Fracture behaviour and toughening mechanism in Zanchor reinforced composites under mode I loading

The mode I interlaminar fracture behaviour and the reinforcing mechanism of Zanchor-reinforced composite laminates were investigated by using DCB (Double Cantilever Beams) and TTT (Through-Thickness Tension) specimens. The DCB test results demonstrated that the Zanchor process was highly effective at improving the mode I fracture toughness of composite laminates, where the fracture toughness increased almost linearly with the Zanchor density. The R-curves of Zanchor composites were approximately divided into the transition and stable regions, where the extent of the transition region was about 15 mm regardless of the Zanchor density. The TTT test results demonstrated that the effect of the Zanchor process could be approximately attributed to ‘wedge effect’ and ‘bridging effect’, where the bridging effect was dominant but the wedge effect was negligible against mode I fracture. The Zanchor process was effective at improving the bridging strength but not effective at improving the maximum strength, where the trend of R-curves could be predicted by considering the bridging effect. Microscopic observations of the DCB and TTT specimens indicated that the Zanchor process induced the fibre bridgings and increased the roughness of fracture surfaces, where the EFB (Entangled Fibre Bundles) played an important role in generating the fibre bridgings, resulting in the increase in mode I fracture toughness.