Bulletin of the American Physical Society
APS March Meeting 2016
Volume 61, Number 2
Monday–Friday, March 14–18, 2016; Baltimore, Maryland
Session X21: Materials in Extremes: Metals at High Strain RatesFocus
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Sponsoring Units: GSCCM DCOMP DMP Chair: Tim Germann, Los Alamos National Laboratory Room: 320 |
Friday, March 18, 2016 8:00AM - 8:12AM |
X21.00001: Elastic-plastic structure of shock waves in single crystal copper R. Ravelo, B.L. Holian, T.C. Germann Large-scale atomistic simulations of shock wave propagation in defect-free copper single crystals exhibit an orientation dependent elastic limit and elastic-plastic two-wave regimes for shock propagation along the (110) and (111) low-index directions but not along (100). By contrast, no orientational difference in the Us-Up profiles of single crystals compared with polycrystalline samples has been reported in shock experiments. The elastic-plastic response of copper shocked along (111) was examined via large-scale non-equilibrium molecular dynamics (NEMD) simulations employing samples of up to 3.5 microns in length and particle velocities between 0.5 and 2.5 km/s (20- 130 GPa). The longer time and length scales allow for a more accurate determination of the elastic limit, longitudinal and plastic wave speeds. Results show a steady elastic precursor for particle velocities below 1.6 km/s, which does not decay in time. The NEMD data for the plastic wave velocity in the split-wave regime extrapolates linearly in particle velocity to the shear-wave speed at zero pressure. [Preview Abstract] |
Friday, March 18, 2016 8:12AM - 8:24AM |
X21.00002: Tantalum strength model incorporating temperature, strain rate and pressure Hojun Lim, Corbett Battaile, Justin Brown, Matt Lane Tantalum is a body-centered-cubic (BCC) refractory metal that is widely used in many applications in high temperature, strain rate and pressure environments. In this work, we propose a physically-based strength model for tantalum that incorporates effects of temperature, strain rate and pressure. A constitutive model for single crystal tantalum is developed based on dislocation kink-pair theory, and calibrated to measurements on single crystal specimens. The model is then used to predict deformations of single- and polycrystalline tantalum. In addition, the proposed strength model is implemented into Sandia's ALEGRA solid dynamics code to predict plastic deformations of tantalum in engineering-scale applications at extreme conditions, e.g. Taylor impact tests and Z machine's high pressure ramp compression tests, and the results are compared with available experimental data. Sandia National Laboratories is a multi program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000. [Preview Abstract] |
Friday, March 18, 2016 8:24AM - 8:36AM |
X21.00003: Molecular simulation of dislocation motion in magnesium alloys under high strain rates Peng Yi, Robert Cammarata, Michael Falk Dislocation motion of \textless $a$\textgreater dislocations on the basal and the prismatic planes under simple shear was studied using molecular simulations in Mg/Al and Mg/Y alloys. The critical resolved shear stress (CRSS) was calculated at temperature from 0K to 500K with solute concentrations from 0 to 7 at.{\%}. The strain rates of 10$^{\mathrm{6}}$-10$^{\mathrm{8}}$ s$^{\mathrm{-1}}$ used in the simulation correspond to experimental strain rates of 10$^{\mathrm{1}}$-10$^{\mathrm{5}}$ s$^{\mathrm{-1}}$ based on Orowan's equation. Basal slip is dominated by the \textless $a$\textgreater edge dislocations. Solute hardening to the CRSS follows a power law, $c^{n}$, where $c$ is the solute concentration. The exponent $n$ transitions from close to 2/3 at low temperature to close to 1 at high temperature. Temperature and strain rate effects on the CRSS are captured by Kocks model based on thermally activated events. Prismatic slip is controlled by the \textless $a$\textgreater screw dislocation that cross-slips between the basal and the prismatic planes, in a locking-unlocking pattern. Temperature affects the slip kinetics through the diffusion of the screw dislocation on the basal plane, which leads to vacancy and loop generation. Solute softening was observed for both Mg/Al and Mg/Y alloys. The softening on prismatic slip is due to the solute pinning effect on the basal plane, and Al is more effective in softening. [Preview Abstract] |
Friday, March 18, 2016 8:36AM - 8:48AM |
X21.00004: Simulating the dynamic response of magnesium alloys Jeffrey Lloyd, Richard Becker Unlike several conventional metals, the mechanical response of magnesium is severely anisotropic for quasistatic and dynamic loading conditions. In this work we present a crystal-based strength model that is the same order of magnitude in computational cost as rate-dependent isotropic strength models, yet is able to capture essential features exhibited by textured magnesium polycrystals. The model demarcates plastic deformation into contributions from basal slip, extension twinning, and non-basal slip mechanisms. Comparisons are made between model predictions and experiments for two magnesium alloys with differing processing histories. The model is then used to explore and quantify the dependence of metallurgical and processing variations for several dynamic experiments that probe propensity for localization and failure under complex loading conditions. [Preview Abstract] |
