High-strength steel rectangular non-bearing accessory corner welding joint fatigue curve obtaining method and application

Abstract

The invention discloses a method for acquiring a fatigue curve of a corner welding joint of a high-strength steel rectangular non-bearing accessory and application. The fatigue curve acquisition method is used for a rectangular non-bearing accessory high-strength steel angle welding joint which realizes a fatigue strength mechanism represented by local micro-area cyclic plastic strain accumulated damage controlled by base metal yield strength through welding optimization. Constraint is formed by sequentially setting the size of a sample, preparing the sample, welding and preparing the sample to be loaded, determining the maximum stress value, the stress step and the stress level number and limiting effective data under each stress level, and data for fitting are obtained through loading according to the constraint. Wherein the application means to further establish a lightweight scheme for thinning the wall thickness of the selected material by determining the characteristic value of the new curve.

Description

Technical Field

[0001] The present invention belongs to the technical field of welding, and specifically relates to a method for obtaining the fatigue curve of a fillet welded joint of a high-strength steel rectangular non-load-bearing accessory and its application. Background Art

[0002] For the fatigue S-N curve of welded joints in the original fatigue design standard of welded structural parts, the basic guiding cognition behind its determination is that the fatigue strength grade of the welded joints of steel materials is not related to the strength level of the materials, but 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 all give a set of S-N curves for fatigue design according to different design forms and quality grades of steel materials under this basic guiding cognition.

[0003] For general dynamic load service structural parts, in addition to the butt joints or fillet joints for positive load-bearing, for the purposes of pipeline or cable layout and fixation, operation monitoring and maintenance, decoration, etc., various-shaped accessories are often welded on the overall structure, mainly rectangular plate-shaped accessories and cylindrical accessories with a central hole. The welds of the accessories themselves do not bear positive fatigue loads, but due to the strong stress concentration at the weld toes or weld roots during the load transfer process, the fatigue performance of the overall structure will be greatly reduced.

[0004] However, the research team of the applicant found through research that: with the increase in the strength level of the base material of the overall part, the fatigue performance of the welded structure of the accessory will also be conditionally improved, rather than the fatigue strength characteristic value being independent of the material characteristics as stipulated in the standard. This discovery after research makes it possible to use high-strength steel for the design and lightweight weight reduction of dynamic load structural parts or non-load-bearing accessories. Based on this, it is necessary to provide a method for obtaining the fatigue curve of a fillet welded joint of a high-strength steel non-load-bearing accessory and its application based on this, so as to accurately obtain the S-N curve of the high-strength steel butt welded joint, standardize the characteristic values, and provide a basic guarantee for the popularization and application of high-strength steel.

[0005] The invention application with the application number: CN2018108462064 discloses "a full-scale fatigue test method for a welded 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 achieve the above technical purposes, the present invention provides a method for obtaining the fatigue curve of a fillet welded joint of a high-strength steel rectangular non-load-bearing accessory and its application. The technical solution is as follows:

[0007] Method for obtaining fatigue curve of fillet welded joint of high-strength steel rectangular non-load-bearing attachment,

[0008] For rectangular non-load-bearing high-strength steel fillet welded joints, the fatigue strength mechanism characterized by the local micro-area cyclic plastic strain accumulation damage controlled by the yield strength of the parent material is achieved through welding optimization.

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

[0010] S1: Setting of specimen size: The size of the base plate and non-load-bearing accessories is set based on the constraint that the specimen can represent the actual service behavior of the non-load-bearing accessories and can be normally loaded by the fatigue testing machine; at the same time, the relationship constraint between the width of the base plate and the thickness of the non-load-bearing accessories is established to eliminate the influence of the edge effect of the small-sized specimen on the non-load-bearing accessories;

[0011] S2: Processing the sample into the parts to be welded: the processing of the base plate to be welded is completed by controlling the roughness of the two cross sections parallel to the loading direction, and the processing of the non-load-bearing accessories to be welded is completed by controlling the fit with the base plate;

[0012] S3: Sample welding: The welding of the base plate and the non-load-bearing accessories is completed in a way that the arc starting and arc ending positions avoid the load loading direction, and at the same time, the weld quality parameters after welding meet the settings;

[0013] S4: within the set number of cycles of the fatigue test, sequentially completing the determination of the maximum stress value of the fatigue test and the setting of stress steps between adjacent stress levels;

[0014] S5: Taking data uniformity as the guiding principle, the determination of the number of stress levels, the determination of the number of valid data at each stress level, and the setting of the valid data screening mechanism at each stress level are completed respectively;

[0015] S6: Conduct a series of stress level full high cycle fatigue life cycle fatigue tests on a fatigue testing machine, and complete data acquisition based on the test results and the above settings.

[0016] Further,

[0017] The bottom plate size in step S1 is specifically: the thickness is set by taking a value within the interval [15, 25] mm, the width is set by taking a value within the interval [70, 105] mm, and the length is set by taking a value within the interval [380, 500] mm;

[0018] The dimensions of the non-load-bearing accessory in step S1 are specifically as follows: The thickness is set by taking values within the range of [15, 25] mm, the width is set by taking values within the range of [30, 60] mm, and the length is set by taking values within the range of [40, 50] mm.

[0019] Furthermore,

[0020] The relationship constraint between the width of the bottom plate and the thickness of the non-load-bearing accessory in step S1 is specifically as follows: The width of the bottom plate is set to be more than four times the thickness of the non-load-bearing accessory.

[0021] Furthermore,

[0022] The roughness of the two cross-sections of the bottom plate to be welded parallel to the loading direction in step S2 is specifically controlled as follows: The roughness is controlled to be less than or equal to 3.2.

[0023] Furthermore,

[0024] The degree of fit between the bottom plate to be welded and the non-load-bearing accessory to be welded in step S2 is specifically controlled as follows: The fit gap between the two is controlled to be less than 1 mm.

