A method for electromagnetic identification of surface defect morphology of metal with protective layer

Abstract

The present invention provides an electromagnetic identification method for the morphology of metal surface defects with a protective layer, which relates to the field of nondestructive testing technology. The method forms an induced eddy current on the metal surface by inputting an excitation coil of a square wave signal excitation source, collects magnetic field signals in a direction perpendicular to the metal surface, transmits them to a signal processing unit for conversion, and displays a real-time image to complete the identification and detection of metal surface defects. The present invention can quickly and effectively obtain morphological information of metal surface defects with a protective layer, including morphological detection of irregular defects, and realizes the identification of the location of defects on the metal surface, thereby improving the accuracy and evaluation capability of traditional electromagnetic nondestructive testing, and can fully meet the target requirements for defect status detection in industrial production, and has good application prospects for nondestructive testing and safety assessment of in-service equipment.

Description

Technical Field

[0001] The invention relates to the technical field of nondestructive testing, and in particular to an electromagnetic identification method for the morphology of metal surface defects with a protective layer. Background Art

[0002] Metal materials are a type of material widely used in industry. Due to frequent use and some inevitable natural factors, the surface of metal materials is often susceptible to external corrosion and damage, resulting in defects, which affects normal work. Therefore, it is of great significance to conduct periodic inspections on products to ensure their integrity. Nondestructive testing technology is based on the analysis and processing of changes caused by the physical properties of materials without destroying the body to achieve defect detection, and its performance has its own advantages and disadvantages. Taking automatic optical testing as an example, this technology is based on optical principles, outputs the reflected light intensity of the object to be tested as a quantitative grayscale value, and compares the grayscale value of the image with the standard image, and finally achieves the purpose of defect detection. Automatic optical testing has high efficiency and reliability, and is simple to operate, and is widely used in many fields. However, due to the disadvantages of optical testing itself, it is often difficult for light to pass through the protective layer on the surface of the object to be tested, and it is easy to misjudge the detection for places where the shadow is not very obvious, which has certain limitations. In addition, penetrant flaw detection technology is also a nondestructive testing technology that is widely used in industry. It mainly uses the capillary principle to detect defects on the surface and near the surface of the object to be tested. Compared with optical testing, penetrant testing technology is suitable for testing all kinds of materials, is almost unaffected by the geometric shape and position of the workpiece, and has intuitive imaging. However, since this type of testing requires the use of penetrant imaging, it is not suitable for the testing of porous loose materials, and can only detect surface opening defects, making it difficult to quantitatively control the detection accuracy.

[0003] If the above technology is used to detect metal materials with protective layers, the protective layer must be removed first to achieve the purpose of defect detection. Such an operation is not only time-consuming, but also causes certain damage to the product itself. Therefore, in order to avoid the limitations of automatic optical detection and penetration technology as much as possible, AC electromagnetic field detection technology can be used for flaw detection. AC electromagnetic field is one of the non-destructive testing methods based on the principle of electromagnetic induction, and is mainly suitable for surface and near-surface detection of conductive materials. It detects the changes in induced eddy currents in the object being tested, thereby implementing non-destructive evaluation of the physical properties of the material or detecting its internal defects. This technology uses non-contact detection, has low requirements for the object being tested, can penetrate the coating, and has high sensitivity and fast detection speed. Compared with other detection technologies, it has obvious advantages. However, traditional AC electromagnetic field detection often uses a single sinusoidal AC signal to study the steady-state frequency response of the object being tested. The information contained in the steady-state output is very limited, so for the evaluation of complex defects, this excitation signal is far from meeting the use requirements. At the same time, the AC electromagnetic field generally obtains the characteristic law reflecting the defect by processing and analyzing the curve changes. The characteristic information reflected by a single curve can only determine the existence and approximate location of a defect. It is difficult to obtain specific information about the defect, so it is often impossible to effectively identify various types of defects. Summary of the invention

[0004] In response to the above-mentioned problems, the present invention provides an electromagnetic identification method for the surface defect morphology of metal with a protective layer, which can solve the above problems, can quickly and effectively obtain the surface defect morphology information of metal with a protective layer, and improve the accuracy and evaluation ability of traditional electromagnetic non-destructive testing.

