Learning Forum

[Jeone]

I did fatigue analysis in SN method, and the life of part was calculated as 16235 cycle. Then, I did EN fatigue analysis due to the low cycle, the life was calculated as 18452. Why the results are different? Can you explain me, please?

Sincerely, 

[danielshaw]

There is a wide variance in fatigue results.  SN and EN use different material properties and different formulations.  I would expect them to produce different results. I am surprised that SN and EN results are that similar.

[Jeone]

Well, should I take into account SN results or EN results? 

[danielshaw]

Whichever one you want.  By numerical fatigue calculation standards, they are essentially the same result.  You always should consider fatigue results in a statistical sense.  There is never one correct answer.  There is a range of correct answers depending on factors such as how comprehensive the fatigue test data is, how well known the applied loading is, how precisely the surface effects are known, etc.  Most commercial design codes apply a large factor of safety on the calculated fatigue life to encompass the known unknowns and the inherent statistical variance.

EN is generally considered to be more mathematically precise than SN, but often the SN test data is more comprehensive, so it can be more accurate.

[Jeone]

Most commercial design codes apply a large factor of safety on the calculated fatigue life to encompass the known unknowns and the inherent statistical variance
Like changing the certainty of survival setting from 50 to 99?

[danielshaw]

I should have mentioned that less than 20,000 cycles is relatively low cycle fatigue if the material is ductile metal.  All other variables being equal, EN should be more accurate that SN for LCF in ductile metals.

[Jeone]

I use ductile material, I will check the analysis again. Thank you so much