Showing papers on "Shear stress published in 1980"

Elastic wave behavior across linear slip interfaces

Abstract: A model for an imperfectly bonded interface between two elastic media is proposed. Displacement across this surface is not required to be continuous. The displacement discontinuity, or slip, is taken to be linearly related to the stress traction which is continuous across the interface. For isotropic interface behavior, there are two complex frequency dependent interface compliances, ηN and ηT, where the component of the slip normal to the interface is given by ηN times the normal stress and the component tangential to the interface is given by ηT times the shear stress and is in the same direction. Reflection and transmission coefficients for harmonic plane waves incident at arbitrary angles upon a plane linear slip interface are computed in terms of the interface compliances. These coefficients are frequency dependent even when the compliances are real and frequency independent. Examples of the effects of buried slip interfaces on reflection coefficient spectra and on Love‐wave dispersion relations are ...

The fluid mechanics of large blood vessels

Abstract: 1. Physiological introduction 2. Propagation of the pressure pulse 3. Flow patterns and wall shear stress in arteries I Straight tubes 4. Flow patterns and wall shear stress in arteries II. Curved tubes 5. Flow patterns and wall shear stress in arteries III. Branched tubes and flow instability 6. Flow in collapsible tubes.

Adaptive regulation of wall shear stress to flow change in the canine carotid artery

Abstract: To study the adaptive response of the vascular wall to blood flow changes, an arteriovenous shunt was constructed between the common carotid artery and the external jugular vein in 12 dogs. Six to eight months postoperatively, the arterial internal radius (r) was determined by angiography and/or the use of pressure-volume relationship. The results showed that r increased with increased flow load (f) and vice versa. Wall shear rate (gamma) was calculated from gamma = 4f/(tau r3), assuming laminar flow. The value of gamma, initially proportional to f, had recovered almost to the control level (within 15%) due to the vessel dilatation or atrophy during the chronic experiment, when f was less than 4 times the control. Transendothelial protein permeability, evaluated at the T-1824-stained surface by a reflectometric method, also showed a close correlation with wall shear (r = 0.934). A local autoregulatory mechanism of wall shear stress involving protein turnover in the vascular wall is suggested.

Frictional heating, fluid pressure, and the resistance to fault motion

Abstract: Expansion of pore fluid caused by frictional heating might have an important effect on the factional resistance and temperature during an earthquake and a controlling influence on the physics of the earthquake process When confined water is heated, the pressure increases rapidly (≳10 bars/°C) As Sibson (1973) has pointed out, this could cause a sharp reduction of effective normal stress and dynamic friction on the fault surface Whether or not this transient stress reduction occurs depends upon the tandem operation of several processes, any of which can break the chain that links frictional heat to frictional stress: the friction must cause an appreciable temperature rise (imposing conditions on the width of the shear zone and rate of conductive transport); the temperature rise must cause an appreciable fluid pressure rise (imposing conditions on the rate of pore dilatation or hydrofracturing, and the rate of Darcian transport); the fluid pressure rise must cause an appreciable reduction of friction (requiring the presence of a continuous fluid phase) Each process depends upon event duration, particle velocity, and the initial value of dynamic friction With the present uncertainty in the controlling parameters (principally permeability, width of the shear zone, initial stress, and factors controlling transient hydrofracture and pore dilatation) a wide variety of fault behavior is possible Limits to fault behavior for various ranges of the controlling parameters can be estimated from the governing equations, however, and results can be summarized graphically If the effective stress law applies and pore dilatation is unimportant, dynamic friction would drop from an initial value of 1 kbar to ∼100 bars when shear strain reached 10 for most earthquakes if the permeability were less than 01 μdarcy; the maximum temperature rise would be only ∼150°C irrespective of final strain If the permeability were ≳100 mdarcies, however, friction would be unaffected by faulting and temperatures could approach melting for shear strains ∼20 For permeabilities ∼1 mdarcy, friction could be reduced appreciably during large earthquakes, but during small ones it could not Combined with thermal effects, dilatational strain of a few percent of pore volume could lead to virtually frictionless faulting or increasing frictional resistance, dependeing upon its sign; unstable propagation of hydrofractures (after fluid pressure exceeded the least principal stress) could cause a sudden increase in fault friction Strengthening due to cooling and Darcian flow at the conclusion of an earthquake could occur in seconds or weeks depending upon event duration, transport parameters, and shear zone width; it might influence the redistribution of stress by aftershocks

