Showing papers by "Technical University of Denmark published in 1989"

Optimal shape design as a material distribution problem

Abstract: Shape optimization in a general setting requires the determination of the optimal spatial material distribution for given loads and boundary conditions. Every point in space is thus a material point or a void and the optimization problem is a discrete variable one. This paper describes various ways of removing this discrete nature of the problem by the introduction of a density function that is a continuous design variable. Domains of high density then define the shape of the mechanical element. For intermediate densities, material parameters given by an artificial material law can be used. Alternatively, the density can arise naturally through the introduction of periodically distributed, microscopic voids, so that effective material parameters for intermediate density values can be computed through homogenization. Several examples in two-dimensional elasticity illustrate that these methods allow a determination of the topology of a mechanical element, as required for a boundary variations shape optimization technique.

Material Failure by Void Growth to Coalescence

Abstract: Publisher Summary This chapter describes the material failure by coalescence of microscopic voids. The voids nucleate mainly at second phase particles, by decohesion of the particle-matrix interface or by particle fracture, and subsequently the voids grow because of plastic straining of the surrounding material. The growth of voids to coalescence by plastic yielding of the surrounding material involves so large geometry changes that finite strain formulations of the field equations are a necessary tool. A convected coordinate formulation of the governing equations is used. Convected coordinates are introduced, which serve as particle labels. The convected coordinate net can be visualized as being inscribed on the body in the reference state and deforming with the material. It is found that after nucleation, cavities elongate along the major tensile axis and that two neighboring cavities coalesce when their length has grown to the order of magnitude of their spacing. This local failure occurs by the development of slip planes between the cavities or simply necking of the ligament. A detailed micromechanical study of shear band bifurcation that accounts for the interaction between neighboring voids and the strongly nonhomogeneous stress distributions around each void has been carried out, and are also elaborated in this chapter.

Computer Aided Process Planning: the state-of-the-art survey

Abstract: SUMMARY Computer Aided Process Planning (CAPP) has been recognized as playing a key role in Computer Integrated Manufacturing (CIM). In the last two decades, a tremendous effort has been made in developing CAPP systems. However, the benefits of CAPP in the real industrial environment are still to be seen. In this paper, a comprehensive summary on the state-of-the-art and projection of future trends in CAPP is presented to help make decisions concerning CAPP implementation today and to aid in guiding research for tomorrow. We systematically overview the historical background of the development of CAPP and discuss the current techniques which includes the implementation approaches, GT technology, application of AI techniques, programming languages, etc., for implementation CAPP systems. About 14 well-known CAPP systems, which are based on the variant, generative or semi-generative approach, are briefly introduced in this paper. In total about 156 currently existing CAPP systems are listed in Table 1 in the ...

A new thiobarbituric acid (TBA) method for determining free malondialdehyde (MDA) and hydroperoxides selectively as a measure of lipid peroxidation

Abstract: A new thiobarbituric acid (TBA) method for selectively determining free malondialdehyde (MDA) and lipid hydroperoxides is described. Partitioning of MDA and hydroperoxides in a Bligh and Dyer extraction was studied with pure substances. It was shown that MDA was present in the methanol-water phase exclusively, and the hydroperoxides (polar as well as nonpolar) were found in the chloroform phase. Therefore, the TBA test on these phases determines MDA directly present in the sample and the MDA formed during a ferric ion-catalyzed cleavage of hydroperoxides, respectively. The MDA present in the methanol-water phase was not bound to amino groups. Hydroperoxide values obtained with the present method corresponded well with a colorimetric determination of peroxides and showed better linearity for higher amounts of hydroperoxides. The possibility of using an iron chelator for preventing hydroperoxide cleavage during the TBA reaction, making the determination selective for MDA without Bligh and Dyer extraction, was investigated. The iron chelator did not completely inhibit peroxide cleavage; therefore, it is necessary to perform the extraction before the TBA determination. The method is suitable for samples with low fat content such as cod mince.

On optimal orientation of orthotropic materials

Abstract: In order to use an anisotropic material effectively it should be oriented optimally with respect to the actual strain condition. Orientations with extreme energy density are obtained for orthotropic materials. It is found that the optimal orientation depends on one non-dimensional material parameter only, plus the ratio of the two principal strains. Complete analytical results are derived, including local as well as global maxima and minima.

