A method for detecting an arbitrary direction crack inside carbon steel based on magnetic resistance disturbance

Abstract

The present application belongs to the technical field of nondestructive defect detection, and particularly relates to a crack detection method for carbon steel in any direction based on magnetic resistance disturbance. The crack detection method magnetizes the carbon steel structure to a high sensitivity stage by applying a static magnetic field; then an alternating current magnetic field is excited within the excitation range drawn on the surface of the carbon steel structure to be detected, and the change of the magnetic permeability of the carbon steel structure is counted to realize effective identification and detection of internal cracks in any direction. The crack detection method comprises: using a static magnetic field B0 to magnetize the carbon steel structure to be detected to the rising stage on the left side of the peak value of the magnetic permeability curve; drawing an excitation range on the surface of the carbon steel structure to be detected, and exciting an alternating current magnetic field B1 within the excitation range; collecting the alternating current magnetic field B1 and the secondary alternating current magnetic field B2 within the excitation range to obtain a signal Bx; obtaining an alternating current component JBx and a direct current component ZBx; moving the alternating current magnetic field B1 within the excitation range to count the change trend of the alternating current component JBx.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of nondestructive defect detection, and particularly relates to a crack detection method for internal cracks in carbon steel in any direction based on magnetic resistance disturbance. BACKGROUND

[0002] Due to the characteristics of high strength and large hardness, carbon steel structures are widely used in the fields of industrial facilities and social infrastructure construction. However, in the process of on-site use, the carbon steel structures are prone to internal or surface cracks due to the influence of complex alternating loads, environment and medium corrosion and other complex factors. Once the cracks expand and cause structural failure, safety accidents will inevitably occur, and in serious cases, the safety of people's production and property may be threatened.

[0003] Further research shows that in the existing nondestructive testing technology for carbon steel structures, the detection technology for surface cracks is relatively mature, while the detection of internal cracks in carbon steel structures still faces many challenges. In particular, the detection of cracks in any direction in the internal carbon steel is more difficult.

[0004] It should be noted that due to the complexity of the geometric characteristics of the cracks and their magnetic response behavior, for example, when the crack opening is narrow or the crack direction is consistent with the excitation magnetic field direction, the generated magnetic leakage signal is often extremely weak and even difficult to be effectively identified and captured. In addition, since the cracks in the carbon steel structure can be distributed at any depth and location in the material, and have uncertain directionality, the existing detection method based on single direction excitation cannot fully stimulate the magnetic abnormal response of the cracks, thereby easily causing missed detection or misjudgment. Therefore, how to realize high sensitivity and high reliability detection of cracks in any direction in the internal carbon steel has become a technical problem to be solved by the technical personnel in the field. SUMMARY

[0005] The present application provides a crack detection method for internal cracks in carbon steel in any direction based on magnetic resistance disturbance. The crack detection method magnetizes the carbon steel structure to a high sensitivity stage by applying a static magnetic field; then an alternating magnetic field is excited within the excitation range drawn on the surface of the carbon steel structure to be detected, and the change in the magnetic permeability of the carbon steel structure is counted; based on the magnetic resistance disturbance of the surface of the carbon steel, effective identification and detection of internal cracks in any direction are realized, which significantly improves the detection efficiency and greatly reduces the risk of missed detection.

[0006] To solve the above technical problems, the present application adopts the following technical scheme:

[0007] A crack detection method for internal cracks in carbon steel in any direction based on magnetic resistance disturbance, comprising the following steps:

[0008] Step S1: use a static magnetic field B0 to magnetize the carbon steel structure to be detected to a magnetic permeability the rising phase on the left side of the curve peak;

[0009] Step S2: defining an excitation range on the surface of the carbon steel structure to be detected, and exciting an alternating magnetic field B1 in the excitation range; wherein the alternating magnetic field B1 induces a secondary alternating magnetic field B2 above the surface of the carbon steel structure to be detected;

[0010] Step S3: calculating the overall magnetic resistance response of the carbon steel structure to be detected in the excitation range; collecting the alternating magnetic field B1 and the secondary alternating magnetic field B2 in the excitation range, and converting to obtain a signal Bx;

[0011] Step S4: calculating the signal Bx to obtain an alternating component JBx and a direct current component ZBx;

[0012] Step S5: moving the alternating magnetic field B1 in the excitation range to count the change trend of the alternating component JBx; wherein the direction of moving the alternating magnetic field B1 in the excitation range is determined by the detection task of the carbon steel structure to be detected;

[0013] The change trend of the alternating component JBx obtained by counting can be used to judge the magnetic resistance disturbance on the surface of the carbon steel, and further realize the detection and evaluation of the internal cracks of the carbon steel in any direction.

