Weld Fatigue

Welds fatigue from the toe, where the geometry concentrates stress. EN 1993-1-9 gives each detail a category — the stress range, in MPa, it survives for 2 million cycles. Below that the curve falls at slope m = 3 to a constant-amplitude limit at 5 million cycles, then at m = 5 to a cut-off at 100 million.

The detail decides the life more than the steel does. A welded detail's category is the same whatever the grade, and on the steep part of the curve life goes as the cube of the stress range — so how a joint is detailed usually matters more than how strong its plate is.

Detail and loading

Constant-amplitude limit ΔσD
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Cut-off limit ΔσL
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Factored range γFf·γMf·Δσ
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Endurance, cycles
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Where it sits on the curve
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Allowable range for a design life

Allowable stress range
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Where it sits on the curve
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Nominal stress method for steel, one stress range at a time. The detail presets are common cases, not a classification: classify a real detail against EN 1993-1-9 Tables 8.1–8.10, use the partial factors from your National Annex (γMf = 1.1 is the UK value for safe-life design), allow for thick-plate size effects, and sum damage with Palmgren-Miner for a real load spectrum. A helper, not a substitute for engineering judgment or the governing code.