Bulk Yield Stress Research Articles

A multiscale model to predict the equation of state of a saturated cement paste

An advanced multiscale analytical approach for predicting the Equation of State (EOS) of fully saturated cement pastes is presented. This approach models the matrix as an elastic–plastic material with linear hardening that includes capillary pores filled with water. The model takes into account both the stochastic variation of pore sizes and their spatial distribution. The microscale level is represented by a spherical medium having the mechanical properties of the cement paste matrix. A single spherical pore that is filled with compressible water is located at the center of the micro-domain. The behavior of the compressible water is modeled by the nonlinear Tait equation of state. The EOS at the macro level is obtained by averaging the micro level domain strains over the pore sizes and pore wall thicknesses. It is shown that the solution depends only on the ratio between the minimum pore wall thickness and the lower limit of capillary pore radius and is independent of each of these parameters separately. The EOS obtained by the proposed model shows good agreement with experimental data for cement paste with water/cement ratio w/c = 0.50. The performed parametric study shows that the bulk modulus and yield stress of undisturbed cement gel matrix significantly affect the EOS, while the variation of Poison ratio, plastic hardening parameter and minimum limit pore size and wall thickness (within physically accepted range) slightly affect the cement paste bulk behavior.

Scratch hardness as a quasi-intrinsic parameter to measure the scratch resistance of polymers

In this work four different polymers (acrylonitrile-butadiene-styrene, high-impact polystyrene, rubber-toughened polybutylene terephthalate, linear low-density polyethylene) were characterized in terms of their bulk (modulus and yield stress) and surface (scratch hardness) mechanical properties. The intrinsic time-dependence of the materials was addressed by performing DMA and compression tests at varying testing speed/frequency, exploiting time-temperature superposition and Eyring's model to obtain data at strain rates compatible with scratch experiments. The latter were performed by applying different loading histories (constant depth or load) and indenters. Scratch hardness was determined using Pelletier's model; it was demonstrated that such a parameter provides a reliable and almost intrinsic (i.e. loading history independent) evaluation of scratch resistance, seen as the resistance the material opposes to indenter penetration. The material compressive yield stress (evaluated at the strain rate relevant to scratch phenomena) was found to be the key controlling factor in determining scratch hardness. It can therefore be taken as a measure of the mechanical scratch resistance when evaluating the possible effects of variables such as material composition, crystallinity, physical ageing … Its relation with other aspects of the scratch phenomenon (in particular deformation recovery) was also explored, accounting for the specific deformation regime imposed by the indenter (transitioning from elastic to predominantly plastic).

Height-to-Diameter Ratio and Porosity Strongly Influence Bulk Compressive Mechanical Properties of 3D-Printed Polymer Scaffolds.

Although the architectural design parameters of 3D-printed polymer-based scaffolds-porosity, height-to-diameter (H/D) ratio and pore size-are significant determinants of their mechanical integrity, their impact has not been explicitly discussed when reporting bulk mechanical properties. Controlled architectures were designed by systematically varying porosity (30-75%, H/D ratio (0.5-2.0) and pore size (0.25-1.0 mm) and fabricated using fused filament fabrication technique. The influence of the three parameters on compressive mechanical properties-apparent elastic modulus Eapp, bulk yield stress σy and yield strain εy-were investigated through a multiple linear regression analysis. H/D ratio and porosity exhibited strong influence on the mechanical behavior, resulting in variations in mean Eapp of 60% and 95%, respectively. σy was comparatively less sensitive to H/D ratio over the range investigated in this study, with 15% variation in mean values. In contrast, porosity resulted in almost 100% variation in mean σy values. Pore size was not a significant factor for mechanical behavior, although it is a critical factor in the biological behavior of the scaffolds. Quantifying the influence of porosity, H/D ratio and pore size on bench-top tested bulk mechanical properties can help optimize the development of bone scaffolds from a biomechanical perspective.

