An extended phase shift fusion imaging method based on phased array leaky surface waves

Abstract

The present invention discloses an extended phase migration fusion imaging method based on phased array leaky surface waves. First, a phased array leaky surface wave detection system is constructed to generate leaky surface wave echo signals. Secondly, a fast Fourier transform interpolation algorithm is applied to fill zeros in the high-frequency part of the wave number domain of the sound field to achieve interpolation of spatial domain data to improve the lateral resolution of the imaging image. Then, imaging is performed by extended phase migration and virtual source extended phase migration algorithms respectively. Finally, the imaging results of different methods are fused by a custom fusion algorithm. The technical effect of the present invention is that the custom fusion imaging result combines the detection advantages of the two imaging methods at different distances, and the imaging time is reduced compared to the traditional time-domain full-focus imaging. Not only the detection range is increased, but also the detection efficiency and detection quality of the leaky surface wave are improved, providing an effective method for metal surface and near-surface defect detection.

Description

Technical Field

[0001] The invention belongs to the technical field of nondestructive testing, and in particular relates to an extended phase shift fusion imaging method based on phased array leaky surface waves. Background Art

[0002] During the long-term processing and use of mechanical equipment, affected by factors such as load, corrosion, and wear, defects such as pitting, cracks, and pores often occur on the surface or near the surface. These defects can cause stress concentration, reduce fatigue strength, weaken load-bearing capacity, and may even cause parts failure, seriously affecting the service life and normal operation of the equipment. Therefore, metal surface defect detection is of great significance to ensure its service safety.

[0003] Ultrasonic surface wave detection is the most widely used nondestructive testing technology for surface damage of mechanical equipment. It has the advantages of high energy, high sensitivity, and long propagation distance along the surface direction. For example, a Chinese patent with authorization announcement number CN 117129568 B and authorization announcement date January 9, 2024 discloses an ultrasonic surface wave detection method for surface defects of boiler header pipe row butt welds. The invention connects the ultrasonic surface wave probe to the ultrasonic flaw detector through a data cable, and then adsorbs the ultrasonic surface wave probe on the surface of the pipe through two magnetic adsorption rollers, and performs circular motion along the weld on the boiler header pipe row. Finally, the weld is judged by the data on the ultrasonic surface wave probe. However, this study mainly analyzes the characteristics of acoustic wave data, and does not realize defect imaging based on surface waves. In addition, the coupling state of the probe has a great influence on surface wave detection. Moving the probe multiple times and frequent coupling cannot guarantee efficiency. Leaky surface wave detection avoids the above problems and can realize non-contact automated scanning. For example, a Chinese patent with authorization publication number CN111781274B and authorization announcement date July 11, 2023 discloses a leaky surface wave ultrasonic imaging method based on block scanning and attenuation compensation. The invention obtains the attenuation curve of the leaky surface wave by collecting defect wave data at different positions, and then uses a four-axis motion platform to perform block-type leaky surface wave ultrasonic B scanning, and performs attenuation compensation on the detection data, and finally superimposes the attenuation compensated data to obtain ultrasonic imaging data. The technical effect of this method is that it can realize non-contact imaging of surface or near-surface defects of parts and realize large-area detection. However, this study only relies on a single probe to collect data. Since a single probe can only collect data in a single direction at a time, its coverage is limited and the resolution is relatively low. The phased array probe is composed of multiple array elements, which can control different array elements to transmit and receive signals, and can collect data sets for the entire detection area. For example, a Chinese patent with application publication number CN117092218A and application publication date November 21, 2023 discloses a method for ultrasonic phased array leaky surface wave full-focus imaging based on root mean square velocity. The invention uses a phased array full-matrix capture method to scan the surface of the test block and collect full-matrix data, and then uses the root mean square velocity model to calculate the flight time of each array element, and combines the root mean square velocity model with the full-focus imaging algorithm to process and image the full-matrix data. Compared with the previous method research, this method improves the efficiency of data acquisition while ensuring the imaging quality, but this method needs to consider the refraction path, the imaging speed is slow, and due to the waveform conversion and propagation attenuation, the echo amplitude of the leaky surface wave is small, which is not conducive to defect detection and imaging.

