Bulletin of the American Physical Society
2005 14th APS Topical Conference on Shock Compression of Condensed Matter
Sunday–Friday, July 31–August 5 2005; Baltimore, MD
Session Y2: Experimental Developments III |
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Chair: Vasant Joshi, Naval Surface Warfare Center, Indian Head Room: Hyatt Regency Constellation C |
Friday, August 5, 2005 8:00AM - 8:15AM |
Y2.00001: Influence of Shock Wave Measurement Technique on the Determination of Hugoniot States C.S. Alexander, T.J. Vogler, W.D. Reinhart, D.E. Grady, M.E. Kipp, L.C. Chhabildas In theory, a shock wave traveling through a material gives rise to a well defined Hugoniot state. However, in practice, the measurement technique used to probe the shocked state imparts on the data a unique set of experimental artifacts which can affect interpretation of this data. Two commonly used methods for acquiring shock wave data, VISAR and inclined-mirror measurements are examined to determine the effects of the measurement technique on the final Hugoniot determination. Recent plate impact experiments on the ceramic silicon carbide are used to calibrate a one-dimensional computer model, which is then used to simulate experimental VISAR and inclined mirror measurements. The results, which highlight potential pitfalls in interpretation of experimental data, will be discussed and solutions to the discrepancies will be proposed. *Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy's National Nuclear Security Administration under Contract DE-AC04-94AL85000. [Preview Abstract] |
Friday, August 5, 2005 8:15AM - 8:30AM |
Y2.00002: Laser-induced Mach waves for ultra-high-pressure experiments Damian Swift Laser-driven experiments are a principal technique for inducing pressures in the terapascal regime and higher. However, when high irradiance laser light interacts with matter, it generates fast electrons and x-rays, which may heat material ahead of hydrodynamic loading waves such as shocks. This preheat limits the scope for investigating properties of initially cold material and potentially reduces the accuracy of measurements. A new configuration for laser experiments is proposed, using convergence and irregular reflection of shocks to induce high pressures without such high laser irradiances. Related Mach wave generators have been developed previously for high- explosive drive; the design considerations for laser-driven Mach wave generators are typically dictated by constraints on the laser pulse duration and differ from high-explosive systems. Relations are presented between the pressures achievable with different variants of the laser drive technique and different combinations of materials in the Mach-interaction region. The prospects for isentropic compression using this type of experiment are discussed. [Preview Abstract] |
Friday, August 5, 2005 8:30AM - 8:45AM |
Y2.00003: The New ICE Age Randy Hickman, Clint Hall, Cris Deeney, Mike Willis Recent advances in the application of pulsed power to the study of dynamic material response have been made on the Sandia Z accelerator. Smoothly increasing multi-megabar pressure loads have been achieved allowing quasi-isentropes for these materials to be inferred. A significant amount of material research does not, however, require such intense pressure loading. In response to this, Sandia National Laboratories is constructing a facility that will house a compact Isentropic Compression Experiment (ICE) pulsed power driver capable of producing currents of $\sim $ 4 Ma, and subsequent pressure loads on 4 samples up to $\sim $1 Mbar to provide a more cost effective, easily accessed machine for dynamic material studies. Additionally, a single stage air gun will be available to support the pulser as well as for standard, low velocity EOS experiments. Instrumentation will include conventional VISAR with ultra low VPF capability, spatially resolved VISAR, flash X-rays, and sample temperature control. Discussion of facility capabilities and preliminary data will be presented. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy under contract DE-ACO4-94AL85000. [Preview Abstract] |
Friday, August 5, 2005 8:45AM - 9:00AM |
Y2.00004: Limitations of the Hopkinson Pressure Bar for High-Frequency Measurements Richard Ames The Hopkinson Pressure Bar is a measurement tool that has been used for a variety of applications, including measurement of high-strain-rate loads, characterization of blast fields, and impulse measurements for ballistic test specimens. The technique is generally used for integrated measurements (such as impulse) but has also been used to measure the time-history of a variety of dynamic loads. The limitations on this type of measurement are strict, however, and generally require bars of extremely small diameter for most measurements of practical interest. Dynamic loads with significant high-frequency energy can become severely distorted as they are propagated down the bar; this phenomenon is typically referred to as signal dispersion. This paper provides a review of the relevant theory behind the Hopkinson Pressure Bar technique and derives relationships that provide practical limitations on the use of the technique for high-frequency measurements. In particular, this paper uses Love's theory to derive the relationship \[ r<\frac{0.465c_0^ }{2\pi \upsilon f} \] where c$_{0}$ is the bar material sound speed, $\nu $ is the Poisson's ratio, f is the frequency of the disturbance, and r is the bar diameter. This equation defines the limitation on bar diameter that will allow propagation with less than 5{\%} dispersion. [Preview Abstract] |
