Mercury Scientific
Volution

Power flowability can be measured using the Volution Powder Flow Tester.

Powder flowability is defined as the ease with which a powder will flow for a specified set of conditions. Powder is generally defined as a collection of individual solid particles surrounded by gas phases. This includes granular materials, bulk solids, pelletised materials, etc. An accepted method for quantifying powder flowability is the Mohr-Coulomb Model. The Mohr-Coulomb Model is a limit state or “Go/ No Go” model and can be used to accurately predict flow behavior. This model quantifies powder flowability with two measurable parameters, Cohesion and Angle of Internal Friction, and two derived parameters, Unconfined Yield Strength and Major Consolidation Stress.

Cohesion is a measure of particle to particle bonding strength. This bonding strength results from various inter-particle forces generated by electrical charges, van der Waals forces, moisture, etc. The Angle of Internal Friction is a measure of the force required to cause particles to move or slide on each other. Internal friction is influenced by many parameters including particle surface friction, particle shape, hardness, particle size, etc. distribution, etc. Cohesion and Angle of Internal friction are determined by measuring a powder’s yield locus. The Yield Locus is a graph of the shear force require to cause a powder to yield or fail relative to compressive load. Cohesion is the intercept of the yield locus and the angle of internal friction is the slope.

Yield Locus
Shear Stress versus Normal Stress

The Unconfined Yield Strength is the shear stress needed to fail or fracture a consolidated powder mass to initialize flow. The force used to consolidate the powder mass is called the Major Consolidation Stress. In other words, the unconfined yield strength is a measure of the strength of a powder mass when the powder experiences major consolidation stress. The Unconfined Yield Strength is calculated using the below formula:

A Flow Function Plot can be generated by plotting a powder’s Unconfined Yield Strength versus Major Consolidation Stress. The flow function plot is a quantitative measure of the flowability of the powder. The inverse of the slope of the flow function plot can be used as a flow index. Generally, the closer a powder’s flow function is to the x-axis, the more easily the powder will flow. The Volution is used to measure a powder’s cohesion and angle of internal friction at various loads to generate its flow function and thus quantify its flow behavior.

The yield locus analysis is designed to determine the angle of internal friction and cohesion for a sample material and then calculate its overall strength under compressive load.This is achieved by measuring the failure strength of a sample under various loads after consolidation under a preset pre-shear load. Plotting the failure strength of the material under different loads generates a yield locus for the sample under the pre-shear load.

The test consists of three parts for every point on the yield locus: consolidation, steady state and failure analysis. Depending on the type of cell used, failure points can be generated on the same sample or fresh sample can be used for each failure point. Generally 3 to 5 points are used to generate the yield locus due to the time required for each point as well as the wear on the sample. If time consolidation is used, a delay time occurs after the steady state step.

In the consolidation step, the sample in the measurement cell is compressed to the preset normal load.With linear cells, this step includes twisting of the lid to help pack the material in the cell to what is called its “critical consolidation”.Critical consolidation is defined as the sample density at which it will reach a steady shear with minimal shear travel.This state in indicated by constant sample density or by a leveling off of the drop in normal load after each twist of the cell lid.For rotational cells, the consolidation step simply consists of compressing the sample until the normal load is reached.

Sample Consolidation
Normal Load versus Time

In the steady state step, shear stress is applied to the sample until the measured shear force and sample volume become stable. With linear cells, the shear stress is applied by moving pushing the lower ring of the cell at a fixed rate relative to the upper ring.For rotational cells, the lid is rotated a fixed rate. The steady state point is the point at which the shear force becomes stable.At the steady state point, the sample has reached a repeatable, stable density relative to the applied compressive load.

Steady State
Shear Force versus Time

In the analysis step, the shear stress is reduced to zero by reversing the shear stress mechanism.The normal load is then reduced to a predetermined level called the shear load and the shear stress is again applied.The shear forces rises as the sample resists shearing until a maximum shear force is reached.At this point the sample fails and the shear force drops rapidly.The generate yield point consists of the maximum shear force and the shear load.

Static Failure Analysis
Shear Force versus Time

By repeating the above sequence 3 to 5 times, a series of yield points are generated from which a yield locus can be plotted.The yield points are selected so that they are in the linear portion of the yield locus.

