Composite material damage thickness measurement method based on terahertz time domain compensation

Abstract

The invention relates to the field of nondestructive testing of composite materials, in particular to a composite material damage thickness measuring method based on terahertz time domain compensation, which comprises the following steps: S1, preparing a plurality of composite material laminated plate samples with variable damage thickness and depth, and obtaining terahertz time domain signals of different samples; s2, establishing a multilayer composite material structure theoretical transmission model, and defining a reflected terahertz response signal; s3, based on terahertz wave response characteristics and a transmission model, proposing a damage thickness measurement thought based on flight time difference of a damage area and a non-damage area; s4, estimating damage flight time information of the composite material by using terahertz signal reference transformation, alignment and cross-correlation methods; and S5, establishing a terahertz signal flight time compensation model, and realizing accurate estimation of the damage thickness. According to the method, the flight time of the damage is simplified into the transmission time difference of the terahertz waves passing through the damage area and the non-damage area, and the influence of human intervention and complex interference can be theoretically avoided.

Description

Technical Field

[0001] The present invention relates to the technical field of nondestructive testing of composite materials, and in particular to a method for measuring the damage thickness of composite materials based on terahertz time domain compensation. Background Art

[0002] Due to the complexity of the service environment and multilayer structure, composite materials may have failure modes of composite material interface damage, which will significantly affect the mechanical properties. Therefore, early detection and accurate characterization of damage parameters are essential for the use of composite materials. Due to its excellent performance, terahertz non-destructive testing technology has shown great potential in non-destructive testing technology and has been successfully applied in the field of non-destructive evaluation of composite materials.

[0003] During terahertz testing, due to dielectric discontinuities in depth, the reflected terahertz signal is defined as the superimposed echoes from various interfaces in the sample. Usually, the flight time of the damage is extracted by the time delay between consecutive damage echoes. However, since terahertz signals are susceptible to complex interferences such as dispersion, multiple reflections, and noise, the performance of flight time estimation may be degraded in these cases.

[0004] For larger damage thickness, the dispersion of terahertz waves due to the increase in transmission distance will lead to pulse broadening of the damage echo. At the same time, signal attenuation may cause the signal-to-noise ratio of the damage echo from the deeper interface to decrease. These factors make it a major challenge to accurately estimate the flight time of damage in complex terahertz signals. Traditional flight time estimation methods can use signal processing methods to improve the flight time estimation performance of complex terahertz signals, such as wavelet transform, which can simultaneously analyze the high-frequency fine features and low-frequency features of the signal, can well suppress the noise in the terahertz signal and obtain the reconstructed terahertz signal, but may fail for complex terahertz signals affected by multiple reflections and overlaps. For example, deconvolution technology models the terahertz test process as a linear time-invariant system, and directly inverse filters the transfer function, but due to the influence of system and environmental noise, the reconstructed pulse response may show obvious ringing artifacts. For example, frequency wavelet domain deconvolution and sparse representation improve deconvolution performance, which can reduce the impact of multiple reflections and effectively solve the dispersion and overlap problems of terahertz signals. However, the best performance is usually obtained by optimizing hyperparameters using manual selection or complex optimization techniques, which is not suitable for processing large amounts of terahertz signals and dealing with complex multi-layer structures or signals with extremely severe overlap and low signal-to-noise ratio. Summary of the invention

[0005] The purpose of the present invention is to propose a composite material damage thickness measurement method based on terahertz time domain compensation to address the problem that terahertz signals in the background technology are easily interfered by complex situations and the performance of time domain estimation may be reduced.

[0006] The technical solution of the present invention is a method for measuring the damage thickness of a composite material based on terahertz time domain compensation, comprising the following steps:

[0007] S1. Prepare multiple composite laminate samples with variable damage thickness and depth, and obtain terahertz time-domain signals of different samples based on the terahertz time-domain spectroscopy system;

[0008] S2. Analyze the influence of lifting distance and sample surface uniformity on terahertz signals, establish a theoretical transmission model for complex multilayer composite materials, and define the reflected terahertz response signal. The distance refers to the distance between the terahertz emitter and the sample surface.

[0009] S3. Based on the response characteristics and transmission model of terahertz waves, a method for measuring the damage thickness of composite materials based on the difference in flight time between damaged and non-damaged areas is proposed;

[0010] S4, using terahertz signal reference transformation, alignment and cross-correlation methods to estimate the flight time information of composite material damage;

[0011] S5. Considering the influence of lifting distance and sample surface uniformity, a terahertz signal flight time compensation model is established to achieve accurate estimation of the damage thickness of the composite material.

[0012] Preferably, in S1, the terahertz time-domain spectroscopy system includes a terahertz source, a control unit, a transmitter, a receiver and a motion platform, the motion directions of the motion platform include X and Y directions, the motion platform acquires 2D information from the sample, the transmitter and the receiver are integrated and installed on the motion platform, and the transmitter and the receiver are responsible for transmitting and receiving terahertz signals; the terahertz source includes a femtosecond laser for generating terahertz pulses.

