When a fluid does not start to flow immediately after being acted on by an external force, but starts to flow when the force reaches a certain level (critical value), such a fluid is called a plastic fluid. Its flow form is called plastic flow.
Assume that the minimum shear stress required before fluid flow occurs is t0 and the total shear stress is t. Then the shear stress after t-t0 is the force that generates and maintains fluid flow.
The definition of plastic viscosity can be seen from the formula.
In plastic flow, the relationship between shear stress and shear rate is linear after plastic failure, and the line drawn in the figure is also a straight line, which intersects the yield value on the shear stress axis (that is, the required At the minimum shear stress—the minimum shear stress required to make the fluid start to flow is called the yield value), the slope of the line is related to the plastic viscosity (see Figure 19-3D).
In overcoming yield value. Fluids that flow only later are properties of plastic fluids.
The rheological curve of a typical plastic fluid can be considered to be composed of two parts. In order to illustrate the situation, we will here
The (D) line in the figure becomes a case of a more detailed figure. It can be seen that: (1) when the shear stress is large enough to overcome the yield value, a section of the line is a curve that does not pass through the origin (that is, the plug flow area); (2) when the shear stress continues to increase, after a certain degree , then its line segment is a straight line or close to a straight line (that is, the part above the plug flow area).
Rheological curve of a typical plastic fluid
In the figure: t0 (that is, f-fa) is the actual yield value and the minimum yield value. fb is the extension line of the straight line, which indicates the theoretical yield value of the plastic body, and fc is the perpendicular line to the starting point of the straight line, which is defined as the maximum yield value.
Fluids that exhibit plastic flow are also called Bingham fluids. Most inks and paints are typical examples of plastic flow.
