Composite material defect nondestructive testing method and system based on terahertz area array transmission

Abstract

The invention discloses a transmission-type nondestructive testing method and system based on a terahertz area array detector, and belongs to the technical field of terahertz imaging nondestructive testing. According to the frequency mixing array imaging system based on the terahertz technology and based on shaping and focusing of terahertz wave beams, a ceramic or glass fiber reinforced composite material sample piece for aerospace is placed between the shaping lens and the imaging lens, transmission type detection is carried out on the sample piece, and the frequency mixing array detector obtains surface area information of the sample piece; and typical defect feature processing and defect identification are carried out, so that the state of the sample piece is rapidly evaluated. The invention aims to solve the problems of low efficiency and low imaging quality in the existing terahertz nondestructive testing technology. By adopting the terahertz area array detector and the optical imaging assembly, rapid and high-resolution nondestructive detection can be realized, and the terahertz area array detector is particularly suitable for application scenes requiring high detection efficiency and imaging quality.

Description

Technical Field

[0001] The present invention relates to the technical field of terahertz imaging, and in particular to a method and system for nondestructive detection of composite material defects based on terahertz array transmission. Background Art

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

[0003] At present, the terahertz surface array detector (FPA) is one of the main detectors used in terahertz imaging technology. In order to improve the sensitivity and imaging quality of FPA, it is usually necessary to use a collimator system to focus the terahertz wave. There are two main types of collimator systems used in existing terahertz surface array detectors: (1) Refractive lens collimator system: The refractive lens collimator system is usually composed of 2 or 3 refractive lenses, which can effectively focus the terahertz wave. However, the transmission loss of the refractive lens material and the interference between different interfaces will reduce the sensitivity and imaging quality of the system. (2) Reflector collimator system: The reflector collimator system is usually composed of 2 or 3 reflectors, which can avoid material transmission loss. However, it is difficult to eliminate aberrations with traditional spherical or aspherical reflectors, resulting in poor imaging quality.

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

[0005] At present, terahertz nondestructive testing systems mainly use a unit detector to perform step scanning on the sample for imaging. This method has some limitations, mainly in terms of low efficiency. Step scanning requires the detector to collect the terahertz signal of the sample point by point, which is a time-consuming process. Especially when high-resolution imaging is required, the scanning area increases and the detection time increases significantly. In addition, the imaging quality of the unit detector is limited by the performance of the detector, including its sensitivity, noise level and dynamic range.

[0006] In terahertz nondestructive testing systems, refractive optical lenses are usually used as optical elements for beam shaping and sample imaging. Refractive lenses focus and shape the terahertz beam through a combination of multiple lenses. However, interference and transmission loss between these lenses will affect the imaging quality of the system. Specifically, the absorption and scattering of the lens material will lead to a decrease in signal intensity, thereby affecting the contrast and sensitivity of the imaging. In addition, manufacturing and alignment errors of the lenses will also introduce aberrations, reducing the clarity and resolution of the imaging. Summary of the invention

[0007] Based on the above-mentioned deficiencies of the prior art, the present invention provides a transmission-type nondestructive testing method and system based on a terahertz surface array detector, aiming to solve the problems of low efficiency of single-point scanning imaging, large system loss, and low imaging quality in the existing terahertz nondestructive testing technology. By adopting a terahertz surface array mixing detector and a specific optical imaging component, the nondestructive testing method can realize transmission detection of composite material samples, process the test results and identify typical defects, and realize fast and high-resolution nondestructive testing, which is particularly suitable for application scenarios such as aerospace composite material production, development, inspection and maintenance that require high detection efficiency and imaging quality. The use of a terahertz surface array mixing detector is the key to improving detection efficiency because it can collect signals from multiple pixel points at the same time, thereby realizing fast imaging. At the same time, the introduction of full-reflection beam shaping technology and off-axis three-reflection optical imaging system is expected to reduce the interference and transmission loss of optical components, improve the imaging quality of thermal insulation tiles and glass fiber reinforced composite material samples, and further ensure the detection results.

