Showing papers by "Technical University of Denmark published in 1992"

Optical Coherence Tomography

Abstract: Optical coherence tomography (OCT) has developed rapidly since its first realisation in medicine and is currently an emerging technology in the diagnosis of skin disease. OCT is an interferometric technique that detects reflected and backscattered light from tissue and is often described as the optical analogue to ultrasound. The inherent safety of the technology allows for in vivo use of OCT in patients. The main strength of OCT is the depth resolution. In dermatology, most OCT research has turned on non-melanoma skin cancer (NMSC) and non-invasive monitoring of morphological changes in a number of skin diseases based on pattern recognition, and studies have found good agreement between OCT images and histopathological architecture. OCT has shown high accuracy in distinguishing lesions from normal skin, which is of great importance in identifying tumour borders or residual neoplastic tissue after therapy. The OCT images provide an advantageous combination of resolution and penetration depth, but specific studies of diagnostic sensitivity and specificity in dermatology are sparse. In order to improve OCT image quality and expand the potential of OCT, technical developments are necessary. It is suggested that the technology will be of particular interest to the routine follow-up of patients undergoing non-invasive therapy of malignant or premalignant keratinocyte tumours. It is speculated that the continued technological development can propel the method to a greater level of dermatological use.

The relation between crack growth resistance and fracture process parameters in elastic-plastic solids

Abstract: CKA~K growth initiation and subsequent resistance is computed for an elastic-plastic solid with an idealized traction separation law specified on the crack plane to characterize the fracture process. The solid is specified by its Young’s modulus, E, Poisson’s ratio, v, initial tensile yield stress, (or, and strain hardening exponent, N. The primary parameters specifying the traction-separation law of the fracture process are the work of separation per unit area, To. and the peak traction, 6. Highly refined calculations have been carried out for resistance curves. K,(Arr), for plane strain, mode I growth in small-scale yielding as dependent on the parameters characterizing the elastic-plastic properties of the solid and its fracture process. With K,, = [El-,/( I ~ v’)] ’ 2 as the intensity needed to advance the crack in the absence ofplasticity, K,J& is presented in terms of its dependence on the two most important parameters, d/nr and N, with special emphasis on initiation toughness and steady-state toughness, Three applications of the results are made : to predict toughnesss when the fracture process is void growth and coalescence, to predict the role of plasticity on interface toughness for similar materials bonded together, and to illuminate the role of plasticity in enhancing toughness in dual-phase solids. The regime of applicability of the present model to ductile fracture due to void growth and coalescence, wherein multiple voids interact within the fracture process zone, is complementary to the regime of applicability of models describing the interaction between a single void and the crack tip. The two mechanism regimes are delineated and the consequence of a transition between them is discussed.

Surface energy and work function of elemental metals.

Abstract: We have performed an ab initio study of the surface energy and the work function for six close-packed surfaces of 40 elemental metals by means of a Green's-function technique, based on the linear-muffin-tin-orbitals method within the tight-binding and atomic-sphere approximations. The results are in excellent agreement with a recent full-potential, all-electron, slab-supercell calculation of surface energies and work functions for the 4d metals. The present calculations explain the trend exhibited by the surface energies of the alkali, alkaline earth, divalent rare-earth, 3d, 4d, and 5d transition and noble metals, as derived from the surface tension of liquid metals. In addition, they give work functions which agree with the limited experimental data obtained from single crystals to within 15%, and explain the smooth behavior of the experimental work functions of polycrystalline samples as a function of atomic number. It is argued that the surface energies and work functions calculated by present day ab initio methods are at least as accurate as the experimental values.

Effects of free long-chain fatty acids on thermophilic anaerobic digestion

Abstract: Low concentrations of the long-chain fatty acids oleate and stearate inhibited all steps of the anaerobic thermophilic biogas process during digestion of cattle manure. The lag phase increased when the concentrations of oleate and stearate were 0.2 g/l and 0.5 g/l, respectively, and no growth was found at concentrations of 0.5 g/l for oleate and 1.0 g/l for stearate. The toxic effect of these acids was permanent as growth did not occur when inhibited cultures were diluted to a non-inhibitory concentration. No adaptation to the fatty acids toxicity was observed by pre-exposing the cultures to non-inhibitory concentrations and the inhibitory response was the same as for cultures not pre-exposed to the fatty acids. Oleate was less inhibitory when added as a neutral oil in the form of the glycerol ester. This indicates that it is the free fatty acid that influences the bacterial activity.

Shape optimization of structures for multiple loading conditions using a homogenization method

Abstract: A formulation for shape optimization of elastic structures subject to multiple load cases is presented. The problem is solved using a homogenization method. When compared to the single load solution strategy, it is shown that the more general formulation can produce more stable designs while it introduces little additional complexity.

