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CHAPTER THREE Structural and Multifunctional Materials CHAPTER SUMMARY The Panel on Structural and Multifunctional Materials focused on emerging materials and the processes used for their fabrication, with special attention to the types of multifunctionality that could be designed into a material. An example might be a composite material in which both the matrix and filaments serve several functions: The matrix might contain microcapsules sensitive to mechanical stress that, upon breaking, would highlight the damaged area by changing color. The strengthening filaments might have two different compositions which, when imbedded in a conducting polymer matrix, would produce a galvanic current. Such a material might be the basis for a new generation of lightweight, long-service electric vehicles. This chapter discusses DoD structural materials development approaches and goals. It highlights the importance of lighter, stiffer, and stronger materials, and the need for materials to operate for long periods at high temperature with predictable degradation. These materials are necessary to improve vehicle mobility, maneuverability, transportability, and survivability. Once all the data were presented, the panel identified four areas of R&D opportunity. In priority order these are Materials design assisted by computation, Service-induced material changes, Composite materials design and development, and Integration of nondestructive inspection and evaluation into the original design.
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These four opportunities are expanded upon, with special emphasis on the design of structural materials that are truly multifunctional. Investments in these research areas should result in advances that would yield many of the necessary new DoD materials. Such advances will Reduce development time and costs, Modernize design criteria, Predict and verify functionality, Continuously monitor in-service health, and Predict residual life. INTRODUCTION The Panel on Structural and Multifunctional Materials looked at (1) emerging materials and processes for fabricating structural materials and (2) multifunctionality that could be built into the structure, such as health monitoring, thermal-load dissipation, and electromagnetic radiation management. This panel concentrated on mesoscopic and macroscopic multifunctionality scales, such as thin laminates, mesoscopic trusses, active fibers, and coatings. The Functional Organic Panel (see Chapter 6) addressed microscopic multifunctionality introduced by atomic or molecular design. Using some of the polymer matrices discussed by that panel can open a fruitful area for future investigation: composite materials in which several levels of multifunctionality are incorporated with the structure. Using data presented by many outstanding scientists and engineers, the panel identified four broad R&D opportunities; in order of priority they are Materials design assisted by computation, Service-induced material changes, Composite materials design and development, and Integration of nondestructive inspection and evaluation into original design. These opportunities, if exploited, can produce many DoD materials of the future. They should also reduce the time and resources required for development of new materials, change the design criteria, and insure that
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functionality is predicted and measured and that behavior of the materials in use is continuously evaluated. The concept of multifunctionality spans all four opportunities. There is a need to maximize the potential for use of materials with intrinsic multifunctionality and to design and fabricate composites with active multifunctional phases. This panel believes that computational materials science is the most important opportunity in the future of materials research. Thus, “materials design assisted by computation” is a major opportunity to Design better materials, Better understand their behavior, Design better structures with them, and Shorten the development cycle from concept to implementation. As a result of the improved understanding emerging from computation-assisted design of materials, it will also become possible to design better structures using these materials. Thus, the second opportunity from this panel is entitled “service-induced material changes” or “how to better use materials.” The rationale for this revolutionary concept is the thought that materials and structures ought to be designed for functionality rather than being based on the material’s initial properties. To design this way it is necessary to understand history-dependent properties and performance evolution. This means the science base for the use of materials must be expanded, and nonequilibrium structures and materials must be understood in order to support a science-based constitutive theory that is explicit in the extensive variables of the material. The advantages of such an approach would be to reduce materials and system development cost by virtual engineering and simulation during design and speed up materials and systems development, enabling higher performance and longer-life designs. Given the timeframe for this study (~2020), the panel did not address possible incremental improvements of materials, unless they could lead to a breakthrough in a system capability. Rather, the focus was on materials in which one might expect major (20-25 percent) performance improvements over the next 15 to 25 years. As Table 3-1 shows, monolithic materials are unlikely to show performance gains of this magnitude; composite materials—or at least materials combinations—are much more likely to do so.
