Electromagnetic acoustic emission excitation device based on double trapezoidal coils
By adopting a double trapezoidal coil design and an electromagnetic shielding shell in the electromagnetic acoustic emission excitation device, the problems of large eddy current range and Lorentz force dispersion in traditional devices are solved, significantly improving the signal-to-noise ratio of the acoustic emission signal and enhancing the detection accuracy of pipeline cracks.
Patent Information
- Application Number
- CN202510083845.X
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- Filing Date
- 2025-01-20
- Publication Date
- 2025-04-29
AI Technical Summary
The traditional electromagnetic acoustic emission excitation device induced a large eddy current range on the surface of the subject and dispersed the Lorentz force, which reduces the signal-to-noise ratio of the acoustic emission signal, which is not conducive to the detection of small defects.
The electromagnetic acoustic emission excitation device based on the double trapezoidal coil is adopted. Through the design of the central area and oblique area of the double trapezoidal coil, a focused eddy current is generated and the Lorentz force is enhanced. The magnetic field distribution is controlled in combination with the electromagnetic shielding shell, which significantly improves the signal-to-noise ratio.
The signal-to-noise ratio of the acoustic emission signal is significantly improved, and the detection accuracy and efficiency of potential cracks in pipeline welds are enhanced.
Smart Images
Figure CN119901820A_ABST
Abstract
Description
Technical Field
[0001] The invention relates to the technical field of nondestructive testing, and in particular to an electromagnetic acoustic emission excitation device based on a double trapezoidal coil. Background Art
[0002] As a key link in the five traditional modes of transportation, pipeline transportation plays an indispensable role in the country's economic development and social progress. However, pipelines often face a series of problems such as crack initiation and corrosion during transportation. These problems lead to frequent pipeline leakage and explosion accidents, posing an extremely serious threat to national property and the lives of the people.
[0003] In recent years, acoustic emission technology has been increasingly valued due to its unique advantages. This technology can not only locate defects and conduct in-depth analysis of metal parts in an all-round way, but also has the ability to monitor the test piece without interrupting the operation of the equipment. Therefore, acoustic emission technology has gradually been adopted as an important means of detecting pipeline weld cracks.
[0004] The generation of acoustic emission signals in traditional acoustic emission technology often shows low repeatability and high unpredictability. Electromagnetic acoustic emission technology, with its unique electromagnetic coupling method, realizes non-contact transfer of energy from the electric field to the test piece, which enables the excitation source to directly stimulate the acoustic emission signal without relying on a coupling agent. However, the eddy current range induced by the traditional electromagnetic acoustic emission excitation device on the surface of the test piece is large and the Lorentz force generated is dispersed, which reduces the signal-to-noise ratio of the acoustic emission signal and is not conducive to the detection of small defects. Therefore, how to optimize the design of the excitation coil has become a key issue that needs to be solved urgently. Summary of the invention
[0005] Therefore, the present invention solves the technical problem in the prior art that the eddy current induced by the electromagnetic acoustic emission excitation device on the surface of the test piece is large in range and the generated Lorentz force is dispersed, which reduces the signal-to-noise ratio of the generated acoustic emission signal and is not conducive to the detection of small defects; the electromagnetic acoustic emission excitation device based on a double trapezoidal coil provided by the present invention can generate focused eddy currents and enhance the Lorentz force, significantly improves the signal-to-noise ratio of the generated acoustic emission signal, can enhance the detection accuracy and efficiency of potential cracks in pipeline welds, and has broad application prospects.
