Showing papers on "Shear stress published in 2015"

Injection‐induced seismicity: Poroelastic and earthquake nucleation effects

Abstract: The standard model of injection-induced seismicity considers changes in Coulomb strength due solely to changes in pore pressure. We consider two additional effects: full poroelastic coupling of stress and pore pressure, and time-dependent earthquake nucleation. We model stress and pore pressure due to specified injection rate in a homogeneous, poroelastic medium. Stress and pore pressure are used to compute seismicity rate through the Dieterich (1994) model. For constant injection rate, the time to reach a critical seismicity rate scales with t ∼ r2/(cfc), where r is distance from the injector, c is hydraulic diffusivity, and fc is a factor that depends on mechanical properties, and weakly on r. The seismicity rate decays following a peak, consistent with some observations. During injection poroelastic coupling may increase or decrease the seismicity rate, depending on the orientation of the faults relative to the injector. If injection-induced stresses inhibit slip, abrupt shut-in can lead to locally sharp increases in seismicity rate; tapering the flux mitigates this effect. The maximum magnitude event has been observed to occur postinjection. We suggest the seismicity rate at a given magnitude depends on the nucleation rate, the size distribution of fault segments, and if the background shear stress is low, the time-varying volume of perturbed crust. This leads to a rollover in frequency-magnitude distribution for larger events, with a “corner” that increases with time. Larger events are absent at short times, but approach the background frequency with time; larger events occurring post shut-in are thus not unexpected.

Shear-driven failure of liquid-infused surfaces.

Abstract: Rough or patterned surfaces infused with a lubricating liquid display many of the same useful properties as conventional gas-cushioned superhydrophobic surfaces. However, liquid-infused surfaces exhibit a new failure mode: the infused liquid film may drain due to an external shear flow, causing the surface to lose its advantageous properties. We examine shear-driven drainage of liquid-infused surfaces with the goal of understanding and thereby mitigating this failure mode. On patterned surfaces exposed to a known shear stress, we find that a finite length of the surface remains wetted indefinitely, despite the fact that no physical barriers prevent drainage. We develop an analytical model to explain our experimental results, and find that the steady-state retention results from the ability of patterned surfaces to wick wetting liquids, and is thus analogous to capillary rise. We establish the geometric surface parameters governing fluid retention and show how these parameters can describe even random substrate patterns.

Vascular remodeling is governed by a VEGFR3-dependent fluid shear stress set point

Abstract: Vascular remodeling under conditions of growth or exercise, or during recovery from arterial restriction or blockage is essential for health, but mechanisms are poorly understood. It has been proposed that endothelial cells have a preferred level of fluid shear stress, or 'set point', that determines remodeling. We show that human umbilical vein endothelial cells respond optimally within a range of fluid shear stress that approximate physiological shear. Lymphatic endothelial cells, which experience much lower flow in vivo, show similar effects but at lower value of shear stress. VEGFR3 levels, a component of a junctional mechanosensory complex, mediate these differences. Experiments in mice and zebrafish demonstrate that changing levels of VEGFR3/Flt4 modulates aortic lumen diameter consistent with flow-dependent remodeling. These data provide direct evidence for a fluid shear stress set point, identify a mechanism for varying the set point, and demonstrate its relevance to vessel remodeling in vivo.

Unsteady boundary layer flow over a permeable curved stretching/shrinking surface

Abstract: The problem of unsteady viscous flow over a curved stretching/shrinking surface with mass suction is studied. A similarity transformation is used to reduce the system of partial differential equations to an ordinary differential equation. This equation is then solved numerically using the function bvp4c from Matlab for different values of the curvature, mass suction, unsteadiness and stretching/shrinking parameters. The physical quantities of interest, such as reduced skin friction, velocity and shear stress are obtained and discussed as functions of these parameters. Results show that for both cases of stretching and shrinking surfaces, multiple (dual, upper and lower branch) solutions exist for a certain range of curvature, mass suction, unsteadiness and stretching/shrinking parameters. This is an opposite situation than that of the plane stretching sheet. In order to establish which of these solutions are stable and which are not, a stability analysis has been performed. It is evident from the results that the pressure inside the boundary layer cannot be neglected for a curved stretching sheet, as distinct from a flat stretching sheet.

