A classification of size effects (SE) in plasticity is attempted. ”Intrinsic” SE are perceived when any internal length scale directly influencing some process or property interferes with the size of the material region where the process is going on or when two internal length scales directly affecting the same process or property interfere. ”Extrinsic” SE arise from the external imposition of spatial gradients in the plastic process or by the building up of internal gradients by the (externally induced) process itself. In dislocation-mediated plasticity plastic strain gradients are resolved by the storage of geometrically necessary dislocations (GND) leading to prominent size effects. Of course, mixed effects with intrinsic and extrinsic contributions can be found as well as superposed effects involving more than two characteristic lengths (i.e., size effects on size effects).The inclusion of both types of SE in continuum or crystallographic theories is commented.

Preliminary ideas are presented on developing coupled, continuum field formulations for composition and mechanics with applications to microelectronic materials. A thermodynamic basis is constructed, reflecting atomic-level transport mechanisms and the associated mechanics. Formal arguments lead to constitutive field relations, and specification of balance laws completes the coupled field description. Early numerical results are also presented.

In nuclear power plants, Zr–based cladding is corroded by the primary coolant. Concomitantly, it undergoes hydrogen pick–up which induces modifications of its mechanical properties, especially creep and recrystallization rates. Below 600K, the deformation in hcp Zr is partially controlled by screw dislocations, which due to their intricate core structure have reduced intrinsic mobility. Here, we address the possible hydrogen induced modifications of the core structure of screw dislocations. We used first–principle calculations based on the density functional theory to evaluate the interaction between hydrogen and screw dislocation cores and also to evaluate the hydrogen induced modifications of the prismatic and basal gamma surfaces. We show that the presence of hydrogen results in significant reductions of the stacking fault energies.

The present paper experimentally studies the cracking phenomena of a TiN thin film on a titanium alloy, and presents a new approach to predict the number of cracks under tensile load. An elastoplastic shear-lag model is developed to obtain the stress distribution caused by the film cracks, which is found to agree well with that calculated with FEM. The number of thin film cracks is predicted using a Monte Carlo simulation using the present approach, and favourably compared with the experimental results.

Recent theoretical calculations and atomistic computer simulations have shown that one-dimensional (1D) glissile clusters of self-interstitial atoms (SIAs) play an important role in the evolution of the microstructure in metals and alloys under cascade damage conditions. Theoretical treatments based on the production bias model (PBM)and cascade-induced source hardening clarify many features of materials behaviour under irradiation by considering 1D glissile SIA clusters. However, further progress in understanding irradiation phenomena demands a detailed knowledge of different reactions where glissile SIA clusters take part. The latest version of the PBM has demonstrated that there is a significant lack of knowledge on mechanisms such as the interaction between mobile clusters and other microstructure components, changes on glide directions or the reduction of cluster mobility. Some aspects of interactions have been recently studied and the formation of immobile complexes of glissile clusters in the bulk and near dislocations has been shown. However, in addition to the cluster - clusters and cluster – dislocation interactions, the interactions of clusters with point defects should be considered. Here we present the first results on interaction between a SIA cluster and vacancy in bcc-iron. The interactions were studied by both atomistic simulations and elasticity theory. Some qualitative differences between the results obtained by these two methods were found, thereby demonstrating the need for the atomistic approach. It is shown that the interaction with vacancies can reduce and, in some cases, suppress the mobility of SIA clusters. The results are discussed in the light of both experimental data and theoretical predictions.

We report on tight-binding molecular dynamics (TBMD) of neck formation processes and atomistic and electronic diffusivity at grain boundaries of nanocrystalline silicon carbide. The TBMD simulations are based on an O(N) algorithm (the Fermi-operator expansion method) for calculating electronic contributions to energy and forces. The code has been fully parallelized on our PC-based parallel machines. The TBMD simulations of collision of SiC nanospheres show that the processes of neck formation depend strongly on contact angles between the two grains. Atomic diffusions are quite different in the necks formed with different angles. Also, the electronic transport property at grain boundary is investigated via a TB representation of an electronic diffusivity. A preliminary result on the diffusivity at a Σ=9 grain boundary of SiC indicates significant enhancement of electron mobility along the grain boundary.

