Unconfined Yield Stress Research Articles

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.

Analysis of operational window in Freeman Technology uniaxial powder tester

The operational window of the recently developed Freeman Technology Uniaxial Powder Tester (UPT) in terms of specimen aspect ratio (H/D), consolidation stress, and consolidation and failure strain rates is investigated for a range of bulk solid materials. The results show that the measured uniaxial unconfined yield stress (uUYS) decreases as aspect ratio increases but this sensitivity of uUYS to H/D reduces significantly for aspect ratios of greater than 1.5. The sensitivity of uUYS to aspect ratio is attributed to the intersection of slip planes with the top and bottom platens. The critical aspect ratio (defined as a ratio beyond which uUYS becomes independent of aspect ratio) is found to be a material-specific property, but it is increased with increasing consolidation stress. A correlation between the internal angle of friction and the critical aspect ratio is found, but it does not follow previously reported work. This discrepancy could be attributed to the differences in the packing fraction of the samples in uniaxial compression and the shear cell tests which were used to measure the internal angle of friction. uUYS is found to be independent of consolidation strain rate for the range investigated here (0.005 to 0.05 s−1). A very high level of repeatability is observed for the measurement of compressibility response of bulk solids with no sensitivity to consolidation strain rate uniaxial compression; powder flow; flowability; unconfined yield stress; aspect ratio; strain rate; compressibility.

Toward the Direct Synthesis of HDPE Powders for Powder Bed Fusion Based Additive Manufacturing

AbstractPowder bed fusion (PBF) suitable polyethylene particles are targeted by means of a direct synthesis, using a bisimino pyridine iron catalyst (BIP FeCl2) supported on microsized silica particles. An active support is generated by treating spray dried silica microparticles with a methyl aluminoxane activated BIP FeCl2 catalyst in toluene. Semibatch ethylene polymerization is mapped with respect to pressure, temperature, and reaction time to find optimal reaction parameters. The optimized synthesis leads to round polyethylene particles with a median size of ≈50 µm which have a sinter window of 8–15 °C and an unconfined yield stress (ffc) of 1.5 in a ring shear tester. The Young modulus of injection molded samples is in the range of 150–250 Pa with an elongation at break of about 30%. The powder flowability is improved by coating with nanosilica powder to an ffc of 3.8. Additivitation with carbon black allows to laser sinter the powder to solid parts in a small scale adapted setup for selective laser sintering (PBF/laser beam (LB)/P). Severe caking is preventing the preparation of CAD model conform parts. Commercial HDPE powder for PBF is additivated the same way to give substantially solid built parts, but with similar caking.

Rheological and mechanical properties of fibre-reinforced cemented paste and foam backfill

ABSTRACT The properties of fibre-reinforced cemented paste backfill (CPB) and foam backfill (FB) were investigated through bench-scale vane viscometer, mini-slump and compression tests. Samples of CPB (no air entrained) and FB (10, 20, 30, and 40% air by volume) were made with four concentrations of sisal, basalt, or Nanofibrillated cellulose fibres (0, 0.2, 0.5, and 0.8%). The results show that air entrainment decreased the unconfined compressive strength (UCS) and yield stress of FB, while the addition of the three types of fibre helped to compensate the UCS and built stronger internal bonds and sound structures, which led to different failure characteristics.

Dynamic ball indentation for powder flow characterization

In industrial processing and manufacturing, characterizing the flowability of particulate solids is of particular importance both for reliable powder flow and for a consistent production rate. Shear testing is the most widely used method for powders subjected to moderate or high stresses, and under quasi-static conditions. However, this method is not suitable for measuring the powder flow properties occurring in dynamic systems, such as powder mixers and screw conveyors. In this study, the rheological behaviour of powders at high shear rates has been evaluated by the ball indentation method. The technique, which simply consists of dropping a ball onto a cylindrical bed of previously consolidated powder, directly measures the material hardness, which is related to the unconfined yield stress by the constraint factor. The impact of the ball on the bed is recorded with a high-speed camera to determine velocity and penetration depth. The hardness against the strain rate is considered for four different materials. Because of their difference in particle size, and by using a range of drop heights and a range of indenter densities, the intermediate regime of flow has been fully analyzed. Although hardness is constant in the quasi-static condition, it results to be strain rate dependent in the intermediate regime of flow. Finally, a predictive correlation that allows the operator to choose the best operating conditions for achieving the desired flow regime is proposed, and the unconfined yield strength of the materials is inferred.

