Method for positioning internal defects of anisotropic material based on laser ultrasonic SAFT technology

Abstract

The invention discloses a method for positioning an internal defect of an anisotropic material based on a laser ultrasonic SAFT technology, and the method specifically comprises the following steps: placing a to-be-detected sample in a laser scanning area, setting a detection point and an excitation point at a certain step length, and exciting ultrasonic waves at the excitation point by a pulse laser, and the laser detection device receives the corresponding ultrasonic signals at the detection points, and the work is repeated until the laser detection device receives all the ultrasonic signals excited by the laser at all the excitation points. And then, analyzing the detected time-domain ultrasonic signal, performing inversion imaging by using an SAFT algorithm developed for the anisotropic material, and filtering a noise signal through windowing processing to improve the accuracy and imaging quality of inversion imaging, thereby realizing detection and positioning of the internal defect of the anisotropic material.

Description

Technical Field

[0001] The invention belongs to the field of non-destructive testing, in particular to a method for locating internal defects of anisotropic materials based on laser ultrasonic SAFT technology. Background Art

[0002] Laser ultrasound is a new ultrasonic testing technology. Compared to traditional ultrasound, it not only enables long-distance, contactless testing but also avoids the contamination of the test object and the generation of interfering harmonic signals caused by the coupling agent used in traditional ultrasonic testing. Laser ultrasonic testing technology also boasts higher energy conversion efficiency. Lasers are characterized by excellent directionality, monochromaticity, and concentrated energy. The ultrasonic waves generated by laser excitation possess the advantages of wide bandwidth and multimodality, enabling higher-precision testing. In today's era of rapidly emerging new materials, laser ultrasonic technology is poised to play a greater role in the field of nondestructive testing.

[0003] Synthetic Aperture Focusing (SAFT) is a key method for improving spatial resolution in acoustic testing. Its core principle is to achieve aperture expansion through array-based signal processing. This technology employs distributed small-aperture units working together to construct an equivalent large-aperture detection system, significantly improving the lateral resolution of the target under test. In laser ultrasonic testing systems, the SAFT over-delay compensation algorithm performs coherent signal superposition processing, enabling precise defect location.

[0004] Previously, some scholars [Wu Wei, Qiu Zongming, Huang Qiuhong. SAFT-based time-domain detection imaging method for internal defects of single-crystal silicon [J]. China Mechanical Engineering, 2016, 27(15): 2075-2079.] used the time-of-flight method and synthetic aperture focusing technology to detect internal defects of single-crystal silicon rods. This method divides the imaging area into grids and then reconstructs the time domain according to the arrival time of the received signal. Although the size of the imaging result is larger than the actual size of the defect, this method also provides a new technical idea for imaging defects in anisotropic materials. Summary of the Invention

[0005] The purpose of the present invention is to overcome the shortcomings of existing ultrasonic non-destructive testing and provide a method for locating internal defects of anisotropic materials using laser ultrasonic SAFT technology.

[0006] In order to achieve the above object, the present invention adopts the following steps: A method for locating internal defects of anisotropic materials based on laser ultrasonic SAFT technology, comprising the following steps:

[0007] Step 1: Set the detection points at a fixed angle, calculate the ultrasonic velocity in each direction based on the arrival time of the echo signal, and draw the ultrasonic velocity distribution map inside the anisotropic material;

[0008] Step 2: Set detection points on the surface of the sample to be tested with a fixed step size. The detection area should occupy more than 60% of the upper surface of the sample to be tested.

[0009] Step 3: Set the laser excitation point according to a certain step size;

[0010] Step 4: The pulsed laser acts on the surface of the sample to be tested to excite ultrasonic waves. The laser detection device completes the reception of the ultrasonic signals according to the set detection points and transmits the collected ultrasonic signals to the computer.

[0011] Step 5: Move the pulse laser to the next excitation point and repeat step 4 until the pulse laser completes ultrasonic excitation at all excitation points and the laser detection device completes receiving the corresponding ultrasonic signal;

[0012] Step 6: Based on the ultrasonic velocity distribution map inside the anisotropic material and the ultrasonic signal obtained in step 5, an inversion calculation is performed using the SAFT algorithm;

[0013] Step 7: After the inversion calculation of all ultrasonic signals is completed, the calculation results are superimposed and accumulated and averaged according to the weights to obtain the inversion imaging results of the internal defects of the sample to be tested.

