Tension fatigue of fibre-dominated and matrix-dominated laminates reinforced with z-pins

The effect of the fibre pattern on the tension fatigue properties of z-pinned composite laminates is experimentally investigated. Carbon-epoxy laminates with fibre-dominated ([0]20 and [0/±45/90]5s) and matrix-dominated ([±45]10s) ply patterns were reinforced in the through-thickness direction with thin z-pins. The tensile elastic modulus of the laminates was not affected significantly by z-pins, however their tensile strength decreased at a linear rate with increasing pin content. The tensile fatigue life of both the fibre-dominated and matrix-dominated laminates was reduced by z-pins, although the mechanisms degrading the fatigue performance are different. The fatigue life of the fibre-dominated laminates was reduced by microstructural damage caused by z-pins, particularly fibre breakage, fibre waviness and resin-rich zones. The fatigue performance of the matrix-dominated laminate was degraded by cracking around the z-pins. The study reveals that the tensile fatigue life of carbon-epoxy is always reduced by z-pins regardless of the fibre pattern. However, the magnitude of the reduction in fatigue performance of z-pinned laminates is dependent on their fibre pattern.