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Mechanical Sciences Division

The goal of the Mechanical Sciences Division is to conceive of and develop transformational research programs in mechanical sciences for the U.S. Army to provide the scientific foundation to create revolutionary capabilities for the future warfighter. The division supports research aligned with the following Army functional concepts: command and control, fires, maneuver, protection, and sustainment.

The division is organized into five programs: Complex Dynamics and Systems, Earth Materials and Processes, Fluid Dynamics, Propulsion and Energetics, and Solid Mechanics. The division’s total budget was $21.9 million for fiscal year (FY) 2019, which includes $1.4 million from Office of Secretary of Defense and Small Business Innovation Research (SBIR)/Small Business Technology Transfer (STTR) programs. It supported a total of 122 single investigator (SI) awards; 19 conference, research instrumentation (RI), and Short-Term Innovative Research (STIR) efforts; and 9 awards to Historically Black Colleges and Universities and Minority Serving Institutions. In addition, the division supported 10 Multidisciplinary University Research Initiative (MURI), 2 Presidential Early Career Award for Scientists and Engineers (PECASE), and 18 Defense University Research Instrumentation Program (DURIP) awards.

Metrics provided for the 3-year period of FY 2017 to FY 2019 indicate that this is a healthy and successful division. There were 568 peer-reviewed publications and 37 significant transitions during this period, and the division supported an average of 297 graduate students and 118 postdoctoral researchers.

COMPLEX DYNAMICS AND SYSTEMS PROGRAM

The vision of the Complex Dynamics and Systems Program is to develop novel analytic and algorithmic methodologies for exploiting the interactions by which high-dimensional dynamical systems store, dissipate, predict, and shape information and energy in dynamically changing environments. This program’s research strategy is to address the following three key scientific questions: (1) What analytical structures capture the most important dynamic features of high-dimensional nonlinear systems and how do one predict, infer, and control them? (2) How do intrinsic information processing, stochasticity, and feedback control modulate the energetics (and vice versa) of nonequilibrium systems? (3) What are the principles by which agile and adaptive cognition, computation, and control are physically encoded within organisms and machines?

Complex dynamics and systems have risen in recent decades to become a study area of both natural science and applied science. The Complex Dynamics and Systems program was divided into three portions by the PM—high-dimensional dynamical systems, nonequilibrium information physics and control, and embodied learning and control.

Overall Scientific Quality and Degree of Innovation

The program has high-quality researchers, some of whom are regarded as deep thinkers in the field. The topics are bold and novel. There is a balance of traditional and new topics. The individual projects generally have potential for impact on the particular area of theory. The program is an interesting basic research enterprise based on extension of traditional models of mechanics of complex systems, indirect influences, elastic stress descriptions and modeling owing to gradients of displacement, noncommutative grouping of elements, statistics of linear and nonlinear stochastic control and learning within nonequilibrium systems, and defining the relationship between embodied dynamics and control in animals and robotics/machines. This body of work appears to be highly valued by the basic research communities represented. The program’s supported teams of researchers are taking on risky areas by exploring the limits of Koopman operators and non-Abelian group operators. Similarly, funded research involving causation entropy and odd elasticity show progress at the basic research level. Funded research on defining dynamical relationships between animals and robots is developing a much-needed community accepted framework, including metrics.

The potential overlap of dynamical systems with solid mechanics and fluid dynamics is very large because problems in continuous media are high-dimensional—in fact, they are infinite dimensional in their primitive form as partial differential equations. The overlap seems to have been largely avoided but not totally, as mentioned above.

Because of the breadth of topics, the program’s impact will likely be smaller than possible. The absence of a precise definition of limitations and pathways of extension of funded research in complex dynamics and systems is a weakness of this program. The program needs to be continued and some important issues needs to be addressed: Where is this research headed? How is success measured in this program? What role does dissipation play in some of the modeling efforts? What is the relationship between the funded research projects and the three pillars of the program?

Scientific Opportunity

The program separates into three components, which individually are unusually broad, as noted above. The connections of the individual projects into smaller groupings and the relation to transitions to the Army were not clarified. The opportunity for greater impact might result from greater focus in the program.

