Chemistry Reference
In-Depth Information
120
100
80
60
23
40
50
120
20
0
0 0 0 0 0 0 0 0
80
ELONGATION (%)
FIGURE 4.25
Tensile stress
strain behavior for a molded sample of a nylon-6,6 at the indicated
temperatures (
C). The arrows indicate the yield points which become more diffuse at
higher temperatures.
strain behavior of polymers is
generally opposite to that of straining rates. This is not surprising in view of the
correspondence of time and temperature in the linear viscoelastic region
(
Section 4.7.2.3
). The curves in
Fig. 4.25
are representative of the behavior of a
partially crystalline plastic.
The influence of temperature on the stress
4.10
Crazing in Glassy Polymers
When a polymer sample is deformed, some of the applied energy can be dissi-
pated by movement of sections of polymer molecules past each other. This yield-
ing process uses energy that might otherwise be available to enlarge preexisting
micro cracks into new fracture surfaces. The two major mechanisms for energy
dissipation in glassy polymers are crazing and shear yielding.
Crazes are pseudocracks that form at right angles to the applied load and
that are traversed by many microfibrils of polymer that has been oriented in
the stress direction. This orientation itself is due to shear flow. Energy is
absorbed during the crazing process by the creation of new surfaces and by
