Confined Blast Loading Research Articles

Influence of Water Cover on the Blast Resistance of Circular Plates

Abstract Explosion containment vessels (ECVs) can effectively limit the range of potential hazards, and improving their blast resistance is an important research topic. Placing ECVs underwater is an excellent and promising method. The effect of water covering on the blast resistance of circular plates was investigated experimentally and numerically in this paper. First, blast experiments of circular water-covered and bare plates were conducted, and strain responses were obtained. The effect of water on the maximum strain as well as the high-level strain crests was investigated based on experimental results. Then, numerical simulations were carried out using ansys/autodyn and validated by experimental results. The displacement and strain response at the center of the circular plate with different water cover heights were analyzed. The experimental and numerical results show that water can effectively reduce the peak dynamic response of the steel plate and increase the vibration period of the steel plate. The center of the circular plate is the most dangerous position under confined blast loading, regardless of whether the plate is covered with water or not. The results from numerical simulations also clearly show that the blast resistance of the steel plate will first be improved and then stable with the increase of the water cover height. The work in this paper can provide a useful reference for the design and protection of explosion containment vessels.

Dynamic response and failure behaviour of thermoplastic fibre–metal​ laminates subjected to confined blast load

Due to the confinement effect, the blast load from a confined space explosion tends to cause more severe damage to structures. When subjected to this type of confined explosion, thermoplastic fibre–metal laminate (TFML) panels would experience repeated shock waves and relatively longer durations of quasi-static pressure. A better understanding of the dynamic response and damage mechanism of TFMLs under confined blast loading can greatly benefit the design of composite structures with improved blast resistance. In the present work, TFMLs with seven different configurations were designed and fabricated. Confined-blast experiments with various masses of trinitrotoluene (TNT) were performed on these TFMLs. The experimental results, including deflection time histories and X-ray computed tomography (CT) images, have been applied to the development of a method for predicting the dynamic response of laminates under confined blast loads. The findings of this work will assist in the rapid assessment of the deformation of fibre–metal laminates and assist in the pre-design of laminate structures in confined explosions.

Dynamic responses of ultralight all-metallic honeycomb sandwich panels under fully confined blast loading

Fatal threats of fully confined blasts to surface battleships must be minimized to avoid catastrophic failure such as ship sinking. In the present study, for enhanced blast resistance, ultralight all-metallic sandwich panels with square honeycomb cores are proposed as an alternative to traditional metallic plates for ship construction. A combined experimental and numerical approach is employed to investigate the dynamic responses of fully-clamped sandwich panel subjected to fully confined blast loading and compare its blast resistance to that of its monolithic counterpart having equal mass. To explore the underlying physical mechanisms, finite element simulations with the Coupled Eulerian-Lagrangian (CEL) approach are performed. The simulation results are validated against experimental measurements for both sandwich and monolithic target plates, with good agreement achieved. It is demonstrated that, with the pressure versus time history on target plate featured by multiple reflected overpressures and a long-duration quasi-static phase with considerably lower pressure amplitude, the proposed sandwich panel exhibits a considerably higher blast resistance than its monolithic counterpart of equal areal density placed at the same standoff distance, due mainly to its consumption of impact energy via global out-of-plane bending, in-plane stretching, and localized core crushing. Results of this study are helpful for designing novel lightweight protection structures with enhanced blast resistance for ship construction.

Large inelastic response of polyurea-coated steel plates to confined blast loading

The large inelastic deformation performance of monolithic steel and polyurea–steel laminate plates to confined blast loading is investigated through a combination of experiments and finite-element modelling. Emphasis is placed on elucidating how the location of the polyurea coating(s) and steel substrate, relative to the direction of loading, affects their inelastic deformation and the impulse and energy transfer to the laminate plates. Experimental results show that monolithic steel plate outperforms – using the maximum central deflection as a criterion – its equivalent polyurea–steel laminate of equal mass in confined blasts. Results from finite element simulations, which will be shown to be in good agreement with the experiments, will reveal that a longer response time for the polyurea–steel laminate attributes to the greater saturation impulse and, in turn, the increased deformation over its monolithic counterpart of equal mass. The internal energy of a laminate plate, unlike its monolithic equivalent, is found to increase even after reaching its maximum central displacement through further out-of-plane deformation that spreads out laterally. The impedance mismatch between the elastomer and steel will be shown to play a key role in the amplification, or attenuation, of the blast wave depending on the placement of the elastomer. A parametric study is performed to elucidate the influence of thickness ratio and mass per unit area on the blast performance of the laminate plates.

