Free-field Blast Research Articles

Characterization of an Advanced Blast Simulator for Investigation of Large Scale Blast Traumatic Brain Injury Studies.

Blast traumatic brain injury (bTBI) is a prominent military health concern. The pervasiveness and long-term impacts of this injury highlight the need for investigation of the physiological outcomes of bTBI. Preclinical models allow for the evaluation of behavioral and neuropathological sequelae associated with bTBI. Studies have implemented rodent models to investigate bTBI due to the relative small size and low cost; however, a large animal model with similar neuroanatomical structure to humans is essential for clinical translation. Small blast simulators are used to induce bTBI in rodents, but a large animal model demands a larger device. This study describes a large advanced blast simulator (ABS4) that is a gas-detonation-driven system consisting of 5 sections totaling 40ft in length with a cross-section of 4 × 4 ft at the test section. It is highly suitable for large animals and human surrogate investigations. This work characterized the ABS4 in preparation of large-scale bTBI testing. An array of tests were conducted with target overpressures in the test section ranging from 10 to 50 psi, and the pressure-time profiles clearly illustrate the essential characteristics of a free-field blast wave, specifically a sharp peak pressure and a defined negative phase. Multiple blast tests conducted at the same target pressure produced very similar pressure profiles, exhibiting the reproducibility of the ABS4 system. With its extensive range of pressures and substantial size, the ABS4 will permit military-relevant translational blast testing.

Blast Wave Simulator for Laminated Glass Panels Experimental Evaluation

The study of blast loads on structures is important due to the potential of significant consequences in various scenarios. From terrorist attacks to industrial accidents, comprehending how structures respond to blast waves is critical for ensuring public safety and designing resilient structures. Studying these effects typically involves two main methods: free-field tests with live explosives and shock tube tests. Although shock tube testing offers certain advantages, both approaches are costly and demand significant space. This research aims to develop a cost-effective and straightforward technique for generating stress waves that closely replicate the progressive and spatial characteristics of free-field or shock tube blast waves. This method was designed to evaluate the dynamic response of laminated glass panels. The stress wave was generated by impacting a piston on the fluid inside a tube, which was connected to a fluid chamber. This setup produced impulsive loads that were distributed across a laminated glass test panel. Moreover, it was used to simulate the shock near filed explosions for a certain part of a structure. High-speed cameras were utilized to analyze the initial velocity of flying glass fragments. The apparatus successfully produced various blast waves and impulsive profiles for different drop weight heights. The initial velocities of randomly selected flying shards ranged from 3 m/s to 4 m/s.

Spatial Intracranial Pressure Fields Driven by Blast Overpressure in Rats

Free-field blast exposure imparts a complex, dynamic response within brain tissue that can trigger a cascade of lasting neurological deficits. Full body mechanical and physiological factors are known to influence the body’s adaptation to this seemingly instantaneous insult, making it difficult to accurately pinpoint the brain injury mechanisms. This study examined the intracranial pressure (ICP) profile characteristics in a rat model as a function of blast overpressure magnitude and brain location. Metrics such as peak rate of change of pressure, peak pressure, rise time, and ICP frequency response were found to vary spatially throughout the brain, independent of blast magnitude, emphasizing unique spatial pressure fields as a primary biomechanical component to blast injury. This work discusses the ICP characteristics and considerations for finite element models, in vitro models, and translational in vivo models to improve understanding of biomechanics during primary blast exposure.

Toward Improvements in Pressure Measurements for Near Free-Field Blast Experiments

This paper proposes two ways to improve pressure measurement in air-blast experimentations, mostly for close-in detonations defined by a small-scaled distance below 0.4 m.kg−1/3. Firstly, a new kind of custom-made pressure probe sensor is presented. The transducer is a piezoelectric commercial, but the tip material has been modified. The dynamic response of this prototype is established in terms of time and frequency responses, both in a laboratory environment, on a shock tube, and in free-field experiments. The experimental results show that the modified probe can meet the measurement requirements of high-frequency pressure signals. Secondly, this paper presents the initial results of a deconvolution method, using the pencil probe transfer function determination with a shock tube. We demonstrate the method on experimental results and draw conclusions and prospects.

