The panel met on December 12-14, 2018, at the National Academies of Sciences, Engineering, and Medicine (National Academies) facility in Washington, D.C., to review the In-House Laboratory Independent Research (ILIR) program projects in materials science conducted in 2018 at the U.S. Army Armament Research, Development, and Engineering Center (ARDEC); U.S. Army Communications–Electronics Research, Development, and Engineering Center (CERDEC); U.S. Army Edgewood Chemical Biological Center (ECBC); U.S. Army Natick Soldier Research, Development, and Engineering Center (NSRDEC); and U.S. Army Tank Automotive Research, Development, and Engineering Center (TARDEC). The panel received overview presentations on the ILIR programs at each RDEC and technical presentations describing the projects. During each set of presentations for a Center, the panel engaged in question-and-answer sessions with the presenters. After the RDEC participants had concluded their presentations and after the panel had formulated initial impressions and developed additional questions during its closed-session deliberations, the panel had a general discussion with senior RDEC leadership.
Project: Atomic and Multiscale Modeling of Tungsten Crystal Lattice Defects Created by Neutron Bombardment for Increasing Fracture Toughness
The goal of this project was to model both the solid solution hardening of tungsten single crystals and the interaction of dislocation movements through molecular dynamics simulations. The project has technological relevance to the Army for ballistic and ordinance applications. The presentation of the literature review on tungsten processing and properties was sufficient to illustrate the purpose of the project research directions. As a generally known solute effect, solid solution hardening may be a promising avenue for strengthening pure tungsten single crystals. The simulation was performed in a
perfect body centered cubic (bcc) lattice with substitutional tantalum (Ta) atoms, which was strained along the directions normal to the cubic unit cell faces <100> directions. The simulation results displayed a reasonable dislocation density and distribution in W-5 percent Ta crystal and, therefore, captured the hardening effects of substitutional Ta atoms. Army Research Laboratory (ARL) experimental tensile tests of pure W and W-5 percent Ta confirmed the simulation hardening prediction. When the experiments were compared, the existing modeling-experimental mismatch was attributed to the much higher strain rate difference in molecular dynamics simulations compared to the experiments.
Future work is being carefully considered to improve W-Ta results and to predict the softening behavior of the W-rhenium (Re) single crystal. Plans also exist for publication(s) in peer-reviewed journals. The collaboration with ARL is ongoing, and molecular dynamics simulations are being performed on ARL’s Centennial Supercomputer. The presenter is actively involved in developing and maintaining a toolset of multiscale methods for materials modeling and has provided support to various ARDEC projects. More interaction with senior researchers could be valuable to this project.
Although the literature review is acceptable, it is worth reconsidering the schematic showing a dislocation reducing its size after interacting with two pinning points1 (shown as part of the presentation), because Orowan strengthening is through particulates (i.e., not through solute atoms). There also needs to be clarification of the unexpected softening behavior in W-Re solid solution.
Project: Dynamic Strain in Composites
This project examines the mechanisms and mechanics of how composites mitigate dynamic strain effects in cylindrical, hybrid (i.e., multimaterial) structures. The researchers performed a literature review of closed-form mathematical solutions to this problem by previous military researchers and then used homogenization and structural enrichment approaches to better define the opportunities for understanding the problem. Multiscale finite element (FE) techniques were used to compare research findings to historic closed-form solutions. Empirical dynamic strain rate tests using Split Hopkinson Pressure Bar tests were conducted at the University of Rhode Island and were used to generate data to support the rather elegant modeling effort. One challenge that is being addressed is obtaining data at a 100 second-1 strain rate. The researchers will be collaborating with the Massachusetts Institute of Technology (MIT) to generate these data in the future.
Once the model is built, supported, and validated with empirical test data, the Army will have a tool to rapidly investigate different materials, geometrical lengths, and diameters in the computer prior to building and testing new designs. This balanced empirical and modeling approach demonstrates the thoughtfulness of the investigators by not only addressing the immediate issues, but also by making a tool that will be useful far into the future. The commendable collaborations with the University of Rhode Island and MIT and other branches of the military (through the Naval Undersea Warfare Center) will avoid duplication of efforts.
Mitigating dynamic strain effects is particularly important where operational conditions place the part in resonance at a natural frequency, because this dramatically increases strain. In industrial products, such as automotive engines, care is taken to make sure that the product can never operate at the natural frequency where strain effects can be very high, because this can result in rapid failure. These precautions are not possible in all systems, some of which are used in the Army and other
1 L. Costa, 2018, “Atomic and Multiscale Modeling of Tungsten Single Crystal Dislocation Interactions with Stress Fields Created by Bigger Solute Atoms,” ARDEC presentation to the Panel on Review of In-House Laboratory Independent Research in Materials Science at the Army’s Research, Development, and Engineering Centers, December 13, Washington, D.C.
branches of the military. Therefore, this work is much more technically challenging than normal mechanical design.
Advanced finite element analysis (FEA) modeling is very useful in designing any structural component, particularly components that encounter dynamic strain effects, such as military vehicles in rough terrain or impact situations. This simulation methodology can be adapted for analyzing numerous situations and, therefore, can be considered a crosscutting design and analysis methodology.
Project: Investigation of Nanosheets Synthesized on Uniaxial Tensile Strained Substrates
The goal of this project is to strain graphene nanosheets by growing them on tensile strained platinum (Pt) substrates. The work shows good promise for military applications (e.g., nanomechanical resonators) in Army priority areas. The theoretical basis for the proposed research is also well understood, and steps have been taken to show the feasibility of graphene synthesis on strained Pt foil.
The project, which consists of the synthesis, straining, and characterization of graphene deposited on strained substrate, is a good niche to use ARDEC’s strengths. The project’s objectives, experimental approaches for data generation and collection, preliminary results, and conclusions are all clear and reasonable.
The research team has access to the equipment required for the synthesis and characterization of graphene nanosheets grown on polycrystalline Pt substrates. The results demonstrated graphene growth with large coverage of the strained Pt substrate. The uniaxial and biaxial strains that were created by the researchers were small but comparable with the literature results. Challenges have been identified in the synthesis of these nanosheets for a better control of deposit areas and their nature. Challenges have also been identified in how to respond to specific needs in Army applications, such as nanomechanical resonators and GPS development,
The future research has been carefully considered and includes a focus on higher quality graphene materials, the transition of synthesis research to the ARDEC laboratory, a better understanding of continued strained growth by coefficient of thermal expansion (CTE) mismatch using Pt and copper (Cu), and an in-depth characterization of graphene nanosheets. Control of final nanosheets (single or multiple layers) and reproducibility of results, along with atomistic modeling, are also planned for future work.
This project also has excellent interactions with seven U.S. and one international university, materials consortia (e.g., the 2D Crystal Consortium at Pennsylvania State University), and one national laboratory. A unique feature of this project is the participation in a strong mentoring program for undergraduate and high school students at Stevens Institute of Technology.
Project: Phosphorene as a Tunable Molecular Beacon
The goal of this project is to develop an efficient method to synthesize single-layer phosphorene two-dimensional materials using a liquid exfoliation process, which starts from a typical black phosphorus (BP) precursor.
Phosphorene is an allotrope of phosphorus. This two-dimensional material is conjectured to be a strong competitor to graphene because, unlike graphene, it has a band gap. Still, a disadvantage of phosphorene is its air stability. It is composed of hygroscopic phosphorus and has an extremely high surface-to-volume ratio, which causes it to react with oxygen and water vapor. When assisted by visible light, phosphorene will degrade within hours. Before fully evaporating as a result of the degradation process, phosphorene (as a solid) will react with water and oxygen to develop liquid phase acid bubbles on the surface. Synthesis of phosphorene also represents a marked challenge. Currently, there are two main ways of producing phosphorene, namely liquid exfoliation and microcleavage.
In this project the first liquid exfoliation process that was attempted made use of dimethyl sulfoxide (DMSO), an important polar aprotic solvent in the form of a colorless liquid. DMSO dissolves both nonpolar and polar compounds. It is also miscible in a broad range of organic solvents and in water.2 The results showed that exfoliated BP had a broad absorption connected to partial exfoliation. There was no signal for true monolayer phosphorene.
The next attempts made use of azolectin and pyrene. In this case, the azolectin and pyrene encapsulated BP, which increased stability to oxygen degradation. Future experiments will further explore pyrene exfoliation, because the technique appears to be ideally suited for inkjet printing of heterostructures.