[0004] Therefore, in order to solve the above problems, it is urgent to design an extended phase shift fusion imaging method based on phased array leaky surface waves. Ultrasonic phased array can effectively solve the problems of small focusing energy and short detection distance in existing single-chip leaky surface wave detection methods through multi-element focusing. At the same time, the extended phase shift imaging method uses Fourier transform to realize frequency domain ultrasonic imaging, avoiding a large amount of propagation time calculation and iterative calculation of refraction points. It has significant advantages over traditional time domain full focusing imaging efficiency. The fast Fourier transform interpolation algorithm is used to solve the problem of low lateral resolution of imaging caused by the limitation of phased array element spacing. By fusing extended phase shift with virtual source extended phase shift imaging through a custom fusion imaging method, defect imaging in a larger area can be achieved, and the detection efficiency and quality of leaky surface waves are improved. Summary of the invention

[0005] In order to solve the above problems, the present invention discloses an extended phase migration fusion imaging method based on phased array leaky surface wave. The feature of the present invention is to first construct a phased array leaky surface wave detection system to generate leaky surface wave echo signals, and then apply the fast Fourier transform interpolation algorithm to fill zeros in the high-frequency part of the acoustic field wave number domain, realize the interpolation of spatial domain data to improve the lateral resolution of the imaging image, and then perform imaging by extended phase migration and virtual source extended phase migration algorithms respectively, and finally fuse the imaging results of different methods by a custom fusion algorithm. The imaging result combines the detection advantages of the two imaging methods at different distances, increases the detection range, improves the detection efficiency and detection quality of leaky surface waves, and provides an effective method for metal surface and near-surface defect detection.

[0006] An extended phase shift fusion imaging method based on phased array leaky surface waves, characterized in that it comprises the following steps:

[0007] Step 1: construct a phased array leaky surface wave detection system to generate a leaky surface wave echo signal;

[0008] Step 2: zero-filling of the high-frequency part of the sound field wave number domain based on fast Fourier transform interpolation;

[0009] Step 3: Leaky surface wave extended phase shift imaging;

[0010] Step 4: Leaky surface wave virtual source extended phase shift imaging;

[0011] Step 5: Customized fusion imaging of leaky surface wave extended phase shift and virtual source extended phase shift.

[0012] In the extended phase shift fusion imaging method based on phased array leaky surface waves, the step 1 of constructing a phased array leaky surface wave detection system to generate a leaky surface wave echo signal comprises the following steps:

[0013] Step 1, the phased array leaky surface wave detection system is composed of a multi-channel ultrasonic research system host, an ultrasonic phased array transducer, an industrial computer and a motion control device; the motion control device includes a motion control card, a servo motor and a hand pulse;

[0014] Step 2: Use Snell’s Law Determining the Rayleigh Angle for Water Detection ,in is the propagation speed of longitudinal waves in water, is the propagation velocity of the surface wave inside the workpiece, and the water depth F, sampling frequency and number of sampling points are set; the motion control device is controlled by an industrial computer to make the ultrasonic phased array transducer The angle is immersed in water, and then the ultrasonic detection command is transmitted to the host of the multi-channel ultrasonic research system through the industrial computer. After receiving the command, the ultrasonic phased array transducer is stimulated to generate ultrasonic pulses. The ultrasonic wave propagates through the water to the surface of the workpiece along the angle direction of the ultrasonic phased array transducer and is converted into a surface wave. After the surface wave propagates to the surface defect of the test block, a defect echo is generated, and during the propagation process, the defect echo will continuously "leak" longitudinal waves into the water, which are then collected by the ultrasonic phased array transducer and transmitted to the host of the multi-channel ultrasonic research system for processing, and the collected signal Realize ultrasonic leak surface wave detection;

[0015] In the extended phase shift fusion imaging method based on phased array leaky surface waves, the zero filling of the high frequency part of the acoustic field wave number domain based on fast Fourier transform interpolation in step 2 comprises the following steps:

[0016] Step 1: The acoustic field at z on the surface of the known workpiece is :

[0017]

[0018] Where: is the length in the wave number domain, and the expression of the sound field can be simplified as follows:

[0019]

[0020] Step 2: Create a Sound field: ,when When it is an odd number:

[0021]

[0022] when When it is an even number:

[0023]

[0024] The sound field is obtained after filling the high-frequency part of the wave number domain with zeros. Performing an inverse Fourier transform on it can realize the interpolation of the sound field in the spatial domain.

