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High-Performance Structural Fibers for Advanced Polymer Matrix Composites 4 Opportunities for DoD-Sponsored Research The high-performance fibers that are being used today in DoD applications are the end result of decades of research and development. The PAN-based carbon fibers continue to be produced using fundamental technology developed in the 1960s. The first product among organic fibers, Kevlar, was introduced commercially in 1972. Although the fundamental properties of filaments quoted in the first patents are very close to today's products, the yarn properties have been improved significantly.1 This trend is continuing and does reflect improvement of commercial processes in both their fundamental design and their control. Both PAN and mesophase pitch carbon fibers have followed similar R&D time lines with both single-filament and bundle properties improving significantly. Given the maturity of these fibers, it seems unlikely that there will be dramatic improvements in fiber properties in the near term (5 to 10 years); rather, the principal R&D opportunities in this period appear to be in efficiently taking advantage of these fiber properties in finished composite structures and in cost reduction. While the DoD-stated performance of commercially available fibers appears sufficient to meet near-term DoD requirements, they may not meet all the needs of the next-generation systems of 2010 and beyond. For example, as outlined by the Future Combat System (FCS) and Objective Force Warrior initiatives, the protective garment of the future will have to protect a soldier from the full spectrum of threats (ballistic, chemical, biological, environmental) and act as the foundation upon which the soldier’s power grid, communication system, and sensor arrays can be built. For the future soldier system to achieve the degree of functionality that will be required in 10 to 15 years, while maintaining or reducing the overall system weight, new multifunctional fiber materials and associated composite systems and processing techniques must be developed, and this development must start now. It can be expected that the development of multifunctional composites for armored vehicles, topside ship structures, and unmanned robotic and aerial vehicles will face similar demands to meet requirements for reduced weight and increased functionality. The creation of such composites will require prudent investment as well as vision on the part of DoD program managers. In view of this time-differentiated array of research opportunities, this chapter is organized into two major sections: (1) opportunities for incremental performance gains and cost reduction (5 to 10 years) and (2) opportunities for more revolutionary gains (more than 10 years). NEAR-TERM (5 TO 10 YEARS) PERFORMANCE GAINS In projecting what might be expected in terms of gains in fiber performance properties in the future, one must separate incremental gains made using current technology from major leaps arising from new non-PAN precursors and/or completely new fiber technologies. This section discusses near-term expectations that would not require a multiyear development period. 1 E.I. du Pont de Nemours and Company. Diverse utilization of DuPont Kevlar. Available at http://www1.dupont.com/dupontglobal/corp/documents/US/en_US/news/releases/media/pdf/kevlar.pdf. Accessed March 2005.
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High-Performance Structural Fibers for Advanced Polymer Matrix Composites Carbon Fiber Improvements PAN-Based Fibers Today’s commercially available PAN-based carbon fibers cover a wide range of properties. The tensile strength ranges from approximately 2.5 to 6.5 GPa (362 to 943 kpsi), while tensile modulus ranges from approximately 220 to nearly 500 GPa (32 to 73 Mpsi). With this broad a product portfolio available, most of the research in the industry is pursuing gains other than improvements in properties. Suppliers of the small-tow carbon fiber (3k to 12k filaments) are working to reduce their costs while maintaining fiber properties. Manufacturers of large-tow fibers (more than 12k and often as much as 320k filaments) are trying to match the performance of the smaller-tow fibers and reduce product variability. Manufacturers of both small- and large-tow carbon fibers are working to improve process control so as to increase process efficiency and improve product uniformity. New monitoring techniques are becoming available that could improve the quality and consistency of both small- and large-tow products. These include high-resolution optical probes for monitoring gels and flaws, portable on-line Raman instruments for monitoring structure, and laser instruments for monitoring fiber size. These and other advanced monitoring techniques are being evaluated on pilot-scale fiber lines in academia and industry. For PAN-based carbon fiber properties, two important research areas have arisen. First, does a wider distribution of properties such as strength, for example, affect the realizable properties of the final part and, if it does, can this effect be predicted reliably and accommodated in the final composite design? This question is important to both the high-performance aerospace user and the much larger-volume applications for infrastructure, transportation, and energy. The effect of a given proportion of low-strength individual filaments in a tow on the strength of a composite made by impregnating the tow with resin is largely unknown. Further, the effect of the initial filament strength distribution on the strength reliability and lifetime of the impregnated tow and, in turn, on the reliability and lifetime of the final part made from many tows, is also not known. Current lifetime models do not consider the statistics of the failure process. Although a model has been developed to accomplish such a prediction,2 no databases exist to verify it. Furthermore, such verification would require an integrated study among fiber producers, prepreg and towpreg companies, and composite part manufacturers. In addition, in situ monitoring of composite part integrity and health could be used to validate such models. The successful outcome of such an effort would result in a lower factor of safety needed for a given reliability, as well as the ability to certify very large parts with minimal or no full-scale testing. Both of these advantages would result in very large cost savings. A second research thrust would be to improve the uniformity of the carbon fiber. For example, reducing the variability in fiber weight per unit length will reduce the fiber areal weight variability in prepreg. Because processing technology is generally tightly held by the companies, some mechanism such as a consortium of universities, government, and companies to produce results useful to all might be a mechanism for achieving this goal. Long-term opportunities to improve the compressive properties of fibers are more difficult to define. Many designs are limited by the compressive strength of the composite. Research has shown that both intra- and intercrystalline disorder must be increased if the compressive properties in both PAN- and pitch-based carbon fibers are to be improved. Pitch-Based Fibers Although pitch-based carbon fibers have been around for some time, they have managed to penetrate only some small niche markets. The primary reason is that many applications for carbon fibers to date have been strength-driven. This has given PAN-based carbon fibers, with their less flaw-sensitive nongraphitic structures, a natural advantage. However, because of their graphitic structure, the thermal conductivities of pitch-based carbon fibers are as much as three times that of copper and orders of magnitude higher than those of PAN-based carbon fibers, and applications are now emerging in which heat transfer is critical. For example, management of excess heat has become a limiting factor in the design of many military aircraft, satellite structures, and electronic packages. During high-speed 2 E.M. Wu and C.S. Robinson. 1998. Computational micro-mechanics for probabilistic failure of fiber composites in tension. Comp. Sci. Tech. 58:1421.
