Search

Startup shear stress data of a well-defined set of binary polystyrene blends consisting of monodisperse blend components were reported recently by Parisi et al. (J Non-Newtonian Fluid Mech 315:105028, 2023). They presented convincing evidence that in the fast flow of melts with narrow molar mass distribution, shear stress undershoot is observed after the overshoot and before approaching the steady state. For blends with broad relaxation time spectra, no undershoot was found. We analyze this data set by comparison with predictions of the rotation zero stretch (RZS) model (Wagner et al. Rheol Acta 63:573–584, 2024), which is generalized here to the multi-mode MRZS model for polymer blends. We confirm that the steady-state shear viscosity of the monodisperse blend components as well as of the binary blends agrees with the viscosity predicted by the Doi-Edwards independent alignment model. As long as there is no undershoot, the RZS model (monodisperse melts) and the MRZS model (binary blends) result in a quantitative description of the full startup curve of the shear stress growth σ 12 + (γ ˙ , t) including overshoot and steady state, based solely on the linear viscoelastic characterization. The shear stress undershoot observed at higher shear rates in melts with narrow molar mass distribution is not described by the RZS or MRZS model. However, the analysis of the experimental data shows clear evidence that undershoot occurs only if after the overshoot, the decreasing shear stress at a higher shear rate undercuts the shear stress at lower rates, i.e., only if ∂ σ 12 + (γ ˙ , t) / ∂ γ ˙ < 0 . For blends with broad relaxation time spectra, ∂ σ 12 + (γ ˙ , t) / ∂ γ ˙ ≅ 0 and no undershoot is observed. The hypothesis is made that undershoot is due to transient shear banding, which is initiated in shear stress regimes characterized by ∂ σ 12 + (γ ˙ , t) / ∂ γ ˙ < 0 and which disappears at larger strains when the shear stress growth σ 12 + (γ ˙ , t) approaches the steady state σ 12 (γ ˙) with ∂ σ 12 (γ ˙) / ∂ γ ˙ > 0 . [ABSTRACT FROM AUTHOR]

This study utilizes molecular dynamics simulation to investigate the complex dynamics of entangled semi-flexible polymer melts. The investigation reveals a significant stress overshoot phenomenon in the systems, demonstrating the intricate interplay between shear rates, chain orientation, and chain stretching dynamics. Additionally, the identification of metastable states, characterized by a dual-plateau phenomenon in the shear stress-strain curve at specific Rouse-Weissenberg number WiR, showcases the system's responsiveness to external perturbations and its transition to stable shear banding states. Moreover, the analysis of flow field deviations uncovers a progression of shear bands with increasing WiR, displaying distinct behaviors in the system's dynamics under different shear rates and chain lengths. These findings challenge established theoretical frameworks and advocate for refined modelling approaches in polymer rheology research. [ABSTRACT FROM AUTHOR]

This study focuses on the rheological behavior of a cellulose nanocrystal gel. This system [5 wt. % cellulose nanocrystal (CNC) + 20 mM NaCl] is proved to be thixotropic, and the detected shear force tightly depends on the growth and break-up of the aggregates of CNC rods. From strain-controlled experiments, a nonmonotonic steady-state flow curve with a minimum stress value of ≈33 Pa is found, and the negative slope of stress versus shear rate suggests the existence of shear bands. From stress-controlled experiments (creep), the "static yield stress" is determined to be 67.5 ± 2.5 Pa. This difference proves that the local minimum stress of the flow curve does not coincide with the "static yield stress" determined by creep tests. However, this minimum stress can maintain flow provided that the material is already in a yielded state. At nominal shear rates below about 100 s−1, shearing is suggested to be localized in a shear band rather than over the whole material. The "dynamic yield stress" is found as "the minimum stress to maintain flow," or the onset of shear banding. Moreover, wall slip also occurs at low nominal shear rates which is related to the interaction between the dynamic microstructure of the CNC gel and the wall: it is hypothesized that the low shear rates allow the CNC aggregates to extensively grow and, thus, the oversized CNC aggregates detach from the asperities of the wall. Our finding of the robust connection between yielding, thixotropy, wall slip, and shear banding shall shed new light on the nature of the nonmonotonic flow curves of yield stress and thixotropic materials. [ABSTRACT FROM AUTHOR]

This paper aims to investigate the nanomechanical properties of amorphous nanolaminates by using the molecular dynamic (MD) simulation of nanoindentation process. To achieve this goal, two metallic glass (MG) nanolaminates with the arrangement of Cu60Zr40/Ni80P20 (CN type) and Ni80P20/Cu60Zr40 (NC type) were constructed. The MD outcomes unveiled that the NC-type sample exhibited a weaker strength at the loading stage and experienced a sharper relaxation at the end of holding stage. The strain maps of samples also revealed that the plastic deformation in the CN type initiated at the interface of substrate/indenter and was generated into the depth of material with a pear-like shape. On the other hand, the NC-type sample showed an accumulation of strain at the top layer normal to the direction of indenting process. This result indicated that the plasticity evolution occurred with a homogenous mode in the bulk of CN-type nanolaminate. [ABSTRACT FROM AUTHOR]

Dynamic shearing banding and fracturing in unsaturated porous media are significant problems in engineering and science. This article proposes a multiphase micro‐periporomechanics (μ$$ \mu $$PPM) paradigm for modeling dynamic shear banding and fracturing in unsaturated porous media. Periporomechanics (PPM) is a nonlocal reformulation of classical poromechanics to model continuous and discontinuous deformation/fracture and fluid flow in porous media through a single framework. In PPM, a multiphase porous material is postulated as a collection of a finite number of mixed material points. The length scale in PPM that dictates the nonlocal interaction between material points is a mathematical object that lacks a direct physical meaning. As a novelty, in the coupled μ$$ \mu $$PPM, a microstructure‐based material length scale is incorporated by considering micro‐rotations of the solid skeleton following the Cosserat continuum theory for solids. As a new contribution, we reformulate the second‐order work for detecting material instability and the energy‐based crack criterion and J‐integral for modeling fracturing in the μ$$ \mu $$PPM paradigm. The stabilized Cosserat PPM correspondence principle that mitigates the multiphase zero‐energy mode instability is augmented to include unsaturated fluid flow. We have numerically implemented the novel μ$$ \mu $$PPM paradigm through a dual‐way fractional‐step algorithm in time and a hybrid Lagrangian–Eulerian meshfree method in space. Numerical examples are presented to demonstrate the robustness and efficacy of the proposed μ$$ \mu $$PPM paradigm for modeling shear banding and fracturing in unsaturated porous media. [ABSTRACT FROM AUTHOR]

