Blast Mitigation Research Articles

Motion-based design approach for a novel variable friction cladding connection used in wind hazard mitigation

Cladding systems typically serve architectural purposes and protect occupants against the external environment. It is possible to leverage these systems to enhance structural resiliency. A common application is the use of blast resistant panels for enhanced protection against man-made hazards, whereas energy is dissipated through sacrificial elements. However, because these protection systems are passive, their mitigation capabilities are bandwidth limited, therefore targeting single types of hazards. The authors have recently proposed a novel variable friction cladding connection (VFCC), which enables the leveraging of cladding inertia for mitigating blast, wind, and seismic hazards. The variation in the friction force is generated by an actuator applying pressure onto sliding friction plates via a toggle system. Previous work has characterized the dynamic behavior of a VFCC prototype, and established design procedures for blast mitigation applications. Here, work is extended for applications to wind mitigation. An analytical model is developed to characterize the dynamic behavior of the VFCC for wind-induced vibrations. A motion-based design framework is developed to enable an holistic integration of the device within the structural design phase. Numerical simulations are conducted on a 24-story building example to demonstrate the motion-based design methodology. Results show that the semi-active cladding system provides significant reduction in the wind-induced inter-story drift and floor acceleration, therefore demonstrating the promise of the VFCC for field applications.

Blast Mitigation Analysis of Semi-Buried Structure

Semi-buried structures are most commonly used at first line of defense along the border between two countries. This demands investigation of their dynamic behaviour under blast loading. Herein, a semi-buried structure with foam sandwiched walls and buttresses to reduce the effect of blast is analysed. The effect of provision of different configurations of buttresses and foam core between two layers of structural wall subjected to explosive loadings is investigated using ABAQUS/Explicit®. Modelling of semi-buried structure is carried out by employing shell elements and soil is modelled using frequency independent spring-dashpot-mass model. The foam core is modelled using brick elements with reduced integration and volumetric hardening. Effect of strain rate on structural steel is modelled by employing Johnson-Cook (J-C) model. Results indicate that geometry of buttresses and foam core type governs structural response to dynamic loading. It is observed that inner wall of the structure is protected by foam provided in between walls and helps in blast mitigation. Further, it is observed that design of such structures is dependent on the correct identification of buttresses type and isolation of inner wall of structure by provision of energy absorbing materials like foam.

Numerical study of a foam-shock trap based blast mitigation strategy

Blast mitigation using various types of foams is of interest to practitioners for the safe design of structures. Experimental and numerical studies have demonstrated the beneficial effects of different foam materials when used as protective soft coatings. However, under certain blast loading conditions and foam dimensions, the load experienced by the target/protected structure was found to be much higher in the presence of foam than in its absence. In this study, a mechanism based on geometric means known as shock trap is used along with foam as a preventive measure against shock amplification. A shock trap is a special arrangement of rigid obstacles with an air gap, designed to offer a tortuous flow path. To analyze the proposed foam-shock trap combination, a popular Lagrangian based Smoothed Particle Hydrodynamics (SPH) is employed. A novel solid wall boundary modeling technique called skew gradient wall boundary treatment is also built into the SPH solver. The material discontinuity is handled by a multimass correction strategy. The blast wave mitigation characteristics of the proposed approach are evaluated using two different foam materials, namely, wet aqueous foam and polystyrene foam. From detailed simulations, it was observed that undesirable shock enhancement effect of foams was found to be completely suppressed by combining it with the shock trap mechanism. The proposed foam shock trap combination significantly reduces the peak load and impulse experienced by the target structure.

Human Skin-Like Composite Materials for Blast Induced Injury Mitigation

Armors and military grade personal protection equipment (PPE) materials to date are bulky and are not designed to effectively mitigate blast impacts. In the current work, a human skin-like castable simulant material was developed and its blast mitigation characteristics (in terms of induced stress reduction at the bone and muscles) were characterized in the presence of composite reinforcements. The reinforcement employed was Kevlar 129 (commonly used in advanced combat helmets), which was embedded within the novel skin simulant material as the matrix and used to cover a representative extremity based human skin, muscle and bone section finite element (FE) model. The composite variations tested were continuous and short-fiber types, lay-ups (0/0, 90/0, and 45/45 orientations) and different fiber volume fractions. From the analyses, the 0/0 continuous fiber lay-up with a fiber volume fraction close to 0.1 (or 10%) was found to reduce the blast-induced dynamic stresses at the bone and muscle sections by 78% and 70% respectively. These findings indicate that this novel skin simulant material with Kevlar 129 reinforcement, with further experimental testing, may present future opportunities in blast resistant armor padding designing.

