Composite material acoustic excitation terahertz nondestructive testing method

Abstract

The application discloses a kind of composite material acoustic excitation terahertz nondestructive testing methods, comprising the following steps: carrying terahertz time-domain spectroscopy nondestructive testing system;With terahertz time-domain spectroscopy nondestructive testing system detection no acoustic excitation loading composite material test piece, obtain no acoustic excitation terahertz time-domain spectroscopy data as reference data Data ref ;Different frequency acoustic excitation signal is set;With terahertz time-domain spectroscopy nondestructive testing system detection in different acoustic excitation field in composite material test piece, obtain after loading acoustic excitation terahertz time-domain spectroscopy data Data ref And loading acoustic excitation terahertz time-domain spectroscopy data respectively after imaging then carry out difference imaging, obtain difference image calculate the contrast of difference image, select the contrast highest one group difference image as final detection result from it.

Description

TECHNICAL FIELD

[0001] The present application belongs to the field of terahertz nondestructive testing, and particularly relates to a composite material acoustic excitation terahertz nondestructive testing method. BACKGROUND

[0002] During the manufacturing process of a composite material to-be-tested sample, various processing defects such as bubbles, holes, interface debonding, micro-cracks, interlayer cracking and various inclusions will inevitably occur, the existence of these defects will change the mechanical state of the entire structure, reduce the material strength and shorten the service life, and the defect size in the composite material ranges from several microns to several centimeters [10.1016 / j.infrared.2019.01.013], the larger defects can be judged by the human eye, but the smaller defects are difficult to find in the macroscopic. When the resonance frequency of the defect area in the composite material is equal to or close to a certain frequency of the continuously changing acoustic excitation, the defect area will have a frequency resonance phenomenon and produce forced vibration [DOI:10.11973 / wsjc202009016], under the resonance condition, the medium amplitude at the defect position is the largest, which achieves the effect of amplifying the defect area, at this time, detecting the data of the sample under the resonance condition by using another detection method can better judge the internal defects of the sample.

[0003] In the existing field of composite material defect detection based on acoustic excitation, electronic speckle and laser speckle technology are mostly used to capture the vibration of the material after being subjected to acoustic excitation. When the measured object is subjected to a certain load (force, heat, vibration), the internal debonding, delamination and other defects of the object will cause the surface of the object to produce greater deformation than normal, this small deformation will be displayed by the speckle interference fringes, which is manifested as the density of the speckle interference fringe pattern, thereby achieving the purpose of nondestructive testing. However, this method is limited by the physical properties of laser speckle and electronic speckle [Yu, Changsong. Research on vibration detection based on electronic speckle shear interferometry [J]. Journal of Changchun University of Science and Technology (Natural Science Edition), 2011, 34(03): 10-12+18.], and cannot penetrate the composite material to directly detect the resonance condition at the defect position, mainly using time-averaging method or stroboscopic method to measure the difference in image light intensity before and after being subjected to acoustic excitation, which leads to that the edges of the defects finally recognized are relatively fuzzy, and it is impossible to achieve more accurate according to the resonance frequency at the defect position of the composite material.

[0004] Terahertz wave is a kind of high frequency electromagnetic wave with a frequency range of 0.1-10THz.Compared with the existing infrared speckle method, terahertz wave has higher spatial resolution, and can measure the detailed vibration of each position of the object in the vibration process, including the central vibration frequency, the second harmonic and the frequency distortion.Compared with optical sensing, the wavelength of terahertz wave is longer.The bonding layer between the composite material and the bonding substrate will cause loss of terahertz wave, and the edge of the bonding defect will affect the scattering effect of terahertz wave, which will affect the intensity distribution of terahertz wave.This characteristic enables terahertz wave to penetrate the material surface to detect the subtle mechanical vibration inside the object, and the more intense the vibration of the object, the higher the energy field generated, and through analysis of the terahertz signal, the shape, defect and edge position inside the object can be obtained.These advantages make terahertz wave have broad application prospects in the fields of defect detection and vibration sensing. SUMMARY

[0005] The application provides a composite material defect acoustic excitation terahertz nondestructive detection method, which is based on a continuously changing acoustic excitation field, uses a reflective terahertz nondestructive detection imaging technology to characterize the vibration of a composite material defect at a corresponding resonance frequency, improves the identification effect of a small defect inside the composite material, and accurately positions the small defect inside a multilayer structure to accurately identify the small defect.

