CN113484421B - Laser ultrasonic internal defect multimode imaging method and system based on synthetic aperture - Google Patents

Laser ultrasonic internal defect multimode imaging method and system based on synthetic aperture Download PDF

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CN113484421B
CN113484421B CN202110749865.8A CN202110749865A CN113484421B CN 113484421 B CN113484421 B CN 113484421B CN 202110749865 A CN202110749865 A CN 202110749865A CN 113484421 B CN113484421 B CN 113484421B
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CN113484421A (en
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钟华
孙凯华
黄姝珂
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Institute of Mechanical Manufacturing Technology of CAEP
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    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N29/00—Investigating or analysing materials by the use of ultrasonic, sonic or infrasonic waves; Visualisation of the interior of objects by transmitting ultrasonic or sonic waves through the object
    • G01N29/04—Analysing solids
    • G01N29/06—Visualisation of the interior, e.g. acoustic microscopy
    • G01N29/0654—Imaging
    • G01N29/069—Defect imaging, localisation and sizing using, e.g. time of flight diffraction [TOFD], synthetic aperture focusing technique [SAFT], Amplituden-Laufzeit-Ortskurven [ALOK] technique
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
    • G01N21/17—Systems in which incident light is modified in accordance with the properties of the material investigated
    • G01N21/1702—Systems in which incident light is modified in accordance with the properties of the material investigated with opto-acoustic detection, e.g. for gases or analysing solids
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N29/00—Investigating or analysing materials by the use of ultrasonic, sonic or infrasonic waves; Visualisation of the interior of objects by transmitting ultrasonic or sonic waves through the object
    • G01N29/04—Analysing solids
    • G01N29/06—Visualisation of the interior, e.g. acoustic microscopy
    • G01N29/0609—Display arrangements, e.g. colour displays
    • G01N29/0645—Display representation or displayed parameters, e.g. A-, B- or C-Scan
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    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N29/00—Investigating or analysing materials by the use of ultrasonic, sonic or infrasonic waves; Visualisation of the interior of objects by transmitting ultrasonic or sonic waves through the object
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    • G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
    • G01N21/17—Systems in which incident light is modified in accordance with the properties of the material investigated
    • G01N21/1702—Systems in which incident light is modified in accordance with the properties of the material investigated with opto-acoustic detection, e.g. for gases or analysing solids
    • G01N2021/1706—Systems in which incident light is modified in accordance with the properties of the material investigated with opto-acoustic detection, e.g. for gases or analysing solids in solids

Abstract

The invention discloses a laser ultrasonic internal defect multimode imaging method and system based on a synthetic aperture, wherein the method comprises the following steps of S1: determining a scanning mode and a scanning path of laser ultrasound according to the size of a sample and a region to be detected; determining the wave speed of ultrasonic waves according to the sample material; s2: calculating a sound field sensitivity map of the imaging area according to the scanning mode of the laser ultrasound determined in the step S1; s3: selecting the optimal ultrasonic wave modes of different imaging areas for imaging by combining the sound field sensitivity distribution of the imaging areas and the propagation characteristics of ultrasonic waves of different modes, and determining a multi-mode combined imaging scheme; wherein the selection of the ultrasonic mode is realized by setting the sound velocity in a synthetic aperture algorithm formula; s4: echo data is obtained by using the scanning mode and the scanning path determined in the step S1, and multi-mode combined synthetic aperture focusing imaging is performed according to the ultrasonic mode selected in the step S3. The invention can reduce dead zone, eliminate artifact and improve imaging quality efficiency.

Description

Laser ultrasonic internal defect multimode imaging method and system based on synthetic aperture
Technical Field
The invention relates to the technical field of laser ultrasonic imaging, in particular to a synthetic aperture-based laser ultrasonic internal defect multimode imaging method and system.
Background
The laser ultrasonic can excite and detect ultrasonic by utilizing laser, has the characteristics of wide frequency band, high resolution, non-contact excitation and detection, simultaneous excitation of various mode waves and the like, can detect components with complex geometric shapes, can be used in toxic, high-temperature, radiation and other severe environments, and is an ultrasonic detection technology with great development and application potential.
Laser ultrasound can be used for detecting surface defects and internal defects, and the current method for detecting surface or subsurface defects by using laser ultrasonic surface waves and lamb waves has higher resolution and detection efficiency and gradually goes from laboratory research to engineering application. However, when the laser ultrasonic body wave is used for detecting internal defects, the problems of weak reflected signals and poor signal to noise ratio exist, and in order to solve the problems, many researchers research on a method for improving the sensitivity and the signal to noise ratio of the laser ultrasonic detection signals. In general, one can start with three aspects, namely improving excitation efficiency, improving detection sensitivity, and post-processing imaging algorithms.
Improved excitation efficiency may be achieved by changing the excitation mode (e.g., increasing laser energy, adding confinement layers, etc.) and spatially modulating the light source. Increasing the excitation light energy can increase the amplitude of the laser excitation ultrasound, but can also damage the surface of the material; the addition of the constraint layer can pollute the surface of the material, and limit the application range of laser ultrasound. Spatial-temporal modulation of the light source may increase the excitation signal to noise ratio but may also increase the complexity and cost of the experimental setup. The improvement of the detection sensitivity can be achieved by selecting an appropriate detection method. Optical detection methods can achieve long-distance non-contact detection, but have low sensitivity and require a smooth sample surface. Therefore, researchers have proposed using non-optical detection methods, and although the sensitivity of the non-optical detection methods is higher, the application requirements of the non-optical detection methods on the surfaces of samples which need to be close (such as electromagnetic ultrasonic transducers and air-coupled transducers) or in contact (such as piezoelectric transducers) can limit the application occasions, and the advantages of laser ultrasonic remote excitation cannot be fully exerted. The use of post-processing imaging algorithms to improve the signal-to-noise ratio and resolution of the defect image is also an effective way, such as full focus and synthetic aperture focus. The synthetic aperture focusing is the same as the data volume required by the traditional laser ultrasonic B scanning, but the image processed by the synthetic aperture focusing has better signal-to-noise ratio and resolution, and can more intuitively display the information of defect position, size and the like. The amount of data to be processed for full focus is much greater than that for synthetic aperture focus, which, although it can achieve better quality images, is less efficient. Thus, the present study selects synthetic aperture focusing for imaging, taking into account both imaging quality and imaging efficiency.
