New Unconfined Yield Stress Method

There is now a new method for measuring Unconfined Yield Strength of Powder, using your existing Texture Analyser.

In industries that handle powders on a regular basis, it is very important to understand how a powder or granular material responds to pressure. In storage, the weight of powder in a container exerts pressure on the particles at the bottom. If the powder has good flow behaviour, it will not consolidate and will flow out of the silo or hopper without sticking – this is very desirable.

Unconfined Yield Stress is a simple technique that can be used to analyse the flow behaviour of many different types of powders and the change in this behaviour with different consolidation stresses and times. The measurement is made by filling a chosen weight of powder sample into a tube and using a compression piston which applies a chosen consolidation force for a specified time. After consolidation, the tube is slid upwards above the consolidation probe and held up via a support disc, before the probe moves back down to compress the freestanding column of powder which “yields”.

Formulae are built into Exponent Connect software which are able to collect the required parameters and calculate the Unconfined Yield Stress which then allows a “flow factor” to be plotted and used to compare between samples. The larger the flow factor, the more easily the powder will flow after any given consolidation pressure.

This assessment of powder characteristics is yet another measurement that the Texture Analyser can perform on powdered materials, alongside the functionality offered by the Powder Flow Analyser.

The probe continues to push on the powder until it reaches a force of 5000g. The more compressible the powder, the further the probe will travel during this time. Here, “compressibility” is defined as the ratio between the final and initial bulk densities:

This is equivalent to the Carr Index often used for powder analysis. In general, a more free flowing powder is less compressible (as the powder particles have already flowed into a more close-packed state). This corresponds to a small increase in bulk density with stress and a low compressibility.

A more cohesive powder tends to show the opposite behaviour and a high compressibility. Holes are present in the compression probe to allow for air pockets to be released during compression from the underlying powder column. Along with the measurement of Compressibility and Bulk Density additional parameters can also be measured within this test after it has been split into three sections: Stiffness, Relaxation and Elastic Modulus. Current users of the PFA projects will notice that projects in Exponent Connect have now been included to measure compressibility and existing projects updated to include the measurement of bulk density when using the split vessel.

To optimise 'walk away time' from the PFA, you may wish to use a new project that ties all three standard PFA tests together (cohesion, Powder Flow Speed Dependence (PFSD) and caking) and runs them sequentially on the sample – this new project is called ‘Three PFA Tests in One’.

This is now included in the sample PFA projects; it is not advisable on a sample that is likely to have a high degree of segregation or attrition, but may otherwise be found useful on many samples if sample quantity and test time are limited. It is highly recommended that the help file for each test is studied before performing the sequential test, so there is a good understanding of what each test does.

The project is set up to perform tests in order with cohesion first, followed by PFSD then caking. This order has been chosen so the least destructive tests are performed before the most destructive (caking) so the tests are most representative of using a new sample each time.