Friday, March 18, 2016 8:48AM - 9:00AM |
X21.00005: \textbf{Deformation twinning in a polycrystalline magnesium alloy during dynamic compression} Caleb Hustedt, Jeffrey Lloyd, Paul Lambert, Vignesh Kannan, Daniel Casem, K.T. Ramesh, Nicholas Sinclair, Richard Becker, Todd Hufnagel We report the results of combined in situ x-ray diffraction studies and crystal plasticity modeling of deformation twinning in polycrystalline magnesium during dynamic compression. Diffraction experiments were conducted at the Dynamic Compression Sector (DCS) of the Advanced Photon Source, on magnesium alloy (AZ31B) specimens (with various crystallographic textures) loaded at strain rates of \textasciitilde 1000~s$^{\mathrm{-1}}$ in a compression Kolsky bar. The diffraction patterns, recorded with temporal resolution of 5-10~microseconds, provide information about the evolution of crystallographic texture during deformation, which we interpret in terms of the twinning mechanism (so-called ``extension'' or ``tensile'' twinning). We compare our observations quantitatively with predictions of the evolution of crystallographic texture from an efficient reduced crystal plasticity model. This model explicitly accounts for basal slip and extension twinning on a rate-independent basis, but treats other mechanisms (pyramidal and prismatic slip) as isotropic, rate-dependent functions. This combination yields substantial improvements in efficiency over full crystal-plasticity models while retaining key aspects of the most important deformation mechanisms. [Preview Abstract] |
Friday, March 18, 2016 9:00AM - 9:12AM |
X21.00006: Deformation twinning activated $\alpha \to \omega $ transformation in titanium under shock compression. Hongxiang Zong, Turab Lookman Materials dynamics, especially the behavior of solids under extreme compression, is a topic of broad scientific and technological interest. However, less is known of the role of grain boundary structures on the shock response of hexagonal-close-packed metals. We use molecular dynamics simulations to study deformation mechanisms in shock compressed Ti bicrystals containing three types of grain boundary (GB) microstructures, i.e., coherent twin boundaries (CTBs), symmetric incoherent twin boundaries (ITB) and \textbraceleft 1-210\textbraceright asymmetric tilt grain boundaries. Our results show that both dislocation activity and the $\alpha \to \omega $ phase transformation in Ti are sensitive to the GB characteristics. In particular, we find that the elastic shock wave can readily trigger the $\alpha \to \omega $ transformation at CTBs but not at the other two GBs, and the activation of the $\alpha \to \omega $ transformation at CTBs leads to considerable wave attenuation (i.e., the elastic precursor decay). Combined with first principle calculations, we find that CTBs can facilitate the overcoming of the energy barrier for the $\alpha \to \omega $ transformation. Our findings have potential implications for interface engineering and materials design under extreme conditions. [Preview Abstract] |
Friday, March 18, 2016 9:12AM - 9:24AM |
X21.00007: Microstructural Effects on Materials under Extreme Dynamic Environments Cyril Williams Studies have shown that microstructure and microstructure evolution can play a major role on the shock response of metals and metallic alloys. When metals and metallic alloys are deformed during shock compression, large numbers of lattice defects such as dislocations can be introduced in the material. These dislocations can lead to strengthening effects such as hardening and/or softening such as dynamic recovery which may consequently change the material behavior. Therefore, to better understand the effects of microstructure and microstructure evolution on the spall response of metals, both in-situ and end-state gas gun plate impact experiments were employed to study 1100-O aluminum. The results show a sharp increase in pullback velocity for 1100-O aluminum with increase in peak shock stress between 4.0 and 8.3 GPa due to shock hardening, followed by a decrease for peak shock stresses up to 12.0 GPa due to softening induced by dynamic recovery. addition, the effects of microstructure on the spall properties of two magnesium alloys fabricated via ECAE (AZ31B-4E) and SWAP (AMX602) were also investigated. The pullback velocities were found to decrease by approximately 15{\%} for AZ31B-4E between 1.7 GPa to 4.6 GPa shock stress. On the contrary, the pullback velocities for AMX602 were found to be random for the same shock stress range studied. Residual microstructure of the post-shocked AZ31B-4E magnesium shows that aluminum-manganese intermetallic inclusions were perhaps responsible for the reduction in pullback velocity. Also, the post-shocked residual microstructure of the AMX602 magnesium revealed features that may have been responsible for its random response. [Preview Abstract] |
Friday, March 18, 2016 9:24AM - 9:36AM |
