Method for obtaining fatigue curve of high-strength steel rectangular non-load-bearing accessory corner butt welded joint and application

Abstract

The application discloses a high-strength steel rectangular non-load-bearing accessory corner joint fatigue curve acquisition method and application. The fatigue curve acquisition method is used for the high-strength steel rectangular non-load-bearing accessory corner joint which is controlled by the local micro area cyclic plastic strain accumulation damage characterization fatigue strength mechanism through welding optimization. The method comprises the following steps: setting a sample size, preparing a sample, preparing a sample to be loaded, determining a stress maximum value, a stress step and a stress level number, limiting effective data under each stress level to form a constraint, and loading fitting data according to the constraint. The application further establishes a light weight scheme of material selection wall thickness reduction by determining characteristic values of the new curve.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of welding, and particularly relates to a high-strength steel rectangular non-load-bearing accessory corner joint welding joint fatigue curve acquisition method and application. BACKGROUND

[0002] The original welding joint fatigue S-N curve in the fatigue design standard of the welded structure part is determined based on the understanding that the fatigue strength level of the steel material welding joint is irrelevant to the material strength level, and is only related to the joint form design, welding quality level and post-welding treatment state. For example, the mainstream EN1993 standard, IIW-2259-15 standard and DNV-RP-C203 standard all give a group of S-N curves that can be used for fatigue design according to different design forms and quality levels of steel materials based on this basic understanding.

[0003] For general dynamic load service structure parts, in addition to the forward bearing butt joint or corner joint, various shaped accessories are often welded on the overall structure for the purpose of pipeline or cable arrangement fixation, operation monitoring and maintenance, decoration and the like, mainly rectangular plate-shaped accessories and cylindrical accessories with central holes. The accessory weld itself does not bear the forward fatigue load, but due to the strong stress concentration at the weld toe or weld root position in the load transmission process, the fatigue performance of the overall structure will be greatly reduced.

[0004] However, the research team of the applicant found that with the improvement of the strength level of the overall part base material, the fatigue performance of the accessory welding structure can also be improved, and the fatigue strength characteristic value specified in the standard is irrelevant to the material characteristics. This research finding makes it possible to apply high-strength steel to the design and lightweight reduction of related dynamic load structure parts or related non-load-bearing accessories. Based on this, it is necessary to provide a high-strength steel non-load-bearing accessory corner joint welding joint fatigue curve acquisition method and application based on this, so as to accurately obtain the high-strength steel butt welding joint S-N curve, standardize the characteristic value, 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-size fatigue test method for a deepwater steel catenary riser welding joint, which comprises the following steps: first step, sample preparation; second step, test preparation; third step, sample loading; fourth step, start fatigue test; fifth step, fatigue test acceptance standard; and sixth step, sample fracture analysis test. SUMMARY

[0006] To achieve the above technical purposes, the application provides a high-strength steel rectangular non-load-bearing accessory corner joint welding joint fatigue curve acquisition method and application, and the technical scheme is as follows:

[0007] Method for obtaining fatigue curve of high-strength steel rectangular non-load-bearing accessory corner joint,

[0008] The application relates to a high-strength steel rectangular non-load-bearing accessory corner joint for realizing a fatigue strength mechanism of local micro-area cyclic plastic strain accumulation damage represented by base material yield strength control through welding optimization.

[0009] The data for fitting are obtained through the following steps, and then the fatigue curve is fitted according to the obtained data:

[0010] S1: setting of sample size: the size of the bottom plate and the non-load-bearing accessory is set so that the sample can represent the service behavior of the actual non-load-bearing accessory and can be normally loaded by the fatigue testing machine; meanwhile, the relationship between the width of the bottom plate and the thickness of the non-load-bearing accessory is established to eliminate the influence of the edge effect of small-size samples on the non-load-bearing accessory;

[0011] S2: sample processing into a piece to be welded: the bottom plate to be welded is processed by controlling the roughness of two sections parallel to the loading direction, and the non-load-bearing accessory to be welded is processed by controlling the adhesion to the bottom plate;

[0012] S3: sample welding: the welding of the bottom plate and the non-load-bearing accessory is completed in a manner that the arc striking and arc collecting positions avoid the loading direction, and the weld quality parameters after welding meet the set requirements;

[0013] S4: the determination of the highest stress value of the fatigue test and the setting of the stress step between adjacent stress levels are sequentially completed within the set number of cycles of the fatigue test;

[0014] S5: the determination of the number of stress levels, the determination of the number of effective data under each stress level and the setting of the effective data screening mechanism under each stress level are respectively completed based on the principle of data uniformity;

[0015] S6: series stress level full-high-cycle fatigue life cycle fatigue test is carried out based on the fatigue testing machine, and the data are obtained based on the test results and the above settings.

[0016] Further,

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

[0018] The non-load-bearing accessory in step S1 is sized, specifically, the thickness is set to be within the interval [15, 25] mm, the width is set to be within the interval [30, 60] mm, and the length is set to be within the interval [40, 50] mm.