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High-Performance Structural Fibers for Advanced Polymer Matrix Composites maneuvers, air drag generates significant heat on flight surfaces, limiting aircraft performance. Satellites are exposed to large thermal gradients that cause warping and dimensional changes. The increasing density of the electronics packages aboard aircraft also makes thermal management a critical design issue. Solutions such as active cooling systems lead to weight penalties and increased system complexity. Passive cooling offers the potential for simplified system designs and reduced weight; pitch-based carbon fiber composites are ideal for these applications. The most promising passive approach consists of using the structure itself (either the airframe or the electronic package) to withdraw excess heat. In this approach, the structure is a composite material reinforced by high-thermal-conductivity, mesophase pitch-based carbon fibers. Applications such as brakes and electronics could create high-volume markets that can be satisfied only by pitch-based carbon fibers. Thus, although pitch-based carbon fibers are unlikely to challenge PAN-based carbon fibers in strength-driven applications any time in the foreseeable future, thermal management may create an even larger market in which pitch-based carbon fibers will dominate. However, for this market to grow, the price of high-thermal-conductivity pitch-based carbon fibers must be reduced significantly. Promising areas for cost reduction in the pitch process are precursor chemistry and the exploration of parameters that control structure during fiber processing, such as fiber property development related to heat treatment. Recent research has shown that fibers produced using more uniform mesophase precursors can develop extremely high thermal conductivities at lower graphitization temperatures than those currently employed in commercial processes. New analytical techniques such as matrix assisted laser desorption ionization (MALDI) mass spectroscopy allow mixtures of complex organics to be characterized. Based on advances such as this in instrumentation, it is now possible to quantify the composition and, thus, monitor the uniformity of the pitch precursor. Optimization and control of molecular orientation during fiber formation also allow the fiber to develop high thermal conductivities at lower temperatures. For example, if the relationship between molecular orientation and flow were better understood, spinnerets could be designed to optimize molecular orientation of the as-spun fiber. Improvements such as these, combined with the more efficient graphitization ovens, offer promise for reducing the price of high-thermal-conductivity pitch-based carbon fibers. As with PAN-based carbon fibers, while incremental gains in properties might be expected, the major effort for pitch-based carbon fibers will most likely be toward price reduction. Success could finally create the high-volume market needed to make pitch-based carbon fibers a commodity product. To create this market, investment is needed to develop a better understanding of how to design with pitch-based composites and of how to process and control the composite microstructure to get the best thermal conductivities. Opportunities Investment in the evaluation and installation of new continuous process monitoring techniques (such as online Raman spectroscopy) could be valuable in the near term by improving the purity of the fiber precursor, enhancing the uniformity of fiber structure, and reducing property variability for both PAN- and pitch-based carbon fibers. Initiate fundamental studies for both PAN- and pitch-based carbon fibers to develop molecular orientation during flow, solidification, and heat treatment. Provide funding for new analytical methods such as MALDI that could lead to improved precursors for pitch-based carbon fibers and carbon-carbon matrices. Invest in studies with revolutionary approaches to aid design and development pitch-based composites for thermal management applications. Organic Fiber Improvements In Chapter 2, the committee expresses an opinion that incremental improvement in tensile properties (especially strength) is still possible, and even likely, for those fibers that are commercially available today. The new developments are likely to be aimed at increased toughness and energy
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High-Performance Structural Fibers for Advanced Polymer Matrix Composites absorption, since this is the area in which these materials offer an advantage over carbon fibers. The development may be a result of research aimed at specific fiber properties and at optimization of ways in which the fibers are incorporated into the final structures. All materials commercially available at this point are a result of many years of product and process development as well as many years of manufacturing experience. All of them operate in a very competitive environment. Thus, one would not expect downward changes in the prices of these materials. Nevertheless, opportunities do exist to lower the cost of utilizing them. These can be realized through better designs of the systems in which they are included. This is an area in which DoD-sponsored research activities would be highly cost-effective. The details are discussed in later sections. Additional efforts toward developing engineering standards for the use of organic fiber composites would also be very beneficial. Such standards are necessitated by the extreme anisotropy of properties of these materials. For example, the modulus of these fibers in the direction parallel to the fiber axis is two orders of magnitude greater than the modulus in the direction perpendicular to the fiber axis. The same is true of strength. Relatively low lateral properties significantly alter the mechanism of failure compared to traditional materials. Even when advanced fibers are exposed to tensile stresses, they fail in shear. The same phenomenon is responsible for their relatively low compressive properties. The increase in lateral interaction observed in aramids compared to polyethylene improves the compressive properties of aramid composites to a noticeable degree. There are several examples in which a systems design approach has helped to circumvent deficiencies in component materials. For example, tires can be designed in such a way that the reinforcing fiber is never put into compression. Use of release technology (to decrease adhesion between the fiber and the resin) to minimize impact of low lateral properties and prevent longitudinal failure is another possibility. These are only two examples of many options that are possible. Research in both of these areas would improve the chances of meeting DoD needs with existing materials for which pricing is defined by competitive forces in the civilian marketplace. Improved Compression Performance of Organic-Based Fibers The exception to the trend of incremental improvements is the newest fiber M5®, which is in late development stages. Although the fiber is expected to be available on a commercial scale in 4 to 5 years, it is likely to reach a large commercial scale a bit later. It has been shown that fibers produced from this material exhibit bidirectional intermolecular hydrogen bonding. This hydrogen bonding should greatly increase the internal shear modulus of the fibers and may result in improvements in mechanical properties in tension and compression. As these properties are realized, a combination of ballistic properties and the ability to function as a structural component has the potential to revolutionize armor development in terms of weight reduction. DoD should fund opportunities to evaluate the properties and applications of M5 once adequate qualified capacity is established. Matrices and Interphase Although the fibers are of great interest, the matrix and interphase are also important constituents of the composite