[0014] Preferably, the static magnetic field B0 in step S1 is excited by a plurality of direct current coils or permanent magnets matched with the carbon steel structure to be detected, and the magnetic field strength is 0.1T to 0.5T.

[0015] Preferably, the alternating magnetic field B1 excited in the excitation range in step S2 satisfies: Formula (1);

[0016] In formula (1), N is the number of turns of the alternating magnetic field excitation coil, and I1 is the amplitude of the alternating magnetic field excitation current.

[0017] Preferably, the overall magnetic resistance of the carbon steel structure to be detected in the excitation range in step S3 satisfies: Formula (2);

[0018] In formula (2), is the surface permeability of the carbon steel structure to be detected; L is the length of the excitation range, and W is the width of the excitation range.

[0019] Preferably, the magnetic field direction of the static magnetic field B0 is the same as the setting direction of the magnetic core for generating the alternating magnetic field B1.

[0020] The present application provides a kind of based on magnetic resistance disturbance's carbon steel internal crack detection method in any direction, specifically includes the following steps: using static magnetic field B0, the carbon steel structure to be detected is magnetized to its permeability The rising stage on the left side of the curve peak value; the excitation range is demarcated on the surface of the carbon steel structure to be detected, and an alternating magnetic field B1 is excited in the excitation range; wherein the alternating magnetic field B1 induces a secondary alternating magnetic field B2 above the surface of the carbon steel structure to be detected; the overall magnetic resistance response of the carbon steel structure to be detected in the excitation range is calculated; the alternating magnetic field B1 and the secondary alternating magnetic field B2 in the excitation range are collected and converted to obtain a signal Bx; the signal Bx is calculated to obtain an alternating component JBx and a direct current component ZBx; the alternating magnetic field B1 in the excitation range is moved, and the change trend of the alternating component JBx is counted. Wherein, the change trend of the alternating component JBx can be used to judge the magnetic resistance disturbance of the surface of the carbon steel, and then the detection and evaluation of the internal arbitrary direction crack of the carbon steel are realized.

[0021] The magnetic resistance disturbance-based carbon steel internal arbitrary direction crack detection method with the above step characteristics has at least the following technical advantages compared with the prior art:

[0022] (1) The magnetic resistance disturbance-based carbon steel internal arbitrary direction crack detection method provided by the present application uses a static magnetic field B0 to magnetize the carbon steel structure to be detected to a magnetic permeability The rising stage on the left side of the curve peak value; in this process, the static magnetic field B0 bypasses the area above the internal crack of the carbon steel structure to be detected, so that the magnetic permeability of the surface of the carbon steel structure to be detected increases along the curve.

[0023] (2) The magnetic resistance disturbance-based carbon steel internal arbitrary direction crack detection method provided by the present application excites an alternating magnetic field B1 in the excitation range, and further induces a secondary alternating magnetic field B2 above the surface of the carbon steel structure to be detected; by counting the change trend of the alternating component JBx in the signal Bx (converted from the alternating magnetic field B1 and the secondary alternating magnetic field B2 in the excitation range), the detection and evaluation of the internal arbitrary direction crack of the carbon steel are realized.

[0024] (3) The magnetic resistance disturbance-based carbon steel internal arbitrary direction crack detection method provided by the present application produces the same order of magnitude of magnetic resistance disturbance for the cracks in any direction, makes up for the deficiency that the existing detection method is difficult to fully excite the magnetic anomaly response of the cracks, effectively reduces the missed detection rate of the internal arbitrary direction cracks of the carbon steel, and has the characteristics of fast detection speed and high effective recognition rate of the internal arbitrary direction cracks of the carbon steel. BRIEF DESCRIPTION OF DRAWINGS

[0025] The drawings are used to provide further understanding of the present application, and constitute a part of the specification, together with the embodiments of the present application, to explain the present application, and do not constitute a limitation on the present application. In the following drawings:

[0026] Figure 1A flowchart of a carbon steel internal arbitrary direction crack detection method based on magnetic resistance disturbance provided by the present application is shown in the figure.

