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Author: Edgar A. Starke, Jr., and James C. Williams
With the abundance of new structural materials, industrial designers and engineers are faced with an ever-growing number of choices for use in products.
Recent advances in structural materials have enabled the development of improved products that have a positive impact on our economic competitiveness, national security, and quality of life. Although the general public tends to hear of such advances only in the context of new consumer devices, high-performance sporting goods, or advanced military aircraft, the materials industry makes continual improvements that affect many fields of endeavor. During the last few decades, new analytical tools for examining microstructures, new sensing devices for process control, and improved modeling capability have increased our knowledge base and ability to tailor materials to specific applications. The improved materials have led to enhanced performance of many products, often times with a reduction in costs.
With the abundance of new structural materials, industrial designers and engineers are faced with an ever-growing number of choices for use in products. Typically, engineers use three major criteria to select materials:
A product team analyzes these criteria in conjunction with specific performance requirements to determine a product’s final configuration and materials content. Some of these decision-making processes are more rigorous than others, but all require constructive interaction between several engineering disciplines.
In recent years, a number of factors have motivated the development and use of improved structural materials and manufacturing processes, including
This article describes several recent examples of the use of improved structural materials and discusses opportunities for future development.
New Product Requirements
The evolution of aircraft has focused on the need to enhance basic capabilities, including range, payload, speed, and operating cost, all of which have been served by improvements in structural materials (Greenwood, 1989). For example, higher air speeds cause increased frictional heating, which, in turn, raises an aircraft’s skin temperature. In response, skin materials have progressed from wood and fabric to advanced alloys of aluminum, titanium, and polymer matrix composite materials containing high-strength carbon fibers. Early aluminum alloy use was hampered by exfoliation corrosion (grain separation due to humid air), but the development of aluminum-clad materials and the use of anodizing resolved this issue. As a result, high-strength aluminum alloys have been the materials of choice for aircraft for several decades. The need to enhance structural efficiency through lower weight led to further improvements in aluminum alloys, many of which were achieved via new manufacturing processes such as double aging, reversion aging, and controlled combinations of heating and mechanical deformation between quenching and aging. These practices, along with tight controls on alloy composition, have increased the strength, durability, and corrosion resistance of aluminum alloys (Starke and Staley, 1996). Today’s aluminum alloys are 1.5 times stronger than the early alloys used for aircraft skin and, taking into account inflation, have essentially the same cost.
Similar advances have been made in aircraft engine materials, where demands for more thrust and better fuel efficiency have led to higher operating temperatures, lower engine weights, and increased rotor operating stresses. Nickel-based superalloys and titanium have replaced steel and aluminum, and various processing methods have been developed to directly shape the alloys into components, including forging, investment casting, directional solidification, and single crystal production (McLean, 1995; Williams, 1995).
In military aircraft, special requirements have driven innovation in both materials and design. For example, the B-2 Spirit bomber, developed for the U.S. Air Force by Northrop Grumman, combines a large payload and exceptionally long range with low radar observability, or stealth technology. Able to carry a payload of 40,000 pounds and fly to almost any point on the globe within hours, the B-2 Spirit uses advanced polymer composite materials and special radar-absorbing surface coatings, coupled with its unique geometric design, the flying wing. Further material and design innovations were required to reduce the radar signature of the engine inlets and to minimize the infrared signature of the exhaust. The B-2 Spirit was the first aircraft program where structural materials were selected for their nonstructural properties, but this emphasis will become more common in the future.
Early commercial aircraft derived much of their technology content from military programs. Today this is less common, but new civilian aircraft systems still require innovative materials solutions to meet market demands. As anyone who has traveled from the United States to the Pacific Rim on a subsonic flight can attest, the trip is long and tiring. A faster aircraft is attractive for these routes, and market projections indicate that substantial demand exists for high-speed civil transport on such long-range flights (National Research Council, 1997b). Studies show that airplanes capable of flying at Mach 2.0-2.4 and carrying 250 to 300 passengers for distances of at least 5,000 nautical miles could have a market large enough to support development costs and compete effectively with next-generation subsonic aircraft. Meeting the constraints for such an aircraft, including strict noise and emission standards, creates challenges unlike any faced before in commercial aviation, and the engineering solution is not yet clear. However, history shows that engineering responses to such needs inevitably emerge, and the country or entity that provides a viable solution may dominate commercial aviation for a significant period of time.
