Materials far from equilibrium : shock-induced deformation and chemistry in RDX and experimental development
Author(s)Dresselhaus-Cooper, Leora Eve
Massachusetts Institute of Technology. Department of Chemistry.
Keith A. Nelson.
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The far-from-equilibrium dynamics that occur during shock waves cause irreversible material changes that are difficult to measure by conventional techniques. Measuring shocked materials requires techniques that can measure all of the relevant details (pressure, velocity, crystalline phase, etc.) in a single acquisition, or shot. Over the last century, experimental technique development has led to advances in our understanding of how shock waves change and destroy materials. The basic science of the initial shock-induced deformations can allow controlled and effective weapons design for future engineers, if new single-shot techniques can determine how established explosive materials initiate, propagate and detonate. This work focuses on the shock-induced dynamics in explosive 1,3,5-triaza-1,3,5-trinitrocyclohexane (RDX), using new multiobservables techniques and algorithms developed here and elsewhere. RDX has many known crystal phases and decomposition mechanisms, but how shock waves induce decomposition remains poorly understood. With in-situ single-shot X-ray diffraction experiments, we were able to measure the dynamic phase diagram of RDX for the first time, observing a phase transition from the a to y phase at 4 GPa occurs within </_2 ns of the shock front. We saw the lattice of the [alpha]-phase compress isotropically before anisotropically transitioning to the y-phase, foreshadowing the reactivity that is predicted to follow. To study the subsequent mechanical deformations and decomposition chemistry, I extended this work to a waveguide quasi-2D cylindrically converging shock experiment. In-situ measurements demonstrated a strong threshold in shock amplitude at 3 mJ of drive energy, corresponding to a change in the decomposition chemistry, void structures, and deformation planes. Additional findings demonstrated a strongly size-dependent deformation mechanism, indicating interactions between the explosive crystal and the surrounding polymer matrix. These shock-induced deformations clearly intensified when shocked a second time. Together, all of these findings give us a new view of how RDX transforms, deforms, and decomposes within the first 500 ns after a shock wave, giving new insight into hotspots. These RDX studies were enabled by a novel imaging and analysis method. In the quasi-2D cylindrically converging shock geometry, I developed a single-shot imaging technique that collects up to 16 frames with as little as 3ns between frames-3 orders of magnitude faster than existing experiments. This technique enabled us to observe shock propagation and the sequence of stochastic phenomena including in-situ fracture, cavitation,4 phase transitions, and decomposition. To extract quantifiable data and uncertainties with mathematical rigor from our image sequences, I collaboratively developed an image-processing algorithm called locally adaptive discriminant analysis (LADA). LADA reveals boundaries between different features in an image (referred to as classes) using machine learning to sort each pixel of an image into its most probable class. LADA located the shock front in our image sequences, and compiled spatially-resolved ANOVA and MLE values to show uncertainty in the images originating from undifferentiable and outlier pixels, respectively. The multi-frame single-shot imaging technique and LADA method give us a powerful new set of tools to obtain a time sequence of the stochastic chemistry induced by shock waves. These advances in understanding the deformation, the voids and the chemistry in RDX-using our new observational and analysis techniques-lead the way for in-situ studies of decomposition chemistry in RDX. Future work extending the imaging and LADA techniques to X-rays can provide the higher resolution required to determine the timescale and structure of the fracture we saw in our initial experiments. An in-situ version of our ultrasmall-angle diffraction can provide time resolution of the sizes and number densities of the hot-spots, showing the kinetics and extent of local thermal decomposition. Developing a new frequency-resolved class of single-shot Raman experiments could also directly measure the temperature in these hot-spots, experimentally quantifying the activation energy required for thermal runaway to detonation. This work lays the foundation for measurements and experimental techniques that will provide new understanding of how shock waves produce uncontrollable chemistry in explosives.
Thesis: Ph. D. in Physical Chemistry, Massachusetts Institute of Technology, Department of Chemistry, 2018.Cataloged from PDF version of thesis. Page 257 blank.Includes bibliographical references.
DepartmentMassachusetts Institute of Technology. Department of Chemistry.
Massachusetts Institute of Technology