Dynamic Collapse of Double Hull Composite Cylinders: Hydrostatic and Shock-Initiated Implosion
An experimental investigation of double-hull composite cylinders during underwater implosion is performed. Composite materials offer several unique advantages for underwater applications, including greater specific strength and stiffness than metals, as well as better corrosion resistance. Double hull composite structures, which feature a low density core between two facesheets of comparatively higher density, increase these benefits further by adding increased bending strength and acoustic attenuation. Despite these advantages, there exists a large knowledge gap with regards to the underwater implosion behavior of double hull composite structures. To that end, a series of experimental studies is performed to address said knowledge gaps and help provide fundamental understanding of the behavior of these structures during implosion. First, the dynamic collapse of hollow and filled double hull composite cylinders is investigated experimentally. Carbon-fiber/epoxy double cylinders with and without parametrically-graded PVC foam cores in between are collapsed in a large-diameter pressure vessel, and dynamic pressure data is used in conjunction with underwater DIC to determine the effect of the double hull structure on implosion mechanics. Buckling initiation and overall collapse behavior of the specimen are studied, as well as the pressure pulse from the implosion released into the fluid. Incidents of the outer tube imploding but not the inner are reported, in addition to cases where both collapse. Results show heavier foam cores increase collapse pressure dramatically, and this increase in collapse pressure is predictively related to the mechanical and geometric properties of the foam cores themselves. Normalized dynamic pressure emitted from implosions is shown to occur in distinct phases, with an additional under- and overpressure region present if the inner tube collapses. When normalized for hydrostatic pressure, fluid impulses from various core densities are shown to remain constant. Energy flux from the implosions, presented as a percentage of available hydrostatic energy, is shown to increase as a function of core density. Increased foam crushing energy at higher core densities, and increased damage observed in post-mortem specimens from heavier cores, are identified as mechanisms responsible for these behaviors. Second, an experimental study is performed which investigates the dynamic collapse of double hull composite cylinders under external hydrostatic pressure and shock loading. All experiments are performed underwater in a 2.1 m diameter semi-spherical pressure vessel that approximates a free-field environment, and Digital Image Correlation (DIC) is used in conjunction with blast transducers to study collapse mechanics. Specimens have carbon-fiber / epoxy facesheets and a PVC foam core that is removed for control in some instances, and are brought to 80% of their natural buckling pressure before being subjected to an underwater explosion (UNDEX) at varying standoff distances. Results show that double hull specimens implode below their natural collapse pressure when subject to explosive loading, but that the addition of the PVC foam core prevents implosion in some cases and substantially increases structural stability in others. The double hull configuration with foam core is also shown to emit significant pressure pulses despite the noisy environment of the pressure vessel.
Nicholas A DeNardo,
"Dynamic Collapse of Double Hull Composite Cylinders: Hydrostatic and Shock-Initiated Implosion"
Dissertations and Master's Theses (Campus Access).