Dynamic Response Of Water-Backed Metallic Sandwich Panels Subjected To Proximity Underwater Explosion

Metallic sandwich structures with two face-sheets and core present extra light and high specific stiffness.Under explosion loading,sandwich structure dissipate energy by plastic deformation of the face-sheets and crushing of core.Configurations of sandwich are adjustable which make it appropriate used in different environments.Sandwich structure was applied in military and civil engineering,especially in protection,aerospace,architectural.?n recent years,with the extension of applications,sandwich structure was used in ocean engineering.A submersible structure presents different deformation resistant capability and anti-shockwave performance when subjected to underwater explosion.?n the novel environments,it is meaningful to perform a systematic investigation on the dynamic mechanical behavior and deformation failure mode of water-backed sandwich structure to shock loading in water.Frist of all,a ballistic pendulum was developed to realize underwater explosion loading on water-backed structures.?n order to makes the sandwich structure in contact with water on both sides,a cabin was placed behind the specimen,which filled with different fluid(water or air).Filling with different fluid,the specimen was water-backed or air-backed respectively.With the Ballistic pendulum,explosion experiment can be perform in laboratory.Distribution of the pressure of the loading shock on the structure was tested with systematic experiments.Based on the experimental results,the Ballistic pendulum can achieve proximity underwater explosion test with good repeatability.Deformation resistant capability and anti-shockwave performance of sandwich structures with honeycomb core were tested with systematic experiments.Face-sheets and honeycomb core were measured by a scanner.Comparisons between scanned results and entities prove that scanned results coincide perfectly with entities.With the scanned result,distribution of compression ratio along the diameter and volumetric strain of the honeycomb core were obtained.Pressure-time history of the secondary shock wave was recorded by a pressure sensor to assess the anti-shockwave performance of the specimen.With respect to time,the secondary shock wave was divided into three parts.Frist part is the transmitted wave.The second part is the convergence of the reflected shock wave.For the problem on hand,the first part of the secondary shock wave is more important than the other two.The constructions behind protective layer are protected objects which may damage by the secondary pressure wave.Therefore,transmitted shock wave intensity is the key parameter to assess the shockwave attenuation performance of sandwich panel.The maximum pressure is a positive relationship with core density,and is an inverse trend with face thickness and core thickness.Compared with core density and face-sheet thickness,change of core thickness has minimal impact on transmitted shock wave intensity.From macro perspective,by using thick face-sheets and honeycomb cores fabricated with thinner aluminum foil,intensity of the transmitted shock wave is distinctly reduced.Namely,shockwave attenuation performance of the sandwich structure is enhanced.Plastic deformation of the back face-sheet is smaller than the front face-sheet.As a result,plastic deformation of the front face-sheet and distribution of compression ratio along the diameter and volumetric strain of the core are the main parameters to assess deformation resistant capability and anti-shockwave performance of sandwich structures.Sandwich with same core,the thinnest front face-sheet presents plastic deformation in reverse direction in an annular region.Therefore,mode MD(mixed deformation mode,including bulged deformation and depressed deformation)was defined.With the increase of face-sheet thickness and yield strength of hineycomb core,deformation mode of the front face-sheet was simplified and compression ratio and densification region of core were decreased.Hence,deflection of the front face-sheet and volumetric strain of core decrease limited.Deflection of the front face-sheet decreases with the increase of face-sheet thickness and core density when the core was partly compressed in the central.When the core was fully compressed in the central,sandwich with thin face-sheet and light core,the front face-sheet presents little deflection due to the deformation in reverse direction.Core thickness is another key parameter of sandwich panel.Sandwich structure with same face-sheet thickness,compression ratio at the central point and volumetric strain of core rapidly decrease with the increase of core thickness,and deflection of the front face-sheet was increased as well.With thick core,there is a cap for the deflection of the front face-sheet and compress ratio of core in the central point.?n a well-designed system,the restraining forces accompanying plastic dissipation of panel and crushing of core are sufficient to arrest the motion of the panel before the core is fully compressed or tearing of the front face plate occur.The maximum vertical displacement of the front face-sheet provide an important guiding significance in the determination of core thickness.Deformation and failure modes of sandwich structure to shock loading in water was investigated with contrast experiments.Under underwater explosion,face-sheets of air-backed sandwich panel produced global deformation with a decrease trend in permanent deflection moving to the boundary.Under same intensity blast loading,back face-sheet of water-backed sandwich panel produced little deformation in the central region which is negligible.For air-backed sandwich panel,deflection of the back face-sheet increases with blast intensity,and finally presents tearing failure and shear failure around the clamped boundary.Deflection of the front face-sheet is a positive relationship with blast intensity.Compression ratio of the core in the central region presents same relationship trend when blast intensity increase,and presents more obvious trend around the clamped boundary.For water-backed sandwich panel,no obvious increase of deflection of the back face-sheet was found with the increase of blast intensity.For water-backed sandwich panels to shock loading in water,volumetric strain is a positive relationship with blast intensity.One can conclude that failure mechanism sandwich panel change with the fluid behind the structure when subjected to underwater explosion.For air-backed sandwich panel,large inelastic deformation is the main deformation mode.Under higher intensity blast,shear failure was found around the clamped boundary.For water-backed sandwich panel,failure mechanism is the deformation of the front face-sheet and compression of core in the central region.As a result,resistance to damage and energy absorption of sandwich panel in water-backed configuration is better than that in air-backed configuration.LS-DYNA software has been used to investigate the dynamic response process of water-backed sandwich panel.With respect to deformed panel and the secondary shock wave,a good agreement between the experimental and computational studies was obtained.Same with the mode for air-backed sandwich panel,with respect to time,dynamic process of water-backed sandwich panel to shock loading in water was divided into three parts:?-interaction of the shock wave and the panel,and acceleration of the front face-sheet,and the generation of the transmitted wave;?-compression of the core,and slowing down of the front face-sheet;?-interaction of the panel and water behind the structure.By using LS-DYNA software,energy absorbed by face-sheets and core have been confirmed.With respect to the simulated results,most energy have been absorbed by deformation process of the front face-sheet and compression of core.Configurations of sandwich panel have been optimized with simulation results.?n the optimized design,sandwich panels comprise thick front face-sheet and thick cores fabricated with thin foil thickness.Deformation resistant capability and anti-shockwave performance of sandwich panel have been boosted enormously.A certain redundant strength is maintained under the explosion intensity in this paper.?n other words,by using the optimized design,sandwich panel is able to stand higher intensity blasts.