MODELING OF THERMAL CRACKING IN ELASTIC AND ELASTOPLASTIC TWO-PHASE SOLIDS

A review is given about fracture mechanical investigations concerning the thermal crack initiation and propagation in one of the segments or in the material interface of two-arid three-dimensional self-stressed two-phase compounds. The resulting boundary value problems of the stationary thermoelasticity and thermoplasticity for the cracked two- and three-dimensional bimaterial structures considered are solved using the finite element method. Furthermore, by applying an appropriate crack growth criterion based on the numerical calculation of the total energy release rate G of a quasistatic mixed-mode crack extension the further development of thermal crack paths starting at the intersection line of the material interface with the external stressfree surface of the two- and three-dimensional elastic bimaterials could be predicted. In the case of the disklike two-phase compounds, the theoretically predicted crack paths show a very good agreement with results gained by associated cooling experiments. Several specimen geometries consisting of different material combinations and subjected to uniform and nonuniform temperature distributions have been studied using the relevant methods of fracture mechanics. Thereby thermal cracks propagating in one segment of an elastic bimaterial only obey the condition G_II = 0, whereas for interface cracks a mixed-mode propagation is always existent where the G_II values play an important role. Moreover, by applying the proposed crack growth criterion the possible crack kinking direction ? of an interface crack tip out of the interface could be predicted by taking into consideration the finite thickness of an interlayer (interphase). In addition, an analysis of the stress and strain fields in the vicinity of thermal interface cracks in the discontinuity area of two- and three-dimensional elastoplastic two-phase compounds has been performed by using the FE-method. Thereby a heat source Q was assumed in one of the two materials in the neighborhood of an interface crack tip. The corresponding stress states in the bimaterial structuresand especiallyin the vicinity of the interface crack tip have been calculated by applying the incremental I₂-plasticity and using a bilinear hardening material law and based on a sequentially coupled solution of the heat transfer and the thermal stress boundary value problems. Finally, the failure assessment has been performed on the basis of the local J-integral which, for three-dimensional interface cracks, was recently generalized by two of the authors.     

Aging and Input Processing in Dual-Task Situations.

The psychological refractory period (PRP) paradigm was used to test whether older participants suffer from input interference in dual-task situations. Young (24 years) and older (57 years) adults gave speeded responses to 2 successively presented stimuli. The results showed increased susceptibility of older participants to input interference. Further experiments revealed that this input interference is related to the salience of the 2nd stimulus and that it is specific to older participants. Our findings indicate that parallel processing at the input stages of dual-task performance requires cognitive control. An age-related decline in the control of input processes should be considered as one source of age effects in dual-task performance. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Convergence properties of bias‐eliminating algorithms for errors‐in‐variables identification

This paper considers the problem of dynamic errors‐in‐variables identification. Convergence properties of the previously proposed bias‐eliminating algorithms are investigated. An error dynamic equation for the bias‐eliminating parameter estimates is derived. It is shown that the convergence of the bias‐eliminating algorithms is basically determined by the eigenvalue of largest magnitude of a system matrix in the estimation error dynamic equation. When this system matrix has all its eigenvalues well inside the unit circle, the bias‐eliminating algorithms can converge fast. In order to avoid possible divergence of the iteration‐type bias‐eliminating algorithms in the case of high noise, the bias‐eliminating problem is re‐formulated as a minimization problem associated with a concentrated loss function. A variable projection algorithm is proposed to efficiently solve the resulting minimization problem. A numerical simulation study is conducted to demonstrate the theoretical analysis. Copyright © 2005 John Wiley & Sons, Ltd.     

Running time analysis of evolutionary algorithms on a simplified multiobjective knapsack problem

In this paper, the expected running time of two multiobjectiveevolutionary algorithms, SEMO and FEMO, is analyzed for a simpleinstance of the multiobjective 0/1 knapsack problem. The considered problem instance has two profit values per item andcannot be solved by one-bit mutations. In the analysis, we make use of two general upper bound techniques, thedecision space partition method and the graph search method. The paperdemonstrates how these methods, which have previously only beenapplied to algorithms with one-bit mutations, are equally applicablefor mutation operators where each bit is flipped independently with acertain probability.     

SPI - a system model for heterogeneously specified embedded systems

Embedded systems typically include reactive and transformative functions, often described in different languages and semantics which are well established in their respective application domains. Additionally, a large part of the system functionality and components is reused from previous designs including legacy code. There is little hope that a single language will replace this heterogeneous set of languages. A design process must be able to bridge the semantic differences for verification and synthesis and should account for limited knowledge of system properties. This paper presents the system property intervals (SPI) model, which employs behavioral intervals and process modes to allow the common representation of different languages and semantics. This model is the basis of a workbench which is targeted at the design of heterogeneously specified embedded systems.     

Centrifugal separation in tube and disc geometries: experiments and theoretical models

The centrifugal separation of particles or droplets in dispersions is important in a variety of applications, e.g. mineral processing, water and waste water treatment, multistep processing of nanomaterials and biotechnology. Despite numerous studies in the past, separation processes of concentrated dispersions driven by gravity or centrifugal forces are not yet completely understood. Centrifuges can be subdivided into two categories: (1) centrifuges operating with tubes of constant cross section and (2) centrifuges operating with disc or cylinder rotors whose cross-sectional area changes with distance along the axis of rotation. This paper investigates the sedimentation process in centrifuges of these two categories experimentally and theoretically. Based on models for the separation of polydisperse dispersions in centrifugal field numerical calculations were carried out including the conversion of the time dependent concentrations profiles into light transmission profiles. The changes in the dispersion concentration profiles during the sedimentation process were monitored in situ by space- and time-resolved NIR light extinction profiles (STEP-Technology) obtained by multisample analytical centrifugation. Cells having constant and increasing cross sections were used. Results show that the velocity of the boundary between supernatant and dispersion as well as the alteration of the concentration measured radially in the centre of the cells do not depend on the geometry of the cells within the experimental errors. The sediment height in cells with increasing cross section is smaller compared to cells with constant cross section. The simulated sedimentation process is in good agreement with measured data for diluted and concentrated silica suspensions. Based on these results the sedimentation processes in disc or cylinder process centrifuges can be predicted from laboratory tests using analytical centrifugation.     

Fission product release and microstructure changes during laboratory annealing of a very high burn-up fuel specimen

A commercial PWR fuel sample with a local burn-up of about 240 MWd/kgHM was annealed in a Knudsen cell mass spectrometer system with a heating rate of 10 K/min up to 2750 K at which temperature the sample was completely vaporized. The release of fission gases and fission products was studied as a function of temperature. In one of the runs the heating was interrupted successively at 900, 1500 and 1860 K and at each step a small fragment of the sample was examined by SEM and analysed by energy dispersive electron probe microanalysis. The release behaviour of volatile, gaseous and other less volatile fission products is presented and analysed with the EFFUS program and related to the structural changes of the fuel.     

Thermochemical characteristics of sugar cane bagasse pellets

Pelletisation facilitates utilisation of sugar cane bagasse as a fuel and storage for year-round electricity generation. The present work determines thermochemical characteristics of bagasse pellets of different sizes and origins, using various temperatures (600, 750 and 900 °C) and gas flow rates (4, 7 and 10 L/min) with varying concentrations of oxygen (5, 10 and 15%) in mixtures with nitrogen. Of major interest are the effects of raw material, origin and size of pellets, and the treatment conditions on the rate of pyrolysis and the structure and reactivity of char in combustion. The char yield of the larger pellets of high-ash content bagasse was practically independent of treatment conditions. Smaller pellets gave better mechanical stability of the char but lower reactivity.