Underwater Dynamic Collapse of Sandwich Composite Structures

An experimental investigation is conducted to study the mechanics of underwater implosion of cylindrical bonded sandwich composite shells. Sandwich structures comprised of concentric carbon-fiber/epoxy shells with PVC foam cores of different densities are imploded in a large-diameter pressure vessel. High-speed photography in conjunction with Digital Image Correlation (DIC) measurements are employed to obtain full-field displacements of the dynamic collapse process. Local dynamic pressure histories are also simultaneously recorded to investigate fluid structure interaction during implosion. Observations of collapse mode, radial displacement and velocity of collapse, interaction between the concentric shells and the foam core and post-buckling failure sequence are made. Increasing foam core shear modulus linearly increases the experimental buckling pressures. Weaker growth of incipient modal deformations are understood to play a pivotal role in obtaining higher critical buckling pressures from bonded sandwich shells than previously studied unbonded sandwich constructions. Three dynamic collapse behaviors as determined by the relative orientation of collapse between the inner and outer shell are observed. Core foam density and imperfections also strongly influence the impulse, energy and pressure pulses released from the implosions.