Chemistry Reference
In-Depth Information
Viscoelasticity
Shearing stopped
Steady
shear
Inelastic
fluid
Viscoelastic
Time
Inelastic fluid
Elastic
recoil
Viscoelastic
fluid
Steady
s
hear
Time
FIGURE 4.33
Comparisons of inelastic and viscoelastic behavior on the cessation of steady shearing.
Viscoelasticity was introduced in
Section 4.7
. A polymer example may be use-
ful by way of recapitulation. Imagine a polymer melt or solution confined in the
aperture between two parallel plates to which it adheres. One plate is rotated at a
constant rate, while the other is held stationary.
Figure 4.33a
shows the time
dependence of the shear stress after the rotation has been stopped.
decays
immediately to zero for an inelastic fluid but the decrease in stress is much more
gradual if the material is viscoelastic. In some cases, the residual stresses may not
reach zero, as in molded or extruded thermoplastics that have been quenched
from the molten state. Such articles contain molded-in stresses that are relieved
τ
