Showing papers in "Modelling and Simulation in Materials Science and Engineering in 2006"
TL;DR: In this paper, an embedded-atom method (EAM) potential was constructed for Ag by fitting to experimental and first-principles data, which accurately reproduces the lattice parameter, cohesive energy, elastic constants, phonon frequencies, thermal expansion, lattice-defect energies, as well as energies of alternate structures of Ag.
Abstract: A new embedded-atom method (EAM) potential has been constructed for Ag by fitting to experimental and first-principles data. The potential accurately reproduces the lattice parameter, cohesive energy, elastic constants, phonon frequencies, thermal expansion, lattice-defect energies, as well as energies of alternate structures of Ag. Combining this potential with an existing EAM potential for Cu, a binary potential set for the Cu–Ag system has been constructed by fitting the cross-interaction function to first-principles energies of imaginary Cu–Ag compounds. Although properties used in the fit refer to the 0 K temperature (except for thermal expansion factors of pure Cu and Ag) and do not include liquid configurations, the potentials demonstrate good transferability to high-temperature properties. In particular, the entire Cu–Ag phase diagram calculated with the new potentials in conjunction with Monte Carlo simulations is in satisfactory agreement with experiment. This agreement suggests that EAM potentials accurately fit to 0 K properties can be capable of correctly predicting simple phase diagrams. Possible applications of the new potential set are outlined.
438 citations
TL;DR: In this paper, the authors developed a finite element scheme to model the individual processes of shear bands that contribute to the macroscopic plasticity behavior, in which the stress-driven increase in the free volume reduces the viscosity and thus leads to the strain localization in the shear band.
Abstract: Inhomogeneous deformation of amorphous alloys is caused by the initiation, multiplication and interaction of shear bands (i.e. narrow bands with large plastic deformation). Based on the free-volume model under the generalized multiaxial stress state, this work develops a finite element scheme to model the individual processes of shear bands that contribute to the macroscopic plasticity behaviour. In this model, the stress-driven increase in the free volume reduces the viscosity and thus leads to the strain localization in the shear band. Using the small-strain and rate-dependent plasticity framework, the plastic strain is assumed to be proportional to the deviatoric stress, and the flow stress is a function of the free volume, while the temporal change in the free volume is also coupled with the stress state. Nonlinear equations from the incremental finite element formulation are solved by the Newton–Raphson method, in which the corresponding material tangent is obtained by simultaneously and implicitly integrating the plastic flow equation and the evolution equation of the free-volume field. This micromechanical model allows us to study the interaction between individual shear bands and between the shear bands and the background stress fields. To illustrate its capabilities, the method is used to solve representative boundary value problems.
119 citations
TL;DR: In this article, the effect of specimen size on the uniaxial deformation response of planar single crystals and polycrystals using discrete dislocation plasticity is investigated. But the authors focus on the case where the dislocations are modeled as line singularities in a linear elastic material.
Abstract: The effect of specimen size on the uniaxial deformation response of planar single crystals and polycrystals is investigated using discrete dislocation plasticity. The dislocations are all of edge character and modelled as line singularities in a linear elastic material. The lattice resistance to dislocation motion, dislocation nucleation, dislocation interaction with obstacles and dislocation annihilation are incorporated through a set of constitutive rules. Grain boundaries are modelled as impenetrable to dislocations. Two types of polycrystalline materials are considered: one that only has grains with a single orientation while the other has a checker-board arrangement of two types of grains which are rotated 90° with respect to each other. The single crystals display a strong size dependence with the flow strength increasing with decreasing specimen size. In sufficiently small single crystal specimens, the nucleation rate of the dislocations is approximately equal to the rate at which the dislocations exit the specimens so that below a critical specimen size the flow strength is set by the strength of the initially present Frank–Read sources. On the other hand, grain boundaries acting as barriers to plastic deformation in polycrystalline specimens of the same size lead to a more diffuse deformation pattern and to a nearly size-independent response.
102 citations
TL;DR: In this article, the influence of phase transformation on the residual stress induced by the welding process, by comparing the results obtained with the described differences in the analyses, was examined, and it was found that both volume changes due to phase transformations and transformation plasticity have a great influence on the in-plane stresses induced by welding process.
