3

Sciences for Lethality and Protection

The Panel on Ballistics Science and Engineering at the Army Research Laboratory (ARL) conducted its review of ARL’s programs on battlefield injury mechanisms, directed energy, and penetration, armor, and adaptive protection on June 5-7, 2017 at Aberdeen Proving Ground, Maryland. This chapter provides an evaluation of that work.

ARL’s research in the area of sciences for lethality and protection during 2017 ranged from basic research that improves our fundamental understanding of the scientific phenomena and technology generation that supports battlefield injury mechanisms in human response to threats and human protective equipment, directed energy programs, and programs that address weapon-target interactions and armor and adaptive protection developments to benefit the warfighter. ARL’s breadth of Sciences for Lethality and Protection Campaign work is performed within the Weapons and Materials Research Directorate (WMRD), the Survivability and Lethality Analysis Directorate (SLAD), and the Sensors and Electron Devices Directorate (SEDD). These directorates working collaboratively execute their mission of leading the Army’s research and technology program and analysis efforts to enhance the protection and lethality of the individual warfighter and advanced weapon systems.

BATTLEFIELD INJURY MECHANISMS

The study of battlefield injury mechanisms is a relatively new area of research at ARL, and ARL has shown greatly improved coordination and focus over the past two years. Excellent progress on many topics by both researchers and management was observed during the review. There has been considerable improvement in prioritization of projects, at least in the near term; while some of the specific goals and timelines of the remaining elements of the program need to be better developed and articulated. The ARL focus on the definition of biological injury in materials- and engineering-relevant terms is a necessary step in moving this critical area forward and may be unique in the field. Other groups studying this area have concentrated either on the biology or the materials aspects but not on both. The focus at ARL on identifying the critical size scale of injury is correct, and the emphasis on the translation of animal data to humans is necessary and positive.

Accomplishments and Advancements

ARL is defining battlefield injury mechanisms in materials science and engineering terms, differentiating itself from the medical community, and thus providing unique input to injury modeling/simulation and armor requirements for injury prevention and assessment. Contributions from researchers and management have significantly increased the focus and coordination of this highly multidisciplinary team. ARL is establishing itself as a leader in this complex area, developing effective collaboration with the relevant medical, materials and engineering communities both within and outside the Department of Defense (DOD).

The focus of the battlefield injury mechanisms research group is to develop an understanding of the translation of traumatic injury from ballistics or blast to the definition of injury mechanisms to allow the design of improved personal armor. Extremity, head, sensory organs, and torso are all of interest, but initial emphasis is on the combat helmet. The approach is to develop a human trauma model from porcine data that ultimately results in an improved helmet design. Major differences between human and porcine skull required transfer function definition of tissue and cell mechanical properties, and threshold conditions for injury. It is assumed that the human and porcine heads will deform similarly, transfer data to predictive model of ballistic injury, and confirm results on a second animal model (which was not defined). The objective of the programs is a 20 percent reduction in weight of soldier armor—an ambitious goal. Projects focus on development of a predictive ballistic injury model; identification of critical injury size scale using pig data; measurement of mechanical properties of tissues, cells, and biological molecules as required input to the injury model; and linking the animal data to humans to predict human response. Overall, the area focus is well described, and integration of the various and diverse technologies to achieve the stated goals is well formulated. However, timelines and resource levels needed for success in defining brain injuries (or other ballistic induced trauma to the human body) were not defined.

ARL management is complimented for creating a comprehensive program, with talented, energetic scientists that cover the skills need to define and make progress in this area. This is an excellent start, and ARL is encouraged to continue to grow this area.

Modeling

The modeling of force response related to ballistics for differing scales of injury is an appropriate use of staff and resources to solve ARL-related issues for soldier injury. Most of the studies are currently creating threshold baseline data to input into models related to molecular, cell, and tissue characterization. ARL researchers are developing custom protocols and techniques to gather proper and relevant data into the models for simulation of deformation of skulls, brain, and limbs. These tools could continue to be supported and developed to help direct efforts toward soldier-related injuries. There seems to be a very good working relationship with collaborators, who are able to provide access to equipment, expertise, and data for input into the models. Overall, the modeling projects within the battlefield injury mechanisms initiatives fit well with ARL priorities. The effort by the research staff and collaborators to produce the proper computational tools shows potential for enabling a better theoretical mechanistic understanding of damage caused by ballistic and blast forces on soldier injuries.

The project on modeling the porcine response to mechanical loading has developed a comprehensive finite element model (FEM) based on material subroutines included in the commercial software, LSDYNA, to simulate the response of impact on a helmet placed on a porcine head. The software is certainly suitable for analyzing the problem if realistic material models and values of parameters in the model are available. The project has modeled the helmet composite, the skin, the skull, and the brain tissue with different material models. This effort has made significant progress in putting together various submodels, studying an impact problem, and getting preliminary results.

The project on modeling surface waves in ceramic armor coupled the photon Doppler velocimetry (PDV) diagnostic to the problem of material strength and fracture under dynamic impact relevant to scenarios of bullet or fragment impact. The researchers on this project are commended for seeking a collaborative effort that includes both experimentation and simulation. The development of damage models—specifically, material fracture—is highly complex, but with significant impact toward the design, development, and specifying requirements for personal protection equipment.

The development of a computational framework to investigate the effects of mechanical loading on voltage-gated K⁺ and Na⁺ using molecular dynamics simulations for studying electrical transport between neurons is relatively a small effort. While K⁺ and Na⁺ are not the ions of greatest interest in characterizing neural damage, they are sufficiently characterized in experiments necessary for model inputs. This work

is largely proof-of-concept for a computational framework to conduct simulations at this scale. The researchers are commended for considering damage markers across length scales in the larger context of brain injury. However, this project was largely disconnected from the other projects due to the lack of possible experimental comparisons and relatively small project funding.

