Correction method for detecting wall thickness of small block defect by pulsed eddy current

Abstract

The invention is applicable to the field of metal material nondestructive testing, and provides a method for correcting wall thickness of a small block defect in pulsed eddy current testing, which comprises the following steps: starting a pulsed eddy current testing probe, and exciting a pulsed magnetic field by the probe to induce an eddy current field in a tested metal conductor; setting a detection step length, gradually scanning and detecting the defect direction of the detected metal conductor small block by a detection probe from a starting point, and automatically calculating and acquiring a wall thickness value at each detection point position; recording an initial change detection point and an end point change detection point of the wall thickness value obtained by scanning detection, calculating the distance DSE between the initial change detection point and the end point change detection point by combining the step length, and then calculating the defect size LDEF of a small block; and the minimum value h of the wall thickness values obtained through scanning detection is taken, and the actual wall thickness value h2 of the small block defect is calculated according to a correction formula. Through the correction method, the block defects smaller than the footprint size can be correctly evaluated, and serious consequences caused by wrong evaluation of small block serious defects are avoided.

Description

TECHNICAL FIELD

[0001] The present application belongs to the field of non-destructive testing of metal materials, and particularly relates to a correction method for detecting the wall thickness of small defects by pulse eddy current. BACKGROUND

[0002] Ferromagnetic metal components are widely used in industry, such as pipelines, rails, special pressure equipment, and building bridges. The ferromagnetic material equipment made of the ferromagnetic metal components may form corrosion defects such as thinning and local holes during service due to the influence of various factors, which threatens the safety and reliability of the equipment. Therefore, it is crucial to regularly conduct effective non-destructive testing and evaluation of the ferromagnetic metal components.

[0003] Currently, the non-destructive testing methods for ferromagnetic metal components include magnetic flux leakage testing, magnetic memory testing, infrared thermal imaging testing, and pulse eddy current testing. Among them, the pulse eddy current testing uses a square wave excitation signal, and its significant feature is that the signal contains rich harmonic components, especially the low-frequency harmonic components can effectively overcome the influence of the skin effect, thereby enabling the detection system to detect deeper defects and providing a solid theoretical basis for the detection of surface and subsurface defects of ferromagnetic metal components.

[0004] The probe part of the pulse eddy current testing mainly consists of an excitation coil and a sensor. The excitation coil is excited by a square wave pulse, and the excitation coil generates a pulse magnetic field and excites an eddy current field in the detected conductor. The eddy current field induces a secondary magnetic field. The magnetic field contains the thickness information of the metal conductor, and the sensor collects the magnetic field signal. By analyzing the magnetic field signal, the thickness value of the detected conductor can be obtained. This is the principle of pulse eddy current testing of the wall thickness of the detected metal conductor.

[0005] The magnetic field excited by the probe projects onto the detected metal and forms an eddy current area, which is called the probe projection footprint (abbreviated as footprint FP). The probe detects the average wall thickness within the footprint. As shown in Figure 1 When the defect area is larger than the footprint, the probe can detect the accurate wall thickness value. When the defect size is smaller than the footprint, the detected wall thickness is the average wall thickness of the defect and the nearby area, so there is a deviation between the result and the actual value. The detection result may cause the wall thickness to be larger, which may not be sufficient for risk assessment. Therefore, the current pulse eddy current testing method has this problem. SUMMARY

[0006] In view of the above problems, the purpose of the present application is to provide a correction method for detecting the wall thickness of small defects by pulse eddy current, which aims to solve the technical problem that the detection result deviates significantly from the actual value when the defect area is smaller than the footprint in the current pulse eddy current testing method.

[0007] The present application adopts the following technical solutions:

[0008] The correction method of the pulse eddy current detection small piece defect wall thickness comprises the following steps:

[0009] Step S1, turn on the pulse eddy current detection probe, the probe excites a pulse magnetic field to induce an eddy current field in the detected metal conductor;

[0010] Step S2, set the detection step, the detection probe starts from the starting point and gradually scans and detects the small piece defect direction of the detected metal conductor, and automatically calculates and obtains the wall thickness value at each detection point;

[0011] Step S3, record the starting change detection point and the ending change detection point of the wall thickness value obtained by scanning and detecting, and calculate the distance D of the two detection points of the starting change and the ending change combined with the step SE , and then calculate the size L of the small piece defect DEF ;

[0012] Step S4, take the minimum value h of the wall thickness value obtained by scanning and detecting, and calculate the actual wall thickness value h2 of the small piece defect according to the correction formula:

[0013]

[0014] Wherein, μ = L DEF / L FP , L FP is the footprint size of the detection probe, and h1 is the wall thickness value of the complete area of the detected metal conductor.

