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Understanding the differences in the shock compression and detonation response of insensitive high explosives (IHEs) and conventional HEs (CHEs) is a long-standing need in HE science and technology. Having previously examined 1,1-diamino-2,2-dinitroethene (FOX-7) IHE single crystals [Winey et al., J. Appl. Phys. 130, 015902 (2021)], the shock and detonation response of hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX)—a widely used CHE—was determined using wave profile measurements in ∼250 μm thick single crystals shock compressed to 63 GPa. In marked contrast to FOX-7, RDX single crystals shocked along the [100] and [111] orientations showed wave profile features consistent with chemical decomposition onset at 15 GPa. These features were more pronounced for [100] RDX, suggesting a higher decomposition rate compared to [111] RDX. At 51 GPa and above, flat-topped single waves were observed for both orientations, showing the classic Chapman–Jouget (C–J) detonation response in which the decomposition is completed within the detonation front. The Hugoniot states and sound speeds determined for the detonation products were similar for both [100] and [111] orientations, showing that the overdriven detonation response for RDX does not depend on crystal orientation. The C–J pressure for RDX single crystals (35 GPa)—determined experimentally—is comparable to that of FOX-7. However, compared to FOX-7, chemical decomposition onset for RDX occurs at much lower pressures and the overdriven C–J detonation response occurs at higher pressures. The present findings constitute the first experimental comparison of the shock and detonation response of conventional and insensitive HE single crystals over a broad pressure range below and above the C–J pressure. [ABSTRACT FROM AUTHOR]

In situ x-ray diffraction (XRD) measurements and their analysis in Al single crystals shock compressed along the [100]-direction were utilized to examine shock wave induced microstructural heterogeneities. High-resolution XRD line profiles for the 200, 400, and 600 Al peaks were measured in uniaxial strain compression states to either 5.6 or 11.7 GPa and partial stress release to 3.5 or 6.6 GPa, respectively. Broadening of the XRD line profiles was analyzed to determine the magnitude of the longitudinal micro-strain distribution (0.195% and 0.28% full width at half maximum for 3.5 and 6.6 GPa stresses, respectively) and the size of coherently diffracting domains (CDDs) (0.125 and 0.07 μm for 3.5 and 6.6 GPa stresses, respectively). From the longitudinal micro-strain distributions, the distribution of local stress differences (or stress deviators) was obtained in the shocked state. The full width at half maximum of this distribution, a measure of the local stress inhomogeneities, is greater than half of the macroscopic stress difference for both 3.5 and 6.6 GPa peak stresses, suggesting considerable variation in local stress deviators. The CDD sizes determined here are comparable to characteristic length scales for defect-free regions determined from defect density measurements in post-shock recovery experiments. The present work represents an important step in understanding material microstructure and inhomogeneities in the shock-compressed state. [ABSTRACT FROM AUTHOR]

The shock wave response of [100] lithium fluoride (LiF) single crystals at high stresses is of long-standing interest due to their extensive use as optical windows in dynamic compression experiments. The report of melting in shock-compressed LiF single crystals between 134 and 152 GPa—based on a single sound speed datum [Liu et al., J. Appl. Phys. 117, 045901 (2015)]—was surprising because good optical transmission was previously demonstrated in LiF shock compressed to ∼200 GPa [Rigg et al., J. Appl. Phys. 116, 033515 (2014)]. To address these apparent differences, we report on plate impact experiments on [100] LiF single crystals shock compressed to 168 GPa. Wave profiles were measured using laser interferometry to determine Hugoniot states and longitudinal sound speeds in shock-compressed LiF. The measured Hugoniot states are in good agreement with those measured in previous studies. However, the measured sound speeds presented here show no evidence of melting up to 168 GPa. In particular, the abrupt drop reported previously in sound speed at 152 GPa was not observed in the present work. Our results establish a lower bound of 168 GPa for the onset of melting in shock-compressed LiF single crystals. [ABSTRACT FROM AUTHOR]

