The Panel on Mechanical Science and Engineering at the Army Research Laboratory (ARL) conducted its review of ARL’s mechanical sciences research and development portfolios at the Aberdeen Proving Ground, Maryland on May 28-30, 2014. The areas of mechanical sciences that were assessed were mechanics, propulsion, and reliability and diagnostics. During the 2013 review, the panel supported the assessment of the autonomous systems; the results of that assessment are presented in Chapter 3.
ACCOMPLISHMENTS AND ADVANCEMENTS
Mechanics
The laboratory facilities are impressive, including the subsonic wind tunnel and facilities supporting work in propulsion and additive manufacturing; these facilities can enable ARL to establish leadership positions in mechanical sciences areas. The facilities would also be attractive to collaborators. Several researchers have industrial backgrounds; this is the strength of the research team.
Recent work on damping augmentation using nanomaterials to tailor interfacial properties of critical vehicle structures is an especially strong activity. The work contributes to useful technology, and the researchers’ approach and vision bridge fundamental questions with applied research.
ARL facilities and personnel at the National Aeronautics and Space Administration (NASA) Glenn Research Center, where research on propulsion is performed, and the NASA Langley Research Center, where helicopter and tilt rotor testing are performed, are among the best in the world in these areas. The facilities have been developed in conjunction with industry and NASA over the past 20 years and can be exploited to support 6.1 and 6.2 research.
Interfacial Strain Energy Dissipation in Hybrid Nanocomposite Beams Under Axial Strain Fields
Overall, the scientific quality of this research reflects originality and is of high quality. The principal investigator is very qualified and is supported well by his team. In general a good awareness of the underlying science and of work conducted elsewhere is evident.
Adaptive Seat Energy Absorber for Active Vehicle Safety
The overall quality of this project is good. The research reflects understanding of the physics and familiarity with other work in the field. Laboratory equipment and modeling work also seem appropriate. The researcher is qualified and is making good use of the test facilities. The project shows a good mix of theory, computation, and experiment.
Structural Damping Modeling for Rotorcraft Comprehensive Analysis
This is a useful and timely project. Hydraulic dampers are the largest operations and support maintenance driver on many helicopters in the Army fleet. They are also critical to operational readiness and airworthiness of the vehicle. Elastomeric dampers are difficult to model and not thoroughly understood from a material constitutive and life prediction perspective. The lead researcher is regarded as an expert in this area. The quality of this work is high and suitable for publication in archival journals. The research is employing appropriate equipment and numerical models.
Computational Fluid Dynamics and Comprehensive Structural Dynamics Correlation with Fuselage and Wind Tunnel Walls
The research reflects understanding of the physics and familiarity with other work in the field. The researcher seems qualified, and the quality of the work seems good, evincing thorough execution. Exploration of coaxial compound rotorcraft is valuable.
Propulsion
The high-pressure, high-temperature, pulsed injection facility is impressive. The high-temperature component/fatigue facility is excellent and has been used to study surface temperature and thermomechanics. The small-engine altitude facility is a very impressive, unique, and useful facility. The tribology laboratory is a very useful facility. It provides a capability for integration of scientific issues. Furthermore, it is capable of replicating practical constraints with respect to loads, temperature, and surface curvature. The ultrasonics method used to find potential cracks is novel and promising. The method is nonintrusive and robust, even at high temperatures. The examination of diesel engines, gas-turbine engines, and associated spray combustion is appropriate.
JP-8 Diesel Injector Experiment
The examination of JP-8 fuel injection is valuable. The plan to extend experimental capability to be able to measure more than penetration length (e.g., droplet size and velocity distributions) is good. Such data are critical for validation of high-fidelity numerical simulation tools used in the design and analysis of the injectors.
Life Cycle Management
The researcher understood well the technical goals and the direction needed to achieve the goals. A good connection exists with Rochester Institute of Technology. The planned follow-up applying statistical analysis will be useful.
Gear Surface
This work was good, and a basic understanding of fundamentals was displayed. A comprehensive list of previous works was identified by the researcher.
Turbine Blade Innovation
The experimental film cooling program aimed to validate Reynolds-averaged Navier-Stokes (RANS) computational prediction. It focuses on the effects of hole geometry and is making good progress.
