Showing papers on "Shear stress published in 1976"

Erosion And Transport Of Bed-Load Sediment

Abstract: In this thesis first a general derivation is given of the 'macro'-equations of mass- and linear-momentum balance that govern the mo'mentum transfer from a Newtonian fluid to rigid particles in a fluid-solid mixture. In particular, attention is paid to a) the attenuation of viscous-momentum transfer from the boundary to the interior of a granular bed subject to a surface flow, b) the drag and lift forces exerted by a turbulent shear flow on particles of the bed surface, and, c) the balance of forces acting on a bed load under uniform-flow conditions. It is shown that filter flow driven by shearing along the boundary of a granular sediment bed exerts a drag force on a layer of only two or three particle diameters within the bed. A drag force on the bulk mass of sediment is only exerted by a pore-pressure gradient. Stability conditions are formulated for a loose granular bed subject to erosive flow, at SHIELDS' grain-movement condition and dUring bed-load transport. 'Macro'-stresses acting along 'wavy' surfaces parallel to the bed are defined for that purpose, and an attenuation factor is introduced for the transmission of turbulent fluid shear from the surface towards the interior of the bed. It is shown that SHIELDS' dimensionless expression for the critical bed shear stress at the threshold of continuous sediment motion, 1/Phi , must reach a constant value for low-shear Reynolds' numbers (Re* < O. 5), as long as there is no cohesion between the particles. It is concluded that the bed load, consisting of particles rolling and saltating over the bed, must reduce the maximum turbulent fluid shear at the bed surface, at sufficiently high bed shear stress, to the critical threshold drag that would lead to the initiation of non-ceasing scour.

Rupture propagation with finite stress in antiplane strain

Abstract: Rupture propagation in antiplane strain is investigated by using both analytic and numerical methods. Under the assumption that a solid will absorb energy irreversibly when it is strained at a sufficiently large shear stress it is found that energy must be absorbed at the rupture front in addition to the work done against the sliding friction stress. The energy absorbed increases with propagation distance, so it is not negligible at any length scale and is much larger than the ideal surface energy of molecular cohesion. The concept of a critical crack length carries over to the case of a finite stress-slip law on a fault plane but does not carry over to a homogeneous inelastic medium. In a dynamic slip event, while a typical value of particle velocity is proportional to stress drop, the peak value near the fault is proportional to material strength.

The lateral migration of a spherical particle in two-dimensional shear flows

Abstract: The lateral migration of a solid spherical particle suspended in a fluid flowing between parallel vertical walls is investigated theoretically using a method developed by Cox & Brenner (1968) Buoyant and neutrally buoyant, freely rotating and non-rotating particles in the fluid flow are considered as is also the case of a sedimenting particle in a quiescent fluid The results obtained are applied to the special cases of plane Poiseuille flow and of plane shear flow, these situations being investigated in detail

Constraints on diffusional cavity growth rates

Abstract: There have been a number of rate equations proposed in the literature for the growth of grain-boundary cavities during creep. Implicit in all these analyses is the assumption that growth occurs freely and that no constraints exist that would attenuate the predicted rates. It is shown that a constraint on growth rate can occur in polycrystals because creep cavities are inhomogeneously distributed amongst the various grain boundaries. This attenuation of growth rate is predicted to be greatest when high-strength alloys with large cavity populations are deformed slowly and least when pure metals with low cavity populations are deformed quickly. The application of these ideas to multiaxial stress states and extrapolation procedures is discussed in the knowledge that the rate of unconstrained cavity growth is directly proportional to the maximum principal tensile stress, whereas constrained growth is proportional to the nth power of the octahedral shear stress.

On the separation of air flow over water waves

Abstract: Conditions leading to the onset of air-flow separation over a mobile air-water interface are discussed. It is argued that, in a frame of reference in which the interfacial boundary assumes a steady shape, the occurrence of separation requires a stagnation point on the interface. In the case of air blowing over water waves, this corresponds to the onset of wave breaking. These arguments are strongly supported by flow visualization and pressure measurements carried out in a laboratory wind-wave flume. Furthermore, the pressure measurements show a greatly enhanced interfacial shear stress for a breaking wave compared with that over an unbroken wave of the same wavelength. The implications of these findings for wind-wave generation are discussed.

