Quasi-static mode II fracture tests and simulations of Z-pinned woven composites

• The mode II fracture toughness of Z-pinned Woven composites was determined.
• There is a critical Z-pin density which maximized fracture toughness.
• A cohesive zone model was implemented to explain the observed experimental results.
• Z-pinning is a viable way to elevate through-the-thickness strength of woven textile composites.

Experimental results from quasi-static mode II ENF (end notch flexure) fracture tests for Z-pinned woven composites are used to investigate the influence of Z-pin density and Z-pin diameter on Mode II fracture toughness. The experimental results show that the 2% Z-pin composite and the 3% Z-pin composite have larger fracture toughness than the 1% Z-pin composite and a composite without Z-pins. Crack propagation changes from unstable propagation to stable propagation (crack arrest) as the Z-pin density increases. The additional fracture toughness provided by Z-pins prevents the primary crack to further propagate, enabling secondary and tertiary cracks to consume additional energy. While the primary crack is held from propagating by the frictional forces provided by Z-pins, the matrix in the composite is driven into the plastic state and enables the composite to absorb more energy. For numerical simulations, a discrete cohesive zone model (DCZM) is used to perform simulations. The simulations in conjunction with experimental results provide critical fracture toughness values.