The effect of stitch distribution on Mode I delamination toughness of stitched laminated composites – experimental results and FEA simulation

In this paper, a tabbed double cantilevered beam test is performed on Vectran® stitched laminated composites with varying stitch distributions. The woven carbon fibre plies were laid-up in a symmetrical manner [0, ±45, 90]2, then stitched in dry preform state followed by a resin film infusion stage. To prevent the stitched specimens from failing in flexure, thick aluminium reinforcing tabs were adhesively bonded to either side of the composite specimen, along with loading blocks. An INSTRON 5567 testing machine was used to test the specimens at a crosshead speed of 0.5 mm/min with crack lengths, crack opening displacements and peak load data being recorded after the crack front had propagated several millimetres. A variation of the compliance method was used to post process the test data. It is revealed that for stitched specimens with identical stitch densities but different stitch distributions, there exists significant variations in critical strain energy release rates, GIc. A 2-node beam and 4-node plate finite element analysis was conducted to model the TDCB experiment with a special 2-node axial, tension only, rod element used to discretely simulate the stitches. The FEA results support the experimental findings identifying stitch distribution as a key parameter in determining the improvement that stitching provides in bridging Mode I delaminations.