Achieving the desired combination of superior detonation performance and insensitivity to shock initiation has been a long-standing goal in high explosive (HE) science and technology. Having previously established the shock insensitivity of 1,1-diamino-2,2-dinitroethene (also known as DADNE or FOX-7) single crystals to 20 GPa (extended to 25 GPa in this work), the FOX-7 detonation response was determined through wave profile measurements in ∼250 μm thick single crystals shock compressed to 64 GPa. Quite unexpectedly, FOX-7 demonstrated the classic Chapman–Jouguet (C–J) detonation response—reaction completion in the detonation front (<0.7 ns) at pressures of 44 GPa and higher—not observed in other insensitive high explosives. The experimentally determined C–J pressure (35 GPa), detonation wave velocities and the detonation products equation of state—together with shock insensitivity to 25 GPa—demonstrate that FOX-7 single crystals display a near-optimal combination of high performance and shock insensitivity, not observed in another HE crystal. [ABSTRACT FROM AUTHOR]

In contrast to relatively pure silica glass (fused silica—FS), commercial silica-rich glasses contain significant fractions of additional oxide components. In particular, soda-lime glass (SLG) consists of approximately 71% SiO2 by weight, which raises the question: what is the effect of additional cations on the shock compression response of silica-rich glasses? To address this question, plate impact experiments were conducted to determine the high-pressure Hugoniot states for shocked SLG (37 to 120 GPa) and compared with recently reported results on FS. Using laser interferometry, particle velocity profiles were measured at the impact surface and at the SLG/LiF window interface. In all experiments, the transmitted profiles show a single shock wave, with no features indicative of a phase transformation. The Hugoniot states determined from the wave profiles are described well using a linear shock velocity–particle velocity relation. Interferometry measurements (using 1550-nm wavelength laser) at the impact surface show that SLG is transparent when shocked to 55 GPa, transparent for tens of nanoseconds after impact between 55 and 81 GPa, and opaque beyond 81 GPa. From impact surface measurements, a linear relationship between the apparent and true particle velocity was observed, resulting in a linear relationship between the refractive index (at 1550 nm) and density. At 120 GPa, the SLG density is nearly twice its ambient value, indicating that SLG can achieve highly dense amorphous states. When compared to FS, shocked SLG is much less compressible and likely does not transform to a crystalline phase. A plausible explanation for this difference is suggested. [ABSTRACT FROM AUTHOR]

Insensitive high explosives (IHEs)—which do not compromise performance—are of considerable interest as a safer alternative to conventional high explosives, such as pentaerythritol tetranitrate and cyclotrimethylenetrinitramine. Despite the strong interest in using IHEs, shock compression experiments on IHE single crystals have not been reported. To address this need, plate impact experiments were conducted to measure wave profiles in 1,1-diamino-2,2-dinitroethene (FOX-7) single crystals—a representative IHE crystal—shocked to 21 GPa longitudinal stress. Particle velocity histories, measured using laser interferometry, show a clear two-wave structure (elastic–inelastic response) at modest stresses (<3.8 GPa). Wave profiles at higher stresses show a single (overdriven) wave. Measured shock velocities and wave profiles provide accurate Hugoniot data to 21 GPa. The measured wave profiles to 21 GPa show no sign of energy release due to chemical decomposition and constitute the first demonstration of an IHE single crystal insensitivity under plane shock compression. Numerical simulations using a phenomenological material model developed for FOX-7 showed good agreement with the measured wave profiles. The experimental findings and continuum simulations presented here constitute a significant first step in gaining an insight into the shock compression response of IHE single crystals. [ABSTRACT FROM AUTHOR]

To gain insights into the relative contributions of different plastic deformation mechanisms, particularly basal slip, for shocked hexagonal close-packed (hcp) metals, magnesium (Mg) single crystals were subjected to shock compression and release along a low-symmetry (LS) orientation to 1.9 and 4.8 GPa elastic impact stresses. LS-axis is a "nonspecific" direction resulting in propagation of quasilongitudinal and quasishear waves. Wave profiles, measured using laser interferometry, show a small elastic wave followed by two plastic waves in compression; release wave profiles exhibited a structured response for the higher stress and a smooth response for the lower stress. The LS-axis wave profiles are significantly different than profiles published previously for c- and a-axes, demonstrating that Mg single crystals exhibit strong anisotropy under shock compression/release. Numerical simulations, using a time-dependent anisotropic modeling framework, show that shock wave loading along the LS-axis involves the simultaneous operation of multiple deformation mechanisms. Shock compression along LS-axis is dominated by basal slip while prismatic slip and pyramidal I { 10 1 ¯ 1 } ⟨ 11 2 ¯ 3 ⟩ slip play a smaller role; coupling between longitudinal and shear deformations was observed. The unloading response is dominated by basal slip with some contribution from prismatic slip; pyramidal I slip is not activated. The present results, unlike results obtained for c- and a-axes, show that the deformation mechanism observed under quasistatic loading conditions along LS-axis is not sufficient to determine the shock response along this orientation. Although requiring numerical simulations for wave analysis, shock propagation along a LS-orientation provides new insights into the plastic deformation response of hcp metal single crystals. [ABSTRACT FROM AUTHOR]

