Pull-out performance of rectangular z-pins in hot-cured carbon fiber reinforced laminates

This paper presents an experimental study on the pull-out performance of circular and rectangular z-pins in hot-cured carbon fiber/epoxy laminates. Pull-out specimens with two different ply stacking sequences (unidirectional and quasi-isotropic) were reinforced with z-pins, arranged in a 4 × 4 pattern and a volume content of 2.0%. Rectangular z-pins were found to provide much higher bridging forces in both unidirectional and quasi-isotropic laminates. Since this enhancement cannot be solely attributed to the larger surface area provided by the rectangular z-pins, optical microscopy was used to investigate the interface between the z-pins and the surrounding laminate. Results showed that a change in the debonding behavior after the post-curing cool-down period leads to higher achievable shear stresses within the pin/laminate interface, and thereby, to superior bridging loads during pin pull-out.