Abstract: In this work, a numerical study of laser beam welding of steel was performed. In particular, phase transformation effects were considered, which consist mainly of volume change and transformation plasticity. Thanks to the possibilities of numerical modelling, additional analyses were performed (a) without taking into account phase transformations and (b) considering only the transformation plasticity phenomenon.The aim of this study was to examine the influence of phase transformation on the residual stress induced by the welding process, by comparing the results obtained with the described differences in the analyses. Finally, the residual stress field computed by the three-dimensional (3D) model was compared with the one computed by a two-dimensional (2D) model in order to estimate the grade of reliability of the more efficient 2D analyses, also in the presence of phase transformations. It was found that both volume changes due to phase transformations and transformation plasticity have a great influence on the residual stress induced by the welding process. 2D numerical models can be used with good accuracy instead of 3D models, if the in-plane stresses are of primary interest. All analyses in this investigation were performed with the finite element code SYSWELD®.
97 citations
TL;DR: Simulations of polycrystalline grain growth with a conventional phase field method and with sparse data structures are compared and it is shown that memory usage and simulation time scale are independent of the number of order parameters when a sparse data structure is used.
Abstract: The concepts of sparse data structures and related algorithms for phase field simulations are discussed. Simulations of polycrystalline grain growth with a conventional phase field method and with sparse data structures are compared. It is shown that memory usage and simulation time scale with the number of nodes but are independent of the number of order parameters when a sparse data structure is used.
85 citations
TL;DR: In this paper, the electronic and structural properties of hexagonal boron nitride (BN) were studied using density functional theory calculations, and three different approximations for the exchange were used to calculate properties such as the bulk modulus, cohesive energy and lattice constants.
Abstract: The electronic and structural properties of hexagonal boron nitride (BN) were studied using density functional theory calculations. Three different approximations for the exchange—correlation energy (the local density and two forms of the generalized gradient)—were used to calculate properties such as the bulk modulus, cohesive energy and lattice constants to determine their relative predictive abilities for this system. In general, calculations using the local density approximation produced properties slightly closer to experimental values than calculations with either generalized gradient approximations. Different stackings, or arrangements of one basal plane with respect to another, were examined to determine the equilibrium stacking(s) and it was found that the different stackings have similar cohesive energies and bulk moduli. Energy versus volume curves were calculated for each stacking using two different methods to determine their relative efficacy. Bulk moduli values obtained assuming no pressure dependence were closer to experimental values than those obtained from three common equations of state. Comparisons between the cohesive energies of hexagonal BN and cubic BN show that the cubic phase is more stable. The pressure/volume dependence of the band structure was studied for several different stackings and all showed similar behaviour, specifically a 3–4.5 eV band gap that was nearly independent of pressure in the −500 to +500 kb regime. These calculated results of the pressure/volume dependence of the band structure are the first reports for this system.
79 citations
TL;DR: Gao et al. as discussed by the authors showed that if the characteristic dimension of a material is below a critical length scale that can be on the order of several nanometres, the Griffith theory of fracture no longer holds.
Abstract: Once the characteristic size of materials reaches nanoscale, the mechanical properties may change drastically and classical mechanisms of materials failure may cease to hold. In this paper, we focus on joint atomistic-continuum studies of failure and deformation of nanoscale materials. In the first part of the paper, we discuss the size dependence of brittle fracture. We illustrate that if the characteristic dimension of a material is below a critical length scale that can be on the order of several nanometres, the classical Griffith theory of fracture no longer holds. An important consequence of this finding is that materials with nano-substructures may become flaw-tolerant, as the stress concentration at crack tips disappears and failure always occurs at the theoretical strength of materials, regardless of defects. Our atomistic simulations complement recent continuum analysis (Gao et al 2003 Proc. Natl Acad. Sci. USA 100 5597–600) and reveal a smooth transition between Griffith modes of failure via crack propagation to uniform bond rupture at theoretical strength below a nanometre critical length. Our results may have consequences for understanding failure of many small-scale materials. In the second part of this paper, we focus on the size dependence of adhesion systems. We demonstrate that optimal adhesion can be achieved by either length scale reduction, or by optimization of the shape of the surface of the adhesion element. We find that whereas change in shape can lead to optimal adhesion strength, those systems are not robust against small deviations from the optimal shape. In contrast, reducing the dimensions of the adhesion system results in robust adhesion devices that fail at their theoretical strength, regardless of the presence of flaws. An important consequence of this finding is that even under the presence of surface roughness, optimal adhesion is possible provided the size of contact elements is sufficiently small. Our atomistic results corroborate earlier theoretical modelling at the continuum scale (Gao and Yao 2004 Proc. Natl Acad. Sci. USA 101 7851–6). We discuss the relevance of our studies with respect to nature's design of bone nanostructures and nanoscale adhesion elements in geckos.
