Method for testing the fatigue crack growth threshold of a weld

Abstract

The application discloses a kind of weld fatigue crack propagation threshold value test methods, comprising: using nondestructive residual stress detection method to obtain the welding residual stress peak value of weld position preparation CT sample, and prefabricated fatigue crack;Setting loading mode is flexibility method and K reduction method, based on flexibility method measurement crack length change, K reduction method keeps average load constant in the process of reducing ΔK;And the notch of test sample is located on the center line of weld, to carry out fatigue crack propagation rate threshold value test, test load uses stress intensity factor ΔK control;The stress intensity factor ΔK of weld CT sample crack tip and weld CT sample fatigue crack propagation rate da / dN are obtained by calculation, measured ΔK-da / dN data is carried out least square linear regression fitting under double logarithmic coordinate system, according to fitting result obtains weld fatigue crack propagation threshold value ΔK th .

Description

Technical Field

[0001] This invention relates to the field of materials analysis and testing technology, and specifically to a method for testing the fatigue crack propagation threshold of welds that takes into account the influence of residual welding stress. Background Technology

[0002] Welded structures are widely used in critical equipment such as rail transit vehicles, ships, marine engineering, and bridges. However, the weld seam, due to its high residual tensile stress distribution and susceptibility to initial defects such as incomplete fusion and slag inclusions, is the weakest link in the fatigue resistance of welded structures. As the service conditions of welded structures become increasingly demanding, preventing fatigue failure has become a key consideration in the design phase of critical engineering equipment. Accordingly, obtaining scientific and accurate fatigue performance data for weld seam locations prone to fatigue failure in critical engineering equipment welded structures is of great significance for the safe service of these structures.

[0003] Fatigue-resistant design based on damage tolerance is a primary method for safety design of welded structures in major engineering equipment in fields such as aerospace, rail transit, and marine engineering. This method acknowledges the existence of initial defects of a certain size within the structure. Then, based on damage tolerance characteristics (primarily load-bearing features) and material property testing results, it determines whether the presence of these initial defects affects the safety of the load-bearing structure under specified service conditions. Fatigue-resistant design based on damage tolerance can ensure that welded structures of major engineering equipment exhibit superior overall performance and load-bearing capacity, while reducing manufacturing and maintenance costs.

[0004] Fatigue crack propagation threshold value ΔK th The weld fatigue crack propagation threshold is a crucial indicator in fatigue-resistant design based on damage tolerance principles. For welded structures, the accuracy of this threshold directly determines their reliability, and it is also a key fatigue life assessment parameter tested by researchers and designers. Despite this, welded structures used in various industries still experience numerous fatigue fractures during service, often resulting in significant economic losses, posing a major threat to operational safety, and presenting significant challenges to the maintenance of load-bearing structures. Therefore, accurate measurement of the weld fatigue crack propagation threshold in welded structures requires a full and scientific consideration of the influence of residual tensile stress in the weld to ensure the long-term safe and reliable service of these critical engineering structures.

[0005] Fatigue crack propagation threshold value ΔK for weld material th The definition is: the fatigue crack propagation rate of the weld, da / dN, equal to 10. -7The ΔK value corresponding to mm / cycle is further calculated from the measured values ​​of crack length 'a' and stress cycle number 'N' in the fatigue crack propagation rate test. The fatigue crack propagation threshold measurement test is a time-consuming process, requiring continuous adjustment of the load to reduce the stress intensity factor range at the crack tip until the corresponding fatigue crack propagation rate is sufficiently small to 10. -7 mm / cycle. Fatigue crack length can be obtained by visual inspection or compliance testing. Visual inspection requires repeated measurement and confirmation by the tester, while compliance testing uses a crack opening displacement gauge (COD gauge) for measurement and computer recording and storage.

[0006] GB / T 6398-2017, "Methods for Fatigue Crack Propagation in Fatigue Testing of Metallic Materials," recommends a procedure for measuring the fatigue crack propagation threshold value, selecting at least five values ​​with an average distribution at 10°C. -7 ~10 -6 For the log(da / dN)-logΔK data pairs in mm / cycle, with log(da / dN) as the independent variable and logΔK as the dependent variable, a linear regression method is used to fit the data points, and then 10 is extracted based on the fitting results. -7 The ΔK value corresponding to mm / cycle is the measured value of the fatigue crack propagation rate threshold. The above method for measuring the fatigue crack propagation rate threshold is based on the compliance method, and the test load is set in a step-down K mode.

