Endurance Limit Modifying Factors

Account for factors that can affect fatigue strength of a structure.

Surface Condition (Finish and Treatment)

Surface condition is an extremely important factor influencing fatigue strength, as fatigue failures nucleate at the surface. Surface finish and treatment factors are considered to correct the fatigue analysis results.

Surface finish correction factor Cfinish is used to characterize the roughness of the surface. It's presented on diagrams that categorize finish by means of qualitative terms such as polished, machined or forged.
Figure 1. Surface finish correction factor for steels*

(*Source: Yung-Li Lee, Jwo. Pan, Richard B. Hathaway and Mark E. Barekey. Fatigue testing and analysis: Theory and practice, Elsevier, 2005)

Surface treatment can improve the fatigue strength of components. NITRIDED, SHOT-PEENED, COLD-ROLLED are considered for surface treatment correction. You can also input a value to specify the surface treatment factor Ctreat.

In general cases, the total correction factor is Csur= Ctreat* Cfinish.

If treatment type is NITRIDED, then the total correction is Csur= 2.0 * Cfinish (Ctreat= 2.0).

If treatment type is SHOT-PEENED or COLD-ROLLED, then the total correction is Csur = 1.0. It means you will ignore the effect of surface finish.

The fatigue endurance limit FL is modified by Csur as: FL' = FL * Csur. For two segment SN curve, the stress at the transition point is also modified by multiplying by Csur.

Fatigue Strength Reduction Factor

In addition to the factors mentioned above, there are various other factors that could affect the fatigue strength of a structure, such as notch effect, size effect, and loading type. Fatigue strength reduction factor Kf is introduced to account for the combined effect of all such corrections. The fatigue endurance limit FL will be modified by Kf as: FL' = FL / Kf.

If both Csur and Kf are specified, the fatigue endurance limit FL will be modified as: FL' = FL * Csur/ Kf.