The presenter also discussed a project for patterning molybdenum disulfide (MoS2) and described how MoS2 grew selectively by powder vaporization in regions exposed to a gallium ion beam on silicon dioxide wafers. The researchers demonstrated that
alterations in the density of surface hydroxyl groups on silicon dioxide substrates can control nucleation and growth in molybdenum disulfide thin films, which are produced by atmospheric pressure chemical vapor deposition. The extent of MoS2 nucleation is linearly correlated to the density of surface hydroxyl groups. Controlling the density of surface hydroxyl groups on the initial substrate provides a method of growing patterned molybdenum disulfide. The researchers established that the surface density of hydroxyl groups on silicon dioxide (SiO2) was altered using conventional gallium focused ion beam (FIB) patterning. Upon gallium-ion beam exposure, the number of hydroxyl groups generated on the surface was directly proportional to the ion dosage. This work established a means of patterning large-area monolayer MoS2 on silicon dioxide substrates, which is a critical step for realizing applications in imaging, catalysis, biosensing, chemical detection, electronics, and optoelectronics.3
Another effort on MoS2 was also presented, which involved
the optimization of complexes between sulfur allotropes and molybdenum species using density functional theory (DFT), which revealed the molecular mechanism of sulfurization. Complete sulfurization of molybdenum trioxide to molybdenum disulfide requires at least three sets of nucleophilic addition-elimination reactions that generate the experimentally observed molybdenum oxysulfide intermediates along the reaction pathway. Each nucleophilic addition reaction of a sulfur allotrope to a molybdenum species gave rise to a molybdenum oxysulfide ring, which can dissociate into a more sulfurized molybdenum intermediate. At the typical growth temperatures used in powder vaporization, the equilibrium constants for these reactions are, essentially, unity. Sulfurization is, therefore, driven by excess sulfur and gas flow through the growth furnace.4
The MoS2 efforts are interesting, but the connection of such work with the efforts on phosphorene is unclear. Also, the title of this project, Phosphorene as a Tunable Molecular Beacon, does not seem appropriate because the project is all about exfoliation of BP into phosphorene. The use of this material as a tunable molecular beacon was not covered in the presentation, and so this title was inappropriate.
There appear to be no publications on this project, and the impact of this work is unclear, because the results appear highly preliminary. With more time and a more focused hypothesis this project can result in unique contributions.
2 NIIR Project Consultancy Services, 2012, Detailed Project Profiles on Chemical Industries, Volume II, 2nd Revised Edition, Delhi, India.
3 S.F. Bartolucci, D. Kaplan, and J.A. Maurer, 2017, Ion beam-induced hydroxylation controls molybdenum disulfide growth, 2D Mater 4:021017.
4 T. Tsafack, S.F. Bartolucci, and J.A. Maurer, 2018, The role of molybdenum oxysulfide rings in the formation of two-dimensional molybdenum disulfide by powder vaporization, Journal of Physical Chemistry A 122:7320–7327.
Project: Low Temperature Production of Oxide-Based Tunable Filters and Phase Shifters
The goal of this project is to manufacture films of BaSrTiO4, barium strontium titanate (BST), on sapphire substrates using a low-temperature microwave sintering process. This goal is in response to the impractical temperatures usually required for the preparation of BST, which are in excess of 800°C. This material is to be produced by spin coating a slurry onto the sapphire substrate, which is subsequently annealed. It was not made clear what temperature is being targeted or what temperatures were used for the films that were produced.
For the films that were manufactured, the measured capacitance decreased by increasing the electric field, and this was interpreted by the researcher as a result of a decrease in the electric permittivity of the dielectric material. The fractional decrease in the capacitance was defined as the tunability. This observation is odd, since the capacitance of the piezoelectric dielectric, which is related to the polarization of the dielectric, is expected to increase with increasing applied electric field. The scientific origin of this capacitance decrease remains unclear. The measured capacitance involves both the dielectric capacitance and the two dielectric-electrode interfacial capacitances in series, and so the assumption made by the presenter that the measured capacitance is entirely due to the dielectric capacitance is flawed. It is possible that the unexpected decrease of the capacitance in relationship to an increasing electric field relates to the effect of the electric field on the interfacial capacitance.
The research cannot be performed effectively without understanding the scientific reasons for the behavior of the films. For the purpose of clarifying the reasons for the film behavior, the contributions of the volumetric and interfacial capacitances to the measured capacitance need to be decoupled. This decoupling has not been performed on this project, but it can be performed by measuring the capacitance for three or more lengths of the dielectric material in the direction of the capacitance measurement.5
No results were presented on the crystal structure of the films, and so no assessment can be made on the correlations between the dielectric properties of the films with respect to the crystallinity in the films. X-ray diffraction needs to be done and the results presented.
Overall, this project is in the extremely preliminary stages of development. Results are incomplete or disconnected, and much more work is required before this project could be considered successful.
Project: Thermochemical Cooling of Directed Energy Weapons
Deployment of directed energy weapons currently hinges on the ability to miniaturize the laser cooling apparatus and its energy requirements. This project therefore stems from an excellent motivation; successful outcomes would produce tremendous miniaturization potential of the cooling system, which would enable more widespread mobile deployment of directed energy systems. These objectives were very clearly described in the presentation and serve as an excellent guide for the project.
The project’s approach is based on a clever concept that would utilize existing onboard fuel mass as a heat-absorbing medium. Rather than simply increasing the heat content of the onboard fuel, a chemical reaction would be triggered to convert thermal energy into chemical energy using an endothermic chemical reaction. Theoretically, a much larger amount of heat per unit mass (or volume) could be
5 A.A. Eddib and D.D.L. Chung, 2018, First report of capacitance-based self-sensing and in-plane electric permittivity of carbon fiber polymer-matrix composite, Carbon 140:413–427.
accommodated by this approach, thereby dramatically enhancing system-level performance-to-weight (i.e., volume) ratio. The potential of this approach is, therefore, theoretically sound and compelling, and this was explained and illustrated well during the review, with good comparisons to nonchemical means of absorbing the heat.
The researchers on the project appear to be well trained in some of the relevant areas. Preliminary results were obtained using a computational model, based on available (catalyst) materials, and these results appear to be encouraging. To ensure progress in this area, several aspects are noted below for consideration.
The heat pulse duration is very short, and it is not clear from the presented material whether the heat extraction rate can match the heat pulse duration. A detailed transient heat balance describing this system would be helpful in determining the characteristic rates and the corresponding physical processes operating on the appropriate time scales. Some intrinsic impedance matching can also be done between different physical processes and components to optimally extract and convert the heat.
Assuming that the project is ultimately successful in extracting heat rapidly and converting it through chemical modification of the fuel, the potential impact of the process on fuel composition and quality needs to be carefully considered, preferably with the input of a turbine manufacturer. Consideration needs to include this question: What are the design latitude and the constraints placed on the system from this angle?
Additional patentable technologies are potentially emerging from this core concept and specific system, and, depending on the desired military and/or civilian applications, further exploration of patentable technologies could be beneficial.
The calculations made to date of the efficiency of catalyst-loaded packed bed are constrained by the chosen shape; however, custom shapes and even chemical compositions of catalysts are obtainable. Customizing the catalyst composition and geometry may prove to be a valuable optimization vector if performed in coordination with the constraints of the turbine on the chemistry of the fuel.
Project: Hierarchical Systems Through Selective Deposition and Growth of Metal-Organic Frameworks on Block Copolymers
This project relates to a Ph.D. thesis of one of the ECBC researchers, whose Ph.D. advisor is at the University of Delaware. The project is nearing the end of its second year. Its goal is to disperse metal-organic framework (MOF) particles into block copolymer (BCP) films or within/on electro-spun nanofibers. There are two primary candidate MOF materials, UiO-66-NH2 and HKUST-1 (plus respective chemical modifiers). These materials are resulting in a set of modified MOFs that are being intensively studied. As is common with MOFs, these materials are being synthesized by combining coordination metals with organic linkers. This can be done ex situ, and then the BCP and MOF can be co-assembled, or the synthesis of the MOF can be done in situ by nucleation and growth of the nanoparticles within a targeted type of BCP (i.e., host) domain. The former approach is succeeding; the latter is not. It employs binding specific ligands to the MOF surface to enable co-assembly into a particular BCP domain type. This basic research project is highly relevant to the mission of ECBC because MOFs have enormous surface areas (> 1,000 m2/g) and huge pore volumes (~ 0.5 cm3/g). The UiO-66-NH2 and HKUST-1 MOFs are excellent absorbers of ammonia, nerve agents, and blister agents, and they are highly selective against H2O/CO2/N2 mixtures.
