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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]

We formulate a particle and force level, activated dynamics-based statistical mechanical theory for the continuous startup nonlinear shear rheology of ultradense glass-forming hard sphere fluids and colloidal suspensions in the context of the elastically collective nonlinear Langevin equation approach and a generalized Maxwell model constitutive equation. Activated structural relaxation is described as a coupled local-nonlocal event involving caging and longer range collective elasticity which controls the characteristic stress relaxation time. Theoretical predictions for the deformation-induced enhancement of mobility, the onset of relaxation acceleration at remarkably low values of stress, strain, or shear rate, apparent power law thinning of the steady-state structural relaxation time and viscosity, a nonvanishing activation barrier in the shear thinning regime, an apparent Herschel–Buckley form of the shear rate dependence of the steady-state shear stress, exponential growth of different measures of a yield or flow stress with packing fraction, and reduced fragility and dynamic heterogeneity under deformation were previously shown to be in good agreement with experiments. The central new question we address here is the defining feature of the transient response—the stress overshoot. In contrast to the steady-state flow regime, understanding the transient response requires an explicit treatment of the coupled nonequilibrium evolution of structure, elastic modulus, and stress relaxation time. We formulate a new quantitative model for this aspect in a physically motivated and computationally tractable manner. Theoretical predictions for the stress overshoot are shown to be in good agreement with experimental observations in the metastable ultradense regime of hard sphere colloidal suspensions as a function of shear rate and packing fraction, and accounting for deformation-assisted activated motion appears to be crucial for both the transient and steady-state responses. [ABSTRACT FROM AUTHOR]

The stress overshoot (or elastic overshoot) and its mechanism during the high-strain rate deformation of metals are systematically and comprehensively investigated by crystal plasticity finite element simulation. The constitutive model by Austin and McDowell (Int J Plast 27(1):1–24, 2011) is adopted to simulate the crystal anisotropic properties and the deformation with a wide range of strain rates by introducing the individual slip system and a new mechanism of dislocation evolution. Based on the validated constitutive model, the effects of strain rate, temperature, and crystal orientation on the stress overshoot phenomenon are studied, and the corresponding physical mechanism is analyzed. The results show that both strain rate and temperature have significant influences on the stress overshoot and all crystal orientations show the stress overshoot. [ABSTRACT FROM AUTHOR]

[Display omitted] • A novel method for testing paste slurry thixotropy was proposed. • The influence of temperature on the paste slurry thixotropy was investigated. • A quantitative indicator of the stress overshoot phenomenon was proposed. • In continuous shear cycles, the variation law of the DYS is generally the same as that of A thix. Paste slurry exhibits significant thixotropy, which has a significant influence on the slurry pipeline transportation resistance. This paper proposes a novel test method for the thixotropy of paste slurry, which can ensure that the slurry reaches flow equilibrium during the test. Accordingly, the influences of the solid mass concentration, cement-to-tailings ratio, and temperature on thixotropy were studied by employing a control variable method, and the influences of the above three factors on the stress overshoot and rheological parameters were evaluated. The results revealed that the solid mass concentration and temperature have significant influence on the thixotropy. Moreover, with an increase in the solid mass concentration, the time required for the slurry to reach flow equilibrium decreases. The cement content and temperature are proportional to the significance of the stress overshoot, and the change in dynamic yield stress can be considered an indicator for the effective quantification of thixotropy. In the continuous shearing cycles, the maximum apparent viscosity at the start of shearing significantly decreases on the time axis. The solid mass concentration has a significant influence on the minimum apparent viscosity in the slurry flow process, whereas the cement-to-tailings ratio and temperature have no significant influences. [ABSTRACT FROM AUTHOR]