[0007] The above testing process requires repeatedly starting and stopping the fatigue testing machine, changing the load and restarting the test, which will lead to the additional crack closure effect of load changes; for the weld fatigue crack propagation rate threshold value, the process of gradually changing the test load ΔK does not take into account the influence of welding residual stress. Summary of the Invention

[0008] The purpose of this invention is to provide a test method for the fatigue crack propagation threshold value of weld considering the influence of welding residual stress. The test method uses a non-destructive residual stress measurement method to measure the peak value of the weld residual stress and uses it as a static load. When testing CT specimens, the compliance method and the K-reduction method are used. The compliance method measures the crack length change, and the K-reduction method keeps the average load constant during the reduction of the stress intensity factor ΔK. The fatigue crack propagation rate threshold value is then tested. The measurement results of this method correspond to the fatigue crack propagation threshold value of the actual weld.

[0009] The objective of this invention is achieved through the following technical solution:

[0010] A method for testing the fatigue crack propagation threshold of welds, taking into account the influence of residual welding stress, includes:

[0011] Step 1: Measure the residual stress distribution at the weld location using a non-destructive residual stress testing method to obtain the peak value of the welding residual stress; prepare a compact tensile (CT) specimen on the weld joint by machining, with the notch of the test specimen located on the weld centerline;

[0012] Step 2: Install the CT specimen prepared according to Step 1 on the fatigue testing machine and pre-fabricate fatigue cracks; after the fatigue cracks are pre-fabricated, set the loading method to constant static load with K-reduction method. The constant static load with K-reduction method includes the use of compliance method and K-reduction method. The crack length change is measured based on compliance method, and the average load is kept constant during the K-reduction method. The average load value is equal to the peak value of the weld residual stress measured in Step 1, thereby carrying out fatigue crack propagation rate threshold value test. The test load is controlled by stress intensity factor ΔK.

[0013] Step 3: Calculate the stress intensity factor ΔK at the crack tip of the weld CT specimen and the fatigue crack propagation rate da / dN of the weld CT specimen. While maintaining a constant average load, gradually decrease the stress intensity factor ΔK. Perform least-squares linear regression fitting on the measured ΔK-da / dN data in a double logarithmic coordinate system. Based on the fitting results, corresponding to 10... -7 The stress intensity factor ΔK value per mm / cycle is used as the fatigue crack propagation threshold value ΔK for the weld. th .

[0014] Furthermore, the peak value of welding residual stress in step one is detected by X-ray diffraction or indentation method.

[0015] Furthermore, the process of installing the CT specimen prepared in step one and pre-inducing fatigue cracks on the fatigue testing machine specifically includes: installing the CT specimen prepared in step one and the crack opening displacement gauge (COD gauge) on the fatigue testing machine; inputting the specimen size and mechanical property parameters of the weld material into the testing software; performing fatigue preloading; and adjusting the elastic modulus value of the weld material input into the computer software to make the crack length obtained by the compliance method consistent with the actual crack length.

[0016] Furthermore, the stress intensity factor ΔK and the fatigue crack propagation rate da / dN of the weld CT specimen mentioned in step three are obtained as follows:

[0017] The stress intensity factor ΔK is calculated according to formula (1):

[0018]

[0019] Where P is the external load, B is the thickness of the CT specimen, α = a / W, θ = πa / 2W, and a and W are the fatigue crack length and specimen width of the CT specimen, respectively.

[0020] The fatigue crack propagation rate da / dN represents the amount of fatigue crack propagation per cycle, calculated according to formula (2):

[0021] da / dN=Δa / N (2)

[0022] Furthermore, in step two, the crack length of the specimen is calculated using the compliance method as shown in the following formula:

[0023]

[0024]

[0025] Where P is the external load, B is the thickness of the CT specimen, a and W are the fatigue crack length and specimen width of the CT specimen, respectively, E is the elastic modulus of the weld material, ν is the displacement of the COD gauge measurement point, C1 to C5 are calculation constants related to the COD gauge measurement position, and a is the fatigue crack length.

