Near-field Underwater Explosion Research Articles

Investigation on the coupling damage effects of ships subjected to near-field underwater explosion loads

Near-field underwater explosions generate complex load forms and cause destructive strikes on ships. To investigate the damage characteristics of ships under the combined effect of multiple loads, local and overall damage modes were first investigated through the combination of experimental tests and numerical simulation. Then a series of numerical simulations of full-scale ships subjected to near-field underwater explosions with different stand-off distances, equivalents, and detonation positions were carried out. The damage effects of shock wave, bubble pulsation, after flow, and water jet loads on ships were analyzed. The results show that the combined effects of different loads lead to different damage modes. Shock waves and after flow loads are the main cause of local damage and hogging damage, while pressure difference and bubble adsorption are the main reasons for the hogging damage. Finally, a criterion was proposed to determine the damage modes of ships under near-field underwater explosions.

Dynamic response of a hollow cylindrical shell subjected to a near-field underwater explosion

To investigate the dynamic response of a hollow cylindrical shell structure subjected to a near-field underwater explosion, underwater explosion experiments were conducted in a 2 × 2 × 2 m water tank, and high-speed cameras were used to record the interactions between the bubbles generated by the underwater explosion and the hollow cylindrical shell. The high-speed photography results showed that the cylindrical shell experienced a minor degree of deformation during the shock-wave stage. However, during the bubble-pulsation stage, the cylindrical shell experienced significant deformation that surpassed the deformation observed during the shock-wave stage. On this basis, combined with the damage results for the cylindrical shell, a numerical model for the hollow cylindrical shell subjected to an underwater explosion was established using LS-DYNA software. The dynamic process and damage mechanism of a hollow cylindrical shell that was subjected to a near-field underwater explosion were revealed by analyzing the pressures and strains of the shell elements, the velocities and displacements of the nodes, and the variations in the energy.

Investigation on cavitation phenomena and load characteristics of fixed square plates subjected to near-field underwater explosion

The cavitation phenomenon caused by near-field underwater explosion imposes a secondary loading on ship structures. Studying the cavitation effects of plates during underwater explosions has implications for ship protection since plate is a typical structure of ships. This paper experimentally investigated the behavior of fixed square plates subjected to near-field underwater explosions. Experimental results indicated that cavitation existed in both shockwave and bubble pulsation stages; and the cavitation closure load at bubble pulsation stage was measured. To further investigate the cavitation phenomenon and the load characteristics at the bubble pulsation stage, finite element models were developed. The numerical results show that cavitation will increase the bubble load impulse by 47 % under specific conditions. These models were then used to investigate the influence of the stand-off distance and charge weight on cavitation area and load characteristics. An empirical formula was finally proposed to predict the coupled impulse of cavitation closure, bubble pulsation and water jet. This research has practical guidelines for the protection design of ship structures.

Damage characteristics of ribbed cylinder in motion under near-field underwater explosion

The damage characteristics of a ribbed cylinder in the torpedo compartment shell is explored. An arbitrary Lagrange–Euler method is used to establish the fluid–structure interaction model for analyzing the ribbed cylinder’s response under near-field underwater explosion while in motion. The influence of detonation direction and standoff distance on the dynamic response of the moving ribbed cylinder is considered. The investigation reveals that the cylinder’s motion causes an uneven distribution of bubble load and secondary load, stemming from cavitation zone collapse, on the shell. This imbalance leads to a notable deflection difference between the shell’s front and rear sections, with maximum deformation concentration at the rear. In addition, in comparison to the lateral condition, static state analysis shows reduced average deflection and increased maximum deflection when the explosion point is above or below the shell, while in the sailing state, both average and maximum deflections increase. Notably, when the charge radius is between 6 and 15 times, the average damage rate in the sailing state consistently remains lower than that in the stationary state, while the maximum damage rate is higher at a specific burst distance.

