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Showing papers on "Shear stress published in 1999"


Journal ArticleDOI
01 Dec 1999-JAMA
TL;DR: The functional regulation of the endothelium by local hemodynamic shear stress provides a model for understanding the focal propensity of atherosclerosis in the setting of systemic factors and may help guide future therapeutic strategies.
Abstract: Atherosclerosis, the leading cause of death in the developed world and nearly the leading cause in the developing world, is associated with systemic risk factors including hypertension, smoking, hyperlipidemia, and diabetes mellitus, among others. Nonetheless, atherosclerosis remains a geometrically focal disease, preferentially affecting the outer edges of vessel bifurcations. In these predisposed areas, hemodynamic shear stress, the frictional force acting on the endothelial cell surface as a result of blood flow, is weaker than in protected regions. Studies have identified hemodynamic shear stress as an important determinant of endothelial function and phenotype. Arterial-level shear stress (>15 dyne/cm2) induces endothelial quiescence and an atheroprotective gene expression profile, while low shear stress (<4 dyne/cm2), which is prevalent at atherosclerosis-prone sites, stimulates an atherogenic phenotype. The functional regulation of the endothelium by local hemodynamic shear stress provides a model for understanding the focal propensity of atherosclerosis in the setting of systemic factors and may help guide future therapeutic strategies.

3,246 citations


Journal ArticleDOI
TL;DR: In this article, the authors derived the general solution of a wave equation describing the dynamics of two-layer viscoelastic polymer materials of arbitrary thickness deposited on solid (quartz) surfaces in a fluid environment.
Abstract: We have derived the general solution of a wave equation describing the dynamics of two-layer viscoelastic polymer materials of arbitrary thickness deposited on solid (quartz) surfaces in a fluid environment. Within the Voight model of viscoelastic element, we calculate the acoustic response of the system to an applied shear stress, i.e. we find the shift of the quartz generator resonance frequency and of the dissipation factor, and show that it strongly depends on the viscous loading of the adsorbed layers and on the shear storage and loss moduli of the overlayers. These results can readily be applied to quartz crystal acoustical measurements of the viscoelasticity of polymers which conserve their shape under the shear deformations and do not flow, and layered structures such as protein films adsorbed from solution onto the surface of self-assembled monolayers.

1,455 citations


Journal ArticleDOI
TL;DR: In this paper, the authors give an account of the development of the idea of yield stress for solids, soft solids and structured liquids from the beginning of this century to the present time.
Abstract: An account is given of the development of the idea of a yield stress for solids, soft solids and structured liquids from the beginning of this century to the present time. Originally, it was accepted that the yield stress of a solid was essentially the point at which, when the applied stress was increased, the deforming solid first began to show liquid-like behaviour, i.e. continual deformation. In the same way, the yield stress of a structured liquid was originally seen as the point at which, when decreasing the applied stress, solid-like behaviour was first noticed, i.e. no continual deformation. However as time went on, and experimental capabilities increased, it became clear, first for solids and lately for soft solids and structured liquids, that although there is usually a small range of stress over which the mechanical properties change dramatically (an apparent yield stress), these materials nevertheless show slow but continual steady deformation when stressed for a long time below this level, having shown an initial linear elastic response to the applied stress. At the lowest stresses, this creep behaviour for solids, soft solids and structured liquids can be described by a Newtonian-plateau viscosity. As the stress is increased the flow behaviour usually changes into a power-law dependence of steady-state shear rate on shear stress. For structured liquids and soft solids, this behaviour generally gives way to Newtonian behaviour at the highest stresses. For structured liquids this transition from very high (creep) viscosity (>106 Pa.s) to mobile liquid (

950 citations


Journal ArticleDOI
TL;DR: In this paper, a two-dimensional (axially symmetric) numerical hydrodynamical calculations of accretion flows induced by a small anomalous azimuthal shear stress are presented.
Abstract: Two-dimensional (axially symmetric) numerical hydrodynamical calculations of accretion flows that cannot cool through emission of radiation are presented. The calculations begin from an equilibrium configuration consisting of a thick torus with constant specific angular momentum. Accretion is induced by the addition of a small anomalous azimuthal shear stress which is characterized by a function ν. We study the flows generated as the amplitude and form of ν are varied. A spherical polar grid which spans more than two orders of magnitude in radius is used to resolve the flow over a wide range of spatial scales. We find that convection in the inner regions produces significant outward mass motions that carry away both the energy liberated by and a large fraction of the mass participating in the accretion flow. Although the instantaneous structure of the flow is complex and dominated by convective eddies, long-time averages of the dynamical variables show remarkable correspondence to certain steady-state solutions. The two-dimensional structure of the time-averaged flow is marginally stable to the Hoiland criterion, indicating that convection is efficient. Near the equatorial plane, the radial profiles of the time-averaged variables are power laws with an index that depends on the radial scaling of the shear stress. A stress in which ν∝r1/2 recovers the widely studied self-similar solution corresponding to an ‘α-disc’. We find that, regardless of the adiabatic index of the gas, or the form or magnitude of the shear stress, the mass inflow rate is a strongly increasing function of radius, and is everywhere nearly exactly balanced by mass outflow. The net mass accretion rate through the disc is only a fraction of the rate at which mass is supplied to the inflow at large radii, and is given by the local, viscous accretion rate associated with the flow properties near the central object.

