Method for acquiring fatigue curve of high-strength steel butt welding joint and application based on method

Abstract

The invention discloses a method for acquiring a fatigue curve of a high-strength steel butt welding joint and application based on the method. According to the curve obtaining method, fitting data are obtained for a butt joint of a fatigue strength mechanism represented by local microcell cyclic plastic strain accumulated damage controlled by base metal yield strength through welding optimization according to the following processing steps that 1, a sample is prepared; 2, setting the number of test cycles; 3, setting the highest stress value of the test and a stress step between adjacent stress levels in sequence by taking the set test cycle number as a limit; 4, determining the number of stress levels, determining the number of effective data under each stress level and setting an effective data screening mechanism under each stress level by taking data uniformity as a guiding principle; and 5, carrying out a series of stress level full-high-cycle fatigue life cycle fatigue test based on a fatigue testing machine, and completing data acquisition based on a test result. The application mainly establishes a lightweight scheme for thinning the wall thickness of the selected material.

Description

Technical Field

[0001] The present invention belongs to the technical field of welding, and particularly relates to a method for obtaining a fatigue curve of a high-strength steel butt welding joint and an application based thereon. Background Art

[0002] In the fatigue S-N curve of the welding joint in the original fatigue design standard for dynamic load structural members, the basic guiding cognition behind its determination is that the fatigue strength grade of the steel material welding joint is not related to the material strength level, and is only related to the joint form design, welding quality grade, and post-weld treatment state. For example: the mainstream EN1993 standard, IIW-2259-15 standard, and DNV-RP-C203 standard are all based on this basic guiding cognition. According to different design forms and quality grades of steel materials, combined with a large amount of test data under the material development level at that time, a set of S-N curves that can be used for fatigue design are given. Specifically: according to the fatigue design requirements and habits of dynamic load structural members, the stress range value corresponding to the cyclic number of 2*10 6 is extracted from each S-N curve and defined as the fatigue strength characteristic value. For example: in the EN1993 standard, for the butt weld joint of double-sided welded full penetration retained welds, the S-N curve corresponds to DC90, and the fatigue strength characteristic value is 90 MPa. Other standards have almost the same definition.

[0003] However, with the continuous progress of metallurgical manufacturing level and welding technology level in recent years, the quality of steel materials themselves and welding manufacturing quality are both continuously improving. More and more research and test results show that: the S-N curves given in the above several mainstream standards are becoming more and more conservative. If still following this basic guiding cognition behind, not only will the conservatism increase significantly, but it also means a significant increase in manufacturing costs.

[0004] Therefore, it is necessary to provide a method for obtaining a fatigue curve of a high-strength steel butt welding joint and an application based thereon, so as to accurately obtain the S-N curve of the high-strength steel butt welding joint and use it for the fatigue design of dynamic load structural members through certain rules, creating conditions for the popularization and application of high-strength steel and the progress of related industrial fields.

[0005] The invention application with the application number: CN2018108462064 discloses "a full-scale fatigue test method for a welding joint of a deepwater steel catenary riser", which includes: the first step, specimen preparation; the second step, test preparation; the third step, specimen loading; the fourth step, starting the fatigue test; the fifth step, fatigue test acceptance standard; the sixth step, specimen fracture analysis test. Summary of the Invention

[0006] To break through the existing cognitive shackles and achieve the above technical objectives, the present invention provides a method for obtaining the fatigue curve of a high-strength steel butt welded joint and an application based thereon. The technical solution is as follows:

[0007] A method for obtaining the fatigue curve of a high-strength steel butt welded joint

[0008] For a high-strength steel butt welded joint with a fatigue strength mechanism characterized by the cumulative damage of local microzone cyclic plastic strain controlled by the yield strength of the base metal through welding optimization

[0009] The acquisition of data for fitting is completed through the following steps, and then the fatigue curve is obtained by fitting the acquired data:

[0010] S1: Prepare the specimens for fatigue tests;

[0011] S2: Set the number of cycles of the fatigue test according to the characteristics of high-cycle fatigue;

[0012] S3: Determine the maximum stress value of the fatigue test and set the stress step between adjacent stress levels in sequence within the limit of the set number of cycles of the fatigue test;

[0013] S4: Determine the number of stress levels, the number of valid data at each stress level, and set the valid data screening mechanism at each stress level respectively under the guiding principle of data uniformity;

[0014] S5: Conduct a series of full high-cycle fatigue life cycle fatigue test measurements at different stress levels based on a fatigue testing machine, and complete the acquisition of data based on the test results in combination with the above settings.

[0015] Furthermore,

[0016] Step S1 specifically includes the following steps:

[0017] S11: Weld to obtain a butt joint specimen for testing according to the welding process that can achieve the fatigue strength mechanism characterized by the cumulative damage of local microzone cyclic plastic strain controlled by the yield strength of the base metal after welding optimization;

[0018] S12: Conduct non-destructive flaw detection on the obtained butt joint specimen for testing after welding with a time delay. If the detection passes, proceed to step S13; otherwise, return to step S11 to re-prepare the butt joint specimen for testing and enter step S12 again for non-destructive flaw detection with a time delay after welding until the detection passes;

[0019] S13: Machine a fatigue specimen with gripping sections adapted to the testing machine fixtures at both ends, a parallel test section loaded with the load of the testing machine in the middle, and a transition section connecting the parallel test section and the gripping sections at both ends.

[0020] Furthermore,

[0021] The butt joint specimen for testing obtained by welding in step S11 is subjected to double-sided submerged arc welding with a root cleaning process.

[0022] Furthermore,

[0023] The non-destructive flaw detection with post-weld time delay in step S12 is specifically as follows: The first non-destructive flaw detection is completed at least 24 hours after welding, and the second non-destructive flaw detection is completed at least 72 hours after welding. The final non-destructive flaw detection result is characterized based on the results of the two non-destructive flaw detections.

[0024] Furthermore,

[0025] The machining in step S13 is specifically wire electrical discharge machining;

[0026] The cutting is carried out to meet the requirement that the surface roughness of the machined cross-section is less than or equal to 3.2, and the acceptance criterion is that no grooves can be observed under a 10-fold magnifying glass.

[0027] Furthermore, before the cutting operation, slag inclusion inspection and cleaning operations are carried out on the weld toe part; the cleaning operation is carried out within the limit of not causing mechanical damage to the weld toe part.

[0028] Furthermore, for the grooves at any cross-section position perpendicular to the weld toe that appear from the start of cutting to the completion of cutting, groove removal operations of grinding + polishing are carried out.