[0013] Preferably, in S1, six 20mmx20mmx2mm composite laminate samples with different damage thicknesses are prepared using 3D printing technology, each sample contains 10 layers, the layer thickness is set to 200um, and glass fiber reinforced polymer is used as the printing material; in the preparation of the damaged samples, the test samples are divided into undamaged areas and damaged areas, and an air gap with a certain thickness is preset to simulate damage. The damage size is 10mmx10mm, and the damage thickness is variable, which are 100um, 200um, 400um, 600um, 800um and 1000um respectively.

[0014] Preferably, in S1, 6 composite laminate samples with a size of 20mmx20mmx1.6mm and different damage depths are prepared, each sample contains 8 layers, the thickness of each layer is set to 200um, and adjacent layers are stacked in sequence by glass fiber reinforced polymer and polylactic acid printing materials. The test sample is divided into an undamaged area and a damaged area, and an air gap with a certain thickness is preset to simulate damage. The damage size of the damaged area is set to 10mmx10mmx0.2mm, the damage depth is set to 200um, 400um, 600um, 800um, 1000um and 1200um, and the damage thickness remains unchanged.

[0015] Preferably, in S2, the reflected terahertz response signal is defined as:

[0016]

[0017] Where y(t) is the reflected terahertz echo, f(t) is the system impulse response function, P and K represent the number of laminate interface reflectors and the number of damaged interface reflectors, respectively, and θ p and θ k denote the model parameters corresponding to the Pth interface reflector and the Kth damage reflector, respectively. e(t) is Gaussian white noise. Δθi=Δθl+Δθe i It is the change of the overall model parameters of the terahertz signal caused by the change of lifting distance and surface uniformity.

[0018] Preferably, in S3, the damage thickness measurement method can be defined as:

[0019] ct 1 =2n 1 d

[0020] ct 2 =2n 2 d

[0021]

[0022] Where d is the damage thickness. 1 =1 is the refractive index of air. 1 is the transmission time of the terahertz wave through a lesion with thickness d. 2 is the refractive index of the sample. 2 is the transmission time of the terahertz wave through the sample with a thickness of d. Δt is the flight time difference, which refers to the transmission time difference of the terahertz wave through the damage with a thickness of d and the sample with a thickness of d, and c is the speed of light.

[0023] Preferably, in S4, the signal reference transformation refers to using the terahertz response signal from the non-damaged area of ​​the same sample as the basic signal for flight time extraction of the damaged area, and the signal alignment uses the time-shifted unit pulse response function as the alignment function to move the terahertz damage signal in the time domain, and accurately align the terahertz reference signal and the first pulse of the damage signal. The aligned terahertz damage signal is defined as:

[0024] y align =y(t)*δ(t+Δt 0 )

[0025] Among them, y align is the aligned terahertz damage signal, * is the convolution operation, Δt 0 is the time shift value of the terahertz damage signal, which can be obtained by the time delay corresponding to the first pulse of the terahertz reference signal and the damage signal.

[0026] Preferably, since the first pulses of the terahertz reference signal and the damaged signal are aligned, the transmission time difference of the terahertz wave can be defined by the time delay difference of the last pulse of the two aligned terahertz signals. The time delay difference of the two signals is determined by the cross-correlation method, specifically: the aligned terahertz signal is subjected to moving average filtering to reduce the noise of the terahertz signal. Secondly, the last pulses of the terahertz reference signal and the damaged signal are truncated into new signal vectors, such as y' ref and y' align , the cutoff interval can be determined by the signal time domain window and then y' ref and y' align The mutual correlation coefficient between them is:

[0027]

[0028] Among them, y' align =y align [p:q],y' ref =y ref [p:q]. p,q is the signal cutoff interval. Furthermore, the initial damage thickness can be obtained based on the obtained cross-correlation coefficient:

[0029] [peak,pos]=max(R)

[0030]

[0031] Where peak,pos is the maximum value of the cross-correlation coefficient R and its index, len(·) is the length of the signal, and f s is the sampling frequency of the terahertz signal, and TOF is the time of flight.

[0032] Preferably, in S5, the positioning parameter h is the difference in transmission distance between the sampling point and the reference point. When h≥0, the transmission distance increases, and the time delay of the first pulse of the terahertz damage signal obtained is less than the time delay from the reference point. The transmission time difference between the terahertz reference signal and the damage signal is defined as:

[0033] Δt=|TOF com -Δt 1

[0034]

[0035] Among them, Δt 1 is the transmission time difference between the sampling point and the reference point caused by the change in transmission distance h, Δt 0 is the transmission time difference caused by the change of transmission distance h, TOF com is the compensated damage flight time, n is the sample refractive index; when h < 0, the transmission distance decreases, the time delay of the first pulse of the obtained terahertz damage signal is greater than the time delay from the reference point, and the overall transmission time difference is defined as:

[0036] Δt=TOF com +Δt 2

[0037]

[0038] Among them, Δt 2 It is the difference in transmission time between the sampling point and the reference point caused by the change in transmission distance h.

