Question:

How is the “Fatigue Time Period” in FTOPT line is related to “Fraction of Design Life” in FTCASE line?

Resolution:

For time history fatigue, this value is related to the "fatigue time period" defined on the fatigue option line “FTOPT".  Essentially the number of cycles and fraction of design life are the same.

For example, if you have a time history simulation of 600s and the probability of occurrence for this case is 0.025, then you can define it in both ways as below. 

First you can define "fatigue time period" as time duration 600s (should be 0.000019026 year on FTOPT), then the "fraction of design life" in FTCASE should be 0.025(this is a fraction).

Also, you can define "fatigue time period" as 1 year on FTOPT, then the ‘fraction of life” in FTCASE should be 1314 (this is the number of occurrence for this case in 1 year which is calculated from 0.025*365*24*3600/600=1314).

Both definitions are the same and would get the same fatigue results.