Method for evaluating influence of airplane wave-absorbing coating damage on low scattering performance of airplane wave-absorbing coating

Abstract

The invention provides a method for evaluating the influence of damage to a wave-absorbing coating of an aircraft on the low scattering performance of the wave-absorbing coating, and belongs to the field of material performance evaluation.The method comprises the steps that regional decomposition is conducted on the aircraft, the aircraft is divided into a plurality of regions according to parts, each region is further decomposed into a plurality of sub-regions, and therefore targeted analysis can be conducted on damage of different parts; the method comprises the following steps: pre-estimating the low scattering performance of an aircraft wave-absorbing coating before damage, and calculating the radiation sectional area of each region by establishing a radiometer radiation sectional area model, so as to determine the low scattering performance reference of an aircraft in an undamaged state; carrying out radiation calculation on the passive millimeter wave absorbing coating; implementing brightness temperature imaging simulation of the passive millimeter wave absorbing coating; the low scattering performance of the airplane wave-absorbing coating after damage is evaluated, the influence of damage on the low scattering performance is quantified through a low scattering effect concept, and a basis is provided for repair decision making.

Description

Technical Field

[0001] The present invention belongs to the field of material performance evaluation, and in particular relates to a method for evaluating the influence of damage to an aircraft radar-absorbing coating on its low-scattering performance. Background Art

[0002] Stealth technology, also known as low observable technology, has been applied to a variety of information platforms and is crucial for enhancing their survivability. Current stealth technologies primarily include radar stealth, infrared stealth, visible light stealth, acoustic stealth, and laser stealth. Radar stealth technology has become a research priority for major military powers worldwide, with numerous aircraft featuring radar stealth capabilities developed. Aircraft stealth technology is primarily categorized as passive and active stealth. Passive stealth involves shaping and absorbing materials. While maintaining the aircraft's aerodynamic shape, the application of absorbing materials can significantly enhance an aircraft's radar stealth capabilities. Currently, typical stealth aircraft worldwide are coated with absorbing materials. However, these coatings are subject to environmental wear, impact, and aging during storage, training, and transportation. This alters the coating's physical and chemical properties, weakening or even eliminating its radar stealth capabilities.

[0003] Damage to an aircraft's radar-absorbing coating, such as from wear, collision, and aging, can affect the aircraft's low-scattering capability to a certain extent. Severe damage requires repair. Before repairing the absorbing coating, the aircraft must be inspected and assessed for damage. There are various methods for detecting damage to aircraft absorbing coatings, with the most common being active detection methods such as ultrasonic technology, eddy current technology, radiography, and infrared thermal imaging. Active detection methods involve transmitting a signal to a target, receiving the reflected signal, and analyzing it to determine damage information. In fact, there is also a category of passive detection methods, which utilizes signals emitted by the target itself for damage detection. A representative example is passive millimeter-wave imaging technology. Passive millimeter-wave imaging receives millimeter-wave radiation emitted by the object itself and can be used for objects with relatively high temperatures.

[0004] Millimeter-wave imaging utilizes electromagnetic waves in the millimeter-wave band, with wavelengths between 1 and 10 mm and frequencies between 30 and 300 GHz. Due to its strong penetrating power, millimeter-wave imaging can penetrate clothing, plastic, and walls, making it commonly used in security screening, such as airport body scans, as well as in military applications such as detecting hidden targets and damage to absorbing coatings. While research on passive millimeter-wave damage detection of flat-plate coatings has been reported, relatively little research has been conducted on the application of passive millimeter-wave radiation to inspect aircraft absorbing coatings. Research on the impact of passive millimeter-wave radiation damage on aircraft low-scattering performance has not been reported. Therefore, it is crucial to continue developing methods to assess the impact of damage to low-scattering aircraft coatings on low-scattering performance using passive millimeter-wave radiation. Summary of the Invention

[0005] To solve the above technical problems, the present invention provides a method for evaluating the impact of damage to an aircraft's radar-absorbing coating on its low-scattering performance. This method, based on passive millimeter-wave radiation detection technology, studies the detection and evaluation of damage to aircraft's radar-absorbing coatings, establishes a method for evaluating the impact of coating damage on the aircraft's low-scattering performance, and provides a basis for aircraft radar-absorbing coating repair technology.

