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,
sample homogeneity, and sample-to-sample variability. Although this example focuses on the challenges associated with ceramics, similar materials-specific complexities arise in composite materials, fibers, and textiles, in concrete and laminated assemblies of multiple materials, and the associated interfaces contained therein.
STATEMENT OF TASK
An ad hoc committee will conduct a study and prepare a report on protection materials for the Army to explore the possibility of a path forward for these materials. Specifically, the committee will:
1. Review and assess the current theoretical and experimental understanding of the major issues surrounding protection materials.
2. Determine the major challenges and technical gaps for developing the future generation of light weight protection materials for the Army, with the goal of valid multi-scale predictive simulation tools for performance and, conversely, protection materials by design.
3. Suggest a path forward, including approach, organizational structure and teaming, including processing, material characterization (composition and microstructure), quasi-static and dynamic mechanical testing and model development and simulation and likely timeframes for the Army to deliver the next generation protection materials.
In considering these questions, the committee should consider the following:
- Shock wave energy dissipative (elastic, inelastic and failure) and management mechanisms throughout the full materials properties spectrum (nano through macro).
- Experimental approaches and facilities to visualize and characterize the response at nano and mesoscales over short time scales.
- Multi-scale modeling techniques to predict energy dissipative mechanisms (twinning, stacking faults, etc.) from the atomic scales and bulk material response.
- Materials and material systems issues including processing and characterization techniques focusing on intrinsic (single crystal) properties and processing controlled extrinsic characteristics (phases, microstructure, interfaces).