75 citations
TL;DR: An explicit Galerkin/least-squares formulation is introduced for a quasilinear transport equation in field dislocation mechanics (FDM) and applied to the study of the kinematics of dislocation density evolution in the following physical contexts: annihilation of dislocations, expansion of polygonal dislocation loop and simulation of a Frank-Read source as discussed by the authors.
Abstract: An explicit Galerkin/least-squares formulation is introduced for a quasilinear transport equation in field dislocation mechanics (FDM) and applied to the study of the kinematics of dislocation density evolution in the following physical contexts: annihilation of dislocations, expansion of a polygonal dislocation loop and simulation of a Frank–Read source. Stability analysis is carried out for the corresponding linear one-dimensional (1D) case. The formulation reduces to the Lax–Wendroff finite difference scheme for the 1D equation when equal weighting is used for the Galerkin and least-squares terms and the shape functions are linear. This conditionally stable method leads to a symmetric well-conditioned system of equations with constant coefficients, making it attractive for large-scale problems.It is shown that the transport equation, in the contexts mentioned above simplifies to the Hamilton–Jacobi equations governing geometrical optics and level-set methods. The weak solutions to these equations are not unique, and the numerical method is able to capture solutions corresponding to shock as well as rarefraction waves by appropriate algorithmic modifications.
71 citations
TL;DR: In this article, the effects of grain orientation on transformation-induced plasticity in multiphase steels are studied through three-dimensional finite element simulations, and the boundary value problems analysed concern a uniaxially loaded sample consisting of a grain of retained austenite surrounded by multiple grains of ferrite.
Abstract: The effects of grain orientation on transformation-induced plasticity in multiphase steels are studied through three-dimensional finite element simulations. The boundary value problems analysed concern a uniaxially-loaded sample consisting of a grain of retained austenite surrounded by multiple grains of ferrite. For the ferritic phase, a rate-dependent crystal plasticity model is used that describes the elasto-plastic behaviour of body-centred cubic crystalline structures under large deformations. In this model, the critical-resolved shear stress for plastic slip consists of an evolving slip resistance and a stress-dependent term that corresponds to the projection of the stress tensor on a non-glide plane (i.e. a non-Schmid stress). For the austenitic phase, the transformation model developed by Turteltaub and Suiker (2006 Int. J. Solids Struct. at press, 2005 J. Mech. Phys. Solids 53 1747–88) is employed. This model simulates the displacive phase transformation of a face-centred cubic austenite into a body-centred tetragonal martensite under external mechanical loading. The effective transformation kinematics and the effective anisotropic elastic stiffness components in the model are derived from lower-scale information that follows from the crystallographic theory of martensitic transformations. In the boundary value problems studied, the mutual interaction between the transforming austenitic grain and the plastically deforming ferritic matrix is computed for several grain orientations. From the simulation results, specific combinations of austenitic and ferritic crystalline orientations are identified that either increase or decrease the effective strength of the material. This information is useful to further improve the mechanical properties of multiphase carbon steels. In order to quantify the anisotropic aspects of the crystal plasticity model, the simulation results for the uniaxially-loaded sample are compared with those obtained with an isotropic plasticity model for the ferritic grains.
67 citations
TL;DR: In this article, an artificial neural network (ANN) model is developed to predict the constitutive flow behavior of austenitic stainless steels during hot deformation, which can be used as a guideline for new alloy development.
Abstract: An artificial neural network (ANN) model is developed to predict the constitutive flow behaviour of austenitic stainless steels during hot deformation. The input parameters are alloy composition and process variables whereas flow stress is the output. The model is based on a three-layer feed-forward ANN with a back-propagation learning algorithm. The neural network is trained with an in-house database obtained from hot compression tests on various grades of austenitic stainless steels. The performance of the model is evaluated using a wide variety of statistical indices. Good agreement between experimental and predicted data is obtained. The correlation between individual alloying elements and high temperature flow behaviour is investigated by employing the ANN model. The results are found to be consistent with the physical phenomena. The model can be used as a guideline for new alloy development.
