Improving the mode I delamination fatigue resistance of composites using z-pins

This paper presents an experimental investigation into the mode I interlaminar fracture toughness and fatigue resistance of carbon fibre–epoxy laminate reinforced in the through-thickness direction with z-pins. The effects of the volume content, diameter and length of z-pins on the interlaminar toughness, fatigue resistance, and crack bridging toughening mechanisms are determined. The delamination crack growth rate under cyclic interlaminar loading slowed rapidly with increasing z-pin content up to a limiting concentration (∼2% by volume), above which no further improvement to the fatigue resistance was achieved due to a transition in the fatigue cracking process. The delamination growth rate also slowed when the z-pin diameter was reduced or the z-pin length was increased. The dependence of the fracture toughness and fatigue resistance on the volume content, diameter and length of the z-pins is related to the crack bridging toughening processes induced by the z-pins.