Callister - An introduction - 8th edition

580 • Chapter 15 / Characteristics, Applications, and Processing of Polymers
Principles of fracture mechanics developed in Section 8.5 also apply to brittle
and quasi-brittle polymers; the susceptibility of these materials to fracture when a
crack is present may be expressed in terms of the plane strain fracture toughness.
The magnitude of K Ic will depend on characteristics of the polymer (molecular
weight, percent crystallinity, etc.) as well as temperature, strain rate, and the external
environment. Representative values of K Ic for several polymers are included in
Table 8.1 and Table B.5, Appendix B.
15.6 MISCELLANEOUS MECHANICAL
CHARACTERISTICS
Impact Strength
The degree of resistance of a polymeric material to impact loading may be of concern
in some applications. Izod or Charpy tests are ordinarily used to assess impact
strength (Section 8.6). As with metals, polymers may exhibit ductile or brittle fracture
under impact loading conditions, depending on the temperature, specimen size,
strain rate, and mode of loading, as discussed in the preceding section. Both semicrystalline
and amorphous polymers are brittle at low temperatures, and both have
relatively low impact strengths. However, they experience a ductile-to-brittle transition
over a relatively narrow temperature range, similar to that shown for a steel
in Figure 8.13. Of course, impact strength undergoes a gradual decrease at still higher
temperatures as the polymer begins to soften. Ordinarily, the two impact characteristics
most sought after are a high impact strength at the ambient temperature
and a ductile-to-brittle transition temperature that lies below room temperature.
Fatigue
Polymers may experience fatigue failure under conditions of cyclic loading. As with
metals, fatigue occurs at stress levels that are low relative to the yield strength.
Fatigue testing in polymers has not been nearly as extensive as with metals; however,
fatigue data are plotted in the same manner for both types of material, and the
resulting curves have the same general shape. Fatigue curves for several common
polymers are shown in Figure 15.11, as stress versus the number of cycles to failure
Figure 15.11 Fatigue curves (stress
amplitude versus the number of
cycles to failure) for poly(ethylene
terephthalate) (PET), nylon,
polystyrene (PS), poly(methyl
methacrylate) (PMMA),
polypropylene (PP), polyethylene
(PE), and polytetrafluoroethylene
(PTFE). The testing frequency was
30 Hz. (From M. N. Riddell, “A
Guide to Better Testing of Plastics,”
Plast. Eng., Vol. 30, No. 4, p. 78, 1974.)
Stress amplitude (MPa)
25
20
15
10
5
PS
PMMA
PP
PE
PTFE
PET
Nylon
(dry)
3
2
1
Stress amplitude (ksi)
0 0
10 3 10 4 10 5 10 6 10 7
Number of cycles to failure