Repeated stressing of metals above certain limits induces inter- and intra-crystalline dislocations and cleavages and eventually cracks which propagate to failure. Some authorities consider crack propagation to be a separate and discrete stage in the failure process. The internal damage is insidiously cumulative and irreversible. This phenomenon was recognized as early as 1829 and was termed fatigue as early as 1839(1). From the beginning of fatigue testing (Wohler, 1858-1870), results have been reported as S-N, S-log N, or log S-log N curves, where N is the number of repetitions of stress S. From a structural design point of view, the purpose of fatigue testing then was to find the endurance or fatigue limit (i.e., fEL) and so to establish the design or working stress (for many steels, fEL came to be regarded as 55 percent of fy, the yield stress, or 46 percent of fu, the ultimate strength).

Report Date


Report Number

No. 318

Digital Object Identifier



Based on original derivations by R. L. Lynch, formerly Research Engineer.