Characterising powder flowability at high shear rates by the ball indentation method

Unreliable powder flow is a major problem during processing of powders. The shear cell is the most widely used method for powders subjected to moderate or high stresses, and under quasi-static conditions, with established methods for designing large bins and hoppers based on the measurement. However, this method is not suitable for measuring the flowability of dynamic systems, such as powder mixing.Here, the ball indentation method is investigated as a technique for evaluating powders in the intermediate and dynamic regime of flow. The method, which simply consists of dropping a ball onto a cylindrical bed of powder previously consolidated, directly measures hardness, which is related to the unconfined yield stress of the powder by the constrain factor (Hassanpour and Ghadiri, 2007). The impact of the ball on the bed is recorded with a high-speed camera to determine velocity and penetration depth. The shear rate is varied by using a range of indenter materials and sizes, and a range of drop heights. The hardness against the strain rate is considered for several materials.It was found that the indenter size does not influence the hardness results, which are consistent with the flowability evaluation achieved with the rheometer. Furthermore the hardness, which is independent of the strain rate in quasi-static conditions, becomes shear rate dependent in intermediate regime of flow. Further work is needed to evaluate hardness in the rapid granular flow regime.

An investigation into the flowability of fine powders used in pharmaceutical industries

This paper investigates into the flow properties of two popular pharmaceutical excipients (Calcium sulphate and Dicalcium phosphate) and their respective feeder hopper design requirements. There are numerous problems related to poor powder flowability that are faced by pharmaceutical industries, due to a lack of fundamental understanding of powder flow properties and their effects on hopper design parameters (Standard, 2008 [1]). Characterization tests confirmed that the Dicalcium phosphate powder displayed a smaller particle size, larger specific surface area and higher value of Hausner ratio compared to Calcium sulphate powder indicating that Dicalcium phosphate powders are markedly more cohesive. This was validated by the instantaneous and time consolidation flow function curves which showed that the Dicalcium phosphate was the more cohesive. Powder flow function and wall friction tests were carried out on a ring type shear tester working on the basis of Jenike's principle. Based on the instantaneous test results, Dicalcium phosphate was found to require a larger hopper outlet dimension for the same hopper half angle compared to Calcium sulphate. The time consolidation test results have shown that both Dicalcium phosphate and Calcium sulphate gained strength during static storage and have become more cohesive. A simple empirical model for powder cohesion and unconfined yield stress has been developed and validated with a number of experimental values derived from common pharmaceutical powders.

Modeling flow properties of fine dry powders using particle morphological properties and its effects on geometry of fly ash evacuation hoppers

ABSTRACTThis paper results from an ongoing effort to correlate the physical properties of powders at a fundamental level with their bulk behavior. Cohesion and unconfined yield stress are the measure of inter-particle forces of attraction in the bulk powder. The existing model for cohesion does not include important bulk properties, such as particle size distribution, tapped density, and prevailing applied stress. In the present paper, flow properties of 25 bulk solids (different cement and fly ash samples) have been evaluated using a ring shear tester and the products have been characterized according to their flowabilities. Models for cohesion and unconfined yield strength have been developed in this study by taking into account the effects of particle size distribution, tapped bulk density and pre-shear stress. The newly developed models have provided good fit to the experimental data. The effect of these flow properties on the design of hoppers have been investigated for fly ash samples collected from seven consecutive electrostatic precipitator (ESP) hoppers of a coal fired thermal power station. The results show that fly ash from the rear end ESP hopper would require higher amount of opening size compared to the first or second field of ESP to ensure proper mass flow condition is achieved at all the ESP hoppers.

