Advanced Composite Materials and Electromagnetic Shielding

In this paper we consider a general electro­ magnetic characterization of advanced composite panels of fiber-reinforced epoxy with or without a bonded-mesh screen embedded in one surface. It is shown that over the frequency range characteristic of the nuclear EMP or lightning, composite panels whose fibers do not possess a preferred orientation may be modeled as uniaxially anisotropic conductive or dielectric media, and that the bonded-mesh screen may be modeled as an equivalent sheet impedance operator which has both resistive and reactive components and which is spatially dispersive. We also present the results of time-domain shielding calculations involv­ ing planar composite panels and cylindrical shells of advanced composites. It is shown that, geometrical considerations aside, a graphite-epoxy composite has a low-pass or integrating characteristic, while a screened poorly conductive composite has a high-pass or differentiating characteristic. Closed regions shielded by composites tend to act as time domain integrators of signals passing from the exterior to the interior in the frequency range below the inter­ nal cavity resonances. Thus the total transfer characteristic between the fields outside and inside a closed region may be understood as approximately representing attenuation and a double integration for conductive (e.g., graphite-epoxy) composites, or simply attenuation for screened nonconductive com­ posites .