[0006] The present invention provides an electromagnetic acoustic emission excitation device based on a double trapezoidal coil, comprising an electromagnetic shielding shell, a rectangular permanent magnet and a double trapezoidal coil arranged above a cracked pipeline specimen; the crack is located on the surface of the cracked pipeline specimen, and the crack is a narrow and long defect with a tip at one end; the double trapezoidal coil is located above the cracked pipeline specimen, and the crack is located below the central axis of the double trapezoidal coil; the double trapezoidal coil includes a central area, an oblique area and an outer area, the current flow direction of the central area is parallel to the expansion direction of the crack, and the projection of the central area on the cracked pipeline specimen covers the crack tip, that is, the crack tip is located at the double trapezoidal coil. The rectangular permanent magnet is located above the double trapezoidal coil and is placed parallel to the crack, and the projection area of the rectangular permanent magnet on the pipe specimen containing the crack covers the projection area of the central area of the double trapezoidal coil on the pipe specimen containing the crack; the electromagnetic shielding shell is located above the pipe specimen containing the crack, and the electromagnetic shielding shell is located outside the rectangular permanent magnet, and the projection area of the electromagnetic shielding shell on the pipe specimen containing the crack covers the projection area of the central area and the oblique area of the double trapezoidal coil on the pipe specimen containing the crack and the projection area of the rectangular permanent magnet on the pipe specimen containing the crack.
[0007] Furthermore, the length of the bottom surface of the rectangular permanent magnet is twice the length of the central area of the double trapezoidal coil, and the width of the bottom surface of the rectangular permanent magnet is 1.5 times the width of the central area of the double trapezoidal coil.
[0008] By adopting the above technical solution: the magnetic field intensity below the outer area of the double trapezoidal coil is very small, which weakens the eddy current induced by the outer area of the double trapezoidal coil on the surface of the cracked pipe specimen and the Lorentz force generated by the eddy current modulated by the magnetic field, thereby weakening the interference of the outer area of the double trapezoidal coil on the oblique area and the central area of the double trapezoidal coil.
[0009] Furthermore, the distance between the rectangular permanent magnet and the double trapezoidal coil is ≤1 mm, and the distance between the rectangular permanent magnet and the cracked pipe specimen is ≤2 mm, so as to ensure that the rectangular permanent magnet generates a sufficiently strong magnetic field on the surface of the cracked pipe specimen.
[0010] Furthermore, the rectangular permanent magnet is composed of two permanent magnets with opposite polarities in the vertical direction. Taking the vertical downward direction as the reference direction, the magnetic field intensity of the magnetic field generated by the left permanent magnet on the cracked pipe specimen is -B S The magnetic field strength of the magnetic field generated by the permanent magnet on the cracked pipe specimen is B S .
[0011] Furthermore, the electromagnetic shielding shell is a hollow rectangular shell with five faces, the height of the electromagnetic shielding shell is greater than the height of the rectangular permanent magnet, and the length of the bottom surface of the electromagnetic shielding shell is equal to the width of the oblique area of the double trapezoidal coil.
[0012] Furthermore, the electromagnetic shielding shell covers the remaining five surfaces of the rectangular permanent magnet except the bottom surface.
[0013] Furthermore, the electromagnetic shielding shell is made of Fe-based nanocrystalline soft magnetic alloy. Since Fe-based nanocrystalline soft magnetic alloy has large magnetic permeability and small magnetic resistance, while the magnetic resistance of air is large, the magnetic lines of force around the rectangular permanent magnet are guided into the electromagnetic shielding shell.
[0014] By adopting the above technical solution: the magnetic field in the electromagnetic shielding shell is mainly in the vertical direction. Combined with the size setting of the rectangular permanent magnet, the magnetic field strength in the central area and the oblique area of the double trapezoidal coil is much greater than the magnetic field strength in the outer area of the double trapezoidal coil, thereby generating a magnetic field focused on the crack tip, and the electromagnetic shielding shell plays a role in controlling the distribution of the magnetic field.
[0015] Furthermore, the double trapezoidal coil is composed of two trapezoidal coils, the two trapezoidal coils are symmetrical about the central axis of the double trapezoidal coil, and the line width and line spacing of the two trapezoidal coils are equal.
[0016] Furthermore, there is a gap between the central area and the outer area of the double trapezoidal coil.