[0006] The purpose of the application is achieved by the following technical scheme:

[0007] A composite material acoustic excitation terahertz nondestructive detection method, comprising the following steps:

[0008] Step one, load a terahertz time domain spectrum nondestructive detection system; the terahertz time domain spectrum nondestructive detection system comprises a terahertz host, a sensor, a terahertz probe, an acoustic excitation generation module, a two-dimensional scanning module and a computer, and the terahertz probe is installed on the two-dimensional scanning module;

[0009] Step two, use the terahertz time domain spectrum nondestructive detection system to detect a composite material to-be-detected sample without acoustic excitation loading, and obtain terahertz time domain spectrum data without acoustic excitation as reference data Data ref ;

[0010] Step three, set acoustic excitation signals with different frequencies;

[0011] Step four, use the terahertz time domain spectrum nondestructive detection system to detect the composite material to-be-detected sample in different acoustic excitation fields, and obtain terahertz time domain spectrum data after loading acoustic excitation

[0012] Step five, compare the reference data Data ref obtained in the step two and the terahertz time domain spectrum data after loading acoustic excitation obtained in the step four respectively, and then differential imaging is performed to obtain differential images

[0013] Step six, calculating the contrast of the differential images, and selecting the differential image with the highest contrast as the final detection result.

[0014] Further, in the terahertz time-domain spectroscopy nondestructive detection system, the terahertz host generates pump light and probe light, which are transmitted into the terahertz probe; the terahertz detection signal collected by the terahertz probe is transmitted into the computer through the sensor; the acoustic excitation generation module is used to adjust the frequency of the acoustic excitation field, so that the defect area in the composite material to be tested produces resonance phenomenon.

[0015] Further, the step two includes: using the two-dimensional scanning module to carry the terahertz probe to detect the composite material to be tested in a full-coverage two-dimensional point-by-point scanning manner, and transmitting the data into the computer to obtain the terahertz time-domain spectroscopy data of the composite material to be tested as the reference data Data ref .

[0016] Further, in the step one, the terahertz host generates pump light and probe light, which are transmitted into the terahertz probe, and the pump light in the terahertz probe is converted into terahertz waves, and the incidence direction of the terahertz waves is perpendicular to the surface of the composite material to be tested.

[0017] Preferably, in the step one, the acquisition step lengths of the rows and columns of the two-dimensional scanning module are respectively m and n; and the acquisition step distance is 0.2mm.

[0018] Further, the step three includes: setting the adjustable frequency interval (f1-Δf, f1+Δf) generated by the acoustic excitation generation module, f1 is the approximate resonance fundamental frequency of the defect area in the composite material to be tested, Δf is the modulation frequency range, f1-Δf is the lower limit of the frequency, and f1+Δf is the upper limit of the frequency; under the acoustic excitation of the resonance fundamental frequency f1, the defect area in the composite material to be tested produces resonance.

[0019] Further, the defect resonance fundamental frequency f1 in the composite material to be tested is calculated by the following formula:

[0020]

[0021] In the formula, h is the defect thickness, a is the defect radius, ρ is the material density, E is the elastic modulus of the material, and u is the Poisson's ratio.

[0022] Further, the step four includes:

[0023] ​4.1) open the sound excitation generating module, set the sound excitation signal frequency to f1-Δf, and generate a sinusoidal wave signal with the frequency of f1-Δf to form a sound excitation field surrounding the composite material sample to be measured;

[0024] 4.2) use the two-dimensional scanning module to carry the terahertz probe to collect the terahertz time-domain spectrum data of the composite material sample to be measured in the f1-Δf sound excitation field in a full-coverage two-dimensional point-by-point scanning manner

[0025] 4.3) increase the frequency of the sinusoidal wave generated by the sound excitation generating module, so that the frequency of the sound field surrounding the composite material sample to be measured is also increased, and the terahertz time-domain spectrum data of the sample is collected again by using the step 4.2)

[0026] 4.4) repeat the step 4.3), when the sound excitation generator generates a sinusoidal wave with the frequency of f1+Δf, collect the terahertz time-domain spectrum data of the sample for the last time k groups of data are obtained in total.

[0027] Further, the step five comprises:

[0028] 5.1) perform time-of-flight imaging on the reference data Data ref and the terahertz time-domain spectrum data after loading the sound excitation to obtain a sound-excitation-free time-of-flight image IMG ref and the images under different frequency sound excitations t represents the detection order;

[0029] 5.2) perform a difference operation on the time-of-flight images of the terahertz data collected under different frequency sound excitations and the time-of-flight IMG ref of the terahertz data collected under sound excitation-free conditions to obtain a difference image

[0030]

[0031] Further, in the step six, the contrast C of all the difference images obtained in the step five is calculated according to the following formula:

[0032]

[0033] In the formula, δ(i,j) = |i-j| is the gray value between adjacent pixels; ρ δ (i,j) is the pixel distribution probability of the gray difference δ between adjacent pixels.

