A method and apparatus for testing crack propagation of a metal material in a fluid environment

Abstract

This application relates to the field of material crack propagation testing technology, and in particular to a method and apparatus for testing crack propagation of metallic materials in a fluid environment. The method includes conducting crack propagation tests in conventional air, room temperature, high temperature, and high pressure fluid environments, simultaneously measuring using a COD gauge, an LVDT sensor, and a force sensor, and calculating C=>C. LL The mapping relationship is then established, and the elastic modulus E of the specimen is corrected using a COD gauge. The compliance coefficient B is corrected using an LVDT sensor. Afterwards, the test environment is established, and the data from the LVDT sensor, the corrected value of the elastic modulus E, and the secondary compliance correction coefficient are measured. This is then processed via C=>C. LL The crack length can be calculated using the mapping relationship and the crack length calculation formula. Through the above steps, the goal of conducting crack propagation tests on metallic materials in a fluid environment is achieved, while ensuring the accuracy of the test results.

Description

TECHNICAL FIELD

[0001] The application relates to the technical field of material crack propagation testing, in particular to a metal material crack propagation testing method and testing device in a fluid environment. BACKGROUND

[0002] Crack propagation testing is one of important testing projects in the study of the fracture mechanics performance of metal materials, and crack propagation length measurement is usually achieved by using two methods, i.e. a flexibility method and a potential method. In the flexibility method, the crack opening displacement of a sample is measured by using a COD gauge, the flexibility is calculated, and finally the crack propagation length is obtained. In the potential method, the crack opening potential of a sample is measured, and the crack propagation length is calculated based on a finite element potential data table. As for the two methods, the flexibility method is more accurate than the potential method in the measurement of crack propagation length, and therefore the flexibility method is more widely used than the potential method.

[0003] An important step of the crack propagation testing by using the flexibility method is the accurate measurement of the crack opening displacement of a sample, and the crack opening displacement of a sample is usually measured by using a COD gauge at home and abroad. However, in a special medium environment, such as a liquid metal environment (for example, a liquid lead-bismuth environment), the existing various COD gauges cannot be normally used, and therefore the crack propagation testing cannot be carried out according to the conventional testing technology. In the potential method, the wire and the environment must be insulated and non-conductive, and in the liquid metal environment, it is difficult to insulate and make the wire and the environment non-conductive, and therefore the potential method cannot be applied, and therefore the crack propagation testing in the medium environment becomes a testing technical problem.

[0004] Based on the above problems, there is an urgent need for a testing device capable of measuring the fatigue crack propagation of a sample in a fluid environment. SUMMARY

[0005] In order to achieve the purpose of being capable of carrying out the fatigue crack propagation testing of a sample in a fluid environment and ensuring the accuracy of the testing results, the application provides a metal material crack propagation testing method and testing device in a fluid environment.

[0006] In one aspect, the application provides a metal material crack propagation testing method in a fluid environment, which adopts the following technical scheme:

[0007] The application provides a metal material crack propagation testing method in a fluid environment, which adopts the following technical scheme:

[0008] A metal material crack propagation testing method in a fluid environment comprises the following steps:

[0009] S1. Prefabricate the stepped C(T) specimen, install the stepped C(T) specimen on the stress corrosion testing machine, and install the COD gauge and LVDT sensor on the stepped C(T) specimen.

[0010] S2. Crack propagation tests were repeatedly conducted under normal air and room temperature conditions. Multiple sets of loading linear displacement V were obtained by simultaneously measuring with a COD gauge, LVDT sensor, and force sensor. LL Multiple sets of opening displacements V and multiple sets of force values ​​F, using V LL By calculating and deriving the data from V and F, we obtain C=>C LL mapping relationship , ;

[0011] S3. Replace the stepped C(T) specimen. After combining the stepped C(T) specimen with the test vessel, install the whole thing on the stress corrosion testing machine, and then install the COD gauge and LVDT sensor on the stepped C(T) specimen.

[0012] S4. The first compliance test was conducted under normal air and room temperature conditions using a COD gauge and an LVDT sensor for simultaneous measurement, using a known crack length a. m The elastic modulus E of the specimen was corrected using COD gauge measurement data, and then the known crack length a was used. m The modified elastic modulus E and the LVDT sensor measurement data are used to make a second correction to the flexibility coefficient B, resulting in a second-order flexibility correction coefficient B0. After the correction is completed, the COD gauge is removed.

[0013] S5. Establish and stabilize the test environment in the test vessel. During the establishment of the test environment, the sample is in a force-holding state.

[0014] S6. Conduct a second compliance test in the test environment, using a known crack length a. m The elastic modulus E of the specimen is further corrected using the second-order correction coefficient B0 of the S4 flexibility and the measurement data of the LVDT sensor, so as to obtain the corrected value of the elastic modulus E of the specimen under the test environment conditions.

[0015] S7. Conduct crack propagation tests normally in the test environment, and calculate the opening displacement compliance C using the data measured by the LVDT sensor.

[0016] S8. Using data A0 and A1 from S2, the second-order compliance correction coefficient B0 from S4, and the opening displacement compliance C=C from S7. i Substitute C=>C LL Mapping formula and The calculated linear displacement compliance C LL ;

[0017] S9, using the mapping relationship formula C=>C LL The regularized crack length a is calculated according to the crack length calculation formula, and a is substituted into the formula The crack length a can be calculated.

[0018] By adopting the above technical scheme, when in use, first, crack propagation tests are carried out in a conventional air, room temperature environment to obtain the mapping relationship formula C=>C LL , then the first flexibility test is carried out in a synchronous measurement mode of the COD gauge and the LVDT sensor to obtain the elastic modulus correction value and the flexibility second correction coefficient, then the COD gauge is removed and the test environment is built, and the second flexibility test is carried out to obtain the elastic modulus correction value in the test environment, and finally, the crack length can be calculated through the mapping relationship formula C=>C LL and the crack length calculation formula by using the elastic modulus correction value in the test environment, the flexibility second correction coefficient and the LVDT sensor calculation data, so as to realize the purpose of carrying out the metal material crack propagation test in a fluid environment, and also ensure the accuracy of the test results.

[0019] Preferably, in S1, before the prefabricated step type C(T) sample, the value range [a min , a max ] of the regularized crack length a in the mapping relationship formula C=>C LL needs to be defined, and the crack propagation test is carried out according to the value range of a.

[0020] By adopting the above technical scheme, when in use, the crack propagation test is carried out according to the value range of a, which effectively ensures the feasibility of the mapping relationship formula C=>C LL , so as to ensure the accuracy of the subsequent test results.

