A method for measuring the axial crack growth rate of pipelines based on the DC potential drop method

Abstract

The present invention discloses a method for measuring the axial crack growth rate of a pipeline based on the DC potential drop method. First, a reference sample containing double cracks and having the same material and geometric dimensions as the sample being tested is prepared, and the crack length of the reference sample is known; then, a DC potential drop measuring device is used to pass a DC current through the reference sample and the sample being tested, respectively, and the voltage between each pair of potential probes is measured and recorded; next, the relationship between the voltage of the sample being tested and time is converted into the relationship between the crack length and time through a calibration formula; finally, based on the relationship between the crack length of the sample being tested and time, the derivative of the crack length with respect to time is solved to determine the crack growth rate of the sample being tested. The method of the present invention has simple equipment and is easy to operate. The calibration formula it relies on is highly universal and reliable.

Description

Technical Field

[0001] The invention belongs to the technical field of experimental testing, and in particular relates to a method for measuring the axial crack growth rate of a pipeline based on a direct current potential drop method. Background Art

[0002] Pipelines, as crucial engineering structures, are widely used in oil and gas transportation, nuclear power, the chemical industry, and other fields. During processing and transportation, pipelines inevitably develop defects and scratches. In actual use, pipeline ruptures are common under various loads, such as high pressure, corrosive media, and extreme temperatures. These can easily lead to large-scale leaks and pollution, and even catastrophic accidents such as fires and explosions. Therefore, timely detection of pipeline cracks to ensure safe operation is a critical issue facing the industry.

[0003] In fact, unstable axial crack propagation is one of the main failure mechanisms in pipelines. During the initial stages of crack initiation and propagation, the axial crack propagation rate of the pipeline can be measured using the DC potential drop method, allowing timely repair measures to be taken, effectively preventing unstable crack propagation. The DC potential drop method is commonly used to measure the crack propagation rate of conductive components. It uses simple equipment, is easy to operate, and has high reliability. The DC potential drop method relies on a reference specimen and a calibration curve. The size and material of the reference specimen are generally consistent with those of the test specimen, and its crack length is known. The calibration curve establishes the relationship between the voltage between the potential probes and the crack length. This curve can be given by a calibration formula or determined through numerical simulation or experiment. For cracked plate components, the calibration curve is often given by the Johnson formula; for cracked pipelines, the calibration curve is often determined through numerical simulation or experiment, resulting in high cost and low versatility. Summary of the Invention

[0004] To overcome the shortcomings of the above-mentioned prior art, the purpose of the present invention is to provide a method for measuring the axial crack growth rate of pipelines based on the DC potential drop method. Based on the DC potential drop method, the present invention uses an analytical method to modify the traditional Johnson formula to obtain a calibration formula suitable for pipeline components containing axial penetrating cracks, and establishes a method for measuring the axial crack growth rate of pipelines.

[0005] The purpose of the present invention is to be achieved through the following technical solutions:

[0006] A method for measuring the axial crack growth rate of a pipeline based on a DC potential drop method includes the following steps:

[0007] The test sample has a first crack penetrating the pipe wall in the axial direction, and a first potential probe, a second potential probe, a third potential probe, a fourth potential probe, a first current probe, and a second current probe are provided on a circumference of the test sample coplanar with a mid-vertical plane of the first crack, wherein the first potential probe and the second potential probe are symmetrically located on either side of the first crack, the first potential probe and the fourth potential probe are symmetrical about the central axis of the test sample, the second potential probe and the third potential probe are symmetrical about the central axis of the test sample, and the first current probe and the second current probe are symmetrical about the central axis of the test sample, and a line connecting the application points of the first current probe and the second current probe is perpendicular to an axial section of the test sample passing through the first crack;

[0008] The reference sample is made of the same material and has the same geometric dimensions as the sample under test. A second crack and a third crack are provided on the reference sample. The second crack and the third crack have the same length. The orientation of the second crack on the reference sample is the same as the orientation of the first crack on the sample under test. The second crack and the third crack are symmetrical about the central axis of the reference sample. A fifth potential probe, a sixth potential probe, a third current probe, and a fourth current probe are provided on the reference sample. The positions of the fifth potential probe, the sixth potential probe, the third current probe, and the fourth current probe on the reference sample correspond to the positions of the first potential probe, the second potential probe, the first current probe, and the second current probe on the sample under test, respectively.

