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In conventional double-hulled submarines, the connecting structures that facilitate the linkage between the two hulls are crucial for load transmission. This paper aims to elucidate the effect of these connecting structures on resistance to shock waves generated by underwater explosions. Firstly, a self-developed numerical solver is built for the one-dimensional water-filled elastically connected double-layer plate model. The shock wave propagation characteristics, shock response of structure, water cavitation, and impact loads transmitted through the gap water and the connecting structures are analyzed quantitatively. The results reveal that the majority of the shock impulse is transmitted by the gap water if the equivalent stiffness of the connecting structures is much less than that of the gap water. Then, a three-dimensional model of the double-hulled, water-filled cylindrical shell is constructed in Abaqus/Explicit, utilizing the acoustic-structural coupling methodology. The analysis focuses on the influence of the thickness and density distribution of the connecting structures on the system's shock response. The results indicate that a densely arranged connecting structure results in a wavy deformation of the outer hull and a notable reduction in both the impact response and strain energy of the inner hull. When the stiffness of the densely arranged connecting structure is comparatively low, the internal energy and plastic energy of the inner hull are decreased by 16.5% and 24.1%, respectively. The findings of this research are useful for assessing shock resistance and for the design of connecting structures within conventional double-hulled submarines. [ABSTRACT FROM AUTHOR]

Numerous studies have shown that explosive sequence loads can cause serious damage to underwater vehicles, especially the bubble surge in the later stage of the explosion, which poses a huge threat to the internal structure of the vehicle. This study explores the damage characteristics of cylindrical shell structures under complete sequence loads based on the Arbitrary Lagrangian–Eulerian (ALE) method. By conducting experiments on the surge characteristics near the damaged plate under explosive action and comparing them with numerical results, the effectiveness of the method is verified. Subsequently, the damage characteristics of single- and double-layered cylindrical shell structures under underwater explosion sequence loads (shock waves, bubbles, surges) were explored, and the failure modes of cylindrical shell structures under various loads were summarized. The results indicate that the damage of shock waves to single-layer cylindrical shell structures is most severe at a blast distance of 0.5 m. For double-layer cylindrical shells, increasing the blast distance will reduce the impact of bubble surge on the pressure-resistant shell. The stress and strain in the central area of the pressure-resistant shell also decrease, and the deflection and Z-direction velocity also decrease accordingly. This study laid the foundation for enhancing the impact resistance of underwater vehicles. [ABSTRACT FROM AUTHOR]

Abstract The similarity test of ship stiffened plate structures under underwater explosions is a cost-effective and efficient method to evaluate the vitality of ships and guide the design of their shock resistance. This study focuses on the nonlinear impact response model tests of ship stiffened plate structures and their similarity laws with actual ships. The vertical motion of the ship stiffened plate structure is characterized by the Hurst index, and an equivalent relationship between the Hurst index of the model and the prototype is derived from classical similarity law. Based on the Hurst index, a similarity transformation relationship between the strain signals of the model and prototype is established. To verify the conclusions, similarity experiments of underwater explosions were conducted on both the model and the prototype. The original signals were grouped by the natural vibration period to determine the variation of the Hurst index over time. The model experiment strain signals for each natural vibration period were converted and compared with the prototype experiment results to verify the method's effectiveness. Simultaneously, the Hurst index of the stiffened plate structure under explosive shock load and its similarity transformation relationship with the prototype were simulated and analyzed. This provides theoretical and technical support for conducting analogous nonlinear response experiments for ship underwater explosions.

In this study, underwater explosion tests with 2.5 g trinitrotoluene explosive under different fixed plates with prefabricated holes were conducted. The experimental results showed that the air inflow from the prefabricated hole caused the bubble to collapse earlier with an increase in the hole diameter. In addition, the deformation mode of the thin plate transitioned from "convex" to "concave" (up to down). Next, the coupled Eulerian-Lagrangian method was used to perform the corresponding numerical simulation. The accuracy of the numerical simulation method was verified through a comparison with the experimental data. In addition, a series of numerical simulations were conducted with different prefabricated-hole diameters, blast distances, and prefabricated-hole shapes. The results showed that the bubble-pulsating water jet substantially influenced the deformation of the thin plate when the diameter of the prefabricated hole was within the theoretical maximum bubble radius. When the blast distance was within the theoretical maximum bubble radius, the thin plate was subjected to only a single bubble pulsation owing to the air inflow from the prefabricated hole. [ABSTRACT FROM AUTHOR]

The calibration of JWL‐Miller equation of state (EOS) parameters for aluminized explosive is a cumbersome but important work in explosive evaluation. Manual calibration is usually adopted while the work may be tedious and the optimal results may be unachievable. An automatic calibrating method was established to optimize this procedure based on genetic algorithm program and finite element software. Optimal JWL‐Miller EOS parameters were achieved by iterative calculation calibrating with cylinder test results and underwater‐explosion experiment results. Cylinder test results were adopted to illustrate the initial phase of explosion, and underwater explosion experiments were conducted to calibrating the Miller term of the equation of state. The results showed that the error between cylinder test and simulation result was less than 1 %, the error of underwater explosion impulse between test and simulation results was less than 3.73 %. The optimized parameters of JWL‐Miller EOS will be useful in the numerical simulation research of aluminized explosives. [ABSTRACT FROM AUTHOR]

ObjectiveThis paper aims to analyzes the shock response characteristics of a large-scale floating shock platform (LFSP) subjected to underwater explosion. MethodsThe acoustic-solid coupling approach is used as the basis for the numerical simulation. The validity of the calculation method used in this paper is verified by comparing the experimental data with simulation, and the shock response characteristics of the location of interest in the LFSP's inner bottm plating is investigated in terms of its longitudinal section distribution as well as the effects of changes in the horizontal distance to the blast source on the whole shock environment of LFSP's inner bottom structure, thereby the LFSP's shock response prediction formula is obtained by fitting method.ResultsThe results indicate that the shock response characteristics of the locations (i.e. outer and inner plates, longitudinal section in centre plane) distributed along the longitudinal section of the inner bottom structure are basically the same. The shock response amplitude of the front and back sides of the floating platform are small, while that of the middle is large. The shock responses of the outer and inner plates differ, and the spectral velocity of the inner plate is greater than that of the outer plate, while the spectral displacement of the outer plate is greater than that of the inner plate.ConclusionsThe measurement points within the inner bottom plating are excited so spikes appear in the shock spectral lines, resulting in the spectral velocity of the inner plate is higher than that of the outer plate. The horizontal distance of bottom plate to the blast source is smaller so the shock response is stronger, bringing about a greater spectral displacement of the outer plate than that of the inner plate.