Rheological behaviour of pure clay and coarse-grained clay suspensions using an inclined blade vane-in-cup

Mixtures of clay and sand suspended in water are complex systems encountered in many industrial and environmental applications, but their understanding and modelling are still limited and require further investigations. In this way, the rheological behaviour of pure non-thixotropic clay and coarse-grained clay suspensions is investigated with a rotational rheometer equipped by an inclined blade vane-in-cup, where the type and the volume fraction of both fine clay and coarse materials are varied. The Herschel–Bulkley model τ = τ y + K γ ̇ n is used to describe successfully the flow curves of suspensions relating the shear stress τ to the shear rate γ ̇ , from which the yield stress τ y , the consistency K and the index n are deduced. The main contributions of this study are (i) to validate the inclined blade vane-in-cup for the estimation of the rheological behaviour of polymer microgels, pure clay suspensions, and coarse-grained clay suspensions, (ii) to conclude on the most appropriate model type to predict the yield stress of pure clay suspensions over a wide range of fine clay volume fractions, and (iii) to highlight how frictional contacts of coarse grains may play a role on the bulk yield stress of coarse-grained clay suspensions. We believe that this work would be useful for improving the rheological description of cohesive suspensions in geophysical and industrial applications. • Rheological measurements are performed on pure clay and coarse-grained clay suspensions. • The inclined blade vane allows to estimate the rheology of complex fluids containing grains. • The most appropriate models are provided to quantify the yield stress of suspensions. • The type of coarse materials may affect the bulk yield stress due to frictional contacts.

Squeeze cementing: Invasion of a yield stress suspension into a pore

Squeeze cementing is a process of fundamental importance in oil & well operations, in which cement slurry is injected under pressure into void spaces in defective sections of a wellbore, The aim is to isolate the borehole from adjacent geological zones and repair defects in the existing cement sheath, e.g. cracks, debonding and micro-annuli. Although cement slurries are often considered a homogeneous fluid, on the scales of relevance the multiphase nature of the cement slurry becomes relevant. In this context, we propose a model in which the cement slurry is considered as a viscoplastic suspension, in which both rheological and particle migration effects can be investigated. This allows us to explore different stoppage mechanisms, that combine bulk yield stress effects, enhancement by the solids fraction and modification by particle migration. Our approach is a continuum model based on the suspension balance model. We use this to model slurry penetration in long ducts, driven by a constant pressure difference. As well as a starting point for a more complex process model, our model provides insight into how shear induced migration combines with yield stress effects to produce non-monotone solids distributions adjacent to plug regions.

Quantitative analysis of artificial dam failure effects on debris flows – A case study of the Zhouqu ‘8.8’ debris flow in northwestern China

Artificial dams are one of the most common hydraulic structures for mitigating debris flow disasters in alpine valley regions. However, performance alteration and failure after successive debris flows can lead to dam failure, releasing large amounts of materials within a very short time; moreover, the contribution of artificial dam failures to debris flows is poorly understood. This study quantitatively analyzed the artificial dam failure effects based on the numerical simulations of the Zhouqu '8.8' debris flow, with three scenarios: all nine dams failed (S1); no dams were ever built (S2); all nine dams remained intact (S3). The results showed that artificial dam failures had a significant amplifying effect on the magnitude of a debris flow. The maximum velocity and flow depth decreased by 20% and 11.2% if all the dams did not collapse; comparison of S1 and S2 showed that discharge and velocity at the front of the debris flow increased by 54.6% and 89%, the bulk density and yield stress increased by 3.3% and 5.7%, due to artificial dam failures. This could increase the destructive capacity of a debris flow and the possibility of a river blockage. A single artificial dam failure could locally amplify the magnitude of debris flow. Overall, on the catchment scale, the magnitude of a debris flow was dominated by topography and channel geometry, which can reduce the amplification effect of dam failures at locations where the channel was curved. However, where the channel was straight and flat, the flow velocity and discharge increased cumulatively by 3 m/s and 637 m3/s due to cascading failure. In addition, a comprehensive scheme combining ecological and engineering measures to mitigate debris flow disasters is discussed. This quantitative study is important and urgent needed to understand the amplification effect of dam failures and to implement debris flow mitigation in alpine valley regions.

