Fracture behaviour of selectively z-pinned composite joints

This study examines the mechanical performance and damage evolution of single-step composite joints reinforced with edge-localised steel z-pins. A combined approach of experimental testing and finite element simulations is employed to evaluate the role of the pins in controlling strength and fracture processes. Results show that z-pinning significantly enhances structural capacity and damage tolerance, increasing ultimate strength and failure strain by approximately 16 % and 25 %, respectively. Both pinned and unpinned joints exhibit a similar damage sequence, initiated by resin cracking near the adherend ends followed by interfacial debonding. However, although debonding begins at similar strain levels in both joint types, the pins markedly reduce crack propagation rates. Visual and microstructural analyses show that the z-pins remain intact during loading while undergoing extensive plastic shear deformation upon final adherend separation. The FE model successfully reproduces the load-strain response and the damage evolution. The simulations show that z-pins enhance joint strength and fracture resistance through a dual mechanism: an initial shielding effect acting before the crack reaches the pins, which reduces crack driving energy by altering the local stress field ahead of the crack tip, followed by a bridging action once the crack propagates beyond the pin row.