Three dimensional microscale characterization of off-axis tunnelling cracks in non-crimp fabric based composites

Microscale fatigue damage evolution of off-axis tunnelling cracks was investigated for non-crimp fabric composites used in wind turbine blades. Test specimens fabricated from a multidirectional laminate of the layup sequence [ 0 b / 60 b / 0 b / − 60 b ] s (with subscript ’b’ representing backing) with glass fibres and epoxy resin, were subjected to cyclic tension-tension loads. Following the testing, small-sized off-axis samples with sides parallel to the major −60° off-axis fibre bundle were cut from the gauge zone of the tested specimens and were investigated by X-ray computed tomography using a dye penetrant. The height of the tunnelling cracks was found to vary in both the one-bundle +60° and the two-bundle −60°2 plies of the laminate. In the presence of the resin rich pockets above or below crack-tips in the thickness direction, the cracks were observed to penetrate with twisted crack planes aligned perpendicular to the overall loading direction. In the presence of backing, the crack-tips either continued to penetrate between fibres along fibre/matrix interface or deflected at near right angles on the fibre, depending on the orientation of backing fibres. The experimental observations of crack penetration and crack deflection mechanisms were analysed with respect to a crack deflection criterion.