CN117269327B - Laser ultrasonic subsurface defect detection positioning method and device - Google Patents

Abstract

The invention discloses a laser ultrasonic subsurface defect detection and positioning method and device. The detection positioning method comprises the steps of setting up a test system, setting up a scanning mode and parameters, scanning a sample, receiving and analyzing signals, wherein the scanning mode comprises one-dimensional scanning and two-dimensional scanning, the two-dimensional scanning is two-dimensional superposition of the one-dimensional scanning on a scanning step distance, and the one-dimensional scanning comprises one-dimensional rapid scanning and one-dimensional fine scanning which are sequentially carried out. The laser ultrasonic subsurface defect detection and positioning method provided by the invention realizes the problems of subsurface defect detection and positioning in a two-dimensional range under an unknown uniform temperature field, and the method is strong in self-adaptability, intuitive in principle, high in positioning precision, and convenient to adjust and maintain, and meanwhile, the detection efficiency and the precision are both considered.

Description

Laser ultrasonic sub-surface defect detection and positioning method and device

Technical Field

The invention relates to the field of ultrasonic nondestructive testing, and in particular to a laser ultrasonic sub-surface defect detection and positioning method and device.

Background Art

Laser ultrasound is a non-contact, non-destructive testing technology that does not require coupling agents. In recent years, it has become an important branch of the field of ultrasonic testing. It is mainly based on thermoelastic or thermal corrosion mechanisms to excite high-frequency ultrasonic signals, and detect ultrasonic signals through interferometers, with high resolution in time and space. Subsurface defects usually refer to defects below the surface of a component, close to the surface. The size of subsurface defects will gradually expand over time, posing a huge safety hazard to in-service components. For example, with the rapid development of metal additive manufacturing technology in recent years, metal parts will inevitably introduce defects such as pores, cracks and inclusions during the manufacturing process, but traditional detection technologies make it difficult to achieve in-situ detection of subsurface defects during the manufacturing process.

Sub-surface defects cannot be detected by appearance inspection, and traditional metallographic inspection, X-ray inspection, magnetic particle inspection, eddy current inspection, electromagnetic ultrasonic transducer and other inspection technologies are also limited by their own shortcomings. Compared with traditional inspection technologies, laser ultrasound has the advantages of non-contact, high sensitivity, and non-destructiveness. It is suitable for harsh environments such as high temperature, high pressure, strong corrosion and strong radiation, and is also suitable for the status inspection of parts with high-speed movement and complex shapes.

At present, there have been many research works on sub-surface defect detection technology based on laser ultrasound, such as using the phase change of surface waves and the reflection of surface waves. However, the phase change of surface waves can only reflect whether sub-surface defects exist, but cannot locate sub-surface defects. The reflection of surface waves will only appear when the laser excitation point and the receiving point are on the same side of the sub-surface defect. Therefore, it is very necessary to design a laser ultrasound-based detection technology that can detect and locate sub-surface defects when the excitation point and the receiving point are on the same side or on the opposite side of the sub-surface defect.

Summary of the invention

In order to overcome the shortcomings of the prior art, the purpose of the present invention is to provide a laser ultrasonic sub-surface defect detection and positioning method and device, which can detect the existence of sub-surface defects and obtain the distribution and location of sub-surface defects in an unknown uniform temperature, pressure and other environments, and also propose a method for determining the uniform sound velocity relationship between laser ultrasonic surface waves and surface-grazing longitudinal waves. The sub-surface defect positioning method of the present invention, in addition to the reflected echo of the surface wave, also provides the mutual conversion wave of the surface wave and the surface-grazing longitudinal wave, which can be used for the detection and positioning of sub-surface defects.

The present invention has the characteristics of strong adaptability, intuitive principle, high positioning accuracy, and taking into account detection efficiency and accuracy as well as convenient adjustment and maintenance.

To achieve the above purpose, the technical solution adopted by the present invention is as follows:

The laser ultrasonic sub-surface defect detection and positioning method of the present invention comprises the following steps:

1) Set up the test system: place the sample, turn on the pulse laser and laser interferometer, the pulse laser emitted by the pulse laser is passed through the beam splitter to obtain a reflected beam and a transmitted beam, the transmitted beam is received by the photodetector as a trigger signal, the reflected beam is focused on the sample surface as the excitation light, the light spot where the excitation light is incident on the sample surface is the excitation point, the laser interferometer emits a continuous laser as the detection light focused on the sample surface and receives the ultrasonic signal, the light spot where the detection light is incident on the sample surface is the receiving point; after adjusting the position of the sample and the laser interferometer, heat the sample evenly.

2) Setting of scanning mode and parameters: According to the detection requirements of sub-surface defects of the sample, set the scanning mode and scanning parameters in sequence.

3) Scanning of the sample: The excitation point and the receiving point are moved step by step on the surface of the sample according to the scanning mode and scanning parameters set in step 2) to start the detection and positioning of sub-surface defects of the sample.

4) Signal reception and analysis: After the ultrasonic signal received by the corresponding laser interferometer and the trigger signal received by the photoelectric detector, the received laser ultrasonic signal is read and analyzed to obtain the location information of the sub-surface defects of the sample.

Specifically, in the step 2), the scanning mode includes one-dimensional scanning or two-dimensional scanning (S3), the two-dimensional scanning (S3) is a two-dimensional superposition of one-dimensional scanning on the scanning step of two-dimensional scanning, and the one-dimensional scanning includes a one-dimensional fast scanning (S1) and a one-dimensional fine scanning (S2) performed sequentially.

The one-dimensional rapid scan (S1) is used to determine whether the laser ultrasonic signal contains sub-surface defect information when the excitation point and the receiving point are far apart: if the laser ultrasonic signal contains sub-surface defect information, a one-dimensional fine scan (S2) is performed on the straight line between the excitation point and the receiving point; if the laser ultrasonic signal does not contain sub-surface defect information, the one-dimensional scan is stopped.

