Daniel Shaw
Ansys Employee

I assume that you are using the Mechanical Fatigue Tool with History data loading.  Some thoughts:

  1. Your SN curve does not seem realistic.  SN only applies to high cycle fatigue (HCF) where the calculated stresses are below yield (Although it is sometimes used for streses slightly above yield).  For ductile materials that typicaly means cycles above 10e4.  You have some very low cycle points on your SN curve.
  2. The SN curve has a big jump in cycles around 100 MPa.
  3. I have never seen SN curves with more than 1e9 cycles.  How did get a SN numer at e28 cycles? 2 MPa is a tiny alternating stress.
  4. I suscpect that the alternating stresses ave very low, so the FT is essentially predicting no fatigue damage.  Sometime FT results are difficult to interpert when the calculated alternating stress is less than the lowest stress from the SN curve.  What are the stress levels in the model and how do they comapre to the SN curve?  You should be able to do a simple hand calculations using the FE and predict what the maximum Salt.  How does that number compare to SN curve.