Numerical and experimental investigation of the fatigue strength of welded thermoplastic FRP structures under consideration of residual stresses (FASTHER)

• Applicant: Professor Dr.-Ing. Sebastian Heimbs, since 9/2022
• Subject Area: Lightweight Construction, Textile Technology ; Engineering Design, Machine Elements, Product Development
• Term: since 2021
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 470592126

Fiber composite structures with a thermoplastic matrix offer several advantages compared to components with thermoset systems. In the case of thermoplastic structures, there is the possibility of using welding processes that ensure joining that is appropriate to the material and at the same time enable weight savings compared to mechanical fasteners. The resistance welding process in particular is one of the most promising variants. As it results from the current state of research, the process was characterized both experimentally and numerically, and the mechanical properties of the joint were investigated to a certain extent. However, there is still a considerable need for research in the investigation of fatigue damage behavior and the influence of the welding process concerning pre-damage to the material. In own preliminary studies, initial knowledge of the resulting residual stresses could be gained. The overall goal of the project is therefore to research the damage behavior of welded thermoplastic FRP structures with regard to cyclic loading, taking into account residual stresses and transferring the knowledge to an extended calculation model.For this purpose, experimental studies on the behavior of fatigue damage are first carried out in the form of Wöhler fatigue tests, and the damage status is recorded. This is done by measuring the stiffness degradation with the help of fiber optic sensors and scanning electron microscope (SEM) recordings for the qualitative assessment of the damage.Building on this, a sub-goal is to develop a method for the cumulative lifetime prediction of welded carbon fiber-reinforced thermoplastic lightweight structures using the finite element method (FEM). In the course of this, a material model is being developed and researched, which relates the physical phenomena during cyclical loading to the progress of damage. This makes a significant contribution to closing the research gap in the area of the mathematical description of the structural durability of such joints.Finally, the experimental investigation of the damage progression mechanisms is the focus of the considerations, which includes both the validation of the created model and the measurement of the residual stresses using fiber optic sensors. Here it is particularly important to research the extent to which the residual stresses can be recorded by the Rayleigh sensors and related to the damage progress.

• DFG Programme: Research Grants
• Co-Investigator: Dr.-Ing. Carsten Schmidt
• Ehemaliger Antragsteller: Professor Dr.-Ing. Peter Horst, until 8/2022