The one-dimensional fine scanning (S2) is used to detect and locate the position of sub-surface defects by analyzing the laser ultrasonic signal when the excitation point and the receiving point are close to each other.

The two-dimensional scanning (S3) is used to determine the position information of sub-surface defects within the scanning area of the two-dimensional scanning.

In step 2), the scanning parameters are mainly scanning step distance and scanning direction:

In the one-dimensional rapid scanning (S1), since the one-dimensional rapid scanning (S1) is a single-point scanning, there is no need to set the scanning step of the one-dimensional rapid scanning before scanning;

In the one-dimensional fine scanning (S2), the scanning step distance of the one-dimensional fine scanning is the distance that the excitation point and the receiving point step simultaneously;

In the two-dimensional scanning (S3), the scanning step of the two-dimensional scanning is the distance between the straight lines of two one-dimensional scannings.

The specific process of the two-dimensional scanning (S3) is as follows:

S3.1) The positions of the excitation point and the receiving point on the surface of the sample (6) are used as the scanning positions, and the scanning parameters of the two-dimensional scanning (S3) are set. The scanning parameters are the scanning area, the scanning step, the scanning initial position and the scanning direction. The scanning area of the two-dimensional scanning is a rectangle, the scanning step of the two-dimensional scanning is 0.5mm, 1mm or 1.5mm, and the scanning initial position is the starting point of the two-dimensional scanning (S3). After the scanning initial position is used as the scanning position, the scanning is started along the scanning direction.

S3.2) Perform a one-dimensional scan at the scanning position and receive a laser ultrasonic signal: If there is a sub-surface defect on the straight line of the one-dimensional scan, the position of the sub-surface defect on the straight line of the one-dimensional scan can be determined from the laser ultrasonic signal.

S3.3) The excitation point and the receiving point are simultaneously moved on the surface of the sample (6) along the scanning direction with the scanning step of the two-dimensional scanning as the step amount. The moved excitation point and the receiving point are used as the next scanning position, and the process returns to step S3.2) and repeats step S3.2).

S3.4) Repeat step S3.3) in the scanning area, and the scanning ends when the scanning position reaches the edge of the scanning area.

S3.5) Process all received laser ultrasonic signals according to the C-scan signal processing method, and extract the information of the conversion of the surface-grazing longitudinal wave into the surface wave (first conversion wave) and the conversion of the surface wave into the surface-grazing longitudinal wave (second conversion wave) at each scanning position according to the scanning step of the two-dimensional scanning from the received laser ultrasonic signals, and based on the relationship between the flight time difference between the surface-grazing longitudinal wave into the surface wave (first conversion wave) and the surface wave and the flight time difference between the surface wave into the surface-grazing longitudinal wave (second conversion wave) and the surface wave and the scanning step of the one-dimensional fine scanning, obtain the sub-surface defect position at each scanning position and the two-dimensional distribution of the sub-surface defects on the sample, and then output all sub-surface defect information in the scanning area.

The specific steps of the one-dimensional rapid scanning (S1) are:

Focus the excitation light and the detection light on the surface of the sample. After collecting the laser ultrasonic signal, extract the time domain signal of the surface wave propagation in the laser ultrasonic signal, and make the following judgments based on the phase change of the surface wave:

If in the time domain signal, a higher amplitude (exceeding the surface wave amplitude) appears earlier than and close to the surface wave ), that is, the surface wave has a phase change, the laser ultrasonic signal contains sub-surface defect information, and there is a sub-surface defect on the straight line between the excitation point and the receiving point, and then continue to perform a one-dimensional fine scan (S2) in this one-dimensional scan along the straight line direction (from the excitation point to the receiving point or from the receiving point to the excitation point) on the straight line to detect and locate the position of the sub-surface defect;

If in the time domain signal, there is no high amplitude (exceeding the surface wave amplitude) at a position earlier than and close to the surface wave ), that is, there is no phase change in the surface wave, then the laser ultrasonic signal does not contain sub-surface defect information, and there is no sub-surface defect on the straight line between the excitation point and the receiving point. The one-dimensional fine scanning (S2) of this one-dimensional scanning is not continued, that is, the one-dimensional scanning is terminated.

The specific steps of the one-dimensional fine scanning (S2) include:

S2.1) Setting parameters: Setting the scanning step of one-dimensional fine scanning, the scanning step of one-dimensional fine scanning is 0.1mm, 0.2mm or 0.5mm;

S2.2) Scanning: Keep the distance between the excitation point and the receiving point unchanged, and use the scanning step of the one-dimensional fine scanning as the step amount between the excitation point and the receiving point. Simultaneously scan point by point on the straight line of the one-dimensional scanning (the straight line where the sub-surface defects are detected in the one-dimensional fast scanning), receive and collect all the laser ultrasonic B-scanning signals, and end this one-dimensional scanning;

S2.3) Signal processing: Extract the signal parameters at the surface wave phase change (sub-surface defect position) from the laser ultrasonic B-scan signal, and analyze and process according to the following formula to obtain the propagation velocity _CR of the surface wave, the propagation velocity _CL of the surface-grazing longitudinal wave, the distance _d1 from the excitation point to the sub-surface defect after the surface wave phase change occurs, and the distance _d2 from the receiving point to the sub-surface defect after the surface wave phase change occurs:

Where _D2 is the distance between the excitation point and the receiving point;

T _R is the surface wave arrival time measured by one-dimensional fine scanning experiment;

k is the absolute value of the slope of the relationship between the change of the surface-grazing longitudinal wave to the surface wave (first converted wave) and the change of the scanning displacement l ₁ or the change of the surface wave to the surface-grazing longitudinal wave (second converted wave) and the change of the scanning displacement l ₂ ;

t ₁ is the time difference between the surface wave and the first converted wave after the phase change of the surface wave occurs during the one-dimensional fine scanning process;

_t2 is the time difference between the surface wave and the second converted wave after the phase change of the surface wave occurs during the one-dimensional fine scanning process;

The sub-surface defect is located at a distance of d ₁ from the excitation point and d ₂ from the receiving point on the straight line between the excitation point and the receiving point after the phase change of the surface wave occurs, thereby finally realizing the detection and positioning of the sub-surface defect.