When the synthetic aperture focusing technology is used for imaging defects, the problems such as artifacts caused by interference of other mode waves, detection blind areas caused by uneven sensitivity distribution and the like exist.
Disclosure of Invention
The invention aims to solve the technical problems that when the synthetic aperture focusing technology and laser ultrasound are used for imaging internal defects in the prior art, imaging blind areas caused by uneven single-mode wave sensitivity distribution and artifacts caused by other mode wave interference exist, the existence of the problems influences the resolution and the signal to noise ratio of the defect images, and the imaging quality is low in efficiency. The invention aims to provide a synthetic aperture-based laser ultrasonic internal defect multimode imaging method and system, which can reduce blind areas, eliminate artifacts and improve imaging quality efficiency by selecting different mode waves for multimode combined synthetic aperture imaging in different imaging areas.
The invention is realized by the following technical scheme:
in a first aspect, the present invention provides a synthetic aperture-based laser ultrasound internal defect multimode combined imaging method comprising:
s1: determining a scanning mode (namely a data acquisition mode) and a scanning path of laser ultrasound according to the size of a sample and a region to be detected; determining the wave speed of ultrasonic waves according to the sample material;
s2: calculating a sound field sensitivity map of the imaging area according to the scanning mode of the laser ultrasound determined in the step S1; the sound field sensitivity map of the imaging area is obtained by coupling sound field distribution of laser excitation ultrasound, interferometer detection ultrasound and defect reflection ultrasound;
s3: combining the sound field sensitivity distribution of the imaging area and the propagation characteristics (such as propagation speed, amplitude and the like) of ultrasonic waves in different modes, selecting the optimal ultrasonic mode of the different imaging areas for imaging, and determining a multi-mode combined imaging scheme; wherein the selection of the ultrasonic mode is realized by setting the sound velocity in a synthetic aperture algorithm formula;
s4: echo data is obtained by using the scanning mode and the scanning path determined in the step S1, and multi-mode combined synthetic aperture focusing imaging is performed according to the ultrasonic mode selected in the step S3.
The working principle is as follows: when the synthetic aperture focusing technology and laser ultrasound are used for imaging internal defects based on the prior art, imaging blind areas caused by uneven single-mode wave sensitivity distribution and artifacts caused by interference of other mode waves exist, and the resolution and the signal to noise ratio of the defect images are affected by the problems, so that the imaging quality is reduced. The invention designs a laser ultrasonic internal defect multimode combined imaging method based on a synthetic aperture, which comprises the steps of firstly determining a scanning mode (namely a data acquisition mode) and a scanning path of laser ultrasonic according to the size of a sample and a region to be detected; secondly, coupling laser excitation ultrasound, detecting ultrasound by an interferometer and sound field distribution of defect reflection ultrasound to obtain a sound field sensitivity map of an imaging area; and then predicting the optimal imaging mode waves of the distributed defects at different positions according to the sound field sensitivity map and the propagation characteristics of the different mode waves, and selecting different mode waves for multi-mode combined synthetic aperture imaging at different areas during imaging. The invention uses the B-scan signal imaging after the subtraction of defect, which can improve the resolution and the signal-to-noise ratio of the image; synthetic aperture imaging with different mode waves for multimode combination is selected for different imaging areas, so that blind areas can be reduced, artifacts can be eliminated, and the imaging quality efficiency can be improved.
Further, the laser source parameters used for laser ultrasound in step S1 include energy density, pulse time, spot radius μm.
Further, the scanning mode of laser ultrasound in step S1 adopts a data acquisition mode of fixed excitation points and multipoint detection to scan and acquire the sample. The sample may be collected by scanning in other ways, and is not limited to the above collection method.
Further, the sound field distribution of the defect reflected ultrasound in step S2 is obtained by the following process:
obtaining the reflection coefficient R of the transverse wave reflected longitudinal wave by adopting a reflection and scattering coefficient matrix model (3) - (8)) according to a given incident wave mode, material parameters and an ultrasonic wave propagation path SL Reflection coefficient R of transverse wave reflection transverse wave SS Reflection coefficient R of longitudinal wave reflection longitudinal wave LL Reflection coefficient R of longitudinal wave reflection transverse wave LS The method comprises the steps of carrying out a first treatment on the surface of the According to the R obtained SL 、R SS 、R LL 、R LS Drawing a sound field distribution diagram of the defect reflected wave; the sound field distribution diagram of the defect reflected wave comprises a defect reflected longitudinal wave, a defect reflected longitudinal wave-to-transverse wave, a defect reflected transverse wave and a defect reflected transverse wave-to-longitudinal wave;
the incident wave modes comprise transverse wave S waves and longitudinal wave L waves, and the material parameters comprise density, young 'S modulus, poisson' S ratio and thermal expansion coefficient.
Specifically, the reflection and scattering coefficient matrix model (including formulas (3) - (8)) is as follows:
for an ideal solid-gas interface, if particle velocity and stress are substituted into continuous conditions, the reflection and scattering coefficient matrices can be obtained as follows:
R＝a/M， (3)
where R is a matrix of reflection and refraction coefficients (related to incident angle, wave velocity and frequency), which is defined as the ratio of the amplitude of the reflected (refracted) wave to the incident wave, M is a matrix related to the angle of reflection, angle of refraction, wave velocity and Ramez constant, and a is a matrix related to the angle of incidence, angle of reflection, wave velocity and Ramez constant.
