A method for evaluating the fatigue strength of a welded joint of a metal material

Abstract

The present invention discloses a method for evaluating the fatigue strength of a welded joint of a metal material, belonging to the technical field of fatigue fracture of welded joints of metal materials. This method determines the maximum fatigue risk factor of a smooth welded joint by testing the fatigue strength of the smooth welded joint and properties such as the external stress and material micro-region strength (microhardness) suffered by the welded joint, and normalizing these performance parameters with a fatigue risk factor model. Finally, the fatigue strength of the welded joint is predicted by combining the material micro-region strength and weld toe defect stress concentration of a general welded joint. By using the present invention, the fatigue strength of a general welded joint can be simply predicted through the fatigue strength of a smooth welded joint, avoiding a large number of laborious fatigue experiments on similar welded joints, reducing the time cost and money cost, having important reference significance for engineering practical applications, and also providing a new theoretical basis and innovative idea for the evaluation and prediction of the fatigue performance of welded joints.

Description

Technical Field

[0001] The present invention relates to the technical field of fatigue fracture of welded joints of metal materials, and particularly relates to a method for evaluating the fatigue strength of welded joints of metal materials. Background Art

[0002] Welding, as an important forming process, is widely used in various key components of high-speed trains. The existence of welded joints inevitably directly affects the safety and reliability of railway vehicles. At the same time, the continuous increase in train speed has also brought great challenges to the welded joints of key components. In particular, the cyclic loads applied to the frame often lead to fatigue damage or even fatigue fracture. Therefore, the fatigue behavior and fracture mechanism of welded joints are important scientific issues.

[0003] Due to complex geometric features and microstructural evolution, the fatigue behavior and fracture mechanism of welded joints are complex and diverse. First, the welding process usually changes the local profile of metal components, which inevitably leads to local stress concentration. The study on the bending fatigue performance of 980 MPa welded ultra-high strength steel shows that the stress concentration caused by the change in weld toe geometry is the reason for fatigue fracture at the weld toe [Shiozaki T, Yamaguchi N, Tamai Y, Hiramoto J, Ogawa K, Effect of weld toe geometry on fatigue life of lap fillet welded ultra-highstrength steeljoints, Int J Fatigue 2018;116:409-20.]. Second, the thermal gradient and cooling rate during the welding process have a significant impact on the microstructure and microhardness of welded joints. At least six microstructures are observed in the heat affected zone of multi-pass welded low alloy C-Mn steel, which produce a severe strength mismatch effect and ultimately affect the structural integrity of the welded joint [Toyoda M, Significance ofprocedure / evaluation of CTOD test ofweldments International M; Document X-1192-89, Institute of Welding(IIW); 1989.]. Compared with the base metal, the fusion zone of the fiber laser welded 316 stainless steel joint has a finer microstructure and higher microhardness [Cui CY, Cui XG, Ren XD, Liu TT, Hu JD, Wang YM, Microstructure andmicrohardness of fiber laser butt welded joint of stainless steel plates, Mater Des 2013;49:761-5.]. Third, welding defects such as microcracks, undercuts and porosity are inevitably introduced during the welding process, and they are important factors affecting the fatigue behavior of welded joints because fatigue cracks are particularly likely to initiate from material defects.Fatigue cracks in the welded joints of medium-carbon steel C45 with poor weldability mainly initiate at volumetric defects such as welding pores and slag inclusions [Wang DQQ, Yao DD, Gao ZB, Wang Q, Zhang ZF, Li XW, Fatigue mechanism of medium-carbon steel welded joint: Competitive impacts of various defects, Int J Fatigue 2021; 151: 106363.]. In addition, the fatigue performance of welded joints is also affected by factors such as the deflection and misalignment of welded joints. In short, the factors affecting the fatigue performance of welded joints are diverse and complex. Therefore, a more reasonable and comprehensive model is needed to evaluate the fatigue performance and fracture mechanism of welded joints under specific working conditions.

