Showing papers by "Polytechnic University of Catalonia published in 1991"

Computer simulation of the structure and mobility of twinning disclocations in H.C.P. Metals

Abstract: The properties of twinning dislocations in [10 1 1}, {11 1 2}, {11 2 2} twin boundaries in hexagonal-close-packed metals have been investigated by atomic-scale computer simulation. Care has been exercised (by use of the concept of bicrystal structure maps) to ensure that all possible stable interface structures have been modelled and the work extends the research reported earlier by us [6] in several ways. First, we have now treated the important case of the {10 1 1} twin Second, the dependence of dislocation energy on the atomic structure of the core has been investigated. Third, the mobility of these interfacial dislocations has been examined by computing the critical resolved shear strain for glide. It has been found that the twinning dislocations corresponding to the {10 1 2} and {11 2 1} twins observed in practice are highly glissile, wheras those for the {10 1 1} and {11 2 2} modes are not. These effects are deduced to be related to the atomic structure of the core (and, in particular, to its width rather than height), and are found to be consistent with the nature of deformation twinning reported for the h.c.p. metals. For {10 1 1} twinning, however, the dislocation of lowest energy and highest mobility does not correspond to the mode observed in practice, implying that twin nucleation may possibly be a controlling factor in that case.

Potential of mean force by constrained molecular dynamics: A sodium chloride ion-pair in water

Abstract: The constrained molecular dynamics simulation method has been used to obtain the mean force and the mean force potential between two particles in solution. The method has been tested for a Lennard-Jones liquid and applied to the study of a Na+-Cl− ion-pair in aqueous solution. A flexible SPC model for water has been assumed. The results have been interpreted in the light of the solvent structure around the ions for separations corresponding to the maxima and minima of the mean force potential. In contrast to earlier studies using rigid water models, the solvent separated ion pair configurations are more stable than the contact ion pair configurations.

Alignment methods for averaging of high-resolution cardiac signals: a comparative study of performance

Abstract: A comparative study of the performance of three alignment methods (the double-level method, a new time-delay estimation method based on normalized integrals, and matched filtering) is presented. A real signal and additive random noise for several signal-to-noise ratios (SNRs) are selected to make an ensemble of computer-simulated beats. The relation between the standard deviation of temporal misalignment versus SNR is discussed. A second study with real ECG signals is also presented. Several morphologies of QRS and P waves are tested. The results are in agreement with the computer simulation study. Nevertheless, the power spectrum of the noise process can affect the results. Matched filter estimation has been tested in the presence of power line interferences (50 Hz), with poor results. An application of the three alignment methods as a function of the SNR is proposed. The new time-delay estimation method has been observed to be robust, even in the presence of nonwhite noise. >

Optimal distance networks of low degree for parallel computers

Abstract: The authors introduce and study a family of interconnection schemes, the Midimew networks, based on circulant graphs of degree 4. A family of such circulants is determined and shown to be optimal with respect to two distance parameters simultaneously, namely maximum distance and average distance, among all circulants of degree 4.. These graphs are regular, point-symmetric, and maximally connected, and one such optimal graph exists for any given number of nodes. The proposed interconnection schemes consist of mesh-connected networks with wrap-around links, and are isomorphic to the optimal distance circulants previously considered. Ways to construct one such network for any number of nodes are shown, their good properties to build interconnection schemes for multicomputers are examined, and some interesting particular cases are discussed. The problem of routing is also addressed, and a basic algorithm is provided which is adequate for implementing the routing policy required to convey messages, traversing shortest paths between nodes. >

Computer simulation of twinning dislocation in magnesium using a many-body potential

Abstract: Twinning is an important plastic deformation mechanism in the response of h.c.p. metals to applied stress, and in the present paper the atomic processes associated with {1011}, {1012}, {1121} and {1122} twinning in Mg are considered. The technique of atomic-scale computer modelling has been used, with atomic interactions described by the many-body potential of the Finnis–Sinclair type recently derived by Igarashi, Khantha and Vitek. The atomic structure, energy and stress state of planar twin interfaces, and the structure and energy of twinning dislocations in these boundaries, have been determined. The ease of glide of these dislocations has also been studied. It is found that the energy and mobility are related to the width of the dislocation core rather than to its height. Hence, dislocations in {1012} and {1121} boundaries are wide, have a low energy and are very glissile, whereas those in {1011} and {1122} boundaries have the opposite characteristics. These results support conclusions drawn ...