To gain insight into the role of orientational order on the shock-induced graphite to diamond phase transformation, three pyrolytic graphite types having different orientational orders were shock-compressed along the average c-axis to peak stresses between 35 and 69 GPa. The materials studied were ZYB-grade highly oriented pyrolytic graphite (HOPG), ZYH-grade HOPG, and as-deposited pyrolytic graphite (PG) having mosaic spreads of 0.8° ± 0.2°, 3.5° ± 1.5°, and ∼45°, respectively. Wave profiles, obtained using laser interferometry, show a multiple-wave structure with a distinct, rapid (<10 ns) rise to the high-pressure phase for each graphite type. Multiple-wave profiles, first observed in this study for the less ordered ZYH-grade HOPG and PG samples, show that somewhat poorly oriented pyrolytic graphites also undergo a well-defined phase transformation. Previously, rapid transformation was reported for ZYB-grade but not ZYH-grade HOPG. The measured wave profiles for both HOPG grades are very similar and both grades show a ∼22 GPa transformation stress. In contrast, the PG wave profiles are quite different and show a ∼46 GPa transformation stress. The continuum results (stress-density states) presented here cannot distinguish between the different high-pressure phases [hexagonal diamond (HD) or cubic diamond] reported in recent x-ray studies. Because ZYB-grade HOPG was recently shown to transform to HD and due to the similar peak states for both HOPG grades, it seems likely that ZYH-grade also transforms into HD. The very different shock responses of PG and HOPG suggest different transformation mechanisms for PG and HOPG, but the high-pressure PG phase remains unclear in the present work. [ABSTRACT FROM AUTHOR]

To understand crystal anisotropy effects on shock-induced elastic-plastic deformation of molybdenum (Mo), results from high-purity single crystals shocked along [110] and [111] orientations to an elastic impact stress of 12.5 GPa were obtained and compared with the [100] results previously reported [A. Mandal and Y. M Gupta, J. Appl. Phys. 121, 045903 (2017)]. Measured wave profiles showed a time-dependent response, and strong anisotropy was observed in the elastic wave attenuation with the propagation distance, elastic limits, shock speeds, and overall structure of the wave profiles. Resolved shear stresses on {110}〈111〉 and {112}〈111〉 slip systems provided insight into the observed anisotropy in elastic wave attenuation and elastic limits and showed that shear stresses, and not longitudinal stresses, are a better measure of strength in shocked single crystals. Under shock compression, resolved shear stresses at elastic limits were comparable to the Peierls stress of screw dislocations in Mo. Elastic wave attenuation was rapid when shear stresses were larger than the Peierls stress. Large differences in the elastic limits under shock and quasi-static loading are likely a consequence of the large Peierls stress value for Mo. Numerically simulated wave profiles, obtained using the dislocation-based plasticity model described in the [100] work, showed good agreement with all measured wave profiles but could not differentiate between the {110}〈111〉 and {112}〈111〉 slip systems. Overall, experimental results and corresponding numerical simulations for the three crystal orientations have provided a comprehensive insight into shock-induced elastic-plastic deformation of Mo single crystals, including the development of a continuum material model. [ABSTRACT FROM AUTHOR]