[0026] Furthermore, during the test in step three, the range of stress intensity factor ΔK at the fatigue crack tip is changed by controlling the test load ΔP value. The stress intensity factor ΔK is linearly related to the test load ΔP. The average value of the test load is kept constant by computer control, and the stress intensity factor ΔK is slowly reduced at a constant rate by computer control. The average value of the test load is set to the peak value of the weld residual stress measured in step one.

[0027] Compared with the prior art, the beneficial effects of the technical solution of the present invention are as follows:

[0028] a) Unlike the national standard recommendation of using the stepwise decreasing K method to measure the fatigue crack propagation threshold of metallic materials, this method can specifically measure the fatigue crack propagation threshold considering the influence of welding residual stress.

[0029] b) During the measurement process of this method, ΔK is gradually reduced under the condition of constant average load, without having to stop the fatigue testing machine, which can effectively avoid the crack tip closure effect caused by additional load settings.

[0030] c) Based on the same residual stress and fatigue crack propagation rate threshold test procedure, the fatigue crack propagation rate threshold value of different regions of the welded joint can be measured specifically under the condition of considering the influence of residual stress. Attached Figure Description

[0031] Figure 1 This is a schematic diagram illustrating the purpose of considering welding residual stress in this invention, wherein the load at the weld position is the superposition of the working load and the welding residual stress;

[0032] Figure 2a Planar dimensional diagram of a compact tensile (CT) specimen prepared for a welded joint. Figure 2b for Figure 2a Side view;

[0033] Figures 3a-3c They are respectively using the constant R load reduction method and the constant K load reduction method. max A schematic diagram of load application for the load reduction method and the static load reduction method K described in this invention;

[0034] Figure 4 shows the use of the constant R load reduction method and constant K load reduction method. max A comparison chart of the weld fatigue crack propagation rate threshold values ​​obtained by the load reduction method and the constant static load K-reduction method described in this invention, wherein... Figure 4a The comparison chart shows the result when R0 = 0.2. Figure 4b The comparison chart shows the result when R0 = 0.3. Figure 4c The comparison chart shows the result when R0 = 0.5. Figure 4d The comparison chart shows the result when R0 = 0.7;

[0035] Figure 5 The relationship between the threshold value and the initial stress ratio is shown under three loading methods;

[0036] Figure 6 The diagram shows the relationship between the slope ratio U of segments B and A of the da / dN-ΔK curve and the initial stress ratio R0 under three loading conditions.

[0037] Figure 7 The constant R-load reduction method and the constant static load reduction K-method described in this invention are shown under the loading conditions of the crack tip K. max A schematic diagram showing the relationship with the critical value. Detailed Implementation

[0038] To make the objectives, technical solutions, beneficial effects, and significant advancements of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described in conjunction with the accompanying drawings provided in the examples of the present invention. Obviously, all the described embodiments are only some embodiments of the present invention, and not all embodiments; based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0039] A method for testing the fatigue crack propagation threshold of welds that takes into account the influence of residual welding stress includes the following steps:

[0040] Step 1: Measure the residual stress distribution at the weld location and prepare the test specimen.

[0041] Welded joints contain residual welding stress. Figure 1This illustrates that under actual working conditions, the load at the weld location is the superposition of the working load and the welding residual stress. Therefore, non-destructive residual stress detection methods, such as X-ray diffraction and indentation, are first used to measure the residual stress distribution at the weld location and obtain the peak value of the welding residual stress. Subsequently, test specimens are prepared on the weld joint using machining methods. Figure 2 shows the prepared compact tensile (CT) specimen, where the CT specimen notch is located on the weld centerline, and the dimensions of the prepared CT specimen meet the requirements of standard ASTM E647-23 (Standard Test Method for Measurement of Fatigue Crack Growth Rates [S]. West Conshohocken: ASTM, 2023.).

[0042] Step 2: Pre-induce fatigue cracks on the CT specimen

[0043] CT specimens and COD gauges are clamped on the fatigue testing machine. The mechanical properties of the weld material and the dimensions of the CT specimens are input into the computer software. Fatigue preloading is performed. By adjusting the elastic modulus value of the weld material input into the computer software, the crack length obtained by the compliance method is made consistent with the actual crack length, ensuring accurate crack length acquisition.