[0017] By adopting the above technical solution: after high-frequency alternating current is simultaneously passed through the two trapezoidal coils constituting the double trapezoidal coil, at the same time, the current direction of the conductor in the center area is opposite to the current direction of the conductor in the outer area. Therefore, the eddy current generated by the conductor in the center area of the double trapezoidal coil on the cracked pipe specimen is opposite to the eddy current generated by the conductor in the outer area on the cracked pipe specimen. The eddy current generated by the conductor in the outer area on the cracked pipe specimen will interfere with the eddy current generated by the conductor in the center area on the cracked pipe specimen, and the gap between the center area and the outer area of the double trapezoidal coil can reduce this interference.
[0018] Furthermore, the length of the central area of the double trapezoidal coil is 1 / 3 of the width of the double trapezoidal coil, and the width of the central area of the double trapezoidal coil is 1 / 6 of the length of the double trapezoidal coil. Combined with the magnetic field focused on the crack tip, the central area of the double trapezoidal coil can generate focused eddy currents at the crack tip.
[0019] Furthermore, the magnetic field strength of the alternating magnetic field generated by the double trapezoidal coil after the high-frequency alternating current is B dThe magnetic induction intensity of the equivalent magnetic field at the right trapezoidal coil in the double trapezoidal coil is B, and the magnetic induction intensity of the equivalent magnetic field at the left trapezoidal coil in the double trapezoidal coil is -B, where B = B S +B d .
[0020] Furthermore, taking the horizontal rightward direction as the reference direction, the Lorentz force f1 = J on the right side of the crack e1 B+2J e2 Bcosθ, Lorentz force on the left side of the crack f2 = -J e1 B-2J e2 B cosθ, where J e1 is the eddy current density on the surface of the cracked pipe specimen below the center of the double trapezoidal coil, J e2 is the eddy current density on the surface of the cracked pipe specimen below the oblique area of the double trapezoidal coil, and θ is the angle between the conductor in the oblique area of the double trapezoidal coil and the horizontal direction.
[0021] By adopting the above technical solution: the Lorentz forces on both sides of the crack tip are in opposite directions, and the Lorentz force pulls the crack tip. At the same time, the Lorentz force generated in the oblique area of the double trapezoidal coil enhances the Lorentz force generated in the central area of the double trapezoidal coil, causing high-frequency vibration of the local particles at the crack tip and stimulating a stronger acoustic emission signal.
[0022] In the above technical solution, the technical effects and advantages provided by the present invention are:
[0023] 1. The present invention provides an electromagnetic acoustic emission excitation device based on a double trapezoidal coil. The central area of the double trapezoidal coil is small and the current directions of the wires are the same at the same time, which can generate focused eddy currents at the crack tip. At the same time, the Lorentz force generated by the wires in the oblique area of the double trapezoidal coil enhances the Lorentz force generated in the central area of the double trapezoidal coil, thereby significantly improving the signal-to-noise ratio of the acoustic emission signal.
[0024] 2. The electromagnetic acoustic emission excitation device based on a double trapezoidal coil provided by the present invention adopts an electromagnetic shielding shell with high magnetic permeability to control the magnetic field distribution so that the magnetic field is focused on the crack tip.
[0025] 3. The present invention provides an electromagnetic acoustic emission excitation device based on a double trapezoidal coil, in which the rectangular permanent magnet is composed of two permanent magnets with opposite polarities in the vertical direction, which can generate Lorentz forces with opposite directions on both sides of the crack tip, pulling the crack tip, causing high-frequency vibration of local particles at the crack tip, and stimulating a stronger acoustic emission signal. BRIEF DESCRIPTION OF THE DRAWINGS
[0026] In order to more clearly illustrate the embodiments of the present application or the technical solutions in the prior art, the drawings required for use in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments recorded in the present invention. For ordinary technicians in this field, other drawings can also be obtained based on these drawings.
