Comparison of dynamic compression behavior of single crystal sapphire to polycrystalline alumina

Due to the considerable interest in the shock loading behavior of aluminum oxide whether it is in the polycrystalline phase or in the single crystal phase well-controlled experiments were conducted to probe differences in shock loading behavior between these two materials. Previous studies concluded that the behavior was similar but careful examination of well-controlled experiments has revealed the two materials are different.Although the experimental results appear to have the same behavior in the shock velocity vs. particle velocity plane, they are considerably different in the stressevolume compression plane and evidence is provided that indicates the single crystal remains crystalline up to the stresses imposed for this analysis. This is an extremely interesting observation since it has many implications including developing dynamic material models capable of transitioning between individual grains and polycrystalline material

Intermediate Strain-Rate Loading Experiments - Technique and Applications to Ceramics

A new test methodology is described which allows access to loading rates that lie between split Hopkinson bar and shock-loading techniques. Gas gun experiments combined with velocity interferometry techniques have been used to experimentally determine the intermediate strain-rate loading behavior of Coors AD995 alumina and 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. This allows access to intermediate loading rates over 5 x 10{sup 3}/s to a few times 10{sup 4}/s

Design of a new liquid cell for shock experiments

Controlled impact methodology has been used on a powdergun to obtain dynamic behavior properties of Tributyl Phosphate (TBP). A novel test methodology is used to provide extremely accurate equation of state data of the liquid. A thin aluminum plate used for confining the liquid also serves as a diagnostic to provide reshock states and subsequent release adiabats from the reshocked state. Polar polymer, polyvinylidene fluoride (PVDF) gauges and velocity interferometer system for any reflector (VISAR) provided redundant and precise data of temporal resolution to five nanoseconds and shock velocity measurements of better than 1%. The design and test methodologies are presented in this paper

Intermediate strain-rate loading experiments -- Techniques and applications

Gas guns and velocity interferometric techniques have been used to determine the loading behavior of AD995 alumina rods 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

Intermediate Strain-Rate Loading - Techniques and Applications

A new test methodology is described which allows access to loading rates that lie between split Hopkinson bar and shock-loading techniques. Gas gun experiments combined with velocity interferometry techniques have been used to experimentally determine the intermediate strain-rate loading behavior of Coors AD995 alumina and 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. This allows access to intermediate loading rates over 5 x 10{sup 3}/s to a few times 10{sup 4}/s

Reshock behavior of silicon carbide

The shear strength of a ceramic is an important material property which is indicative of its behaviour under impact loading condition. The brittle nature of ceramics suggest that impact loading of a ceramic attained due to subsonic impact could change the initial shear strength of the ceramic. If so, the subsequent behaviour of the ceramic may not be dependent on the the initial strength. Thus, it is necessary to determine the shear strength of a cermaic under single and multiple impacts. The present work determines the change in the shear strength of silicon carbide under a single shock, shock-reshock, and shock-release wave propagation, respectively. The material used in this work is marketed by CERCOM Inc. as SiC-B. The results of this work indicate that shear strength of SiC-B increases under shock-reshock

Launch capabilities to 16 km/s

A systematic study is described that has led to the successful launch of thin flier-plates to velocities of 16 km/s. In this paper, the authors describe a novel technique that has been implemented to enhance the performance of the Sandia Hyper Velocity Launcher (HVL). This technique of creating an impact-generated acceleration reservoir, has allowed the launch of 0.5 mm to 1.0 mm thick titanium (Ti-6Al-4V) and aluminum (6061-T6) alloy plates to record velocities up to 15.8 km/s. These are the highest metallic projectile plate velocities ever achieved for macroscopic masses in the range of 0.1 g to 1 g

Simultaneous PVDF/VISAR measurement technique for isentropic loading with graded density impactors

A simultaneous PVDF/VISAR measurement technique was used for isentropic-loading experiments with a polymethyl methacrylate (PMMA) specimen. The experiments used a graded density impactor accelerated onto a tantalum driver backed with PMMA and then lithium fluoride windows for each experiment. Simultaneous measurements made at each window interface provided precise transit time and particle velocity measurements which can be used to determine the stress-vs-strain loading path using Lagrangian analysis techniques. The experimental technique provides access to 40 GPa stress levels in PMMA under isentropic-loading conditions

Using Time-resolved Wave Profile Measurements to Determine Elusive Phase Transitions in Molybdenum

AbstractThe purpose of this study was to investigate the dynamic strength of molybdenum (Mo) at high pressures, particularly at the location of a solid-solid phase transition within Mo prior to entering the melt regime. The intent was to build on previous work by conducting both symmetric and non-symmetric impact experiments using a two-stage light gas gun and VISAR diagnostic system to examine molybdenum behavior up to pressures of 305GPa. The approach required compensating for the wave interaction due to the low impedance LiF window, but provided detailed information regarding the release state. The main effort consisted of a series of Mo symmetric impact experiments having a fixed ratio of impactor thickness (1mm) to target thickness (4.25mm) and variation of the impact velocity which resulted in changes in both shock and release wave velocity. Based on the geometry, any structural change resulting in a change in release wave velocity would be noticeable through changes in the amount of time at the Hugoniot state or dwell time. As expected, the shock velocity increased proportionally with increased impact velocity, but a noticeable change in dwell time occurred at approximately 190GPa. Additional analysis of the strength variation of Mo showed an increase in strength from 1.3GPa to 3GPa at approximately 190GPa the location of phase transition, followed by a dramatic drop in strength for stresses above 190GPa. Strength increased again for stresses of 300 and 305GPa. The data acquired in this effort indicate the phase transition occurs at a slightly lower pressure than previously indicated and there appears to be a mixed phase region. This paper describes the strategy, experimental method, and corresponding results which are used to draw conclusions about the dynamic strength of molybdenum at high pressure