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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,
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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).
