Nondestructive testing method for terahertz composite material

Abstract

The invention belongs to the field of nondestructive testing, and relates to a terahertz composite material nondestructive testing method which comprises the following steps: S1, pretreating a to-be-tested material sample; s2, judging whether the to-be-detected material sample responds to terahertz waves or not, and if not, ending; s3, measuring the thickness of the to-be-detected material sample; s4, carrying out terahertz spectrum testing on the to-be-detected material sample, and determining a delay line range; s5, setting a delay line step length; s6, setting an imaging scanning range; s7, setting an imaging scanning step length; s8, clamping the to-be-detected material sample; s9, determining the focus position of the terahertz wave; s10, enabling the center of the to-be-seen material sample to coincide with the focal point of the terahertz wave, and carrying out scanning imaging; s11, calculating the defect depth of the to-be-detected material sample; and S12, carrying out terahertz time domain slice contrast imaging.

Description

Technical Field

[0001] The invention belongs to the field of nondestructive testing of material detection, testing and failure analysis, and relates to a nondestructive testing method for terahertz composite materials. Background Art

[0002] At present, the commonly used non-destructive testing methods for composite materials are mainly ultrasonic method, X-ray method, computer tomography method, thermal imaging method and visual method. The ultrasonic method can detect the looseness, stratification, inclusions, voids and other defects inside the composite material, but the wavelength of ultrasonic wave is long, it is easy to diffract at small cracks, and it needs to use coupling agent, and more probes need to be replaced for different defects; the X-ray method can detect surface and internal defects, but X-rays are harmful to the human body, which limits its application scenarios; computer tomography has a high spatial resolution and can be three-dimensional imaging, but the detection efficiency is low, the cost is high, and it is not suitable for on-site detection of flat thin plates and large components; the thermal imaging method has requirements on the thermal conductivity of the material, and the sample thickness cannot be too thick; the visual method is simple, but it is greatly affected by the experience of workers and can only detect surface defects.

[0003] Terahertz waves are electromagnetic waves between millimeter waves and infrared light, with a frequency range of 0.1 to 10 THz. They are characterized by penetrability, broadband, coherence, instantaneity, and non-destructiveness. THz waves have strong penetrating power for many non-metallic and non-polar dielectric materials, and can realize non-contact detection and analysis of microstructures and microdefects in composite materials. THz spectrometers and imaging systems based on ZnTe crystals can realize rapid non-destructive detection of thermal protection structures of spacecraft and stealth coating protection structures of fighter jets, and directly observe defects such as microcracks, fiber delamination, interface cracking, fiber curling, and holes in composite materials. Terahertz non-destructive testing uses the reflectivity and penetration of terahertz electromagnetic signals combined with various imaging technologies to detect defects in materials. By scanning and imaging the absorption peaks and refractive index of the scanned sample in the terahertz frequency band, the state of the object being tested can be judged, and non-contact detection and analysis of microstructures and microdefects of composite materials and accurate quantification can be realized.

[0004] Reflective terahertz nondestructive testing uses the characteristics of terahertz waves being very sensitive to the interface between defects and normal ones to detect the depth information of defects. Interfaces will be generated between materials with different refractive indices, and each interface will reflect a part of the incident terahertz pulse. The time delay between the terahertz echoes reflected by the two interfaces corresponds to the thickness of each layer. Terahertz waves will reflect terahertz echoes at the interface between the normal composite material substrate and defects (delamination, debonding, inclusions). The depth and thickness of the defects can be measured by the time delay between these terahertz pulse echoes. Transmission terahertz nondestructive testing uses the characteristics of different responses of THz waves to penetrate the composite material substrate and defects, and uses a THz pulse scanning imaging system to achieve two-dimensional imaging of defects in composite material plates. However, the application of this technology is currently limited to laboratory testing. Since there is no clear test method for the detection of composite materials, it cannot be applied to products at present. Summary of the invention

[0005] Purpose of the present invention: To solve the problem that internal defects of composite material plates cannot be accurately located, the present invention proposes a terahertz composite material nondestructive testing method for detecting and locating internal defects of composite material plates.

