Near-field blast loading and transient target response : a collaboration between Sheffield and Cape Town

Near-field blast loads are high in magnitude, short in duration, and non-uniformly distributed across the loaded face of a structural element. Experimental characterisation of near-field blast loading and the resulting deformation of a blast loaded target is made difficult by conflicting requirements, namely: robustness to survive the extreme loading conditions; and sensitivity to accurately measure transient behaviour at high sampling frequencies. As such, there is little definitive experimental data in the literature, and the deformation behaviour of plates subjected to non-uniform impulsive loading is yet to be properly quantified. This paper presents an update on the ongoing collaborative research effort between the University of Sheffield, UK, and the University of Cape Town, South Africa. Direct experimental measurements of blast pressure and impulse using an array of Hopkinson pressure bars (Sheffield), and high-fidelity transient plate deformation measurements using digital image correlation (Cape Town) are jointly-used to assess, and develop predictive methods for, the response of blast loaded plates. Simplified predictive methods, based on knowledge of the applied load rather than an assumed distribution, have been developed which show very good correlation with the experiments and physics-based numerical models

Deformation of thin plates subjected to impulsive load : Part III – an update 25 years on

In 1989, Nurick and Martin published two review papers on the deformation of thin steel plates subjected to impulsive air-blast loading. The state of the art has progressed significantly in the following 25 years, and this review paper restricts itself to experimental studies that investigate the response of monolithic metal plates subjected to air-blast loading generated by detonating plastic explosive. From the large number of experiments reported, it is shown that the failure progressions in circular and quadrangular plates are similar and can be adequately described by three “failure modes” – namely large plastic deformation (mode I), tensile tearing (mode II) and shearing (mode III) although the severity and location of these failures on the plates is primarily determined by spatial distribution of the blast loading across the plate surface, and that boundary conditions significantly influence the onset of shearing and tearing failures due to variation in the in-plane movement of the plate material. The non-dimensional analysis approaches used by Nurick and Martin have been expanded to include the effects of load localisation and stand-off distance, and show good correlation with the expanded sets of test data published since 1989. It is concluded that these approaches still hold merit as simple tools for evaluating the likely effect of a close proximity air blast load on a flat metal plate

Perforated Plates as Passive Mitigation Systems

This paper presents the results of tests on fully-clamped circular plates subjected to blastloading directed down a tube. Four series of tests were performed. In one set of experiments,the blast wave was allowed to progress unhindered down the tube to impinge upon the plate,and in the other tests, perforated plates were placed in the path of the blast wave to hinderprogression down the tube, disrupting the blast and absorbing some of the kinetic energy.Results of the tests indicate that the perforated plates can be used as a form of passive mitigation

Influence of venting on the response of scaled aircraft luggage containers subjected to internal blast loading

This paper concerns the mitigation of damage in aircraft luggage containers subjected to internal blast loading. It reports findings of experimental and computational work on the influence of venting on the blast response of scaled unit load devices. The internal geometry of the structure was based on a 1:6 scale version of the commonly used LD-3 unit load device. To simplify the problem, only the face closest to the aircraft primary structure could deform whilst the other walls were kept rigid. Small, spherical, charges of PE4 plastic explosive were detonated inside the scaled structures. The fully confined blast tests exhibited the highest permanent displacements and were the only tests to produce rupture of the target plate. Introducing venting reduced the target plate displacement significantly. Computational simulations were developed using LS-Dyna to provide additional insight into the blast loading and its interaction with the structure beyond what could be measured experimentally. Venting appeared to have no effect on the pressure peak, but it was effective at removing the late-time pressure reflections. The influence of the side venting was slightly obscured in the experiments due to boundary pulling-in effects at higher charge masses, but the simulations showed that venting from two sides was slightly more effective in reducing target plate deformation than single-sided venting. The paper demonstrates the potential benefit of using LD-3 ULDs unit load device with canvas sides (rather than solid ones) and venting lengthwise along the aircraft body to redirect the loading away from vulnerable locations

The damage and impulse transfer characteristics of flexible steel V-structures with large bend radii