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TABLE 3-1 Potential for Achieving Property Improvements of 20 to 25 Percent over Current State of the Art for Various Classes of Materials by 2020 Strength Toughness Stiffness Density Environmental Resistance High Temperature Capability Metals No No No No Yes Increase by 200°F MMCsa Yes No Yes Yes Yes No Ceramics Yes No No No No No CMCsa Yes Yes Yes Yes No Yes Polymers Yes Yes Yes No No No PMCsa Yes Yes Yes Yes Yes Yes aMMC = metal matrix composite; CMC = ceramic matrix composite; PMC = polymer matrix composite. Processing of these materials is critical to improvements in the properties indicated. Since the fractions of fibers comprising a composite material may be varied over a wide range, these materials may be designed with a broad range of density, stiffness, and strength values, because the filaments used are very strong and have large elastic moduli. Many filamentary materials also have lower density than metals, so metal matrix composites, for example, can be less dense than the parent metal but have greater strength and stiffness, increasing both specific strength and specific stiffness. These points are especially relevant for polymer matrix composites where very high specific properties can be obtained for nominal temperature applications. Thus, “composite materials design and development” is a compelling approach that merits study and refinement, because reduced weight is a primary design criterion in many structures. In addition to extensive composite material development efforts, this panel believes that opportunities exist for “integrating non-destructive inspection and evaluation into the original design” of both materials and structures. This would allow for continuous monitoring of the health of all newly designed structures. Integrating sensors into the structure requires that they be very small, so many new types of sensors must be created. In addition, small portable advanced sources, such as X-ray and neutron sources, will be needed to allow field evaluation of structures and some sources should be incorporated into the internal structure in places that would be difficult to examine with an outside source. This chapter continues with a discussion of DoD needs for materials that are lighter, stiffer, and stronger than those available today, and materi-
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als capable of long-term high-temperature exposure without excessive degradation. These materials are needed to improve vehicle mobility, maneuverability, transportability, and survivability. The following sections describe applications of computational approaches to materials design, the integration and optimization of materials systems, multifunctional materials, materials with self-healing abilities, materials with thermal or electrical conductivity spanning the range from conductors to insulators, and advanced coatings and adhesives. The chapter concludes by drawing out crosscutting materials R&D opportunities that will be vital to critical DoD systems in the coming decades. DOD NEEDS FOR MULTIFUNCTIONAL STRUCTURAL MATERIALS The materials needs of DoD are all-inclusive. Though DoD uses every known type of structural material, it has a continuing need for new materials because the military must always strive to be better prepared than any potential enemy, and potential enemies are themselves also striving to become better prepared. In general, the military needs materials that are lighter, stronger, stiffer, and usable at higher temperatures. This allows equipment to be more mobile, maneuverable, transportable, and to last longer. For the military’s air arms, the goals have always been to fly higher, farther, and faster. Recently, DoD has emphasized the total life-cycle cost of all types of equipment and materials of construction. There has been great interest in “smart materials”—e.g., materials that will monitor and report on their own health. This requires the development of many new sensors, some of which must be an integral part of the material. New instruments to activate and query these sensors are also required. Finally, there have recently been demands for multifunctional materials, e.g., a composite with high strength and stiffness in which the strengthening filaments can supply battery power. The significance of materials costs to integrated systems costs is discussed in Appendix C, but the essential points are covered here. Forty years ago, Westbrook1 noted that structural materials vary in price by seven orders of magnitude and that the usage of a material in pounds per annum is inversely related to the cost per pound; for example, reducing 1 Westbrook, J.H., Internal General Electric Report, General Electric, Schenectady, NY, 1962.
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TABLE 3-2 Market for Structural Materials Value of Pound of Weight Saved Over Life of Vehicle ($) Number of Units Sold Per Year Market Size ($ billion) Automobiles and trucks 2 30 million 600 Commercial aircraft 200 2,500 150 Spacecraft 20,000 100 20 materials cost by a factor of two can result in a four-fold increase in usage. For a vehicle, another important factor is the value of weight saved over its service life. Table 3-2 summarizes the impact on the market when gasoline is $2.00 per gallon, an automobile has a 100,000-mile life, commercial aircraft has a 100,000-hour life, and spacecraft goes into orbit once. The cost of materials is a relatively small fraction of the fabricated cost of a structure, typically 10 to 20 percent. Thus, combining these fabricated costs of the structure with the value of a pound saved gives the average maximum cost that can be tolerated in a particular application. For the automobile example, where the value of a pound saved is $2.00 times a 20 percent material cost as a fraction of total cost, the upper limit is $0.40 per pound for the primary structural material of the automobile, which is about the cost of automobile-quality steel. It is also possible to conclude that aluminum will not be a cost-effective substitute for steel in automobiles until gasoline costs $4.00 per gallon.2 As noted in Appendix C, these calculations must be fine-tuned to align with factors such as the speed at which the object moves and the complexity of the structure fabricated. For example, the value of a pound saved in the rotating part of the gas turbine in an airplane is 10 times the value of a pound saved in the fuselage. Also, materials costs for complex composite structures are as little as 2-5 percent of the total fabricated cost. The importance of reducing materials weight is that all vehicles, engines, and aircraft can be lighter and require less energy to run, thus saving fuel or enabling them to carry larger payloads. For military vehicles, fuel savings are especially critical. Figures from the U.S. Army indicate that 2 P. Bridenbaugh, Alcoa (retired), private communication, October 11, 2001.
