Terahertz surface array transmission-based composite material defect nondestructive testing method and system

Abstract

The application discloses a kind of based on terahertz surface array detector transmission type nondestructive testing method and system, belong to terahertz imaging nondestructive testing technical field.The application is based on the mixing array imaging system of terahertz technology and based on the shaping and focusing of terahertz beam, by being placed between shaping lens and imaging lens with ceramic or glass fiber reinforced composite material sample piece for aerospace, the transmission type detection is carried out to sample piece, and the surface area information of sample piece is obtained by mixing array detector, and typical defect feature processing and defect identification are carried out, to rapidly evaluate the state of sample piece.The application aims to solve the problem of low efficiency and low imaging quality in existing terahertz nondestructive testing technology.By using terahertz surface array detector and optical imaging assembly, fast, high-resolution nondestructive testing can be achieved, especially suitable for application scenarios requiring high detection efficiency and imaging quality.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of terahertz imaging, and in particular to a composite material defect nondestructive detection method and system based on terahertz surface array transmission. BACKGROUND

[0002] Terahertz waves (THz) are electromagnetic waves between microwaves and infrared light, with a frequency range of 0.1 THz to 10 THz and a wavelength of 3 mm to 30 μm. Terahertz waves have unique advantages such as strong penetration, non-contact detection, and wide spectral lines, and have broad application prospects in the field of nondestructive testing. In recent years, terahertz imaging technology has been widely applied in aerospace, electronic products, food safety, and biomedical fields.

[0003] Currently, terahertz focal plane array detectors (FPA) are one of the main detectors used in terahertz imaging technology. In order to improve the sensitivity and imaging quality of FPA, a quasi-optical system is usually used to focus terahertz waves. The existing quasi-optical systems for terahertz focal plane array detectors mainly have two types: (1) Refractive lens quasi-optical system: Refractive lens quasi-optical system usually consists of 2 or 3 refractive lenses, which can effectively focus terahertz waves. However, the transmission loss of refractive lens materials and the interference between different interfaces will reduce the sensitivity and imaging quality of the system. (2) Reflective mirror quasi-optical system: Reflective mirror quasi-optical system usually consists of 2 or 3 reflective mirrors, which can avoid material transmission loss. However, traditional spherical or aspherical mirrors are difficult to eliminate aberrations, resulting in poor imaging quality.

[0004] Non-destructive testing (NDT) technology is an integral part of materials science, manufacturing, and quality control, which can evaluate the integrity and performance of materials without damaging the sample. Terahertz (THz) wave technology has become an emerging technology in the field of non-destructive testing due to its unique spectral characteristics. Terahertz waves can penetrate many non-polar materials such as plastics, ceramics, and glass fiber reinforced composites, while being sensitive to water molecules and certain chemicals, making it advantageous in detecting hidden defects and foreign objects.

[0005] Currently, the terahertz non-destructive testing system mainly uses a single detector to step scan the sample for imaging. This method has some limitations, mainly in low efficiency. Step scanning requires the detector to collect terahertz signals of the sample point by point, which takes a long time, especially when high-resolution imaging is required, the scanning area increases, and the detection time will significantly increase. In addition, the imaging quality of the single detector is limited by the performance of the detector, including its sensitivity, noise level, and dynamic range.

[0006] In a terahertz non-destructive testing system, the optical elements for beam shaping and sample imaging are usually refractive optical lenses. Refractive lenses focus and shape the terahertz beam through the combination of multiple lenses. However, the interference and transmission loss between these lenses can affect the imaging quality of the system. Specifically, the absorption and scattering of the lens material can cause a decrease in signal strength, affecting the contrast and sensitivity of the imaging. In addition, the manufacturing and alignment errors of the lenses can also introduce aberrations, reducing the clarity and resolution of the imaging. SUMMARY

[0007] Based on the deficiencies of the prior art described above, the present application provides a transmission type non-destructive testing method and system based on a terahertz area array detector, aiming to solve the problems of low single-point scanning imaging efficiency, large system loss and low imaging quality in existing terahertz non-destructive testing technology. By using a terahertz area array mixing detector and a specific optical imaging assembly, the non-destructive testing method can realize transmission detection of composite material samples, process the detection results and identify typical defects, achieving fast and high-resolution non-destructive testing, especially suitable for aerospace composite material production, research and development, detection and maintenance applications that require high detection efficiency and imaging quality. The use of a terahertz area array mixing detector is the key to improving detection efficiency, as it can simultaneously collect signals from multiple pixels, enabling fast imaging. At the same time, the introduction of a total reflection type beam shaping technology and an off-axis three-mirror optical imaging system is expected to reduce the interference and transmission loss of optical elements, improve the imaging quality of the insulation tiles and glass fiber reinforced composite material samples, and further ensure the detection results.

