Response of an underwater cylindrical composite shell to a proximal implosion

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An experimental investigation is conducted to study the dynamic underwater response of a cylindrical composite shell under near critical hydrostatic pressure, to the implosion of another shell in proximity. A primary cylindrical composite shell is imploded in proximity to a secondary shell which is similar in all respects except for the secondary shell having a smaller length. Length differences of 10% and 20% are chosen to simulate variations in collapse pressures occurring in shells from real life manufacturing defects and/or degradation during operational use. The response of the secondary shell is investigated to understand if and how its collapse occurs in addition to studying the Fluid-Structure Interaction (FSI) phenomenon. The pair of shells are subjected to underwater hydrostatic loading using a large pressure vessel suitable for high-speed photography in conjunction with 3D Digital Image Correlation (DIC). 3D-DIC is employed to obtain full-field displacement measurements of both the shells, and local dynamic pressure histories are also simultaneously recorded. The primary shell always imploded first, causing a dynamic loading on the secondary shell. In cases of implosion of the secondary shell, although the transient radial deformations occurred in mode 2, the failure itself occurred with a localized failure of the shell walls. It is observed that there exists an inner critical stand-off distance for the secondary shell to fail catastrophically upon the implosion of the primary shell, and an outer critical stand-off distance beyond which the secondary shell does not implode. A critical stand-off distance is found to exist only in the case of the 10% smaller secondary shell length. If the secondary shell stand-off distance is more than the outer critical distance or when length of the secondary shell is 20% smaller, the secondary shell responds with bending and breathing modes and no visible damage is recorded. When the stand-off distance is in between the inner and the outer critical distances, the relative orientation of the incipient modal shapes of the two shells is the factor governing the collapse of the secondary shell. A method is also developed to decouple full-field 3D-DIC measurements into bending and breathing deformation measurements.

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Journal of the Mechanics and Physics of Solids