Study on structural damage mechanisms and defence of rectangular closed-section simplified hull girders with thinner plates under near-field underwater explosions

Understanding the damage mechanisms of typical ship structures under fatal underwater explosion loads is crucial for developing effective defensive measures. To provide references for enhancing the survivability of ships, this paper conducts analyses of the damage mechanisms of simplified hull girders (SHGs) using a combination of experimental and simulation methods, and proposes targeted structural improvements. Initially, two types of thin-walled rectangular closed-section SHGs are subjected to near-field underwater explosion experiments. Corresponding simulation models are established, and the validity of the computational results is verified. The progressive damage processes of SHGs in hogging and sagging are examined from a structural perspective. Furthermore, the thin-walled plate deformation transfer processes are analyzed. Finally, based on the analyzed damage mechanisms, targeted defensive structures for overall damage are designed. The study results show that enhancing the anti-deformation capability of the upper and lower edges of the side plates can effectively improve the overall bending resistance of the SHG.

Experimental Study on the Underwater Explosion Bubble Near deformable boundary

Studying near-field underwater explosions is important for the research of submersible and underwater explosive weapons. In this study, we conducted experiments to investigate the coupling of near-field underwater explosion bubbles with titanium alloy plates and steel plates. Our findings show that the boundary of a titanium alloy plate causes the first pulsation period of a bubble to be longer than in the free field, while the boundary of a steel plate causes the first pulsation period of a bubble to be shorter than in the free field. Furthermore, we simulated the process of the explosion and found that changes in the period may be caused by ventilation.

Dynamic response and vulnerability analysis of pier under near-field underwater explosion

Marine reinforced concrete constructions (suspended tunnels, undersea pipelines, and cross-sea bridges) are inherently at risk from underwater explosions due to the rising incidence of worldwide terrorism. Sea-crossing bridges are as an important land-sea connection channel, whose anti-explosion performance cannot be ignored. In this paper, the dynamic response and damage mechanism of piers of sea-crossing bridge under underwater explosion loads are studied by the arbitrary Lagrangian-Eulerian (ALE) method. Firstly, the correctness of the explosive load and reinforced concrete material models is verified by the Zamyshlyaev formula and underwater explosion test, respectively. Secondly, the finite element (FE) model of a pier under an underwater explosion load is established. Taking the residual vertical bearing capacity of the pier as the performance index, the damage evaluation method and four damage levels of the pier under explosion load are put forward. The study of different parameters such as explosive weight, explosion location, explosion height, and explosion distance shows that the damage to the pier is the most serious when the detonation point is directly below the projection of the pier cap. The residual bearing capacity of the pier falls exponentially as explosive weight increases. Different detonation heights have significant effects on the damage to the pier. Additionally, it is found using the explosion vulnerability analysis approach that the pier is very susceptible to collapse when the explosive weight is 350 kg.

SPH numerical simulation of peak pressure in near-field underwater explosion with moving boundary method using virtual particles

A new moving boundary method using virtual particles in smoothed particle hydrodynamics is designed and applied to numerical simulation of underwater explosion. The wave structure in underwater explosion was semi-analyzed, which provides references for the understanding of numerical results in various dimensions. The results show that the boundary point between the near-field and middle-field, with the specific distance R/a = 6, is a demarcation point in the wave structure. Piecewise power fitting computational results in the near-field underwater explosion are in good agreement with the numerical results.

The effects of superstructure form on damage characteristics of ship subjected to underwater explosion

To investigate the influence of superstructure on the damage characteristics of ships subjected to a near-field underwater explosion, two hull girder models with different superstructures were designed basing on the principle of similarity. The experiment and numerical simulation were conducted to attain the dynamic response of the models. In addition, this paper discussed the impact of the structure and layout modifications of the superstructure on the hull girder’s damage and simply analyzed the overall and local coupling damage characteristics of the hull. The results show that the presented numerical simulation could accurately predict the hull girder’s overall damage mode and deformation. Under the same explosion condition, the presence of the superstructure increases the hull girder’s resistance to the overall bending deformation and minimizes the damage of grillage at the hull bottom. When the abrupt change of cross-section location is within the range of 1/10 girder length from the girder’s midspan (to the bow or stern), the failure mode of the hull girder is sagging damage. The length of the superstructure of the hull girder should be limited to 0.4–0.8 times the hull’s length; hence, the superstructure and the main hull can resist bending deformation more effectively.

