The demand for Carbon Fiber Reinforced Thermoplastic (CFRTP) composites in advanced engineering applications is increasing, particularly in the aerospace and automotive industries. CFRTP has accelerated the development of efficient, high-strength joining techniques. Ultrasonic welding has gained attention as a promising joining method for CFRTP due to its short welding times, ease of automation, and ability to maintain material integrity without additional adhesives.

On the other hand, the integration of thermosetting and thermoplastic composites, such as CF/epoxy and CF/PEEK, is becoming increasingly important for applications that require both high thermal resistance and rapid, efficient joining capabilities. Several studies have reported that a strong and reliable joint between CF/epoxy laminates and CF/PEEK laminates can be achieved through a co-cured gradient interphase between epoxy and a polyetherimide (PEI) thermoplastic layer.

Our group has proposed a method for rapidly manufacturing skin-rib panels by joining CFRTP pultruded square rods to CFRTP laminates. This method has a problem where the square rods and laminates may melt and deform during the welding process. Therefore, the approach of joining epoxy and PEEK through a PEI layer may allow for joining without melting the base materials, which could significantly improve quality.

This study aims to extend the applicability of thermosetting and thermoplastic composite structures by using a welding layer composed of carbon nanotube-doped polyetherimide (CNT/PEI) to enhance the adhesion between CF/Epoxy and CF/PEEK laminates or square rods. We evaluated the mechanical property of ultrasonic welding with this CNT/PEI layer. Additionally, functionality assessments were conducted to evaluate the joint’s capability for monitoring the welding quality and detecting damage through electrical resistance change methods.