Reliability and Diagnostics
According to the Office of the Assistant Secretary of the Army, Acquisitions Logistics & Technology, four out of five U.S. Army systems fail to achieve reliability requirements. Correspondingly, the costs to operate and maintain Army systems are not sustainable. Fatigue is a dominant concern in aircraft reliability. Today, modeling, inspection, and replacement of components subject to fatigue failure are critical to maintaining airworthiness.
ARL researchers are conducting research and developing an approach to design of aircraft structural components with goals to reduce fatigue failures and sustainment costs associated with maintenance. The program has the potential to revolutionize the achievement of high-reliability systems and includes the following elements: stronger materials that will lead to more durable structures; intelligent materials and structures that are self-sensing (providing materials state awareness) and, in some cases, potentially self-healing, with improved mechanical properties; physics-based models of failure (including uncertainty quantification) that will provide understanding of potential damage precursors and improved diagnostics and prognostics; diagnostics and prognostics that will relate materials/structure state to the distribution of remaining life of the structure; and control systems that will, given state awareness, keep aircraft in a safe state.
There is much scientific and engineering research that needs to be completed to demonstrate the feasibility of this approach. ARL scientists and engineers are on the path to achieving this. Early results presented by the researchers are promising.
Damage Tolerance of Novel Composite Materials
This research has demonstrated the feasibility of improving structure fracture toughness and resistance to delamination of composites through the use of needling or interleaves. Tests have been conducted and showed benefits of needling and interleaves.
Dynamic Response of Topologically Interlocking Structures
This project involves the use of additive manufacturing techniques to create interlocking elements that can be assembled to sustain impact loads greater than those sustained by traditional materials. If
successful, this research could lead to the design of a new class of high-impact-resistant materials. This research has made progress by using interlocking mechanisms to improve impact resistance on a simple configuration.
Additive Manufacturing of Extremely Lightweight, Adaptive, Durable, Damage-Tolerant (XLADD) Structures
Additive manufacturing of composite materials or multifunctional composites would create a new approach for making advanced composites or intelligent composites. This research has successfully demonstrated the use of 3D printing machines to fabricate composite materials.
Statistical Estimation of Sensitivity in Modeling Parameters
The main accomplishment of this project is the development of a new probability-ratio-based method for estimating sensitivity of a system with an intractable response surface that requires computationally intensive Monte Carlo evaluation. The new approach can significantly improve the computational time and cost for estimating material responses from limited test data.
Advanced Sensor Fusion
The proposed work, once completed, could lead to a good approach for life-cycle monitoring of structural integrity and lead to the zero maintenance goal. This research has successfully demonstrated diagnostic and prognostic capabilities with a suite of various sensors for structural components under static and dynamic loads. The approach is based on the utilization of a particle-filtering algorithm to compute Bayesian posterior probabilities of remaining life.
Electrical Impedance Spectroscopy for Assessing Remaining Useful Life
The goal of this project is to use electrical measurements across carbon fibers passing through a structure to detect change in loading or other incipient damage. Previous work had looked only at the real (resistance) part of the impedance to detect such changes. The research has demonstrated that the phase angle can be more sensitive to subtle changes that cannot be detected by resistance alone.
XLADD Structures
This work is developing lightweight, high-strength, multifunctional materials that can be used to integrate self-healing and sensing-based capabilities for condition monitoring and damage tolerance. This work is making good progress.
Virtual-Risk-Informed Agile Maneuver Sustainment (VRAMS)
The overall vision presented is very good and is necessary for successful implementation of online monitoring and control.
Materials State Awareness: Identification of Damage Precursors
The goal of this work is to explore smart materials to enable intelligent built-in sensing capabilities to detect damage precursors. To this end, magnetostrictive particles were embedded into composite structures. Fractographic analysis results obtained to date indicate that changes in magnetic field flux intensity have the potential to provide useful precursor information.
Mechanics
Publication in archival journals is necessary and very valuable for establishing a world- class research reputation and developing early-career researchers. The ratio of journal to conference papers needs to be appropriate.
Research tasks appear to be somewhat disconnected, and there is no common vision. It was not clear how the current topics are related to ARL’s long-term vision nor was it clear how the current topics were selected. On many tasks, the duration and schedule of paths forward were also not clear. This information would help focus the teams on milestones and the management on providing timely guidance. It seems appropriate for ARL researchers to work on programs that are more than 1-2 years in duration.