[0006] Technical Solution

[0007] A nondestructive testing method for terahertz composite materials comprises the following steps:

[0008] S1: Pre-treat the sample of the material to be tested;

[0009] S2: Determine whether the material sample to be tested responds to the terahertz wave, if not, end;

[0010] S3: Measure the thickness of the material sample to be tested

[0011] S4: Perform terahertz spectrum test on the material sample to be tested to determine the delay line range;

[0012] S5: Set the delay line step size;

[0013] S6: Set the imaging scanning range;

[0014] S7: Set the imaging scan step length;

[0015] S8: Clamp the material sample to be tested;

[0016] S9: Determine the focal position of the terahertz wave;

[0017] S10: aligning the center of the sample of the material to be tested with the focus of the terahertz wave to perform scanning imaging;

[0018] S11: Determine the defect depth of the material sample to be tested;

[0019] S12: Perform terahertz time-domain slice contrast imaging.

[0020] Furthermore, since terahertz waves have strong penetration ability for non-polar materials, but for some conductive materials, the high concentration of free carriers inside them have strong absorption and reflection ability for terahertz, it is impossible to obtain information below the surface through terahertz spectroscopy. Therefore, before conducting terahertz testing, it is necessary to test whether the sample material to be tested responds to terahertz waves. In S2, the terahertz system spectrum range is set to 0.1THz-3THz, and the delay line moving distance is set to 100mm; the material sample to be tested is tested to observe whether a reflection peak appears in the terahertz spectrum; if the terahertz wave peak amplitude exceeds 5% of the full scale, it is judged that the material sample to be tested responds to the terahertz wave.

[0021] Furthermore, in order to accurately test the location and thickness of the defect, the thickness of the material sample to be tested needs to be accurately measured. The thickness test process is as follows:

[0022] For reflection testing, the terahertz wave is controlled to be incident on the surface of the material sample to be tested at an angle of 45°; for transmission testing, the terahertz wave is controlled to be incident on the surface of the material sample to be tested at an angle of 90°; the delay time is measured; and the thickness d of the material sample to be tested is calculated according to the delay time;

[0023] The formula is as follows:

[0024]

[0025] Where c is the speed of light in vacuum, Δt is the delay time, n c is the sample refractive index, n 0 Represents the refractive index of air.

[0026] Furthermore, for reflection testing, the delay line range mainly determines the measurement range of the terahertz time domain spectrum. This range must ensure that the time window of the time domain signal during the terahertz test must contain the main peak of the reference signal and the main peak of the sample signal. The process of determining the delay line range is as follows:

[0027] Set the delay line moving range to the upper and lower limits of the one-dimensional moving platform moving range, and the moving step length to 0.015mm;

[0028] Perform reflective terahertz time-domain spectroscopy test on the material sample to be tested, and obtain the surface peak position x1 and the bottom peak position x 2 , the delay line range is x 1 ~x 2 .

[0029] Furthermore, for transmission testing, the delay line range mainly determines the measurement range of the terahertz time-domain spectrum. This range must ensure that the time window of the time domain signal during the terahertz test must contain the main peak of the reference signal and the main peak of the sample signal. The process of determining the delay line range is as follows:

[0030] When no sample of material to be tested is placed, set the delay line moving range to the minimum to maximum value and determine the reference signal peak position x 1 mm;

[0031] Clamp the material sample to be tested and set the delay line moving range to x 1 +1mm to the maximum value, and measure the peak position x of the transmission signal of the sample to be tested 2 , the delay line range is determined as x 1 ~x 2 .

[0032] Furthermore, in S5, the delay line moving step determines the time resolution of the terahertz time domain spectrum. The larger the delay line moving step, the worse the time resolution of the terahertz time domain spectrum. Reducing the delay line moving step can improve the time resolution, but it will also increase the test time. It is generally set to shorten the test time while meeting the resolution. The delay line step setting method is as follows:

[0033] First, the delay line moving step size is set to 0.001 mm and the reference signal is tested to obtain a clear terahertz pulse waveform;

[0034] Then gradually increase the step size and test the reference signal until the step size increases to the point where the difference Δt between the peak position of the terahertz pulse signal and the peak position of the terahertz pulse signal at a step size of 0.001 mm cannot meet the depth accuracy requirement: Δt×c / 2>d 0 ,d 0 is the required depth resolution;

[0035] Then the maximum value of the delay line moving step is Δt×c / 2; Δt is the half-peak width of the terahertz pulse in air, and c is the speed of light in air.