[0039] Preferably, considering the uniformity of the sample surface, the damage thickness d is accurately obtained by flight time compensation. The overall calculation method is as follows:

[0040]

[0041] Compared with the prior art, the present invention has the following beneficial technical effects: the flight time of damage is simplified to the transmission time difference between the damaged area and the non-damaged area. In order to better obtain the flight time of damage, a flight time compensation model is established, which takes into account the influence of the lifting distance and surface uniformity of the sample. In this case, a series of numerical and experimental results, including different damage thicknesses and depths, show the effectiveness of the proposed method for damage thickness estimation. These results highlight the robustness of the method to complex interference and external environment during terahertz testing, and can effectively improve the accuracy of damage thickness estimation in different scenarios. Compared with other classical flight time extraction methods, the proposed method does not require any prior knowledge of damage and can obtain better damage thickness estimation performance. In future work, since the proposed method is very sensitive to the transmission time difference through the sample, it can not only be used to accurately estimate the damage thickness inside the composite material, but also has great potential to be extended to various thickness estimation fields, such as additive manufacturing, damage formation (TGO in thermal barrier coatings), material wear, etc. BRIEF DESCRIPTION OF THE DRAWINGS

[0042] Figure 1 It is the overall flow chart of the present invention;

[0043] Figure 2 Set up the working principle for the experimental device of the present invention;

[0044] Figure 3 is a diagram of the lifting distance and surface uniformity of the sample of the present invention;

[0045] Figure 4 This is a sample diagram prepared under different damage thicknesses of the present invention;

[0046] Figure 5 It is a diagram of the terahertz reference and damage signals obtained under different damage thicknesses of the present invention;

[0047] Figure 6 The damage thickness estimation results under different damage thicknesses of the present invention;

[0048] Figure 7 The sample images prepared at different damage depths of the present invention;

[0049] Figure 8 It is a diagram of the terahertz reference and damage signals obtained at different damage depths of the present invention;

[0050] Fig. 9 It is the damage thickness estimation result at different damage depths of the present invention;

[0051] Fig.10 This is the influence of the sample surface uniformity on the measured terahertz signal of the present invention. DETAILED DESCRIPTION

[0052] Embodiment 1, as Figure 1 As shown, the present invention proposes a composite material damage thickness measurement method based on terahertz time domain compensation, comprising the following steps:

[0053] S1. Prepare multiple composite laminate samples with variable damage thickness and depth, and obtain terahertz time-domain signals of different samples based on the terahertz time-domain spectroscopy system;

[0054] S2. Analyze the influence of lifting distance and sample surface uniformity on terahertz signals, establish a theoretical transmission model for complex multilayer composite materials, and define the reflected terahertz response signal. The distance refers to the distance between the terahertz emitter and the sample surface.

[0055] S3. Based on the response characteristics and transmission model of terahertz waves, a method for measuring the damage thickness of composite materials based on the difference in flight time between damaged and non-damaged areas is proposed;

[0056] S4, using terahertz signal reference transformation, alignment and cross-correlation methods to estimate the flight time information of composite material damage;

[0057] S5. Considering the influence of lifting distance and sample surface uniformity, a terahertz signal flight time compensation model is established to achieve accurate estimation of the damage thickness of the composite material.

[0058] Embodiment 2, the present invention proposes a composite material damage thickness measurement method based on terahertz time domain compensation. Compared with embodiment 1, this embodiment introduces S1 in detail.

[0059] like Figure 2 As shown in the figure, this experiment uses an integrated terahertz time-domain spectroscopy system to obtain the terahertz signal of the sample. The effective frequency range of the system ranges from 10 GHz to 4.5 THz. In order to obtain better depth resolution, the reflection mode is adopted. The terahertz time-domain spectroscopy system consists of a terahertz source, a control unit, an integrated transmitter and an xy motion platform. Among them, the terahertz source containing a femtosecond laser is used to generate terahertz pulses; the integrated transmitter and receiver are responsible for transmitting and receiving terahertz signals and are installed on the xy motion platform; the xy motion platform obtains 2D information from the sample. During the operation of the terahertz time-domain spectroscopy system, the femtosecond laser is first used to generate pump pulses and probe pulses through a beam splitter. The pump pulse is used to stimulate the terahertz pulse through the photoconductive antenna, and the probe pulse is used as a coherent femtosecond pulse to detect the terahertz signal reflected by the terahertz receiver. In addition, the optical delay line is used to adjust the time delay of the terahertz signal, and the xy motion platform performs raster scanning to collect the full-field terahertz signal of the sample.

