Damage from low-temperature irradiation and the subsequent degradation of materials performance pose sig-nificant challenges for the storage of radioactive materials and for peripheral components in some nuclear reactor designs. Fully understanding the mechanical behavior of such materials requires test data for strain rates in both the quasi-static (< 10/s) and dynamic (>> 10/s) regimes. While dynamic testing has generally been avoided in the past for neutron irradiated (contamination concerns) and ion irradiated (insufficient volume) materials, surface-sensitive Richtmyer-Meshkov instability (RMI) tests were used in the present work to overcome these limitations. Here, nanopillar compression, nanoindentation, and RMI testing data from a helium implanted surface layer (similar to 10 mu m thick) were compiled to explore the effects of helium bubbles on the materials strength of high-purity copper at strain rates of 0.001/s - 10(8)/s. While nano-mechanical testing revealed increases in yield strength and hardness with increasing helium dose from 1000 to 4000 appm He, RMI indicated no significant changes in strength as compared to unimplanted copper. This discrepancy in behavior was rationalized through a combination of recent literature and follow-on molecular dynamics (MD) simulations, leading to the conclusion that the nanoscale helium bubbles acting as dispersed barriers to dislocation motion at quasi-static strain rates collapse under shock loading and cease to be effective barriers at high strain rates.

Inert simulant materials, or “mocks”, are often used as surrogates for plastic-bonded explosives (PBX) in non-detonative tests in order to mitigate hazards. Mocks should reproduce as many non-detonative properties of the explosive as possible, including structural behavior in a variety of thermal and mechanical environments. Recently, the molecular crystal idoxuridine (IDOX) has been identified as an ideal mock for the main component in the explosive polymer-matrix composite PBX 9501, and has performed favorably under quasistatic loading conditions. Here, in order to assess robustness over a range of mechanical environments, plastic-bonded IDOX was compression tested from 0.001/s to 2000/s strain rates and compared to PBX 9501 historical data. Plastic-bonded IDOX showed good agreement to PBX 9501 across these strain rates, justifying continued development and production as a mock.

Interfacial friction is a key aspect to understanding and modelling dynamic processes in which materials interact. However, friction is a complex phenomenon that depends on a multitude of factors, including sliding velocity. Understanding how friction behavior changes as a function of sliding rate is thus crucial for accurately simulating dynamic processes. Recent literature has shown that the split-Hopkinson pressure bar can be adapted for friction measurements associated with high sliding rates. The present work introduces an insert designed to be transferrable between a quasi-static load frame and a compression split-Hopkinson bar, enabling friction measurements across a wide range of sliding velocities (10 -4 – 20 m/s). Here, the split-Hopkinson pressure bar setup is modelled using a multiphysics research code (FLAG), developed at Los Alamos National Laboratory (LANL), to identify and reduce potential issues in the configuration prior to experimental implementation.

Phase transformations play an important role in the mechanical behavior of materials subjected to extreme loading conditions. A series of shock-reshock experiments were fielded to determine whether the phase transitions in materials are significantly enhanced or inhibited by preexisting microstructural features. Polycrystalline zirconium samples were shock loaded using gas-gun plate impact and soft recovered to examine microstructure using electron backscatter diffraction (EBSD). Drive conditions were varied to study the (hcp) alpha to (hexagonal) omega solidsolid phase transformation. Recovered samples were then subjected to a second shock loading event to determine the change in material behavior as a function of pre-shock microstructure. Crystallography of phase fragments in the final microstructure showed that prior twinning (formed during the shock to a peak stress below the transition threshold) appeared to suppress omega formation/retention after reshock. Conversely, when a material was initially shocked into the omega phase field, retained-omega was not found to have a large impact on subsequent omega formation during reshock. This suggests that nucleation and growth of omega phase are important processes, and the relative activity of nucleation vs. growth processes is modified by a pre-existing substructure. Additionally, orientation relationships reveal a reverse transformation pathway (omega to alpha) dominates the final microstructure, suggesting significant grain growth in the omega phase field is possible even for dynamic timescales.