Space programs offer significant opportunities to use new structural materials to improve existing systems.
For example, one consideration in building the international space station is the number of space shuttle missions necessary to transport materials and components for its assembly in space. An obvious way to reduce this number is to increase the payload per mission by reducing the weight of the shuttle system itself. Such weight reductions can best be accomplished by increasing the structural efficiency and using stronger, lighter materials. The space shuttle’s large expendable cryogenic tank, used to carry the liquid hydrogen rocket engine fuel, was selected as a weight reduction candidate. It has been known for some time that alloying aluminum with lithium decreases the material density and increases the stiffness, but only quite recently have melting and processing methods been developed to put this to practical use. The shuttle’s improved hydrogen tank uses a new aluminum-lithium alloy that is 5 percent lighter and 30 percent stronger than the aluminum alloy 2219 of the original tank. In addition, the tank was redesigned to be more structurally efficient. The combination of the new design and the new material provided a 6,800-pound reduction in weight, which translates directly into a comparable increase in the shuttle payload (Wagner, 1998).
Altering specific material properties to optimize a component is a major part of materials engineering. Properties such as stiffness or strength can be enhanced by the controlled introduction of a second phase constituent. The resulting multiphase material derives its properties from both constituent phases, and the combination, or composite, is superior to either component alone. Selective reinforcement of aluminum, titanium, and intermetallic alloys can be achieved with hard, stiff particulates such as silicon carbide, or with monofilaments of boron, silicon carbide, and carbon to obtain performance properties as much as two times better than those of unreinforced alloys.
Titanium-matrix composites containing silicon-carbide monofilaments have been shown to have high strength, stiffness, and thermal stability, and they represent a superior class of materials for use in aircraft components. In aircraft engines, these materials permit the design of smaller, more efficient rotating components and can allow for higher thrust-to-weight ratios. A number of potential applications exist for titanium-matrix composites in military turbine engines (Peel, 1996), but these composites are expensive and must be used selectively to reinforce structural components while achieving a balance between performance and cost. Typically, a numerical factor called the economic trade factor is used to define the additional cost that is acceptable to achieve a pound of weight reduction. For military aircraft the acceptable cost is higher than for civilian applications; even so, the current cost of titanium-matrix composites is prohibitively high. To achieve extensive use, the material costs must be reduced to approximately $500 per pound, down from the current $2,000 to $2,500 per pound.
Cost is less of an issue in the production of sporting goods, where new structural materials can also have a significant impact on performance. In one example, a shape-memory alloy called Zeemet has been specifically developed by Memry Corporation for golf club head inserts (NASA, 1997). Shape-memory alloys have the capability to reversibly change shape with changes in temperature. Zeemet is also superelastic and has high damping attributes that affect the dynamics between the club and the ball. When a Zeemet club insert contacts a golf ball, it undergoes a split-second change in its metallurgical structure, keeping the golf ball on the club face longer and thus supplying more spin. Anyone who has played golf can appreciate the benefit that more "bite" provides in controlling the ball when it lands.
Shape-memory alloys are also used in various medical devices, including stints, which are used to prevent the reblockage of blood vessels after angioplasty. Memry’s stints are made from Nitinol, a roughly equiatomic alloy of nickel and titanium. A stint is inserted into the region of the blockage inside the blood vessel where it is then expanded by a balloon. At this point it is in a soft, low-temperature, martensitic condition. Warm (40?C) saline solution is injected into the blood vessel to induce a phase change, stiffening the material. The phase-change temperature has enough hysteresis to prevent the stint shape from transforming back when the body temperature returns to normal.
