Yield Stress Measurements Research Articles

Strength-hardness relationship for AlSi10Mg alloy produced by laser powder bed fusion: An experimental study

In this work, significant strength and ductility variations are reported for AlSi10Mg parts fabricated at different orientations using laser powder bed fusion (LPBF). Hardness and surface roughness of the specimens at different orientations were measured. Tensile testing together with digital image correlation technique were conducted on the specimens. XY specimens showed the highest yield stress and ultimate tensile strength while XZ specimens showed the highest ductility. Hardness measurements for different specimens were in accordance with the tensile test results, following the same order as the UTS values, XY specimens being the highest and XY-45° (out-of-plane) specimens being the lowest. Fractography of the broken surfaces of the specimens under tensile testing revealed the microstructural features and various defects in the tensile fracture. The anisotropy in mechanical properties is attributed to the microstructural anisotropy as well as presence of various types of defects induced by the AM process, which affects the deformation and failure mechanism of the parts. Linear relationships between experimental Vickers hardness versus yield stress and UTS measurements were developed. In case of material selection for different applications, these relationships can be used as a simple tool for converting hardness and yield stress (or UTS) values to each other. An equivalent strain-hardness relationship was also proposed which can be used for health monitoring of parts subject to tensile loading.

Application of Superparamagnetic Nanoparticle (SPM-NP) Heating in Wax Removal during Crude Oil Pipeline Pigging

Mechanical pigging is a major wax removal technique in crude oil pipeline transportation. To lower the risk of pig stalling accidents in pigging, a superparamagnetic nanoparticle (SPM-NP) pigging system that consists of an electromagnetic pig and nanopaint is designed to soften the wax deposit and reduce the wax resistive force. During pigging, the induction coil on the pig generates an alternating magnetic field, which induces heat production of the nanopaint on the pipe wall. The COMSOL software is used to simulate the temperature distribution of the wax layer. Yield stress measurements reveal that the wax layer yield stress is nearly linearly correlated to temperature. Compared with a regular pig, the SPM-NP pigging system reduces the wax resistive force by up to 20~40%, with an acceptable electrical current intensity of about 20 A in the given cases. Multiple factors affecting the wax layer temperature were analyzed. SPM-NP heating could be a promising technique for wax remediation and hydrate prevention.

Effect of operational factors on reproducibility of yield stress measurement based on the vane method for cemented paste backfill

• Precise measure of yield stress is critical for CPB and vane method is widely used. • The reproducibility is subject to instrumental, analytical and operational influence. • Various vane positions, standing times and rotation speeds were tested. • A satisfactory reproducibility requires consistent preparation and operation. • A lower rotation speed facilitates a closer result to the true value. • Equilibrium time of 1 min and rotation speed of 0.1 rpm are suggested for CPB. Precise determination of yield stress with the desired reproducibility is critical for cemented paste backfill (CPB). An overview of the instrumental, analytical and operational factors was carried out to evaluate the key factors in CPB measurement using the vane method. Therewith the effects of vane inserted depth (10 to 40 mm), standing time (0 to 6 min), and rotational speed (0.05 to 5 rpm) were tested. The results show that an increased rotation speed or prolonged standing time leads to higher measured values and poorer reproducibility. The ESEM images indicate that larger yield stress values correspond to denser microstructures and the deterioration in structural homogeneity leads to pronounced deviation in results. Increasing the rotation speed enhances the role of viscous resistance and leads to an overestimation of yield stress. It is recommended that a consistent operation should be followed with the vane centered in the sample, resting for less than 1 min, and rotating below 0.1 rpm.

STATIC YIELD STRESS MEASUREMENTS OF FINE AGGREGATES

Fine grained concrete with a maximum grain size of D = 2 mm can be designed as high-performance concrete with outstanding properties. Typical applications are 3Dprinting, grouts, textile-reinforced concrete and thin high-performance structures. Studies reported in the literature that with decreasing grain size the impact of the granulometric properties of the aggregates is increasing. These properties are not only influencing the water demand but also the rheological behavior, such as the static yield stress. In order to investigate and quantify the effects of granulometry on static yield stress measurements six different sands of threedeposits within the range of d/D = 0/2 mm and industrial made glass spheres were tested. The granulometric properties of the materials have been investigated using digital image analyzing. The measurement device was a rotational viscosimeter (Viskomat NT) equipped with a Vane-paddle. Factors as the sample preparation (compaction) and applied shear rateunder a constant shear rate protocol (CSR) were analyzed. An optimum measurement procedure for the grain mixtures was found in using 0.2 rpm and a layerwise static compaction. The analysis showed the importance of the grain size distribution of comparable grain mixtures regarding the static yield torque.

