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Student Number : 9608998A - MSc dissertation - School of Electrical and Information Engineering - Faculty of Engineering and the Built Environment / The history of flux compression is relatively short. One of the founders, a Russian physicist, Sakharov developed the idea of compressing a magnetic field to generate high magnetic fields and from this he also developed a generator to produce current impulses. Most of this initial work was performed in military research laboratories. The first open source literature became available in the 1960s and from there it has become an international research arena. There are two types of flux compression generators, field generators and current generators. These are discussed along with the basic theory of flux compression generators and related physics. The efficiency of generators is often quite low. However in many generators high explosives are used and because of their high energy density, the current or field strength produced is substantially greater then the initial source. This of course limits the locations possible for experimental work and subsequently limits the industrial applications of flux compression generators . This research presents a theoretical design for a non-explosive flux compression generator. The generator is designed to produce a current impulse for tests in laboratory and remote locations. The generator has the advantage of being non-destructive, therefore reducing costs, and allowing for repeatable experiments. The design also reduces the possibilities or many of the loss mechanisms.

Alguns altos explosivos foram caracterizados por espectroscopia Raman e no Infravermelho, e o efeito da temperatura sobre os espectros Raman foi medido in-situ. Foram estudados os produtos comerciais TNT, HMX, RDX e PETN, os quais, com exceção do HMX e RDX, pertencem a classes químicas distintas e também foi investigado o TATP sintetizado no laboratório. As amostras foram inicialmente caracterizadas por FT-IR, FT-IR/ATR e espectroscopia Raman com excitação no visível (632,8 nm) e no NIR (1064 nm) visando determinar se a técnica de amostragem exercia algum efeito, especificamente transições de fase e degradação, sobre os espectros. ATR e FT-Raman forneceram os espectros a partir dos quais foi feita a atribuição de bandas, a qual foi suportada por simulações teóricas (DFT, B3PW91). Cada amostra foi aquecida até uma temperatura abaixo do ponto de fusão, na qual o comportamento do espectro com o aquecimento era reversível. No caso do PETN e TNT esse valor foi bem próximo do ponto de fusão e no caso do HMX e RDX, foi substancialmente inferior. As bandas mais afetadas pela temperatura devem ser aquelas envolvidas nas rotas de relaxação de energia em explosivos. Os resultados obtidos sugerem que o PETN sofra decomposição térmica através da ruptura da ligação C-ONO2, enquanto que no HMX e RDX a ligação N-N deve ser rompida. As mudanças no espectro do TNT indicam que vibrações envolvendo os grupos NO2 e a ligação C-N são as mais sensíveis à temperatura. TATP sublima à temperatura de 70°C e até essa temperatura o espectro não é afetado pelo calor. Provavelmente a energia é utilizada no processo de sublimação. / Selected high explosives were characterized by Raman and Infrared spectroscopies and the effect of temperature was followed in-situ by Raman spectroscopy. TNT, HMX, RDX, PETN (commercial products) and TATP belong to distinct chemical families (except HMX and RDX) and their response to heating was evaluated. The samples were first characterized by FT-IR, FT-IR/ATR and Raman with excitation in the visible (632.8 nm) and in the NIR (1064 nm) aiming at the detection of sampling effects in the obtained spectra, specifically phase transitions and degradation. ATR and FT-Raman were the techniques of choice to provide the spectra for band assignment, which was assisted by theoretical simulations (DFT). Each sample was heated up to a temperature well below its melting point, in order to avoid thermal decomposition. The bands most affected by temperature were taken as the routes for energy relaxation in explosives. The obtained results lead to the conclusion that PETN decomposes through the rupture of the C-ONO2 bond, whereas in HMX and RDX the N-N bond is broken. TNT spectra indicates that the NO2 and C-N vibrations are the most sensitive to temperature and TATP sublimated at 70°C and no bands were affected by temperature. The results are agreement with the literature or theoretical simulations.

