Showing papers on "Shear stress published in 2001"

Shear coefficients for Timoshenko beam theory

Abstract: The Timoshenko beam theory includes the effects of shear deformation and rotary inertia on the vibrations of slender beams. The theory contains a shear coefficient which has been the subject of much previous research. In this paper a new formula for the shear coefficient is derived. For a circular cross section, the resulting shear coefficient that is derived is in full agreement with the value most authors have considered best. Shear coefficients for a number of different cross sections are found.

Erodibility of cohesive streambeds in the loess area of the midwestern USA

Abstract: Excess stress parameters, critical shear stress (τ c ) and erodibility coefficient (k d ), for degrading channels in the loess areas of the midwestern USA are presented based on in situ jet-testing measurements. Critical shear stress and k d are used to define the erosion resistance of the streambed, The jet-testing apparatus applies hydraulic stresses to the bed and the resulting scour due to the impinging jet is related to the excess stress parameters. Streams tested were primarily silt-bedded in texture with low densities, which is typical of loess soils. Results indicate that there is a wide variation in the erosion resistance of streambeds, spanning six orders of magnitude for τ c and four orders of magnitude for k d . Erosion resistance was observed to vary within a streambed, from streambed to streambed, and from region to region. An example of the diversity of materials within a river system is the Yalobusha River Basin in Mississippi. The median value of τ c for the two primary bed materials, Naheola and Porters Creek Clay Formations, was 1.31 and 256 Pa, respectively. Streambeds composed of the Naheola Formation are readily eroded over the entire range of shear stresses, whereas only the deepest flows generate boundary stresses great enough to erode streambeds composed of the Porters Creek Clay Formation. Therefore, assessing material resistance and location is essential in classifying and modelling streambed erosion processes of these streams.

Structure Development during Shear Flow Induced Crystallization of i-PP: In Situ Wide-Angle X-ray Diffraction Study

Abstract: In situ synchrotron wide-angle X-ray diffraction (WAXD) was used to monitor crystallization of isotactic polypropylene (i-PP) in the subcooled melt at 140 °C after step shear. The melt was subjected to a shear strain of 1430% at three different shear rates (10, 57, and 102 s-1) using a parallel-plate shear apparatus. WAXD results were used to determine the type (α- and β-crystals), orientation, and corresponding mass fractions of i-PP crystals. It was found that formation of oriented α-crystals occurred immediately after application of the shear field. Subsequently, growth of primarily unoriented β-crystals was observed. WAXD patterns clearly showed that β-crystals grew only after the formation of oriented α-crystals in the sheared i-PP melt. The contribution of β-crystals to the total crystalline phase was as high as 65−70% at high shear rates (57 and 102 s-1) and low (20%) at low shear rates (10 s-1), which was attributed to the different amount of surface area of oriented α-crystal cylindrites generate...

Endothelial cellular response to altered shear stress

Abstract: Endothelial cells are normally exposed constantly to mechanical forces that significantly influence their phenotype. This symposium presented recent information concerning endothelial cell responses to shear stress associated with blood flow. Endothelial cell shear stress mechanosensors that have been proposed include membrane receptor kinases, integrins, G proteins, ion channels, intercellular junction proteins, membrane lipids (e.g., those associated with caveolae), and the cytoskeleton. These sensors are linked to signaling cascades that interact with or result in generation of reactive oxygen species, nitric oxide, and various transcription factors among other responses. Endothelial cells adapt to sustained shear stress, and either an increase or decrease from normal shear leads to signaling events. In vitro models for the study of endothelial cell responses must consider the pattern of shear stress (e.g., steady vs. oscillatory flow), the scaffold for cell growth (e.g., basement membrane or other cell types such as smooth muscle cells), and the extent of flow adaptation. These cellular responses have major relevance for understanding the pathophysiological effects of increased shear stress associated with hypertension or decreased shear stress associated with thrombotic occlusion.