[0044] The crack length of the specimen is calculated using the compliance method as shown in formulas (3) and (4):

[0045]

[0046]

[0047] Where P is the external load, B is the thickness of the CT specimen, a and W are the fatigue crack length and specimen width of the CT specimen, respectively, E is the elastic modulus of the weld material, ν is the displacement of the COD gauge measurement point, C1 to C5 are calculation constants related to the COD gauge measurement position, and a is the fatigue crack length.

[0048] Step 3: After the fatigue crack is pre-formed, the loading method is set to constant static load reduction K method, that is, the reduction ΔK method is based on the compliance method to measure the crack length change and set the average load (static load) to a constant value. During the reduction ΔK process, the static load is kept constant; where the static load is equal to the peak value of the weld residual stress obtained in step 1.

[0049] Fatigue crack propagation rate threshold tests were conducted. The test load required for the test was controlled by the stress intensity factor ΔK. The initial stress ratios R0 were set to 0.2, 0.3, 0.5, and 0.7, respectively. To ensure that the threshold measurement process was within the fatigue fracture range and to maintain the integrity of the test curve, the ΔK reduction procedure started before the curve showed an inflection point. In this embodiment, ΔK = 300 MPa / mm was selected based on the material properties. 0.5 This is the starting point for the ΔK reduction program.

[0050] The stress intensity factor ΔK is calculated according to formula (1):

[0051]

[0052] Where P is the external load, B is the thickness of the CT specimen, α = a / W, θ = πa / 2W, and a and W are the fatigue crack length and specimen width of the CT specimen, respectively.

[0053] The fatigue crack propagation rate da / dN represents the amount of fatigue crack propagation per cycle, calculated according to formula (2):

[0054] da / dN=Δa / N (2).

[0055] The recommended value for setting the termination fatigue crack length in the weld fatigue crack propagation threshold test is equal to the specimen width W. The test should then begin according to the set control method until the continuously acquired fatigue crack propagation rate (da / dN) is between 10. -7 mm / cycle~10 -6 The test can be stopped when there are at least 5 pairs of da / dN-ΔK within the mm / cycle range.

[0056] The measured ΔK-da / dN data were fitted using least squares linear regression in a log-log coordinate system. Based on the fitting results, da / dN = 10 was calculated. -7 The ΔK value corresponding to mm / cycle is used as the fatigue crack propagation rate threshold ΔK. th There is no need to stop the test and change the fatigue load during the test.

[0057] This embodiment implements the constant R load reduction method and the constant K method. max The threshold values ​​were determined under three loading methods: the load reduction method, the static load reduction K-method, and the dead static load reduction K-method. The test values ​​under the three loading methods were the same as those under the dead static load reduction K-method. Specifically, the dead static load reduction K-method... max K in the load reduction method max From 400MPa / mm 0.5 Up to 1000MPa / mm 0.5 Threshold value under certain conditions.

[0058] Using the constant R load reduction method and constant K as described in this invention max The load programs for the three loading methods—reduction load method, static load reduction K method, and static load reduction K method—are as follows: Figures 3a-3c As shown. Under the static load reduction K-method, ΔK is linearly related to the test load ΔP. The average value of the test load is kept constant by computer control, and its value is set to the peak value of the weld residual stress measured in step one. The stress intensity factor ΔK is slowly reduced at a constant rate by computer control until at least 5 pairs of crack propagation rates (da / dN) are continuously obtained between 10. -6 ~10 -7 Data pairs of ΔK-da / dN between mm / cycle.

[0059] Figure 4 shows the fatigue crack propagation rate test results under these three loading methods and different initial stress ratios. It can be seen that in the region near the crack propagation threshold, the stress intensity factor ΔK and the crack propagation rate da / dN have a nonlinear relationship. The value of da / dN deviates downward from the linear relationship as ΔK decreases. Referring to the BS 7910—2019 standard (Guide to Methods for Assessing the Acceptability of Flaws in Metallic Structures [S]. London: BSI, 2019.), the curve is divided into two segments, A and B, and fitted based on the Paris formula. The fitting parameters are shown in Table 1.