The mechanical behavior of unsaturated porous media under non-isothermal conditions plays a vital role in geo-hazards and geo-energy engineering (e.g., landslides triggered by fire and geothermal energy harvest and foundations). Temperature increase can trigger localized failure and cracking in unsaturated porous media. This article investigates the shear banding and cracking in unsaturated porous media under non-isothermal conditions through a thermo–hydro–mechanical (THM) periporomechanics (PPM) paradigm. PPM is a nonlocal formulation of classical poromechanics using integral equations, which is robust in simulating continuous and discontinuous deformation in porous media. As a new contribution, we formulate a nonlocal THM constitutive model for unsaturated porous media in the PPM paradigm in this study. The THM meshfree paradigm is implemented through an explicit Lagrangian meshfree algorithm. The return mapping algorithm is used to implement the nonlocal THM constitutive model numerically. Numerical examples are presented to assess the capability of the proposed THM mesh-free paradigm for modeling shear banding and cracking in unsaturated porous media under non-isothermal conditions. The numerical results are examined to study the effect of temperature variations on the formation of shear banding and cracking in unsaturated porous media. [ABSTRACT FROM AUTHOR]

Extensive research has shown that nanolayered structures are capable of suppressing the shear banding in metallic glass in nanoindentation experiments. However, the specific mode and mechanism of the shear banding underneath the indenter remains unknown. Also, the quantification of shear banding-induced strain localization is still a challenge. Herein, the size-dependent shear banding behavior of a CuTiZrNb high-entropy alloy-based nanolayered glass with individual layer thicknesses (h) ranging from 5 to 80 nm was systematically investigated by nanoindentation tests. It was found that the hardness of the designed structure was almost size-independent. Yet, a clear transition in the deformation modes from the cutting-like shear bands to the kinking-like ones was discovered as h decreased to 10 nm. Moreover, multiple secondary shear bands also appeared, in addition to the primary ones, in the sample with h = 10 nm. The transition leads to an obvious strain delocalization, as clearly illustrated by the proposed theoretical model, which is based on the assumption of a pure shear stress state to quantify the shear banding-induced strain localization. The strain delocalization results from the higher density of amorphous/amorphous interfaces that exhibit the change in morphology with a refined layered glass structure. [ABSTRACT FROM AUTHOR]

The steady pipe flow of thixotropic yield stress fluids has been investigated theoretically based on a modified isotropic kinematic hardening (mIKH) model. Analytical solution is derived for a specific case (m = n = 1) and a general semi-analytical solution is put forward as well. The effect of thixotropic yield stress on shear rate and velocity profiles is illustrated by comparing to other well-known solutions. Moreover, the influences of model parameters are examined. It is worth noting that shear banding may occur at the yielded surface in case of a sufficiently large Bingham number, thixotropic number, and flow index, but a sufficient small value of structure-related exponent. [ABSTRACT FROM AUTHOR]

Graphite IG-110 is a synthetic polycrystalline material used as a neutron moderator in reactors. Graphite is inherently brittle and is known to exhibit a further increase in brittleness due to radiation damage at room temperature. To understand the irradiation effects on pre-existing defects and their overall influence on external load, micropillar compression tests were performed using in situ nanoindentation in the Transmission Electron Microscopy (TEM) for both pristine and ion-irradiated samples. While pristine specimens showed brittle and subsequent catastrophic failure, the 2.8 MeV Au2+ ion (fluence of 4.378 × 1014 cm−2) irradiated specimens sustained extensive plasticity at room temperature without failure. In situ TEM characterization showed nucleation of nanoscale kink band structures at numerous sites, where the localized plasticity appeared to close the defects and cracks while allowing large average strain. We propose that compressive mechanical stress due to dimensional change during ion irradiation transforms buckled basal layers in graphite into kink bands. The externally applied load during the micropillar tests proliferates the nucleation and motion of kink bands to accommodate the large plastic strain. The inherent non-uniformity of graphite microstructure promotes such strain localization, making kink bands the predominant mechanism behind unprecedented toughness in an otherwise brittle material. [ABSTRACT FROM AUTHOR]

The rheological properties of fresh concrete are a direct function of the interaction behaviour of the granular inventory of the concrete (i.e., gravel, sand and cement) and especially of the colloidal fractions of cement. Under low shear stresses, agglomeration of colloidal particles is observed, while at high shear stresses, dispersion of these agglomerates occurs. Besides the agglomeration state, the formation of shear banding, zones with concentrated shear flow, is another controlling mechanism of the flow behaviour of cement suspensions. Rheological creep tests in this study are focused on investigating the influence of shear history and hydration process on thixotropy of cement suspension. In this paper, the meaning of the word thixotropy is slightly extended to additionally encompass rheological aging and hydration effects. Selected samples were analyzed by coupling a rheometer to synchrotron X‐ray tomography to gain insight into the shear‐induced microstructural changes during shear start‐up tests. The observations show heterogeneities in the velocity profile in the shear gap and the development of shear banding. [ABSTRACT FROM AUTHOR]

When it comes to the measurement of yield stress, the experimental procedure appears to play a significant role. Using a series of experiments, in which the effects of time dependence and shear banding were identified and taken into account, we determined the dynamic and static yield stresses of the materials as unique, test-independent properties. We studied the shear rheological properties of an aqueous suspension of Laponite®, which is a highly time-dependent (thixotropic) material. To minimize the irreversible effect of aging on its material properties, the Laponite® dispersion was aged for 347 days under a controlled environment. For comparison, an aqueous solution of Carbopol®—a slightly time-dependent material—was also investigated. The peak values of the shear stress evolution in constant shear rate tests were compared with the static and dynamic yield stress values. We noticed that, as the shear rate is reduced the peak stress value tends asymptotically to the dynamic yield stress for the slightly time-dependent material, but to slightly above the static yield stress for the thixotropic material. For the Laponite® suspension, at relatively low shear rates, we observed that peak stresses are influenced by shear banding. By simulating stress evolution curves using stress step-changes, we eliminated the influence of shear banding and discovered that the lowest yielding point coincides with the static yield stress. In addition, we provided the complete flow curve for the Laponite® suspension, showing the role of the static and dynamic yield stresses, and the unattainable zone which is closely related to steady shear banding effects. [ABSTRACT FROM AUTHOR]