Three questions are listed to describe the strategy to lead future scientific discovery. They appear to relate more to the area of embodied learning and control. It is not clear whether this implies an intended emphasis of that third area with de-emphasis of the other two areas. No argument was given for greater opportunity in any area.

Significant Accomplishments

The Complex Dynamics and Systems Program is the biggest of those in the Mechanical Sciences Division, having an FY 2019 budget of about 31 percent of the total and per-project funding of about 127 percent of the division average. Peer-reviewed publications at about 1.3 per project-year and graduate student-postdoctoral support of about 1.1 per project-year are each modest.

Partnerships and Transitions

Transition was broadly defined, including, for example, co-authored papers and takeover of funding by another industrial organization. Nevertheless, the purchase of a company started under ARO

sponsorship by Delphi for $450 million is impressive. In addition, ARL’s Vehicle Technology Directorate showed interest in two of the ARO research projects. There were no other indications of transition from 6.1 level research to the 6.2 or higher level. Potential transitions were also not clarified—that is, the relation of the research to enabling technology was usually not obvious.

Level of Effort

The program leaned toward the theoretical side, although some experimental research was included. The program has healthy-size components in both the single investigator and MURI areas.

While the breadth of the program is a positive feature, care is urged to examine the program for excessive fragmentation that leads to a loss of focus.

Other

The program vision described analysis and methodology for high-dimensional dynamical systems in dynamically changing environments. The appearance of dynamically changing environments was not obvious in highlighted projects. More importantly, it is not generally required for classification as a complex system.

The highlighted topics in high-dimensional systems often dealt with continuum problems that might profit from interactions with other programs such as solid and fluid mechanics and materials—for example, elasticity, metamaterials, and phase transition.

EARTH MATERIALS AND PROCESSES PROGRAM

The vision of the Earth Materials and Processes Program is to enable maneuvering, communication, and situational awareness in all terrain through understanding and prediction of the physical and mechanical properties and behaviors of rocks, soil, and man-made earth surfaces and their interactions with their surrounding environment. This program’s research strategy is to address the following two key scientific questions: (1) How do grain-scale features influence bulk properties in unconsolidated earth materials? (2) How can earth surface interaction with air and water be predicted at warfighter-relevant spatiotemporal scales—microns to hundreds of kilometers?

The program supports three Army functional concepts: to understand the mechanical behavior of granular and fine particle systems for maneuver; to understand physical interactions in the dense urban environment for command and control; and for intelligence with a plan to explore mountain communications for command and control.

Overall Scientific Quality and Degree of Innovation

The research completed within the program is novel and broad, similar to activities potentially supported at DOE and NSF, but with relevance to the Army. Research specifically highlighted related to granular mechanics investigations of granular assemblages with the effects of grain roughness, mineralogy, and comminution accommodated to define evolving rheology, to develop realistic and efficient digital elevation models (DEMs) to accommodate the complex response of angular granular assemblages, and to understand the geophysical signatures of such assemblages. Related activities included understanding the entrainment, transport, and deposition of dilute granular suspensions, relevant to alluvial and aeolian systems—in particular, with models capable of accommodating realistic grain geometries and domains of sufficient dimension or grain-numbers to test scientific hypotheses of

aggregate response. A current MURI on coarse-grained systems and a forthcoming MURI on fine-grained granular systems offer potential for important advances in these areas. Specifically, such projects that link single investigators with complementary skills in analysis, imaging, and observation is an effective method in boosting scientific value from a small team and its broadened scientific perspective.

This overall theme of the response of heterogeneous systems extends to other projects. One such project is the behavior of ice and ice-laden materials involving phase change owing to melting and the evolution of rheology. Another such project is the integration of transport and disaggregation of earth materials across a broad range of scales in contributing to the understanding of stratified flows together with the impact of moisture and state of materials on their geophysical signatures. These projects, broadly representative of earth-water-atmosphere interactions, have complementary linkages to the mechanics of such systems. These projects that define the program portfolio provide coherence between the dual themes of granular mechanics and earth-water-atmosphere interactions.

