Bulletin of the American Physical Society
15th APS Topical Conference on Shock Compression of Condensed Matter
Volume 52, Number 8
Sunday–Friday, June 24–29, 2007; Kohala Coast, Hawaii
Session M7: Inelastic Deformation-Mostly Strain |
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Chair: Vitali Nesterekno, University of California, San Diego Room: Fairmont Orchid Hotel Promenade III |
Wednesday, June 27, 2007 10:30AM - 10:45AM |
M7.00001: High strain rate response of an elastomer. Tong Jiao, Rodney Clifton, Stephen Grunschel Pressure-shear plate impact experiments have been conducted to study the mechanical response of an elastomer (polyurea) at very high strain rates: 10$^{5}$ - 10$^{6}$ s$^{-1}$. Thin samples are cast between two hard steel plates. Longitudinal waves reverberating through the sample are used to determine the slope of the isentrope at compressive stresses greater than, say, 500 MPa - the pressure at impact. Release wave experiments, combined with plane wave simulations, are used to extend the isentrope into the tensile regime. Because the shearing resistance of polyurea depends strongly on pressure, two approaches are used to investigate the regime of high shearing rate and low pressure. First, an unloading longitudinal wave reflected from the rear surface of the target assembly is made to arrive at the sample midway through its loading by the incident shear wave. As a result, the sample is sheared at high strain rates and both high and low pressure during a single experiment. Second, the thickness of the flyer and front plates are selected such that the compressive pulse passes through the cast-in-place sample before the shear wave arrives, allowing the shearing resistance to be measured at zero pressure. Results of these experiments and their simulation will be presented. [Preview Abstract] |
Wednesday, June 27, 2007 10:45AM - 11:00AM |
M7.00002: Finite element based micromechanical modeling of brittle materials under compressive loading Reuben Kraft, Jean-Fran\c{c}ois Molinari, K.T. Ramesh The performance of brittle materials is tightly linked to damage mechanisms at microstructural length scales. Thus, robust micro-level models are needed to adequately describe macro-level performance for materials many times subjected to extreme loading conditions. With a focus on brittle failure under compressive loading, this presentation discusses the results of a numerical framework designed to model damage evolution at the microstructural level. A two-dimensional plane strain finite element model has been developed in which intergranular cracking is explicitly modeled using cohesive interfaces with well-characterized material parameters and an optimized contact algorithm. Effects of confinement, friction, strain rate, and spatial distribution of flaws on the macroscopic strength will be presented. In addition, the inhomogenity of damage evolution is observed through use of the microstructure's dual graph providing valuable insight into the damage process. [Preview Abstract] |
Wednesday, June 27, 2007 11:00AM - 11:15AM |
M7.00003: A Novel integrated experimental-numerical method for characterisation of materials at high strain rates. Ben Elliott, Arin Jumpasut, Nik Petrinic, Clive Siviour Accurate prediction of material response at high strain rates necessarily requires an integrated approach to developing, calibrating and validating constitutive models. Experimental characterisation is a challenging task and simplified analyses inherently contain a number of unrealistic assumptions. These lead to results that are insufficiently accurate for use in challenging industrial design such as that found in aerospace applications. These problems can be avoided by the use of an integrated experimental-numerical approach which explicitly models non-ideal aspects of the characterisation procedures. This paper will demonstrate such an approach, where the problem is addressed by solving a related inverse problem. Calibration experiments and instrumentation thereon must be carefully chosen to provide appropriate information for a suitable numerical model of the material. In this case, Hopkinson bar experiments at various temperatures were used in conjunction with high speed photography and image processing to provide accurate experimental data. These were used directly within numerical models of the experiments in order to form a problem that could be accurately solved using inverse methods to yield useful physical material information. The properties obtained and material models chosen were validated using an experiment sufficiently complex to be industrially meaningful. [Preview Abstract] |
Wednesday, June 27, 2007 11:15AM - 11:45AM |
