Guillaume Michal

Atomistic simulation of nanoindentation of iron with different indenter shapes

Abstract In this article molecular dynamics (MD) simulations have been conducted to investigate the effect of the indenter shape on the nanoindentation behaviours of iron with body centred cubic structure. Two types of indenters (hemispherical indenter and pyramidal indenter) with dimensions of several nanometres have been modelled. The simulation results have shown that the indenter shape significantly influences the nanoindentation behaviours at such small scale. The indentation force increases with indentation depth during loading for the hemispheri-cal indenter, while the indentation force is low in values for the pyramidal indenter. To validate the MD model nanoindentation experiments have been carried out. The calculated indentation hardness of the hemispherical indenter is in reasonable agreement with the experimental value.

Molecular dynamics study on the atomic mechanisms of coupling motion of (0 0 1) symmetric tilt grain boundaries in copper bicrystal

Recent research has revealed that some grain boundaries (GBs) can migrate coupled to applied shear stress. In this paper, molecular dynamics (MD) simulations were performed on sixteen [0 0 1] symmetric tilt GBs of bicrystal Cu to identify atomic-scale GB migration mechanisms and investigate their dependence on GB structure. The misorientation angles (θ) of the sixteen GBs cover the interval from 0° to 90° and a wide range of Σ values. A general method was proposed to explore the possible GB structures for each misorientation angle. Molecular statics simulation at a temperature of 0 K was carried out first to determine the equilibrium and some possible metastable structures of the sixteen investigated [0 0 1] GBs. MD simulations were then conducted on the bicrystal models at equilibrium by applying a shear strain parallel to the GB plane. Shear deformation caused the tangential translation of the grain and induced normal motion of the GBs. This boundary coupling motion was present in the entire range of misorientation angles. Different mechanisms of coupled boundary motion at atomic scale were carefully examined in this work. The common feature of these mechanisms can be regarded as the displacement of local atoms and rotation of certain structure unit. Structure phase transformation of GB was found during the migration of Σ17 (4 1 0) and Σ73 (8 3 0) GBs.

Optimisation of dispersion parameters of Gaussian plume model for CO2 dispersion

The carbon capture and storage (CCS) and enhanced oil recovery (EOR) projects entail the possibility of accidental release of carbon dioxide (CO2) into the atmosphere. To quantify the spread of CO2 following such release, the ‘Gaussian’ dispersion model is often used to estimate the resulting CO2 concentration levels in the surroundings. The Gaussian model enables quick estimates of the concentration levels. However, the traditionally recommended values of the ‘dispersion parameters’ in the Gaussian model may not be directly applicable to CO2 dispersion. This paper presents an optimisation technique to obtain the dispersion parameters in order to achieve a quick estimation of CO2 concentration levels in the atmosphere following CO2 blowouts. The optimised dispersion parameters enable the Gaussian model to produce quick estimates of CO2 concentration levels, precluding the necessity to set up and run much more complicated models. Computational fluid dynamics (CFD) models were employed to produce reference CO2 dispersion profiles in various atmospheric stability classes (ASC), different ‘source strengths’ and degrees of ground roughness. The performance of the CFD models was validated against the ‘Kit Fox’ field measurements, involving dispersion over a flat horizontal terrain, both with low and high roughness regions. An optimisation model employing a genetic algorithm (GA) to determine the best dispersion parameters in the Gaussian plume model was set up. Optimum values of the dispersion parameters for different ASCs that can be used in the Gaussian plume model for predicting CO2 dispersion were obtained.

Investigation of the Effects of Pipe Wall Roughness and Pipe Diameter on the Decompression Wave Speed in Natural Gas Pipelines

The shock tube experimental results have shown clearly that the decompression wave was slowed down in a pipe with a rough inner surface relative to that in a smooth pipe under comparable conditions. In the present paper a one-dimensional dynamic simulation model, named EPDECOM, was developed to investigate the effects of pipe wall roughness and pipe diameter on the decompression wave speed. Comparison with experimental results showed that the inclusion of frictional effects led to a better prediction than that of the widely used model implemented in GASDECOM. EPDECOM simulation results showed that the effect of roughness on the decompression wave speed is significant for pipe diameters less than 250 mm. However the decompression wave speed is nearly independent of the roughness for diameters above 250 mm as the frictional effect becomes negligible at such diameters.Copyright