Oxidation of ferrous ions by ozone in acidic solutions

Abstract: The oxidation of ferrous ions by ozone in acidic solutions of pD 0-2 was studied using a stopped-flow spectro-photometer. The reaction can be characterized as an oxygen atom transfer from O 3 to Fe 2+ . An intermediate product assigned to be the ferryl ion, FeO 2+ , was found and its UV-vis spectrum measured.

Atmospheric heavy metal deposition in Northern Europe 1990

Abstract: Atmospheric heavy metal deposition in northern Europe including Denmark, Estonia, Finland, Iceland, Latvia, Lithuania, Norway, Sweden and adjacent areas of Russia, was monitored by moss technique Total concentrations of cadmium, chromium, copper, iron, lead, nickel vanadium and zinc were analysed Results are presented in the form of maps The regional deposition pattern shows a decreasing gradient from relatively high values in the southern parts of Scandinavia to low values towards the North The gradient is steep for cadmium, lead and vanadium, whereas the concentrations of chromium, copper, iron, nickel and zinc show weaker gradients Lead, which originates from anthropogenic emission sources, is found in lower concentrations in the arctic regions of Svalbard and Iceland Chromium, copper, iron, and vanadium, originating also from soil dust or volcanic activity, are found in high concentrations in sparsely vegetated arctic regions Important local enhancements of the concentrations in moss were found superimposed on the regional background pattern in northern Sweden at the smelter at Roennskaer, in Norway at Odda, in Finland at the copper and nickel smelter in Harjavalta and at the steel mill in Tornio, as well as in Russia in the western part of the Kola peninsula, especially at the great smelting combinates in Nikel and Monchegorsk Higher levels of metals are found in the industrial north-eastern area of Estonia, close to Liepaja (steel mill) and Riga in Latvia and at Mazeikiai (oil refinery) in Lithuania, and in the metropolitan area of St Petersburg as well as industrial areas in the NW, SW and SE of the Leningrad oblast A significant decrease in the concentrations of most elements was found (AB) (14 refs)

Multiple ordered phases in a block copolymer melt

Abstract: A poly(ethylenepropylene)-poly(ethylethylene) (PEP-PEE) diblock copolymer conctaining 65 vol% PEP was studied by SANS and rheological measurements. Four distinct phases were identified as a function of temperature: three ordered phase at low temperatures and a disordered phase at high temperatures. The order-disorder and disorder-order transitions were determined to be first-order.

Analyses of Plastic Flow Localization in Metals

Abstract: The continuum mechanics framework for analyzing plastic flow localization is reviewed. The prediction of the localization of deformation into shear bands is sensitive to the constitutive description. The classical isotropic hardening elastic-plastic solid with a smooth yield surface and normality is very resistant to localization, but deviations from these idealizations have a strong effect. Thus, a material that forms a sharp vertex on the yield surface, as predicted by crystal plasticity, shows flow localization at quite realistic levels of strain, and even the formation of a rounded vertex on the yield surface has an important influence. Also softening induced by material damage or by the heating due to plastic dissipation have significant influence in promoting the onset of flow localization. In a practical situation one effect, such as thermal softening under high deformation rates, may be the dominant cause of localization, but often the interaction of different effects appears to be the more realistic explanation of observed flow localization. Some relevant constitutive models are reviewed and the effect of the different material models on localization predictions is illustrated. Important information on localization behavior in uniformly strained solids is obtained by a relatively simple material stability analysis, but often failure bymore » flow localization occurs in nonuniformly strained regions, where numerical solution procedures are necessary to obtain theoretical predictions. The numerical results reviewed cover localization under dynamic as well as quasi-static loading conditions. 81 refs., 16 figs.« less

Carbon and energy metabolism of atp mutants of Escherichia coli.

Abstract: The membrane-bound H(+)-ATPase plays a key role in free-energy transduction of biological systems. We report how the carbon and energy metabolism of Escherichia coli changes in response to deletion of the atp operon that encodes this enzyme. Compared with the isogenic wild-type strain, the growth rate and growth yield were decreased less than expected for a shift from oxidative phosphorylation to glycolysis alone as a source of ATP. Moreover, the respiration rate of a atp deletion strain was increased by 40% compared with the wild-type strain. This result is surprising, since the atp deletion strain is not able to utilize the resulting proton motive force for ATP synthesis. Indeed, the ratio of ATP concentration to ADP concentration was decreased from 19 in the wild type to 7 in the atp mutant, and the membrane potential of the atp deletion strain was increased by 20%, confirming that the respiration rate was not controlled by the magnitude of the opposing membrane potential. The level of type b cytochromes in the mutant cells was 80% higher than the level in the wild-type cells, suggesting that the increased respiration was caused by an increase in the expression of the respiratory genes. The atp deletion strain produced twice as much by-product (acetate) and exhibited increased flow through the tricarboxylic acid cycle and the glycolytic pathway. These three changes all lead to an increase in substrate level phosphorylation; the first two changes also lead to increased production of reducing equivalents. We interpret these data as indicating that E. coli makes use of its ability to respire even if it cannot directly couple this ability to ATP synthesis; by respiring away excess reducing equivalents E. coli enhances substrate level ATP synthesis.

