Cyclic loading causes fatigue cracking in a ship's welded structural details. If these are not designed to resist fatigue cracking, the ship's profitability may be affected by repair costs and its economic life shortened. Fatigue cracks also may lead to fractures in the ship's primary hull structure, an event resulting in catastrophic failure. It is therefore necessary that structural designers use techniques that will minimize fatigue damage, ensuring structural integrity for the ship's intended service life.

One technique for predicting and assessing fatigue cracking uses empirical data derived from laboratory tests of representative structural details. After details undergo fatigue tests, test data are analyzed in terms of stress applied to each detail and the number of cycles required to reach failure. The test results are commonly referred to as S-N data and are presented in S-N curves.