Buckling and Postbuckling Behaviors of Composite Panels with a Delamination

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Interlaminar delaminations are introduced easily in the interlaminar area of composite panels by the impact loadings, fatigue etc owing to its weak interlaminar fracture toughness. The decrease of the bending rigidity due to the delaminations causes significant drops of the buckling load and the compressive strength of the panel. This problem is called CAI (Compression After Impact)". CAI is an urgent issue for the wider application of the composite materials to the airplane structures, because it often becomes critical for the design of the composite structures of airplanes. The objective of the present research is to get the data to improve the properties of the composite materials against CAI. The effects of delamination size and position on the buckling and postbuckling behaviors are studied analytically and experimentally about unidirectionally reinforced composite square panels in the first year and woven composite laminates in the second year. The square panels are clamped on the four sides and studied as a two dimensional panel in order to clarify the effects of the delamination on the more realistic structure. The complex mechanisms of the compressive property degradation owing to the delamination are observed and investigated. The analysis developed in the present research based on the Rayleigh-Ritz approximation can explain well not only the buckling behaviors but also the very complex postbuckling behaviors including secondary buckling.