Showing papers by "Technical University of Denmark published in 1984"

An analysis of ductile rupture in notched bars

Abstract: Ductile fracture in axisymmetric and plane strain notched tensile specimens is analyzed numerically, based on a set of elastic-plastic constitutive relations that account for the nucleation and growth of microvoids. Final material failure by void coalescence is incorporated into the constitutive model via the dependence of the yield function on the void volume fraction. In the analyses the material has no voids initially; but as the voids nucleate and grow, the resultant dilatancy and pressure sensitivity of the macroscopic plastic flow influence the solution significantly. Considering both a blunt notch geometry and a sharp notch geometry in the computations permits a study of the relative roles of high strain and high triaxiality on failure. Comparison is made with published experimental results for notched tensile specimens of high-strength steels. All axisymmetric specimens analyzed fail at the center of the notched section, whereas failure initiation at the surface is found in plane strain specimens with sharp notches, in agreement with the experiments. The results for different specimens are used to investigate the circumstances under which fracture initiation can be represented by a single failure locus in a plot of stress triaxiality vs effective plastic strain.

Carbon-13 nuclear magnetic resonance data for oligosaccharides

Abstract: Publisher Summary This chapter presents a compilation of 13 C-nuclear magnetic resonance (NMR) data for oligosaccharides in the form of tables. The literature-survey presented in the chapter covers most of the data of 1982, and all of the information given had been measured for solutions in D 2 O unless stated otherwise. The data for peracetates of xylobioses and of glucobioses were recorded for solutions in CDCl 3 . All data have been copied from the original articles. For mutarotated mixtures of trisaccharides and larger oligosaccharides, the complete set of data is given for the α anomer, but only the chemical shifts for the reducing end of the anomer are given, provided that all other chemical shifts (for the remaining units) are identical. The sampling and assignment techniques used for the measurement of 13 C-NMR spectra of oligosaccharides are identical to those for monosaccharides. Particularly for oligosaccharides, it is important to measure the spectra at the same temperature—that is, with a precision better than ±0.5 p.p.m., when data are to be compared accurately.

Cadmium soil sorption at low concentrations. I: Effect of time, cadmium load, pH, and calcium

Abstract: Sorption of Cd at low concentrations onto two Danish soils (loamy sand, sandy loam) was examined in terms of kinetics and governing factors. From an environmental point of view soil sorption of Cd is a fast process: More than 95% of the sorption takes place within 10 min, equilibrium is reached in 1 hr, and exposures up to 67 wk did not reveal any long term changes in Cd sorption capacities. The soils have very high affinity for Cd at pH = 6.00 (10−3 M CaCl2) exhibiting distribution coefficients in the order of 200 to 250 (soil Cd concentration/solute Cd concentration). However, the sorption isotherms describing the distribution of Cd between soil and solute are slightly curvelinear. In the pH-interval 4 to 7.7, the sorption capacity of the soil approximately increases 3 times for a pH increase of one unit. Increasing the Ca concentration from 10−3 to 10−2 M reduces the sorption capacity of the sandy loam to one third.

Stability analysis for a semiconductor laser in an external cavity

Abstract: A detailed theoretical analysis of stability is presented for a semiconductor laser in an external cavity. The limits of stable operation are determined as a function of the external cavity parameters and the linewidth enhancement factor α. Instability is related to jumps of the laser frequency between external cavity modes (frequency bistability) or to feedback-induced intensity pulsations due to the carrier density dependence of the refractive index. The limit of bistability is derived from the steady-state solutions of the rate equations and the intensity pulsation limit is obtained from a small-signal analysis. This analysis also gives the location of zeros in the system determinant and the resulting FM noise spectrum. For practical applications we emphasize the determination of the stable tuning range for the phase in the external cavity and the classification of the possible types of instability for various feedback levels.

Review of Weibull Statistics for Estimation of Wind Speed Distributions

Abstract: A statistical distribution commonly used for describing measured wind speed data is the Weibull distribution. A review of the methods found in the statistical literature for the purpose of estimation of the parameters in Weibull distributions is given, with a special emphasis on the efficiency of the different methods. From this review, the most appropriate method for a given application can be chosen. The general conclusion is that maximum likelihood estimators should be used due to their large sample efficiency. However, they require an iterative minimization. The recommended closed form estimators when there are few observations (say, less than 25) are the least-squares estimators. If wind speed data with a good speed resolution are available, the closed form ten fractile estimators are very good. The simplest are the two fractile estimators. They, however, require around three times as many observations as the maximum likelihood estimators in order to be of the same accuracy. If mean and stan...

