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regarding the interpolation methods (Fatigue Loading)I suggest you to read as much as you can about this important phenomenon. As you know it is observed that repeated loading and unloading weakens objects over time. After a number of cycles, the object becomes so weak that it fails.
Basically it is the way your load changes in amplitude over the time. Now you can have:
1) Constant Amplitude Loading. It is fully defined by an alternating stress, mean stress, stress ratio and number of cycles.
2) Variable Amplitude Loading. It is a load history record that defines the fluctuation history of a load. Usually it is loaded into Cosmos Works from a field test data acquisition system.
S-N Curve is the behavior of a datum material characterized by the relation between the alternating stresses and the corresponding number of cycles to failure. This diagram is very important. You must pay attention on how the diagram was obtained. I mean this diagram depends of the material and the loading typology applied to obtain the data. For Low Cycle Fatigue (presents significant plastic deformations) I don't think Cosmom Simulation has the mathematical ability and the code to approach this class of problems.
Regarding the High Cycle Fatigue you can use the formulation of Mischke with the equation of Mason-Coffin.
A good approximation is given by Sf=1000N exp(-0.0738) where N is the number of cycle you want to calculate the Fatigue Limit at. So you could build a diagram (it is an approximation) for different number of cycles.
Have you tried to check on this web site: Material Database
Your last question.
First of all you must calculate the Ultimate Limit Fatigue from Marin's equation and then you verify your results with Gerber, Goodman or ASME Elliptic criteria.
Hope it is helpful.