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Interfacial fatigue of reinforced concrete under tension fatigue loading is simulated based on the shear-lag model. The effects of degradation of concrete strength, the loading frequency and the stress ratio as well as corrosive environment on the interfacial fatigue are considered and discussed. It is found that: (1) degradation of concrete strength and corrosive environment result in opposite effects on interfacial fatigue. (2) Degradation of concrete strength will result in larger effects on interfacial damage in low stress ratio than that in high stress ratio. (3) With a decrease of loading frequency, interfacial corrosion damage will remarkably increase though loading frequency has a small effect on fatigue crack growth in inert media.

Kinetic equations are analysed for thermal degradation of polymers. The governing relations are based on the fragmentation–annihilation concept. Explicit solutions to these equations are derived in two particular cases of interest. For arbitrary values of adjustable parameters, the evolution of the number-average and mass-average molecular weights of polymers is analysed numerically. Good agreement is demonstrated between the numerical results and experimental data. It is revealed that the model can correctly predict observations in thermo-gravimetric tests when its parameters are determined by matching experimental data for the decrease in molecular weight with exposure time.

The mechanical integrity of single-walled carbon nanotubes for bending and torsional deformations is evaluated using molecular dynamics simulations. A cantilever type nanotube loaded by a follower lateral force at the free end of the tube exhibits an unstable local buckling near the fixed end. A hysteresis loop is observed, accompanied by an irreversible thermodynamically stable topological transition from four hexagons to two pairs of pentagons and heptagons. The finite deformation can be described by a linear elastic solution almost up to the point of buckling. The initial torsional deformation gives a linear relationship between the torsional moment and the angle of twist per unit length. After buckling, an anti-plane distorted structure is generated, whose rigidity is weaker that of the original structure. The critical bending stress and the shear stress in torsion depend not only on the geometry of the cross section but also on the tube length.

Neural networks (NNs) are employed to study the deformation characteristics of sintered aluminium preforms. The proposed NN model has used the measured parameters, namely the load, the aspect ratio and the initial preform fractional density ratio to predict multiple material characteristics, namely the axial stress, the hoop stress, the hydrostatic stress, the axial strain, the hoop strain and the Poisson's ratio. The model is based on a 'four layered NN' with back propagation learning algorithm. The experimental set-up available in the laboratory has been used to get the training data for the sintered aluminium with various preform densities and different aspect ratios (0.50, 0.75 and 1.00) using MoS₂ as lubricant. The predicted values from the proposed NN coincide well with the experimental values. In addition, a comparative study between the regression analysis and the NN revealed that the NN can predict the material characteristics of sintered aluminium preform better than regression polynomials within a few per cent error.

A new method of computing grain boundary energies for arbitrary misorientation and inclination angles has been proposed, based on an atomistic approach (molecular statics) using a semi-empirical atomic potential (2NN MEAM). The procedure was applied to computation of the [110] symmetric tilt boundary energy of pure Al. It is shown that the computed grain boundary energy and especially its misorientation dependence are in good agreement with the information in the experimental literature. The probable sources and amounts of computation error and the applicability to the computation of inter-phase interface boundary energy are also discussed.

The distribution of (elastic) lattice strains following plastic deformation of an aluminium alloy is examined using finite element simulations of aggregates with individually resolved crystals. The aggregates consist of face centred cubic crystals with initial orientations assigned by sampling the measured crystallographic texture. The simulations show that the lattice strains within the crystals are influenced by various microstructural features, material parameters and modelling assumptions. A hierarchy among these various effects is established, based on applications where low probability events are crucial, such as in fatigue or fracture problems. The simulation results are discussed in detail and compared with experimental data for the macroscopic stress, the lattice strains in the unloaded state and the associated uncertainties.

A new Al potential with improved stacking fault energy is constructed using the force-matching method. The potential is fitted to an ab initio forces database and various experimental data. By using a slightly larger cut-off, we found that the new potential gives the relaxed stacking fault energy in the experimental range without changing the excellent thermal and surface properties of the original force-matching Al potential given by Ercolessi and Adams (1994 Europhys. Lett.26 583).

