Method for evaluating fatigue behavior of welded joint

Abstract

This invention relates to a method for evaluating the fatigue behavior of welded joints, specifically for welded joints of high-strength steel structural components operating under dynamic loads. The method includes the following steps: butt welding two identical substrates to obtain a butt joint with a butt weld seam; placing a vertical plate on the front side of the butt weld seam and performing a fillet weld to obtain a fillet weld seam, resulting in a specimen with both a fillet joint and a butt joint. The welding direction of the fillet weld seam is perpendicular to the welding direction of the butt weld seam, and the center of the fillet weld seam intersects with the center of the butt weld seam; subjecting the specimen to a three-point bending load fatigue test, placing the side of the specimen with the fillet weld seam and the butt weld seam on the tension surface. When the indenter meets the maximum displacement threshold, the specimen is considered to have failed, and the corresponding number of load cycles is taken as the fatigue life of the specimen. This invention can simulate the interaction and fatigue failure behavior of butt weld seams and fillet weld seams under actual dynamic loads, providing indirect fatigue behavior evaluation for large high-strength steel structural components.

Description

Technical Field

[0001] This invention relates to a method for evaluating the fatigue behavior of welded joints, and particularly to a method for evaluating the fatigue behavior of welded joints of high-strength steel structural components in dynamic load applications. Background Technology

[0002] Welding is a key technology and major process in the on-site production of high-strength steel structural components. During the welding process, non-equilibrium solid-state phase transformation caused by non-equilibrium heating and cooling makes the welded joint a weak link in the entire structure. Various structural failures during manufacturing and service are often related to the weakening of welded joint performance. High-strength steel structural components are mostly used in dynamic load applications, such as engineering machinery, marine structures, rail vehicles, and heavy energy equipment. Fatigue failure of welded joints in high-strength steel structural components is a major form of structural failure. Therefore, it is necessary to evaluate the fatigue behavior of welded joints in high-strength steel structural components to obtain fatigue performance indicators for welded joints in dynamic load applications.

[0003] The fatigue behavior of welded joints differs from that of a homogeneous metal matrix. Macroscopic or microscopic discontinuities generated during welding often become initial fatigue crack initiation sites, especially microscopic defects at the weld toe, which can directly propagate without crack initiation. Simultaneously, residual tensile stress generated during welding, weld toe geometry, and stress concentrations arising from microscopic defects during service promote the propagation of initial fatigue cracks, accelerating the fatigue failure process, primarily manifested as a reduction in fatigue strength and fatigue life. GB / T 3075-2008, "Methods for Axial Force Control in Fatigue Testing of Metallic Materials," is a commonly used industry standard for fatigue performance evaluation and testing. However, this standard primarily targets homogeneous base metals. If welded joints are evaluated based on this standard while retaining weld reinforcement, the influence of residual welding stress on the fatigue behavior of the welded joint cannot be examined. Furthermore, the sharp transitions at the sample edges, especially at the weld toe, increase stress concentration, which differs significantly from the dynamic service characteristics of general high-strength steel structural components and cannot accurately reflect the actual fatigue behavior of welded joints in high-strength steel structural components. In particular, high-strength steel structural components currently in service under typical dynamic loads are generally designed as closed box structures or open multi-dimensional composite welded structures. Regardless of the form, they involve a combination of butt joints and corner joints (including T-joints and cross joints). During the welding process, the residual stress fields of the corner joints and butt joints are nonlinearly superimposed, creating a complex residual stress state in the welded structure, which will have a significant impact on the early fatigue service safety of high-strength steel structural components. At the same time, double-sided fillet welds are generally non-penetration welds, which, even if they do not bear principal stress, are prone to becoming initial fatigue crack initiations during fatigue service. Single-sided fillet welds are more prone to early fatigue failures such as root cracking. However, current relevant standards do not include evaluation methods for the fatigue behavior and failure of structural components in actual dynamic load applications when butt welds and fillet welds interact.