[0005] To achieve the above object, the technical solution adopted by the present invention is:

[0006] The present invention provides an electromagnetic identification method for the morphology of metal surface defects with a protective layer, which is characterized by comprising a detection device and a signal processing unit; the detection device comprises an excitation coil and a sensor unit; the method steps are as follows:

[0007] S101: inputting a square wave signal excitation source into the excitation coil, and the generated electromagnetic field passes through the protective layer on the metal surface, forming a uniform induced eddy current on the metal surface;

[0008] S102: acquiring a disturbance magnetic field signal perpendicular to the metal surface through a sensor unit, and sending the disturbance magnetic field signal to a signal processing unit, wherein the sensor unit includes a plurality of circular planar coils arranged along an XY plane;

[0009] S103: According to the transient response of the disturbance magnetic field when the square wave signal value changes, the signal processing unit converts the received magnetic field change into a proportional image for real-time display.

[0010] The electromagnetic identification method for the morphology of metal surface defects with a protective layer provided by the present invention, preferably, the square wave signal in step 1 is a given rising edge time t r , Falling edge time t f , pulse width t w square wave signal.

[0011] The electromagnetic identification method for the morphology of defects on the surface of a metal with a protective layer provided by the present invention preferably further comprises:

[0012] S104: When the signal processing unit displays the defect image, adjust the direction position of the detection probe on the XY plane until the image is displayed clearly.

[0013] In the electromagnetic identification method for the morphology of defects on the surface of a metal with a protective layer provided by the present invention, preferably, the excitation coil in step S101 is a rectangular coil.

[0014] The present invention provides an electromagnetic identification device for metal surface defect morphology, characterized in that it includes a detection device and a signal processing unit; the detection device is electrically connected to the signal processing unit; the detection device includes an excitation coil and a sensor unit fixedly arranged up and down; the detection device is used to generate an induced eddy current in the XY plane; the sensor unit is used to collect magnetic field signals in the Z direction; and the signal processing unit is used to convert the magnetic field signals into image information.

[0015] In the electromagnetic identification device for metal surface defect morphology provided by the present invention, preferably, the sensor unit comprises a plurality of circular planar coils arranged along an XY plane; all of the plurality of circular planar coils are evenly arranged.

[0016] In the electromagnetic identification device for metal surface defect morphology provided by the present invention, preferably, the excitation coil is a rectangular coil.

[0017] The above technical solution has the following advantages or beneficial effects:

[0018] The present invention provides an electromagnetic identification method for the morphology of metal surface defects with a protective layer, comprising a detection device and a signal processing unit; the detection device comprises an excitation coil and a sensor unit; the method steps are as follows:

[0019] S101: inputting a square wave signal excitation source into the excitation coil, and the generated electromagnetic field passes through the protective layer on the metal surface, forming a uniform induced eddy current on the metal surface;

[0020] S102: acquiring a disturbance magnetic field signal perpendicular to the metal surface through a sensor unit, and sending the disturbance magnetic field signal to a signal processing unit;

[0021] S103: According to the transient response of the disturbance magnetic field when the square wave signal value changes, the signal processing unit converts the received magnetic field change into a proportional image for real-time display.

[0022] The present invention can quickly and effectively obtain the morphological information of metal surface defects with a protective layer, including the morphological detection of irregular defects, and realize the identification of the location of defects on the metal surface, thereby improving the accuracy and evaluation ability of traditional electromagnetic nondestructive testing, and can fully meet the target requirements for defect status detection in industrial production, and has good application prospects for nondestructive testing and safety assessment of in-service equipment. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] The present invention and its features, configurations and advantages will become more apparent by reading the detailed description of non-limiting embodiments with reference to the following drawings. The same reference numerals indicate the same parts throughout the drawings. The drawings are not drawn to scale, but rather to illustrate the subject matter of the present invention.

