The effects of through-the-thickness stitching on the Mode I interlaminar fracture toughness of flax/epoxy composite laminates

The effects of through-the-thickness stitching using natural fibres on the interlaminar fracture toughness and tensile properties of flax fibre/epoxy composite laminates were experimentally studied. Flax yarn and cotton thread were used to stitch preforms of woven fabric and unidirectional flax fibre laminates in various stitch densities. Tensile and Double Cantilever Beam (DCB) tests were conducted to characterise the in-plane tensile properties and the Mode I interlaminar fracture toughness, respectively. The effects of the in-plane fibre architecture on the delamination resistance of the composite laminates were also investigated. The results showed that the reduction in tensile properties of the composites due to imperfections caused by stitching was almost identical for both stitch materials. The DCB test results revealed that cotton thread stitch does not necessarily improve the interlaminar fracture toughness of the composite, however stitching with flax yarn can improve it by at least 10% at the lowest stitch fibre areal fraction.