New views of granular mass flows

Abstract: Concentrated grain-fluid mixtures in rock avalanches, debris flows, and pyroclastic flows do not behave as simple materials with fixed rheologies. Instead, rheology evolves as mixture agitation, grain concentration, and fluid-pressure change during flow initiation, transit, and deposition. Throughout a flow, however, normal forces on planes parallel to the free upper surface approximately balance the weight of the superincumbent mixture, and the Coulomb friction rule describes bulk intergranular shear stresses on such planes. Pore-fluid pressure can temporarily or locally enhance mixture mobility by reducing Coulomb friction and transferring shear stress to the fluid phase. Initial conditions, boundary conditions, and grain comminution and sorting can influence pore-fluid pressures and cause variations in flow dynamics and deposits.

Crustal stress field in southern California and its implications for fault mechanics

Abstract: We present a new, high spatial resolution image of stress orientation in southern California based on the inversion of earthquake focal mechanisms. We use this image to study the mechanics of faulting in the plate boundary region. The stress field contains significant spatial heterogeneity, which in some cases appears to be a result of the complexity of faulting and in other cases appears to be a cause. Temporal changes in the stress field are also observed, primarily related to major earthquakes. The observed 15° (±10°) rotation of the stress axes due to the 1992 M7.3 Landers mainshock implies that the deviatoric stress magnitude in the crust is low, of the order of 10 MPa. This suggests that active faults in southern California are weak. The maximum principal stress axis near the San Andreas Fault is often at ∼50° to the fault strike, indicating that the shear stress on the fault is comparable to the deviatoric stress. The San Andreas in southern California may therefore be a weak fault in a low-strength crust.

Comparison of Brain Responses Between Frontal and Lateral Impacts by Finite Element Modeling

Abstract: This study was conducted to investigate differences in brain response due to frontal and lateral impacts based on a partially validated three-dimensional finite element model with all essential anatomical features of a human head. Identical impact and boundary conditions were used for both the frontal and lateral impact simulations. Intracranial pressure and localized shear stress distributions predicted from these impacts were analyzed. The model predicted higher positive pressures accompanied by a relatively large localized skull deformation at the impact site from a lateral impact when compared to a frontal impact. Lateral impact also induced higher localized shear stress in the core regions of the brain. Preliminary results of the simulation suggest that skull deformation and internal partitions may be responsible for the directional sensitivity of the head in terms of intracranial pressure and shear stress response. In previous experimental studies using subhuman primates, it was found that a lateral impact was more injurious than a frontal impact. In this study, shear stress in the brain predicted by the model was much higher in a lateral impact in comparison with a frontal impact of the same severity. If shear deformation is considered as an injury indicator for diffuse brain injuries, a higher shear stress due to a lateral impact indicate that the head would tend to have a decreased tolerance to shear deformation in lateral impact. More research is needed to further quantify the effect of the skull deformation and dural partitions on brain injury due to impacts from a variety of directions and at different locations.

Conceptual and physical clarification of rate and state friction: Frictional sliding as a thermally activated rheology

Abstract: We observed slow frictional slip occurring at a constant shear stress below the nominal friction level and compared it with the time-dependent strengthening of the frictional interface, which was also tracked experimentally. It was found that slip velocity decreases as the interface strengthens due to aging, while it increases with the applied shear stress. These dependencies were both exponential and were of similar magnitudes, as implied by the framework law of rate- and state-dependent friction. In the spirit of the adhesion theory of friction the dependence of slip velocity on interface strength is understood to be the result of the change of the shear stress acting on frictional junctions due to the change of junction population, though the observed dependence was somewhat stronger than a simple model based on this idea predicts. By correcting the observed slip velocity for the effect of the change of the interface strength, we could obtain a unique relationship between stress and slip velocity, which may be readily compared with a standard rheological formulation. Thus the obtained relationship between stress and slip velocity showed a reasonable agreement with the absolute rate theory over a temperature range of 25-800°C for the present experimental condition (fine albite powder, 20 MPa normal stress, no pore water).