Scientific Opportunity

The program has been successful in establishing a broad theme linking the mechanics of complex earth materials with processes and textures that evolve from earth-water-atmosphere interactions at a wide range of length scales. These areas are rich for discovery—in particular, the proposed extension to explore the more complex mechanics of fine-grained materials is an exciting one that will probe the important impacts of chemical, biological, and complex fluid-solid interactions. The currently proposed directions of the program are largely in continuation of the successful and productive direction of the current program—understanding the complex response of multi-phase materials, across the scales and with specific application to earth-water-atmosphere interactions—a very broad suite of potential disciplines. Proposed extensions to this are to tailor the large-scale interactions of earth-water-atmosphere to include implications for the built environment and to explore controls on information transmission in challenging environments via seismic acoustic and electromagnetic signals. The former of these is linked to urban environments and presents important scientific challenges. The second proposed focus, on information transmission, is also rich in potential discovery, with application to both wireless communication and in process-based understanding of remote sensing signals that is logically linked to the mechanics-based elements of the existing and evolving program.

Significant Accomplishments

Accomplishments described in the review represent significant scientific advances. In particular, the program appears to host a large number of early- through mid-career researchers with a diverse and creative portfolio of investigations. In particular, the program accommodates this diverse array of projects under the dual themes of the complex mechanics of multiphase earth materials and earth-water-land interactions—essentially defining behavior across the scales. Scaling micromechanical analyses to engineering-relevant representative-volumes is noteworthy. Such calculations intrinsically limited by the grain number, the grain-grain interactions, and limits on computational resources are noteworthy. Although it is difficult to determine the ingenuity and impact of the work done by the investigators involved based on limited project details provided by ARO, members of the panel are familiar with the work done by many of these principal investigators (PIs), and it is of very high quality. Integrated metrics of papers published, numbers of students and postdoctoral researchers supported, and a broad array of transitions is indicative of a well-directed program.

Partnerships and Transitions

This program has been particularly successful in developing follow-on linkages with other DoD offices—notably, with the Cold Regions Research and Engineering Laboratory (CRREL) but also with broader government agencies, such as National Oceanic and Atmospheric Administration (NOAA). These linkages include internally funded basic research projects, joint field campaigns, and intellectual linkages through summer internships. Together, these transitions identify the broad relevance of the program in transitioning its scientific vision to engineering applications.

Level of Effort

In addition to the intrinsic scientific outcomes, the program covers an unusually broad spectrum of spatial and temporal length-scales and complex solid-liquid-gas/land-water-atmosphere interactions and integrates laboratory and analytical investigations that span micrometer to kilometer scales. The large number of successful transitions with awards to other DoD programs identify the broad applicability and relevance of the supported research.

Other

The program has been successful in developing a coherent and interleaved research program between multiple investigators that leverages the existing resources and maximizes scientific impact—the program is producing important results and discoveries. This thoughtful and well-executed development of an interdisciplinary research program is commendable. The development of the program included grantees meetings and workshops as two potential mechanisms to retain momentum, broaden the catchment of research topics, and to expand the success.

There are clear and close linkages of the Earth Materials and Processes Program with the Solid Mechanics and Fluid Dynamics Programs and potentially with the Complex Dynamics and Systems Program.

FLUID DYNAMICS PROGRAM

The vision of the Fluid Dynamics Program is to develop frameworks for understanding and exploitation of nonlinear flow interactions. This program’s research strategy is to address the following three key scientific questions: (1) Does turbulence possess a “structure”? If so, can it provide a useful description of turbulent flow behaviors and permit control? (2) Can the complexity of the Navier-Stokes equations be reduced while maintaining essential physics of a given flow? (3) What novel strategies allow computation of flow physics, balancing accuracy and efficiency, without simply relying on massive parallelization?

The general aim of the Fluid Dynamics Program is to improve the current understanding of flow phenomena via theory, computation, and experiment. Specific objectives are the efficient prediction of flow physics, the discovery of novel flow phenomena, and the generation of new strategies for flow control. Interest in this general area is related to the Army’s Functional Concepts for maneuver (e.g., projectiles direction in flight, vertical lift vehicles, and attack reconnaissance aircraft), sustainment (e.g., the endurance required to operate in sufficient scale over ample duration), and fires (e.g., precision strike missiles and extended range cannon artillery).

