LCR7000 & RHEOSIGHTS P/N: 974194 |
Rev: A
|
ECO: 52468
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In summary the Weissenberg Rabinowitsch shear rate correction accounts for the fact that
the true shear rate is often larger than the apparent shear rate for non-Newtonian
materials. The true shear rate can be calculated using the following equation:
Where:
n = the tangent slope of the log true shear stress vs. log apparent
shear rate curve at the apparent shear rate being corrected.
•
γ
= the true shear rate
•
a
γ
= the apparent shear rate described previously.
Note that the shift in shear rate can be substantial causing a trace of ln (viscosity) vs. ln
(shear rate) to move down and far to the right on the plot. The total viscosity change
(viscosity drop at a fixed shear rate), however, very rarely exceeds 15 % and occurs only in
the non-Newtonian range of the material.
The Bagley correction method is used to calculate true stress. To obtain the true shear
stress perform the following procedure:
Use a minimum of two dies (though preferably three or more) having the same entrance
angle and same diameter (D), but with differing capillary lengths (L). Collect steady-state
flow data on shear rate and test pressure (or plunger force). (At least, one L/D ratio should
be 5 or less and at least one should be greater than 16.) Prepare a plot of Pressure (or
Plunger Force) versus the Length to Diameter (L/D) ratio of the dies used. For points at
constant apparent shear rate draw the best straight line through the data and determine
the intercept with the Pressure axis (P
0
) or Force axis (F
0
). To obtain the true shear stress
use the following equation:
13.
Corrected Shear Stress
τ
=
−
=
−
(
)
(
)
P
P
D
L
F
F
D
L
A
0
0
B
4
4