[0025] Furthermore,

[0026] The weld quality parameters in step S3 consist of the fillet weld leg length of the bottom plate side for positive load transfer, the transition angle of the fillet weld of the bottom plate side for positive load transfer, the radius of the transition arc at the four corners of the circumferential fillet weld, and the undercut parameters of the circumferential fillet weld of the non-load-bearing accessory on the positive load transfer side.

[0027] Furthermore,

[0028] The fillet weld leg length of the bottom plate side for positive load transfer is controlled within the range of [8, 16] mm, and at the same time, it should also be greater than one-third of the thickness of the non-load-bearing accessory;

[0029] The transition angle of the fillet weld of the bottom plate side for positive load transfer is controlled to be greater than 135°;

[0030] The radius of the transition arc at the four corners of the circumferential fillet weld is controlled to be greater than 10 mm, and it should also be greater than two-thirds of the thickness of the non-load-bearing accessory;

[0031] The undercut parameters of the circumferential fillet weld of the non-load-bearing accessory on the positive load transfer side include the undercut depth and the total undercut length. The undercut depth is controlled to be less than 0.5 mm, and the total undercut length is controlled to be less than 3 mm.

[0032] Furthermore,

[0033] The welding in step S3 is carried out with the angular deformation of the bottom plate after welding being less than 5°.

[0034] Furthermore,

[0035] For the welding in step S3, control the welding current between 170 - 240 A, control the welding voltage between 18 - 26 V, and control the welding speed between 160 - 280 mm / min.

[0036] Furthermore,

[0037] In step S4, determine the maximum stress value of the fatigue test, specifically: take values when the stress value falls within the interval [30% R p , 55% R p to complete the determination.

[0038] R in the above p : The actual yield strength of the high-strength steel base material, unit: MPa.

[0039] Furthermore,

[0040] In step S4, set the stress step between adjacent stress levels, specifically: set the stress step between adjacent stress levels to be greater than 15 MPa.

[0041] Furthermore,

[0042] In step S5, determine both the number of stress levels and the number of valid data at each stress level with at least 3 as the requirement.

[0043] Furthermore,

[0044] In step S4, set the effective data screening mechanism at each stress level, specifically completed through the following formula:

[0045]

[0046] N i : Any valid data at the current stress level;

[0047] The mean value of the valid data at the current stress level.

[0048] Furthermore,

[0049] When fitting the fatigue curve based on the acquired data, convert the maximum stress value of the fatigue test into the fatigue cyclic stress range according to the relationship of Δσ = 0.5σ max ;

[0050] In the above,

[0051] Δσ: Fatigue test stress range, unit: MPa;

[0052] σ max: The maximum stress value of the fatigue test, unit: MPa.

[0053] Furthermore,

[0054] When fitting the fatigue curve based on the obtained data, the fitting of the straight-line segment in the finite life interval is carried out with a fixed slope, specifically:

[0055] For the as-welded rectangular non-load-bearing appendages, a slope of 3 is adopted;

[0056] For the rectangular non-load-bearing appendages subjected to high-frequency mechanical impact treatment or local grinding of the weld toe, a slope of 5 is adopted.

[0057] Regarding the application of the fatigue curve obtained by the method for obtaining the fatigue curve of the fillet welded joint of high-strength steel rectangular non-load-bearing appendages,

[0058] First, determine the fatigue strength characteristic values of the fillet welded joint of high-strength steel rectangular non-load-bearing appendages according to the fitted curve. Then, in combination with the fatigue strength characteristic values of the non-load-bearing fillet joint under the existing structural member fatigue standard, determine the parameter setting of the lightweight material selection with high-strength steel as the material; specifically:

[0059] 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 fillet welded joint of high-strength steel rectangular non-load-bearing appendages, and determine the wall thickness reduction percentage of the high-strength steel welded structural member according to the following formula,

[0060]

[0061] In the above formula,

[0062] r: The wall thickness reduction percentage of the high-strength steel welded structural member;

[0063] FAT w : The fatigue strength characteristic value of the fillet joint of high-strength steel rectangular non-load-bearing appendages, unit: MPa.

[0064] A method for obtaining the fatigue curve of the fillet welded joint of high-strength steel rectangular non-load-bearing appendages and its application based on this invention are based on the fillet welded joint of high-strength steel rectangular non-load-bearing appendages that can realize the 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, and establish the corresponding curve standard for the popularization and use of this characteristic welded structure and the setting basis for product thinning during specific use. Description of the Drawings

[0065] Figure 1 It is the step sequence diagram for obtaining the data used for fitting in the present invention;

[0066] Figure 2 The front welding view of the bottom plate and the rectangular non-load-bearing accessory in the present invention;

[0067] Figure 3 is Figure 2 the top view of;

[0068] Figure 4 The schematic diagram of the secondary welding mode of the circumferential fillet welds on the sides of the accessory starting and ending arcs separately in the present invention;

[0069] Figure 5 The schematic diagram of the fillet weld leg length and transition angle of the positive load transfer fillet weld on the side of the bottom plate in the present invention;

[0070] Figure 6 The schematic diagram of the transition arc radius at the four sides of the circumferential fillet weld in the present invention;

[0071] Figure 7 The front welding view of the bottom plate and the rectangular non-load-bearing accessory in the embodiment of the present invention;

[0072] Figure 8 is Figure 7 the top view of;

[0073] Figure 9 The schematic diagram of the S-N fatigue curve in the embodiment of the present invention.

[0074] In the figure,

[0075] 1 - Bottom plate;

[0076] 2 - Rectangular non-load-bearing accessory;

[0077] 3 - Circumferential fillet weld of the accessory. Specific embodiments

[0078] Next, according to the attached drawings of the specification and the specific embodiments, the method for obtaining the fatigue curve of the fillet welded joint of the high-strength steel rectangular non-load-bearing accessory and its application in the present invention will be further specifically described.

