Method and device for testing crack propagation of metal material in fluid environment

Abstract

The invention relates to the technical field of material crack propagation testing, in particular to a metal material crack propagation testing method and device in a fluid environment, and the method comprises the following steps: carrying out a crack propagation test in conventional air, room temperature, high temperature and high pressure fluid environments, carrying out synchronous measurement by using a COD gauge, an LVDT sensor and a force value sensor, and calculating to obtain C = gt; the method comprises the following steps: calculating the mapping relation of CLL and COD, correcting the elasticity modulus E of a sample by using a COD gauge, correcting a flexibility coefficient B by using an LVDT sensor, establishing a test environment, and measuring data, a corrected value of the elasticity modulus E and a flexibility secondary correction coefficient by using the LVDT sensor through C = gt; the crack length can be calculated according to the CLL mapping relational expression and the crack length calculation formula. Through the steps, the purpose of carrying out crack propagation test on the metal material in a fluid environment is achieved, and meanwhile, the accuracy of a test result is ensured.

Description

Technical Field

[0001] The present application relates to the technical field of material crack propagation testing, and particularly to a method and device for testing the crack propagation of metal materials in a fluid environment. Background Art

[0002] The crack propagation test is one of the important test items in the study of the fracture mechanics properties of metal materials. The determination of the crack propagation length is usually carried out by two methods: the compliance method and the potential method at home and abroad. Among them, the compliance method is to measure the crack opening displacement of the specimen using a COD gauge, then calculate the compliance, and finally obtain the crack propagation length; the potential method is to measure the potential at the crack mouth of the specimen and then calculate the crack propagation length based on the finite element potential data table. For the above two methods, in terms of the measurement accuracy of the crack propagation length, the compliance method is higher than the potential method, so the compliance method is more widely used than the potential method.

[0003] An important link in carrying out the crack propagation test by the compliance method is the accurate measurement of the crack opening displacement of the specimen. At present, the COD gauge is usually used at home and abroad to measure the opening displacement of the specimen. However, in a special medium environment, such as a liquid metal environment (such as a liquid lead-bismuth environment, etc.), various existing COD gauges cannot be used normally, and the crack propagation test cannot be carried out according to the usual test techniques. When using the potential method to measure the crack length, it is necessary to ensure that the wire and the environment are insulated and non-conductive. In a liquid metal environment, it is difficult to insulate the wire and the environment and make them non-conductive, so the potential method cannot be applied. Therefore, carrying out the crack propagation test in such a medium environment has become a test technical problem.

[0004] Based on the above problems, there is an urgent need for a test device that can measure the fatigue crack propagation of the specimen in a fluid environment. Summary of the Invention

[0005] In order to achieve the purpose of being able to carry out the fatigue crack propagation test on the specimen in a fluid environment while ensuring the accuracy of the test results, the present application provides a method and device for testing the crack propagation of metal materials in a fluid environment.

[0006] On the one hand, a method for testing the crack propagation of metal materials in a fluid environment provided by the present application adopts the following technical solution: The present application provides a method for testing the crack propagation of metal materials in a fluid environment, adopting the following technical solution: A method for testing the crack propagation of metal materials in a fluid environment includes the following steps: S1. Pre-fabricate a stepped C(T) specimen, install the stepped C(T) specimen on a stress corrosion testing machine, and install a COD gauge and an LVDT sensor on the stepped C(T) specimen; S2. Repeatedly conduct crack propagation tests under normal air and room temperature conditions. Through the synchronous measurement method using a COD gauge, an LVDT sensor, and a force sensor, multiple sets of loading line displacements V LL , multiple sets of opening displacements V, and multiple sets of force values F are obtained. Using the V LL , V, and F data for calculation and derivation, the mapping relationship C => C LL is obtained, where C LL = B 1 * (A 0 + A 1 * C i ), and B 1 = 1 + (1 - B 0 ) * (α i-1 - α max ) / α max ; S3. Replace the stepped C(T) specimen. After combining the stepped C(T) specimen with the test autoclave, the whole is installed on a stress corrosion testing machine, and then the COD gauge and the LVDT sensor are installed on the stepped C(T) specimen; S4. Conduct the first compliance test under normal air and room temperature conditions using the synchronous measurement method with a COD gauge and an LVDT sensor. Use the known crack length a m and the COD gauge measurement data to correct the elastic modulus E of the specimen. Then use the known crack length a m , the corrected elastic modulus E, and the LVDT sensor measurement data to perform a secondary correction on the compliance coefficient B to obtain the secondary corrected compliance coefficient B 0 . After the correction is completed, remove the COD gauge; S5. Establish and stabilize the test environment in the test autoclave. During the process of establishing the test environment, the specimen is in a force-holding state; S6. Conduct the second compliance test in the test environment. Use the known crack length a m , the secondary corrected compliance coefficient B 0 in S4, and the LVDT sensor measurement data to further correct the elastic modulus E of the specimen to obtain the corrected value of the elastic modulus E of the specimen under the test environment conditions; S7. Normally conduct crack propagation tests in the test environment. Use the LVDT sensor measurement data to calculate the opening displacement compliance C; S8. Use the data A 0 and A 1 in S2, the secondary corrected compliance coefficient B 0 in S4, and the opening displacement compliance C = C i in S7, and substitute them into the mapping formula C => C LL : C LL = B 1 * (A0 +A 1 *C i ) and B 1 =1+(1 - B 0 )*(α i-1 -α max ) / α max The loading line displacement compliance C is calculated LL ; S9. Use the loading line displacement compliance C in S8 LL The normalized crack length α is calculated according to the crack length calculation formula, and substituting α into the formula a = α * W, the crack length a can be calculated

[0007] By adopting the above technical solution, during use, first conduct a crack propagation test under normal air and room temperature conditions to obtain the mapping relationship of C => C LL , then adopt the synchronous measurement method of COD gauge and LVDT sensor to conduct the first compliance test to obtain the elastic modulus correction value and the compliance secondary correction coefficient. After that, remove the COD gauge and establish the test environment, and conduct the second compliance test to obtain the elastic modulus correction value under the test environment. Finally, use the elastic modulus correction value, compliance secondary correction coefficient, and LVDT sensor calculation data under the test environment. Through the mapping relationship of C => C LL and the crack length calculation formula, the crack length can be calculated, thus achieving the purpose of conducting a crack propagation test on metal materials in a fluid environment, and at the same time ensuring the accuracy of the test results

[0008] Preferably, in S1, before prefabricating the stepped C(T) specimen, it is necessary to first define the value range [α LL of the normalized crack length α in the mapping relationship of C => C min , α max , and conduct a crack propagation test according to the value range of α

[0009] By adopting the above technical solution, during use, conducting a crack propagation test according to the value range of α effectively ensures the feasibility of the mapping relationship of C => C LL , thus ensuring the accuracy of subsequent test results

