A terahertz detection method for residual stress in coatings

Abstract

The present invention discloses a terahertz detection method for residual stress in coatings, comprising steps S1: projecting terahertz waves onto the surface of the coating to be tested using a terahertz detector and collecting a first batch of spectra; step S2: applying stress to the coating; step S3: projecting terahertz waves onto the surface of the coating to be tested using the terahertz detector and collecting a second batch of spectra; step S4: plotting the spectrum; step S5: identifying resonance peaks; step S6: peak analysis; step S7: comparison and reference; step S8: peak assignment; and step S9: stress calculation. The terahertz detection method for residual stress in coatings disclosed by the present invention overcomes the problem in the prior art that residual stress detection primarily relies on lossy methods and suffers from insufficient detection accuracy.

Description

Technical Field

[0001] The present invention belongs to the technical field of residual stress detection, and in particular relates to a terahertz detection method for residual stress of a coating. Background Art

[0002] Residual stresses are caused by internal forces within a material, which can cause distortions in the lattice structure. These forces can arise from a variety of sources, including thermal expansion or contraction, phase changes, and mechanical deformation. When these forces are not fully released, the material may contain regions of compressive or tensile stress, which can significantly affect its properties and behavior. This stress can also cause changes in the material's lattice structure, resulting in alterations in crystal orientation, lattice spacing, and interatomic distances.

[0003] Taking aircraft coatings as an example, residual stress within the coating can cause deformation or warping of the aircraft structure, affecting its aerodynamics and overall performance. It can also lead to cracking, delamination, or other types of damage to the coating and / or substrate, compromising the aircraft's structural integrity and affecting the durability and service life of the coating, leading to increased maintenance costs and downtime for the aircraft.

[0004] Residual stress in a coating can decrease over time or remain permanently, depending on the specific conditions and properties of the coating and substrate. Factors such as the coating's thickness and composition, the type and properties of the substrate, and the environmental conditions to which the coating material is exposed all influence the evolution of residual stress in the coating. In some cases, residual stress may relax or redistribute over time due to changes in temperature, humidity, or other factors, while in other cases, it may remain relatively stable over time.

[0005] Therefore, airlines and aircraft operators need to follow manufacturer recommendations, industry standards, and regulatory guidelines to determine the optimal recoating cycle for their specific aircraft and coating systems. Regular inspections, proper maintenance, and timely recoating of coatings are essential to ensure the integrity and performance of the coating system and protect the underlying substrate. Therefore, it is crucial to accurately measure and manage residual stress in aircraft surface coatings to ensure their safe and efficient operation.

[0006] Currently, commonly used methods for measuring residual stress within coatings are destructive and non-destructive. Destructive methods include coating stripping tests and microscopic observation, while non-destructive methods include X-ray diffraction, neutron diffraction, micro-Raman spectroscopy, and curvature measurement. Each technique has its advantages and disadvantages and is suitable for different materials and coatings. The choice of technique depends on the specific application and requirements. To achieve accurate test results, destructive methods are still the main method.

[0007] Therefore, further improvements are made to the above problems. Summary of the Invention

[0008] The main purpose of the present invention is to provide a terahertz detection method for coating residual stress, overcoming the problem that residual stress detection in the existing technology mainly relies on lossy methods and has insufficient detection accuracy. An accurate method using terahertz (THz) spectroscopy to obtain the residual stress of the substrate surface coating and the degradation of the coating in harsh environments (such as strong ultraviolet rays, corrosion, etc.) is proposed.

[0009] Another object of the present invention is to provide a terahertz detection method for residual stress in coatings, using terahertz spectroscopy. Terahertz spectroscopy is a non-destructive analysis method for analyzing the molecular structure and vibration modes of materials. In coating research, terahertz spectroscopy is used to measure the stress conditions in coatings. Since molecules are composed of atoms, atoms in molecules move in certain vibration modes. These vibration modes correspond to different molecular vibration frequencies. Stress affects the vibrational rotational phonon modes and frequencies of molecules. Under the influence of stress, the frequency of molecular vibrations changes, and this change can be observed in the terahertz spectrum. Therefore, terahertz spectroscopy can measure the residual stress in the coating by analyzing the frequency shift and broadening of the terahertz resonance peak, and detect the residual stress distribution of the coating under strong ultraviolet rays, corrosion, and high temperature environments.

