Skin stiffener debonding of top-hat stiffened composite structures

Abstract

Top-hat stiffened plates provide an efficient structure for engineering applications. During service debonding between the stiffener and the plate is a common failure mechanism. Therefore, an extensive understanding of the residual strength is required to rapidly and efficiently determine precautions to be taken to ensure the safety of the structure. Critical assessment of necessary repairs reduces the through life costs and in design damage assessment can lead to optimisation through tolerance of common damage incidents. Research on damaged stiffened structures to date is primarily focused on airframe applications and considers open sections with co-cured stiffeners which are not typical of marine structures. These studies have shown that debond size and location have a significant effect on the damage mode of the panel. However, they do not consider the interaction of failure modes or ultimate failure. Typical marine composite joints are manufactured by post-curing sub-components using a chopped strand mat layer at the interface. To predict failure of these joints requires accurate assessment of the material and fracture properties and a consistent set of data which is lacking in the literature. Therefore, the research in this thesis considers the damage tolerance of top-hat stiffened panels containing a debond between the stiffener and plate through numerical and experimental work. The focus of the work is post-cured top-hat multi-stiffened panels used in large marine applications manufactured from heavy weight glass vinylester woven roving.

An automated tool using non-linear finite element analysis capable of modelling debond damage and assessing the ultimate and the residual strength of the panel is verified. A parametric study investigating panel topology, damage parameters and stiffener type show the complexities of the damage case. Results show that top-hat stiffened panels exhibit a trend between ultimate strength and the debond size with crack initiation not necessarily propagating as geometric imperfections accelerate buckling but can provide an arrest point for crack propagation. Nominal lateral pressures are shown to significantly increase the damage tolerance. Full characterisation of typical materials is conducted experimentally providing a complete data set of mechanical characterisation and fracture data for both co-cured interfaces, typical of mid-laminate debonds and sub-component joints. Tensile, compressive, shear and exural tests are conducted and a model for the non-linearity of the woven roving in tension and shear is proposed. The fracture results show the post-cured joint exhibits a 20% increase in mode I and II strain energy release rates. The experimental data is used in a number of studies to further verify and optimise the finite element model. Mode I and II tests are simulated to ascertain the cohesive element interface strengths and Turon's interface parameters. The material data is shown to give accurate results for the structural response, crack initation and debonding of an as built large scale top-hat stiffened panel which is tested experimentally under four point bend. Therefore, the effect of skin-stiffener debonding has been investigated for top-hat stiffened panels, providing improved characterisation of the material and interfaces and guidance on the damage tolerance to damage and design parameters.

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ORCID for Adam Sobey: orcid.org/0000-0001-6880-8338

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