[0079] To fully understand the technical solution, the following progressive method will be adopted: first, give a technical overview of the technical solution, and then explain the specific working process and principle.

[0080] Technical Overview Section

[0081] For the high-strength steel fillet welded joints of rectangular non-load-bearing appendages that adopt a fatigue strength mechanism characterized by local microzone cyclic plastic strain accumulation damage controlled by the yield strength of the base material through welding optimization, the original S-N curve standard is no longer applicable. This technical solution aims to provide a method for determining such a new standard curve. To complete the determination of this new standard curve, the difficulties lie in two points: 1. How to ensure that the preparation process of the loaded specimens does not cause additional impacts on the results. 2. How to obtain effective data for fitting.

[0082] Regarding the first of the above difficulties, this technical solution establishes control measures from multiple aspects, including the size setting of the specimens, the processing of the specimens, the welding treatment of the specimens, the limitation of the maximum stress value in the fatigue test, and the setting of the stress steps between adjacent stress levels. Regarding the second of the above difficulties, this technical solution takes the standard deviation as the guiding ideology, sets up a mechanism for screening effective data, and at the same time limits the number of stress levels and the number of effective data at each stress level. The corresponding steps are as follows, see Figure 1 :

[0083] S1: Setting of specimen size: The sizes of the base plate and the non-load-bearing appendage are set with the constraint that the specimen can both characterize the service behavior of the actual non-load-bearing appendage and be normally loaded by the fatigue testing machine; at the same time, by establishing a relationship constraint between the width of the base plate and the thickness of the non-load-bearing appendage, the influence of the edge effect of the small-size specimen on the non-load-bearing appendage is eliminated;

[0084] S2: Processing the specimen into a workpiece to be welded: The base plate for welding is processed by controlling the roughness of the two cross-sections parallel to the loading direction, and the non-load-bearing appendage for welding is processed by controlling the degree of fit with the base plate;

[0085] S3: Welding the specimen: The base plate and the non-load-bearing appendage are welded in a way that the starting and ending arc positions avoid the load loading direction, and at the same time, the weld quality parameters after welding meet the settings;

[0086] S4: Determine the maximum stress value of the fatigue test and set the stress steps between adjacent stress levels in sequence within the limit of the set number of cycles of the fatigue test;

[0087] S5: Guided by data uniformity, determine the number of stress levels, the number of effective data at each stress level, and set the effective data screening mechanism at each stress level respectively;

[0088] S6: Conduct a series of full high-cycle fatigue life cycle fatigue tests at different stress levels based on the fatigue testing machine, and complete the acquisition of data based on the test results combined with the above settings.

[0089] Specific Working Principle and Process Section

[0090] Method for obtaining fatigue curve of fillet welded joint of high-strength steel rectangular non-load-bearing accessory and application based thereon

[0091] The actual S-N curve of the welded joint of the high-strength steel rectangular non-load-bearing accessory is obtained through a designed specific test method, and the fatigue strength characteristic value is extracted as the design basis for the non-load-bearing accessory in the dynamic load service structural member. Based on the mechanism that the fillet joint fatigue strength of the non-load-bearing accessory can be conditionally improved with the increase of the material yield strength, the specific technical principle of this mechanism is as follows:

[0092] The accessory structure on the large dynamic load structural member mainly plays an auxiliary role during service. Generally, it is connected to the bottom plate of the overall structure in the form of a fillet weld. The fillet weld does not directly bear the normal fatigue load, but plays a role in transferring the load. The actual load-bearing is the base metal of the structural member. However, during the process of the fillet weld of the accessory transferring the load, the stress concentration caused by the geometric shape factor of the fillet weld is superimposed on the external load, increasing the stress amplitude actually borne by the overall structure. At the same time, the inevitable macroscopic and microscopic welding defects in the fillet weld of the accessory can become the initial fatigue crack source, increasing the possibility of early fatigue cracking. This series of reasons leads to the fatigue behavior of the accessory welded joint being different from that of the base metal. Through welding optimization treatment, the adverse effects of the above factors on the fatigue performance of the welded joint can be minimized or eliminated, and the fatigue failure mechanism of the welded joint can be converted into a local microzone cyclic plastic strain cumulative damage fatigue failure mechanism close to that of the uniform base metal controlled by the yield strength, so as to exert the high fatigue performance potential of the high-strength steel base metal and realize the conditional improvement of the fatigue performance of the high-strength steel welded joint.

[0093] The method for obtaining the fatigue curve and application of the welded joint of the high-strength steel rectangular non-load-bearing accessory designed based on the above fatigue mechanism is aimed at the rectangular non-load-bearing accessory in the large thick-walled dynamic load service structural member. Through the design of the fatigue evaluation specimen of the accessory welding structure, the selection of the accessory welding method and the optimization of the welding process technology, the design and control of the accessory welding quality parameters, the fatigue evaluation of the accessory welding structure at a series of stress levels and the acquisition of the S-N fatigue curve with a high survival rate, the applicability determination of the fatigue strength characteristic value based on the actual S-N curve and the formulation of the application criterion, etc., a fatigue performance evaluation method for the non-load-bearing accessory welding structure is formed, which can obtain the actual fatigue performance index after the accessory welding of the dynamic load structural member through a simple and fast test, providing an important basis and technical support for the design and application of the high-strength steel dynamic load structural member, especially for the lightweight weight reduction of the structure. The specific technical solutions are as follows:

[0094] Step 1: Design and processing of the fatigue evaluation specimen of the accessory welding structure

