Sublaminate Damage Mechanisms in Composite Structures

This interim report briefly summarizes the objectives, accomplishments and current direction of research on sublaminate damage mechanisms in composite structures. A Mode I suppression series of experiments and a companion analysis have been completed for graphite-epoxy double cracked-lap-shear specimens. The results show that a very small transverse normal force, 1-3% of the axial tensile force, is all that is needed to suppress Mode I action in interlaminar fracture. Complementary experiments on low cycle fatigue in tension and compression and on Mode I suppression in tension have been performed. The former confirm earlier static findings. The latter are also in agreement with static findings and illustrate the effectiveness of Mode I suppression technology. A practical working approach to analysis and design against delamination has been formulated. A simple fracture law is found to represent the existing data on four graphite-epoxy material systems and one glass-epoxy system created by a number of investigators on a variety of types of specimens. It has an intrinsic means of accounting for the observed high scatter of high Mode II (or Mode III), low Mode I delamination found in experiments. Keywords: Delamination, Interlaminar fracture, Damage in composites, Composite materials.