[0010] Preferably, the calculation steps in S2 are as follows First, conduct a crack propagation test under normal air and room temperature. Through the synchronous measurement of the COD gauge, LVDT sensor, and force sensor, obtain the loading line displacement V LL , the opening displacement V, and the force value F. Secondly, use the data of V LL , V, and F to conduct a first - order linear fitting calculation to obtain the loading line displacement compliance C LL and the opening displacement compliance C, and synchronously record CLL and C to complete the test; then analyze the measured data of C and C LL . As the crack length extends, C and C LL show a linear correlation, and the expression of the linear relationship is f(x)=A 0 +A 1 *x, and calculate the first-power fitting constant A 0 and the first-power fitting coefficient A 1 , so as to obtain the mapping relationship C=>C LL : C LL =A 0 +A 1 C; after obtaining the mapping relationship of C=>C LL , repeatedly conduct crack propagation tests under the same conditions. According to the multiple groups of load line displacement compliance C LL and multiple groups of opening displacement compliance C i obtained from the repeated tests, and use the mapping relationship of C=>C LL in S2 to calculate the regularized crack length α LVDT and α COD respectively, so as to obtain the comparison curve of the regularized crack length data α LVDT and α COD ; finally, further correct the mapping relationship of C=>C LL according to the test results. The corrected formula one is: C LL =B 1 *(A 0 +A 1 *C i ); In formula one, A 0 and A 1 are the first-power fitting constant and the first-power fitting coefficient obtained in S2; B 1 is the immediate correction coefficient of C LL , and its calculation formula two is B 1 =1+(1-B 0 )*(α i-1 -α max ) / α max ; C i is the opening displacement compliance measured in the crack propagation test; In formula two, B 0 is the compliance secondary correction coefficient; α i-1 is the calculated value of the previous regularized crack length measurement; α max is the maximum value of the regularized crack length α value range corresponding to the mapping relationship of C=>C LL .

[0011] By adopting the above technical solution, during use, a new C=>C is obtained through comparison of multiple crack propagation testsLL The mapping relation formula eliminates the errors caused by different installation states of the specimen and the LVDT sensor in multiple tests, and further ensures the accuracy of the test results.

[0012] Preferably, in the mapping relation calculation, if B 0 is equal to 1, then B 1 is equal to 1; if B 0 is greater than 1 and B 1 is less than 1, then B 1 is equal to 1; if B 0 is less than 1 and B 1 is greater than 1, then B 1 is equal to 1.

[0013] By adopting the above technical solution, during use, by limiting B 0 and B 1 , the influence of special situations on the test results is eliminated, and the accuracy of the subsequent calculation results is ensured.

[0014] Preferably, every time the stepped C(T) specimen is replaced, and after the connection between the COD gauge and the LVDT sensor and the stepped C(T) specimen is completed, a flexibility test is required to evaluate the installation effect between the COD gauge, the LVDT sensor and the stepped C(T) specimen. After the test passes, the subsequent test can be carried out.

[0015] By adopting the above technical solution, before the formal test, by evaluating the installation effect of the COD gauge and the LVDT sensor, the errors caused by the inconsistent installation states of the COD gauge and the LVDT sensor are eliminated, thus ensuring the accuracy of the detection results during the formal test process.

[0016] Preferably, in the stretching process, the rotation influence of the LVDT measuring point position is minimized, and no correction is considered.

[0017] By adopting the above technical solution, the force influence at the connection between the LVDT sensor and the test crack during the test is eliminated, and the error caused by the LVDT installation state on the test results is reduced, thus ensuring the normal progress of the subsequent test.

[0018] Preferably, the correction steps for the elastic modulus E and the flexibility coefficient B of the specimen in S4 are as follows: S41. Preset the initial value of the elastic modulus E, the increment △E, and the allowable deviation β for crack length verification; S42. Substitute the initial value of the elastic modulus E and the loading line displacement flexibility C LL measured by the COD gauge into the crack length calculation formula to calculate the regularized crack length α. Compare α with the known regularized crack length α 0 . If α > α0 , then the increment ΔE takes a positive value, otherwise ΔE takes a negative value; S43. Take E = E + ΔE, and calculate the normalized crack length α according to the crack length calculation formula. Calculate the normalized crack length deviation Δα = |α - α 0 |; S44. Compare Δα with β. If Δα is greater than β, repeat step S43 until Δα is less than β. At this time, the value of E is the corrected value of the specimen elastic modulus E; S45. Preset the secondary compliance correction coefficient B 0 = 1.0, the increment ΔB, and the allowable deviation β of the crack length verification; S46. Use the secondary compliance correction coefficient B 0 , and calculate the instantaneous compliance correction coefficient B 1 according to Equation 2 in S2, where α i-1 takes the known normalized crack length α 0 , use B 1 and the opening displacement compliance C measured by the LVDT sensor i , and calculate the loading line displacement compliance C LL according to Equation 1 in S2. Use the corrected value of the elastic modulus E and C LL , and calculate the normalized crack length α according to the crack length calculation formula; S47. Compare α in S46 with α 0 , if α > α 0 , then the increment ΔB takes a negative value, otherwise ΔB takes a positive value; S48. Take B 0 = B 0 + ΔB, repeat S46 to calculate the normalized crack length α again, and calculate the normalized crack length deviation Δα = |α - α 0 |; S49. Compare Δα with β. If Δα is greater than β, repeat step S48 until Δα is less than β. At this time, the value of B 0 is the secondary compliance correction coefficient.

[0019] By adopting the above technical solution, the error range between the corrected value of the elastic modulus E and the secondary compliance correction coefficient B 0 and the actual value is less than the maximum value of the allowable deviation of the crack length verification, thus ensuring the accuracy of the test results.

[0020] Preferably, the steps for further correcting the specimen elastic modulus E in S6 are: S61. Preset the increment ΔE and the allowable deviation β of the crack length verification; S62. Use the corrected value of the specimen elastic modulus E in S44 and the secondary compliance correction coefficient B in S4 0, the opening displacement compliance C is measured by the LVDT sensor i , and according to Equation 2 in S2, the compliance immediate correction coefficient B is calculated 1 ; Then, according to Equation 1 in S2, the loading line displacement compliance C is calculated LL , and finally, according to the crack length calculation formula, the normalized crack length α is calculated. Compare α with the known normalized crack length α 0 . If α is greater than α 0 , then the increment △E takes a positive value; otherwise, △E takes a negative value. S63. Take E = E + △E, repeat the calculation steps of S62, and calculate the normalized crack length α again. Calculate the normalized crack length deviation △α = |α - α 0 |; S64. Compare △α with the crack length verification allowable deviation β. If △α is greater than β, repeat S63 until △α is less than β. At this time, the value of E is the corrected value of the elastic modulus E under the test environmental conditions; The crack length calculation formula is: Formula 3: The normalized crack length α = a / W = C 0 + C 1 U X + C 2 U X 2 + C 3 U X 3 + C 4 U X 4 + C 5 U X 5 ; Formula 4: U X ={[BEV X / F] 1 / 2 + 1} -1 ; In Formula 4, E is the elastic modulus; B is the specimen thickness; V X / F is the loading line displacement compliance C LL ; In Formula 3, C 0 = 1.0002, C 1 = -4.0632, C 2 = 11.242, C 3 = -106.04, C 4 = 464.33, C 5 = -650.68.

[0021] By adopting the above technical solution, before crack propagation in the test environment, the elastic modulus is corrected twice to eliminate the influence of the environment on the test results of crack propagation, thereby reducing the error caused by the test environment.

[0022] Preferably, after obtaining a in S9, the crack length of the specimen needs to be actually measured, and a is detected and verified through the actual measurement results.

