Showing papers on "Shear stress published in 1984"

A Simple Higher-Order Theory for Laminated Composite Plates

Abstract: A higher-order shear deformation theory of laminated composite plates is developed. The theory contains the same dependent unknowns as in the first-order shear deformation theory of Whitney and Pagano (1970), but accounts for parabolic distribution of the transverse shear strains through the thickness of the plate. Exact closed-form solutions of symmetric cross-ply laminates are obtained and the results are compared with three-dimensional elasticity solutions and first-order shear deformation theory solutions. The present theory predicts the deflections and stresses more accurately when compared to the first-order theory.

Induction of human vascular endothelial stress fibres by fluid shear stress

Abstract: Endothelial cells of the arterial vascular system and the heart contain straight actin filament bundles, of which there are few, if any, in the venous endothelium1–4. Since stress fibre-containing endothelial cells within the vascular system tend to be located at sites exposed to particularly high shear stress of blood flow, we have investigated, in an experimental rheological system (Fig. 1), the response of the endothelial actin filament skeleton to controlled levels of fluid shear stress. Here we report that endothelial stress fibres can be induced by a 3-h exposure of confluent monolayer cultures of human vascular endothelium to a fluid shear stress of 2 dynes cm−2, approximately the stress occurring in human arteries in vivo. Fourfold lower levels of shear stress that normally occur only in veins, had no significant effect on the endothelial actin filament system. The formation of endothelial stress fibres in response to critical levels of fluid shear stress is probably a functionally important mechanism that protects the endothelium from hydrodynamic injury and detachment.

Influence of hemodynamic forces on vascular endothelial function. In vitro studies of shear stress and pinocytosis in bovine aortic cells.

Abstract: The relationships between fluid shear stress, a physiologically relevant mechanical force in the circulatory system, and pinocytosis (fluid-phase endocytosis) were investigated in cultured bovine aortic endothelial cells using a specially designed apparatus. Continuous exposure to steady shear stresses (1-15 dyn/cm2) in laminar flow stimulated time- and amplitude-dependent increases in pinocytotic rate which returned to control levels after several hours. After 48 h continuous exposure to steady shear stress, removal to static conditions also resulted in a transient increase in pinocytotic rate, suggesting that temporal fluctuations in shear stress may influence endothelial cell function. Endothelial pinocytotic rates remained constant during exposure to rapidly oscillating shear stress at near physiological frequency (1 Hz) in laminar flow. In contrast, however, a sustained elevation of pinocytotic rate occurred when cells were subjected to fluctuations in shear stress amplitude (3-13 dyn/cm2) of longer cycle time (15 min), suggesting that changes in blood flow of slower periodicity may influence pinocytotic vesicle formation. As determined by [3H]thymidine autoradiography, neither steady nor oscillating shear stress stimulated the proliferation of confluent endothelial cells. These observations indicate that: (a) alterations in fluid shear stress can significantly influence the rate of formation of pinocytotic vesicles in vascular endothelial cells, (b) this process is force- and time-dependent and shows accommodation, (c) certain patterns of fluctuation in shear stress result in sustained elevation of pinocytotic rate, and (d) shear stresses can modulate endothelial pinocytosis independent of growth stimulation. These findings are relevant to (i) transendothelial transport and the metabolism of macromolecules in normal endothelium and (ii) the role of hemodynamic factors in the localization of atherosclerotic lesions in vivo.