To gain insight into inelastic deformation mechanisms for shocked hexagonal close-packed (hcp) metals, particularly the role of crystal anisotropy,magnesium (Mg) single crystals were subjected to shock compression and release along the a-axis to 3.0 and 4.8 GPa elastic impact stresses. Wave profiles measured at several thicknesses, using laser interferometry, show a sharply peaked elastic wave followed by the plastic wave. Additionally, a smooth and featureless release wave is observed following peak compression. When compared with wave profiles measured previously for c-axis Mg [Winey et al., J. Appl. Phys. 117, 105903 (2015)], the elastic wave amplitudes for a-axis Mg are lower for the same propagation distance, and less attenuation of elastic wave amplitude is observed for a given peak stress. The featureless release wave for a-axis Mg is in marked contrast to the structured features observed for c-axis unloading. Numerical simulations, using a time-dependent anisotropic modeling framework, showed that the wave profiles calculated using prismatic slip or (1012) twinning, individually, do not match the measured compression profiles for a-axis Mg. However, a combination of slip and twinning provides a good overall match to the measured compression profiles. In contrast to compression, prismatic slip alone provides a reasonable match to the measured release wave profiles; (1012) twinning due to its uni-directionality is not activated during release. The experimental results and wave profile simulations for a-axis Mg presented here are quite different from the previously published c-axis results, demonstrating the important role of crystal anisotropy in the time-dependent inelastic deformation of Mg single crystals under shock compression and release. [ABSTRACT FROM AUTHOR]

The thermodynamic response of liquid nitrogen has been studied extensively, in part, due to the long-standing interest in the high pressure and high temperature dissociation of shocked molecular nitrogen. Previous equation of state (EOS) developments regarding shocked liquid nitrogen have focused mainly on the use of intermolecular pair potentials in atomistic calculations. Here, we present EOS developments for liquid nitrogen, incorporating analytical models, for use in continuum calculations of the shock compression response. The analytical models, together with available Hugoniot data, were used to extrapolate a low pressure reference EOS for molecular nitrogen [R. Span et al., J. Phys. Chem. Ref. Data 29, 1361 (2000)] to high pressures and high temperatures. Using the EOS presented here, the calculated pressures and temperatures for single shock, double shock, and multiple shock compression of liquid nitrogen provide a good match to the measured results over a broad range of P-T space. These calculations provide the first comparison of EOS developments with recently measured P-T states under multiple shock compression. The present EOS developments are general and are expected to be useful for other liquids that have low pressure reference EOS information available. [ABSTRACT FROM AUTHOR]

Previous reports of rapid phase transformation above 18 GPa [Erskine and Nellis, Nature 349, 317 (1991)] and large elastic waves below 18 GPa [Lucas et al., J. Appl. Phys. 114, 093515 (2013)] for shock-compressed ZYB-grade highly oriented pyrolytic graphite (HOPG), but not for less oriented ZYH-grade HOPG, indicated a link between the orientational order dependence of the HOPG response above and below the phase transformation stress. To gain insight into this link and into the mechanical response of HOPG shocked to peak stresses approaching the phase transformation onset, the compressibility of ZYB- and ZYH-grade HOPG in the shocked state was examined using front surface impact experiments. Particle velocity histories and sound velocities were measured for peak stresses reaching 18 GPa. Although the locus of the measured peak stress-particle velocity states is indistinguishable for the two grades of HOPG, the measured sound velocities in the peak state reveal significant differences between the two grades. Specifically (1) the measured sound velocities are somewhat higher for ZYH-grade HOPG compared with ZYB-grade HOPG; (2) the measured sound velocities for ZYH-grade HOPG increase smoothly with compression, whereas those for ZYB-grade HOPG exhibit a significant reduction in the compression dependence from 12 GPa to 17GPa and an abrupt increase from 17GPa to 18 GPa; and (3) the longitudinal moduli, determined from the measured sound velocities, are smaller than the calculated bulk moduli for ZYB-grade HOPG shocked to peak stresses above 15 GPa, indicating the onset of an elastic instability. The present findings demonstrate that the softening of the longitudinal modulus (or elastic instability) presented here is linked to the large elastic waves and the rapid phase transformation reported previously—all observed only for shocked ZYB-grade HOPG. The elastic instability in shocked ZYB-grade HOPG is likely a precursor to the rapid phase transformation observed above 18 GPa for this HOPG grade. [ABSTRACT FROM AUTHOR]

