R. S. Hixson

Direct measurements of the α-ϵ transition stress and kinetics for shocked iron

Iron undergoes a polymorphic phase transformation from the α-phase (bcc) to the ϵ-phase (hcp) when compressed to stresses exceeding 13 GPa. Because the epsilon phase is denser than the alpha phase, a single shock wave is unstable and breaks up into an elastic wave, a plastic wave, and a phase transition wave. Examination of this structured wave coupled with various phase transformation models has been used to indirectly examine the transition kinetics. Recently, multimillion-atom molecular dynamics (MD) simulations have been used to examine the shock-induced transition in single crystal iron illustrating an orientation dependence of the transition stress, mechanisms, and kinetics. The objective of the current work was to perform plate impact experiments to examine the shock response of polycrystalline and single crystal iron with nanosecond resolution for impact stresses spanning the α-ϵ transition. The current data reveal an orientation dependence of the transition stress coupled with a transition time t...

Quasielastic release in shock‐compressed solids

Shock‐ and release‐wave measurements are reported for 6061‐T6 aluminum [J. R. Asay and L. C. Chhabildas, in Shock Waves and High‐Strain‐Rate Phenomena in Metals, edited by M. A. Meyers and L. E. Murr (Plenum, New York, 1981), pp. 417–431], oxygen‐free‐electronic copper, and a Si‐bronze alloy. Significant departure from ideal elastic‐plastic response is observed in all three materials. Experimentally determined release‐wave profiles show evidence for the onset of reverse plastic flow immediately upon release from the shocked state. This phenomenon is analyzed in terms of internal stresses acting on straight dislocation pileups and pinned dislocation loops created by the shock‐compression process. Following shock compression and prior to release, the internal stresses are opposed by the applied shear stress; that is, they exactly balance each other and no plastic flow occurs. As the applied stress is reduced in the unloading wave, reverse plastic flow occurs immediately due to internal reverse stresses acti...

Determining the refractive index of shocked [100] lithium fluoride to the limit of transmissibility

Lithium fluoride (LiF) is a common window material used in shock- and ramp-compression experiments because it displays a host of positive attributes in these applications. Most commonly, it is used to maintain stress at an interface and velocimetry techniques are used to record the particle velocity at that interface. In this application, LiF remains transparent to stresses up to 200 GPa. In this stress range, LiF has an elastic-plastic response with a very low (<0.5 GPa) elastic precursor and exhibits no known solid-solid phase transformations. However, because the density dependence of the refractive index of LiF does not follow the Gladstone-Dale relation, the measured particle velocity at this interface is not the true particle velocity and must be corrected. For that reason, the measured velocity is often referred to as the apparent velocity in these types of experiments. In this article, we describe a series of shock-compression experiments that have been performed to determine the refractive index of LiF at the two most commonly used wavelengths (532 nm and 1550 nm) between 35 and 200 GPa to high precision. A modified form of the Gladstone-Dale relation was found to work best to fit the determined values of refractive index. In addition, we provide a direct relationship between the apparent and true particle velocity to correct experimentally obtained wave profiles by others using these velocimetry techniques.

Overdriven‐detonation and sound‐speed measurements in PBX‐9501 and the ‘‘thermodynamic’’ Chapman–Jouguet pressure

Sound speeds, at pressure, and the overdriven Hugoniot were measured for the plastic‐bonded explosive PBX‐9501. The two curves intersect at the Chapman–Jouguet (CJ) state because of the sonic condition D=c+u. This permitted a novel determination of the ‘‘thermodynamic’’ CJ pressure. A value of 34.8±0.3 GPa was obtained. The data permit a direct experimental determination of the isentropic gamma, γS=−(∂lnP/∂lnV)S, and the Gruneisen parameter, γ=V(∂P/∂E)V, in the overdriven pressure range.

Transient impact response of three polymers

Experimental data are presented for the transient response of three polymers, Estane, Estane with a nitro-plasticizer, and adiprene, subject to flat-plate impact. These data are analyzed in terms of a generalized Maxwell model and the theory of characteristics for the dynamical motion. Comparison of theory and experiment quantifies the high-frequency component in the Maxwell description; for Adiprene the high-frequency shear modulus and relaxation time are 300 MPa and approximately 50 ns, respectively. For estane the corresponding quantities are 300 MPa and approximately 200 ns. Low-pressure Hugoniot data are also presented for estane. An estimate of the low-pressure Hugoniot is given for Adiprene. Some experimental data are also presented for the Estane/nitro-plasticizer combination; this material is used as a binder in plastic-bonded explosives. Theory is presented for a thermodynamically consistent calculation of temperature in high-rate loading of polymers whose mechanical properties are represented b...