Strain Rate Sensitivity of the Additive Manufacturing Material Scalmalloy\xae

This work investigates the influence of strain rate on the stress/strain behaviour of Scalmalloy. This material is an aluminium–scandium–magnesium alloy, specifically developed for additive manufacturing. The bulk yield stress of the material processed by Selective Laser Melting is approximately 340 MPa which can be increased by heat-treating to approximately 530 MPa. These numbers, combined with the low mass density of 2.7 g/cm3, make Scalmalloy an interesting candidate for lightweight crash-absorbing structures. As this application is inherently dynamic, it is of interest to study the loading rate sensitivity, which is difficult to predict: Al–Sc alloys exhibit classic strain rate sensitivity with an increased yield stress at elevated strain rates. However, Al–Mg alloys are known to show the contrary effect, they exhibit less strength as strain rate is increased. To answer the question how these effects combine, we study the dynamic behaviour at four different strain rates ranging from 10−3 to 1000 /s using servo-hydraulic and split-Hopkinson testing methods. The resulting data is analysed in terms of strain rate sensitivity of tensile strength and failure strain. A constitutive model based on a simplified Johnson–Cook approach is employed to simulate the tensile tests and provides good agreement with the experimental observations.

Wall slip and bulk yielding in soft particle suspensions

We simulate a dense athermal suspension of soft particles sheared between hard walls of a prescribed roughness profile, using a method that fully accounts for the fluid mechanics of the solvent between the particles, and between the particles and the walls, as well as for the solid mechanics of changes in the particle shapes. We thus capture the widely observed phenomenon of elastohydrodynamic wall slip, in which the soft particles become deformed in shear and lift away from the wall slightly, leaving behind a thin lubricating solvent layer of high shear. For imposed stresses below the material's bulk yield stress, we show the observed wall slip to be dominated by this thin solvent layer. At higher stresses, it is augmented by an additional contribution arising from a fluidisation of the first few layers of particles near the wall. By systematically varying the roughness of the walls, we quantify a suppression of slip with increasing wall roughness. For smooth walls, slip radically changes the steady state bulk flow curve of shear stress as a function of shear rate, by conferring a branch of apparent (slip-induced) flow even for $\sigma<\sigma_y$, as seen experimentally. We also elucidate the effects of slip on the dynamics of yielding following the imposition of a constant shear stress, characterising the timescales at which bulk yielding arises, and at which slip first sets in.

Microstructure design of CTAC:FA and BTAC:FA lamellar gels for optimized rheological performance utilizing automated formulation platform.

The main objective of this paper was to optimize hair conditioner performance through variation of composition utilizing automated cosmetic formulation platform and advanced characterization techniques as well as develop understanding of how performance (wet combing and wet lubrication) of hair conditioner is affected by its rheology (i.e. yield stress) and controlled breakdown of the formulations (dilution). The experimental results show that yield stress greatly impacts rheology, stability and performance of the lamellar gels for hair conditioning. All samples were prepared on the Chemspeed Flex Formax. A mechanical rheometer was used to measure bulk viscosity and yield stress in each sample. Dia-stron tensile tester was used to measure the lamellar gels ability to reduce combing force. Potential stronger lamellar gel network formation in the formed lamellar gels potentially leads to higher yield stress exhibited. Viscosity values were also measured after a controlled breakdown (i.e. dilution) of each sample. This was also carried out using a mechanical rheometer. Yield stress of the formulations was engineered through composition variation and was recorded in each system. The highest yield stress value is 251.179Pa at a BTAC/CA ratio of 6:10, and the lowest yield stress is 50.14Pa at a BTAC/CA ratio of 6:5. The highest yield stress value is 50.14Pa at a CTAC/CA ratio of 6:10, and the lowest yield stress is 19.98Pa at a CTAC/CA ratio of 2:10. The highest overall yield stress values can also be observed in the BTAC/CA system, whereas the CTAC/CA system has relatively lower yield stress values. Dilution of each formulation caused a breakdown in viscosity of each formulation with the formulations with highest yield stress maintaining higher viscosity than the other formulations. The formulations with highest yield stress in each system which also maintains the highest dilution viscosity (6% BTAC/10% CA and 6% CTAC/10% CA) have the best effect on reducing overall combing force, that is from dry hair tress to wet hair tress and after product is rinsed off. At a BTAC/CA system of ratio 6:5, there is an 89% reduction in combing force and a 95% reduction in combing force in the BTAC/CA system of ratio 6:10. At a CTAC/CA system of ratio 2:10, there is a 65% reduction in combing force and a 88% reduction in combing force in the CTAC/CA system of ratio 6:10. A 'conditioned' soft feel was observed on each hair tress as the sample was applied and after it was rinsed off. The overall performance of the lamellar gels for hair conditioning can be engineered through optimization of the formulation microstructure and formulation microstructure breakdown on dilution.