Drilling fluids perform a number of important functions during a drilling operation, including that of lifting drilled cuttings to the surface and balancing formation pressures. Drilling fluids are usually designed to be structured fluids exhibiting shear thinning and yield stress behavior, and most drilling fluids also exhibit thixotropy. Accurate modeling of drilling fluid rheology is necessary for predicting friction pressure losses in the wellbore while circulating, the pump pressure needed to resume circulation after a static period, and how the fluid rheology evolves with time while in static or near-static conditions. Although modeling the flow of thixotropic fluids in realistic geometries is still a formidable future challenge to be solved, considerable insights can still be gained by studying the viscometric flows of such fluids. We report a detailed rheological characterization of a water-based drilling fluid and an invert emulsion oilbased drilling fluid. The micro structure responsible for thixotropy is different in these fluids which results in different thixotropic responses. Measurements are primarily focused at transient responses to step changes in shear rate, but cover also steady state flow curves and stress overshoots during start-up of flow. We analyze the shear rate step change measurements using a structural kinetics thixotropy model.

Magnetorheological gel (MRG) excels in the material properties in term of adjustability and sedimentation performance, which could upgrade the performances of the current magnetorheological fluid based adjustable devices for structural control and vibration mitigation. However, the characterization and modelling of the stress-strain hysteresis responses of MRG has not been reported in the past, which are fundamental steps towards engineering applications. In this study, the stress-strain hysteresis of polyurethane based MRG sample with 60% carbonyl iron particle weight fraction was characterized under sinusoidal shear excitations with broad ranges of strain amplitude (5%–100%), excitation frequency (0.1 Hz–2 Hz) and magnetic fields (0–0.91 T). Significant stress overshooting phenomenon were observed under the application of low magnetic fields (0.27 T). A structurally-simple and accurate phenomenological model has been established to capture this unique nonlinearity. By validating the experimental results, the proposed model accurately predicts the hysteretic behaviour and the overshoot of the MRG under the excitation scenarios and the magnetic fields considered. Finally, the support vector machine (SVM) was implemented to provide the solution to the model generalization. The SVM-assisted model showed good agreement with the experimental data and can benefit the efficiency and viability in developing controllable MRG-based devices and system.

Using non-equilibrium molecular dynamics simulations we study the yielding behaviour of a model cluster crystal formed by ultrasoft particles under shear. We investigate the evolution of stress as a function of strain for different shear rates, γ, and temperatures. The stress-strain relation displays a pronounced maximum at the yielding point; the height of this maximum, σp, increases via a power law with an increasing shear range and tends to saturate to a finite value if the limit shear rate goes to zero (at least within the considered temperature range). Interestingly, this behaviour can be captured by the Herschel-Bulkley type model which, for a given temperature, allows us to predict a static yield stress σp0 (in the shear rate tending to zero limit), a characteristic timescale τc, and the exponent α of the above-mentioned power-law decay of the σp at high shear rates. Furthermore, for different temperatures, the σp can be scaled as functions of onto a single master curve when scaled by corresponding τc and σp0. Moreover, for a given shear rate, σp displays a logarithmic dependence on temperature. Again, the σp-T curves for different shear rates can be scaled on a single logarithmic master curve when scaled by a corresponding fitting parameters. [ABSTRACT FROM AUTHOR]

Abstract Drilling fluids are complex materials that display elastoviscoplastic and thixotropic characteristics. Rotational viscometers that impose discrete rotational velocities, such as the Fann 35A, are traditionally employed to evaluate the rheological properties of drilling fluids in field operations due to their robustness and low cost. These viscometers are used to measure the steady-state shear stresses by imposing different shear rates and the transient stress overshoot as an estimation of the fluid's gel strength. In the current work, two synthetic drilling fluids and two bentonite suspensions were tested in a Fann 35A viscometer and in a rotational rheometer to check how accurate the Fann 35A viscometer results can be in comparison to rheometer data. Although the steady-state shear stresses measured by the Fann 35A were corroborated by the corresponding values obtained from the rheometer, the accuracy of Fann 35A on measuring the stress overshoots depends on the fluid rheological characteristics. Two hypotheses were proposed to explain the difference on the stress overshoots: (i) wall slip and (ii) the inappropriate control of the shear rate. [ABSTRACT FROM AUTHOR]