Economic Selection of Quality Manufactured Product

Abstract: In this article an economic analysis is presented for the problem of selecting the most favorable quality distribution for an industrial process. An important special case is that of selecting the best mean (target) value. The analysis, which takes into account the stochastic nature of the process, is illustrated by considering the problem of choosing the optimal amount of overfill in a filling operation. A detailed solution is given for situations in which the distribution of the quality measurements is approximately normal; for that case a table is provided that simplifies the solution of the problem in practice. Industrial examples are considered. Lognormal and Poisson distributions are also discussed.

Modelling Velocity Spectra in the Lower Part of the Planetary Boundary Layer

Abstract: Principles used when constructing models for velocity spectra are reviewed. Based upon data from the Kansas and Minnesota experiments, simple spectral models are set up for all velocity components in stable air at low heights, and for the vertical spectrum in unstable air through a larger part of the planetary boundary layer. Knowledge of the variation with stability of the (reduced) frequency % MathType!MTEF!2!1!+-% feaafiart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9% vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x% fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaaccaGae8NKby% kaaa!37B5!\[f\] m for the spectral maximum is utilized in this modelling. Stable spectra may be normalized so that they adhere to one curve only, irrespective of stability, and unstable w-spectra may also be normalized to fit one curve. The problem of using filtered velocity variances when modelling spectra is discussed. A simplified procedure to provide a first estimate of the filter effect is given. In stable, horizontal velocity spectra, there is often a ‘gap’ at low frequencies. Using dimensional considerations and the spectral model previously derived, an expression for the gap frequency is found.

On the creep constrained diffusive cavitation of grain boundary facets

Abstract: Creep rupture in a polycrystalline metal at a high temperature, by cavity growth on a number of grain boundary facets, is studied numerically. An axisymmetric model problem is analysed, in which a cavitating facet is represented as disk-shaped, and the model dimensions are taken to represent spacings between neighbouring cavitating facets. For the grains both power law creep and elastic deformations are taken into account, and the description of cavity growth is based on an approximate expression that incorporates the coupled influence of grain boundary diffusion and power law creep. The cases considered include creep-constrained cavity growth at low stresses, where the voids link up to form grain boundary cracks at relatively small overall strains, as well as the power law creep dominated behaviour at higher stress levels, where rupture occurs at large overall strains. The numerical results are compared with results based on various simplified analyses.

Wettability and Contact Angles

Abstract: In everyday life one encounters many different systems where it is apparent that various processes take place at the line of contact between a liquid phase and a solid phase. The molecules in the liquid phase can move larger distances than those in the solid phase. Therefore, since the molecules in a solid phase are well fixed, this gives rise to the fact that it is in general not possible to study the surface forces of the solid phase in the same way we have described for the liquid phase elsewhere in this book. In spite of this, there exist many examples where we have to attribute a definite value of surface tension to the solid surface, in order to be able to describe the different reactions taking place at the solid surface in contact with liquid phase.

The nucleotide sequence of the gene encoding the K99 subunit of enterotoxigenic Escherichia coli

Abstract: The nucleotide sequence of the gene encoding the K99 fimbrial subunit of enterotoxigenic Escherichia coli was determined. It appeared that the subunit is composed of 159 amino acid residues preceded by a N-terminal signal sequence of 22 amino acid residues. The secondary structure of the mature K99 polypeptide and the location of potential antigenic determinants were predicted. A comparison was made between the amino acid sequence of the K99 subunit and the subunits of other fimbrial adhesins.

Solitary waves on nonlinear elastic rods. I

Abstract: In continuation of an earlier study of propagation of solitary waves on nonlinear elastic rods, numerical investigations of blowup, reflection, and fission at continuous and discontinuous variation of the cross section for the rod and reflection at the end of the rod are presented. The results are compared with predictions of conservation theorems for energy and momentum.