This article presents an implementation of periodic boundary conditions (PBC) for Dislocation Dynamics (DD) simulations in three dimensions (3D). We discuss fundamental aspects of PBC development, including preservation of translational invariance and line connectivity, the choice of initial configurations compatible with PBC and a consistent treatment of image stress. On the practical side, our approach reduces to manageable proportions the computational burden of updating the long-range elastic interactions among dislocation segments. The timing data confirms feasibility and practicality of PBC for large-scale DD simulations in 3D.

In this work we report the self-affinity analysis of the fracture surfaces of a polymeric semicrystalline material and an opal glass. In the case of the plastic material, samples of isotactic polypropylene (i-PP) were prepared by varying the cooling rate from the melt; this resulted in different spherulite sizes. Samples were then broken in bend test after being immersed in liquid nitrogen. In the case of the opal glass, samples with different sizes of the opacifying particles, obtained by different thermal treatments, were broken in a punch test. In both cases the fracture surfaces were analyzed by both Scanning Electron Microscopy (SEM) and Atomic Force Microscopy (AFM) in the contact mode. Self-affinity analysis was performed by the variable bandwidth method, covering a range of length scales spanning from a few nanometers up to ten micrometers. The roughness exponents are found to be of similar values close to ζ = 0.8 with the correlation length corresponding to the size of the spherulites in the plastic material and to the size of the opacifying particles in the opal glass. These results should be taken into account in the development of multiscale models and simulations of the fracture process of real heterogeneous materials.

An approach is presented for the determination of the residual stresses and elastic moduli of particle systems resulting from computer simulations of particle or atomic deposition. The proposed technique is based on fundamental concepts of elasticity and is capable of capturing the variation of stresses and moduli as functions of position within the system. Application to a simple particle system consisting of a deposited thin film is demonstrated.

The mechanics of generalized continua provides an efficient way of introducing intrinsic length scales into continuum models of materials. A Cosserat framework is presented here to descrine the mechanical behavior of crystalline solids. The first application deals with the problem of the stress field at a crak tip in Cosserat single crystals. It is shown that the strain localization patterns developping at the crack tip differ from the classical picture : the Cosserat continuum acts as a bifurcation mode selector, whereby kink bands arising in the classical framework disappear in generalized single crystal plasticity. The problem of a Cosserat elastic inclusion embedded in an infinite matrix is then considered to show that the stress state inside the inclusion depends on its absolute size lc. Two saturation regimes are observed : when the size R of the inclusion is much larger than a characteristic size of the medium, the classical Eshelby solution is recovered. When R is much small than the inclusion, a much higher stress is reached (for an inclusion stiffer than the matrix) that does not depend on the size any more. There is a transition regime for which the stress state is not homogeneous inside the inclusion. Similar regimes are obtained in the study of grain size effects in polycrystalline aggregates of Cosserat grains.

A bridging law for fiber reinforced composites under dynamic crack propagation conditions has been derived. Inertial effects in the mechanism of fibre pullout during dynamic propagation of a bridged crack are critically examined for the first time. By reposing simple shear lag models of pullout as problems of dynamic wave propagation, the effect of the frictional coupling between the fibres and the matrix is accounted for in a fairly straightforward way. The solutions yield the time-dependent relationship between the crack opening displacement and the bridging traction. Engineering criteria and the role of material and geometrical parameters for significant inertial effects are identified.

Dynamic Monte-Carlo simulations of three dimensional ion motions are carried out based on a multi-well potential model. A parallel plate capacitor is modeled with an ionic conducting polymer having two ideal blocking electrodes. Positive ions (maximum 1000) are located on a cubic lattice with a maximum size of 100×100×100 locations. A negative background charge, constant in space and time, is used to provide charge neutrality. The positive ions can perform hops between neighboring sites with a probability corresponding to distributed energy barriers. To calculate the potentials due to an interaction of the ions the method of images is used. The steady state and the dynamic properties are studied after application of a voltage step in dependence on the sample thickness, the ion concentration and the voltage. A simulation of a space charge polarization yields an explanation for the experiments in the time and in the frequency domain.