The experimental approach has involved an attempt to disperse the MOFs into films of polystyrene (PS), PS-polyisoprene-PS triblock copolymers, and polyethylene oxide films. Dispersion and
incorporation of the MOFs using electrospinning was done with polyacrylonitrile (PAN) and polyvinylidene fluoride (PVDF) nanofibers. The use of homopolymer films instead of block copolymers, however, does not appear to be related to the research theme, because no patterning can be expected. Another apparent issue is the relative sizes of the MOF particles versus the BCP domain size of the respective host material, as well as the relative size of the MOF particles versus the diameters of the electrospun PAN and PVDF fibers. Many of the MOF particles have been synthesized under growth conditions that result in MOF sizes that are too large for accommodation and patterning. The wide-angle X-ray scattering results show that the synthesized MOFs are of good quality. The modification of the MOFs by acyl chloride chemistry has been demonstrated and provides versatility for sensing and uptake functions.
The project is highly relevant for ECBC, and the concept of patterning MOFs using BCPs is sound. Some results are promising; however, more work needs to be done to achieve the correct nanoparticle size and surface ligands, so as to be able to direct the location and patterning of the MOFs within a BCP film. Results to date on homopolymer film and electrospun nanofibers are not promising and also not in line with the stated project’s title, Hierarchical Systems Through Selective Deposition and Growth of Metal-Organic Frameworks on Block Copolymers. Additionally, the ability to function will require the MOFs to be at or near the top surface of the BCP films. This means that the targeted BCP domain needs to, ideally, pattern the top surface of the BCP film, as would be the case, for example, in vertically oriented, hexagonally packed cylindrical microdomains. Such BCP organization is well known, and there are specific BCP compositions, molecular weights, solvent processing steps, and substrate surface treatments that can be adapted. The project needs focus on vertically aligned cylinders of a large enough diameter and MOFs with surface ligands compatible to the cylindrical domains (and of a sufficiently smaller size than the cylinder diameter so as to be attracted to these domains). If the film is also quite thin (i.e., only two or three times the cylinder-cylinder spacing), then the MOFs will be at or near the top surface and will be able to rapidly respond to environmental stimuli. Use of atomic force microscopy (AFM) to study the surface patterning also needs to be pursued.
This research has produced two publications, one patent application, and invited talks that include an upcoming Gordon Conference presentation. Because the principal investigator (PI) is pursuing his Ph.D., additional publications in fundamental science and engineering journals are expected.
Project: Modeling and Quantifying Nano-Enhanced Scattering in Polymers
This is a very successful project, completed after 3 years and $1.7 million of effort. The project has three thrusts: nano-enhanced scattering for enhanced light absorption in thin films, bioinspired control of scattering and polymer-based enhancement for infrared (IR) transparency, and forward scattering of IR radiation. All three areas are interrelated and have resulted in excellent publications (six papers published and one manuscript in review) and a half-dozen presentations at top conferences. Some of the ideas and materials developed are likely to be transitioned to industry. Top measurement facilities, the finite difference time domain (FDTD) software modeling development at the NSRDEC for material processing, and extensive optical characterization have been key to the success of this project. Several strong collaborations helped this project markedly, including the Defense Agency for Technology and Quality (DTaQ), South Korea; Northeastern University; University of Massachusetts, Lowell; and the Institute for Soldier Nanotechnologies at MIT, as well as with researchers at the Lincoln Laboratory. Overall, this is a very well organized, highly collaborative, and highly productive project.
The goals of the project are to discover new means to control scattering from the visible to the IR, so as to revolutionize a host of applications, many of which are Army-specific. This is done through the development of novel optical material systems by combining nanoparticle size, shape, dielectric properties, and system design using lightweight polymer films and fibers. The work is extremely relevant to Army needs because it encompasses soldier survivability, especially signature management, solar collection, and communication. This project also helps to strengthen and broaden the Army’s rapidly expanding state-of-the-art efforts in metamaterial research. Broadly, the research seeks to understand the basic mechanisms of scattering of nanoscale optical materials in order to create greatly improved material systems (metamaterials) for novel applications, particularly those that can be embodied in textile fibers and in thin films of lightweight materials.
The first area exploits plasmonic materials to greatly boost the optical absorption of underlying thin films. This enhancement can be used in solar collection, heat transfer, and in managing the emissive properties of materials. The approach uses silver or other metallic nanoparticles to interact with and couple long wavelength radiation into a thin film substrate. This substrate’s thickness is much less than the incident wavelength, resulting in greatly increased absorption in the thin film (e.g., amorphous silicon [a-Si]). Purposeful surface texturing to increase light-matter interactions is shown to be very effective, as is regulation of the metallic particle size and shape (e.g., nanourchins and nanoprisms), interparticle spacing, and fill factor (for controlling resonances). Patterning from nearly random to highly periodic was shown to further control the optical behavior. NSRDEC’s collaboration with Lincoln Laboratory has been important for obtaining high-quality patterns. The ability to couple long wavelength radiation to the thin film substrate (amorphous silicon [a-Si]) affords opportunities for IR sensing and increased efficiencies. Scaling up these systems into inexpensive, large area films by roll-to-roll manufacturing in collaboration with the University of Massachusetts, Lowell, is a noteworthy endeavor.
Bioinspired optical research is the second area and involves cephalopod exemplars, where the spatial arrangements of the iridophores (reflecting) and, especially, the chromatophores (absorbing and scattering) of the animal are mimicked to provide bioimitating, broad spectrum, highly forward scattering composite materials. A striking publication on these material systems appeared in Advances in Optical Materials (2018),6 which described how actual squid pigment granules were extracted and purified to make high index, strongly absorbing additives that were incorporated into linear low density polyethylene (LLDPE) fibers. When combined with a back reflecting material these fibers demonstrated the ability to tailor color.
Polymer-based, strong IR-scattering materials constitute the third research area with applications to thermal and signature management, and increased energy efficiency for photovoltaic devices through the enhanced coupling of a wide range of solar wavelengths. The influence of adding inorganic particles to ultrahigh molecular weight polyethylene (UHMWPE) to enhance forward scattering of IR wavelengths was explored in order to make a highly transparent IR polymer. Particles ranged from 30 nm diameter zinc oxide to 50 nm silicon (Si) to 1,000 nm Si. The UHMWPE/particle material was gel spun and drawn to extreme draw ratios (~ 80x) to provide a very high crystallinity sample with very low IR absorption.
Overall, the breadth of multiscale experiments contributed to the success of these various subthemes. This project was very relevant to the NSRDEC mission. The concepts developed are of general utility and impact a wide range of researchers in the Department of Defense (DoD) and beyond. Many new directions for future research are emerging from this project.
6 A. Kumar, R.M. Osgood III, S.R. Dineen, B.D. Koker, R. Peng, and L.F. Deravi, 2018, Color-changing biomimetic films: Natural light-scattering nanoparticles enable visible through short-wave infrared color modulation, Advanced Optical Materials 6(8).
Project: Novel Materials for High Frequency Sensing
The goal of this project is to develop functionalized carbon nanotubes for applications in high frequency sensing. The issue to solve is the chemical damage that surfaces of carbon nanotubes can suffer during functionalization because the molecules that are used for functionalization are too reactive during synthesis. In addition, the carbon nanotubes are incompatible with most solvents used after the functionalization reaction, so new solvents need to be developed.
The functionalization is done for a variety of organic molecules. For example, incorporation of amidine on the surfaces of the nanotubes can be used for carbon dioxide sensing. In this case, the sensing mechanism is the conversion of the amidine into amidinium bicarbonate. The reaction is meant to be reversible, so that the sensor can be used repeatedly. The theoretical basis for the project and the innovation in the design are well established. The sensor array that has been developed has a detection limit of 0.03 percent; is reversible (i.e., multiuse); and works in air, at room temperature, and with atmospheric humidity—it appears to meet all of the requirements. The researchers have also tested for the detection of dimethylmethylphosphonate (DMMP), a simulant for sarin gas, and cyclohexanone, which is found on RDX (cyclotrimethylene trinitramine) explosives.
The technical approach is to disperse the carbon nanotubes and then attach them onto a glass substrate. The anchoring of the nanotubes on the glass substrate required functionalization of the silica surface with alkyl bromides through organosilanization. Once on the substrate, the array becomes a series of microantenna that can send signals and be used for the remote detection of gas.
A variety of testing protocols and characterization techniques have been used to determine optimal manufacturing of the sensor arrays and the sensing performance. Testing includes nuclear magnetic resonance (NMR), ultraviolet–visible (UV-vis) spectroscopy, Raman spectroscopy, Fourier transform infrared spectroscopy, and X-ray photoelectron spectroscopy. The project resulted in the filing of a provisional patent,7 which was filed on September 17, 2018, two peer-reviewed publications,8,9 and two conference presentations.10,11
The key technical goal of attaching carbon nanotubes with interchangeable chemical sensing groups to interchangeable surfaces has been achieved. The sensor array that was developed is the most sensitive chemiresistive CO2 sensor that has been reported. This project is a success and a model project for how to develop technologies that are relevant to the Army.