system. In general, advanced resin formulations are needed that feature low cost, room-temperature cure, low viscosity, improved toughness, dimensional stability, and long-term environmental durability. Further, it is particularly important for the resins to be compatible with lower-cost manufacturing methods such as VARTM and e-beam curing. VARTM is being considered by all services as an affordable process. The Army requires VARTM resins that can be infused at room temperature, be cured at low temperature, and provide improved multi-hit ballistic and structural performance in ground vehicle hull structures. In parallel, studies on sizings for tailored ballistic and structural properties for the new resins are needed. Navy ship applications require resins that can be processed through VARTM with improved fire, smoke, and toxicity performance. The new resins should be developed in parallel with new fire-resistant core materials, with an emphasis on adhesion since sandwich construction is a common design feature. There are some significant opportunities for concurrent investment in refining current chemistries (phenolics) and developing new ones (e.g., low-viscosity preceramic polymers). In addition, the use of carbon fiber with vinyl ester VARTM resins for ship structure requires additional study of sizings to
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High-Performance Structural Fibers for Advanced Polymer Matrix Composites promote adhesion and durability of the composite. The new class of low-viscosity cyclic thermoplastics that polymerize after infusion offers the potential for improved toughness in VARTM-fabricated structures. Electron-beam (e-beam) curing is a nonthermal process that uses high-energy electrons and/or X-rays to initiate polymerization and crosslinking reactions at controlled rates at room temperature. The primary challenges facing the current state-of-the-art e-beam resins are the lack of toughness, hot-wet operating temperature limits, and consolidation rheology. Long-term durability of composites may also require optimization of the fiber-matrix interface that forms during e-beam processing. Composites As stated earlier, gains in fiber performance over the next 5 to 10 years are likely to be incremental rather than revolutionary (with the possible exception of M5). The same can be said for matrix resins and for composites made using these matrices and fibers. Although there are longer-term opportunities for greater performance leaps (discussed later), the next decade will generally be characterized by continuous improvement of existing composite systems. Types of improvements might include incremental increases in strength and stiffness, better fiber-matrix interface strength and/or durability, enhanced damage resistance and damage tolerance properties, and processing advancements. One exception is the emerging area of multifunctional composite materials where composites, as man-made materials, are the ideal host to incorporate a full spectrum of functionality for personnel protection (ballistic and extremity protection, biological and chemical protection) and structural applications (ballistic and blast protection) at minimum weight. Multifunctional composites can also provide the foundation for the integration of signature management, communication systems, power grids, and sensor arrays. It is recognized that the development of multifunctional composites for armored vehicles, topside ship structures, and unmanned robotic and aerial vehicles will require continuous DoD investment. In the near term, current fibers, resins, and processes can be used to create first-generation multifunctional composites. Ideally, each layer in the composite will serve multiple functions. The near-term challenges are twofold: (1) design methods to predict the optimum grading of layer functionality to achieve performance requirements; (2) advances in processing science to enable integration of functionality and manufacturing for scale-up into affordable components and structures for test and validation. The creation of such composites will require prudent investment as well as vision on the part of DoD program managers. Multifunctional Composite Armor The U.S. Army is undergoing a transformation to a more agile, lighter, and more lethal force. Current acquisition focuses on the Medium Brigade, which will consist of C-130-transportable wheeled vehicles using existing armor technologies (principally metals and composite spall liners). In the mid-term, the Future Combat System (FCS) will be fielded and will become a "system of systems" integrating land, air, and soldier missions. The plans for block upgrades of technology offer significant opportunities for the insertion of new materials and affordable processes. Current state-of-the art armor used in the Composite Armored Vehicle Advanced Technology Vehicle combines ceramics and composites to provide structure and ballistic protection at areal densities of 25 pounds per square foot. FCS vehicles will require significant reductions (20 to 50 percent) in areal density for various platforms and a level of multifunctionality not achieved in previous systems. In addition, crew protection kits for tactical vehicles are needed to increase protection levels. Steel kits currently being deployed in Iraq are too heavy and exceed the gross vehicle weight, thus leading to increased maintenance, repair, and replacement. Multifunctional ultralightweight armor that provides structure and improved protection against projectiles and improvised explosive devices is needed in the near term. Significant DoD investment is necessary to enable multifunctional composite armor incorporating lower-density fibers such as carbon and organics (e.g., Kevlar, Spectra, and PBO) that provide adequate structure with improved ballistic performance to achieve these objectives. The M5 fiber offers a potential to combine improved compression properties with excellent ballistic performance that should be evaluated for structural armor applications. In addition, three-dimensional preforms that can be hybridized with multiple fibers offer great promise for structural armor and blast protection. Significant
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High-Performance Structural Fibers for Advanced Polymer Matrix Composites investment in materials development concurrent with the design and manufacturing of these new classes of multifunctional armors and structures is needed to ensure that affordable technologies are ready for insertion into current and future tactical vehicles and other FCS systems, as well as Navy hull and ship topside structures. In aircraft structures, it is widely recognized that material costs are a small percentage of total acquisition cost. Although composites, by their nature, lend themselves to single operation co-curing of large and complex structures, assembly costs still represent a significant contribution to overall cost. When assembly of subcomponents is necessary, good fit (without the need for rework or additional operations) is essential. Improved dimensional tolerance via materials and processing would provide a significant cost reduction in assembly and thus the use of composites. The cost of refit can be up to 30 percent of the total composite part cost. Recent activities promise an excellent start in understanding how to improve dimensional tolerance and determining which parameters are critical in controlling dimensional tolerance.3 These studies determined that fiber variability from lot to lot (density, filament diameter, filaments per tow, etc.) plays a significant role in fit and assembly issues and identified prepreg processing steps that could alleviate some problems. Further studies into the root causes of poor dimensional repeatability would help to address this concern and identify which parameters are major contributors. A current trend across the industry is the assessment of VARTM to replace more costly