Ball indentation on powder beds for assessing powder flowability: Analysis of operation window

The characterisation of bulk behaviour of cohesive powders is very important in processing of particulate solids, e.g. for reliable powder flow out of storage vessels. For filling and dosing of small quantities of powders in capsules and for dispersion in dry powder inhalers, the interest is on the behaviour of loosely-compacted powders in small quantities and under very low applied loads. Furthermore at the early stages of drug development, the quantity of the powder available is often very small and the traditional bulk testing methods are neither possible nor applicable. In this work we evaluate a method to infer powder flowability by ball indentation. This technique provides a measure of flow resistance which can be related to the unconfined yield stress. It can be applied at very low loads and requires only a small sample quantity, typically a few mm3. The operational window in the ball indentation method in terms of minimum sample size, penetration depth and indenter properties (such as size, shape, friction and Young's modulus) has been analysed and reported here.

Flowability of lignocellusic biomass powders: influence of torrefaction intensity

The poor flowability of powders produced from raw lignocellulosic biomass may be an economically issue for the production of second-generation biofuels. Torrefaction is a pre-treatment step of the gasification process that improves the physical characteristics of biomass by making it more coal-like. Particularly, the loss of resilience allows a reduction of the grinding energy consumption and is likely to improve the flow behaviour of woody powders. In this study, we investigated the effect of particle size and shape distribution on flow properties (unconfined yield stress and flowability factor) of powder from raw and torrefied biomass (Picea abies). Several intensities of torrefaction were tested, and its extent was quantified by the global mass loss, chosen as synthetic indicator of torrefaction intensity (its accounts for both the temperature level and the residence time). The intensity of torrefaction shifts the particle size distribution towards smaller sizes. An effect on the circularity and aspect ratio was also observed. A strong, positive correlation was obtained between the measured flowability of biomass powders at different consolidation stresses and the intensity of heat treatment. These results confirm the interest of torrefaction as a pre-treatment step and aim to provide new knowledge on rheological properties of biomass powders.

Axisymmetric passive lateral earth pressure of retaining walls

This paper attempted to examine the passive lateral earth pressure of axisymmetric retaining walls. The Stress Characteristics Method (SCM) was employed for analysis. The stress equilibrium equations were obtained through the finite difference method along the characteristic lines. Effects of backfill slope, wall angle, and radius of retaining wall on passive stress distribution on the retaining wall were analyzed. Furthermore, the wall was considered frictional and adhesive. Apart from the axisymmetric retaining wall, the analysis covered a wall under plane strain condition. The validity of the results was confirmed through developing and comparing a number of models through the finite element method versus the SCM. This study showed that the level of passive lateral pressure of retaining walls in axial symmetry was higher than that of walls under plane strain condition. Moreover, axial symmetry or plane strain type of wall did not affect the level of unconfined yield stress, which was evident in the finite element model.

The effects of physical and chemical preprocessing on the flowability of corn stover

Continuous and reliable feeding of biomass is essential for successful biofuel production. However, the challenges associated with biomass solids handling are commonly overlooked. In this study, we examine the effects of preprocessing (particle size reduction, moisture content, chemical additives, etc.) on the flow properties of corn stover. Compressibility, flow properties (interparticle friction, cohesion, unconfined yield stress, etc.), and wall friction were examined for five corn stover samples: ground, milled (dry and wet), acid impregnated, and deacetylated. The ground corn stover was found to be the least compressible and most flowable material. The water and acid impregnated stovers had similar compressibilities. Yet, the wet corn stover was less flowable than the acid impregnated sample, which displayed a flow index equivalent to the dry, milled corn stover. The deacetylated stover, on the other hand, was the most compressible and least flowable examined material. However, all of the tested stover samples had internal friction angles >30°, which could present additional feeding and handling challenges. All of the “wetted” materials (water, acid, and deacetylated) displayed reduced flowabilities (excluding the acid impregnated sample), and enhanced compressibilities and wall friction angles, indicating the potential for added handling issues; which was corroborated via theoretical hopper design calculations. All of the “wetted” corn stovers require larger theoretical hopper outlet diameters and steeper hopper walls than the examined “dry” stovers.

Comparison of three rotational shear cell testers: Powder flowability and bulk density

Developed to aid in the design of hoppers and silos, the shear cell is now frequently used to rank the flowability of powders relative to one another. While standards, such as ASTM D6773 and D6128, exist for shear cell tests, there are still differences between commercially available shear cell testers, such as cell geometry and size. In this work, we used two materials, a free-flowing alumina and a cohesive alumina, to compare measurements from three commercially available rotational shear cells. Results were collected and compared for cohesion, unconfined yield stress, major principal stress, pre-shear stress, flow function coefficient, bulk density, effective angle of internal friction, and the angle of internal friction. ANOVA methods were used to determine the statistical significance and relative size of each of these effects. This work has found that while, as expected, the material type has the largest effect on the shear cell results, the consolidation at which the material was tested and the tester type are also statistically significant effects. These results indicate that care should be taken when comparing the results between different shear cells.

