Pre-cracking method of WOL sample for stress corrosion test of welded joint

Abstract

The invention provides a pre-cracking method of a WOL sample for a stress corrosion test of a welded joint, which comprises the following steps: step 1, preliminary analysis: observing the morphology of a fracture with failed pre-cracking of the welded WOL sample, and preliminarily analyzing the cause of the fracture morphology; step 2, verification analysis: judging whether cause judgment is accurate or not; if yes, executing the step 3; if not, repeating the step 2; step 3, obtaining an influence factor K; 4, manual intervention: setting manual intervention parameters; 5, detecting the intervened sample, and judging whether the crack is qualified or not; if yes, obtaining a qualified precrack, and carrying out a constant displacement stress corrosion test on a WOL sample of the required welding joint to measure a KISCC value with high precision; according to the pre-cracking method of the WOL sample for the stress corrosion test of the welded joint, the pre-cracking success rate of the welded sample can be greatly improved, and the pre-cracking success of a great part of the WOL sample of the welded joint is realized; and the KISCC value of the welded joint of the ship body structure can be measured.

Description

Technical Field

[0001] The present invention relates to the technical field of stress corrosion detection of metal materials, and in particular to a pre-cracking method for a WOL specimen used for stress corrosion testing of welded joints. Background Art

[0002] Welding is the most common method of connecting metal materials and is widely used in ship structures. Metal welded joints are subject to disturbances from waves and currents as well as corrosion from Cl- when serving in marine environments. As structural weaknesses, they are subject to potential stress corrosion failure risks under the coupling of force and environment. Hull structural materials, especially welded joints, will inevitably develop defects during use. The constant displacement stress corrosion test is a test method established based on the principle that crack-like defects are sensitive to stress corrosion cracking. It can quantitatively measure the critical stress intensity factor K of stress corrosion cracking sensitivity. ISCC This value mainly provides necessary parameters for the design of hull structural parts and verifies whether the material is suitable for working in seawater environment. It has important guiding significance for the safe service of hull structural materials.

[0003] The modified wedge-open (WOL) specimen is commonly used in constant displacement loading stress corrosion testing. The test is conducted in accordance with GB / T 15970.6-2007, "Corrosion stress corrosion testing of metals and alloys—Part 6: Preparation and application of pre-cracked specimens under constant load or displacement." First, a pre-crack is created in the WOL specimen. Then, the v-value is preliminarily calculated based on the specimen and crack size. The specimen is then loaded based on the v-value. Finally, the specimen is immersed in a corrosive solution. After a period of time, the specimen is pulled apart to observe whether stress corrosion propagation occurs in the crack. Pre-crack preparation of the WOL specimen is the first step in the entire experiment and is crucial to its success.

[0004] Currently, laboratory pre-cracking involves creating a fatigue crack at the crack tip using the K-reduction (stress corrosion factor) method on a fatigue testing machine. The crack size is checked using a COD gauge, and pre-cracking is stopped when the specimen reaches the preset size. After pre-cracking, a small crack appears at the notch tip of the specimen. When crack propagation is deemed to have ceased, the specimen is broken and the maximum and minimum pre-crack lengths are measured, with a measurement error of no more than 0.5% W. The final stress corrosion crack length is also measured with the same accuracy at three locations on one side: 0.25B, 0.50B, and 0.75B. The average of these five measurements is used as the effective crack length for calculating KISCC. The pre-crack is considered invalid if: a) the difference between any two of these three measurements exceeds 2.5% W; b) the difference between the maximum and minimum crack lengths exceeds 5% W; or c) any portion of the fatigue crack plane deviates from the notch plane by more than 10°.