Injury and death to armored passenger-vehicle occupants and ground personnel from explosive shock waves

This study evaluated the risks for injury and death to occupants from blast waves to the side and underbody of an armored passenger-vehicle and to ground personnel from free-field blast waves. The Kingery-Bulmash empirical relationships for explosive shock waves were augmented by the Swisdak empirical relations for stand-off distances up to Z = 39.8 m/kg1/3 to tabulate shock-wave characteristics using the Friedlander wave-shape. A 15 kg, hemispherical explosion was analyzed in detail for the shock wave velocity and compression of air behind the wave front. An armored SUV was analyzed with Z = 1.6 m/kg1/3 (4 m) standoff distance from pressure loading on the near-side, far-side and underbody. The rigid body displacement was 0.36 m and 7.8° yaw for a side loading. When a segment of the occupant compartment accelerates inward, there are risks for injury from the intrusion. Energy is transferred to the occupant by deformation of their body (Ed) and by velocity increasing the kinetic energy of the body region (Ek). Body deformation injures an occupant by exceeding the tolerable compression (crush mechanism) or exceeding the rate-dependent tolerance, which is defined by the rate times the extent of compression (viscous mechanism). The risk for injury and death to ground personnel was analyzed for free-field blast waves by stand-off distance and TNT weight. A 15 kg charge posed a 99% risk of death at 3.9 m, 50% risk at 5.2 m, 1% risk at 7.8 m and injury threshold at 8.2 m. A 100 kg charge posed a 99% risk of death at 8.5 m, 50% risk at 11.6 m, 1% risk at 17.3 m and injury threshold at 18.0 m. The study describes the steps to analyze blast loading of an armored passenger-vehicle for risks of occupant injury. It describes the steps to analyze injury risks to ground personnel from blast wave pressure.

Experimental and numerical investigation on the attenuation of blast waves in concrete induced by cylindrical charge explosion

Concrete material is widely used in military facilities to resist the deliberate attacks by earth penetration weapon (EPW). The charge of EPW is more nearly cylindrical than spherical in geometry, and its explosion usually occurs in a certain depth of burial (DoB). To quantify the influences of charge shape and DoB on blast waves in concrete, experimental and numerical investigation on the attenuation of blast waves in concrete induced by cylindrical charge explosion is carried out in the present study. Firstly, a set of blast wave test in concrete with cylindrical charges is conducted to provide fundamental data including the stress-time histories and failure in concrete with the considerations of aspect ratio and DoB. Then based on the concrete material model by Kong and Fang and LS-DYNA's multi-material ALE algorithm, the attenuation of free-field blast waves in concrete generated from spherical charges and cylindrical charges with different aspect ratios is numerically investigated in detail. Finally, the influence of DoB on the peak stress directly below cylindrical charges is discussed and a coupling factor for peak stress is proposed. Several empirical formulas are presented based on dimensional analysis and curve-fitting the numerical data, including the critical distance beyond which the charge shape effect could be neglected, coupling factor for peak stress and peak stress from cylindrical charges with varied aspect ratio under different DoBs, all of which are useful for blast-resistant design.

Silencing a free field blast simulator to meet residential standards

The Naval Medical Research Center in Silver Spring, Maryland, plans to install a new free field blast simulator in an existing building on campus. The new blast simulator will be an addition to two current blast simulators and is capable of generating higher dynamic pressure levels, up to 35 psi, than the existing simulators by igniting a mixture of oxygen and acetylene to create the simulated blast. In order to accurately simulate the blast, the outlet of the simulator must be exhausted to the outdoors which will produce noise levels in excess of 140 dBA at five feet. The campus lies within 1800 feet of the nearest residential neighborhood and 950 feet of the nearest commercial neighborhood. The nearest outdoor occupied area on campus, a parking lot, is only 90 feet away. Sound pressure levels produced by the simulator as well as the exhaust requirements were evaluated in order to design a mitigation scheme to reduce noise to meet acceptable residential levels in the parking lot area. Phoenix Noise and Vibration worked with Vibro Acoustics to develop a silencer design to meet these criteria which will be reviewed in the presentation.

