Energy transmission and heat accumulation mechanisms governing process robustness in continuous ultrasonic welding of continuous carbon fiber-reinforced PEEK (CCF/PEEK) composites

Continuous ultrasonic welding (CUW) is a promising technology for assembling large-scale thermoplastic composite structures; however, achieving robust joint quality remains challenging due to the complex dynamic thermal equilibrium involved. In this study, the CUW process of continuous carbon fiber-reinforced PEEK (CCF/PEEK) laminates was investigated using Response Surface Methodology (RSM), in-situ thermal monitoring, ultrasonic C-scan inspection, and instantaneous power-signal characterization. A strong interaction between vibration amplitude and welding speed was identified, showing that joint quality depends on the balance between effective energy input and thermal exposure during the moving welding process. The optimized processing condition produced cohesive failure and achieved a maximum lap shear strength of 33.14 MPa. Quantitative analysis of temperature and power curves further revealed that high nominal energy input does not necessarily lead to high joint strength. Under high amplitude and insufficient pressure conditions, intermittent acoustic contact caused hammering-related instability, in which part of the input energy was dissipated through impact-dominated mechanism rather than interfacial heating. The coefficient of variation of power, , was introduced to evaluate power-signal stability: a low corresponded to stable acoustic coupling, whereas pronounced power drops and higher values indicated intermittent decoupling. These findings demonstrate that CUW quality is governed by effective energy transmission, thermal history, and acoustic coupling stability rather than nominal energy input alone. The identified process window provides guidance for robust CCF/PEEK continuous ultrasonic welding and supports the use of power-signal monitoring for in-situ process assessment.