[0019] Further,

[0020] The relationship between the width of the base plate and the thickness of the non-load-bearing accessory in step S1 is established, specifically, the width of the base plate is set to be greater than or equal to four times the thickness of the non-load-bearing accessory.

[0021] Further,

[0022] The roughness of the base plate to be welded in two sections parallel to the loading direction in step S2 is controlled, specifically, the roughness is controlled to be less than or equal to 3.2.

[0023] Further,

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

[0025] Further,

[0026] The weld quality parameters in step S3 are composed of the base plate side forward load transfer fillet weld fillet length, the base plate side forward load transfer fillet weld transition angle, the circumferential fillet weld four-side angle transition arc radius, and the forward load transfer side non-load-bearing accessory circumferential fillet weld undercut parameter.

[0027] Further,

[0028] The base plate side forward load transfer fillet weld fillet length is controlled to be within the interval [8, 16] mm, and also greater than one-third of the thickness of the non-load-bearing accessory;

[0029] The base plate side forward load transfer fillet weld transition angle is controlled to be greater than 135°;

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

[0031] The forward load transfer side non-load-bearing accessory circumferential fillet weld undercut parameter includes 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] Further,

[0033] The welding in step S3 is performed with the post-weld base plate angle deformation being less than 5°.

[0034] Further,

[0035] The welding in step S3 is performed with the welding current controlled between 170-240 A, the welding voltage controlled between 18-26 V, and the welding speed controlled between 160-280 mm / min.

[0036] Further,

[0037] The determination of the fatigue test maximum stress value in step S4 is performed by taking the stress value falling into the interval [30% R p , 55% R p ] to complete the determination.

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

[0039] Further,

[0040] The setting of the stress step between adjacent stress levels in step S4 is performed by setting the stress step between adjacent stress levels to be greater than 15 MPa.

[0041] Further,

[0042] The number of stress levels and the number of effective data at each stress level in step S5 are both required to be determined to be at least 3.

[0043] Further,

[0044] The setting of the effective data screening mechanism at each stress level in step S4 is performed by the following formula:

[0045]

[0046] N i : any one effective data at the current stress level;

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

[0048] Further,

[0049] When the fatigue curve is obtained by fitting based on the obtained data, the fatigue test maximum stress value is converted into the fatigue cycle stress range according to the relationship Δσ = 0.5σ max ;

[0050] In the above,

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

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

[0053] Further,

[0054] In the fatigue curve fitting based on the obtained data, the fitting of the finite life interval straight line segment adopts a fixed slope, specifically:

[0055] For the welded state rectangular non-load-bearing accessory, the slope is 3;

[0056] For the rectangular non-load-bearing accessory treated by high-frequency mechanical impact or locally polished by the weld toe, the slope is 5.

[0057] Based on the fatigue curve obtained by the high-strength steel rectangular non-load-bearing accessory corner joint welding joint fatigue curve acquisition method,

[0058] First, the fatigue strength characteristic value of the high-strength steel rectangular non-load-bearing accessory corner joint welding joint is determined according to the fitted curve, and then the fatigue strength characteristic value of the non-load-bearing corner joint under the existing structure fatigue standard is combined to determine the lightweight parameter setting of the high-strength steel as the selected material; Specifically:

[0059] The stress range value corresponding to the fatigue cycle number of 2*10 6 times on the fitting curve is defined as the fatigue strength characteristic value of the high-strength steel rectangular non-load-bearing accessory corner joint welding joint, and the high-strength steel welding structure wall thickness reduction percentage is determined according to the following formula,

[0060]

[0061] In the formula,

[0062] r: high-strength steel welding structure wall thickness reduction percentage;

[0063] FAT w : fatigue strength characteristic value of high-strength steel rectangular non-load-bearing accessory corner joint, unit: MPa.

[0064] The high-strength steel rectangular non-load-bearing accessory corner joint fatigue curve acquisition method and the application based thereon of the application are based on the high-strength steel corner joint of the rectangular non-load-bearing accessory which can realize the fatigue strength mechanism of the local micro area cyclic plastic strain accumulation damage represented by the base material yield strength control through welding optimization. The establishment of the corresponding curve standard and the setting basis of product thinning during use for the popularization and use of such welding structure with this characteristic. BRIEF DESCRIPTION OF DRAWINGS

[0065] Figure 1 The step sequence diagram for obtaining data for fitting in the application;

[0066] Figure 2 Welding main view of the bottom plate and the rectangular non-load-bearing accessory in the application;

[0067] Figure 3 Welding main view of the bottom plate and the rectangular non-load-bearing accessory in the application; Figure 2 Top view of the welding main view;

[0068] Figure 4 Schematic diagram of the secondary welding mode of the accessory circumferential fillet weld side respectively starting and ending arc in the application;

[0069] Figure 5 Schematic diagram of the bottom plate side positive load transfer fillet weld angle length and transition angle in the application;

[0070] Figure 6 Schematic diagram of the circumferential fillet weld four-side angle transition arc radius in the application;

[0071] Figure 7 Welding main view of the bottom plate and the rectangular non-load-bearing accessory in the application;

[0072] Figure 8 Welding main view of the bottom plate and the rectangular non-load-bearing accessory in the application; Figure 7 Top view of the welding main view;

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

[0074] In the figure,

[0075] 1-bottom plate;

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

[0077] 3-accessory circumferential fillet weld. DETAILED DESCRIPTION

[0078] Below, according to the description of the drawings and the specific embodiment, the high-strength steel rectangular non-load-bearing accessory corner joint fatigue curve acquisition method and application of the application are further specifically described.

