Unformatted text preview: 260 ● Chapter 9 / Failure fatigue failure. High-cycle fatigue is associated with fatigue lives greater than about 104 to 105 cycles. 9.11 FATIGUE IN POLYMERIC MATERIALS 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 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 9.27, as stress versus the number of cycles to failure (on a logarithmic scale). Some polymers have a fatigue limit. As would be expected, fatigue strengths and fatigue limits for polymeric materials are much lower than for metals. The fatigue behavior of polymers is much more sensitive to loading frequency than for metals. Cycling polymers at high frequencies and/or relatively large stresses can cause localized heating; consequently, failure may be due to a softening of the material rather than as a result of typical fatigue processes. 9.12a CRACK INITIATION AND PROPAGATION [DETAILED VERSION (CD-ROM)] 9.12b CRACK INITIATION AND PROPAGATION (CONCISE VERSION)
The process of fatigue failure is characterized by three distinct steps: (1) crack initiation, wherein a small crack forms at some point of high stress concentration; (2) crack propagation, during which this crack advances incrementally with each stress cycle; and (3) ﬁnal failure, which occurs very rapidly once the advancing FIGURE 9.27 Fatigue curves (stress amplitude versus the number of cycles to failure) for polyethylene terephthalate (PET), nylon, polystyrene (PS), polymethyl 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.) 25 PS 20
Stress amplitude (MPa) PET 3 15 PMMA 2 10 PP PE 5 PTFE Nylon (dry) 1 0 103 104 105 Number of cycles to failure 106 107 0 Stress amplitude (ksi) ...
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