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The influence of two-dimensional finite element modeling assumptions on the debonding prediction for skin-stiffener specimens was investigated. Geometrically nonlinear finite element analyses using two-dimensional plane-stress and plane-strain elements as well as three different generalized plane-strain type approaches were performed. The computed skin and flange strains, transverse tensile stresses and energy release rates were compared to results obtained from three-dimensional simulations. The study showed that for strains and energy release rate computations the generalized plane-strain assumptions yielded results closest to the full threedimensional analysis. For computed transverse tensile stresses the plane-stress assumption gave the best agreement. Based on this study it is recommended that results from plane-stress and plane-strain models be used as upper and lower bounds. The results from generalized plane-strain models fall between the results obtained from plane-stress and plane-strain models. Two-dimensional models may also be used to qualitatively evaluate the stress distribution in a ply and the variation of energy release rates and mixed mode ratios with delamination length. For more accurate predictions, however, a three-dimensional analysis is required.

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The interlaminar fracture toughness of a plain-weave carbon-epoxy composite was studied under Mode I, Mode II and Mixed-Mode I and II loading. The objective was to compare results for a composite material with two different fiber forms: unidirectional tape and plain-weave fabric (PW). Tests were performed using the double-cantilever beam (DCB), four-point end-notch flexure (4ENF) and mixed-mode bending (MMB) configurations. Delamination initiation values were obtained as well as propagation values. The specimens were made of 24 layers of IM7/8552 plain-weave fabric. The results were compared with tests performed by Hansen and Martin [1] on IM7/8552 unidirectional tape. For all Mode I, Mode II and Mixed-Mode tests, the woven initiation Gc are quite similar to the corresponding initiation values for the tape material. The woven Gc propagation values, however, are much higher (2.6 times in Mode I and 3.1 times in Mode II) than the corresponding propagation values for the tape material. Furthermore, these propagation values may have more significance to structural components than propagation values for tape laminates.