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

与热固性系统的组件相比，热塑性基体的纤维复合材料结构具有几个优点。在热塑性结构的情况下，有可能使用焊接工艺，以确保与材料相适应的连接，同时与机械紧固件相比，可以减轻重量。特别是电阻焊工艺是最有前途的一种变体。基于目前的研究现状，对该过程进行了实验和数值表征，并对接头的力学性能进行了一定程度的研究。然而，在材料预损伤的疲劳损伤行为和焊接工艺的影响研究方面，仍有相当大的研究需求。在自己的初步研究中，可以获得所产生的残余应力的初步知识。因此，该项目的总体目标是研究焊接热塑性FRP结构在循环加载下的损伤行为，考虑残余应力并将知识转化为扩展计算模型。为此，首先以Wöhler疲劳试验的形式进行疲劳损伤行为的实验研究，并记录损伤状态。这是通过在光纤传感器和扫描电子显微镜（SEM）记录的帮助下测量刚度退化来完成的，从而对损伤进行定性评估。在此基础上，子目标是开发一种使用有限元法（FEM）预测焊接碳纤维增强热塑性轻量化结构累积寿命的方法。在此过程中，开发和研究了将循环加载过程中的物理现象与损伤过程联系起来的材料模型。这对于缩小此类节点结构耐久性数学描述领域的研究空白具有重要意义。最后，对损伤进展机制的实验研究是考虑的重点，包括建立的模型的验证和使用光纤传感器的残余应力测量。在这里，研究残余应力在多大程度上可以被瑞利传感器记录，并与损伤过程相关，就显得尤为重要。

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.