Study Shows Why Common Explosive PETN sometimes fails

The explosive PETN has been around for a century and is used by everyone from miners to the military, but it took new research by Sandia National Laboratories to begin to discover key mechanisms behind what causes it to fail at small scales.

“Despite the fact explosives are in widespread use, there’s still a lot to learn about how detonation begins and what properties of the explosive define the key detonation phenomena,” said Alex Tappan of Sandia’s Explosives Technology Group.

Explosives are typically studied by pressing powders into pellets; tests are then done to determine bulk properties. To create precise samples to characterize PETN at the mesoscale, the researchers developed a novel technique based on physical vapor deposition to create samples with varying thicknesses. That allowed them to study detonation behavior at the sub-millimeter scale and to determine that PETN detonation fails at a thickness roughly the width of a human hair. This provided a clue into what physical processes at the sub-millimeter level might dominate the performance of PETN (pentaerythritol tetranitrate).

The idea is that by understanding the fundamental physical behavior of an explosive and the detonation process, researchers will improve predictive models of how explosives will behave under a variety of conditions.

Right now, “if we want to model the performance of an explosive, it requires parameters determined from experiments under a particular set of test conditions. If you change any of the conditions, those models we have for predictions don’t hold up any more,” said Rob Knepper of Sandia’s Energetic Materials Dynamic and Reactive Sciences organization.

The tests use less explosive than what’s inside a .22-caliber bullet, and researchers wearing safety glasses and ear protection can stand next to the experiment in a protective enclosure, Tappan said.

It adds new information for a very old explosive. “What we brought to the table is a new experiment that allowed samples to be made that are small enough to measure this critical thickness property,” Tappan said. “Other research been done on PETN in a different form or when it had a binder added to it. This is the first time these data have been done on the critical detonation geometry for pure, high-density PETN.

“What we brought to the table is a new experiment that allowed samples to be made that are small enough to measure this critical thickness property,” Tappan said. “Other research been done on PETN in a different form or when it had a binder added to it. This is the first time these data have been done on the critical detonation geometry for pure, high-density PETN.”