[0004] Chinese Patent 201110047097.8 discloses a method for evaluating the fatigue characteristics of the T-joint portion of a T-welded joint structure. This method can indirectly evaluate the fatigue performance of the T-joint without conducting actual tests. However, it cannot evaluate or predict the fatigue performance of butt joints that primarily bear normal stress in structural components. Chinese Patent 201510963648.3 discloses a fatigue specimen for a cross-shaped welded joint. This specimen includes an integrally formed cross-shaped base with fillet welds at right-angle corners. It is only suitable for testing and observing fillet weld cracks and is not applicable to evaluating the fatigue behavior of butt joints bearing principal stress. Chinese patent 201611076258.5 discloses a method for preparing fatigue specimens of plate-shaped welded components and its application. This method involves attaching strain gauges to the welded component and determining the minimum width of the fatigue specimen containing residual stress and the maximum width of the fatigue specimen without residual stress based on the changes in residual stress during the individual cutting of the welded structural component. While this method can reflect the influence of welding residual stress on the fatigue performance of the specimen, it is labor-intensive, involves a cumbersome residual stress monitoring procedure, and is costly. Furthermore, the fatigue specimens used in this method are not representative of typical high-strength steel structural components operating under dynamic loads in enclosed box-type structures. In summary, none of the aforementioned patents evaluate the fatigue behavior of welded joints in high-strength steel structural components operating under dynamic loads, and they cannot accurately reflect the fatigue service characteristics under conditions of interaction between butt joints and corner joints with intersecting welds. Summary of the Invention

[0005] The purpose of this invention is to provide a method for evaluating the fatigue behavior of welded joints. For high-strength steel structural components in dynamic load environments, a simple and easy-to-use sample is used as a small structural component. Combined with bending load fatigue test, it can simulate the interaction and fatigue failure behavior of butt welds and fillet welds in actual dynamic load service environments, and provide indirect fatigue behavior evaluation for large high-strength steel structural components of various strength levels.

[0006] This invention is implemented as follows:

[0007] A method for evaluating the fatigue behavior of welded joints, targeting welded joints of high-strength steel structural components operating under dynamic loads, includes the following steps:

[0008] Step 1: Use two identical substrates to weld together to obtain a butt joint with a butt weld seam;

[0009] Step 2: Set a vertical plate of the same material as the substrate on the front side of the butt weld and perform fillet welding to obtain a fillet weld, and obtain a sample with a corner joint and a butt joint. The welding direction of the fillet weld is perpendicular to the welding direction of the butt weld, and the center of the fillet weld intersects with the center of the butt weld.

[0010] Step 3: The specimen undergoes a three-point bending load fatigue test. The side of the specimen with the fillet weld and the butt weld is placed on the tension surface. When the indenter meets the maximum displacement threshold, the specimen is determined to have failed, and the corresponding number of load cycles is taken as the fatigue life of the specimen.

[0011] In step one, the sample size satisfies the following relationship:

[0012] W = (10 - σ) Y / 345)×B

[0013] L=(3.5+σ Y / 345)×W

[0014] In the formula, W is the sample width, L is the sample length, B is the substrate thickness, and σ Y It represents the yield strength of the substrate metal.

[0015] In step one, the bevel of the butt welding is a single-sided bevel or a double-sided bevel.

[0016] The bevel can be a V-shaped bevel, a Y-shaped bevel, or a U-shaped bevel.

[0017] In step one, the welding process method for butt welding includes manual shielded metal arc welding, gas metal arc welding, tungsten inert gas welding, submerged arc welding, plasma arc welding, or laser welding.

[0018] In step two, the fillet weld joint is either a double-sided fillet weld joint or a single-sided fillet weld joint.

[0019] In step two, the welding process for the fillet weld is the same as that for the butt weld.

[0020] In step three, the maximum bending stress at the center of the tensioned surface and the moment of inertia corrected by the corner joint satisfy the following relationship:

[0021]

[0022]

[0023]

[0024] S = (6~7) × B

[0025] In the formula, σ max M is the maximum bending stress at the center of the tension surface. max denoted as , where is the maximum bending moment at the center of the tensioned surface, B is the substrate thickness, I is the moment of inertia at the center of the tensioned surface, F is the indenter loading force, S is the span of the three-point bending specimen, W is the specimen width, and h is the height of the corner joint plate.

[0026] In step three, the load is controlled by a sine wave.

[0027] Step two also includes performing a macroscopic surface inspection and manual ultrasonic non-destructive testing on the sample, wherein the machining roughness R on both ends of the sample is... a Not exceeding 12.5.