Blast mitigation performance of cladding using square dome-shape kirigami folded structure as core

Structural response of the sacrificial cladding with square dome-shape kirigami (SDK) structure as core under blast loads is investigated in this study. A sample of SDK core folded from a pre-cut aluminium sheet is crushed under quasi-static loading condition. A numerical model is then developed and calibrated using experimental data. The calibrated model of SDK foldcore cladding is then placed on to a rigid block as a sacrificial layer to resist blast loading for structure protection. To evaluate the blast mitigation capacities, the parameters such as peak load transmitted to the protected structure, energy absorption, centre crushed distance and loading duration are compared among the claddings with different cores. Compared to square honeycomb, superior performance of blast mitigation is demonstrated for the proposed SDK foldcore sacrificial cladding by yielding a uniform collapsing similar to aluminium foam. Significant increase in energy absorption of the core is observed for the sacrificial cladding with SDK foldcore. It also yields a higher plateau stress than aluminium foam of the same density and is applicable to a wider range of blast loadings. The peak transmitted load to the protected structure is reduced by more than 70% comparing with the case without cladding. SDOF analysis of the sacrificial cladding systems is carried out and validated using numerical results. Based on the SDOF analysis, complete solution is derived and then used to obtain simplified design charts to show the suitable range of blast load scenarios where the sacrificial claddings are effective.

Experimental and numerical investigation of a hollow cylindrical water barrier against internal blast loading

A promising potential method of mitigating the blast effects caused by terrorist explosions is constructing a water barrier encircling an explosive found in public places. Eight in situ experiments were carried out to investigate the response and mitigation efficiency of hollow cylindrical water barriers against internal blast loading. Both the peak overpressure and the impulse along the side toward the base of the barrier were reduced mainly through reflection and diffraction of the blast wave. Numerical models corresponding to the experiments were developed using the commercial software AUTODYN and validated by the overpressure–time histories recorded in the experiments. The numerical results demonstrated that the water barrier enhanced the blast loading over the barrier, although it mitigated the blast loading along the side toward the base of the barrier. Parametric studies were carried out to comprehensively investigate the influences of the thickness, the height, and the inner diameter of the water barrier on the blast mitigation or blast enhancement effects. Adding a water cover on the top of the barrier was proved to be an effective way to eliminate the blast enhancement effects and efficiently improve the blast mitigation performance since it can substantially increase the kinetic energy extracted by the water barrier.

Challenges in blast protection research

Abstract This paper presents a view of some of the challenges that are presented in investigating protection methodologies against explosive blast effects. In particular, the paper is concerned with experimental efforts that can aid in the understanding of complex blast effects in typical real world scenarios. Current progress in the implementation of blast mitigation methodologies in the landward defence environment is reviewed.

Blast mitigation effect of the layered concrete structure with an air gap: A numerical approach

A novel layered concrete structure with an air gap was proposed to resist explosion attacks from terrorist bombings or conventional weapons. The proposed layered structure potentially benefits mitigating the blast wave induced from detonation in the predamaged concrete shelter layer by projectile penetration, because the wave impedance between air and concrete material is far different. To quantitatively evaluate the blast mitigation effects and reveal the blast-resistant mechanism, a fine numerical model of the proposed layered structure was built in LS-DYNA, in which trinitrotoluene explosive was detonated in predamaged concrete shelter at different depths. The developed model was validated by field tests of concrete slab under contact explosion. Failure mode of concrete shelter and pressure time histories on the protective structure layer were captured and compared. The distinct blast mitigation effect of the proposed layered structure was demonstrated. A parametric study on the perforation limit, height, and residual depth of concrete shelter was also performed. The calculated results revealed that trinitrotoluene explosive detonated at the impact perforation limit of concrete shelter by projectile penetration is a dangerous critical condition that may cause negative effects on blast mitigation. The blast mitigation rates can always exceed 99%, as long as the concrete shelter is not blasted to perforation. A fitting formula to obtain a higher blast mitigation rate in guiding the design of a layered concrete structure with an air gap was put forward.