[0079] In order to fully understand the technical solution, the following is to make a technical summary of the technical solution, and then explain the progressive way of the specific working process and principle.

[0080] Technical Summary Section

[0081] For the rectangular non-bearing accessory high-strength steel corner joint adopting the fatigue strength mechanism characterized by the local micro-area 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, and the technical solution aims to provide a method for determining such a new standard curve. To complete the determination of such a new standard curve, the difficulties lie in the following two points: 1. How to prepare the loading sample without causing additional effects on the results. 2. How to obtain effective data for fitting.

[0082] For the first point in the above difficulties, the technical solution establishes control processes from the size setting of the sample, the processing of the sample, the welding of the sample, the limitation of the highest stress value of the fatigue test, and the setting of the stress step between adjacent stress levels. For the second point in the above difficulties, the technical solution sets an effective data screening mechanism with standard deviation as the guiding ideology, and limits the number of stress levels and the number of effective data at each stress level. The corresponding step sequence is as follows, see Figure 1 :

[0083] S1: Setting the size of the sample: the size of the bottom plate and the non-bearing accessory is set to constrain the actual service behavior of the non-bearing accessory and to be normally loaded by the fatigue testing machine; at the same time, the relationship between the width of the bottom plate and the thickness of the non-bearing accessory is established to eliminate the influence of the edge effect of the small size sample on the non-bearing accessory;

[0084] S2: Process the sample into a welded piece: process the bottom plate to be welded to control the roughness of the two sections parallel to the loading direction, and process the non-bearing accessory to be welded to control the fit with the bottom plate;

[0085] S3: Welding of the sample: weld the bottom plate and the non-bearing accessory in a way that the arc striking and arc collecting positions avoid the loading direction, and at the same time make the weld quality parameters after welding meet the set requirements;

[0086] S4: Determine the highest 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;

[0087] S5: Based on the principle of data uniformity, determine the number of stress levels, the number of effective data at each stress level, and the setting of the effective data screening mechanism at each stress level;

[0088] S6: Perform a series of stress level full high cycle fatigue life fatigue test based on the fatigue testing machine, and based on the test results and the above settings, complete the data acquisition.

[0089] Detailed Working and Procedure Section

[0090] The application discloses a fatigue curve acquisition method of a high-strength steel rectangular non-load-bearing accessory corner joint and application thereof.

[0091] The application is based on the mechanism that fatigue strength of the non-load-bearing accessory corner joint can be improved with the increase of the yield strength of the material, and the specific technical principle of the mechanism is as follows.

[0092] The accessory structure on a large dynamic load structure mainly plays an auxiliary role in the service process, is connected with a bottom plate of the whole structure in a corner joint mode, and the corner weld does not directly bear a positive fatigue load, but plays a role in load transmission, and the structure base material actually bears the load. However, in the process of load transmission of the accessory corner weld, stress concentration caused by the geometric shape factor of the corner weld is superposed with the external load, and the stress amplitude actually borne by the whole structure is increased. Meanwhile, macro and micro welding defects inevitably existing in the accessory corner weld can become initial fatigue crack sources, and the possibility of early fatigue cracking is increased. The fatigue behavior of the accessory welded joint is different from that of the base material due to the above reasons. Through welding optimization treatment, the adverse effects of the above factors on the fatigue performance of the welded joint can be maximally reduced or eliminated, the fatigue failure mechanism of the welded joint is converted into a local micro area cyclic plastic strain cumulative damage fatigue failure mechanism close to the yield strength control of the uniform base material, the fatigue performance potential of the high-strength steel base material is developed, and the fatigue performance of the high-strength steel welded joint is conditionally improved.

[0093] The fatigue curve acquisition method and application of the high-strength steel rectangular non-load-bearing accessory welded joint designed based on the fatigue mechanism are used for the rectangular non-load-bearing accessory in a large thick-walled dynamic load service structure, fatigue evaluation sample design of the accessory welded structure, accessory welding method selection and welding process technology optimization, accessory welding quality parameter design and control, series stress level accessory welded structure fatigue evaluation and high-survival-rate S-N fatigue curve acquisition, fatigue strength characteristic value applicability judgment and application criterion formulation, and the like, so that a non-load-bearing accessory welded structure fatigue performance evaluation method is formed. The fatigue performance index of the dynamic load structure after the accessory is welded can be acquired through simple and fast test, important basis and technical support are provided for high-strength steel dynamic load structure design and application, and in particular, structural lightweight reduction.