[0006] In order to achieve the above object, the present invention adopts the following technical solutions:

[0007] A method for evaluating the effect of damage to an aircraft radar-absorbing coating on its low-scattering performance comprises the following steps:

[0008] Step 1: Decompose the aircraft into regions according to its components. Each region is further decomposed into several sub-regions to conduct targeted analysis of damage in different parts.

[0009] Step 2: Calculate the radiation cross-sectional area of ​​each region and estimate the low scattering performance of the aircraft's radar-absorbing coating before damage;

[0010] Step 3: Consider the impact of external factors on emissivity and combine it with the reflectivity calculation formula to determine the millimeter wave emissivity received by the radiometer;

[0011] Step 4: Implement brightness temperature imaging simulation of passive millimeter wave absorbing coating based on brightness temperature tracking method;

[0012] Step 5: Evaluate the low-scattering performance of the damaged aircraft radar-absorbing coating and quantify the low-scattering effect C. The low-scattering effect C is used to represent the impact of the damage on the low-scattering performance, providing a basis for repair decisions.

[0013] Beneficial effects:

[0014] This invention uses a passive millimeter-wave radiation imaging simulation method to detect damage to an aircraft's radar-absorbing coating. The aircraft is decomposed into 13 regions, each of which is further divided into 25 to 30 subregions. For each region, a model of the aircraft's millimeter-wave radiation cross-sectional area is established. The emission and reflection characteristics of millimeter waves on the radar-absorbing coating are used to construct a calculation model for the millimeter-wave emissivity of the radar-absorbing coating. Combined with the brightness temperature tracking method, brightness temperature simulations and scattering patterns are generated for the aircraft scene, aircraft region, damaged subregion, and undamaged subregion. The millimeter-wave radiation cross-sectional area of ​​the damaged aircraft is calculated, and the concept of a low-scattering effect is proposed. This low-scattering effect is used to assess the impact of damage on the aircraft's low-scattering properties, providing technical support for the field of radar-absorbing coating repair. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Figure 1 The present invention is a flow chart of a method for evaluating the impact of damage to an aircraft radar-absorbing coating on its low-scattering performance.

[0016] Figure 2 This is a schematic diagram of the typical aircraft area decomposition.

[0017] Figure 3 This is a flow chart of the brightness temperature tracking method.

[0018] Figure 4 It is a schematic diagram of the ray tracing model.

[0019] Figure 5 This is a diagram showing the impact of regional damage on the aircraft's low-scattering performance. DETAILED DESCRIPTION

[0020] In order to make the objectives, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only intended to illustrate the present invention and are not intended to limit the present invention. In addition, the technical features involved in the various embodiments of the present invention described below may be combined with each other as long as they do not conflict with each other.

[0021] like Figure 1 As shown, a method for evaluating the impact of damage to an aircraft radar-absorbing coating on its low-scattering performance of the present invention comprises the following steps:

[0022] Step S1: Regional decomposition of typical damaged parts of the aircraft;

[0023] Step S2: Estimation of the low scattering performance of the aircraft's radar absorbing coating before damage;

[0024] Step S3: passive millimeter wave absorbing coating radiation calculation;

[0025] Step S4: Brightness temperature imaging simulation of passive millimeter wave absorbing coating;

[0026] Step S5: evaluating the low scattering performance of the aircraft's radar absorbing coating after damage.

[0027] Preferably, in step S1, a typical aircraft is selected and divided into 13 regions according to components, namely, wings, tail, fuselage, navigation lights, vertical tail, horizontal tail, air inlet interior, radar cabin, air inlet lip, cockpit, wing leading edge, wing upper surface, and wing lower surface (corresponding to Figure 2 13 circles), such as Figure 2 As shown, each region is further decomposed into 25~30 sub-regions.

[0028] Preferably, in step S2, the radiation cross-sectional area of ​​the radiometer is introduced for each area:

[0029] ;

[0030] in, Indicates the radiation cross-sectional area of ​​the radiometer in each area before coating damage, is the regional radiation cross-sectional area, Represents the brightness temperature difference between the area and the background environment, expressed as:

[0031] ;

[0032] in, is the brightness temperature of the region, is the brightness temperature of the background environment.

[0033] Low scattering performance of aircraft radar-absorbing coatings before damage It can be expressed as:

[0034] ;

[0035] Where i represents the i-th region, represents the radiation cross-sectional area of ​​the radiometer before the coating is damaged in the i-th region, is the radiation cross-sectional area of ​​the i-th region, is the brightness temperature difference between the ith region and the background environment.

