Fatigue Class S-N curve

A fatigue-class S-N curve relates the cyclic stress range (S) a welded detail sees to the number of cycles to failure (N), giving each weld geometry a fatigue class so designers can predict crack initiation under repeated wave and cargo loading. Plotted log-log, the curve is a straight line of slope m (typically 3 below the knee), with a transition in slope at about 10^7 cycles in DNV-RP-C203. The detail class, B through W in the IIW and class systems, drops with worsening weld geometry and stress concentration; a smooth butt weld outranks a fillet-welded attachment. Hot-spot or nominal stress feeds the chosen curve. It is the backbone of the Common Structural Rules fatigue check.