[0027] Figure 1 It is a structural schematic diagram of the present invention;
[0028] Figure 2 for Figure 1 A top view of
[0029] Figure 3 It is a schematic diagram of magnetic field distribution when the present invention is not provided with an electromagnetic shielding shell;
[0030] Figure 4 A schematic diagram of the magnetic field distribution after the electromagnetic shielding shell is installed in the present invention;
[0031] Figure 5 is a distribution diagram of eddy currents generated at the crack tip after the double trapezoidal coils in the present invention are energized;
[0032] Figure 6 is an analysis diagram of the Lorentz force on the crack tip in the present invention;
[0033] Figure 7 Schematic diagram of the Lorentz force generated at the crack tip at three different moments of the present invention;
[0034] Description of reference numerals:
[0035] 1. Electromagnetic shielding shell; 2. Rectangular permanent magnet; 3. Double trapezoidal coil; 4. Pipe specimen; 5. Crack. DETAILED DESCRIPTION
[0036] In order to enable those skilled in the art to better understand the technical solution of the present invention, the present invention will be further described in detail below with reference to the accompanying drawings.
[0037] Embodiment 1:
[0038] This embodiment provides an electromagnetic acoustic emission excitation device based on a double trapezoidal coil, such as Figure 1 , Figure 2 As shown, it includes an electromagnetic shielding shell 1, a rectangular permanent magnet 2 and a double trapezoidal coil 3 arranged above a pipeline specimen 4 containing a crack 5; the crack 5 is located on the surface of the pipeline specimen 4 containing the crack 5, and the crack 5 is a narrow and long defect with a sharp end at one end; the double trapezoidal coil 3 is located above the pipeline specimen 4 containing the crack 5, and the crack 5 is located below the central axis of the double trapezoidal coil 3; as shown Figure 1As shown, the double trapezoidal coil 3 includes a central area, an oblique area and an outer area, the current flow direction of the central area is parallel to the expansion direction of the crack 5, and the projection of the central area on the pipeline specimen 4 containing the crack 5 covers the tip of the crack 5, that is, the tip of the crack 5 is located directly below the central area of the double trapezoidal coil 3; the rectangular permanent magnet 2 is located above the double trapezoidal coil 3 and is placed parallel to the crack 5, and the projection area of the rectangular permanent magnet 2 on the pipeline specimen 4 containing the crack 5 covers the projection area of the central area of the double trapezoidal coil 3 on the pipeline specimen 4 containing the crack 5; the electromagnetic shielding shell 1 is located above the pipeline specimen 4 containing the crack 5, and the electromagnetic shielding shell 1 is located outside the rectangular permanent magnet 2, and the projection area of the electromagnetic shielding shell 1 on the pipeline specimen 4 containing the crack 5 covers the projection area of the central area and the oblique area of the double trapezoidal coil 3 on the pipeline specimen 4 containing the crack 5 and the projection area of the rectangular permanent magnet 2 on the pipeline specimen 4 containing the crack 5.
[0039] Specifically, Figure 2 As shown, the length of the bottom surface of the rectangular permanent magnet 2 is twice the length of the central area of the double trapezoidal coil 3 , and the width of the bottom surface of the rectangular permanent magnet 2 is 1.5 times the width of the central area of the double trapezoidal coil 3 .
[0040] In this embodiment, the magnetic field strength below the outer area of the double trapezoidal coil 3 is very small, which weakens the eddy currents induced by the outer area of the double trapezoidal coil 3 on the surface of the pipe specimen 4 containing the crack 5 and the Lorentz force generated by the eddy currents modulated by the magnetic field, thereby weakening the interference of the outer area of the double trapezoidal coil 3 on the oblique area and the central area of the double trapezoidal coil 3.
[0041] Specifically, the distance between the rectangular permanent magnet 2 and the double trapezoidal coil 3 is ≤1 mm, and the distance between the rectangular permanent magnet 2 and the pipe specimen 4 containing the crack 5 is ≤2 mm, so as to ensure that the rectangular permanent magnet 2 generates a sufficiently strong magnetic field on the surface of the pipe specimen 4 containing the crack 5.
[0042] Specifically, Figure 1 As shown, the rectangular permanent magnet 2 is composed of two permanent magnets with opposite polarities in the vertical direction. Taking the vertical downward direction as the reference direction, the magnetic field intensity of the magnetic field generated by the left permanent magnet on the pipe specimen 4 containing the crack 5 is -B S The magnetic field strength of the magnetic field generated by the permanent magnet on the pipe specimen 4 containing the crack 5 is B S .