[0036] Further, in S6, the imaging scanning range determines the image range of the terahertz wave detection, and generally should be able to display the allowable defect size in one image. The imaging scanning range is 50% larger than the allowable defect size.

[0037] Furthermore, in S7, the imaging scanning step size refers to the interval between different pixel points when the sample is scanned and imaged using a transmission terahertz pulse scanning imaging system. The imaging scanning step size has an important impact on the test results. If the imaging scanning step size is too large, the defect size may be smaller than the step size and be skipped, that is, the imaging accuracy is low and the defect cannot be effectively identified; if the imaging scanning step size is too small, the test time may be extended, and the effect of improving the imaging accuracy is not obvious, thereby reducing the test efficiency. The imaging scanning step size should be able to ensure that the same defect can be found in two repeated scans. The imaging scanning step size should be less than 50% of the allowable defect size.

[0038] Furthermore, in S9, the terahertz focus position is the position where the terahertz wave signal is the strongest, and generally the center of the sample area of ​​the material to be tested should be coincident with the terahertz focus. The process of determining the terahertz wave focus position is as follows: first, without placing a sample, measure the reference signal and move the delay line position to the position corresponding to the signal peak;

[0039] The metal sheet with holes is pasted to the sample holder, and the horizontal axis (X) moving range of the two-dimensional moving platform is set to be the length of one side of the material sample to be tested, and the vertical axis (Z) moving range is set to be the length of one side of the material sample to be tested, and the scanning step length is set to 0.5 mm, and scanning is performed;

[0040] Finally, the coordinates of the pixel with the strongest signal are found in the scanned two-dimensional image as the terahertz focus position.

[0041] Further, in S10, the X-axis moving range and the Z-axis moving range are set according to the material sample to be inspected, and scanning imaging is performed.

[0042] Further, in S11, the surface reflection peak and the bottom reflection peak are determined in the terahertz time domain spectrum, and the peak between the surface reflection peak and the bottom reflection peak is observed as the defect reflection peak. The delay time between the defect reflection peak and the surface reflection peak is calculated from the terahertz spectrum, and then the distance of the defect from the upper surface of the sample is calculated by the thickness of the material sample to be tested.

[0043] Further, in S12, according to the terahertz transmission time domain spectra of the substrate and the defect obtained by the test, the delay line of the terahertz time domain spectrum is fixed at the position where the intensity difference is maximum and then the terahertz pulse scanning is performed to obtain the time domain slice contrast imaging image.

[0044] Beneficial effect: The present invention provides a terahertz nondestructive testing method for composite materials. According to the material parameters and spectral response of the composite material plate, the terahertz wave signal is determined, and the composite material plate is further subjected to reflective or transmissive terahertz time-domain spectroscopy testing. Finally, image processing is performed, and defects are accurately located and measured, thereby quickly completing nondestructive testing of defects in the composite material plate. BRIEF DESCRIPTION OF THE DRAWINGS

[0045] The present invention includes 5 drawings, and the drawings and the descriptions of the drawings are as follows:

[0046] Figure 1 This is a flow chart of the terahertz nondestructive testing method;

[0047] Figure 2 is the terahertz spectrum; Figure 2 (a) Schematic diagram of the spectrum with reflection peak. Figure 2 (b) Schematic diagram of the spectrum without reflection peak;

[0048] Figure 3 Schematic diagram of the incident geometry of the optical path;

[0049] Figure 4 This is a schematic diagram of the transmission terahertz detection tooling;

[0050] Figure 5 This is the terahertz wave time domain spectrum of the defects of the material sample to be tested.

[0051] Figure 6 This is a schematic diagram of time-domain slicing imaging of terahertz time-domain spectroscopy; DETAILED DESCRIPTION

[0052] Embodiment 1

[0053] Testing environment: 20℃, air condition, air humidity is less than 70%;

[0054] The first step is to prepare the material samples and equipment to be tested, clean and dry the samples for use, and connect the equipment; clean the surface of the samples with an organic solvent.

[0055] The second step is to set the terahertz system spectrum range to 0.1THz-3THz, the delay line moving distance to 100mm (usually the material refractive index is <10), and test whether there is a reflection peak in the terahertz spectrum. If the terahertz wave peak amplitude exceeds 5% of the full scale, it is judged that the material sample to be tested responds to the terahertz wave. Figure 2 When the terahertz reflection peak appears in the spectrum, the next step can be performed.