[0060] Table 1: Test results of fatigue crack propagation rate in sections A and B

[0061]

[0062] As an important parameter for determining whether fatigue cracks will propagate, the fatigue crack propagation threshold value ΔK th The value of ΔK corresponding to the crack propagation rate da / dN being close to zero is usually da / dN = 1 × 10 -7 ΔK at mm / cycle is used as the fatigue crack propagation threshold value. th As shown in Table 1, when the constant R load reduction method is used, compared to R0 = 0.2, the threshold values ​​measured under R0 = 0.3, 0.5, and 0.7 conditions decrease by 28.3%, 43.4%, and 48.1%, respectively, with the reduction gradually increasing; when the constant K load reduction method is used... max When using the "load reduction method", as the initial stress ratio R0 increases (corresponding to K), maxWith the increase of the initial stress ratio R0 (corresponding to the increase of the absolute value of static load during the load reduction process), compared with R0 = 0.2, the measured threshold values ​​under the conditions of R0 = 0.3, 0.5 and 0.7 decreased by 33.4%, 43.7% and 44.6% respectively; when the "constant static load reduction K method" described in this invention is used, with the increase of the initial stress ratio R0 (corresponding to the increase of the absolute value of static load during the load reduction process), compared with R0 = 0.2, the measured threshold values ​​under the conditions of R0 = 0.3, 0.5 and 0.7 decreased by 43.1%, 44.3% and 44.6% respectively. Under this load reduction method, when R0 is higher than 0.3, there is no significant difference in the measured value of fatigue crack propagation threshold.

[0063] For the use of constant R load reduction method and constant K max Measurement of fatigue crack propagation threshold ΔK using the load reduction method and the static load reduction K method th In other words, the different loading details of the three measurement methods lead to differences in the maximum stress intensity factor K. max They exhibit completely different trends during the measurement process; while K max As an important parameter in fatigue crack propagation, it has a significant impact on crack propagation near the crack threshold.

[0064] Based on the above data analysis, the fatigue threshold values ​​measured under different initial stress ratios under the three loading methods are as follows: Figure 5 As shown, the measured threshold values ​​under all three loading methods decrease with increasing initial stress ratio, and the measured value of the constant static load reduction method is always the lowest, while the measured value of the constant R load reduction method is always the highest. Since the constant R load reduction method has a smaller load in the near threshold region, the measured threshold value will be higher than the true value of the weld crack propagation threshold value containing tensile residual stress.

[0065] Numerically, the fatigue crack propagation threshold value (corresponding to da / dN = 1 × 10) -7 The crack propagation threshold (mm / cycle) is determined by the slope change of the test data curve. As ΔK decreases, a larger change in the slope of the da / dN-ΔK curve indicates a higher crack propagation threshold. To visually compare the difference in slope changes between the two curve segments under different loading conditions, the slope ratio U = k for segments B and A of the da / dN-ΔK curve is defined. B / k A The values ​​of U under the three initial stress ratios R0 are as follows: Figure 6 As shown, the U value is greater than 1 under different loading methods, and U decreases as the initial stress ratio increases. When R0 is 0.2, 0.3, and 0.5, the U values ​​differ significantly among the three loading methods, while they are similar when R0 is 0.7. Furthermore, the U value is higher under the "constant R load reduction method," reflecting a significant change in the slope of the curve at both ends. In contrast, the slope changes relatively less with R0 under the "constant static load reduction method" described in this invention, indicating better stability in the threshold test results.

[0066] Depend on Figure 7 As shown, under the constant static load reduction K-method loading conditions described in this invention, when the initial stress ratio R0 is 0.7 and 0.5, during the loading process, K... max Always greater than the critical value of 500 MPa / mm 0.5 Under the condition that R0 is 0.2, K max Always below 500 MPa / mm 0.5 For the case where R0 is 0.3, when using the static load reduction K method, the initial K... max All below 500 MPa / mm 0.5 Critical value, but as the crack propagates, K... max It gradually increases until it exceeds the critical value. However, under the constant R-load reduction method, K only becomes significant when R0 is 0.7. max The value is close to 500 MPa / mm 0.5 Critical value.

[0067] Therefore, when using the static load reduction method (K method) with initial stress ratios R0 of 0.3, 0.5, and 0.7, and when using the constant R reduction method with R0 of 0.7, the measured threshold values ​​are relatively close and low. Given the superposition of residual tensile stress during welding, the crack tip K... max The values ​​are often high, and the crack tip K value is higher when using the constant static load reduction K method. max The continuously rising threshold value is more consistent with the actual situation of cracks in welds, and the measured threshold value is more valuable for reference.

