Shock Physics

Helen - Well, another useful way of understanding explosions and explosive materials is to understand the shockwaves that they create and how these affect other materials, including living tissue. Dr. Bill Proud divides his time between the Cavendish labs here in Cambridge and the Institute of Shock Physics at Imperial College in London, and he's found some time to join us today. Hi, Bill. Thanks for coming along.

Bill - I'm very happy to be here.

Helen - Now, we've already mentioned the word shockwave, but what are these things?

Bill - Well, I think the easiest way to consider them is if you make any kind of sound in the room, any kinds of noise, you're sending out pressure pulses through the air, and people have generally heard of a thing called the speed of sound, and most people doing A levels, for example, say it's 330 metres a second. The thing about the shockwave though is that the pressure pulse is supersonic, with respect to the materialit's going through.

Helen - It's going faster than the speed of sound?

Bill - It's going faster than the speed of sound and the thing here is in the uncompressed material. So, what you're doing is, these pulses for example under normal explosive loading, you can take the density up to a factor of two times higher. You're compressing things by a factor of two, compared to what they would be like under normal conditions. And if you're going to nuclear weapons, if you're going really crazy, then you can compress things to five times their normal volume. And as well as this massive increase in density, the temperature of which those materials are can jump up several thousand degrees centigrade, but on a time scale of less than a microsecond.

Helen - So these are happening very, very quickly. So how on earth do we go about actually studying them?

比尔,人们一直在努力研究effects of these things for a variety of reasons and things like hauling and mining, mineral extraction and demolition of buildings et cetera, all these use explosives. Historically, even back in the Tudor times, people were trying to kill each other using quite large quantities of explosives. Up until say, about the end of the 19th century, people generally blew things up and looked at the after effects. But if you are interested in the photography at all, there's Eadweard Muybridge who looked at horses galloping, all four hooves off the ground at the same time, in the 1870s. He was using cameras which were very much like normal cameras, normal film cameras, which probably will be regarded as old fashioned technology these days, and these were developed and advanced. A gentleman called Courtney Pratt, who was working in the Cavendish Laboratory in the 1940s and '50s, developed a camera called an image converter camera and that could take an image on a microsecond timescale. At the present time, there are cameras made in the UK and this is one of the leading countries in terms of this type of camera development. They can take images at a billion frames a second. So, in one-thousand millionth of a second basically, you can capture an image.

Helen - So, these cameras are now letting you start to see much, much more detail of the anatomy of these explosions.

Bill - Exactly. You can see where materials break. You can see how they change shape, how long it takes for things to happen. Because one of the important differences between doing things at a low rate and doing things at a high rate is that at a high rate, things happen differently. You can imagine getting some shopping from a supermarket and you get one of those really flimsy plastic bags because you don't want to pay for the reusable bag, and you walk down the street and the handles stretched, and that's called creep. That's a very low rate, I would say. It takes a long time to happen.

Helen - That's quite a slow process, yes.

Bill - Very slow process.

Helen - Right.

Bill - Whereas a shock will be a case of trying to yank that handle on the plastic bag apart and then when it does break, after you scream because you actually discover the plastic's really strong at that point, you then discover that it breaks. It doesn't stretch. It has fractures across the surface and this is the important difference. As you go to these very high rates, you'll find that the material breaks in a different fashion because it doesn't have the time to break in a fashion that you will consider the normal way it's going to behave. And that's what makes it an interesting field of study.

Helen - Okay, so we've got these cameras that are now extremely fast. But how are we actually using these to study shockwaves? How are we employing these to understand more about what's happening when explosions happen?

Bill - Well, the cameras are very useful because they give you a full field view of what's going on and there's also gauges that you can put into it. So these are little sensors that you put inside the target and they give you pressure versus time. The main thing that you do experimentally is try to simplify the scenario. So take things from say, a three-dimensional case where things have very regular shapes make them have nice flat plates of material, and do impacts on those. The other thing you can do that simplifies the situation is - instead of using explosives (and for those people so interested, explosives have a sort of triangular pressure shape) you can use a thing called a plate impact gun. You fire a flat-ended projectile at a target and that puts in a square shaped pressure pulse, and you can control the height of that pressure pulse and the duration of that pressure pulse, independently of one another. So you can really probe the space. The pressure, temperature of volume, space that the material can occupy under these very high loadings and then relate it to the more complicated three-dimensional case of a structure or a building being exploded or being hit by an object, in order to understand it. If you can understand how things work in one dimension, you've got a chance in three dimensions. If you don't understand one dimension, as soon as you get to the real world, things are going to get terribly difficult quite quickly.

Helen - And are you mostly concerned with buildings and structures, and things that we create or also living matter and people as well.

Bill - Well, both in the Cavendish and in Imperial for a number of years now, we've been looking at a whole variety of materials. So, metals are quite commonly looked at, composites that are used in aircrafts, bird strike, things like that. We were quite interested in bird strike. That's an interesting subject because there's a little cycle happens there. People for a few years actually used dead birds to hit aircrafts with.

Helen - Frozen chickens, I heard.

Bill - Yes, that's the great story about howwe have to defrost the chicken out of Sainsbury's or wherever you buy it before you fire it at the train otherwise it does horrendous damage! So people used real birds and then they got a bit worried about that because it's a bit messy and smelly and you just swap to plastic bags full of gelatine and wood. The wood represents the backbone of the bird and they go through that. And then after a while, they cycle back to using real birds - it depends who retires and when they swap. So as well as doing bird strike, you might consider how do shockwaves genetically modify the material under very low, much lower impact conditions. Hit something hard enough - it'll take up the temperature. You'll kill it just by the temperature or the pressure will rupture the cell membranes. If you use lower pressures, you can modify the materials ever so slightly. Most of those modifications will of course ultimately kill the living organism. Here, we're thinking about cells, thinking about spores, and even in some cases, DNA, and that kind of material.

Helen - So, we want to deliberately use this as a way of actually killing things like bugs.

Bill - There is actually a greater push in things like the bread industry. In the food industry. You've got these very long tubes full of liquids, powders, and in some cases, gases. Now they're all used in food production and if something goes wrong there, how do you sterilise? You can pass chemicals down it, but there's also a big aim in things like flour production industries to pulse pressure the materials and knock the microbes out by destroying their membranes.