[0009] Using the DC potential drop measurement method, a DC current of magnitude I0 is passed through the reference sample through the third current probe and the fourth current probe, and the voltage U(a0) between the fifth potential probe and the sixth potential probe is measured;

[0010] Using a DC potential drop measurement method, a DC current of magnitude I is passed through the test sample via a first current probe and a second current probe, and the relationship between the voltage U(a) between the first potential probe and the second potential probe and the relationship between the voltage U(0) between the third potential probe and the fourth potential probe during the first crack propagation process are recorded.

[0011] The time-varying relationship of the length of the first crack is determined by using the time-varying relationship of the current I0, the voltage U(a0), the current I, the voltage U(a), and the time-varying relationship of the voltage U(0);

[0012] According to the relationship between the length of the first crack and time, the derivative of the length of the first crack with respect to time is solved to determine the expansion speed of the first crack on the tested sample.

[0013] Preferably, the midpoint of the first crack is located in the middle of the axial direction of the tested sample.

[0014] Preferably, a three-dimensional rectangular coordinate system is established with the pipeline axis as the Z axis, one end surface of the pipeline as the XY plane, and the center of the end surface as the origin. The pipeline is the test sample and the reference sample, so that the projection of the first crack on the XY plane is located on the positive half axis of the X axis. The length of the pipeline is 2W, the diameter is D, and the coordinates of the midpoint of the first crack are (D / 2, 0, W);

[0015] The time-varying relationship of the length of the first crack is determined by using the time-varying relationship of the voltage U(a) and the time-varying relationship of the voltage U(0) through the calibration formula;

[0016] The calibration formula is as follows:

[0017]

[0018]

[0019] I1=I-I2

[0020]

[0021] Where: a is half the length of the first crack; W is half the length of the tested sample; y is the arc length between the first potential probe and the center of the first crack; a0 is half the length of the second and third cracks.

[0022] Preferably, the propagation rate of the first crack is calculated by the following formula:

[0023]

[0024] Where: v is the growth velocity of the first crack; t is time.

[0025] Preferably, the lengths of the second crack and the third crack are known.

[0026] Preferably, both the reference sample and the tested sample are tubular samples.

[0027] Preferably, the first crack is an axial through-crack located in the tested sample, and the second crack and the third crack are axial through-cracks located in the reference sample.

[0028] Preferably, the measurement of the pipeline axial crack growth rate is achieved by a DC potential drop measuring device.

[0029] Preferably, the DC potential drop measuring device comprises a DC power supply, a first nanovoltmeter, a second nanovoltmeter and a computer;

[0030] Using a DC potential drop measurement method, a DC current of magnitude I is passed through a first current probe and a second current probe to a test sample, and a time-varying relationship of a voltage U(a) between the first potential probe and the second potential probe, as well as a time-varying relationship of a voltage U(0) between the third potential probe and the fourth potential probe, is recorded during a first crack extension process: the two poles of a DC power supply are connected to the first current probe and the second current probe, respectively, the first potential probe and the second potential probe are connected to the two poles of a first nanovoltmeter, the third potential probe and the fourth potential probe are connected to the second nanovoltmeter, and the first nanovoltmeter and the second nanovoltmeter are both connected to a computer;

[0031] Using the DC potential drop measurement method, a DC current of magnitude I0 is passed through the reference sample through the third current probe and the fourth current probe. When measuring the voltage U(a0) between the fifth potential probe and the sixth potential probe: the two poles of the DC power supply are connected to the third current probe and the fourth current probe respectively, the fifth potential probe and the sixth potential probe are connected to the two poles of the first nanovoltmeter respectively, and the first nanovoltmeter is connected to a computer.

