We present the results of a combined experimental and theoretical study performed using non-linear mechanics schemes to investigate the structural behaviour of a composite macroscopic material. A simple model is considered to define the order of the complexity of the real system represented by a graphite peek polymer under static and dynamic load In particular a relationship has been found between the critical points in the energy-time diagram and in the bifurcation plot.

This study was initiated to investigate the structural efficiency of the “bessbeetle” (Odontotaenius Disjunctus) as compared to typical “man-made” or synthetic composites. This synthetic laminate is configured according to standard composites design practices. In fact, an orthotropic unidirectional T300 tape with circular fibers and 60% fiber volume fraction is used. The specific ply orientation of the symmetric man-made composite is [0/+45/-45/90]2S. The natural laminate studied is unsymmetric with the individual plies made of the same orthotropic unidirectional T300 tape but with different ply orientation. Contrary to previous beliefs the natural composite did not assume a balance of inplane and bending properties which in essence mimic the behavior of isotropic behavior. The results of the efficiency of the natural composite for tension did not show any noticeable advantages while the pure bending case resulted in higher values for the principal stresses in the unsymmetric laminate.

An atomistic computer simulation of mode I crack extension in cubic silicon carbide has been performed using a realistic many–body interatomic potential computed by Tersoff. The crack front is parallel to the [110] direction and the crack plane lies in the (111) plane. The stable crack tip configurations were calculated and the effective stress intensity factor and the effective crack tip position were evaluated in the relaxed atomic configuration by the least-square method. The crack was stable over a wide range of the stress intensity factors from 0. 6KG to 3. 4KG, where KG is the Griffith critical stress intensity factor. At 3.5KG an interatomic bond near the tip across the (001) plane ruptured and the crack advanced. When the crack is stable, the effective K is larger than the given K by nearly 0. 2KG to 0.4KG. Crack tip process was also simulated over a range of temperatures. At 1000K. secondary cracks were nucleated and grew like voids around the main crack, and thus the main crack was blunted.

A lattice type model is used for simulating fracture processes in concrete and sandstone. Two type of lattices were adopted: a regular triangular lattice and a random lattice. With both lattices the fracture process in four point shear specimens can be simulated. The results are in good agreement with experimental observations. With the regular lattice, where disorder is implemented via a generated grain structure, the typical failure mechanism for granular materials, crack face bridging, is retrieved. With the random lattice, these details cannot be simulated, but the macroscopic crack pattern is obtained correctly with an enormous decrease in the number of elements.

In this study, the Embedded Atom Method is combined with Monte Carlo and molecular dynamics simulations to study the fracture properties of Σ9 and Σ11 tilt boundaries in nickel. The Monte Carlo simulations are used to simulate the exposure of the bicrystal to a hydrogen environment at 300° C. These simulations establish the equilibrium distribution of hydrogen at the boundaries as a function of far-field concentration. The effect of the hydrogen on the fracture process is then studied with molecular dynamics. It will be shown that the fracture stress of the Σ9 boundary is affected over a wider range of far-field concentrations than the Σ11 boundary, although the Σ11 boundary shows that catastrophic failure occurs when the sample is charged beyond a certain far-field concentration.

The fracture results of an amorphous epoxy resin and polycrystalline silicon nitride tested under monotonically increasing crack mouth opening displacements were compared. A conventional compact tension specimen was used for the epoxy. A new loading system and specimen was designed for the ceramic to permit stable crack growth. Tests conducted on both materials revealed multiple events of subcritical crack growth, dynamic initiation, dynamic growth, and subsequent arrest. The elastic energy release rate at initiation, G11, and arrest, G1a, was evaluated for each event. The value of G11 was found to be higher than G1a for both materials.

Also common was the systematic existence of slow growth prior to dynamic initiation. Additionally, both materials showed differences between the fracture surface morphology for slow, 10-6 m/s, and fast, 103 m/s, growth. Although a constant G1a was recorded for the epoxy, large scatter and a possible trend was recorded for G1a in the ceramic.

