Non-Newtonian fluids (most drilling fluids fit this general classification) do not show a direct proportionality between shear stress and shear rate. The ratio of shear stress to shear rate (viscosity) varies with shear rate and the ratio is called "effective viscosity", but this shear rate must be identified for each effective viscosity value.
An example of a non-Newtonian shear stress/shear rate (slope of the dashed lines) shear rate relationship is shown in Figure 4. Note that the ratio of shear stress to shear rate differs at each shear rate. The viscosity of non-Newtonian fluids is known as effective viscosity and the shear rate must be specified. Figure 5 shows that the effective viscosity decreases as the shear rate increases. This effect is called "shear thinning".
Figure 4
Rheogram Showing Non-Newtonian Behavior
Figure 4
Rheogram Showing Non-Newtonian Behavior
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