transformation, and twinning and dislocation generation. These are very general mechanisms of plastic deformation that occur in most crystalline ceramic materials. Plasticity is enhanced at the contact point by further constraining the material by inducing compressive stress over the entire outside layer of the ceramic. This can be done by tighter wrapping of the cloth that encases the laminar composite. In addition to a plastic zone, a cone crack develops at the point of contact and propagates into the solid and eventually completely through the ceramic plate. The highly fractured zone of ceramic material, which generates from the rear surface of the ceramic plate, forms primarily within the boundary of the cone crack. This cone crack plays an important role in transferring momentum from the bullet to the backup plate. The cone is filled with plastically deformed and crushed ceramic material before the impact event is complete.
For the bullet to penetrate into the ceramic plate, ceramic material that is under the bullet flows around the bullet and sprays into the air on the impact side of the plate. As the ceramic material flows away from the front of the bullet, it breaks into small particles of ceramic (10-100 µm). These particles erode the bullet as the crushed ceramic flows past the bullet and sprays into the air on the impact side of the armor. In the most favorable scenario, the bullet is completely eroded away and—if within the design parameters of the insert—eliminated as a fatal threat to the person wearing the vest.
Finally, and for purposes of this committee, the incoming momentum of the bullet has to be transferred to the target. This is first done by momentum transfer to the cone of crushed and deformed ceramic. The force is picked up by the backup plate, which catches the moving ceramic cone. As the base of the cone is very much larger than the apex (1 mm vs. 25 mm radius), the pressure at the base is about 1,000 times less at the base than at the apex. The backup plate then deforms, further absorbing the impact force of the bullet. The final transfer of momentum is to the person wearing the protective vest. This absorption of force ends up in blunt trauma injury, sometimes severe enough to topple the person but not to kill him. It should be noted that the momentum transfer of a bullet is only a hundredth that of severe head contact in American football.