Method for evaluating the influence of damage to an aircraft wave-absorbing coating on its low-scattering properties

Abstract

The application provides an evaluation method for the influence of aircraft wave-absorbing coating damage on low scattering performance, and belongs to the field of material performance evaluation, and comprises the following steps: regionally decomposing the aircraft, dividing the aircraft into multiple regions according to components, and further decomposing each region into a plurality of sub-regions, so as to analyze the damage of different parts; estimating the low scattering performance of the aircraft before the wave-absorbing coating is damaged, establishing a radiometer radiation cross-section model, calculating the radiation cross-section of each region, and determining the low scattering performance benchmark of the aircraft in the undamaged state; performing passive millimeter wave wave-absorbing coating radiation calculation; implementing passive millimeter wave wave-absorbing coating brightness temperature imaging simulation; evaluating the low scattering performance of the aircraft after the wave-absorbing coating is damaged, quantifying the influence of the damage on the low scattering performance through a low scattering effect concept, and providing a basis for repair decision-making.

Description

TECHNICAL FIELD

[0001] The present application belongs to the field of material performance evaluation, and particularly relates to a method for evaluating the influence of damage to an aircraft wave-absorbing coating on its low-scattering performance. BACKGROUND

[0002] Stealth technology, also known as low detectability technology, has been applied to various information platforms and is of great significance to improving the survivability of information platforms. The current stealth technology mainly includes radar stealth, infrared stealth, visible light stealth, sound stealth, laser stealth, etc. At present, radar stealth technology has become the focus of research by military powers around the world, and a number of aircraft with radar stealth capabilities have been developed. Aircraft stealth technology mainly includes passive stealth and active stealth, and passive stealth includes shape conforming and wave-absorbing materials. When the aerodynamic shape of the aircraft remains unchanged, the application of wave-absorbing materials can further greatly improve the radar stealth capability of the aircraft. At present, typical foreign stealth aircraft are coated with wave-absorbing materials, but these coatings will be affected by the environment during storage, training and transportation, and will be damaged, collided and aged, etc., which will change the physical and chemical properties of the wave-absorbing coating, and thus weaken or lose the radar stealth function of the coating.

[0003] When the aircraft wave-absorbing coating is damaged by wear, collision and aging, etc., it will affect the low-scattering capability of the aircraft to a certain extent, and when the damage is serious, the wave-absorbing coating needs to be repaired. Before repairing the wave-absorbing coating, the damage detection and evaluation of the aircraft must be carried out. There are many methods for detecting the damage of the aircraft wave-absorbing coating, and the mainstream methods include active detection methods such as ultrasonic technology, eddy current method, ray method and infrared thermal imaging method. The so-called active detection method is to emit a signal to the target, receive the reflected signal, and obtain the damage information of the target by analyzing the reflected signal. In fact, there is also a passive detection method, i.e. passive millimeter wave imaging technology, which receives the millimeter wave radiation emitted by the object itself, and can be used for objects with high temperature.

[0004] Millimeter wave imaging uses millimeter wave electromagnetic waves with a wavelength of 1 to 10 millimeters and a frequency of 30 to 300 GHz. Millimeter wave imaging has strong penetration ability, such as the ability to penetrate clothing, plastic, walls, etc., so it is often used for security checks, such as airport body scanners, and military concealed target detection, such as damage detection of wave-absorbing coatings. There have been some reports on passive millimeter wave radiation damage detection of flat coating samples, but there are relatively few studies on how to apply passive millimeter wave radiation to detect the damage of aircraft wave-absorbing coatings. There is no research on the influence of the damage of wave-absorbing coatings on the low-scattering performance of aircraft after passive millimeter wave radiation is applied to detect the damage of wave-absorbing coatings. Therefore, it is very important to continue to develop an evaluation method for the influence of the damage of low-scattering aircraft coatings on the low-scattering performance based on passive millimeter wave radiation. SUMMARY

[0005] To address the aforementioned technical problems, this invention provides an assessment method for the impact of damage to aircraft absorbing coatings on their low scattering performance. This method is based on passive millimeter-wave radiation detection technology, studies the detection and assessment of damage to aircraft absorbing coatings, establishes an assessment method for the impact of coating damage on aircraft low scattering performance, and provides a basis for aircraft absorbing coating repair technology.

