Accounting for stress concentration in fatigue analysis
Accounting for stress concentration in fatigue analysis
(OP)
Hey everybody,
I have a question that may seem obvious but I can't find any info that can answer it...
Say I run a FEA simulation on a notched specimen with a stress concentration of Kt=3. If I want to see if the specimen will survive an "infinite" number of cycles, do I check my actual FEA stress against an SN curve for an unnotched (Kt=1) specimen OR an SN curve for a notched specimen (Kt=3) if the data is available? MMPDS provides SN curves for different Kt values.
When using a classical approach (assuming Kt=Kf), either the endurance limit is factored by Kt or the actual stress found using hand calculations is factored by Kt.
Because the FEA model already accounts for the stress concentration, I would think that I should check the actual stress against the SN curve for an unnotched specimen (Kt=1). Is this logic correct?
Are the notched SN curves only useful for stresses found using hand calculations, as they account for notch sensitivity?
Thanks
I have a question that may seem obvious but I can't find any info that can answer it...
Say I run a FEA simulation on a notched specimen with a stress concentration of Kt=3. If I want to see if the specimen will survive an "infinite" number of cycles, do I check my actual FEA stress against an SN curve for an unnotched (Kt=1) specimen OR an SN curve for a notched specimen (Kt=3) if the data is available? MMPDS provides SN curves for different Kt values.
When using a classical approach (assuming Kt=Kf), either the endurance limit is factored by Kt or the actual stress found using hand calculations is factored by Kt.
Because the FEA model already accounts for the stress concentration, I would think that I should check the actual stress against the SN curve for an unnotched specimen (Kt=1). Is this logic correct?
Are the notched SN curves only useful for stresses found using hand calculations, as they account for notch sensitivity?
Thanks





RE: Accounting for stress concentration in fatigue analysis
RE: Accounting for stress concentration in fatigue analysis
I am doing something similar right now. I have an FEA of a fitting with a stress concentration, and I have the peak principal nodal stress; it's hard to envision a far field stress. I have a pretty small mesh size so I'm satisfied with the accuracy of the model. So I plan to use Kt = 3 SN data, reading in at 1/3 of the FEA stress. Now I'm lucky in that even if the stress was 40% greater than the FEA prediction, I still have an acceptable safe life, with a very conservative safe-life factor (10, 'cause we won't do a fatigue test of these fittings).
another day in paradise, or is paradise one day closer ?
RE: Accounting for stress concentration in fatigue analysis
Brian
www.espcomposites.com
RE: Accounting for stress concentration in fatigue analysis
Your statement:
"Because the FEA model already accounts for the stress concentration, I would think that I should check the actual stress against the SN curve for an unnotched specimen (Kt=1). Is this logic correct? "
is correct if you mean by "actual stress" the FEA predicted hot spot stress.
Assuming long life fatigue limit region, elastic conditions etc.
As the other contributors have pointed out most situations are not simple
plates with a hole and thus it is best to also plot any real test data of
the notched component if available.
RE: Accounting for stress concentration in fatigue analysis
another day in paradise, or is paradise one day closer ?