The ballistic limit velocity is the velocity when there is 50% probability of complete penetration. Determination of the ballistic limit velocity in different materials is performed according to one of several military test standards. There are several definitions of ballistic limit velocity but the military test standards use the Protection criteria, from which follows v50BL(P) - v50 Ballistic Limit (Protection). The perforation according to v50BL(P) is determined by the condition of a witness plate, usually a thin aluminium plate, placed behind the object to be tested. A complete or partial penetration of the object depends on whether the witness plate has cracks or holes that allow light to pass through it or not. The limit velocity is the velocity at which there is a 50% probability of complete penetration. This experimental series has determined the limit velocity for this type of cracks to appear in two types of aluminium plates when hit by 1.1 g fragment simulating projectiles (FSP). For a 0.5 mm aluminium plate 2024-T4 the limit velocity is 78 m/s and for a 1.0 mm aluminium plate EN-AW 1050A (SS4007) the limit velocity is 99 m/s.