Showing papers on "Shear stress published in 1981"

The dynamic response of vascular endothelial cells to fluid shear stress.

Abstract: We have developed an in-vitro system for studying the dynamic response of vascular endothelial cells to controlled levels of fluid shear stress. Cultured monolayers of bovine aortic endothelial cells are placed in a cone-plate apparatus that produces a uniform fluid shear stress on replicate samples. Subconfluent endothelial cultures continuously exposed to 1-5 dynes/cm2 shear proliferate at a rate comparable to that of static cultures and reach the same saturation density (congruent to 1.0-1.5 X 10(5) cells/cm2). When exposed to a laminar shear stress of 5-10 dynes/cm2, confluent monolayers undergo a time-dependent change in cell shape from polygonal to ellipsoidal and become uniformly oriented with flow. Regeneration of linear "wounds" in confluent monolayer appears to be influenced by the direction of the applied force. Preliminary studies indicate that certain endothelial cell functions, including fluid endocytosis, cytoskeletal assembly and nonthrombogenic surface properties, also are sensitive to shear stress. These observations suggest that fluid mechanical forces can directly influence endothelial cell structure and function. Modulation of endothelial behavior by fluid shear stresses may be relevant to normal vessel wall physiology, as well as the pathogenesis of vascular diseases, such as atherosclerosis.

Conditional statistics of Reynolds stress in rough-wall and smooth-wall turbulent boundary layers

Abstract: Quadrant analysis has been used to investigate the events contributing to the Reynolds shear stress in zero-pressure-gradient turbulent boundary layers over regularly arrayed rough surfaces of several different densities, and over a smooth surface. By partitioning the stress into ejections, sweeps, and inward and outward interactions, it is shown that sweeps account for most of the stress close to rough surfaces, and that the relative magnitude of the sweep component increases both with surface roughness and with proximity to the surface. The sweep-dominated region delineates a ‘roughness sublayer’ with a depth of up to several roughness element heights, in which the turbulence characteristics depend explicitly on the roughness. In the remainder of the inner (or constant-stress) layer, and in the outer layer, the flow obeys familiar similarity laws with respect to surface roughness.The difference ΔS0 between the fractional contributions of sweeps and ejections to the stress is shown to be well related everywhere to the third moments of the streamwise and normal velocity fluctuations. Experimental proportionalities are established between the third moments and δS0, and are shown to agree with predictions made from cumulant-discard theory.The time scale for the passage of large coherent structures past a fixed point, T, is assumed proportional to the mean time between occurrences in a specified quadrant of an instantaneous stress u'w’ at least H times the local mean stress u'w’, where H is a threshold level. For both the ejection and sweep quadrants and for any choice of H, it is found that T scales with the friction velocity u* and the boundary-layer thickness δ, such that Tu*/δ is invariant with change of surface roughness.

The stress tensor in a granular flow at high shear rates

Abstract: The stress tensor in a granular shear flow is calculated by supposing that binary collisions between the particles comprising the granular mass are responsible for most of the momentum transport. We assume that the particles are smooth, hard, elastic spheres and express the stress as an integral containing probability distribution functions for the velocities of the particles and for their spatial arrangement. By assuming that the single-particle velocity distribution function is Maxwellian and that the spatial pair distribution function is given by a formula due to Carnahan & Starling, we reduce this integral to one depending upon a single non-dimensional parameter R: the ratio of the characteristic mean shear velocity to the root mean square of the precollisional particle-velocity perturbation. The integral is evaluated asymptotically for R [Gt ] 1 and R [Lt ] 1 and numerically for intermediate values. Good agreement is found between the stresses measured in experiments on dry granular materials and the theoretical predictions when R is given the value 1·7. This case is probably the one for which the present analysis is most appropriate. For moderate and large values of R, the theory predicts both shear and normal stresses that are proportional to the square of the particle diameter and the square of the shear rate, and depend strongly on the solids volume fraction. A provisional comparison is made between the stresses predicted in the limit R → ∞ and the experimental results of Bagnold for shear flow of neutrally buoyant wax spheres suspended in water. The predicted stresses are of the correct order of magnitude and yield the proper variation of stress with concentration. When R [Lt ] 1, the shear stress is linear in the shear rate, and the analysis can be applied to shear flow in a fluidized bed, although such an application is not developed further here.