[0008] To achieve the above-mentioned purpose, the technical scheme provided by the present application is as follows:

[0009] On the one hand, the present application provides a composite material defect non-destructive testing method based on terahertz area array transmission, which comprises the following steps:

[0010] S1, setting parameters for the terahertz area imaging system, setting the frequency modulation range of the terahertz frequency modulation source to 0.3THz to 0.4THz, the maximum output power to 40mw, and the output wave to a continuous wave, setting the array size of the terahertz area array detector to 32x32, 64x64, 128x128 or 256x256, and the array unit size to 0.4mmx0.4mm;

[0011] S2, placing the sample to be detected on an electric four-dimensional displacement table and adjusting it to the initial position;

[0012] S3, start the terahertz frequency modulation source, the divergent terahertz Gaussian beam is collimated through the off-axis collimating mirror of the terahertz total reflection beam shaping lens, the collimated beam is transmitted to the first group of microarray off-axis mirrors, the first group of microarray off-axis mirrors include a plurality of sub off-axis parabolic mirrors, each sub off-axis parabolic mirror focuses the incident beam to a plane parallel to it, thereby dividing the parallel beam into a plurality of focused sub-beams, then transmitted to the second group of microarray off-axis mirrors, again divergent to form a plurality of sub-beams incident on the off-axis integral mirror, all the sub-beams will coincide on the focal plane after passing through the off-axis integral mirror, thereby obtaining a uniform shaped spot, then reflected by a plane mirror;

[0013] S4, the shaped terahertz beam is incident from one side of the sample to be detected, is reflected on the first mirror of the terahertz off-axis three-mirror imaging lens after internal fold reflection and scattering in the sample to be detected to form the first reflected beam; the first reflected beam is incident on the second mirror and reflected to form the second reflected beam; the second reflected beam is incident on the third mirror and reflected to form the third reflected beam; finally, the terahertz area array detector located at the convergent spot of the third reflected beam is used for receiving and imaging;

[0014] S5, based on the area of the sample to be detected, the scanning area size is determined according to the needs, then the scanning step and the scanning route are set, based on the setting of the scanning parameters and the control of the upper computer, the electric four-dimensional displacement table is controlled to move, the scanning imaging of the large area of the sample to be detected is realized, and a plurality of scanning images are obtained;

[0015] S6, the obtained plurality of scanning images are spliced, the feature points in each image are extracted, the feature points are matched, the mapping matrix between the adjacent two images is calculated and fused, when extracting the feature points in each image, a plurality of feature points are extracted first, then the plurality of feature points are judged, the feature points with the terahertz detection image characteristics are retained, more than four feature points are selected from the obtained feature points and substituted into the homography transformation matrix, the matrix parameter value is solved to realize the fusion operation of the image, and a complete image of the sample to be detected is obtained;

[0016] S7, the complete image of the sample to be detected is image enhanced, the Unet neural network model is used to extract the signal characteristics of the detected sample, the visible light band image and the test image are taken as input for image enhancement, and the image enhancement formula is as follows:

[0017]

[0018] In the formula, I get represents the obtained complete image, which is an optical image, I optical is used as the original input in the quality reduction model, I enhancedrepresents an enhanced image, and ε is a quality reduction parameter, represents a convolution operation, represents a down-sampling rate, and n represents noise of the system;

[0019] S8, inputting the enhanced image after signal amplification and filtering into a defect recognition model to realize target defect detection of the sample to be detected.

[0020] Preferably, the terahertz surface array imaging system comprises a terahertz surface array detector, a terahertz frequency modulation source, a terahertz total reflection beam shaping lens, a terahertz off-axis three-mirror imaging lens, and an electric four-dimensional displacement table.

[0021] The frequency modulation range of the terahertz frequency modulation source is 0.3 THz to 0.4 THz, the maximum output power is 40 mw, and the output wave is a continuous wave.

[0022] The array scale of the terahertz surface array detector is 32x32, 64x64, 128x128, or 256x256, and the array unit size is 0.4 mm x 0.4 mm.

