CN115046872A - Fatigue crack real-time measuring method - Google Patents
Fatigue crack real-time measuring method Download PDFInfo
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Abstract
The invention relates to a fatigue crack real-time measuring method, S1, selecting a CT sample; s2, positioning the DCPD system to measure fatigue cracks through a direct current voltage drop method; s3, selecting COMSOL software to carry out finite element simulation calibration and establishing a model; s4, determining the optimal voltage probe spacing, deriving the probe potential difference of each crack length, drawing a relation curve of the crack length and the voltage drop, and obtaining the real-time crack length from the output voltage according to the crack length-voltage curve obtained by calibration in the subsequent test, and then calculating the real-time crack propagation rate; s5, carrying out the overload test of unimodal tension under constant amplitude and different overload ratios by using various materials, and calculating and processing the test result. The invention has the following advantages: the structural strength and the assembly precision of the arc sections are ensured. The error of a test system is reduced, the reliability and the stability of a test result are improved, and the crack length and the fatigue life precision obtained through measurement are high.
Description
The technical field is as follows:
the invention relates to the field of fatigue cracks, in particular to a real-time measurement method for fatigue cracks.
Background art:
the current fatigue crack measuring methods mainly comprise an eye measuring method, a machine vision method, an optical measuring method, an acoustic emission method, a flexibility method and an alternating current voltage drop method. The visual inspection method adopts naked eyes or a microscope, and an electron microscope carries out direct image measurement on the length of the crack, and has very limited application occasions and no guarantee on accuracy because real-time automatic observation cannot be realized, the tiny crack cannot be identified, and the measurement cannot be carried out in severe environments such as high temperature and high pressure; the machine vision method is based on DIC technology, uses computer to assist and carries on the large-scale calculation, it is more accurate to compare with visual method, but can't measure the crack inside the structure too and need expensive hardware support; the optical measurement method has very high requirements on system hardware, and can only measure surface cracks; the acoustic emission method is complex in device and cannot measure cracks in real time; the flexibility method needs to use the relation between the crack length and the crack opening displacement in fracture mechanics for auxiliary calculation, and because the introduced actual variables are too much, the theoretical calculation is often greatly deviated from the actual condition; the alternating current voltage drop method can overcome the defects of the methods according to the skin effect of alternating current, is suitable for severe environments, can achieve real-time measurement, and has the obvious defects that the method is easy to cause huge errors due to the fact that the method is sensitive to crack length and slight influence, and noise influencing a measurement result is generated due to the fact that alternating current is unstable.
The invention content is as follows:
the invention aims to overcome the defects, and provides a fatigue crack real-time measuring method, which reduces the error of a test system, increases the reliability and stability of a test result, and has high crack length and fatigue life precision.
The purpose of the invention is realized by the following technical scheme: a fatigue crack real-time measuring method comprises the following specific steps:
s1, selecting a CT sample, wherein the CT sample is made of various metal materials, the CT sample is clamped in front of a fatigue testing machine, two sides of the CT sample are polished, guide grooves with the depth of 5% -6% of the thickness are formed in the two sides of the CT sample, and a plurality of screw holes are processed on the CT sample to fix probes on a DCPD system, so that poor contact and even falling off of the probes caused by vibration in the test are prevented;
s2, positioning a DCPD system to perform fatigue crack measurement through a direct current voltage drop method, wherein the DCPD system comprises a voltmeter for acquiring DCPD signals, a multi-channel acquisition system for acquiring various required output signals and being in butt joint with a computer for storage, and a direct current power supply system for providing reversible steady direct current;
s3, selecting COMSOL software to carry out finite element simulation calibration, establishing a model, setting insulation on each surface of a CT sample, setting a zero potential energy surface of the CT sample, carrying out grid division according to the size of a COMSOL superfine unit, and manually encrypting a grid at the tip of a crack; simulating that the crack starts from the initial crack length by using a parameter scanning function, forwards expands by 15mm-16mm, records point position data every 0.5mm-0.6mm, uses different potential probe distances for each simulation according to a control variable method, researches the influence of the changed potential probe distance on the crack length-voltage calibration result, finds the optimal potential probe distance which is good in linearity of the calibration curve in the whole crack expansion range and sensitive to crack change as the screw hole punching distance of a subsequent test;
s4, determining the optimal voltage probe spacing, deriving the probe potential difference of each crack length, drawing a relation curve of the crack length and the voltage drop, and obtaining the real-time crack length from the output voltage according to the crack length-voltage curve obtained by calibration in the subsequent test, and then calculating the real-time crack propagation rate;
s5, carrying out unimodal stretching overload tests under constant amplitude and different overload ratios by using various materials, controlling a measurement module of the DCPD system by using a peak-valley mode, carrying out fatigue crack prefabrication and constant amplitude fatigue tests by using a K-lowering method, calculating Paris formula constants C and m of constant amplitude loading, analyzing and calculating the fatigue life and the crack propagation rate under the condition of unimodal overload, and calculating and processing test results.
