Fraunhofer Institute for Mechanics of Materials IWMCarbon-fiber-reinforced thermoplastics (CF-PAEK) offer enormous potential for lightweight construction in aviation, wind energy, and automotive engineering—yet their fatigue behavior beyond 10⁷ load cycles is largely unknown. It is precisely this VHCF range that is critical for components with a 30-year service life. The project combines innovative high-frequency testing techniques (0.2–20 kHz) with a novel mechanism-based damage model to enable, for the first time, reliable service life predictions up to 10⁹ cycles. This will allow CFRP structures to be designed less conservatively in the future—while maintaining higher safety. For manufacturers, this means fully leveraging material efficiency, achieving weight savings, and using materials more sustainably.
Characterization and modeling of the fatigue properties of thermoplastic CFRP from the HCF to the VHCF range using high-frequency methods
Ongoing research project
Project description
Market Demand and Challenges
Carbon-fiber-reinforced plastics with a thermoplastic matrix are considered future-oriented materials for wind turbines, aerospace, and automotive manufacturing—with annual growth rates exceeding 10%. Compared to thermosets, thermoplastic matrices offer significant advantages in terms of recyclability, formability, and reparability. CFRP structures subjected to high cyclic loads—such as wind turbine rotor blades or aircraft fuselage structures—must withstand service lives of up to 30 years and thus load cycle counts well beyond 10⁸. However, there is very little reliable data on the fatigue behavior of thermoplastic CFRP in this VHCF (Very High Cycle Fatigue) range. The reason: Conventional testing systems operate at a maximum of 10 Hz, meaning a single test involving up to 10⁹ cycles would take 3 years to complete. As a result, CFRP structures are currently designed with extreme conservatism—the full potential for lightweight construction remains untapped, material is wasted, and structures are unnecessarily heavy.
The Project’s Approach
The project combines two innovative high-frequency testing methods—microbending resonance (0.2–2 kHz at Fraunhofer IWM) and ultrasonic fatigue (20 kHz at INATECH)—to systematically characterize, for the first time, the fatigue behavior of CF-PAEK laminates up to 10⁹ load cycles. Through multistage in-situ damage detection (thermography, vibrometry, microscopy, nonlinear acoustics), the relevant damage mechanisms are identified and quantified. In parallel, a novel, mechanism-based continuum damage model is being formulated at Fraunhofer IWM and implemented as an FEM material routine, enabling a consistent life-cycle prediction from the LCF through the HCF to the VHCF range.
Contribution to Addressing the Challenges
The project closes a fundamental knowledge gap regarding the long-term fatigue behavior of thermoplastic CFRP and provides both experimental methods and a practical simulation tool. As a result, CFRP components can be designed in the future with lower safety factors—while simultaneously achieving higher reliability. This enables lighter wind turbine rotor blades, weight-optimized aircraft structures (lower kerosene consumption), and an overall more sustainable use of materials. Direct validation using a wind turbine rotor blade as a case study demonstrates the industrial applicability.
Fraunhofer IWM’s work packages in the project:
Fatigue database for CFRP extending into the VHCF range—frequency-resolved and mechanism-based
The Fraunhofer IWM is compiling an experimental database that provides Wöhler curves for CFRP laminates up to 10⁹ cycles in multiple fiber orientations—including continuously quantified damage progression over the entire test duration.
Industrially applicable continuum damage model for CFRP—valid from static load to VHCF
Fraunhofer IWM is developing a novel damage model that describes all relevant CFRP failure mechanisms—fiber breakage, interfiber breakage (tension, compression, shear), and delamination—in a physics-based manner and with separate mechanisms, and is being implemented as a ready-to-use user-material subroutine in the commercial FE environment Abaqus. In addition, a simplified postprocessing model is being developed in the form of a generalized Miner rule for direct use in industrial design practice.
A design tool for industry, demonstrated and validated on a rotor blade
Fraunhofer IWM is concluding the project with a fully validated model, the performance of which is demonstrated in a case study on a wind turbine rotor blade—in collaboration with Prof. Montesano of the University of Waterloo—under real-world structural conditions.
Based on the project results, Fraunhofer IWM can offer industrial companies the following research and development services:
1. VHCF fatigue characterization of CFRP and fiber-reinforced composites using micro-bending resonance
Fraunhofer IWM determines fatigue data for fiber-reinforced composites up to 10⁹ load cycles—in just a few days rather than several years. Using its unique micro-flexural resonance apparatus, the institute records Wöhler curves in multiple test directions with negligible specimen heating, reliably covering the entire VHCF range.
2. Mechanism-Based Life Prediction for CFRP Components via FEM Simulation
Fraunhofer IWM performs numerical life-cycle assessments for customer-specific laminate structures and loading scenarios—based on a novel continuum damage model that describes all relevant failure mechanisms (fiber breakage, interfiber breakage, delamination) in a physics-based and mechanism-separated manner and is available as a ready-to-use subroutine in Abaqus.
3. Evaluation of Frequency Influence and Transferability of High-Frequency Test Data to Operating Conditions
Fraunhofer IWM quantifies whether and to what extent fatigue data obtained at high frequencies can be extrapolated to real operating frequencies.
4. In-situ damage detection and quantification in composite materials
Fraunhofer IWM offers combined measurement techniques for the early detection and continuous quantification of fatigue damage in fiber-reinforced composites—as an experimental service and as a methodological basis for condition monitoring concepts.
5. Qualification of thermoplastic CFRP systems for long-term applications in the VHCF range
Fraunhofer IWM qualifies new thermoplastic CFRP systems based on PAEK, PEEK, or PPS matrices for their suitability for VHCF applications—thereby providing a validated materials science basis for material selection in safety-critical long-term applications.