Bed Load Transport in a River Meander

Abstract: Bed load transport in Muddy Creek, Wyoming, a sand-bedded meandering river with equilibrium bottom topography, was found to consist of a zone of maximum sediment flux that shifted across the channel from near the inside bank in the upstream part of the bend toward the pool at the minimum radius of curvature. Significant net cross-stream transport continued even through the crossings between the bends. The downstream bed load transport field for the bend which was studied in greatest detail was the same as that computed from bed form migration measurements and can be predicted from appropriate boundary shear stress data and the Yalin bed load equation. The zone of maximum bed load transport followed an outward-shifting region of maximum boundary shear stress, although in the downstream end of the bend the sediment transport maximum tended to stay closer to the centerline than the boundary shear stress maximum due to particle size influences. Net cross-stream transport varied with particle size and was effected by three major processes: topographically-induced, near-bed, cross-stream flow; trough wise flow along obliquely oriented bed forms; and rolling or avalanching of particles on bed form lee faces plus rolling or mass sliding on a steep cross-stream point bar side slope. Coarse particles were carried outward over the top of the point bar by a near-bed cross-stream flow that was induced by downstream shoaling. These particles then rolled and slid on the point bar side slope and eventually were carried toward the outer bank by troughwise transport along oblique dunes. In the upstream part of the bend, fine particles were carried inward by the channel curvature-induced, near-bed flow and by troughwise transport along lee faces of oblique bed forms. Fine particles and coarse particles crossed paths on top of the point bar, where weak troughwise flow moved just the finer particles toward the convex bank, and on the point bar face, where coarse particles rolled against the secondary circu- lation that was carrying the finer particles. Net cross-stream bed load transport was toward the pool and was on average about 10% of the downstream bed load transport. The Engelund equation predicts reasonably well the general pattern of net cross-stream transport through the meander, but it does not account for the substantial troughwise transport caused by oblique bedforms at several locations in the bend. Our observations suggest that equilibrium bed topography occurs when there is net outward transport into the outward shifting zone of maximum boundary shear stress.

The kinematics of the plate boundary zone through New Zealand: a comparison of short- and long-term deformations

Abstract: Summary Surveys of shear strain rates derived from repeated triangulation cover about one-quarter of the area of the plate boundary zone through New Zealand and are well distributed within it. Using a method developed by Haines relating shear strains to the total strain and velocity fields, the dilatational and rotational strain rates and the velocities within the deformed zone are derived. The plate boundary zone is shown to be two-dimensional, for the most part, in that gradients of the velocity along the length of the zone are small or zero. The shear strain rates are not uniform, however; they reach a maximum along a central axis and decrease gradually to zero toward the sides. Nor are the two independent components of strain, one involving shortening (or extension) perpendicular, and the other shear parallel, to the zone equally developed everywhere; there is a clear geographic separation with shortening developed over the subduction thrust and shear developed behind in a zone of wrench faulting. Time variations in the strain field in part of the region above the subduction interface are related to changes in the degree of coupling across the interface. A velocity field is computed: the relative velocity of the two plates from this data agree very well with that derived from seafloor spreading data. These strain and velocity fields derived from geodetic studies give the short-term kinematics of the plate boundary zone. The long-term kinematics of the zone are estimated from: (1) uplift and subsidence data which are related to changes in crustal thickness and hence to dilatational strain, and (2) variation in the declination of primary magnetization in rocks of known age, related to the rotational strain. The rate of rotational strain has increased in time from about 2 to 3° Myr-1, 10 Myr ago, to about 9° Myr-1 today. The increase can be understood as a secondary effect of a developing compressional component increasing the coupling between the plates so that more of the motion parallel to the zone is accommodated by distributed shear and less by aseismic slip. The rates of strain averaged over the last 1 Myr or so, and at a scale length of a few tens of kilometres, are closely similar in distribution and magnitude to the strain rates derived from short-term kinematics. While, therefore, at scale lengths of a few hundred metres or so faulting is very important and the deformation is discontinuous with unstrained blocks faulted, with large but uncertain slip, against adjacent blocks, the deformation at these longer-scale lengths is continuous and regular. The two consequences of this study are that: (1) strain from geodetic data can be directly related to the average long-term deformation and therefore can be an important tool in the study of deformation in regions of current tectonic activity, and (2) at scale lengths of a few tens of kilometres the deformation of a region can be described as the deformation of a continuum and we can ignore the effects of faulting.

Stress Ratio Effects on Shear Modulus of Dry Sands

Abstract: Resonant column tests involving anisotropic confining conditions were conducted to evaluate static stress condition, especially stress ratio effects on the low‐amplitude dynamic shear modulus, G0, of three clean, dry sands. Six static stress paths, involving both triaxial compression and triaxial extension, developed the desired ranges of stress ratios and stress histories. The results of these tests determined that increasing the stress ratio decreased G0, up to 20%–30%, but the reduction was not significant for stress ratios below about 2.0. Stress history caused a reduction in G0, but when the final stress ratio was the maximum value attained, the loading, unloading, and reloading path followed to develop this maximum value had little influence. The results also indicated that G0, depends about equally on the static principal stresses in the direction of wave propagation and in the direction of particle motion. The effect of the third principal stress appears to be unimportant. Two empirical equations ...

