Improving the structural properties and damage tolerance of bonded composite joints using z-pins

The effect of z-pin reinforcement on the structural properties and damage tolerance of composite joints is experimentally studied in this article. The ultimate load, failure strain, and absorbed energy capacity of carbon fiber-epoxy T-joints and single lap joints increased rapidly with the volume content of z-pins. The improvement to these properties was due to the capacity of z-pins to resist unstable (rapid) delamination growth along the bonded region to the T-joints (induced by interlaminar mode I stress) and lap joints (induced by interlaminar mode II stress) by forming a crack bridging traction zone. Large increases to the damage tolerance of T-joints and lap joints containing pre-existing bond-line cracks were also achieved by z-pins. The z-pins formed a bridging traction zone across the delaminated bond-line which retained high strength in the joints, even in the presence of very long cracks which severely weakened conventional joints.