Showing papers on "Shear stress published in 2013"

Measurements of mechanical anisotropy in brain tissue and implications for transversely isotropic material models of white matter

Abstract: White matter in the brain is structurally anisotropic, consisting largely of bundles of aligned, myelin-sheathed axonal fibers. White matter is believed to be mechanically anisotropic as well. Specifically, transverse isotropy is expected locally, with the plane of isotropy normal to the local mean fiber direction. Suitable material models involve strain energy density functions that depend on the I 4 and I 5 pseudo-invariants of the Cauchy–Green strain tensor to account for the effects of relatively stiff fibers. The pseudo-invariant I 4 is the square of the stretch ratio in the fiber direction; I 5 contains contributions of shear strain in planes parallel to the fiber axis. Most, if not all, published models of white matter depend on I 4 but not on I 5 . Here, we explore the small strain limits of these models in the context of experimental measurements that probe these dependencies. Models in which strain energy depends on I 4 but not I 5 can capture differences in Young's (tensile) moduli, but will not exhibit differences in shear moduli for loading parallel and normal to the mean direction of axons. We show experimentally, using a combination of shear and asymmetric indentation tests, that white matter does exhibit such differences in both tensile and shear moduli. Indentation tests were interpreted through inverse fitting of finite element models in the limit of small strains. Results highlight that: (1) hyperelastic models of transversely isotropic tissues such as white matter should include contributions of both the I 4 and I 5 strain pseudo-invariants; and (2) behavior in the small strain regime can usefully guide the choice and initial parameterization of more general material models of white matter.

Low Coseismic Shear Stress on the Tohoku-Oki Megathrust Determined from Laboratory Experiments

Abstract: Large coseismic slip was thought to be unlikely to occur on the shallow portions of plate-boundary thrusts, but the 11 March 2011 Tohoku-Oki earthquake [moment magnitude (Mw) = 9.0] produced huge displacements of ~50 meters near the Japan Trench with a resultant devastating tsunami. To investigate the mechanisms of the very large fault movements, we conducted high-velocity (1.3 meters per second) friction experiments on samples retrieved from the plate-boundary thrust associated with the earthquake. The results show a small stress drop with very low peak and steady-state shear stress. The very low shear stress can be attributed to the abundance of weak clay (smectite) and thermal pressurization effects, which can facilitate fault slip. This behavior provides an explanation for the huge shallow slip that occurred during the earthquake.

Non-pneumatic tire

Abstract: A structurally supported tire includes a ground contacting tread portion, a reinforced annular band disposed radially inward of the tread portion, and a plurality of web spokes extending transversely across and radially inward from the reinforced annular band and anchored in a wheel or hub. The reinforced annular band comprises an elastomeric shear layer, at least a first membrane adhered to the radially inward extent of the elastomeric shear layer and at least a second membrane adhered to the radially outward extent of the elastomeric shear layer. Each of the membranes has a longitudinal tensile modulus sufficiently greater than the shear modulus of the shear layer so that when under load the ground contacting portion of the tire deforms to a flat contact region through shear strain in the shear layer while maintaining constant the length of the membranes, the web spokes transmitting load forces between the annular band and the hub through tension in the web spokes not connected to the ground contacting portion of the tire.

On the micro mechanics of one-dimensional normal compression

Abstract: Discrete-element modelling has been used to investigate the micro mechanics of one-dimensional compression. One-dimensional compression is modelled in three dimensions using an oedometer and a large number of particles, and without the use of agglomerates. The fracture of a particle is governed by the octahedral shear stress within the particle due to the multiple contacts and a Weibull distribution of strengths. Different fracture mechanisms are considered, and the influence of the distribution of fragments produced for each fracture on the global particle size distribution and the slope of the normal compression line is investigated. Using the discrete-element method, compression is related to the evolution of a fractal distribution of particles. The compression index is found to be solely a function of the strengths of the particles as a function of size.

Nonlocal rheology of granular flows across yield conditions.