[0060] For the actual terahertz test process, the terahertz transmitter is first focused on a polished metal plate to obtain a reference signal. Then, the test sample is placed horizontally on the metal plate. By adjusting the focus position of the terahertz transmitter, a refocusing process is performed on the test sample. Finally, the terahertz test process is performed by moving the xy scanning stage through appropriate steps, and the terahertz signals from all sampling points of the sample are collected.

[0061] like Figure 4-Figure 5 As shown, further, samples to be tested with different damage thicknesses were first prepared, and the corresponding terahertz response signals were obtained through the terahertz time-domain spectroscopy system. In order to obtain different terahertz damage signals from different damage thicknesses, six 20mmx20mmx2mm composite laminate samples with different damage thicknesses were prepared using 3D printing technology. Each sample contains 10 layers, and the layer thickness is set to 200um. Glass fiber reinforced polymer is used as the printing material, and its corresponding refractive index is calibrated to about 1.65. In the preparation of the damaged sample, an air gap with a certain thickness is preset to simulate the damage. The damage size is 10mmx10mm, and the damage thickness is variable, which are 100um, 200um, 400um, 600um, 800um and 1000um respectively. In order to increase the complexity of the terahertz signal, the damage is placed at the bottom of the sample, as shown in Figure 4 As shown. Then, the terahertz time-domain spectroscopy system is used to collect terahertz damage signals from different samples. For each test sample, five different terahertz damage signals are obtained from five sampling points in the damaged area, and the corresponding damage-free terahertz signal is obtained from the non-damaged area of ​​the sample, and the damage-free terahertz signal is used as a comparison signal. Figure 5 The terahertz signals obtained for different samples are shown. It can be found that at different damage thicknesses, the terahertz damage signal and the non-damage signal are similar, and both signals have zero-point drift. The difference is that the time delay of the first pulses of the two signals is different, which is caused by the surface inhomogeneity of the sample. In addition, since the damage occurs at the bottom of the sample, the damage echo component from the interface below the damage corresponds to the last pulse in the terahertz signal. When the damage thickness is small, the damage echo component from the interface above the damage is submerged in the terahertz signal, increasing the difficulty of estimating the damage thickness. As the damage thickness increases, the damage echo component becomes more obvious, and the time delay of the last pulse between the terahertz damage and non-damage signals gradually increases.

[0062] like Figure 7-Figure 8As shown, further, similar to the preparation of various damage thickness test samples, 6 composite laminate samples with a size of 20mmx20mmx1.6mm and different damage depths were prepared. Each sample contains 8 layers, and the thickness of each layer is set to 200um. Adjacent layers are stacked in sequence by glass fiber reinforced polymer and polylactic acid printing materials. The average refractive index of the test sample is about 1.9. Figure 7 As shown in the figure, the test samples are divided into undamaged areas and damaged areas. The damage size of the damaged area is set to 10mmx10mmx0.2mm, and the damage depth is set to 200um, 400um, 600um, 800um, 1000um and 1200um. The damage thickness remains unchanged. For each sample, five different sampling points are selected in the damaged area for better evaluation. For the samples to be tested with different damage depths, the measured terahertz damage signals and non-damage signals are shown in Figure 2. Figure 8 As shown in the figure, it can be found that the first pulse of the terahertz signal still has a zero offset and a time delay difference. For terahertz damage signals from different damage depths, the damage echo components overlap, and the corresponding time delay gradually increases with the increase of the damage depth. When the damage depth exceeds 800um, due to the increase of the terahertz wave transmission distance, the dissipation and absorption effects are enhanced, and the damage echo components will be submerged in the terahertz signal. This phenomenon is attributed to the dissipation and absorption effects of the terahertz wave as the transmission distance increases, as shown in Figure 1. Figure 8 (f) as shown.

[0063] like Fig. 9 Figure 2 shows the estimated damage thickness results for different damage depths. For each sample with a fixed damage depth, 5 tests were performed at 5 different sampling points. Fig. 9 (a)-(e) are the estimated damage thickness values ​​of 5 sampling points at different damage depths. It can be seen that the estimated damage thickness values ​​of different sampling points are very close to the true values. In addition, with the increase of damage depth, the estimation error does not increase significantly. For sampling point 5, when the damage depth is 800um, the estimation error is the worst, which is 7.33%. In addition, Fig. 9 (f) shows the average estimation error of 5 different sampling points at different damage depths. For a damage depth of 600 μm, the maximum average estimation error is 5.05%, which further demonstrates the effectiveness of the proposed method in damage estimation performance at different damage depths.

[0064] Embodiment 3, the present invention proposes a composite material damage thickness measurement method based on terahertz time domain compensation. Compared with embodiment 2, this embodiment introduces S2 in detail.

[0065] like Figure 3As shown in the figure, in terahertz detection, the lifting distance and sample surface uniformity are two key factors affecting the transmission characteristics of terahertz waves. The lifting distance refers to the distance between the terahertz emitter and the sample surface, which is determined by the focal length of the terahertz lens and the test requirements. The sample surface uniformity refers to the irregularity or fluctuation of the sample surface, which is related to the roughness and surface damage of the sample.