X21.00008: Damage Tolerant Microstructures for Shock Environments Ellen Cerreta, Darcie Dennis-Koller, Juan Pablo Escobedo, Saryu Fensin, Steve Valone, Carl Trujillo, Curt Bronkhorst, Ricardo Lebensohn While dynamic failure, due to shock loading, has been studied for many years, our current ability to predict and simulate evolving damage during dynamic loading remains limited. One reason for this is due to the lack of understanding for the linkages between process-induced as well as evolved microstructure and damage. To this end, the role of microstructure on the early stages of dynamic damage has been studied in high purity Ta and Cu. This work, which utilizes plate-impact experiments to interrogate these effects, has recently been extended to a subset to Cu-alloys (Cu-Pb, Cu-Nb, and Cu-Ag). These multi-length scale studies, have identified a number of linkages between damage nucleation and growth and microstructural features such as: grain boundary types, grain boundary orientation with respect to loading direction, grain orientation, and bi-metal interfaces. A combination of modeling and simulation techniques along with experimental observation has been utilized to examine the mechanisms for the ductile damage processes such as nucleation, growth and coalescence. This work has identified differing features of importance for damage nucleation in high purity and alloyed materials, lending insight into features of concern for mitigating shock induced damage in more complicated alloy systems. [Preview Abstract] |
Friday, March 18, 2016 9:36AM - 9:48AM |
X21.00009: Rate-dependent scaling laws for spall failure Justin Wilkerson, KT Ramesh Here we derive simple bounds on the growth rate of voids considering the combined retarding effects of micro-inertia and dislocation kinetics. We make use of these bounds to derive simple scaling laws capable of predicting the strong rate-dependence of spall strength. We show that the rate-sensitivity exponent for spall strength is bounded to below 6/7 when micro-inertia is the dominant retarding effect on void growth. However, under conditions in which the void growth is predominately governed by dislocation kinetics the rate-sensitivity exponent may rise to a maximum value of 1. With these scaling laws in hand, we go on to further explore the role of microstructure on spall strength. Though simple, the derived scaling laws compare well with experimental measurements and prove useful in shedding light on some of the more perplexing observations associated with spall failure. In particular, the scaling laws are helpful in understanding the somewhat anomalous dependence of spall strength on pre-existing microstructure, e.g. grain size and purity content. [Preview Abstract] |
Friday, March 18, 2016 9:48AM - 10:00AM |
X21.00010: Diffusion of Dissipative Correlation in the Dynamic Failure of Solids Dennis Grady A property identified as the dissipative action has found application as a unifying attribute underlying the dynamic failure of solid materials. Failure modes include tensile spall, impact-induced dynamic shear, shock compaction and steady shock-wave compression. The present work explores the possible application of Langevin dynamics and related statistical mechanical implications as underlying the extreme dynamic failure of solids. [Preview Abstract] |
Friday, March 18, 2016 10:00AM - 10:12AM |
X21.00011: A model for ductile metal friction at high velocities J. E. Hammerberg, R.J. Ravelo, T.C. Germann We describe a meso-macro scale model for the frictional force at ductile metal interfaces for high velocities and large compressions. The model incorporates the micro-mesoscopic growth and refinement of material microstructure in a highly strained region at the sliding interface and incorporates both rate dependent plasticity and thermal conduction. The model compares favorably with recent large scale (1.8 billion atom) simulations to 50 ns of 3-dimensional polycrystalline 13-50 nm grain size Al-Al interfaces at pressures of 15 GPa using the SPaSM NonEquilibrium Molecular Dynamics (NEMD) simulation code. [Preview Abstract] |
Friday, March 18, 2016 10:12AM - 10:24AM |
X21.00012: Novel Feed-through Richtmyer-Meshkov Instability (RMI) Experiment for Characterization of Dynamic Material Response Saul Opie, Sudrishti Gautam, Elizabeth Fortin, Jenna Lynch, Eric Loomis, Pedro Peralta Hydrodynamic instabilities occur often in applications where forces act across a bimaterial interface. In Rayleigh-Taylor (RT) instabilities, surface perturbations grow exponentially under opposing pressure and density gradients. In the closely related Richtmyer-Meshkov (RM) instability, the same perturbations grow linearly due to an impulsive acceleration, e.g., a passing shock wave. These effects are often analyzed with linear fluid theory, but it is well known that for materials possessing shear strength the perturbation evolution can be significantly affected. A challenge in modeling these effects is that existing knowledge of the interplay between strength and hydrodynamic instabilities in solids is limited for the loads and strain rates that are typically used to study them. We have developed novel feed-through RM instability experiments that are useful to understand and model this interplay. We will describe the experimental setup and show simulations that agree well with experimental results in two materials, one-phase copper, and iron loaded above and below the alpha-epsilon phase boundary, where modeling used a phase-aware strength model. In copper, the growth of surface perturbations is quite sensitive to strength model parameters, and so is the amplitude of the shock front perturbations. This is also observed in iron, along with an additional sensitivity in the modeling results to the parameters used to describe phase change kinetics. [Preview Abstract] |
