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Nine graphite/epoxy panels were tested in uniaxial tension to examine the ability of stiffening strips to redirect propagating damage. The materials used were Hercules A370-5H/3501-6 prepreg fabric and AS4/3501-6 prepreg unidirectional tape. The layup of the unstiffened regions of the panels was four plies of fabric with the quasi-isotropic layup [0f/45s]f. Three different four-ply unidirectional tape stiffener layups parallel to the applied load were tested. The specimen width was 203 mm with stiffener widths of either 48 or 64 mm. Slits of three lengths, 51, 71, and 102 mm (1.0 mm wide), were precut into the panels perpendicular to the direction of loading. The panels were tested under load control. Stress-strain and photoelastic data were taken. The finite element code ADINA was used to examine the two-dimensional response of the panels. The three slit lengths were modeled to gain an understanding of the local stress and strain response ahead of the slit in the stiffened region. Contour plots of maximum strain and the orientation of these maximum values with respect to the load direction near slits and stiffened regions were generated. These analytical models showed higher strains in the stiffened regions ahead of the slit compared to the strains in the unstiffened regions. The local orientation of the maximum tensile strains in the stiffened regions ahead of the slit remained perpendicular to the slit and parallel to the applied load. At failure, propagating damage progressed directly across the stiffener either perpendicular to the applied load or at 45° with respect to the applied load. It is concluded that the maximum tensile strain and its orientation ahead of the slit, as derived from the analysis, plays a key role in the prediction of subsequent damage propagation in these laminates. It is also concluded that the structural and material couplings, due to the unsymmetric layups in the stiffener regions, have little, if any, affect on the ability of the stiffener to redirect damage propagation, since in all cases damage progressed through the stiffeners.
graphite/epoxy fabric composites, stiffened panels, stiffening strips, stress concentrations, damage propagation, two-dimensional finite-element analysis
Aeronautics engineer, Boeing Helicopter, Philadelphia, PA
Boeing assistant professor of aeronautics and astronautics, Technology Laboratory for Advanced Composites, Massachusetts Institute of Technology, Cambridge, MA
Associate professor, Technology Laboratory for Advanced Composites, Massachusetts Institute of Technology, Cambridge, MA