Propagation and linkage of oceanic ridge segments

Abstract: We have investigated the propagation of spreading ridges and the development of structures that link ridge segments using an analogy between ridges and cracks in elastic plates. The ridge-propagation force and a path factor that controls propagation direction were calculated for echelon ridge segments propagating toward each other. The ridge-propagation force increases as ridge ends approach but then declines sharply as the ends pass, so ridge segments may overlap somewhat. The sign of the path factor changes as ridge ends approach and pass, so the overlapping ridge ends may diverge and then converge following a hook-shaped path. The magnitudes of shear stresses in the plane of the plate and orientations of maximum shear planes between adjacent ridge segments were calculated to study transform faulting. For different loading conditions simulating ridge push, plate pull, and ridge suction, we identify a zone of intense mechanical interaction between adjacent ridge ends in which stresses are concentrated. For all conditions, the shear stress in the interaction zone increases as ends approach and remains large as the ends overlap; thus crust in this zone may fracture and weaken in preparation for the formation of a through-going transform fault. The calculated shear planes rotate toward an orientation about 90° from the strike of ridges as the ends pass, thus favoring the orthogonal arrangement of ridges and transforms. The magnitudes of mean stresses in the plane of the plate and orientations of principal stress planes were also calculated. The mean stress is tensile in the interaction zone, so basins may form there, except in the case of ridge push loading. The planes across which the maximum tension acts are oblique to ridges, thus favoring obliquely oriented normal faults bounding the transform valley.

Undrained anisotropy and principal stress rotation in saturated sand

Abstract: This Paper describes an investigation carried out in a new hollow cylinder apparatus into anisotropy and the effects of principal stress rotation in a medium-loose sand, tested under undrained conditions. Principal stresses have been rotated at a constant shear stress during both monotonie and cyclic loading. The experimental observations are described and then explained within a framework based on the concept of state boundary surfaces. The over-riding importance of initial anisotropy in determining the response of the sand to rotations in principal stress directions is demonstrated. Pore pressures are shown to be generated by rotation of principal stress directions at constant shear stress and their accumulation during cyclic principal stress rotation can lead to failure. L'article presente une etude de l'anisotropie effectuee dans un nouvel appareil creux cylindrique et decrit les effets de la rotation de la contrainte principale dans un sable moyennement lâche teste dans des conditions non-drainees. L...

Boundary Shear in Symmetrical Compound Channels

Abstract: Experimental results are presented concerning the boundary shear stress and boundary shear force distributions in a compound section comprising of one rectangular main channel and two symmetrically...

A Model for Flow in Meandering Streams

Abstract: Erosion and deposition in streams and tidal channels depend on the divergence of the sediment-flux field, and this is governed by the distribution of boundary shear stress. As a consequence, erosion arid deposition patterns in such systems are sensitive to spatial variations in boundary shear stress, which in turn, can be induced by the complex interplay between the flow and bed and bank topography. A method whereby variations in boundary shear stress can be calculated is presented herein for flows that are broad in relation to their depth and for which the bed slopes are small. The formal scheme employs a regular perturbation expansion around a zero-order state that includes vertically integrated, topographically induced convective accelerations. Use of a zero-order velocity field that includes these accelerative effects yields a model that is applicable to streams with typical bed slopes and channel curvatures, one that can be employed in the majority of situations of interest to geomorphologists, sedimentologists, and hydraulic engineers concerned with fluvial and estuarine systems. In order to verify the model, our calculations are shown to reproduce, with reasonable accuracy, the free-surface topography and boundary shear stress distribution measured by Hooke (1975) when applied to a channel of the geometry that he investigated. The latter part of the paper describes how the interplay of the various components of flow through a curved channel with bar-pool topography is affected by the parameters of the problem.

Boundary Shear in Smooth Rectangular Channels

Abstract: The results of some experiments are reported concerning the boundary shear stress and boundary shear force distributions in a smooth channel of rectangular cross section. An empirically derived equation is presented which gives the percentage of the total shear force carried by the walls as a function of the breadth/depth ratio. Ancillary equations are presented giving the mean wall, bed, and bed center line shear stresses as functions of aspect ratio. A comparison is made between boundary shear stress distributions in open channel and closed conduit flows at comparable aspect ratios. Certain differences are noted in both the distributions and the mean resistance coefficients on account of different secondary flow structure. The results will be useful to those engaged in resistance, sediment, or dispersion studies.

