Properties of Explosives and Their Application Part 2

From the first post in this series I wrote about determining the performance of an explosive using the Cylinder Expansion test, or Cylex as it's commonly known. As I mentioned the radial velocity of an expanding cylinder (copper for the cylex test) is measured. This is done a number of different ways. The earliest method used a streak camera.

A streak camera is a one dimensional camera that uses a rotating mirror to "streak" an image across the film plane. The space dimension is controlled by only allowing the light to pass through a slit which gives you a measurement in space as a function of mirror rotational velocity, or time. How they work and the set-up for a cylex test are shown in the next two drawings.

Schematic of  Streak Camera Functionality.

Cylex Set-Up and Film Record

Another method by which the wall velocity can be measured is by using an interferometer. One system is known as a VISAR (Velocity Interferometer for Any Reflector) which was developed at Sandia National Lab. The advantage of an interferometer set-up is that the velocity is measured directly while the streak camera record has to be converted from space-time to velocity-time which requires some extra assumptions. At any rate, we now have a wall velocity for the cylinder and it looks something like this.

Cylex Wall Velocity.

From this basic experiment a wealth of performance properties can be deduced and the subsequent ability of the explosive to do work can be analyzed. The Gurney Energy for the explosive can be directly calculated from the wall velocity since the charge to mass ratio is known. I'll cover using this information and other relationships calculated from it in explosive systems design in future posts.