Weapons and Materials Research Directorate
The Army Research Laboratory’s (ARL’s) Weapons and Materials Research Directorate (WMRD) was reviewed by the Panel on Armor and Armaments of the Army Research Laboratory Technical Assessment Board (ARLTAB) at Aberdeen Proving Ground, Maryland, during July 27-30, 2009, and August 16-18, 2010. The theme of the 2009 review was warfighter protection and survivability; the 2010 review was focused on lethality research and development (R&D).
The Army Research Laboratory is the corporate laboratory underpinning the operational commands for the U.S. Army, and its Weapons and Materials Research Directorate serves as the bridge to the science and technology (S&T) on materials issues supporting warfighter protection and lethality S&T efforts. (Table A.1 in Appendix A characterizes the staffing profile for WMRD.) The overviews presented to the panel in the 2009-2010 period outlined the breadth and scope of WMRD’s mission: (1) to support fundamental research (both in-house and through collaborations with academia); (2) to perform evolving research and development; (3) to serve as a “spin-out” resource to develop testbeds, prototypes, and development centers; and (4) to support eventual industrial production of components and systems in the areas of protection and lethality.
CHANGES SINCE THE PREVIOUS REVIEW
WMRD’s principal scope and vision for the future continue to be the importance of materials and manufacturing science and technology for providing the Army with advanced materials and manufacturing science-based solutions that increase lethality and survivability. The 2009 review of the protection portfolio examined a broad overview of how ARL is using its materials science and engineering enterprise to develop technology to increase warfighting ground vehicle survivability and thus protect the warfighter. WMRD articulated the materials-by-design approach employed within ARL, which dem-
onstrated that the directorate’s scientists and engineers understand the complex relationship between structure, processing, and properties. Using the various computational tools to help predict material performance shows the staff’s excellent desire to increase their fundamental understanding of material behavior and to maximize experimental capabilities and effectiveness.
The importance of high-performance computing in linking experimental facilities to modeling and its validation to WMRD’s programs was discussed in the context of a three-legged stool: theory, experiments, and computation, for both the protection and the lethality thrust areas. The 2009 review presentations clearly described the diverse challenges of WMRD as it strives to address near-term ballistic protection to counteract the improvised explosive device (IED) threats facing U.S. troops overseas while still striving to support ongoing R&D and also to reinvigorate longer-term materials R&D. This balanced approach is appropriate: addressing first and foremost the pressing deliverables to support warfighters in the field while continuing to support and grow the fundamental and applied R&D to support the warfighter in conflicts of the future. Clearly the transition of focus within ARL from the Future Combat Systems (FCS) to the Ground Combat Vehicle (GCV) program scope has forced some changes in course within WMRD in the protection area. Complex and shifting requirements issues drive this transition, and WMRD has been commendably flexible in adapting to these changes in focus and scope within its armor protection R&D thrust.
WMRD’s overview of its lethality portfolio, presented to the panel in 2010, emphasized the importance of warfighter outcomes (WFOs) in maintaining adversary overmatch, minimizing collateral damage, and providing non-line-of-sight scalable lethality. The breadth of the force application mission was outlined: it spans soldier ground tactics, aviation, fire support and non-line-of-sight, networked systems, scalable effects from nonlethal to lethal—all the while maintaining overmatch.
The high-level emphasis on tuning effects to targets as the focus moves from structures to individuals was detailed for the panel; WMRD also described how this changing emphasis demands variable-scalable lethality. The expressed goal of the lethality program is the right lethality at the right time and in the right place without putting the warfighter in harm’s way. The described concept of an armed wingman, an armed robotic entity that separates the warfighter from the energetics and gun mechanisms, is very forward thinking and innovative.
WMRD incorporated the results of discussions with panel members during the 2009 review into its 2010 program in such significant examples as the following: continued emphasis on multiscale modeling and bridging scales, focus on numeric and physics issues in modeling, emphasis on differential verification and validation (V&V), and work on developing a suite of standard validation metrics for the computer codes and simulations.
The previous ARLTAB report1 suggested that WMRD had not appeared to be striking an appropriate balance between experiment and computational efforts, with too little emphasis on computational and modeling areas. During the 2009-2010 reviews, the balance was found to have improved considerably.
The degree of integration of modeling and simulation with testing and evaluation (T&E) has increased substantially over the past 2 years. Significant progress was evident in the extent to which a balanced view of both modeling and experimental verification was included in most topics; this more effective integration is very positive and should be encouraged. In contrast to previous reviews, for the projects reviewed in 2009-2010 computations were more intimately connected to the research effort—a positive and noteworthy change. There was also greater recognition of the need for V&V. Modeling at different length scales is important, and WMRD evinced positive signs in this area in both the protec-
tion and the lethality arena. WMRD should conduct more end-to-end simulations (involving simulation of entire processes) and simulations as a function of resolution and timescales; these are challenging problems.
In both the protection and the lethality thrusts, WMRD also presented an enhanced recognition of the systems approach to problem definition and an increased ability to think outside traditional “boxes.” This has clearly led to new realizations of the factors limiting performance and new approaches to addressing old problems. The integration of the protection and lethality R&D thrusts with warfighter tactics, techniques, and procedures (TTPs) is also very proactive, and WMRD is encouraged to continue this emphasis.
WMRD and ARL have restarted a basic energetics synthesis program. This is an exciting and important reinvestment for the entire country, including the Department of Defense (DoD) and national defense programs. ARL’s visionary stance and investment in the future are commendable.
