Dependence Of Shear Stress Research Articles

Growth of ductile opening-mode fractures in geomaterials

Abstract Opening-mode fractures in clinker and opal-CT chert spheroids form by growth and coalescence of pores, and are associated with extensive textural and compositional changes in the host material. Extensive inelastic deformation outside the immediate vicinity of fracture tips characterizes these fracture processes as ductile. Fracture formation in clinker is concurrent with high-temperature combustion alteration of diatomaceous mudstone. Fracture formation in chert spheroids is associated with the opal-CT to quartz transition in the same host material during early marine diagenesis. In both cases, growth of elongate pores is attributed to the combined effects of diffusive-fracture growth and flow by solution-precipitation creep. Pore growth and coalescence occur preferentially ahead of fracture tips along two directions oblique to the mean macroscopic fracture direction. This growth process, referred to as side-lobe damage, is interpreted to reflect the shear-stress dependence of pore growth by solution-precipitation creep. The tendency for oblique fracture growth is suppressed by global stress and strain-boundary conditions forcing the fracture along a characteristic zig-zag propagation path that is macroscopically perpendicular to the loading direction. These examples of ductile fracture demonstrate that macroscopic fracture formation is not uniquely associated with damage processes by microfracture at low-temperature ‘brittle’ subsurface conditions. Instead, fracture is a deformation process that can occur due to various inelastic-deformation mechanisms under diverse crustal environments, which include high-temperature conditions.

Influence of Shear Stress on Cationic Surfactant Uptake by Anionic Gels.

Kinetic studies of cationic surfactant uptake by anionic polymer gel membrane under various shear flow have been performed, varying the alkyl chain length of surfactant, the ionic strength of surfactant solution, and the charge density of gel. By exposing the gel surface to a shear flow of ca. 1 Pa, the rate of surfactant uptake is distinctly enhanced, while the maximum binding ratio to the gel is not influenced. A linear relationship between the surfactant initial flux and shear stress has been established. At a high ionic strength, the effect of shear stress is suppressed, suggesting that the enhancement of surfactant uptake under shear flow is caused by a decrease in the surface electrostatic potential of the negatively charged polyelectrolyte gel, which favors the uptake of the positively charged surfactant. From the Nernst and Planck equation, a relationship between the surfactant uptake kinetics and the electrostatic field is derived that allows us to estimate the shear stress dependence of the change in the electrostatic field on the gel surface. The origin of the shear-induced surface electrostatic field change is discussed.

Influence of non-glide stresses on plastic flow: from atomistic to continuum modeling

The Schmid law, which is accurate for face-centered-cubic (fcc) metals, assumes that only the shear stress acting in the slip plane in the slip direction controls the plastic deformation. Hence, it is implicitly assumed that the critical resolved shear stress (CRSS) for the slip is not affected by any other components of the applied stress tensor. This rule is almost ubiquitously utilized in large-scale continuum computations of plastically deforming single and polycrystals. On the other hand, in materials with more complex structures and for some orientations of the dislocation line the cores can spread onto several non-parallel planes. The most widely-known example is the screw dislocation in bcc metals, though this phenomenon is quite universal in structures that are not close packed. Signatures of such core configurations commonly include unexpected deformation modes and slip geometries, strong and unusual dependence of flow stresses on temperature and strain rate, a high Peierls stress, and, in general, a breakdown of the Schmid law. In this paper, we first summarize results of atomistic computer simulations of the response of 1/2〈1 1 1〉 screw dislocations to the applied shear stresses. The calculations have been made using central-force many-body potentials and tight-binding based bond-order potentials for molybdenum. While the core structure found is not the same for the two descriptions of atomic interactions, both lead to a very similar orientation dependence of the critical resolved shear stress for the dislocation motion which takes place along the most highly stressed {1 1 0} plane. This dependence reveals the break down of the Schmid law invoked by the effect of shear stresses acting in another {1 1 0} plane, which are called non-glide stresses. These results are then transferred to macroscopic level by formulating single crystal yield criteria that include the effects of non-glide components of the stress tensor. These criteria form a basis for multislip yield criteria and flow relations for continuum analyses. Using this approach we demonstrate that the effects of non-glide stresses that have their origin at the level of individual dislocations also have significant effect on polycrystalline response, including a significant tension-compression asymmetry.

