• Video
  • 5 mins
  • Level 1: Introductory

Explosive accuracy

Updated Monday 11th October 2010

Explosives expert Dr Sidney Alford joins Jem to explain the art of effective explosions.

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Dr Sidney Alford demonstrates how accurate metal cutting can be achieved using a minimum of explosive.

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Jem Stansfield
Dr Sidney Alford designs ways of channelling the power of an explosion to achieve very accurate results with the minimum amount of explosive.

Dr Sidney Alford

A simple crude way of severing steel is just to put a ribbon of explosive across it, but you need a relatively massive amount of explosive and it deforms the steel and it blows a bit off the back.  It’s a very ugly and inefficient way of doing it. 

I came up, quite some years ago now, more than 20 years ago, with an idea for shaping the shockwave.  What I do is put explosive on the back of one of these elements.  It’s magnetic rubber, which is a great aid to placement and the explosive went immediately on the back and the shockwave from this side went down that way and the shockwave from this side went down that way, so not surprisingly they crossed over.  Now, if you put it here and put explosive on the back they cross over in the steel, and what happens is you have a shockwave going down that way and a shockwave that way and where they cross, they’re already zones of extremely high pressure, you’ve got high pressure times two which suddenly relaxes.  Now, under those circumstances, steel cracks.  I would crack, you would crack!  Then the crack actually proceeds down from the surface to the back of the steel and if it reaches the back of the steel and it compresses it and then stretches it hard enough that will just fall in two parts.

Jem Stansfield
Wow!  I’ll be astonished to see that crack a piece of steel like that.

Dr Sidney Alford

I’ll be pleased because I haven’t done it for a few years, but we’ll give it a whirl.  That is very little explosive to cut that.

Jem Stansfield
Yeah.

Dr Sidney Alford

So, what I’ll do is take the end of a detonator in there and retire and we’ll press the button.

Jem Stansfield
Fabulous, I can’t wait to see it.

Dr Sidney Alford

Okay. Firing; four, three, two, one. 

Jem Stansfield
Detonation of high explosives like the sort Sidney is using creates a shockwave that travels faster than the speed of sound through the block of explosive and then through the material next to it.  Using high speed photography to slow the process down 250 times you can see the shockwave travelling through the air above the explosive distorting it like a mirage and radiating out like a bubble.  It is the power of this that Sidney’s rubber prism will hopefully have focused into a thin line along the block of steel. 

Look at that!

 

Dr Sidney Alford

Oh well, that shows you the principle.

Jem Stansfield
Wow, you can see there the crack’s quite clean, it’s really quite clean.

Dr Sidney Alford

Yeah.  You can see at the initiation end it didn’t go.

Jem Stansfield
Why does it become more powerful as it moves away from the detonator?

Dr Sidney Alford

It takes a definite distance for the detonation wave to accelerate up to a maximum value.  It probably would have severed completely if we’d initiated in the middle but in a long piece or let’s say you’re cutting up the side of an oil tanker, if an inch or two isn’t severed that’s neither here nor there.

Jem Stansfield
Yeah.

Dr Sidney Alford

And very little deformation of the plate.

Jem Stansfield
Yeah, it’s just a crack.

Dr Sidney Alford

Hmm that’s right.  This is in fact, when you get it right, it is the most efficient way I know of severing steel.  It is not cutting steel but it’s severing it.  That suffices to give a dead straight cut, so if you cut the wrong bit of steel it makes it easier to weld up again.

Jem Stansfield
I like that.

3’59”

 

 

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