Can anyone tell me how to draw stress profile curve in FLAC 3D along path AB
Zone → Profile plot item.
There you can pick what value to plot and where the profile line is going to be.
Unfamiliar with FLAC3D v6, but for FLAC3D v7, it is outlined in the software manual here.
In summary: Build Plot > Zone Profile. Then select the zone value and component for your desired stress, set the beginning and end x,y,z and number of points you require along the profile. You are then also able to export this profile from FLAC3D.