Appendix A

Background and Statement of Task

Current operations in Iraq and Afghanistan unambiguously demonstrate the need for threat-specific, ultra-lightweight transparent and opaque armor in many Army systems, constructed facilities, and personnel protection. As the threats have escalated and become more varied, the challenges for rapidly developing optimized threat-specific, passive lightweight armor packages have grown complex because of the interplay of issues involving energy absorption and momentum transfer issues. Critical components for further accelerating the optimization of these material systems are the development of validated predictive-performance computer models, materials design tools, and integrated structural design to take advantage of advanced materials technology. This approach is based on the determination and quantification of the various impact energy absorption mechanisms, including the various deformation modes, damage nucleation and accumulation processes, and the resulting eventual failure of materials at high rates under very high impact stress (shock wave).

Over the past few years there have been major initiatives that bear on this activity. In 2007, the Army Research Office, in conjunction with other Army Research Laboratory groups, convened the workshop “Impact Damage on the Performance of Armor Ceramics.” More recently, the National Research Council’s National Materials Advisory Board completed a study entitled Integrated Computational Materials Engineering: A Transformational Discipline for Improved Competitiveness and National Security. In addition, the Basic Energy Sciences Office of the U.S. Department of Energy convened a high-level study committee on “Directing Matter and Energy: Five Challenges for Science and the Imagination” and another on “Basic Research Needs for Materials under Extreme Environments.” In May 2008, the U.S. Army’s Engineer Research and Development Center (ERDC) and the Army Research Office (ARO) conducted a workshop to share emerging fundamental discoveries in experimentation, theory, and computational methods for the mechanics of cementitious and ceramic materials. Then on September 22-24, 2008, a major Army Research Laboratory workshop, “Multi-Scale Materials Behavior in Ultra-High Loading Rate Environments,” focused on multiscale materials and mechanics for dynamic energy management at the macro- and microscale.

In order to design and produce impact-resistant advanced materials and systems, validated, robust multiscale physics-based models (atomistic to polycrystalline to continuum) are needed to simulate reliably the mechanical response of such materials and systems in extreme environments. It is well known, for example, that variation in material characteristics (phases, microstructure, and defects) including grain boundaries and intergranular films can significantly affect the quasi-static, mechanical behavior of structural ceramics. There are, however, many significant differences between the high-rate and quasi-static stress environments, including differences in the following areas: stressed volume; overstressed condition; propagation and rate of stress waves (compression, tensile, and shear); kinetic effects; mixed, spatially varying macrostress states; activation of new micromechanical mechanisms; and possibility of phase transformations, among others. Ultimate failure is a function of the temporal and spatial interaction of the macrostresses with the ceramic materials at the microstructural and nanostructural scales, including elastic and inelastic (plastic) deformation, damage nucleation, and evolution and resulting failure from the macroscale (top down) or from the nanoscale (bottom up). The macromechanical responses (constitutive equations), assuming homogeneous, defect-free mechanically isotropic bodies, are very well known, but the spatial micromechanical responses and stochastic variability are not nearly as well established. As computing power and speed continue to increase, the ability to simulate the mechanical response at the microstructural and mesostructural level will become much more important. Many existing models and codes, being extrapolations from metal behavior, exclude defects, microcracking, ceramic plasticity, ceramic-specific failure mechanisms, high-pressure phase transformations,