Coalescence of Drops

Abstract: This review examines different stages of the coalescence process of liquid drops on a planar interface under different conditions. Depending on the application, drops coalescence under the influence of applied external shear stress. The focus of this review is on the effect of the viscous stress, Marangoni stress, and electric field stress on the outcome of this process, particularly on the time of coalescence and partial coalescence. Theoretical progress and experiments of this phenomenon are examined, and a future outlook of this area of research is given.

Experimental and Numerical Analysis of the Shear Behaviour of Cemented Concrete–Rock Joints

Abstract: The shear behaviour of cemented concrete–rock joints is a key factor affecting the shear resistance of dam foundations, arch bridge foundations, rock socketed piles and rock bolts in rock engineering. This paper presents an experimental and numerical investigation of the shear behaviour of cemented concrete–rock joints by direct shear tests. In this study we focused on the bond strength of cemented concrete–rock joints, so limestone with smooth surfaces was used for samples preparation to reduce the roughness effect. The experimental results show that the shear strength of joints with good adhesion is strongly dependent on the bond strength of the cohesive interfaces when the applied normal stress is less than 6 MPa. In addition, the sudden and gradual bond failure processes of the cohesive interfaces were observed with an increase of the normal stress. A simple, yet realistic, model of cemented concrete–rock joint is proposed to simulate the observed behaviour, including elastic behaviour of the bond before peak shear stress and post-peak behaviour due to bond failure and friction increase. Finally, the parameters analysis and calibration of the proposed model are presented.

Strain localisation and grain breakage in sand under shearing at high mean stress: insights from in situ X-ray tomography

Abstract: This work presents results from a series of triaxial compression tests on two quartz sands (differing principally in grain shape), at confining pressures high enough to cause grain breakage during shearing. Tests are performed inside an X-ray scanner, which allows specimens to be imaged non-destructively as they deform. Observation of the acquired images clearly shows different mechanisms of deformation, including shearing, dilation, compaction and grain breakage. These mechanisms are investigated quantitatively through 3D measurements of local porosity, as well as strain (obtained by 3D Digital image correlation), which is analysed in terms of volumetric and shear components. These tools allow the transition between macroscopically dilative (typically of a dense sand at low mean stress) and compactive behaviour to be investigated. The analysis reveals that at the high end of the confining pressure range studied (100–7,000 kPa), the more rounded sand deforms with highly localised shear and volumetric strain—the porosity fields show a dilative band within which a compactive region (due to grain crushing) grows. The more angular material shows shear strain localisation; however, its faster transition to compactive behaviour (due to a higher propensity for individual grains to crush) translates to much more distributed compactive volumetric strain.

The importance of entrainment and bulking on debris flow runout modeling: Examples from the Swiss Alps

Abstract: . This study describes an investigation of channel-bed entrainment of sediment by debris flows. An entrainment model, developed using field data from debris flows at the Illgraben catchment, Switzerland, was incorporated into the existing RAMMS debris-flow model, which solves the 2-D shallow-water equations for granular flows. In the entrainment model, an empirical relationship between maximum shear stress and measured erosion is used to determine the maximum potential erosion depth. Additionally, the average rate of erosion, measured at the same field site, is used to constrain the erosion rate. The model predicts plausible erosion values in comparison with field data from highly erosive debris flow events at the Spreitgraben torrent channel, Switzerland in 2010, without any adjustment to the coefficients in the entrainment model. We find that by including bulking due to entrainment (e.g., by channel erosion) in runout models a more realistic flow pattern is produced than in simulations where entrainment is not included. In detail, simulations without entrainment show more lateral outflow from the channel where it has not been observed in the field. Therefore the entrainment model may be especially useful for practical applications such as hazard analysis and mapping, as well as scientific case studies of erosive debris flows.