Surface-tension-driven evolution of a viscoplastic liquid coating the interior of a cylindrical tube

One mechanism for airway closure in the lung is the surface-tension-driven instability of the mucus layer which lines the airway wall. We study the instability of an axisymmetric layer of viscoplastic Bingham liquid coating the interior of a rigid tube, which is a simple model for an airway that takes into account the yield stress of mucus. An evolution equation for the thickness of the liquid layer is derived using long-wave theory, from which we also derive a simpler thin-film evolution equation. In the thin-film case we show that two branches of marginally yielded static solutions of the evolution equation can be used to both predict the size of the initial perturbation required to trigger instability and quantify how increasing the capillary Bingham number (a parameter measuring yield stress relative to surface tension) reduces the final deformation of the layer. Using numerical solutions of the long-wave evolution equation, we quantify how the critical layer thickness required to form a liquid plug in the tube increases as the capillary Bingham number is increased. We discuss the significance of these findings for modelling airway closure in obstructive conditions such as cystic fibrosis, where the mucus layer is often thicker and has a higher yield stress.

Dual-platform micromechanical characterization of soils: Oscillation shear rheometry and spherical indentation

The dynamics of soil structure is caused by biotic and abiotic processes, with the onset and magnitude of deformation controlled by soil rheological and mechanical properties. Quantification of such properties is challenging because soil behaviour changes with soil moisture, but common rheological tests are not applicable over all consistency ranges. Here, we combine oscillation shear rheometry with spherical indentation mechanical measurements of soil to obtain greater characterization over a broader range of water contents. The elastic modulus could be measured with either approach, with good agreement found for measured silt and clay textured remoulded agricultural soils. For shear rheometry, plastic viscosity, complex modulus and shear yield stress were also obtained. The spherical indentation provided measurements of hardness and yield stress. Although yield stress was correlated between approaches, the values were orders of magnitude greater for the indenter (0.54 ± 0.33 kPa vs. 34.4 ± 31.2 kPa), presumably because of different loading and failure conditions. At drier water contents, yield stress varied more between the two tests on the clay soil, which corresponded with brittle fracture creating artefacts in shear rheometry measurements. Spherical indentation has not been widely applied to the testing of soils, but the good agreement over a wide water content range between elastic modulus obtained from spherical indentation measurements (0.66 ± 0.27 MPa in wetter zone to 4.45 ± 2.53 MPa in drier zone) and shear rheometry (0.47 ± 0.11 MPa in wetter zone to 2.02 ± 0.98 MPa in drier zone) is promising. Moreover, spherical indentation can be applied to materials varying from brittle to viscous and allows testing on structurally intact soil aggregates. The geometry of a spherical indenter may more closely mimic contacting soil aggregates, so scope exists to extend the approach to explore the slumping of aggregated seedbeds produced by tillage.

Generalized models of slump and spread in combination for higher precision in yield stress determination

The cone slump test is a simple alternative for on-site measure of yield stress, and on this principle, cylinder mold is employed owing to its easy accessibility. As an extension of the previous investigation which has validated the cone's cylinder-like behavior in the slump geometry, this work adopted an equivalent cylinder to facilitate comparison in the spread geometry, together with cylinders of various dimensions to explore dimensionless spread to yield stress relation. Good consistency was observed in both slump profiles and spread contours between the mini cone and its equivalent cylinder. The spread diameters after normalization displayed a definite common correlation with dimensionless yield stress. After integrating the corrective coefficient β, the models of slump and spread were generalized to both cylinder and cone molds with desired accuracy, alleviating the overall deviation of 14.88 % by individual slump height to 10.25 % combining with spread and enhancing the robustness of predicted values.