[0014] Compared with the existing technology, the significant advantages of the present invention are: it can quickly and conveniently obtain the propagation speed of ultrasound in all directions inside anisotropic materials, and can accurately inversely calculate the position and size of defects inside anisotropic materials, overcoming the difficulties of traditional ultrasonic imaging caused by material anisotropy. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Figure 1 It is a schematic structural diagram of the laser ultrasonic detection system of the present invention.

[0016] Figure 2 This is the principle diagram of laser ultrasonic SAFT.

[0017] Figure 3 This is the SAFT algorithm superposition calculation principle diagram. DETAILED DESCRIPTION

[0018] Traditional laser ultrasonic SAFT algorithms are only suitable for the detection of isotropic materials and lack the ability to detect anisotropic materials. This invention uses an optimized SAFT algorithm to detect and image internal defects in anisotropic materials by setting detection points with different angle gradients to obtain ultrasonic wave velocities propagating in all directions.

[0019] A method for detecting internal defects of anisotropic materials using a novel SAFT algorithm comprises the following steps:

[0020] (1) Setting detection points at fixed angles, calculating the ultrasonic velocity in each direction based on the arrival time of the echo signal, and drawing a distribution map of the ultrasonic velocity inside the anisotropic material;

[0021] (2) Setting detection points on the surface of the sample to be tested with a fixed step size, and the detection area should occupy more than 60% of the upper surface of the sample to be tested;

[0022] (3) Setting the laser excitation point according to a certain step size;

[0023] (4) The pulsed laser acts on the surface of the sample to be tested to stimulate ultrasonic waves. The laser detection device completes the reception of ultrasonic signals according to the set detection points and transmits the collected ultrasonic signals to the computer;

[0024] (5) Move the pulse laser to the next excitation point and repeat step 4 until the pulse laser completes ultrasonic excitation at all excitation points and the laser detection device completes receiving the corresponding ultrasonic signal;

[0025] (6) Based on the ultrasonic velocity distribution map inside the anisotropic material and the ultrasonic signal obtained in step 5, an inversion calculation is performed using the SAFT algorithm;

[0026] (7) After the inversion calculation of all ultrasonic signals is completed, the calculation results are superimposed and accumulated and averaged according to the weights to obtain the inversion imaging results of the internal defects of the sample to be tested.

[0027] Furthermore, the sample to be tested is an anisotropic material such as single crystal silicon.

[0028] Furthermore, the angle set in step (1) is 0.5° to 1°.

[0029] Furthermore, the detection point step size set in step (2) is 0.1 mm to 0.5 mm.

[0030] Furthermore, in step (3), the excitation point step size is set to 1 mm to 2 mm.

[0031] In order to make the purpose, technical solutions and advantages of the present invention more clear, the present invention is further described below with reference to the accompanying drawings and embodiments.

[0032] Example

[0033] The detection system structure of the present invention is as follows Figure 1As shown, the apparatus includes a pulsed laser 1, a laser detection device 2, a sample to be tested 4, and a computer 3. The pulsed laser 1 applies laser light to an excitation point on the upper surface of the sample to be tested 4 to excite ultrasonic waves. The laser detection device 2 receives the ultrasonic signal at a preset detection point and transmits the ultrasonic signal to the computer 3. In this embodiment, the sample to be tested is a single crystal silicon block.

[0034] The sample to be tested is placed in the laser scanning area, and detection and excitation points are set at a specific step size. A pulsed laser excites ultrasonic waves at the excitation points, and the laser detection device receives the corresponding ultrasonic signals at the detection points. This process is repeated until the laser detection device has received all the ultrasonic signals generated by the laser at all excitation points. The detected time-domain ultrasonic signals are then analyzed and inversion imaging is performed using the SAFT algorithm developed for anisotropic materials. Windowing is used to filter out noise signals, improving the accuracy and quality of the inversion imaging, thereby enabling the detection and location of internal defects in anisotropic materials.

