Blast resistance of air-backed RC slab against underwater contact explosion

Reinforced concrete(RC)structures are common in engineering,and usually exposed to air or water,may be subjected to various blast scenarios.This paper aims to investigate the blast resistance of an airbacked RC slab against underwater contact explosions(UWCEs).A detailed numerical model based on CLE method considering explosive,water,air,and RC slab is developed to examine the structural behavior of the air-backed RC slab due to UWCEs.At first,the reliability of the numerical method is validated by comparing the numerical results of an UWCE test with experimental data.Then,the difference in dynamic behavior of air-backed and water-backed RC slabs due to UWCEs is explored with the calibrated model.The results indicate that the blast response of the air-backed slab induced by UWCE is fiercer than that of water-backed slab with equal charge mass.In addition,parametric studies are also conducted to explore the effects of the charge mass,standoff distance,reinforcement spacing,concrete compression strength,and boundary condition on the blast performance of the air-backed RC slab.

Underwater Explosion Treatment of Marine Soft Foundation

The consolidation of soft foundations is a problem that commonly appears in harbor construction engineering on muddy coasts. Introduced in this paper is the experimental study on underwater explosion treatment of soft foundations, including the regulation of explosion hollow development and medium movement, the law of similarity of densification and settlement of filled stone mass by shock wave pressure and explosion energy and their construction technologies, etc. A number of projects have been completed by applying this new technique, and the technique itself has been proved practicable.

3-D NUMERICAL SIMULATION OF UNDERWATER EXPLOSION BUBBLE

The problem is solved by use of the boundary integral equation method and the high order finite element discretization on the assumption that the flow of water is inviscid, incompressible and irrotational, and the bubble gas obeys the isoentropic rule. The evolution of bubble is simulated by means of mixed Euler-Lagrangian method. The comparison of the numerical results with the 'exact solution' of a spheric bub ble and experimental data shows that the mathematical model is reasonable and the 3-D numerical method has high accuracy. Important behaviour of bubble, such as immigration and jetting, and dynamic load on an ellipsoid near explosion are derived.

Investigation on charge parameters of underwater contact explosion based on axisymmetric SPH method

This paper investigates the effects of charge parameters of the underwater contact explosion based on the axisymmetric smoothed particle hydrodynamics （SPH） method. The dynamic boundary particle is proposed to improve the pressure fluctuation and numerical accuracy near the symmetric axis. An in-depth study is carried out over the influence of charge shapes and detonation modes on the near-field loads in terms of the peak pressure and impulse of shock waves. For different charge shapes, the cylindrical charge with different length-diameter ratios may cause strong directivity of peak pressure and impulse in the near field. Compared with spherical charge, the peak pressure of cylindrical charge may be either weakened or enhanced in different directions. Within a certain range, the greater the length-diameter ratio is, the more obvious the effect will be. The weakened ratio near the detonation end may reach 25% approximately, while the enhanced ratio may reach around 20% in the opposite direction. However, the impulse in different directions seems to be uniform. For different detonation modes, compared with point-source explosion, the peak pressure of plane-source explosion is enhanced by about 5%. Besides, the impulse of plane-source explosion is enhanced by around 5% near the detonation end, but close to those of the point-source explosion in other directions. Based on the material constitutive relation in the axisymmetric coordinates, a simple case of underwater contact explosion is simulated to verify the above conclusions, showing that the charge parameters of underwater contact explosion should not be ignored.

Shock wave and bubble characteristics of underwater array explosion of charges

In this study, we focused on the effect of the underwater explosion parameters of multi-point array explosion. The shock wave and bubble parameters of aggregate charge, two charges, and four charges were measured through an underwater explosion test, and their influence on the explosion power field of charge quantity and array distance was analyzed. Results show that the multi-shock wave collision of array explosion can be approximated to a linear superposition, and the interaction of delayed shock wave can be deemed as the increase of the shock wave baseline. Shock wave focusing and delayed superposition increase the shock wave peak pressure. Compared with the aggregate charge, the greater the number of array explosion points is, the higher the impulse and the gain of the bubble peak pressure are. At the same array distance, the smaller the charge quantity is, the higher the bubble impulse will be. At the same charge quantity, the smaller the array distance is, the higher the bubble impulse will be. The bubble period decreases gradually with the increase of the charge quantity, but the test orientation has little effect on the bubble period.