[0032] The present invention has the following beneficial effects:

[0033] This paper establishes a method for measuring the axial crack growth rate in pipelines based on the DC potential drop method. Using a reference specimen containing two cracks, a DC potential drop measurement device is used to measure the time-varying relationship between the potential probes on the pipeline. This relationship is then converted into a time-varying relationship between the crack length using a theoretically derived calibration formula, which is then differentiated to obtain the crack growth rate. This measurement method requires simple equipment and is easy to operate. The calibration formula it relies on is highly versatile and reliable. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] Figure 1 This is a schematic diagram of measuring the pipeline axial crack growth rate based on the DC potential drop method of the present invention;

[0035] Figure 2 This is a schematic diagram of the current probe and potential probe placement device on the test sample of the present invention;

[0036] Figure 3 It is a schematic diagram of the installation positions of the current probe and the potential probe on the reference sample of the present invention.

[0037] In the figure, 1-test sample, 2-first current probe, 3-third potential probe, 4-fourth potential probe, 5-second current probe, 6-first potential probe, 7-second potential probe, 8-first crack, 9-reference sample, 10-third current probe, 11-third crack, 12-fourth current probe, 13-fifth potential probe, 14-sixth potential probe, 15-second crack, 16-DC power supply, 17-first nanovoltmeter, 18-second nanovoltmeter, 19-computer. DETAILED DESCRIPTION

[0038] The present invention is further described in detail below with reference to the accompanying drawings:

[0039] See also Figures 1 to 3 The present invention discloses a method for measuring the axial crack growth rate of a pipeline based on a DC potential drop method, which involves a test sample 1, a reference sample 9 and a DC potential drop measuring device.

[0040] The DC potential drop measurement device includes a DC power supply 16, a current probe, a potential probe, a nanovoltmeter, a power cord, a signal cord, and a computer 19. The current probe is fixed at a specific position on the sample (test sample 1, reference sample 9) and connected to the DC power supply via a power cord, providing a constant DC current to the sample. The potential probe is fixed at a specific position on the sample (test sample 1, reference sample 9) and connected to the nanovoltmeter via a signal cord. The potential probe is used to collect the temporal relationship of the potential at a specific position on the sample during the crack propagation process. The collected temporal relationship of the potential is transmitted to the computer to determine the voltage between each pair of potential probes, thereby obtaining the temporal relationship of the voltage.

[0041] The test specimen 1 of the present invention contains a first crack 8 extending axially along the test specimen 1. During the experiment, the first crack 8 continuously expands, and the length 2a (mm) of the first crack 8 continuously changes. A reference specimen 9 is made of the same material and has the same geometric dimensions as the test specimen 1. Reference specimen 9 contains two cracks of equal length, located symmetrically about the pipeline axis: a third crack 11 and a second crack 15. The crack lengths of the third crack 11 and the second crack 15 are known, both 2a0 (mm). During the experiment, the lengths of the third crack 11 and the second crack 15 on the reference specimen remain unchanged.

[0042] A pair of current probes (i.e., first current probe 2 and second current probe 5) and two pairs of potential probes are arranged on the test sample 1. The first pair of potential probes includes a first potential probe 6 and a second potential probe 7, and the second pair of potential probes includes a third potential probe 3 and a fourth potential probe 4. A pair of current probes (i.e., third current probe 10 and fourth current probe 12) and a pair of potential probes (i.e., fifth potential probe 13 and sixth potential probe 14) are arranged on the reference sample 9.