Friday, August 5, 2005 9:00AM - 9:15AM |
Y2.00005: Intermediate and High Strain-Rate testing of Soft Materials S.P. Anderson, E. Palamidi, J.J. Harrigan Strain-gauged bars are often employed as load cells for direct impact testing of materials and are incorporated within the split Hopkinson pressure bar (SHPB). Low impedance bars (e.g. magnesium or polymer bars) are desirable when testing soft specimens such as various energetic materials and cellular solids. However, due to the rheological properties of polymer bars, wave dispersion and attenuation occurs. For relatively large diameter bars and high frequency waves, geometrical wave dispersion due to radial inertia also occurs. As a result, the elementary one-dimensional wave theory can only predict accurately the propagation of stress waves along the bar within a limited frequency range. The spectral element method (SEM) is a powerful tool for the analysis of propagating stress waves in structures. In this paper the SEM is applied to the SHPB to obtain the stress-strain curves of specimens under investigation. To demonstrate the technique experimental results for balsa wood and aluminium foam using both magnesium alloy and PMMA bars are provided. Approximate four-mode rod theory applicable to viscoelastic bars is used to correct for geometrical dispersion. The results are compared with lower-order approximations. [Preview Abstract] |
Friday, August 5, 2005 9:15AM - 9:30AM |
Y2.00006: Measurements of strain propagation in Hopkinson bar specimens Clive Siviour, David Williamson, William Proud The research presented in this paper uses the recent application of the speckle technique to the split Hopkinson pressure bar (SHPB), to make measurements of strain propagation in elongated specimens. Specimens of length 20 mm were deformed in an SHPB at impact speeds of approximately 5 ms$^{-1}$. The SHPB was used to measure the stress transmitted through the specimen, and its average deformation. The speckle technique allowed measurements of the deformation field throughout the length of the specimen as the stress wave passed through it. From this field it was possible to calculate the strain associated with the propagating stress wave, and therefore to calculate an effective Young's modulus for the material at high strain rates. [Preview Abstract] |
Friday, August 5, 2005 9:30AM - 9:45AM |
Y2.00007: Measurement of strength of EN3B Mild Steel using lateral gauges. Ruth Hammond, Ron Winter, Ernie Harris Hammond and co-workers, (APS SCCM, 2003, p1125), measured the resistance change of lateral manganin gauges mounted in EN3B mild steel samples. All of the experiments were conducted at an impact velocity of 400m/s giving a shock pressure of about 7GPa. Although the main aim of the study was to determine the effect of seven different thicknesses of protective padding on the resistance-time profile, the results also provide data from which the strength of the sample material can be derived. The method used to estimate the strength of the steel is based on a picture of the mechanisms at play during gauge operation as provided by high resolution hydrocode simulations. First, existing calibration curves were used to estimate the stress at each gauge corresponding to the measured resistance change. Hydrocode simulations were then used to determine the material strength that matched the observed stress at the gauges. An elastic-perfectly plastic model was assumed for both the sample and the manganin. [Preview Abstract] |
Friday, August 5, 2005 9:45AM - 10:00AM |
Y2.00008: An Automated Test Bed for VISAR Probe Characterization Terry Salyer, Noor Khalsa, Larry Hill Accurate characterization of VISAR probes is helpful for their effective fielding on a given experiment. Much stands to be gained through optimal placement and choice of probe as well as optimal target surface preparation. Revelations through a series of dynamic shots can be time consuming, expensive, and inefficient. An automated system to measure probe illumination and return characteristics independent of the VISAR helps to alleviate these problems. Motion of a target reflector is simulated via linear traverses and a rotation stage. Laser illumination is provided to yield a probe response measured with sensitive optical power detectors. A beam profiler is used for 2-D analysis of the illumination spot over the full range of target travel. Furthermore, the whole system is automated through LabVIEW software control. A proposed standardized probe test consists of the 1-D axial response, sensitivity to target angle, sensitivity to target surface preparation, and the illumination spot characteristics. As the community recognizes the need for more specialized probes, such a tool enables the rapid development of new designs as well as the cataloging of current ones. [Preview Abstract] |
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