[0004] Evaluating the fatigue performance of welded joints has always been a challenging task. In the 1960s, a method (nominal stress method) was proposed, that is, cyclic loads were applied to a specific material until the fatigue life and fatigue strength were obtained, and finally the S-N curve ( curve) was plotted. The disadvantages of this method are high cost, time-consuming, and the data obtained lack general applicability to other materials and welded joints [Schutz W, A history of fatigue, Eng Fract Mech 1996; 54: 263-300.]. Secondly, the structural stress method takes into account the influence of the geometric characteristics of the weld, and integrates the S-N curve family and fatigue grades determined by the nominal stress method for different welded joints into a single master S-N curve based on the equivalent structural stress. However, this evaluation method only considers the influence of the geometric mutation at the weld toe and ignores the stress concentration effect of the notch. Moreover, there are also subjective differences in finite element analysis, and it is impossible to comprehensively evaluate the fatigue behavior of welded joints [Xiao ZG, Yamada K, A method of determining geometric stress for fatigue strength evaluation of steel welded joints, Int J Fatigue 2004; 26: 1277-93.]. In recent years, the notch stress can be directly used to calculate the maximum notch stress affected by the weld toe or weld root to evaluate the fatigue strength of welded joints. The disadvantage of this method is that it only considers the stress concentration effect at notches such as the weld toe and ignores the non-uniform microstructure and stress gradient distribution of welded joints [Bruder T, K, Baumgartner J, Hanselka H, Evaluation of nominal and local stress based approaches for the fatigue assessment of seam welds, Int J Fatigue 2012; 34: 86 - 102.]。

[0005] Considering the defects of existing traditional evaluation methods, it is necessary to explore new evaluation methods for the fatigue performance of welded joints in order to more simply and quickly evaluate the fatigue performance of welded joints, providing a new theoretical basis and innovative ideas for the evaluation of the fatigue performance of welded joints. Summary of the Invention

[0006] Aiming at the complexity of the fatigue performance and fracture mechanism of welded joints and the defects of existing evaluation methods, the purpose of the present invention is to provide a method for evaluating the fatigue strength of welded joints of metal materials. This method combines mechanical experiments such as microhardness and fatigue, as well as the characterization of fatigue fracture surfaces and surface morphologies, and finds that the fatigue performance of general welded joints is significantly lower than that of smooth welded joints due to the existence of stress concentration at the weld toe notch and the degradation of the microstructure of welded joints. Thus, a criterion considering multiple factors is proposed to predict the fatigue strength of general welded joints through the fatigue strength of smooth welded joints. This method avoids a large number of time-consuming and laborious fatigue experiments on similar welded joints, reducing the time cost and financial cost, and providing a new theoretical basis and innovative ideas for the evaluation and prediction of the fatigue performance of welded joints.

[0007] To achieve the above object, the technical solution adopted by the present invention is as follows:

[0008] A method for evaluating the fatigue strength of welded joints of metal materials: The method includes the following steps:

[0009] (1) Carry out butt welding treatment on a certain metal material to obtain a welded joint with welding reinforcement and non-uniform microstructure and property gradient, and perform stress relief annealing treatment. Then, prepare microhardness specimens, plate-shaped welding fatigue specimens perpendicular to the welding direction, and surface morphology characterization specimens, etc.

[0010] (2) Test the surface microhardness of the welded joint on the cross-section perpendicular to the welding direction to obtain the microhardness HV distribution curve; test the high-cycle fatigue performance of the smooth welded joint after removing the reinforcement and draw the stress-life S-N curve to obtain the fatigue strength σ -1光滑接头 ; The stress concentration coefficient K t distribution curve can be obtained according to the surface geometric shape changes and defects of the welded joint.

[0011] The microhardness HV distribution curve obtained according to step (2) can be converted by formula (2) to obtain the material strength σ I distribution curve;

[0012]

[0013] The fatigue strength σ of the smooth welded joint obtained according to step (2) -1光滑接头 Substituting into formula (3) can obtain the external stress σ E光滑接头 distribution curve;

[0014] σ E光滑接头 = 2σ -1光滑接头 (3).

[0015] The stress concentration factor K distribution curve obtained according to step (2). For a smooth welded joint, the surface stress concentration factor is always equal to 1. For a welded joint with a weld toe, the surface stress concentration factor has a mutation at the weld toe and surface defects, and the average value can be calculated through surface topography characterization. t distribution curve, for a smooth welded joint, the surface stress concentration factor is always equal to 1, for a welded joint with a weld toe, the surface stress concentration factor has a mutation at the weld toe and surface defects, and the average value can be calculated through surface topography characterization.

[0016] (3) According to the fact that material fatigue is essentially the result of the competition between the external load σ E (external factor) and material properties σ I (internal factor), combined with the local stress concentration factor K on the material surface t the fatigue risk factor R can be obtained f model, as shown in formula (1);

[0017]

[0018] In formula (1), σ I is the material strength;

[0019] (4) Combining formula (1) in step (3) and the parameter curves obtained in step (2), the fatigue risk factor R f curve and the maximum fatigue risk factor R fmax , R fmax The position of is the fatigue source where fatigue fracture is most likely to occur.