Dynamic fracture and spallation in ductile solids

Abstract: A mathematical model of ductile hole growth under the application of a mean tensile stress is developed and applied to the problem of spallation in solids. The object is to describe dynamic ductile fracture under a wide range of tensile loading conditions. The mathematical model presented here describes both plate‐impact spallation (as observed by postshot examination and time‐resolved pressure measurements) and explosively produced spallation (as observed by dynamic x‐radiographic techniques) in copper. It is found to be inapplicable to ductile fracture of expanding rings, even in the absence of possible adiabatic shear banding and classical necking instabilities, because of the fact that the mean tensile stress (void growth) and the deviatoric stress (homogeneous plastic shear strain) are not independent. A phenomenological model of void growth under uniaxial stress conditions is developed independently and applied to the numerical finite‐difference solution of fracture in an expanding ring. The initial...

The steady state of deformation

Abstract: The steady state of deformation for any mass of particles is that state in which the mass is continuously deforming at constant volume, constant normal effective stress, constant shear stress, and constant velocity. The steady state of deformation is achieved only after all particle orientation has reached a statistically steady-state condition and after all particle breakage, if any, is complete, so that the shear stress needed to continue deformation and the velocity of deformation remain constant. The similarities and differences between steady-state deformation and the current use of the term critical state are described. A special undrained triaxial test on a sand is presented to demonstrate clearly that a special flow structure exists during steady-state deformation, which is quite different from the initial structure, and which is credited to a nonrandom, i.e., statistically oriented, arrangement of the sand grains.

An empirical equation of the relative viscosity of polymer melts filled with various inorganic fillers

Abstract: Based on Maron-Pierce's equation, an empirical equation was suggested, which relates the relative viscosity (ηr) of the polymer melt filled with various inorganic filler, such as glass fiber, carbon fiber, talc, precipitated- and natural-calcium carbonate powder, and glassy small sphere, to the volume fraction (φ) of the filler. The equation isηr = (1 −φ/A)−2, whereA is a parameter relating to the packing geometry of the filler, which is similar to the parameterφ0 in Maron-Pierce's equation. In the equationηr is defined as the ratio of the viscosity of the filledsystem to that of the medium at the same shear stress not the shear rate. The applicability of the equation is above the shear stress about 104 dyne/cm2. The equation has a simple form and is considered to have a practical utility for filled-polymer melt systems.

On the mixing of a rectangular jet

Abstract: Hot-wire measurements in an incompressible rectangular jet, issuing into a quiet environment at ambient conditions, are presented. A blow-down-type air supply system was used to provide the airflow to a cylindrical settling chamber 1.75 m in length and 0.6 m in diameter. The measurements were made with constant-temperature anemometers in conjunction with linearizers. The two signals from the linearizers were sent through a sum and difference unit which was calibrated from dc to 100 kHz. The distributions of mean velocity and the turbulence shear stresses were measured in the two central planes of the jet stations up to 115 widths downstream of the nozzle exit. Three distinct regions characterized the jet flow field: a potential core origin, a two-dimensional-type region, and an axisymmetric type region. The onset of the second region appeared to be at a location where the shear layers separated by the short dimension of the nozzle meet; and the third region occurred at a downstream location where the two shear layers from the short edges of the nozzle meet. In the central plane, similarity was found both in the mean velocity and shear stress profiles beyond 30 widths downstream of the nozzle exit; profiles of rms velocity showed similarity in the second, but not the third region.

The Effects of Probe-Induced Flow Distortion on Atmospheric Turbulence Measurements.

Abstract: We present a theory for probe-induced flow distortion which is applicable in the atmosphere at heights greater than about 10 times the obstacle size We use the theory to calculate the behavior of Reynolds shear stress and velocity variances ahead of a cylinder and a sphere The stress is found to be most seriously distorted, the extent depending on the nature of the trailing wake We show that the linear form of the theory should be adequate for most surface-layer applications, and we discuss how the theory can be applied to more complex geometries We show that the “tilt correction” approach to the problem, which has been used by some workers, is incorrect in principle since it violates vorticity conservation, and is not even a good approximation in general

Boundary Shear in Smooth and Rough Channels

Abstract: A series of flume experiments are reported in which the walls and bed were differentially roughened, normal depth flow set, and measurements made of the boundary shear stress distribution An empirically derived equation is presented giving the percentage of the shear force carried by the walls as a function of the breadth/depth ratio and the ratio between the Nikuradse equivalent roughness sizes for the bed and the walls The results are compared with other available data for the smooth channel case and some disagreements noted The systematic reduction in the shear force carried by the walls with increasing breadth/depth ratio and bed roughness is illustrated Further equations are presented giving the mean wall and bed shear stress variation with apsect ratio and roughness parameters Although the experimental data is somewhat limited, the equations are novel and thought to indicate the general behavior of such open channel flows with some success

On the structure of turbulent flow over a progressive water wave: theory and experiment in a transformed, wave-following co-ordinate system