Terverticillate penicillia: chemotaxonomy and mycotoxin production

Abstract: A total of 4346 isolates of terverticillate Penicillium species was examined for production of my? cotoxins and other important secondary metabolites. Nearly all taxa and chemotypes (38 in all) produced one or more known mycotoxins consistently. Profiles of secondary metabolites were specific for each taxon, but several mycotoxins were produced by more than one species, suggesting a high degree of relatedness in the terverticillate penicillia. A high number of misidentified Penicillium isolates were reidentified, clarifying connections between taxa and profiles of mycotoxins and other secondary me? tabolites. Confirmed production of several mycotoxins in different taxa is reported for the first time. Penicillium chrysogenum, P. atramentosum, P. coprophilum, P. crustosum, P. expansum, P. glandicola var. glandicola and var. glaucovenetum, P. griseofulvum, P. hirsutum var. hirsutum, var. albocoremium, var. allii and var. hordei, and P. vulpinum are reported for the first time to be consistent producers of roquefortine C. Griseofulvin is reported from P. coprophilum. Tremorgen penitrem A was consistently produced by P. crustosum, P. glandicola var. glandicola and P. clavigerum. Terrestric acid is reported from P. hirsutum and its varieties, and P. crustosum. P. hirsutum var. hirsutum and P. solitum produce compactin. Penicillium mononematosum produces cyclopaldic acid and isochromantoxin. P. expansum was found to produce the mycotoxin chaetoglobosin C and P. atramentosum produces rugulovasine A. Several new varieties, chemotypes and combinations are proposed, based on profiles of secondary metabolites, morphology and physiological characters. Living ex type and authentic cultures of taxa believed to be synonyms of important species were examined and revised lists of synonyms are pre? sented.

Dosimetry for Radiation Processing

Abstract: During the past few years significant advances have taken place in the different areas of dosimetry for radiation processing, mainly stimulated by the increased interest in radiation for food preservation, plastic processing and sterilization of medical products. Reference services both by international organizations (IAEA) and national laboratories have helped to improve the reliability of dose measurements. Several dosimeter systems like calorimetry, perspex, and radiochromic dye films are being improved and new systems have emerged, e.g. spectrophotometry of dichromate solution for reference and sterilization dosimetry, optichromic dosimeters in the shape of small tubes for food processing, and ESR spectroscopy of alanine for reference dosimetry. In this paper the special features of radiation processing dosimetry are discussed, several commonly used dosimeters are reviewed, and factors leading to traceable and reliable dosimetry are discussed.

Inelastic scattering in resonant tunneling

Abstract: The exact resonant-tunneling transmission probability for an electron interacting with phonons is presented in the limit that the elastic coupling to the leads is independent of energy. The phonons produce transmission sidebands but do not affect the integrated transmission probability or the escape rate of the electron from the resonant site. In the Appendixes, we evaluate the Green function that appears in the expression for the transmission probability.

Critical loads for nitrogen deposition on forest ecosystems

Abstract: Critical loads for N deposition are derived from an ecosystem's anion and cation balance assuming that the processes determining ecosystem stability are soil acidification and nitrate leaching. Depending on the deposition of S, the parent soil material, and the site quality critical N deposition rates will range between 20 to 200 mmol m−2 yr−1 (3 to 14 kg ha−1 yr−1) on silicate soils and reach 20 to 390 mmol m−2 yr−1 (3 to 48 kg ha−1) on calcareous soils.

Phytoplankton growth rate and nitrogen content: implications for feeding and fecundity in a herbivorous copepod

Abstract: Observations of natural feedlng and egg-production rates of planktonic copepods have revealed distinct responses, Independent of phytoplankton blomass, to oceanographic processes that ferhlize the photic layer Are such responses caused by changes ln phytoplankton growth rate, influencing feeding behaviour, and/or by changes In the chemical composition of the phytoplankton, lnfluenclng fecundlty? The dlatom Thalassioslra welssflogu, grown in continuous cultures at different d~ luhon rates and different nltrogen concentrations In the growth medlum, was offered to the copepod Acarha tonsa at a concentrahon of 1 5 p p m Over more than 2 orders of magnitude Increase in algal growth rate, the Ingested cell volume increased by less than a factor of 2, ingested carbon remalned constant whereas ingested nltrogen as well as rate of egg production increased by a factor of ca 6 Vanahon in mgested cell volume was interpreted as a behavioural feedlng response to vanations In algal growth condltlons, vanation in ingested mtrogen and egg production, as being caused malnly by vanatlon In algal nltrogen content It 1s concluded that vanable fecundlty may potentially be explained by vanable chemical composition of the algae while the observed feeding response is probably msufflclent to account for the vanabllitv observed In the sea

Regularization, GSVD and truncated GSVD

Abstract: The purpose of this paper is to analyze Tikhonov regularization in general form by means of generalized SVD (GSVD) in the same spirit as SVD is used to analyze standard-form regularization. We also define a truncated GSVD solution which is of interest in its own right and which sheds light on regularization as well. In addition, our analysis gives insight into a particular numerical method for solving the general-form problem via a transformation to standard form.