[0024] Figure 1 It is a schematic diagram of the distribution of induced current on the conductor surface under the action of the excitation source;

[0025] Figure 2 The magnetic field distribution curves and images of four different morphological defects along the x-axis and z-direction provided in Example 1 of the present invention are as follows;

[0026] Figure 3 The irregular defect and the Bz image formed by the irregular defect provided in Example 1 of the present invention;

[0027] Figure 4 It is a Bz image formed by triangular defects with azimuth angles of 0°, 90°, 180° and 270° provided in Example 1 of the present invention;

[0028] Figure 5 The cylindrical coil structure model and its corresponding Bz image provided in Example 1 of the present invention;

[0029] Figure 6 is a circular planar coil structure model and its corresponding Bz image provided by Example 1 of the present invention;

[0030] Figure 7 It is a schematic diagram of the electromagnetic identification device for metal surface defect morphology provided in Example 2 of the present invention. DETAILED DESCRIPTION

[0031] The present invention will be further described below in conjunction with the accompanying drawings and specific embodiments, but they are not intended to limit the present invention.

[0032] Embodiment 1:

[0033] According to Faraday's law of electromagnetic induction, when an excitation source with alternating current approaches a metal conductor, an induced current will be generated on the surface of the conductor; Figure 1 The figure shows the schematic diagram of the induced current distribution on the conductor surface under the action of the excitation source. When there is no defect on the conductor surface, the induced current is evenly distributed, and the induced magnetic field is zero in the z direction of space. When there is a circular defect on the flat surface, the current flow direction at the defect is deflected due to the change in the surface condition of the conductor. The overall distribution of the induced current is no longer uniform, and the induced magnetic field also changes accordingly, and a non-zero magnetic field component appears in the z direction. Figure 1 As shown in the figure, on the outermost surface of the circular defect, the current bends, and the bending deformation is the largest at this position. Therefore, the disturbing magnetic field in the z direction is the largest at this time, which is displayed as a peak on the disturbance curve. Similarly, on the other side of the circular defect, the disturbing magnetic field in the z direction also changes significantly, but the direction of the magnetic field is exactly opposite to that of the other side.

[0034] The disturbance magnetic field collected along the x-axis path ① is shown in the curve, where the maximum value is reached at the positions a and e on the edge of the circular defect; similarly, the disturbance magnetic field on the upper surface of the plate is collected along path ②, and a Bz curve similar to the previous one can still be obtained, but the disturbance peak and disturbance position at this time have changed; the change in the peak value of the disturbance magnetic field is explained by the change in the eddy current flow direction shown in the figure. The eddy current at the position shown by point a is the convergence of all eddy currents in the left half plane of the circle after changing direction, and the eddy current on the right side of the position shown by point a' is only a part of the eddy current at point a. Therefore, the disturbance peak corresponding to a' is weaker than the disturbance peak corresponding to point a. Secondly, the disturbance position has also moved, and this position just matches the geometric shape of the circular defect. Therefore, the change in the peak value of the Bz disturbance magnetic field at different positions corresponding to the defect morphology constitutes the theoretical basis for the electromagnetic image recognition of the defect morphology.

[0035] An electromagnetic identification method for the morphology of metal surface defects with a protective layer provided in an embodiment of the present invention includes a detection device and a signal processing unit 2; the detection device includes a rectangular excitation coil 12 and a sensor unit 13; the method steps are as follows:

[0036] S101: Input a square wave signal excitation source to the rectangular excitation coil 12. The generated electromagnetic field passes through the protective layer on the metal surface and forms a uniform induced eddy current on the metal surface. The square wave signal has a given rising edge time t r , Falling edge time t f , pulse width t w Square wave signal;

[0037] S102: acquiring a disturbance magnetic field signal perpendicular to the metal surface through the sensor unit 13, and sending the disturbance magnetic field signal to the signal processing unit 2;

[0038] S103: according to the transient response of the disturbance magnetic field when the square wave signal value changes, the signal processing unit 2 converts the received magnetic field change into a proportional image for real-time display;

[0039] S104: When the signal processing unit 2 displays the defect image, adjust the direction position of the detection probe on the XY plane until the image is displayed clearly.

[0040] In this example, defects with surface morphology of elongated rectangle, circle, square, and isosceles triangle were selected for research. The geometric parameters of each defect were selected as follows: the length and width of the elongated rectangle defect were 15 mm × 1 mm, the diameter of the circular defect was 2 mm, the side length of the square defect was 1 mm, and the waist length and base length of the isosceles triangle defect were In order to avoid the interference of defect depth on morphological recognition, the depth of all defects was set to 1 mm.