[0029] Furthermore, the fatigue specimens formed by machining are carried out in a way that retains the weld reinforcements on both the front and back sides,

[0030] Constrained by the effectiveness of the fracture position, the stability of the test process and results, and the size of the testing machine fixture, the dimensions of the clamping section, the parallel test section, and the transition section are determined;

[0031] Specifically: Constrained by the effectiveness of the fracture position, the width of the parallel test section, the width of the clamping section, and the transition radius of the transition section are set; Constrained by the stability of the test process and results, the length of the parallel test section is set; Constrained by the size of the testing machine fixture, the length of the clamping section adapted to the size of the testing machine fixture is set.

[0032] Furthermore, the width of the parallel test section is set according to the thickness of the plate,

[0033] The width of the clamping section is set to be greater than or equal to 1.75 times the width of the parallel test section;

[0034] The transition radius of the transition section is set to be greater than 100 mm;

[0035] The length of the parallel test section is defined according to the following range:

[0036]

[0037] wherein,

[0038] l: the length of the parallel test section, unit: mm;

[0039] b: the width of the parallel test section, unit: mm.

[0040] Furthermore, the deformation angle of the fatigue specimen obtained by processing in step S13 is less than 5°.

[0041] Furthermore, the number of cycles of the fatigue test in step S2 is set to 1×10 5 -1×10 7 times, and accordingly, the maximum stress value of the fatigue test in step S3 is set to fall within the interval [35% R p , 70% R p , and the stress step between adjacent stress levels is set to fall within the interval [1.5% R p , 7.5% R p or fall within the interval [10 MPa, 20 MPa],

[0042] wherein R p : the actual yield strength of the high-strength steel base material, unit: MPa.

[0043] Furthermore, in step S4, the determination of the number of stress levels is completed under the guiding principle of data uniformity, and it is completed with at least 6 stress levels.

[0044] Furthermore, in step S4, the determination of the number of valid data at each stress level is completed under the guiding principle of data uniformity, and it is completed with at least 5 valid data.

[0045] Furthermore, in step S4, the setting of the valid data screening mechanism at each stress level is completed under the guiding principle of data uniformity, specifically:

[0046] First, with the number of cycles set in step S2 and the number of valid data at each stress level in step S4 as common constraints, the determination of the data set that meets the common constraints is completed,

[0047] Second, the median value in the data set is determined,

[0048] Again, according to the determined median value, establish the determination and screening for the other data except the median value based on whether the ratio of any value in the dataset to the median value falls within the interval [0.31, 3.2]. Retain the data that falls within this interval and eliminate the data that does not fall within this interval.

[0049] Then, with the number of data after determination and screening still meeting the number of valid data under each stress level in step S4 as the limit, form the dataset under this stress level for participating in data fitting.

[0050] Further, when the number of valid data under each stress level is determined to be 5, if the ratio of any value in the dataset to the median value all falls within the interval [0.5, 2.0], then select three median values to participate in curve fitting, or select 5 valid data to participate in curve fitting; otherwise, complete the data selection according to the scheme where all 5 data participate in fitting.

[0051] Further, when obtaining the fatigue curve by fitting based on the acquired data, convert the highest stress value of the fatigue test into the fatigue cyclic stress range according to the relationship of Δσ = 0.9σ max ;

[0052] In the above,

[0053] Δσ: Fatigue test stress range, unit: MPa;

[0054] σ max : Highest stress value of the fatigue test, unit: MPa.

[0055] An application of a method for obtaining the fatigue curve of high-strength steel butt welded joints

[0056] First, determine the fatigue strength characteristic value of the high-strength steel welded joint according to the fitted curve. Then, combine the fatigue strength characteristic value of the full penetration butt joint under the existing structural member fatigue standard to determine the parameter setting of lightweight selection with high-strength steel welded structural members as the material; specifically:

[0057] Define the stress range value corresponding to the fatigue cycle number of 2×10 6 cycles on the fitted curve as the fatigue strength characteristic value of the high-strength steel welded joint, and determine the wall thickness reduction percentage of the high-strength steel welded structural member according to the following formula,

[0058]

[0059] In the above formula,

[0060] r: Wall thickness reduction percentage of the high-strength steel welded structural member;

[0061] FAT w:Characteristic value of fatigue strength of high-strength steel welded joint, unit: MPa;

[0062] FAT90: Characteristic value of fatigue strength of full penetration butt joint in current fatigue design standard of structural members, unit: MPa;

[0063] α: Safety factor.

[0064] Furthermore,

[0065] The determination of the safety factor α starts with an initial value of 0.8 and is determined based on real-time verification and correction. Specifically:

[0066] Set the safety factor α to 0.8 and establish a fatigue loading test based on specimens with large-size retained welding residual stress.

[0067] Set the test stop condition as the number of cycles reaching 2×10 6 times;

[0068] When fatigue cracks appear during the test, correct the safety factor and reload until no fatigue cracks appear during the test, thus completing the determination of the safety factor; otherwise, determine the safety factor as 0.8.

[0069] Furthermore,

[0070] The so-called "based on large size" specifically means: set the width of the specimen to five times the actual plate thickness, and the specimen uses a full-thickness retained weld specimen.

[0071] A method for obtaining the fatigue curve of a high-strength steel butt welded joint and its application based on this invention are for the application of high-strength steel butt joints with stable and reliable welding process and good welding quality in the manufacturing end of structural members in dynamic load structural members. Fatigue strength characteristic values significantly higher than the standard S-N curve are obtained, providing technical support for the application of high-strength steel in the field of dynamic load structural members and the lightweight reduction of structural materials. The specific beneficial effects are as follows:

[0072] (1) Based on the mechanism that the fatigue strength of full penetration high-strength steel butt joints can be conditionally improved as the yield strength of the material increases, this invention obtains the high-survival-rate S-N curve and fatigue strength characteristic values through a series of tests for the fatigue design of high-strength steel dynamic load welded structural members, breaking through the limitations of existing mainstream design standards. On the premise of ensuring fatigue service safety, it realizes the application of high-strength steel in fatigue service occasions and lightweight reduction, helps reduce the overall manufacturing cost of structural members, and leads to the improvement of the industry's manufacturing level and overall technological progress;

[0073] (2) Based on the existing fatigue test standards and combined with the unique characteristics of the fatigue behavior of high-strength steel welded joints, through a series of technical means of control, the present invention has obtained the result of significantly improved fatigue strength characteristic values, subverting the understanding of the fatigue performance of welded joints in the industry for many years. Compared with the existing standards and technologies, it has remarkable creativity;

[0074] (3) The present invention provides technical support for the application and promotion of high-strength steel in multiple industrial fields involving dynamic load service. The replacement of ordinary carbon steel with high-strength steel has the effect of green and low-carbon both in the manufacturing process and the service process in the long run, and it will make a lasting contribution to the industrial field in achieving energy conservation and carbon reduction. BRIEF DESCRIPTION OF THE DRAWINGS