Ordered aggregates of ultrafine iron oxide particles: ‘Super crystals’

Abstract: Ultrafine iron oxide particles with a mean diameter of 6.9 nm and a very narrow size distribution have been studied by electron microscopy, Mossbauer spectroscopy, and X-ray diffraction. Electron micrographs and electron diffraction patterns show that the particles form ‘super crystals’, that is, three-dimensional ordered close-packed structures, with a nearest neighbour distance of 9.1 nm.

The connection between the Penicillia and Aspergilli and mycotoxins with special emphasis on misidentified isolates.

Abstract: Species of Penicillium and Aspergillus are the most potent mycotoxin producers, but the connection between species and profiles of mycotoxins has been obscured by many misidentifications of fungal isolates, especially in Penicillium. Available producers of known mycotoxins in the two important genera were examined concerning identity and production of secondary metabolites using thin-layer chromatography and high performance liquid chromatography with diode array detection. 152 isolates of Penicillium were reclassified, leaving the connection between species and mycotoxin profiles much clearer in this genus. Earlier data on mycotoxin production by different taxa were confirmed by analyzing several isolates of each taxon.

Substrate interactions during aerobic biodegradation of benzene.

Abstract: This study dealt with the interactions with benzene degradation of the following aromatic compounds in a mixed substrate: toluene, o-xylene, naphthalene, 1,4-dimethylnaphthalene, phenanthrene, and pyrrole. The experiment was performed as a factorial experiment with simple batch cultures. The effect of two different types of inocula was tested. One type of inoculum was grown on a mixture of aromatic hydrocarbons; the other was grown on a mixture of aromatic hydrocarbons and nitrogen-, sulfur-, and oxygen-containing aromatic compounds (NSO compounds), similar to some of the compounds identified in creosote waste. The culture grown on the aromatic hydrocarbons and NSO compounds was much less efficient in degrading benzene than the culture grown on only aromatic hydrocarbons. The experiments indicated that toluene- and o-xylene-degrading bacteria are also able to degrade benzene, whereas naphthalene-, 1,,4-dimethylnaphthalene-, and phenanthrene-degrading bacteria have no or very little benzene-degrading ability. Surprisingly, the stimulating effect of toluene and o-xylene was true only if the two compounds were present alone. In combination an antagonistic effect was observed, i.e., the combined effect was smaller than the sum from each of the compounds. The reason for this behavior has not been identified. Pyrrole strongly inhibited benzene degradation even at concentrations of about 100 to 200 micrograms/liter. Future studies will investigate the generality of these findings.

The matters that really matter for hypertext usability

Abstract: We compare 92 benchmark measurements of various usability issues related to hypertext which have been published in the hypertext literature in order to find which ones have shown the largest effects.

Construction and decoding of a class of algebraic geometry codes

Abstract: A class of codes derived from algebraic plane curves is constructed. The concepts and results from algebraic geometry that were used are explained in detail; no further knowledge of algebraic geometry is needed. Parameters, generator and parity-check matrices are given. The main result is a decoding algorithm which turns out to be a generalization of the Peterson algorithm for decoding BCH decoder codes. >

Modeling absorption kinetics of subcutaneous injected soluble insulin

Abstract: Absorption of subcutaneously injected soluble insulin deviates markedly from simple first-order kinetics and depends both on the volume and concentration of the injected solution. This paper presents a model of the absorption process in which insulin is presumed to be present in subcutis in a low molecular weight form, a high molecular weight form, and an immobile form where the molecules are bound to the tissue. The model describes how diffusion and absorption gradually reduce the insulin concentrations in the subcutaneous depot and thereby shift the balance between the three forms in accordance with usual laws of chemical kinetics. By presuming that primarily low molecular weight insulin penetrates the capillary walls, the model can account for experimentally observed variations in the absorption rate over a wide range of volumes and of concentrations. The model is used to determine the effective diffusion constant D for insulin in subcutis, the absorption rate constant B for low molecular weight insulin, the equilibrium constant Q between high and low molecular weight insulin, the binding capacity C for insulin in the tissue, and the average life time T for insulin in its bound state. Typical values for a bolus injection in the thigh of fasting type I diabetic patients are D = 0.9 x 10(-4) cm2/min, B = 1.3 X 10(-2)/min, and Q = 0.13 (ml/IU)2. Binding of insulin in the tissue is significant only at small concentrations. The binding capacity is of the order of C = 0.05 IU/cm3 with a typical average life time in the bound state of T = 80.0 min. Combined with a simplified model for distribution and degradation of insulin in the body, the absorption model is used to simulate variations in plasma free insulin concentrations with different delivery schedules, i.e., bolus injection and dosage by means of an infusion pump. The simulations show that a pump repetition frequency of 1-2 per hr is sufficient to secure an almost constant plasma insulin concentration.