[0006] To achieve the above objectives, the present invention adopts the following technical solution:

[0007] A method for evaluating the impact of damage to an aircraft's absorbing coating on its low scattering performance includes the following steps:

[0008] Step 1: Decompose the aircraft into regions. Divide the aircraft into multiple regions according to its components, and further decompose each region into several sub-regions to conduct targeted analysis of damage to 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: Considering the influence of external factors on emissivity, and combining the reflectivity calculation formula, determine the millimeter-wave emissivity received by the radiometer;

[0011] Step 4: Simulate the brightness temperature of the passive millimeter-wave absorbing coating using the brightness temperature tracking method;

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

[0013] Beneficial effects:

[0014] This invention utilizes a passive millimeter-wave radiation imaging simulation method to detect damage to aircraft absorbing coatings. The aircraft is decomposed into 13 regions, each further subdivided into 25-30 sub-regions. For each region, a millimeter-wave radiation cross-sectional area model of the aircraft is established. Utilizing the emission and reflection characteristics of millimeter waves in the absorbing coating, a model for calculating the emissivity of millimeter waves in the coating is constructed. Combined with a brightness temperature tracking method, brightness temperature simulation diagrams and scattering patterns are generated for the aircraft scene, aircraft regions, damaged sub-regions, and undamaged sub-regions. The millimeter-wave radiation cross-sectional area of ​​the aircraft after damage is calculated, and the concept of low scattering effect is proposed. This low scattering effect is used to assess the impact of damage on the aircraft's low scattering characteristics, providing technical support for the field of absorbing coating repair. Attached Figure Description

[0015] Figure 1 This is a flowchart of a method for evaluating the impact of damage to an aircraft absorbing coating on its low scattering performance, according to the present invention.

[0016] Figure 2 is a typical aircraft region decomposition schematic diagram.

[0017] Figure 3 is a flowchart of the brightness temperature tracking method.

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

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

[0020] In order to make the objectives, technical solutions and advantages of the present application clearer, the present application is further described in detail below with reference to the accompanying drawings and examples. It should be understood that the specific examples described herein are only used to explain the present application and do not limit the present application. In addition, the technical features involved in each embodiment of the present application described below can be combined with each other as long as they do not conflict with each other.

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

[0022] Step S1: region decomposition of typical damage components of the aircraft;

[0023] Step S2: pre-estimation of the low scattering performance of the aircraft radar-absorbing coating;

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

[0025] Step S4: passive millimeter wave radar-absorbing coating brightness temperature imaging simulation;

[0026] Step S5: evaluation of the low scattering performance of the aircraft radar-absorbing coating after damage.

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

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

[0029] ;

[0030] wherein, ​A i represents the radiometric cross section area of each region before the coating is damaged, A i represents the radiometric cross section area of each region before the coating is damaged, ΔT i represents the brightness temperature difference between the region and the background environment, and is expressed as:

[0031] ;

[0032] wherein, T i represents the brightness temperature of the region, T b represents the brightness temperature of the background environment.

[0033] Low scattering performance of an aircraft wave-absorbing coating before the coating is damaged may be expressed as:

[0034] ;

[0035] wherein, i represents the i-th region, A i represents the radiometric cross section area of the i-th region before the coating is damaged, A i represents the radiometric cross section area of the i-th region before the coating is damaged, ΔT i represents the brightness temperature difference between the i-th region and the background environment.

[0036] Preferably, in the step S3, the emissivity of the generally measured target is affected by both internal factors and external factors, the internal factors including the roughness of the target itself, the absolute temperature of the target itself, and the dielectric constant, etc., and the external factors including the observation direction of the radiometer, the wavelength, the polarization, etc., and here, the influence of the internal factors is ignored, and only the external factors are considered.

[0037] When an electromagnetic wave is incident on the surface of an object, absorption, reflection, and projection phenomena occur, and assuming that the object is in a state of thermal equilibrium, in this state, the emissivity of the object is equal to the absorption rate, and at this time, the reflection, projection, and emission of the surface of the object satisfy the normalization relationship, i.e.:

[0038] ;

[0039] wherein, ε represents the emissivity, ρ represents the reflectivity, τ represents the transmissivity. Here, the influence of the transmission is ignored, i.e. Therefore, the emissivity may be simplified as:

[0040] ;

[0041] For the reflectivity of a smooth plane, the Fresnel reflectivity may be used to calculate:

[0042] ;

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

[0044] The millimeter wave emissivity received by the radiometer is therefore :

[0045] .

[0046] Preferably, in the step S4, the brightness temperature tracking method is used for the radiometric imaging simulation. Under high frequency conditions, the electromagnetic wave can be regarded as a local plane wave. When the path of the propagation and scattering of the electromagnetic wave is processed, the geometric optical analysis method can be used. The brightness temperature tracking method is based on the geometric optical analysis method and analyzes the propagation trajectory of the brightness temperature in the radiometric scene.