[0075] Figure 1 It is a step sequence diagram for obtaining data for fitting in the present invention;

[0076] Figure 2 It is a schematic diagram of a rectangular fatigue specimen with the weld reinforcement retained in the present invention;

[0077] Figure 3 is Figure 2 top view;

[0078] Figure 4 It is a schematic diagram of the groove form and dimensions of double-sided submerged arc welding of Q500MD high-strength steel in the embodiment of the present invention;

[0079] Figure 5 It is a schematic diagram of the specific form of the fatigue specimen of the butt welded joint of Q500MD high-strength steel in the embodiment of the present invention;

[0080] Figure 6 It is a schematic diagram of the fatigue S-N curve of the butt joint series stress levels of Q500MD high-strength steel in the embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0081] Next, a method for obtaining the fatigue curve of a high-strength steel butt welded joint and its application based on this will be further specifically described according to the drawings in the specification and the specific embodiments.

[0082] To form a full understanding of the overall technical solution, the necessity and technical overview of the present technical solution are first described separately, and then the specific working principle and process of the present technical solution are described on this basis.

[0083] Necessary part of this technical solution

[0084] Curve fitting in reality is all based on numerical analysis. The reason for establishing protection for the method of obtaining the fatigue curve of high-strength steel butt welded joints in this case is not to protect a curve fitting method, but for high-strength steel butt welded joints with a fatigue strength mechanism that characterizes the cumulative damage of cyclic plastic strain in local micro-regions controlled by the yield strength of the base metal through welding optimization. The original S-N curve standard is no longer applicable. A new S-N curve standard needs to be formulated for this new type of welded joint. This technical solution aims to provide a method for determining this new standard curve.

[0085] Technical overview part

[0086] To complete the method for determining such a new standard curve mentioned above, the difficulty does not lie in which specific numerical analysis method to adopt, but in how the specimen preparation process does not cause additional impacts on the results, and at the same time how to obtain effective data for fitting and minimize the discreteness of the effective data to reflect the actual engineering situation.

[0087] This technical solution starts from the above two difficulties and finally completes the determination of this technical solution, so that the finally determined standard curve can be widely used as the standard for this type of welded joint.

[0088] Regarding the first difficulty, this technical solution establishes corresponding treatment measures from multiple aspects, including ensuring the handling of precautions for welding operations; using submerged arc welding with a root cleaning process to ensure the penetration quality at the center of the plate; establishing a detection process based on non-destructive testing to ensure the joint quality of subsequent fatigue specimens; determining the dimensions of the clamping section, parallel test section, and transition section with the effectiveness of the fracture position, the stability of the test process and results, and the dimensions of the testing machine fixture as constraints; reducing the influence of the machining accuracy of the cross-section of the specimen test section on the stability of the test results; and eliminating the angular deformation of the fatigue specimen so as not to generate additional bending stress, which may affect the stability of the results. Among them, determining the dimensions of the clamping section, parallel test section, and transition section with the effectiveness of the fracture position, the stability of the test process and results, and the dimensions of the testing machine fixture as constraints specifically means: setting the width of the parallel test section, the width of the clamping section, and the transition radius of the transition section with the effectiveness of the fracture position as the constraint; setting the length of the parallel test section with the stability of the test process and results as the constraint; and setting the length of the clamping section adapted to the dimensions of the testing machine fixture with the dimensions of the testing machine fixture as the constraint. The specific establishment process of the corresponding treatment measures in the above-mentioned multiple aspects can be seen in the following specific working principle and process section.

[0089] For the second difficulty, the present technical solution establishes the following data acquisition process, whose basic guiding ideology is: meeting the high-cycle fatigue characteristics + meeting the data uniformity. See Figure 1 .

[0090] First, set the number of cycles of the fatigue test according to the high-cycle fatigue characteristics;

[0091] Second, determine the maximum stress value of the fatigue test and set the stress step between adjacent stress levels in sequence within the set number of cycles of the fatigue test;

[0092] Third, guided by the data uniformity principle, determine the number of stress levels, the number of valid data at each stress level, and set the valid data screening mechanism at each stress level respectively;

[0093] Finally, conduct a series of full high-cycle fatigue life cycle fatigue tests on the stress levels based on the fatigue testing machine, and complete the acquisition of data based on the test results combined with the above settings. The specific processing process is detailed in the following specific working principle and process section.

[0094] Specific working principle and process part

[0095] A method for obtaining the fatigue curve of a high-strength steel butt welded joint and an application based thereon according to the present invention.

[0096] Obtain the actual S-N curve of the high-strength steel welded joint and extract the fatigue strength characteristic value through the designed specific test method, which is used as the design basis for the dynamic load service structural member. Based on the mechanism that the fatigue strength of a structurally complete full-penetration butt joint can be conditionally improved as the yield strength of the material increases, the specific technical principle of this mechanism is as follows:

[0097] The fatigue behavior of a structurally complete full-penetration high-strength steel butt joint is different from that of the homogeneous base metal. It is more affected by three major factors: stress concentration caused by the weld geometry, initial fatigue crack sources formed by macroscopic or microscopic defects on the weld toe surface or inside due to welding, and welding residual tensile stress, which exceed the yield strength of the base metal itself and become the most influential factors for the joint fatigue behavior. If through technical measures such as joint type design, welding process optimization, and weld forming quality control, the adverse effects of the above three factors on the fatigue performance of the welded joint are minimized or eliminated to the greatest extent, then the fatigue failure mechanism of the welded joint is close to the local microzone cyclic plastic strain cumulative damage fatigue failure mechanism controlled by the yield strength of the homogeneous base metal, so as to be able to exert the high fatigue performance potential of the high-strength steel base metal and achieve the conditional improvement of the fatigue performance of the high-strength steel welded joint.