Sand liquefaction in large-scale simple shear tests

Abstract: Samples used in the study were approx 90-in. long by 42-in. wide and 4-in. thick and they were subjected to uniform stress cycles at a frequency of 4 cps. Measurements of changes in pore-water pressure and shear strain development are presented for samples prepared at relative densities of 54%, 68%, 82%, and 90%, and the results are interpreted to determine the values of stress ratio causing initial liquefaction and different degrees of shear strain, giving appropriate consideration to the effects of system compliance. It is shown that a condition of initial liquefaction could be induced in all samples, but for samples with relative densities greater than about 45%, initial liquefaction was accompanied by limited shear strains, the limiting strain potential decreasing with the relative density. The test results are compared with data from other investigations and conclusions are drawn concerning the accuracy of tests on small-scale samples and shaking table investigations.

Oceanic lithosphere and asthenosphere - Thermal and mechanical structure

Abstract: A coupled thermal and mechanical solid state model of the oceanic lithosphere and asthenosphere is presented. The model includes vertical conduction of heat with a temperature dependent thermal conductivity, horizontal and vertical advection of heat, viscous dissipation or shear heating, and linear or nonlinear deformation mechanisms with temperature and pressure dependent constitutive relations between shear stress and strain rate. A constant horizontal velocity u sub 0 and temperature t sub 0 at the surface and zero horizontal velocity and constant temperature t sub infinity at great depth are required. In addition to numerical values of the thermal and mechanical properties of the medium, only the values of u sub 0, t sub 0 and t sub infinity are specified. The model determines the depth and age dependent temperature horizontal and vertical velocity, and viscosity structures of the lithosphere and asthenosphere. In particular, ocean floor topography, oceanic heat flow, and lithosphere thickness are deduced as functions of the age of the ocean floor.

A physical model for the rate of deposition of fine-grained sediments in the deep sea

Abstract: A model for the net entrapment of fine suspended sediment in the viscous sublayer of a turbulent boundary layer yields an expression of the same form as one derived previously on an empirical basis. The expression is modified by a factor that takes into account the reduction in rate of deposition caused by occasional erosion, bottom roughness, and organic resuspension. A curve for the variation in critical deposition shear stress with particle settling velocity is constructed using the assumption that the critical deposition stress is equal to the critical erosion stress for fine noncohesive material. Application of the model to postglacial deposition rates using modern concentration data gives reasonable agreement and suggests values of the factor little less than unity. The model involves assumption of a constant near-bed suspended sediment concentration. An alternative case of decrease in concentration with distance along the flow path shows that the grain-size modes in the deposited sediment resemble the pattern of size modes found north of the Carnegie Ridge in the Panama Basin.

Flow and sedimentary processes in the meandering river South Esk, Glen Clova, Scotland

Abstract: Channel geometry, flow and sedimentation in a meander bend of the River South Esk were studied from bankfull stages (January–February) to low water stages (May) in 1974. Bed topography varied little over the study period, showing a typical pool and ripple geometry. Variation of mean depth and velocity with discharge differed from section to section around the bend, due primarily to locally varying flow resistance with stage. The flow pattern for all stages was dominated by a single spiral over the point bar, with a development zone at the bend entrance. Deviation of bed shear stress from the mean flow direction was in general accord with theory, especially for high stages. The use of a uniform longitudinal water surface slope in the calculation of bed shear stress is not justified because of a complicated water surface topography, also such calculated shear may not represent effective bed shear on grains, as it accounts also for energy losses associated with secondary flows. Dunes covered much of the bar at high stages, with increasing proportions of ripples, sand ribbons and lower phase plane beds at low stages. Local flow resistance generally decreases from dunes, diminished and ripple-backed dunes, ripples, sand ribbons to plane beds, and bed forms are predicted quite well by the stream power-grain size scheme. Mean size, sorting and skewness of sediment over the bed changes little with stage. In general, size decreases, sorting improves and skewness changes from positive to negative from the talweg to the inner bank, and in the downstream direction. Allen's (1970a, b) force balance equation for moving bed load particles is supported for bankfull stage, with some reservations, and textural characteristics are explained by progressive sorting in the direction of sediment transport. Large-scale trough cross stratification (with some flat bedding) formed at high stage by dunes (and lower phase plane beds) dominates the point bar sediments. Alternations of fine-medium sand (often cross-laminated) and vegetation-rich layers result from periodic deposition on the grassed upper bar surface. Fining upwards sequences produced by lateral channel migration are modified by a coarsening upward subsequence in the upstream bar region where spiral flow is developing from the bend upstream.

