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Ballistics Sciences: Terminal Ballistics

INTRODUCTION

The Panel on Ballistics Science and Engineering at the Army Research Laboratory conducted its review of ARL’s terminal ballistics program, on August 20-22, 2013. This chapter provides an evaluation of that work, recognizing that it represents only a portion of ARL’s ballistics sciences core technology competency portfolio.

This year’s presentations to the Panel outlined the breadth and scope of ARL’s terminal ballistics scientific and engineering research efforts during 2013. These programs span the gap between basic research that improves fundamental understanding of scientific phenomena and technology generation that supports terminal ballistic developments and fielded system upgrades. ARL’s terminal ballistics mission scope is principally centered within the Weapons and Materials Research Directorate (WMRD) and the Survivability and Lethality Analysis Directorate (SLAD). These directorates execute their mission of leading the Army’s research and technology program and analysis efforts to enhance the protection and lethality of the individual soldier and advanced weapon systems.

ACCOMPLISHMENTS AND ADVANCEMENTS

The Army Research Laboratory has a strong record of achievement and timely support of the warfighter in developing advanced capabilities for defeating many types of enemy targets and platforms over many years, and the recent and ongoing work described in the review of terminal ballistics demonstrated how ARL continues to build on its tradition of excellence in protecting the warfighter. The review was divided into topic areas, including technical keynote presentations and posters covering materials for terminal ballistics, penetration mechanics, humans in extreme ballistic environments, and computational terminal ballistics. The speakers and the presenters of posters demonstrated considerable knowledge of the technical areas addressed, displayed strong enthusiasm for their work, and showed dedication to the missions of ARL, supporting the warfighters and national defense. ARL’s efforts in terminal ballistics address both fundamental and urgent Army warfighter needs of great importance. The linkages between the research and technology presented and the ties to Army military vehicles and weapons were clearly demonstrated. Specific accomplishments and advancements in each of the topical areas are summarized below.

Materials for Terminal Ballistics

The overview presentations for the materials for terminal ballistics area were very impressive and provided a rationale for the diverse materials issues under investigation; the researchers have gained from recent combat experience and lessons learned from in-theatre observations. The study of small munitions, specifically striving to build linkages between materials and ballistic performance, was very positively viewed; ARL is encouraged to continue to pursue this direction as a pathway to increased predictive



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2 Ballistics Sciences: Terminal Ballistics INTRODUCTION The Panel on Ballistics Science and Engineering at the Army Research Laboratory conducted its review of ARL’s terminal ballistics program, on August 20-22, 2013. This chapter provides an evaluation of that work, recognizing that it represents only a portion of ARL’s ballistics sciences core technology competency portfolio. This year’s presentations to the Panel outlined the breadth and scope of ARL’s terminal ballistics scientific and engineering research efforts during 2013. These programs span the gap between basic research that improves fundamental understanding of scientific phenomena and technology generation that supports terminal ballistic developments and fielded system upgrades. ARL’s terminal ballistics mission scope is principally centered within the Weapons and Materials Research Directorate (WMRD) and the Survivability and Lethality Analysis Directorate (SLAD). These directorates execute their mission of leading the Army’s research and technology program and analysis efforts to enhance the protection and lethality of the individual soldier and advanced weapon systems. ACCOMPLISHMENTS AND ADVANCEMENTS The Army Research Laboratory has a strong record of achievement and timely support of the warfighter in developing advanced capabilities for defeating many types of enemy targets and platforms over many years, and the recent and ongoing work described in the review of terminal ballistics demonstrated how ARL continues to build on its tradition of excellence in protecting the warfighter. The review was divided into topic areas, including technical keynote presentations and posters covering materials for terminal ballistics, penetration mechanics, humans in extreme ballistic environments, and computational terminal ballistics. The speakers and the presenters of posters demonstrated considerable knowledge of the technical areas addressed, displayed strong enthusiasm for their work, and showed dedication to the missions of ARL, supporting the warfighters and national defense. ARL’s efforts in terminal ballistics address both fundamental and urgent Army warfighter needs of great importance. The linkages between the research and technology presented and the ties to Army military vehicles and weapons were clearly demonstrated. Specific accomplishments and advancements in each of the topical areas are summarized below. Materials for Terminal Ballistics The overview presentations for the materials for terminal ballistics area were very impressive and provided a rationale for the diverse materials issues under investigation; the researchers have gained from recent combat experience and lessons learned from in-theatre observations. The study of small munitions, specifically striving to build linkages between materials and ballistic performance, was very positively viewed; ARL is encouraged to continue to pursue this direction as a pathway to increased predictive 14

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capability. Continued modeling and simulation (M&S) efforts to bridge the boundaries between mesoscale and microscale are encouraged. The organizational effort to encourage students in the science, technology, engineering, and mathematics (STEM) fields and to provide existing personnel with international and university connections is also considered to be very positive. Many of the materials for ballistics programs reviewed were very impressive. For example, investigation of next-generation aluminum alloy armor and the evolution of the Eglin armor steel are both promising research topics. Aluminum alloy armor design and the materials manufacturing technology of these alloys with superior ballistic performance are key to controlling material and fabrication costs while supporting lighter weight technologies for the Army. Research to develop an available Al alloy with desirable performance but with reduced costs is key to this strategic direction in armor and vehicle design. The use of THERMOCALC, a state-of- the-art thermodynamics modeling program, to modify the Al alloy 2139 composition, particularly reducing the silver content, is very promising. Continuing to partner with industry on alloy development to achieve an Al alloy with similar properties, yield strength, fracture toughness, and formability at a lower cost is clearly the right direction for this research. Altering the alloy chemistry of cast Eglin armor steel with the aim of using this material for underbody blast resistance is a very promising technology to address both increased blast performance and reduced manufacturing and assembly costs. Development of the manufacturing capability for net-shape single-piece underbody manufacturing was very impressive. Simulations of the solidification during casting and, after that, the blast performance using currently available M&S tools, along with experimental testing as an integral part of the development process, were both technologically state of the art and clearly aimed at addressing important Army vehicle needs. Exploration of the utilization of nanocrystalline alloys for shaped-charge liners appears to be a very promising avenue of research. Nanocrystalline metals offer the possibility of improved properties (strength, ductility) for shaped-charge applications. Fabricating these materials in bulk by means of powder processing is challenging because grain growth occurs even at low temperatures. In this project, the investigators exploit a thermodynamic approach to stabilizing nanocrystalline grains by populating grain boundaries with a solute element that decreases grain boundary free energy. To achieve this goal, the investigators have developed a simple thermodynamic model for grain boundary free energy and applied it, pairwise, across the periodic table. From this pairing of binary alloys, copper-tantalum was chosen as a candidate material. Ductility was better than that of microcrystalline samples. A warhead prototype has been fabricated that may represent one of the largest bulk components ever fabricated with a uniformly nanocrystalline grain structure. Next, the warhead will be tested. This achievement represents a significant advance in the nanostructured materials field and an impressive achievement for ARL. ARL work involving the multiscale modeling of noncrystalline ceramics and glass is seeking to develop a physics-based multiscale modeling capability to predict the performance and optimize the design of noncrystalline ceramics and glasses not yet synthesized. A specific goal is to develop a fundamental understanding of why certain chemically substituted glasses exhibit enhanced resistance to penetration by shaped- charge jets and other ballistic threats. This research relates very strongly to the glass research effort, which is focused on shaped charge jet/glass interactions; it is possible that at some point certain results from this study could support the glass research activity. Of particular note was the team’s ability to leverage work from other institutions, including new results in nanotechnology, applying experimental equipment from geophysics—for example, the diamond anvil cell—and interacting with glass manufacturing research and development (R&D) teams. This team is striving to work across multiple scales, from nano- to mesoscale, and there is significant opportunity in ARL’s efforts to integrate the basic science described during the review with the glass research application work and the experimental tools. Since 2007, ARL has been developing novel fabrication technologies to advance three- dimensional through-thickness reinforcement (3D-TTR) woven fabrics and composites; the goal of the work has been to enhance ceramic composite armor performance by reducing the ballistic damage zone around the impact point. This research is focused on integrated manufacturing and modeling and 15

