Characterisation of Blast Loading: Current Research at The University of Sheffield

The Blast & Impact Research Group at the University of Sheffield is currently involved in several projects aimed at providing a better understanding of the blast pressure acting on targets under different threats. These projects fall broadly under two distinct scenarios: the combined soil-throw/blast load acting on a vehicle underside resulting from the detonation of a shallow-buried improvised explosive device; and the free-air blast load acting on a structural component which either wholly or partly forms a non-infinite reflecting surface. The research is largely experimentally based, and is augmented with numerical analysis. This paper provides a brief overview of the work conducted to date

Secondary shock delay measurements from explosive trials

Following detonation of an explosive material, a series of rarefaction expansion waves collapse inwards from the interface between the explosive and the surrounding air. These rarefaction waves coalesce at the centre of the explosive and reflect as a shock wave. Whilst these successive shocks are small in magnitude compared to the primary shock and are often ignored, the inward reflected shock immediately following the primary shock wave, typically referred to as the ‘secondary shock’, is a noticeable feature on blast pressure histories and usually arrives after the beginning of the negative phase. This paper presents results from medium and large scale surface blast tests where accurate measurements of secondary shock delay (time after arrival of the primary shock) are obtained for various explosives at various scaled distances. A method is presented for adjusting the secondary shock delay time by the product of the velocity of detonation divided by the cube-root of the packing density of the explosive. The relationship between this new secondary shock delay parameter and scaled distance is then found to be consistent for all explosives considered. This gives a new empirical method for estimating the yield of an explosive, or determining the velocity of detonation, based only on measurements of the secondary shock delay

TNT equivalence of C-4 and PE4: a review of traditional sources and recent data

Since standard engineering-level blast models are typically developed to predict airblast parameters (pressure and impulse) from TNT bursts, prediction of airblast from other materials uses an equivalence factor by which an equivalent TNT weight is computed and used in the source term of the model. This approach is widespread in the industry and has been codified in numerous manuals, books, and papers. A recent effort co-sponsored by TSWG (U.S.) and FSTD (Singapore) collected and compiled equivalence data for a wide variety of explosive materials (both military grade as well as home-made) into a single software tool named STREET. The database thus assembled provides a comprehensive and expandable repository for equivalence data. Two of the main achievements in STREET are the consideration of equivalence as a function of scaled standoff (rather than a scalar), and the documentation of uncertainty in the estimated value. In this paper, we consider specifically the manual- and test-derived data related to Composition C-4, and as a first step, we draw some judgments regarding the equivalence implicit in blast curves provided by UFC 3-340-02, for both pressure and impulse. Next, we consider PE4, which is similar in composition to C-4 and is used widely in the UK. A significant body of blast data for this explosive has been generated, from which equivalence is computed and is compared to the available data for C-4, with a view towards determining whether these two materials can in fact be considered as a single explosive (with two alternate names). Finally, considering the combined data for both C-4 and PE4, new curve fits are provided that represent the pressure and impulse equivalence of the C-4/PE4 material (and its uncertainty) as a function of scaled standoff

Single-Degree-of-Freedom response of finite targets subjected to blast loading – The influence of clearing

When evaluating the dynamic response of a structure subjected to a high explosive detonation, it is common to simplify both the target properties and the form of the blast pressure load - a standard approach is to model the target as an equivalent Single-Degree-of-Freedom (SDOF) system with the blast load idealised as a pulse which decays linearly with time. Whilst this method is suitable for cases where the reflecting surface is large, it is well known that for smaller targets, the propagation of a rarefaction 'clearing' wave from the edges of the target may cause a premature reduction in the magnitude of the blast pressure and hence reduce the total impulse acting on the structure. In this article, a simple method for calculating clearing relief, based on an acoustic approximation of the rarefaction wave, is coupled with an SDOF model to investigate the influence of clearing on the dynamic response of elastic targets. Response spectra are developed for a range of target sizes and blast events that may be of interest to the engineer, enabling the effects of blast wave clearing to be evaluated and situations where blast wave clearing may increase the peak displacement of the target to be determined. When the natural period of the target is large compared to the duration of loading, the reduction in positive phase impulse leads to significantly lower values of peak displacement when compared to an identical system subjected to a triangular blast load. For systems where the natural period is comparable to the duration of the loading, the early onset of negative pressure (attributed to blast wave clearing) can coincide with the rebound of the target and result in greater peak displacements. It is concluded that blast wave clearing should be evaluated and its influence quantified in order to ensure that blast resistant designs are efficient and safe. © 2012 Elsevier Ltd

