The performance of thin-plate structural systems is known to be greatly influenced by the effect of buckling and post-buckling behaviour. The main factors, which affect the buckling, and post-bucking characteristics and consequently the ultimate performance of thin plate structural components, are noted to be structural geometry, material properties, loading and boundary conditions and geometric imperfections. Present day knowledge and understanding of the buckling and post-buckling behaviour of thin plate structures is at a fairly sophisticated level due to the intensive research that has been carried out over the years in this field of study. This is particularly true for the case of compressive loading whereby the collapse and unloading failure mechanics of thin plate structures has been well documented for this case. The same is not true for the case of shear loading and although much work has been carried out there is a lack of knowledge and in-depth understanding of shear post-ultimate conditions which essentially defines the initiation and progressive development of the plastic failure mechanisms of thin plate structural system. This thesis makes a contribution to the area of study by taking advantage of the developments in recent years of computational technology and computing power to develop finite element modelling strategies and solution procedures using the commercially available FE package PATRAN/NASTRAN to describe in detail the post-buckled shear failure of thin plate structural systems. The work of this thesis provides an in-depth understanding of the complex post-buckled failure mechanics associated with thin-plate structures subjected to in-plane shear displacement and combined shear and compression loading. Simply supported in-plane normal stress free and straight edge boundaries are employed to examine the shear performance as well as the failure mechanisms of thin and stocky web plates. Finite element modelling strategies are developed, which are able to describe the complete loading history from the onset of initial buckling through the nonlinear elastic post-buckling to initial material yielding and its further propagation throughout the structure leading to the development of an appropriate failure mechanism that causes final plastic collapse and subsequent load drop-off. The post-buckled failure response of the thin plate structures is determined with due consideration being given to the effects of geometric and material nonlinearities. The effect of stiffeners on structural performance is detailed for single and multiple asymmetrical and symmetrically attached stiffeners. The degrading influence on the structural performance of cut-outs as well as the considerable redeeming effect due to reinforcements attached at the cut-out boundaries is highlighted in this thesis. The work of the thesis covers the in-plane shear displacement loading of thin web plates, thin web plates with transverse stiffeners, web plates with cut-outs, web plates with stiffened cut-outs and the interactive shear and compressive loading of transversely stiffened web structures. The in-plane shear displacement loading of these structures using the multipoint constraint loading strategy in the finite element modelling procedures has shown to be highly successful in being able to provide an in-depth understanding of the failure mechanics of these structures to a level not to be found in the existing literature.

Submarine landslides retrogressively fail from intact material at the headwall and then become fluidized by strain weakening; the final deposits of these flows have low porosity, which controls their character in seismic reflection data. Submarine landslides occur on the open slope and also localized areas including margins of turbidite channel-levee systems. I develop and quantify this model with 3-D seismic reflection data, core and log data from Integrated Ocean Drilling Program Expedition 308 (Ursa Basin, Gulf of Mexico), flume experiments, and numerical modeling. At Ursa, multiple submarine slides over the last 60 ky are preserved as mass transport deposits (MTDs). Retrogression proceeded from an initial slope failure that created an excavated headwall, which reduced the horizontal stress behind the headwall and resulted in normal faults. Fault blocks progressively weakened until the gravitational driving stress imposed by the bed slope exceeded soil strength, which allowed the soil to flow for more than 10 km away from the source area. The resulting MTDs have lower porosity (higher bulk density) relative to non-failed sediments, which ultimately produces high amplitude reflections at the base and top of MTDs. In the laboratory, I made weak (low yield strength) and strong flows (high yield strength) from mixtures of clay, silt, and water. Weak flows generate turbidity currents while moving rapidly away from the source area. They create thin and long deposits with sinuous flow features, and leave behind a relatively smooth and featureless source area. In contrast, strong flows move slowly, do not generate a turbidity current, and create blocky, highly fractured source areas and short, thick depositional lobes. In Pleistocene turbidite channels of the Mississippi Fan, deep-seated rotational failures occurred in the flanking levees. The rotational failures displaced material into the channel from below where it became eroded by turbidity flows. This system achieved a delicate steady state where levee deposition and displacement along the fault into the channel was balanced by erosion rate of turbidity flows. This work enhances our understanding of geohazards and margin evolution by illuminating coupled processes of sedimentation, fluid flow, and deformation on passive continental margins. / text

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