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simulation efforts that, if successful, will result in materials-by-design tools that enable development of lightweight protection systems. This is more likely to be a structures-by-design development than a materials-by-design achievement, but the work can be useful for the development of 3D-TTR hybrid composite armor. This research is forward-looking and promises to achieve practical armor system design using advanced concepts of 3D reinforcement. Achievement of this goal will require ARL to develop its own internal weaving capability to implement the architecture suggested by modeling or to team with industry. It will be important for ARL to strategically determine which of these two courses of action it deems most promising. ARL has a long history of projects aimed at elucidating the property–performance relationship of armor ceramics and their applicability to armor design and ballistic enhancement. The armor ceramic projects are pursuing both an understanding of damage evolution mechanisms in silicon carbide-new (SiC-N) under dynamic loading and the use of nondestructive testing to quantify microstructure features within the ceramic, in particular the glassy phase along grain boundaries. SiC-N has been shown to fail under dynamic loading via intergranular fracture. This observation, coupled with its superior ballistic behavior compared to other armor ceramics, led the researchers to conclude that the intergranular grain boundary film (IGF) is key to better ballistic performance for boron carbide (B 4 C). Linkage of these observations with further utilization of in situ diagnostics seems a promising approach to quantifying the details of how these ceramics operate during ballistic impact. It will be beneficial to link the damage evolution studies with other research where nondestructive testing using impedance spectroscopy has been shown to be able to identify overperforming and underperforming SiC-N. Using scanning probe microscopy, the researchers were able to map the conductivity of grain boundary phases in the ceramic studied. Quantification of the relationships among microstructure, defect type and distribution of nondestructive characterization data, and ballistic behavior in armor ceramic materials is a laudable goal if used to support lot-acceptance testing for ceramic armor components. This nondestructive testing needs to be closely tied with both traditional ballistic testing and postmortem material damage analysis. Penetration Mechanics ARL presented an array of evolving fundamental and applied projects focused on the science of penetration mechanics; the development and implementation of new imaging and velocimetry diagnostics aimed at the quantification of penetration linked to lethality; and a view into the continuum-level models under development for ceramic material response that attempts to bridge the scale from meso to macro. The utilization of advanced diagnostics to quantify the time-dependent penetration behavior of ceramics is both innovative and crucial to the development of models capturing the physics involved in armor penetration and thereby seminally important to design from the perspectives of both survivability and lethality. ARL’s team designed a multiple-head flash x-ray system for real in situ observations of projectile penetration into a ceramic armor surrogate. Rate of observation has been enhanced to obtain better imaging resolution. Because absorption scales with sample thickness, the team has also developed a novel photon doppler velocimetry (PDV) technique to track projectile penetration travel into the sample, enabling larger target studies. The x-ray technique was used to determine dwell time during initial penetration and how that can be used to design stacked ceramic armor. ARL’s ceramic material model development work was highlighted in several poster presentations. Innovative mesoscale models from actual material reconstructions are under development to inform macroscale continuum models. Improvements were made in coupling of constitutive models to the host codes to better handle the failure and fracture of materials. In one example, a predictive tool using the Kayenta ceramic model has been developed to predict the response surface associated with material shear deformation as a function of load. Results of limited ballistic tests performed to test the model showed good correlation with model predictions. This modeling effort is of a high standard, as demonstrated by the authors’ peer-reviewed journal article. In addition, work involving finite-element modeling (FEM) of tungsten carbide (WC) penetration into silicon carbide (SiC) was well integrated with 16

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experiments performed at various rates and with increasing complexity that favorably predicted dwell transition and penetration velocities in the high-rate loading regime. Adaptation of the plasticity model (Kayenta), originally developed for geological materials, to model the mechanical response (tensile failure) of WC reflects innovative modeling through incorporation of the material model development into shock physics finite-element analysis. This work is well connected to material modeling work conducted at Sandia National Laboratories and the University of Utah. It is important to step up efforts to demonstrate how this knowledge and these insights will contribute to the design of armors effective in defeating WC projectiles. Research into the development of depleted uranium alternative projectiles, including in a segmented-rod form, has been an area of focused research at ARL for over a decade, when it became clear that cleanup of depleted uranium after warfare is both hazardous and costly. There has been little choice politically, therefore, but to investigate means to further enhance the ballistic-impact performance of conventional tungsten heavy alloy (WHA). A significant achievement has been the development of a rigid-body penetrator, in which the strong and tough WHA contains one or more embedded inserts of very hard tungsten carbide/cobalt (WC/Co). Extensive impact testing has demonstrated that the location of each insert in a segmented-rod WHA is critical to achieving optimal ballistic performance at oblique angles of attack. The use of aligned short segments minimizes or eliminates the susceptibility of a long rod to the bending stresses experienced following oblique impact. Excellent progress has been made in this challenging area. In particular, the embedded WC/Co insert appears to be a solution to the oblique- angle impact problem. Building on this achievement, there is the prospect of further improvements in ballistic-impact performance via the use of inserts of multimodal-structured WC/Co and diamond-hard- faced WC/Co. Notwithstanding this progress, there are strategic needs for further development in this area, given the evolving stratagems of the Army surrounding future weapons and vehicles; these needs are addressed later in this chapter. ARL’s successful application of new experimental instruments and diagnostics in new size and time-scale regimes—including optical photography, flash x-ray cineradiography, and new imaging techniques from other institutions such as the national laboratories—to the quantification of in situ penetration into armor is exciting, and ARL is to be congratulated for actively pursuing these diagnostics. Collaboration with the national laboratories has included the application of models and codes and the use of experimental facilities and instrumentation techniques, both of which are very positive; the project using Los Alamos National Laboratory’s proton radiography facilities and applying Lawrence Livermore National Laboratory’s PDV technique are particularly noteworthy. Both efforts appear to be especially successful. The principal opportunity (and challenge to ARL management) is how to effectively expand and accelerate this work. The focus of the imaging and velocimetry technique development for impact studies is to identify, enhance, evolve, and develop current state-of-the-art diagnostics to increase information gathered about material state, structure, deformation, kinetics, and dynamics during impact and penetration experiments. Specifically, this work is addressing imaging diagnostics that push toward greater spatial and temporal resolution, laser-based interferometry diagnostics that probe interactions at enhanced temporal resolution, and diagnostics that can identify material state in a multiple-material mixed environment. Techniques being addressed include high-speed flash x-ray cineradiography, proton radiography, x-ray phase contrast imaging, and multicolor flash x-ray computed tomography that has the potential to resolve multiple materials in a reconstructed 3D space that is critical to predictive model development. This effort can be expected to enhance ARL capabilities important to advancing fundamental understanding of impact/penetration phenomena and is strongly encouraged. Research investigating phase field modeling (PFM) of fracture and twinning in brittle solids addresses an area relevant to the fracture of ceramic armor materials. This is good and interesting fundamental materials work. The driving focus behind this research is motivated by the observation that polycrystalline armor materials such as ceramics and metals often demonstrate twinning and transgranular fracture at the single crystal scale. In very high strain rate situations, even brittle materials can undergo plastic deformation, by dislocation motion and by deformation twinning, as well as fracture. In order to 17