We have carried out a series of atomistic simulations on arrays of about 10,000 atoms containing an atomically sharp crack and subjected to increasing stress levels. The ordered stoichiometric alloys B2 NiAl, B2 RuAl and A15 Nb3AI have been studied at different temperatures and stress levels, as well as the elements Al, Ni, Nb and Ru. The many body interactions used in the simulations were derived semi-empirically, using techniques related to the Embedded Atom Method. Trends in dislocation generation rates and crack propagation modes will be discussed and compared to experimental indications where possible, and some of the simulations will be demonstrated in the form of computer movies.

Edge dislocations were created on a surface of a small copper single crystal. Very sharp yield stress was observed when a partial dislocation was created. Edge dislocations in copper were split into Heidenreich–Shockley partial dislocations connected with the stacking fault.

Tight-binding, total-energy (TBTE) methods have successfully predicted surface atomic geometries for a variety of semiconducting and insulating materials that are well described by a nearest-neighbor model of interatomic interactions. However, little work has been done on distant-neighbor models, which are required to study many important mineral oxides. In this paper we demonstrate one way in which the TBTE methodology can be extended to these materials. To illustrate this approach, we calculate surface atomic structures for cassiterite SnO2 (110), β-cristobalite SiO2 (110) and βtridymite SiO2 (10TO).

Polar and non-polar interfaces of grain boundaries in compound semiconductors can be defined by the stoichlometry in the interface region, and It Is possible to construct two polar and one non-polar Interfaces for a symmetrical tilt grain boundary In the zinc-blende structure of which the Interface Is polar surfaces. The atomic and electronic structures of polar and non-polar interfaces of the {122} Σ=9 grain boundary in SIC have been examined by using the SCTB method coupled with the supercell technique. By using the calculated binding energies, the relative stability of the polar and non-polar interfaces has been analyzed through calculations of the thermodynamic potentials as a function of the atomic chemical potentials. It has been shown that the wrong bonds and the stoichlometry much Influence the stability and properties of grain boundaries in SiC. The stability of polar Interfaces In heterovalent compound semiconductors has been discussed.

The structure and dynamics of Cu(100), Cu(110) and Cu(111) have been studied by Molecular Dynamics simulations using the potential derived from effective medium theory. The formation energy for adatoms is lower for Cu(llO) than for Cu(lO0) or Cu(lll). The relatively high concentration of defects on Cu(11O) makes this surface premelt near the bulk melting point. The surface diffusivity is high for all three surfaces at elevated temperatures, the diffusion mechanism at the disordered surfaces appears to be quite complicated.

A new quantum-mechanical method capable of determining atomic configurations of extended defects has been developed. The Hamiltonian is constructed using the first-principles tight-binding linear muffin-tin orbital method in the atomic-sphere approximation. A semiempirical correction is employed to describe the effect of electrons in the interstitial region and the repulsive part of the total energy is described by a pair potential. The forces on atoms are calculated by differentiating the corresponding energy terms with respect to atomic positions. The equilibriun configuration is then found by minimizing the total energy. In this approach, the electronic and chemical aspects of defect structures can be accounted for more adequately than when using central-force pair or many-body potentials. In particular, the angular dependence of interatomic forces, which is important in transition metals with unfilled d-bands, is described correctly within the present quantum-mechanical treatment.

The (110) surface of gold is known experimentally to undergo a 1 × 2 reconstruction which is believed to be of the “missing row” type. There have been a number of theoretical studies of the energetics and geometry of this reconstruction. In this work, we present new results obtained using two versions of the Equivalent Crystal Theory (ECT). The ECT has proven to be quite accurate in providing predictions of surface energies and planar relaxations. The extended version of the ECT has been modified to better describe such quantities as elastic constants and shear energies. We have computed the energetics and geometry of the missing-row reconstructed surface, in both unrelaxed and relaxed form, using a Monte Carlo energy minimization technique, allowing relaxation of the positions of atoms up to three layers deep. We compare these results with previous theoretical work and with available experimental data.

The atomic structure of Σ = 3 tilt boundaries with variously inclined boundary planes has been studied using the many body potentials for gold. A chain unit model, analogous to the structural unit model, describes the relationship between different boundaries. The basic units in this model are units of the (111) and (112) twin boundaries and the model corresponds to atomic level faceting into these boundaries.

