Method for testing aging degree of aluminum alloy aircraft plate with epoxy coating

Abstract

The invention relates to the technical field of processing, testing or inspection of airplane parts, and provides a method for testing the aging degree of an aluminum alloy airplane plate with an epoxy coating. The method comprises the following steps: placing a coating sample in a coastal atmospheric environment to be exposed for many years, respectively measuring the glossiness, chromaticity and multi-liquid contact angle of the samples with different ages, calculating the surface free energy according to the contact angle, and comprehensively evaluating the aging degrees of the aged and exposed samples with different exposure ages according to the gloss loss rate, the color change rate and the surface free energy. The aging degree of the aluminum alloy aircraft plate with the epoxy coating is obtained, and quantitative judgment of the aging grade of the coating is achieved. The method gives consideration to optical and surface energy indexes, the evaluation result is stable, and a reliable basis can be provided for airplane plate maintenance decision making.

Description

Technical Field

[0001] This invention relates to the technical fields of aircraft component processing, testing, and inspection, and in particular to a method for testing the aging degree of epoxy-coated aluminum alloy aircraft panels. Background Technology

[0002] Epoxy-coated aluminum alloys are commonly used as protective materials for aircraft skins and structural components, widely applied to aircraft surfaces to improve the corrosion resistance and service life of the aluminum alloy substrate. During long-term service, especially in the high-salt, high-humidity coastal environment, the coating is susceptible to aging and degradation due to the combined effects of ultraviolet light, temperature and humidity cycles, and sea salt deposition. This leads to a gradual decline in surface and protective properties, ultimately affecting the structural safety and reliability of the entire aircraft. Currently, most aging monitoring methods for aircraft epoxy coatings use single performance parameters, such as color changes. While these methods can reflect the aging trend of the coating to some extent, their evaluation indicators are singular and lack comprehensiveness, making it difficult to fully depict the overall changes in coating performance. This can easily lead to misjudgments or omissions, and cannot effectively support maintenance strategy formulation. Furthermore, some methods do not perform multi-point sampling on the sample surface, resulting in results influenced by local surface conditions, lacking statistical stability, and often ignoring the changes in surface energy during the aging process, failing to reveal the impact of microstructural degradation on macroscopic performance.

[0003] In summary, existing testing technologies for the aging degree of epoxy-coated aluminum alloy aircraft panels have several technical problems, including a single evaluation index, lack of comprehensiveness, susceptibility to misjudgment or omission, inability to effectively support maintenance strategy formulation, lack of statistical stability, and inability to reveal the impact of microstructural degradation on macroscopic performance. Summary of the Invention

[0004] To address the shortcomings of the existing technologies, this invention provides a method for testing the aging degree of epoxy-coated aluminum alloy aircraft panels. This method combines changes in coating optical properties with changes in surface physicochemical properties to construct a representative and stable aging evaluation system, thereby improving the accuracy and operability of determining the service reliability of epoxy-coated aluminum alloy aircraft panels.

[0005] The present invention provides a method for testing the aging degree of epoxy-coated aluminum alloy aircraft panels, comprising: Aluminum alloy aircraft plate samples with epoxy coatings were exposed to the coastal atmospheric environment for different years to form aging exposed samples with different exposure years. Gloss was measured on aged samples with different exposure years. At least five different locations were randomly selected on the surface of each aged sample to measure the gloss, and the average gloss of each aged sample was obtained. The gloss loss rate was calculated compared with that of the unexposed sample. Colorimetric measurements were performed on aged samples with different exposure years. The colorimetric measurements included brightness, chroma, and hue. The colorimetric values ​​of each aged sample were obtained, and the color change rate compared with the unexposed sample was calculated. Contact angle measurements were performed on aging samples with different exposure years. At least 20 locations were randomly selected on the surface of each aging sample and various liquids were added. The contact angle formed by the droplets at each location was measured and the average value was calculated. The surface free energy of the coating was then calculated based on the average contact angle. The aging degree of the epoxy-coated aluminum alloy aircraft panels is comprehensively evaluated based on the surface free energy of the coating, the discoloration rate compared with the unexposed sample, and the gloss loss rate compared with the unexposed sample, in order to obtain the aging degree of the epoxy-coated aluminum alloy aircraft panels.

