Thesis
Quantitative analysis of shock-induced shear localization in energetic organic crystals
University of Iowa
Master of Science (MS), University of Iowa
Spring 2024
DOI: 10.25820/etd.007489
Abstract
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.
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.
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.
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.
Details
- Title: Subtitle
- Quantitative analysis of shock-induced shear localization in energetic organic crystals
- Creators
- Luke Matthias Weger
- Contributors
- H.S. Udaykumar (Advisor)Jia Lu (Committee Member)Shaoping Xiao (Committee Member)
- Resource Type
- Thesis
- Degree Awarded
- Master of Science (MS), University of Iowa
- Degree in
- Mechanical Engineering
- Date degree season
- Spring 2024
- Publisher
- University of Iowa
- DOI
- 10.25820/etd.007489
- Number of pages
- ix, 59 pages
- Copyright
- Copyright 2024 Luke Weger
- Language
- English
- Date submitted
- 04/18/2024
- Description illustrations
- Illustrations, tables, graphs, charts
- Description bibliographic
- Includes bibliographical references (pages 56-59).
- Public Abstract (ETD)
Energetic materials are materials that have large amounts of stored energy in their physical structure, like propellants (which are dangerous if improperly used). When these materials are hit at high speeds, the internal energy can cluster and lead to their ignition. These regions of gathered energy are known as shear bands, which are a leading force towards ignition. There are well-reported studies surrounding shear bands within metals, but little is known about this behavior in propellants, where it is crucial to the material’s predictable performance and safe handling. To better understand how hotspots in propellants develop when a hole in the material is struck, we investigated shear band patterning for two types of propellants. This behavior is compared to what is known of shear band patterning in metals to uncover new characteristics of propellants. We conducted a study on images of the hotspot formation in order to explain several components that define the development of shear bands, including the spacing between neighboring bands and the growth rate of the band lengths. To produce a more complete analysis, we also used a machine learning approach to predict the bands’ development at different experimental conditions. We found that patterns and scaling relationships agreed well with the predicted behavior in metals. Through this project, we now better understand the extent and manner to which shear bands contribute to igniting energetic materials, which will work to support a safer, more efficient, and more predictable use of the material.
- Academic Unit
- Mechanical Engineering
- Record Identifier
- 9984647256402771
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