3D numerical simulation on fluid-structure interaction of structure subjected to underwater explosion with cavitation

In the underwater-shock environment, cavitation occurs near the structural surface. The dynamic response of fluid-structure interactions is influenced seriously by the cavitation effects. It is also the difficulty in the field of underwater explosion. With the traditional boundary element method and the finite element method （FEM）, it is difficult to solve the nonlinear problem with cavitation effects subjected to the underwater explosion. To solve this problem, under the consideration of the cavitation effects and fluid compressibility, with fluid viscidity being neglected, a 3D numerical model of transient nonlinear fluid-structure interaction subjected to the underwater explosion is built. The fluid spectral element method （SEM） and the FEM are adopted to solve this model. After comparison with the FEM, it is shown that the SEM is more precise than the FEM, and the SEM results are in good coincidence with benchmark results and experiment results. Based on this, combined with ABAQUS, the transient fluid-structure interaction mechanism of the 3D submerged spherical shell and ship stiffened plates subjected to the underwater explosion is discussed, and the cavitation region and its influence on the structural dynamic responses are presented. The paper aims at providing references for relevant research on transient fluid-structure interaction of ship structures subjected to the underwater explosion.

Parallel Computing of the Underwater Explosion Cavitation Effects on Full-scale Ship Structures

As well as shock wave and bubble pulse loading, cavitation also has very significant influences on the dynamic response of surface ships and other near-surface marine structures to underwater explosive loadings. In this paper, the acoustic-structure coupling method embedded in ABAQUS is adopted to do numerical analysis of underwater explosion considering cavitation. Both the shape of bulk cavitation region and local cavitation region are obtained, and they are in good agreement with analytical results. The duration of reloading is several times longer than that of a shock wave. In the end, both the single computation and parallel computation of the cavitation effect on the dynamic responses of a full-scale ship are presented, which proved that reloading caused by cavitation is non-ignorable. All these results are helpful in understanding underwater explosion cavitation effects.

Underwater explosion effects of 60 mm H.E. mortar bomb on a cylindrical concrete structure-PIT

PIT tests are usually performed when a mass distribution of High Explosive(H.E) projectile fragments is required. This paper shows the underwater detonation effects of 60 mm, M90 H.E. mortar bomb filled with Comp. B on cylindrical concrete structure(concrete pipe closed at one end-similar to a PIT test)which is 2 m high(inner height) with inner diameter of also 2 m. Thickness of both wall and bottom of a pipe is 0.35 m. Detailed characteristics of concrete which is used for manufacturing of a pipe are specified. Mortar bomb is submerged directly in to the water(no free airspace around the bomb) with the nose pointing to the bottom of a pipe. Number and mass of fragments after detonation are presented by table and photographs. Fragments of dummy fuze, through which blasting cap was protruded, are collected and reassembled to form a shape of a fuze after detonation where expanding of fuze material due to a detonation products is visualized. After underwater detonation, detonation of the same mortar bomb is performed in an empty pipe and the effects of this kind of detonation are observed. Distance at which fragments generated from submerged mortar bomb will not reach concrete pipes wall is also determined.

Experimental investigations on small-and full-scale ship models with polyurea coatings subjected to underwater explosion

Nowadays, the mitigation of damage to a ship caused by the underwater explosion attracts more and more attention from the modern ship designers. In this study, two kinds of scale tests were conducted to investigate the effects of polyurea coatings on the blast resistance of hulls subjected to underwater explosion. Firstly, small-scale model tests with different polyurea coatings were carried out. Results indicate that polyurea has a better blast resistance performance when coated on the front face, which can effectively reduce the maximum deflection of the steel plate by more than 20% and reduce the deformation energy by 35.7%-45.4%. Next, a full-scale ship(approximately 50 m × 9 m) under loadings produced by the detonation of 33 kg of spherical TNT charges was tested, where a part of the ship was coated with polyurea on the front face(8 mm + 24 mm) and not on the contrast area. Damage characteristics on the bottom were statistically analyzed based on a 3D scanning technology, indicating that polyurea contributes to enhancing the blast protection of the ship. However, damage results of this test were different from those of the small-scale tests. Moreover, the deformation area of the bottom with polyurea was greatly increased by 40.1% to disperse explosion energy, a conclusion that cannot be drown from the small-scale tests.

Investigation on the Determination of Initial Shock Pressure at Near Interface Field of TNT Charge and Water for Underwater Explosion

There are few report on the directly measurement of the initial shock pressure of explosive charge at its interface of water for underwater explosion. The special technologies have been taken to the measurement system with manganin piezoresistive gauge （PRG） in order to measure the initial shock pressure at the interface and its near field of TNT chare and water. The free-holding PRG film gauge can directly determine the shock peak pressure at the interface and near field of TNT charge up to 12.85 GPa, which is satisfying for the good agreement to the 12.97 GPa with one dimensional theoretical analysis and 12.86 GPa with numerical simulation. The maximum discrepancy is 0.93 %. The results show that it is precise and reliable to determine the initial shock pressure of underwater explosion charge with the PRG technology.