Fine powder flow under humid environmental conditions from the perspective of surface energy

The influence of humidity on surface energetics and flow behavior of fine pharmaceutical powders was investigated. Amorphous and crystalline fine powders with hydrophilic (Corn starch and Avicel PH105) and hydrophobic (ibuprofen) nature were considered for this study. The surface energy was determined using surface energy analyzer and flow behavior was measured in terms of unconfined yield stress (UYS) using a shear tester. The study showed that unlike hydrophobic ibuprofen powder, surface energy and flow of hydrophilic excipient powders were affected by relative humidity (RH). The Lifshitz–van der Waals dispersive (γLW) component of surface energy barely changed with varying RH for all pharmaceutical powders. For hydrophilic excipients, the specific component of surface energy (γSP) was found to increase with increasing RH. Furthermore, for these excipients, flow deterioration at elevated RH was observed due to increased capillary bridge formation. Detailed analysis showed that γSP component of surface energy can be an effective indicator for flow behavior of fine powders under varying humid conditions. The present study also brought out the existence of different regimes of probable interparticle forces which dictate the bulk flow behavior of fine hydrophilic powder under humid conditions.

Correlation between Powder Flow Properties Measured by Shear Testing and Hausner Ratio

Shear testing provides rigorous estimates of flow properties relevant to the characterization, handling, and processing of powders, and is a necessary test procedure in the formal design of powder storage facilities. However, despite the automation of modern test equipment, it can be time consuming and expensive. In contrast, measurement of bulk density is straightforward and less laborious, and tapping devices are cheaper. Here we explore the relationship between Hausner ratio and cohesion and also examine correlation between Hausner ratio, σc/σy, and σpre for a suite of 13 milled and 2 spray-dried lactose powders, 3 sand samples and 3 samples of refractory dust; Hausner ratio is the ratio of tapped bulk density to loose bulk density, σc is major consolidation stress, σy is unconfined yield stress and σpre is preconsolidation stress. Cohesion and flow function were measured with an annular shear cell at values of σpre up to 5kPa. Loose poured bulk density was measured following a modified New Zealand standard and tapped density measurement was based on a method for dry dairy products and the European Pharmacopoeia; Hausner ratio at 1250 taps was used. Our results show that cohesion at σpre of 0.31kPa, 0.61kPa, 1.20kPa, 2.41kPa, and 4.85kPa correlates linearly with Hausner ratio; the slope and intercept of the correlation are functions of σpre. A plot of σc/σy against Hausner ratio shows an exponential decay trend and regression yields two fitting parameters that correlate well with σpre. These correlations are potentially useful for assessing flow characteristics when shear testing cannot be performed.

Roll wave appearance in bentonite suspensions flowing down inclined planes

As in Newtonian fluids, unconfined yield stress fluid slender flows are prone to become unstable and host travelling waves. We propose a non-local dimensionless number ψ, obtained from a physical order-of-magnitude analysis, to predict, for the first time, the transition that marks the onset of roll waves in unconfined discharges of bentonite, whose rheology was assumed as a Bingham plastic. Involved variables include the discharge volume flow, Bingham viscosity, mixture density, gravity acceleration, plane inclination and yield stress. Experimental verification considering various discharge flow-plane inclinations and concentrations of the mixture indicate that for values of ψ exceeding the unity, single or multiple roll waves appear.