Research on Local Structure Plastic Damage of Cylindrical Shell under Near-field Underwater Explosion

The local structure of the cylindrical shell produces plastic deformation under the near-field underwater explosion load, based on the fact that the ring rib is the strong support of reinforced cylindrical shell plate, this paper simplifies the reinforced cylindrical shell model into a rigid slatted beam model at both ends, and lists the equations of slatted beams under the action of shock wave and bubble pulsation load. This equation takes the initial damage caused by shock wave into account, and compares the calculation results obtained by the theoretical method with the experimental data, which verifies the correctness of this mechanical model. At the same time, it is explained that this calculation method can better predict the plastic damage of the local structure of the cylindrical shell under the action of near-field underwater explosion.

Investigation on bubble load characteristics of near-field underwater explosion

The wall pressure generated by near-field underwater explosion bubble involves significant nonlinear problems. This has been the focus and difficulty of the research on the anti-explosion design of battleships. To quantify the load characteristics of bubble pulsation and water jet pressure, underwater explosion tests were conducted beneath a clamped square plate subjected to 2.5g TNT at different distances (R = 10, 15, 20, 22.5, and 25 cm). Results showed that as the stand-off distance increased to the maximum radius of the bubble, there was a peak value of the impulse for bubble pulsation and water jet. A finite element model was then established and benchmarked. The benchmarked model was used to further explore the load characteristics of the bubble pulsation and water jet, considering the effect of the stand-off distance, plate dimension, horizontal distribution, and explosive equivalent. It was found that: a) as the plate dimension increased, the pressure of the shock wave to the plate was basically unchanged, while the period and impulse of the bubble pulsation and water jet decreased; b) the impact of the water jet pressure was concentrated within 6.72 times the radius of the explosive. Finally, an empirical formula was proposed to predict the impulse of the bubble pulsation and water jet. This research is able to provide guidance to investigate the blast resistance of ships.

Multi-peak phenomenon of large-scale hull structural damage under near-field underwater explosion

The underwater explosion (UNDEX) may cause more serious damage to the hull structure under certain detonation conditions. In this study, through a near-filed UNDEX test with a full-scale barge in a natural environment, a detailed simulation of the entire UNDEX process applying the S-ALE method is conducted and validated. Several near-field UNDEX simulations of the barge with different detonation distances are then conducted to study the response characteristics and damage results. The double-peak phenomenon of ship bottom deformation and the single-peak phenomenon of residual strength under different detonation distances, which is referred to as the multi-peak phenomenon, are identified based on the results of local deformation and residual strength, and their mechanisms are further analyzed and clarified.

Damage mechanisms of full-scale ship under near-field underwater explosion

The near-field underwater explosion produced by the torpedo and other weapons was one of the most critical threats to naval ships. However, most underwater explosion tests were scale models, which would bring dissimilarities between the gravity and strain rate. We carried out a near-field underwater explosion test on a full-scale ship and recorded the dynamic response processes to make up for this shortcoming. The ship damage was mainly embodied in the loss of overall strength and damage of local structures. The Coupled Eulerian–Lagrangian (CEL) method was used to establish the model of the near-field underwater explosion, and the effectiveness of the calculation method was validated. Based on the analysis results, the near-field underwater explosion could be divided into three stages: shock wave and after flow, bubble pulsation, and water jet. The overall strength of the ship was lost under the combined action of these three stages. The “negative pressure” caused by bubble pulsation was the key to the sagging deformation of the ship. This study would guide the production of underwater weapons and ship protection design.

Study on the characteristics of initial shock waves generated by cylindrical charge for underwater explosion

In order to provide a theoretical basis for studying the shock wave propagation and evolution generated by cylindrical charge of near-field underwater explosion, the characteristics of initial shock waves generated by cylindrical charge are studied. First, a new theoretical model for initial shock waves is proposed by the polar curve method. Based on the theoretical model and the simulated results, the influences of incident angle, length-to-diameter ratio and type of charge on the initial shock waves are analysed. Then the experiments of underwater explosion load tests are carried out to verify the theoretical and simulation results. Finally some instructive conclusions are drawn: i) The two-dimensional theoretical model can calculate the initial underwater shock wave pressure and its direction of cylindrical charge through detonation velocity, detonation pressure, explosive density and incident angle, which is provides a theoretical basic for studying the shock wave propagation and evolution studies. By comparing the theoretical results with the numerical results verified by experiments, the maximum error is not more than 9.93%. ii) Increasing the incident angle α 0 will reduce the initial shock wave pressure of cylindrical charge and make its direction deflect towards the material interface. When the incident angle α 0 is in the range of 0 - 10°, increasing the incident angle makes the initial shock wave pressure decrease rapidly as a power function. With the increasing of the incident angle, the attenuation rate of the initial shock wave firstly decreases and then increases when the incident angle α 0 is in the range of 10 - 72°. This phenomenon explains the reason that the axial and radial initial shock wave pressures are much higher than those in other directions after the central initiation of cylindrical charge. iii) According to the geometric relationship, the length-to-diameter ratio of the charge will limit the range of the incident angle. The length-to-diameter ratio will affect the initial shock wave pressure at the same initiation time. The pressure decreases with the increase of the length-to-diameter ratio when the initial shock wave is formed on the cylindrical surface of the charge.

