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Anomalous hardening under shock compression in (021)-oriented cyclotrimethylene trinitramine single crystals Atomic-scale analysis of defect dynamics and strain relaxation mechanisms in biaxially strained ultrathin films of face-centered cubic metalsThe propagation of shock waves normal to ͑111͒ in the energetic molecular crystal cyclotrimethylene trinitramine ͑RDX͒ has been studied using large-scale molecular dynamics simulations. Partial dislocation loops with Burgers vector 0.16͓010͔ are nucleated homogeneously on ͑001͒ at Rankine-Hugoniot shock pressures greater than 1.3 GPa. Calculations of the ͓010͔ cross-section of the ͑001͒ generalized stacking fault energy surface as a function of applied pressure along ͓001͔ 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 ͑111͒-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 ͑111͒-oriented RDX single crystals observed in recent experiments by giving rise to an anomalous plastic hardening.

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.

An experimental study of the elastic and plastic properties of sucrose single crystals, which can be considered to be a model material for both pharmaceutical excipients and explosives, has been carried out using nanoindentation. Instrumented indentation was used to characterize the properties of both habit and cleavage planes on the (100) and (001) orientations; the elastic modulus on the (100) is 38 GPa, while the modulus on the (001) is 33 GPa. The hardness of sucrose is approximately 1.5 GPa. Nanoindentation enables assessment of the onset of plastic deformation on cleaved surfaces, and a maximum shear stress of 1 GPa can be supported prior to plastic deformation. The deformation in this material is crystallographically dependent, with pileup around residual indentation impressions showing evidence of preferential slip system activity.

We recently proposed that the change observed in the elastic-plastic response of (111)-oriented cyclotrimethylene trinitramine (RDX) crystals under shock compression is caused by an anomalous hardening that is mediated by the homogeneous nucleation of partial dislocation loops with Burgers vector 0.16[010] on (001) {Cawkwell et al., [J. Appl. Phys. 107, 063512 (2010)]}. The orientation dependencies of the (001)[010] slip system suggested that (021)-oriented RDX crystals should also display an anomalous hardening. Molecular dynamics simulations of (021)-oriented RDX crystals confirm that this slip system is activated at a shock pressure 1.34&lt;P≤1.54 GPa. Plate impact experiments on (021)-oriented RDX single crystals show a two-wave elastic-plastic response at 1.0 GPa and an almost overdriven response at 2.25 GPa that is entirely consistent with the theoretical prediction.

Plate impact experiments were performed on oriented single crystals of the energetic material cyclotrimethylene trinitramine (RDX). The experiments were performed to determine the anisotropic dynamic yield point for the RDX crystal, as well as to provide data for continuum modeling efforts. Impact was on the (111), (210), and (100) planes to access 3, 2, and 0 slip systems, respectively. Velocity history profiles were measured using Doppler interferometry. Impacts on the (210) plane resulted in nominally conventional results, with distinct elastic and plastic waves, stress relaxation, elastic precursor decay, and increasing wave separation with propagation distance. Velocity profiles from impacts on the (111) plane had no discernable precursor, although an inflection seen in the thicker samples might be the nearly overdriven elastic wave. Wave arrival times signaled a slower elastic wave speed in the (111) profiles. Several unexpected features were observed in the elastic precursor of the profiles from impacts on the (100) plane. Up to three distinct step features were resolved in these profiles in the region of the elastic precursor; these features are not understood. In preparing samples for these experiments, it was noted that the (100) crystal slabs were exceptionally brittle. Wave speeds determined from the shock experiments were consistent with both pulse-echo wave speed measurements and wave speeds calculated from the measured elastic tensor. The elastic limit, as indicated by the peak of the leading wave, was relatively isotropic.

We have developed a model for the finite deformation thermomechanical response of α-cyclotrimethylene trinitramine (RDX). Our model accounts for nonlinear thermoelastic lattice deformation through a free energy-based equation of state developed by Cawkwell et al. (2016) in combination with temperature and pressure dependent elastic constants, as well as dislocation-mediated plastic slip on a set of slip systems motivated by experimental observation. The kinetics of crystal plasticity are modeled using the Orowan equation relating slip rate to dislocation density and the dislocation velocity developed by Austin and McDowell (2011), which naturally accounts for transition from thermally-activated to dislocation drag limited regimes. Evolution of dislocation density is specified in terms of local ordinary differential equations reflecting dislocation-dislocation interactions. The paper presents details of the theory and parameterization of the model, followed by discussion of simulations of flyer plate impact experiments. Impact conditions explored within this combined simulation and experimental effort span shock pressures ranging from 1 to 3 GPa for four crystallographic orientations and multiple specimen thicknesses. Simulation results generated using this model are shown to be in strong agreement with velocimetry measurements from the corresponding plate impact experiments. Finally, simulation results are used to motivate conclusions about the nature of dislocation-mediated plasticity in RDX.

Crystalline molecular explosives are key components of engineered explosive formulations. In precision applications a high degree of consistency and predictability is desired under a range of conditions to a variety of stimuli. Prediction of behaviors from mechanical response and failure to detonation initiation and detonation performance of the material is linked to accurate knowledge of the material structure and first stage of deformation: elasticity. The elastic response of pentaerythritol tetranitrate (PETN), cyclotrimethylene trinitramine (RDX), and cyclotetramethylene tetranitramine (HMX), including aspects of material and measurement variability, and computational methods are described in detail. Experimental determinations of elastic tensors are compared, and an evaluation of sources of error is presented. Computed elastic constants are also compared for these materials and for triaminotrinitrobenzene (TATB), for which there are no measurements.