STATIC YIELD STRESS MEASUREMENTS OF FINE AGGREGATES

Fine grained concrete with a maximum grain size of D = 2 mm can be designed as high-performance concrete with outstanding properties. Typical applications are 3Dprinting, grouts, textile-reinforced concrete and thin high-performance structures. Studies reported in the literature that with decreasing grain size the impact of the granulometric properties of the aggregates is increasing. These properties are not only influencing the water demand but also the rheological behavior, such as the static yield stress. In order to investigate and quantify the effects of granulometry on static yield stress measurements six different sands of threedeposits within the range of d/D = 0/2 mm and industrial made glass spheres were tested. The granulometric properties of the materials have been investigated using digital image analyzing. The measurement device was a rotational viscosimeter (Viskomat NT) equipped with a Vane-paddle. Factors as the sample preparation (compaction) and applied shear rateunder a constant shear rate protocol (CSR) were analyzed. An optimum measurement procedure for the grain mixtures was found in using 0.2 rpm and a layerwise static compaction. The analysis showed the importance of the grain size distribution of comparable grain mixtures regarding the static yield torque.

Rheology of mature fine tailings

Rheological characteristics of mature fine tailings (MFTs) were examined in both the linear and non-linear viscoelastic regimes. MFT samples exhibited thixotropic behavior as well as apparent slip, which was suppressed by using sandpaper of grit 80 (200 μm) at the wall of the parallel-plate geometry. The real yield stress of MFTs with shear-thinning behavior was retrieved after fitting to a Herschel–Bulkley equation. Creep and oscillatory shear tests are also used to verify the yield stress measurement. The yield stress of two MFT suspensions possessing similar volume fractions was found to be similar; however, a kaolinite suspension (formulated to mimic/match the MFT rheology) showed more shear thinning and less thixotropy. The presence of bitumen remaining in the MFTs (up to 2 wt. %) suppressed the apparent slip to a large extent due to the immobilization of the particles at the interface. Finally, the effect of temperature on MFT rheology was studied in detail over the range of 0–50 °C, showing a minimum viscosity and yield stress at about 20 °C (demonstrating a non-monotonic increase with the increase in temperature).

Rheological properties of concentrated allophane, halloysite, and kaolinite suspensions

This study investigated the rheological properties of concentrated suspensions of allophane, halloysite, and kaolinite, which are representative of a series of weathering products of volcanic glass and feldspar minerals in volcanic rocks and deposits, to elucidate the mechanism of landslide generation involving these materials. All suspensions generally showed shear thinning during flow curve measurements, as represented by a steady decrease of viscosity with increasing shear strain rate from 0.1 to 1000 s−1. A difference among them was that the kaolinite suspension showed unstable flow behavior at high solid concentration, and the halloysite suspension had a similar behavior at low solid concentration. The shear stress at the beginning of flow—which represents the yield stress of the suspension—showed a power law correlation with the solid volumetric fraction, and that for kaolinite agreed well with previous static yield stress measurements. The stress of the halloysite suspension deviated from the power law curve at solid fraction < ~0.15. Dynamic viscoelasticity tests indicated that all suspensions showed a power law correlation between the modulus value and solid fraction. The crossover strain (when a solidlike-to-liquidlike transition occurs) of the kaolinite suspension was almost constant within the investigated solid fraction range, whereas that of the halloysite suspension decreased noticeably at solid fraction < ~0.15. Overall, the allophane suspension had the weakest strength and lowest resistance to oscillatory strain at a given solid fraction condition. The rheological properties of halloysite observed here may be related to the occurrence of flow-like, long-distance landslides in volcanic areas containing weathered tephra.

Evaluation of a Uniaxial Powder Tester and Comparison with an Annular Shear Tester

