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Validation of New Crack Monitoring Technique for Victoria Class High-Pressure Air Bottles

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Technical Report

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DRDC - Atlantic Research Centre Dartmouth NS Canada

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High-pressure air bottles are used in the Victoria class submarines to supply breathing air and to provide pressurized air for surfacing. The bottles are exposed to cyclic loading, which may result in the initiation and growth of fatigue cracks. An internal crack-like indication in a high-pressure air bottle instigated this study to better understand the growth of potential fatigue cracks. This will enable better management of the remaining operating life of high-pressure air bottles with detected cracks. In this study, the feasibility of using externally-mounted strain gauges to monitor the growth of known internal cracks in high-pressure air bottles was examined. A combination of numerical modelling and experiments was used to validate this technique. Two experimental samples were internally pressurized and external strain was measured. Reasonable agreement of measured strain with finite element analyses of the samples suggests the high-pressure air bottle finite element analyses results are reliable. Finite element analyses and experiments showed an area on the exterior of test samples opposite an internal notch with a modified strain field. The numerical analyses indicated that changes in the strain field associated with crack growth may be detected using external strain gauges. These results are consistent with a previous numerical analysis of a high-pressure air bottle which indicated the growth of internal cracks can be monitored with external strain gauges. Digital image correlation was used to measure the strain field in the area opposite the internal notch. It was useful for identifying the precise location of the notch, but further improvements to its use are necessary to ensure reliable results. It shows promise as a method to identify the location of high-pressure air bottle internal cracks and to complement other non-destructive examination techniques.

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