[0036] Preferably, in step S3, the emissivity of the target being measured is usually affected by both internal factors and external factors. Internal factors include the target's own roughness, absolute temperature, and dielectric constant, etc., and external factors include the observation direction, wavelength, and polarization of the radiometer. Here, the influence of internal factors is ignored and only external factors are considered.

[0037] When electromagnetic waves are incident on the surface of an object, absorption, reflection, and projection phenomena will occur. Assuming that the object is in a thermal equilibrium state, in this state, the emissivity of the object is equal to the absorptivity. At this time, the reflection, projection, and emission of the object surface satisfy the normalized relationship, that is:

[0038] ;

[0039] in, is the emissivity, is the reflectivity, is the transmittance. Here we ignore the effect of transmission, that is, , so the emissivity can be simplified to:

[0040] ;

[0041] For the reflectivity of smooth surfaces, Fresnel reflectivity can be used To calculate:

[0042] ;

[0043] in, is the incident angle of the electromagnetic wave on the surface of the absorbing coating, Represents the angle of refraction in the coating. and represent the magnetic permeability and relative complex permittivity of the absorbing coating, respectively.

[0044] Therefore, the millimeter wave emissivity received by the radiometer is for:

[0045] .

[0046] Preferably, in step S4, the brightness temperature tracking method is used for the radiation imaging simulation. Under high-frequency conditions, electromagnetic waves can be considered local plane waves. Geometric optics analysis methods can be used to analyze the propagation and scattering paths of electromagnetic waves. The brightness temperature tracking method, based on geometric optics analysis methods, analyzes the propagation trajectory of brightness temperature in the radiation scene.

[0047] The process of brightness temperature tracking method is as follows Figure 3 As shown, first, the scene modeling is performed to establish a scene model for electromagnetic wave propagation and scattering. The scene model needs to restore the environmental background, perform parameter assumptions, and generate rays. Next, reverse ray tracing is performed. Starting from the receiving point in the scene, ray scanning is performed within the set field of view, and the path of the ray is traced in reverse. After reflection and transmission, it is traced to the emission source of the electromagnetic wave and the propagation path information of the electromagnetic wave is obtained. Then, based on the obtained propagation path and other information, an inversion operation is performed to establish an inversion model and perform brightness temperature calculation. Finally, the simulated data is analyzed and visualized (i.e. Figure 3 Visual extraction of results in

[15] , including brightness temperature simulation maps and scattering patterns of the scene, aircraft area, damaged sub-area, and undamaged sub-area.

[0048] Preferably, in step S5, each sub-region of the 13 regions of the aircraft is divided into a damaged region and an undamaged region, and a radiometer radiation cross-sectional area is introduced for each sub-region:

[0049] ;

[0050] in, represents the radiometer radiation cross-sectional area of ​​each sub-area after coating damage, is the radiation cross-sectional area of ​​the damaged area, Represents the brightness temperature difference between the damaged area and the undamaged area:

[0051] ;

[0052] in, is the brightness temperature of the damaged area, is the brightness temperature of the undamaged area.

[0053] Low scattering performance of aircraft radar-absorbing coating after damage It can be expressed as:

[0054] ;

[0055] Where i represents the i-th region, and j represents the j-th subregion of the i-th region.

[0056] In order to characterize the effect of coating damage on the low-scattering performance of an aircraft, the present invention introduces the low-scattering effect C (in dB), which can be expressed as:

[0057] ;

[0058] Among them, the smaller the value of C, the smaller the impact of coating damage on the low scattering performance of the aircraft.

[0059] Example:

[0060] This embodiment provides a method for evaluating the impact of damage to an aircraft's radar-absorbing coating on its low-scattering performance. The aircraft is divided into 13 regions according to its components, and each region is further decomposed into 25 to 30 sub-regions. For each region, an aircraft millimeter-wave radiation cross-sectional area model is established. Based on passive millimeter-wave imaging technology and brightness temperature tracking, brightness temperature simulation maps and scattering patterns of the aircraft scene, aircraft region, damaged sub-region, and undamaged sub-region are established. The aircraft millimeter-wave radiation cross-sectional area before and after damage is calculated, and a low-scattering effect is proposed to evaluate the impact of damage on the aircraft's low-scattering characteristics. The component-level regional damage low-scattering impact map is visualized. The process is as follows: Figure 1 As shown, the specific steps include:

[0061] Step S1: Regional decomposition of typical damaged aircraft parts.