To understand inelastic deformation mechanisms for shocked hexagonal-close-packed (hcp) metals, shock compression and release wave profiles, previously unavailable for hcp single crystals, were measured for c-axis magnesium crystals. The results show that the elastic-inelastic loading response is strongly time-dependent. Measured release wave profiles showed distinct peaked features, which are unusual for inelastic deformation during unloading of shocked metals. Numerical simulations show that pyramidal slip provides a reasonably good description of the inelastic loading response. However, {1012} twinning is needed to explain the unloading response. The results and analysis presented here provide insight into the relative roles of dislocation slip and deformation twinning in the response of shocked hcp metals. [ABSTRACT FROM AUTHOR]

Plate impact experiments were conducted to determine the dynamic compression response of two ballistic gel concentrations (10 wt.% and 20 wt.%) subjected to uniaxial strain loading to 1.3GPa. Shockless and shock wave compression of gel samples was achieved through flyer plate impacts on fused silica and z-cut quartz plates, respectively, placed ahead of the samples. Laser interferometry was used to measure the input compression wave profile and the propagated wave profile in each experiment. Using established wave analysis methods, the longitudinal stress-density compression results were obtained for each type of loading. The 20 wt.% gel showed a stiffer compression response than the 10 wt.% gel; the difference corresponded to the gel properties at ambient conditions. The shockless experiments provided both the loading paths and the peak states for this type of loading. Although the differences were small, the peak states achieved in shockless compression were below the shock wave (Hugoniot) data for both gel concentrations. The shockless compression results presented here are the first such results on gels, and provide the gel response at loading rates that are intermediate between the shock wave response and lower loading rate response. As such, they are expected to be useful for incorporating the dynamic compression response of gels in numerical simulations of impact phenomena. [ABSTRACT FROM AUTHOR]

Liquid nitrogen was subjected to multiple shock compression to examine its response to pressures (15-40 GPa) and temperatures (1800-4000 K) previously unexplored in static and shock compression studies. Raman spectroscopy measurements were used to characterize the molecular bond response and to determine temperatures in the peak state. By extending our analysis to include other Raman spectroscopy measurements, an empirical relation was developed that describes the pressure and temperature dependence of the Raman shift (of the 2330 cm-1 mode) for both shock and static compression. Based on the P-T dependence of the Raman shifts, the liquid nitrogen molecular response is best understood by considering three temperature regimes: below 1500 K, 1500-4000 K, and above 4000 K. For the pressures and temperatures accessed in the present work, liquid nitrogen remains a molecular fluid, and becomes a grey-body emitter at the highest pressures. [ABSTRACT FROM AUTHOR]

Understanding and modeling the response of hcp metals to high stress impulsive loading is challenging because the lower crystal symmetry, compared to cubic metals, results in a significantly more complex material response. To gain insight into the inelastic deformation of hcp metals subjected to high dynamic stresses, shock wave compression of single crystals provides a useful approach because different inelastic deformation mechanisms can be examined selectively by shock compression along different crystal orientations. As a representative example, we report, here, on wave propagation simulations for beryllium (Be) single crystals shocked along the c-axis, a-axis, and several low-symmetry directions to peak stresses reaching 7 GPa. The simulations utilized a time-dependent, anisotropic material model that incorporated dislocation dynamics, deformation twinning, and shear cracking based descriptions of inelastic deformation. The simulation results showed good overall agreement with measured wave profiles for all the different crystal orientations examined [Pope and Johnson, J. Appl. Phys. 46, 720 (1975)], including features arising from wave mode coupling due to the highly anisotropic inelastic response of Be. This good agreement demonstrates that the measured profiles can be understood in terms of dislocation slip along basal, prismatic, and pyramidal planes, together with deformation twinning along {10̄12} planes. Our results show that the response of shocked Be single crystals involves the simultaneous operation of multiple, distinct inelastic deformation mechanisms for all orientations except the c-axis. For shocked c-axis Be, the measured wave profiles do not provide good discrimination between pyramidal slip and other inelastic deformation mechanisms, such as shear cracking. The findings presented here provide insight into the complex inelastic deformation response of shocked Be single crystals and are expected to be useful for other hcp crystals. More broadly, the present work demonstrates the potential of shock wave propagation along low-symmetry directions to examine, and discriminate between, different inelastic deformation mechanisms in crystalline solids. [ABSTRACT FROM AUTHOR]

The objective of this work was to determine how variations in composition influence the shock wave response of bulk amorphous alloys (BAAs). Toward this end, (Hf,Zr)-based bulk amorphous alloy specimens (nominal composition of (Hf0.5Zr0.5)56.7Cu15.3Ni12.5Nb5.0Al10.0Y0.5) were subjected to peak stresses of 4-16 GPa in plate impact experiments and their response was compared to a previously studied Zr-based BAA (nominal composition of Zr56.7Cu15.3Ni12.5Nb5.0Al10.0Y0.5). The (Hf,Zr)-based BAA displayed a Hugoniot elastic limit (HEL) of ∼7.4 GPa corresponding to an elastic strain of 4.3%. (Hf,Zr)-based BAAs shock compressed above the HEL exhibited distinct two wave structures, small elastic precursor relaxation, non-steady plastic waves, and strength loss. All of these features are qualitatively similar to previous observations in various Zr-based BAAs. One dimensional wave propagation simulations incorporating a strain-softening strength model, developed previously for a Zr-based BAA, showed excellent agreement between measured and simulated particle velocity histories for the (Hf,Zr)-based BAA. The only significant differences in the shock wave responses of the (Hf,Zr)-based alloy and the Zr-based alloy are elastic shock velocity and plastic shock velocity differences which are due to the different ambient densities and different elastic moduli for the two alloy compositions. These findings demonstrate that, apart from differences related to ambient density and elastic stiffness, the substitution of Hf for 50% of the Zr did not significantly alter the shock compression response of the (Hf,Zr)-based BAA as compared to the previously examined Zr-based BAA. Based on the results of this study and other relevant data in the literature, it is expected that monolithic BAAs displaying brittle quasi-static compression behavior will likely display shock compression response that is comparable to Zr-based and (Hf,Zr)-based BAAs. [ABSTRACT FROM AUTHOR]

Analytic developments are presented to determine the strength of shock compressed single crystals from real-time x-ray diffraction (XRD) measurements. Both linear elastic and nonlinear elastic analysis methods are considered. Material strength in the shocked (constant) state may be determined using one of two approaches: from measurements of longitudinal and lateral lattice strains; or from measurements of longitudinal lattice strains and longitudinal wave profiles. The second approach is demonstrated for aluminum single crystals following shock compression along [100] to peak impact stresses of 5.5-12.7 GPa and partial release (reflection from the window material) to final stresses of 3.5-7.1 GPa. The material strength of the Al(100) in the final state was found to increase with peak stress or plastic strain. The material strength at the Hugoniot elastic limit was 0.025 GPa and the material strength in the final state was 0.52 GPa for the highest stress experiment. Because of the large final stresses, incorporating nonlinear elasticity into the analysis was necessary to obtain accurate values of the material strength; for the highest stress experiment, the material strength in the final state determined using the linear elastic analysis overestimated the strength by approximately 80%. The use of XRD measurements and the nonlinear elastic analysis method for strength determination in the shocked state is expected to be particularly useful for extreme pressures and temperatures where continuum methods for strength determination may face experimental limitations. [ABSTRACT FROM AUTHOR]

The acoustic properties of four polymers compressed to high pressures in a diamond anvil cell were determined using the impulsive stimulated light scattering (ISLS) method. Despite the weak scattering efficiency of these polymers, good signal quality was obtained by using a continuous wave probe and an optical heterodyne detection. We provide, for the first time, longitudinal acoustic velocities up to 5 GPa for two thermoplasts: poly(methyl-methacrylate) and poly(styrene), and two elastomers: poly(butadiene) and triblock copolymer of polystyrene-block-polybutadiene-block-polystyrene. The longitudinal acoustic velocities for all of these polymers displayed nonlinear pressure dependence. Despite the significant differences in the initial acoustic velocities these velocities converged above 2.5 GPa. This convergence is associated with the ultimate reduction of free volume in the studied polymers. We explored the possibility of measuring shear acoustic waves in these polymers using ISLS in a depolarized geometry. The data obtained here are important for modeling the response of polymers at extreme conditions. [ABSTRACT FROM AUTHOR]