[0056] The third step is to measure the thickness of the composite material plate. In the terahertz reflection mode system, the terahertz wave is incident on the sample surface at an angle of 45°, and the optical path changes ( Figure 3 ), the new delay formula is as follows by calculating the system geometry:

[0057]

[0058] Where c is the speed of light in vacuum, Δt is the delay time, n c is the sample refractive index, n 0represents the refractive index of air, and d is the sample thickness. The sample thickness is calculated by measuring the flight time in reflection mode and combining the sample optical parameters.

[0059] Step 4: Set the delay line moving range to the upper and lower limits of the one-dimensional moving platform moving range, set the moving step length to 0.015 mm, perform reflective terahertz time-domain spectroscopy test, and obtain the surface peak position x 1 and the bottom peak position x 2 , the delay line range is x 1 ~x 2 .

[0060] The fifth step is to set the delay line step size. First, set the delay line step size to 0.001mm and test the reference signal to obtain a clear terahertz pulse waveform. Then gradually increase the step size and test the reference signal until the step size increases to the point where the difference Δt between the peak position of the terahertz pulse signal and the peak position of the terahertz pulse signal at a step size of 0.001mm cannot meet the depth accuracy requirement (Δt×c / 2>d0, d0 is the required depth resolution). The delay line step size is at most Δt×c / 2, where Δt is the half-peak width of the terahertz pulse in air, and c is the speed of light in air. Generally speaking, the half-peak width of a terahertz pulse is 1.5×10 -12 s, the speed of light in air is about 3×10 8 m / s, so the delay line moving step should be no more than 0.225mm. On this basis, the moving step can be appropriately reduced according to the needs of measurement accuracy, and the test time can be extended accordingly. In order to balance the test accuracy and time, it is recommended that the delay line moving step is not less than 0.001mm. The delay line moving step is set in the range of 0.001 to 0.225mm.

[0061] Step 6: Set the imaging scanning range according to the size of the defect to be found, so that a complete defect size can be found. For example, if the defect size to be found is 6mm, set the imaging scanning range to 9mm.

[0062] Step 7: Set the imaging scan step length. The scan step length should be smaller than the minimum defect size. At the same time, consider the test time and select a suitable imaging scan step length. If the defect size to be found is 6mm, set the imaging scan step length to 3mm.

[0063] Step 7: Clamp the sample to ensure that the terahertz wave is incident at 45°.

[0064] Step 8: Determine the terahertz focal position. First, measure the reference signal without placing a sample, and move the delay line position to the position corresponding to the signal peak. Paste the metal sheet with holes to the sample holder, set the horizontal axis (X) movement range of the two-dimensional moving platform to +42.5mm~-42.5mm, the vertical axis (Z) movement range to +42.5mm~-42.5mm, and the scanning step length to 0.5mm for scanning. Finally, find the coordinates of the pixel with the strongest signal in the scanned two-dimensional image, which is the terahertz focal position.

[0065] In the ninth step, the center of the sample area to be tested is aligned with the terahertz focus, and the X-axis moving range is set to 30 mm to 42 mm, and the Z-axis moving range is set to 12 mm to 24 mm for scanning imaging.

[0066] Step 10: Determine the surface reflection peak and the bottom reflection peak in the terahertz time domain spectrum. The peak between the surface reflection peak and the bottom reflection peak is the defect reflection peak. Calculate the delay time between the defect reflection peak and the surface reflection peak from the terahertz spectrum, and then calculate the distance between the defect and the upper surface of the sample by the thickness of the material sample to be tested. Figure 5 shown.

[0067] Step 11: According to the terahertz transmission time domain spectra of the substrate and defects obtained by the test, the delay line of the terahertz time domain spectrum is fixed at the position where the intensity difference is the largest, and then the terahertz pulse scanning is performed to obtain the time domain slice contrast imaging image. Figure 6 shown.