[0021] Preferably, the calculation steps in S2 are:

[0022] First, the crack propagation test is carried out in a conventional air, room temperature environment, and the loading line displacement V LL , the opening displacement V and the force value F are obtained through the synchronous measurement of the COD gauge, the LVDT sensor and the force value sensor; secondly, the first linear fitting calculation is carried out by using the V LL , V and F data to obtain the loading line displacement flexibility C LL and the opening displacement flexibility C LL , and C LL and C are recorded synchronously to complete the test; then the measured data of C and C LL are analyzed, and C and C LL are linearly related with the crack length expansion, the linear relationship expression is , and the first power fitting constant A0 and the first power fitting coefficient A1 are calculated to obtain the mapping relationship formula C=>CLL mapping relationship ; to get C=>C LL After establishing the mapping relationship, crack propagation tests were repeatedly conducted under the same conditions. Based on the multiple sets of loading line displacement compliance C obtained from the repeated tests... LL and multiple sets of opening displacement compliance C i and use C=>C in S2 LL The mapping relationship is used to calculate the regularized crack length α. LVDT and α COD To obtain regularized crack length data α LVDT and α COD The comparison curves; finally, based on the results of this experiment, the value of C=>C was determined. LL The mapping relationship is further modified, and the modified Formula 1 is: ;

[0023] In Equation 1, A0 and A1 are the first-order power fitting constants and first-order power fitting coefficients obtained in S2; B1 is C LL The instantaneous correction factor is calculated using formula two. C i The crack propagation test yields the opening displacement compliance.

[0024] In Equation 2, B0 is the second-order correction coefficient for compliance; α i-1 This is the calculated value from the previous regularized crack length measurement; α max For C=>C LL The maximum value of the regularized crack length α corresponding to the mapping relationship.

[0025] By adopting the above technical solution, and through multiple crack propagation tests during use, a new C=>C value is obtained by comparison. LL The mapping formula eliminates errors caused by different sample installation states and LVDT sensor installation states in multiple tests, further ensuring the accuracy of the test results.

[0026] Preferably, in the mapping relationship calculation, if B0 equals 1, then B1 equals 1; if B0 is greater than 1 and B1 is less than 1, then B1 equals 1; if B0 is less than 1 and B1 is greater than 1, then B1 equals 1.

[0027] By adopting the above technical solution, the influence of special circumstances on the test results is eliminated by limiting B0 and B1, thus ensuring the accuracy of subsequent calculation results.

[0028] Preferably, each time the step-shaped C(T) sample is replaced, and after the COD gauge and the LVDT sensor are connected with the step-shaped C(T) sample, the installation effect between the COD gauge, the LVDT sensor and the step-shaped C(T) sample is evaluated and tested through the flexibility test, and after the test is passed, the subsequent test is carried out.

[0029] By adopting the technical scheme, before the formal test, the installation effect evaluation test is carried out on the COD gauge and the LVDT sensor, so as to eliminate the error caused by the inconsistent installation state of the COD gauge and the LVDT sensor, thereby ensuring the accuracy of the detection result in the formal test process.

[0030] Preferably, the rotation of the LVDT measurement point position in the stretching process is minimized in design, and no correction is considered.

[0031] By adopting the technical scheme, the stress influence of the connection between the LVDT sensor and the test crack in the test process is eliminated, and the error caused by the installation state of the LVDT to the test result is reduced, thereby ensuring the normal progress of the subsequent test.

[0032] Preferably, the correction steps of the elastic modulus E and the flexibility coefficient B in S4 are as follows:

[0033] S41, presetting the initial value of the elastic modulus E, the increment ΔE and the crack length verification allowable deviation β;

[0034] S42, substituting the initial value of the elastic modulus E and the loading line displacement flexibility C measured by the COD gauge into the crack length calculation formula to calculate the regularized crack length α, and comparing α with the known regularized crack length α0, if α>α0, then the increment ΔE takes a positive value, otherwise ΔE takes a negative value; LL S43, taking E=E+△E, and calculating the regularized crack length α according to the crack length calculation formula, and calculating the regularized crack length deviation △α=|α-α0|;

[0035] S43, taking E=E+△E, and calculating the regularized crack length α according to the crack length calculation formula, and calculating the regularized crack length deviation △α=|α-α0|;

[0036] S44, comparing △α and β, if △α is greater than β, repeating the step S43 until △α is less than β, and at this time, the value of E is the correction value of the elastic modulus E of the sample;

[0037] S45, presetting the flexibility second correction coefficient B0=1.0, the increment △B and the crack length verification allowable deviation β;

[0038] S46, using the flexibility second correction coefficient B0, calculating the flexibility real-time correction coefficient B1 according to the formula two in S2, wherein α i-1 Taking the known regularized crack length α0, using B1 and the opening displacement flexibility C measured by the LVDT sensor to calculate the flexibility correction coefficient B2, and calculating the flexibility correction coefficient B2 according to the formula three in S2, wherein α i, the loading line displacement compliance C is calculated according to the formula one in S2 LL , the elastic modulus E correction value and C LL , the regularized crack length a is calculated according to the crack length calculation formula;

[0039] S47, compared to a and a0 in S46, if a>a0, the increment△B takes a negative value, otherwise△B takes a positive value;

[0040] S48, take B0=B0+△B, repeat S46 to calculate the regularized crack length a again, and calculate the regularized crack length deviation△a=|a-a0|;

[0041] S49, compare△a and β, if△a is greater than β, repeat S48 until△a is less than β, and the B0 value is the compliance second correction coefficient at this time.

[0042] By adopting the above technical scheme, the error range of the elastic modulus E correction value and the compliance second correction coefficient B0 and the actual value is less than the maximum value of the crack length calibration allowable deviation, so that the accuracy of the test result is ensured.

[0043] Preferably, the step of further correcting the elastic modulus E of the test sample in S6 is:

[0044] S61, preset the increment△E and the crack length calibration allowable deviation β;

[0045] S62, use the test sample elastic modulus E correction value in S44, the compliance second correction coefficient B0 in S4, and the opening displacement compliance C measured by the LVDT sensor i , the compliance real-time correction coefficient B1 is calculated according to the formula two in S2;

[0046] Then, the loading line displacement compliance C is calculated according to the formula one in S2 LL Finally, the regularized crack length a is calculated according to the crack length calculation formula, and a and the known regularized crack length a0 are compared, if a>a0, the increment△E takes a positive value, otherwise△E takes a negative value;

[0047] S63, take E=E+△E, repeat the calculation step of S62, and calculate the regularized crack length a again, and calculate the regularized crack length deviation△a=|a-a0|;

[0048] S64, compare△a with the crack length calibration allowable deviation β, if△a is greater than β, repeat S63 until△a is less than β, and the E value is the elastic modulus E correction value under the test environment condition at this time;

[0049] The crack length calculation formula is:

[0050] Formula three: regularized crack length ;

[0051] Formula four: {U}X = { { {BEVX / F}1 / 2 + 1}}-1 ;

[0052] In formula four, E is the elastic modulus; B is the thickness of the sample; V X / F is the loading line displacement flexibility C LL ; in formula three, C0=1.0002, C1=-4.0632, C2=11.242, C3=-106.04, C4=464.33, C5=-650.68.