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70 percent by weight of the expendable supplies needed on a battlefield are fuels. The cost of a gallon of fuel, transported halfway around the world, stored and transported to forward fuel depots, and finally delivered by air to vehicles at the battlefront may be as much as $400 per gallon, although the more usual price was between $13 and $30 per gallon (DSB, 2001). Weight reduction coupled with strength increase is doubly important for vehicle or engine weight and for allowable engine size. For rotating or oscillating components, both mass and strength are important, but the impact of reducing rotating or oscillating mass cannot be minimized. In any engine, reduction of moving mass makes it possible to reduce the mass of shafts, bearings, and bearing support structures; thus a simple weight reduction in moving mass can cascade through the engine to a dramatic total weight decrease. These same factors apply to flywheel-type energy storage systems and to rail-gun systems. Material stiffness is a property that is especially important for extensive structures that must hold their shape and in tube and sheet structures where buckling propensity is directly related to the elastic modulus (Seely and Smith, 1955). Specific modulus, a performance index of a material, is defined as the elastic modulus divided by density. In many sheet or column structures where the material has a low modulus, to ensure sufficient component rigidity the thickness of the structure must be increased above that required to achieve the specified design strength. In the design of these structures, the most important quantity is the elastic modulus. The structural stresses may not be the significant quantities—that is, the stresses may not limit the loads that can be applied to the member without causing structural damage, and hence the strength properties of the material (such as yield stress) are not of primary importance. Note that the specific moduli of Fe, Al, and Mg are almost identical, so simply substituting one material for another does not change this (ASM, 1961). One area where elastic modulus is critical is in the design and construction of gun tubes. High stiffness allows the propellant charge to expand and accelerate a projectile to maximum velocity, but does not allow the expanding gases to leak past the projectile and thus undermine efficiency. New high-energy propellants require that gun tubes be stiffer; this can be achieved using a composite reinforced by filamentary windings. These propellants also demand that the refractory interior surfaces of the gun tube be resistant to erosion and corrosion at temperatures higher than those that are currently encountered.
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In monolithic materials, stiffness is very difficult to change by alloying and is hardly affected by microstructure. In fact, alloying and heat treatment work better to decrease stiffness than to increase it. Thus, designers of new monolithic materials have only two options: The material must be made lighter so that structural efficiency is improved or it must be made stronger. These, then, are the most promising directions for monolithic materials research. In the design of composite materials, however, density, stiffness, and strength are almost independent quantities (Schaffer et al., 1995). The filaments used are quite strong and often have elastic moduli about twice the modulus of metals, so increasing stiffness is a real design opportunity. Many filamentary materials also have low density; incorporating them into a metal or alloy will both decrease density and increase strength, thus increasing both the specific strength and the specific stiffness. It is these qualities that make new composite materials compelling opportunities for the future. Because ceramic or polymer matrix materials can produce very strong, lightweight structures, research on these materials is of primary importance. The drawbacks are that the fabrication costs of composite materials often exceed those of metals and alloys, and their ductility and fracture toughness are usually lower. Where cost is not of primary importance and low ductility and global toughness are not the primary causes of failure, composite materials are prime candidates for structural design. However, R&D on composite materials must proceed in many directions simultaneously. The most important needed advances are in the processing and scale-up of multiple material combinations. Current processing methods are slow, involving hand lay-up of binder-impregnated composite sheets or the processing of only small batches of material. Other needed advances include adequate exterior coatings for nonoxide composites, fiber coatings to prevent fiber-matrix chemical diffusion at high temperatures, and control of the interface between filaments and matrix to optimize properties. It is also essential to understand the mechanics of fracture in each type of material. Especially in the design and manufacture of composite materials, there are opportunities for integrating nondestructive investigation and evaluation sensors into the original design of both materials and structures. This would allow for continuous health monitoring of all newly designed structures. The integration of sensors into the structure requires that they be very small, so many new types of sensors will be needed. The integration of sensors into the surface of DoD land vehicles is a critical step in the formulation of smart armor (TACOM, 2001). The surface
