Ductilité | EPFL Graph Search

La ductilité est la capacité d'un matériau à se déformer plastiquement sans se rompre. La rupture se fait lorsqu'un défaut (fissure ou cavité) devient critique et se propage. Un matériau qui présente une grande déformation plastique à rupture est dit ductile, sinon il est dit fragile. C'est une propriété dite « purement géométrique » : elle ne caractérise qu'un allongement à la rupture (sans unité, ou l'allongement en mètre si la longueur pour l'essai de ductilité est normalisée), indépendamment de l'énergie ou de la contrainte nécessaire à cette rupture. L'origine de la ductilité d'un métal est la mise en mouvement des dislocations dont il est le siège. Cependant, cette mise en mouvement génère d'autres dislocations, qui se gênent mutuellement ce qui durcit le matériau mais néanmoins le rend plus fragile : c'est le phénomène d'écrouissage. La ductilité est une propriété conditionnée par la malléabilité. — Buffon La ductilité désigne surtout la capacité d'une matière à résister à

Long-term behaviour and ductility analysis of standard and fibre-reinforced concrete beams

Michele Albertini

Long-term concrete behaviour are two subjects of current interest in many studies published over the last decade. This thesis project is dedicated to the study of both the above mentioned topics, developing a comprehensive work that can further bring a higher degree of knowledge of the subject and the state of the art today. The first part of the work is devoted to the study of creep phenomena on reinforced concrete beams, creating a model that describes the real behavior of the structure, evaluating strains and long-term deformations, which cause phenomena of stress redistribution and delayed breakage over time. In this regard, a numerical model will be presented to evaluate the effect of the phenomena of increased deformations linked to time, such as linear creep, non-linear creep and shrinkage. This model was then applied to a case study, which allowed the evaluation of the degree of reliability of the proposed model. The second part of the work, instead, is dedicated to the study of the phenomenon of ductile breakage of beams subjected to bending. In particular the study concerned the influence of metal fibers on the degree of ductility of the structure. A test campaign was carried out, which made it possible, through an analysis of the results obtained, to assess the effect of the fibres on the breaking of the compressed area of the beams. The laboratory tests on fibre-reinforced concrete beams conducted as part of this study have shown that the presence of metal fibres within the concrete matrix increases the degree of ductility of the structure and also the displacement capacity after breakage.

2020

Local structural effects in fiber-reinforced polymer web-core sandwich structures

Sonia Yanes Armas

Glass fiber-reinforced polymer (GFRP) pultruded decks and sandwich panels currently represent two of the most extensive applications of FRP materials for load-bearing structural components in the bridge and building domains. Based on the state of the art, the global structural behavior of both systems has been fairly well investigated. Nonetheless, local effects governing in most cases the global behavior have been barely addressed. Selected local structural effects relevant to the global structural performance of pultruded GFRP bridge decks and GFRP-foam web-core sandwich structures are therefore investigated in this research. The effect of the core geometry of pultruded GFRP decks on the systemâs behavior in its transverse-to-pultrusion direction was experimentally investigated. The experimental work conducted on two deck designs with trapezoidal- and triangular-cell cross sections showed that the transverse structural performance depends on the cell geometry. Furthermore, the systemsâ transverse bending and in-plane shear stiffness were evaluated and the results indicated that a triangular core causes a more pronounced bi-directional behavior of the deck when it is subjected to concentrated loads. The local behavior of the web-flange junctions (WFJs) of the pultruded deck with trapezoidal cells was experimentally investigated regarding energy dissipation capacity and recovery subsequent to unloading. The experimental responses reported for two junction types with similar geometry and fiber architecture but different initial imperfections demonstrated that dissimilar imperfections could significantly affect WFJ behavior and change it from brittle to ductile. The time-dependent recovery and energy dissipation mechanisms of the WFJs exhibiting a ductile response were evaluated; the viscoelastic effects were found to be small in both cases. The rotational behavior of all WFJ types present in the trapezoidal-core deck was characterized. An experimental procedure based on three-point bending and cantilever experiments conducted on the web elements was developed and used for this purpose. The rotational stiffness, strength and failure modes of the WFJs differed depending on the web type, location of the WFJ within the deck profile, existing initial imperfections and direction of the applied bending moment. Numerical simulations of the full-scale deck were performed to demonstrate the validity of the experimental moment-rotation (M-Ï) relationships and simplified M-Ï curves provided. The effects of creep on the load-bearing behavior of GFRP-foam web-core sandwich structures were investigated. A study of the creep behavior of polyurethane (PUR) foams was conducted and showed that in order to assess the long-term structural performance of the sandwich system, the foam anisotropy, density and loading type should be considered. The creep behavior of web-core sandwich panels, and specifically the structural aspects affected by the web-core interaction, were analyzed using the GFRP-PUR sandwich roof of the Novartis Campus Main Gate Building as case study and currently available design guidelines. The resulting sandwich designs depended on the applied design recommendations. Finally, provisions for the cross-sectional design of the hybrid web-core were proposed.

EPFL2017

Seismic vulnerability of cultural heritage buildings in Switzerland

Mylène Devaux Baudraz

Seismic risk for cultural heritage buildings has been underestimated for years in Switzerland. A strong earthquake can occur at any time in this region, as it has been shown during the last centuries. As reminder the 1356 seismic event that destroyed the city of Basel can be quoted; this event, whose intensity is assessed as having been I=IX on the European Macroseismic Scale (EMS-98), is recorded as the most violent earthquake that struck central and northern Europe. Since 2003 or even 1989, the seismic safety of common buildings is well defined through modern building codes (Swisscodes (SIA)). However, this is not the case of cultural heritage buildings whose seismic vulnerability has been only partly addressed. This PhD thesis was initiated in order to fill in this void of knowledge by creating a methodology for assessing the seismic vulnerability of historical edifices. Surveys carried out on cultural heritage buildings after seismic event showed that such kind of structures are particularly vulnerable under seismic actions. This vulnerability is essentially due to their particular structure that is characterized by slender components (pillars), wide open spaces, quite big masses located at high levels (vaults and their filling, lantern towers, etc.) on one hand; on the other hand, they generally have few components that can resist the lateral actions perpendicular to the nave and their masonry fabric is non-ductile. Moreover the non-existence of a stiff horizontal component (or a deformable one in case of wooden ceilings) and heavy masses concentrated in walls rather at floors make it impossible to apply models that have been developed for common buildings. The methodology that is presented in this report is composed of four steps. It gives the possibility to first sort out the sacred edifices that are seismically vulnerable from the ones that are not vulnerable. The second step allows engineers to determine the seismic vulnerability of a given edifice by the use of simplified models that give valuable results. In case more accurate results are required, more sophisticated models can be applied, as the Finite Element Method. In the last step, the seismic vulnerability of the edifice is put in parallel with the expected peak ground acceleration of the region. This last phase permits to set if the given edifice can be damaged by earthquakes, which damage is expected and which retrofitting is required.

EPFL2008