In the one-dimensional rapid scan (S1), the distance _D1 between the excitation point and the receiving point remains unchanged, and in the one-dimensional fine scan (S2), the distance _D2 between the excitation point and the receiving point remains unchanged. The relationship between the distance _D1 between the excitation point and the receiving point in the one-dimensional rapid scan (S1) and the distance _D2 between the excitation point and the receiving point in the one-dimensional fine scan (S2) is: _D1 > _D2 .

Preferably, _D1 is 40 mm, 50 mm or 60 mm, and _D2 is 15 mm, 20 mm or 25 mm.

In the one-dimensional fine scanning (S2), the specific process of processing the laser ultrasonic signal is as follows:

I. Extract the surface wave propagation time _TR in the laser ultrasonic B scanning signal, the relationship between the surface-grazing longitudinal wave converted to the surface wave (first converted wave) and the scanning displacement change _l1 , and the relationship between the surface wave converted to the surface-grazing longitudinal wave (second converted wave) and the scanning displacement change _l2 , and obtain the propagation velocity _CR of the surface wave and the propagation velocity _CL of the surface-grazing longitudinal wave;

Preferably, the propagation velocity of the surface wave is calculated by the following formula:

Where _D2 is the distance between the excitation point and the receiving point, and _TR is the surface wave arrival time measured in one-dimensional fine scanning.

The relationship between the surface-grazing longitudinal wave and the surface wave (first conversion wave) changes with the scanning displacement is calculated by the following _formula :

Wherein, d is the scanning displacement after the phase change of the surface wave occurs during the one-dimensional fine scanning process, and _t1 is the time difference between the surface wave and the first converted wave after the phase change of the surface wave occurs during the one-dimensional fine scanning process.

The relationship between the surface wave converted into the surface-grazing longitudinal wave (second converted wave) and the change in scanning displacement is calculated by the following _formula :

Where _D2 is the distance between the excitation point and the receiving point, d is the scanning displacement after the phase change of the surface wave occurs during the one-dimensional fine scanning process, and _t2 is the time difference between the surface wave and the second converted wave after the phase change of the surface wave occurs during the one-dimensional fine scanning process.

The information of relation _l1 and relation _l2 is symmetrical. The relationship between the propagation velocity of the surface wave _CR and the propagation velocity of the surface-grazing longitudinal wave _CL can be obtained by equation 2) or equation 3):

Where k is the absolute value of the slope of the relationship l ₁ or the relationship l ₂ .

According to equations 1) and 4), the propagation speed of the surface wave can be obtained: and the propagation speed of surface-grazing longitudinal waves

Since the above process does not involve variables that affect the sound velocity, such as temperature and pressure, it can obtain the surface wave sound velocity and surface-grazing longitudinal wave sound velocity of a sample containing subsurface defects in an unknown uniform state.

II. Extract the surface wave propagation time _TR , the propagation time _TLR of the surface-grazing longitudinal wave to the surface wave (first conversion wave), and the propagation time _TRL of the surface wave to the surface-grazing longitudinal wave (second conversion wave) in the laser ultrasonic signal. Using the distance _D2 between the excitation point and the receiving point and the solved propagation velocity _CR of the surface wave and the propagation velocity _CL of the surface-grazing longitudinal wave, the positional relationship between the scanning position and the sub-surface defect can be obtained.

The distance _d1 between the excitation point and the subsurface defect after the surface wave phase change occurs is calculated by the following formula:

Where _D2 is the distance between the excitation point and the receiving point, _CL is the propagation velocity of the surface-grazing longitudinal wave, _CR is the propagation velocity of the surface wave, and _TLR is the propagation time for the surface-grazing longitudinal wave to convert into the surface wave (the first converted wave).

The distance _d2 between the receiving point and the subsurface defect after the surface wave phase change occurs is calculated by the following formula:

Where _D2 is the distance between the excitation point and the receiving point, _CR is the propagation velocity of the surface wave, _CL is the propagation velocity of the surface-grazing longitudinal wave, and _TRL is the propagation time for the surface wave to be converted into the surface-grazing longitudinal wave (the second converted wave).

Furthermore, after the surface wave phase change occurs, the relationship between the distance _d1 between the excitation point and the sub-surface defect and the distance _d2 between the receiving point and the sub-surface defect is:

d ₁ +d ₂ =D ₂ 7)

Where _D2 is the distance between the excitation point and the receiving point.

Preferably, in combination with equation 1) and equation 5), the distance _d1 between the excitation point and the sub-surface defect after the surface wave phase change occurs is calculated by the following equation:

Where _CR is the propagation velocity of the surface wave, _CL is the propagation velocity of the surface-grazing longitudinal wave, and _t1 is the time difference between the surface wave and the first converted wave after the phase change of the surface wave occurs during the one-dimensional fine scanning process.

Combining equations 1) and 6), the distance _d2 between the receiving point and the sub-surface defect after the surface wave phase change occurs is calculated by the following equation:

Where _CR is the propagation velocity of the surface wave, _CL is the propagation velocity of the surface-grazing longitudinal wave, and _t2 is the time difference between the surface wave and the second converted wave after the phase change of the surface wave occurs during the one-dimensional fine scanning process.

After the surface wave phase change occurs, the relationship between the distance _d1 between the excitation point and the sub-surface defect and the distance _d2 between the receiving point and the sub-surface defect is as follows: (7) The specific position of the sub-surface defect on the straight line of the one-dimensional scan can be obtained from _d1 , _d2 and the positions of the excitation point and the receiving point when the surface wave phase change occurs.