Experimental and numerical investigation on the dynamic response and damage of large-scale multi-box structure under internal blast loading

Internal blast is different from external or air blast for the reason of internal blast load is much more complicated and more destructive to target structures. The present work presents a large-scale experiment on multiple steel box structure under confined blast loading. A multi-box structure specimen with a dimension of 7.5 m × 2.9 m × 2.2 m was designed and field blast test with 1.782 kg TNT detonated inside this model was carried out. The damage features and dynamic response of the specimen were analyzed. The plates near the explosive suffer from strong impact and result in some fragments with high velocity which are destructive to other structural members. Meanwhile, the dynamic response processes were recorded by using the high-speed photography system. The response process of the whole model can be divided into two different stages, namely, localized damage response stage and integral vibration stage. In addition, numerical model considering both double-sided and single-sided fillet welding of this large-scale experiment were built, the dynamic response and the propagation of internal blast wave were analyzed. The numerical studies show that, to achieve a good consistency with the experiment, the failure strain for double-sided and single-sided fillet welding should be set to 0.1 and 0.08 respectively for such large-scale structure under explosive load. Finally, several suggestions for the design of multi-box structure are put forward based on the experimental and numerical investigations.

Effect of Stiffener Strength on the Failure Mode of Stiffened Plate under Confined Explosion: Experimental Studies

The objective of this work was to investigate the effect of stiffener strength on the failure mode of an outer-stiffened plate subjected to confined blast loading. A relatively rigid box with one open side was designed, to provide a confined space, and the stiffened plate was fixed onto the open side. Various field blast experiments of stiffened plates with different dimensions were conducted. Transducers were placed on typical points to record the overpressure history. The post-explosion deformation was drawn utilizing the 3D scanner technique, and the failure modes of the stiffened plates were examined in detail. The effect of plate thickness, stiffener thickness, stiffener height, and stand-off distance on the failure mode of the stiffened plate is discussed. It was shown that two typical failure modes were observed in the stiffened plates, namely uniform global dome deformation and nonuniform dome deformation, with local lattice along the stiffeners. The transformation of these two deformation modes originated from the relative strength of the stiffener compared to the plate, hence a relative strength factor was proposed to clarify the division.

Confined blast loading of steel plates with and without pre-formed holes

This paper presents experimental and numerical results for fully-clamped square steel plates to elucidate how the deformation and failure are affected by the presence of pre-formed holes, and the pre-formed holes’ geometry, under confined blast loading. The geometry of the pre-formed holes will be shown to play a crucial role in the deformation and failure of a plate. Numerical simulations successfully predict the inelastic deformation and crack propagation observed in experiments; and, it will be shown that venting by the pre-formed holes does not alleviate the peak pressure of the primary blast wave, but is effective in attenuating the secondary blast waves arising from multiple reverberations within the test chamber. Pressure-venting by the pre-formed holes will be shown to be ineffectual in reducing the maximum transient midpoint displacement due to impulse saturation. The crack path that develops in a plate with pre-formed square holes was found to be affected by stress triaxiality.

Predicting the Deflection of Square Plates Subjected to Fully Confined Blast Loading

The main objective of this study is to conveniently and rapidly develop a new dimensionless number to characterize and predict the deflection of square plates subjected to fully confined blast loading. Firstly, based on the Kirchhoff–Love theory and dimension analysis, a set of dimensionless parameters was obtained from the governing equation representing the response of a thin plate subjected to impact load. A new dimensionless number with a definite physical meaning was then proposed based on dimensional analysis, in which the influence of bending, torsion moment and membrane forces on the dynamic response of the blast-loaded plate were considered along with the related parameters of the blast' energy, the yield strength of the material, the plate thickness and dimensions of the confined space. By analyzing the experimental data of plates subjected to confined blast loading, an approximately linear relationship between the midpoint deflection–thickness ratio of the target plate and the new dimensionless number was derived. On this basis, an empirical formula to predict the deflection of square plates subjected to fully confined blast loading was subsequently regressed, and its calculated results agree well with the experimental data. Furthermore, numerical simulations of square plates subjected to blast loading in a cuboid chamber with different lengths were performed. The numerical results were compared with the calculated data to verify the applicability of the present empirical formula in different scenarios of blast loading from explosions in a cuboid space. It is indicated that the new dimensionless number and corresponding empirical formula presented in this paper have good applicability and reliability for the deflection prediction of plates subjected to fully confined explosions in a cuboid chamber with different lengths, especially when the plates experience a large deflection–thickness ratio.

