Publications

Development of well-controlled hypervelocity launch capabilities is the first step to understand material behavior at extreme pressures and temperatures not available using conventional gun technology. In this paper, techniques used to extend both the launch capabilities of a two-stage light-gas gun to 10 km/s and their use to determine material properties at pressures and temperature states higher than those ever obtained in the laboratory are summarized. Time-resolved interferometric techniques have been used to determine shock loading and release characteristics of materials impacted by titanium and aluminum fliers launched by the only developed three-stage light-gas gun at 10 km/s. In particular, the Sandia three stage light gas gun, also referred to as the hypervelocity launcher, HVL, which is capable of launching 0.5 mm to 1.0 mm thick by 6 mm to 19 mm diameter plates to velocities approaching 16 km/s has been used to obtain the necessary impact velocities. The VISAR, interferometric particle-velocity techniques has been used to determine shock loading and release profiles in aluminum and titanium at impact velocities of 10 km/s.

Gas guns and velocity interferometric techniques have been used to determine the loading behavior of an AD995 alumina rod 19 mm in diameter by 75 mm and 150 mm long, respectively. Graded-density materials were used to impact both bare and sleeved alumina rods while the velocity interferometer was used to monitor the axial-velocity of the free end of the rods. Results of these experiments demonstrate that (1) a 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, and (2) the intermediate loading rates obtained in this configuration lie between split Hopkinson bar and shock-loading techniques.

Planar impact experiments and wave profile measurements provided single and double shock equation of state data to 30 GPa. Both compression wave wave profile structure and release wave data were used to infer time-dependent strength and equation of state properties for soda-lime glass.

Understanding high-pressure material behavior is crucial to address the physical processes associated with a variety of hypervelocity impact events related to space sciences such as orbital-debris impact on a debris shield. At very high impact velocities material properties will be dominated by phase-changes, such as melting or vaporization, which cannot be achieved at lower impact velocities. Development of well-controlled and repeatable hypervelocity launch capabilities is the first step necessary to improve our understanding of material behavior at extreme pressures and temperatures not currently available using conventional two-stage light-gas gun techniques. In this paper, techniques used to extend the launch capabilities of a two-stage light gas gun to 16 km/s are described. It is anticipated that this technology will be useful in testing, evaluating, and design of various debris shields proposed for use with many different spacecrafts before deployment.

In this paper a brief description of dynamic techniques commonly available for determining material property studies is presented. For many impact applications, the material generally experiences a complex loading path. In most cases, the initial loading conditions can be represented by the shock commonly referred to as the Hugoniot state. Subsequent loading or release structure, i.e., off-Hugoniot states would however be dependent on the physical processes dominating the material behavior. The credibility of the material model is tested by the accuracy of predictions of off-Hugoniot states. Experimental techniques commonly used to determine off-Hugoniot states are discussed in this survey.

An experimental technique is described to launch an intact ``chunk,`` i.e. a 0.3 cm thick by 0.6 cm diameter cylindrical titanium alloy (Ti-6Al-4V) flyer, to 10.2 km/s. The ability to launch fragments having such an aspect ratio is important for hypervelocity impact phenomenology studies. The experimental techniques used to accomplish this launch were similar but not identical to techniques developed for the Sandia HyperVelocity Launcher (HVL). A confined barrel impact is crucial in preventing the two-dimensional effects from dominating the loading response of the projectile chunk. The length to diameter ratio of the metallic chunk that is launched to 10.2 km/s is 0.5 and is an order of magnitude larger than those accomplished using the conventional hypervelocity launcher. The multi-dimensional, finite-difference (finite-volume), hydrodynamic code CTH was used to evaluate and assess the acceleration characteristics i.e., the in-bore ballistics of the chunky projectile launch. A critical analysis of the CTH calculational results led to the final design and the experimental conditions that were used in this study. However, the predicted velocity of the projectile chunk based on CTH calculations was {approximately} 6% lower than the measured velocity of {approximately}10.2 km/S.

