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2 Fundamentals of Lightweight Armor Systems As described in Chapter 1, the path forward for de- Ad = Weight of the armor system/Area being protected velopment of protection materials must consider the armor The units are kilograms per square meter (kg/m2) or, systems that form the context in which those protection materials are used. This chapter presents a brief overview of more commonly in the United States, pounds per square a few armor systems, including the threats to them and the foot. Note that areal density is a physical characteristic of designs for them, to give the reader enough information to the armor and does not indicate if that armor is effective. The inform the discussion. effectiveness of two armor systems can only be assessed by The first section of this chapter discusses how armor sys- comparing their performance against the same threat. The tems are characterized and tested. However, while a general effectiveness of a given armor system is called its mass discussion such as this is valid for all classes of armor sys- effectiveness, Em, a dimensionless quantity that is simply tems, the threats and the design philosophy are completely the ratio of the areal density of rolled homogeneous armor dependent on how the armor system is used. Accordingly, the (RHA), a common steel for tank armor (see Box 2-1 for its following discussion covers the three applications of armor composition) that will stop a particular threat, to the areal systems considered in this study: (1) personnel protection, density of the given armor that will stop that same threat: which includes body armor and helmets, (2) vehicle armor, and (3) transparent armor.1 For each of these applications, Em (Armor) = Ad(RHA)/Ad(Armor) very specific constraints drive the armor design and thus the ultimate choice of protection materials. This chapter pro- The mass effectiveness of an armor system does indeed vides, within the security guidelines discussed in the final indicate how effective it is against a specific threat and section, a general description of the threats and the armor generally suggests whether the system may be considered designs against those threats as well as a brief description lightweight—that is, the higher the Em value, the lighter the of some systems fielded as of 2011. weight of the armor system. However, one of the complica- tions of armor is that Em does not translate from one threat to another; it is even possible that two armor systems will ARMOR SYSTEM PERFORMANCE AND TESTING IN reverse their relative effectiveness against different threats. GENERAL Definition of Armor Performance BOX 2-1 The complexities of armor systems make even the as- Composition of Rolled Homogeneous Armor [L] sessment of weight situationally dependent: What is light- (MIL-DTL-12560) weight for vehicles is extremely heavy for personnel. Thus, in assessing whether an armor system is sufficiently light- • L ow-alloy (Ni-Cr-Mo), high-strength steel (0.26-0.28 percent weight, one cannot look at the absolute weight of the system. C). Rather, because armor is used to protect a particular area, its • Q uenched and tempered (Stage III, 500°C-600°C) material, practical weight is best described by its areal density, Ad: cementite strengthening precipitate:/tempered martensite struc- ture. 1Transparent armor is the technical term for protective transparent mate - rial systems commonly called ballistic-resistant windows. 12

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13 FUNDAMENTALS OF LIGHTWEIGHT ARMOR SYSTEMS FIGURE 2-1 Partial and complete ballistic penetration. In a partial penetration the projectile stops within the armor structure, whereas in a complete penetration, it exits the armor structure. Note that the clay is not part of the armor structure but is placed behind the armor to record its deformation. BFD, back-face deformation. Testing of Armor Systems (DoD) Office of the Inspector General4 described the Army’s testing to certify armor. Although the purchase specifica- This section describes the testing and analysis of com- tion for body armor might seem insensitive, it allows for an plete armor systems. The experimental approaches used “acceptable number of complete and partial penetrations,” to understand the behavior and measure the properties of as shown in Figure 2-1. An additional parameter for body individual materials are discussed in Chapters 3 through 5. armor certification is the maximum depth of the back-face Measurement of both partial and complete penetration deformation for partial penetrations. (Back-face deformation by threats of the separate material composing the system is the depth of the crater left by each partial penetration in and of the full armor system is key to understanding how the clay placed behind the armor during testing with threats. materials are selected for use in armor systems to protect It represents the blunt force trauma inflicted on the wearer, against ballistics. In the case of body armor, in addition to which can contribute to injury or even death.) The accepted the ability of the armor to stop the projectile, there is another deformation of the back face of an armor system is currently requirement—namely, that the deflection of the backside of 44 mm (1.73 in.) or less5 (see Figure 2-1). the armor toward the wearer be small. To assess the different threats against a particular ar- The specifics of the tests used to qualify armor systems mor system, two key measurements, V0 and V50, are made. for field use are well documented and will not be described at V0, the ballistic limit, is “the maximum velocity at which a length here. As an example, the very elaborate requirements particular projectile is expected to consistently fail to pen- for the testing of body armor are described in great detail in etrate armor of given thickness and physical properties at a the National Institute of Justice (NIJ) standard.2 In addition, specified angle of obliquity.”6 If the measured V0 exceeds a recent National Research Council (NRC) report examined the maximum velocity for a particular threat (see Table 2-1) specific aspects of the techniques used to evaluate body ar- the armor system is said to defeat that threat. Essentially, the mor.3 Yet another recent report by the Department of Defense 4Inspector General, Department of Defense. 2009. DoD Testing Re- quirements for Body Armor, Report No. D-2009-047. Available online at http://www.dodig.mil/audit/reports/fy09/09-047.pdf. Last accessed April 2Department of Justice. 2008. Ballistic Resistance of Body Armor, NIJ 15, 2011. 5Department of Justice. 2008. Ballistic Resistance of Body Armor, NIJ Standard–0101.06. Available online at http://www.ncjrs.gov/pdffiles1/ nij/223054.pdf. Last accessed April 15, 2011. Standard–0101.06. Available online at http://www.ncjrs.gov/pdffiles1/ 3NRC. 2009. Phase I Report on Review of the Testing of Body Armor nij/223054.pdf. Last accessed April 15, 2011. 6Department of Defense. 1997. Department of Defense Test Method Materials for Use by the U.S. Army: Letter Report. Washington, D.C.: The National Academies Press. Available online at http://www.nap.edu/catalog. Standard: V50 Ballistic Test for Armor, MIL-STD-662F, December 18. php?record_id=12873. Accessed April 7, 2011. Aberdeen Proving Ground, Md.: U.S. Army Research Laboratory.

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14 OPPORTUNITIES IN PROTECTION MATERIALS SCIENCE AND TECHNOLOGY FOR FUTURE ARMY APPLICATIONS TABLE 2-1 National Institute of Justice (NIJ) Ballistic Aberdeen Test Center (ATC), projectile velocity is measured Threat Standards with optical screens and electronic counters before, inside, and after passing the target.7 The ATC range also has high- Kinetic speed cameras that can capture 6,688 frames per second at Energy Weight Velocity (Relative to full resolution and up to 100,000 frames per second at lower Level Projectile (g) (m/s) Type IIA) resolutions. In addition, flash x-rays can provide a three- dimensional reconstruction of a material’s deformation and Type IIA 9 mm full-metal- 8.0 373± 9.1 1.0 jacketed round nose failure during a ballistic event. 8 It is clear that researchers (FMJ RN) wish for additional real-time measurements on ballistic time .40 S&W FMJ 11.7 352 ± 9.1 1.3 scales both locally and globally in relation to the point of im- Type II 9 mm FMJ RN 8.0 398 ± 9.1 1.1 pact. The ability to make quantitative measurements across .357 magnum 10.2 436 ± 9.1 1.7 many properties would necessitate approaches and methods jacketed soft point wholly beyond those that are currently known. (JSP) Figure 2-2, taken from an earlier NRC study,9 shows a Type IIIA .357 SIG FMJ flat 8.1 448 ± 9.1 1.5 typical range at ATC as well as one at New Lenox Machine nose (FN), Co. .44 magnum 15.6 436 ± 9.1 2.7 semijacketed hollow point (SJHP) Exemplary Threats and Armor Designs Type III 7.62 mm FMJ, steel- 9.6 847 ± 9.1 6.2 Although the testing and definitions described above (rifles) jacketed bullets (U.S. military designation hold for all classes of armor systems, the threats and the M80) design philosophy are completely dependent on how the Type IV .30 caliber armor- 10.8 878 ± 9.1 7.5 armor is used. Thus, each of the three applications focused (armor- piercing (AP) bullets on in this report (personnel, vehicle, and transparent armors) piercing rifle) (U.S. military are treated separately. It should be noted that military armor designation M2 AP) systems are currently purchased according to performance specifications that are classified. Descriptions of threats and designs in this study are taken from the open literature and qualification tests described above ensure that V0 exceeds the documents approved for public release. As such, they are performance specification. only illustrative of current threats and designs. However, the expense of firing and the inability to control projectile velocity exactly makes the determination of 0 percent penetration statistically problematic during the PERSONNEL PROTECTION experimental phase of armor development. The determina- tion of V0 is therefore generally reserved for the final stages Threat of development and qualification. Modern armor for personnel protection includes both For research and development purposes, the use of body armor and combat helmets. The threats for which V50, “the velocity at which complete penetration and par- personnel armor is designed are small-caliber projectiles, tial penetration are equally likely to occur,” is much more including both bullets and fragments. The level of ballistic prevalent. These tests are done with a configuration similar protection of personnel armor is taken as the total kinetic to that in Figure 2-1 but without the clay, which is replaced energy of a single round that the armor can stop.10 The stan- by a “witness plate” placed at a distance behind the armor configuration. A complete penetration event takes place when a thin witness plate is fully penetrated, or perforated, by the projectile; partial (or no) penetration takes place when no perforation of the witness plate is observed. To calculate V50, the highest partial/no penetration velocities and the 7Rooney, J.P. 2008. Army Aberdeen Test Center Light Armor Range Complex. ITEA Journal 29: 347-350. lowest complete penetration velocities are used, generally 8An example of using flash x-rays to observe the sample and projectile with at least 4 and often as many as 10 shots—enough to changes during a penetration event is shown in Figure 2-6, which is dis- make sure there are at least two partial/no and at least two cussed later in this chapter. complete penetrations. 9NRC. 2009. Phase I Report on Review of the Testing of Body Armor During the development of armor systems, it is much Materials for Use by the U.S. Army: Letter Report. Washington, D.C. : The National Academies Press. Available online at http://www.nap.edu/catalog. more important to understand what is actually occurring php?record_id=12873. Accessed April 7, 2011. during the penetration event than it is to simply measure V0 10Montgomery, J.S., and E.S. Chin. 2004. Protecting the future force: or V50. To this end, ballistic ranges are often equipped with A new generation of metallic armors leads the way. AMPTIAC Quarterly an array of sophisticated diagnostic tools. For example, at 8(4): 15-20.

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15 FUNDAMENTALS OF LIGHTWEIGHT ARMOR SYSTEMS FIGURE 2-2 Indoor firing ranges. Depicted are (left) the gun barrel (foreground) and Oehler screens at the light armor range complex, which measure velocity midway between the barrel and target. The target box contains the target being shot at and debris. The red panel collects behind-armor debris. Depicted at right is an alternative setup for a commercial indoor firing range at New Lenox Machine Co. SOURCE: Adapted from John Wallace, Technical Director, ATC, “Body armor test capabilities,” presentation to the Committee to Review the Testing of Body Armor Materials for Use by the U.S. Army, on March 10, 2010. FIGURE 2-3 Examples of 7.62 mm (.30 cal) small arms projectiles. SOURCE: Courtesy of Robert Skaggs. dards set by the NIJ shown in Table 2-111 are for typical bal- an additional constraint on body armor systems. (See the listic threats, although not specifically those for military body preceding discussion on back-face deflection.) armor, which are classified. Note that a Type IV projectile has more than 7.5 times the energy of a Type IIA projectile. Design Considerations for Fielded Systems In addition to surviving the impact of specific projec- tiles (see Figure 2-3), there is generally a requirement to The design of armor for personnel protection depends withstand multiple hits on the same armor panel. For armor on the specific threat. For fragments and lower velocity pen- meeting NIJ Type IIA and Type III standards, panels must etrators, vests are typically made from polymer fibers (see demonstrate the ability to survive six hits without failure. Chapter 5). Advances in fibers for personnel armor began Only Type IV has no multi-hit requirements.12 Personnel with the use of fiberglass and nylon. These were followed protection armor is also often designed against fragments. in the late 1960s by polyaramid fibers (DuPont PRD 29 and Finally, for body armor, as previously mentioned, stop- PRD 49), now called Kevlar. Later, high molecular weight ping penetration is not the only issue. It is also important that polyethylene fibers, made of Spectrashield and Dyneema, when stopping the projectile, the armor itself does not deflect were also used as backing in vests. Zylon, made of polyben- to an extent that would severely injure the wearer. This puts zobisoxazole (PBO), has also been considered. Figure 2-4 depicts how the evolution of fibers has steadily improved the performance of polymer vests. Thus, the primary factor 11Department of Justice. 2008. Ballistic Resistance of Body Armor, in the design of armor for vests is the selection of the fiber. NIJ Standard–0101.06. Available online http://www.ncjrs.gov/pdffiles1/ When the threat increases to rifle rounds, including nij/223054.pdf. Last accessed April 15, 2011. 12Ibid.

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16 OPPORTUNITIES IN PROTECTION MATERIALS SCIENCE AND TECHNOLOGY FOR FUTURE ARMY APPLICATIONS FIGURE 2-4 Increase in ballistic performance as a function of improved fibers. This figure depicts how the V 50 of fiber-based vests has increased as new fibers have been introduced over the years. SOURCE: Philip Cunniff, U.S. Army Natick Soldier Research, Development and Engineering Center, “Fiber research for soldier protection,” presentation to the committee, March 10, 2010. ballistic plates and enhanced side ballistic insert plates are armor-piercing projectiles (see Table 2-1, Types III and IV), inserted into plate carrier pockets in the polymeric vest. ballistic fabric alone is insufficient. Stopping these threats These plates can withstand multiple small-arms hits, includ- requires adding a ceramic plate to the outside of the vest. The ing armor-piercing rounds.16 hard ceramic blunts and/or erodes the projectile nose, which IBA can stop small-arms ballistic threats and fragments, increases the projected area of the projectile and spreads the load across more of the fabric.13 It is the combination thus reducing the number and severity of wounds. An im- provement, the X small-arms protective insert, is designed of two independently developed materials—a ceramic face- for “potential emerging small arms ballistic threats.”17 plate and a fiber fabric—that constitutes the armor system The deltoid and axillary protectors, an integral compo- and provides overall protection. The combination creates a nent of the improved outer tactical vest, extend protection complex system where the performance of the ceramic and against fragments and 9-mm rounds to the upper arm areas the polymer backing (vest) are intimately connected. An (see Figure 1-1).18 extended discussion of ceramics and polymer protection The combination of ceramic inserts and polymeric fibers materials can be found in Chapter 5. in the IBA vest is an example of how particular arrange- The currently fielded body armor, the Interceptor body ments of specific materials make up a typical armor system. armor (IBA), makes use of the combination of ceramic and fiber described above and shown in Figure 2-5.14 The main The complexity goes even further: A change in threat can drastically change the performance of a given armor system. component of this armor is the improved outer tactical vest, Figure 2-6 shows how the Nammo 7.62-mm M993 tungsten which provides protection against fragments and 9-mm rounds.15 Enhanced small-arms protective insert (ESAPI) carbide projectile, with a velocity of 970 m/sec, more easily defeats a B4C ceramic plate than does the Type IV APM2 threat. This indicates how armor systems solutions are inter- 13Montgomery, J.S., and E.S. Chin. 2004. Protecting the future force: twined with the specific threat they are intended to defeat. A new generation of metallic armors leads the way. AMPTIAC Quarterly Because helmets and vests demand similar levels of pro- 8(4): 15-20. 14Inspector General, Department of Defense. 2009. DoD Testing Require- 16U.S. Army. 2010. Interceptor Body Armor (IBA) brochure, October. ments for Body Armor. Report No. D-2009-047, January 29. Available online at http://www.dtic.mil/cgi-bin/GetTRDoc?AD=ADA499208&Loca Available online at https://peosoldier.army.mil/FactSheets/PMSPIE/ tion=U2&doc=GetTRDoc.pdf. Last accessed April 29, 2011. SPIE_SPE_IBA.pdf. Last accessed April 29, 2011. 15Figure 2-5 shows the version of tactical vest before the improved outer 17Ibid. 18Ibid. tactical vest was introduced.

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17 FUNDAMENTALS OF LIGHTWEIGHT ARMOR SYSTEMS FIGURE 2-5 Interceptor body armor. Shown are the various components that make up the Interceptor body armor system (see DoD Inspector General’s Report No. D-2009-047, January 29, 2009). The outer tactical vest, the deltoid axillary protectors, and the carrier for the ESAPI inserts (not shown) are made of Cordura, Kevlar, and/or Twaron fabric. The ESAPI ballistic inserts are composite ceramic plates with bal - listic fiber backing (see the Interceptor body armor [IBA] brochure of the Program Executive Office, Soldier, October 2010). SOURCE: DoD Inspector General. 2009. DoD Testing Requirements for Body Armor. Report No. D-2009-047, January 29. Available online at http://www. dtic.mil/cgi-bin/GetTRDoc?AD=ADA499208&Location=U2&doc=GetTRDoc.pdf. Last accessed April 29, 2011. tection, primary ballistic protection is also based on the per- it must also protect against blunt forces. Equally important, formance of the fiber. However, the currently fielded helmet, the helmet must provide comfort and thermal management the advanced combat helmet (see Box 2-2 for materials of without degrading vision or hearing and be able to interface construction), must not only provide ballistic protection, but with other equipment, including night vision goggles and FIGURE 2-6 Effect of a ballistic threat on perfor- mance. This figure shows X-ray exposures during two impacts on boron carbide plates, each with a different type of projectile. In the top set, a 7.62- mm Type IV APM2 has not yet fully penetrated the ceramic after 25 microseconds. In the bottom set, in the same time frame, the 7.62-mm M993 projectile has begun to exit the ceramic. This is striking evidence of the effect of different threats on the performance of ballistic armor. SOURCE: Adapted from William Gooch, Jr., U.S. Army Re- search Laboratory, “Overview of the development of ceramic armor technology—Past, present and the future,” presentation at the 30th International Con- ference on Advanced Ceramics and Composites, Cocoa Beach, Florida, January 24, 2006.

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18 OPPORTUNITIES IN PROTECTION MATERIALS SCIENCE AND TECHNOLOGY FOR FUTURE ARMY APPLICATIONS used to fashion improvised explosive devices. Countries such as Iran have taken it upon themselves to manufacture many BOX 2-2 sizes of projectiles that are nominally concave metal disks Construction of the Advanced Combat Helmet propelled by large cylindrical high-explosive charges. Specific requirements for the multithreat environment Component materials: to which truck and tactical wheel systems are exposed are • H elmet shell: aramid fabric + resin. defined by the Army’s long-term armor strategy specifica- • C hin strap: Cotton/polyester webbing and foam nape pad, or tions, which are classified. nylon webbing and leather nape pad; foam pads are made of polyurethane. Design Considerations for Fielded Systems SOURCE: U.S. Army. 2010. Advanced Combat Helmet (ACH) brochure, The design of armor systems for vehicles depends on the October. Available online at https://peosoldier.army.mil/Factsheets/PMSPIE/ size of the vehicle, the threat or threats the vehicle is likely SPIE_SPE_ACH.pdf. Last accessed April 29, 2011. to encounter, and, equally important, the weight of the armor that the vehicle can handle. Since the early days of tanks in World War I, metal has been the primary armor material used for large combat vehicles. Table 2-2 gives selected examples of such materials and their applications. weapons.19,20 Ultimately, the weight of the helmet is limited Figure 2-8 depicts the various classes of armor that are by the ability of the neck to bear weight, especially over long in use or under consideration for combat vehicles. This study periods of time. considers only the passive armor systems; electromagnetic, energetic, and smart armor are beyond its scope, as are reac- tive armor systems. VEHICLE ARMOR As with personnel protection, passive vehicle protection While vehicle armor is generally understood to encom- is generally a complicated arrangement of material layers, pass armor systems to protect all classes of vehicles, this each serving a different role in the overall protection sched- study will focus on armor protection for land vehicles such ule. Figure 2-9 schematically depicts one such arrangement as the M1A1/M1A2 Abrams main battle tank, the Bradley that comprises six layers of various materials, including fighting vehicle, the Stryker combat vehicle, and the high- ceramics, metals, and polymers.22 Note that the entire sys- mobility multipurpose wheeled vehicle (HMMWV, or Hum- tem serves many more functions than just protection against vee) (see Figure 2-7). projectiles. Unlike designs for protecting personnel, armor designs for vehicles are less constrained in thickness. This allows for Threat a concept known as “spaced armor,” another option for the Like personnel armor, vehicle armor is also typically arrangement of armor. In spaced armor, a thin armor plate required to protect against small-caliber projectiles and is separated from the main armor system with the goal of fragments. In addition, however, it is required to stop a host breaking up or disrupting the projectile, thus making it easier of other threats. These include medium- and large-caliber for the remainder of the armor to stop it. This concept was ballistic threats (20-140 mm);21 shaped charge munitions, as used by the Germans in World War II23 and in various armor depicted in Box 2-3; and chemical energy munitions. Rocket- configurations since. It should also be noted that, even if the propelled grenades are ubiquitous in the world of terrorists threat does not completely exit the armor, pieces of the back owing to the efforts of the countries that manufacture them face can be accelerated by the shock wave, creating spall, to market them to developing countries. Because little effort which can have sufficient velocity to considerably damage was made to destroy ammunition dumps during the invasion people and equipment inside the vehicle. Thus, armor design of Iraq, the artillery projectiles left behind have since been must minimize behind-the-armor damage, which can ad- 19U.S. Army. 2010. Advanced Combat Helmet (ACH) brochure, Octo - ber. Available online at https://peosoldier.army.mil/Factsheets/PMSPIE/ SPIE_SPE_ACH.pdf. Last accessed April 29, 2011. 20Walsh, S.M., B.R. Scott, T.L. Jones, K. Cho, and J. Wolbert. 2008. 22William Gooch, Jr., U.S. Army Research Laboratory, “Overview of the A materials approach in the development of multi-threat warfighter head protection, December. Available online at http://www.dtic.mil/cgi-bin/Ge development of ceramic armor technology—Past, present and the future,” tTRDoc?AD=ADA504397&Location=U2&doc=GetTRDoc.pdf. Last ac- presentation at the 30th International Conference on Advanced Ceramics cessed April 29, 2011. and Composites, Cocoa Beach, Fla., January 24, 2006. 21Normandia, M.J., J.C. LaSalvia, W.A. Gooch Jr., J.W. McCauley, and 23A. Hurlich. 1950. Spaced Armor. Available online at http://www.dtic. A.M. Rajendran. 2004. Protecting the future force: Ceramics research leads mil/cgi-bin/GetTRDoc?AD=ADA954865&Location=U2&doc=GetTRD to improved armor performance. AMPTIAC Quarterly 8(4): 21-27. oc.pdf. Last accessed April 29, 2011.

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19 FUNDAMENTALS OF LIGHTWEIGHT ARMOR SYSTEMS FIGURE 2-7 Examples of Army combat vehicles. This figure portrays a subset of combat vehicles for which ballistic and/or blast protection is a critical consideration. SOURCE: Photo courtesy of the U.S. Army. TABLE 2-2 Metallic Armor Materials Military BOX 2-3 Metal Specification Application Shaped Charge Characteristics Rolled MIL-DTL-12560 M1A1/M1A2 Abrams homogeneous light armored vehicle, above beltline Shaped charges are made by inverting a soft metal cone (typically armor copper) that will be propelled by an explosive charge to velocities High- MIL-DTL-46100 M1A1/M1A2 Abrams near 10,000 m/s. The copper slug is hydrodynamically driven into the hardness steel light armored vehicle, below beltline plastic regime and stretches continuously as it is propelled forward armor toward the target. At some point it starts to separate into a string of Aluminum MIL-DTL-46027 M113 armored personnel carrier liquidlike particles, but before this occurs it is just like a long rod alloy 5083- M109 Paladin self-propelled howitzer penetrator that is traveling at supersonic speed and will penetrate great H131 Bradley fighting vehicle, lower half thicknesses. The optimum standoff distance for a chemical energy Aluminum MIL-DTL-46063 Bradley fighting vehicle, upper half penetrator is between 2.4 and 4 cone diameters. At this distance, the alloy 7039- penetrator will not have started to fragment before it hits the target. T64 The example below shows (1) copper liner, (2) charge, (3) body, (4) SOURCE: Montgomery, J.S., and E.S. Chin. 2004. Protecting the future booster, and (5) initiation charge. force: A new generation of metallic armors leads the way. AMPTIAC Quarterly 8(4): 15-20. versely affect the survival of the crew even if the projectile is stopped.24 Before the start of the current conflicts, light vehicles (e.g., Humvees and light trucks) were lightly armored if at all. However, unanticipated threats began to be seen—for ex- ample, rocket-propelled grenades and improvised explosive devices—causing a rethinking of that approach. Programs to quickly up-armor the Humvees and other vehicles were 24Prakash, A. 2004. Virtual Experiments to Determine Behind-Armor Debris for Survivability Analysis, December. Available online at http:// www.dtic.mil/cgi-bin/GetTRDoc?AD=ADA433014&Location=U2&doc= GetTRDoc.pdf. Last accessed April 29, 2011.

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20 OPPORTUNITIES IN PROTECTION MATERIALS SCIENCE AND TECHNOLOGY FOR FUTURE ARMY APPLICATIONS FIGURE 2-8 Examples of vehicle protection. This figure shows the many types of protection systems that are used or under consideration for Army combat vehicles. This study looks at only those materials that passively protect the vehicle from ballistics and blast threats. SOURCE: Christopher Hoppel, Chief, High Rate Mechanics and Failure Branch, Army Research Laboratory, “Multi-scale modeling of armor materi - als,” presentation to the committee, March 10, 2010. established. Since August 2004, all Marine Corps vehicles operating outside the forward operating bases have had their armor protection upgraded.25 Consequently, there is now a very large array of armor combinations, often with one kit laid on top of the other, making the scheme shown in Fig- ure 2-9 simple by comparison. Since a bomb blast severely damaged the U.S.S. Cole on October 12, 2000, taking 19 lives, the Navy has also shown FIGURE 2-9 Schematic of vehicle armor protection system. The armor is made of many layers, each with a different overall function. more interest in developing structures that can survive a In this construct, ballistic protection is obtained primarily through blast. A Navy multidisciplinary research program known as the ceramic tile and composite backing. The composite faceplate Integrated Cellular Materials Approach to Force Protection also contributes to the protective properties of the vehicle armor, is developing complex, topologically designed sandwich while the ballistic components contribute to the structural integrity panels for this application. Like vehicle armor, these panels of the armor. Other configurations (not shown) might include a must protect against both ballistic and other threats. structure designed primarily for blast resistance. SOURCE: Wil- liam Gooch, Jr., U.S. Army Research Laboratory, “Overview of the development of ceramic armor technology—Past, present and TRANSPARENT ARMOR the future,” presentation at the 30th International Conference on Advanced Ceramics and Composites, Cocoa Beach, Fla., January Threat 24, 2006. The windshields and side windows of vehicles such as Humvees and trucks are an important application for trans- electromagnetic fields or lasers. This study, however, will parent armor. Currently, such windows are designed to pro- cover only the ballistic requirements of transparent armor. tect against armor-piercing threats as well as high-velocity The specifications for transparent armor are called out fragments. In addition, they must be able to withstand mul- in Army Tank Purchase Description (ATPD) 2352P, July 7, tiple hits and to fracture in a way that maintains their struc - 2008,26 which describes the general characteristics that trans- tural integrity and transparency. Advanced applications of parent armor must possess to qualify for purchase. These transparent armor often demand additional protection against 25Gen. William L. Nyland, Assistant Commandant of the Marine Corps, 26ATPD 2352P, July 7, 2008, supersedes ATPD 2352N, January 3, 2008. and Major General (Select) William D. Catto, Commanding General Marine ATPD 2352 defines a standardized four-shot pattern and is used throughout Corps Systems Command, Statement before the House Armed Services the Army to provide consistent criteria for evaluating multiple impacts on Committee on Marine Corps vehicle armoring and improvised explosive transparent armor. ATPD 2352P is available at https://aais.ria.army.mil/ device countermeasures, June 21, 2005. AAIS/award_web_09/W52H0909A00030000/Award_attach/Attach1.pdf.

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21 FUNDAMENTALS OF LIGHTWEIGHT ARMOR SYSTEMS FIGURE 2-10 Example of transparent armor for a vehicle window. SOURCE: Stephan Bless, Institute for Advanced Technology, University of Texas at Austin, “Transparent armor research issues,” presentation to the committee, March 10, 2010. FROM ARMOR SYSTEMS TO PROTECTION specs also set forth criteria for the environmental effects MATERIALS of transparent armor. As with other documents on applica- tions of armor, DTA184044, the document that describes The goal of armor system development is (1) to continu- the threats that must be defeated, is contained in a classified ally decrease the weight—that is, to increase Em—required to appendix and so cannot be elaborated on here. protect against a given threat or (2) to not increase the weight required to protect against a greater threat. According to the Design Considerations for Fielded Systems Army, new armor systems can in fact be delivered to the field relatively quickly. However, this is generally because new In contrast to conventional opaque ceramic armors, the armor configurations and materials are not radically different design of transparent armor is often driven by the multi-hit from those that have already been demonstrated to be effec- requirement, a requirement mostly achieved by layering tive. This is best illustrated by looking at how one presently (see Figures 2-10 and 5-14). A typical transparent armor designs an armor system in response to a new threat. uses a layer of glass or glass ceramic followed by a layer of While there is no unique way to design an armor sys- polycarbonate and then other similar layers until seven or tem, Figure 2-11 reflects what the study committee heard more have been stacked and bonded with polyvinyl butyral from several presenters representing the Army Research adhesive layers. While the backing of transparent armor is Laboratory and from other invited speakers. Figure 2-11 also primarily polycarbonate, other polymeric materials, such as demonstrates several major limitations that impact protection polyurethane, are showing some potential. materials research. These will be addressed in subsequent Most current armor windows are laminates of glass and chapters. plastic.27 The three main transparent ceramic candidates are currently aluminum oxynitride (AlON), magnesium alumi- Existing Paradigm nate spinel (MgAl2O4), commonly referred to as spinel, and single-crystal aluminum oxide (Al2O3-sapphire).28 These In response to a new threat against which current ar- materials are described further in Chapter 5. mor systems fail, a new armor system concept—including geometry, configuration, and materials—is chosen that, from experience, designers hope will defeat the new threat. Changes in geometry can be as simple as adding thicknesses 27Patel, P.J., G.A. Gilde, P.G. Dehmer, and J.W. McCauley. 2000. Trans - to various layers in an existing configuration; possible, but parent armor. The AMPTIAC Newsletter 4(3): 1, 2-5, 13. less likely, is an entirely new design. Materials are chosen 28Sands, J.M., P.J. Patel, P.G. Dehmer, A.J. Hsieh, and M.C. Boyce. 2004. from a set of available materials whose ballistic and blast Protecting the force: Transparent materials safeguard the Army’s vision. performance have already been proven both as individual AMPTIAC Quarterly 8(4): 28-36.

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22 OPPORTUNITIES IN PROTECTION MATERIALS SCIENCE AND TECHNOLOGY FOR FUTURE ARMY APPLICATIONS NEW THREAT Armor Concept Select from Materials Research (Geometry Available and Development Configuration) Materials Fail Pass Make & Shoot Ballistic Shoot or Evaluation Model Model Fail Modeling and Select from M&S Pass Simulation Available Evaluation Research and Models/Codes Development NEW ARMOR FIGURE 2-11 Current paradigm for armor design. In response to a new threat, a new concept and materials are chosen and then tested or modeled depending on several factors. Armor that fails ballistic testing is redesigned and the (costly) process begins again. The goal is to reduce repetitive looping by making better use of modeling and simulation. It should be noted that, while computations are sometimes used, the shoot-and-look mode is much more common. In addition, the materials research and development community and the modeling and simulation community are not particularly well connected. there are limitations to the effectiveness of modeling and materials and combined with other materials. While much simulation. If the properties of the materials—that is, the excellent materials research is under way, emerging research constitutive relations needed to run computational material materials are seldom, if ever, chosen for new armor because models for them—are not known, then the modeling would there is no way to directly tie how they perform in a re- have to use information from the most similar existing mate- search environment to how they will perform in the actual rial, making the result uncertain. This is another reason why armor configuration. Moreover, most nonarmor applications armor designers do not consider using research materials materials are chosen according to their bulk quasi-static that have not yet been sufficiently characterized under ap- properties, such as hardness, strength, and toughness, even propriate dynamic conditions (see Chapter 4). Consequently, though such properties do not always predict the materials’ modeling and simulation are often used more as a guide to ballistic or blast performance. This issue will be discussed identify trends due to design changes than as a source of extensively in Chapter 3. absolute results. Thus, even configurations that survive the The next step is deciding how to evaluate the candidate modeling and simulation step may fail ballistic testing. Chap- armor system. Although it might seem intuitive to run simu- ter 4 elaborates on the limitations of how well the material lations before expensive testing, the decision on how to test can be modeled and addresses shortcomings in the models the new configuration actually depends on several factors. If themselves. the armor varies only slightly from existing armor, then the Armor that fails ballistic testing is redesigned and the most expedient method might well be to build the armor and process begins again. Once there is a successful ballistic go straight to a ballistic evaluation. However, if the armor test, the armor will be constructed in sufficient quantities design is significantly different from current armor systems,

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23 FUNDAMENTALS OF LIGHTWEIGHT ARMOR SYSTEMS to qualify the new configuration for fielding. It can only be designs against specific threats. These limitations extend hoped that a new armor design comes about without too to the ability to test armor systems with militarily relevant many loops (as shown in the Figure 2-11 diagram), which are threats. The details of specific threats and design are gener- costly in terms of effort. More important, repetitive looping ally not published in the open literature. Even the availability delays the fielding of new armors, which in turn adversely of information on armor systems that is not proprietary or impacts the safety of troops. At the same time, speed must classified is often restricted by DoD to DoD and contractors be offset by the need to make sure the armor will perform as to DoD. The underlying technical basis for these restrictions expected in the field. may be available to researchers working on armor under con- As described, the current armor design paradigm clearly tract to DoD, but it is not generally available to researchers makes it difficult to incorporate a new material into existing outside of that context. armor systems or to use it in an entirely new design. Any path In addition, there are export control restrictions that forward for future generations of lightweight armor materi- generally limit the distribution of information to U.S. citi - als must alter this paradigm, although it will not be an easy zens and lawful permanent residents. The restrictions that undertaking. As has been shown in this chapter, the ultimate apply to armor materials cover almost all of the relevant performance of an armor system depends on the materials protection materials, including ceramics near theoretical used and on the geometric arrangement of those materials, density—among them B4C (boron carbide), SiC (silicon both of which vary according to threat and the application. carbide), and Al2O3 (aluminum oxide, or alumina), discussed The challenge is to represent the complications inherent in in Chapter 5—composite materials, arrays of woven cloth, materials in a way that allows designers to focus on the ma- metals, and ceramics, and layers of metals. terials independently of the specific armor system design or Information in the public domain as defined in 22 CFR threat that the armor is intended to thwart. As will be shown 120.11 is generally not subject to International Traffic in in Chapter 3, meeting this challenge will require an under- Arms Regulations (ITAR). The definition of “technical data” standing of how to relate the behavior of a material—espe- that is subject to the export control regulations does not in- cially its failure behavior—during ballistic or blast events clude “information concerning general scientific, mathemati- to its initial structure and composition. Chapter 4 describes cal or engineering principles commonly taught in colleges and universities or information in the public domain.”29 approaches for successfully predicting the theoretical and experimental failure behavior of a protection material for the The combination of security regulations and ITAR benefit of the materials research community. Finally, it will makes it extremely difficult for fundamental research in pro- be important to convey information about armor performance tection materials to connect to the development of restricted to those developing armor materials. armor systems. Ultimately much of work on armor is restrict- ed. For example, a quick search of the Defense Technical Information Center database for “vehicle armor” indicates SECURITY AND EXPORT CONTROLS that only about 30 percent of the technical documents from It is important to acknowledge the security restrictions 2005 through 2010 are approved for public release. Clear, that surround protection materials. Such limitations are up-to-date boundaries need to be specified between restricted prudent and necessary but require periodic review to ensure and unrestricted information and related research. Such a they are consistent with the current state of open knowledge review, however, is beyond the scope of this report. and do not unnecessarily restrict the exchange of information This report is a public document, and its content is with an open research community when such an exchange limited to general descriptions of threats, performance, and would be beneficial to national security. design that may be discussed without restriction. While the The information content of this study, which deals with restrictions discussed above suffice for the needs of this armor systems and armor performance, is bound by both study, it is important to note that they can significantly com - security regulations and export control law. The security plicate the use of available information in basic research. limitations generally imposed by the Army in this area re- 2922 CFR 120.10(a)(5). strict the discussion of performance of certain armor system