4.2.2. Fractographic and Heat-Affected Zone Analysis

The fracture surface of the disassembled joints was visually observed after the procedure (Figure 11). Samples are shown as they were immediately after the tests; the welds were not manually separated to avoid influencing the appearance of the fractured joints. The red dashed lines indicate the location of the overlap under heating. Comparison with fracture surfaces at room temperature is shown on the right-hand side. Due to the temperatures reached during disassembly, all welds exhibited ply squeeze out (fiber squeeze out with polymer), as more clearly observed right above the upper dashed line in Figure 11a (110 ◦C and 130 ◦C), Figure 11b (110 ◦C), and Figure 11c (130 ◦C and 150 ◦C). The visible fracture surfaces exhibit a combination of cohesive and substrate failure modes: intralaminar failure through upper plies, broken fibers in the GF/PP adherend, and failure within the nanocomposite film. The MWCNT/PP films melted at the interface, with the most visible examples marked by the red circled areas in Figure 11b,c.

Figure 12 shows SEM micrographs of fracture surfaces from Figure 11b to further analyze microstructure. Welds fractured at room temperature (Figure 12a) mostly displayed broken fibers from the upper plies of the GF/PP adherends, with matrix-fiber debonding and some matrix torn from the fibers' surface. When disassembly temperature increased from 110 ◦C to 150 ◦C (Figure 12b–d), fracture surfaces also showed bare, broken fibers, but the thermoplastic matrix exhibited severe softening and drawing behavior with areas that underwent melting and separation from the fibers. As observed in Figure 11b,c, the presence of melted nanocomposite film was confirmed (left-hand SEM images in Figure 12c,d). The porous morphology was shown to be characteristic of the PP matrix under strain [55]. Overall, disassembly at high temperature was facilitated by a combination of melted nanocomposite film at the interface, matrix softening in the adherends, and fiber– matrix debonding.

An indication of the extent of the heat-affected zone through the GF/PP adherends' thickness is their change in color and opacity (between the dashed, red lines). The polypropylene matrix became transparent at its melting point (150 ◦C), which is more clearly recognized in Figure 11b at 130 ◦C and 150 ◦C. The heat-affected area was quantified using image analysis and is reported in Figure 13 for three images under each parameter combination. The HAZ area increased by up to 97% with MWCNT content and with initial surface temperature.
