Fatigue behavior of ultrasonically welded CF/PPS single-lap joints compared with bulk PPS using entropy-based finite element analysis

In this study, carbon-fiber-reinforced polyphenylene sulfide (CF/PPS) laminates were joined by ultrasonic welding using a PPS interlayer to form single-lap joints, and their temperature-dependent fatigue behavior was investigated within an entropy-based fracture framework. A fatigue damage model based on entropy generation was implemented in finite element analysis (FEA), and the responses of bulk PPS and welded joints were compared in relation to local stress concentration and viscoelastic energy dissipation. Static tensile and tensile fatigue tests conducted at multiple temperatures were used to evaluate the model and to examine the effect of temperature on fatigue life, damage localization, and fracture characteristics. The numerical predictions showed good agreement with the experimental results, supporting the applicability of the proposed approach. Although the welded joints exhibited lower fatigue lives than bulk PPS, the main distinction was not limited to the fatigue-life level itself but was also reflected in the different S–N slopes and fracture morphologies, indicating distinct fatigue-damage accumulation mechanisms. These results suggest that the fatigue behavior of welded joints cannot be interpreted solely from bulk material properties and that local entropy generation associated with joint geometry and temperature-dependent dissipation plays an important role in governing fatigue failure.