processes in DoD and commercial applications. VARTM has been the process of choice for fabrication of large-scale ship prototype structures. The VARTM process is being critically evaluated for aerospace applications as an alternative to RTM. VARTM is also the leading candidate to manufacture the multifunctional composite armor previously discussed. VARTM is widely accepted as an affordable process to fabricate prototypes. VARTM is a low-pressure process that offers reduced tooling costs. Fiber volume fractions and associated properties are lower than for higher-pressure processes, so parts processed using VARTM might have to be heavier to meet the same performance requirements as parts fabricated by a traditional process. VARTM uses one-sided tooling, and strict control of geometry is not yet possible and may add to assembly costs. The process is largely a manual process lacking quality control and automation. Due to the widespread interest in the VARTM process, it is recommended that DoD initiate a program with university-industry-government participation. Virtual manufacturing and simulation should play an important role in accelerated insertion of materials and processes into DoD systems. Advancements in intelligent processing will allow for risk and cost reduction, but advances in three-dimensional flow simulation in porous media are also required, as well as models to predict input properties such as the permeability tensor as a function of fiber architecture, compaction, and distortion. A key element of this effort will be the transfer of technologies developed in universities into the industrial base. This will require education and training as well as software and hardware deployment. Industry will conduct manufacturing trials and quantify the cost and performance trade-offs. Research in automation using simulation, sensing, and control systems should be pursued to advance this process from prototype to a production-ready process. OPPORTUNITIES TO IMPROVE PROGRAMMATIC EFFICIENCY Improved Building Block Certification Approach Mechanical testing of full-scale structures or large components can be extremely costly in terms of both program resources and program risk. Large-scale testing facilities require expensive, specialized equipment, and the test article itself is costly to fabricate and instrument. In addition, a single test of a large component may take weeks or even months to set up and complete. Thus, savings can be gained by reducing the quantity of these types of tests. From the standpoint of risk, a program may incur a severe setback in terms of redesign, material substitutions, or process changes if large-scale tests do not produce the required or expected results. 3 Prime contracts referenced include Precision Assembly for Composite Structures, F33615-96-2-50051 (Boeing), Processing for Dimensional Control, F33615-97-C-5006 (Boeing), and Reduced Dimensional Variation in OMC Lay-Ups, F33615-97-C-5007 (GE Aircraft Engines).
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High-Performance Structural Fibers for Advanced Polymer Matrix Composites Clearly, for complex composite products, the large-scale or full-scale test should serve as the final check on the entire process of product and process development, and there should be little risk of surprises at this stage. To both reduce the number of large-scale tests and have confidence that a product will meet design requirements, a structured approach to the qualification and certification process is needed. Such an approach has been used, particularly in the aircraft industry, and has become known as the “building block” approach because it comprises a number of stages (or blocks). Each block consists of testing and analysis that increase in complexity with each successive block and builds on knowledge gained in previous blocks. Although the concept of the building block approach is widely acknowledged in the composites industry, it is applied with varying degrees of rigor, and details are far from universal.4 This lack of standardization has hindered exploitation of the full potential of this approach. Ideally, a building block program is aimed at obtaining information at the lowest possible level of complexity and using analysis rather than testing as much as possible to minimize cost. Cost-efficiency is achieved by testing greater numbers of less expensive small specimens and fewer of the more expensive components and full-scale articles. At the same time, the process should provide a means of assessing technology risks early in a program to reduce the probability of unanticipated results in the final stages of certification. Although there have been some attempts at detailing the specifics of the building block approach and standardizing the methodology,5 more work needs to be done in this area. Based on the above, DoD funding could be directed in several ways to promote the standardization and efficiency of the building block process. One area that needs study is statistical continuity throughout the process. Currently, there are rigorous statistical methods for selecting the numbers of specimens tested at the lowest building block level (coupon testing to obtain material property basis values). However, this statistical precision has not been carried forward to the element, subcomponent, and component levels. Quantifying the statistical significance of numbers of test articles at the higher levels could help to reduce the amount of these more expensive tests. A related subject for research is the development of a well-defined methodology for maximizing the use of data obtained at lower building block levels in order to minimize the quantity of higher-level testing. In addition to specific research projects, continued and expanded funding of the MIL-HDBK-17 coordination activity would serve to maintain an industry-wide forum for review of building block methodology. Part of the limitation in developing the correlation from the coupon level to the subcomponent and component level is our poor understanding of how the stochastics of the failure process in a small coupon translate into stochastics of failure at the component level. To gain this understanding, a rather rigorous and large program of experimentation and modeling is required, and DoD funding in this area would be an excellent investment. Improved Mechanical Test Selectivity When a composite material is qualified to a specification and characterized to provide basic lamina and laminate mechanical properties, testing is performed with a variety of loading types at several environmental conditions. Typically various tension, compression, and shear tests are conducted in four or five different dry and wet environments. Although the current testing may provide very complete characterization, it is not clear that all of these tests are necessary. Composite material procurement specifications represent one area in which unneeded testing may be imposed. Procurement specifications frequently state a required value for nearly every mechanical property in multiple environments. In its simplest form, a procurement specification has only two functions: (1) to describe the material adequately so that there is no doubt about what is being purchased and (2) to provide a means of assuring that quantities of material purchased over time are the same (within a given tolerance) as the material originally qualified. Since certain properties and conditions tend to be more reliable indicators of material variability than others, the committee believes that specifications could be simplified to include fewer tests and conditions if an optimum set of tests were defined. For example, test frequency could be reduced once qualified materials are in production. 4 MIL-HDBK-17, 1998. 5 See, for example, R.S. Whitehead, H.P. Kan, R. Cordero, and E.S. Saether. 1987. Certification testing methodology for composite structures. NADC-87042-60. Warminster, Pa.: Naval Air Development Center.