[0068] Therefore, the weld fatigue crack propagation threshold test method using the constant static load reduction K method described in this invention can take into account the influence of welding residual stress, resulting in more stable threshold data, and the values ​​are lower than those obtained by the constant R reduction method and the constant K method. max The measured value K by the load reduction method is lower, and the measured threshold value of the weld fatigue crack propagation rate is lower, which is more conservative and safer for the assessment of the remaining fatigue life of welded structures.

[0069] The above embodiments are only used to illustrate the technical solutions of the present invention, and are not intended to limit them. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention. Non-essential improvements, adjustments or substitutions made by those skilled in the art based on the content of this specification are all within the scope of protection claimed by the present invention.

Claims

1. A method for testing the threshold value of fatigue crack propagation in welds, characterized in that, include: Step 1: Measure the residual stress distribution at the weld location using a non-destructive residual stress testing method to obtain the peak value of the welding residual stress; prepare a compact tensile (CT) specimen on the weld joint using a machining method, with the notch of the test specimen located on the weld centerline; Step 2: Install the CT specimen prepared according to Step 1 on the fatigue testing machine and pre-fabricate fatigue cracks; after the fatigue cracks are pre-fabricated, set the loading method to constant static load reduction K method. The constant static load reduction K method includes the use of compliance method and reduction K method. The crack length change is measured based on compliance method, and the average load is kept constant during the reduction of stress intensity factor ΔK by reduction K method. The average load value is equal to the peak value of the residual stress of the weld measured in step one, thereby carrying out the fatigue crack propagation rate threshold test. The test load is controlled by the stress intensity factor ΔK. The process of installing the CT specimen prepared in step one and pre-fabricating fatigue cracks on the fatigue testing machine specifically includes: installing the CT specimen prepared in step one and the crack opening displacement gauge on the fatigue testing machine; inputting the specimen size and mechanical property parameters of the weld material into the testing software; performing fatigue preloading; and adjusting the elastic modulus value of the weld material input into the computer software to make the crack length obtained by the compliance method consistent with the actual crack length. Step three: obtain the crack tip stress intensity factor ΔK of the weld CT specimen and the fatigue crack propagation rate da / dN of the weld CT specimen by calculation, under the condition of maintaining constant average load, gradually reduce the stress intensity factor ΔK, and perform least square linear regression fitting of the measured ΔK-da / dN data pair in the double logarithmic coordinate system. According to the fitting result, the stress intensity factor ΔK value corresponding to 10 -7 mm / cycle is taken as the weld fatigue crack propagation threshold ΔK th ; In the third step of the test, the stress intensity factor range ΔK at the fatigue crack tip is changed by controlling the test load ΔP. The stress intensity factor ΔK is linearly related to the test load ΔP. The average value of the test load is kept constant by computer control, and the stress intensity factor ΔK is slowly reduced at a constant rate by computer control. The average value of the test load is set to the peak value of the weld residual stress measured in the first step. In the constant static load reduction K method, the constant rate of decrease of the stress intensity factor ΔK is achieved by controlling the deceleration rate of the test load ΔP, so that during the reduction of the stress intensity factor ΔK, the maximum stress intensity factor K at the crack tip is minimized. max As the crack propagates, it continues to show a monotonically increasing trend; The stress intensity factor ΔK and the fatigue crack propagation rate da / dN of the weld CT specimen mentioned in step three are obtained as follows: The stress intensity factor ΔK is calculated according to formula (1): （1） Where P is the external load, B is the thickness of the CT sample, and α = a / W, , where a and W are the fatigue crack length and specimen width of the CT specimen, respectively; The fatigue crack propagation rate da / dN represents the amount of fatigue crack propagation per cycle, calculated according to formula (2): （2）； In step two, the crack length of the specimen is calculated using the compliance method, as shown in the following formula: （3） （4） Where P is the external load, B is the thickness of the CT specimen, a and W are the fatigue crack length and specimen width of the CT specimen, respectively, E is the elastic modulus of the weld material, v is the displacement of the COD gauge measurement point, C1~C5 are calculation constants related to the COD gauge measurement position, and a is the fatigue crack length.

2. The method for testing the fatigue crack propagation threshold value of welds according to claim 1, characterized in that, The peak value of welding residual stress in step one is detected by X-ray diffraction or indentation method.

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