Bulletin of the American Physical Society
APS March Meeting 2016
Volume 61, Number 2
Monday–Friday, March 14–18, 2016; Baltimore, Maryland
Session Y21: Materials in Extremes: Metals at High Strain Rates IIFocus
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Sponsoring Units: GSCCM DCOMP DMP Chair: Ramon Ravelo, University of Texas, El Paso Room: 320 |
Friday, March 18, 2016 11:15AM - 11:27AM |
Y21.00001: The relaxation of shear stress in a metal alloys with a wide grain size distribution under shock loadings Evgeniya G. Skripnyak, Vladimir V. Skripnyak, Nataliya V. Skripnyak The influence of a grain size distribution on the relaxation of shear stress in the metal alloys under shock wave loading was investigated by numerical simulation. The model takes into account the influence of a grain size distribution and a precipitation concentration on the kinetics of shear stress relaxation. The relaxation rate of shear stress in shock waves depends on the specific volume of nano- and ultra-fine grains in the FCC and HCP metal alloys. A wide distribution of grain size reduces the relaxation rate of elastic precursor in HCP alloys. The relaxation of the elastic precursor depends on size and volume concentration of precipitates in metal alloys. Results of simulation show that the rate of plastic deformation in the shock wave exceeds significantly that of the elastic precursor at the same value of shear stresses. [Preview Abstract] |
Friday, March 18, 2016 11:27AM - 11:39AM |
Y21.00002: Strain Functionals for Characterizing Atomistic Geometries Edward Kober, Sven Rudin The development of a set of strain tensor functionals that are capable of characterizing arbitrarily ordered atomistic structures is described. This approach defines a Gaussian-weighted neighborhood around each atom and characterizes that local geometry in terms of n-th order strain tensors, which are equivalent to the moments of the neighborhood. Fourth order expansions can distinguish the cubic structures (and deformations thereof), but sixth order expansions are required to fully characterize hexagonal structures. Other methods used to characterize atomic structures, such as the Steinhardt parameters or the centrosymmetry metric, can be derived from this more general approach. These functions are continuous and smooth and much less sensitive to thermal fluctuations than other descriptors based on discrete neighborhoods. They allow material phases, deformations, and a large number of defect structures to be readily identified and classified. Applications to the analysis of shock-loaded samples of Cu, Ta and Ti will be presented. This strain functional basis can also then be used for developing interatomic potential functions, and an initial application to Cu will be presented. [Preview Abstract] |
Friday, March 18, 2016 11:39AM - 11:51AM |
Y21.00003: Dual Domain Material Point Method for Materials in Extreme Duan Zhang, Tilak Dhakal Dual domain material point method is the latest version of the material point method designed to overcome many numerical difficulties of the original material point method with an increased numerical accuracy. In this talk, after reviewing the numerical theory of the method, we apply this method to cases involving extreme material deformation, shock propagation, and pulverization based on continuum theories. We will compare this method to other similar particle methods, and then examine the applicability and needed modification of the continuum theory for cases involving strong thermodynamic non-equilibrium. The history of the material deformation is often important in such systems. We will explore the Lagrangian capability brought by the use of particles in the dual domain material point method and introduce a multiscale scheme taking advantages of the particle-mesh communications in the method to study history dependent thermodynamically non-equilibrium systems, caused by extreme material deformations, such as hypervelocity impact and shock loading. We will also discuss the history tracking capability, analyze numerical advantages and difficulties, and show the results obtained from this numerical scheme. [Preview Abstract] |
Friday, March 18, 2016 11:51AM - 12:03PM |
Y21.00004: Multiscale Modeling using Molecular Dynamics and Dual Domain Material Point Method Tilak Dhakal, Duan Zhang For problems with very large material deformation rate, the time scale of material deformation can be shorter than the time that the material takes to reach a thermodynamic equilibrium. In these situations constitutive relation for the material becomes difficult to obtain. Furthermore, for these non-equilibrium problems, the history dependency of the material becomes important. A numerical method capable of tracking material deformation history is needed in a numerical simulation effort. In this work we use the dual domain material point (DDMP) method, which uses Lagrangian material points to track the history of the material where as Eulerian grids are used to calculate the gradients in continuum level. Molecular dynamics (MD) calculations are performed in the material points to calculate the closure quantities such as stress bypassing the need for a constitutive relation. Since the material points do not need to directly communicate among each other, the MD calculations can be done in parallel. In this work, GPUs are used to accelerate MD calculations. Examples of shock wave propagation in monoatomic gas and in Cerium metal are presented. [Preview Abstract] |
Friday, March 18, 2016 12:03PM - 12:15PM |
