Progress in Transparent Armor

There is a growing need for transparent armor that can provide good visibility with ballistic protedion. 0 Thesoldiermaneuveringaground combat vehicle through hostile fire must be able to see outside to navigate. The windshields of military helicopters must be both transparent and ballistically pr&ective. As terrorist violence increases, the need grows for armored limou-sines that completely shield their executive and diplomatic passengers. Banks and command centers need security enclosures that provide protection while allowing personnel to monitor the surrounding area. Since 1965, significant advances in armor design have been part of the mission of the Army Materials and Mechanics Research Center (AMMRC) in Watertown, Massachusetts, five miles west of Boston. In its charter as the Army’s lead laboratory for materials research and development, the AMMRC is unsurpassed in the nation, and probably in the world, in the field of transparent armor materials. Earlier Developments In the 1940s and 19509, transparent armors were laminated glass assemblies popularly known as “bulletproof glass”. Their uses were limited: attaining sufficient ballistic protection required materials of great weight and thickness, with consequent low light transmission. By the 19609, the laminated glass armors used in WWII and the Korean conflict were not practical for the new lightweight vehicle concepts designed for increased payload and mobility. Following the principles of o- paque armor design introduced in the 19608, transparent armor evolved from laminated glass to glasdplas-tic laminates which offered significant reductions in weight and thickness. Tests on transparent ceramic materials, such as single-crystal aluminum oxide laminatedto plastic, have shown high ballistic performance levels. However, the lack of availability in sufficient sizes and quantities of transparent ceramics has precluded their use in these applications. Current process development is endeavoring to establish the manufacturing technology to produce adequate sizes of transparent ceramics at acceptable cost and production rates. Meanwhile, the Army’s persistent requirements for improved optics, greater ballistic protection, and lighter weight are being addressed by development of more efficient glass/plastic armor systems. Stopping Projectiles Both private industry and U.S. Army laboratories have conducted armor development programs to investigate materials and their geometric synergism for enhancing d e feat of kinetic-energy projectiles. Results have generally indicated that the optimum configuration is as shown in figure 1. The exterior of transparent armor consists of either one ply or a number of laminated plies of glass selected for its ballistic performance. Except for the introduction of new kinds of glass materials, this face of the armor is similar to laminated “bulletproof glass.” However, the old glass armor assemblies had a glass interior face, and impads often caused dangerous secondary projectiles, or spall, to break off from the inner layer and endanger personnel. To counteract this problem, the new transparent armor incorporates a tough plastic back plate bonded to the glass laminate, which serves a dual role. First, it acts as a “catchjuly-august 1985 21 rpall emerged from the back of this new plastlc-glasr ectile at a range of 250 meters. The projectile is tung8 appreciable thickness of laminated soda-lime glass. This limitation is worsened as ambient light decreases, in such cases as overcast sky or near-dusk conditions. The requirement for high performance, lightweight armored vehicles has compounded the visibility handicap, b e cause increased road speed and advanced reconnaissance and combat requirements make greater demands on the vision of operating personnel. Prototype glass/plastic vision blocks developed by AMMRC to replace the conventional laminated glass blocks have shown impressive optical and ballistic improvements. Comparing the two, the prototype glass/plastic block weighs 15.7 pounds, versus the 18.4 pounds of the earlier system. The new blocks have an in-line light transmission of 75 per cent, compared to 63 per cent on the old blocks. And the newer blocks will protect against 14.4-mm (BS41 API) and 20-mm (HVAP-T) Soviet rounds at a 45 per cent obliquity. (see Fig. 4, above) F’igure 4 shows the prototype glass/ plastic vision blocks and the projectiles they are designed to defeat. The block shown on the right has successfully defeated a 20-mm HVAP-T tungsten carbide projectile at a range of 250 meters. Evolutionary kpvementsin transparent glass/plastic laminates will B I ;te . aminate vision block despite being n carbide and is seen at far left with continue, keeping in pace with the Army’s increasingrequirements. The future of transparent armor includes: New types of glass, both amorphous and crystallized, and transparent ceramics should be pursued for improvements in ballistic efficienCY. 0 Coatings must be developed to make the plastic rear face armor more durable by increasing ita resistance to scratching and chemical attack. The development of new inter-layers promises to reduce manufacturing costs. 0 Analytical methods are now being investigated to correlate construction parameters with ballistic performance for opaque materials. Extension of these correlations to transparent materials may minimize trial and error in determining the most efficient composite construction. 0 Innovative approaches will be developed to counter new threats, such as direded energy devices and weapons. Transparent armor development should continue evolving: to enhance the survivability of the soldier and the materiel upon which both he and the mission depend; and to be pre pared to defeat the threats anticipated throough the end of this century. I hit by a 20-mm HVAP autocannon I a .30-cal ball round for comparison.

GORDON R. PARSONS,

a veteran mechanical engineer with 16 years’ service at the U.S. Army Materials and Mechanics Research Center, has been a major participant in thedevelopment of armored transparencies for helicopters and ground vehicles, and has published many papers on the subject. He holds a BS degree and a masters in mechanical engineering (materials) from Northeastern University. Currently, he is a supervisory materials engineer in the Composite Development Division of AMMRC’s Organic Materials Laboratory. 22