A series of experiments has been performed to evaluate the effectiveness of a Whipple bumper shield to orbital space debris at impact velocities of ∼ 10 km/s. Upon impact by a 19 mm (0.87 mm thick, L/D ∼0.5) flier plate, the thin aluminum bumper shield disintegrates into a debris cloud. The debris cloud front propagates axially at velocities of ∼14 km/s and expands radially at a velocity of ∼7 km/s. Subsequent loading by the debris on a 3.2 mm thick aluminum substructure placed 114 mm from the bumper penetrates the substructure completely. However, when the diameter of the flier plate is reduced to 12.7 mm, the substructure, although damaged is not perforated. Numerical simulations performed using the multi-dimensional hydrodynamics code CTH also predict complete perforation of the substructure by the subsequent debris cloud for the larger flier plate. The numerical simulation for a 12.7 mm flier plate, however, shows a strong dependence on assumed impact geometry, i. e., a spherical projectile impact geometry does not result in perforation of the substructure by the debris cloud, while the flat plate impact geometry results in perforation. © 1993.

A series of experiments has been performed on the Sandia HyperVelocity Launcher (HVL) to evaluate the effectiveness of a Whipple bumper shield to orbital space debris at impact velocities in excess of 10 km/s. Upon impact by a 0.67 g (0.87 mm thick) flier plate, the thin aluminum bumper shield disintegrates into a debris cloud. The debris cloud front propagates axially at velocities of [approximately]14 km/s and expands radially at a velocity of [approximately]7 km/s. Subsequent loading on a 3.2 mm thick aluminum substructure by the debris penetrates the substructure completely. However, when the mass of the flier plate is reduced to 0.33 g, the substructure, although damaged, is not perforated over the duration of the experiment. Numerical simulations performed using the multi-dimensional hydrodynamics code CTH also predict complete penetration of the substructure by the subsequent debris cloud for a 0.87 g flier plate. The numerical simulations for a 0.33 g flier plate show a strong dependence on assumed impact geometry. For the assumption of a spherical projectile impact geometry, perforation of the substructure by the subsequent debris cloud is not predicted by CTH.

In this paper, measurements on the quasi-isentropic compression of tungsten to stress levels of 250 GPa are reported. Results of these experiments have been compared to those obtained under shock loading conditions to comparable stresses. These experiments have allowed the determination of temperature, pressure, and loading rate effects on the dynamic yield strength of tungsten up to 250 GPa. These results show that the dynamic yield strength of tungsten is dependent on the loading rate with the strength being higher for the relatively slower rates of loading along the quasi-isentropic. The pressure dependence of the yield strength of tungsten is determined nearly independent of temperature effects from quasi-isentropic loading experiments to 250 GPa, because the temperature rise in an quasi-loading experiment is much lower than those associated with shock loading experiments.

Very high driving pressures (tens or hundreds of GPa), are required to accelerate flier plats to hypervelocities. This loading pressure pulse on the fiber plates must be nearly shockless to prevent the plate from melting or vaporizing. This is accomplished by using graded-density impactors referred to as ``pillows.`` When this graded-density material is used to impact a flier-plate in a modified two-stage light gas gun, nearly shockless megabar pressures are introduced into the flier plate. The pressure pulses must also be tailored to prevent spallation of the flier-plate. This technique has been used to launch nominally 1-mm-thick aluminum, magnesium and titanium (gram-size) intact plates to 10.4 km/s, and 0.5-mm-thick aluminum and titanium (half-gram size) intact plates to 12.2 km/s. This is the highest mass-velocity capability attained with laboratory launchers to data, and should open up new regimes of impact physics and lethality studies related to space sciences for laboratory investigations. 14 refs.

A systematic study is described which addresses the technical issues associated with launching flier-plates intact to hypervelocitites. First, very high pressures are needed to launch the flier plates to hypervelocitites, and second this high pressure loading must be uniform and nearly shockless. To achieve both these criteria, a graded-density material referred to as a pillow'' is used to impact a flier plate. When this graded-density material is used to impact a flier plate at high velocities on a two-stage light-gas gun, nearly shockless megabar pressure pulses are introduced into the flier plate. Since the loading on the flier plate is shockless, melting of the flier plate is prevented. This technique has been used to launch a 2-mm thick titanium alloy (Ti-6Al-4V) plate to a velocity of 8.1 km/s, and a 1-mm thick aluminum alloy (6061-T6) plate to a velocity of 10.4 km/s. A method is described by which the flier plate velocities could be further augmented to velocities approaching 14 km/s. 18 refs., 16 figs.