Brookfield DV3T Rheometer User Manual
Page 85
Brookfield Engineering Labs., Inc.
Page 85
Manual No. M13-2100
VII.2 The Herschel-Bulkley Model
(τ
= shear stress,
τ
o
= yield stress, k = consistency index,
= shear rate, and n = flow index)
What does it tell you?
The Herschel-Bulkley model is simply the Power Law model with the addition of τ
o
for yield
stress. Yield stress, τ
o
, denotes how much shear stress is required to initiate flow. This model also
provides a consistency index, k, which is a product’s viscosity at 1 reciprocal second, and a flow
index, n, which indicates the degree with which a material exhibits non-Newtonian flow behavior.
Since Newtonian materials have linear shear stress vs. shear rate behavior and n describes the
degree of non-Newtonian flow, the flow index essentially indicates how “non-linear” a material
is. For Herschel-Bulkley fluids, n will always be greater than or less than 1.
When n < 1 the product is shear-thinning or Pseudoplastic. This means the apparent viscosity
decreases as shear rate increases. The closer n is to 0, the more shear thinning the material is.
When n > 1 the product is shear-thickening or Dilatant. It’s apparent viscosity increases as shear
rate increases.
VII-2