[0027] Figure 2 A magnetization curve diagram of a carbon steel structure to be detected magnetized by using a static magnetic field B0 is shown in the figure.

[0028] Figure 3 A magnetic permeability disturbance distribution diagram generated above the internal crack after the carbon steel structure to be detected is magnetized by using a static magnetic field B0 is shown in the figure.

[0029] Figure 4 A detection signal result diagram (after normalization processing) finally obtained by the carbon steel internal arbitrary direction crack detection method based on magnetic resistance disturbance provided by the present application is shown in the figure.

[0030] Figure 5 A detection signal result diagram (after normalization processing) finally obtained by the existing detection method based on single direction excitation is shown in the figure.

[0031] Figure 6 A detection device structure diagram for verifying the carbon steel internal arbitrary direction crack detection method based on magnetic resistance disturbance provided by the present application is shown in the figure.

[0032] Reference signs:

[0033] 10, AC signal generator; 20, DC signal generator; 30, DC excitation coil group; 301, first winding; 302, second winding; 40, carbon steel structure to be detected; 50, signal Bx; 60, U-shaped magnetic core; 70, AC excitation coil; 80, amplification and filtering circuit; 90, array magnetic field sensor. DETAILED DESCRIPTION

[0034] The present application provides a carbon steel internal arbitrary direction crack detection method based on magnetic resistance disturbance. The crack detection method magnetizes the carbon steel structure to a high sensitivity stage by applying a static magnetic field. Then, an AC magnetic field is excited within the excitation range drawn on the surface of the carbon steel structure to be detected, and the magnetic permeability change of the carbon steel structure is counted. Based on the magnetic resistance disturbance of the carbon steel surface, the internal crack in any direction is effectively recognized and detected, which significantly improves the detection efficiency and greatly reduces the risk of missed detection.

[0035] The present application provides a carbon steel internal arbitrary direction crack detection method based on magnetic resistance disturbance, as shown in the figure, including the following steps: Figure 1

[0036] Step S1: using a static magnetic field B0 to magnetize the carbon steel structure to be detected to the rising stage on the left side of the peak value of the magnetic permeability curve.

[0037] ​​Specifically, in a preferred embodiment of the present invention, the static magnetic field B0 can be formed by multiple DC coils or permanent magnets adapted to the carbon steel structure under test, with a magnetic field strength of 0.1T to 0.5T, specifically used to magnetize the carbon steel structure under test to its permeability. The rising phase to the left of the curve peak, see reference as follows: Figure 2 As shown.

[0038] The static magnetic field B0 bypasses the region above the internal crack in the carbon steel structure under test, causing disturbance. This disturbance affects the surface permeability of the carbon steel structure. Along The curve becomes larger; please refer to the following for details. Figure 3 As shown.

[0039] Based on completing step S1, further implement step S2. Specifically, step S2: delineate an excitation range on the surface of the carbon steel structure to be inspected, and generate an alternating magnetic field B1 within the excitation range; wherein, the alternating magnetic field B1 induces a secondary alternating magnetic field B2 above the surface of the carbon steel structure to be inspected.

[0040] It is worth noting that the alternating magnetic field B1 (with a frequency not exceeding 10 kHz) excited within the static magnetization range in step S2 specifically satisfies: Equation (1). In Equation (1), N is the number of turns of the AC magnetic field excitation coil, and I1 is the amplitude of the AC magnetic field excitation current.

[0041] In addition, it is preferable to set the magnetic core that generates the alternating magnetic field B1 in the same direction as the magnetic field direction of the static magnetic field B0.

[0042] Step S3: Calculate the overall magnetoresistive response of the carbon steel structure under test within the excitation range; collect the AC magnetic field B1 and the secondary AC magnetic field B2 within the excitation range, and convert them into signal Bx.

[0043] Based on step S2, step S3 is further implemented. In implementing step S3, the excitation range of the AC magnetic field B1 is first determined. The length of the excitation range is set as L, and the width of the excitation range is set as W. Further derivation yields that the surface magnetoresistance of the carbon steel structure under test within this excitation range satisfies: Equation (2).

[0044] In formula (2), Let L be the surface permeability of the carbon steel structure under test; L be the length of the excitation range; and W be the width of the excitation range. It is worth noting that since L and W are both fixed values, the surface magnetoresistance of the carbon steel structure under test within this excitation range is actually determined by the surface permeability of the carbon steel structure under test. It is determined by (the change). When the surface magnetic permeability of the carbon steel structure under test... As the value increases, the surface magnetic resistance R of the carbon steel structure under test within the excitation range will decrease.

