John D. Yeager

In situ investigation of the dynamic response of energetic materials using IMPULSE at the Advanced Photon Source

The mechanical and chemical response of energetic materials is controlled by a convolution of deformation mechanisms that span length scales and evolve during impact. Traditional methods use continuum measurements to infer the microstructural response whereas advances in synchrotron capabilities and diagnostics are providing new, unique opportunities to interrogate materials in real time and in situ. Experiments have been performed on a new gas-gun system (IMPact system for Ultrafast Synchrotron Experiments) using single X-ray bunch phase contrast imaging (PCI) and Laue diffraction at the Advanced Photon Source (APS). The low absorption of molecular materials maximizes x-ray beam penetration, allowing measurements in transmission using the brilliance currently available at APS Sector 32. The transmission geometry makes it possible to observe both average lattice response and spatially heterogeneous, continuum response (1-4 um spatial resolution over ~2 × 2 mm area, 80 ps exposure, 153 ns frame-rate) in energetic materials ranging from single crystals to plastic-bonded composites. The current work describes our progress developing and using these diagnostics to observe deformation mechanisms relevant to explosives and the first experiments performed with explosives on IMPULSE at APS.

Dynamic experiment using IMPULSE at the Advanced Photon Source

The ability to examine the dynamic response of materials at extreme conditions requires diagnostics that can provide real-time, in situ, spatially resolved measurements at the appropriate length scale. Recent advances in synchrotron sources and diagnostics coupled to dynamic loading platforms are transforming the dynamic compression field to allow for such investigations. In the current work, recent experimental efforts on the IMPULSE (IMPact System for ULtrafast Synchrotron Experiments) capability at the Advanced Photon Source (Argonne, IL) will be highlighted to describe its development and use to examine phenomena including jet-formation in metals, compaction, crack formation and propagation, and material strength and failure. These experimental results have relied in part on: 1) the development of a robust optically multiplexed intensified detector configuration to obtain the first shock movies and 2) gun system improvements to better synchronize the impact event with the 80-ps width X-ray bunch. The IMPULSE capability is expected to continue to reveal novel phenomena for materials subjected to high strain rate loading while developing the required knowledge base to ensure success for future facilities including the Dynamic Compression Sector at the Advanced Photon Source and LANLs MaRIE.

Jet formation in cerium metal to examine material strength

Examining the evolution of material properties at extreme conditions advances our understanding of numerous high-pressure phenomena from natural events like meteorite impacts to general solid mechanics and fluid flow behavior. Recent advances in synchrotron diagnostics coupled with dynamic compression platforms have introduced new possibilities for examining in-situ, spatially resolved material response with nanosecond time resolution. In this work, we examined jet formation from a Richtmyer-Meshkov instability in cerium initially shocked into a transient, high-pressure phase, and then released to a low-pressure, higher-temperature state. Ceriums rich phase diagram allows us to study the yield stress following a shock induced solid-solid phase transition. X-ray imaging was used to obtain images of jet formation and evolution with 2–3 μm spatial resolution. From these images, an analytic method was used to estimate the post-shock yield stress, and these results were compared to continuum calculations that...

Nanoindentation of explosive polymer composites to simulate deformation and failure

Abstract Crack initiation and propagation is a common concern for molecular composites such as plastic bonded explosives (PBXs) and pharmaceutical tablets. Under compressive stresses, cracks form at contacts between crystals and propagate along crystal-binder interfaces, causing composite failure. To investigate this process, crystal-binder interfaces have been characterised and their mechanical properties tested. Here, samples were created with interfaces representative of those in PBXs and characterised with surface energy measurements and neutron reflectometry (NR). Nanoindentation was performed to simulate the deformation and cracking that occurs at crystal–crystal contacts through the binder. NR revealed that use of a plasticiser disrupts typical crystal–binder intermixing and results in a mechanically weaker interface. During nanoindentation, a plasticised binder was observed by atomic force microscopy to delaminate around indentation impressions, whereas a non-plasticised binder did not. Differences in interfacial adhesion and incompatible strain, dictated by the elastic–plastic film compliances, were used to explain the contrasting delamination behaviours.

Investigation of Dynamic Material Cracking with In Situ Synchrotron-Based Measurements

The development of time-resolved techniques that provide in situ spatially resolved measurements of the dynamic response of materials is a long-standing scientific need. Traditional methods use continuum measurements to infer the microstructure response of materials subjected to high strain rate loading; whereas advances in synchrotron capabilities and diagnostics provide unique opportunities to interrogate materials in situ. Recently we have implemented and performed experiments on a gas-gun system using single X-ray bunch phase contrast imaging (PCI) at the Advanced Photon Source to examine shock-induced phenomena. PCI is especially well suited for observing interfaces, something for which common shock physics and impact diagnostics are not. Here we present an overview of the capability and results from PCI investigations of in situ damage, including cracking and spall.