ACCOMPLISHMENTS AND ADVANCEMENTS
Protection and Survivability
Lightweight Materials for Armor
As a program of alloy development, the Al-2139 is an outstanding example of alloy development aimed at a specific application. WMRD deserves credit for articulating the role of silver (Ag) in the evolution of 2139’s microstructure and the strengthening that results from this microstructure. However, in its current state, the program does not quite rise to the level of materials by design, although there is a good basis here to evolve the program to one of microstructural design and possibly one of materials design. Toward this end, the investigators will need to adopt broader objectives. Rather than rationalizing the origin of measured properties, a more fundamental question must be addressed—namely, what is it about (the atomic structure of) Ag, as opposed to, say, copper (Cu), or gold (Au), that leads to the remarkable properties of this alloy? An answer to this question would allow one to identify other alloying elements, and possibly alloy systems, that might share or surpass the strength of Al-2139.
Trimodal aluminum (Al) was another thrust in the use of Al for protection. This technical thrust area has the potential to increase the strength of Al alloys by up to 75 percent while maintaining good ductility. This is an extraordinary engineering goal and generally would not be believable if the concept had not been demonstrated in a pilot program. The problem over the past 2 years has been to reproduce these outstanding results. It is claimed that the product has been replicated over the past few months. This project evolved from decades of WMRD research into improved Al alloys for armor applications. After successfully producing an ultrafine-grain-sized Al with poor ductility, the investigators used a rule-of-mixtures empirical approach to improve ductility while sacrificing strength. Very surprisingly, they produced a product with both very exceptional strength and good ductility. This material has the potential to reduce the weight of aluminum armor by a factor of two! For the past 2 years, the principal effort was the reproduction of these exceptional results. This effort has led to a better understanding of mechanisms for obtaining higher-strength Al alloys. The composition is far from optimized, but if the strength and ductility have truly been reproduced (which is not yet fully demonstrated), this work has the potential to transform the use of Al for the needs of the Army, Navy, and Air Force as well as civilian aerospace needs. If reproducibility can be demonstrated, this work should receive DoD-wide expansion through the Army Research Office (ARO), the Office of Naval Research (ONR), the Air Force Office of Scientific Research (AFOSR), and the Defense Advanced Research Projects Agency (DARPA). This
work has the potential to open the pathways for ultrahigh-strength Al as the investigation of martensite and bainite did for steel 80 years ago.
The magnesium (Mg) alloy and process development program within WMRD is clearly a positive approach to examining lighter metals and alloys for structural and protective components in a systems-based approach to armor protection. Mg is lower density but lacks ductility and strength compared to structural steel and is subject to corrosion under certain operational regimes. The use of Mg as part of a systems approach to armor protection therefore seems warranted rather than its use as a monolithic armor. WMRD is to be commended for examining the utilization of Mg and for its follow-through in developing an armor specification for AZ31 magnesium.
Brittle Materials and Related Technologies for Armor
The ceramic armor materials efforts within WMRD presented for the panel details of the armor concepts under development, particularly in the areas of effective plasticity and multiscale modeling. This presentation detailed the overall context to the work being done in ceramics and transparent materials. Two topics were discussed at greater length:
A new project attempting to apply modeling at different length scales (quantum through continuum) to describe the fracture behavior of AlON, combined with some experimental verification at each length scale. The intention is to move from AlON to SiC or B4C if this project is successful. Although it appears too early to judge the success of this program, the attempt to connect these efforts is laudable.
An attempt to use the optical response to ballistic impact in AlON and other transparent materials to understand stress, strain, and defects ahead of the actual fracture front, particularly the phenomenon described as “effective plasticity.”
Numerous references were made by the panel to the lack of definitive success criteria, which makes it hard to know what properties or structures should be optimized. (For example, what are the desired fragmentation size distributions for ceramic armor? Also, what is the desired mixture of static and dynamic failure prevention in ceramic armor?) Without crisp success criteria, it remains difficult to understand the real goals of the substantial modeling and experimental efforts that are underway, or how they will be leveraged by the more applied engineering groups.
Accomplishing the implementation of ceramics and other brittle materials in protection systems requires the ability to do the following: (1) find a quantitative way to correlate hardness and ballistics results and nondestructive evaluation (NDE) measures of ceramic-tile quality; (2) provide tools for inline manufacturing quality control as well as for field assessment of accidentally damaged tiles; and (3) predict their performance, in particular their fracture response, through predictive models. The use of NDE techniques has shortened the laboratory’s experimental cycle for ceramic armor evaluation. Preliminary correlations developed between NDE performance of ceramic tiles under ballistic testing have established that the technique can detect defects during the manufacturing process, and WMRD is working on improving the implementation of the technique for manufacturing and field use. The ability to ensure ceramic-tile performance in an armor assembly is essential to the ARL mission, and this work seems to be a good approach to a problem that no one else will solve for the Army.
WMRD’s focus on dynamic failure is centered on experimental observation of crack tip propagation and bifurcation, the creation of appropriate constitutive models, and the implementation and validation of such models through computations employing the finite element method. This effort was described
as part of a plan to develop a multiscale modeling capability; a number of collaborators are assisting in that larger effort. WMRD did describe having developed two failure-capable finite element models (FEMs)—Ortiz’s cohesive zone model and Belyschko’s xFEM model—both in two dimensions. WMRD has also begun experimental measurement of crack propagation and plans to employ techniques such as Rosakis’ coherent gradient scattering technique in future work to validate the models. The program described is an important one, being central to Army goals of survivability and lethality; the specific continuum computational objectives are appropriate. The challenge of developing a physics-based fracture model that can be trusted (in the usual verify and validate sense) is a significant one that has occupied the careers of many theoreticians and experimentalists at the Department of Energy (DOE) laboratories. ARL’s level of commitment to this important area should be increased as soon as possible.
