Endurance limit, fatigue

Cumulative Damage. Pressure vessels may be subjected to a variety of stress cycles during service some of these cycles have ampHtudes below the fatigue (endurance) limit of the material and some have ampHtudes various amounts above it. The simplest and most commonly used method for evaluating the cumulative effect of these various cycles is a linear damage relationship in which it is assumed that, if cycles would produce failure at a... 

The data presented in Figure 19.7 were obtained on a Sonntag-Universal machine which flexes a beam in tension and compression. Whereas the acetal resin was subjected to stresses at 1800 cycles per minute at 75°F and at 100% RH, the nylons were cycled at only 1200 cycles per minute and had a moisture content of 2.5%. The polyethylene sample was also flexed at 1200 cycles per minute. Whilst the moisture content has not been found to be a significant factor it has been observed that the geometry of the test piece and, in particular, the presence of notches has a profound effect on the fatigue endurance limit. 

Fig. 8.71 Corrosion fatigue endurance limits for various alloys in aerated salt solutions or...

Fatigue data are normally presented as a plot of the stress (S) versus the number of cycles (N) that cause failure at that stress the data plotted defined as an S-N curve (Fig. 2-43). The use of an S-N curve is used to establish a fatigue endurance limit strength. The curve asymptotically approaches a parallel to the abscissa, thus indicating the endurance limit as the value that will produce failure. Below this limit the material is less susceptible to fatigue failure. 

Fig. 2-43 S-N curve establishes fatigue endurance limit strength.

Two conclusions can be drawn from an inspection of the S-N curve (1) the higher the applied material stress or strain, the fewer cycles the specimen can survive and (2) the curve gradually approaches a stress or strain level called the fatigue endurance limit below which the material is much less susceptible to fatigue failure. Different materials may... 

The flexural fatigue endurance limit (ASTM D671) provides Information on the ability of a material to resist mechanical... 

Beryllium is too expensive to be widely used as a metal by itself or as the main constituent of alloys. It is claimed that 2 5 per cent of beryllium added to copper is useful for springs, giving a sixfold tensile strength and higher fatigue endurance limit especially under conditions of corrosion. One per cent added to silver is said to make it resistant to tarnish. The alloy is heated in hydrogen to 400° with a little water vapour whereby a thin protective film of oxide is produced. 

The structural fatigue due to vibration at a single frequency is calculated from the RMS stress amplitude. For comparison with fatigue endurance limits which are based on peak stresses, the RMS stress is multiplied by a factor of 3.5 [peak stresses do not exceed 3.5 times the RMS stress (Ref. 3)]. 

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... 

Table 7. Fatigue endurance limits for various amorphous and crystalline steel filaments ...

As mentioned above, the fatigue behavior with damage development should be assessed by the Wohler curve and the fatigue endurance limit. The latter value is the most important parameter for design and construction purposes. For the description of the Wohler curves, different methods can be selected according to the plotting (semi- or double logarithmic scale). 

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. 

Table 3-9. a) Fatigue Endurance Limit Data of Reinforced Thermoplastics per ASTM D 671 at 1,800 cycles/min. 

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