Project: Tunable Temperature Responsive Liquid Crystal Nanofibers
This project is currently in its second year. The aim is to use electron-spinning of a liquid crystal (LC)-containing polymer fiber so as to enable color tunability through changes in both the ambient and
7 T. Swager, B. Yoon, and G. Walsh, Switchable single-walled carbon nanotube-polymer composites for CO2 sensing, Application Number 62/732,541.
8 B. Yoon, S. Choi, T. Swager, and G. Walsh, 2018, Switchable single-walled carbon nanotube-polymer composites for CO2 Sensing, ACS Appl. Mater. Interfaces 10(39):33373–33379.
9 F. Bruno, R. Nagarajan, W. Kiratitanavit, Z. Farhana, B. Yoon, S. Fossey, and M. Bernabei, 2018, Novel enzymatically synthesized substituted polyaniline with high conjugation and conductivity, MRS Advances 3(27):1519–1524.
10 F. Bruno, R. Nagarajan, W. Kiratitanavit, Z. Farhana, B. Yoon, S. Fossey, and M. Bernabei, 2017, Novel enzymatically synthesized substituted polyaniline with high conjugation and conductivity, Oral presentation at the MRS Fall Meeting and Exhibit, Boston, Mass., November 26-December 1.
11 F. Bruno, R. Nagarajan, W. Kiratitanavit, N. Favreau-Farhadi, B. Yoon, S. Fossey, and M. Bernabei, 2018, Synthesis and evaluations of substituted polyaniline with high conjugation and conductivity, Oral presentation at ACS National Meeting and Exhibition, Boston, Mass., August.
physiological temperature, leading to smart textiles for soldier uniforms. The project has collaborators at Virginia Commonwealth University and the Naval Research Laboratory. It builds on NSRDEC’s longstanding expertise in textiles, specifically in electrospinning of polymer fibers. The approach includes a set of different spinning conditions to create LC-containing fibers of different types. The LC phase is concentrically spun with a polymer to create coaxial LC core-polymer cladding, or spun as an LC emulsion in the polymer or as LC-polymer blend. The research focus is to see how the size and boundary conditions of the LC within the surrounding polymer influence the appearance of the color of the fiber as a function of temperature through the change of the LC pitch.
The concept of the inclusion of a temperature-sensitive LC within a polymer for a nonwoven fabric is quite relevant to NSRDEC. The project has several novel characterization techniques in addition to the usual UV-vis spectroscopy, polarizing microscopy, and differential scanning calorimetry (DSC). These are Raman spectroscopy, atomic force microscopy (AFM), and quartz crystal microbalance with dissipation monitoring (QCM-D). The researchers have acquired experience with electrospinning, are now familiar with their characterization tools, and are working to correlate the observations enabled by the various techniques in order to provide insight into the molecular level mechanisms influencing the fiber color versus the temperature. To enable the fluids to be electrospun, two polymers are used, polyvinylpyrrolidone (PVP) and polystyrene (PS), along with two LC formulations, LC-1 and LC-2, and two solvents, toluene and acetone.
The results show that while the PVP polymer and LC system can be spun into 10 micron diameter fibers, the PS polymer and LC fibers are nearly 10 times larger—well beyond the diameter of what qualifies as a textile fiber. With the flow field, the change in solvent content versus time will be highly dependent on the fiber diameter, and processing needs to focus on producing consistent fiber diameters for the chosen set of compositional parameters (e.g., polymer type, LC composition, and solvent content). Quantification of color will depend on the orientation of the pitch axis relative to the viewing angle, so beyond Raman spectroscopy and polarized light microscopy, the use of X-ray diffraction to probe the orientation of the molecular director in the LC phase is needed. Because the reflected color is dependent not only on the pitch, but also on the viewing angle and the number of turns of the pitch, future studies will need to quantify these important parameters as a control and for variously confined LC phases. Perhaps the use of highly drawn glass capillaries would afford easy quantification of LC color within a cylindrical confinement of known radius. The QCM-D is an intriguing new characterization method for following the behavior of the LC phase during temperature changes. The researchers will need to validate their theory that the reorientation of the LC molecules is likely the source of energy dissipation and will also need to correlate the QCM-D signal with changes in both color and X-ray patterns.
Other issues to explore are the time dependent loss of solvents after spinning. The fiber-to-fiber fusion is evident in the optical micrographs and likely reflects the presence of solvent in the fibers right after spinning. The solidification process of the polymer + solvent + LC system is complex. It would be informative to do cross-sectional microscopy to determine the nature of the LC domains within the various types of fiber and how these depend on the choice of composition and spinning conditions. This would need to be done while still maintaining cognizance that the surface of the fiber will be cooled by rapid solvent evaporation. It would also be useful to determine whether any of the components in the LC formulations are miscible with the PVP or the PS.
Additionally, the surrounding polymer fiber is likely to be birefringent due to the molecular orientation of the polymer during electrospinning. The influence of a birefringent enclosure on the optical appearance of the confined LC needs to be explored. It would also be beneficial to examine the influence of the oriented polymer on the anchoring conditions to the LC component because the orientation of the cholesteric pitch axis is normal to the orientation of the director. It is also important to monitor how incorporation of an LC phase within the polymer fiber influences the mechanical properties of the fiber.
This project is pursuing novel LC-containing polymer fibers; progress to date is modest but will probably accelerate because the team has now successfully made materials and done preliminary characterization. While there is excellent expertise at NSRDEC with regard to the modeling of optical properties of materials, connections to optical modelers could improve the interpretations of existing experimental data and may allow suggestions on new types of samples to fabricate and characterize.
Project: Computational Design and Development of Polyurea-Based Adhesives
This project is connected to a TARDEC researcher’s Ph.D. thesis and is near the end of its first year. The project’s goal is to create polyurea adhesives that mechanically outperform other polymer-based adhesives for the adhesive joining of components that can contribute to the lightweighting of Army vehicles. The approach is “materials by design”12 of new polyurea-based nanocomposites for high strain rate, high stiffness, and toughness applications. Multiscale modeling will range from the molecular scale through the nanometer scale microphase, with separation into hard and soft segment domains, all the way up to the centimeter scale for shear lap joint test specimens.
The project is quite relevant to the TARDEC mission and involves an industry collaboration with PPG, Inc. The concept of employing nanofillers to enhance the mechanical behavior of polyureas is not as well explored as other polymer systems. There are many useful paradigms already known on how to best incorporate the nanoadditive to benefit the polymer matrix. The choice of polyureas is motivated by the high strain rate behavior of certain polyurea compositions that have been used as coatings of rigid structures (e.g., concrete walls) for blast resistance. The hope is to develop a new adhesive material that is in regime I of the National Aeronautics and Space Administration (NASA) Materials and Processes Technical Information System (MAPTIS) adhesive database, where the ultimate shear strength is > 10 megapascal (MPa) and the displacement at failure is > 3.7 mm.
The project is very ambitious, with a huge span of activities that include mechanical modeling of complex polymers and nanoadditive interactions over time and length scales, chemical synthesis and compounding of nanocomposites, nanoscale and microscale characterization, finite element (FE) modeling, and mechanical testing at the macroscale. The modest progress shows the challenges of trying to master such a broad range of subjects and activities. Many new skills have been acquired by the researchers (e.g., dynamic mechanical analysis [DMA], differential scanning calorimetry [DSC], thermogravimetric analysis [TGA], and chemical synthesis). Cooperation with PPG, Inc. has also been helpful with polyurea adhesive formulations. The candid presentation of limited project progress and problems encountered, and of the additional detailed information on the project in the supplemental presentation slides, was appreciated.
Because of the lack of background in chemical synthesis, there have been many problems in making successful samples of polyureas and of polyurea nanocomposites. Cure kinetics need to be better monitored and characterized. This could be accomplished through analyzing in situ samples using Fourier-transform infrared spectroscopy [FTIR] spectra against cure time and temperature, and modeling isothermal cure (i.e., thin samples between good thermal conductors such as aluminum plates) and adiabatic cure (i.e., thicker samples that are thermally insulated). The addition of nanoadditives, with
12 D.A. Tzelepis, 2018, “Computational Design and Development of Polyurea-Based Adhesives,” TARDEC, presentation to the Panel on Review of In-House Laboratory Independent Research in Materials Science at the Army’s Research, Development, and Engineering Centers, December 12, Washington, D.C.
their large surface area and thermal transport, can influence chemical reactions and kinetics. Homogeneous dispersion of additives requires careful surface modification and attention to mixing during the synthesis of the polyurea. Additives that have been explored to date include carbon nanotubes and Halloysite (a clay-based nanotube material) in rather low concentrations (1.0 and 0.1 percent weight).
