The use of pre-hole Z-pinning (PHZ) method to improve the impact resistance and post-impact flexural performance of composite skin/stringer joints

Carbon fiber reinforced plastic (CFRP) composites have been widely used in the last thirty years. The main issue in using CFRP composite joints is the bond strength between skin and stringer. This paper presents the results of an experimental investigation into the effect of pre-hole z-pinning technique (PHZ) on the impact resistance and post-impact flexural properties of skin/stringer composite joint with different z-pin configuration. Unpinned and pinned carbon/epoxy specimens are subjected to low-velocity impacts resulting in barely visible damage. The responses to impact of the laminates are characterized in terms of damage evolution, load-carrying capacity, and energy absorption. It is found that z-pinning maintains the integrity of the skin/stringer structure, and it improves the impact resistance and is capable of delaying the onset of delamination. Z-pin shifts the impact damage from interface debonding to matrix cracks on the impacted surface. Z-pin significantly improve the residual flexural properties. Skin crush occurs to the specimens with the highest z-pin Vol., as the bonding strength gets so improved that the weakness of the joint transfers from interface bonding strength to skin in-plane strength. These results can provide some guidance for designing z-pin reinforced joints.