Repeated stress-relaxation experiments are used to characterize the deformation parameters in 3 types of single crystals (Cu, Ni3Al and Ge) in which different dislocation mechanisms are known to operate. Mobile dislocation exhaustion rates and the amplitude of the yield point at reloading after stress-relaxation are measured. These two parameters are discussed in terms of the work-hardening coefficient in monotonic tests.

Thermal activation of <123> germanium single crystal deformation is studied by using stress-relaxation experiments. These exhibit a non logarithmic dependence of stress as a function of time. The athermal component of stress and the apparent activation volume are determined. The role of dislocation multiplication is discussed in terms of specific features of the monotonic and transient deformation behaviour.

Drying or curing of a coating after vitrification or gelation is accompanied by stress development. Evaporation of solvent, polymerization, cross-linking, and cooling all cause shrinkage, but adhesion of the coating to the substrate prevents shrinkage to a stress-free state. The interaction of shrinkage and restraint creates strain and stress. If the local stress grows above the local strength of the coating, it can produce cracking, delamination, or other defects.

A large deformation elastic model based on the Galerkin/finite element method is developed to analyze stress development in coatings subject to uniform shrinkage. The model is used to analyze effects of delamination and surface cracks. The strain energy release rates in both delamination and surface cracking are computed at different crack lengths. In both cases, results show that thicker coatings have larger energy release rates and are more vulnerable to cracking. A key conclusion of this modeling is that a crack can propagate only from an inherent flaw greater than a certain size. If the coating is thin enough so that the maximum energy release rate is less than the crack growth resistance, then no inherent flaws in the coating can grow into a crack, and so the coating remains crack-free. The model also shows how to calculate a critical coating thickness, i.e., the maximum thickness of a coating that can remain crack-free.

Interfacial defects like grain boundaries and phase boundaries play an important role in the mechanical behaviour of engineering alloys. In this work the problem of a crack on a bi-crystal interface is studied at the atomic scale, with the goal of elucidating the effects of varrying interatomic interaction on crack behaviour and to assess the suitability of existing fracture criteria to the anisotropic bi-crystal case. Calculations are performed using the Quasicontinuum (QC) method [1]. Using suitable approximations, some of the existing fracture criteria were used to predict ductile or brittle fracture and compared to the QC results.

A hybrid molecular-dynamics/finite-element simulation scheme is applied to describe multiscale phenomena in nanodevices. The quality of both static and dynamic coupling between atomistic and continuum regions is studied. The hybrid scheme is used for the Si/Si3N4 interface problem (static coupling), and for the projectile impact on Si problem (dynamic coupling). Excellent agreement is found between hybrid and full molecular dynamics simulation results in the static case, and no wave reflections are found at the atomistic/continuum hand-shake in the dynamic case. The hybrid scheme is thus validated a powerful and cost effective method for performing multiscale simulations of nanodevices.

We simulate the microscopic details of brittle fracture in silicon by dynamically coupling empirical-potential molecular dynamics of a strained sample to a quantum-mechanical description of interatomic bonding at the crack tip. Our simulations show brittle fracture at loads comparable to experiment, in contrast with empirical potential simulations that show only ductile crack propagation at much higher loading. While the ductility of the empirical potentials can be attributed to their short range, it is unclear whether the increased range of the tight-binding description is sufficient to explain its brittle behavior. Using the multiscale method we show that at a temperature of 1100 K, but not at 900 K, a dislocation is sometimes nucleated when the crack tip impinges on a vacancy. While this result is too limited in length and time scales to directly correspond to experimental observations, it is suggestive of the experimentally observed brittle to ductile transition.