[0023] By adopting the above technical solution, after the test is completed, the data measured in the test is compared with the actually measured data to further detect the accuracy of the test results, which is beneficial to practical applications.

[0024] On the other hand, the test device for the crack propagation test method of metal materials applied in a fluid environment provided by the present application adopts the following technical solution: A test device for the crack propagation test method of metal materials applied in a fluid environment includes a kettle lid, a gravitational frame connected to the kettle lid, a C(T) specimen arranged on the gravitational frame, a clamping assembly for clamping and positioning the C(T) specimen, a pull rod for transmitting a load to the C(T) specimen, and an LVDT detection device. A COD gauge is installed at the loading line of the C(T) specimen. There are two groups of LVDT detection devices, 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) specimen. The LVDT detection device includes an LVDT sensor and an extension rod installed on the kettle lid, and an LVDT signal collector arranged on the extension rod. One end of the LVDT signal collector is detachably connected to the C(T) specimen. The other end of the LVDT signal collector is provided with a connection hole penetrating along the force application direction of the pull rod. One end of the extension rod is connected to the iron core of the LVDT sensor, and the other end of the extension rod passes through the kettle lid and is rotatably connected to the connection hole through a rotating pin.

[0025] By adopting the above technical solution, on the one hand, through the cooperation of the test kettle, the kettle lid, the gravitational frame, the pull rod and the clamping assembly, on the premise of ensuring the normal progress of the crack propagation test, it is beneficial for the staff to establish different test environments; on the other hand, through the rotating pin connection of the extension rod, the LVDT signal collector and the C(T) specimen, the influence of the horizontal rotation resistance received by the C(T) specimen during the crack propagation process is reduced, thereby ensuring the accuracy of the test results and being more conducive to use.

[0026] In summary, the present application includes at least one of the following beneficial technical effects: First, carry out a crack propagation test in a conventional air and room temperature environment to obtain C => C LLThe mapping relation formula, and then the first flexibility test is carried out by the synchronous measurement method of the COD gauge and the LVDT sensor to obtain the elastic modulus correction value and the flexibility secondary correction coefficient. After that, the COD gauge is removed and the test environment is established, and the second flexibility test is carried out to obtain the elastic modulus correction value under the test environment. Finally, the elastic modulus correction value under the test environment, the flexibility secondary correction coefficient, and the LVDT sensor calculation data are used, and through C => C LL The mapping relation formula and the crack length calculation formula can be used to calculate the crack length, so as to achieve the purpose of carrying out the metal material crack propagation test in the fluid environment, and at the same time ensure the accuracy of the test results; By repeatedly carrying out the crack propagation test under the same conditions, and according to the results obtained from multiple tests, for C => C LL The mapping relation is further corrected to eliminate the errors caused by different sample installation states and LVDT sensor installation states in multiple tests, and further ensure the accuracy of the test results; By the combined use of the LVDT sensor and the COD gauge, the deviation between the crack length data measured by the LVDT and the crack length data measured by the COD gauge is ≤ ±2%, and the deviation between the crack length data measured by the LVDT and the actually measured crack length data is ≤ ±4%. Brief Description of the Drawings

[0027] Figure 1 is a flowchart showing the overall test process in Embodiment 1 of the present application; Figure 2 is a schematic diagram showing the structure of the stepped C(T) specimen in Embodiment 1 of the present application; Figure 3 is to show C and C in Embodiment 1 of the present application LL A schematic diagram of the first-order linear fitting curve; Figure 4 is to show α in Embodiment 1 of the present application LVDT and α COD A schematic diagram of the comparison curve; Figure 5 is a schematic diagram showing the flexibility test hysteresis curve in Embodiment 1 of the present application; Figure 6 is an axonometric schematic diagram mainly showing the overall structure in Embodiment 2 of the present application; Figure 7 is an axonometric schematic diagram mainly showing the structure of the gravitational frame in Embodiment 2 of the present application; Figure 8 is a sectional view mainly showing the structure of the tie rod installation in Embodiment 2 of the present application; Figure 9 is an axonometric schematic diagram mainly showing the structure of the clamping assembly in Embodiment 2 of the present application; Figure 10It is an axonometric schematic diagram mainly showing the structure of the LVDT detection device in the second embodiment of the present application; Figure 11 It is an axonometric schematic diagram mainly showing the structure of the LVDT signal collector in the second embodiment of the present application.

[0028] Reference numerals: 1, kettle lid; 2, gravitational frame; 21, fixing block; 22, connecting rod; 3, specimen; 4, clamping assembly; 41, lower specimen fixture; 42, lower specimen fixing shaft; 43, upper specimen fixture; 44, upper specimen fixing shaft; 5, pull rod; 6, LVDT detection device; 61, LVDT sensor; 62, extensometer rod; 63, LVDT signal collector; 64, pivot pin; 7, COD gauge; 8, bellows. Detailed implementation manners

[0029] The following further elaborates on the present application in conjunction with FIGS. 1 - Figure 11 Make a further detailed description of the present application.

[0030] The embodiments of the present application disclose a method and a device for testing crack propagation of metal materials in a fluid environment. Embodiment

[0031] Referring to Figure 1 , a method for testing crack propagation of metal materials in a fluid environment includes the following steps: S1: Define the value range of α, prefabricate and install a stepped C(T) specimen; The specific steps are as follows: Define the mapping relationship of C => C LL and regularize the value range [α min , α max of the crack length α. 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 fixedly install two LVDT sensors on both sides of the notch surface of the stepped C(T) specimen.

[0032] Referring to Figure 2 , in the crack rate expansion test, on the one hand, it is necessary to consider the space size inside the test kettle during the synchronous measurement of the LVDT sensor and the COD gauge, that is, the specimen specification size cannot be too large; on the other hand, it is necessary to consider the installation space required by the LVDT sensor and the COD gauge, that is, the specimen specification size cannot be too small; in addition, it is necessary to consider the loading force range (50 kN) of the existing test device. Therefore, in this embodiment, a stepped C(T) specimen with a width W = 50.8 mm, a thickness B = 12.7 mm, a net side groove thickness B N = 10 mm, and a machined length a m = 25 mm is selected.

[0033] S2: Repeatedly conduct multiple crack propagation tests under normal conditions to obtain the first-order fitting coefficient A 0 and A 1 and the mapping relationship after correction C => C LL ; The specific steps are as follows: Specific steps are as follows: S21. Under normal air and room temperature, conduct a crack propagation test according to the range of the regularized crack length α defined in S1. Obtain the load line displacement V LL , the opening displacement V, and the force value F through synchronous measurement using a COD gauge, an LVDT sensor, and a force value sensor. And use the V LL and F data to perform a first-order linear fitting calculation to obtain the load line displacement compliance C LL , use the V and F data to perform a first-order linear fitting calculation to obtain the opening displacement compliance C, and synchronously record C LL and C to complete the test; S22. Analyze the measured data of C and C LL . As the crack length expands, it is found that C and C LL are linearly correlated, and their linear relationship expression is f(x) = A 0 + A 1 * x, and calculate the first-order power fitting constant A 0 and the first-order power fitting coefficient A 1 , thereby obtaining the mapping relationship C => C LL C LL = A 0 + A 1 C; S23. Repeatedly conduct multiple crack propagation tests under the same conditions. According to the multiple groups of load line displacement compliances C LL and multiple groups of opening displacement compliances C i obtained from the repeated tests, and use the mapping relationship C => C LL in S2 to calculate the regularized crack length α LVDT and α COD respectively, to obtain the comparison curve of the regularized crack length data α LVDT and α COD . At the same time, according to the test results, correct the mapping relationship C => C LL . The mapping relationship after correction C => C LL is as follows: Formula 1: C LL = B 1 *(A 0 + A 1 * C i ); Formula 2: B 1 = 1 + (1 - B 0 )*(α i-1 - αmax ) / α max ; In Equation (1), A 0 and A 1 are the first - power fitting constant and the first - power fitting coefficient obtained in S22, C i is the opening displacement compliance measured in the current crack propagation test, B 1 is the immediate correction coefficient for C LL ; In Equation (2), B 0 is the secondary compliance correction coefficient, obtained by analysis after the first compliance test in S4; α i-1 is the calculated value of the previous regularized crack length measurement; α max is the maximum value of the range of the regularized crack length α corresponding to the mapping relationship of C => C LL ; and in the calculation of the above B 1 , if B 0 = 1, or B 0 > 1 and B 1 < 1, or B 0 < 1 and B 1 > 1, then B 1 = 1.

