Modeling the thermoforming process of fiber-reinforced thermoplastics

热塑性纤维增强复合材料（TPFRC）可以重复加热，成型和冷却，与传统的热固性复合材料相比。由于热塑性塑料具有许多优点（周期短、易于储存和处理、韧性增加和可回收性），热塑性塑料系统的市场一直以每年5%的平均速度增长。很多年了热成型特别适用于加工TPFRC。然而，工艺稳定性往往难以保证。由于热梯度、纤维-基质相互作用以及固化聚合物基质的复杂结晶行为，残余应力在几个尺度上发生，这可能导致不期望的部件变形。传统的试错法很难避免这些缺陷。相反，数值模型是一个很有前途的替代方案。因此，本研究项目的总体目标是开发一个模拟工具的热成型过程TPFRC。为此，预测非均匀温度场和热流是至关重要的。此外，热塑性基质的结晶过程必须基于热条件来确定，并且另外必须考虑由于该放热反应而产生的额外热量。这就需要解决一个完全耦合的瞬态热机械问题。在这个项目的过程中，提出了以下主要创新：一方面，半结晶热塑性塑料的粘塑性材料模型开发的整个范围内的工艺相关的温度和结晶度。此外，结晶动力学模型提出，该帐户的纤维的影响。基于这些模型，各向异性，热粘塑性本构关系的开发，以模拟复合材料的各个层。对于模型参数的实验识别，将在整个温度和结晶度范围内进行大量的常规测试方法。此外，新的实验方法的开发，特别是调查在低结晶度和温度以上的玻璃化transition.The开发的模型允许详细分析的局部结晶度和由此产生的（可能是异质的）材料性能以及残余应力的演变的材料行为。预计这将导致新的调查结果的发病和发展的不希望有的部分扭曲，从而个人建议的最佳工艺参数。

Thermoplastic fiber reinforced composites (TPFRC) can be repeatedly heated, formed and cooled, in contrast to conventional thermoset composites. Due to its many advantages (short cycle times, ease of storage and handling, increased toughness, and recyclability) the market for thermoplastic systems has been growing at an above-average rate of 5% p. a. for years. Thermoforming is particularly suitable for processing TPFRC. However, the process stability is often difficult to guarantee. Due to thermal gradients, the fiber-matrix interactions, and the complex crystallization behavior of the solidifying polymer matrix, residual stresses occur on several scales, which can lead to undesired part distortions. Conventional trial-and-error approaches are hardly suitable to avoid these defects. Instead, numerical models represent a promising alternative.The overarching goal of this research project is therefore the development of a simulation tool for the thermoforming process of TPFRC. To this end, it is crucial to predict the heterogeneous temperature fields and heat flows. Furthermore, the crystallization process of the thermoplastic matrix has to be determined based on the thermal conditions and the additional heat due to this exothermic reaction must be accounted for, in addition. This requires the solution of a fully coupled, transient thermo-mechanical problem.In the course of this project, the following main innovations are proposed: On the one hand, a visco-plastic material model for semi-crystalline thermoplastics is developed for the entire range of process-relevant temperatures and degrees of crystallinity. Furthermore, a crystallization kinetics model is proposed, which accounts for the influence of the fibers. Based on these models, an anisotropic, thermo-visco-plastic constitutive law is developed in order to model the individual plies of the composite. For the experimental identification of the model parameters, a large number of conventional test methods will be carried out over the entire range of temperatures and degrees of crystallinity. In addition, novel experimental methods are developed, in particular to investigate the material behavior at low degrees of crystallinity and temperatures above the glass transition.The developed model allows for a detailed analysis of the local degree of crystallinity and the resulting (possibly heterogeneous) material properties as well as for the evolution of residual stresses. It is expected that this will lead to new findings regarding the onset and development of undesired part distortions and thus to individual recommendations regarding the optimal process parameters.