Dear Andrew,
You say you’ve computed times to failure, lifetime damage, and DELs for various OpenFAST outputs. Presumably you mean you computed these by running many load case simulations across a range of operation and parked/idling conditions and you’ve weighted each simulation using the wind speed probability distribution within MLife. Is that correct?
How did you select the ultimate design load for each component (LUlt in MLife) in this process?
The OpenFAST load outputs are defined at various cross sections of the turbine (e.g,. TwrBsMyt is at the tower base). For structures composes of simple cylindrical shapes and isotropic materials such as the tower and shaft, computing the time to failure at various cross sections of those components should give you a sense of the overall lifetime of those components. (You’d likely also need to confirm ultimate strength and resistance to buckling.)
For noncylindrical components such as the hub, nacelle and composite blades, the process is more complicated because you’ll have to use the loads output from OpenFAST as an input into a detailed FEA analysis of those components to determine the resulting state of stress, from which failure can be predicted.
Best regards,