Electro-Mechanical Response of Ultrasonically Welded Thermoplastic Composite Interfaces under Static and Cyclic Flexural Loads Using Nanocomposites

In this work, glass fiber/polypropylene adherends were ultrasonically welded with multifunctional nanocomposite polymer films containing multi-walled carbon nanotubes (MWCNTs), embedded at the interface. Their potential for strain and damage monitoring under flexural loading for two different joint configurations was assessed via electrical resistance changes. Cyclic bending tests at various interval durations were first performed on welded single-lap joints to analyze the stability of different MWCNT weight contents (5 wt % to 20 wt %). Subsequently, 3-point bending joints (3PBJs) were welded with single- and double-film configurations and then subjected to static and cyclic loading to assess the monitoring capability through electro-mechanical responses. The 3PBJs welded with nanocomposite films led to an increase in flexural strength by 20.6%. Single-film 3PBJs showed a stable increase in resistance, but load changes corresponding to damage initiation in the specimen were not captured. Pre-cracked single-film welds and double-film welds experienced crack propagation at the welded interface, confirmed through optical microscopy, which resulted in corresponding changes in the resistance curves by up to 300%. Double-film welds were tested under low cyclic flexural loading, which translated into cyclic changes in electrical resistance for the nanocomposite film under tension. Overall, the proposed nanocomposite films show potential to enable ultrasonic welding and integrated sensing at the interface of thermoplastic composite joints.