Improved Resistance to Mode I/ <scp>II</scp> Fracture of Carbon/Epoxy Laminates Reinforced by U‐Shaped z‐Pins and Carbon Nanotubes

ABSTRACT U‐shaped z‐pins are designed to improve the fracture toughness of composite laminates with integrations of carbon nanotubes (CNTs). Laminates with only U‐shaped z‐pins, with both U‐shaped z‐pins and carbon nanotubes, as well as blank laminates without reinforcement, are designed. Mode I delamination and Mode II delamination tests are conducted. Mode I delamination tests show that U‐shaped z‐pins can greatly improve fracture toughness, and significant R‐curve behavior is exhibited. Compared to blank specimens, the introduction of only z‐pins and the simultaneous implantation of z‐pins and carbon nanotube films increase the fracture toughness by 413% and 279%, respectively. Mode II delamination tests show that carbon nanotube films significantly improve fracture toughness. Compared to blank specimens, the introduction of only z‐pins and the simultaneous implantation of z‐pins and carbon nanotube films increase the fracture toughness by 35% and 141%, respectively. In addition, the delamination failure mechanism is revealed through SEM analysis of the fracture surface. The cohesive zone model based on the bilinear constitutive relation and integrating bridging resulted in an R‐curve used to simulate delamination propagation with the synergistic effect of U‐shaped z‐pins and carbon nanotubes. Good consistency between simulations and test results indicates the applicability of the established numerical model.