Characterizing mode II delamination cracks in stitched composites

This paper deals with characterizing the bridging mechanisms developed acro ss delamination cracks by through-thickness reinforcement, using stitched c arbon/epoxy laminates under mode II loading as a prime example. End Notched Flexure (ENF) tests are performed which show that stitching can provide st able crack growth. The bridging law, which characterizes the bridging actio n of the stitches, is deduced from both crack profile measurements and load vs. deflection curves. Consistent results are obtained from the two method s. The inferred laws imply that delamination cracks will commonly grow in c onditions that are neither accurately nor properly described by linear elas tic fracture mechanics. Large scale bridging calculations are required, in which the essential material property is the bridging traction law. The lev el of detail in which the law must be determined can be inferred from the s ensitivity of predicted crack growth to variations in the law. It is recomm ended that the required parametric traction law be deduced in engineering p ractice from load vs, deflection data from the standard ENF (or similar) te st, with due regard to selecting the notch size and other specimen dimensio ns to ensure that crack growth is stable in the test.