The invention is further improved in that: in step S5, the K-lowering method controls the load size according to the following formula:
wherein the content of the first and second substances,
in order to be the magnitude of the stress intensity factor,
the gradient c is kept constant for the corresponding maximum stress intensity factor amplitude at the beginning of the K-drop.
The invention is further improved in that: in step S5, the overload test method specifically includes: let Δ K from
Down to
The length of the prefabricated crack is 3 mm; keeping the delta K unchanged and extending to 5mm, immediately applying a single-peak overload with the frequency of 10Hz after the program judges that the delta K is extended to 5mm, and continuously using the delta K for loading after the overload; and when the real-time expansion rate displayed by the DCPD software returns to the normal rate and 10000 cycles are kept stable, manually stopping the test, and exporting test data.
The invention is further improved in that: in step S1, the multiple materials are low-carbon steel Q460C, high-carbon steel 1080, medium-carbon steel 1045, and 12NiCr6 nichrome steels with two different heat treatment modes; wherein, two different heat treatment modes of the 12NiCr6 nickel-chromium steel are as follows: (1) keeping the temperature at 880 ℃ for 1 hour, and normalizing; (2) keeping the temperature at 880 ℃ for 1 hour, and performing water quenching and tempering at 500 ℃.
The invention is further improved in that: in step S5, before the overload test, the pin in contact with the CT sample was wrapped with an insulating tape, and then temperature compensation was performed, with an input current of 1.5A and a gap between potential probes of 18mm, and the apparatus, the CT sample, the probes, and the wires were fixed to start the test.
The invention is further improved in that: the temperature compensation method is to use a piece of metal with the same material as a reference sample to eliminate the temperature influence.
The invention is further improved in that: in step S2, the real-time crack length is calculated according to the formula
：
In the above formula, the first and second carbon atoms are,
in order to be the length of the crack,
is half the distance of the voltage probe,
in order to be the width of the test piece,
for the purpose of the initial crack length,
and (4) calculating the real-time crack propagation rate according to the cycle number after the real-time crack length is measured for the voltage drop after standardization.
The invention is further improved in that: in step S2, the dc power supply system is equipped with a solid-state relay bridge.
Compared with the prior art, the invention has the following advantages:
finite element analysis was performed using COMSOL software and experimentally compared to determine the potential probe position and nominal voltage-crack length curve required for measurements using the DCPD method. Five materials are used for unimodal tensile overload tests under constant amplitude and different overload ratios, Paris formula coefficients C and m of the materials are calculated, and the accuracy and the stability of the DCPD method for real-time fatigue crack measurement under the conditions of constant amplitude and overload are verified. The test method and the DCPD measurement method are reliable, the data noise is low, the DPCD method is verified to be good in stability, the signal noise is low, and the measured crack length and fatigue life precision are high.