[0066] In terahertz testing, it is usually assumed that the lifting distance of the terahertz reference signal obtained from the polished metal plate is the same as the lifting distance of the reflected terahertz response signal obtained from the sample, but in actual testing, they are not the same. Due to manual operation errors, the focus position of the terahertz transmitter will change, which will cause the terahertz response signal to change in the time domain. For all sampling points in a given sample, the reflected terahertz response signal is defined as:

[0067]

[0068] Among them, y 1 (t) is the reflected terahertz echo, f 1 (t) is the impulse response function when the lifting distance varies, P and K represent the number of laminate interface reflectors and the number of damaged interface reflectors, respectively, θ p and θ k denote the model parameters corresponding to the Pth interface reflector and the Kth damage reflector, respectively, e(t) is Gaussian white noise, Δθ l is the change in model parameters caused by the change in lift distance. In particular, in the same measurement, Δθ of all sampling points l All the same.

[0069] In addition, the surface uniformity of the sample will also affect the reflected terahertz response signal. For different sampling points in the tested sample, the corresponding terahertz signal will change with the change of surface uniformity. Given a sampling point in the test sample, the corresponding reflected terahertz response signal is expressed as:

[0070]

[0071] Among them, y 2 (t) is the reflected terahertz echo, f 2 (t) is the impulse response function caused by the change of surface uniformity at the sampling point, P and K represent the number of laminate interface reflectors and the number of damaged interface reflectors, respectively, and θ p and θ k denote the model parameters corresponding to the Pth interface reflector and the Kth damage reflector, respectively, e(t) is Gaussian white noise, Δθe iis the change in model parameters caused by the change in surface uniformity of the sampling point. Generally speaking, for different sampling points, it may be different due to the change in sample surface uniformity.

[0072] Finally, the reflected terahertz response signal at the sampling point can be defined as:

[0073]

[0074] Where y(t) is the reflected terahertz echo, f(t) is the system impulse response function, P and K represent the number of laminate interface reflectors and the number of damaged interface reflectors, respectively, and θ p and θ k denote the model parameters corresponding to the Pth interface reflector and the Kth damage reflector, respectively. e(t) is Gaussian white noise. Δθi=Δθl+Δθe i It is the change of the overall model parameters of the terahertz signal caused by the change of lifting distance and surface uniformity.

[0075] Embodiment 4, the present invention proposes a composite material damage thickness measurement method based on terahertz time domain compensation. Compared with embodiment 3, this embodiment introduces S3 in detail.

[0076] Traditional terahertz damage thickness measurement methods mainly rely on accurately extracting the flight time of the damage from the terahertz response signal. However, terahertz waves are susceptible to complex interferences during propagation, such as dispersion, overlap, multiple reflections, noise, etc. These interferences can cause aliasing of the damage echo component in the terahertz signal. Although some traditional terahertz damage thickness measurement methods are committed to solving this problem, they often require manual selection of hyperparameters, and their performance is limited by manual experience and prior knowledge. In general, these hyperparameters are sensitive to changes in the terahertz signal. For terahertz signals from different sampling points, manual adjustment is required to obtain optimal performance, which is very time-consuming for samples with a large number of sampling signals. Therefore, a new damage thickness measurement method based on the difference in flight time between damaged and non-damaged areas is proposed. Specifically, based on the transmission characteristics of terahertz waves in samples, for composite layer samples without damage, the transmission time of terahertz waves through the sample can be defined as the time delay difference between the first pulse (from the upper interface of the sample) and the last pulse (from the lower interface of the sample) of the terahertz response signal. When damage occurs inside the sample, the transmission time of the terahertz wave through the sample will change. The difference in THz wave transmission time between the undamaged area and the damaged area is directly related to the damage thickness. In particular, since the energy of the THz signal is mainly concentrated in the first and last pulses, the other pulse echoes, including those from the damage, show lower amplitudes due to the high reflectivity of the THz wave at the upper and lower interfaces of the sample. In this work, the damage thickness is estimated by analyzing the difference in transmission time between the undamaged area and the damaged area. Ideally, assuming that the sample surface uniformity and the test parameters do not change, the proposed damage thickness measurement method can be defined as:

[0077] ct 1 =2n 1 d

[0078] ct 2 =2n 2 d (4)

[0079]

[0080] Where d is the damage thickness. 1 =1 is the refractive index of air. 1 is the transmission time of the terahertz wave through a lesion with thickness d. 2 is the refractive index of the sample. 2 is the transmission time of the terahertz wave through the sample with a thickness of d. Δt is the flight time difference, which refers to the transmission time difference of the terahertz wave through the damage with a thickness of d and the sample with a thickness of d, and c is the speed of light.