[0053] By adopting the above technical scheme, the elastic modulus is twice corrected before crack propagation in the test environment, so as to eliminate the influence of the environment on the test crack propagation test result, and further reduce the error caused by the test environment.

[0054] Preferably, after a is calculated in S9, the crack length of the sample is measured, and a is detected and verified through the measured result.

[0055] By adopting the above technical scheme, after the test is completed, the test data and the measured data are compared, and the accuracy of the test result is further detected, which is beneficial to practical application.

[0056] On the other hand, the test device for the metal material crack propagation test method applied in the fluid environment provided by the application adopts the following technical scheme:

[0057] A test device for a metal material crack propagation test method applied in a fluid environment, comprising a cover, a gravity frame connected to the cover, a C(T) sample provided on the gravity frame, a clamping assembly for clamping and positioning the C(T) sample, a pull rod for transmitting load to the C(T) sample, and an LVDT detection device, wherein a COD gauge is installed at the loading line of the C(T) sample, the LVDT detection device is provided with two groups, and the two groups of LVDT detection devices are correspondingly connected to the upper and lower sides of the crack opening of the C(T) sample;

[0058] The LVDT detection device comprises an LVDT sensor and an extension rod installed on the cover, and an LVDT signal collector provided on the extension rod, one end of the LVDT signal collector is detachably connected to the C(T) sample, the other end of the LVDT signal collector penetrates a connecting hole in the force application direction of the pull rod, one end of the extension rod is connected to the core of the LVDT sensor, and the other end of the extension rod is connected to the connecting hole through a rotating pin after penetrating the cover.

[0059] By adopting the technical scheme, on one hand, through cooperation of the test kettle, the kettle cover, the gravity frame, the pull rod and the clamping assembly, different test environments can be established by the workers under the premise of ensuring normal crack propagation test; on the other hand, through the extension rod, the LVDT signal collector and the rotary pin connection of the C(T) sample, the influence of the horizontal rotation resistance of the C(T) sample in the crack propagation process is reduced, so that the accuracy of the test result is ensured, and the use is more favorable.

[0060] In summary, the present application includes at least one of the following beneficial technical effects:

[0061] First, the crack propagation test is carried out in a conventional air, room temperature environment to obtain the mapping relationship of C=>C LL , then the COD gauge and the LVDT sensor are used to perform the first flexibility test, the elastic modulus correction value and the flexibility second correction coefficient are obtained, then the COD gauge is removed and the test environment is built, the second flexibility test is performed, the elastic modulus correction value in the test environment is obtained, finally, the crack length is calculated through the mapping relationship of C=>C LL and the crack length calculation formula using the elastic modulus correction value in the test environment, the flexibility second correction coefficient and the LVDT sensor calculation data, so as to realize the purpose of metal material crack propagation test in a fluid environment, and also ensure the accuracy of the test result;

[0062] By repeatedly performing crack propagation tests under the same conditions, and further correcting the mapping relationship of C=>C LL according to the results obtained from multiple tests, the errors caused by different sample installation states and LVDT sensor installation states in multiple tests are eliminated, and the accuracy of the test result is further ensured;

[0063] Through cooperation of the LVDT sensor and the COD gauge, the deviation of the crack length data measured by the LVDT from the crack length data measured by the COD gauge is ≤±2%, and the deviation of the crack length data measured by the LVDT from the actual crack length data is ≤±4%. DETAILED DESCRIPTION

[0064] Figure 1 is a flowchart embodying the overall test process in the first embodiment of the present application;

[0065] Figure 2 is a schematic diagram embodying the structure of the stepped C(T) sample in the first embodiment of the present application;

[0066] Figure 3 is a schematic diagram embodying the C and C LL linear fitting curves in the first embodiment of the present application;

[0067] Figure 4 is a schematic diagram of the α LVDT and α COD a schematic diagram of the comparison curve;

[0068] Figure 5 is a schematic diagram of the hysteresis curve of the flexibility test in the embodiment one of the present application;

[0069] Figure 6 is a schematic diagram of the axial view of the overall structure in the embodiment two of the present application;

[0070] Figure 7 is a schematic diagram of the axial view of the gravity frame structure in the embodiment two of the present application;

[0071] Figure 8 is a sectional view of the pull rod mounting structure in the embodiment two of the present application;

[0072] Figure 9 is a schematic diagram of the axial view of the clamping assembly structure in the embodiment two of the present application;

[0073] Figure 10 is a schematic diagram of the axial view of the LVDT detection device structure in the embodiment two of the present application;

[0074] Figure 11 is a schematic diagram of the axial view of the LVDT signal collector structure in the embodiment two of the present application.

[0075] The drawing mark: 1, kettle cover; 2, gravity frame; 21, fixed block; 22, connecting rod; 3, sample; 4, clamping assembly; 41, sample lower clamp; 42, sample lower fixed shaft; 43, sample upper clamp; 44, sample upper fixed shaft; 5, pull rod; 6, LVDT detection device; 61, LVDT sensor; 62, extension rod; 63, LVDT signal collector; 64, rotating pin; 7, COD gauge; 8, bellows. DETAILED DESCRIPTION

[0076] The following will be combined with the drawing 1-attached Figure 11 The present application is further described in detail.

[0077] The embodiment of the present application discloses a metal material crack propagation test method and test device in a fluid environment.

[0078] Embodiment 1:

[0079] Referring to Figure 1 A metal material crack propagation test method in a fluid environment, comprising the following steps:

[0080] S1: defining the α value range, pre-preparing and installing the step type C(T) sample;

[0081] The specific steps are: Define C => C LL The range of the regularized crack length α in the mapping relationship [α min ,α max Prefabricate a stepped C(T) specimen, install the stepped C(T) specimen on a stress corrosion testing machine, install the COD gauge on the built-in knife edge of the loading line of the stepped C(T) specimen, and fix two LVDT sensors on both sides of the notch surface of the stepped C(T) specimen.

[0082] Reference Figure 2 In crack rate propagation tests, on the one hand, the space inside the test vessel needs to be considered for simultaneous measurement by the LVDT sensor and COD gauge, meaning the sample size cannot be too large; on the other hand, the installation space required for the LVDT sensor and COD gauge also needs to be considered, meaning the sample size cannot be too small. Furthermore, the loading range of the existing testing equipment (50kN) needs to be taken into account. Therefore, in this embodiment, a width W = 50.8mm, a thickness B = 12.7mm, and a net side groove thickness B are selected. N =10mm, machining length a m =25mm stepped C(T) specimen.

