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, and its review of energetics, interior, and exterior ballistics on July 15-17, 2014. This chapter provides an evaluation of ARL’s ballistics (including energetics) sciences core technology competency portfolio.
ARL’s ballistics scientific and engineering research efforts during 2013 and 2014 span both basic research that improves fundamental understanding of scientific phenomena and technology generation that supports interior, exterior, and terminal ballistic and energetic developments and fielded system upgrades. ARL’s energetics and ballistics mission scope is 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 the Army’s advanced weapon systems.
ACCOMPLISHMENTS AND ADVANCEMENTS
The ARL has a strong record of achievement and timely support of the warfighter as it develops advanced capabilities for defeating many types of enemy targets and platforms over many years, and the recent and ongoing work described in this review of ballistics demonstrates how ARL continues to build on its tradition of excellence in protecting the warfighter.
The reviews in 2013 and 2014 were divided into topic areas described in technical keynote presentations and posters covering materials for interior, exterior, and terminal ballistics, energetics, penetration mechanics, humans in extreme ballistic environments, and computational terminal ballistics. The oral and poster presenters demonstrated considerable knowledge of the technical areas addressed, displayed
strong enthusiasm for their work, and dedication to the missions of ARL supporting the warfighters and national defense. ARL’s efforts in energetics, interior, exterior, and 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. The Research@ARL monograph series on energy and energetics1 and materials modeling at multiple scales 2 are commendable. Specific accomplishments and advancements in each of the topical areas during the 2013 and 2014 reviews 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 knowledge 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 impressive; ARL is encouraged to continue to pursue this direction as a pathway to increased predictive 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 very positive.
Many of the materials for ballistics programs reviewed were very impressive. For example, investigation of next-generation Al alloy armor and the evolution of the Eglin armor steel are both promising research topics. Aluminum alloy armor design and the materials manufacturing technology for 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 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 by reducing its silver content, is very promising. Continuing to partner with industry on alloy development to achieve an Al alloy with desired yield strength, fracture toughness, and formability at a lower cost is 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. The manufacturing capability developed 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 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,
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1 Army Research Laboratory, 2012, Research@ARL: Energy & Energetics, June, http://www.arl.army.mil/www/pages/172/docs/Research_at_ARL_2012_s.pdf.
2 Army Research Laboratory, 2014, Research@ARL: Materials Modeling at Multiple Scales, July, http://www.arl.army.mil/www/pages/172/docs/research-at-arl.vol3-issue2.pdf.
the investigators 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 the work of other institutions, including new results in nanotechnology, applying experimental equipment from geophysics—for example, the diamond anvil cell—and interacting with glass manufacturing 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 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 than a materials-by-design achievement, but the work can be useful for the development of 3D-TTR hybrid composite armor. It 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 do it by teaming 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 (B4C). 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 adapted 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 was 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 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 timescale 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. The Army Research Laboratory Technical Assessment Board (ARLTAB) strongly encourages this effort, which is expected to enhance ARL capabilities important to advancing fundamental understanding of impact/penetration phenomena.
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 motivation behind this research is 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 deformation twinning as well as fracture. To 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 its academic partners, introducing stochastic variation in the fibers and reductions in stiffness and strength due to the weaving process, are innovative and worthy of continued investment. Work is needed to effectively apply the experimental results to further refine the model and to verify its validity and value.
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 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 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).
The project on evaluation of the effects of blast and ballistic protection on soldier performance included modifications to two soldier equipment items that 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. Various available aramid yarns, knits, and felt constructs are being systematically investigated for groin ballistic protection,
starting with insights gleaned from the U.K. underwear options already deployed. The scientific and engineering approach to addressing this near-term warfighter need offers very promising options, and exploration of additional fabrics and weave alternatives 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 FEM for head impacts while wearing a helmet is 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 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 can discern neither 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 to protect the soldier in the field. It will be important to apply the ORCA code to all 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 to validate 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 in the context of 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 is generally attributed to a dilatancy (bulking) effect, but the excellent experimental and computational work described builds on prior knowledge, particularly work conducted 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 might be 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 modeling to FEM and is an excellent start to 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 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 that 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.
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.
Energetics
The advanced energetics program that has been undertaken by ARL is an ambitious effort to achieve the next generation of high-energy energetics, which can lead to greatly improved energy densities and new formulations supporting core Army needs. Energetics in the energy density range above that of conventional explosives is a strong strategic choice central to ARL’s mission, and it is an area where relatively modest investment, if continued over a number of years, could make the laboratory a national leader. In terms of staffing and program direction, it is worth exploring the potential for tapping into the knowledge and experience of synthetic chemistry professionals, particularly by hiring from within the pharmaceutical industry. In the area of synthesis there is much to gain from leveraging outside resources, both industry and universities, and working to build ties to them.
The early results from the program are encouraging, and the staff currently in the program are excellent. It is important, however, that ARL expand support for this area through new hires and support for external collaborators. One of the primary goals of the advanced energetics initiative was to attract early-career scientists into this field, and ARL has been successful in hiring new explosive formulation and synthesis expertise into its workforce. In addition, ARL’s investment in multiscale modeling of energetics as an enabling technology is impressive, and ARL is certainly among the leaders nationally in this effort, alongside Los Alamos National Laboratory and Sandia National Laboratories. Continued development of these tools and of a coupled theory–experimental–simulation–validation program is important to strengthen and nurture the multiscale energetics modeling efforts.
ARL’s energetics team outlined a rational approach to the important problem of identifying candidate molecules for novel energetics. This group has recently been formed, but early progress is promising. Tetranitroglycoluril (TNGU) appears to be a good starting point for a more comprehensive investigation, and this work is encouraged. Explosives based on TNGU and related molecules have demonstrated potential for reducing sensitivity to accidental initiation while maintaining high energy density. In addition, the x-ray analysis of the amorphous polymorph of poly-CO produced in the diamond anvil cell revealed a short range order that was reproduced with a density functional theory (DFT) model. This is a significant step in understanding this elusive material.
