Bulletin of the American Physical Society
21st Biennial Conference of the APS Topical Group on Shock Compression of Condensed Matter
Volume 64, Number 8
Sunday–Friday, June 16–21, 2019; Portland, Oregon
Session 1E: Experimental Techniques and Diagnostics |
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Room: Broadway I/II |
Sunday, June 16, 2019 4:00PM - 4:15PM |
1E.00001: Programmable velocity trap for triggering gun diagnostics Gareth Tear, William Proud Diagnostics that require precise temporal triggering require a gun and triggering system with low jitter. A programmable and calibrated velocity trap for triggering diagnostics accurately without requiring accurate and predictable muzzle velocities has been developed. The design, applications and limits will be presented. Cutting edge measurements often require precise temporal triggering. For example a framing camera with an interframe time of 15ns requires a pre trigger around 1.8 microseconds prior to imaging (to clear the CCD), with a precision of $+$/-5ns for the imaging frames. This equates to a fractional tolerance of $+$/-0.3{\%} in the timing. Gas and powder guns typically have high jitter when triggered due to uncertainty in the muzzle velocity. One way of compensating for this is with a velocity trap, a device which measures the speed of the projectile and immediately adjusts the trigger delay to match the velocity difference. These devices have fallen out of favour when compared to digital pre-triggering available on many commercial cameras and oscilloscopes because they are difficult to calibrate and time consuming to setup. We have developed a programmable velocity trap to allow easier triggering of diagnostics on gun experiments. [Preview Abstract] |
Sunday, June 16, 2019 4:15PM - 4:30PM |
1E.00002: Imaging the Reactive Flow Structure Evolution in Shocked Nitromethane and Nitromethane with Additives Erin Nissen, Mithun Bhowmick, Dana Dlott We used a tabletop laser driven flyer plate to generate planar shock waves in cuvettes to produce detonations in liquid nitromethane (NM), and NM with sensitizing and inert additives. The liquid is sandwiched between an optical window and a thin aluminum lid. Photon Doppler velocimetry was used to track the flyer and particle velocity at the lid/NM interface. Images were taken using a 5-ns gated sCMOS camera at different times to analyze the flow structure evolution as a function of impact velocity and additive. Three distinct structure regimes were found to be controlled by impact velocity in pure NM, while the additives in NM control the size of the cellular structures. This may be used to calculate the reaction zone length to corroborate PDV measurements. Mechanical defects at the lid/NM interface were also investigated. There were no changes in structures between rough or smooth aluminum or steel lids, signifying the structures are a property of NM and not propagating from the lid. Molecular layers that inhibit or enhance the shock chemistry were patterned on the lid interface to control the shock to detonation time and cellular structures. [Preview Abstract] |
Sunday, June 16, 2019 4:30PM - 4:45PM |
1E.00003: Probing Shocked Materials with Time-resolved Raman Scattering Dmitro Martynowych, Keith Nelson We present methods to acquire time-resolved Raman spectra of shocked materials in a single shot. A thin layer (5-50 \textmu m) of material is pressed between two glass plates, confining it to a planar geometry. A sub-nanosecond laser pulse is focused into a circular ``ring'' pattern of 200 \textmu m radius, launching a shock wave that propagates within the plane of the sample and focuses toward the circle's center. This converging geometry generates regions of high pressure at its focus. We couple this technique with single-shot femtosecond stimulated Raman scattering which yields a full stokes an anti-stokes Raman spectra. A spectral broad (\textasciitilde 100 fs) white-light probe pulse, and a spectral narrow (\textasciitilde 1 ps) pump pulse are spatiotemporally overlapped on the sample. These pulses drive stimulated Raman transitions which manifest as stimulated loss and gain on the broadband probe pulse. [Preview Abstract] |
Sunday, June 16, 2019 4:45PM - 5:00PM |
1E.00004: Micromechanical approach to model deformation response of granular materials using FEM considering meso-structure from X-ray computed tomography Mohmad Mohsin Thakur, Dayakar Penumadu The discrete nature of granular materials results in complex mechanical interactions such as non-affine deformations including slippage at grain contacts, force chain buckling and shear banding. Even until now, geomechanics community largely relies on triaxial testing as the basis for constitutive behavior at continuum scale. However, the continued evidence with the lack of suitable predictive models with reasonable number of parameters to capture phenomenological effects makes us believe that this problem is too complicated for any continuum-based approach. The urgent need is to incorporate mesoscale effects which are discrete and non-repeating in the numerical modeling, and hence FEM and X-ray CT imaging are explored concurrently. The 3D X-ray CT images of Ottawa sand are transformed into a 3-D FEM mesh to solve a boundary value problem using actual grain and pore microstructure. The variation in contact interaction properties such as limiting shear stress and elastic slip stiffness between the surface of grains is investigated. Additionally, development of force chains and shear bands on cylindrical specimen is presented with jamming/unjamming of force chains evident in kinetic energy and deviatoric stress oscillations [Preview Abstract] |
Sunday, June 16, 2019 5:00PM - 5:15PM |
1E.00005: Using Free Surface Velocity and X-Ray Imaging to Monitor the Closure of a Cylindrical Hole in Copper and Tantalum for Strength Measurements Under Pressure (symp) Andrew Robinson, Jonathan Lind, Matthew Nelms, Nathan Barton, Mukul Kumar The flow stress in a metal is dependent on a variety of factors such as strain, strain rate, microstructure, and temperature. Experiments (i.e. quasi-static tensile testing or Kolsky bar testing) with well characterized stress states have been used to determine the relation between flow stress and these many factors. However, for higher strain rates (\textgreater 10$^{\mathrm{5}}$/s) there is a dearth of high-fidelity data. Here, we present results of a recent in-situ gas-gun experimental technique that can probe strength effects at strain rates of \textgreater 10$^{\mathrm{5}}$/s. By measuring the diameter of a long cylindrical hole using x-ray imaging in conjunction with back surface velocimetry while the sample is subjected to controlled dynamic loading, the factors affecting flow stress can be inferred. Models of the experiment indicate that a relatively large volume of material around the hole experiences strain rates above 10$^{\mathrm{5}}$/s. Materials with higher dynamic flow stresses tend to exhibit less diameter reduction than materials with lower flow stresses all else being equal. Experimental results indicate that the hole diameter reduction is also dependent on the peak pressure of the loading pulse and the duration of the pressure pulse. [Preview Abstract] |
Sunday, June 16, 2019 5:15PM - 5:30PM |
1E.00006: Feasibility Studies of the Use of Inelastic X-ray Scattering as a Temperature Diagnostic of Transiently Compressed Matter Oliver Karnbach, David McGonegle, Gianluca Gregori, Justin Wark Recent experiments at LCLS have demonstrated the feasibility of using femtosecond x-ray pulses to inelastically scatter from phonons in a solid[1]. In principle, measuring the relative intensities of the Stokes and anti-Stokes peaks could provide a direct measure of the temperature without recourse to needing to know the Debye temperature. However, the number of inelastically scattered photons is low, and thus absolute temperature measurements on laser-compressed samples will need to accumulate data over many shots. We present here simple calculations of the cross section, compare them with the data provided in [1], and comment on the long-term feasibility of using this technique at the European XFEL. We further consider the degree of elastic scattering with which the inelastic signal will need to compete owing to intrinsic and shock-induced defects in samples of interest. Synthetic phonon spectra and scattering signals are calculated in various materials under dynamic compression using large-scale molecular dynamics simulations. [1] E.E. McBride et al., Rev. Sci. Instrum. 89, 10F104 (2018) [Preview Abstract] |
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