Fragment tracking—insights into what happens in explosions

A bang and a swirl of dust from detonating 9 pounds of plastic explosive in the desert signaled the beginning of tests that—thanks to advances in high-speed cameras, imaging techniques and computer modeling—will help Sandia National Laboratories researchers study fragmenting explosives in ways that weren't possible before.

"The details matter," said Mark Anderson, principal investigator on Sandia's fragment tracking project, which began explosive experiments in May. "Explosives are very complex to understand and to use, and they continually keep us humble."

Researchers want to know how pipe bombs and other improvised explosive devices come apart and how much destruction they cause to learn how to mitigate that damage. They'd like to create computer models of explosive phenomena for broader studies since it's impossible to do experiments for every possible situation, said Phillip Reu, team lead for diagnostic development on the three-year study and a member of Sandia's Diagnostics Science and Engineering department.

The project marries modern cameras, diagnostic technology and the latest computer algorithms to gather more data. Past techniques and equipment couldn't provide detailed enough experimental data, forcing modelers to make assumptions about how materials failed or where fragments flew.

From a diagnostic standpoint, the dream is to be able to watch an exploding device expand, come apart and become fragments, then see the fragments fly and trace where they are in space and in which directions they go. That understanding could lead to better models and "what if" scenarios.

"We have to understand what creates the environment to mitigate the environment," said Measurement Science and Engineering department researcher Tim Miller, who is developing algorithms to measure the shape and trajectory of explosive fragments.

Modelers, experimentalists exchange data

Modelers and experimentalists work in parallel. "The modelers give us a predicted outcome, which helps the experimentalists set up for the test," said Anderson, a researcher in Sandia's Energetic Systems Research department. "Then we take our measurements and feed them back to the modeler. The interchange is extremely powerful because we do better experiments having some modeling insights. And the modelers better see how their code works, and more importantly, where it doesn't work, so they can unravel why that is."

The improvements in diagnostic equipment allow researchers to better measure what goes on in an explosion. "We know that fragments travel at high speed, but we don't really know the aerodynamic drag," Anderson said. "When we see the size and shape of the fragment and we see it as it tumbles and rotates, we know the drag changes."

Better instrumentation gives researchers new insights into such characteristics as mass, orientation, length-width ratio, velocity and tumble rate. "Is it one big chunk? Is it a bacon strip, is it a cornflake, is it a sausage link? Maybe it's a steak coming at you," said Engineering Sciences researcher Steve Attaway. "That's the shape characterization."

High-performance computing has improved the ability to calculate fracturing and fragments. "We're running calculations now that 10 years ago we wouldn't dream of attempting," he said. "That said, every calculation is incomplete in what can be included, so the ability to gain more insight on how to make approximations to better mimic the fracture and fragmentation is important."

How fast a fragment moves and whether it spins make a difference in the damage it inflicts. Simply assuming fragments are one size or behave one way skews the assessment of possible damage and ways to lessen it.

"If you think the average fragment is going to go a half mile, but you get one that's shaped just so and it flies 2 miles, then that's a problem, that begins to illustrate the range safety aspect of fragmenting explosive devices," Attaway said. "We're trying to get the best idea we can of that distribution."