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investigate the competition between twinning and fracture, a PFM has been developed and tested on single-twin and single-fracture events. For twinning, the free-energy functional includes the elastic field (which changes nontrivially upon twinning) competing with a twin/matrix interfacial free energy. For a single twin forming under an indenter, this model captures both reversible and irreversible twinning. For fracture, the free-energy functional involves a balance between the elastic energy released and the surface formation energy—that is, a Griffith criterion for fracture. Crack initiation and opening were demonstrated in various notched sample configurations. This research is a new application of PFM and offers a promising method for probing shock behavior in complex microstructures. Although the ultimate payoff may be several years in the future, it is an effort worth pursuing. This fundamental research project is building a foundation for future modeling and is considered promising. PFM is a good addition to ARL’s suite of computational capabilities. Composite model development to support ballistics predictive capability is being pursued via numerical models aimed at understanding how the woven portion of the armor package can be optimized to increase penetration resistance. This research specifically addresses implementation of a woven fabric model to simulate the response of soft armor to the impact of a debris cloud generated by buried charge, such as that from an improvised explosive device (IED). Improvements being made to the material model based on experimental work by ARL and academic partners, introducing stochastic variation in the fibers and reductions in stiffness and strength due to the weaving process, are considered innovative and worthy of continued investment. Work is needed to effectively apply the experimental results to further model refinement and to verify the validity and value of the model. Determination of the mechanisms controlling penetration in lightweight materials is key to achieving future lightweight armors for both personnel and vehicle protection. Results presented for aluminum alloy 1100-O showed that for a 30 percent cold-rolling reduction, a dislocation cell structure was observed; for 70 percent reduction, the cell density increased and a laminar microstructure began to emerge; and for 80 percent reduction, a fully developed laminar structure was formed. This correlation enabled the variation of spallation pullback velocity with shock resistance, with peak shock stress to be investigated for each Al-alloy microstructure. For the 30 percent reduction, the variation with shock stress was not monotonic, whereas for the microstructures with higher dislocation content, the variation in shock resistance increased or at least did not decrease with increasing shock stress. This work provides a possible window into the effect of microstructure on blast resistance. The interaction with university and international research partners was a strong point. The project demonstrates a solid step toward developing an understanding of the effects of microstructure on Al-alloy armor blast resistance using modeling and simulation (M&S) tools. The work reflects good leveraging of interactions outside ARL. Humans in Extreme Ballistic Environments The humans in extreme ballistic environments activities appear to be well organized, the technical strategy is well posed, and the current state of science and technology in this area is well defined. The design, modeling, and testing of the warrior injury assessment manikin to test the effects of extreme acceleration and loading effects associated with underbody vehicle blast is clearly an area unique to Army mission challenges and well connected to warfighter needs. This innovative and collaborative effort to collect data required for predicting injuries to support the design and sensor placement on anthropomorphic test devices is to be commended. Ties to the medical communities to map current wartime injuries and subsequently inform vehicle and warfighter equipment to reduce injuries and enhance survivability are excellent. ARL is to be commended on the excellent partnering with university and external experts related to how experiments are conducted and data collected. The program appears to be well run and technically sound, but there appears to be insufficient collaborative activity on the physiological effects of kinetics to inform research on what kinetics can be tolerated—for example, the limits for traumatic brain injury (TBI). 18

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The project on evaluation of the effects of blast and ballistic protection on soldier performance included modifications to two soldier equipment items that positively improved warfighter protection. These items included a helmet support device (to address the tendency of the head to drop forward under the burden of the helmet and night vision goggles) to maintain the helmet in an optimized position for protection and a mandible guard addition to the helmet. The team demonstrated that the mandible guard interferes with common weapon aiming and firing and thus presents an integration challenge. These investigations included both live soldier tests and laboratory assessments. The live soldier tests were performed on a soldier sitting in a chair and a soldier navigating an Army obstacle course. Both projects represent innovative and timely attention to addressing warfighter needs and are examples of excellent integrated research and technology applied to short-term warfighter needs. The project on soft protection/continuous fiber woven composites is addressing a critical near- term warfighter need for groin protection that balances protection, comfort, and flexibility. Systematic investigation of various available aramid yarns (yarn denier), knits, and felt constructs starting with insights gleamed from the U.K. underwear options already deployed are under evaluation for groin ballistic protection. The scientific and engineering approach addressing this near-term warfighter need encompasses very promising options, and exploration of additional fabrics and weave options is encouraged. Teaming with industry appears particularly crucial to this endeavor. Two projects, one theoretical and one experimental, are addressing head protection through strongly coupled modeling. The integrated approach for improving low-velocity-impact head protection via an ARL-developed finite-element model (FEM) for head impacts while wearing a helmet is clearly responsive to a key Army priority; such low-velocity impacts may be a result of falling or of exposure to an explosive event. Present helmet pads are effective for impacts at about 10 fps, but the objective of the ongoing work is to increase impact energy absorption from <<10 fps up to tens of fps to 150 g. To date, the model has been validated with experimental results in the range 10-14 fps, with interest in extending the validation for <1 to 20 fps. The modeling results presented have indicated pad characteristics that may meet goals, primarily for frangible or frangible elastic materials. Alternatively, an external helmet load-bearing fixture has been conceived. Both novel concepts have been prototyped, and there has been some initial testing. Such out-of-the-box thinking is to be lauded, but it is also reasonable to question whether a helmet is ultimately the correct approach or whether some form of back- or shoulder-mounted head protection device would perhaps be a more effective solution. This project appears to be an excellent example in which the numerical model supports experimental concepts and corresponding experiments verify the model and concept. What makes it a special case is that this work informed out- of-the-box conceptual thinking about external supports for the helmet and even a replacement of the helmet with shoulder- or back-supported head protection. ARL is encouraged to continue pursuing this area of science and engineering. The work on modeling of the head/helmet system subjected to blast and ballistic loads is leading to the development of a computational framework to define loading response to the head and the interaction with helmets as input to neuro-network analysis. Improvement and further development of the computational effort for both the helmet and the coupling to the head is encouraged. This is in line with the view of the ARL team, which recognizes the limitation of the current model and the importance of exploring new ideas for improvement and linking them to the g-force-loading helmet design project. The use of a torsional Kolsky bar to evaluate high-strain-rate characteristics and quantify the mechanical properties of viscoelastic polymers at very high strain rates has been reported in the scientific literature. This project is in support of quantification of the high-rate mechanical response of human tissues to facilitate the development of constitutive models to describe such tissue subjected to extreme loading. Such polymers could be used as synthetic surrogates for biological tissues and are therefore of interest to experimental and modeling efforts looking at ballistic and blast effects on the body. The experimental measuring techniques are complex, and the Army is on target in attempting to develop this capability. Unfortunately the Army investigators could not replicate the analysis reported in the literature. The finding, if true, is disappointing and important, because mechanical characterization methods at these strain rates are difficult and few. While it is understandable that alternatives are not readily available, it 19