[0034] The present application has the following beneficial effects:​

[0035] The application provides a composite material acoustic excitation terahertz nondestructive testing method, which can be applied to the field of terahertz nondestructive testing and is used for detecting nonpolar materials such as foams, resins and rubbers. The method is characterized in that the measured sample of the composite material is placed in an acoustic wave field of a specific frequency through an external loading acoustic excitation field mode, resonance phenomenon is generated, a micro off-surface displacement is generated in a defect area of the material and the change of the internal energy field of the material is caused, the terahertz nondestructive testing is performed on the sample by combining the terahertz technology, the nondestructive testing on the sample is realized under the common excitation of terahertz and acoustic waves, and a new detection mode is provided for the field of terahertz nondestructive testing.

[0036] Compared with detection technologies such as digital speckle, the method can obtain the information of each layer inside the measured sample of the composite material by analyzing the terahertz echo at different depths of the sample, has stronger detection capability for defects such as debonding and weak adhesion, can obtain information such as the spatial position, size and shape of the defects through data analysis, and improves the identification capability of the micro defects inside the composite material. BRIEF DESCRIPTION OF DRAWINGS

[0037] Figure 1 A composite material terahertz acoustic excitation defect detection method flow chart is described in the embodiments of the application.

[0038] Figure 2 A terahertz time-domain spectroscopy nondestructive testing system composition principle diagram used in the composite material terahertz acoustic excitation defect detection method described in the embodiments of the application.

[0039] Figure 3 It is a terahertz time-domain image.

[0040] Fig. 4(a) is a terahertz time-of-flight difference image of the measured sample of the composite material under the acoustic excitation of a frequency of 30 Hz.

[0041] Fig. 4(b) is a terahertz time-of-flight difference image of the measured sample of the composite material under the acoustic excitation of a frequency of 50 Hz.

[0042] Fig. 4(c) is a terahertz time-of-flight difference image of the measured sample of the composite material under the acoustic excitation of a frequency of 70 Hz.

[0043] In the drawings:

[0044] 1-terahertz main machine; 2-sensor; 3-terahertz probe; 4-acoustic excitation generation module; 5-two-dimensional scanning module; 6-measured sample of the composite material; 7-computer. DETAILED DESCRIPTION

[0045] In order to make the purpose, technical scheme and advantages of the application clearer and more apparent, the application is further described in detail below with reference to the drawings.

[0046] AsFigure 1 As shown, this embodiment takes a composite material sample 6 with pre-existing defects as an example to provide a method for detecting terahertz acoustic excitation defects in composite materials, including the following steps:

[0047] Step 1: Equip a terahertz time-domain spectroscopy nondestructive testing system:

[0048] like Figure 2 As shown, the terahertz time-domain spectroscopy nondestructive testing system mainly includes a terahertz host 1, a sensor 2, a terahertz probe 3, an acoustic excitation generation module 4, a two-dimensional scanning module 5, and a computer 7. The terahertz host 1 generates pump light and probe light, which are transmitted to the terahertz probe 3, which is mounted on the two-dimensional scanning module 5. The terahertz detection signal collected by the terahertz probe 3 is transmitted to the computer 7 via the sensor 2. The acoustic excitation generation module 4 is used to adjust the frequency of the acoustic excitation field to induce resonance in the defect area of ​​the composite material sample 6.

[0049] Step 2: Use a terahertz time-domain spectroscopy nondestructive testing system to test the composite material sample subjected to silent excitation, and obtain silent-excited terahertz time-domain spectral data as reference data:

[0050] The terahertz host 1 generates pump light and probe light, which are transmitted to the terahertz probe 3. The pump light in the terahertz probe 3 is converted into a terahertz wave. The incident direction of the terahertz wave must be perpendicular to the surface of the composite material sample 6, i.e., the terahertz wave must be incident directly onto the surface of the composite material sample 6. The two-dimensional scanning module 5, carrying the terahertz probe 3, performs a full-coverage two-dimensional point-by-point scan of the composite material sample 6. The row and column acquisition step sizes of the two-dimensional scanning module 5 are set to m and n, respectively, with a step size of 0.2 mm. The terahertz detection signal acquired by the terahertz probe 3 is transmitted to the computer 7 via the sensor 2 to obtain the terahertz time-domain data of the composite material sample 6, which serves as reference data (Data). ref .