[0095] Figure 2 、 3The figure shows the design form of a fatigue evaluation specimen for the welded structure of a rectangular accessory located on a thick-walled base plate, which is used to simulate the accessory structure in an actual large-scale thick-walled dynamic load service structure for fatigue evaluation. This design form includes a base plate 1 (thick-walled base material), a rectangular non-load-bearing accessory 2, and a circumferential fillet weld 3 of the accessory. The design dimensions of the fatigue evaluation specimen for the welded accessory structure need to consider both the representativeness compared with the actual structure and the tonnage of commonly used dynamic load fatigue testing machines. If the specimen size is too small, it is not sufficient to reflect the service behavior of the accessory in an actual large-scale thick-walled dynamic load structure, and the evaluation results are not representative. If the specimen size is too large, the tonnage of currently commonly used dynamic load fatigue testing machines cannot achieve normal loading tests. In view of this, the design dimensions of the fatigue evaluation specimen for the welded accessory structure are as follows: the thickness T of the thick-walled base plate is 15 - 25 mm, the width W of the base plate is 70 - 105 mm, and the length L of the base plate is 380 - 500 mm. To ensure the representativeness of the fatigue grade of the rectangular accessory welded structure, the length l of the rectangular accessory is 40 - 50 mm, the thickness t is 15 - 25 mm, and the width w is 30 - 60 mm. To ensure the independence of the rectangular accessory welded structure on the thick-walled base plate and retain the welding residual stress, and to avoid the influence of the edge effect of small-sized specimens on the fatigue behavior of the accessory structure, it is also necessary to satisfy W > 4t. The weld form of the accessory is a circumferential closed fillet weld, and no bevels are opened on both the accessory and the thick-walled base plate.

[0096] The thick-walled base plate is machined to ensure that the surface roughness Ra of two cross-sections parallel to the loading direction is ≤ 3.2. At the same time, ensure that its angular deformation ɑ < 5°. The rectangular accessory can be machined or cut by thermal processing, but it is necessary to ensure the flatness of the contact surface with the thick-walled base plate, and the fitting gap is less than 1 mm. To ensure the welding quality of the circumferential fillet weld of the accessory, before welding, the relevant areas including the thick-walled base plate and the accessory need to be ground and cleaned to remove pollutants such as rust and oil stains that may cause a reduction in weld quality.

[0097] Step 2: Selection of accessory welding method and optimization of welding process technology

[0098] Considering the usage habits and convenience at the manufacturing site of dynamic load structural parts, a semi-automatic welding method with a flux-cored wire for gas shielded metal arc welding is selected to complete the welding of the circumferential closed fillet weld of the fatigue evaluation specimen for the rectangular accessory welded structure. To ensure the quality of the circumferential fillet weld of the accessory that transmits the load in the positive direction and avoid starting and stopping arcs at this position, a secondary welding mode of starting and stopping arcs laterally as shown in Figure 4 is adopted. The main technical control points are as follows:

[0099] (1) The starting and stopping arc positions are both located at the lateral center of the accessory, avoiding starting and stopping arcs in the positive direction of the transmitted load;

[0100] (2) Except for the starting arc point and the ending arc point located at the lateral center position of the accessory, continuous welding shall be ensured at other positions;

[0101] (3) Through appropriate oscillation of the welding torch, ensure that the circumferential fillet weld of the accessory spreads evenly and smoothly on the thick-walled base metal floor plate, avoiding the formation of undercut and sharp transition weld beads. At the same time, ensure that the weld beads at the four corners are evenly and continuously transitioned;

[0102] (4) By controlling the welding heat input or adopting methods such as local rapid cooling on the back side, ensure that the overall angular deformation ɑ of the thick-walled base metal floor plate after welding is less than 5°.

[0103] To successfully complete the welding of the circumferential fillet weld of the above-mentioned accessory structure, a commonly used AWS A5.29 E71T1 gas shielded flux-cored wire with a diameter of 1.2 mm is used, and the following optimized welding process parameters are applied:

[0104] Welding current I = 170 - 240 A, welding voltage U = 18 - 26 V, welding speed v = 160 - 280 mm / min, and the welding shielding gas adopts an argon-rich mixed gas of 80% Ar + 20% CO 2 . The gas flow rate f = 16 - 30 L / min. According to the actual molten pool flow characteristics and weld bead forming characteristics, perform necessary oscillation of the welding torch to ensure that the circumferential fillet weld of the accessory structure spreads evenly and avoid the formation of large-size undercut and sharp transition weld beads on the side of the thick-walled base metal floor plate.

[0105] Step 3. Design and control of the welding quality parameters of the accessory

[0106] The welding quality parameters of the circumferential fillet weld of the accessory welding structure for fatigue performance evaluation include the fillet weld leg length h of the circumferential fillet weld on the side of the thick-walled base metal floor plate for positive transfer load f , the transition angle β of the circumferential fillet weld on the side of the thick-walled base metal floor plate for positive transfer load, the corner transition arc radius R of the four sides of the circumferential fillet weld, and the undercut size of the circumferential fillet weld of the accessory on the positive transfer load side. As Figure 5 and Figure 6 shown. The limitations of the main welding quality parameters for the circumferential fillet weld of the accessory welding structure are as follows:

[0107] (1) The fillet weld leg length h of the circumferential fillet weld on the side of the thick-walled base metal floor plate for positive transfer load f = 8 - 16 mm, and h f > t / 3;

[0108] (2) The transition angle β of the circumferential fillet weld on the side of the thick-walled base metal floor plate for positive transfer load > 135°;

[0109] (3) The corner transition arc radius R of the four sides of the circumferential fillet weld > 2t / 3, and R > 10 mm;

[0110] (4) The undercut depth of the circumferential fillet weld on the load side of the forward transmission accessory is less than 0.5 mm, and the total length is less than 3 mm.

[0111] Considering that the side fillet welds on both sides of the accessory have little effect on stress concentration during service loading, and thus have little effect on the overall fatigue behavior of the welded structure of the accessory. Therefore, no special restrictions are imposed on the relevant quality parameters of the side fillet welds on both sides of the accessory.