65 citations
TL;DR: In this article, a series of finite element simulations are used to simulate the effects of particle shock compression on Al+Fe2O3 thermite systems (Al+Fe3) composed of micron-size particles suspended in a polymer binder.
Abstract: The shock compression of spatially-resolved particle systems is studied at the mesoscale through a series of finite element simulations. The simulations involve propagating shock waves through aluminium–iron oxide thermite systems (Al+Fe2O3) composed of micron-size particles suspended in a polymer binder. Shock-induced chemical reactions are not considered in this work; the particle systems are modelled as inert mixtures. Eulerian formulations are used to accommodate the highly dynamic nature of particulate shock compression. The stress–strain responses of the constituent phases are modelled explicitly at high strain rates and elevated temperatures. Dynamic behaviour of the model system is computed for a set of mixtures (20% and 50% epoxy content by weight) subjected to a range of loading conditions (particle velocities that span 0.300–1.700 km s−1). Spatial profiles of pressure and temperature obtained from the numerical simulations provide insight into thermomechanical responses at the particle level; such resolution is not available in experiments. Finally, Hugoniot data are calculated for the particle mixtures. Stationary pressure calculations are in excellent agreement with experiments, while shock velocity calculations exhibit larger deviations due to the 2D approximation of the microstructure.
TL;DR: In this article, the authors performed preliminary simulations of simple shear deformation and indentation simulations using different radii of a spherical indenter using molecular dynamics in order to uncover the internal stress state for elastic deformation.
Abstract: Preliminary simulations of simple shear deformation and indentation simulations using different radii of a spherical indenter are performed using molecular dynamics in order to uncover the internal stress state for elastic deformation and subsequent initial plasticity under nano-indentation. An atomic single-crystalline aluminium model containing up to 1,372,000 atoms and an ideal friction-free spherical indenter are presented in a set of simulations. Effects of the stress distribution using several kinds of spherical radii of indenters on the critical condition of dislocation emissions are discussed with much emphasis. The critical shear stress for the dislocation emission under indentation is well accorded with the shear strength under the simple shear deformation exposed to the equivalent external stresses to the indentation-induced stress states. It is confirmed that shear strength strongly depends on the external stress component and therefore, high compressive stress states generated beneath the indenter lead to the much higher critical shear stress than μ/2π.
TL;DR: In this article, the toluidine blue (TB) aggregation phenomenon in water solutions was investigated by UV-Vis absorption spectroscopy in a wide range of concentrations, from 10−9 to 10−2 M. The experimental data were fitted by assuming that the spectrum was due to the overlapping of the bands of six different aggregation species simultaneously present in solution.
Abstract: The toluidine blue (TB) aggregation phenomenon in water solutions was investigated by UV-Vis absorption spectroscopy in a wide range of concentrations, from 10−9 to 10−2 M. The experimental data were fitted by assuming that the spectrum was due to the overlapping of the bands of six different aggregation species simultaneously present in solution. The model and the peak centres of the different species were hypothesized on the basis of exciton theory. All the spectra obtained were simultaneously processed thus allowing the derivation of the K constant, which was assumed to be the same for all the equilibria and which was found equal to 10 500. The e spectra of the different aggregation species were also obtained. The overall TB spectrum may be mainly attributed to the H-type aggregation, although some of the species also show the J type bands. The structural features of TB and its dimeric aggregate were also investigated by means of molecular mechanics semi-empirical potential energy calculations. The results obtained for the dimer confirm the H type aggregation and show that the lowest energy model of the four possible is the one obtained by a mirror plane of symmetry containing the aromatic rings, in agreement with the x-ray diffraction data which will be presented in a forthcoming paper.
TL;DR: In this paper, the singular and non-singular parts of the image stress are dealt with separately, while the remaining nonsingular part is treated using the standard finite element method.
