LCR7000 & RHEOSIGHTS P/N: 974194 |
Rev: A
|
ECO: 52468
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From lab to production,
providing a window into the process
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Newton's law states that the shear stress on a material is directly proportional to the rate
at which it is sheared. Using the simple shear deformation model described previously, this
means the force necessary to move the top plate will increase proportionally with the
increase in speed of the top plate. Graphically this means a plot of linear (as opposed to
log) shear stress vs. linear shear rate will yield a straight line, as illustrated in the next
Figure. The slope at any given point on the curve is the viscosity of the material at that
shear rate.
Because the magnitude of the change in viscosity is frequently large, it is often more
convenient to make a log-log plot.
The Newtonian region for materials on a log-log plot of shear stress vs. shear rate is where
the slope is equal to 1.0. For most polymers, the slope falls to less than 1.0 as the shear rate
increases.
It is often easier to see if the material is behaving like a Newtonian material by looking at a
viscosity vs. shear rate plot. Viscosity is constant (slope=0.0) for both log-log and linear-
linear plots of viscosity vs. shear rate as shown in the examples below.