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High-Performance Structural Fibers for Advanced Polymer Matrix Composites This same concept applies to the testing performed to characterize the generic lamina and laminate properties in the early stages of a building block certification program. Since particular properties and conditions tend to become the important design drivers, there is little need to extensively test noncritical loadings and environments. Although small-specimen coupon testing is not as costly as higher-level building block tests, there may be opportunities for significant savings if the quantity of such testing could be reduced. To achieve this, DoD-funded research could be directed toward determining which types of tests and environments are insignificant as design drivers for given classes of applications. The result of such research would be recommendations for elimination of certain tests for particular applications. Such recommendations would have to be based again on a large-scale study using both experiments and modeling. In addition, DoD-funded research could be focused on standardization of procurement specifications. While this goal has been attempted by a number of organizations over the years, there has been no nationally funded program to sustain the effort and bring about its implementation. Use of Historical Test Data to Support Development of Analytical Methods As stated earlier, the use of analysis is generally less expensive than conducting mechanical tests. Although some level of testing will always be necessary to verify analytical predictions, advances in analysis methods are continually needed to drive the testing requirement to a minimum. However, to develop and validate new and robust analytical methods, a body of test data is needed that covers various types of materials, laminate configurations, loading conditions, and environments. Although much testing has been conducted for various DoD and Federal Aviation Administration (FAA) programs during recent years, these data are not available in a readily usable form. If these data could be compiled, categorized, and documented, the developers of analytical methods could, in some cases, draw on these data rather than conducting new tests. Areas in which current analysis methods are weak, and where historical data would be beneficial, include compression after impact (both solid laminate and sandwich structure), mechanically fastened joints, delamination growth, and fatigue. To achieve this, DoD might consider funding an effort to compile coupon, element, and subcomponent test data from recent certification programs and make it publicly available in a standard format. The payoff would be the acceleration of analysis method development to further reduce the need for numerous larger-scale tests. LONG-TERM (MORE THAN 10 YEARS) PERFORMANCE GAINS In the following sections, the committee identifies strategic research areas in fibers, matrices, interfaces, and composites that may require a decade or more to bear fruit. Although these research thrusts carry a higher risk of failure than the nearer-term opportunities identified above, they would, if they are successful, produce dramatic performance gains for composite structures. Nanoscale Fibers Using current technology and precursors, only incremental improvements in fiber properties will be achieved. Fibers with controlled nanostructure have the potential, however, to achieve step changes in properties. In addition, nanoscale fibers may have niche applications in sensors or other low-volume applications. Nanoscale fibers are those with a diameter of less than 100 nanometers. Their surface area is more than 100-fold that of conventional fibers. The two major classifications of nanofibers are those processed from polymers, and those inorganic fibers that are grown (including carbon nanotubes). Polymer nanoscale fibers can be woven into unique fabrics with high surface area and small pore size. This makes nanofiber textiles excellent candidates for barrier materials to protect against chemical and biological weapons as well as environmental threats such as wind and rain.6 This large amount of surface area also provides a convenient platform to place arrays of sensors. Conductive nanofibers 6 K. Graham, H. Schreuder-Gibson, and M. Gogins. 2003. Incorporation of electrospun nanofibers into functional structures. Presented at International Nonwovens Technical Conference 2003, sponsored by the Association of the Nonwoven Fabrics Industry (INDA) and the Technical Association of the Pulp and Paper Industry (TAPPI), Baltimore, Md., September 15-18.
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High-Performance Structural Fibers for Advanced Polymer Matrix Composites woven throughout a soldier’s uniform could serve as a grid to bring power to—and to convey information from—the host of nanoscale sensors that will be part of the future soldier system. Polymer nanofibers also have potential use in ballistic protection applications. On average, the defect size in a nanometer-diameter fiber is smaller than the average defect size in a conventional fiber. Therefore the critical stress required to fracture a nanofiber could be much greater than that of a conventional fiber. The high surface area of nanofiber textiles will also greatly help to transfer the load of an impact to the surrounding fabric, promoting energy dissipation over a larger area. By producing nanofiber textiles from high-performance polymers such as Kevlar KM2 and PBO, whose surface can be functionalized to perform multiple duties, there is the potential to make a single, breathable, multifunctional fabric that will defend the soldier from the full spectrum of threats. One limitation of this application is that production rates for nanofibers are inherently slow; therefore, nanofibers might be better used in small-scale niche applications or woven into traditional fabrics, thus providing the sensing and power functionality only. Multifunctional armor is also an essential component of a future soldier’s system and will rely heavily on organic fibers that can be converted into protective clothing. The Interceptor system combines a Kevlar vest with ceramic armor inserts. The Land Warrior soldier system is the first fully integrated system that provides protection from the full spectrum of threats (ballistic, chemical, biological, environmental), but it is moderately heavy (92 pounds). The Objective Force Warrior (OFW) is the Army’s current effort in soldier system design led by a lead system integrator. The goal of OFW is to design with the goal of taking the advances achieved in the Land Warrior program to the next level of development by reducing the soldier’s overall load to 35 percent of his body weight and providing greater capability in signature management, threat protection, information gathering, and troop coordination. In support of the OFW program, the DoD funds a wide variety of academic and industrial research programs7 that address power generation, advanced sensor development, robotics, and microclimate control. Success in each of these areas is being driven by revolutionary advances in microelectronics, biochemistry, and materials processing on increasingly smaller length scales, approaching the nanoscale regime. This added functionality is likely to be integrated into the soldier’s clothing fabricated with organic fibers in the submicron range. The most recent developments indicate that commercial sources of such materials will become available in the near future. Significant investment of DoD resources into nanofibers, materials, and devices is warranted to meet the long-term objectives for soldier protection. Carbon Nanotubes Carbon nanotubes are quasi-one-dimensional, nearly single-crystalline (axially), hollow, graphitic carbon structures. Two varieties of carbon nanotubes exist: single-walled nanotubes (SWNTs) and multiwalled nanotubes (MWNTs). The first nanotubes observed were multiwalled nanotubes. MWNTs consist of two or more concentric cylindrical shells of graphene sheets arranged coaxially around a central hollow core with interlayer separation as in graphite (0.34 nm).8 In contrast, single-shell or single-walled nanotubes9,10 are made of single graphene (one layer of graphite) cylinders and have a very narrow size distribution (1-2 nm). Often many (tens of) single-shell nanotubes pack into larger ropes. Both types of nanotubes have the physical characteristics of solids and are microcrystals, although their diameters are close to molecular dimensions. In nanotubes, the hexagonal symmetry of the carbon atoms in planar graphene sheets is distorted because the lattice is curved and must match along the edges (with dangling bonds) to make perfect cylinders. This leads to a helical arrangement of carbon atoms in the nanotube shells. Depending on the helicity and dimensions of the tubes, the electronic structure changes considerably.11,12 Hence, although graphite is a semimetal, carbon nanotubes can be either metallic or 7 Examples include Multidisciplinary University Research Initiatives (MURIs) through the Army Research Office (ARO), Collaborative Technology Alliances (CTAs) through the Army Research Laboratory (ARL), and contracts through the Defense Advanced Research Projects Agency (DARPA). 8 P.M. Ajayan and T.W. Ebbesen. 1997. Nanometre-size tubes of carbon. Reports on Progress in Physics 60:1025. 9 S. Iijima and T. Ichihashi. 1993. Single-shell carbon nanotubes of 1-nm diameter. Nature 363:605. 10 D.S. Bethune, C.H. Kiang, M.S. de Vires, G. Gorman, R. Savoy, J. Vazquez, and R. Beyers. 1993. Cobalt-catalyzed growth of carbon nanotubes with single-atomic-layer walls. Nature 363:605. 11 J.W. Mintmire, B. Dunlap, and C.T. Carter. 1992. Are fullerene tubules metallic? Phys. Rev. Lett. 68:631.
