Understanding Explosions

24 January 2010

Interview with 

Dr Graham McShane, Cambridge University


Chris -  Graham, welcome to The Naked Scientists.  Good to have you with us.

Graham -   Thanks, Chris.

Chris -   So tell us first of all, what actually happens when something explodes?  How do you try and understand that?

Graham -   Okay.  Well the starting point for any explosion is a lump of solid explosive.   Gasoline explosions, simulating bomb drops at an airshow.When that explosive detonates, it rapidly converts into high pressure, high temperature gas and that gas then expands and it pushes on its surroundings.  That generates a pressure pulse or a shockwave which propagates away from the explosion and will strike some kind of structure that's nearby.  The type of loading that explosion will generate depends strongly on the medium that's surrounding that explosive.  We've looked at a couple of different types [of medium], and one type might be water.  The explosive might be a bomb or a detonation under the water next to a ship.  When that gas expands, it pushes on the water, compresses the water, and sends a shockwave towards your ship's structure.

Chris -   So a depth charge?

Graham -   A depth charge or something like that.  That's right.

Chris - In the context of a submarine or similar, this would be important.

Graham -   That's right.  Water is particularly dangerous from that point of view because it's a very stiff material.  It's very difficult to compress.  That means the pressures that can build up around the explosive are very large, so you can get shock pulses with very high peak pressures.

Chris -   So, if a depth charge does go off within a reasonably close proximity of a ship's hull or submarine, how do you work out how much force is then transmitted onto the hull of the vessel?

Graham -   Well that's very difficult to do.  In terms of a research exercise, there are various things we could do to try and analyze what the loading that that blast imparts to the structure is.  So, one thing we can do is we can try to simulate it in the lab.  We can try to experimentally simulate an underwater shock pulse.

Chris -   You'd have to build a warship or something.

Graham -   Well, not quite a warship.  We can reduce it down to maybe a small piece of a panel of a side of a ship and we can simulate the type of pressure pulse that you see in water near to an explosion.  Then we can measure directly what the momentum, the velocity that's imparted to that structure is.  Another thing we can do is do calculations, so we can run calculations where we simulate the pressure loading on a structure and then we can extract all sorts of information like the deformation of the structure and the forces that are required to hold the panel in place.

Chris -   Does this mean that the people are now able to build better ships and better subs so that when these kind of shockwaves slam into them, they dissipate the energy better so that you don't get a hole punched in the side?

Graham -   Yes.  We've certainly come up with a number of ideas for different types of materials that can help to protect ships better.  So, some examples are things like sandwich structures which you'd sometimes find in airplanes to provide you with a very light and stiff structure.  In these types of structures you have solid face sheets with a lightweight deformable core in between them and that core can crush and dissipate some energy.  These types of structures are very good at mitigating the effects of a blast near to the structure.  But then, these things are much more expensive than conventional materials.  So there's always a question of how much money you're prepared to spend to protect your ship.

Chris -   But then, given the massive cost of a ship, it probably must be worth that because if you lose the vessel, you've lost a huge amount haven't you?

Graham -   That's right.  That's right.  But then the cost could be enormous.  It could be a huge amount extra. These types of materials that we've looked at are very much more complicated than a solid steel panel.  You have to think about how to fabricate these things, how to make a ship-scale structure out of them, and then how to maintain them as well.  So there are lots of added costs, but there are solutions that work well.

Chris -   And looking on land, can you correlate what you understand and what happens in the water, the aquatic environment, to what happens on land?  Obviously, air is a fluid too,  it's just a slightly thinner one.

Graham -   That's right, yes.  I mean, an explosion in air shares many characteristics with an explosion in water.  It starts off with the same basic physical principles.  You start off with an explosive which converts to a gas and expands, and it compresses the air.  The big difference is that the air doesn't compress so much, so the pressures that you generate are different, and also the way that the shock pulse interacts with the structure is also different in air and in water.  In water, when the shock pulse reflects off a ship, it can cause the water to open apart, to cavitate.  That doesn't really happen in air.  So, there are different phenomena going on and that affects what your optimal solutions are for your material design.

Chris -   Presumably also, when something explodes on land, it also doesn't just have liquid to play with.  It's also got whatever is in the environment around it.  So if you're detonating a landmine there will be a huge amount of material from the ground, emitted upwards and projected up alongside that rush of air, which in itself is going to be devastatingly harmful because it presumably will be like sandpaper a thousand fold over, won't it?

Graham -   Absolutely.  A landmine detonation is very different from an explosion in free air, far away from anything else.  In a landmine explosion, your explosive is initially buried under a layer of soil.  So, that gas, when it tries to expand, it has to compress that soil and shock waves pass through the soil, but the soil then expands out and it sprays at the structure at very high velocities.  So the loading is very much more complicated for a landmine and it's taken a lot of research effort to understand what that loading is.  Never mind thinking about what materials are best for protecting vehicles against that type [of explosion].