Description of the drawings:
FIG. 1 is a plan view of a finite element model in step S3 according to the present invention.
FIG. 2 is a schematic diagram of the setting of the zero potential energy surface in step S3 according to the present invention.
Fig. 3 is a schematic diagram of the grid division in step S3 according to the present invention.
FIG. 4 is a diagram illustrating potential simulation results of the CT sample in step S3 according to the present invention.
FIG. 5 is a schematic diagram showing the relationship between the normalized voltage drop and the crack length calculated by the formula of the DCPD system and the finite element method in the invention and the comparison of the actual measurement result in the fatigue crack propagation test.
FIG. 6 is a graphical representation of normalized voltage drop and crack length without passing the voltage probe spacing in step S4 of the present invention.
FIG. 7 is a graph illustrating the crack length versus normalized voltage drop of the present invention.
FIG. 8 is a diagram illustrating the K-lowering method in step S5 according to the present invention.
Fig. 9 is a schematic diagram of the load applied in step S5 according to the present invention.
FIG. 10 is a graph showing the crack growth rate curve of the low carbon steel Q460C test in accordance with the present invention.
FIG. 11 is a graphical representation of two material parameters of the low carbon steel Q460C based on a set of data points in accordance with the present invention.
Fig. 12 to 14 are graphs showing the grouping results of the three groups of test crack propagation rates of the low carbon steel Q460C in the invention.
FIG. 15 is a schematic diagram of two material parameters of the low carbon steel Q460C based on a small sample method.
FIG. 16 is a graph showing the crack propagation life curve of the low carbon steel Q460C according to the present invention at an overload ratio of 2.
FIG. 17 is a graph showing the crack propagation life curve of the low carbon steel Q460C at an overload ratio of 2.2 in accordance with the present invention.
Fig. 18 is a graph showing the crack propagation life curve of the low carbon steel Q460C according to the present invention at an overload ratio of 2.5.
FIG. 19 is a graph of crack propagation life for mild steel Q460C at different overload ratios according to the present invention.
FIG. 20 is a graph showing experimental calculated crack growth rates of the low carbon steel Q460C after overload in the present invention.
The specific implementation mode is as follows:
for the purpose of enhancing the understanding of the present invention, the present invention will be further described in detail with reference to the following examples and the accompanying drawings, which are only used for explaining the present invention and are not to be construed as limiting the scope of the present invention.
The method for measuring the fatigue crack in real time comprises the following specific steps:
s1, selecting a CT sample, wherein the CT sample is made of various metal materials, the CT sample is clamped in front of a fatigue testing machine, two sides of the CT sample are polished, guide grooves with the depth of 5% -6% of the thickness are formed in the two sides of the CT sample, and a plurality of screw holes are processed on the CT sample to fix probes on a DCPD system, so that poor contact and even falling off of the probes caused by vibration in the test are prevented;
the materials are low-carbon steel Q460C, high-carbon steel 1080, medium-carbon steel 1045 and 12NiCr6 nickel-chromium steel with two different heat treatment modes; wherein, two different heat treatment modes of the 12NiCr6 nickel-chromium steel are as follows: (1) keeping the temperature at 880 ℃ for 1 hour, and normalizing; (2) keeping the temperature at 880 ℃ for 1 hour, and performing water quenching and tempering at 500 ℃.
S2, positioning a DCPD system to measure fatigue cracks through a direct current voltage drop method, wherein the DCPD system comprises a voltmeter for acquiring the DCPD signals and a computer for acquiring various required output signals and butt-jointing and storing the signals with the computerThe system comprises a multi-channel acquisition system and a direct current power supply system for providing reversible stable direct current, wherein the direct current power supply system is loaded with a solid relay bridge, when the stable direct current passes through a crack area of a CT sample, the change of the length of a crack changes the resistance of the crack area, so that the potential difference among a plurality of probes is changed, the real-time length of the crack is reflected by measuring the potential difference at two sides of the crack, and the crack propagation rate is measured; calculating the real-time crack length according to a formula
：
In the above formula, the first and second carbon atoms are,
in order to be the length of the crack,
is half the distance of the voltage probe,
in order to be the width of the test piece,
for the purpose of the initial crack length,
and (4) calculating the real-time crack propagation rate according to the cycle number after the real-time crack length is measured for the voltage drop after standardization.
