Interlaminar fracture toughness of a quasi 3D braided composite

Fiber reinforced composite materials are a heavily sought after material for next generation vehicles for light-weighting components due to their high specific strength and stiffness. However, when these composite materials are manufactured into laminates, they have relatively weak interlaminar strength and are prone to delamination. This is especially the case when a delamination crack already exists. Quasi-3D (Q3D) braided composites seek to solve this issue by weaving the bias tows into the adjacent (above and below) plies. The plies are physically connected through fiber tows as opposed to being bonded simply by the epoxy, and the composite will achieve a higher interlaminar strength due to fiber failure being required for crack propagation as opposed to simply matrix failure. The [0◦/60◦/-60◦] UD and Q3D carbon composites are investigated in this study for their better in-plane isotropy. Both laminates were manufactured using the vacuum assisted resin transfer molding (VARTM) method with API SC-15 toughened epoxy. The tensile property of the Q3D composite has been found to be competitive with UD composite. Mode I and Mode II interlaminar fracture toughness tests were conducted on UD and Q3D samples. In Mode I experiments, the samples were continuously loaded to full beam split using the double cantilever beam method to obtain the fracture toughness throughout the sample. The Q3D composite shows a large increase in fracture toughness once the crack stabilized and the interlaminar tows become engaged. The Q3D composites also shows a larger energy absorption when the area under the load-extension curve is taken into account. In Mode II testing under end-notch flexural test conditions, the Q3D composite shows