There is strong opportunity for more work related to control of noise in the vehicle’s interior. This fits well in the protect-the-soldier mission, aligns with published technology goals to reduce interior noise associated with lightweight drive systems and airframe, and might dovetail well with innovative damping technologies being pursued by multiple researchers in the division. There is good potential for both fundamental and applied research contributions.
The mechanics area needs a more robust workforce with sufficient experienced staff to mentor recently hired, less experienced staff. There is a need to delineate a clear path connecting development, laboratory testing, and full-scale testing of components and systems. ARL work is not clearly differentiated from external contractor work, and the ARL workforce is challenged by the requirement to manage local, existing ARL research and monitor external contractor work. There is a need to accelerate the full operational capability of all primary research facilities—for example, the subsonic wind tunnel. There is also a need for a local ARL capability to conceive, develop, test, and implement computational fluid dynamics (CFD) and simulation tools for increasing complex airframe configurations and flight regimes. Although it may be difficult to develop advanced multiphysics CFD capabilities from scratch, the cognizant staff need to be sufficiently knowledgeable to assess and evaluate off-the-shelf CFD software for accuracy and predictive capability.
Interfacial Strain Energy Dissipation in Hybrid Nanocomposite Beams Under Axial Strain Fields
Previous (1990s-2000s) work on characterization of elastomeric materials and associated device damping may provide some beneficial structure for the work. ARL’s current research focuses on physical and numerical modeling on carbon-nanotube-reinforced composites. Future exploration involving bench and rotor testing is needed if there is going to be application of these materials. Appropriate facilities and laboratory equipment need to be engaged to best support the research path. ARL needs to consider the implications of amplitude dependence on operational condition. Low damping at higher amplitudes can limit cycle behavior. ARL needs to consider internal noise reduction as a potential application for
damping. The preparation of descriptive papers is commendable, and submission to peer-reviewed journals will be appropriate as work continues to mature. The aeroelastic rotor experimental system (ARES) rotor test stand and the wing and rotor aeroelastic testing system (WRATS) tilt rotor test stand, both located at NASA Langley Research Center, need to be exploited further. These are world-class facilities that were developed by ARL in conjunction with industry.
Many projects reflect appropriate system-level thinking; continued evolution along this path is needed. Even fundamental research can be motivated by and grounded in system considerations. Doing so will enhance the ability to identify thrust areas that have high payoff for the Army, the Department of Defense (DoD), and the vehicle community at large.
Rotorcraft Capability Assessment and Trade-off Tool
The ARL contribution to this project and the role of Georgia Institute of Technology are not clear. Also unclear are whether the project is using existing methodology or developing new methods to expand capabilities and how the resulting tool will be validated. Without this clarity, industry and government users will not have confidence in the results. Additional research into learning what other groups are doing with trade study tools is needed. It is not clear how numerical models grounded in correlations based on past data will be useful for future designs. Special care is warranted here. The qualifications of the research team may be below the required critical mass and training level. A single principal investigator with at least a master’s degree would be an acceptable start, but additional staff, perhaps with Ph.D. training, is needed. The researchers need to consider reliability, cost, and maintainability factors in their work. They also need to engage industry and other elements within the DoD as much as possible. Working on this project in an insular fashion can undermine the impact of a very important project.
Performance Bounds for Micro Autonomous Vehicles
The overall topic is useful for approximating the endurance of micro autonomous vehicles (MAVs) at various scales. Developing additional detail on the project’s objectives and path forward would be helpful. Time frame, priorities, and workforce requirements need to be quantified. Considerable work has been done on MAV performance throughout the country, including some funded by the Army, but insufficient information was provided to permit judging whether the researchers have a broad understanding of the science and what other groups have done. It was also difficult, based on the information provided, to assess whether appropriate laboratory equipment or numerical models were being employed. Team members seem qualified. A conference paper has recently been written, but no details were provided, so it is not clear whether this work is intended to be published in a journal.
