John D. Molitoris

Dynamic fragmentation of cellular, ice-templated alumina scaffolds

We examine the dynamic failure of ice-templated freeze-cast alumina scaffolds that are being considered as biomimetic hierarchical structures. Three porosities of alumina freeze-cast structures were fabricated, and a systematic variation in microstructural properties such as lamellar width and thickness was observed with changing porosity. Dynamic impact tests were performed in a light-gas gun to examine the failure properties of these materials under high strain-rate loading. Nearly complete delamination was observed following impact, along with characteristiccracking across the lamellar width. Average fragment size decreases with increasing porosity, and a theoretical model was developed to explain this behavior based on microstructural changes. Using an energy balance between kinetic, strain, and surface energies within a single lamella, we are able to accurately predict the characteristic fragment size using only standard material properties of bulk alumina.

TIME-SEQUENCED X-RAY OBSERVATION OF A THERMAL EXPLOSION

The evolution of a thermally‐initiated explosion is studied using a multiple‐image x‐ray system. HMX‐based PBX 9501 is used in this work, enabling direct comparison to recently‐published data obtained with proton radiography [1]. Multiple x‐ray images of the explosion are obtained with image spacing of ten microseconds or more. The explosion is simultaneously characterized with a high‐speed camera using an interframe spacing of 11 μs. X‐ray and camera images were both initiated passively by signals from an embedded thermocouple array, as opposed to being actively triggered by a laser pulse or other external source. X‐ray images show an accelerating reacting front within the explosive, and also show unreacted explosive at the time the containment vessel bursts. High‐speed camera images show debris ejected from the vessel expanding at 800–2100 m/s in the first tens of μs after the container wall failure. The effective center of the initiation volume is about 6 mm from the geometric center of the explosive.

DETAILED COMPARISON OF BLAST EFFECTS IN AIR AND VACUUM

The role of air as an energy transfer medium was examined experimentally by subjecting identical large‐area rectangular witness plates to short‐range blast effects in air and vacuum (∼50 mtorr) at 25 °C. The expanding reactant front of 3 kg C4 charges was observed by fast camera to be cylindrically symmetric in both air and vacuum. The horizontal component of the reactant cloud velocity (perpendicular to the witness plates) was constant in both cases, with values of 3.0 and 5.9 km/s for air and vacuum, respectively. As a result of the blast, witness plates were plastically deformed into a shallow dish geometry, with local maxima 30 and 20 mm deep for air and vacuum, respectively. The average plate deflection from the air blast was 11 mm, ∼10% deeper than the average vacuum plate deflection. Shock pressure estimates were made with a simple impedance‐matching model, and indicate peak values in the 30–50 MPa range are consistent with the reactant cloud density and velocity. However, more detailed analysis is...

Experimental and Multiscale Numerical Studies of Woven Fabric Carbon Composite Cylinder Subjected to Internal Pressure Loading

Carbon composite cylinders were fabricated using woven fabric composite sheets, and a mechanical device was designed and fabricated to apply internal pressure loading to the composite cylinders. This device can be used with a uniaxial testing machine without any fluid to generate internal pressure loading. Experimental tests were conducted for the composite cylinders to determine the failure loading using the device. Then, a multiscale modeling technique was utilized to predict the failure strength which was determined using the failure criterion based on the stresses and strains at the constituent material level such as fibers and matrix materials. Both numerical and experimental results agreed well. The subsequent studies to be conducted are physical tests and multiscale modeling and simulations to design carbon composite cylinders for extreme loading such as cases and containers for high explosives.

Mix and instability growth from oblique shock

We have studied the formation and evolution of shock-induced mix resulting from interface features in a divergent cylindrical geometry. In this research a cylindrical core of high-explosive was detonated to create an oblique shock wave and accelerate the interface. The interfaces studied were between high-explosive/aluminum, aluminum/plastic, and plastic/air. Surface features added to the aluminum were used to modify this interface. Time sequence radiographic imaging quantified the resulting instability formation from the growth phase to over 60 μs post-detonation, thus allowing the study of the onset of mix and evolution to turbulence. The plastic used here was porous polyethylene. Radiographic image data are compared with numerical simulations of the experiment.

Studies of Dynamic Failure of Steel Pipes using X-ray Radiography

The failure of a steel pipe subjected to shock loading was observed using x ray imaging. We describe and analyze the x ray images in detail. We see radiographic evidence that most of the fractures were due to shear rather than brittle failure. We also make quantitative comparisons between static radiographs and simulations but do not see perfect agreement. The sources of the current lack of agreement are discussed, as well as future work planned.

Pre- and poststrike atmospheric assessment system

The tactical motivation for a compact and easily deployable remote sensing unit for hazardous material releases is described. The most important measurements such a unit could make for accurately determining the extent of a plume from a target or potential target is a range resolved profile of the wind within the vicinity of the strike zone. However, to make such measurements a new type of instrument is required which is inherently robust, compact, and efficient. To demonstrate that such an instrument is feasible, we have designed, built, tested, and performed an initial laboratory evaluation of a compact Doppler lidar system for tactical applications.