[0112] Step 4: Fatigue evaluation of the welded structure of the accessory at a series of stress levels and obtaining the S-N fatigue curve with a high survival rate

[0113] For Figure 2 、 3 The fatigue evaluation specimens of the rectangular accessory welded structure completed in design and welding production as shown are subjected to high-cycle fatigue tests at a series of stress levels. Considering the fatigue failure characteristics of the accessory welded structure and the relative uniformity of the fatigue life under the same stress level, no less than 3 groups of stress levels are used to complete the high-cycle fatigue test evaluation, and the number of effective test data under each stress level is no less than 3. At the same time, considering the representativeness of the selected stress levels and the tonnage of the fatigue testing machine, under the fatigue test condition of stress ratio R = 0.5, the principle for selecting the maximum stress value of the fatigue test is σ max = 30% - 55% R p , where R p is the actual yield strength (MPa) of the high-strength steel base material. When conducting high-cycle fatigue tests at a series of stress levels, the stress step S σ > 15 MPa between adjacent stress levels. Here, the stress step S σ is defined as: S σ = σ n - σ n-1 , where σ n and σ n-1 respectively represent the adjacent two maximum stress values of the high-cycle fatigue test at a series of stress levels. A high-frequency resonance fatigue testing machine is used to conduct high-cycle fatigue life cycle fatigue test measurements at a series of stress levels, with a stress ratio R = 0.5, and the test stop condition is that the test frequency drops significantly due to the occurrence of fatigue cracks in the specimen or the number of cycles reaches 1 * 10 7 times. For the determination of the effectiveness of the high-cycle fatigue test results and test data at a series of stress levels, the following principles are followed:

[0114] (1) Ensure that the fatigue life of each fatigue specimen under the condition of a series of stress levels, counted by the number of fatigue cycles, is 1 * 10 5 - 1 * 10 7 times;

[0115] (2) Under the same stress level, the difference between any one fatigue life data and the mean value of three effective fatigue life data must be less than 30% of the mean value, that is: Here, N i represents any one fatigue life data at the same stress level, represents the mean value of three effective fatigue life data at the same stress level, that is: If this principle cannot be satisfied, it is necessary to analyze the fatigue test process, fracture morphology, microscopic characteristics, etc., and explain the abnormal data from the fatigue failure mechanism and key influencing factors to clarify the reason for the data abnormality. Otherwise, the data needs to be discarded and the sample needs to be replicated until this principle is satisfied;

[0116] (3) The fatigue specimen of the welded structure of the accessory must undergo fatigue failure or fracture at the weld toe or weld root of the fillet weld in the forward transfer load. If the specimen fractures at other positions, the test results are considered invalid and the sample replication test needs to be carried out again;

[0117] (4) If the specimen does not undergo fracture failure at a certain stress level, the test results are considered invalid, and it is necessary to appropriately increase the stress level and conduct the fatigue test again until the specimen undergoes fatigue failure at the weld toe or weld root of the fillet weld.

[0118] Apply the cumulative number of no less than 9 effective fatigue life data at no less than three stress levels determined to be valid above to fit the S-N fatigue curve. Use a fixed slope to fit the straight-line segment in the finite life range of the S-N curve. For the fatigue test of the welded structure of the accessory in the as-welded state, the slope m = 3 is applied. For the fatigue test of the welded structure of the accessory with high-frequency mechanical impact treatment or local grinding of the weld toe, the fatigue life can be significantly improved, and the slope m = 5 is applied. Refer to the standard methods of GB / T 24176 or ISO 12107, and translate 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 highest stress value σ of the fatigue test max is converted into the stress range Δσ. Under the condition of stress ratio R = 0.5, Δσ = 0.5σ max .

[0119] Step Five: Applicability Judgment and Application Criteria for Fatigue Strength Characteristic Values Based on the Actual S-N Curve

[0120] According to the general requirements and practices of the 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 200万 is defined as the fatigue strength characteristic value of the high-strength steel welded joint or welded structure, denoted as FAT W , that is, FAT W = Δσ 200万According to the current mainstream fatigue design standards for structural components, regardless of the material strength level, for rectangular accessory welded structures with a length in the load-transferring direction not exceeding 50 mm, the fatigue strength characteristic value is uniformly taken as FAT = 80 MPa, denoted as FAT80. In view of this, under the same service conditions of the accessory welded structure, the relative increase ratio of the fatigue strength characteristic value FAT W obtained from the actual fatigue test of the high-strength steel thick-wall base plate accessory welded structure to FAT80 is used as the basis for lightweighting and wall thickness reduction in the design and material selection of high-strength steel welded structural components. To improve the safety during the service of the structure, a safety factor of 0.9 is introduced for the increase ratio of the fatigue strength characteristic value of the accessory welded structure. That is: define the percentage reduction in the wall thickness of the high-strength steel welded structural component as r, then:

[0121]

[0122] Example

[0123] The fatigue curve acquisition method and application of this technical solution are implemented. Using a Q500MD high-strength steel base plate with a wall thickness of 20 mm, the fatigue evaluation specimen design and processing of the accessory welded structure, the selection of the accessory welding method and the optimization of the welding process technology, the design and control of the accessory welding quality parameters, the fatigue evaluation of the accessory welded structure under a series of stress levels and the acquisition of the high-survival-rate S-N fatigue curve, the applicability determination of the fatigue strength characteristic value based on the actual S-N curve and the recommendation of the application criterion are successively completed according to the specific steps described in the present invention. Finally, the material lightweighting weight reduction ratio when using the Q500MD high-strength steel base plate for the design and manufacturing of the accessory welded structure in fatigue service scenarios is obtained as follows:

[0124] Step 1: Fatigue evaluation specimen design and processing of the accessory welded structure

[0125] Figure 7 、 8 The specific design form of the fatigue evaluation specimen of the rectangular accessory welded structure located on the Q500MD high-strength steel thick-wall base plate is shown, which is used to simulate the accessory structure in the actual large-scale thick-wall dynamic load service structural component for fatigue evaluation. The design dimensions of the fatigue evaluation specimen of the accessory welded structure take into account both the representativeness compared with the actual structure and the tonnage of the commonly used dynamic load fatigue testing machine, ensuring the independence of the rectangular accessory welded structure on the thick-wall base plate and retaining the welding residual stress, and avoiding the influence of the edge effect of the small-size specimen on the fatigue behavior of the accessory structure. The accessory weld form is a circumferential closed fillet weld, and no bevel is opened on both the accessory and the thick-wall base plate.