[0094] Step one, accessory welded structure fatigue evaluation sample design and processing

[0095] Figure 2 、 3The fatigue evaluation sample design form of the rectangular accessory welding structure on the thick-walled bottom plate is shown, which is used for simulating the accessory structure in the actual large thick-walled dynamic load service structure for fatigue evaluation. The design form includes the bottom plate 1 (thick-walled base metal), the rectangular non-load-bearing accessory 2, and the accessory circumferential fillet weld 3. The fatigue evaluation sample design size of the accessory welding structure needs to consider the representativeness compared with the actual structure and the commonly used dynamic load fatigue testing machine tonnage. If the sample size is too small, it is not enough to reflect the service behavior of the accessory in the actual large thick-walled dynamic load structure, and the evaluation result is not representative. If the sample size is too large, the current commonly used dynamic load fatigue testing machine tonnage cannot realize normal loading test. In view of this, the fatigue evaluation sample design size of the accessory welding structure is as follows: the thickness of the thick-walled bottom plate T = 15-25 mm, the width of the bottom plate W = 70-105 mm, and the length of the bottom plate L = 380-500 mm. In order to ensure the representativeness of the fatigue grade of the rectangular accessory welding structure, the length of the rectangular accessory l = 40-50 mm, the thickness t = 15-25 mm, and the width w = 30-60 mm. In order to ensure the independence of the rectangular accessory welding structure on the thick-walled bottom plate and retain the welding residual stress, it is necessary to avoid the influence of the edge effect of the small size sample on the fatigue behavior of the accessory structure, while meeting W > 4t. The accessory welding form is a circumferential closed fillet weld, and neither the accessory nor the thick-walled bottom plate is beveled.

[0096] The thick-walled bottom plate is machined to ensure that the roughness Ra of the two sections parallel to the loading direction is ≤3.2. At the same time, the angular deformation ɑ is ensured to be <5°. The rectangular accessory can be machined or hot-processed, but the contact surface with the thick-walled bottom plate needs to be flat and the fitting gap needs to be less than 1 mm. In order to ensure the quality of the circumferential fillet weld of the accessory, the relevant areas including the thick-walled bottom plate and the accessory need to be polished and cleaned before welding to remove contaminants such as rust and oil that may cause weld quality degradation.

[0097] Step two, selection of accessory welding method and optimization of welding process

[0098] Considering the usage habit and convenience of the dynamic load structure manufacturing site, the circumferential closed fillet weld of the rectangular accessory welding structure fatigue evaluation sample is welded by using the semi-automatic welding of flux-cored gas shielded arc welding. In order to ensure the quality of the accessory circumferential fillet weld that transmits the load in the positive direction, the two-welding mode of side starting and ending arc is adopted as shown in Figure 4 The main technical control points are as follows:

[0099] (1) The starting and ending arc positions are located at the center of the accessory side to avoid starting and ending arc in the positive direction that transmits the load;

[0100] (2) Except the arc starting point and the arc ending point located in the lateral center of the accessory, the continuity of welding is ensured in other positions;

[0101] (3) By proper welding gun oscillation, the circumferential fillet weld of the accessory is ensured to spread evenly and smoothly on the thick base plate, avoiding the formation of undercut and sharp transition welds. At the same time, the welds at the four corners are ensured to transition evenly and continuously;

[0102] (4) By controlling the welding heat input or using local rapid cooling on the back, the overall angular deformation of the thick base plate after welding is ensured to be less than 5°.

[0103] In order to successfully complete the circumferential fillet welding of the accessory structure, the 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] The welding current I is 170-240 A, the welding voltage U is 18-26 V, the welding speed v is 160-280 mm / min, the welding protective gas is 80% Ar + 20% CO2 rich argon mixed gas, and the gas flow rate f is 16-30 L / min. According to the actual molten pool flow characteristics and weld forming characteristics, necessary welding gun oscillation is carried out to ensure the uniform spreading of the circumferential fillet weld of the accessory structure, avoiding the formation of large size undercut and sharp transition welds on the thick base plate side.

[0105] Step three, accessory welding quality parameter design and control

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

[0107] (1) The thick base plate side positive load transfer fillet weld angle length h f = 8-16 mm, and h f > t / 3;

[0108] (2) The thick base plate side positive load transfer fillet weld transition angle β > 135°;

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

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

[0111] Considering that the fillet welds on both sides of the attachment have less effect on stress concentration during service loading, the influence on the overall fatigue behavior of the attachment welding structure is also small. Therefore, no special restrictions are made on the quality parameters related to the fillet welds on both sides of the attachment.