For transverse and longitudinal wave incidence, M remains unchanged, and is the following formula:
Figure BDA0003144112420000031
wherein lambda is 1 Sum mu 1 Ramez constant, alpha, for sample material rL 、α rS 、β tL The angles of the reflected longitudinal wave, reflected transverse wave and refracted longitudinal wave, c L1 And c S1 Propagation velocities of longitudinal waves and transverse waves in the sample, respectively.
For transverse wave incidence, there are:
Figure BDA0003144112420000032
wherein R is SL 、R SS The reflection coefficients of the transverse wave reflected longitudinal wave and transverse wave reflected transverse wave are respectively D SL The refractive index of the transverse wave is the refractive index of the longitudinal wave.
Figure BDA0003144112420000041
Wherein alpha is in Is the angle of the incident wave.
For longitudinal wave incidence, there are:
Figure BDA0003144112420000042
wherein R is LL 、R LS The reflection coefficients of the longitudinal wave reflected by the longitudinal wave and the longitudinal wave reflected by the transverse wave are respectively D LL The refractive index of the longitudinal wave is the refractive index of the longitudinal wave.
Figure BDA0003144112420000043
Given the incident wave mode, material parameters and ultrasonic propagation path, R can be found according to formulas (3) - (8) SL 、R SS 、R LL 、R LS Thus, the sound field distribution diagram of the defect reflected wave can be drawn (fig. 5).
The three directivity coupling can obtain the sound field sensitivity graph of four mode waves, as shown in fig. 6, the colors in the graph represent the amplitude intensity, and the numerical values are shown in the right legend. As can be seen from fig. 6, the ultrasonic waves of different modes have different sensitivity distributions, and under the same display range, the sensitivity range of the SS wave is the widest, the sensitivity pattern amplitude of the LL wave and the LS wave is lower for the SL wave. There is one linear dead zone in the LL wave and LS wave sensitivity maps, and three linear dead zones in the SL wave and SS wave sensitivity maps, which are caused by the directionality of the laser excitation ultrasonic wave under the thermoelastic mechanism.
Further, the sound field sensitivity map of the imaging region includes a defect reflection longitudinal wave sensitivity map, a defect reflection longitudinal wave to transverse wave sensitivity map, a defect reflection transverse wave sensitivity map, and a defect reflection transverse wave to longitudinal wave sensitivity map.
Further, the different mode ultrasonic waves in step S3 include SS wave, SL wave, LL wave, LS wave, wherein: the SS wave is a transverse wave reflected transverse wave, the SL wave is a transverse wave reflected longitudinal wave, the LL wave is a longitudinal wave reflected longitudinal wave, and the LS wave is a longitudinal wave reflected transverse wave.
Further, the propagation characteristics of the ultrasonic waves in the different modes in the step S3 include amplitude and propagation speed.
Further, the selection of the ultrasonic mode in step S3 is achieved by setting the sound velocity in the synthetic aperture algorithm formula; wherein, the formula of the synthetic aperture algorithm is set as follows:
Figure BDA0003144112420000044
wherein I (x, z) is the amplitude at the focus point (x, z), N is the number of scan steps of the excitation source, v 1 、v 2 Respectively the propagation velocity of the selected mode wave in the material, d i1 And d i2 The distances between the excitation point and the detection point and the focusing point are respectively;
when the focusing point (x, z) is a defect, the probe light is
Figure BDA0003144112420000051
The signal S (M i T) will appear a peak caused by the defect reflection, and by superposition, the amplitude of the point (x, z) in the reconstructed image can be enhanced, while the amplitude of the rest of the non-defective points is not significantly changed.
In a second aspect, the present invention also provides a synthetic aperture-based laser ultrasonic internal defect multimode combined imaging system supporting the synthetic aperture-based laser ultrasonic internal defect multimode combined imaging method, the system comprising:
the laser ultrasonic data acquisition unit is used for determining a scanning mode (namely a data acquisition mode) and a scanning path of laser ultrasonic according to the size of the sample and the region to be detected; determining the wave speed of ultrasonic waves according to the sample material;
the sound field sensitivity map unit is used for calculating a sound field sensitivity map of the imaging area according to the scanning mode of the laser ultrasound determined by the laser ultrasound data acquisition unit; the sound field sensitivity map of the imaging area is obtained by coupling sound field distribution of laser excitation ultrasound, interferometer detection ultrasound and defect reflection ultrasound;
the ultrasonic mode selection unit is used for selecting the optimal ultrasonic mode of different imaging areas for imaging by combining the sound field sensitivity distribution of the imaging areas and the propagation characteristics (such as propagation speed, amplitude and the like) of ultrasonic waves of different modes, and determining a multi-mode combined imaging scheme; wherein the selection of the ultrasonic mode is realized by setting the sound velocity in a synthetic aperture algorithm formula;
multimode combined imaging unit: and the laser ultrasonic data acquisition unit is used for acquiring echo data by using the scanning mode and the scanning path determined by the laser ultrasonic data acquisition unit, and performing multi-mode combined synthetic aperture focusing imaging according to the ultrasonic mode selected by the ultrasonic mode selection unit.
Further, the sound field distribution of the defect reflection ultrasound is obtained by the following process:
obtaining the reflection coefficient R of the transverse wave reflected longitudinal wave by adopting a reflection and scattering coefficient matrix model (3) - (8)) according to a given incident wave mode, material parameters and an ultrasonic wave propagation path SL Reflection coefficient R of transverse wave reflection transverse wave SS Reflection coefficient R of longitudinal wave reflection longitudinal wave LL Reflection coefficient R of longitudinal wave reflection transverse wave LS The method comprises the steps of carrying out a first treatment on the surface of the According to the R obtained SL 、R SS 、R LL 、R LS Drawing a sound field distribution diagram of the defect reflected wave; the sound field distribution diagram of the defect reflected wave comprises a defect reflected longitudinal wave, a defect reflected longitudinal wave-to-transverse wave, a defect reflected transverse wave and a defect reflected transverse wave-to-longitudinal wave;
the incident wave modes comprise transverse wave S waves and longitudinal wave L waves, and the material parameters comprise density, young 'S modulus, poisson' S ratio and thermal expansion coefficient.
