Hierarchical organization | EPFL Graph Search

A hierarchical organization or hierarchical organisation (see spelling differences) is an organizational structure where every entity in the organization, except one, is subordinate to a single other entity. This arrangement is a form of a hierarchy. In an organization, the hierarchy usually consists of a singular/group of power at the top with subsequent levels of power beneath them. This is the dominant mode of organization among large organizations; most corporations, governments, criminal enterprises, and organized religions are hierarchical organizations with different levels of management, power or authority. For example, the broad, top-level overview of the general organization of the Catholic Church consists of the Pope, then the Cardinals, then the Archbishops, and so on. Members of hierarchical organizational structures chiefly communicate with their immediate superior and with their immediate subordinates. Structuring organizations in this way is useful partly because it

Microtensile failure mechanisms in lamellar bone: Influence of fibrillar orientation, specimen size and hydration

Daniele Casari, Tatiana Kochetkova, Johann Michler

A mechanistic understanding of bone fracture is indispensable for developing improved fracture risk assessment in clinics. Since bone is a hierarchically structured material, gaining such knowledge requires analysis at multiple length scales. Here, the tensile response of cortical bone is characterized at the lamellar length scale under dry and hydrated conditions with the aim of investigating the influence of bone's microstructure and hydration on its microscale strength and toughness. For individual lamellae, bone strength strongly correlates with the underlying mineralized collagen fibrils orientation and shows a 2.3-fold increase compared to the macroscale. When specimen size is increased to a few lamellae, the influence of fibril orientation and the size effect on strength are significantly reduced. These findings highlight the critical influence of defects, such as canaliculi and interlamellar interfaces, when assessing larger volumes. Hydration leads up to a 3-fold strength decrease but activates several toughening mechanisms enabling inelastic deformation. In axial specimens, toughening is seen through fibril bridging and crack kinking. In transverse specimens, water presence leads to a progressive but stable crack growth parallel to the fibril orientation, suggesting crack-tip plasticity at the fibrillar interfaces. This work offers a better understanding of the role of interfaces, porosity, and hydration in crack initiation under tensile loading, which is a crucial step towards improved clinical management of disease-related bone fractures through multiscale modeling approaches. Statement of significance Bone features a complex hierarchical structure which gives rise to several toughening mechanisms across several length scales. To better understand bone fracture, particularly the changes associated with age and disease, it is essential to investigate bone mechanical response at different levels of its hierarchical structure. For the first time, we were able to observe the nucleation of a single crack in hydrated bone lamellae under well-controlled uniaxial tensile loading conditions. These experiments highlight the role of water, interfaces, defects, and the ratio of defect to specimen size on bone's apparent strength and toughness. Such knowledge can be used in the future to develop multiscale models enabling improved clinical management of disease-related bone fractures. (C) 2021 The Authors. Published by Elsevier Ltd on behalf of Acta Materialia Inc.

ELSEVIER SCI LTD2021

Building Electricity Demand Forecasting with Temporal Hierarchies

Lorenzo Donadio

Accurate building electricity load forecasts play a major role in the energy transition, as they facilitate flexibility deployment, grid stability and overall reduce costs and CO2 emissions. This research leaverages forecasting and reconciliation of temporal hierarchies to achieve coherent and accurate forecasts for different time scales. Two different hierarchical structures and their variants were studied, a daily and hourly hierarchy. Four different forecasting algorithms were implemented and compared, as well as a variety of reconciliation methods, their improvements of base forecasts were quantified and assessed, with up to ten percent accuracy gains for certain levels of the hierarchical structure. This thesis constitutes a comprehensive comparison of and the developed tools for temporal hierarchical forecasting and reconciliation.

2021

Self-cleaning and wear-resistant polymer nanocomposite surfaces

Feyza Karasu Kiliç, Yves Leterrier, Luca Alois Egon Müller

Superhydrophobic self-cleaning and wear-resistant nanocomposite surfaces were produced by mimicking the hierarchical structure of the lotus leaf using a combination of rapid self-assembly and a UV nano-imprint lithography (UVNIL) process with a silicone master. Two different acrylate formulations containing acrylated silica nanoparticles and an acrylated silicone surfactant were used. The presence of the silicone master did not suppress the spontaneous migration of the surfactant to the polymer surface, which increased its hydrophobic character. Adding acrylated silica particles considerably increased the viscosity of the acrylate suspensions and led to a shear-thinning behavior. However the particles did not prevent the fast migration process of the surfactant and further increased the hydrophobicity of the material, due to increased nanoscale roughness of the nanocomposite surface. The largest increase of hydrophobicity was achieved for the UVNIL printed lotus surfaces using the acrylate formulation with lowest viscosity. These surfaces became superhydrophobic for the highest investigated concentration of silica. These nanocomposite lotus surfaces were, in addition, very hard with a microhardness above 400 MPa and particularly wear-resistant, and were self-cleaning with respect to hydrophobic contamination.

2018

Microtensile failure mechanisms in lamellar bone: Influence of fibrillar orientation, specimen size and hydration

Daniele Casari, Tatiana Kochetkova, Johann Michler

ELSEVIER SCI LTD2021