Abstract: An investigation of the turbulent flow structure over a progressive water wave, as well as the structure of the wave-induced flow field in a transformed wave-following frame, is reported. Experimental results are given for a free-stream velocity of 2·4 m s−1 over a 1 Hz mechanically generated deep-water wave. The velocity components were measured with a cross hot-film probe oscillating in a transformed wave-following frame. The amplitude and phase of the wave-induced velocity components are deduced by correlation to the generated water wave. The mean flow tends to follow the wave form so that the water wave should not be regarded as surface roughness. The mean velocity profile is basically log-linear and is similar to that over a smooth plate, because ripples riding on the waves do not produce sufficient roughness to interfere with the wind field. The wave-induced motion in the free stream is irrotational; but, in the boundary layer, it has strong shear behaviour related to the wave-associated Reynolds stress. The shear stress production as well as the energy production from the mean flow is concentrated near the interface. A phase jump of 180° in the wave-induced turbulent Reynolds stresses in the middle of the boundary layer was observed. The relationships between the induced turbulent Reynolds stresses and the induced velocities are of an eddy-viscosity type.

Determination of the Shear Strength of Shock Compressed 6061-T6 Aluminum

Abstract: The strength of 6061-T6 aluminum was assessed over the stress range of 8–40 GPa using velocity interferometry to measure reloading and unloading profiles from the initial shocked state. These results show that the shear stress which can be supported in the shocked state increases by about a factor of five over this range. This observation is in agreement with previous investigations. However, an important new observation is that a substantial increase in shear stress occurs during reloading, resulting in a well-defined elastic precursor. This result indicates a significant departure from the elastic-plastic model and suggests that softening occurs during initial shock compression.

Creep and fabrics of polycrystalline ice under shear and compression

Abstract: Creep tests were performed in torsion and torsion–compression on polycrystalline ice at temperatures near the melting point Syntectonic recrystallization occurs at strains of the order of 2–3%, leading to a rapid increase in strain-rate It is shown that the increase of creep-rate during tertiary creep arises from the development of fabrics favouring the glide on basal planes but also from the softening processes associated with recrystallization The c-axis fabric of recrystallized ice developed in simple shear consists of two-maxima, one at the pole of the permanent shear plane and the other between the normal of the second plane of maximum shearing stress and the principal direction of compression In torsion–compression, a three- or four-maximum fabric is formed according to the intensity of different components of the stress tensor The maxima are clustered around the principal direction of compression Processes of fabric formation are discussed The experimentally developed fabrics are probably produced by the strain-induced recrystallization, for which the driving force is provided by differences in stored plastic strain energy However the degree of preferred orientation of ice c-axes must be a function of the total strain when syntectonic recrystallization becomes less important In this case, fabrics are principally formed by plastic flow and a steady state is obtained for very high strains

On the relationship between engineering properties and delamination of composite materials

Abstract: The influence of the coefficient of mutal influence, Poisson's ratio and coefficients of thermal and moisture expansion on delamination is studied. Engineering theories are compared to finite element and experimental results. It is shown that the mismatch in coefficients of mutual influence can have a strong influence on delamination with fiber angles in the 10-15 degree range being critical for adjacent layer combinations. The mismatch in coefficient of mutual influence is reduced by a factor of two and the interlaminar shear stress is reduced significantly when the + or - adjacent layers are interspersed between 0 and 90 degree layers. It is shown how the results can be used for design of composite laminates.

Silicon pressure sensor

Abstract: A monolithic silicon pressure sensor employing a four-terminal resistive element is formed in a thin monocrystalline silicon diaphragm. The resistive element is a diffused resistor having current contacts at the ends and two voltage contacts located midway between the current contacts and on opposite sides of a current axis defined between the two current contacts. The thin silicon diaphraghm has a square shape and is oriented in a (100) silicon surface with its sides parallel to a [110] crystal orientation. The resistor is oriented with its current axis parallel to a [100] crystalline direction and at 45 degrees with respect to the edge of the diaphragm to maximize sensitivity of the resistor to shear stresses generated by flexure of the diaphragm resulting from pressure differentials across the diaphragm. With a current flowing between current contacts, a shear stress acting on the resistor generates a voltage which appears at the voltage contacts and which is proportional to the magnitude of the shear stress.

Strain controlled vs stress controlled hydrogen induced fracture in a quenched and tempered steel

Abstract: Experiments on commercial and laboratory heats of a 5 pct Ni steel have delineated a wide range of hydrogen-induced cracking behavior. Both precracked and notched specimens were tested in 0.21 MPa H2 gas at room temperature; the microstructure and hardness were held essentially constant. One extreme of behavior was exhibited by specimens with negligible amounts of intergranular weakening due to impurity segregation; here, strain-controlled, plasticity-related cracking occurred along surfaces of maximum shear stress within the prior austenitic grains. As impurity segregation increased (due to 480 °C aging of samples with Mn and Si) increasing amounts of stress-controlled cracking occurred along prior austenite grain boundaries. The latter produced a steady decrease in the stress intensity for crack extension and the local stress for fracture of the notched bars. The mechanisms involved and the practical implications of these phenomena are discussed.