Following the evolution of hard sphere glasses in infinite dimensions under external perturbations: compression and shear strain.

Abstract: We consider the adiabatic evolution of glassy states under external perturbations. The formalism we use is very general. Here we use it for infinite-dimensional hard spheres where an exact analysis is possible. We consider perturbations of the boundary, i.e., compression or (volume preserving) shear strain, and we compute the response of glassy states to such perturbations: pressure and shear stress. We find that both quantities overshoot before the glass state becomes unstable at a spinodal point where it melts into a liquid (or yields). We also estimate the yield stress of the glass. Finally, we study the stability of the glass basins towards breaking into sub-basins, corresponding to a Gardner transition. We find that close to the dynamical transition, glasses undergo a Gardner transition after an infinitesimal perturbation.

Energy dissipation due to interfacial slip in nanocomposites reinforced with aligned carbon nanotubes.

Abstract: Interfacial slip mechanisms of strain energy dissipation and vibration damping of highly aligned carbon nanotube (CNT) reinforced polymer composites were studied through experimentation and complementary micromechanics modeling. Experimentally, we have developed CNT-polystyrene (PS) composites with a high degree of CNT alignment via a combination of twin-screw extrusion and hot-drawing. The aligned nanocomposites enabled a focused study of the interfacial slip mechanics associated with shear stress concentrations along the CNT-PS interface induced by the elastic mismatch between the filler and matrix. The variation of storage and loss modulus suggests the initiation of the interfacial slip occurs at axial strains as low as 0.028%, primarily due to shear stress concentration along the CNT-PS interface. Through micromechanics modeling and by matching the model with the experimental results at the onset of slip, the interfacial shear strength was evaluated. The model was then used to provide additional insig...

The interplay between anisotropyand strain localisation in granular soils: a multiscale insight

Abstract: This paper presents a multiscale investigation on the interplay among inherent anisotropy, fabric evolution and strain localisation in granular soils, based on a hierarchical multiscale framework with rigorous coupling of the finite-element method (FEM) and discrete-element method (DEM). DEM assemblies with elongated particles are generated to simulate inherent anisotropy and are embedded to the Gauss points of the FEM mesh to derive the required constitutive relation. Specimens preparedwith different bedding plane angles are subjected to biaxial shear under either smooth or rough loading platens. Key factors and physical mechanisms contributing towards the occurrence and development of strain localisation are examined. The competing evolutions of two sources of anisotropy, one related to particle orientations and the other related to contact normals, are found to underpin the development of the shear band. A single band pattern is observed under smooth boundary conditions, and its orientation relative to the bedding plane depends critically on the relative dominance between the two anisotropies. Under rough boundary conditions, the non-coaxial material response and the boundary constraint jointly lead to cross-shaped double shear bands. The multiscale simulations indicate that the DEM assemblies inside the shear band(s) undergo extensive shearing, fabric evolution and particle rotation, and may reach the critical state, while those located outside the shear band(s) experience mild loading followed by unloading. The particle-orientation-based fabric anisotropy needs significantly larger shear and dilation for mobilisation than the contact-normal based one. The asynchrony in evolution of the two fabric anisotropies can cause non-coaxial responses for initially coaxial packings, which directly triggers strain localisation.

Estimation of the bed shear stress in vegetated and bare channels with smooth beds

Abstract: The shear stress at the bed of a channel influences important benthic processes such as sediment transport. Several methods exist to estimate the bed shear stress in bare channels without vegetation, but most of these are not appropriate for vegetated channels due to the impact of vegetation on the velocity profile and turbulence production. This study proposes a new model to estimate the bed shear stress in both vegetated and bare channels with smooth beds. The model, which is supported by measurements, indicates that for both bare and vegetated channels with smooth beds, within a viscous sublayer at the bed, the viscous stress decreases linearly with increasing distance from the bed, resulting in a parabolic velocity profile at the bed. For bare channels, the model describes the velocity profile in the overlap region of the Law of the Wall. For emergent canopies of sufficient density (frontal area per unit canopy volume a≥4.3 m−1), the thickness of the linear-stress layer is set by the stem diameter, leading to a simple estimate for bed shear stress.

