Mode I Interlaminar Fracture Toughness of Natural Fiber Stitched Flax/Epoxy Composite Laminates – Experimental and Numerical Analysis

The mode I interlaminar fracture toughness of stitched and unstitched composite laminates of continuous UD flax fiber and epoxy matrix was experimentally and numerically investigated. A twistless flax fiber yarn was used for stitching purpose. The mode I interlaminar energy release rate of the unstitched and stitched laminates was experimentally determined through Double Cantilever Beam (DCB) test. Tensile test was conducted to characterize the tensile properties of the dry and resin impregnated forms of the stitch yarn. A 3D Finite Element (FE) model of DCB specimen to analyze delamination propagation of stitched flax fiber laminate was developed using cohesive element with nonlinear softening law and embedded 2-node beam element. The out-of-plane flax yarn stitching was found to generate a twofold increase in the delamination resistance of the composite laminate at a medium stitch density. The FE analysis results agreed well with the experimental results, where a good fit between the predicted and experimental R-curves were achieved.