[0025] The extended phase shift fusion imaging method based on phased array leaky surface waves, the leaky surface wave extended phase shift imaging in step three comprises the following steps:

[0026] Step 1: Set the phased array at The medium interface is located at Where is the propagation distance of ultrasound in water; each array element of the phased array transmits ultrasonic signals in turn, and all array elements receive echo signals each time they are transmitted, and the collected A group of echo signals, forming a full matrix data set ,in is the number of transmitting array elements, is the number of receiving array elements, Represents the echo signal transmitted by m array elements and received by n array elements;

[0027] When the array element m is launched, the surface coordinates of the workpiece The sound field at is:

[0028] , ;

[0029] Where: , is the speed of ultrasonic wave propagation in water; , is the leaky surface wave velocity of ultrasonic wave propagating on the workpiece; is a conjugate complex number;

[0030] Step 2: According to step 2, the emission sound field and receiving sound field The high frequency part of the wave number domain is filled with zeros to realize the fast Fourier transform interpolation and obtain Expand in direction times the sound field and ;

[0031] Step 3: Sound field can be viewed as a function of depth The scatterer in The sound field generated by the emission It can be considered that the phased array is located at depth In The two are related and there is no phase difference. The correlation of the frequency domain sound field after inverse Fourier transform can be expressed as:

[0032] ;

[0033] Perform an inverse Fourier transform to obtain:

[0034] ;

[0035] when When is the focused imaging amplitude when the array element m is used as the transmitting array element:

[0036] ;

[0037] Step 4: The focused imaging amplitudes obtained when each array element is emitted are accumulated to obtain the final imaging amplitude based on the full matrix data:

[0038] ;

[0039] The extended phase shift fusion imaging method based on phased array leaky surface waves, the leaky surface wave virtual source extended phase shift imaging in step 4 comprises the following steps:

[0040] Step 1: The number of array elements that make up the virtual source is , the array element spacing is , the virtual source is located on the center line of the array, and the focal length is , at this time, it can be regarded as a virtual source located at the focal point emitting ultrasonic waves downward, and the number of array elements is A phased array can be generated by exciting different continuous array elements Virtual sources, all virtual sources constitute a virtual array; when collecting data, each virtual source emits ultrasonic signals in turn, and all array elements receive echo signals to obtain Group Dimensional data, where is the sampling length;

[0041] Step 2: When the virtual source When transmitting, all array elements receive and collect Data, two-dimensional Fourier transform of the data, get The receiving sound field at , on the workpiece surface The receiving sound field at is:

[0042] ;

[0043] Step 3: Receive the sound field according to step 2 of Zero padding is performed in the direction to obtain Expand in direction times the receiving sound field , perform two-dimensional inverse Fourier transform to obtain ;

[0044] Step 4: When the sound field Is a virtual source The focused imaging amplitude when used as a transmitting array element:

[0045] ;

[0046] Where: Indicates that the ultrasonic wave comes from the virtual source To the scatterer The propagation time;

[0047] For convenience Calculate the focal length , that is, the virtual source is set at the interface of the medium. Since the process of generating the virtual source by the phased array element also takes time, the propagation time is:

[0048] ;

[0049] Where: is the horizontal coordinate of the location of the virtual source m;

[0050] Step 5: Accumulate the focused imaging amplitude obtained when each virtual source is emitted to obtain the final imaging amplitude. It is expressed as:

[0051] ;

