Endurance limit fatigue strength

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.

The fatigue life is the number of cycles a specimen can remain at a specified strain or stress before specimen failure occurs. As stress is decreased, there is a point beyond which failure does not occur regardless of the number of cycles the specimen experiences. This stress value is called the endurance limit or endurance strength. 

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. 

ASTM Special Technical Publication No. 91 discusses in detail the important ramifications to be considered in the various statistical aspects of fatigue testing. Most often, the fatigue curves as well as the tabulated values of endurance strengths and endurance limits are based on the 50% probability curve. As a result, designers do not resort to using scatter-band... 

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