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1.
“Banded structures” of macroscopic dimensions can be induced by simple shear flow in many different types of soft matter systems.
Depending on whether these bands extend along the gradient or vorticity direction, the banding transition is referred to as
“gradient banding” or “vorticity banding,” respectively. The main features of gradient banding can be understood on the basis of a relatively simple constitutive equation. This minimal model for gradient banding will
be discussed in some detail, and its predictions are shown to explain many of the experimentally observed features. The minimal
model assumes a decrease of the shear stress of the homogeneously sheared system with increasing shear rate within a certain
shear-rate interval. The possible microscopic origin of the severe shear-thinning behaviour that is necessary for the resulting
nonmonotonic flow curves is discussed for a few particular systems. Deviations between experimental observations and predictions
by the minimal model are due to obvious simplifications within the scope of the minimal model. The most serious simplifications
are the neglect of concentration dependence of the shear stress (or on other degrees of freedom) and of the elastic contributions
to the stress, normal stresses, and the possibility of shear-induced phase transitions. The consequences of coupling of stress
and concentration will be analyzed in some detail. In contrast to predictions of the minimal model, when coupling to concentration
is important, a flow instability can occur that does not require strong shear thinning. Gradient banding is sometimes also
observed in glassy- and gel-like systems, as well as in shear-thickening systems. Possible mechanisms that could be at the
origin of gradient-band formation in such systems are discussed. Gradient banding can also occur in strongly entangled polymeric
systems. Banding in these systems is discussed on the basis of computer simulations. Vorticity banding is less well understood and less extensively investigated experimentally as compared to gradient banding. Possible scenarios
that are at the origin of vorticity banding will be discussed. Among other systems, the observed vorticity-banding transition
in rod-like colloids is discussed in some detail. It is argued, on the basis of experimental observations for these colloidal
systems, that the vorticity-banding instability for such colloidal suspensions is probably related to an elastic instability,
reminiscent of the Weissenberg effect in polymeric systems. This mechanism might explain vorticity banding in discontinuously
shear-thickening systems and could be at work in other vorticity-banding systems as well. This overview does not include time-dependent phenomena like oscillations and chaotic behaviour. 相似文献
2.
The non-monotonic shear flow of a viscoelastic equimolar aqueous surfactant solution (cetylpyridinium chloride-sodium salicylate)
is investigated rheologically and optically in a transparent strain-controlled Taylor Couette flow cell. As reported before,
this particular wormlike micellar solution exhibits first a shear thinning and then a pronounced shear-thickening behavior.
Once this shear-thickening regime is reached, a transient phase separation/shear banding of the solution into turbid and clear
ring-like patterns orientated perpendicular to the vorticity axis, i.e., stacked like pancakes, is observed (Wheeler et al.
1998; Fischer 2000). The solution exhibit several unique features as no induction period of the shear induced phase, no structural
build-up at the inner rotating cylinder, jumping pancake structure of clear and turbid ringlike phases, and oscillating shear
stresses appear once the pancake structure is present. According to our analysis this flow phenomenon is not purely a mechanical
or rheological driven hydrodynamic instability but one has to take into account structural changes of the oriented micellar
aggregates (flow induced non-equilibrium phase transition) as proposed by several authors. Although this particular flow behavior
and the underlying mixture of shear induced phases and mechanical instabilities is not fully understood yet, some classification
characteristics based on a recent theoretical approach by Schmitt et al. (1995) and Porte et al. (1997) where a strong coupling
between the flow instability (non-homogeneous flow profile due to the bands) and the structural changes causes the observed
transient phenomena can be derived. In reference to the presented model the observed orientation of the rings is typical for
complex fluids that undergo a spinodal phase separation coupled with a thermodynamic flow instability. In contrast to other
shear banding phenomena, this one is observed in parallel plate, cone-plate, and Couette flow cell as well as under controlled
stress and controlled rate conditions. Therefore, it adds an additional aspect to the present discussion on shear banding
phenomena, i.e., the coupling of hydrodynamics and phase transition of rheological complex fluids.
Received: 8 January 2001 Accepted: 15 May 2001 相似文献
3.