[0008] In order to achieve the above object, the technical solution provided by the present invention is as follows:

[0009] In one aspect, the present invention provides a method for nondestructive detection of composite material defects based on terahertz array transmission, which comprises the following steps:

[0010] S1. Set the parameters of the terahertz array imaging system, set the frequency modulation range of the terahertz frequency modulation source to 0.3THz to 0.4THz, the maximum output power to 40mw, the output wave to a continuous wave, set the array size of the terahertz array detector to 32×32, 64×64, 128×128 or 256×256, and the array unit size to 0.4mm×0.4mm;

[0011] S2. Place the sample to be tested on the electric four-dimensional translation stage and adjust it to the initial position;

[0012] S3, start the terahertz frequency modulation source, and the divergent terahertz Gaussian beam is collimated by the off-axis collimating reflector of the terahertz total reflection beam shaping lens, and the collimated beam is transmitted to the first group of micro-array off-axis reflectors, which includes a plurality of sub-off-axis parabolic mirrors, each of which focuses the incident beam onto a plane parallel to it, thereby dividing the parallel beam into a plurality of focused sub-beams, and then transmitting it to the second group of micro-array off-axis reflectors, and diverging again to form a plurality of sub-beams that are incident on the off-axis integrating reflector, and all the sub-beams will overlap at the focal plane after passing through the off-axis integrating reflector, thereby obtaining a uniform light spot after shaping, and then reflecting by a plane reflector;

[0013] S4, the shaped terahertz beam is injected from one side of the sample to be detected, and after being refracted, reflected and scattered inside the sample to be detected, it is reflected on the first reflector of the terahertz off-axis three-reflector imaging lens to form a first reflected beam; the first reflected beam is irradiated on the second reflector and reflected to form a second reflected beam; the second reflected beam is irradiated on the third reflector and reflected to form a third reflected beam; finally, the terahertz surface array detector located at the convergence spot of the third reflected beam is used for receiving imaging;

[0014] S5. Based on the area of ​​the sample to be detected, the size of the scanning area is first determined according to the needs, and then the scanning step and scanning route are set. Based on the setting of scanning parameters and the control of the host computer, the electric four-dimensional translation stage is controlled to move, so as to realize the scanning imaging of a large area of ​​the sample to be detected and obtain multiple scanned images;

[0015] S6, performing image stitching on the obtained multiple scanned images, extracting feature points in each image, matching feature points, calculating the mapping matrix between two adjacent images and performing fusion processing, first extracting multiple feature points when extracting feature points in each image, then judging the multiple feature points, retaining feature points with terahertz detection image features, selecting four or more feature points from the obtained feature points and substituting them into the homography transformation matrix, solving the matrix parameter value to realize the image fusion operation, and obtaining a complete image of the sample to be detected;

[0016] S7. Perform image enhancement on the complete image of the sample to be tested. Use the Unet neural network model to extract the signal features of the sample to be tested. Use the visible light band image and the test image as input for image enhancement. The image enhancement formula is as follows:

[0017]

[0018] In the formula, I get Represents the complete image obtained, which is an optical image, I optical For the original input in the quality reduction model, I enhancedrepresents the enhanced image, ε is the quality reduction parameter, Indicates the convolution operation. represents the downsampling rate, n represents the noise of the system;

[0019] S8. After signal amplification and filtering, the enhanced image is input into a defect recognition model to achieve target defect detection of the sample to be inspected.

[0020] Preferably, the terahertz area array imaging system comprises a terahertz area array detector, a terahertz frequency modulation source, a terahertz total reflection beam shaping lens, a terahertz off-axis three-reflection imaging lens and an electric four-dimensional translation stage;

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

[0022] The terahertz area array detector has an array size of 32×32, 64×64, 128×128 or 256×256, and an array unit size of 0.4 mm×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 reflector, a second reflector and a third reflector, and the free-form surface equation of the XY polynomial is:

[0024]

[0025] Where r is the radial distance of the light at the incident point; 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 aspheric terms;

[0026] The terahertz total reflection beam shaping lens comprises an off-axis collimating reflector, an off-axis micro-array reflector, an off-axis integrating reflector and a plane reflector, and the off-axis micro-array reflector comprises a first group of micro-array off-axis reflectors and a second group of micro-array off-axis reflectors;

[0027] The electric four-axis translation stage includes an x-axis linear translation stage, a y-axis linear translation stage, a z-axis linear translation stage and a θ-axis rotational translation stage. The measuring ranges of the x-axis linear translation stage, the y-axis linear translation stage and the z-axis linear translation stage are all ≥500mm, and the angle variation range of the θ-axis rotational translation stage is ±20 degrees.

