Showing papers by "Technical University of Denmark published in 1988"

Generating optimal topologies in structural design using a homogenization method

Abstract: Optimal shape design of structural elements based on boundary variations results in final designs that are topologically equivalent to the initial choice of design, and general, stable computational schemes for this approach often require some kind of remeshing of the finite element approximation of the analysis problem. This paper presents a methodology for optimal shape design where both these drawbacks can be avoided. The method is related to modern production techniques and consists of computing the optimal distribution in space of an anisotropic material that is constructed by introducing an infimum of periodically distributed small holes in a given homogeneous, i~otropic material, with the requirement that the resulting structure can carry the given loads as well as satisfy other design requirements. The computation of effective material properties for the anisotropic material is carried out using the method of homogenization. Computational results are presented and compared with results obtained by boundary variations.

Microcracking in Ceramics Induced by Thermal Expansion or Elastic Anisotropy

Abstract: The effect of crystal anisotropy on the formation of grain-boundary microcracks is analyzed, by considering a planar array of hexagonal grains as a model of a polycrystalline ceramic. The stress singularities at triple-grain junctions are analyzed by an asymptotic method as well as by a numerical solution, and the critical size of a grain-boundary defect is investigated by a crack analysis. It is found that elastic anisot-ropies can significantly increase the stress levels near triple points, which results in a smaller critical grain size for microcracking.

Distribution coefficients of Cd, Co, Ni, and Zn in soils

Abstract: SUMMARY Batch adsorption experiments were conducted with a mixture of solutes at low equilibrium concentrations of Cd (0.7-12.6 μg1−1), Co (18-118μg1−1, Ni (22-330 μg 1−1), and Zn (40-1480 μg1−1) in 38 different soils. Statistical correlations indicated that metal sorption onto the soils was influenced by the presence of clays and hydrous oxides of Fe and Mn. Based on calculated distribution coefficients for these metals, Co will generally exhibit the highest mobility in soils, but the mobility of Zn will increase faster with decreasing pH. Two types of empirical relationships are developed from these data to estimate values for the distribution coefficients.

The iteration of cubic polynomials Part I: The global topology of parameter space

Abstract: Fonctions univalentes en dynamique analytique complexe. Torsion de la structure complexe. Topologie globale de l'espace parametre

Void growth and failure in notched bars

Abstract: Void growth and ductile failure in the nonuniform multiaxial stress fields of notched bars are studied numerically and experimentally. U-notched bars with different notch acuities are made from partially consolidated and sintered iron powder compacts with various residual porosities. The materials are modelled using an elastic-viscoplastic constitutive relation that accounts for strength degradation resulting from the growth of microvoids. The matrix stress-strain relation and the initial void volume fractions used in the calculations are determined experimentally. The remaining parameters in the constitutive equations are evaluated from micromechanical models. Comparisons of the calculations with experimental results indicate that the constitutive model can provide good estimates of the evolution of the void volume fraction and of the strength reduction induced by void growth under a variety of nonuniform stress histories.

Subharmonic energy-gap structure in superconducting weak links

Abstract: We present corrected calculations of the subharmonic energy-gap structure using the model of Octavio, Tinkham, Blonder, and Klapwijk, which includes the effect of normal scattering in the weak link. We show that while the overall predictions of this model do not change qualitatively, the details of the predicted curves are different and in better agreement with experiment. We also present calculation of the current-voltage characteristics and of the excess currents for T = 0, as the normal scattering parameter Z is varied. We also show how the calculation can be shortened using symmetry arguments and discuss how this model improves the understanding of the connection between weak links and tunnel junctions.

Disordering and melting of aluminum surfaces.

Abstract: We report on a molecular-dynamics simulation of an Al(110) surface using the effective-medium theory to describe the interatomic interactions. The surface region is found to start melting \ensuremath{\cong}200 K below the bulk melting temperature with a gradual increase in the thickness of the disordered layer as the temperature approaches the bulk melting point. The more close-packed Al(111) surface shows a much weaker disordering below the melting temperature.

Effect of N‐substituents on the 13C NMR parameters of azoles

Abstract: The 13C chemical shifts and a large collection of coupling constants have been measured for 169 N-substituted azoles with no other substituents on the ring. Simple additive models have been used to discuss both the chemical shifts and the coupling constants. The chemical shift of the substituent depends on the nature of the azole which can, accordingly, be classified into three to six families. Some structural problems (ring-chain isomerism of 1-trifluoromethylsulphonyl-1,2,3-triazole, silylotropy and stannotropy) are also discussed.