Overall Scientific Quality and Degree of Innovation

The overall scientific quality of the program is very good, with a proper mix of very capable established and young investigators, and theoretical, computational, and experimental projects. Several researchers supported by the program—nearly 30 percent—have received significant awards. The general focus of much of the research, with its emphasis on turbulence, the development of reduced-order models for fluid flow and computing, is rather predictable, but, in an established field such as fluid dynamics, it could not be otherwise. While these are the dominant topics of the program, appropriate breadth is achieved by supporting research in other areas as well, such as particulate and biological flows, harnessing flow for material assembly, and the prediction of unsteady boundary layer separation. Compressible flows—structure of supersonic flow, shock-boundary layer interaction—are also an important and appropriate component of the program, although the character of the program in this area is somewhat less innovative than in some of the others. Side-by-side with established methods of investigation, new methods are being developed, an example being the application of machine learning to pursue a reduction in the complexity of fluid flows.

The program currently supports three MURIs—two devoted to the development of novel computational methods and one exploring flow in the glymphatic system of the brain. These are all high-quality projects in the hands of very capable investigators and one can expect significant fruits from these investments.

Scientific Opportunity

Several investigators are trying to open new research pathways in unconventional directions and there is a certain amount of risk associated with these efforts. While machine learning and, more generally, big data ideas and methods are finding their way in the broad field of fluid dynamics and can be considered, therefore, a rather safe bet, the jury is still out for others, such as the application of network theory to fluid flow problems or the development of hyperbolic Navier-Stokes equation methods. Close attention needs to be paid to these projects, and a critical evaluation of their results is appropriate. The need to pursue niche opportunities and support work not supported by the larger, in a sense competing, programs mentioned before, is understandable, but an excessive reliance on this strategy is also a risky proposition.

The support of novel and sophisticated experimental methods—for example, luminescent micro-beads for particle-image velocimetry, molecular tagging velocimetry in liquid helium, micro skin-friction sensors, and highly resolved tomographic particle image velocimetry—is an interesting facet of this program. Computation is also an essential component of modern fluid dynamics. ARO’s research program in this area is properly based on the recognition that standard approaches will never be sufficient owing to the basic limitations of the existing and future computing equipment. For this reason, in addition to the further improvement of standard approaches, the program includes less-traditional components—such as operator-based methods, hyperbolic Navier-Stokes equations, and fractional order methods for conservation laws—to which the previous comments are applicable.

A suitable research opportunity may exist in the area of compressible flow turbulence, which would seem highly appropriate for ARO’s Fluid Dynamics Program. Additional research opportunities in the biofluids area might also be considered.

Significant Accomplishments

It is understandable that the major accomplishments to date have been achieved in what may be considered the more established and, perhaps, traditional areas of the program. New ideas and methods will take a longer time to bear comparable fruits. Examples of significant accomplishments to date are the refinement of large-eddy simulation methods by means of a clever use of information on the local flow

physics, high-order overset methods for accurate computational fluid dynamics (CFD) solutions in complex geometries, and the stabilization and control of projectile trajectory by an improved understanding of aerodynamic force generation.

Partnerships and Transitions

The program has generated some interesting transitions and partnerships. NASA is in the process of adopting the hyperbolic Navier-Stokes equations for its FUN3D code. An overset method developed under support of the program has been adopted for the Army’s rotorcraft code HELIOS. A cooperative agreement has been entered which supports the long-range distributed and collaborative engagement ERP.

Level of Effort

Several other research programs in the fluid dynamics area are in existence supported by other government organizations (e.g., NSF, NASA, AFOSR) as well as private entities (e.g., the Boeing Corporation). The ARO’s effort cannot compete with many of these in terms of scale. Rather, it tries to find niche opportunities of particular relevance to ARO approaching new investigators and trying to promote research collaborations.

