Revealing the failure modes of Z-pins with varying pre-curing degrees and their influence on CFRPs

Z-pinning is an effective technique to improve the resistance of delamination crack growth in CFRP (Carbon Fiber Reinforced Polymers) laminates. The interfacial strength between Z-pin and laminates is a critical factor affecting the bridging behavior. The active functional groups on the surface of partially cured Z-pins could facilitate the co-curing interfacial bonding. In this work, Z-pin with various pre-curing degrees (1, 0.8, 0.6, 0.4) was fabricated through fitting the cure kinetics equations. The constitutive laws of partially cured Z-pin were obtained through the multiple mode bridging tests. The results indicate that decreasing pre-curing degrees enhances the Z-pin/laminate interfacial strength. The failure modes varied from the debonding at the Z-pin/CFRPs interface for fully cured Z-pins to the internal Z-pin splitting and/or the fracture in the resin-rich area for partially cured ones, resulting in a decrease in bridging traction energy. By appropriately adjusting the cure degree of Z-pins, the interlaminar fracture toughness of Z-pinned CFRPs was improved with the G_Ic increased by 307%-631%. Additionally, the secondary curing effect of Z-pins revealed that the resin accumulation within Z-pins with a too low pre-curing degree led to significant deviations and non-linear tensile behaviour of Z-pins, and it is important to ensure an initial cure degree that surpasses the gel point.