Joe Flow - Yield Stress Determination
The yield stress of a sample is not a material constant but is very important in practical work. Many rheological methods have been developed to determine the yield stress.
What is the yield stress?
Substances with a yield stress only start to flow once the outside forces acting on them are larger than their internal structural forces. Below the yield point the substance shows elastic behavior (gel character). It behaves like a solid that reacts to load with a small deformation which completely regenerates when the load is removed. The material therefore behaves like a spring having a recoiling force which is proportional to the deflection. When shear forces are increased, the yield stress can be overcome so that irreversible deformations are present and the sample starts to flow.
The reason for the yield stress are intra-molecular interactions which cause a three-dimensional network of forces that give the substance a certain structural strength. This network of forces is destroyed when the yield point is overcome.
Why is the yield stress important?
The yield stress is an important parameter for many practical issues and applications. It is used to characterize the dispersion and gel stability or the pumping behavior during filling processes.