[0098] The method for obtaining and applying the fatigue curve of the high-strength steel welded joint designed based on the above fatigue mechanism can obtain the fatigue strength characteristic value significantly higher than the standard S-N curve through the preparation of the high-strength steel butt joint, the design and production of the fatigue specimen form of the welded joint, the design and test of the series of stress level fatigue tests within the full high-cycle fatigue life cycle, the acquisition of the high survival rate S-N fatigue curve, the formulation of the application criterion for the fatigue strength characteristic value based on the actual S-N curve, and the safety verification of the fatigue strength characteristic value based on the actual S-N curve. It can be used for the fatigue design of high-strength steel dynamic load welded structural parts, thus helping to break through the limitations of the existing structural fatigue design standards and providing technical support for the application of high-strength steel in the field of dynamic load structural parts and the weight reduction of structural materials. The specific technical solutions are as follows:

[0099] Step 1: Preparation of the high-strength steel butt joint

[0100] According to the manufacturing conditions and requirements of the structural parts with good welding quality applied to dynamic load occasions, design the groove form suitable for double-sided submerged arc automatic welding, generally the double Y-shaped groove, with a root face of 2 - 6 mm reserved in the middle. The groove angle is differentially designed according to the plate thickness and the design requirements of specific structural parts. According to the manufacturing and welding needs of different industrial fields, it can be designed as a double-sided symmetric groove form or a double-sided asymmetric groove form. The welding groove can be processed by thermal cutting (flame cutting or plasma cutting) and mechanical cutting. If the thermal cutting processing method is adopted, the oxide scale on the cutting surface needs to be mechanically ground before welding. To ensure the welding quality, the two sides of the groove need to be mechanically ground before welding to remove rust, oil stains, etc. that have an adverse effect on the welding quality.

[0101] According to actual needs, the gas metal arc welding method is used for tack welding, and then single-wire or multi-wire submerged arc automatic welding can be adopted. No matter which submerged arc welding method is adopted, in order to ensure the penetration quality of the plate center, strict root cleaning treatment is required, using mechanical grinding or carbon arc gouging + mechanical grinding. On the premise of ensuring the welding quality, the welding process parameter range can be specifically determined according to the on-site manufacturing construction conditions, including but not limited to welding preheating measures, post-heating measures, post-weld heat treatment measures, welding current, welding voltage, welding speed, etc. To ensure the comprehensive performance of the welded joint, the welding materials adopt theoretical equal-strength matching.

[0102] To ensure the joint quality of subsequent fatigue test sampling, two ultrasonic non-destructive inspections of the welded joint are carried out 24 hours and 72 hours after welding respectively. According to the relevant standard requirements, if the inspection is qualified, the subsequent fatigue test sampling can be carried out. If delayed cracks are found, the reasons need to be analyzed and the welding process needs to be adjusted, and then welding and non-destructive inspection are carried out again.

[0103] Step 2: Design and production of the fatigue specimen form of the welded joint

[0104] In order to ensure the consistency between the fatigue test results of welded joints and the service conditions of actual welded structural parts, so that the fatigue test results and the obtained fatigue S-N curve are representative, the design and manufacturing method of fatigue specimens for welded joints is as follows:

[0105] (1) Design of fatigue specimens for welded joints

[0106] Rectangular specimens with a parallel test section machined in the middle are used, and the weld reinforcements on both the front and back sides are retained, as shown in Figure 2 、 3 . At the same time, considering the representativeness of the tonnage of existing fatigue testing machines and the specimen size, the specimen thickness t = 15 - 30 mm, which is the original thickness of the plate used for welding tests. The width b of the parallel test section of the fatigue specimen is b = 1 - 1.5t, and the width B of the clamping end is B ≥ 1.75b, so as to ensure that there is sufficient stress concentration in the test section and fatigue fracture occurs here, rather than an invalid test result of fracture at the clamping end due to the randomness of the fatigue test results. The length l of the parallel test section has an important influence on the stability of the fatigue test process and results of welded joints. If l is too long, the overall stiffness of the specimen decreases, and it is difficult to achieve high-frequency resonance during the fatigue test. If the specimen length is too short, the contingency of the fatigue fracture position of the specimen will increase. Therefore, the length l of the parallel test section is limited to The clamping end length L is designed according to the fixture size of the fatigue testing machine, and generally L ≥ l is required. In order to ensure a smooth transition from the clamping end to the test section and avoid stress concentration and fracture at the transition position during the test, the transition radius R > 100 mm, which refers to the arc radius of the transition section. In addition, in order to reduce the influence of the machining accuracy of the cross-section of the specimen test section on the stability of the test results, the surface roughness Ra ≤ 3.2. Since the inevitable welding residual stress during the plate welding process will cause the deformation of the welded plate, thus introducing angular deformation in the fatigue specimen, and specimens with too large angular deformation will generate additional bending stress during the axial tensile fatigue loading process, which is not conducive to the stability of the test results. Therefore, the deformation angle ɑ of the fatigue specimen < 5°.

[0107] (2) Manufacturing of fatigue specimens for welded joints

[0108] The fatigue specimens are fabricated by wire electrical discharge machining (WEDM) + polishing to avoid work hardening in the test section and its influence on the subsequent fatigue test process to the greatest extent. Since there are often small surface welding slag on both sides of the submerged arc weld, which affects the electrical conductivity during WEDM, it may even cause the cutting molybdenum wire to break, and a deep wire-breaking groove is formed at the cutting position of the cross-section perpendicular to the weld toe. This groove is exactly perpendicular to the subsequent fatigue test loading direction. Coupled with the stress concentration brought by the weld toe itself, it is easy to cause early fatigue fracture at this position, affecting the accuracy of the test results. Therefore, before cutting the fatigue specimens by WEDM, the small slag inclusions at the weld toe are thoroughly cleaned first, but no mechanical damage should be caused to the weld toe. If a groove is formed at the cross-section perpendicular to the weld toe due to wire breakage during the machining process, the groove must be removed by grinding + polishing. The acceptance standard for the cross-section quality of the fabricated fatigue specimens is that no obvious groove can be observed under a 10x magnifying glass.

[0109] Step 3: Design and testing of fatigue tests at a series of stress levels within the full high-cycle fatigue life cycle

[0110] Considering the high-cycle fatigue characteristics of the actual welded joints of low-alloy high-strength steel, the fatigue test cycle number (conditional fatigue life) of 1×10 5 ~1×10 7 cycles is defined as the full high-cycle fatigue life cycle. If the fatigue test cycle number is less than 1×10 5 cycles, obvious local macroscopic strain generally occurs, that is, it enters the low-cycle fatigue category. If the fatigue test cycle number is higher than 1×10 7 cycles, it basically enters the infinite fatigue life category, thus having no influence on the structural cumulative damage based on the S-N curve.

[0111] Meanwhile, considering that the full-cycle high-cycle fatigue life of the high-strength steel welded joint must correspond to a certain fatigue test cyclic stress, under the condition of stress ratio R = 0.1, it is specified that the maximum stress value σ max of the fatigue test is 35% - 70%R p , where R p is the actual yield strength (MPa) of the high-strength steel base metal. To achieve the purpose of fatigue tests at a series of cycle numbers within the full high-cycle fatigue life cycle, fatigue tests at a series of stress levels need to be carried out, and the stress step S σ between adjacent stress levels is 1.5% - 7.5%R p or 10 - 50 MPa. Here, the stress step S σ is defined as: S σ = σ n - σ n-1 , where σ n and σ n-1They respectively represent the highest stress values of two adjacent groups in the high-cycle fatigue test of a series of stress levels. It should be noted that: the selection of the highest stress value in the fatigue test is to achieve the test results of the above full high-cycle fatigue life cycle, that is: to ensure that the fatigue test cycle times under the series of stress levels are 1×10 5 ~1×10 7 times.