[0045] Step S4: Calculate the signal Bx to obtain the AC component JBx and the DC component ZBx.

[0046] Based on completing step S3, step S4 is further implemented. Specifically, by filtering and amplifying the signal Bx, the AC component JBx and the DC component ZBx can be obtained.

[0047] Step S5: Move the AC magnetic field B1 within the excitation range and statistically analyze the changing trend of the AC component JBx.

[0048] Based on step S4, step S5 is further implemented. The direction of the AC magnetic field B1 within the excitation range is specifically determined by the inspection task of the carbon steel structure to be inspected. For example, if the inspection task is the fillet weld of the carbon steel structure to be inspected, the AC magnetic field B1 within the excitation range is usually set in the same direction as the fillet weld; in this case, the AC magnetic field B1 within the excitation range is moved along the extension direction of the fillet weld, that is, the moving direction of the AC magnetic field B1 within the excitation range is in the same direction as the setting direction of the AC magnetic field B1. If the inspection task is the circumferential weld of the carbon steel structure to be inspected, the static magnetic field B0 is usually set perpendicular to the circumferential weld; the setting direction of the AC magnetic field B1 within the excitation range is also perpendicular to the circumferential weld; in this case, the AC magnetic field B1 within the excitation range is moved along the extension direction of the circumferential weld, that is, the moving direction of the AC magnetic field B1 within the excitation range is perpendicular to the setting direction of the AC magnetic field B1.

[0049] Then, the changing trend of the AC component JBx is statistically analyzed to detect cracks in the carbon steel structure under inspection. (See reference...) Figure 4 As shown, Figure 4 This is a schematic diagram of the (normalized) detection signal result obtained by the method for detecting arbitrary-direction cracks inside carbon steel based on magnetoresistive perturbation provided by the present invention. When there are no defects, the background value of JBx is 0. When a significant trough appears in JBx, it indicates the presence of a crack in that area.

[0050] Further analysis revealed that the above detection results exhibited good sensitivity to cracks parallel and perpendicular to the static magnetic field B0 direction. Specifically, Figure 5 A schematic diagram of the detection signal result obtained by an existing detection method based on unidirectional excitation is provided. Compared to... Figure 4 It can be observed that, Figure 5 The existing technology shown can only provide a good sensitive response to cracks perpendicular to the magnetic field direction, but it is difficult to respond to cracks parallel to the magnetic field direction.

[0051] In another aspect, in order to facilitate the understanding of the present application by those skilled in the art, a detection device is further provided for verifying the effect. Referring to FIG. 1, the detection device mainly comprises an alternating current signal generator 10, a direct current signal generator 20, a direct current excitation coil set 30, a carbon steel structure to be detected 40, a signal Bx 50, a U-shaped magnetic core 60, an alternating current excitation coil 70, an amplification filtering circuit 80, and an array magnetic field sensor 90. Figure 6

[0052] In the process of implementing the detection, first, a constant current source output by the direct current signal generator 20 is loaded into the direct current excitation coil set 30 to generate a static magnetic field B0 of 0.2T. Then, the carbon steel structure to be detected 40 is placed in the static magnetic field B0 generated by the direct current excitation coil set 30, so that it is magnetized to the rising stage on the left side of the curve peak value. Further, the U-shaped magnetic core 60 is placed on the surface of the carbon steel structure to be detected 40 at the center of the static magnetic field B0. The distance between the two legs of the U-shaped magnetic core 60 is 20mm, and the width is 10mm. The alternating current excitation coil 70 is wound around the crossbeam of the U-shaped magnetic core 60. The alternating current signal generator 10 generates a sine signal of 1kHz frequency and loads it to the alternating current excitation coil 70, so as to generate an alternating magnetic field B1 parallel to the direction of the static magnetic field B0 in the range of L=20mm, W=10mm on the surface of the carbon steel structure to be detected 40. The alternating magnetic field B1 will induce a secondary alternating magnetic field B2 above the surface of the carbon steel structure to be detected 40. Finally, 3x2 evenly distributed array magnetic field sensors 90 are arranged in the range of L=20mm, W=10mm. The array magnetic field sensors 90 are specifically used to collect the alternating magnetic field B1 and the secondary alternating magnetic field B2 in the excitation range, so as to obtain a plurality of electrical signals (which are superimposed and converted to obtain the signal Bx).