Heterogeneity, suspension, and yielding in sparse microfibrous cellulose gels 1. Bubble rheometer studies

Microstructural effects on suspension and yielding are studied in aqueous dispersions of bacterial cellulose fibers using a small suspended air bubble as a sensitive probe particle. An external pressure field is used to control the applied stress and characterize very early stages of fluid yielding, as well as more developed flow. The bubble allows sensitive measurement of small yield stress values but also indicates a discrepancy between bulk and microscale yield stress values. Image analysis and flow visualization provide a measurement of the deformation, yielding, and flow of low-concentration microfiber dispersions at length scales comparable to that of the fibers. Tracking of trapped tracer particles indicates local restructuring occurs in fiber networks, driving heterogeneous yielding and flow. The observed heterogeneity effects decreased as fiber concentration increased, reducing network restructuring. The size of the yielded region in the gels varied inversely with fiber concentration, but did not fully account for the bulk-microscale discrepancies, indicating the gels are restructuring, responsive fluids. We suggest a two-fluid description of sparse fiber gels is necessary to fully account for the heterogeneity and suspension performance.

An approach to develop printable strain hardening cementitious composites

New additive manufacturing methods for cementitious materials hold a high potential to increase automation in the construction industry. However, these methods require new materials to be developed that meet performance requirements related to specific characteristics of the manufacturing process. The appropriate characterization methods of these materials are still a matter of debate. This study proposes a rheology investigation to systematically develop a printable strain hardening cementitious composite mix design. Two known mixtures were employed and the influence of several parameters, such as the water-to-solid ratio, fibre volume percentage and employment of chemical admixtures, were investigated using a ram extruder and Benbow-Bridgwater equation. Through printing trials, rheology parameters as the initial bulk and shear yield stress were correlated with variables commonly employed to assess printing quality of cementitious materials. The rheology properties measured were used to predict the number of layers a developed mixture could support. Selected mixtures had their mechanical performance assessed through four-point bending, uni-axial tensile and compressive strength tests, to confirm that strain hardening behaviour was obtained. It was concluded that the presented experimental and theoretical framework are promising tools, as the bulk yield stress seems to predict buildability, while shear yield stress may indicate a threshold for pumpability.

The Impact of Rheology on the Transition From Stick‐Slip to Creep in a Semibrittle Analog

AbstractFaults can release energy via a variety of different slip mechanisms ranging from steady creep to fast and destructive earthquakes. Tying the rheology of the crust to various slip dynamics is important for our understanding of plate tectonics and earthquake generation. Here we propose that the interplay of fractures and viscous flow leads to a spectrum between stick‐slip and creep. We use an elasto‐visco‐plastic rock analog (Carbopol U‐21) where we vary the yield stress to investigate its impact on slip dynamics in shear experiments. The experiments are performed using a simple shear apparatus, which provides distributed shear across the entire width of the experiment and allows in situ observations of deformation. We record force and displacement during deformation and use time lapse photography to document fracture development. A low yield stress (25 Pa) leads to creep dynamics in the absence of fractures. An intermediate yield stress (144 Pa) leads to the development and interaction of opening (mode I) and shear (mode II) fractures. This interaction leads to a spectrum in slip dynamics ranging from creep to stick‐slip. A high yield stress (357 Pa) results in the development of many mode I fractures and a deformation signal dominated by stick‐slip. These results show that bulk yield stress, fracture formation, and slip dynamics are closely linked and can lead to a continuum between creep and stick‐slip. We suggest that rheology should be considered as an additional mechanism to explain the broad range of slip dynamics in natural faults.