[0112] A high-frequency resonance fatigue testing machine is used to conduct the full high-cycle fatigue life cycle fatigue test of a series of stress levels. The stress ratio R = 0.1, and the test stop condition is that the specimen generates a fatigue crack, resulting in a significant decrease in the test frequency or the cycle times reaching 1×10 7 times. After the test, check the fracture position of the fatigue specimen. If the fatigue crack occurs at the weld toe, the test result is considered valid. If the specimen fractures at any position other than the weld toe, including but not limited to the base metal at the clamping end, the base metal in the transition section, and the weld metal, the test result is considered invalid and the test needs to be redone.

[0113] Step Four: Obtaining the high-survival-rate S-N fatigue curve for structural design

[0114] Based on the fatigue test data of a series of stress levels within the full high-cycle fatigue life cycle, obtain the high-survival-rate S-N fatigue curve. The key lies in the selection of the fatigue test cycle times data under the series of stress levels, which should not only ensure the uniformity and representativeness of the data within the full high-cycle fatigue life cycle, but also reduce the discreteness and volatility of the fatigue cycle times data at each stress level. In view of this, the following restrictions are imposed on the fatigue cycle times data of the series of stress levels participating in the S-N fatigue curve fitting:

[0115] (1) To ensure the uniformity of the fatigue test cycle times data within the full high-cycle fatigue life cycle, the number of stress levels at equal intervals or unequal intervals (stress steps) is not less than 6, and the number of valid data at each stress level is not less than 5. Considering the unique discreteness and volatility of the welding joint fatigue test data, the meaning of valid data, in addition to meeting the requirements for valid test results described in Step Three, also needs to meet the requirement of numerical uniformity: the ratio of any actual measured cycle times data to the median data at each stress level is between 0.31 and 3.2, that is: N i / N m = 0.31 - 3.2. Here, N i represents any actual measured cycle times data at each stress level, and N mIt represents the median of the actual test cycle number data at each stress level; the theoretical basis behind the determination of the median judgment scheme here is: although the fatigue test data of welded joints presents a discrete characteristic as a whole, for any inevitable event, under the condition of a large sample size, the test data always tends to be normally distributed, and the establishment of the median judgment scheme is a further constraint scheme that follows this central limit theorem.

[0116] The specific implementation process of median determination is as follows:

[0117] First, the set number of cycles and the number of valid data at each stress level are used as common constraints to determine the data set that meets the common constraints.

[0118] Next, determine the median value in the data set.

[0119] Next, based on the determined median, the data other than the median is judged and screened based on whether the ratio of any value in the data set to the median falls within the interval [0.31, 3.2]. The data that falls within the interval is retained, and the data that does not fall within the interval is eliminated.

[0120] Then, the number of data after judgment and screening still meets the number of valid data at each stress level, forming a data set at the stress level for data fitting. The specific processing process here is: for the data set screened according to the median ratio constraint, determine whether it meets the number of valid data. If it meets the requirement, the final required data set is formed. If it does not meet the requirement, it is necessary to continue to re-load multiple groups of experiments, re-acquire valid data, re-determine the median, and then repeat this process until it is satisfied.

[0121] (2) For each stress level, there are no less than 5 valid data. If the number of valid data is set to 5, the specific number of data involved in the fitting can be determined according to the specific situation. If N i / N m = 0.5 to 2.0, three intermediate values ​​are selected to participate in SN curve fitting, or 5 valid data are used for SN curve fitting; if N i / N m Beyond the range of 0.5 to 2.0, that is: N i / N m =0.31~0.5 or 2.0~3.0, then 5 valid data must be used to participate in SN curve fitting;

[0122] The fitting of the S-N fatigue curve refers to the standard methods of GB / T 24176 or ISO 12107, and is translated downward by an appropriate multiple of the estimated standard deviation to ensure a high survival rate of the fitted S-N curve. Since the fatigue cyclic stress range Δσ is generally used in the fatigue design of welded structures, when fitting the S-N curve, the maximum stress value σ of the fatigue test is max converted into the stress range Δσ. Under the condition of stress ratio R = 0.1, Δσ = 0.9σ max .

[0123] Step Five: Application Criteria for Fatigue Strength Characteristic Values Based on the Actual S-N Curve

[0124] According to the general requirements and practices of current fatigue design of welded structures, the stress range value Δσ corresponding to the fatigue cycle number of 2×10 6 cycles on the S-N curve obtained through actual testing and fitting is 200万 defined as the fatigue strength characteristic value of high-strength steel welded joints, denoted as FAT W , that is, FAT W =Δσ 200万 . According to the current mainstream structural fatigue design standards, regardless of the material strength level, for fully penetrated butt joints, the fatigue strength characteristic value is taken as FAT = 90 MPa, denoted as FAT90. In view of this, under the same service conditions of welded structural parts, the relative increase ratio of the fatigue strength characteristic value FAT W obtained from the actual test of high-strength steel welded joints to FAT90 is used as the basis for lightweight and wall thickness reduction in the design and material selection of high-strength steel welded structural parts. In order to improve the safety during the service of the structure, a safety factor of 0.8 is introduced into the increase ratio of the fatigue strength characteristic value. That is: Define the wall thickness reduction percentage of high-strength steel welded structural parts as r, then:

[0125]

[0126] Step Six: Safety Verification of Fatigue Strength Characteristic Values Based on the Actual S-N Curve

[0127] To ensure sufficient safety and reliability when using the fatigue strength characteristic value of high-strength steel welded joints obtained from actual tests for structural fatigue design, a fatigue loading test verification is carried out using large-sized specimens with welding residual stresses retained. The large-sized specimens used for fatigue performance safety verification still adopt full-thickness weld specimens with a width 5 times the thickness, so as to effectively retain the welding residual stresses and accurately simulate the actual service process of structural parts. A large-tonnage electro-hydraulic servo fatigue testing machine is used for fatigue verification tests, with stress ratio R = 0.1, Δσ = FAT W , and the fatigue verification test stop condition is set as the cycle number 2×10 6times. If no fatigue crack appears in the large-sized specimen during the test, it is considered that the fatigue strength characteristic value application criterion described in Step 5 is safe and reliable. Otherwise, it is necessary to analyze the cause of the failure of the verification test and conduct necessary duplicate sample tests. If failure still occurs within 2 million fatigue cycles, it is necessary to optimize the safety factor described in Step 5 and re-conduct the fatigue service safety verification test.

