48 research outputs found
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
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
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 âundrained strengthâ 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
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
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
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
Finite element simulation of plates under non-uniform blast loads using a point-load method: Buried explosives
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. When a high explosive is shallowly buried in soil, the added confinement given by the geotechnical material results in a blast which is predominantly directed vertically. This imparts an extremely high magnitude, spatially non-uniform load on the target structure. A recently commissioned experimental rig designed by the authors has enabled direct measurements of the blast load resulting from buried explosive events. These direct measurements have been processed using an in-house interpolation routine which evaluates the load acting over a regular grid of points. These loads can then be applied as the nodal-point loads in a finite element model. This paper presents results from a series of experiments where a free-flying plate was suspended above a shallow buried explosive. Dynamic and residual deformations are compared with finite element simulations of plates using the experimentally recorded, and interpolated, nodal point-loads. The results show very good agreement and highlight the use of this method for evaluating the efficacy of targets subjected to non-uniform blast loads
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A multilevel neo-institutional analysis of infection prevention and control in English hospitals: coerced safety culture change?
Despite committed policy, regulative and professional efforts on healthcare safety, little is known about how such macro-interventions permeate organisations and shape culture over time. Informed by neo-institutional theory, we examined how inter-organisational influences shaped safety practices and inter-subjective meanings following efforts for coerced culture change. We traced macro-influences from 2000 to 2015 in infection prevention and control (IPC). Safety perceptions and meanings were inductively analysed from 130 in-depth qualitative interviews with senior- and middle-level managers from 30 English hospitals. A total of 869 institutional interventions were identified; 69% had a regulative component. In this context of forced implementation of safety practices, staff experienced inherent tensions concerning the scope of safety, their ability to be open and prioritisation of external mandates over local need. These tensions stemmed from conflicts among three co-existing institutional logics prevalent in the NHS. In response to requests for change, staff flexibly drew from a repertoire of cognitive, material and symbolic resources within and outside their organisations. They crafted 'strategies of action', guided by a situated assessment of first-hand practice experiences complementing collective evaluations of interventions such as 'pragmatic', 'sensible' and also 'legitimate'. Macro-institutional forces exerted influence either directly on individuals or indirectly by enriching the organisational cultural repertoire