A composite material is composed primarily of a body constituent, or matrix, which gives it its bulk form. Additional structural constituents determine the in- ternal structure. These include flakes, fibers, particles, laminates, and fillers, as depicted in Fig. 7.19. The resultant properties of a combination of materials are determined in one of three ways. First, the net characteristics can be a simple sum- mation of the individual properties. An obvious example is a laminar composite, in which case electrical and thermal conductivity are given by a summation rule. A second way in which the properties of a composite can differ from those of the original materials is complementation. Each component contributes separate and distinct properties. Laminates and clad materials are examples. Finally, the third way is interaction. Each constituent supplements the others, and the final result is a material that has properties intermediate between those of the constituents or higher than those of both.
The four most common composites used in the aircraft industry are graphite/ epoxy, boron/epoxy, aramid/epoxy, and glass/epoxy. 17Extensive use of graphite/ epoxy is incorporated into the F/A-18 wing skins, horizontal and vertical tail fins, fuselage dorsal cover and avionics bay door, and speed brake. Graphite/epoxy is also in widespread use in commercial aircraft as well. The trend toward composites is not new; even older designs, such as the Lockheed L-1011, Boeing 727, and McDonnell-Douglas DC-10, made use of graphite/epoxy. Essentially, the entire structure of the Lear Fan 2100 is graphite/epoxy.
Scattering from a composite is determined as it is for any "pure" material; it is simply a matter of specifying/Zr, Er, tan 8,, and tan 8u. The complexity of the composite structure and constituents usually results in constitutive parameters that are at least mildly anisotropic and inhomogeneous. For example, fiber-reinforced materials have a dielectric constant that is slightly different for the electric field vector parallel to the fiber compared to the electric field vector perpendicular to the fiber. Likewise, the dielectric constant of each layer in a laminar composite can vary. Frequently, both these cases are treated by using an equivalent or effective permittivity and permeability. The electrical characteristics of several composite materials are summarized in Appendix E
The use of composites has both advantages and disadvantages insofar as the RCS is concerned. The (specular) reflection coefficients of these materials are usually less than those of metals. Traveling waves are also less of a problem because the surface impedance has a larger real part than a good conductor and the reactive part is not inductive. A disadvantage is that composites are penetrable; significant field strengths can exist in the interior of composite bodies. Interior metal structures that would normally be shielded by a conducting enclosure will scatter.
The penetration of waves through composites not only affects the RCS but also presents the potential for electromagnetic interference (EMI) between electronic systems. Care must be taken to shield and ground components properly, not only to reduce EMI but also for lightning protection. The latter usually requires embedding a mesh or wire grid inside the composite, which will also affect the RCS.