Understanding behind helmet blunt trauma injury using the mini-pig model to study and quantify trauma to human brain is a potentially powerful analogue. Custom apparatus (sensors implanted) and techniques to make in situ measurements have been developed to gather data. A baseline model has been created that can be used to evaluate human data. Clearly this fits into ARL core mission priorities to provide the resources and staff to improve protective equipment standards for soldiers and discover baseline data of actual forces that cause injury to the brain. This investigation has been successful in showing that protocols can be developed with custom-designed equipment to measure comparable locations of the force of a single impact on a helmet. The continued support of these types of projects that develop tools can lead to applicable deliverables that could improve protection of soldiers and provide leadership in adaptive characterization methods.

The anatomical variability and posturing modeling initiative makes an important contribution to ARL’s modeling effort by providing a general method for modifying existing finite element models of the human body to incorporate variations in gender, size, and posture to be expected in any real-world scenario. This method is a cost-effective way to provide estimates on the variability of model outputs due to these input distributions without laboriously remeshing the base model for each change. The project is well conceived and competently executed. The work uses appropriate modeling tools and the computing resources are adequate for the task. Results to date on both simple scaling of size and on more complex positional changes are consistent with experience and warrant continued development and validation.

The investigation of helmet response to ballistic loading was undertaken to develop models and methods to measure impacts on materials used in helmet protection. Clearly this fits into ARL core mission priorities to provide the resources and staff to improve protective equipment standards for soldiers. This project evaluates current ultrahigh molecular weight polyethylene (UHMWPE) and Kevlar materials to establish baseline data to resist deformation caused by actual forces from impact and penetration. A model was developed that correlates very well with experimental data for the blunt force absorbed into the polymer laminate composites (delamination or ricochet depending on the angle of impact). The model was predictive for flat panel versus curved structures to assess UHMWPE and Kevlar. It is advisable to continue support for these types of projects that develop tools that provide leadership in adaptive characterization methods and that could improve protection of soldiers.

The project on mechanics of soft armor is aimed at modeling the ballistic response of soft armor (Kevlar) utilizing a finite element approach. The advantages of knit structures are the increased stretch of the resulting fabrics, less mobility restrictions, and increased comfort when compared to woven structures. Knit soft armor may offer significant improvements in extremity protection without sacrificing soldier mobility. The finite element model developed has proven effective in predicting how changes in fiber chemistry or knit construction impact the ballistic protection of the resulting soft armor. Collaboration with the U.S. Army Soldier Research, Development and Engineering Center (Natick) shows effective interactions with relevant Army laboratories.

Experimental

The experimental program investigating neuron health after being subjected to various strain and strain rate environments as inferred from visible doping techniques showed an interruption of protein transport from individual neurons following mechanical deformation. This project was part of a larger effort seeking to understand damage markers at a cellular level for traumatic brain injury (TBI). The problem is an ambitious, but potentially high-reward, investigation into a suspected critical aspect of diagnosing mild TBI. Potential outcomes include a definition of cellular damage thresholds relevant to Army-specific hazards. A clearly defined threshold for TBI that could be used to guide the design and

development of test protocols for personal protection equipment would be a transformative accomplishment for future prevention of behind-helmet blunt trauma injuries. The state of the art in diagnosing TBI is very rudimentary, and discovery of neuron response with mechanical insult is a reasonable approach.

ARL has acquired state-of-the-art equipment for applying digital image correlation (DIC); this is especially true for conducting stereo-vision DIC at high speed using the newest digital camera technology. An example of DIC use is characterizing the constitutive behavior of the skull subjected to ballistic loading conditions, which is a crucial step in the development of predictive models for blunt trauma. The experimental program has developed a methodology to characterize the material strength of skull bone matter by coupling DIC with the quasi-static compression experiments to obtain this type of data. The multilayer aspects of skull structure suggest a modeling approach with discrete layers of different strength. The diagnostic data collected is well suited to immediate use of continuum material models (e.g., homogenized property definitions) or sophisticated mesoscale models of porosity and multimaterials (e.g., spatially variant property definitions). The project is being conducted with clear impact toward system-level simulations of behind-helmet blunt trauma injuries. The experiments have successfully provided needed experimental data of material strength in surrogate (porcine) samples, and the results have been extended toward human skull structure. There are many excellent examples of the DIC being applied at ARL to a variety of experimental programs.

The research focus on the detection of impact-induced nano-structured changes in brain tissue is a unique technique that describes the development of an ultra-structural analysis tool. The utility of this tool within this study produces scan data using X-ray diffraction of a rat optical nerve and full brain slice scans. These scans can be used to evaluate “ordered” biological specimens and, with contrast agents, “less ordered” proteins. This study investigated the regulatory proteins that are ordered within single optical nerve fibers. The goal was to measure and develop the ability to characterize nanometer space changes in the layered nerve structures after impact.

There are different testing geometries between civilian data (car accident) and military data that do not allow direct comparison of injuries to the lower leg. Thus, the Army has taken a leadership role in investigating military-related injuries. The investigation of lower leg blast injuries was undertaken through the design and development of methods to measure the impulse on lower leg simulants to provide baseline data that could be used in the design of boot protection. Clearly, this fits into ARL core mission priorities to provide the resources and staff to improve protective equipment standards for soldiers. Multiple impacts (4) from a floor panel were studied on a control lower leg (no boot versus a boot) with regard to lower leg injury. This investigation has been successful in showing that a properly designed boot can protect against lower leg injury of a soldier in a neutral seated posture. The modeling program related to these tests focused on ankle and tarsal fractures in foot location under high pressure. It is advisable to continue support for these types of projects that develop tools that can lead to improved protection of soldiers and provide leadership in adaptive characterization methods.

Quantification of the ballistic response of ceramics is an important area of study experimentally and computationally, and one in which ARL is among the leaders in both experiments and simulations. Taking a two-prong approach, better experimental monitoring of the performance of protective systems, both brittle and ductile, coupled with the state-of-the-art computations, has increased Army tools for prediction of armor behavior, with an emphasis on multihit phenomena. This is an area that is deserving of strong ARL support, both with infrastructure and personnel, if both predictive design and performance capability for ceramic armor is to be achieved.