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sensor must pick up the speed and direction of the penetrating projectile and initiate the appropriate response in the armor package to defeat the threat. Shape charge jets travel the fastest and require a defeat mechanism different from the long rod penetrator. The material at the base of the armor to capture the resultant debris needs to be lightweight but strong enough to prevent perforation. The defeat mechanism of smart armor requires the storage and distribution of power, as discussed in Chapter 4. Much effort has been expended in recent years on materials for propulsion and power generation systems, because these offer the greatest potential benefits. For example, the operating efficiency of a gas-turbine engine will increase by more than 1 percent for every 10 degrees centigrade increase in the turbine-inlet gas temperature (Sims, Stoloff, and Hagel, 1987). The increase in efficiency may be either increased power or decreased fuel usage. Substantial fuel savings in aircraft and ground vehicle engines and in stationary turbines used for local power generation can be achieved by using new materials that can accommodate the temperature increase. All devices that operate at high temperature demand special materials that respond to an extensive set of design requirements. Strength and stiffness are just two of these. Others include creep and fatigue resistance coupled with good oxidation and corrosion properties. In-service conditions provide additional challenges, including ingestion of debris. Thus, the ability to absorb damage without compromising safety is a crucial characteristic. Because a common measure of aircraft turbine engine efficiency, especially in military systems, is the thrust-to-weight ratio, R&D strategies often use material elements to increase thrust (through higher-temperature alloys) and decrease engine weight (with low-density materials). Higher turbine-inlet gas temperatures have become possible for three fundamental reasons. First, single crystal turbine blades were introduced into the early turbine stages. These blades can operate at higher temperatures because elements added to strengthen and stabilize grain boundaries lowered the alloy melting temperature. More recently, new higher temperature alloys contain increasing amounts of Ta and Re, both heavy elements, so the density of these blade materials has increased by about 25 percent (Antolovich, 1992; Kissinger, 1996; Pollock, 2000). Finally, thermal barrier coatings are used to insulate metallic material from the heat of flowing gases. Improvements in the insulating characteristics of the thermal barrier coatings should be pursued. However, the other techniques mentioned are
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approaching the limit of their usefulness. Adding dense elements to the blade is self-defeating because the heavier rotating blades increase stresses on the supporting disk. Thus, the disk, the shaft, and the bearing support structure must be strengthened, adding still more weight to the engine. Significant progress in turbine engine efficiency is likely to come from materials that allow engine weight to decrease. New high-temperature materials are also needed for nuclear reactors. The hottest materials in a gas fluid reactor may be as high as 1000°C. Solid oxide fuel cells are also typically operated at about 1000°C, a temperature at which fuel cell reactions occur efficiently, although lower temperature operation (to ~600°C) is being pursued to reduce material and manufacturing costs. For military purposes, where the input is presumed to be diesel fuel, cells operating at the higher temperatures will still be required. In these systems there is also a great need for coatings for refractory metals and for technology to join refractory metals to baser metals. In some applications, such as propulsion for ships, materials must perform at these very high temperatures for very long periods without maintenance (for example, modern naval nuclear plants operate for their nominal 30-year life without refueling). Other areas where marked progress is possible are metallic and ceramic-based composites (NRC, 1998), perhaps using discontinuous reinforcements and amorphous and nanoscale matrix materials. Current efforts could lead to the emergence of such materials. These efforts should be guided by computational design approaches, and should also consider material degradation in service as part of alloy selection and treatment. Finally, since periodic inspection of parts already in an assembled engine is not easy, the new materials should have NDI/NDE elements integrated into their structure. New high-temperature materials are needed for continuous exposure to 1400°C. These materials must balance the properties needed for use in complex systems. In the current environment the necessary bywords, in addition to high temperature, stiffness, and strength, are affordability, durability, and reliability. Reasonable goals for materials development by 2020 would be a 25 percent increase in tensile, creep, and fatigue strength for a whole range of materials, coupled with a 25 percent decrease in density. Of necessity there can be no degradation in oxidation resistance at temperatures to about 1000°C. Materials usable to about 1200-1400°C with the same density as superalloys (not more than ~10 g/cc) will also be required. High