Probabilistic analysis of blast–obstacle interaction in a crowded internal environment

Recent terror events such as the Manchester Arena bombing and Brussels and Istanbul airport attacks featured improvised explosive devices detonated in crowded internal spaces. A blast wave that propagates in the presence of obstacles will have fundamentally different properties to those of an unimpeded blast wave. Physical processes such as reflection, diffraction, and superposition of multiple wave fronts result in highly complex and situational-dependent loading characteristics which cannot be predicted using simple tools such as those for predicting free-field blast parameters. The influence of blast–obstacle interaction within an internal environment has not yet been studied. This article uses computational fluid dynamics within a probabilistic framework to quantify the influence of obstacle density and positioning on blast loading characteristics. Two mechanisms which alter the properties of a blast wave are studied: ‘channelling’ and ‘shielding’. It is shown that channelling effects are highly localised and result in increased loading near the explosive, the effect of which increases with obstacle density. Shielding is shown to be a cumulative effect which increases with distance from the explosive, and with increasing obstacle density. Whole-domain cumulative density functions are used to derive quantitative descriptors of the loading characteristics and how they vary relative to simple benchmark cases, with a view to providing clear guidance on the development of future predictive tools.

Experimental Performance Assessment of Thermobaric Explosives in Free Field and Internal Blast Tests

Experimental Performance Assessment of Thermobaric Explosives in Free Field and Internal Blast Tests

Experimental Performance Assessment of Thermobaric Explosives in Free Field and Internal Blast Tests

Experimental Performance Assessment of Thermobaric Explosives in Free Field and Internal Blast Tests

Changes of arterial blood gas indexes of free-field primary blast lung injury of pigs and its application value

To observe the changes of arterial blood gas indexes in pigs with the free-field primary blast lung injury (PBLI) model, and to explore the value of arterial blood gas indexes in predicting moderate to severe PBLI. Nine adult healthy Landrace pigs were selected to construct the pig free-field PBLI model. Arterial blood samples were taken 15 minutes before the explosion (before injury) and 10, 30, 60, 120, and 180 minutes after the explosion (after injury). Arterial blood gas indexes and pulse oxygen saturation (SpO2) were measured, compare the changes of blood gas analysis indexes and SpO2 levels at different time points, and observe the changes of gross injury scores and pathological injury scores of lung tissue. Analyze the correlation between the blood gas indicators. As time prolonged, at each time point, pH, arterial partial pressure of oxygen (PaO2), and SpO2 were lower than those before the injury, and blood lactic acid (Lac) and arterial partial pressure of carbon dioxide (PaCO2) were higher than those before the injury. Compared with that before the injury, the pH value in the blood decreased significantly 10 minutes after the injury (7.39±0.06 vs. 7.46±0.02, P < 0.05), and the Lac increased significantly (mmol/L: 3.61±2.89 vs. 1.10±0.28, P < 0.05), and lasts until 180 minutes after injury (pH value: 7.37±0.07 vs. 7.46±0.02, Lac (mmol/L): 2.40±0.79 vs. 1.10±0.28, both P < 0.05); while PaO2 and SpO2 decreased significantly at 180 minutes after injury [PaO2 (mmHg, 1 mmHg = 0.133 kPa): 59.40±10.94 vs. 74.81±9.39, P < 0.05; SpO2: 0.75±0.11 vs. 0.89±0.08, P < 0.05], PaCO2 increased significantly (mmHg: 56.17±5.38 vs. 48.42±4.93, P < 0.05). Correlation analysis showed that the gross injury score of lung blast injury animals was positively correlated with the pathological injury score (r = 0.866, P = 0.005); PaO2 and SpO2 were positively correlated (r = 0.703, P = 0.000); pH value and Lac were negative Correlation (r = -0.400, P = 0.006); pH value is negatively correlated with PaCO2 (r = -0.844, P = 0.000). This study successfully established a large mammalian free-field PBLI model, arterial blood gas analysis is helpful for the early diagnosis of PBLI, whether SpO2 can be used to evaluate the severity of lung injury remains to be further verified.