Structural Composites for Armor Applications
Composite sandwich armor panels as structural units constitute one area of ARL’s composite program. The main system examined consists of a woven composite support backplate, ceramic hexagonal bricks as core, and a composite faceplate. Complementary numerical models were developed, but to date they appear to deal with mainly elastic behavior, with the tracking of evolution of delamination ongoing. Overall, this is a good effort that is clearly required to meet Army programmatic needs. The beam experiments performed are a good start, but tests under quasi-static and impact loads on complete panels must follow as soon as practical. Questions include these: Will the panel maintain structural integrity after first impact? After multiple impacts? What is the effect of blast loading on the panels? How does the panel degrade due to the vibration of the vehicle? What NDE methods will be used to evaluate panels in service? What criteria will be used to render panels nonoperational?
The sandwich system being examined currently appears to be a transitional concept for WMRD. New concepts that involve peripheral support of ceramic bricks with specially woven fiber composites appear to be coming onboard in the near future. The same type of effort will have to be expended for such panels. Here, the governing mechanisms will probably be different, governed by nonlinear behavior and progressive damage of the fibrous composite support structure.
WMRD’s research to develop woven ballistic fabric models, and the use of these models to optimize the design and performance of woven ballistic fabrics for use in personnel armor and as backing materials and shock isolation systems or ceramic laminate armors, show strong promise and growth. This is a combined theoretical and experimental research project that has a complementary manufacturing technology program. The models being developed in this research are being used to address what weave geometry the manufacturers should make for each application of interest. The research appears to have a very solid underpinning, and there is a clear need for such models, because a large number of parameters can be varied in a three-dimensional woven fabric, and there is a need to be able to optimize ballistic performance for a given armor weight.
The related technical challenges appear to be in the areas of textile modeling and three-dimensional weaving. This research thrust is being conducted by a team that includes WMRD, several universities, and U.S. industry. Three levels of materials models are being pursued: filament-level fabric models, yarn-level composite models, and layer-level composite models. Experimental fabric impact testing has shown some differences between the measured and predicted displacement that is thought to be due to energy loss caused by fiber transverse plastic deformation, friction, and/or a progressive failure of the material.
The yarn-based models being developed in this research are currently being integrated in LS-DYNA, and this code is being used to conduct three-dimensional composite material modeling and ballistic simulations. Although the research is still underway, it is already delivering results that are being used
to guide the development of encapsulated ceramic laminate armors that include three-dimensional woven fabrics. This research is producing valuable results that are expected to contribute to the design of higher-performance encapsulated ceramic laminate armors for defeating a range of threats.
Armor Protective Systems to Support Current Operations
Armor has clearly evolved in various ways over the years as the threats have also evolved. In particular, IEDs are placing new demands on armor. Advances in materials and computational tools have made possible new approaches to address passive armor protection. WMRD highlighted a number of armor technologies and designs that were quickly developed and applied to a variety of combat vehicles used to support current U.S. operations in Iraq and Afghanistan. The armor work highlighted by WMRD clearly showed the value of ballistic protection technology and development capabilities to the Army. The results and accomplishments shown were very impressive and clearly demonstrate ARL’s preeminent position in the United States in armor development.
The approaches taken are most often empirical and often have somewhat of a trial-and-error nature, with an increasing level of physically based modeling and simulation clearly being used. WMRD’s approach to solving these short-term problems is sound in that it brings modern protection techniques to armor. A continued weakness of this approach, although clearly understandable given the needs to respond rapidly to U.S. warfighters’ needs, is the modeling effort, which should continue to strive to build in more and more robust physics and be resolved numerically.
WMRD and ARL management should consider whether ballistic protection support to current operations should continue to be focused within WMRD in the longer term or whether the responsibility for this type of support work should be transitioned to the Tank Automotive Research, Development, and Engineering Center (TARDEC). Although WMRD has demonstrated that it can do this type of support work and do it extremely well, it is not clear that this is the best use of ARL scientific resources, because many of these same researchers could be used to plan and conduct longer-range, high-payoff armor research initiatives that could potentially lead to more effective ways to defeat future threats.
The WMRD Lethality Division has established a lethality strategic vision that serves as a motivator and focal point for all WMRD lethality science and technology efforts. The lethality S&T program has undergone a major restructuring during the past 3 or 4 years, and approximately 30 percent of the program represents new starts aimed at addressing some of the most important perceived deficiencies of the program. The lethality S&T program is currently focused on five research areas: energetics materials and propulsion, affordable precision munitions, projectiles and multifunctional warheads, materials and manufacturing science for lethality, and advanced weapon concepts. The lethality strategic vision addresses a number of user-identified deficiencies arising from current operations in Iraq and Afghanistan, and WMRD pointed out some of the current important lethality deficiencies and user needs in the current warfighting environment. Consideration of user-identified deficiencies has led the WMRD staff to identify new technology opportunities that may offer a way to fill some of these needs. The vision appears to be broad enough that it should be fairly easy for all staff members to see how their individual efforts can support the overall program; however, it appears that some of the newer staff members do not yet see how their work fits into the bigger picture.