Static Failure Points
Shear Force versus Time

A least squares regression is performed to calculate a linear function for the yield locus.The slope of the calculated line is the angle of internal friction.The intercept of the line is the cohesion.From the cohesion, angle of internal friction and steady state point, the unconfined yield strength and major consolidation stress are calculated using Mohr Coulomb equations.

Static Yield Locus
Shear Force versus Normal Load

Because the yield points are generated by measuring several steady states for the same sample, the steady state point used for the strength calculation is the average of all the steady state points.In addition, to account for the effect of the steady state on the measured shear force during failure analysis, the measured shear force can be adjusted based on whether its steady state was higher or lower than the average.This is called prorating and can correct for variations in sample density for each yield point measurement.

Compressibility is calculated using the sample’s initial density and density after the consolidation step.

Static yield analysis generates the strength of a static or not-moving sample.This would be the condition in a silo or chute when the sample is at rest.Therefore, to get the sample to flow, the force used to move the sample must be greater than the static yield strength.

The wall friction analysis is designed to determine the kinematic angle of surface friction for a sample material against a container material. This is achieved by measuring the friction force between the container material and the sample material under different loads to generate a wall yield locus. The analysis consists of three parts: consolidation, steady state and analysis. All parts are automatic.

In the consolidation step, the sample in the measurement cell is compressed to the preset starting load. With linear cells, this step includes twisting of the lid to help pack the material in the cell to what is called its “critical consolidation”. Critical consolidation is defined as the sample density at which it will reach a steady friction with minimal shear travel. This state in indicated by constant sample density or by a leveling off of the drop in normal load after each twist of the cell lid. For rotational cells, the consolidation step simply consists of compressing the sample until the normal load is reached.

Sample Consolidation
Normal Load versus Time

In the steady state step, shear stress is applied to the sample until the measured friction force and sample volume become stable. With linear cells, the shear stress is applied by moving pushing the container material at a fixed rate relative to the upper ring. For rotational cells, the lid is rotated a fixed rate. The steady state point is the point at which the shear force becomes stable. At the steady state point, the sample has reached a repeatable, stable density relative to the applied compressive load.

Steady State
Shear Force versus Time

In the analysis step, the friction force under the starting load is maintained until it is stable. The load on the sample is then reduced to a preset level and the friction force is again maintained until it is stable. This is repeated several times to produce a friction value for several applied loads.

Friction Points
Shear Force and Load vs Time

The shear versus load data is then plotted to create a wall yield locus. A least squares regression is performed to calculate a linear function for the yield locus. The slope of the calculated line is the kinematic angle of surface friction.

Friction Yield Locus
Shear Force vs Normal Load

If you need a shear tester, the Volution Powder Flow Tester (VFT) is the one to get. The VFT offers the following advantages over other other powder shear testers on the market:

Low Cost: The VFT is very affordable compared to other shear testers. The reason is that we designed the instrument ourselves. We do not pay university licensing fees or royalties because we designed it using our 20 years of experience in the powder flow business. We also did not use external engineering companies which further reduces our costs. These savings are passed on to users.

Large Pressure Range – Due to our heavy duty frame and drive system, the VFT can deliver up to 50 kg of vertical force. That’s about 6 times more than competing instruments.

Automatic Sample Weighing: The VFT weighs the sample automatically during the measurement eliminating the need for an external balance and the time required to weight the sample.

Normal Load Correction Due To Sample Density: The VFT automatically adjusts the normal force applied to the sample lid to correct for the force from the powder mass above the shear zone. This is very important for dense powders. Other systems do not make this adjustment resulting in shear force that are artificially high.

True Time Testing: The analysis cells of the VFT can be removed and kept under load off of the instrument. This means time tests can be performed while other samples are being run on the instrument. Other shear testers have no capability to run time tests or you must leave the sample on the instrument for hours and hours so no other testing can be done.

Can Test Powders and Granular Materials : Due to the geometry of the test cell, the Volution can test both powders and granular materials. Other shear testers cannot. The reason is that the dimensions of the test cells for other instruments are too small to allow large particles to be measured. It is generally recommended that a layer of a minimum of 20 particles separate shear planes from cell edges. Some cells are not deep enough. Other cells have vanes will not allow large particles to enter or will only a thin layer.