Surface Factor

Surface Factor

The endurance limit (https://www.fea-solutions.co.uk/s-n-curves/) of a material is either obtained from a fatigue test (https://www.fea-solutions.co.uk/static-test-vs-fatigue-test/) or estimated using the ultimate tensile strength of the material if the test data is not available. Such an endurance limit does however not take into account the effects of the surface finish of an actual component.

That is why the surface factor, or surface condition factor is introduced, given by the following equation:

The coefficients a and b are dependent on the surface finish used, which can be one of the following types:

The surface factor has to be used with caution as wrong values can dramatically change the endurance limit. For example, if a material has an ultimate strength of 520MPa, the factor for a ground surface is 0.929, but for a forged surface just 0.540.

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