• Total strain is responsible for breakage, however, the net microstructure change depends on shear-rate. • The same parameter but different shear historiespredicts diverging zero-shear-rate viscosity and viscosity plateau at a low shear rate. • It predicts breakage-dependent transient and aging-dependent steady-state shear banding, viscosity-bifurcation, delayed start. • Also predict waiting time dependent overshoot, stress hysteresis, sudden stepdown shear rate results, etc. • At a steady state, the model reduces to either Bingham, herschel Bulkley-type or Newtonian model. Most earlier thixotropic elasto-visco-plastic models follow the quasi-static pre-yielding linear elastic assumption of Oldroyd's 1946 model. To develop a new model, the present work considers a more realistic pre-yielding non-linear visco-elastic and plastic deformation. Our model is valid for reversible (finite thixotropic time scale) and irreversible (infinite thixotropic time scale) thixotropic materials. Despite being a simple algebraic equation, our model appropriately explains both the viscosity plateau at low shear rates and the diverging zero shear rate viscosity, using the same parameters but different shear histories. Our model also predicts experimentally observable transient shear banding due to microstructure breakage by shear rejuvenation and steady-state shear banding due to aging. Furthermore, our model predicts initial gel structure (waiting time) dependent stress overshoot during shear rate startup flow, different stress hysteresis in shear-rate ramps, sudden stepdown shear rate test results, and viscosity bifurcation during creeping flow phenomena effectively. Depending on shear histories, at steady state, our model reduces to either Bingham model, Herschel Bulkley type model with shear rate dependent yield stress, or Newtonian fluids model. Our model requires only four parameters for the irreversible and five for the reversible thixotropic-elasto-visco-plastic (TEVP) model obtainable from the rheometer test. Our model favourably predicts a series of recent experimental results. The current framework has the potential to provide a possible physical interpretation of the Bingham model. It also has the capability to predict a delayed flow start using an appropriate structure degradation kinetic. [Display omitted] [ABSTRACT FROM AUTHOR]

Using non-equilibrium molecular dynamics simulations we study the yielding behaviour of a model cluster crystal formed by ultrasoft particles under shear. We investigate the evolution of stress as a function of strain for different shear rates, , and temperatures. The stress-strain relation displays a pronounced maximum at the yielding point; the height of this maximum, σp, increases via a power law with an increasing shear range and tends to saturate to a finite value if the limit shear rate goes to zero (at least within the considered temperature range). Interestingly, this behaviour can be captured by the Herschel-Bulkley type model which, for a given temperature, allows us to predict a static yield stress σp0 (in the shear rate tending to zero limit), a characteristic timescale τc, and the exponent α of the above-mentioned power-law decay of the σp at high shear rates. Furthermore, for different temperatures, the σp can be scaled as functions of onto a single master curve when scaled by corresponding τc and σp0. Moreover, for a given shear rate, σp displays a logarithmic dependence on temperature. Again, the σp–T curves for different shear rates can be scaled on a single logarithmic master curve when scaled by a corresponding fitting parameters.

This study examines the influence of the short arms in model asymmetric 3-arm star polymers, in which the short arm is unentangled while the other two long arms are entangled, on linear and nonlinear shear rheological behavior. As increasing the short arm length of the asymmetric star polymers, the linear viscoelastic rubber plateau decreases, indicating the relaxed short arms act as "solvent" that dilute the star polymers. This idea is examined by the relationship between the plateau moduli of polymer solutions G N (φ) and the corresponding polymer melts G N (1) for entangled linear polymers, G N (φ) = G N (1) φ 1+α, where φ is the volume fraction of the polymer. The "solvent" effect of the short arms is also supported by the nonlinear rheology results showing weaker damping function and larger peak strain at stress overshoot. With the above clarification of the short-arm contributions, this work provides a guidance of molecular design for tuning linear and nonlinear rheology of star polymers. [Display omitted] • The influence of the short arms in model asymmetric star polymers on shear rheology is identified. • The short arms act as "solvents", leading to lower plateau modulus just like entangled polymer solutions. • The dilution effect of the short arms on nonlinear rheology is represented by larger peak strain and weaker damping response. [ABSTRACT FROM AUTHOR]

