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Civil engineering, Medical engineering
Viscoelastic polyurethane foam is widely used in wheelchair cushions as it offers good pressure relieving capabilities. However, the behaviour of this material is largely un-quantified, by comparison with conventional elastomeric materials. Consequently, in many cases, inadequate cushioning is provided to wheelchair users with complex seating requirements. This thesis characterises and numerically models viscoelastic polyurethane foam. Temperature-dependent static compression and simple shear test procedures are conducted on a range of viscoelastic polyurethane foams and selected results are utilised to identify Ogden Hyperfoam material model parameters. Time-dependent creep and stress relaxation test procedures are conducted and test results are used in conjunction with Time-Temperature-Superposition (TTS), William-Landel- Ferry (WLF) and Arrhenius theories to generate long-term predictions of material behaviour. Appropriate spring-dashpot models are utilised to model predicted long-term viscoelastic performance. Thermal conductivity parameters are obtained using Glicksman’s theoretical model. The accuracy of predictions obtained using TTS and WLF theories has been proven. Validation has also been achieved for the temperature-dependent Hyperfoam, long-term viscoelastic and thermal conductivity parameters. The range of fully validated material model parameters were utilised to simulate the in-service seating behaviour of polyurethane foam. Simulation results were relatively compared and analysed with respect to relevant pressure ulcer risk factors. From analyses of the FE simulations, results lend support to findings from clinical trials particularly with respect to the relationship between shear and direct pressure in wheelchair seating. Other FE results disagree with accepted seat prescription timings used in current clinical practises.
Briody, C. (2012) The characterisation and numerical modelling of viscoelastic polyurethane foams for use in custom wheelchair seating. Doctoral Thesis, Dublin Institute of Technology. doi:10.21427/D7T60Q