[0034] In the application of the above mapping relationship, it should be noted that the ranges of the α value domain in the finally measured mapping relationship are different, and the corresponding A 0 and A 1 values are also different. If the length of the machined notch of the specimen changes or the test range changes and exceeds the mapping range, then the mapping relationship is redone; in addition, the mapping relationship obtained from the test can only represent one specification of the specimen used in the test, that is, the mapping relationship of the 1T specimen cannot be applied to the 0.5T specimen, and similarly, the mapping relationship of the 0.5T specimen cannot be applied to the 1T specimen.

[0035] In this embodiment, taking two crack propagation tests as an example for description, the specific implementation steps are as follows: The first step: Install a stepped CT specimen, install a COD gauge at the specimen loading line, and install an LVDT sensor at the specimen notch, and conduct the test according to the following test parameters; Table 1: Test parameters

[0036] The second step: Conduct a crack propagation test under normal air and room temperature, synchronously measure the loading - line displacement V LL and the opening displacement V through the COD gauge and the LVDT sensor, use the loading - line displacement V LL to calculate the regularized crack - length data α COD , and use the loading - line displacement V LLThe compliance coefficient C of the loading line displacement of the COD gauge is calculated from the opening displacement V LL and the compliance coefficient C of the opening displacement measured by the LVDT sensor 1 , and record C LL and C 1 synchronously to complete the test; Table 2: Compliance data for the first crack propagation test (partial)

[0037] Step 3: Organize the compliance coefficient C of the loading line displacement of the COD gauge LL and the compliance coefficient C of the opening displacement measured by the LVDT sensor 1 , that is, based on the measured C 1 and C LL data, it is found that C 1 and C LL have a first-order linear correlation, and the first-order linear fitting curve is as Figure 3 shown, and the linear relationship is expressed as: Linear equation: f(x)=A 0 +A 1 *x, where: A 0 =-2.77846E-1, A 1 =4.74155E-1; The first-order fitting linear correlation: R = 0.999878. By examining the first-order linear fitting equation and calculating the correlation coefficient, it can be seen that the compliance C measured by the LVDT and the compliance C measured by the COD gauge LL are highly linearly correlated; Step 4: To verify the first-order linear relationship between C and C LL , replace the stepped CT specimen, and conduct the crack propagation test again under normal air and room temperature. The loading line displacement V LL and the opening displacement V are synchronously measured by the COD gauge and the LVDT sensor. Use the loading line displacement V LL to calculate the regularized crack length data α COD , and use the loading line displacement V LL and the opening displacement V to calculate the compliance coefficient C of the loading line displacement of the COD gauge LL and the compliance coefficient C of the opening displacement measured by the LVDT sensor 2 ; Step 5: Recheck the mapping relationship. Use the compliance coefficient C of the loading line displacement of the COD gauge LL to calculate the crack length α COD and the compliance coefficient C of the opening displacement measured by the LVDT sensor 2 , and calculate the compliance coefficient C of the loading line displacement LL ' of the loading line displacement through the mapping relationship in S23, and then use the compliance coefficient C of the loading line displacementLL 'Calculate the crack length α LVDT , and calculate α LVDT and α COD deviation; The test parameters used in the second test are the same as those in the first test; the second test applies the mapping relationships of C 2 and C LL obtained in the first test, synchronously measure the opening displacement V and the loading line displacement V LL and calculate the corresponding compliance C 2 and C LL , and use C 2 and C LL to calculate the regularized crack length data α LVDT and α COD respectively; Table 3: Crack propagation data in the second test (partial)

[0038] Step 6: Further obtain the comparison curves of α LVDT and α COD as shown in Figure 4 ; it is found through comparison that the two curves do not coincide in the initial stage of crack propagation, but gradually coincide as the test progresses. It is preliminarily judged that this is caused by the difference in the initial installation states (specimen installation, LVDT installation) between the second test and the first test. This also shows that the change in the initial installation state affects the measurement accuracy of small opening displacements, but weakens as the opening displacement gradually increases; Step 7: Modify the mapping relationship of C => C LL according to the results of the first test and the second test. The modified mapping relationship is: C LL = B 1 *(A 0 + A 1 * C i ).

[0039] During the above test process, it is assumed that during the process of applying load to the specimen, the crack propagation of the specimen is a pure mode I crack action, that is, the crack surface develops linearly, that is, the error caused by the irregular crack development to the result calculation is removed; during the tensile process, the rotation influence of the LVDT measuring point position is minimized in design and no correction is considered, that is, the influence of the plane rotation resistance between the LVDT sensor and the specimen connection on the test result is reduced during the crack propagation process of the specimen; the LVDT installation state is stable during the test process, and the compliance calculation range takes the middle linear part, that is, the influence of external factors such as the installation state during the test on the test result is reduced to ensure the accuracy of the test result.

[0040] S3. Replace the specimen and install the test kettle; The specific operation steps are as follows: Replace the stepped C(T) specimen. After installing the stepped C(T) specimen together with the kettle lid of the test kettle, install the whole on the stress corrosion testing machine, install the COD gauge at the loading line of the stepped C(T) specimen, and install the LVDT sensor at the notch of the specimen.

