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Web Content Editor Prof. Structural composites are of interest in aerospace applications and in numerous industrial and consumer uses in which light weight, high strength, long fatigue life, and enhanced corrosion resistance are critical. Much needs to be done to advance processibility and durability, to provide a more comprehensive database, and to improve the economics of these systems.
A wide range of future needs encompasses synthesis, characterization, processing, testing, and modeling of important polymer matrix composite systems. In general, the future of polymer matrix composites is bright. The engineering community is now in the second generation of applications of composites, and primary structures are now being designed with these materials.
There is a growing confidence in the reliability and durability of polymer composites and a growing realization that they hold the promise of economic as well as engineering gain. Commercial programs such as high-speed civil transport will not succeed without the use of polymer composites. Integrated synthesis, processing, characterization, and modeling will allow the use of molecular concepts for the. A more precise understanding of the manufacturing, processing, and component design steps will greatly accelerate the acceptance of these advanced materials.
New horizons for properties and performance, for example, in smart and intelligent materials, actuators, sensors, high-temperature organic materials, and multicomponent hybrid systems, will involve the potential of introducing a new age of economic success and technical excellence. Advanced polymer matrix composites have been used for more than 20 years, for example, on the B-1 bomber and for many top-of-the-line Navy and Air Force jet fighters. For military purposes, the high performance and stealthiness of composites have often outweighed issues of durability and even safety.
Building lighter, more maneuverable tanks, trucks, and armored vehicles might be an area for future military growth.
- Mathematical elasticity. 3-dimensional elasticity.
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However, as the Pentagon's budget shrinks, efforts to transform these materials into civilian uses are under way Pasztor, Problems include the need to identify significant nondefense companies that will use advanced composites. For nearly 30 years, it has been suggested that aircraft designers around the world would rapidly utilize these new materials. Unfortunately, those predictions have not been realized, and U.
For a number of reasons, there is continued reticence to employ these advanced materials in many areas, particularly in commercial aviation. Costs, processibility, and durability appear to be the major issues.
To this point, this area has been considered a technical success but not a financial success. Nevertheless, aircraft in various stages of development have composites as some fraction of their structural weight. For example, 15 percent of the Boeing , 6 percent of the MD Trijet, and 15 percent of the MD are estimated to be composites.
European aviation firms have begun flight-testing an all-composite tail rotor for a helicopter, and Japanese efforts are under way to develop a military helicopter that has a very high composite content. It has been predicted that in the future, fiber-reinforced composites FRCs will partially replace conventional materials in civil engineering applications. These could include buildings, bridges, sewage and water treatment facilities, marine structures, parking garages, and many other examples of infrastructure components.
Composite materials are also expected to help replace conventional materials such as steel and concrete in many future projects. The polymer matrix resin composites discussed above have already made inroads in areas such as antenna coverage and water treatment plants. Less expensive fiber-reinforced.
Sheet molding compounds, which are used extensively in automobiles and housing, are not considered by many structural engineers to be suitable for infrastructure replacement owing to their relatively low strength. Advanced polymer composites, on the other hand, which often consist of continuously reinforced fiber materials, have superior strength and stiffness. The liquid crystalline nature of stiff polymer molecules in solution was predicted by Onsager in , further refined by Flory in , and experimentally verified through aramid investigations at the Du Pont Company in the s.
Flory suggested that as the molecular chain becomes more rodlike, a critical aspect ratio is reached, above which the molecules necessarily line up to pack efficiently in three dimensions. Liquid crystal polymer concepts have been extended to encompass a vast number of homopolymer and copolymer compositions that exhibit either lyotropic or thermotropic behavior.
Industrially, most of the effort has been focused on the main-chain nematic polymers. These polymers combine inherently high thermal and mechanical properties with processing ease and versatility. Processing ease originates from the facile way that molecular rods can slide by one another, the very high mechanical properties come from the "extended chain" morphology present in the solid state, and the thermal stability derives from the highly aromatic chain chemistry. Inherent in this structure is a high level of structural, and hence property, anisotropy for example, the axial modulus is 1 to 2 orders of magnitude higher than the transverse modulus.
The direction of molecular chain orientation is coincident with the direction of covalent bonding in the chain; normal to the orientation direction the bonding is secondary van der Waals, hydrogen bonding, and so on. Low orientation in these materials means global but not local randomness, and properties within "domains" are highly anisotropic.
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A useful spin-off of the study of liquid crystal polymers was the recognition of the importance of mesophases in the development of structure in conventional polymers. Examples of this include the stiffening of polyimide backbones to reduce the expansion coefficient and improve processibility and the recognition of the importance of a pseudo-hexagonal rotator, transient nematic phase in the crystallization of oriented polymer melts.
Increasing the end-to-end distance of conventional polymers through the application of either mechanical or electromagnetic fields can lead to the formation of structure equivalent to that achieved by the manipulation of molecularly stiff molecules. Fibers from lyotropic para-aramid polymers Figure 3. The fibers are dry-jet wet spun from percent sulfuric acid solution with sufficient. An annealing step may be performed to improve structural perfection, resulting in an increase of fiber modulus.
These fibers have very high modulus and tensile strengths as well as excellent thermal and environmental stability. Weaknesses include low compressive properties endemic with all highly uniaxially oriented polymers and a significant moisture regain. Worldwide fiber production capacity is about 70 million pounds Selling prices vary according to grade i. Consumption worldwide in was about 50 million pounds, somewhat trailing capacity.
Major markets include reinforcement for rubber and composites, protective apparel, ropes and cable, and asbestos replacement. The use of para-aramid fiber is projected to grow at greater than 10 percent per year worldwide over the next 5 years. The environmental issues involved in the handling and disposal of large quantities of sulfuric acid or other solvents may make thermotropic approaches more attractive in the future.