The highly transient nature of shock loading and pronounced microstructure effects on dynamic materials response call for in situ, temporally and spatially resolved, x-ray-based diagnostics. Third-generation synchrotron x-ray sources are advantageous for x-ray phase contrast imaging (PCI) and diffraction under dynamic loading, due to their high photon fluxes, high coherency, and high pulse repetition rates. The feasibility of bulk-scale gas gun shock experiments with dynamic x-ray PCI and diffraction measurements was investigated at the beamline 32ID-B of the Advanced Photon Source. The x-ray beam characteristics, experimental setup, x-ray diagnostics, and static and dynamic test results are described. We demonstrate ultrafast, multiframe, single-pulse PCI measurements with unprecedented temporal (<100 ps) and spatial (∼2 μm) resolutions for bulk-scale shock experiments, as well as single-pulse dynamic Laue diffraction. The results not only substantiate the potential of synchrotron-based experiments for addressing a variety of shock physics problems, but also allow us to identify the technical challenges related to image detection, x-ray source, and dynamic loading.

The explosive molecular crystal cyclotetramethylene tetranitramine was studied in three orientations in a set of plate impact experiments; the orientations studied were {110}, {011}, and {010} in ${\text{P2}}_1\mathrm{/n}$ space group. The elastic–plastic shock response was measured using laser interferometry. The measured particle velocity profiles showed elastic precursor decay typical of a stress relaxing material. There is anisotropy in elastic shock strength and decay. The amount of precursor decay with propagation distance and stress relaxation behind the elastic shock varied among the orientations. The {010} orientation had larger elastic precursors than did the other two orientations; the {010} crystal does not have the regular plastic deformation mechanisms available to it. Elastic Hugoniots were obtained from the measurements. The inelastic deformation mechanisms may vary with orientation.

Investigation of deformation beginning with elasticity and continuing through the elastic–plastic transition to incipient cracking has been conducted for (210), (021) and (001) oriented single crystals of the explosive cyclotrimethylene trinitramine, commonly known as “RDX”. Nanoindentation was performed with a conical tip over a range of loads. The resulting load–depth data exhibited distinct, reproducible, orientation-dependent load excursions demonstrating elastic–plastic transitions. Indent impressions were imaged by atomic force microscopy revealing deformation features consistent with slip on six planes. Impressions on the (210) and (001) planes showed deformation pile-up features associated with the zone axes of slip planes. Slip traces were evident on the (210) plane indicating slip on four planes and suggesting cross-slip. Height data, for impressions formed by progressively increasing loads, indicated one additional slip system consistent with (010) slip. All of the orientations exhibited cracking thresholds at very low loads. The reduced elastic moduli were anisotropic and the hardness values were isotropic indicating limited plasticity. Maximum shear stresses estimated from a Hertzian model, at load excursions, were within 1/15 to 1/10 of published shear moduli, indicating deformation initiated near the theoretical yield strength, presumably by homogeneous nucleation of dislocations. The material strength parameters and deformation pathways inferred from this work are compared to previous microhardness investigations in which the ambiguity of results can be attributed to the effects of cracking and simultaneous slip on multiple systems. A mechanistic explanation for the hindered plasticity, and cracking, observed for RDX is offered in terms of compatibility conditions.

The propagation of shock waves normal to (111) in the energetic molecular crystal cyclotrimethylene trinitramine (RDX) has been studied using large-scale molecular dynamics simulations. Partial dislocation loops with Burgers vector 0.16[010] are nucleated homogeneously on (001) at Rankine–Hugoniot shock pressures greater than 1.3 GPa. Calculations of the [010] cross-section of the (001) generalized stacking fault energy surface as a function of applied pressure along [001] reveals that the stacking fault enclosed by the partial dislocation loops is rendered metastable by a stress-induced change in molecular conformation. Furthermore, large-scale molecular dynamics simulations performed on quasi-two-dimensional (111)-oriented single crystals show a two-wave elastic-plastic response with a “galloping” plastic wave. We propose that the onset of homogeneous dislocation nucleation accounts for the abrupt change in the elastic-plastic response of macroscopic (111)-oriented RDX single crystals observed in recent experiments by giving rise to an anomalous plastic hardening.