We performed molecular dynamics simulations of the maximum angle of stability of a bi-disperse granulate with cohesion (either springs or constant inter-particular forces) in a rotating drum. Segregation has been avoided by computing only the first angle of maximum stability after the rotating drum started its rotation. The results show that larger cohesive forces have to be applied in order to recover the same avalanche angle as for a mono-disperse granulate.

This article addresses a theoretical analysis of the 'modified maximum force criterion' (MMFC) introduced by Hora et al (1996 A prediction method for ductile sheet metal failure in FE-simulation Proc. Numisheet'96 Conf. pp 252–6). In this work a computational framework for the MMFC is given and it will be shown that the MMFC contains an important singularity which emerges if the yield locus used in the necking simulation exhibits straight line segments and leads to singularities in the predicted forming limit curve or even in total failure in forming limit diagram prediction. It is shown that the singularity emerges in commercial sheet alloys and, therefore, the general reliability of the MMFC appears questionable. It is thus concluded that an extension of the MMFC is required in order to fix the singularity problem.

The formation energies of Pu–X (X = Al, Ga, In) compounds are calculated in the framework of density functional theory within the local spin-density approximation and non-local corrections to the exchange-correlation functional. The factors that govern the stability of δ-(Pu–X) solid solutions are analysed using fcc superstructures for which total energy calculations are performed. The cluster variation method is used to determine the influence of chemical short range order on the structural stability of δ-(Pu–X) solid solutions as a function of temperature and composition. The differences between the three systems are discussed. The calculated Pu–X (X = Al, Ga, In) phase diagrams in the Pu-rich side are compared with the experimental ones.

In order to analyse the interactions of non-metallic (non-conducting) inclusions in (conducting) liquid metals in the presence of an electromagnetic (em) field, a simple system of two spherical particles was studied. This study dealt with particles in a row in an oblique configuration with respect to the dc em-field.

The interaction force between the particles and the migration force exerted on them were calculated, taking into account the effect of electromagnetically driven flow (em-flow) of the liquid.

The state of em-flow had essentially the same features as the non-oblique cases; however, a strong downward flow was generated on the particle side. In contrast to the bulk em force the magnitude of the migration force became lower. A small (relative) repulsive force existed between the two particles, which is related to the anti-clockwise rotating forces of the particle pair.

The results obtained give valuable insight into the case that takes into account the particle coagulation and transportation in the em processing of metals.

The isothermal decomposition of austenite into ferrite is investigated using a two-dimensional cellular automaton (CA) algorithm. This CA model provides a simple solution for the difficult moving boundary problem that governs the ferrite grain growth. In this model, the growth of ferrite grains is controlled by both the austenite–ferrite (γ–α) interface mobility and the carbon diffusion in the retained austenite. The competition between the γ–α interface dynamics and the carbon diffusion in the austenite results in a non-equilibrium γ–α interface condition. In order to predict the growth kinetics of ferrite grains, the carbon diffusion coupled with the γ–α interface dynamics is resolved numerically. The driving force for the γ–α interface mobility is calculated using a regular solute sub-lattice model. The kinetics of ferrite transformation predicted by this CA method is compared with that of a JMA model and experiments in the literature. The simulation provides an insight into the carbon diffusion in austenite and the microstructure evolution during transformation. Meanwhile, the results also show that the γ–α interface under the present non-equilibrium condition is numerically stable in two dimensions and the simulated morphology of the ferrite grains is an almost equiaxed polygon.

The thermal response of thick plate weldments under multi-layer and multi-block welding sequences is studied by the use of three-dimensional finite element models. The moving isotherm approach is applied to the modelling of inserted arc power and the temperature dependence of the material's thermal properties is considered as well. The addition of welded material that normally occurs during the welding is also simulated by the use of the element rebirth technique. Arc movement, welder speed, arc moving direction and the time lag between the sequences are also considered in this research. The results show that the number of layers in the welding of thick plates plays a significant role in the formation of the thermal profile, especially for those points that are located near the weld. For the far points this effect is very small and can be neglected. In the case of multi-block welding, the results showed that this kind of welding sequence not only changes the magnitudes of maximum temperatures but also significantly affects the distribution of isothermal surfaces in the whole domain. Moreover, the outcomes are practically feasible and the model can be recommended for analysis of thermal fields in thick plate weldments.