The high-strain-rate response of pure magnesium and AZ31B magnesium alloy is examined in compression and in a forced shear-loading top-hat configurations. Compression specimens loaded in the direction normal to the plane of the rolled plate (TT) display higher-strain-rate sensitivity than specimens that were loaded within the plane of the rolled plate (IP). This effect is more pronounced for pure magnesium as compared to the alloy, due to increased twinning in the IP direction as compared to the TT. Additionally, top-hat shear specimens loaded at high strain rates are observed to display stable deformation during loading, and the development of adiabatic shear bands is not observed. We hypothesize that this result is due to adiabatic heating during deformation, which enhanced the contribution of slip, lessened the role twinning, and possibly activated dynamic recrystallization processes, thus, preventing the formation of distinct shear bands.

Natural kamacite samples (Fe92.5Ni7.5) from a fragment of the Gibeon meteorite were studied as a proxy material for terrestrial cores to examine phase transition kinetics under shock compression for a range of different pressures up to 140 GPa. In situ time-resolved X-ray diffraction (XRD) data were collected of a body-centered cubic (bcc) kamacite section that transforms to the high-pressure hexagonal close-packed (hcp) phase with sub-nanosecond temporal resolution. The coarse-grained crystal of kamacite rapidly transformed to highly oriented crystallites of the hcp phase at maximum compression. The hcp phase persisted for as long as 9.5 ns following shock release. Comparing the c/a ratio with previous static and dynamic work on Fe and Fe-rich Fe-Ni alloys, it was found that some shots exhibit a larger than ideal c/a ratio, up to nearly 1.65. This work represents the first time-resolved laser shock compression structural study of a natural iron meteorite, relevant for understanding the dynamic material properties of metallic planetary bodies during impact events and Earth’s core elasticity.

Iron is a key constituent of planets and an important technological material. Here, we combine in situ ultrafast x-ray diffraction with laser-induced shock compression experiments on Fe up to 187(10) GPa and 4070(285) K at 10^{8}  s^{-1} in strain rate to study the plasticity of hexagonal-close-packed (hcp)-Fe under extreme loading states. {101[over ¯]2} deformation twinning controls the polycrystalline Fe microstructures and occurs within 1 ns, highlighting the fundamental role of twinning in hcp polycrystals deformation at high strain rates. The measured deviatoric stress initially increases to a significant elastic overshoot before the onset of flow, attributed to a slower defect nucleation and mobility. The initial yield strength of materials deformed at high strain rates is thus several times larger than their longer-term flow strength. These observations illustrate how time-resolved ultrafast studies can reveal distinctive plastic behavior in materials under extreme environments.

Spall fracture is a high strain-rate damage phenomenon associated with shock or impulsive loading events. When a material that has been subjected to shock compression is allowed to release, rarefaction waves propagate into the sample and reduce the internal stress to zero. If multiple rarefaction waves intersect, they generate tension which, if sufficient, can nucleate voids in the material. It has been observed in several works investigating spall fracture that although the shock-wave profiles suggested spall occurred, imaging of the recovered sample revealed no voids or cracks. In this study, we aim to determine whether a second shock event could recompact existing spall damage, and if so, what form does the microstructure at the recompaction interface have? Through a series of gas-gun flyer-plate impact experiments, we demonstrate that modest shock stresses of 2 GPa–3 GPa are enough to both fully compact a damaged copper target back to a state of zero porosity and, furthermore, drive recrystallization of the interface such that there is a new bond formed where the free surfaces were brought together.

Understanding shock wave propagation behavior in coatings is valuable for predicting their performance in high-velocity impacts. Samples of stainless steel were deposited by low pressure plasma spray and cold gas dynamic spraying. The sound speeds of the deposits were measured, and the deposits were tested in flyer plate impact tests in a gas gun to determine the shock propagation and porosity compaction properties. The most porous sample was tested to determine its equation of state, while the other samples were tested to measure shock wave profiles as a function of deposition parameters. Comparison of the results between the coatings and to reference wrought stainless steel shows the effect of deposition conditions on the dynamic behavior of the deposits. Higher deposit density leads to higher sound speed. The shock speed for the highest porosity plasma sprayed deposit was below that of wrought stainless steel. The Hugoniot elastic limit stress for cold sprayed deposits is higher than that for plasma sprayed deposits due to the higher degree of cold work. Porosity crush up times were found to vary with the total porosity of the deposit. Better understanding of the behavior of these deposits under extreme impact conditions is demonstrated.

