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Sound speed measurements in shock compressed solids have long been valuable for the development of equations of state at extreme conditions, shock-induced phase transformations, and a comprehensive characterization of the thermophysical response of high-pressure standards. We present results from plate impact experiments to 190 GPa to determine the longitudinal sound speed in copper—an important high-pressure standard. Surprisingly, the sound speeds determined using the two most common experimental techniques—the front surface impact (FSI) approach and the release wave overtake (RWO) approach—diverge significantly for stresses greater than ∼100 GPa. Further analyses, including numerical simulations, show that the FSI experiments provide the correct sound speeds and that fundamental assumptions underlying the RWO method are likely violated due to the complex release response of shock compressed copper. The sound speeds determined using the FSI approach provide for a more accurate high-pressure description of copper in dynamic compression experiments. The present findings are in contrast to the results for shock compressed silver [Wallace et al., Phys. Rev. B 104, 214106 (2021)], where both methods provided consistent sound speed results. Thus, the findings presented here demonstrate the need to experimentally verify the validity of the RWO method on a case-by-case basis. Finally, we note that even at the high stresses in the present work, the copper unloading response shows a time-dependent, quasielastic response often observed in metals at lower stresses. [ABSTRACT FROM AUTHOR]

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]

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]

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]

Thin polycrystalline aluminum (1050 and 6061-T6 alloys) samples were shocked to 4 GPa to examine elastic wave attenuation not observed in thicker samples (1–10 mm). Using laser interferometry in plate impact experiments, particle velocity histories were obtained for 0.08–1 mm thick samples, thinned from bulk material. Unlike past work on thicker samples, thin 1050 Al samples reveal large and rapidly attenuating elastic wave amplitudes, indicating a time-dependent elastic-plastic response. Extrapolation of measured elastic wave amplitudes to larger sample thicknesses agrees well with previously observed amplitudes for thicker 1050 and 1060 Al samples. Thus, all of the results for relatively pure polycrystalline Al can be reconciled into a single consistent picture: elastic wave attenuation, due to time-dependent elastic-plastic response, is confined to material close to the impact surface. In contrast to the 1050 Al results, thin 6061-T6 Al samples reveal an elastic wave amplitude of ∼0.7 GPa with no attenuation, in quantitative agreement with previous results for thick 6061-T6 Al samples. The lack of elastic wave attenuation even in thin samples suggests that elastic wave amplitudes in shocked 6061-T6 Al are governed by different plastic deformation mechanisms than those for shocked pure Al. [ABSTRACT FROM AUTHOR]

To examine the effect of compressive loading rate on the elastic limit of brittle solids, shockless and shock wave uniaxial strain experiments were conducted on x-cut quartz to a peak stress of 11 GPa. Using a compact pulsed power generator, x-cut quartz crystals were subjected to shockless compression (loading rate of ∼3×105/s). Plate impact experiments were used to subject samples to shock wave compression (loading rate >=4×107/s). Particle velocity histories, measured at propagation distances of 1.5–3.5 mm, demonstrated that the elastic limit of x-cut quartz under shockless compression was 85%–90% higher than the elastic limit under shock wave compression. The substantial increase in the elastic limit with decreasing loading rate is contrary to the expected loading rate dependence of material strength. Mechanistic implications of this finding are discussed. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