[0053] Further, the U-shaped magnetic core 60 is placed on the surface of the carbon steel structure to be detected 40 at the center of the static magnetic field B0. The distance between the two legs of the U-shaped magnetic core 60 is 20mm, and the width is 10mm. The alternating current excitation coil 70 is wound around the crossbeam of the U-shaped magnetic core 60. The alternating current signal generator 10 generates a sine signal of 1kHz frequency and loads it to the alternating current excitation coil 70, so as to generate an alternating magnetic field B1 parallel to the direction of the static magnetic field B0 in the range of L=20mm, W=10mm on the surface of the carbon steel structure to be detected 40. The alternating magnetic field B1 will induce a secondary alternating magnetic field B2 above the surface of the carbon steel structure to be detected 40. Finally, 3x2 evenly distributed array magnetic field sensors 90 are arranged in the range of L=20mm, W=10mm. The array magnetic field sensors 90 are specifically used to collect the alternating magnetic field B1 and the secondary alternating magnetic field B2 in the excitation range, so as to obtain a plurality of electrical signals (which are superimposed and converted to obtain the signal Bx).

[0054] Further, the U-shaped magnetic core 60 is placed on the surface of the carbon steel structure to be detected 40 at the center of the static magnetic field B0. The distance between the two legs of the U-shaped magnetic core 60 is 20mm, and the width is 10mm. The alternating current excitation coil 70 is wound around the crossbeam of the U-shaped magnetic core 60. The alternating current signal generator 10 generates a sine signal of 1kHz frequency and loads it to the alternating current excitation coil 70, so as to generate an alternating magnetic field B1 parallel to the direction of the static magnetic field B0 in the range of L=20mm, W=10mm on the surface of the carbon steel structure to be detected 40. The alternating magnetic field B1 will induce a secondary alternating magnetic field B2 above the surface of the carbon steel structure to be detected 40. Finally, 3x2 evenly distributed array magnetic field sensors 90 are arranged in the range of L=20mm, W=10mm. The array magnetic field sensors 90 are specifically used to collect the alternating magnetic field B1 and the secondary alternating magnetic field B2 in the excitation range, so as to obtain a plurality of electrical signals (which are superimposed and converted to obtain the signal Bx). ​

[0055] Further, the alternating magnetic field B1 in the excitation range is moved (for example, the positions of the direct current excitation coil set 30 and the alternating current excitation coil 70 are adjusted along the length direction of the carbon steel structure 40 to be detected), and the change trend of the signal Bx 50 and the alternating component JBx output by the array magnetic field sensor 90 is counted. By analyzing the change trend of the alternating component JBx, the magnetic resistance disturbance of the carbon steel structure 40 to be detected can be determined, and the detection and evaluation of the internal crack can be realized. When the alternating component JBx appears the trough feature, it is determined that the crack is detected.

[0056] So far, the carbon steel internal arbitrary direction crack detection method based on magnetic resistance disturbance provided by the present application realizes the detection and evaluation of the carbon steel internal arbitrary direction crack by using the static magnetic field B0 and exciting the alternating magnetic field B1 in the excitation range, and counting the change trend of the signal Bx. In this process, the alternating magnetic field B1 in the excitation range and the signal Bx converted from the secondary alternating magnetic field B2 can produce the same order of magnetic resistance disturbance to the crack in any direction, so it can also compensate for the technical defects that the existing detection method is difficult to fully excite the magnetic anomaly response of the crack in any direction.

[0057] The present application provides a carbon steel internal arbitrary direction crack detection method based on magnetic resistance disturbance, which specifically comprises the following steps: using a static magnetic field B0 to magnetize a carbon steel structure to be detected to a magnetic permeability The rising stage on the left side of the curve peak value; the excitation range is determined on the surface of the carbon steel structure to be detected, and the alternating magnetic field B1 is excited in the excitation range; wherein the alternating magnetic field B1 induces a secondary alternating magnetic field B2 above the surface of the carbon steel structure to be detected; the overall magnetic resistance response of the carbon steel structure to be detected in the excitation range is calculated; the alternating magnetic field B1 and the secondary alternating magnetic field B2 in the excitation range are collected to convert the signal Bx; the signal Bx is calculated to obtain the alternating component JBx and the direct current component ZBx; the alternating magnetic field B1 in the excitation range is moved, and the change trend of the alternating component JBx is counted. The change trend of the alternating component JBx can be used to determine the magnetic resistance disturbance of the carbon steel surface, and the detection and evaluation of the carbon steel internal arbitrary direction crack can be realized.