[0083] S2: Repeated crack propagation tests were conducted under normal conditions to obtain initial fitting coefficients A0 and A1, and corrected C=>C. LL The mapping relationship;

[0084] The specific steps are as follows:

[0085] S21. Under normal air and room temperature conditions, conduct crack propagation tests according to the regularized crack length α range defined in S1. Obtain the loading linear displacement V by simultaneously measuring with a COD gauge, LVDT sensor, and force sensor. LL The opening displacement V and the force F, and using V LL The loading line displacement compliance C is obtained by performing a linear fitting calculation with the F data. LL The opening displacement compliance C is calculated using a linear fit with V and F data, and C is recorded simultaneously. LL Together with C, complete the experiment;

[0086] S22, Analyze C and C LL The measured data showed that C and C2 were obtained as the crack length expanded. LL They are linearly correlated, and their linear relationship expression is: And calculate the first power fitting constant A0 and the first power fitting coefficient A1, thus obtaining C=>C LL mapping relationship ;

[0087] S23. Repeatedly conduct multiple crack propagation tests under the same conditions, and calculate the multiple sets of loading line displacement compliance C obtained from the repeated tests. LL and multiple sets of opening displacement compliance C i and use C=>C in S2 LL The mapping relationship is used to calculate the regularized crack length α. LVDT and α COD To obtain regularized crack length data α LVDT and α COD The comparison curves, and based on the results of this experiment, the value of C=>C LL The mapping relationship is corrected, and after correction, C=>C LL The mapping relationship is as follows:

[0088] Formula 1: ; Formula 2: ;

[0089] In Equation 1, A0 and A1 are the first-order power fitting constants and first-order power fitting coefficients obtained in S22, and C i B1 represents the opening displacement compliance measured in the current crack propagation test, where C is the value of C. LL Real-time correction coefficient;

[0090] In Equation 2, B0 is the second-order compliance correction coefficient, which is obtained in S4 after analysis following the first compliance test; α i-1 This is the calculated value from the previous regularized crack length measurement; α max For C=>C LL The maximum value of the regularized crack length α corresponding to the mapping relationship; and in the above calculation of B1, if B0=1, or B0>1 and B1<1, or B0<1 and B1>1, then B1=1.

[0091] In applying the above mapping relationship, it should be noted that the values ​​of A0 and A1 will be different depending on the range of α values ​​in the final measured mapping relationship. If the length of the notch in the sample machining changes or the test range changes and exceeds the mapping range, then the mapping relationship should be redone. In addition, the mapping relationship obtained from the experiment can only represent one specification of sample used in the experiment. That is, the mapping relationship of 1T sample cannot be applied to 0.5T sample, and similarly, the mapping relationship of 0.5T sample cannot be applied to 1T sample.

[0092] This embodiment uses two crack propagation tests as an example for description. The specific implementation steps are as follows:

[0093] Step 1: Install the stepped CT specimen, install the COD gauge at the specimen loading line, and install the LVDT sensor at the specimen notch. Conduct the test according to the following test parameters.

[0094]

[0095] Step 2: Conduct crack propagation tests under normal air and room temperature conditions, and simultaneously measure the loading linear displacement V using a COD gauge and an LVDT sensor. LL And the opening displacement V, using the loading line displacement V LL Calculate the regularized crack length data α COD and using the applied linear displacement V LL The compliance coefficient C of the COD gauge loading line displacement is obtained by calculating the opening displacement V. LL The LVDT sensor measures the opening displacement compliance coefficient C1, and C is recorded synchronously. LL Together with C1, complete the experiment;

[0096]

[0097] Step 3: Adjust the compliance coefficient C of the COD gauge for loading linear displacement. LL The LVDT sensor measures the opening displacement compliance coefficient C1, that is, based on the measured C1 and C LL Data reveals C1 and C LL It has a linear correlation of the first degree, and the linear fitting curve is as follows: Figure 3 As shown, the linear relationship is expressed as:

[0098] Linear equations: Where: A0 = -2.77846E-1, A1 = 4.74155E-1;

[0099] The linear correlation of the first-order fit is R = 0.999878. Examining the linear fit equation and calculating the correlation coefficient, we can see the measurement flexibility C of LVDT and COD gauge. LL Highly linearly correlated;

[0100] Step 4: To verify C and C LL The linear relationship was established. A stepped CT sample was replaced, and crack propagation tests were conducted again under normal air and room temperature conditions. The loading linear displacement V was simultaneously measured using a COD gauge and an LVDT sensor. LL And the opening displacement V, using the loading line displacement V LL Calculate the regularized crack length data α COD and using the applied linear displacement V LL The compliance coefficient C of the COD gauge loading line displacement is obtained by calculating the opening displacement V. LL The LVDT sensor measures the opening displacement compliance coefficient C2;

[0101] Step 5: Verify the mapping relationship and apply the linear displacement compliance coefficient C using the COD gauge. LL Calculate the crack length αCOD and LVDT sensor measures the opening displacement flexibility coefficient C2, through the mapping relationship in S23 to calculate the loading line displacement flexibility coefficient C LL ', and then use the loading line displacement flexibility coefficient C LL ' to calculate the crack length a LVDT , and calculate a LVDT and a COD deviation;

[0102] The second test uses the same test parameters as the first test; the second test applies the mapping relationship of C2and C LL obtained from the first test, and simultaneously measures the opening displacement V and the loading line displacement V LL and calculates the corresponding flexibility C2and C LL , and C2and C LL respectively calculate the regularized crack length data a LVDT and a COD ;

[0103]

[0104] Step 6: According to the test results above, further obtain the comparative curves of a LVDT and a COD , as shown in Figure 4 ; through comparison, it is found that the two curves do not coincide at the initial stage of crack propagation, but gradually coincide as the test develops, which is preliminarily judged to be caused by the difference between the initial installation state of the second test (sample installation, LVDT installation) and the initial installation state of the first test, which also shows that the change of the initial installation state has an impact on the accuracy of small opening displacement measurement, but it weakens as the opening displacement gradually increases;

[0105] Step 7: According to the results of the first test and the second test, the mapping relationship of C=>C LL is corrected, and the corrected mapping relationship is: .

[0106] In the above test process, it is assumed that during the process of applying load to the sample, the crack propagation of the sample is purely I-type crack action, i.e. the crack surface develops linearly, i.e. to eliminate the error caused by the irregular development of the crack to the result calculation; the rotation of the LVDT measurement point position is minimized in the design, and no correction is considered, i.e. to reduce the influence of the plane rotation resistance between the LVDT sensor and the sample connection during the crack propagation process of the sample on the test results; the LVDT installation state is stable during the test process, and the flexibility calculation range is taken as the middle linear part, i.e. to reduce the influence of external factors such as installation state on the test results during the test process, and to ensure the accuracy of the test results.

[0107] S3, replace the sample and install the test kettle;

[0108] The specific operation steps are: replace the stepped C(T) sample, after the stepped C(T) sample and the kettle cover of the test kettle are installed together, the whole is installed on the stress corrosion testing machine, and the COD gauge is installed at the loading line of the stepped C(T) sample, and the LVDT sensor is installed at the notch of the sample.

