Experimental assessment of the damage tolerance of z-pinned T-stiffened composite panels

Stiffened composite panels used in aircraft, buildings and other light-weight structural applications are susceptible to unstable bond-line cracking that results in poor damage tolerant properties. This paper presents an experimental study into the improvement to the damage tolerance of T-shaped stiffened carbon fibre/epoxy panels when reinforced with thin z-pins. Stiffener pull-off tests revealed that the ultimate failure load and absorbed energy capacity of the skin-to-stiffener flange connection to the panels increased rapidly with the volume content of z-pins due to suppression of bond-line cracking. Improvements to the ultimate load and absorbed energy capacity of over 75% and 600%, respectively, was achieved when the skin–stiffener flange connection to the panels were reinforced at the relatively modest z-pin content of 4% by volume. Experimental analysis also showed that z-pins were highly effective at improving the damage tolerance of T-stiffened panels that contained a single bond-line delamination crack or multiple cracks along and near the bond-line caused by impact loading. The residual mechanical properties of damaged z-pinned panels were much higher (typically in the range of 100–500%) than of the unpinned panel, and based on this research z-pinning is shown to be an effective technique for increasing the damage tolerance of T-stiffened composite panels for load-bearing structural applications.