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In 2016, we published a method for processing dual-energy computed tomography (DECT) data called system-independent rho-e/Z-e ( $\rho _{\mathrm {e}}/Z_{\mathrm {e}}$ ) or SIRZ. Using data from multiple DECT systems and spectra, SIRZ estimated the electron density $\rho _{\mathrm {e}}$ and effective atomic number $Z_{\mathrm {e}}$ (based on published X-ray cross sections), for a set of known specimens. However, the decomposition process required complex spectral modeling of the DECT system, which made SIRZ difficult to implement and automate. This paper describes the subsequent work on “SIRZ-2” to simplify the spectral modeling, automate the process, and improve its range and versatility. The SIRZ-2 basis functions are more accurate for the X-ray energy range ( $Z_{\mathrm {e}}$ of 6–20) were scanned with multiple spectral pairs (up to 200 keV) on seven different DECT systems, including a commercial airport luggage scanner. For these varied tests, the average SIRZ-2 relative errors for $\rho _{\mathrm {e}}$ estimates were four times lower (0.7% compared to 3.0% for SIRZ), while average $Z_{\mathrm {e}}$ relative errors were comparably low for both methods ( $\rho _{\mathrm {e}}$ , $Z_{\mathrm {e}}$ ) that can be directly compared across DECT systems and over time.

The U.S. Army Research Laboratory, in conjunction with Lawrence Livermore National Laboratory, is developing a Multi-Energy Flash Computed Tomography (MEFCT) diagnostic that will be used to capture tomographic image(s) of dynamic impact and detonation events. To accomplish dynamic tomography, the diagnostic uses numerous source–detector pairs to accumulate up to fifteen two-dimensional images, which are subsequently used to compute up to three three-dimensional tomographic reconstructions. The diagnostic is designed to provide either: a single-frame, three-dimensional tomographic reconstruction that delineates material specificity throughout the field, or a three-frame tomographic reconstruction movie spaced in time, while lacking the information pertaining to the material specificity. This work assesses aspects of the diagnostic development including structural design, dynamic capability, instrument resolution and computational reconstruction. Examples of real-time measurements are provided from static phantom fiducials, as well as a dynamic experiment depicting a non-symmetric ballistic penetration to demonstrate the usefulness of the capability.

An increase in the strength of concrete when loaded dynamically has been noted in the testing literature since the early twentieth century. The origins or mechanisms leading to this increase, despite having been observed in a variety of tests, are not satisfactorily established. Aspects of test setup, specimen design, etc., have been shown to influence the outcome of any given test. More recently, computer representations of concrete have been tasked with analyzing or predicting the dynamic behavior of structures. Computers have also enabled an inward look at the same empirical tests, showing that some strength increase in compression can be captured by implementation of the proper plasticity model. The major factor touted for strength increase is the well known pressure sensitivity of concrete and a mechanism known as ‘inertial confinement’. The present work proposes a new mechanism for dynamic strength increase, focusing on the failure mechanism of concrete in compression known as shear faulting. The faulting process and its associated plastic deformation mode is compared using several material models. Adjustments are made to some parameters within these models to study their effect on dynamic and inertial plastic response. Shear dilation, which does little to increase dynamic strength at moderate strain rates, is identified as a key component of a concrete material model subject to high strain rates. Shear dilation’s effects can be seen in the range of strain rates that are practically attainable in a laboratory by using the split Hopkinson pressure bar apparatus. They may also have an increasingly important effect on problems featuring even higher strain rates, such as blast, impact, and penetration through concrete slabs.

A novel but relatively easy-to-implement mitigation concept to enforce standoff distance and reduce shock loading on a vertical, partially-submerged structure is evaluated experimentally using scaled aquarium experiments and numerically using a high-fidelity finite element code. Scaled, watertamped explosive experiments were performed using aquariums of different sizes. The effectiveness of different mitigation configurations, including air-filled media and an air gap, is assessed relative to an unmitigated detonation using the same charge weight and standoff distance. Experiments using an airfilled media mitigation concept effectively dampen the explosive response of an aluminum plate and reduce the final displacement at plate center by approximately half. Experiments using an air-gap resulted in a focused water slug hitting the plate, an effect we hypothesize to be due to water encasement of the charge. Finite element simulations used for the initial experimental design compare very well to experiments ...

Structural failures, such as the MacArthur Maze I-880 overpass in Oakland, California and the I-35 bridge in Minneapolis, Minnesota, are recent examples of our national infrastructure's fragility and serve as an important reminder of such infrastructure in our everyday lives. These two failures, as well as the World Trade Center's collapse and the levee failures in New Orleans, highlight the national importance of protecting our infrastructure as much as possible against acts of terrorism and natural hazards. This paper describes a process for evaluating the vulnerability of critical infrastructure to large blast loads using a fully-coupled finite element approach. A description of the finite element software and modeling technique is discussed along with the experimental validation of the numerical tools. We discuss how such an approach can be used for specific problems such as modeling the progressive collapse of a building.

Both steel and reinforced concrete bridges are vulnerable to fire caused by a multitude of events such as tanker truck accidents, wildfires, arson or terrorism. This paper addresses bridges with steel super-structure and steel reinforced concrete or composite piers. After a brief summary of fire effects on bridges, the paper discusses the fire-induced collapse of two spans of the MacArthur Maze, a steel elevated freeway in Oakland, California. The two spans collapsed on April 29th, 2007 due to a fire caused by a tanker truck that overturned on the bridge. This paper discusses solutions and technologies that can mitigate the fire hazard in steel bridges. Based on the field investigation and analysis of collected data as well as fire analysis results, the following conclusions were reached: (1) the collapse could be prevented if the bridge was studies for the fire risk which would have led to recognition of high fire hazard at this intersection and vulnerability of the exposed steel girders,(2) the steel bridges and overpasses can be fire-protected using a variety of technologies and products successfully used in buildings; and (3) the current analytical tools could predict the time of the collapse accurately.

At approximately 3:41 AM on the morning of April 29, 2007, a tractor-trailer rig carrying 8,600 gallons (32.6 m3 ) of fuel overturned on Interstate 880 in Oakland, CA. The resultant fire weakened the surrounding steel superstructure and caused a 50-yard (45.7 m) long section of the above connecting ramp from Interstate 80 to Interstate 580 to fail in approximately 18 minutes. In this study, we performed a loosely-coupled thermal-structural finite element analysis of the freeway using the LLNL Engineering codes NIKE3D, DYNA3D and TOPAZ3D. First, we applied an implicit structural code to statically initialize the stresses and displacements in the roadway at ambient conditions due to gravity loading. Next, we performed a thermal analysis by approximating the tanker fire as a moving box region of uniform temperature. This approach allowed for feasible calculation of the fire-to-structure radiative view factors and convective heat transport. We used a mass scaling methodology in the thermal analysis to reduce the overall simulation time so an explicit structural analysis could be used, which provided a more computationally efficient simulation of structural failure. Our approach showed structural failure of both spans due to thermal softening under gravity loading at approximately 20 minutes for a fixed fire temperature of 1200°C and fixed thermal properties. When temperature-dependent thermal properties were applied, the south and north spans collapsed at approximately 10 minutes and 16 minutes, respectively. Finally, we performed a preliminary fully-coupled analysis of the system using the new LLNL implicit multi-mechanics code Diablo. Our investigation shows that our approach provides a reasonable first-order analysis of the system, but improved modeling of the transport properties and the girder-box beam connections is required for more accurate predictions.Copyright © 2008 by ASME