Keywords: aerospace structures, composite, buckling, delamination, exact strip, Wittrick-Williams algorithm.

Composite materials are widely used in aerospace structures. Although these materials offer many advantages over metals in terms of strength, stiffness and low mass, additional types of failure can be encountered. One of the most serious of these is delamination, which reduces the resistance to compressive and shear loads. Therefore, understanding the effects of delamination on buckling behaviour is essential for the correct design and safe use of composite materials in aircraft.

This paper uses exact stiffness analysis for the critical buckling analysis of composite plates with through-the-length delaminations. The usual discretisation of the finite element and finite strip methods is avoided by using analytical solutions of the member stiffness equations, the resulting transcendental eigenproblems being solved by the Wittrick-Williams algorithm. Accuracy is enhanced by coupling sinusoidal responses to give an infinitely long plate model.

Numerical results show that, as the delamination width is increased, there is a reduction in the critical buckling load, accompanied by a transition from a global mode to a local mode of the delaminated portion. This transition tends to be sudden for longitudinal and shear loading, and more gradual for transverse loading. The local buckling load decreases as the delamination is moved towards the surface of the plate, becoming negligibly small for wide delaminations of only a few plies. The effect of multiple through-the-length delaminations is dominated by local buckling of the thinnest portion of the delaminated region. The chosen example was insensitive to the widthwise location of the delamination, and gave qualitatively similar results for both simply supported and clamped edge conditions.

Because the method uses much smaller models than finite element analysis, solution times are very competitive. This approach is therefore suitable for parametric studies in the preliminary design of aerospace structures.

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