[0062] An aircraft's low-scattering performance depends on a variety of factors, including its layout, frequency characteristics, azimuth characteristics, and the presence of damage to the radar-absorbing coating. A comprehensive analysis of an aircraft's low-scattering performance would require extensive testing and analysis, generating tens of gigabytes of data and hindering frontline maintenance. Therefore, for aircraft damage detection and assessment, it's not necessary to perform complete low-scattering performance testing every time. Instead, component-by-component testing and assessment can be performed to reduce testing costs.

[0063] The present invention selects a typical low-scattering aircraft and divides the aircraft into 13 areas according to its components, including wings, tail, fuselage, navigation lights, vertical tail, horizontal tail, air inlet interior, radar cabin, air inlet lip, cockpit, wing leading edge, wing upper surface, and wing lower surface. Figure 2 As shown, each region is further decomposed into 25~30 sub-regions.

[0064] Step S2: Estimation of the low scattering performance of the aircraft's radar-absorbing coating before damage.

[0065] Step S3: Calculation of radiation of the passive millimeter wave absorbing coating.

[0066] Almost all objects in nature exhibit incomplete absorption and incomplete radiation of incident radiation. Therefore, nature can be regarded as a gray body. Under the condition of the same radiation brightness, the absolute temperature corresponding to the gray body is greater than the absolute temperature of the black body.

[0067] For the millimeter wave band, assuming the bandwidth , the radiant brightness of the gray body at temperature T is , its equivalent blackbody radiation temperature It can be expressed as:

[0068] ;

[0069] in, represents the wavelength, represents the incident direction of the incident electromagnetic wave, It represents the angle between the radiation propagation direction and the surface normal direction. is the azimuth; is the Boltzmann constant.

[0070] The emissivity of an object is defined as the ratio of the brightness of a gray body to the brightness of a black body at the same temperature. express:

[0071] ;

[0072] Generally speaking, the emissivity of the target being measured is affected by both internal and external factors. Internal factors include the target's own roughness, absolute temperature, and dielectric constant, while external factors include the radiometer's observation direction, wavelength, and polarization. Here, we ignore the influence of internal factors and only consider external factors. The calculation process is as above.

[0073] Step S4: Brightness temperature imaging simulation of passive millimeter wave absorbing coating.

[0074] Under high-frequency conditions, electromagnetic waves can be regarded as local plane waves. When dealing with the propagation and scattering paths of electromagnetic waves, we can use the analysis method of geometric optics. The brightness temperature tracking method is based on the geometric optics analysis method and analyzes the propagation trajectory of brightness temperature in the radiation scene. The process of the brightness temperature tracking method is as follows: Figure 3 shown.

[0075] The ray tracing model generally starts with a radiometer and performs ray scanning within the decomposed aircraft component area. Each time a ray is emitted, the propagation path of the ray is traced. During the ray tracing process, the aircraft surface coating is considered smooth. When the ray hits the plane of the aircraft decomposition area, the ray is reflected on the surface until it reaches the outside of the field of view. The ray tracing is stopped and the next ray tracing step is started. When all the set ray tracing is completed, the ray tracing model is established, such as Figure 4 As shown in the figure, three simple propagation modes in reverse ray tracing are shown: (1) The ray is directly incident from the radiometer to the ground background and reflected to the sky, corresponding to Figure 4 in curve; (2) The ray is emitted from the radiometer to the surface of the aircraft decomposition area and reflected to the sky, corresponding to Figure 4 in curve; (3) The ray is incident on the side of the aircraft decomposition area, then reflected to the ground, and then reflected to the sky, corresponding to Figure 4 in curve. represents the incident angle on the surface of the ground medium, Represents the ground scattering angle.

[0076] The brightness temperature inversion model in the present invention ignores the influence of factors such as multi-layer media, diffuse reflection and diffuse transmission.

[0077] Step S5: evaluating the low scattering performance of the aircraft's radar-absorbing coating after damage.

[0078] The visual diagram of the impact of damage on the low scattering performance of the aircraft can be given according to different damage modes according to the decomposed component areas of the aircraft, such as Figure 5 As shown. Figure 5 The damage extent of aircraft components can be quickly determined to determine whether repairs are needed.