Influence of impurities on the α to ω phase transition in zirconium under dynamic loading conditions

Experiments have been carried out to investigate the α to ω phase transition in polycrystalline zirconium (Zr) metal. Samples with three levels of impurities were shock loaded using both gas-driven and powder-driven guns and ramp loaded using magnetic drive to investigate the effects of impurities on the properties of this solid-solid phase transition. Samples were backed by either sapphire or LiF windows and data were obtained using a VISAR to record the particle velocity at the sample/window interface. The results of these experiments showed significant differences in both the transition stress and kinetics for samples with different impurity levels demonstrating that impurities play an important role in the transformation mechanism in zirconium.

Release path temperatures of shock-compressed tin from dynamic reflectance and radiance measurements

Dynamic reflectance and radiance measurements were conducted for tin samples shock compressed to 35 GPa and released to 15 GPa using high explosives. We determined the reflectance of the tin samples glued to lithium fluoride windows using an integrating sphere with an internal xenon flashlamp as an illumination source. The dynamic reflectance (R) was determined at near normal incidence in four spectral bands with coverage in visible and near-infrared spectra. Uncertainties in R/R0 are <2%, and uncertainties in absolute reflectance are <5%. In complementary experiments, thermal radiance from the tin/glue/lithium fluoride interface was recorded with similar shock stress and spectral coverage as the reflectance measurements. The two sets of experiments were combined to obtain the temperature history of the tin surface with an uncertainty of <2%. The stress at the interface was determined from photonic Doppler velocimetry and combined with the temperatures to obtain temperature-stress release paths for tin. W...

Sound speed measurements in tantalum using the front surface impact technique

Shock compression experiments were performed on tantalum to determine the longitudinal sound speed on the Hugoniot from 36 to 105 GPa. Tantalum samples were impacted directly on to lithium fluoride windows at velocities ranging from 2.5 to 5.0 km/s and the resulting particle velocity profiles at the sample/window interface were recorded using optical velocimetry techniques. The time of arrival of the rarefaction wave from the back surface of the tantalum sample was then used to determine the longitudinal sound speed at the corresponding impact stress. In contrast to recently reported work, we see no evidence of a phase transition in the tantalum in this stress range.

Spall response of single-crystal copper

We performed a series of systematic spall experiments on single-crystal copper in an effort to determine and isolate the effects of crystal orientation, peak stress, and unloading strain rate on the tensile spall strength. Strain rates ranging from 0.62 to 2.2 × 106 s−1 and peak shock stresses in the 5–14 GPa range, with one additional experiment near 50 GPa, were explored as part of this work. Gun-driven impactors, called flyer plates, generated flat top shocks followed by spall. This work highlights the effect of crystal anisotropy on the spall strength by showing that the spall strength decreases in the following order: [100], [110], and [111]. Over the range of stresses and strain rates explored, the spall strength of [100] copper depends strongly on both the strain rate and shock stress. Except at the very highest shock stress, the results for the [100] orientation show linear relationships between the spall strength and both the applied compressive stress and the strain rate. In addition, hydrodynam...

Dislocation‐drag contribution to high‐rate plastic deformation in shock‐loaded tantalum

Time‐resolved plastic waves in plate‐impact experiments give information on the relationship between applied shear stress and plastic strain rate at low plastic strain. This information is essentially different from that obtained at intermediate strain rates using Hopkins on bar techniques, because in the former case the material deformation state is driven briefly into the regime dominated by dislocation drag rather than thermal activation. Two VISAR records of the particle velocity at the tantalum/sapphire (window) interface are obtained for symmetric impact producing peak in situ longitudinal stresses of approximately 75 kbar and 111 kbar. The risetimes of the plastic waves are about 100 ns and 60 ns, respectively, with peak strain rates of about 2×105/s and 1×106/s, respectively, as determined by weak‐shock analysis [Wallace, Phys. Rev. B 22, 1487 (1980), and Tonks, Los Alamos DataShoP Report LA‐12068‐MS (1991)]. These data show a much stronger dependence of plastic strain rate on applied shear stress...