[0041] like Figure 2 As shown, (a) the Bz curve and Bz image of the defect shape is an elongated rectangle; (b) the Bz curve and Bz image of the defect shape is a circular hole; (c) the Bz curve and Bz image of the defect shape is a square; (d) the Bz curve and Bz image of the defect shape is a triangle.

[0042] The perturbed magnetic field on the three-dimensional cross section at t=0-0.01s is selected to obtain the magnetic field distribution curves and images of the above four types of defects of different shapes along the x-axis z direction. It can be seen from the perturbed magnetic field distribution curve that there is a large difference between the response curves at t=0 and t=0.01s. The curve at t=0.01s has two positive and negative peaks, while the response curve at t=0 has four positive and negative peaks. Analysis of the reasons for the formation of the peaks shows that the peak presented at t=0.01s is generated by the source field of the excitation coil, and its position reflects the geometric size of the excitation coil and has nothing to do with the surface defects of the plate. The four peaks presented at t=0, of which the formation of the left and right peaks is also directly related to the excitation source, and has the same formation mechanism as at t=0.01s, and its occurrence position is exactly the same as at t=0.01s. The four peaks presented at t=0, of which the formation of the two middle peaks has no direct connection with the excitation source, and is only caused by the surface defects of the plate. It can be seen from the peak amplitude that the peak formed by the defect is several times higher than the peak formed by the excitation source, which is very effective for image detection and can reduce the adverse effect of the excitation source on defect imaging. In addition, from the disturbance magnetic field curves and Bz images corresponding to t = 0 and t = 0.01s, it can be seen that for transient analysis, it is very important to choose the response at the appropriate time. The excitation signal at t = 0 suddenly changes, and the instantaneous response can fully reflect the inherent state properties of the object being measured.

[0043] When there is a defect, due to the influence of magnetic field disturbance, the location of the defect will be distorted, showing as two peaks in the Bz curve. As the geometric shape of the defect changes, the distance between the two peaks will also change accordingly. However, when the defects are circular, square, and isosceles triangle, the peak spacing of the three Bz curves is relatively close and difficult to distinguish. In contrast, the Bz image can present the shape of an elongated rectangle, circle, square, and isosceles triangle respectively. Therefore, by simply analyzing the Bz curve, we can only determine whether a defect exists or not, but it is difficult to summarize the characteristic rules for its specific shape. The Bz image can very intuitively and clearly present the morphological characteristics of the defect and the position information on the metal surface.

[0044] Comprehensive analysis Figure 2 The following characteristics can be summarized for the disturbance magnetic field Bz imaging: (1) The disturbance magnetic field Bz imaging is clear at t=0, with almost no other interference; (2) The disturbance magnetic field Bz imaging is symmetrical; (3) The disturbance magnetic field Bz imaging has no display image in the area near the symmetry axis. Therefore, the above three characteristics need to be combined to fully restore the specific shape of the defect.

[0045] The above is the disturbed magnetic field image obtained when the defect shape is regular. In order to further verify the effectiveness of this method, an irregular crack is used as the measured object. Figure 3 The left picture shows the constructed crack morphology. Figure 3 The right picture is the Bz image formed by the crack. Through verification, it can be found that this method is not only effective for regular defects, but also for the morphological detection of some irregular defects, and can fully meet the target requirements for defect status monitoring in industrial production.

[0046] In the process of structural component status detection and fault diagnosis, it is often necessary to understand the azimuth characteristics of the actual defects in order to understand the specific degree of damage to the object being tested. This embodiment takes an equilateral triangle defect as an example, and rotates it clockwise around the center of the aluminum plate by a certain angle to obtain the Bz image formed by defects at different azimuth angles. The defect geometric parameters are selected as follows: the side length of the triangle is 1mm, and in the initial position, one side of the triangle is parallel to the x-axis, and the remaining triangles are rotated 90° in sequence, such as Figure 4 (a)-(d) are shown in the upper left corner.