Other

Historically, fluid dynamics has played an important role in the development of methods for, and understanding of, complex systems. A well-known example is the Lorenz model and its role in the early days of chaos theory. Thus, natural crosscutting opportunities for the Fluid Dynamics Program exist with the Complex Dynamics and Systems Program. For example, applications of the Koopman operator are being pursued by the fluid dynamics research community, and this is one of the areas supported by the Complex Dynamics and Systems Program. Fluid problems are naturally high-dimensional systems, and this is another research area of interest to the Complex Dynamics and Systems Program.

Another natural crosscutting opportunity exists with information sciences, given the strong current interest on the part of the fluid dynamics community in the general area of big data. In spite of extensive research conducted over the past 30 years, predictive models of the dynamics of turbulent flows over a wide range of scales and frequencies remains an unmet demand. Machine learning and neural networks techniques are currently being used with some degree of success to analyze the dynamics of nonlinear complex turbulent flows.

PROPULSION AND ENERGETICS PROGRAM

The vision of the Propulsion and Energetics Program is to develop the ability to control chemical energy release rates in energetic materials and fuels via the understanding of phenomena governing initiation, burning, reaction, and extinction. This program’s research strategy is to address the following three key scientific questions: (1) What are the chemical mechanisms that control ignition and initiation in high energy density systems? (2) How can researchers manipulate processes in materials and material interfaces to achieve control over reactions and reaction rates? and (3) What modeling frameworks enable predictive, computationally efficient models of large-scale processes? The Propulsion and Energetics Program aims to perform basic research (6.1) to create revolutionary capabilities for the future warfighter. This vision, while unspecific, is well suited to drive novel scientific advancements. A capability of the

U.S. Army to tailor energetic material performance in terms of delivery time scale, location, and total energy has unique opportunities to facilitate adversary overmatch for the warfighter. A modern demonstration of this vision would be expected to include crosscutting projects that bridge areas such as solid mechanics, interface transport phenomena, turbulent combustion, or advanced manufacturing.

Overall Scientific Quality and Degree of Innovation

The program includes high-quality research projects, yet some concerns do exist, as discussed here. In some cases, strong claims have been made without supporting evidence.

One research project seeks to develop a liquid-phase chemical reaction mechanism for the explosive compound RDX, intended for application as a burn modifier. The confined rapid thermolysis (CRT) experimental method is limited to detection of rapid thermolysis type reactions from the liquid phase. RDX is solid at ambient conditions, meaning that the reaction mechanism does not include any kinetics of condensed-phase reactions, surface and interfacial reactions, gas/solid reactions, and even gas/gas reactions and focuses exclusively on unimolecular decomposition reactions. In real processes, decomposition species evolution occurs in time, yet these time dependencies are not possible to segregate by the CRT method. These limitations suggest caution when applying the mechanism to combustion models where complex physiochemical processes exist or to models at detonation pressures. What the program failed to make clear is the transformative opportunities and innovative nature of this RDX reaction mechanism that has limitations in capturing processes from materials initially at ambient, solid-state conditions. Additional detail about the transition of the HMX reaction model to in-house codes and Army customers may help to address this concern.

The research project to advance kinetic mechanisms of Army fuels using shock tube methods seeks to address the auto ignition behavior that causes knock in engines. While the shock tube method has been used quite extensively to study the gas-phase kinetics of various fuel/oxidizer mixtures, the method is not able to represent the fuel spray dynamics and the in-cylinder temperature and pressure most relevant to engines. This work has not yet been applied to Army-relevant fuels (e.g., JP-8) and fuel blends, nor does the work indicate that kinetic mechanisms to support the flame speed measurements will be an outcome of this project. The researchers need to clarify the relationship of the objectives of this project to compliment other Army TARDEC-funded research occurring in more engine-relevant conditions and with Army-relevant fuels and fuel blends.

There are some concerns about the projects addressing the key scientific question on frameworks for predictive models. Emphasis was placed on the higher dimension of the manifold rather than the higher dimension of the physics or the addition of new physics. One-dimensional manifolds are not limited to 1D physics; they might have higher dimensionality in terms of physics than 2D manifolds; and they might have more embedded physics than 2D manifolds. The emphasis on the manifold leads the observer to infer that the work is not extending the physics or the physical dimensionality of the flamelet configuration. Extending the manifold without extending the physics only increases computational cost without any clear gain. This comment has relevance to the project on adaptive modeling of cool flame-assisted ignition and combustion where mention is also made of 2D manifolds.