Large Step Shear Strain Experiments with Parallel‐Disk Rotational Rheometers

Abstract: A new method is proposed and tested for step shear strain experiments with parallel‐disk rotational rheometers. The method is applicable to large strains, i.e., outside the linear viscoelastic region. The nonhomogeneity of the strain in the parallel‐disk rheometer is accounted for by a correction term which is similar to the well‐known Rabinowitsch correction in capillary rheometry. The transient shear relaxation moduli of a low‐density polyethylene (150°C) and of a polystyrene (180°C) from this method agree very well with equivalent data from a cone‐and‐plate rheometer. The two different geometries give an overlapping set of data; small‐strain data (γ=0.1−5) from cone‐and‐plate and large‐strain data (γ=0.4−25) from parallel disk. The step strain data support the separability of the relaxation modulus into time‐ and strain‐dependent functions. The strain dependence is well approximated by a sigmoidal function. The data were obtained with a Rheometrics dynamic spectrometer having a maximum angular displace...

Compressive and torsional behaviour of Kevlar 49 fibre

Abstract: The mechanical anisotropy of an aromatic polyamide fibre, Kevlar 49, was studied in tension, compression and torsion. A new technique involved applying small and defined compressive strains to filaments by bonding them to one side of a beam which is subsequently bent to compress the fibres. Using scanning electron and optical microscopy, fibres were shown to form regularly-spaced helical kink bands at 50 to 60° to the fibre axis after the application of small axial compressive strains. Tensile tests of previously-compressed fibres revealed only a 10% loss in tensile strength, after application of as much as 3% compressive strain. A torsion pendulum apparatus was used to measure the shear modulus and an apparent shear strength of fibres. A loss of tensile strength after the application of large (> 10%) torsional shear strains coincided with a loss in recoverable shear strain due to longitudinal fibre splitting. Ratios of tensile-to-compressive strength, tensile-to-shear strength and tensile-to-shear moduli of 5∶1, 17∶1, and 70∶1, respectively, were measured for Kevlar 49.

Turbulent wave boundary layers: 2. Second‐order theory and mass transport

Abstract: The solution for the turbulent near-bottom boundary layer produced by a progressive wave train is advanced to second order in wave steepness. As in the first-order analysis (part 1) the effective viscosity is assumed to be the product of a vertical length scale and the first few Fourier components of a shear velocity based on the instantaneous, local bed shear stress. An analytical solution for the second-order flow field is obtained, with attention directed primarily toward the second-order, wave-induced steady current or mass transport. The mass transport is found to depend critically on temporal variation of the effective viscosity. The most dramatic result of the analysis is a predicted reversal of the mass transport produced by relatively long waves. This result, for which supporting experimental evidence is presented, has not been predicted previously and cannot be obtained by a time-invariant eddy viscosity model. Implications of the present results for related problems are discussed.

Orientation effects and rheology of short glass fiber-reinforced thermoplastics

Abstract: The rheology of polyolefines, polyamide 6, and polystyrene-acrylonitrile filled with glass fibers of different concentrations and aspect ratios have been investigated in simple shear flow, capillary rheometry, and uniaxial elongation. A comparison is made with unfilled and glass bead-filled melts. Fiber orientation was investigated by X-ray microradiography. Steady-state viscosities are obtained on fibers aligned parallel to the direction of flow. Entrance pressure losses, the shape of the viscosity function, and the appearance of a yield stress are depending on the fiber aspect ratio. The temperature dependence of the viscosity function is not significantly different from that of the unfilled melt. Transient shear stresses were measured on samples of different initial orientations of the fibers. The change of fiber orientation during shear flow gives rise to a pronounced overshoot of shear stress and normal stress difference. Elastic strains in shear are increased by the fibers but elastic elongational strains are reduced. Mechanisms are proposed to explain the experimental observations.

Effects of Fluid Flow on Living Vascular Cells

Abstract: Shear stress produced by a flowing fluid has been found to cause changes in the structure and function of vascular endothelial cells. They tend to become elongated and aligned with the direction of flow. They respond to changes in fluid shear stress (either from low shear to high shear or vice versa) by transiently increasing fluid-phase endocytosis. And they are capable of producing intracellular actin and myosin filaments that are oriented in the flow direction.

