Fatigue life prediction method considering residual stress after welding and repair welding

Abstract

The invention provides a fatigue life prediction method considering residual stress after welding and repair welding, and relates to the technical field of welding. Comprising the steps that finite element software is used for calculating the transverse residual stress of a weld toe in the welding and repair welding process, and the transverse residual stress of the weld toe is extracted; performing linearization processing on the extracted transverse residual stress of the weld toe part according to the BS7910 standard so as to calculate average stress; the average stress is incorporated into calculation of a fatigue load, and an optimized stress ratio is obtained; calculating membrane stress and bending stress of the welding joint; calculating according to the membrane stress and the bending stress to obtain a structural stress and bending ratio; calculating equivalent structural stress by optimizing a dimensionless function of the stress ratio, the structural stress and the bending ratio; and predicting the fatigue life according to the equivalent structural stress to obtain a life prediction result. The problem of low accuracy of life prediction of the fatigue test piece in the prior art is solved.

Description

Technical Field

[0001] The present invention relates to the field of welding technology, and particularly to a fatigue life prediction method considering residual stresses after welding and repair welding. Background Art

[0002] In the field of performance research of materials and engineering structures, fatigue life prediction has always been an extremely crucial topic. Traditional fatigue life prediction methods mainly rely on the structural stress method for prediction. The core operation of this method is to calculate the equivalent structural stress and then predict the fatigue life of welded specimens based on the obtained results. However, it should be noted that there is a significant limitation in this prediction process, that is, the influence generated by the factor of repair welding, which is relatively common in actual engineering applications, is not taken into account. As a means of repairing welding defects and locally strengthening welded structures, repair welding has an important role in changing the local geometry of welded joints, the distribution state of residual stresses, etc., and these changes are very likely to have complex and profound effects on the fatigue life of welded specimens.

[0003] Although traditional fatigue life prediction methods consider the influence of residual stresses on life, they do not consider the stress field after repair welding and do not incorporate the influence of residual stresses into the calculation of stress ratio. Residual stresses will form a complex stress field in the welded area and its surrounding regions, greatly affecting the mechanical properties of the structure. For this reason, after actual welding and in the case of different numbers of repair welds, the test points obtained from fatigue tests are relatively scattered, which not only increases the difficulty of data analysis but also greatly reduces the accuracy of life prediction for fatigue specimens after repair welding. The inability to accurately estimate the life of fatigue specimens may lead to misjudgment of the actual serviceability of structures in engineering design and applications, thereby leaving potential safety hazards and affecting the quality and service life of the entire project. Summary of the Invention

[0004] In order to overcome the deficiencies of the prior art, the purpose of the present invention is to provide a fatigue life prediction method considering residual stresses after welding and repair welding, and the present invention solves the problem of low accuracy of life prediction for fatigue specimens in the prior art.

[0005] To achieve the above purpose, the present invention provides the following solution:

[0006] A fatigue life prediction method considering residual stresses after welding and repair welding, comprising:

[0007] Using finite element software to calculate the transverse residual stresses at the weld toes during welding and repair welding, and extracting the transverse residual stresses of the weld toes;

[0008] Linearize the transverse residual stress at the weld toe extracted according to the BS7910 standard to calculate the average stress;

[0009] Incorporate the average stress into the calculation of the fatigue load to obtain the optimized stress ratio;

[0010] Calculate the membrane stress and bending stress of the welded joint;

[0011] Calculate the structural stress and bending ratio based on the membrane stress and bending stress;

[0012] Calculate the equivalent structural stress through the dimensionless functions of the optimized stress ratio, structural stress and bending ratio;

[0013] Predict the fatigue life based on the equivalent structural stress to obtain the life prediction result.

[0014] Preferably, the calculation formula for the transverse residual stress is:

[0015] σ R = σ(x) = σ Y (A + A 1 x + A 2 x 2 + A 3 x 3 );

[0016] where A, A1, A2, and A3 are the first coefficient, second coefficient, third coefficient, and fourth coefficient of the fitting function respectively, σ Y is the yield stress, σ R is the transverse residual stress, and x is the distance from the extraction point to the upper surface.

[0017] Preferably, the calculation formula for the optimized stress ratio is:

[0018]

[0019] where σ max , σ min are the maximum structural stress and minimum structural stress of the fatigue load respectively, R is the optimized stress ratio, σ RS is the average stress at the weld toe, and σ yield is the material yield strength.