Geometrically necessary dislocations in a c-axis oriented cylindrical bar of basal-slipping hexagonal crystals deformed in torsion

A crystallographic analysis of the size-dependent geometrically necessary dislocations (GND) stored in a cylindrical hexagonal single crystal deformed by torsion with its c-axis aligned with the torsion axis has been made. The only activation of basal slip and strain rate dependence of critical resolved shear stresses have been assumed.

Tension–shear coupling in slip and decohesion of iron crystals

Results from atomistic simulations are used to parameterize constitutive relations involving the Frenkel sinusoidal dependence of shear stress on sliding displacement and a Rose–Ferrante–Smith universal binding form for dependence of tensile stress on opening displacement, for cases where the crystal symmetry dictates that a polarity in slip displacement exists.

Modeling shear-induced phase transitions of binary polymer mixtures

The shear-induced phase transitions of binary polymer mixtures have been studied. We solved the two-fluid model numerically in a high Weissenberg number and infinite Peclet number flow regime without linearization of the diffusive equation or the adiabatic approximation. Our simulations have captured key features of flow-induced phase transitions, such as the shift in the phase boundary from the equilibrium phase diagram under flow conditions. Both shear-induced mixing and demixing phenomena have been reproduced. The dynamic phase behavior of polymer mixtures is governed by parameters including φA and χ for thermodynamic forces, τ′ and G′ for the degree of dynamical asymmetry in polymer mixtures, and the imposed nominal shear rate. Dynamic phase diagrams are obtained for steady state flow. We have identified correlations between the microstructure evolution and rheology, and a mechanism to reach the steady state. The rheological signature of demixing fluids under shear is that the dependence of the steady state shear stress on shear rate changes from nonlinear to linear within that range of shear rates in which the demixing occurs.

Rheological behavior and microstructure of magnetic dispersions with varying nonmagnetic particle content

Abstract We examine the frequency, stress, and strain dependencies of the dynamic shear moduli of magnetic dispersions containing mixtures of varying amounts of magnetic cobalt-modified γ-Fe 2 O 3 and nonmagnetic cobalt-modified α-Fe 2 O 3 particles, and we interpret these results in terms of a network structure in the dispersions. The storage modulus exhibits a plateau, providing evidence for such a network. The critical strain amplitude indicating the transition from linear to nonlinear behavior is independent of magnetic particle fraction. This implies that the topology of the network structure formed by magnetic interactions between the particles is independent of nonmagnetic particle content. However, the critical stress amplitude indicating the transition to nonlinear behavior does depend on magnetic particle fraction. This indicates that the strength of the network depends on nonmagnetic particle content. The loss modulus exhibits a minimum when considered as a function of frequency. This minimum can be rationalized in terms of network elasticity at low frequencies and local viscous behavior at high frequencies. Furthermore, the ‘network elasticity’ extracted from small-amplitude oscillatory measurements scales the magnetic particle fraction dependence of shear stress–shear rate data for steady shear flow onto a single master curve.

Method for determining the shear stress in cylindrical systems.

We develop a method for determining the elements of the pressure tensor at a radius r in a cylindrically symmetric system, analogous to the so-called "method of planes" used in planar systems [B. D. Todd, Denis J. Evans, and Peter J. Daivis, Phys. Rev. E 52, 1627 (1995)]. We demonstrate its application in determining the radial shear stress dependence during molecular dynamics simulations of the forced flow of methane in cylindrical silica mesopores. Such expressions are useful for the examination of constitutive relations in the context of transport in confined systems.