[0047] The flow of the brightness temperature tracking method is shown in Figure 3 . First, the scene modeling is performed to establish the scene model of the propagation and scattering of the electromagnetic wave. The scene model needs to restore the environmental background, perform the parameter assumption, and generate the rays. Next, the reverse ray tracking is performed. The rays are scanned in the set field of view range from the receiving point in the scene. The path of the ray is tracked in reverse. After the reflection and transmission, the emission source of the electromagnetic wave is tracked until the propagation path information of the electromagnetic wave is obtained. Then, the inversion operation is performed according to the obtained propagation path information to establish the inversion model and perform the brightness temperature calculation. Finally, the simulated data is analyzed and visualized (i.e., the result visualization extraction in Figure 3 ). The analysis and visualization include the scene, the aircraft region, the sub-region damage, the sub-region undamaged brightness temperature simulation diagram, and the scattering mode, etc.

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

[0049] ;

[0050] wherein, denotes the radiometer radiation cross-sectional area of each sub-region after the coating is damaged, is the radiation cross-sectional area of the damaged region, denotes the brightness temperature difference between the damaged region and the undamaged region:

[0051] ;

[0052] wherein, is the brightness temperature of the damaged region, is the brightness temperature of the undamaged region.

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

[0054] ;

[0055] wherein i represents the i-th region, and j represents the j-th sub-region of the i-th region.

[0056] The present application introduces a low scattering effect C (unit: dB) to characterize the influence of coating damage on the low scattering performance of the aircraft, which can be expressed as:

[0057] ;

[0058] The smaller the value of C, the less the influence of coating damage on the low scattering performance of the aircraft.

[0059] Embodiment:

[0060] The evaluation method provided by the embodiment for the influence of aircraft radar absorbing coating damage on the low scattering performance of the aircraft divides the aircraft into 13 regions according to parts, and each region is further divided into 25-30 sub-regions. For each region, a millimeter wave radiation cross-sectional area model of the aircraft is established, a brightness temperature simulation diagram and a scattering mode of a flight scene, an aircraft region, a sub-region damage, and a sub-region undamaged are established based on passive millimeter wave imaging technology and brightness temperature tracking method, millimeter wave radiation cross-sectional areas of the aircraft before and after damage are calculated, the influence of damage on the low scattering characteristics of the aircraft is evaluated by a low scattering effect, and a low scattering influence diagram of a part-level region damage is visualized. The flow is as shown in Figure 1 , and specifically includes the following steps:

[0061] Step S1: division of typical damage component regions of the aircraft.

[0062] The low scattering performance of the aircraft is related to various factors, including layout form, frequency characteristics, azimuth characteristics, and whether the radar absorbing coating is damaged or not. If it is necessary to completely analyze the low scattering performance of the aircraft, a large amount of testing and analysis needs to be performed, and tens of G of massive data are generated, which is not conducive to the use and maintenance of the front-line troops. Therefore, for damage detection and evaluation of the aircraft, it is not necessary to perform complete low scattering performance testing every time, and the testing and evaluation can be performed according to parts to reduce the testing cost.

[0063] The present application selects a typical low scattering aircraft, divides the aircraft into 13 regions such as wings, tail, fuselage, navigation lights, vertical tail, horizontal tail, inlet internal, radar cabin, inlet lip, cockpit, wing leading edge, wing upper surface, and wing lower surface according to parts, as shown in Figure 2 , and each region is further divided into 25-30 sub-regions.

[0064] Step S2: Estimation of low scattering performance of aircraft wave-absorbing coating before damage.

[0065] Step S3: Passive millimeter wave wave-absorbing coating radiation calculation.

[0066] Almost all objects in nature exhibit incomplete absorption and incomplete radiation of incident radiation, so 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 that of the black body.

[0067] For the millimeter wave band, it is assumed that the bandwidth The radiation brightness of the gray body at temperature T is The equivalent black body radiation temperature can be expressed as:

[0068] ;

[0069] wherein, denotes the wavelength, denotes the incident direction of the incident electromagnetic wave, denotes the included angle between the radiation propagation direction and the surface normal direction, is the azimuth angle. is the Boltzmann constant.

[0070] The emissivity of the object is defined as the ratio of the brightness of the gray body to the brightness of the black body at the same temperature, which is denoted by :

[0071] ;

[0072] Generally, the emissivity of the measured target is affected by internal factors and external factors. The internal factors include the roughness of the target itself, the absolute temperature of the target itself, and the dielectric constant, etc. The external factors include the observation direction of the radiometer, the wavelength, the polarization, etc. Here, the influence of the internal factors is ignored, and only the external factors are considered, and the calculation process is as described above.

[0073] Step S4: Passive millimeter wave wave-absorbing coating brightness temperature imaging simulation.

[0074] Under high frequency conditions, electromagnetic waves can be regarded as local plane waves. When dealing with the propagation and scattering path of electromagnetic waves, the geometric optical analysis method can be used. The brightness temperature tracking method is based on the geometric optical analysis method, and the propagation trajectory of the brightness temperature in the radiation scene is analyzed. The process of the brightness temperature tracking method is shown in Figure 3 .