The formation of flow-induced, oriented structures in two-phase systems, as in this study, is a phenomenon of considerable interest to the scientific and industrial sectors. The main difficulty in understanding the formation of bands of droplets is the simultaneous interplay of physicochemical, hydrodynamic, and mechanical effects. Additionally, banded structure materials frequently show multiple length scales covering several decades as a result of complex time-dependent stress fields. Here, to facilitate understanding a subset of these structures, we studied water in oil emulsions and focused on the effects of three variables specifically: the confinement factor   (C o = 2 R / H) , the viscosity ratio (p) , and the applied shear rate (γ ˙) . The confinement (C o) is the ratio between the drop's diameter ( 2 R ) and the separation of (the gap between) the circular rotating disks ( H ) containing the emulsion. We carried out (a) observations of the induced structure under different simple shear rates, as well as (b) statistical and morphological analysis of these bands. At low shear rates, the system self-assembles into bands along the direction of the flow and stacked normal to the velocity gradient direction. At higher shear rates is possible to observe bands normal to the vorticity direction. Here, we show that a detailed analysis of the dynamics of the band structures is amenable, as well as measurements of flow field anomalies simultaneously observed. The local emulsion viscosity varies in time, increasing in regions of higher droplet concentration and subsequently inducing velocity components perpendicular to the main flow direction. Thus, the emulsion morphology evolves and changes macroscopically. A relatively plausible explanation is attributed to the competitive effects of coalescence and the rupture of drops, where p values less than one predominate coalescence. [ABSTRACT FROM AUTHOR]

The temporal and spatial evolution of shear banding during startup and steady-state shear flow was studied for solutions of entangled, linear, monodisperse polyethylene C 3000 H 6002 dissolved in hexadecane and benzene solvents. A high-fidelity coarse-grained dissipative particle dynamics method was developed and evaluated based on previous NEMD simulations of similar solutions. The polymeric contribution to shear stress exhibited a monotonically increasing flow curve with a broad stress plateau at intermediate shear rates. For startup shear flow, transient shear banding was observed at applied shear rates within the steady-state shear stress plateau. Shear bands were generated at strain values where the first normal stress difference exhibited a maximum, with lifetimes persisting for up to several hundred strain units. During the lifetime of the shear bands, an inhomogeneous concentration distribution was evident within the system, with higher polymer concentration in the slow bands at low effective shear rate; i.e., γ ˙ < τ R − 1 , and vice versa at high shear rate. At low values of applied shear rate, a reverse flow phenomenon was observed in the hexadecane solution, which resulted from elastic recoil of the molecules within the slow band. In all cases, the shear bands dissipated at high strains and the system attained steady-state behavior, with a uniform, linear velocity profile across the simulation cell and a homogeneous concentration. [ABSTRACT FROM AUTHOR]

Energy-efficient materials are key to combating the high energy costs and climate change. The manufacturing temperatures of industrially important Zr-based bulk metallic glasses (BMGs) relative to steels are low, and exist between the liquidus temperature Tl (∼850 °C) and glass transition temperature Tg (∼400 °C). However, these materials show limited plastic deformability (ductility) at room temperature (strains typically less than 3%); moreover they soften but exhibit limited ductility at high processing temperatures. Their low ductility should be improved because it impedes fatigue resistance and machinability, such as via cold (plastic) forming. In this study, chemical composition changes, which reduced Tg, resulted in remarkably ductile BMGs with extreme deformations of over 70% under compression, thereby enabling their energy-efficient processing at low temperatures. In contrast to previously reported conclusions on the high GFA and deformation-induced nanocrystallization being the precursors to ductility, formation of a low amount of meso-crystallites within the glassy material during cooling efficiently hindered the propagation of shear bands and microcracks under loading, thus increasing significantly ductility. This characteristic, in addition to optimal chemical composition, played an important role in improving the ability of BMGs to undergo solid-state processing at low temperatures and increased deformation rates.

The formation and evolution of a heterogeneous flow and flow reversal are examined in highly elastic, gel-like wormlike micelles (WLMs) formed from an amphiphilic triblock poloxamer P234 in 2M NaCl. A combination of linear viscoelastic, steady shear, and creep rheology demonstrate that these WLMs have a yield stress and exhibit viscoelastic aging, similar to some soft glassy materials. Nonlinear shear rheology and rheoparticle tracking velocimetry reveal that these poloxamer WLMs undergo a period of strong elastic recoil and flow reversal after the onset of shear startup. As flow reversal subsides, a fluidized high shear rate region and a nearly immobile low shear rate region of fluid form, accompanied by wall slip and elastic instabilities. The features of this flow heterogeneity are reminiscent of those for aging yield stress fluids, where the heterogeneous flow forms during the initial stress overshoot and is sensitive to the inherent stress gradient of the flow geometry. Additionally, macroscopic bands that form transiently above a critical shear rate become "trapped" due to viscoelastic aging in the nearly immobile region. This early onset of the heterogeneous flow during the rapidly decreasing portion of the stress overshoot differs from that typically observed in shear banding WLMs and is proposed to be necessary for observing significant flow reversal. Exploring the early-time, transient behavior of this WLM gel with rheology similar to both WLM solutions and soft glassy materials provides new insights into spatially heterogeneous flows in both of these complex fluids. [ABSTRACT FROM AUTHOR]

Lamellar gel networks based on mixtures of cetostearyl alcohol and a cationic surfactant, cetyl-trimethylammonium chloride, were studied using a combination of rheometry and optical coherence tomography (OCT) velocimetry. Experiments were conducted in a stress-controlled rheometer with a parallel plate geometry. Each formulation was found to exhibit a yield stress and thixotropy. The shear start-up behavior in response to a constant stress was directly observed using OCT velocimetry. Close to the yield stress, the velocity had a power law behavior with time after an initial period of transience. At larger stresses, the velocity undergoes two successive increases in power law scaling with time. When sheared at low, constant, shear rates 1–5 s−1, the fluids exhibit plug flow with strong wall slip at both rheometer plates. At rates of 10–150 s−1, the fluids separate into a distinctive three shear band morphology while the wall slip reduces. These rheological properties can be explained by a multilamellar vesicle to planar lamellae transition. [ABSTRACT FROM AUTHOR]

