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What do rocks and explosives have in common?

Two scientists are applying what they know about the granular microstructure of rocks to high explosives—a novel approach that could predict how explosives age.
December 12, 2019
Microstructures of a rock, the explosive PBX, and the explosive PETN.

“Explosives are useful to us because—like rocks—they have microstructure,” says physicist Carly Donahue. “And like rocks, high explosives have curious acoustic behavior.”CREDIT: James Ten Cate, Cary Skidmore, Kathryn Brown


“Like rocks, high explosives have curious acoustic behavior.”- Carly Donahue

By J. Weston Phippen

The definitive way to test an aging detonator in the U.S. nuclear stockpile is to fire it. But this destroys the detonator, and it doesn’t reveal much about what impact time has on the high explosives inside.

For decades, the weapons in our stockpile have been exposed to high and low humidity, and to intense cold and heat depending on where they’re kept, which makes the aging process unique to each. “Getting information on the high explosive inside those detonators is either really difficult or not possible,” says explosives chemist Peter Schulze. Even so, answering this question—will an aged detonator still work as intended?—is vital to the Lab’s mission.

Here is where the partnership of two Lab disciplines is paying off. For 20 years, the Elasticity, Vibrations, and Acoustics team at Los Alamos has used ultrasound to study rock microstructure, which is full of cracks. It turns out that high explosives share a similar microstructure. “That’s what gives explosives their explosive properties,” says physicist Carly Donahue, who has studied the microstructure of everything from moon rocks to medicines. “Also like rocks, high explosives have curious acoustic behavior.”

Last year, Schulze and Donahue applied for a Laboratory Directed Research and Development grant to study this curious acoustic behavior. By pulsing an ultrasonic wave through a pressed high-explosive pellet (the kind found in detonators), they hoped to learn what role time plays in the performance of these devices—and they wanted to do it without destroying the detonator. Here’s how it worked:

When an ultrasonic wave meets a crack in the explosive’s microstructure, the wave changes slightly. As you increase the wave’s vibrational intensity—its amplitude—it will deform as it encounters microscopic cracks in the pellet. Eventually, the frequency shifts lower and becomes flat. The wave becomes nonlinear.

It’s a concept similar to someone playing a musical note on a guitar, and as the note is plucked harder, it becomes more out of tune. Under normal circumstances, a note played at any volume should remain the same. But not with rocks and high explosives.

Schulze and Donahue theorized that as high explosives age, their microstructure alters to make them more prone to this frequency change. To test their theory, they used heat to artificially age pressed high explosives, exposing different batches of pellets to the heat for different lengths of time. They then tested each batch with ultrasound, slowly increasing the amplitude until the frequency changed. Then they fired the detonators to compare results.

What they discovered was that the high-explosive pellets that demonstrated the frequency change at a lower amplitude were the same pellets that, later, when destroyed, had become more sensitive with age. They exploded too easily.

As the two set out to find, the test determines the quality of detonators without destroying them. It’s inexpensive, easily repeatable and, eventually, Donahue says, “We hope it will tell us more about how high explosives age so we can understand what this means for our nuclear weapons.”

Donahue and Shulze’s work will be published in early 2020 in Propellants, Explosives, Pyrotechnics.