Multi-Scale Modelling of Thermoplastic Fibre Reinforced Composites during Thermoforming

标准纤维增强聚合物包含热固性聚合物基质，聚合物通过化学反应不可逆地固化。新型热塑性复合材料具有良好的反复加热、成型和冷却能力。进一步的优势包括更短的部件周期时间、易于存储和处理、更高的韧性和可回收性。正因为如此，热塑性复合材料的应用越来越广泛。最终零件生产者将坯料加热到熔化温度以上，并形成最终零件形状。然而，工艺稳定性是一个主要问题。由于纤维与基体之间的温度梯度和热膨胀系数不匹配造成的残余应力可能会导致最终零件的形状变形。为了抵消这种影响，必须仔细确定加热和冷却速度、刀具形状和零件定位等工艺参数。这通常是通过试错来完成的，这在一定程度上削弱了潜在的时间和成本优势。为了改善这种情况，需要能够预测复合材料零件残余应力和变形的热成形过程的计算模型。现有的型号只能部分满足这些要求。残余应力的积累必须在多个尺度上考虑，包括纤维水平和丝束水平，而丝束包括数千条纤维。必须考虑聚合物的整个加工温度范围，从室温到高于熔融温度。最后，必须充分描述和考虑聚合物在纤维存在下的熔融和固化。该项目将产生一个具有三个主要创新的比例桥接热成形模拟。首先，建立了整个热成型温度范围的矩阵本构模型。它将在纤维和丝束水平上得到应用。其次，建立了固体和液体两种基质相的TOW模型。最后，纤维和丝束的模型将在整个温度范围内进行全面的实验表征。预计该项目的最终结果是一个多尺度的热成型模拟工具，具有新的保真度水平。

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.