Geometric damage tensor based on microplane model

Abstract: An appealing approach to formulate constitutive models for characterizing distributed damage due to microcracks and voids is continuum damage mechanics with the concepts of effective stress and strain equivalence. In that approach, in which damage is imagined to characterize the reduction of the net stress‐transmitting cross‐section area of the material, the constitutive model is separated into two independent parts, one for damage and the other for elastic and inelastic behavior (rheology) other than damage, which, if combined appropriately, give the overall constitutive behavior. However, the existing multidimensional formulations for damage are quite complex, and practical implementations capable of fitting experimental data are hard to obtain. The microplane models, by contrast, provide conceptual simplicity and close fits of multiaxial test data for concrete, soils, etc., although, as formulated in the past, various kinds of physical phenomena were mixed in the definition of the microplane stress‐str...

Geometrical Setting of Time-Dependent Regular Systems:. Alternative Models.

Abstract: We analyse exhaustively the geometric formulations of the time-dependent regular dynamical systems, both the Hamiltonian and the Lagrangian formalisms. We study the equivalence between the different models and, in each case, between the Lagrangian and the Hamiltonian formulations, giving the suitable definitions of the Legendre transformation. In addition, we include the variational formalisms as well as the Klein formalism.

The crystallography and atomic structure of line defects in twin boundaries in hexagonal-close-packed metals

Abstract: The crystallographic analysis of line defects in interfaces is discussed and applied to the particular case of twinning dislocations in hexagonal-close-packed (hop) metals, which have been studied here by atomistic simulation. Two crystallographic approaches are used; first, the concept of bicrystal structure maps is developed for the case of interfaces between crystals having multiple-atom bases, and second, the topological theory of line defects based on symmetry theory is used. On the basis of the atomistic calculations, some general conclusions concerning the relative contribution to the total energy of dislocations made by their elastic fields and core structures are presented.

A dynamical equivalent to the equilateral libration points of the earth-moon system

Abstract: Consider the Earth-Moon-particle system as a Restricted Three Body Problem. There are two equilateral libration points. In the actual world system, those points are no longer relative equilibrium points mainly due to the effect of the Sun and to the noncircular motion of the Moon around the Earth. In this paper we present the problem as a perturbation of the RTBP and we look for the dynamical equivalent of L 4,5. It turns out to be a quasiperiodic orbit. It is obtained for a simplified model but the procedure to obtain it is general and can be carried out with an additional computational effort.

Vosperian and superconnected Abelian Cayley digraphs

Abstract: A digraphX is said to be Vosperian if any fragment has cardinality either 1 or|V(X)| ? d + (X) ? 1. A digraph is said to be superconnected if every minimum cutset is the set of vertices adjacent from or to some vertex. In this paper we characterize Vosperian and superconnected Abelian Cayley directed graphs. Our main tool is a difficult theorem of J.H. Kemperman from Additive Group Theory. In particular we characterize Vosperian and superconnected loops network (also called circulants).

