Terahertz detection method for residual stress of coating

Abstract

The invention discloses a terahertz detection method for residual stress of a coating. The terahertz detection method comprises the following steps: S1, projecting terahertz waves to the surface of a to-be-detected coating through a terahertz wave detector, and collecting a first batch of spectrums; s2, stress is applied to the coating; s3, projecting terahertz waves to the surface of the to-be-detected coating through a terahertz wave detector, and collecting a second batch of spectrums; s4, drawing a spectrogram; s5, identifying formants; step S6, peak value analysis; step S7, comparing and referencing; step S8, peak value distribution; and step S9, stress calculation. According to the terahertz detection method for the residual stress of the coating, disclosed by the invention, the problems that in the prior art, residual stress detection mainly depends on a lossy method, and the detection precision is not enough are solved.

Description

Technical Field

[0001] The 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 that cause the lattice structure to deform. These internal forces can arise from a variety of sources, including thermal expansion or contraction, phase changes, and mechanical deformation. When these forces are not fully relieved, the material may contain areas of compressive or tensile stress, which can have a significant impact on the material's properties and behavior. This stress can also cause changes in the material's lattice structure, resulting in changes 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, thus affecting its aerodynamics and overall performance. It may also cause cracking, delamination or other types of damage to the coating and / or substrate, thereby compromising the structural integrity of the aircraft and affecting the durability and service life of the coating, leading to increased aircraft maintenance costs and increased downtime.

[0004] Residual stress in a coating can decrease over time or remain in the coating forever, depending on the specific conditions and properties of the coating and substrate. Factors such as the thickness and composition of the coating, the type and properties of the substrate, and the environmental conditions to which the coating material is exposed can all affect 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 long periods of 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 system, regularly inspect, properly maintain and timely recoat the coating to ensure the integrity and performance of the coating system and protect the underlying substrate. Therefore, it is very important to accurately measure and manage the residual stress of aircraft surface coatings to ensure its safe and efficient operation.

[0006] At present, the commonly used methods for detecting residual stress inside coatings are divided into two types: destructive and non-destructive. Destructive methods include coating peeling experiments and microscope observations; non-destructive methods include X-ray diffraction, neutron diffraction, micro-Raman spectroscopy, curvature measurement, etc. Each technology has its advantages and disadvantages and is suitable for different types of materials and coatings. The choice of technology depends on the specific application and requirements. In order to obtain accurate test results, the current detection methods are still mainly destructive methods.

[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 residual stress of coatings, to overcome the problem that residual stress detection in the prior art mainly relies on lossy methods and has insufficient detection accuracy, and to propose an accurate method for obtaining the residual stress of coatings on the substrate surface and the degradation of coatings in harsh environments (such as strong ultraviolet rays, corrosion, etc.) using terahertz (THz) spectroscopy.

[0009] Another object of the present invention is to provide a terahertz detection method for residual stress of coatings, using terahertz spectroscopy. Terahertz spectroscopy is a non-destructive analysis method for analyzing the molecular structure and vibration mode of materials. In coating research, terahertz spectroscopy is used to measure the stress in the coating. Since molecules are composed of atoms, atoms move in certain vibration modes in molecules. These vibration modes correspond to different molecular vibration frequencies. Stress affects the vibration rotation phonon mode and frequency of molecules. Under the influence of stress, the frequency of molecular vibration will change, 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 of a coating, 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 an optical path length L0, a refractive index n0 and a 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 spectrum data, and storing the first spectrum data (terahertz broadband) of the coating to be tested in a database;

[0012] Step S2: applying stress to the coating to simulate relevant impacts 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 an optical path length L1, a refractive index n1, and a peak frequency f1, rotating the coating (sample) or rotating the angle of the polarizer on one side of the terahertz wave detector, collecting corresponding second spectrum data, and storing the (terahertz broadband) second spectrum data of the coating to be tested in a database;

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

[0015] Step S5: Identify the resonance peaks and find the 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 technology 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 peak 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, drawing the frequency shift curve of the terahertz peak, and calculating the influence of stress on the molecular vibration frequency by analyzing the frequency shift curve, so as to infer the stress state in the coating, and analyzing the coating degradation by comparing with the database and extracting the resonance peak change, and calculating the residual stress in combination with the material parameters. The calculation method is:

[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 residual strain is calculated as:

[0023] εr=Δf / f0;

[0024] Among them, 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 influences 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 that residual stress detection in the prior art mainly relies on lossy methods and has insufficient detection accuracy. The present invention proposes a method for obtaining residual stress of a 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 changes in the resonance peaks on the samples of the present invention.