420 citations


Journal ArticleDOI
TL;DR: The observed decrease in compressive and shear stiffness for alginate gel after exposure to Na+ was significant and indicated that physiological conditions will soften the gel over a time period of up to 7 days after gelation.
Abstract: The equilibrium and viscoelastic properties of alginate gel crosslinked with Ca2+ were determined as a function of alginate concentration and duration of exposure to physiologic concentrations of NaCl. Compressive and shear stress relaxation tests and oscillatory shear tests were performed to measure the material properties at two time periods after storage in NaCl compared to no NaCl exposure. The effect of concentration was determined by testing 1-3% alginate gel in a bath of physiological NaCl and CaCl2. After 15 h of exposure to NaCl, the compressive, equilibrium shear, and dynamic shear moduli decreased by 63, 84, and 90% of control values, respectively. The material properties exhibited no further changes after 7 days of exposure to NaCl. The loss angle and amplitude of the relaxation function in the shear also decreased, indicating less viscous behaviors in both dynamic and transient configurations. All moduli, but not the loss angle, significantly increased with increasing alginate concentration. The observed decrease in compressive and shear stiffness for alginate gel after exposure to Na+ was significant and indicated that physiological conditions will soften the gel over a time period of up to 7 days after gelation. The alginate gel retains significant solid-like behaviors, however, as measured by a loss angle of approximately 3 degrees. This study provides the first available data for material properties of alginate gel tested in physiological saline.

397 citations


Journal ArticleDOI
TL;DR: In this article, the authors calculate the ideal shear strength of Al and Cu at zero temperature using pseudopotential density functional theory within the local density approximation, which allows for structural relaxation of all five strain components other than the imposed shear strain.
Abstract: The ideal shear strength is the minimum stress needed to plastically deform an infinite dislocation-free crystal and is an upper bound to the strength of a real crystal. We calculate the ideal shear strengths of Al and Cu at zero temperature using pseudopotential density functional theory within the local density approximation. These calculations allow for structural relaxation of all five strain components other than the imposed shear strain and result in strengths on ${111}$ planes of 1.85 and 2.65 GPa for Al and Cu, respectively ( $8%$-- $9%$ of the shear moduli). In both Al and Cu, the structural relaxations reduce the ideal shear strengths by $35%$ to $45%$, but the directions of relaxation strain in each are qualitatively different.

396 citations


Journal ArticleDOI
TL;DR: Examination of topographical patterns of NF-kappaB, Egr-1, c-Jun, and c-Fos activation in a specially designed in vitro disturbed laminar shear stress model indicates that regional differences in blood flow patterns in vivo may represent important local modulators of endothelial gene expression at anatomic sites predisposed for atherosclerotic development.
Abstract: The vascular endothelium is exposed to a spectrum of fluid mechanical forces generated by blood flow; some of these, such as fluid shear stress, can directly modulate endothelial gene expression. Previous work by others and in our laboratory, using an in vitro uniform laminar shear stress model, has identified various shear stress response elements (SSREs) within the promoters of certain endothelial genes that regulate their expression by interacting with various transcription factors, including nuclear factor-kappaB (NF-kappaB), early growth response-1 (Egr-1), and activator protein-1 (AP-1, composed of c-Jun/c-Jun and c-Jun/c-Fos protein dimers). In the current study, we have examined the topographical patterns of NF-kappaB, Egr-1, c-Jun, and c-Fos activation in a specially designed in vitro disturbed laminar shear stress model, which incorporates regions of significant spatial shear stress gradients similar to those found in atherosclerosis-prone arterial geometries in vivo (eg, arterial bifurcations, curvatures, ostial openings). Using newly developed quantitative image analysis techniques, we demonstrate that endothelial cells subjected to disturbed laminar shear stress exhibit increased levels of nuclear localized NF-kappaB, Egr-1, c-Jun, and c-Fos, compared with cells exposed to uniform laminar shear stress or maintained under static conditions. In addition, individual cells display a heterogeneity in responsiveness to disturbed flow, as measured by the amount of NF-kappaB, Egr-1, c-Jun, and c-Fos in their nuclei. This differential regulation of transcription factor expression by disturbed versus uniform laminar shear stress indicates that regional differences in blood flow patterns in vivo-in particular, the occurrence of spatial shear stress gradients-may represent important local modulators of endothelial gene expression at anatomic sites predisposed for atherosclerotic development.