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modulus materials are also needed for space frames, vehicle body stock, tubular construction materials, etc. By 2020 many materials with modulus increases of 20-40 percent will be required. It is likely that these modulus goals can only be attained with composites. SPECIFIC AREAS OF OPPORTUNITY Materials Design Assisted by Computation A critical limitation to the introduction of new materials into military systems is the extreme length of the development cycle, often as long as 15 years. At the front end of the development cycle is the arduous task of creating a material with superior properties. Not only must the composition of the material be established, but—of equal importance—so must the process by which this material is made. Processing of structural materials is critical because the microstructure developed during processing is a primary determinant of the resultant mechanical properties. Complicating the design process is the fact that a material has many mechanical and physical properties, e.g., elastic modulus, yield strength, ductility, fracture toughness, fatigue resistance, density, weldability, and corrosion resis-tance. Improvement of one property often causes degradation of another. Hence material design is a compromise. Traditionally, the development cycle is long because it involves sequential synthesis of the material and then extensive testing of many properties and combinations. Computation holds the promise of shortening the development cycle by eliminating much experimental synthesis and testing. However, to replace experiment, computation must be reliable; the computational tools must consistently predict a suite of mechanical, physical, and thermodynamic properties of materials. Specifically, not only must the properties of single crystal multicomponent materials be predictable; the properties of multiphase, nonequilibrium materials as a function of processing conditions must also be predictable. Not only is this task not easy, at this time it is not possible. Some current limitations of computational materials science at the atomistic scale are Lack of understanding of the underlying atomic mechanisms that determine phase transformations and mechanical properties, e.g., strength, durability, and damage tolerance;
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TABLE 3-3 Examples of Multifunctional Capabilities of Targeted Structural Materials Smart Material Composite Functionality Piezoelectric Material Shape Memory Alloy Polymer Foam Polymer Matrix Composite Metal Matrix Composite Ceramic Matrix Composite Health monitoring Sensor element Coated damage sensors Optical fibers Self-healing As composite or foam Crack healing SMA foam Via polymer chemistry SMA 2nd phase Electromagnetic and acoustic wave management Acoustic Layered and reinforced Erosion resistance Actuation Single crystals Films, magnetic Active polymer valves SMA/piezoelectric fibers layers SMA 2nd phase Thermal Insulation Thermal management Discontinuously reinforced Infrared management Electrical Nanoreinforced systems, circuit boards Discontinuously reinforced Magnetic Magnetic activated Data storage Permanent magnetic Permanent magnetic
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TABLE 3-4 Examples of Military Applications Likely to Benefit from Revolutionary Advances in Multifunctional Structural Materials Application Functionality Armor Stealth Antennas and Arrays Engines Structure Power Launchers Health monitoring Remote sensing Remote sensing Remote sensing Remote sensing Remote sensing Remote sensing Remote sensing Self-healing Shape memory alloys and foams Stealth coating Conformal and directional Corrosion resistant Corrosion resistant Electromagnetic and acoustic wave management Embedded nanoscale networks for shielding Actuation Smart armor Embedded conformal antennas Thermal Infrared matching Higher operating temperature Higher operating temperature Higher operating temperature Electrical Smart armor Millimeter wave reduction Photovoltaics Power storage Magnetic Permanent magnetics
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because of their high surface-to-volume ratio. Further understanding of fabrication and response of thin-film SMAs will make it possible to use them in microelectromechanical systems (MEMS) and even nanoelectromechanical systems (NEMS) devices. The materials are currently very experimental due to problems of maintaining uniformity in fabrication, understanding of fatigue mechanisms, and lack of constitutive descriptions at small scales. Exploration of shape memory effects, which have been demonstrated for some polymeric solids, is a promising avenue to be explored. Porous SMA materials offer the advantages of other metallic foam structures enhanced by material response due to the activity of the SMA matrix material. Porous SMAs have been fabricated successfully under less than ideal conditions, resulting in promising structures of high porosity, though with extremely uneven properties. With emphasis on processing methods, microstructure control and optimization, and modeling to address an active material, SMA foams offer great potential for lightweight, damage-tolerant, self-healing structural components. The last category of SMAs with potential to emerge as a vital material system in the future is composites, where the SMA could be a secondary phase to provide self-healing. The self-healing aspect is discussed more fully in the next section. Electroactive polymers and ion-exchange polymer-metal composites are active material systems that show large deformations in the presence of a low applied voltage, mimicking biological tissue. The polymers have conductivities in a wide range, from values comparable to semiconductors to values as high as copper. Their conductivity and color can be reversed by controlled chemical or electrical stimuli. Electroactive polymers can be used both as light and chemical sensors and as actuators, but because of their low elastic modulus they are not typically used as structural members. Nevertheless, they are certainly multifunctional materials with enormous growth potential and could play a role in self-healing composite materials. Composites Composites offer the greatest opportunity for significant advances in material design and function. Because of their multiphase nature and fabrication methods these materials offer simple routes for embedded sensors, actuators, and other elements that provide multifunctionality. In the traditional ceramic matrix composite (CMC), metal matrix composite (MMC), and polymer matrix composite (PMC) materials, evolutionary