[0005] At present, the parent material WOL specimens are pre-cracked using the equipment-controlled K reduction method, with a success rate of nearly 100%. However, the same method is used for pre-crack of welded joint WOL specimens. However, due to the crack expansion in the middle and not on the sides, the COD gauge cannot accurately detect the crack of the specimen through the flexibility method. Ultimately, the success rate of pre-crack of welded joints is less than 10%, resulting in the K reduction of welded joint specimens. ISCC The determination of the stress corrosion K value has reached a bottleneck. However, a large number of welded joints are used in marine metal materials, especially titanium alloy equipment, and it is urgent to determine the stress corrosion K value of the joints. ISCC In particular, pre-crack is the most critical step in the test, and the success rate of pre-crack in welding WOL samples is extremely low, which seriously affects the K ISCC The determination of the value of the sample crack value makes it impossible to provide the necessary parameters for the design of the hull structure and verify whether the material is suitable for working in the seawater environment. Therefore, it is necessary to study how to improve the accuracy of sample crack detection, thereby increasing the success rate of pre-crack detection of welded joints and ensuring the realization of the sample stress corrosion K ISCC The determination of the value is one of the problems that need to be solved urgently.

[0006] Patent CN118969147A discloses a method for analyzing the sensitivity of fracture toughness prediction model parameters, including the following steps: S1. Obtaining fracture toughness data from a specimen through a fracture toughness test and using this data to train a fracture toughness prediction model; S2. Sampling the fracture toughness data to obtain sampled data; S3. Inputting the sampled data into the trained fracture toughness prediction model to predict the corresponding fracture toughness and storing it in an analysis data set; S4. Using the Sobol global sensitivity analysis method, calculating the sensitivity index of each physical parameter of the specimen in the analysis data set to the fracture toughness, and sorting each physical parameter in descending order based on the sensitivity index. This method can accurately analyze the degree to which each physical parameter affects fracture toughness based on the corresponding sensitivity index. However, since this analysis method requires the establishment of a prediction model and the required index data that can only be measured after the model is trained, it can easily lead to complex crack measurement methods and high costs. Summary of the Invention

[0007] In view of this, the present invention aims to propose a pre-crack method for WOL specimens used for stress corrosion testing of welded joints, so as to solve the problem in the prior art that due to the uneven structure of the welded joint, the welded joint expands in the middle but not on the sides during pre-crack, resulting in specimen failure and inability to calculate K ISCCThe problem of K value is solved; in this way, the success rate of pre-crack of welding specimens can be greatly improved, and the pre-crack of most welding joint WOL specimens can be successfully completed; it can also effectively solve the problem that due to the uneven structure of the welding joint, the welding joint expands in the middle but not on both sides during pre-crack, resulting in specimen failure and the inability to calculate K ISCC The problem of K value; so that the hull structure welded joints can also be ISCC Value determination.

[0008] To achieve the above object, the technical solution of the present invention is achieved as follows:

[0009] The present invention relates to a pre-cracking method for a WOL specimen for stress corrosion testing of a welded joint, the method comprising the following steps:

[0010] Step 1: Preliminary analysis: Observe the fracture morphology of the welded WOL specimen where the pre-crack failed, and preliminarily analyze the cause of the fracture morphology;

[0011] Step 2: Verification and Analysis: Verify the analyzed causes one by one through n independent repeated tests to determine whether the cause judgment is accurate; if yes, proceed to step 3; if no, repeat step 2;

[0012] Step 3: Obtain the influencing factor K: According to the determined causes of the fracture morphology, find out the factor K that affects the crack qualification rate;

[0013] Step 4: Manual intervention: When setting the pre-crack parameters, perform manual intervention parameter settings;

[0014] Step 5: Test the sample after intervention to determine whether the crack is qualified; if it is qualified, obtain the pre-crack, conduct constant displacement stress corrosion test on the required weld joint WOL sample, and measure the K with high accuracy. ISCC value; otherwise, return to step 1.