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High-Performance Structural Fibers for Advanced Polymer Matrix Composites semiconducting. Nanotubes are closed by fullerene-like end caps that contain topological defects (pentagons in the hexagonal lattice). The electronic character of the ends of these tubes differs from that of the cylindrical parts of the tubes and is more metallic due to the presence of defects in these regions.13 Both SWNTs and MWNTs are currently available in micron lengths, and longer lengths have been made in the laboratory. The topology of nanotubes indicates extremely high thermal and electrical conductivity in the axial direction, and the measured longitudinal elastic modulus is in the terapascal range—higher than that of any known material. The combination of high aspect ratio, small size, strength, stiffness, low density, and high conductivity makes nanotubes extremely intriguing candidates as fillers in polymer composites or in polymer precursors for graphite or aramid fibers. Multiwalled hollow nanofibers (with about eight layers of nanotubes) are also being made in a configuration much like steel wool using a catalytic chemical vapor deposition technique.14 Several small companies are currently producing SWNT in quantities of kilograms per day.15 Current purified material costs around $500 per gram. These prices have the potential to decrease dramatically as volume increases and other continuous processing methods are developed. A complication here is that the perfection—and thus the properties of the nanotubes—is strongly affected by such processing methods as pulsed laser deposition, electric-arc discharge, or high-pressure carbon monoxide conversion. The high price, combined with dispersion issues and size distribution issues, makes the use of nanotubes as reinforcing fibers on their own a very long term project. There are, however, other opportunities for nanotubes that take advantage of their high electrical conductivity and large surface area. For example, the addition of less than 1 weight percent of nanotubes to polymers can increase their conductivity by eight orders of magnitude and can increase the glass transition temperature by tens of degrees. This can justify the use of nanotubes to modify matrix materials. In addition, nanotubes could be used in other combinations to make unique fibers. There have been reports of nanotube composite nanofibers with a combination of strength and modulus that cannot be achieved with other fibers.16 Continued investment in long-term research on the use of nanotubes to modify matrix behavior can only lead to more of these innovations. Nanostructured Micron-Scale Fibers Nanoscale fillers offer an opportunity to control the nanostructure of fibers. Work is already going on in pitch-based fibers17 and PAN precursors.18,19 Current pitch precursors lead to extended graphitic structures that make the fibers flaw sensitive. By adding small quantities of nanotubes to mesophase precursors, the balance of properties might change.20 One could expect the nanotubes to nucleate domains within the mesophase. Smaller domains might be expected to decrease the flaw sensitivity of the fiber and increase its strength. Initial results demonstrate that the domain size is indeed reduced. However, the nanotubes were not dispersed uniformly in the mesophase and the expected improvement in fiber strength was not observed. While this initial study provides no conclusive proof that combining nanotubes and mesophase can improve the balance of properties for pitch-based fibers, it offers some 12 N. Hamada, S. Sawada, and A. Oshiyama. 1992. New one-dimensional conductors: Graphitic microtubules. Phys. Rev. Lett. 68:1579. 13 D.L. Carroll, P. Redlich, P.M. Ajayan, J.-C. Charlier, X. Blase, A. De Vita, and R. Car. 1997. Electronic structure and localized states at carbon nanotube tips. Phys. Rev. Lett. 78:2811. 14 For example, by Hyperion Catalyst International, Cambridge, Mass. 15 For example, by Carbolex in Lexington, Ky., and Carbon Nanotechnologies, Inc., in Houston, Tex. 16 A.B. Dalton, S. Collins, E. Munoz, J.M. Razal, V.H. Ebron, J.P. Ferraris, J.N. Coleman, B.G. Kim, and R.H. Baughman. 2003. Super-tough carbon-nanotube fibres. Nature 423(6941):703. 17 R. Andrews, D. Jacques, A.M. Rao, T. Rantell, F. Derbyshire, Y. Chen, J. Chen, and R.C. Haddon. 1999. Nanotube composite carbon fibers. Applied Physics Letters 75(9):1329-1331. 18 T.V. Sreekumar, T. Liu, B.G. Min, H. Guo, S. Kumar, R.H. Hauge, and R.E. Smalley. 2004. Polyacrylonitrile single-walled carbon nanotube composite fibers. Advanced Materials 16(1):58-61. 19 F. Ko, Y. Gogotsi, A. Ali, N. Naguib, H. Ye, G. Yang, C. Li, and P. Willis. 2003. Electrospinning of continuous carbon nanotube-filled nanofiber yarns. Adv. Mat. 15:1161-1163. 20 T. Cho, Y.S. Lee, R. Rao, A.M. Rao, D.D. Edie, and A.A. Ogale. 2003. Structure of carbon fiber obtained from nanotube-reinforced mesophase pitch. Carbon 41(7):1419-1424.