[0047] Figure 4(a)-(d) are Bz images formed by triangular defects at azimuth angles of 0°, 90°, 180° and 270° respectively. It can be found through observation that although defects at different azimuth angles can be shown in the image, there are still certain differences in the clarity and completeness of the image. When the triangle is at the initial position and 180°, the Bz image is relatively blurred, the corners of the triangle are partially missing, and the recognition is low. In contrast, when it is at 90° and 270°, the integrity and clarity of the image are higher, and the shape of the defect can be almost restored.

[0048] It can be concluded that the defect azimuth will have a certain impact on the clarity and completeness of image recognition. Taking an equilateral triangle defect as an example, when one of the symmetry axes of the triangle is consistent with the direction of the induced current on the metal surface, the clarity and completeness of the Bz image are the highest, and the detection effect is the best; as the azimuth changes, the Bz imaging effect gradually decreases. In general, the larger the angle formed by the defect boundary and the induced current, the more complete the image formed by the disturbing magnetic field Bz and the higher the clarity. Therefore, when using the disturbing magnetic field Bz imaging to realize defect morphology recognition, if the defect image is not very clear, certain measures can be taken, such as adjusting the probe azimuth until a clear image is obtained, so as to achieve effective detection. Of course, since the generated image is related to the direction of the eddy current formed by the excitation source on the metal surface, the identification of the position of the defect on the metal surface can also be achieved based on this characteristic.

[0049] In order to analyze the influence of coil structure on image recognition and explore the excitation source structure suitable for image recognition, we established Figure 5 (a) Figure 6 (a) The excitation coil structure model shown in Figure 1. Coils of different structures are all passed with a square wave excitation signal with a rising edge and falling edge time of 0.001s and a pulse width of 0.01s, and a square defect with a side length of 1mm.

[0050] Figure 5 (b) Figure 6 (b) The Bz images of the two different coil structures mentioned above. It can be observed that the specific shape of the defect cannot be clearly displayed in the image, and it is difficult to distinguish the shape of the defect. By continuously adjusting the position of the defect to obtain the corresponding Bz image, the approximate position and outline of the defect can be barely inferred, as shown by the dotted line in the figure. It is known from previous studies that using a rectangular coil as an excitation source can obtain an obvious defect image. Therefore, based on the image characteristics of various coil structures, it can be concluded that using a rectangular coil as the structure of the excitation coil can obtain a better image recognition effect.

[0051] The deep mechanism of the above phenomena and conclusions can be summarized into the following two points: (1) The excitation coils with different structures generate different directions of source magnetic fields. Different source magnetic fields thus generate eddy currents with different directions and sizes on the surface of the metal plate, which is the cause of the large differences in Bz imaging; (2) From the perspective of magnetic field disturbance, for a regularly arranged eddy current, a very small disturbance will cause a significant change in the magnetic field; while for an eddy current with inconsistent arrangement rules, the degree of disturbance will appear less obvious. Based on the above analysis, the cylindrical coil generates an eddy current in the shape of a ring on the surface of the object being measured, and its size and direction are symmetrically distributed about the axial direction. Due to the different degrees of obstruction of the defects at different positions of the eddy current in the shape of a ring, the disturbance generated is also irregularly distributed. Therefore, it is difficult for the disturbance magnetic field Bz imaging to reflect the specific morphological characteristics of the defects.

[0052] Embodiment 2:

[0053] like Figure 7 As shown, the present embodiment provides an electromagnetic identification device for metal surface defect morphology, comprising a detection device and a signal processing unit 2; the detection device is electrically connected to the signal processing unit 2; the detection device comprises a rectangular excitation coil 12 and a sensor unit 13 fixedly arranged up and down; the sensor unit 13 comprises a plurality of circular planar coils arranged along the XY plane; all the plurality of circular planar coils are evenly arranged; the detection device is used to generate induced eddy currents in the XY plane; the sensor unit 13 is used to collect magnetic field signals in the Z direction; the signal processing unit 2 is used to convert the magnetic field signals into image information.