Thixotropy is a common behavior of soft matters widely used in various industries. Numerous models, both empirical and theoretical, have been proposed for different soft matters. In this paper the Maxwell viscoelastic model has been modified by decoupling the second order tensor type equation into vector type equations, without introducing new material parameters or assuming the dependence of the viscosity and/or elasticity on time and/or shear rate. The proposed Maxwell model is known as the vector type Maxwell model while the original one as the second order tensor type Maxwell model. The vector type Maxwell model was applied to both the stationary and the single step shear rate shear flow and the numeric results were compared with the experiments in the literature. It is found that the vector type Maxwell model predicted better both the shear and the normal stresses in the stationary shear flow than the second order tensor type Maxwell model, and the vector type Maxwell fluids may show thixotropic behavior in the single step shear rate flow as observed in experiments.

The uniaxial compression tests were performed to research the deformation behavior of Zr 41.2 Ti 13.8 Cu 12.5 Ni 10 Be 22.5 (Vit1) bulk metallic glass (BMG) in a wide range of strain-rates and temperatures. The true stress-strain curves in the supercooled liquid region (SLR) fitted by Maxwell-pulse constitutive model have been compared with the experimental data. It was found that there were larger deviation between the true stress-strain curves obtained from compression tests and Maxwell-pulse model. In order to describe the whole flow process more accurately, the influences of ambient temperature and strain rate on viscous flow and stress overshoot related to structural relaxation, which were ignored in Maxwell-pulse model, were introduced to construct a new constitutive relationship of Vit1 alloy in SLR. The constitutive models before and after modified were then written into the finite element (FE) simulation software, respectively. It was provided that the fitting results simulated by the new constitutive model were in better agreement with the uniaxial compression test data than those of Maxwell-pulse. [ABSTRACT FROM AUTHOR]

The rheological characterization of drilling fluids is crucial to ensure an adequate performance of drilling operations. These materials show elastoviscoplastic and thixotropic behaviors simultaneously so that its gel strength is highly affected by the aging time to which the sample is subjected. This work presents experimental evidences of a correlation between the stress overshoot observed in flow start-up rheometric experiments after different aging times and the storage modulus, G′, monitored during a low amplitude oscillatory sweep over the aging time. A comparison of the correlation with the measured data at five different shear rates show that results lie within error bands of 10%. The results also suggest that the stress overshoot for different aging times can be estimated by using only two rheometric tests: an oscillatory sweep and a controlled shear rate experiment. [ABSTRACT FROM AUTHOR]

Polystyrene solutions are the widely used model system to study polymer dynamics and rheology. In the strong nonlinear region, we shear polystyrene solutions of uniform or bimodal distribution up to unprecedented high rates. The unique shear strain hardening takes place on such solutions with high T g solute, irrespective of dispersity. Non-Gaussian stretching of chains is suggested by full recovery even after a shear strain of 60. Shear strain hardening in uniform solutions happens at a universal Rouse-Weissenberg number, 150, regardless of the number of entanglements. The critical stress between traditional stress overshoot and hardening is around 104 Pa, for 10% solutions regardless of dispersity or number of entanglements. This systematic study charts the parameters for shear strain hardening. [Display omitted] • Systmatic study of shear strain hardening in solution of uniform and bimodal distributed solutions. • A universal Rouse-Weissenberg number for the onset of hardening is identified and the critical stress is 10000 Pa. • Full recovery after a shear strain of 60 demonstrates its elastic origin: non-Gaussian stretching. [ABSTRACT FROM AUTHOR]

A solid-like particle network structure induced by steady shear flow is observed in polymer nanocomposites for the first time. This unique structural evolution is evidenced by the stress overshoot during start-up of steady shear. After the steady shear ordering event, the enhanced rubbery response is measured using small amplitude oscillatory shear. The particle surface chemistry dominates the formation of the percolation structure: the shear-induced reinforcement effect is observed at lower particle loadings for hydrophobic filler than for hydrophilic filler. These results indicate that reinforcement in nanocomposites can be controlled by filler surface functionality and shearing effects, enabling improved mechanical properties with low filler loadings. [ABSTRACT FROM AUTHOR]

