Page 203 - Manufacturing Engineering and Technology - Kalpakjian, Serope : Schmid, Steven R.

I 82 Chapter 7 Polymers: Structure, General Properties, and Applications

Creep and Stress Relaxation. Because of their viscoelastic behav-
|_0W-denSity High-impact ior, thermoplastics are particularly susceptible to creep and stress
polyethylene polypropylene relaxation, and to a larger extent than metals. The extent of these
phenomena depends on the polymer, stress level, temperature, and
time. Thermoplastics exhibit creep and stress relaxation at room
-.gy
§ Polyvinyl chloride temperature; most metals do so only at elevated temperatures.
"'
Q Crazing. When subjected to tensile or bending stresses, some ther-
3 moplastics (such as polystyrene and PMMA) develop localized,
E wedge-shaped, narrow regions of highly deformed material, a
process called crazing. Although they may appear to be like cracks,
Polymethylmethacrwate crazes are spongy material, typically containing about 50% voids.
With increasing tensile load on the specimen, these voids coalesce to
form a crack, which eventually can lead to a fracture of the polymer.
'18 0 32 Crazing has been observed both in transparent, glassy polymers
TGITIPGVGIUFG (°C) and in other types. The environment (particularly the presence of
solvents, lubricants, or water vapor) can enhance the formation of
crazes (environmental-stress cracking and solvent crazing).
Residual stresses in the material also contribute to crazing and
FIGURE 7.l2 Effect of temperature on the
impact strength of various plastics. Small changes cracking of the polymer; radiation (especially ultraviolet radiation)
in temperature ean have 3 Significant effeet on can increase the crazing behavior in certain polymers.
A phenomenon related to crazing is stress whitening. When
impact Strength Soufgg; After P_(j_ 1>0We11_
subjected to tensile stresses (such as those caused by folding or
bending), the plastic becomes lighter in color, a phenomenon usual-
ly attributed to the formation of microvoids in the material. As a result, the material be-
comes less translucent (transmits less light), or more opaque. This behavior easily can
be demonstrated by bending plastic components commonly found in colored binder
strips for report covers, household products, and toys.

Molecules are
100 _ being oriented

80 -
E
g
73' 60 -
9
5
40

20 If

0
O 25 50 75 100 125
Elongation (mm)
(G) (b)

FIGURE 1.13 (a) Stress-elongation curve for polycarbonate, a thermoplastic. Source:
Courtesy of R.P. Kambour and R.E. Robertson. (b) High-density polyethylene tensile-test
specimen, showing uniform elongation (the long, narrow region in the specimen).