Fatigue assessment

Fatigue assessment estimates whether a welded structural detail will survive its design life under cyclic wave loading, using S-N curves and Miner’s linear damage rule. The cumulative damage D = sum of (n_i / N_i) over all stress-range blocks must stay below 1.0, where n_i is the cycles at stress range i and N_i is the cycles to failure from the detail’s S-N curve. Ship rules express the long-term stress range as a Weibull distribution over about 10^8 cycles for a 20 to 25 year North Atlantic life. Hot-spot or notch stress at the weld toe, found by fine-mesh FE, feeds the curve. Welded joints, not the base plate, govern, so detail geometry and weld quality dominate fatigue life.