The objective of the present study is to experimentally investigate the transient rheological behavior of concentrated xanthan gum solutions in start-up shear flow fields. Using a strain-controlled rheometer, a number of constant shear rates were suddenly imposed to aqueous xanthan gum solutions with different concentrations and the resultant shear stress responses were measured with time. The main findings obtained from this study can be summarized as follows: (1) For all shear rates imposed, however low it may be, the shear stress is rapidly increased with time (stress overshoot) upon inception of steady shear flow before passing through the maximum stress value and then gradually decreased with time (stress decay) until reaching a steady state flow. (2) As the imposed shear rate is increased, a more pronounced stress overshoot takes place and the maximum stress value becomes larger, whereas both times at which the maximum stress is observed and needed to reach a steady state flow are shortened. (3) The maximum shear stress is linearly increased with shear rate in a double logarithmic scale and becomes larger with increasing concentration at equal shear rates. In addition, the time at which the maximum stress occurs exhibits a linear relationship with the inverse of shear rate in a double logarithmic scale for all xanthan gum solutions, regardless of their concentrations. (4) The shear stress is sharply increased with an increase in strain until reaching the maximum stress at small range of deformations. The maximum stress is observed at similar strain values, irrespective of the imposed shear rates lower than 10 1/s. (5) The Bird-Leider model can be successfully used with regard to quantitatively predicting the transient behavior of concentrated xanthan gum solutions. However, this model has a fatal weakness in terms of describing a decrease in shear stress (stress decay). [ABSTRACT FROM AUTHOR]

Based on the shear-transformation-zone (STZ) theory, we propose a constitutive model for describing homogeneous elastoplastic deformation of amorphous solids where the interaction of shear transformations and free volume dynamics is incorporated. This theoretical model can reproduce the stress overshoot behavior that shows the dependence of strain rate, temperature, STZ population and dilatancy of systems. It reveals that the stress overshoots its steady state value due to the delayed activation of shear transformations that results from the insufficient free volume in the system. However, the subsequent strain softening (stress drop) is attributed to the shear-induced dilatation that is a result of the positive interplay between shear transformations and free volume creation, the latter playing the dominant role. Our analysis also demonstrates that the STZs, as basic carriers of amorphous plasticity, govern the yielding of the system, whereas the free volume dynamics significantly affects the post-yielding behaviors. [ABSTRACT FROM AUTHOR]

The transient response in metallic glass deformation is investigated via a mesoscale simulation by shear transformation zone dynamics. We directly connect the microscopic processes, i.e. shear transformation-induced free volume creation and diffusive rearrangement-induced free volume annihilation, to the macroscopic transient response. Notably, the dynamic competition between these processes gives rise to the signature stress overshoot in the homogeneous deformation regime at elevated temperatures; and the suppression of the annihilation process facilitates strain localization into a shear band at low temperatures. [ABSTRACT FROM AUTHOR]

Rheological analysis was used to understand the structure-property relations of polymer nano-composites based on ethylene vinyl acetate. Two geometrically different nano-particles (sphere of CaCO and platelet of montmorillonite) having the same energetic attractions with ethylene vinyl acetate were studied for concentrations between 2.5 and 15 wt%. Three phenomena were studied: the appearance of a solid-like behavior in the linear viscoelastic domain, the limits of linear viscoelasticity, and the presence of stress overshoot in step shear tests. In particular, stress overshoot was investigated based on the tube concept of polymeric chains. Also, differences related to nano-particle geometry (platelet vs. spherical) were investigated based on a filler-network mechanism. Due to higher physical contacting probability, platelet particles can better interact and create a network structure, which dominates the rheological response. On the other hand, although spherical particles can limit the motion of polymeric chains under flow, a strong physical network was not formed. For platelets, scaling behavior was well described by fractal model which considers direct aggregation, and such scaling was not observed for spherical particles. The filler-network mechanism was validated by image analysis. [ABSTRACT FROM AUTHOR]