[0015] Further, the detection step range is set to be between 10mm-50mm.

[0016] Further, the distance D SE is equal to the number of steps between the starting change and the ending change two detection points multiplied by the step length.

[0017] The beneficial effects of the present application are: when the defect size is smaller than the footprint, the detected wall thickness is the average wall thickness of the defect and the nearby area, the correction method of the present application can roughly judge the size of the defect in the direction according to the gradual scanning and detection of the detection probe along the defect direction, and then the actual wall thickness is inversely deduced according to the defect size and the footprint size. Through this correction method, the block defect smaller than the footprint size can be correctly evaluated, and the wrong evaluation of the small block defect is avoided to avoid serious consequences. BRIEF DESCRIPTION OF DRAWINGS

[0018] Figure 1 is a schematic diagram of the relationship between the footprint and the defect;

[0019] Figure 2 is a schematic diagram when the probe footprint FP does not enter the defect area;

[0020] Figure 3is a schematic diagram of the probe footprint FP entering the defect area;

[0021] Figure 4 is a curve diagram of the probe scanning defect result;

[0022] Figure 5 is a schematic diagram of the calculation formula of the actual wall thickness value h2. DETAILED DESCRIPTION

[0023] In order to make the objects, technical solutions and advantages of the present application clearer, the present application will be further described in detail below in combination with the drawings and examples. It should be understood that the specific examples described herein are only used to explain the present application and do not limit the present application.

[0024] In order to illustrate the technical solutions of the present application, the following specific examples are used to illustrate the present application.

[0025] The footprint area of the detection probe is in the non-defect area and covers the defect, and the detection signal will change obviously. The present application gradually scans and detects in the direction of the defect, and then roughly judges the size of the defect in the direction. After the size of the defect is determined, the actual wall thickness value is inversely calculated by proportional operation with the size of the footprint, so that the correction of the small defect wall thickness can be realized. Specifically, the pulse eddy current detection small defect wall thickness correction method provided by the present embodiment comprises the following steps:

[0026] Step S1, turn on the pulse eddy current detection probe, and the probe excites a pulse magnetic field to induce an eddy current field in the detected metal conductor.

[0027] The principle of the pulse eddy current detection probe has been described in the background art, and will not be repeated here. The detection probe excites a pulse magnetic field to induce an eddy current field in the detected metal conductor. The sensor in the detection probe collects the secondary magnetic field data formed by the eddy current field, so as to calculate the wall thickness of the detected metal conductor.

[0028] Step S2, set a detection step, and the detection probe starts from the starting point and gradually scans and detects in the direction of the small defect of the detected metal conductor, and automatically calculates and obtains the wall thickness value at each detection point.

[0029] The detected metal conductor is a metal plate, as shown in Figure 2 The footprint area (EP shown in the figure) of the detection probe does not cover the defect. The detection probe excites a pulse magnetic field to induce an eddy current field in the metal plate AB area. A suitable value between 10mm-50mm is selected as the detection step for scanning and detection, and the scanning and detection is gradually performed in the direction of the small defect of the metal plate.

[0030] Step S3, record the start change detection point and the end change detection point of the wall thickness value obtained by scanning detection, and calculate the distance D of the two detection points according to the step length SE Then calculate the size L of the small defect DEF .

[0031] For example Figure 4 A specific detection example is recorded, which has a total of 18 detection points, and a detection wall thickness value is automatically calculated at each detection point. When the footprint FP just starts to intersect with the small defect, that is, the B point enters the defect area, the detection result will change, and the data at this time are as shown in the S point in Figure 4 , which is the start point of the detection probe entering the defect area, that is, the start change detection point. Similarly, the detection probe continues to scan forward at an equal step length, as shown in Figure 3 , at this time the probe footprint FP has entered the defect area, and then continues to scan, and after the footprint FP just does not intersect with the defect, that is, the A point leaves the defect area, the detection result will change, and the data at this time are as shown in the E point in Figure 4 , which is the end point of the detection probe leaving the defect area, that is, the end change detection point, and then the data will tend to be stable.

