Analytic techniques for sizing the walls of advanced composite electronics enclosures

Abstract

Electronic enclosures for space or avionics application can be designed using laminated composites to reduce weight, provide a modular design that has equal or better thermal and mechanical performance, and has a lower cost per enclosure than the standard “black” aluminum design. Initial sizing of an enclosure to determine the number of plies and ply orientation can be accomplished by subdividing the structure into simple shapes and analytic closed-form equations used to calculate bending stresses and deflections, uniaxial and shear buckling allowables, and natural frequencies. This initial sizing was performed on a three-sided enclosure with integral mounting flanges . The walls were analyzed using static equivalent of random vibration loads in closed-form analytic approximate or exact equations and compared with those using finite element analysis (FEA). Depending on the degree of orthotropy, i.e., how close the off-diagonal flexural stiffnesses are to zero, the analytic predictions for laminae stresses vary with finite element results . Two different hybrid PAN/pitch fiber/epoxy laminates and a carbon fabric/epoxy laminate with varying degrees of orthotropy were chosen for comparison. The margins of safety for the analytic results was within 5% of the FEA results for the orthotropic laminate but was different by factors of 1.5 to 13 for the non-orthotropic laminates. There was good comparison between analytic solutions and FEA for buckling, natural frequency, deflection, and stresses. In all cases the analytic predictions were conservative. These analytic equations were used for initial sizing of an enclosure, and a detailed FEA was performed on the electronics enclosure under actual random vibration loads. The final enclosure was fabricated and tested under these random vibration loads.