4.2.1. Tensile Test Results

Feasibility of the disassembly procedure was assessed and quantified using a tensile testing machine. The lap shear strength (LSS) from load–displacement curves was calculated using the maximum load and the overlap area (25.4 mm × 127 mm). For each MWCNT weight fraction, disassembly was initiated at three adherend's surface temper-

atures: 110 ◦C, 130 ◦C and 150 ◦C. Figure 9 summarizes the calculated LSS for all cases and the range of LSS reduction when compared to room temperature tests. It is observed that, as surface temperature increased to 150 ◦C, the required strength for disassembly was reduced by up to 94%, corresponding to an applied tensile load below 250 N. The lowest surface temperature (110 ◦C) led to a considerable reduction in LSS, but as the interface temperature likely did not reach melting point, it is not as effective as higher temperatures. At 130 ◦C, there is a considerable difference between 15 wt.% MWCNT and 20 wt.% or 25 wt.% MWCNT films. There are potentially two causes for this behavior: (1) it was observed that an increase in MWCNT content could lead to lower toughness at the interface for welded joints [36,39,40]; (2) due to the slow crosshead speed during disassembly tests (1.3 mm/min), the temperature likely continued to increase at the interface, which may have been more significant at higher MWCNT loadings.

**Figure 9.** Comparison between lap shear strength of GF/PP welded joints during disassembly procedure when surface temperature reached 110 ◦C, 130 ◦C and 150 ◦C. Interface contained 15 wt.%, 20 wt.% and 25 wt.% MWCNT. Room temperature values are used as a reference, as reported in [36].

To further investigate the joints' behavior during disassembly, the shear stress and surface temperature curves were simultaneously plotted with respect to time, as seen in Figure 10. Representative curves are shown for all weight fractions on Figure 10a–c (15 wt.%, 20 wt.% and 25 wt.% MWCNT), at one surface temperature (110 ◦C, 130 ◦C and 150 ◦C). The heat up and disassembly phases are labeled to show the duration of each one. All tests were initiated after less than two minutes (120 s), with the fastest heat up phase for 25 wt.% MWCNT/PP films. Similarly, the disassembly phase generally lasted less than two minutes. As the applied load increased at the beginning of the disassembly phase, the surface temperature, and by extension, interface temperature, continued to increase as well because the contact at the weld line was not yet severed. However, after failure initiation (at the stress peak), the temperature slowly started to decrease as the integrity of the interface was compromised, leading to fewer conductive paths between MWCNTs. Since the applied voltage was kept constant throughout the disassembly procedure, the temperature consequently decreased. In some cases, as seen in Figure 10c, the disassembly phase displayed an inconsistent stress curve. One possible explanation is that, upon closer inspection of the specimens after disassembly, a small crack defect along the direction of the fibers was found in the adherends at the overlap. As all adherends were visually inspected after welding and no such defect was detected, it is reasonable to assume the crack was created during the disassembly process, likely explaining the inconsistent curves

**Figure 10.** Representative lap shear stress curves (solid lines) of GF/PP welded joints during disassembly procedure with interface containing (**a**) 15 wt.% MWCNT at adherend's surface temperatures of 110 ◦C; (**b**) 20 wt.% MWCNT at adherend's surface temperatures of 130 ◦C; and (**c**) 25 wt.% MWCNT at adherend's surface temperatures of 150 ◦C. Corresponding surface temperature curves are shown as dashed lines.