Abstract: In this paper, we quantitatively analyze the extent of structure build-up at rest in carbon nanotubes (CNTs) dispersed in an epoxy by a set of transient flow measurements. Stress overshoots appeared at very small deformations during forward and reverse flow measurements by providing some rest time between the two consecutive flows in opposite directions, during which the suspensions structure was reconstructed. The rest time required to build-up the structure completely was much longer than the values reported in literature (about 1h). Moreover, unlike fiber suspensions or some nano-composites, it was shown that the Brownian motion plays an important role in the structure build-up of the CNT suspensions in the absence of flow. We observed critical shear rates at low and intermediate concentrations above which some nanotube entanglements broke down; this resulted in lower elasticity of the suspensions and partial structure build-up at rest during transient flow reversal measurements. This phenomenon and the possible mechanisms of structure evolution during flow and rest were further investigated by comparing the experimental results with the predictions of a structural model. [Copyright &y& Elsevier]

Abstract: Rheological experiments on relaxation of shear stress in a diluted ferrofluid with clustered iron oxide nanoparticles ( vol%) have been performed. Changes of the stress appearing in the fluid at constant magnetic field strength after a stepwise change of shear rate have been measured. It has been found that the time of the transient, until the shear stress in the fluid will be steady, depends strongly on the strength of the applied magnetic field and shear rate. For vanishing magnetic field slow relaxation has not been observed. The time of the transient in the presence of a magnetic field can reach several minutes. The change of the transient viscosity as a function of the steady viscosity shows a linear behavior and depends on magnetic field strength. Those effects can be attributed to the process of structure formation and destruction due to the simultaneous action of an applied magnetic field and shear flow. A similar behavior is known from the rheology of complex fluids like polymer melts or aggregating colloids. We propose a model of the rheological effects based on the assumption that the clusters form linear chains which size distribution is determined by applied magnetic field and shear rate. [Copyright &y& Elsevier]

Rheological responses of hydrophobic fumed silica powders, whose surface silanol groups were modified by hexadecane, suspended in 1,4-dioxane at lower silica concentrations than 6.8 vol% have been investigated as a function of the silica concentration. Transient shear stress behavior before attaining the steady-state shear stress could be classified into three regimes as follows, irrespective of the silica concentration: at the lower shear rates than ca. 0.3 s-1 a stress overshoot was observed, at the shear rate ranges from 0.3 to 30 s-1 sustained oscillations in shear stresses were exhibited and these oscillations were first observed for the suspensions at the low particle concentrations, and beyond the shear rate of 30 s-1 a sigmoid decrease of the shear stress with increasing time, that is, structural breakdown, was observed. At the steady state the silica suspensions showed shear thickening. Small angle neutron scattering (SANS) measurements of the silica suspension under shear flow provided that changes in the SANS intensities were well correlated with the shear thickening behavior. However, shear thinning behavior at higher shear rates did not cause any changes in the SANS intensities. [ABSTRACT FROM AUTHOR]

Abstract: Stress overshoot related to the variation in free volume is an important phenomenon in cylinder compression tests of bulk metallic glasses (BMGs). A Maxwell-pulse constitutive model was proposed according to the overshoot of Zr55Cu30Al10Ni5 BMG in the supercooled liquid region (SCLR). Compression experiments and the constitutive model showed good agreement, especially with regard to the stress overshoot phenomenon. Furthermore, micro-backward extrusion experiments and numerical simulation were conducted in SCLR. The results showed that the constitutive model describes the mechanical behavior of the BMG well. [ABSTRACT FROM AUTHOR]

Abstract: High-temperature mechanical properties and stress relaxation of amorphous Zr41.2Ti13.8Cu12.5Ni10Be22.5 alloy (number indicate at.%) taken from thin sections of commercial golf club heads were investigated at high temperatures. Stress overshooting was observed below the glass transition temperature, and the viscous flow behavior was revealed in the supercooled liquid temperature region. The stress was found to increase after high-temperature interruption due to crystallization at high temperatures. Below the glass transition temperature, not only stress overshooting, but also a stress increase was observed. The presence of stress overshooting may be linked to a stress-induced rearrangement of solute atoms in the matrix of the amorphous phase. The stress relaxation rate below the glass transition temperature was very low whereas it was very fast at temperatures between the glass transition temperature and the crystallization temperature T x. The stress relaxation rate decreases above the crystallization temperature, T x, which can be attributed to the difficult atomic rearrangement and the low diffusivity in intermetallics. [Copyright &y& Elsevier]