[0028] Preferably, the terahertz area array detector can directly perform real-time detection and imaging of the surface area of ​​the sample. For the detection area exceeding the array scale, the array scale can be selected according to the size of the scanning area, and the large-area scanning detection can be completed by setting the corresponding scanning step and scanning path in combination with the electric four-axis translation stage.

[0029] Preferably, the homogenized 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%.

[0030] Preferably, the scanning process adopts a transmission optical path, and the terahertz wave is injected from one side of the sample to be detected, and is received by the terahertz surface array detector after being refracted, reflected and scattered inside the object.

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

[0032] Preferably, the off-axis collimating reflector is an off-axis parabolic reflector with a focal length of 120 mm and an off-axis eccentricity of 120 mm, 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 scale of 17*17, and the off-axis integrating reflector is a reflector with a focal length of 1143 mm and an off-axis eccentricity of 1143 mm.

[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-based extraction method to perform threshold segmentation on the image, sets parameters, obtains the contour of the image defect, and calculates and identifies the defect size and area.

[0035] On the other hand, the present invention provides a nondestructive testing system for the above-mentioned composite material defect nondestructive testing method based on terahertz array transmission, which includes a terahertz array detector, a terahertz frequency modulation source, a terahertz total reflection beam shaping lens, a terahertz off-axis three-reflection imaging lens and an electric four-dimensional translation stage;

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

[0037] The terahertz area array detector has an array size of 32×32, 64×64, 128×128 or 256×256, and an array unit size of 0.4 mm×0.4 mm;

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

[0039]

[0040]

[0041] Where r is the radial distance of the light at the incident point; 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 aspheric terms;

[0042] The terahertz total reflection beam shaping lens comprises an off-axis collimating reflector, an off-axis micro-array reflector, an off-axis integrating reflector and a plane reflector;

[0043] The electric four-axis translation stage includes an x-axis linear translation stage, a y-axis linear translation stage, a z-axis linear translation stage and a θ-axis rotational translation stage. The measuring ranges of the x-axis linear translation stage, the y-axis linear translation stage and the z-axis linear translation stage are all ≥500mm, and the angle variation range of the θ-axis rotational translation stage is ±20 degrees.

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

[0045] (1) The nondestructive testing system of the present invention adopts a free-form surface design method, which can effectively eliminate aberrations and improve imaging quality. The reflective unit adopts an off-axis three-reflection optical design scheme to reduce the volume and weight of the system and increase the effective aperture. The off-axis three-reflection optical imaging system is suitable for terahertz surface array detectors, which can improve the sensitivity and imaging quality of the terahertz imaging system. The present invention has broad application prospects 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 invention can achieve fast imaging and significantly improve the efficiency of nondestructive testing by using a terahertz surface array detector. Through optimized optical design and polynomial free-form surface imaging lens, the system can provide high-resolution imaging results. The combination of terahertz total reflection beam shaping lens and off-axis three-reflection imaging lens ensures the uniformity and contrast of imaging.

[0047] (3) The host computer software in the nondestructive testing system of the present invention realizes the automatic control of the entire nondestructive testing process, simplifies the operation process, and reduces human errors. The electric four-dimensional translation stage enables the system to adapt to the testing needs of samples of different sizes and shapes, further improving the testing efficiency.

[0048] (4) The nondestructive testing method and system provided by the present invention provide a complete terahertz-based nondestructive testing process, which can be applied in multiple scenarios, and utilizes unique image fusion and processing methods to ensure that a large area can be scanned and tested in one go while ensuring the accuracy of the test, thus providing a new method for large-area nondestructive testing. BRIEF DESCRIPTION OF THE DRAWINGS

[0049] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art are briefly introduced below.

[0050] Figure 1 It is a schematic diagram of the overall process of the nondestructive testing method of the present invention;

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

[0052] Figure 3 A top view of the nondestructive testing system of the present invention;

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

[0054] Figure 5 is a light path diagram of the nondestructive testing system of the present invention;

[0055] Figure 6 is a structural diagram of an off-axis three-mirror imaging lens in an embodiment of the present invention;

[0056] Figure 7 is a structural diagram of a terahertz total reflection shaping lens in an embodiment of the present invention;

[0057] Figure 8a-8h They are respectively original images of triangles, squares, pentagons and circles of different sizes in the embodiment of the present invention and simulation imaging results using a 64×64 terahertz surface array detector.

