Dear Dayuan,
FAST v7 or the ElastoDyn module of FAST v8 does not calculate a total blade stiffness matrix per se because the equations of motion consider nonlinearities. For blades, stiffness arises both from elasticity and geometric nonlinearities (centrifugal stiffness). The elastic component is linear, based on the specified distributed bending stiffness and curvature of the specified mode shapes. The geometric stiffness is never formulated as a stiffness matrix directly in FAST. However, the effective stiffness brought about by centrifugal effects is explained in Section 3.2 of my old Master’s thesis, although not all of the information is up-to-date (it is about 13 years old), it doesn’t cover everything, and there a few errors: nrel.gov/docs/fy04osti/34755.pdf (see Eq. (3.77)).
More information on the FAST theory basis is provided in the following forum topic: Coupled blade modes in FAST.
Best regards,