Y21.00005: \textbf{Study on the oblique perforation of thin steel pates by flat and ogival projectiles } Zitao Guo, Wei Zhang, Peng Ren This paper presents a numerical study on the oblique perforation of thin steel plates. Numerical simulations of 1 mm single A3 steel plates impacted by flat and ogival projectiles at 0°, 15°, 30°, 45° and 60° angles over a range of velocities from 50 to 250 m/s were performed using the finite element code ABAQUS, where a modified versions of the J-C constitutive relation and fracture criterion based on a series of quasi-static and dynamic tensile tests with smooth and notched axisymmetric specimens were adopted to approximate behaviors of target material. Corresponding oblique perforation experiments were also conducted in order to be compared and calibrated. Initial-residual velocity curves and ballistic limits of targets under different angle impact were determined and compared, and the effects of projectile nose shape and obliquity on the ballistic resistance and failure models of targets were investigated. Results show that the nose shape of the projectile and oblique angles severely affected both the energy absorption and the failure mode of the target plate during perforation. Good agreement is found between the numerical simulations and experimental results. [Preview Abstract] |
Friday, March 18, 2016 12:15PM - 12:27PM |
Y21.00006: Ductility of metal alloys with grain size distribution in a wide range of strain rates Vladimir V. Skripnyak, Nataliya V. Skripnyak, Evgeniya G. Skripnyak Ductility of ultrafine grained (UFG) metal alloys with a distribution of grain size was investigated in wide loading conditions by numerical simulation. The multiscale models with a unimodal and a bimodal grain size distributions were developed using the data of structure research of hexagonal close packed and face center cubic UFG alloys. Macroscopic fracture is considered as a result of the formation of percolation clusters of damage at the mesoscopic level. The critical fracture strain of UFG alloys on the mesoscale level depends on the relative volumes of coarse grains. The nucleation of damages at quasi-static and dynamic loading is associated with strain localization in UFG partial volumes with bimodal grain size distribution. The concentration of damages arise in the vicinity of the boundaries of coarse and ultrafine grains. The occurrence of a bimodal grain size distributions causes the increase of UFG alloys' ductility, but decrease of their tensile strength. [Preview Abstract] |
Friday, March 18, 2016 12:27PM - 12:39PM |
Y21.00007: Experimental analysis of quasi-static and dynamic fracture initiation toughness of gy4 armor steel material Peng Ren, Zitao Guo Quasi-static and dynamic fracture initiation toughness of gy4 armour steel material are investigated using three point bend specimen. The modified split Hopkinson pressure bar (SHPB) apparatus with digital image correlation (DIC) system is applied to dynamic loading experiments. Full-field deformation measurements are obtained by using DIC to elucidate on the strain fields associated with the mechanical response. A series of experiments are conducted at different strain rate ranging from 10-3 s-1 to 103 s-1, and the loading rate on the fracture initiation toughness is investigated. Specially, the scanning electron microscope imaging technique is used to investigate the fracture failure micromechanism of fracture surfaces. The gy4 armour steel material fracture toughness is found to be sensitive to strain rate and higher for dynamic loading as compared to quasi-static loading. [Preview Abstract] |
Friday, March 18, 2016 12:39PM - 12:51PM |
Y21.00008: Lithium Melt Line Determined using the Z Method Marvin Zocher, Leonid Burakovsky The Z method is a numerical procedure that can be used for the construction of phase diagrams. In the present work the method is discussed and its use is demonstrated through a determination of the melt line for lithium. [Preview Abstract] |
Friday, March 18, 2016 12:51PM - 1:03PM |
Y21.00009: A fitting empirical potential for NiTi alloy and its application. Guowu Ren, Tiegang Tang, Huseyin Sehitoglu Due to its superelastic behavior, NiTi shape memory alloy receives considerable attentions over a wide range of industrial and commercial applications. Limited to its complex structural transformation and multiple variants, semiempirical potentials for performing large-scale molecular dynamics simulations to investigate the atomistic mechanical process, are very few. In this work, we construct a new interatomic potential for the NiTi alloy by fitting to experimental or ab initio data. The fitting potential correctly predicts the lattice parameter, structural stability, equation of state for cubic B2(austenite) and monoclinic B19'(martensite) phases. In particular the elastic properties( three elastic constants for B2 and thirteen ones for B19') are in satisfactory agreement with the experiments or ab initio calculations. Furthermore, we apply this potential to conduct the molecular dynamics simulations of the mechanical behavior for NiTi alloy and the results capture its reversible transformation. [Preview Abstract] |
Friday, March 18, 2016 1:03PM - 1:15PM |
Y21.00010: Highly Accurate Calculations of the Phase Diagram of Cold Lithium Luke Shulenburger, Andrew Baczewski The phase diagram of lithium is particularly complicated, exhibiting many different solid phases under the modest application of pressure. Experimental efforts to identify these phases using diamond anvil cells have been complemented by ab initio theory, primarily using density functional theory (DFT). Due to the multiplicity of crystal structures whose enthalpy is nearly degenerate and the uncertainty introduced by density functional approximations, we apply the highly accurate many-body diffusion Monte Carlo (DMC) method to the study of the solid phases at low temperature. These calculations span many different phases, including several with low symmetry, demonstrating the viability of DMC as a method for calculating phase diagrams for complex solids. Our results can be used as a benchmark to test the accuracy of various density functionals. This can strengthen confidence in DFT based predictions of more complex phenomena such as the anomalous melting behavior predicted for lithium at high pressures. [Preview Abstract] |