Geometrically nonlinear isogeometric analysis of laminated composite plates based on higher-order shear deformation theory

Abstract: In this paper, we present an effectively numerical approach based on isogeometric analysis (IGA) and higher-order shear deformation theory (HSDT) for geometrically nonlinear analysis of laminated composite plates. The HSDT allows us to approximate displacement field that ensures by itself the realistic shear strain energy part without shear correction factors (SCFs). IGA utilizing basis functions namely B-splines or non-uniform rational B-splines (NURBS) enables to satisfy easily the stringent continuity requirement of the HSDT model without any additional variables. The nonlinearity of the plates is formed in the total Lagrange approach based on the small strain assumptions. Numerous numerical validations for the isotropic, orthotropic, cross-ply and angle-ply laminated plates are provided to demonstrate the effectiveness of the proposed method.

An optically transparent membrane supports shear stress studies in a three-dimensional microfluidic neurovascular unit model

Abstract: We report a microfluidic blood-brain barrier model that enables both physiological shear stress and optical transparency throughout the device. Brain endothelial cells grown in an optically transparent membrane-integrated microfluidic device were able to withstand physiological fluid shear stress using a hydrophilized polytetrafluoroethylene nanoporous membrane instead of the more commonly used polyester membrane. A functional three-dimensional microfluidic co-culture model of the neurovascular unit is presented that incorporates astrocytes in a 3D hydrogel and enables physiological shear stress on the membrane-supported endothelial cell layer.

An experimental study on the effects of temperature and magnetic field strength on the magnetorheological fluid stability and MR effect.

Abstract: In this study, the stability and rheological properties of a suspension of carbonyl iron microparticles (CIMs) in silicone oil were investigated within a temperature range of 10 to 85 °C. The effect of adding two hydrophobic (stearic and palmitic) acids on the stability and magnetorheological effect of a suspension of CIMs in silicone oil was studied. According to the results, for preparing a stable and efficient magnetorheological (MR) fluid, additives should be utilized. Therefore, 3 wt% of stearic acid was added to the MR fluid which led to an enhancement of the fluid stability over 92% at 25 °C. By investigating shear stress variation due to the changes in the shear rate for acid-based MR fluids, the maximum yield stress was obtained by fitting the Bingham plastic rheological model at high shear rates. Based on the existing correlations of yield stress and either temperature or magnetic field strength, a new model was fitted to the experimental data to monitor the simultaneous effect of magnetic field strength and temperature on the maximum yield stress. The results demonstrated that as the magnetic field intensified or the temperature decreased, the maximum yield stress increased dramatically. In addition, when the MR fluid reached its magnetic saturation, the viscosity of fluid depended only on the shear rate.

Spatial heterogeneities in tectonic stress in Kyushu, Japan and their relation to a major shear zone

Abstract: We investigated the spatial variation in the stress fields of Kyushu Island, southwestern Japan Kyushu Island is characterized by active volcanoes (Aso, Unzen, Kirishima, and Sakurajima) and a shear zone (western extension of the median tectonic line) Shallow earthquakes frequently occur not only along active faults but also in the central region of the island, which is characterized by active volcanoes We evaluated the focal mechanisms of the shallow earthquakes on Kyushu Island to determine the relative deviatoric stress field Generally, the stress field was estimated by using the method proposed by Hardebeck and Michael (2006) for the strike-slip regime in this area The minimum principal compression stress (σ3), with its near north–south trend, is dominant throughout the entire region However, the σ 3 axes around the shear zone are rotated normal to the zone This result is indicative of shear stress reduction at the zone and is consistent with the right-lateral fault behavior along the zone detected by a strain-rate field analysis with global positioning system data Conversely, the stress field of the normal fault is dominant in the Beppu–Shimabara area, which is located in the central part of the island This result and the direction of σ3 are consistent with the formation of a graben structure in the area