Another application for shape-memory alloys is an anti-scald device, called MemrySafe. Tap water scald injuries have been cited as the second most common cause of serious burn injuries, often occurring in children and the elderly. With MemrySafe, a valve reacts to the water temperature, reducing the flow if the water becomes too hot and restoring the flow when the water temperature reaches a safe level.
Extending Product Life
Many aircraft operated by the U.S. Air Force are being used much longer than originally intended. Aircraft with a planned lifetime of 10-20 years may remain in service for up to four times as long, and many planes have encountered age-related problems such as fatigue cracking, corrosion, and wear. To ensure continued airworthiness and flight safety, the structural components which have high failure probability must be repaired or replaced (National Research Council, 1997a).
New alloys, materials, and processing technologies, developed since these aircraft were originally designed, are being used to produce better components with significantly lower life-cycle costs. The bulkheads and ventral fins on the F-16 fighter are good examples to illustrate this trend. One of the F-16’s three bulkheads supports the vertical stabilizer and typically fails before its specified service life of 8,000 hours. Its material is being replaced with the aluminum-lithium alloy 2097, which has 3 times the fatigue life, 5 percent lower density, and 7 percent higher stiffness than the original material, alloy 2024. Because the replacement alloy is more fatigue resistant, it decreases the frequency and cost of downtime for bulkhead replacement, at an estimated cost savings of over $76.5 million for the F-16 fleet (Austin et al., 1999).
The ventral fins located under the aft section of the F-16’s fuselage provide added stability during tight, high-speed turns, and are subject to high stresses from severe buffeting and turbulence. The conventional aluminum-alloy fins fail in less than 400 hours of flight time, and the material is being replaced with a new metal-matrix composite (MMC), an aluminum alloy reinforced with silicon-carbide particles, which is about 50 percent more stiff than monolithic aluminum. The life of the MMC fins is projected to exceed 8,000 hours -- more than 17 times the life of the original fins -- at an approximate savings of $20.7 million (Austin et al., 1999).
New Manufacturing Methods
Performance-driven applications are often the motivation for introducing new materials and processes. In the automotive industry, two factors drive improvements in fuel efficiency: regulation and competition. Early improvements in fuel efficiency were prompted by government regulation of car manufacturers’ average fleet economies. These improvements usually came with an increase in vehicle cost, which can be a disadvantage, especially in the United States where gasoline prices are low and provide little incentive to pay for a more fuel-efficient car.
In 1994 the U.S. government initiated the Partnership for the Next Generation of Vehicles (PNGV), an alliance of government agencies and the three major auto manufacturers. The aim of PNGV is to develop a midsize vehicle capable of getting 80 miles per gallon and meeting current safety and emission requirements. Since weight is a critical factor in determining gas mileage, reducing the weight of structural materials is a major thrust in this program. Based on extensive research by aluminum producers and automobile manufacturers in the 1980s, new technologies enable aluminum cars to meet all of the expectations of today’s drivers, yet weigh several hundred pounds less than their steel counterparts. The advantages of aluminum for cars, as for aircraft, are its light weight and high strength-to-weight ratio. When built of aluminum instead of steel, the weight of automotive load-bearing structures can be cut almost in half.
Ford has built 40 aluminum-intensive automobiles based on the Taurus/Sable platform for test purposes, and General Motors recently announced an aluminum - structured electric vehicle. The Audi A8 has an aluminum spaceframe structure that uses formed extrusions which are fusion welded to coupling units made by a high-ductility casting process. The structure is completed with aluminum panels, and all the closures are made of stamped aluminum sheet.
Some of the concerns associated with aluminum car construction include cost, lack of expertise in aluminum design and manufacturing, inadequate repair capability, and public perceptions about the safety of aluminum structures. None of these issues is insurmountable, but each must be addressed as new product development progresses.
Another opportunity for introducing new materials is in biomedical applications such as measuring devices, surgical instruments, and implants. Titanium alloys are often used for joint and long-bone implants because they have lower modulus that closely matches that of bone. Stainless steel or alloys of chromium and cobalt also are used extensively (Fuller and Rosen, 1986).