S3, selecting COMSOL software to carry out finite element simulation calibration, building a model as shown in figure 1, setting insulation on each surface of a CT sample as shown in figure 2, setting a zero potential energy surface of the CT sample as shown in figure 3, carrying out mesh division through the size of a COMSOL superfine unit, and manually encrypting a mesh at the crack tip; simulating that the crack starts from the initial crack length by using a parameter scanning function, forwards expands by 15mm-16mm, records point position data every 0.5mm-0.6mm, uses different potential probe distances for each simulation according to a control variable method, researches the influence of the changed potential probe distance on the crack length-voltage calibration result, finds the optimal potential probe distance which is good in linearity of the calibration curve in the whole crack expansion range and sensitive to crack change as the screw hole punching distance of a subsequent test, takes the crack expansion of 1mm as an example in the embodiment, and shows the potential simulation result of the CT sample as shown in FIG. 4;
specifically, the comparison between the relationship between the normalized voltage drop and the crack length calculated by the formula of the DCPD system and the finite element method and the actual measurement result in the fatigue crack propagation test is shown in fig. 5, because the formula of the DCPD system ignores the influence of the initial crack height of the CT sample, the relationship between the voltage drop and the crack length measured in the actual test and the calibration relationship obtained by the formula of the DCPD system have certain deviation, and the coincidence degree of the result between the finite element method and the actual measurement is high and is within the allowable range of the measurement error, so the crack length can be calculated by using the crack length-voltage drop relationship obtained by the calibration by the finite element method;
s4, after verifying the accuracy of the finite element result, the optimum voltage probe spacing needs to be determined, and the relationship between the normalized voltage drop and the crack length corresponding to the gap without voltage probe spacing obtained by using the finite element calculation is shown in fig. 6, and the following conclusion can be reached: an increase in crack length results in an increase in voltage drop; an increase in the potential probe distance results in a slight decrease in the potential drop, i.e. in the measurement sensitivity; the linearity of a calibration curve is more excellent due to the increase of the distance of the potential probe; according to the conclusion, the larger the probe distance is, the more excellent the linearity of the calibration curve is, and the shorter the probe distance is, the higher the sensitivity of the calibration curve is; since the two potential probes are determined by the screw hole positioning, the minimum distance is necessarily existed between the centers of the two screw holes. According to the analysis and the actual conditions, the sensitivity and the linearity of the optimal probe distance need to be considered at the same time, the optimal potential probe distance is selected to be 18mm, namely, the unilateral distance is 9mm, and the distance can ensure the accuracy and the reliability of the DCPD calibration;
aiming at the condition that DV =18mm is selected, the potential difference of the two probes can be derived when each crack length is selected, a relation curve of the crack length and the voltage drop is drawn, as shown in FIG. 7, a subsequent test can obtain the real-time crack length from the output voltage according to a crack length-standardized voltage drop curve obtained by calibration, and then the real-time crack propagation rate is calculated;
s5, carrying out unimodal stretching overload tests under constant amplitude and different overload ratios by using various materials, controlling a measurement module of the DCPD system by using a peak-valley mode, carrying out fatigue crack prefabrication and constant amplitude fatigue tests by using a K-lowering method, specifically, calculating Paris formula constants C and m of constant amplitude loading, analyzing and calculating the fatigue life and the crack propagation rate under the condition of unimodal overload, and calculating and processing test results, wherein the operation is shown in FIG. 8;
the K-lowering method controls the load size according to the following formula:
wherein the content of the first and second substances,
in order to be the magnitude of the stress intensity factor,
the gradient c is kept constant for the corresponding maximum stress intensity factor amplitude at the beginning of the K-drop.
