Degree

Doctor of Philosophy (PhD)

Department

The Department of Mechanical and Industrial Engineering

Document Type

Dissertation

Abstract

Thermoplastic composites (TPCs) have gained widespread use, particularly in large or integrated structural components, necessitating effective joining techniques. Fusion bonding (welding) has emerged as a suitable method for TPCs due to their ability to be reshaped through heating and cooling, eliminating the need for mechanical fasteners, long curing times, and extensive surface preparation. Among the welding techniques, ultrasonic welding (USW) stands out for its fast-cycling time and potential for automating large-scale structures, thereby reducing energy consumption. However, limited industrial applications of USW in this context require further knowledge to instill confidence in the process. Moreover, composite structures are susceptible to damage, making damage monitoring critical during their service life. Additionally, the interest in composite repair techniques has grown to maintain their structural integrity.

This dissertation focuses on damage monitoring of advanced TPC joints using embedded multifunctional films at the bond line. The research investigated the impact of multifunctional films on the USW process for TPC joints and their mechanical performance. The addition of 15 and 20 wt% multi-walled carbon nanotube (MWCNT) had negligible effects on the welding process, while 25 wt% led to 39% reduction in lap shear strength (LSS) due to brittleness. Furthermore, incorporating 15 wt% MWCNT increased flexural strength. Subsequently, welded joints with nanocomposite films containing optimized MWCNT concentrations (15 wt%) showed potential for damage monitoring through real-time electrical resistance changes at the weld interface under various loading conditions.

The study also developed novel repair techniques for welded TPC joints, including successive repair with neat and nanocomposite films using and ultrasonic-assisted method and one-step repair by USW and resistance heating. Neat polypropylene (PP) films resulted in a significant strength recovery of LSS after each repair cycle, while nanocomposite films restored the strength to a lesser extent. Comparing the two one-step repair methods, the ultrasonic-assisted technique yielded a stronger and more uniform repair interface than resistance heating. Importantly, nanocomposite films exhibited promise in monitoring the initiation and propagation of damage within repaired joints, even after multiple repair cycles.

This comprehensive investigation contributes valuable insights into the damage monitoring and repair of advanced TPC joints, paving the way for more reliable and efficient applications of the TPCs in structural components across industries.

Date

10-24-2023

Committee Chair

Palardy, Genevieve

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