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. / Ph. D.

Thin, elastic, shallow shells having uniform thickness and rectangular boundaries are investigated. The boundary conditions are either simply supported or claped, and the shell is subjected to a uniformly distributed load applied over either the full shell area or a central region. The thickness, material properties, edge lengths, and surface area of the shell are specified, and the objective is the determination of the shell shape which will maximize the buckling load. Marguerre's two, coupled, non-linear equations of equilibrium are used to describe prebuckling deformations and stresses. Considering small vibrations about the equilibrium state, two, coupled, linear equations of motion are derived. Subsequently, by recognizing that at buckling the lowest frequency of vibration goes to zero, the buckling equations are obtained. Finally, the Lagrange multiplier technique is employed to formulate an augmented objective function, and the calculus of variations is applied in order to derive the governing set of equations. The resulting system of equations is solved numerically by the finite difference method. Results for shells with various surface areas are presented. For each surface area the investigation is performed on shells having either clamped or simply supported boundary conditions and either a square or a rectangular boundary. The applied uniform load covers either the full shell area or a partial central region. The shell form, buckling load, and buckling modes of the optimal forms are compared with those of the reference form (double sine) having the same surface area, and changes are noted. Also, comparisons with respect to forms, buckling load, and type of buckling are made between the optimal form of a shell subjected to a full uniform load and the optimal form of the same shell subjected to a partial uniform load. In some cases, the buckling load of the optimal form is sensitive to imperfections in the form or in the loading distribution as well as to changes in the design. In these cases, some of the apparent advantages of the optimum form may be diminished. Thus, the frequencies of vibration at buckling, the corresponding buckling modes, and the presence of adjacent equilibrium states are monitored in order to evaluate the sensitivity of the optimal form to imperfections and to design changes. / Ph. D. / incomplete_metadata