Full-field measurements of strain localisation in sandstone by neutron tomography and 3D-volumetric Digital Image Correlation

AbstractRecent studies have demonstrated that the combination of x-ray tomography during triaxial tests (“in-situ” tests) and 3D- volumetric Digital Image Correlation (3D-DIC) can provide important insight into the mechanical behaviour and deformation processes of granular materials such as sand. The application of these tools to investigate the mechanisms of failure in rocks is also of obvious interest. However, the relevant applied confining pressures for triaxial testing on rocks are higher than those on sands and therefore stronger pressure containment vessels, i.e., made of thick metal walls, are required. This makes in-situ x-ray imaging of rock deformation during triaxial tests a challenge. One possible solution to overcome this problem is to use neutrons, which should better penetrate the metal-walls of the pressure vessels. In this perspective, this work assesses the capability of neutron tomography with 3D-DIC to measure deformation fields in rock samples. Results from pre- and post-deformation neutron tomography of a Bentheim sandstone sample deformed ex-situ at 40MPa show that clear images of the internal structure can be achieved and utilised for 3D-DIC analysis to reveal the details of the 3D strain field. From these results the character of the localised deformation in the study sample can thus be described. Furthermore, comparison with analyses based on equivalent x-ray tomography imaging of the same sample confirms the effectiveness of the method in relation to the more established x-ray based approach

Fast 4D imaging of fluid flow in rock by high‐speed neutron tomography

High‐speed neutron tomographies (1‐min acquisition) have been acquired during water invasion into air‐filled samples of both intact and deformed (ex situ) Vosges sandstone. Three‐dimensional volume images have been processed to detect and track the evolution of the waterfront and to calculate full‐field measurement of its speed of advance. The flow process correlates well with known rock properties and is especially sensitive to the distribution of the altered properties associated with observed localized deformation, which is independently characterized by Digital Volume Correlation of X‐ray tomographies acquired before and after the mechanical test. The successful results presented herein open the possibility of in situ analysis of the local evolution of hydraulic properties of rocks due to mechanical deformation

Worldwide surveillance of self-reported sitting time: a scoping review

Abstract: Background: Prolonged sitting time is a risk factor for chronic disease, yet recent global surveillance is not well described. The aims were to clarify: (i) the countries that have collected country-level data on self-reported sitting time; (ii) the single-item tools used to collect these data; and (iii) the duration of sitting time reported across low- to high-income countries. Methods: Country-level data collected within the last 10 years using single-item self-report were included. The six-stage methodology: (1) reviewing Global Observatory for Physical Activity! Country Cards; (2–4) country-specific searches of PubMed, the Demographic and Health Survey website and Google; (5) analysing the Eurobarometer 88.4; and (6) country-specific searches for World Health Organization STEPwise reports. Results: A total of 7641 records were identified and screened for eligibility. Sixty-two countries (29%) reported sitting time representing 47% of the global adult population. The majority of data were from high-income (61%) and middle income (29%) countries. The tools used were the International Physical Activity Questionnaire (IPAQ; n = 34), a modified IPAQ (n = 1) or the Global Physical Activity Questionnaire (GPAQ; n = 27). The median of mean daily sitting times was 4.7 (IQR: 3.5–5.1) hours across all countries. Higher-income countries recorded a longer duration of sitting time than lower-income countries (4.9 vs 2.7 h). Conclusions: This study provides an updated collation of countries collecting self-reported sitting time data. The daily sitting time findings should be interpreted cautiously. Current surveillance of sitting time is limited by a lack of coverage. Measures of population sitting time that are valid, feasible and sensitive to change should be embedded within global surveillance systems, to help guide future policy, research and practice. Trial registration: Not applicable

Etude expérimentale sur la localisation des déformations dans les grès poreux

Cette étude expérimentale traite la localisation des déformations sur un grès poreux: le grès de Vosges. Un nombre des essais triaxiaux sont effectués sous des pressions de confinement (i.e., 20 MPa - 190 MPa) et des déformations axiales différentes pour mieux comprendre la réponse mécanique de ce grès. La localisation des déformations a été étudiée dans des différentes échelles en appliquant une variation de mesures de champs (full-field methods) comme la Tomographie Ultrasonore (en 2D), les Emissions Acoustiques (en 3D), les Rayons X (en 3D), et la Corrélation des Images (en 3D). Les méthodes expérimentales ont été appliquées avant, pendant et après les essais triaxiaux. Des coupes fine ont été observées sous le microscope optique et électronique (SEM). La combinaison des multiples techniques expérimentales, qui ont des différentes sensitivités et résolutions, a décrit la procédure de la formation et l évolution des bands de déformation observées sur le grès de Vosges. Des bandes de cisaillement ont été formées sous des pressions intermédiaires et des bandes de compaction sous des pressions élevées. Des bandes de compactions pure n ont pas été observées.Les bandes de déformations observées se sont caractérisées comme des zones de déformation de cisaillement localisée et/ou de compaction. En plus, elles se sont caractérisées comme des structures de fable vitesse ultrasonore, des places d origine des fissures inter- ou intra- granulaires, et des places des densités de matériel élevées.Deux mécanismes principales ont été observées au niveau de grain dans les bandes de cisaillement et de bandes de compaction (shear-enhanced compaction bands): d un cote c est la fissuration des grains (endommagement) et de l autre cote c est la réduction de porosité (sur la forme de compaction). Les deux mécanismes i présent différences sur leurs proportions et leur ordre d occurrence dans le temps.This PhD thesis presents a laboratory study aiming at a better understanding of the stress-strain response of the Vosges sandstone (porous rock) tested at a range of confining pressures (i.e., 20-190 MPa) and different axial strain levels. Localised deformation was captured at different scales by a combination of full-field experimental methods, including Ultrasonic Tomography (2D), Acoustic Emissions (3D), X-ray Tomography (3D), and 3D volumetric Digital Image Correlation, plus thin section and Scanning Electron Microscope observations (2D). These experimental methods were performed before, during and after a number of triaxial compression tests. The combined use of the experimental techniques, which have different sensitivity and resolution, described the processes of shear band and shear-enhanced compaction band generation, which formed at low to intermediate and relatively high confining pressures, respectively. Pure compaction bands were not identified. The deformation bands were characterised as zones of localised shear and/or volumetric strain and were captured by the experimental methods as features of low ultrasonic velocities, places of inter- and intra-granular cracking and structures of higher density material. The two main grain-scale mechanisms: grain breakage (damage) and porosity reduction (compaction) were identified in both shear band and shear-enhanced compaction band formation, which presented differences in the proportions of the mechanism and their order of occurrence in time.SAVOIE-SCD - Bib.électronique (730659901) / SudocGRENOBLE1/INP-Bib.électronique (384210012) / SudocGRENOBLE2/3-Bib.électronique (384219901) / SudocSudocFranceF