Showing papers by "University of Stuttgart published in 1984"

Non-Associated Plasticity for Soils, Concrete and Rock

Abstract: With reference to practical engineering problems it is shown that considerable differences may be encountered between the results from associated and those from nonassociated plasticity theories. Next, the need for a non-associated plasticity theory is demonstrated by considering test results for sand, concrete and rock. Elementary material parameters are discussed such as Young's modulus and Poisson's ratio for the description of the elastic properties; and a cohesion and a friction angle for the determination of the strength. The salient difference from associated plasticity theory concerns the introduction of a dilatancy angle which controls the inelastic (plastic) volume changes. This dilatancy angle is not only a suitable parameter for the description of soils, but also appears to be useful for concrete and rock. Basically, the paper consists of three parts as we consider three types of models of increasing complexity. The first model is a perfectly-plastic model, which employs the five aforementioned parameters. It is based on test data rather than on Drucker's hypothesis of material stability. The consequences thereof are examined. The second model is a straightforward extension of the first model by augmenting it with friction hardening and cohesion softening. This novel idea is introduced to account for the degradation of the cohesion of cemented granular materials with increasing inelastic deformation. The model is employed in an analysis which shows that plastic deformations tend to localize in thin shear bands, which may occur even before peak strength is reached. Finally, a review is given of concepts for modelling hysteresis and strain accumulation in cyclic loading. The concept of a bounding surface in addition to a yield surface is discussed and is adapted for use in a sophisticated model for loose and cemented granular materials under cyclic loading.

Quantum theory of the damped harmonic oscillator

Abstract: A phenomenological stochastic modelling of the process of thermal and quantal fluctuations of a damped harmonic oscillator is presented. The divergence of the momentum dispersion associated with the Markovian limit is removed by a Drude regularization. The variances of position and momentum are evaluated in closed form at arbitrary temperature and for arbitrary damping. Properties of real and imaginary time correlation functions are discussed, and a spectral decomposition of the equilibrium density matrix is given.

Bistable systems: Master equation versus Fokker-Planck modeling

Abstract: Relaxation and fluctuations of nonlinear macroscopic systems, which are frequently described by means of Fokker-Planck or Langevin equations, are studied on the basis of a master equation. The problem of an approximate Fokker-Planck modeling of the dynamics is investigated. A new Fokker-Planck modeling is presented which is superior to the conventional method based on the truncated Kramers-Moyal expansion. The new approach is shown to give the correct transition rates between deterministically stable states, while the conventional method overestimates these rates. An application to the Schl\"ogl models for first- and second-order nonequilibrium phase transitions is given.

Quantum Tunneling in Dissipative Systems at Finite Temperatures

Abstract: A quantum system which can tunnel out of a metastable state, and which interacts with an environment at temperature $T$, is considered. It is found that heat enhances the tunneling probability at $T=0$ by a factor $\mathrm{exp}[A(T)M{\ensuremath{\omega}}_{0}\frac{{q}_{0}^{2}}{\ensuremath{\hbar}}]$, where $M$ is the mass of the system, ${\ensuremath{\omega}}_{0}$ is the frequency of small oscillations about the metastable state, and ${q}_{0}$ is the tunneling distance. For an undamped system $A(T)$ is exponentially small, $A(T)\ensuremath{\propto}\mathrm{exp}(\frac{\ensuremath{-}\ensuremath{\hbar}{\ensuremath{\omega}}_{0}}{{k}_{B}T})$, whereas for a dissipative system $A(T)$ grows algebraically with temperature.

Property changes of UHMW polyethylene hip cup endoprostheses during implantation.

