Quantitative analysis of shock-induced shear localization in energetic organic crystals
Creators: name: Weger, Luke Matthias
Publisher: name: University of Iowa
Year: 2024
Subjects: Energetic materials
Hotspots
Shear bands
Shocks
Physics

description: Energetic materials such as octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX), and cyclotrimethylene trinitramine (RDX), play important roles in military and space applications and are routinely used as solid fuel in rocket propellants. It is important that these materials are stable enough to withstand thermal, mechanical, or electrical stimuli during storage and transportation to avoid inadvertent initiation, but they must also reliably perform on demand. These materials have microstructural heterogeneities, such as voids and cracks, and shock wave interaction with these heterogeneities leads to localized heated regions known as hotspots. Hotspot formation is regarded as the critical first step in shock initiation and eventual detonation. The focus of this thesis was on an energy localization mechanism that influences the development of hotspots known as shear bands.
description: Shear bands are thought to play a significant role in hotspot formation towards the ignition of energetic crystals loaded under the high-pressure, high strain-rate conditions of shock waves. Both modeling and experimental studies have shown evidence of shear bands that are present around collapsing pores due to inconsistencies in the material’s microstructure. While shear bands are well studied and visualized in inert metals, they are less well understood in reactive low-symmetry organic (CHNO) energetic crystals, meaning that their ignition likelihood is unpredictable. To date, models for such materials have lacked the fidelity to capture physically realistic shear bands. In this work, recently developed atomistics-consistent materials models were employed for HMX and RDX, two examples of energetic crystals which have been shown to produce shear bands in excellent agreement with molecular dynamics (MD) simulations. Shock-induced pore collapse in HMX and RDX is calculated for a wide range of pore sizes and shock strengths.
description: A Python package was developed for a quantitative pattern analysis to extract key metrics, such as shear band spacings at various instants of pore collapse, shear band propagation velocities, etc. These metrics were compared across materials, and with scaling relationships for shear bands that have been developed mostly in the context of metals to provide powerful insight into the ignition behavior of energetic crystals. Additionally, I leveraged a physics-aware recurrent convolutional neural network known as PARCv2, to learn the evolutionary behavior of hotspots at the micro-scale to inform hotspot formation predictions at the macro-scale.
description: In my work, I found the characteristics and scaling relationships of shear bands aligned well with instability theory in metals. This finding, along with a faster method to inform a multi-scale approach to classifying energetic material sensitivity, helps us to better understand shear bands’ role in hotspot formation in shocked energetic crystals. This in turn will help us to more efficiently and predictably harness the material’s energetic potential.