Fresnel coil electromagnetic ultrasonic transducer for defect and stress detection and detection method

Abstract

The application discloses a Fresnel coil electromagnetic ultrasonic transducer for defect and stress detection and a detection method, and belongs to the technical field of ultrasonic flaw detection and stress detection.The application solves the problems of low detection efficiency, poor precision and corrosion on the surface of large-scale precision rotary equipment caused by the existing ultrasonic method.The application comprises a cover, a shell and a printed circuit board which are sequentially fixed from top to bottom, wherein the upper surface of the printed circuit board is processed with a Fresnel annular coil, a permanent magnet is arranged in the shell, and the permanent magnet is oppositely arranged with the Fresnel annular coil.The application realizes ultrasonic focusing through the Fresnel coil, improves the signal strength, narrows the detection area, and thus improves the spatial resolution of stress detection.Meanwhile, the electromagnetic ultrasonic technology does not cause corrosion on the surface of the large-scale precision rotary equipment to be detected, does not need to smear a coupling agent, and effectively improves the detection efficiency.

Description

TECHNICAL FIELD

[0001] The present application relates to a Fresnel coil electromagnetic ultrasonic transducer for defect and stress detection and a detection method, and belongs to the technical field of ultrasonic flaw detection and stress detection. BACKGROUND

[0002] Large-scale precision rotating equipment such as an aero-engine or a gas turbine is assembled by multiple rotors. Frequent starting, accelerating, decelerating and stopping of the engine cause high and low cycle fatigue and mechanical friction of the rotors under the action of alternating stress and torsional stress and other cyclic stresses. In such a harsh working environment, the effect of every tiny defect in the rotor is amplified, leading to imbalance and even failure of the entire engine rotor system, and causing aviation accidents. At the same time, the assembly quality of the rotors has a great influence on the performance of the aero-engine or gas turbine. In the assembly process, if there is unevenness of contact stress at the rotor connecting interface, the deformation amount generated by the aero-engine or gas turbine at high speed will be uneven, and the imbalance of the rotor will change greatly, eventually leading to vibration of the aero-engine or gas turbine during operation. More than 90% of the faults of a turbofan aero-engine are caused by vibration, which is one of the reasons why the aero-engine needs to be overhauled after several hundred hours of operation. In addition, the rotor is in a high-speed rotating state of tens of thousands of revolutions per minute, and is also subjected to thermal shock from high-temperature, high-pressure and highly corrosive gas in the turbine chamber at a temperature of up to 2000 degrees Celsius. Therefore, in view of the diversity of faults and high failure rate of large-scale precision rotating equipment such as an aero-engine or a gas turbine, it has been a hot issue in the aviation industry to research high-precision and reliable non-destructive defect and stress detection technology. The existing defect and stress detection methods have the following problems:

[0003] 1. The existing defect detection method (water immersion focusing method) has great environmental restrictions, complex water immersion structure, low detection efficiency, and can damage the surface of the large-scale precision rotating equipment. This method needs to be operated in water, which has great environmental restrictions. Water immersion focusing requires an additional water tank or pool for immersion, which increases the complexity and cost of operation. In addition, water immersion may cause corrosion or contamination of the surface of the large-scale precision rotating equipment. Ultrasonic waves propagating in water can be affected by water absorption and scattering, which can cause signal attenuation and distortion, thereby reducing the accuracy and efficiency of detection.

[0004] 2. The existing stress measurement method has low detection precision and spatial resolution, and cannot reflect the real contact situation. Currently, there are two methods to measure the contact stress. One method is to set a pressure-sensitive film or apply red powder on the contact interface of large-scale precision rotary equipment. The contact situation is judged by observing the changes of the pressure-sensitive film or red powder after assembly, and then the contact stress is calculated. This method changes the contact state of the interface, and has low measurement precision. The other method is ultrasonic method, which is divided into water immersion focusing method and non-water immersion non-focusing method. Both methods will damage the surface of the measured part. The difference is that the spatial resolution of the non-water immersion non-focusing method is low. It is difficult to reflect the stress state of small area.

[0005] 3. The coupling agent damages the surface. Traditional ultrasonic technology usually uses contact transducer. In order to ensure high sensitivity and reliability, various ultrasonic coupling agents are usually used. Ultrasonic wave needs a certain transition time when passing through the coupling agent, and interference harmonic wave is generated, which brings instability to the detection. The use of coupling agent increases the additional workload, resulting in low detection efficiency. More seriously, it will cause certain corrosion and damage to the surface of large-scale precision rotary equipment. Therefore, the traditional ultrasonic technology is limited in practical application.