[0043] To facilitate the description of the positions of the current probe and the potential probe, the axis of the pipeline (referring to the tested sample 1 and the reference sample 9) is taken as the Z axis, and the axis of one end of the pipeline ( Figure 2 、 Figure 3 A three-dimensional rectangular coordinate system is established with the bottom surface (in the figure) as the XY plane and the center of the end surface as the origin, so that the projection of the first crack 8 on the test sample 1 on the XY plane is exactly on the positive half of the X axis. Assuming the pipeline length is 2W (mm), the pipeline diameter is D (mm), and the midpoints of the cracks (referring to the first crack 8, the second crack 15, and the third crack 11) are equidistant from the two ends of the pipeline, that is, the Z coordinate of the crack midpoint is W.

[0044] The coordinates of the first current probe 2 on the test sample 1 are (0, D / 2, W), and the coordinates of the second current probe 5 are (0, -D / 2, W). Positions of the two pairs of potential probes on the test sample 1: (1) The Z coordinates of the first potential probe 6 and the second potential probe 7 are both W (mm); (2) Positions of the first potential probe 6 and the second potential probe 7 on the XY plane: distributed on both sides of the first crack 8, with the arc lengths of the first potential probe 6 and the second potential probe 7 from the center of the first crack 8 being y (mm); The position of the third potential probe 3 on the XY plane is symmetrical with the position of the second potential probe 7 about the Z axis, and the position of the fourth potential probe 4 on the XY plane is symmetrical with the position of the first potential probe 6 about the Z axis.

[0045] The coordinates of the third current probe 10 on the reference specimen 9 are (0, D / 2, W), and the coordinates of the fourth current probe 12 are (0, -D / 2, W). Positions of a pair of potential probes (i.e., the fifth potential probe 13 and the sixth potential probe 14) on the reference specimen 9: (1) The Z coordinates of the fifth potential probe 13 and the sixth potential probe 14 are both W (mm); (2) Positions on the XY plane: The fifth potential probe 13 and the sixth potential probe 14 are distributed on both sides of one of the two cracks (i.e., the second crack 15 and the third crack 11) on the reference specimen 9, and the fifth potential probe 13 and the sixth potential probe 14 are 1 / 4 of the distance from the crack (referring to the second crack 15 or the third crack 11, Figure 3 The arc length of the center of the second crack 15) is shown in y (mm).

[0046] A DC current of I (mA) is passed through the test sample 1, and the voltage between the first pair of potential probes is measured as U(a) (mV), and the voltage between the second pair of potential probes is measured as U(0) (mV). A DC current of I0 (mA) is passed through the reference sample 9, and the voltage between the potential probes is measured as U(a0) (mV).

[0047] The calibration formula establishes the relationship between crack length and voltage. Through the calibration formula, the change in voltage over time of the test sample can be converted into the change in crack length over time.

[0048] The calibration formula is as follows:

[0049]

[0050]

[0051] I1=I-I2 (3)

[0052]

[0053] Where:

[0054] W is half of the pipe length (mm);

[0055] y is the distance from the potential probe to the crack center (mm);

[0056] a is half the length of the first crack 8 on the test sample 1 (mm);

[0057] a0 is half the length of the second crack 15 and the third crack 11 (mm);

[0058] I is the magnitude of the DC current flowing through the test sample 1 (mA);

[0059] I0 is the magnitude of the DC current passed through reference sample 9 (mA);

[0060] U(a) is the voltage (mV) between the first potential probe 6 and the second potential probe 7 on the test sample 1;

[0061] U(0) is the voltage between the third potential probe 3 and the fourth potential probe 4 on the test sample 9 (mV);

[0062] U(a0) is the voltage (mV) between the fifth potential probe 13 and the sixth potential probe 14 on the reference sample 9.

[0063] The formula for calculating the crack growth rate is as follows:

[0064]

[0065] Where:

[0066] v is the growth rate of the first crack 8 (mm / s);

[0067] t is time (s).

