Search

High-energy pulsed X-ray momentum coupling is a promising technology for early deflection of NEOs (Near Earth Objects) that might impact Earth. Analytic models for the radiation interactions can often preclude the need for large hydrocode analyses, and offer the advantage of many simple calculations that reveal important features of the nonlinear phenomena, e.g., thresholds, peak coupling, and high-energy scaling limits. However, model validation is an important element. One such model is used to analyze relevant experiments conducted on the Sandia Z-pinch machine. Samples were exposed to X-ray pulses approximating a 200-eV blackbody at fluences of ~1 kJ/cm². Target momenta were measured. Model calculations give impulse couplings somewhat greater than the data, but a more appropriate value for the one uncertain model parameter (the effective target decomposition energy), can account for this discrepancy. The analytic model is thus appropriate for system-level parameter studies that will be important constituents of all NEO mitigation investigations. [ABSTRACT FROM AUTHOR]

Velocity-time histories of free- or windowed-surfaces have been used to calculate wave speeds and hence deduce the shear moduli for materials at high pressure. This is important to high velocity impact phenomena, e.g. shaped-charge jets, long rod penetrators, and other projectile/armour interactions. Historically the shock overtake method has required several experiments with different depths of material to account for the effect of the surface on the arrival time of the release. A characteristics method, previously used for analysis of isentropic compression experiments, has been modified to account for the effect of the surface interactions, thus only one depth of material is required. This analysis has been applied to symmetric copper impacts performed at Sandia National Laboratory's Star Facility. A shear modulus of 200GPa, at a pressure of ~180GPa, has been estimated. These results are in broad agreement with previous work by Hayes et al. [ABSTRACT FROM AUTHOR]

Using a line-imaging VISAR to infer the position (x) and time (t) dependent distension of a spalling sample requires two assumptions: (1) a calculated velocity surface v[no spall] (x, t) for the no-spall case to compare with the observed v [observed](x, t) surface, and (2) a lack of significant wave processing by the near-surface microstructure. We have designed and are conducting a matrix of experiments to evaluate these assumptions. In each experiment, we use a line-imaging VISAR to measure the velocity history of carefully characterized tantalum and copper samples taken to an incipient spall condition. The pre-shot characterization included spatially resolved mapping of grain locations and orientations by electron backscatter diffraction (EBSD). These samples are then soft-recovered and sectioned along the same line as monitored by the line-imaging VISAR. An initial pair of experiments provided ∼1 mm of spall separation; we are preparing further experiments with incipient spall conditions. [ABSTRACT FROM AUTHOR]

A suite of impact experiments was conducted to assess spatial and shot-to-shot variability in dynamic properties of tantalum. Samples had a uniform refined ∼ 20 micron grain structure with a strong axisymmetric [111] crystallographic texture. An HEL amplitude of 2.8 GPa (corresponding to Y ≈ 1.5 GPa) was observed. Free-surface spall experiments showed clear wave attenuation and spallation phenomena. Here, loading stresses were ∼ 12.5 GPa and various ratios of impactor to target thicknesses were used. Spatial and shot-to-shot variability of the spall strength was ±20%, and of the HEL, ±10%. Experiments conducted with smaller diameter flyer plates clearly showed edge effects in the line and point VISAR records, indicating lateral release speeds of roughly 5 km/s. © 2006 American Institute of Physics [ABSTRACT FROM AUTHOR]

We present new data on the unreacted approximate isentrope of the HMX-based explosive LX-04, measured to 170 kbar, using newly developed long pulse isentropic compression techniques at the Sandia National Laboratories Z Machine facility. This study extends in pressure by 70% the previous state of the art on unreacted LX-04 using this technique. This isentrope will give the unreacted Hugoniot from thermodynamic relations using a Gruneisen equation of state model. The unreacted Hugoniot of LX-04 is important in understanding the structure of the reaction front in the detonating explosive. We find that a Hugoniot given by US = 2.95 km/s + 1.69 uP yields for an isentrope a curve which fits our LX-04 ICE data well. © 2004 American Institute of Physics [ABSTRACT FROM AUTHOR]