[0023] The terahertz off-axis three-mirror imaging lens is an XY polynomial free-form surface off-axis three-mirror imaging lens, comprising a first mirror, a second mirror, and a third mirror, and the free-form surface equation of the XY polynomial is:

[0024]

[0025] In the formula, r is the radial distance of the light ray at the incident point; c is the curvature radius of the beam shaping mirror; k is the quadratic surface coefficient of the beam shaping mirror; m and n are the orders of the aspherical terms;

[0026] The terahertz total reflection beam shaping lens comprises an off-axis collimating mirror, an off-axis microarray mirror, an off-axis integral mirror, and a plane mirror, and the off-axis microarray mirror comprises a first group of microarray off-axis mirrors and a second group of microarray off-axis mirrors.

[0027] The electric four-axis displacement table comprises an x-axis linear displacement table, a y-axis linear displacement table, a z-axis linear displacement table, and a θ-axis rotary displacement table, and the range of the x-axis linear displacement table, the y-axis linear displacement table, and the z-axis linear displacement table is all ≥500 mm, and the angle change range of the θ-axis rotary displacement table is ±20 degrees.

[0028] Preferably, the terahertz surface array detector can directly perform real-time detection imaging on the surface area of a sample, and for a detection area larger than the array scale, the array scale can be selected according to the size of the scanning area, and the corresponding scanning step and scanning path are set through the electric four-axis displacement table to complete large-area scanning detection.

[0029] Preferably, the terahertz total reflection type beam shaping lens obtains a homogenized light spot size greater than or equal to 50mm*50mm, and a relative light intensity distribution uniformity greater than or equal to 70%.

[0030] Preferably, the scanning process adopts a transmission type optical path, and the terahertz wave is emitted from one side of the sample to be detected, and is received by the terahertz surface array detector after internal fold reflection and scattering of the object.

[0031] Preferably, the sample to be detected is a ceramic or glass fiber reinforced composite material.

[0032] Preferably, the off-axis collimating mirror is an off-axis parabolic mirror with a focal length of 120mm and an off-axis eccentricity of 120mm, the first group of microarray off-axis mirrors and the second group of microarray off-axis mirrors are both off-axis array mirrors with an array size of 17*17, and the off-axis integral mirror is a mirror with a focal length of 1143mm and an off-axis eccentricity of 1143mm.

[0033] Preferably, the homography transformation matrix in step S6 is a 3*3 matrix, which is used for image correction and image stitching.

[0034] Preferably, in step S8, the defect recognition model uses an edge extraction based method to set parameters for threshold segmentation, obtain the contour of the image defect, and calculate and identify the defect size and area.

[0035] In another aspect, the present application provides a nondestructive testing system for the above-mentioned terahertz surface array transmission based composite material defect nondestructive testing method, which comprises a terahertz surface array detector, a terahertz frequency modulation source, a terahertz total reflection type beam shaping lens, a terahertz off-axis three-mirror type imaging lens, and an electric four-dimensional displacement platform.

[0036] The frequency modulation range of the terahertz frequency modulation source is 0.3THz to 0.4THz.

[0037] The array size of the terahertz surface array detector is 32*32, 64*64, 128*128 or 256*256, and the array unit size is 0.4mm*0.4mm.

[0038] The terahertz off-axis three-mirror type imaging lens is an XY polynomial free-form surface off-axis three-mirror type imaging lens, which comprises a first mirror, a second mirror and a third mirror, and the free-form surface equation of the XY polynomial is:

[0039]

[0040]

[0041] In the formula, r is the radial distance of the light at the point of incidence; c is the radius of curvature of the beam shaping mirror; k is the quadratic surface coefficient of the beam shaping mirror; m and n are the orders of the aspherical terms;

[0042] The terahertz total reflection type beam shaping lens comprises an off-axis collimating mirror, an off-axis microarray mirror, an off-axis integral mirror and a plane mirror.

[0043] The electric four-axis displacement table comprises an x-axis linear displacement table, a y-axis linear displacement table, a z-axis linear displacement table and a theta-axis rotary displacement table, the range of the x-axis linear displacement table, the y-axis linear displacement table and the z-axis linear displacement table is greater than or equal to 500 mm, and the angle change range of the theta-axis rotary displacement table is ±20 degrees.

[0044] Compared with the prior art, the present application has the following advantages:

[0045] (1) The nondestructive testing system of the present application adopts a free-form surface design method, which can effectively eliminate aberration and improve imaging quality. The reflecting unit adopts an off-axis three-mirror optical design scheme, which reduces the volume and weight of the system and improves the effective aperture. The off-axis three-mirror optical imaging system is suitable for a terahertz area array detector, which can improve the sensitivity and imaging quality of the terahertz imaging system. The present application has a wide application prospect and can be used for nondestructive testing in the fields of aerospace, electronic products, food safety, biomedicine, etc.