Experimental Study of Explosion Mitigation by Deployed Metal Combined with Water Curtain

In this paper, protective barriers made of perforated plates with or without a water cover were investigated. In urban areas, such barriers could be envisaged for the protection of facades. An explosive-driven shock tube, combined with a retroreflective shadowgraph technique, was used to visualize the interaction of a blast wave profile with one or two plates made of expanded metal. Free-field air blast experiments were performed in order to evaluate the solution under real conditions. Configurations with either one or two grids were investigated. The transmitted pressure was measured on a wall placed behind the plate(s). It was observed that the overpressure and the impulse downstream of the plate(s) were reduced and that the mitigation performance increased with the number of plates. Adding a water layer on one grid contributed to enhance its mitigation capacity. In the setup with two plates, the addition of a water cover on the first grid induced only a modest improvement. This blast mitigation solution seems interesting for protection purposes.

Explosion mitigation by metal grid with water curtain

This paper examines blast mitigation based on geometric means, namely a perforated plate or a chain mail, with or without a water film cover. This method may be used for the protection of structures, and its effectiveness and limitations were assessed. First, a shock tube was used to visualize the interaction of a blast wave profile with a metallic grid or with a metallic grid covered by a layer of water. Secondly, free-field air blast experiments were performed in order to evaluate the protection system under real conditions. Three types of grids were tested. The first was a metallic plate having small round holes, the second had large round holes, and the third had square holes. A chain mail made of steel rings was also tested. The porosity of the grids ranged from 48 to 69%. In case of a collision between a shock wave and a grid, it was observed that one part of the incident shock wave was reflected by the plate. The remaining part was transmitted through the plate. The overpressure and the impulse downstream from the grid were reduced, and the reduction increased when the porosity decreased. When a film of water covered the grid, it was observed that the water film disintegrated into droplets long after the passage of the blast wave. Filling the holes with water enhanced the overpressure and impulse reduction as it contributed to the reflection of the shock wave.

Blast enhancement from metalized explosives

Experiments are carried out to determine the effects of particle size and mass loading on the free-field blast wave from spherical, constant volume metalized explosive charges. The charges are comprised of gelled nitromethane with uniformly embedded aluminum, magnesium, or glass particles. Particle sizes are varied over an order of magnitude with particle mass fractions up to 50%. Peak blast overpressures are directly measured within the fireball with piezoelectric pressure gauges and outside the fireball are inferred by tracking the velocity of the blast wave and using the Rankine–Hugoniot relation. With the addition of inert particles, the peak blast overpressure is initially mitigated, but then recovers in the far field. For charges with reactive particles, the particles react promptly with oxidizers in the detonation products and release energy as early as within the first few hundred microseconds in all cases. The particle energy release enhances the peak blast overpressures in the far field by up to twice the values for a constant volume charge of the baseline homogenous explosive. By plotting the peak blast overpressure decay as a function of energy-scaled distance, it is inferred that at least half of the particle energy release contributes to the blast overpressure in the far field of higher mass loadings, and nearly all of the particle energy for a particle mass fraction of 10%. For aluminum, the blast augmentation is not a systematic function of particle size. This observation implies that conventional models for particle combustion that depend on particle surface area are not appropriate for describing the rapid aluminum reaction that occurs in the extreme conditions within the detonation products, which influences the blast wave propagation.