Force Distributions near Jamming and Glass Transitions

Abstract: We calculate the distribution of interparticle normal forces P(F) near the glass and jamming transitions in model supercooled liquids and foams, respectively. P(F) develops a peak that appears near the glass or jamming transitions, whose height increases with decreasing temperature, decreasing shear stress and increasing packing density. A similar shape of P(F) was observed in experiments on static granular packings. We propose that the appearance of this peak signals the development of a yield stress. The sensitivity of the peak to temperature, shear stress, and density lends credence to the recently proposed generalized jamming phase diagram.

Factors Influencing Blood Flow Patterns in the Human Right Coronary Artery

Abstract: Evidence suggests that atherogenesis is linked to local hemodynamic factors such as wall shear stress. We investigated the velocity and wall shear stress patterns within a human right coronary artery (RCA), an important site of atherosclerotic lesion development. Emphasis was placed on evaluating the effect of flow waveform and inlet flow velocity profile on the hemodynamics in the proximal, medial, and distal arterial regions. Using the finite-element method, velocity and wall shear stress patterns in a rigid, anatomically realistic model of a human RCA were computed. Steady flow simulations (ReD=500) were performed with three different inlet velocity profiles; pulsatile flow simulations utilized two different flow waveforms (both with Womersley parameter=1.82, mean ReD=233), as well as two of the three inlet profiles. Velocity profiles showed Dean-like secondary flow features that were remarkably sensitive to the local curvature of the RCA model. Particularly noteworthy was the "rotation" of these Dean-like profiles, which produced large local variations in wall shear stress along the sidewalls of the RCA model. Changes in the inlet velocity profiles did not produce significant changes in the arterial velocity and wall shear stress patterns. Pulsatile flow simulations exhibited remarkably similar cycle-average wall shear stress distributions regardless of waveform and inlet velocity profile. The oscillatory shear index was very small and was attributed to flow reversal in the waveform, rather than separation. Cumulatively, these results illustrate that geometric effects (particularly local three-dimensional curvature) dominate RCA hemodynamics, implying that studies attempting to link hemodynamics with atherogenesis should replicate the patient-specific RCA geometry.

Shear response of layered silicate nanocomposites

Abstract: The linear and nonlinear melt state viscoelastic properties for a series of layered silicate based intercalated polymer nanocomposites are studied to elucidate the role of highly anisotropic nanometer thick layers in altering the flow properties of such hybrids. The steady shear viscosities for the nanocomposites exhibit enhanced shear-thinning at all shear rates, with the viscosity at high shear rates being almost independent of silicate loading and comparable to that of the unfilled polymer. Further, the elasticity, as measured by the first normal stress difference, when compared at constant shear stress is surprisingly independent of the silicate loading and identical to that of the unfilled polymer. This unique combination of unfilled polymerlike viscosity and elasticity for these filled nanocomposites, is attributed to the ability of the highly-anisotropic layered silicates to be oriented in the flow direction and results in a minimal contribution by the silicate layers to both the viscosity and the ...

Shear level influences resistance artery remodeling: wall dimensions, cell density, and eNOS expression.

Abstract: The magnitude of shear stimulus has been shown to determine the level of growth factor expression in cell culture. However, little is known regarding what effect shear level has on specific arteria...

Derivation of plastic stress–strain relationship from ball indentations: Examination of strain definition and pileup effect

Abstract: The ball indentation technique has the potential to be an excellent substitute for a standard tensile test, especially in the case of small specimens or property-gradient materials such as welds. In our study, the true stress–true strain relationships of steels with different work-hardening exponents (0.1–0.3) were derived from ball indentations. Four kinds of strain definitions in indentation were attempted: 0.2sinγ, 0.4hc/a, ln[2/(1 + cosγ)], and 0.1tanγ. Here, γ is the contact angle between the indenter and the specimen, hc is the contact depth, and a is the contact radius. Through comparison with the standard data measured by uniaxial tensile testing, the best strain definition was determined to be 0.1tanγ. This new definition of strain, in which tanγ means the shear strain at contact edge, reflected effectively the work-hardening characteristics. In addition, the effects of pileup or sink-in were considered in determining the real contact between the indenter and the specimen from the indentation load–depth curve. The work-hardening exponent was found to be a main factor affecting the pileup/sink-in phenomena of various steels. These phenomena influenced markedly the absolute values of strain and stress in indentation by making the simple traditional relationship Pm/σR ≈ ≈ 3 valid for the fully plastic regime.