Energetics Research and Development
The Energetic Materials and Propulsion Program is broadly focused on developing new approaches to the storage and release of chemical energy, with the goal of enabling the next generation of munitions and propulsion systems. This focus is mission-appropriate and offers the potential for high payoff for the Army and DoD.
As described, the program is reliant on the development of a fundamental scientific understanding of the principles underlying the storage and release of chemical energy. Because energy release, in particular, is poorly understood owing to a complexity that involves multiple length scales and timescales, it is recognized that, to be successful, the program must integrate modeling and experiment across these same length scales and timescales. The most prominent effort in this area is the 6-year Multiscale Reactive Modeling of Insensitive Munitions project, which will develop modeling capabilities at the atomic scale (modeling chemical decomposition), the mesostructural scale (modeling dislocation dynamics and single crystal plasticity), and the microstructural scale (modeling polycrystalline and continuum response). The models at each level are to be verified experimentally.
Although the project is making good progress at the atomic level—for example, using quantum chemical methods to cull the list of potential high-energy systems before attempting synthesis and to characterize reaction paths and rate expressions relevant to liquid rocket propellants—there was little indication of comparable progress at other length scales. This is not surprising, as the multilength-scale part of the project is extremely ambitious and unlikely to be realized in full over the project’s lifetime and with the current funding level. However, the objectives are laudable even if scaled back, and given sufficient time it should produce dramatic payoffs for DoD.
As presented, the energetics material modeling program appears to emphasize quantum chemical modeling and experimental investigation heavily, and verification appears to recieve less emphasis. The energetics material modeling effort should be focused on a few challenging problems selected from appropriate length scales. In identifying these, both the resultant modeling capabilities and experiments needed for their verification should be articulated upfront. More attention should be directed toward maturing models to provide useful design information. For example, can quantum chemical methods be employed not only to identify potential high-energy molecules but also to develop a strategy by which they may be synthesized?
The synthesis and modeling of energetics are clearly important in the area of insensitive munitions. Although WMRD explained to the panel that there are no insensitive explosives, only insensitive munitions (IM), the goal is for munitions that are safer for those deploying them. The munitions must survive certain insults: impact by bullet, fragment, or shaped-charge jet; conditions of slow or fast cook-off (in fire); and nearby detonations (sympathetic detonation). The ARL approach is not novel, but it is likely to achieve the desired result in the allotted time. All of the services attempt to achieve IM by a combination of new materials and new casings. The general approach with casings is to have pressure relief of sorts during fire threat so that the munition breaks open and leaks or burns but does not detonate. For the most part, the new materials discussed are not really new compounds but new formulations: high bulk density nitroguanidine (NTO) and dinitroaniline as the melt-cast replacement for TNT; or eutectics of nitrate salts (diethylenetriamine trinitrate, ethylene diamine dinitrate with nitroguanidine and methylnitroguanidine—DEMN). The ionic liquids are relatively new; the munitions group at Edwards Air Force Base has been studying them for over a decade. These comments are not meant to fault the present program at WMRD; it is a daunting problem, and a solution is wanted now. No doubt the modeling program examining properties and the synthesis program creating new materials will, in the long term, provide better answers than those available today.
Energetics Synthesis and Disruptive Energetics
WMRD is a leader in the field of advanced energetics materials. The DOE nuclear community and other branches of DoD have had active programs in this area in the past, but most have scaled back or eliminated such efforts. WMRD personnel indicated that the Office of the Secretary of Defense (OSD) has recognized and supported ARL’s efforts in this area.
WMRD is, commendably, encouraging early-career chemists to pursue the synthesis of new molecules for explosives. ARL should send its synthetic chemists to spend time at the Los Alamos or the Livermore National Laboratory with the current U.S. explosives synthesis chemists. In addition, at least three outstanding chemists who spent their careers making explosive compounds are now retired and may be available for consultation.
Theoretical Predictions and Modeling of Energetics Material Properties
The goal of the theoretical energetics program is to predict the physiochemical properties of energetics materials using ab initio methods (density functional theory [DFT]) and molecular dynamics, which will build on the results of the DFT calculations to construct appropriate force fields. As well as providing information for force field development, the DFT calculations are being explored for their usefulness in predicting heats of formation and the crystal structure of energetic molecules.
The use of DFT and other quantum mechanical calculations to determine heats of formation and crystal structures of potential high-energy materials will likely be successful. Verifying the calculations against the structures of known systems is appropriate and will establish an error range (much like an experimental error) on the calculations. The use of force field models to extract information about phenomena such as shock sensitivity is much more ambitious and is unlikely to see near-term results.
Although the overall thrust of the program was detailed for the panel, the objectives of the force field modeling were not clearly stated. Are the force fields to be used to model the shock wave in a decomposing energetics material, or are the goals more reasonable—for example, to predict mesostructure parameters? Also, there was no evidence of a carefully constructed experimental program to look at energetics materials beyond the atomic level. WMRD should clearly articulate the goals for the force field modeling of energetics and provide an experimental method to verify the results. An appropriate effort might be to predict the elastic constants of a crystal of energetics material and when successful to study the effects of shear on molecular conformation.