[0079] The foregoing description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Those skilled in the art will readily appreciate that various modifications and variations of the present invention are possible. Any modifications, equivalent substitutions, or improvements made within the spirit and principles of the present invention are intended to be within the scope of protection of the present invention.

Claims

1. A method for evaluating the impact of damage to an aircraft's radar-absorbing coating on its low-scattering performance, characterized in that: The following steps are involved: Step 1: Decompose the aircraft into regions according to its components. Each region is further decomposed into several sub-regions to conduct targeted analysis of damage in different parts. Step 2: Calculate the radiation cross-sectional area of ​​each region and estimate the low scattering performance of the aircraft's radar-absorbing coating before damage; Step 3: Consider the impact of external factors on emissivity and combine it with the reflectivity calculation formula to determine the millimeter wave emissivity received by the radiometer; Step 4: Implement brightness temperature imaging simulation of passive millimeter wave absorbing coating based on brightness temperature tracking method; Step 5: Evaluate the low-scattering performance of the damaged aircraft radar-absorbing coating and quantify the low-scattering effect C. The low-scattering effect C is used to represent the impact of the damage on the low-scattering performance, providing a basis for repair decisions.

2. The method for evaluating the impact of damage to an aircraft radar-absorbing coating on its low-scattering performance according to claim 1, characterized in that: In step 1, the aircraft is divided into 13 areas.

3. The method for evaluating the impact of damage to an aircraft radar-absorbing coating on its low-scattering performance according to claim 2, characterized in that: Each region is decomposed into 25~30 sub-regions.

4. The method for evaluating the impact of damage to an aircraft radar-absorbing coating on its low-scattering performance according to claim 1, characterized in that: In step 2, the radiometer radiation cross-sectional area of ​​each area before coating damage is calculated based on the radiation cross-sectional area of ​​the area and the brightness temperature difference between the area and the background environment, thereby calculating the low scattering performance of the aircraft absorbing coating before damage.

5. The method for evaluating the impact of damage to an aircraft radar-absorbing coating on its low-scattering performance according to claim 1, characterized in that: In step 3, only the influence of external factors on the emissivity is considered, and the reflectivity is calculated in combination with the Fresnel reflectivity formula to determine the millimeter wave emissivity received by the radiometer.

6. The method for evaluating the impact of damage to an aircraft radar-absorbing coating on its low-scattering performance according to claim 1, characterized in that: In step 4, the brightness temperature tracing method includes scene modeling, inverse ray tracing, inversion model establishment and data visualization to generate brightness temperature simulation maps and scattering patterns of damaged and undamaged aircraft scenes, regions, and sub-regions.

7. The method for evaluating the impact of damage to an aircraft radar-absorbing coating on its low-scattering performance according to claim 1, characterized in that: In step 5, the radiometric radiation cross-sectional area of ​​each sub-area is calculated using the brightness temperature difference between the damaged area and the undamaged area to obtain the low scattering performance of the damaged aircraft radar absorbing coating.

8. The method for evaluating the impact of damage to an aircraft radar-absorbing coating on its low-scattering performance according to claim 4, characterized in that: The step 2 includes: The radiation cross-sectional area of ​​the radiometer is introduced for each area: ； in, Indicates the radiation cross-sectional area of ​​the radiometer in each area before coating damage, is the regional radiation cross-sectional area, Indicates the brightness temperature difference between the area and the background environment; Low scattering performance of aircraft radar-absorbing coatings before damage Expressed as: ； Where i represents the i-th region, represents the radiation cross-sectional area of ​​the radiometer before the coating is damaged in the i-th region, is the radiation cross-sectional area of ​​the i-th region, is the brightness temperature difference between the ith region and the background environment.

9. The method for evaluating the impact of damage to an aircraft radar-absorbing coating on its low-scattering performance according to claim 8, characterized in that: The step 5 comprises: The radiation cross-sectional area of ​​the radiometer is introduced for each sub-area: ； in, represents the radiometer radiation cross-sectional area of ​​each sub-area after coating damage, is the radiation cross-sectional area of ​​the damaged area, represents the brightness temperature difference between the damaged area and the undamaged area; Low scattering performance of aircraft radar-absorbing coating after damage Expressed as: ； Where i represents the i-th region, and j represents the j-th subregion of the i-th region.

10. The method for evaluating the impact of damage to an aircraft radar-absorbing coating on its low-scattering performance according to claim 9, characterized in that: The calculation formula for the low scattering effect C is: ； The unit of the low scattering effect C is dB.

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