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The aim of this study was to investigate the dynamic collapse of composite shells in a constant external pressure water environment that is representative of a naval underwater structure. Laminated carbon/epoxy composite shells with diameters of 1.735 in., wall thickness of 0.041 in, length-to-diameter ratios ranging for 2.8 to 12 and [55/-55/(90)3/-55/55] layup were collapsed in a custom pressure testing facility that provided a constant pressure water environment. Buckling was sudden, dynamic, led to failure and fragmentation of the shells; the whole event lasted only a couple of ms. The dynamic collapse of the shells was recorded using high-speed digital imaging and dynamic pressure sensors synchronized with the camera were used to monitor the emanating pressure waves. All shells buckled in mode 2 at pressure levels predicted by models adopted. Collapse led to a localization zone in the central section of the shells, approximately spanning on a 4D length for the longer ones, and shorter for the shorter shells. A single axial crack developed in the collapsing section, which propagated 2 to 4 diameters depending on the length of the specimen. The axial crack was located on the extrados for long shells, and on the intrados for shorter ones. Helical cracks initiated from the tips of the axial crack, propagated outwards, and were responsible for the collapse and fragmentation of the two outer sections. The receding walls of the central localizing zone caused a dynamic drop in pressure that lasted until the inward motion was arrested by contact. This was followed by a sharp, short duration positive pressure pulse associated with an outward expansion wave. The pressure pulse varied to some degree around the circumference with the highest peak occurring opposite the initial crack. The final result of such dynamic events was catastrophic failure and fragmentation of the shell into small shreds. / text

The objective of the present work is to investigate the behavior of laminated fiber -reinforced polymer matrix composite shell structures under bending load with the help of a modified finite element computer code which was previously developed for the analysis of pseudo-layered single material shells. The laminates are assumed to be orthotropic and the formulation is adapted to first order shear deformation theory. The aim is to determine the large deformation characteristics numerically, and to predict the modes of failure by the illustration of the critical elements of the model. Therefore, several failure theories are also integrated to the code to detect first ply failure. Triangular shell elements are used and all the related data are generated from the mid-plane. Laminates under transverse loading are analyzed through several boundary conditions and ply orientations. To verify the numerical results obtained, a commercial finite element program is used to compare the outputs of the study, and the comparison is found to have shown good agreement. The onset of damage is investigated by using different failure criteria consisting of maximum stress, Tsai-Wu, and Tsai- Hill theories and close results are obtained.

The objective of this thesis is to investigate the progressive failure behavior of laminated fiber reinforced composite shell structures under different loading conditions. The laminates are assumed to be orthotropic and the first order shear deformation theory is applied. Three-node layered flat-shell elements are used in the analysis. To verify the numerical results obtained, experimental and analytical results found in literature are compared with the outputs of the study, and the comparison is found to have shown good agreement with the previous work. Rectangular graphite/epoxy composite laminates under transverse loading are analyzed through several boundary conditions and stacking sequences. Maximum stress criteria, Hashin&rsquo / s criteria and Tsai Wu criteria are employed to detect the failure and progressive failure methodology is be implemented according to instantaneous degradation approach. First ply failure, final failure loads, corresponding deformations and failure patterns are presented and compared.

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