Blast injury to the dismounted soldier, via shock coupling to the body of the warfighter, remains a persistent and complex problem. Measuring the buried charge effects (primary) to test protective armor in addition to aerosols and debris (secondary) is well justified. This fits into ARL’s core mission priorities to provide the resources and staff to improve protective equipment standards for soldiers. This project further evaluates high velocity soil propelled by buried explosives on the degradation of current uniform fabrics. It reflects a blast injury problem identified by military surgeons in which ARL is uniquely qualified to investigate. This study evaluated computational models for blast delivery velocities using a

shock tube and fabric model simulations that generated data from experiments to input into soil and fabric models. This allowed for development of standard blast methodology that can be used to develop new fabrics.

Challenges and Opportunities

ARL is to be complimented on having created a well-focused, unique, multidisciplinary research team focused on a critical, highly complex, multifaceted research area. The overall coordination and focus of work has greatly improved over the past two years. ARL needs to continue to focus on coordination across this area. The emphasis on identification of critical size scale of injury is correct, and further emphasis in this aspect represents an opportunity for further study. ARL focus on the definition of biological injury in materials and engineering terms may be unique in the field and is needed to move injury protection forward.

ARL’s research group is maturing but has not yet developed as a cohesive group. Therefore, a continued focus on team building and diverse competencies is needed. Integration of computation and experiment appears to be well done, but the level of modeling and its integration in the research does not seem as sophisticated as that in other areas within ARL’s science and technology (S&T) portfolio. Increased emphasis on modeling in the battlefield injury mechanisms projects is strongly suggested.

The battlefield injury mechanisms portfolio seems well connected to outside groups, although the nature of collaborations was unclear. ARL needs to pursue activities aimed at impacting and organizing the larger external community in this area, taking a leadership position in sectors of the injury mechanisms S&T area within the DOD and the larger academic community. As one example, ARL’s focus on quantifying and translating animal data to humans appears strongly warranted, necessary, and positive. ARL is probably a leader in this aspect of the field and could represent one area for ARL to lead the external research community. Two types of pigs were discussed, and the response of the two types is not the same. Therefore, there is the challenge of pig-to-pig transfer function as well as pig-to-human transfer function. This approach of developing transfer functions is very important for making substantial progress on inferred injury mechanisms to humans based on animal studies.

A challenge to the battlefield injury mechanisms team lies in finding a common language that transcends the disciplinary range of the team. The opportunity is to provide unique solutions to the prevention of injuries caused by nonpenetrating ballistic impact such as traumatic brain injuries and compressive force injuries to the extremities. Effective definition of priorities, timelines, and strategic goals in this complex area are continuing challenges.

Finally, ARL’s battlefield injury mechanisms programs have reached the level of maturity where timelines, well-defined goals, and a well-defined overall program roadmap are critical to guide future work.

Modeling

Modeling the porcine response to mechanical loading is very challenging, and its satisfactory solution requires expertise in the areas of constitutive modeling, estimating values of material parameters from test data (assuming that it is available under test conditions likely to prevail in the impact event), numerical analysis, and interpreting results of numerical simulations. The development of the software, LSDYNA, is based on several assumptions. The user needs to be aware of these assumptions and various subtleties in using the software. Once the use of the software has been mastered and reasonable values of material parameters and so on have been determined from test data, then it will be a powerful tool not only for analyzing and ascertaining the damage induced by impact loads but also for designing new armor.

In the project on modeling surface waves in ceramic armor due to the stochastic nature of brittle fracture, experimental design of model validation is a challenge. In this case, the burden is on the

experimentalist to quantify experimental uncertainties. The large-model development effort presented in the project on ballistics response of ceramics could be reevaluated to better leverage the modeling capabilities of the ALEGRA computational framework with the experimental conditions uniquely of interest to the Army.

The development of a computational framework to investigate the effects of mechanical loading on voltage-gated K⁺ and Na⁺ using molecular dynamics simulations for studying electrical transport between neurons represents an intriguing scientific challenge. The largest challenge to this project is related to the lack of experimental data for the Ca⁺ ion, the ion of major interest, and conducting simulations at a scale sufficient to draw conclusions about the operability of systems of neurons. Connecting the outcomes from this project (at molecular scale) to the project studying porcine brain response (at the single-neuron scale) is currently not possible. The research framework may benefit from a reevaluation of the best use of ARL resources and team skill sets.

Understanding behind-helmet blunt trauma injury using the mini-pig model to study and quantify trauma to the human brain is a potentially powerful analogue. The major challenge is the comparison of the animal model to what may occur in a human. There is a need to discover what forces are causing micro-tissue and vascular damage even though the helmet can successfully stop the projectile. Also, there is a need to plan and develop infrastructure for survival surgeries of animal models to discover long-term outcomes. There are opportunities to try to change the protocols to measure animal survival health and possibly lead to an investigation of what areas of the brain are being affected by the injury (motor functions). Data measured can be used to guide new standards in characterization of blunt trauma injury and development of models to improve animal-to-human correlations.

In the anatomical variability and posturing modeling thrust, although the level of effort is modest, considerable thought could be given to a thorough validation protocol given the expected widespread adoption of this adaptation to human models for many purposes.

The investigation of helmet response to ballistic loading was undertaken to develop models and methods to measure impacts on materials used in helmet protection. The testing of materials under varying environmental conditions (temperature, etc.) could also be explored to discover changes in material properties as a function of the environment. There are opportunities to develop a high throughput assessment of newer polymers and composites for helmet protection, and to compare these to existing materials being used in current helmets. Baseline data measured can be used to guide new standards in characterization of blunt trauma helmet protection models.

The project on mechanics of soft armor is aimed at modeling the ballistic response of soft armor (Kevlar) utilizing a finite element approach. The project may benefit by teaming with university research groups with extensive fabric mechanics expertise—for example, North Carolina State University or Clemson University.

It needs to be noted that modeling the response of ceramic armor is very important, and extremely challenging, given that ceramic armor has been studied since the late 1960s. Although progress has been made in fracture initiation and propagation, a robust and highly accurate computational ceramic constitutive model for multiple impacts remains elusive. The same can be said for some of the substrate materials (e.g., UHMWPE, Kevlar, etc.). Some of the new diagnostics described in the “Penetration, Armor, and Active Protection” section have promise in elucidating fundamental mechanics of failure and propagation.