We present a method to use real time, synchrotron x-ray diffraction measurements to determine the strength of shocked single crystals following compression and release during uniaxial strain loading. Aluminum and copper single crystals shocked along [111] were examined to peak stresses ranging from 2 to 6 GPa. Synchrotron x rays were used to probe the longitudinal lattice strains near the rear free surface (16 and 5 μm depths for Al and Cu, respectively) of the metal crystals following shock compression and release. The 111 diffraction peaks showed broadening indicating a heterogeneous microstructure in the released state. The diffraction peaks also shifted to lower Bragg angles relative to the ambient Bragg angle; the magnitude of the shift increased with increasing impact stress. The Bragg angle shifts and appropriate averaging procedures were used to determine the macroscopic or continuum strength following compression and release. For both crystals, the strengths upon release increased with increasing impact stress and provide a quantitative measure of the strain hardening that occurs in Al(111) and Cu(111) during the shock and release process. Our results for Al(111) are in reasonable agreement with a previous determination based solely on continuum measurements. Two points are noteworthy about the developments presented here: Synchrotron x rays are needed because they provide the resolution required for analyzing the data in the released state; the method presented here can be extended to the shocked state but will require additional measurements. [ABSTRACT FROM AUTHOR]

Photoluminescence (PL) spectra of bound excitons were measured in uniaxially strained GaP by performing shock-wave experiments at liquid nitrogen temperatures. GaP samples doped with sulfur or nitrogen were compressed up to 3 GPa when subjected to uniaxial strains along the [100] crystallographic orientation. PL lines from shallow sulfur donors redshifted upon compression, tracking the reduction in the indirect band gap. PL lines related to the isoelectronic NN1 pairs, in contrast, exhibited splitting and nonlinear blueshift. An empirical approach was used to model the NN1 behavior. It was shown that the splitting pattern is consistent with the previously proposed symmetry of NN1 defects and nonlinearities resulting from the reduction in the exciton binding energy. At high stresses, the NN1 lines disappeared due to the ionization of bound excitons. [ABSTRACT FROM AUTHOR]

Using a recently developed compact pulsed power generator (CPPG), z-cut quartz samples were subjected to shockless uniaxial strain compression to 7 GPa peak stress. Over this stress range, the behavior of z-cut quartz under shock compression [J. Appl. Phys. 88, 5671 (2000)] is nonlinear elastic and in good agreement with predictions based on the measured values of the second and third order elastic constants. In-material and free surface velocities were measured in the present work and analyzed, using a Lagrangian analysis, to provide a continuous loading curve for z-cut quartz. The wavelet speed-particle velocity (Cu-u, where Cu≡(Δh/Δt)u) results were somewhat sensitive to the CPPG panel design details, as well as the velocity data acquisition and reduction techniques. Improvements to provide optimal data were discussed and used to provide accurate longitudinal stress-density response to 7 GPa (6% density compression). The Cu-u curves, a more stringent measure of the material response to nonlinear compression, were within 0.6% of the values predicted using shock compression measurements. This excellent agreement demonstrates that the shockless compression response of quartz is indistinguishable from the shock response in the nonlinear elastic regime and provides a good foundation for comparing the inelastic response of solids under shockless and shock compression. The present work also demonstrated the use of the CPPG as a laboratory capability for examining the shockless compression of materials. [ABSTRACT FROM AUTHOR]

We describe the use of a third generation synchrotron facility to obtain in situ, real-time, x-ray diffraction measurements in plate impact experiments. Subnanosecond duration x-ray pulses were utilized to record diffraction data from pure and magnesium-doped LiF single crystals shocked along the [111] and [100] orientations. The peak stresses were 3.0 GPa for the [111] oriented LiF and between 3.0 and 5.0 GPa for the [100] oriented LiF. For these stresses, shock compression along [111] results in elastic deformation and shock compression along [100] results in elastic-plastic deformation. Because of the quality of the synchrotron x-ray pulses, both shifting and broadening of the diffraction data were obtained simultaneously. As expected, shifts for elastic compression and elastic-plastic compression in shocked LiF were consistent with uniaxial and isotropic lattice compression, respectively. More importantly, diffraction patterns from crystals shocked along [100] exhibited substantial broadening due to elastic-plastic deformation. The broadening indicates that the shocked LiF(100) crystals developed substructure with a characteristic size for coherently diffracting domains (0.1–10 μm) and a distribution of (100) microlattice-plane rotations (∼1° wide). In contrast to the LiF(100) results, broadening of the diffraction pattern did not occur for elastically deformed LiF(111). Another important finding was that the amount of lattice disorder for shocked LiF(100) depends on the loading history; the broadening was larger for the magnesium-doped LiF(100) (large elastic precursor) than for ultrapure LiF(100) (small elastic precursor) shocked to the same peak stress. The data are simulated by calculating the diffraction pattern from LiF(100) with a model microstructure consisting of coherently diffracting domains. The lattice orientation and longitudinal strain is assumed uniform within domains, but they vary from domain to domain with Gaussian distributions. Simulations using such a model are in good agreement with the measured diffraction patterns. The principal finding from the present work is that synchrotron x-rays can provide real-time data regarding microstructure changes accompanying shock-induced deformation and structural changes. [ABSTRACT FROM AUTHOR]