[0020] According to the position characteristics of R fmax obtained in step (4) and the K t always equal to 1 on the smooth surface, it can be found that the position of the fatigue source is the position where the microhardness HV min is the lowest, then the maximum fatigue risk factor R f max光滑接头 of the smooth welded joint can be expressed as formula (4):

[0021]

[0022] In formula (4), HV min is the minimum microhardness.

[0023] (5) According to the definitions and calculations in steps (3) and (4), the location of the maximum fatigue risk factor of a general welded joint is generally at the weld toe defect. Then, the maximum fatigue risk factor R of the welded joint fmax焊接接头 can be expressed as formula (5):

[0024]

[0025] In formula (5), HV 焊趾 is the microhardness at the weld toe, and K t焊趾 is the stress concentration coefficient at the weld toe.

[0026] Let the maximum fatigue risk factor R of a general welded joint fmax焊接接头 reach the maximum fatigue risk factor R of a smooth welded joint fmax光滑接头 , that is, R fmax光滑接头 = R fmax焊接接头 ; then, by simultaneously solving formulas (4) and (5), the fatigue strength σ of the general welded joint can be obtained -1焊接接头 , as shown in formula (6);

[0027]

[0028] (6) According to the method of testing the fatigue risk factor R f (x) curve and the maximum fatigue risk factor R f max and other fatigue risk factor models of a smooth welded joint, combined with the microhardness distribution of the welded joint and the stress concentration coefficient at the weld toe, the cyclic stress when the fatigue risk factor of a general welded joint with a weld toe reaches R fmax can be predicted, that is, the fatigue strength of this welded joint.

[0029] The design mechanism and beneficial effects of the present invention are as follows:

[0030] 1. The design mechanism of the present invention: Fatigue failure refers to the fatigue crack initiation and propagation behavior of metal after cumulative damage under cyclic or alternating loads, which means that for many years, the high-cycle fatigue behavior of materials is usually described by The fatigue performance of materials is described by curves and the Basquin equation, and fitting parameters such as the fatigue strength coefficient and fatigue strength exponent are defined to evaluate the fatigue performance of materials [Cui W, A state-of-the-art review on fatigue life prediction methods for metal structures, J Mar Sci Technol 2002; 7: 43-56.]. However, these parameters for different materials and components need to be obtained through a large number of tests, which prompts researchers to develop and explore more feasible evaluation methods. To evaluate the fatigue performance of materials, it is necessary to identify the main factors that affect the fatigue performance of materials. For general homogeneous metal materials, the microstructure and the external cyclic load are the internal and external factors affecting the fatigue performance of materials. For welded joints, fatigue fractures often occur at locations with a high fatigue risk, such as complex composition segregation and microstructures, or stress concentration areas of geometric defects on the joint surface. Therefore, the fatigue performance evaluation of welded joints is a very difficult theoretical problem. Some previous studies proposed a fatigue risk factor model based on the idea of competition between the micro-mechanical properties (internal factor) and multiple external stresses (external factor) of materials, and successfully evaluated and predicted the fatigue crack initiation locations of spring steels with surface gradient microstructures [Wang DQQ, Wang Q, Zhu YK, Zhang P, Zhang ZJ, Ren CX, Li XW, Zhang ZF, Evaluating the fatigue cracking risk of surface strengthened 50CrMnMoVNb spring steel with abnormal life time distribution, Mater Sci Eng A 2018; 732: 192-204]. The basic form of the fatigue risk factor model is:

[0031]

[0032] In the above formula, R f represents the fatigue risk factor, K t represents the local stress concentration coefficient on the material surface, σ E is the external stress applied to the material, σ R is the residual stress, σ I represents the material strength of the internal micro-region of the material.

[0033] Combined with the fatigue fracture mechanism and characteristics of welded joints, this model can comprehensively consider the fatigue fracture risk of welded joints of metal materials under multiple factors such as external loads, material strength in complex micro-regions of welded joints, and surface geometric morphology, intuitively evaluate the fatigue performance of welded joints, and ultimately achieve the purpose of predicting their fatigue strength.