Molecular dynamics simulation on generalized stacking fault energies of FCC metals under preloading stress

Molecular dynamics (MD) simulations are performed to investigate the effects of stress on generalized stacking fault (GSF) energy of three fcc metals (Cu, Al, and Ni). The simulation model is deformed by uniaxial tension or compression in each of [111], [11-2], and [1-10] directions, respectively, before shifting the lattice to calculate the GSF curve. Simulation results show that the values of unstable stacking fault energy (γusf), stable stacking fault energy (γsf), and unstable twin fault energy (γutf) of the three elements can change with the preloaded tensile or compressive stress in different directions. The ratio of γsf/γusf, which is related to the energy barrier for full dislocation nucleation, and the ratio of γutf/γusf, which is related to the energy barrier for twinning formation are plotted each as a function of the preloading stress. The results of this study reveal that the stress state can change the energy barrier of defect nucleation in the crystal lattice, and thereby can play an important role in the deformation mechanism of nanocrystalline material.

A Study of Crack Propagation in BCC Iron by Molecular Dynamics Method

In this paper, molecular dynamics method has been employed to model mode I crack propagation in body center cubic (BCC) single iron crystal. To maximize the simulation efficiency the parallel computing was performed. Six cases with different lattice orientations have been simulated to investigate the crack propagation behaviors at atomic level. The strain distributions have been calculated to indicate the density of dislocation. It has been found that the lattice orientation significantly affects the propagation behaviors. The crack in BCC iron propagates more readily along the direction <111> on the plane {1-10}.

Tension/compression asymmetry of grain boundaries with non-planar structure

Molecular dynamics simulations were carried out to investigate the mechanical property and the deformation mechanisms of Cu bicrystal with non-planar structured grain boundaries (GBs) under uniaxial tension and compression. The simulation results showed that the non-planar GBs could change their equilibrium configurations under the applied stress, and the deformation mechanisms varied when altering the misorientation angle. The stacking fault energy curve was affected by the stress perpendicular to the slip plane and therefore has an influence on the dislocation nucleation mechanisms. Previous studies have revealed a ubiquitous tension/compression (T/C) strength asymmetry of many ultra-fine or nanocrystalline materials, and a higher compressive strength was usually reported. However, in the present study, the bicrystal samples with non-planar structured GBs show a higher tensile strength than the compressive one. The unusual T/C asymmetry property has an implication that the GBs with non-planar structure can play a significant role in affecting the mechanical properties of nanostructured materials.

CFD Simulation of CO2 Dispersion in a Real Terrain

A deeper understanding of CO2 dispersion resulting from accidental releases is essential to evaluate the risk associated with the Carbon Capture and Storage (CCS) technology. The dispersion patterns of CO2, which is a heave gas, may vary according to local conditions. In this study the possibility of simulating the dispersion of CO2 clouds over a real topographically complex area in Australia by a general purpose Computational Fluid Dynamics (CFD) code is investigated. The study may offer a viable method for assessment of risks associated with CCS.

Hertz Contact at the Nanoscale with a 3D Multiscale Model

This paper presents a three-dimensional multiscale computational model, which is proposed to combine the simplicity of FEM model and the atomistic interactions between two solids. A significant advantage of the model is that atoms are populated in the contact regions, which saves significant computation time compared to fully MD simulations. The model is used in the case of asperity contact. The normal displacement, contact radius and pressure distribution are compared with those from Hertz’s solution and atomistic simulations in the literature. Some important features of nanoscale contacts obtained by MD simulations can be caught by the model with acceptable accuracy and low computational cost.

Finite-Temperature Multiscale Simulations for 3D Nanoscale Contacts

A finite-temperature analysis of a multiscale model, which couples finite element and molecular dynamics, is presented in this paper. The model is evaluated by the patch test and demonstrates its capacity. Then, the multiscale scheme is used to study 3D nanoscale contacts. The linear relationship between the contact area ratio and load is observed at small loads, but the temperature effect is small. However, the change in the root mean square (RMS) of heights depends on the temperature at high loads.