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seems that a more rigorous follow-up is warranted. The Army is one of a very few organizations with a mission need for such data. Without more analysis of the Army modeling efforts and plans, one cannot discern the absolute necessity for such data nor the degree of accuracy required, but it seems certain that competence in this area is vital for the military. ARL is encouraged to continue to explore both experimental techniques/diagnostics and constitutive model development in the area of tissue mechanical behavior. A finite-element approach was developed to numerically model the forces of a bottom explosion on the warfighter’s foot and leg below the knee. The resolution of the computational elements supported modeling of all the bones and the soft tissue. Existing data were consistent with the model, so that both the model and data have been shown to yield a result indicating minimal foot damage for a short impulse of low-amplitude and low acceleration but major damage for a much larger amplitude impulse over a longer loading time. The team expects to refine the model and further compare it with experimental data; however, it is nearing sufficient validity to support examination of floor protection concepts that could reduce the impact from under-vehicle blast loading on the soldier. Work to explore extension of the model to evaluate blast effects on the upper leg and torso and potential means for mitigating those effects seems promising. This project presents an excellent example of a combined theoretical and experimental approach to developing a basis for relatively timely and inexpensive engineering trade-offs of concepts to improve vehicle design and safety systems. The project on methodology for evaluating small-caliber systems involves the application of a previously developed modeling tool to a newer small-caliber weapon. The predictions of the work are comparable to experimental results to the degree necessary for the field. The speed and ease with which this work was completed is ample evidence of the utility of the model for addressing practical military ballistic and warfighter weapon needs. It is, however, difficult to see a clearly defined research component in the current work. Any innovative steps in the construction of the model are years in the past and were not presented. This does not detract from the accomplishments or the successes of this project, but it is not clear what further fundamental development of the model is planned or needed. The project applying survivability analysis to body armor decisions using the operational- requirements-based casualty assessment (ORCA) code analyzed the torso for vulnerability to frontal ballistic trauma. The analysis was repeated for two body armor configurations. This analysis provided data that could be used to compare the protective benefits of the larger armor against the drawbacks of weight and bulk. A similar analysis was used to compare injury and disability with and without protective undergarments. These data help bolster the case for these safety devices needed to protect the soldier in the field. It will be important to apply the ORCA code to all the classes of warfighter protective equipment deployed in theatre as well as to new equipment being designed and tested, and to clearly link the applications to the effort at validating the ORCA code. Computational Terminal Ballistics The lethal mechanisms and the blast and ballistic protection projects provided an interesting and reasonably comprehensive review of the broad scope of ARL work in these areas. ARL has a strong record of achievement over many years in developing advanced capabilities for defeating many types of enemy targets, and the recent and ongoing work described is building upon its tradition of excellence. The ARL effort to examine small combat units and scalable effects that extend through new and effective systems appears particularly well-conceived and thoughtfully planned. The glass research presentation described in depth work being conducted to develop an advanced fundamental understanding of the fracture behavior of glasses during penetration by a shaped charge jet and details of the interactions between the jet and the fragmenting glass. The effectiveness of glass, whether self-confined or mechanically confined by other materials, to resist penetration by shaped-charge jets has long been known and generally attributed to a dilatancy (bulking) effect, but the excellent experimental and computational work described builds on prior knowledge, particularly work conducted 20

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at ARL more than 20 years ago. This project involves a research strategy using highly resolved experimental investigations and high-fidelity computational modeling. The project incorporates state-of- the-art constitutive mechanical models developed at ARL aimed at discovering a mechanism for disruption of shaped-charged jets in glass targets. As such it is establishing a suite of experimental and computational tools that may be potentially applicable to a variety of extended studies. This is outstanding work exemplifying how experimental study and modeling can effectively use discovery science and research to drive innovation. This research is comparable in technical quality to that of other leading laboratories. The computational terminal ballistics overview described exciting new work focused on the effects of electromagnetic (EM) fields on the formation and breakup of shaped-charge jets. The phenomenon, initially discovered through computational analyses and subsequently examined computationally in some detail as well, will be better understood through systematic investigation in a series of well-structured experiments. The presentation had two major components: a broad overview of computational ballistics and specific results for computational model employment and development for EM armor applications. ARL has enhanced the ALEGRA multiphysics code from Sandia National Laboratories to incorporate ceramics modeling (Kayenta), extended FEM, Lagrangian material tracking, coupled optimization software (Dakota), and magnetohydrodynamics robustness and new materials. In the computational modeling effort for EM armor, specific accomplishments included successfully applying the enhanced ALEGRA model to assess the behavior of EM armor, identifying correspondence and important differences with experimental results, and developing a prototype design for a compact power source. This project exemplifies how ARL is utilizing and extending the best National Nuclear Security Administration modeling tools to address Army mission projects and deliverables. Coupling of these predictive tools with the combat vehicle vulnerability analysis modeling appears to be an area where a game-changing predictive modeling tool suite could be developed; it could positively impact phenomenological and operational system implementation and performance modeling of the future ground combat vehicle (GCV). The EM “squish” phenomenon was newly recognized as having potential value in helping to make an advanced capability more effective. The basic physical mechanism is understood, and the Sandia model is used to explore alternative configurations aimed at optimizing the effect. This is the same model, however, used for the jet-induced plasma investigation, which is known to have a discrepancy that may also be relevant to this effect. One expects that the requisite modification of the model mentioned for the jet-induced plasma will also be required to achieve significant further progress in exploring the squish phenomenon. The project on flow strength of polymers modeling focuses on atomistic to FEM and is an excellent start toward interfacing atomistic and continuum models of polymer mechanical behavior. Expansion of this modeling multi-length-scale approach is strongly encouraged as a path forward to address the distinctive behavioral differences at high strain rates exhibited by polymeric materials. The modeling and simulation of military operations on urban terrain (MOUT) target penetration project has completed some target analysis and quantified the margin of error. In order to match experimental data, researchers had to divide the solution space and solve the equations using two different techniques. That they were able to predict results within 10 percent is considered to be a strong and very promising technical ARL accomplishment. The project on reduced-order modeling of underbody blast is an in-house effort that developed a simplified modeling approach amenable to rapid determination of blast-loading histories on critical Army targets. Simplified assumptions are made that attempt to represent the essential aspects of impulse loading without resorting to a detailed three-dimensional (3D) computation of the blast response. This modeling is useful for rapid turnaround system evaluations. Linkage of this modeling to a testing program that evaluates the effectiveness of the modeling is clearly necessary to validate the accuracy and to quantify the margin and uncertainty of the model. The project has also determined a set of analytical solutions that could be used for verification of the simplified numerical model and its mathematical implementation. This work represents a step beyond pure empirical modeling that may be appropriate for 21

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Monte Carlo or system analysis. The simplifications that are used impose a degree of uncertainty in defining loading histories, however, because impulse is an integrated quantity and the uncertainties may be acceptable for some system evaluations. Key to this effort is determining the limits of the applicability of the simplified model approach. The jet-induced plasma characterization project clearly represents a discovery science project. It is based on a particular concept that guides the parameters of interest. It includes an experimental investigation to characterize the plasma. A Sandia model was employed by Sandia collaborators to capture the characterization in a model of the plasma jet. This comparison and modeling and experiment resulted in the discovery of an apparent discrepancy. Further experimental measurements have begun to determine the source of the apparent discrepancy. As more data are obtained, Sandia plans to revise the computer code and expects that modification to require significant effort. In the meantime, the experimental results have provided evidence that can advance the concept. While much remains to be done to complete the investigation, the next step would be to explore a practical implementation approach to armor protection. Demonstration of the utilization of reduced-order modeling of underbody blast for estimating and evaluating lower limb soldier injuries in vehicles subjected to blast loading is both important and timely to inform new vehicle design considerations. The project illustrated completed-scale impulse tests of flat plates and V-hulls to validate underbody models, which were used to support analyses of alternatives for joint light tactical vehicles and to inform design strategies for the GCV. Reduced-fidelity models to support system engineering trades and program planning and execution decisions are an extremely important line of model development. The project on novel penetrator efficiencies is focused on segmented penetrators. It also involves the development of extending rod penetrators. Segmented penetrators were the topic of intensive study at least 20 years ago, but the largely proof-of-concept effort was of limited success. One of the challenges is defining appropriate and credible baselines for comparison, which are greatly needed. The researchers on this current updated look at segmented penetrators appear to understand the importance of developing credible baselines for comparison. Some results to date with respect to achieving and maintaining desired separation in flight and segment colinearity during penetration are promising. The potential benefits of segmented rods may become increasingly evident as impact velocities extend well beyond the current conventional ordnance velocity regime of ≤1,600 m/s. A particularly interesting means for extending the rod close to the target and locking the segments together has recently been transitioned to the U.S. Army Armament Research, Development and Engineering Center (ARDEC) for possible application in next- generation kinetic energy (KE) and depleted uranium (DU) replacement programs. As noted for segmented penetrators, it is imperative that credible baselines be established for performance comparisons to monolithic, nonextending rods. In the vehicle protection armor modeling project, the goal is to explore armor concepts using modeling and simulation to gain a fundamental understanding of the mechanisms at work and how ARL can exploit them to defeat current and future threats to Army platforms. Proven modeling and simulation tools can be extremely useful in exploring advanced armor concepts. Such tools have been in a continual state of evolution for many years, with much of the work being conducted at the Department of Energy national laboratories. The overall validity with respect to both large-scale deformations and specific material behaviors, as well as the ability of the models to effectively model target/threat interactions for a range of threat types (KE, shaped charges, explosively formed penetrators [EFP], and blast), is critical. This is important work and clearly will be helpful in guiding ARL armor concept development efforts and setting the stage for follow-on, well-defined, proof-of-concept experiments and subsequent advances. Establishing and maintaining a strong link between this modeling work and system testing as validation is key to the development of effective predictive design capabilities. Implementing existing multiphysics modeling capabilities to simulate explosive armor performance, exploring several design possibilities, and conducting appropriate comparative experiments as a basis for modifying the model parameters represent a promising start to the development of a tool for designing explosive reactive armor. 22

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Development of modeling tools for both metallic armor and 3D hybrid composite protection systems appears to be an outstanding contribution to practical armor system design using advanced strength and damage models for metallics and ceramics and concepts of 3D composite reinforcement. The metallic modeling provides computationally based guidance for alloy development for armor applications. Combining strength and damage models followed by a parameter sensitivity analysis to determine which material parameters are most important for reducing penetrator damage in an aluminum (Al) and a magnesium (Mg) alloy represents a strong systematic approach to providing insight into armor design and performance. This analysis demonstrated that the work-hardening parameters characteristic of these materials are most important for new materials design, with failure strain ranked as next in importance. To implement the architecture suggested by 3D composite modeling, it will be important to strategically address the development of weaving capability within ARL. OPPORTUNITIES AND CHALLENGES An important overarching consideration in assessing specific research activities ongoing at ARL is whether the work can reasonably be expected to solve short-term critical warfighter needs encountered in theatre or is focused on the long term to have some potential to make a significant contribution to the eventual development of advanced capabilities important to meeting the operational Army’s warfighting, peacekeeping, and perhaps other mission needs. If these goals cannot be met, attention can be redirected to other areas. The opportunities and challenges are presented here in two categories: (1) overarching topics related to ARL’s overall science and technology (S&T) enterprise in terminal ballistics and (2) specific succinct opportunities and challenges tied to particular terminal ballistics thrust topics or projects. Overarching ARL Topics The materials presented did not always provide details of the programmatic ties and interplay with the ARL integration into the 6.1 (basic research) to 6.7 (operational system development) S&T infrastructures. These details would provide a richer context in which to assess the potential ability of the research to meet current Army needs and support the Army of the future. Further, how ARL is leveraging the Army Research Office’s (ARO’s) investment to support the near-term and long-term Army strategic vision was not always clear across all the projects presented. Examples of how individual projects fit into Army overall goals and relate to one another and to other ARL projects would facilitate the ARLTAB assessment of the quality of ARL’s S&T work. Model validation, which requires concurrent research of materials properties and performance, was clearly insufficiently defined and elucidated during the review for the majority of the projects presented. Some excellent examples of validation were shown at some level, such as in the MOUT project, but this was not seen throughout the review. Too often, just a computer-based visualization of a model was presented with little or no quantitative comparisons to data. Details of complex material and structural models matter, but these, along with the basis for choosing model parameter values, were seldom discussed. When considerable simplification of geometry or assumptions of material behavior is made, it is important to provide data justifying such approximations. The success of a model in reproducing a visual image of the overall phenomenology is not validation. It is important to achieve delineation on a project by project basis. Is validation sought for that project's scope of work to determine whether a detailed comparison with quantitative data is warranted, or is validation for this project deemed to be the ability to predict trends in response or performance so as to map out regions that would define and limit experiments? A rigorous formal internal validation program is needed within ARL to quantify the extent to which the physics within the broad spectrum of ballistics models is being developed to accurately describe the physics operative. Given the importance of such models to develop predictive 23

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design capability in support of current Army programs and future system, platform, and equipment development, increased emphasis on validation is warranted. In addition to the need for an ARL-wide strategic approach to model validation, methods are needed to quantify the margin of uncertainty (QMU) for these models. For example, it is not clear how the ORCA and MUVES-S2 1 models are validated. The reviews often lack sufficient details on how ARL’s models are formulated and validated; the sensitivity, if known, to key parameters and variables; and the statistical variations to be expected. Also, it is necessary to present the error bars in comparisons between models and data. ARL’s staff is not as visible in professional technical societies and technical conferences to the extent that their accomplishments and scientific expertise warrant. Obviously the sequestration and travel restrictions have negatively affected staff interactions with the outside R&D community. Lack of interactions through conferences and professional associations will have a deleterious effect on collaborative efforts and on maintaining the edge in areas of expertise. This has already affected morale and opportunity cost, and it will pose serious consequences for retaining and hiring staff in the future. In the poster presentations, there were examples of technical work that suffers from a lack of external collaboration. Moreover, ARL’s strategic focus on innovation through adoption and development of scientific ideas and insights from the scientific community cannot be applied to solve Army problems if it is forced inward. If this situation is sustained, a not-invented-here syndrome will be nearly impossible to avoid, leading to internal reinvention of wheels that would be better brought in from outside. ARL’s damage and failure modeling across the spectrum of materials of relevance is less technically evolved and therefore less predictive than the strength and equation-of-state modeling capabilities within ARL presented during the review. It is important to increase efforts in this area, given its importance to ballistics science and technology. Physically based damage modeling needs to include the statistical aspects of how and where damage evolution and failure occur in a material. This includes identification and modeling of the damage and failure mechanisms in biological and soft materials that as a newer field represent a challenging scientific problem. It is also important to explore strengthening the staffing and collaboration in this area with external university and national research laboratories and the medical community. Materials for Terminal Ballistics It appears crucial for ARL to develop strategic thinking behind internal investments, program and mission deliverables, and staffing planning to support the ability of the Army to meet the national security mission needs of the future. This strategic planning appears particularly poignant as the future GCV design pathways are fixed. For example, while glass, effectively confined, is known to have potential for contributing to the defeat of shaped-charged jets, explosive reactive armor (ERA) and even nonexplosive reactive armor (NERA) have greater potential, and ERA is already being utilized with great effectiveness. Ceramics similarly can be very effective, but only when very effectively confined, which currently makes them too expensive for implementation in vehicle protection applications. The key questions are therefore these: Which of ARL’s current areas of S&T are sufficiently mature in the area of materials for terminal ballistics to meet current and projected performance criteria in specific applications? Which have been found, for reasons of performance or cost, not to warrant further continued effort at the expense of new innovative S&T areas? Better characterizing and qualifying the materials ARL receives from various suppliers will help to make engineered systems deliver the expected performance. It is important to identify the microstructural features to measure and the property or properties in next-generation aluminum alloy armor that correlate with ballistic performance. It may be strength and (quasi-static) fracture toughness as measured so far, but that remains to be verified. Assessing the ballistic performance of the developed alloy is crucial to determining whether research on this alloy should continue. 1 MUVES-S2 (Modular UNIX-based Vulnerability Estimation Suite) is a software-based modeling tool. 24

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Mechanical performance of nanocrystalline alloys for shaped-charge liners will certainly be a function of microstructure, which in turn arises from processing. The research would benefit from a grain scale modeling component, including both microstructural evolution (sintering and grain growth) and mechanical response (ductility). When combined, these models can not only predict resulting structures but can also suggest optimized microstructures. This may be a much more efficient approach than iteratively reprocessing to achieve optimized material properties. The Office of Naval Research (ONR) has some interest in these systems. The possibility of partnering with the Navy on this topic is worth investigating. It is worthwhile to expand the research to include variations in the volume fractions of the constituent phases. Near 50:50 compositions are likely to develop bicontinuous nanostructured composites, in which the constituent nanophases are interpenetrating in three dimensions. Such composite structures are extraordinarily resistant to grain coarsening at high temperatures, thus opening an opportunity for high-strain-rate superplastic formation, as observed for a tri-continuous oxide ceramic. The 3D through-thickness reinforcement (TTR)-hybrid composite armor development effort appears to be a structures-by-design development project rather than a materials-by-design achievement, although this research is viewed as having merit. Since this effort has been under way for more than 5 years, however, it is reasonable to ask what significant achievements it has recorded to date. Has clear proof-of-concept been established? This armor system has a very complex structure and geometry that will be extremely time-consuming to model at the level of the fiber or even the yarn. Considerable simplification will be required, and each level of simplification will require validation by some carefully designed experiments. This level of validation has not yet been done and has not even been planned. Without this, the utility of modeling for further refinement of such woven composite systems is compromised. All composite armor studies utilize existing fiber chemistries and processes, unchanged by the fiber industry for the past several decades. Translating the 3D-TTR effort from structure-by-design to materials-by-design will require the incorporation of fiber chemistry and processing expertise, either developed in-house or accessed externally. Recognizing the paucity of new fiber development by fiber manufacturers, next- generation materials will likely need to be developed in-house at ARL. The deliverables to be gleaned from elucidating the property-performance relationship of armor ceramics were insufficiently defined to show what the prior program accomplished. What results have been obtained that suggest this program will provide results useful to the Army? While one possible use could be to support lot-acceptance testing for ceramic armor components, it seems unlikely that it could replace traditional ballistic testing for this purpose. Ballistic testing remains a key acceptance/rejection basis for ceramic-enhanced small arms protective inserts (ESAPI) plates used in body armor. The strategic direction of this project needs to be evaluated. The project on ceramic microstructures for enhanced ballistic protection appeared to be retreading old ground. The work has shown that ceramics with fine grain size and IGFs have better ballistic performances than those with coarser grain sizes and limited or no IGFs. This work would be significantly enhanced by the use of transmission electron microscopy (TEM) to characterize grain boundary structure and chemistries, because the IGFs are believed to be key to intergranular fracture. Grain size and IFG effects on fracture have been extensively studied, and the researchers need to integrate the knowledge amassed in this extensive literature into their analysis. This program covers a large array of materials ranging from commercial aluminas (why these aluminas were chosen was not clear) to B 6 O, AlB 12 , AlMgB 14, and composites. At present there is little fundamental perspective. What is new and promising about this work? Are ARL researchers aware of previous work reported in the open literature or in government reports that has been done assessing microstructure vs. ballistic performance correlations? The goal of the project on advanced materials and processing for soldier protection is to identify the high-rate mechanisms, materials, and architectures and the innovative processes and concepts for enabling quantifiable improvement in key aspects of soldier-borne protection for both head and body. The focus on improved composite designs for helmets, which is exploring the effects of existing, commercial polymer yarn constructs for better ballistic protection while using state-of-the-art modeling to 25

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identify improved yarn ply orientation patterns, is very positive and forward thinking. While integration of this modeling with other types of body armor or lightweight vehicle armor was not discussed, it needs to be strongly encouraged even if the current goal of defeating of a 7.62-mm small arms threat represents a perhaps insurmountable objective in a helmet of a tolerable weight. In the advanced materials and processing for soldier protection project, the focus was on a ballistic helmet capable of defeating theater- relevant small arms threats, new insight and approaches to mitigating the shock and adverse impulses associated with impact, and an ESAPI system solution capable of meeting the objective threats at a 10 percent lighter areal density. The goal of achieving a ballistic helmet capable of defeating 7.62-mm small arms threats, which is very likely only achievable at a total helmet weight that is intolerable to a user, poses a virtually insurmountable challenge. A 10 percent reduction in areal density (AD) for ESAPI body armor is a realistic goal, but the strategic planning needed to achieve this goal was not described. One stated planned accomplishment was initial multiscale technology integration to demonstrate small arms protection at effective AD of 3 lb/ft2. This is almost certainly not going to happen. The Defense Advanced Research Projects Agency (DARPA) spent many millions of dollars trying to do this for body armor (3.5 lb/ft2 goal) a decade or so ago and accomplished virtually nothing. The objectives of this project need to be evaluated. Penetration Mechanics ARL’s penetration mechanics program is an ambitious effort aimed at merging state-of-the-art modeling with new experimental diagnostics. This is a great challenge that could advance the science of penetration mechanics. Predictive capability, however, will only be achieved if bridging the scales from a modeling perspective is strongly pursued, coupled with a strong program in material damage evolution and failure modeling. The time-dependent penetration behavior of ceramics project described application of a flash x- ray and PDV to reverse ballistic testing of metallic penetrators into subscale ceramic targets; this effort represents a positive application of evolving diagnostics to Army problems. PDV appears to be a useful new tool for large-sample studies able to track particle velocities over longer time intervals than velocity interferometer system for any reflector. This body of work provides real-time data that are critically needed for model development and, thereafter, verification and validation. Although the x-ray technique has been used for years, its use in materials studies remains critical. The PDV work appears to be a key new tool in future ballistic testing, but only if tied to quantitative analysis of the deformation and fracture mechanisms during terminal ballistic experiments and then as input to improving computational models applicable to lethality and protection technologies. Dwell was first recognized as a notable consideration in the performance of hard-faced armors at Lawrence Livermore National Laboratory (LLNL) in the late 1960s. ARL initiated work focused on dwell many years ago. A critical question is: What has dwell- centric research to date achieved toward the development of superior ceramic armor materials? It is necessary that a strategic overview of this topic be undertaken. Ceramic material model development is an activity of critical importance at ARL if it can lead to creation of a predictive modeling capability for application of ceramics and other materials in Army armor and lethality systems. Significant efforts have been conducted by a number of organizations over many years, including focused work supported by DARPA, that have not achieved the goal stated for the modest ARL effort. ARL claims that improvements have been made in the coupling of constitutive models to the host codes in order to better handle the failure and fracture of materials. What advances with respect to predictive capability have been achieved? ARL also claims that a variety of simplified ballistic experiments that examine the time-dependent failure of materials have been conducted to validate the improved material models and codes. What have these experiments shown? No details that would elucidate these questions were presented. 26

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Research in depleted uranium alternative projectiles is a project crying out for strategic planning and context definition for future Army needs and ties to Army strategic planning linked to new vehicle designs. The presenter stated that significant progress continues in developing nanocrystalline tungsten (W)-based composites as depleted uranium (DU) substitute materials; however, few specific accomplishments were cited. Research to develop non-DU projectile materials having at least comparable performance has been under way for more than three decades. Other ARL work included in the DU-replacement effort that is directed toward improving the performance of (sheathed) tungsten carbide (WC)-based projectiles against oblique targets may be of some value. The researchers need to consider the applicability of diamond-hard-faced tungsten carbide/cobalt (WC/Co) alloys as inserts in segmented WHAs. These materials have been under continuous development for decades, and today they are the materials of choice for drill bits in the oil- and gas-exploration industries. They are available commercially in disc-shaped forms for drag bits and as profiled inserts for roller-cone bits. The diamond hard-facing is actually bonded with Co, as is the underlying compositionally graded WC/Co, thus imparting fracture toughness (bend strength) to the graded composite material. Another option is a multimodal-structured WC/Co, which can be fabricated via liquid-phase sintering of mixed powder compacts, even though the Co content is <2.0 wt-percent; normally, at least 10 wt-percent Co is required to ensure complete densification, which incurs a weight penalty. A denser WC/Co insert that is harder and tougher should be advantageous. For kinetic energy (KE) penetrator applications, presenters did not explain what they have gained by recently focusing on nanocrysalline materials. The researchers noted that the engineering properties of these new materials remain quite poor. They exhibit minimal ductility and toughness and resist efforts to integrate them into KE projectiles. It may be that this challenge cannot be surmounted. Work with sheathed penetrators was also mentioned. This area was also explored extensively at least as far back as the early 1980s. The presenters did not demonstrate much awareness of prior work in this research area or of lessons learned contributing to the present effort. It is time for some focused strategic thinking on the objectives, the Army needs, and specific goals in this research area rather than continual repetition of past approaches. Phase field modeling (PFM) of fracture and twinning in brittle solids is tied to the observations that polycrystalline armor materials such as ceramics and metals often demonstrate twinning and transgranular fracture at the single crystal scale. In this work, phase field theory and numerical simulation are used to model these phenomena; this may provide a payoff for the Army in the long run. This project would benefit from interaction with ab initio or empirical atomistic modeling as well as experimental work; it could supply data for input (surface energies, for example) as well as information for validation (twin size, nucleation mechanisms). This work would benefit from integration with the academic phase field modeling community. Collaboration and insights into the state of-the art currently available in this area are yet another casualty of the ill-advised government policy that restricts conference travel. In PFM, interfaces are diffuse, which may affect fracture propagation (by smearing the crack tip discontinuity). The effect of the diffuse interface on fracture predictions merits attention. The extent to which this work might ultimately benefit the ARL mission needs to be articulated. Assessment of the quality and ties to strategic Army objectives of the composite model project is difficult because development of the work is at such an early stage. The goal of creating a method for evaluation of optimal, feasible, and cost-effective fabrics is appropriate. The stated steps to improve and validate the model are essential but have not yet been taken. Examination of the composite model development to date leads to several strategic investment questions. Will the model represent knitted materials as well as woven? Will it be possible to validate this model for nonrepeatable experiments or experiments with a large QMU? Will the model be able to effectively represent laminates of materials? The researchers on the project on tailored mechanisms for light armor ballistics articulated the goal: to develop a fundamental understanding of the deformation mechanisms and failure processes active under shock loading conditions for light armor materials such as aluminum and magnesium and then, using key discoveries, to control ballistic performance. Dynamic fracture testing, using plate impact assemblies, was conducted on as-received 1100-O aluminum cold-rolled to 30, 70, and 80 percent 27

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reduction to study the effects of microstructural evolution on spallation response. While an understanding of the relation between processing and microstructure and blast resistance for aluminum alloys is interesting physical metallurgy, its relation to improved armor was not defined. Further, the real purpose of this work or its value to the Army are unclear. After decades of working with metals such as aluminum for armor applications, the M134, Sheridan tank, and the Bradley fighting vehicle and of seeing their vulnerabilities to mines, rocket-propelled grenades, KE threats, and IEDs, it seems appropriate that ARL is finally looking to develop a fundamental understanding. Humans in Extreme Ballistic Environments The strategic, integrated system approaches to both the warrior injury assessment manikin (WIAMan) and humans in extreme ballistic environments seem headed toward significant near-term improvements in soldier protection. The fundamental underlying research was not described in detail, so it is not clear whether a breakthrough in the understanding—for example, of the cause(s) in traumatic brain injury—might lead to further breakthroughs in armor protection. Linkages to more modeling and simulation are encouraged as a way to facilitate more predictive performance capability development. Data on physical differences between male and female skeleton and body structure are now required to complete a female war fighter manikin development program. The project on evaluation of the effects of blast and soldier protection measures on soldier performance faces several challenges. The research was not connected to research at other institutions (e.g., aviator helmet research across the Department of Defense (DoD) or sports helmet research) to foster the best innovation The metrics for physical performance were insufficiently defined, and no quantitative results were presented. There was no sign of substantive interactions with other institutions performing human performance modeling, testing, and simulation. Overall, what was presented was a series of demonstrations rather than a description of basic scientific research or engineering development. This line of investigation is important, and if the quantitative rigor of the work can be enhanced there is great potential for it to make a significant contribution to the field and to the engineering of soldier equipment. The soft protection continuous-fiber woven composites project is strongly tied to yarn and fabric mechanics expertise, which is not available in-house but could be brought in through consultants. It was unclear how much deformation of the fabrics studied would be equivalent to fabric penetration; this is important for model validation. Understanding of the complex parameters that lead to fabric comfort is also expertise that does not exist in-house but it could be accessed by engaging consultants. A question arises: Would it be worthwhile for ARL to consider developing a broader in-house manufacturing capability to support related projects and equipment development in the future? The project on an integrated approach for improving head protection against low-velocity impacts is focused on the need for energy dissipation over a broad range of low- velocity head impacts. This has resulted in the helmet pad investigation; it has also led to a novel shoulder-supported fixture and has called into question whether in the long term a helmet is the optimized solution for warfighters. This opens the door to new ideas for devices supported not only by the neck but also by or only by the shoulders or back (a space helmet) of the warfighter. Such approaches might help solve the low-velocity problem, might proved support for increased helmet weight necessitated by cameras and electronics, provide the basis for increased ballistic protection, and perhaps open the possibility of supporting more electronic functionality. Continued research in this area is encouraged. Linkages between this project and the modeling effort addressing the head/helmet system subjected to blast and ballistic loads are suggested as a positive avenue of research. Assessing the validity of neuro-network analysis is so challenging that it is not likely to produce short-term applications. The project on applying survivability analysis to body armor decisions appears to be simply using an existing design tool for design analysis. The model did not produce quantitative data that were not self- evident. Being shot in the torso (or femoral artery) is bad, and the closer to the heart and lungs, the worse is the effect. Smaller armor protects less of the torso. Wearing protective shorts prevents groin area 28

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injuries more effectively than not wearing them. The case for applying computational models (instead of mere design rules) to these problems needs to be made much more strongly. ORCA does not appear to include modeling of armor and its effects; to simulate armor, the projectile velocity is simply attenuated based on data from experiments or other models. Coupling ORCA to ballistics models would extend its utility as a design tool. Integrating ORCA with some of the more physical models being developed in the WIAMan project efforts appear to be a fruitful avenue of research. Computational Terminal Ballistics The computational terminal ballistics presentation could have been made more effective by systematically addressing the modeling of kinetic energy penetrators, shaped-charge warheads, and EFPs in sequence—specifically, stressing the differences in their lethal penetration mechanisms and clarifying why each type of penetrator is effective against certain targets. This would have effectively set the stage for the blast and ballistic protection overview that followed the presentation. The fact that the ARL ballistics modeling uses approximately 87 percent of Army’s high-performance computing (HPC) resources and approximately 65 percent of DoD resources overall is a serious challenge for future computational modeling. Although a plot of HPC resource growth was displayed, there was no analysis to show that the future computing capabilities would be adequate for ARL needs, much less new HPC needs that might emerge across the DoD enterprise. For the project on vulnerability analysis of ground combat vehicles, more information on consideration of operational systems would have been helpful. Throughout many of the terminal ballistics briefings and posters, reducing the weight of combat vehicles through lighter armor and faster and more effective lower- caliber munitions was clearly central to ARL’s strategic vision. However, it appeared there is not a clear set of objectives associated with the operational concept options, just an interest in providing options for performance versus size and weight to the requirements community. It seems that a common vision for cost and weight reduction, while at least maintaining capability would have provided a useful context for assessing this work and perhaps for the researchers. The jet-induced-plasma characterization effort is simultaneously a high-risk and potentially high- payoff project. The physical characterization is still under way, so the potential payoff is a long way off. Even if successful and able to move to a higher level of technology readiness, the concept would necessarily be only one element of a layered capability. The project is an ongoing collaboration between ARL and Sandia. The results of experiments show signs of promise, but the phenomena have not yet been fully characterized, and continued modeling and validation are strongly encouraged. If the EM squish phenomenon turns out to be promising, and if a follow-on exploration with a laboratory prototype shows it could be feasible, this approach can enhance other experimental effects. The effort is presently a numerical investigation using a model shown in another project to be lacking in a key area of physical characterization. Until the model is corrected and there is an understanding of the impact of the correction on the model’s accuracy, this phenomenon needs to also be investigated experimentally. Even then, the relative improvement in advanced capability may not be as significant as predicted by the original concepts for the amount of additional equipment required. The project on flow strength of polymers, covering the length scales from atomistic through continuum, would benefit by collaboration with existing polymer rheology and molecular modeling communities. The results need to be applied to anisotropic systems and other chemistries. Validation of the model against experimental data is crucial. Both of the projects on reduced-order underbody blast modeling displayed many simplifications in the modeling. Further development may be necessary to expand the range of applicability of the modeling approach. There was no clear indication of specific progress on these projects since a review conducted by the ARLTAB in May 2012. Plans for model validation were not discussed, and plans for future accreditation by the U.S. Army Test and Evaluation Command were not satisfactorily explained. 29

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The LF2XA explosive model parameterization approach is appropriate for ideal explosives; however, this explosive is likely a nonideal energetic material. Hence the variable reactive burn modeling needs to be regarded with skepticism. The calibration of this model was done using highly resolved CTH Eulerian computations. 2 However, the re-parameterization of the model in ALEGRA to replicate the CTH results may be the result of insufficient numerical resolution. Further verification of the modeling is required. Model calibration is linked to sustained planar shock experiments, and the applications to other shock loading conditions—that is, thin pulse or nonplanar projectile loading—may be far enough removed from the states of the model calibration conditions. This work follows a traditional approach in computation of shock-initiated reactive flow. Although there are recognized weaknesses in this approach, it may be sufficient for many studies. The researchers working on novel penetrator efficiencies who look at segmented penetrators appear to understand the importance of developing credible baselines for comparison, and some results to date are promising with respect to achieving and maintain desired separation in flight and segment co- linearity during penetration. The potential benefits of segmented rods may become increasingly evident as impact velocities extend well beyond the current conventional ordnance velocity regime of ≤1,600 m/s. The work on extending the rod penetrator is also interesting, reflecting progress from earlier investigations into their potential. A particularly interesting means for extending the rod close to the target and locking the segments together has recently been transitioned to ARDEC for possible application in next-generation KE and DU replacement programs. As noted for segmented penetrators, it is imperative that credible baselines be established for performance comparisons to monolithic, nonextending rods, and validation to experiments is crucial. Strategic planning of S&T to support future Army needs for advanced KE is also crucial. The armor modeling efforts described are important work and clearly will be helpful in guiding the development of ARL armor concepts while setting the stage for follow-on, well-defined, proof-of- concept experiments and subsequent advances. Finding measurable performance parameters so that the model predictions and experiments can be quantitatively compared is very important. Quantitative validation of the modeling, or at least of its ability to qualitatively predict changes in penetration resistance with changes in design parameter, is also seminal to this effort. Use of the results of the modeling to develop simpler—that is, much less computationally intensive—predictions of penetration resistance that can be used for vehicle-level assessments appears to be an important avenue to pursue. Researchers on the project on armor material modeling and optimization stated that their goal was to determine which material properties have the most significant influence on ballistic performance of lightweight military specification metals; they noted that the goal will be realized by taking a design-of- experiments approach to modeling and simulation. Overall, the optimization effort seems sound, but to have an impact it will be important to translate the findings to the materials community for implementation. OVERALL TECHNICAL QUALITY OF THE WORK The overall quality of ARL’s applied research and development is very high. There is, as realized by ARL management, a need to focus more on the basic research that will underlie future developments. ARL’s existing S&T work in the ballistics area is very well served by the current Aberdeen Proving Grounds infrastructure and facilities. There was clear evidence of a speedy response to changing needs to support the warfighter with innovations in ballistic survivability and lethality. ARL’s experimental programs concerning threats are quite detailed and demonstrate commendable knowledge of the evolving threats. The spectrum of armor design demonstrated a broad array of technical approaches and flexible and rapid response. ARL’s staff is clearly motivated and competent, and all the staff members articulated 2 CTH is a multimaterial, Eulerian, large deformation, strong shock wave, solid mechanics code developed at Sandia National Laboratories. 30

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a well-defined line of sight from their research to the mission of the Laboratory and to the warfighter. All the briefings and poster presentations were well presented by the researchers. For the majority of posters, the work was state of the art and was properly juxtaposed with research at other institutions. For example, this aspect of the project on multiscale modeling of non-crystalline ceramics (glass) was impressive: The team is drawing on new results in nanotechnology, applying experimental equipment from geophysicists, and interacting with glass manufacturing R&D teams. In the Board's judgment, overall the basic science and the modeling of glass were first rate. Many of the posters presented displayed in-depth collaborations with outside organizations, including other DoD laboratories, academia, and especially the national lnaboratories. ARL collaborated with the national laboratories on both the application of models/codes and the use of experimental facilities/instrumentation techniques. Some of the new analytical techniques and diagnostics developed to follow projectile penetration were very impressive. For example, the project described in the poster on the development of imaging and velocimetry techniques for impact studies used the Los Alamos National Laboratory’s (LANL’s) proton radiography facilities and applied LLNL’s photon doppler velocimetry techniques. The observation of penetration phenomena at ever-smaller scales and faster times is crucial to the development of predictive modeling capability in the area of terminal ballistics and penetration mechanics. ARL is to be congratulated for seeking out the application of new diagnostic techniques to provide in situ data on penetrator–target interactions as a means to both discovery science and validation of the model for penetration mechanics. Additionally, the use of impedance spectroscopy and scanning probe microscopy for mapping grain boundaries in SiC-N was impressive. Many of the principal investigators of research projects at ARL have working relationships with universities and national laboratories. This is praiseworthy and deserving of encouragement by management. Beyond that we see an opportunity to enrich the experience of the principal investigators by establishing further collaborations and short sabbaticals where they could become directly involved in cooperative research at allied institutions. One-to-one interactions on a daily basis would almost certainly enhance productivity and possibly generate new ideas for further productive research. A good starting point for such a sabbatical might be one or two weeks every year. Of course, the reverse arrangement could also benefit a visiting researcher. The computational activities are in general well integrated into a large proportion of the research presented. Large, complex, and intensive calculations benefited from the use of externally developed, state-of-the-art code platforms, many developed at DOE. Many have been used in collaboration with other groups or national facilities, but some outstanding examples were developed in-house. There were extensive modeling efforts over a variety of length scales to follow penetrator–target interactions. Reduced-order modeling is an in-house-driven program that is clearly producing results for systems modeling. There was a clear demonstration of the interplay between materials and design of armor systems, which showed that close collaboration between materials, design, and computation efforts would be required to optimize performance. ARL is making good use of funds allocated to Small Business Innovation Research (SBIR) projects. Several administrators and senior technical staff cited positive experiences with various sponsored projects. In one case, a small-business entity has demonstrated, for the first time, growth of single crystals of aluminum oxynitride. This achievement has opened up exciting opportunities for basic research at ARL. It appears that the new processing technology can also be applied to other difficult-to- process materials. ARO also gains substantial leveraging from its Small Business Technology Transfer (STTR) projects. 31