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Appendix A Background and Statement of Task Current operations in Iraq and Afghanistan unam- September 22-24, 2008, a major Army Research Laboratory biguously demonstrate the need for threat-specific, ultra- workshop, “Multi-Scale Materials Behavior in Ultra-High lightweight transparent and opaque armor in many Army Loading Rate Environments,” focused on multiscale materi- systems, constructed facilities, and personnel protection. als and mechanics for dynamic energy management at the As the threats have escalated and become more varied, the macro- and microscale. challenges for rapidly developing optimized threat-specific, In order to design and produce impact-resistant advanced passive lightweight armor packages have grown complex be- materials and systems, validated, robust multiscale physics- cause of the interplay of issues involving energy absorption based models (atomistic to polycrystalline to continuum) and momentum transfer issues. Critical components for fur- are needed to simulate reliably the mechanical response of ther accelerating the optimization of these material systems such materials and systems in extreme environments. It is are the development of validated predictive-performance well known, for example, that variation in material char- computer models, materials design tools, and integrated acteristics (phases, microstructure, and defects) including structural design to take advantage of advanced materials grain boundaries and intergranular films can significantly technology. This approach is based on the determination affect the quasi-static, mechanical behavior of structural and quantification of the various impact energy absorption ceramics. There are, however, many significant differences mechanisms, including the various deformation modes, dam- between the high-rate and quasi-static stress environments, age nucleation and accumulation processes, and the resulting including differences in the following areas: stressed vol- eventual failure of materials at high rates under very high ume; overstressed condition; propagation and rate of stress impact stress (shock wave). waves (compression, tensile, and shear); kinetic effects; Over the past few years there have been major initia- mixed, spatially varying macrostress states; activation of tives that bear on this activity. In 2007, the Army Research new micromechanical mechanisms; and possibility of phase Office, in conjunction with other Army Research Labora- transformations, among others. Ultimate failure is a function tory groups, convened the workshop “Impact Damage on of the temporal and spatial interaction of the macrostresses the Performance of Armor Ceramics.” More recently, the with the ceramic materials at the microstructural and nano- National Research Council’s National Materials Advisory structural scales, including elastic and inelastic (plastic) Board completed a study entitled Integrated Computational deformation, damage nucleation, and evolution and resulting failure from the macroscale (top down) or from the nanoscale Materials Engineering: A Transformational Discipline for Improved Competitiveness and National Security. In addi- (bottom up). The macromechanical responses (constitutive tion, the Basic Energy Sciences Office of the U.S. Depart- equations), assuming homogeneous, defect-free mechani- ment of Energy convened a high-level study committee on cally isotropic bodies, are very well known, but the spatial “Directing Matter and Energy: Five Challenges for Science micromechanical responses and stochastic variability are not and the Imagination” and another on “Basic Research Needs nearly as well established. As computing power and speed for Materials under Extreme Environments.” In May 2008, continue to increase, the ability to simulate the mechanical the U.S. Army’s Engineer Research and Development Center response at the microstructural and mesostructural level will (ERDC) and the Army Research Office (ARO) conducted become much more important. Many existing models and a workshop to share emerging fundamental discoveries in codes, being extrapolations from metal behavior, exclude experimentation, theory, and computational methods for the defects, microcracking, ceramic plasticity, ceramic-specific mechanics of cementitious and ceramic materials. Then on failure mechanisms, high-pressure phase transformations, 111

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112 OPPORTUNITIES IN PROTECTION MATERIALS SCIENCE AND TECHNOLOGY FOR FUTURE ARMY APPLICATIONS sample homogeneity, and sample-to-sample variability. 3. Suggest a path forward, including approach, organiza- tional structure and teaming, including processing, material Although this example focuses on the challenges associated characterization (composition and microstructure), quasi- with ceramics, similar materials-specific complexities arise static and dynamic mechanical testing and model develop- in composite materials, fibers, and textiles, in concrete and ment and simulation and likely timeframes for the Army to laminated assemblies of multiple materials, and the associ- deliver the next generation protection materials. ated interfaces contained therein. In considering these questions, the committee should STATEMENT OF TASK consider the following: An ad hoc committee will conduct a study and prepare a • hock wave energy dissipative (elastic, inelastic and S report on protection materials for the Army to explore the failure) and management mechanisms throughout the full possibility of a path forward for these materials. Specifically, materials properties spectrum (nano through macro). the committee will: • xperimental approaches and facilities to visualize and E 1. Review and assess the current theoretical and ex- characterize the response at nano and mesoscales over perimental understanding of the major issues surrounding short time scales. protection materials. • ulti-scale modeling techniques to predict energy dissipa- M 2. Determine the major challenges and technical gaps for tive mechanisms (twinning, stacking faults, etc.) from the developing the future generation of light weight protection atomic scales and bulk material response. materials for the Army, with the goal of valid multi-scale • aterials and material systems issues including process- M predictive simulation tools for performance and, conversely, ing and characterization techniques focusing on intrinsic protection materials by design. (single crystal) properties and processing controlled ex- trinsic characteristics (phases, microstructure, interfaces).