Dear Muddassir,
I’m not sure I understand your question. What do you want equations for DTTorSpr and DTTorDmp based on…on materials/geometry (this will depend on the configuration of the drivetrain), on natural frequencies, or on something else? DTTorSpr and DTTorDmp are inputs to FAST, but FAST doesn’t require specific equations to derive them.
You may find the following related forum topics useful: Drivetrain 2.2Hz frequency and natural frequency 0.625Hz def.
Best regards,