[0112] Step four, fatigue evaluation of attachment welding structure at series stress levels and obtaining of high-survival S-N fatigue curve

[0113] For the rectangular attachment welding structure fatigue evaluation sample shown in Figure 2 , 3 , the design and welding are completed, and series stress level high-cycle fatigue test evaluation is carried out. Considering the fatigue failure characteristics of the attachment welding structure and the relative uniformity of fatigue life under the same stress level, at least 3 groups of stress levels are used to complete the high-cycle fatigue test evaluation, and the effective test data under each stress level is not less than 3. At the same time, considering the representativeness of the selected stress level and the tonnage of the fatigue testing machine, under the condition of stress ratio R=0.5, the principle of selecting the highest stress value of the fatigue test is selected as σ max =30%~55%R p , here, R p is the actual yield strength (MPa) of the high-strength steel base material. When performing series stress level high-cycle fatigue tests, the stress step S σ >15MPa 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 represent the highest stress values of the adjacent two groups of series stress level high-cycle fatigue tests. High-frequency resonance fatigue testing machine is used to perform series stress level high-cycle fatigue life cycle fatigue test, stress ratio R=0.5, and the test stopping condition is that the fatigue crack of the sample causes the test frequency to drop significantly or the cycle number reaches 1*10 7 times. For the series stress level high-cycle fatigue test results and the effectiveness of the test data, the following principles are followed:

[0114] (1) Ensure that the fatigue life of each fatigue sample under the condition of series stress level is 1*10 5 ~1*10 7 times in terms of fatigue cycle number;

[0115] (2) Under the same stress level, the difference between any one fatigue life data and the average of three effective fatigue life data must be less than 30% of the average, 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 met, the abnormal data needs to be explained and described from the fatigue failure mechanism and key influencing factors through the analysis of fatigue test process, fracture morphology, microstructure, etc., to clarify the reason for the abnormal data. Otherwise, the data needs to be discarded and retested until the principle is met;

[0116] (3) The fatigue test sample of the attached part welding structure must occur fatigue failure or fracture at the position of the welding toe or root of the angular weld. If the sample is fractured at other positions, the test result is considered invalid, and the retest needs to be performed;

[0117] (4) If the sample does not occur fracture failure at a certain stress level, the test result is considered invalid, and the fatigue test needs to be performed again with an appropriate increase in the stress level until the fatigue failure occurs at the position of the welding toe or root of the angular weld.

[0118] The S-N fatigue curve is fitted by applying not less than three groups of stress levels under which not less than nine effective fatigue life data are accumulated. The fixed slope is used to fit the straight line segment of the finite life interval of the S-N curve. For the fatigue test of the welding structure of the attached part, the slope m=3 is applied. For the fatigue test of the attached part with high-frequency mechanical impact treatment or local grinding of the welding toe, the fatigue life can be significantly improved, and the slope m=5 is applied. Refer to the standard method of GB / T 24176 or ISO 12107, and shift downward by an appropriate multiple of the estimated standard deviation to ensure the high survival rate of the fitted S-N curve. Since the fatigue cycle stress range Δσ is generally used in the fatigue design of the welding structure, the highest stress value σ max of the fatigue test is converted into the stress range Δσ. Under the condition of stress ratio R=0.5, Δσ=0.5σ max .

[0119] Step five, applicability determination and application criteria of fatigue strength characteristic value based on actual S-N curve

[0120] According to the general requirements and practices of the current welding structure fatigue design, the stress range value Δσ 6 corresponding to the fatigue cycle number of 2*10 200万 of the S-N curve obtained by actual test and fitting is defined as the fatigue strength characteristic value of the high-strength steel welded joint or welding structure, denoted as FAT W , that is, FAT W = Δσ 200万According to the current mainstream structure fatigue design standard, no distinction is made between material strength levels, and for a rectangular accessory welded structure with a length of not more than 50 mm in the load transmission direction, the fatigue strength characteristic value is FAT = 80 MPa, denoted as FAT80. In view of this, under the same service condition of the accessory welded structure, the fatigue strength characteristic value FAT W The relative promotion ratio of FAT80 is used as a basis for lightweighting and wall thickness reduction in the design and material selection process of high-strength steel welded structures. In order to improve the safety of the structure during service, a safety factor of 0.9 is introduced for the fatigue strength characteristic value promotion ratio of the accessory welded structure. That is, define the wall thickness reduction percentage of the high-strength steel welded structure as r, then:

[0121]

[0122] Embodiment

[0123] According to the fatigue curve acquisition method and application of the technical solution, a Q500MD high-strength steel base plate with a wall thickness of 20 mm is used, and the accessory welded structure fatigue evaluation sample design and processing, accessory welding method selection and welding process optimization, accessory welding quality parameter design and control, series stress level accessory welded structure fatigue evaluation and high-survival S-N fatigue curve acquisition, fatigue strength characteristic value applicability determination and application criterion recommendation based on the actual S-N curve are sequentially completed according to the specific steps described in the present application. Finally, the material lightweighting and weight reduction ratio for the design and manufacture of accessory welded structures in fatigue service occasions using Q500MD high-strength steel base plates is obtained, which is as follows:

[0124] Step one, accessory welded structure fatigue evaluation sample design and processing

[0125] Figure 7 、 8 As shown in the figure, it is a specific design form of a rectangular accessory welded structure fatigue evaluation sample on a Q500MD high-strength steel thick wall bottom plate, which is used to simulate the accessory structure in actual large thick wall dynamic load service structures for fatigue evaluation. The design size of the accessory welded structure fatigue evaluation sample takes into account the representativeness compared with the actual structure and the commonly used dynamic load fatigue testing machine tonnage, ensuring the independence of the rectangular accessory welded structure on the thick wall base plate and retaining the welding residual stress, avoiding the influence of edge effect of small size sample on the fatigue behavior of the accessory structure. The accessory weld form is a circumferential closed fillet weld, and neither the accessory nor the thick wall bottom plate is beveled.