[0043] Specifically, Figure 1 and Figure 2As shown, the electromagnetic shielding shell 1 is a hollow rectangular shell with five faces. The height of the electromagnetic shielding shell 1 is greater than the height of the rectangular permanent magnet 2 , and the length of the bottom surface of the electromagnetic shielding shell 1 is equal to the width of the oblique area of the double trapezoidal coil 3 .
[0044] Specifically, the electromagnetic shielding shell 1 covers the remaining five surfaces of the rectangular permanent magnet 2 except the bottom surface.
[0045] Specifically, the electromagnetic shielding shell 1 is made of Fe-based nanocrystalline soft magnetic alloy. Since the Fe-based nanocrystalline soft magnetic alloy has a large magnetic permeability and a small magnetic resistance, while the magnetic resistance of air is large, the magnetic lines of force around the rectangular permanent magnet 2 are guided into the electromagnetic shielding shell 1.
[0046] In this embodiment, Figure 3 This is a schematic diagram of the magnetic field distribution when the electromagnetic shielding shell is not added to the present invention. Figure 4 This is a schematic diagram of the magnetic field distribution after the electromagnetic shielding shell is installed in the present invention. It can be seen that after the electromagnetic shielding shell 1 is installed, the magnetic field at the double trapezoidal coil 3 is mainly in the vertical direction. Combined with the size setting of the rectangular permanent magnet 2, the magnetic field strength in the central area and the oblique area of the double trapezoidal coil 3 is much greater than the magnetic field strength in the outer area of the double trapezoidal coil 3, thereby generating a magnetic field focused on the tip of the crack 5, and the electromagnetic shielding shell 1 plays a role in controlling the distribution of the magnetic field.
[0047] Specifically, Figure 1 and Figure 2 As shown, the double trapezoidal coil 3 is composed of two trapezoidal coils, the two trapezoidal coils are symmetrical about the central axis of the double trapezoidal coil 3, and the line width and line spacing of the two trapezoidal coils are equal.
[0048] Specifically, there is a gap between the central area and the outer area of the double trapezoidal coil 3.
[0049] In this embodiment, after the two trapezoidal coils constituting the double trapezoidal coil 3 are simultaneously supplied with high-frequency alternating current, at the same time, the current direction of the conductor in the central area is opposite to the current direction of the conductor in the outer area. Figure 5 The distribution diagram of the eddy current generated at the crack tip after the double trapezoidal coil is energized in the present invention is shown in FIG. Figure 5 As shown, the eddy currents generated by the conductors in the central area of the double trapezoidal coil 3 on the pipe specimen 4 containing cracks 5 are in opposite directions to the eddy currents generated by the conductors in the outer area on the pipe specimen 4 containing cracks 5. The eddy currents generated by the conductors in the outer area on the pipe specimen 4 containing cracks 5 will interfere with the eddy currents generated by the conductors in the central area on the pipe specimen 4 containing cracks 5, and the gap between the central area and the outer area of the double trapezoidal coil 3 can reduce such interference.
[0050] Specifically, Figure 2As shown, the length of the central area of the double trapezoidal coil 3 is 1 / 3 of the width of the double trapezoidal coil 3, and the width of the central area of the double trapezoidal coil 3 is 1 / 6 of the length of the double trapezoidal coil 3. Combined with the magnetic field focused on the tip of the crack 5, as shown Figure 5 As shown, the central area of the double trapezoidal coil 3 can generate a focused eddy current at the tip of the crack 5.
[0051] Specifically, the magnetic field strength of the alternating magnetic field generated by the double trapezoidal coil 3 after the high-frequency alternating current is B d The magnetic induction intensity of the equivalent magnetic field at the right trapezoidal coil in the double trapezoidal coil 3 is B, and the magnetic induction intensity of the equivalent magnetic field at the left trapezoidal coil in the double trapezoidal coil 3 is -B, where B = B S +B d .