[0028] The present invention provides a method for evaluating the fatigue behavior of welded joints. First, a small structural component is obtained by optimizing the design of the sample size to simulate the application scenarios of weld intersections of butt joints and corner joints, as well as areas with high fatigue failure sensitivity. The fatigue behavior of the small structural component is evaluated through a three-point bending load test. This method can accurately reflect the influence of residual stress state, loading mode, fatigue failure initiation location, and fatigue life of the large structural component on the fatigue life of the welded joint during actual service. It can be used to indirectly evaluate the fatigue behavior of welded joints of high-strength steel structural components actually in dynamic load conditions, and has good applicability.

[0029] Secondly, this invention eliminates the need for fatigue behavior evaluation of large structural components. The samples used are easy to implement and use, and the cost is low. It can quickly evaluate the fatigue behavior of welded joints of high-strength steel structural components of various strength levels using different welding processes, which helps to provide technical guidance for the safe service of structural components. The evaluation results obtained by the method are highly reliable, while improving efficiency and reducing costs.

[0030] Compared with the prior art, the present invention has the following advantages: the indirect evaluation results are close to the actual service fatigue behavior of high-strength steel structural components, and have good usability, universal applicability and representativeness. It can be implemented simply and quickly, with high efficiency and low cost. Attached Figure Description

[0031] Figure 1 This is a schematic diagram illustrating the steps of the fatigue behavior evaluation method for welded joints of the present invention;

[0032] Figure 2 This is a top view of the sample of the present invention having a double-sided weld joint;

[0033] Figure 3 for Figure 2 A schematic diagram of the side view structure;

[0034] Figure 4 This is a top view of the specimen of the present invention having a single-sided weld joint;

[0035] Figure 5 for Figure 4 A schematic diagram of the side view structure;

[0036] Figure 6This is a schematic diagram of the loading method for the specimen of the present invention in a three-point bending load fatigue test.

[0037] In the diagram, 1 is the base plate, 2 is the vertical plate, 3 is the butt weld, and 4 is the corner weld. Detailed Implementation

[0038] The present invention will be further described below with reference to the accompanying drawings and specific embodiments.

[0039] A fatigue behavior evaluation method for welded joints, specifically for welded joints of high-strength steel structural components operating under dynamic loads, see [reference]. Figure 1 The method includes the following steps:

[0040] Step 1: Two identical substrates 1 are butt-welded to obtain a butt joint with a butt weld 3. Depending on the actual requirements, the butt weld bevel can be single-sided or double-sided, and the bevel form includes, but is not limited to, V-groove, Y-groove, or U-groove. Furthermore, representative welding processes should be selected based on the specific industrial application scenario. Butt welding processes include manual shielded metal arc welding (SMAW), gas metal arc welding (GMAW), tungsten inert gas welding (TIG), submerged arc welding (SAW), plasma arc welding (PAW), or laser welding (LW), to ensure the surface and internal welding quality of the joint.

[0041] Step two: A vertical plate 2 is placed on the front side of the butt weld and fillet welded to obtain a fillet weld 4, resulting in a sample with both a corner joint and a butt joint. The welding direction of the fillet weld is perpendicular to the welding direction of the butt weld, and the center of the fillet weld intersects with that of the butt weld. The material of the vertical plate 2 is the same as that of the base plate 1.

[0042] Considering the correlation between the residual stress state in welded structures and the length, width, and substrate thickness of the specimen, finite element simulation calculations were used to compare and determine the residual stress state of intersecting welds in typical high-strength steel structural components, which can more accurately reflect the actual situation. Therefore, see [reference needed]. Figures 2-5 Preferably, the sample dimensions, including the sample width, sample length, and substrate thickness (all in mm), are optimized to satisfy the following relationship:

[0043] W = (10 - σ) Y / 345)×B

[0044] L=(3.5+σ Y / 345)×W

[0045] In the formula, W is the sample width (i.e., parallel to the butt weld direction), L is the sample length (i.e., parallel to the fillet weld direction), B is the substrate thickness, and σ Y It represents the yield strength of the substrate metal.