Compared with the prior art, the invention has the following advantages and beneficial effects:
1. according to the invention, firstly, laser excitation ultrasound is coupled, an interferometer detects sound field distribution of ultrasound and defect reflection ultrasound to obtain a sound field sensitivity map, and then optimal imaging mode waves of defects distributed at different positions are predicted according to the sound field sensitivity map and propagation characteristics of different mode waves, and different areas are selected for multi-mode combined synthetic aperture imaging during imaging.
2. The invention uses the B-scan signal imaging after the subtraction of defect, which can improve the resolution and the signal-to-noise ratio of the image.
3. According to the invention, different mode waves are selected for multi-mode combined synthetic aperture imaging in different imaging areas, so that blind areas can be reduced, artifacts can be eliminated, and the imaging quality efficiency can be improved.
Drawings
The accompanying drawings, which are included to provide a further understanding of embodiments of the invention and are incorporated in and constitute a part of this application, illustrate embodiments of the invention. In the drawings:
FIG. 1 is a flow chart of a laser ultrasonic internal defect multimode combined imaging method based on a synthetic aperture.
Fig. 2 is a laser excited ultrasound directivity pattern of the present invention.
FIG. 3 is a diagram of the present invention interferometer for detecting ultrasound directivity patterns.
FIG. 4 is a schematic view of the interface incidence of the present invention.
Fig. 5 is a plot of the defect diffuse sound field distribution of the present invention.
Fig. 6 is a sound field sensitivity graph of the present invention.
FIG. 7 is a schematic diagram of defect location and area division according to the present invention.
FIG. 8 is a scanning schematic diagram of the present invention.
FIG. 9 is a diagram of a B-scan signal according to the present invention.
FIG. 10 is a diagram of a multi-mode combined imaging of different position defects in accordance with the present invention.
Detailed Description
For the purpose of making apparent the objects, technical solutions and advantages of the present invention, the present invention will be further described in detail with reference to the following examples and the accompanying drawings, wherein the exemplary embodiments of the present invention and the descriptions thereof are for illustrating the present invention only and are not to be construed as limiting the present invention.
Example 1
As shown in fig. 1, the laser ultrasonic internal defect multimode combined imaging method based on the synthetic aperture comprises the following steps:
s1: determining a scanning mode (namely a data acquisition mode) and a scanning path of laser ultrasound according to the size of a sample and a region to be detected; determining the wave speed of ultrasonic waves according to the sample material;
s2: calculating a sound field sensitivity map of the imaging area according to the scanning mode of the laser ultrasound determined in the step S1; the sound field sensitivity map of the imaging area is obtained by coupling sound field distribution of laser excitation ultrasound, interferometer detection ultrasound and defect reflection ultrasound;
s3: combining the sound field sensitivity distribution of the imaging area and the propagation characteristics (such as propagation speed, amplitude and the like) of ultrasonic waves in different modes, selecting the optimal ultrasonic mode of the different imaging areas for imaging, and determining a multi-mode combined imaging scheme; wherein the selection of the ultrasonic mode is realized by setting the sound velocity in a synthetic aperture algorithm formula;
s4: echo data is obtained by using the scanning mode and the scanning path determined in the step S1, and multi-mode combined synthetic aperture focusing imaging is performed according to the ultrasonic mode selected in the step S3.
In specific implementation, the laser source parameters adopted by the laser ultrasound in the step S1 include energy density, pulse time and light spot radius μm.
The scanning mode of laser ultrasound in the step S1 adopts a data acquisition mode of fixed excitation points and multipoint detection to scan and acquire samples. The sample may be collected by scanning in other ways, and is not limited to the above collection method.
In specific implementation, the specific steps of step S2 are as follows:
when the excitation detection separation laser ultrasonic body wave is used for detecting the internal defects, the sensitivity of a sound field at a certain point (x, z) in the sample can be expressed by the following formula:
Figure BDA0003144112420000071
wherein the subscript m=s or L represents an excitation transverse wave or longitudinal wave, n=s or L represents a detection transverse wave or longitudinal wave, N is the number of scanning points, E mn (x, z) represents the sound field sensitivity at point (x, z), θ g Representation ofThe angle theta between the point and the normal direction of the excitation point d Represents the angle between the point and the normal direction of the detection point, G m (θ g ) Indicating the directionality of the laser excited ultrasonic wave, R mn Representing the reflection coefficient of the defect reflected wave, D n (θ d ) Indicating the directionality of the detected ultrasonic waves by the interferometer. From the above equation, the sound field sensitivity distribution is affected by three factors, namely the directionality of the excited ultrasound, the directionality of the detected ultrasound by the interferometer, and the sound field distribution after the action of the ultrasound and the defect.
The directionality of laser excited ultrasound and interferometer detected ultrasound can be expressed by the following formula:
Figure BDA0003144112420000072
Figure BDA0003144112420000073
wherein formula (1) is the relation between the amplitudes and angles of the laser excited compression wave (i.e. longitudinal wave) and the shear wave (i.e. transverse wave) under the thermoelastic mechanism, formula (2) is the relation between the longitudinal wave and transverse wave components and angles of the surface displacement caused by the out-of-plane point load, and can also be used for describing the detection directivity, subscript L, S represents the longitudinal wave and transverse wave respectively, and k is the ratio of the wave speeds of the compression wave and the shear wave in the sample. According to the formulas (1) and (2), a directivity pattern (fig. 2 and 3) can be obtained, wherein (a) in fig. 2 is a longitudinal wave pattern, and (b) in fig. 2 is a transverse wave pattern; fig. 3 (a) shows a longitudinal wave pattern, and fig. 2 (b) shows a transverse wave pattern.