An Improved Mathematical Model for Relating Shear Stress to Shear Rate in Drilling Fluids and Cement Slurries

Abstract: The Newtonian, Bingham, and power law models previously have been used to approximate the rheology of drilling fluids and cements. The proposed yield-pseudoplastic model provides more consistently accurate descriptions of the rheology of such fluids. Simple explicit relationships between the wall shear rate and the volumetric flow rate in both pipe and annualar flow have been derived from this model for use in engineering calculations.

The growth of gravity-capillary waves in a coupled shear flow

Abstract: The growth rates and phase speeds of gravity-capillary wind waves are investigated through numerical solution of a linear, viscous, coupled, shear-flow perturbation model. Numerical results are obtained by transforming the boundary-value problem of a perturbed mean laminar shear flow into a matrix-eigenvalue problem using standard finite-difference methods.Detailed calculations are performed for a basic state composed of a logarithmic-linear mean flow profile in the air and a linear-logarithmic mean flow profile in the water. We exclude turbulent Reynolds stresses. Calculated growth rates show excellent agreement with corresponding experimental growth rates. This implies that the initial growth of gravity-capillary wind waves is almost certainly due to the instability of the coupled laminar shear flow in the air and water.The investigation also demonstrates that the shear flow in the water cannot be ignored in wave growth studies, since the usual 3-4%, highly sheared, wind-induced surface drift produces a significant increase in the growth of wind-generated gravity-capillary waves.

Shear strength of low-rise walls with boundary elements

Abstract: Results of tests on eight specimens representing low-rise shear walls with boundary elements are reported and analyzed. The principal variables included amount of flexural reinforcement, amount of horizontal wall reinformcement, amount of vertical wall reinforcement, and height-to-horizontal-length ratio. Flexural reinforcement was varied from 1.8% to 6.4% of the boundary element area; horizontal wall reinforcement and vertical wall reinforcement were varied from 0% to 0.5% of the wall area; and height-to-horizontal-lenght ratio was varied from 1:4 to 1:1. The test program was designed to determine the effect of load reversals. One specimen was repaired and retested. Results indicate that current design procedures underestimate the strength of lowrise shear walls, even when the walls are subjected to reversed load. A suggested design procedure is presented.

Grain boundary cavitation under various states of applied stress

Abstract: Submicrometre grain boundary cavities are produced in Nimonic 80A when plastic deformation in any of three different stress states is followed by a short anneal. Tension, torsion and compression specimens were plastically strained in a systematic manner and then annealed for 2 h at 750 °C. Detailed quantitative observations with a 1 MV microscope showed that the number of cavities per unit volume was a function of the shear strain and independent of the stress state. Furthermore the measurements revealed the surprising result that most cavities were on those grain boundaries which were parallel to the maximum principal stress axis. However, Preferential cavity growth occurred during subsequent tensile creep and cavities on these parallel boundaries either remained constant in size or diminished while those on boundaries which were orthogonal to the applied stress axis grew relatively quickly, thus producing the usual appearance of cavitated tensile samples. Plastic strain was more detrimental to torsional creep ductility when the direction of torque between plastic deformation and creep was reversed which is in accordance with the anisotropic cavitated boundary distribution. The results are compatible with the hypothesis that cavities are produced by grain interior slip and stabilized by plastic deformation induced internal tensile stresses.