Cadmium soil sorption at low concentrations: VIII. correlation with soil parameters

Abstract: Cadmium distribution coefficients, K d were determined at low Cd concentrations (solute: 02 to 30 μg Cd dm−3, soil: 0044 to 11 mg Cd kg−1) for 63 Danish agricultural soils The K d values ranged from 15 to 2450 L kg−1 About 40% of the soils had K d values below 200 L kg−1 The observed K d values correlated very well with soil pH (r 2 = 072) Introducing soil organic matter content as a second parameter improved the correlation some (r 2 = 079) No further improvements were obtained by introducing traditional soil parameters as clay, silt, fine sand, coarse sand and CEC or ‘reactive’ parameters as oxyhydroxides of Mn, Fe and Al The identified regression equation for predicting K d values indicates that K d approximately doubles for each 05 unit increase in pH or 2% increase (weight basis) in organic matter content

Dantale: A New Danish Speech Material

Abstract: A new Danish speech material (DANTALE) for clinical and experimental speech audiometry is digitally recorded on compact disc (CD). The speech material is designed to meet present audiological requirements at Danish hearing centres. One channel of the CD contains the speech signals and the other a masking noise. The CD also contains various calibration signals recorded on both channels at the end of the CD. The speech material compromises: 1) Digit triplets for the measurement of speech reception threshold (SRT). 2) Lists of monosyllabic words for the measurement of discrimination score (DS) for adults, children and small children. The word lists for the adults are equalized with regard to important phonetic and "visual" elements and the word lists for the children consist of minimal pairs. 3) Continuous speech for the measurement of the most comfortable loudness level (MCL), assessment of hearing aid fitting and the like. The masking noise is an amplitude-modulated, speech-shaped noise signal, which is designed to simulate a 4-person speech babble in order to assess both the frequency selectivity and the temporal resolution. The speech material is described and the long-term power spectra and modulation spectra are given.

Braids and coverings : selected topics

Abstract: This book is based on a graduate course taught by the author at the University of Maryland, USA. The lecture notes have been revised and augmented by examples. The work falls into two strands. The first two chapters develop the elementary theory of Artin Braid groups both geometrically and via homotopy theory, and discuss the link between knot theory and the combinatorics of braid groups through Markov's Theorem. The final two chapters give a detailed investigation of polynomial covering maps, which may be viewed as a homomorphism of the fundamental group of the base space into the Artin braid group on n strings. This book will be of interest to both topologists and algebraists working in braid theory.

Softening by void nucleation and growth in tension and shear

Abstract: The effect of void nucleation and growth on overall stress-strain behavior is investigated for solids undergoing plastic straining under axisymmetric and shearing conditions. Contact between the void surface and the nucleating particle is taken into account and is found to be important under shear and under axisymmetric straining when the stress triaxiality is low. The notion of the macroscopic stress drop due to nucleation of a void is defined and computed, both for isolated voids and for voids in periodic arrays. The stress drop for an isolated void in an infinite matrix can be used to predict softening due to void nucleation when the void concentration is dilute. Interaction between voids in shear during nucleation is analysed numerically and softening effects are calculated along with large strain aspects of void deformation during subsequent growth.

Configurations in graphs of large minimum degree, connectivity, or chromatic number

Abstract: Wagner [42] proved that a graph of large chromatic number contains a subgraph that can be contracted into a large complete graph. Modifying Wagner's proof, Dirac [lo] and Jung [17] proved that, for every natural number k there exists a natural number f , ( k ) such that every graph G of chromatic number at leastf,(k) contains a subdivision of the complete graph K, . These results have inspired several investigations on configurations, in particular, paths, cycles, and subdivision, in graphs or digraphs of large minimum degree, connectivity, or chromatic number. In the undirected case there is a richness of such results, while one very quickly encounters unsolved questions, counterexamples, and NP-complete problems in the directed case. Reference [39] gives a survey of these results and problems, and in the present paper we indicate more possible directions in this area. We also consider infinite graphs. In particular, we give a short proof of the recent result of Thomas [30] that the tree-width (to be defined later) of an infinite graph (of finite tree-width) is the maximum of the tree-widths of its finite subgraphs. Combined with results of Robertson and Seymour [ZS] this proves the conjecture of the author [38], as pointed out by Seese and Nesetril and Seymour (private communication) that every infinite graph of sufficiently large connectivity has a subgraph that is contractible to any prescribed finite planar graph.