Multi-Scale Modelling of Thermoplastic Fibre Reinforced Composites during Thermoforming

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Standard fibre-reinforced polymers contain a thermoset polymer matrix, where the polymer is cured irreversibly by a chemical reaction. Novel thermoplastic composites excel in the ability to undergo repeated heating, forming, and cooling. Further advantages include shorter part cycle times, ease of storage and handling, increased toughness, and recyclability. For these reasons the application of thermoplastic composites becomes more and more popular. The final part producer heats up the blank above the melting temperature and forms it into the final part shape.However, process stability is a major problem. Residual stresses due to thermal gradients and coefficient of thermal expansion mismatches between fibre and matrix might lead to shape distortions of the final part. To counteract this effect, process parameters such as heating and cooling rates, tool shape, and part positioning must be carefully determined. This is typically done by trial and error, partly vitiating potential time and cost advantages. To improve the situation, computational models of the thermoforming process are needed which can predict residual stresses and distortions of the composite parts. Existing models meet these requirements only partly. The build-up of residual stresses must be considered at multiple scales, at the fibre level as well as at the tow level, whereas the tow includes thousands of fibres. The entire process temperature range of the polymer must be considered, from room temperature to above the melting temperature. Finally, the melting and solidifying of the polymer in the presence of fibres must be fully characterised and considered.The project will result in a scale-bridging thermoforming simulation with three major innova-tions. First of all, a matrix constitutive model for the entire temperature range of thermoforming shall be developed. It will find application at the fibre and at the tow level. Secondly, a tow model will be created for both solid and liquid matrix phases. At last, the models for fibre and tow will be fully characterised experimentally over the entire temperature range. It is expected that the final result of the project is a multi-scale simulation tool for thermoforming with a new level of fidelity.