Analysis of Multiple-Impact Ballistic Performance of a Tempered Glass Laminate with a Strike Face Film

Multiple high velocity impacts are of particular concern to bullet resistant glass systems incorporating commercial tempered or heat strengthened glass as exterior or strike faces. Initial impact by a single projectile typically produces a quasi-symmetrical crater exhibiting the typical mirror, mist, and hackle regions with radial cracks propagating across the entire strike face. These radial cracks effectively reduce system survivability during subsequent impacts. Should an impact from a second projectile occur on the damaged glass, loosely constrained fragments would be forced in a direction away from the strike face resulting in less resistance to projectile penetration. Since a greater amount of material is ejected from the strike face, the second crater is often significantly larger than the first and exhibits extreme geometrical variation. The sequence of multiple impacts could be deduced from the size and geometry of the existing craters as these characteristics are relatively dependent on the radial crack pathways from the previous impacts. If the glass fragments were constrained in the strike face, it may be possible to increase the extent to which they interact with the projectile, each other, and the remaining material in lateral directions, further facilitating energy dissipation while reducing crater size. One proposed solution explored the simple application of a window film to the strike face with the purpose of increasing penetration resistance by way of limiting material loss. A three-dimensional laser scanner and analytical software were employed to capture a point cloud of each crater and facilitated dimensional analysis and comparison. Digital quantification of crater characteristics supported the findings that utilizing a film to confine fragments on the strike face effectively decreased the average crater size and improved the material performance during subsequent ballistic impacts in protective glass systems.