The S-N curves are specified by the standard and take into account the type of the material, the connection or connection welds , the stress direction and the surrounding environment. As an example, see the S-N curves for the air environment from DNVGL-RP-C Edition April : S-N category depends on the type of constructional detail non-welded, welded and stress direction parallel to the weld, perpendicular to the weld and shear. In the finite element model, the required information is missing and in result, the task of setting a classification can be time-consuming. In SDC Verifier the Weld Finder tool recognizes welds automatically and converts stresses from local element direction into weld direction, which helps to set classification in different weld stress directions.
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The design was sufficient to meet requirements of 2. However the loads have changed and a single case is showing a design ratio in excess of FL. The model shows relatively complex geometry and loading which might make the standard S-N curves less valid anyway.
I am fairly sure that if I model in the welds I can reduce the peak stresses to an acceptable level, particularly using the Notch methodology covered in D. However I can see no guidance to show that it is appropriate to replace local stress with notch stress when justifying the omission of detailed fatigue analysis. Does anyone have any experience with this regard? You can include the welds and compare the new stress to the fatigue limit.
But remember to compare the hotspot stress to the fatigue limit for the appropriate SN curve.. For a detailed FE analysis you already include the geometric part of the stress, i. For this reason it is typically appropriate to apply the D-curve in a hotspot analysis.
The SCF embedded in the D-curve is 1. But be careful! If you are dealing with anything else than double bevel butt welds you might need to consider a worse SN classification.
Are you considering extrapolation, as described in CHapter 4 of C? Another thing: Why apply solid elements?
If you apply shells you can use the hot spot stress as extrapolated according to chapter 4 in C This stress can be compared to the fatigue limit directly.
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