[0046] Depending on the actual service environment and structural form required for evaluation, the corner joint can be selected as a double-sided welded corner joint or a single-sided welded corner joint, without beveling. To simulate the impact of the corner joint on the overall structure, and considering the tonnage of a typical testing machine, the height of the corner joint's vertical plate 2 is preferably h = 6~8mm.

[0047] The welding process for fillet welds is the same as that for butt welds to ensure the surface and internal welding quality of the weld joint. Before fillet welding, the intersection of the upright plate and the butt weld seam needs to be ground smooth.

[0048] In addition, after obtaining the samples, a macroscopic surface inspection and manual ultrasonic non-destructive testing are performed to determine whether the welding quality meets the fatigue test requirements. Acceptance criteria for surface and internal weld defects generally vary depending on the application. Typically, they are based on... Figures 2-5 When processing and manufacturing test specimens, the smoothness of the specimen surface is considered to affect the final fatigue test results. In particular, the roughness of the cross section has an important impact on the stability of the fatigue test results. Specifically, poor cross section machining accuracy can easily cause initial fatigue crack initiation. Initial fatigue crack initiation can cause unpredictable preferential cracking during the fatigue test, resulting in an inaccurate reflection of the fatigue performance of the material and the weld joint itself. Therefore, the machining roughness Ra of the two cross sections (i.e., the two side surfaces of the specimen along the length L of the specimen) is required to not exceed 12.5. This can effectively avoid initial fatigue crack initiation caused by poor cross section machining accuracy.

[0049] Step 3, see Figure 6 The specimen was subjected to a three-point bending load fatigue test. The side of the specimen with the fillet weld and the butt weld was placed on the tension surface. When the indenter met the maximum displacement threshold, the specimen was determined to have failed, and the corresponding number of load cycles was taken as the fatigue life of the specimen.

[0050] Preferably, considering the influence of a corner joint of a certain height on the maximum bending stress (tensile stress) at the center of the tensioned surface, the maximum bending stress at the center of the tensioned surface and the moment of inertia corrected by the corner joint satisfy the following relationship:

[0051]

[0052]

[0053]

[0054] S = (6~7) × B

[0055] In the formula, σ max M is the maximum bending stress at the center of the tension surface. max, where B is the maximum bending moment at the center of the tensioned surface, I is the moment of inertia at the center of the tensioned surface, F is the indenter loading force, S is the span of the three-point bending specimen in mm, W is the specimen width, and h is the height of the corner joint plate.

[0056] Specifically, in the three-point bending load fatigue test, the bending load is controlled by a sine wave with a stress ratio of 0.1 and a loading frequency of 10Hz. The maximum displacement of the indenter during loading is used as the failure criterion. In conventional low-frequency fatigue tests, the displacement increment generally does not exceed 5%. Considering that the maximum displacement increment must exceed the conventional value to a certain extent for fatigue crack propagation failure to be considered, in this embodiment, the specimen is considered to have failed due to fatigue when the maximum displacement increment is 10%, and the corresponding number of load cycles is taken as the fatigue life of the specimen under certain load conditions.

[0057] This invention addresses the design characteristics of high-strength steel structural components operating under dynamic loads, which involve numerous butt joints and non-penetration corner joints (including T-joints and cross joints) forming intersecting welds. Considering the high sensitivity of these intersecting welds to failure during dynamic load operation in relevant industrial sectors, the invention optimizes the design of test specimens for fatigue behavior evaluation. It examines the interaction between butt joint and corner joint test welds during specific fatigue loading processes to indirectly evaluate the fatigue behavior of typical high-strength steel welded structures used in dynamic load applications. See also... Figure 2 and Figure 4 The butt weld and fillet weld intersect perpendicularly, with the weld intersection located at the center of the specimen. This design fully considers different fillet welding methods in actual applications and their impact on fatigue failure modes. It can simulate the influence of the intersection of butt weld and fillet weld on the overall residual stress state and failure mode of high-strength steel structural components during actual service.