Alguns altos explosivos foram caracterizados por espectroscopia Raman e no Infravermelho, e o efeito da temperatura sobre os espectros Raman foi medido in-situ. Foram estudados os produtos comerciais TNT, HMX, RDX e PETN, os quais, com exceção do HMX e RDX, pertencem a classes químicas distintas e também foi investigado o TATP sintetizado no laboratório. As amostras foram inicialmente caracterizadas por FT-IR, FT-IR/ATR e espectroscopia Raman com excitação no visível (632,8 nm) e no NIR (1064 nm) visando determinar se a técnica de amostragem exercia algum efeito, especificamente transições de fase e degradação, sobre os espectros. ATR e FT-Raman forneceram os espectros a partir dos quais foi feita a atribuição de bandas, a qual foi suportada por simulações teóricas (DFT, B3PW91). Cada amostra foi aquecida até uma temperatura abaixo do ponto de fusão, na qual o comportamento do espectro com o aquecimento era reversível. No caso do PETN e TNT esse valor foi bem próximo do ponto de fusão e no caso do HMX e RDX, foi substancialmente inferior. As bandas mais afetadas pela temperatura devem ser aquelas envolvidas nas rotas de relaxação de energia em explosivos. Os resultados obtidos sugerem que o PETN sofra decomposição térmica através da ruptura da ligação C-ONO2, enquanto que no HMX e RDX a ligação N-N deve ser rompida. As mudanças no espectro do TNT indicam que vibrações envolvendo os grupos NO2 e a ligação C-N são as mais sensíveis à temperatura. TATP sublima à temperatura de 70°C e até essa temperatura o espectro não é afetado pelo calor. Provavelmente a energia é utilizada no processo de sublimação. / Selected high explosives were characterized by Raman and Infrared spectroscopies and the effect of temperature was followed in-situ by Raman spectroscopy. TNT, HMX, RDX, PETN (commercial products) and TATP belong to distinct chemical families (except HMX and RDX) and their response to heating was evaluated. The samples were first characterized by FT-IR, FT-IR/ATR and Raman with excitation in the visible (632.8 nm) and in the NIR (1064 nm) aiming at the detection of sampling effects in the obtained spectra, specifically phase transitions and degradation. ATR and FT-Raman were the techniques of choice to provide the spectra for band assignment, which was assisted by theoretical simulations (DFT). Each sample was heated up to a temperature well below its melting point, in order to avoid thermal decomposition. The bands most affected by temperature were taken as the routes for energy relaxation in explosives. The obtained results lead to the conclusion that PETN decomposes through the rupture of the C-ONO2 bond, whereas in HMX and RDX the N-N bond is broken. TNT spectra indicates that the NO2 and C-N vibrations are the most sensitive to temperature and TATP sublimated at 70°C and no bands were affected by temperature. The results are agreement with the literature or theoretical simulations.

<div>Understanding the fundamental physics involved in the high strain rate deformation of high explosives (HE) is critical for developing more efficient, reliable, and safer energetic materials. When HE are impacted at high velocities, several thermo-mechanical processes are activated, which are responsible for the ignition of these materials. These processes occur at different time and length scales, some of them inaccessible by experimentation. Therefore, computational modeling is an excellent alternative to study multiscale phenomena responsible for the ignition and initiation of HE. This thesis aims to develop a continuum model of HMX to study the anisotropic behavior of HE at the mesoscale, including fracture evolution and plastic deformation. This thesis focus on three types of simulations. First, we investigate dynamic fracture and hotspot formation in HMX particles embedded in Sylgard binder undergoing high strain rate compression and harmonic excitation. We use the phase field damage model (PFDM) to simulate dynamic fracture. Also, we implement a thermal model to capture temperature increase due to fracture dissipation and friction at both cracks and debonded HMX/Sylgard interface. In our simulations, we observe that crack patterns are strongly dominated by initial defects such as pre-existing cracks and interface debonding. Regions with initial debonding between HMX particles and the polymer are critical sites where cracks nucleate and propagate. Heating due to friction generates in these regions too and caused the formation of critical hotspots. We also run simulations of a HMX particle under high-frequency harmonic excitation. As expected, higher frequencies and larger amplitudes lead to an increase in the damage growth rate. The simulations suggest that the intensity of the thermal localization can be controlled more readily by modifying the bonding properties between the particle and the binder rather than reducing the content of bulk defects in the particle. </div><div><br></div><div>Second, we present simulations of shock compression in HMX single crystals. For this purpose, we implemented a constitutive model that simulates the elastoplastic anisotropic response of this type of material. The continuum model includes a rate-dependent crystal plasticity model and the Mie-Gruneisen equation of state to obtain the pressure due to shock. Temperature evolves in the material due to plastic dissipation, shock, and thermo-elastic coupling. The model is calibrated with non-reactive atomistic simulations to make sure the model obeys the Rankine-Hugoniot jump conditions. We compare finite element (FE) and molecular dynamic (MD) simulations to study the formation of hot spots during the