[0006] Compared with the prior art, the beneficial effects of this invention are as follows: This invention provides a method for testing the aging degree of epoxy-coated aluminum alloy aircraft panels. The method involves exposing epoxy-coated aluminum alloy aircraft panel samples to a coastal atmospheric environment for different durations, creating aged samples with varying exposure years. Gloss is measured on each aged sample at least five randomly selected locations on the surface of each sample to obtain the average gloss value. The gloss loss rate compared to unexposed samples is then calculated. Colorimetry is also measured on the aged samples for different exposure years, including brightness, chroma, and hue. The colorimetric value for each aged sample is obtained, and the color change rate compared to unexposed samples is calculated. Contact angle measurements were performed on aging exposed samples for a certain number of years. At least 20 locations were randomly selected on the surface of each aging exposed sample, and various liquids were dropped on them. The contact angle formed by the droplets at each location was measured and the average value was calculated. The surface free energy of the coating was then calculated based on the average contact angle. Based on the surface free energy of the coating, the discoloration rate compared to the unexposed sample, and the gloss loss rate compared to the unexposed sample, the aging degree of aging exposed samples with different exposure years was comprehensively evaluated to obtain the aging degree of epoxy-coated aluminum alloy aircraft panels. This allows for the construction of a representative and stable aging evaluation system that combines changes in coating optical properties with changes in surface physicochemical properties, thereby improving the accuracy and operability of judging the service reliability of epoxy-coated aluminum alloy aircraft panels. Attached Figure Description

[0007] The accompanying drawings, which are included to provide a further understanding of the invention and form part of this invention, illustrate exemplary embodiments of the invention and are used to explain the invention, but do not constitute an undue limitation of the invention. Some specific embodiments of the invention will be described in detail below with reference to the accompanying drawings in an exemplary and non-limiting manner. The same reference numerals in the drawings designate the same or similar parts or components. It should be understood by those skilled in the art that these drawings are not necessarily drawn to scale. In the drawings: Figure 1 This is a schematic flowchart of a method for testing the aging degree of epoxy-coated aluminum alloy aircraft plates according to an embodiment of the present invention. Figure 2 This is a schematic diagram of various environmental parameters at a coastal test site in an embodiment of the present invention. Detailed Implementation

[0008] To enable those skilled in the art to better understand the present invention, the technical solutions of this embodiment will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are merely some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort should fall within the scope of protection of the present invention.

[0009] See Figures 1-2 This embodiment provides a method for testing the aging degree of epoxy-coated aluminum alloy aircraft panels, including the following steps: S101. Exposed aluminum alloy aircraft plate samples with epoxy coatings to coastal atmospheric exposure environment for different years to form aging exposure samples with different exposure years. S102. Perform gloss measurement on aged exposed samples with different exposure years. Measure gloss at least five different locations on the surface of each aged exposed sample to obtain the average gloss of each aged exposed sample, and calculate the gloss loss rate compared to the unexposed sample. When calculating the gloss loss rate compared to the unexposed sample, the average gloss of the unexposed sample can be used as a reference to obtain accurate data on the gloss loss rate, thereby improving the accuracy and stability of gloss measurement.

[0010] S103. Perform colorimetric measurements on aged samples with different exposure years. The colorimetric measurements include brightness, chroma, and hue. Obtain the colorimetric value of each aged sample and calculate the color change rate compared with the unexposed sample. S104. The contact angle of the aging exposed samples with different exposure years is measured. At least 20 positions are randomly selected on the surface of each aging exposed sample and various liquids are dropped. The contact angle formed by the droplets at each position is measured and the average value is calculated. The surface free energy of the coating is then calculated based on the average contact angle. S105. Based on the surface free energy of the coating, the discoloration rate compared to the unexposed sample, and the gloss loss rate compared to the unexposed sample, the aging degree of the aged exposed samples with different exposure years is comprehensively evaluated to obtain the aging degree of the epoxy-coated aluminum alloy aircraft sheet.

