ANALYSIS OF STITCHED LAMINATED ENF SPECIMENS FOR INTERLAMINAR MODE-IIFRACTURE-TOUGHNESS

Two simple micromechanics based models are proposed to predict the eff ect of through-thickness reinforcement (stitching) on the improvement of delamination crack growth resistance in end-notched flexure (ENF) s pecimens. In the first model, it is assumed that stitches stretch elas tically and then rupture when the load carried approaches the failure load. In the second model, it is assumed that stitches are discontinuo us and that the stitch thread-matrix interface is completely frictiona l. Approximate closed form solutions for energy release rates are obta ined, and the effects of stitch density, matrix-stitch thread interfac ial shear stress, stitch thread diameter, volume fraction of stitches, critical energy release rate and Young's modulus are then examined. A simple design study for sizing the ENF specimen to minimise geometric nonlinear response is presented. The influences of interlaminar shear deformation and friction between the crack surfaces on the strain ene rgy release rate are examined.