365 citations


Journal ArticleDOI
TL;DR: In this paper, the effects of short term shearing on the subsequent crystallization of polydisperse Ziegler−Natta isotactic polypropylene are observed using in situ optical measurements and ex situ microscopy.
Abstract: The effects of “short term shearing” on the subsequent crystallization of a polydisperse Ziegler−Natta isotactic polypropylene are observed using in situ optical measurements and ex situ microscopy. Imposition of brief intervals of shear (0.25−20 s, less than a thousandth of the quiescent crystallization time) can reduce the crystallization time by 2 orders of magnitude (e.g., at 141 °C with a wall shear stress of 0.06 MPa). With increasing shearing time, the crystallization time saturates and highly anisotropic growth ensues. This transition to oriented growth correlates with changes in the transient behavior during flow and the semicrystalline morphology observed ex situ. During flow, we observe the generation of long-lived, highly oriented structures (evident in the transient birefringence) under all conditions that induce subsequent growth of highly oriented crystallites. In turn, the development of oriented crystallites observed in situ after cessation of flow correlates with development of a “skin-core” morphology (highly oriented skin on a spherulitic core) observed ex situ. Interestingly, the long-lived structures generated during flow appear at shorter times with increasing temperature (at fixed shear stress), the opposite of the trend one would expect on the basis of the temperature dependence of quiescent crystallization.

357 citations


Journal ArticleDOI
TL;DR: In this paper, the effect of radiation on the natural convection flow of an optically dense incompressible fluid along a uniformly heated vertical plate with a uniform suction was analyzed using a series solution for small values of ξ (a scaled streamwise coordinate); an asymptotic solution for large ξ ; and a full numerical solution.

307 citations


Journal ArticleDOI
TL;DR: In this article, an extensive experimental investigation aimed at evaluating the significant parameters that influence the magnitude of this size effect in shear was carried out and it was found that the reduction in sheer stress at failure was related more directly to the maximum spacing between the layers of longitudinal reinforcement rather than the overall member depth.
Abstract: The current American Concrete Institute (ACI) shear design procedures can be very unconservative if applied to large, lightly reinforced members because these procedures do not recognize that as the size of such members increases, the shear stress required to cause failure decreases. This paper describes an extensive experimental investigation aimed at evaluating the significant parameters that influence the magnitude of this size effect in shear. It was found that the reduction in shear stress at failure was related more directly to the maximum spacing between the layers of longitudinal reinforcement rather than the overall member depth. High-strength concrete members displayed a more significant size effect in shear than normal strength concrete members. Some simple modifications to the ACI shear design procedures are suggested that will result in a more consistent level of safety across the possible range of concrete strengths and member sizes.

307 citations


Journal ArticleDOI
TL;DR: In this paper, three analytical models have been developed for rock bolts: one for bolts subjected to a concentrated pull load in pullout tests, another for bolts installed in uniformly deformed rock masses, and one for bolt subjected to the opening of individual rock joints.

Journal ArticleDOI
TL;DR: In this paper, the role of ageing in the rheology of soft materials was studied theoretically and several generalized rheological response functions suited to ageing samples were defined, in which time translation invariance was lost.
Abstract: We study theoretically the role of ageing in the rheology of soft materials. We define several generalized rheological response functions suited to ageing samples (in which time translation invariance is lost). These are then used to study ageing effects within a simple scalar model (the "soft glassy rheology" or SGR model) whose constitutive equations relate shear stress to shear strain among a set of elastic elements, with distributed yield thresholds, undergoing activated dynamics governed by a "noise temperature", $x$. (Between yields, each element follows affinely the applied shear.) For $1

