Mechanisms and Characterisation of Explosions (MaCE)

Lead Research Organisation: University of Sheffield
Department Name: Civil and Structural Engineering

Abstract

Analysis of the effects of high explosive blast loading on structures has applications in transport security, infrastructure assessment and defence protection. Engineers must utilise materials in efficient and effective ways to mitigate loads of extreme magnitudes, acting over milliseconds. But there is a fundamental problem which hampers research and practice in this field; we still do not fully understand the loads generated by a high explosive blast.

Scientific characterisation of blast loading was a pressing issue in the middle of the last century, as researchers developed methods to predict the loading from large conventional blasts, and from atomic weapons at relatively long distances from targets. The huge amount of effort expended on this work, and the involvement of some of the world's leading physicists and mathematicians (G.I. Taylor, John von Neumann) reflected the existential nature of that threat. This work was predominately based on studying blast loading on targets at relatively long distances from detonations (far-field).

Over the past few decades, whilst great advances have been made in understanding and designing materials to withstand extraordinary loads, experimental characterisation of blast loading itself has not kept pace in three key areas, which this project directly aims to address:

Firstly, we don't know the magnitudes of explosive loading on targets very close to a high explosive detonation. Today's terrorist threats are frequently from smaller, focused, close-range explosions. Scenarios such as bombs smuggled onto aircraft, or targeted attacks on key items of critical infrastructure are ones in which such "near-field" loading is potentially devastating. But there is an almost total absence of high quality experimental work on characterising near-field blast loading. Predictions in these safety-critical areas currently rely on extrapolation of simple far-field models, or the use of inadequately validated numerical models. The project will provide new, properly validated, numerical models based on high quality experimental work to address this.

This raises the second knowledge gap. Our current models of detonation-to-blast-wave mechanisms are based on simplified assumptions, such as that energy is released essentially instantaneously on detonation. Whilst this appears to work well for the far-field, there are major doubts over its validity in the near-field. This project will bring together blast engineers, high-temperature experimentalists, and energetic chemistry researchers to identify the role of early-stage post-detonation chemical reactions between the explosive fireball and the atmospheric oxygen in releasing energy, and how that affects the subsequent blast loading. The data gathered in the project will allow a new conceptual blast model to be created based on novel experimental analysis.

The final knowledge gap is the question of whether blast loading in well-controlled scientific experiments is essentially deterministic or chaotic in nature. Addressing this issue is vital if the blast loading research community is to have the equivalent of a standard wind tunnel or shaking table test. Our preliminary work has led to the hypothesis that there is a region at the boundary between the near- and far-fields, where instabilities in the fireball will lead to large and random spatial and temporal variations in pressure loading, but that either side of this, the loading should be deterministic and determinable. The project will provide the data to validate this hypothesis, thus being able to provide guidance to other researchers in the field.

Addressing these gaps, through a programme of multi-disciplinary experimental research, will produce a step change in our understanding of blast loading and our ability to protect against blast threats.

Planned Impact

The findings from MaCE are expected to have wide and deep impact for blast protection engineering practitioners in scenarios ranging from aviation security, building design and major event security planning. The immediate impact will be the provision of badly-needed, high-quality and detailed experimental blast load data which can be used both directly by engineers in their analyses, and as a benchmark for the validation of detailed numerical modelling prediction tools. A longer-term impact will emerge from the enhanced fundamental understanding of blast loading mechanisms in the particularly dangerous region immediately adjacent to an explosion, which can be used to develop the next generation of protection systems.

The proposers of this project have a long and successful track record of directing fundamental research towards impact in the field of blast protection engineering. We will build on this background, to ensure that the findings of MaCE are quickly, widely and effectively disseminated to blast protection specialists. The team is well-known and well connected throughout the UK and international blast protection practitioner community. The project will have a steering committee, comprising partners from leading Govt. and industry organisations, which will meet at 6-monthly intervals, and through which results and findings will be immediately disseminated. This will provide an immediate impact, through the availability of high-quality data for use in design of protection systems and validation of modelling approaches, and will give a longer-term direction to the development of novel protection systems as the understanding of the magnitudes and mechanisms of loading become clearer. Project updates will be presented at the annual Home Office sponsored Gathering of Experts in Mitigating Systems (GEMS), the UK's leading technical symposium for blast protection specialists.

A series of dedicated seminars will also be run on the theme of Blast Effects Acting on Materials and Structures (BEAMSem). These events will bring together leading academics and practitioners to quickly disseminate results and to aid in the integration of threat and protection solution research. It is hoped that knowledge of the mechanisms of blast loading may also lead to novel material development to be able to mitigate close-in explosions, and hence lead to improved safety in both military and civilian arenas.

A key output from MaCE will be a fast-running engineering model (FREM) that will allow users to quickly and accurately predict blast load parameters for a range of scenarios. To achieve the widest possible impact two approaches will deliver this FREM to the user community. The first will be to integrate our findings in the US Army FREM ConWep, access to which is difficult to attain. The proposers have strong connections with the US Army Engineers Research and Development Center (ERDC) who develop ConWep to facilitate this. We will also use the recently-launched Energetics Materials Blast Information Group (EMBIG) to develop and disseminate a separate FREM to its suitably cleared members, who hitherto have been at a distinct disadvantage. The implementation of the physics of the conceptual model into widely used modelling software such as LS-DYNA will also ensure the developments reach the widest possible audience.

International collaboration is vital in this field. The proposers will build on existing links with leading researchers and practitioners in the USA, Germany, South Africa and Australia, sharing results and findings (via the APSC), and using this to help define possibilities and future challenges for the field of blast protection engineering. A further key development will be the curation of an experimental results database which not only houses the project results, but other suitably well controlled tests from research centres worldwide to develop the definitive globally accepted dataset on blast loading parameters.

Publications

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