[0041] S4. Conduct the first compliance test under normal environment to obtain the corrected value of the elastic modulus and the secondary correction coefficient of compliance; The specific operation steps are as follows: Conduct the first compliance test by synchronously measuring with the COD gauge and the LVDT sensor under normal air and room temperature. Use the known crack length a m (i.e., the machined length) and the measurement data of the COD gauge to correct the elastic modulus E of the specimen. Then use the known crack length a m , the corrected elastic modulus E, and the measurement data of the LVDT sensor to conduct a secondary correction on the compliance coefficient B to obtain the secondary correction coefficient B 0 of compliance. After the correction is completed, remove the COD gauge; In this embodiment, in practical applications, the corrected value of the elastic modulus E of the specimen and the secondary correction coefficient B 0 of compliance are both completed by the software using the approximation calculation method. The specific steps are as follows: Steps for correcting the elastic modulus E of the specimen: S41. Preset the initial value of the elastic modulus E (manually input value before the experiment), preset the increment △E = ±0.0001E, and preset the allowable deviation for crack length verification as β. The smaller the value of β, the smaller the approximation calculation error and the more accurate the final measurement result. In this embodiment, β is taken as 0.0001; S42. Substitute the initial value of the elastic modulus E and the loading line displacement compliance C LL measured by the COD gauge into the crack length calculation formula to calculate the normalized crack length α. Compare α with the known normalized crack length α 0 (α 0 = a m / W). If α > α 0 , then the increment △E = +0.0001E, otherwise △E = -0.0001E; S43. Take E = E + △E, and calculate the normalized crack length α again according to the crack length calculation formula. Calculate the normalized crack length deviation △α = |α - α 0 |; S44. Compare △α with β. If △α is greater than β (0.0001), repeat step S43 and compare the newly obtained △α with β again. Repeatedly perform the above calculations until △α is less than β. At this time, the value of E is the corrected value of the elastic modulus E of the specimen; Flexibility secondary correction coefficient B 0 Derivation steps: S45. First, preset the flexibility secondary correction coefficient B 0 = 1.0, increment △B = ±0.0001, and the allowable deviation β of crack length verification is 0.0001; in this step, the flexibility 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 flexibility coefficient B is 1. Therefore, the flexibility secondary correction coefficient B 0 is preset to 1.0; S46. Substitute B 0 into formula two B 1 = 1 + (1 - B 0 ) * (α i-1 - α max ) / α max to calculate the instantaneous flexibility correction coefficient B 1 , where α i-1 takes the known regularized crack length α 0 (α 0 = a m / W); then use B 1 , the opening displacement flexibility C i measured by the LVDT sensor, and A 0 and A 1 calculated in S2, substitute them into formula one C LL = B 1 * (A 0 + A 1 * C i ) to calculate the loading line displacement flexibility 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; S47. Compare the α calculated in S46 with the known regularized crack length α 0 . If α > α 0 , then the increment △B = -0.0001, otherwise △B = +0.0001; S48. Take B 0 = B 0 + △B, repeat S46 to calculate the regularized crack length α again, and calculate the regularized crack length deviation △α = |α - α 0 |; S49. Compare △α with β. If △α is greater than β (0.0001), repeat step S48, and compare the newly obtained △α with β again. Repeat the above calculation until △α is less than β. At this time, the value of B 0 is the flexibility secondary correction coefficient.

[0042] S5. Establish a test environment through a test autoclave; The specific operation steps are as follows: Establish and stabilize a test environment (i.e., a fluid environment) through a test autoclave. During the establishment of the test environment, it is necessary to ensure that the stepped C(T) specimen is in a force-maintaining state to keep the installation state of the LVDT sensor unchanged, thereby reducing the influence of the installation state of the LVDT sensor on the test results.

[0043] S6. Conduct the second compliance test to obtain the corrected value of the elastic modulus under the test environment conditions; The specific operation steps are as follows: Conduct the second compliance test in the test environment, using the known crack length a m , the secondary compliance correction coefficient B obtained in S4 0 , and the measurement data of the LVDT sensor to further correct the elastic modulus E of the specimen, and obtain the corrected value of the elastic modulus E of the specimen under the test environment conditions; In this embodiment, the correction process of the elastic modulus E of the specimen under the test environment conditions is completed by the software using the approximation calculation method. The specific operation steps are as follows: S61. Preset the initial value of the elastic modulus E (manually input before the experiment), preset the increment △E = ±0.0001E, and preset the allowable deviation β of the crack length verification to be 0.0001. In this step, the initial value of E can be reset, or the corrected value of the elastic modulus E of the specimen obtained in S44 can be used. The two calculation methods are the same, only the number of calculations is different. In this embodiment, it is preferred to use the corrected value of the elastic modulus E of the specimen in S44; S62. Use the secondary compliance correction coefficient B obtained in S4 0 , substitute it into formula two B 1 = 1 + (1 - B 0 ) * (α i-1 - α max ) / α max , and calculate the instantaneous compliance correction coefficient B 1 . In the formula, α i-1 takes the known regularized crack length α 0 (α 0 = a / W); then use B 1 , the opening displacement compliance C measured by the LVDT sensor i , substitute it into formula one C LL = B 1 * (A 0 + A 1 * C i ) to calculate the loading line displacement compliance C LL ; finally, use the corrected value of the specimen elastic modulus E and C LL , calculate the regularized crack length α according to the crack length calculation formula, and compare α with the known regularized crack length α0 Compare. If α is greater than α 0 , then the increment ΔE = +0.0001E; otherwise, ΔE = -0.0001E; S63. Take E = E + ΔE, repeat the calculation steps of S62, calculate the regularized crack length α again, and calculate the regularized crack length deviation Δα according to the formula Δα = |α - α 0 |; S64. Compare Δα with the allowable deviation β (0.0001) of the crack length verification. If Δα is greater than β, repeat the steps of S63, and calculate repeatedly until Δα is less than β. At this time, the value of E is the corrected value of the elastic modulus E under the test environmental conditions; In S42, S43, S45, S46, S62, and S9, the crack length calculation formula used is: Formula Three: U X ={[BEV X / F] 1 / 2 +1} -1 ; Formula Four: The regularized crack length α = a / W = C 0 +C 1 U X +C 2 U X 2 +C 3 U X 3 +C 4 U X 4 +C 5 U X 5 ; In Formula Three, E is the elastic modulus; B is the specimen thickness; V X / F is the loading line displacement compliance C LL ; In Formula Four, C 0 =1.0002, C 1 =-4.0632, C 2 =11.242, C 3 =-106.04, C 4 =464.33, C 5 =-650.68.

[0044] S7. Normally conduct the crack propagation test to obtain the opening displacement compliance; The specific test steps are as follows: First, normally conduct the crack propagation test in the test environment, and record the data V of the LVDT 1 sensor installed above the specimen loading line during the test, 1 and the data V of the LVDT 2 sensor installed below the specimen loading line2 ; Secondly, use data V 1 and V 2 , substitute into the formula V = V 1 -V 2 to calculate the opening displacement V; Finally, use the opening displacement V and the loading force value F, substitute into the formula C = V / F to calculate the opening displacement compliance C.

[0045] S8. Use the above test data to convert the opening displacement compliance into the loading line displacement compliance; The specific operation steps are as follows: Use data A 0 and A 1 in S2, the compliance secondary correction coefficient B 0 in S4, the opening displacement compliance C = C i in S7, substitute into C =>C LL in the corrected mapping formula to calculate the loading line displacement compliance C LL ; The calculation steps are as follows: S81. Use the compliance secondary correction coefficient B 0 obtained in S4, the known quantity α i-1 , the known quantity α max , substitute into formula two B 1 = 1+(1 - B 0 )*(α i-1 -α max ) / α max to calculate B 1 ; S82. Use data A 0 and A 1 obtained in S2, B 1 obtained in S81, the opening displacement compliance C (i.e., C i ) in S7, substitute into formula one C LL = B 1 *(A 0 +A 1 *C i ) to calculate the loading line displacement compliance C LL .

[0046] S9. Use the loading line displacement compliance to calculate the crack length; The specific operation steps are as follows: Use the loading line displacement compliance C LL obtained in S8, first calculate the normalized crack length α according to the crack length calculation formula, and then substitute α into the formula a = α*W to calculate the crack length a; where W is the specimen width.