New laboratory-scale methods for quantification of the explosive performance using laser-induced shock waves show great promise. The laser-generated shock-wave test to measure explosive performance of milligram quantities of energetic materials has demonstrated good correlation between measured blast wave velocities with conventional large-scale measurements of detonation velocity for a wide range of
previously-studied energetic materials. The method has also been utilized for the characterization of new energetic materials available only in milligram quantities, yielding results that are consistent with the expected performance of new, disruptive energetic materials synthesized by ARL. This work has provided ARL, and perhaps other laboratories, with an inexpensive, very-small-sample-size screening test for the experimental energetic materials being developed. Although the scientific basis for it is still being established, this method appears to be a promising tool for the identification and quantification of new energetic materials.
An optical method to monitor the reactions of metals in explosive materials has been developed and demonstrated with a boron/potassium nitrate (B-KNO3) powder mixture. The two-camera method with wavelength filters was used to monitor boron oxide (BO2), as well as material incandescence. This has enabled the first visualization of a key B reaction in real time. This work provides a new tool for optimizing metal-energetic material mixtures and can be applied to systems beyond the B-KNO3 test case. This research is impressive as a scientific study that also has very useful applications to energetic material research; the researcher is an expert in his field.
The modeling efforts across the energetics research area are showing growth and progress. The coarse-grained modeling research aimed at predicting the response of microstructured energetic materials demonstrates the need for this approach based on the large computing time required for a direct atomistic simulation. If successful, this development could reduce computation time by one or two orders of magnitude. The concepts in this research have been developed in the soft matter/polymer communities, and this is the first effort at extending them to hard energetic materials. The basic approach is to idealize the material as a hard sphere that interacts with its neighboring spheres by forces that are derived from an atomistic analysis of the compound in question. The idealization loses information about the interactions, and effects of the lost information are accounted for in the model by introducing random forces between the spheres. A model for chemical reactions, assuming the sphere is a perfectly stirred batch reactor, is also incorporated so that detonation can be modeled. A problem of a planar shock was analyzed using the model with 384,000 spheres and was compared with a full atomic simulation with 8,064,000 atoms; good agreement was found.
The large-scale DFT analysis of nanodiamonds to investigate molecular surface configurations that were then subjected to collision and implosion conditions is interesting research. The numerical modeling for large-scale DFT analysis demonstrated that this is a useful technique and gave encouraging results. Although these extended molecular solid models did not include stored strain energy, future work is being planned to use this modeling approach as a framework for investigating potential energy release mechanisms, and this extension needs to be pursued. Further efforts in the development of physically based energetics models are needed.
The High Performance Computing Institute (HPCI) research, funded by the U.S. Army’s ARDEC, represents a new idea in modeling that aims to eliminate the system-specific models used in existing models. The goal is to define a physics-based multiscale model starting at the atomistic level and progressing through a succession of scales to the continuum level. ARL is currently addressing issues from atomistic through mesoscale to meet criteria established by ARDEC. ARL has completed and validated the atomistic model against a finite element simulation. The coarse graining method development needed to reach the mesoscale is in process, but the microscale issues have not been fully solved and need to be further examined.
The optically measured explosive impulse project represents the first nonintrusive, high-resolution optical measurement of the blast wave pressure profiles. The idea is to use a high-speed digital video camera to visualize by Schlieren methods the decay of an explosively generated blast wave. Careful calibration of the Schlieren system and image processing is used to quantitatively interpret the images
of the blast wave. This test configuration required compensating for optical system response, the light source intensity distribution, and fabrication of multigram, spherical test charges. The result is a sequence of high-resolution images of the spatial distribution of index of refraction behind the leading shock wave. The index of refraction distributions can be further interpreted to provide pressure-time histories that can be integrated to provide impulse. This technique appears to offer significant advantages over the conventional methods of using an array of piezoelectric pencil gauges, particularly in the near field, where protecting the gauges from vibration and impact is a challenge. The technique is being validated against conventional pencil gauge data, and continued efforts to develop quantitative optical measurements of blast waves are strongly encouraged.
Interior Ballistics
Much progress has been made in ARL’s core technologies germane to experimental investigations and modeling of gun interior ballistics. The experimental programs designed to investigate large-caliber gun ammunition (ammo) vulnerability and hot climate ammo responses represent important technological areas for ARL research. In the case of ammo vulnerability, it was determined that direct jet injection was the major factor in the detonation of an ammo compartment. However, penetration of the compartment did not always create catastrophic events, and to date, based on the experimental data collected, an empirical model via curve fit to the data has been constructed. The development of physics-based models aimed at capturing the controlling mechanisms of ammo compartments subjected to off-normal events is crucial to the attainment of a true predictive capability rather than the current curve-fitting approach.
The optical methods of measurement extending from the shock wave impulse to muzzle flash and muzzle blast are intriguing areas of research. The projects in this area represented ingenious use of classical optical methods with modern cameras and imaging processing. The results tie in nicely with other projects on modeling muzzle blast and flash, and they provide experimental data on explosive performance for very small amounts (milligrams) of experimental explosives.
The in-house modeling efforts in the interior ballistics have greatly benefited from the long history of efforts at ARL, which is the leader in interior-ballistics modeling. ARL modeling tools are being effectively used for advancing gun technologies and related efforts. Extension of interior ballistic models to muzzle blast and mechanics of gun barrels/deformable projectiles shows good promise. Predictive model development in this area could be enabling for design innovation that can lead to significant improvement in gun performance and the ability to take advantage of potential improvements in propellant performance that are being pursued in other projects.