Experimental

Significant challenges exist in terms of relating neuron response to the manifestation of post-traumatic stress disorder, mild TBI, and severe TBI that is challenged further by limited staff availability and skills. Due to the problem complexity, progress may be likely only from a focused research thrust that is supported through a team of researchers with a multifunctional skill set consisting of microbiologists, biomedical engineers, shock physicists, and mechanical engineers. While the individual researchers could

communicate the interconnectivity of different research projects, as a whole, the effort appeared too small (both in terms of staffing and funding) to make meaningful contributions to this field within a short time frame. The team stated that funding for the current multiyear program was ending this fiscal year. There are significant opportunities to capitalize upon the recent emphasis toward mitigating additional TBIs, the unique Army hazards, and the immature state of the art for ARL to dominate the research of TBIs from a materials and engineering perspective. ARL is at the cusp of establishing a transformative research program coupling engineers, microbiologists, and physicists advancing experimental methods and computational models for the future prevention of behind-helmet blunt trauma injuries. ARL’s program would benefit from a larger strategic effort with transparent and clearly articulated goals that closely couple the Army medical researchers, the ARL research team, and other external researchers (e.g., sports injury researchers), perhaps within a multiagency research consortium. Some brief mentions were provided of external collaborations, but presented information remained largely focused on the local ARL researchers, resulting in an assessment that potential programmatic reward will be limited in scope.

Characterizing the constitutive behavior of the skull subjected to ballistic loading conditions is a crucial step in the development of predictive models for blunt trauma. Challenges exist in relating quasi-static material properties to the simulation of dynamic impacts along with the development of material strength models suitable for representing the large human population from singular experiments. The applicability of the material strength models being developed under low strain-rate conditions could be considered relative to the strains expected under conditions of behind helmet blunt trauma. A useful undertaking would be for ARL representatives to participate within the external DIC community. Also, ARL needs to increase its focus on uncertainty quantification. Efforts toward understanding the interplay between experimental factors and the measurement uncertainty of quantities were largely unreported. Characterizing skull material strength has the ability to impact the warfighter directly through improved personal protection equipment and also the larger knowledge base of human physiology. ARL’s unique position to contribute to this understanding from a mechanics of materials perspective is unrivaled by other agencies. It would be beneficial for ARL to lead the experimental research in this effort. Because of the mesoscale variations of skull materials, the ability to advance model development from homogenized properties toward spatially resolved predictions could be exploited by growth in the area of mesoscale computational model development.

The research focus on the detection of impact-induced nano-structured changes in brain tissue is a unique technique that describes the development of an ultra-structural analysis tool. A significant challenge to this work is the measurement of the shear forces on the structures within the brain and the resulting mechanical response to tissue degeneration. The next steps could include the investigation and characterization of the compromised regulatory structure force response to translate from the single fiber structure (nerve) to bundle (nerves) to brain tissue model. The project may also find it fruitful to investigate control/direct loading versus impact loading on nanometer space changes between layers. There is extensive research being conducted related to football and other sports injuries on TBI and ARL needs to be knowledgeable of and strongly coordinating with that community.

The investigation of lower leg blast injuries was undertaken through the design and development of methods to measure the impacts on lower legs to provide baseline data that could be used in the design of boot protection. Trying to institute new boot standard specifications may not be popular with troops. There are opportunities to try to scale models of boots desired by troops for comfort and protection. Data measured can be used to guide new standards in specification of military boot to provide protection from injury.

Blast injury to the dismounted soldier, via shock coupling to the body of the warfighter, remains a persistent and complex problem. The challenge is to measure the injuries sustained by a soldier due to deteriorated fabric exposed to debris. There are opportunities to study operationally relevant experiments to measure shock and debris kinetics on fabrics. This also allows the investigation of the critical role in the failure of fabrics due to wear patterns to material type and distance from the blast.

DIRECTED ENERGY

The ARL directed energy (DE) program focuses on radio frequency (RF)-DE and laser-DE. ARL leadership and research teams successfully implemented some of the recommendations of the previous Army Research Laboratory Technical Assessment Board (ARLTAB) report. Specifically, the collection of presented projects demonstrated a coordinated strategy across the enterprise for the laser-related DE work, with indications of much greater collaboration with the Navy and Air Force. Internal to ARL, principal investigators (PIs) from the DE sessions demonstrated greater awareness of work done with threat warning and countermeasures. The quality of the programs will continue to benefit from even deeper and more frequent collaborations both internal and external to ARL to foster rapid innovation with operational and contextual relevance.

The DE work presented aligns with the overarching goal of developing new knowledge and technologies associated with lowering size, weight, power and cost (SWAPC) of army-vehicle mounted offensive and defensive DE systems. Low size, weight, and power (SWAP) improvements for the protection and hardening of platform apertures were also investigated.

Although ARL is not working to develop fully functional DE systems, the work presented aims to make unique impacts in the areas of compact power, propagation, and target response. Other novel or high-risk areas of work include beam control, thermal management, and material effects for Army-centric operational scenarios and demonstrations. Use of high peak power ultra-short laser pulses to “burn” their way through the air and obscurants to create quasi-steady-state waveguides called filaments is a complex, high-risk area of basic research. The mechanism is based on super-heated air (plasma) depositing energy in the air causing heat-induced changes in the index of refraction on the order of millisecond time scales. Secondary pulses or quasi-continuous wave (CW) DE propagating along the same path “sees” the thermal disruption path as a lower loss waveguide.

In contrast, the overall strategy for the RF-DE work was not as evident based on the topics presented. Specifically, The RF work appeared to remain fragmented and unrelated to the overall mission. For example, the operational context of the cognitive radar and the counter-unmanned aircraft system (UAS) work was not apparent. Progress made over the course of the last two years was not evident in two projects—cognitive radar and counter UAS.

Accomplishments and Advancements

Overall, the DE research program demonstrated positive trending in adding value to body of research and development (R&D) knowledge with relevant operational focus and alignment to the ARL enterprise strategy.