[0020] Preferably, the calculation formula for the structural stress is:

[0021] σ s = σ m + σ b ;

[0022] where σ s is the structural stress, σ mis the membrane stress, σ b is the bending stress.

[0023] Preferably, the calculation formula of the bending ratio is:

[0024]

[0025] where r is the bending ratio.

[0026] Preferably, the expression of the dimensionless function of the bending ratio is:

[0027]

[0028] where is the dimensionless function of the load bending ratio r, and m is the crack growth exponent.

[0029] Preferably, when the fatigue test load is tensile stress, the calculation formula of the equivalent structural stress is:

[0030]

[0031] where ΔS s is the equivalent structural stress, and t is the thickness of the butt joint.

[0032] Preferably, when the fatigue test load is compressive stress, the calculation formula of the equivalent structural stress can also be expressed as:

[0033]

[0034] Preferably, the calculation formula of the life prediction result is:

[0035] N = (ΔS s / C d ) -1 / h ;

[0036] where C d , h are the first test constant and the second test constant respectively, and N is the life prediction result.

[0037] The present invention discloses the following technical effects:

[0038] The present invention provides a fatigue life prediction method considering residual stresses after welding and repair welding, including: calculating the transverse residual stress at the weld toe during the welding and repair welding processes using finite element software, and extracting the transverse residual stress of the weld toe; linearizing the extracted transverse residual stress at the weld toe according to the BS7910 standard to calculate the mean stress; incorporating the mean stress into the calculation of the fatigue load to obtain an optimized stress ratio; calculating the membrane stress and bending stress of the welded joint; calculating the structural stress and bending ratio based on the membrane stress and bending stress; calculating the equivalent structural stress through a dimensionless function of the optimized stress ratio, structural stress, and bending ratio; predicting the fatigue life based on the equivalent structural stress to obtain a life prediction result. By linearizing the stress, the present invention converts the residual stress at the weld toe into the mean stress, realizes the conversion of the residual stress curve into the mean stress, and considers the influence of the residual stresses of welding and repair welding, thus achieving a more accurate prediction of the fatigue life of welded specimens. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] In order to more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required for use in the embodiments. Obviously, the drawings described below are only some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can be obtained based on these drawings.

[0040] Figure 1 It is a flowchart of a fatigue life prediction method considering residual stresses after welding and repair welding provided by an embodiment of the present invention;

[0041] Figure 2 It is a schematic diagram of a strategy for a fatigue life prediction method considering residual stresses after welding and repair welding provided by an embodiment of the present invention;

[0042] Figure 3 It is a schematic diagram of the weld toe part provided by an embodiment of the present invention;

[0043] Figure 4 It is an S-N curve diagram obtained by calculating the equivalent structural stress using the traditional method provided by an embodiment of the present invention;

[0044] Figure 5 It is an S-N curve diagram obtained by calculating the equivalent structural stress provided by an embodiment of the present invention. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0045] The following will clearly and completely describe the technical solutions in the embodiments of the present invention with reference to the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0046] To make the above objects, features, and advantages of the present invention more obvious and understandable, the present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0047] As Figure 1 shown, the present invention provides a fatigue life prediction method considering residual stress after welding and repair welding, including:

[0048] Step 100: Use finite element software to calculate the transverse residual stress at the weld toe during the welding and repair welding processes, and extract the transverse residual stress of the weld toe.

[0049] Specifically, as Figures 2 - 3 shown, calculate and simulate the welding and repair welding processes through finite element software such as MSC.Marc and ABAQUS. Since cracks are most likely to occur at the weld toe during the actual service process of welded components and only the transverse residual stress is in the same direction as the fatigue test load, the transverse residual stress at the weld toe in the finite element result file is extracted.

[0050] Step 200: Linearly process the extracted transverse residual stress at the weld toe according to the BS7910 standard to calculate the mean stress.

[0051] Specifically, fit the numerical points of the extracted transverse residual stress at the weld toe according to the BS7910 standard and linearly process the stress to obtain the mean stress.

[0052] Step 300: Incorporate the mean stress into the calculation of the fatigue load to obtain an optimized stress ratio.

[0053] Specifically, the optimized stress ratio adds the mean stress compared to the stress ratio of the fatigue load, and incorporates the factor of residual stress into the prediction of fatigue life. Calculate the mean stress, and calculate the optimized stress ratio according to the fatigue test load and the mean stress at the weld toe.