[0052] The extended phase shift fusion imaging method based on phased array leaky surface waves, wherein the customized fusion imaging of leaky surface wave extended phase shift and virtual source extended phase shift in step 5 comprises the following steps:

[0053] Step 1: It is known that the imaging amplitude obtained by the leaky surface wave extended phase shift imaging method in step 3 is The imaging amplitude obtained by the extended phase shift imaging method of the leaky surface wave virtual source in step 4 is , find the difference between the imaging amplitudes corresponding to the two imaging methods:

[0054] ;

[0055] Step 2: Set the threshold , when the difference The absolute value is greater than or equal to the threshold When the fused image takes the maximum value, The absolute value is less than the threshold When , the fused image takes the minimum value, then the imaging amplitude of the fused image is It is expressed as:

[0056] ;

[0057] Step 3: Use the imfilter function to Perform smoothing to obtain smoother fused image amplitude .

[0058] Since the present invention adopts the above technical solution, compared with the prior art, the present application has the following beneficial effects:

[0059] 1. Through leaky surface wave detection, the influence of coupling state of traditional contact detection can be avoided; through ultrasonic phased array multi-element focused imaging, it has better defect characterization ability and higher detection accuracy, which can effectively solve the problems of small focusing energy and short detection distance in the existing single-wafer leaky surface wave detection method; through the extended phase shift imaging method, the Fourier transform is used to realize frequency domain ultrasonic imaging, avoiding a large amount of propagation time calculation, which has significant advantages over the traditional time domain full focusing imaging efficiency, and can greatly improve the detection efficiency while ensuring the detection quality;

[0060] 2. The leaky surface wave signal is filled with zeros in the high-frequency region of the wave number domain through the fast Fourier transform interpolation algorithm to achieve spatial interpolation, which solves the problem of low imaging resolution caused by the limitation of the phased array element spacing, improves the lateral resolution of the imaging image, and realizes the accurate characterization of the shape of smaller defects;

[0061] 3. By fusing the extended phase shift and virtual source extended phase shift imaging results through a customized fusion imaging method, the detection advantages of the two imaging methods at different distances can be integrated, the average defect size error can be reduced, and the detection range can be increased. BRIEF DESCRIPTION OF THE DRAWINGS

[0062] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0063] Figure 1 It is a schematic diagram of a phased array leaky surface wave detection system disclosed in the present invention;

[0064] Figure 2 It is a schematic diagram of extended phase shift imaging based on leaky surface wave detection model disclosed in the present invention;

[0065] Figure 3It is a schematic diagram of virtual source extended phase shift imaging based on an equivalent model disclosed in the present invention;

[0066] Figure 4 It is a schematic diagram of the inspection test block and defect distribution disclosed in the present invention;

[0067] Figure 5 : is a comparison diagram of the method disclosed in the present invention and the traditional full-focus imaging results, wherein: (a) is the traditional full-focus imaging; (b) is the extended phase shift imaging in step three; (c) is the virtual source extended phase shift imaging in step four; (d) is the custom fusion imaging in step five;

[0068] Figure 6 : is a comparison chart of the evaluation indexes of the method disclosed in the present invention and the traditional full-focus imaging effect, wherein: (a) is the imaging amplitude; (b) is the signal-to-noise ratio; (c) is the imaging time;

[0069] Figure 7 It is a flow chart of an extended phase shift fusion imaging method based on phased array leaky surface waves disclosed by the present invention; DETAILED DESCRIPTION

[0070] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0071] Step 1: Construct a phased array leaky surface wave detection system to generate a leaky surface wave echo signal. The specific steps are as follows:

[0072] Step 1: See Figure 1 The phased array leaky surface wave detection system of this example includes: a Vantage64LE multi-channel ultrasonic research system host (Verasonics), a 64-element ultrasonic phased array transducer with a center frequency of 5MHz, an array element spacing of 0.5mm, and an array element width of 0.4mm (Imasonic), a computer with 32G memory and Intel® Core™ i7-8700K CPU @ 3.70GHz, and a motion control device including a three-axis scanning platform, a DMC-1846 motion control card, a servo motor, and a hand pulse; see Figure 4 In this example, the defect test block selected is the phased array standard B-type test block, the material is 45 steel, the diameter of the detection hole is 0.1mm, the detection area is marked with a black frame, and the size is 20×33.5mm;