ObjectiveTo address the issue of assessing structural breach damage in ships under underwater explosion, a breach prediction method based on the PCA-BOA-KNN model is established. MethodsFirst, finite element models for five-compartment and seven-compartment segments are constructed, and explosion simulation analysis is carried out for 21 sets of underwater explosion conditions. Subsequently, principal component analysis (PCA) is employed to reduce the dimensionality of the peak acceleration, peak velocity, peak displacement, peak stress and peak overpressure values, resulting in two principal features. Finally, the PCA results are integrated into a Bayesian optimization algorithm (BOA) K-Nearest Neighbors (KNN) model. The established breach prediction model is used to predict the breach conditions at different ship cross-sections under a set of conditions. ResultsThe results show that by using PCA to extract the first two factors, the cumulative contribution rate is 85.165%. Therefore, the first two factors can represent the primary information of the five features. The results obtained using the PCA-BOA-KNN breach prediction model are generally consistent with the simulation results. ConclusionThe proposed prediction model approach is effective for predicting ship structural breaches and has reference value for predicting breachs in ship structures with different principal dimensions.

ObjectiveThis paper studies the damage modes of targets simultaneously subjected to multiple damage elements by analyzing the damage characteristics of a cylindrical shell structure subject to a double underwater explosion (UNDEX). MethodsFirst, a finite element model of the cylindrical shell is established, then the arbitrary Lagrangian−Eulerian (ALE) method is used to investigate its damage characteristics when subject to UNDEX with single or dual explosion sources, and the damage effects are evaluated in terms of the structural fracture area, maximum vertical displacement and extent of plastic strain area, and the 500 kg TNT charge is calculated and analyzed. ResultsThe results show that when the standoff distance of dual explosion source is 1 m, the damage to the outer cylindrical shell is more serious when the standoff distance of dual explosion sources is equal to that of the single explosion source, but the damage to the inner cylindrical shell is less than that of the single explosion source. At the same time, with the increase in standoff distance of the two TNT charges, especially when the standoff distance is greater than 3 m, the damage effects on the cylindrical shell structure become weaker. ConclusionsBy calculating the damage to a cylindrical shell structure by UNDEX under different initial parameters, the general rules of damage to cylindrical shell structures by single and dual explosion sources are given, which can provide references for enhancing the power of underwater weapons.

ObjectiveIn order to explore the effects of different stiffening configurations on the bearing capacity and anti-blast performance of double stiffened cylindrical shells, a numerical study is made of the responses of four kinds of double stiffened cylindrical shells under hydrostatic pressure and underwater explosion load.MethodsFirst, finite element models of stiffened cylindrical shells with different structural forms are established. Next, finite element software ANSYS is used to calculate and analyze the influence of each structural form on the strength, stability and ultimate bearing capacity of the cylindrical shell. Finally, finite element software Abaqus/Explicit is used to calculate and analyze the influence of each structural form on the deformation deflection and plastic strain of the cylindrical shell under explosion load. ResultsCompared with a traditional single stiffened cylindrical shell with the same mass, the I-shaped double stiffened cylindrical shell has a great advantage in carrying capacity. When the explosion load level is low, the anti-blast performance of a double cylindrical shell with small rib spacing is similar, but decreases when the explosion load level is high.ConclusionThe reasonable design of double stiffened cylindrical shells can yield better design schemes for bearing capacity and anti-blast performance.

Abstract Underwater explosions (UNDEX) generate shock waves that interact with the air–water interface and structures, leading to the occurrence of rarefaction waves and inducing cavitation phenomena. In deep-water explosions, complex coupling relationships exist between shock wave propagation, bubble motion, and cavitation evolution. The shock wave initiates the formation of cavitation, and their growth and collapse are influenced by the pressure field. The collapsing bubbles generate additional shock waves and fluid motion, affecting subsequent shock wave propagation and bubble behavior. This intricate interaction significantly impacts the hydrodynamic characteristics of deep-water explosions, including pressure distribution, density, and phase changes in the surrounding fluid. In this paper, we utilize a two-fluid phase transition model to capture the evolution of cavitation in deep-water explosions. Our numerical results demonstrate that the introduction of a two-phase vapor–liquid phase change model is necessary to accurately capture scenarios involving prominent evaporation or condensation phenomena. Furthermore, we find that the cavitation produced by the same charge under different explosion depths exhibits significant differences, as does the peak value of cavitation collapse pressure. Similarly, the cavitation produced by different charge quantities under the same explosion depth varies, and the relationship between cavitation volume and charge quantity is not a simple linear increase. The research methods and results presented in this paper provide an important reference for studying the dynamic characteristics of deep-water explosions.

This paper examines the response characteristics of a warship's double-layer plates under a secondary near-field explosion after the ship's outer plate has been perforated by shaped metal jets. First, the effectiveness of the Coupled Eulerian–Lagrangian (CEL) method was validated, showing numerical simulations to be well aligned with experimental results. Subsequently, the damage inflicted on the outer plate by metal jets was simplified to a prefabricated orifice, further studying the explosive impact response of double-layer plates under different inter-compartmental water levels and charge distances. Our findings indicated the following: (1) shockwave and bubble pulsation loads are the main causes of deformation in the outer plate; (2) the driving of the outer plate and the flooding water between compartments are the main causes of deformation in the inner plate; and (3) deformation in the outer plate will decrease as the water level in the compartment increases, while deformation in the inner plate will increase with the increasing water level. Consequently, under certain specific damage, the ingress of water into a compartment effectively enhances the explosion resistance of the double-layer plates. [ABSTRACT FROM AUTHOR]