The Thermal and Microstructural Effect of Plasticizing HMX-Nitrocellulose Composites

ABSTRACT Thermal ignition via self-heating (cook-off) of cyclotetramethylene-tetranitramine (HMX)-containing plastic-bonded explosives (PBXs) is driven by the β → δ phase transition in the HMX, which is affected if not dominated by microstructure. Here, the HMX-binder interface and phase transition were studied for several variations of PBX 9404 (HMX with plasticized nitrocellulose [NC] binder). Neutron reflectometry was used to examine the interface under several conditions—pristine, after aging, and after thermal treatment. The initial interfacial structure depended on the plasticizer, but the interface homogenized over time. Thermal and optical analyses showed that all formulated materials had higher transition temperatures than neat HMX. This effect increased with NC content.

Probing interfaces between pharmaceutical crystals and polymers by neutron reflectometry.

Pharmaceutical powder engineering often involves forming interfaces between the drug and a suitable polymer. The structure at the interface plays a critical role in the properties and performance of the composite. However, interface structures have not been well understood due to a lack of suitable characterization tool. In this work, we have used ellipsometry and neutron reflectometry to characterize the structure of such interfaces in detail. Ellipsometry provided a quick estimate of the number of layers and their thicknesses, whereas neutron reflectometry provided richer structural information such as density, thickness, roughness, and intermixing of different layers. The combined information allowed us to develop an accurate model about the layered structure and provided information about intermixing of different layer components. Systematic use of these characterization techniques on several model systems suggests that the nature of the polymer had a small effect on the interfacial structure, while the solvent used in polymer coating had a large effect. These results provide useful information on the efforts of engineering particle properties through the control of the interfacial chemistry.

Development of inert density mock materials for HMX

ABSTRACT Inert surrogates or mocks for high explosives are commonly used in place of the real material for complex experiments or in situations where safety is a concern. Here, several materials were tested as potential mocks for HMX in terms of density, thermal stability, and processability. Selection criteria were developed and a literature search was conducted primarily using the Cambridge Structural Database. Out of over 200 potentially acceptable materials, six were chosen for crystallization experiments and a suite of analytical characterization. Of these six, 5-iodo-2ˊ-deoxyuridine, N,Nˊ-bis(2,3,4,5,6-pentafluorophenyl)oxamide, and 2,3,4,5,6-pentafluorobenzamide all were found to be thermally stable at 150°C, matched HMX density as a pressed pellet, and could be crystallized to appropriate particle sizes. These three materials are considered suitable inert density mocks for HMX and will be the subject of future testing.

Characterization of hypervelocity metal fragments for explosive initiation

The fragment impact response of two plastic-bonded explosive (PBX) formulations was studied using explosively driven aluminum fragments. A generic aluminum-capped detonator generated sub-mm aluminum particles moving at hypersonic velocities. The ability of these fragments to initiate reaction or otherwise damage two PBX materials was assessed using go/no-go experiments at standoff distances of up to 160 mm. Lower density PBX 9407 (RDX-based) was initiable at up to 115 mm, while higher density PBX 9501 (HMX-based) was only initiable at up to 6 mm. Several techniques were used to characterize the size, distribution, and velocity of the particles. Witness plate materials, including copper and polycarbonate, and backlit high speed video were used to characterize the distribution of particles, finding that the aluminum cap did not fragment homogeneously but rather with larger particles in a ring surrounding finer particles. Finally, precise digital holography experiments were conducted to measure the three-dim...

Characterizing the propensity of hypervelocity metal fragments to initiate plastic bonded explosives

The off-normal detonation behavior of two plastic-bonded explosive (PBX) formulations was studied using explosively-driven aluminum fragments created by two types of detonators. A generic aluminum-cupped detonator contained 100 mg of PETN which was sufficient to fragment the aluminum into hundreds of sub-mm particles moving at hypersonic velocity; for comparison, a Teledyne RISI® RP-80 was also tested which generated a more substantial flyer plate of aluminum. Low-density polystyrene foam was used as a witness material with subsequent computed tomography analysis to characterize the distribution of particles post-test. Precise digital in-line holography experiments were conducted in situ to measure three-dimensional shape and size of the fastest-moving fragments as they impacted PBXs. Fragments showed significant variability in size, shape and distribution or clustering. Depending on the shot, single or multiple shock impacts could be imparted to the PBX from the generic detonator fragments, or clusters, ...