[0128] Embodiment

[0129] Implement the method for obtaining the fatigue curve of the butt welding joint of high-strength steel and its application according to the present invention. Use Q500MD high-strength steel with a wall thickness of 20 mm, and sequentially complete the preparation of the high-strength steel butt joint, the design and manufacture of the fatigue specimen form of the welded joint, the design and testing of the series of stress level fatigue tests within the full high-cycle fatigue life cycle, the acquisition of the high survival rate S-N fatigue curve, the application of the fatigue strength characteristic value based on the actual S-N curve, and the safety verification of the fatigue strength characteristic value based on the actual S-N curve according to the specific steps described in the present invention. Finally, obtain the material lightweight reduction ratio when designing and manufacturing structural parts in fatigue service occasions using Q500MD high-strength steel. Specifically, as follows, for the effect of the example, the number of stress levels in this embodiment is specifically set to 8, and the number of valid data at each stress level is 5, but this does not represent the limitation of the number of stress levels and the number of valid data at each stress level:

[0130] Step 1. Preparation of the high-strength steel butt joint

[0131] According to the manufacturing conditions and requirements of welded structural parts with good welding quality applied to dynamic load occasions, design a double Y-shaped groove suitable for double-sided submerged arc automatic welding. The specific groove dimensions are as Figure 4 shown. The welding groove is processed by mechanical cutting. To ensure the welding quality, the two sides of the groove are mechanically ground before welding to remove rust, oil, etc. that have an adverse effect on the welding quality.

[0132] Use the gas metal arc welding method for tack welding. The matching welding wire is GB / T 8110-2008 ER50-G, and the wire diameter is 1.2 mm. Then, use single-wire submerged arc automatic welding to complete the main butt joint. The matching type of the submerged arc welding wire and flux is GB / T 5293-2018 F5A4-SUG. To ensure the penetration quality at the center of the plate, the method of carbon arc air gouging + mechanical grinding is used for root cleaning. Table 1 shows the tack welding and submerged arc welding process parameters of the Q500MD high-strength steel butt joint.

[0133] Table 1 Tack welding and submerged arc welding process parameters of the Q500MD high-strength steel butt joint

[0134]

[0135] Remark:

[0136] 1. GMAW represents gas metal arc welding, SAW represents submerged arc welding, and DCEP represents that the welding wire is connected to the positive pole of the welding power supply;

[0137] 2. For root pass welding, 100% CO 2 gas is used, and the flow rate of the shielding gas is 15 - 20 L / min;

[0138] 3. Before filling welding, the submerged arc welding flux needs to be dried at 300 - 350 °C for not less than 2 hours.

[0139] In order to ensure the joint quality of the samples taken for subsequent fatigue tests, two ultrasonic non-destructive inspections of the welded joints are carried out at 24 hours and 72 hours after welding respectively. No welding cracks are found in both ultrasonic non-destructive inspections, and the conditions for taking samples for fatigue tests are met.

[0140] Step 2: Design and manufacture of fatigue specimen forms for welded joints

[0141] The specific form of the fatigue specimen for the butt welded joint of Q500MD high-strength steel is as Figure 5 shown. Here, b = t = 20 mm, B = 35 mm, the length of the parallel test section l = 70 mm, the width of the clamping end L = 75 mm, the transition radius R = 110 mm, the surface roughness Ra = 3.2, and the weld reinforcement on both the front and back sides is retained. At the same time, the deformation angle ɑ of the fatigue specimen < 5°.

[0142] The fatigue specimens are manufactured by wire electrical discharge machining + polishing to avoid work hardening of the test section and its influence on the subsequent fatigue test process to the greatest extent. Since there are often small surface slag deposits on both sides of the submerged arc weld, which affect the conductivity during wire electrical discharge machining, and even cause the cutting molybdenum wire to break, and a deep wire break groove is formed at the cutting position of the cross-section perpendicular to the weld toe. This groove is exactly perpendicular to the subsequent fatigue test loading direction, and together with the stress concentration brought by the weld toe itself, it is easy to cause early fatigue fracture at this position, affecting the accuracy of the test results. Therefore, before wire electrical discharge machining of the fatigue specimens, the small slag inclusions at the weld toe are first thoroughly cleaned, but no mechanical damage should be caused to the weld toe. If a groove is formed at the cross-section perpendicular to the weld toe due to wire breakage during the machining process, the groove must be removed by grinding + polishing. The acceptance standard for the cross-section quality of the machined fatigue specimens is that no obvious groove can be observed under a 10-fold magnifying glass.

[0143] Step 3: Design and test of fatigue tests with a series of stress levels within the full high-cycle fatigue life cycle

[0144] First, according to the yield strength R of Q500MD high-strength steel pThe maximum stress value for the selected series of fatigue tests was 585 MPa, and the stress ratio R = 0.1. Here, the maximum stress value σ of the fatigue test max = 35% - 70% R p , and the stress step between adjacent stress levels satisfied S σ = 1.5 - 7.5% R p or 10 - 50 MPa. A series of full high-cycle fatigue life cycle fatigue tests were conducted using a high-frequency resonance fatigue testing machine. The test stop condition was that the test frequency decreased significantly due to the generation of fatigue cracks in the specimen or the number of cycles reached 1×10 7 cycles. After the test, the fracture location of the fatigue specimen was inspected. If the fatigue crack occurred at the weld toe, the test result was considered valid. If the specimen fractured at any location other than the weld toe, including but not limited to the base metal at the clamping end, the base metal in the transition section, and the weld metal, the test result was considered invalid and the test needed to be repeated.

[0145] Table 2 shows the fatigue test results of the Q500MD high-strength steel butt joint at a series of stress levels. The fatigue test cycle numbers (conditional fatigue lives) were all in the range of 1×10 5 to 1×10 7 cycles. The fracture location of all fatigue specimens was the weld toe, and the test results were valid.

