Multi-stage loading uniaxial fatigue accumulated damage assessment method in corrosion environment

Abstract

The invention relates to a fatigue damage assessment method, in particular to a multi-stage loading uniaxial fatigue accumulated damage assessment method in a corrosion environment, which comprises the following steps: S1, designing a fatigue pattern according to a standard or a requirement; s2, carrying out multi-stage loading fatigue tests on the fatigue sample in a corrosion environment and a non-corrosion environment; s3, fitting parameters obtained by the multi-stage loading fatigue test in the corrosion environment to obtain a material single-tension fatigue strength coefficient sigma fc and a material single-tension fatigue strength index bc; fitting parameters obtained by the multi-stage loading fatigue test in the non-corrosive environment to obtain a material single-tension fatigue strength coefficient sigma f and a material single-tension fatigue strength index b; s4, calculating to obtain an environmental corrosion influence weight coefficient lambda i; s5, calculating to obtain a numerical value of a loading sequence influence factor mu i; s6, calculating a multi-stage loading uniaxial fatigue accumulated damage value in the corrosion environment; the factors such as the corrosion environment, the loading sequence and the load level can be comprehensively considered, and the cumulative damage can be evaluated.

Description

Technical Field

[0001] The invention relates to a fatigue damage assessment method, and more specifically to a multi-stage loading uniaxial fatigue cumulative damage assessment method under a corrosive environment. Background Art

[0002] During vehicle operation, the size and type of loads on structural parts will change due to factors such as road conditions. The periodic alternation of different types of loads constitutes multi-level fatigue loads, causing cumulative damage to the components. In addition, affected by different service environments, the structural parts of vehicles are often affected by corrosion. As the vehicle is used for a longer time, the corrosion problem of its metal structure becomes increasingly prominent, reducing the fatigue resistance of the material and thus affecting the safety of vehicle use. Therefore, it is necessary to construct a multi-level loading damage assessment method under the combined action of corrosion and fatigue, realize the prediction of fatigue life under multi-level loading, and provide support for the safe operation of the vehicle. Summary of the invention

[0003] The purpose of the present invention is to provide a method for evaluating the cumulative damage of uniaxial fatigue under multi-stage loading in a corrosive environment, which can comprehensively consider factors such as the corrosive environment, loading sequence and load level to achieve the evaluation of the cumulative damage.

[0004] The purpose of the present invention is achieved through the following technical solutions:

[0005] A method for evaluating uniaxial fatigue cumulative damage under multi-stage loading in a corrosive environment, the method comprising the following steps:

[0006] Step S1: Design fatigue pattern according to standards or requirements;

[0007] Step S2: performing a multi-stage loading fatigue test under a corrosive environment and a multi-stage loading fatigue test under a non-corrosive environment on the fatigue specimen;

[0008] Step S3: By formula The single tensile fatigue strength coefficient σ of the material is obtained by fitting the parameters obtained from the multi-stage loading fatigue test under corrosive environment fc , material single tension fatigue strength index b c ; Through the formula The single tensile fatigue strength coefficient σ of the material is obtained by fitting the parameters obtained from the multi-stage loading fatigue test under non-corrosive environment f , material single tension fatigue strength index b;

[0009] Step S4: By formula λ i =σ aeq,i / σ a,i (2) and formula Calculate the environmental corrosion influence weight coefficient λ i ;

[0010] Step S5: According to the formula The loading order influencing factor μ is calculated i The value of

[0011] Step S6: By formula Calculate the accumulated damage value of uniaxial fatigue under multi-level loading in a corrosive environment;

[0012] The M represents the loading level of the fatigue test, n i Indicates the number of cycles of the i-th level fatigue load, N f,i It represents the life when only the i-th level fatigue load acts;

[0013] The lambda i is the weight coefficient of corrosion influence of the i-th level loading environment, which is used to characterize the influence of corrosion on fatigue life;

[0014] The formula λ i =σ aeq,i / σ a,i (2), where σ a,i represents the stress amplitude corresponding to the i-th level fatigue loading under corrosive environment, σ aeq,i Indicates σ in corrosive environment a,i Stress amplitude in air environment when fatigue life under load alone is equal;

[0015] The μ i Defined as the loading sequence influence factor, it is used to consider the effect of loading sequence and load level on fatigue life;

[0016] The formula It represents the maximum axial stress amplitude when the fatigue load is loaded to the i-th level;

[0017] The corrosive environment is a liquid corrosive environment or a salt spray corrosive environment;

[0018] The formula The σ fc Indicates the single tensile fatigue strength coefficient of the material under corrosive environment, b c Represents the single tensile fatigue strength index of the material in a corrosive environment and satisfies

[0019] The formula In n is the uniaxial fatigue stress range, σ a represents the uniaxial fatigue stress amplitude, σ f is the material single tension fatigue strength coefficient, b represents the material single tension fatigue strength index;

[0020] The multi-stage loading fatigue test includes two-stage and three-stage loading, and the nonlinear damage accumulation model formulas are:

[0021]

[0022] The beneficial effects of the present invention are:

[0023] It can comprehensively consider factors such as corrosion environment, loading sequence and load level, realize the evaluation of cumulative damage, and predict fatigue life under multi-level loading, providing support for the safe operation of the vehicle. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] The present invention is further described in detail below with reference to the accompanying drawings and specific implementation methods.