Parallel complexity in the design and analysis of concurrent systems

Abstract: We study the parallel complexity of three problems on concurrency: decision of firing sequences for Petri nets, trace equivalence for partially commutative monoids, and strong bisimilarity in finite transition systems. We show that the first two problems can be efficiently parallelized, allowing logarithmic time Parallel RAM algorithms and even constant time unbounded fan-in circuits with threshold gates. However, lower bounds imply that they cannot be solved in constant time by a PRAM algorithm. On the other hand, strong bisimilarity in finite labelled transition systems can be classified as P-complete; as a consequence, algorithms for automated analysis of finite state systems based on bisimulation seem to be inherently sequential in the following sense: the design of an efficient parallel algorithm to solve any of these problems will require an exceedingly hard algorithmic breakthrough.

Microplane Model for Triaxial Deformation of Saturated Cohesive Soils

Abstract: A microplane model, in which the constitutive properties are characterized independently on planes of various orientations (microplanes), is presented. It is found that the basic scheme previously developed for concrete is also valid for clays, but with certain modifications. The micro‐macro constraint is kinematic, and the stresses on each microplane are defined as explicit functions of the volumetric and deviatoric normal and shear components of the macroscopic strain tensor on the microplane. To account for undrained behavior, a pore water pressure term that affects only the volumetric equations is introduced. This makes possible the uncoupling of the stress‐strain and the pore water pressure formulations. The model is calibrated and verified by comparisons with numerous data for both drained and undrained tests, and good agreement is attained, including volume changes, pore water pressure evolution, and various stress‐strain diagrams. Although the model involves nine material parameters, four (or five...

A general methodological analysis for optimum design

Abstract: DAO, a computer aided optimum design system by the finite element method, has been developed. The system can solve efficiently 2D and 3D structural fixd-geometry and shape optimization problems. The power and viability of this methodology is illustrated by the solution to a structural optimization problem. The shape of the central section of an arch dam is optimized.

Second-species solutions in the circular and elliptic restricted three-body problem

Abstract: An analytical proof of the existence of some kinds of periodic orbits of second species of Poincare, both in the Circular and Elliptic Restricted three-body problem, is given for small values of the mass parameter. The proof uses the asymptotic approximations for the solutions and the matching theory developed by Breakwell and Perko. In the paper their results are extended to the Elliptic problem and applied to prove the existence of second-species solutions generated by rectilinear ellipses in the Circular problem and nearly-rectilinear ones in the Elliptic case.

Protamines in prosobranchian gastropods (mollusca) vary with different modes of reproduction

Abstract: We have studied the protamines from different families of prosobranch gastropods. In those with external fertilization we find a simple pattern of sperm nuclear basic proteins, consisting of one major protamine. This protamine has between 75 and 105 amino acid residues in the ten species of the order Archaeogastropoda that we have examined. In six species of the order Patellogastropoda (Kozloff, Marine Invertebrates of the Pacific Northwest, Univ. of Wash. Press, Seattle, pp. 193–232, 1987), the protamine-like component contains 220 ± 10 amino acid residues. However, the protamines from the archaeogastropod Monodonta turbinata and the patellogastropod Patella cerulea show a very similar amino acid composition. Cytochemically, four species of archaeogastropods and three species of patellogastropods show intermediate type 3 sperm basic proteins in the classification scheme of Bloch (Genetics 61:93–111, 1969; Handbook of Genetics, Plenum, New York, pp. 136–167, 1976). The sperm basic protein profile in six internally fertilizing species of the orders Mesogastropoda and Neogastropoda is more complex since several protamine bands are found. In all cases the major protamines have high electrophoretic mobilities and the number of amino acid residues is estimated to be from 40 to 60. The combined set of major protamines in the mesogastropod Littorina neritoides and the neogastropod Murex brandaris display a similar amino acid composition that differs significantly from that in the externally fertilizing gastropods. In one mesogastropod and three neogastropod species cytochemical staining indicates the possible presence of type 2 stable protamines. Protamines from a neogastropod have a capacity for compacting DNA that is similar to that of an externally fertilizing archaeogastropod. We propose that changes in protamine features amongst different prosobranch gastropods are correlated with changes in nuclear condensation patterns and the fertilization biology of these mollusks.