Fatigue characteristics of stainless steel pin-reinforced composite hat joints

Z-pinned composite hat joints were tested to investigate the effects of z-pinning on the static and fatigue behaviours of the joints under pull-off loading. Stainless steel pins with 0.5-mm diameters were inserted into the overlapped area with pin areal densities of 0.25%, 0.5%, 2.0%, and 4.0%. The pin surfaces were chemically treated to increase the friction between the pins and the laminate. Test results showed that z-pinning significantly increased the static and fatigue strengths of hat joints. The static strength of the joint with a 4.0% pin density increased up to 27% compared with that of unpinned joints. The effect of z-pinning on fatigue strength was more evident. The fatigue strength of the joint at a 1,000,000-cycle loading increased up to 48% in the joints with a 4.0% pin density.