In the last 30 days

• Open/close Designing meaningful density functional theory calculations in materials science—a primer

  Ann E Mattsson et al 2005 Modelling Simul. Mater. Sci. Eng. 13 R1 Tag this article

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  Density functional theory (DFT) methods for calculating the quantum mechanical ground states of condensed matter systems are now a common and significant component of materials research. The growing importance of DFT reflects the development of sufficiently accurate functionals, efficient algorithms and continuing improvements in computing capabilities. As the materials problems to which DFT is applied have become large and complex, so have the sets of calculations necessary for investigating a given problem. Highly versatile, powerful codes exist to serve the practitioner, but designing useful simulations is a complicated task, involving intricate manipulation of many variables, with many pitfalls for the unwary and the inexperienced. We discuss several of the most important issues that go into designing a meaningful DFT calculation. We emphasize the necessity of investigating these issues and reporting the critical details.

• Open/close Nucleation of elementary and twinning dislocations at a twin boundary in hexagonal close-packed crystals

  J Wang et al 2012 Modelling Simul. Mater. Sci. Eng. 20 024001 Tag this article

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  In this paper, we study the kinetics and energetics involved in the nucleation and propagation of and twinning dislocations (TDs) in Mg using atomistic simulations. We demonstrate that for both twins, a 2-layer TD of mixed character nucleates as a result of the interaction of a basal dislocation with a twin boundary. The favorability of the 2-layer TD over a 4-layer TD of edge character can be explained by its greater mobility. The twin boundary likely translates by nucleating and propagating 2-layer TDs with opposite-signed screw components in equal amounts on average.

• Open/close Phase-field models in materials science

  Ingo Steinbach 2009 Modelling Simul. Mater. Sci. Eng. 17 073001 Tag this article

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  The phase-field method is reviewed against its historical and theoretical background. Starting from Van der Waals considerations on the structure of interfaces in materials the concept of the phase-field method is developed along historical lines. Basic relations are summarized in a comprehensive way. Special emphasis is given to the multi-phase-field method with extension to elastic interactions and fluid flow which allows one to treat multi-grain multi-phase structures in multicomponent materials. Examples are collected demonstrating the applicability of the different variants of the phase-field method in different fields of materials science.

• Open/close Atomic structures of symmetric tilt grain boundaries in hexagonal close packed (hcp) crystals

  J Wang and I J Beyerlein 2012 Modelling Simul. Mater. Sci. Eng. 20 024002 Tag this article

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  Using molecular dynamics (MD) simulations, the dislocation structures of symmetric tilt grain boundaries (STGBs) in hexagonal close packed (hcp) crystal structures are studied. STGBs over the entire range of possible rotation angles θ from 0° to 90° are found to have an ordered atomic structure. Formation energy calculations reveal four local minimum-energy boundaries that correspond to coherent grain boundaries (GBs). Deviations in tilt from the basal plane (θ = 0°, , prismatic plane (θ = 90°, , or one of these four minimum-energy boundaries, , result in the formation of a tilt wall (edge-type grain boundary dislocations, GBDs) superimposed on the nearest GB structure in θ-space. As θ deviates far from the rotation angle of one and draws closer to that of an adjacent , an abrupt transition in STGB base boundary structure and GBD Burgers vector occurs. For all θ, the sign and spacing of GBDs depend on θ, and their Burgers vector is either one or two times the interplanar spacing of P _B. We present a simple model that generalizes the results to other c/ a ratios. Subsequent MD simulations show that (1) the model forecasts the STGB structure to first-order and (2) STGBs with two distinct atomic structures can have remarkably different responses when interacting with basal lattice dislocations originating from the adjoining crystals.

• Open/close A scheme to combine molecular dynamics and dislocation dynamics

  Steffen Brinckmann et al 2012 Modelling Simul. Mater. Sci. Eng. 20 045001 Tag this article

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  Many engineering challenges occur on multiple interacting length scales, e.g. during fracture atoms separate on the atomic scale while plasticity develops on the micrometer scale. To investigate the details of these events, a concurrent multiscale model is required which studies the problem at appropriate length- and time-scales: the atomistic scale and the dislocation dynamics scale. The AtoDis multiscale model is introduced, which combines atomistics and dislocation dynamicsinto a fully dynamic model that is able to simulate deformation mechanisms at finite temperature. The model uses point forces to ensure mechanical equilibrium and kinematic continuity at the interface. By resolving each interface atom analytically, and not numerically, the framework uses a coarse FEM mesh and intrinsically filters out atomistic vibrations. This multiscale model allows bi-directional dislocation transition at the interface of both models with no remnant atomic disorder. Thereby, the model is able to simulate a larger plastic zone than conventional molecular dynamics while reducing the need for constitutive dislocation dynamics equations. This contribution studies dislocation nucleation at finite temperature and investigates the absorption of dislocations into the crack wake.