[0011] It should be noted that in coastal atmospheric exposure environments, different environmental parameters can affect the aging rate of epoxy-coated aluminum alloy aircraft panels. See [link / reference]. Figure 2 , Figure 2 The results show various environmental parameters of a test site on the coast.

[0012] It should be noted that the testing method for the aging degree of epoxy-coated aluminum alloy aircraft panels in this embodiment aims to address key issues in existing technologies, such as the single evaluation dimension, unstable test results, difficulty in reflecting microscopic changes, and inability to support maintenance strategy formulation. Specifically, existing methods typically focus only on a single surface performance parameter, such as color or gloss changes, lacking systematic analytical means, which easily leads to misjudgments or omissions. Furthermore, due to the lack of multi-point sampling, the measurement results are easily affected by local defects or contamination, resulting in poor reliability. In addition, traditional methods ignore the change law of surface free energy of the coating during aging, failing to reveal the correlation between microstructural degradation and macroscopic performance evolution from a physicochemical mechanism perspective. To address these issues, this embodiment constructs a representative and stable aging assessment system based on three key dimensions. First, in terms of optical performance, by measuring the gloss and color of samples exposed for different years, the gloss loss rate and color change rate indices are obtained, reflecting the degradation trend of the coating surface's specular reflectivity and color composition. Secondly, regarding physicochemical properties, contact angle tests were conducted on the sample surface using various standard liquids to calculate the surface free energy value of the coating. This comprehensively reflects the changes in polar groups and surface energy states of the coating, revealing the impact of microstructure degradation on the material's surface properties. Furthermore, to improve the statistical stability of the data, this embodiment sets clear multi-point sampling requirements for each measurement: gloss is measured at least five different locations randomly selected on the surface of each aged and exposed sample; multiple liquids are added at least 20 random locations on the surface of each aged and exposed sample; and colorimetry testing also employs multi-point measurement and averaging to eliminate local abnormal interference at the source, ensuring more representative results. Finally, based on the surface free energy of the coating, the discoloration rate compared to the unexposed sample, and the gloss loss rate compared to the unexposed sample, the aging degree of the aged exposed samples with different exposure years is comprehensively evaluated to obtain the aging degree of the epoxy-coated aluminum alloy aircraft panels. This reflects the transformation process of the coating from an intact state to different aging degrees, providing clear data support for maintenance cycle setting and condition assessment. By achieving collaborative collection and fusion analysis of multi-dimensional indicators, the accuracy of identifying the aging behavior of epoxy-coated aluminum alloys is improved, the stability and application reliability of the testing method are enhanced, and it can effectively support the monitoring and maintenance management of the service status of aircraft coatings.

[0013] Preferably, the exposure environment of the aging test specimens is an outdoor exposure site within 1-2 kilometers of the coastline. Each exposed specimen is fixed on an exposure frame with an inclined angle of 30°±5°. The exposed specimens are installed facing the ocean direction, and the exposure frame is made of corrosion-resistant stainless steel. The spacing between each specimen is kept consistent, thereby ensuring that specimens with different exposure years have the same environmental exposure conditions, avoiding the influence of differences in exposure location on the aging process of the specimens, and improving the comparability and reliability of subsequent aging degree assessment results.

[0014] Preferably, the instrument used for gloss measurement is a standard gloss meter with an incident angle of 60°. Each measurement position is measured continuously for no less than 3 times and the average value is taken. During the measurement process, the measurement window of the gloss meter is kept in close contact with the sample surface. Before the measurement, the sample surface is cleaned with alcohol and non-woven cloth to remove dirt and dust from the sample surface, thereby avoiding interference from dirt and dust on the measurement results.