The project needs to downselect from the vast sets of polyureas and nanoadditives available in order to focus on the notion that the high strain rate mechanical properties of polyureas will lead to excellent adhesives for Army applications. Given that the PI is pursuing his Ph.D., there are likely to be publications in fundamental science and engineering journals. This project would benefit greatly from building an expanded team of subject matter experts to oversee, consult, and narrow the focus of the research.
Project: Demonstration of New Time-Dependent Reliability Method on Military Vehicle Suspension Components Using Up-Crossing and Joint Up-Crossing Rate
The goal of the project is to improve understanding and predictive capability of component failure in ground vehicles subject to stochastic loading histories. The project focuses specifically on cyclic fatigue failure of suspension springs. Field data for suspension displacements are analyzed and represented in terms of a linear combination of functions that assume that the data are random and follow a Gaussian distribution. An eigenvalue analysis is used to determine the number of random variables needed to characterize the process. Time realizations of the random process are generated and their amplitudes are computed using a rainflow counting method. A cumulative linear damage model and Miner’s rule for fatigue failure are combined with the number of cycles of each amplitude to predict component lifetime.
This project is relevant to the Army’s goal to build up its in-house competency in reliability testing and analysis, which will ultimately reduce the need for physical tests and increase reliance on validated computational models. The project is being conducted in parallel with an experimental investigation (Development of Test Methodology for Suspension Component Characterizations in the Laboratory) of spring failure under simulated random loading histories. The methods are appropriate to achieving project goals. Although no conclusions were explicitly stated, the project appears to have successfully demonstrated the use of the method by comparing, or perhaps calibrating to, experimental data emerging from the parallel program.
Predicting fatigue failure under random loadings represents a Holy Grail of sorts in component reliability. It has been an ongoing challenge across many engineering fields. The present project appears to be making good progress toward developing the tools needed to address this goal. As with all analyses of this type, a critical assessment of the degree of correlation between failure predictions and failure data is difficult because of the many variables involved and the potential for many nonunique solutions to emerge. In turn, extrapolating predictions outside the calibrated domain, which is one goal of the present study, needs to be done with caution. Plans to compare the current predictions with experimental data outside the calibration regime are a good next step in assessing the fidelity of predictions. Explicit documentation of input material properties and their variations needs to be included in future presentations of this work.
A further step in assessing the predictions might involve purposely nonrandom loadings, such as those that might occur when a vehicle drives over washboard roads and trusses. The simulations could be used to guide critical experiments, perhaps with only two or three loading amplitudes. This type of activity would be predicated on the availability of resources needed for testing and on the limitations on accessibility to the test equipment, both of which appear to represent limitations on the potential scope of the experimental work.
Project: Development of Test Methodology for Suspension Component Characterizations in the Laboratory
The Army relies heavily on armored and other vehicles to perform in adverse conditions over a period of many years. Premature wear of suspension components can lead to failure in the field, compromising mission integrity and putting lives at risk. Therefore, it is important to have a clear understanding of the fatigue behavior of critical components of the suspension in order to inform the design and maintenance of these components. Because of the long service life of the vehicles and their components, it is important to find means of accelerating the testing, while at the same time understanding how representative accelerated testing modes are of true service conditions and wear. This project aims to develop a methodology for predicting the failure of structural components in real-world use from a relatively small number of test points in a controlled environment.
In suspension components, nonuniform stress distribution can lead to crack initiation and propagation, resulting in premature failure. It is, therefore, important to model stress distribution in these components and to disambiguate the impact on stress concentration of intrinsic part geometry and load path from other sources of stress, which include defects incorporated into the part during manufacture or aspects of assembly and maintenance. The objective of this research project is well directed and will have important implications if properly achieved. Successful achievement of the stated research goals would accelerate and enhance the ability to predict and improve the operational life and utility of military equipment.
The project focused on examining stress and strain distribution in a suspension spring. One major finding was to confirm that strain is non-uniformly distributed along the spring; this is a significant, albeit known, issue. The principal novelty of the finding was in the specific distribution. The presentation, however, could have done a better job of discussing instances of similar effects in published or patent literature. It was not made clear how much of the effect was intrinsic to the geometry of the part and load path (to zeroth or first order) and how much was attributable to how the part was manufactured and assembled. Closer coordination with finite element analysis (FEA) efforts would help to better understand the specific load paths and to adapt the measurements of strain to validate the models.
It is known that the cumulative damage to a structure determines its effective service life and is caused by a combination of maximum load, frequency, and duration of impulses. Because understanding of these phenomena is so commercially important, multiple industries (e.g., aerospace, automotive, structural, and energy) have developed means of accelerated testing. Many procedures have been developed by the various commercial entities, but relatively little has been published. The less-known and unknown effects of nonuniform strain that could potentially be postulated and isolated remain a subject of curiosity. For example, it would be logical to test for frequency-dependence, or effects of systematically varying net displacement or preload, on the discrepancy between strains at different locations in the spring. These relationships presumably impact the extent to which accelerated testing would be representative of real-world conditions. The team needs to connect with experts in academia and industry to gain a better view of both the published and unpublished work in this area.
The presented chart entitled “measuring strain data at different pre-load levels generates the load vs. life curve,”13 raised some unaddressed questions: Were these points generated experimentally? For how many decades in each axis is this trend expected to continue? These points were germane to
13 I. Baseski, 2018, “Development of Test Methodology for Suspension Component Characterizations in the Laboratory,” ARDEC, presentation to the Panel on Review of In-House Laboratory Independent Research in Materials Science at the Army’s Research, Development, and Engineering Centers, December 12, Washington, D.C.
understanding the validity of the conclusions but were not made clear. Using a logarithmic scale with real units on the axes would have helped the translation of these data.
It was also unclear from the presentation whether the rate of testing impacts the temperature of the part being tested and, if so, what impact that has on estimated versus real fatigue life. It was also unclear from the presentation whether this is basic research. Going forward, it would be instructive to apply additional means (e.g., X-ray powder diffraction [XRD] and microanalysis) to characterize the built-in stresses due to processing and mounting.
Vehicles, structures, and their components used in military service can experience very different loads compared with those of commercial equivalents. There is, therefore, a need to develop reliable ways of assessing the accumulation of damage that allows for accurate predictions of the realistic (i.e., unaccelerated) service life of military vehicles, as well as to fast track the testing procedures. The goals of this project are worthwhile and important to the mission of the Army. Taking into account the comments made above would benefit the project.
Project: Exploring Graphene’s Potential for Enhanced Chemical Reactivity Induced by Regions of Local Conformational Deformation on Rough Iron Surfaces: Kinky Chemistry Applied
The long-range objective for this project is to develop a boundary lubrication model allowing for further fundamental understanding of tribochemical reactions. Such a model could potentially contribute to innovative tribological solutions for the Army in this mission critical area. As a first step toward this long-term objective, investigators are exploring the tribological properties of graphene under dynamic boundary lubrication conditions using reactive force field (ReaxFF). The term “kinky chemistry” in the title of the project refers to the purpose of the project: to explore the effects of surface kinks on graphene’s tribological behavior in boundary lubrication conditions.
During the first year of the project the PI demonstrated proof of concept by simulating reasonable responses for both pure and functionalized graphene on pristine (i.e., crystallographically planar) and rough iron surfaces. The simulations provide intriguing insights into the atomic interactions and structures that give rise to tribological properties. The work merits continued investigation using a more complete set of force fields to reflect the broader range of chemical environments of real lubricated surfaces. The development of these force fields is expected to proceed in the second year of this project.
Every opportunity needs to be taken to develop confidence in the model. The PI needs to consider using the model to predict experimentally accessible phenomena, such as contact angles, or simply the change in contact angles as a result of functionalization. In addition to the conventional methods of rheological testing, AFM may be used because it can provide friction imaging. A closer collaboration with density functional theory (DFT) and hybrid DFT force field modeling needs to be considered.
Project: Fundamental Study on the Suppression of Dendrite Growth in Lithium-Metal Batteries via Carbon Nanoribbons through In-Situ Optical Microscopy
The focus of this work is to understand the fundamental phenomena of dendrite formation in lithium metal batteries under different external (e.g., temperature and charge/discharge) conditions and the mechanism of the dendritic growth suppression. Carbon nanoribbons (CNRs) will be used as barriers to dendritic growth with the final goal of quantifying the growth suppression through using optical microscopy. The ultimate goal is to improve the cycle life and safety of lithium metal batteries.