Abstract: The rheology of dense granular flows is studied numerically in a shear cell controlled at constant pressure and shear stress, confined between two granular shear flows. We show that a liquid state can be achieved even far below the yield stress, whose flow can be described with the same rheology as above the yield stress. A nonlocal constitutive relation is derived from dimensional analysis through a gradient expansion and calibrated using the spatial relaxation of velocity profiles observed under homogeneous stresses. Both for frictional and frictionless grains, the relaxation length is found to diverge as the inverse square root of the distance to the yield point, on both sides of that point.

Stiffness of sands through a laboratory test database

Abstract: Deformations of sandy soils around geotechnical structures generally involve strains in the range small (0·01%) to medium (0·5%). In this strain range the soil exhibits non-linear stress–strain behaviour, which should be incorporated in any deformation analysis. In order to capture the possible variability in the non-linear behaviour of various sands, a database was constructed including the secant shear modulus degradation curves of 454 tests from the literature. By obtaining a unique S-shaped curve of shear modulus degradation, a modified hyperbolic relationship was fitted. The three curve-fitting parameters are: an elastic threshold strain γe, up to which the elastic shear modulus is effectively constant at G0; a reference strain γr, defined as the shear strain at which the secant modulus has reduced to 0·5G0; and a curvature parameter a, which controls the rate of modulus reduction. The two characteristic strains γe and γr were found to vary with sand type (i.e. uniformity coefficient), soil state (i....

Stiffness of clays and silts: Normalizing shear modulus and shear strain

Abstract: An analysis is presented of a database of 67 tests on 21 clays and silts of undrained shear stress-strain data of fine-grained soils Normalizations of secant G in terms of initial mean effective stress p′ (ie, G/p′ versus log γ) or undrained shear strength cu (ie, G/cu versus log γ) are shown to be much less successful in reducing the scatter between different clays than the approach that uses the maximum shear modulus, Gmax, a technique still not universally adopted by geotechnical researchers and constitutive modelers Analysis of semiempirical expressions for Gmax is presented and a simple expression that uses only a void-ratio function and a confining-stress function is proposed This is shown to be superior to a Hardin-style equation, and the void ratio function is demonstrated as an alternative to an overconsolidation ratio (OCR) function To derive correlations that offer reliable estimates of secant stiffness at any required magnitude of working strain, secant shear modulus G is norma

Flow detection and calcium signalling in vascular endothelial cells.

Abstract: Blood vessels alter their morphology and function in response to changes in blood flow, and their responses are based on blood flow detection by the vascular endothelium. Endothelial cells (ECs) covering the inner surface of blood vessels sense shear stress generated by flowing blood and transmit the signal into the interior of the cell, which evokes a cellular response. The EC response to shear stress is closely linked to the regulation of vascular tone, blood coagulation and fibrinolysis, angiogenesis, and vascular remodelling, and it plays an important role in maintaining the homoeostasis of the circulatory system. Impairment of the EC response to shear stress leads to the development of vascular diseases such as hypertension, thrombosis, aneurysms, and atherosclerosis. Rapid progress has been made in elucidating shear stress mechanotransduction by using in vitro methods that apply controlled levels of shear stress to cultured ECs in fluid-dynamically designed flow-loading devices. The results have revealed that shear stress is converted into intracellular biochemical signals that are mediated by a variety of membrane molecules and microdomains, including ion channels, receptors, G-proteins, adhesion molecules, the cytoskeleton, caveolae, the glycocalyx, and primary cilia, and that multiple downstream signalling pathways become activated almost simultaneously. Nevertheless, neither the shear-stress-sensing mechanisms nor the sensor molecules that initially sense shear stress are yet known. Their identification would contribute to a better understanding of the pathophysiology of the vascular diseases that occur in a blood flow-dependent manner and to the development of new treatments for them.