Flow Characteristics of Cemented Paste Backfill

Cemented paste backfill (CPB) is primarily used for backfilling underground voids at George Fisher Mine (Mount Isa, Australia). The objective of this paper is to summarise the geotechnical characterisation of the tailings and the rheological properties of the CPB as determined from a laboratory testing program undertaken at James Cook University. Two binders were examined [a General Purpose cement and a slag blend cement] over a range of dosages from 0 to 6% and CPB mix solids content in the range of 72–78%. The slump tests were carried out using the standard cone (ASTM C 143) used for concrete and a cylinder with 110 mm (diameter) × 110 mm (height), whereas the yield stress was measured using a shear vane (Brookfield vane spindle V-73). The index characteristics of the tailings including the grain size distribution, liquid limit, plastic limit, specific gravity were determined as per ASTM standards. This paper will then discuss the interrelationships among the solid content, slump, saturated density and the yield stress of the CPB. It is shown that there is strong correlation between the two different slump test devices used in this study. The smaller cylindrical device appears to have good potential for slurries like mine tailings or dredged mud that have high water content for slump test. There is also strong inter-relationship among solid content, slump, yield stress, and bulk density. Increasing the solid content increases the bulk density and yield stress, but reduces the slump. While there is hardly any difference between the two binder types used in this study in terms of flow parameters, namely the yield stress and slump, the binder dosage has an effect. At any specific solid content, higher binder dosages lead to a drop in the slump and increase in the yield stress. The difference is more pronounced in dense slurries. It is also strongly believed that the trends and relationships developed in this study may be valuable for the other mining operations using CPB.

Rheology of microgels in single particle confinement

In this work, we investigate the shear rheology of Carbopol 981 microgel particle suspensions, confined between shearing plates with gap separations from 5 to 100 μm. We show that even for confining gaps smaller than that of the gel particle size, the yielding of concentrated microgel suspensions is delayed to stress levels above the bulk yield stress. Furthermore, for stresses below this new yield point, slip is described by elastohydrodynamic lubrication theory as long as the direct confinement of the single gel particles between the shearing surfaces is limited to a Hertzian deformation. For a strong, non-Hertzian particle deformation, the slip layer breaks down and leads to a frictional interaction of the single confined particle with the two shearing surfaces, depending on their surface roughness. Lubrication pressures and friction coefficients have been quantified with in situ normal force measurements on the confined particles, which have also been utilized to unambiguously determine the relevant swollen particle dimensions.

Both protein adsorption and aggregation contribute to shear yielding and viscosity increase in protein solutions

A combination of sensitive rotational rheometry and surface rheometry with a double-wall ring were used to identify the origins of the viscosity increase at low shear rates in protein solutions. The rheology of two high molecular weight proteins is discussed: Bovine Serum Albumin (BSA) in a Phosphate Buffered Saline solution and an IgG1 monoclonal antibody (mAb) in a formulation buffer containing small quantities of a non-ionic surfactant. For surfactant-free BSA solutions, the interfacial viscosity dominates the low shear viscosity measured in rotational rheometers, while the surfactant-laden mAb solution has an interfacial viscosity that is small compared to that from aggregation in the bulk. A viscoelastic film forms at the air/water interface in the absence of surfactant, contributing to an apparent yield stress (thus a low shear viscosity increase) in conventional bulk rheology measurements. Addition of surfactant eliminates the interfacial yield stress. Evidence of a bulk yield stress arising from protein aggregation is presented, and correlated with results from standard characterization techniques used in the bio-pharmaceutical industry. The protein film at the air/water interface and bulk aggregates both lead to an apparent viscosity increase and their contributions are quantified using a dimensionless ratio of the interfacial and total yield stress. While steady shear viscosities at shear rates below ∼1 s(-1) contain rich information about the stability of protein solutions, embodied in the measured yield stress, such low shear rate data are regrettably often not measured and reported in the literature.

Specimen- and grain-size dependence of compression deformation behavior in nanocrystalline copper

