Getting the stress right in a creep test

Creep tests are the best way of obtaining the zero-shear viscosity of your sample, which in turn can be used to support stability assessments, sag and slump predictions, molecular weight comparisons etc. However, all too often rheometer users give up with creep tests because they find it difficult to choose the right stress to apply to get meaningful results for a particular sample.

An easy way of identifying a suitable value is to first run a quick oscillation stress sweep. A typical set up would be:

• Method: Oscillatory Stress Sweep
• Frequency: 1Hz (6.28 rad/s)
• Start stress: 0.001 Pa
• End stress: 100 Pa
• Logarithmic data collection.

Plot: Elastic modulus vs stress, log-log scaling.

Below is a typical result for a viscoelastic liquid, clearly showing the linear viscoelastic behaviour plateau up to a yield point, whereupon the elastic modulus drops rapidly.

Now, if you identify a stress that is well below this yield point, and ideally just above the noisy data, you will have a stress that you can comfortably apply in a subsequent creep test, without fear of clobbering your sample.