Experimental and numerical studies on the dynamic response of stiffened plates under confined blast loads

Abstract Stiffened plate widely exists in ships, steel girders and wing of airplanes for its good bending resistance and lighter weight. The main purpose of this work is to investigate the dynamic response of stiffened plate subjected to confined blast loading through both experimental and numerical approaches. Firstly, several field blast experiments of box-shaped specimens with different dimensions and different explosive weight were conducted. The damage features of both inner-stiffened and outer-stiffened plate were obtained and investigated. Then, numerical models were built and validated by the experimental results, and the dynamic response processes were analyzed. Both the experimental and numerical results show that outward deformations produced in both inner-stiffened and outer-stiffened plate, while the ultimate deflection of outer-stiffened plate is smaller than that of inner-stiffened. In the meantime, overall bending deformations are observed in the outer stiffeners, while in-plane bulking of the flanges and instability failure of the web are observed in the inner stiffeners. In addition, the different dynamic response of stiffened plate under confined and unconfined blast loading were analyzed and compared using the validated numerical method, and the tendency of the dimensionless deflection of inner and outer stiffened plates subjected to confined blast were discussed.

The analysis of the equivalent bare charge of aluminum cased charge exploding in confined space

The blast loadings from a cased charge, which include a cluster of fragments and shock waves, would cause severe damage to the structures, especially in the scenario of a confined space. In this paper, experiments of the dynamic response of steel plates subjected to the confined blast loading from a cased charge were performed, in which two set warhead models of aluminum cased TNT charge were employed. By using the explicit dynamic code AUTODYN and the Smoothed Particle Hydrodynamic (SPH) method, the rapture process of the aluminum shell was reproduced and the equivalent bare charge was determined by analyzing the results from numerical simulations. In addition, the dynamic response of steel plates subjected to confined blast loadings from cased charges were calculated numerically. Then, the method for calculating the equivalent bare charge that related to the deflection of blast loaded plates was proposed. In addition, this method was further used to determine the approximate mass of TNT charge that contributing to the blast loading of shock wave and subsequent quasi-static pressure in the confined space, in which the energy released by the burning of the detonation products and aluminum shell were taken into account. The present paper presented a reliable method to analyze the response of steel plates under confined blast loadings from a cased charge, which would be useful in the design of the protective structures.

An experimental study on the mitigation effects of fine water mist on confined-blast loading and dynamic response of steel plates

The blast loading from an explosion of a condensed explosive in a confined space is quite different from that in an open environment. Due to the confinement effect, a confined-blast load usually causes more severe damage. The detonation products from a fuel-rich explosive, such as Trinitrotoluene (TNT), can react with the surrounding air under specific conditions and release additional energy, resulting in a significant increase of reflected shockwaves and quasi-static pressure in a confined space. Mitigation of the confined-blast loading is an effective way to protect structures from destruction. In this paper, a detailed experimental investigation of the influence of water mist on both the mitigation of blast load and dynamic response of the structure is presented. Six cases of experimental tests, with different charge masses of 80, 120 and 160 g of TNT explosive, with and without the presence of water mist, are performed in a confined chamber, with the shock pressure and deflection-time history curves of blast-loaded steel plates being recorded, as well as quasi-static pressure and temperature of the contained gas. By analysing the experimental data, it is found that the water mist decreases the reflected blast wave and quasi-static pressure by restraining the combustion effect and subsequent energy release of detonation products. As a consequence, both the first peak deflection and the residual deformation of the blast-loaded steel plates decrease greatly. Furthermore, based on a non-dimensional analysis of the experimental results for plates with different thickness subjected to different loading conditions, a new empirical relationship between non-dimensional deflection of the plate and a non-dimensional load parameter (for fully confined blast loading) was obtained. By employing this new empirical relation, the mitigation effect of fine water mist on confined blast load was determined quantitatively. The use of water mist is a very good alternative to mitigate the damage of structures subjected to a confined-blast load in the design of anti-blast structures.