Some ARL researchers are pursuing complex, high-risk areas of basic research, and this is encouraged to continue. This work includes novel waveguide concepts investigating a high-risk, low-technology readiness level (TRL) technique to improve laser propagation through atmospheric mediums using high peak power ultra-short laser pulses to “burn” their way through the air and obscurants to create quasi-steady-state waveguides called filaments. Further, ARL presented two additional projects that encountered materials properties challenges that the principal investigators (PIs) were aware of, and they are encouraged to be persistent in pursuing solutions. Those projects focused on low-loss fiber and ultra-low quantum resonantly pumped illumination.

Particularly outstanding laser DE work is being performed. The work reported on exploiting Raman lasers to greatly improve fiber power output is exceptional and is an archetype for research at ARL that compliments other DOD laboratories while not duplicating academic or industrial research. The key evaluation metrics were exceeded in the presentation of this work and include excellent contextual reference with alignment to the overall strategy; definition of quantitative objectives based on models (e.g., using the chart that identified Raman gain predictions); and establishing aggressive benchmarks for success and demonstrating consistent progress. Another example of exceptional work was multiple

chromophore mixtures for sensor protection focused on technology to protect and harden the sensors on Army assets against DE threats. The method uses nonlinear optical materials and coatings that are frequency agile in the visible spectrum to passively protect Army optical sensors from DE-laser threats such as that from straight damage, jamming, dazzling, and so on.

ARL has made substantive progress over last 2 years, addressing many of the shortfalls identified in previous ARLTAB reports, including increased collaboration with a diversity of research partners to validate and compare results or address experimental and modeling gaps; growth in the knowledge depth of its personnel; prioritized research based on operational relevance in streamlined focus areas; establishing a strategic vision by looking at operationally relevant drivers and trade-offs to determine where discovery is really needed; system-level or event-driven discovery rather than disparate seemingly random and siloed projects. While most of these improvements directly apply to the laser-DE program, the overall quality of work is good and on an upward trend with examples of exceptional work.

The overall scientific quality of the research is comparable to that at leading national and international institutions, with several projects demonstrating impressive quality and uniqueness with realistic potential for high payoffs, such as diode clad pumped broadband Raman fiber lasers working to achieve potential power scaling in the ballpark of 80-100 KW from a single fiber aperture, and nonlinear optical materials and coatings that are frequency agile in the visible spectrum to passively protect army optical sensors from DE laser threats such as that from straight damage, jamming, dazzling, and so on.

ARL demonstrated a willingness to push and even lead the technological edge by pursuing a focused selection of exciting, high-risk work that the ARLTAB encourages ARL to continue. Select laser-DE work demonstrated exceptionally disciplined research approaches, while some projects could benefit from a more disciplined approach to model validation. More fundamentally, RF work can benefit from vision and focus demonstrated with laser work in order to show continuity and measureable progress.

An exceptional accomplishment is the work on exploiting Raman laser to greatly improve fiber power output and is an archetype for research at ARL that compliments other DOD laboratories while not duplicating academic or industrial research. Additional exceptional work is the nonlinear optical materials and coatings that are frequency agile in the visible spectrum to passively protect Army optical sensors from DE-laser threats such as that from straight damage, jamming, dazzling, and so on.

Challenges and Opportunities

In order to provide a more comprehensive assessment of research activities, improvements in the presentation logistics and structure in future ARLTAB panel meetings is needed. The ARLTAB had a difficult time in assessing two of the major components of the evaluation, specifically “quality of the research” and the “qualifications of the research team.” Therefore, it was difficult to make those assessments when some principal investigators were not present. Substitute presenters did well but were unable to answer detailed questions—for example, on cognitive radar; lasers through the atmosphere; and efficient Er-based mid-infrared (mid-IR) laser. Essential classified material or underlying classified concept of operations needs to be presented in classified venues—for example, on counter-UAS, ultrashort radar, and cognitive radar. Presenters need to clearly articulate the contextual reference by including the operational context, research goals, and progress toward more model-based benchmarks. By following the model of those presentations identified as exceptional, future presentations may include quantitative objectives or analytically derived requirements.

The positive trend toward operational focus could be applied to the RF-related work. The RF work appeared to remain fragmented and unrelated to the overall mission. For example, the operational context of the cognitive radar and the counter-UAS work was not apparent.

There are indications of much greater collaboration with Navy and Air Force. In addition, internal to ARL principal investigators from the DE sessions were aware of work done with threat warning and countermeasures. However, greater collaboration could uncover additional innovation and is strongly encouraged.

Several of the projects lacked quantitative objectives or analytically derived requirements—for example, counter-UAS lacked tactically significant range targets. The projects could benefit from a more disciplined approach to model validation.

The work on utilizing the thermal signature of an ultrashort pulsed laser filament for guiding fiber lasers investigates a high-risk, low-TRL technique to improve laser propagation through atmospheric mediums using high peak power ultra-short laser pulses to “burn” their way through the air and obscurants to create quasi-steady-state waveguides called filaments is a complex, high-risk area of basic research. The work is encouraged to continue maturing novel waveguide concepts.

Some key opportunities to enhance the DE research program remain. There is a greater attempt to tie the projects to a coherent set of mission areas; however, the mission-driven strategy alone is not sufficient. There needs to be a tighter coupling of the research topics to the overarching problems that ARL has chosen to investigate. When asked how projects were prioritized, the answer was incoherent, with support for demonstrations being the key drivers for the research. This was illustrated in the opening presentation describing swarm, counter-swarm and counter-UAS. The demonstrations were nice but were lean on fundamental research objectives. Model validation continues to be an ad hoc process in many of the projects. Without contextual reference, the study of laser eye safety in an aerosol environment could await system development.

PENETRATION, ARMOR, AND ADAPTIVE PROTECTION

ARL continues to demonstrate a strong record of achievement in the fundamental and applied sciences and the engineering of penetration, armor, and adaptive protection. The ongoing work described in the 2017 review continues to highlight how ARL is building on its history of excellence to provide the knowledge basis for future Army needs in the area of warfighter protection. This is an absolutely critical and core competency that underlies Army capabilities.

