Fiber-reinforced polymer composites have gained relevance in the aerospace industry due to their great potential for weight reduction. This, thanks to their outstanding specific properties and ability to be tailored to different applications. Developing efficient composite joining techniques is a challenge that requires great attention as it is key to a more sustainable industry. Thermoplastic composites present the great advantage that they can be joined via fusion bonding, a joining technique in which the interface is virtually erased and is generally faster than other available techniques such as mechanical fastening and adhesion bonding. A fusion bonding technology that stands out for its short processing times and cost-efficiency is ultrasonic welding, a process in which the joint is developed via low-amplitude and high-frequency mechanical vibrations that heat up the interface. Although the effect of different parameters is well known, the particular effect of the part -commonly referred to as adherend- thickness is yet not fully understood.

This thesis investigated the effect of the adherends’ thickness on the process response and weld evolution on static ultrasonic welding of CF/LMPAEK thermoplastic composites. Different characterization techniques were used to assess the latter, such as the output from the welding equipment (power and displacement), temperature readings, microscopy analysis, high-speed camera recordings, and numerical models. The results showed that increasing only the top adherend’s thickness gradually increases the heating in this adherend up to a point where significant fiber squeeze-out is observed at early stages of the process. This overheating was associated with hammering and the differences in compliance when increasing the thickness. It is hypothesized that this hammering may contribute to overheating. Welding with a higher force significantly decreased the overheating in the top adherend as hammering is reduced, whereas changing the amplitude did not show to be as influential. In contrast, increasing only the bottom adherend’s thickness did not have as much effect on the process. The dissipated power and cooling rate were the two variables that showed to be most affected, and both were associated with more bulk viscoelastic dissipation as the thickness increases. Finally, changing from fabric to UD reinforcement and welding only different top adherend’s thicknesses resulted in less hammering and fewer differences in the process response and weld evolution between thin and thick adherends, which was associated with the variation in compliance.

The findings of this study indeed contribute to the understanding of the effect of the adherends' thickness. However, more research is required to fully grasp this effect on ultrasonic welding of thermoplastic composites. For example, developing a process envelope that quantifies the thickness limitations for different process parameters and investigating the effect of the thickness in continuous ultrasonic welding are some of the next steps towards a more robust and well-understood joining technology for thermoplastic composites.