Damage characteristics of cabin in navigational state subjected to near-field underwater explosion

An arbitrary Lagrange-Euler (ALE) method is used to establish a numerical model for the fluid-structure interaction of near-field underwater explosion while cabins are under sailing conditions. First, the ALE method is used to calculate shock wave and bubble pulsation loads of free field underwater explosion. The results show that the values of shock wave peak, bubble fluctuation period, maximum radius of bubble are similar to empirical formula calculation, which verifies the accuracy of ALE method in calculating explosion loads. On this basis, from the aspects of local damage, longitudinal strength and impact environment of cabin structure, the dynamic response characteristics of cabin structure under the coupling of impact and bubble load at a certain speed and different explosion distance are analyzed and compared with that at static condition. The results show that compared with the static state, the local damage is weakened, the longitudinal strength is increased, the difference of longitudinal strength between navigation and static state is 7%–12%, the shock spectrum value of the compartment corresponding to the explosion source is reduced, and the shock spectrum value of the adjacent compartment is slightly increased; When the cabin is sailing, the increase of the explosion distance within the maximum radius of bubble pulsation increases the action area of bubble pulsation load, and the effect of bubble pulsation load is enhanced, but the coupling enhancement of explosion load is difficult to appear, so that the damage of cabin structure is reduced. The working condition of the cabin suffering from near-field underwater explosion damage under the navigational state analyzed in this paper is closer to the reality, and more truly reflects the process of underwater explosion shock wave and bubble load coupling applied to cabins, which lays a foundation for the fuze warhead coordination design and relevant parameter setting when the torpedo and other underwater weapons give full play to the damage efficiency.

Research on the progressive sagging damage and plastic hinge line mechanism of basic simplified hull girder subjected to near-field underwater explosion bubble

Near-field underwater explosion bubbles are a major threat to the survivability of ships and may lead to severe and destructive accidents. Therefore, in this study, the damage mechanism of a basic simplified hull girder(SHG) under a near-field underwater explosion load is analysed using the Coupled Eulerian–Lagrangian (CEL) method. First, the calculation accuracy of the established finite element model is verified. Then, the damage development process of SHGs to near-field underwater explosions is investigated from the perspective of progressive sagging damage and plastic hinge lines. In addition, theoretical derivations are carried out based on the constructed plastic hinge line mode. Finally, some laws on the simulation data are discussed in terms of the plastic absorption energy and bending moment. The results show that a stable crease mode will appear in the midship when the SHGs are sagging damage under the near-field underwater explosion bubble load. These creases are necessary conditions for sagging damage, which are mainly W-shaped in this study. The analytical solutions of the W-shape size agree well with the simulation results.

Numerical Study on the Sagging Damage of the Simplified Hull Girder Subjected to Underwater Explosion Bubble

The pulsation of the bubbles resulting from underwater explosions can lead to severe damage to the structure of the ship’s hull, and even to its sinking. To study the damage mechanism of a simplified hull girder (SHG) subjected to near-field underwater explosion bubble, the Coupled Eulerian–Lagrangian (CEL) method based on verifications of the calculation accuracy was used to simulate 11 SHG structures. The sagging bend mechanism of SHGs was analyzed from the perspective of plastic hinge lines. Moreover, the length formula of the potential bend zone was studied through the assumed plastic hinge lines. The influence of transverse bulkheads on bending mode and total longitudinal strength was investigated. The results show that SHGs’ sagging damage is composed of regular plastic hinge lines, mainly depending on side plates’ folding—W-shaped in this paper. When facing the near-field underwater explosion bubble, the distant transverse bulkheads influence the total longitudinal strength and do not always play a positive role.