[0054] Step 400: Calculate the membrane stress and bending stress of the welded joint.

[0055] Step 500: Calculate the structural stress and bending ratio according to the membrane stress and bending stress.

[0056] Step 600: Calculate the equivalent structural stress through the dimensionless function of the optimized stress ratio, structural stress, and bending ratio.

[0057] Specifically, in the structural stress method, the influence of the welding residual stress on the fatigue specimen life prediction can be improved through the optimized stress ratio, and the equivalent structural stress is calculated.

[0058] Step 700: Predict the fatigue life based on the equivalent structural stress to obtain the life prediction result.

[0059] Furthermore, the calculation formula for the transverse residual stress is:

[0060] σ R = σ(x) = σ Y (A + A 1 x + A 2 x 2 + A 3 x 3 );

[0061] Even further, the calculation formulas for the membrane stress, bending stress, and self-equilibrating stress are as follows respectively:

[0062]

[0063] Among them, A, A1, A2, and A3 are the first coefficient, second coefficient, third coefficient, and fourth coefficient of the fitting function respectively, σ Y is the yield stress, σ R is the transverse residual stress, x is the distance from the extraction point to the upper surface, σ sb is the self-equilibrating stress, and σ R = σ m + σ b + σ sb . Stress linearization is a technique that simplifies the complex stress distribution to a linear distribution through mathematical methods. Stress linearization means decomposing σ R into the sum of the membrane stress, bending stress, and self-equilibrating stress, where the membrane stress and bending stress are constant values, and the self-equilibrating stress is a function of x. In this patent, only the membrane stress and bending stress are used, where the membrane stress is the average stress, and the sum of the membrane stress and bending stress is the structural stress.

[0064] Furthermore, the calculation formula for the optimized stress ratio is:

[0065]

[0066] Among them, σ max , σ min are the maximum structural stress and minimum structural stress of the fatigue load respectively, R is the optimized stress ratio, σ RS is the average stress at the weld toe, and σ yield is the material yield strength.

[0067] Furthermore, the calculation formula for the structural stress is:

[0068] σ s = σ m + σ b ;

[0069] where σ s is the structural stress, σ m is the membrane stress, and σ b is the bending stress.

[0070] Furthermore, the calculation formula for the bending ratio is:

[0071]

[0072] where r is the bending ratio.

[0073] Furthermore, the expression for the dimensionless function of the bending ratio is:

[0074]

[0075] where is the dimensionless function of the load bending ratio r, and m is the crack growth exponent.

[0076] Furthermore, when the nominal stress for fatigue is tensile stress, the calculation formula for the equivalent structural stress is:

[0077]

[0078] where ΔS s is the equivalent structural stress, and t is the thickness of the butt joint.

[0079] Furthermore, when the nominal stress for fatigue is compressive stress, the calculation formula for the equivalent structural stress can also be expressed as:

[0080]

[0081] Furthermore, the calculation formula for the life prediction result is:

[0082] N = (ΔS s / C d ) -1 / h ;

[0083] where C d , h are the first test constant and the second test constant respectively, and N is the life prediction result.

[0084] Specifically, Table 1 is the value table of C d , h for aluminum alloy, and Table 1 is as follows:

[0085] Table 1 Cd , h Aluminum Alloy Value Table

[0086]

[0087] Table 2 is for C d , h Steel Value Table, Table 2 is as follows:

[0088] Table 2 C d , h Steel Value Table

[0089]

[0090] Furthermore, the fatigue life S-N curve diagram calculated by the traditional method and the fatigue life curve diagram calculated by the method of the present invention are as Figures 4 - 5 shown. By comparison, it is found that due to considering the influence of residual stress, the stress state of the actual joint is reflected more truly and accurately. The more accurate equivalent structural stress is calculated, the equivalent structural stress corresponding to the life of the test points in the curve diagram increases, and a more accurate fatigue life S-N curve diagram is obtained.

[0091] In this test, due to adopting strict control of welding processes and methods, the quality of the welded joints is in a good state. When not considering the residual stress, the fatigue data points of the joints are relatively scattered, on both sides of the upper 95% line. When considering the residual stress, the data points are more concentrated, and all the data points are above the upper 95% line, showing more excellent performance.