• Underwater explosion induced cavitation near sacrificial coatings are studied. • RKDG-FE-GFM-IsenCav is developed to study the cavitation effects. • Five benchmark problems are studied. • Effects of geometrical and physical properties of the model are discussed. The load resulting from cavitation collapse is a major threat to structures exposed to underwater explosions. Protective structures for warships, such as sacrificial coatings, which are sandwich structures attached to the wet face of the ship hull. They have energy absorbing cores made of cellular or composite materials that can significantly influence the effects of cavitation. In this study, a physical model consisting of water, an explosion bubble, and a sacrificial coating is established to investigate the effects of cavitation near sacrificial coatings subjected to underwater explosions. A numerical method is developed that couples the Runge-Kutta discontinuous Galerkin method, the finite element method, a Ghost Fluid Method-based numerical technique, and an isentropic one-fluid cavitation model to capture the physical phenomena of shock wave propagation, cavitation inception, expansion and collapse, and explosion bubble expansion and contraction. To ensure the accuracy of the mathematical model, five benchmark problems and a convergence test of the numerical solution are studied. The study comprehensively discusses the influences of core strengths, face sheet thickness, stand-off distances, and structure curvature on the evolution of the cavitation region, pressure profiles near the structure, and structure responses. [ABSTRACT FROM AUTHOR]

The dynamic compressive responses of a multi–layer sandwich structures with the truss core were studied experimentally and numerically under the underwater explosion shock loading. A three–layer truss-core sandwich specimen was designed and fabricated from the stainless steel. The density gradient between core layers is realized by changing the cross-sectional size of the truss members in the core layer. A water tank explosion testing device was employed to carry out the underwater explosion experiment. The finite element method was used to investigate the influences of the layered strategy of structures and the density gradient between core layers on the dynamic response. The results reveal that the density–decreasing sandwich panels CBA has the optimal underwater explosion resistance, and the explosion resistance increases as the decrease of the density gradient ratio. [ABSTRACT FROM AUTHOR]

The article presents the results of in situ tests of a concrete massive buttress dam under dynamic impacts of varying intensity. The study analyzes the dynamic response of the structure to impulse excitation from a vibration generator, from underwater explosions, and from the action of minor earthquakes. The dynamic buttress dam characteristics are evaluated. The damping parameters and natural frequencies of the structure are specified. [ABSTRACT FROM AUTHOR]

To study the load-transfer law and shock environment of a pipe–float system, an experimental model was designed, manufactured, and installed on a floating shock platform. Two underwater explosion cases were studied, focusing on vertical and horizontal shock cases. The experimental results show that the structure of the system on the floating raft significantly influences the load acceleration transfer and the composition of the vertical and horizontal components. Furthermore, the flexible connector can effectively block the response behavior of the two ends of the pipe, and the peak acceleration difference between the two pipe ends is 98.9%. The vertical and horizontal components of the shock-wave load affect the stress concentration locations on the pipe. The main frequency in the shock environment at the connection point between the floating raft and the upper equipment shifts toward middle and low frequencies under the influence of the upper equipment, and it will be closer to the natural frequency of the upper equipment. [ABSTRACT FROM AUTHOR]

The article compares the results of empirical expressions for the maximum peak pressures of an underwater explosion and numerical models made in hydrocode software LS-DYNA. Spherical charge of 136.08 kg TNT explosive was chosen and observed distance was up to 15 m. An overview of empirical expressions is given and the results according to several authors are compared. The Cole expression was chosen as a reference for comparison. For the purpose of the numerical modelling, the influence of the size of the finite elements of the explosive, the size and shape of the volume of water and the size of the finite elements of water was examined. The size of the finite elements of the medium (water) was shown to be the most significant parameter that affects the magnitude of the pressure results, and recommendations for modelling were given.

ObjectivesIn order to enhance the anti-shock performance of a double-hull submarine against non-contact underwater explosions, a method of plastering anti-shock coating on the surface of double-hull ships is proposed, and the shock response of the inner hull is used to evaluate the shock mitigation effects of different anti-shock coating approaches. MethodsBy employing the acoustic-solid coupling method, a nonlinear fluid-structure coupling dynamic model is established for the external flow field, outer hull, gapped water and inner hull. Three approaches to laying anti-shock coating on the hull are compared, namely covering it on both the inner and outer hulls (Case 2), on the inner hull (Case 3) and on the outer hull (Case 4). ResultsAs the results show, in Case 2, the average reduction rates in the peak values of acceleration, velocity and displacement are 94.1%, 81.2% and 23.3% respectively, while the average reduction rates in the peak values of circumferential and axial micro-strain are 67.7% and 88.3% respectively; in Case 3, the three values are 60.4%, 45.4% and −2.1%, and the two values are 34.6% and 68.0%; and in Case 4, the three values are 86.7%, 75.1% and 20.3%, and the two values are 68.6% and 77.8%. ConclusionsCovering the hull with anti-shock coating can effectively enhance the anti-shock performance of double-hull ships, especially in terms of the acceleration and velocity results, and coating the outer hull plays a primary protective role.

In conventional double-hulled submarines, the connecting structures that facilitate the linkage between the two hulls are crucial for load transmission. This paper aims to elucidate the effect of these connecting structures on resistance to shock waves generated by underwater explosions. Firstly, a self-developed numerical solver is built for the one-dimensional water-filled elastically connected double-layer plate model. The shock wave propagation characteristics, shock response of structure, water cavitation, and impact loads transmitted through the gap water and the connecting structures are analyzed quantitatively. The results reveal that the majority of the shock impulse is transmitted by the gap water if the equivalent stiffness of the connecting structures is much less than that of the gap water. Then, a three-dimensional model of the double-hulled, water-filled cylindrical shell is constructed in Abaqus/Explicit, utilizing the acoustic-structural coupling methodology. The analysis focuses on the influence of the thickness and density distribution of the connecting structures on the system’s shock response. The results indicate that a densely arranged connecting structure results in a wavy deformation of the outer hull and a notable reduction in both the impact response and strain energy of the inner hull. When the stiffness of the densely arranged connecting structure is comparatively low, the internal energy and plastic energy of the inner hull are decreased by 16.5% and 24.1%, respectively. The findings of this research are useful for assessing shock resistance and for the design of connecting structures within conventional double-hulled submarines.