[0058] The carbon steel internal arbitrary direction crack detection method based on magnetic resistance disturbance with the above step characteristics has at least the following technical advantages compared with the prior art:

[0059] (1) The carbon steel internal arbitrary direction crack detection method based on magnetic resistance disturbance provided by the present application uses a static magnetic field B0 to magnetize a carbon steel structure to be detected to a magnetic permeability The rising stage on the left side of the curve peak value; in this process, the static magnetic field B0 bypasses the area above the internal crack of the carbon steel structure to be detected, so that the magnetic permeability along The curve becomes larger.

[0060] (2) The method for detecting the internal arbitrary direction crack of carbon steel based on magnetic resistance disturbance provided by the application excites an alternating magnetic field B1 in an excitation range, and further induces a secondary alternating magnetic field B2 above the surface of the carbon steel structure to be detected; the change trend of the alternating component JBx in the signal Bx (converted from the alternating magnetic field B1 in the excitation range and the secondary alternating magnetic field B2) is counted, so that the internal arbitrary direction crack of the carbon steel is detected and evaluated.

[0061] (3) The method for detecting the internal arbitrary direction crack of carbon steel based on magnetic resistance disturbance provided by the application generates the same order of magnetic resistance disturbance to the crack in any direction, makes up for the deficiency that the existing detection method is difficult to fully excite the magnetic anomaly response of the crack, effectively reduces the missed detection rate of the internal arbitrary direction crack of the carbon steel, and has the characteristics of fast detection speed and high effective recognition rate of the internal arbitrary direction crack of the carbon steel.

[0062] The above is only a specific embodiment of the application, but the protection scope of the application is not limited to this. Any person skilled in the art can easily think of changes or replacements within the technical range disclosed by the application, which should be covered within the protection scope of the application. Therefore, the protection scope of the application should be subject to the protection scope of the claims.

Claims

1. A method for detecting an arbitrary direction crack inside a carbon steel based on a magnetic reluctance disturbance, characterized in that, The method comprises the following steps: Step S1 : Using a static magnetic field B0, magnetize the carbon steel structure under test to its magnetic permeability the rising phase to the left of the curve peak; Step S2: defining an excitation range on the surface of the carbon steel structure to be detected, and exciting an alternating magnetic field B1 in the excitation range; wherein the alternating magnetic field B1 induces a secondary alternating magnetic field B2 above the surface of the carbon steel structure to be detected; Step S3: calculating the overall magnetic resistance response of the carbon steel structure to be detected in the excitation range; collecting the alternating magnetic field B1 and the secondary alternating magnetic field B2 in the excitation range, and converting to obtain a signal Bx; Step S4: calculating the signal Bx to obtain an alternating component JBx and a direct current component ZBx; Step S5: moving the alternating magnetic field B1 in the excitation range, and counting the change trend of the alternating component JBx; wherein the change trend of the alternating component JBx can be used to judge the magnetic resistance disturbance of the surface of the carbon steel, and thus the detection and evaluation of the internal cracks of the carbon steel in any direction can be realized. The magnetic field direction of the static magnetic field B0 is the same as the setting direction of the magnetic core for generating the alternating magnetic field B1.

2. The method for detecting an arbitrary direction crack in a carbon steel according to claim 1, wherein The static magnetic field B0 in the step S1 is excited by a plurality of direct current coils or permanent magnets matched with the carbon steel structure to be detected, and the magnetic field strength is 0.1T to 0.5T.

3. The method for detecting an arbitrary direction crack in a carbon steel according to claim 1, wherein The alternating magnetic field B1 excited in the excitation range in the step S2 satisfies: Formula (1); In formula (1), N is the number of turns of the alternating magnetic field excitation coil, and I1 is the amplitude of the alternating magnetic field excitation current.

4. The method for detecting an arbitrary direction crack in a carbon steel according to claim 1, wherein The overall magnetic resistance of the carbon steel structure to be detected in the excitation range in the step S3 satisfies: Formula (2); In formula (2) thereof, μ0is the permeability of free space, μs is the surface permeability of the carbon steel structure under test, L is the length of the excitation range, and W is the width of the excitation range.

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