[0046] (2) The nondestructive testing system of the present application can realize fast imaging and significantly improve the efficiency of nondestructive testing due to the use of a terahertz area array detector. Through optimized optical design and a polynomial free-form surface imaging lens, the system can provide high-resolution imaging results. The combination of the terahertz total reflection type beam shaping lens and the off-axis three-mirror type imaging lens ensures the uniformity and contrast of imaging.

[0047] (3) The upper computer software in the nondestructive testing system of the present application realizes automatic control of the entire nondestructive testing process, simplifies the operation process and reduces human error. The electric four-dimensional displacement table enables the system to adapt to the detection requirements of samples of different sizes and shapes, further improving the detection efficiency.

[0048] (4) The nondestructive detection method and system provided by the present application provide a complete terahertz-based nondestructive detection process, which can be applied in multiple scenarios. The unique image fusion and processing method ensures that large-area scanning detection can be realized while ensuring detection accuracy, providing a new method for large-area nondestructive detection. BRIEF DESCRIPTION OF DRAWINGS

[0049] In order to more clearly illustrate the technical solutions in the embodiments of the present application or the prior art, the drawings required to be used in the embodiments or prior art description will be briefly introduced as follows.

[0050] Figure 1 It is a schematic diagram of the whole process of the nondestructive testing method of the present application.

[0051] Figure 2 It is a structural schematic diagram of the nondestructive testing system of the present application.

[0052] Figure 3 It is a top view of the nondestructive testing system of the present application.

[0053] Figure 4 It is a front view of the nondestructive testing system of the present application.

[0054] Figure 5 It is an optical path diagram of the nondestructive testing system of the present application.

[0055] Figure 6 It is a structural diagram of the off-axis three-mirror imaging lens in the embodiments of the present application.

[0056] Figure 7 It is a structural diagram of the terahertz total reflection type shaping lens in the embodiments of the present application.

[0057] Figures 8a-8h It is the original image and the simulation imaging result of the 64x64 terahertz area array detector of the triangle, square, pentagon and circle of different sizes in the embodiments of the present application.

[0058] Some reference numerals in the drawings are as follows:

[0059] 1, terahertz frequency modulation source; 2, terahertz total reflection type beam shaping lens; 3, electric four-dimensional displacement table; 4, terahertz off-axis three-mirror imaging lens; 5, terahertz area array detector; 41, first mirror; 42, second mirror; 43, third mirror. DETAILED DESCRIPTION

[0060] In order to make the purpose, technical solutions and advantages of the present application more clear and understandable, the present application will be further described in detail below with embodiments. It should be understood that the specific embodiments described herein are only used to explain the present application and not to limit the present application.

[0061] In one aspect, the present application provides a composite material defect nondestructive testing method based on terahertz area array transmission, as shown in Figure 1 which includes the following steps:

[0062] S1, parameter setting is performed on the terahertz planar array imaging system, the frequency modulation range of the terahertz frequency modulation source is set to 0.3 THz to 0.4 THz, the maximum output power is 40 mw, the output wave is a continuous wave, the array scale of the terahertz planar array detector is set to 32*32, 64*64, 128*128 or 256*256, and the array unit size is 0.4 mm*0.4 mm.

[0063] S2, the sample to be detected is placed on the electric four-dimensional displacement table, and is adjusted to an initial position.

[0064] S3, the terahertz frequency modulation source is started, the divergent terahertz Gaussian beam is subjected to beam collimation through the off-axis collimating reflector of the terahertz total reflection type beam shaping lens, the collimated beam is transmitted to the first group of microarray off-axis reflectors, the first group of microarray off-axis reflectors include a plurality of sub-off-axis parabolic mirrors, each sub-off-axis parabolic mirror focuses the incident beam onto a plane parallel to it, thereby dividing the parallel beam into a plurality of focused sub-beams, which are then transmitted to the second group of microarray off-axis reflectors, and again diverge to form a plurality of sub-beams incident on the off-axis integral reflector, all the sub-beams coincide on the focal plane after passing through the off-axis integral reflector, thereby obtaining a shaped uniform spot, and then the plane reflector is used for light path reflection, thereby shortening the distance between the emission end and the measured object.