A two-fluid, phase transition-based multiphase flow model is employed to simulate the dynamics of phase transition between liquid and vapor phases during shock wave and rarefaction wave propagation in underwater explosions. The aim is to understand the influence of a rigid wall on the cavitation evolution process and the cavitation collapse load, considering various charge quantities and water depths. The evolution of crucial physical qualities, such as the density, pressure, and the cavitation domain, within the flow field are analyzed and summarized. The presence of a rigid wall is found to significantly impact the cavitation evolution process in underwater explosions. It affects the shape, size, and dynamics of the cavitation domain, as well as the interaction between the explosion and the surrounding fluid. Specifically, the reflected wave on the wall influences the cavitation collapse load, leading to notable differences in the collapse time and collapse pressure compared to free-field conditions. Under different operating conditions, the size and position of the cavitation domain exhibit distinct changes. The proximity of the rigid wall results in unique patterns of cavitation domain evolution, which in turn lead to variations in the pressure distribution and the emergence of new cavitation regions. The findings of this study provide valuable insights into the behavior of cavitation and atomization induced by underwater explosions near the free surface. The understanding gained from these investigations can contribute to the development of effective safety measures and protective strategies in marine and underwater engineering applications. By accurately predicting and mitigating the effects of cavitation, it is possible to enhance the design and operation of underwater structures, ensuring their integrity and minimizing the potential risks associated with underwater explosions. [ABSTRACT FROM AUTHOR]

The article compares the results of empirical expressions for the maximum peak pressures of an underwater explosion and numerical models made in hydrocode software LS-DYNA. Spherical charge of 136.08 kg TNT explosive was chosen and observed distance was up to 15 m. An overview of empirical expressions is given and the results according to several authors are compared. The Cole expression was chosen as a reference for comparison. For the purpose of the numerical modelling, the influence of the size of the finite elements of the explosive, the size and shape of the volume of water and the size of the finite elements of water was examined. The size of the finite elements of the medium (water) was shown to be the most significant parameter that affects the magnitude of the pressure results, and recommendations for modelling were given. [ABSTRACT FROM AUTHOR]

Cavitation resulting from underwater explosions in compressible multiphase or multicomponent flows presents significant challenges due to the dynamic nature of shock–cavitation–structure interactions, as well as the complex and discontinuous nature of the involved interfaces. Achieving accurate resolution of interfaces between different phases or components, in the presence of shocks, cavitating regions, and structural interactions, is crucial for modeling such problems. Furthermore, pressure convergence in simulations involving shock–cavitation–structure interactions requires accurate algorithms. In this research paper, we employ the diffuse interface method, also known as the interface-capturing scheme, to investigate cavitation in various underwater explosion test cases near different surfaces: a free surface and a rigid surface. The simulations are conducted using the unstructured compressible Navier–Stokes (UCNS3D) finite-volume framework employing central-weighted essentially non-oscillatory (CWENO) reconstruction schemes, utilizing the five-equation diffuse interface family of methods. Quantitative comparisons are made between the performance of both models. Additionally, we examine the effects of cavitation as a secondary loading source on structures, and evaluate the ability of the CWENO schemes to accurately capture and resolve material interfaces between fluids with minimal numerical dissipation or smearing. The results are compared with existing high-order methods and experimental data, where possible, to demonstrate the robustness of the CWENO schemes in simulating cavitation bubble dynamics, as well as their limitations within the current implementation of interface capturing. [ABSTRACT FROM AUTHOR]

In order to improve the explosion characteristics of emulsion explosives, titanium hydride was added to emulsion explosives to produce a new type of hydrogen storage emulsion explosives. Charges with different contents of titanium hydride were evaluated through underwater explosion experiments and detonation velocity tests. The tests on underwater explosion and detonation velocity reveal that compared to pure emulsion explosives, the detonation parameters of emulsion explosives containing titanium hydride showed a trend of first increasing and then decreasing. When the mass ratio of titanium hydride in the emulsion explosive is 1 % to 3 %, all detonation parameters have been improved to a certain extent. When the mass ratio of titanium hydride in the emulsion explosive is 3 % to 10 %, only part of the detonation parameters (specific impulse, specific shock energy, specific total energy and volume energy density) has been improved. The maximum increase of specific impulse, specific shock energy, specific total energy and volume energy density of emulsion explosive containing titanium hydride is 7.06 %, 8.95 %, 3.97 % and 8.22 %, respectively. Based on the analysis, it is evident that though powdered TiH2 participates in the detonation reaction process of the emulsion explosive, the majority of TiH2′s energy is released during the secondary reaction occurring after the detonation wave front. Therefore, the detonation performance of emulsion explosives can be effectively improved by adding a certain mass ratio of titanium hydride. [ABSTRACT FROM AUTHOR]

The manuscript analyzes the impact of a non-contact underwater explosion on the foundation of a 35 mm naval cannon mounted on board a Project 258 minehunter. The finite element method was used to complete the task. Cole's empirical formulas were used to describe the distribution of the pressure wave from the explosion of the TNT charge in water as a function of distance, time, and mass. The hull geometry was reflected based on technical documentation as a shell structure reinforced with beam-bar elements. Devices with large weights were represented as rigid bodies. Simplifications were used to minimize the number of degrees of freedom. The construction of ship's hull is made of non-magnetic austenitic steel. The dynamic characteristics of this steel were determined based on static and dynamic tensile tests. The Johnson-Cook constitutive model was used to describe the material properties of steel. As part of the work, the impact resistance study of marine structures was presented, how it is defined by the existing regulations in the Polish Navy was considered, and the scope of their applicability was given. The scientific innovation of the presented work consists of checking and specifying the guidelines for designing and constructing warships. [ABSTRACT FROM AUTHOR]