Wall Effect of Underwater Explosion Load Based on Wave Motion Theories

Owing to the existence of the flow field boundary, the shock wave load near the boundary is different from the freefield shock wave load. In the present paper, the hull plate load subjected to underwater shock wave is investigated based onwave motion theories; in addition, the experimental study of the hull plate load is carried out. According to the theoreticalanalysis of the hull plate pressure, we find that the hull plate pressure oscillates repeatedly and decays rapidly with timepassing, the maximum hull plate pressure is 2/（1＋n） times the maximum free field pressure, where n is the ratio ofimpedance, and the impulse is much smaller than the free field impulse. Compared with the experimental study, thetheoretical results agree well with the experimental data.

Prolonged simulation of near-free surface underwater explosion based on Eulerian finite element method

n the area of naval architecture and ocean engineering,the research about the underwater xplosion problem is of great significance.To achieve prolonged simulation of near-free surface underwater explosion,the underwater explosion transient numerical model is established in this paper based on compressible Eulerian finite element method(EFEM).Compared with Geers Hunter formula,EFEM is availably validated by simulating the free-field underwater xplosion case.Then,the bubble pulsation and flow field dynamic characteristics of the cases with different underwater explosive depth are compared in this work.Lastly,the height of the water hump and the pressure of flow flied are analyzed quantitatively through the simulation results.

Forensic Seismology and Boundary Element Method Application vis-à-vis ROKS Cheonan Underwater Explosion

On March 26,2010 an underwater explosion(UWE)led to the sinking of the ROKS Cheonan.The official Multinational Civilian-Military Joint Investigation Group(MCMJIG)report concluded that the cause of the underwater explosion was a 250 kg net explosive weight(NEW)detonation at a depth of 6 9 m from a DPRK"CHT-02D"torpedo.Kim and Gitterman(2012a)determined the NEW and seismic magnitude as 136 kg at a depth of approximately 8m and 2.04,respectively using basic hydrodynamics based on theoretical and experimental methods as well as spectral analysis and seismic methods.The purpose of this study was to clarify the cause of the UWE via more detailed methods using bubble dynamics and simulation of propellers as well as forensic seismology.Regarding the observed bubble pulse period of 0.990 s,0.976 s and 1.030 s were found in case of a 136NEW at a detonation depth of 8 m using the boundary element method(BEM)and 3D bubble shape simulations derived for a 136kg NEW detonation at a depth of 8 m approximately 5 m portside from the hull centerline.Here we show through analytical equations,models and 3D bubble shape simulations that the most probable cause of this underwater explosion was a 136 kg NEW detonation at a depth of 8m attributable to a ROK littoral"land control"mine(LCM).

Dynamic Adaptive Finite Element Analysis of Acoustic Wave Propagation Due to Underwater Explosion for Fluid-structure Interaction Problems

In this paper, an investigation into the propagation of far field explosion waves in water and their effects on nearby structures are carried out. For the far field structure, the motion of the fluid surrounding the structure may be assumed small, allowing linearization of the governing fluid equations. A complete analysis of the problem must involve simultaneous solution of the dynamic response of the structure and the propagation of explosion wave in the surrounding fluid. In this study, a dynamic adaptive finite element procedure is proposed. Its application to the solution of a 2D fluid-structure interaction is investigated in the time domain. The research includes：a） calculation of the far-field scatter wave due to underwater explosion including solution of the time-depended acoustic wave equation, b） fluid-structure interaction analysis using coupled Euler-Lagrangian approach, and c） adaptive finite element procedures employing error estimates, and re-meshing. The temporal mesh adaptation is achieved by local regeneration of the grid using a time-dependent error indicator based on curvature of pressure function. As a result, the overall response is better predicted by a moving mesh than an equivalent uniform mesh. In addition, the cost of computation for large problems is reduced while the accuracy is improved.

Similarity research of anomalous dynamic response of ship girder subjected to near field underwater explosion

The final anomalous sag distortion of the ship girder subjected to the near field underwater explosion （undex） below the middle ship is studied. The sinking exercise of Spruance class destroyer DD973 sunk by one MK48 torpedo is first presented, and a simulation model is established. The exponential attenuation phase, the reciprocal attenuation phase, the post reciprocal attenuation phase, and the negative pressure phase of the undex load are precisely applied in this model. The fluid-solid interaction, the added water mass, the gravity, and the change of buoyancy are also taken into account. The similarity theory is then used to analyze the dynamic response of the ship girder. Similarity parameters and theory prediction formulae of the dynamic response of the ship girder are presented, The physical meaning and influences of these similarity parameters are analyzed.