The laser ultrasonic sub-surface defect detection and positioning device in the present invention comprises a pulse laser, a beam splitter, a cylindrical lens, a photodetector, a reflector, a first X-Z displacement platform, a laser interferometer, and a second X-Z displacement platform. The pulse laser emitted by the pulse laser is reflected and transmitted by the beam splitter and split into a reflected light beam and a transmitted light beam. The optical paths of the reflected light beam and the transmitted light beam are perpendicular to each other. The transmitted light beam is collected by the photodetector. The reflected light beam is sequentially transmitted by the cylindrical lens and reflected by the reflector, and then focused as an excitation light on an excitation point on the surface of the sample fixed on the first X-Z displacement platform. The excitation light excites surface waves and surface-grazing longitudinal waves on the surface of the sample. The surface waves and surface-grazing longitudinal waves propagate through the surface of the sample and are received by the laser interferometer. The laser interferometer is fixed on the second X-Z displacement platform, and the detection light emitted by the laser interferometer is focused on the receiving point on the surface of the sample.

Specifically, the excitation point and the receiving point are located on the same surface of the sample, and the direction of the straight line between the excitation point and the receiving point is perpendicular to the depth direction of the sub-surface defect.

Specifically, the wavelength of the pulse laser emitted by the pulse laser is preferably 1064 nm, the repetition frequency is preferably 5 KHz, the pulse energy is preferably 1.2 mJ, and the wavelength of the detection light emitted by the laser interferometer is preferably 1064 nm.

The detection and positioning device also includes a data acquisition card and a computer, the photoelectric detector and the laser interferometer are respectively connected to the data acquisition card, the first X-Z displacement platform and the second X-Z displacement platform are both provided with a control card, the displacements of the first X-Z displacement platform and the second X-Z displacement platform are both controlled by the control card, and the data acquisition card, the control card of the first X-Z displacement platform and the control card of the second X-Z displacement platform are all connected to the computer.

The detection and positioning device also includes an electromagnetic induction heater, and the sample is placed on the electromagnetic induction heater. The electromagnetic induction heater is used to heat the sample and can evenly heat the sample to above 600°C.

The laser ultrasonic sub-surface defect detection and positioning method of the present invention can determine whether the laser ultrasonic signal contains sub-surface defect information when the excitation point and the receiving point are far apart through one-dimensional rapid scanning. If there is sub-surface defect information, a one-dimensional fine scanning is performed with the excitation point and the receiving point close to each other to achieve accurate positioning of the sub-surface defect. This detection and positioning method uses a combination of one-dimensional rapid scanning and one-dimensional fine scanning to achieve large-spacing sub-surface defect judgment and small-spacing and small-step sub-surface defect positioning. The present invention takes into account both detection efficiency and detection accuracy, and realizes problems such as sub-surface defect detection and positioning within a two-dimensional range under an unknown uniform temperature field. The conversion of the surface-grazing longitudinal wave to the surface wave (first conversion wave) and the conversion of the surface wave to the surface-grazing longitudinal wave (second conversion wave) mentioned in the present invention provide a reference characteristic waveform for the positioning of sub-surface defects. At the same time, the present invention also provides a method for determining the uniform sound velocity relationship between laser ultrasonic surface waves and surface-grazing longitudinal waves, which can determine the sound velocity of surface waves and surface-grazing longitudinal waves under unknown uniform temperature, pressure and other environments.

Compared with the prior art, the present invention has the following beneficial effects:

1) The present invention has strong adaptability and can adapt to the influence of temperature, pressure and other factors on the change of sound speed in the current environment;

2) The principle of the present invention is intuitive, and provides a characteristic waveform of the mutual transformation between the surface wave and the surface-grazing longitudinal wave passing through the sub-surface defect;

3) The present invention has high positioning accuracy and can locate sub-surface defects based on the first converted wave and the second converted wave;

4) The method of the present invention takes into account both detection efficiency and accuracy, and is easy to adjust and maintain.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a system schematic diagram of a laser ultrasonic sub-surface defect detection and positioning device according to an embodiment of the present invention.

FIG. 2 is a flow chart of an implementation of a laser ultrasonic sub-surface defect detection and positioning method according to an embodiment of the present invention.

FIG. 3 is a schematic diagram of a one-dimensional rapid scan and a special point experimental signal diagram of Example 1 of the present invention.

FIG. 4 is a schematic diagram of a one-dimensional fine scan and a special point experimental signal diagram of Example 1 of the present invention.

FIG. 5 is B-scan experimental data of one-dimensional fine scanning according to Example 1 of the present invention.

FIG. 6 is a schematic diagram of one-dimensional fine scanning and B-scan experimental data of Example 2 of the present invention.

In the figure: 1. pulse laser; 2. beam splitter; 3. cylindrical lens; 4. photodetector; 5. reflector; 6. sample; 7. first X-Z displacement platform; 8. laser interferometer; 9. second X-Z displacement platform; 10. data acquisition card; 11. computer; 12. electromagnetic induction heater.

DETAILED DESCRIPTION

The technical solution of the present invention is further described below in conjunction with the accompanying drawings and embodiments.

The laser ultrasonic sub-surface defect detection and positioning method in this specific embodiment includes the following steps:

1) Set up the test system: After placing the sample, turn on the pulse laser 1 and the laser interferometer 8. The pulse laser emitted by the pulse laser 1 passes through the beam splitter 2 to obtain a reflected beam and a transmitted beam. The transmitted beam is received by the photodetector 4 as a trigger signal. The reflected beam is focused on the surface of the sample 6 as an excitation light. The laser interferometer 8 emits a continuous laser as a detection light to focus on the surface of the sample 6 and receive the ultrasonic signal. The positions of the sample 6 and the laser interferometer 8 are adjusted respectively by controlling the movement of the first X-Z displacement platform 7 and the second X-Z displacement platform 9, so that the speed change of the displacement vibration on the surface of the sample 6 measured by the laser interferometer 8 is minimized, thereby improving the receiving sensitivity. Depending on whether the sample needs to be heated, set whether to turn on the electromagnetic induction heater 12 and set the heating temperature. Use the electromagnetic induction heater 12 to uniformly heat the sample 6. When using laser ultrasound to detect and locate sub-surface defects of the sample 6, the sample 6 is in a steady-state uniform temperature field.