[0015] Furthermore, step one includes:

[0016] Step S11: Preliminary analysis: Select titanium alloy samples of the same material and observe the fracture morphologies of qualified welding base material WOL samples and unqualified welding base material WOL samples after pre-crack;

[0017] Step S12: comparing the difference in fracture morphology between the failed and successful pre-cracked WOL specimens of the welded base material to obtain the difference in surface morphology of the failed pre-cracked WOL specimens;

[0018] Step S13: Preliminary analysis of the causes of fracture morphology: After preliminary and in-depth analysis of the fracture morphology of the failed and successful pre-cracked WOL specimens of the welding base material, the causes of the fracture morphology are obtained.

[0019] Furthermore, step three specifically includes: obtaining the influencing factor K: according to the determined cause of the fracture morphology and the characteristic of the metal sample that the middle expands first and the two sides expand later, the factor K that affects the crack qualification rate is obtained.

[0020] Furthermore, step four includes:

[0021] Step S41: Manual intervention: When pre-setting the pre-crack parameters, the manual intervention parameter factor K is set. Within the preset time t1, the K value is increased according to the first formula so that the crack propagation rate on the surface of the welded WOL specimen reaches a crack propagation rate of a0%-b0% per t0s, where t0, a0, and b0 are all positive numbers;

[0022] Step S42: Prefabricate cracks by reducing the K value, reducing the K value according to the second formula;

[0023] Step S43: Monitor the crack propagation range in real time and determine whether the crack propagation at the surface of the welded WOL specimen has reached d%-e%. If so, terminate the crack propagation prematurely. Although the crack at the prefabrication site is shorter than the crack in the parent material WOL specimen, it meets the fatigue pre-crack requirements. If not, repeat step S43. Where d and e are both positive numbers.

[0024] Further, in step S41, the first formula is: K=cK0, c is a positive number, and c is the K value increase rate, and K0 is the K value number generally set in the initial stage.

[0025] Furthermore, in step S42, the second formula is: K=fK0, where f is a negative number and f is the rate at which the K value decreases, and K0 is the K value conventionally set in the initial stage.

[0026] Furthermore, the value of t0 is 10.

[0027] Furthermore, the value of a0 is 2, and the value of b0 is 3.

[0028] Furthermore, the value of d is 50.

[0029] Furthermore, the value of e is 60.

[0030] Compared with the prior art, the pre-cracking method for WOL specimens for stress corrosion testing of welded joints described in the present invention has the following beneficial effects:

[0031] By setting up the method, it is possible to pre-crack by combining fatigue equipment with manual monitoring, thereby ensuring that the success rate of pre-crack of welding specimens is greatly improved, and that the pre-crack of most welded joint WOL specimens is successful; and the success rate of pre-crack of welding WOL specimens is currently close to 100%. Specifically, the original equipment K reduction method for pre-crack is improved, and manual intervention is added to greatly improve the success rate of pre-crack of welding specimens, breaking through the limitations of the original pre-crack method, and providing a K reduction method for welding specimens. ISCC In addition, it can also effectively solve the problem that due to the uneven structure of the welded joint, the welded joint expands in the middle but not on both sides during the pre-crack, resulting in sample failure and the inability to calculate K. ISCC The problem of K value; so that the hull structure welded joints can also be ISCC Value determination. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] The accompanying drawings, which constitute part of the present invention, are provided to provide a further understanding of the present invention. The exemplary embodiments of the present invention and their descriptions are used to explain the present invention and do not constitute an improper limitation of the present invention. In the accompanying drawings:

[0033] Figure 1a Schematic diagram of the fracture morphology of the qualified pre-crack in the WOL specimen of the base material;

[0034] Figure 1b Schematic diagram of the fracture morphology of unqualified pre-crack in the WOL specimen of the welded joint;

[0035] Figure 2a This is a schematic diagram of the first-view structure of a 20mm thick constant displacement stress corrosion WOL specimen;

[0036] Figure 2b Schematic diagram of the structure of a 20mm thick constant displacement stress corrosion WOL specimen from the second perspective;

[0037] Figure 3a Schematic diagram of the fracture morphology of the sample after the test of the 1-5 groups of welded WOL samples in Example 2;

[0038] Figure 3b Schematic diagram of the fracture morphology of the sample after the test of the 6-9 groups of welded WOL samples in Example 2;