M7.00004: Atomistic simulation of plasticity, spall damage and fracture of crystalline and polycrystalline metals under high strain rate Invited Speaker: Modeling and simulation of dynamic atomistic phenomena and processes in condensed matter are considered, which accompany intensive shock compression and release, uniaxial and hydrostatic stretching. Standards are presented for molecular dynamics (MD) modeling and simulation of relaxation processes: (1) the choice of system sizes, particle numbers and boundary conditions is discussed with respect to the spatial and temporal requirements and restrictions imposed by correlation lengths and correlation times of the processes to be modeled; (2) instantaneous and time averaged diagnostics are considered, spatial resolution included. The diagnostics includes study of (a) time evolutions of distributions of macroparameters (stress, temperature etc.) and structural characteristics (dislocation motion, void growth); (b) deviations of atom velocity and position distributions from the equilibrium ones etc. A hierarchy of dynamic and stochastic processes is introduced by the comparison of time scales with the dynamic memory time (predictability limit) which appears as a result of the Lyapunov instability of particle trajectories. Some theoretical MD based multi-scale approaches are presented which could be used to extend the MD results to the larger spatial and temporal scales. Examples are presented for Al, Fe, pure and Al doped Cu, and some other species for both perfect and defected crystals. The EAM potentials are mostly deployed. Comparisons with the experimental data available as well as with the simulation results of other authors are given. [Preview Abstract] |
Wednesday, June 27, 2007 11:45AM - 12:00PM |
M7.00005: Atomistic simulations of fracture in nanocrystalline copper under high strain rates Alexey Yanilkin, Alexey Kuksin, Genri Norman, Vladimir Stegailov Structural features of nanocrystalline materials attract continuous attention due to their unique properties and prospects for various technological applications. In this work microscopic mechanisms of plasticity and fracture in nanocrystalline copper are considered at the atomistic level by molecular dynamics method. The EAM potential model [Y. Mishin et al // PRB 63 (2001) 224106] is used to describe interatomic potential. The initial structure is created by filling random Voronoi polyhedra with different orientations of lattice and subsequent equilibration. Three ways of the high strain rate ($10^{8}$ -- $10^{10}$ s$^{- 1}$) plastic deformation and fracture processes modeling are compared: hydrostatic and uniaxial strain and shock wave loading in the impacor-target model. The dependence of the results on the average grain size, orientation and shape is studied. [Preview Abstract] |
Wednesday, June 27, 2007 12:00PM - 12:15PM |
M7.00006: Modeling of Al crystal fracture under high-rate strain based on atomistic simulations Alexey Kuksin, Genri Norman, Vladimir Stegailov, Alexey Yanilkin The work presents the kinetic model of fracture under high-rate strain based on the results of molecular dynamic (MD) simulations. Kinetic parameters for the model as functions of strain and temperature are obtained via statistical averaging over the multiple MD runs of (a) void nucleation in a crystal and (b) void growth under stretching. In the EAM model of monocrystal Al at temperatures close to the melting point the void formation is shown to be a process of crystal homogeneous melting and further cavitation in the melt formed. With the help of the model developed shock-wave loading is modeled and dynamic spall strength of the defectless Al crystal is calculated. The results obtained are compared with the experimental data [G.I. Kanel et al. // J. Phys.: Cond. Mat. 16 (2004) S1007]. While good agreement is observed in the high-temperature region, it becomes worse when temperature decreases. This fact could manifest the increasing role of defects. The dependence of the spall strength on loading duration and on strain rate is governed mostly by nucleation rate of voids and not by their growth rate. [Preview Abstract] |
Wednesday, June 27, 2007 12:15PM - 12:30PM |
M7.00007: Atomistic study of nanoprecipitates influence on plasticity and fracture of crystalline metals Vladimir Stegailov, Alexey Kuksin, Genri Norman, Alexey Yanilkin The recent experimental results [G.I.Kanel et al., 2006] show the essential influence of the nanoprecipitates on spall strength of copper single crystals. In this work we address this issue by the molecular dynamics study. The models under consideration are the EAM systems of Al nanoclusters in the Cu matrix and Cu clusters in the Al matrix. We consider these two cases as the representative examples of nanocluster-matrix difference in shear strength. Three ways of the high strain rate deformation modeling are studied: hydrostatic and uniaxial strain and shock wave loading in the impactor-target model. The preexisting edge dislocation interaction with the precipitate under shear deformation is addressed. The effect of the precipitate size is considered. [Preview Abstract] |
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