MULTIFUNCTIONAL FILMS FOR FUSION BONDING AND STRUCTURAL HEALTH MONITORING OF THERMOPLASTIC COMPOSITE JOINTS

摘要

Ultrasonic welding is a promising method to quickly bond thermoplastic composite parts. In this process, a thermoplastic film, called an "energy director", is placed between two pieces of composite materials, and melted during the welding process to facilitate bonding. This study seeks to develop a multifunctional energy director suitable for three purposes: 1) ultrasonic welding, 2) structural health monitoring (SHM) via electrical resistance measurements, and 3) heating element for disassembly and repair of the damaged bond. Various materials and manufacturing methods were investigated to develop multifunctional films: 1) magnetic or conductive nanoparticles deposited onto polymer films, and 2) multi-walled carbon nanotubes (MWNTs) dispersed into polymers. The latter was selected as a promising candidate because of its ease of manufacturability through compression molding and its compatibility with the welding process. The thermo-electrical and electro-mechanical behavior of compression molded films containing up to 20 wt% MWNT in a polypropylene (PP) matrix was assessed via a sourcemeter and dynamic mechanical analyzer. To demonstrate multifunctionality, glass fiber/PP specimens were ultrasonically welded with the MWNT/PP films as energy directors. While the multifunctional films reduced lap shear strength by up to 10%, they did not significantly affect the welding process. Moreover, welded specimens with the MWNT/PP films were successfully tested under cyclic bending for SHM through resistance measurements.