Comparative study of metal and composite z-pins for delamination fracture and fatigue strengthening of composites

The influence of the material properties of z-pins on the mode I delamination properties of carbon-epoxy laminates is investigated. Improvements to the delamination fracture toughness and fatigue resistance of laminates reinforced in the through-thickness direction with z-pins made of metal (copper, titanium or stainless steel) or unidirectional carbon fibre composite is determined. Irrespective of the material, the z-pins are highly effective at increasing the fracture toughness and fatigue resistance by forming a large-scale bridging zone along delamination cracks. However, the fracture toughening and fatigue strengthening capacity of the z-pins is strongly dependent on their material properties, and increased in the order: copper (least effective), titanium, stainless steel and carbon fibre (most effective). The fracture and fatigue failure modes of the z-pins are also dependent on their material properties. The study reveals that improvements to the mode I fracture toughness and fatigue resistance can be optimized via a judicious choice of the z-pin material. Z-pins with high stiffness, strength and fatigue resistance (such as carbon fibre) provide the greatest improvement to the delamination toughness and fatigue strength.