Repeated Stressing Mechanical Fatigue

FIGURE 30 Growth of an edge crack in a test piece of a natural rubber vulcanizate stretched repeatedly to 46% extension. (From Greensmith, Mullins, and Thomas [26].) 

If the crack grows to many times its original depth, so that / /q before fracture ensues, then the corresponding fatigue life may be obtained by setting / = oo in Eq. (27) yielding 

The different crack growth laws for strain-crystallizing and noncrystallizing elastomers thus lead to quite different fatigue life relations. For a noncrystallizing elastomer, the fatigue life is much more dependent on the size of 

FIGURE 31 Fatigue life versus depth of initial cut for test pieces of natural rubber and SBR stretched repeatedly to 50% extension. (From Lake and Lindley [55].) 

FIGURE 10.33 Fatigue life for test pieces of natural rubber versus minimum extension, min-Ae, denotes the additional strain imposed repeatedly. (From Cadwell et al. (1940).) 

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One of the major causes of failure in mbber is the development of cracks at the surface. The growth of these cracks under repeated deformation, or fatigue, leads to catastrophic failure. This fatigue failure is initiated at minute flaws where stresses are high and mechanical mpmre at such points can lead to the development of cracks. Similarly, attack by ozone can cause cracks to occur at the surface whose rate of growth is directly proportional to the ozone concentration. 

Property Summary Nylons are recommended for general-purpose gears and other mechanical components. Acetals for maximum fatigue life, for highly accurate parts, or exposure to extremely humid conditions. Phenolic-fabric laminates for low-cost, thin stamped gears or parts. Polycarbonates for intermittent, very high impacts (not recommended for applications involving repeated cyclical stress). TFE-filled acetals for heavy-duty applications. 

Failure and decay of mechanical properties after repeated applications of stress or strain are known as fatigue. Generally the "fatigue life" is defined as the number of cycles of deformation required to bring about rupture (see also Chap. 13). 

Fatigue is the decay of mechanical properties after repeated application of stress and strain. Fatigue tests given information about the ability of a material to resist the development of cracks or crazes resulting from a large number of deformation cycles. 

Stress, fatigue and mechanical damage. These are results of the use of the plastic objects and could comprise frequent bending of a PVC soft toy leading to its failure or abrasion of the surface of a vinyl record as it is repeatedly pulled out of its sleeve to play, resulting in its inability to produce perfect sound. 

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