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

[0059] 1. Terahertz frequency modulation source; 2. Terahertz total reflection beam shaping lens; 3. Electric four-dimensional translation stage; 4. Terahertz off-axis three-reflection imaging lens; 5. Terahertz surface array detector; 41. First reflector; 42. Second reflector; 43. Third reflector. DETAILED DESCRIPTION

[0060] In order to make the purpose, technical solution and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with the embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0061] On the one hand, the present invention provides a method for nondestructive detection of composite material defects based on terahertz array transmission, such as Figure 1 As shown, it includes the following steps:

[0062] S1. Set the parameters of the terahertz array imaging system. Set the frequency modulation range of the terahertz frequency modulation source to 0.3THz to 0.4THz, the maximum output power to 40mw, the output wave to a continuous wave, and the array scale of the terahertz array detector to 32×32, 64×64, 128×128 or 256×256, and the array unit size to 0.4mm×0.4mm.

[0063] S2. Place the sample to be tested on the electric four-dimensional translation stage and adjust it to the initial position.

[0064] S3, start the terahertz frequency modulation source, and the divergent terahertz Gaussian beam is collimated by 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, which includes multiple 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 multiple focused sub-beams, which are then transmitted to the second group of microarray off-axis mirrors, diverged again to form multiple sub-beams that are incident on the off-axis integrating mirror. All the sub-beams will overlap at the focal plane after passing through the off-axis integrating mirror, thereby obtaining a uniform light spot after shaping, and then a plane mirror is used to reflect the return light path to shorten the distance between the transmitting end and the object under test.

[0065] S4. The shaped terahertz beam is incident from one side of the sample to be detected, and after refraction, reflection and scattering inside the sample to be detected, it is reflected on the first reflector of the terahertz off-axis three-reflector imaging lens to form a first reflected beam; the first reflected beam is irradiated on the second reflector and reflected to form a second reflected beam; the second reflected beam is irradiated on the third reflector and reflected to form a third reflected beam; finally, the terahertz surface array detector located at the converging spot of the third reflected beam is used for receiving and imaging.

[0066] S5. Based on the area of ​​the sample to be tested, the size of the scanning area is first determined according to the needs, and then the scanning step and scanning route are set. Based on the setting of the scanning parameters and the control of the host computer, the electric four-dimensional translation stage is controlled to move to achieve scanning and imaging of a large area of ​​the sample to be tested, and multiple scanned images are obtained.

[0067] S6. Perform image stitching on the multiple scanned images obtained, extract feature points in each image, perform feature point matching, calculate the mapping matrix between two adjacent images and the corresponding fusion processing, first extract multiple feature points when extracting feature points in each image, then judge the feature points, extract feature points with terahertz detection image characteristics, select four or more feature points from the obtained feature points and substitute them into the homography transformation matrix, solve the matrix parameter values ​​to realize the image fusion operation, and obtain a complete image of the sample to be detected; wherein, the homography transformation matrix is ​​a 3*3 matrix, which is used for image correction and image stitching.

[0068] S7. Perform image enhancement on the complete image of the sample to be tested. Use the Unet neural network model to extract the signal features of the sample to be tested. Use the visible light band image and the test image as input for image enhancement. The image enhancement formula is as follows:

[0069]

[0070] In the formula, I get Represents the complete image obtained, which is an optical image, I optical For the original input in the quality reduction model, I enhanced represents the enhanced image, ε is the quality reduction parameter, Indicates the convolution operation. represents the downsampling rate, and n represents the noise of the system.

[0071] S8, after amplifying and filtering the enhanced image, the enhanced image is input into the defect recognition model to detect the target defect. In step S8, the defect recognition model uses edge extraction to extract the image, sets parameters for threshold segmentation, obtains the outline of the image defect, and calculates and identifies the defect size and area.

[0072] The homogenized spot size obtained by the terahertz total reflection 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 micro-array off-axis reflectors and the second group of micro-array off-axis reflectors are both off-axis array reflectors with an array scale of 17*17. The off-axis integrating reflector is a reflector with a focal length of 1143mm and an off-axis eccentricity of 1143mm.