Importance of Unattached Bacteria and Bacteria Attached to Sediment in Determining Potentials for Degradation of Xenobiotic Organic Contaminants in an Aerobic Aquifer

Abstract: The bacterial abundance, distribution, and degradation potential (in terms of degradation versus lack of degradation) for four xenobiotic compounds in an aerobic aquifer sediment have been examined in laboratory and field experiments. The xenobiotic compounds studied were benzene, toluene, o-xylene, and naphthalene (all at concentrations of approximately 120 micrograms/liter). The aerobic degradation experiments ran for approximately 90 days at 10 degrees C, which corresponded to the groundwater temperature. At the end of the experiment, the major part of the microbial biomass, quantified as acridine orange direct counts, was attached to the groundwater sediment (18 x 10(6) to 25 x 10(6) cells per g [dry weight], and only a minor part was unattached in the groundwater (0.6 x 10(6) to 5.5 x 10(6) cells per ml). Experiments involving aquifer sediment suspensions showed identical degradation potentials in the laboratory and in the field. However, laboratory experiments involving only groundwater (excluding aquifer sediment) showed less degradation potential than in situ experiments involving only groundwater, indicating that the manipulation or approach of the laboratory experiments could affect the determination of the degradation potentials. No differences were observed between the groundwater-only and the sediment compartments in the in situ experiments in the ability to degrade the compounds, but the maximum degradation rates were substantially lower in the groundwater-only compartment. Preparations used in laboratory experiments for studying the degradation potential for xenobiotic organic contaminants should contain sediment to obtain the highest numbers of bacteria as well as the broadest and most stable degradation.(ABSTRACT TRUNCATED AT 250 WORDS)

Surface magnetism in iron, cobalt, and nickel.

Abstract: We have calculated magnetic moments, work functions, and surface energies for several of the most closely packed surfaces of iron, cobalt, and nickel by means of a spin-polarised Green's-function technique based on the linear muffin-tin orbitals method within the tight-binding and atomic sphere approximations. We find enhanced spin moments at all the surfaces considered except for Ni fcc(111), where the moment at the surface reverts to its bulk value. This is in close agreement with earlier slab calculations

Correlation of binary polyolefin phase behavior with statistical segment length asymmetry

Abstract: The segment-segment interaction parameter χ is dreported as a function of temperature for four pairs of polyolefin based on polyethylene, poly(ethylenepropylene), poly(ethylethylene) and poly(ethylene-co-ethylethylene). These results are not anticipated by the traditional Flory-Huggins heat of mixing. However, a correlation between the magnitude of χ and the statistical segment length asymmetry was established. This contribution to χ is attributed to the mismatch in polymer conformational size that must be accomodated in order to achieve a homogeneous mixture

Equivalent displacement based formulations for maximum strength Truss topology design

Abstract: Maximum strength elastic truss structural design is conveniently formulated in terms of displacements and bar volumes. The resulting problem is nonconvex, and for topology design very large, as one seeks the optimal topology as a subset of a large number of potential bars connecting all nodal points of an initially chosen set. In this paper we present a number of equivalent formulations in the displacements only, taking full advantage of the structure of the optimization problem. The equivalent formulations are of min-max type or are quadratic programming problems in the displacements, reducing in some cases even to linear programming problems.

Environmental cadmium in Europe.

Abstract: The environmental fate and effects of cadmium have been reviewed and evaluated by the Ecotoxicology Section of the Scientific Advisory Committee on Toxicology and Ecotoxicology of Chemical Compounds (CSTE), and advice has been given to the Commission of European Communities in 1981 (Commission of the European Communities, 1981) on its impact on the environment and on ecosystems, and on the identified risks connected to the levels of cadmium exposure on man and the environment.

Recursive forgetting algorithms

Abstract: Fn the first part of this paper, a general forgetting algorithm is formulated and analysed. It contains most existing forgetting schemes as special cases. Conditions are given ensuring that the basic convergence properties will hold. In the second part of the paper, the results are applied to a specific algorithm with selective forgetting. Here, the forgetting is non-uniform in time and space. The theoretical analysis is supported by a simulation example demonstrating the practical performance of this algorithm.

Elastic Fracture Mechanics of the Peel-Test Geometry

Abstract: The results of a linear-elastic analysis to determine the phase angle at the tip of an interface crack in the peel test are presented in this paper. The phase angle is fairly insensitive to the peel angle and, when the film and substrate have identical elastic properties, the mode-I and mode-II components of the crack-tip stress field are approximately equal. The phase angle has some dependence on both the elastic mismatch and the applied force; it is very sensitive to any residual strains in the film.