[0092] In the actual welding operation scenario, the situation is much more complicated and various problems will be encountered. From the fluctuation of the impurity content of the welding material, to the change of the temperature and humidity of the welding environment, and then to the difference in the proficiency of the welding workers' operation techniques, etc., many factors are intertwined, resulting in uneven welding quality. At some joints with poor welding quality, defects such as pores, cracks, and incomplete penetration may occur. At this time, it is particularly crucial to consider the residual stress of the welded joint after welding. The existence of residual stress is very likely to further cause stress concentration at these defective parts. In the long run, it will seriously reduce the fatigue strength, brittle fracture resistance, and dimensional stability of the welded structure, and ultimately pose a major threat to the safety and reliability of the entire welded structure. Therefore, in the case of uneven actual welding quality, it is a very necessary link to deeply study and fully consider the residual stress of the welded joint for predicting the fatigue life of the structure.

[0093] The various embodiments in this specification are described in a progressive manner. Each embodiment focuses on the differences from other embodiments. For the same and similar parts between the embodiments, reference can be made to each other.

[0094] In this text, specific examples are used to illustrate the principle and implementation of the present invention. The description of the above embodiments is only used to help understand the method and its core idea of the present invention; at the same time, for those of ordinary skill in the art, according to the idea of the present invention, there will be changes in the specific implementation and application scope. In summary, the content of this specification should not be construed as a limitation to the present invention.

Claims

1. A fatigue life prediction method considering residual stress after welding and repair welding, characterized in that: include: Finite element software is used to calculate the transverse residual stress at the weld toe during welding and repair welding, and the transverse residual stress at the weld toe is extracted; According to BS7910 standard, the extracted transverse residual stress at the weld toe is linearized to calculate the average stress; Incorporating the average stress into the calculation of fatigue load to obtain an optimized stress ratio; Calculate membrane stress and bending stress of welded joints; Calculating the structural stress and bending ratio according to the membrane stress and bending stress; The equivalent structural stress is calculated by optimizing the dimensionless function of stress ratio, structural stress and bending ratio; The fatigue life is predicted according to the equivalent structural stress to obtain a life prediction result.

2. A fatigue life prediction method considering residual stress after welding and repair welding according to claim 1, characterized in that: The calculation formula of the transverse residual stress is: s R =σ(x)=σ Y (A+A1x+A2x 2 +A3x 3 ); Among them, A, A1, A2, A3 are the first coefficient, second coefficient, third coefficient and fourth coefficient of the fitting function respectively, σ Y is the yield stress, σ R is the transverse residual stress, and x is the distance from the extraction point to the upper surface.

3. A fatigue life prediction method considering residual stress after welding and repair welding according to claim 1, characterized in that: The calculation formula of the average stress is: s RS =s m Among them, σ RS is the average stress at the weld toe, σ m is the membrane stress at weld toe.

4. A fatigue life prediction method considering residual stress after welding and repair welding according to claim 2, characterized in that: The calculation formula of the optimized stress ratio is: Among them, σ max ,σ min are the maximum structural stress and the minimum structural stress under fatigue load, R is the optimized stress ratio, σ RS is the average stress at the weld toe, σ yield is the yield strength of the material.

5. A fatigue life prediction method considering residual stress after welding and repair welding according to claim 4, characterized in that: The calculation formula of the structural stress is: s s =s m +s b ; Among them, σ s is the structural stress, σ m is the membrane stress, σ b is the bending stress.

6. A fatigue life prediction method considering residual stress after welding and repair welding according to claim 5, characterized in that: The calculation formula of the bending ratio is: Where r is the bending ratio.

7. A fatigue life prediction method considering residual stress after welding and repair welding according to claim 6, characterized in that: The expression of the dimensionless function of the bending ratio is: in, is a dimensionless function of the load-bending ratio r, and m is the crack extension index.

8. A fatigue life prediction method considering residual stress after welding and repair welding according to claim 7, characterized in that: When the fatigue test load is tensile stress, the calculation formula of the equivalent structural stress is: Among them, ΔS s is the equivalent structural stress and t is the thickness of the butt joint.

9. A fatigue life prediction method considering residual stress after welding and repair welding according to claim 8, characterized in that: When the fatigue test load is compressive stress, the calculation formula of the equivalent structural stress can also be expressed as:

10. A fatigue life prediction method considering residual stress after welding and repair welding according to claim 9, characterized in that: The calculation formula of the life prediction result is: N=(ΔS s / C d ) -1 / h ; Among them, C d , h are the first test constant and the second test constant respectively, and N is the life prediction result.

Images