[0073] Step 2: Propagation speed of longitudinal waves in water m / s, steel test block surface wave propagation velocity m / s, the second critical angle is calculated to be 30.03° according to Snell's law, the water acoustic distance is set to 14mm, and in order to fully ensure that leaky surface waves can be generated during the detection process, the phased array probe is immersed in water at an angle of 31° to the vertical direction, the sampling frequency is set to 62.5MHz, the number of sampling points is set to 4096, and the leaky surface wave signal is obtained. ;

[0074] Step 2: Zero filling of the high frequency part of the sound field wave number domain based on fast Fourier transform interpolation. The specific steps are as follows:

[0075] Step 1: Wavenumber domain length , the acoustic field at z on the workpiece surface is :

[0076]

[0077] The expression of the sound field can be simplified as:

[0078]

[0079] Step 2: Create a sound field with a length of 340: , is an even number, so:

[0080]

[0081] The sound field is obtained after filling the high-frequency part of the wave number domain with zeros. Performing an inverse Fourier transform on it can realize the interpolation of the sound field in the spatial domain.

[0082] Step 3: Leaky surface wave extended phase shift imaging, the specific steps are as follows:

[0083] Step 1: See Figure 2 , the phased array is located at The medium interface is located at Where is the propagation distance of ultrasound in water; each array element of the phased array transmits ultrasonic signals in turn, and all array elements receive echo signals each time they are transmitted, and the collected A group of echo signals, forming a full matrix data set ,in is the number of transmitting array elements, is the number of receiving array elements, Represents the echo signal transmitted by m array elements and received by n array elements;

[0084] When the array element m is launched, the surface coordinates of the workpiece The sound field at is:

[0085] , ;

[0086] Where: ; ; is a conjugate complex number;

[0087] Step 2: According to step 2, the emission sound field and receiving sound field The high frequency part of the wave number domain is filled with zeros to realize the fast Fourier transform interpolation and obtain Expand in direction times the sound field and ;

[0088] Step 3: Sound field can be viewed as a function of depth The scatterer in The sound field generated by the emission It can be considered that the phased array is located at depth In The two are related and there is no phase difference. The correlation of the frequency domain sound field after inverse Fourier transform can be expressed as:

[0089] ;

[0090] Perform an inverse Fourier transform to obtain:

[0091] ;

[0092] when When is the focused imaging amplitude when the array element m is used as the transmitting array element:

[0093] ;

[0094] Step 4: Accumulate the focused imaging amplitudes obtained when the 64 array elements are transmitted to obtain the final imaging amplitude based on the full matrix data:

[0095] ;

[0096] Step 4: Leaky surface wave virtual source extended phase shift imaging, the specific steps are as follows:

[0097] Step 1: See Figure 3 , the number of array elements that make up the virtual source is 5, the virtual source is located on the center line of the array, and the focal length is At this time, it can be regarded as a virtual source located at the focal point emitting ultrasonic waves downward. The phased array with 64 array elements can generate 60 virtual sources by exciting different continuous array elements. All virtual sources constitute a virtual array. When collecting data, each virtual source emits ultrasonic signals in turn, and all array elements receive echo signals to obtain 60 sets of 4094×64 size data.