[0126] For the thick-walled base plate of Q500MD, machining is adopted to ensure that the surface roughness Ra of two cross-sections parallel to the loading direction is ≤ 3.2. At the same time, the angular deformation ɑ < 5° is ensured. The rectangular accessory also adopts the machining method to ensure the flatness of the contact surface between the bottom of the accessory and the thick-walled base plate, and the fitting gap is less than 1 mm. To ensure the welding quality of the circumferential fillet weld of the accessory in the subsequent process, the relevant areas including the thick-walled base plate and the accessory are ground and cleaned before welding, removing contaminants such as rust and oil that may cause the reduction of weld quality, and the metal luster is shown on the welding parts.

[0127] Step 2: Selection of accessory welding method and optimization of welding process technology

[0128] Considering the usage habits and convenience at the manufacturing site of dynamic load structural parts, the circumferential closed fillet weld of the rectangular accessory welding structure fatigue evaluation specimen is welded by the semi-automatic welding with flux-cored wire of gas shielded arc welding. To ensure the quality of the circumferential fillet weld of the accessory for positive load transfer and avoid starting and stopping arcs at this position, the secondary welding mode of starting and stopping arcs laterally as shown Figure 2 is adopted. First, the left semi-circumferential fillet weld is welded, and then the right semi-circumferential fillet weld is welded. The main technical control points are as follows:

[0129] (1) The starting and stopping arc positions are both located at the lateral center of the accessory, avoiding starting and stopping arcs in the positive direction of load transfer;

[0130] (2) Except for the starting and stopping arc points located at the lateral center of the accessory, continuous welding is ensured at other positions;

[0131] (3) Through appropriate oscillation of the welding torch, it is ensured that the circumferential fillet weld of the accessory spreads evenly and smoothly on the thick-walled base plate, avoiding undercut and the formation of sharp transition weld beads on the fillet weld on the positive load transfer side. At the same time, the uniform and continuous transition of the weld beads at the four corners is ensured;

[0132] (4) By appropriately reducing the welding heat input, it is ensured that the overall angular deformation ɑ < 5° of the thick-walled base plate of Q500MD after welding.

[0133] To successfully complete the welding of the circumferential fillet weld of the above accessory structure, the flux-cored wire of gas shielded arc welding AWS A5.29 E71T1 (CHT71 of Atlantic Company) with a diameter of 1.2 mm is adopted, and the following optimized welding process parameters are applied:

[0134] Welding current I = 180 - 200 A, welding voltage U = 20 - 22 V, welding speed v = 250 - 270 mm / min, and the welding shielding gas adopts 80% Ar + 20% CO 2An argon-rich mixed gas with a gas flow rate f = 24 - 26 L / min. According to the actual molten pool flow characteristics and weld formation characteristics, necessary welding torch oscillation is carried out to ensure the uniform spreading of the circumferential fillet welds of the accessory structure, and to avoid large-sized undercut and sharp transition weld beads on the fillet welds on the bottom plate side of the Q500MD thick-walled base material.

[0135] Step 3: Design and control of welding quality parameters for accessories

[0136] The welding quality parameters for the circumferential fillet welds of the accessory welding structure for fatigue performance evaluation include the fillet weld leg length h of the positive transfer load fillet weld on the bottom plate side of the thick-walled base material f , the transition angle β of the positive transfer load fillet weld on the bottom plate side of the thick-walled base material, the transition arc radius R at the four corners of the circumferential fillet weld, and the undercut size of the circumferential fillet weld of the accessory on the positive transfer load side. As Figure 5 and Figure 6 shown. Through the optimization of welding process technology, the main welding quality parameter limits for the circumferential fillet welds of the accessory welding structure in this embodiment are as follows:

[0137] (1) The fillet weld leg length h of the positive transfer load fillet weld on the bottom plate side of the thick-walled base material f = 10 - 13 mm, and h f > t / 3;

[0138] (2) The transition angle β of the positive transfer load fillet weld on the bottom plate side of the thick-walled base material > 150°;

[0139] (3) The transition arc radius R at the four corners of the circumferential fillet weld > 2t / 3, and R > 10 mm;

[0140] (4) The undercut depth of the circumferential fillet weld of the accessory on the positive transfer load side is less than 0.5 mm, and the total length is less than 3 mm.

[0141] Considering that the side fillet welds on both sides of the accessory have a relatively small effect on stress concentration during service loading, and thus have a relatively small impact on the overall fatigue behavior of the accessory welding structure. Therefore, no special restrictions are imposed on the relevant quality parameters of the side fillet welds on both sides of the accessory.