[0075] The ray tracing model generally takes the radiometer as a starting point, performs ray scanning in the range of the decomposed aircraft component area, traces the propagation path of each ray, and stops the ray tracing when the ray is outside the field of view. After the set of ray tracing is completed, the ray tracing model is established, as shown in FIG. 1, which shows three simple propagation modes in reverse ray tracing: (1) the ray is directly incident from the radiometer to the ground background and reflected to the sky, corresponding to the curve in FIG. 2; (2) the ray is emitted from the radiometer to the surface of the decomposed aircraft area and reflected to the sky, corresponding to the curve in FIG. 3; and (3) the ray is incident to the side of the decomposed aircraft area, then reflected to the ground, and then reflected to the sky, corresponding to the curve in FIG. 4. Figure 4 Figure 4 Figure 4 Figure 4

[0076] The brightness temperature inversion model in the present application ignores the influence of multi-layer medium, diffuse reflection and diffuse transmission and other factors.

[0077] Step S5: low scattering performance evaluation of the aircraft wave-absorbing coating after damage.

[0078] The influence of damage on the low scattering performance of the aircraft wave-absorbing coating can be visualized according to different damage modes in the decomposed component area of the aircraft, as shown in FIG. 5. The damage degree of the aircraft component can be quickly obtained by analyzing FIG. 5, and it is determined whether repair is needed. Figure 5 Figure 5

[0079] The above only describes the preferred embodiments of the present application and is not used to limit the present application. For those skilled in the art, the present application can have various modifications and changes. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present application shall be included in the protection scope of the present application.​​​​​​​​​​​

Claims

1. A method of evaluating the effect of damage to a radar absorbing coating of an aircraft on its low-scattering properties, characterized in that, The method comprises the following steps: Step 1, regional decomposition of the aircraft, the aircraft is divided into multiple regions according to parts, each region is further decomposed into several sub-regions, so as to analyze the damage of different parts; Step 2, calculate the radiation cross section area of each region, estimate the low scattering performance of the aircraft before the damage of the wave-absorbing coating; Step 3, considering the influence of external factors on emissivity, combined with the reflectivity calculation formula, determine the millimeter wave emissivity received by the radiometer; Step 4, implement passive millimeter wave coating brightness temperature imaging simulation based on the brightness temperature tracking method; Step 5, evaluate the low scattering performance of the aircraft after the damage of the wave-absorbing coating, quantify the low scattering effect C, which represents the influence of the damage on the low scattering performance, and provide a basis for repair decision, including: The radiometer radiation cross section area is introduced for each sub-region: ； wherein, represents the radiometer radiation cross section area of each sub-area after coating damage, is the radiation cross section area of the damaged area, represents the difference in brightness temperature between the damaged area and the undamaged area; Low-scattering properties of damaged aircraft wave-absorbing coating is represented as: ； Wherein, i represents the i th region, j represents the j th sub-region of the i th region; The calculation formula of the low scattering effect C is: ； Wherein, the unit of the low scattering effect C is dB.

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

3. The method of claim 2, wherein the method further comprises: Each region is decomposed into 25-30 sub-regions.

4. The method of claim 1, wherein the aircraft wave-absorbing coating is damaged. In the step 2, the radiometer radiation cross section area of each region before the damage of the coating is calculated by the radiation cross section area of the region, the brightness temperature difference between the region and the background environment, so as to calculate the low scattering performance of the aircraft before the damage of the wave-absorbing coating.

5. The method of claim 1, wherein the aircraft wave-absorbing coating is damaged. In the step 3, only the influence of external factors on emissivity is considered, the reflectivity is calculated combined with the Fresnel reflectivity formula, and then the millimeter wave emissivity received by the radiometer is determined.

6. The method of claim 1, wherein the aircraft wave-absorbing coating is damaged. In the step 4, the brightness temperature tracking method includes scene modeling, reverse ray tracing, inversion model establishment and data visualization, generates the brightness temperature simulation diagram and scattering mode of the aircraft scene, region, sub-region damage and undamaged.

7. The method of claim 1, wherein the aircraft wave-absorbing coating is damaged. In the step 5, the low scattering performance of the aircraft after the damage of the wave-absorbing coating is obtained by using the radiometer radiation cross section area of each sub-region calculated by the brightness temperature difference between the damaged region and the undamaged region.

8. The method of claim 4, wherein the method further comprises: The step 2 comprises: The radiometer radiation cross section area is introduced for each region: ； wherein, represents the radiometric radiation cross section area of each region before the coating is damaged, is the region radiometric radiation cross section area, represents the difference in brightness temperature between the region and the background environment; Low scattering properties of aircraft wave absorbing coatings before damage is represented as: ； where i represents the i-th region, represents the radiometer radiation cross section of the i-th region before coating damage, is the radiation cross section of the i-th region, is the brightness temperature difference between the region and the background environment of the i-th region.

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