Design,Analysis and Fabrication of a Graphite/Epoxy Electronics Enclosure Flanged Aperture,with Supporting Electromagnetic Interference Test Data

Abstract

The structural design of a rectangular metal electronics enclosure for airborne or space applications is straightforward and contained in a number of texts. However, the design of a composite electronics enclosure, optimized for weight and structural and thermal load paths, has not been addressed. In particular, the design of flanges that attach the walls to one another or the enclosure to its cover is an important topic because of the need for EMI shielding and flange structural stability in a random vibration environment. Several carbon fabric/epoxy flanged aperture (two overlapping flanges with gap) designs were considered for integration into a composite electronics enclosure. The design evaluation was based on structural integrity, EMI shielding effectiveness, ease of fabrication, development time, number of parts, total cost, and total weight. The most promising design consisted of two overlapping "L" flanges with a double EMI gasket in the aperture. This design was analyzed for instability of the flanged aperture, local buckling of the individual flanges between fasteners, and bearing and shearout of the flanged aperture. Laminate configurations used in the structural analysis consisted of [(0°)]₃, [(0°), (45°)]_s, and [(0°), (45°), 0.076 mm thick syntactic foam core, (45°), (0°)]. The preliminary analysis indicated that the [(0°)]₃ laminate configuration would fail by local buckling between flanges. The [(0°), (45°), 0.076 mm thick syntactic foam core, (45°), (0°)] laminate had the highest margins of safety in overall flanged aperture instability, local buckling, bearing, and shearout. This configuration was thought to be the best lay-up because large margins of safety would be needed in random vibration to compensate for the unknown torsional and bending loads in the flanges. EMI shielding tests on a Ni plated carbon fabric flanged aperture from 200 MHz to 40 GHz showed that the geometry of the flange alone provided significant shielding over that of an open hole. The shielding effectiveness of the Ni plated carbon fabric/epoxy flanged aperture with EMI gasket was equivalent to that of a continuous solid Ni plated carbon fabric/epoxy panel. The [(00), (450)], and [(00), (450), 0.076 mm thick syntactic foam core, (45°), (00)] laminate configurations are currently being incorporated into a finite element model of the electronics enclosure.

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