Guidance, Navigation, and Control of Flight Bodies
The experimental results and simulations used to analyze the path of projectiles stabilized by gyroscopic control were very impressive. The analysis was well conceived, and it provided clear evidence for the value of the proposed control mechanism (taking advantage of a control spinner from the rear). A by-product of this work was an analysis of the flight profiles of retrofitted projectiles. The simulations did an excellent job of recovering the profiles as affected by the use of canards at the front end. They also showed clearly that this control mechanism could be expected to provide weak control of the trajectories. This project was successful on several fronts: WMRD designed a reduced system of ordinary differential equations (ODEs) that is capable of reproducing the controlled trajectories well and hence may lend itself better to onboard calculation. The design that followed provided the use of a spinner to stabilize and control projectile trajectories. WMRD provided a clear analysis (by customer request) of the flight dynamics of a retrofitted projectile and an explanation of why it does not lend itself
to adequate control. An opportunity in this project area lies in the design of retrofits that would provide the requisite guidance, navigation, and control constrained by the requirement that only the nose cone can be modified. WMRD’s experimental and modeling efforts in this area are considered to represent the state of the art in DoD and are to be commended.
Another flight-control project considered the modeling of magnetic fields, nominally those of Earth, but as observed inside a spinning metal body in the presence of pulsed electrical circuitry, as a path to utilizing this approach to flight control. The approach was described as hierarchical—analytical solutions for simple geometries validated a finite element analysis (FEA), and the FEA technique was then applied to more complex configurations. A Monte Carlo strategy was used to assess acceptable tolerances of hypothetical manufacturing processes. Overall, the approach was well founded and the results impressive. One area of note was the discussion of very small deviations between the FEA calculations and a set of experimental measurements. The WMRD program plan suggested that with more careful experimentation, these errors would diminish. This is certainly possible, but it is also possible that the numerical calculations were in error. Elementary grid refinement studies that might support the belief in the FEA computation as being more reliable were not presented. An awareness of the limitations of the numerical techniques appeared underappreciated by WMRD; the issue should be more closely examined.
Gun and Rocket Propulsion Research and Development
The reaction mechanisms employed for gun and rocket propulsion modeling (outside of ARL) are highly simplified constructs with empirically adjusted parameters, limiting their applicability as well as their ability to accurately simulate highly transient events such as thrust and throttling. The purpose of this research is to reduce the number of empirical parameters by using first-principle methods to determine the mechanism for propellant reactions, which in turn will be used to simulate combustion-chamber dynamics. More than a hundred mechanisms were identified using first principles. This large number creates a kinetic complexity so vast that it is difficult to believe the combustion simulations that follow. To provide confidence in the calculations, WMRD should model a simple starter system (one with a few reactions and less complex kinetics) that can be interrogated experimentally. Next, it should demonstrate that the model is consistent with the experimental results before proceeding to more complex problems.
Further, WMRD is applying its expertise in computational chemistry to develop a model for the burning of a specific propellant, which can in turn be used as the constitutive model within a continuum code to compute the performance characteristics of candidate rocket combustion-chamber designs. The propellant has many components, and the possible reactions number around five hundred. Quantum methods are used to compute the thermodynamic and kinetic constants that enter into the rate equations that govern the propellant burn. This is a huge task in its own right, involving enormous amounts of computational time. Selected individual reaction rates have been validated against experimental data. To scale the description to the level needed for chamber design and assessment, the number of rate equations needs to be significantly reduced by a systematic process that eliminates the least important. This is an essential step in the multiscaling of this system, because otherwise the number of rate equations would be far too many to employ in the macroscopic computational model in which each spatial point is governed by its own system of rate equations.
WMRD has invested heavily in developing computational fluid dynamics (CFD) models for the design and analysis of hypergolic liquid bipropellant rocket engines with selectable thrust capabilities as well as hybrid rocket engines (i.e., solid fuel with hypergolic liquid oxidizer) in FY 2010 and FY 2011. In FY 2011, detailed chemical kinetics mechanisms for these fuels will be coupled with the ARL CFD model and validated using test-stand data. Subsequently, the model will be used in FY 2011 to complete
the design testing of the Army’s hybrid tactical missile engine at the Aviation and Missile Research, Development and Engineering Center, and in FY 2012 it will be used to aid in the flight test of a hypergolic liquid bipropellant rocket engine for tactical-battlefield-class missile platforms. If successful, this project will provide an excellent demonstration of the power of multiscale modeling. Moreover, the general approach should have broad applicability. WMRD is to be commended for this effort.
WMRD presented experimental results and its programmatic thrust on blasts from TNT-Al mixtures, with the Al phase having different textures: spherical or granular. The measurements included pyrometry, front velocity (with a framing camera), pressure, and gas composition. The goal was the creation of a data set to inform future computer modeling efforts, which might subsequently be used to optimize so-called scalability of explosions. The experiments themselves were appropriate, though conducted without consultation with modelers. Consequently, one might wonder whether the observed quantities adequately test the computational approach, or provide necessary constitutive properties. This dialogue should occur as the project evolves so that a more complete understanding of the operative physics can be achieved.
Thereafter, given a specific blast, as specified by the explosive pressure pulse released in a room for example, the question addressed is: What is the pressure pulse experienced by an individual’s chest cavity when that person is standing at different locations in the room? The chest has been modeled by a one-dimensional spring-mass-dashpot system with inputs from biomechanical data. The objective is to understand the likelihood that the individual can survive the blast, or the contrary, and it is part of WMRD’s broader effort to develop scalable weapons with limited collateral damage. The simulations can account for the shape of the blast pulse, and this ties into the project on multiphase blast, because the aim of that project is to produce blast pulses of different shapes. The project could benefit from additional inputs of two types. First, there is work that significantly addresses the pressure pulse interaction considered here and which would probably provide analytical results.2 Second, greater collaboration with a biomechanics expert on blast damage to living beings would seem to be called for, given the objectives of the project. Although probably important to questions related to scalability, the project is not particularly ambitious.
