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