[0052] Specifically, taking the horizontal rightward direction as the reference direction, the Lorentz force f1=J on the right side of the crack 5 e1 B+2J e2 Bcosθ, Lorentz force f2 on the left side of crack 5 = -J e1 B-2J e2 Bcosθ, where J e1 is the eddy current density on the surface of the pipe specimen 4 with crack 5 below the center of the double trapezoidal coil 3, J e2 is the eddy current density on the surface of the pipe specimen 4 containing the crack 5 below the oblique area of the double trapezoidal coil 3, and θ is the angle between the conductor in the oblique area of the double trapezoidal coil 3 and the horizontal direction.
[0053] In this embodiment, θ=45. Figure 6 The analysis diagram of the Lorentz force on the crack tip in the present invention is shown in FIG. Figure 6 As shown, where the Lorentz force f 11 、f 13 、f 21 、f 23 The size is equal to J e2 B, Lorentz force f 12 、f 22 The size is equal to J e1 B. The Lorentz forces acting on crack 5 in the vertical direction cancel each other out, and the net force is zero. In the horizontal direction, the Lorentz force on the right side of crack 5 is f1=f 12 +(f 11 +f 13 )×cos45. That is The Lorentz force on the left side of crack 5 is f2 = f 22 +(f 21 +f 23 )×cos45. , that is
[0054] In this embodiment, Figure 7 Schematic diagram of the Lorentz force generated at the crack tip at three different times in the present invention, as shown in FIG. Figure 7 As shown, the Lorentz force generated by using the double trapezoidal coil 3 is focused on the crack 5 tip.
[0055] Specifically, the Lorentz forces on both sides of the crack tip are in opposite directions, and the Lorentz force pulls the crack tip. At the same time, the Lorentz force generated in the oblique area of the double trapezoidal coil 3 enhances the Lorentz force generated in the central area of the double trapezoidal coil 3, causing high-frequency vibration of the local particles at the crack tip, and stimulating a stronger acoustic emission signal.
[0056] In summary, the double trapezoidal coil 3 proposed in the present invention has the characteristics of eddy current focusing and Lorentz force enhancement in the central area. Combined with the electromagnetic shielding shell 1 that can control the magnetic field distribution, a stronger acoustic emission signal can be generated at the tip of the crack 5, which significantly improves the signal-to-noise ratio of the acoustic emission signal, can enhance the detection accuracy and efficiency of potential cracks in pipeline welds, and has broad application prospects.
[0057] The above description is only by way of illustration of certain exemplary embodiments of the present invention. It is undoubted that those skilled in the art can modify the described embodiments in various ways without departing from the spirit and scope of the present invention. Therefore, the above drawings and descriptions are illustrative in nature and should not be construed as limiting the scope of protection of the claims of the present invention.
Claims
1. An electromagnetic acoustic emission excitation device based on a double trapezoidal coil, characterized in that: The invention comprises a pipeline test piece (4), wherein a double trapezoidal coil (3) is arranged on the pipeline test piece (4); a rectangular permanent magnet (2) is arranged above the double trapezoidal coil (3); an electromagnetic shielding shell (1) is arranged outside the rectangular permanent magnet (2); and a crack (5) is arranged on the pipeline test piece (4), wherein the crack (5) is located below the central axis of the double trapezoidal coil (3).
2. The electromagnetic acoustic emission excitation device based on a double trapezoidal coil according to claim 1 is characterized in that: The crack (5) is a long and narrow defect with a pointed end.
3. The electromagnetic acoustic emission excitation device based on a double trapezoidal coil according to claim 2 is characterized in that: The double trapezoidal coil (3) comprises a central area, an oblique area and an outer area, the current flow direction of the central area is parallel to the expansion direction of the crack (5), and the projection of the central area on the pipeline test piece (4) covers the tip of the crack (5), that is, the tip of the crack (5) is located directly below the central area of the double trapezoidal coil (3); the projection area of the rectangular permanent magnet (2) on the pipeline test piece (4) covers the projection area of the central area of the double trapezoidal coil (3) on the pipeline test piece (4); the projection area of the electromagnetic shielding shell (1) on the pipeline test piece (4) covers the projection area of the central area and the oblique area of the double trapezoidal coil (3) on the pipeline test piece (4) and the projection area of the rectangular permanent magnet (2) on the pipeline test piece (4).
