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SHEAR

When forces act parallel to the surface area of the matter, internal stress (force / surface area) between adjacent layers of surface resist the relative movement of layers.

SHEAR and SHEAR RATE:(Figure 6-01)

shear

With the application of force, if a layer moves with velocity V at a distance of r then

Shear rate = V / r

VISCOSITY:

Viscosity is a way to describe how much resistance a fluid - melted polymer - exhibits when an attempt is made to make it flow. High viscosity means the melt is thick and resistance to flow is high. Low viscosity means resistance flow is less.

Just as solid surfaces in contact exert frictional force on each other, the relative motion of layers of liquid is also restricted by friction. This internal resistance to flow existing between two liquid layers when they are moved relative to each other is called viscosity.

Consider the flow of liquid through a pipe and imagine that liquid moves in layers. The liquid moves when there exists the pressure difference between the pipe ends. The liquid in the center of pipe would be moving with higher velocity than the liquid layer at the internal wall of the pipe. This is because the frictional forces act between the layers of liquid. Therefore there exists a velocity gradient between pipe wall and center of pipe. Such motion of liquid is known as laminar flow. The velocity of particles at a given distance from the center of motion is constant. If the layers intermix we get turbulent flow.

velocity gradient

A streamline is a curve whose tangent always lies along the direction of motion of fluid at that point. The streamlines never cross and in laminar flow they do not change with time.

It is observed that frictional force is directly proportional to the product of velocity gradient and cross sectional area.

F=h A x velocity gradient = h A x dv/dr

h = coefficient of viscisity. A = cross sectional area. dv/dr= velocity gradient.

               

       Tangential stresses        Shear stress
h = ------------------------------   =  --------------------
       Velocity gradient            Shear rate

       
                 Force      distance                 
Units is ------------- x ------------------  =   Pascal sec.= Pa s.
                 Area        distance / sec.
                  

Viscosity varies with temperature.

Fluid

Viscosity

Pas at 200C

Fluid

Viscosity

Pas at 200C

Air

1.8 x 10-5

Blood

3 - 4 x 10-3

Glycerol

8.3 x 10-1

Water

1 x 10-3

Castsor Oil

2.42

Mercury

1.5 x 10-3

SHEAR STRESS / SHEAR RATE CURVE: (Figure 6-02)

stress strain relation


From the above plastic flow curve it is observed that the viscosity reduces as the shear rate increases. Different polymers will exhibit different curve of the same nature.

SHEAR THINING at compression zone of SCREW

Shear in screw

Shearing in Nozzle

Shear in nozzle
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