The relative orientations between the lattices of the low pressure (B1) and high pressure (B2) phases of shock compressed KCl single crystals were examined using plate impact loading along the [111] and the [110] directions. The B2-phase lattice planes, perpendicular to the loading direction, were determined from transient x-ray diffraction measurements. Two closely spaced diffraction peaks were observed for the [111] loading direction. The lower Bragg angle peak is consistent with a 200 peak of a cubic B2-phase unit cell giving the orientation relation [111]B1 is parallel to [100]B2. The higher Bragg angle peak is not consistent with any peak from either a cubic B1 or a cubic B2 unit cell; the origin of this peak is unknown. Other experiments found no orientation relations; for the [111] loading direction, [111]B1 is not parallel to [211]B2 and for the [110] loading direction, [110]B1 is not parallel to [100]B2. The orientation relation determined for the [111] loading in this work is incompatible with a previously determined orientation relation obtained for the [100] loading [T. d’Almeida and Y. M. Gupta, Phys. Rev. Lett. 85, 330 (2000)]. This finding suggests that the transformation pathway between the B1 and B2 lattices in KCl crystals depends on the shock compression direction for the B1 phase. The results are discussed in terms of the compatibility between the macroscopic uniaxial strain imposed by shock wave loading and the microscopic rearrangement of atoms leading to the observed orientation relations. [ABSTRACT FROM AUTHOR]

To gain insight into the instability and trajectory change in projectiles penetrating dry sand at high velocities, two dimensional plane strain mesoscale simulations were carried out using representative models of a particulate system and of a small projectile. A program, ISP-SAND, was developed and used to generate the representative particulate system with mean grain sizes of 60 and 120 μm as well as ±30% uniform size distribution from the mean. Target porosities ranged from 30% to 40%. The penetration of ogive nose steel projectiles with caliber radius head of 3.5 and length-to-diameter (l/d) ratio of 3.85 was simulated using the updated Lagrangian explicit parallel finite element code ISP-TROTP. Deformation of the projectile and individual sand grains was analyzed using a nonlinear elastic-inelastic model for these materials. Grain-grain and grain-projectile interactions were analyzed using a contact algorithm with and without friction. Projectile instability was quantified and compared using the lateral displacement of the center of mass, lateral force acting on the projectile, and its rotational momentum about the center of mass. The main source of projectile instability and the ensuing trajectory change in the penetration simulations was found to be the inhomogeneous loading of the projectile due to the heterogeneities and randomness inherent in a particulate media like sand. The granularity of the media has not been considered explicitly in previous work. Projectile instability increased with impact velocity, as expected. However, it also increased for the case of elastic impactor that preserved the nose shape, with an increase in grain size, and for uniform grain sizes. Moreover, friction, inherently present in geologic materials, was found to be a major contributor to instability. Conclusions derived from one projectile depth simulations were confirmed by two deeper penetration simulations considering up to three full lengths of penetration (requiring a larger sand target). The deep penetration simulation predicted considerable instability with a trajectory change of approximately 45° when friction was considered in the dry sand medium. An overall conclusion of this work is that projectile penetration studies in geologic materials need to explicitly consider the heterogeneous or particulate nature of these materials. [ABSTRACT FROM AUTHOR]