This paper reports results from an experimental and computational study on the influence of bend radius and internal angle on the damage and impulse transfer characteristics of flexible steel V-structures subjected to localized explosion loading. This issue has bearing on the manufacturing of V-hulls used for Mine Resistant Ambush Protected vehicles used around the world. Global impulse transfer, damage and transient deformation were measured during small-scale explosive detonations on 1:8-scale V-structures. The work found that increasing the bend radius to values that can be used in practical manufacturing generated damage that was less localized than the damage observed in V-structures with tighter bend radii. High-speed imaging was able to measure transient deformation that was maximal in the centre, and lower elastic post-peak vibration magnitudes at high charge masses. The impulse transfer increased as the bend radius increased and the internal V-angle increased. Since V-structures with tighter bend radii exhibit less permanent deformation and higher deformation gradients, they will be more prone to localized ruptures when deployed for blast protection, whereas structures with larger tip radii will need a larger region of the V-structure repaired after a blast event but may be less prone to rupturing when the blast loading is localized

Predicting the response of plates subjected to near-field explosions using an energy equivalent impulse

Recent experimental work by the current authors has provided highly spatially and temporally resolved measurements of the loading imparted to, and the subsequent dynamic response of, structures subjected to near-field explosive loading [1]. In this article we validate finite element models of plates subjected to near-field blast loads and perform a parametric study into the relationship between imparted load and peak and residual plate deformation. The energy equivalent impulse is derived, based on the theory of upper bound kinetic energy uptake introduced herein, which accounts for the additional energy imparted to a structure from a spatially non-uniform blast load. Whilst plate deflection is weakly correlated to total impulse, there is shown to be a strong positive correlation between deflection and energy equivalent impulse. The strength of this correlation is insensitive to loading distribution and mode of response. The method developed in this article has clear applications for the generation of fast-running engineering tools for the prediction of structural response to near-field explosions

Behaviour of a blast-driven ball bearing embedded in rear detonated cylindrical explosive

This paper presents insights into the flight characteristics of a ball bearing embedded in a rear detonated cylindrical charge, which represents an idealised piece of shrapnel from an improvised explosive device. A novel experimental technique was developed to quantify the loading from a blast-driven ball bearing. The impulse contributions from the blast pressure and the ball bearing impact were separately identifiable in the experimental data. Computational simulations, validated using experimental data, were used to elucidate additional detail about the momentum transfer and damage in the ball bearings during the blast event. The results show the critical influence of charge mass and aspect ratio on the development of the detonation pressure profile, its interaction with the embedded bearing, and the flight characteristics of the bearing. Length-to-diameter ratios below a critical value were more efficient in transferring momentum to the embedded bearings. These findings provide unique and detailed insights that will prove valuable to blast protection engineers considering the effects of embedded projectiles in improvised explosive devices

Influence of ball bearing size on the flight and damage characteristics of blast-driven ball bearings

This paper presents insights into the influence of ball size on the flight characteristics and damage of a ball bearing embedded in a rear detonated cylindrical charge. It includes results from a post-test damage analysis of ball bearings from previously reported experiments. Computational simulations using Ansys Autodyn were used to provide extra information about the velocity variation during flight and the damage sustained by the ball bearings during the blast event. The influence of bearing size (diameter and mass) was investigated using the validated simulation models to extend the dataset beyond the initial experimental work. The peak bearing velocity is influenced by the charge mass to ball bearing mass ratio and the aspect ratio of the charge. Larger ball bearings require extra momentum to accelerate them to higher velocities, but their higher surface area means a greater portion of the explosive charge is involved in transferring kinetic energy to the projectile. Tensile spalling was to be the major damage mechanism within the ball bearings. The charge aspect ratio also influenced the hydrostatic pressure propagation within the ball bearing itself, affecting the location and degree of internal cracking within the bearings. These findings will prove valuable to blast protection engineers considering the effects of embedded projectiles in improvised explosive devices

An all-Mach number HLLC-based scheme for multi-phase flow with surface tension

This paper presents an all-Mach method for two-phase inviscid flow in the presence of surface tension. A modified version of the Hartens–Lax–van Leer Contact (HLLC) solver is developed and combined for the first time with a widely used volume-of-fluid (VoF) method: the compressive interface capturing scheme for arbitrary meshes (CICSAM). This novel combination yields a scheme with both HLLC shock capturing as well as accurate liquid–gas interface tracking characteristics. It is achieved by reconstructing non-conservative (primitive) variables in a consistent manner to yield both robustness and accuracy. Liquid–gas interface curvature is computed via height functions and the convolution method. We emphasize the use of VoF in the interest of interface accuracy when modelling surface tension effects. The method is validated using a range of test-cases available in the literature. The results show flow features that are in sensible agreement with previous experimental and numerical work. In particular, the use of the HLLC-VoF combination leads to a sharp volume fraction and energy field with improved accuracy