Journal ArticleDOI
TL;DR: In this paper, a series of high-resolution laboratory experiments have been performed on the nucleation of propagating slip failure on preexisting faults having different surface roughness to demonstrate how the size scale and duration of shear rupture nucleation are affected by geometric irregularity of the rupturing surfaces.
Abstract: A series of systematic, high-resolution laboratory experiments have been performed on the nucleation of propagating slip failure on preexisting faults having different surface roughnesses to demonstrate how the size scale and duration of shear rupture nucleation are affected by geometric irregularity of the rupturing surfaces. On the basis of the experimental results it has been discussed theoretically how consistently scale-dependent physical quantities inherent in shear rupture are scaled. The experiments led to conclusive results that the nucleation process consists of two phases (phase I, an initial, quasi-static phase, and phase II, a subsequent accelerating phase) and that the nucleation process is greatly affected by geometric irregularities on the rupturing surfaces. In phase I the rupture grows at a slow, steady speed which is independent of the rupture growth length L. In contrast, during phase II the rupture develops at accelerating speeds V, which increase with an increase in L, obeying a power law V/VS = α(L/λc)n, where VS is the shear wave velocity, λc is the characteristic length representing the geometric irregularity of the fault surfaces, and α and n are constants (α = 8.87 × 10−29 and n = 7.31). Scale dependency of scale-dependent physical quantities, including the nucleation zone size and its duration, is commonly ascribed to scale dependency of the slip-dependent constitutive law parameter Dc, which is in turn governed by λc. It has been discussed that a unified comprehension can be provided for shear rupture of any size scale if the constitutive law for shear rupture is formulated as a slip-dependent law.

Journal ArticleDOI
TL;DR: In this paper, the influence of particle size distribution (PSD) and interparticle friction μp on the nature of deformation in granular fault gouge was examined by constructing assemblages of particles with self-similar size distributions defined by the two-dimensional power law exponent.
Abstract: Two-dimensional numerical simulations were conducted using the distinct element method (DEM) to examine the influences of particle size distribution (PSD) and interparticle friction μp on the nature of deformation in granular fault gouge. Particle fracture was not allowed in this implementation but points in PSD space were examined by constructing assemblages of particles with self-similar size distributions defined by the two-dimensional power law exponent D. For these numerical “experiments,” D ranged from 0.81 to 2.60, where D=1.60 represents the two-dimensional equivalent of a characteristic PSD to which cataclastically deforming gouge is thought to evolve. Experiments presented here used μp values of 0.10 and 0.50 and were conducted using normal stress σn on the shear zone walls of 70 MPa. Shear strain within these simulated assemblages was accommodated by intermittent displacement along discrete slip surfaces, alternating between broadly distributed deformation along multiple slip planes and highly localized deformation along a single, sharply defined, subhorizontal zone of slip. Slip planes corresponded in orientation and sense of shear to shear structures observed in natural gouge zones, specifically Riedel and Y shears; the oblique Riedel shears showed more extreme orientations than typical, but their geometries were consistent with those predicted for low-strength Coulomb materials. The character of deformation in the shear zone varied with PSD due to changes in the relative importance of interparticle slip and rolling as deformation mechanisms. A high degree of frictional coupling between large rolling particles in low D (coarse-grained) assemblages resulted in wide zones of slip and broadly distributed deformation. In higher D assemblages (D >= 1.60), small rolling particles self-organized into columns that separated large rolling particles, causing a reduction in frictional resistance within the deforming assemblage. This unusual particle configuration appears to depend on a critical abundance of small particles achieved at D ≈ 1.60 and may enable strain localization in both real and simulated granular assemblages.

Journal ArticleDOI
TL;DR: In this article, a new laboratory technique for coupled shear-flow tests of rock joints was developed and used to investigate the coupled effect of joint shear deformation and dilatancy on hydraulic conductivity of the rock joints.

Journal ArticleDOI
TL;DR: In this paper, the authors studied the friction of simulated fault gouge as a function of normal stress (σn = 25 to 70 MPa) and load point velocity (V = 0.001 to 10 mm/s).
Abstract: During earthquake rupture, faults slip at velocities of cm/s to m/s. Fault friction at these velocities strongly influences dynamic rupture but is at present poorly constrained. We study friction of simulated fault gouge as a function of normal stress (σn = 25 to 70 MPa) and load point velocity (V = 0.001 to 10 mm/s). Layers of granular quartz (3 mm thick) are sheared between rough surfaces in a direct shear apparatus at ambient conditions. For a constant σn, we impose regular step changes in V throughout 20 mm net slip and monitor the frictional response. A striking observation at high velocity is a dramatic reduction in the instantaneous change in frictional strength for a step change in velocity (friction direct effect) with accumulated slip. Gouge layers dilate for a step increase in velocity, and the amount of dilation decreases with slip and is systematically greater at higher velocity. The steady state friction velocity dependence (a-b) evolves from strengthening to weakening with slip but is not significantly influenced by V or σn. Measurements of dilation imply that an additional mechanism, such as grain rolling, operates at high velocity and that the active shear zone narrows with slip. Data from slow (μm/s) and fast (mm/s) tests indicate a similar displacement dependent textural evolution and comparable comminution rates. Our experiments produce a distinct shear localization fabric and velocity weakening behavior despite limited net displacements and negligible shear heating. Under these conditions we find no evidence for the strong velocity weakening or low friction values predicted by some theoretical models of dynamic rupture. Thus certain mechanisms for strong frictional weakening, such as grain rolling, can likely be ruled out for the conditions of our study.

