Modeling the thermoforming process of fiber-reinforced thermoplastics

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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.