In the case of implants, the primary requirement is biocompatibility, which precludes the use of many familiar structural materials. The ideal implant material would thus be pure titanium, which is chemically inert, but it is too weak when produced by conventional means, and its common alloying elements are potentially toxic. However, researchers at Los Alamos National Laboratory have developed a new manufacturing method for producing nanocrystalline titanium that may replace alloys for bone implants. These ultrafine-grained nanopowder materials are light in weight, 10 times stronger than their conventionally manufactured counterparts, and superplastic, with high formability at certain temperatures. Their strength makes it possible to use hollow hip implants so as to better match the stiffness of surrounding bone (Los Alamos National Laboratory, 1996).
Another promising application for new materials is infrastructure repair. An estimated 180,000 bridges in the United States are structurally deficient or functionally obsolete, and their repair is costly, time-consuming, and disruptive. However, aluminum’s light weight, high strength, durability, and low maintenance requirements offer much promise for rapid, cost-efficient rehabilitation of bridges. One example of its use is the renovation of the Corbin Bridge in Huntingdon County, Pennsylvania. A prefabricated aluminum deck was used to replace the existing steel and asphalt deck, increasing the load limit from 7 to 20 tons (Aluminum Association, 1998). In other examples, wider aluminum decks have replaced narrower ones made of reinforced concrete. Because an aluminum superstructure is lighter, it can be wider and still use the existing bridge foundation. In another innovative application, composite wraps can be placed on the piers and supports of existing bridges for earthquake protection.
Cost Competition and Economic Security
The aerospace industry is critically important to the economic security of the United States, being the largest positive contributor to the balance of payments. Exports for this sector often exceed imports by $25 billion per year. Recently, however, the U.S. commercial aircraft industry has seen its world market share decrease steadily -- a cause for significant concern. In response to such concerns, the new Boeing 777 series airplane was developed with a market-driven strategy designed to increase the plane’s value.
The 777 has many competitive features, including long range, low operating and maintenance costs, and improved reliability, passenger capacity, and fuel efficiency. These features were achieved by combining new materials and design techniques that together contribute directly to the operator’s bottom line. To keep fuel and maintenance costs low, the 777 takes advantage of new composite materials and aluminum alloys to decrease weight and improve corrosion and fatigue resistance (National Research Council, 1996). The 777 weighs 6,000 pounds less than earlier midsize plane models. Based on the current fuel price of $0.86 per gallon, this translates to over $1 billion in savings for a typical fleet over its normal lifetime. The engines for the Boeing 777 are the quietest and most fuel efficient ever produced. These engines are quiet because of the large fan diameter, and are fuel efficient because of the higher operating temperatures in the engine and the high bypass ratio of the fan.
The 777 represents an example of a high-technology product that still must compete in the marketplace on the basis of cost. Performance improvement is limited by cost and can only be achieved if the means of performance improvement meets acceptable target cost values. Similar concerns also affect military applications, where increased emphasis is being placed on system affordability due to declining budgets, increased competition for international sales, and the desire to transfer defense technology into the private sector. For these reasons, future materials development activities must achieve both enhanced performance and improved affordability.
Future Structural Materials
The future will see a wide variety of new and improved structural materials. There will be incremental improvements and tailored properties through process simulation and modeling, including functionally graded materials and layered structures. A number of new materials products will become part of the structural materials inventory, including bulk amorphous metallic glasses, nano-structured materials, metallic forms for ultralightweight structures, multifunctional materials, and "smart" materials. There will also be a significant reduction in the time from material development to application due to the use of improved simulation and modeling and scaling from simple experiments. More faithful cost models will also help to determine the viability of manufacturing new materials before the commitment is made to enter R&D scale-up or production.
New structural materials present abundant choices for engineers and industrial designers. The product development cycle will continually become shorter requiring that design, materials, and manufacturing engineering become more fully integrated. The resulting products will be both improved and more affordable.
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