Online evaluation of ultrasonically welded carbon fiber reinforced thermoplastic laminates by means of infrared vibrothermography

Recent advances in thermoplastic matrix composite manufacturing are enabling their increased use in primary structural applications, owing to advantages such as processability, weldability, and recyclability [1]. Among the available joining techniques, ultrasonic welding is particularly attractive for producing high-quality joints with short cycle times and high repeatability. However, its adoption in primary structures requires reliable and efficient non-destructive testing (NDT) methods to ensure weld integrity. This work examines ultrasonic spot welding (UW) of thermoplastic composite laminates in a sequential, online process. Welding is achieved using sonotrodes that apply pressure and high-frequency vibrations to promote interfacial bonding, assisted by a thermoplastic energy director layer. The proposed IR-NDT approach reuses the same welding setup and sonotrode to periodically apply lower-energy vibrations, generating a thermal response suitable for infrared-based online NDT inspection. The method is inspired by conventional Vibrothermography IR-NDT [2], where mechanical excitation produces thermal signatures that reveal defective zones. The application in the UW context is found to provide signatures allowing the identification of welded area, as well as the assessment of weld quality. Partially welded regions exhibit localized heating, while fully consolidated welds appear colder due to more efficient heat dissipation between adherends [3].