A novel double cantilever beam test for stitched composite laminates

Through-the-thickness stitching dramatically improves the interlaminar frac ture toughness of laminated composites. Currently the Double Cantilever Bea m (DCB) test is being used for determining the Mode I interlaminar fracture toughness of unstitched and lightly stitched composite laminates. But the standard DCB test method is not suitable for laminates with medium to high density s titches. Due t o the high bending moment at the crack-tip, the sp ecimens fail due to compressive stresses before the delamination could prop agate. In order to overcome this limitation, a new fixture has been develop ed and used to perform Mode I fracture toughness tests on laminates with hi gh density stitches. An analysis method has been developed to determine the load-deflection and the load-energy release rate relationships in the new test. The new test method has been found to be successful in testing graphi te/epoxy laminates containing Kevlar stitches with a density of 64 stitches per square inch. The apparent fracture toughness of these specimens is abo ut 45 times that of unstitched specimens.