The distribution of the sound field reflected by the defect is directional. According to the propagation principle of ultrasonic waves, when ultrasonic waves propagate at an angle to the interface of two materials, reflection and refraction occur and new waves are generated. The interface incidence diagram is shown in FIG. 4, assuming that a circular hole defect is located in the far field region and the defect size is far smaller than the distance of the acoustic wave propagation path, an ultrasonic wave is necessary from the excitation point G when the excitation point, the detection point and the ultrasonic mode are determined 1 Incident on the surface of the circular hole defect at a point H 1 And reflectUltrasonic detected point D 1 And (5) receiving.
For an ideal solid-gas interface, if particle velocity and stress are substituted into continuous conditions, the reflection and scattering coefficient matrices can be obtained as follows:
R＝a/M， (3)
where R is a matrix of reflection and refraction coefficients (related to incident angle, wave velocity and frequency), which is defined as the ratio of the amplitude of the reflected (refracted) wave to the incident wave, M is a matrix related to the angle of reflection, angle of refraction, wave velocity and Ramez constant, and a is a matrix related to the angle of incidence, angle of reflection, wave velocity and Ramez constant.
For transverse and longitudinal wave incidence, M remains unchanged, and is the following formula:
Figure BDA0003144112420000081
wherein lambda is 1 Sum mu 1 Ramez constant, alpha, for sample material rL 、α rS 、β tL The angles of the reflected longitudinal wave, reflected transverse wave and refracted longitudinal wave, c L1 And c S1 Propagation velocities of longitudinal waves and transverse waves in the sample, respectively.
For transverse wave incidence, there are:
Figure BDA0003144112420000082
wherein R is SL 、R SS The reflection coefficients of the transverse wave reflected longitudinal wave and transverse wave reflected transverse wave are respectively D SL The refractive index of the transverse wave is the refractive index of the longitudinal wave.
Figure BDA0003144112420000083
Wherein alpha is in Is the angle of the incident wave.
For longitudinal wave incidence, there are:
Figure BDA0003144112420000084
wherein R is LL 、R LS The reflection coefficients of the longitudinal wave reflected by the longitudinal wave and the longitudinal wave reflected by the transverse wave are respectively D LL The refractive index of the longitudinal wave is the refractive index of the longitudinal wave.
Figure BDA0003144112420000091
Given the incident wave mode, material parameters and ultrasonic propagation path, R can be found according to formulas (3) - (8) SL 、R SS 、R LL 、R LS Therefore, the sound field distribution diagram of the defect reflection wave can be drawn (fig. 5), wherein (a) in fig. 5 is a defect reflection longitudinal wave diagram, (b) in fig. 5 is a defect reflection longitudinal wave to transverse wave diagram, (c) in fig. 5 is a defect reflection transverse wave diagram, and (d) in fig. 5 is a defect reflection transverse wave to longitudinal wave diagram.
By coupling the three directivities, a sound field sensitivity map of four mode waves can be obtained, as shown in fig. 6, in which (a) is a defect reflection longitudinal wave sensitivity map, in which (b) is a defect reflection longitudinal wave to transverse wave sensitivity map, in which (c) is a defect reflection transverse wave sensitivity map, in which (d) is a defect reflection transverse wave to longitudinal wave sensitivity map, in which the colors represent amplitude intensity, and the numerical values are shown in the right-hand legend. As can be seen from fig. 6, the ultrasonic waves of different modes have different sensitivity profiles, and the sensitivity of the SS wave is better than that of the SL wave and the sensitivity of the LL wave is better than that of the LS wave in the same display range. There is one linear dead zone in the LL wave and LS wave sensitivity maps, and three linear dead zones in the SL wave and SS wave sensitivity maps, which are caused by the directionality of the laser excitation ultrasonic wave under the thermoelastic mechanism.
In step S1, multiple mode waves are excited by laser ultrasound, and laser source parameters adopted by the laser ultrasound include energy density, pulse time and spot radius, and the specific parameters are shown in table 3.
In specific implementation, step S3 includes the following steps:
based on the example, the sample material was aluminum (the properties of the aluminum sample material are shown in Table 2), the size was 10mm×8mm, the defects were side holes, only one defect was contained in each of the nine samples, the sizes of the defects were the same (d 1 mm) but the positions were different (see FIG. 7, table 1), and the gray rectangular areas in FIG. 7 were not imaged. The excitation point is fixed at x=3mm, and the detection range starts from x= -3mm to x=3mm, and the step size is 0.05mm. (FIG. 8) the clear defect reflection signal is obtained by subtracting the defect-free B-scan signals. Wherein the B-scan signal diagram is shown in fig. 9. Fig. 9 (a) is a B-scan containing defect D5, fig. 9 (B) is a B-scan containing no defect D5, and fig. 9 (c) is a B-scan after subtraction of defect-free D5. Fig. 9 (a) and (B) are B-scan graphs of the defect echo obtained by calculating the values of the defect-containing 5 and the defect-free 5, respectively, with the horizontal axis representing the position of the probe point and the vertical axis representing the propagation time. d-L, d-R in the figure respectively represents a direct grazing surface longitudinal wave and a direct surface wave, and LL, LS, SL and SS respectively represent a defect reflection longitudinal wave to longitudinal wave, a defect reflection longitudinal wave to transverse wave, a defect reflection transverse wave to longitudinal wave, and a defect reflection transverse wave to transverse wave. LS, SL and SS can be seen in blur in FIG. 9 (a), whereas the defect reflection signal cannot be seen in FIG. 9 (b). Since the parameters are identical except for whether the defect is included in the two-time value calculation, the two B-scan images are subtracted to obtain clear four-mode defect reflected waves, as shown in fig. 9 (c).
From the occurrence of the defect reflection echo in fig. 9 (c), it can be judged that a defect exists inside the sample, but the position of the defect cannot be determined. To obtain more information of the defect and analyze the imaging effect of different mode waves on the defect, SAFT reconstruction of four mode waves is performed on the defects D1-D9 respectively, and the obtained results are shown in FIG. 10 (blue circles in the figure represent the positions and sizes of the defects).
