Densité | EPFL Graph Search

La densité (ou densité relative) d'un corps est le rapport de sa masse volumique à la masse volumique d'un autre corps pris comme référence.

Pour les liquides et les solides, le corps de référence est l'eau pure à .
Pour les gaz, le corps de référence est l'air, à la même température et sous la même pression.

La densité d'un corps est une grandeur sans dimension et sa valeur s'exprime sans unité de mesure. Il ne faut pas confondre la densité d'un corps, sans dimension, avec la densité d'une grandeur qui est le rapport statistique, dimensionnel, d'une grandeur physique donnée sur un élément d'espace (longueur, surface ou volume), utilisé pour apprécier la répartition de cette grandeur, comme c'est le cas par exemple pour la densité surfacique d'une population, la masse surfacique, la charge surfacique, la concentration surfacique, etc. Histoire et terminologie La densité d'un corps est une notion de physique ancienne, apparue au . Cette grandeur caractéristique d'un

Postbuckling behavior and imperfection sensitivity of elastic-plastic periodic plate-lattice materials

Fani Derveni

Advances in additive manufacturing have enabled a new generation of materials with advantageous properties inherent to their architecture. Recently, architected materials with periodic arrangements of plates, called plate-lattice materials, have been developed to reach theoretical least upper bounds for stiffness and strength of isotropic materials. This work investigates the buckling behavior of several plate-lattice architectures with relative densities between 0.5% to 25% when subjected to uniaxial compression. Finite element unit cell models with shell elements and periodic boundary conditions are used to simulate the buckling and post-buckling behavior of plate-lattices made from an elastic-perfectly plastic parent material. Five plate-lattice architectures with cubic symmetry are investigated - three anisotropic architectures (simple cubic, body-centered cubic, face-centered cubic) and two isotropic architectures (simple cubic and body-centered cubic combination, simple cubic and face-centered cubic combination). Geometric imperfections of varying amplitude are applied to determine the imperfection sensitivity of these plate-lattice materials, and to calculate their buckling knockdown factors. Consistent with the behavior of the elastic-plastic plate building blocks that comprise the lattice, this investigation reveals that plate-lattice materials are generally imperfection insensitive when their constituent plates become thinner and first bifurcation occurs in the elastic range away from the yield stress. When this occurs, the initial post-buckling behavior is stable and the ultimate strength can be many times higher than the initial buckling stress. (C) 2021 Published by Elsevier Ltd.

ELSEVIER2022

Quantitative microstructural characterisation of concrete cured under realistic temperature conditions

Xinyu Zhang

The curing temperature is known to influence the strength and durability of concrete. In general, curing at elevated temperature results in high early strength gain, but impairs the long-term strength and transport properties. Previous investigations have not provided a complete understanding of these effects; in particular, no quantitative relation between microstructural and macroscopic changes has been developed. This work has adopted a combination of macro and micro characterisation to further a comprehensive and quantitative understanding of the property development under different curing conditions, through approaches which include both qualitative and quantitative microstructural analysis with SEM, EDS, XRD, NMR and TG. A systematic study on several mix designs and curing regimes has been carried out, to relate microstructural properties such as degree of hydration, C-S-H composition, C-S-H relative density and capillary porosity to compressive strength and water sorptivity. The results show that the Ca/(Si+Al) ratio of C-S-H is constant with temperature, and the increase in relative C-S-H brightness is thus attributed to the higher C-S-H density and lower chemically bound water at higher temperatures. The early hydration of concretes is accelerated by high temperatures, but the ultimate value is not limited, as opposed to widely reported lower values in the literature. This discrepancy is explained by the fact that previous workers have assumed that the bound water is constant for different curing temperatures whereas this is not the case. Linear relationships were established between the degree of hydration and the compressive strength of concretes, which depend on curing temperatures and mix design. Through their influence on the density and distribution of hydration products, elevated temperatures increase the ultimate capillary porosity of concretes. The relationship between capillary porosity and compressive strength seems independent of temperature to a first approximation, but no unique relationship is shown between sorptivity and compressive strength. Materials which were exposed to higher temperatures for only short periods of time, such as might be experienced in prefabrication or in the curing of large masses, showed some recovery in both microstructure and mechanical properties from the detrimental effects of high temperature.

EPFL2007

Mechanical properties and cytocompatibility of dense and porous Zn produced by laser powder bed fusion for biodegradable implant applications

Karel Lietaert, Andreas Mortensen, Ludger Weber

In this work, the macrotexture of dense Zn produced by laser powder bed fusion (LPBF) was studied and the mechanical properties for different tensile bar orientations were measured. The compressive strength of LPBF Zn scaffolds with five different unit cells was measured for a relative density of 20-51%. In addition, the response of mesenchymal stem cells to the LPBF Zn scaffolds was studied. The elastic modulus and yield strength of dense LPBF Zn were 110.0 +/- 0.2 GPa and 78.0 +/- 0.4 MPa, respectively in the vertical and 81.0 +/- 0.4 GPa and 55.0 +/- 0.7 MPa in the horizontal direction. This could be explained by the preferential orientation of the < 0001 > direction in the building plane. For LPBF Zn scaffolds, the plateau stress for the different unit cells varied between 8 and 33 MPa for a 30% relative density. Calcein staining, lactate production and DNA measurements over a 13-day period showed that mesenchymal stem cell viability was low for Zn scaffolds. This work forms a basis for further research into the LPBF texture formation of metals with hexagonal crystal structure, guides implant designers in scaffold unit cell and relative density selection and motivates further research into the cytocompatibility of LPBF Zn. Statement of Significance Laser powder bed fusion (LPBF) is a manufacturing technology which allows the seamless combination of porous and non-porous volumes in a metallic implant and is used in the orthopedic manufacturing industry today. The production of highly dense Zn with LPBF has been described earlier, but the mechanical properties of the resulting material have not been studied in detail yet. This study is the first to report on (i) the influence of different scanning strategies on the macrotexture of dense LPBF Zn and the resulting anisotropy of its mechanical properties, (ii) the relationship between the relative density and strength for LPBF Zn scaffolds with five different unit cells and (iii) the in vitro response of mesenchymal stem cells to these scaffolds. (C) 2020 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

ELSEVIER SCI LTD2020