[0034] 2. Beneficial effects of the present invention: The present invention proposes a new method that uses mechanical properties such as the microhardness and high-cycle fatigue of welded joints, as well as the local stress concentration on the surface of welded joints, to simply predict the fatigue strength of other general welded joints with weld toes by the fatigue strength of smooth welded joints. This method establishes a quantitative relationship between the fatigue performance of welded joints and the surface geometric morphology, non-uniform microstructural organization, and complex material micro-region strength of welded joints, comprehensively evaluating the fatigue performance of welded joints under the influence of multiple factors. It avoids a large number of laborious and time-consuming fatigue experiments on similar welded joints, reduces time costs and financial costs, and provides a new theoretical basis and innovative idea for the evaluation and prediction of the fatigue performance of welded joints. Brief Description of the Drawings

[0035] Figure 1 It is the surface microhardness distribution curve and nephogram on the cross-section perpendicular to the welding direction of the welded joint of S355J2W steel.

[0036] Figure 2 It is the tensile-compressive fatigue life distribution diagram (S-N curve) of the general welded joint specimen and smooth welded joint specimen with weld toes of S355J2W steel.

[0037] Figure 3 It is the typical fish-scale pattern surface morphology diagram at the weld toe of the general joint specimen of S355J2W steel: (a) three-dimensional surface profile; (b) two-dimensional surface morphology; (c) schematic diagram of the texture curve of the typical fish-scale pattern and the approximate ellipse of the notch.

[0038] Figure 4 It is the simplified material strength curve and external stress curve of the general welded joint and smooth welded joint specimens of S355J2W steel.

[0039] Figure 5 It is the fatigue risk coefficient curve of the general welded joint and smooth welded joint specimens of S355J2W steel. Detailed Embodiment

[0040] The following describes the present invention in more detail in conjunction with embodiments. These examples are only descriptions of the best implementation modes of the present invention and do not limit the scope of the present invention in any way.

[0041] The present invention is a method for evaluating the fatigue strength of welded joints of metal materials: This method includes the following steps:

[0042] (1) Carry out butt welding treatment on a certain metal material to obtain a welded joint with weld reinforcement and non-uniform microstructure and property gradient, and then carry out stress relief annealing treatment. Then, prepare microhardness specimens, plate-shaped welding fatigue specimens perpendicular to the welding direction, surface topography characterization specimens, etc.

[0043] (2) Test the surface microhardness of the welded joint on the cross-section perpendicular to the welding direction to obtain the microhardness HV distribution curve; test the high-cycle fatigue performance of the smooth welded joint after removing the weld reinforcement and draw the S-N curve, and measure the fatigue strength σ -1光滑接头 of the smooth welded joint; the stress concentration coefficient K t distribution curve can be obtained according to the surface geometric shape change and defects of the welded joint.

[0044] The material strength σ I (x) distribution curve can be obtained by conversion through formula (2) according to the microhardness HV distribution curve obtained in step (2);

[0045] Substitute the fatigue strength σ -1光滑接头 of the smooth welded joint obtained in step (2) into formula (3) to obtain the external stress σ E distribution curve;

[0046] According to the stress concentration coefficient K t distribution curve obtained in step (2), for the smooth welded joint, the surface stress concentration coefficient is always equal to 1, and for the welded joint with weld toes, the surface stress concentration coefficient has a mutation at the weld toes and surface defects, and the average value can be calculated through surface topography characterization.

[0047] (3) According to the fact that material fatigue is essentially the result of the competition between the external load σ E (external factor) and material properties σ I (internal factor), combined with the local stress concentration coefficient K t on the material surface, the fatigue risk factor R f model formula (1) can be obtained;

[0048]

[0049] In formula (1), σ I is the material strength;

[0050] (4) Combining formula (1) in step (3) and the parameter curves obtained in step (2), the fatigue risk factor R f curve of the overall welded joint and the maximum fatigue risk factor R fmax , R fmax The position of is the fatigue source where fatigue fracture is most likely to occur.

[0051] According to step (4), R fmax Positional features and K of smooth surfaces t It is always equal to 1, and the location of the fatigue source can be found, that is, the lowest microhardness HV min The maximum fatigue risk factor of the smooth welded joint is R f max光滑接头 It can be expressed as formula (4):

[0052]

[0053] In formula (4), HV min is the minimum microhardness.

[0054] (5) According to the definitions and calculations in steps (3) and (4), the maximum fatigue risk factor of a welded joint is generally located at the weld toe defect. Therefore, the maximum fatigue risk factor of a welded joint is R fmax焊接接头 It can be expressed as formula (5):

[0055]

[0056] In formula (5), HV 焊趾 K is the microhardness at the weld toe, t焊趾 is the stress concentration factor at the weld toe.