[0039] Figure 3c Schematic diagram of the fracture morphology of the sample after the test of the 10-14 groups of welded WOL samples in Example 2;

[0040] Figure 3d Schematic diagram of the fracture morphology of the sample after the test of the 15-18 groups of welded WOL samples in Example 2;

[0041] Figure 4Schematic diagram of the flow chart of the pre-cracking method. DETAILED DESCRIPTION

[0042] The inventive concepts of the present disclosure will be described below using terms commonly used by those skilled in the art to convey the essence of their work to other persons skilled in the art. However, these inventive concepts can be embodied in many different forms and should not be considered limited to the embodiments described herein.

[0043] It should be noted that, in the absence of conflict, the embodiments of the present invention and the features in the embodiments may be combined with each other.

[0044] The present invention will be described in detail below with reference to the accompanying drawings and in conjunction with embodiments.

[0045] In the existing technology, materials such as titanium alloys and high-strength steel used in marine engineering have been in service in marine environments for a long time, and there is a hidden danger of stress corrosion failure under the coupling of force and environment. Therefore, some metal materials used in marine engineering need to undergo a large number of stress corrosion tests at the beginning of their design to measure the critical stress intensity factor K of their stress corrosion cracking sensitivity. ISCC , one is to verify whether the material is suitable for service in seawater environment, and the other is to provide design parameters for subsequent component construction. However, due to the uneven structure of the welded joint, the welded joint expands in the middle but not on the sides when the crack is prefabricated, resulting in specimen failure and the inability to calculate K ISCC value.

[0046] In order to solve the problem in the prior art that the welded joint is uneven in structure, the welded joint is in a state of expansion in the middle and not on both sides when the crack is prefabricated, which leads to the failure of the specimen and the inability to calculate K ISCC The present embodiment provides a method for pre-cracking a WOL specimen for stress corrosion testing of a welded joint, the method comprising the following steps:

[0047] Step 1: Preliminary analysis: Observe the fracture morphology of the welded WOL specimen where the pre-crack failed, and preliminarily analyze the cause of the fracture morphology;

[0048] Step 2: Verification and Analysis: Verify the analyzed causes one by one through n independent repeated tests to determine whether the cause judgment is accurate; if yes, proceed to step 3; if no, repeat step 2;

[0049] Step 3: Obtain the influencing factor K: According to the determined causes of the fracture morphology, find out the factor K that affects the crack qualification rate;

[0050] Step 4: Manual intervention: When setting the pre-crack parameters, perform manual intervention parameter settings;

[0051] Step 5: Test the sample after intervention to determine whether the crack is qualified; if it is qualified, obtain the pre-crack, conduct constant displacement stress corrosion test on the required weld joint WOL sample, and measure the K with high accuracy. ISCC If no, return to step 1.

[0052] In this embodiment, the method described in this application can provide a pre-cracking method for welded joint WOL specimens tested according to the GB / T 15970.6-2007 standard.

[0053] By combining fatigue equipment with manual monitoring to pre-crack, the success rate of pre-crack of welding specimens is greatly improved, and the pre-crack success rate of most welded joints WOL specimens is achieved; and the success rate of pre-crack of welding WOL specimens is close to 100%. Specifically, the original equipment K reduction method for pre-crack is improved, and manual intervention is added to greatly improve the success rate of pre-crack of welding specimens, breaking through the limitations of the original pre-crack method, and providing a K reduction method for welding specimens. ISCC In addition, it can also effectively solve the problem that due to the uneven structure of the welded joint, the welded joint expands in the middle but not on both sides during the pre-crack, resulting in sample failure and the inability to calculate K. ISCC The problem of K value; so that the hull structure welded joints can also be ISCC The determination of the value has important guiding significance for the design of components of marine engineering materials.

[0054] Step one includes:

[0055] Step S11: Preliminary analysis: Select titanium alloy samples of the same material and observe the fracture morphology of qualified welding base material WOL samples and unqualified welding base material WOL samples after pre-crack; Figure 1a 、 Figure 1b Shown in the yellow area.

