Joule heating is a promising heating method used in the composites industry, which is characterised by high heating efficiencies and low power consumption. However, imperfect contact at the electrode/material interfaces results in contact resistances. Contact resistances play a major role in Joule heating systems, therefore, identifying contact resistances provides a better understanding of the heating process. Furthermore, the accuracy of the process models of heating systems can be increased by taking contact resistances into account. In this study, contact resistances of the Joule heating section in a powder-epoxy based towpregging line were explored. For different process parameters, such as production speed, tension and temperature, contact resistances occurring between metal roller electrodes and dry carbon fibre tows (i.e. without powder-epoxy) were measured to gain an insight into the heating process. Moreover, the effect of contact resistance heating on the overall temperature profile of the carbon fibre tows was determined with an infrared thermal camera. A simplified finite element model was developed to calculate the temperature distribution along the moving carbon fibre tow, which showed a good agreement with the experimental data. Results suggest that contact resistances in the towpregging line were significant and contact resistance heating changed the heating profile.
This paper explores the performance of low-cost unidirectional carbon fibre towpregs with respect to line production speed and fibre volume fraction. Using an automated production line, towpregs were produced at different production speeds, resulting in modified fibre volume fractions. The towpregs were used to manufacture unidirectional composite plates, which were then tested to evaluate mechanical performance. The fibre straightness and interfacial void ratio of the composite plates were determined by statistical analysis of the samples’ optical micrographs. The results demonstrate that adjusting the line production speed enables targeted fibre volume fractions (FVF) to be reached, resulting in the composites having different mechanical performances (2039 MPa and 2186.7 MPa tensile strength, 1.26 and 1.21 GPa flexural strength for 59.8% and 64.4% FVF, respectively). It was shown that at lower production speeds and FVF, composites exhibit good consolidation and low porosity, which is highlighted by the better interlaminar shear strength performances (8.95% increase), indicating the limitations of manufacturing very high FVF composites. Furthermore, it was concluded that fibre straightness plays a key role in mechanical performance, as samples with a lesser degree of fibre straightness showed a divergence from theoretical tensile properties.
The purpose of this study is to examine the effects of exchange current density, as dependent on operating parameters, on the activation polarization in an HTPEM fuel cell. In line with this purpose, cell performance tests were performed for different relative humidity levels (from 0 to 20% in steps of 5%) and six different acid doping levels (4.26, 4.53, 4.95, 5.51, 7.24, 7.51 H 3 PO 4 /RPU PBI) at constant cell temperature (120 • C). The exchange current densities, as dependent on acid doping level and relative humidity, were determined by fitting a zero-dimensional steady-state mathematical model to the cell performance test results. Finally, to investigate the activation polarization in the cell, the exchange current densities obtained were incorporated within a numerical model. The results show that the effects of relative humidity on the exchange current density are relatively limited at all acid doping levels when the relative humidity is greater than 5%. The exchange current densities exhibit a decreasing trend with increasing acid doping level. Furthermore, it was found that the acid doping level and relative humidity have a strong influence on the anode overpotential, whereas no significant effect on the cathode overpotential was observed.
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