Multi-scale approach of the instrumented indentation technique on the fracture toughness estimation

Instrumented Indentation Technique (IIT) is widely used to determine the mechanical properties of materials. The elastic modulus is usually determined by applying the methodology proposed by Oliver and Pharr [1] who supposed that its value is independent of the indentation depth. However, some authors [2, 3] have observed a decrease of the elastic modulus when the indenter displacement increases which allowed them to introduce a continuous damage theory used afterwards to estimate the fracture toughness of ductile materials. The assumption made by the authors is that a damage in the region very close to the bottom of the indent results in the formation of microvoids which leads to the variation of the elastic modulus as a function of the indenter displacement. Starting from this observation, Lee et al. [2] proposed an energy model based on the Griffith’s theory and the continuous damage mechanics (CDM) which states that the elastic modulus variation is related to the fraction void volume through a variable damage, introduced by Kachanov [4], related to the surface density of the microdefects. On the other hand, the works carried out over ductile materials by Li et al. [3] have been performed only with nanoindentation data preventing a discussion on the scale-­‐effect. (...

Design and construction of a shell-nexorade hybrid timber structure

International audienceThis paper presents the design and fabrication of an innovative structure, spanning 6.5m with a structural thickness of 12cm. The structural system derives from nexorades, also called reciprocal frames by some authors, obtained by a novel form-finding method based upon translation of the edges. It also allows for the covering with planar facets if the geometry before transformation is covered with planar facets. This property is used for the pavilion, covered with planar timber panels used as bracing elements. The benefit of this structural system, called shell-nexorade hybrid is discussed through a comparative study between the braced and un-braced nexorade (the stiffness is multiplied by ten adding only 30% mass). Finally, the fabrication process, achieved through robotized milling of the beams, is shown, illustrating the potential of pluri-disciplinary approach for conceptual design

Multi-scale approach of the instrumented indentation technique on the fracture toughness estimation

Instrumented Indentation Technique (IIT) is widely used to determine the mechanical properties of materials. The elastic modulus is usually determined by applying the methodology proposed by Oliver and Pharr [1] who supposed that its value is independent of the indentation depth. However, some authors [2, 3] have observed a decrease of the elastic modulus when the indenter displacement increases which allowed them to introduce a continuous damage theory used afterwards to estimate the fracture toughness of ductile materials. The assumption made by the authors is that a damage in the region very close to the bottom of the indent results in the formation of microvoids which leads to the variation of the elastic modulus as a function of the indenter displacement. Starting from this observation, Lee et al. [2] proposed an energy model based on the Griffith’s theory and the continuous damage mechanics (CDM) which states that the elastic modulus variation is related to the fraction void volume through a variable damage, introduced by Kachanov [4], related to the surface density of the microdefects. On the other hand, the works carried out over ductile materials by Li et al. [3] have been performed only with nanoindentation data preventing a discussion on the scale-­‐effect. (...

8. Pétrole : les différents carburants

Quel que soit le mode de transport, il requiert une énergie que l’on a coutume d’appeler « carburant » lorsque l’on alimente des engins mobiles (certains l’appelleront « combustible » mais ce terme est plutôt dédié aux énergies pour les unités fixes). Les carburants sont aujourd’hui majoritairement liquides et proviennent directement du pétrole à 94 %, mais ils peuvent aussi être gazeux, de synthèse, fabriqués à partir de biomasse (les biocarburants, cf. V.9). Quelle que soit leur origine, le..

Caracterisation et reactivite des intermediaires reactionnels dans la decomposition du methanol a pression atmospherique

SIGLECNRS T Bordereau / INIST-CNRS - Institut de l'Information Scientifique et TechniqueFRFranc

Quels VEGétaux et systèmes de production durAbles pour satisfaire les besoins en bioénergies, synthons et matériaux biosourcés ?

Atelier de Réflexion Prospective VegACe document est le fruit d’un travail collectif, réalisé dans le cadre de l’Atelier de Réflexion Prospective « Quels VEGétaux et systèmes de production durAbles pour satisfaire les besoins en bioénergie, synthons et matériaux biosourcés ? » Les participants ont rédigé 13 documents de synthèse ou livrables, correspondants aux différents groupes de travail, mis en place dans l’ARP VegA. Chaque groupe de travail a eu pour objectif de cerner l’état des connaissances et identifier des pistes de recherche : il a travaillé avec des frontières poreuses pour ne pas contraindre les réflexions, aux dépens de problématiques parfois difficilement attribuables à un seul groupe. Une synthèse a été établie, à partir de ces 13 documents de travail, avec un regroupement en trois chapitres principaux : • Le premier chapitre identifie les attentes et les besoins en énergies, synthons et matériauxbiosourcés, comparativement aux usages actuels du carbone fossile, • Le deuxième explore la diversité des solutions végétales susceptibles de répondre qualitativement à ces besoins, • Le troisième aborde la conception, l’évaluation et la mise en pespective de systèmes de production durables, fondées sur ces solutions végétales et répondant aux attentes et besoins du premier chapitre

Voss surfaces: A design space for geodesic gridshells

International audienceThe design of envelopes with complex geometries often leads to construction challenges. To overcome these difficulties, resorting to discrete differential geometry proved successful by establishing close links between mesh properties and the existence of good fabrication, assembling and mechanical properties. In this paper, the design of a special family of structures, called geodesic shells, is addressed using Voss nets, a family of discrete surfaces. The use of discrete Voss surfaces ensures that the structure can be built from simply connected, initially straight laths, and covered with flat panels. These advantageous constructive properties arise from the existence of a conjugate network of geodesic curves on the underlying smooth surface. Here, a review of Voss nets is presented and particular attention is given to the projection of normal vectors on the unit sphere. This projection, called Gauss map, creates a dual net which unveils the remarkable characteristics of Voss nets. Then, based on the previous study, two generation methods are introduced. One enables the exploration and the deformation of Voss nets while the second provides a more direct computational technique. The application of theses methodologies is discussed alongside formal examples

Discrete Voss Surfaces: Toward the design of Geodesic Gridshells

The design and construction of doubly-curved structures often reveals to be challenging and can result in complex manufacturing and assembly. A recent strategy to tackle this difficulty consists in exploiting the connection between discrete differential geometry and constructive properties to identify curve networks with good fabrication or mechanical properties. Following this approach, a family of surfaces, called Voss surfaces, is here presented. Among other features, they can be built from flat panels, initially flat straight strips, and hinged connections. These properties arise from the existence of a conjugate network of geodesic curves. Two generation methods are presented to shape discrete Voss surfaces: the first allows their exploration through linear spaces; the second provides a unique solution by means of a direct computation