[0068] The steps of using the pipeline axial crack growth rate measurement method based on the DC potential drop method of the present invention are as follows:

[0069] Step 1: Prepare a reference specimen 9 made of the same material and with the same geometric dimensions as the test specimen 1. Reference specimen 9 contains two cracks of equal length and symmetrically positioned about the pipe axis (i.e., the second crack 15 and the third crack 11). The lengths of these two cracks on reference specimen 9 are known, both 2a0 (mm).

[0070] Step 2: Using a DC potential drop measurement device, pass a DC current of I0 (mA) through the reference sample 9 via the current probes, and measure the voltage U(a0) between the potential probes on the reference sample;

[0071] Step 3: Using a DC potential drop measurement device, pass a DC current of I (mA) through the current probe to the test sample 1, and record the change in the voltage U(a) and U(0) between the two pairs of potential probes over time during the crack propagation process;

[0072] Step 4: Convert the relationship between the voltage of the tested sample and time into the relationship between the crack length and time through the calibration formula (Formula (1) to Formula (4));

[0073] Step 5: Based on the relationship between the crack length of the tested sample and time, solve the derivative of the crack length with respect to time (Equation (5)) to determine the crack growth rate of the tested sample.

Claims

1. A method for measuring the axial crack growth rate of a pipeline based on the DC potential drop method, characterized in that: The process includes the following: The test sample (1) has a first crack (8) penetrating the pipe wall thickness in the axial direction, and a first potential probe (6), a second potential probe (7), a third potential probe (3), a fourth potential probe (4), a first current probe (2) and a second current probe (5) are provided on the circumference of the test sample (1) coplanar with the mid-vertical plane of the first crack (8), wherein the first potential probe (6) and the second potential probe (7) are symmetrically located on both sides of the first crack (8), the first potential probe (6) and the fourth potential probe (4) are symmetrical about the central axis of the test sample (1), the second potential probe (7) and the third potential probe (3) are symmetrical about the central axis of the test sample (1), the first current probe (2) and the second current probe (5) are symmetrical about the central axis of the test sample (1), and the line between the application points of the first current probe (2) and the second current probe (5) is perpendicular to the axial section of the test sample (1) passing through the first crack (8); The reference sample (9) is made of the same material and has the same geometric dimensions as the sample to be tested (1). A second crack (15) and a third crack (11) are provided on the reference sample (9). The second crack (15) and the third crack (11) have the same length. The orientation of the second crack (15) on the reference sample (9) is the same as the orientation of the first crack (8) on the sample to be tested (1). The second crack (15) and the third crack (11) are symmetrical about the central axis of the reference sample (9). A fifth potential probe (13), a sixth potential probe (14), a third current probe (10) and a fourth current probe (12) are provided on the reference sample (9). The positions of the fifth potential probe (13), the sixth potential probe (14), the third current probe (10) and the fourth current probe (12) on the reference sample (9) correspond to the positions of the first potential probe (6), the second potential probe (7), the first current probe (2) and the second current probe (5) on the sample to be tested (1). Using the DC potential drop measurement method, a current of magnitude of I 0 DC current, measure the voltage between the fifth potential probe (13) and the sixth potential probe (14) U ( a 0); Using the DC potential drop measurement method, a current of magnitude of I The DC current is recorded during the expansion of the first crack (8) and the voltage between the first potential probe (6) and the second potential probe (7) is recorded. U ( a ) over time, and the voltage between the third potential probe (3) and the fourth potential probe (4) U (0) Relationships over time; Using current I 0. Voltage U ( a 0) Current I ,Voltage U ( a ) changes with time and voltage U (0) Determine the relationship between the length of the first crack (8) and its change over time; According to the relationship between the length of the first crack (8) and time, the derivative of the length of the first crack (8) with respect to time is solved to determine the expansion speed of the first crack (8) on the test sample; A three-dimensional rectangular coordinate system is established with the pipeline axis as the Z axis, one end face of the pipeline as the XY plane, and the center of the end face as the origin. The pipeline is the test sample (1) and the reference sample (9), so that the projection of the first crack (8) on the XY plane is located on the positive half axis of the X axis. The length of the pipeline is 2 W , diameter is D , the coordinates of the midpoint of the first crack (8) are ( D / 2, 0, W ); By calibrating the formula, using the voltage U ( a ) changes with time and voltage U (0) Determine the relationship between the length of the first crack (8) and its change over time; The calibration formula is as follows: Where: a is half the length of the first crack (8); W Half the length of the tested sample (1); y is the arc length between the first potential probe (6) and the center of the first crack (8); U ( a ) is the voltage between the first potential probe (6) and the second potential probe (7) on the test sample (1); U ( a 0) is the voltage between the fifth potential probe (13) and the sixth potential probe (14) on the reference sample (9); I 0 is the magnitude of the DC current flowing through the reference sample (9); I is the magnitude of the DC current flowing through the test sample (1); U (0) is the voltage between the third potential probe (3) and the fourth potential probe (4) on the test sample (9); a 0 is half the length of the second crack (15) and the third crack (11).