Numerical investigation of particle–blast interaction during explosive dispersal of liquids and granular materials

Experiments show that when a high-explosive charge with embedded particles or a charge surrounded by a layer of liquid or granular material is detonated, the flow generated is perturbed by the motion of the particles and the blast wave profile differs from that of an ideal Friedlander form. Initially, the blast wave overpressure is reduced due to the energy dissipation resulting from compaction, fragmentation, and heating of the particle bed, and acceleration of the material. However, as the blast wave propagates, particle–flow interactions collectively serve to reduce the rate of decay of the peak blast wave overpressure. Computations carried out with a multiphase hydrocode reproduce the general trends observed experimentally and highlight the transition between the particle acceleration/deceleration phases, which is not accessible experimentally, since the particles are obscured by the detonation products. The dependence of the particle–blast interaction and the blast mitigation effectiveness on the mitigant to explosive mass ratio, the particle size, and the initial solid volume fraction is investigated systematically. The reduction in peak blast overpressure is, as in experiments, primarily dependent on the mass ratio of material to explosive, with the particle size, density, and initial porosity of the particle bed playing secondary roles. In the near field, the blast overpressure decreases sharply with distance as the particles are accelerated by the flow. When the particles decelerate due to drag, energy is returned to the flow and the peak blast overpressure recovers and reaches values similar to that of a bare explosive charge for low mass ratios. Time–distance trajectory plots of the particle and blast wave motion with the pressure field superimposed, illustrate the weak pressure waves generated by the motion of the particle layer which travel upstream and perturb the blast wave motion. Computation of the particle and gas momentum flux in the multiphase flow generated during explosive particle dispersal indicates that the particle momentum flux is the dominant term in the near field. Both the gas and particle loading must be taken into account when determining the damage to nearby structures following the detonation of a high-explosive charge surrounded by a material layer.

Experimental investigation of blast mitigation and particle–blast interaction during the explosive dispersal of particles and liquids

The attenuation of a blast wave from a high-explosive charge surrounded by a layer of inert material is investigated experimentally in a spherical geometry for a wide range of materials. The blast wave pressure is inferred from extracting the blast wave velocity with high-speed video as well as direct measurements with pressure transducers. The mitigant consists of either a packed bed of particles, a particle bed saturated with water, or a homogeneous liquid. The reduction in peak blast wave overpressure is primarily dependent on the mitigant to explosive mass ratio, M/C, with the mitigant material properties playing a secondary role. Relative peak pressure mitigation reduces with distance and for low values of M/C (< 10) can return to unmitigated pressure levels in the mid-to-far field. Solid particles are more effective at mitigating the blast overpressure than liquids, particularly in the near field and at low values of M/C, suggesting that the energy dissipation during compaction, deformation, and fracture of the powders plays an important role. The difference in scaled arrival time of the blast and material fronts increases with M/C and scaled distance, with solid particles giving the largest separation between the blast wave and cloud of particles. Surrounding a high-explosive charge with a layer of particles reduces the positive-phase blast impulse, whereas a liquid layer has no influence on the impulse in the far field. Taking the total impulse due to the blast wave and material impact into account implies that the damage to a nearby structure may actually be augmented for a range of distances. These results should be taken into consideration in the design of explosive mitigant systems.