2) Scanning mode and parameter settings: According to the sub-surface defect detection requirements of the sample, set the scanning mode and scanning parameters in sequence;

3) Scanning of the sample: The light spots of the excitation light and the detection light on the surface of the sample 6, i.e., the excitation point and the receiving point, are controlled by moving the first X-Z displacement platform 7 and the second X-Z displacement platform 9, and the surface of the sample 6 is stepped and moved according to the scanning mode and scanning parameters set in step 2), and the detection and positioning of the sub-surface defects of the sample 6 are started;

4) Signal reception and analysis: After the ultrasonic signal received by the corresponding laser interferometer 8 and the trigger signal received by the photodetector 4, all received laser ultrasonic signals are collected by the data acquisition card 10 and transmitted to the computer 11, which reads and analyzes the received laser ultrasonic signals.

The specific process of two-dimensional scanning (S3) is as follows:

S3.1) The positions of the excitation point and the receiving point on the surface of the sample 6 are used as the scanning positions, and the scanning parameters of the two-dimensional scanning S3 are set. The scanning parameters are the scanning area, the scanning step, the scanning initial position and the scanning direction. The scanning area is set to a rectangle, and the scanning initial position is used as the scanning position and the scanning is started along the scanning direction;

S3.2) Perform one-dimensional scanning at the scanning position and receive laser ultrasonic signals. If there is a sub-surface defect on the straight line where the one-dimensional scanning is performed, the position of the sub-surface defect on the straight line where the one-dimensional scanning is performed can be determined;

S3.3) The sample surface is recorded as the M plane, and the distance between the excitation point and the receiving point is kept unchanged. The excitation point and the receiving point are simultaneously moved in the M plane along the scanning direction with the scanning step of the two-dimensional scanning as the step amount. The positions of the excitation point and the receiving point after the movement are used as the scanning positions of the next one-dimensional scanning, and the process returns to step S3.2) and repeats step S3.2);

S3.4) Repeat step S3.3) in the scanning area, and the scanning ends when the scanning position reaches the edge of the scanning area;

S3.5) Process all received laser ultrasonic signals according to the C-scan signal processing method, and extract the first conversion wave information of the surface-grabbing longitudinal wave into the surface wave and the second conversion wave information of the surface wave into the surface-grabbing longitudinal wave at each scanning position in different directions according to the scanning step of the two-dimensional scanning, and according to the relationship between the flight time difference between the surface-grabbing longitudinal wave and the surface wave, and the flight time difference between the surface wave and the surface wave and the scanning step of the one-dimensional fine scanning, obtain the sub-surface defect positions at each scanning position in different directions and the two-dimensional distribution of the sub-surface defects on the sample, and then output the position information of all sub-surface defects in the scanning area.

The steps of 1D fast scan (S1) include:

The excitation light and the detection light are focused on the surface of the sample 6. After the laser ultrasonic signal is collected, the time domain signal of the surface wave propagation in the laser ultrasonic signal is extracted. The phase change of the surface wave is used to determine whether there is a sub-surface defect on the straight line between the excitation point and the receiving point. If there is a sub-surface defect, then the one-dimensional fine scan (S2) of this one-dimensional scan is continued on the straight line along the straight line direction to detect and locate the position of the sub-surface defect; if there is no sub-surface defect, then the one-dimensional scan is terminated.

The specific steps of one-dimensional fine scanning (S2) include:

S2.1) Setting parameters: Setting the scanning step length of one-dimensional fine scanning;

S2.2) Scanning: Keep the distance between the excitation point and the receiving point unchanged, and use the scanning step of the one-dimensional fine scanning as the step amount between the excitation point and the receiving point. Simultaneously, perform step-by-step scanning on the straight line where the sub-surface defects are detected in the one-dimensional fast scanning (S1), receive and collect all the laser ultrasonic B-scan signals, and end this one-dimensional scanning;

S2.3) Signal processing: Extract the signal parameters at the position of the sub-surface defect where the surface wave phase changes from the laser ultrasonic B-scan signal, and analyze and process according to the following formula to obtain the propagation velocity _CR of the surface wave, the propagation velocity _CL of the surface-grazing longitudinal wave, the distance _d1 from the excitation point to the sub-surface defect after the surface wave phase change occurs, and the distance _d2 from the receiving point to the sub-surface defect after the surface wave phase change occurs:

Where _D2 is the distance between the excitation point and the receiving point;

T _R is the surface wave arrival time measured by one-dimensional fine scanning experiment;

k is the absolute value of the slope of the relationship between the change of the surface-grazing longitudinal wave to the surface wave (first converted wave) and the change of the scanning displacement l ₁ or the change of the surface wave to the surface-grazing longitudinal wave (second converted wave) and the change of the scanning displacement l ₂ ;

t ₁ is the time difference between the surface wave and the first converted wave after the phase change of the surface wave occurs during the one-dimensional fine scanning process;

_t2 is the time difference between the surface wave and the second converted wave after the phase change of the surface wave occurs during the one-dimensional fine scanning process.

The relative positions of the sub-surface defect, the excitation point, and the receiving point on the measured surface of the sample 6 are: excitation point-sub-surface defect-receiving point, that is, the sub-surface defect is located on the straight line between the excitation point and the receiving point corresponding to the current one-dimensional fine scan (S2), at a distance of _d1 from the excitation point and _d2 from the receiving point, thereby finally realizing the detection and positioning of the sub-surface defect.