[0047] During the above tests, it should be noted that: for each installation of the specimen, COD gauge, and LVDT sensor, an installation effect evaluation test should be carried out; during each flexibility test, the alternating load force should not be too large to cause crack growth of the specimen; during the establishment of the test environment, the holding force of the specimen should not be too large to cause crack growth of the specimen.

[0048] During the installation effect evaluation test, through multiple installation practices, the following quantified inspection data for the installation effect are established: For the quantified inspection requirements of the installation effect of the COD gauge, borrowing the flexibility test software, the force and loading line displacement hysteresis curves require that the loading and unloading curves visually coincide. Within the flexibility calculation range of 30% - 70%, the statistical coefficient of variation of the flexibility for multiple loading and unloading is less than 0.1%; For the quantified detection requirements of the installation effect of the LVDT sensor, borrowing the flexibility test software, the force and opening displacement hysteresis curves require that the loading and unloading curves visually coincide within the range of 10% - 90% of the force value for 30% - 70% within the flexibility calculation range, and the statistical coefficient of variation of the flexibility for multiple loading and unloading is less than 0.2%.

[0049] The specific test for checking the installation effect through the flexibility test is as follows: The flexibility test parameters are: the waveform is a sine wave, the initial loading rate (V0) is 2 mm / min, the force - controlled loading rate (V1) is 500 N / s, the force peak value (Fmax) is 4000 N, the load ratio (R) is 0.1, the loading frequency (f) is 1.0 Hz, the number of loading cycles is 30, and the known crack length is 25 mm; Table 4: Data Sheet of Flexibility Test (Partial)

[0050] Refer to Figure 5 , according to the above calculation and inspection results, it can be seen that the loading and unloading hysteresis curves of the COD gauge visually coincide, and the statistical coefficient of variation of the flexibility CV = 0.100%. According to the quantified rules for checking the installation effect, the installation effect of the COD gauge is good; the loading and unloading hysteresis curves of the LVDT visually coincide (within the force value range of 10% - 90%), and the statistical coefficient of variation of the flexibility CV = 0.186%. According to the quantified rules for checking the installation effect, the installation effect of the LVDT is good.

[0051] S10. Measure the actual crack length of the specimen, and verify the a obtained in S9 through the measurement results; The actual measurement steps are as follows: S101. Take out the stepped C(T) specimen after the crack propagation test, and mark the initial machining length position A, the pre - fabricated crack position B, and the specimen back position C; S102. Measure the pixels at positions A, B, and C respectively; S103. Use the known quantity AC = W - a m Substitute the numerical values of A, B, and C in S102 into the formulas AB = (A - B) / (A - C) * AC and a 0 = AB + a m to obtain the prefabricated crack length AB and the initial crack length a 0 ; S104. Use the measured prefabricated crack length AB and the initial crack length a 0 Compare and calculate with the crack length W*(a / W) reported by the software and the reported prefabricated crack length in the S9 test to obtain the crack length measurement deviation and the crack growth measurement deviation

[0052] In summary, based on the crack growth test data obtained by the LVDT sensor, the following evaluations are carried out (1) Using a double LVDT sensor cannot completely replace the COD gauge. The COD gauge is still required to establish the mapping relationship between the opening displacement compliance C and the loading line displacement compliance CLL, and the COD gauge is needed to calibrate the elastic modulus at the initial stage of the test (2) Although using a double LVDT sensor cannot completely replace the COD gauge, the compliance C measured by the double LVDT in air at normal temperature and pressure can be relatively well converted into the loading line displacement compliance CLL, and the crack growth curve can be obtained based on the LVDT measurement data (3) In a special environment, as long as the LVDT sensor can measure normally and stably and the initial installation state remains the same, it is feasible to use the LVDT sensor in combination with the COD gauge for the crack growth test. Moreover, the crack growth data shows good performance. The deviation between the crack length data and the measured data is only 1.098%, which is lower than the expected target deviation of 5%. The compliance measurement is stable during the crack growth test, and the correlation R of the compliance calculation by one - time fitting is > 0.9999

[0053] Example 2 Refer to Figure 6 and Figure 7 A test device for a metal material crack growth test method in the fluid environment of Example 1, its structure includes a horizontally placed test kettle, a kettle cover 1 buckled on the test kettle, and a gravitational frame 2 connected to the kettle cover 1. A stepped C(T) specimen 3 is arranged on the gravitational frame 2. The gravitational frame 2 is composed of a fixed block 21 and several connecting rods 22. In this embodiment, the connecting rods 22 are preferably set to 4, and the four connecting rods 22 are spaced around the fixed block 21. The top of the connecting rod 22 is threadedly connected to the kettle cover 1 of the test kettle, and the bottom of the connecting rod 22 is fixed to the fixed block 21 by bolts. When in use, after assembling the gravitational frame 2 on the kettle cover 1, then buckle the kettle cover 1 on the test kettle. At this time, the gravitational frame 2 extends into the test kettle

[0054] Referring 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 installed at the loading line of the C(T) specimen 3. Among them, the clamping assembly 4 is used to clamp and fix the C(T) specimen 3 on the gravity frame 2. The top end of the pull rod 5 is connected to the driving end of the stress corrosion testing machine. The pull rod 5 is used to transfer the load to the C(T) specimen 3. Both the LVDT detection device 6 and the COD gauge 7 are used to detect the crack propagation of the C(T) specimen 3. The difference is that the COD gauge 7 is only used for testing under normal temperature and atmospheric pressure in the atmosphere, and the test results are used to assist in calculating the test results of the LVDT detection device 6. The LVDT detection device 6 can detect crack propagation both in the normal temperature and atmospheric pressure environment and in the high-temperature corrosive fluid environment. In this application, two groups of LVDT detection devices 6 are provided.

[0055] Referring to Figure 6 and Figure 8 ,the top end of the pull rod 5 is located above the kettle cover 1. The top end of the pull rod 5 is used to connect the stress corrosion testing machine. The bottom end of the pull rod 5 passes through the kettle cover 1 and is located below the kettle cover 1. The pull rod 5 can slide on the kettle cover 1, and a sealing structure for sealing the gap between the pull rod 5 and the kettle cover 1 is arranged between the pull rod 5 and the kettle cover 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 fixed to the kettle cover 1 by bolts. The sealing ring is located between the bottom end of the bellows 8 and the kettle cover 1, and the sealing ring is used to seal the connection gap between the bellows 8 and the kettle cover 1.

[0056] Referring to Figure 6 and Figure 8 ,during use, the stress corrosion testing machine provides a tensile force to the pull rod 5 to drive the pull rod 5 to slide along its own axis. During the sliding process of the pull rod 5, through the combined use of the sealing ring and the bellows 8, on the premise of ensuring that the sliding of the pull rod 5 is not affected, the purpose of sealing the gap between the pull rod 5 and the kettle cover 1 is achieved, which is beneficial to the subsequent establishment of the high-temperature corrosive fluid environment.

