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 hotcured 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 4x4 pattern and a volume content of 2.0%. Rectangular z-pins were found to provide much higher bridging forces in both unidirectional and quasiisotropic 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.