Modeling the thermoforming process of fiber-reinforced thermoplastics

与传统的热固性复合材料相比，热塑性纤维增强复合材料(TPFRC)可以重复加热、成型和冷却。由于其许多优点(周期短、易于储存和处理、增加韧性和可回收)，热塑性塑料系统的市场多年来一直以每年5%的高于平均速度增长。热成型特别适用于TPFRC的加工。然而，工艺稳定性往往难以保证。由于温度梯度、纤维-基质相互作用以及固化聚合物基质的复杂结晶行为，残余应力在多个尺度上发生，这可能导致不希望看到的零件变形。传统的试错法很难避免这些缺陷。因此，本研究项目的首要目标是开发一种TPFRC热成形过程的模拟工具。为此，预测非均匀的温度场和热流是至关重要的。此外，必须根据热条件确定热塑性基质的结晶过程，此外还必须考虑该放热反应产生的额外热。这就需要解决一个完全耦合的、暂态的热力问题。在这个项目的过程中，提出了以下主要创新：一方面，建立了半结晶热塑性材料的粘塑性模型，该模型适用于整个工艺相关的温度和结晶度范围。在此基础上，建立了考虑纤维影响的结晶动力学模型。在这些模型的基础上，建立了各向异性的热粘塑性本构关系，以模拟复合材料的各向异性。对于模型参数的实验识别，将在整个温度范围和结晶度范围内进行大量的常规测试方法。此外，还开发了新的实验方法，特别是在低结晶度和玻璃化转变以上温度下的材料行为。所开发的模型允许详细分析局部结晶度和由此产生的(可能是异质的)材料性质以及残余应力的演变。预计这将导致关于不受欢迎的部分扭曲的发生和发展的新发现，从而产生关于最佳工艺参数的个别建议。

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.