ON THE DYNAMIC COLLAPSE OF CYLINDRICAL SHELLS UNDER COMBINED HYDROSTATIC AND IMPULSIVE PRESSURE LOADINGS

The dynamic collapse of submerged cylindrical shells subjected to lateral impulsive pressure loads caused by underwater explosions is studied via coupled experimental and numerical work. Aluminum tubes with 50.6mm outside diameter and diameter-to-thickness ratio of 32 were tested in a 5m x 5m x 1.6mdeep water tank under various explosive charges placed at different distances. Explosive charges and standoff distances were combined so as to eventually cause collapse of the specimens. Subsequently, the parent problem of the dynamic collapse of such structures under hydrostatic pressure is also investigated to determine the collapse and propagation pressures. Additional experiments were then conducted combining hydrostatic pressure and impulsive pressure loads. In these cases, hydrostatic pressure was applied quasi-statically and kept nearly constant. Subsequently, an explosive charge was detonated inside the pressure vessel. Dynamic pressure sensors were placed in various locations in the water around the tube in order to monitor the pressure wave propagation. In both sets of experiments, dynamic pressure and strain measurements were recorded using a fit-for-purpose data acquisition system with sampling rates of up to 1 mega samples/sec per channel. The characteristics of the pressure pulses and the charges necessary to collapse the pipe under different hydrostatic pressure levels were then compared. In parallel, finite element models were developed using commercially available software to simulate underwater explosion, the pressure wave propagation, its interaction with a cylindrical shell and the subsequent onset of dynamic collapse. The surrounding fluid was modeled as an acoustic medium, the shells as J(2) flow theory based materials with isotropic hardening, and proper fluid-structure interaction elements accounting for relatively small displacements of the boundary between fluid and structure were used. Finally, the physical experiments were numerically reproduced with good correlation between results.