| Microwave absorbing materials as a main method to accomplish low observable technology have received much attention recently. Meanwhile microwave absorbing materials is also considered promising in civil fields for its extensively application as electromagnetic interference shielding materials. Fiber reinforced microwave absorbing composites is one kind of advanced composites material with desirable mechanical properties and high temperature resistance. And now it has become a candidate selection for special use of stealthy aircraft.Two kinds of typical structures for fiber reinforced microwave absorbing composites with parallel and plain fabric fibers were investigated in this thesis. By mean of FEM numerical simulation, the effect of structural and electromagnetic characteristics on the microwave reflection of composites were systematically analyzed. The location where the energy loss occurs and its mechanism were discussed. In order to analysis the absorption performance of materials when oblique incident electromagnetic wave irradiate, FITD method is selected for RCS prediction.Based on sufficient analysis to the geometrical characteristics of the parallel and plain structures, two-dimensional model with periodic boundary condition is used to simulate the microwave reflection of parallel structure, while three-dimensional model with perfect electric conductor and perfect magnetic conductor boundaries were applied to plain structure. The reflection and RCS indexes of composites were selected to characterize the microwave absorption performance, and the basic calculation theories were clarified. Benchmark parameters for reflection and RCS calculation of materials were selected.Except for some similarities on reflection change rules between the parallel and plain structures, the results shows that plain structure has more variability than parallel structure,for plain structural material is formed by two part of fibers that one is parallel to electric field and the other is vertical. It is showed that the distance between fibers parallel to electric field has effects on the width and depth of both two frequency absorption peaks,while the distance between vertical ones only affect the position of higher frequencyabsorption peak. When the fibers parallel to electric field has an electric conductivity of 40S/m and vertical ones has an electric conductivity of 20 S/m, a relatively better absorption performance of composites will be implemented. And plain structure has a better performance on polarization stability, while its RCS index is a little higher than parallel structure.The distribution and mechanism of energy loss in parallel and plain structural composites were analyzed. Results showed that the main part of energy loss is contributed by the induced current in fibers. And the displacement current in parallel structure reduces with the increasing of electromagnetic wave propagation depth, while the displacement current of plain structure exists mainly at the interface between fibers and substrates.The computational electromagnetic modeling and analysis of presentative parallel and plain composite structure is helpful to optimization and comprehensive modification for fiber reinforced microwave absorbing composites. It gives some important guidelines for the application and design of microwave absorbing structures. |