Pore and eroding fluid influences on surface erosion of soil-closure

Abstract: Comments are made regarding the influence of the pore fluid composition on soil structure, the dispersion-cohesion boundary and the Pinhole Test, and determinations of the critical shear stress of core samples. It is noted that different clay minerals have different boundary curves, which is presumably a factor in the width of the "Dispersive or Nondispersive" band. The uncertainity inherent in the Pinhole Test is also a factor which contributes to the band of uncertainity. This test is an empirical test based on subjective evaluation of the occurrence of dispersion, and its use to evaluate the critical shear stress on soil is inaccurate. The rotating cylinder test, using loss of weight as a measure of erosion, provides accurate determinations of the critical shear stress of core samples or remolded samples of cohesive soils, so long as vertical variations in soil properties are not significant over the length of the sample.

Axially symmetric turbulent boundary layers on cylinders: mean velocity profiles and wall pressure fluctuations

Abstract: Experimental measurements of the mean velocity profiles produced by axially symmetric turbulent boundary layers on cylinders of various diameters are described. The profile measurements were made with very small hot wires developed for this investigation. Measurements of the wall shear stress on cylinders ranging from 0.02 to 2.0 in. in diameter are also reported. In the boundary layer on cylinders, well-defined regions exist in which the two-dimensional law of the wall and a three-dimensional wake law are valid. There was no evidence that the boundary layer was not fully turbulent even on the cylinders of smallest diameter. Measurements of wall pressure fluctuations beneath the boundary layer on a 1 in. diameter cylinder are also described. The results were much the same as those previously reported by Willmarth & Yang (1970) for a 3 in. diameter cylinder. The only difference was the discovery that the wall pressure was correlated in the transverse direction approximately half-way around the cylinder. This was not true on the 3 in. diameter cylinder.

Shear flow instability in nematic liquids : theory steady simple shear flows

Abstract: 2014 We study shear flow instabilities in nematic liquids of planar geometry (molecules parallel to the plates limiting the sample) and where molecules are aligned (along Ox) perpendicular to the shear flow direction (along Oy). An external magnetic field reinforces the alignement imposed by surface effects. We develop a bidimensional model which takes into account exactly boundary conditions on the director and the velocity fields. The two types of instability which have been observed by Guyon and Pieranski (homogeneous distortion and convective rolls parallel to Oy) are interpreted within the framework of this model. We obtain instability thresholds together with the value of the critical wave vector qcx at threshold. Under weak fields the homogeneous distortion (qcx = 0) is achieved, whereas in high fields a roll instability develops (qcx ~ 0). The crossover from one regime to the other takes place at H ~ 1 kG. Moreover, we show that in either case, as far as the director is concerned, the aspect of the distortion along the z-direction is dominated by only one wave vector qz. Contrary to the case of the homogeneous distortion where qz varies strongly with H, for the roll instability qz remains nearly constant and close to 03C0/2a (2 a is the thickness of the sample). This allows us to develop an approximation which leads to results in good agreement with those obtained exactly. LE JOURNAL DE PHYSIQUE TOME 37, AVRIL 1976, Classification Physics Abstracts 6 . 300 7 . 130

Mantle plumes - A boundary layer approach for Newtonian and non-Newtonian temperature-dependent rheologies

Abstract: Stress is placed on the temperature dependence of both a linear Newtonian rheology and a nonlinear olivine rheology in accounting for narrow mantle flow structures. The boundary-layer theory developed incorporates an arbitrary temperature-dependent power-law rheology for the medium, in order to facilitate the study of mantle plume dynamics under real conditions. Thermal, kinematic, and dynamic structures of mantle plumes are modelled by a two-dimensional natural-convection boundary layer rising in a fluid with a temperature-dependent power-law relationship between shear stress and strain rate. An analytic similarity solution is arrived at for upwelling adjacent to a vertical isothermal stress-free plane. Newtonian creep as a deformation mechanism, thermal anomalies resulting from chemical heterogeneity, the behavior of plumes in non-Newtonian (olivine) mantles, and differences in the dynamics of wet and dry olivine are discussed.