Temporal Gradients in Shear, but Not Spatial Gradients, Stimulate Endothelial Cell Proliferation

Abstract: Background—The effect of temporal and spatial gradients in shear on primary human endothelial cell (HUVEC) proliferation was investigated. The sudden-expansion flow chamber (SEFC) model was used to differentiate the effect of temporal gradients in shear from that of spatial gradients. With a sudden onset of flow, cells are exposed to both temporal and spatial gradients of shear. The temporal gradients can be eliminated by slowly ramping up the flow. Methods and Results—HUVEC proliferation in the SEFC remained unstimulated when the onset of flow was slowly ramped. Sudden onset of flow stimulated a 105% increase of HUVEC proliferation (relative to ramped onset) within the region of flow reattachment. To further separate temporal and spatial gradients, a conventional parallel-plate flow chamber was used. A single 0.5-second impulse of 10 dyne/cm 2 increased HUVEC proliferation 5463% relative to control. When flow was slowly ramped over 30 seconds, HUVEC proliferation was not significantly different from controls. Steady laminar shear over 20 minutes inhibited HUVEC proliferation relative to controls regardless of step (3668%) or ramp (2165%) onsets of flow. Conclusions—The results indicate that temporal gradients in shear stress stimulate endothelial cell proliferation, whereas spatial gradients affect endothelial proliferation no differently than steady uniform shear stress. (Circulation. 2001;103: 2508-2513.)

The ideal strength of tungsten

Abstract: Using pseudopotential density functional theory within the local-density approximation, we calculate the ideal shear strengths of W for slip on {110}, {112} and {123} planes allowing for complete structural relaxation orthogonal to the applied shear. The strengths in the weak directions on all planes are found to be very nearly equal (about 18GPa, or 11% of the shear modulus G). Moreover, the shear instability occurs at approximately the same applied shear strain (17–18%). This unusual isotropy is explained in terms of the atomic configurations of high-energy saddle points reached during shear. Analysis of these saddle points may also offer a simple explanation for the prevalence of the pencil glide of dislocations on planes containing a (111) direction in bcc metals. Finally, we calculate the ideal cleavage strengths of W on {100} and compare our calculated ideal shear and cleavage strengths with experimental nanoindentation and whisker measurements. All these results can be rather simply unders...

Selectin receptor–ligand bonds: Formation limited by shear rate and dissociation governed by the Bell model

Abstract: We have studied the principles that govern the formation and dissociation of an adhesive bond between a cell moving in shear flow and a substrate and tested different theories of how force affects bond dissociation. Viscosity relates the kinematics of fluid movement (shear rate, units of time(-1)) to shear stress (units of force/area, the product of shear rate and viscosity). At different medium viscosities, the formation of receptor-ligand bonds between a cell in the flowstream and P-selectin on the vessel wall showed a similar efficiency as a function of shear rate but not of shear stress. Therefore, bond formation was a function of shear rate and hence of the kinematics of receptor and ligand movement. By contrast, the kinetics of bond dissociation was a function of shear stress and hence of force on the bond. The different requirements for bond formation and dissociation allowed dissociation kinetics to be measured at higher forces on the bond by increasing medium viscosity. Data over an extended range of forces on the bond therefore could be collected that enabled five different proposed equations, relating force to bond dissociation, to be compared for fit to experimental data. The relationship proposed by Bell [Bell, G. I. (1978) Science 200, 618-627] fit the data significantly the best and also predicted an off-rate in the absence of force that best matched an independent measurement [Mehta, P., Cummings, R. D. & McEver, R. P. (1998) J. Biol. Chem. 273, 32506-32513].

Shear stress induces a time- and position-dependent increase in endothelial cell membrane fluidity

Abstract: Blood flow-associated shear stress may modulate cellular processes through its action on the plasma membrane. We quantified the spatial and temporal aspects of the effects of shear stress (τ) on th...