Abstract: In this study it is demonstrated that the combined chemical and mechanical influences of the implant situation cause property changes of ultra-high-molecular-weight polyethylene (UHMW PE) hip joint cups. Nearly 30 out of 48 loosened cups, retrieved 3 weeks to 11 years after implantation, were investigated. Density measurements show a density increase with implantation time and a dependence of these changes from implant position and loading conditions. The rate of extractable constituents also increases with course of time. An increased in vivo conditioned oxidation of the UHMW PE can be demonstrated by infrared (IR) spectrometry. The density increase can be explained by post-crystallization, which is the result of oxidative chain scission. This leads to a reduction of the average molecular weight of the PE and to an increased extractability of constituents. Since these changes have been recognized as the reasons for aging and failing of UHMW PE, the methods of material characterization used in this study for retrieved implants will help to develop suitable in vitro testing and simulating methods. They are the prerequisite for the necessary improvements of the material properties of UHMW PE.

Crossover from Thermal Hopping to Quantum Tunneling

Abstract: Consider a macroscopic system rendered unstable by both thermal fluctuation and quantum tunneling. Kramer's classical theory of the rate of decay by thermal activation is extended to lower temperatures where quantum tunneling prevails. By means of a functional-integral approach, a general formula for the decay rate is derived which describes the transition between the high- and low-temperature regimes. The influence of dissipation on the decay rate is emphasized.

High-power millimetre-wave mode converters in overmoded circular waveguides using periodic wall perturbations

Abstract: This work reports on measurements and calculations (coupled mode equations) on the conversion of circular electric TE0n gyrotron mode compositions (TE01 to TE04) at 28 and 70 GHz to the linearly polarized TE11 mode by means of mode converter systems using periodic waveguide wall perturbations. Mode transducers with axisymmetric radius perturbations transform the TE0n gyrotron mode mixture to the more convenient TE01 mode for long-distance transmission through overmoded waveguides. Proper matching of the phase differences between the TE0n modes and of lengths and perturbation amplitudes of the several converter sections is required. A mode converter with constant diameter and periodically perturbed curvature transfers the unpolarized TE01 mode into the TE11 mode which produces an almost linearly polarized millimetre-wave beam needed for efficient electron cyclotron resonance heating (ECRH) of plasmas in thermonuclear fusion devices. The experimentally determined TE0n -to-TE01 conversion efficiency...

Diffusion of gold in silicon studied by means of neutron-activation analysis and spreading-resistance measurements

Abstract: In- and out-diffusion of gold in silicon were investigated with the aid of a neutronactivation analysis in combination with mechanical sectioning or by the spreadingresistance technique. In-diffusion profiles in the range 1371–1073 K show that Au diffuses in Si mainly via the so-called kick-out mechanism. From the Au diffusion and solubility measurements the interstitialcy contributionDISD to the Si self-diffusion coefficient was determined, which shows that the self-diffusion occurs to a considerable extent via selfinterstitials. Out-diffusion profiles at 1173 K were measured on wafers homogeneously supersaturated with Au. The observed decrease of the electrical activity of Au in the bulk indicates that during the out-diffusion anneal the majority of Au atoms originally dissolved substitutionally changes its configuration.

Spark Ignition: Its Physics and Effect on the Internal Combustion Engine

Abstract: The interest in a better understanding of the processes involved in ignition and self-sustained flame propagation is as old as the internal combustion engine itself. In order to gain a better insight into these processes, the main efforts have been concentrated on experiments in the very engine aiming at a direct and quick success. This, however, also presented its major drawback. The complexity of the mutually dependent influences of the large number of different operation parameters of the real engine prevented any significant breakthrough in the past. Too much room had still to be left by this approach for speculations and interpretations due to serious lacks of accessible information.

Critical simply connected algebras

Abstract: It is well-known that simply connected algebras are uniquely determined by a graded tree.Reversely,each graded tree gives rise to a not necessarily representation-finite algebra. We call an algebra critical provided it is not representation-finite, but any proper convex full subalgebra is.All critical algebras arising from graded trees are classified.