Journal ArticleDOI
TL;DR: In this article, a large eddy simulation (LES) code was developed and applied to study the effect of spatially variable surface properties on the areally averaged surface shear stress at the land-atmosphere interface.
Abstract: A large eddy simulation (LES) code of the atmospheric boundary layer (ABL) has been developed and applied to study the effect of spatially variable surface properties on the areally averaged surface shear stress at the land-atmosphere interface. The LES code simulates the space and time evolution of the large-scale turbulent eddies and their transport effects in the ABL. We report here on simulations of flow over spatially variable roughness fields. The dynamics are simulated, and the resulting space-time fields are averaged to explore the effects of the surface variability length scales on the average surface shear stress, as used in large-scale models to estimate scalar fluxes, such as evaporation. We observe asymmetrical response of the smooth-to-rough and rough-to- smooth transitions, such that the effects of the transitions accumulate rather than cancel. It is shown that the presence of abrupt changes in surface roughness and the atmosphere's response to these patches create a marked dependence of the statistical structure of surface shear stress on the length scale of the surface patches. An increase in regionally averaged surface stress for decreasing horizontal patch length scale is found.

Journal ArticleDOI
TL;DR: The results obtained showed that smooth muscle contraction affects the residual strains which exist in a ring segment cut out from the artery and exposed to no external load, and basal muscular tone reduces the strain gradient in the arterial wall and yields a near uniform stress distribution.
Abstract: To study the effects of smooth muscle contraction and relaxation on the strain and stress distribution in the vascular wall, a mathematical model was proposed. The artery was assumed to be a thick-walled orthotropic tube made of nonlinear, incompressible elastic material. Considering that the contraction of smooth muscle generates an active circumferential stress in the wall, a numerical study was performed using data available in the literature. The results obtained showed that smooth muscle contraction affects the residual strains which exist in a ring segment cut out from the artery and exposed to no external load. When the ring specimen is cut radially, it springs open with an opening angle. The predicted monotonic increase of the opening angle with increasing muscular tone was in agreement with recent experimental results reported in the literature. It was shown that basal muscular tone, which exists under physiological conditions, reduces the strain gradient in the arterial wall and yields a near uniform stress distribution. During temporary changes in blood pressure, the increase in muscular tone induced by elevated pressure tends to restore the distribution of circumferential strain in the arterial wall, and to maintain the flow-induced wall shear stress to normal level.

Journal ArticleDOI
TL;DR: Three well-defined laminar flow profiles were created to distinguish the influence of a gradient in shear and steady shear on platelet-derived growth factor A (PDGF-A) and monocyte chemoattractant protein-1 (MCP-1) expression in human endothelial cells, and suggest distinct roles of temporal gradient inShear and Steady shear in atherogenesis in vivo.
Abstract: —Three well-defined laminar flow profiles were created to distinguish the influence of a gradient in shear and steady shear on platelet-derived growth factor A (PDGF-A) and monocyte chemoattractant protein-1 (MCP-1) expression in human endothelial cells. The flow profiles (16 dyne/cm 2 maximum shear stress) were ramp flow (shear stress smoothly transited at flow onset), step flow (shear stress abruptly applied at flow onset), and impulse flow (shear stress abruptly applied for 3 s only). Ramp flow induced only minor expression of PDGF-A and did not increase MCP-1 expression. Step flow increased PDGF-A and MCP-1 mRNA levels 3- and 2-fold at 1.5 hours, respectively, relative to ramp flow. In contrast, impulse flow increased PDGF-A and MCP-1 expression 6- and 7-fold at 1.5 hours, and these high levels were sustained for at least 4 hours. These results indicate that a temporal gradient in shear (impulse flow and the onset of step flow) and steady shear (ramp flow and the steady component of step flow) stimulates and diminishes the expression of PDGF-A and MCP-1, respectively. NO synthase inhibitor N G -amino-l-arginine (L-NAA) was found to markedly enhance MCP-1 and PDGF-A expression induced by step flow, but decrease their expression induced by impulse flow, in a dose-dependent manner. NO donor spermine-NONOate (SPR/NO) dose-dependently reduced the MCP-1 and PDGF-A expression induced by impulse flow. Moreover, impulse flow was found to stimulate sustained (4 hours) IκB-α degradation and egr -1 mRNA induction. L-NAA prevented IκB-α degradation, whereas SPR/NO increased IκB-α resynthesis 2 hours after impulse flow. Both L-NAA and SPR/NO inhibited the impulse flow inducibility of egr -1 4 hours after the flow stimulation. The results show that both NO induced by steady shear and NO donor inhibit temporal gradient in shear-induced MCP-1 and PDGF-A expression by downregulation of their respective transcription factors NFκB and egr -1, whereas NO induced by impulse flow stimulates MCP-1 and PDGF-A expression by upregulation of the transcription factors. The above findings suggest distinct roles of temporal gradient in shear and steady shear in atherogenesis in vivo.

