Computation of 3-D stress singularities for multiple cracks and crack intersections by the scaled boundary finite element method

Among the various possible ways of dealing with notch and crack situations, the scaled boundary finite element method [SBFEM, (Wolf and Song in Finite element modelling of unbounded structures. Wiley, Chichester, 1996; Wolf in The scaled boundary finite element method. Wiley, Chichester, 2003)] has been adopted in this work. This method has been proved to be versatile, much less time consuming than the finite element method and generates highly accurate numerical predictions in cases of structures with notches and cracks. The SBFEM gives the advantage of boundary element method by reducing one dimension in modelling the structures but the mathematical formulations are more related to conventional displacement based finite element method. This method requires a certain scalability of the given structure with respect to a point called similarity center. Like in the case of the boundary element method, the structure needs to be discretized only at the surface where standard displacement based isoparametric finite element formulations are adequate. Unlike in the boundary element method, however, no fundamental solution is required by the scaled boundary finite element method. The similarity or scalability of the method requires separation of coordinates such that in the radial direction (i.e. scaling direction) it yields simple differential equations that can be solved analytically. So this approach can be considered as a semi-analytical method. Several two-dimensional examples have been analysed for crack and notch situations that are well known cases in fracture mechanics. A number of three-dimensional cases have been considered for different crack configurations that yield high order of singularity. The results, according to the authors’ knowledge are up to now unpublished in the open literature. Parametric studies are conducted for structures with bi-material interfaces.     

Oxidation of Silicon Carbide Fibers During Static Fatigue in Air at Intermediate Temperatures

The static fatigue of SiC-based fiber bundles and single fibers has been examined in previous papers, with emphasis placed on the analysis of the stress–rupture time data, and on the modelling of delayed failure from slow crack growth. The present paper investigates the oxidation of the fibers during static fatigue, at temperatures in the intermediate temperature range (500°–800°C). Two oxidation-induced phenomena have been evidenced: the formation of a thin silica film at the surface of fibers and the delayed failure of fiber bundles and single filaments. The stress–rupture time data are interpreted with respect to the chemical and structural characteristics of fibers, and to the oxide film growth rate. The structural analysis of the fibers was carried out using scanning electron microscopy and Auger electron spectroscopy. Delayed failure was found to result from slow crack propagation from surface defects, as a result of the consumption of the free carbon at grain boundaries and the local stresses induced by the SiC→SiO₂ transformation at the crack tip. The respective contributions of these phenomena to static fatigue are discussed.     

Forum The built environment through the cultural–architectural lens

The Building Research & Information special issue ‘Developing Theories on the Built Environment’ (36(3) 2008) raises the question of whether there could be a common framework to combine a wide range of interests and provide insightful answers. This commentary explores the significance of cultural contexts for theories on the built environment. Deficiencies in both architectural theory and history have left cultural associations and values inarticulated. Increasingly, architectural theory and history have become marginalized in the widening debate about the built environment as their focus has become too limited and separated from everyday and urgent needs. The cultural ‘footprint’ remains the strongest force and determines patterns of behaviour and expectations. It is argued that a cultural lens is the primary key in providing both insight into an ideal and consciousness of (intellectual and practical) habits influencing our actions. In order to grasp the complexity of the built environment and provide a useful model for altering our collective actions, it is vital to study the transformational chains, for example, from the cultural to the ecological.

Ce numéro spécial de Building Research & Information consacré aux théories de développement du milieu bâti (volume 36(3), 2008) pose la question de la création possible d'un cadre commun capable de combiner une grande palette d'intérêts et de fournir des réponses intuitives. Cet article analyse le sens des contextes culturels de théories relatives au milieu bâti. Les lacunes que l'on constate dans la théorie et dans l'histoire de l'architecture n'ont pas permis d'expliquer les valeurs et les associations culturelles. De plus en plus, la théorie et l'histoire de l'architecture ont été marginalisées dans le débat qui s'élargit sur le milieu bâti car leurs points d'intérêt sont devenus trop limités et à l'écart des besoins quotidiens et urgents. L'empreinte culturelle demeure la force la plus importante et détermine les tendances de comportement et des attentes. On prétend qu'une loupe grossissante de la culture est la clef principale pour fournir à la fois un idéal et une conscience des habitudes (intellectuelles et pratiques) ayant une influence sur nos actions. Pour saisir la complexité du milieu bâti et fournir un modèle utile en vue de modifier nos actions collectives, il est essentiel d'étudier les chaînes transformationnelles, par exemple, de la culture à l'écologie.

Mots clés: théorie de l'architecture, milieu bâti, cadre conceptuel, sagesse pratique, formulation de théories, elaboration de théories     

Energy transfer in hydrogen/air and methane/air mixtures

This article presents gas kinetic calculation methods for the energy transport in hydrogen/ air and methane/ air mixture. The total molecular heat conductivity as well as its various shares are calculated and discussed for a temperature range of 400 to 3500 K and for a pressure of 10⁵ Pa. The variation of the air/ fuel ratio under the conditions of chemical equilibrium is also investigated. As opposed to our previous article, an extension of the Chapman-Knskog method which goes beyond the classical 1st approximation for elastic collisions is applied for the evaluation of suitable calculation methods. This is carried out following the method for strong relaxing thermal nonequilibrium according to Brun. The equations applied in this work are simplified formulations for the proximity to equilibrium. The method for the evaluation of the parameters of the inelastic collision and some chosen results are presented. A discussion of the various shares of the molecular heat conductivity emphasizes the considerable influence of the diffusion and the thermal diffusion processes in fuel/ air mixtures compared to the Fourier heat conductivity.