Experimental and Numerical Investigations of Pressure Field of Curved Shell Structure Subjected to Interior Blast

A terrorist attack on a long‐span spatial structure would cause horrible results. Therefore, it is important to determine the characteristics of blast pressure fields to protect such structures. In this study, fully confined blast loading tests were conducted using a rigid curved shell model, which had an inner space similar to that of a reticulated dome. Four different scenarios were carried out to record the blast loading on five typical positions. The blast pressure‐time data were compared and analyzed. In addition, a suitable numerical simulation method was proposed for the issues involved in interior blast loading. This numerical model was verified by comparing with the test data. A parametrical analysis of the interior blast simulations was conducted based on this numerical method. The blast loading values at specific positions were obtained with the key parameters varied within a reasonable scope. The blast loading from blast tests and simulations were presented. On this basis, the interior blast loading could conveniently be predicted by using the method and data in this paper, which could be used in the protective design of other reticulated domes.

Experimental and numerical studies on the dynamic response of steel plates subjected to confined blast loading

This paper presents the results of experimental and numerical investigations into the response of unstiffened and stiffened steel plates under confined blast loading. Experimental tests were carried out on square mild steel plates with different stiffener configurations under the loading produced by cylindrical TNT charge explored in a specially designed blast chamber. The permanent deformation profiles of specimens were recorded. In general, the plates show large inelastic global deformations with the maximum deflection occurring at the center of the plate. Through the comparison to the impulse in a free air blast with the same charge, the equivalent impulse applied to the plate could increase by up to 4.03–5.63 times in the confined blast cases due to the confinement effect of blast chamber. Besides, numerical simulations are conducted to gain further insight into the deformation modes of plates using ANSYS/AUTODYN. Very good correlation is obtained with regards to midpoint deflections and deformation profiles. The validated numerical models were further used to investigate the influence of the stiffener and vents on deformation modes of stiffened and unstiffened plates under confined blast loading. The results from numerical simulations demonstrate that the maximum plastic deformation decreases greatly with increasing the mass ratio of the stiffener to the steel plate. It was found that the stiffener location (on the side or opposite side of loading) has an insignificant influence on the deformation modes of the stiffened plates under confined blast loading. The numerical results also clearly show that the vents can reduce the degree of confinement of the blast chamber as well as the impulse applied to the plates in the confined blast cases, which results in the decrease of structural deformation.

Dynamic response of multibody structure subjected to blast loading

Abstract The response of multibody structures with plastic hinges subjected to confined blast loading is investigated through experimental tests, theoretical calculation and numerical simulation in this paper. The time histories of plastic hinges’ rotation angles have been obtained using a time-to-digital converter (TDC) in experiments during the deformation process. These tests were simulated using ABAQUS/Explicit, with a strain rate-dependent, critical plastic strain fracture criterion employed to model tearing failure of plastic hinges. Based on the rigid plastic assumption, a dynamics analytical model for blast loaded multibody structures is presented, in which the strain hardening and strain rate effects are considered. The FE models and theoretical method were validated by comparing their predictions against the corresponding experimental results, and were then used to gain insight into the effects of blast loading and thickness of plastic hinges on the deformation processes of the multibody structure.

Dynamic Deformation of Clamped Circular Plates Subjected to Confined Blast Loading

AbstractIn this paper, the dynamic deformation of thin metal circular plates subjected to confined blast loading was studied using high‐speed three‐dimensional Digital Image Correlation (3D DIC). A small‐scale confined cylinder vessel was designed for applying blast loading, in which an explosive charge was ignited to generate blast loading acting on a thin metal circular plate clamped on the end of the vessel by a cover flange. The images of the metal plates during the dynamic response were recorded by two high‐speed cameras. The 3D transient displacement fields, velocity fields, strain fields and residual deformation profiles were calculated by using 3D DIC. Some feature deformation parameters including maximum out‐of‐plane displacement, final deflection, maximum principal strain and residual principal strain were extracted, and the result was in good agreement with that simulated by AUTODYN. A dimensionless displacement was introduced to analyse the effects of plate thickness, material types and charge mass on the deflection of metal plates. DIC is also proven to be a powerful technique to measure dynamic deformation under blast loading.