[0034] Figure 4 It 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 It is a diagram showing the changes in the measurement results after the material coating of the present invention has aged. DETAILED DESCRIPTION

[0037] The following description is used to disclose the present invention so that those skilled in the art can implement the present invention. The preferred embodiments described below are only examples, and those skilled in the art can think of other obvious variations. The basic principles of the present invention defined in the following description can be applied to other embodiments, variations, improvements, equivalents, and other technical solutions that do not deviate from the spirit and scope of the present invention.

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

[0039] Preferred embodiments.

[0040] During the flight of an aircraft, the coating will decompose and become thinner due to external forces, corrosion, etc., and the optical and electrical parameters will also change as the coating degrades. The change in optical path length (L) is a direct and reliable parameter derived from the time domain spectrum and can be used to correlate with changes in coating degradation. The change in optical distance of each layer can be obtained from the deconvoluted peak. Figure 1 Typical deconvoluted peaks are shown. From the peak distances, the optical distance can be calculated. The optical path length is measured directly 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 make these measurements regularly so that trends can be tracked.

[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. For example Figure 2 shown.

[0045] 4. Result analysis. The change in optical distance is related to the degradation level of the coating. By quantifying the change in optical distance, the degree of coating degradation or wear can be inferred. By extracting the coating material signal and calculating the material property parameters, information related to the coating material can be obtained, such as coating material decomposition, thickness change, etc.

[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 an optical path length L0, a refractive index n0 and a 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 spectrum data, and storing the first spectrum data (terahertz broadband) of the coating to be tested in a database;

[0048] Step S2: applying stress to the coating to simulate relevant impacts 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 an optical path length L1, a refractive index n1, and a peak frequency f1, rotating the coating (sample) or rotating the angle of the polarizer on one side of the terahertz wave detector, collecting corresponding second spectrum data, and storing the (terahertz broadband) second spectrum data of the coating to be tested in a database;

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

[0051] Step S5: Identify the resonance peaks and find the 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 technology 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 peak 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 (the change of the sample under stress or the aging degree of the sample material itself can be observed);

[0055] Step S9: Stress calculation, by measuring the terahertz spectrum data of the coating under different strain conditions, drawing the frequency shift curve of the terahertz peak, and calculating the influence of stress on the molecular vibration frequency by analyzing the frequency shift curve, so as to infer the stress state in the coating, and analyzing the coating degradation by comparing with the database and extracting the resonance peak change, and calculating the residual stress in combination with the material parameters. The calculation method is:

[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 residual strain is calculated as:

[0059] εr=Δf / f0;

[0060] Among them, 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 is evenly applied to the silicon-based sample, which is then heated at 100 degrees for 2 minutes and then rapidly quenched. Quenching causes rapid cooling, causing different parts of the material to experience different thermal contractions, which results in residual stress.

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

[0068] Figure 4 It is the result of measuring after rotating the measuring point at multiple angles. At this time, the material has not yet aged, the sample has not shown anisotropy, and the measurement results of the sample at different angles do not differ significantly.

[0069] The red coated sample was tempered. Tempering can relieve or reduce residual stress 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 been subjected to several heating annealing treatments. After several heating cycles, the anisotropy of the material has also changed, indicating that the material has aged and degraded, the material has lost its uniformity, and local damage, such as cracks, delamination or loss of adhesion, requires re-coating.

[0071] It is worth mentioning that the coating and other technical features involved in the patent application of this invention should be regarded as the prior art. The specific structure, working principle and possible control method and spatial arrangement method of these technical features can be selected by conventional methods 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 substitutions for some of the technical features therein. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the protection scope of the present invention.

Claims

1. A terahertz detection method for residual stress of coating, 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, thereby obtaining first information including an optical path length L0, a refractive index n0 and a peak frequency f0, rotating the coating or rotating the angle of the polarizer on one side of the terahertz wave detector, collecting corresponding first spectrum data, and storing the first spectrum data of the coating to be tested in a database; Step S2: applying stress to the coating to simulate relevant impacts 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, thereby obtaining second information including an optical path length L1, a refractive index n1, and a peak frequency f1, rotating the coating or rotating the angle of the polarizer on one side of the terahertz wave detector, collecting corresponding second spectrum data, and storing the second spectrum data of the coating to be tested in a database; Step S4: plotting a spectrum diagram, plotting an amplitude or intensity spectrum according to a frequency axis, thereby obtaining a graphical representation of the terahertz TDS spectrum; Step S5: Identify the resonance peaks and find the 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 technology 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 materials; 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, drawing the frequency shift curve of the terahertz peak, and calculating the influence of stress on the molecular vibration frequency by analyzing the frequency shift curve, so as to infer the stress state in the coating, and analyzing the coating degradation by comparing with the database and extracting the resonance peak change, and calculating the residual stress in combination with the material parameters. The calculation method is: 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 residual strain is calculated as: εr=Δf / f0; Among them, 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 residual stress of coating according to claim 1, characterized in that: The relevant effects of step S2 include load and corrosion.

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

5. The terahertz detection method for residual stress of coating 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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