Session X17: Matter in Extreme Environments: Dynamic Compression

Watching extreme materials through the ultrafast shock compression microscope

We have developed a microscope that looks into solids and liquids as they are subjected to controlled high velocity impacts. These impacts generate shock waves that propagate a few kilometers per second, creating intense mechanical and thermal effects that can trigger new kinds of chemistry. One of these impacts can create pressures of 200,000 atm and temperatures of 4000K while compressing matter to half its density. I will describe the shock compression microscope and the peripheral high-speed optical diagnostics that measure pressure, temperature, density, composition and structure in real time. A few applications will be discussed: shock initiation and detonation of high explosives, shock attenuation by molecular frameworks, and cchemistry in extreme states of water.
  

Results of Rayleigh-Taylor material strength experiments at high pressure and high strain rates on NIF and Omega

We are studying material strength at high pressures (upto 8 Mbar), high strain rates (~10⁷ s^-1) and high strains (> 30%) under ramped compression condition using Rayleigh-Taylor instability properties. Understanding plastic deformation dynamics of materials under these extreme conditions is an area of research of high interest to a number of fields, including meteor impact dynamics and advanced inertial confinement fusion designs. We have studied various metals such as tantalum, lead and copper using laser driven high pressure platform. Our studies show that the work hardening dominates in this regime. We will describe the experimental results of the high pressure, high rate plastic deformation dynamics of various metals from Omega and NIF in comparison with the various strength models including Livermore Multiscale Model [2].

[1] H. –S. Park et al., Phys. Rev. Lett. 114, 065502 (2015).
[2] N. Barton, et al., J. App. Physics, 109, 073501 (2011).   

In-Situ X-Ray Probes of Water-Saturated Granular Media under Dynamic Compaction

Granular systems are ubiquitous in our everyday world and influence many scientific problems including mine blasts, projectile penetration, and astrophysical collisions. Despite its significance, a fundamental understanding of granular media’s behavior falls short of its solid counterpart, limiting predictive capabilities. Granular response is complex in part to the intricate interplay between numerous degrees of freedom not present in its solid equivalent. To address the role of geophysically relevant water-saturation in granular media, previous studies use VISAR or PDV, diagnostics that focus on the aggregate effect leaving the principal interactions of these multiple degrees of freedom too entangled to elucidate. This study uses a gas gun platform coupled to in-situ X-ray probe diagnostics to probe the role of water-saturation in dynamic compaction. Analyses include evaluating displacement fields, grain fracture, and diffraction profiles. Results herein are directly compared to previous studies that were unable to include in-situ X-ray diagnostics.   

Laser-induced Microparticle Impact Experiments on Viscoelastic Gels

High-velocity impact testing is used to study fundamental aspects of materials behavior under high strain rates as well as in applications ranging from micrometeorite detection to the development of novel drug delivery platforms. In this work, we study the high-velocity micro-particle impact response of viscoelastic gels, including hydrogels and synthetic polymer gels with non-aqueous solvents. In an all-optical laser-induced projectile impact test, micro-particles are accelerated through a laser ablation process. Depending on the laser energy, particles reach speeds up to 1 km/s in free space. Steel and silica particles are monitored during impact with an ultrahigh-speed multi-frame camera that can record up to 16 images with time resolution of each frame as short as 5 ns. We present images and movies capturing individual particle impact and penetration in gels and discuss the observed dynamics under a typical strain rate of 10⁸ s^-1. The results can provide direct input for modeling of high-velocity impact responses and high strain rate deformation in gels and other soft materials.   

Lithium Fluoride optical properties at 825 GPa and the propagation of uncertainty to windowed equation of state measurements

Lithium fluoride (LiF) is an optically transparent material that tamps reverberations and preserves targets from residual gas in dynamic compression equation of state (EOS) measurements. However, LiF's non-trivial refractive index distorts interferometry measurements of target-window interface velocity. An accurate optical correction to this distortion is crucial in the determination of isentropes in other experiments. We present a measurement of LiF refractive index for stress up to 825 GPa from a shock-ramp experiment at the National Ignition Facility. We argue that a relationship between true and apparent velocity of the target-window interface is more valuable than refractive index to the EOS community. Finally, we present simulated data of dynamically compressed tin and LiF to demonstrate the propagation of the optical uncertainty from this work to EOS measurements. Simulations in which the tin-LiF interface reaches a peak stress of 825 GPa show that the tin isentrope can be determined up to a peak stress of 1.5 TPa with a 0.2% uncertainty in density due to the optical response of LiF.   

Damage mechanics from high velocity micro-particle impacts on polyethylene tape.