The laser ultrasonic sub-surface defect detection and positioning device in this specific embodiment is shown in FIG1 , where the bold line is the optical path of the pulsed laser and the continuous laser used for detection, specifically:

The detection and positioning device is used for detecting and positioning sub-surface defects of a sample 6, and comprises a pulse laser 1, a beam splitter 2, a cylindrical lens 3, a photodetector 4, a reflector 5, a first X-Z displacement platform 7, a laser interferometer 8, and a second X-Z displacement platform 9. The pulse laser emitted by the pulse laser 1 is reflected and transmitted by the beam splitter 2 and split into a reflected light beam and a transmitted light beam, the optical paths of the reflected light beam and the transmitted light beam are perpendicular to each other, the transmitted light beam is collected by the photodetector 4, the reflected light beam is sequentially transmitted by the cylindrical lens 3 and reflected by the reflector 5, and then focused as an excitation light on an excitation point on the surface of the sample 6 fixed on the first X-Z displacement platform 7, the excitation light excites a surface wave and a surface-grazing longitudinal wave on the surface of the sample 6, the surface wave and the surface-grazing longitudinal wave propagate through the surface of the sample 6 and are received by the laser interferometer 8; the laser interferometer 8 is fixed on the second X-Z displacement platform 9, and the laser interferometer 8 emits a detection light focused on a receiving point on the surface of the sample 6.

The wavelength of the pulse laser emitted by the pulse laser 1 is 1064 nm, the repetition frequency is 5 KHz, the pulse energy is 1.2 mJ, and the wavelength of the detection light emitted by the laser interferometer 8 is 1064 nm.

The detection and positioning device also includes a data acquisition card 10 and a computer 11; the photoelectric detector 4 and the laser interferometer 8 are respectively connected to the data acquisition card 10, the first X-Z displacement platform 7 and the second X-Z displacement platform 9 are both provided with a control card, the displacement of the first X-Z displacement platform 7 and the second X-Z displacement platform 9 are both controlled by the control card, and the data acquisition card 10, the control card of the first X-Z displacement platform 7 and the control card of the second X-Z displacement platform 9 are all connected to the computer 11.

The detection and positioning device also includes an electromagnetic induction heater 12 for heating the sample 6. The sample 6 is placed on the electromagnetic induction heater 12. The electromagnetic induction heater 12 can evenly heat the sample 6 to above 600°C.

The specific embodiments of the present invention are as follows:

Example 1

The sample to be tested is processed by CNC. The sample of Example 1 is steel with a size of 150 mm × 60 mm × 30 mm. The subsurface defect is a circular hole with a distance of 1 mm from the upper surface, a diameter of 1 mm, and a depth of 15 mm. The schematic diagram is shown in (a) of FIG3 .

The detection and positioning method of this embodiment is shown in FIG2 , and includes the following steps:

1) Set up the test system: turn on the pulse laser 1 and the laser interferometer 8, place the sample 6 on the first X-Z displacement platform 7, adjust the first X-Z displacement platform 7 and the second X-Z displacement platform 9 to minimize the speed change of the displacement vibration of the sample surface measured by the laser interferometer 8.

2) Setting of scanning mode and parameters: According to the sub-surface defect detection requirements of sample 6, the scanning mode of sample 6 is set.

3) Sample scanning, signal reception and analysis

S1) One-dimensional rapid scanning and signal analysis: The distance between the excitation point and the receiving point is set to D ₁ = 50 mm, and the time domain signal containing the surface wave propagation in the laser ultrasonic signal is extracted.

In this embodiment, the time domain signals of different sub-surface defect information of one-dimensional rapid scanning at room temperature are shown in Figure 3. Figure 3 (a) is a schematic diagram of one-dimensional rapid scanning in Example 1. Figure 3 (b) is a typical signal of a region without sub-surface defects in Figure 3 (a), such as point A, and Figure 3 (c) is a typical signal of a region with sub-surface defects in Figure 3 (a), such as point B. By comparing and analyzing Figure 3 (b) and Figure 3 (c), it can be seen that the laser ultrasonic signal passing through the region with sub-surface defects will change in phase, which can determine that the scanning area at point B contains sub-surface defects, and the one-dimensional fine scanning in this one-dimensional scanning can be continued to locate the sub-surface defects.

S2) One-dimensional fine scanning and signal analysis.

After determining that there is a sub-surface defect in a certain direction (on the straight line of one-dimensional rapid scanning), set the distance between the excitation point and the receiving point to D ₂ = 20mm, the scanning step distance = 0.1mm, and the scanning length (D ₁ -D ₂ ) = 30mm. Scan the direction point by point with the scanning step distance, collect signal data, obtain the B-scan signal in the direction, and detect and locate the position of the sub-surface defect. The specific analysis steps are as follows:

S2.1) Determine the sound velocity of the surface wave and the surface-grazing longitudinal wave under unknown temperature and pressure. Extract the surface wave propagation time _TR in the laser ultrasonic B-scan signal, the relationship _l1 between the surface-grazing longitudinal wave and the surface wave (first conversion wave) and the scanning displacement, and the relationship _l2 between the surface wave and the surface-grazing longitudinal wave (second conversion wave) and the scanning displacement. According to the distance _D2 between the excitation point and the receiving point, the absolute value k of the slope of the relationship _l1 or the relationship _l2 , the propagation velocity of the surface wave is obtained. The propagation speed of the surface-grazing longitudinal wave

S2.2) Determine the location of the sub-surface defect. Extract the time difference _t1 between the surface wave and the first converted wave after the phase change of the surface wave in the laser ultrasonic B scanning signal, and the time difference _t2 between the surface wave and the second converted wave after the phase change of the surface wave. According to the propagation speed _CR of the surface wave and the propagation speed _CL of the surface-grazing longitudinal wave, obtain the distance from the excitation point after the phase change of the surface wave to the sub-surface defect. The distance between the receiving point and the sub-surface defect after the surface wave phase change occurs

In this embodiment, the time domain signals of different sub-surface defect information of one-dimensional fine scanning at room temperature are shown in Figure 4. Figure 4 (a) is a schematic diagram of one-dimensional fine scanning in this embodiment, Figure 4 (b) is a typical signal in which the excitation point and the receiving point are located on the same side of the sub-surface defect in Figure 4 (a), such as point C, Figure 4 (c) is a typical signal in which the excitation point and the receiving point are located on the opposite sides of the sub-surface defect in Figure 4 (a), and the excitation point is close to the sub-surface defect, such as point D, and Figure 4 (d) is a typical signal in which the excitation point and the receiving point are located on the opposite sides of the sub-surface defect in Figure 4 (a), and the receiving point is close to the sub-surface defect, such as point E.