Friday, March 18, 2016 1:15PM - 1:27PM |
Y21.00011: Cold melting of Li under pressure: Perspectives from first-principles molecular dynamics simulations Weiyi Xia, Weiwei Gao, Xiang Gao, Peihong Zhang Despite much work (experiment and theory), the pressure-dependent melting temperature of Li is still under debate. In particular, there is still controversy and significant uncertainty in determining the melting temperature of Li at pressures around 50 GPa. An earlier report [1] suggests that Li melts at as low as 190 K between 40 and 64 GPa. Such a low melting temperature is not likely unless quantum effects of lattice vibration play a significant role. Later experiment [2], on the other hand, reports that Li melts above 300 K under pressured up to 64 GPa and does not seem to support the view that lattice quantum effects to play any important role. In this talk, we will present results from large-scale (large systems and long simulation times) first-principles molecular dynamics simulations and phonon free energy calculations, aiming at resolving some of the issues. [1] C.L. Guillaume et al, Nature Phys. 7, 211 (2011). [2] A. M. J. Schaeffer et al, Phys. Rev. Lett. 109, 185702 (2012). [3] F.A. Gorelli et al, Phys. Rev. Lett. 108, 055501 (2012). [Preview Abstract] |
Friday, March 18, 2016 1:27PM - 1:39PM |
Y21.00012: ABSTRACT WITHDRAWN |
Friday, March 18, 2016 1:39PM - 1:51PM |
Y21.00013: Nature of Pressure-induced Insulating States in Simple Metals Ivan Naumov, Russell Hemley As experimentally established, all the alkali metals and heavy alkaline earth metals (Ca, Sr and Ba) become progressively less conductive on compression, at least up to some critical limit over a broad pressure range. Of these metals, Li and Na clearly undergo pressure-induced metal-insulator transitions, which may also be called reverse Mott transitions. Here, using group theory arguments and first-principles calculations, we show that such transitions can be understood in terms of band representations introduced by Zak. The valence bands in the insulating states are described by simple and composite band representations constructed from localized Wannier functions centered on points unoccupied by atoms [1]. The character of the Wannier functions is closely related to the degree of s-p(-d) hybridization and reflects multi-center chemical bonding in these insulating states. The conditions under which an insulating state is allowed for structures having an integer number of atoms per primitive unit cell as well as re-entrant (i.e., metal-insulator-metal) transition sequences are detailed, resulting in predictions of semimetallic phases with flat surface states. The general principles developed are tested and applied to the alkali and alkaline earth metals, including elements where high-pressure insulating phases have been identified or reported (e.g., Li, Na, and Ca). This research was supported by EFree, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences under Award DESC0001057. [1] I. I. Naumov and R. J. Hemley, PRL, \textbf{114}, 156403 (2015). [Preview Abstract] |
Friday, March 18, 2016 1:51PM - 2:03PM |
Y21.00014: Simulation texture development of polycrystalline aluminum under dynamic loading Xiaomian Hu, Hao Pan, Zihui Wu Effect of texture to dynamic response of polycrystalline metals under dynamic loading attracted much attention because of interesting phenomena and great challenges to experiment and simulation. This paper uses a crystal plasticity finite element method (CPFEM) with a dislocation based hardening law to model the texture development of polycrystalline aluminum under simple compression, uniaxial strain ramp loading and shock wave loading. Strain hardening under the three compression conditions is also compared. The simulation results show that the preferred orientation during of the polycrystalline aluminum under the three compression conditions has some different. It caused normalized stress-strain profiles of state of 1D stress and 1D strain are different when strain is over 5\% and strain rate is same. [Preview Abstract] |
Friday, March 18, 2016 2:03PM - 2:15PM |
Y21.00015: Cooling rates dependence of medium range order development in metallic glasses C. Z. Wang, Y. Zhang, F. Zhang, M. I. Mendelev, M. J. Kramer, K. M. Ho Rapid cooling from metallic liquids is a widely used approach to synthesize novel alloys with desirable properties because such rapid cooling drives phase selection away from equilibrium phases resulting in new metastable phases and morphologies. However, molecular dynamics simulation of such rapid solidifications faces a well-known time-scale challenge that the cooling rate is several orders of magnitude faster than experiments. We propose an efficient cooling strategy in which most of the computer time is spent on a prolonged isothermal process slightly below the glass-transition temperature $T_{g}$. Such a sub-$T_{g}$ annealing reduces the effective cooling rates in MD simulations to \textasciitilde 10$^{\mathrm{7}}$ K/s. The effects of lowering cooling rates on the evolution of short-range and medium-range orders are investigated. The glassy samples prepared in this way demonstrate significant energetic stability, slow dynamics, and well-developed short-range and medium-range orders. [Preview Abstract] |
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