To predict the complex rheology and shear-rate inhomogeneities of lamellar gel networks, we propose a simple thixotropic constitutive model with an elastoplastic stress and a smoothly decreasing modulus near a solid boundary, motivated by the analysis of the effect of confinement and shear flow on lamellae orientation near surfaces. We show that the model qualitatively captures the important features of the lamellar gel shear rheology observed in experiments [Datta et al., J. Rheol. 64(4), 851–862 (2020)]. These include thixotropic shear thinning that is intermediate between constant viscosity and constant stress, a power-law slow creep under small constant shear stress and abrupt transition to fast creep at higher stress, as well as partial recovery of strain upon stress removal. In addition, the model correctly predicts a gap-dependent rheology and roughly predicts the amplitude dependence of storage and loss moduli in oscillatory tests despite having only a single thixotropic time constant. Most importantly, the introduction of the modulus gradient enables the model to predict the unique shear-banding phenomenon of lamellar gel networks wherein a thin, fast-shearing band exists near the boundary that widens only slowly with increased apparent shear rate until a very high rate is reached, while the bulk moves as a plug [Datta et al., J. Rheol. 64(4), 851–862 (2020)]. We discuss the influence of a lower modulus near the boundary and its possible origin in the underlying lamellar structure of the material. [ABSTRACT FROM AUTHOR]

Shear banding in entangled polymer solutions is an elusive phenomenon in polymer rheology. One recently proposed mechanism for the existence of banded velocity profiles in entangled polymer solutions stems from a coupling of the flow to banded concentration profiles. Recent work [Burroughs et al., Phys. Rev. Lett.126, 207801 (2021)] provided experimental evidence for the development of large gradients in concentration across the fluid. Here, a more systematic investigation is reported of the transient and steady-state banded velocity and concentration profiles of entangled polybutadiene in dioctyl phthalate solutions as a function of temperature (T) , number of entanglements (Z), and applied shear rate (W i a p p ), which control the susceptibility of the fluid to unstable flow-concentration coupling. The results are compared to a two-fluid model that accounts for coupling between elastic and osmotic polymer stresses, and a strong agreement is found between model predictions and measured concentration profiles. The interface locations and widths of the time-averaged, steady-state velocity profiles are quantified from high-order numerical derivatives of the data. At high levels of entanglement and large W i a p p , a significant wall slip is observed at both inner and outer surfaces of the flow geometry but is not a necessary criterion for a nonhomogeneous flow. Furthermore, the transient evolution of flow profiles for large Z indicate transitions from curved to "stair-stepped" and, ultimately, a banded steady state. These observed transitions provide detailed evidence for shear-induced demixing as a mechanism of shear banding in polymer solutions. [ABSTRACT FROM AUTHOR]

We investigate the effects of micellar entanglement number on the kinetics of shear banding flow formation in a Taylor–Couette flow. Three sets of wormlike micellar solutions, each set with a similar fluid elasticity and zero-shear-rate viscosity, but with varying entanglement densities, are studied under the startup of steady shear. Our experiments indicate that in the set with low fluid elasticity, the transient shear banding flow is characterized by the formation of a transient flow reversal in a range of entanglement densities. Outside of this range, the transient flow reversal is not observed. For the sets of medium and high elasticities, the transient flow reversals exist for relatively small entanglement densities and disappear for large entanglement densities. Our analysis shows that wall slip and elastic instabilities do not affect the transient flow feature. We identify a correlation between micellar entanglement number, the width of the stress plateau, and the extent of the transient flow reversal. As the micellar entanglement number increases, the width of the stress plateau first increases; then, at a higher micellar entanglement number, the plateau width decreases. Therefore, we hypothesize that the transient flow reversal is connected to the micellar entanglement number through the width of the stress plateau. [ABSTRACT FROM AUTHOR]

One of the key factors, which hampers the application of metallic glasses as structural components, is the localization of deformation in narrow bands of a few tens up to one hundred nanometers thickness, the so-called shear bands. Processes, which occur inside shear bands are of central importance for the question whether a catastrophic failure of the material is unavoidable or can be circumvented or, at least, delayed. Via molecular dynamics simulations, this study addresses one of these processes, namely the local temperature rise due to viscous heat generation. The major contribution to energy dissipation is traced back to the plastic work performed by shear stress during steady deformation. Zones of largest strain contribute the most to this process and coincide with high-temperature domains (hottest spots) inside the sample. Magnitude of temperature rise can reach a few percent of the sample's glass transition temperature. Consequences of these observations are discussed in the context of the current research in the field. [ABSTRACT FROM AUTHOR]

The formation of flow-induced, oriented structures in two-phase systems, as in this study, is a phenomenon of considerable interest to the scientific and industrial sectors. The main difficulty in understanding the formation of bands of droplets is the simultaneous interplay of physicochemical, hydrodynamic, and mechanical effects. Additionally, banded structure materials frequently show multiple length scales covering several decades as a result of complex time-dependent stress fields. Here, to facilitate understanding a subset of these structures, we studied water in oil emulsions and focused on the effects of three variables specifically: the confinement factor (Co=2R/H), the viscosity ratio (p), and the applied shear rate (γ˙). The confinement (Co) is the ratio between the drop’s diameter (2R) and the separation of (the gap between) the circular rotating disks (H) containing the emulsion. We carried out (a) observations of the induced structure under different simple shear rates, as well as (b) statistical and morphological analysis of these bands. At low shear rates, the system self-assembles into bands along the direction of the flow and stacked normal to the velocity gradient direction. At higher shear rates is possible to observe bands normal to the vorticity direction. Here, we show that a detailed analysis of the dynamics of the band structures is amenable, as well as measurements of flow field anomalies simultaneously observed. The local emulsion viscosity varies in time, increasing in regions of higher droplet concentration and subsequently inducing velocity components perpendicular to the main flow direction. Thus, the emulsion morphology evolves and changes macroscopically. A relatively plausible explanation is attributed to the competitive effects of coalescence and the rupture of drops, where p values less than one predominate coalescence.

Due to sedimentation, irregular particles of sand arrange anisotropically in nature. Anisotropy diversifies the internal friction angles between relative research planes at different directions in soil material. Thus, to analyze the mechanical mechanism of strength anisotropy, except for conventional inherent anisotropy of the material, stress distribution caused by external loads should also be considered in addition. The level and direction of the inherent strength anisotropy of geomaterials can be measured by a modified anisotropic variable, which is derived from the stress tensor and the fabric tensor. It is assumed that the overall shear strength of the material depends on the shear strength of the most mobilized plane, which is the spatially mobilized plane (SMP) and has a closely relationship with the stress state according to the SMP criterion. Therefore, the orientation of the most mobilized plane builds a bridge that connects the anisotropy of geomaterials and the magnitude and direction of stress. On this basis, a general description of anisotropic strength, considering both the magnitude and directions of the principal stresses, is given, of which the parameters can be easily obtained by triaxial tests. Besides, the variation of the sliding plane along with the direction of the major principal stress is approached. Furthermore, three types of tests, including the direct shear test, the true triaxial test and torsion shear test, were performed to prove the reliability of the new criterion. It shows that both the anisotropic shear strength and the orientation of the sliding plane can be predicted accurately by the newly built anisotropic strength criterion. [ABSTRACT FROM AUTHOR]

With the increased use of toughened epoxy adhesives in current transportation lightweighting efforts, it is critical that damage mechanisms, such as strain whitening, are understood and quantified. Damage quantification is needed for the constitutive models used in structural design; however, thin bond lines in adhesive joints limit direct observation. In this study, microscope observations of bulk toughened epoxy adhesive specimens subjected to tensile loading were linked to damage. Cracks on the surface opened during loading, leading to strain whitening at the crack tips and the initiation and propagation of shear bands. The stresses approximated at the crack tips suggested that particle cavitation could be occurring in these regions. Changes in specimen stiffness were linked to crack growth and the formation of shear bands. Material damage calculated using traditional load-unload stiffness (D ~ 35%) was higher than other methods such as change in material strength (D ~ 18%) and damage from changes in stiffness during load-reload (D ~ 19%). The differences were attributed to short-term viscoelastic effects. A new approach calculated damage from the strain whitening on the free surface (D ~ 21%). Values were in agreement with damage figures from other methods. The technique can quantify damage over the loading history and identify areas of damage localization. [ABSTRACT FROM AUTHOR]

We investigate the start-up dynamics under a stress-controlled flow field in aging soft materials, with the aid of an inelastic structural-kinetic model that exhibits non-monotonic steady-state shear stress–shear rate flow curve. In particular, we focus on the occurrence of both transient and steady-state shear banding in such systems. We find that even when the applied stress is greater than the minimum value of the stress associated with the non-monotonic flow curve, the system could eventually undergo structural arrest, resulting in a complete stoppage of flow. For further increase in stress, the system progressively shows transient shear banding as well as apparent steady-state shear banding. At sufficiently high shear stresses, the model shows an eventual homogeneous steady state. We delineate the effects of inertia and waiting time on the transient and apparent steady-state shear banding. [ABSTRACT FROM AUTHOR]

We present a strong nonlocal hydromechanical model formulated based on state-based peridynamics for simulating strain localisation in unsaturated geomaterials. The governing equations are integro-differential equations in which length scales are assumed for both the skeleton deformation and pore fluid flow. Recently proposed hydromechanical correspondence principle is adopted to implement the classical local constitutive model for the solid skeleton, and the generalised Darcy's law for unsaturated water flow into the nonlocal hydromechanical model. Numerical simulations of shear bands in unsaturated geomaterials were conducted to investigate the effect of the hydromechanical length scale and the mechanical loading rate on the formation of shear bands. The numerical results have shown that both the deformation and pore water pressure are concentrated in the shear bands with a finite thickness. The numerical results have demonstrated that both the hydromechanical length scale and the mechanical loading rate affect the formation of shear bands in unsaturated geomaterials and the peak value of the load capacity of unsaturated geomaterials. [ABSTRACT FROM AUTHOR]

This study reports the influence of the dead zone (DZ) formed around obstacles due to the viscoelastic fluid flow on the flow behavior and properties. It is known that when a viscoelastic fluid, which is a semi-dilute polymer solution, is infused into a channel with structures resembling a pillar, a DZ is formed around the structure owing to the shear-variation-induced viscosity changes. In this study, hydrolyzed polyacrylamide solutions were infused into a microchannel with a triangle-shaped pillar array. Furthermore, the continuous contraction and expansion flow around the pillars were investigated using a micro-particle image velocimetry. The flow measurement results showed that the shape of DZ changes depending on the Weissenberg number, and accordingly, the flow behavior around and inside the DZ changes. In particular, it was confirmed that the velocity fluctuations trend in the entire flow field increased with the growth in DZ. Moreover, it was found that the shear caused by the velocity difference between the DZ and the other region induces a non-uniform viscosity distribution in the DZ, resulting in the formation of a complex flow containing both extension and rotation behavior inside the DZ. [ABSTRACT FROM AUTHOR]

This article presents data regarding the research paper entitled “Hierarchically activated deformation mechanisms to form ultra-fine grain microstructure in carbon containing FeMnCoCr twinning induced plasticity high entropy alloy [1]”. In this article we provide supporting data for describing the associated mechanisms in microstructure evolution and grain refinement of a carbon-doped TWIP high-entropy alloy (HEA) during thermomechanical processing. Microstructural characterization before and after deformation was performed using scanning electron microscope (SEM) outfitted with EBSD detector and transmission electron microscopy (TEM) were used for microstructure observation and investigation of nanostructure evolution during deformation. Inverse pole figure (IPF) map, grain boundary map and kernel average misorientation map (KAM) were used for systematic analysis of nanostructural evolution and deformed heterostructure consisting of hierarchical mechanical twinning, shear-banding, microbanding and formation of strain-induced boundaries (SIBs).

Soft glasses of colloidal rods (fd-virus particles) with orientational domains were recently shown to exhibit inhomogeneous flow profiles [Dhont et al., Phys. Rev. Fluids 2, 043301 (2017)]: fracture and accompanied plug flow at small shear rates, which transits to gradient shear-banding on increasing the shear rate, while a uniform flow profile develops at sufficiently high shear rates. These flow profiles coexist with Taylor-vorticity bands. The texture of such glasses under flow conditions consists of domains with varying orientations. The observed gradient shear-banding was solely attributed to the strong shear thinning behavior of the material inside the domains (henceforth abbreviated as domain-interior), without considering the texture stress that is due to interactions between the glassy domains. Here, we present new experiments on the shear-banding transition to assess the role played by the texture stress in comparison to the domain-interior stress. For a large concentration, well into the glassy state, it is found that both texture stress and domain-interior stress contribute significantly to the gradient shear-banding transition in the shear-rate region where it occurs. On the other hand, for a small concentration close to the glass-transition concentration, the domains are shown to coalesce within the shear-rate range where gradient shear-banding is observed. As a result, the texture stress diminishes and the domain-interior stress increases upon coalescence, leading to a stress plateau. Thus, a subtle interplay exists between the stresses arising from the structural order on two widely separated length scales from interactions between domains and from the rod-rod interactions within the domain-interior for both concentrations. [ABSTRACT FROM AUTHOR]

Shear banding is an important deformation and failure mechanism in metallic systems, especially at high-rate straining. Dynamic recrystallization was often reported to account for the refined microstructure of shear bands but rarely confirmed using direct quantitative measurement. Here, we employ quantitative precession electron diffraction analysis to uncover shear band microstructure in pure titanium. The results reveal that the microstructure is dominated by early stages of grain subdivision process. Dynamic recrystallization is not as prevalent as perceived conventionally. Our results offer key insights into understanding shear banding and highlight the need for quantitative analyses of shear band microstructure.

Rheo-NMR velocimetry was used to study shear banding of a 6 wt.% cetylpyridinium chloride (CPCl) worm-like micelle solution under shear startup conditions with and without pre-shear. 1D velocity profiles across the fluid gap of a concentric cylinder Couette shear cell were measured every 1 s following shear startup for four different applied shear rates within the stress plateau. Fitting of the velocity profiles allowed calculation of the shear banding characteristics (shear rates in the high and low shear band, the interface position and apparent slip at the inner rotating wall) as the flow transitioned from transient to steady state regimes. Characteristic timescales to reach steady state were obtained and found to be similar for all shear banding characteristics. Timescales decreased with increasing applied shear rate. Large temporal fluctuations with time were also observed and Fourier transform of the time and velocity autocorrelation functions quantified the fluctuation frequencies. Frequencies corresponded to the elastically driven hydrodynamic instabilities, i.e. vortices, that are known to occur in the unstable high shear band and were dependent upon both applied shear rate and the pre-shear protocol.

[Display omitted] Under specific conditions, rod-like cellulose nanocrystals (CNC) can assemble into structurally ordered soft glasses (SGs) with anisotropy that can be controlled by applying shear. However, to achieve full structural control of SGs in real industrial processes, their response to mixed shear and extensional kinematics needs to be determined. We hypothesise that by knowing the shear rheology of the CNC-based soft glass and adopting a suitable constitutive model, it is possible to predict the structure-property relationship of the SG under mixed flows. We use an aqueous suspension with 2 wt% CNC at 25 mM NaCl to form a structurally ordered SG composed of a CNC network containing nematic domains. We combine rheometry and microfluidic experiments with numerical simulations to study the flow properties of the SG in shear, extension, and mixed flow conditions. Extensional flow is investigated in the Optimised Shape Cross-slot Extensional Rheometer (OSCER), where the SG is exposed to shear-free planar elongation. Mixed flow kinematics are investigated in a benchmark microfluidic cylinder device (MCD) where the SG flows past a confined cylinder in a microchannel. The SG in the MCD displays a velocity overshoot (negative wake) and a pronounced CNC alignment downstream of the cylinder. Simulations using the thixotropic elasto-visco-plastic (TEVP) model yield near quantitative agreement of the velocity profiles in simple and mixed flows and capture the structural fingerprint of the material. Our results provide a comprehensive link between the structural behaviour of a CNC-based SG and its mechanistic properties, laying foundations for the development of functional, built-to-order soft materials. [ABSTRACT FROM AUTHOR]

We analyze transient dynamics during the shear start-up in viscoelastic flows between two parallel plates, with a specific focus on the signatures for the onset of transient shear banding using the Johnson–Segalman, nonstretching Rolie-Poly, and Giesekus models. We explore the dynamics of the shear start-up in monotonic regions of the constitutive curves using two different methodologies: (i) the oft-used "frozen-time" linear stability (eigenvalue) analysis, wherein we examine whether infinitesimal perturbations imposed on instantaneous stress components (treated as quasisteady states) exhibit exponential growth, and (ii) the more mathematically rigorous fundamental-matrix approach that characterizes the transient growth via a numerical solution of the time-dependent linearized governing equations, wherein the linearized perturbations coevolve with the start-up shear flow. Our results reinforce the hitherto understated point that there is no universal connection between the overshoot and subsequent decay of shear stress in the base state and the unstable eigenvalues obtained from the frozen-time stability analysis. It may, therefore, be difficult to subsume the occurrence of transient shear banding during the shear start-up within the ambit of a single model-independent criterion. Our work also suggests that the strong transients during the shear start-up seen in earlier work could be sensitive to the small solvent viscosity values considered in the absence of otherwise negligible terms such as fluid inertia. [ABSTRACT FROM AUTHOR]

Complex systems often exhibit shear banding—the coexistence of two different states characterized by their internal structuring and local shear rates. For some of them, the heterogeneous flow corresponds to the final steady-state response, while for others, shear banding can only be transient, the banding structure healing back to a homogeneous flow in the ultimate steady state after long-lived periods. In order to explain the diversity of observations, Moorcroft and Fielding have established general criteria for the onset of banding in time-dependent flows of complex systems, ranging from polymeric fluids to soft glassy materials [Moorcroft et al., Phys. Rev. Lett. 110, 086001 (2013)]. The proposed criteria are based on the time evolution of the bulk rheological response function of the system to a given time-dependent flow protocol and are associated with a specific signature in the mechanical response. In this contribution, we test the validity of these criteria in the case of two common time-dependent flow protocols: a step stress and a shear startup. Two types of fluids are examined. On the one hand, a wormlike micelles system exhibiting steady shear banding is studied experimentally, using rheometry coupled with direct visualizations and particle image velocimetry. On the other hand, we analyze previous literature on yield stress fluids exhibiting transient shear banding. Under conditions of a constant imposed stress, for both types of fluids, the onset of banding arises in a time window compatible with the Moorcroft–Fielding criterion. However, the mechanical signature, i.e., the shape of the bulk mechanical signal as a function of time, is not the one expected within some of the specific models with which the general Moorcroft–Fielding criteria were tested numerically. Under shear startup, both types of fluids behave differently. The criterion holds for yield stress fluids, the onset of banding arising just after the stress overshoot, as expected. On the contrary, for wormlike micelles, the window of instability is delayed, even if the overshoot clearly plays a crucial role in the nucleation of shear-induced structures. Regardless of the flow protocol or the system, wall slip seems to go hand in hand with banding indicating that it is a key ingredient to take into account. [ABSTRACT FROM AUTHOR]

We report on the response of semidilute shear-banding wormlike micelles to time-dependent flow protocols such as step stress and shear startup. We use global rheology coupled with direct optical visualizations in the Taylor–Couette flow to provide a detailed 2D description of the spatiotemporal dynamics of the shear banding flow under imposed stress. We compare these dynamics to their extensively studied counterpart under an imposed shear rate. We show that, for a given asymptotic state, the onset of banding is delayed and the transients are longer when the stress is used as a control parameter. The way the banding structure is built up strongly depends on the control parameter. In the two cases, secondary flows develop on top of the shear-banding instability: after shear startup, the interface between bands is formed in the bulk of the system and migrates inward to its stationary position in the gap of the Taylor–Couette device, before being destabilized due to elastic instability; under creep flow, the interface between bands forms at the inner wall and becomes wavy while it is still migrating outward to its steady position, leading to permanent rearrangements of the flow structure until a steady state is reached, potentially explaining longer transients. We also outline the possible mechanical signature of the onset of shear banding in terms of shape of the bulk mechanical signals, which is the subject of Part II of this work [Briole et al., J. Rheol., 65, 1201–1217 (2021)]. [ABSTRACT FROM AUTHOR]

Long-chained surfactant solutions have found widespread use in the oil and gas industry due to a host of attractive properties. In this paper, we characterize one such commercially used viscoelastic surfactant that forms a wormlike micellar gel at room temperature and a viscoelastic solution at higher temperatures. We probe both states by conducting linear and nonlinear rheological tests and analyze their behavior under the framework of micellar rheology. Our study outlines departure from behavior exhibited by more conventional micellar systems and uncovers interesting dynamics like shear-induced fracture and possible shear-banding in these materials. In doing so, we provide a detailed understanding of a novel class of wormlike micellar solutions. [ABSTRACT FROM AUTHOR]

Non-equilibrium molecular dynamics are used in a coarse-grained scale to simulate the simple shear flow of telechelic and non-telechelic associative polymer solutions. The work includes the study of inception of flow and stress relaxation in transient flows and the constitutive curve at steady state. The instabilities (plateau stress, stress maxima) and the formation of bands with different velocity gradient and concentration depend on the location of the hydrophobic sites (sticker sites, SS) in the chain. Results allow to shed light on the origin of banded flow within the transient state, related to depletion of bridges between aggregates. This situation induces instabilities in the velocity profiles and concentration inhomogeneities along the velocity gradient during the stress overshoot. As the steady state is reached, instabilities disappear for systems where the SS are in the middle of the chain. The conformation relaxation occurs during a time scale larger than that of the stress, but this is not sufficient condition for the presence of banded flow in telechelic polymer solutions, as it holds in polymer melts. The difference in concentration among the bands suggests the existence of a shear-gradient concentration coupling mechanism under banded flow conditions, such as spatial non-homogeneities in the network density, induces chain diffusivity in the gradient direction and increasing normal stresses, especially in telechelic chains. The role of normal stresses on the flow-concentration coupling instability is analyzed in detail. [ABSTRACT FROM AUTHOR]

Manipulation of the atomic-scale structure of two-dimensional (2D) interfaces have been shown to provide nanocomposites with enhanced strength, deformability, and radiation damage resistance. In comparison with 2D interfaces, here we investigate the mechanical response of nanocomposites containing three-dimensional (3D) Cu/Nb interfaces consisting of a chemical/structural gradient separating pure Cu and Nb layers, through which the lattice mismatch between face-centered cubic Cu and body-centered cubic Nb is accommodated over a distance of several nanometers. It is demonstrated that 3D interfaces increase the yield and flow strength by 50% and 22%, respectively, over composites containing 2D interfaces at similar layer thicknesses. After 14% compressive strain, the onset of shear banding results in co-deformation of both Cu and Nb phases within and outside of the shear band. We conclude with a discussion of the role of interface structure in shear band formation and growth in 3D Cu/Nb. Image, graphical abstract [ABSTRACT FROM AUTHOR]

We report experiments on spatiotemporal evolution of the velocity profiles in shear-banding wormlike micelles upon inception of the flow in a Taylor–Couette (TC) cell. Both moderately entangled and highly entangled solutions are considered over a broad range of fluid elasticity E. Fluid elasticity, E = Wi/Re, characterizes the relative importance of the elastic to inertial effects. For both moderately and highly entangled solutions, upon inception of the shear, and during the stress decay period, fluid moves in the opposite direction to that of the imposed motion in a subset of the gap beyond critical thresholds of elasticity and flow ramp up rate, which depend on the fluid entanglement density. Surprisingly, beyond a second critical threshold of the fluid elasticity, the transient backflow disappears in moderately entangled solutions, highlighting the importance of the micellar entanglement on transient evolution of the flow in shear banding systems. More interestingly, we report the formation of multibanded quasisteady velocity profiles under certain conditions of fluid elasticity and flow ramp up rate. The multibanded profiles are characterized by a low shear band near the inner cylinder, a high shear band in the middle of the TC gap, and another low shear band near the outer cylinder. Finally, we show that the apparent wall slip at the inner cylinder of the TC cell is more pronounced for highly entangled solutions and decreases as the fluid elasticity increases. Experimental observations are compared with the existing simulations of the Vasquez–Cook–McKinley model, and several suggestions are made for future simulations. [ABSTRACT FROM AUTHOR]

Shear banding is an important deformation and failure mechanism in metallic systems, especially at high-rate straining. Dynamic recrystallization was often reported to account for the refined microstructure of shear bands but rarely confirmed using direct quantitative measurement. Here, we employ quantitative precession electron diffraction analysis to uncover shear band microstructure in pure titanium. The results reveal that the microstructure is dominated by early stages of grain subdivision process. Dynamic recrystallization is not as prevalent as perceived conventionally. Our results offer key insights into understanding shear banding and highlight the need for quantitative analyses of shear band microstructure. A critical quantitative assessment of shear band microstructures is made using precession beam diffraction imaging. Specifically, grain subdivision, as opposed to dynamic recrystallization, drives the shear band microstructure evolution. [ABSTRACT FROM AUTHOR]

