Research On Large Deformation Of Sandwich Structure And Resistance To Underwater Blast

The blast-resistance of the ship structure in underwater explosion is one of the most concerning issues for many coountries.Metallic sandwich structure has been atttracting attention for its outstanding performance in blast resisting.Focusing on the sandwich structure,the interaction mechanism between the underwater shock wave and the sandwich structure has been studied in this thesis.And a systematic design guide of sandwich structure with excellent blast resistance has been provided for engineering application.Based on the extended high order sandwich panel theory(EHSAPT)and the small deformation assumption,the static mechanical response of elastoplastic sandwich has been established theoretically.The normal and shear stresses follow the bilinear constitutive stress-strain relation in the core.According to the disscussion on the possible stress states in the core,the strain energy have been obtained respectively and the governing equations have been derived.Numerical methods have been adopted to solve the equations.Finte element simlation and experiment for the threepoint bending test have verified the theoretical model.Secondly,based on static EHSAPT and the assumption of large displacement with moderate rotation,the dynamic mechanical response of sandwich structure has been established theoretically.The geometric and material nonlinearities have been considered simultaniously.The face sheets are isotropic and the core is orthotropic.The Green-Lagrangian strain has been adopted in the face sheets and core.According to the disscussion on the possible stress states in the face sheets and the core,a systematic strategy for the multidimensionally elastoplastic dynamic analysis of sandwich structures has been subsequently developed.The strain energy has been obtained subsequently and the governing equations have been derived via the principle of minimum potential energy.Numerical methods have been adopted to solve the dynamic equations.Based on the proposed nonlinear dynamic EHSAPT,the elastoplastic response of a simply-supported sandwhic beam under blast load has been studied.It reveals that the maximum displacement?shear and normal stresses show a decrease and the time periods are getting longer when the core bilinear ratio declines.It suggests that the strain hardening has significant effects on the dynamic response of sandwich beams made from elastoplastic materials.At the same time,the comparison with the finite element simulations shows that the proposed nonlinear dynamic EHSAPT can accurately predict the nonlinear mechanical behavior of the sandwich structure and can be further applied in theoretical assessment.Thirdly,based on EHSAPT and the assumption of large deformation,the static and dynamic mechanical response of sandwihc structure has been established theoretically.The face sheets and core are orthotropic and the materials are elastic.The total lagrange method and the principle of virtual work have been utilized to derive the static and dynamic governing equations of sandwich beam in large deformation.The explicit form of the structural stiffness has been given.The equations have been solved by Newton-Raphson method and Newmark-beta method.The conventional finite element simulations have validated the proposed EHSAPT in large deformation.Fourthly,the mechanical response of sandwich structure in underwater blast has been theoretical acquired and the fluid-structure interaction model has been set up.Through load analysis,the dynamic response problem of the sandwich structure in underwater shock wave can be transformed into the dynamic problem of the sandwich structure with a structural damping in the vacuum,under the combined loads of the incident and reflected waves.At the same time,by considering only the transverse displacements of the top face and the core,a simplified set of governing equations for the sandwich structure in underwater shock waves has been obtained.The equations have been solved to obtain the displacement and velocity of the top face and the core,and further the impulse that has to be transmitted to sandwich structure has been acquired.For the convenience of engineering application,a further simplified form of the transferred impulse has been obtained by ignoring the core stiffness.Comparison with literature and finite element simulations shows that these two transferred impulse proposed in this thesis provide better predictions.Simultaniously,the theoretical analysis in this thesis shows that the key factors affecting the fluidstructure interaction include: the magnitude and decay time of the shock wave;the density of the fluid,the speed of sound in the fluid;the mass of the top face,and the density of the core.The maximum impulse transmitted does not lie on the yield strength and the strain hardening of the core.To provide a sandwich structure resistant to underwater shocks,a relatively low-density core should be selected.Finally,an optimization on the metallic sandwich structure plastered on ship sides to mitigate the underwater shock has been given.Considering the effects of strain rate,a systematic design guidance has been given in determining the number of core layers,thickness,relative density,yield strength,and the thickness of the face sheets.So that the designed metal sandwich structure can completely absorb the energy from underwater explosions and the pressure transmitted to the hull is minimum.Verified by finite element simulation,the designed metallic sandwich structure delays the impact on the hull and reduces the magnititude.It protects the hull and is proven to have good resistance to underwater blast.