Atomistic Monte Carlo simulations utilizing many-body embedded atom method (EAM) potentials have been carried out for a series of symmetrical [001] twist boundaries in Pt-3 at.% Ni and Ni-3 at.% Pt alloy bicrystals at 850 K throughout the misorientation range 0° to 45°. The results demonstrate enhancement of the solute-atom concentration at twist boundaries for both alloys; the interfacial Gibbsian excess is a factor of two greater on the Ni-rich side of the phase diagram. The spatial distributions of solute atoms in the vicinity of the interfaces are found to be markedly different on the two sides of the Ni-Pt phase diagram. For low-angle boundaries on the Pt-rich side solute atoms tend to segregate in hourglass-like regions along the cores of the primary grain boundary screw dislocations, while in the case of Ni-3 at.% Pt they occupy bipyramidal regions centered on the cells of the dislocation grid. The former behavior explains the oscillatory solute-atom concentration profiles nomnal to the plane of an intetface.

We report the results of computer simulations of phase transitions in noble-gas clusters. The calculations were performed on a MasPar MP-l massively parallel computer with 8,192 processing elements (PE's). We discuss the efficient implementation of molecular dynamics algorithms for small clusters on this type of architecture. The simulations are based on a classical Lennard-Jones pair potential and follow the temporal evolution of the system by numerically integrating Newton's equations of motion using the Gear algorithm. Because the number of particles is much smaller than the number of PE's, optimal partitioning of the processing element array is an essential and non-trivial task.

In nonequilibrium surface growth processes, such as molecular beam epitaxy and chemical vapor deposition, kinetic roughening plays a significant role in determining surface morphology. The stochastic dynamics in the processes induce novel scaling in surface roughness with respect to growth time and system sizes. We present here a series of numerical simulation studies of simple theoretical models, such as ballistic deposition model and biased solid-on-solid model, to demonstrate the dynamical scaling and phase transitions in a class of surface growth problems in the context of the Kardar-Parisi-Zhang theory. Different dynamical processes, such as desorption and surface diffusion, are shown to have dramatic effects on the scaling and thus the surface structures.

We present molecular dynamics results on the diffusion properties of Ag monomers and dimers on the (111) surface of Pt. In particular we focus on the behavior of dimers upon collision on the surface with impact energies of 0.5eV and 5eV. We found that the dimer has the largest probability of being found stable on the surface when the temperature is in the range 150K<T<300K, while at T=OK a metastable configuration featuring monomers and dimers is likely to form.

Gas-surface reactions of the type that contribute to growth during the chemical vapor deposition (CVD) of diamond films are generally completed in picoseconds, well within timescales accessible by molecular dynamics (MD) simulations. For low-pressure deposition, however, the time between collisions for a surface site can be microseconds, which makes direct modeling of CVD crystal growth impossible using standard MD methods. To effectively bridge this discrepancy in timescales, the gas-surface reactions can be modeled using MD trajectories, and then this data can be used to define probabilities in a Monte Carlo algorithm where each step represents a gas-surface collision. We illustrate this approach using the reaction of atomic hydrogen with a diamond (111) surface as an example, where we use abstraction and sticking probabilities generated using classical trajectories in a simple Monte Carlo algorithm to determine the number of open sites as a function of temperature. We also include models for the thermal desorption of hydrogen that predict that growth temperatures are not restricted by the thermal loss of chemisorbed hydrogen.

A Monte Carlo model has been developed to describe the gas phase formation and sintering of nanoclusters. Sintering was incorporated into the model by modifying Kadanoff's algorithm [1] for random particle walks on the surface of the cluster. The effect of restructuring and sintering on cluster-cluster aggregation have been investigated via 2d simulations. In the initial stages the sintering process results in clusters which are compact on small length scales. As time progresses and the clusters become larger, the sintering process slows down and the fractal dimension of the clusters is decreased. The model shows the effect of time and temperature on the specific surface area and size distribution of clusters.