[0109] S4, perform the first flexibility test in a conventional environment to obtain the corrected value of the elastic modulus and the second correction coefficient of the flexibility;

[0110] The specific operation steps are: in a conventional air, room temperature, the first flexibility test is performed in a synchronous measurement mode of COD gauge and LVDT sensor, the elastic modulus E of the sample is corrected using the known crack length a m (machining length) and the COD gauge measurement data, and the flexibility coefficient B is secondly corrected using the known crack length a m , the corrected elastic modulus E, and the LVDT sensor measurement data, to obtain the second correction coefficient B0 of the flexibility, and the COD gauge is removed after the correction is completed;

[0111] In the embodiment, the sample elastic modulus E correction value and the flexibility second correction coefficient B0 are completed by using the approximation calculation mode in the actual application, and the specific steps are as follows:

[0112] The sample elastic modulus E correction step:

[0113] S41, preset the initial value of the elastic modulus E (manually input the value before the experiment), preset the increment DE = ± 0.0001E, and preset the crack length verification allowable deviation β, the smaller the value of β, the smaller the approximation calculation error, and the more accurate the final measurement result, in the embodiment, β is 0.0001;

[0114] S42, the initial value of the elastic modulus E, the loading line displacement flexibility C LL is substituted into the crack length calculation formula, the regularized crack length a is calculated, a and the known regularized crack length a0 (a0 = a m / W) are compared, if a > a0, then the increment DE = + 0.0001E, otherwise DE = - 0.0001E;

[0115] S43, take E = E + DE, and calculate the regularized crack length a again according to the crack length calculation formula, and calculate the regularized crack length deviation DE = |a-a0|;

[0116] S44, compare △α and β, if △α is greater than β (0.0001), repeat S43, and compare the newly obtained △α with β again, repeat the above calculation until △α is less than β, at this time the E value is the corrected value of the elastic modulus E of the sample;

[0117] Step of deriving the second correction coefficient B0 of compliance:

[0118] S45, preset the second correction coefficient B0 of compliance as 1.0, the increment △B as ±0.0001, and the allowable deviation β of crack length verification as 0.0001; in this step, the compliance coefficient B is the error caused by the different installation states of the LVDT sensor and the COD gauge, if there is no error, the initial value of the compliance coefficient B is 1, so the second correction coefficient B0 of compliance is preset as 1.0;

[0119] S46, substitute B0 into formula two to obtain the real-time correction coefficient B1 of compliance, wherein α i-1 Take the known regularized crack length α0 (α0=a m / W); then substitute B1, the opening displacement compliance C i measured by the LVDT sensor, A0 and A1 calculated in S2 into formula one to obtain the loading line displacement compliance C LL ; finally, use the corrected value of the elastic modulus E and C LL to calculate the regularized crack length α according to the crack length calculation formula;

[0120] S47, compare α calculated in S46 with the known regularized crack length α0, if α>α0, then the increment △B=-0.0001, otherwise △B=+0.0001;

[0121] S48, take B0=B0+△B, repeat S46 to calculate the regularized crack length α again, and calculate the deviation △α=|α-α0| of the regularized crack length;

[0122] S49, compare △α and β, if △α is greater than β (0.0001), repeat S48, and compare the newly obtained △α with β again, repeat the above calculation until △α is less than β, at this time the B0 value is the second correction coefficient of compliance.

[0123] S5, build the test environment through the test kettle;

[0124] The specific operation steps are: build the test environment (i.e. fluid environment) through the test kettle and stabilize, and ensure that the stepped C(T) sample is in a force maintaining state during the process of building the test environment, so as to maintain the installation state of the LVDT sensor unchanged, thereby reducing the influence of the installation state of the LVDT sensor on the test results.

[0125] S6, performing the second flexibility test to obtain the elastic modulus correction value under the test environment condition;

[0126] The specific operation steps are as follows: performing the second flexibility test in the test environment, using the known crack length a m , the flexibility second correction coefficient B0 obtained in S4, and the LVDT sensor measurement data, further correcting the sample elastic modulus E to obtain the correction value of the sample elastic modulus E under the test environment condition;

[0127] In the embodiment, the correction process of the sample elastic modulus E under the test environment condition is completed by software using the approximation calculation method, and the specific operation steps are as follows:

[0128] S61, presetting the initial value of the elastic modulus E (manually input before the experiment), presetting the increment DE = ±0.0001E, and presetting the crack length checking allowable deviation β = 0.0001. In this step, the initial value of E can be re-preset, or the sample elastic modulus E correction value obtained in S44 can be used. The calculation methods of the two are the same, only the calculation times are different. In the embodiment, the sample elastic modulus E correction value in S44 is preferably used;

[0129] S62, using the flexibility second correction coefficient B0 obtained in S4, substituting into formula two to calculate the flexibility real-time correction coefficient B1, wherein a i-1 The known regularized crack length a0 (a0 = a / W) is taken; then B1, the opening displacement flexibility C i measured by the LVDT sensor are used, and formula one is substituted to calculate the loading line displacement flexibility C LL ; finally, the sample elastic modulus E correction value and C LL are used, and the regularized crack length a is calculated according to the crack length calculation formula. a and the known regularized crack length a0 are compared. If a is greater than a0, then the increment DE = +0.0001E, otherwise DE = -0.0001E;

[0130] S63, taking E = E + DE, repeating the calculation step of S62, and calculating the regularized crack length a again. The regularized crack length deviation a is calculated according to the formula a = |a-a0|;

[0131] S64, comparing a and the crack length checking allowable deviation β (0.0001). If a is greater than β, repeating the step of S63, and repeating the calculation until a is less than β. At this time, the value of E is the elastic modulus E correction value under the test environment condition;

[0132] In S42, S43, S45, S46, S62 and S9, the crack length calculation formula used is: Formula three: {U}X= { { {BEVX / F}1 / 2+1}}-1 ;

[0133] Formula four: regularized crack length ;

[0134] In formula three, E is the elastic modulus; B is the sample thickness; V X / F is the loading line displacement flexibility C LL ; in formula four, C0=1.0002, C1=-4.0632, C2=11.242, C3=-106.04, C4=464.33, C5=-650.68.

[0135] S7, normally carry out crack propagation test, get the opening displacement flexibility;

[0136] The specific test steps are: first, normally carry out crack propagation test in the test environment, record the LVDT1 sensor data V1 installed above the loading line of the sample and the LVDT2 sensor data V2 installed below the loading line of the sample during the test; second, use the data V1 and V2 to calculate the opening displacement V by substituting into the formula V=V1-V2; finally, use the opening displacement V and the load value F to calculate the opening displacement flexibility C by substituting into the formula C=V / F.

[0137] S8, use the above test data to convert the opening displacement flexibility into the loading line displacement flexibility;

[0138] The specific operation steps are: use the data A0 and A1 in S2, the flexibility second correction coefficient B0 in S4 and the opening displacement flexibility C=C i in S7, substitute into the mapping formula C=>C LL corrected to calculate the loading line displacement flexibility C LL ;

[0139] The calculation steps are as follows:

[0140] S81, use the flexibility second correction coefficient B0 obtained in S4, the known quantity α i-1 , the known quantity α max Substitute into formula two to calculate B1;

[0141] S82, use the data A0 and A1 obtained in S2, B1 obtained in S81 and the opening displacement flexibility C (i.e. C i ) in S7, substitute into formula one , the loading line displacement flexibility C LL .