ObjectivesThe structural parameters of the stiffened plate have an essential influence on its resistance to underwater explosion. It is necessary to analyze the specific influence degree of different parameters such as the stiffeners' height, number, and thickness. MethodsA series of numerical simulations were performed using the CEL method. Then, the influence of stiffened plates' structural parameters on their resistance to underwater explosion was analyzed. Based on the energy absorption law of stiffened plates underwater explosion load, a coefficient was introduced to characterize the influence degree of stiffeners. A calculation method of equivalent thickness was proposed considering stiffener coefficients. ResultsThe height of the stiffener, the panel thickness, and the stiffener's thickness have remarkable effects on the resistance to underwater explosion. In contrast, the thickness of the wing plate has an insignificant impact. Under the condition of the same amount of structural mass change, the influence of the height of the transverse/longitudinal stiffener on the damaged area of the stiffened plate is 2.5 times that of the wing panel thickness. The impact of the panel thickness and the thickness of the transverse/longitudinal stiffener on the damaged area is twice that of the wing panel thickness. With the increase of the stiffeners' height, number, and thickness, the structural strength and resistance to underwater explosion of the stiffened plate are enhanced, and the damage degree of the stiffened plate is reduced. The correlation coefficient of the proposed equivalent thickness calculation method considering the stiffener coefficient is −0.94. The method can adequately reflect the stiffened plate's resistance to underwater explosion. ConclusionsThe research results can be used as the basis for evaluating the ship structure against underwater explosion load and provide a reference for the design of the ship's anti-explosion structure.

Numerous studies have shown that explosive sequence loads can cause serious damage to underwater vehicles, especially the bubble surge in the later stage of the explosion, which poses a huge threat to the internal structure of the vehicle. This study explores the damage characteristics of cylindrical shell structures under complete sequence loads based on the Arbitrary Lagrangian–Eulerian (ALE) method. By conducting experiments on the surge characteristics near the damaged plate under explosive action and comparing them with numerical results, the effectiveness of the method is verified. Subsequently, the damage characteristics of single- and double-layered cylindrical shell structures under underwater explosion sequence loads (shock waves, bubbles, surges) were explored, and the failure modes of cylindrical shell structures under various loads were summarized. The results indicate that the damage of shock waves to single-layer cylindrical shell structures is most severe at a blast distance of 0.5 m. For double-layer cylindrical shells, increasing the blast distance will reduce the impact of bubble surge on the pressure-resistant shell. The stress and strain in the central area of the pressure-resistant shell also decrease, and the deflection and Z-direction velocity also decrease accordingly. This study laid the foundation for enhancing the impact resistance of underwater vehicles.

Over the past century, the safety of dams has gradually attracted attention from all parties. Research on the dynamic response and damage evolution of dams under extreme loads is the basis of dam safety issues. In recent decades, scholars have studied the responses of dams under earthquake loads, but there is still much room for improvement in experimental and theoretical research on small probability loads such as explosions. In this paper, a 50-m-high concrete gravity dam is used as a prototype dam, and a water explosion model test of a 2.5-m-high concrete gravity dam is designed. The water pressure and the acceleration response of the dam body in the test are analysed. The pressure characteristics and dynamic response of the dam body are assessed. Taking the dam damage test as an example, a numerical model of concrete gravity dam damage is established, and the damage evolution of the dam body is analysed. By combining experiments and numerical simulations, the damage characteristics of the dam body under the action of different charge water explosions are clarified. The integrity of the dam body is well maintained under the action of a small-quantity water explosion, and the dynamic response of the dam body is mainly caused by the shock wave. Both the shock wave and the bubble pulsation cause the dam body to accelerate, and the peak acceleration of the dam body under the action of the bubble pulsation is only one percent of the peak acceleration of the dam body under the action of the shock wave. When subjected to explosions in large quantities of water, the dam body is seriously damaged. Under the action of a shock wave, the dam body produces a secondary acceleration response, which is generated by an internal interaction after the dam body is damaged. The damage evolution process of the dam body under the action of a large-scale water explosion is analysed, and it is found that the shock wave pressure of the water explosion causes local damage to the dam body facing the explosion. After the peak value of the shock wave, the impulse continues to act on the dam body, causing cumulative damage and damage inside the dam body.

Based on the static and dynamic test results of steel fiber synergistically reinforced cellular concrete (SFSRCC) under a complex stress state, the yield surface equation and strain rate effect are modified to construct a modified Holmquist–Johnson–Cook (HJC) model suitable for the dynamic compression behavior characteristics of SFSRCC, and the modified HJC model is verified. At the same time, a finite width SFSRCC protective layer–roller compacted concrete (RCC) slab–water–explosive multimedia coupling model is established to explore the influence of setting different thicknesses of the SFSRCC protective layer (0, 0.1, 0.2, and 0.3 m) on the explosion‐proof performance and effect of the RCC slab structure. The results show that the heavy stress–strain curves and dynamic failure modes of the SFSRCC under different strain rates are in good agreement with the experimental results. The maximum errors of peak stress and peak strain between the simulation results and the experimental results are 1.04% and 6.6%, respectively, which confirms the validity of the HJC correction model and the accuracy of the simulation results. After setting different thicknesses of the protective layer, the RCC slab blast crater depth is reduced by 56.7%, 80%, and 93.3% compared with that at 0.18 m without a protective layer program, and almost no damage to the back explosion surface occurs. The failure volume ratio of the protected RCC slab structure gradually decreases from 38.53% to 3.24% with increasing protective layer thickness, and the protected RCC slab shows only slight damage after setting the protective layer. When the thickness of the protective layer is 0.3 m, the damage of the protected RCC slab structure is distributed in the surface area, which has little effect on the structural safety. These results show that the additional SFSRCC protective layer under the condition of underwater near‐field explosion can significantly reduce the damage degree of the slab structure and improve the explosion‐proof performance of the protected RCC slab structure. This research offers a theoretical reference for the explosion‐proof material selection and design of hydraulic structures and explosion‐proof application of SFSRCC under the action of underwater explosion impact loads. [ABSTRACT FROM AUTHOR]

