Topical inquiries for the Swedish Defence Research Agency (FOI) include analysis of explosive substances in different sample types. Research into explosives in complex matrixes can provide an analytical support function for forensic investigation i.e. tools for areas such as finding bomb factories, identification and risk analysis of home-made explosives (HME) and improvised explosive devices (IED) as well as preventive measures against maliciously intended use of explosives. Additionally, the research may lay the groundwork for indications of health- and environmental hazards. Utilizing state-of-the-art equipment and years of extensive expertise, FOI is able to carry out these types of research tasks to provide security and sustainability for society. The aim of this thesis project is to establish and validate developed methods for collecting, extracting, separating, and detecting trace amounts of explosives in various bodies of water using a solid-phase extraction (SPE) robot and a high-resolution (HR) mass spectrometer (MS) connected to an ultra-high-performance liquid chromatograph (UHPLC). Particular areas of interest include locations in the Stockholm archipelago where experimental detonations of explosives have taken place. Overall, UHPLC-HRMS analysis provides a powerful tool for analyzing explosives in complex matrixes with unambiguous and reliable measurement data. The compounds of investigation were hexogen (RDX), octogen (HMX), pentyl (PETN), and trotyl (TNT). To summarize, during the course of the thesis, trace amounts of some explosives were detected and quantified in various bodies of water. Furthermore, the applied method for the project was successful in qualitatively and quantitatively analyze the compounds of interest with limit of detection ranging between 0.33–11 μg/L (ppb) in various water sources.

Mesoscale models are needed to predict the effect of changes to the microstructure of plastic-bonded explosives on their shock initiation and detonation behaviour. This thesis describes the considerable progress that has been made towards a mesoscale model for two HMX-based explosives PBX9501 and EDC37. In common with previous work in the literature, the model is implemented in hydrocodes that have been designed for shock physics and detonation modelling. Two relevant physics effects, heat conduction and Arrhenius chemistry, are added to a one-dimensional Lagrangian hydrocode and correction factors are identified to improve total energy conservation. Material models are constructed for the HMX crystals and polymer binders in the explosives, and are validated by comparison to Hugoniot data, Pop-plot data and detonation wave profiles. One and two-dimensional simulations of PBX9501 and EDC37 microstructures are used to investigate the response of the bulk explosive to shock loading. The sensitivity of calculated temperature distributions to uncertainties in the material properties data is determined, and a thermodynamic explanation is given for time-independent features in temperature profiles. Hotspots are widely accepted as being responsible for shock initiation in plastic-bonded explosives. It is demonstrated that, although shock heating of crystals and binder is responsible for temperature localisation, it is not a feasible hotspot mechanism in PBX9501 and EDC37 because the temperatures generated are too low to cause significant chemical reaction in the required timescales. Critical hotspot criteria derived for HMX and the binders compare favourably to earlier studies. The speed of reaction propagation from hotspots into the surrounding explosive is validated by comparison to flame propagation data, and the temperature of the gaseous reaction products is identified as being responsible for negative pressure dependence. Hotspot size, separation and temperature requirements are identified which can be used to eliminate candidate mechanisms in future.