4. The electromagnetic acoustic emission excitation device based on a double trapezoidal coil according to claim 3 is characterized in that: The length of the bottom surface of the rectangular permanent magnet (2) is twice the length of the central area of the double trapezoidal coil (3), the width of the bottom surface of the rectangular permanent magnet (2) is 1.5 times the width of the central area of the double trapezoidal coil (3), the distance between the rectangular permanent magnet (2) and the double trapezoidal coil (3) is ≤1 mm, and the distance between the rectangular permanent magnet (2) and the pipeline test piece (4) is ≤2 mm.
5. The electromagnetic acoustic emission excitation device based on double trapezoidal coils according to claim 4 is characterized in that: The rectangular permanent magnet (2) is composed of two permanent magnets with opposite polarities in the vertical direction. Taking the vertical downward direction as the reference direction, the magnetic field intensity of the magnetic field generated by the left permanent magnet on the pipe test piece (4) containing the crack (5) is -B S The magnetic field strength of the magnetic field generated by the permanent magnet on the pipe specimen (4) containing the crack (5) is B S .
6. The electromagnetic acoustic emission excitation device based on double trapezoidal coils according to claim 5 is characterized in that: The electromagnetic shielding shell (1) is a hollow rectangular shell having five faces, the height of the electromagnetic shielding shell (1) is greater than the height of the rectangular permanent magnet (2), and the length of the bottom surface of the electromagnetic shielding shell (1) is equal to the width of the oblique area of the double trapezoidal coil (3); the electromagnetic shielding shell (1) is made of Fe-based nanocrystalline soft magnetic alloy.
7. The electromagnetic acoustic emission excitation device based on a double trapezoidal coil according to claim 6 is characterized in that: The double trapezoidal coil (3) is composed of two trapezoidal coils, the two trapezoidal coils are symmetrical with respect to the central axis of the double trapezoidal coil (3), and the line width and line spacing of the two trapezoidal coils are equal; after high-frequency alternating current is simultaneously passed through the two trapezoidal coils, at the same time, the current direction of the conductor in the central area is opposite to the current direction of the conductor in the outer area; and there is a gap between the central area and the outer area of the double trapezoidal coil (3).
8. The electromagnetic acoustic emission excitation device based on double trapezoidal coils according to claim 7 is characterized in that: The width of the central area of the double trapezoidal coil (3) is 1 / 3 of the width of the double trapezoidal coil (3), and the length of the central area of the double trapezoidal coil (3) is 1 / 6 of the length of the double trapezoidal coil (3).
9. The electromagnetic acoustic emission excitation device based on double trapezoidal coils according to claim 8, characterized in that: The double trapezoidal coil (3) generates an alternating magnetic field with a magnetic field strength of B after a high-frequency alternating current is passed through it. d The magnetic induction intensity of the equivalent magnetic field at the right trapezoidal coil in the double trapezoidal coil is B, and the magnetic induction intensity of the equivalent magnetic field at the left trapezoidal coil in the double trapezoidal coil is -B, where B = B S +B d .
10. The electromagnetic acoustic emission excitation device based on double trapezoidal coils according to claim 9, characterized in that: Taking the horizontal rightward direction as the reference direction, the Lorentz force f1=J on the right side of the crack (5) e1 B+2J e2 Bcosθ, the Lorentz force f2 on the left side of the crack (5) = -J e1 B-2J e2 B cosθ, where J e1 is the eddy current density on the surface of the pipe specimen (4) with crack (5) below the center area of the double trapezoidal coil (3), J e2 The eddy current density on the surface of a pipe test piece (4) containing a crack (5) below the oblique area of the double trapezoidal coil (3), wherein θ is the angle between the conductor of the oblique area of the double trapezoidal coil (3) and the horizontal direction.