Abstract: Dislocations intersecting free surfaces present a challenge for numerical implementation of traction-free boundary conditions in dislocation dynamics simulations. The difficulty arises when singular analytic expressions of dislocation stress fields need to be used in combination with numerical methods to calculate image stress fields due to the free surfaces. A new hybrid method is developed here in which the singular and non-singular parts of the image stress are dealt with separately. The analytic solution for a semi-infinite straight dislocation intersecting the surface of elastic half-space is used to account for the singular part of the image stress, while the remaining non-singular part is treated using the standard finite element method. The numerical advantages of this decomposition are demonstrated with examples.
TL;DR: In this paper, the equivalency of the two indenters by performing extensively both 3D and 2D finite element analyses to simulate the load-displacement response of a wide range of elasto-plastic materials obeying power law strain-hardening during indentation for both Berkovich and conical indenters, respectively.
Abstract: The Berkovich indenter, which is one of the most commonly used indenter tips in instrumented indentation experiments, requires a tedious 3D finite element simulation. The indenter is widely idealized as a conical indenter of 70.3° half-angle to enable a substantially less demanding 2D axisymmetric modelling. Although the approach has been commonly adopted, limited studies have been performed to investigate possible deviations due to this simplification. The present study attempts to address the equivalency of the two indenters by performing extensively both 3D and 2D finite element analyses to simulate the load-displacement response of a wide range of elasto-plastic materials obeying power law strain-hardening during indentation for both Berkovich and conical indenters, respectively. It is demonstrated that the equivalency between these two indenters in terms of curvature of the loading curve is not valid across the range of material properties under study. However, it is established that if only the ratio of the remaining work done (WR) and the total work done (WT) of the load-indentation curve is of interest, this simplification can be adopted with satisfactory results.
TL;DR: In this paper, two different texture-dependent material models based on the Taylor assumption are discussed and applied to the simulation of deep drawing operations of aluminium, which allows to reduce the sharpness of the crystallite orientation distribution function related to single crystals or texture components.
Abstract: In this paper two different texture-dependent material models based on the Taylor assumption are discussed and applied to the simulation of deep drawing operations of aluminium. From the numerical point of view, large-scale FE computations based on the Taylor model are very time-intensive and storage-consuming if the crystallographic texture is approximated by several hundred discrete crystals. Furthermore, the Taylor model in its standard form, which is based on discrete crystal orientations, has the disadvantage that the anisotropy is significantly overestimated if only a small number of crystal orientations are used. We quantitatively analyse this overestimation of anisotropy and suggest two Taylor-type models which allow us to reduce the sharpness of the crystallite orientation distribution function related to single crystals or texture components. One model is an elastic-viscoplastic Taylor model based on discrete orientations. The sharpness is reduced by modelling the isotropic background texture by an isotropic material law. The other model is a rigid-viscoplastic material one, which is based on continuous model functions on the orientation space. This model allows for a direct incorporation of the scattering around an ideal texture component since the model contains the half-width as a microstructural parameter which can be biased. These models are used to compute yield stresses, R values and earing profiles. The predictions are compared with experimental data.
TL;DR: In this paper, a series of molecular dynamics simulations using the embedded atom method is conducted to investigate crack propagation under mode I loading in a Ni single crystal with and without defects, and critical loads and strain energy distributions around the crack tip are obtained.
Abstract: A series of molecular dynamics simulations using the embedded atom method is conducted to investigate crack propagation under mode I loading in a Ni single crystal with and without defects. The crack system (0 0 1)[1 0 0] in a slab of 160 000 atoms was studied. Defects consisting of lines of vacancies were introduced near the crack tip. Critical loads and strain energy distributions around the crack tip are obtained. Our results indicate that the critical strain necessary for crack propagation is dependent on the defect configuration and can either increase or decrease relative to the defect-free system.
TL;DR: In this article, Li et al. measured the temperature dependence of elastic (Young's) modulus of two different grades of optical glasses to measure their stress response to the applied strain at lens molding temperatures.
Abstract: Cylindrical compression experiments were performed on two different grades of optical glasses to measure their stress response to the applied strain at lens moulding temperatures. The temperature dependence of elastic (Young's) modulus of the two glasses was measured using the technique of Brillouin light-scattering, from ambient temperature to approximately 250 °C above the glass transition temperature. The measured high temperature material data were used as input to finite element method simulation of cylindrical compression as well as to the precision lens moulding process.Good agreement was obtained between stress relaxation patterns predicted from cylinder compression simulations and analytical models. The close agreement between predicted and experimental loads during lens moulding validates the viscoelastic material model.