[0057] Referring to Figure 6 and Figure 9, the clamping assembly 4 is composed of a specimen lower fixture 41, a specimen lower fixed shaft 42, a specimen upper fixture 43 and a specimen upper fixed shaft 44. The bottom end of the specimen lower fixture 41 is fixed to the fixed block 21 by bolts. The top end of the specimen lower fixture 41 is provided with a lower clamping groove for clamping the C(T) specimen 3. The specimen lower fixed shaft 42 is inserted through the top end of the specimen lower fixture 41 and passes through the lower clamping groove. The top end of the specimen upper fixture 43 is fixedly connected to the bottom end of the pull rod 5 by bolts. The bottom end of the specimen upper fixture 43 faces the specimen lower fixture 41, and an upper clamping groove for clamping the C(T) specimen 3 is provided at the bottom end of the specimen upper fixture 43. The specimen upper fixed shaft 44 is inserted through the bottom end of the specimen upper fixture 43 and passes through the upper clamping groove.

[0058] Refer to Figure 6 and Figure 9 , during use, a through hole one for the specimen upper fixed shaft 44 to pass through and a through hole two for the specimen lower fixed shaft 42 to pass through 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 specimen lower fixture 41 and fixed and limited by the specimen lower fixed shaft 42. The upper end of the C(T) specimen 3 is placed into the upper clamping groove of the specimen upper fixture 43 and fixed and limited by the specimen upper fixed shaft 44, thereby completing the clamping and fixing of the C(T) specimen 3.

[0059] Refer to Figure 6 and Figure 10 , two groups of LVDT detection devices 6 are respectively arranged corresponding to the upper and lower ends of the C(T) specimen 3. Each group of LVDT detection devices 6 includes an LVDT sensor 61, an extensometer rod 62 and an LVDT signal collector 63. Among them, the LVDT sensor 61 is fixed to the upper end face of the kettle cover 1 by bolts. The top end of the extensometer rod 62 is threadedly connected to the iron core of the LVDT sensor 61. The bottom end of the extensometer rod 62 passes through the kettle cover 1 and is connected to the LVDT signal collector 63 located below the kettle cover 1.

[0060] Refer to Figure 10 and Figure 11 , one end of the LVDT signal collector 63 is detachably connected to the C(T) specimen 3 by bolts. The other end of the LVDT signal collector 63 is provided with a connection hole penetrating in the vertical direction. The bottom end of the extensometer rod 62 is inserted into the connection hole through a rotating pin 64. The axis of the rotating pin 64 is perpendicular to the axis of the connection hole. The rotating connection between the extensometer rod 62 and the LVDT signal collector 63 is realized through the rotating pin 64, thereby reducing the resistance of the C(T) specimen 3 to the horizontal rotational force during the crack propagation process.

[0061] Refer to Figure 6, in addition, in the present application, an O-ring is also provided between the extension rod 62 and the kettle cover 1. During use, the gap between the extension rod 62 and the kettle cover 1 is sealed by the O-ring, so as to facilitate the establishment of a high-temperature fluid environment between the kettle cover 1 and the test kettle subsequently.

[0062] The implementation principle of the embodiment of the present application is as follows: During use, first assemble the reaction frame, LVDT sensor 61, extension rod 62, LVDT signal collector 63, upper specimen fixture 43, lower specimen fixture 41, pull rod 5, COD gauge 7, stepped C(T) specimen 3 and the kettle cover 1 together, and assemble the whole onto the stress corrosion testing machine. Then, first in the normal temperature and atmospheric pressure environment, through the synchronous measurement method of the COD gauge 7 and the LVDT sensor 61, obtain the primary mapping relationship between the compliance of the loading line displacement measured by the COD gauge 7 and the compliance of the opening displacement measured by the LVDT sensor 61, as well as the elastic modulus correction value. Then remove the COD gauge 7, establish a high-temperature corrosive fluid environment through the test kettle. In the high-temperature corrosive fluid environment, only perform compliance testing on the C(T) specimen 3 through the LVDT test device, so as to measure the opening displacement compliance of the C(T) specimen 3 and the elastic modulus correction in the high-temperature corrosive fluid environment. Then perform fatigue loading on the C(T) specimen 3 to carry out a crack propagation test. Measure the opening displacement of the C(T) specimen 3 through the LVDT sensor 61, and calculate the opening displacement compliance data according to the mapping relationship between the loading line displacement compliance and the opening displacement compliance. Finally, use the test data and further calculate through the crack length calculation formula to obtain the crack length of the C(T) specimen 3, so as to achieve the purpose of performing crack propagation testing on the C(T) specimen 3 in the high-temperature corrosive fluid environment, and at the same time ensure the accuracy of the test results.

[0063] It should be noted that in this embodiment, the technical features "C(T) specimen, COD gauge, LVDT sensor and kettle cover" refer to the same content as the corresponding technical features involved in the method steps in Embodiment 1.

[0064] The above are all the preferred embodiments of the present application, and do not limit the protection scope of the present application accordingly. Therefore, all equivalent changes made according to the structure, shape, and principle of the present application should be covered within the protection scope of the present application.

Claims

1. A method for testing crack growth of metal materials in a fluid environment, characterized in that: The steps include: S1. Prefabricate the step-type C(T) specimen, install the step-type C(T) specimen on the stress corrosion testing machine, and install the COD gauge and LVDT sensor on the step-type C(T) specimen; S2. Repeat the crack extension test in normal air and room temperature, and obtain multiple sets of loading line displacements V by synchronous measurement of COD gauge, LVDT sensor and force sensor. LL , multiple sets of opening displacements V and multiple sets of force values ​​F, using V LL , V and F data are calculated and deduced to obtain C=>C LL The mapping relationship C LL =B1*(A0+A1*C i )、B1=1+(1-B0)*(α i-1 -α max ) / α max ; S3. Replace the step-type C(T) specimen, combine the step-type C(T) specimen with the test kettle, install the whole on the stress corrosion testing machine, and then install the COD gauge and LVDT sensor on the step-type C(T) specimen; S4. Perform the first compliance test in normal air and room temperature by synchronously measuring with COD gauge and LVDT sensor, using a known crack length a m The elastic modulus E of the specimen is corrected using the measured data of the COD gauge, and then the known crack length a is used m , the corrected elastic modulus E, and the LVDT sensor measurement data are used to perform secondary correction on the flexibility coefficient B to obtain the flexibility secondary correction coefficient B0. After the correction is completed, the COD gauge is removed; S5. Establish and stabilize the test environment in the test kettle. During the establishment of the test environment, the specimen is in a force-holding state; S6. Perform a second compliance test in the test environment using a known crack length a. m , the flexibility secondary correction coefficient B0 in S4, and the LVDT sensor measurement data are used to further correct the elastic modulus E of the specimen to obtain the corrected value of the elastic modulus E of the specimen under the test environment conditions; S7. Carry out crack growth test normally in the test environment, and calculate the opening displacement compliance C using the LVDT sensor measurement data; S8, using the data A0 and A1 in S2, the secondary flexibility correction coefficient B0 in S4, and the opening displacement flexibility C=C in S7 i , substitute C=>C LL The mapping formula C LL =B1*(A0+A1*C i ) and B1=1+(1-B0)*(α i-1 -α max ) / α max The loading line displacement flexibility C is calculated LL ; S9, use the load line displacement compliance C in S8 LL The regularized crack length α is calculated according to the crack length calculation formula, and α is substituted into the formula a=α*W to calculate the crack length a.

2. The method for testing crack growth of metal materials in a fluid environment according to claim 1, characterized in that: In S1, before prefabricating the step-type C(T) specimen, it is necessary to define C=>C LL The range of the regularized crack length α in the mapping relationship [α min ,α max ] and carry out crack growth tests according to the value range of α.

