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Brookeld Engineering Laboratories, Inc. Page 35 Manual No. M/07-022-D0613

II.12.4 The Casson Model

€

t = t

+ h

(

€

= shear stress,

€

= yield stress, h = plastic viscosity, and D = shear rate)

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 ow after yielding.

The Casson model indicates the product’s yield stress (

€

), which is the amount of shear stress

required to initiate ow, and the product’s plastic viscosity,

h, which is the viscosity of the product

after it yields.

Figure II-46

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 ow. This model is most suitable

for uids that exhibit Pseudoplastic or shear thinning, ow behavior after yielding.

These uids have a non-linear ow curve. The point at which it crosses the y-axis is the product’s

yield stress (

€

). To protect 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 low shear rates.

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