Modifying factors of the SN-curve

Many factors can influence the SN-curve for a given material. The basic SN-curve data is usually obtained from rotating bending tests of polished laboratory specimens. The surface roughness, size, type of loading, and mean stress level can all influence the fatigue life behavior of components as compared to laboratory specimens. The different modifying factors are often collectively called non-local effects. See the section on Non-local and surface effects for details.

Factors are often used to correct for these observations. These factors tend to have a greater affect at longer lifetimes, when stresses are lower. In Specialist Durability, the surface roughness factor and size factor are multiplied by the points S1 and SE in the diagram, to effectively shift the SN-curve (see the figure below).

The factors are defined as the ratio of the endurance limit of the component to the endurance limit of the lab specimen:

nsize · nroughness = (SE)component / (SE)specimen

The size factor can account for differences in fatigue life expected from differences in size between the component and the laboratory specimen from which the basic SN-curve was determined. Observations have typically shown that larger specimens have a lower endurance limit than small laboratory specimens.

The size effect factor could be less than or larger than one, depending on the size of the component as compared to the size of the laboratory specimen. The numerical value of this factor to use in an analysis will depend on prior experience and insight into the component fatigue behavior.

Similarly, the surface roughness factor can account for differences in fatigue life from difference in surface roughness between the component and the laboratory specimen from which the basic SN-curve was determined. It has been observed that fatigue life is reduced as the surface roughness increases. Surface scratches such as machining marks can serve as stress concentrations and more quickly initiate cracks than polished specimens. This factor could also be less than or greater than one, depending on the relative roughness between the component and laboratory specimen. Again, the value of this factor to use in an analysis will depend on prior experience and insight into the component fatigue behavior.

Note:

For taking into account additional effects, the product of all influence factors can be used in one of the parameters.

If the SN-curve is defined with two points, then

S1' = S1 · nsize · nroughness

SE' = SE · nsize · nroughness

Otherwise, only the endurance limit is modified.

Effect of Modifying Factors on the SN-Curve

Specialist Durability has the provision to define a joint correction factor for surface and size.

Learn more

Defining SN-curves

Logarithmic nature of fatigue

Determination of SN-curves

How does the SN-curve definition via universal slope work?

Mean stress effects

Variable amplitude loading

Stress-life analysis in Specialist Durability

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Modifying factors of the SN-curve, Simcenter 3D 2021.1 Series

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