[0146] Table 2 Fatigue Test Results of Q500MD High-Strength Steel Butt Joint at a Series of Stress Levels

[0147]

[0148]

[0149] Step 4: Obtaining the high survival rate S-N fatigue curve

[0150] Based on the fatigue test data at a series of stress levels within the full high-cycle fatigue life cycle, the high survival rate S-N fatigue curve was obtained. The key lies in the selection of the fatigue test cycle number data, which should not only ensure the uniformity and representativeness of the data within the full high-cycle fatigue life cycle but also reduce the discreteness and volatility of the fatigue cycle number data at each stress level. Accordingly, the fatigue test results of the Q500MD high-strength steel butt joint at a series of stress levels shown in Table 2 were evaluated. The number of stress levels was 8, and the number of valid data at each stress level was not less than 5, and it satisfied the requirement of N i / N m = 0.31 - 3.2, and at the same time satisfied N i / N m= 0.5 to 2.0. Select data from this range for S-N curve fitting, as shown in Table 3. The fitting of the S-N fatigue curve follows the standard methods of GB / T 24176 or ISO12107, and is translated downward by an appropriate multiple of the estimated standard deviation to ensure a high survival rate for the fitted S-N curve. Under the condition of stress ratio R = 0.1, perform conversion according to Δσ = 0.9σ max and perform S-N curve fitting using the fatigue cyclic stress range Δσ. Figure 6 Shown is the fatigue S-N curve of the Q500MD high-strength steel butt joint series at different stress levels (Note: The red solid and dashed lines in the figure are the S-N curves with different survival rates actually tested and fitted for the Q500MD high-strength steel butt joint, and the blue solid line is the representative IIW (International Institute of Welding) standard S-N design curve in the current industry).

[0151] Table 3 Fatigue test data of the Q500MD high-strength steel butt joint series for S-N curve fitting

[0152]

[0153] Step Five: Application of fatigue strength characteristic values based on the actual S-N curve

[0154] According to the general requirements and practices of current welded structure fatigue design, the stress range value Δσ 6 corresponding to 2*10 200万 cycles on the S-N curve obtained through actual testing and fitting is defined as the fatigue strength characteristic value of the high-strength steel welded joint, denoted as FAT W , that is, FAT W = Δσ 200万 . According to the current mainstream structural member fatigue design standards, such as the IIW (International Institute of Welding) standard, regardless of the material strength level, for fully penetrated butt joints, the fatigue strength characteristic value is uniformly taken as FAT = 90 MPa, denoted as FAT90, as Figure 6 shown. According to the 97.7% survival rate S-N curve of the Q500MD high-strength steel butt joint shown in Figure 6 , extract the fatigue strength characteristic value, FAT W = 115 MPa.

[0155] Under the same service conditions of welded structural members, take the relative increase ratio of the fatigue strength characteristic value FAT W obtained from the actual test of the high-strength steel welded joint to FAT90 as the basis for lightweighting and wall thickness reduction in the design and material selection of high-strength steel welded structural members. To improve the safety during the service of the structure, a safety factor of 0.8 is introduced for the increase ratio of the fatigue strength characteristic value. That is: Define the wall thickness reduction percentage of the high-strength steel welded structural member as r, then:

[0156] Accordingly, r = 22.2%. This indicates that for dynamic load structural components based on Q500MD high-strength steel, a material design scheme with a 22.2% reduction in wall thickness can be adopted, providing a guiding basis for lightweight design in the industrial field.

[0157] Step Six: Safety Verification of Fatigue Strength Characteristic Values Based on the Actual S-N Curve

[0158] To ensure sufficient safety and reliability when using the fatigue strength characteristic values of high-strength steel welded joints obtained from actual tests for structural fatigue design, fatigue loading tests are carried out using large-sized specimens with welding residual stresses retained. The large-sized specimens for fatigue performance safety verification still use full-thickness specimens with welds retained, as Figure 1 shown. t = 20 mm, b = 5t = 100 mm, l = 180 mm, L = 200 mm, transition radius R = 120 mm, B = 200 mm, thus effectively retaining welding residual stresses and accurately simulating the fatigue service process of actual structural components. To reduce the influence of the machining accuracy of the specimen test section on the stability of test results, the surface roughness Ra ≤ 3.2. At the same time, to reduce the adverse influence of the angular deformation of the fatigue specimen on the stability of test results, the deformation angle ɑ of the fatigue specimen < 5°.

[0159] A large-tonnage electro-hydraulic servo fatigue testing machine is used for fatigue verification tests, with a stress ratio R = 0.1, Δσ = FAT W = 115 MPa, and the fatigue verification test stop condition is set as the number of cycles 2*10 6 times. The results show that under the fatigue loading verification test conditions of such large-sized specimens with welding residual stresses retained, no fatigue cracks occurred. It can be considered that the fatigue strength characteristic value application criterion described in Step Five is safe and reliable.

Claims

1. A method for obtaining the fatigue curve of a butt welded joint of high-strength steel, characterized in that: For the butt welded joint of high-strength steel with a fatigue strength mechanism that realizes the characterization of cumulative damage of cyclic plastic strain in the local micro-region controlled by the yield strength of the base metal through welding optimization, The acquisition of data for fitting is completed through the following steps, and then the fatigue curve is obtained by fitting the acquired data: S1: Prepare the specimens for fatigue tests; S2: Set the number of cycles of the fatigue test according to the characteristics of high-cycle fatigue; S3: Determine the maximum stress value of the fatigue test and set the stress step between adjacent stress levels in sequence within the limit of the set number of cycles of the fatigue test; S4: Guided by the principle of data uniformity, determine the number of stress levels, the number of valid data at each stress level, and set the valid data screening mechanism at each stress level respectively; S5: Conduct a series of full high-cycle fatigue life cycle fatigue test measurements at different stress levels based on a fatigue testing machine, and complete the acquisition of data based on the test results in combination with the above settings.

2. The method for obtaining the fatigue curve of a butt welded joint of high-strength steel according to claim 1, characterized in that: Step S1 specifically includes the following steps: S11: Weld to obtain a butt joint specimen for testing according to the welding treatment that can realize the fatigue strength mechanism of characterizing the cumulative damage of cyclic plastic strain in the local micro-region controlled by the yield strength of the base metal after welding optimization; S12: Conduct non-destructive flaw detection on the obtained butt joint specimen for testing after welding with a time delay. When the detection passes, proceed to step S13; otherwise, return to step S11 to re-prepare the butt joint specimen for testing and enter step S12 again for non-destructive flaw detection with a time delay after welding until the detection passes; S13: Machine to form a fatigue specimen with gripping sections adapted to the testing machine fixtures at both ends, a parallel test section loaded by the testing machine in the middle, and a transition section connecting the parallel test section and the gripping sections at both ends.

3. The method for obtaining the fatigue curve of a butt welded joint of high-strength steel according to claim 2, characterized in that: For the butt joint specimen for testing obtained by welding in step S11, submerged arc welding with back gouging treatment is used.

4. The method for obtaining the fatigue curve of a butt welded joint of high-strength steel according to claim 2, characterized in that: The non-destructive flaw detection with a time delay after welding in step S12 is specifically: Conduct the first non-destructive flaw detection at least 24 hours after welding, conduct the second non-destructive flaw detection at least 72 hours after welding, and characterize the final non-destructive flaw detection result based on the results of the two non-destructive flaw detections.