Precision Simulation Environment Initiative
The precision simulation environment initiative within WMRD grew from internal capabilities of the Lethality Division and was stimulated by the Very Affordable Precision Projectile (VAPP) Program. Essentially, a collection of models (based on physics, empirical data, and statistics) has been integrated to allow for the early assessment of VAPP prototypes during both initial development and refinement prior to demonstration. The simulation environment also supports a complete hardware loop that includes an initial loading of mission parameters into the warhead, in-flight telemetry and real-time stimulation of onboard actuators, and performance data acquisition and analysis. This simulation environment has contributed to the success of the VAPP Program. The precision simulation environment is a useful application of integrated models (with different bases) coupled with a hardware interface to test prototypes that provides a complete simulation of a family of new munitions. Such an approach is recommended for future munitions development, with the caveat that the simulation environment be made as robust
and accessible as possible, with the least dependency on a specific individual for its routine use and adaptation to new systems.
Multiscale Modeling of Lethality Materials
WMRD has applied reasonable, state-of-the-art density functional theory methods for gaining an understanding of the structure of the grain-boundary interface as a function of dopants. (The dopants arise from the sintering aids employed in creating the Si3N4 ceramics that underlie the tiled materials.) The primary result so far is the prediction of the likely positions in which dopants are to be found on the surface, assuming uniform models for the adjoining ceramic. These distances and several metrics for related clusters have been properly validated by experiments. There are two schools of thought as to how this research may proceed. According to one school, the future stages of this work will require a better understanding of the molecular dynamics at the interface, their reactivity, and detailed interaction with the adjoining ceramic. This will require the use of new methods, ranging from reactive force fields (e.g., REAXFF), to the use of molecular dynamics codes (e.g., LAMMPS), to other methods to explore rare events. The other school conjectures that cohesive enhancement results from local interactions that extend only to first- and perhaps second-neighbor interactions around the impurity site. In such a case, one may be able to identify cohesive enhancers as elements with a set of identifiable atomic properties, such as d electron count. Thus, a detailed simulation of dynamic properties is unnecessary. It is notable that WMRD is pursuing research in this area with an eye toward both possibilities. Regardless of which line of thought is borne out, ARL would likely benefit from interaction and collaboration with other groups studying interfacial cohesion and from a more thorough review of the literature.
Warheads and Projectiles—Ongoing R&D and New Thrusts
WMRD’s S&T program in warheads and projectiles has changed over the past decade, and in particular the last few years, moving from a strong focus on tank-on-tank engagements to the spectrum of problems faced by small units fighting distributed engagements, especially in urban environments. These distributed engagements take place in military operations both in urban terrain and in more open environments. A few examples from the WMRD portfolio, provided below, illustrate this point, as well as showing ARL’s lead and excellence in addressing challenges to the Army posed by very quickly evolving threats and therefore evolving needs for warheads with tailored performance. The focus of the warheads and projectiles S&T program has been shifted to scalable/adaptable effects, weapon effects in urban operations, next-generation kinetic energy projectiles, and increasing soldier lethality. The capability to tailor weapon effectiveness through various approaches (e.g., designed grooves, dual-purpose energetics) is interesting and offers the possibility of reducing collateral damage in complex urban environments. Working on various approaches to scale and adapt warhead performance using careful experimentation and computational tools is encouraged and could have high potential payoff in the future.
M855A1 Round The WMRD-led S&T effort that resulted in the type classification of the M855A1 round provides an example that shows how the S&T program has changed. The M855A1 solves a user need for an improved 5.56 mm round that can deliver more consistent antipersonnel lethality in a variety of operational scenarios and that can also deliver adequate performance against light armor and be environmentally friendly (green). The WMRD effort focused on gaining a detailed understanding of the causes of the performance issues associated with the current M855 round from an integrated viewpoint involving aeroballistic, terminal ballistic, and personnel incapacitation concerns. This improved under-
standing permitted a WMRD-led team to identify a new design concept that could provide improved performance and also be more environmentally friendly than the currently fielded M855 round is. The WMRD warheads and projectiles S&T program also includes, among other efforts, research in areas such as a multipurpose artillery round that can produce selectable energy output, extensible rods and/or segmented penetrators, and advanced tungsten alloy penetrator materials that can be used in place of depleted uranium with no loss of performance. The warheads and projectiles S&T program appears to be focused on appropriate objectives that, if achieved, will make a difference to warfighters.
