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

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

Repeatability of buried charge testing

The relationship between the geotechnical parameters and the impulse delivered by a charge buried shallowly (<100 mm) within a soil mass has received much attention in recent times. It has previously been demonstrated that for uniform soils a high degree of repeatability in the delivered impulse can be achieved when carefully controlling the geotechnical conditions. In this paper the authors explore the recommendations given in AEP55 regarding the testing of surrogate mine blasts using both minepots and the STANAG standard sandy gravel. With moisture content and bulk density being intrinsically coupled, using bulk density as a measure of geotechnical control and hence repeatability is questionable. A methodology for the careful preparation of the Stanag sandy gravel soil is presented along with comparative results from minepots demonstrating the comparative repeatability of both methodologies. These results are then compared with a number of other soils to allow gen-eral conclusions about the repeatability of specific soil characteristics to be drawn

Experimental studies of the effect of rapid afterburn on shock development of near-field explosions

Many conventional high explosives do not contain sufficient internal oxygen to fully combust the gaseous products which result from detonation of the explosive material. Because of this, under-oxygenated explosives continue to burn after detonation. This process, called afterburn, is known to influence the late-time pressure and energy released by the explosive, which has particular significance for confined explosives. Recent experimental work at the University of Sheffield, along with a small number of previous studies, has shown that some afterburn occurs at timescales commensurate with the development of the shock wave. This article presents the results from a series of tests measuring the reflected pressure acting on a rigid target following the detonation of small explosive charges. High-speed video is used to capture the emerging structure of the detonation products and air shock, while the spatial and temporal distributions of the reflected pressure are recorded using an array of 17 Hopkinson pressure bars set flush with an effectively rigid target. Tests are conducted in inert atmospheres and oxygen-rich atmospheres in order to assess the contribution of rapid afterburn on the development of the shock front and interaction with a rigid target situated close to the explosive charge. The results show that early-stage afterburn has a significant influence on the reflected shock parameters in the near-field

Localised variations in reflected pressure from explosives buried in uniform and well-graded soils

Recent experiments into characterisation of the loading resulting from detonation of a shallow buried explosive have highlighted the complex underlying physical mechanisms present at the face of a target situated above the soil surface. This paper presents the results from such experiments, where the localised blast pressure and impulse is measured using an array of Hopkinson pressure bars at specific points on the target surface. Two different soil types are tested; a relatively uniform sand, and well-graded sandy-gravel. It is observed that the variability in localised loading is intrinsically linked to the particle size distribution of the soil medium; the uniform soil produces repeatable data with little variation whereas the well-graded soil demonstrates considerable spread. The cause of this spread is quantified and discussed with reference to the distinct loading mechanisms acting on the target as seen in the experimental data

Reflected pressures from explosives buried in idealised cohesive soils

Recent work has concentrated on the characterisation of the temporal and spatial impulse distribution of blast form buried charges. A new soil container preparation methodology has been created to allow for the generation of highly repeatable, tightly controlled clay beds which will allow clays of different undrained strengths to be generated. Tests using these well controlled beds has allowed for an improved understanding into which geotechnical parameters govern the impulse delivered by a buried charge. Namely in the current programme of work this is an investigation into the ‘undrained strength’ of a cohesive material as an indicator of potential impulse output. Initial results are compared against previously published work on cohesionless soils (sands) to try to establish the full range of loading which can be generated by a buried charge

Predicting the role of geotechnical parameters on the output from shallow buried explosives

Experiments have been conducted to quantify the effect the geotechnical conditions surrounding a buried charge have on the resulting output. From the results obtained the critical importance of moisture content in governing the magnitude of impulse delivered is highlighted. This has led to the development of a first-order predictive model for the impulse delivered from a buried charge, based on bulk density and moisture content, allowing rapid assessment of the effect of varying the geotechnical conditions. The work utilised a half-scale impulse measurement apparatus which incorporated a deformable target plate. Impulse, peak and residual target deflections were recorded for each test. No variations the charge geometry, mass of explosive, burial depth or stand-off were considered, with the focus solely being on the effect of the geotechnical conditions on the magnitude of loading and structural response. Five different types or grades of soils were used in the work, with both cohesive and cohesionless soils represented. The effect of air voids on the impulse generated was also investigated which showed that while strongly correlated, air voids alone is a poorer predictor of impulse than moisture content

Influence of particle size distribution on the blast pressure profile from explosives buried in saturated soils

The spatial and temporal distribution of pressure and impulse from explosives buried in saturated cohesive and cohesionless soils has been measured experimentally for the first time. Ten experiments have been conducted at quarter-scale, where localised pressure loading was measured using an array of 17 Hopkinson pressure bars. The blast pressure measurements are used in conjunction with high-speed video filmed at 100,000 fps to investigate in detail the physical processes occurring at the loaded face. Two coarse cohesionless soils and one fine cohesive soil were tested: a relatively uniform sand, a well-graded sandy-gravel, and a fine-grained clay. The results show that there is a single fundamental loading mechanism when explosives are detonated in saturated soil, invariant of particle size and soil cohesion. It is also shown that variability in localised loading is intrinsically linked to the particle size distribution of the surrounding soil

Measuring the spatial and temporal pressure variation from buried charges

The effect of changing geotechnical parameters on the impulse generated from a shallow buried charge has been the topic of a large amount of scientific interest in recent years. Many previous researchers have utilised a free flying mass experimental approach to capture the impulse imparted from such an event. This methodology has also been used for a parametric study conducted at the University of Sheffield Blast and Impact laboratory A new approach which aims to better capture the loading from shallow buried charges uses a fixed plate with data recorded via load transducers and spatially and temporally resolved via an array of Hopkinson pressure bars. This paper outlines the revised experimental approach for the capture of spatially and temporally resolved impulse data at the blast-target interface. Issues encountered during the commissioning tests using charges bur-ied in silica sand are discussed, and initial results from the original and revised Hopkinson pressure bar arrays are presented

In Things We Trust? Towards trustability in the Internet of Things

This essay discusses the main privacy, security and trustability issues with the Internet of Things