Characterization and modeling of the fatigue properties of thermoplastic CFRP in the HCF and VHCF regime using high frequency methods

• Applicants: Professor Dr.-Ing. Frank Balle; Privatdozent Dr.-Ing. Jörg Martin Hohe; Dr.-Ing. Thomas Straub
• Subject Area: Polymeric and Biogenic Materials and Derived Composites
• Term: since 2024
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 547463967

Continuously carbon fiber reinforced composites (CFRP) are important high-performance materials in many fields of lightweight design. Due to their inherent specific stiffness and strength and their high recycling capability, composites with thermoplastic matrix systems gain increasing importance. Furthermore, these materials in general exhibit a superior fatigue resistance with S-N-curves showing small slopes in both the LCF and the HCH regime. Their fatigue response in the VHCF regime at extremely large numbers of cycles such as in wind energy is subject to investigation. It is not known whether a rigorous fatigue limit exists or whether the HCF characteristics continue into the VHCF regime due to the excessive testing times at 108 cycles and beyond. For numerical fatigue assessment in most cases simplified Woehler-Miner type approaches are employed. Fully 3D models capturing multi axial stress states and generalized cyclic load histories are limited to few approaches in progress. Up to date, no general integrated formulation capturing the entire HCF and VHCF ranges is available. The present project aims on improvement in both, knowledge about the relevant fatigue mechanisms and their numerical modeling. In an experimental approach, the fatigue damage and degradation behavior of thermoplastic CFRP (CF-PAEK with UD and MD stacking sequence) will be investigated. Objective is to gain a comprehensive insight into fatigue and degradation under VHCF loads including possible transitions in fatigue mechanisms. In a first step, the necessary experimental techniques will be developed and established. Two complementary techniques will be used. Micro specimen tests (IWM) enable a unique optical in-situ observation in high frequency experiments combining high resolution and reliable results, thus providing a novel and unique method for detection of nucleation and growth of fatigue micro cracks till specimen failure. The VHCF regime will be investigated by INATECH using the ultrasonic fatigue testing techniques first developed in DFG priority program SPP 1466 (Infinite life). Both techniques are not established in detail for CFRP and thus need to be re-developed accordingly. The methods will be benchmarked, including the questions of gauge section volume and frequency effects considering the time temperature superposition principle. The ultrasonic fatigue technique will be applied to elevated isothermal test conditions for the first time. The fatigue experiments will be completed by an extensive analytical and fractographic analysis of the micromechanical damage and failure mechanisms. The experiments will be complemented by the development and finite element implementation of a damage material model describing material degradation and failure. The model enables a reliable prediction of the fatigue degradation throughout the HCF and VHCF regimes. Its application will be demonstrated in a relevant case study in cooperation with two visiting scholars.

• DFG Programme: Research Grants