Affordable Precision Munitions The Army has an urgent operational need for affordable, precision, indirect-fire munitions (artillery and mortars) that cause low collateral damage. The focus of the current Army efforts has been on precision 105 mm and 155 mm artillery munitions and 120 mm mortar munitions. The Army has fielded Excaliber, a 155 mm precision artillery munition, but this munition has a very high unit cost (approximately $60,000 or more) that limits its availability. The Army currently has a need to be able to retrofit the existing large stockpile of 105 mm and 155 mm artillery projectiles at low cost by adding relatively inexpensive guidance kits to current “dumb” projectiles. The kits that are currently in development, however, do not provide as much accuracy as the user desires at all ranges of interest. The WMRD affordable precision munitions technology program attempts to address the user’s need for more affordable precision munitions by developing and demonstrating new technologies that go beyond those considered by other researchers. The WMRD program focuses on several areas: reduced-state guidance, navigation, and control; unsteady aerodynamics; structural dynamics; and precision munitions technology demonstrations, including guide-to-hit tests. The precision munitions technologies that are being pursued in the WMRD program have been selected in order to break the current high-cost paradigm. Both the WMRD initiative and its approach are appropriate: the technical approach being pursued features a combination of analytical and computer modeling, laboratory/bench experiments, and full-scale field testing, including guide-to-hit tests. WMRD has made impressive progress in this area and should continue to push the state of the art. This is a challenging technical problem, but there may be a near-term opportunity for WMRD to make a real difference to the Army by providing more cost-effective technologies for low-cost precision munitions.
Scalable Binary Annular Munition Scalable Binary Annular Munition (SBAM) is aimed at conceptualizing and demonstrating a novel, explosive-based munition that can provide several (at least two) selectable energy outputs. This novel explosive containment and initiation concept would form the basis for a lethal, scalable munition that can tailor the energy output (fragment speed and mass, as well as blast effects) delivered to the target. Proof-of-concept testing has been conducted, demonstrating that two different output modes can be achieved. At this point, it is too early to tell whether these two particular output modes will provide useful target effects. Additional work will be needed to quantify the effects that can be achieved, the target types that can be addressed, and the potential impact on collateral damage.
Materials and Manufacturing Science for Lethality
WMRD should continue investments in developing materials and processes for new warheads and projectiles and for increased lethality in support of the warfighter. Specific thrust areas include the following:
Depleted Uranium Replacement Program (Nanocrystalline Tungsten Particulate Processing): WMRD’s thrust is aimed at achieving an effective replacement for the depleted uranium (DU)
used in kinetic energy penetrators by showing that fully dense, nanostructured tungsten (W) will exhibit the suitable ballistic launch properties and adiabatic shearing at impact that give DU its superior ballistic performance. To replace DU, a combination of high density, dynamic shear localization, engineering properties, and a viable manufacturing route is needed. WMRD is pursuing two approaches: equal channel angular extrusion (ECAE) and a bottom-up approach by means of powder-processing techniques starting with nanoscale tungsten. Sintering curves were obtained that showed the time-temperature-densification relationships for milled tungsten powders. The researchers have identified the most promising materials and processing routes for continued development. This information will serve as a guide to the optimization of all the necessary properties, after which small-caliber gun tests will begin. The trial-and-error work is somewhat Edisonian, but the required properties have been well identified, and the parametric work shows how the properties can be changed. This gives the project a high probability of success in the near term. The calculations by Johns Hopkins University on the shear-banding of the bimodal mixtures is interesting. More theory and simulation could help guide the approaches taken by WMRD.
Equal Channel Angular Extrusion as an Alternate Processing Route for Penetrators: ECAE is a process for forming a rod of any shape by taking it through a 90° bend in the process, shearing it severely. The WMRD research group has developed an ECAE facility consisting of a custom device that operates in a large-capacity press. The facility is used to deform W extruded rods at elevated temperature (600°C to 1200°C) intended as armor penetrators replacing depleted uranium. Commercially available W rods have relatively large (approximately 200 mm) grains along their axis and associated yielding anisotropies. These features have been shown to result in splitting axial cracks in Hopkinson bar experiments that are debilitating in high-penetration projectiles designed with W. Overall, the ECAE capability constitutes a strength area for WMRD. The application of the method to W is worthwhile, and the study of failure modes is useful to the Army’s efforts to replace DU with W.
Gun Liner Emplacement by Elastomeric Materials (GLEEM) Processing: WMRD is examining GLEEM processing for lining and/or autofrettaging a gun barrel to enhance its service life. The initial demonstration has been with a CoCr alloy, smooth-bore tube formed over a mandrel. Chromium (Cr) in an alloy is acceptable, but not in plating, because of Cr toxicity issues. The tube is slipped inside the gun barrel, and then a set of stacked elastomer plugs is inserted. One plug at a time is tamped down with a stainless steel rod, to impart stress onto the side-walls of the tube, exceeding the deformation stress of both the tube and the interior surface of the gun barrel itself. WMRD researchers have measured the bond strength achieved at about 3,000 psi, which is comparable to or in excess of current liners inserted using shrink-fit operations. WMRD’s next step will be to send the barrel-and-liner to a company to machine the rifle bore and then test it, as well as to work on other materials like Ta-10W (which has increased hot strength). WMRD researchers believe that the major challenge to scaling up will be controlling the friction in the process. This project seems to be very applied engineering with little evidence of systematic research and development or modeling to guide the effort. WMRD should examine whether this development project can be designed to incorporate a more coupled experimental-modeling emphasis.
Ceramic Gun Barrel Materials: Ceramic gun barrels are once more being considered by the Army as an alternative to metal gun barrels. The advantage that ceramics provide is that they can reduce the weight carried by soldiers in the field. Apparently such efforts have some history at ARL, where ceramic tubes with internal rifling were previously designed and fabricated. They were subsequently wrapped with carbon-fiber-epoxy composite layers pre-tensioned so as to place the
ceramic tubes under compression. During the firing of projectiles through such tubes, the pressure levels approached the strength of the ceramic, raising questions about the feasibility of the concept. Recent developments at ARL have shown that the strength and toughness of Si3N4 and SiAlON ceramics can be significantly increased by the addition of small amounts of rare-earth oxides (strength > 1 GPa, toughness > 10 MPam0.5). The improvements have been shown to result from grain boundary modifications that are introduced by the added metals (which segregate at the grain boundaries and couple with SiO2 to create amorphous intergranular films). The improvements are sufficient to make the ceramic barrel scenarios plausible. This project includes the mechanical evaluation of the new ceramics and the development of new barrel-manufacturing efforts, while simultaneously pursuing quantum mechanical and atomistic modeling of the effect of the added metals on the grain boundaries.