[0015] Preferably, the colorimetric measurement employs a spectrophotometer with a D65 light source and a 10° observation angle. Measurements are taken at no fewer than five different locations for each exposed sample, and the average value is calculated for each location. Before colorimetric measurement, the surface of the exposed sample is cleaned by wiping it with various liquids and allowed to dry before measurement. The luminance L*, chroma C*, and hue angle h of the exposed sample are then recorded. The color difference value ΔE of the exposed sample is calculated by comparing the luminance L*, chroma C*, and hue angle h with the corresponding parameters of the unexposed sample. Standard deviation analysis is then performed on the color difference value ΔE to eliminate outliers. Finally, the color change rate of each exposed sample is calculated by comparing the color difference value ΔE of the exposed sample after outlier elimination with the color difference reference value of the unexposed sample. This improves the reliability and repeatability of the colorimetric measurement, making it more stable and reliable. It is understood that aged coatings often exhibit localized blistering and erosion. Simply averaging ΔE is easily skewed by extreme values. In this embodiment, by combining multi-point sampling with the ΔE standard deviation elimination strategy, local erosion interference can be quantitatively eliminated, thereby improving statistical reliability and making the output color change rate more representative of the overall coating state rather than local defects.

[0016] Preferably, when calculating the color difference value ΔE of the exposed sample in the difference calculation, the calculation formula is as follows: ; ;In the formula, Indicates the brightness of the unexposed sample. Indicates the chroma of the unexposed sample. Indicates the hue angle of the unexposed sample; It refers to the red-green axis components of the color space calculated based on the chroma C* and hue angle h of the exposed sample; It refers to the yellow-blue axis component of the color space calculated based on the chroma C* and hue angle h of the exposed sample; This refers to the chroma of the unexposed sample. And tone angle The calculated red-green axis components; This refers to the chroma of the unexposed sample. And tone angle The calculated yellow-blue axis components.

[0017] It should be noted that, typically, color difference assessment mainly utilizes the ΔE value output by the instrument, which cannot uniformly process the brightness, chroma, and hue angle data from different spectrophotometers on-site, leading to significant cross-device comparison errors. The calculation formula provided in this embodiment overcomes the differences in instrument data formats, enabling rapid and low-computational-power color quantification in the hangar, significantly improving the interoperability of test results between different batches and devices. The explicit correction in radians in the calculation formula ensures numerical stability of low-brightness coatings under high-salt and high-humidity environments. Coastal environments easily cause coating surfaces to turn gray and lose brightness, amplifying numerical fluctuations in traditional ΔE calculations under low-brightness scenarios. The formula explicitly specifies that the h value is first converted to radians before participating in trigonometric function calculations, avoiding the amplification of angle input errors in low-gloss samples, thereby effectively suppressing low-brightness noise and ensuring the stability and comparability of ΔE results on heavily aged samples.

[0018] Preferably, the contact angle measurement is performed using an optical contact angle measuring instrument. The droplet volume of various liquids is controlled within 3μL±0.2μL. After adding the droplet at each measurement position, it is kept still for no less than 10 seconds before measurement. During the measurement process, the camera captures the droplet profile, and the contact angle is calculated using a fitting method. Each measurement position is measured no less than 3 times, and the average value is taken. This improves the accuracy and repeatability of the contact angle measurement and avoids fluctuations in the measurement results caused by factors such as changes in droplet volume and shape.

[0019] Preferably, the surface free energy is calculated using the following formula: ; ;in, This represents the average static contact angle measured for the j-th liquid. This represents the total surface tension of the j-th liquid. Represents the dispersive component of the j-th liquid. Represents the polar component of the j-th liquid. The dispersive component representing the surface free energy of the coating. The polar component of the surface free energy of the coating. This represents the surface free energy.

[0020] It should be noted that during the coating aging process, polar functional groups (such as hydroxyl and carboxyl groups) are easily damaged by ultraviolet light and hydrolysis, and measuring only the contact angle cannot distinguish between changes in polarity and dispersion. In this embodiment, the multi-liquid angle measurement is substituted into the following formula: The total surface free energy is clearly decomposed into polar and dispersive components, which can be used to infer the chemical degradation path during aging. This not only quantifies the change in free energy, but also identifies whether the main cause of aging is the destruction of polar structures or damage to the hydrophobic skeleton, which helps to optimize the formulation of protective materials and analyze the aging sources.