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developments in embedded optical sensors for local strain interrogation are likely, as are small improvements in the use of embedded smart materials (SMAs or piezoelectric elements) as actuators. Revolutionary advances in traditional composites should result from use of nanotechnology and wireless technology and incorporation of self-healing mechanisms. Nanotechnology offers exciting opportunities for radical changes in composite functionality. One area is to incorporate carbon nanotubes or other nanoparticulates into the matrix material to reach the percolation threshold at relatively low volumes (e.g., 1 percent or less). If the orientation and patterning of the nanoscale reinforcement could be controlled, integrated, and organized, the networks could add functions to health monitoring, surveillance, and stealth. A high impact of nanoscale reinforcements is foreseen, however, only in PMCs, where they also offer potential for improved mechanical properties. In particular, nanoscale reinforcement of the polymer could enhance matrix properties to the point where compression strength is improved; entangled networks of nanotubes may also toughen the material. Understanding the mechanisms and the degree of property improvement possible needs significant research attention. In particular, work should focus on surface modification of reinforcements; control of matrix-reinforcement adhesion; processing methods to control dispersement and alignment of reinforcements; hybridizing of nano- and microscale reinforcements; and integrating atomistic micromechanics and continuum modeling for predictive capability. Initial research into nanoparticles, such as exfoliated graphite and even cellulose microfibrils, indicate that such extremely low-cost systems offer multifunctionality enhancements comparable to carbon nanotubes; these opportunities should be fully explored, as should electrospinning of polymeric and other reinforcements that can produce submicron microfibrillar mats. In contrast to the high probability of success in nanostructured PMCs, the potential for significant advances in nanocrystalline metals seems much more limited because the nanoscale grain size lacks stability with temperature changes and there are difficulties with processing and impurities. Much greater progress is likely with amorphous metals, where the best properties might be found in composite materials formed of an amorphous metal matrix with ductile dendritic phase reinforcements. Use of simple and fast thermoplastic forming techniques for these materials, the known glass transition temperature, and the thermal stability of the final amorphous composite merit further study. The panel recommends computa-
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tional work to predict new potential alloy systems and create improved amorphous matrix composites (e.g., using SMA inclusions) to improve work hardening. In health monitoring, use of wireless interrogation and networking technology should significantly improve remote access to localized structure information. Additional comments on health monitoring are found in the research priority section below, where NDI/NDE represent one of the major opportunity areas. The self-healing capabilities of polymer-based composites have recently received wide attention due to work by researchers at the University of Illinois at Urbana-Champaign (White et al., 2001). Composite self-healing mimics nature’s autonomic healing response. In polymers and polymer composites, the effect has been achieved by chemical triggers (catalyst) and microcapsules of fluid repair agent in the polymer matrix. When localized damage in the form of a microcrack reaches and bursts a microcapsule, the repair agent wicks into the crack, where it is polymerized by reaction with the catalyst. Tests on healed samples demonstrate no significant loss in material strength. This breakthrough in engineered self-healing for polymer-based materials deserves significant research emphasis, as it could lead to a wide array of self-healing polymers, composites, and adhesives for structures. It is critical to decrease to nearly instantaneous the time required for healing response. Also under investigation for self-healing are metal-based composites, which include SMAs. At present, true self-healing has not been demonstrated; the focus has been on damage tolerance rather than self-healing. Nevertheless, intelligent material architecture has the potential to design materials with significantly improved damage tolerance and self-healing capacity by using SMAs that undergo phase transformation in the presence of the stress concentration from a crack. Also proposed is using SMAs for finite deformation mechanisms (as defined by the lattice transformation) for materials like amorphous metals used for work hardening. There has been great progress in fire-safe polymers that can be used alone or as matrix materials for composites (Sorathia et al., 1997, 2001). Incorporating these materials into aircraft, spacecraft, ships, and other vehicles will decrease weight and improve safety. Fire-safe matrix materials will also lower the hazards of graphite fiber released into the air when aircraft built with such composites burn.