Plastic deformation behaviour in Ni 3Ti single crystals with D0 24 structure

The plastic deformation behaviour of Ni 3Ti single crystals with the D0 24 structure was investigated in compression tests. Two slip planes of the type {1 1 ̄ 00} and (0001) were selected depending on crystal orientation. Temperature dependences of the critical resolved shear stress (CRSS), strain rate sensitivity of flow stress, activation volume and dislocation structure were examined. Anomalous strengthening slightly appeared for the {1 1 ̄ 00}<11 2 ̄ 0> prism slip system in a narrow temperature range between 300 and 500 °C. Accompanying the yield stress anomaly (YSA) were small jogs formed on 1/3<11 2 ̄ 0> screw dislocations, acting as pinning points for motion of the dislocations and resulting in a zigzag shape of dislocation lines inclined to the Burgers vector by ±15~20°. In contrast, a remarkable YSA was found for the (0001) basal slip system over a wide temperature range between −196 and 800 °C. At low temperatures at and below 300 °C, 1/3<2 1 ̄ 1 ̄ 0> superlattice dislocations were dissociated into two 1/3<10 1 ̄ 0> super-Shockley partials bounded by an SISF after unloading. Above 300 °C, the dissociation mode of 1/3<2 1 ̄ 1 ̄ 0> superlattice dislocations changed to APB-type splitting accompanied by a large increase in CRSS. The strong YSA was caused by the Kear-Wilsdorf locking of 1/3<2 1 ̄ 1 ̄ 0> screw dislocations from the (0001) slip plane onto the {01 1 ̄ 0} planes.

Effect of compositional order upon ductile shear failure and structural relaxation of Fe (80− x) Ni xB 20 metallic glass ribbons

Experimental results of the compositional dependence of the critical shear stress at low temperatures and of several structural relaxation characteristics at elevated temperatures are compared and examined with respect to singularities of the behavior at certain ‘stoichiometric’ compositions in a series of Fe 80− x Ni x B 20 ( x=10, 20, 30, 40, 50 and 60) metallic glass ribbons. The shear stress for ductile failure has been determined in uniaxial tension; structural relaxation has been followed by the change of the vibration frequency during annealing with a constant heating rate up to crystallisation in a vibrating-reed set-up. Minima in all measured characteristics are observed for x=40 (and possibly for x=60). This is explained in the framework of the polycluster model of amorphous solids by small values of the critical stresses for local plastic shear along intercluster boundaries in the Fe 40Ni 40B 20 metallic glass due to the low concentration of coincident nodes at these boundaries in comparison with those in metallic glasses of nearby compositions.

Indentation hardnesses of semiconductors and a scaling rule

The Vickers hardnesses of 15 semiconductors, namely Si, Ge, SiC, AlN, GaN, GaP, GaAs, GaSb, InP, InAs, InSb, ZnO, ZnSe, ZnTe and CdTe, have been investigated from room temperature to their melting points. The temperature dependences of the Vickers hardness H V of 11 of these semiconductors, namely those with a cubic structure, obey a universal relationship when H V and the temperature T are scaled respectively by the shear modulus G and by Gb 3 / k B, with b being the magnitude of the Burgers vector. The scaling rule is the same as that found for the temperature dependence of the critical shear stress c for the {111}<110> slip system, satisfying the relationship H V = (70-100) c .

Tensile deformation and fracture behaviour in NbSi 2 and MoSi 2 single crystals

Tensile deformation behaviour in NbSi 2 and MoSi 2 single crystals with the C40 and C11 b structure, respectively, was examined in a temperature range up to 1500 °C and compared with that under compression, focusing on the temperature dependence of the critical resolved shear stress (CRSS), elongation/fracture strain and crack initiation process. Two different types (Type A and Type B) of geometry of NbSi 2 tensile specimens were used to evaluate elongation and fracture behaviour. No difference in the CRSS for the 0001 2 1 ̄ 1 ̄ 0 slip of NbSi 2 and that for the {011)<100] slip in MoSi 2 was found in both modes of tension and compression. In contrast, the ductile-brittle transition temperature (DBTT) depended strongly on the loading mode and the type of geometry. Elongation and process for crack initiation in NbSi 2 changed remarkably depending on the deformation temperature. Four new slip systems of 10 1 ̄ 0 1 3 1 2 ̄ 10 , 10 1 ̄ 1 1 3 1 2 ̄ 10 , 10 1 ̄ 0 0001 and 11 2 ̄ Y 112X (X×Y=−6 ) were found near the crack path in NbSi 2 deformed at 1000 °C.