[0021] Preferably, in the comprehensive aging assessment process, when normalizing the three indicators of gloss loss rate, discoloration rate, and surface free energy change, preset thresholds for gloss loss rate, discoloration rate, and free energy change are selected as references, and a linear normalization formula is used to convert the measured value of each of the three indicators into a dimensionless value between 0 and 1. Then, according to the different importance of the three indicators in contributing to the aging degree, different weighting coefficients are assigned to obtain a comprehensive aging index.

[0022] It should be noted that in existing technologies, after measuring gloss loss, color change, or surface free energy, each is typically given as a percentage or absolute value separately. Then, subjective judgments of mild, moderate, and severe aging are made based on human experience. In this embodiment, a linear normalization formula is used to convert the measured value of each of the three indicators into a dimensionless value between 0 and 1. This eliminates the scale effect caused by differences in measurement range, unit, and instrument model. Data obtained from any location and any device can be directly compared horizontally, establishing a unified health scale for panels from different fleets and years. Different weighting coefficients are assigned based on the different importance of the three indicators in contributing to the degree of aging, allowing for emphasis on different indicators based on aircraft type, coating formulation, or failure mechanisms in specific environments. For example, the surface free energy weight is increased in sea salt erosion areas, and the color change rate weight is increased in high UV areas, enabling the model to have scene adaptability. Moreover, in this embodiment, only three interdisciplinary indicators (optics, colorimetry, and interface energy) are retained, and each indicator is limited to the 0-1 range by normalization. The weights are then determined after the scale is unified to prevent the phenomenon of a single indicator being vetoed or redundant weighting due to large differences in magnitude or collinearity among multiple indicators. The change in the aging index reduces the bias of any single indicator, thus balancing robustness and sensitivity.

[0023] Preferably, the weighting coefficients of the gloss loss rate, discoloration rate, and surface free energy change are determined by the analytic hierarchy process (AHP). Specifically, based on the relative importance of each indicator to aging contribution determined by expert review, the weighting coefficients of each indicator are determined by matrix calculation. This ensures that the comprehensive aging index calculation method is more scientific and reasonable, avoids subjective errors caused by manually determining the weights, and makes the aging evaluation results more objective and reliable.

[0024] Preferably, the aluminum alloy substrate used in the aging exposure test specimens has a thickness of 1mm-3mm and a coating thickness of 25-45μm. The uniform size of the specimens is 100mm×150mm. Each specimen undergoes a uniform surface pretreatment before coating, including degreasing, acid etching, and chromate passivation. It should be noted that if the aluminum substrate is too thin, sea salt penetration will cause back stress warping during accelerated testing, resulting in a shift in the gloss meter's incident angle; if it is too thick, thermal inertia will inhibit the coating degradation rate. In this embodiment, limiting the substrate thickness to the range of 1mm-3mm maintains the same level as the thermal mass and elastic modulus of real aircraft skin, avoiding false signals in gloss and contact angle caused by thickness differences. At the same time, the coating thickness range of 25-45μm covers the thickness of the two-coat spraying process commonly used in aviation, without obscuring microcracks due to excessive coating thickness, ensuring the sensitivity of the aging indicators. Furthermore, when the uniform sample size is 100mm × 150mm, it exceeds the geometric dimensions of the spectrophotometer and contact angle meter's viewfinder, while still allowing for 20 randomly placed sampling points without overlap. Moreover, the size is close to that of a typical patch for a civil aircraft, facilitating comparison with the actual aircraft body. Additionally, each sample undergoes a uniform surface pretreatment before coating, including degreasing and cleaning to remove oil film, improving acid etching efficiency, roughening the substrate with acid etching to form a uniform hydrophilic surface, and chromate passivation to generate an Al-Cr-O composite film, providing self-healing chromate and improving coating adhesion.

[0025] Furthermore, the epoxy coating on the surface of the aging exposed sample is prepared by electrostatic spraying. During the spraying process, the uniformity of the coating thickness is controlled, and the curing conditions are a temperature of 120℃-140℃ and a curing time of 30min-60min, thereby ensuring the uniformity of the coating's physical properties, avoiding the influence of coating thickness differences on the consistency of aging test results, and improving coating uniformity.