Sébastien Manneville 《Rheologica Acta》2008,47(3):301-318
Recent experimental techniques used to investigate shear banding are reviewed. After recalling the rheological signature of
shear-banded flows, we summarize the various tools for measuring locally the microstructure and the velocity field under shear.
Local velocity measurements using dynamic light scattering and ultrasound are emphasized. A few results are extracted from
current works to illustrate open questions and directions for future research.
相似文献
Sébastien MannevilleEmail: |
4.
Paul T. Callaghan 《Rheologica Acta》2008,47(3):243-255
The phenomenon of shear banding in complex fluids has been investigated using NMR velocimetry and NMR spectroscopy, mostly
in wormlike micelle systems, but more recently in colloidal systems and multilayer vesicles. A particular advantage of NMR
is the ability to simultaneously investigate structural ordering and to compare such ordering with local strain rates. In
this paper, we describe the basics of Rheo-NMR and summarise its recent application to the study of shear banding. 相似文献
5.
Shear banding (SB) is manifested by the abrupt “demixing” of the flow into regions of high and low shear rate. In this paper, we first relate analytically the rheological parameters of the fluid with the range of shear rates and stresses of SB occurrence. For this, we accept that the origin of shear banding is constitutive, and adopt a non-linear viscoelastic expression able to accommodate the double-valuedness of the stress with flow intensity, under certain conditions. We then implement the model for the case of pressure driven flow through a cylindrical pipe; we derive approximate expressions for the velocity profile in the two-banded regions (core and outer annular), the overall throughput in the presence or absence of “spurt”, and the radial location limits of the shear rate discontinuity. 相似文献
6.
N. Hu 《Journal of the mechanics and physics of solids》2004,52(3):499-531
In this paper, the phenomenon of strain localization, i.e., shear banding, in densely distributed metallic assemblies has been studied. A discrete element methodology for analyzing metallic granular materials has been put forward. In this numerical model, elastoplastic contact, as well as friction, rolling resistance and cohesion between spheres, are explicitly taken into account. The calculations reveal that the shear banding mechanism in dense assemblies can be thought as an instability triggered by initial imperfections. Within a band, the motion, deformation and rearrangement of spheres soften the resistance of the aggregate, as these mechanisms create additional geometric imperfections. Additionally, the simulations showed that the shear-band width does not change conclusively with the friction, rolling resistance and plasticity parameters. However, cohesive strength, even in small amounts, drastically increased the shear-band width. Finally, the shear-band thickness and inclination angles are strongly dependent on the degree of initial imperfection. Whereas for a perfect assembly the shear band inclinations were consistently around 60°, more heterogeneous assemblies lead to shear band angles closer to the continuum mechanics solution, which is 45°. This was found to be in agreement with recent experimental observations. 相似文献
7.
A shear-compression specimen for large strain testing 总被引:5,自引:0,他引:5
A new specimen geometry, the shear-compression specimen (SCS), has been developed for large strain testing of materials. The
specimen consists of a cylinder in which two diametrically opposed slots are machined at 45° with respect to the longitudinal
axis, thus forming the test gage section. The specimen was analyzed numerically for two representative material models, and
various gage geometries. This study shows that the stress (strain) state in the gage, is three-dimensional rather than simple
shear as would be commonly assumed. Yet, the dominant deformation mode in the gage section is shear, and the stresses and
strains are rather uniform. Simple relations were developed and assessed to relate the equivalent true stress and equivalent
true plastic strain to the applied loads and displacements. The specimen was further validated through experiments carried
out on OFHC copper, by comparing results obtained with the SCS to those obtained with compression cylinders. The SCS allows
to investigate a large range of strain rates, from the quasi-static regime, through intermediate strain rates (1–100 s−1), up to very high strain rates (2×104s−1 in the present case). 相似文献
8.
9.
