Low velocity impact performance of stitched flax/epoxy composite laminates

This paper presents an experimental investigation of the effect of through-thickness natural fibre stitches on the low-velocity impact response of the woven flax/epoxy composite laminates with the purpose of extending their use to higher performance applications. Two impact energy levels were selected to produce a perforated and a non-perforated damage state in stitched/unstitched composites for study. Twistless flax yarn and twisted cotton thread were used to stitch at an equivalent stitch areal fraction for all laminates of the same thickness. Unstitched cross-plies [0/90]4s of continuous flax fibres were also manufactured at a similar thickness for benchmarking of energy absorption and fracture mechanisms. Comparison of the damage sustained in the unstitched and stitched natural fibre composites showed that while delamination was not the predominant damage mode in both laminates, stitching does facilitate the propagation of in-plane cracks. The experimental findings revealed that stitching with thicker yarn (Flax) led to a lower ratio of absorbed energy per area of damage as well as energy absorbed for full penetration.