A thermodynamic model for predicting wax formation in crude oils

Abstract: Most crude oils contain high molecular weight components, which at low temperatures may precipitate as a wax phase. This may cause plugging of pipes and numerous other problems. This paper presents a solid-liquid equilibrium based model for the description of wax formation. The model for the Gibbs energy contains a contribution based on Flory's theory of multicomponent polymer solutions and a contribution from a metastable subcooled state which oil mixtures may attain. The latter is formulated in terms of the surface tension of the wax phase. Experimental wax appearance points (temperatures), WAP's, are reported for 17 different stabilized North Sea crude oils. The values predicted by the new model are in very good agreement with the experimental WAP's.

Product inhibition of butyrate metabolism by acetate and hydrogen in a thermophilic coculture.

Abstract: Studies on product inhibition of a thermophilic butyrate-degrading bacterium in syntrophic association with Methanobacterium thermoautotrophicum showed that a gas phase containing more than 2 × 10−2 atm (2.03 kPa) of hydrogen prevented growth and butyrate consumption, while a lower hydrogen partial pressure of 1 × 10−3 to 2 × 10−2 atm (0.1 to 2.03 kPa) gradually inhibited the butyrate consumption of the coculture. No inhibition of butyrate consumption was found on the addition of 0.75 × 10−3 atm (76 Pa) of hydrogen to the gas phase. A slight inhibition of butyrate consumption by the coculture occurred at an acetate concentration of 16.4 mM. Inhibition gradually increased with increasing acetate concentration up to 81.4 mM, when complete inhibition of butyrate consumption occurred. When the culture contained an acetate-utilizing methanogen in addition to M. thermoautotrophicum, the inhibition of the triculture by acetate was gradually reversed as the acetate concentration was lowered by the aceticlastic methanogen. The results show that optimal growth conditions for the thermophilic butyrate-degrading bacterium depend on both hydrogen and acetate removal.

Rectilinear drawings of graphs

Abstract: We consider graphs drawn in the plane such that every edge crosses at most one other edge. We characterize, in terms of two forbidden sub-configurations, which of these graphs are equivalent to drawings such that all edges are straight line segments. As a consequence we obtain a complete characterization of the pairs of dual graphs that can be represented as geometric dual graphs such that all edges except one are straight line segments.

On the presence of disjoint subgraphs of a specified type

Abstract: We say that a graph family ℱ has the Erdos-Posa property if there exists a function f(k) such that any graph G contains either k disjoint subgraphs each isomorphic to a member of ℱ, or contains a set S of at most f(k) vertices such that G — S contains no graph in ℱ. We derive a general sufficient condition for a family of graphs to have the Erdos-Posa property. In particular, for any fixed natural number m the collection of cycles of length divisible by m has the Erdos-Posa property. As a by-product, we obtain a polynomially bounded algorithm for finding a cycle of length divisible by m. On the other hand, we describe a general class of planar graphs H such that a collection of subdivisions of H does not have the Erdos-Posa property. In fact, H may even be a tree.

Cloning and expression in Escherichia coli of the gene for extracellular phospholipase A1 from Serratia liquefaciens.

Abstract: From a genomic library of Serratia liquefaciens, a cloned DNA fragment comprising a two-gene operon was isolated and expressed in Escherichia coli. One of the gene products was identified as a phospholipase A1, and the enzyme was found to be excreted to the outer environment from S. liquefaciens as well as from E. coli. Both genes were sequenced, and the relationship between open reading frames in the DNA sequence and in vitro-expressed polypeptides was established. The length of the phospholipase polypeptide was found to be 319 amino acids. In the amino-terminal end of the coding sequence was a stretch of about 20 hydrophobic amino acids, but, in contrast to consensus signal peptides, no basic residues were present. The length of the second polypeptide was 227 amino acids. It was found that expression of the phospholipase gene in both E. coli and S. liquefaciens was growth phase regulated (late expression).