A few years ago, there were severe restrictions on attending and participating in technical conferences and symposia. It was good to see that this disturbing trend has been reversed, and the staff is now obtaining approval to present elements of its technical work at venues such as the Society for Experimental Mechanics, the American Physical Society’s Shock Physics Conference, the International Symposium on Ballistics, and the American Ceramic Society’s annual exposition and conference. These and related conferences are great settings for professional development, for the staff to present its work and receive feedback from technical peers, and engage in technical discussions. Such technical dialogues often lead to further insight, and spark new ideas and avenues of investigation. Management needs to continue to encourage participation in technical societies and associated meetings, as peer review and the ability to always know where the state of the art is moving is critical to this area, especially as so much of the leading-edge research is classified or limited-release.

Accomplishments and Advancements

A major research initiative has been launched as the result of a war-gaming exercise that identified a potential vulnerability. A well-developed, multifaceted research effort has been established and is being executed. The advanced penetrator work is a potential game-changer and is viewed as an exceptional accomplishment. Integral to this initiative is the use of classified computations to support analysis of possible mitigation concepts. ARL researchers have taken high-performance computing (HPC) to a whole new level—an impressive display of using computational sensitivity analysis of a very, very large problem with multiple length scales. The sensitivity analyses have resulted in identification of unknowns and issues, quantification of design variations, and establishing where basic information is required for higher fidelity simulations. As examples, the computational analyses have identified where fundamental information is lacking, such as thermal conductivity at elevated temperatures in a plasma. It has been

determined that computational results are dependent upon the calculated temperature distribution, and a parametric study of various constitutive strength models showed that the prediction of temperature as a function of time and location depends upon the assumed model.

Although the computational study has identified weaknesses, and fundamental research initiatives are being formulated, decisions are required in the short term to address and mitigate the identified vulnerabilities. This, then, is a significant challenge faced by ARL researchers. A balanced technical effort of HPC combined with well-planned experiments with appropriate diagnostics could provide insights and guidance for progress while a longer-term research effort addresses the fundamental unknowns.

It is interesting to note that the goal of ARL is to conduct the research required to sustain the Army 30 (or 50) years in the future. Yet, the preceding is a major initiative that has a relatively short time frame. Clearly, rapid response to uncovered threats and vulnerabilities needs to be (and is) a part of ARL’s portfolio; thus, the mission statement could reflect the realities of the real world, while emphasizing the need for long-term research.

Modeling

As part of their HPC initiative, ARL has begun to quantify uncertainties in computational results. Uncertainty quantification (UQ) was not part of the ARL technical initiative one or two years ago, and ARL was criticized by the ARLTAB for the lack of UQ. ARL has responded, and has demonstrated a good start. As an example, researchers examined the uncertainty in computational results for two state-of-the-art transient (dynamic) wave-propagation, solid mechanics computer programs. The researchers determined the uncertainties in the results through parametric studies, which included changes in material parameters and changes in the computational domain (for example, changing mesh size and parsing the problem to different processors). While some results were not surprising (for example, the depth of penetration depends upon the strength of the target material), the researchers found one example where the results differed greatly as the computational domain was adjusted. The root cause was that the algorithm to distribute the computation mesh onto multiple processors was not invariant to how the domain was distributed to the various processors. This issue is being addressed, but it demonstrates the importance of UQ.

Diagnostics and Collaboration

The advanced diagnostics work at ARL has great potential in helping to bring illumination and insight to unresolved problems in dynamic response and failure of traditional and advanced materials. One example is the quantification of material strength at high strain rates by constructing smaller and smaller split-Hopkinson pressure bars (SHPBs). The smallest SHPB utilizes specimens 50 microns in diameter and 25 microns long, and can achieve strain rates of 5 × 10⁵ s^-1. Challenges have included development of instrumentation to interrogate the response of the specimen; achieving linearity of the input bar, specimen, and output bar; and even preparing specimens. For example, on a somewhat larger system than the micro SHPB, specimens were prepared by milling or centerless grinding. Since the samples were small, the relative volume of work-hardened material was high, which significantly strengthened the smaller samples. Samples are now prepared through electrical discharge machining, and the sudden increase in strength at very high strain rates is no longer observed for some specific materials.

Another example of advanced diagnostic work at ARL is the development of time-resolved, in situ imaging. ARL researchers are conducting experiments using phase-contrast X-ray imaging at the Advanced Photon Source (Argonne National Laboratory [ANL]). This is a unique, national resource, which the ARL is taking advantage of. The objective is to record in situ observations of microstructural evolution, damage, and fracture in terminal ballistic events. Also, the high-voltage in situ diagnostic

radiography apparatus (HIDRA), which consists of 14 flash X-ray sources and includes two synchronized ultra-high-speed cameras used in stereo imaging, has been developed and is being applied to impact of ceramic targets. Integral to the effort is quantitative comparison with computational simulations, which has shown some agreement, but has also highlighted important differences near the impact site, which suggests deficiencies in the understanding of the failure processes. An advantage of HIDRA is that it is an in-laboratory resource that is not limited to availability and scheduling of the Advanced Photon Source (however, see the following concerning the availability of range time).

A collaboration with Ernst-Mach-Institut and Lawrence Livermore National Laboratory has the objective of developing a multienergy flash X-ray computed tomography diagnostic. This diagnostic, once completed, will image events in three dimensions and at three points in time. At this point, a tomogram reconstruction has been accomplished using only four source/detector pairs. Already lessons have been learned about shielding and X-ray film requirements for quantitative assessments.

Penetration Mechanics and Adaptive Protection

Adaptive protection is a subcategory of the Sciences for Lethality and Protection Campaign, with the objective of developing mass-efficient, novel threat defeat mechanisms.

One project is notable because of its success while highlighting a concern. The study used a modeling and simulation tool set that had been developed over the last few years to facilitate mine blast simulations. The technical effort was an HPC study to investigate and optimize a cellular core structure that demonstrated considerable enhanced performance over legacy structural systems to mine blast. At this point, no experiments have been conducted, but an Army partner is planning a full-scale demonstration experiment.

Another subcategory of the Sciences for Lethality and Protection Campaign is penetration mechanics. A novel adaptation of fundamental penetration mechanics has the potential of being a “game changer” in terminal ballistics and lethality of long-rod projectiles. Approximately one-quarter-scale testing has demonstrated greatly increased penetration, and a few design iterations have been devised to control unwanted and undesirable effects.