• Open/close A review of extended/generalized finite element methods for material modeling

  Ted Belytschko et al 2009 Modelling Simul. Mater. Sci. Eng. 17 043001 Tag this article

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  The extended and generalized finite element methods are reviewed with an emphasis on their applications to problems in material science: (1) fracture, (2) dislocations, (3) grain boundaries and (4) phases interfaces. These methods facilitate the modeling of complicated geometries and the evolution of such geometries, particularly when combined with level set methods, as for example in the simulation growing cracks or moving phase interfaces. The state of the art for these problems is described along with the history of developments.

• Open/close A simple finite element model of diffusion, finite deformation, plasticity and fracture in lithium ion insertion electrode materials

  A F Bower and P R Guduru 2012 Modelling Simul. Mater. Sci. Eng. 20 045004 Tag this article

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  We describe a finite element method for modeling deformation, diffusion, fracture and electrochemical reactions in materials used as lithium ion insertion electrodes. With a view to modeling high-capacity composite electrode materials such as silicon or tin, the model accounts for finite deformations and plastic flow, and models the evolving electrochemical boundary conditions resulting from the creation of new fracture surfaces using a cohesive zone. In addition, a simple mixed element is used to account for the driving force for diffusion arising from stress gradients. In this approach, the equations for diffusion and deformation are fully coupled, and can be integrated using a stable implicit time-stepping scheme. The method is illustrated by modeling plastic flow and fracture during cyclic lithiation of a thin-film Si electrode and a simple model of a battery microstructure.

• Open/close Micromechanics prediction of the effective elastic moduli of graphene sheet-reinforced polymer nanocomposites

  Xiang-Ying Ji et al 2010 Modelling Simul. Mater. Sci. Eng. 18 045005 Tag this article

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  We investigate the stiffening effect of graphene sheets dispersed in polymer nanocomposites using the Mori–Tanaka micromechanics method. The effective elastic moduli of graphene sheet-reinforced composites are first predicted by assuming that all the graphene sheets are either aligned or randomly oriented in the polymer matrix while maintaining their platelet-like shape. It is shown that a very low content of graphene sheets can considerably enhance the effective stiffness of the composite. The superiority of graphene sheets as a kind of reinforcement is further verified by a comparison with carbon nanotubes, another promising nanofiller in polymer composites. In addition, we analyze several critical physical mechanisms that may affect the reinforcing effects, including the agglomeration, stacking-up and rolling-up of graphene sheets. The results reveal the extent to which these factors will negatively influence the elastic moduli of graphene sheet-reinforced nanocomposites. This theoretical study may help to understand the relevant experimental results and facilitate the mechanical characterization and optimal synthesis of these kinds of novel and highly promising nanocomposites.

• Open/close Modeling of graphene–polymer interfacial mechanical behavior using molecular dynamics

  Amnaya P Awasthi et al 2009 Modelling Simul. Mater. Sci. Eng. 17 015002 Tag this article

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  Carbon nanotube (CNT) polymer-matrix composites exhibit promising properties as structural materials for which appropriate constitutive models are sought, to predict their macroscale behavior. The reliability of determining the homogenized response of such materials depends upon the ability to accurately capture the interfacial behavior between the nanotubes and the polymer matrix. In this work, molecular dynamics simulations, using the Consistent Valence Force Field (CVFF) to describe the atomistic interactions, are used to study nanoscale load transfer between polyethylene and a graphene sheet, a model system chosen to characterize the force-separation behavior between CNTs and the polymer matrix. Separation studies are conducted for both opening as well as sliding modes and cohesive zone parameters such as peak traction and energy of separation are evaluated for each mode. Studies are also carried out to investigate the effect of tension and compression on sliding mode separation. Size dependence studies are conducted utilizing different sizes of the computational domain and different boundary conditions, to obtain the representative volume element and connect to continuum level properties. These results set the stage for continuum length-scale micromechanical models which may be used in determining the overall material response, incorporating interfacial phenomena.

• Open/close Application of crystal plasticity to an austenitic stainless steel

  Youngung Jeong et al 2012 Modelling Simul. Mater. Sci. Eng. 20 024009 Tag this article

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  The visco-plastic self-consistent (VPSC) model is applied to a 304 austenitic stainless-steel sheet sample to investigate its suitability as a constitutive model for sheet metal forming simulations. The factors to which the model prediction is sensitive are explored. In particular, the influence of the textures obtained at three different depths on the in-plane variation of the r-value (Lankford coefficient) and the yield stress is investigated. Then, the grain number sensitivity in the representative volume element is analyzed. Moreover, the influence of the linearization scheme for the crystal constitutive power law on the simulation result is discussed. For simulation, discrete crystallographic orientation distributions are calculated from the pole figures obtained at different depths. Several mechanical experiments are conducted, namely, uniaxial tension, in-plane biaxial and the bulge tests. The mechanical response of the material is simulated using the VPSC model and compared with the model predictions.