Chris -   So it's not simply just the explosive force of the mine going off.  It's the fact that all that material gets ejected too and it totally changes the dynamics of an explosion.

Graham -   Absolutely.  There are a number of factors in it.  First off, when the soil starts to move, it starts to move extremely quickly, so it compresses the air around it and it'll send a shockwave ahead of it.  So there is a bit of an air blast going on there as well.  Following that, you got the sand flying behind it at very high speed.  And then behind that, you've got all the gases, the remnants of the explosive which follow behind that.  So it's a very complicated loading scenario.

Chris -   And how do you study that?

Graham -   Well again, we've got this combination of simple experimental techniques to try and capture the physics of what's going on and computational methods that can maybe capture more of the real landmine event.  We can attempt to some extent to simulate a landmine in the lab, but the engineering department is a city centre location!  It's not really suitable for setting off landmines.  So we have to think of other ways of throwing soil at structures at high speed.  So that's one aspect of the research and again, we can do a similar thing [to how we study explosions in water].  We can measure how the structure deforms and responds.  We can also use computation as well where we can try and simulate the sort of sand flows that are generated by these things and look at how different types of materials will behave.

Chris -   Is this why these improvised explosive devices are so devastating in places like Afghanistan?  Because when a car, or land rover, goes over it, if it hasn't got appropriate protection, then it's having to cope with this incredible force which is all directed at one point on the vehicle.

Graham -   Precisely.  It's very difficult to protect vehicles against this extreme type of loading.  You can design vehicles to try and protect the troops inside.  You can have very large vehicles with very heavy floors, very far off the ground because the further away from the landmine you can get, the lower the loads that the landmine imparts on your vehicle.  But for a lightweight vehicle, a Land Rover or something like that, it's very difficult to do.  You have to try and think of some lightweight solutions and it's very difficult to combine the light weight that you need in a mobile vehicle and also protection against this rather severe loading.

Chris -   And the strategy you've outlined for ships of having this sort of multilaminar structure, that would work, but just the weight would be a problem? or not?

Graham -   Well, no.  That would work as well.  You've got this sandwich structure, this layered structure.  You're going to think about how heavy is it, how much protection can you provide for the same mass of armour.  So if I had a solid steel plate, I could compare that with a sandwich plate of the same mass, and it will be more expensive to make, but it's likely to be stiffer.  The core can absorb some energy, so it's likely to give you some performance benefits overall.

Chris - The prediction is that there's something like 110 million landmines scattered around the earth.  People are stepping on them all the time, not just driving land rovers over them and they're often civilians.

Graham -   Yes.

Chris -   Is there anything we can do for people who live in at-risk areas? Could they wear something to avoid this 'sandpapering' of their legs and basically stripping all the flesh off which is what these mines are doing to them?

Graham -   Yes, that's right.  That's a very difficult problem to solve.  The problem is the intensity of the loading and when you step on a mine, you're very, very close to it.  As I say, the further you can get away from a landmine, the better you are.  To step on a landmine, it's a very severe loading.  It's very hard to think of material solutions to that.  The best solution is to try to remove the landmines somehow.  To try to detect them and to improve vehicles. You can improve vehicles so people can get into these areas, find the landmines and remove them.  I think that's a more effective solution.

Chris - Well just before we wrap this up with Graham, Helen you've actually had some experience of this haven't you? Explosion up close and personal.

Helen - That's right, actually it's the underwater explosions that I've unfortunately had a very close encounter with and it really hit home to me, just how quickly and how far underwater explosions reach.  I was diving, several years ago in Malaysia and unfortunately it was in the vicinity of a fish bomb that was thrown in the water. I should just explain briefly that across South-East Asia this is quite a prevalent way of catching fish.  It's actually illegal in many countries because it's very damaging not only to the people that occasionally have bombs going off in their hands but also to the environment.  I was in the water and this enormously loud "bang" sound ripped though the water.  I felt it all the way through my insides and it was absolutely terrifying.  I guess I knew what it was as I knew this could happen so I got out of the water as quickly and safely as I could and then noticed the fisherman who put this bomb in was miles away! He was actually right across on the horizon and I could just about see him.  I just couldn't believe that this had travelled quite so far and was still so powerful as to make me feel like if it had come any closer it might have deafened me or something.  It was terrifying.

Graham - Yes, it sounds like a pretty terrifying experience.  It just goes to show how efficiently a blast pulse can propagate through water.  Although that fisherman was quite some distance away, even his small bit of explosive would have created quite big pressures that would have propagated for quite a large distance.

Helen - And I assume if he had been any closer I would have been at risk of my ear drums rupturing and even my organs being damaged if it had been very close.

Graham - Absolutely. Yes, a shock pressure hitting the body will do all sorts of damage and it's best avoided so I can understand why they're keen to ban that kind of practice.

Helen - I know now what those poor fish are feeling as well! So yes it was a terrifying experience indeed.


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