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

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

Approach to Developing Design Charts for Quantifying the Influence of Blast Wave Clearing on Target Deformation

If a structural component is located close to the free edge of a building, clearing of the blast wave around the target edge may significantly influence the temporal characteristics of the applied pressure. Because of this, traditional analysis methods assuming a linear decaying load may not be valid, particularly if the blast event imparts a relatively large impulse from the negative phase. Treatment of this phenomenon is brief in the literature, and its influence is usually neglected. This article presents an approach to quantifying the influence of clearing on target deformation, through rigorous analysis of elastic–perfectly-plastic equivalent single-degree-of-freedom (SDOF) systems. The cleared load is evaluated for structural components situated at various distances from the free edge of a reflecting surface using the Hudson acoustic approximation. The results from the SDOF analyses are then used to draw up design charts for determination of the likely influence clearing may have on the design of blast-resistant structural components. Four regions are identified: areas where clearing is beneficial, where clearing has no effect, where clearing is acting adversely, or where clearing is acting highly adversely. The method presented herein provides clear demarcation of these regions

Finite element simulation of plates under non-uniform blast loads using a point-load method: Blast wave clearing

There are two primary challenges associated with assessing the adequacy of a protective structure to resist explosive events: firstly the spatial variation of load acting on a target must be predicted to a sufficient level of accuracy; secondly, the response of the target to this load must also be quantified. If a target is embedded within a finite reflecting surface then the process of blast wave clearing will occur. Diffraction of the blast around the target edge causes a low pressure relief wave to propagate inwards towards the centre of the target, reducing the late-time development of pressure and resulting in high spatial non-uniformity of the blast load. This paper presents experimental measurements of the dynamic displacement-time histories of steel plates subjected to blast loads where the plate was situated within a finite reflecting surface to allow for clearing effects to take place. Associated finite element modelling is presented, where coupled blast-target interaction is modelled explicitly using the Arbitrary Lagrangian-Eulerian solver in LS-DYNA. An alternative method is presented, where the loading is applied as discrete load predictions at individual nodes. The results show that vast computational savings can be made when modelling the load in this manner, as well as better agreement with the experimental measurements owing to a more accurate representation of the applied load

Geotechnical causes for variations in output measured from shallow buried charges

The role of the geotechnical conditions on the impulse delivered by a shallow buried charge has received much attention in recent times. As the importance of the soil in these events has become better understood, the control over the geotechnical conditions has improved. While previous work has investigated directly the role of geotechnical conditions on the magnitude of the impulse from a buried charge, the current work aims to identify how these same conditions also affect the repeatability of testing using soils. In this paper the authors draw together their work to date for a wide range of different soil types and moisture contents to investigate the variation in output from nominally identical tests. The methodology for the preparation of soil beds and the measurement of impulse is described along with the measured variations in peak and residual deflections of a target plate fixed to the impulse measurement apparatus

Double marking revisited

In 2002, the Qualifications and Curriculum Authority (QCA) published the report of an independent panel of experts into maintaining standards at Advanced Level (A-Level). One of its recommendations was for: ‘limited experimental double marking of scripts in subjects such as English to determine whether the strategy would signi-ficantly reduce errors of measurement’ (p. 24). This recommendation provided the impetus for this paper which reviews the all but forgotten literature on double marking and considers its relevance now

Displacement timer pins: An experimental method for measuring the dynamic deformation of explosively loaded plates

The measurement of dynamic deformation of an explosively loaded plate is an extremely onerous task. Existing techniques such as digital image correlation are expensive and the equipment may be damaged by explosively driven debris/ejecta, particularly if it is necessary to locate such equipment close to loaded elements which are likely to fail. A new, inexpensive and robust measurement technique for use in full-scale blast testing is presented, which involves the placement of displacement timer pins (DTPs) at pre-defined distances from the rear surface of the centre of a plate. A strain gauge on the perimeter of each pin records the time at which the plate comes into contact with the end of each DTP and hence has deformed to that value of displacement, giving a direct measure of the time-varying deformation at a discrete point on the plate. An experimental proof-of-concept was conducted and the results are compared with numerical displacements determined using LS-DYNA. The numerical and experimental results were in very good agreement, which suggests that the proposed experimental method offers a valuable means for determining the full-scale response of structures subjected to blast loads in aggressive environments. Further improvements to the experimental procedure are outlined, along with applications where the DTPs are particularly suited. 2015 Elsevier Ltd. All rights reserved

Effects of strain rate and moisture content on the behaviour of sand under one-dimensional compression

The influence of strain rate and moisture content on the behaviour of a quartz sand was assessed using high-pressure quasi-static (0.001 /s) and high-strain-rate (1000 /s) experiments under uniaxial strain. Quasi-static compression to axial stresses of 800 MPa was carried out alongside split Hopkinson pressure bar (SHPB) experiments to 400 MPa, where in each case lateral deformation of the specimen was prevented using a steel test box or ring, and lateral stresses were recorded. A significant increase in constrained modulus was observed between strain rates of 0.001 /s and 1000 /s, however a consistently lower Poisson's ratio in the dynamic tests minimised changes in bulk modulus. The reduction in Poisson’s ratio suggests that the stiffening of the sand in the SHPB tests is due to additional inertial confinement rather than an inherent strain-rate dependence. In the quasi-static tests the specimens behaved less stiffly with increasing moisture content, while in the dynamic tests the addition of water had little effect on the overall stiffness, causing the quasi-static and dynamic series to diverge with increasing moisture content

Experimentation and modelling of near field explosions

Repeatable experimental results and numerical work has shown that using the Jones-Wilkins-Lee (JWL) equation of state (EOS) will give very accurate results of peak pressures and impulse delivered to a rigid target at large scaled distances. However, recent experiments/numerical modelling at small scaled distances show that the JWL will overpredict peak pressures and impulse due to the assumption of (near) instantaneous energy release from detonation. The results of this experimental/numerical study are presented herein. In the experimental work PE4 spheres at two different scaled distances have been tested using an array of Hopkinson Pressure Bars (HPB) at specific points on a rigid target to measure the local pressure-time histories. From the HPB measurements, it appears that below certain scaled distances there are chemical-physical mechanisms that do not have sufficient time to contribute to the energy driving the loading mechanisms, explaining the over-prediction of the JWL. Importantly though, the experimental results show that at very small scaled distances (0.172 m/kg1/3) the test to test percentage variation is very low (5.1%); whilst at larger scaled distances (0.819 m/kg1/3) it is much higher (23.1%). This paper presents a model which describes the process by which experimental results move from repeatable to variable to repeatable as scaled distance increases from the extreme near field to far field