Understanding damage mechanics is important when designing materials that will be exposed to high velocity particle impacts, such as polymer-based erosion coatings for helicopter rotor blades or impact shields for satellites. Here, we investigate the high-velocity impact deformation response of ultra-high molecular weight polyethylene (UHMWPE) tape to determine the fundamental impact dynamics and to evaluate how material processing can modify the tape’s mechanical behaviors. Using a laser-induced projectile impact test, we conduct impact experiments using steel particles (~15-30 μm diameter) against UHMWPE tape with linearly structured fibrals at velocities up to 500 m/s. The particles are monitored pre- and post-impact with an ultra-high-speed 16-frame camera with nanosecond time resolution. Post-mortem damage morphologies are assessed as a function of particle impact speed. Based on real-time and post-mortem observations, we discuss the damage mechanisms of the UHMWPE tape under supersonic micro-particle impacts.   

Dynamic experiments to examine the high-pressure, solid phases of cerium

The ability to understand and predict the response of matter at extremes requires knowledge of a materials equation-of-state including the location of phase boundaries, transition kinetics, and the evolution of material strength. Cerium metal exhibits a rich phase diagram at moderate pressures that continues to attract scientific interest as an ideal material for studies focused on the dynamic multiphase properties of materials. Recent dynamic experiments have provided information on the shock-melt transition, and the Hugoniot that spans two solid phases and the liquid. Despite these efforts, the high-pressure, solid region of the phase diagram remains largely unexplored dynamically. Static data have identified the ε phase which exists up to Mbar pressures along a room temperature isotherm. At higher temperatures (greater than 600 K), a direct α-ε transition has been reported although there are disagreements in both slope and location of the boundary. In this work, double-shock loading was used to access the α-ε region of the phase diagram to obtain equation-of-state (EOS) information, and to determine the location of the epsilon phase boundary for shock loading (LA-UR-18-30153).   

Proton radiography experiment probing the double shock nature around the α-ε phase boundary of cerium

Traditionally, to understand the density variations in matter at extreme conditions the standard Rankine-Hugoniot jump conditions are applied to experiments to infer the density. Proton radiography allows the direct measurement of density. The reintroduction of the 40-mm powder driven gas gun into the proton radiography facility at Los Alamos National Laboratory has given us the opportunity to probe the α-ε phase transition states in cerium metal. A couple of experiments were fielded measuring the in-situ material density of the shocked material. A two-phase model which was developed to understand the shock states in double shocked experiments was compared with the as obtained material densities. The density comparison with the model and the interpretation of the data will be discussed. In general, there is a good agreement between the calculations and the experimental densities obtained.
  

Shock compression of additive manufactured metals

Additive manufacturing allows production of novel structures, not possible with conventional methods. These can take the form of complex geometries, unusual microstructures, or even functionally graded materials. The benefits that these could bring to industries such as aerospace and defense have resulted in a large push to develop AM techniques for structural metals. However – the mechanical response of such parts is often far removed from that of their conventional counterparts. Here, we present results on the shock compression of a range of AM metals, including Ti6Al4V, stainless steel, and tantalum, investigating how they respond to dynamic loading. Comparisons of their equation-of-state and damage tolerance to that of wrought samples will highlight the importance of thorough testing and qualification of AM parts, and reveal some of the challenges that need to be addressed before they can replace traditional methods.   

Backward and Forward Analyses of Shocked and Ramp-Compressed Metals to 5 Mbars

In this report, we present a series of shocked and ramp compressed data on various metals including tantalum and tin. These samples were shocked and ramp-compressed to pressures as high as 5 Mbars with graded density impactors (GDI). To analyze these data, we utilize both backward (characteristics) and forward analyses. The former method does not require a priori knowledge of a pressure drive, and often fails in the presence of strength or phase transition. By using both of these analysis techniques, we can explore the possibility of looking at phase transition and strength during ramp compression. This study also gives us insight into the in-situ properties of GDI during ramp compression. The results are compared to non-destructive ultra sound scans of GDIs.   

Update on multi-megabar shockless compression at the Z machine

Quasi-isentropic, shockless ramp-wave experiments promise accurate equation-of-state (EOS) data of materials in the solid phase at relatively low temperatures and multi-megabar (100’s GPa) pressures. In this range of pressure, isothermal diamond-anvil techniques have limited pressure accuracy due to reliance on theoretical EOS of calibration standards, thus accurate quasi-isentropic compression data would help immensely in constraining EOS models. Multi-megabar shockless compression experiments using the Z Machine at Sandia as a magnetic drive with stripline targets have been performed on many solid materials over the past decade. An update is given on recent results and developments, including experimental techniques, analysis methods, and uncertainty quantification.