EFFECT OF MAGNETIC HYSTERESIS OF THE SOLID PHASE ON THE RHEOLOGICAL PROPERTIES OF MR FLUIDS

An experimental investigation is described concerning the effect of the existence of a remanent magnetization of the dispersed particles on the rheological properties of magnetorheological fluids (MRF). Two MRF's were used: (1) solid phase: cobalt ferrite particles + silica gel (1.5% w/w); liquid phase: silicone oil (viscosity 20 mPa·s); and (2) solid phase: carbonyl iron + silica gel; liquid phase; silicone oil. The cobalt ferrite particles were synthetized as monodisperse colloidal spheres with an average diameter of 850 nm. The dependence of the dimensionless shear stress (τ⋆/ϕ) vs. Mason number (Mn) fails to scale when a "magnetorheological hysteresis procedure" is followed, specially for the higher volume fractions used (≈ 7.5%). The yield stress (τy) is first estimated from successive rheograms obtained decreasing the external field (H0) values for different ϕ. A more precise determination can be done by applying a stress ramp in the oscillatory regime. The critical stress amplitude (τc) needed to exceed the viscoelastic linear region (VLR) is obtained. It is found that both τy and τc strongly depend on the magnetic history of the sample. As expected, the previous results were not obtained in a classical MRF of carbonyl iron particles since they do not present magnetic hysteresis. We conclude that cobalt ferrite suspensions are an other kind of MRF which works at low fields (0 – 17.8 kA/m) with the opposite effect: decrease of the yield stress with the field. This property can be improved using particles with stronger remanent magnetization.

COMPARATIVE INVESTIGATIONS ON ER FLUIDS WITH DIFFERENT POLARIZATION MECHANISMS

The rheological and electric properties of various ER fluids in strong electric fields have been investigated simultaneously. From the measured data the dependences of shear stress and real and imaginary part of the high field permittivity on field strength, temperature and frequency have been derived. One group of ER fluids containing perovskite type particles obeys a polarization mechanism which is attributed to the local displacement of ions in the crystal structure of the particles. The contribution of conduction to ER activity is low and the electric properties of these ER fluids are nearly constant. The increase of shear stress with frequency is in accordance with the polarization model. In contrast, another group of ER fluids containing zeolite or ion conducting polymer particles shows a strong dependence of the complex permittivity on field strength and temperature demonstrating the pronounced nonlinearity of the electric properties. It is assumed that both conduction and polarization contribute to the ER activity in these systems. The relative extent of the contributions changes with external conditions like field strength and temperature.

Three alternative experimental methods to determine the antiphase-boundary energies of the γ′ precipitates in superalloys

Abstract The {111} antiphase-boundary (APB) energies Γ of the L12 long-range-ordered γ′ phases that precipitation strengthen the commercial nickel-based superalloys Nimonic PE16 and Nimonic 105 were measured by three independent methods. Γ was derived, firstly, from the dependence of the critical resolved shear stress on the dispersion of the γ′ precipitates in these superalloys, secondly, from the minimum size of Orowan dislocation loops which can be sustained by γ′ precipitates and, thirdly, from the separations of the superpartial dislocations bounding APB faults in the single-phase γ′ alloys. The latter method is based on state-of-the-art transmission electron microscopy investigations and is considered as the most direct. The results determined by the three different methods show (with one exception) satisfactory agreement. This proves the validity of all three experimental approaches and thus supports the theoretical models on which the first two methods are based. Since Γ governs the strength of su...