Prediction of air blast mitigation in an array of rigid obstacles using smoothed particle hydrodynamics

The mitigation of blast waves propagating in air and interacting with rigid barriers and obstacles is numerically investigated using the mesh-free smoothed particle hydrodynamics method. A novel virtual boundary particle procedure with a skewed gradient wall boundary treatment is applied at the interfaces between air and rigid bodies. This procedure is validated with closed-form solutions for strong and weak shock reflection from rigid surfaces, supersonic flows over a wedge, formation of reflected, transverse, and Mach stem shocks, and also earlier experiments on interaction of a blast wave with concrete blocks. The mitigation of the overpressure and impulse transmitted to the protected structure due to an array of rigid obstacles of different shapes placed in the path of the blast wave is thereafter determined and discussed in the context of the existing experimental and numerical studies. It is shown that blockages having the shape of a right facing triangle or square placed in tandem or staggered provide better mitigation. The influence of the distance between the blockage array and protected structure is assessed, and the incorporation of a gap in the blockages is shown to improve the mitigation. The mechanisms responsible for the attenuation of air blast are identified through the simulations.

Epoxy-glass Microballoon Syntactic Foams for Blast Mitigating Applications

Polymeric syntactic foams refer to a class of cellular material created using preformed hollow spheres bound together with a polymeric matrix. These cellular materials possess exceptional ability to respond against high impact dynamic loads. This paper is an attempt to fabricate polymeric syntactic foams of epoxy containing hollow glass microballoon at varying loading (40 % - 60 %) and explore their potential towards blast mitigation. The tensile, compressive and flexural strength were found to be inversely proportional to the microballoon loading in the quasi-static regime. The strain rate sensitivity of the foams was confirmed by performing high strain rate studies using split hopkinson pressure bar. The flow stress of these foams was found to increase with increasing strain rates. The syntactic foams were subjected to controlled transient blast loadings using a shock tube. The samples remained intact and no strain was observed on the strain gauge, even under a blast load of ~ 90 psi, which clearly highlight their potential as core materials for blast mitigating applications.

Performance-based design procedure of a novel friction-based cladding connection for blast mitigation

Cladding systems are conventionally designed to provide buildings with environmental protection against wind, temperature, humidity, moisture, etc. Recently, researchers have proposed to leverage these systems to provide additional protection against manmade (e.g., blast) and natural (e.g., earthquakes, hurricanes) hazards. This can be achieved, for example, by redesigning the connection between the cladding and the structural system to provide energy dissipation via friction. While promising, the use of flexible cladding connection has only been considered for singular hazards. In this study, the authors propose a novel semi-active damping system to connect the cladding to the structure via a variable friction mechanism. By varying the normal force applied on friction plates through a system of adjustable toggles, it is possible to mitigate vibrations over a wide frequency range, therefore enabling mitigation of different types of hazards (i.e. to achieve multi-hazard resistance). In its passive in-situ mode, the device is designed to provide very high stiffness and friction resistance to mitigate the effects of blast.The objective of this paper is to enable a holistic integration of said device within the structural design process by developing a performance-based design procedure. The study will focus on the passive in-situ mode of the device, which will provide a stepping stone for the development of performance-based design procedures for its semi-active (i.e. actuated) capabilities. The proposed performance-based design procedure consists of the following: 1) determine the design performance criteria, including the blast properties and allowable connection gap between the cladding and structure; 2) select design properties for the cladding connection, including stiffness and damping; and 3) design a rubber impact bumper located between the structure and the cladding in order to mitigate slamming of the cladding into the structure for very high blast loads.

Polyurea Coated and Plane Reinforced Concrete Panel Behavior under Blast Loading: Numerical Simulation to Experimental Results

This research investigated the behavior of reduced scale standard reinforced concrete panels coated with various polyurea systems under blast loading. The blast mitigation performance of four coatings including two plain polyureas and two discrete fiber-reinforced polyurea (DFRP) systems was evaluated. Chopped E-glass fibers were discretely integrated in with the polyurea to develop a DFRP system. The addition of glass fiber to a polymer coating provides improved stiffness and strength to the composite system while the polyurea base material provides ductility according to a fiber length optimization study conducted by Carey and Myers (2011) [1]. Reduced scale panel blast testing was conducted. In addition, the finite element program LS-DYNA was used to model panel and coating response under blast loading. Several modeling solutions were undertaken and compared for concrete formulation. Winfrith concrete material, model and Surface Cap model, which are widely used to simulate the concrete behavior under dynamic loading, were examined. Modeling results were analyzed and compared to the experimental work to validate the conclusions.

