Dear Leonardo,
If you’ve changed the chord, presumably you are changing the AeroDyn input file of FAST, but this input file is only used to define the aerodynamic properties of the simulation. I would expect cracks to impact the blade stiffness rather than the aerodynamics. Thus, you should change the structural blade input file from the ElastoDyn module (both stiffness and mode shapes).
That said; regarding blade failure, FAST can be used to calculate the reaction loads that would lead to blade failure, but the actual failure event is not generally modeled directly e.g. see: Making fault - #2 by Jason.Jonkman.
Best regards,