[0081] like Figure 6As shown in Figure 2, the damage thickness estimation results under different damage thicknesses are shown in Figure 2. Based on the proposed method, the damage thickness is estimated and the estimated damage thickness of 5 different sampling points under different damage thicknesses is obtained, as shown in Figure 2. Figure 6 As shown in (a)-(e), the estimated damage thickness values ​​at different sampling points are very close to the true values. In addition, the estimation error decreases with the increase of damage thickness. For sampling point 3, when the damage thickness is 200um, the estimation performance is the worst, and the estimated damage thickness is 215.69um, with an error of up to 7.84%. Figure 6 (f) shows the average estimation error of 5 different sampling points with different damage thicknesses. The results show that the average estimation error of all damage thicknesses is less than 5%, which proves the effectiveness of the proposed method for damage thickness estimation under different damage thicknesses.

[0082] Embodiment 5, the present invention proposes a composite material damage thickness measurement method based on terahertz time domain compensation. Compared with embodiment 4, this embodiment introduces S4 in detail.

[0083] First, in order to suppress the effect of lift-off distance on thickness estimation performance, signal reference transformation is completed. Usually, the terahertz reference signal is obtained by collecting the reflected echo of the metal plate. The traditional thickness estimation method uses this reference signal as the basis for damage flight time extraction. However, in terahertz testing, changes in lift-off distance may cause differences between the reference signal and the sample signal. In this case, the reference signal may not be the best choice for accurately extracting damage thickness. To solve this problem, in the proposed damage thickness measurement method, the terahertz response signal from the non-damaged area of ​​the same sample is used as the basic signal for damage flight time extraction. The special thing is that the transformed terahertz reference signal and the terahertz damage signal from the damaged area show similar characteristics and change with the change of lift-off distance. Therefore, the signal reference transformation not only alleviates the impact of lift-off distance changes on thickness estimation performance, but also compensates for the fluctuation of the test environment or parameter changes during the terahertz test.

[0084] Secondly, after completing the signal reference transformation, in order to improve the accuracy of damage thickness measurement, the signal alignment and cross-correlation flight time extraction process are completed for the preliminary estimation of the damage flight time. This method aims to automatically determine the time delay difference between the terahertz reference signal and the damage signal. Initially, the detrend function is used to solve the zero drift problem that occurs in the terahertz test system. Due to the influence of surface uniformity, the first pulses of the terahertz reference signal and the damage signal will be time-shifted, which will reduce the accuracy of the flight time estimation. Therefore, the signal alignment operation is completed. Specifically, the unit pulse response function of the time shift is used as the alignment function to move the terahertz damage signal in the time domain, and the first pulses of the terahertz reference signal and the damage signal are accurately aligned.

[0085] y align =y(t)*δ(t+Δt 0 ) (5)

[0086] Among them, y align is the aligned terahertz damage signal, * is the convolution operation, Δt 0 is the time shift value of the terahertz damage signal, which can be obtained by the time delay corresponding to the first pulse of the terahertz reference signal and the damage signal.

[0087] Then, after completing the signal alignment, the initial estimation process of the damage's flight time is realized by calculating the transmission time difference between the terahertz damage signal and the reference signal. In this case, since the first pulses of the terahertz reference signal and the damage signal are aligned, the transmission time difference of the terahertz wave can be defined by the time delay difference of the last pulse of the two aligned terahertz signals. In order to solve the time delay difference of the terahertz signal, the cross-correlation method, as a signal similarity measurement tool, can determine the time delay difference between the two signals by calculating the correlation coefficient, which provides great potential for damage flight time estimation. Specifically, the aligned terahertz signal is first subjected to a moving average filter to reduce the noise of the terahertz signal. Secondly, the last pulse of the terahertz reference signal and the damage signal are truncated into new signal vectors, such as y' ref and y' align , the cutoff interval can be determined by the signal time domain window and then y' ref and y' align The mutual correlation coefficient between them is:

[0088]

[0089] Among them, y' align =y align [p:q],y' ref =y ref [p:q]. p,q are the cutoff intervals of the signal.

[0090] Furthermore, the initial damage thickness can be obtained according to the obtained cross-correlation coefficient:

[0091] [peak,pos]=max(R)

[0092]

[0093] Where peak,pos is the maximum value of the cross-correlation coefficient R and its index, len(·) is the length of the signal, and f s is the sampling frequency of the terahertz signal, and TOF is the time of flight.

[0094] Example 6: The present invention proposes a method for measuring the damaged thickness of a composite material based on terahertz time domain compensation. Compared with Example 5, this example introduces S5 in detail.