[0051] Step 3: Set acoustic excitation signals of different frequencies:

[0052] The adjustable frequency range (f1-Δf, f1+Δf) of the acoustic excitation generation module 4 is set, where f1 is the approximate resonant fundamental frequency of the defect region of the composite material test sample 6, Δf is the modulation frequency range, f1-Δf is the lower limit of the frequency, and f1+Δf is the upper limit of the frequency. The resonant fundamental frequency f1 of the defect in the composite material test sample 6 is shown in Equation (1), where h is the defect thickness, a is the defect radius, ρ is the material density, E is the elastic modulus of the material, and u is Poisson's ratio.

[0053]

[0054] The defect region in the composite material sample 6 under test resonates under the acoustic excitation at the resonant fundamental frequency f1.

[0055] Step four, using the terahertz time-domain spectroscopy nondestructive testing system to detect the composite material sample under test in different acoustic excitation fields, and obtaining the terahertz time-domain spectroscopy data after loading the acoustic excitation:

[0056] 4.1) Turn on the acoustic excitation generation module 4, set the acoustic excitation signal frequency to f1-Δf, and generate a sine wave signal with a frequency of f1-Δf to form an acoustic excitation field surrounding the composite material sample 6 under test.

[0057] 4.2) Use the two-dimensional scanning module 5 to carry the terahertz probe 3 to collect the terahertz time-domain spectroscopy data of the composite material sample 6 under test in the f1-Δf acoustic excitation field in a full-coverage two-dimensional point-by-point scanning manner

[0058] 4.3) Increase the frequency of the sine wave generated by the acoustic excitation generation module 4, so that the frequency of the acoustic field surrounding the composite material sample 6 under test also increases, and the terahertz time-domain spectroscopy data of the sample is collected again using the steps in 4.2)

[0059] 4.4) Similarly, repeat the detection steps in 4.3), and when the acoustic excitation generator generates a sine wave with a frequency of f1+Δf, collect the terahertz time-domain spectroscopy data of the sample for the last time k sets of data are obtained in total.

[0060] Step five, image the reference data obtained in step two and the terahertz time-domain spectroscopy data after loading the acoustic excitation obtained in step four, respectively, and perform differential imaging to obtain a differential image:

[0061] 5.1) Perform time-of-flight imaging on the terahertz detection data Data ref of the composite material sample 6 under test without acoustic excitation and the terahertz detection data of the composite material sample 6 under test in the acoustic wave field at each frequency to obtain images IMG ref under different frequency acoustic excitations and the time-of-flight image IMG ref without acoustic excitation, where t represents the detection order. When terahertz propagates in different media, reflection and transmission occur, which are manifested as troughs and peaks in the time domain. The distance between different peaks or troughs is the time of flight T, as shown in Figure 3 .

[0062] 5.2) Perform differential operation on the time-of-flight images IMG of the terahertz data collected under different frequency acoustic excitations using formula (2) ref and the time-of-flight image IMG ref of the terahertz data collected without acoustic excitation to obtain a differential image IMG As shown in Figure 4.

[0063]

[0064] Step 6: Calculate the difference image The contrast ratio is calculated, and the set of difference images with the highest contrast ratio is selected as the final detection result.

[0065] Calculate all the difference images obtained in step 5.2) according to formula (3). Contrast C:

[0066]

[0067] Where δ(i,j)=|ij| is the gray value between adjacent pixels, ρ δ (i,j) represents the pixel distribution probability with a gray-level difference of δ between adjacent pixels; the set of data with the highest contrast is used as the final detection result.

[0068] To illustrate the effectiveness of this invention, the method of this invention was used to detect composite materials containing pre-existing defects, obtaining terahertz time-of-flight differential images in the range of 30Hz to 70Hz. Calculations showed that the composite sample under test exhibited the highest contrast for pre-existing defects within the 50Hz acoustic excitation field. Therefore, 50Hz is the resonant fundamental frequency of the debonding defect region in the composite material. Defect imaging at this frequency is better than imaging results at other frequencies; hence, this image was used as the final detection result.

