Ultrasonic welding of fully and hybrid thermoplastic composite structures

Abstract

Composite structures have been becoming increasingly popular in multiple industries such as the automotive, marine and aerospace sectors, due to its ability to be lightweight yet retaining its specific stiffness properties. Moreover, thermoplastics have been highly preferred due its resin having the ability to become softer once heated, while retaining its properties after cooling down. This makes them the perfect candidate for the ultrasonic welding joining method. Not only does ultrasonic welding provide excellent bonding between joints, it is also cost and time effective compared to other joining processes. Elium® is a novel acrylic thermoplastic resin that was utilized in this project, a first of its kind resin that that is able to cure at room temperature.

The objective of this final year project was to investigate the welding characteristics of fully and hybrid thermoplastic composites manufactured using novel resin Elium®. The thermoplastic composite parts were manufactured at laminate scales using vacuum assisted resin infusion process, and trials of ultrasonic welding were performed thereafter. Lap shear tests were then conducted in order to determine the weld strength of each of the different configurations, as well as in-depth and scanning electron microscopy (SEM) to understand the weld characteristics and failure mechanisms behind a good or bad welded samples. Samples were also adhesively joined and tested in order to compare between both joining methods.

From the results of ultrasonic welding, weld PES/EL_PES/EL had the highest maximum LSS of 4.94MPa in pure thermoplastic configurations, while weld C-UHMWPE/EL_C-UHMWPE/EL had the highest maximum LSS of 8.99MPa in hybrid thermoplastic configurations. This shows a 45% improvement in weld strength when using hybrid configurations. Moreover, most configurations from adhesive joining and ultrasonic welding joining methods show similar LSS results, with the exception of adhesive C-UHMWPP/EL_C-UHMWPP/EL. Fiber pullout, fiber imprints, rough surfaces and shear cusps were observed from failure surfaces of samples and SEM investigation, indicating strong bonds between adherends. Hybrid thermoplastic fibers are looking promising in several applications and industries. Although ultrasonic welding is challenging to use for thermoplastic fibers, it is definitely a good alternative for adhesive joining if the welding parameters are well optimized.