This work reports on a recent advancement in three-dimensional dislocation dynamics modelling. A method is presented for the treatment of dislocation image stresses resulting from the presence of nearby traction-free surfaces. The image stress-field of a dislocation segment below a finite-sized free surface is obtained by the distribution of N generally prismatic rectangular or square dislocation loops padding the area (the external bounding surfaces of the simulation box). The unphysical tractions created at surface collocation points by sub-surface crystal dislocations are annulled by proper determination of the loops' Burgers vectors. The image stresses on a dislocation segment are simply those stresses resulting from the surface loops. The accuracy can be improved by increasing the number of collocation points (i.e. surface loop density).

Polycrystalline piezoelectric materials are aggregations of crystal grains and domains with uneven forms and orientations. Therefore, the macroscopic ferroelectric property should be characterized by introducing a microscopic inhomogeneity in the crystal morphology. In this study, a multi-scale finite element modelling procedure based on a crystallographic homogenization method has been proposed for describing a macroscopic property of polycrystalline ferroelectrics with consideration of the crystal morphology at a microscopic scale. The proposed procedure has been applied to two kinds of piezoelectric materials, BaTiO₃ and PbTiO₃ polycrystals; the influence of microscopic crystal orientations on macroscopic ferroelectric properties was verified numerically. From the computational results, it has been shown that piezoelectric constants of polycrystalline ferroelectrics can be maximized by design of microscopic crystal morphology.

Atomistic simulation is a useful method for studying material science phenomena. Examination of the state of a simulated material and the determination of its mechanical properties is accomplished by inspecting the stress field within the material. However, stress is inherently a continuum concept and has been proven difficult to define in a physically reasonable manner at the atomic scale. In this paper, an expression for continuum mechanical stress in atomistic systems derived by Hardy is compared with the expression for atomic stress taken from the virial theorem. Hardy's stress expression is evaluated at a fixed spatial point and uses a localization function to dictate how nearby atoms contribute to the stress at that point; thereby performing a local spatial averaging. For systems subjected to deformation, finite temperature, or both, the Hardy description of stress as a function of increasing characteristic volume displays a quicker convergence to values expected from continuum theory than volume averages of the local virial stress. Results are presented on extending Hardy's spatial averaging technique to include temporal averaging for finite temperature systems. Finally, the behaviour of Hardy's expression near a free surface is examined, and is found to be consistent with the mechanical definition for stress.

This paper reviews the continuum notions of average mechanical properties such as stress and strain and the meaning of such notions when molecular dynamics (MD) is used to compute them. The focus of this paper is a discussion of how boundary conditions are actually enforced in MD and what corresponding notion of averages is therefore appropriate. Finally, we discuss the concepts as they relate to the Parrinello–Rahman approach as a specific example.

In order to better understand the mechanisms of tungsten (W) film delamination from the silicon (Si) substrate, a three-dimensional molecular dynamics (MD) simulation is being conducted to investigate the formation of residual stress during the film deposition process. For the purpose of simplicity, a Morse pair potential is proposed in this paper to simulate the interactions between W and Si atoms during the film deposition process. It appears from numerical solutions that the residual stress field in the W film is very sensitive to the W–Si interfacial potential model proposed for the MD simulation. By calibrating the controlling parameters in the interfacial potential model using the comparison between the simulated stresses and experimental data, the film stress transition from tension to compression during the film deposition process could be qualitatively simulated via the proposed simulation procedure. The numerical results presented in this paper provide a better insight into the effect of interfacial atomic potential on the stress transition in thin films. In addition, it can be seen from the MD simulation that there might exist a phase transition from the crystalline Si to amorphous W–Si structure to crystalline W around the interface area. Well-designed experiments are required to verify the simulation results.