[0057] Let the maximum fatigue risk factor R of a general welded joint be fmax焊接接头 The maximum fatigue risk factor R for smooth welded joints is achieved fmax光滑接头 , that is, R fmax光滑接头 =R fmax焊接接头 ; Combining formulas (4) and (5), we can get the fatigue strength σ of a general welded joint: -1焊接接头 , as shown in formula (6);

[0058]

[0059] (6) Based on the fatigue risk factor R of the smooth welded joint tested f Curve and maximum fatigue risk factor R fmax The fatigue risk factor model method, combined with the microhardness distribution of the welded joint and the stress concentration factor at the weld toe, can predict the fatigue risk factor of a general welded joint with a weld toe when the fatigue risk factor reaches R fmax The cyclic stress at that time is the fatigue strength of this welded joint.

[0060] Embodiment 1:

[0061] This example uses S355J2W steel as an example. The fatigue strength of a general welded joint with a weld toe is predicted by the fatigue strength of a smooth welded joint and compared with the measured fatigue strength. The specific steps are as follows:

[0062] Step 1: The base material S355J2W steel is subjected to butt welding on a plane to obtain a welded test plate with welding reinforcement, and stress relief annealing treatment is carried out; part of the welded test plate is mechanically ground and polished to remove the weld toe to obtain a smooth welded test plate. Then, microhardness specimens, plate-shaped welding fatigue specimens perpendicular to the welding direction, and surface topography characterization specimens of the general welded joint with weld toe and the smooth welded joint without weld toe are prepared respectively.

[0063] Step 2: The surface microhardness of the two welded joints on the cross-section perpendicular to the welding direction is tested to obtain the microhardness HV distribution curve, as Figure 1 shown; the high-cycle fatigue performance of the smooth welded specimen with the reinforcement removed and the welded specimen with the weld toe retained is tested, and the S-N curve is plotted, and the fatigue strength σ -1光滑接头 of the smooth welded specimen and the fatigue strength σ -1焊接接头 (measured value) of the welded specimen with weld toe are obtained, as Figure 2 shown.

[0064] Step 3: Calculate the stress concentration coefficient K t1 at the weld toe of the welded specimen with weld toe, which can be expressed by formula (7):

[0065]

[0066] Calculate the stress concentration coefficient K t2 of the weld ripple defect at the weld toe, which can be expressed by formula (8):

[0067]

[0068] The total stress concentration coefficient K t at the weld toe can be expressed as K t = K t1 K t2 . (The meanings of the parameters in formulas (7) and (8) and the average values of measurement and calculation are listed in Table 1, Figure 3 showing the notch morphology and approximate ellipse schematic diagram of the weld ripple at the weld toe characterized by a white light interferometer).

[0069] Step 4: Through the relationship the material strength σ I distribution curves of the two specimens can be obtained, as Figure 4 the black curve; combined with the stress concentration characteristics on the surface of the specimens, through the relationship σ E = 2σ -1 the K t σ E distribution curves of the two specimens can be obtained, as Figure 4 the red (smooth welded specimen) and blue (welded specimen with weld toe) curves.

[0070] Step 5: According to formula (1), the fatigue risk factors R of the two specimens can be plotted f curves, such as Figure 5 the red (smooth welded specimen) and blue (weld toe welded specimen) curves. The maximum fatigue risk factors R of the two specimens fmax are both greater than 0.7. The fatigue source of the smooth welded specimen is located at the minimum microhardness of the welded joint, and the fatigue source of the weld toe welded specimen is located at the weld toe notch. The fatigue sources of both specimens are located at the position of the maximum fatigue risk factor.

[0071] Step 6: According to formula (5), through the fatigue strength, minimum microhardness, weld toe microhardness and weld toe stress concentration of the smooth welded specimen, calculate the fatigue strength of the weld toe welded specimen, and obtain a predicted value of 105 MPa, which is about 5% larger than the measured value of 100 MPa. The comparison parameters of the measured and calculated fatigue strengths of the welded specimens are shown in Table 2. Therefore, the prediction effect of the fatigue strength of the general welded joint with a weld toe predicted by the fatigue risk factor model is in good agreement.