[0073] The scanning process uses a transmission optical path. The terahertz wave is injected from one side of the sample to be tested, and is received by the terahertz surface array detector after refraction, reflection and scattering inside the object. The terahertz surface array detector can directly perform real-time detection and imaging of the surface area of ​​the sample. For the detection area that exceeds the array scale, the array scale can be selected according to the size of the scanning area, and the electric four-axis translation stage can be combined to set the corresponding scanning step and scanning path to complete the large area scanning detection.

[0074] On the other hand, the present invention provides a transmission type nondestructive testing system based on a terahertz surface array detector, such as Figures 2 to 7 As shown, it includes a terahertz surface array detector 5, a terahertz frequency modulation source 1, a terahertz total reflection beam shaping lens 2, a terahertz off-axis three-reflection imaging lens 4, an electric four-dimensional translation stage 3 and a host computer.

[0075] In a specific application, the frequency modulation range of the terahertz frequency modulation source 1 is 0.3THz to 0.4THz; the array scale of the terahertz surface array detector 5 is 32×32, 64×64, 128×128 or 256×256, and the array unit size is 0.4mm×0.4mm.

[0076] like Figure 6 As shown, the terahertz off-axis three-mirror imaging lens 4 is an XY polynomial free-form surface off-axis three-mirror imaging lens, including a first reflector 41, a second reflector 42 and a third reflector 43. The free-form surface equation of the XY polynomial is:

[0077]

[0078] Where r is the radial distance of the light at the incident point; 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.

[0079] The terahertz total reflection beam shaping lens 2 comprises an off-axis collimating reflector, an off-axis micro-array reflector, an off-axis integrating reflector and a plane reflector.

[0080] The electric four-axis translation stage 3 includes an x-axis linear translation stage, a y-axis linear translation stage, a z-axis linear translation stage, and a θ-axis rotation translation stage. The measuring ranges of the x-axis linear translation stage, the y-axis linear translation stage, and the z-axis linear translation stage are all ≥500 mm, and the angle variation range of the θ-axis rotation translation stage is ±20 degrees.

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

[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, and is received by the terahertz surface array detector 5 after being reflected and scattered inside the object.

[0083] In a specific application, the system also includes a host computer for controlling the movement of the electric four-dimensional translation stage and the terahertz frequency modulation source, a data processing and imaging analysis module, and a signal amplification and filtering module. The host computer is used to control the movement of the electric four-dimensional translation stage 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 beam shaping lens can reduce the optical path loss. After the terahertz beam is emitted, it passes through an off-axis collimating mirror with a diameter of 120mm for beam collimation. The collimated beam passes through an off-axis micro-array mirror and an off-axis integrating mirror to obtain a uniform light spot. Both the off-axis micro-array mirror and the off-axis integrating mirror include multiple micro-mirror arrays. The array size is selected according to the spot diameter. The off-axis micro-array mirror divides the incident light into a series of sub-beams, which are incident on the off-axis integrating mirror after reflection. All the sub-beams will overlap on the focal plane after passing through the integrating mirror. The tiny unevenness within the range of each sub-beam will be smoothed in the process of overlap to obtain a uniform light spot. Specific embodiments

[0086] In this embodiment, a nondestructive testing system based on a terahertz surface array detector is provided, the system includes a terahertz surface array detector, a terahertz frequency modulation source, a terahertz total reflection beam shaping lens, a terahertz off-axis three-reflection imaging lens, an electric four-dimensional translation stage and a host computer. The array scale of the terahertz surface array detector can be selected as 32×32, 64×64, 128×128 or 256×256 or higher according to the specific application scenario, and the array unit size is uniformly 0.4mm×0.4mm.