The first converted wave and the second converted wave are related to the relative positions of the excitation point, the receiving point and the sub-surface defect. Figure 5 (a) is the B-scan experimental data (2us-12us) of one-dimensional fine scanning. It can be found from the figure that in addition to the surface wave, there are characteristic signals such as surface wave echo, the first converted wave and the second converted wave; Figure 5 (b) is the B _- scan experimental data (2us-6.8us) of one-dimensional fine scanning. The relationship between the propagation velocity _CR of the surface wave and the propagation velocity _CL of the surface-grazing longitudinal wave can be obtained through the absolute value k of the slope of the relationship l 1 or the relationship l ₂ , and then the distance d ₁ and d ₂ between the excitation point and the receiving point from the sub-surface defect after the surface wave phase change (sub-surface defect position) can be obtained.

S3) Two-dimensional scanning and signal analysis and processing method. Determine the scanning parameters of the two-dimensional scanning, such as the scanning area, scanning step, scanning initial position and scanning direction. Set the scanning step to 1mm, start from the scanning initial position, and perform one-dimensional fast scanning and one-dimensional fine scanning in sequence along the scanning direction until the scanning position reaches the edge of the scanning area, and collect all laser ultrasonic signals in the scanning area. Using the collected signal, superimpose the one-dimensional fine scanning on the scanning step of the two-dimensional scanning, extract the information of the surface-grazing longitudinal wave converted to the surface wave (first conversion wave) and the surface wave converted to the surface-grazing longitudinal wave (second conversion wave) in different directions according to the scanning step, obtain the sub-surface defect positions in different directions, and output all sub-surface defect information in the scanning area.

Example 2

The sample to be tested is processed by CNC. The sample of Example 2 is aluminum with a size of 160mm×60mm×35mm. The sub-surface defects are three circular holes with a distance of 1mm from the upper surface, a diameter of 1mm, and depths of 18mm, 25mm, and 25mm respectively. The spacing between adjacent circular holes is 25mm. The schematic diagram is shown in (a) of Figure 6.

The steps of the detection and positioning method of this embodiment are the same as those of Embodiment 1, and provide the results of one-dimensional fine scanning of samples of different materials and different numbers of defects at room temperature.

In the one-dimensional fine scanning, the distance between the excitation point and the receiving point is set to D ₂ =20 mm, the scanning step is set to 0.2 mm, and the scanning length is set to (D ₁ -D ₂ )=80 mm.

Figure 6 (a) is a schematic diagram of one-dimensional fine scanning of this embodiment, Figure 6 (b) is the B-scan experimental data (2us-12us) of one-dimensional fine scanning, and Figure 6 (c) is the B-scan experimental data (2us-6.66us) of one-dimensional fine scanning. From the figure, we can see three first conversion waves. By calculating the positions of the three first conversion waves and the absolute value k of the slope of the relationship l ₁ , we can get the distance d ₁ between the excitation point and the sub-surface defect after three surface wave phase changes (three sub-surface defect positions).

The laser ultrasonic sub-surface defect detection and positioning method of the present invention utilizes a combination of one-dimensional rapid scanning and one-dimensional fine scanning to achieve large-spacing sub-surface defect judgment and small-spacing and small-step sub-surface defect positioning, while taking into account both detection efficiency and detection accuracy. At the same time, the present invention also provides a method for determining the uniform sound velocity relationship between laser ultrasonic surface waves and surface-grazing longitudinal waves, which can determine the sound velocity of surface waves and surface-grazing longitudinal waves in unknown uniform temperature, pressure and other environments.

The embodiments described above are only preferred implementation schemes of the present invention, but they are not intended to limit the present invention. Ordinary technicians in the relevant technical field can make various changes and modifications without departing from the spirit and scope of the present invention. For example, the laser can be focused into a line light source using a cylindrical lens, or it can be focused into a point light source using a spherical lens, that is: the pulsed laser emitted by the pulsed laser is focused into a point source laser through a spherical lens, irradiated on the surface of the workpiece and stimulated ultrasonic waves; the two X-Z displacement platforms can also be replaced by two galvanometer scanning groups, wherein the first galvanometer scanning group is used to adjust the irradiation position of the pulsed laser emitted by the pulsed laser on the sample surface, and the second galvanometer scanning group is used to adjust the measurement position of the laser interferometer on the sample surface. Therefore, all technical solutions obtained by equivalent replacement or equivalent transformation should be included in the protection scope of the present invention.

