Materials for Blast and Penetration Resistance

While only one part of the entire spectrum of structural protection technologies, materials is certainly one of its cornerstones. The focus of this article is on materials instrumental to providing protection against explosive blasts and projectile or fragment penetration of critical structures. Armors, or penetration resistant materials, are the largest classification of BPRMs, but the term doesn’t do justice to what materials and materials systems are available for today’s protective shielding. Traditionally, when we think of armor, heavy military tanks on a battlefield roll into our imagination. Armor connotes a thick layer, typically steel, employed as a shield from projectiles fired from various weaponry. While such images accurately depict traditional armor systems, they are severely dated and do not reflect the advanced protective materials and systems available today. Protection against blast and penetrating fragments is accomplished with a plethora of materials and material combinations; sophisticated systems engineered to mitigate equally sophisticated projectiles and explosives. The demand for well-engineered structures has dictated drastic improvements in ductility, fracture toughness, corrosion resistance and machinability of constituent materials. This in turn has driven materials engineers to conquer new problems. The threat of terrorist attack has heightened awareness of the need for better buildings. These improved structures will protect occupants from the shock pressure of a blast, and the biggest dangers to personnel in a building: fragmenting structural materials, breaking glass, and building collapse. For buildings, weight plays some role, but cost is typically the driving force. Another key consideration in structural protection of buildings is that in most cases, blast and penetration protection schemes are installed as retrofits. Beyond the limitations of cost, the occupied volume of the installed materials and its impact on building form, fit and function is also critical and a major materials selection concern. Barring building collapse, fragmenting wall structures and flying window glass would be the main cause of occupant casualties. Primary structural support in most buildings is provided by regularly spaced, reinforced concrete support columns, connecting rigid concrete and steel floor sections. The interior and exterior walls are typically not load bearing and are usually made of hollow concrete blocks, or metal or wood frames, covered by metal, wood, or gypsum board panels. All of these materials will deform, displace, and/or completely disintegrate when exposed to significant overpressures associated with bomb blasts, posing the greatest threat to loss of life. There is ongoing research to replace or cover many of these materials with polymer, fabric, steel, or laminate sheeting that would maintain the aesthetic appearance of the building, while standing up to the shock pressure and not fragmenting or containing fragments. Combinations of these materials with foam cores may even help to mitigate the shock loading and further protect occupants from blast overpressures. For existing buildings, some researchers have proposed spray-on coatings that could achieve many of these goals without drastic remodeling. In the Defense community, recent routine exposure of military personnel to operations in urban settings has raised the question of how to mitigate acts of terrorism in close proximity. The issue becomes one of retrofitting existing buildings quickly without serious degradation of mobility or performance. This is especially critical, as most domestic American civilian and government buildings currently offer little in the way of protection from explosions. As we discuss the materials herein, you will begin to see how they may be incorporated into a facility for protection, based on the individual needs of each structure. Those protection needs are dictated by the desired level of protection the building must provide to its occupants or its ability to function. The three levels of enhanced protection described in Table 1 outline in broad strokes what the needs of a building might be. These levels give a general view of the danger occupants might face, but they also relate to what types of solutions would have to be employed to make them blast resistant. For instance, the Level 1 building will have to employ expensive structural hardening measures to make up for its lack of standoff. The Level 2 building is less likely to be targeted because of active security measures, but if it were hit damage would be extensive.