[0006] 4. Currently, the defect and stress detection of large-scale rotary equipment usually uses two sets of equipment to detect defects and stress respectively, which not only increases the initial investment, but also increases the long-term operating cost. The operator needs to be familiar with the operation process of the two kinds of equipment. The measured parts may need to be clamped and adjusted several times during the detection process, which prolongs the detection period. Since the two sets of equipment may come from different manufacturers, different technical standards and data processing methods are adopted, and the detection results may be inconsistent. Such inconsistency may affect the accuracy of data analysis and decision-making. SUMMARY

[0007] The present application is to solve the problems of low detection efficiency, poor precision and corrosion to the surface of large-scale precision rotary equipment in the existing ultrasonic method, and further provides a Fresnel coil electromagnetic ultrasonic transducer for defect and stress detection and a detection method.

[0008] The technical scheme adopted by the present application to solve the above technical problems is:

[0009] A Fresnel coil electromagnetic ultrasonic transducer for defect and stress detection, comprising a cover, a shell and a printed circuit board which are sequentially fixed from top to bottom, wherein the upper surface of the printed circuit board is processed with a Fresnel annular coil, a permanent magnet is arranged inside the shell, and the permanent magnet is arranged vertically opposite to the Fresnel annular coil.

[0010] Further, the Fresnel annular coil comprises a plurality of concentric circular coils.

[0011] Further, the radius r of each circle in the Fresnel annular coil n The following formula is obtained:

[0012]

[0013] Wherein, F0 is focal length, λ is ultrasonic wavelength, n is the number of coil rings, n is odd.

[0014] Further, the shell comprises an upper shell and a lower shell arranged in a convex shape, a through hole is vertically formed in the shell, the permanent magnet is sleeved in the through hole, the cover is fixedly connected with the upper shell, and the printed circuit board is fixedly connected with the lower shell.

[0015] Further, the permanent magnet is arranged at the same height as the shell.

[0016] Further, the cover and the shell are fixedly connected through a plurality of first screws.

[0017] Further, the shell and the printed circuit board are fixedly connected through a plurality of second screws.

[0018] A detection method of the above-mentioned Fresnel coil electromagnetic ultrasonic transducer, comprising the following steps:

[0019] Step one: positioning the Fresnel coil electromagnetic ultrasonic transducer to the detection position L(i) of the large-scale precision rotary equipment;

[0020] Step two: passing current through the Fresnel annular coil to generate ultrasonic waves inside the large-scale precision rotary equipment and focus them on the focal point area;

[0021] Step three: the Fresnel coil electromagnetic ultrasonic transducer collects the reflected ultrasonic wave signal S(i) inside the large-scale precision rotary equipment;

[0022] Step four: analyzing and extracting the waveform index W(i) of the ultrasonic wave signal S(i), judging whether there is a defect at the current detection position according to the waveform index W(i), if there is, according to the corresponding relationship between the waveform index W(i) and the defect characteristic D(i), obtaining the specific information of the defect;

[0023]

[0024] Wherein, N is the sampling point number of the ultrasonic wave signal S(i), j takes 1, 2, …, N;

[0025] Step five: analyzing and extracting the propagation time difference Δt(i) of the ultrasonic wave signal S(i), and according to the corresponding relationship between the propagation time difference Δt(i) and the stress σ(i), obtaining the specific information of the stress at the detection position L(i);

[0026] Step six: repeat steps one to five, sequentially detect defects and stress information at all positions required to be detected by large precision rotary equipment, and draw a defect distribution map and a stress field cloud chart.

[0027] Further, in step four, the correspondence between the waveform index W(i) and the defect feature D(i) is:

[0028] D(i) = F(W(i))

[0029] Where F(·) represents the function between the waveform index W(i) and the defect feature D(i), which is obtained through calibration experiments.

[0030] Further, in step five, the correspondence between the propagation time difference Δt(i) and the stress σ(i) is obtained through calibration experiments:

[0031] σ(i) = K·Δt(i)

[0032] Where K is the stress coefficient.

[0033] Compared with the prior art, the present application has the following effects:

[0034] The defect and stress detection Fresnel coil electromagnetic ultrasonic transducer of the present application is printed on the upper surface of the printed circuit board, that is, the excitation coil in the form of Fresnel ring structure and the coil manufacturing technology of the printed circuit are used, the excitation coil in the form of Fresnel ring structure is an improved version of the runway-shaped coil, which has the characteristics of exciting and receiving body waves as the runway-shaped coil, and has the characteristics of focusing ultrasonic waves as the Fresnel ring, so that the sound waves are focused in the focal region, the signal strength is improved and the detection area is reduced, thereby improving the stress detection precision and spatial resolution.

[0035] The defect and stress detection Fresnel coil electromagnetic ultrasonic transducer of the present application adopts an integrated mounting method, the permanent magnet and the printed circuit board with the Fresnel ring-shaped coil are mounted together to form a whole. The traditional electromagnetic ultrasonic transducer is mostly separated, which is more complex to operate in actual application, and is easy to be accidentally touched, thereby causing the relative position of the permanent magnet and the excitation coil to change, thereby causing measurement error. The printed circuit coil manufacturing technology adopted by the present application has the optimal comprehensive characteristics, can reduce the coil resistance, is easy to manufacture, has higher dimensional accuracy, is easy to use, makes the structure of the electromagnetic ultrasonic transducer more compact, and can adapt to more complex measurement environments.

[0036] The defect and stress detection Fresnel coil electromagnetic ultrasonic transducer of the application adopts electromagnetic ultrasonic technology without coupling agent. The defect and stress detection Fresnel coil electromagnetic ultrasonic transducer of the application can directly excite ultrasonic waves on the surface to be measured of the large precision rotary equipment, and the coupling agent is not needed during measurement, so as to avoid the influence of the coupling agent on the detection result, improve the defect and stress detection precision, and meanwhile, the electromagnetic ultrasonic technology will not cause corrosion to the surface to be measured of the large precision rotary equipment, and the coupling agent is not needed to be applied, so as to effectively improve the detection efficiency. BRIEF DESCRIPTION OF DRAWINGS

[0037] Fig. 1 It is an explosion schematic view of the defect and stress detection Fresnel coil electromagnetic ultrasonic transducer of the application.

[0038] Fig. 2 It is a three-dimensional structure schematic view of the defect and stress detection Fresnel coil electromagnetic ultrasonic transducer of the application.

[0039] Fig. 3 It is an application position schematic view of the defect and stress detection Fresnel coil electromagnetic ultrasonic transducer of the application applied to the large precision rotary equipment.

[0040] In the drawings:

[0041] 1, cover; 2, shell; 21, upper shell; 22, lower shell; 23, through hole; 3, printed circuit board; 4, Fresnel annular coil; 5, permanent magnet; 6, first screw; 7, second screw; 8, Fresnel coil electromagnetic ultrasonic transducer; 9, large precision rotary equipment. DETAILED DESCRIPTION

[0042] Specific implementation one: combination Figs. 1-3 It is obvious that the described implementation is only a part of the implementation of the application, but not all the implementation. Based on the implementation of the application, all the other implementation obtained by the person skilled in the art without creative labor belongs to the protection range of the application.

[0043] It should be noted that the description of the application about "front", "back", "left", "right", "inner", "outer", "left side", "right side", "upper part", "lower part", "top", "bottom" and the like is defined based on the position or relationship shown in the drawings, and is only for the convenience of describing the application and simplifying the description, and is not intended to indicate or imply that the described structure must be constructed and operated in a particular position, therefore, it cannot be understood as a limitation to the application. In the description of the application, the meaning of "multiple" is two or more, unless otherwise specifically limited.

[0044] In the description of the present application, unless otherwise explicitly specified and limited, the terms "mounting", "connecting", "connection" should be understood in a broad sense, for example, can be fixed connection, can also be detachable connection, or integrally connected; can be directly connected, or indirectly connected through an intermediate medium, can be the internal communication of two elements. For those skilled in the art, the specific meaning of the above terms in the present application can be understood according to the specific circumstances.

[0045] A defect and stress detection Fresnel coil electromagnetic ultrasonic transducer, comprising a cover 1, a shell 2 and a printed circuit board 3 which are sequentially and fixedly connected from top to bottom, wherein the upper surface of the printed circuit board 3 is processed with a Fresnel annular coil 4, a permanent magnet 5 is arranged inside the shell 2, and the permanent magnet 5 is arranged vertically opposite to the Fresnel annular coil 4.

[0046] The Fresnel annular coil 4 refers to the structure form of the coil, which is not a single structure, that is, the Fresnel annular coil 4 and the printed circuit board 3 are an integral whole, which are made by the coil manufacturing technology of the printed circuit.

[0047] The permanent magnet 5 is packaged in the shell 2 through the cover 1 and the printed circuit board 3.

[0048] The permanent magnet 5 is arranged vertically opposite to the Fresnel annular coil 4, so that the center of the permanent magnet 5 coincides with the center of the Fresnel annular coil 4. A bias magnetic field perpendicular to the surface to be detected of the large precision rotary equipment is generated by the permanent magnet 5.

[0049] When detecting defects and stress on the surface of the large precision rotary equipment or other devices, the defect and stress detection Fresnel coil electromagnetic ultrasonic transducer of the present application is placed on the surface to be detected. The Fresnel annular coil 4 is excited by a pulse or alternating current source to generate an alternating electromagnetic field on the surface to be detected of the large precision rotary equipment, and according to the Biot-Savart law, eddy current is induced, which is subjected to Lorentz force in the bias magnetic field generated by the permanent magnet 5, causing high-frequency vibration of the particles on the surface to be detected of the large precision rotary equipment, and further generating ultrasonic waves.

[0050] The defect and stress detection Fresnel coil electromagnetic ultrasonic transducer of the present application adopts electromagnetic ultrasonic technology which does not need coupling agent. The defect and stress detection Fresnel coil electromagnetic ultrasonic transducer of the present application can directly excite ultrasonic waves on the surface to be detected of the large precision rotary equipment, and does not need coupling agent during measurement, thereby avoiding the influence of the coupling agent on the detection result, improving the defect and stress detection precision, and at the same time, the electromagnetic ultrasonic technology will not cause corrosion to the surface to be detected of the large precision rotary equipment, and the detection efficiency is effectively improved without applying coupling agent.

[0051] The defect and stress detection Fresnel coil electromagnetic ultrasonic transducer of the application, the upper surface of the printed circuit board 3 is processed with the Fresnel annular coil 4, that is, the excitation coil in the form of Fresnel ring structure and the coil manufacturing technology of the printed circuit, the excitation coil in the form of Fresnel ring structure is an improved version of the runway-shaped coil, which has the characteristics of exciting and receiving body waves like the runway-shaped coil, and has the characteristics of focusing ultrasonic waves like the Fresnel ring, so that the sound waves are focused in the focal point area, the signal strength is improved and the detection area is reduced, thereby improving the stress detection precision and spatial resolution.

[0052] The defect and stress detection Fresnel coil electromagnetic ultrasonic transducer of the application adopts an integrated installation mode, the permanent magnet 5 and the printed circuit board 3 processed with the Fresnel annular coil 4 are installed together to form a whole. The traditional electromagnetic ultrasonic transducer is mostly separated, which is more complex to operate in actual application, and is easy to be accidentally touched, so that the relative position of the permanent magnet and the excitation coil changes, thereby causing measurement error. The printed circuit coil manufacturing technology adopted by the application has the optimal comprehensive characteristics, can reduce the coil resistance, is convenient to manufacture, has higher size accuracy, is easy to use, makes the structure of the electromagnetic ultrasonic transducer more compact, and can adapt to more complex measurement environments.

[0053] Ultrasonic defect detection is a non-destructive testing method that uses ultrasonic waves to detect internal defects of materials. Its principle is based on the interaction between ultrasonic waves and internal defects, interfaces or other inhomogeneities in materials when the ultrasonic waves propagate in the materials, resulting in reflection, scattering, refraction and other phenomena. By analyzing the waveform index W of the echo signal, the defect condition of the material can be detected and evaluated to determine the type, position, shape and size of the material defect.

[0054] Ultrasonic contact stress measurement is a method for measuring internal stress distribution of materials using ultrasonic technology. Its principle is based on the fact that the propagation time of ultrasonic waves changes when they propagate in materials under stress. By measuring the propagation time difference Δt (the difference between the propagation time of ultrasonic waves under stress and the propagation time of ultrasonic echoes under no stress) and the stress coefficient K (the stress coefficient K is determined by calibration test), the distribution of internal stress of the material can be inferred.

[0055] The Fresnel annular coil 4 includes a plurality of concentric circular coils. That is, the Fresnel annular coil 4 is composed of a series of concentric circles, and conforms to the distribution of the Fresnel annular array. In this way,

[0056] The radius r of each circular ring in the Fresnel annular coil 4 n The following formula is obtained:

[0057]

[0058] Wherein, F0 is focal length, λ is ultrasonic wavelength, n is coil ring number, n is odd number. In this way, the width of the circular ring is Δr n And it meets the Fresnel annular array distribution. The area below the Fresnel annular coil 4 generates ultrasonic waves, and the area below the coil gap does not generate ultrasonic waves. The distance from the edge of the two adjacent circular rings of the coil to the focal point differs by half the ultrasonic wavelength (π phase), so that the ultrasonic waves are focused at the focal point (half-wavelength principle).

[0059] The shell 2 comprises an upper shell 21 and a lower shell 22 arranged in a convex shape. A through hole 23 is vertically formed in the shell 2, and the permanent magnet 5 is arranged in the through hole 23. The cover 1 is fixedly connected with the upper shell 21, and the printed circuit board 3 is fixedly connected with the lower shell 22. In this way, the through hole 23 provides a space for the permanent magnet 5, and the upper shell 21 and the lower shell 22 arranged in a convex shape facilitate the arrangement of the connection structure between the cover 1 and the shell 2 and between the shell 2 and the printed circuit board 3. The step formed between the upper shell 21 and the lower shell 22 facilitates the fixed connection between the shell 2 and the printed circuit board 3. Meanwhile, the convex shell 2 can also reduce the weight of the transducer as a whole.

[0060] The permanent magnet 5 is arranged at the same height as the shell 2. In this way, the height of the permanent magnet 5 can be limited by the cover 1 and the printed circuit board 3, so that the permanent magnet 5 always maintains direct contact with the printed circuit board 3 during the measurement process. This further ensures the accuracy of the measurement structure.

[0061] The cover 1 and the shell 2 are fixedly connected by a plurality of first screws 6. In this way, the first screws 6 are short screws, and the number of the first screws 6 is preferably four and is evenly distributed circumferentially around the cover 1. The first screws 6 pass through the cover 1 and are threadedly connected with the upper part of the shell 2.

[0062] The shell 2 and the printed circuit board 3 are fixedly connected by a plurality of second screws 7. In this way, the second screws 7 are long screws, and the number of the second screws 7 is preferably four and is evenly distributed circumferentially around the shell 2. The second screws 7 pass through the shell 2 and are threadedly connected with the printed circuit board 3, or the second screws 7 pass through the shell 2 and the printed circuit board 3 in sequence and are fixedly connected by nuts.

[0063] A detection method of the above-mentioned Fresnel coil electromagnetic ultrasonic transducer, comprising the following steps:

[0064] Step one: positioning the Fresnel coil electromagnetic ultrasonic transducer 8 to the to-be-detected position L(i) of the large-scale precision rotary equipment 9;

[0065] Step two: passing current through the Fresnel annular coil 4 to generate ultrasonic waves inside the large-scale precision rotary equipment 9 and focus them on the focal point area; the focal point area is the area where ultrasonic energy converges, i.e. the measurement area.

[0066] Step three: the Fresnel coil electromagnetic ultrasonic transducer 8 collects the reflected ultrasonic signal S(i) in the large precision rotary equipment 9;

[0067] Step four: analyze and extract the waveform index W(i) of the ultrasonic signal S(i), determine whether there is a defect at the current detection position through the waveform index W(i), if there is, according to the corresponding relationship between the waveform index W(i) and the defect feature D(i), the specific information of the defect is obtained;

[0068]

[0069] Wherein, N is the sampling point number of the ultrasonic signal S(i), j takes 1, 2,..., N; that is, from the first sampling point, until the N sampling points in the ultrasonic signal are calculated.

[0070] Step five: analyze and extract the propagation time difference Δt(i) of the ultrasonic signal S(i), according to the corresponding relationship between the propagation time difference Δt(i) and the stress σ(i), the specific information of the stress at the detection position L(i) is obtained;

[0071] Step six: repeat steps one to five, detect all positions of the large precision rotary equipment required to detect defects and stress information in turn, and draw a defect distribution map and a stress field cloud chart.

[0072] The detection method of the present application can realize the defect and stress detection of large rotary equipment at the same time, effectively reduce the detection and equipment operation cost, improve the detection efficiency, and overcome the problem of poor data consistency and compatibility between multiple sets of equipment.

[0073] In step four, the corresponding relationship between the waveform index W(i) and the defect feature D(i) is:

[0074] D(i)=F(W(i))

[0075] Wherein F(·) represents the function between the waveform index W(i) and the defect feature D(i), which is obtained through calibration experiment.

[0076] In step five, the corresponding relationship between the propagation time difference Δt(i) and the stress σ(i) is obtained through calibration experiment:

[0077] σ(i)=K·Δt(i)

[0078] Wherein, K is the stress coefficient.

[0079] The above merely describes preferred specific embodiments of the present application, but the protection scope of the present application is not limited thereto, and any person skilled in the art, according to the technical solution and inventive concept of the present application, makes equivalent replacement or change within the technical range disclosed by the present application, which should be covered within the protection scope of the present application.

Claims

1. A method of inspection of a defect and stress inspection using a Fresnel coil electro-magnetic acoustic transducer, characterized by: The electromagnetic ultrasonic transducer of the Fresnel coil comprises a cover (1), a shell (2) and a printed circuit board (3) which are sequentially fixed from top to bottom, wherein the upper surface of the printed circuit board (3) is processed with a Fresnel annular coil (4), the shell (2) is internally provided with a permanent magnet (5), and the permanent magnet (5) is arranged in vertical opposition with the Fresnel annular coil (4); the Fresnel annular coil (4) comprises a plurality of concentrically arranged circular annular coils. The detection method comprises the following steps: Step one: positioning the electromagnetic ultrasonic transducer of the Fresnel coil (8) to the to-be-detected position L(i) of the large-scale precision rotary equipment (9); Step two: passing current into the Fresnel annular coil (4) to generate ultrasonic waves inside the large-scale precision rotary equipment (9) and focus them on the focal point area; Step three: the electromagnetic ultrasonic transducer of the Fresnel coil (8) collects the reflected ultrasonic wave signal S(i) inside the large-scale precision rotary equipment (9); Step four: analyzing and extracting the waveform index W(i) of the ultrasonic wave signal S(i), judging whether there is a defect at the current to-be-detected position according to the waveform index W(i), and if there is, obtaining specific defect information according to the corresponding relationship between the waveform index W(i) and the defect feature D(i); wherein N is the sampling point number of the ultrasonic wave signal S(i), and j takes the value of 1, 2, …, N; Step five: analyze and extract the propagation time difference Δt(i) of the ultrasonic signal S(i), and according to the corresponding relationship between the propagation time difference Δt(i) and the stress Step six: repeating steps one to five to detect defects and stress information at all positions required to be detected by the large-scale precision rotary equipment in turn, and drawing a defect distribution diagram and a stress field cloud diagram. (i), obtain the specific information of the stress at the position L(i) to be detected. The shell (2) comprises an upper shell (21) and a lower shell (22) arranged in a convex shape, a through hole (23) is vertically formed in the shell (2), the permanent magnet (5) is fitted in the through hole (23), the cover (1) is fixed with the upper shell (21), and the printed circuit board (3) is fixed with the lower shell (22).

2. The detection method of claim 1, wherein: The radius of each circle in the Fresnel toroidal coil (4) r n This is obtained by the following equation: wherein F 0 is the focal length, The permanent magnet (5) is arranged in the same height as the shell (2). is the ultrasonic wavelength, n is the number of coil turns, n is an odd number.

3. The method of claim 1, wherein: The cover (1) and the shell (2) are fixed by a plurality of first screws (6).

4. The method of claim 1, wherein: The shell (2) and the printed circuit board (3) are fixed by a plurality of second screws (7).

5. The method of claim 1, wherein: In step four, the corresponding relationship between the waveform index W(i) and the defect feature D(i) is:

6. The method of claim 1, wherein: D(i)=F(W(i)) 7. The method of claim 1, wherein: wherein F(·) represents the function between the waveform index W(i) and the defect feature D(i), which is obtained through calibration experiments. In step five, the stress field cloud diagram is obtained by the following formula: wherein K is a stress coefficient.

8. The method of claim 1, wherein: In step five, the corresponding relationship between the time difference of propagation Δt(i) and the stress ​ (i) is obtained through calibration experiments as follows: ​ (i) = K - Δt(i) ​

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