Analysis of explosion in enclosure based on improved method of images

The aim of this paper is to present an improved method to calculate the pressure loading on walls during a confined explosion. When an explosion occurs inside of an enclosure, reflected shock waves produce multiple pressure peaks at a given wall location, especially at the corners. The effects of confined blast loading may bring about more serious damage to the structure due to multiple shock reflection. An approach, first proposed by Chan to describe the track of shock waves based on the mirror reflecting theory, using the method of images (MOI) is proposed to simplify internal explosion loading calculations. An improved method of images is proposed that takes into account wall openings and oblique reflections that cannot be considered with the standard MOI. The approach, validated using experimental data, provides a simplified and quick approach for loading calculation of a confined explosion. The results show that the peak overpressure tends to decline as the measurement point moves away from the center, and increases sharply as it approaches the enclosure corners. The specific impulse increases from the center to the corners. The improved method is capable of predicting pressure–time history and impulse with an accuracy comparable to that of three-dimensional AUTODYN code predictions.

Deformation and Failure of Pre-Notched Thin Metal Plates Subjected to Confined Blast Loading

In this paper, the dynamic deformation and rupture of pre-notched thin metal plates subjected to confined blast loading were investigated. The thin copper plates with cross-shape pre-notch were clamped on the end of a confined cylinder vessel by a cover flange. An explosive charge with a mass of 4g was detonated in the vessel center to generate blast load acting on the metal plates. The images of metal plates were recorded by two high-speed cameras. The displacement and strain fields during the deformation and rupture process were measured by using 3D digital image correlation (3D DIC). The effects of pre-notches on the dynamic deformation and rupture of thin metal plates were analyzed. The microstructure of fracture surface was examined The 3D DIC technique is proven to be an effective method to conduct dynamic full-field deformation measurement.

Prediction of Confined Blast Loading in Single-Layer Lattice Shells

Single-layer lattice shells (also known as gridshells) are widely used for architecturally innovative structures. When an explosion occurs inside such a structure, confined blast loading on the structural components will be seriously affected by different factors, such as charge locations and weight, structural types and forms. Moreover, slight changes of blast loading perhaps result in various responses for such a complicated structure. In this paper, blast loads on single-layer lattice shell are calculated by AUTODYN software package. The effect of scaled distance, ratio of rise to span and ratio of height to span are investigated. Simplification of blast loading is studied, and the principles of equivalent loading process are validated with a 40 meters single-layer Kiewitt-8 reticulated dome. In order to predict the blast loading, a precise and simple model is derived from numerical results, which is suitable for a wide scope of single-layer lattice shells. Two applications with different charge weight, structural spans and forms are worked out by using the blast prediction model. Good agreements of comparisons are achieved between prediction model and numerical results.

Characteristics of Confined Blast Loading in Unvented Structures

Confined blast loading occurs in many scenarios and the effects of confined blast loading may result in more serious damage to buildings due to multiple shock reflections (Shi et al. 2009). However, spherical charges are assumed for all confined explosive-effects computations in modern standards for blast-resistant design such as UFC-3-340-02 (2008) and the soon-to-be published ASCE Standard for the Blast Protection of Buildings (ASCE forthcoming) without consideration of effects of charge shape on the distribution of reflected overpressure and impulse. As confinement is an aggravation factor of explosion effects, analysis and design of infrastructure under critical scenarios of confined blast loading should take the aggravation factor into consideration. This paper is to develop a numerical model for prediction of blast loads inside unvented structures as a result of variation of the charge shape, charge orientation, geometries and volumes of confined chambers. A finite element program, AUTODYN (Century Dynamics, 2003), is utilized extensively to generate a model which is capable of being calibrated with the experimental results conducted by Wu et al. (2010) in external conditions and by Zyskowski et al. (2004) in a confined small box. The calibrated AUTODYN model is then used to conduct parametric studies to analyze the effects of the variation of charge shape, charge orientation, chamber geometry and chamber volume on the peak reflected overpressure and impulse on the walls of the chamber. The quasi-static overpressure for fully confined blast loading is characterized and the simulated results are used to derive the relationships between the quasi-static overpressure and scaled distance for the fully confined blast loading. Discussion is made on characteristics of fully confined blast loading inside chambers.

A Study of Blast Effects Inside an Enclosure

A study of blast effects inside an enclosure was performed for a 1-lb C4 charge placed at the center of a rectangular bunker. The prediction based on the Euler equations shows good agreement with the test data. Predictions calculated by the method of images were also obtained to help illustrate the internal shock reflection effects. The results show that the confined blast loading is more complex than the free field loading due to internal shock reflections and interactions. Focusing of multiple reflections is possible and may cause significant late time loadings.