Back-face deformation (BFD)

The indentation left in Roma Plastilina No. 1 clay backing material behind an armour panel after a bullet impact that does not penetrate. Maximum 44 mm under NIJ 0101.06. Exceeding this limit constitutes test failure because the blunt trauma to the wearer's thorax at this deformation level exceeds the 25% serious-injury probability threshold from Wayne State University biomechanical studies.