Table 1 side Kong Quexian parameter table
Sample Reflector Coordinates
1 D1 (zone 1) (-2，-2)
2 D2 (zone 2) (0，-2)
3 D3 (zone 3) (2，-2)
4 D4 (zone 4) (-2，-4.5)
5 D5 (zone 5) (0，-4.5)
6 D6 (zone 6) (2，-4.5)
7 D7 (region 7) (-2，-7)
8 D8 (zone 8) (0，-7)
9 D9 (zone 9) (2，-7)
TABLE 2 Material Properties of aluminum samples
Parameters (parameters) Value of
Density/(kg.m) 3 ) 2.7×10 3
Young's modulus/MPa 7×10 4
Poisson's ratio 0.33
Coefficient of thermal expansion/K -1 2.3×10 -7
TABLE 3 excitation light parameters
Parameters (parameters) Value of
Energy density/(W.m) -2 ) 1×10 12
Pulse time/ns 5
Spot radius/μm 100
In specific implementation, step S4 includes the following steps:
echo data (namely B scanning signals for imaging) are obtained by using the scanning mode and the scanning path determined in the step S1, and multi-mode combined synthetic aperture focusing imaging is carried out according to the ultrasonic mode selected in the step S3; the formula for carrying out multi-mode combined synthetic aperture focusing imaging is as follows:
Figure BDA0003144112420000111
wherein I (x, z) is the amplitude at the focus point (x, z), N is the number of scan steps of the excitation source, v 1 、v 2 Respectively the propagation velocity of the selected mode wave in the material, d i1 And d i2 The distances between the excitation point and the detection point and the focusing point are respectively;
when the focusing point (x, z) is a defect, the probe light is
Figure BDA0003144112420000112
The signal S (M i T) will appear a peak caused by defect reflection, the amplitude of the point (x, z) position in the reconstructed image can be enhanced by superposition, the amplitude of the rest non-defective points has no obvious change, and compared with the ultrasonic B scanning result, the method realizes->
Figure BDA0003144112420000113
The signal-to-noise ratio is improved by times, and the contrast and the transverse resolution of the defect image are improved.
The velocity of the longitudinal wave is selected for the following v1 and v2 when (x, z) is in region 1-3, and the velocity of the transverse wave is selected for the following v1 and v2 when (x, z) is in region 4-9.
Wherein, the selection of the mode is: defects 1-3 are closer to the upper surface of the sample, and artifacts are generated by using four-mode wave imaging, but the LL wave propagation time is shortest and can be separated from other mode waves in the time domain, so that defects 1-3 are imaged by using only the part containing the LL wave in the B-scan diagram. And (4) imaging the defects 4-9 by using SS waves.
The synthetic aperture imaging results of the multimode combination using the selected mode combination are shown in fig. 10. In fig. 10, (a) is a defect 1, (b) is a defect in fig. 10, (c) is a defect 3, (d) is a defect 4, (e) is a defect 5, and (f) is a defect in fig. 10, (g) is a defect in fig. 10, (h) is a defect 8, and (i) is a defect 9 in fig. 10.
Therefore, the SS wave is selected to image the defects D4-D9, and the image signal to noise ratio of the four mode waves is not high for the defects D1-D3, but the LL wave is selected to image the defects D1-D3 because the propagation time of the LL wave is shortest and has certain amplitude intensity, the LL wave can be distinguished from other mode waves in the B-scan diagram, and only the part containing the LL wave in the B-scan diagram can be intercepted for imaging. The multimode combined imaging results using this mode combination are shown in fig. 10.
Therefore, the method solves the problems of weak reflected signals and poor signal to noise ratio when the laser ultrasonic body wave detects internal defects, selects the longitudinal wave reflection longitudinal wave (LL wave) to image the defects (defects D1-D3) near the surface according to the sound field sensitivity map obtained by coupling laser excitation ultrasonic directivity, interferometer detection ultrasonic directivity, sound field distribution directivity after the ultrasonic wave and defect action and LL wave propagation characteristics, selects the transverse wave reflection transverse wave (SS wave) to image the defects (defects D4-D9) below the near surface, and compared with the traditional single-mode imaging, the multi-mode combined imaging method can realize artifact elimination, blind area reduction and imaging quality improvement. The multimode combined synthetic aperture imaging method combined by the modes of the figure (10) can realize high-quality imaging of defects at different positions, can reduce artifacts of defect images, eliminate blind areas and improve imaging quality of the defects.
Example 2
As shown in fig. 1 to 10, the present embodiment is different from embodiment 1 in that the system supports the laser ultrasonic internal defect multimode combination imaging method based on the synthetic aperture described in embodiment 1, and the system includes:
the laser ultrasonic data acquisition unit is used for determining a scanning mode (namely a data acquisition mode) and a scanning path of laser ultrasonic according to the size of the sample and the region to be detected; determining the wave speed of ultrasonic waves according to the sample material;
the sound field sensitivity map unit is used for calculating a sound field sensitivity map of the imaging area according to the scanning mode of the laser ultrasound determined by the laser ultrasound data acquisition unit; the sound field sensitivity map of the imaging area is obtained by coupling sound field distribution of laser excitation ultrasound, interferometer detection ultrasound and defect reflection ultrasound;
the ultrasonic mode selection unit is used for selecting the optimal ultrasonic mode of different imaging areas for imaging by combining the sound field sensitivity distribution of the imaging areas and the propagation characteristics (such as propagation speed, amplitude and the like) of ultrasonic waves of different modes, and determining a multi-mode combined imaging scheme; wherein the selection of the ultrasonic mode is realized by setting the sound velocity in a synthetic aperture algorithm formula;
multimode combined imaging unit: and the laser ultrasonic data acquisition unit is used for acquiring echo data by using the scanning mode and the scanning path determined by the laser ultrasonic data acquisition unit, and performing multi-mode combined synthetic aperture focusing imaging according to the ultrasonic mode selected by the ultrasonic mode selection unit.