Assessment of Compression Driven Shock Tube Designs in Replicating Free-Field Blast Conditions for Traumatic Brain Injury Studies.

Compression driven shock tubes are indispensable in studies of blast-induced traumatic brain injury (bTBI). The ability of shock tubes in faithfully recreating free-field blast conditions is of enormous interest and has a direct impact on injury outcomes. Toward this end, the evolution of blast wave inside and outside of the compression driven shock tube has been studied using validated, finite element based shock tube models. Several shock tube configurations (uniform cross-section, transition, conical, suddenly expanded, and end plate) have been considered. The finite element modeling approach has been used to simulate the transient, dynamic response of blast wave propagation. The response is studied for longer durations (40-100 msec) compared with the existing literature. We demonstrate that locations inside and outside of the shock tube can generate free-field blast profile in some form, but with numerous caveats. Our results indicate that the locations inside the shock tube are affected by higher underpressure and corresponding kinetic energy yield compared with free-field blast. These effects can be minimized using optimized end plate configuration at the exit of the shock tube, yet this is accompanied by secondary loading that is not representative of the free-field blast. Blast wave profile can be tailored using transition, conical, and suddenly expanded sections. We observe oscillations in the blast wave profile for suddenly expanded configuration. Locations outside the shock tube are affected by jet-wind effects because of the sudden expansion, barring a narrow region at the exit. For the desired overpressure yield inferred in bTBI, obtaining positive phase durations of <1 msec inside the shock tube, which are sought for studies in rodents, is challenging. Overall, these results underscore that replicating free-field blast conditions using a shock tube involves tradeoffs that need to be weighed carefully and their effect on injury outcomes should be evaluated during laboratory bTBI investigations.

Blast wave modification by detonator placement

The blast wave generated by a high explosive detonation is dependent not only on the shape of the charge but on the location of the detonator or detonators. Even for a spherical charge, the blast wave can be very asymmetric if the initiation is not at the center of the charge. The asymmetry is even greater for cylindrical charges. High-resolution, high-fidelity calculations of the blast wave generated by several spherical and cylindrical charges have been used to quantify these differences for free-field blast propagation. Except for the center-detonated spherical charge, the blast wave never becomes spherical. Several examples are shown to demonstrate and quantify the asymmetries and to illustrate that the blast wave, once asymmetric, can never regain spherical symmetry. At distances of over 40 charge radii, the asymmetries are clearly defined. As the overpressure at the shock front decays below half a bar, the propagation velocity approaches ambient sound speed. At half a bar (50 kPa), the Mach number of the shock is 1.19 and at a tenth of a bar (10 kPa) the shock velocity is only Mach 1.04. If the shock front is asymmetric at low overpressures, all parts of the shock front are moving at very nearly the same velocity and can therefore never “catch up” to other parts of the front: once asymmetric, always asymmetric. Results of several calculations have been analyzed to determine the quantitative differences in shock properties resulting from detonator placement and charge shape. Properties are significantly different behind the shock fronts, especially in the density distribution.

Peak overpressure and impulse due to diffraction over a cylinder and/or multi-reflection of a shock wave in structural design- Part I

The design and planning of structural components, such as columns, to resist blast loads is a complex task. Often standard free-field blast loads, specified in design codes or other literature, are used for the analysis of the object. These loads are then further increased or decreased depending on the topology of the surrounding geometry (Mach-Stem), the shape of the impacted object itself, and blast wave reflection. However, most of these research works focus purely on the assessment of the structural components itself, ignoring a complex fluid mechanics phenomenon such as diffraction, which is of particular interest when circular columns are standing next to each other or placed in front of a façade. The article addresses three main topics. First, the article answers the question whether the area behind the column is protected, meaning constituting a shadowed area. We present findings that the close area behind a column is subjected to higher pressure and impulse values as there would be without the column. Hence, the incident pressure sees significant pressure buildup due to diffraction. This pressure buildup is quantified using pressure increase factors and presented together with the accompanying impulse. Second, this pressure buildup is of relevance for realistic design of a façade behind the column, which is not covered in current design codes at all. We discuss relevant parameters in the design process. Third, directly coupled with the assessment of the pressure buildup behind the column due to diffraction is the assessment of pressure and impulse in the area behind the column due to multi-wave reflection at a façade, leading to a significant pressure and impulse scale-up, which might be relevant for design of a column and/or a façade. This article identifies gaps in understanding diffraction and subsequent multi-reflection of blast wave within a structural design framework and provides insights on how to establish safe design accounting for these effects.