[0056] Step S12: Compare the difference in fracture morphology between the failed and successful pre-cracked WOL specimens of the welded base material to obtain the difference in surface morphology of the failed pre-cracked WOL specimens; specifically, Figure 1a As shown in Figure 2, the crack straightness of the WOL specimen of the parent material after pre-cracked by the fatigue equipment is good, which meets the validity judgment conditions of the crack in the standard GB / T 15970.6-2007; Figure 1b As shown in the figure, the pre-crack fracture of the welding specimen has almost no expansion on both sides of the crack, and only the middle of the crack expands. The crack straightness is not enough to meet the validity judgment conditions.

[0057] Step S13: Preliminary analysis of the causes of fracture morphology: After preliminary in-depth analysis of the fracture morphology of the failed and successful pre-cracked WOL specimens of the welding base material, the causes of the fracture morphology are obtained. Figure 1a 、 1b After an in-depth analysis of the fracture morphologies of the failed and successful pre-cracked WOL specimens of the welded base material, it was found that the metallographic structures of the two were extremely different. The structure of the base material WOL specimen was uniform, while the structure of the welded WOL specimen was uneven, and the precipitated phase and grain size were also relatively disordered. Therefore, when the welded WOL specimen was pre-cracked on the fatigue machine, the stress at different parts was very different, and uniformity could not be guaranteed.

[0058] By setting up each step in step one, it is easy to preliminarily determine the cause of the failure of pre-cracks, and then to find the specific cause for the cause. It is also beneficial to simplify the simplicity of the method of pre-cracks, reduce the cost of pre-cracks that meet the requirements, improve the efficiency of pre-cracks that meet the requirements, and enhance the flexibility and reliability of the operation of pre-cracks that meet the requirements.

[0059] Step three specifically includes: obtaining the influencing factor K: according to the causes of the determined fracture morphology, combined with the characteristics of the metal sample that the middle first expands and the two sides expand later, the factor K that affects the crack qualification rate is obtained; wherein, the increase of the K value can promote the expansion of cracks at the surface. Specifically, due to the uneven metallographic structure of the welding sample, the sample adopts the normal pre-crack parameters, resulting in unqualified cracks that expand in the middle but do not expand on both sides, so the first thing to solve is how to promote the expansion of cracks at the surface. In fact, when the welding sample adopts the same pre-crack method, the core often expands too long and the two sides do not expand. From this, it can be judged that the load required for the expansion of the two sides of the sample is much greater than that of the core. Therefore, when the K reduction method is used to pre-crack, the value of the cracking load that has not been reached on both sides is further reduced, so that the cracks on both sides cannot expand. Therefore, it is not difficult to see that the change of the factor K value can affect the pre-crack qualification rate.

[0060] Step 4 includes:

[0061] Step S41: Manual intervention: When pre-setting the pre-crack parameters, the manual intervention parameter factor K is set. Within the preset time t1, the K value is increased according to the first formula to promote the expansion of the crack on the surface of the welded WOL specimen; the speed of crack expansion on the surface of the welded WOL specimen reaches a crack expansion of a0%-b0% per t0s, where t0, a0, and b0 are all positive numbers; the values of t0, a0, and b0 are set accordingly according to the specific conditions of the different required specimens.

[0062] Step S42: Prefabricate cracks by reducing the K value, reducing the K value according to the second formula to prevent the crack at the core from extending too long;

[0063] Step S43: Monitor the crack propagation range in real time and determine whether the crack propagation at the surface of the welded WOL specimen has reached d%-e%. If so, terminate the crack propagation prematurely. Although the crack at the prefabrication site is shorter than the crack in the parent material WOL specimen, it meets the fatigue pre-crack requirements. If not, repeat step S43. Both d and e are positive numbers, and their specific values can be set as needed.