Interactive software for calculating the principal stresses of compacted cohesive powders with the Warren-Spring equation

The Unconfined Yield Stress ( σ c) and Major Consolidation Stress ( σ 1) of a cohesive powder′s compact are found by constructing two Mohr semicircles that are tangential to the Yield Loci Curve (YLC); the first passing through the origin (0,0) and the second at the consolidation conditions ( σ 0, τ 0). When the YLC can be described by the Warren-Spring equation ( τ/ C) n = ( σ + Τ)/ Τ or an alternative algebraic expression, this translates into finding the solution of two pairs of simultaneous equations that set the conditions for the tangential YLC and corresponding Mohr semicircles to have the same value and slope at their respective contact points. Once the Mohr semicircle′s equation that corresponds to the consolidation conditions has been found, the Effective Angle of Internal Friction ( δ) is calculated in a similar manner. The numerical calculation procedure has been automated in a freely downloadable program posted on the web as a Wolfram Project Demonstration. It allows the user to choose and adjust the values of C, T, n and σ 0, and the plot′s scales, by moving sliders on the computer screen. The program calculates and displays the corresponding values of σ c, σ 1 and δ, and plots the YLC, two Mohr semicircles and the line that defines δ. Since a linear YLC is just a special case of the model where n = 1, the program can be used with input parameters originally obtained by linear regression. But although the program can render reasonable estimates of the principal stresses σ c , σ 1 and δ in this case too, the physical meaning of C, and especially T, is unclear when calculated by extrapolation instead of being determined experimentally.

Characterisation of flowability of cohesive powders by testing small quantities of weak compacts

Characterisation of flowability of small quantities of cohesive powders is of great industrial interest, particularly for the pharmaceutical sectors, where a limited amount of material is available or the material is not easily accessible. In this paper, ball indentation on a powder bed is evaluated for flow characterisation of small quantities of cohesive powders at low levels of consolidation pressures. In parallel, the bulk powder is subjected to the common test method of unconfined direct compression. Analogous to indentation hardness testing, for which the indentation pressure is related to the flow stress, a constraint factor ( C) is defined relating the unconfined yield stress and indentation pressure. The constraint factors for the test powders have been evaluated, enabling an easy method to characterize the flow behaviour of cohesive powders.

Characterisation of Flowability of Loosely Compacted Cohesive Powders by Indentation

AbstractIn this paper we report on our investigations of the flowability of bulk powders using indentation and unconfined direct compression testing on a small assembly of powders compacted at low pressure levels. Samples of α‐lactose, magnesium carbonate and glass beads (made cohesive by silanization) have been consolidated to different pressure levels (ranging from 3 to 20 kPa) and the indentation hardness and unconfined yield stress of the compacts were then characterised. The latter was obtained by unconfined direct compression. The experimental results show that there is a linear relationship between the yield stress and consolidation pressure for the cases of α‐lactose and magnesium carbonate, from which it is possible to deduce the powder flow behaviour. The results of yield stress obtained from indentation and unconfined compression measurements are similar for the cases of α‐lactose and magnesium carbonate, whilst the results are different for the case of cohesive glass beads. Furthermore simulations were carried out using the Distinct Element Method to predict the bulk behaviour of the cohesive powders based on single particle mechanical properties. The simulation results qualitatively agree with the experiment, i.e. there is a linear relationship between the unconfined yield stress and the consolidation pressure, from which the role of inter‐particle adhesion on the flow behaviour may be deduced.

Proposal of an approximation equation for the yield locus to evaluate powder properties

The yield locus (YL) of powder bed can be used to determine many mechanical properties of a powder such as cohesion, unconfined yield stress, stress ratio, etc. Generally, the YL of powder beds is obtained by fitting the results of shear tests to linear approximations based on the Coulomb equation or to curved approximations based on the Warren–Spring equation. Meanwhile, the yielding characteristics of a powder bed are expressed by the Roscoe condition diagram. In this diagram, the YL appears orthogonal to the normal stress axis at both ends corresponding to tensile and compressive strength. However, the YL approximated by the Coulomb or Warren–Spring equations is not orthogonal to the normal stress axis at both ends, and is not the same shape as the YL shown in Roscoe condition diagrams. Thus, the abovementioned mechanical properties obtained from the YL of a powder bed are likely to be affected by the approximate expression for the YL. Despite this, no one has investigated how the mechanical properties of powder beds such as stress ratios are affected by the approximation method for the YL. In this paper, we propose a new approximation equation for the YL that conforms both to the shape of the YL in the Roscoe condition diagrams and experimental results. Then, these YL obtained by our equation, and by the Coulomb and Warren–Spring equations are used to determine the mechanical and flow properties of powder beds. These values are compared with each other in order to discuss the validity of our equation.