We introduce a novel approach to calculating the Peierls energy barrier (and Peierls stress) based on the analysis of the dislocation migration dynamics, which we apply to 1/2a⟨111⟩ screw dislocations in bcc Ta. To study the migration of screw dislocations we use molecular dynamics with a first principles based embedded-atom method force field for Ta. We first distinguish the atoms belonging to the dislocation core based on their atomic strain energies, defining the dislocation core as the 12 atoms with higher strain energies per Burgers vector. We then apply this definition to the moving dislocations (following the dynamics of a [1−10] dipole of 1/2⟨111⟩ screw dislocations at 0.001 K) and extract their Peierls energy barrier (E_P) and Peierls stress (τ_P). >From the dynamics of a dislocation dipole, we determine E_P = 0.032 eV (and τ_P = 790 MPa) for twinning shear and E_P = 0.068 eV (and τ_P = 1430 MPa) for anti-twinning shear, in good agreement with the results by applying direct shear stresses. This dislocation dynamics method provides insights regarding the dislocation migration process, allowing us to determine the continuous path of dislocation migration. We find that under twinning shear the screw dislocation moves along a path at an angle of only 8.5° with the [1−10] direction while for anti-twinning shear it moves along a path at an angle of 29.5° with the [1−10] direction, documenting the magnitude of the violation of the Schmid Law.

Large-scale atomistic simulations are performed to study different mechanisms of plastic deformation in uncapped submicron thin polycrystalline copper films. It was recently shown that diffusional mass transport along grain boundaries in thin films leads to the formation of a novel defect identified as a diffusion wedge (Gao et al 1999 Acta Mater.47 2865–78). Eventually, a crack-like stress field develops near the grain boundary–substrate junction as tractions along the grain boundaries relax under the constraint that the adhesion between film and substrate prohibits strain relaxation close to the interface. The emergence of crack-like stress concentration causes nucleation of an unexpected class of dislocations near the root of the grain boundary on glide planes parallel to the film surface. These dislocations are unexpected because there is no driving force for parallel glide (PG) in the overall biaxial stress field. In this work, we demonstrate that PG dislocations dominate plasticity in polycrystalline submicron thin films when tractions along the grain boundaries are relaxed by diffusional creep. We illustrate that partial dislocations play an important role in plasticity of nanostructured thin films and that the grain boundary structure has a significant influence on dislocation density in neighbouring grains. To allow modelling of thicker films, we propose a discrete dislocation model of diffusional creep to investigate the effect of PG on the flow stress of submicron films. A deformation map summarizes the range of dominance of different strain relaxation mechanisms in ultra-thin films. We show that besides the classical 'threading dislocation' regime, there are numerous novel mechanisms once the film thickness approaches the nanoscale.

Charging many metal films or powders with hydrogen results in the growth of second-phase metal hydride particles. A large volume difference between hydride and metal can induce plastic deformation and result in a significant hysteresis upon removal of the hydrogen. For a palladium hydride system (with an 11% volume change), we estimate this hysteresis with calculations of the elastic and plastic works of multiple forward and reverse transformations using a two-dimensional finite-element model and find reasonable agreement between finite-element estimates and experimental observations. We also use a three-dimensional phase-field model to determine the effects of particle interactions, but for the isotropic dilation in the Pd/PdH system, these interactions do not apparently have a significant effect.

This paper presents an experimental analysis and a numerical simulation of the mechanical behaviour experienced during the tensile test by both cylindrical and strip specimens of different materials for which the classical so-called Bridgman's procedure aimed at predicting the stress distribution at the necking zone cannot be directly applied. A set of experiments has been carried out in order to derive the elastic and hardening parameters that characterize the material response. The simulation of the deformation process during the whole test is performed with a finite element large strain elastoplasticity-based formulation. Finally, the results obtained with the simulation are experimentally validated.

We develop a many-body force field (FF) for tantalum based on extensive ab initio quantum mechanical (QM) calculations and illustrate its application with molecular dynamics (MD). As input data to the FF we use ab initio methods (LAPW-GGA) to calculate: (i) the zero temperature equation of state (EOS) of Ta for bcc, fcc, and hcp crystal structures for pressures up to ∼500 GPa, and (ii) elastic constants. We use a mixed-basis pseudopotential code to calculate: (iii) volume-relaxed vacancy formation energy also as a function of pressure. In developing the Ta FF we also use previous QM calculations of: (iv) the EOS for the A15 structure; (v) the surface energy bcc (100); (vi) energetics for shear twinning of the bcc crystal. We find that, with appropriate parameters, an embedded atom model FF (denoted as qEAM FF) is able to reproduce all this QM data. We illustrate the use of the qEAM FF with MD to calculate such finite temperature properties as the melting curve up to 300 GPa and thermal expansivity in a wide temperature range. Both our predictions agree well with experimental values.