Understanding the friction behavior between two sliding bodies can inform the design of machines, processing of materials, and simulation of dynamic processes. Kinetic friction is a complex phenomenon that depends on a multitude of factors such as sliding velocity, normal force, contact area, surface roughness, material properties, lubrication conditions, and thermal effects. This literature review covers the major known effects of sliding velocity, normal load, and surface roughness on the measured kinetic friction coefficient in the context of microscopic friction phenomena. Classic macroscale friction models are reviewed to illustrate approaches for simulating friction behavior. Prominent experimental friction setups within the literature are discussed with respect to achievable velocity and pressure regimes. The background information gathered here will be used to inform experimental procedures and modeling strategies of the exploratory research (ER) project titled “Measurement of Dynamic Friction via Kolsky Bar” (20200418ER).

High purity single crystal titanium (Ti) under shock wave loading is modeled under both onedimensional and three-dimensional cylindrical conditions. Cylinder sizes of 10 um and 20 radius are both considered in order to assess influence of boundary conditions. A thermodynamically consistent single crystal model for application to shock conditions is presented. The model accounts for the coupled non-linear elastic, dislocation slip, deformation twinning, and structural phase transformation response of the titanium material. Plate impact experiment results using a copper flyer are used to compare against the simulations for crystals oriented in [0001] and [10 (1) over bar1] crystallographic directions. The one-dimensional and three-dimensional simulations of the two differently oriented single crystals indicate differences between the onedimensional and three-dimensional representation, especially for the [10 (1) over bar1] oriented single crystal. This orientation breaks the relative orientation symmetry between the crystal and cylinder which otherwise exists for the [0001] oriented single crystal. A significant amount of heterogeneity in the field response of the [10 (1) over bar1] oriented simulation was demonstrated due to the highly coupled nature of the deformation. A bi-crystal model composed of both the [0001] and [10 (1) over bar1] orientations with the boundary between the two along the axis of the cylinder is also considered for cylinder model sizes of 10 and 20 mu m. The results indicate a strong interaction between the two grains that affects the omega phase volume fraction achieved relative to the single crystal calculations.

Additive manufacturing of metal components results in unique microstructures with, necessarily, mechanical properties that are distinct from conventionally produced components. In this work, four distinct microstructural features associated with directed energy deposition of 304L stainless steels, their stability, and their influences on flow strength were examined. These were (1) high dislocation density comparable with deformed materials, (2) increased ferrite content, (3) local chemical heterogeneity, and (4) tortuous grain morphology. In situ neutron diffraction measurements were used to monitor the evolution of the as-built microstructure during post-build heat treatment and relate the specific microstructural features to the strength behavior of the material following the heat treatment. The increased flow strength of the additively manufactured material relative to wrought counterparts is found to be due primarily to an increased dislocation density in the as-built material. However, the increased dislocation density does not completely account for the increased strength and it is hypothesized that some of the additional strength is related to the unique AM grain structure.

Under plate impact experiments (uniaxial strain) single crystal explosives exhibit elastic-plastic mechanical behavior, however at quasi-static rates (uniaxial stress) they are brittle. We have conducted Split-Hopkinson Pressure Bar (SHPB) experiments to bridge the strain rate gap between the two extremes in an effort to tease out the effects of strain rate and pressure on the plasticity, and to probe the mechanisms of failure in single crystal RDX, PETN and PBX9501. Samples were compressed in different crystallographic orientations to promote different proposed deformation and fracture mechanisms, while utilizing in-situ synchrotron X-ray diffraction, phase contrast imaging, or high speed visible light imaging. Researchers have postulated that in sub-shock impacts, the mechanisms of stress dissipation an explosive possesses are very important to “hot spot” formation—which initiates the first chemical reactions within an energetic. What is “unpredictable” or “stochastic” for an observer at the macroscale, might in fact, be deterministic when a complete understanding of the microscale physics is understood. This paper will focus on the development of the mini- and micro-Kolsky bars utilized in order to maximize the strain rate within the samples, the initial results on the mechanical behavior and fracture mechanisms of these high explosives, and the challenges we have encountered and overcome.

