Ballistic-grade armor used to be specified based primarily on material hardness, hence the pervasive use of steel plates in body armor and vehicular plating. When a projectile strikes a hard surface it can either ricochet, shatter, or penetrate the material. The ideal situation is that a bullet will impact the hard surface of the plate and fracture. The impacting force will disperse as the shrapnel from the shattered bullet spreads out, protecting the safe side of the plate. Yet there is no guarantee that bullets will strike armor panels with the ideal trajectory.