A new, non-radiographic diagnostic has been developed that appears to provide information on multiple spall and damage layers in metals. The velocities of multiple layers (up to 5 in copper) can be determined using this method, with additional information possible on damaged material between layers at densities less than the bulk metal value. Metals that are melted on release (tin) also seem to exhibit a distinctive signature that is quite different from conventional multi-layer spall. Experimental results on these metals and proton radiographs confirming these results are also presented. © 2004 American Institute of Physics [ABSTRACT FROM AUTHOR]

Experimental results of shock wave effects from high intensity (70 -215 GW) soft X-ray irradiation on several meteorite targets are presented. From inhomogeneous materials, useful data on particle velocity and in-situ velocity were obtained and permitted the computation of the yield stress, shock wave velocity, compression, as well as the momentum and energy coupling coefficients. [ABSTRACT FROM AUTHOR]

Multi-dimensional impact tests were conducted on the ferroelectric ceramic PZT 95/5 and alumina-loaded epoxy (ALOX) encapsulants, with the purpose of providing benchmarks for material models in the ALEGRA wavecode. Diagnostics used included line-imaging VISAR (velocity interferometry), a key diagnostic for such tests. Results from four tests conducted with ALOX cylinders impacted by nonplanar copper projectiles were compared with ALEGRA simulations. The simulation produced approximately correct attenuations and divergence, but somewhat higher wave velocities. Several sets of tests conducted using PZT rods (length:diameter ratio = 5:1) encapsulated in ALOX, and diagnosed with line-imaging and point VISAR, were modeled as well. Significant improvement in wave arrival times and waveforms agreement for the two-material multi-dimensional experiments was achieved by simultaneous multiple parameter optimization on multiple onedimensional experiments. Additionally, a variable friction interface was studied in these calculations. We conclude further parameter optimization is required for both material models. [ABSTRACT FROM AUTHOR]

Three sets of LX-04 samples of 0.18 and 0.49 mm nominal thicknesses were all dynamically loaded by Sandia's Z-accelerator with a ramp compression wave with a 200 ns rise time and about 160 kb peak stress. The LX-04/lithium fluoride samples interface velocities were measured using VISAR's. Comparisons of experimental and computational results will be given. Compression and release isentropes both show some reaction and kinetic behavior of the LX-04. Experiments were also performed on ultra-fine TATB where PMMA windows were used. Future experiments on single crystals of HMX that are designed to measure the phase transition at high pressures will be discussed. [ABSTRACT FROM AUTHOR]

Gas gun experiments combined with velocity interferometric techniques have been used to experimentally determine the loading behavior of sleeved Cercom silicon carbide rods. Graded density materials have been used as impactors, thereby eliminating the tension states generated by the radial stress components during the loading phase. Results of these experiments demonstrate that the time-dependent stress pulse generated during impact allows an efficient transition from the initial uniaxial strain loading to a uniaxial stress state as the stress pulse propagates through the rod allowing access to intermediate loading rates. Intermediate loading rates obtained in this configuration lie between split Hopkinson bar and shock-loading techniques. [ABSTRACT FROM AUTHOR]

Shock-induced depoling of the ferroelectric ceramic PZT 95/5 is utilized in a number of pulsed power devices. Several experimental and theoretical efforts are in progress in order to improve numerical simulations of these devices. In this study we have examined the shock response of normally poled PZT 95/5 under uniaxial strain conditions. On each experiment the current produced in an external circuit and the transmitted waveform at a window interface were recorded. The peak electrical field generated within the PZT sample was varied through the choice of external circuit resistance. Shock pressures were varied from 0.6 to 4.6 GPa, and peak electrical fields were varied from 0.2 to 37 kV/cm. For a 2.4 GPa shock and the lowest peak field, a nearly constant current governed simply by the remanent polarization and the shock velocity was recorded. Both decreasing the shock pressure and increasing the electrical field resulted in reduced current generation, indicating a retardation of the depoling kinetics. [ABSTRACT FROM AUTHOR]