Logarithmic nature of fatigue

From the log-log representation of the SN-curve, you can easily identify how to calculate the number of cycles for arbitrary load amplitudes S:

This may also be written as (for the case SE ≤ S < SMax):

resp.

The effect of this exponential influence of increasing load may be seen in the following table:

Load factor	k=3life factor	k=5life factor	k=7life factor
1.0	1.0	1.0	1.0
1.1	0.75	0.62	0.51
1.2	0.58	0.4	0.27
1.3	0.45	0.27	0.16

This means that for well-shaped steel components (k=7), an increase of the load of 30% diminishes the life by a factor of more than 6. For other especially light-weight materials, the exponents can be even larger.

Learn more

Defining SN-curves

Determination of SN-curves

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

Modifying factors of the SN-curve

Mean stress effects

Variable amplitude loading

Stress-life analysis in Specialist Durability

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Logarithmic nature of fatigue, Simcenter 3D 2021.1 Series

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