[0010] To achieve the above objectives, the present invention provides a terahertz detection method for residual stress in coatings, comprising the following steps:

[0011] Step S1: Projecting a terahertz wave onto the surface of the coating to be tested through a terahertz wave detector and collecting a first batch of spectra, thereby obtaining first information including the optical path length L0, the refractive index n0, and the peak frequency f0; rotating the coating (sample) or rotating the angle of the polarizer on one side of the terahertz wave detector; collecting corresponding first spectral data; and storing the first spectral data (terahertz broadband) of the coating to be tested in a database;

[0012] Step S2: applying stress to the coating to simulate the relevant impact in the flight environment;

[0013] Step S3: Projecting a terahertz wave onto the surface of the coating to be tested through a terahertz wave detector and collecting a second batch of spectra, thereby obtaining second information including the optical path length L1, the refractive index n1, and the peak frequency f1; rotating the coating (sample) or rotating the angle of the polarizer on one side of the terahertz wave detector to collect corresponding second spectral data; and storing the second spectral data (terahertz broadband) of the coating to be tested in a database;

[0014] Step S4: plotting a spectrum graph, plotting the amplitude or intensity spectrum according to the frequency axis, thereby obtaining a graphical representation of the terahertz TDS spectrum;

[0015] Step S5: identifying resonance peaks and finding prominent peaks in the spectrum;

[0016] Step S6: Peak analysis, analyzing the resonance peak by determining the frequency, amplitude, and shape of the resonance peak, using a peak fitting algorithm or mathematical modeling technique to accurately extract relevant features, and calculating the optical path length change ΔL and the frequency change Δf;

[0017] Step S7: comparison and reference, comparing the observed resonance peaks with relevant reference materials (known reference spectra or literature values ​​of similar materials or molecular vibrations);

[0018] Step S8: Peak assignment, assigning the resonance peak to a specific vibration or electronic transition based on the frequency of the resonance peak and the known spectral characteristics of the material or molecule under study;

[0019] Step S9: Stress calculation. By measuring the terahertz spectrum data of the coating under different strain conditions, the frequency shift curve of the terahertz peak is plotted. By analyzing the frequency shift curve, the influence of stress on the molecular vibration frequency is calculated, thereby inferring the stress state in the coating. By comparing with the database, the coating degradation is analyzed and the resonance peak changes are extracted. The residual stress is calculated in combination with the material parameters. The calculation method is as follows:

[0020] σ r =Eε r ;

[0021] Among them, σ r is the residual stress of the material, E is the elastic modulus of the material, ε r is the residual strain;

[0022] The calculation method of residual strain is:

[0023] εr=Δf / f0;

[0024] Where Δf = f1-f0 is the change in frequency before and after stress is applied, and f0 is the initial vibration frequency under stress-free conditions.

[0025] As a further preferred technical solution of the above technical solution, the relevant impacts of step S2 include load and corrosion.

[0026] As a further preferred technical solution of the above technical solution, in step S5, the resonance peak corresponds to a frequency at which the coating exhibits strong absorption or reflection of terahertz radiation.

[0027] As a further preferred technical solution of the above technical solution, the relevant features include peak parameters, identification frequency shift, and broadening.

[0028] As a further preferred technical solution of the above technical solution, step S7 is used to determine specific resonances associated with certain materials or functional groups.

[0029] The beneficial effects of the present invention are:

[0030] The present invention overcomes the problem in the prior art that residual stress detection mainly relies on lossy methods and has insufficient detection accuracy. The present invention proposes a method for obtaining residual stress of substrate surface coating using terahertz (THz) spectroscopy, which can perform non-contact high-precision measurement. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 It is a schematic diagram of sparse vectors corresponding to the optical path change of the present invention.

[0032] Figure 2 It is a schematic diagram of the relationship between the optical path length, the incident angle, and the actual distance of the coating in the present invention.

[0033] Figure 3 It is a diagram showing the change of resonance peaks on the sample of the present invention.

[0034] Figure 4 This is a diagram of measurement results after the measurement points on the sample of the present invention are rotated at multiple angles.

[0035] Figure 5 It is a stress release diagram of the present invention.

[0036] Figure 6 This is a diagram showing changes in measurement results after aging of the material coating of the present invention. DETAILED DESCRIPTION

[0037] The following description is intended to disclose the present invention so that those skilled in the art can implement the present invention. The preferred embodiments described below are for illustrative purposes only, and those skilled in the art will readily appreciate other obvious variations. The basic principles of the present invention defined in the following description may be applied to other embodiments, variations, improvements, equivalents, and other technical solutions that do not depart from the spirit and scope of the present invention.

[0038] In the preferred embodiment of the present invention, those skilled in the art should note that the coatings and the like involved in the present invention may be considered as prior art.

[0039] Preferred embodiment.

[0040] During flight, coatings degrade and thin due to external forces, corrosion, and other factors. Optical and electrical parameters also change as the coating degrades. Changes in optical path length (L) are a direct and reliable parameter derived from time-domain spectroscopy and can be used to correlate changes in coating degradation. Changes in optical distances between layers can be obtained from the deconvoluted peaks. Figure 1 Typical deconvoluted peaks are shown. Based on the peak distance, the optical distance can be calculated. The optical path length is directly measured from the time-domain spectrum.