collapse of nano-size void in a HMX energetic crystal. The FE simulations captured the transition from viscoelastic collapse for relatively weak shocks to a hydrodynamic regime for strong shocks. The overall temperature distributions and the rate of pore collapse are similar to MD simulations. We observe that the void collapse rate and temperature field are strongly dependent on the plasticity model, and we quantify these effects. We also studied the collapse of a micron size void in HMX impacted at different crystal orientations and impact velocities. The simulation results of void collapse are in good agreement with a gas gun void collapse experiment. While the void size and crystal orientation do not affect the area ratio rate, they strongly affect the void collapse regime and temperature. Also, increased plastic activity when the crystal is impacted on the plane (110) renders higher temperature fields.</div><div><br></div><div>Finally, we studied shock compression and dynamic fracture in polycrystalline HMX using the same model implemented for shocks in single crystals. The goal of this study is to understand the role of crystal anisotropy and how it affects other hotspot formation mechanisms such as frictional heating. To simulate fracture, we used a phase field damage model implemented for large deformations. We first perform simulations of sustained shocks in polycrystalline HMX, where the grains are perfectly bonded to understand the effect of plastic deformation and hotspot formation due to plastic heating. Then, we simulate shocks in polycrystalline HMX with dynamic fracture. Simulations capture fracture evolution and frictional heating at cracks. In the polycrystalline case, we study heat generation due to shock and plastic deformation. A heterogeneous temperature field forms when the shock wave travels in the material. Temperature increases more in crystals that showed a higher magnitude of accumulated slip. When weak grain boundaries are included in the simulations, frictional heating becomes the dominant hotspot formation mechanism. As the crystals' interfaces break and crack surface sliding occurs, temperature increases due to friction at cracks. Hotspots tend to form at cracks oriented 45 deg from the shock direction. For this case, crystal anisotropy does not play an important role in temperature generation due to plastic dissipation. However, the random orientation of the crystals creates heterogeneous deformation and stress fields that cause the formation of a higher number of hotspots than the case where all the grains are oriented in the same direction.</div>

En physique des explosifs, la postcombustion désigne la phase de combustion qui intervient après la fin de la détonation lorsque l’explosif considéré est initialement déficient en oxydant. Les produits de détonation, qui apparaissent sous la forme d’une boule de feu, peuvent alors à leur tour être oxydés, ce qui permet de libérer une quantité supplémentaire d’énergie dans l’écoulement et d’augmenter le souffle. Ce phénomène complexe est piloté par l’interaction entre des ondes de chocs, une zone de mélange turbulente créée par des instabilités hydrodynamiques de type Rayleigh-Taylor et Richtmyer-Meshkov, et une flamme de diffusion. Compte tenu de son effet significatif sur la performance d’une explosif, une bonne compréhension de la postcombustion est nécessaire afin de pouvoir la modéliser et déterminer avec précision les effets d’une charge donnée. A cette fin, des travaux, à la fois numériques et expérimentaux, ont été menés afin de mieux comprendre le processus de mélange intervenant dans les boules de feu puis le phénomène dans son ensemble. Afin de contourner les difficultés liées à la caractérisation des produits de détonation, cette étude s’est concentrée sur l’explosion de capacités sphériques sous pression qui permet de produire un écoulement similaire à celui provoqué par une détonation sphérique. Les résultats obtenus sont semblables à ceux de la littérature sur la postcombustion des explosifs et apportent un éclairage nouveau sur l’influence de certains paramètres tels que la masse de l’explosif ou les propriétés des perturbations initiant les instabilités. / In the field of high explosives, the afterburning corresponds to the combustion processes occurring right after the end of a detonation, when the explosive used is originally oxidizer-deficient. Its detonation products, which appears as a fireball, can then be oxidised. The additional energy that their combustion generates enhances the blast and improves the explosive performance. This complex phenomenon is driven by the interaction between shock waves, a turbulent mixing layer caused by the emergence of Raylegh-Taylor and Richtmyer-Meshkov instabilities, and a diffusion flame. Because of its significant influence on the blast, a good understanding of the afterburning is thus necessary in order to model and predict accurately the effects of a given explosive device. To this end, an experimental and numerical work was conducted in order to, first, better understand the mixing process inside fireballs and, then, the whole phenomenon. In order to avoid the difficulties due to the imprecise characterisation of the detonation products, this study focused on the explosions of pressurised vessels which produces a flow similar to the one following a spherical detonation. The results are in good agreement with the ones found in the literature about the afterburning of high explosives. They also shed a new light on the influence of some parameters such as the mass of the charge or the properties of the perturbations initiating the instabilities.