On the existence of a simple yield stress fluid behavior

Abstract: Materials such as foams, concentrated emulsions, dense suspensions or colloidal gels, are yield stress fluids Their steady flow behavior, characterized by standard rheometric techniques, is usually modeled by a Herschel–Bulkley law The emergence of techniques that allow the measurement of their local flow properties (velocity and volume fraction fields) has led to observe new complex behaviors It was shown that many of these materials exhibit shear banding in a homogeneous shear stress field, which cannot be accounted for by the standard steady-state constitutive laws of simple yield stress fluids In some cases, it was also observed that the velocity fields under various conditions cannot be modeled with a single constitutive law and that nonlocal models are needed to describe the flows Doubt may then be cast on any macroscopic characterization of such systems, and one may wonder if any material behaves in some conditions as a Herschel–Bulkley material In this paper, we address the question of the existence of a simple yield stress fluid behavior We first review experimental results from the literature and we point out the main factors (physical properties, experimental procedure) at the origin of flow inhomogeneities and nonlocal effects It leads us to propose a well-defined procedure to ensure that steady-state bulk properties of the materials are studied We use this procedure to investigate yield stress fluid flows with MRI techniques We focus on nonthixotropic dense suspensions of soft particles (foams, concentrated emulsions, Carbopol gels) We show that, as long as they are studied in a wide (as compared to the size of the material mesoscopic elements) gap geometry, these materials behave as ‘simple yield stress fluids’: they are homogeneous, they do not exhibit steady-state shear banding, and their steady flow behavior in simple shear can be modeled by a local continuous monotonic constitutive equation which accounts for flows in various conditions and matches the macroscopic response

Uniform Bottom Shear Stress and Equilibrium Hyposometry of Intertidal Flats

Abstract: Hypsometry is the distribution of horizontal surface area with respect to elevation. Recent observations of tidal flat morphology have correlated convex hypsometry with large tide ranges, long-term accretion and/or low wave activity. Concave hypsometry, in turn, has been correlated with small tide ranges, long-term erosion and/or high wave activity. The present study demonstrates that this empirical variation in tidal flat hypsometry is consistent with a simple morphodynamic model which assumes tidal flats to be at equilibrium if maximum bottom shear stress ('c) is spatially uniform. Two general cases are considered: (i) dominance of 'c by tidal currents, where 'c is equal to maximum tidally-generated shear stress ('CT), and (ii) dominance by wind waves, where 'c is equal to maximum wave-generated shear stress ('Cw). Analytic solutions indicate that a tidal flat which slopes linearly away from a straight shoreline does not produce a uniform distribution of '!7 T or '!;W. If the profile is adjusted until either '!7 T or '!;w is uniform, then domination by tidal currents favors a convex hypsometry, and domination by wind waves favors a concave hypsometry. Equilibrium profiles are also derived for curved shorelines. Results indicate that an embayed shoreline significantly enhances convexity and a lobate shoreline significantly enhances concavity so much so that the potential effect of shoreline curvature on equilibrium hypsometry is of the same order as the effect of domination by '!7 T or '!7 W. Mixing in Estuaries and Coastal Seas Coastal and Estuarine Studies Volume 50, Pages 405-429 Copyright 1996 by the American Geophysical Union

Inversion of basal friction in Antarctica using exact and incomplete adjoints of a higher‐order model

Abstract: Basal friction beneath ice sheets remains poorly characterized and yet is a fundamental control on ice mechanics. Here we use a complete map of surface velocity of the Antarctic Ice Sheet to infer the basal friction over the entire continent by combining these observations with a three-dimensional, thermomechanical, higher-order ice sheet numerical model from the Ice Sheet System Model open source software. We demonstrate that inverse methods can be readily applied at the continental scale with appropriate selections of cost function and of scheme of regularization, at a spatial resolution as high as 3 km along the coastline. We compare the convergence of two descent algorithms with the exact and incomplete adjoints to show that the incomplete adjoint is an excellent approximation. The results reveal that the driving stress is almost entirely balanced by the basal shear stress over 80% of the ice sheet. The basal friction coefficient, which relates basal friction to basal velocity, is, however, significantly heterogeneous: it is low on fast moving ice and high near topographic divides. Areas with low values extend far out into the interior, along glacier and ice stream tributaries, almost to the flanks of topographic divides, suggesting that basal sliding is widespread beneath the Antarctic Ice Sheet. ©2013. American Geophysical Union. All Rights Reserved.