High purity single crystal titanium (Ti) under shockwave loading in plate impact experiment is modeled and simulated, with help of the finite element. A thermodynamically consistent system of equations is formulated in the frame of large deformation to consider material anisotropy, rate dependence, and multi-physics including nonlinear elasticity, dislocation based plastic slip, deformation twinning, and phase transformation. A novel kinematics is proposed to consider phase transformation in the α twin variants and also to consider the dislocation based slip in all of components of parent material, primary twins and the ω phase. The thermodynamically consistent driving forces for plastic slip, twinning, and phase transformation are developed based upon the second law of thermodynamics. For nonlinear elasticity, the stiffness matrix is dependent on the volume fractions of all components, and the volumetric part of Cauchy stress is obtained from the nonlinear equation of state. Dislocation based plastic slip in multi-variant and multiphase heterogeneous materials is developed and interactions of dislocations among all slip modes are taken into account. A mechanism for dislocation density evolution during twinning and phase transformation is proposed. The shock loading along the [0001] and [101¯1] directions of single crystal high purity Ti is investigated computationally. Very good correspondence between simulation and experiment is obtained, which includes pole figures, volume fractions of components, free surface velocity, peak pressure, and phase transformation pressure. Multiple experimental phenomena are interpreted based upon the progression of dislocation slip, deformation twinning, and phase transformation. In compression with the [101¯1] crystal, a higher volume fraction in the primary twins but a lower secondary twin volume fraction in the [0001] crystal in experiment was observed. The higher propensity for phase transformation occurs in the [0001] crystal is reproduced. In addition to material texture, distributions of temperature, stresses, and plastic strains dependent on the impact loading directions are revealed.

Under sufficient stresses, such as during dynamic loading, titanium experiences a phase transformation from hcp alpha phase to hexagonal omega phase. Omega phase is often retained in the microstructure after unloading, and has a strong influence on subsequent mechanical properties. Simulations suggest there are multiple pathways and underlying mechanisms for this transformation. Due to the incredibly short timescales involved, experimental measurements for model validation have been difficult. However, new capabilities at the Advanced Photon Source have enabled diffraction measurements during plate impact experiments to study the evolution of titanium during transformation. These high-rate data allow us to probe the mechanism and kinetics of phase transformations in new ways. Recent results will be presented and compared to post-mortem characterization of soft-recovered shocked specimens. Comparisons are made with previous tests where material was shock-loaded and soft recovered for microstructural analysis. Together these techniques create a consistent picture of material behavior during the shock-induced ff–! phase transformation in titanium.

Additively manufacturing tantalum is a challenging process in which obstacles are stemming from the high melting temperature and susceptibility to oxidation of tantalum. Several combinations of deposition parameters were considered in an effort to obtain fully dense additively manufactured tantalum produced on an EOSINT M280 DMLS system. Deposition parameters significantly affect the resulting microstructure of additively manufactured tantalum, altering grain morphology, grain size, crystallographic preferred orientation, and deposition porosity. Due to the nature of the laser sintering process applied, which implies large directional temperature gradients, the resulting microstructures were strongly columnar along the building direction. Microstructural differences for different deposition conditions manifested themselves in both grain morphology and preferred crystal orientations in the columnar grains. Deposition speed and laser power were important parameters to consider for obtaining porosity-free material. Stripe width had the most significant effect on grain growth.

Coupled elasticity, plastic slip, and twinning in high-purity single crystal titanium loaded by split-Hopkinson pressure bar (SHPB) are investigated, in the framework of large deformation and by utilizing the finite element method (FEM). A thermodynamically consistent system of equations for combined plastic slip and twinning is formulated. Novel kinematics is proposed to develop the driving forces for slip and twinning processes and to consider plastic slip in twins. Dislocation based crystal plasticity is proposed, with an emphasis on the interactions among all slip modes in multi-variant/multiphase heterogeneous materials. A mechanism for dislocation density evolution during twinning is proposed. The impact loading along the [0001] and [101¯1] directions of single crystal high purity titanium is investigated. The evolution of stress-strain field, dislocation density, and volume fraction for each variant in the sample during the entire loading process are obtained and discussed in detail. Results in simulations show that for the [0001] specimen due to the existence of six symmetry primary twin variants with respect to the loading direction and fifteen different slip systems in each variant, the fields of stress and dislocation density are very homogenous in the sample. The simulation results are compared with experimental data for the [0001] and [101¯1] specimens. The experimental results are explained and interpreted for both specimens, which include the pole figures, the stress-strain curves, and volume fraction of variants. The simulation results provide an important insight into mechanical responses of high-purity single crystal titanium under high rate loading, and this paper advances the mesoscale modeling and simulations in coupled plastic slip and twinning.