Ductile metallic glass foams (DMGFs) are a new type of structural material with a perfect combination of high strength and toughness. Owing to their disordered atomic-scale microstructures and randomly distributed macroscopic voids, the compressive deformation of DMGFs proceeds through multiple nanoscale shear bands accompanied by local fracture of cellular structures, which induces avalanche-like intermittences in stress–strain curves. In this paper, we present a statistical analysis, including distributions of avalanche size, energy dissipation, waiting times and aftershock sequence, on such a complex dynamic process, which is dominated by shear banding. After eliminating the influence of structural disorder, we demonstrate that, in contrast to the mean-field results of their brittle counterparts, scaling laws in DMGFs are characterized by different exponents. It is shown that the occurrence of non-trivial scaling behaviours is attributed to the localized plastic yielding, which effectively prevents the system from building up a long-range correlation. This accounts for the high structural stability and energy absorption performance of DMGFs. Furthermore, our results suggest that such shear banding dynamics introduce an additional characteristic time scale, which leads to a universal gamma distribution of waiting times. [ABSTRACT FROM AUTHOR]

Lattice inhomogeneities, i.e. nitrogen interstitial-induced hexagonal-close-packed martensite (HCP-ε N) and shear bands, can form in face-centred cubic nitrogen-expanded austenite (FCC-γ N) synthesised on Fe-Cr-Mn and Fe-Cr-Ni austenitic stainless steel (ASS) using triode-plasma nitriding (TPN). Homogenous elemental distribution between HCP-ε N and FCC-γ N supports the displacive shear transformation mechanism proposed for a high-Mn, low stacking fault energy ASS under nitrogen interstitial-induced deformation. While resulting from transformation-induced plasticity, HCP-ε N exhibits similar lattice expansion behaviour to the parent FCC-γ N. However, inhomogeneous elemental distributions in the shear bands formed in γ N layers on a 400°C TPN-treated high-Ni ASS indicate local migration of substitutional elements. Image, graphical abstract [ABSTRACT FROM AUTHOR]

In the present work, we define material functions at yielding in a perspective where elasticity and thixotropy can play an important role. These complex materials need to be characterized with a broader approach than the simple single value of the shear yield stress in a viscometric condition. Special emphasis is given to the distinction between static and dynamic yield stresses along with the distinction of viscometric and extensional yielding. The definition of static properties has no parallel with the usual material functions in nonyielding systems, like polymeric liquids, for example, where the words viscometric and extensional have a clear kinematic representation. Instead, static properties have to be measured by means of a stress state, since different stress states can be achieved where no motion takes place. We introduce the definition of material functions at yielding by specifying the yield stress tensor, an entity recently introduced by Thompson et al. [J. Nonnewton. Fluid Mech. 261, 211–219 (2018)], relative to a particular state. In this regard, we define the material functions at yielding as the components of the static and dynamic yield stress tensors in viscometric, in uniaxial extension, in biaxial extension, and in planar extension conditions. [ABSTRACT FROM AUTHOR]

The viscosity of a shear-banding wormlike micelle solution at high shear rates is investigated by capillary rheology and particle streak velocimetry. Measurements of the flow profile and pressure gradient show an extended entrance region, which exceeds a length to diameter ratio of 100, to reach a fully developed flow. We characterized this entrance region for capillaries with different cross sections and used the results to select a downstream portion of the capillary, where viscosity measurements can be made on fully developed flow. Measurements from this portion of the channel show a shear-thinning power-law behavior for all channel geometries from shear rates of 1000– 120 000 s − 1 . Varying the surfactant concentration shows two distinct power-law behaviors that depend on both the shear rate and the concentration and are an indication of change in the micelle length. [ABSTRACT FROM AUTHOR]

Metallic glasses commonly demonstrate softening due to excessive free volume or hardening through partial nanocrystallization when subjected to irradiation. However, this study demonstrates a unique strengthening effect on plasticity without sacrificing hardness in Zr-based metallic glass under low-dose helium ion irradiation in the absence of nanocrystals. Nanoindentation hardness measurements reveal an initial increase in hardness with low irradiation doses, followed by a rapid decrease at higher doses. In situ mechanical transmission electron microscopy experiments confirmed the switch from localized shear banding to homogeneous shear flow with low-dose helium ion irradiation. Notably, a remarkable increase in tensile plasticity ∼18 % was achieved without compromising the strength ∼1.74 GPa. The improvement can be attributed to the formation of local dense packing regions filled with helium atoms, which elevate the critical stress for plastic deformation. These regions interact with the loosely packed surroundings and facilitate shear band formation. Our findings offer invaluable atomic-scale insight into the structure-property relationships in irradiated metallic glasses, thereby supporting their potential applications in advanced fission and fusion reactors. [ABSTRACT FROM AUTHOR]

We investigated the shear banding phenomena in the non-isothermal simple-shear flow of a viscoelastic-fluid-based nanofluid (VFBN) subject to exothermic reactions. The polymeric (viscoelastic) behavior of the VFBN was modeled via the Giesekus constitutive equation, with appropriate adjustments to incorporate both the non-isothermal and nanoparticle effects. Nahme-type laws were employed to describe the temperature dependence of the VFBN viscosities and relaxation times. The Arrhenius theory was used for the modeling and incorporation of exothermic reactions. The VFBN was modeled as a single-phase homogeneous-mixture and, hence, the effects of the nanoparticles were based on the volume fraction parameter. Efficient numerical schemes based on semi-implicit finite-difference-methods were employed in MATLAB for the computational solution of the governing systems of partial differential equations. The fundamental fluid-dynamical and thermodynamical phenomena, such as shear banding, thermal runaway, and heat transfer rate (HTR) enhancement, were explored under relevant conditions. Important novel results of industrial significance were observed and demonstrated. Firstly, under shear banding conditions of the Giesekus-type VFBN model, we observed remarkable HTR and Therm-C enhancement in the VFBN as compared to, say, NFBN. Specifically, the results demonstrate that the VFBN are less susceptible to thermal runaway than are NFBN. Additionally, the results illustrate that the reduced susceptibility of the Giesekus-type VFBN to the thermal runaway phenomena is further enhanced under shear banding conditions, in particular when the nanofluid becomes increasingly polymeric. Increased polymer viscosity is used as the most direct proxy for measuring the increase in the polymeric nature of the fluid.