Impulsive stimulated thermal scattering (ISTS) was used to determine the complete set of second-order elastic constants for pentaerythritol tetranitrate (PETN) and cyclotrimethylene trinitramine (RDX) single crystals. Despite the weak scattering efficiency of these materials, excellent signal quality was obtained by using an optical heterodyne detection approach. The elastic constants for PETN agree well with previous values obtained from ultrasonic velocity measurements. The elastic constants for RDX are consistent with previous values obtained from ultrasonic velocity measurements and from resonant ultrasound spectroscopy, but show significant differences with values obtained from Brillouin scattering data. The present results demonstrate that the ISTS method, with optical heterodyne detection, provides a useful and accurate approach for determining the elastic constants of energetic crystals. [ABSTRACT FROM AUTHOR]

Time-resolved x-ray diffraction measurements were used to examine the lattice deformation during elastic-plastic deformation in Mg-doped (approximately 100 ppm) LiF single crystals shocked along [100]. The magnesium impurities significantly increase the elastic limit of the LiF crystals, as compared to the low values observed for ultrapure LiF crystals, leading to a large amplitude elastic wave and significant stress relaxation behind the elastic wave. The objective of the current work was to examine lattice deformation throughout this wave profile using time-resolved, x-ray diffraction methods (2 ns resolution) for plate impact experiments to gain insight into time-dependent, elastic-plastic deformation at the microscopic level. The diffraction data were analyzed using an x-ray model coupled to an existing wave propagation code that incorporated dislocation mechanisms for elastic-plastic deformation including stress relaxation. All experimental results revealed a uniaxial lattice compression at the elastic wave front followed by a rapid transition toward isotropic unit cell compression during stress relaxation. Furthermore, comparison between the experimental data and the calculated streak records indicated that the lattice transition proceeds at a faster rate than predicted by the model. Further implications of these results are discussed. [ABSTRACT FROM AUTHOR]

Plate impact experiments were conducted to produce two and three step shock wave loadings in [100] ultrapure, lithium fluoride (LiF) crystals to examine the role of loading history on optical window response in laser interferometry measurements. Peak compressive stresses ranged between 5.0 and 17.5 GPa, and the window response was characterized by measuring the difference between the apparent and actual velocities of reflecting surfaces by using a velocity interferometer. In some experimental configurations, this velocity correction was obtained independently from the projectile velocity. Our results show that the velocity correction in [100] lithium fluoride windows can be described in all cases by a single linear relation, Δu=(0.2739±0.0016)u. Because this correction is independent of the loading history, it is applicable to arbitrary loading, which includes ramp-wave or shockless compression. By using the velocity correction and the measured particle and shock velocities, we have also determined the density dependence of the refractive index for [100] lithium fluoride at 532 nm to be n=(1.2769±0.0024)+(0.0443±0.000 82)ρ. [ABSTRACT FROM AUTHOR]

Plate-impact experiments were performed to examine the influence of grain size on the dynamic tensile (or spall) behavior of shocked polycrystalline aluminum. Ultrapure and commercially pure 1050 aluminum plates were cold rolled to 80% strain and heat treated under predetermined conditions to produce recrystallized samples with average grain sizes varying between 49 and 453 μm. Well-characterized samples were subjected to plane wave loading at peak compressive stresses of 4 and 21 GPa, and free-surface velocity profiles were obtained using velocity interferometry. At 4 GPa, the observed pullback velocity, a characteristic feature of the spall response, was similar for different grain sizes of 1050 and ultrapure Al, suggesting that the preferential failure mode is intragranular. At 21 GPa, the spall response (i.e., the pullback velocity and the signal structure) depended on the alloy content; the pullback velocity of ultrapure Al increased with increase in grain size, while it remained constant for 1050 Al. In addition, the structure of pullback signals showed a well-defined change in slope for different grain size samples in ultrapure Al, while no such feature was observed for 1050 Al. For the grain sizes examined, the σHEL was nearly independent of the grain size for 1050 Al and beyond a certain grain size for ultrapure Al. [ABSTRACT FROM AUTHOR]