[0098] Step 2: When the virtual source When transmitting, all array elements receive and collect 4094×64 data. The data is transformed into two-dimensional Fourier transform to obtain The receiving sound field at , on the workpiece surface The receiving sound field at is:

[0099] ;

[0100] Step 3: Receive the sound field according to step 2 of Zero padding is performed in the direction to obtain Expand in direction times the receiving sound field , perform two-dimensional inverse Fourier transform to obtain ;

[0101] Step 4:

[0102] When the sound field Is a virtual source The focused imaging amplitude when used as a transmitting array element:

[0103] ;

[0104] Where: is the horizontal coordinate of the location of the virtual source m;

[0105] Step 5: Accumulate the focused imaging amplitude obtained when each virtual source is emitted to obtain the final imaging amplitude. It is expressed as:

[0106] ;

[0107] Step 5: Customized fusion imaging of leaky surface wave extended phase shift and virtual source extended phase shift. The specific steps are as follows:

[0108] Step 1: It is known that the imaging amplitude obtained by the leaky surface wave extended phase shift imaging method in step 3 is The imaging amplitude obtained by the extended phase shift imaging method of the leaky surface wave virtual source in step 4 is , find the difference between the imaging amplitudes corresponding to the two imaging methods:

[0109] ;

[0110] Step 2: Set the threshold , when the difference When the absolute value of is greater than or equal to 0.05, the fused image takes the maximum value. When the absolute value of is less than 0.05, the fused image takes the minimum value, and the imaging amplitude of the fused image is It is expressed as:

[0111] ;

[0112] Step 3: Use the imfilter function to Perform smoothing to obtain smoother fused image amplitude .

[0113] Comparison of imaging results of the imaging method of steps 3 to 5 in the specific embodiment of the present invention and the conventional full-focus imaging method Figure 5 As shown, it can be seen that the problem of low imaging resolution caused by the limitation of phased array element spacing can be solved by the fast Fourier transform interpolation method, and accurate characterization of the shape of smaller defects can be achieved; the customized fusion imaging method proposed in the present invention can effectively make up for the shortcomings of insufficient far-field detection of extended phase shift imaging and insufficient near-field detection of virtual source extended phase shift, and can effectively increase the detection range compared with traditional full-focus imaging.

[0114] To quantitatively evaluate the imaging results, Figure 6 The comparison chart of the effect evaluation index of leakage surface wave through the method proposed in the present invention and traditional full-focus imaging shows that when the propagation distance of the leakage surface wave is less than 5mm, the defect amplitude of the customized fusion imaging image is greater than that of the virtual source extended phase shift imaging, which is consistent with the extended phase shift imaging; when the propagation distance of the leakage surface wave is greater than 5mm, the defect amplitude of the customized fusion imaging image is greater than that of the extended phase shift imaging, which is consistent with the virtual source extended phase shift imaging. Therefore, the customized fusion imaging method proposed in the present invention can integrate the detection advantages of extended phase shift imaging and virtual source extended phase shift imaging at different distances; in the customized fusion imaging method proposed in the present invention, the average signal-to-noise ratio of defects in holes 1 to 6 is improved by 5.11 dB, 9.48 dB and 4.47dB compared with extended phase shift imaging, virtual source extended phase shift imaging and traditional full-focus imaging, respectively, and the efficiency of customized fusion imaging is improved compared with full-focus imaging.

[0115] In summary, a series of indicators show that the extended phase shift fusion imaging method based on phased array leaky surface waves proposed in the present invention can accurately identify the surface and near-surface defect shapes of the workpiece in the image, which not only effectively increases the detection range, but also improves the detection efficiency and detection quality of leaky surface waves, providing an effective method for metal surface and near-surface defect detection.

[0116] The above is a detailed introduction to the present invention. Specific examples are used in this article to illustrate the principles and implementation methods of the present invention. The description of the above embodiments is only used to help understand the methods and core ideas of the present invention. It should be pointed out that for ordinary technicians in this technical field, without departing from the principles of the present invention, several improvements and modifications can be made to the present invention, and these improvements and modifications also fall within the scope of protection of the claims of the present invention.