[0142] S9, using the loading line displacement flexibility to calculate the crack length;

[0143] The specific operation steps are: using the loading line displacement flexibility C LL , first, the regularized crack length a is calculated according to the crack length calculation formula, and then a is substituted into the formula , the crack length a can be calculated; in the formula, W is the sample width.

[0144] During the above test process, it should be noted that: the installation effect evaluation test should be carried out every time the sample, COD gauge and LVDT sensor are installed; the alternating load force should not be too large to cause the crack growth of the sample during each flexibility test; the holding force of the sample should not be too large to cause the crack growth of the sample during the establishment of the test environment.

[0145] During the installation effect evaluation test, the following installation effect quantitative inspection data is established through many installation practices:

[0146] The quantitative inspection requirement of the installation effect of the COD gauge is that, by using the flexibility test software, the force and the loading line displacement hysteresis curve requires that the loading and unloading curves are visually coincided, the statistical variation coefficient of the flexibility of multiple loading and unloading is less than 0.1% within the flexibility calculation range of 30%~70%;

[0147] The quantitative inspection requirement of the installation effect of the LVDT sensor is that, by using the flexibility test software, the force and the opening displacement hysteresis curve requires that the loading and unloading curves are visually coincided within the force value range of 10%~90% within the flexibility calculation range of 30%~70%

[0148] , the statistical variation coefficient of the flexibility of multiple loading and unloading is less than 0.2%.

[0149] The specific test for checking the installation effect by flexibility test is as follows:

[0150] The flexibility test parameters are: the waveform is a sine wave, the initial loading rate (V0) is 2mm / min, the force control loading rate (V1) is 500N / s, the force peak value (Fmax) is 4000N, the load ratio (R) is 0.1, the loading frequency (f) is 1.0Hz, the loading cycle number is 30, and the known crack length is 25mm;

[0151]

[0152] Referring to Figure 5According to the above calculation check result, the COD gauge loading and unloading hysteresis curve visually coincides, the flexibility statistical variation coefficient CV = 0.100%, according to the installation effect check quantitative rule, the COD gauge installation effect is good; the LVDT loading and unloading hysteresis curve visually coincides (the force value range is 10%~90%), the flexibility statistical variation coefficient CV = 0.186%, according to the installation effect check quantitative rule, the LVDT installation effect is good.

[0153] S10, the crack length of the sample is measured, and a obtained in S9 is detected and verified through the measured result;

[0154] The measurement steps are as follows:

[0155] S101, the stepped C(T) sample after crack propagation test is taken out, and the initial machining length position A, the prefabricated crack position B and the sample back position C are marked;

[0156] S102, the pixel of A, B and C positions is measured respectively;

[0157] S103, using known quantity AC = W-a m , the A, B and C values in S102 are substituted into the formula 、 , the prefabricated crack length AB and the initial crack length a0 are obtained;

[0158] S104, using the measured prefabricated crack length AB, the initial crack length a0 and the crack length , the software reported prefabricated crack length comparison calculation, the crack length measurement deviation and crack propagation measurement deviation are obtained.

[0159] In summary, according to the test data obtained by the crack propagation test of the LVDT sensor, the following evaluation is carried out:

[0160] (1) the double LVDT sensor cannot completely replace the COD gauge, and the COD gauge is still needed to establish the mapping relationship between the opening displacement flexibility C and the loading line displacement flexibility CLL, and the COD gauge is needed to calibrate the elastic modulus in the initial stage of the test;

[0161] (2) although the double LVDT sensor cannot completely replace the COD gauge, the flexibility C measured by the double LVDT sensor in normal temperature and pressure air can be better converted into the loading line displacement flexibility CLL, and the crack propagation curve can be obtained based on the LVDT measurement data;

[0162] (3) In special environment, as long as the LVDT sensor can measure normally and stably, the initial installation state is consistent, and the LVDT sensor is used with the COD gauge for the crack propagation test, which has feasibility, and the crack propagation data is good, the deviation of the crack length data and the measured data is only 1.098%, which is lower than the expected target deviation of 5%, and the flexibility measurement is stable during the crack propagation test process, and the correlation R of the flexibility calculation is greater than 0.9999.

[0163] Embodiment 2

[0164] With reference to Figure 6 and Figure 7 , a test device applied to the metal material crack propagation test method in the fluid environment in embodiment 1, the structure of the test device comprises a test kettle placed horizontally, a kettle cover 1 buckled on the test kettle, a gravity frame 2 connected to the kettle cover 1, and a stepped C(T) sample 3 arranged on the gravity frame 2. The gravity frame 2 is composed of a fixed block 21 and a plurality of connecting rods 22. In this embodiment, the connecting rods 22 are preferably arranged as four, and the four connecting rods 22 are distributed at intervals on the periphery of the fixed block 21. The top end of the connecting rod 22 is threadedly connected with the kettle cover 1 of the test kettle, and the bottom end of the connecting rod 22 is fixed on the fixed block 21 by bolts. In use, the gravity frame 2 is assembled on the kettle cover 1, and then the kettle cover 1 is buckled on the test kettle. At this time, the gravity frame 2 extends into the test kettle.

[0165] With reference to Figure 6 and Figure 7 , a clamping assembly 4, a pull rod 5 and an LVDT detection device 6 are arranged on the gravity frame 2, and a COD gauge 7 is arranged at the loading line of the C(T) sample 3. The clamping assembly 4 is used for clamping and fixing the C(T) sample 3 on the gravity frame 2. The top end of the pull rod 5 is connected with the driving end of the stress corrosion testing machine, and the pull rod 5 is used for transmitting load to the C(T) sample 3. The LVDT detection device 6 and the COD gauge 7 are both used for crack propagation detection of the C(T) sample 3. The difference is that the COD gauge 7 is only used for testing in the normal temperature and pressure atmospheric environment, and the test result is used for assisting the calculation of the test result of the LVDT detection device 6. The LVDT detection device 6 can be used for crack propagation detection in the normal temperature and pressure atmospheric environment and the high-temperature corrosion fluid environment. In this application, the LVDT detection device 6 is provided with two groups.

[0166] With reference to Figure 6 and Figure 8The top end of the pull rod 5 is located above the vessel lid 1. The top end of the pull rod 5 is used to connect to the stress corrosion testing machine. The bottom end of the pull rod 5 passes through the vessel lid 1 and is located below the vessel lid 1. The pull rod 5 can slide on the vessel lid 1. A sealing structure is provided between the pull rod 5 and the vessel lid 1 to seal the gap between the pull rod 5 and the vessel lid 1. The sealing structure includes a bellows 8 and a sealing ring. The bellows 8 is sleeved on the pull rod 5, and the top end of the bellows 8 is welded and fixed to the side wall of the pull rod 5. The bottom end of the bellows 8 is fixedly connected to the vessel lid 1 by bolts. The sealing ring is located between the bottom end of the bellows 8 and the vessel lid 1. The sealing ring is used to seal the connection gap between the bellows 8 and the vessel lid 1.

