Z-Pin composites: Aerospace structural design considerations

Z-pins are increasingly used to enhance the delamination toughness and impact damage tolerance of composite aircraft structures. An important consideration in the design of z-pinned structures is the deterioration of the in-plane mechanical properties of the composite material because of the pins. Experimental property data presented in this paper reveal that large improvements to the delamination toughness of carbon-epoxy composite gained with z-pins also result in an unavoidable reduction to the in-plane tension, compression, bending, interlaminar shear, and fatigue properties. The data show that increasing the volume fraction of z-pins in carbon-epoxy to increase the delamination resistance causes a corresponding deterioration to the in-plane properties, and this is a key consideration in the design of z-pinned aircraft structures for damage tolerance. The data reveal that the reduction to the in-plane mechanical properties caused by z-pins is usually modest (typically less than 5-15%) compared to the very large improvements in delamination toughness (up to nearly 500%).