In this embodiment, the sound field distribution of the defect reflected ultrasound is obtained by the following process:
obtaining the reflection coefficient R of the transverse wave reflected longitudinal wave by adopting a reflection and scattering coefficient matrix model (3) - (8)) according to a given incident wave mode, material parameters and an ultrasonic wave propagation path SL Reflection coefficient R of transverse wave reflection transverse wave SS Reflection coefficient R of longitudinal wave reflection longitudinal wave LL Reflection coefficient R of longitudinal wave reflection transverse wave LS The method comprises the steps of carrying out a first treatment on the surface of the According to the R obtained SL 、R SS 、R LL 、R LS Drawing a sound field distribution diagram of the defect reflected wave; the sound field distribution diagram of the defect reflection wave comprises defect reflection longitudinal waves and defect reflectionThe longitudinal wave is converted into transverse wave, the defect reflected transverse wave and the defect reflected transverse wave are converted into longitudinal wave;
the incident wave modes comprise transverse wave S waves and longitudinal wave L waves, and the material parameters comprise density, young 'S modulus, poisson' S ratio and thermal expansion coefficient.
The execution process of each unit is performed according to the synthetic aperture-based laser ultrasound internal defect multimode combined imaging method flow described in the embodiment, and in this embodiment, details are not repeated.
According to the invention, firstly, laser excitation ultrasound is coupled, an interferometer detects sound field distribution of ultrasound and defect reflection ultrasound to obtain a sound field sensitivity map, and then optimal imaging mode waves of defects distributed at different positions are predicted according to the sound field sensitivity map and propagation characteristics of different mode waves, and different areas are selected for multi-mode combined synthetic aperture imaging during imaging. According to the invention, the ultrasonic modes with good imaging effects in different areas are selected for multi-mode combined imaging according to the sensitivity distribution of the sound field, so that the problems are solved, blind areas can be reduced, artifacts can be eliminated, and the imaging quality efficiency can be improved.
It will be appreciated by those skilled in the art that embodiments of the present application may be provided as a method, system, or computer program product. Accordingly, the present application may take the form of an entirely hardware embodiment, an entirely software embodiment, or an embodiment combining software and hardware aspects. Furthermore, the present application may take the form of a computer program product embodied on one or more computer-usable storage media (including, but not limited to, disk storage, CD-ROM, optical storage, and the like) having computer-usable program code embodied therein.
The present application is described with reference to flowchart illustrations and/or block diagrams of methods, apparatus (systems) and computer program products according to embodiments of the application. It will be understood that each flow and/or block of the flowchart illustrations and/or block diagrams, and combinations of flows and/or blocks in the flowchart illustrations and/or block diagrams, can be implemented by computer program instructions. These computer program instructions may be provided to a processor of a general purpose computer, special purpose computer, embedded processor, or other programmable data processing apparatus to produce a machine, such that the instructions, which execute via the processor of the computer or other programmable data processing apparatus, create means for implementing the functions specified in the flowchart flow or flows and/or block diagram block or blocks.
These computer program instructions may also be stored in a computer-readable memory that can direct a computer or other programmable data processing apparatus to function in a particular manner, such that the instructions stored in the computer-readable memory produce an article of manufacture including instruction means which implement the function specified in the flowchart flow or flows and/or block diagram block or blocks.
These computer program instructions may also be loaded onto a computer or other programmable data processing apparatus to cause a series of operational steps to be performed on the computer or other programmable apparatus to produce a computer implemented process such that the instructions which execute on the computer or other programmable apparatus provide steps for implementing the functions specified in the flowchart flow or flows and/or block diagram block or blocks.
The foregoing description of the embodiments has been provided for the purpose of illustrating the general principles of the invention, and is not meant to limit the scope of the invention, but to limit the invention to the particular embodiments, and any modifications, equivalents, improvements, etc. that fall within the spirit and principles of the invention are intended to be included within the scope of the invention.

Claims (9)

1. The laser ultrasonic internal defect multimode combined imaging method based on the synthetic aperture is characterized by comprising the following steps of:
s1: determining a scanning mode and a scanning path of laser ultrasound according to the size of a sample and a region to be detected; determining the wave speed of ultrasonic waves according to the sample material;
s2: calculating a sound field sensitivity map of the imaging area according to the scanning mode of the laser ultrasound determined in the step S1; the sound field sensitivity map of the imaging area is obtained by coupling sound field distribution of laser excitation ultrasound, interferometer detection ultrasound and defect reflection ultrasound;
s3: selecting the optimal ultrasonic wave modes of different imaging areas for imaging by combining the sound field sensitivity distribution of the imaging areas and the propagation characteristics of ultrasonic waves of different modes, and determining a multi-mode combined imaging scheme; wherein the selection of the ultrasonic mode is realized by setting the sound velocity in a synthetic aperture algorithm formula;
s4: echo data is obtained by using the scanning mode and the scanning path determined in the step S1, and multi-mode combined synthetic aperture focusing imaging is carried out according to the ultrasonic mode selected in the step S3;
the selection of the ultrasonic mode in the step S3 is realized by setting the sound velocity in the synthetic aperture algorithm formula; wherein, the formula of the synthetic aperture algorithm is set as follows:
Figure FDA0004109724220000011
wherein I (x, z) is the amplitude at the focus point (x, z), N is the number of scan steps of the excitation source, v 1 、v 2 Respectively the propagation velocity of the selected mode wave in the material, d i1 And d i2 The distances between the excitation point and the detection point and the focusing point are respectively;
when the focusing point (x, z) is a defect, the probe light is
Figure FDA0004109724220000012
The signal S (M i T) will appear a peak caused by the defect reflection, and by superposition, the amplitude of the point (x, z) in the reconstructed image can be enhanced, while the amplitude of the rest of the non-defective points is not significantly changed.
2. The synthetic aperture based laser ultrasound internal defect multimode combination imaging method of claim 1, wherein the laser source parameters employed for laser ultrasound in step S1 include energy density, pulse time, spot radius.