[0026] Preferably, when comprehensively evaluating the aging degree of aging samples with different exposure years, the process includes: establishing trend curve models for the changes in gloss loss rate, discoloration rate, and surface free energy with the number of exposure years, and determining the objective weights of the three indicators—gloss loss rate, discoloration rate, and surface free energy—based on the entropy weight method. When determining the weights using the entropy weight method, the information entropy value of each indicator on the aging samples with each exposure year is first calculated. Then, the weight coefficients of the three indicators in the comprehensive evaluation are determined based on the information entropy value of each indicator. Finally, the three indicators are weighted and summed according to the determined weight coefficients to obtain a comprehensive aging index for accurately evaluating the aging degree of the coating.

[0027] It should be noted that in existing multi-index evaluation schemes, fixed or manual empirical weights are typically used. Even in the few schemes that use entropy weighting, one-time test data is often directly input into the algorithm, outputting static weights. In this embodiment, a time-trend curve is first established for the three indicators of gloss loss rate, discoloration rate, and surface free energy, and then the entropy value at each time node is calculated. The entropy value reflects the novelty of the indicator information at different service stages in real time. For example, in the early aging stage, color difference changes drastically before gloss, so the entropy weight automatically increases the proportion of discoloration rate; if surface energy drops sharply in the middle and late stages, the weight will shift. This dynamic objective mechanism breaks through the conventional practice of single static entropy weighting in existing technologies, enabling the comprehensive index to accurately capture the migration of the aging-dominant mechanism over time. The entropy weighting method essentially measures dispersion using information entropy. By inputting time series data, the high correlation of the three indicators in certain time intervals can be detected and their weights can be automatically reduced, effectively weakening the repeated scoring of gloss loss rate and discoloration rate when they are highly collinear in the middle and late stages, improving the discrimination sensitivity and physical explanatory power of the aging index, and surpassing the effect achieved by fixed weights or one-time entropy weighting. Information entropy values ​​are directly calculated from experimental data, requiring no expert scoring and containing no environmental empirical parameters. Localized weights can be generated by substituting the respective trend curves for different aircraft models or climate zones, without the need for parameter readjustment, significantly improving the method's transferability and standardization potential.

[0028] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for testing the aging degree of epoxy-coated aluminum alloy aircraft panels, characterized in that, include: Aluminum alloy aircraft plate samples with epoxy coatings were exposed to the coastal atmospheric environment for different years to form aging exposed samples with different exposure years. Gloss was measured on aged samples with different exposure years. At least five different locations were randomly selected on the surface of each aged sample to measure the gloss, and the average gloss of each aged sample was obtained. The gloss loss rate was calculated compared with that of the unexposed sample. Colorimetric measurements were performed on aged samples with different exposure years. The colorimetric measurements included brightness, chroma, and hue. The colorimetric values ​​of each aged sample were obtained, and the color change rate compared with the unexposed sample was calculated. Contact angle measurements were performed on aging samples with different exposure years. At least 20 locations were randomly selected on the surface of each aging sample and various liquids were added. The contact angle formed by the droplets at each location was measured and the average value was calculated. The surface free energy of the coating was then calculated based on the average contact angle. The aging degree of the epoxy-coated aluminum alloy aircraft panels is comprehensively evaluated based on the surface free energy of the coating, the discoloration rate compared with the unexposed sample, and the gloss loss rate compared with the unexposed sample, in order to obtain the aging degree of the epoxy-coated aluminum alloy aircraft panels.

2. The method for testing the aging degree of epoxy-coated aluminum alloy aircraft panels according to claim 1, characterized in that, The aging exposure test specimens are exposed in an outdoor exposure area within 1-2 kilometers of the coastline. Each exposed specimen is fixed on an exposure frame with an inclined angle of 30°±5°. The exposed specimens are installed facing the ocean direction, and the exposure frame is made of stainless steel with anti-corrosion treatment. The spacing between each specimen is consistent.

3. The method for testing the aging degree of epoxy-coated aluminum alloy aircraft panels according to claim 1, characterized in that, The instrument used for gloss measurement is a standard gloss meter with an incident angle of 60°. Each measurement position is measured continuously for no less than 3 times and the average value is taken. During the measurement process, the measurement window of the gloss meter is kept in close contact with the sample surface. Before the measurement, the sample surface is cleaned with alcohol and non-woven cloth to remove dirt and dust from the sample surface.