Bulk flowability measurement of particulate systems for quality control and product development purposes requires a fast, repeatable, accurate and affordable characterization approach. In the present study, two versions of the recently developed Freeman Technology Uniaxial Powder Tester (UPT) are compared to the Schulze Ring Shear Tester and the Johanson Hang-Up Indicizer. The UPT instruments were found to have excellent repeatability in measurements of uniaxial unconfined yield stress and compressed bulk density across a broad range of materials. Amongst the testing devices investigated in this study, the computer-controlled Advanced UPT (AUPT) instrument showed higher repeatability than the Manual UPT (MUPT) and the Johanson Indicizer. The Schulze Ring Shear Tester (RST) had slightly better repeatability than the AUPT, but there were a few situations in which its automated software did not produce a value of unconfined yield stress, resulting in a need to manually process the data. For most of the test cases, the AUPT measured a higher value of uniaxial unconfined yield stress than the MUPT. This was attributed to the stress relaxation that the powder bed experiences in the MUPT, where the consolidation stress is not automatically maintained on the sample. The AUPT measured lower values of unconfined yield stress compared to the Schulze RST for all the cases in this study. This is a well-recognized occurrence as the Schulze RST and other biaxial testers, in contrast to the AUPT, brings the sample to the critical consolidation state before measuring the shear strength. The AUPT produced its flow function data in about half the time as the Schulze RST.

Slow penetration for characterizing concrete for digital fabrication

Digital fabrication processes with concrete require a specific yield stress evolution after placing. This paper shows that this goal can be achieved with displacement controlled continuous slow penetration methods, and gives a model understanding and specific suggestions for measurement. It is found that the force recorded in continuous and point-wise penetration tests with a conical tool moving at a slow rate scales linearly and over multiple decades with independent measurements of yield stress carried out using uniaxial compression and vane test. A model adapted from soil-mechanical stability predicts the bearing capacity factor that relates the penetration force to yield stress. The experimental value agrees with the predicted one for an undrained/plastic material. The measurements indicate a yield stress validity range of 1–200 kPa and as low as 0.1 kPa with consideration of depth dependency. This range is the one of greatest interest for structural build-up in digital fabrication during production.

Yield stress measurement techniques: A review

This contribution is to add to the timely celebration of Professor R. B. Bird's outstanding career and accomplishments. Following introductory remarks on material/fluid types, the paper reviews the concept of yield stress. Although yield stress has been studied for several decades, it is still very much a topic of current interest. This paper covers phenomenological yield stress models as well as experimental techniques to measure yield stress. It also discusses, in particular, problems associated with very low yield stress measurements that occur with bio-fluids such as blood.

Rheology of enzyme liquefied corn stover slurries: The effect of solids concentration on yielding and flow behavior.

The measurement of yield stress and shear thinning flow behavior of slurries formed from unpretreated corn stover at solids loadings of 100-300 g/L provides a key metric for the ability to move, pump, and mix this lignocellulosic slurry, particularly since corn stover slurries represent a major potential feedstock for biorefineries. This study compared static yield stress values and flow hysteresis of corn stover slurries of 100, 150, 200, 250, and 300 g/L, after these slurries were formed by adding pellets to a cellulase enzyme solution (Celluclast 1.5L) in a fed-batch manner. A rotational rheometer was used to quantitate relative yield stress and its dependence on processing history at insoluble solids concentrations of 4%-21% (wt/vol). Key findings confirmed previous observations that yield stress increases with solids loadings and reaches ~3000 Pa at 25% (wt/vol) solids concentration compared to ~200 Pa after enzyme liquefaction. While optimization of slurry forming (i.e., liquefaction) conditions remains to be done, metrics for quantifying liquefaction extent are needed. The method for obtaining comparative metrics is demonstrated here and shows that the yield stress, shear thinning and shear thickening flow behaviors of enzyme liquefied corn stover slurries can be analyzed using a wide-gap rheometry setup with relative measuring geometries to mimic the conditions that may exist in a mixing vessel of a bioreactor while applying controlled and precise levels of strain.

(Invited) Process for Continuous Electro-Osmotic Dewatering of Phosphatic Clay Suspensions