2. The method for measuring the axial crack growth rate of a pipeline based on the DC potential drop method according to claim 1 is characterized in that: The midpoint of the first crack (8) is located in the middle of the axial direction of the test sample (1).

3. The method for measuring the axial crack growth rate of a pipeline based on the DC potential drop method according to claim 1 is characterized in that: The growth rate of the first crack (8) is calculated by the following formula: Where: v is the growth rate of the first crack (8); t For time.

4. The method for measuring the axial crack growth rate of a pipeline based on the DC potential drop method according to claim 1, characterized in that: The lengths of the second crack (15) and the third crack (11) are known.

5. The method for measuring the axial crack growth rate of a pipeline based on the DC potential drop method according to claim 1, characterized in that: The reference specimen (9) and the tested specimen (1) are both tubular specimens.

6. The method for measuring the axial crack growth rate of a pipeline based on the DC potential drop method according to claim 1, characterized in that: The first crack (8) is an axial penetration crack located in the test sample (1), and the second crack (15) and the third crack (11) are axial penetration cracks located in the reference sample (9).

7. The method for measuring the axial crack growth rate of a pipeline based on the DC potential drop method according to claim 1, characterized in that: The measurement of the pipeline axial crack growth rate is achieved by using a DC potential drop measuring device.

8. The method for measuring the axial crack growth rate of a pipeline based on the DC potential drop method according to claim 7, characterized in that: The DC potential drop measuring device comprises a DC power supply (16), a first nanovoltmeter (17), a second nanovoltmeter (18) and a computer (19); Using the DC potential drop measurement method, a current of magnitude of I The DC current is recorded during the expansion of the first crack (8) and the voltage between the first potential probe (6) and the second potential probe (7) is recorded. U ( a ) over time, and the voltage between the third potential probe (3) and the fourth potential probe (4) U (0) When the relationship changes with time: connect the two poles of the DC power supply (16) to the first current probe (2) and the second current probe (5) respectively, connect the first potential probe (6) and the second potential probe (7) to the two poles of the first nanovoltmeter (17) respectively, connect the third potential probe (3) and the fourth potential probe (4) to the second nanovoltmeter (18) respectively, and connect the first nanovoltmeter (17) and the second nanovoltmeter (18) to the computer (19); Using the DC potential drop measurement method, a current of magnitude of I 0 DC current, measure the voltage between the fifth potential probe (13) and the sixth potential probe (14) U ( a 0): connect the two poles of the DC power supply (16) to the third current probe (10) and the fourth current probe (12) respectively, connect the fifth potential probe (13) and the sixth potential probe (14) to the two poles of the first nanovoltmeter (17) respectively, and connect the first nanovoltmeter (17) to the computer (19).

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