What is yield stress? measurement principle

In the case of fluids, yield stress is defined as the minimum shear stress required to initiate flow. It is also often referred to as "Bingham stress", and materials exhibiting yield stress are often referred to as "Bingham plasticity" or viscoplasticity. While easy to define, this quantity is not so easy to measure. However, its importance in coating phenomena is widely accepted.

The most direct way to measure this stress is through shear creep experiments. This can be done in so-called stress-controlled rheometers. The minimum stress that can be exerted on a sample varies by instrument type, but through judicious use of geometry, stresses (shear forces) in the range of 1 to 5 dynes/cm2 can be applied. This is the range of yield stress exhibited by most low solids coatings. However, traffic detection is not simple. In the traditional sense, when flow occurs, the strain measured in the sample needs to be linear in time. This may require long measurements.

Estimates of yield stress can be obtained from constant strain rate measurements of stress and viscosity. When viscosity is plotted against stress, its magnitude appears to approach infinity at low stress. The asymptote on the stress axis gives an estimate of the yield stress.

Another method used is the stress relaxation measurement after applying a step strain. For materials exhibiting viscoplasticity, the stress decays to a nonzero value as an estimate of the yield stress.