scholarly journals Anisotropic nature of the capillary stress tensor

2021 ◽  
Vol 249 ◽  
pp. 11010
Author(s):  
Mojtaba Farahnak ◽  
Richard Wan ◽  
Mehdi Pouragha

The paper describes a micromechanical approach that explores the anisotropic nature of the capillary stress tensor and its evolution in pendular granular materials via Discrete Element Modeling (DEM) simulations. Dimensionless parameters are used to address the conditions under which the contribution of capillarity (or cohesive interparticle forces) to the stress transmission within a Representative Elementary Volume (REV) is expected to be considerable. From a series of suction-controlled conventional triaxial tests, numerical results show that the significance of the capillary stress and the relative magnitude of its mean to deviatoric components is directly connected to the characteristic particle size and applied stress. In addition, it is shown that the anisotropic character of the capillary stress tensor intensifies with increasing suction. Furthermore, a simple shear test is conducted at constant mean stress to reveal the development of deviatoric capillary stresses in the absence of any change in mean stress, which cannot be captured by the commonly used Bishop’s stress expression.

2021 ◽  
Author(s):  
Serwaa Omowale ◽  
Andrea Casas ◽  
Yu-Hsuan Lai ◽  
Sarah Sanders ◽  
Ashley Hill ◽  
...  

BACKGROUND Stress has been associated with adverse birth and postpartum health outcomes. Few studies have explored racial differences in maternal stress in a birthing population longitudinally in the United States (U.S.) during the ongoing COVID-19 pandemic. OBJECTIVE This study aimed to: (1) assess changes in reported stress before, during, and after initial emergency declarations (e.g., stay-at-home orders) were in place due to the COVID-19 pandemic and (2) assess Black-White differences in reported stress in a pregnant and postpartum population from Southwestern Pennsylvania. METHODS We leveraged data from the ongoing Postpartum Mothers Mobile Study (PMOMS) which surveys participants in real-time throughout the pregnancy and postpartum periods via ecological momentary assessment (EMA) and smartphone technology. We analyzed data from a subset of PMOMS participants (n=85) who were either Black or White, and who submitted EMA responses regarding stress between November 1, 2019 and August 31, 2020, the timeframe of this study. Data were divided into four phases based on significant events during the COVID-19 pandemic: a pre-phase (baseline), early-phase (first case of COVID-19 reported in U.S.), during-phase (stay-at-home orders), and post-phase (stay-at-home orders eased). We assessed mean stress levels at each phase using mixed-effects models and post-hoc contrasts based on the models. RESULTS Overall mean stress (min: 0, max: 4 as measured by a modified Cohen Perceived Stress Scale) during the pre-phase was 0.8 for Black and White participants [range for Black participants: 0-3.9, range for White participants: 0-2.8]. There was an increase of 0.26 points (t=5.19, d.f.=5649, p<.0001) in the during-phase as compared with the pre-phase, and an increase of 0.19 points (t=3.09, d.f.=5649, p=0.002) in the post-phase compared with the pre-phase (n=85). No difference was found between Black and White participants in the change in mean stress from the pre-phase to the during-phase (δ ̂= -0.016, p=0.867). There was a significant difference between Black and White participants in the change in mean stress from the during-phase to the post-phase (δ ̂ = -0.39, p<0.0001). CONCLUSIONS There was an overall increase in mean stress levels in this subset of pregnant and postpartum participants during the same time as the emergency declarations/stay-at-home orders in the US. Compared to baseline, mean stress levels remained elevated when stay-at-home orders eased. We found no significant difference in the mean stress levels by race. Given that stress is associated with adverse birth outcomes and postpartum health, stress induced by the ongoing COVID-19 pandemic may have adverse implications for birthing populations in the U.S. INTERNATIONAL REGISTERED REPORT RR2-10.2196/13569


2000 ◽  
Vol 627 ◽  
Author(s):  
Dmitri V. Grinev ◽  
Sam F. Edwards

ABSTRACTWe develop the statistical-mechanical theory that delivers the fundamental equations of stress equilibrium for static arrays of rigid grains. The random geometry of static granular packing composed of rigid cohesionless particles can be visualised as a network of intergranular contacts. The contact network and external loading determine the network of intergranular forces. In general, the contact network can have an arbitrary coordination number varying within the system. It follows then that the network of intergranular forces is indeterminate i.e. the number of unknown forces is larger than the number of Newton's equations of mechanical equilibrium. Thus, in order for the network of intergranular forces to be determined, the number of equations must equal the number of unknowns. We argue that this determines the contact network with a certain fixed coordination number. The complete system of equations for the stress tensor is derived from the equations of intergranular force and torque balance, given the geometric specification of the packing. The granular material fabric gives rise to corrections to the Euler-Cauchy equation that become significant at mesoscopic lengthscales. The stress-geometry equation establishes the relation between various components of the stress tensor, and depends on the topology of the granular array.


2009 ◽  
Vol 33 (10) ◽  
pp. 1289-1313 ◽  
Author(s):  
L. Scholtès ◽  
P.-Y. Hicher ◽  
F. Nicot ◽  
B. Chareyre ◽  
F. Darve

SPE Journal ◽  
2016 ◽  
Vol 21 (03) ◽  
pp. 1041-1049 ◽  
Author(s):  
P. H. Winterfeld ◽  
Yu-Shu Wu

Summary For processes such as production from low-permeability reservoirs and storage in subsurface formations, reservoir flow and the reservoir stress field are coupled and affect one another. This paper presents a thermal/hydrological/mechanical (THM) reservoir simulator that is applicable to modeling such processes. The fluid- and heat-flow portion of our simulator is for general multiphase, multicomponent, multiporosity systems. The geomechanical portion consists of an equation for mean stress, derived from linear elastic theory for a thermo-poroelastic system, and equations for stress-tensor components that depend on mean stress and other variables. The integral finite-difference method is used to solve these equations. The mean-stress and reservoir-flow variables are solved implicitly, and the remaining stress-tensor components are solved explicitly. Our simulator is verified by use of analytical solutions for stress- and strain-tensor components and is compared with published results.


2013 ◽  
Vol 247 ◽  
pp. 161-171 ◽  
Author(s):  
Mojtaba Farahnak Langroudi ◽  
Abbas Soroush ◽  
Piltan Tabatabaie Shourijeh ◽  
Roozbeh Shafipour

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