[0072] Table 1 Average values of relevant parameters of stress concentration factor

[0073]

[0074] Table 2 Comparison of measured and calculated fatigue strengths of general welded joint specimens

[0075]

Claims

1. A method for evaluating the fatigue strength of a welded joint of a metal material, characterized in that: The method comprises the following steps: (1) Perform butt welding on a certain metal material in a plane to obtain a welded joint with a weld reinforcement and uneven microstructure and property gradients, and then perform stress relief annealing; then, prepare a microhardness specimen, a plate-shaped welding fatigue specimen perpendicular to the welding direction, and a surface topography characterization specimen; (2) Test the surface microhardness of the welded joint on the cross-section perpendicular to the welding direction, and measure the distribution curve of the microhardness HV; test the high-cycle fatigue performance of the smooth welded joint after removing the reinforcement, and plot the stress-life S-N curve to obtain the fatigue strength σ of the smooth welded joint. -1光滑接头 ; The stress concentration factor K can be obtained according to the surface geometric shape change and defects of the welded joint. t Distribution curve; (3)Based on the fact that material fatigue is essentially the result of the competition between the external load σ E and the material property σ I , combined with the local stress concentration factor K t on the material surface, the fatigue risk factor R f model can be obtained, as shown in formula (1); In formula (1), σ I is the material strength; (4) Combine formula (1) in step (3) and each parameter curve obtained in step (2) to plot the fatigue risk factor R of the overall welded joint f curve and the maximum fatigue risk factor R fmax , R fmax The position of which is the fatigue source most prone to fatigue fracture; (5) According to the definitions and calculations in steps (3) and (4), the location of the maximum fatigue risk factor of a general welded joint with a weld toe is generally at the weld toe defect. Then, the maximum fatigue risk factor R of the welded joint can be obtained. fmax焊接接头 ; Let the maximum fatigue risk factor R fmax焊接接头 of the general welded joint reach the maximum fatigue risk factor R fmax光滑接头 of the smooth welded joint. Then, the fatigue strength σ -1焊接接头 of the general welded joint with a weld toe and the fatigue strength σ -1光滑接头 of the smooth welded joint can be obtained. (6) According to the fatigue risk factor R of the tested smooth welded joint f curve and the maximum fatigue risk factor R fmax fatigue risk factor model method, combined with the microhardness distribution of the welded joint and the stress concentration coefficient at the weld toe, can predict the cyclic stress of a general welded joint with a weld toe when the fatigue risk factor reaches R fmax That is, the fatigue strength of this welded joint.

2. The fatigue strength evaluation method for the welded joint of the metal material according to claim 1, characterized in that: The microhardness HV distribution curve obtained in step (2) can be converted through the relationship of formula (2) to obtain the material strength σ I distribution curve; 3. The method for evaluating the fatigue strength of a welded joint of a metallic material according to claim 1, characterized in that: Substitute the fatigue strength σ of the smooth welded joint obtained in step (2) -1光滑接头 into formula (3) to obtain the external stress σ E光滑接头 distribution curve; σ E光滑接头 = 2σ -1光滑接头 (3).

4. The method for evaluating the fatigue strength of a welded joint of a metallic material according to claim 1, characterized in that: The stress concentration factor K obtained in step (2) t Distribution curve. For smooth welded joints, the surface stress concentration factor is always equal to 1. For general welded joints with weld toes, the surface stress concentration factor has mutations at the weld toes and surface defects, and the average value can be calculated through surface topography characterization.

5. The method for evaluating the fatigue strength of a welded joint of a metallic material according to claim 1, characterized in that: The R obtained in step (4) fmax position feature and the K of the smooth surface t is always equal to 1, and it can be found that the position of the fatigue source is the position where the microhardness is the lowest HV min . Then, the maximum fatigue risk factor R of the smooth welded joint fmax光滑接头 can be expressed by Equation (4): In formula (4), HV min is the minimum microhardness.

6. The method for evaluating the fatigue strength of a welded joint of a metal material according to claim 5, characterized in that: The maximum fatigue risk factor R of the general welded joint with a weld toe in step (5) fmax焊接接头 can be expressed by formula (5): In formula (5), HV 焊趾 is the microhardness at the weld toe, and K t焊趾 is the stress concentration factor at the weld toe.

7. The method for evaluating the fatigue strength of a welded joint of a metallic material according to claim 6, characterized in that: The fatigue strength σ of the general welded joint with weld toes in step (5) -1焊接接头 and the fatigue strength σ of the smooth welded joint -1光滑接头 are related as shown in formula (6):

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