Technology Identification of High-Performance Vertical Takeoff and Landing Tail-Sitter Aircraft
Publication of two recent conference papers is a good sign of productivity. The research did not reflect knowledge of previous analytical or numerical research on tail-sitter performance. The only historical reference was to old designs and schematics of new ones. FLIGHTLAB1 analysis would be sufficient. Qualifications of the researcher seem compatible with the research challenges. It is not clear
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1 FLIGHTLAB is a flight vehicle modeling and analysis tool developed by Advanced Rotorcraft Technology, Inc., Sunnyvale, California.
whether cruise performance of a tail-sitter is a meaningful research question. The design seemed similar to a standard aircraft with a canard. The research challenge lies in the transition from cruise to hover and in low-speed operational maneuvers. Apart from the Defense Advanced Research Projects Agency (DARPA) X-Plane, it is not clear whether anyone in the DoD is seriously considering a tail-sitter.
Adaptive Seat Energy Absorber for Active Vehicle Safety
The dynamic model of the occupant seemed to be a very simplified one-dimensional simulation of the crash event. There was some uncertainty about the lumbar load time histories, and it was not clear why the loads changed sign prior to impact. Also, concerns persist regarding weight trade-offs for the technology.
CFD/Comprehensive Structural Dynamics Correlation with Fuselage and Wind Tunnel Walls
Computer equipment and numerical models seem adequate, but approximately 40 hours per run on 128 cores makes design iteration difficult. Detailed goals and the timeline were not made clear. Implementation of Helios in CAMRAD II would be a step forward, if that is indeed the goal of this program. Utilization of this methodology for design evaluations of future vertical-lift concepts is going to be very time consuming with that level of fidelity. Specific questions need to be identified and the work coordinated with the Aeroflightdynamics Directorate of the U.S. Army Aviation and Missile Research Development and Engineering Center. More collaboration with the wider Army and technical communities and understanding of the challenges associated with high-speed coaxial compound configurations would be valuable.
Structural Damping Modeling for Rotorcraft Comprehensive Analysis
There is opportunity in the reliability and durability area to engage experimental facilities such as the mechanical test rigs and temperature chambers. This project provides a good opportunity for collaborations between mid-career and early-career researchers. The appropriate mix of theory, computation, and experimentation are being applied, but experimental testing to validate advanced damper models is required. The ARES rotor test stand and the WRATS tilt rotor test stand, both located at the NASA Langley Research Center, need to be exploited further.
Propulsion
The consideration of new applications associated with smaller machinery and longer operations, especially for diesel engines, is necessary. A systems overview study is required to identify specific opportunities and needs for research, especially for diesel engines. In some areas, cooperation with designers from industry or NASA can focus the research and optimize impacts under practical constraints.
High-Pressure, High-Temperature, Pulsed Injection Facility
The droplet sizing capability needs to be developed.
High-Temperature Component/Fatigue Facility
In this recently commissioned facility, surface temperature and thermomechanics have been studied so far, but the facility can be used for other studies—for example, study of the thermal and momentum boundary layer.
Small-Engine Altitude Facility
This impressive facility, which is being used to study diesel engines, can also be used for testing gas-turbine secondary power units and components (for example, compressors).
High-Speed Bearing Laboratory
This laboratory has capabilities beyond those it is presently using, which include studying the extension of time between overhauls. A plan for more fundamental research is needed.
JP-8 Diesel Injector Experiment
There is a need to identify from a systems perspective which aspects of diesel engines need improvement and to then use the findings to define research projects. There is also a need to identify new uses for diesel engines and the research that might be useful to advance the new application. It would be useful to characterize each experimental realization with the relevant nondimensional group values—Reynolds number, Weber number, gas-to-liquid viscosity ratio, and density ratio. The effects of change in upstream pressure on flow through an individual orifice in a connected group of orifices need to be explored. The literature from the diesel industry and previous research on liquid injectors and round jets needs to be studied carefully.
Diesel Injector Computations
Examination of JP-8 injection is very valuable. As noted above, there is a need to identify from a systems perspective which aspects of diesel engines need improvement and to then use the findings to define research projects. There is also a need to use the nondimensional groupings. Recent advances in high-fidelity computational tools scalable on state-of-the-art high-performance computers need to be explored. Because the length scales vary over three or more orders of magnitude, the interesting and relevant physics cannot be described with limited computational resources. A strategy is needed in which good subgrid models are developed and used to describe the final stages of atomization and vaporization.