[0095] like Fig.10 As shown in Figure 1, when the surface uniformity of the sample changes, the damage flight time estimated by formula (7) may be inconsistent with the actual transmission time difference between the terahertz reference signal and the damage signal. In order to further improve the estimation accuracy of the damage thickness, a terahertz signal flight time compensation model is established. Specifically, Fig.10 The figure shows the terahertz wave propagation process at different sampling points of the damaged sample to be tested. Point O is the sampling point of the terahertz reference signal, and points A and B are the sampling points of the terahertz damage signal. It can be found that due to the different uniformity of the sample surface, the terahertz wave transmission distance at different sampling points is different, which will cause the terahertz wave transmission time at different sampling points to change. Fig.10 , h is the difference in transmission distance between the sampling point and the reference point. When h ≥ 0, that is, sampling point A, the transmission distance increases, and the time delay of the first pulse of the obtained terahertz damage signal is less than the time delay from the reference point O. Therefore, the overall transmission time difference between the terahertz reference signal and the damage signal contains two parts: one is the transmission time difference caused by the change of the transmission distance h, which can be defined as The second is the transmission time difference caused by damage, which is related to the damage thickness. For sampling point A (h ≥ 0), the overall total transmission time difference can be defined as:

[0096] Δt=|TOF com -Δt 1

[0097]

[0098] Among them, Δt can be obtained by formula (7), Δt 1 TOF is the transmission time difference between the sampling point and the reference point caused by the change in transmission distance h. com is the compensated damage flight time, and n is the sample refractive index.

[0099] On the contrary, when h < 0, that is, sampling point B, the transmission distance decreases, the time delay of the first pulse of the obtained terahertz damage signal is greater than the time delay from the reference point O, and the overall transmission time difference is defined as:

[0100] Δt=TOF com +Δt 2

[0101]

[0102] Among them, Δt 2It is the difference in transmission time between the sampling point and the reference point caused by the change in transmission distance h.

[0103] Finally, considering the uniformity of the sample surface, the damage thickness d can be accurately obtained by flight time compensation:

[0104]

[0105] In summary, this paper proposes a new physically induced terahertz time-of-flight compensation method to achieve accurate thickness measurement of damage in composite laminates. The core of the proposed method is to simplify the flight time of damage to the transmission time difference between the damaged area and the non-damaged area. In order to better obtain the flight time of damage, a flight time compensation model is established, which takes into account the influence of the lifting distance and surface uniformity of the sample. In this case, a series of numerical and experimental results, including different damage thicknesses and depths, show the effectiveness of the proposed method for damage thickness estimation. These results highlight the robustness of the method to complex interference and external environment during terahertz testing, and can effectively improve the accuracy of damage thickness estimation in different scenarios. Compared with other classical time-of-flight extraction methods, the proposed method does not require any prior knowledge of damage and can obtain better damage thickness estimation performance. In future work, since the proposed method is very sensitive to the transmission time difference through the sample, it can not only be used to accurately estimate the damage thickness inside the composite material, but also has great potential to be extended to various thickness estimation fields, such as additive manufacturing, damage formation (TGO in thermal barrier coatings), material wear, etc.

[0106] The embodiments of the present invention are described in detail above with reference to the accompanying drawings, but the present invention is not limited thereto, and various changes can be made within the knowledge scope of technicians in the relevant technical field without departing from the purpose of the present invention.

Claims

1. A composite material damage thickness measurement method based on terahertz time domain compensation, characterized in that: The following steps are involved: S1. Prepare multiple composite laminate samples with variable damage thickness and depth, and obtain terahertz time-domain signals of different samples based on the terahertz time-domain spectroscopy system; S2. Analyze the influence of lifting distance and sample surface uniformity on terahertz signals, establish a theoretical transmission model for complex multilayer composite materials, and define the reflected terahertz response signal. The distance refers to the distance between the terahertz emitter and the sample surface. S3. Based on the terahertz wave response characteristics and transmission model, a method for measuring the damage thickness of composite materials based on the difference in flight time between damaged and non-damaged areas is proposed; S4, using terahertz signal reference transformation, alignment and cross-correlation methods to estimate the flight time information of composite material damage; S5. Considering the influence of lifting distance and sample surface uniformity, a terahertz signal flight time compensation model is established to achieve accurate estimation of the damage thickness of the composite material.

2. The composite material damage thickness measurement method based on terahertz time domain compensation according to claim 1 is characterized in that: In S1, the terahertz time-domain spectroscopy system includes a terahertz source, a control unit, a transmitter, a receiver and a motion platform. The motion directions of the motion platform include X and Y directions. The motion platform obtains 2D information from the sample. The transmitter and the receiver are integrated and installed on the motion platform. The transmitter and the receiver are responsible for transmitting and receiving terahertz signals. The terahertz source includes a femtosecond laser to generate terahertz pulses.

3. The composite material damage thickness measurement method based on terahertz time domain compensation according to claim 1 is characterized in that: In S1, six composite laminate samples with different damage thicknesses of 20mmx20mmx2mm were prepared using 3D printing technology. Each sample contained 10 layers, and the layer thickness was set to 200um. Glass fiber reinforced polymer was used as the printing material; In the preparation of damaged samples, the test samples are divided into undamaged areas and damaged areas. An air gap with a certain thickness is preset to simulate damage. The damage size is 10mmx10mm, and the damage thickness is variable, namely 100um, 200um, 400um, 600um, 800um and 1000um.

