Hydrostatic and shock-initiated instabilities in double-hull composite cylinders

The dynamic collapse of hollow and foam-filled double hull composite cylinders is investigated experimentally. Concentric carbon-fiber/epoxy double cylinders with and without parametrically-graded PVC foam cores are collapsed in a large-diameter pressure vessel under critical hydrostatic pressure as well sub-critical hydrostatic pressure and shock loading. Dynamic pressure data is used in conjunction with underwater Digital Image Correlation (DIC) to determine the effect of the double hull structure on implosion mechanics. Buckling initiation and overall collapse behavior are studied, as well as the pressure pulses released during the dynamic event. Incidents of partial collapse are reported, in addition to cases where the entire structure collapses. The physical mechanisms responsible for this behavior are identified, and the time between inner and outer cylinders collapsing is related to buckling phase angle. For hydrostatically initiated implosions, results show heavier foam cores increase critical collapse pressure linearly with foam crushing strength. Pressure pulses emitted during collapse are shown to occur in distinct phases, with an additional under- and overpressure region present if the inner cylinder collapses. Impulse is demonstrated to be primarily a function of collapse pressure, with energy released increasing with core density. For shock initiated cases, specimens are shown to implode below their natural collapse pressure when subject to explosive loading, with the addition of a foam core substantially increasing structural stability and sometimes preventing collapse. Specimens with foam cores are shown to undergo prolonged vibrations before collapsing. Post-mortem specimens are used to elucidate fracture and failure mechanisms.