Finite-Element Studies on Factors Influencing the Response of Underground Tunnels Subjected to Internal Explosion

The role of underground metro tunnels in a modern urban society is indispensable. Nowadays, these populous metro tunnel networks are becoming a prime target for terror strikes around the world. Considering these terror threats, it is crucial to study the vulnerability of tunnels when subjected to an internal explosion. This paper evaluates the vulnerability of underground metro tunnels exposed to an internal explosion using the explicit finite element (FE) method. The developed FE model has been validated with the field data reported in the literature for free-field blast wave propagation. Parametric studies were also carried out using the developed models to investigate the effect of lining material, lining thickness, lining shape, and type of surrounding of soil on the blast response of tunnels. A typical metro tunnel of 5.0 m internal diameter embedded at a depth of 9.0 m below the ground level is considered for the analysis. Numerical analysis shows that reinforced cement concrete (RCC) tunnels suffer severe damage when compared with the cast-iron tunnels. Box tunnels are extremely vulnerable to internal explosion when compared with circular and horseshoe tunnels. RCC tunnels embedded in medium dense sand are less vulnerable when compared with soft saturated clayey soil.

Characterization of debris throw from masonry wall sections subjected to blast

The failure of structural components, e.g. masonry walls, due to accidental or intentional explosions exhibits a considerable risk to the health of persons, operational safety, and surrounding structures. The debris throw originating from overloaded structural elements poses a significant threat to structures and persons in the surrounding environment in distances, which may exceed the hazard-range of the blast wave itself. Insights into the break-up process during structural failure and the resultant debris throw characteristics expressed in terms of fragment mass distributions, initial velocities and launch angles, are thus essential to assess the significant potential explosion hazard. However, to date the data basis for hazard assessment and model development on debris throw of masonry structures is limited. In this article, we present the results from shock-tube experiments of single-span masonry walls subjected to dynamic blast loads characteristic for far-field explosions and free-field blast propagation. Reflected peak overpressures in the tests ranged from 100 to 150 kPa with corresponding maximum impulses of approximately 2000 kPa ms to 3000 kPa ms. A new methodology based on high-speed stereo video imaging allows to analyze the break up process and the resulting debris sizes, launch velocities and three-dimensional launch angles before the debris hit the ground. The data derived can be used for empirical predictions of debris launch characteristics as well as for the validation of numerical simulations aimed at the proper assessment of hazardous secondary blast effects from masonry failure.

Approximating a free-field blast environment in the test section of an explosively driven conical shock tube

This paper presents experimental data on incident overpressures and the corresponding impulses obtained in the test section of an explosively driven $$10^\circ $$ (full angle) conical shock tube. Due to the shock tube’s steel walls approximating the boundary conditions seen by a spherical sector cut out of a detonating sphere of energetic material, a 5.3-g pentolite shock tube driver charge produces peak overpressures corresponding to a free-field detonation from an 816-g sphere of pentolite. The four test section geometries investigated in this paper (open air, cylindrical, $$10^\circ $$ inscribed square frustum, and $$10^\circ $$ circumscribed square frustum) provide a variety of different time histories for the incident overpressures and impulses, with a circumscribed square frustum yielding the best approximation of the estimated blast environment that would have been produced by a free-field detonation.