Compression Failure Mechanisms of Uni-ply Composite Plates with a Circular Cutout

The paper discusses the experimental and finite element investigation into the failure mechanisms of uniply graphite-epoxy composite plates with circular holes loaded in uniaxial compression. The effect of hole size on the failure mode is established. Test specimens composed of a single layer of uniply graphite-epoxy sandwiched between two layers of transparent polyetherimide plastic were designed and produced. This unique stacking geometry enabled the investigators to carry out a nondestructive evaluation of the failure mechanisms associated with the uniply. A wide range of hole sizes was tested in order to document the dependance of failure modes to specimen geometry. The experimental work made use of real time holographic interferometry and photomicrography to study the initiation of failure in the test specimens. A Finite Element Analysis (FEA) was carried out for those specimens which exhibited in-plane failure initiated at the edge of the hole. The analysis modelled the graphite-epoxy uniply in an attempt to reproduce the results obtained experimentally. Experimental results revealed failure initiation to occur a t the edge of the hole, in the form of fiber microbuckling/ kinking, followed by delamination, for specimens with hole sizes varying from the largest down to a cutoff hole size. Below this range, failure either initiated at the outer free edges, in the forms of cracks, or was in the form of global (total) delamination. The FEA results agreed very well with the buckled mode shapes from the experimental results, although the estimated buckling loads were larger than those obtained *Graduate Research Assistant, Student Member AIAA t Asst. Professor of Aerospace Engineering, Member AIAA. Copyright c 1992 by Anthony M. Waas. Published by the American Institute of Aeronautics and Astronautics, Inc. with permission. from experiments, probably due to imperfections in loading and material composition in the experiments. The trends for buckling modes and loads agree quite well for the two phases, revealing the inverse behaviour between buckling load and hole size.