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At least in concept, it is possible to design an almost unlimited number of composites with varying degrees of multifunctionality (tri-, tetra-, pentafunctionality, etc.) by employing both multifunctional matrices and multifunctional strengthening reinforcements. Some examples might include: A composite with a matrix containing microcapsules sensitive to thermal, electrical, or mechanical stress that on breaking would indicate the area of in-service damage to the material. Self-healing in the damaged area could then be accomplished by another family of microcapsules (as described above). The strengthening phases could be two different filaments that, in a conducting polymer matrix, could function as a battery. A microcellular structural foam used in the matrix. Some of these materials may be radar-absorbing, conducting, or light-emitting. With these, the strengthening filaments might have sonic sensing ability. Improved stealth coating. Stealth coatings today have several layers; roughness and wear are a critical problem. Composite technology, including nanoscale patterning, has the potential to create macroscale monolithic stealthy materials, removing the tremendous maintenance burden of layered structures. A photovoltaic military uniform cloth also containing kevlar body armor woven in at vulnerable places. During periods of sunlight the derived electricity could be used to maintain or replace charge in the batteries needed to power many of the devices to be carried by the infantryman of the future. Adhesives and Coatings Adhesives and coatings are used almost everywhere in military systems, most commonly to join components, seal, protect or insulate, produce patterns via photolithography, and in general to enhance the function and manufacturing of military systems. Adhesives are also used as matrix materials for all types of composites, including solid propellants and explosives as well as high-performance structural materials. Advances in the science and engineering of adhesion, including how such systems fail, will continue to yield improvements in adhesion performance and reliability. Substantial improvements over the next 20 years are expected; they
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will enable use of adhesives in ways not currently contemplated. Some likely advances and their possible applications are described here. The ability to measure and characterize the chemical and topographical features of surfaces is important. Modern methods of chemical instrumentation make it possible to quantify the types of chemical groups active on surfaces. One can then design adhesive systems to bridge gaps between surfaces with different chemical functionality. These tools have also made it possible to modify the chemistry of surfaces to promote wetting or adhesion; previous methods, such as high-energy plasma treatments or oxidation, have not permitted much control and often damage treated regions. Today, chemical modification of surfaces is extensive; applications range from surface coatings to promote coupling to chemical reactions to alter the chemical makeup of surfaces. With modern lithographic methods, surfaces can be designed with topographic features that control wetting and promote adhesion. However, not all adhesion is chemical; some types of bonding are more physical, especially in porous materials and on surfaces with complex topography or woven fibers. In many of these systems mechanical interlocking is possible. Understanding of this type of adhesion is also advancing. Progress in understanding of the failure of adhesive systems has come about largely from advances in the fracture mechanics of adhesives and improved methods of engineering analysis of adhesive systems. These methods improve understanding of the role of residual internal stresses, such as curing and thermal stresses, that can play a dominant role in many adhesive systems, as well as the response of the system to external loads and time. These advances in failure analysis, coupled with improved material characterization—including methods of representing aging—will continue to bring improvements in the reliability of all types of adhesion systems. The stiffness of many adhesive systems can greatly affect the properties of composite materials. The maximum measured compressive strength obtainable for fiber-reinforced composite materials, caused by local elastic instability and often a limiting design feature, is dictated by the shear modulus of the matrix adhesive binding the system together, as it is in laminated sheet structures. Because most polymeric glasses, such as epoxies, all have about the same shear modulus and the spatial sizes are so small, attempts to improve matrix stiffness via composite reinforcement have not been pursued. With the advent of nanosized reinforcement, such as carbon nanotubes, it should be possible to substantially improve the stiffness of matrix materials by reinforcing them at the nanoscale level,
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thereby increasing the measured compressive strength of the fiber-reinforced composite. The concept of repairable adhesives is hovering on the horizon: Adhesives are commonly used in chip attachment in microelectronics. Modern chips are so expensive that their repair in complex circuits is not only justified but necessary. This has led to the need for adhesive systems that can be repaired by selectively destroying the bond via chemical or thermal means so that damaged parts can be removed and replaced. Embedded sensors that can help quantify the life of adhesive systems, coupled with design of repairable systems, should greatly add to the reliability and use of adhesives. Significant advances in high-temperature adhesives and sealants are expected with the evolution of hybrid organic/inorganic materials that will also extend the utility