[0087] When using, first, configure the system parameters and set the frequency modulation range of the terahertz frequency modulation source to 0.3THz to 0.4THz to meet different detection requirements. Place the sample to be tested on the electric four-dimensional translation stage and adjust it to the initial position. Set the frequency modulation and output power parameters of the terahertz frequency modulation source, the detector sampling time and contrast, and the movement parameters of the electric translation stage through the host computer software. Start the terahertz frequency modulation source, and the divergent terahertz Gaussian beam passes through the total reflection beam shaping lens to achieve a homogenized flat-top collimated beam. The shaping spot size is 100mm×100mm. After passing through the sample, the shaped terahertz beam 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 translation stage moves the sample according to the predetermined path, the detector synchronously collects data, and is monitored in real time by the host computer to achieve spliced ​​large-field sample transmission imaging. The processed image is displayed on the host computer to analyze the internal structure and potential defects of the sample. The terahertz detector is a surface array detector. It can select the array scale according to the size of the scanning area and complete high-efficiency scanning detection by setting the corresponding scanning step and scanning path. Figure 8a-8h They are respectively original images of triangles, squares, pentagons and circles of different sizes in the embodiment of the present invention and simulation imaging results using a 64×64 terahertz surface array detector.

[0088] The embodiments described above are only descriptions of the preferred implementation modes of the present invention, and are not intended to limit the scope of the present invention. Without departing from the design spirit of the present invention, various modifications and improvements made to the technical solutions of the present invention by ordinary technicians in this field should all fall within the protection scope determined by the claims of the present invention.

Claims

1. A nondestructive detection method for composite material defects based on terahertz array transmission, characterized in that: It includes the following steps: S1. Set the parameters of the terahertz array imaging system, set the frequency modulation range of the terahertz frequency modulation source to 0.3THz to 0.4THz, the maximum output power to 40mw, the output wave to a continuous wave, set the array size of the terahertz array detector to 32×32, 64×64, 128×128 or 256×256, and the array unit size to 0.4mm×0.4mm; S2. Place the sample to be tested on the electric four-dimensional translation stage and adjust it to the initial position; S3, start the terahertz frequency modulation source, and the divergent terahertz Gaussian beam is collimated by the off-axis collimating reflector of the terahertz total reflection beam shaping lens, and the collimated beam is transmitted to the first group of micro-array off-axis reflectors, which includes a plurality of sub-off-axis parabolic mirrors, each of which focuses the incident beam onto a plane parallel to it, thereby dividing the parallel beam into a plurality of focused sub-beams, and then transmitting it to the second group of micro-array off-axis reflectors, and diverging again to form a plurality of sub-beams that are incident on the off-axis integrating reflector, and all the sub-beams will overlap at the focal plane after passing through the off-axis integrating reflector, thereby obtaining a uniform light spot after shaping, and then reflecting by a plane reflector; S4, the shaped terahertz beam is injected from one side of the sample to be detected, and after being refracted, reflected and scattered inside the sample to be detected, it is reflected on the first reflector of the terahertz off-axis three-reflector imaging lens to form a first reflected beam; the first reflected beam is irradiated on the second reflector and reflected to form a second reflected beam; the second reflected beam is irradiated on the third reflector and reflected to form a third reflected beam; finally, the terahertz surface array detector located at the convergence spot of the third reflected beam is used for receiving imaging; S5. Based on the area of ​​the sample to be detected, the size of the scanning area is first determined according to the needs, and then the scanning step and scanning route are set. Based on the setting of scanning parameters and the control of the host computer, the electric four-dimensional translation stage is controlled to move, so as to realize the scanning imaging of a large area of ​​the sample to be detected and obtain multiple scanned images; S6, performing image stitching on the obtained multiple scanned images, extracting feature points in each image, matching feature points, calculating the mapping matrix between two adjacent images and performing fusion processing, first extracting multiple feature points when extracting feature points in each image, then judging the multiple feature points, retaining feature points with terahertz detection image features, selecting four or more feature points from the obtained feature points and substituting them into the homography transformation matrix, solving the matrix parameter value to realize the image fusion operation, and obtaining a complete image of the sample to be detected; S7. Perform image enhancement on the complete image of the sample to be tested. Use the Unet neural network model to extract the signal features of the sample to be tested. Use the visible light band image and the test image as input for image enhancement. The image enhancement formula is as follows: In the formula, I get Represents the complete image obtained, which is an optical image, I optical For the original input in the quality reduction model, I enhanced represents the enhanced image, ε is the quality reduction parameter, Indicates the convolution operation. represents the downsampling rate, n represents the noise of the system; S8. After signal amplification and filtering, the enhanced image is input into a defect recognition model to achieve target defect detection of the sample to be inspected.