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High-Performance Structural Fibers for Advanced Polymer Matrix Composites hope that this may be possible. If nothing else, it proves that the addition of nanotubes can reduce the domain structure of mesophase pitch. This alone could allow better control of the matrix structure in carbon-carbon composites formed from mesophase matrices. In PAN precursors, the addition of SWNTs leads to improved thermal stability and higher modulus of the precursor fiber.21 The carbonized fiber has higher modulus than pure PAN-based carbon fibers and the same strength. There is evidence that the approach of incorporating single-walled nanotubes into precursors is highly effective and worthy of further investigation in order to obtain a step change in fiber properties. Significant investment in processing and structure control is necessary before such fibers could be commercialized. Attention to potential environmental, safety, and health concerns is also warranted in light of past experience with other small fibers. Advances in Fiber Processing Because of their characteristic fibrillar structure, polymeric precursor fibers are preferable for producing high-strength carbon fibers. Although prices for carbon fiber from SAF- and textile-based precursors prices have dropped to an all-time low, current PAN-based processes are unlikely to allow further cost reduction. Alternative, potentially low-cost routes for producing this class of carbon fibers should be explored. These should include more efficient processes for producing PAN, rather than the current wet-spinning technique, and exploring alternate methods that can decrease stabilization times and/or reduce the amount of oxygen added during stabilization and, thus, increase overall process conversion. Also, potentially low-cost precursor polymers with higher carbon contents should be evaluated. Melt Spinning of Mesophase Pitch Fibers High-thermal-conductivity fibers are melt-spun from a liquid crystalline mesophase pitch precursor. The current domestic process for producing mesophase pitch involves thermal treatment of an isotropic pitch precursor. The molecular weight distribution of the resulting product is relatively broad. Although the process is relatively simple, research has shown that more uniform mesophases develop higher degrees of molecular orientation during fiber formation and graphitize more readily during final heat treatment. The development of cost-effective new processes for producing mesophase precursors and techniques controlling the structure of the mesophase during fiber formation could reduce final heat treatment temperatures by as much as 600°C. This alone would reduce production costs by a factor of 10, making high-thermal-conductivity carbon fibers attractive for high-volume applications such as consumer electronics and automotive brakes. Nanofiber Processing A number of processing methods can be used to obtain polymer nanofibers. These include electrospinning, various types of melt blowing, controlled phase separation of polymer blends during melt spinning, and self-assembly of biomolecules into fibrous structures. All of these processing techniques are in relatively early stages of development, and there is much work yet to be done. Modeling and Real-time Characterization of Fiber Spinning Processes Thanks to the rapid advances in digital technology that have occurred over the last decade, it has become possible to obtain real-time spectroscopic (infrared, X-ray scattering, Raman) and imaging data for a variety of polymer processing techniques including fiber spinning. Such data provide insight into the influence of processing variables on the molecular ordering and phase transitions that occur during fiber spinning. This molecular ordering (or lack thereof) ultimately determines the mechanical properties of high-performance fibers. More accurate theoretical models can be developed based on this information, which can then be used to optimize processing conditions in order to produce the highest-quality fibers. These real-time characterization techniques provide a feedback mechanism on the molecular scale that 21 T.V. Sreekumar, T. Liu, B.G. Min, H. Guo, S. Kumar, R.H. Hauge, and R.E. Smalley. 2004. Polyacrylonitrile single-walled carbon nanotube composite fibers. Advanced Materials 16(1):58-61.
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High-Performance Structural Fibers for Advanced Polymer Matrix Composites until recently was either not possible or prohibitively expensive. Such feedback will only increase in importance as the individual fibers themselves become composites through the addition of nanoparticulates such as layered clay silicates and carbon nanotubes. Wise investment in this type of basic research will lead to a reduction in the time required to develop new fibers and will also accelerate the optimization of current commercially available fibers. Opportunities Investment in more efficient processes for producing PAN precursor fibers (such as melt spinning rather than the current wet-spinning technique) and alternate methods that can decrease stabilization times and/or reduce the amount of oxygen added during stabilization—and thus increase overall process conversion—should be considered. Also, potentially low-cost precursor polymers with higher carbon contents should be evaluated. Investment in these areas could yield long-term cost and performance benefits for DoD systems. Investment in programs developing new nanostructured (micron-scale fibers with nanoscale structure) and nanoscale-diameter fibers is critical to fibers exhibiting a step change in properties. Investment should be made in fundamental studies that couple online monitoring of the development of molecular orientation during flow, solidification, and heat treatment to the verification of fundamental process and product models that incorporate molecular detail. This type of basic research, now possible because of growing computer power and advances in computational materials science, could greatly reduce the time required to develop new classes of fibers and lead to improved properties in current commercially available fibers. Improved techniques for measuring the composition of pitch and more cost-effective new processes for producing mesophase precursors should be developed, and techniques for controlling the structure of the mesophase should be funded. Advances in Matrixes Controlled architecture of matrix materials is possible using bio-inspiration and self-assembly techniques. While this is an important area for future investment, there has already been significant investment in this area. One opportunity for improving resins is the addition of nanoscale filler. This would lead to a hierarchical composite with nanocomposite resin embedding traditional fiber structures. The main advantage of nanofillers is that they can provide multifunctionality to the resin (e.g., improved toughness and conductivity or improved toughness and UV absorption). The challenges include increased resin viscosity and the possibility of filtering out nanofillers leading to inhomogeneous distribution. Nevertheless, there are hints of success in this area. A recent paper showed that the addition of multiwalled carbon nanotubes to epoxy for use as an adhesive to join two graphite-epoxy composites resulted in about a 40 percent improvement in the average shear strength of the lap shear joint. The failure occurred inside the graphite fiber composite instead of in the adhesive. This success was observed at a loading of 5 weight percent MWNT.22 Nitrogen-doped nanotubes have been found to increase the glass transition temperature of epoxy by as much as 25°F. Increases in electrical conductivity of as much as eight orders of magnitude at 1 weight percent of filler have been observed.23,24 22 K.-T. Hsiao, J. Alms, and S.G. Advani. 2003. Use of epoxy/multiwalled carbon nanotubes as adhesives to joint graphite fiber reinforced polymer composites. Nanotechnology 14:791-793. 23 A. Eitan, L.S. Schadler, J. Hansen, P.M. Ajayan, R.W. Siegel, R. Andrews, and M. Terrones. 2002. Processing and thermal characterization of nitrogen doped MWNT/epoxy composites. Proceedings of the 10th US-Japan Conference on Composite Materials, Stanford University, Stanford, Calif., September 16-18. 24 J. Coleman, A. Dalton, S. Curran, A. Rubio, A. Davey, A. Drury, B. McCarthy, B. Lahr, P. Ajayan, S. Roth, R. Barklie, and W. Blau. 2000. Phase separation of carbon nanotubes and turbostratic graphite using a functional organic polymer. Adv. Mater. 12:213.