[0065] S4, the shaped terahertz beam is incident from one side of the sample to be detected, is reflected on the first reflector of the terahertz off-axis three-mirror type imaging lens after internal fold reflection and scattering in the sample to be detected to form a first reflected beam; the first reflected beam is incident on the second reflector and is reflected to form a second reflected beam; the second reflected beam is incident on the third reflector and is reflected to form a third reflected beam; finally, the terahertz planar array detector located at the converging light spot of the third reflected beam is used for receiving and imaging.

[0066] S5, based on the area of the sample to be detected, the scanning area size is determined according to the requirement, then the scanning step and the scanning route are set, based on the setting of the scanning parameters and the control of the upper computer, the electric four-dimensional displacement table is controlled to move, thereby realizing the scanning and imaging of the large area of the sample to be detected, and a plurality of scanning images are obtained.

[0067] S6, image stitching is performed on the obtained multiple scanning images, feature points in each image are extracted, feature point matching is performed, a mapping matrix between two adjacent images and corresponding fusion processing are calculated, when the feature points in each image are extracted, a plurality of feature points are first extracted, then the feature points are judged, the feature points with terahertz detection image characteristics are extracted, four or more feature points are selected from the obtained feature points and substituted into a homographic transformation matrix, matrix parameter values are solved to realize image fusion operation, and a complete image of the sample to be detected is obtained; wherein the homographic transformation matrix is a 3*3 matrix, which is used for image correction and image stitching.

[0068] S7, image enhancement is performed on the complete image of the sample to be detected, a Unet neural network model is used to extract signal characteristics of the detected sample, and a visible light band image and a test image are used as input for image enhancement, and the image enhancement formula is as follows:

[0069]

[0070] In the formula, I get represents the obtained complete image, which is an optical image, I optical is used as the original input in the quality reduction model, I enhanced represents an enhanced image, and ε is a quality reduction parameter. represents a convolution operation, represents a down-sampling rate, and n represents the noise of the system.

[0071] S8, after the enhanced image is input into a defect recognition model after signal amplification and filtering, target defect detection is realized. In step S8, the defect recognition model uses an edge extraction-based method to perform image processing, sets parameters for threshold segmentation, obtains the outline of the image defect, and calculates and identifies the defect size and area.

[0072] The uniform light spot size obtained by the terahertz total reflection type beam shaping lens is greater than or equal to 50mm*50mm, and the relative light intensity distribution uniformity is greater than or equal to 70%. The off-axis collimating reflector is an off-axis parabolic reflector with a focal length of 120mm and an off-axis eccentricity of 120mm. The first group of microarray off-axis reflectors and the second group of microarray off-axis reflectors are both off-axis array reflectors with an array size of 17*17. The off-axis integral reflector is a reflector with a focal length of 1143mm and an off-axis eccentricity of 1143mm.

[0073] The scanning process adopts a transmission light path, the terahertz wave is emitted from one side of the sample to be detected, and is received by the terahertz area array detector after being folded, reflected, scattered and diffused in the object. The terahertz area array detector can directly perform real-time detection and imaging on the surface area of the sample. For a detection area larger than the array size, the array size can be selected according to the size of the scanning area, and the corresponding scanning step and scanning path are set by the electric four-axis displacement table to complete the large-area scanning detection.

[0074] In another aspect, the present application provides a transmission type non-destructive testing system based on a terahertz area array detector, as shown in the figure, which comprises a terahertz area array detector 5, a terahertz frequency modulation source 1, a terahertz total reflection type beam shaping lens 2, a terahertz off-axis three-mirror type imaging lens 4, an electric four-dimensional displacement table 3 and an upper computer. Figures 2 to 7

[0075] In specific applications, the frequency modulation range of the terahertz frequency modulation source 1 is 0.3THz to 0.4THz; the array size of the terahertz area array detector 5 is 32x32, 64x64, 128x128 or 256x256, and the array unit size is 0.4mmx0.4mm.

[0076] As shown in the figure, the terahertz off-axis three-mirror type imaging lens 4 is an XY polynomial free-form surface off-axis three-mirror imaging lens, which comprises a first mirror 41, a second mirror 42 and a third mirror 43, and the free-form surface equation of the XY polynomial is: Figure 6

[0077]

[0078] In the formula, r is the radial distance of the light at the incident point; c is the curvature radius of the beam shaping mirror; k is the quadratic surface coefficient of the beam shaping mirror; m and n are the orders of the aspherical terms.