The compression deformation behavior of electrodeposited nanocrystalline copper pillars with average grain sizes (d) of 360, 100, and 34nm has been investigated as a function of specimen size (D). The yield stress for nanocrystalline pillars with d=360 and 100nm does not depend on specimen size, exhibiting essentially the bulk yield stress until the specimen size is reduced down to the critical values ((D/d)∗=35 and 85), below which the yield stress decreases with the decrease in specimen size. In contrast, the yield stress for nanocrystalline pillars with d=34nm does not depend much on specimen size, exhibiting the bulk yield stress value for all specimen sizes investigated. The dominant deformation mechanism changes from dislocation glide for pillars with d=360 and 100nm to grain boundary diffusional creep for pillars with d=34nm. Grain-size induced softening occurs for pillars with d=34nm being consistent with the occurrence of change in deformation mechanisms, whereas the bulk yield stress for pillars with d=360 and 100nm increases with the decrease in grain size according to the classical Hall–Petch relationship. The critical (D/d)∗ values determined for nanocrystalline Cu pillars with d=360 and 100nm increases with the decrease in grain size so as to conform to the same power law scaling obtained for coarse-grained Cu polycrystals. This is the first indication that the specimen size-induced softening extends from micrometer to nanometer scales as far as the dominant deformation mechanism is dislocation glide. The considerably large critical (D/d)∗ values determined for nanocrystalline Cu pillars with d=360 and 100nm are discussed in terms of strain continuity among neighboring grains and the generation of geometrically necessary dislocations to maintain strain continuity at the grain boundaries.

Influence of visco-elastic binder properties on ram extrusion of a hardmetal paste

The influence of the viscous and elastic components of a visco-elastic binder phase on the extrusion behaviour of a tungsten carbide–cobalt hardmetal paste was investigated by studying the paste over a range of temperatures, 30–42 °C, where the binder changed from a semi-solid gel to a viscous liquid. The extrusion behaviour of the paste was studied by ram extrusion and quantified by the Benbow–Bridgwater (BB) method. The elastic and viscous components of the binder were studied separately by means of oscillatory and steady shear techniques in a controlled stress rheometer using rough parallel plates. The paste behaviour fitted the four parameter BB model well: the initial bulk yield stress parameter, σ0, increased linearly with the binder elastic modulus, G′, while both the velocity dependence parameters α and β were linearly related to the binder steady shear viscosity. Analysis of paste-wall slip parameters gave estimated slip layer thicknesses of 2–5 particle diameters.

Stick-slip control of the Carbopol microgels on polymethyl methacrylate transparent smooth walls

Microgel suspensions such as Carbopol are commonly used as viscoplastic fluid models in fluid mechanics experiments. Polymethyl methacrylate (PMMA) walls are also frequently used in these experiments since they enable flow visualizations via their transparency properties. When sheared with smooth surfaces, the microgels slip on the surfaces and an apparent motion of the fluid occurs below the bulk yield stress τy. This motion stops at a sliding yield stress τs. The occurrence of this apparent slip below τy can significantly modify the flow features. This article presents a PMMA treatment to inhibit the slip of the Carbopol microgels. The method is simple to perform, independent of the treatment conditions and stable over time.

Discussion of the dependence of the effect of size on the yield stress in hard materials studied by microcompression of MgO

Microcompression has attracted considerable interest in the study of size effects, mainly in soft metals. Little data is available in the literature on experiments on materials with a higher bulk flow stress, although it has been shown that the technique can be successfully employed to suppress cracking due to the small specimen dimensions. Here, microcompressions on MgO were carried out to demonstrate the possibility of individually activating different slip systems. The yield stresses obtained in conjunction with transmission electron microscopy show that both the hard and soft slip system in MgO can be characterised individually. Microcompression is, therefore, a potential alternative to macroscopic testing of brittle materials under confining pressure or at high temperatures. To determine the influence of size on such measurements, results on the two slip systems in MgO and from the literature are compared. It is found that the bulk yield stress of a material might be used to estimate the effect of size on its yield stress at the microscale.

Foam slip on surfaces of intermediate or low wettability

Wall slip of foams is important in several practical contexts. If the surface is not perfectly wetted by the interstitial liquid, the foam–wall interface may exhibit a yield stress. This paper represents the first quantitative study of this phenomenon. A simple model of the interface between disorganized foam and a solid surface is presented. This model treats the interface as a network of soap film contact lines. Experimental data are presented on the tension at soap film contact lines, and the stress at the foam–solid interface, at very slow translation speeds and for several surfaces. These agree broadly with theoretical predictions up to parity of the interface yield stress with the bulk yield stress of the foam. The experimental data include the yield tension for a soap film contact line pulled in an oblique direction with respect to the contact line normal. These data are also broadly consistent with theoretical predictions.