Shear‐Induced Structure in a Concentrated Suspension of Solid Spheres

Abstract: Two novel phenomena were observed in steady and transient shear measurements which were made in a Couette device of a R‐17 Weissenberg Rheogoniometer with suspensions of polystyrene spheres, 40–50 μm in diameter, suspended in a mixture of silicone oils at volume fractions 0⩽φ0.55. When φ⩾0.3, the steady‐shear viscosity at a given shear rate was found to drift for many hours to an asymptotic value which, in contrast to the scatter of the initial measurements, was very reproducible. Again, when φ⩾0.3, the shear stress showed a memory for the direction of previous shearing when the shear was stopped for a while and then restarted with either the same or the opposite sign. Moreover, during oscillatory shear experiments, these suspensions exhibited a nonlinear response which in fact could be predicted from their response to a sudden reversal of the direction of steady shear. It would appear, therefore, that such concentrated two‐phase systems cannot be modeled as isotropic fluids having a scalar effective viscosity unless the solids concentration is low.

Shear band inclination and shear modulus of sand in biaxial tests

Abstract: The spontaneous shear band formation in the biaxial test on dry sand samples with constant cell pressure is treated as a bifurcation problem. The constitutive response of sand is described in terms of mobilized friction and dilatancy. Dilatancy is looked upon as an internal constraint and the hardening rule is expressed in terms of an adequate dimensionless stress measure. Owing to fail of normality in sand, localization always occurs in the hardening regime. The theoretical solution of the shear band inclination is a geometrical mean of the classical Coulomb and Roscoe solutions and is in good agreement with the experimental data. The incipient shear modulus is proportional to the stress level and can be estimated to be also proportional to these cant modulus.

Investigation of a Reattaching Turbulent Shear Layer: Flow Over a Backward-Facing Step

Abstract: The paper studies incompressible flow over a backward-facing step in order to investigate the flow characteristics in the separated shear layer, the reattachment zone, and the redeveloping boundary layer after reattachment. It is shown that turbulent intensities and shear stress reach maxima in the reattachment zone, followed by rapid decay near the surface after reattachment. In addition, it is found that downstream of reattachment, the flow returns very slowly to the structure of an ordinary turbulent boundary layer.

Unstable slippage across a fault that separates elastic media of different elastic constants

Abstract: In this paper it is demonstrated that relatively slow, quasi-static slippage on a fault that separates two half spaces of different elastic constants can become unstable if the slippage is governed by the Amontons-Coulomb friction law (the shear stress across a fault required for slipping motion is proportional to the normal compressive stress across the fault). If the two half spaces have identical properties, unstable slippage is not possible under this friction law. The unstable slippage that is investigated in this paper is a consequence of the existence of a short-range normal traction stress that gliding edge dislocations produce across an interface between two half spaces of different elastic constants, This normal traction stress does not exist if the two half spaces have identical properties. (Recent work of Dundurs and Comninou and their co-workers has revealed the importance of the short-range traction stress components to crack problems.)

Shear flow rheological properties, fiber damage, and mastication characteristics of aramid-, glass-, and cellulose-fiber-reinforced polystyrene melts

Abstract: An experimental study of the viscosity and principal normal stress difference of a polystyrene melt filled with aramid (Kevlar), glass, and cellulose fibers is reported. The influence of loading level and mastication on the rheological properties is discussed. The effects of mixing and mastication on fiber damage are considered. Glass fibers break down rapidly to very small aspect ratios, while aramid shows a “kinked” structure, with kinks occurring every 100 μm. A mechanism is proposed for fiber breakage based on buckling during rotation in shear flow. It is found that addition of fibers increases the viscosity in the same manner as a reduction in temperature, and data may be superposed by reduced plotting. This indicates that the viscosity increase is due solely to enhanced viscous dissipation in the matrix and not to interparticle forces as is the case with smaller particles. The principal normal stress difference increases at fixed shear stress with fiber loading. The extent of increase depends upon fiber loading, aspect ratio, and modulus.

Long slender drops in a simple shear flow

Abstract: We study theoretically the slow viscous motion of a long slender drop placed in a simple shear flow, the drop having a low viscosity compared with that of the suspending fluid. As a simplifying approximation, the cross-section of the drop is taken to be circular. An equilibrium shape with the drop nearly aligned with the flow is found for all shear rates, although the equilibrium is only stable to small disturbances for shear rates below some critical value. The stable equilibria just below the critical shear rate are found to be accessible only if the shear rate is increased slowly.

