Tom F. Thornhill

Equation of state measurements of materials using a three-stage gun to impact velocities of 11 km/s

Results of an experimental series performed utilizing a three-stage gun to obtain precise material property equation of state (EOS) data for a titanium alloy (Ti6-Al-4V) at extreme pressure states that are not currently attainable using conventional two-stage light-gas gun technology is reported herein. What is new is the technique being implemented for use at engagement velocities exceeding 11 km/s. Shock-velocity in the target is being determined using 100 μm diameter fiber-optic pins and measuring shock transit times over a known distance between two parallel planes. These fiber-optic pins also indicate that the flyer-plate bow and tilt is comparable to two-stage light-gas gun technology. The thermodynamic state of the flyer plate prior to impact has also been determined both experimentally and calculationally. In particular, the temperature, and hence the density of the flyer-plate is also well known prior to impact. Results of these studies indicate that accurate Hugoniot information can be obtained using the three-stage light gas gun. This new test-methodology has extended the EOS of Ti6-Al-4V titanium alloy to stresses up to approximately 250 GPa.

DYNAMIC COMPACTION OF SAND

The dynamic compaction of sand was investigated experimentally and computationally to stresses of 1.8 GPa. Experiments were performed in the partial compaction regime at impact velocities from 0.25 to 0.75 km/s. Multiple velocity interferometry probes were used on the rear surface of a stepped target to obtain an accurate measurement of shock velocity, and impedance matching was used to deduce the shock Hugoniot state. Wave profiles were further examined for estimates of reshock states, and a relationship between stress and rise time of the shock was deduced. Experimental results were used to fit parameters for the P‐α and P‐λ models for porous materials in CTH.

Shock Response of Dry Sand

The dynamic compaction of sand was investigated experimentally and computationally to stresses of 1.8 GPa. Experiments have been performed in the powders partial compaction regime at impact velocities of approximately 0.25, 0.5, and 0.75 km/s. The experiments utilized multiple velocity interferometry probes on the rear surface of a stepped target for an accurate measurement of shock velocity, and an impedance matching technique was used to deduce the shock Hugoniot state. Wave profiles were further examined for estimates of reshock states. Experimental results were used to fit parameters to the P-Lambda model for porous materials. For simple 1-D simulations, the P-Lambda model seems to capture some of the physics behind the compaction process very well, typically predicting the Hugoniot state to within 3%.

Polycrystalline Aluminum Oxynitride Hugoniot and Optical Properties

Aluminum oxynitride (AlON) is an interesting ceramic because it is both polycrystalline and transparent. We have conducted plate impact experiments to measure the Hugoniot up to 100 GPa and the spall strength up to the HEL. AlON appears to loose spall strength completely under certain conditions, perhaps due to the propagation of a failure wave. It remains transparent, however, up to at least 5 GPa allowing refractive index measurements over this regime.

Pressure-shear experiments on granular materials.

Pressure-shear experiments were performed on granular tungsten carbide and sand using a newly-refurbished slotted barrel gun. The sample is a thin layer of the granular material sandwiched between driver and anvil plates that remain elastic. Because of the obliquity, impact generates both a longitudinal wave, which compresses the sample, and a shear wave that probes the strength of the sample. Laser velocity interferometry is employed to measure the velocity history of the free surface of the anvil. Since the driver and anvil remain elastic, analysis of the results is, in principal, straightforward. Experiments were performed at pressures up to nearly 2 GPa using titanium plates and at higher pressure using zirconium plates. Those done with the titanium plates produced values of shear stress of 0.1-0.2 GPa, with the value increasing with pressure. On the other hand, those experiments conducted with zirconia anvils display results that may be related to slipping at an interface and shear stresses mostly at 0.1 GPa or less. Recovered samples display much greater particle fracture than is observed in planar loading, suggesting that shearing is a very effective mechanism for comminution of the grains.