Journal ArticleDOI
TL;DR: The partial disassembly of adherens junctions the authors have observed during shear induced changes in endothelial cell shape may have important implications for control of the endothelial permeability barrier and other aspects of endothelialcell function.
Abstract: Endothelial cells exhibit profound changes in cell shape in response to altered shear stress that may require disassembly/reassembly of adherens junction protein complexes that mediate cell-cell adhesion. To test this hypothesis, we exposed confluent porcine aortic endothelial cells to 15 dyne/cm(2) of shear stress for 0, 8.5, 24, or 48 hours, using a parallel plate flow chamber. Cells were fixed and stained with antibodies to vascular endothelial (VE) cadherin, alpha-catenin, beta-catenin, or plakoglobin. Under static conditions, staining for all proteins was intense and peripheral, forming a nearly continuous band around the cells at cell-cell junctions. After 8.5 hours of shear stress, staining was punctate and occurred only at sites of continuous cell attachment. After 24 or 48 hours of shear, staining for VE-cadherin, alpha-catenin, and beta-catenin was intense and peripheral, forming a band of "dashes" (adherens plaques) that colocalized with the ends of stress fibers that inserted along the lateral membranes of cells. Staining for plakoglobin was not observed after 24 hours of shear stress, but returned after 48 hours. Western blot analysis indicated that protein levels of VE-cadherin, alpha-catenin, and plakoglobin decreased, whereas beta-catenin levels increased after 8.5 hours of shear stress. As cell shape change reached completion (24 to 48 hours), all protein levels were upregulated except for plakoglobin, which remained below control levels. The partial disassembly of adherens junctions we have observed during shear induced changes in endothelial cell shape may have important implications for control of the endothelial permeability barrier and other aspects of endothelial cell function.

Journal ArticleDOI
TL;DR: In this paper, a substantially augmented data set of earthquakes for which can be estimated, mostly from in situ stress measurements, for comparison with laboratory stick-slip friction events led to the hypothesis that.
Abstract: Apparent stress τa is defined as , where is the average shear stress loading the fault plane to cause slip and η is the seismic efficiency, defined as Ea/ W, where Ea is the energy radiated seismically and W is the total energy released by the earthquake. The results of a recent study in which apparent stresses of mining-induced earthquakes were compared to those measured for laboratory stick-slip friction events led to the hypothesis that . This hypothesis is tested here against a substantially augmented data set of earthquakes for which can be estimated, mostly from in situ stress measurements, for comparison with τa. The expanded data set, which includes earthquakes artificially triggered at a depth of 9 km in the German Kontinentales Tiefbohrprogramm der Bundesrepublik Deutschland (KTB) borehole and natural tectonic earthquakes, covers a broad range of hypocentral depths, rock types, pore pressures, and tectonic settings. Nonetheless, over ∼14 orders of magnitude in seismic moment, apparent stresses exhibit distinct upper bounds defined by a maximum seismic efficiency of ∼0.06, consistent with the hypothesis proposed before. This behavior of τa and η can be expressed in terms of two parameters measured for stick-slip friction events in the laboratory: the ratio of the static to the dynamic coefficient of friction and the fault slip overshoot. Typical values for these two parameters yield seismic efficiencies of ∼0.06. In contrast to efficiencies for laboratory events for which η is always near 0.06, those for earthquakes tend to be less than this bounding value because Ea for earthquakes is usually underestimated due to factors such as band-limited recording. Thus upper bounds on appear to be controlled by just a few fundamental aspects of frictional stick-slip behavior that are common to shallow earthquakes everywhere. Estimates of from measurements of τa for suites of earthquakes, using , are found to be comparable in magnitude to estimates of shear stress on the basis of extrapolating in situ stress data to seismogenic depths.