Experimental investigations of shear and elongational flow properties of polystyrene melts reinforced with calcium carbonate, titanium dioxide, and carbon black

Abstract: Shear and elongational flow measurements on polystyrene melts reinforced with small particles were carried out. The influences of loading level, particle size and surface treatment on shear viscosity, principal normal stress difference, and elongational viscosity were discussed. These systems exhibited yield values for both shear and elongational flow. Experimental values for the ratio of the tensile to the shear yield stress give satisfactory agreement with the predictions of the von Mises yield criterion. The yield value appears to increase with decreasing particle size and may be varied with surface treatment. The principal normal stress difference at fixed shear stress decreases with volume loading of particulates. The results are interpreted in terms of a system forming a gel due to interparticle forces, which is disrupted by a critical distortional strain energy.

Stress measurements at depth in the vicinity of the San Andreas Fault: Implications for the magnitude of shear stress at depth

Abstract: Using the hydraulic fracturing technique, we have made a systematic series of in situ stress measurements in wells drilled near the San Andreas fault. In an attempt to provide constraints for the magnitude of shear stress on the San Andreas fault at depth we have measured both the variation of stress with distance from the fault in relatively shallow (∼230 m) wells and the variation of stress with depth in a ∼1-km-deep well located 4 km from the fault. The shallow wells are located along profiles roughly perpendicular to the fault in the western Mojave desert near Palmdale and in central California where the fault is creeping. In both areas the direction of maximum compression was found to be approximately 45° from the local trend of the San Andreas. The two stress profiles show very similar results: (1) shear stress (on planes parallel to the San Andreas) increases with distance from the fault, more markedly in the western Mojave, (2) the far-field shear stress at ∼200 m depth is ∼50 bars, and (3) the horizontal principal stresses as well as shear stress increase with depth more rapidly in the wells farthest from the fault. The ∼1-km-deep well, also located in the western Mojave desert, shows increases of both horizontal principal stresses and shear stress with depth. Shear stress increases from about 25 bars at 150–300 m to about 80 bars at 750–850 m. Although this rapid increase of shear stress with depth suggests that the mean shear stress on the fault at seismogenic depths exceeds several hundred bars, the principal stresses increase with depth in a steplike manner. As this may be a near-surface effect, extrapolation of the measurements to much greater depths may not be warranted.

The entrainment of cohesive sediments in freshwater

Abstract: Presented in this study are experimentally measured characteristics of the entrainment rate and settling speeds of fine-grained, cohesive sediments in freshwater. The investigation focused on three different sediments of varying mineral composition which are representative of the sediments of Lake Erie. The influences of the bulk sediment water content and of the mineral and size composition on the entrainment rate and equilibrium concentration of these suspended sediments were determined. Linear increases in the bulk sediment water content resulted in logarithmic increases in the entrainment rate and equilibrium concentration. For a limited range of shear stresses near that needed to initiate noticeable entrainment the entrainment rate and equilibrium concentration increased logarithmically as the applied shear stress was increased linearly. For larger stresses, ar increases in the applied shear stress caused approximately linear increases in the entrainment rate and equilibrium concentration. At fixed values of water content and shear stress the entrainment rate and equilibrium concentration increased as the clay mineral content of the sediment increased and as the median particle size decreased. Settling speed experiments demonstrated that the median settling speed of the suspended sediment particles in a quiescent fluid increased as the ionic strength of the solution increased. At amore » fixed ionic strength the settling speed decreased with increased clay mineral content.« less

Toward a viscoelastic modelling of the injection molding of polymers

Abstract: By using theLeonov viscoelastic constitutive equation, an idealized problem has been solved for onedimensional, unsteady, non-isothermal flow of polymer between two parallel plates and the subsequent non-isothermal relaxation following cessation of flow. Numerical results are presented for the time dependence of the pressure gradient, the gapwise distribution of linear velocity, shear rate, shear stress and normalstress differences, together with the components of birefringence in different planes. Comparison of the present predictions for the pressure gradient with results based upon an “inelastic” model indicate close agreement whereas the corresponding predictions for normal-stress differences are found to be markedly different from those for the “inelastic” case.

Shear heating and the state of stress on faults

Abstract: Evidence for shear heating associated with crustal faulting is described for a number of fault zones in a wide variety of tectonic environments. It is shown that a thermal anomaly associated with the fault has resulted in the metamorphism of adjacent rocks to higher grades than can be accounted for by considering other heat sources. If relative displacements across these faults occurred at plate tectonic rates or slower, then shear stresses of a kilobar or so must have existed on the faults in order to produce the observed shear heating.