A recent spill of wastewater at a former phosphate mining facility in Piney Point Florida, reported in March 2021, highlights the need for alternatives to creating large impoundments of the dilute 2-3 wt.% suspension of phosphatic clay produced as a waste stream in the beneficiation of phosphate ore. A 1997 report highlights releases in Florida.1 In 1994, the earthen berm of a clay settling area operated by Hopewell Mine failed along a section of approximately 100 feet. Approximately 482 million gallons of water were lost. A series of releases of large volumes of turbid water from clay settling areas into nearby surface waters occurred between 1989 and 1993 at the MMM Nichols Phosphate Mine. In 1994, the dike of an inactive settling area was breached over a section of approximately 150 linear feet, causing very rapid dewatering of the impoundment.A typical Florida phosphate mining operation produces more than 6,300 L/s of phosphatic clay. The clay-water suspension is pumped into large impoundments called clay settling areas in which separation is achieved by hindered settling and self-consolidation. As it settles, the supernatant water is recycled for use in the beneficiation plant. A top crust is formed after a few years, but the clay beneath the crust has a large water content and a pseudo-plastic character that limits the amount of weight the settling area can support. Clay settling areas cover roughly 390 square kilometers of land in Florida. Uses for the land are limited by the properties of the clay that leave the settling areas unstable, even after 50 years of elapsed time.This presentation will describe the results of a research project, funded by Mosaic, to create an electrokinetic alternative to creation of clay settling areas. Research in use of electrokinetic phenomena to accelerate the dewatering process has been active for more than 50 years.2 The use of an electric field to separate the water from the solids is attractive because the inherent stability of the clay suspension is due, in part, to the surface charges residing on the platelets. A single-stage design was developed for continuous electro-osmotic dewatering of phosphatic clay suspensions that demonstrated efficient production of a dewatered cake with a solids content of 35 wt.% at a dry-clay production rate of 4.5 kg/h m2 from a feed clay of 10 wt.%.3,4 The results were supported by measurements of yield stress and a mathematical model that accounted for compressibility of the cake and for both electro-osmotic and hydraulic permeability.5 References Damage Cases and Environmental Releases from Mines and Mineral Processing Sites, U.S. Environmental Protection Agency, Washington, DC, 1997.Dizon and M. E. Orazem, "Advances and Challenges of Electrokinetic Dewatering of Clays and Soils," Current Opinion in Electrochemistry, 22 (2020), 17-24.Kong, A. Dizon, S. Moghaddam, and M. E. Orazem, “Development of Fully-Continuous Electrokinetic Dewatering of Phosphatic Clay Suspensions,” in Electrochemical Engineering: From Discovery to Product, Volume XVIII of Advances in Electrochemical Science and Engineering, R. Alkire, P. N. Bartlett, and M. Koper, editors, John Wiley & Sons, Hoboken, 2018, 159-192.Dizon and M. E. Orazem, “Efficient Continuous Electrokinetic Dewatering of Phosphatic Clay Suspensions,” Electrochimica Acta, 298 (2018), 134-141.Dizon and M. E. Orazem, “Mathematical Model and Optimization of Continuous Electro-Osmotic Dewatering,” Electrochimica Acta, 304 (2019) 42-53.

Extrusion rheometry of collagen dough

Although collagen is widely used (for example, in the food industry, in the pharmaceutical industry and in biomedicine), the rheological properties of the material are not well known for high concentrations (8% collagen, 90% water). Rheological properties were measured using a capillary-slit rheometer (an extrusion process), where the tested sample of collagen matter was pushed by a hydraulically driven piston through a narrow rectangular slit at very high shear rates of 50–6 000 s&lt;sup&gt;–1&lt;/sup&gt;. The Herschel-Bulkley (HB) constitutive equation and a new correlation taking into account the finite gap width was used to evaluate the rheological properties (n = 0.2, K = 879 Pa s&lt;sup&gt;n&lt;/sup&gt;, τ&lt;sub&gt;0&lt;/sub&gt; = 2 380 Pa). Use was made of a new yield stress measurement method evaluating τ&lt;sub&gt;0&lt;/sub&gt; 'post mortem' after extrusion stops. The effects of wall slip and of air bubbles, which caused apparent compressibility of the 'silly putty' collagen material, were also studied. Corrections of the wall slip effect were implemented using sliding layer thickness δ.

Interaction of Mobile Dislocation with Additives in Ionic Crystals

The mechanical properties of materials are affected by the interaction of mobile dislocation with additive ions. Although this has been widely investigated by various methods (yield and proof stress measurements, microscopy observations of dislocation, internal friction measurements, stress relaxation tests and so on), it is difficult to obtain the information on mobile dislocation-additives interaction during plastic deformation of bulk.