4. The method for testing the aging degree of epoxy-coated aluminum alloy aircraft panels according to claim 1, characterized in that, The colorimetric measurement was performed using a spectrophotometer with a D65 light source and a 10° observation angle. At least five different positions were measured for each exposed sample, and the average value of the results was taken for each position. Before colorimetric measurement, the surface of the exposed sample was wiped clean with various liquids and allowed to dry before measurement. The luminance L*, chroma C*, and hue angle h of the exposed sample were then recorded. The color difference value ΔE of the exposed sample was calculated by comparing the luminance L*, chroma C*, and hue angle h with the corresponding parameters of the unexposed sample. Standard deviation analysis was performed on the color difference value ΔE of the exposed sample to eliminate outliers. The color change rate of each exposed sample was calculated by comparing the color difference value ΔE of the exposed sample after outlier elimination with the color difference reference value of the unexposed sample.

5. The method for testing the aging degree of epoxy-coated aluminum alloy aircraft panels according to claim 4, characterized in that, When calculating the color difference value ΔE of the exposed sample, the formula is as follows: In the formula, Indicates the brightness of the unexposed sample. Indicates the chroma of the unexposed sample. Indicates the hue angle of the unexposed sample; It refers to the red-green axis components of the color space calculated based on the chroma C* and hue angle h of the exposed sample; It refers to the yellow-blue axis component of the color space calculated based on the chroma C* and hue angle h of the exposed sample; This refers to the chroma of the unexposed sample. And tone angle The calculated red-green axis components; This refers to the chroma of the unexposed sample. And tone angle The calculated yellow-blue axis components.

6. The method for testing the aging degree of epoxy-coated aluminum alloy aircraft panels according to claim 1, characterized in that, The contact angle measurement was performed using an optical contact angle measuring instrument. The droplet volume of various liquids was controlled at 3μL±0.2μL. After adding the droplet at each measurement position, it was kept still for no less than 10 seconds before measurement. During the measurement, the camera captured the droplet profile, and the contact angle was calculated using a fitting method. Each measurement position was measured no less than 3 times, and the average value was taken.

7. The method for testing the aging degree of epoxy-coated aluminum alloy aircraft panels according to claim 1, characterized in that, The surface free energy is calculated using the following formula: in, This represents the average static contact angle measured for the j-th liquid. This represents the total surface tension of the j-th liquid. Represents the dispersive component of the j-th liquid. Represents the polar component of the j-th liquid. The dispersive component representing the surface free energy of the coating. The polar component of the surface free energy of the coating. This represents the surface free energy.

8. The method for testing the aging degree of epoxy-coated aluminum alloy aircraft panels according to claim 1, characterized in that, In the comprehensive aging assessment process, when normalizing the three indicators of gloss loss rate, discoloration rate, and surface free energy change, preset thresholds for gloss loss rate, discoloration rate, and free energy change are selected as references, and a linear normalization formula is used to convert the measured value of each of the three indicators into a dimensionless value between 0 and 1. Then, according to the different importance of the three indicators in contributing to the aging degree, different weighting coefficients are assigned to obtain a comprehensive aging index.

9. The method for testing the aging degree of epoxy-coated aluminum alloy aircraft panels according to claim 8, characterized in that, The weighting coefficients for the loss rate, discoloration rate, and surface free energy change were determined using the analytic hierarchy process (AHP). Specifically, the weighting coefficients for each indicator were determined by matrix calculation based on the relative importance of each indicator to the aging contribution as determined by expert review.

10. The method for testing the aging degree of epoxy-coated aluminum alloy aircraft panels according to claim 1, characterized in that, The aluminum alloy substrate used for the aging exposure test specimens has a thickness of 1mm-3mm and a coating thickness of 25-45μm. The uniform size of the test specimens is 100mm×150mm. Each test specimen undergoes uniform surface pretreatment before coating, which includes degreasing and cleaning, acid etching and chromate passivation.

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