Enric Santanach Carreras Nadia El Kissi Jean-Michel Piau Fabrice Toussaint Sophie Nigen 《Rheologica Acta》2006,45(3):209-222
In the present work, the effects of pressure on the viscosity and flow stability of four commercial grade polyethylenes (PEs)
have been studied: linear-low-density polyethylene copolymer, high-density polyethylene, metallocene polyethylenes with short-chain
branches (mPE-SCB), and metallocene polyethylenes with long chain branching (mPE-LCB). The range of shear rates considered
covers both stable and unstable flow regimes. “Enhanced exit-pressure” experiments have been performed attaining pressures
of the order of 500×105 Pa at the die exit. The necessary experimental conditions have been clearly defined so that dissipative heating can be neglected
and pressure effects isolated. The results obtained show an exponential increase in both shear and entrance-flow pressure
drop with mean pressure when shear rate is fixed and as long as flow is stable. These pressure effects are described by two
pressure coefficients, βS under shear and, βE under elongation, that are calculated using time–pressure superposition and that are independent of mean pressure and flow
rate. For three out of four PE, pressure coefficient values can be considered equal under shear and under elongation. However,
for the mPE-LCB, the pressure coefficient under elongation is found to be about 30% lower than under shear. Flow instabilities
in the form of oscillating flows or of upstream instabilities appear at lower shear rates as mean pressure increases. Nevertheless,
the critical shear stress at which they are triggered remains independent of mean pressure. Moreover, it is found that the
βS values obtained for stable flows do not differ much from the values obtained during upstream instability regimes, and differ
really from pressure effects observed under oscillating flow and slip conditions. 相似文献
10.
Shear banding occurs in the flow of complex fluids: various types of shear thinning and shear thickening micelle solutions
and liquid crystals. In order to cope with the strongly inhomogeneous interface between the bands, constitutive models used
in standard rheology must be supplemented by non-local terms. This leads rather generally to non-linear partial differential
equations of the reaction-diffusion type. We use this formalism in order to explain some observed experimental features and
as a guide for future research in this field.
Received: 17 May 1999/Accepted: 3 August 1999 相似文献
11.
Amy L. Rechenmacher 《Journal of the mechanics and physics of solids》2006,54(1):22-45
A variety of experimental techniques have been used to advance understanding of strain localization phenomena in sands. However, all of these methods have fallen short in characterizing the evolution of the grain-scale processes that necessarily control shear band formation and growth in sands. This paper presents results of application of the non-destructive displacement measurement technique of digital image correlation (DIC) to measure two- and three-dimensional surface displacements on plane strain and axisymmetric sand specimens over short time steps. The abundance of local displacement data, high level of accuracy, and nearly continuous (spatially and temporally) record of displacement evolution afforded by the DIC technique has finally enabled a means to quantify local displacements to particulate-scale intensity. The data have been used to evaluate the local displacement mechanisms leading to the triggering of the formation of persistent shear bands, the timing of shear band formation with regard to the achievement of peak stress, and the character of displacements within fully formed shear bands. Insights are offered regarding the relation between strain localization and global stress-strain behavior, and the ensuing interpretations of shear banding as a hardening or softening phenomenon. Comparison of behavior between plane strain and triaxial tests offer additional perspective on the influences of three-dimensional stresses and boundary conditions on shear banding. The results further shed light on the micro-deformation mechanisms (i.e. buckling columns) responsible for the observed local strain non-uniformities that characterize “steady-state” shear band evolution. 相似文献
12.
Song Suncheng 《Acta Mechanica Solida Sinica》1994,7(4):285-291
A modified technique called compatible stress iterative procedure isproposed in finite element analysis,which has well improved the conventionalweighted-residual method and was successful in dealing with the formation andlocalization process of shear banding. 相似文献
13.
Christian Sailer Martin Weber Helmut Steininger Ulrich A. Handge 《Rheologica Acta》2009,48(5):579-588
In this work, we studied the melt rheology of multigraft copolymers with a styrene–acrylonitrile maleic anhydride (SANMA)
terpolymer backbone and randomly grafted polyamide 6 (PA 6) chains. The multi-grafted chains were formed by interfacial reactions
between the maleic anhydride groups of SANMA and the amino end groups of PA 6 during melt blending. Because of the phase separation
of SANMA and PA 6, the grafted SANMA backbones formed nearly circular domains which were embedded in the PA 6 melt with a
diameter in the order of 20 to 40 nm. The linear viscoelastic behaviour of PA 6/SANMA blends at a sufficiently large SANMA
concentration displayed the characteristics of the critical gel state, i.e. the power relations G′ ∝ G′′ ∝ ω
0.5. In elongation, the PA 6/SANMA blend at the critical gel state showed a non-linear strain hardening behaviour already at
a very small Hencky strain. In contrast to neat PA 6, the elasticity of the PA 6/SANMA blends was strongly pronounced, which
was demonstrated by recovery experiments. Rheotens tests agreed with the linear viscoelastic shear oscillations and the measurements
using the elongational rheometer RME. Increasing the SANMA concentration led to a larger melt strength and a reduced drawability.
