Dear Enes,
The theoretical development of the mass matrix used in FAST v7 and the ElastoDyn module of FAST v8 is documented in the “unofficial FAST theory manual” discussed in the following forum topic: FAST theory manual - #72 by Jason.Jonkman. Please send me an e-mail (or your e-mail address) and I’ll reply back with the “unofficial FAST theory manual”.
Best regards,