Numerical investigation of supercritical Taylor-vortex flow for a wide gap

Abstract: For a wide gap ( R 1 / R 2 = 0.5) and large aspect ratios L/d , axisymmetric Taylor-vortex flow has been observed in experiments up to very high supercritical Taylor (or Reynolds) numbers. This axisymmetric Taylor-vortex flow was investigated numerically by solving the Navier–Stokes equations using a very accurate (fourth-order in space) implicit finite-difference method. The high-order accuracy of the numerical method, in combination with large numbers of grid points used in the calculations, yielded accurate and reliable results for large supercritical Taylor numbers of up to 100 Ta c (or 10 Re c ). Prior to this study numerical solutions were reported up to only 16 Ta c . The emphasis of the present paper is placed upon displaying and elaborating the details of the flow field for large supercritical Taylor numbers. The flow field undergoes drastic changes as the Taylor number is increased from just supercritical to 100 Ta c . Spectral analysis (with respect to z ) of the flow variables indicates that the number of harmonics contributing substantially to the total solution increases sharply when the Taylor number is raised. The number of relevant harmonics is already unexpectedly high at moderate supercritical Ta . For larger Taylor numbers, the evolution of a jetlike or shocklike flow structure can be observed. In the axial plane, boundary layers develop along the inner and outer cylinder walls while the flow in the core region of the Taylor cells behaves in an increasingly inviscid manner.

Observability canonical (phase-variable) form for non-linear time-variable systems

Abstract: An observability canonical form for non-linear time-variable systems, [xdot]=f(x,u,t), y=h(x,u,t), is introduced by analogy with the corresponding linear phase-variable forms. The transformation into observability canonical form follows from the nonlinear observability map, whose jacobian must be assumed to be a regular matrix in the considered domains of state x, input u and time t. If this observability matrix can be inverted analytically or numerically, the transformation into the observability canonical coordinates can be achieved directly. As opposed to linear systems, the non-linear observability canonical form with input depends, additionally, on the time derivatives of the input. This restricts a practical implementation.

Spectroscopy of the intermediate states of the solid state polymerization reaction in diacetylene crystals

Abstract: This contribution gives a review of recent spectroscopic investigations concerning the photophysical and photochemical primary and secondary processes of the solid state polymerization reaction in diacetylene single crystals. It will be shown, that diacetylenes are an unique model system for the study of the reaction mechanism of a solid state chemical reaction which is characterized by a variety of reaction intermediates. The polymerization reaction in these crystals is of special importance, due to the resulting polymer single crystals, which exhibit extraordinary anisotropic physical properties.

Heavy Metal Ionophores: Correlations Among Structural Parameters of Complexed Nonpeptide Polyamino Acids

Abstract: Nonpeptide polyamine acid ligands such as ethylenediaminetetraacetic acid (EDTA) and diethylenetriaminepentaacetic acid (DTPA) are members of a family of nonspecific heavy metal ionophores. Drawing upon the Cambridge Crystallographic Data Files and the results of a crystal structure determination for BaNd(III)DTPA·3H2O, we have looked for correlations between structural parameters and the physical and chemical properties of the metal ions complexed. Good correlations were found for primary metal-ligand (PMtL) interaction parameters (parameters involving only atoms interacting directly with the metal). Meaningful correlations were also found for secondary metal–ligand (SMtL) interaction parameters and for parameters involving only ligand atoms (L parameters). The correlations observed are interpreted as indicating that (a) contact distances between oxygen atoms in the metal ion coordination sphere are important to the determination of average Mt–O bond distances, (b) the Mt–ethylenediamine rings display considerably less sensitivity to contact distances in the coordination polyhedron than do Mt–amino acid rings and (c) there appears to be a meaningful correlation between the Mt–O bond distance and the amount of single bond character in the associated CO (carboxylate) bond.

Modulatory effects of different temperatures and Ca2+ concentrations on gangliosides and phospholipids in monolayers at air/water interfaces and their possible functional role.