In-lineultrasonic characterization of shear dispersion processes of polydisperse fillers in polymer melts

Shear break-up processes of polydisperse fractal clusters areinvestigated by the ultrasound scattering technique. Within theframework of fractal aggregation and the hybrid approach modelfor polydisperse correlated scatterers, the concept of variancein the local filler concentration is used to derive a newexpression for the scattering cross-section for polydispersefractal aggregates in the Rayleigh scattering regime. Considering the scaling laws for the shear-induced disruption ofthe clusters, the shear stress dependence of the ultrasoundscattered intensity for polydisperse fractal aggregates is alsoderived. The fractal scattering regime is further discussed forboth monodisperse and polydisperse clusters of size larger thanthe wavelength. In-line ultrasonic measurements for the sheardisruption processes of silica fume fillers compounded with polypropylene during extrusion are investigated. A criticaldisaggregation shear stress is determined and is found todecrease with the filler surface treatment concentration. Thisstress is representative of the particle adhesiveness andaggregate dispersion in the matrix. This is confirmed by theimprovement in impact resistance tests. On the basis of thescaling laws and the self-consistent-field approximationusually used in the microrheological models, the shear-thinningbehaviour of silica fume clusters is successfully simulated.

On the temperature dependence of the critical resolved shear stress of the γ ′-strengthened superalloy NIMONIC PE16

The temperature dependence of γ ′-strengthening of the commercial nickel-base superalloy NIMONIC PE16 has been investigated by measuring the critical resolved shear stress in the temperature range 373–1173 K and observing the resulting slip line patterns.

Effect of fluid shear stress on the permeability of the arterial endothelium.

The localization of atherosclerotic lesions is due, in part, to regional variations in the permeability of arterial endothelium to macromolecules. In turn, endothelial permeability may be influenced by fluid shear stresses. The spatial variation in endothelial permeability is reviewed and evidence for shear stress dependence upon permeability is presented. These results are examined in light of various signaling mechanisms that increase permeability by increasing the transport of water and macromolecules through the junctions separating endothelial cells. Signaling pathways cause a change in the dense peripheral band of actin and actin stress fibers or alter the phosphorylation of junction proteins which affects their ability to localize in junctions. Future directions to clarify the effect of shear stress on permeability are considered.

Étude rhéo-acoustique de la rupture d'agrégates de particules en suspension dans un champ de cisaillement Application à la désagrégation des globules rouges

Rheo-acoustic study of particle aggregate break-up in a shear flow. Application to red blood cell aggregates. Shear-induced disruption of reversible flocs in a concentrated suspension is investigated by ultrasound backscattering in the low shear regime. Fractal flocs are considered as non-Brownian scatterers much smaller than the wavelength, with acoustic properties close to those of the surrounding medium, so that the attenuation of the coherent field is weak and multiple scattering can be neglected. The concept of variance in local particle volume fraction is used to deduce a first order expression of the ultrasound scattering coefficient (cross-section per unit volume) for Rayleigh scatterers in a dense suspension. On the basis of a scaling law for the shear-induced disruption of aggregates, the shear stress dependence of the ultrasonic scattered intensity from a dense suspension of flocs is derived. In a second part, the shear break-up of hardened red blood cell aggregates is investigated in a plane—plane flow geometry by ultrasound scattering. Rheo-acoustical experiments are analysed within the framework of the self consistent field approximation and the scaling laws currently used in microrheological models. Finally, the ability of ultrasonic, light reflectometry and viscometry methods to provide quantitative information about red blood cell aggregation and membrane adhesiveness is discussed.

On the plasticity of short-range ordered and long-range ordered alloys

The plastic behaviour of short-range ordered and long-range ordered alloys shows remarkable similarities but also differences, e.g. in the temperature dependence of the critical shear stress, in the work hardening of single and polycrystals, in the slip characteristics and in the development of dislocation microstructures. These features are compared and illustrated by experimental data on concentrated fcc alloys showing tendencies to short-range ordering (Cu–Al) and on bcc intermetallic (long-range ordered) compounds (CuZn, Fe 3Al, Fe 3Si). In spite of strong differences of the developed dislocation microstructures and fine structures of slip distribution on the micro scale due to different dislocation configurations in fcc and bcc lattices, quite similar patterns of local shear occur on larger scales, e.g. slip band evolution and instabilities of slip in certain temperature regimes, indicating more general reasons. Even the frequently discussed ‘stress anomaly’ in the temperature dependence of the critical shear stress shows striking similarities in short- and long-range ordered alloys suggesting that at least part of its causes may lie in the counteracting processes of a kind of obstacle destruction mechanism by gliding dislocations and a restoration process of diffusional origin.