Ram Gopisetti
Ansys Employee

hi , IDAM controls the mechanism of damage where as MAT_ADD_DAMAGE controls the criteria for the material on when damages forms. for MAT_24 with IDAM 1 has to be used and it is coherent to plastic strain defined in that material card and you dont need to define the FS vale again in the section cards. Moreover, MAT_24 is not good for brittle materials, i hope your research should answer it , there are material models like *MAT_BRITTLE_DAMAGE,*MAT_JOHNSON_HOLMQUIST_CERAMICS and concreat related material for handellign brittle materials. you can explore show deleated elemnts in the view option and see if it helps , 

cheers, ram