Lalit Chhabildas

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

TIME RESOLVED OPTICAL SIGNATURES FOR HUGONIOT STATE MEASUREMENTS IN SHOCK COMPRESSED COMPOSITION‐B

Broadband photo‐diodes sensitive over the visible and near infrared electromagnetic spectrum are used to monitor impact flash luminosity versus time. Based on careful experimental layout and impact timing the prompt portion of the impact flash signatures reveal the shock propagation timing through a Composition‐B target plate. Application of impedance matching techniques and Rankine‐Hugoniot Jump equations to this waveform timing provides apparent Hugoniot state measurements of shock compressed Composition‐B in the 25 to 50 GPa range. This data will be discussed in detail, along with comparison to previous work below the Composition‐B detonation pressure.

INVESTIGATION OF 6061‐T6 ALUMINUM STRENGTH PROPERTIES TO 160 GPa

Shock compression experiments were performed on 6061‐T6 aluminum up to 160 GPa to probe aluminum strength in the shocked state as it passes through the melt regime. A careful set of experiments, using established two and three stage flyer plate launch techniques were conducted using symmetric impact loading conditions to compress the aluminum through the solid to the liquid phase boundary. Velocity interferometry provides the fine structure of shock loading and release behavior almost as an in situ particle velocity wave profile at the aluminum/lithium fluoride window interface. Results are detailed in terms of wave speeds and estimates of strength of the material in the shocked state.

Shock Loading of polycrystalline alumina and sapphire--a comparative study

There is considerable interest in the shock loading behavior of aluminum oxide whether it is in the polycrystalline phase or in the single crystal phase. Results of well‐controlled experiments conducted recently on polycrystalline alumina and Z‐cut sapphire are summarized to conduct a comparative study. Although the experimental results appear to have the same behavior in the shock‐velocity vs. particle‐velocity plane, they are considerably different in the stress‐volume compression plane. This is an extremely interesting observation and cannot be explained merely by the differences in the strength of the material in the shocked state.

Feasibility of an Explosive End-Projector for Conducting Shock Loading Experiments

This paper focuses on examining the feasibility of using an explosive end-projector device to conduct shock loading experiments. The concept employs an explosive end-projector to accelerate a bonded bimetallic impactor toward a stationary target material in order to conduct complex shock loading experiments. A high-impedance material, tantalum, was specifically selected to generate high stresses in targets. The driver plate materials were varied to create an impedance spectrum from 2 to 4 which would encompass materials from aluminum to zirconium. Techniques for launching single density metal plates exist. [1] The objective of this paper is to further refine an existing explosive technique to launch bimetallic plates which would generate complex shock/reshock or shock/release waves when used as a impactor. Although a bimetallic impactor will generate complex wave loading within a target material upon impact, it must first survive intact the explosive shock acceleration stress history imposed during launch in order to obtain the terminal velocity for use as an impactor. Initial computational studies using the Lagrangian finite element code EPIC were promising. [2] Based on the concern of the plate spallation during launch [2], an air gap between explosive and the backside of the bimetallic plate was modeled to reduce the magnitude of the initial shock resulting from direct explosive-metal interaction and to further gain insight into its effects on pressure, terminal velocity, and planarity of the plate. Initially, a single, homogenous plate of aluminum was modeled to examine the effects of the air gap without the complication of bimetallic wave reverberation. The air gap did reduce the intensity of pressure by 30% with only a 4% loss in terminal velocity. Planarity was exacerbated by the air gap leading to increased warping. With the introduction of bimetallic plates and the corresponding wave reverberation pressure reductions ranged between 20% to 30% with corresponding losses in terminal velocity of between 5% to 7%. Planarity was improved for the configurations in which the high density material, tantalum, was on the free surface or impacting side of the impactor but warping increased when the order was reversed. This results from the low-impedance material serving as a buffer to allow quasi-isentropic and monotonic increase in loading of the tantalum plate. However, when the high density material was placed on the back side of the impactor, the loading history was reversed and the low density material was subjected to high pressures. Overall, the technique appears to be a feasible alternative for conducting shock loading experiments. The air gap reduces pressure with only a minimal loss in terminal velocity, and the air gap in conjunction with plate geometry changes appears to mitigate spall. The air gap does not improve planarity with the explosive end-projector design used in this investigation but optimized designs would improve the flexibility of the technique.Copyright