Cross-scale finite element analysis of machined CF/PEEK: Evaluating the impact of interfacial damage and crystallinity on frictional performance

The carbon-fiber-reinforced-thermoplastic composites (CFRTPs) are widely used in the aerospace industries due to their enhanced toughness as well as improved reparability and recyclability. The surface damages and defects generated in the manufacturing process directly affect the mechanical properties of CFRTPs, leading to a degradation in the wear resistance of the machined surface, consequently affecting the service performance of the jointed structures. Accurately predicting the mechanical properties of machined surfaces has always been an essential and challenging goal, especially for composites with heterogeneous properties. This work introduces a novel framework for predicting the mechanical performance of machined surfaces, effectively bridging the gap between the machining process and the in-service behavior of CFRTPs. For the first time, a multi-scale finite element analysis (FEA) method is proposed to evaluate the friction properties of CF/PEEK machined surfaces, incorporating the effects of machining-induced damage and crystallinity variation. Firstly, 3D microscale FEA model of UD-CF/PEEK milling was established to analyze the effects of different parameters on the fiber–matrix interfacial damage. Diverse loading conditions were applied to the structural representative volume element (RVE) to investigate the effects of interfacial damage and crystallinity variation on the macroscopic elastic properties of UD-CF/PEEK. Finally, the friction properties of the machined surface were investigated using a macroscopic FEA model, exploring the effects of machining-induced damage and crystallinity on the machined surface of UD-CF/PEEK. It is found that the mechanical properties of the machined surface are influenced by machining-induced interfacial damage and crystallinity variation. While increased interfacial damage degrades mechanical performance, higher crystallinity enhances it. Notably, increasing the equivalent cutting depth intensifies subsurface interfacial damage, which has a more significant effect on friction and wear than the beneficial influence of crystallinity.