Stress intensity factors and COD in an orthotropic strip

Abstract: The elasticity problem for an orthotropic strip or a beam with an internal or an edge crack under general loading conditions is considered. The numerical results are given for four basic loading conditions, namely, uniform tension, pure bending, three point bending, and concentrated surface shear loading. For the strip with an edge crack additional results regarding the crack opening displacements are obtained by using the plastic strip model. A critical quantity which is tabulated is the maximum compressive stress in the plane of the crack. It is shown that this stress may easily exceed the yield limit in compression and hence may severely limit the range of application of the plasticity results.

Investigation into the existence of edge—face coagulated structures in Na-montmorillonite suspensions

Abstract: The rheological and coagulation characteristics of Na-montmorillonite suspensions have been investigated as a function of pH and NaCl concentration to ascertain whether edge—face coagulation occursContrary to the current hypothesis of particle interactions in this system, dilute suspensions ( 03 mol dm–3 The extrapolated shear stresses of the salt contaminated systems exhibited a minimum in the pH range 7–9When the concentration of Na-montmorillonite was increased above 1 % w/w, at NaCl concentrations below 10–3 mol dm–3, the suspensions became pseudoplastic and a shear modulus became measurable The shear modulus and the extrapolated shear stress both increased exponentially with clay concentration These effects are attributed to the interpenetration of electrical double layers because the addition of small amounts of NaCl, by compressing the electrical double layers, markedly decreased the magnitude of both parameters However, increasing the NaCl concentration above 5 × 10–3 mol dm–3 brought about edge—edge coagulation demonstrated by an increase in the shear modulus, the appearance of a yield stress in the flow curves and the onset of thixotropic character

Structural failure in selected raw fruits and vegetables

Abstract: A torsion test was developed for studying the structural failure of selected raw fruits and vegetables. Apple, melon and raw potato flesh were tested at a shear strain rate of approximately 0.26s-1 in torsion and uniaxial compression. Low strain modulus values were determined in addition to shear stresses and normal strains at failure. Results corroborated the maximum normal strain failure criterion proposed by Segerlind and Dal Fabbro (1978) for apples and suggested its application to potatoes and melons if true strains are used rather than engineering strains. The maximum shear stress theory also seemed to be a possible failure criterion for potatoes. Results comparing compressible and incompressible materials suggest that bulk strain affects the shear stress at failure. The observed failure planes supported the quantitative results for stresses at failure. Scanning electron micrographs indicated that the cellular failure occurred in the cell wall, regardless of whether it was due to tension, Varying specimen lengths or diameters had negligible effects on the uniaxial compression modulus but did affect the shear stress at failure in a manner yet to be satisfactorily explained.

Cyclic Stress-Strain History and Shear Characteristics of Clay

Abstract: Effects of some factors on the excess pore pressure are clarified, followed by an examination of the reliability of cyclic pore pressure measurement, the normalization of stresses, and the failure due to cyclic loading of clays. An equation of the residual excess pore pressure, represented as a function of both the maximum cyclic shear strain and the overconsolidation ratio, is deduced. An overconsolidated state due to the cyclic stress-strain history is similar in strength to one due to the ordinary overconsolidation history. In spite of the temporary loss in strength and deformation modulus immediately after cyclic loading, the dissipation of pore pressures leads to higher strength and deformation modulus than the initial ones before cyclic loading. It is implied that cyclic stress-strain history can be one of the factors causing natural ground to become lightly overconsolidated.

Fatigue-crack-tip plastic strains by the stereoimaging technique

Abstract: The stereoimaging technique is an accurate, high-resolution means of measuring the in-plane displacements resulting from the deformation of a specimen so that the corresponding components of the strain tensor can be computed independently of the stresses. The example used in this paper is a fatigue-cracked specimen of a microscopically homogeneous experimental powder-metallurgy aluminum alloy, analyzed to determine the displacement and strain fields accompanying the opening of the fatigue crack. The displacement measurements are processed by a computer program which compensates for measurement fluctuations in the displacement data by smoothing, and derives the strain magnitudes. The principal strains and the maximum shear strain are determined using Mohr's circle, and the latter strain is then used to estimate the plastic-zone size. The crack-opening mode may be inferred from the displacement map, and the state of stress (plane stress or plane strain) inferred by applying the in-plane compatibility equation.