The claim of greater efficiency than given by tabular storage can be achieved by calculating manifold solutions only as needed and then storing for repeated use is unsubstantiated and is questionable. When a set of quantities from one calculation is stored to five or more significant digits, they will likely never be repeated exactly over space and time in the large-eddy simulation. So, when are they reused and why are they saved? They would only be useful for storage if one were willing to interpolate between stored values. The question then arises whether the stored data allows optimal interpolation. A table can be arranged to give optimal interpolation. It can also be limited to cover the range of needed inputs and outputs. Furthermore, the concern about memory cost for tabular storage was addressed years ago by

Ihme et al. at Stanford University.¹ Deep learning neural networks require low memories and provide better means for interpolation than traditional tabular storage.

The work on adaptive modeling of cool flames is interesting and is widely believed to be relevant to knock problems in diesel engines. A concern is that computations for cool flames have been strongly dependent on boundary conditions, and therefore it must be inferred that they cannot be viewed as independent flames. Rather, their stability is dependent on a global environment. Perhaps a solution has been found but clarification is needed.

The research on adaptive modeling and diagnostics of cool flame-assisted ignition and comb claims that an order of magnitude reduction in computational time is achieved compared to traditional chemical solver solutions. It is not explained what that method is other than with the undefined “PFA” label. If the method is the use of flamelet theory, then that is old news, established by many other researchers. The use of neural networks for flamelet theory also is not new, as noted above.

In summary, several claims have been made in justifying the launching of some work. More justification is needed for such work.

Scientific Opportunity

The scientific opportunities of the current program are difficult to discern. This results from a program heavily weighted on gas-phase combustion that involves long-established researchers in the field or projects making strong, unsubstantiated claims. In many projects, the reliance on existing experimental facilities that are evolving incrementally to reach extreme test conditions may be limiting entrance of innovative ideas and a diversity of researchers.

The focus to unravel complex kinetic mechanisms is noteworthy and commendable. The ability to reduce those detailed kinetics into strategic transitions to the Army was not fully explained.

Significant Accomplishments

The current program contains projects with low technical risk exposure. That is, the projects generally encompass incremental advancements to established experimental or computational methods or application of methods to materials already in existence. As a result, the accomplishments of the program are not anticipated to drive significant transformation in their respective field. This program could benefit from shifting focus to new projects with higher technical risk.

Partnerships and Transitions

Transitions are broadly defined to include research deliverables and personnel for internal and external Army customers. Of the eight transitions listed for the review period, only half of listed transitions show immediate delivery to the Army.

For the project on the RDX reaction mechanism, the project accomplishments include the validation of an HMX reaction mechanism. It is not clear if the reported transition of an HMX reaction model to Army in-house codes is a direct result of ARO funding. For the project on the dynamic response of reactive materials, the deliverables were not clear.

The relationship of future potential transitions to the Army modernization priorities were not clear.

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¹ M. Ihme, C. Schmitt, and H. Pitsch, 2009, Optimal artificial neural networks and tabulation methods for chemistry representation in LES of a bluff-body swirl-stabilized flame, Proceedings of the Combustion Institute 32(1):1527-1535, https://doi.org/10.1016/j.proci.2008.06.100.

Level of Effort

The current program is heavily focused on gas phase kinetics for the purpose of developing or improving existing kinetic mechanisms for liquid fuels and possible propellant burn modifiers. As a result, the current program does not differ significantly in its goals and focus from a program that would have been reviewed more than 10 years ago.

Other

The program seeks to address three key scientific questions. These questions are broadly posed such that they do not provide a useful discriminator for proposal evaluation and prioritization. The first question—What are the chemical mechanisms that control ignition and initiation in high energy density systems?—suggests that mechanisms of condensed explosives is of interest, yet much of the research supporting this question is in liquid fuels and engine applications. The second question—How can researchers manipulate processes in materials and material interfaces to achieve control over reactions and reaction rates?—is supported by a project that could be considered more appropriate for applied research (6.2). The third question—What modeling frameworks enable predictive, computationally efficient models of large-scale processes?—was missing a discussion of technical gaps in existing DOE/DoD codes that already perform simulations across scales.