TL;DR: In this article, an approximate predictive model is developed for the evaluation of the interfacial thermal stresses in a bi-material assembly with a low-yield-stress bonding material, which is considered linearly elastic at a strain level below the yield point and ideally plastic at higher strains.
Abstract: An approximate predictive model is developed for the evaluation of the interfacial thermal stresses in a bi-material assembly with a low-yield-stress bonding material. This material is considered linearly elastic at a strain level below the yield point and ideally plastic at higher strains. The results of the analysis can be used for the assessment of thermally induced stresses in bonding materials in some laser packages and in similar micro- and opto-electronic assemblies.
TL;DR: In this paper, a 3D mathematical model was developed to simulate the weld pool development in a moving A-TIG weld pool with different oxygen and sulfur concentrations, and it was shown that the surface active elements (oxygen and sulfur) can change the temperature coefficient of surface tension from a negative value to a positive one, which can cause significant changes in fluid flow patterns and the weld penetration.
Abstract: A 3D mathematical model was developed to simulate the weld pool development in a moving A-TIG weld pool with different oxygen and sulfur concentrations. It is shown that the surface active elements—oxygen and sulfur, which change the temperature coefficient of surface tension from a negative value to a positive one, can cause significant changes in fluid flow patterns and the weld penetration. When surface active element content increases, the weld penetration and depth/width ratio increase sharply and then remain nearly a constant. Positive temperature coefficient of surface tension dominates the fluid flow and the weld pool is narrow and deep. The further increasing surface active element content leads to an inappreciable difference in the weld pool size and shape when the oxygen content increases beyond 280 ppm and sulfur content beyond 125 ppm. Positive and negative temperature coefficients of surface tension co-exist in the weld pool when surface active element content is less than the critical value. The fluid flows in the weld pool change apparently with different surface active element. Depending upon the oxygen and sulfur concentrations, three, one or two vortexes that have different positions, strength and directions may be found in the weld pool. The vortexes with opposite direction caused by positive temperature coefficient of surface tension can efficiently transfer the thermal energy from the arc, creating a deep weld pool.
TL;DR: In this article, a comparison between the decomposed virial formula and the Tsai formula is made, and it is shown that the latter is more adequate for calculating both the overall average stress and local stress distribution.
Abstract: A systematic comparison between the decomposed virial formula and the Tsai formula shows that they are mathematically equivalent in calculating the overall average stress of an atomistic system. But in the case of calculating local stress distribution, the former gives ambiguous results, e.g. it gives nonzero normal stress at free surfaces and it typically 'underestimates' the inhomogeneity of microstructures and deformations in material. With a highly degenerate atomic chain model, we show mathematically that the results obtained by the decomposed virial formula are accurate only if the deformation is homogeneous within the neighbourhood of an interaction-cutoff radius, centred at the atomic site considered. Thus it is worth noting that the Tsai formula is more adequate for calculating both the overall average stress and local stress distribution.
TL;DR: In this paper, the effect of the number and orientation of available slip systems on the indentation of planar single crystals is investigated and the relationship between the nominal indentation hardness and the slip system strength depends on the number of soft slip systems.
Abstract: The effect of the number and orientation of available slip systems on the indentation of model planar single crystals is investigated. The crystal is taken to have three slip systems oriented at 60° relative to each other. The material is characterized by viscoplastic continuum slip theory. The number of potentially active slip systems is controlled by specifying values of the flow strength on each of the systems. Attention is confined to indentation of a block by a sticking rigid sharp indenter. Crystals with one, two or three soft (and therefore potentially active) slip systems are considered. We also investigate the effect of the orientation of the soft slip system relative to that of the indenter. Results are presented for the hardness, the mode of slip, the induced lattice rotations and the stress distribution. The relation between the nominal indentation hardness and the slip system strength depends on the number of soft slip systems and can differ significantly from that for a plastically isotropic solid. With two or three soft slip systems, circumstances are found where the deformation mode involves regions of predominantly single slip. Particularly when there is one soft slip system, regions of positive mean normal stress can develop, the location of which depends on the orientation of the soft slip system relative to the indenter.
TL;DR: In this article, a template of the full atomistic displacement fields within a radius of R-core = 50 angstrom is shown to significantly reduce the magnitude and range of the spurious forces, permitting higher accuracy in the multiscale simulations as well as reduced size of the atomistic region.