Computer simulation of the screw dislocation motion in b.c.c. metals under the effect of the external shear and uniaxial stresses

Abstract: An atomistic study of the motion of the 1/2 [111] screw dislocation was carried out for a shear stress applied on {112} planes and for uniaxial stresses along [012], [001] and [111]. Central force interactions described by the three different empirical potentials used in the previous work (Duesbery, Vitek & Bowen 1973) were assumed. The distortions of the core and the subsequent dislocation motion always reflected the twinning-antitwinning asymmetry of shear on {112} planes. The non-shear components of the stress tensor introduced further asymmetries which vary with interatomic forces. The application of the results of this study to the theory of slip and twinning in b. c. c. metals, is discussed.

Slip processes in the deformation of polystyrene

Abstract: Two slip processes are characterized in the deformation of atactic polystyrene by compression. In the optical microscope, one appears as intensive shear bands and the other as diffuse shear zones. But in the electron microscope, the latter reveals itself in the form of two sets of numerous, fine, discontinuous shear bands intersecting at nearly right angles. In addition to their differences in appearance, the coarse slip band propagates fast along a localized path, inclines at less than 45‡ with the compression axis, and invariably produces shear fracture when it extends across the specimen. On the other hand, the fine slip bands spread slowly by multiplication mainly along the maximum shear stress direction, contribute to almost all the macroscopic strain and cause shape changes of the specimen. Hence the coarse band process is a brittle mode and the fine band process a ductile mode. The relative abundance of these bands depends on the thermal history of the specimen, the loading condition, and the deformation temperature. The average shear strain inside either band is about 1.5 and is recoverable upon annealing.

Analysis of RC Shear Panels under Cyclic Loading

Abstract: A nonlinear constitutive model for plain concrete subjected to cyclic biaxial stresses is described in brief. The model takes account of the compressive softening of concrete prior to reaching maximum compressive stress. Two reinforced concrete shear wall structures, one loaded monotonically and the second subjected to large cyclic load reversals are analyzed and compared with test data. Simulation of cyclic loading response is greatly improved compared to previous elastoplastic models. In addition to the improved material characterization in compression, the treatment of tensile cracking is felt to be a significant factor in the good match obtained with experimental data.

Studies on multilayer film coextrusion I. The rheology of flat film coextrusion

Abstract: Multilayer blown film coextrusion was studied, both experimentally and theoretically. For the experimental study, an annular die with a feed-port system was designed and multilayer blown films were produced by rotating the inner mandrel with a one horsepower variable-speed drive at speeds from nearly 2 to 6 rpm, and by inflating the tubular molten film with air. The die has 16 feed slots and melt pressure transducers are mounted along the axial direction of the outer wall of the annular flow channel. The transducers were used to determine the pressure gradient in the annular flow channel, which then permitted determination of the reduction in pressure drop when different combinations of two polymer systems were coextruded. Polymers used for b own film coextrusion were: (1) low-density polyethylene with ethylene-vinyl acetate; (2) low-density polyethylene with high-density polyethylene; (3) low-density polyethylene with polypropylene; (4) high-density polyethylene with ethylene-vinyl acetate. For the theoretical study, stratified helical flow was analyzed using a power-law non-Newtonian model. A computational procedure was developed to predict the number of layers, layer thickness, and the volumetric flow rate as functions of certain processing variables (namely, the pressure drop in the die, and the angular speed of rotation of the inner mandrel of the die) and the rheological parameters of the individual polymers concerned. Comparison was made of the theoretical prediction of volumetric flow rate with experimental ones. Some representative results are presented of the theoretically predicted axial and angular velocity distributions, shear stress profiles, and shear rate profiles.

Reflection, Transmission and Attenuation of Elastic Waves at a Loosely-Bonded Interface of Two Half Spaces

Abstract: Summary The problem of reflection and transmission of a longitudinal plane periodic wave incident on the interface between loosely-bonded half spaces is investigated by assuming that the interface behaves like a dislocation which preserves the continuity of traction while allowing a finite amount of slip. The wave equations are solved by imposing further the interface conditions that the normal displacement is continuous and that the shearing stress is proportional to velocity of slip. The results of numerical calculations are given in the form of graphs for different degrees of bonding, ranging from no bonding (zero shear stress) to full bonding (welded contact) between the half spaces. It is found that the elastic wave energy is attenuated at a loosely-bonded interface except at normal incidence, grazing incidence and at (approximately) critical incidence of the longitudinal wave at the interface.