The overload test method comprises the following specific steps: let Δ K from
Down to
The length of the prefabricated crack is 3 mm; keeping the delta K unchanged and extending to 5mm, immediately applying a single-peak overload with the frequency of 10Hz when the program judges that the delta K extends to 5mm as shown in figure 9, and continuously using the delta K for loading after the overload; real-time expansion rate returning to normal speed when displayed by DCPD softwareAfter the rate is kept stable for 10000 cycles, the test is manually stopped, test data is derived, and the following table lists test parameters of five materials used in the test with different overload ratios:
TABLE 2.2 test Loading parameters
The constant amplitude test results are as follows: after the test is finished, the collected test data are collected and analyzed, and 3 groups of test data of the low-carbon steel Q460C with the stress ratio of 0.1 are fitted, and the result is shown in fig. 10 and 11;
due to the fact that the samples are inconsistent due to the fact that the microscopic structures of the materials are unevenly distributed, the fatigue crack propagation rates measured by different samples made of the same materials possibly have a large statistical dispersion phenomenon, according to a small sample processing method, single sample data can be calculated, the confidence coefficient is 95%, the reliability is 99.9% and serves as calculation standard values of C and m, low-carbon steel Q460C is taken as an example, the second-stage crack propagation rate data measured by three groups of tests are shown in figures 12, 13 and 14, and the data obtained by the small sample processing method are shown in figure 15;
the analysis result of the method can be obtained from a small sample of three groups of Q460C samples, and the data point of the test is very stable and basically has no noise interference, so the concentration is good. There was also little difference in results between the data for the three Q460C samples, which ensured good homogeneity of the materials used in the experiments. Meanwhile, C and m obtained by fitting by using two fitting methods are 4.1748, -9.8892 and 4.1744, -9.8887 respectively, and the results of the two data processing methods are very small in difference in experiments. Comparison of C and m with three sets of tests independently fitted gives rise to tests with negligible sample differences within the error range. The accuracy and the reliability of the test result are verified, and the reliability of the DCPD method in real-time measurement of the fatigue crack length is also verified.
Overload test results: in the overload test, the amplitude of the stress intensity factor is fixed for each material, the overload ratio of the single-peak overload is changed, so that by taking Q460C as an example, three groups of test results with the overload ratios of 2, 2.2 and 2.5 are used for drawing crack propagation life curves, as shown in figures 16, 17 and 18, and square data are the constant amplitude loading test results for reference;
it can be obtained from the test data that before overload is applied, because the applied load condition is completely consistent with the constant amplitude stage, the three groups of data are completely consistent with the constant amplitude loaded data, when overload is applied, the crack propagation rate does not drop immediately, but gradually drops from the initial basic rate, after the overload is applied for a period of time, the crack propagation rate drops to the lowest point, then along with the increase of the load cycle number, the fatigue crack propagation rate gradually and slowly rises to the initial rate level, at the moment, the overload effect completely disappears, and the subsequent propagation behavior is consistent with the stable propagation behavior before overload. Comparing the three sets of data allows a graph to be plotted of crack propagation life at different overload ratios, as shown in FIG. 19;
after a crack propagation life a-N curve is obtained, calculating the crack propagation rate by a moving average method: the crack growth rate curve can be obtained by dividing the crack growth length increased every 10000 cycles by 10000 cycles as the average crack growth rate of the 10000 cycles, as shown in fig. 20.