[0064] In step S41, the first formula is: K = cK0, where c is a positive number and represents the rate of increase of the K value, and K0 is the K value conventionally set in the initial stage. In step S42, the second formula is: K = fK0, where f is a negative number and represents the rate of decrease of the K value, and K0 is the K value conventionally set in the initial stage. Furthermore, in this embodiment, the value of t0 is 10; the value of a0 is 2; and the value of b0 is 3. That is, the setting of the K value in the method must meet the requirement that the crack length expands by 2%-3% every 10 seconds in the initial stage, and the specific parameters vary depending on the specimen. In step S43, the value of d is 50, and the value of e is 60.

[0065] Different from the existing phenomenon that the two sides of the weld joint specimen do not expand but only the middle, which easily causes the problem of unqualified pre-cracks, the present application increases the K value through manual intervention through the setting of step S41, thereby promoting the expansion of cracks close to the surface of the specimen and away from the core of the specimen. Step S42 is used to prevent the crack in the core of the specimen from expanding too long. When the crack expansion reaches the required degree, it is stopped in advance through manual intervention, thereby realizing manual intervention adjustment of the pre-crack, improving the qualified rate of the pre-crack, and enhancing the uniformity of the pre-crack on the specimen.

[0066] Specifically, by increasing the K value, the core crack and the cracks on both sides of the specimen can be propagated together at the beginning of pre-cracking. When pre-cracking a WOL specimen of the parent material, the crack length at the initial stage is 1% per 10 seconds; this data is detected by a COD gauge and can be fed back on the fatigue testing machine's display. However, the same rate is not suitable for welded WOL specimens. The crack length at the initial stage of welded WOL specimens must expand by 2%-3% per 10 seconds before the cracks on both sides can expand. Therefore, increasing the K value within a preset time can promote the expansion of the cracks on both sides. In addition, the pre-cracking method uses a decreasing K value. During the pre-cracking process, the K value gradually decreases, and the load on the specimen also gradually decreases. Based on the cracks of unqualified specimens, it can be found that the core crack requires a smaller load to expand. As the K value decreases, the core crack continues to expand, while the cracks on the sides gradually stop expanding. Therefore, in order to make the crack qualified, it is proposed to artificially intervene in the crack expansion and terminate the expansion in advance when the crack expands to 50%-60%. At this time, although the prefabricated crack is slightly shorter than the crack of the parent material sample, it meets the requirements of fatigue pre-crack.

[0067] By setting up each step in step four, the pre-cracks of the WOL specimens of the welded structure can be effectively controlled. By manually intervening in the K value, the pre-cracks can be expanded on demand, and the success rate of the pre-cracks of the WOL specimens of the welded joints can be greatly improved, providing important technical support for the constant displacement stress corrosion test of the WOL specimens of metal materials.

[0068] Example 1

[0069] First, a pre-cracking method for WOL specimens used for stress corrosion testing of welded joints is used. After the pre-cracking of the welded joint WOL specimens is qualified, constant displacement stress corrosion testing can be performed on WOL specimens of welded joints of different metal materials. The following test of a WOL specimen of a material is used as an example.

[0070] Step 1: Select TC4 as the test material;

[0071] Step 2: Select a 3.5% NaCl solution as the corrosion solution;

[0072] Step 3: Set the ambient temperature to 23-25°C and the relative humidity to 40%-50%;

[0073] Step 4: Select the WOL sample type. The specific dimensions of the WOL sample are shown in Figure 2.

[0074] Step 5: Determine the test item as a constant displacement stress corrosion test on a 20 mm thick welded WOL specimen;

[0075] Step 6: The method for conducting the test comprises the following steps:

[0076] ① Fatigue pre-crack: On an electro-hydraulic servo fatigue testing machine, a fatigue crack was preformed on a 20 mm thick welded WOL specimen, K = 34.0 MPa·m 1 / 2 , the stress ratio R is set to 0.1, and the loading frequency f is set to 8 Hz.