of adhesive systems. RESEARCH AND DEVELOPMENT PRIORITIES This chapter discusses the most notable DoD structural materials needs. From these, the panel has extracted four broad R&D priorities. Investment in these priority areas should provide enabling materials technologies to DoD for use by 2020. Materials Design Assisted by Computation In the design of materials assisted by computation, the goal is to implement new materials by integrating constitutive models into a framework that employs FEM calculations. The first-level potentials (electron, atom, dislocation, and microstructure) have to be described to yield time, temperature, and size-dependent models of material behavior for all classes of materials, monolithic and composite. This effort will require more precise understanding of physical phenomena and better computer equipment that can extract the important data from a calculation and also analyze these first-level data. A successful effort would predict possible material properties before development costs are incurred and predict material properties and behavior so that the cost of characterization (e.g., temperature dependence of polymer-based materials) could be reduced. This would not only reduce costs but also accelerate material development, especially the introduction of new materials and materials systems into DoD systems. The best
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approach may be to pick a sample application and then assemble teams of specialists—computational materials experts, structural analysis experts, systems experts, and materials scientists—to demonstrate atoms-to-structures design. Service-Induced Material Changes As a result of the improved understanding emerging from computation-assisted design of materials it will become possible to use these materials to design better structures. Thus, the second opportunity identified by this panel involves service-induced material changes or how to use materials. The rationale for this concept is that materials and structures ought to be designed based on material functionality in actual service rather than on the material’s initial properties. This means understanding history-dependent properties and performance evolution. The science base for the use of materials must therefore be expanded, and nonequilibrium structures and materials must be better understood. This will be the basis for a science-based constitutive theory that is explicit about the extensive variables of the material. Among the benefits of such an approach would be history-based reliability prediction for individual components and replacement of “factors of ignorance” in design with deterministic model-based sensing and prediction. Other benefits are robust multidisciplinary virtual design environments that reduce cost and speed design, testing, prototyping, manufacturing, and deployment. The results would be reduced vehicle weight, enhanced performance, and fuel savings. To be successful, this effort must be complemented by efforts to predict in-service loads, which in turn would require new model-based life-cycle sensing and design methodology based on computational and phenomenological models of material state evolution. Multifunctional Composite Materials R&D on composite materials needs to advance in several areas simultaneously. The most important general advances needed are in the processing and scale-up of material combinations. Advances needed for MMCs and CMCs include adequate exterior coatings for nonoxide com-
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posites, fiber coatings to prevent fiber-matrix chemical diffusion at higher temperature, and interface control to tailor bonding between filaments and matrix. Advances needed for PMCs incorporating high-performance polymer fibers include improvements in compressive strength and matrix stiffness. Other necessary advances for composites are in the area of structural multifunctionality, for purposes of NDI/NDE integration as well as adding thermal management, actuation ability, etc. This exciting possibility employs some of the multifunctional monolithic polymers identified in Chapter 6 as the matrix phase in a structural composite. Benefits of this effort by 2020 would be improvements of 20-25 percent in strength, toughness, stiffness, density, environmental resistance, and high-temperature capability, leading to enhanced mobility, maneuverability, survivability, and transportability of DoD systems. New multifunctional composites would increase warfighter capabilities while integrating formerly discrete systems into a single package. Integrating Nondestructive Inspection and Evaluation into Design As a complement to composite material development, this panel believes that there are opportunities for integrating NDI/NDE into the original design of both materials and structures. This would allow for continuous health monitoring of all new structures. Sensors integrated into structure would have to be very small, so many new types of sensors will be needed, as will be portable advanced sources, such as X-ray and neutron sources, to allow field evaluation of structures. Some sources should be incorporated into internal structure wherever the structures will be difficult to examine with an outside source. Salient results of this effort would be real-time recovery of information from material (especially multifunctional) systems that can be used to evaluate safety, reliability, need for maintenance or replacement, or remaining strength and life. This information would support management decision-making at all stages of engineering design and development. It would depend on new methodologies for recovering, interpreting, and using information from integrated sensors or sensor arrays in material systems, and the actual recovery of such information.
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Representative terms from entire chapter: