Cable lead seal crack evaluation method and system based on subharmonic and phased array imaging

Abstract

The invention discloses a subharmonic and phased array imaging-based cable lead seal crack evaluation method and system. The method comprises the following steps: an ultrasonic transmitter generates strong ultrasonic waves to generate fundamental frequency and subharmonic waves and injects the fundamental frequency and subharmonic waves into a cable lead seal; a phased array sensor is adopted to receive ultrasonic waves scattered back by the cable lead sealing material, and beam forming and focusing are carried out according to a delay rule; performing digital filtering processing on the focused ultrasonic wave based on a band-pass filter, and separating fundamental frequency and subharmonic components; imaging the fundamental frequency component and the subharmonic component respectively, indicating an opening part of the crack by using a fundamental frequency image, and indicating a closed part of the crack by using a subharmonic image; and measuring the depth of the crack by comparing the fundamental frequency image with the subharmonic image. According to the invention, the characteristic that the subharmonic only scatters at the closed crack is utilized, so that the closed crack defect of the lead seal of the high-voltage cable can be detected, and the depth of the closed crack in the thickness direction of the lead seal of the cable can be measured.

Description

Technical Field

[0001] The invention belongs to the technical field of cable seal defect detection, and relates to a cable seal crack assessment method and system based on subharmonic and phased array imaging. Background Art

[0002] High-voltage cable terminal seals are an important part of cable accessories. Due to poor installation quality or external forces during operation, there may be pore defects or cracks inside, which pose a hidden danger to the safe operation of the power grid.

[0003] In order to eliminate the hidden dangers of lead seal defects to the safe operation of the power grid, a study has proposed a method for detecting lead seal defects using an ultrasonic phased array. This method obtains information about the inside of the lead seal by collecting the fundamental amplitude information of the ultrasonic echo in the scatterer. However, the above method can only be used to detect pore defects and open crack defects, and cannot effectively detect closed crack defects. If closed cracks are underestimated and ignored, catastrophic accidents may occur, because closed cracks may be expanded by huge external stresses, thereby developing into electrical breakdown failures. Summary of the invention

[0004] In order to solve the deficiencies in the prior art, the present invention provides a cable seal crack assessment method and system based on subharmonics and phased array imaging. By utilizing the characteristic that subharmonics are scattered only at closed cracks, the closed crack defects of high-voltage cable lead seals can be detected, and the closed crack depth in the cable seal thickness direction can be measured.

[0005] The present invention adopts the following technical solution.

[0006] The first aspect of the present invention provides a cable seal crack assessment method based on subharmonic and phased array imaging, comprising:

[0007] The ultrasonic transmitter generates strong ultrasonic waves to generate fundamental frequency and subharmonics and inject them into the cable seal;

[0008] A phased array sensor is used to receive the ultrasonic waves scattered back by the cable sealing material, and beam forming and focusing are performed according to the delay law;

[0009] The focused ultrasonic wave is digitally filtered based on a bandpass filter to separate the fundamental frequency and subharmonic components;

[0010] The fundamental frequency and subharmonic components are imaged separately, the fundamental frequency image is used to indicate the opening part of the crack, and the subharmonic image is used to indicate the closed part of the crack;

[0011] By comparing the fundamental frequency image and the subharmonic image, the crack depth is measured.

[0012] Preferably, the ultrasonic transmitter is a LiNbO3 single crystal transmitter, which is used to generate a strong ultrasonic signal with a frequency of 7 MHz.

[0013] Preferably, the phased array sensor determines the layout and number of array elements according to the size of the detection object and the required resolution; matches the frequency response range of the sensor according to the acoustic characteristics of the detected material; adjusts the focus depth according to the possible location of the crack; synchronizes the transmission and reception signals of all sensor elements based on timing circuits and control logic; uses a coupling medium that does not contain bubbles or other impurities to establish acoustic contact with the detected material; uses digital filters and other signal processing techniques to reduce noise and interference and improve the signal-to-noise ratio; and uses a high-speed data acquisition system to capture the large amount of data generated by the phased array sensor.

[0014] Preferably, the beamforming and focusing are performed according to the delay law, specifically as follows:

[0015] The delay time difference Δt between adjacent sensors is calculated according to the delay law, and beamforming and focusing are performed according to the delay time difference Δt, where the calculation formula of the delay time difference Δt is:

[0016]

[0017] Among them, t m ,t m-1 is the delay time of the mth and m-1th sensors, F is the distance from the center of the phased array sensor to the focal point, t0 is a constant, C is the wave speed, θ is the sound beam deflection angle, M is the number of sensors, and d is the array element spacing.

[0018] Preferably, the frequency response of the bandpass filter is:

[0019]

[0020] Where: f low represents the lower limit frequency of the passband, f high represents the upper frequency limit of the passband;

[0021] When used to separate the fundamental frequency component, [f low ,f high ]=[f 0,low ,f 0,high ]=[f0-Δf,f0+Δf];

[0022] When used to separate subharmonic components, [f low ,f high ]=[f i,low ,f i,high ]=[f i -Δf i ,f i +Δfi ];

[0023] f is the signal frequency; ω is the normalized angular frequency;

[0024] f 0,low ,f 0,high Indicates the lower and upper frequency limits of the passband when used to separate the fundamental frequency component;

[0025] f i,low ,f i,high Indicates the lower and upper frequency limits of the passband when used to separate subharmonic components;

[0026] f0 is the fundamental frequency, Δf is the fundamental frequency bandwidth, f i is the subharmonic frequency, Δf i is the subharmonic bandwidth.

[0027] Preferably, the frequency response of the bandpass filter is defined by a discrete Fourier transform of an impulse response h[n], as follows:

[0028]

[0029] Where: h[n] is the impulse response of the filter, n is the index of the filter sample in the time domain, and the length is N.

[0030] Preferably, the impulse response is:

[0031] h[n]=h ideal [n]·w[n]

[0032]

[0033] Among them, h ideal [n] is the ideal filter impulse response; ω c is the cutoff angular frequency; w[n] is the window function.

[0034] Preferably, the window function is:

[0035]

[0036] Where: I0(·) is the zero-order modified Bessel function, and β is the parameter that controls the main lobe width and side lobe attenuation.

[0037] The second aspect of the present invention provides a cable seal crack assessment system based on subharmonic and phased array imaging, comprising:

[0038] Ultrasonic transmitting module, used for ultrasonic transmitter to generate strong ultrasonic waves to generate subharmonics and inject into cable seals;

[0039] An ultrasonic receiving module is used to receive ultrasonic waves scattered back by the cable sealing material using a phased array sensor, and perform beam forming and focusing according to a delay law;

[0040] A filtering processing module is used to perform digital filtering processing on the focused ultrasonic wave based on a bandpass filter to separate the fundamental frequency and subharmonic components;

[0041] An imaging module, used to image the fundamental frequency and subharmonic components respectively, using the fundamental frequency image to indicate the opening part of the crack, and using the subharmonic image to indicate the closed part of the crack;

[0042] The depth measurement module is used to measure the depth of cracks by comparing the fundamental frequency image and the subharmonic image.

[0043] A third aspect of the present invention provides a terminal, comprising a processor and a storage medium; the storage medium is used to store instructions; the processor is used to operate according to the instructions to execute the steps of the method.

[0044] A fourth aspect of the present invention provides a computer-readable storage medium having a computer program stored thereon, which implements the steps of the method when executed by a processor.

[0045] Compared with the prior art, the beneficial effects of the present invention include at least:

[0046] The bandpass filter designed and adopted in the present invention can realize free control of transition bandwidth and stopband attenuation. By adjusting β, higher stopband attenuation or narrower transition band can be achieved. The fundamental frequency and subharmonic components can be accurately separated and then accurately imaged. The detection accuracy of crack defects is improved, and the 20mm error existing in traditional methods in extreme cases is reduced to 1mm.

[0047] The present invention is based on the phenomenon that subharmonics are only generated in closed crack defects, and realizes accurate measurement of closed crack defects that cannot be detected by fundamental wave ultrasound.

[0048] The present invention uses fundamental frequency and subharmonic images simultaneously, can evaluate the opening and closed parts of the crack, provides more comprehensive crack characteristics, can more accurately identify and evaluate cracks, helps to improve the safety of the structure, and prevent possible catastrophic accidents. BRIEF DESCRIPTION OF THE DRAWINGS

[0049] Figure 1 It is a geometrical schematic diagram of setting the delay law of the present invention to achieve accurate beam forming and focusing;

[0050] Figure 2 It is a basic flow chart of the cable seal crack assessment method based on subharmonic and phased array imaging of the present invention;

[0051] Figure 3 It is a schematic diagram of the principle of the cable seal crack assessment method based on subharmonic and phased array imaging of the present invention. DETAILED DESCRIPTION

[0052] In order to make the purpose, technical scheme and advantages of the present invention clearer, the technical scheme of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. The embodiments described in this application are only part of the embodiments of the present invention, not all of them. Based on the spirit of the present invention, other embodiments obtained by ordinary technicians in this field without creative work are all within the scope of protection of the present invention.

[0053] Embodiment 1 of the present invention provides a cable seal crack assessment imaging method based on subharmonics and phased array algorithm, such as Figure 1 As shown, the following steps are included:

[0054] Step 1: The ultrasonic transmitter generates strong ultrasonic waves to generate fundamental frequency and subharmonics and inject them into the cable seal;

[0055] Further preferably, a LiNbO3 single crystal transmitter is used to generate strong ultrasonic waves. LiNbO3 single crystal transmitter refers to an ultrasonic transmitter made of lithium niobate (LiNbO3). This material has good piezoelectric effect and high frequency response characteristics, and can generate high-intensity ultrasonic waves; the single crystal structure gives it high sensitivity and resolution; sufficient sound wave intensity will generate subharmonic waves when interacting with cracks in the material.

[0056] Step 2: Use a phased array sensor to receive the ultrasonic waves scattered back by the cable sealing material, and perform beam forming and focusing according to the delay law;

[0057] Further preferably, using a phased array sensor to receive ultrasonic waves and focusing them according to a delay law comprises:

[0058] 1) Ensure that the layout and number of phased array sensor elements are appropriate for the size of the object being detected and the required resolution;

[0059] 2) Ensure that the frequency response range of the sensor matches the acoustic properties of the material being tested;

[0060] 3) Correctly set the delay law to achieve accurate beamforming and focusing;

[0061] 4) Adjust the focus depth to match the possible location of the crack;

[0062] 5) Ensure that the transmit and receive signals of all sensor elements are synchronized to avoid phase errors;

[0063] 6) Use appropriate timing circuits and control logic to synchronize sensor elements;

[0064] 7) Use appropriate coupling media to ensure good acoustic contact between the sensor and the material being tested;

[0065] 8) Ensure that the coupling medium does not contain bubbles or other impurities;

[0066] 9) Use digital filters and other signal processing techniques to reduce noise and interference and improve the signal-to-noise ratio (SNR);

[0067] 10) Use high-speed data acquisition systems to capture the large amounts of data generated by phased array sensors.

[0068] Correctly set the delay law for accurate beamforming and focusing:

[0069] Assume that the ultrasonic phased array has a total of M array elements (sensors). Figure 1 From the geometric relationship in, we can see that:

[0070]

[0071] Where F is the distance from the center of the array to the focal point, t m is the delay time of the mth array element, t0 is to ensure t n is a constant set to be positive, C is the wave velocity, θ is the sound beam deflection angle, and d is the array element spacing.

[0072] From equation (1), the focusing delay time of the mth array element can be derived:

[0073]

[0074] Substituting m=0 into equation (2), we can obtain the start-up time of the first array element:

[0075]

[0076] Substituting equation (3) into equation (2), we can obtain the specific expression of the focusing delay time of the mth array element:

[0077]

[0078] Then the delay time difference of the mth array element relative to the m-1th array element is:

[0079]

[0080] The ultrasonic phased array deflects and focuses by controlling the above-mentioned delay time difference Δt to achieve beam forming and focusing.

[0081] Step 3: Perform digital filtering on the focused ultrasonic wave to separate the fundamental frequency and subharmonic components;

[0082] Further preferably, the received wave is digitally filtered to separate the fundamental frequency and subharmonic components, including:

[0083] 1) Design an appropriate bandpass filter based on the frequency characteristics of the fundamental frequency and subharmonics. Ensure that the filter can clearly distinguish the fundamental frequency and subharmonics while minimizing the crossover between frequency bands;

[0084] 2) Accurately identify and understand the frequency range of the fundamental frequency and subharmonics to ensure that the filter cutoff frequency is set correctly;

[0085] If you want to keep the wave signals with frequencies f / 2 and f, you need to choose an appropriate cutoff angular frequency ω c , defines the frequency range that the filter allows to pass.

[0086] First, convert the target frequency to the normalized angular frequency ω = 2πf / f s , where f s Sampling frequency. For example, if the target frequencies are f / 2 = 50 Hz and f = 100 Hz, and the sampling frequency f s =400Hz, then the corresponding angular frequency is ω f / 2=0.25π and ω f =0.5π. In order to retain these two frequencies, ω c Set to be no less than ωf\omega_f, usually slightly larger than ω f , for example c =0.55π.

[0087] Next, using the impulse response formula of an ideal low-pass filter h ideal [n] constructs a filter, where ω c is the designed cutoff frequency, and N is the filter length. In this way, the filter can effectively retain the frequency components of f / 2 and f.

[0088] 3) Determine the filter order to achieve the desired filtering effect while avoiding over-filtering, which may cause phase distortion or ringing effects;

[0089] Where A is the stopband attenuation and Δω is the normalized transition bandwidth.

[0090] 4) Maintain signal integrity during filtering to avoid excessive attenuation of useful signal components;

[0091] 5) Ensure the stability of digital filters and avoid instability or oscillation in numerical calculations;

[0092] 6) Without introducing new noise or distortion, the filter should effectively suppress noise and improve the signal-to-noise ratio.

[0093] Specifically, in order to process the signal, it is usually necessary to retain the signal in a specific frequency range (such as the fundamental frequency and subharmonics) through a bandpass filter, while filtering out other frequency components (such as noise and high-frequency interference).

[0094] Design goals:

[0095] Determine the passband of the filter (the frequency range that the filter allows to pass, mainly covering the fundamental frequency f0 and the frequencies of subharmonics) and the stopband (the frequency range that the filter suppresses, mainly high-frequency signals and low-frequency interference beyond subharmonics).

[0096] Frequency response of a bandpass filter:

[0097]

[0098] Where: f low represents the lower limit frequency of the passband, f high Indicates the upper frequency limit of the passband.

[0099] In ultrasonic applications the fundamental frequency range is

[0100] [f 0,low ,f 0,high ]=[f0-Δf,f0+Δf].

[0101] The subharmonic range is

[0102] [f i,low ,f i,high ]=[f i -Δf i ,f i +Δf i ].

[0103] Where: f0 is the fundamental frequency, Δf is the bandwidth, f i is the subharmonic frequency.

[0104] All passband ranges are combined to determine the overall passband range of the filter.

[0105] The frequency response of the filter is defined by the discrete Fourier transform of the impulse response h[n]:

[0106]

[0107] Where: h[n] is the impulse response of the filter, n is the index of the filter sample in the time domain, length N, and ω is the normalized angular frequency;

[0108] The original filter design method is the raised cosine window method, which controls the impulse response that cuts off the ideal frequency response through the raised cosine window function w[n].

[0109] The ideal filter impulse response is:

[0110]

[0111] Control truncation:

[0112] h[n]=h ideal [n]·w[n]

[0113] Raised cosine window function:

[0114]

[0115] The use of the raised cosine window function to control truncation has the problems of wide transition band, insufficient stopband attenuation, and inflexible frequency response. Therefore, the present invention introduces a new window function to achieve the work of controlling truncation:

[0116]

[0117] Where: I0 is the zero-order modified Bessel function, and β is an important parameter for controlling the main lobe width and side lobe attenuation.

[0118] The above improvements can realize free control of transition bandwidth and stopband attenuation, and higher stopband attenuation or narrower transition band can be achieved by adjusting β.

[0119] Step 4: Imaging the fundamental frequency and subharmonic components respectively, using the fundamental frequency image to indicate the opening part of the crack, and using the subharmonic image to indicate the closed part of the crack;

[0120] Further preferably, the fundamental frequency and subharmonic images are used to indicate the opening part and the closing part of the crack respectively:

[0121] 1) When an ultrasonic wave with a frequency of f acts on an open crack defect, the frequency of the scatterer echo is also f; when an ultrasonic wave with a frequency of f acts on a closed crack defect, the scatterer echo has multiple frequencies, such as f, f / 2, 2f, etc. Based on this, when a wave with a frequency of f acts on a lead seal sample, the received echo contains fundamental waves, subharmonics, and superharmonics, which proves that the lead seal sample contains closed crack defects, otherwise it does not.

[0122] 2) Since subharmonics are mainly generated in the closed part of the crack, they have good selectivity for closed cracks. This selectivity enables subharmonics to indicate the closed part of the crack more effectively.

[0123] 3) The digital filter separates the subharmonics received by the phased array sensor. By comparing the fundamental frequency image with the subharmonic image, the former can only show the crack opening part, while the subharmonic image can provide more accurate information about the crack depth.

[0124] Step 5: By comparing the fundamental frequency image and the subharmonic image, the depth of the crack is measured. From the image, the depth of the crack is the length of the crack in the image.

[0125] A cable seal crack assessment imaging method based on subharmonics and phased array algorithm in this embodiment specifically includes:

[0126] a) System configuration and crack simulation;

[0127] A LiNbO3 single crystal transmitter is prepared to generate a strong ultrasonic signal with a frequency of 7 MHz; a 32-element phased array sensor is configured to receive ultrasonic waves scattered by cracks; a digital filter is set to separate the fundamental frequency and subharmonic components in the received signal; a simulated closed fatigue crack is formed in a 40 mm thick lead block sample, and the crack depth is about 10 mm.

[0128] b) Ultrasonic testing process;

[0129] A three-cycle 7MHz ultrasonic signal is emitted to the sample through a LiNbO3 single crystal transmitter; a phased array sensor is used to receive the ultrasonic signal scattered by the crack; a digital filter is applied to separate the fundamental frequency and subharmonic components; the fundamental frequency component is used to generate an image of the crack opening part; and the subharmonic component is used to generate an image of the crack closing part.

[0130] c) Result analysis and verification;

[0131] The fundamental frequency and subharmonic images are compared to determine the depth and position of the crack; the subharmonic image shows the closed part of the crack, while the fundamental frequency image shows the open part of the crack; the crack depth obtained by the subharmonic phased array imaging method of the present invention is compared with the crack depth directly measured after cutting the sample; the verification results show that the crack depth error measured by the subharmonic phased array imaging method is within 1 mm, which is much higher than the 20 mm error of the traditional method.

[0132] Embodiment 2 of the present invention provides a cable seal crack assessment system based on subharmonic and phased array imaging, comprising:

[0133] Ultrasonic transmitting module, used for ultrasonic transmitter to generate strong ultrasonic waves to generate subharmonics and inject into cable seals;

[0134] An ultrasonic receiving module is used to receive ultrasonic waves scattered back by the cable sealing material using a phased array sensor, and perform beam forming and focusing according to a delay law;

[0135] A filtering processing module is used to perform digital filtering processing on the focused ultrasonic wave based on a bandpass filter to separate the fundamental frequency and subharmonic components;

[0136] An imaging module, used to image the fundamental frequency and subharmonic components respectively, using the fundamental frequency image to indicate the opening part of the crack, and using the subharmonic image to indicate the closed part of the crack;

[0137] The depth measurement module is used to measure the depth of cracks by comparing the fundamental frequency image and the subharmonic image.

[0138] Embodiment 3 of the present invention provides a terminal, including a processor and a storage medium; the storage medium is used to store instructions;

[0139] The processor is configured to operate according to the instructions to execute the steps of the method.

[0140] Embodiment 4 of the present invention provides a computer-readable storage medium having a computer program stored thereon, which implements the steps of the method when executed by a processor.

[0141] Compared with the prior art, the beneficial effects of the present invention include at least:

[0142] The bandpass filter designed and adopted by the present invention can realize free control of transition bandwidth and stopband attenuation. By adjusting β, higher stopband attenuation or narrower transition band can be achieved, thereby improving the detection accuracy of crack defects and reducing the 20mm error existing in traditional methods to 1mm in extreme cases.

[0143] The present invention is based on the phenomenon that subharmonics are only generated in closed crack defects, and realizes accurate measurement of closed crack defects that cannot be detected by fundamental wave ultrasound.

[0144] The present invention uses fundamental frequency and subharmonic images simultaneously, can evaluate the opening and closed parts of the crack, provides more comprehensive crack characteristics, can more accurately identify and evaluate cracks, helps to improve the safety of the structure, and prevent possible catastrophic accidents.

[0145] The present disclosure may be a system, a method and / or a computer program product. The computer program product may include a computer-readable storage medium carrying computer-readable program instructions for causing a processor to implement various aspects of the present disclosure.

[0146] A computer-readable storage medium may be a tangible device that can hold and store instructions used by an instruction execution device. A computer-readable storage medium may be, for example, but not limited to, an electrical storage device, a magnetic storage device, an optical storage device, an electromagnetic storage device, a semiconductor storage device, or any suitable combination of the foregoing. More specific examples of computer-readable storage media (a non-exhaustive list) include: a portable computer disk, a hard disk, a random access memory (RAM), a read-only memory (ROM), an erasable programmable read-only memory (EPROM or flash memory), a static random access memory (SRAM), a portable compact disk read-only memory (CD-ROM), a digital versatile disk (DVD), a memory stick, a floppy disk, a mechanical encoding device, such as a punch card or a raised structure in a groove on which instructions are stored, and any suitable combination of the foregoing. As used herein, a computer-readable storage medium is not to be interpreted as a transient signal per se, such as a radio wave or other freely propagating electromagnetic wave, an electromagnetic wave propagating through a waveguide or other transmission medium (e.g., a light pulse through a fiber optic cable), or an electrical signal transmitted through a wire.

[0147] The computer-readable program instructions described herein can be downloaded from a computer-readable storage medium to each computing / processing device, or downloaded to an external computer or external storage device via a network, such as the Internet, a local area network, a wide area network, and / or a wireless network. The network can include copper transmission cables, optical fiber transmissions, wireless transmissions, routers, firewalls, switches, gateway computers, and / or edge servers. The network adapter card or network interface in each computing / processing device receives the computer-readable program instructions from the network and forwards the computer-readable program instructions for storage in the computer-readable storage medium in each computing / processing device.

[0148] The computer program instructions for performing the operation of the present disclosure may be assembly instructions, instruction set architecture (ISA) instructions, machine instructions, machine-related instructions, microcode, firmware instructions, state setting data, or source code or object code written in any combination of one or more programming languages, including object-oriented programming languages-such as Smalltalk, C++, etc., and conventional procedural programming languages-such as "C" language or similar programming languages. Computer-readable program instructions may be executed completely on a user's computer, partially on a user's computer, as an independent software package, partially on a user's computer, partially on a remote computer, or completely on a remote computer or server. In the case of a remote computer, the remote computer may be connected to the user's computer via any type of network-including a local area network (LAN) or a wide area network (WAN), or may be connected to an external computer (e.g., using an Internet service provider to connect via the Internet). In some embodiments, an electronic circuit, such as a programmable logic circuit, a field programmable gate array (FPGA), or a programmable logic array (PLA) may be personalized by utilizing the state information of a computer-readable program instruction, and the electronic circuit may execute a computer-readable program instruction, thereby realizing various aspects of the present disclosure.

[0149] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention rather than to limit it. Although the present invention has been described in detail with reference to the above embodiments, ordinary technicians in the relevant field should understand that the specific implementation methods of the present invention can still be modified or replaced by equivalents, and any modifications or equivalent replacements that do not depart from the spirit and scope of the present invention should be covered within the scope of protection of the claims of the present invention.

Claims

1. A cable seal crack assessment method based on subharmonic and phased array imaging, characterized in that: include: The ultrasonic transmitter generates strong ultrasonic waves to generate fundamental frequency and subharmonics and inject them into the cable seal; A phased array sensor is used to receive the ultrasonic waves scattered back by the cable sealing material, and beam forming and focusing are performed according to the delay law; The focused ultrasonic wave is digitally filtered based on a bandpass filter to separate the fundamental frequency and subharmonic components; The fundamental frequency and subharmonic components are imaged separately, the fundamental frequency image is used to indicate the opening part of the crack, and the subharmonic image is used to indicate the closed part of the crack; By comparing the fundamental frequency image and the subharmonic image, the crack depth is measured.

2. A cable seal crack assessment method based on subharmonic and phased array imaging according to claim 1, characterized in that: The ultrasonic transmitter is a LiNbO3 single crystal transmitter, which is used to generate a strong ultrasonic signal with a frequency of 7 MHz.

3. The cable seal crack assessment method based on subharmonic and phased array imaging according to claim 1 is characterized in that: The phased array sensor determines the layout and number of array elements according to the size of the detected object and the required resolution; matches the frequency response range of the sensor according to the acoustic characteristics of the detected material; adjusts the focus depth according to the possible location of the crack; and synchronizes the transmission and reception signals of all sensor elements based on timing circuits and control logic; Use a coupling medium that does not contain bubbles or other impurities to make acoustic contact with the material being tested; use digital filters and other signal processing techniques to reduce noise and interference and improve the signal-to-noise ratio; Use high-speed data acquisition systems to capture the large amounts of data generated by phased array sensors.

4. The cable seal crack assessment method based on subharmonic and phased array imaging according to claim 1 is characterized in that: The beam forming and focusing are performed according to the delay law, as follows: The delay time difference Δt between adjacent sensors is calculated according to the delay law, and beamforming and focusing are performed according to the delay time difference Δt, where the calculation formula of the delay time difference Δt is: Among them, t m ,t m-1 is the delay time of the mth and m-1th sensors, F is the distance from the center of the phased array sensor to the focal point, t0 is a constant, C is the wave speed, θ is the sound beam deflection angle, M is the number of sensors, and d is the array element spacing.

5. The cable seal crack assessment method based on subharmonic and phased array imaging according to claim 1 is characterized in that: The frequency response of the bandpass filter is: Where: f low represents the lower limit frequency of the passband, f high represents the upper frequency limit of the passband; When used to separate the fundamental frequency component, [f low ,f high ]=[f 0,low ,f 0,high ]=[f0-Δf,f0+Δf]; When used to separate subharmonic components, [f low ,f high ]=[f i,low ,f i,high ]=[f i -Δf i ,f i +Δf i ]; f is the signal frequency; ω is the normalized angular frequency; f 0,low ,f 0,high Indicates the lower and upper frequency limits of the passband when used to separate the fundamental frequency component; f i,low ,f i,high Indicates the lower and upper frequency limits of the passband when used to separate subharmonic components; f0 is the fundamental frequency, Δf is the fundamental frequency bandwidth, f i is the subharmonic frequency, Δf i is the subharmonic bandwidth.

6. The cable seal crack assessment method based on subharmonic and phased array imaging according to claim 4 is characterized in that: The frequency response of the bandpass filter is defined by the discrete Fourier transform of the impulse response h[n] as follows: Where: h[n] is the impulse response of the filter, n is the index of the filter sample in the time domain, and the length is N.

7. A cable seal crack assessment method based on subharmonic and phased array imaging according to claim 6, characterized in that: The impulse response is: h[n]=h ideal [n]·w[n] Among them, h ideal [n] is the ideal filter impulse response; ω c is the cutoff angular frequency; w[n] is the window function.

8. The cable seal crack assessment method based on subharmonic and phased array imaging according to claim 7 is characterized in that: The window function is: Where: I0(·) is the zero-order modified Bessel function, and β is the parameter that controls the main lobe width and side lobe attenuation.

9. A cable seal crack assessment system based on subharmonic and phased array imaging, using the method according to any one of claims 1 to 8, characterized in that: The system comprises: Ultrasonic transmitting module, used for ultrasonic transmitter to generate strong ultrasonic waves to generate subharmonics and inject into cable seals; An ultrasonic receiving module is used to receive ultrasonic waves scattered back by the cable sealing material using a phased array sensor, and perform beam forming and focusing according to a delay law; A filtering processing module is used to perform digital filtering processing on the focused ultrasonic wave based on a bandpass filter to separate the fundamental frequency and subharmonic components; An imaging module, used to image the fundamental frequency and subharmonic components respectively, using the fundamental frequency image to indicate the opening part of the crack, and using the subharmonic image to indicate the closed part of the crack; The depth measurement module is used to measure the depth of cracks by comparing the fundamental frequency image and the subharmonic image.

10. A terminal comprising a processor and a storage medium; characterized in that: The storage medium is used to store instructions; The processor is configured to operate according to the instructions to execute the steps of the method according to any one of claims 1-8.

11. A computer-readable storage medium having a computer program stored thereon, characterized in that: When the program is executed by a processor, the steps of the method according to any one of claims 1 to 8 are implemented.

Images