MicroRNAs in flow-dependent vascular remodelling.

Abstract: Changes in haemodynamic forces in the vascular system result in an altered expression of miRs, which play important gene-regulatory roles by pairing to the mRNAs of protein-coding genes to fine-tune post-transcriptional repression. The development and structure of blood vessels are highly adapted to haemodynamic forces, such as shear stress, cyclic stretch, and circumferential wall stress, generated by the conductance of blood. Thus, fluctuations in shear stress contribute to miR-regulated differential gene expression in endothelial cells (ECs), which is essential for maintenance of vascular physiology. Several microRNAs have been identified that are induced by high shear stress mediating an atheroprotective role, such as miR-10a, miR-19a, miR-23b, miR-101, and miR-143/145. While changes in the expression profile of miR-21 and miR-92a by high shear stress are associated with an atheroprotective function, low shear stress-induced expression of miR-21, miR-92a, and miR-663 results in a pathological EC phenotype. MiR-155 fulfils pleiotropic functions in different regions of vasculature, when exposed to different modes of shear stress. Thus, changes in shear stress result in differential expression of numerous miRs, triggering the balance between susceptibility and resistance to cardiovascular diseases. Further elucidating the regulation of miRs by flow may allow future clinical applications of miRs as diagnostic and therapeutic tools.

Low-dimensional intrinsic material functions for nonlinear viscoelasticity

Abstract: Rheological material functions are used to form our conceptual understanding of a material response. For a nonlinear rheological response, the possible deformation protocols and material measures span a high-dimensional space. Here, we use asymptotic expansions to outline low-dimensional measures for describing leading-order nonlinear responses in large amplitude oscillatory shear (LAOS). This amplitude-intrinsic regime is sometimes called medium amplitude oscillatory shear (MAOS). These intrinsic nonlinear material functions are only a function of oscillatory frequency, and not amplitude. Such measures have been suggested in the past, but here, we clarify what measures exist and give physically meaningful interpretations. Both shear strain control (LAOStrain) and shear stress control (LAOStress) protocols are considered, and nomenclature is introduced to encode the physical interpretations. We report the first experimental measurement of all four intrinsic shear nonlinearities of LAOStrain. For the polymeric hydrogel (polyvinyl alcohol - Borax) we observe typical integer power function asymptotics. The magnitudes and signs of the intrinsic nonlinear fingerprints are used to conceptually model the mechanical response and to infer molecular and microscale features of the material.

Intraventricular Flow Velocity Vector Visualization Based on the Continuity Equation and Measurements of Vorticity and Wall Shear Stress

Abstract: We have developed a system to estimate velocity vector fields inside the cardiac ventricle by echocardiography and to evaluate several flow dynamical parameters to assess the pathophysiology of cardiovascular diseases. A two-dimensional continuity equation was applied to color Doppler data using speckle tracking data as boundary conditions, and the velocity component perpendicular to the echo beam line was obtained. We determined the optimal smoothing method of the color Doppler data, and the 8-pixel standard deviation of the Gaussian filter provided vorticity without nonphysiological stripe shape noise. We also determined the weight function at the bilateral boundaries given by the speckle tracking data of the ventricle or vascular wall motion, and the weight function linear to the distance from the boundary provided accurate flow velocities not only inside the vortex flow but also around near-wall regions on the basis of the results of the validation of a digital phantom of a pipe flow model.