The project is Army-relevant for existing batteries for starting, lighting, and ignition. It will also support future vehicle concepts such as all-electric tanks. The hypothesis is that CNRs will suppress
dendrite growth through plating metallic lithium and will prevent dendrite formation through entrapment. The high electrical conductivity of CNRs also has benefits for electron transfer to cell current collectors.
The experimental approach consists of building testing cells with CNR materials; characterizing dendrite formation behavior by appropriate electrochemical testing (e.g., electrochemical impedance spectroscopy [EIS] and cyclic voltammetry [CV]); and in-situ optical microscopy studies with time-lapse photography. The latter is an extension of TARDEC’s capabilities to provide this project with the quantification of dendrite structure and growth. The results were presented for four carbon nanoscale formulations through scanning electron microscope (SEM) observations.
Significant experience and equipment exist at the TARDEC Energy Storage Team to use and test the CNRs’ effects in lithium battery cells. An ongoing transfer of knowledge and skills to junior scientists seems to be occurring, particularly in the area of electrochemical testing. An important collaborator is Central Michigan University, which provided the CNR materials and will be a partner in developing the CNR application methods.
A more fundamental study of nanoscale materials (considering type, size, size distribution, shape, and directionality) in nucleation and dendritic growth phenomena at the subsurface of lithium metal could enhance the understanding of the suppression efficiency. Additionally, although the junior researchers have developed good experimental skills, they could benefit from closer oversight and mentoring from senior scientists. These factors are critical to conducting research in a scientifically meaningful fashion by understanding the proposed work; better defining its innovation; producing more meaningful results; and producing better presentations of the research, conclusions, and directions for future research. There are currently no publications; however, some are planned for 2019.
Project: Integrated Micromixers and Acoustic Streaming to Prevent Reverse Osmosis Membrane Fouling
The Army relies heavily on reverse osmosis (RO) to provide clean and fresh water for its operations, both for potable and non-potable use. A major performance limitation for RO systems, particularly in the context of military applications, however, is the premature fouling of the membrane due to low-quality feed water. An improvement in the longevity of RO membranes by reducing fouling or the impact that fouling has on filtration performance would, therefore, hold tremendous potential to enhance Army personnel health, operational capabilities, and equipment performance and longevity. Successful implementation and improvement of the technology could also have a tremendous impact on civilian infrastructure, water quality, and, in particular, security. These factors provided strong motivation for this project.
The presentation noted that fouling correlated with dead zones of low flow and mixing.14 The approach therefore assumes that the elimination of dead zones can help distribute the fouling sites more uniformly across the membrane. The PI arrived at an interesting and otherwise counterintuitive approach to prevent fouling by particles through introducing microscale barriers. The reason this is presumed to work is that increasing the velocity of the flow field by the barriers prevents the lodging of the particles. The barriers constitute secondary structures on the membrane (i.e., microlouvers) that allow an opportunity to optimize length scale, shape, and distribution of the microstructures, which have been shown to impact the long-term performance of the membrane.
14 J. Walker, 2018, “Integrated Micromixers and Acoustic Streaming to Prevent Reverse Osmosis Membrane Fouling,” TARDEC, presentation to the Panel on Review of In-House Laboratory Independent Research in Materials Science at the Army’s Research, Development, and Engineering Centers, December 12, Washington, D.C.
The study examined scalable fabrication methods used to control the shape and then proceeded to experimentally and computationally measure the effect of geometry on performance. This was explained and illustrated in the presentation, showing the effect of three different microbarrier geometric variations in the shape of a chevron. The choice of the chevron shape was well motivated, and one of the shapes that was examined performed better than the others. A brief comparison was made to other work in the field using flow barriers and indicated an improvement over prior art. A very promising finding was that the oscillation of bubbles in the flow was improving the cleaning action.
Several questions and suggestions arose during the review. The fouling arising from particulate ingress occurs by a different mechanism than fouling by mineralization. It would be good to clarify which of these mechanisms are optimally selected or mitigated by chevron structures with the given surface energy, length scale, and flow rates. Increasing flow velocity on the one hand improves particle entrainment, which allows better distribution across the membrane and reduces the bottleneck effect, while at the same time it may, in fact, enhance advection or transport of the generative species.
Questions to consider are: Do the feed spacers compare to the chevrons in their scale, and is there interference between these two types of objects? Stagnation behind the spacer (in the pocket of the spacer) does not foul, which suggests a potential strategy to collocate membrane spacers with the natural stagnation zones that are free from accumulation.
An example was shown of an oscillating bubble having a cleaning action, which was very intriguing and encouraging. Would it be possible to purposely induce distributed cavitation to help dislodge deposits? If so, what are the conditions that could favor this occurring in a controlled manner? There is also a need within this project for more extensive referencing of prior art and for marketable performance requirements.
Project: Multifunctional Laminated Structures with Self-Sensing Capabilities
This project uses electrically conductive through-thickness reinforcement (z-pins) in a continuous fiber polymer-matrix structural laminate for both reinforcement and sensing functions, thereby making the composite multifunctional. The sensing is based on electrical resistance measurement.
The project is relevant to the organization’s mission and purpose. The multifunctional concept is novel, although there is much prior work (since 1989) on multifunctionality, with resistance-based sensing, in continuous carbon fiber polymer-matrix composite laminates in the absence of z-pins. The concept is relevant to structural health monitoring (damage monitoring), which is needed for military vehicles and other ground systems. Visual inspection is not adequate to provide the monitoring. Ultrasonic inspection is superior to visual inspection, but it is not adequately sensitive to minor damage in the absence of well-defined cracks. In addition, the through-thickness reinforcement alleviates the issue related to the weakness of the interlaminar interface in the laminate, though this aspect of the use of the through-thickness reinforcement is not novel.
Since through-thickness reinforcement is not typically present in a structural composite, this concept is not applicable to the vast majority of existing structures. The concept is, however, applicable to new structures that involve this special three-dimensional (3D) composite material. In contrast, carbon fiber composites without the through-thickness reinforcement provide sensing functions and are widely used in existing structures.
The project has provided modeling that supports the concept, but experimental work is critical to showing the feasibility of the concept. No experimental work, however, is included in the research plan for this project. Moreover, how the resistance measurement can be carried out in practical implementation of the technology was not considered. In addition, how sensing using the z-pin can be used along
with previously reported sensing (using the in-plane carbon fiber polymer-matrix composite) was not considered. Such considerations are critically relevant to the practicality and applicability of the concept.
The modeling considers z-pins in the form of rods of diameter 2 mm or 0.8 mm. The through-thickness reinforcement of such a large diameter would significantly disturb the in-plane continuous fibers, which have a much smaller diameter (e.g., 10 μm). As a result, the in-plane mechanical properties would degrade, even though the interlaminar shear strength is increased. In addition, the interfaces between the through-thickness reinforcement and the surrounding composite material would tend to become sites for the initiation of fatigue. The presence of excessive polymer matrix at this interface would alleviate the negative effect of the in-plane continuous carbon fibers, which are conductive, on the sensing performance of the conductive z-pins, but it is expected to also weaken this interface. The consideration of through-thickness reinforcement in the form of through-thickness fibers incorporated in the laminate by 3D weaving would be beneficial because such reinforcement is much smaller in diameter than the z-pins.
In the modeling, the electrical conductivity of the z-pin is assumed to be reduced by a factor of 103 due to cracking (in the case of the titanium z-pin) or fiber breakage (in the case of the carbon fiber polymer-matrix composite z-pin). This assumption appears to be arbitrary, and work needs to be done to obtain a reliable estimate or likely range.
The finding that the carbon fiber polymer-matrix composite z-pin is more effective for structural health monitoring than the titanium z-pin is expected. The finding that the z-pin of either type arrests cracks is also expected. The continuous fibers are primarily intended to be those that are not conductive (e.g., glass and Kevlar fibers) as opposed to carbon fibers, which are conductive. Carbon fibers are used, however, in high-performance structures, due to their high elastic modulus. Therefore, the inclusion of the scenario of carbon fibers in the research would be beneficial. Because of the conductivity of carbon fibers, their presence may affect the sensing effectiveness of the conductive z-pins.
One conference publication, based on the concept of this project involving the z-pin, has resulted from this research. The other two publications that were listed during the presentation did not involve the concept of this project. The research involving solely modeling, however, seems not to be limited by resources of staff, equipment, opportunities to publish, or other factors.
