The strength of composite bonded T-joints transversely reinforced by carbon pins

In this paper, an improved ultrasonically assisted z-pinning (UAZ) process was adopted that uses an acrylic board with holes as a mould to hold the pins instead of a polymer foam carrier. Using the process, 29 carbon/epoxy composite T-joints with three different areal pin densities and three different pin diameters were fabricated and tested in pull-off loading. The test results show that the strength of the composite T-joints can be increased by the z-pinning technique and that it is directly related to the total contact area between the pins and the laminate. The ultimate strength of the joint with a 4.0% pin density and 0.5 mm diameter pins increased by more than 70% compared with the unpinned joints. It was also found that there was a minimum critical condition required for the pins to improve the performance of the joints. In all of the tested joints, the first crack occurred in the intersection region or at the ends of the flanges.

► Failure of composite T-joints reinforced by carbon z-pins is investigated.
► First crack occurs in the intersection region or at the ends of the flanges.
► Major failure mode is delamination in the skin.
► Pull-off strength is dominated by the total contact area between pins and laminate.
► Z-pins increases the joint strength by up to 70% compared with the unpinned joint.