[0126] The Q500MD thick-wall base plate is machined to ensure that the roughness Ra of the two sections parallel to the loading direction is less than or equal to 3.2. At the same time, the angular deformation a is less than 5°. The rectangular accessory is also machined to ensure the flatness of the contact surface between the bottom of the accessory and the thick-wall base plate, and the fitting gap is less than 1 mm. In order to ensure the quality of the circumferential fillet weld of the subsequent accessory, the relevant areas including the thick-wall base plate and the accessory are polished before welding to remove contaminants such as rust and oil that may cause weld quality degradation, and the welding site shows a clear metallic luster.

[0127] Step two, accessory welding method selection and welding process optimization

[0128] Considering the usage habits and convenience of the dynamic load structure manufacturing site, the circumferential closed fillet weld of the rectangular accessory welded structure fatigue evaluation specimen is welded by using the semi-automatic welding of flux-cored wire with gas shielded arc welding. In order to ensure the quality of the circumferential fillet weld of the accessory for forward load transmission, and to avoid starting and stopping the arc at this position, a two-step welding mode is adopted as shown in Figure 2 . First, the left half circumferential fillet weld is welded, and then the right half circumferential fillet weld is welded. The main technical control points are as follows:

[0129] (1) The starting and stopping positions of the arc are located at the lateral center of the accessory, avoiding starting and stopping the arc in the forward direction for load transmission;

[0130] (2) In addition to the starting and stopping points located at the lateral center of the accessory, other positions ensure continuous welding;

[0131] (3) By appropriate welding gun swinging, the circumferential fillet weld of the accessory is evenly and smoothly spread on the thick-wall base plate, avoiding the formation of edge biting and sharp transition welds on the fillet weld for forward load transmission. At the same time, it ensures that the welds at the four corners are evenly and continuously transitioned;

[0132] (4) By appropriately reducing the welding heat input, the overall angular deformation a of the Q500MD thick-wall base plate after welding is less than 5°.

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

[0134] The welding current I = 180-200A, the welding voltage U = 20-22V, the welding speed v = 250-270mm / min, the welding protective gas is 80% Ar + 20% CO2 rich argon mixed gas, and the gas flow f = 24-26L / min. According to the actual molten pool flow characteristics and the weld forming characteristics, necessary welding gun swinging is carried out to ensure that the circumferential fillet weld of the accessory structure is uniformly spread, and large size undercut and sharp transition weld of the Q500MD thick base plate side fillet weld is avoided.

[0135] Step three, accessory welding quality parameter design and control

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

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

[0138] (2) The thick base plate side positive load transfer fillet weld transition angle β> 150°;

[0139] (3) The circumferential fillet weld four-side angle transition arc radius R> 2t / 3, and R> 10mm;

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

[0141] Considering that the two-side fillet welds of the accessory have less effect on stress concentration during service loading, the influence on the overall fatigue behavior of the accessory welding structure is also small. Therefore, no special limit is made to the quality parameters related to the two-side fillet welds of the accessory.

[0142] Step four, series stress level accessory welding structure fatigue evaluation and high survival rate S-N fatigue curve acquisition

[0143] For Figure 7 , 8The rectangular attachment welded structure fatigue evaluation specimen shown to be completed design and welded production, a series of stress level high cycle fatigue test evaluation. Considering the attachment welded structure fatigue failure characteristics and the relative uniformity of fatigue life under the same stress level, at least 3 groups of stress level are used to complete the high cycle fatigue test evaluation, and the effective test data under each stress level is not less than 3. At the same time, considering the representativeness of the selected stress level and the tonnage of the fatigue testing machine, under the condition of stress ratio R = 0.5, the principle of selecting the highest stress value of the fatigue test is σ max = 30% ~ 55% R p , here, R p is the actual yield strength (MPa) of the high-strength steel base material. When a series of stress level high cycle fatigue tests are carried out, the stress step S σ between adjacent stress levels is greater than 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 highest stress values of the adjacent two groups of series stress level high cycle fatigue tests. A high frequency resonance fatigue testing machine is used to carry out a series of stress level high cycle fatigue life cycle fatigue test, the stress ratio R = 0.5, and the test stopping condition is that the fatigue crack of the specimen causes the test frequency to drop significantly or the cycle number reaches 1*10 7 times. Table 1 shows the stress level selection and test results of the attachment welded structure fatigue performance evaluation test of the example. According to the series stress level high cycle fatigue test results and the effectiveness determination principle of the test data, all the test results and data are determined to be effective.