Challenges and Opportunities

In 2014, ARL reorganized its research portfolio into four functional campaigns: Human Sciences, Sciences for Lethality and Protection, Sciences for Maneuver, and Informational Sciences. ARL also identified four crosscutting campaigns: Extramural Basic Research, Computational Sciences, Materials Research, and Analysis and Assessment. Within a functional campaign, ARL has identified essential research areas (ERAs). This reorganization has permitted ARL to develop an integrated, multifaceted research program to address current and future Army requirements. There are a number of examples of researchers spread among four or five projects, but there is commonality because the projects are related or integrated into a bigger picture. This assists in communication and cross-fertilization. However, some researchers in this area noted that they are spread among four or five projects and that this results in a decrease in depth of the work. This “integrated” approach can be contrasted to researchers being spread over a number of projects that have little-to-no commonality; consequently, the researchers are now spread thin with the result of decreased depth in their research. ARL management is encouraged to weigh the trade-offs that each approach offers.

A few items were identified that the ARL staff found to be especially irritating and are impediments to productivity. In particular, the Army procurement system is not designed to handle efficiently the requirements of procurement for a research environment, where items are purchased in very small (sometimes single-unit) quantities. Several researchers identified purchasing as a significant impediment to research. To purchase research equipment like a laser takes about one year due to the “red tape.” It

appears that the process for purchasing research equipment is the same as requisition for the Army in general. There were instances cited where it took a year from submitting a purchase request to receiving the item. Progress grinds to a halt if that item is an essential piece of equipment for an experimental program (for example, a laser with specific requirements). Although it is recognized that this is an issue that is beyond the control of ARL, ARL needs to aggressively pursue an approach for developing streamlined procurement procedures applicable to a research environment.

Some operations (e.g., debugging diagnostics) could likely be performed safely with two people, as would be required for indoor laboratories. The number of personnel required for each type of firing-site operation could be reviewed by management to explore avenues to improve efficiency and productivity without increasing safety concerns.

Some important research initiatives are being delayed because testing ranges are shared between several projects. This is not a complaint that people cannot work together, but various projects have different range and diagnostic requirements. Sharing range time often means tearing down the diagnostic setup to permit the next project to set up its respective diagnostics and conduct experiments. Thus, setups and teardowns disrupt continuity; progress is delayed by not having access to the range while another group is conducting its set of experiments. This greatly affects productivity. ARL management is encouraged to investigate what can be done to allocate dedicated ranges to some of its important efforts in the penetration, armor, and adaptive protection program areas.

There is a start of work in the UQ area of research, but there is a long way to go. It was unclear, however, what level of research is being conducted at ARL, or the number of people involved. ARL needs to continue an emphasis in UQ. Further, ARL could purse the integration of UQ into its data-to-decision workflow that includes modeling and simulation, experimentation, and design. To accelerate integration and given the complexity of its objectives, ARL scientists and engineers could leverage software and methodologies developed at other Department of Energy (DOE) and DOD laboratories.

Regarding the development of anti-tank guided missile (ATGM) soft-kill countermeasures, the current staff has a physics background, which is insufficient for making rapid strides in engineering and development of the project. Perhaps engineers with suitable backgrounds could be recruited to facilitate rapid design and development.

ARL needs to resist going to full-scale demonstrations without any or minimal small-scale testing. Such a rapid rush to conduct “admiral’s tests” limits development of core competency in terms of fundamental knowledge. Further, integrated full-scale testing without the underlying physics understanding does not develop predictive design capability or accurate performance modeling. Additionally, the rush to full-scale testing can lead to very expensive surprises and unforeseen application problems. The cellular core structure underbody blast application was noted as a concern in this area. The structure was designed on computational science. However, the fabricated structure cores were never tested for material response at small scale to verify if the assumed response was obtainable. No fundamental study or validation experimental was performed before the structure was sent for field testing.

Modeling

Thus, ARL has a good start in UQ, where none had existed a couple of years ago. The concept of UQ needs to be embraced by all the researchers and integrated into their data-to-decision workflow, which includes modeling and simulation, experimentation, and design. To accelerate integration and given the complexity of the objectives, ARL scientists and engineers need to leverage methodologies and software developed at the DOE weapons laboratories.

ARL embraced one of the suggestions of the ARLTAB by establishing an ERA to understand failure of materials, explicitly failure of materials under high rates of loading and, often, high pressures. With understanding, the objective then becomes one of designing materials to manipulate failure and to exploit mechanisms. Predictive modeling of failure requires insight into initiation, nucleation, and propagation

mechanisms, which occur at various length and time scales that often differ by orders of magnitude. An essential element to making substantial progress in the physics of failure is the developing of advanced diagnostics that can interrogate material response in extreme dynamic environments.

Diagnostics and Collaboration

The current work using the very small SHPBs, which includes aluminum, Ti-6Al-4V, and tantalum, could be expanded to include a face-centered cubic (FCC) metal (such as FCC copper or a FCC steel), which others have shown to have strong strain-rate strengthening at rates above 10⁴ s^-1. This work needs to continue, and results compared with plate impact and pressure/shear experiments. This will probably be challenging in interpretation because of the high pressures in plate impact experiments and the use of different diagnostic measuring devices.

Much remains to be done for the X-ray tomography diagnostic that is being developed, including modification to include spectral analysis, which will allow identification of material type by using three different energy broadband sources. The photon flux versus energy of the sources needs to be established and the algorithms need to be developed to compute material specificity, with all being integrated into post-processing software. The challenges are nontrivial, and it is highly likely that they will require a sustained effort as unanticipated problems arise and are resolved. However, this diagnostic, once on-line, will be a unique addition to ARL’s suite of experimental diagnostics to interrogate the in situ response of materials in an extreme, dynamic environment.