Dynamic tensile response and fracture of a Zr-based bulk amorphous alloy (BAA) were examined by subjecting samples to uniaxial tensile strain in plate-impact experiments. Following elastic compressive loading to peak stresses ranging between 3.9 and 6.1 GPa, wave interactions produced tensile loading resulting in spallation in the BAA samples. Rear-surface velocity histories, obtained using laser interferometry, provided a real-time measure of the tensile response including spallation. The initial tensile loading was elastic (loading rates approximately 8×105 s-1) and the data were analyzed to obtain a nonlinear, tensile stress-strain relation. Tensile fracture or spall, observed in all experiments, was initiated at a tensile stress of 3.8±0.3 GPa; this initiation value was independent of the impact stress and is significantly higher than that observed for crystalline metals. A phenomenological tensile fracture model was incorporated into one-dimensional wave propagation simulations to gain insight into the BAA tensile response and damage. Good agreement was obtained between the numerical simulations and the experimental measurements. With increasing impact stress, the BAA samples exhibited a change from ductile to brittle tensile response. [ABSTRACT FROM AUTHOR]

Two-dimensional (2D) mesoscale simulations of planar shock compression, followed by either reloading or unloading, are presented to examine and understand the quasielastic response observed experimentally in shocked polycrystalline aluminum. The simulations included a realistic representation of the grain ensembles in polycrystalline samples to identify heterogeneous deformation features deemed important to model the continuum measurements. The simulations were carried out using a 2D Lagrangian finite element code (ISP-TROTP) that incorporated elastic-plastic deformation in grain interiors and utilized a contact/cohesive methodology to analyze the response of finite strength grain boundaries. Local heterogeneous response due to mesoscale features was quantified by calculating appropriate material variables along in situ Lagrangian tracer lines and comparing the temporal variation of their mean values with results from 2D continuum simulations. A series of initial calculations ruled out effects due to finite element size and width of the representative volume element used in our simulations. Simulations using a variety of heterogeneities were performed to identify the heterogeneities that were most important for simulating the experimentally observed quasielastic response. These were inclusions, hardened grain boundaries, and microporosity. Mesoscale simulations incorporating these effects demonstrate that the shock-deformed state in polycrystalline aluminum is strongly heterogeneous with considerable variations in lateral stresses. The simulated velocity profiles for a representative reloading and unloading experimental configuration were found to agree well with experimental data, and suggest that hardened grain boundaries are the most likely source of mesoscale heterogeneities in shocked 6061-T6 aluminum. The calculated shear strength and shear stresses were also found to be in good agreement with the reported experimental values. [ABSTRACT FROM AUTHOR]

Plane shock wave experiments were performed on bulk amorphous alloy (BAA) samples having a nominal composition of Zr56.7Cu15.3Ni12.5Nb5.0Al10.0Y0.5. Peak compressive stresses ranged from 4 to 16.4 GPa. Piezoelectric pins and a velocity interferometer were used to measure elastic shock speeds and particle velocity histories, respectively. The elastic Hugoniot curve was determined and the Hugoniot elastic limit (HEL) was measured to be approximately 7.0 GPa, a value significantly higher than expected from quasistatic uniaxial stress data. Impact loading beyond the HEL results in a distinct two wave structure due to elastic-plastic deformation. Our data also show clear evidence for strength loss under shock loading above the HEL. Unlike most metals, the present data show distinct elastic response during unloading. We present a continuum model to describe the deformation response of the BAA to shock loading. Simulations using this time-dependent, strain-softening strength model were able to successfully match the measured wave profiles. The calculated profiles indicate that the characteristic time for stress relaxation is very small (few nanoseconds) and confirm that a significant loss of strength occurs as the plastic wave propagates through the material. When the material is shocked to a peak stress of 16.4 GPa, our model indicates that the yield stress in the shocked state is reduced by at least 0.7 GPa from its value of 2.7 GPa at the HEL. [ABSTRACT FROM AUTHOR]