Journal ArticleDOI
12 Feb 1999-Science
TL;DR: Atomistic simulations show that dislocations can move faster than the speed of sound if they are created as supersonic dislocation at a strong stress concentration and are subjected to high shear stresses.
Abstract: It is thought that dislocations cannot surpass the sound barrier at the shear wave velocity because the energy spent in radiation has a singularity there. Atomistic simulations show that dislocations can move faster than the speed of sound if they are created as supersonic dislocations at a strong stress concentration and are subjected to high shear stresses. This behavior is important for the understanding of low-temperature deformation processes such as mechanical twinning and may be relevant for the dynamics of tectonic faults. The motion of the dislocations at a speed of 2 times the shear wave velocity can be understood from a linear elastic analysis, but many of the peculiarities of the supersonic dislocations are dominated by nonlinear effects that require a realistic atomistic description.

Journal ArticleDOI
TL;DR: The data successfully predict the threshold wall shear stress below which rolling does not occur, which is a special case of the more general regulation by shear of the number of bonds, in which theNumber of bonds falls below one.
Abstract: Wall shear stress in postcapillary venules varies widely within and between tissues and in response to inflammation and exercise. However, the speed at which leukocytes roll in vivo has been shown to be almost constant within a wide range of wall shear stress, i.e., force on the cell. Similarly, rolling velocities on purified selectins and their ligands in vitro tend to plateau. This may be important to enable rolling leukocytes to be exposed uniformly to activating stimuli on endothelium, independent of local hemodynamic conditions. Wall shear stress increases the rate of dissociation of individual selectin–ligand tether bonds exponentially (1, 4) thereby destabilizing rolling. We find that this is compensated by a shear-dependent increase in the number of bonds per rolling step. We also find an increase in the number of microvillous tethers to the substrate. This explains (a) the lack of firm adhesion through selectins at low shear stress or high ligand density, and (b) the stability of rolling on selectins to wide variation in wall shear stress and ligand density, in contrast to rolling on antibodies (14). Furthermore, our data successfully predict the threshold wall shear stress below which rolling does not occur. This is a special case of the more general regulation by shear of the number of bonds, in which the number of bonds falls below one.

Journal ArticleDOI
TL;DR: The Cohesive Strength Meter (CSM) as discussed by the authors is a second-generation erosion instrument designed to measure the critical erosion shear stress (τoCr) of intertidal sediments in situ.
Abstract: The shear resistance of muddy sediments directly governs the susceptibility of the sediment to erosion by tidal and wave induced currents. Measurements of the natural erosion shear stress are important in modelling estuarine systems and in determining the possible impact of human disturbance. A second-generation erosion instrument the Cohesive Strength Meter (CSM) designed to measure the critical erosion shear stress (τoCr) of intertidal sediments in situ is described. The CSM allows measurements of small-scale spatial and temporal variation of sediment stability to be made on a scale and rapidity previously unachievable. New features of specialized hardware are described and the instrument is empirically calibrated in terms of an equivalent horizontal bed shear stress using quartz sand. Some example results from recent field trials on the Konigshafen (Sylt-Romo Bight) are presented. The erosion characteristics of this intertidal flat varied on a cm scale due to the presence of patchy diatom biofilms. Areas with a diatom biofilm were more stable than those without, giving a biostabilization index of 6·2.

Journal ArticleDOI
TL;DR: In this paper, a finite-beam element for modeling the shear behavior and its interaction with the axial force and the bending moment in reinforced concrete (RC) beams and columns is presented.
Abstract: After a few years of successful application of the fiber beam element to the analysis of reinforced concrete (RC) frames, the introduction of the mechanisms of shear deformation and strength appears to be the next necessary step toward a realistic description of the ultimate behavior of shear sensitive structures This paper presents a new finite-beam element for modeling the shear behavior and its interaction with the axial force and the bending moment in RC beams and columns This new element, based on the fiber section discretization, shares many features with the traditional fiber beam element to which it reduces, as a limit case, when the shear forces are negligible The element basic concept is to model the shear mechanism at each concrete fiber of the cross sections, assuming the strain field of the section as given by the superposition of the classical plane section hypothesis for the longitudinal strain field with an assigned distribution over the cross section for the shear strain field Transve