[0144] Table 1 Stress level selection and test results of the attachment welded structure fatigue performance evaluation test of the example in the as-welded state

[0145]

[0146] The above-determined effective stress level of not less than three groups is used to fit the S-N fatigue curve with cumulative effective fatigue life data of not less than 9. The fixed slope is used to fit the straight line segment of the S-N curve in the finite life interval. Referring to the standard method of GB / T 24176 or ISO 12107, and through the appropriate multiple downward translation of the estimated standard deviation, the high survival rate of the fitted S-N curve is ensured. Since the fatigue cycle stress range Δσ is generally used in the fatigue design of welded structures, the highest stress value σ max of the fatigue test is converted into the stress range Δσ when fitting the S-N curve. Under the condition of stress ratio R = 0.5, Δσ = 0.5σ maxFor the welded bracket fatigue test, the slope m = 3 is applied. For comparison, a set of 3 high frequency mechanical impact treated bracket welded structure samples are tested simultaneously, and the slope m = 5 is applied for S-N curve fitting, so as to evaluate the improvement effect of the post-welding life extension treatment on the fatigue performance of the bracket welded structure. Figure 9 The S-N fatigue curve of the Q500MD thick wall base plate bracket welded structure is shown in the embodiment.

[0147] Step five, fatigue strength characteristic value applicability determination and application criteria based on actual S-N curve

[0148] According to the general requirements and practices of the current welded structure fatigue design, the stress range value Δσ corresponding to the fatigue cycle number of 2*10 6 200万 The fatigue strength characteristic value of the high strength steel welded joint or welded structure is defined as FAT W , that is, FAT W = Δσ 200万 . According to the current mainstream structure fatigue design standard, without distinguishing the material strength level, for the rectangular bracket welded structure with a length of not more than 50mm in the load transmission direction, the fatigue strength characteristic value is FAT = 80MPa, which is denoted as FAT80. In view of this, under the same service condition of the bracket welded structure, the relative improvement ratio of the fatigue strength characteristic value FAT W of the Q500MD high strength steel thick wall base plate bracket welded structure obtained in the actual fatigue test to FAT80 is taken as the basis for lightweight and wall thickness reduction in the design of high strength steel welded structures. In order to improve the safety during the service of the structure, the safety factor of 0.9 is introduced into the fatigue strength characteristic value improvement ratio of the bracket welded structure. That is, the wall thickness reduction percentage r of the high strength steel welded structure is defined as follows:

[0149]

[0150] Based on Figure 9 The fatigue strength characteristic value FAT W = 85MPa is extracted from the 97.7% survival rate S-N fatigue curve of the Q500MD thick wall base plate bracket welded structure shown in the embodiment. Accordingly, r = 5.6%. This shows that the thick wall base plate bracket welded structure based on Q500MD high strength steel for service in dynamic load occasions can adopt a material design scheme with a wall thickness reduction of 5.6%, which provides a guidance basis for lightweight design in the industrial field.​

Claims

1. A method for obtaining fatigue curves of corner welded joints of high-strength steel rectangular non-load-bearing accessories, characterized in that: This study focuses on high-strength steel corner joints for rectangular non-load-bearing attachments, which utilize welding optimization to achieve fatigue strength characteristics based on the fatigue strength mechanism of local micro-region cyclic plastic strain accumulation damage controlled by the yield strength of the base material. The following steps are used to obtain the data for fitting, and then the fatigue curve is obtained by fitting the obtained data: S1: Setting the specimen size: The size of the base plate and the non-load-bearing attachment is set with the constraint that the specimen can both characterize the service behavior of the actual non-load-bearing attachment and be normally loaded by the fatigue testing machine; at the same time, the influence of the edge effect of small-sized specimens on the non-load-bearing attachment is eliminated by establishing the relationship between the width of the base plate and the thickness of the non-load-bearing attachment. S2: Sample processing into weldable parts: The base plate to be welded is processed by controlling the roughness of the two sections parallel to the loading direction, and the non-load-bearing accessories to be welded are processed by controlling the fit with the base plate. S3: Sample welding: Weld the base plate and non-load-bearing accessories by avoiding the load loading direction at the arc start and end positions, while ensuring that the weld quality parameters meet the set requirements. S4: Determine the highest stress value of the fatigue test and set the stress step between adjacent stress levels sequentially, within the limit of the set number of fatigue test cycles; S5: Guided by the principle of data uniformity, determine the number of stress levels, the number of valid data at each stress level, and the setting of the valid data screening mechanism at each stress level. S6: Conduct a series of full-cycle fatigue life tests at various stress levels using a fatigue testing machine. Based on the test results and the above settings, complete the data acquisition.

2. The method for obtaining the fatigue curve of the corner weld joint of a high-strength steel rectangular non-load-bearing accessory according to claim 1, characterized in that: The base plate dimensions in step S1 are 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 [70, 105] mm, and the length is set by taking values ​​within the range of [380, 500] mm. The dimensions of the non-load-bearing accessory in step S1 are 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.