[0041] The change in optical distance can be used to correlate with coating degradation by:

[0042] 1. Establish a baseline. Based on the original known thickness (d), record or measure the terahertz optical path length (L) of the original coating and obtain the refractive index (n). L = n × d. This can be used as a benchmark.

[0043] 2. Regular measurements. Use a terahertz measurement system to regularly measure the optical length of the coating over time. It is best to perform these measurements regularly to track trends.

[0044] 3. Multi-angle incident measurement. Perform multi-angle incident measurement to obtain the optical path length (L) and improve measurement accuracy. Multi-angle measurement can also be used for degradation evaluation. For example, using two different incident angles: θ1 and θ2, there will be two optical time delays measured. Δt1cos(θ1) = Δt2cos(θ2). Therefore, Δt1 / Δt2 will be constant as a degradation moment. This constant value Δt1 / Δt2 will change as the degradation progresses. Figure 2 shown.

[0045] 4. Results Analysis. Changes in optical distance correlate with the level of coating degradation. By quantifying changes in optical distance, the extent of coating degradation or wear can be inferred. Extracting coating material signals and calculating material property parameters can provide information related to the coating material, such as coating material decomposition and thickness changes.

[0046] like Figure 3-6 As shown, the present invention discloses a terahertz detection method for residual stress of a coating, comprising the following steps:

[0047] Step S1: Projecting a terahertz wave onto the surface of the coating to be tested through a terahertz wave detector and collecting a first batch of spectra, thereby obtaining first information including the optical path length L0, the refractive index n0, and the peak frequency f0; rotating the coating (sample) or rotating the angle of the polarizer on one side of the terahertz wave detector; collecting corresponding first spectral data; and storing the first spectral data (terahertz broadband) of the coating to be tested in a database;

[0048] Step S2: applying stress to the coating to simulate the relevant impact in the flight environment;

[0049] Step S3: Projecting a terahertz wave onto the surface of the coating to be tested through a terahertz wave detector and collecting a second batch of spectra, thereby obtaining second information including the optical path length L1, the refractive index n1, and the peak frequency f1; rotating the coating (sample) or rotating the angle of the polarizer on one side of the terahertz wave detector to collect corresponding second spectral data; and storing the second spectral data (terahertz broadband) of the coating to be tested in a database;

[0050] Step S4: drawing a spectrum graph, drawing an amplitude or intensity spectrum according to a frequency axis, thereby obtaining a graphical representation of the terahertz TDS spectrum (the spectrum graph includes corresponding data of the first spectrum data and the second spectrum data);

[0051] Step S5: identifying resonance peaks and finding prominent peaks in the spectrum;

[0052] Step S6: Peak analysis, analyzing the resonance peak by determining the frequency, amplitude, and shape of the resonance peak, using a peak fitting algorithm or mathematical modeling technique to accurately extract relevant features, and calculating the optical path length change ΔL and the frequency change Δf;

[0053] Step S7: comparison and reference, comparing the observed resonance peaks with relevant reference materials (known reference spectra or literature values ​​of similar materials or molecular vibrations);

[0054] Step S8: Peak assignment, assigning the resonance peak to a specific vibration or electronic transition based on the frequency of the resonance peak and the known spectral characteristics of the material or molecule under study (this can be used to observe changes in the sample under stress or the aging degree of the sample material itself);

[0055] Step S9: Stress calculation. By measuring the terahertz spectrum data of the coating under different strain conditions, the frequency shift curve of the terahertz peak is plotted. By analyzing the frequency shift curve, the influence of stress on the molecular vibration frequency is calculated, thereby inferring the stress state in the coating. By comparing with the database, the coating degradation is analyzed and the resonance peak changes are extracted. The residual stress is calculated in combination with the material parameters. The calculation method is as follows:

[0056] σ r =Eε r ;

[0057] Among them, σ r is the residual stress of the material, E is the elastic modulus of the material, ε r is the residual strain;

[0058] The calculation method of residual strain is:

[0059] εr=Δf / f0;

[0060] Where Δf = f1-f0 is the change in frequency before and after stress is applied, and f0 is the initial vibration frequency under stress-free conditions.

[0061] Specifically, the relevant impacts of step S2 include load and corrosion.

[0062] More specifically, in step S5 , the resonance peak corresponds to a frequency at which the coating exhibits strong absorption or reflection of terahertz radiation.

[0063] Furthermore, the relevant features include peak parameters, identification frequency shift, and broadening.