The occurrence of the critical gel state can be interpreted by the cooperative motion of molecules which develops between
the grafted PA 6 chains of neighbouring micelle-like SANMA domains. 相似文献
14.
M. Gregory Forest Sebastian Heidenreich Siegfried Hess Xiaofeng Yang Ruhai Zhou 《ournal of non Newtonian Fluid Mechanics》2008
Nano-rod dispersions in steady shear exhibit persistent transient responses both in experiments and simulations. The rotational contribution from shear flow couples with orientational diffusion, excluded-volume interactions, and distortional elasticity to yield complex dynamics and gradient morphology of the rod ensemble. The classification of sheared responses has mostly focused on “nematodynamics” of the collective particle response known as tumbling, wagging and kayaking; in heterogeneous simulations, one monitors the variability in nematodynamics across the domain. In this paper, we focus on flow coupling and non-Newtonian feedback in transient heterogeneous simulations, and in particular on a remarkable effect: the formation of localized, pulsating jet layers in the shear gap. We solve the Navier–Stokes momentum equations coupled through an orientation-dependent stress to three different orientational models (a kinetic Smoluchowski equation and two tensor models, one from kinetic closure and another from irreversible thermodynamics). A similar spurt phenomenon was reported in 1D simulations of a model for planar nematic liquids by Kupferman et al. [R. Kupferman, M. Kawaguchi, M.M. Denn, Emergence of structure in models of liquid crystalline polymers with elasticity, J. Non-Newt. Fluid Mech. 91 (2000) 255–271], which we extend to full orientational configuration space. We show: the pulsating jet layers correlate, in space and time, with the formation of a non-topological “oblate defect phase” in which the principal axis of orientation spreads from a unique direction to a circle; the jet-defect layers form where the local nematodynamics transitions from finite oscillation (wagging) to continuous rotation (tumbling), and when neighboring directors lose phase coherence; and, a negative first normal stress difference develops in the pulsating jet-defect layers. Finally, we extend one model algorithm to two space dimensions and show numerical stability of the jet-defect phenomenon to 2D perturbations. 相似文献
15.
The rheological characterisation of concentrated shear thickening materials suspensions is challenging, as complicated and occasionally discontinuous rheograms are produced. Wall slip is often apparent and when combined with a shear thickening fluid the usual means of calculating rim shear stress in torsional flow is inaccurate due to a more complex flow field. As the flow is no longer “controlled”, a rheological model must be assumed and the wall boundary conditions are redefined to allow for slip. A technique is described where, by examining the angular velocity response in very low torque experiments, it is possible to indirectly measure the wall slip velocity. The suspension is then tested at higher applied torques and different rheometer gaps. The results are integrated numerically to produce shear stress and shear rate values. This enables the measurement of true suspension bulk flow properties and wall slip velocity, with simple rheological models describing the observed complex rheograms. 相似文献
16.
Understanding notch-related failure is crucial for the design of reliable engineering structures. However, substantial controversies exist in the literature on the notch effect in bulk metallic glasses (BMGs), and the underlying physical mechanism responsible for the apparent confusion is still poorly understood. Here we investigate the physical origin of an inverse notch effect in a Zr-based metallic glass, where the tensile strength of the material is dramatically enhanced, rather than decreased (as expected from the stress concentration point of view), by introduction of a notch. Our experiments and molecular dynamics simulations show that the seemingly anomalous inverse notch effect is in fact caused by a transition in failure mechanism from shear banding at the notch tip to cavitation and void coalescence. Based on our theoretical analysis, the transition occurs as the stress triaxiality in the notched sample exceeds a material-dependent threshold value. Our results fill the gap in the current understanding of BMG strength and failure mechanism by resolving the conflicts on notch effects and may inspire re-interpretation of previous reports on BMG fracture toughness where pre-existing notches were routinely adopted. 相似文献
17.