Abstract: 1. Gangliosides are neuraminic acid-containing glycolipids preferently localized in nervous membranes and showing physicochemical peculiarities, e.g., drastically changing amphiphilic properties by Ca2+ binding. On account of this they are favorite compounds to act as modulators of membraneous organization and functions during synaptic transmission. Lipid monolayers are suitable experimental systems for the study of the surface behavior of amphipatic molecules and therefore are useful to interpret membraneous organization. 2. The surface pressure/area isotherms of monolayers of different individual gangliosides (GM1, GD1a, GD1b, GT1b) of an artificial reconstituted and a natural ganglioside mixture from bovine brain and of ganglioside mixtures from different brain parts of summer- and winter-adapted dsungarian hamsters were compared at three temperatures (11, 20, and 37°C) with egg phosphatidylcholine (PC) and phosphatidylserine (PS) monolayers. The monolayers were formed in a Teflon trough on a triethanolamine/HCl-buffered (pH 7.4) subphase, in some cases containing different amounts of CaCl2. 3. The surface pressure/area isotherms of ganglioside monolayers, in contrast to phospholipids, generally showed slowly rising slopes, with transitions from the liquidexpanded to the liquid-condensed state at a surface pressure of 20–30 mN/m. Ganglioside monolayers, in particular from GD1a or GT1b versus GD1b or from mixtures from summer- versus winter-adapted hamster brain, were differently affected by temperature and/or by Ca2+. PS monolayers were slightly condensed only by Ca2+. PC monolayers, however, were influenced neither by temperature nor by Ca2+. In mixed monolayers of the unpolar natural lipid cholesterol (Ch) and the disialoganglioside GD1a, intermolecular interactions were indicated. 4. Ganglioside monolayers, in contrast to phospholipids, were shown to be easily modulated by temperature and/or Ca2+ ions, thus enabling gangliosides to act as possible membrane modulators, e.g., during synaptic transmission. In particular, the differences concerning the influences of temperature and/or Ca2+ on the surface behavior of ganglioside mixtures from the brain of summer- compared with winteradapted hamsters are correlated with other physiologically relevant data.

A combination of pseudopotentials and density functionals: Results for Cun, Cun+, Agn, and Agn+ clusters (n ≤ 4)

Abstract: Structures and binding energies have been calculated for neutral and singly ionized Cu and Ag clusters with up to four atoms. The valence-electron system is treated at the ab initio SCF level; core-valence interaction is accounted for by semi-empirical pseudopotentials, corrected for core polarization; valence correlation is included using a local spin-density-functional approximation, corrected for self-interaction. The results are compared to previous calculations for Li, Na and K clusters, and it is shown that, in spite of the different bulk crystal structure, small group 1a and 1b clusters have many properties in common.

Negative-U properties for defects in solids

Abstract: A defect has negative-U properties if it can trap two electrons (or holes) with the second bound more strongly than the first. The system can be thought of as an extrinsic Cooper pair, the defect providing an environment in which a net attraction can develop between the otherwise Coulombically repulsive carriers. Evidence previously cited for this behavior in selected liquid and solid state systems will be reviewed. Recently, the first direct and unambiguous demonstration of the phenomenon in a solid has been supplied for two simple point defects in crystalline silicon—the lattice vacancy and interstitial boron. The experiments leading to this identification are described and mechanisms for this remarkable phenomenon are discussed.

Reaction kinetics in metal hydride reaction beds with improved heat and mass transfer

Abstract: Periodically operating absorption engines, i.e. absorption heat pumps, heat transformers and refrigerators based on metal-hydrogen reactions, are becoming of great interest in technical applications. In order to keep the material inventory small and to design compact machines, short cycle times are necessary. In this paper, experimental investigations of the technical reaction kinetics of various AB5 alloys are presented and it is shown that it is equally important to improve both the hydrogen flow and the heat transfer in a reaction bed.