Abstract: Spurious forces are shown to arise when dislocations interact with atom/continuum interfaces in some classes of multiscale models due to the use of linear elasticity in continuum descriptions of the material deformations and/or the singular dislocation fields. For Al, such forces can reach 500MPa for dislocations within a few Angstroms of the interface and can remain significant at distances of similar to 20 angstrom on the atomistic side and similar to 15 angstrom on the continuum side of the interface, inhibiting the creation of truly seamless coupling. Replacement of the continuum representation of the dislocation displacement fields by a 'template' of the full atomistic displacement fields within a radius of R-core = 50 angstrom is shown to significantly reduce the magnitude and range of the spurious forces. Implementation of the template method permits dislocations to approach within less than 10 angstrom of the interface from both atomistic and continuum sides, permitting higher accuracy in the multiscale simulations as well as reduced size of the atomistic region.
TL;DR: In this paper, the fluid in the upstream region acts as a fluidized piston (FP) that continuously presses and supplies the downstream fluid, which can treat dense and polyatomic fluid flow driven by a pressure gradient efficiently with the prescribed inlet streaming velocity at high speed.
Abstract: We have developed a new method, the fluidized piston model (FPM), to drive dense fluid flow in molecular dynamics (MD) simulations Fluid in the upstream region acts as a fluidized piston (FP) that continuously presses and supplies the downstream fluid The FPM can treat dense and polyatomic fluid flow driven by a pressure gradient efficiently with the prescribed inlet streaming velocity at high speed, which is usually used for hydrodynamic MD studies Furthermore, the temperature inside the sampling region is kept constant without direct control We apply the FPM to the 100 m s−1 water flow inside (20, 20), (12, 12), (10, 10), (8, 8) and (6, 6) carbon nanotubes (CNTs) The results show that the appropriate density in the sampling region is adaptively obtained during the simulation and the desired streaming velocity is obtained except inside the (6,6) CNT The deviation of the obtained streaming velocity from the desired value inside the (6,6) CNT is related to the current technical limit of the temperature control method applied in the upstream FP region and the highly discrete nature of the single-file water flow
TL;DR: In this paper, an amorphous Ni-Al binary alloy is made by a usual melt-quench simulation and subjected to tension by means of molecular dynamics simulation and the positiveness of elastic stiffness coefficients, Bijkl, is discussed for all atoms.
Abstract: The elastic limit of a crystal can be evaluated by the positiveness of elastic stiffness coefficients, Bijkl. We had demonstrated that the nucleation of lattice defects such as dislocation and cleavage cracking can be predicted by the atomic Bijkl at each atom point. Amorphous metals and bulk metallic glasses draw intense interest whether the criteria are applicable or not since they are regarded as the ultimate of lattice defects. In the present study, an amorphous Ni–Al binary alloy is made by a usual melt–quench simulation and subjected to tension by means of molecular dynamics simulation. During simulations, the positiveness of atomic Bijkl is discussed for all atoms. Contrary to an Ni–Al crystal, many atoms show negative value even in the initial equilibrium of the amorphous before loading. These unstable atoms turn out to be the non-clustered atom or the outer-shell of the local cluster such as 12(0, 0, 12, 0) icosahedron. On the other hand, the centre atoms of the local clusters show high stability resulting in the positive Bijkl of the whole system. It is also demonstrated that the change in the atomic Bijkl can reveal the collapse and re-configuration of local clusters during the deformation.
TL;DR: In this paper, a new methodology is presented for computer simulations of microstructures that incorporates realistic complex particle morphologies/shapes and a specified realistic two-point correlation function.
Abstract: A new methodology is presented for computer simulations of microstructures that incorporates realistic complex particle morphologies/shapes and a specified realistic two-point correlation function. The technique also enables simulations of sufficiently large microstructural windows that include short-range (on the order of particle size) as well as long-range (few hundred times the particle size) microstructural heterogeneities and spatial patterns. Incorporation of such simulated microstructural windows in the finite element-based computations leads to local stresses and strains that have distributions statistically similar to those resulting from the corresponding real microstructure. Therefore, these microstructure simulations provide realistic representative volume elements for computational parametric studies.