To conduct a precise shock assessment of marine diesel engines, a 200 t floating shock platform was utilized to simulate realistic testing conditions. The testing generated the acceleration time curve and the shock response spectrum for the diesel engine. According to the applicable standards, the spectral velocity was chosen as the evaluation index, and an evaluation of the longitudinal, transverse, and vertical shock environment of the diesel engine was conducted. The shock factor interpolation method was corrected using the confidence interval based on normal distribution, and the interpolated confidence interval of the shock factor was determined. The findings reveal that shock waves were identified as the primary external force, and it was found that the influence of bubble pulsation can be disregarded when assessing a floating shock platform. This paper proposes the use of normal-distribution-based shock factor confidence intervals, which can accurately predict multidirectional shock factors and offer improved shock safety compared to the traditional method of unidirectional shock factor interpolation. The results and methods obtained in this study can provide valuable guidance and assistance for predicting the shock environment of large shipboard machinery on significant floating shock platforms. [ABSTRACT FROM AUTHOR]

The interaction of the two coupling bubble with a floating structure is modelled to investigate the dynamic behavior of the warship under an underwater explosion (UNDEX) by coupled Eulerian-Lagrangian (CEL) method. In this study, the CEL method is first verified by the experimental results for free field underwater explosion. Then, a series of cases for various initial distance and delay intervals between the bubble pair are implemented to analyze the deformation and damage modes of the floating structure. It is observed that for large initial distance between the bubble pair synchronously generated, the floating structure can be damaged with longitudinal bending buckling in the middle domain. When the initial distance between the bubble pair decreases to a critical value, the floating structrure is damaged with a remarkable crevasse besides the longitudinal bending buckling in the middle domain. However, under two bubbles with different phase differences, the floating structure a the deformation of the whipping motion without remarkable local structural damage. This is because there is the conspicuous despression effect between two out-of-phase bubbles, which weakens the bubble pulsation load. These phenomena present a theoretical reference for further studies on the destruction of underwater explosion on the ship structure caused by multiple bubbles generated with various phase differences. [ABSTRACT FROM AUTHOR]

This paper starts with the importance and basic physical phenomena of underwater explosion, explaining the background and significance of ship damage and protection research under underwater explosion loads, research progress and status, as well as key challenges. To address these challenges, the paper elaborates on popular underwater explosion theories, models and methods. In terms of theoretical and computational research, a unified theory of bubble dynamics is presented, as well as a transient strong non-linear gas-liquid-solid fully coupled model and numerical method for underwater explosion, which have been used to develop a fundamental industrial software FSLAB capable of solving practical problems in fluid-structure interaction. In terms of experimental research, underwater explosion surrogate experimental methods and model testing methods are elaborated. Based on this, theoretical analysis, computational and experimental results in the field of ship damage and protection under underwater explosion loads are presented and discussed, aiming to provide references for underwater explosion-related research.

Underwater explosions that threaten the hull integrity and proper operation of navy ships' systems and subsystems are considered from the viewpoint of damage estimation. The commercial code LS-DYNA with the Arbitrary Lagrangian-Eulerian numerical technique is used to analyze the shock effects on structural components of a ship-like form for three different scenarios. Computed maximum permanent deformations are matched with the corresponding keel shock factor (KSF) values for searching a functional relationship of a linear form. Although observed for limited simulations and therefore should be viewed with caution, the linear dependency of the maximum permanent deformations on the KSF values implies that the KSF may be a simple indicator for estimating the extent of damage due to underwater blasts.

ObjectiveIn order to improve the explosion and impact resistance of the protective structures of unmanned underwater vehicles (UUVs), autonomous underwater vehicles (AUVs), air bottles, etc., the structural response and failure modes of carbon fiber reinforced plastic (CFRP) cylindrical shells under underwater explosion and high hydrostatic pressure are investigated. MethodA computational model of CFRP cylindrical shell implosion under the combined action of hydrostatic pressure and impact load is established using ABAQUS software and the coupled Euler-Lagrange (CEL) method. The effectiveness of the numerical simulation method is then verified by comparison with the experimental results. On this basis, the failure modes and parametric effects of CFRP cylindrical shell implosion are obtained. ResultsThe underwater implosion of composite cylindrical shells can be divided into three stages: buckling, wall contact and failure propagation. Reducing the length-to-diameter ratio of the CFRP cylindrical shell can improve the impact resistance ability and affect the failure mode of the structure. With the increase in the number of fiber layers, the static water bearing capacity and impact resistance ability of the shell structure increase. With the increase in the impact block velocity, the wall boundary contact and failure propagation of the cylindrical shell become more obvious, matrix fractures occur more frequently and the cracks show an obviously increasing trend in the lengthwise direction of the cylindrical shell. ConclusionThe results of this study can provide data guidance for the structural design of underwater vehicles and promote the application of composite materials in the field.

This paper examines the response characteristics of a warship’s double-layer plates under a secondary near-field explosion after the ship’s outer plate has been perforated by shaped metal jets. First, the effectiveness of the Coupled Eulerian–Lagrangian (CEL) method was validated, showing numerical simulations to be well aligned with experimental results. Subsequently, the damage inflicted on the outer plate by metal jets was simplified to a prefabricated orifice, further studying the explosive impact response of double-layer plates under different inter-compartmental water levels and charge distances. Our findings indicated the following: (1) shockwave and bubble pulsation loads are the main causes of deformation in the outer plate; (2) the driving of the outer plate and the flooding water between compartments are the main causes of deformation in the inner plate; and (3) deformation in the outer plate will decrease as the water level in the compartment increases, while deformation in the inner plate will increase with the increasing water level. Consequently, under certain specific damage, the ingress of water into a compartment effectively enhances the explosion resistance of the double-layer plates.

To study the load-transfer law and shock environment of a pipe–float system, an experimental model was designed, manufactured, and installed on a floating shock platform. Two underwater explosion cases were studied, focusing on vertical and horizontal shock cases. The experimental results show that the structure of the system on the floating raft significantly influences the load acceleration transfer and the composition of the vertical and horizontal components. Furthermore, the flexible connector can effectively block the response behavior of the two ends of the pipe, and the peak acceleration difference between the two pipe ends is 98.9%. The vertical and horizontal components of the shock-wave load affect the stress concentration locations on the pipe. The main frequency in the shock environment at the connection point between the floating raft and the upper equipment shifts toward middle and low frequencies under the influence of the upper equipment, and it will be closer to the natural frequency of the upper equipment.