[0167] Reference Figure 6 and Figure 8 In use, the stress corrosion testing machine applies tension to the tie rod 5 to drive it to slide along its own axis. During the sliding process of the tie rod 5, the sealing ring and the bellows 8 work together to seal the gap between the tie rod 5 and the vessel cover 1 while ensuring that the sliding of the tie rod 5 is not affected. This is beneficial for the establishment of a high-temperature corrosive fluid environment in the future.

[0168] Reference Figure 6 and Figure 9 The clamping assembly 4 consists of a lower sample clamp 41, a lower sample fixing shaft 42, an upper sample clamp 43, and an upper sample fixing shaft 44. The bottom end of the lower sample clamp 41 is fixed to the fixing block 21 by bolts. The top end of the lower sample clamp 41 is provided with a lower clamping groove for clamping the C(T) sample 3. The lower sample fixing shaft 42 passes through the top end of the lower sample clamp 41 and through the lower clamping groove. The top end of the upper sample clamp 43 is fixedly connected to the bottom end of the pull rod 5 by bolts. The bottom end of the upper sample clamp 43 faces the lower sample clamp 41 and is provided with an upper clamping groove for clamping the C(T) sample 3. The upper sample fixing shaft 44 passes through the bottom end of the upper sample clamp 43 and through the upper clamping groove.

[0169] Reference Figure 6 and Figure 9 In use, a through hole 1 for the upper fixing shaft 44 of the specimen and a through hole 2 for the lower fixing shaft 42 of the specimen are respectively opened on the upper and lower sides of the loading line of the stepped C(T) specimen 3. Then, the lower end of the C(T) specimen 3 is placed into the lower clamping groove of the lower specimen clamp 41 and fixed and limited by the lower fixing shaft 42. The upper end of the C(T) specimen 3 is placed into the upper clamping groove of the upper specimen clamp 43 and fixed and limited by the upper fixing shaft 44, thereby completing the clamping and fixing of the C(T) specimen 3.

[0170] Reference Figure 6 and Figure 10Two groups of LVDT detection devices 6 are respectively arranged at the upper and lower ends of the C(T) sample 3, and each group of LVDT detection devices 6 comprises an LVDT sensor 61, an extension rod 62 and an LVDT signal collector 63. The LVDT sensor 61 is fixed on the upper end surface of the cover 1 by bolts, the top end of the extension rod 62 is connected to the core of the LVDT sensor 61 by threads, and the bottom end of the extension rod 62 is connected to the LVDT signal collector 63 located below the cover 1 after penetrating through the cover 1.

[0171] With reference to Figure 10 and Figure 11 One end of the LVDT signal collector 63 is detachably connected to the C(T) sample 3 by bolts, and the other end of the LVDT signal collector 63 penetrates a connecting hole in the vertical direction, the bottom end of the extension rod 62 penetrates the connecting hole through a rotating pin 64, the axis of the rotating pin 64 is perpendicular to the axis of the connecting hole, and the rotating pin 64 realizes the rotating connection between the extension rod 62 and the LVDT signal collector 63, so as to reduce the resistance of the C(T) sample 3 to the horizontal rotating force during the crack propagation process.

[0172] With reference to Figure 6 In addition, in the present application, an O-ring is also arranged between the extension rod 62 and the cover 1, and in use, the gap between the extension rod 62 and the cover 1 is sealed by the O-ring, so as to facilitate the establishment of a high-temperature fluid environment through the cover 1 and the test kettle.

[0173] The implementation principle of the embodiment of the application is as follows: in use, first, the counterforce frame, the LVDT sensor 61, the extension rod 62, the LVDT signal collector 63, the upper clamp 43, the lower clamp 41, the pull rod 5, the COD gauge 7, the stepped C(T) sample 3 and the cover 1 are assembled together and are integrally assembled to a stress corrosion testing machine, then, under the normal temperature and pressure atmospheric environment, the COD gauge 7 and the LVDT sensor 61 are synchronously measured to obtain a one-time mapping relationship between the loading line displacement flexibility measured by the COD gauge 7 and the opening displacement flexibility measured by the LVDT sensor 61 and an elastic modulus correction value; then, the COD gauge 7 is removed, the high-temperature corrosion fluid environment is established through the test kettle, under the high-temperature corrosion fluid environment, only the C(T) sample 3 is tested for flexibility by the LVDT testing device, so that the opening displacement flexibility of the C(T) sample 3 and the elastic modulus correction under the high-temperature corrosion fluid environment are measured; then, the C(T) sample 3 is subjected to fatigue loading to carry out a crack propagation test, the opening displacement of the C(T) sample 3 is measured by the LVDT sensor 61, the opening displacement flexibility data are calculated according to the mapping relationship between the loading line displacement flexibility and the opening displacement flexibility, finally, the test data are used to further calculate through a crack length calculation formula, so that the crack length of the C(T) sample 3 is obtained, thereby achieving the purpose of testing the crack propagation of the C(T) sample 3 under the high-temperature corrosion fluid environment, and ensuring the accuracy of the test result.

[0174] It should be noted that the technical features "C(T) sample, COD gauge, LVDT sensor and cover" in the embodiment are the same as the corresponding technical features involved in the method steps in the embodiment 1.

[0175] The above are preferred embodiments of the application, and do not limit the protection scope of the application, therefore: all equivalent changes made according to the structure, shape, principle of the application should be covered in the protection scope of the application.