The program did not plan for adjustments in FY 2021 to the key scientific questions. The program’s vision and key scientific questions need to be reviewed in order to refocus the program to modern opportunities and technical knowledge gaps, to remove barriers for accessing new research opportunities, and to improve transparency in proposal selection. The program could strongly benefit from increased breadth in researchers via crosscutting initiatives bridging chemistry, materials science, fluid dynamics, and solid mechanics.

SOLID MECHANICS PROGRAM

The Solid Mechanics Program aims to uncover the physical processes responsible for deformation, damage initiation and propagation, and failure of material systems—particularly under extreme pressure, strain rate, and repetitive loading—ultimately leading to the creation of lightweight, resilient, and adaptable soldier and system protections. This program’s research strategy is to address the following four key scientific questions: (1) How can a material system’s response to dynamic or complex loads be analytically described and predicted? (2) How can dynamic crack growth be visualized and predicted? (3) How do material defects, system morphology, and temperature affect damage propagation? and (4) What can researchers learn from biological and geological systems to strengthen and toughen material systems?

Overall Scientific Quality and Degree of Innovation

The goal of the program is light, resilient, and adaptable materials for protection of both the warfighter and vehicles or other systems. Of principal concern are high-pressure loadings, high strain rates, and repetitive loadings. Lightweight but strong materials can reduce the weight of personnel protection in body armor, thus enhancing maneuver and protection for the soldier. Better understanding of material defects and fracture can both increase the strength and durability of protective systems. Improved performance under repetitive loads can extend the service life of protective systems and vehicles, thus enhancing maneuver and resilience.

The new PM has done an excellent job in rationalizing the existing portfolio of projects. The portfolio has been reorganized around three themes: constitutive response; visualization and prediction of fracture patterns; and defect, morphology, and temperature effects. The quality of the individual projects and PIs is strong on average but variable. The program hosts several well-known and accomplished researchers, and has a good mix of young and capable researchers at lesser-known universities. Some projects may not be at the cutting edge of important problems in solid mechanics, but the PM needs the leeway in reformulating the portfolio.

Scientific Opportunity

The current transition to new leadership offers a moment of opportunity to direct and focus the research of the program. This will not happen in a sharp turn but in a progression of terminating projects and judiciously funding new ones. The questions to be decided are how best to focus research opportunities to achieve Army objectives of protection, maneuver, resilience, and sustainment; and what areas of work to de-emphasize in the transition. The current ARO thinking that response to dynamic loads and crack growth and visualization may be de-emphasized in future awards seems consistent with current opportunities in the field.

The areas of focus for the program in the near future have tentatively been identified as (1) damage across scales—hierarchical materials and structures; (2) the isolation of defects and inhomogeneities to control damage; and (3) nature-inspired design. Each of these builds in part on existing strengths of the program. The first two were briefed in the presentation, but the third was only mentioned and it is not clear to what extent it constitutes a separate thrust for the future. Nonetheless, this third focus may represent the boldest shift and biggest opportunity, but one caution is that this field is not entirely new and the mechanics of biological systems has been an active field for some time. A concern is the program moving away from solid mechanics to design. Of course, these fields are related, but the question is where is the emphasis going to be placed? The concept that there are important new lessons to be learned from geomaterials might be questioned, and did not seem well supported. Many of the current projects in the portfolio relate to one or more of these new directions, but not all. A decision will have to be made on what current directions to de-emphasize. This transition in program direction is a critical moment of opportunity and needs to be both carefully planned and critically reviewed. The projects that have already been awarded in the new thrust areas of hierarchical structures and inhomogeneities appear modest steps in the new directions. To be successful, bolder projects may need to be funded.

Significant Accomplishments

The Solid Mechanics Program is the smallest of those in the Mechanical Sciences Division, having an FY 2019 budget of about 8 percent of the total and per-project funding of about 80 percent of the division average. Peer-reviewed publications at about 1.4 per project-year and graduate student-postdoctoral support of about 1.5 per project-year are each modest. The raw number of peer-reviewed published papers is, of course, an inadequate metric for research quality. The venues of publication would be useful additional information.

