Back face deformation (BFD), also known as “back face signature” or “back face deflection,” refers to the deformation that occurs to the wear side of armor when struck with a projectile. It is an important factor in assessing the effectiveness of body armor in stopping or mitigating the impact of ballistic threats.
When a bullet strikes body armor, the energy from the impact is absorbed and dispersed throughout the armor material. Some of this energy is transmitted through the armor and can cause the material on the opposite side (the side facing the wearer’s body) to deform. This deformation can potentially cause injury or even death to the wearer if it is excessive, even if the bullet itself did not penetrate the armor. This is why BFD is so important.
Back face deformation itself is measured in millimeters and is used as a standard metric for evaluating the safety and performance of body armor. Due to its importance, The National Institute of Justice (NIJ) has established maximum allowable limits for back face deformation at 44mm or 1.7inch. These limits ensure that the resulting deformation from stopping a rated ballistic threat remains within safe limits.