3. The synthetic aperture-based laser ultrasonic internal defect multimode combination imaging method of claim 1, wherein the scanning mode of laser ultrasonic in step S1 adopts a data acquisition mode of fixed excitation point and multipoint detection to scan and acquire a sample.
4. The synthetic aperture-based laser ultrasound internal defect multimode combination imaging method of claim 1, wherein the sound field distribution of the defect-reflected ultrasound in step S2 is obtained by:
obtaining a reflection coefficient R of transverse wave reflected longitudinal wave by adopting a reflection and scattering coefficient matrix model according to a given incident wave mode, material parameters and an ultrasonic wave propagation path SL Reflection coefficient R of transverse wave reflection transverse wave SS Reflection coefficient R of longitudinal wave reflection longitudinal wave LL Reflection coefficient R of longitudinal wave reflection transverse wave LS The method comprises the steps of carrying out a first treatment on the surface of the According to the R obtained SL 、R SS 、R LL 、R LS Drawing a sound field distribution diagram of the defect reflected wave; the sound field distribution diagram of the defect reflected wave comprises a defect reflected longitudinal wave, a defect reflected longitudinal wave-to-transverse wave, a defect reflected transverse wave and a defect reflected transverse wave-to-longitudinal wave;
the incident wave modes comprise transverse wave S waves and longitudinal wave L waves, and the material parameters comprise density, young 'S modulus, poisson' S ratio and thermal expansion coefficient.
5. The synthetic aperture-based laser ultrasonic internal defect multimode combination imaging method of claim 4, wherein the acoustic field sensitivity map of the imaging region comprises a defect reflection longitudinal wave sensitivity map, a defect reflection longitudinal wave to transverse wave sensitivity map, a defect reflection transverse wave sensitivity map, and a defect reflection transverse wave to longitudinal wave sensitivity map.
6. The synthetic aperture based laser ultrasonic internal defect multimode combination imaging method of claim 1, wherein the different modes of ultrasonic waves in step S3 comprise SS wave, SL wave, LL wave, LS wave, wherein: the SS wave is a transverse wave reflected transverse wave, the SL wave is a transverse wave reflected longitudinal wave, the LL wave is a longitudinal wave reflected longitudinal wave, and the LS wave is a longitudinal wave reflected transverse wave.
7. The synthetic aperture-based laser ultrasonic internal defect multimode combination imaging method of claim 1, wherein the propagation characteristics of the different modes of ultrasonic waves in step S3 include amplitude and propagation speed.
8. A synthetic aperture based laser ultrasound internal defect multimode combined imaging system supporting a synthetic aperture based laser ultrasound internal defect multimode combined imaging method according to any one of claims 1 to 7, the system comprising:
the laser ultrasonic data acquisition unit is used for determining a scanning mode and a scanning path of laser ultrasonic according to the size of the sample and the region to be detected; determining the wave speed of ultrasonic waves according to the sample material;
the sound field sensitivity map unit is used for calculating a sound field sensitivity map of the imaging area according to the scanning mode of the laser ultrasound determined by the laser ultrasound data acquisition unit; the sound field sensitivity map of the imaging area is obtained by coupling sound field distribution of laser excitation ultrasound, interferometer detection ultrasound and defect reflection ultrasound;
the ultrasonic mode selection unit is used for selecting the optimal ultrasonic mode of different imaging areas for imaging by combining the sound field sensitivity distribution of the imaging areas and the propagation characteristics of ultrasonic waves of different modes, and determining a scheme of multi-mode combined imaging; wherein the selection of the ultrasonic mode is realized by setting the sound velocity in a synthetic aperture algorithm formula;
multimode combined imaging unit: the laser ultrasonic imaging device comprises a laser ultrasonic data acquisition unit, a multi-mode combined synthetic aperture focusing imaging unit, a laser ultrasonic data processing unit and a laser ultrasonic data processing unit, wherein the laser ultrasonic data acquisition unit is used for acquiring an ultrasonic data of a scanning mode and a scanning path determined by the laser ultrasonic data acquisition unit, and performing multi-mode combined synthetic aperture focusing imaging according to an ultrasonic mode selected by the ultrasonic mode selection unit;
the selection of the ultrasonic mode is achieved by setting the sound velocity in a synthetic aperture algorithm formula, wherein the synthetic aperture algorithm formula is set as follows:
Figure FDA0004109724220000021
wherein I (x, z) is the amplitude at the focus point (x, z), N is the number of scan steps of the excitation source, v 1 、v 2 Respectively the propagation velocity of the selected mode wave in the material, d i1 And d i2 The distances between the excitation point and the detection point and the focusing point are respectively;
when the focusing point (x, z) is a defect, the probe light is
Figure FDA0004109724220000031
The signal S (M i T) will appear a peak caused by the defect reflection, and by superposition, the amplitude of the point (x, z) in the reconstructed image can be enhanced, while the amplitude of the rest of the non-defective points is not significantly changed.
9. The synthetic aperture based laser ultrasound internal defect multimode combination imaging system of claim 8, wherein the sound field distribution of the defect reflected ultrasound is obtained by:
obtaining a reflection coefficient R of transverse wave reflected longitudinal wave by adopting a reflection and scattering coefficient matrix model according to a given incident wave mode, material parameters and an ultrasonic wave propagation path SL Reflection coefficient R of transverse wave reflection transverse wave SS Reflection coefficient R of longitudinal wave reflection longitudinal wave LL Reflection coefficient R of longitudinal wave reflection transverse wave LS The method comprises the steps of carrying out a first treatment on the surface of the According to the R obtained SL 、R SS 、R LL 、R LS Drawing a sound field distribution diagram of the defect reflected wave; the sound field distribution diagram of the defect reflected wave comprises a defect reflected longitudinal wave, a defect reflected longitudinal wave-to-transverse wave, a defect reflected transverse wave and a defect reflected transverse wave-to-longitudinal wave;
the incident wave modes comprise transverse wave S waves and longitudinal wave L waves, and the material parameters comprise density, young 'S modulus, poisson' S ratio and thermal expansion coefficient.
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