Experimental investigation on dynamic response and damage models of circular RC columns subjected to underwater explosions

Reinforced concrete(RC) columns are widely used as supporting structures for high-piled wharfs.The study of damage model of a RC column due to underwater explosion is a critical issue to assess the wharfs antiknock security.In this study,the dynamic response and damage model of circular RC columns subjected to underwater explosions were investigated by means of scaled-down experiment models.Experiments were carried out in a 10.0 m diameter tank with the water depth of 2.25 m,under different explosive quantities(0.025 kg-1.6 kg),stand-off distances(0.0 m-7.0 m),and detonation depths(0.25 m-2.0 m).The shock wave load and dynamic response of experiment models were measured by configuring sensors of pressure,acceleration,strain,and displacement.Then,the load distribution characteristics,time history of test data,and damage models related to present conditions were obtained and discussed.Three damage models,including bending failure,bending-shear failure and punching failure,were identified.In addition,the experie nce model of shock wave loads on the surface of a RC column was proposed for engineering application.

Interaction of underwater explosion bubble with complex elastic-plastic structure

Considering the elastic-plasticity of the structure, the combination of boundary element method and finite element method （FEM） is employed to present the calculation method for solving the complex coupled dynamic problem of bubble, elastic-plastic structure and the free surface, and the complete three-dimensional calculation program is developed as well. The error between the calculated result and the experimental result is within 10%. Taking a surface ship for example, the three-dimensional calculation program is extended to engineering filed. By employing the program, the response of the ship under the bubble loading is analyzed. From the stress-time history curves of typical elements of the structure, it can be seen that the pressure reaches its maximum when the bubble collapses and this validates that the pressure generated by the bubble collapse and the jet can cause serious damage on the ship structure. From the dynamic process of the interaction between the three-dimensional bubble and the ship, the low order vertical mode of the ship is provoked and the ship presents whip-shaped motion. And the ship does elevation and subsidence movement with the expansion and shrinkage of the bubble. Some rules and conclusions which can be applied to the engineering problems are obtained from the analysis in this paper.

An Analytical Solution for Dynamic Response of the Plate with Different Impedances Subjected to Underwater Explosion

In this paper, the transmitted part of the incident wave is considered to revise Taylor＇s solution, which is used to extend its application for analytical models to predict the response of the plate with different material properties. The influence of the material properties and the boundary condition of the plate on fluid and structural dynamics is systematically investigated. The analytical results are compared with those of detailed dynamic FE simulations and the two are in good agreement. The results indicate that the analytical method is valid and suitable for the plates with different material properties subjected to underwater explosion. It is found that Taylor＇s results of the plate with small impedance are invalid, which indicates a potential application field for the analytical method.

Underwater Explosion Technology for Offshore Engineering Applications

- This article briefs how to improve the holding power of a drilling vessel's anchor and to remove those unreasonable structures by such a technology - underwater-explosion technology. The article gives details of the structure, characteristics and applications of a rocket-launched anchor; as well as the principle, properties, applications of directional explosion cutting.

Dynamic Response of Floating Body Subjected to Underwater Explosion Bubble and Generated Waves with 2D Numerical Model

The low frequency load of an underwater explosion bubble and the generated waves can cause significant rigid motion of a ship that threaten its stability.In order to study the fluid-structure interaction qualitatively,a two-dimensional underwater explosion bubble dynamics model,based on the potential flow theory,is established with a double-vortex model for the doubly connected bubble dynamics simulation,and the bubble shows similar dynamics to that in 3-dimensional domain.A fully nonlinear fluid-structure interaction model is established considering the rigid motion of the floating body using the mode-decomposition method.Convergence test of the model is implemented by simulating the free rolling motion of a floating body in still water.Through the simulation of the interaction of the underwater explosion bubble,the generated waves and the floating body based on the presented model,the influences of the buoyancy parameter and the distance parameter are discussed.It is found that the impact loads on floating body caused by underwater explosion bubble near the free surface can be divided into 3 components:bubble pulsation,jet impact,and slamming load of the generated waves,and the intensity of each component changes nonlinearly with the buoyance parameter.The bubble pulsation load decays with the increase in the horizontal distance.However,the impact load from the generated waves is not monotonous to distance.It increases with the distance within a particular distance threshold,but decays thereafter.