The University of Sheffield’s department of civil and structural engineering has been involved with the experimental characterisation of blast loading since the mid-2000s when it was contracted by the UK Defence Science and Technology Laboratory (Dstl) to develop experimental techniques for measuring landmine explosions.
The team realised that there was a lot to learn about the explosion process, particularly close-in to the explosion, and decided to study in depth what happens in air, which is a simpler situation than buried explosives.
This is the subject of the four-year, £1.2m Mechanism and Characterisation of Explosions (MACE) project funded by the Engineering and Physical Sciences Research Council (EPSRC ).
Lecturer in blast and impact engineering Sam Rigby talked to Berenice Baker about the aims of the project and how the team captures data in the extreme conditions of a blast.
Berenice Baker: What are the challenges of studying explosions close-up?
Sam Rigby: A lot of the historical context comes from work done in the 1940s and 1950s to do with the large-scale explosions and the nuclear trials they were doing in the US. They developed some very comprehensive understanding of what was going on, but it tended to be in the situations where you were far enough from the explosion where things were behaving regularly in that they were predictable and understandable through more basic physics.
As you step closer to an explosion, the magnitude is so high and the durations are so short that until recently there weren’t any ways to experimentally measure or observe this.
BB: Why do we need to know what happens in the heart of the blast?
SR: Because of how an explosion expands in the air, it loses a lot of energy very quickly so once you are a few radii away from the charge the pressures have dropped by orders of magnitude; it’s very non-linear. When the explosive detonates, the fireball expands to a certain size and position then it stops expanding. The shockwave detaches and does damage as it propagates.
We’re interested in what happens very close in. You’ve got this combined loading made up of very highly-pressurised, highly-shocked air, but it’s also got the products of detonation that are travelling at very high velocities of several times the speed of sound. That region is where these really interesting effects related to the chemistry of the explosion happen as well.
BB: What instruments do you use?
SR: We’re using an instrument called a Hopkinson pressure bar which is over 100 years old. It was invented by Bertram Hopkinson in 1914 as a way of measuring high-amplitude dynamic forces. It’s essentially a steel cylinder.
The equipment we use has an array of these Hopkinson pressure bars; in the current configuration there’s 17 of them, all placed within a 200mm diameter instrumented region to trap a pressure signal. In the same way that a sound wave is a disturbance that’s travelling through air – if you speak your voice box vibrates, it sets the air particles in motion, and they cascade into one another which sets up this train of information – exactly the same thing happens in a Hopkinson pressure bar.
BB: What do you hope to characterise through this project?
SR: We’re looking at developing a compiled dataset of explosions measurements. We can use scaling laws to express the blast wave and blast loading parameters at a certain scale, which means we can compare results with lots of different experiments that were conducted with lots of different sizes of explosives.
Our aim is to set up an international database for blast loading and blast wave parameters. Ours will be the first data but we want to open this up to other researchers so they can say we’ve carried out this test at this distance and here are our parameters. Somebody can type in “I’ve got this size explosion at this distance” then it will refer to our data and provide them with an estimation of what the pressure is.
BB: Who will benefit from this research?
SR: Part of the reason why we started this project is because it’s fairly wide-reaching and has lots of potential different beneficiaries. We’ve worked a lot on looking at a better understanding of what might happen is someone were to bring an explosive device onto an aircraft. It also applies to military defence, so what’s the likely loading from an RPG or an IED detonated near a military vehicle. It also extends into the civil and structural engineering domains, so how can we design structures and building components to be resilient against bomb attacks, either accidental or malicious in the case of terrorist devices.
BB: What kind of explosives facilities do you have?
SR: We’ve got a test site out in the Derbyshire hills just outside Buxton with loads of reinforced concrete bunkers that were part of an old munitions store in WWII. We’ve got the bunkers that are designed with explosions in mind, and it’s far enough away from our nearest neighbour that we can set off explosives in the order of a few kilograms, verging on real-sized explosive of the size a person might have on their body.
We’ve got a range of diagnostic techniques and as part of this grant we’re adding high-pressure transducers and we’re hoping to branch out into optical measurements too, so infra-red cameras and high-speed video cameras.
BB: What is the ultimate aim of this research?
SR: Our approach to blast protection engineering needs to be proactive rather than reactive. I don’t think it’s good enough for us to wait for something to happen, cause a lot of death and damage and then we look at it and say I don’t understand that, what do I need to do to improve that next time? As blast protection engineers we need to be ahead of the game; we need to be in a position where we are one step ahead of the people who are trying to cause damage with these sorts of reasons.