FVA 1008 I - Efficient material characterization and modeling of short-fiber-reinforced thermoplastics

In recent years, fiber-reinforced thermoplastic composites (FRTP) have increasingly gained prominence in technical applications, surpassing traditional thermoset composites. This upward trend can be attributed to various advantages of FRTP, such as their recyclability and simpler processing methods, which are becoming more significant in the context of sustainable and resource-efficient products. Of particular importance are short-fiber-reinforced thermoplastic composites as they combine favorable mechanical properties with cost-effective and efficient manufacturing processes. However, the technical application faces challenges, particularly due to their complex and highly nonlinear mechanical behavior. Engineers require material properties and suitable material models to accurately represent this behavior in computer-aided simulations when designing components. One critical aspect that demands attention is creep– the deformation of a component over time under a constant load – as it significantly affects long-term performance and may lead to component failure during use. Currently, state-of-the-art experimental characterization involves extensive testing programs. This is especially true for long-term tests aimed at determining creep behavior, which often require prolonged and costly testing periods lasting weeks to months.

The project "Effective Material Characterization" (FVA 1008 I) aims to develop an experimental procedure for time and thus resource efficient determination of the long-term behavior. The starting point is an innovative testing and modeling methodology developed at IVW for continuous-fiber-reinforced thermoplastics (cFRTP) (see https://doi.org/10.1016/j.compositesb.2023.110734). This approach employs a stepwise relaxation test to conduct a comprehensive and holistic material screening, with the resulting material data directly utilized for a corresponding material model. The methodology is based on the mechanical concept of a time-independent equilibrium curve and a time-dependent overstress. The equilibrium curve represents the "true" material behavior and can be understood as the long-term material response. The material behavior measured during the experiment is a combination of the equilibrium curve and the time- or rate-dependent overstress, where the overstress describes deviations from the "true" material behavior. Through an extrapolation procedure, the equilibrium curve can be approximated using the stepwise relaxation test, achieving significant time savings with a typical test duration of only 1- 2 days. The associated material model can be calibrated solely based on the stepwise relaxation test, enabling the accurate description of the complex material behavior. This approach has the potential to replace lengthy creep tests with model-based predictions, providing a highly efficient alternative.

In the FVA project "Effective Material Characterization," this approach is now being tested and adapted for short-fiber-reinforced thermoplastics. The goal is to generalize the methodology to enable broader application. This aims to facilitate synergistic and highly efficient characterization and modeling of short-fiber-reinforced thermoplastic composites, thereby allowing the high potential of this material class to be better utilized in the future.