Submerged structures in naval applications are susceptible to underwater explosion and predicting the transient response and damage is a prime concern for the designer. Coupled Arbitrary Lagrangian-Eulerian (ALE) based numerical investigation using LS-DYNA for the inelastic dynamic response and microstructure failures of composite hull subjected to UNDEX is presented. As a prelude to the study, shock pressure in fluid domain and strain values obtained on the composite pressure hull is compared with that available in literature. The numerical study is extended to predict the large deformation, puncture and microstructural behaviours of the hull by varying the charge weight of the explosive. The results of the predicted large deformation, puncture of the hull and the microstructural failures such as fibre tension, fibre compression, matrix shear, matrix tension, and matrix compression based on the Chang- Chang composite failure model, Tsai-Wu failure theory and maximum stress theory are presented. [ABSTRACT FROM AUTHOR]

As a significant detection task, underwater explosion events have piqued the interest of geophysicists. On September 26, 2022, two underwater explosions occurred in the Baltic Sea region involving the Nord Stream pipeline. Subsequent reports from the media and the European-Mediterranean Seismological Centre deduced that one event was detected at 17:03:49 GMT, exhibiting a magnitude of 3.1 on the Richter scale, whereas the second event lacked comprehensive information. To meticulously investigate the details of the two Nord Stream incidents, we employed 14 proximate stations for subsequent analysis. Using a linear inversion technique, we determined that the two events occurred at 00:03:30 and 17:03:49, with corresponding coordinates of 54.87°N, 15.52°E and 55.59°N, 15.80°E, respectively, and Richter magnitudes of 2.3 and 3.1, respectively. Spectral analysis corroborates that both events were underwater explosions, and the presence of anomalous signals within the frequency domain may provide valuable insights into source parameters and characteristics, particularly for the first event. [ABSTRACT FROM AUTHOR]

The consequences of dam damage caused by explosions, wars, and terrorist attacks are extremely serious, and they can cause casualties among downstream residents. Studying the damage behaviors of dams is a prerequisite for improving their anti-knock performance. Researchers view the dam as homogeneous for research; but in reality, the concrete strength of the dam decreases from bottom to top. The partitioning of dam concrete strength can meet the different functional and economic requirements of a concrete gravity dam (referred to as concrete strength partition gravity dam (CSPGD)). Therefore, CSPGD shows a more complex dynamic performance and failure characteristics under the impact load of an underwater explosion. First, by investigating the current status of anti-knock research on CSPGDs, a fully coupled finite element numerical model for an underwater explosion of CSPGD was established. Considering the initial stress such as the self-weight of the dam, the upstream reservoir hydrostatic pressure, and the uplift pressure of the dam foundation during the service period, the anti-knock performance of CSPGD was studied. The results showed that the interface of CSPGD had a strain rate effect under the action of blast load, and it was easy to produce tensile failure at a low strain rate. In addition, the dynamic response and damage characteristics under different explosion scenarios such as explosive charge weight (w), detonation depth (D), and standoff distance (R) were further studied. The dam crest was always a weak anti-knock part, and the foundation anti-sliding stability was also very important to dam safety. Therefore, it was proposed and suggested to use the crack length of the dam crest and dam foundation to evaluate the overall anti-knock capacity of CSPGD. The study also found that the detonation depth affected the response time of dam damage and had a significant impact on the anti-knock performance of CSPGD. [ABSTRACT FROM AUTHOR]

A gravity dam monolith bounded by the transverse contraction joints on both sides is usually recognized as an independent element in various dynamic analyses. However, in most cases a gravity dam monolith is actually subdivided by longitudinal joints for mass concrete construction requirements. Previous studies of gravity dams subjected to underwater explosion (UNDEX) shock loadings have not dealt with the longitudinal-joint issue in much detail. In this work, a numerical simulation of the dynamic response of an old gravity dam without good longitudinal-joint connection integrity in an UNDEX event is performed. The coupled acoustic-structural analysis procedure with ABAQUS/Explicit is employed. The reliability of the procedure is validated by numerical simulations of two UNDEX impact tests in the literature. Nine calculation cases are considered to compare different responses of the dam with different connection states between monolith sections and underwater detonation depths. The results of this numerical investigation show that the longitudinal joints play an important role in the UNDEX response of the dam. The longitudinal joints greatly weaken the flexural rigidity of the dam and make the dam damaged in a piecemeal manner. This study adds to the rapidly expanding field of the UNDEX resistance performance evaluation of gravity dams by accounting more for the longitudinal-joint issue. The antiterrorism situation worldwide is experiencing constant changes, making the world continually face multiple unstable factors. Dams are easy attack targets for their enormous social and economic values. Among various dam types, gravity dams are commonly recognized to have the best blast resistance performance owing to their monumental body sizes. A gravity dam is routinely divided into several monoliths by transverse joints. Each monolith is then subdivided by longitudinal joints for mass concrete construction requirements. After the completion of a dam monolith, the longitudinal joints should be grouted to ensure the integrity of the monolith. For some dams serving several decades, their longitudinal-joint connection integrity might deteriorate. Such structural integrity losses should lead to blast resistance performance degradation. With this in mind, the authors carried out a series of numerical simulations of an old gravity dam when it is experiencing underwater explosion (UNDEX) shock loadings. Nine cases are considered concerning different longitudinal-joint connection states and UNDEX scenarios. This work clarifies the mechanism that the longitudinal-joint connection failure deteriorates the UNDEX resistance performance of the dam. Great efforts on reinforcing old dams through enhancing their longitudinal-joint connection integrity are thus needed to ensure their adequate blast resistance performance. [ABSTRACT FROM AUTHOR]

To investigate the overall damage characteristics and failure modes of a warship subjected to an underwater non-contact near-field explosion, a hull girder with a trapezoidal cross-section was designed, manufactured, and tested. The design criteria and parameters were determined according to the similarity criterion. Dynamic responses of the girder freely floating on water were obtained under varying conditions, including stand-off distance, charge mass, and position of attack. Damage morphologies of the girder model were obtained. Based on our analysis, basic conditions for sagging damage of the hull girder are proposed. The aim of this study was to determine an efficient method of attack resulting in the most severe damage to the ship hull. The experimental results show that the girder mainly exhibits a first-order response when the first wet frequency of the girder is close to the frequency of the explosion bubble pulsation. The largest deformation was observed when the underwater explosion occurred directly below the midspan of the girder compared to other explosions of the same intensity at different attack positions. When the ratio of stand-off to maximum bubble radius (λ) satisfies 0.7 ≤ λ＜2, the bubble mainly causes sagging damage instead of hogging. As λ decreases (1≤ λ＜2), the sagging damage increases under the same charge mass. However, as λ decreases further (0.7≤ λ＜1), the sagging deformation decreases. This is likely due to the impact of the liquid jet formed by the collapsing bubble, which causes the girder deformation to shift from sagging back to hogging deformation. The initial shock wave excites the high-frequency response of the girder structure but contributes very little to the overall velocity and displacement. However, bubble pulsation typically causes a low-frequency response, which will affect the velocity and displacement of the girder. The low-pressure region of the flow field formed by bubble pulsation and resonant coupling between the girder and the bubble are the predominant causes of damage to the overall girder structure.