Abstract: Forward and reverse stress growth experiments have been conducted on polypropylene/organoclay nanocomposites containing the same clay loading but characterized by different microstructures. Stress overshoots have been observed for the initial start-up experiments and for the following reverse start-up experiments after a certain rest time. The amplitude of these overshoots increased with the applied shear rate and rest time, but the overshoots occurred at the same strain of about 1.7. The overshoots are related to the structure of the nanocomposites, in particular the magnitude of the overshoots increased with the degree of the clay exfoliation in the matrix. Two models, initially developed for colloidal suspensions and fiber suspensions, have been used to describe the observed phenomena. The overshoots are fairly well predicted by the first (structure network) model and explained by the competing effects of the structure breakdown under flow and reorganization during rest time. However, the model predicts that the shear stress following the overshoot decreases and reaches steady-state too rapidly. The second model developed for ellipsoid suspensions describes quite well the stress overshoots for the initial forward flow, but no effect of rest time is predicted. A modified version has been proposed by adding a molecular diffusivity contribution in the Folgar–Tucker equation. The effect of the particle disorientation is qualitatively predicted, but the kinetics is too slow compared to that deduced from experiments. [Copyright &y& Elsevier]

The statistical theory of temporary polymer networks developed in parts I and II of this work is closed by a discussion on dynamical effects, including comparison with experiments. After a short review of the theory developed in parts I and II we discuss the situation in which a prescribed velocity gradient is imposed on the physical system formed by the temporary network and the solvent. The dynamical equation for the 2nd moments (derived from a generalized bead-spring model) contains the configuration-dependent transition probability, which depends on the second moments in a complex way. An approximate solution of these equations is obtained from a computer program. It shows the experimentally observed behavior, in particular the stress overshoot maximum after a sudden start of the flow. The derivation of the above equations contains simplifying assumptions which, however, leave the essential physics intact. The most important assumptions concern the relaxation time approach and the decay and formation processes. These were supposed to be dilute enough to break up the many-junction processes into one-junction processes. Both assumptions are analogous to frequently used assumptions in Boltzmann's kinetic theory. [ABSTRACT FROM AUTHOR]

Magnetorheological gel (MRG) excels in the material properties in term of adjustability and sedimentation performance, which could upgrade the performances of the current magnetorheological fluid based adjustable devices for structural control and vibration mitigation. However, the characterization and modelling of the stress-strain hysteresis responses of MRG has not been reported in the past, which are fundamental steps towards engineering applications. In this study, the stress-strain hysteresis of polyurethane based MRG sample with 60% carbonyl iron particle weight fraction was characterized under sinusoidal shear excitations with broad ranges of strain amplitude (5%–100%), excitation frequency (0.1 Hz–2 Hz) and magnetic fields (0–0.91 T). Significant stress overshooting phenomenon were observed under the application of low magnetic fields (0.27 T). A structurally-simple and accurate phenomenological model has been established to capture this unique nonlinearity. By validating the experimental results, the proposed model accurately predicts the hysteretic behaviour and the overshoot of the MRG under the excitation scenarios and the magnetic fields considered. Finally, the support vector machine (SVM) was implemented to provide the solution to the model generalization. The SVM-assisted model showed good agreement with the experimental data and can benefit the efficiency and viability in developing controllable MRG-based devices and system. Unlabelled Image • Comprehensive experimental characterization of the stress-strain hysteresis relationship of magnetorheological gel. • Discovered the stress overshoot behaviour in magnetorheological gel upon on the application of low level magnetic field. • Establishment of a four-parameter overshoot model to portray the nonlinear behaviour of magnetorheological gel. • Generalization of the four-parameter overshoot model by support vector machine method. [ABSTRACT FROM AUTHOR]