FASTHER

Numerical and Experimental Investigation of the Fatigue Strength of Welded Thermoplastic FRP Structures under Consideration of Residual Stresses

Thermoplastic matrices in fiber-reinforced composite structures have increasingly become the focus of research in recent years, as they offer a decisive advantage over thermoset matrices: weldability. This offers significant potential, for example with regard to weight reduction, shorter cycle times, and the recyclability that is particularly important today. Although various welding processes are the subject of research and are already partly used in industrial applications, the fundamental operating principle of these processes is very similar. By introducing heat into the region of the intended joining zone, the matrix is melted. Under the influence of externally applied pressure on the joining partners during the cooling phase, the components are subsequently joined to form a single part. In in-house numerical simulations carried out within the framework of the JoinThis project, it has already been shown that residual stresses occur in the joining zone and its surrounding area as a result of the cooling process, leading to pre-damage of the matrix. Particularly in cyclically loaded components, this can result in potentially life-reducing pre-damage, which must therefore be taken into account in the design and dimensioning of such components.

The objective of the FASTHER project is to systematically investigate the damage behaviour of welded thermoplastic fiber-reinforced composite structures under cyclic loading while taking thermal residual stresses into account. The central research hypothesis is that a computational method for predicting the service life of welded fiber-reinforced thermoplastics can be developed based on the finite element method. Upon completion of the project, a detailed modelling approach for the fatigue damage of welded thermoplastic fiber-reinforced composite structures will be available for the first time, providing valuable insights for the computational dimensioning of aircraft structures. In addition, the understanding of the numerical modelling of fatigue in fiber-reinforced composites will be expanded, and a significant contribution will be made to methods for the experimental acquisition of validation data using fiber-optic sensors.