ObjectivesDuring the mission, the ship may be simultaneously subjected to the combined wave load and underwater explosion bubble pulsation load, resulting in the "superposition effect" of hull response and the loss of total strength of the ship. Therefore, it is necessary to explore the dynamic response law of hull girder under the combined action of above-mentioned loads. MethodsFirst, a simplified hull girder model is established by theoretical analysis, and the bubble pulsation load of underwater explosion and wave load are solved. Then, based on the Hamilton Principle, the differential equations of motion of hull girder at both free ends subjected to the two loads are derived separately and jointly. Finally, based on the solution of the motion differential equation, the hull girder's free vibration response and the simplified model's motion response under the three working conditions of external load combination are analyzed. ResultsThe results show that under the combined action of the two loads, the motion response of the hull girder is 15% larger than the linear superposition of the motion response of the two loads alone. ConclusionsThe research done can provide a reference for developing an algorithm to analyze the motion response of ship structures under combined loads.

ObjectivesThis paper aims to address the numerical simulation problems of the dynamic response of ships subject to near-, medium- and far-field underwater explosions by establishing several numerical methods and calculation models. MethodsFirst, load and fluid-structure interaction models are established on the basis of the Eulerian finite element method and acoustic finite element method using the field-split technique, and FSLAB fluid-structure interaction software is developed. Next, near-, medium- and far-field underwater explosions are numerically simulated respectively. The shock wave propagation law, bubble shape and load evolution characteristics of near free-surface and near-wall underwater explosions are obtained, and the shock response characteristics of a spherical shell and ship subject to far-field underwater explosions are analyzed. Finally, the FSLAB software results are compared with the analytical solutions, reference solutions and experimental data. ResultsThe results show that the FSLAB fluid-structure interaction software developed in this paper is effective and accurate in simulating the impact damage of underwater explosions on warships. ConclusionThis study can provide a basis and support for the power assessment of underwater anti-explosion and shock design of warships.

Background: This research arises from the concern of damage to submersible marine structures such as underwater vehicles and pipelines and the need to understand dynamic failure. This work focuses on analyzing the dynamic bubble-to-structure interaction of curved metallic plates subjected to air-backed underwater explosive loading. Objective: This research aims to expand the current understanding of gas bubble formations during nearfield underwater explosion events with experimental analysis. Methods: The experiments were performed in an underwater explosive facility using high-speed cameras to measure full-field displacement and velocities during deformation through a digital image correlation technique. In addition, during the experiments, pressure transducers were used to record the pressure pulses emanated from the explosive charge. Experiments were performed for two plate curvatures (112 and 305 mm) and three standoff distances from the plate's surfaces (55, 73, and 110 mm) for each curvature. Results: The experiments show that deformations are higher if the explosive standoff is smaller or the structural radius of curvature is higher. In addition, the increase in structural deformation also leads to distorted bubble shapes that increase their repulsion from the structural specimen and decrease the bubble collapse period. Conclusions: Nearfield bubble dynamics are highly sensitive to the proximity of nearby structures and the compliance of such structures. Though they have higher deformations in general, complicated structures may be able to mitigate bubble attachment or jetting for specific explosive charges. [ABSTRACT FROM AUTHOR]

The overall mechanical behavior of the structure of a gravity dam is comprehensively reflected by modal information included in the measured vibration response. In this study, the dynamic elastic modulus of the dam body and foundation materials were inverted via an underwater explosion, and analyzed. First, the first three main frequencies and basic mode shapes of the dam were obtained based on the acceleration response induced by the explosion. Then, the support vector regression model optimized with the particle swarm optimization algorithm was used to invert the mechanical parameters. The results indicate that the objective function that considers dominant frequency and mode shape has a greater advantage in terms of anti-noise compared to when only the dominant frequency is considered. The inverted dynamic elastic modulus is about 1.5 times that of the static elastic modulus of the corresponding material, and the calculated modal values using the inversion parameters are basically consistent with the measured values. This method provides a new way of determining the dynamic parameters of gravity dams. [ABSTRACT FROM AUTHOR]

The strong nonlinear interactions between underwater explosion and water surface were numerically investigated using a phase transition model based on a four-equation system, which can deal with the complex deformable interface among different phases, including water, air, explosion bubble, and cavitation. The numerical method is verified by comparing the numerical results with experimental results, and good agreements are found. This study considers an ideal sine wave for simulating the shape of water surface. Two examples of different detonation depths of charge are investigated. In each example, the first case is the basic simulation without surface wave, and the other three cases are the simulations with sine waves of different wavelengths. Unique characteristics of the interactions, such as shock wave propagation, explosion bubble expansion, and the generation, development, and collapse of cavitation, are observed in the numerical simulations. By capturing the detailed density and pressure contours during the interaction process, we can better understand the underlying mechanisms of the explosion bubble, cavitation, and surface waves. These numerical results demonstrate that geometric nonlinearity impacts cavitation evolution and the explosion bubble movement mechanism. Additionally, the secondary cavitation phenomenon has been found in the cases without surface wave, and its fundamental physical mechanism is presented in detail. The present results can expand the existing database of multiphase flow in the underwater explosion and provide an insight into the strong nonlinear interaction between the underwater explosion and water surface. [ABSTRACT FROM AUTHOR]