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Experimental Investigation of the Strength of Damaged Pressure Hulls - Phases 5 & 6: The Influence of Out-of-Circularity on Collapse

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Technical memorandum

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Collapse tests were performed on twelve small-scale ring-stiffened cylinders. The test specimens were machined from aluminium tubing, and then mechanically deformed in order to introduce more realistic levels of out-of-circularity OOC in the critical collapse mode. Six of the test specimens had additional damage in the form of artificial corrosion thinning, which was introduced by machining away some of the shell plating. Corrosion damage was found to affect the strength of cylinders in different ways, depending on its orientation with respect to the OOC shape. When the corrosion damage was aligned with an inward lobe of the applied OOC shape, the effects of thinning and imperfections were additive and led to significant decreases in collapse pressure. When the hull thinning was collocated with outward OOC lobes, the corrosion damage tended to reduce the overall OOC, and only a small reduction in collapse pressure was noted, primarily due to the high stresses in the thinned shell itself. Finite element models were used to simulate the mechanical procedure used to apply OOC to the cylinders. The predicted residual stress field was in the elastic range of the material, and subsequent collapse analysis indicated that those residual stresses resulted in a 3 reduction in collapse pressure compared to a stress-relieved model. It is not thought that residual stresses have significantly affected the collapse behaviour of the actual test specimens. Finite element models based on the measured shape and material properties of forty specimens tested in the current and previous testing phases were able to predict the experimental collapse pressures with an accuracy of 9.5, with 95 confidence.

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  • Submarine Engineering

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