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High-Performance Structural Fibers for Advanced Polymer Matrix Composites Another fruitful area of research is the use of self-healing polymers as matrices in composites. In these systems, microcracking has been identified as one of the first damage modes, and self-healing polymers will be effective in increasing the long-term durability and life of composites.25-27 Interface and Interphases A designed interface is crucial to controlling composite properties. Several areas are worthy of investment here. First, interdisciplinary studies are needed in which molecular-scale modeling is coupled with new chemistry to create sizings that have controlled coupling between matrix and fiber, and to create an interphase region that contributes to optimized properties. Recent studies have shown that incorporating nanoparticles into fiber sizings offers improvements of strength and energy absorption for structural armor applications and represents a new mechanism for tailoring properties. This interdisciplinary approach is critical to success. Second, combined studies are needed involving the chemical industry, the fiber industry, and new chemistry, to test the effect of sizings at the bulk scale. Composites In 10 to 20 years, next-generation composites are likely to include the maturation of current materials and processes as well as the introduction of new fibers of increasingly small diameters, nanoreinforced resins and interphases that will provide levels of multifunctionality that cannot be achieved with current materials. In both cases, improvements in design methodology are needed. Current materials can be optimized to extract their full potential as discussed below. However, next-generation materials will require new multiscale modeling methods to bridge the molecular interactions between constituents and the overall performance of the composite systems. It is also recognized that the processing science of multifunctional composites materials incorporating nanoscale constituents and devices must be developed. Investment of funding in these areas now is necessary to develop the design and processing methods of the future. Developing a Better Design Methodology One of the clearest messages that came from the committee's study is that designers are not taking full advantage of current composite systems. Available materials systems are not optimized, and the properties of composites are not understood well enough to design properly. In addition, because of the variability in composite properties, an extremely conservative design is used that typically adds weight and cost. Finally, the part design is often completed without the material system in mind. In order to develop a better design methodology, the following will be necessary: Understand the role of each constituent in controlling properties, particular damage development and failure. This requires developing an experimental database from carefully controlled systems in which only the interface, or only the fiber, or only the matrix is changed, to determine the role of each. In addition, these data must be obtained for specific processing methods. Incorporate statistical mechanics into property predictions at the micromechanical level. This should include constitutive laws that model polymer deformation and failure appropriately. Develop new design tools that incorporate information from the database and the new models. 25 S.R. White, N.R. Sottos, P.H. Geubelle, J.S. Moore, M.R. Kessler, S.R. Sriram, E.N. Brown, and S. Viswanathan. 2001. Autonomic healing of polymer composites. Nature 409:794-797. 26 S.R. White, N.R. Sottos, P.H. Geubelle, J.S. Moore, M.R. Kessler, S.R. Sriram, E.N. Brown, and S. Viswanathan. 2002. Autonomic healing of polymer composites. Nature 415:817. 27 E.N. Brown, S.R. White, and N.R. Sottos. 2004. Microcapsule induced toughening in a self-healing polymer composite. Journal of Materials Science 39:1703-1710.
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High-Performance Structural Fibers for Advanced Polymer Matrix Composites Understand the residual stress gradients and defects that develop under specific manufacturing processes. The first opportunity requires some significant, focused funding that involves the fiber manufacturers, resin manufacturers, composite processors, and either academia or a large neutral testing facility. A carefully designed set of experiments is needed that would alter fiber properties, fiber strength distributions, interface properties (using some of the new interfaces developed in academia), and matrix properties and then characterize the resulting interface behavior, as well as the macroscopic tensile, compressive, fatigue, creep, and toughness behavior as a function of each variable. Although a few studies have attempted a portion of this, no large-scale study has been conducted. The second opportunity is in the area of modeling of properties. Continuum models must be linked to micromechanical models and perhaps even molecular-level models. The fiber and interphase property variations must be included and the nonlinear behavior of the matrix must not be ignored. The modeling must consider the thermal and stress gradients that develop for a given processing method, as well as the thickness and curvature of the final part. Realistic modeling of the processing defects that occur must be included. Finally, the stochastics of the failure process and how it translates from small samples to the component level must be considered. This is a difficult task, but a great deal of progress has been made in this area recently.28 Interdisciplinary and Interinstitutional Development A common theme in the committee’s discussions was the appropriate role of academic research in the development of the fiber and composites industry. There has often been a disconnect between academic and industrial research efforts in the composites area. For example, the academic community has studied interfaces in great detail, but often the technology developed—and the fundamentals learned—are not used by industry, either because of poor transfer of technology, poor use of the literature by companies, or the inability to scale the interfaces developed. Even when industry is aware of developments in academia, the time and resources required to redevelop the expertise in the industry are often considered too high. On the other hand, most academic facilities cannot produce fibers, interfaces, matrices, and composites and thus rely on industrial materials. Although the field is highly proprietary (making it difficult for academics to get information to study their materials properly or for industry to share), there are some precompetitive issues that would best be addressed by a collaborative, interdisciplinary or interinstitutional research program facilitated by government. The committee did not specifically explore the form that such interinstitutional collaborations might take: it merely notes that the current paradigm of academia studying model materials—or materials that it is not allowed to completely characterize—leads to less than ideal science. 28 A.M. Sastry, C.W. Wang, and L. Berhan. 2001. Deformation and failure in stochastic fibrous networks: Scale, dimension and application. Key Engineering Materials, Trans Tech Publications 200:229-250.
Representative terms from entire chapter: