85
4
5 Additives—Additives in the form of fillers increase the viscosity
of polymer melt. Additives in the form of plasticizers decrease
the viscosity by breaking the long molecular chains of the
polymer, thus reducing the molecular weight too.
5 Reinforcement—Reinforcement causes a physical barrier; therefore, they increase the viscosity of the composite mix.
4.1.1.5 Normal Stresses
In non-Newtonian fluids, some radial outside force is exerted by the
fluid in the pipe through which flow is taking place. This force is
normal to the flow and is considered a normal stress exerted on the
fluid or body. For the purpose of illustration, a surface ABCD is
used, and this surface is in the yz plane perpendicular to the x-axis.
τ xx , τ yy , and τ zz are the normal stresses working on the surface or
body, as shown in . Fig. 4.2.
If total pressure acting on the body is p, the following equations
for normal stresses may be written as
τ
τ
xx
xx
p
= − +
(4.8)
τ
τ
yy
yy
p
= − +
(4.9)
τ
τ
zz
zz
p
= − +
(4.10)
Normal stresses τ xx , τ yy , and τ zz act away from the body. Two normal
stresses are denoted as N 1 and N 2 . N 1 is the first normal stress difference (FNSD), and N 2 is second normal stress difference (SNSD), so
A
D
C
B
z
y
x
τ yz
τ yy
τ yx
τ xy
τ xx
τ xz
. Fig. 4.2 Normal stresses working on a body
4.1 · Fundamentals of Rheology