2. The method for nondestructive detection of composite material defects based on terahertz array transmission according to claim 1, characterized in that: The terahertz array imaging system comprises a terahertz array detector, a terahertz frequency modulation source, a terahertz total reflection beam shaping lens, a terahertz off-axis three-reflection imaging lens and an electric four-dimensional translation stage; The terahertz frequency modulation source has a frequency modulation range of 0.3THz to 0.4THz, a maximum output power of 40mw, and an output wave of continuous wave; The array scale of the terahertz area 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 reflector, a second reflector and a third reflector, and the free-form surface equation of the XY polynomial is: Where r is the radial distance of the light at the incident point; 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 aspheric terms; The terahertz total reflection beam shaping lens comprises an off-axis collimating reflector, an off-axis micro-array reflector, an off-axis integrating reflector and a plane reflector, and the off-axis micro-array reflector comprises a first group of micro-array off-axis reflectors and a second group of micro-array off-axis reflectors; The electric four-axis translation stage includes an x-axis linear translation stage, a y-axis linear translation stage, a z-axis linear translation stage and a θ-axis rotational translation stage. The measuring ranges of the x-axis linear translation stage, the y-axis linear translation stage and the z-axis linear translation stage are all ≥500mm, and the angle variation range of the θ-axis rotational translation stage is ±20 degrees.

3. The method for nondestructive detection of composite material defects based on terahertz array transmission according to claim 1, characterized in that: The terahertz area array detector can directly perform real-time detection and imaging of the surface area of ​​the sample. For the detection area exceeding the array scale, the array scale can be selected according to the size of the scanning area, and the large-area scanning detection can be completed by setting the corresponding scanning step and scanning path in combination with the electric four-axis translation stage.

4. The method for nondestructive detection of composite material defects based on terahertz array transmission according to claim 1, characterized in that: The homogenized spot size obtained by the terahertz total reflection 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%.

5. The method for nondestructive detection of composite material defects based on terahertz array transmission according to claim 1, characterized in that: The scanning process uses a transmissive optical path. The terahertz wave is emitted from one side of the sample to be tested, and is received by the terahertz surface array detector after refraction, reflection and scattering inside the object.

6. The method for nondestructive detection of composite material defects based on terahertz array transmission according to claim 1, characterized in that: The sample to be tested is ceramic or glass fiber reinforced composite material.

7. The method for nondestructive detection of composite material defects based on terahertz array transmission according to claim 1, characterized in that: The off-axis collimating reflector is an off-axis parabolic reflector with a focal length of 120 mm and an off-axis eccentricity of 120 mm. 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 scale of 17*17. The off-axis integrating reflector is a reflector with a focal length of 1143 mm and an off-axis eccentricity of 1143 mm.

8. The method for nondestructive detection of composite material defects based on terahertz array transmission according to claim 1, characterized in that: In step S6, the homography transformation matrix is ​​a 3*3 matrix, which is used for image correction and image stitching.

9. The method for nondestructive detection of composite material defects based on terahertz array transmission according to claim 1, characterized in that: In step S8, the defect recognition model uses an edge-based extraction method to perform threshold segmentation on the image, sets parameters, obtains the outline of the image defect, and calculates and identifies the defect size and area.

10. A nondestructive testing system for the composite material defect nondestructive testing method based on terahertz array transmission as claimed in claim 1, characterized in that: It includes a terahertz surface array detector, a terahertz frequency modulation source, a terahertz total reflection beam shaping lens, a terahertz off-axis three-reflection imaging lens, and an electric four-dimensional translation stage; The frequency modulation range of the terahertz frequency modulation source is 0.3THz to 0.4THz; The terahertz area array detector has an array size of 32×32, 64×64, 128×128 or 256×256, and an array unit size of 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 reflector, a second reflector and a third reflector, and the free-form surface equation of the XY polynomial is: Where r is the radial distance of the light at the incident point; 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 aspheric terms; The terahertz total reflection beam shaping lens comprises an off-axis collimating reflector, an off-axis micro-array reflector, an off-axis integrating reflector and a plane reflector; The electric four-axis translation stage includes an x-axis linear translation stage, a y-axis linear translation stage, a z-axis linear translation stage and a θ-axis rotational translation stage. The measuring ranges of the x-axis linear translation stage, the y-axis linear translation stage and the z-axis linear translation stage are all ≥500mm, and the angle variation range of the θ-axis rotational translation stage is ±20 degrees.

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