Daniel E. Hooks

Direct observation of plasticity and quantitative hardness measurements in single crystal cyclotrimethylene trinitramine by nanoindentation

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.

Elastic–plastic wave profiles in cyclotetramethylene tetranitramine crystals

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 P21/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.

Homogeneous dislocation nucleation in cyclotrimethylene trinitramine under shock loading

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...

Elastic-plastic shock wave profiles in oriented single crystals of cyclotrimethylene trinitramine (RDX) at 2.25GPa

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 im...

Resonant ultrasound spectroscopy measurement of the elastic constants of cyclotrimethylene trinitramine

We used a resonant ultrasound spectroscopy technique to measure the nine independent elastic constants of cyclotrimethylene trinitramine C3H6N6O6 also known by its code designation RDX. The measurements were made on a single crystal grown from acetone containing RDX in solution. The elastic constants in gigapascal units are C11=25.6, C22=21.3, C33=19.0, C23=6.4, C31=5.72, C12=8.67, C44=5.38, C55=4.27, and C66=7.27.

Isentropic loading experiments of a plastic bonded explosive and constituents

The plastic bonded explosive PBX 9501 and its constituents [cyclotetramethylene tetranitramine (HMX) crystals, nitroplasticized Estane ®5703 and a fine-crystallite HMX laden binder mixture] were subjected to a ramped quasi-isentropic compression load using the Z machine at Sandia National Laboratories to determine equation of state and constitutive property data. Various sample thicknesses of these materials were subjected to an identical ramp loading history up to 4.5 GPa over 350 ns and particle velocities were measured using a velocity interferometry technique to assess material response. Upon defining appropriate constitutive relationships for the individual constituents, a topologically disordered model of the composite material was numerically simulated and details of the mesoscale simulation indicate that much of the plastic deformation first occurs locally at the large HMX crystal contacts points and subsequently by the deformation of the interstitial fine-crystallite/binder material.

Anomalous hardening under shock compression in (021)-oriented cyclotrimethylene trinitramine single crystals

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<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.

Determination of second-order elastic constants of cyclotetramethylene tetranitramine (β-HMX) using impulsive stimulated thermal scattering

The second-order elastic constants for cyclotetramethylene tetranitramine (β-HMX) single crystals were determined using the impulsive stimulated thermal scattering (ISTS) method. Despite the low symmetry of these crystals, the complete set of 13 elastic constants were determined accurately from acoustic velocity measurements using samples cut parallel to three different crystal planes. Our acoustic velocities are consistent with the limited sound speed data available from ultrasonic measurements. However, significant differences are observed between the elastic constants determined from our experiments and those obtained previously using Brillouin scattering. Our results demonstrate the usefulness and efficiency of the ISTS method for determining the full set of elastic constants of low-symmetry molecular crystals, including energetic crystals.

Analysis of wave profiles for single-crystal cyclotetramethylene tetranitramine

Wave profiles measured in the β-polymorph of single-crystal cyclotetramethylene tetranitramine display the characteristic response of an elastic-plastic material, an elastic precursor followed by a plastic wave. Moreover, the elastic precursor decays with the length of run. Numerical simulations with a rate-dependent elastic-plastic model are used to account for nonlinear and transient wave behavior. In addition, to account for the measured anisotropy in propagation, parameters of an isotropic model are fit for two propagation directions, normal to the (011) and (010) planes of the P21∕n space group. Equation of state parameters are constrained by data for the longitudinal sound speed and hydrostatic compression. The fits show that the effective yield strength varies with direction from 0.18GPa for the (011) orientation to 0.31GPa in the (010) orientation.

Time-Resolved Spectroscopic Measurements of Shock-Wave Induced Decomposition in Cyclotrimethylene Trinitramine (RDX) Crystals: Anisotropic Response

Plate impact experiments on the (210), (100), and (111) planes were performed to examine the role of crystalline anisotropy on the shock-induced decomposition of cyclotrimethylenetrinitramine (RDX) crystals. Time-resolved emission spectroscopy was used to probe the decomposition of single crystals shocked to peak stresses ranging between 7 and 20 GPa. Emission produced by decomposition intermediates was analyzed in terms of induction time to emission, emission intensity, and the emission spectra shapes as a function of stress and time. Utilizing these features, we found that the shock-induced decomposition of RDX crystals exhibits considerable anisotropy. Crystals shocked on the (210) and (100) planes were more sensitive to decomposition than crystals shocked on the (111) plane. The possible sources of the observed anisotropy are discussed with regard to the inelastic deformation mechanisms of shocked RDX. Our results suggest that, despite the anisotropy observed for shock initiation, decomposition pathways for all three orientations are similar.