Shock experiments on low density polyurethane foams reveal evidence of reaction at low impact pressures. However, these reaction thresholds are not evident over the low pressures reported for historical Hugoniot data of highly distended polyurethane at densities below 0.1 g/cc. To fill this gap, impact data for PMDI foam with a density of 0.087 g/cc were acquired for model validation. An equation of state (EOS) was developed to predict the shock response of these highly distended materials over the full range of impact conditions representing compaction of the inert material, low-pressure decomposition, and compression of the reaction products. A tabular SESAME EOS of the reaction products was generated using the JCZS database in the TIGER equilibrium code. In particular, the Arrhenius Burn EOS, a two-state model which transitions from an unreacted to a reacted state using Arrhenius kinetics, as implemented in the shock physics code CTH, was modified to include a statistical distribution of states. Hence, a single EOS is presented that predicts the onset to reaction due to shock loading in PMDI-based polyurethane foams. This methodology was also used to predict the anomalous compaction of PMDI foams over published data sets from 0.087 to 0.87 g/cc, and solid Polyurethane at a theoretical maximum density (TMD) of 1.264 g/cc. Likewise, similar modeling techniques were used to predict the performance of SX-358 foam, an RTV-based stress cushion material at a nominal density of 0.41 g/cc, and the matrix material, with properties similar to Sylgard, at 1.1 g/cc. At the start of this study, data were only available at a single impact condition below the threshold for reaction; hence, the decomposition of this material at higher pressures was revealed as a significant finding of this work. The decomposition of SX-358 at higher impact pressures to product species including solid, liquid, and gaseous molecules was estimated with thermochemical equilibrium calculations using TIGER. with thermochemical equilibrium calculations using TIGER. This modeling approach, developed for PMDI foam, was shown to predict gas gun data, acquired as part of this study, up to pressures of 14 GPa. Furthermore, additional phase transitions were predicted in the product species under shock compression. To date, this study is the first known to the authors that demonstrates and successfully predicts the decomposition of these low-density polymer-based foams using a single model applicable to a broad range of impact loading conditions.
A microscale model of the brain was developed in order to understand the details of intracranial fluid cavitation and the damage mechanisms associated with cavitation bubble collapse due to blast-induced traumatic brain injury (TBI). Our macroscale model predicted cavitation in regions of high concentration of cerebrospinal fluid (CSF) and blood. The results from this macroscale simulation directed the development of the microscale model of the superior sagittal sinus (SSS) region. The microscale model includes layers of scalp, skull, dura, superior sagittal sinus, falx, arachnoid, subarachnoid spacing, pia, and gray matter. We conducted numerical simulations to understand the effects of a blast load applied to the scalp with the pressure wave propagating through the layers and eventually causing the cavitation bubbles to collapse. Collapse of these bubbles creates spikes in pressure and von Mises stress downstream from the bubble locations. We investigate the influence of cavitation bubble size, compressive wave amplitude, and internal bubble pressure. The results indicate that these factors may contribute to a greater downstream pressure and von Mises stress which could lead to significant tissue damage.
Blast waves from an explosion in air can cause significant structural damage. As an example, cylindrically-shaped charges have been used for over a century as dynamite sticks for mining, excavation, and demolition. Near the charge, the effects of geometry, standoff from the ground, the proximity to other objects, confinement (tamping), and location of the detonator can significantly affect blast wave characteristics. Furthermore, nonuniformity in the surface characteristics and the density of the charge can affect fireball and shockwave structure. Currently, the best method for predicting the shock structure near a charge and the dynamic loading on nearby structures is to use a multidimensional, multimaterial shock physics code. However, no single numerical technique currently exists for predicting secondary combustion, especially when particulates from the charge are propelled through the fireball and ahead of the leading shock lens. Furthermore, the air within the thin shocked layer can dissociate and ionize. Hence, an appropriate equation of state for air is needed in these extreme environments. As a step towards predicting this complex phenomenon, a technique was developed to provide the equilibrium species composition at every computational cell in an air blast simulation as an initial condition for hand-off to other analysis codes for combustion fluid dynamics or radiation transport. Here, a bare cylindrical charge of TNT detonated in air is simulated using CTH, an Eulerian, finite volume, shock propagation code developed and maintained at Sandia National Laboratories. The shock front propagation is computed at early times, including the detonation wave structure in the explosive and the subsequent air shock up to 100 microseconds, where ambient air entrainment is not significant. At each computational cell, which could have TNT detonation products, air, or both TNT and air, the equilibrium species concentration at the density-energy state is computed using the JCZS2i database in the thermochemical code TIGER. This extensive database of 1267 gas (including 189 ionized species) and 490 condensed species can predict thermodynamic states up to 20,000 K. The results of these calculations provide the detailed three-dimensional structure of a thin shock front, and spatial species concentrations including free radicals and ions. Furthermore, air shock predictions are compared with experimental pressure gage data from a right circular cylinder of pressed TNT, detonated at one end. These complimentary predictions show excellent agreement with the data for the primary wave structure.
Compaction waves in porous energetic materials have been shown to induce reaction under impact loading. In the past, simple two-state burn models such as the Arrhenius Burn model have been developed to predict slapper initiation in Hexanitrostilbene (HNS) pellets; however, a more sophisticated, fundamental approach is needed to predict the shock response during impact loading, especially in pellets that have been shown to have strong density gradients. The intergranular stress measures the resistance to bed compaction or the removal of void space due to particle packing and rearrangement. A constitutive model for the intergranular stress is needed for closure in the Baer-Nunziato (BN) multiphase mixture theory for reactive energetic materials. The intergranular stress was obtained from both quasi-static compaction experiments and from dynamic compaction experiments. Additionally, historical data and more recently acquired data for porous pellets compacted to high densities under shock loading were used for model assessment. Predicted particle velocity profiles under dynamic compaction were generally in good agreement with the experimental data. Hence, a multiphase model of HNS has been developed to extend current predictive capability.
A new approach to explosive sample preparation is described in which microelectronics-related processing techniques are utilized. Fused silica and alumina substrates were prepared utilizing laser machining. Films of PETN were deposited into channels within the substrates by physical vapor deposition. Four distinct explosive behaviors were observed with high-speed framing photography by driving the films with a donor explosive. Initiation at hot spots was directly observed, followed by either energy dissipation leading to failure, or growth to a detonation. Unsteady behavior in velocity and structure was observed as reactive waves failed due to decreasing channel width. Mesoscale simulations were performed to assist in experiment development and understanding. We have demonstrated the ability to pattern these films of explosives and preliminary mesoscale simulations of arrays of voids showed effects dependent on void size and that detonation would not develop with voids below a certain size. Future work involves experimentation on deposited films with regular patterned porosity to elucidate mesoscale explosive behavior.
A joint experimental and computational study was performed to evaluate the capability of the Sandia Fire Code VULCAN to predict thermocouple response temperature. Thermocouple temperatures recorded by an Inconel-sheathed thermocouple inserted into a near-adiabatic flat flame were predicted by companion VULCAN simulations. The predicted thermocouple temperatures were within 6% of the measured values, with the error primarily attributable to uncertainty in Inconel 600 emissivity and axial conduction losses along the length of the thermocouple assembly. Hence, it is recommended that future thermocouple models (for Inconel-sheathed designs) include a correction for axial conduction. Given the remarkable agreement between experiment and simulation, it is recommended that the analysis be repeated for thermocouples in flames with pollutants such as soot.