[0142] Step 4: Fatigue evaluation of the accessory welding structure under a series of stress levels and obtaining the high-survival-rate S-N fatigue curve

[0143] For Figure 7 , 8The completed rectangular accessory welded structure fatigue evaluation specimen for design and welding fabrication is subjected to a series of high-cycle fatigue tests at different stress levels for evaluation. Considering the fatigue failure characteristics of the accessory welded structure and the relative uniformity of fatigue life under the same stress level, no less than 3 stress levels are used to complete the high-cycle fatigue test evaluation, and the number of valid test data for each stress level is no less than 3. At the same time, considering the representativeness of the selected stress levels and the tonnage of the fatigue testing machine, under the fatigue test condition with a stress ratio R = 0.5, the principle for selecting the maximum stress value of the fatigue test is σ max = 30% - 55%R p , where R p is the actual yield strength (MPa) of the high-strength steel base material. When conducting a series of high-cycle fatigue tests at different stress levels, the stress step S σ between adjacent stress levels is > 15 MPa. Here, the stress step S σ is defined as: S σ = σ n - σ n-1 , where σ n and σ n-1 represent the maximum stress values of two adjacent groups in the series of high-cycle fatigue tests at different stress levels respectively. A high-frequency resonance fatigue testing machine is used to conduct the series of high-cycle fatigue life cycle fatigue test measurements, with a stress ratio R = 0.5. The test stop condition is that the specimen generates a fatigue crack resulting in a significant decrease in the test frequency or the number of cycles reaches 1×10 7 cycles. Table 1 shows the selection of stress levels and test results for the fatigue performance evaluation test of the accessory welded structure in the embodiment. According to the series of high-cycle fatigue test results and the validity determination principle of the test data, all test results and data are determined to be valid.

[0144] Table 1 Selection of stress levels and test results for the fatigue performance evaluation test of the accessory welded structure in the as-welded state in the embodiment

[0145]

[0146] Using the cumulative no less than 9 valid fatigue life data under no less than three stress levels determined to be valid as above for the fitting of the S-N fatigue curve. The straight line segment in the finite life range of the S-N curve is fitted with a fixed slope. Referring to the standard methods of GB / T 24176 or ISO 12107, and shifting 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 max of the fatigue test is converted into the stress range Δσ. Under the condition of a stress ratio R = 0.5, Δσ = 0.5σ maxFor the fatigue test of the welded structure of as-welded accessories, the applied slope m = 3. For comparison purposes, a set of 3 specimens of the welded structure of high-frequency mechanical impact-treated accessories were used for S-N curve fitting simultaneously, with the applied slope m = 5, so as to evaluate the improvement effect of post-weld life extension treatment on the fatigue performance of the welded structure of accessories. Figure 9 The S-N fatigue curve of the welded structure of the bottom plate accessories of the thick-walled base metal of Example Q500MD is shown.

[0147] Step Five: Applicability Judgment and Application Criteria for Fatigue Strength Characteristic Values Based on the Actual S-N Curve

[0148] According to the general requirements and practices of current welded structure fatigue design, 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 or welded structures, 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 the welded structure of rectangular accessories with a length in the load transfer direction not exceeding 50 mm, the fatigue strength characteristic value is taken as FAT = 80 MPa, denoted as FAT80. In view of this, under the same service conditions of the welded structure of accessories, the relative increase ratio of the fatigue strength characteristic value FAT W of the welded structure of the bottom plate accessories of the thick-walled base metal of high-strength steel in Example Q500MD obtained in the actual fatigue test and FAT80 is used as the basis for lightweight and wall thickness reduction in the design and material selection of high-strength steel welded structures. In order to improve the safety during the service of the structure, a safety factor of 0.9 is introduced for the increase ratio of the fatigue strength characteristic value of the welded structure of accessories. That is: Define the percentage reduction in the wall thickness of high-strength steel welded structures as r, then:

[0149]

[0150] Based on Figure 9 the 97.7% survival rate S-N fatigue curve of the welded structure of the bottom plate accessories of the thick-walled base metal of Example Q500MD shown, the fatigue strength characteristic value FAT W = 85 MPa is extracted. Accordingly, r = 5.6%. This shows that for the welded structure of the bottom plate accessories of the thick-walled base metal based on Q500MD high-strength steel used in dynamic load applications, a material design scheme with a 5.6% reduction in wall thickness can be adopted, providing a guiding basis for lightweight weight reduction design in the industrial field.

Claims

1. Method for obtaining fatigue curve of fillet welded joint of high-strength steel rectangular non-load-bearing accessory, Characterized in that: For the fillet welded joint of high-strength steel rectangular non-load-bearing accessory with a fatigue strength mechanism that realizes the characterization of cumulative damage of cyclic plastic strain in local micro-regions controlled by the yield strength of the base material 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: Setting of specimen size: The sizes of the bottom plate and the non-load-bearing accessory are set with the constraint that the specimen can both characterize the service behavior of the actual non-load-bearing accessory and be normally loaded by the fatigue testing machine; at the same time, by establishing the relationship constraint between the width of the bottom plate and the thickness of the non-load-bearing accessory, the influence of the edge effect of the small-size specimen on the non-load-bearing accessory is eliminated. S2: Processing the specimen into a workpiece to be welded: The bottom plate for welding is processed by controlling the roughness of two cross-sections parallel to the loading direction, and the non-load-bearing accessory for welding is processed by controlling the fitting degree with the bottom plate. S3: Welding the specimen: The bottom plate and the non-load-bearing accessory are welded in a way that the starting and ending arc positions are avoided from the load loading direction, and at the same time, the weld quality parameters after welding meet the setting. S4: Determine the maximum stress value of the fatigue test and set the stress step between adjacent stress levels in sequence with the set number of cycles of the fatigue test as the limit. S5: 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. S6: Conduct a series of full high-cycle fatigue life cycle fatigue test measurements at different stress levels based on the 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 the fillet welded joint of the high-strength steel rectangular non-load-bearing accessory according to claim 1, Characterized in that: The size of the bottom plate in step S1 is specifically: the thickness is set by taking values within the range of [15, 25] mm, the width is set by taking values within the range of [70, 105] mm, and the length is set by taking values within the range of [380, 500] mm; The size of the non-load-bearing accessory in step S1 is specifically: the thickness is set by taking values within the range of [15, 25] mm, the width is set by taking values within the range of [30, 60] mm, and the length is set by taking values within the range of [40, 50] mm.

