When high explosives are subjected to sudden shock waves, such as from an impact or detonation, tiny regions of intense heat - called hot spots - form at microstructural defects such as pores. These hot spots play a critical role in determining whether the explosive will initiate and fully detonate. Understanding how hot spots form and behave across length scales is key to developing predictive models for the safety and performance of explosives used in defense, mining and other applications.
While computational modeling has been a powerful tool for investigating hot spots since the 1960s, full atomically resolved simulations of hot spots have been limited to very small pores - until now. Using LLNL's Sierra supercomputer, a Lawrence Livermore National Laboratory (LLNL) team has made significant progress in understanding how microscopic hot spots form in insensitive high explosives based on TATB (1,3,5-triamino-2,4,6-trinitrobenzene). Their research appears in The Journal of Physical Chemistry C.
The researcher team, led by computational materials scientist Matt Kroonblawd, performed extensive molecular dynamics (MD) simulations at unprecedented scales, ultimately reaching a multi-micron domain with a 300-nanometer diameter pore. Their goal was to observe precisely how hot spots form when pores within the explosive collapse after being shocked. These simulations involved up to 600 million atoms, making them the largest reported MD simulations ever performed for an explosive material.
Hot-spot formation involves highly coupled material mechanics, transport, phase transitions and chemistry, which makes MD a well-suited tool to model this phenomenon. This is because MD allows for minimal physical approximations - a typical modeling practice where models use mathematics to approximate (or assume) how a material may respond under different conditions. These assumptions can sometimes lead to errors when extrapolating predictions to different sizes or conditions.
To complement their MD simulations, the team also used continuum-based ALE3D simulations to extend their analysis to even larger pores and bridge the gap between atomistic and microstructural scale modeling. ALE3D is a multiphysics finite-element code that allows researchers to adjust specific material properties independently, offering a different perspective than MD.
By comparing results from the two approaches, the team uncovered an unexpected trend in how hot spots behave across different size scales and a physical explanation for those results.
Shockingly, the MD simulations revealed that when hot spots form at pores larger than 20 nanometers, they exhibit scale-invariant features - those which are independent of size. This means the hot-spot temperature distributions and structural features were found to be consistent regardless of the pore size - if the pores are above the 20-nanometer threshold.
Using ALE3D simulations, the team identified that hot-spot scale invariance is linked to TATB's mechanical strength, which controls its stress-strain response. At the ultrafast strain rates involved in pore collapse, the stress-strain response for TATB crystals is not affected by the rate of deformation. Thus, the ALE3D simulations confirmed that the stress-strain response is sufficient to explain the behavior observed in MD simulations.
These findings simplify the modeling of hot spots and offer confidence and guidance on applying insights from MD simulations to larger length scales.
"Such work positions molecular dynamics simulations as a foundation for developing more general multiscale models of insensitive high explosives and can potentially guide the development of new explosive materials with improved safety and performance properties," Kroonblawd said.
LLNL coauthors of the paper include Brandon Zimmerman, Brad Steele, Joel Christenson, Amitesh Maiti, Ryan Austin and Larry Fried.
-Shelby Conn
