Georgia Institute of Technology
Christopher Miller enjoys digging into classical properties of materials, such as how they deform and resist fracture. For his Ph.D. research with Min Zhou at the Georgia Institute of Technology, he has examined the structural mechanics of highly energetic materials, such as explosives. A better understanding of how they respond to impacts – from the molecular level to the macroscale – can prevent accidental detonations.
Miller is most interested in what happens at the mesoscale level – the interaction of energetic crystals the size of sand grains. Because that scale, measured in microns, bridges the gap between molecular dynamics and bulk material response, his work incorporates chemistry, materials science, solid mechanics and wave propagation.
As shock waves spread through energetic composites, the materials deform and crack. Those physical changes generate heat, which starts chemical chain reactions. Testing these processes experimentally is time-consuming and expensive, Miller says, and researchers can’t record many of the physical changes at the explosive’s center, which are essential to understanding how it detonates. High-performance computing is invaluable to overcome these challenges and to study existing explosives and develop new ones. Miller says he “can run 1,000 simulations overnight of some of these impact tests rather than an experiment that takes multiple hours to set up.”
A range of physical processes occur as an explosion begins: pores collapse, materials deform, and friction between grains generates heat. Modeling these systems requires simulating each of these processes, weighing their effects, and matching those combinations with experimental results. “One of the largest unsolved questions of the shock physics community is how relatively important each of these effects are,” Miller says.
At Georgia Tech, he and his colleagues use an algorithm called Cohesive Dynamics for Explosives (CODEX) to study explosive grain structure, examining how embedded aluminum shavings can stabilize the materials by improving their structural integrity and making them less likely to detonate. “As the stress wave propagates through the material, tiny aluminum particles help to redistribute the pressure,” he says.
Miller’s also worked with other codes in practicums at two national laboratories. Working with Laurence Fried at Lawrence Livermore National Laboratory in 2016, Miller used the ALE3D code to model the effects of friction within explosives. In 2018, he and Cole Yarrington used Sandia National Laboratories’ CTH code to incorporate the effects of holes and pores on explosives at the mesoscale. The codes have various tradeoffs in time versus accuracy and in how well they can scale up for larger simulations, Miller notes. “This is a research problem that will require a number of codes to fully answer. I can examine each physical process using the best possible tool for the job.”
Miller would like to integrate results from various simulations and codes to answer important questions and make predictions. For example, he’s observed that smaller individual grains within explosives make the materials more sensitive, primarily because of the increased surface area. He’s also examining how the aluminum shavings desensitize explosives without compromising power. “They don’t react in initiation, but when the explosive detonates, they’ll go.”