Claims (5)

1. A laser ultrasonic subsurface defect detection positioning method is characterized in that: the detection positioning method comprises the following steps:

1) Setting up a test system: placing a sample, opening a pulse laser (1) and a laser interferometer (8), obtaining a reflected light beam and a transmitted light beam after the pulse laser emitted by the pulse laser (1) passes through a beam splitter (2), receiving the transmitted light beam by a photoelectric detector (4), focusing the reflected light beam on the surface of the sample (6) as excitation light, focusing a light spot on the surface of the sample (6) as excitation point by the excitation light, focusing continuous laser emitted by the laser interferometer (8) on the surface of the sample (6) as detection light and receiving ultrasonic signals, and uniformly heating the sample (6) after adjusting the positions of the sample (6) and the laser interferometer (8) by the light spot on the surface of the sample (6);

2) Scanning mode and parameter setting: setting scanning modes and scanning parameters in sequence;

3) Scanning of samples: the excitation point and the receiving point move step by step on the surface of the sample (6) according to the scanning mode and the scanning parameters set in the step 2), and detection and positioning of subsurface defects of the sample (6) are started;

4) Receiving and analyzing signals: after receiving the ultrasonic signals from the laser interferometer (8) and the trigger signals from the photoelectric detector (4), reading and analyzing the received laser ultrasonic signals;

In the step 2), the scanning mode comprises one-dimensional scanning or two-dimensional scanning (S3), wherein the two-dimensional scanning (S3) is two-dimensional superposition of one-dimensional scanning on a scanning step distance of the two-dimensional scanning, and the one-dimensional scanning comprises one-dimensional rapid scanning (S1) and one-dimensional fine scanning (S2) which are sequentially carried out; in the step 2), the scanning parameters comprise a scanning step distance and a scanning direction;

the specific steps of the one-dimensional fine scanning (S2) comprise:

s2.1) setting parameters: setting a scanning step distance of one-dimensional fine scanning;

s2.2) scanning: keeping the distance between the excitation point and the receiving point unchanged, taking the scanning step distance of the one-dimensional fine scanning as the stepping amount, synchronously scanning point by point, receiving and collecting all laser ultrasonic B scanning signals, and ending the one-dimensional scanning;

S2.3) signal processing: signal parameters are extracted from the laser ultrasonic B scanning signals, and the signal parameters are analyzed and processed according to the following formula to obtain the propagation speed C _R of the surface wave, the propagation speed C _L of the glancing surface longitudinal wave, the distance d ₁ of an excitation point from a subsurface defect after the surface wave phase change and the distance d ₂ of a receiving point from the subsurface defect after the surface wave phase change:

Wherein D ₂ is the distance between the excitation point and the receiving point;

T _R is the arrival time of the surface wave measured by the one-dimensional fine scanning experiment;

k is the absolute value of the slope of the relationship l ₁ that the glancing surface longitudinal wave changes with the scanning displacement or the relationship l ₁ that the surface wave changes with the scanning displacement;

t ₁ is the time difference between the surface wave and the first converted wave after the phase change of the surface wave occurs in the one-dimensional fine scanning process;

t ₂ is the time difference between the surface wave and the second converted wave after the phase change of the surface wave occurs in the one-dimensional fine scanning process;

Subsurface defects are located on a straight line between the excitation point and the receiving point, at a distance of excitation point d ₁ and at a distance of receiving point d ₂.

2. The laser ultrasonic subsurface defect detection and localization method according to claim 1, wherein the method comprises the following steps: in the one-dimensional quick scanning (S1), a scanning step distance of the one-dimensional quick scanning is not required to be set; in the one-dimensional fine scanning (S2), the scanning step distance of the one-dimensional fine scanning is 0.1mm, 0.2mm or 0.5mm; in the two-dimensional scanning (S3), the scanning step distance of the two-dimensional scanning is 0.5mm, 1mm or 1.5mm.

3. The method for detecting and positioning the laser ultrasonic subsurface defect according to any one of claims 1 or 2, wherein the method comprises the following steps: the specific process of the two-dimensional scanning (S3) is as follows:

S3.1) taking the positions of the excitation point and the receiving point on the surface of the sample (6) as scanning positions, setting scanning parameters of two-dimensional scanning (S3), and starting scanning along a scanning direction after taking a scanning initial position as the scanning position, wherein the scanning parameters are a scanning area, a scanning step distance, a scanning initial position and a scanning direction;

S3.2) one-dimensional scanning is carried out on the scanning position, and laser ultrasonic signals are received;

s3.3) the excitation point and the receiving point simultaneously move on the surface of the sample (6) along the scanning direction by taking the scanning step distance of two-dimensional scanning as the stepping amount, the moved excitation point and the moved receiving point serve as the next scanning position, and the step S3.2) is returned to the step S3.2) to repeat the step S3.2);

S3.4) repeating the step S3.3) in the scanning area, and finishing scanning after the scanning position reaches the edge of the scanning area;

S3.5) extracting glancing surface longitudinal wave information, namely first converted wave information and second converted wave information, of all scanning positions from the received laser ultrasonic signals according to scanning steps of two-dimensional scanning, so as to obtain subsurface defect positions of all scanning positions and two-dimensional distribution conditions of subsurface defects on a sample (6), and further outputting all subsurface defect information in a scanning area.

4. The laser ultrasonic subsurface defect detection and localization method according to claim 1, wherein the method comprises the following steps: the step of one-dimensional fast scanning (S1) comprises:

Focusing excitation light and detection light on the surface of a sample (6), collecting laser ultrasonic signals, extracting time domain signals of surface wave propagation in the laser ultrasonic signals, and judging:

if the amplitude exceeds the surface wave amplitude in the time domain signal at a position earlier than and close to the surface wave The phase of the surface wave changes, the laser ultrasonic signal contains subsurface defect information, subsurface defects exist on a straight line between an excitation point and a receiving point, and then one-dimensional fine scanning in the one-dimensional scanning is continuously performed on the straight line (S2);

if the amplitude of the surface wave does not exceed the amplitude of the surface wave at a position earlier than and close to the surface wave in the time domain signal If the phase of the surface wave is unchanged, the laser ultrasonic signal does not contain subsurface defect information, subsurface defects do not exist on a straight line between an excitation point and a receiving point, and the one-dimensional scanning is finished.

5. The method for detecting and positioning the laser ultrasonic subsurface defect according to any one of claims 1 and 4, wherein the method comprises the following steps: in the one-dimensional rapid scanning (S1), the distance between an excitation point and a receiving point is D ₁; in the one-dimensional fine scanning (S2), the distance D ₂ between the excitation point and the receiving point is kept unchanged; the relationship of D ₁ to D ₂ is: d ₁>D₂.