Abstract
Laminated composites, such as graphite-epoxy, are vulnerable to delamination. Through-the-thickness stitching can dramatically improve the interlaminar strength and fracture toughness of laminated composites. In some instances the fracture toughness is so high that conventional DCB tests cannot propagate the crack, instead the specimen fails in flexure. Recently a novel fixture has been developed and used successfully to measure Mode I fracture toughness of such stitched composites. Due to the stable propagation, the location of the crack front and the corresponding load can be precisely measured. The nonlinear differential equations of equilibrium of the new specimen were solved using an iterative procedure to obtain the strain energy release rate as a function of load and crack length. Results include, bending moment, transverse deflection and slope as a function of crack length. The analytical method in conjunction with experimental measurements has been successfully used to determine the instantaneous GIC of the specimen. It was found that the apparent fracture toughness of the specimens tested was roughly constant along the stitched zone, and was about sixty times that of unstitched specimen.
