Studies of explosive charges have usually been done at national laboratories using a gun to fire a flat bullet into an explosive charge inside a thick-walled chamber that contains the fierce blast. Such tests require large facilities.

Now, researchers at the University of Illinois at Urbana-Champaign have recreated the flat-plate impact in miniature on a tabletop. In the process, they may also have made contributions to the field of energetic materials by recreating conditions inside a bomb and achieving measuring them.

Researchers have recreated a flat-plate impact—key to precisely recreate the conditions inside a detonating explosive—in miniature on a tabletop. The results inform a topic known as the "hot-spot model" of explosive initiation by revealing the dynamics of hot spot growth. That’s important because the long-term goal is to make safer explosives, and eliminating hot spots is one way to do that.

“Because we can do this so many times a day with a small-scale device on a benchtop that has highly sophisticated optical diagnostics, we see what is happening during the millionth of a second the charge is exploding,” says Dana Dlott, professor of chemistry.

“This is an important verification of the hot-spot model of explosive initiation, and also it’s important because we measured time scales and rates never before seen,” says graduate student Will Bassett.

Hot spots play a role in the phase cycle of explosions as well as in controlling explosions and their after-effects. And although much has been written about hot spots, they are not often observed.

The Illinois researchers used a pulsed laser to launch a flat bullet 0.5 millimeters in diameter at speeds up to 12 times the speed of sound. This "flyer plate" hits an explosive charge that is less than one millionth the size, yet is capable of producing the same conditions as a larger detonating explosive.

Previously, the team observed that the powdered form of a standard test explosive known as HMX material explodes in two phases. First, there is an immediate, prompt explosion at impact. Then, a delayed explosion occurs some 300 nanoseconds later.

The research extends this earlier work to show that initially a small part of the explosive starts to react, and that this spatially limited reaction spreads throughout the HMX.

The team next aims to validate its instrument and apply test results to improve safety outcomes of explosion science.