As can be seen from FIG. 20, in the test with the overload ratio of 2, the crack growth rate after the overload was minimized to 1.3X 10-5 (mm/cycles); the crack propagation rate is reduced to 7.94 x 10-6(mm/cycles) in the test with the overload ratio of 2.2; the crack propagation rate of the specimen with the overload ratio of 2.5 is reduced to 3.54X 10-6(mm/cycles) at the lowest. In combination with the above analysis, a single peak tensile overload can be divided into 4 stages: a stable expansion stage before overload, a descending stage after overload (the crack expansion rate gradually descends to the lowest point after the overload occurs), a hysteresis stage (the expansion rate gradually slowly ascends after reaching the lowest point), and a stable expansion stage after overload. It should be noted that the three sets of test results with the overload ratio of 2.5 in the figure only reach the lag phase and do not fully recover to the standard rate, because in the test with the mild steel Q460C material overload ratio of 2.5, the test is terminated based on the latest test safety requirements, and although the rate is not recovered, the existing data is sufficient to reflect the overload phenomenon. Of these 3 different overload ratios, the test with overload ratio 2 returned to the standard rate after 24 ten thousand cycles on average, the test with overload ratio 2.2 returned to the standard rate after 38 ten thousand cycles, and the test with overload ratio 2.5 did not return to the standard rate until the end, but eventually returned to the standard rate according to the trend of the rate increase and the comparison of the test data with overload ratios 2 and 2.2.
From the comparison between the three sets of data, it can be concluded that: overload changes the fatigue life and crack growth rate of the material, and when the material is the same as the sample, the larger the overload ratio, the higher the delayed fatigue life of the material, the longer the length required to return to the standard rate (i.e., the overload influence length), the more the crack growth rate is reduced after overload, and the more obvious the hysteresis phenomenon is.
Further, in step S5, before the overload test, the pin contacting the CT sample is wrapped with an insulating tape, and then temperature compensation is performed by using a metal of the same material as a reference sample to eliminate the temperature influence, inputting a current of 1.5A, setting the distance between potential probes to be 18mm, and fixing the apparatus, the CT sample, the probes, and the wires to start the test.
The Paris constant coefficients C and m with the confidence coefficient of 95% and the reliability of 99.9% are calculated by using an integral calculation method and a small sample processing method. The result shows that the consistency of the materials used in the test is good, the test method and the DCPD measurement method are reliable, the data noise is low, and the Paris constant coefficients C and m calculated by the two methods are consistent within the error range. The DPCD method is good in stability, small in signal noise, and high in crack length and fatigue life precision obtained through measurement.
In addition, the present invention analyzes the crack propagation behavior after a single peak overload and calculates the crack propagation rate after the overload from the DPCD measurements. The calculation result shows that the fatigue life and the propagation behavior of the material are changed by overload, and the crack propagation behavior after the overload can be divided into four stages: before overload stable expansion stage, after overload descending stage, delay stage and after overload stable expansion stage. The larger the overload ratio, the greater the effect of the overload on fatigue propagation and the higher the delayed fatigue life of the material.
It will be understood by those skilled in the art that the present invention is not limited to the embodiments described above, which are described in the specification and illustrated only to illustrate the principle of the present invention, but that various changes and modifications may be made therein without departing from the spirit and scope of the present invention, which fall within the scope of the invention as claimed. The scope of the invention is defined by the appended claims and equivalents thereof.
