Since the beginning of armed conflict, armor has played a significant role in the protection of warriors. In present-day conflicts, armor has inarguably saved countless lives. Over the course of history—and especially in modern times—the introduction of new materials and improvements in the materials already used to construct armor have led to better protection and a reduction in the weight of the armor. Body armor, for example, has progressed from the leather skins of antiquity, through the flak jackets of World War II to today’s highly sophisticated designs that exploit ceramic plates and polymeric fibers to protect a person against direct strikes from armor-piercing projectiles (Figure 1-1). The advances in vehicle armor capabilities have similarly been driven by new materials, as shown in Figures 1-2 and 1-3.
But even with such advances in materials, the weight of the armor required to manage threats of ever-increasing destructive capability presents a huge challenge. For example, body armor, which presently constitutes almost 30 percent of a soldier’s fighting load,1 is the single largest weight carried by an Army rifle squad. For vehicles, up-armored Humvees have reached the limit beyond which armor cannot be added without “compromising essential vehicle capabilities.”2
The challenge for protective material developers, made clear by current military engagements, is twofold: (1) to ensure the rapid (re)design and manufacture of armor systems optimized against specific threats and (2) at the same time, ensure that these systems are as lightweight as possible. As described above, many of the advances in the performance of lightweight armor have historically come from the introduction of new or improved materials. However, it has become increasingly difficult to produce new materials with properties that allow the design of complex new armor systems or the rapid iterations of such designs. Not only must a material be quickly identified, but it must also be reliably produced,
1Dean, C. 2008. The modern warrior’s combat load: Dismounted operations in Afghanistan. 2003. Medicine and Science in Sports & Exercise 40(5): 60.
2Inspector General, U.S. Department of Defense. 2009. Procurement and delivery of joint service armor protected vehicles. Report No. D-2009-046. Available online at http://www.dodig.mil/audit/reports/fy09/09-046.pdf. Accessed April 7, 2001.
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1 Overview INTRODUCTION Since the beginning of armed conflict, armor has played a significant role in the protection of warriors. In present-day conflicts, armor has inarguably saved countless lives. Over the course of history—and especially in modern times—the introduction of new materials and improvements in the materials already used to construct armor have led to better protection and a reduction in the weight of the armor. Body armor, for example, has progressed from the leather skins of antiquity, through the flak jackets of World War II to today’s highly sophisticated designs that exploit ceramic plates and polymeric fibers to protect a person against direct strikes from armor-piercing projectiles (Figure 1-1). The advances in vehicle armor capabilities have similarly been driven by new materials, as shown in Figures 1-2 and 1-3. But even with such advances in materials, the weight of the armor required to manage threats of ever-increasing de- structive capability presents a huge challenge. For example, body armor, which presently constitutes almost 30 percent of a soldier’s fighting load,1 is the single largest weight carried by an Army rifle squad. For vehicles, up-armored Humvees have reached the limit beyond which armor cannot be added without “compromising essential vehicle capabilities.”2 FIGURE 1-1 A soldier wearing protective equipment. SOURCE: Adapted from Gaston Bathalon, Commander, U.S. Army Research Institute of Environmental Medicine, “The Soldier as a Decisive The Challenge Weapon: USAMRMC soldier focused research,” presentation to The challenge for protective material developers, made the Board on Army Science and Technology on February 15, 2011. clear by current military engagements, is twofold: (1) to en- sure the rapid (re)design and manufacture of armor systems optimized against specific threats and (2) at the same time, ensure that these systems are as lightweight as possible. As described above, many of the advances in the performance of lightweight armor have historically come from the introduc- 1Dean, C. 2008. The modern warrior’s combat load: Dismounted op - erations in Afghanistan. 2003. Medicine and Science in Sports & Exercise tion of new or improved materials. However, it has become 40(5): 60. increasingly difficult to produce new materials with proper- 2Inspector General, U.S. Department of Defense. 2009. Procurement and ties that allow the design of complex new armor systems or delivery of joint service armor protected vehicles. Report No. D-2009-046. the rapid iterations of such designs. Not only must a material Available online at http://www.dodig.mil/audit/reports/fy09/09-046.pdf. be quickly identified, but it must also be reliably produced, Accessed April 7, 2001. 7
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8 OPPORTUNITIES IN PROTECTION MATERIALS SCIENCE AND TECHNOLOGY FOR FUTURE ARMY APPLICATIONS Security,3 describes how, like advances in armor, the “vast majority of disruptive technologies since the start of the in- dustrial revolution” have been due to materials innovations, but that “the insertion of new materials technologies has become much more difficult and less frequent” as materials development fails to keep pace with the rapid design pro- cess. This describes exactly the problems experienced with development of the new protection materials that are the focus of this study. The Integrated Computational Materials Engineering (ICME) report cites many advances and several examples of successful implementation. It advocates pushing the large body of existing computational materials science to the next step. Unfortunately, while “the optimization of the materials, manufacturing processes, and component design” is well described in the ICME report, the path forward for FIGURE 1-2 Up-armored high-mobility multipurpose wheeled protection materials is far more complicated, since designs vehicle (HMMWV, or Humvee). SOURCE: Available at http:// must deal with highly nonlinear and large deformations typi- www.militaryfactory.com/armor/imgs/hmmwv-m1114uah.jpg. Courtesy of U.S. DoD. cally not encountered in commercial products, where applied stresses are kept well below the elastic limit in the linear re- gime. Simply put, the key materials properties—for example, which is not currently possible with the extensive, costly, tensile strength and toughness—that inform the design of and time-consuming practice that is perhaps best described commercial structures and devices are well established and as “build it, shoot it, and then look at it.” This problem, in- extensively measured. Such is not the case for armor. cluding specific recommendations for areas of investigation, The armor that protects U.S. fighting forces is seldom will be addressed further at the end of Chapter 3. a single, homogeneous material. More often than not, what This seeming technological inability to keep up with is called “armor” is actually a complex system constructed evolving needs is not exclusive to protection materials. A of several, often quite different, materials arranged in a very recent National Research Council (NRC) study, Integrated specific configuration designed to protect against a particular Computational Materials Engineering: A Transformational threat. As will be discussed extensively in this study, the Discipline for Improved Competitiveness and National properties and behavior of a protection material must be considered in the specific context of how it will be used in the construction of a particular armor system. Further, there is often little understanding of how to link specific material properties to the actual behavior of the materials and armor systems during the many types of ballistic and blast events. It is often the case that new protection materials have not been well characterized with respect to strain rates, pressures, and the like under appropriate conditions, either alone or as part of an armor system, and databases for materials’ performance and constitutive relationships are often not available. This is especially true at the high strains and very high strain rates relevant to ballistic and blast threats. This gap in knowledge greatly limits the ability of simulation codes to play a sig - nificant role in guiding the development of new materials. Moreover, the design philosophy is completely dependent on how the armor system is to be used. FIGURE 1-3 Areal density of armor versus time, demonstrating In this study, the committee was guided by military ap- that new lightweight materials such as titanium, aluminum, and plications that necessitate lightweight armor, with particular ceramics have provided increased protection at a lower weight per emphasis on (1) personnel protection, which includes body unit area over time. The flattening curve illustrates that the chal- armor and helmets, (2) vehicle armor, and (3) transparent lenge for the future is to be able to continue to decrease the areal density of the armor despite increased threats. SOURCE: Adapted from Fink, B.K. 2000. Performance Metrics for Composite Inte- 3NRC. 2008. Integrated computational systems engineering: A transfor- gral Armor. ARL-RP-8. Aberdeen Proving Ground, Md.: Army mational discipline for improved competitiveness and national security. Research Laboratory. Washington, D.C.: The National Academies Press.
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9 OVERVIEW armor for face shields, vehicle windows, and other applica- The sponsor further requested that the study not include tions requiring transparency. For each of these applications, restricted material so as to permit wide dissemination of system-level constraints affect armor design and, ultimately, study results to the research and development communities. the design and choice of protection materials. The committee viewed the need for strong coupling between armor system STUDY METHODOLOGY designers and protection materials developers as the most difficult challenge to be addressed. The study consisted of six full two- or three-day com- mittee meetings held mostly in Washington, D.C., but also included a three-day meeting held near Aberdeen Proving SCOPE OF THE STUDY Ground, in Maryland, one day of which was devoted to visit- The Assistant Secretary of the Army (Acquisition, Lo- ing the U.S. Army Research Laboratory and observing some gistics, and Technology) requested that the NRC’s Board on of the relevant experimental testing facilities. The committee Army Science and Technology and its National Materials received briefings from academic, industrial, military, and Advisory Board collaborate to form an ad hoc study com- government presenters covering lightweight materials for mittee to investigate opportunities in protection materials warfighter protection as well as vehicle protection. Topics science and technology for the Army. ranged from ballistic threats to blast threats and from very The committee was given the following statement of hard to relatively soft armor materials and included a brief task: from the National Aeronautics and Space Administration on protection of space vehicles against hypervelocity impacts from meteors. The committee met in closed sessions to de- Statement of Task velop conclusions and recommendations responsive to the An ad hoc committee will conduct a study and prepare a study task, drawing upon the materials presented in open report on protection materials for the Army to explore the sessions and additional published materials cited throughout possibility of a path forward for these materials. Specifically, the report. the committee will: 1. Review and assess the current theoretical and ex- Report Organization perimental understanding of the major issues surrounding protection materials. The report contains 6 chapters and 10 appendixes. This 2. Determine the major challenges and technical gaps for first chapter provides the introduction and background to developing the future generation of light weight protection the study and defines the overall perspective of the report. materials for the Army, with the goal of valid multi-scale Chapter 2 introduces the reader to some armor systems and predictive simulation tools for performance and, conversely, gives examples relating to the key concept of reducing the protection materials by design 3. Suggest a path forward, including approach, organiza- areal density of the protection materials while improving tional structure and teaming, including processing, material the performance of armor against ever-increasing threats. characterization (composition and microstructure), quasi- Chapter 2 also makes the important distinction between static and dynamic mechanical testing and model develop- armor systems and material systems. ment and simulation and likely timeframes for the Army to Chapters 3, 4, and 5 provide the technical details of the deliver the next generation protection materials. committee’s assessment of current knowledge and discuss the gaps and opportunities meriting high priority in future The sponsor requested that in considering the questions research. In order to appreciate the task for designing ma- posed by the task statement, the committee should consider terials for armor, Chapter 3 covers the complex interacting the following: mechanisms and processes that take place during deforma- tion and failure when a material is impacted by a high- • hock wave energy dissipative (elastic, inelastic and S velocity penetrator. failure) and management mechanisms throughout the full Chapter 4 addresses the computational and experimental materials properties spectrum (nano through macro). approaches to armor material design and the need to better • xperimental approaches and facilities to visualize and E couple and integrate these activities to create materials by de- characterize the response at nano and mesoscales over sign and armor systems by design. Multiscale modeling and short time scales. • ulti-scale modeling techniques to predict energy dissipa- M simulation are reviewed for a few key scenarios for threat- tive mechanisms (twinning, stacking faults, etc.) from the protection materials, illustrating the considerable challenge atomic scales and bulk material response. of accurately capturing the extreme deformations involved in • aterials and material systems issues including process- M penetration. The goal is to enable much more rapid advances ing and characterization techniques focusing on intrinsic in both materials and systems and, accordingly, a much faster (single crystal) properties and processing controlled ex- and better response to changing threats. trinsic characteristics (phases, microstructure, interfaces).
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10 OPPORTUNITIES IN PROTECTION MATERIALS SCIENCE AND TECHNOLOGY FOR FUTURE ARMY APPLICATIONS Other Issues Chapter 5 provides a broad perspective on the structure and composition of exemplary protection materials including The sponsor asked that the committee suggest both an ceramics, polymers, metals, and composites. It highlights the organizational structure and a teaming approach as part of most exciting opportunities in materials research—opportu- the path forward. In considering the sponsor’s request that nities that may lead to revolutionary advances in protection the study report not include restricted material so as to en- and a significant reduction in areal density. This chapter is able wide dissemination to the research and development extensively appended with descriptions and processing for communities, the committee recognized a broader issue— specific materials. namely, that restricted information is a barrier to research Chapter 6 suggests a path forward and recommends collaborations.4 Chapter 2 addresses armor system design future research tied to the conclusions of the earlier chapters. at the unrestricted level but closes with a comment on the To realize all the potential gains for protection materials extensive regime of security and export control restrictions noted in the report, an important new paradigm is proposed, that affects research on protection materials. Several speak- along with an organizational plan for its implementation. ers from industry, government, and academic organizations Collectively, these chapters provide technical recom- told the committee that these restrictions make it extremely mendations and a proposed way forward for long-term difficult for fundamental research in protection materials to research directed at the development of the following: be usefully communicated among the various organizations and to be connected to the development of armor systems, • A fundamental understanding of how a ballistic which entails restricted information. It notes that a review object or a blast interacts with a material—in other of classification guidelines and export control restrictions words, the material’s performance. This would in- would facilitate clearer, more up-to-date boundaries for the clude an understanding of which time and length necessary control of information. Chapter 6 proposes an scales are important and how controlling the mate- organizational structure to bridge this gap. rial’s composition and microstructure, and hence its mechanical behavior, contributes to altering the de- Overarching Recommendation formation mechanisms and improving performance; • Experimental approaches to identify and quantita- The committee’s key recommendations are presented tively characterize the mechanisms and processes in Chapter 6, with ancillary recommendations found in that lead to damage during these dynamic events; Chapters 3 and 4. The overall thrust of this report, however, • Quantitative relationships for the evolution of the is evident in the following overarching recommendation: damage during a high-deformation event and extend- ing these relationships to account for multiple events, Overarching Recommendation. Given the long-term im- termed multi-hit relationships; portance of lightweight protection materials to the Depart- • Computational approaches—coupled with synergis- ment of Defense (DoD) mission, DoD should establish the tic experiments that inform and validate—to predict defense initiative protection materials by design (PMD), the performance of specific protection materials in an with associated funding lines for basic and applied research. integrated armor configuration; Responsibility for this new initiative should be assigned to • Model-driven methods to design new materials or one of the Services, with participation by other DoD com- improve existing ones to meet the behavior criteria ponents whose missions also require advances in protection for successful protection; materials. The PMD initiative should include a combina- • Model-driven synthesis, processing, and manufactur- tion of computational, experimental, and materials testing, ing capabilities to produce affordable materials in characterization, and processing research conducted by quantities needed for defense applications; and government, industry, and academia. The program director • An environment that allows successful interplay and should be given the authority and resources to collaborate collaboration among the DoD, government labs, with the national laboratories and other institutions in the industry, and academe while at the same time ad- use of unique facilities and capabilities and to invest in DoD dressing security and organizational matters. infrastructure where needed. Appendix A includes the Statement of Task, Appendix B This overarching recommendation requires actions in provides biographical sketches of the committee members, four important elements of the PMD initiative: and Appendix C lists committee meetings and speaker topics. Appendixes D through J contain much additional detailed in- formation on protection materials, supplementing the points made in Chapters 3, 4, and 5. 4A detailed discussion of the effects on research of classification guide - lines, security, and export controls is beyond the scope of this study.
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11 OVERVIEW The chapters that follow develop the rationale and • Element 1. Fundamental understanding of mecha- conclusions that underpin the detailed recommendations in nisms of deformation and failure due to ballistic and Chapter 6 and identify actions that are needed to address the blast threats. four elements of the initiative. The committee is unanimous • Element 2. Advanced computational and experimen- in its support of these recommendations. tal methods. • Element 3. Development of new materials and mate- rial systems. • Element 4. Organizational approach.