Andreas M. Kempf 《Flow, Turbulence and Combustion》2008,80(3):351-373
This paper addresses issues relating to the validation of LES from experiments. It highlights the differences between large
eddy simulations (LES) and Reynolds averaged Navier–Stokes simulations (RANS) and how LES puts higher demands on the validation
process than RANS. Some popular validation experiments are outlined, which are part of the Workshop on Turbulent Non-Premixed
Flames. Important features of validation experiments are discussed to help modellers with the selection of suitable experiments,
and experimentalists with the design. An LES validation using the typical approach is then presented, together with some unusual,
more innovative approaches that deviate from common RANS practice. The final section considers experimental errors, with examples
of their effects, and how LES can contribute to their analysis and compensation. This paper does not aim to provide a complete
review of the field, but rather to communicate some ideas that were presented at an invited Workshop presentation. 相似文献
18.
Identification of Dynamic (Young’s and Shear) Moduli of a Structural Adhesive Using Modal Based Direct Model Updating Method 总被引:1,自引:1,他引:0
In this paper, mechanical characteristics (Young’s modulus and shear modulus) of an adhesive are identified using modal based
direct model updating method and experimental modal data. The results show that both Young’s and shear moduli of adhesive
are frequency dependent. Also, it is demonstrated that the thickness and length of the adhesive-line have influence on these
properties. All experiments and subsequent identifications are conducted both in bending and shear modes, and it has been
shown that the shear modulus of adhesive is more sensitive to length and thickness variations. The repeatability and consistency
of method is proved through repeating the process several times and with different adherends. 相似文献
19.
Recent experiments have shown that nano-sized metallic glass (MG) specimens subjected to tensile loading exhibit increased ductility and work hardening. Failure occurs by necking as opposed to shear banding which is seen in bulk samples. Also, the necking is generally observed at shallow notches present on the specimen surface. In this work, continuum finite element analysis of tensile loading of nano-sized notched MG specimens is conducted using a thermodynamically consistent non-local plasticity model to clearly understand the deformation behavior from a mechanics perspective. It is found that plastic zone size in front of the notch attains a saturation level at the stage when a dominant shear band forms extending across the specimen. This size scales with an intrinsic material length associated with the interaction stress between flow defects. A transition in deformation behavior from quasi-brittle to ductile becomes possible when this critical plastic zone size is larger than the uncracked ligament length. These observations corroborate with atomistic simulations and experimental results. 相似文献
20.
The shear orientation of hexagonal and lamellar liquid crystalline phases of polymeric surfactants was investigated by rheo-optical
techniques (flow birefringence (Δn), small-angle light scattering) as well as by nuclear magnetic resonance and optical microscopy. The evolution of birefringence
in the hexagonal phase is discussed for simple and oscillatory shear, and an alignment of rodlike micelles along the flow
direction was found. A shear induced formation of vesicles (“onions”) is observed with the lamellar phase. They displayed
a characteristic four-lobe pattern in depolarized light scattering. Above a critical shear stress vesicles were degraded and
perpendicularly aligned lamellae (i.e. with their normal along the vorticity direction) were obtained. A comparison of experiments
performed at constant stress and constant rate revealed that the vesicle to planar lamellae transition occurred above a critical
shear stress. The behavior of the polysoap lyotropic mesophases under shear, i.e. the strain dependent alignment in the hexagonal
phase, the shear induced formation of vesicles, and a transition to planar lamellae in the lamellar phase, is very similar
to the behavior of lyotropic mesophases formed by low molar mass surfactants or amphiphilic block copolymers. The geometrical
constraints that are introduced when amphiphilic side groups are fixed to a polymer backbone do not significantly alter the
response of the mesophase to a shear deformation.
Received: 4 May 1999 /Accepted: 19 July 1999 相似文献