Claims (8)
1. A fatigue crack real-time measuring method is characterized by comprising the following specific steps:
s1, selecting a CT sample, wherein the CT sample is made of various metal materials, the CT sample is clamped in front of a fatigue testing machine, two sides of the CT sample are polished, guide grooves with the depth of 5% -6% of the thickness are formed in the two sides of the CT sample, and a plurality of screw holes are processed on the CT sample to fix probes on a DCPD system, so that poor contact and even falling off of the probes caused by vibration in the test are prevented;
s2, positioning a DCPD system to perform fatigue crack measurement through a direct current voltage drop method, wherein the DCPD system comprises a voltmeter for acquiring DCPD signals, a multi-channel acquisition system for acquiring various required output signals and being in butt joint with a computer for storage, and a direct current power supply system for providing reversible steady direct current;
s3, selecting COMSOL software to carry out finite element simulation calibration, establishing a model, setting insulation on each surface of a CT sample, setting a zero potential energy surface of the CT sample, carrying out grid division according to the size of a COMSOL superfine unit, and manually encrypting a grid at the tip of a crack; simulating that the crack starts from the initial crack length by using a parameter scanning function, forwards expands by 15mm-16mm, records point position data every 0.5mm-0.6mm, uses different potential probe distances for each simulation according to a control variable method, researches the influence of the changed potential probe distance on the crack length-voltage calibration result, finds the optimal potential probe distance which is good in linearity of the calibration curve in the whole crack expansion range and sensitive to crack change as the screw hole punching distance of a subsequent test;
s4, determining the optimal voltage probe spacing, deriving the probe potential difference of each crack length, drawing a relation curve of the crack length and the voltage drop, and obtaining the real-time crack length from the output voltage according to the crack length-voltage curve obtained by calibration in the subsequent test, and then calculating the real-time crack propagation rate;
s5, carrying out unimodal stretching overload tests under constant amplitude and different overload ratios by using various materials, controlling a measurement module of the DCPD system by using a peak-valley mode, carrying out fatigue crack prefabrication and constant amplitude fatigue tests by using a K-lowering method, calculating Paris formula constants C and m of constant amplitude loading, analyzing and calculating the fatigue life and the crack propagation rate under the condition of unimodal overload, and calculating and processing test results.
2. The method for real-time measuring fatigue cracks according to claim 1, wherein in step S5, the K-drop method controls the load size according to the following formula:
wherein, the first and the second end of the pipe are connected with each other,
in order to be the magnitude of the stress intensity factor,
the gradient c is kept constant for the corresponding maximum stress intensity factor amplitude at the beginning of the K-drop.
3. The method for real-time measurement of fatigue cracks according to claim 2, wherein in step S5, the overload test method comprises the following specific steps: let Δ K from
Down to
The length of the prefabricated crack is 3 mm; keeping the delta K unchanged and extending to 5mm, immediately applying a single-peak overload with the frequency of 10Hz when the program judges that the delta K is extended to 5mm, and continuously using the delta K for loading after the overload; and when the real-time expansion rate displayed by the DCPD software returns to the normal rate and 10000 cycles are kept stable, manually stopping the test, and exporting test data.
4. The method of claim 3, wherein in the step S1, the plurality of materials are low-carbon steel Q460C, high-carbon steel 1080, medium-carbon steel 1045, and 12NiCr6 NiCr steel with two different heat treatment modes; wherein, two different heat treatment modes of the 12NiCr6 nickel-chromium steel are as follows: (1) keeping the temperature at 880 ℃ for 1 hour, and normalizing; (2) keeping the temperature at 880 ℃ for 1 hour, and performing water quenching and tempering at 500 ℃.
5. The method of claim 4, wherein in step S5, before the overload test, the pins contacting with the CT sample are wrapped by an insulating tape, and then temperature compensation is performed, the input current is 1.5A, the distance between potential probes is 18mm, the device, the CT sample, the probes and the wires are fixed, and the test is started.
6. A method for real-time measurement of fatigue cracks according to claim 5, wherein the temperature compensation method is to use a metal of the same material as the reference sample to eliminate the temperature influence.
7. A method for real-time fatigue crack measurement according to any of claims 1-6, wherein in step S2, the real-time crack length is calculated according to the formula
：
In the above formula, the length of the crack,
is half the distance of the voltage probe,
in order to be the width of the test piece,
for the purpose of the initial crack length,
and (4) calculating the real-time crack propagation rate according to the cycle number after the real-time crack length is measured for the voltage drop after standardization.
8. The method according to claim 1, wherein in step S2, the dc power supply system is equipped with a solid-state relay bridge.
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