[0077] ② Calculate the WOL specimen loading value: Calculate the preload value based on the measured specimen size and pre-crack length.

[0078] ③WOL specimen loading: Set the preload value in the data acquisition system, place the specimen in the vise and adjust it to the appropriate position to clamp it, place the COD gauge on the top edge of the specimen, and then click the "Start" button of the torque loading device to load. When the data acquisition system captures the COD gauge opening value that is the same as the preset value, the test automatically stops.

[0079] ④WOL specimen immersion: The loaded specimen was placed in 3.5% NaCl, with the solution covering the pre-cracked area of the specimen, and immersed for 75 days.

[0080] ⑤Calculate K ISCC Value: After the sample has been soaked for a sufficient time, the sample is pulled apart on the electro-hydraulic servo fatigue testing machine and the maximum value of the fatigue pre-crack a is measured using a tool microscope. max and the minimum value a min , and measure the fatigue pre-crack length at 0.25B, 0.50B, and 0.75B on both sides and one side, and take the average value of the five position measurements as the calculation K Ⅰ The effective crack length a of the value, this test uses the initial method to calculate K ISCC value.

[0081] K Ⅰ The calculation formula is:

[0082]

[0083] In the formula: Y=30.96(a / w)-195.8(a / w) 2 +730.6(a / w) 3 -1186.3(a / w) 4 +754.6(a / w) 5 .

[0084] Step 7: According to the judgment conditions, test the samples of different brands after the test and determine whether they are expanded.

[0085] The test is considered invalid if:

[0086] Condition 1: The difference between any two of the last three crack length measurements exceeds 2.5%W;

[0087] Condition 2: the difference between the maximum and minimum crack lengths is greater than 5%W;

[0088] Condition 3: The plane where any part of the fatigue crack surface is located deviates from the notch plane by more than 10°;

[0089] Condition 4 is factor 2.5 (K Ⅰ / Rp 0.2 ) 2 Greater than the thickness of the specimen and / or the crack length.

[0090] According to the above test, the results of the test on 8 samples of different brands are shown in Table 1 below:

[0091] Table 1

[0092]

[0093] After the samples were pulled apart, the cracks in the samples were verified. All samples did not meet the validity judgment conditions of conditions 1, 2, and 3. As can be seen from Table 1, stress corrosion expansion occurred in samples 1-2, but not in samples 3-8. Sample 1-2 did not meet the plane strain condition (factor 2.5 (K Ⅰ / R p0.2 ) 2 Greater than the thickness of the specimen and / or crack length), so according to the test conditions and judgment conditions, the K ISCC Test results of value conditions:

[0094] This group of samples K ISCC =61.80MPa·m 1 / 2 .

[0095] Example 2

[0096] Since this method is designed to provide a pre-crack method for WOL specimens of welded joints and needs to be universal, 18 different groups of welded WOL specimens in the test process were used as samples to perform fatigue pre-cracks. The fracture morphology and crack size of the specimens after pre-cracks were as follows: Figures 3a-3d As shown in Table 2.

[0097] Table 2 Pre-crack lengths of different groups of welded WOL specimens

[0098]

[0099]

[0100] from Figures 3a-3d It can be seen that after the new method of pre-cracking is adopted, the length of the crack on the fracture of the welded WOL specimen is relatively straight, and the cracks on both sides are almost consistent with the crack in the middle. From the crack measurement results in Table 2, it can also be confirmed that after the new method of pre-cracking is adopted, the welded joint WOL specimen can be pre-cracked. Therefore, it is further proved that the pre-crack method of the present application has the characteristics of a wide range of applications, and the qualified pre-crack method of the welded joint WOL specimen is achieved through equipment control and manual intervention, which can ensure the qualified rate of the pre-cracks of different welded joint WOL specimens and improve the success rate of the pre-cracks of the welded joint WOL specimens; thereby, the K of the welded joint WOL specimen can be achieved. ISCC The accuracy of the measurement value.