Ducted Rotor
This is an interesting project for which new research is needed. The appropriate parameters need to be identified and used to characterize each experimental realization, for example, disk loading, duct length/duct diameter ratio, fan pressure ratio, and lip radius/duct diameter ratio. Familiarity with turbofan research would be helpful. The ground effect needs to be examined; higher power might be needed near the ground. There is also a need to explore low-speed behavior before generalizing about the effect of blade twist.
Life-Cycle Management
It is questionable whether a crack can practically be tolerated for any duration of time after it is first discovered, and this needs to be investigated further.
Gear Surface
The work would be strengthened by interactions with designers to determine the ranges of parameters of interest and constraints on them.
Hybrid Gear
The mode analysis in this interesting work is very good, but the focus needs to extend beyond the vibrational modes. An improved understanding of material limitations would be helpful. A plan for nondestructive inspection of structural integrity is needed.
Compact High-Efficiency Centrifugal Compressor
The observed behavior is the opposite of what was expected and is not understood: Efficiency decreases and stall margin worsens. There is an opportunity to explore what causes this behavior.
Turbine Blade Innovation
There is a need to examine the vast literature on film cooling. More comprehensive characterization could be helpful—for example, velocity measurements and momentum/flux ratios could be considered. More information about flow upstream in the cooling hole would be helpful.
Reliability and Diagnostics
In many of the current projects it will be important to pay more attention to the underlying physics of the problem (e.g., how sensors interface with the material and the structure). The ability to better predict the remaining useful life of a structure will depend critically on the ability to develop multiphysics, multiscale models (including uncertainties) and sensors that will provide relevant real-time data. The development of intelligent real-time diagnostics needs to follow from knowledge of physics models and sensor data. The team studying virtual risk-informed agile maneuver sustainment will need to develop a roadmap for what will be needed to advance from concept to application. To develop this roadmap, the team will need to identify milestones, timelines, and goals.
Damage Tolerance of Novel Composite Materials
The proposed method, if successful, could lead to a reduction of composite weight or an increase in load-carrying capacity. Both methods have been tried before and reported in the literature and have shown mixed benefits. Extensive experimentation and physics-based modeling will be required to examine the failure mechanisms adequately.
Dynamic Response of Topologically Interlocking Structures
The idea in this project is to use additive manufacturing techniques to create interlocking elements that can be assembled to sustain greater impact loads than traditional materials. The problem needs to be clearly defined, particularly with respect to the application, in order to size the materials properly. In-depth modeling is necessary to understand the fundamental underlying physics. It is not clear how the approach can be generalized.
Additive Manufacturing of XLADD Structures
Additive manufacturing would be a new approach for making multifunctional composites or intelligent composites. The addition of continuous fibers by additive manufacturing still needs to be demonstrated. Considerable modification of additive manufacturing machines will be necessary to achieve the project goal. Materials, processing, and integration issues related to additive manufacturing need to be carefully studied, not only the manufacturing machine. It is important to begin the move to additive manufacturing with real materials that would be used in military systems.
Statistical Estimation of Sensitivity in Modeling Parameters
The exact computational approach was not made clear. It is important that there be a mathematical or statistical demonstration of the accuracy of the method instead of relying on Monte Carlo computational results. There have been many advances recently in the quantification of uncertainty, particularly at the Department of Energy laboratories, that need to be studied and leveraged.
Nonlinear Structural Response Under Multiaxial Dynamic Loading
In this project, procedures and methods have been developed and used to conduct a series of multiaxial component tests on electronic packaged materials. The tests to date have demonstrated that the simple superposition model is not adequate to describe certain damage mechanisms and that more sophisticated models (and corresponding physical explanations) will need to be developed to predict cumulative damage of structures that experience multiaxial loading.
There is a need to develop new methods to characterize material failure mechanisms more effectively and efficiently under various loading conditions. It was not made clear whether the project is to focus on developing new test methods or developing new modeling techniques. The potential outcome of the project needs to be clearly defined.
Advanced Sensor Fusion
The structure of the model relating sensor data to life predictions was not made clear. There was no clear description of the extent to which the model is based on physics rather than empirically fitted functions. Without a physics basis, life-prediction models will be severely limited in their application. It is questionable whether the modified Paris law is really physics based.