<p>  </p> <p>Polymer bonded explosives (PBX) are a particle reinforced composite containing a high solids loading of explosive particulates bound in a polymer matrix. Commercially produced energetic particulates contain some percentage of flaws in the form of contaminants, porosity, and preexisting fractures. Additional large-scale porosity within the composite is generated during PBX formulation. The introduction of novel additive manufacturing techniques to the energetics field alters the known composite structure and introduces a porosity variable that has not been fully characterized. Porosity collapse during deformation is believed to be a predominant mechanism for hotspot formation, which dominates shock initiation behaviors. These phenomena are difficult to experimentally characterize due to inherent small spectral and temporal scales, and as such numerical and computational models are relied upon to inform fundamental physics. Experimental characterization of the behaviors of energetic materials during deformation is necessary to better inform computational studies and improve our understanding of hotspot formation mechanisms. </p> <p>This dissertation experimentally evaluates the high-rate deformation of porosity in individual explosive particulates and within the overall composite structure. This has included the development of a novel micromachining technique for pore generation in energetic single crystals using the focused ion beam (FIB), resulting in precise and controllable porosity generation that is easily reproducible in collaboration with computational studies. FIB was shown to be an effective pore generation technique, verified by assessing surface roughness and pore quality compared to contemporary manufacturing methods. Three experimental subsets are evaluated: surface cracks in HMX single crystals, polygonal pores in HMX single crystals, and large-scale porosity variations in mock vibration assisted print (VAP) produced composites of borosilicate glass beads and Sylgard 184® binder. A single stage light gas gun was used to impact the samples at 400 m/s and the impact event and resultant material response were observed in real time using x-ray phase contrast imaging (PCI). Machined surface cracks were shown to have negligible effect on the final fracture behaviors of HMX crystals. In polygonal pores fractures were shown to originate due to stress concentration during impact followed by otherwise expected brittle fracture behaviors. For wedge-like pores, the shockwave culminates on the front face of the pore and contributed to early fracture in some samples as well as a consistent open fracture opposite the impact along the shockwave direction in later stages of impact. For the blunt rectangular-like pores two differing behaviors were observed, wherein either the pore condensed and fracture at the pore was not seen during the impact event or large open fractures formed at the pore corners opposite the shockwave. The variance in response is attributed to the energy of fracture dissipating somewhere else in the material bulk, like the behaviors observed in the milled slot samples. Finally, additively manufactured PBX deformation behaviors were observed to be dominated by the collapse of the existing ordered porosity in the bulk which occurred at an increased rate relative to the bulk material compression. This resulted in a three-stage progression of deformation, consisting of a rapid collapse of large-scale ordered porosity, followed by the densification of the remaining features, and ultimately ending in compaction of the bulk as the impact projectile fully compressed the samples. Future work includes exploration of further FIB produced pore effects on dynamic fractures, evaluation of printed material deformation behaviors at additional rates, as well as application and evaluation of additional VAP printed material formulations.  </p>

<p dir="ltr">Energetic materials contain microstructural defects like cracks, voids, grain boundaries, and interfaces which act as nucleation sites for ignition and detonation when shocked. Finite element (FE) models are currently unable to capture explicit microstructure with voids, cracks, and randomly oriented grains with representative mechanics, thermal conduction, and reactivity that exhibit the full shock to detonation transition (SDT). Modern computational efforts seek to accurately model material response while also balancing efficiency and speed. Work presented in this thesis will highlight all of these microstructural features, investigate mechanical and thermal response of each microstructure, connect these results to what is observed in other experimental and computational work, and bring computational modeling even closer to an efficient model that contains all processes necessary to replicate SDT.</p><p dir="ltr">In energetic materials (EM), voids are irregular in shape, but most computational work has focused on circular void collapse behavior. However, geometries that contain irregularities or corners are more likely to act as initiation sites due to stress concentrations. Validation and calibration of void simulations with experimental lengthscales and loading conditions is still limited. Plus, pore collapse modeling efforts at low impact velocities do not model fracture, and it is known that cracks cause more extreme temperatures than pores.</p><p dir="ltr">Other microstructure characteristics like cracks and grains have sub-micrometer length scale, and influence the mechanical and thermal response of materials under extreme conditions. However, approximations and coarse-graining must be applied to continuum FE simulations to fit length and timescales required to capture phenomena such as detonations that occur at a millimeter scale. With the use of machine learning (ML), numerical models can be trained on results of small-scale microstructure simulations and applied to larger length and time-scale simulations. The ML model follows Microstructure-Informed Shock-induced Temperature net (MISTnet) model and is trained upon stress, strain, temperature, pressure, and slip data and includes crystal plasticity, fracture, friction, an equation of state, and heat conduction. The ML model is able to predict temperature fields behind the shock, concentrations at grain boundaries, and the influence of grain orientation.</p><p dir="ltr">Accurate temperature values are extremely important to modeling EM because thermal hot spots (HS) are the main cause of ignition. Critical HS cause the chemical reactions which transition the shock front into a detonation, but many continuum models do not include chemistry in their framework. A 1-step Arrhenius reaction model is added to FE mechanics model to investigate the relationship HS have on the run to detonation (RTD).</p>