Penetration Mechanics and Adaptive Protection

Adaptive protection is a subcategory of the Sciences for Lethality and Protection Campaign, with the objective of developing mass-efficient, novel threat defeat mechanisms. The challenge is real-time threat sensing and identification (while minimizing false positives), and defeat of the threat. Individual projects have little cross-fertilization (the nature of these small- and medium-funding level projects are very diverse, with the primary commonality being to neutralize a threat). Some of the projects that fall into this category include the optical threat warner, ATGM soft-kill countermeasures, trajectory alteration, and tailored explosive response. Some of the projects have well-integrated numerical simulations and experiments, while others appear to be demonstrations with little theory or advancing fundamental knowledge. These projects tend to be “stove-pipe,” in contrast to a well-integrated, synergistic research plan like humans in extreme ballistic environments or manipulating physics of failure. While the technical efforts fall under the purview of ARL, many small, remotely connected projects provide management challenges for a well-integrated program.

One project used a modeling and simulation tool set that had been developed over the last few years to facilitate mine blast simulations. The caution is to resist going to full-scale demonstration prior to some confirmation experiments. This work could be linked to an experimental program that is done at smaller scale to validate assumptions in the computational model and to ensure performance of the cellular core structure. ARL could negotiate with the Army partner to move forward a little more cautiously.

Another subcategory of the Sciences for Lethality and Protection Campaign is penetration mechanics. The ARL is encouraged to put an emphasis on this novel approach with continued small-scale testing to explore robustness, and uncover and resolve potential issues. Numerical simulations currently are not able to replicate accurately the experimental results, and a priority could be given to resolving this issue. Initial full-scale testing could be implemented on select designs to ensure that issues with scaling up the concept are understood and resolved.

OVERALL QUALITY OF THE WORK

In the battlefield injury mechanisms area, the integration of experiment and modeling is well done, although the level of modeling sophistication does not seem to match that of other ARL research groups. The group is at an early stage of defining key mechanical impact limits of injury, and this has been correctly identified as a critical end point for the group. The ARL team is to be complimented on having built effective bridges to outside research groups both within the Army and with universities. ARL needs to explore collaboration with investigators studying football and other head injuries. By nature of their approach, ARL is assuming a leadership position in this difficult and critical area of study. Focusing efforts on brain injury and helmets is justified and allows research focus in a very broad area of study. Overall, the group is talented and enthusiastic, and is maturing as a team that has not reached its full potential. Continued focus on team building and definition of deliverables and criteria for acceptable progress is still a work in progress. The scope of the work undertaken by this group is very broad, and it is not apparent that sufficient resources, especially in key areas of biological and computational sciences, are available. There is an apparent issue in communication between the various disciplines in this highly interdisciplinary research group (generally true when bringing physical, biological, and engineering assets together). This represents an opportunity for ARL to catalyze educational programs designed to facilitate communication between diverse scientific disciplines.

In the DE area, the overall scientific quality of the research is comparable to that at leading national and international institutions, with several projects demonstrating impressive quality and uniqueness with realistic potential for high pay-offs. The DE program focuses on radio frequency (RF)-DE and laser-DE. Internal to ARL, principal investigators from the directed energy sessions demonstrated greater awareness of work done with threat warning and countermeasures. The quality of the programs will continue to benefit from even deeper and more frequent collaborations both internal and external to ARL to foster rapid innovation with operational and contextual relevance. Specifically, the collection of presented projects demonstrated a coordinated strategy across the enterprise for the laser-related DE work, with indications of much greater collaboration with the Navy and Air Force. In contrast, the overall strategy for the RF-DE work was not as evident based on the topics presented. Specifically, the RF work appeared to remain fragmented and unrelated to the overall mission. For example, the operational context of the cognitive radar and the counter-UAS work was not apparent. Progress made over the course of the last two years was not evident in two projects—cognitive radar and counter-UAS.

In the penetration, armor, and adaptive protection area, ARL continues to demonstrate a strong record of achievement in fundamental and applied sciences as well as engineering. The ongoing work described in the 2017 review continue to highlight how ARL is building on its history of excellence to provide the knowledge basis for future Army needs in the area of warfighter protection. This is an absolutely critical and core competency that underlies Army capabilities. A major research initiative has been launched as the result of a war-gaming exercise that identified a potential vulnerability. A well-developed, multifaceted research effort has been established and is being executed. Integral to this initiative is the use of classified computations to support analysis of possible mitigation concepts. ARL researchers have taken HPC to a whole new level—an impressive display of using computational sensitivity analysis of a very, very large problem with multiple length scales. The sensitivity analyses have resulted in identification of unknowns and issues, quantification of design variations, and establishing where basic information is required for higher fidelity simulations.

CONCLUSIONS AND RECOMMENDATIONS

ARL’s research on sciences for lethality and protection ranges from basic research that improves its basic understanding of scientific phenomena to the generation of technology that supports battlefield injury mechanisms, human response to threats, and human protective equipment; directed energy programs; and penetration, armor, and adaptive protection developments.

The ARL 6.1 research management team needs to acknowledge that true knowledge research has to be allowed to fail often. Acceptance of the outcome as acceptable and expected for high-risk research is an iterative process for research and is totally necessary. For example, one to two orders of magnitude improvement in performance requires new approaches, and one needs to allow many of them time to fail before they succeed. ARL management needs to be tolerant of such failures.

The battlefield injury mechanisms portfolio seems well connected to outside groups, although the nature of collaborations was unclear. As one example, ARL’s focus on quantifying and translating animal data to humans appears strongly warranted, necessary, and positive. ARL is probably a leader in this aspect of the field and could represent one area for ARL to lead the external research community.

The directed energy program focuses on radio frequency (RF)-DE and laser-DE. ARL leadership and research teams successfully implemented some of the recommendations of the previous ARLTAB report. Specifically, the collection of presented projects demonstrated a coordinated strategy across the enterprise for the laser-related DE work, with indications of much greater collaboration with the Navy and Air Force. Internal to ARL, principal investigators from the directed energy sessions demonstrated greater awareness of work done with threat warning and countermeasures. Also, the radio frequency work needs vision and focus demonstrated with laser work in order to show continuity and measureable progress.

In the penetration, armor, and adaptive protection area, ARL has begun to quantify uncertainties in computational results. UQ was not part of the ARL technical initiative one or two years ago, and ARL was criticized by the ARLTAB for the lack of UQ. ARL has responded, and has demonstrated a good start, but there is a long way to go. It was unclear, however, what level of research is being conducted at ARL, or the number of people involved.