3. The method for testing crack growth of metal materials in a fluid environment according to claim 2, characterized in that: The calculation steps in S2 are: First, crack growth tests were carried out in normal air and room temperature, and the loading line displacement V was obtained by synchronous measurement of the COD gauge, LVDT sensor and force sensor. LL , opening displacement V and force F; secondly, use V LL , V and F data are linearly fitted to obtain the loading line displacement compliance C LL and the opening displacement flexibility C, and synchronously record C LL and C, complete the experiment; then analyze C and C LL The measured data of C and C are obtained as the crack length increases. LL It is linearly related, and its linear relationship expression is f(x)=A0+A1*x, and the first-order power fitting constant A0 and the first-order power fitting coefficient A1 are calculated, so that C=>C LL The mapping relationship C LL =A0+A1C; get C=>C LL After mapping the relationship, the crack extension test was repeated under the same conditions. According to the multiple groups of loading line displacement compliance C obtained by repeated tests, LL and multiple groups of opening displacement flexibility C i , and use C=>C in S2 LL The mapping relationship of the regularized crack length α is calculated separately. LVDT and α COD , to obtain the regularized crack length data α LVDT and α COD Finally, according to the test results, C=>C LL The mapping relationship is further modified, and the modified formula 1 is: C LL =B1*(A0+A1*C i ); In formula 1, A0 and A1 are the first-order power fitting constant and first-order power fitting coefficient obtained in S2; B1 is C LL The instant correction coefficient is calculated by the following formula: B1=1+(1-B0)*(α i-1 -α max ) / α max ; C i is the opening displacement compliance measured in the crack growth test; In formula 2, B0 is the secondary correction coefficient of flexibility; α i-1 is the calculated value of the last regularized crack length measurement; α max C=>C LL The maximum value of the regularized crack length α corresponding to the mapping relationship.

4. The method for testing crack growth of metal materials in a fluid environment according to claim 3, characterized in that: In the mapping relationship calculation, if B0 is equal to 1, then B1 is equal to 1; if B0 is greater than 1 and B1 is less than 1, then B1 is equal to 1; if B0 is less than 1 and B1 is greater than 1, then B1 is equal to 1.

5. The method for testing crack growth of metal materials in a fluid environment according to claim 3, characterized in that: Each time the step-type C(T) specimen is replaced and the connection between the COD gauge and LVDT sensor and the step-type C(T) specimen is completed, the installation effect between the COD gauge, LVDT sensor and the step-type C(T) specimen must be evaluated through a flexibility test. Subsequent tests can be carried out only after the test passes.

6. The method for testing crack growth of metal materials in a fluid environment according to claim 3, characterized in that: The rotation effect of the LVDT measuring point position during the stretching process is minimized and no correction is considered.

7. The method for testing crack growth of metal materials in a fluid environment according to claim 4, characterized in that: The correction steps for the elastic modulus E and flexibility coefficient B of the specimen in S4 are: S41, preset the initial value of elastic modulus E, increment △E, and crack length verification allowable deviation β; S42, the initial value of elastic modulus E, the loading line displacement compliance C measured by COD gauge LL Substitute into the crack length calculation formula to calculate the regularized crack length α, and compare α with the known regularized crack length α0. If α>α0, then the increment △E takes a positive value, otherwise △E takes a negative value; 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|; S44, compare △α and β, if △α is greater than β, repeat step S43 until △α is less than β, at which point 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 △B, crack length verification allowable deviation β; S46, using the flexibility secondary correction coefficient B0, according to S2 in the second formula to calculate the flexibility instantaneous correction coefficient B1, where α i-1 Take the known regularized crack length α0 and the opening displacement compliance C measured by B1 and LVDT sensor i , the loading line displacement flexibility C is calculated according to formula 1 in S2 LL , using the corrected value of elastic modulus E 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, the increment △B takes a negative value, otherwise △B takes a positive value; S48, taking B0=B0+△B, repeating S46 to calculate again to obtain the regularized crack length α, and calculating the regularized crack length deviation △α=|α-α0|; S49, compare △α and β. If △α is greater than β, repeat step S48 until △α is less than β. At this time, the B0 value is the secondary flexibility correction coefficient.

8. The method for testing crack growth of metal materials in a fluid environment according to claim 7, characterized in that: The steps for further correcting the elastic modulus E of the specimen in S6 are: S61, preset increment △E and crack length verification allowable deviation β; S62, using the sample elastic modulus E correction value in S44, the flexibility secondary correction coefficient B0 in S4, and the LVDT sensor to measure the opening displacement flexibility C i , according to formula 2 in S2, the flexibility instant correction coefficient B1 is calculated; Then, according to the formula 1 in S2, the loading line displacement flexibility C is calculated: LL Finally, the regularized crack length α is calculated according to the crack length calculation formula, and α is compared with the known regularized crack length α0. If α is greater than α0, the increment △E takes a positive value, otherwise △E takes a negative value; S63, taking E=E+△E, repeating the calculation step S62, calculating again to obtain the regularized crack length α, and calculating the regularized crack length deviation △α=|α-α0|; S64, compare △α with the crack length verification allowable deviation β, if △α is greater than β, repeat S63 until △α is less than β, at which time the E value is the elastic modulus E correction value under the test environment conditions; The formula for calculating the crack length is: Formula 3: Regularized crack length α=a / W=C0+C1U X +C2U X 2 +C3U X 3 +C4U X 4 +C5U X 5 ; Formula 4: U X ={[BEV X / F] 1 / 2 +1} -1 ; In formula 4, E is the elastic modulus; B is the sample thickness; V X / F is the displacement flexibility of the loading line C LL ; In formula 3, C0=1.0002, C1=-4.0632, C2=11.242, C3=-106.04, C4=464.33, C5=-650.

68.

9. The method for testing crack growth of metal materials in a fluid environment according to claim 1, characterized in that: After a is calculated in S9, the crack length of the sample needs to be measured and a needs to be tested and verified through the measured results.

10. A testing device for the metal material crack extension testing method in a fluid environment according to claims 1 to 9, characterized in that: The invention comprises a kettle cover (1), a gravity frame (2) connected to the kettle cover (1), a C(T) sample (3), a clamping assembly (4) for clamping and positioning the C(T) sample (3), a pull rod (5) for transmitting load to the C(T) sample (3), and an LVDT detection device (6) arranged on the gravity frame (2), a COD gauge (7) being installed at the loading line of the C(T) sample (3), and two groups of the LVDT detection device (6) being arranged, and the two groups of the LVDT detection device (6) being connected to the upper and lower sides of the crack opening of the C(T) sample (3) respectively; The LVDT detection device (6) comprises an LVDT sensor (61) and an extension rod (62) installed on the kettle cover (1), and an LVDT signal collector (63) arranged on the extension rod (62); one end of the LVDT signal collector (63) is detachably connected to the C(T) sample (3); the other end of the LVDT signal collector (63) is provided with a connecting hole extending along the force application direction of the pull rod (5); one end of the extension rod (62) is connected to the iron core of the LVDT sensor (61); and the other end of the extension rod (62) passes through the kettle cover (1) and is rotatably connected to the connecting hole via a rotating pin (64).

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