[0079] The terahertz total reflection type beam shaping lens 2 comprises an off-axis collimating mirror, an off-axis microarray mirror, an off-axis integral mirror and a plane mirror.

[0080] The electric four-axis displacement table 3 comprises an x-axis linear displacement table, a y-axis linear displacement table, a z-axis linear displacement table and a theta-axis rotary displacement table. The range of the x-axis linear displacement table, the y-axis linear displacement table and the z-axis linear displacement table is all greater than or equal to 500mm, and the angle change range of the theta-axis rotary displacement table is ±20 degrees.

[0081] ​​In one embodiment, the distance between the lowermost ray of the incident light of the terahertz off-axis three-mirror imaging lens 4 and the lower edge of the second mirror is greater than 10 mm; the distance between the reflected light of the third mirror and the upper edge of the second mirror is greater than 10 mm. The focal length of the lens is 80 mm, the F number is 0.6-1.2, and the light aperture is 50 mm-150 mm. The distance between the center of the first mirror of the off-axis three-mirror imaging lens and the center of the second mirror is 349.468 mm; the distance between the center of the second mirror and the center of the third mirror is 217.684 mm; and the distance between the center of the third mirror and the center of the plane of the detector is 400.744 mm.

[0082] In a specific application, the system adopts a transmission optical path, and the terahertz wave is emitted from one side of the object to be detected, is reflected and scattered inside the object, and is received by the terahertz planar array detector 5.

[0083] In a specific application, the system further includes an upper computer for controlling the movement of the electric four-dimensional displacement table and the terahertz frequency modulation source, a data processing and imaging analysis module, and a signal amplification and filtering module. The upper computer is used to control the movement of the electric four-dimensional displacement table and the operation of the terahertz frequency modulation source. The data processing and imaging analysis module is used to splice and enhance the image, and the signal amplification and filtering module is used to perform signal amplification and filtering processing.

[0084] The terahertz total reflection type beam shaping lens can reduce the optical path loss. After the terahertz beam is emitted, it is collimated by the off-axis collimating mirror with a diameter of 120 mm. The collimated beam passes through the off-axis microarray mirror and the off-axis integral mirror to obtain a uniform light spot. The off-axis microarray mirror and the off-axis integral mirror each include a plurality of micro mirror arrays. The array size is selected according to the diameter of the light spot. The off-axis microarray mirror divides the incident light into a series of sub-beams, which are reflected to the off-axis integral mirror. All the sub-beams will coincide on the focal plane after passing through the integral mirror. The slight unevenness in each sub-beam range will be smoothed during the coincidence process to obtain a uniform light spot. Embodiments

[0086] In this embodiment, a non-destructive testing system based on a terahertz planar array detector is provided. The system includes a terahertz planar array detector, a terahertz frequency modulation source, a terahertz total reflection type beam shaping lens, a terahertz off-axis three-mirror imaging lens, an electric four-dimensional displacement table, and an upper computer. The array size of the terahertz planar array detector can be selected as 32x32, 64x64, 128x128, or 256x256 or higher according to the specific application scenario, and the array unit size is uniform at 0.4 mm x 0.4 mm.

[0087] In use, first, the system is parameter configured, the frequency modulation range of the terahertz frequency modulation source is set to 0.3THz to 0.4THz to adapt to different detection requirements. The sample to be detected is placed on the electric four-dimensional displacement table and is adjusted to the initial position. The frequency modulation and output power parameters of the terahertz frequency modulation source, the sampling time and contrast of the detector, and the movement parameters of the electric displacement table are set through the upper computer software. The terahertz frequency modulation source is started, the divergent terahertz Gaussian beam realizes the homogenized flat-top collimated beam after the full reflection type beam shaping lens, the shaping spot size is 100mm*100mm, the terahertz beam after shaping is imaged by the off-axis three-reflection imaging lens, the focal length of the off-axis three-reflection imaging lens is 80mm, the F number is 1, and the detector array collects the transmitted terahertz wave. The electric displacement table moves the sample according to the predetermined path, the detector synchronously collects data, and the upper computer is monitored in real time to realize the spliced large field of view sample transmission imaging. The processed image is displayed on the upper computer to analyze the internal structure and potential defects of the sample. The terahertz detector is a surface array detector, which can select the array size according to the size of the scanning area, and complete efficient scanning detection through setting the corresponding scanning step and scanning path, Figures 8a-8h The original images and the simulation imaging results of the 64*64 terahertz surface array detector of different sizes of triangles, squares, pentagons and circles in the embodiment of the application are respectively shown in the following table.