Shear stress-strain-time behaviour of clays

Abstract: Shear stress–strain–time models for soils were examined in terms of undrained triaxial compression tests, with pore water pressure measurement, using reconstituted specimens of kaolinite and Cucaracha shale. Multiple-step constant load and constant rate of deformation tests were performed. The testing variables included final equal all-round consolidation pressure (100–800 lb/in2), overconsolidation ratio (1–8) and the magnitude of the first load increment (20–82% in terms of shear stress level). Additional constant load test data reported in the literature were analysed. The parameters of an exponential stress–strain model were expressed and interpreted in terms of the parameters of the hyperbolic stress–strain model. The parameters of both models can be expressed in terms of the undrained modulus to undrained shear strength ratio Eu/Su and the axial strain at failure ef. The creep parameter λ which controls time or strain rate effects was correlated with Eu/Su. It appears that any soil has a potential t...

Effective granular modulus to model pavement responses

Abstract: This report presents the results of a research study to further investigate the initial findings of an earlier research project concerning the prediction of pavement deflections. The principal objective of this study was to explore the consistent lack of agreement between field-measured deflections and those computed by using elastic-layered theory coupled with nonlinear dynamic modulus tests. During several periods of the year, surface deflections were measured with the Thumper testing vehicles on the same three Maryland flexible pavement sections that were previously studied. These field deflections were predicted mathematically for a large number of specific test conditions. Although these predicted deflections failed to match the measured values, a consistent trend in the ratio of the corresponding deflections was detected. As suggested in a previous study, an adjustment factor was applied to the granular base modulus to cause the deflection ratio to approach one. Linear log-log relationships were derived between this factor and increasing measured deflection values. From this analysis, it was surmised that current laboratory methods of granular material characterization appear to be inadequate for modeling in situ behavior, regardless of the measuring device. Based on the findings of recent seismic research, further analysis was made to determine whether a relationship exists between the adjustment factor and the induced shear strain in the granular layer. A clear curvilinear plot was produced, which indicated that the adjustment to the granular modulus is definitely related to the shear strain that develops in response to the surface loading. As a result, a procedure was presented for correcting for the effective in situ granular base modulus. (Author)

Viscoelastic Analysis of Adhesively Bonded Joints

Abstract: In this paper an adhesively bonded lap joint is analyzed by assuming that the adherends are elastic and the adhesive is linearly viscoelastic. After formulating the general problem a specific example for two identical adherends bonded through a three parameter viscoelastic solid adhesive is considered. The standard Laplace transform technique is used to solve the problem. The stress distribution in the adhesive layer is calculated for three different external loads namely, membrane loading, bending, and transverse shear loading. The results indicate that the peak value of the normal stress in the adhesive is not only consistently higher than the corresponding shear stress but also decays slower.

The Effect of Longitudinal Strain on the Shear Stress of an Ice Sheet: In Defence of Using Stretched Coordinates

Abstract: Thickness changes of ice sheets are, except perhaps at the snout region, small as compared to unity. This suggests using a coordinate stretching so as to make the surface changes in the new coordinates of order one. The explicit occurrence of the smallness parameter in the governing equations then allows us to search for perturbation solutions in various problems. Here, it is shown that the classical formula for the basal shear stress follows easily from such a perturbation procedure. Furthermore it can be improved to account for longitudinal strain effects. As compared to previous work in this area, these formulae are explicit and allow us to take vertical variations of material properties into account in a straightforward manner.

Hydraulics of channels with flood-plains

Abstract: This paper presents the results of an experimental study on the interaction between the flow in a “wide” main channel and the flood-plain. This interaction region has been shown to resemble a shear...

A Reynolds-stress closure model of turbulence applied to the calculation of a highly curved mixing layer

Abstract: The measurements in a highly curved mixing layer reported by Castro & Bradshaw (1976) are used to evaluate the performance of a calculation method based on the solution of modelled transport equations for the Reynolds stresses and the dissipation rate of turbulent energy. The model reproduces the suppression of turbulence by stabilizing curvature and, downstream of the curved region, where the flow returns asymptotically to being a plane mixing layer, calculated values of turbulent intensity and shear stress overshoot the plane-layer values in accordance with the experimental observations. The results are compared with those obtained by Townsend (1980) from a rapid-distortion model which correctly predicts the streamwise variation of the shear stress to intensity ratio. By contrast, calculations based on a conventional two-equation eddy-viscosity model fail badly to account for curvature effects on this flow.