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Fatigue and the S–N curve

Fatigue failure occurs under varying (cyclic) load, even when the stress stays below the yield strength — cracks grow a little with each load cycle until the material finally breaks. A Wöhler curve (S–N curve) plots stress amplitude against number of cycles to failure, and many steels have a fatigue limit where the curve flattens out.

S–N curve: σa vs. Nf\text{S–N curve: } \sigma_a \text{ vs. } N_fstress amplitude against number of cycles to failure

Symbols

σa\sigma_astress amplitudeMPa
NfN_fnumber of cycles to failure

Example

A component that easily withstands static load can still fail after millions of load cycles at a stress far below the yield strength.

A fatigue limit exists for many steels, but rarely for aluminum — there, strength keeps decreasing with more cycles.
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Part of Materials Technology: Mechanical properties.