4. The composite material damage thickness measurement method based on terahertz time domain compensation according to claim 1 is characterized in that: In S1, 6 composite laminate samples with a size of 20mmx20mmx1.6mm and different damage depths were prepared. Each sample contained 8 layers, and the thickness of each layer was set to 200um. Adjacent layers were stacked in sequence by glass fiber reinforced polymer and polylactic acid printing materials. The test samples were divided into undamaged areas and damaged areas. An air gap with a certain thickness was preset to simulate damage. The damage size of the damaged area was set to 10mmx10mmx0.2mm, and the damage depth was set to 200um, 400um, 600um, 800um, 1000um and 1200um, and the damage thickness remained unchanged.

5. The composite material damage thickness measurement method based on terahertz time domain compensation according to claim 1, characterized in that: In S2, the reflected terahertz response signal is defined as: Where y(t) is the reflected terahertz echo, f(t) is the system impulse response function, P and K represent the number of laminate interface reflectors and the number of damaged interface reflectors, respectively, and θ p and θ k denote the model parameters corresponding to the Pth interface reflector and the Kth damage reflector, respectively. e(t) is Gaussian white noise. Δθi=Δθl+Δθe i It is the change of the overall model parameters of the terahertz signal caused by the change of lifting distance and surface uniformity.

6. The composite material damage thickness measurement method based on terahertz time domain compensation according to claim 1, characterized in that: In S3, the damage thickness measurement method can be defined as: ct1=2n1d ct2=2n2d Where d is the damage thickness. n1=1 is the refractive index of air. t1 is the transmission time of the terahertz wave through the damage with a thickness of d. n2 is the refractive index of the sample. t2 is the transmission time of the terahertz wave through the sample with a thickness of d. Δt is the flight time difference, which refers to the transmission time difference of the terahertz wave through the damage with a thickness of d and the sample with a thickness of d, and c is the speed of light.

7. The composite material damage thickness measurement method based on terahertz time domain compensation according to claim 1, characterized in that: In S4, signal reference transformation refers to using the terahertz response signal from the non-damaged area of ​​the same sample as the basic signal for flight time extraction of the damaged area. Signal alignment uses the time-shifted unit pulse response function as the alignment function to move the terahertz damage signal in the time domain and accurately align the first pulse of the terahertz reference signal and the damage signal. The aligned terahertz damage signal is defined as: y align =y(t)*δ(t+Δt0) Among them, y align is the aligned terahertz damage signal, * is the convolution operation, and Δt0 is the time shift value of the terahertz damage signal, which can be obtained by the time delay corresponding to the first pulse of the terahertz reference signal and the damage signal.

8. The composite material damage thickness measurement method based on terahertz time domain compensation according to claim 7, characterized in that: Since the first pulses of the terahertz reference signal and the damaged signal are aligned, the transmission time difference of the terahertz wave can be defined by the time delay difference of the last pulse of the two aligned terahertz signals. The time delay difference of the two signals is determined by the cross-correlation method, specifically: the aligned terahertz signal is subjected to moving average filtering to reduce the noise of the terahertz signal. Secondly, the last pulse of the terahertz reference signal and the damaged signal are truncated into new signal vectors, such as y' ref and y' align , the cutoff interval can be determined by the signal time domain window and then y' ref and y' align The mutual correlation coefficient between them is: Among them, y' align =y align [p:q],y' ref =y ref [p:q]. p,q is the signal cutoff interval. Furthermore, the initial damage thickness can be obtained based on the obtained cross-correlation coefficient: [peak,pos]=max(R) Where peak,pos is the maximum value of the cross-correlation coefficient R and its index, len(·) is the length of the signal, and f s is the sampling frequency of the terahertz signal, and TOF is the time of flight.

9. The composite material damage thickness measurement method based on terahertz time domain compensation according to claim 8, characterized in that: In S5, the positioning parameter h is the difference in transmission distance between the sampling point and the reference point. When h ≥ 0, the transmission distance increases, and the time delay of the first pulse of the terahertz damage signal obtained is less than the time delay from the reference point. The transmission time difference between the terahertz reference signal and the damage signal is defined as: Δt=|TOF com -Δt1 Among them, Δt1 is the transmission time difference between the sampling point and the reference point caused by the change of the transmission distance h, and Δt0 is the transmission time difference caused by the change of the transmission distance h. TOF com is the compensated damage flight time, n is the sample refractive index; when h < 0, the transmission distance decreases, the time delay of the first pulse of the obtained terahertz damage signal is greater than the time delay from the reference point, and the overall transmission time difference is defined as: Δt=TOF com +Δt2 Wherein, Δt2 is the transmission time difference between the sampling point and the reference point caused by the change in the transmission distance h.

10. The composite material damage thickness measurement method based on terahertz time domain compensation according to claim 9, characterized in that: Considering the uniformity of the sample surface, the damage thickness d is accurately obtained by flight time compensation. The overall calculation method is as follows:

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