Research on the dynamic response of pressurized cylindrical shell structures subjected to a near-field underwater explosion

To study the dynamic response of a pressurized thin-walled circular tube structure subjected to a near-field underwater explosion, deformation and damage tests of a pressurized cylindrical tube shell with different internal pressures and wall thicknesses with the effects of an underwater explosion from 2 g of explosives at different stand-off distances were carried out in a water tank. LS-DYNA finite element software was used to carry out relevant numerical simulations to explore the anti-explosion mechanism of a pressurized cylindrical shell structure affected by factors including the initial internal pressure, the thickness of the cylindrical shell, and the stand-off distance. The simulation inputs are high-speed photography images and the damage results of the cylindrical shell. The simulation results highlight the bubble pulsation, action process deformation, and energy change of the cylindrical shell. The results indicate that an increase in the initial pressure of the cylindrical shell can effectively improve its anti-explosion ability, while the different stand-off distances of r = 12 cm and r = 9 cm correspond to the same initial internal pressure. Increasing the initial internal pressure also causes the deflection difference of the cylindrical shell to decrease. The modeling results also indicate that an increase in the wall thickness may weaken the anti-explosion ability of the cylindrical shell, which has an initial internal pressure of Pr = 0.7 MPa. After an underwater explosion, the maximum displacement difference of a cylindrical shell with thicknesses of h = 1 mm and h = 1.5 mm is less than the maximum rebound distance difference. With this decrease in the distance, the proportion of the high-pressure cylindrical shell subjected to a shock wave increases while the proportion of bubble pulsation decreases. Specifically, the maximum proportions of bubble pulsation are 30% and 92% at the stand-off distances of 3 and 12 cm, respectively.

Evaluating the shape and orientation effect of non-spherical charge on the pressure distribution of underwater explosion: Finite element analysis

Limited studies available are presented to investigate the shape and orientation effect of pressure distribution and peak pressure of non-spherical charge in near-field underwater explosion (UNDEX). A detailed 3-D finite element model based on arbitrary Lagrange-Euler (ALE) algorithm is used to study the near-field pressure distribution of UNDEX within infinite water field. The numerical results of the model are verified against the empirical results. The influence of the shape and orientation effect on the pressure distribution was also analyzed based on the numerical data. It is found that the model is suitable for the reproduction of the near-field shock waves for the non-spherical explosives within infinite water field. The shape and orientation effect have significant impact on the pressure distribution of near-field UNDEX. The deviation of the incipient pressure of the non-spherical charges from the constant-mass spherical charge (C0) increases with the length-to-diameter ratio increasing. Compared with the cylindrical charge (C1), the peak pressure of spherical charge continue to reduce with lower attenuation speed in the same propagation time from 0.02 ms to 0.22 ms. However, the peak pressure of cuboid charge is not monotone changing with the same propagation time. The pressure distribution and peak pressure of UNDEX have direct relation with the detonation modes, especially in the near-field region. A reference formula for the peak pressure of the non-spherical charge is also obtained to provide a reference for engineering design.

Effects of fracture models on structural damage and acceleration in naval ships due to underwater explosions

This study examined the structural damage quantitively due to a near-field underwater explosion (UNDEX) using a more reasonable fracture model. Technical backgrounds of the Geers-Hunter UNDEX model and combined HC-LN fracture model based on Hosford-Coulomb (HC) and localized necking (LN) were reviewed. A finite element analysis (FEA) model for which the UNDEX experiment was performed was generated. Two fracture models were applied to the FEA model: constant strain and HC-LN fracture models. A user subroutine was developed to implement the HC-LN fracture model. The material constants for the aluminum alloy that was the model ship material were calculated using tensile tests and their FEAs. The HC-LN fracture model predicted a similar extent of damage to that in the experiment. The constant strain fracture model predicted an unrealistic fracture in the bottom shells that reduced the hull girder bending capacity and thus the increased acceleration history. A damage extent can be estimated accurately only by UNDEX FEAs with a more reasonable fracture model.