R. T. Olson

A multiscale strength model for extreme loading conditions

We present a multiscale strength model in which strength depends on pressure, strain rate, temperature, and evolving dislocation density. Model construction employs an information passing paradigm to span from the atomistic level to the continuum level. Simulation methods in the overall hierarchy include density functional theory, molecular statics, molecular dynamics, dislocation dynamics, and continuum based approaches. Given the nature of the subcontinuum simulations upon which the strength model is based, the model is particularly appropriate to strain rates in excess of 104 s−1. Strength model parameters are obtained entirely from the hierarchy of simulation methods to obtain a full strength model in a range of loading conditions that so far has been inaccessible to direct measurement of material strength. Model predictions compare favorably with relevant high energy density physics (HEDP) experiments that have bearing on material strength. The model is used to provide insight into HEDP experimental ...

Effects of shock-breakout pressure on ejection of micron-scale material from shocked tin surfaces

This effort investigates the relation between ejecta production and shock-breakout pressure (PSB) for Sn shocked with a Taylor shockwave (unsupported) to pressures near the solid-on-release/partial melt-on-release phase transition region. The shockwaves were created by detonation of high explosive (HE) PBX-9501 on the front side of Sn coupons. Ejecta production at the backside or free side of the Sn coupons was characterized through use of piezoelectric pins, optical shadowgraphy, x-ray attenuation radiography, and optical-heterodyne velocimetry. Ejecta velocities, dynamic volume densities, and areal densities were then correlated with the shock-breakout pressure of Sn surfaces characterized by roughness average of Ra=16 μin or Ra=32 μin.

Probing the underlying physics of ejecta production from shocked Sn samples

This effort investigates the underlying physics of ejecta production for high explosive (HE) shocked Sn surfaces prepared with finishes typical to those roughened by tool marks left from machining processes. To investigate the physical mechanisms of ejecta production, we compiled and re-examined ejecta data from two experimental campaigns [W. S. Vogan et al., J. Appl. Phys. 98, 113508 (1998); M. B. Zellner et al., ibid. 102, 013522 (2007)] to form a self-consistent data set spanning a large parameter space. In the first campaign, ejecta created upon shock release at the back side of HE shocked Sn samples were characterized for samples with varying surface finishes but at similar shock-breakout pressures PSB. In the second campaign, ejecta were characterized for HE shocked Sn samples with a constant surface finish but at varying PSB.

Dynamic comparisons of piezoelectric ejecta diagnostics

We investigate the quantitative reliability and precision of three different piezoelectric technologies for measuring ejected areal mass from shocked surfaces. Specifically we performed ejecta measurements on Sn shocked at two pressures, P≈215 and 235 kbar. The shock in the Sn was created by launching a impactor with a powder gun. We self-compare and cross-compare these measurements to assess the ability of these probes to precisely determine the areal mass ejected from a shocked surface. We demonstrate the precision of each technology to be good, with variabilities on the order of ±10%. We also discuss their relative accuracy.

Second shock ejecta measurements with an explosively driven two-shockwave drive

We develop and apply an explosively driven two-shockwave tool in material damage experiments on Sn. The two shockwave tool allows the variation of the first shockwave amplitude over range 18.5 to 26.4 GPa, with a time interval variation between the first and second shock of 5 to 7 μs. Simulations imply that the second shock amplitude can be varied as well and we briefly describe how to achieve such a variation. Our interest is to measure ejecta masses from twice shocked metals. In our application of the two-shockwave tool, we observed second shock ejected areal masses of about 4 ± 1 mg/cm2, a value we attribute to unstable Richtmyer-Meshkov impulse phenomena. We also observed an additional mass ejection process caused by the abrupt recompression of the local spallation or cavitation of the twice shocked Sn.

Method to separate and determine the amount of ejecta produced in a second-shock material-fragmentation event

The authors consider a mathematical method to separate and determine the amount of ejecta produced in a second-shock material-fragmentation process. The technique is theoretical and assumes that a material undergoing a shock release at a vacuum interface ejects particulate material or fragments as the initial shock unloads and reflects at the vacuum-surface interface. In this case it is thought that the reflected shock may reflect again at the source of the shock and return to the vacuum-surface interface and eject another amount of fragments or particulate material.

Explosively driven two-shockwave tools with applications

We present the development of an explosively driven physics tool to generate two mostly uniaxial shockwaves. The tool is being used to extend single shockwave ejecta models to account for a second shockwave a few microseconds later. We explore techniques to vary the amplitude of both the first and second shockwaves, and we apply the tool experimentally at the Los Alamos National Laboratory Proton Radiography (pRad)facility. The tools have been applied to Sn with perturbations of wavelength λ = 550 μm, and various amplitudes that give wavenumber amplitude products of kh {3/4,1/2,1/4,1/8}, where h is the perturbation amplitude, and k = 2π/λ is the wavenumber. The pRad data suggest the development of a second shock ejecta model based on unstable Richtmyer-Meshkov physics.

High-resolution measurements of shock behavior across frictional Be/Cu interfaces

A longstanding question in the field of multi-material behavior pertains to the treatment of interfaces possessing finite frictional strength under high dynamic pressures and shear. Here, we examine the effects of constrained interface sliding on local deformation near the boundary using new, high-resolution measurements combined with simulations to infer friction strength. The experiments use laser driven plate impacts at the Los Alamos National Laboratory TRIDENT Laser Facility to launch a shock wave into a target consisting of a central cylindrical plate of Be and an outer ring of Cu oriented, such that the shock propagates at nearly a 90° angle to the interface normal producing a large velocity gradient across the material boundary. Impact experiments were performed on targets that underwent diffusion bonding of the two materials and on targets that were only press fit together. Friction-induced surface deformation was diagnosed using line-imaging velocity interferometry and surface Transient Imaging Displacement Interferometry in the immediate region of the interface. In these studies, we observed a significant behavioral change in both simulations and experiments between targets with diffusion bonded interfaces and those that were press fit. Bonded targets exhibited a mutual dragging between the Be and Cu parts throughout the entire experiment, whereas unbonded targets displayed a surface slope reversal on the Cu side of the interface, which simulations suggest arise due to altered wave interactions from a 3× lower frictional force compared to the bonded interface.

Investigation of Ejecta Production in Tin Using Plate Impact Experiments

Experiments to investigate ejecta production in shocked tin have been performed using plate impact facilities at Los Alamos National Laboratory. Three primary diagnostics — piezoelectric pins, Asay foils, and low energy X‐ray radiography — were fielded simultaneously in an attempt to quantify the amount of ejecta produced in tin as the shock wave breaks out of the free surface. Results will be presented comparing and contrasting all three diagnostics methods. Advantages and disadvantages of each method will be discussed.

Pressure effects on the ejection of material from shocked tin surfaces

Los Alamos National Lab (LANL) is actively engaged in the development of a model to predict the formation of micron‐scale fragments ejected (ejecta) from shocked metals that have surface defects. The LANL ejecta model considers that the amount of ejecta is mainly related to the materials phase on shock release at the free‐surface. This effort investigates the relation between ejecta production and shock‐breakout pressure for Sn shocked with high explosives to pressures near the solid‐on‐release/partial‐liquid‐on‐release phase transition region. We found that the amount of ejecta produced for shock‐breakout pressures that resulted in partial‐liquid‐on‐release increased significantly compared to that which resulted in solid‐on‐release. Additionally, we found that the amount of ejecta remained relatively constant within the partial‐liquid‐on‐release, regardless of shock‐breakout pressure.