Modeling of tank ammo utilizing the German technology of perforated, coated propellant grains that feature temperature compensation to ensure peak pressure within safety limits in hot climates is one example of a promising line of research. The long-standing problem of high ambient temperature (hot climates) resulting in muzzle pressures that are significantly higher and can exceed design limits continues to be a key issue relating to ammo safety. The Germans have developed a perforated propellant grain configuration with each perforation having a specific filling material and heat compensating end-coatings. This is applied to a particular experimental 120 mm round. The results of firing tests showed the desired moderation of peak pressure over a wide range of ambient temperatures. However, the physics of the operative mechanisms within this technology remain poorly understood, and the models are not predictive. A multiphase, multidimensional model is therefore being developed to better understand the physics of this design for ambient temperature insensitivity and to also serve as a basis to explore more affordable alternative grain configurations for this and other types of rounds. The modeling results to date have been experimentally correlated with test data on the behavior of individual grains and with a
representative multigrain large-caliber round configuration. This line of research appears both fruitful and central to ARL’s mission.
The NGen code has been utilized in conjunction with the Abaqus commercial finite element code to analyze tapered-bore guns. Tapered-bore guns offer the possibility of greater muzzle velocity and compact length with no parasitic mass (no Sabot). A tapered-bore gun consists of a full-bore section and a tapered section. The effects of the length of the full bore and the tapered sections, and the taper angle, need to be assessed and predictive models developed. To explore this, an internal ballistics code, NGen, was coupled with a commercial solid mechanics code, Abaqus, explicitly to analyze projectile deformation. The results demonstrate that this is a way to identify the effect of base pressure and swage pressure on the deformation of the projectile and to determine the muzzle velocity. Modeling the interaction between two complex nonlinear processes poses a significant challenge. Initial results from this first coupling of an internal ballistics code with a structural mechanics code are promising.
Modeling of the complex chemical dynamics operative in solid rocket propellants represents a compelling area of research. A complex model of the chemical kinetics and fluid dynamics of the many reactions that define the explosive reaction of ammonium perchlorate-hydroxyl-terminated polybutadiene (AP-HTPB) has been developed. The model is detailed enough to define conditions under which the explosive reaction can be safely released under pressure as high as 15,000 psi (the current safe limit is 2,000 psi), making an increase of pressure to 5,000 psi attractive. The work is an excellent example of computational chemistry that adds significantly to our understanding of chlorine oxide chemistry and is relevant to other problem areas of halogen-oxygen reactivities. The methods employed are relevant to the understanding of other energetic material combustion kinetics and will aid in the optimization of explosive systems.
Exterior Ballistics
The presentations and posters provided in the area of exterior ballistics were excellent. The work represents solid applied R&D that is thoughtful and innovative. The balanced use of empirical/experimental and analytic/modeling approaches is exemplary. Although there are challenges with validation, most of the posters presented modeling informed by empirical testing and vice versa. Moreover, the staff is diligently applying codes—for example, computational fluid dynamics (CFD)—developed outside ARL rather than incur the expense of internal development.
The work on global positioning system (GPS)-guided munitions, specifically M-code receiver testing, is an area where ARL has established a leadership position. At present, the effort appears focused on testing that uses key expertise at ARL. This activity could inform decisions on which critical 6.1 and 6.2 problems to solve in the future.
The efforts on Tobit Kalman filter and the effects of canard interactions are particularly noteworthy. These efforts represent new techniques for guidance systems that are independent of the GPS challenge problem. This project developed simulation using commercial software (CFD++) of subsonic aerodynamics of a model projectile (unclassified shape) with canards and fins and the computed aerodynamic coefficients and determined classical aerodynamic stability coefficients. It also determined that interaction of the wake of canards with fins destabilizes the original configuration and has proposed changes in the geometry of the canards and fins to eliminate this instability. The project then verified the effect of predictions with numerical simulations and validated the results against wind tunnel testing. This program represents a solid effort in aerodynamic engineering with a payoff for guided projectile design. It is a good opportunity for providing staff with basic research topics in fluid dynamics that can help
them build their careers and enhance ARL’s foundational research for next-generation ballistic designs and models.
The project on coupled jet interactions for maneuvering flight behavior has developed simulations using commercial software (CFD++) to simulate the coupled aerodynamics of a maneuvering body (supersonic M = 2 finned projectile) with 6 degrees of freedom (DOF) rigid body dynamics. The simulations have incorporated maneuvering accomplished by transient supersonic jet injected transversely to the flow near the front of the projectile. Comparison of the fully coupled simulation with a traditional aerodynamic response coefficient matrix has been used as force and moment inputs to 6 DOF rigid body simulations. These simulations examined interactions of the jet wake (vortex pair) with the fins and predicted steady effects on the aerodynamic coefficients. The effective angle of attack compares reasonably well between the fully coupled and response coefficient computations; significant differences were observed for angular orientation (yaw/pitch angle trajectories) in certain cases. The project has demonstrated that the coupling effect may be important in some parameter ranges. This is a good start on a project in aerodynamic engineering that would have a good payoff for guided projectile design. The success of this project requires validation with careful experiments. Separating fundamental flow physics issues from vehicle design would be another excellent opportunity for providing staff with basic research topics in fluid dynamics that could help them build their career and enhance ARL’s foundational research for next-generation ballistic designs and models.
The project on compressive sensing infrared (IR) is exploring the application of compressive sensing encoding to spinning projectiles. There appear to be three motivators: using sensors with a small number of pixel elements (current technology in high-speed IR); compression of information for rapid transmission over a data link; and imaging in a rotating reference frame. The ideas used are an extension of standard compressive sensing methods to a rotating sensor. A concept has been identified and some numerical modeling carried out to examine issues in the proposed encoding scheme associated with various sensor configurations and encoding schemes. This is an exploratory project at an early stage. Compressive sensing is an appropriate research topic, and the methodology is technically sound.