Claims

1. A method for acoustically excited terahertz nondestructive testing of composite materials, characterized in that, Includes the following steps: Step 1: Install a terahertz time-domain spectroscopy non-destructive testing system; the terahertz time-domain spectroscopy non-destructive testing system includes a terahertz host, sensor, terahertz probe, acoustic excitation generation module, two-dimensional scanning module and computer, and the terahertz probe is installed on the two-dimensional scanning module; Step 2: Use a terahertz time-domain spectroscopy nondestructive testing system to test the composite material sample subjected to silent excitation, and obtain silent-excited terahertz time-domain spectral data as reference data. ref ; Step 3: Set acoustic excitation signals of different frequencies; Step 3 includes: The adjustable frequency range (f1-Δf, f1+Δf) of the acoustic excitation generation module is set, where f1 is the fundamental resonant frequency of the defect region in the composite material sample under test, Δf is the modulation frequency range, f1-Δf is the lower limit of the frequency, and f1+Δf is the upper limit of the frequency. The fundamental resonant frequency f1 of the defect in the composite material sample under test is calculated by the following formula: In the formula, h is the defect thickness, a is the defect radius, ρ is the material density, E is the elastic modulus of the material, and u is Poisson's ratio; Under acoustic excitation at the fundamental frequency f1, the defect region within the composite material sample under test resonates. Step 4: Use a terahertz time-domain spectroscopy nondestructive testing system to test the composite material samples under different acoustic excitation fields to obtain terahertz time-domain spectral data after acoustic excitation. Step 5: Process the reference data obtained in Step 2. ref and the terahertz time-domain spectral data obtained after acoustic excitation in step four. After imaging each image separately, differential imaging is performed to obtain a differential image. Step 6: Calculate the difference image The contrast ratio is calculated, and the set of difference images with the highest contrast ratio is selected as the final detection result.

2. The acoustic-excited terahertz nondestructive testing method for composite materials as described in claim 1, characterized in that, The terahertz time-domain spectroscopy nondestructive testing system consists of a terahertz host that generates pump light and probe light, which are transmitted to the terahertz probe; the terahertz detection signal collected by the terahertz probe is transmitted to the computer via a sensor; and the acoustic excitation generation module is used to adjust the frequency of the acoustic excitation field to induce resonance in the defect area of ​​the composite material sample.

3. The acoustic-excited terahertz nondestructive testing method for composite materials as described in claim 1, characterized in that, Step two includes: using a two-dimensional scanning module carrying a terahertz probe to detect the composite material sample in a full-coverage two-dimensional point-by-point scanning manner, and transmitting the data to a computer to obtain the terahertz time-domain spectral data of the composite material sample as reference data. ref .

4. The acoustic-excited terahertz nondestructive testing method for composite materials as described in claim 3, characterized in that, In step one, the terahertz host generates pump light and probe light, which are transmitted to the terahertz probe. The pump light in the terahertz probe is converted into a terahertz wave, and the incident direction of the terahertz wave is perpendicular to the surface of the composite material sample to be tested.

5. The acoustic-excited terahertz nondestructive testing method for composite materials as described in claim 3, characterized in that, In step one, the acquisition step size of the row and column of the two-dimensional scanning module is set to m and n, respectively; The sampling step size is 0.2 mm.

6. The acoustically excited terahertz nondestructive testing method for composite materials as described in claim 1, characterized in that, Step four includes: 4.1) Turn on the acoustic excitation generation module, set the acoustic excitation signal frequency to f1-Δf, and generate a sine wave signal with a frequency of f1-Δf to form an acoustic excitation field surrounding the composite material sample under test. 4.2) Using a two-dimensional scanning module carrying a terahertz probe, terahertz time-domain spectral data of the composite material sample under test in the f1-Δf acoustic excitation field are acquired in a full-coverage two-dimensional point-by-point scanning manner. 4.3) Increase the frequency of the sinusoidal wave generated by the acoustic excitation module, so that the frequency of the acoustic field surrounding the composite material sample also increases accordingly, and repeat step 4.2) to acquire the terahertz time-domain spectral data of the sample. 4.4) Repeat step 4.3) when the acoustic excitation generator produces a sine wave with frequency f1+Δf, and collect the last sample terahertz time-domain spectral data. A total of k sets of data were obtained.

7. The acoustic-excited terahertz nondestructive testing method for composite materials as described in claim 1, characterized in that, Step five includes: 5.1) Regarding the reference data Data ref and terahertz time-domain spectral data after acoustic excitation Perform time-of-flight imaging to obtain silent-excited time-of-flight images (IMG). ref and images under acoustic excitation at different frequencies t represents the detection order; 5.2) The time-of-flight images of terahertz data acquired under acoustic excitation at different frequencies are obtained using the following formula. Time of flight of terahertz data acquired under silent excitation (IMG) ref Perform a difference operation to obtain the difference image.

8. The acoustic-excited terahertz nondestructive testing method for composite materials as described in claim 1, characterized in that, In step six, all the difference images obtained in step five are calculated according to the following formula. Contrast C: In the formula, δ(i,j)=|ij| represents the grayscale value between adjacent pixels; ρ δ (i,j) represents the pixel distribution probability with a gray level difference of δ between adjacent pixels.

Images