Impact of Processing Conditions on Rheology, Tribology and Wet Lubrication Performance of a Novel Amino Lipid Hair Conditioner

The objective of this work was to carry out a comprehensive evaluation of the performance of a novel cationic amino lipid surfactant, Brassicyl Valinate Esylate (BVE), in contrast to conventional alkyl quaternary ammonium surfactants (quats), through a study of the effects of process mixing speed on its overall rheological, tribological and wet lubrication performance in comparison to BTAC and CTAC, two cationic surfactants widely used in cosmetics. The major cosmetic application of cationic surfactants is in the preparation of hair conditioners. Hence, this analysis was done firstly by conducting tensile combing tests to evaluate reduction in wet lubrication which translates to conditioning performance. The combing results serve as a testing metric that adequately corresponds to consumer perception of conditioned hair. To correlate this technically, yield stress measurements were conducted to establish rheologic profiles of the conditioner formulations, and in vitro tribological testing of the emulsion systems between two steel surfaces were done to technically simulate the spreading and rubbing of conditioner on the hair. The effect of processing conditions on the formulations was then evaluated. BVE was found to be an effective conditioning surfactant suitable as an eco-friendly replacement for BTAC and CTAC in hair conditioner formulations. The results showed that higher shear mixing rates during formulation lead to poorer performance effects evident through decreased yield stress values, lower percentage reduction in combing force and a higher coefficient of friction.

Understanding the effects of processing conditions on the formation of lamellar gel networks using a rheological approach

Lamellar gel networks are multiphase systems which form the basis of many cosmetic and pharmaceutical cream products, thanks to their superior stability compared to typical oil-in-water emulsions, and highly desirable rheological properties inferred by the interconnected structure. There has previously been considerable interest in the formulation of lamellar gel networks, but little interest has been given to the effects of processing conditions on the formation of the desired structure, or the possibility for process optimisation through understanding power consumption. The evolution of the microstructure of an incipient lamellar gel network during processing was investigated by varying the temperature, vane speed and time using a rheometer equipped with a four-bladed vane in cup geometry. Torque and vane speed measurements were recorded at 2 Hz for the duration of the experiment, from which apparent viscosity (taken at a reference shear rate of 200 s−1) and power input were calculated. Samples were then characterised by yield stress and flow curve measurements to determine the impact of processing conditions on the final product microstructure. Increasing vane speed increased the maximum apparent viscosity achieved and yield stress of the sample, and reduced the time taken to reach the peak apparent viscosity. However, the increased power requirements from the higher vane speed were not counteracted by shorter processing times. Increasing the temperature reduced the rate of apparent viscosity increase but did not affect the yield stress of the final lamellar gel network, offering a reduction in power consumption due to a lower apparent viscosity for the majority of the process.

Assessing powder flowability at low stresses using ball indentation method: Evaluation of constraint factor

Powders can exhibit different flow behaviour resulting from a combination of physical properties of the material and equipment design. Problems with powder flow are ubiquitous in process industry and become prominent when dealing with fine and cohesive powders. It is therefore of great importance to characterise the flowability of cohesive materials for better process control. Powder flowability is commonly assessed under relatively high preconsolidation loads using shear cell and uniaxial compression methods by which the unconfined yield strength (Y) is evaluated as a function of the applied load. However, these techniques are typically limited to applied stresses greater than 1 kPa and require a relatively large quantity of powder. To overcome these limitations, the recently developed Ball Indentation Method (BIM) is used in this work for assessing powder flow behaviour at low stress levels. The unconfined yield strength (Y) is inferred from the resistance to ball penetration into the surface of a powder bed, based on the method for measurement of hardness (H). This requires the flow resistance, represented by hardness, to be related to the unconfined yield strength by a proportionality factor termed the constraint factor, C, following the analogy with yield stress measurement in continuum solids, i.e. Y=H/C. The constraint factor for silanised glass ballotini, calcium carbonate, α-lactose monohydrate, Avicel and limestone is evaluated and reported here. It is shown that the unconfined yield strength inferred by this method correlates well with those from the uniaxial compression and shear cell measurement. The characterisation of the constraint factor makes it possible to use BIM for powder flowability testing at low stress levels and using a very small powder quantity. This is highly desirable for applications such as capsule filling, tableting and dry powder inhaler devices.