Journal ArticleDOI
TL;DR: Shear stress-induced cell alignment and stress fiber formation were inhibited by the dominant-negative mutants of Rho and p160ROCK, but not by the Dominant-negative mutant of Cdc42, indicating that the Rho-p160R ROCK pathway regulates the cytoskeletal reorganization in response to shear stress.
Abstract: Shear stress, the tangential component of hemodynamic forces, plays an important role in endothelial remodeling. In this study, we investigated the role of Rho family GTPases Cdc42 and Rho in shear stress-induced signal transduction and cytoskeleton reorganization. Our results showed that shear stress induced the translocation of Cdc42 and Rho from cytosol to membrane. Although both Cdc42 and Rho were involved in the shear stress-induced transcription factor AP-1 acting on the 12-O-tetradecanoyl-13-phorbol-acetate-responsive element (TRE), only Cdc42 was sufficient to activate AP-1/TRE. Dominant-negative mutants of Cdc42 and Rho, as well as recombinant C3 exoenzyme, attenuated the shear stress activation of c-Jun NH2-terminal kinases (JNKs), suggesting that Cdc42 and Rho regulate the shear stress induction of AP-1/TRE activity through JNKs. Shear stress-induced cell alignment and stress fiber formation were inhibited by the dominant-negative mutants of Rho and p160ROCK, but not by the dominant-negative mutant of Cdc42, indicating that the Rho-p160ROCK pathway regulates the cytoskeletal reorganization in response to shear stress.

Journal ArticleDOI
TL;DR: In this paper, the influence of mixing shear rate on the rheological properties of cement paste was investigated, and the extent of structural breakdown within the paste was evaluated as a function of mixer type and speed, based on hysteresis loop area.

Journal ArticleDOI
02 Sep 1999-Chaos
TL;DR: A model with no local simplifying force rules is introduced, that allows for an exact computation of contact forces for given granular texture and boundary conditions and shows that the forces below the mean do not contribute to the shear stress.
Abstract: We present the results of a systematic numerical investigation of force distributions in granular packings. We find that all the main features of force transmission previously established for two-dimensional systems of hard particles hold in three-dimensional systems and for soft particles, too. In particular, the probability distribution of normal forces falls off exponentially for forces above the mean force. For forces below the mean, this distribution is either a decreasing power law when the system is far from static equilibrium, or nearly uniform at static equilibrium, in agreement with recent experiments. Moreover, we show that the forces below the mean do not contribute to the shear stress. The subnetwork of the contacts carrying a force below the mean thus plays a role similar to a fluid surrounding the solid backbone composed of the contacts carrying a force above the mean. We address the issue of the computation of contact forces in a packing at static equilibrium. We introduce a model with no local simplifying force rules, that allows for an exact computation of contact forces for given granular texture and boundary conditions.

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TL;DR: In this paper, the effects of shear on the potential energy landscape of ductile glasses were examined with respect to the mechanical properties of ductiles. And the authors found that shear strain can cause the disappearance of local potential energy minima, rendering the system mechanically unstable and forcing the system to move to alternate local minima.
Abstract: The effects of shear on the potential energy landscape are examined with regard to the mechanical properties of ductile glasses. Shear strain is found to cause the disappearance of local potential energy minima, rendering the system mechanically unstable and forcing the system to move to alternate local minima. These mechanical instabilities are characterized by the decrease to zero of several features of the potential energy landscape: a barrier height, the curvature at both the barrier and the local minimum, and the distance from the barrier to the local minimum. From an analysis of the results of different size systems, the mechanical instabilities are shown to be localized to a small number of atoms. The consequences of the mechanical instabilities on the mechanical properties are stress drops which give rise to the yielding and plastic-flow behavior characteristic of ductile glasses; these stress drops are discontinuous at the molecular level, but become continuous at the macroscopic level. A simple ...

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TL;DR: A new model is used to analyze the fully coupled problem of pulsatile blood flow through a compliant, axisymmetric stenotic artery using the finite element method, and pressure drop results are found to compare well to an experimentally based theoretical curve, despite the assumption of laminar flow.
Abstract: A new model is used to analyze the fully coupled problem of pulsatile blood flow through a compliant, axisymmetric stenotic artery using the finite element method. The model uses large displacement and large strain theory for the solid, and the full Navier-Stokes equations for the fluid. The effect of increasing area reduction on fluid dynamic and structural stresses is presented. Results show that pressure drop, peak wall shear stress, and maximum principal stress in the lesion all increase dramatically as the area reduction in the stenosis is increased from 51 to 89 percent. Further reductions in stenosis cross-sectional area, however, produce relatively little additional change in these parameters due to a concomitant reduction in flow rate caused by the losses in the constriction. Inner wall hoop stretch amplitude just distal to the stenosis also increases with increasing stenosis severity, as downstream pressures are reduced to a physiological minimum. The contraction of the artery distal to the stenosis generates a significant compressive stress on the downstream shoulder of the lesion. Dynamic narrowing of the stenosis is also seen, further augmenting area constriction at times of peak flow. Pressure drop results are found to compare well to an experimentally based theoretical curve, despite the assumption of laminar flow.