Claims

1. A method for testing crack propagation in metallic materials under fluid conditions, characterized in that, Includes the following steps: S1. Prefabricate the stepped C(T) specimen, install the stepped C(T) specimen on the stress corrosion testing machine, and install the COD gauge and LVDT sensor on the stepped C(T) specimen. S2. First, crack propagation tests were repeatedly conducted under normal air and room temperature conditions. The loading linear displacement V was obtained by synchronous measurement using a COD gauge, LVDT sensor, and force sensor. LL First, the opening displacement V and the force F; then, V is used. LL A linear fitting calculation is performed on the V and F data to obtain the loading linear displacement compliance C. LL And the opening displacement compliance C, C is recorded synchronously. LL And C, complete the experiment; then analyze C and C. LL The measured data showed that C and C2 were obtained as the crack length expanded. LL They are linearly correlated, and their linear relationship expression is: And calculate the first power fitting constant A0 and the first power fitting coefficient A1, thus obtaining C=>C LL mapping relationship ; to get C=>C LL After establishing the mapping relationship, the sample was replaced, and crack propagation tests were repeatedly conducted under the same conditions. Based on the multiple sets of loading line displacement compliance C obtained from the repeated tests... LL and multiple sets of opening displacement compliance C i and using mapping relationships Each calculated the regularized crack length α. COD and α LVDT To obtain regularized crack length data α COD and α LVDT The comparison curves; finally, based on the results of this experiment, the value of C=>C was determined. LL The mapping relationship is further modified, and the modified Formula 1 is: ; In Equation 1, A0 and A1 are the first-order power fitting constant and the first-order power fitting coefficient; B1 is the C LL The instantaneous correction factor is calculated using formula two. C i The crack propagation test yields the opening displacement compliance. In Equation 2, B0 is the second-order correction coefficient for compliance; α i-1 This is the calculated value from the previous regularized crack length measurement; α max For C=>C LL The maximum value of the regularized crack length α corresponding to the mapping relationship; S3. Replace the stepped C(T) specimen. After combining the stepped C(T) specimen with the test vessel, install the whole thing on the stress corrosion testing machine, and then install the COD gauge and LVDT sensor on the stepped C(T) specimen. S4. The first compliance test was conducted under normal air and room temperature conditions using a COD gauge and an LVDT sensor for simultaneous measurement, using a known crack length a. m The elastic modulus E of the specimen was corrected using COD gauge measurement data, and then the known crack length a was used. m The modified elastic modulus E and the LVDT sensor measurement data are used to perform a second correction on the flexibility, resulting in a second-order flexibility correction coefficient B0. After the correction is completed, the COD gauge is removed. S5. Establish and stabilize the test environment in the test vessel. During the establishment of the test environment, the sample is in a force-holding state. S6. Conduct a second compliance test in the test environment, using a known crack length a. m The elastic modulus E of the specimen is further corrected using the second-order correction coefficient B0 of the S4 flexibility and the measurement data of the LVDT sensor, so as to obtain the corrected value of the elastic modulus E of the specimen under the test environment conditions. S7. Conduct crack propagation tests normally in the test environment, and calculate the opening displacement compliance C using the data measured by the LVDT sensor. S8. Using data A0 and A1 from S2, the second-order compliance correction coefficient B0 from S4, and the opening displacement compliance C=C from S7. i Substitute C=>C LL Mapping formula and The calculated linear displacement compliance C LL ; S9. Use the linear displacement compliance C applied in S8. LL The regularized crack length α is calculated using the crack length calculation formula, and then α is substituted into the formula. The crack length a can then be calculated, where W is the sample width.

2. The method for testing crack propagation in metallic materials under fluid conditions according to claim 1, characterized in that: In S1, before prefabricating the stepped C(T) specimen, C=>C must be defined first. LL The range of the regularized crack length α in the mapping relationship [α min ,α max ], and conduct crack propagation tests based on the range of α.

3. The method for testing crack propagation in metallic materials under fluid conditions according to claim 2, characterized in that: Each time the stepped C(T) specimen is replaced and the connection between the COD gauge and LVDT sensor and the stepped C(T) specimen is completed, a compliance test must be conducted to evaluate the installation effect between the COD gauge, LVDT sensor and the stepped C(T) specimen. Subsequent tests can only be carried out after the test is passed.

4. The method for testing crack propagation in metallic materials under fluid conditions according to claim 2, characterized in that: The design minimizes the influence of rotation at the LVDT measuring point position during the stretching process, without considering corrections.

5. The method for testing crack propagation in metallic materials under fluid conditions according to claim 1, characterized in that, The steps for correcting the elastic modulus E and compliance of the specimen in S4 are as follows: S41. Preset initial value of elastic modulus E, increment ΔE, and allowable deviation β for crack length verification; S42. The initial value of elastic modulus E and the compliance value of loading line displacement C obtained by COD gauge measurement. LL Substitute the values ​​into the crack length calculation formula to calculate the regularized crack length α. Compare α with the known regularized crack length α0. If α > α0, then the increment ΔE is positive; otherwise, ΔE is negative. S43. Take E=E+△E, and calculate the regularized crack length α according to the crack length calculation formula. Calculate the regularized crack length deviation △α=|α-α0|. S44. Compare Δα and β. If Δα is greater than β, repeat step S43 until Δα is less than β. At this time, the E value is the corrected value of the elastic modulus E of the sample. S45, preset flexibility secondary correction coefficient B0=1.0, increment ΔB, crack length verification allowable deviation β; S46. Using the second-order compliance correction factor B0, calculate the instantaneous compliance correction factor B1 according to formula 2 in S2, where α i-1 Given a known regularized crack length α0, the opening displacement compliance C is measured using B1 and LVDT sensors. i The compliance coefficient C of the loaded linear displacement is calculated according to Equation 1 in S2. LL Using the elastic modulus E correction value and C LL The regularized crack length α is calculated according to the crack length calculation formula; S47. Compare α and α0 in S46. If α > α0, then the increment ΔB takes a negative value; otherwise, ΔB takes a positive value. S48. Take B0 = B0 + ΔB, repeat S46 to calculate the regularized crack length α again, and calculate the regularized crack length deviation Δα = |α - α0|. S49. Compare △α and β. If △α is greater than β, repeat step S48 until △α is less than β. At this point, the value of B0 is the second-order compliance correction coefficient.

6. The method for testing crack propagation in metallic materials under fluid conditions according to claim 5, characterized in that, The step in S6 to further correct the elastic modulus E of the specimen is as follows: S61, Preset increment ΔE and crack length verification allowable deviation β; S62. Using the corrected value of the elastic modulus E of the specimen in S44, the second-order correction coefficient of compliance B0 in S4, and the LVDT sensor, the opening displacement compliance C is measured. i According to Formula 2 in S2, the instantaneous compliance correction coefficient B1 is calculated. Then, the compliance coefficient C of the loading line displacement is calculated according to Equation 1 in S2. LL Finally, the regularized crack length α is calculated according to the crack length calculation formula. α is compared with the known regularized crack length α0. If α is greater than α0, the increment ΔE is positive; otherwise, ΔE is negative. S63. Take E = E + ΔE, repeat the calculation steps in S62, and calculate the regularized crack length α again. Calculate the regularized crack length deviation Δα = |α - α0|. S64. Compare △α with the allowable deviation β for crack length verification. If △α is greater than β, repeat S63 until △α is less than β. At this time, the E value is the correction value of elastic modulus E under the test environment conditions. The formula for calculating crack length is: Formula 3: Regularized Crack Length ； Formula 4: ; In Equation 4, E is the elastic modulus; B is the sample thickness; V X / F represents the compliance factor C under load linear displacement. LL In Equation 3, C0 = 1.0002, C1 = -4.0632, C2 = 11.242, C3 = -106.04, C4 = 464.33, and C5 = -650.

68.

7. The method for testing crack propagation in metallic materials under fluid conditions according to claim 1, characterized in that: After calculating 'a' in S9, the crack length of the sample needs to be measured, and the measured results are used to verify 'a'.

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