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

具有热塑性基体的纤维复合材料结构与具有热固性系统的部件相比具有若干优点。在热塑性结构的情况下，有可能使用焊接工艺，该焊接工艺确保适合于材料的连接，同时与机械紧固件相比能够减轻重量。电阻焊接工艺尤其是最有前途的变体之一。由于它的结果从目前的研究状况，该过程的特点是实验和数值模拟，并在一定程度上的接头的力学性能进行了研究。然而，仍然有相当大的研究需要在调查的疲劳损伤行为和影响的焊接工艺有关的预损伤的材料。在自己的初步研究中，可以获得有关残余应力的初步知识。因此，本项目的总体目标是研究焊接热塑性FRP结构在循环载荷作用下的损伤行为，考虑残余应力，并将知识转移到扩展的计算模型中。为此，首先以Wöhler疲劳试验的形式对疲劳损伤行为进行了实验研究，并记录了损伤状态。这是通过测量刚度退化的帮助下，光纤传感器和扫描电子显微镜（SEM）记录的定性评估的damage.Building在此基础上，一个子目标是开发一种方法的累积寿命预测焊接碳纤维增强热塑性轻质结构使用有限元法（FEM）。在此过程中，正在开发和研究一种材料模型，该模型将循环加载期间的物理现象与损伤的进展联系起来。这使得一个显着的贡献，以关闭在该地区的数学描述的结构耐久性等joints.Finally，损伤进展机制的实验研究是重点的考虑，其中包括所创建的模型的验证和使用光纤传感器的残余应力的测量。因此，研究瑞利传感器能够记录到的残余应力与损伤过程的关系就显得尤为重要。

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.