Additional shear resistance from fault roughness and stress levels on geometrically complex faults

Abstract: [1] The majority of crustal faults host earthquakes when the ratio of average background shear stress τb to effective normal stress σeff is τb/σeff≈0.6. In contrast, mature plate-boundary faults like the San Andreas Fault (SAF) operate at τb/σeff≈0.2. Dynamic weakening, the dramatic reduction in frictional resistance at coseismic slip velocities that is commonly observed in laboratory experiments, provides a leading explanation for low stress levels on mature faults. Strongly velocity-weakening friction laws permit rupture propagation on flat faults above a critical stress level τpulse/σeff≈0.25. Provided that dynamic weakening is not restricted to mature faults, the higher stress levels on most faults are puzzling. In this work, we present a self-consistent explanation for the relatively high stress levels on immature faults that is compatible with low coseismic frictional resistance, from dynamic weakening, for all faults. We appeal to differences in structural complexity with the premise that geometric irregularities introduce resistance to slip in addition to frictional resistance. This general idea is quantified for the special case of self-similar fractal roughness of the fault surface. Natural faults have roughness characterized by amplitude-to-wavelength ratios α between 10−3 and 10−2. Through a second-order boundary perturbation analysis of quasi-static frictionless sliding across a band-limited self-similar interface in an ideally elastic solid, we demonstrate that roughness induces an additional shear resistance to slip, or roughness drag, given by τdrag=8π3α2G∗Δ/λmin, for G∗=G/(1−ν) with shear modulus Gand Poisson's ratio ν, slip Δ, and minimum roughness wavelength λmin. The influence of roughness drag on fault mechanics is verified through an extensive set of dynamic rupture simulations of earthquakes on strongly rate-weakening fractal faults with elastic-plastic off-fault response. The simulations suggest that fault rupture, in the form of self-healing slip pulses, becomes probable above a background stress level τb≈τpulse+τdrag. For the smoothest faults (α∼10−3), τdrag is negligible compared to frictional resistance, so that τb≈τpulse≈0.25σeff. However, on rougher faults (α∼10−2), roughness drag can exceed frictional resistance. We expect that τdrag ultimately departs from the predicted scaling when roughness-induced stress perturbations activate pervasive off-fault inelastic deformation, such that background stress saturates at a limit (τb≈0.6σeff) determined by the finite strength of the off-fault material. We speculate that this strength, and not the much smaller dynamically weakened frictional strength, determines the stress levels at which the majority of faults operate.

Shape memory alloy wire-based smart natural rubber bearing

Abstract: In this study, two types of smart elastomeric bearings are presented using shape memory alloy (SMA) wires. Due to the unique characteristics of SMAs, such as the superelastic effect and the recentering capability, the residual deformation in SMA-based natural rubber bearings (SMA-NRBs) is significantly reduced whereas the energy dissipation capacity is increased. Two different configurations of SMA wires incorporated in elastomeric bearings are considered. The effect of several parameters, including the shear strain amplitude, the type of SMA, the aspect ratio of the base isolator, the thickness of SMA wire, and the amount of pre-strain in the wires on the performance of SMA-NRBs is investigated. Rubber bearings are composed of natural rubber layers bonded to steel shims as reinforcement. Results show that ferrous SMA wire, FeNiCuAlTaB, with 13.5% superelastic strain and a very low austenite finish temperature (?62??C), is the best candidate to be used in SMA-NRBs subjected to high shear strain amplitudes. In terms of the lateral flexibility and wire strain level, the smart rubber bearing with a cross configuration of SMA wires is more efficient. Moreover, the cross configuration can be implemented in high-aspect-ratio elastomeric bearings since the strain induced in the wire does not exceed the superelastic range. When cross SMA wires with 2% pre-strain are used in a smart NRB, the dissipated energy is increased by 74% and the residual deformation is decreased by 15%.

Rheological Hysteresis in Soft Glassy Materials

Abstract: The nonlinear rheology of a soft glassy material is captured by its constitutive relation, shear stress versus shear rate, which is most generally obtained by sweeping up or down the shear rate over a finite temporal window. For a huge amount of complex fluids, the up and down sweeps do not superimpose and define a rheological hysteresis loop. By means of extensive rheometry coupled to time-resolved velocimetry, we unravel the local scenario involved in rheological hysteresis for various types of well-studied soft materials. We introduce two observables that quantify the hysteresis in macroscopic rheology and local velocimetry, respectively, as a function of the sweep rate $\ensuremath{\delta}{t}^{\ensuremath{-}1}$. Strikingly, both observables present a robust maximum with $\ensuremath{\delta}t$, which defines a single material-dependent time scale that grows continuously from vanishingly small values in simple yield stress fluids to large values for strongly time-dependent materials. In line with recent theoretical arguments, these experimental results hint at a universal time scale-based framework for soft glassy materials, where inhomogeneous flows characterized by shear bands and/or pluglike flow play a central role.