Time‐resolved biofilm deformation measurements using optical coherence tomography

Abstract: The interaction of shear stress with the biofilm leads to a dynamic deformation, which is related to the structural and material characteristics of biofilms. We show how optical coherence tomography can be used as an imaging technique to investigate the time-resolved deformation on the biofilm mesoscale as well as to estimate mechanical properties of the biofilm. For the first time time-resolved deformation from cross-sectional views of the inner biofilm structure could be shown. Changes in the biofilm structure and rheological properties were calculated from cross sections in real-time and time-lapsed measurements. Heterotrophic biofilms were grown in a flow cell set-up at low shear stress of τw = 0.01 Pa. By applying higher shear stress elastic and viscoelastic behavior of biofilms were quantified. Deformation led to a change in biofilm conformation and allowed to estimate rheological properties. Assuming an ideal wall shear stress calculation, the shear modulus G = 29.7 ± 1.7 Pa and the Young's modulus E = 36.0 ± 2.6 Pa were estimated.

Large deformation finite-element modelling of progressive failure leading to spread in sensitive clay slopes

Abstract: The occurrence of large landslides in sensitive clays, such as spreads, can be modelled by progressive development of large inelastic shear deformation zones (shear bands). The main objective of the present study is to perform large deformation finite-element modelling of sensitive clay slopes to simulate progressive failure and dislocation of failed soil mass using the coupled Eulerian Lagrangian (CEL) approach available in Abaqus finite-element software. The degradation of undrained shear strength with plastic shear strain (strain-softening) is implemented in Abaqus CEL, which is then used to simulate the initiation and propagation of shear bands due to river bank erosion. The formation of horsts and grabens and dislocation of soil masses that lead to large-scale landslides are simulated. This finite-element model explains the displacements of different blocks in the failed soil mass and also the remoulding of soil around the shear bands. The main advantages of the present finite-element model over othe...

Modeling injection-induced seismicity with the physics-based earthquake simulator RSQSim

Abstract: Although the phenomenon of earthquakes induced by the subsurface injection of fluids has been recognized, and the basic mechanisms understood, for many decades (e.g., Healy et al. , 1968), the recent increase in seismicity associated with oil and gas development, including large damaging events (e.g., Ellsworth, 2013; Keranen et al. , 2013; Hough, 2014; Rubinstein et al. , 2014) makes clear the need to better understand the processes controlling such seismicity and to develop techniques to mitigate the associated seismic hazard. The relationship of fault stress, fault strength, and fluid pressure at the onset of fault slip in the most basic form is given by the modified Coulomb criterion, ![Graphic][1] (1)in which τ and σ are the shear and normal stress, respectively, acting on the fault surface, P is the pore‐fluid pressure, and μ is the coefficient of fault friction. The term ( σ − P ) is the effective normal stress (Terzaghi, 1925). From equation (1), a fault can be brought to a critical state through an increase of shear stress τ , a decrease of the normal stress σ , an increase of fluid pressure P , or some combination of the three. Increase of pore‐fluid pressure is the most widely cited cause of earthquakes induced by human activities (National Research Council, 2012). Consequently, investigations and models of induced seismicity have tended to focus mainly on spatial changes of fluid pressures (Hsieh and Bredehoeft, 1981; Shapiro and Dinske, 2009). Although the immediate cause of injection‐induced earthquakes is the increase of fluid pressure that brings a fault to a critical stress state, models of the spatial changes of fluid pressure alone are insufficient to either predict or understand the space–time characteristics of induced earthquakes. Comprehensive system‐level models that couple physics‐based simulations of seismicity with reservoir simulations of fluid … [1]: /embed/inline-graphic-1.gif