Nonlinear response and failure characteristics of internally pressurized composite cylindrical panels

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Virginia Polytechnic Institute and State University

Results of an experimental and analytical study of the nonlinear response and failure characteristics of internally pressurized 4- to 16-ply-thick graphite-epoxy cylindrical panels are presented. Specimens with clamped boundaries simulating the skin between two frames and two stringers of a typical transport fuselage were tested to failure. Failure results of aluminum specimens are compared with the graphite-epoxy test results. The specimens failed at their edges where the local bending gradients and interlaminar stresses are maximum. STAGS nonlinear two-dimensional shell analysis computer code results are used to identify regions of the panels where the response is independent of the axial coordinate. A geometrically nonlinear one-dimensional cylindrical panel analysis was derived and used to determine panel response and interlaminar stresses. Inclusion of the geometric nonlinearity was essential for accurate prediction of panel response. Measurements of panel radius and edge circumferential displacements associated with specimen slipping were also required in the one-dimensional analysis for good correlation between analytical and experimental results.

Some panels failed with significant damage in the form of tensile fiber breaks and ply delaminations preceding the ultimate pressure. Other panels failed suddenly without any apparent damage preceding the ultimate pressure. The failure usually occurred along one edge of the panel leaving the other edge intact. The damage on the panel surfaces and through-the-thickness were examined to determine the failure characteristics of the panels. Various failure criteria were applied to the stresses predicted from the one-dimensional analysis. The maximum stress failure criterion applied to the predicted tensile stress in the fiber direction agreed best with the experimentally determined first damage pressures. Results indicate that all panels tested would support applied internal pressures well above fuselage proof pressures.