Partnerships and Transitions

The number of transitions in the Solid Mechanics Program is modest compared to the other programs. This, in part, can likely be ascribed to the lack of permanent leadership in recent years, to the broad funding of seed projects, and to limited overall funding. Four transitions to customers are described, of

which two are co-authored publications. No MURI or PECASE opportunities have followed from the current portfolio.

Collaborations with other ARO programs are possible and could leverage resources and expertise. There is one present project involving the geomechanics of Berea sandstone with an obvious connection to the Earth Materials and Processes Program. However, it is not in one of the thrust areas of that other program. Effort could be invested in reviewing for opportunities in the Materials Science Division areas. A particularly attractive area of collaboration could exist in the area of biomechanics, from the perspectives of nature-inspired materials, warfighter interactions with protective systems, and injury prevention.

Level of Effort

Solid mechanics is by definition a broad discipline, and the scope of the current portfolio of projects reflects that breadth. The program is in transition from a multiyear period of acting leadership to one with a recently recruited but permanent PM. Thus, the current portfolio of projects represents, by its history and intention, a collection with an unusual number of seed opportunities awaiting a centralizing vision. The recent recruitment of a new PM could facilitate defining that vision.

Other

The current transition in the program has led to a rationalizing of the structure of the portfolio and a tentative identification of new areas of focus, both building on existing projects and developing new ones. This is a promising time to assemble workshops to bring the principal investigator community together to help inform planning for the new portfolio. This needs to be a high-priority initiative.

OVERALL ASSESSMENT

In general, the scientific quality of the work funded is of sufficiently high quality and is not of concern. As expected, this fundamental research program of the ARO, when considered as a whole, supports a large number of smaller projects that have a distribution from very high risk, unproven concepts (e.g., dynamic analysis frameworks) to very low risk, historically vetted methods (e.g., shock tube methods). The majority of the questions are aimed at understanding the methodology for PM-selected focus areas within their proposal. In general, the PM appears to have significant autonomy in adjusting the focus areas of the research portfolio—it is the PM who can target potential PIs, manage the proposal review process, assemble proposal review scores, and make final recommendations as to prioritization of funded projects. The individual PM-centric approach for managing division portfolios raised questions related to transparency and methodology of proposal solicitation, proposal review and final assessment, and proposal selection for risk balancing and strategic alignment. This level of PM independence could impede ARO’s top-down distillation of Army needs into research thrusts for funding.

In addition to technical diversification or collaboration between projects, some portfolios would also benefit from increased diversity of research PIs to include early-career PIs and less long-term continued funding provided to late-career PIs.

As demonstrated by the newer PM, focus questions were adjusted at review time in order to give the research portfolio a cohesive focus. This indicates that the portfolios are not being managed by a strategic plan with a long-term timeline; instead, the goals of any given year are adjusted on demand. This has implications for the autonomy of the PM to follow research that may not be best aligned with the long-term ARO strategy.

Recommendation 10: The Army Research Office (ARO) management should establish processes that help to ensure that proposed research is unique, pioneering, and/or novel. ARO management should place emphasis on envisioning and conducting workshops or other events that reach beyond the current cadre of funded principal investigators to explore fields broadly and define new directions and new investigators for the programs.

In a number of divisions, areas of missed opportunity for interdivision collaboration and an apparent stovepipe of projects under each PM were identified. There were certainly examples where this is not the case, but in an agile and responsive research portfolio, more interdisciplinary projects are expected. The MURI projects provide a good example of interdisciplinary projects, yet these are not readily accessible to most projects within a PM’s portfolio. Efforts to promote improved collaboration across ARO divisions and scientific disciplines would be beneficial.

Recommendation 11: The Army Research Office (ARO) management should develop mechanisms that facilitate interactions within the Mechanical Sciences Division and with the Materials Science, Chemical Sciences, and Physics Divisions. ARO should focus these interactions to be on funding projects with aligned priorities within the programs, be they within the same division or across divisions.