Although the absorbing material has good absorbing performance, only one or one layer of structure absorbing material can not achieve the effect of complete stealth, so a multilayer structure of absorbing material should be designed. A complex honeycomb structure developed by Lockheed is composed of seven layers: the first layer is a surface layer made of glass cloth impregnated with epoxy resin, and the second to fifth layers are composed of glass cloth, absorption layer and The honeycomb absorption layer is made of honeycomb, the sixth layer is honeycomb, and the seventh layer is thin aluminum plate. The layers are bonded with epoxy resin. This multilayer material not only has sufficient rigidity, strength and high temperature resistance, but also has low quality. It is suitable for the stealth skin of aircraft. The following discusses the possible structural design of structural absorbing materials.
(1) Corrugated board sandwich structure
The corrugated board can be made of structural wave-absorbing material, or it can be coated with wave-absorbing paint. The corrugated board is a structure in which two inclined planes intersect, which is conducive to multiple wave absorption.
(2) Pyramid sandwich structure
The pyramid as the interlayer is a structural absorbing material, and it can also be coated with absorbing paint. The four slopes of the pyramid intersect, and the height of the pyramid (the thickness of the absorber) is different, and the effective absorption range is different. The apex angle of the pyramid sandwich structure is preferably about 40°.
(3) Honeycomb sandwich structure
Honeycomb manufacturing is relatively mature, and you can consider applying absorbing coatings on the sandwich core, or using structural absorbing materials to make honeycombs.There are many types of honeycomb shapes, and you should choose a shape that is beneficial to absorbing.
(4) Filling structure of absorbing material
The honeycomb sandwich structure of the wave-transmitting material is filled with a wave-absorbing material. The wave-absorbing material can be flocculent, foamed, spherical or fibrous, and hollow spheres are more effective as absorbers.
(5) Multilayer absorbing structure
The upper part is the honeycomb, and the lower part is the absorbing material. As shown in Figure 2-36, the honeycomb is made of wave-transmitting material, and the absorbing material adopts a multi-layer structure.
(6) Add a wave-absorbing layer structure in the ply
The composite material is sandwiched into the absorbing layer in the composite material layer to make the structure absorbing material.
(7) Bonding or mechanical connection structure
Use bonding or mechanical means to sandwich the pre-prepared structural absorption diagram 237 into the layered wave absorbing material and combine the composite material into a layered body, and the total thickness is controlled at 1/2 of the radar wavelength (according to the lowest frequency in the structure In the structural design of absorbing materials, it is difficult for a single structural form to achieve complete stealth, and a comprehensive design of multiple structural forms can be used to achieve the best absorbing effect.
(8) Matters needing attention in structural design
① When the absorbing layer is applied to an aircraft, its mass and volume will be strictly limited. It is very difficult to meet the requirements of broadband absorbing performance under this condition. Radar Absorbing Structure (RAS) is a multifunctional composite material, which can not only absorb radar waves, but also has the advantages of light weight and high strength as a structural part. RAS provides a certain leeway for impedance matching design in terms of thickness, and is a promising absorbing material. Of course, the design of RAS must consider both electrical and mechanical properties to meet the design requirements.
②The influence of the dispersion effect. The magnetic permeability and permittivity of the material both have the frequency dispersion effect, which is called the electromagnetic parameter, which changes with the surface of the frequency. Compared with the dielectric constant, the magnetic permeability has a more obvious dispersion effect. The dispersion effect is an inherent characteristic of the material. The study of the dispersion effect is of practical significance for broadening the frequency band and improving the accuracy of the design.
③ Absorption characteristics of oblique incident waves. RAM is rarely strictly perpendicular to the incident direction in practical applications. Some RAMs have good absorption perpendicular to the incident direction, and the performance deteriorates sharply as the incident angle increases. Therefore, it is necessary to calculate and test RAM under oblique incidence. The absorption characteristics of the product, and use it as one of the indicators for evaluating and testing RAM. Theoretical calculation shows that whether it is horizontal polarization or vertical polarization, the law of microwave absorption performance with the angle of incidence is similar. Oblique incidence has a critical incidence angle of 00. If the incidence angle is 0<00, the reflection coefficient in the reflection direction is approximately unchanged, or smaller than that of normal incidence; if 0>00, the reflection coefficient increases with the increase of 0 Get bigger. In addition, calculations show that the absorption performance of vertically polarized waves is greater than that of horizontally polarized waves, and the oblique incidence performance of the double-layer material is better than that of the single-layer material.