Methodology for adaptive accelerated fatigue testing of thermoplastic composites considering the self-heating effect

• Applicant: Professor Dr.-Ing. Frank Balle
• Subject Area: Polymeric and Biogenic Materials and Derived Composites
• Term: since 2026
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 582095008

Thermoplastic composites reinforced with carbon fibers have become highly relevant to industries, particularly in aviation and aerospace applications due to their unique properties, and as one sustainable alternative to metallic alloys and thermoset composites. Despite their superior properties, these materials remain susceptible to mechanical fatigue, requiring detailed investigation, especially given their long service life. Thermoplastics can endure millions of load cycles, but what happens afterward? In practical terms: how will composite aircraft components perform after 20 or even 30 years? How can their fatigue life be predicted and this knowledge used to design structural components that meet sustainability criteria? Finally, how can the fatigue behavior of materials be studied with such long service lives? Through collaboration between the University of Freiburg (Germany) and the Silesian University of Technology (Poland), we are developing a methodology to accelerate fatigue testing by applying ultrasonic-frequency loading, reducing test duration from years to weeks. A major challenge at such frequencies is the self-heating effect, generated by the viscoelastic properties of polymers composites. This effect dominates the material response, accelerates structural degradation, and prevents direct comparison of fatigue results with those obtained at lower frequencies. To address this challenge, the project team investigates the thermomechanical behavior of thermoplastics during fatigue at low and ultrasonic frequencies, with comprehensive analysis of structural degradation and damage accumulation. Planned fatigue tests in both regimes will identify critical self-heating temperature intervals that can be treated as material properties representing fatigue durability. The project also aims to answer the question of transferring results between loading regimes by developing a model that enables proper interpretation of data obtained under ultrasonic loading. This will support the development of a composite fatigue degradation model that integrates low-, high-, and very-high-cycle fatigue into a single S-N curve, typically used to represent fatigue performance of materials. Preliminary and collaborative investigations confirmed the relevance of these studies. The project will implement adaptive fatigue testing using fuzzy controllers, enabling final validation of the methodology. From a practical perspective, the developed methodology greatly accelerates fatigue testing of thermoplastic materials while ensuring reliable results through adaptive control of the self-heating effect, thereby avoiding its undesirable influence. Another outcome is a method to determine critical self-heating temperature intervals, which can serve as a fatigue durability measure and significantly reduce the number of experiments needed to characterize composite fatigue performance and design criteria.

• DFG Programme: Research Grants
• International Connection: Poland
• Partner Organisation: Narodowe Centrum Nauki (NCN)
• Cooperation Partner: Professor Dr. Andrzej Katunin