Development of a methodology for efficient, mechanical material characterization and modeling of continuous fiber-reinforced thermoplastics under long-term loading

• Applicant: Dr.-Ing. Sebastian Schmeer
• Subject Area: Polymeric and Biogenic Materials and Derived Composites ; Mechanics
• Term: since 2024
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 541861448

Continuous fiber-reinforced thermoplastics (cFRTP) can make an important contribution to the issue of sustainability and climate protection thanks to their high lightweight quality and material recycling at compound level. However, for the reliable and optimized design of such components, precise knowledge of the mechanical material behavior and its accurate description in the form of numerical material models are essential. A major issue is the complex, highly nonlinear material behavior of cFRTP, which is the result of various effects occurring simultaneously within the material. Methods commonly used in practice, such as classical tensile tests or creep tests, are not able to separate individual effects such that tedious test schedules are required for the material characterization. This is especially delicate for the characterization of the long-term material response due to a single test duration in the range of several weeks or even months. Furthermore, due to the high experimental effort, the mechanical material behavior under biaxial long-term loading is mostly unknown and therefore poses an open research question. Consequently, with respect to material characterization, the need arises for a methodology that allows the experimental investigation of the complex material phenomenology in an efficient manner. Moreover, an accurate material model is required which is capable of capturing the complex mechanical material response. In preliminary work, a methodology has been devised that provides an approach for the efficient material characterization and a corresponding material model for cFRTP. This approach is based on the mechanical concept of an “equilibrium curve” which clearly separates the material response into a rate independent equilibrium portion and a rate dependent overstress contribution. Using the developed stepwise relaxation test (SRT) in an exemplary material characterization, it was possible to clearly separate both contributions and to determine stress relaxation, damage and plasticity from just one test. Utilizing an extrapolation method for the relaxations periods of the SRT it was possible to significantly reduce the overall test time. By using the experimental data of the SRT, a full calibration of the corresponding material model was possible such that also the model calibration is facilitated. In the preliminary work, it was exemplary demonstrated that the material model offers the potential to substitute tedious creep tests with model predictions. Based on the demonstrated high potential of the approach proposed in preliminary work, this project aims to generalize and thoroughly validate this method for application to cFRTP. An essential aspect of the project is also the extension of this method, initially developed for uniaxial loading, to biaxial loading conditions in order to provide a profound understanding of the material response.

• DFG Programme: Research Grants