Comparative study of the mode I and mode II delamination fatigue properties of z-pinned aircraft composites

This paper compares improvements to the mode I and mode II delamination fatigue resistance of an aerospace composite material achieved by z-pin reinforcement. Mode I (cyclic crack opening) and mode II (cyclic crack sliding) interlaminar fatigue tests were performed on a carbon fibre reinforced epoxy composite reinforced in the through-thickness direction with different volume contents and diameters of z-pins. Paris curves obtained from displacement-controlled fatigue tests reveal that z-pins are more effective at resisting the initiation and growth of delamination cracks under mode I than mode II conditions. Both the mode I and II fatigue resistance increase with the z-pin content due to the formation of a large-scale extrinsic crack bridging toughening zone, although fatigue strengthening is greater for mode I. Improvements to the mode I and mode II delamination fatigue resistance are also dependent on the z-pin diameter. Similarities and differences in the mode I and mode II fatigue properties are related to the fatigue strengthening mechanisms induced by z-pins for the two load conditions.