3. The method for obtaining the fatigue curve of the fillet welded joint of the high-strength steel rectangular non-load-bearing accessory according to claim 1, Characterized in that: The establishment of the relationship constraint between the width of the bottom plate and the thickness of the non-load-bearing accessory in step S1 is specifically: the width of the bottom plate is set to be more than four times the thickness of the non-load-bearing accessory.

4. The method for obtaining the fatigue curve of the fillet welded joint of the high-strength steel rectangular non-load-bearing accessory according to claim 1, Characterized in that: Controlling the roughness of the two cross-sections of the bottom plate for welding parallel to the loading direction in step S2 is specifically: controlling the roughness to be less than or equal to 3.

2.

5. The method for obtaining the fatigue curve of the fillet welded joint of the high-strength steel rectangular non-load-bearing accessory according to claim 1, characterized in that: In step S2, controlling the fitting degree between the bottom plate to be welded and the non-load-bearing accessory to be welded specifically means: controlling the fitting gap between the two to be less than 1 mm.

6. The method for obtaining the fatigue curve of the fillet welded joint of the high-strength steel rectangular non-load-bearing accessory according to claim 1, characterized in that: The weld quality parameters in step S3 are composed of the fillet weld leg length of the positive load transfer angle weld on the bottom plate side, the transition angle of the positive load transfer angle weld on the bottom plate side, the transition arc radius of the four-side angle of the circumferential fillet weld, and the undercut parameters of the circumferential fillet weld of the non-load-bearing accessory on the positive load transfer side.

7. The method for obtaining the fatigue curve of the fillet welded joint of the high-strength steel rectangular non-load-bearing accessory according to claim 6, characterized in that: The fillet weld leg length of the positive load transfer angle weld on the bottom plate side is controlled within the range of [8, 16] mm, and at the same time, it should also be greater than one-third of the thickness of the non-load-bearing accessory; The transition angle of the positive load transfer angle weld on the bottom plate side is controlled to be greater than 135°; The transition arc radius of the four-side angle of the circumferential fillet weld is controlled to be greater than 10 mm, and it should also be greater than two-thirds of the thickness of the non-load-bearing accessory; The undercut parameters of the circumferential fillet weld of the non-load-bearing accessory on the positive load transfer side include the undercut depth and the total undercut length. The undercut depth is controlled to be less than 0.5 mm, and the total undercut length is controlled to be less than 3 mm.

8. The method for obtaining the fatigue curve of the fillet welded joint of the high-strength steel rectangular non-load-bearing accessory according to claim 1, characterized in that: In step S3, the welding is carried out with the angular deformation of the bottom plate after welding being less than 5°.

9. The method for obtaining the fatigue curve of the fillet welded joint of the high-strength steel rectangular non-load-bearing accessory according to claim 6, characterized in that: In step S3, the welding is carried out by controlling the welding current between 170 - 240 A, controlling the welding voltage between 18 - 26 V, and controlling the welding speed between 160 - 280 mm / min.

10. The method for obtaining the fatigue curve of the fillet welded joint of the high-strength steel rectangular non-load-bearing accessory according to claim 1, characterized in that: In step S4, the determination of the maximum stress value of the fatigue test is completed, specifically: taking the stress value falling within the interval [30%R p , 55%R p for value-taking, thereby completing the determination. R in the above p : actual yield strength of high-strength steel base metal, unit: MPa.

11. The method for obtaining the fatigue curve of the fillet welded joint of the high-strength steel rectangular non-load-bearing accessory according to claim 1, characterized in that: In step S4, setting the stress step between adjacent stress levels specifically means that the stress step between adjacent stress levels is set to be greater than 15 MPa.

12. The method for obtaining the fatigue curve of the fillet welded joint of the high-strength steel rectangular non-load-bearing accessory according to claim 1, characterized in that: In step S5, the number of stress levels and the number of valid data at each stress level are both determined with the requirement of at least 3.

13. The method for obtaining the fatigue curve of the fillet welded joint of the high-strength steel rectangular non-load-bearing accessory according to claim 1, characterized in that: In step S4, setting the effective data screening mechanism at each stress level is specifically completed through the following formula: N i : Any valid data at the current stress level; The mean of the valid data at the current stress level.

14. The method for obtaining the fatigue curve of the fillet welded joint of the high-strength steel rectangular non-load-bearing accessory according to claim 1, characterized in that: 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 = 0.5σ max ; In the above, Δσ: Stress range of fatigue test, unit: MPa; σ max : The maximum stress value in the fatigue test, unit: MPa.

15. The method for obtaining the fatigue curve of the fillet welded joint of the high-strength steel rectangular non-load-bearing accessory according to claim 1, characterized in that: When fitting the fatigue curve based on the obtained data, a fixed slope is used for fitting the straight-line segment in the finite life interval, specifically: For the as-welded rectangular non-load-bearing accessory, a slope of 3 is used; For the rectangular non-load-bearing accessory subjected to high-frequency mechanical impact treatment or local grinding of the weld toe, a slope of 5 is used.

16. An application of the fatigue curve obtained based on the method according to claim 1, characterized in that: First, determine the fatigue strength characteristic value of the fillet welded joint of the high-strength steel rectangular non-load-bearing accessory according to the fitted curve, and then, in combination with the fatigue strength characteristic value of the non-load-bearing fillet joint under the existing structural member fatigue standard, determine the parameter setting of the lightweight material selection with high-strength steel as the material; 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 fillet welded joint of high-strength steel rectangular non-load-bearing appendages, and determine the wall thickness reduction percentage of high-strength steel welded structural parts according to the following formula, In the above formula, r: Percentage reduction in wall thickness of the high-strength steel welded structural member; FAT w : Fatigue strength characteristic value of the corner joint of high-strength steel rectangular non-load-bearing accessories, unit: MPa.

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