Turbulent wave boundary layers: 1. Model formulation and first‐order solution

Abstract: Oscillatory turbulent flow produced near a rough seabed by linear surface waves is analyzed. An analogy to the wall region of steady turbulent flow is shown to be valid very near the bed and provides the basis for a model in which the eddy viscosity is the product of a vertical length scale and the first two Fourier components of a shear velocity based on the instantaneous, local bed shear stress. With this time-varying eddy viscosity model, an approximate closed-form solution for the boundary layer flow is obtained. Comparison with available laboratory measurements indicates that the eddy viscosity does in fact vary with time and that the theoretical approach taken here is physically sound. The effect of a time-varying viscosity on quantities of practical interest is found to be potentially important.

Jumps in layered miscible fluids

Abstract: Experiments with hydraulic jumps in a layered flow with a small density difference at the interface are described. Two different configurations are examined, with shear between the fluids either upstream or downstream of the jump. It is shown that when the shear stress on all interfaces is small enough for there to be no mixing either the theory which assumes hydrostatic pressure on the face of the jump or that which assumes energy conservation in one of the layers describes the results. In the experiments for a jump in the lee of a towed obstacle this condition is always satisfied, but for a jump advancing into stationary layers it is only satisfied when the ratio of the height behind the jump to that in front is less than about 2. Beyond this limit there is mixing behind the first wave of the undular jump and the flow behaves like the head of a gravity current. The theory with energy conservation in one layer is extended to the case of a stationary jump, and for this case it is shown that for a given downstream control an approximate value of the fluid entrained can be computed.

High velocity ring shear tests on sand

Abstract: Ring shear tests on sand materials have been carried out to show whether there is a change in the behaviour of these materials at high shearing rates and normal stresses. This question is relevant to the problem of the mobility of large rock avalanches (sturzstroms), but is also of more general academic interest. A ring shear apparatus of unusual, although simple, design has been constructed, capable of achieving circumferential velocities of 2 m/s at normal stresses of up to 200 kPa. Tests were carried out on two types of coarse sand, wet or dry, sand-rock flour mixtures and polystyrene beads. Perfect frictional behaviour was observed in all the tests, uninfluenced by either velocity or normal stress over the entire range of these variables. Change in frictional behaviour due to high rate of shearing therefore cannot be used to explain the high observed mobility of sturzstroms. Des essais de cisaillement circulaire par torsion ont ete effectes afin de decouvrir si un changement ait lieu dans le comportem...

Non-Newtonian Fluid Model Incorporated Into Elastohydrodynamic Lubrication of Rectangular Contacts

Abstract: A procedure is outlined for the numerical solution of the complete elastohydrodynamic lubrication of rectangular contacts incorporating a non-Newtonian fluid model. The approach uses a Newtonian model as long as the shear stress is less than a limiting shear stress. If the shear stress exceeds the limiting value, the shear stress is set equal to the limiting value. The numerical solution requires the coupled solution of the pressure, film shape, and fluid rheology equations from the inlet to the outlet. Isothermal and no-side-leakage assumptions were imposed in the analysis. The influence of dimensionless speed, load, materials, and sliding velocity and limiting-shear-strength proportionality constant on dimensionless minimum film thickness was investigated. Fourteen cases were used in obtaining the minimum-film-thickness equation for an elastohydrodynamically lubricated rectangular contact incorporating a non-Newtonian fluid model. Computer plots are also presented that indicate in detail pressure distribution, film shape, shear stress at the surfaces, and flow throughout the conjunction.

Boundary skin friction and sediment transport about an animal-tube mimic

Abstract: A flume study was made of fluid flow, boundary skin friction, and sediment transport about an animal-tube mimic. The effect of a tube on momentum transfer depends most strongly on its height. A tube increases the net boundary skin friction locally and tends to promote sediment entrainment near its base. Flow in this region appears to be governed by the same similarity laws that apply to cylinders with much larger body Reynolds number. Particles travelling as suspended load may be deposited immediately downstream of the tube. The net sedimentological effect of any tube (i.e. deposition or scour) will depend both on its height and on the boundary shear stress imposed by the external flow. The near-wake region of a tube is dominated by a strong cross-stream exchange of momentum. Wake perturbations decay downstream seemingly in accordance with similarity laws which also govern mounted, two-dimensional structures.