[0035] The following is a detailed description of the detection process of locating internal defects of anisotropic materials using laser ultrasonic SAFT in conjunction with the accompanying drawings:

[0036] (1) Figure 2 Where x is the horizontal axis distance from the excitation point to the defect, ' is the horizontal axis distance from the detection point to the defect, y is the vertical axis distance from the excitation point to the defect, and the ultrasonic propagation speed is c L , L is the ultrasonic signal excited by the laser, L' is the ultrasonic signal after being scattered by the defect, and the propagation speed of the reflected ultrasonic wave is c ' L , then the arrival time of the detection can be calculated

[0037] (2) Figure 3 In the example, the number of grids is M×N, where M is the number of grid rows, N is the number of grid columns, the grid size is (Δx, Δy), the unit coordinates of the excitation point are (i0, j0), and the unit coordinates of the detection point are (i D ,j D ), the coordinates of the grid to be imaged are (i, j). According to the positions of the excitation point and the detection point, the propagation angle α of the incident longitudinal wave and the propagation angle β of the reflected longitudinal wave can be calculated, and then the time t when the ultrasonic signal reaches the detection point can be obtained. L '(i,j), the amplitude of the ultrasonic signal received by the detection point corresponding to the moment is S(i,j). Assuming that there are Q detection points on the upper surface, and the corresponding ultrasonic amplitudes are obtained in the above way, the amplitude of the grid area is

[0038]

[0039] (3) Finally, the ultrasonic signals obtained at all excitation points are inverted and calculated, and the settlement results are superimposed and accumulated to achieve focused imaging of the entire imaging area, thereby completing the positioning and imaging display of the internal defects of the sample to be tested.

[0040] In this example, the single crystal silicon block sample size was 10.00 mm × 5.00 mm, with an excitation step size of 1.00 mm and a detection step size of 0.1 mm. Laser light, acting as a surface heat source, was incident perpendicular to the surface under a thermoelastic mechanism. The laser detection device was adjusted to receive echo signals at the detection point. Using a novel SAFT algorithm for superposition calculation, the inverted defect transverse axis dimensions remained consistent with the actual dimensions.

Claims

1. A method for locating internal defects of anisotropic materials based on laser ultrasonic SAFT technology, characterized in that: The following steps are involved: Step 1: Set the detection points at a fixed angle, calculate the ultrasonic velocity in each direction based on the arrival time of the echo signal, and draw the ultrasonic velocity distribution map inside the anisotropic material; Step 2: Set detection points on the surface of the sample to be tested with a fixed step size. The detection area should occupy more than 60% of the upper surface of the sample to be tested. Step 3: Set the laser excitation point according to a certain step size; Step 4: The pulsed laser acts on the surface of the sample to be tested to excite ultrasonic waves. The laser detection device completes the reception of the ultrasonic signals according to the set detection points and transmits the collected ultrasonic signals to the computer. Step 5: Move the pulse laser to the next excitation point and repeat step 4 until the pulse laser completes ultrasonic excitation at all excitation points and the laser detection device completes receiving the corresponding ultrasonic signals; Step 6: Based on the ultrasonic velocity distribution map inside the anisotropic material and the ultrasonic signal obtained in step 5, an inversion calculation is performed using the SAFT algorithm; Step 7: After the inversion calculation of all ultrasonic signals is completed, the calculation results are superimposed and accumulated and averaged according to the weights to obtain the inversion imaging results of the internal defects of the sample to be tested.

2. The method for locating internal defects of anisotropic materials based on laser ultrasonic SAFT technology according to claim 1, characterized in that: The sample to be tested is an anisotropic material.

3. The method for locating internal defects of anisotropic materials based on laser ultrasonic SAFT technology according to claim 2, characterized in that: The sample to be tested is single crystal silicon.

4. The method for locating internal defects of anisotropic materials based on laser ultrasonic SAFT technology according to claim 1, characterized in that: The angle set in step 1 is 0.5° to 1°.

5. The method for locating internal defects of anisotropic materials based on laser ultrasonic SAFT technology according to claim 1, characterized in that: In step 2, the detection point step size is set to 0.1mm to 0.5mm.

6. The method for locating internal defects of anisotropic materials based on laser ultrasonic SAFT technology according to claim 1, characterized in that: In step 3, the excitation point step size is set to 1 mm to 2 mm.

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