Surface roughness effects in near-bed turbulence: Implications to sediment entrainment

Abstract: In this study, the characteristics of near-bed turbulence were experimentally investigated for three distinct roughness regimes, namely (1) isolated; (2) wake interference; and (3) skimming. Spherical particles of the same size and density were placed upon a rough sediment bed to simulate the three regimes. Experimental runs for the aforementioned regimes were performed in a tilting water-recirculating flume. Flow measurements atop the spherical particles were performed by means of a 3D laser Doppler velocimeter. The aim of the tests was to provide further evidence that the structure of turbulence is affected throughout the boundary layer by the presence of roughness geometry. The measurements reported here include velocity profiles of the mean streamwise and vertical velocity components and of the Reynolds shear stress distribution. To further quantify the differences in turbulent structure under various surface roughnesses, a quadrant analysis was performed.

Modelling three‐dimensional flow structures and patterns of boundary shear stress in a natural pool–riffle sequence

Abstract: Fluid-sediment interactions control river channel forms and processes. Analysis of spatial hydraulic patterns and the resulting boundary shear stress are required to aid understanding of river system behaviour. In this paper, the hydraulic processes active in a natural pool-riffle sequence are simulated using a three-dimensional computational fluid dynamics (CFD) model. Methods employed for the prescription of model boundary conditions are outlined. Model calculations are assessed using comparisons with field observations acquired over a range of flows. Simulations are then used to illustrate flow structures and patterns of boundary shear stress for a near-bankfull and an intermediate flow event. Results are used to assess existing theories that seek to explain the development and maintenance of pool-riffle sequences. Simulated results suggest that near-bed velocities and bed shear stresses decrease on riffles and increase in pools as discharge increases. Model simulations indicate that secondary flow acts to route near-bed flow over the downstream side of riffles and into the pool-head away from the centre of pools. Implications for sediment transport and pool maintenance are discussed.

Shear correction factors in Timoshenko's beam theory for arbitrary shaped cross-sections

Abstract: In this paper shear correction factors for arbitrary shaped beam cross-sections are calculated. Based on the equations of linear elasticity and further assumptions for the stress field the boundary value problem and a variational formulation are developed. The shear stresses are obtained from derivatives of the warping function. The developed element formulation can easily be implemented in a standard finite element program. Continuity conditions which occur for multiple connected domains are automatically fulfilled.

Atomistic study of non-Schmid effects in the plastic yielding of bcc metals

Abstract: In this paper we investigate by computer modelling, using many-body central force potentials, the response of the core of 1 (111) screw dislocations in bcc transition metals to externally applied stresses. The objective is to identify those components of the applied stress tensor which play the main role in the breakdown of the Schmid law and to establish the dependence of the critical stress needed for dislocation motion upon these stress components. This development lays the ground for constitutive relations that reect correctly the non-Schmid character of plasticow in bcc metals that are needed in continuum approaches to plasticity of these materials. First, we investigate the eŒ ect of pure shear stress acting parallel to the Burgers vector. This study involves calculation of the critical resolved shear stress (CRSS) for various orientations of the maximum resolved shear stress plane. The results show the dependence on the sense of shearing, exhibit the so-called twinning± antitwinning asymmetry and reveal quantitatively the overall deviation from the Schmid law. The next step is investigation of the eŒ ect of tensile and compressive stresses for a number of diŒ erently oriented tension or compression axes. These calculations demonstrate that shear stresses parallel to the Burgers vector are not suc cient to explain the variation in the CRSS with the orientation of the loading axis and suggest that other components of the stress tensor are aŒ ecting the dislocation behaviour. Finally, a combined eŒ ect of the shear stresses parallel and perpendicular to the Burgers vector is investigated. The resultant dependence of the CRSS on the shear stress perpendicular to the Burgers vector explains the orientation dependence found in the tension and compression studies. The present atomistic calculations establish that gliding of the 1 (111) screw dislocation in bcc metals depends on shear stresses both parallel and perpendicular to the Burgers vector that act not only in the slip plane but all three {110} and also {112} planes of the (111) zone. The results of these calculations determine the functional dependence of the CRSS on these shear stresses.