Project: Noise and Error Assessment for 3D Digital Image Correlation Systems Set Up for Simultaneous Measurement of Multiple Sides
The goal of the project is to determine measurement error of a digital image correlation (DIC) system configured to measure displacements of two perpendicular surfaces simultaneously. The project concluded its third year, with the preceding years focused on the development and investigation of the experimental DIC setup and the investigation of a four-camera DIC setup. Over the past year the project consisted mainly of studying the effects of the subset size used in the correlation operation on bias and noise in two sets of measurements. One measurement set focused on a tensile specimen, which was translated by a prescribed amount, and the other focused on the specimen when it was loaded to one strain level in the elastic domain. Data analysis consisted of attempting to correlate bias and noise with subset size and camera aperture. The reported conclusions were that the thickness-side of DIC measurements differs more from theoretical strain value than front-side DIC measurements, and that various factors may influence the inherent bias/deviation of double-side DIC setup.15 The purported relevance
15 B. Sia, 2018, “Noise and Error Assessment for 3D Digital Image Correlation Systems Set Up for Simultaneous Measurement of Multiple Sides,” TARDEC, presentation to the Panel on Review of In-House Laboratory Independent Research in Materials Science at the Army’s Research, Development, and Engineering Centers, December 12, Washington, D.C.
to TARDEC’s mission is to provide an alternative method of strain measurement to contact gauges for laboratory-scale material property characterization and as a solution for structural health monitoring.
The project was lacking on several fronts. There was no theoretical basis or hypothesis underpinning the work, and no scientific goals or objectives were presented. Theoretical analysis was used in the interpretation of the measurements, and there were not any substantive conclusions. The experimental data set was also sparse. The work lacked comparisons with baseline measurements (i.e., under conditions in which only one face was imaged). The lack of mention of the speckle size and the pixel size relative to relevant specimen dimensions revealed a lack of understanding of the importance of these connections in the fidelity of DIC data. The connection between this work and structural health monitoring was also not evident.
Project: On the Development of a Load-Agnostic Structural Lightweighting Design Optimization Methodology
The goal of this project is to develop design optimization methodologies that enable rapid insertion of new lightweighting technologies into Army ground vehicles. The approach employs normalized performance metrics based on stiffness, strength, and natural vibration frequencies of existing components. Shape optimization is achieved using gradient-based search methods implemented in an FE solver. These methods are appropriate for achieving project goals. The methodology was demonstrated for a control arm assembly of a suspension system of the Stryker fighting vehicle. The calculations yielded a design in which the component was 15 percent lighter and exhibited 10-30 percent improvements in the performance metrics.
The project is relevant to the Army’s goal to improve mobility of ground vehicles through lightweighting. It is expected to enable emerging additive manufacturing technologies, which have the potential to significantly expand the geometric design space.
Although the research approach employed in this project is derivative of optimization approaches used by others and falls closer to applied rather than to basic research, it is likely to yield findings of use in the Army design community. It would also be useful to expand the scope of the activity from one-off demonstrations of the methodology to the evaluation of more generic, perhaps simpler, component geometries, with the goal of developing greater fundamental understanding of underlying design principles.
Project: On the Feasibility of Electromagnetic Treatments for Relieving Residual Stress in Welded Armor Plates
The project is an empirical evaluation of the opportunity for electromagnetic treatments to relieve residual stresses in welded armor plates (both MIL–DTL 12560 and 46100 steel armor alloys). Plates of both alloys with a thickness of 0.5 inch were welded and subsequently measured for as-welded stress in three locations by X-ray diffraction. The exact same plates were then subjected to either 1.5 KV or 3.0 KV pulse/pulses. Plates were given either 1 pulse or 10 total pulses that were spaced 5 minutes apart. The plates were subsequently remeasured for residual stress in the same areas previously tested. In aggregate, it was observed that little residual stress relief occurred due to the electromagnetic pulses.
Having a method to relieve residual stress or, ideally, impart a slight compressive stress for retarding fatigue or guarding against overload crack initiation, is highly valuable for improving both component and overall structural life. This is particularly important for large welded components, such as welded military vehicles, which are too large for traditional thermal stress relief in a furnace. Aside from the benefits to improving welded armor performance, there is substantial potential benefit to the U.S. economy through diverse applications in stress-relieving large structures ranging from automobiles to offshore oil rigs.
Over more than 30 years there have been numerous attempts to relieve residual stress in welded structures using various potential technologies. In addition to the electromagnetic method being explored in this study, other examples include ultrasonics and acoustic resonance. That being said, techniques using thermal methods or mechanical deformation (e.g., shot peening, burnishing, and, to a lesser extent, friction stir processing) are regarded as statistically reliable methods to consistently and quantifiably relieve residual stresses.
In the study, X-ray diffraction was used to characterize residual stress. This technique has a shallow sampling depth, so surface conditions (even light abrasion with fine grit sandpaper to clean a surface) can influence the resulting stress measurement. It may be beneficial to use a technique such as neutron diffraction to obtain a larger sampling volume, a more accurate stress measurement, and even depth profiles of the residual stress distribution in the welded sample. Oak Ridge National Laboratory would be an ideal collaborator for neutron diffraction measurement on this project.
In the field of residual stress relief, it is important to fully understand the metallurgical mechanism(s) of residual stress relief or recovery at small distance scales. Moving dislocations through thermal treatment or plastic deformation using transmission electron microscopy (TEM) have been confirmed many times in the literature to relieve residual stress. The investigators also used this microscopy technique, but they did not show dislocation structures being moved by electromagnetic pulses. It may be beneficial to revisit the TEM literature on topics in which residual stress relief occurs, such as in annealing and normalization. In this project there needs to be a fundamental mechanistic understanding of how electromagnetic pulses could possibly relieve residual stress at the atomic and tens of nanometer scales. It may be that at a very high KV, heating occurs, so a threshold KV value exists for applying this technique for stress relief. Other factors to potentially investigate in the literature are the magnetic properties of the material being processed. If a small-scale mechanistic hypothesis of residual stress relief by electromagnetic pulses is to be developed after careful literature review, it can be further tested. However, for now, the empirical experimental evidence of the investigators suggests electromagnetic pulses result in, at best, little stress relief in these armor alloys. This negative result is, in itself, valuable.
Residual stress is a crosscutting scientific issue in that it can influence the mechanical performance of different type of structures—stress is stress regardless of its source (e.g., load, residual stress, and thermally induced stress due to thermal expansion) and is a very important issue to study. Examples of other projects in which residual stress is important are those on friction stir welding (FSW) (relevant to the project Parametric Optimization of Thermal Controlled Friction Stir Welding of Thick Al-Cu-Mg-Ag Alloy Plates), suspension component testing and reliability (relevant to the projects Development of Test Methodology for Suspension Component Characterizations in the Laboratory and Demonstration of New Time Dependent Reliability Method on Military Vehicle Suspension Components Using Up-Crossing and Joint Up-Crossing Rate), and also in the design process (relevant to the project On the Development of a Load-Agnostic Structural Lightweighting Design Optimization Methodology).
Project: Parametric Optimization of Thermal Controlled Friction Stir Welding of Thick Al-Cu-Mg-Ag Alloy Plates
This project involves friction stir welding (FSW) of thick 2139-T8 Al-Li plates at an isothermal temperature using active, closed-loop tool speed control. Actively monitoring temperature of the weld using a thermocouple on the shoulder of the tool gives local temperature feedback on the weld itself. Controlling temperature throughout the welding process is believed to be one of the most important variables for controlling microstructure and the mechanical properties of the weld.
This research is very relevant to the Army. Aluminum–lithium (Al-Li) alloys have the lowest density of all common aluminum alloy families, and their generally high strength and good fatigue and fracture
toughness make them an ideal material for lightweighting applications. Lightweighting is needed in many army research projects. The FSW of dissimilar aluminum alloys, particularly those with high strength-to-weight ratios that are traditionally difficult to weld, expands the design opportunities for applying these alloys into vehicle structures. It also improves the manufacturability of lightweight vehicles.
The research started with welding the 6061-T6 plate to establish basic welding parameters. Subsequent welding of the 2139-T8 plate was conducted at three different temperatures of 490°C, 500°C, and 510°C. The grain structure and strengthening precipitate were characterized using electron back-scattered diffraction (EBSD) and TEM bright field selective area diffraction (SAD). Significant microstructural differences were observed through the thickness of the weld. These were attributed to temperature control; however, the amount of chromium (Cr) used to control grain structure and the total strain imparted to the material could also be variables that contributed to the dynamic grain recrystallization during the welding event. Joint efficiencies of approximately 81.6 percent, as defined by welded strength and base metal strength, were achieved.
The research approach was empirical in nature and was conducted in collaboration with Pacific Northwest National Laboratory (PNNL). It was apparent that this partnership with the FSW laboratory at PNNL was very effective for obtaining significant data and knowledge with a very limited investment of researcher time (only 10 percent and 26 percent of two researchers’ time).
