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Several three-dimensional element analyses of (0/ϑ/−ϑ), graphite epoxy laminates, where ϑ = 15, 20, 25, 30, and 45°, subjected to axial tensile load, were performed. The interlaminar stresses in the ϑ/−ϑ interface were calculated with and without a matrix crack in the central −ϑ plies. The interlaminar normal stress changes from a small compressive stress when no matrix crack is present to a high tensile stress at the intersection of the matrix crack and the free edge. The analysis of local delamination from the −ϑ matrix crack indicates a high strain energy release rate and a localized Mode I component near the free edge, within one-ply distance from the matrix crack. To examine the stress state causing the matrix cracking, the maximum principal normal stress in a plane perpendicular to the fiber direction in the −ϑ ply was calculated in an uncracked laminate. The corresponding shear stress parallel to the fiber was also calculated. The principal normal stress at the laminate edge increased through the ply thickness and reached a very high tensile value at the ϑ/−ϑ interface indicating that the crack in the −ϑ ply may initiate at the ϑ/−ϑ interface. Predicted crack profiles on the laminate edge in the −ϑ ply were constructed from the principal stress directions. The cracks were found to be more curved for layups with smaller ϑ angles, which is consistent with experimental observations in the literature.
Research scientist, Analytical Services and Materials, Inc., Hampton, VA
Senior scientist, NASA Langley Research Center, Hampton, VA
Stock #: CTR10360J