C H A P T E R T H R E E
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) ad- dressed 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
Structural and Multifunctional Materials
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 materi- als.” 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 under- stand 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 improve- ments 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.
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 applica- tions. 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- 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
Environmental High Temperature Strength Toughness Stiffness Density Resistance 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.
Structural and Multifunctional Materials
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 materi- als, materials with self-healing abilities, materials with thermal or electrical conductivity spanning the range from conductors to insulators, and ad- vanced 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 materi- als 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
1Westbrook, J.H., Internal General Electric Report, General Electric, Schenectady, NY, 1962.
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 $4.00 per gallon.2 As noted in Appendix C, these calculations must be fine-tuned to align with factors such as the speed at which the object moves and the complexity of the structure fabricated. For example, the value of a pound saved in the rotating part of the gas turbine in an airplane is 10 times the value of a pound saved in the fuselage. Also, materials costs for complex composite structures are as little as 2-5 percent of the total fabricated cost.
The importance of reducing materials weight is that all vehicles, engines, and aircraft can be lighter and require less energy to run, thus saving fuel or enabling them to carry larger payloads. For military vehicles, fuel savings are especially critical. Figures from the U.S. Army indicate that TABLE 3-2 Market for Structural Materials
Value of Pound of
Weight Saved Over Number of Units
Life of Vehicle ($) Sold Per Year Market Size ($ billion)
Automobiles and trucks 2 30 million 600
Commercial aircraft 200 2,500 150
Spacecraft 20,000 100 20
2P. Bridenbaugh, Alcoa (retired), private communication, October 11, 2001.
Structural and Multifunctional Materials
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 impor- tant 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 in- creased above that required to achieve the specified design strength. In the design of these structures, the most important quantity is the elastic modu- lus. 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 substitut- ing one material for another does not change this (ASM, 1961).
One area where elastic modulus is critical is in the design and con- struction 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 wind- ings. 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.
In monolithic materials, stiffness is very difficult to change by alloying and is hardly affected by microstructure. In fact, alloying and heat treat- ment 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 simulta- neously. 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 materi- als, 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
Structural and Multifunctional Materials
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 centi- grade 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 condi- tions provide additional challenges, including ingestion of debris. Thus, the ability to absorb damage without compromising safety is a crucial charac- teristic. 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 tempera- tures because elements added to strengthen and stabilize grain boundaries lowered the alloy melting temperature. More recently, new higher tem- perature 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
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 manufactur- ing 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 proper- ties 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