The project enabling vision for projectile-image-based navigation seeks to identify important performance characteristics of an optical seeker on a mortar round against a moving target. Key challenges to the design of such a directed projectile include very high g forces on the electronics at firing, the initial field of regard covering the uncertainty ellipse from earlier targeting data, image updates of the designated target as the round rotates, and potential impacts of atmospheric, background, and motion effects on the optical image resolution. Although in the early stages of characterization, progress has been made laying out key operational parameters and performance characterizations.
An 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 with some potential to contribute significantly 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 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 ARL’s 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. One area the overall strategy does not address is the need to identify core, differentiated capabilities where ARL experiences minimal competition and the ARL capability is excellent. Some of these areas are obvious: for example, energetics and interior, exterior, and terminal ballistics. However, the strategy needs to devote dedicated and concerted efforts to highlighting these capabilities. Although the advance of technology places a premium on selecting new areas in which to develop capabilities and expertise, there is a danger of overlooking the existence of challenging research problems in established areas.
ARL has for a long time been primarily consumed by the needs of the warfighters engaged in ongoing conflict. That meant that most programs and projects were directed at near-term deliverables, usually engineering solutions to battlefield problems. Evaluating the quality of such work is a matter of looking at the technical quality and the compromises necessary to deliver the product in a timely manner. Now ARL has effectively transformed its programs into strategic thrusts that support Department of Defense (DoD) strategic areas. The speed of this transition and the way the new directions were presented and developed are impressive. However, longer-term strategic programs demand a more detailed description of the implementation strategy. ARL is not the only player in these programs, and it is not known where the critical responsibilities lie. During its assessment of ballistics sciences the ARLTAB has had trouble differentiating ARL’s real goals from its aspirations. The engineering projects are integrated into larger programs, but the time scales or criticality of the project elements were not clearly elucidated. Even long-term science programs need some reasonable near-term objectives. In the future more attention could be devoted to these issues in the program overviews, where the ARL role and the critical paths could be shown.
Model validation, which requires concurrent research of materials properties and performance, was 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. Validation of models remains spotty, and at times just a computer-based visualization of a model was presented with few or no quantitative comparisons to data. Often it seemed that validation was being carried out, but the specifics of the validation approach were not presented. Presentations instead provided qualitative arguments based on visual comparison of model and experimental outputs. This is a prime area in which ARL needs to develop and promulgate general principles for model validation on which all researchers at ARL can draw. There is need for an ARL-wide approach to the utilization of models to conduct sensitivity studies. This is crucial to provide guidance in the design of experimental studies, particularly for the selection of materials and variation of parameters.
Details of complex material and structural models matter, but these, along with the basis for choosing model parameter values, were seldom discussed. When there is considerable simplification of geometry or assumptions about material behavior, 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 the same as validation. It is important to achieve validation 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 a predictive design capability in support of current Army programs and future systems, 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-S23 models are validated. During the 2014 review, specific details of the validation methodology remained inconsistent, although more connectivity between theory, experiment, and modeling was shown. Presentations provided insufficient 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. Uncertainties need to be consistently presented in the form of error bars, confidence intervals, or similar measures when comparing models and data. There is a need for sensitivity studies to identify key parameters. For example, sensitivity analysis appears to be strongly warranted in the modeling of the tapered barrel research project aimed at exploring the importance of bullet constitutive behavior as it relates to friction.
ARL staff continue to be less visible in professional technical societies and technical conferences than their accomplishments and scientific expertise warrant. The continuing budget restrictions and uncertainties, sequestration cuts, and travel restrictions have negatively affected staff interactions with the outside R&D community, albeit 2014 showed an improvement over 2013 even as it remains significantly below levels that existed prior to implementation of the restricted conference attendance policy. Lack of interactions through conferences and professional associations will continue to have a deleterious effect on collaborative efforts and on maintaining the edge in its areas of expertise. This will continue to negatively affect morale and opportunity cost, and it will pose serious consequences to staffing retention and hiring in the future if the situation is not reversed. 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 their focus is predominantly forced inward. If this situation is sustained, a not-invented-here syndrome will be nearly impossible to avoid in the future, leading to in-house reinvention of wheels that would be better brought in from outside. Continued efforts to push back on this situation is in ARL’s and the Army’s interests.
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.
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3 Modular UNIX-based Vulnerability Estimation Suite (MUVES-S2) is a software-based modeling tool.
Materials for Terminal Ballistics
It appears crucial for ARL to develop strategic thinking behind internal investments, program and mission deliverables, and staffing to support the ability of the Army to meet the national security mission needs of the future. This strategic thinking 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 S&T areas? Better characterizing and qualifying the materials ARL receives from various suppliers will help ARL to make engineered systems that 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 a developed alloy is crucial to determining whether research on the alloy needs to continue.
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 also suggest optimized microstructures. This may be a much more efficient approach than iteratively reprocessing to optimize 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 would be worthwhile to expand the research to include variations in the volume fractions of the constituent phases. Near 50:50 compositions are likely to deliver 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 tricontinuous oxide ceramic.
The 3D-TTR hybrid composite armor development effort appears to be a structures-by-design development effort rather than a materials-by-design effort, although the 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 such diligence, the utility of modeling for further refinement of these 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 material-by-design will require the incorporation of fiber chemistry and processing expertise, either developed in-house or accessed externally. Given 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 suggests 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) 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 IGF 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 B6O, AlB12, AlMgB14, and composites. At present there is little fundamental perspective. What is new and promising about this work? Are ARL researchers aware of previous work published in the open literature or in government reports that has been done assessing microstructure vs. ballistic performance?
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 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 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, although if this engenders outside-the-box thinking it may be a boon to future research. 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 achievable only at a total helmet weight that is intolerable to a user, poses a virtually insurmountable challenge. A 10 percent reduction in areal density for ESAPI body armor is a realistic goal, but the strategic planning needed to achieve this goal was not described in detail. The Defense Advanced Research Projects Agency (DARPA) spent many millions of dollars trying to reduce body armor weight to circa 3.5 lb/ft2 goal a decade or so ago, and minimal reductions were achieved. The objectives of this project need to be evaluated on a continuing basis.
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 a 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? A portion of ARL’s research in this topical area could not be briefed to the panel except at a baseline level due to classification restrictions. This suggests opening discussions with the National Research Council (NRC) to arrange for a focused classified technical assessment of the science within this topical area.
The development of models for ceramic materials is an activity of critical importance at ARL if it can lead to the creation of a predictive capability for the application of ceramics and other materials in Army armor and lethality systems. Much work has been carried out 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, and perhaps, as mentioned above, this suggests establishment of a focused effort to provide a scientific assessment of this research area in a classified venue.
Research in DU alternative projectiles is a project crying out for strategic planning and context definition for future Army needs and ties to Army strategic planning for new vehicle designs. The presenter stated that significant progress continues in developing nanocrystalline W-based composites as replacements for DU materials. However, few specific accomplishments were cited, perhaps because classification restrictions limited the briefings. Nevertheless, 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) WC-based projectiles against oblique targets may be of some value. The researchers need to consider the applicability of diamond-hard-faced 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 by the liquid-phase sintering of mixed powder compacts, even though the Co content is <2.0 wt-%; normally, at least 10 wt-% Co is required to ensure complete densification, which incurs a weight penalty. A denser WC/Co insert that is harder and tougher would be advantageous.
For KE penetrator applications, presenters did not explain what they have gained by recently focusing on nanocrysalline materials. The researchers noted in this briefing 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 evince much awareness of prior work in this research area or of lessons learned pertinent to the present effort, albeit again the technical content of the ARL presentation may have been restricted because of classification issues, suggesting the need for additional assessment at the appropriate classification level.
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, which could 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 PFM community. Collaboration and insights into the state of the art currently available in this area is 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 of the composite model project and its ties to strategic Army objectives is difficult because 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?
Researchers on the project on tailored mechanisms for light armor ballistics articulated their 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 reduction to study the effects of microstructural evolution on spallation response. While an understanding of the relationship between processing and the microstructure and blast resistance of aluminum alloys is interesting as physical metallurgy, how it could lead to improved armor was not spelled out. Further, the real purpose of this work—in other words, its value to the Army—is unclear. After decades of working with metals such as aluminum for armor applications, the M134, the 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 simula-
tion 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 DoD or to sports helmet research elsewhere) 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 information 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 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. It 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 optimal 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 provide support for the increased helmet weight necessitated by cameras and electronics, could provide a basis for increased ballistic protection, and could support 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 for research. Assessing the validity of neuronetwork 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 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 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 appears 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 KE 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 a particular target. 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 GCVs, 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 central to ARL’s strategic vision. However, it appeared there is no 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.
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 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 follow-on exploration with a laboratory prototype shows the approach could be feasible, this approach could enhance other experimental efforts. 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 that 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, for the amount of additional equipment required, the improvement in advanced capability may not be as significant as predicted by the original concepts.
The project on flow strength of polymers, covering the length scales from atomistic through continuum, would benefit from collaboration with the 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 contained many simplifications in the modeling. Further development may be necessary to expand the range of applicability for 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.
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.4 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.
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4 CTH is a multimaterial, Eulerian, large deformation, strong shock wave, solid mechanics code developed at Sandia National Laboratories.
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 conditions of the model calibration. 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 maintaining desired separation in flight and segment colinearity 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 since earlier investigations of its 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 to allow comparing performance to that of 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, much less computationally intensive, predictions of penetration resistance that can be used for vehicle-level assessments appears to be an important avenue of pursuit.
Researchers on the project on armor material modeling and optimization stated that their goal was to determine which material properties have the most influence on the 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.
Energetics
Striking the right balance when considering disruptive energetics is important. The advertised orders of magnitude gain in performance (in terms of energy release per unit mass) potential seems challenging; enhancing energy densities by 30 to 100 percent within a 5- to 10-year time frame seems ambitious. Since there has been relatively little gain in energy density since CL-205 was discovered nearly 30 years ago, it appears to be a formidable task to surpass these energy densities in a relatively short time. Perhaps aiming for gains of a few percent would be more realistic. Nonetheless, thinking outside the box is needed to advance this field. On the other hand, gains of up to 50 percent are plausible with a long-term, focused effort supported by sustained additions in staffing and further investments in infrastructure. Caution needs to be applied to avoid overselling this program and raising expectations too high. Increases of only 10-15 percent in density with CHNO explosives or propellants would represent a major improvement with significant implications for combat systems.
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5 CL-20 is a nitroamine explosive.
Extended solids, of which poly CO is but one example, are intriguing materials that could provide the basis for an entirely new and more powerful class of energetic materials. Little is known, however, about many of the properties that will determine their practicality let alone the methods for large-scale production. In addition, the current emphasis on poly-CO seems excessive compared to that warranted by other efforts, such as investing in scale-up versus discovery targeted at more practical systems or theoretical efforts aimed at establishing more useful bounds on possible energy. This is high-risk, high-payoff research that belongs in the portfolio with more moderate risk approaches. Given the difficulties of scaling up high-pressure synthesis, it would be prudent to aggressively pursue alternative synthetic routes such as plasma discharge or laser shock. If one of these should be successful it would have a tremendous impact on all phases of the work. The list of unknowns in this area is formidable; not least is the understanding of how to release the energy inherent in the material within the desired time scale. There are many problems associated with formulating a practical product, and the fundamentals of doing so are largely unknown. The modeling is too premature to assess whether sufficient energy can be obtained from the release of strain energy. ARL staff members are well aware of these issues and appear capable of addressing them. However, a more balanced strategy developed jointly by staff and ARL management would be highly warranted.
It remains a challenge to prove that a substantial gain in energy release can be realized with many of the new materials under investigation. This is particularly the case for nanodiamond energy. Considering the nanodiamond data to date, there appears to be little evidence from the modeling that these materials release substantial strain energy. The kinetic barriers to formation are unknown and difficult to assess. Optical access to the larger volume cell is difficult, which means that there is no way to measure the progress of the reaction, while larger diamonds that would allow in situ measurements are prohibitively expensive. Data mining of the numerical simulations needs to be done to assess the potential for energy release. In addition to releasing energy, an advanced energetic concept such as nanodiamonds must also release a copious quantity of gas to be useful as a propellant or warhead component. It is unclear whether this material will truly represent a viable energetic material as traditionally used in a fragmenting warhead or as a propellant additive. At this point, it appears uncertain whether this is a reasonable route for an enhanced energetic.
Similar approaches are being initially applied to explore the performance improvement of nonhydrogenic boron. Designing the spherical samples with sufficient materials for uniform ignition remains problematic. A more sensitive, higher speed camera is on order to increase data rate and measurement space; the spherical samples and the camera both appear to be critical for this effort. The spectroscopic and photographic approaches of other work, reported during the poster session, can provide complementary data. The spectroscopy can be performed in parallel with the optical Schleiren approach. Leveraging all of these techniques consistently across all the energetic development efforts within ARL would provide a basis for cross comparison to support value and performance assessments.
The orthoamides present a particularly challenging target class of molecules. There are very few synthetic approaches in the literature, and few other groups are working on them. Consequently, complete success is not assured, but the risk is moderated by the fact that any knowledge generated will be of lasting value to the field and the potential payoff could be substantial. ARL needs to take a leadership role in this area. The fact that others are not working in this area makes it even more important for ARL to maintain this effort because of its fundamental importance to the ARL mission. Even though early results are encouraging and the investigators seem very capable, it is difficult to speed up progress. Progress on this research topic will probably be limited by the small pool of trained and creative chemists working in ARL on this material. Beyond the obvious means of adding staff, ways need to be found to enlarge the community engaged in related research. Perhaps support for students or postdoctoral researchers
in academia could be targeted to this area. More attention to this work at the appropriate conferences could also help to increase the number of collaborators.
The coarse-grained modeling effort needs to be extended in several ways to be more realistic. For example, directionality (i.e., anisotropy) and nonuniformity of the material needs to be accounted for. A method of incorporating defects would be a crucially important addition. Defects appear paramount to understanding and modeling the nanodiamond and tetranitroglycoluril (TNGU) materials. In coupling reduced-scale modeling to the continuum scale, the link from the mesoscale to the statistical scale has not been included. This is the scale where effects like defects and localization predominate. For example, energy localization to form hot spots is due to interactions of internal boundaries, and these are extreme states in statistical representations. Capturing these effects is critical to understanding reactive behavior. Coarse-grain modeling with the inputs of information from the smaller scales (i.e., atomistics) does not provide the complete link to traditional continuum models. More thought and development at this level needs to be done. The ultimate aim is to incorporate information from a coarse-grain model into a continuum-scale model for analyzing a wide range of geometries and loading conditions. This is a major challenge.
It will be important to identify at what stage and for what range of conditions comparison with experiment rather than with another numerical model needs to be carried out. In doing this validation, a quantitative comparison is needed under conditions that test the predictability of the model. It would be useful to identify specific milestones and an estimate of when this might be achieved in order to be able to assess progress toward the overall goal. From the standpoint of HPCI, challenges exist in adding back effects lost when moving through the coarse-grain scale.
Interior Ballistics
The physics in interior ballistics is complex and can appear daunting; substantial work has been done in this area, but further study of certain crucial areas is critical to developing new predictive capabilities or enhancing current ones. One example is the modeling of the fracture and the localized effects of contact with propellant grains; another example is the need for mesh adaptivity as interior ballistic models are being used to treat more complex propellant geometries and extended to muzzle blast prediction. Tools for adaptive mesh refinement exist in the open literature that could be readily included in this modeling effort. Although several applications of the interior ballistic modeling were shown, comparisons against experimental data tended to be qualitative rather than quantitative. There is a need for quantitative validation and a systematic treatment of uncertainty rather than simply comparing observational data with simulation results. Based on the past and present activities at ARL, there appears to be sufficient experimental data for model validation in the interior ballistics area, and systematic quantitative comparisons with the database are strongly encouraged.
In the area of large-caliber tank propellant, further work to complete the validation could provide a basis for developing designs that are less expensive than the German grain design (e.g., via 3D printing) and that are scalable to other caliber weapons. To some extent, experimental validation is expected to be based on developmental optical and spectroscopic test methods. These much-needed methods are also under development. Continued progress is expected, but it is not clear when the research can move to the exploration of alternative concepts. Another researcher is expected to join this research area soon. It may be desirable to further increase available resources based on the potential for substantial savings in production costs and performance improvements over a wide range of ambient temperatures for a variety of round types.
The experimental programs on small-caliber green primers and ammunition compartment vulnerability both appear to warrant increased emphasis. The green primer project is small and entirely customer-funded. Customers are unlikely to fund continued data collection, especially once their primers are seen to be noncompetitive. This is an excellent beginning of a potentially valuable effort that could be continued at low cost. The work is efficient and effective. This project seems to be managed by very competent and well-focused researchers given the constrained tasking for the project. The effort is unique and seems deserving of more freedom of scope to capitalize on the current investment. The ammunition compartment vulnerability effect is crucial to warfighter survivability and requires significant effort if predictive capability and not just curve-fitting is to be achieved. The effects of spall fragment interactions and subsequent venting configurations need to be quantitatively characterized for inclusion in the model. If the model is to be utilized for the long term and achieve predictive capability, the project will need to re-verify the assumptions when new materials are stowed within the compartment and the exact geometries of the loading are quantified. The effect on the ammo compartment of vehicle under-body blast also needs to be considered in detail. The research seems appropriate and critical to warfighter safety, but if no parameters are provided as to how accurate the model is expected to be or the end goals of the effort identified, it seems unlikely that true progress will be achieved.
In the area of advanced kinetic round modeling, important challenges involve understanding the effects of projectile shape change and incorporating a material model that can accurately simulate shape changes in the circumstances encountered, which include high pressure, large deformations, and temperature generation due to plastic dissipation. One important task is to provide a quantitative comparison with experimental results to validate the model predictions. Another is to understand the sensitivity of the predictions to material parameters and the characterization of friction between the bore and the projectile so that the minimum data needed for predictive simulations can be identified. In particular, sensitivity studies need to be conducted to determine which performance features are sensitive to material and friction properties (and in what range) and which are not. Further down the line, barrel wear and possible projectile failure (fracture and large localized deformations that adversely affect projectile performance) also need to be considered.
Exterior Ballistics
The flight sciences and the guidance, navigation, and control (GN&C) overview presentation did not communicate the motivation for the subsequent posters clearly enough to place all the exterior ballistics research in context and define the interconnections. Moreover, the low-cost thrust is not a readily credible motivator of research. It would be more accurate to describe this as research into guided munitions with relaxed component performance and tolerances. If the problem were simply a matter of introducing less costly components, the production contractor would as a matter of course pursue this approach to increase its profit margin on fixed-price production contracts. In addition, the GN&C techniques presented in the overview were not placed in context of the intended long-term objectives or ARL strategy. There are new methods and old methods and different scopes of applicability—what works in a surface-to-air missile may be inappropriate in a gun-launched round. Presumably, ARL’s GN&C scope includes all Army air and ground vehicles. Should the munitions GN&C efforts dominate ARL research because the WMRD is focused on ballistics?
A number of the junior staff presenting in the exterior ballistics poster session would be well served by participating in a mechanism whereby they can learn of past R&D in missiles and munitions guidance and flight dynamics. Two potential approaches include mentoring by senior staff and regular peer review sessions with other researchers from across the U.S. Army Research, Development and Engineer-
ing Command (RDECOM) at both the senior and entry levels. Examples include exposure to DARPA’s and other DoD laboratories’ automatic target recognition research, which has been under way for over two decades, and GN&C approaches in currently deployed (and in some case retired; e.g., Pershing II guidance) missiles and munitions.
The image-based exterior ballistics projects are aimed at characterizing a system concept for a mortar round with an optical sensor to home on and intercept a moving land vehicle. This is a problem that ARL is uniquely qualified to explore. There is little previous work on missile targeting and guidance for this short-range, short-time engagement with such a low-cost round. The initial characterization is self-consistent in defining and circumscribing the system concept to meet the challenge. This includes defining the seeker field of regard, which covers the evolving ambiguity ellipse from the time of receipt of the (moving) target coordinates to when the seeker begins to look; the requisite optical resolution for target recognition; and the effects of atmospheric flow, turbulence, and vibrations on image quality. The next research steps identified by ARL include exploration of effects and requisite technologies in more detail. Using imaging to guide rotating projectiles is a component of the ARL long-term strategy of placing GN&C in all projectiles and represents an opportunity to extend GN&C to a large number of projectiles. Some of the challenges are to make imaging sufficiently robust for the projectile launch environment, useful for a wide range of targets, and to lower its cost. There is opportunity to develop optical sensing techniques that would enable guidance on spin-stabilized projectiles—a large subset of ARL munitions. It is unclear what role this effort will play in the overall strategy of developing GN&C solutions for current and future Army munitions.
However, a more detailed understanding of the operational environment (e.g., variety of targets, expected countermeasures, and the red force/blue force mix) could better inform which technical risks should become GN&C research priorities and which risks are likely to be resolved with present technologies. For example, if both red and blue tanks are in proximity, as seems to be the example target set of the poster presentation, differences in configuration and atmospheric degradation may imply a significantly higher sensor resolution than determined in the initial analysis. Overall, this is an excellent area to explore as the Army moves beyond the recent land wars, and the work is of very high quality. It would be helpful for management to be more explicit about its vision of the battlefield of the future to better focus follow-on research priorities.
As for the development of advanced aerodynamic performance control algorithms, the basic flow physics of canard wake–vehicle interaction remains to be elucidated. The challenge is to have a fundamental focus when examining generic issues of how the wake interacts with the boundary layer on the body or fin to change the pressure distribution as well as cause flow separation. Upcoming experiments planned in the water tunnel could be used to gain an in depth understanding of this. Quantitative data are needed to validate numerical simulations beyond looking at spin-up rate. There are substantial opportunities for interactions with the subsonic aerodynamics community, both commercial and academic, to leverage past work on guided missiles and launch vehicles.
