Good information about How Composites is useful as a Stealth Materials to reduce RCS
Many modern military aircraft incorporate some type of surface treatment that provides radar cross section reduction to thereby transform these aircraft into "low observable" or "stealth" airplanes. Generally, these treatments employ materials that absorb or conduct incident radar energy, and typically include adhesive bonding or spray-paint-like processes for material adherence. Where materials (e.g. caulks, paints, adhesives) requiring a wet application are used, inherent undesirable requirements include surface preparation, mixing, cure time, presence of volatiles and hazardous materials, use of personal protection devices, and acquisition of special application equipment. In addition to being quite inconvenient, the application of these materials requires an inordinate amount of time at both the manufacturing event and at any repair event in the field. Correspondingly, because of these time factors, labor expenses escalate significantly.
Electromagnetic radiation absorbent/shielding materials and structures are well-known. Such electromagnetic radiation absorbent/shielding materials and structures are commonly used in electromagnetic capability/electromagnetic interference (EMC/EMI) test cells to eliminate reflection and interference during testing. Electromagnetic radiation absorbent materials and structures are also utilized in electromagnetic anechoic chambers for testing high frequency radar, in antennas, and in Low Observable (LO) structures.
Radar absorbing material (RAM) currently in military and commercial use are typically composed of high concentrations of iron powders in a polymer matrix. These materials are both very heavy and very costly, two key limitations to their adoption for many applications. Various attempts to overcome these problems have involved the creation of artificial dielectrics, including ones based on conductive fiber-filled composites. While successful in many ways, these composites are beset by their own technical difficulties. Uniformity and consistency, critical attributes for a successful RAM, are difficult to achieve with fiber-filled composites as mixing and distribution of the fibers is opposed by the natural tendency of the fibers to clump. Also, there is not as great a cost reduction as expected with fiber-filled composites.
Composite materials have a wide variety of commercial and industrial uses, ranging from aircraft and automobile to computer parts. Composite materials have many advantages which make them attractive to different industries. For instance, composite materials can reduce heat transfer, resist conduction of electricity, limit reflection of radar waves, are flexible but strong, and can be fairly easily formed into complex shapes during manufacturing. Some examples of commercial applications include the complex shapes of certain automobiles, airplanes, and boats which would be difficult to form with metal materials. Another important use of composite materials is the creation of stealth aircraft which minimize their radar cross section through the use of radar absorbing composite materials that form the majority of the aircraft's structure.
One example of an aircraft made largely from composite materials is the F-22 Raptor, the world's premier tactical aircraft, designed and manufactured by Lockheed Martin Tactical Aircraft Systems. The Raptor's composite parts are formed with flexible graphite fibers, called a ply, that are impregnated with epoxy or BMI resins which harden when subjected to the application of heat. The uncured plies are placed on tools, each tool corresponding to a composite part of the Raptor. Thus, when the graphite resin mixture hardens over the tool, the composite part is formed with the proper shape.
A number of production techniques are available for forming composite parts. Again, using the Raptor as an example, once the plies are placed over the tool, a vacuum bag is used to hold the plies securely to the tool during curing of the resin. The vacuum bag forces the material to the tool and prevents the formation of bubbles and other material deformities. The tools are then placed in an autoclave for heating. An autoclave is essentially a large oven with the ability to precisely control the thermal energy applied to tools during curing of composite parts. An autoclave operator can monitor and control the amount of thermal energy applied to the tools to maintain a predetermined heating rate of the composite parts.
soruce : www_globalsecurity.org/military/world/stealth-aircraft-material.htm