X-ray diffraction measurements, utilizing multiple and single diffraction methods, were used to examine lattice compression of magnesium doped (approximately 100 ppm) LiF single crystals shocked along the [100] direction. Unlike ultrapure LiF crystals, examined in previous x-ray diffraction studies, magnesium doped crystals display large elastic wave amplitudes under shock wave compression. Analysis of multiple diffraction data from the (200) and (202) planes and single diffraction data from the (200) planes showed that the crystal lattice for doped crystals was compressed isotropically in the peak state, similar to that observed for ultrapure crystals. This agreement demonstrates that the large elastic wave amplitude and subsequent stress relaxation observed in wave propagation measurements for the doped LiF crystals are a transient phenomenon and do not appear to affect the lattice compression in the final state. Implications of this finding are discussed. [ABSTRACT FROM AUTHOR]

A nonlinear anisotropic continuum framework for describing the thermoelastic-plastic response of single crystals shocked along arbitrary orientations has been developed. Our modeling approach incorporates nonlinear elasticity, crystal plasticity, and thermodynamic consistency within an incremental tensor formulation. Crystal plasticity was incorporated by considering dislocation motion along specified slip planes. The theoretical developments presented here are sufficiently general to also accommodate other types of inelastic deformation mechanisms. As representative applications of the theoretical developments, numerical simulations of shock wave propagation in lithium fluoride (LiF) and copper single crystals are presented and compared to wave profile data for several crystal orientations. Simulations of shock wave propagation along low-symmetry directions, where data are not available, are also presented to examine the propagation of quasilongitudinal and quasishear waves in crystals undergoing elastic-plastic deformation. Temperature calculations for the shocked single crystals are discussed. [ABSTRACT FROM AUTHOR]

Plane shock wave experiments were performed to examine optical transmission in sapphire single crystals (c cut, a cut, and r cut) compressed to longitudinal stresses ranging between 119 and 260 kbar. Peak stress and particle velocity values in our experiments were obtained by compiling and analyzing published continuum data on shocked sapphire crystals. Time-resolved measurements, with nanosecond resolution, showed time-, stress-, and orientation-dependent changes in optical transmission beyond the Hugoniot elastic limit. Over the 300–680-nm range examined in our work, no wavelength dependence was observed. Loss of optical transmission in the stress range examined is due to inelastic deformation in shocked sapphire crystals. The present data reveal that inelastic deformation mechanisms are different in r-cut sapphire compared to a-cut and c-cut sapphire, and may be different for all three orientations examined. Although quantitative analysis of the time-dependent optical transmission data is not possible at present, optical transmission results have provided insight into inelastic deformation mechanisms in shocked sapphire. Present results are expected to be valuable for use of sapphire windows in shock wave experiments. [ABSTRACT FROM AUTHOR]

Plate impact experiments were carried out to understand inelastic deformation in soda-lime glass shocked between 3 and 10.8 GPa. In-material, wave profile measurements were obtained using longitudinal and lateral stress gauges (4.6–10.8 GPa), and electromagnetic particle velocity gauges (2.9–6 GPa) at comparable sample thicknesses. The 4.6 and 6 GPa experiments revealed time-dependent material inelastic response along with time-dependent loss of material strength. Because of the unsteady, two wave structure observed in the longitudinal wave profiles in conjunction with the time-dependent changes in the lateral stress data, previous interpretations of the shocked soda-lime glass response in terms of a propagating failure wave are not valid. At higher peak stresses (∼10 GPa), the experimental results do not display time-dependent strength loss. The shock wave response of soda-lime glass over the 4–10 GPa range is complex, and material strength and inelastic deformation features depend significantly on the peak stress. Using the experimental results, a phenomenological continuum model incorporating the various material phenomena was developed. Wave profile simulations using the continuum model show reasonable overall agreement with the experimental profiles at different stress levels. Because of the approximate nature of the continuum model, all of the experimental details were not reproduced in the wave propagation simulations. It is likely that around and above 10 GPa, other material phenomena not included in our model may need to be considered. [ABSTRACT FROM AUTHOR]

An anisotropic continuum framework for describing the nonlinear thermoelastic response of shocked single crystals has been developed. Using this framework, the propagation of large amplitude stress waves along arbitrary directions in crystals of any symmetry can be examined. We developed an incremental approach, where the reference state for the dynamical variables and the material properties is updated throughout the calculation. Results from our incremental approach are in excellent agreement with the results from a finite strain formulation. Using a finite-difference wave propagation code, we performed numerical simulations of large amplitude elastic wave propagation in single crystals. Results of impact loading simulations for quartz and sapphire single crystals are presented. The framework developed was also useful for examining the propagation of pure mode nonlinear waves for various crystal symmetries. Our calculations showed that pure longitudinal waves resulted from impact loading along any rotational symmetry axis for all crystal symmetry classes. Differences between pure mode wave propagation for linear and nonlinear elastic deformation are discussed. [ABSTRACT FROM AUTHOR]

Experimental methods to permit continuous, time-resolved x-ray diffraction measurements in plate impact experiments were developed and used to examine lattice deformation in shock compressed LiF single crystals. Using an x-ray streak camera diffraction data with 2-4 ns resolution were obtained from crystals subjected to both shock and ramp wave loading along the [111] direction. Because of the penetration depth of x rays into the sample, interpretation of the ramp wave loading data required an analytic model to simulate the results. The penetration depth used in the model was determined experimentally from the time-resolved shock wave loading data. Good agreement between the simulations and experimental data was found for both loading conditions, suggesting that the analytic model has broad applicability. [ABSTRACT FROM AUTHOR]

The ordinary refractive index of sapphire, at 532 nm, was measured in single crystals subjected to elastic, uniaxial tensile (and compressive) strain along the c axis in plate impact experiments. The refractive index under both tension and compression was determined from the difference between the uncorrected particle velocities from velocity interferometer measurements and the known particle velocities in symmetric impact experiments. Results to a peak stress of 100 kbar in tension and compression show that the refractive index has an overall monotonic, nonlinear dependence on density that is asymmetric about the ambient density. In compression, the refractive index decreases by an extremely small amount; in contrast, the change is relatively larger in tension. These refractive index results at large tensile stresses suggest that sapphire polarizability is quite different under compression and tension. Compression measurements by themselves can be approximated well by a linear refractive index-density relation, resulting in a window correction (at 532 nm) that is a constant fraction of the true particle velocity up to at least 115 kbar longitudinal stress. The present data also provide high-precision shock velocity results in compression and demonstrate the appropriateness of using second and third order elastic constants for calculating the elastic response of sapphire under uniaxial tensile strain loading. [ABSTRACT FROM AUTHOR]

Initial stage kinetics of the cadmium sulfide (CdS) phase transition was investigated using picosecond time-resolved electronic spectroscopy in plate-impact shock wave experiments. Real-time changes in the electronic spectra were observed, with 100 ps time resolution, in CdS single crystals shocked along a and c axes to stresses ranging between 35 and 90 kbar, which is above the phase-transition threshold stress of approximately 30 kbar. Significant difference in the transformation kinetics was observed for the two crystal orientations. At sufficiently high instantaneous stress, above approximately 60 to 70 kbar for a axis and 50 kbar for c axis, transformation to a metastable state appears to reach a constant state within the 100 ps time resolution. At lower instantaneous stresses, an incubation period on the order of several nanoseconds is observed prior to the onset of electronic changes that mark the onset of the structural change. The subsequent increase in absorbance was quite rapid, with a constant state being reached within the first few nanoseconds after the onset of the structural changes. These results suggest that the nucleation process determines the transformation rate. This insight into transformation kinetics, along with the transformation mechanism obtained from the high-stress experiments, was used to develop a phenomenological model, incorporating ideas of nucleation and growth in martensitic transformations, to simulate the time-dependent extinction of light observed in our experiments. The calculational results incorporating both extinction due to light absorption by the daughter phase volumes and scattering of light by small volumes of the daughter phase were in good agreement with experimental observations. Finally, the orientational differences observed in the transformation kinetics were interpreted in terms of the differences in the elastic-plastic response for the two orientations. [ABSTRACT FROM AUTHOR]

