Composite are now widely used in the aircraft industry. The use of light-weight, high-temperature resistant composite materials will allow for aircraft to have high-performance and economical designs. The composites used in the aircraft industry can be classified into two types: Thermosets and thermoplastics. Thermosets are polymers that are obtained by curing, where there are chemical cross-links between the polymer chains. Wings, fuselages, and bulkheads are some applications in aerospace which are made of thermoset composites. The interest in and the application of thermoplastic composites is increasing in the aerospace industry due to their advantages over thermoset composites. They have the advantage that they can be re-molten and reshaped upon heating, that they are recyclable, and have a high material toughness. The first advantage can lead to major cost-benefits since it makes efficient forming and welding techniques possible. As a result, different welding techniques have been developed for joining thermoplastic composite structures such as resistance, induction, and ultrasonic welding. Ultrasonic welding is generally the fastest and most energy-efficient welding technique.

This thesis explores the continuous ultrasonic welding (CUW) of unidirectional carbon fiber-reinforced thermoplastic composites, with a focus on the impact of adherend thickness on the welding process and joint quality. As the demand for lightweight, high-performance materials in industries such as aerospace and automotive continues to rise, efficient and scalable welding techniques for thermoplastic composites are becoming essential. CUW presents a promising method for joining large composite structures, offering speed and energy efficiency advantages.

Through experimental investigations, this study evaluates the effects of increasing adherend thickness on defects such as overheating and fiber squeeze-out. Two different clamping setups, the bar clamp and picture frame jigs, were used to assess their impact on weld quality. It was concluded that the welds made on the bar-clamped jig overall had a much higher energy requirement compared to those made on the picture frame jigs. The picture frame jig, due to its lack of constraints on the thick adherends, resulted in much better parallelism throughout the process. The welds made using the picture frame jig also experienced less overheating at the interface compared to the bar-clamped jig. The resulting weld uniformity and strength were also higher when using this jig. However, one issue with the picture frame jig was the extent of top surface scraping, which was found to be prevalent regardless of the weld speed. The changes in process parameters were also studied. Contrary to findings from static welds, increasing the welding force resulted in through-the-thickness heating when the thickest adherend was welded and did not have much effect on the weld strength of the thickest adherend on the picture frame jig. The findings from this thesis result in the need for further research to mitigate the effects of the thicker adherends.