Criteria for Shear Banding in Time-Dependent Flows of Complex Fluids

Abstract: We study theoretically the onset of shear banding in the three most common time-dependent rheological protocols: step stress, finite strain ramp (a limit of which gives a step strain), and shear startup. By means of a linear stability analysis we provide a fluid-universal criterion for the onset of banding for each protocol, which depends only on the shape of the experimentally measured time-dependent rheological response function, independent of the constitutive law and internal state variables of the particular fluid in question. Our predictions thus have the same highly general status, in these time-dependent flows, as the widely known criterion for banding in steady state (of negatively sloping shear stress vs shear rate). We illustrate them with simulations of the Rolie-Poly model of polymer flows, and the soft glassy rheology model of disordered soft solids.

Tectonic overpressure in weak crustal-scale shear zones and implications for the exhumation of high-pressure rocks

Abstract: [1] A two-dimensional numerical simulation of lithospheric shortening shows the formation of a stable crustal-scale shear zone due to viscous heating. The shear zone thickness is controlled by thermomechanical coupling that is resolved numerically inside the shear zone. Away from the shear zone, lithospheric deformation is dominated by pure shear, and tectonic overpressure (i.e., pressure larger than the lithostatic pressure) is proportional to the deviatoric stress. Inside the shear zone, deformation is dominated by simple shear, and the deviatoric stress decreases due to thermal weakening of the viscosity. To maintain a constant horizontal total stress across the weak shear zone (i.e., horizontal force balance), the pressure in the shear zone increases to compensate the decrease of the deviatoric stress. Tectonic overpressure in the weak shear zone can be significantly larger than the deviatoric stress at the same location. Implications for the geodynamic history of tectonic nappes including high-pressure/ultrahigh-pressure rocks are discussed.

A new mechanism for the formation of ply wrinkles due to shear between plies

Abstract: The formation of out-of-plane ply deformation causes significant reductions in the mechanical properties of composites. In-plane fibre misalignments also cause reductions in the compressive strength, yet the origins of these defects are misunderstood. This paper presents a new mechanism for the formation of wrinkles, which is based upon the shear forces generated as a result of mismatches in the coefficient of thermal expansion of composite and tool, as well as the process of ply slippage that occurs during consolidation into radii. Using a U-shaped tool, defects in composite spars have been characterised using light microscopy, showing that the tool geometry and prepreg bridging leads to “instability sites,” which lead to wrinkles up to 750 μm in height as well as in-plane misalignment of 0° plies of up to 50°. Increasing the frictional shear stress through omission of release film prevents the formation of wrinkles, supporting the mechanism presented in this paper.

In-plane force fields and elastic properties of graphene

Abstract: Bond stretching and angle bending force fields, appropriate to describe in-plane properties of graphene sheets, are derived using first principles' methods. The obtained force fields are fitted by analytical anharmonic potential energy functions, providing efficient means of calculations in molecular mechanics simulations. Using both molecular dynamics simulations and first principles' methods, numerical results regarding the mechanical behavior of graphene monolayers under various loads, like uniaxial tension in different directions or hydrostatic tension, are presented and compared. Graphene's response in shear stress is also investigated using molecular dynamics, where a noticeable asymmetric mechanical behavior is found. Stress-strain curves and elastic constants, such as, Young modulus, Poisson's ratio, bulk modulus, and shear modulus, are calculated. Our results are compared with available experimental estimates, as well as, with corresponding theoretical calculations. Finally, the effects of the anharmonicity of the extracted bond stretching and angle bending potentials on the mechanical properties of graphene are discussed.