[0054] Directly above the metal specimen 1 is a rectangular excitation coil, which is located at the relative magnetic permeability μ r =1, in air with relative dielectric constant ε=1; the coil is evenly wound on a rectangular frame with a length of 40mm, a width of 25mm, and a height of 25mm, with a thickness of 1mm, and the number of coil turns is set to 1000 turns. The excitation source is input through the excitation coil to generate a magnetic field; the electromagnetic field will pass through the protective layer, thereby generating uniform induced eddy currents on the surface of the specimen; the given rise time t r , Falling edge time t f Both are 0.001s, pulse width t w = 0.01s square wave signal; since the image formed by the defect morphology is caused by the disturbing magnetic field in the z direction, a circular planar coil in the xy plane is used to obtain the z direction magnetic field on the upper surface of the defect; the PC unit is responsible for converting the disturbing magnetic field into image information and displaying the image reflecting the defect morphology on the screen.

[0055] During the detection process, the probe of the detection device is attached to the surface of the metal protective layer to be tested, and the handheld probe is moved parallel to the metal surface; each time the probe moves to a position, it remains in place and pauses for a period of time; during this period of time, the sensor unit inside the detection device collects the disturbing magnetic field in the z direction of the area and transmits it to the signal processing unit; the signal processing unit processes the signal and sends it to the computer, and the computer software converts the magnetic field into a proportional image; the user can observe the status of metal surface defects in real time through the computer PC.

[0056] In summary, the electromagnetic identification method for the surface defect morphology of metal with a protective layer provided by the present invention can quickly and effectively obtain the surface defect morphology information of the metal with a protective layer by static detection, thereby improving the accuracy and evaluation capability of traditional electromagnetic nondestructive testing.

[0057] Those skilled in the art should understand that those skilled in the art can implement the above-mentioned variations by combining the prior art and the above-mentioned embodiments, which will not be described in detail here. Such variations do not affect the essential content of the present invention, and will not be described in detail here.

[0058] The above describes the preferred embodiments of the present invention. It should be understood that the present invention is not limited to the above-mentioned specific embodiments, and the devices and structures that are not described in detail should be understood to be implemented in a common manner in the art; any technician familiar with the art can make many possible changes and modifications without departing from the technical solution of the present invention, or modify them into equivalent embodiments with equivalent changes, which does not affect the essential content of the present invention. Therefore, any simple modifications, equivalent changes and modifications made to the above embodiments based on the technical essence of the present invention without departing from the content of the technical solution of the present invention are still within the scope of protection of the technical solution of the present invention.

Claims

1. A method for electromagnetic identification of surface defect morphology of metal with protective layer, characterized in that: The method comprises a detection device and a signal processing unit; the detection device comprises an excitation coil and a sensor unit; the method comprises the following steps: S101: inputting a square wave signal excitation source into the excitation coil, and the generated electromagnetic field passes through the protective layer on the metal surface, forming a uniform induced eddy current on the metal surface; S102: acquiring a disturbance magnetic field signal perpendicular to the metal surface through a sensor unit, and sending the disturbance magnetic field signal to a signal processing unit, wherein the sensor unit includes a plurality of circular planar coils arranged along an XY plane; S103: According to the transient response of the disturbance magnetic field when the square wave signal value changes, the signal processing unit converts the received magnetic field change into a proportional image for real-time display; S104: When the signal processing unit displays the defect image, adjust the direction position of the detection probe on the XY plane until the image is displayed clearly; The excitation coil in step S101 is a rectangular coil.

2. The electromagnetic identification method of the surface defect morphology of metal with protective layer as claimed in claim 1, characterized in that: The square wave signal in step 1 is given a rising edge time t r , Falling edge time t f , pulse width t w square wave signal.

3. A device for electromagnetically identifying metal surface defect morphology based on any one of claims 1-2, characterized in that: It includes a detection device and a signal processing unit; The detection device is electrically connected to the signal processing unit; the detection device includes an excitation coil and a sensor unit fixedly arranged up and down; the detection device is used to generate an induced eddy current in the XY plane; The sensor unit is used to collect magnetic field signals in the Z direction; The signal processing unit is used to convert the magnetic field signal into image information; Wherein, the excitation coil is a rectangular coil.

4. The electromagnetic identification device for metal surface defect morphology according to claim 3, characterized in that: The sensor unit includes a plurality of circular planar coils arranged along an XY plane; all of the plurality of circular planar coils are evenly arranged.

Images