TL;DR: In this article, the microscopic mechanism of stress-induced martensitic transformation of Ni-Ti alloys was observed for tensile simulation using four different strain rates, and the relationship between stress and martensite ratio does not depend on the strain rate.
Abstract: Molecular dynamics simulation (MDS), using the embedded atom method for interatomic interactions, is performed to reveal the microscopic mechanism of stress-induced martensitic transformation of Ni–Ti alloys. Stress-induced martensitic transformation was observed for tensile simulation using four different strain rates. The relationship between stress and martensite ratio does not depend on the strain rate. Investigation of the results of MDS for the transformation pathway makes it clear that there are multiple pathways between the parent phase and the martensite phase regardless of the strain rate. Multiple pathways appear owing to differences in the relationship (angle) between the tensile direction and the specific lattice lengths of the parent or martensite unit cell. Increase and decrease in the martensite ratio during tensile simulation varies according to the pathways, depending on the strain rate.
TL;DR: In this paper, molecular dynamics simulations have been performed for one-and two-phase polymeric materials created on the computer and an external force was applied to the materials and their response followed as a function of time; the simulated materials exhibit a realistic response: the behaviour is more rigid and brittle when the force increases at a higher rate.
Abstract: Deformation brings out important features of viscoelastic behaviour in polymers. To achieve a better understanding of the underlying phenomena, molecular dynamics simulations have been performed for one- and two-phase polymeric materials created on the computer. An external force was applied to the materials and their response followed as a function of time.The mechanical properties were found to be strongly affected by the loading conditions, particularly the force increase rate. The simulated materials exhibit a realistic response: the behaviour is more rigid and brittle when the force increases at a higher rate. The material is able to partially recover in a viscoelastic manner if the force is removed after deformation. There are both quantitative and qualitative differences between the engineering stress and true stress. The presence of a rigid phase in polymer liquid crystals (PLCs) significantly influences their mechanical properties. Higher liquid crystalline (LC) phase concentrations increase stiffness while they make the polymer more brittle. The viscoelastic phase shift is smaller in PLCs than in one-phase amorphous polymers; the LC-rich islands in the LC-poor matrix make the material more elastic.When a creep force is applied for some time and then removed, the material exhibits partial viscoelastic recovery. The extent of that recovery is dependent on the magnitude of the creep force; a higher applied force results in less recovery. It also depends on the time during which the force was applied; longer times will result in less recovery. These results could be expected, confirming the model's validity. Unexpectedly the deformation mechanisms at higher stress levels were found to be different from those taking place at lower force levels. This reflects on a more localized deformation for higher creep force levels.
TL;DR: In this paper, the numerical implementation of a Lagrangian-based approach for the determination of the stress-strain behavior of solids via molecular dynamics (MD) via continuum homogenization is presented.
Abstract: This paper presents the numerical implementation of a Lagrangian-based approach for the determination of the stress–strain behaviour of solids via molecular dynamics (MD). This approach is based on continuum homogenization and it offers a framework in which the notions of effective stress and effective deformation for a particle system can be said to have the same meaning that they have in a continuum context. Since the effective stress response of the system is not based on the notion of virial stress, the paper presents three MD calculations to demonstrate how the continuum-based notion of effective stress differs from that of virial stress.
TL;DR: In this article, the effects of the substitutional element copper in solution in?-iron on glide of a? 111 { 110 } edge dislocation are investigated by atomic-scale computer simulation.
Abstract: The effects of the substitutional element copper in solution in ?-iron on glide of a ? 111 { 110 } edge dislocation are investigated by atomic-scale computer simulation. Under static conditions (temperature T = 0?K), single copper atoms and nearest-neighbour pairs in the first atomic plane below the dislocation slip plane provide the strongest barrier to slip, in partial agreement with continuum theory. This contrasts with recent simulation results for the Ni?Al fcc alloy (Rodary et al 2004 Phys. Rev. B 70 054111), where Al atoms displaced into nearest-neighbour coordination across the slip plane form the strongest obstacles. The dynamics of dislocation glide in Fe?Cu solid solution at T > 0?K are determined as a function of solute concentration. Parameters such as velocity, critical stress and drag coefficient are analysed. Again, there are differences from the Ni?Al system. The results are discussed in terms of the static strength of solute configurations and the different crystal structure of iron and nickel.