[0058] Examples 1-6

[0059] Hot-rolled Q550E steel plates with wall thicknesses of 8mm, 12mm, and 16mm were selected. Based on the mainstream solid wire gas shielded automatic welding (GMAW) process specifications for on-site structural component manufacturing, the described method was applied to complete butt welding and fillet welding, and the weld joints were inspected for welding quality. Each wall thickness specification included both double-sided and single-sided fillet weld joints. A fixed maximum tensile stress value of 0.5 to 0.6 times the specified minimum yield strength of the Q550E steel plate was selected as the maximum tensile stress value for conditional fatigue life evaluation; here, 300MPa was chosen, i.e., σmax = 300MPa, and the stress ratio R = 0.1. Considering the discreteness of fatigue performance data, three-point bending load fatigue tests were performed on three specimens for each embodiment.

[0060] Table 1 lists the specimen dimensions and corresponding three-point bending load fatigue test parameters for Examples 1-6. Table 2 lists the three-point bending load fatigue test results for Examples 1-6.

[0061] Table 1

[0062]

[0063] Table 2

[0064]

[0065] As shown in Table 2, the fatigue life under a certain load condition gradually decreases with the increase of steel plate wall thickness, which is consistent with the actual situation in dynamic load service. Meanwhile, the consistency of the fatigue life data obtained from the three samples in each embodiment is also good.

[0066] The fatigue behavior evaluation method for welded joints of this invention is a common technology for evaluating the fatigue behavior of welded joints in high-strength steel structural components. It has good universal applicability and representativeness in various industrial fields involving dynamic load fatigue service, with a wide coverage and high promotion value.

[0067] The above are merely preferred embodiments of the present invention and are not intended to limit the scope of protection of the present invention. Therefore, any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for evaluating fatigue behavior of a welded joint of a high-strength steel structural member serving in a dynamic load environment, characterized in that: The method comprises the following steps: Step one, butt welding two same substrates to obtain a butt joint with a butt weld; Step two, setting a vertical plate with the same material as the substrate on the front of the butt weld and performing corner welding to obtain a corner weld, and obtaining a sample with a corner joint and a butt joint, the welding direction of the corner weld being perpendicular to the welding direction of the butt weld, and the corner weld intersecting the center of the butt weld; Step three, performing a three-point bending load fatigue test on the sample, placing the side of the sample with the corner weld and the butt weld on the tensile side, determining the failure of the sample when the indenter meets the maximum displacement threshold, and taking the corresponding load cycle number as the fatigue life of the sample; In the step two, the sample size satisfies the following relationship: W = (10 - σ Y / 345) x B L = (3.5 + σ Y / 345) x W where W is the width of the specimen, L is the length of the specimen, B is the substrate thickness, σ Y is the yield strength of the substrate base metal; In the step three, the maximum bending stress at the center of the tensile side and the corrected moment of inertia through the corner joint satisfy the following relationship: S = (6-7) x B where σ max is the maximum bending stress at the center of the tensile surface, M max is the maximum bending moment at the center of the tensile surface, B is the substrate thickness, I is the moment of inertia at the center of the tensile surface, F is the indenter loading force, S is the three-point bend specimen span, W is the specimen width, and j is the corner joint post height.

2. The method of evaluating the fatigue behavior of a welded joint according to claim 1, characterized in that: In the step one, the groove of the butt welding is a single-sided groove or a double-sided groove.

3. The method of evaluating the fatigue behavior of a welded joint according to claim 2, characterized in that: The form of the groove is a V-shaped groove, a Y-shaped groove or a U-shaped groove.

4. The method of evaluating the fatigue behavior of a welded joint according to claim 1, characterized in that: In the step one, the welding process of the butt welding includes manual electrode arc welding, gas shielded arc welding, tungsten inert gas welding, submerged arc welding, plasma arc welding or laser welding.

5. The method of evaluating the fatigue behavior of a welded joint according to claim 1, characterized in that: In the step two, the corner joint is a double-sided fillet joint or a single-sided fillet joint.

6. The method of evaluating the fatigue behavior of a welded joint according to claim 1, characterized in that: In the step two, the welding process of the corner welding is the same as the welding process of the butt welding.

7. The method of evaluating the fatigue behavior of a welded joint according to claim 1, characterized in that: In the step three, the load is controlled by a sine wave.

8. The method of evaluating the fatigue behavior of a welded joint according to claim 1, characterized in that: The step two further comprises surface macroscopic inspection and manual ultrasonic nondestructive flaw detection on the sample, wherein the machining roughness R a does not exceed 12.5.

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