[0032] Therefore, the distance D between the start change and the end change of the two detection points can be obtained by multiplying the step length by the number of steps between the two detection points SE .

[0033] Therefore, the size L of the defect DEF = D SE -L FP ; L FP is the footprint size of the detection probe.

[0034] Step S4, take the minimum value h of the wall thickness value obtained by scanning detection, and calculate the actual wall thickness value h2 of the small defect according to the correction formula:

[0035]

[0036] Wherein, μ = L DEF / L FP , L FP is the footprint size of the detection probe, and h1 is the wall thickness value of the complete area of the detected metal conductor.

[0037] For the above formula, as shown in Figure 5 , the minimum wall thickness value h actually detected is the equivalent thickness of the defect position, and the area S of the polygon AGHBKJ is:

[0038] S = L FP *h1-L DEF *(h1-h2) = L FP *h

[0039] h1-μ*(h1-h2)=h

[0040]

[0041] Therefore, the above calculation formula can be obtained.

[0042] Here is a specific experimental example:

[0043] The experimental object was a Q235 steel plate measuring 1000mm in length, 50mm in width, and 10mm in thickness. A defect measuring 50mm in length, 30mm in width, and 6mm in thickness was located far from the edge of the steel plate. The size L of the probe footprint FP was also measured. FP =70mm. h1=10mm. The probe scans along the defect centerline from left to right in 10mm increments. Data is shown in Table 1.

[0044] Table 1 Scanning Data

[0045] Detection point 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 Detection wall thickness 9.9 10 9.3 8.8 8.4 7.8 7.6 7.4 7.6 7.7 7.8 8.3 8.7 9.2 9.9 9.9

[0046] like Figure 4 As shown, point S is the 3rd scan point, and point E is the 16th scan point. The number of steps between the two points is 12, therefore we can obtain D. SE =10mm * 12 = 120mm, defect size L DEF =D SE -L FP =50mm, based on the data in Table 1, the minimum wall thickness was measured to be h = 7.4mm, μ = L DEF / L FP =0.714. Based on the formula for calculating the actual wall thickness h2, the calculated defect wall thickness h2 = 6.36 mm. The actual thickness of the steel plate is 6 mm, which is closer to the actual value of 6 mm than the detected value of 7.4 mm. Therefore, it can play a corrective role for the detection of small defects. However, the overall error is still slightly large because the lateral dimension of the defect is only 30 mm, which is smaller than the probe footprint size. This correction method is a primary correction, and the small lateral dimension of the defect affects the calculation deviation. If the lateral dimension of the defect is larger than the footprint size, the calculation result will be very close to the actual defect wall thickness.

[0047] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method of calibrating a pulsed eddy current inspection of a small defect wall thickness, characterized by, The correction method comprises the following steps: Step S1, turn on the pulsed eddy current detection probe, and the probe generates a pulsed magnetic field to induce an eddy current field in the detected metal conductor; Step S2, set a detection step, and the detection probe starts from the starting point and gradually scans and detects in the direction of the small block defect of the detected metal conductor, and the wall thickness value is automatically calculated and obtained at each detection point; Step S3, record the start change detection point and the end change detection point of the wall thickness value detected by the scanning detection, and calculate the distance D of the two detection points of the start change and the end change in combination with the step length SE Then calculate the size L of the small block defect DEF ; Step S4, take the minimum value h of the wall thickness value obtained by scanning and detecting, and calculate the actual wall thickness value h2 of the small block defect according to the correction formula: where μ = L DEF / L FP , L FP is the footprint size of the detection probe, and h1 is the wall thickness value of the complete region of the metal conductor under test.

2. The method of claim 1, wherein the pulse eddy current testing is performed on a small piece of the wall thickness to be inspected. The detection step is set to be between 10mm-50mm.

3. The method of claim 2, wherein the pulse eddy current testing is performed on a wall thickness of a small piece of a defect. Distance D SE is equal to the number of steps between the two detection points of the start change and the end change multiplied by the step length.

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