Further, interaction of the jet wake with fins and body is crucial to understanding how a jet influences aerodynamic forces beyond simple force balance due to the jet’s momentum. A key open issue in predicting flight dynamics is the adequacy of a quasi-steady model based on traditional aerodynamic coefficients vis-à-vis a model that treats flow as fundamentally unsteady. Including fundamental unsteady effects will be important for high-speed maneuvers and jet durations shorter than the time scales needed to set up steady flow over the body. There are important issues for designing GN&C algorithms that rely on physics modeling of aerodynamics. A key challenge lies in assuring the reliability of unsteady CFD modeling of 3D flow over a very large range of length scales, particularly for the near-body and fin region with interaction of jet wake with boundary layers. Planned activities in validation will be
essential to ARL’s success in this area. Opportunity exists for contributing to the fundamental understanding of jet wake–boundary layer interaction flow physics. The challenge is to have a fundamental focus in examining the generic issues of how the wake interacts with the boundary layer on the body or fin to change the pressure distribution as well as to cause flow separation. If it is essential to understand extreme unsteadiness, then experiments that require significant investment will need to be carried out to time resolve these details. It is essential to partner as soon as possible with other organizations that have technical resources, including high-speed flow facilities, advanced diagnostics like 3D digital particle image velocimetry (DPIV) and surface pressure measurements with pressure-sensitive paint.
OVERALL TECHNICAL QUALITY OF THE WORK
Within the DoD and in the area of ballistic science and technology ARL has an unequaled record of achievement and timely support of the warfighter through its sustained development of advanced capabilities for defeating many types of enemy targets and platforms, and its development of increasingly lethal munitions to place adversary personnel and assets at risk while satisfying the spectrum of national security missions engaged in by the Army. ARL’s efforts in ballistic science address both fundamental understandings and urgent warfighter needs of great importance to national security. ARL’s personnel, facilities, and programs are the clear go-to place for the DoD and the entire defense agency enterprise in the area of ballistic science and engineering. As such, ARL is central to the national defense and needs to be supported by government at all levels.
The overall quality of ARL’s applied research and development is very high. There is, as ARL management realizes, a need to focus more on the basic research that will underpin future developments, particularly now that the Army may soon no longer be fighting two wars. The time is ripe, say ARL’s managers, for ARL and the Army to place emphasis on thinking strategically about what the Army wants and needs to be able to do 5 to 30 years from now. This is a good time to work on future groundbreaking advances and to emphasize incorporating progressive 6.1 research into the overall research portfolio. A plan for transitioning from 6.1 to 6.2 to 6.3 research needs to be clearly articulated and disseminated. Given the constraints on funding, a hierarchy of research priorities needs to be defined relative to the overall strategy. Projects need to be terminated when they no longer show promise or no longer play a significant role in the overall strategy.
ARL’s existing S&T work in energetics and ballistics is very well served by the current Aberdeen Proving Grounds infrastructure and facilities. There was clear evidence of speedy responses to changing needs to support the warfighter with innovations in ballistic survivability and lethality. ARL’s experimental program concerning threats is quite detailed and demonstrates commendable knowledge of how these threats are evolving. The spectrum of armor design demonstrated a broad array of technical approaches and flexible and rapid response. ARL’s staff are clearly motivated and competent, and all the staff members articulated a well-defined line-of-sight from their research to the mission of the ARL and to the warfighter. All the briefings and posters 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. As one example, this aspect of the project on multiscale modeling of noncrystalline 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 such as Corning’s. Similarly, the novel energetics synthesis, experimental characterization, and modeling efforts are excellent examples of a well-coordinated and integrated research program.
Many of the posters displayed in-depth collaborations with outside organizations, including other DoD laboratories, academia, and especially the National Laboratories. Collaboration with the National
Laboratories included the application of models and codes and the use of experimental facilities and instrumentation techniques. Some of the new analytical techniques and diagnostics developed to follow projectile penetration and test novel energetics were very impressive. For example, the project described in the poster on developing imaging and velocimetry techniques for impact studies used the Los Alamos National Laboratory’s proton radiography facilities and applied Lawrence Livermore National Laboratory’s photon Doppler velocimetry (PDV) techniques. The objective—to observe 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. The laser-generated shock wave test to measure explosive performance of milligram quantities of energetic materials, which was developed by ARL, is also very impressive. ARL is to be congratulated for seeking out the application of these new diagnostic techniques to provide in situ data on penetrator–target interactions as a means to model penetration mechanics, modeling validation, and characterization of energetics in a cost-effective manner. Additionally, the use of impedance spectroscopy and scanning probe microscopy for mapping grain boundaries in SiC-N was impressive.
The overall program on novel energetics is a strong strategic choice for ARL. Its thrust areas are central to ARL’s mission and investment in them, if continued over the next decade, could make ARL a national and international leader. The early results from this program are encouraging, and the staff currently working in the program are excellent. Expanding activity in this area, with concomitant support in the form of new hires and external collaborators, is warranted.
The computational activities are in general well integrated into a large proportion of the research presented. Large, complex, and/or intensive calculations benefited from the use of externally developed, state-of-the-art code platforms, many from the Department of Energy. 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, muzzle blast, and interior ballistics. Reduced-order modeling is an in-house-driven program that is producing results for systems modeling. The interplay between materials and design of armor systems was well presented, as were energetics synthesis development and linkages to properties, which showed that the issue requires close collaboration between materials development, design, structure/property characterization, and computational efforts to optimize performance. To advance the state of the art, particularly in material modeling and failure modeling under conditions encountered in regimes pertinent to ARL problems and in the CFD area germane to exterior ballistics, ARL needs to collaborate on tool development if the tools are to evolve to meet Army needs. The importance of ARL’s developing a laboratory-wide validation methodology, formalism, and implementation program cannot be emphasized enough. Sensitivity studies, beyond providing a way to guide and refine where experiments should be concentrated and on which materials, need to also model how systems can fail.
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, the growth of single crystals of aluminum oxynitride. This achievement has opened up exciting opportunities for basic research at ARL. It appears that the new technology can also be applied to other difficult-to-process materials.
