The Effect of PA66 Nanofibrous Interlayers on Mode II Delamination Behavior of Filament-wound CFRP Laminates at Room and Cryogenic Temperatures

This work aims to assess the potential of commercially available PA 6,6 nanofibrous mats when incorporated to large scale filament winding process. The conventional wet winding process was employed on a specially designed flat mandrel to manufacture uni-directional composite laminates. A49-12 K carbon fibers and cryogenic-compatible CTD 7.1 epoxy resin was employed. The winding process was temporarily paused at the mid-plane thickness to introduce a pre-crack using a 12 μm non-adherent film and to place PA66 nanofibers with an aerial weight of 3 g/m². The winding process then resumed. Laminate curing was performed in an autoclave oven for 3 h at 80^oC under nitrogen environment. Flat wound laminates were then cut into end notched flexure (ENF) test samples in accordance with ASTM D7905/D7905M-19. ENF tests were performed at room temperature (RT) and cryogenic conditions in a liquid nitrogen bath. Test results suggested that mode II strain energy (G_IIc) of interlayered laminates were 35% higher than the one of neat laminates when tested at room temperature. On the contrary, addition of polymeric nanofibrous interlayers reduced G_IIc by 40% in cryogenic conditions. Fractographic analysis suggested that the improvement at RT was primarily due to (i) toughening at the resin rich pockets inherent by the tow-undulation effect in wet winding (ii) crack deflection in irregular tow-tow interfaces. The reduction in G_IIc was attributed to synchrony of several factors, namely dominance of fiber/matrix debonding due to thermal contraction at fiber/resin interfaces, elevated brittleness of the polymeric nanofibers and pre-mature cracking due to nanofiber/resin debonding.