5. The method for obtaining the fatigue curve of a butt welded joint of high-strength steel according to claim 2, characterized in that: The machining in step S13 is specifically wire electrical discharge machining; The cutting is carried out to meet the requirement that the surface roughness of the machined section is less than or equal to 3.2, and the acceptance criterion is that no grooves can be observed under a 10-fold magnifying glass.

6. The method for obtaining the fatigue curve of a butt welded joint of high-strength steel according to claim 5, characterized in that: Before the cutting operation, slag inclusion inspection and cleaning operations are carried out on the weld toe part; the cleaning operation is carried out within the limit of not causing mechanical damage to the weld toe part.

7. A method for obtaining the fatigue curve of a high-strength steel butt welding joint according to claim 5, characterized in that: For the grooves at the cross-sectional position perpendicular to the weld toe that appear at any time from the start of cutting to the completion of cutting, grinding + polishing groove removal operations are carried out.

8. A method for obtaining the fatigue curve of a high-strength steel butt welding joint according to claim 2, characterized in that: The fatigue specimens formed by machining are carried out in a manner that retains the weld reinforcements on both the front and back sides, constrained by the effectiveness of the fracture position, the stability of the test process and results, and the size of the testing machine fixture, to determine the dimensions of the clamping section, the parallel test section, and the transition section; Specifically: Constrained by the effectiveness of the fracture position, the width of the parallel test section, the width of the clamping section, and the transition radius of the transition section are set; Constrained by the stability of the test process and results, the length of the parallel test section is set; Constrained by the size of the testing machine fixture, the length of the clamping section adapted to the size of the testing machine fixture is set.

9. A method for obtaining the fatigue curve of a high-strength steel butt welding joint according to claim 8, characterized in that: The width of the parallel test section is set according to the thickness of the plate, The width of the clamping section is set to be greater than or equal to 1.75 times the width of the parallel test section; The transition radius of the transition section is set to be greater than 100 mm; The length of the parallel test section is limited according to the following range: where, l: the length of the parallel test section, unit: mm; b: the width of the parallel test section, unit: mm.

10. A method for obtaining the fatigue curve of a high-strength steel butt welding joint according to claim 2, characterized in that: The deformation angle of the fatigue specimen obtained by machining in step S13 is less than 5°.

11. A method for obtaining the fatigue curve of a high-strength steel butt welding joint according to claim 1, characterized in that: The number of cycles of the fatigue test in step S2 is set to 1×10 5 -1×10 7 times, and accordingly, the maximum stress value of the fatigue test in step S3 is set to fall within the interval [35%R p , 70%R p , and the stress step between adjacent stress levels is set to fall within the interval [1.5%R p , 7.5%R p or to fall within the interval [10 MPa, 20 MPa]. R in the above p : The actual yield strength of the high-strength steel base material, unit: MPa.

12. A method for obtaining the fatigue curve of a high-strength steel butt welding joint according to claim 1, characterized in that: In step S4, the determination of the number of stress levels is completed under the guiding principle of data uniformity, and it is completed with at least 6 stress levels.

13. A method for obtaining the fatigue curve of a high-strength steel butt welding joint according to claim 1, characterized in that: In step S4, the determination of the number of valid data at each stress level is completed under the guiding principle of data uniformity, and it is completed with at least 5 valid data.

14. A method for obtaining the fatigue curve of a high-strength steel butt welding joint according to claim 1 or 13, characterized in that: In step S4, the setting of the valid data screening mechanism at each stress level is completed under the guiding principle of data uniformity, specifically: First, constrained by the number of cycles set in step S2 and the number of valid data at each stress level in step S4, the determination of the data set that meets the common constraints is completed, Secondly, the median value in the data set is determined, Next, based on the determined median, the data other than the median is judged and screened based on whether the ratio of any value in the data set to the median falls within the interval [0.31, 3.2]. The data that falls within the interval is retained, and the data that does not fall within the interval is eliminated. Then, a data set at the stress level for data fitting is formed, subject to the number of data after determination and screening still satisfying the number of valid data at each stress level in step S4.

15. The method for obtaining fatigue curve of high-strength steel butt welded joint according to claim 14, Features: When the number of valid data at each stress level is determined to be 5, if the ratio of any value to the median in the data set falls within the interval [0.5, 2.0], three intermediate values ​​are selected to participate in curve fitting, or 5 valid data are selected to participate in curve fitting; otherwise, the data selection is completed according to the scheme that all 5 data participate in the fitting.

16. The method for obtaining fatigue curve of high-strength steel butt welded joint according to claim 1, Features: When obtaining a fatigue curve by fitting based on the acquired data, the maximum stress value of the fatigue test is converted into a fatigue cyclic stress range according to the relationship of Δσ = 0.9σ max ; In the above, Δσ: fatigue test stress range, unit: MPa; σ max : The maximum stress value in the fatigue test, unit: MPa.

17. An application based on the fatigue curve according to claim 1, Features: First, the fatigue strength characteristic value of the high-strength steel welded joint is determined based on the fitted curve. Then, the fatigue strength characteristic value of the fully penetrated butt joint under the existing structural fatigue standard is combined to determine the lightweight parameter setting for high-strength steel welded structural parts as the material selection; specifically: Define the stress range value corresponding to the fatigue cycle number of 2×10 6 times on the fitted curve as the fatigue strength characteristic value of the high-strength steel welded joint, and determine the wall thickness reduction percentage of the high-strength steel welded structural member according to the following formula In the above formula, r: percentage of wall thickness reduction of high-strength steel welded structural parts; FAT w : Fatigue strength characteristic value of high-strength steel welded joint, unit: MPa; FAT90: Fatigue strength characteristic value of full penetration butt joint in the current structural parts fatigue design standard, unit: MPa; α: safety factor.

18. The application based on fatigue curve according to claim 17, Features: The safety factor α is determined with an initial value of 0.8 and is determined based on real-time verification and correction, specifically: The safety factor α is set to 0.8, and a fatigue loading test based on a large-size specimen with retained welding residual stress is established. Set the test stop condition to the loop count reaching 2×10 6 times; When fatigue cracks appear during the test, the safety factor is corrected and reloaded until fatigue cracks do not appear during the test, thus completing the determination of the safety factor; Otherwise the safety factor is determined to be 0.

8.

19. The application based on fatigue curve according to claim 18, Features: The large-size-based method is as follows: the width of the test piece is set to be five times the actual plate thickness, and the test piece adopts a full-thickness weld retention specimen.

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