3. The method for obtaining the fatigue curve of the corner weld joint of a high-strength steel rectangular non-load-bearing accessory according to claim 1, characterized in that: The constraint on the relationship between the width of the base plate and the thickness of the non-load-bearing attachments in step S1 is as follows: the width of the base plate is set to be at least four times the thickness of the non-load-bearing attachments.

4. The method for obtaining the fatigue curve of the corner weld joint of a high-strength steel rectangular non-load-bearing accessory according to claim 1, characterized in that: In step S2, the roughness of the base plate to be welded on the two sections parallel to the loading direction is controlled as follows: the roughness is controlled to be less than or equal to 3.

2.

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

6. The method for obtaining the fatigue curve of the corner weld joint of a high-strength steel rectangular non-load-bearing accessory according to claim 1, characterized in that: The weld quality parameters in step S3 consist of the weld length of the fillet weld on the bottom plate side with positive load transfer, the transition angle of the fillet weld on the bottom plate side with positive load transfer, the radius of the transition arc of 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.

7. The method for obtaining the fatigue curve of the corner weld joint of a high-strength steel rectangular non-load-bearing accessory according to claim 6, characterized in that: The length of the fillet weld for positive load transfer on the bottom plate side is controlled within the range of [8, 16] mm, and must also be greater than one-third of the thickness of the non-load-bearing accessory. The transition angle of the fillet weld for positive load transfer on the base plate side is controlled to be greater than 135°. The radius of the transition arc at the four corners of the circumferential fillet weld should be greater than 10mm and should also be greater than two-thirds of the thickness of the non-load-bearing accessory. The undercut parameters for the circumferential fillet welds of non-load-bearing accessories 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 corner weld joint of a high-strength steel rectangular non-load-bearing accessory according to claim 1, characterized in that: The welding in step S3 shall be carried out such that the angular deformation of the base plate after welding is less than 5°.

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

10. The method for obtaining the fatigue curve of the corner weld joint of a high-strength steel rectangular non-load-bearing accessory according to claim 1, characterized in that: Step S4 completes the determination of the highest stress value in the fatigue test, specifically: the stress value falls within the range [30%R]. p 55%R p The value is then selected to complete the determination. R in the above p : Actual yield strength of high-strength steel base material, unit: MPa.

11. The method for obtaining the fatigue curve of the corner weld joint of a high-strength steel rectangular non-load-bearing accessory according to claim 1, characterized in that: Step S4 involves setting the stress step between adjacent stress levels, specifically by setting the stress step between adjacent stress levels to be greater than 15 MPa.

12. The method for obtaining the fatigue curve of the corner weld joint of a 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 points under each stress level must be determined with a minimum of 3 data points.

13. The method for obtaining the fatigue curve of the corner weld joint of a high-strength steel rectangular non-load-bearing accessory according to claim 1, characterized in that: The effective data filtering mechanism for each stress level in step S4 is set using the following formula: N i Any valid data point at the current stress level; The mean of valid data at the current stress level.

14. The method for obtaining the fatigue curve of the corner weld joint of a high-strength steel rectangular non-load-bearing accessory according to claim 1, characterized in that: When fitting the fatigue curve based on the acquired data, the highest stress value of the fatigue test is set according to Δσ = 0.5σ. max The relationship is converted into the fatigue cyclic stress range; In the above, Δσ: Stress range in fatigue testing, unit: MPa; σ max : Maximum stress value in fatigue test, unit: MPa.

15. The method for obtaining the fatigue curve of the corner weld joint of a high-strength steel rectangular non-load-bearing accessory according to claim 1, characterized in that: When fitting the fatigue curve based on the acquired data, a fixed slope is used for fitting the straight line segment within the finite life range, specifically: For welded rectangular non-load-bearing attachments, a slope of 3 is used; For rectangular non-load-bearing attachments that have undergone high-frequency mechanical impact treatment or local grinding of weld toes, a slope of 5 is adopted.

16. An application method based on the fatigue curve obtained by the method as described in claim 1, characterized in that: First, based on the fitted curve, the fatigue strength characteristic value of the corner weld joint of the high-strength steel rectangular non-load-bearing accessory is determined. Then, combined with the fatigue strength characteristic value of the non-load-bearing corner joint under the existing structural fatigue standard, the parameter settings for lightweighting using high-strength steel as the material are determined; specifically: The fatigue cycle number corresponding to the fitted curve is 2×10. 6 The stress range value is defined as the fatigue strength characteristic value of the corner weld joint of a high-strength steel rectangular non-load-bearing accessory, and the wall thickness reduction percentage of the high-strength steel welded structure is determined according to the following formula. In the above formula, r: Percentage of wall thickness reduction in high-strength steel welded structural components; FAT w Fatigue strength characteristic value of corner joint of high-strength steel rectangular non-load-bearing accessory, unit: MPa.

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