A Novel Double Cantilever Beam Test for Stitched Composite Laminates

Abstract

Through-the-thickness stitching dramatically improves the interlaminar fracture toughness of laminated composites. Currently theDouble Cantilever Beam (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 stitches. Due to the high bending moment at the crack-tip, the specimens fail due to compressive stresses before the delamination could propagate. In order to overcome this limitation, a new fixture has been developed and used to perform Mode I fracture toughness tests on laminates with high 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 graphite/epoxy laminates containing Kevlar stitches with a density of 64 stitches per square inch. The apparent fracture toughness of these specimens is about 45 times that of unstitched specimens.