High precision Hugoniot measurements on statically pre-compressed fluid helium

The capability for statically pre-compressing fluid targets for Hugoniot measurements utilizing gas gun driven flyer plates has been developed. Pre-compression expands the capability for initial condition control, allowing access to thermodynamic states off the principal Hugoniot. Absolute Hugoniot measurements with an uncertainty less than 3% on density and pressure were obtained on statically pre-compressed fluid helium utilizing a two stage light gas gun. Helium is highly compressible; the locus of shock states resulting from dynamic loading of an initially compressed sample at room temperature is significantly denser than the cryogenic fluid Hugoniot even for relatively modest (0.27–0.38 GPa) initial pressures. The dynamic response of pre-compressed helium in the initial density range of 0.21–0.25 g/cm3 at ambient temperature may be described by a linear shock velocity (us) and particle velocity (up) relationship: us = C0 + sup, with C0 = 1.44 ± 0.14 km/s and s = 1.344 ± 0.025.

Hypervelocity Impact Flash for Missile- Defense Kill Assessment and Engagement Analysis: Experiments on Z

Kill assessment continues to be a major problem for the nations missile defense program. A potential approach for addressing this issue involves spectral and temporal analysis of the short-time impact flash that occurs when a kill vehicle intercepts and engages a target missile. This can provide identification of the materials involved in the impact event, which will, in turn, yield the data necessary for target identification, engagement analysis, and kill assessment. This report describes the first phases of a project under which we are providing laboratory demonstrations of the feasibility and effectiveness of this approach. We are using two major Sandia facilities, the Z-Pinch accelerator, and the two- and three-stage gas guns at the Shock Thermodynamics and Applied Research (STAR) facility. We have looked at the spectral content of impact flash at velocities up to 25 km/s on the Z-Pinch machine to establish the capability for spectroscopy for these types of events, and are looking at similar experiments at velocities from 6 to 11 km/s on the gas guns to demonstrate a similar capability for a variety of research-oriented and applied materials. The present report describes only the work performed on the Z machine.

High Velocity Uniaxial Strain Response of ERG Aerospace Aluminum Foam

We report the results of uniaxial strain experiments of ERG Aerospace aluminum foam, at 50% relative density up to 10 GPa. The reverse ballistic plate reverberation technique was used to obtain shock compression states of the material. In these tests, 6061 T-6 aluminum, oxygen free homogenous copper (OFHC), and tantalum were used as standard material targets and were shocked by an aluminum foam projectile traversing up to 2.0 km/s. The response of the target plates were monitored by three different velocity interferometers positioned at three different locations on the witness plate. This provided us with the compaction behavior of the foam material in three discrete locations per sample, due to the presence of porosity in the foam material.Copyright

Multi‐Dimensional Hydrocode Analyses of Penetrating Hypervelocity Impacts

The Eulerian hydrocode, CTH, has been used to study the interaction of hypervelocity flyer plates with thin targets at velocities from 6 to 11 km/s. These penetrating impacts produce debris clouds that are subsequently allowed to stagnate against downstream witness plates. Velocity histories from this latter plate are used to infer the evolution and propagation of the debris cloud. This analysis, which is a companion to a parallel experimental effort, examined both numerical and physics‐based issues. We conclude that numerical resolution and convergence are important in ways we had not anticipated. The calculated release from the extreme states generated by the initial impact shows discrepancies with related experimental observations, and indicates that even for well‐known materials (e.g., aluminum), high‐temperature failure criteria are not well understood, and that non‐equilibrium or rate‐dependent equations of state may be influencing the results.

Expansion into vacuum of a shocked tungsten carbide-epoxy mixture.

The behavior of a shocked tungsten carbide / epoxy mixture as it expands into a vacuum has been studied through a combination of experiments and simulations. X-ray radiography of the expanding material as well as the velocity measured for a stood-off witness late are used to understand the physics of the problem. The initial shock causes vaporization of the epoxy matrix, leading to a multi-phase flow situation as the epoxy expands rapidly at around 8 km/s followed by the WC particles moving around 3 km/s. There are also small amounts of WC moving at higher velocities, apparently due to jetting in the sample. These experiments provide important data about the multi-phase flow characteristics of this material.