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When should you use it?
This model should be used with non-Newtonian materials that have a yield stress and then behave in a Newtonian
fashion once they begin to flow. As a result, the shear stress shear rate plot forms a straight line after yielding.
(Products that have a yield stress only begin to flow after a certain amount of shear stress is applied. They are also
called "viscoplastic". Their shear stress vs. shear rate graphs intersect the y-axis at a point greater than 0.)
An Example of the Bingham Model at Work
A manufacturer of drilling fluid applies the Bingham
Model to ensure the quality of their product. Results from
a recent batch, shown in figure 6-3, showed that the yield
stress and plastic viscosity were both below the pass/fail
criteria, which would cause the fluid to insufficiently hold-
up the cuttings. The shipment was canceled and the root
cause of the problem was identified.
6.4. The Casson model
√τ
√τ
√ ηD
=
+
(τ = shear stress, τ
0
What does it tell you?
The Casson model provides parameters similar to that of the Bingham model. However, unlike the Bingham model,
it was developed for materials that exhibit non-Newtonian flow after yielding. The Casson model indicates the
product's yield stress (τ
viscosity, h, which is the viscosity of the product after it yields.
When should you use it?
The Casson model should be used with non-Newtonian materials that have a yield stress and that do not exhibit a
"Newtonian-like" behavior once they begin to flow. This model is most suitable for fluids that exhibit Pseudoplastic or
shear thinning, flow behavior after yielding.
These fluids have a non-linear flow curve. The point at which it crosses the y-axis is the product's yield stress (τ
project the point at which the curve will intersect with the y-axis, the Casson model linearizes or straightens the plot
by taking the square root of the data. To ensure accurate extrapolation to yield stress it is best to take some data at
M19-2101 REVISION 2.0
= yield stress, η = plastic viscosity and D = shear rate)
0
0
) which is the amount of shear stress required to initiate flow, and the product's plastic
D V N E X T R H E O M E T E R - O P E R AT I O N A L M A N U A L
Drilling Fluid
Plastic Viscosity (η) = 6622 cP
Yield Stress (τ
Figure 6-4
) = 116 dynes/cm
2
0
0
). To
83
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