Observations on the Melt Rheology of Thermotropic Aromatic Polyesters

Abstract: Rheological measurements were made on a variety of substantially aromatic copolyesters. All of them differ from flexible chain polymers in having unusually long relaxation times, estimated from shear rate dependence of viscosity and from melt elasticity. This behaviour is not accounted for by an unusual molecular weight distribution (MWD); limited measurements indicate that the MWD is somewhat narrower than that of conventional condensation polymers. Numerous anomalous flow phenomena have been observed, though not necessarily with all samples. These include the occurrence of a yield stress, large secondary shear stress maximum after start-up of steady shear, transient negative normal stress, and shear-thickening of melt viscosity. Although the melts are highly elastic, they do not exhibit swelling after extrusion from a capillary. The melt rheology is often very sensitive to temperature and dependent upon thermal history. In one material this dependence has been linked to the destruction and/or growth of crystallites formed from blocks of one comonomer.

Time dependent nonlinear shear stress effects in simple liquids: A molecular dynamics study

Abstract: The effects of large amplitude shearing rates on 108 particle amorphous Lennard‐Jones systems have been examined. A resolution of the structural and dynamical properties reveals that there is a tendency for shear to reorganize the liquid into layers to facilitate flow and hence reduce shear viscosity. The first steps have been made to determine those physical phenomena that are necessary to develop a theory of time dependent nonlinear shear viscoelastic effects. The form of the distortion giving rise to nonlinear shear stresses is well understood although its magnitude is intimately linked with parallel structural changes in an, at present, intractable way.

Some constraints on levels of shear stress in the crust from observations and theory

Abstract: In situ stress determinations in North America, southern Africa, and Australia indicate that on the average the maximum shear stress increases linearly with depth to at least 5.1 km measured in soft rock, such as shale and sandstone, and to 3.7 km in hard rock, including granite and quartzite. Regression lines fitted to the data yield gradients of 3.8 MPa/km and 6.6 MPa/km for soft and hard rock, respectively. Generally, the maximum shear stress in compressional states of stress for which the least principal stress is oriented near vertically is substantially greater than in extensional stress regimes, with the greatest principal stress in a vertical direction. The equations of equilibrium and compatibility can be used to provide functional constraints on the state of stress. If the stress is assumed to vary only with depth z in a given region, then all nonzero components must have the form A+Bz, where A and B are constants which generally differ for the various components. This implies that the deviatoric stress changes linearly with depth, and the general solution also allows the directions of the horizontal principal stresses to change monotonically with depth. Solutions to the equations, a churning stress to vary with both z and x, were fit to the observations of Zoback and Roller, who measured stress along a horizontal profile near Palmdale, California. The results indicate that the average shear stress in the upper 8 km of the fault zone is about 3.5 MPa less than the shear stress in the far field, but this far field term, which is part of the solution and has the form A+Bz, cannot be evaluated using the existing constant depth data.

Principal Stress Rotation: A Missing Parameter

Abstract: Changes in the directions of principal stresses occur in all ground work associated with engineering works, as well as earthquakes; an important topical example is the cyclic loading from waves on the foundation soil of offshore structures. A review of the shortcomings of existing laboratory shear apparatus is followed by a description of a new plane strain device, the Directional Shear Cell, which avoids strain constraints while enabling chosen rotations of principal stress directions to be imposed on a uniformly stressed cubical sample. These rotations may be single jumps of any magnitude from 0? to 90? or continuous cyclic variations in direction up to 75—. Data from tests on sand in the Directional Shear Cell show the severe effects of some principal stress rotations on stress-strain relationships. The cyclic tests with large continuous direction change showed an unrelenting increase in cumulative strain at low mobilised shear strength.

Experimental investigation of the influence of molecular weight distribution on the rheological properties of polypropylene melts

Abstract: An experimental study of steady shear and elongational flow Theological properties of a series of polypropylene melts of varying molecular weight and distribution is reported. Broadening the molecular weight distribution increases the non-Newtonian character of the shear viscosity function and increases the principal normal stress differences at fixed shear stress. The behavior is compared to earlier rheological property-molecular weight studies. Correlations are developed for these properties in terms of molecular structure. Elongational flow studies indicate that for commercial and broader molecular weight distribution samples, ready failure by neck development occurs and the elongational viscosity appears to decrease with increasing elongation rate. For narrower molecular weight distribution samples, the elongational viscosity is an increasing function of elongation rate, The implication of these experimental results to viscoelastic fluid constitutive equations and polymer melt processing is developed.