Delamination properties and in situ damage monitoring of z-pinned carbon fiber/epoxy composites

Carbon-fiber-reinforced composites with layer stacking structures are sensitive to delamination crack. To improve the interlaminar properties and further explore the capability of in situ damage monitoring of the laminate, CCF300 and CCF800 carbon fiber pins were selected to fabricate the z-pinned composites. Compared with the control sample, the GIC values of CCF300 and CCF800 z-pinned composites are increased by 398 and 378%, respectively. This indicates that the delamination resistance improvement of the laminates is dominated by the z-pin debonding and pull-outs. The electrical resistance of the laminates was utilized to in situ monitor the crack propagation within the composite laminate. The results show that the presence of the z-pins enhances the sensitivity of damage detection. The ΔR/R0 of CCF800 z-pinned composite is nearly three times that of the control sample at the moment the crack length reaches 110 mm. Crack length-displacement curves were obtained according to the relationship between ΔR and Δa, which clearly displayed the steady or stick-slip crack growth of the laminates with or without z-pins, respectively. Visualization of the crack growth process can provide a novel method for the delamination failure analysis of the composite.