[0088] The above-described embodiments only describe the preferred embodiments of the application, and do not limit the scope of the application. Without departing from the design spirit of the application, various modifications and improvements of the technical solutions of the application made by those skilled in the art shall fall within the protection scope of the claims of the application.

Claims

1. A method for non-destructive testing of defects in composite materials based on terahertz area array transmission, characterized in that: It comprises the following steps: S1, parameter setting is performed on the terahertz surface array imaging system, the frequency modulation range of the terahertz frequency modulation source is set to 0.3THz to 0.4THz, the maximum output power is 40mw, the output wave is a continuous wave, the array scale of the terahertz microarray detector is set to 32*32, 64*64, 128*128 or 256*256, and the array unit size is 0.4mm*0.4mm; S2, the sample to be detected is placed on the electric four-dimensional displacement table, and is adjusted to the initial position; S3, the terahertz frequency modulation source is started, the divergent terahertz Gaussian beam is subjected to beam collimation through the off-axis collimating reflector of the terahertz total reflection type beam shaping lens, the collimated beam is transmitted to the first group of microarray off-axis reflectors, the first group of microarray off-axis reflectors comprises a plurality of sub-off-axis parabolic mirrors, each sub-off-axis parabolic mirror focuses the incident beam onto a plane parallel thereto, thereby dividing the parallel beam into a plurality of focused sub-beams, which are then transmitted to the second group of microarray off-axis reflectors, and again diverge to form a plurality of sub-beams incident on the off-axis integral reflector, all the sub-beams coincide on the focal plane after passing through the off-axis integral reflector, thereby obtaining a uniform light spot after shaping, and then reflecting by using a plane reflector; S4, the shaped terahertz beam is incident from one side of the sample to be detected, is reflected on the first reflector of the terahertz off-axis three-mirror imaging lens after internal fold reflection and scattering in the sample to be detected to form a first reflected beam; the first reflected beam is incident on the second reflector and is reflected to form a second reflected beam; the second reflected beam is incident on the third reflector and is reflected to form a third reflected beam; finally, the terahertz microarray detector located at the converging light spot of the third reflected beam is used for receiving and imaging; S5, based on the area of the sample to be detected, the scanning area size is determined according to the requirements, then the scanning step and the scanning route are set, the electric four-dimensional displacement table is controlled to move based on the setting of the scanning parameters and the control of the upper computer, thereby realizing the scanning and imaging of the large area of the sample to be detected, and a plurality of scanning images are obtained; S6, the obtained plurality of scanning images are spliced, the feature points in each image are extracted, the feature points are matched, the mapping matrix between the adjacent two images is calculated and fusion processing is performed, a plurality of feature points are extracted when the feature points in each image are extracted, then the plurality of feature points are judged, the feature points with the terahertz detection image characteristics are retained, more than four feature points are selected from the obtained feature points and are substituted into the homography transformation matrix, the matrix parameter value is solved to realize the fusion operation of the images, and a complete image of the sample to be detected is obtained; S7, the complete image of the sample to be detected is subjected to image enhancement, the Unet neural network model is used to extract the signal characteristics of the detected sample, the visible light band image and the test image are used as inputs to perform image enhancement, and the image enhancement formula is as follows: where I get denotes the obtained complete image, which is an optical image, I optical denotes the original input used in the quality reduction model, I enhanced denotes the enhanced image, and ε is a quality reduction parameter, denotes that a convolution operation is performed, denotes a down-sampling rate, and n denotes the noise of the system; S8, the enhanced image is input into a defect recognition model after signal amplification and filtering, and target defect detection of the sample to be detected is realized.