Underwater oblique shock wave reflection

Domains of regular and irregular reflection for underwater oblique shock waves have been identified theoretically. We compare the Tait and Mie-Gr\uneisen equations of state in their ability to demonstrate physics of shock wave reflection and find implications for designing blast mitigation devices.

Blast mitigation by water in a bag on a tunnel floor

Mitigation of blast wave, caused by explosion of explosives, from a straight tube using water in a bag (water bag) was evaluated. The length of the tube was 330 mm and the cross-section area was 30 x 30 mm2. One end of the tube was closed. The water bag was placed on the floor or closed end wall of the tube near the explosive. The thickness of water was 3 mm. A specially designed small detonator, which contains lead azide of 100 mg, was ignited near the closed end wall of the tube. The blast pressure outside the tube was measured and examined. The blast wave was remarkably mitigated by the water bag. Equivalent ratio analysis revealed that the glass beads absorbed 33%-45% of explosion energy.

Energy absorption of the ring stiffened tubes and the application in blast wall design

Thin-walled mental tubes under lateral crushing are desirable and reliable energy absorbers against impact or blast loads. However, the early formations of plastic hinges in the thin cylindrical wall limit the energy absorption performance. This study investigates the energy absorption performance of a simple, light and efficient energy absorber called the ring stiffened tube. Due to the increase of section modulus of tube wall and the restraining effect of the T-stiffener flange, key energy absorption parameters (peak crushing force, energy absorption and specific energy absorption) have been significantly improved against the empty tube. Its potential application in the offshore blast wall design has also been investigated. It is proposed to replace the blast wall endplates at the supports with the energy absorption devices that are made up of the ring stiffened tubes and springs. An analytical model based on beam vibration theory and virtual work theory, in which the boundary conditions at each support are simplified as a translational spring and a rotational spring, has been developed to evaluate the blast mitigation effect of the proposed design scheme. Finite element method has been applied to validate the analytical model. Comparisons of key design criterions such as panel deflection and energy absorption against the traditional design demonstrate the effectiveness of the proposed design in blast alleviation.

Numerical Investigation of Blast Mitigation Mechanism of Accumulated Particles inside Straight Tube

Numerical Investigation of Blast Mitigation Mechanism of Accumulated Particles inside Straight Tube

Investigation of strain‐rate effects in Al foams and Ni/Al hybrid foams on different scales

AbstractOpen‐cell metal foams are a new class of cellular materials with structural features resembling those of lightweight load‐bearing materials such as cancellous bones and wood. Their high stiffness‐to‐weight ratio coupled with their typical long, flat stress‐strain response make them ideal candidates as cost‐effective shock energy absorbers in crashworthiness, impact loading and blast mitigation strategies. The macroscopic mechanical properties of foams are strongly influenced by both the mechanical behaviour of single pores at the mesoscopic level and the struts and their structure at the microscopic length‐scale, based on a strong structure‐property relationship. This is shown in the present contribution where an experimental‐numerical investigation has been conducted demonstrating the existence of strain‐rate effects at different hierarchical scales. Micro inertia effects arising due to the pore geometry as well as further strain‐rate effects stemming from the rate‐sensitivity of the Ni coating in Ni/Al hybrid foams are also outlined. (© 2017 Wiley‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Blast mitigation with fluid Containers: Effect of mitigant type

The effect of filled external containers on the deformation induced in a steel plate under near-field blast loading has been investigated through a combined numerical and experimental study. Six different fill materials (mitigants) were considered for inducing near-field blast mitigation. The mitigants evaluated were bulk water, aerated water, sand, expanded polystyrene (EPS), a combination of EPS and water, and shear thickening fluid. The performance of the mitigants depended on their mass, with sand providing the best mitigation and EPS the worst for a given volume. Bulk water provided the greatest reduction of the peak deformation per unit of added mass. The mitigant material also had a significant effect on the deformation-time history of the steel plate. The sand and ½ EPS + ½ water containers were found to significantly delay the arrival of the pressure wave at the target surface due to their compressibility and low sound speed. Numerical analysis reveals that different mechanisms induce blast mitigation, and these are identified for each of the different mitigant materials.