Steady non-ideal detonations in cylindrical sticks of expolsives

Numerical simulations of detonations in cylindrical rate-sticks of highly non-ideal explosives are performed, using a simple model with a weakly pressure dependent rate law and a pseudo-polytropic equation of state. Some numerical issues with such simulations are investigated, and it is shown that very high resolution (hundreds of points in the reaction zone) are required for highly accurate (converged) solutions. High resolution simulations are then used to investigate the qualitative dependences of the detonation driving zone structure on the diameter and degree of confinement of the explosive charge. The simulation results are used to show that, given the radius of curvature of the shock at the charge axis, the steady detonation speed and the axial solution are accurately predicted by a quasi-one-dimensional theory, even for cases where the detonation propagates at speeds significantly below the Chapman-Jouguet speed. Given reaction rate and equation of state models, this quasi-one-dimensional theory offers a significant improvement to Wood-Kirkwood theories currently used in industry

Shock Wave Dynamics of Novel Aluminized Detonations and Empirical Model for Temperature Evolution from Post-Detonation Combustion Fireballs

This research characterizes the blast wave and temperature evolution of an explosion fireball in order to improve the classification of aluminized conventional munitions based on a single explosive type such as RDX. A drag model fit to data shows initial shock velocities of 1.6-2.8 km/s and maximum fireball radii ranging from 4.3-5.8 m with most of the radii reached by 50 ms upon detonation. The Sedov-Taylor point blast model is fitted to data where a constant release (s=1) of energy upon detonation suggests shock energies of 0.5-8.9 MJ with blast dimensionalities indicative of the spherical geometry (n3) observed in visible imagery. An inverse correlation exists between blast wave energy and overall aluminum content in the test articles. Using a radiative cooling term and a secondary combustion term, a physics-based empirical model is able to reduce 82 data points to five fit parameters to describe post-detonation combustion fireballs. The fit-derived heat of combustion has a 96% correlation with the calculated heat of combustion but has a slope of 0.49 suggesting that only half of the theoretical heat of combustion is realized. Initial temperature is not a good discriminator of detonation events but heat of combustion holds promise as a potential variable for event classification

Diagnostic techniques in deflagration and detonation studies

Advances in experimental, high-speed techniques can be used to explore the processes occurring within energetic materials. This review describes techniques used to study a wide range of processes: hot-spot formation, ignition thresholds, deflagration, sensitivity and finally the detonation process. As this is a wide field the focus will be on small-scale experiments and quantitative studies. It is important that such studies are linked to predictive models, which inform the experimental design process. The stimuli range includes, thermal ignition, drop-weight, Hopkinson Bar and Plate Impact studies. Studies made with inert simulants are also included as these are important in differentiating between reactive response and purely mechanical behaviour

Analysis and simulation of small scale microwave interferometer experiments on non-ideal explosives

Small scale experiments for non-ideal and homemade explosives (HMEs) were investigated, analyzed, and subsequently modeled in an attempt to develop more predictive capabilities for the threat assessment of improvised explosive devices (IEDs), as well as to provide new analysis capabilities for other investigators in the field. Non-ideal explosives and HMEs are challenging to characterize because of the nearly limitless parameter space (e.g. sample composition, density, particle morphology, etc.) which gives rise to a broad range of explosive sensitivity and performance. Large scale tests, such as rate stick and gap tests, are not feasible for characterizing every HME of interest due to limitations in time and cost. These small scale experiments utilize a 35 GHz microwave interferometer to measure the instantaneous shock and failing detonation wave velocities in explosives. Only those explosives which are transparent to the microwave radiation are evaluated, including ammonium nitrate plus fuel oil (ANFO). It is shown here for the first time that the small scale measurements may be related to large scale sensitivity and performance for a large enough sample size and level of confinement. Specifically, four different experimental configurations were explored that require only 1-5 g of material. By varying the charge diameter, as well as the thickness and sound speed of the confining material, the failure rate and shock front curvature of an overdriven failing detonation may be tailored. The detailed experimental data is also highly repeatable, provided that the initial sample density is uniform and consistent from test to test. Results from the MI data also reveal the existence of an inflexion point in velocity, which is thought to be related to the measurements obtained from larger rate sticks. The different MI experiments were subsequently modeled in 2d as well as 3d using the shock physics hydrocode CTH. An ignition and growth reactive burn (IGRB) model was developed for non-ideal explosives, and shown to be relevant to capturing the behavior of some of the overdriven failing detonation waves. Many simplifying assumptions were made, so that the MI data might possibly be used for model calibration and validation. It was determined that an intermediate level of confinement utilizing low sound speed polyvinyl chloride (PVC) is most relevant for fitting the IGRB model constants, which were then used to predict the other MI experiments with partial success. Overall, the CTH simulations provide much more information than what is available from the MI measurements alone. These simulations were used to investigate pressure waves in the explosive and confiner materials, and to show that the reactive waves are likely transitioning from supersonic to subsonic deflagration, where thermal effects, compaction behavior, and material strength are important. Consequently, these simulations are not able to match the weaker confinement and smaller diameter experiments over the full duration of the tests. The calibrated IGRB model was then used to make several predictions for shock sensitivity, changes to the initial density, and other large scale tests. Future work is suggested to validate these predictions and to improve the model development. Overall, the high level of integration between experimental and modeling efforts shown in this work is critical to better understand HMEs and to design new small scale experiments

Homogeneous explosion and shock initiation for a three-step chain-branching reaction model

The role of chain-branching cross-over temperatures in shock-induced ignition of reactive materials is studied by numerical simulation, using a three-step chainbranching reaction model. In order to provide insight into shock initiation, the simpler problem of a spatially homogeneous explosion is first considered. It is shown that for ratios of the cross-over temperature to the initial temperature, T-B, sufficiently less than unity, the homogeneous explosion can be quantitatively described by a widely used two-step model, while for T-B sufficiently above unity the homogeneous explosion can be effectively described by the standard one-step model. From the matchings between these homogeneous-explosion solutions, the parameters of the reduced models are identified in terms of those of the three-step model. When T-B is close to unity, all the reactions of the three-step model have a leading role, and hence in this case the model cannot be reduced further. In the case of shock initiation, for T-B (which is now the ratio of the cross-over temperature to the initial shock temperature) sufficiently below unity, the three-step solutions are qualitatively described by those of the matched two-step model, but there are quantitative differences due to the assumption in the reduced model that a purely chain-branching explosion occurs instantaneously. For T-B sufficiently above unity, the matched one-step model is found to effectively describe the way in which the heat release and fluid dynamics couple. For T-B close to unity, the competition between chain branching and chain termination is important from the outset. In these cases the speed at which the forward moving explosion wave that emerges from the piston is sensitive to T-B, and changes from supersonic to subsonic for a value of T-B just below unity