A consistent thermomechanical material model was developed for unreacted ammonium perchlorate (AP) single crystals for shock compression normal to the (210) and (001) crystal planes. Building on previous work, the mechanical response for both orientations was described using a single isotropic elastic-plastic model and an overstress model to describe rate-dependent yielding. Velocity interferometer measurements to 12 GPa were performed to extend the AP Hugoniot curve to higher stresses. The specific heat c[sub v], the coefficient of thermal pressure (∂P/∂T)[sub V], and the isothermal bulk modulus B[sub T] were determined from Hugoniot and isothermal compression curves, along with available data at atmospheric pressure. Time-resolved Raman spectroscopy experiments were carried out under stepwise loading to obtain temperatures in the shocked state. Calculated temperatures using our material model are in good agreement with the temperatures obtained from our experiments, thus providing validation for our modeling approach. © 2002 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Sapphire crystals were shocked to 190 kbar along the a axis to characterize their use as optical windows, for velocity interferometry measurements, up to their Hugoniot elastic limit. When partially polarized light is incident on the samples, birefringence in the material is manifested as a beat frequency in the probe light that is returned from the specimens. Proper procedures for interpreting the velocity interferometry data for various polarization conditions were developed. The refractive indices at 514.5 nm wavelength decreased linearly with the density. The data were analyzed to yield three photoelastic coefficients: p[sub 12], p[sub 31], and p[sub 41]. Calibration is developed for any polarization state of the probe light. Particle velocity wave forms are consistent with elastic behavior up to 170 kbar shock stress, and evidence of deviation from elastic behavior is present at 190 kbar impact stress. High precision shock velocity measurements are reported to 170 kbar stress along the a axis. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Compression and shear wave experiments using plate impact loading were conducted on polycrystalline silicon carbide (SiC). The material was subjected to combined compression-shear loading to peak compressive stresses ranging from 3 to 18 GPa. The compression (shock) wave profiles and the propagation velocities of shear and longitudinal release waves in the shocked SiC were measured using in situ, electromagnetic velocity gauges. The Hugoniot elastic limit (HEL) of the material was found to be 11.5±0.4 GPa. The measured wave velocities were used to determine the elastic moduli of the material as functions of density compression in the shocked state. The data were further analyzed to obtain the mean stress response of the SiC under uniaxial-strain compression. The longitudinal and mean stress results completely characterize the material stress state. Numerical simulations were also carried out to verify the peak-state data analysis. Our results show that the Poisson's ratio of the material increases with elastic shock compression from an ambient value of 0.161 to 0.192 at the HEL. Above the elastic limit, the maximum shear stress supported by the material increases from 4.5 to 6.4 GPa at a peak stress of 18 GPa. This finding verifies independently the results from lateral manganin gauge measurements in the same material [R. Feng et al., J. Appl. Phys. 83, 79 (1998)]. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Presents a study that examined the loading and unloading response of manganin gauges oriented parallel and perpendicular to the shock front and embedded in an aluminum-oxide matrix. Details of the application of an analytic model for piezoresistance response coupled with electromechanical constants in calculating the gauge response; Analysis of residual resistance measurements; Results and implications.