[0064] Furthermore, step S7 is used to determine specific resonances associated with certain materials or functional groups.

[0065] The present invention is specifically implemented as follows:

[0066] The red coating was evenly applied to a silicon-based sample, which was then heated at 100 degrees Celsius for two minutes and then rapidly quenched. Quenching causes rapid cooling, resulting in different thermal contractions in different parts of the material, which in turn creates residual stress.

[0067] Collect multi-point terahertz spectra on the sample, and the results are as follows Figure 3 As shown in Figure 2, two minutes after the first heating, residual stress is generated in the sample (the frequency of the resonance peak shifts and the width of the resonance peak broadens).

[0068] Figure 4 This is the result of measuring the sample at multiple angles. At this point, the material has not aged, the sample has not exhibited anisotropy, and there is no significant difference in the measurement results at different angles.

[0069] The red coated sample was tempered. Tempering can reduce or alleviate residual stresses in quenched material. During tempering, the material is reheated and then slowly cooled.

[0070] observe Figure 5 as well as Figure 6 It can be seen that the sample has undergone several heating and annealing treatments. After several heating cycles, the anisotropy of the material has also changed, indicating that the material has aged and degraded, with loss of material uniformity and local damage such as cracks, delamination or loss of adhesion, requiring re-coating.

[0071] It is worth mentioning that the technical features such as coatings involved in the patent application of this invention should be regarded as prior art. The specific structure, working principle and possible control method and spatial arrangement method of these technical features can be selected by conventional means in the field and should not be regarded as the inventive point of this patent. This patent will not be further elaborated.

[0072] For those skilled in the art, it is still possible to modify the technical solutions described in the aforementioned embodiments, or to make equivalent replacements for some of the technical features therein. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A terahertz detection method for residual stress in coatings, characterized in that: The following steps are involved: Step S1: Projecting a terahertz wave onto the surface of the coating to be tested through a terahertz wave detector and collecting a first batch of spectra to obtain first information including the optical path length L0, the refractive index n0, and the peak frequency f0; rotating the coating or rotating the angle of the polarizer on one side of the terahertz wave detector to collect corresponding first spectral data; and storing the first spectral data of the coating to be tested in a database; Step S2: applying stress to the coating to simulate the relevant impact in the flight environment; Step S3: Projecting a terahertz wave onto the surface of the coating to be tested through a terahertz wave detector and collecting a second batch of spectra to obtain second information including the optical path length L1, the refractive index n1, and the peak frequency f1; rotating the coating or rotating the angle of the polarizer on one side of the terahertz wave detector to collect corresponding second spectral data; and storing the second spectral data of the coating to be tested in a database; Step S4: plotting a spectrum graph, plotting the amplitude or intensity spectrum according to the frequency axis, thereby obtaining a graphical representation of the terahertz TDS spectrum; Step S5: identifying resonance peaks and finding prominent peaks in the spectrum; Step S6: Peak analysis, analyzing the resonance peak by determining the frequency, amplitude, and shape of the resonance peak, using a peak fitting algorithm or mathematical modeling technique to accurately extract relevant features, and calculating the optical path length change ΔL and the frequency change Δf; Step S7: comparison and reference, comparing the observed resonance peaks with relevant reference data; Step S8: Peak assignment, assigning the resonance peak to a specific vibration or electronic transition based on the frequency of the resonance peak and the known spectral characteristics of the material or molecule under study; Step S9: Stress calculation. By measuring the terahertz spectrum data of the coating under different strain conditions, the frequency shift curve of the terahertz peak is plotted. By analyzing the frequency shift curve, the influence of stress on the molecular vibration frequency is calculated, thereby inferring the stress state in the coating. By comparing with the database, the coating degradation is analyzed and the resonance peak changes are extracted. The residual stress is calculated in combination with the material parameters. The calculation method is as follows: s r =Ee r ; Among them, σ r is the residual stress of the material, E is the elastic modulus of the material, ε r is the residual strain; The calculation method of residual strain is: εr=Δf / f0; Where Δf = f1-f0 is the change in frequency before and after stress is applied, and f0 is the initial vibration frequency under stress-free conditions.

2. The terahertz detection method for coating residual stress according to claim 1, characterized in that: The relevant impacts of step S2 include load and corrosion.

3. The terahertz detection method for coating residual stress according to claim 1, characterized in that: In step S5 , the resonance peaks correspond to frequencies at which the coating exhibits strong absorption or reflection of terahertz radiation.

4. The terahertz detection method for coating residual stress according to claim 1, characterized in that: Relevant features include peak parameters, identification frequency shift, and broadening.

5. The terahertz detection method for coating residual stress according to claim 1, characterized in that: Step S7 is used to determine specific resonances associated with certain materials or functional groups.

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