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.