Electrical Impedance Spectroscopy for Assessing Remaining Useful Life
It is possible that the carbon fibers can break before any damage has accrued. Perhaps using some scalar function of the two impedance components would be better than using either one alone.
XLADD Structures
There is need for physics-based models at three different length scales. Such models are important because bridging of the scales is critical to understanding damage precursors. There is a need to move this research to the study of realistic materials. There needs to be cross-collaboration with the materials group.
Virtual Risk-Informed Agile Maneuver Sustainment
Identification of damage precursors is a key component of the VRAMS concept. It will be important to identify research needs in related areas such as sensing with embedded sensors or self-sensing material, sensor instrumentation and signal processing, and relating sensor physics to damage features.
Materials State Awareness: Identification of Damage Precursors
There was no discussion about why magnetostrictive material was used. The underlying physics and the actual physical phenomenon were not made clear. Such understanding will be important to the discovery and development of effective and useful precursors. Stiffness reduction and strength reduction are serious issues that need to be addressed.
OVERALL TECHNICAL QUALITY OF THE WORK
The combined research program in mechanics, propulsion, and reliability and diagnostics has several projects whose quality vary from average to outstanding (interfacial strain energy dissipation in hybrid nanocomposite beams under axial strain field is in the outstanding category). The continued development of critically important and valuable ground test facilities like the quite subsonic wind tunnel is notable. The recruitment and hiring of outstanding new staff members greatly increases the potential of developing future research achievement.
The quality of the research would be accelerated by infusing the existing workforce with sufficient senior staff to mentor junior staff. Clearer pathways connecting basic research and full-scale testing of components and systems would also improve research quality. Critique with recommendations through exposure of research to peer review in archival journal publications will also improve research quality. Complete full operational capability of primary research facilities provides a unique resource and advantage to capture research of the highest quality.
Interfacial strain energy dissipation in hybrid nanocomposite beams under axial strain field is an exceptional area. The work here includes the characterization of carbon-nanotube-reinforced composites and associated device damping. This work is driven by physical and numerical modeling. It reflects system-level thinking and modeling. Internal noise reduction, including for rotorcraft, is a potential application for damping. Increased aeroelastic stability of rotorcraft main rotor blades is also a potential application of this research.
The quiet subsonic wind tunnel and the high-altitude test chamber are high-quality facilities.
Mechanics
Overall, the technical quality is good, and there is great potential for the mechanics team to continue to work toward excellence. The facilities and personnel at the Aberdeen Proving Ground are making progress. Many of the recently hired early-career researchers are beginning to become productive contributors and generate high-quality technical work. It is important to provide opportunities for development and effective supervision of the early-career researchers. The new facilities in Maryland have been under design and development for the past few years, and when these are coupled with adequate numbers of qualified research staff, the team could become very productive.
The researchers are doing thorough and competent work but are not generally leading the field. However, there is significant potential to do so, because the group is very talented. They could become leaders by identifying niche areas and technology solution ideas and by developing and validating the associated computational, experimental, and design tools required to answer specific key research questions. It was not clear how many researchers are engaged in theory, computation, and experiment. There appears to be more emphasis on applied research than on fundamental research. Overall, the mechanics program is not sufficiently staffed to accomplish its mission.
Propulsion
The experimental facilities are excellent and offer some unique opportunities. There is good applied research but little competitive fundamental research. If more fundamental research is desired, each laboratory experiment and computational program needs to be designed and constructed to answer specific fundamental scientific questions. In this way, the researchers will be performing more than exploratory testing or computational exercises based on questionable physics models. Researchers need to be in command of existing work on related topics. Many more journal publications, with first-rate peer reviews, are needed to make a greater impact and to challenge researchers to perform at their full potential. Better mentoring of early-career researchers is needed.
Reliability and Diagnostics
The overall technical quality of ARL’s applied research and development in the area of reliability and diagnostics is very high. The VRAMS team has a strong contingent of enthusiastic, highly capable researchers. The potential to produce fundamentally important results to improve the reliability and survivability of Army systems is high. In order to demonstrate the high quality of the research and to get important feedback from other experts in the fields, it is important that research papers be submitted to good-quality archival research journals.