[0101] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A pre-cracking method for WOL specimens used for stress corrosion testing of welded joints, characterized in that: The method comprises the following steps: Step 1: Preliminary analysis: Observe the fracture morphology of the welded WOL specimen where the pre-crack failed, and preliminarily analyze the cause of the fracture morphology; Step 2: Verification and Analysis: Verify the analyzed causes one by one through n independent repeated tests to determine whether the cause judgment is accurate; if yes, proceed to step 3; if no, repeat step 2; Step 3: Obtain the influencing factor K: According to the determined causes of the fracture morphology, find out the factor K that affects the crack qualification rate; Step 4: Manual intervention: When setting the pre-crack parameters, perform manual intervention parameter settings; Step 5: Test the sample after intervention to determine whether the crack is qualified; if it is qualified, obtain the pre-crack, conduct constant displacement stress corrosion test on the required weld joint WOL sample, and measure the K with high accuracy. ISCC value; otherwise, return to step 1.

2. The pre-cracking method for a WOL specimen for stress corrosion testing of a welded joint according to claim 1, characterized in that: The step one comprises: Step S11: Preliminary analysis: Select titanium alloy samples of the same material and observe the fracture morphologies of qualified welding base material WOL samples and unqualified welding base material WOL samples after pre-crack; Step S12: comparing the difference in fracture morphology between the failed and successful pre-cracked WOL specimens of the welded base material to obtain the difference in surface morphology of the failed pre-cracked WOL specimens; Step S13: Preliminary analysis of the causes of fracture morphology: After preliminary and in-depth analysis of the fracture morphology of the failed and successful pre-cracked WOL specimens of the welding base material, the causes of the fracture morphology are obtained.

3. The pre-cracking method for WOL specimens for stress corrosion testing of welded joints according to claim 1, characterized in that: The step three specifically includes: obtaining the influencing factor K: according to the determined cause of the fracture morphology and the characteristic of the metal sample that the middle expands first and the two sides expand later, obtaining the factor K that affects the crack qualification rate.

4. The pre-cracking method for WOL specimens for stress corrosion testing of welded joints according to claim 1, characterized in that: The fourth step includes: Step S41: Manual intervention: When pre-setting the pre-crack parameters, the manual intervention parameter factor K is set. Within the preset time t1, the K value is increased according to the first formula so that the crack propagation rate on the surface of the welded WOL specimen reaches a crack propagation rate of a0%-b0% per t0s, where t0, a0, and b0 are all positive numbers; Step S42: Prefabricate cracks by reducing the K value, reducing the K value according to the second formula; Step S43: Monitor the crack propagation range in real time and determine whether the crack propagation at the surface of the welded WOL specimen reaches d%-e%. If yes, terminate the propagation in advance. Although the crack at the prefabrication is shorter than the crack at the parent material WOL specimen, it meets the fatigue pre-crack requirement. If no, repeat step S43. Wherein, d and e are both positive numbers.

5. The pre-cracking method for WOL specimens for stress corrosion testing of welded joints according to claim 4, characterized in that: In step S41, the first formula is: K=cK0, where c is a positive number and c is the rate of increase of the K value, and K0 is the K value conventionally set in the initial stage.

6. The pre-cracking method for WOL specimens for stress corrosion testing of welded joints according to claim 4, characterized in that: In step S42, the second formula is: K=fK0, where f is a negative number and f is the rate at which the K value decreases, and K0 is the K value conventionally set in the initial stage.

7. The pre-cracking method for WOL specimens for stress corrosion testing of welded joints according to claim 4, characterized in that: The value of t0 is 10.

8. The pre-cracking method for WOL specimens for stress corrosion testing of welded joints according to claim 4, characterized in that: The value of a0 is 2, and the value of b0 is 3.

9. The pre-cracking method for WOL specimens for stress corrosion testing of welded joints according to claim 4, characterized in that: The value of d is 50.

10. The pre-cracking method for WOL specimens for stress corrosion testing of welded joints according to claim 4, characterized in that: The value of e is 60.

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