Magnetorheological Elastomers

Lin Chen — Mon, 09 May 2011 02:09:52 PDT

The damping property of magnetorheological (MR) elastomers is characterized by a modified dynamic mechanical-magnetic coupled analyzer. The influences of the external magnetic flux density, damping of the matrix, content of iron particles, dynamic strain and driving frequency on the damping properties of MR elastomers were investigated experimentally. The experimental results indicate that the damping properties of MR elastomers greatly depend on the interfacial slipping between the inner particles and the matrix. Different from general composite materials, the interfacial slipping in MR elastomers is affected by the external applied magnetic field.

Investigation on Magnetorheological Elastomers Based on Natural Rubber

Lin Chen — Mon, 09 May 2011 02:04:39 PDT

Magnetorheological Elastomers (MR Elastomers or MREs) are a kind of novel smart material, whose mechanical, electrical, magnetic properties are controllable under applied magnetic fields. They have attracted increasing attentions and broad application prospects. But conventional MREs are limited to wide applications because their MR effects and mechanical performances are not high enough. This paper aims to optimize the fabrication method and to fabricate good natural rubber based MREs with high modulus by investigating the influences of a variety of fabrication conditions on the MREs performances, such as matrix type, external magnetic flux density, and temperature, plasticizer and iron particles. Among these factors, the content of iron particles plays a most important contribution in shear modulus. When the iron particle weight fraction is 80% and the external magnetic flux density is 1 T, the field-induced increment of shear modulus reaches 3.6MPa, and the relative MR effect is 133%. If the iron weight fraction increases to 90%, the field-induced increment of shear modulus is 4.5MPa. This result has exceeded the best report in the literatures researching the MREs on the same kind of matrix. The dynamic performances of MREs were also experimentally characterized by using a modified Dynamic Mechanical Analyzer (DMA) system. The effects of strain amplitude and driving frequency on viscoelastic properties of MREs were analyzed.

Friction and Adhesion in Rigid Surface Indentation of Nitrile Rubber

Stephen Jerrams — Mon, 09 May 2011 02:04:37 PDT

When a rigid body in the form of a plane strain indentor is forced into an elastomer, the asperities on the surface of the indentor are filled by the softer material. As depth of ingress increases, the rubber displaced into the indentor asperities exhibits stick-slip behaviour. The rubber adheres to the rigid body and if the depth of ingress is held at a maximum, the level of adhesion remains constant despite short-term load relaxation occurring in the rubber.

This text describes the influence of a range of factors on indentation forces and adhesion in rigid indentation of hydrogenated nitrile rubbers. Blocks of rubber in four hardness grades were subjected to plane strain indentation using mild steel plate indentors. The edges forced into the elastomers were radiused to produce ingress of a semi-circular profile into the blocks and this allowed subsequent finite element modelling of the indentor as a continuum. During physical testing, indentation rates and indentor surface finish were varied and load/displacement characteristics, adhesion and short-term load relaxation were measured. The correlation between indentation loads at the common maximum depth of ingress and the adhesion theory of friction for different surface finishes was examined. Nonlinear finite element stress analyses, employing adaptive meshing, alternative friction algorithms and competing strain energy density functions were used to model the indentation process and comparisons of surface profiles with test results are included.

Contradictions in Irish Academic Research

Stephen Jerrams et al. — Mon, 09 May 2011 02:04:36 PDT

The conditions that govern academic research vary greatly from country to country and research in the Republic of Ireland was and remains markedly different from that of its larger European neighbours and the United States. Despite the quality of its education system and the excellent reputation of its universities, until recently Ireland had relatively low levels of academic research. Pinnacles of excellence could be found in certain disciplines, but state funding was low and issues relating to industrial collaborations, international partnerships, commercialisation and the exploitation of Intellectual Property (IP) rarely arose. Even today the Irish Government’s spending on academic research, though only slightly less than the European average based on GNP, is dwarfed by the Research and Development (R & D) budgets of individual multinational companies. Nonetheless, rapid economic growth has led to a heightened awareness of the need for strategically planned research. The ‘Lisbon Objective’ proposes to make Europe “the most dynamic knowledge-driven economy in the world by 2010”. Consequently research is heavily influenced by this policy and so a range of unfamiliar problems are posed for managers of Irish academic research. Key to successful operational planning and growth is the need to reconcile a number of contradictions at the heart of R & D in Third Level Institutes[1].

[1] ‘Third Level Institute’ is a term used in Ireland to describe any higher education institute