Deterministic chaos in the beer production-distribution model

Abstract: Depending upon the assumed ordering policy, the classical Beer Distribution Model shows a great variety of complex dynamic behaviors, including limit cycle oscillations of different periodicities and deterministic chaos. This article presents an overview of these phenomena. The ordering policy is expressed in terms of two parameters that measure the fraction of the supply line and the fraction of the anticipated shipments accounted for in the placement of orders. Our results indicate that, in certain regions of parameter space, any neighborhood to a given solution will contain qualitatively different solutions. The complexity of the system is revealed in two- and three-dimensional reconstructions of chaotic attractors as well as in Poincare sections obtained from such reconstructions.

Two-level semantics and code generation

Abstract: We present a two-level denotational metalanguage that is suitable for defining the semantic of PASCAL-like languages. The two levels allow for an explicit distinction between computations taking place at compile-time and computations taking place at run-time. While this distinction is perhaps not absolutely necessary for describing the input-output semantics of programming languages, it is necessary when issues like data flow analysis and code generation are considered. For an example stack-machine we show how to generate code for the run-time computations and still perform the compile-time computations. Based on an example it is argued that compiler-tricks like the use of activation records suggest how to cope with certain syntactic restrictions in the metalanguage. The correctness of the code generation is proved using Kripke-like relations and using a modified machine that can be made to loop when a certain level of recursion is encountered.

A Flow-line Model for Calculating the Surface Profile and the Velocity, Strain-rate, and Stress Fields in an Ice Sheet

Abstract: A flow-line model is presented for calculating the surface profile and the velocity, strain-rate, and stress fields in an ice sheet with given base-elevation profile, ice thickness at the dome (divide), flow-law parameters, mass-balance distribution, and convergence/divergence conditions along the flow line. The model, which is based on a “quasi-similarity” hypothesis as regards the horizontal velocity-depth profiles, accounts for changes along the flow line in the depth distributions of temperature, normal stress deviators, and possible enhanced flow of deep ice of Wisconsin origin. A curvilinear coordinate system is applied with horizontal axes along flow lines and surface-elevation contours, respectively. The flow equations are reduced to two differential equations, one for the surface-elevation profile, and the other for a profile function that determines the depth distributions of velocities and strain-rates. The two equations are coupled through a profile parameter that communicates the influence of velocity-profile changes to the surface-profile equation. It is shown that the variation along the flow line of this parameter should also be considered when deriving flow-law parameters from ice-sheet flow-line data. For a symmetric dome, explicit expressions are derived for the depth distributions of the vertical velocity, strain-rates, and stresses. The strain-rate profiles display an inflection about half-way down the ice sheet, and, in the case of isothermal ice, have surface values 2.2 times their depth-averaged values. The depth distribution of the vertical velocity indicates that a relatively thick layer of almost stagnant ice is present at the ice-sheet base below a dome.

An analysis of the temperature and rate dependence of Charpy V-notch energies for a high nitrogen steel

Abstract: The brittle-ductile transition for a high nitrogen steel is investigated by numerical analyses of the Charpy impact test. The material is described in terms of an elastic-viscoplastic constitutive model that accounts for the nucleation and growth of micro-voids, leading to ductile fracture, as well as for cleavage failure by micro-crack nucleation. The temperature dependence of flow strength and strain hardening is included in the model, and this leads to the prediction of a transition from cleavage fracture to predominantly ductile fracture as the temperature increases. For the particular steel considered it is found that the variation of strain hardening with temperature has a strong effect on the failure mode transition. Both slow loading and impact loading of the Charpy specimen are analyzed. Most of the computations are based on a quasi-static formulation since, even at the strain rates encountered in the Charpy impact test, material strain rate sensitivity is the main time effect. The influence of material inertia is investigated in a few transient analyses.

Dependence of lipid membrane phase transition temperature on the mismatch of protein and lipid hydrophobic thickness

Abstract: A two-component solution theory is studied which incorporates hydrophobic matching as a major contribution to the lipid-protein interactions in biological membranes. A special geometrical constraint has been discovered which has important implications for the quantitative interpretation of physical effects to lipid-protein interactions. The theory has an advantage over conventional Landau-type phenomenological descriptions in that it accounts for phase separation. A certain class of experimental systems, photosynthetic reaction centre and antenna proteins reconstituted into synthetic lipid membranes of different hydrophobic thicknesses, are considered with a view to determining the parameters of the theory. The theoretical predictions are found to be in good agreement with experimental measurements of shifts in the phase transition temperature.