Characterization of Multifunctional MWCNT/PP Nanocomposite Films for Use in Structural Health Monitoring and Disassembly of Ultrasonically Welded Single Lap Joints

Abstract

In this study, multi-walled carbon nanotubes (MWCNT) nanocomposites were characterized and used as energy directors (EDs) in ultrasonic welding (USW) to test their viability as embedded sensors for structural health monitoring (SHM) and as heating elements for disassembly of thermoplastic composite joints. Three MWCNT loadings were used in this study to manufacture MWCNT/polypropylene (PP) nanocomposites, 15, 20, and 25 wt% while only one MWCNT loading was used to manufacture MWCNT/Nylon-6 (PA-6) nanocomposites, 15 wt%. The masterbatch pellets were compression molded into 0.06, 0.25, and 0.50 mm thick films for characterization and welding. For the purposes of using these MWCNT nanocomposites as EDs in USW for SHM and disassembly, the parameters that were investigated were the electrical conductivity, resistive heating performance, melting temperature, specific heat, storage modulus, loss modulus, and gauge factor. Electrical characterization showed that the quenched 15 wt% MWCNT/PA-6 nanocomposites had the lowest but most consistent conductivity while the air cooled MWCNT/PP nanocomposites showed less ohmic behavior and increasing conductivity with increasing MWCNT loadings. Resistive heating characterization showed that more conductive films could reach higher temperatures under similar voltages. Results from differential scanning calorimeter (DSC) analysis showed that the melting temperature and specific heat of the as received and post processed nanocomposites did not significantly vary with MWCNT loadings. The storage and loss moduli both increased with higher MWCNT loadings, until the loading reached 25 wt%, whereupon both moduli behaved more similarly to that of the pure polymer. Electrodes were taped to the steel grips of the dynamic mechanical analyzer (DMA) to measure the gauge factor (GF) of each xii nanocomposite in tension. After using the PP nanocomposites as EDs in USW, their GF was measured as the single lap specimens were pulled apart via tension. The GFs of the nanocomposites in the weld interface were approximately two orders of magnitude lower than the films in tension, indicating that the nanocomposite films in their current form are not yet suitable as strain sensors. However, the resistance signal of the nanocomposite films in the weld interface could detect damage.