Overall, the project has a good balance between basic and applied research. A weakness is the apparently limited projected use of ceramic barrels because of the slow cooling rate of the wrapped ceramic concept. The issue of their resilience to impact also needs to be addressed and remains a significant barrier to scale-up and end use in the field.
OPPORTUNITIES AND CHALLENGES
The Weapons and Materials Research Directorate is pursuing an appropriate mission that is well suited to its excellence in S&T in the areas of protection and lethality and is serving the short-term tactical needs of the warfighter as well as adhering to an S&T vision to prepare for wars of the future. That said, there remain opportunities and challenges.
During the 2009-2010 review cycle, WMRD continued to demonstrate increased emphasis on coupling experimental and modeling efforts within its programmatic efforts; achieving and maintaining this balance constitute a worthy goal. Neverthless, error convergence in all modeling and simulation efforts should still be pursued and can be improved. In addition, verification and validation, while more obvious in the current review cycle than ever before, should continue to receive attention. More case studies in which the accuracy of the codes is checked against validation experiments should be strongly encouraged by WMRD management.
An increase in the number of new, early-career staff is clearly reinvigorating the staffing in the directorate. This hiring trend should be continued as the path to sustainable excellence in the areas of protection and lethality.
WMRD is making an investment in energetics synthesis, a national asset to support both DoD and DOE programs. WMRD should continue to pursue its building of a core program in this area. WMRD’s control of the intellectual property in the area of precision design projects is an excellent approach to the development of new munitions, because it helps to maintain expertise within DoD through such means as patents and publications. Holding the intellectual property within the Army and within DoD should be encouraged across an increasing number of technical S&T areas. The low-cost precision munition work is a strong, exemplary success story.
Much of WMRD’s energetics materials modeling program appears to emphasize quantum chemical modeling and experimental investigation heavily; verification appears to receive less emphasis. The energetics materials modeling effort should be focused on a few challenging problems selected from appropriate length scales. In identifying these, both the resultant modeling capabilities and experiments needed for their verification should be articulated upfront. More attention should be directed toward maturing models to provide useful design information. For example, can quantum chemical methods be
employed not only to identify potential high-energy molecules but also to develop a strategy by which they may be synthesized?
The WMRD program in advanced weapons concepts is designed to identify projects that have high risk and high payoff for the Army. Innovative ideas are sought from WMRD researchers and leadership that bear on the mission of the Lethality Division and meet immediate or perceived future needs of the Army. The number of proposals considered has grown from 4 in FY 2009, to 11 in FY 2010, to 31 for FY 2011, demonstrating the success in stimulating idea generation within the division. Interaction with the warfighter has occurred in evaluating the utility of some proposals. Input has also been sought from the U.S. Army Training and Doctrine Command and other Army customers.
Based on the success of this program in advanced weapons concepts in the Lethality Division, it appears that similar programs should be launched in other areas within WMRD’s portfolio, so long as the criteria for funding are such that high-risk, high-payoff projects are likely to be funded over those that are deemed to be more conventional and that program funding is not used to augment or supplant standard funding mechanisms. Researchers in the Lethality Division should be encouraged to identify customer proponents to enhance the likelihood that standard project funding follows closely on the success of the initial project.
WMRD should consider conducting a comprehensive trade-off study for the SBAM munition (if this has not already been done) and using the results of such a study to help guide and optimize the munition design concept.
OVERALL TECHNICAL QUALITY OF THE WORK
The Weapons and Materials Research Directorate continues to conduct science and technology of very wide breadth and great depth, protecting warfighters and providing them with robust lethal instruments to carry out their mission objectives.
The fact that even in time of war in Iraq and Afghanistan when short-term tactical problems such as protecting the warfighter from IEDs have been given to WMRD, the directorate has maintained an excellent series of S&T programs to invest in science and engineering programs for meeting future Army needs. WMRD’s integrated expertise in warfighter protection and the development of lethal devices, systems, and platforms to support the warfighter remain excellent—a shining example of balancing fundamental science and engineering with the short-term tactical needs of the Army and DoD.
High-quality research is being carried out in almost all WMRD areas of interest: materials development and characterization thrusts, model development, and simulation. The WMRD-led S&T effort on the M855A1 round and the affordable precision munitions program are examples of the strong technical expertise embodied in WMRD. WMRD is strongly encouraged to continue its focus on capturing and controlling the intellectual property and modeling and simulation expertise in the protection and lethality areas.
As the path to the development of advanced modeling and simulation tools aimed at predictive capability to support future systems, WMRD is strongly encouraged to continually refine models coupled to systematic validation experiments over a range of scales and to be mindful of quantitative assessment of the margins and uncertainties in their numerics and simulations.
WMRD and ARL have restarted a basic energetics synthesis program. This is a very exciting development and an important reinvestment for the entire country, including DoD and national defense programs in general. ARL’s vision and investment in the future are commendable.