[0068] Embodiment 2

[0069] Test environment: 20℃, air condition, air humidity is less than 70%;

[0070] The first step is to prepare the material samples and equipment to be tested, clean and dry the samples for use, and connect the equipment;

[0071] The second step is to set the terahertz system spectrum range to 0.1THz-3THz, the delay line moving distance to 100mm (usually the material refractive index is <10), and test whether the transmission peak appears in the terahertz spectrum; if the terahertz wave peak amplitude exceeds 5% of the full scale, it is judged that the material sample to be tested responds to the terahertz wave. Figure 2 Figure (a) is a spectrum with a transmission peak, and Figure (b) is a spectrum without a transmission peak.

[0072] The third step is to accurately measure the thickness of the composite material sheet. In the terahertz reflection mode system, the terahertz wave is incident on the sample surface at an angle of 90°, and the optical path changes ( Figure 3 ), the new delay formula is as follows by calculating the system geometry:

[0073]

[0074] Where c is the speed of light in vacuum, Δt is the delay time, n c is the sample refractive index, n 0 Represents the refractive index of air, and d is the sample thickness. The sample thickness can be accurately calculated by measuring the flight time in reflection mode combined with the sample optical parameters.

[0075] Step 4: When no sample is placed, set the delay line moving range from the minimum to the maximum value and determine the reference signal peak position x 1 After determining the reference signal peak position, clamp the sample and set the delay line moving range to x 1 +1mm to the maximum value, measure the sample transmission signal peak position x 2 The delay line range is determined as x 1 ~x 2 .

[0076] The fifth step is to set the delay line step size. First, set the delay line step size to 0.001mm and test the reference signal to obtain a clear terahertz pulse waveform. Then gradually increase the step size and test the reference signal until the step size increases to the point where the difference Δt between the peak position of the terahertz pulse signal and the peak position of the terahertz pulse signal at a step size of 0.001mm cannot meet the depth accuracy requirement (Δt×c / 2>d0, d0 is the required depth resolution). The delay line step size is at most Δt×c / 2, where Δt is the half-peak width of the terahertz pulse in air, and c is the speed of light in air. Generally speaking, the half-peak width of a terahertz pulse is 1.5×10 -12 s, the speed of light in air is about 3×10 8 m / s, so the delay line moving step should not be greater than 0.225mm. The moving step can be appropriately reduced according to the measurement accuracy and the test time can be extended accordingly. In order to balance the test accuracy and time, it is recommended that the delay line moving step should not be less than 0.001mm. In summary, the delay line moving step should be in the range of 0.001 to 0.225mm.

[0077] Step 6: Set the imaging scanning range according to the size of the defect to be found, so that a complete defect size can be found. For example, if the defect size to be found is 6mm, set the imaging scanning range to 9mm.

[0078] The seventh step is to set the imaging scanning step length. The scanning step length should be smaller than the minimum defect size. At the same time, the appropriate imaging scanning step length should be selected after comprehensively considering the test time.

[0079] Step 8. Clamp the sample to ensure that the terahertz wave is incident at 90°.

[0080] The ninth step is to determine the terahertz focal position. First, without placing a sample, measure the reference signal and move the delay line position to the position corresponding to the signal peak. Paste the metal sheet with holes to the sample holder, set the horizontal axis (X) movement range of the two-dimensional moving platform to +42.5mm~-42.5mm, the vertical axis (Z) movement range to +42.5mm~-42.5mm, and the scanning step length to 0.5mm for scanning. Finally, find the pixel coordinates with the strongest signal in the scanned two-dimensional image, which is the terahertz focal position.

[0081] The tenth step is to make the center of the sample area to be tested coincide with the terahertz focus, set the X-axis moving range to 30mm~42mm, and the Z-axis moving range to 12mm~24mm, and perform scanning imaging.

[0082] Step 11: Determine the surface reflection peak and the bottom reflection peak in the terahertz time domain spectrum. The peak between the surface reflection peak and the bottom reflection peak is the defect reflection peak. Calculate the delay time between the defect reflection peak and the surface reflection peak and the delay time between the surface reflection peak and the bottom reflection peak from the terahertz spectrum. Then calculate the distance between the defect and the upper surface of the sample by the thickness of the material sample to be tested. Figure 5 shown.

[0083] In the twelfth step, according to the terahertz transmission time domain spectra of the substrate and the defect obtained by the test, the delay line of the terahertz time domain spectrum is fixed at the position where the intensity difference is the largest and then the terahertz pulse scanning is performed to obtain the time domain slice contrast imaging image. Figure 6 shown.