Friday, March 18, 2016 10:24AM - 10:36AM |
X21.00013: A method using the surface perturbation to determine the strength of materials Jianwei Yin, Hao Pan, Zihui Wu, Xiaomian Hu The determination of the dynamic strength of materials at high pressure and strain rate has been focused by the shock dynamics community for many years. This paper simulated the surface perturbation problems under high pressure and strain rate loading. By adjusting the characteristic geometric variables as wave number, amplitude of initial perturbation at the free surface, we studied the growth of the perturbations in the improved forms of the theoretical results given by Piriz et al. (Phys. Rev. E 78, 056401, 2008). The critical condition that the surface perturbation was restrained from unstable grow was also deduced. In the stable region of perturbation growth, the relationship among the relative velocities, displacements of the key positions at the perturbed surface and the material strength was studied. The experimental feasibility of using the improved relationship to determine the strength of materials was also discussed. [Preview Abstract] |
Friday, March 18, 2016 10:36AM - 10:48AM |
X21.00014: Dynamic Failure Mode Transitions in 7075Al Expanding rings driven by Electromagnetic loading Mingtao Liu, Tiegang Tang, Zhaoliang Guo, Cheng Fan Dynamic failure mode transitions are observed in 7075Al electromagnetic expanding rings with a typical size of 3mm in thickness and 0.5mm in height. The rings are driven to maximum expanding velocities ranged from 60m/s to 180m/s, corresponding to strain rates of about 3000 to 9000 per second. At lower strain rates, the fractures of the rings are dominated by the hoop tensile stress, and the cracks are along the radial direction. At higher strain rates, the fractures of the rings are dominated by the maximum shear stress, and the cracks are lie along with an angle of about 45 degree with the radial direction. While the rings deform at medium strain rates, a mixed failure mode is observed, which simultaneously consists of tensile fracture and shear fracture. The failure strains of the specimen and the numbers of the fragmentations were measured after testing. The failure strains show a maximum value as the strain rate increasing, but the numbers of the fragmentations increase firstly, then decrease and then increase again. These phenomena were found to have a close relationship with the dynamic failure mode transitions. [Preview Abstract] |
Friday, March 18, 2016 10:48AM - 11:00AM |
X21.00015: Study of expanding fracture behavior of a copper cylinder under hollow explosive loading. Cheng Fan, Zhaoliang Guo, Mingtao Liu, Tiegang Tang We study the expanding fracture behavior of a copper cylinder under hollow explosive loading. Besides the tensile fracture along the circumferential direction, spall fracture along radial direction occurs, which is evidenced by the step-like behavior with three velocity jumps in the free surface velocity curves and microstructure study of the soft recovered fragments. After considering the spall fracture mechanism, a numerical simulation is carried our and the result shows good agreement with the experiment data. [Preview Abstract] |
Friday, March 18, 2016 11:00AM - 11:12AM |
X21.00016: Compressible Heating in the Condense Phase due to Pore Collapse in HMX Ju Zhang, Thomas Jackson Axisymmetric pore collapse in HMX is studied numerically by solving multi-phase reactive Euler equations. The generation of hot spots in the condense phase due to compressible heating is examined. The motivation is to improve the understanding of the role of embedded cavities in the initiation of reaction in explosives, and to investigate the effect of hot spots in the condense phase due to compressible heating alone, complementing previous study on hot spots due to the reaction in the gas phase and at the interface. It is found that the shock-cavity interaction results in pressures and thus temperatures that are substantially higher than the post-shock values in the condense phase. However, these hot spots in the condense phase due to compressible heating alone do not seem to be sufficiently hot to lead to ignition at shock pressures of 1-3 GPa. Thus, compressible heating in the condense phase may be excluded as a mechanism for initiation of explosives. It should be pointed out that the ignition threshold for the temperature, the so-called ``switch-on'' temperature, of hot spots depend on chemistry kinetics parameters. Switch-on temperature is lower for faster reaction rate. The current chemistry kinetics parameters are based on previous experimental work. [Preview Abstract] |
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