2. The terahertz plane array transmission-based composite material defect nondestructive detection method according to claim 1, characterized in that: the terahertz plane array imaging system comprises a terahertz plane array detector, a terahertz frequency modulation source, a terahertz total reflection beam shaping lens, a terahertz off-axis three-mirror imaging lens and an electric four-dimensional displacement table; the frequency modulation range of the terahertz frequency modulation source is 0.3 THz to 0.4 THz, the maximum output power is 40 mw, and the output wave is a continuous wave; the array scale of the terahertz plane array detector is 32*32, 64*64, 128*128 or 256*256, and the array unit size is 0.4 mm*0.4 mm; the terahertz off-axis three-mirror imaging lens is an XY polynomial free-form surface off-axis three-mirror imaging lens, comprising a first mirror, a second mirror and a third mirror, and the free-form surface equation of the XY polynomial is: wherein r is the radial distance of a light ray at an incident point; c is the curvature radius of the beam shaping mirror; k is the quadratic surface coefficient of the beam shaping mirror; m and n are the orders of the aspherical terms; the terahertz total reflection beam shaping lens comprises an off-axis collimating mirror, an off-axis microarray mirror, an off-axis integral mirror and a plane mirror, the off-axis microarray mirror comprises a first group of microarray off-axis mirrors and a second group of microarray off-axis mirrors; the electric four-dimensional displacement table comprises an x-axis linear displacement table, a y-axis linear displacement table, a z-axis linear displacement table and a theta-axis rotary displacement table, the range of the x-axis linear displacement table, the y-axis linear displacement table and the z-axis linear displacement table is greater than or equal to 500 mm, and the angle change range of the theta-axis rotary displacement table is ±20 degrees.

3. The terahertz area array transmission based non-destructive testing method for composite material defects according to claim 1, characterized in that: The terahertz plane array detector can directly perform real-time detection imaging on the surface area of a sample, for a detection area larger than the array scale, the array scale can be selected according to the size of the scanning area, and the corresponding scanning step and scanning path are set through the electric four-axis displacement table to complete large-area scanning detection.

4. The terahertz area array transmission based non-destructive testing method for composite material defects according to claim 1, characterized in that: The uniformization spot size obtained by the terahertz total reflection beam shaping lens is greater than or equal to 50 mm*50 mm, and the relative light intensity distribution uniformity is greater than or equal to 70%.

5. The terahertz area array transmission based non-destructive testing method for composite material defects according to claim 1, characterized in that: The scanning process adopts a transmission type optical path, the terahertz wave is emitted from one side of the sample to be detected, and is received by the terahertz plane array detector after being folded and scattered in the object.

6. The terahertz area array transmission based non-destructive testing method for composite material defects according to claim 1, characterized in that: The sample to be detected is a ceramic or glass fiber reinforced composite material.

7. The terahertz area array transmission based non-destructive testing method for composite material defects according to claim 1, characterized in that: The off-axis collimating mirror is an off-axis parabolic mirror with a focal length of 120 mm and an off-axis eccentricity of 120 mm, the first group of microarray off-axis mirrors and the second group of microarray off-axis mirrors are off-axis array mirrors with an array scale of 17*17, and the off-axis integral mirror is a mirror with a focal length of 1143 mm and an off-axis eccentricity of 1143 mm.

8. The terahertz area array transmission based non-destructive testing method for composite material defects according to claim 1, characterized in that: The homography transformation matrix in step S6 is a 3*3 matrix, which is used for image correction and image stitching.

9. The terahertz area array transmission based non-destructive testing method for composite material defects according to claim 1, characterized in that: In step S8, the defect recognition model uses an edge extraction-based method to set parameters for threshold segmentation, obtain the contour of the image defect, and calculate and identify the defect size and area.

10. A non-destructive testing system for use in the terahertz face array transmission based non-destructive testing method of composite material defects according to claim 1, characterized in that: It includes a terahertz surface array detector, a terahertz frequency modulation source, a terahertz total reflection type beam shaping lens, a terahertz off-axis three-mirror type imaging lens and an electric four-dimensional displacement table; The frequency modulation range of the terahertz frequency modulation source is 0.3 THz to 0.4 THz; The array scale of the terahertz surface array detector is 32*32, 64*64, 128*128 or 256*256, and the array unit size is 0.4mm*0.4mm; The terahertz off-axis three-mirror type imaging lens is an XY polynomial free-form surface off-axis three-mirror type imaging lens, which comprises a first mirror, a second mirror and a third mirror, and the free-form surface equation of the XY polynomial is: In the formula, r is the radial distance of light at the incident point; c is the curvature radius of the beam shaping mirror; k is the quadratic surface coefficient of the beam shaping mirror; m and n are the order of aspherical terms; The terahertz total reflection type beam shaping lens comprises an off-axis collimating mirror, an off-axis microarray mirror, an off-axis integral mirror and a plane mirror; The electric four-dimensional displacement table comprises an x-axis linear displacement table, a y-axis linear displacement table, a z-axis linear displacement table and a theta-axis rotary displacement table, the range of the x-axis linear displacement table, the y-axis linear displacement table and the z-axis linear displacement table is all greater than or equal to 500mm, and the angle change range of the theta-axis rotary displacement table is ±20 degrees.

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