Claims

1. A nondestructive testing method for terahertz composite materials, characterized by: The following steps are involved: S1: Pre-treat the sample of the material to be tested; S2: Determine whether the material sample to be tested responds to the terahertz wave, if not, end; S3: Measure the thickness of the material sample to be tested; S4: Perform terahertz spectrum test on the material sample to be tested to determine the delay line range; S5: Set the delay line step size; S6: Set the imaging scanning range; S7: Setting the imaging scan step length; S8: Clamp the material sample to be tested; S9: Determine the focal position of the terahertz wave; S10: aligning the center of the sample of the material to be tested with the focus of the terahertz wave to perform scanning imaging; S12: Calculate the defect depth of the material sample to be tested; S11: Perform terahertz time-domain slice contrast imaging.

2. The method according to claim 1, characterized in that: In S2, the terahertz system spectrum range is set to 0.1THz-3THz, and the delay line moving distance is set to 100mm; the material sample to be tested is tested to observe whether a reflection peak appears in the terahertz spectrum; if the terahertz wave peak amplitude exceeds 5% of the full scale, it is determined that the material sample to be tested responds to the terahertz wave.

3. The method according to claim 2, characterized in that: For reflective type, the thickness test process is as follows: Control the terahertz wave to be incident on the surface of the material sample to be tested at an angle of 45°, and measure the delay time; calculate the thickness d of the material sample to be tested according to the delay time; The formula is as follows: Where c is the speed of light in vacuum, Δt is the delay time, n c is the refractive index of the sample, and n0 represents the refractive index of air.

4. The method according to claim 2, characterized in that: For transmission type, the thickness test process is as follows: Control the terahertz wave to be incident at an angle of 90° on the surface of the material sample to be tested, and measure the delay time; calculate the thickness d of the material sample to be tested according to the delay time; The formula is as follows: Where c is the speed of light in vacuum, Δt is the delay time, n c is the refractive index of the sample, and n0 represents the refractive index of air.

5. The method according to claim 3, characterized in that: For reflective testing, the delay line range is determined as follows: Set the delay line moving range to the upper and lower limits of the one-dimensional moving platform moving range, and the moving step length to 0.015mm; A reflective terahertz time-domain spectroscopy test is performed on the material sample to obtain the surface peak position x1 and the bottom peak position x2, and the delay line range is x1 to x2.

6. The method according to claim 4, characterized in that: For transmission testing, the process for determining the delay line range is as follows: When no sample of the material to be tested is placed, the moving range of the delay line is set from the minimum value to the maximum value, and the peak position of the reference signal is determined to be x1 mm; Clamp the material sample to be tested, set the delay line moving range to x1+1mm to the maximum value, measure the transmission signal peak position x2 of the material sample to be tested, and determine the delay line range to be x1~x2.

7. The method according to claim 5 or 6, characterized in that: In S5, the delay line step size is set as follows: First, the delay line moving step size is set to 0.001 mm and the reference signal is tested to obtain a clear terahertz pulse waveform; Then, the step size is gradually increased and the reference signal is tested until the step size is increased to the point where the difference Δt between the peak position of the terahertz pulse signal and the peak position of the terahertz pulse signal at a step size of 0.001 mm cannot meet the depth accuracy requirement: Δt×c / 2>d0, d0 is the required depth resolution; Then the maximum value of the delay line moving step is Δt×c / 2; Δt is the half-peak width of the terahertz pulse in air, and c is the speed of light in air.

8. The method according to claim 7, characterized in that: In S6, the imaging scanning range is 50% larger than the allowable defect size.

9. The method according to claim 8, characterized in that: In S7, the imaging scanning step length is less than 50% of the allowable defect size.

10. The method according to claim 9, characterized in that: In S9, the process of determining the focal position of the terahertz wave is as follows: first, without placing a sample, measure the reference signal, and move the delay line position to a position corresponding to the signal peak; The metal sheet with holes was pasted onto the sample holder, and the horizontal axis (X) moving range of the two-dimensional moving platform was set to be the length of one side of the material sample to be tested, and the vertical axis (Z) moving range was set to be the length of the other side of the material sample to be tested, and the scanning step was set to 0.5 mm, and scanning was performed; Finally, the coordinates of the pixel with the strongest signal are found in the scanned two-dimensional image as the terahertz focus position.

Images