To predict the lifetime of a component, it is necessary to define the properties at the beginning of life, and also to realize that these properties will begin to change almost as soon as the part is put in service, with some changes occurring as soon as the part is fabricated if it is exposed to air or moisture.
How these microstructural changes affect macrostructural properties is important, as is the degree to which macrostructural changes can be tolerated and useful structural and other properties maintained. Many factors play a role in selecting materials for applications with specific durability requirements.
Consideration must be given to such factors as the extent of the existing database, coolant compatibility, expected time in service, and resistance to microcracking. All these factors vary with the environment. It is important to note that most experiments to establish a material's properties are usually done under ambient conditions. The variety of nonambient conditions tend to make testing in the many possible combinations unreasonably expensive.
For this reason, experimental approaches are not viewed as a good way to establish reliability in every possible combination of conditions. Instead, sophisticated modeling approaches, verified and validated through the judicious use of a small amount of intelligently designed data collection, are more likely to succeed.
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While adequate numbers of composite materials are available today, there is always a need to design better directionally specific properties in to the material. A stronger, lighter, more durable composite is always wanted.
New types of reinforcements with effects on the nanoscale, new methods of processing, and new chemistries and structures are all promising ways to improve properties. PMCs are desirable primarily because they provide lightweight and high-strength materials solutions. See, for example, Figure , which compares the specific strength-specific stiffness of polymer composites and monolithic metallic materials.
Potentially more important than specific strength,. A multifunctional structural material may be self-interrogating 5 , 6 , 7 or self-healing, 8 , 9 or it may provide stealth or protect against enemy fire. Multifunctional materials might combine sensing, moving, analyzing, communicating, and acting, in addition to providing structure.
Flexibility in processing composite materials will be needed to enable these advances. This trend can be seen in the amount of PMCs used in fighter. Zhao and H.
Phases in composites
Two-dimensional strain mapping in model fiber-polymer composites using nanotube Raman sensing. Sensing with carbon fibres in polymer composites.
Materials Science Research International 8 2 Sittner and R. Developing hybrid polymer composites with embedded shape-memory alloy wires. JOM 52 10 White, N. Sottos, P. Geubelle, J. Moore, M. Kessler, S. Sriram, E. Brown, and S. Autonomic healing of polymer composites.
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Nature ; Correction: Nature Brown, S. White, and N. Microcapsule induced toughening in a self-healing polymer composite. Journal of Materials Science 39 5 In the s, for example, fighter aircraft such as the F, F, and F had about 2 to 4 percent by weight carbon-epoxy composites. Similar trends are shown in Figure for commercial aircraft. Typical PMC applications in both commercial and military aircraft have been horizontal and vertical stabilizers, small fuselage sections, and some wing skins see Figure Currently this usage is approaching 70 percent on the Eurofighter and other advanced aircraft, with entire airframes and support structures being constructed of PMCs.
Commercial aircraft currently under development such as the Airbus A and the Boeing are projecting extensive usage of PMCs throughout the aircraft see Figure Similar trends can be seen in rotor aircraft. Whereas PMCs were used for rotor blades and side conformal fuel tanks in the s and s, they now make up the whole airframe of the V Successful application of these materials has increased the structural capability of aircraft and reduced weight, resulting in better performance.
As applications and materials mature, they continue to be considered for even more extreme applications. The combination of light weight and stiffness makes PMCs attractive candidates for structural components in both military and civilian applications. In aerospace, where thin structures and complex shapes are needed to save weight, the ability to create a unitized structure with integral stiffeners and skin significantly reduces the number of parts and fasteners.
While PMCs tend to be more costly to fabricate because of a large amount of hand labor, innovations in automated tape lay-up machines, such as improved heads and expanded databases, and the application of these machines on primary structures are helping to reduce this cost.
PMCs are becoming competitive with metallic structures in part due to unitized structures, which are more easily produced from PMCs. An example of this can be seen in the redesign of the C horizontal stabilizer. This critical component of the tail is approximately 7 feet wide and 35 feet long and was historically constructed of aluminum. Redesigning this component to use PMCs resulted in a 20 percent savings in weight, a 69 percent reduction in tooling, and an 81 percent reduction in the number of fasteners, for a net cost saving of 48 percent over the aluminum component.
Fewer fasteners has the added benefit of reducing sources of corrosion as this is the area where corrosion most frequently occurs. Redesign using PMCs and cast aluminum reduced the total to almost 15,, leading to a 53 percent savings in both weight and in labor hours. Miller and K. It is believed, for example, that in some resin systems, entrapped water turns to steam and can result in debonding. However, if the temperature capability of these materials could be increased at a reasonable cost, the application window would be significantly larger an example is shown in Figure Advanced materials have given designers some but not yet all the solutions they need.
Examples of applications where PMCs could be used more are control flaps and structure in airplanes that receive heating from engine exhaust wash. These are currently made of titanium. Because they are control flaps, they must bear aerodynamic loads similar to the wing. Research programs in progress are investigating the feasibility of making these structures out of oxide ceramics; however, a low-cost PMC with a protective coating could be an attractive alternative.
Engine nacelles of transport aircraft are typically made of aluminum with thick insulation blankets to keep the aluminum structure from overheating. With the development of more efficient, higher-thrust engines, the operating temperature of these nacelles is increasing. This results in increased insulation and structure and, therefore, increased structural weight of the entire system.
An alternative would be to make them from titanium, but this leads to an increased weight penalty, and increased weight reduces both range and payload, resulting in diminished capability. Key design requirements include impact resistance, acoustic and vibration loads, and chemical compatibility with lubricants. In vehicles with speeds of Mach 3 or higher, airframes and engine ducts are typically constructed of titanium or superalloys to withstand exposure to the aerodynamic heating of components and the engine heat.
The aeroshell, which includes the leading edges and control flaps, must also be capable of all-weather operation and must withstand erosion from rain, hail, and airborne particles. Engine ducts must also be resistant to erosion by the. This is currently designed using a titanium alloy and might be replaceable by a higher-temperature-capable PMC.
Typical properties needed in these applications are strength, thermal shock resistance, and fatigue resistance.
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Thus, the typical composite can be characterized as inhomogeneous and anisotropic. The astute reader will know of many other materials that can also be characterized as inhomogeneous and anisotropic at some length scale; for example, polycrystalline metals by virtue of their grain size and orientation. The difference between typical structural PMCs and polycrystalline metal components such as steel I beams or superalloy turbine blades lies in the degree of anisotropy and degree of heterogeneity. Similarly, the ratio of coefficients of thermal expansion of the two phases is typically to 1 with the matrix being the larger.
These disparities mean the composite material is a complex structure that will display large anisotropic effects at the microstructural level—that is, at length scales comparable to the fiber diameter. Further, the extreme differences in properties of the two phases make the interfacial bonding between the phases enormously important.