This report responds to a request by the Assistant Secretary of the Army (Acquisition, Logistics, and Technology) to the National Research Council (NRC) to examine the current theoretical and experimental understanding of the key issues surrounding protection materials, identify the major challenges and technical gaps for developing the future generation of lightweight protection materials, and recommend a path forward for their development. While underscoring the paramount need for lightweight materials, the charge included requirements to consider multiscale shockwave energy transfer mechanisms and experimental approaches for their characterization over short timescales, as well as multiscale modeling techniques to predict mechanisms for dissipating energy.
Accordingly, two NRC boards—the National Materials Advisory Board1 and the Board on Army Science and Technology—established the Committee on Opportunities in Protection Materials Science and Technology for Future Army Applications to investigate opportunities in protection materials science and technology for the Army. What follows is the evaluation developed by that committee.
The report considers exemplary threats and design philosophy for the three key applications of armor systems: (1) personnel protection, including body armor and helmets, (2) vehicle armor, and (3) transparent armor. For each of these applications, specific constraints drive the armor design and thus the ultimate choice of protection materials.
In developing its recommendations, the committee assessed current knowledge and gaps in that knowledge as it sought to prioritize the various types of lightweight protective materials and armor systems for future research. Key areas and research challenges for protection materials discussed in these pages include the following:
1In January 2011 the National Materials Advisory Board (NMAB) and the Board on Manuacturing and Engineering Design combined to form the National Materials and Manufacturing Board. The move underscored the importance of materials science to innovations in engineering and manufacturing.
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Summary This report responds to a request by the Assistant Sec- • Penetration mechanisms in metals and alloys, ceram- retary of the Army (Acquisition, Logistics, and Technology) ics and glasses, and polymeric materials (Chapter 3). to the National Research Council (NRC) to examine the cur- • Failure mechanisms in cellular-sandwich materials rent theoretical and experimental understanding of the key due to blast (Chapter 3). issues surrounding protection materials, identify the major • Current capabilities for modeling and simulation of challenges and technical gaps for developing the future gen- protection materials and material systems on scales eration of lightweight protection materials, and recommend ranging from the atomic to the macroscopic, includ- a path forward for their development. While underscoring ing a discussion of state-of-the-art modeling and the paramount need for lightweight materials, the charge simulation tools (Chapter 4). included requirements to consider multiscale shockwave • The state of the art in experimental methods, includ- energy transfer mechanisms and experimental approaches ing defining the length and timescales of interest, for their characterization over short timescales, as well as evaluating material behavior at the relevant high- multiscale modeling techniques to predict mechanisms for strain rates, and investigating shock physics, dy- dissipating energy. namic failure processes, and impact phenomenology Accordingly, two NRC boards—the National Materi- (Chapter 4). als Advisory Board1 and the Board on Army Science and • Ceramic armor materials, including crystalline and Technology—established the Committee on Opportunities amorphous ceramics, ceramic powders, processing in Protection Materials Science and Technology for Future and fabrication techniques, and transparent crystal- Army Applications to investigate opportunities in protection line ceramics (Chapter 5). materials science and technology for the Army. What follows • Fibers, including the effect of fiber diameter on is the evaluation developed by that committee. strength in high-performance fibers, microstruc- The report considers exemplary threats and design phi- tural advances to approach the theoretical maximum losophy for the three key applications of armor systems: (1) t ensile strength and modulus, and the need for personnel protection, including body armor and helmets, (2) mechanical tests at high strain rates and pressures vehicle armor, and (3) transparent armor. For each of these (Chapter 5). applications, specific constraints drive the armor design and • Ballistic fabrics, including ballistic testing, failure thus the ultimate choice of protection materials. mechanisms, and interactions among fibers and In developing its recommendations, the committee among yarns during loading (Chapter 5). assessed current knowledge and gaps in that knowledge • Metals and metal-matrix composites and their desir- as it sought to prioritize the various types of lightweight able attributes, especially those of low-density metals protective materials and armor systems for future research. such as magnesium alloys (Chapter 5). Key areas and research challenges for protection materials • Fabrication and assembly of armor systems, with discussed in these pages include the following: an emphasis on adhesives for armor and transparent armor, including (1) general considerations for se- lecting an adhesive interlayer and (2) testing, simula- 1In January 2011 the National Materials Advisory Board (NMAB) and the Board on Manuacturing and Engineering Design combined to form tion, and modeling of adhesives and armor systems the National Materials and Manufacturing Board. The move underscored (Chapter 5). the importance of materials science to innovations in engineering and manufacturing. 1
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2 OPPORTUNITIES IN PROTECTION MATERIALS SCIENCE AND TECHNOLOGY FOR FUTURE ARMY APPLICATIONS NEW THREAT Material .. Characteristics of Threat Material 2 Material 1 Armor Concept Select from Materials Research (Geometry Available and Development/ Configuration) Materials Design Fail Characterization Pass Make & Shoot Ballistic Shoot or Canonical Microstructure Evaluation Model Model Mechanisms Make & Shoot Model Fail Rapid Modeling and Iterations Select from M&S Pass Simulation Available Evaluation Research and Models/Codes Development Increased Fidelity Characteristics of Armor Performance NEW ARMOR FIGURE S-1 New paradigm for armor development. The new design path for armor provides enhanced and closer coupling of the materials research and development community and the modeling and simulation community, resulting in significantly reduced time for development of new armor. This new approach connects the armor design process to the materials research and development community through canonical models to deal with the restricted information problem. The elements of armor system design are not themselves new, but the emphasis shifts from design-make-shoot-redesign to rapid simulation iterations, and from designing with off-the-shelf materials to designing that exploits materials for their protective properties. The feedback loop between armor system design and material design contrasts with current practice, in which a one-way flow puts new materials on the shelf to be tried in the make-shoot-look process. is accomplished through canonical models that translate Findings and recommendations pertaining to these areas armor system requirements (often data with restricted ac- and research challenges appear in Chapters 3 through 5. cess) into characterizations, microstructures, behaviors, and The single overarching recommendation is repeated here in deformation mechanisms that an open research community the summary, along with the four key recommendations in can use in designing new lightweight protection materials. the main text. The principal objective of this new paradigm is to enable the design of superior protection materials and to accelerate OVERARCHING RECOMMENDATION their implementation in armor systems. This new paradigm will build upon the multidisciplinary collaboration concepts The conclusion of this study is that the ability to design and lessons from other applications documented in the report and optimize protection material systems can be acceler- Integrated Computational Materials Engineering.2 It can be ated and made more cost effective by operating in a new focused on the most promising opportunities in lightweight paradigm of lightweight protection material development protection materials, bringing such current products as ce- (Figure S-1). In this new paradigm, the current armor ramic plates and polymer fiber materials well beyond their system design practice, which relies heavily on a design- make-shoot iterative process, is replaced by rapid iterations of modeling and simulation, with ballistic evaluation used 2NRC. 2008. Integrated Computational Systems Engineering: A Trans - selectively to verify satisfactory designs. Strong coupling formational Discipline for Improved Competitiveness and National Secu- with the materials research and development community rity. Washington, D.C.: The National Academies Press.
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3 SUMMARY present state of performance and opening the possibility for • Relating material performance to deformation and radically new armor system solutions to be explored and failure mechanisms. Developing models and data for optimized in tens of months rather than tens of years. choosing materials based on their ability to inhibit or avoid failure mechanisms as opposed to choosing Overarching Recommendation. Given the long-term im- them based on bulk properties as measured in quasi- portance of lightweight protection materials to the Depart- static and dynamic tests. ment of Defense (DoD) mission, DoD should establish a • Developing superior armor materials by identifying defense initiative for protection materials by design (PMD), compositions, crystalline structures, and microstruc- with associated funding lines for basic and applied research. tures that counteract observed failure mechanisms Responsibility for this new initiative should be assigned to and by establishing processing routes to the synthesis one of the Services, with participation by other DoD com- of these materials. ponents whose missions also require advances in protection • Reducing the cost of production of protection mate- materials. The PMD initiative should include a combination rials by improving the processes and yields and by of computational, experimental, and materials testing, char- enhancing the ability to manufacture small lots. acterization, and processing research conducted by govern- ment, industry, and academia. The program director of the Element 2—Advanced Computational and Experimental initiative should be given the authority and resources to col- Methods laborate with the national laboratories and other institutions in the use of unique facilities and capabilities and to invest The second element of the PMD initiative would be to in DoD infrastructure where needed. advance and exploit the capabilities of the emerging compu- This overarching recommendation requires actions in tational and experimental methods discussed in Chapter 4. four important elements of the PMD initiative. The first objective is to predict the ballistic and blast per- formance of candidate materials and materials systems as a prelude to the armor design process. The second objective is RECOMMENDATIONS to define requirements that will guide the synthesis, process- ing, fabrication, and evaluation of protection materials. The Element 1—Fundamental Understanding of Mechanisms PMD initiative would develop the next generation of of Deformation and Failure Due to Ballistic and Blast Threats • DoD advanced protection codes that incorporate The first element of the PMD initiative would be to de- experimentally validated, high-fidelity, physics- velop better fundamental understanding of the mechanisms based models of material deformation and failure, as of high-rate3 material deformation and failure in various well as the necessary high-performance computing protection materials, discussed in Chapter 3. As part of the infrastructure; new paradigm, armor development should be considered not • Experimental facilities and capabilities to assess and from the viewpoint of conventional bulk material properties certify the performance of new protection materials but from the viewpoint of mechanisms. The deeper funda- and system designs, as well as provide insight into mental understanding could lead to the development of more fundamental material behaviors under relevant con- failure-resistant material compositions, crystal structures, ditions with unprecedented simultaneous high spatial and microstructures and to protective materials with better and temporal resolution; and performance. Moreover, by identifying the operative mecha- • Collaborative infrastructure for encouraging direct nisms and quantifying their activity, mathematical damage communication and improved cooperation between models can be written that may allow computational armor modelers and experimenters, through both (1) the design. Chapter 3 discusses failure mechanisms for the sev- establishment of collaborative environments and (2) eral classes of materials. requirements in proposals when the specific research topic is well served by such collaboration. Recommendation S-1/6-1. The Department of Defense should establish a program of sustained investment in basic The high-priority opportunities identified in Chapter and applied research that would facilitate a fundamental 4 will need sustained investment and program direction to understanding of the mechanisms of deformation and failure advance computational and experimental capabilities. The due to ballistic and blast events. This program should be es- envisioned computational capabilities must be developed tablished under a director for protection materials by design, in partnership with a strong experimental effort that identi- with particular emphasis on the following: fies the dynamic mechanisms of material behavior. These mechanisms must be understood and modeled for the activity to be successful, the material characteristics and properties must be known for the simulations to be carried out, and the 3Ballistic velocities typically range from several hundred to several outcomes of the computational modeling must be validated. thousand meters per second and can lead to strain rates of up to 105 s–1.
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4 OPPORTUNITIES IN PROTECTION MATERIALS SCIENCE AND TECHNOLOGY FOR FUTURE ARMY APPLICATIONS Recommendation S-2/6-2. The Department of Defense should designate a custodian for this database and should establish a program of sustained investment in basic arrange for experimental results of the PMD program and applied research in advanced computational and experi- to be provided to the database and shared with the mental methods under the director of the protection materials research community. The database should include by design (PMD) initiative, with particular emphasis on the ceramics, polymers, metals, glasses, and composite following: materials in use today and should be expanded as new materials are developed. • Dynamic mechanism characterization. Identify and —Opaque and transparent ceramics and ceramic characterize (1) the failure mechanisms underlying powders. The intrinsic properties of opaque and damage to a material caused by projectiles from transparent ceramics and ceramic powders are weapons and detonations and (2) the compositional not yet fully realized in armor systems. There is and microstructural features of each constituent of need for understanding at the atomic, nano-, and the material, as well as the material’s overall struc- micron levels of how powders and processing ture. An enhanced experimental infrastructure will can be designed and manipulated to maximize be needed to make progress in high-resolution (time the intrinsic benefits of dense ceramic armor and and space) experiments on material deformation and reduce production costs. failure characterization. —Polymeric, carbon, glass, and ceramic fibers. • Code validation and verification. Focus on mul- There is an opportunity to develop finer diameter tiscale, multiphysics material models, integrated and more ideally microstructured polymeric and simulation/experimental protocols, prediction with carbon fibers with potentially a two- to fivefold quantified uncertainties, and simulation-based quali- improvement in specific tensile strength over the fication to help advance the predictive science for current state of the art. Such improvements would protection systems. significantly reduce the weight of body armor. • Challenges and canonical models. Periodically pro- —Polymers. In addition to polymer fibers, ther- pose open challenges comprising design, simulation, moplastic and thermoset polymers are used as and experimental validation that will convincingly monolithic components and also serve as matrixes demonstrate the PMD. Each challenge problem must in various composites. Improved measurements of address the corresponding canonical model and must and models for the deformation mechanisms and result in quantifiable improvements in performance failure processes are needed for thermoplastic- within that framework. and thermoset-based protection materials. —Magnesium alloys. The very low density of magnesium provides potential for the develop- Element 3—Development of New Materials and Material ment of very lightweight alternatives to tradi- Systems tional metallic materials in protection material The third element of the PMD initiative is the develop- systems. The basic understanding of strengthening ment and production of new materials and material systems mechanisms in magnesium should be advanced, whose characteristics and performance can achieve the especially the development of ultra-fine-grained behavior validated in modeling and simulation of the new magnesium alloys through severe plastic deforma- armor system. The recommendations in this element target tion. Magnesium-based fibers are also worthy of the most promising opportunities identified in Chapter 5. exploration. • Adhesives and active brazing/soldering materi - Recommendation S-3/6-3. The Department of Defense als. Development of adhesives and active brazing/ should establish a program of sustained investment in basic soldering materials and their processing methods and applied research in advanced materials and processing, to match the elastic impedance of current materials under the director of the PMD initiative program, with par- while minimizing the thermal stresses will improve ticular emphasis on the following: the ballistic and blast performance of panels made of bonded armor, including transparent armor. • Test methods. Advances are needed in test methods • A sustained effort to develop a database of high- for determining the high strain rates (103 to 106 s–1) strain-rate materials for armor. Material behavior and dynamic properties must be measured and char- and dynamic failure processes of (especially) fibers, acterized over the range of strains, strain rates, and polymers, and ceramics. Results should be passed stress states in the context of penetration and blast on to the designated database of materials with high- events. Develop a comprehensive database of materi- strain-rate behavior. als that exhibit high-strain-rate behavior and consider • Material characterization. The characterization of, them as materials of interest. The PMD director composition, crystalline structure, and microstruc-
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5 SUMMARY the authority to direct funding and set PMD priorities. The ture at appropriate length scales is a key task that program will require committed funding to ensure long-term will need more attention to take advantage of the success and should be subject to periodic external reviews improved experimental tools for quantifying initial to ensure that high standards of achievement are established and deformed microstructures. and maintained. To meet these requirements, the commit- • Cost reduction. Advances are needed to reduce the tee recommends the notional DoD organizational approach cost of producing protection materials by improving depicted in Figure S-2. their processing and yield and by improving small-lot manufacturing capability. Recommendation S-4/6-4. In order to make the major ad- • Processing science and intelligent manufacturing. vances needed for the development of protection materials, Advances are needed in basic understanding of and the Department of Defense should appoint a PMD program ability to model the consequences of material pro- director, with authority and resources to accomplish the cessing for performance and other characteristics following: of interest. Intelligent manufacturing sensing and control capabilities are needed that can maintain low • Plan and execute the PMD initiative and coordinate variance and produce affordable protection materials, PMD activities across the DoD. even in relatively low volumes. • Select an existing facility to be the DoD center for PMD and fund a research director and the staff, Element 4—Organizational Approach e quipment, and programs needed by the PMD initiative; The fourth element of the PMD initiative is an organi- • Award a competitive contract for an open access zational construct for multidisciplinary collaboration among PMD center whose mission would be to host and academic researchers, government laboratories, and indus- foster open collaboration in research and develop- try, in both restricted-access and open settings. The PMD ment of protection materials; initiative will need strong top-level leadership with insight into both the open and restricted research environments and Program Review Board Director $ $ Visiting Researchers Open PMD Restricted DoD PMD Canonical Collaboration Collaboration Models Center Center Test Services and Models / Codes Universities Other Government Labs #... Industry #2 Industry #1 Industry FIGURE S-2 PMD initiative organizational structure involving academic researchers, government laboratories, and industry.
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6 OPPORTUNITIES IN PROTECTION MATERIALS SCIENCE AND TECHNOLOGY FOR FUTURE ARMY APPLICATIONS research collaborations.4 Such limitations are prudent and • Establish an external review board to conduct peri- odic reviews of programs in both centers; and necessary but require periodic review to ensure they are • Provide liaison with the Department of Energy, the consistent with the current state of open knowledge and do National Institute of Standards and Technology, and not unnecessarily restrict the exchange of information with other government laboratories on matters related to an open research community when such an exchange would PMD. be beneficial to national security. The chapters that follow develop the rationale and The sponsor asked that the committee suggest an or- conclusions that underpin the detailed recommendations in ganizational structure for the path forward and a teaming Chapter 6 and identify needed actions in the four elements approach for it. In considering the sponsor’s request that the of the initiative. study report not include restricted material, which would 4A detailed discussion of the effects on research of classification guide - have precluded wide dissemination to the research and devel- lines, security, and export control is beyond the scope of this study. opment communities, the committee recognized the broader issue of the role restricted information plays in impeding