Load-transfer mechanisms and damage evolution of double-layer structures with different interlayer media subjected to underwater explosions

An experimental and numerical investigation was conducted to clarify the load-transfer mechanisms and damage evolution of double-layer plates subjected to underwater explosions (UNDEX). Air-backed (AB), semi-water-backed (SB), and water-backed (WB) conditions were examined under far-field and close-in explosions using high-speed imaging, pressure and strain measurements, and fluid-structure interaction simulations. Under the far-field condition, the air layer substantially attenuated loading on the inner plate, whereas the SB and WB conditions were characterized by localized water-hammer loading and repeated pressure-wave transmission, respectively. The continuous water path in the WB condition generated MPa-level internal loads and plastic deformation of the inner plate. Under close-in explosion, rupture of the outer plate created a direct pathway for shock waves, detonation products, and external-water ingress. When rupture was avoided, load transfer was governed by plate-driven water-hammer and cavitation closure loading in the SB condition, but by transmitted pressure waves and cavitation-induced loading in the WB condition. Bubble-related loading occurred as a temporally separated event under the far-field condition, whereas bubble evolution, cavitation and structural response were strongly coupled under close-in explosion. For the tested close-in explosion cases, the SB condition reduced the final outer-plate deflection by 42.7% relative to AB and the final inner plate deflection by 57.1% relative to WB. These findings provide an interpretation of load redistribution in the investigated double-layer structure. Broader design conclusions require further variation of filling ratio, plate spacing, structural dimensions, and loading conditions.