Evaluation of applicability of 2D iron core model for two-limb configuration of GOLEM tokamak

This paper presents evaluation of applicability of 2D iron core model for highly non-axisymmetric two limb configuration of GOLEM tokamak (former CASTOR). Presented results explain the long-term discrepancy between measured magnitudes of external poloidal field and those calculated by air-core approach on this tokamak. The model has been applied to two poloidal planes at different toroidal angles in the vacuum vessel region and has shown that close to central column of the transformer, it is possible to correct for 3D effects by variation of chosen dimensions of axisymmetric iron core model. Satisfactory agreement of the 2D model results with the measured distribution of B_R field component was achieved.     

Compound Size Effect in Composite Beams with Softening Connectors.?I: Energy Approach

The size effect on the nominal strength of steel-concrete composite beams caused by shear failures of connectors such as welded studs is analyzed by two different approaches: (1) In this paper (Part I) by a fracture type analysis of the energy release caused by propagation of the zone of failed connectors along the beam; and (2) in a companion paper (Part II) by a direct solution of the load-deflection diagrams from the differential equations of beam bending theory. The former can capture the large size asymptotic size effect and yields simple formulas suitable for design, whereas the latter can provide the solution for small beam sizes for which the connector failure zone is not much shorter than the span. The force-slip diagram of the connectors exhibits postpeak softening, which engenders an energetic size effect on the nominal strength of the connector. If the connectors are geometrically scaled with the beam, the size effect in the shear failure of connectors (mesoscale) is superimposed on the size effect due to propagation of the zone of connector failures along the beam (macroscale), producing in the beam as a whole a compound size effect that is stronger than in linear elastic failure mechanics. If the connector sizes and the interface area per connector are not scaled with the overall dimensions of the composite beam, the size effect law proposed by Ba?ant in 1984 is applicable. Comparisons with available test results are presented in Part II.     

Fracture Mechanics of ASR in Concretes with Waste Glass Particles of Different Sizes

Using waste glass as an aggregate in concrete can cause severe damage because of the alkali-silica reaction (ASR) between the alkali in the cement paste and the silica in the glass. Recent accelerated 2-week tests, conducted according to ASTM C 1260, revealed that the damage to concrete caused by expansion of the ASR gel, which is manifested by strength reduction, depends in these tests strongly on the size of the glass particles. As the particle size decreases, the tensile strength first also decreases, which is expected because of the surface-to-volume ratio of the particles, and thus their chemical reactivity increases. However, there exists a certain worst (pessimum) size below which any further decrease of particle size improves the strength, and the damage becomes virtually nonexistent if the particles are small enough. The volume dilatation due to ASR is maximum for the pessimum particle size and decreases with a further decrease of size. These experimental findings seem contrary to intuition. This paper proposes a micromechanical fracture theory that explains the reversal of particle size effect in the accelerated 2-week test by two opposing mechanisms: (1) The extent of chemical reaction as a function of surface area, which causes the strength to decrease with a decreasing particle size; and (2) the size effect of the cracks produced by expansion of the ASR gel, which causes the opposite. The pessimum size, which is about 1.5 mm, corresponds to the case where the effects of both mechanisms are balanced. For smaller sizes the second mechanism prevails, and for sizes <0.15 mm no adverse effects are detectable. Extrapolation of the accelerated test (ASTM C 1260) to real structures and full lifetimes will require coupling the present model with the modeling of the reaction kinetics and diffusion processes involved.     

Continuous Relaxation Spectrum for Concrete Creep and its Incorporation into Microplane Model M4

Efficient numerical finite-element analysis of creeping concrete structures requires the use of Kelvin or Maxwell chain models, which are most conveniently identified from a continuous retardation or relaxation spectrum, the spectrum in turn being determined from the given compliance or relaxation function. The method of doing that within the context of solidification theory for creep with aging was previously worked out by Ba?ant and Xi in 1995 but only for the case of a continuous retardation spectrum based on the Kelvin chain. The present paper is motivated by the need to incorporate concrete creep into the recently published Microplane Model M4 for nonlinear triaxial behavior of concrete, including tensile fracturing and behavior under compression. In that context, the Maxwell chain is more effective than the Kelvin chain, because of the kinematic constraint of the microplanes used in M4. The paper shows how to determine the continuous relaxation spectrum for the Maxwell chain, based on the solidification theory for aging creep of concrete. An extension to the more recent microprestress-solidification theory is also outlined and numerical examples are presented.     

Copper-Tungsten Composites Sprayed by HVOF

Copper and copper-tungsten composite coatings were produced by high-velocity oxy-fuel spraying (HVOF). After initial optimization of the spraying parameters, coatings of various compositions were made and their structure, composition, mechanical, thermal, and electrical properties were characterized. The HVOF technique was able to produce rather dense coatings with minimal oxide content and relatively good mechanical and thermal properties compared to, for example, plasma-sprayed coatings; however, the achieved tungsten content was quite low.     

Cohesive Crack Model with Rate-Dependent Opening and Viscoelasticity: II. Numerical Algorithm, Behavior and Size Effect

In the preceding companion paper (Bažant and Li, 1995), the solution of an aging viscoelastic law was structure containing a cohesive crack with a rate-dependent stress-displacement softening law was reduced to a system of one-dimensional integro-differential equations involving compliance functions for points on the crack faces and the load point. An effective numerical algorithm for solving these equations, which dramatically reduces the computer time compared to the general two-dimensional finite element solution, is presented. The behavior of the model for various loading conditions is studied. It is shown that the model can closely reproduce the available experimental data from fracture tests with different loading rates spanning several orders of magnitude, and tests with sudden changes of the loading rate. Influence of the loading rate on the size effect and brittleness is also analyzed and is shown to agree with experiments. This revised version was published online in July 2006 with corrections to the Cover Date.     

Effect of noble metals in the decomposition of nitrous oxide over Fe-ferrierites

The decomposition of nitrous oxide was studied over Fe-ferrierite, Me-ferrierites and Fe/Me-ferrierites (Me: Pt, Rh and Ru). Flow as well as batch experiments were carried out and showed a synergy between Fe and Me ions. Ions of noble metals in Fe-ferrierite increased the catalytic activity in the sequence Pt < Rh ≅ Ru. Addition of NO substantially decreased the decomposition of N₂O over Rh/ferrierite and Ru/ferrierite, but not over bimetallic ferrierites. NO _x species created during the decomposition of nitrous oxide alone as well as with addition of NO, and employment of nitrous oxide labeled with ¹⁸O allowed us to assume a changing decomposition mechanism in the presence of Me ions in Fe-ferrierites.     

Holz im italienischen Flugzeugbau

Zusammenfassung Grunds?tzliche Betrachtungen über die Verwendung von Holz im Flugzeugbau. Der Holzflugzeugbau in Italien. Werkstoffe, Fertigungsweisen und Erfahrungen. Vgl. A. Guglielmetti, Entwicklungsrichtlinien der italienischen Luftfahrttechnik. Z. Luftwissen Bd. 8 (1941) S. 209 bis 215.