Claims

1. An extended phase shift fusion imaging method based on phased array leaky surface waves, characterized in that: The following steps are involved: Step 1: construct a phased array leaky surface wave detection system to generate a leaky surface wave echo signal; Step 2: zero-filling the high-frequency part of the sound field wave number domain based on fast Fourier transform interpolation, characterized in that the specific steps of the step include: Step 1: The acoustic field at z on the surface of the known workpiece is : ； Where: is the length in the wave number domain, and the expression of the sound field can be simplified as follows: ； Step 2: Create a Sound field: ,when When it is an odd number: ； when When it is an even number: ； The sound field is obtained by filling the high frequency part of the wave number domain with zeros. The inverse Fourier transform can realize the interpolation of the sound field in the spatial domain. Step 3: leaky surface wave extended phase shift imaging, characterized in that the specific steps of the step include: Step 1: Set the phased array at The medium interface is located at Where is the propagation distance of ultrasound in water; each array element of the phased array transmits ultrasonic signals in turn, and all array elements receive echo signals each time they are transmitted, and the collected A group of echo signals, forming a full matrix data set ,in is the number of transmitting array elements, is the number of receiving array elements, Represents the echo signal transmitted by m array elements and received by n array elements; When the array element m is launched, the surface coordinates of the workpiece The sound field at is: ， ； Where: , is the speed of ultrasonic wave propagation in water; , is the leaky surface wave velocity of ultrasonic wave propagating on the workpiece; is a conjugate complex number; Step 2: According to step 2, the emission sound field and receiving sound field The high frequency part of the wave number domain is filled with zeros to realize the fast Fourier transform interpolation and obtain Expand in direction times the sound field and ; Step 3: Sound field can be viewed as a function of depth The scatterer in The sound field generated by the emission It can be considered that the phased array is located at depth In The two are related and there is no phase difference. The correlation of the frequency domain sound field after inverse Fourier transform can be expressed as: ； Perform an inverse Fourier transform to obtain: ； when When is the focused imaging amplitude when the array element m is used as the transmitting array element: ； Step 4: The focused imaging amplitudes obtained when each array element is emitted are accumulated to obtain the final imaging amplitude based on the full matrix data: ； Step 4: Leaky surface wave virtual source extended phase shift imaging, characterized in that the specific steps of the step include: Step 1: The number of array elements that make up the virtual source is , the array element spacing is , the virtual source is located on the center line of the array, and the focal length is , at this time, it can be regarded as a virtual source located at the focal point emitting ultrasonic waves downward, and the number of array elements is A phased array can be generated by exciting different continuous array elements Virtual sources, all virtual sources constitute a virtual array; when collecting data, each virtual source emits ultrasonic signals in turn, and all array elements receive echo signals to obtain Group Dimensional data, where is the sampling length; Step 2: When the virtual source When transmitting, all array elements receive and collect Data, two-dimensional Fourier transform of the data, get The receiving sound field at , on the workpiece surface The receiving sound field at is: ； Step 3: Receive the sound field according to step 2 of Zero padding is performed in the direction to obtain Expand in direction times the receiving sound field , perform two-dimensional inverse Fourier transform to obtain ; Step 4: When the sound field Is a virtual source The focused imaging amplitude when used as a transmitting array element: ； Where: Indicates that the ultrasonic wave comes from the virtual source To the scatterer The propagation time; For convenience Calculate the focal length , that is, the virtual source is set at the interface of the medium. Since the process of generating the virtual source by the phased array element also takes time, the propagation time is: ； Where: is the horizontal coordinate of the location of the virtual source m; Step 5: Accumulate the focused imaging amplitude obtained when each virtual source is emitted to obtain the final imaging amplitude. It is expressed as: ； Step 5: Customized fusion imaging of leaky surface wave extended phase migration and virtual source extended phase migration, characterized in that the specific steps of the step include: Step 1: It is known that the imaging amplitude obtained by the leaky surface wave extended phase shift imaging method in step 3 is The imaging amplitude obtained by the extended phase shift imaging method of the leaky surface wave virtual source in step 4 is , find the difference between the imaging amplitudes corresponding to the two imaging methods: ； Step 2: Set the threshold , when the difference The absolute value is greater than or equal to the threshold When the fused image takes the maximum value, The absolute value is less than the threshold When , the fused image takes the minimum value, then the imaging amplitude of the fused image is It is expressed as: ； Step 3: Use the imfilter function to Perform smoothing to obtain smoother fused image amplitude .

Images