Electronic structure and bonding in the RhC molecule by all-electron ab initio HF–Cl calculations and mass spectrometric measurements

Abstract: In the present study we present all‐electron ab initio Hartree–Fock (HF) and configuration interaction (CI) calculations of the 2Σ+ ground state as well as of 16 excited states of the RhC molecule. The calculated spectroscopic constants of the lowest lying states are in good agreement with the experimental data. The chemical bond in the electronic ground state is mainly due to interaction of the 4d orbitals of Rh with the 2s and 2p orbitals of C. The bond is a triple bond composed of two π bonds and one σ bond. The 5s electron of Rh hardly participates in the bond formation. It is located in a singly occupied, nonbonding orbital. The chemical bond in RhC is polar with a charge transfer from Rh to C giving rise to a dipole moment of 2.82 D at the experimental equilibrium distance. Mass spectrometric equilibrium measurements over the temperature range 1970–2806 K have resulted in the selected dissociation D°0 = 576.0±3.8 kJ mol−1 for RhC(g).

Cadmium soil sorption at low concentrations: II. Reversibility, effect of changes in solute composition, and effect of soil aging

Abstract: Desorption of Cd from two Danish soils (loamy sand, sandy loam) previously exposed to low concentrations of Cd was examined in terms of reversibility, effect of changes in solute composition, and effects of aging of the soils for extended time prior to accomplishing the desorption experiments. The loamy sand exhibited full reversibility of Cd sorption in 10−3 M CaCl2 at pH = 6.0, while the sandy loam at the same conditions exhibited a partial irreversibility of the order of 1 μg Cd g−1 soil. However, this partial irreversibility was not caused by any strong binding of Cd since no irreversibility was observed in 10−2 M CaCl2 at the same pH. Changing the solute composition with respect to Zn, CaCl2, pH and the presence of organic ligands (EDTA) caused significant desorption of Cd from the soils. The degree of desorption was proven with respect to variations in CaCl2 and pH to be governed by the sorption equilibrium conditions independently of the previous distribution of Cd between soil and solute. Aging of the soil at a temperature of 1 °C (after exposure to Cd) for 35 and 67 wk did not reveal any changes with respect to desorption of Cd at pH = 6.0 in 10−3 M CaCl2.

G -closure of some particular sets of admissible material characteristics for the problem of bending of thin elastic plates

Abstract: In Ref. 1, we considered theG-closure of some initially given arbitrary setU of the positive-definite, symmetrical plane tensorsD of the 2nd rank, connected with the differential operator ∇ ·D · ∇ in two dimensions. Here, theG-closure procedure is applied to the 4th-order operator ∇∇ ··D ·· ∇∇ in a plane, arising in the theory of plates and containing self-adjoint tensorsD of the 4th rank. The paper generalizes some results obtained earlier in Refs. 2 and 3. The complete solution of the general problem of regularization, which presupposes the arbitrary character of the initially given setU, is not yet obtained.

Observation of Oxygen Frenkel Disorder in Uranium Dioxide above 2000 K by Use of Neutron-Scattering Techniques

Abstract: Diffraction and coherent diffuse quasielastic scattering of neutrons have been used to investigate Frenkel disorder of the oxygen sublattice in single crystals of stoichiometric U${\mathrm{O}}_{2}$. Measurements were made up to 2900 K using a special high-temperature furnace. The results provide the first direct evidence for the occurrence of Frenkel disorder in U${\mathrm{O}}_{2}$ and enable estimates of the defect concentration and activation energy to be made.

A Theoretical and Experimental Study of Undertow

Abstract: It is well known that on a three-dimensional beach large volumes of water carried shorewards by the breakers feed longshore currents, which eventually escape back through the breaker line, often as rip currents. In a steady two-dimensional situation, however, the mass flux represented by (among other things) the surface roller in the breakers returns as a seaward current close to the bottom. This current is called the undertow. In this paper theoretical results for the undertow are compared with the results of recent laboratory experiments.

G-closure of a set of anisotropically conducting media in the two-dimensional case

Abstract: This paper deals with the problem of the invariant description of a set of conducting materials which may be obtained from some given initial compounds by the process of mixing. Complete solution to this problem is provided for a two-dimensional case.