Dear Amirhossein Hosseinpour,
Without changes to the source code, FAST cannot model a sudden blade failure through cracking, delamination, etc., which I presume you’d want to model as a sudden change in blade stiffness. I have not attempted this myself, but one challenge I foresee will be that such blade properties are defined as parameters (constants) at the beginning of the simulation that are not meant to be changed. You can probably get around this by adding a scale factor that changes the stiffness at a given time in the simulation. Perhaps a bigger problem is that the blade mode shapes are derived from the stiffness (among other factors), so, changing the stiffness should also impact the mode shape, which will be more difficult to adjust in the middle of a simulation.
Best regards,