Endurance limit dynamic fatigue

The dynamic fatigue can be good to excellent for certain grades if care is taken to limit the strains by restricting the stresses to values in keeping with the low modulus. For example, PTFE is used to manufacture bellows with excellent dynamic endurance. 

Dynamic fatigue b more widefy recognized as a cause of firacture, and most handbools provide fritigue data in the form of S-N curves, as iUustrated in Figure 8.20. These curves usualfy flatten b ond 10 les, in which case it b possible to define a fatigue endurance limit, le. the minimum stress required to cause failure within 10 cycles. Both the static... 

Dynamic fatigue is more widely recognized as a cause of fracture, and most handbooks provide fatigue data in the form of S-N curves, as illustrated in Fig. 8.20. These curves usually flatten out beyond 10 cycles, in which case it is possible to define a fatigue endurance limit, i.e. the minimum st ress required to cause failure within 10 cycles. Both the static and the dynamic tests reflect the response of the material to the small defects that are present in the unnotched specimens, and in a sense are measuring the distribution of intrinsic flaw sizes as much as the fracture resistance of the polymer. Consequently, when the component is to be used in a critical application, there is a good case for mounting a full-scale fracture mechanics study. 

Lewis et al. [16,17] and Wake et al. [18,19] have examined in some detail the concept of an endurance limit for static fatigue tests, i.e. a value of the applied stress below which joint failure will not occur. Such a concept is obviously of considerable benefit to the design engineer and is analogous to that discussed above for dynamic fatigue failure. Indeed, Wake et al. have considered this concept both for the case of simple static fatigue and for the case when the joint is first subjected to a number of load cycles and then the residual tensile fracture stress measured. 

With prepreg epoxy systems, the first two values are lower by at least 0.5%. As seen from Table 2, these values are much lower than the fracture strain for most composites. Thus, the epoxy fractures much earlier than the fibers. This situation is stressed under dynamic load on carbon- or Kevlar fiber composites, since polymers possess a much lower fatigue-endurance limit than carbon- and Kevlar fiber composites this is not the case for fiber-glass composites. 

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