The solid technical approach to the start of this project opens many opportunities for further research. To better understand the fundamental aspects of deformation temperature during FSW, hot torsion data on this alloy could be investigated. Of particular value would be data on strain to failure and flow stress as a function of temperature. Deforming an aluminum alloy at a temperature where it is most ductile may result in optimal grain structures. Additionally, these data may provide insight into why the highest temperature exhibited cracking, possibly due to reduced ductility at this temperature. Further work on the influence of both grain size and precipitate reversion and/or coarsening on strength could be investigated. Generally, finer grain structures have higher (not lower) strength due to the classic Hall–Petch relationship. Therefore, some of the reduced strength in the friction stir zone may be due to precipitate reversion (dissolution), which is indicated by a lack of precipitate diffraction spots in the TEM SAD images. If precipitate reversion did occur, the solute may be able to be reprecipitated by applying a post-weld aging treatment. Obviously, the viability of a post-weld age depends on the size of the part being welded.
The primary hypothesis of the project is that controlling welding temperature is important for controlling both the microstructure and resulting properties of the welded structure. Temperature is so important that it may be useful to validate it (as measured using one thermocouple in the shoulder of the FSW tool itself) both empirically and through computer modeling.
One empirical technique is to embed fine thermocouples into the plate itself and friction stir weld through this thermocouple array. Another empirical approach is to image the weld with a thermal camera properly calibrated to the emissivity of the aluminum plate being welded. This empirical data could then be compared with a computer model of the heat distribution throughout the weld. The value of having a validated computer model is that subsequent hypotheses can be tested quickly prior to additional welding trials.
This work is intended to be presented at two different conferences in 2019. These events may facilitate additional opportunities for collaboration with other external researchers on this valuable work.
Project: Quantum Modeling of Woven 3D Graphene Through Computational Materials Science
The objective for this project was to investigate the possibility that graphene sheets could be interlinked using chemical vapor deposition on a poly-Ni (polycrystalline nickel) substrate. It was
conjectured that manipulating the microstructure of the substrate could produce woven graphene sheets. This investigation was to be conducted using DFT approaches and, specifically, the Quantum Expresso (QE) package.
This project was confronted with a number of challenges, the most consequential of which was that the time allotted to the task (0.3 full-time equivalent of the investigator’s time) was far too little to justify the breadth and scope of the proposed investigation. The effort, as originally envisioned, required the installation and maintenance of QE on a cluster—a significant effort in and of itself. This was followed by the time required for the researcher to become competent with a software package with which he had no previous experience. The ultimate conclusion that the QE package was inadequate for the proposed investigation points more to the inexperience of the investigator than a true deficiency of the package, which has been extensively used to model graphene.
In summary, the lack of progress associated with the a major research effort where the initial learning curve was quite steep appears to have precipitated a sense of urgency in the investigator resulting in rushed and superficial analysis, which did not conform to a well thought out investigative plan.
TARDEC Crosscutting Findings
Across several projects it was observed that TARDEC needs to broaden its collaborations with the scientific research community, both through connecting to outside researchers and to those within other Army laboratories. There were numerous instances when researchers failed to make adequate contacts with those in academia, industry, and/or national and defense laboratories with expertise directly applicable to the subject matter of the investigation. Such oversights are particularly grievous when the needed expertise is found at ARL or in a university investigator funded by the Office of Naval Research.
For the project Computational Design and Development of Polyurea-Based Adhesives there are DoD experts and academicians who need to be consulted to provide guidance to the project. Enhanced collaboration with domain experts outside of the RDECs would be beneficial for the project Integrated Micromixers and Acoustic Streaming to Prevent Reverse Osmosis Membrane Fouling. Several publishable and patentable results were obtained by the researcher, and the filing and dissemination of these results to the broader scientific and engineering community could help to foster these connections. The project on the Development of a Load-Agnostic Structural Lightweighting Design would benefit from collaborations involved in manufacturing, to check the feasibility of designs being proposed, and, more generally, to establish and integrate manufacturing constraints in the optimization process. The project Feasibility of Electromagnetic Treatments for Relieving Residual Stress in Welded Armor Plates could be improved by outside collaborations, and, as mentioned, Oak Ridge National Laboratory would be an ideal collaborator for neutron diffraction measurement.
TARDEC projects could also benefit from enhanced collaborations with other Army laboratories (e.g., ARL, ARO, and ARDEC), and TARDEC needs to establish communication channels that allow for the better sharing of resources, both intellectual and physical, among Army-funded programs. For the project Quantum Modeling of Woven 3D Graphene through Computational Materials Science an opportunity was missed while planning the investigation. Researchers at ARL have an active program in computer-aided design and fabrication of cross-linked polymer networks. This program includes DFT modeling of graphene and functionalized graphene sheets. The investigation reviewed would have benefited from interactions with these researchers. The project Exploring Graphene’s Potential for Enhanced Chemical Reactivity Induced by Regions of Local Conformational Deformation on Rough Iron Surfaces: Kinky Chemistry Applied could have benefited from more collaborations with other Army laboratories and the broader research community.
Several projects also reflected a lack of sufficient literature searches. The project Computational Design and Development of Polyurea-Based Adhesives missed relevant literature that could have been used to support the project. The project Fundamental Study on the Suppression of Dendrite Growth in Lithium-Metal Batteries via Carbon Nanoribbons Through In-Situ Optical Microscopy could have benefited from a more in-depth literature search on various formulations of carbonaceous materials to better support the uniqueness of CNR selection and further the understanding of its role in dendrite suppression. In the project Noise and Error Assessment for 3D Digital Image Correlation Systems Set Up for Simultaneous Measurement of Multiple Sides the apparent lack of familiarity with related literature undoubtedly contributed to project shortcomings. Over the past decade, DIC has been employed extensively in academia, industry, and government laboratories. It has become a standard tool used in conjunction with mechanical testing. The issue being addressed in this project, bias and noise in DIC measurements, has been previously studied and is broadly understood. Most of the pertinent literature on DIC methods dates back 5 to 10 years and could have been employed to achieve solutions for this study. No presentations or publications associated with this work were reported in the presentation. The project On the Feasibility of Electromagnetic Treatments for Relieving Residual Stress in Welded Armor Plates could potentially be enhanced by investigating the literature on the magnetic properties of the material being processed.
Recommendation: To advance greater scientific understandings and enhance its projects, TARDEC should promote more extensive interactions of its researchers with the broader scientific community, should consider stimulating greater interactions with other RDECs and Army laboratories, and should broaden its scientific literature searches.
Recommendation: TARDEC should provide better mentorship and oversight to junior staff members.
The quality and quantity of output normalized by the number of projects from the ILIR programs at ARDEC (4 projects) and NSRDEC (3 projects) were significantly greater than those at TARDEC (12 projects). The panel only reviewed 1 project from ECBC and only 2 from CERDEC, so no general observations are offered for these centers.
Some RDECs grow their talent in-house; they hire Bachelor of Science- or Master of Science-level personnel and then support the personnel while they pursue Ph.D. studies at local universities on a part-time basis. The majority of Ph.D.’s and Ph.D. candidates at TARDEC were reported to fall in this category. Most personnel at the other RDECs, it was reported, arrive with their Ph.Ds. in hand, often from reputable schools outside the vicinity of their respective laboratories. There is great value in students’ being fully immersed in a Ph.D. program and having frequent and constant interactions with other students and with multiple faculty. The homegrown Ph.D.’s do not have that opportunity. Differences in research quality appear to correlate with this characteristic.
Rigor in the development and evaluation of ILIR project proposals appears to differ greatly across the RDECs. For example, NSRDEC requires 8- to 10-page proposals (single-spaced, 10 point font) and has all of its proposals reviewed by people outside of NSRDEC (i.e., in academia and in other government agencies). TARDEC, on the other hand, requires 3-page proposals that are reviewed by a panel of senior technical experts, presumably in-house. The former model seems to correlate with higher project
quality. This raises the following question: Do proposal requirements and review procedures adopted by the individual RDECs need to be subjected to greater scrutiny?
Junior researchers in both ARDEC and TARDEC could benefit from more interaction and closer oversight with senior researchers. TARDEC researchers could also benefit from greater mentoring from senior researchers. This was particularly apparent on the projects Fundamental Study on the Suppression of Dendrite Growth in Lithium-Metal Batteries via Carbon Nanoribbons Through In-Situ Optical Microscopy, and Noise and Error Assessment for 3D Digital Image Correlation Systems Set Up for Simultaneous Measurement of Multiple Sides.
Recommendation: ARDEC and TARDEC should provide greater mentorship and oversight to junior staff members.