Fatigue

Fatigue is failure under repeated loading at stresses well below the static strength. Each load cycle grows a tiny crack a little; after hundreds of thousands or millions of cycles the crack reaches a critical size and the member fractures, often with little warning. Bridges, crane girders, wind-loaded masts, offshore structures and rail components all carry the millions of cycles that make fatigue a design case.

Test data are summarised as S-N curves: stress range against number of cycles to failure, plotted on log scales. For welded steel details the curves are nearly straight lines with a slope constant m of about 3, so halving the stress range multiplies life by about eight. What matters is the stress range, not the peak stress, and the detail category: a plain rolled plate lasts far longer than a fillet-welded attachment, because welds introduce notches, residual stresses and small defects where cracks start.

Real loading has many different ranges. Miner's rule adds the damage linearly: each block of n cycles at a range whose life is N uses up n over N of the life, and failure is predicted when the sum reaches one. It is simple and widely used, though actual sequences can make it slightly unconservative or conservative. Cycle counting methods such as rainflow turn an irregular stress history into blocks of ranges.

Good fatigue design is mostly good detailing: avoid abrupt changes of section, grind weld toes where needed, keep attachments away from high-stress regions, and use bolted or better-categorised welded details. Inspection plans then target the details with the least margin. Fracture mechanics, which tracks crack growth from an initial flaw, is used to set inspection intervals and to assess cracks found in service.