The hull (and maybe the turret) should take atleast some damage from the fully penetrating AP-round. Also an AP-round penetrating crew compartments usually kills or atleast severely injures the crew members because of shrapnell and sudden pressure & temperature changes inside the tank.
Video of 120mm APFSDS (and HEAT) round effects outside and inside a tank:
http://www.youtube.com/watch?v=yHbf-Eb3xak