I would really appreciate your help.
I am playing around with the NX Durability Wizard and went through the documentation.
I know how the Strength Safety Factor (SSF) is calculated, but please explain to me how the
1) Fatigue Safety Factor (FSF)
2) Fatigue Life (FL)
are calculated for the specified node (see attached PDF)?
3) How would the results change if I used a Fatigue Strength Factor (K_f)?
4) In which case should I select “Infinite Life” or “Cycles to Failure”?
5) What are the “Number of Fatigue Duty Reversals”?
I would appreciate it so much if you could explain this to me.
Thanks in advance and kind regards.
the durability wizard allows to get an idea of the fatigue behavior under simple loads, in your case under alternating betwenn your load case and applying the same load with negative value (alternating load).
1) Several Fatigue Models introduce an endurance limit, meaning a stress amplitude under which no damage occurs. The FSF for infinite life gives you the (local) margin of safety, that is the factoc that I may apply to the load to exactly reach this endurance limit. Take into acccount that the durabilit ywizard uses the largest principal stress value to estimate this value.
2) Fatigue Life uses the SN curve data you enetered at the stress amplitude calculated like in (1) and gives you the number of cycles from the Sn curve data.
3) There are many local effects that have an influence on the fatigue behavior, like surface roughness, notch sharpness, surface treatment etc. To take those into account while using the material SN curve the Kf factor may be used . It is applied to the Stress Amplitude before using it in (1) and (2).
4) Infinite Life means a design that will never fail (assuming there is an endurance limit for a material). The Cycles To Failure criterion means I want to design for a given number of aplications of the given load cycle. By aware that this only applies for the given load, and the given SN curve. In reality many loads are acting on a structure.
5) The Number of Fatigue Duty Reversals is the design value which is taken in the case you choose Cycles to Failure, this means you first look up the stress amplitude that the material can endure for "Number of Fatigue Duty Reversals" cycles in the SN curve. Than you compare your stress amplitude from (1) with this value instead of the endurance limit.