Numerical predictions of the negative phase

The field of blast protective design emerged in the late 1940s and focussed mainly on large scale (nuclear) explosive loading massive structures. In these situations, positive phase effects were seen to dominate and the negative phase could effectively be ignored. Recently, however, the threat has moved to smaller scale explosives and increasingly lightweight structures. Here, the negative phase becomes important, however despite this the negative phase is often overlooked. This research presents a numerical investigation on the negative phase, with a primary focus on an accurate numerical scheme for modelling the negative phase blast pressure. Numerical tests are performed on deformable targets to determine fully reflected blast parameters, with associated numerical modelling conducted using Abaqus/Explicit. Moreover, the failure modes are obtained for light-weight panel employing the Perzyna model for metallic materials. The computational methods are adapted for better representation of the negative phase, including mesh refinement strategies, modelling of the explosive event and accurate description of the air behaviour. The results herein can be used to inform blast resistant designers on how to accurately model negative phase effects

A comprehensive comparison of methods for clearing effects on reflected airblast impulse

Having calculated the free-field pressure history at the location of a building, an engineer engaged in design or assessment of that building must then calculate the loads on the various surfaces of the structure. Numerous engineering methods have been developed that provide approximate (and generally conservative) approaches towards the calculation of these loads. Of greatest importance is the load on the front face (i.e., the building surface directly facing the explosion source). Depending on the size of the building and the blast load duration, clearing effects due to the building’s boundaries may reduce the reflected impulse on the front face from the fully reflected value predicted by standard blast models. Unfortunately, there are many methods available in the literature for evaluating clearing effects, each using somewhat similar, yet distinctly different, equations. One approach given in UFC 3-340-02 (and reproduced in UFC 3-340-01) has gained widespread acceptance; another is presented in a set of guidelines published by ASCE and used for industrial applications; and lastly, a formerly classified study dating back to 1955 which, although declassified in 1998, seems to have escaped the notice of the blast community. The focus of the present paper is to evaluate all three of these methods empirically, by comparing their results against a series of blast tests with varying charge weights and scaled reflecting building dimensions. A comparative evaluation is then made of the strengths and weaknesses of each approach, with recommendations for future use by researchers and blast engineers

Modelling split-Hopkinson pressure bar tests on quartz sand

FE modelling of a confined split Hopkinson pressure bar (SHPB) test on dry quartz sand was carried out using LS-DYNA in order to assess whether Material Model 5 could replicate experimental results, which would enable a more detailed investigation of the stress state in SHPB specimen. Quasi-static test data was used to select the material model input, and the model SHPB was set up to replicate the experimental conditions. The results show that Material Model 5 replicates the volumetric response provided as input data, but fails to predict the shear response observed in the quasi-static experiments. This was found to be due to the model treating the shear modulus as a constant rather than it increasing with strain, a feature which makes the Material Model 5 unsuitable for modelling SHPB tests on sand

Recommendations for cubicle separation in large-scale explosive arena trials

In large-scale arena blast testing, a common and economical practice undertaken is to position several cubicle targets radially around a central charge. To gain maximal benefit from this, targets should be positioned at their minimum permissible separation at which no blast wave interference is sustained from neighbouring obstructions. This interference typically occurs either when targets positioned at the same stand-off range are too close creating an amplification effect where a superposition forms between the incident blast wave and the reflected wave off the cubicle, or, where a target is positioned in the region behind another target, which causes a shadowing effect with decreased magnitudes of pressure and impulse. A comprehensive computational modelling study was undertaken using the hydrocode Air3D to examine the influence of cubicle positioning at different ranges on the surrounding blast wave pressure-time fields. A systematic series of simulations were conducted to show the differences in incident peak overpressure and positive phase impulse between free-field and obstructed-field simulation configurations. The predictions from the modelling study indicated that the presence of cubicle target obstructions resulted in differences in peak incident overpressure and positive phase impulse in nearby pressure waves. In all cases, at close separation distances, there were greater differences in peak pressure than positive phase impulse. However, with increased separation, peak pressure returned to free-field conditions sooner whilst differences in impulse remained significant, thus governing separation distance recommendations. The simulations showed that, for targets at the same stand-off range, clear separations of between 3.88 m and 6.92 m were required to achieve free-field equivalency, depending on the distance from the charge to the target. For targets at different stand-off ranges an angle greater than 54.2° from the front corner of the cubicle has been shown to ensure free-field equivalent conditions. A bespoke recommendation table has been generated to provide precise positioning for cubicles at different stand-off ranges in a look-up matrix format that can be readily used by engineers in the field