Plate impact experiments were conducted to examine the dynamic tensile response of Zr-based bulk amorphous alloys (BAAs) having a nominal composition of Zr56.7Cu15.3Ni12.5Nb5.0Al10.0Y0.5. The experimental configuration used in our work permitted soft recovery of the samples to allow a careful examination of the fractured samples along with real-time measurements of the sample free-surface velocity (FSV) histories. Tensile loading was preceded by elastic compressive loading to peak stresses in the 3.6 to 6.0 GPa range. Tensile damage in the recovered samples was examined using optical and electron microscopy. The microscopy results showed that the BAA samples exhibit a brittle behavior (as a glass) at the macroscopic level and a ductile behavior (as a metal) at the microscopic level; in addition, corrugations and bumps are observed at the nanoscale. The observed fracture morphologies are related to three key features present in our spall experiments: preceding compressive stress (3.6–6.0 GPa), high tensile loading rate (∼106/s), high mean tensile stress (∼2.3 GPa); and are intrinsically related to the amorphous glassy structure of the BAAs (free volume content). We propose that the compressive stress depletes the free volume content. With increasing compressive stress, the available free volume decreases causing a suppression of shear stresses during tension. Thus, the mean tensile component becomes more dominant at higher stresses. Consequently, the observed surface morphology results from brittle cleavage, causing an increased damage localization in the recovered samples spalled at higher stresses. These observations support the inferences made from measurements of FSV histories. The high tensile loading rate is proposed to be responsible for cracking by multiple shear band propagation and interception, rendering the observed serrated surface morphology. Finally, we proposed that the corrugations are created due to a succession of arrest and propagation of mode I cracks. A subsequent dilatation, due to the effect of the tensile mean stress, caused the corrugations to evolve to bump-type features with sizes in the range of 10–100 nm. Our proposed mechanisms, although qualitative, constitute a systematic attempt to provide an explanation for the fracture morphologies observed in spalled BAA samples. [ABSTRACT FROM AUTHOR]

Plane shock wave experiments were conducted to determine the strength and elastic response of natural and synthetic diamond single crystals shocked along [100] to peak elastic stresses of ∼90 and ∼120 GPa. Velocity interferometry was used to measure particle velocity histories and shock velocities in the diamond samples. The maximum elastic wave amplitudes (89±3 GPa) for both crystal types were comparable. This value corresponds to shear stresses of 30 and 35 GPa (∼G/15) for the (111) [110] and (111) [211] slip systems, respectively. Surprisingly, the elastic limit (57±3 GPa) was lower for the higher peak stress. The elastic constant C111 was experimentally determined to be -7804±653 GPa. [ABSTRACT FROM AUTHOR]

An anisotropic continuum material model was developed to describe the thermomechanical response of unreacted pentaerythritol tetranitrate (PETN) single crystals to shock wave loading. Using this model, which incorporates nonlinear elasticity and crystal plasticity in a thermodynamically consistent tensor formulation, wave propagation simulations were performed to compare to experimental wave profiles [J. J. Dick and J. P. Ritchie, J. Appl. Phys. 76, 2726 (1994)] for PETN crystals under plate impact loading to 1.2 GPa. Our simulations show that for shock propagation along the [100] orientation where deformation across shear planes is sterically unhindered, a dislocation-based model provides a good match to the wave profile data. For shock propagation along the [110] direction, where deformation across shear planes is sterically hindered, a dislocation-based model cannot account for the observed strain-softening behavior. Instead, a shear cracking model was developed, providing good agreement with the data for [110] and [001] shock orientations. These results show that inelastic deformation due to hindered and unhindered shear in PETN occurs through mechanisms that are physically different. In addition, results for shock propagation normal to the (101) crystal plane suggest that the primary slip system identified from quasistatic indentation tests is not activated under shock wave loading. Overall, results from our continuum simulations are consistent with a previously proposed molecular mechanism for shock-induced chemical reaction in PETN in which the formation of polar conformers, due to hindered shear, facilitates the development of ionic reaction pathways. [ABSTRACT FROM AUTHOR]