[0025] Figure 1 It is the overall flow chart of the method for evaluating uniaxial fatigue cumulative damage under multi-stage loading in a corrosive environment of the present invention;

[0026] Figure 2 It is a schematic diagram of the dimensions of the multi-stage loading fatigue specimen of the present invention. DETAILED DESCRIPTION

[0027] The present invention is further described in detail below with reference to the accompanying drawings.

[0028] like Figure 1 to Figure 2 As shown, in order to achieve the technical effect of "being able to comprehensively consider factors such as the corrosion environment, loading sequence and load level to achieve the evaluation of cumulative damage", the steps and functions of a method for evaluating uniaxial fatigue cumulative damage under multi-stage loading in a corrosion environment are described in detail below;

[0029] The following takes the salt spray corrosion environment as an example and takes the 6061-T6 aluminum alloy, a common material for automobile chassis components, as the research object to illustrate the specific implementation of the method of the present invention. The process of the patent method of the present invention is as follows: Figure 1 As shown;

[0030] First, according to the ASTM E466-15 fatigue test standard, the following Figure 2 The multi-stage loading uniaxial fatigue specimen shown has its long axis direction consistent with the rolling direction of the material. All specimens are selected from the same aluminum alloy plate, and each cross section of the specimen is polished to 2000 mesh before the test. This embodiment repeats the experiment for each group of specimens under the same conditions for many times, and the average value is taken as the result;

[0031] In this embodiment, two-stage (M=2) and three-stage (M=3) uniaxial fatigue tests were carried out, the load spectrum used was a sine wave, the stress ratio was 0.1, the loading frequency was 10 Hz, and the other test parameters were shown in Table 1. For the two-stage loading test, "LH" represents the maximum stress value σ max,1 =230MPa,σ max,2=250MPa, indicating a two-level load spectrum from low stress value to high stress value, "HL" is the opposite. For three-level loading, "LMH" indicates the maximum stress value σ max,1 =230MPa,σ max,2 =250MPa,σ max,3 =270MPa, "HML" is the opposite. "A" means the test environment is air, that is, there is no corrosion, "S" means the in-situ corrosion-fatigue test is carried out, and the corrosion condition is 3.5wt.% NaCl salt spray environment. For the two-stage loading fatigue test, n1 represents the number of loading cycles set by the first-level uniaxial load, and the number of fatigue cycles experienced by the second-level loading until the specimen breaks is recorded as n2. For the three-stage loading fatigue test, n3 represents the number of fatigue cycles experienced by the third-level load until the specimen breaks;

[0032] Table 1. Specific parameters of secondary and tertiary loading fatigue tests

[0033]

[0034]

[0035] Tables 2 and 3 of this embodiment respectively give the results of the secondary and tertiary uniaxial non-corrosion fatigue tests and the in-situ corrosion-fatigue tests. f,i represents the life when only the i-th level fatigue load acts, D f,i =n i N f,i represents the damage value of the i-th level fatigue load under the M-level loading condition, It represents the total life of the specimen under M-level loading conditions. It represents the total damage value of M-level loading calculated according to the Palmgren-Miner criterion. For secondary loading, M=2, that is, i=1,2; for tertiary loading, M=3, that is, i=1,2,3.

[0036] Table 2. Secondary loading fatigue test results

[0037]

[0038] Table 3. Three-level loading fatigue test results

[0039]

[0040] The weight coefficient of corrosion influence of the i-th loading environment λ i Calculation: First, based on the relationship between normal stress amplitude and fatigue life, as shown in formula (1):

[0041] In formula (1), Δσ n is the uniaxial fatigue stress range, σa represents the uniaxial fatigue stress amplitude, σ f is the material single tension fatigue strength coefficient, b represents the material single tension fatigue strength index;

[0042] Combining the uniaxial fatigue test results of 6061-T6 aluminum alloy under multi-stage loading in Table 2 and Table 3, the parameter values ​​of formula (1) under corrosive environment and non-corrosive environment are fitted, as shown in Table 4. fc Indicates the material single tension fatigue strength coefficient, b c Indicates the material single tension fatigue strength index;

[0043] Table 4. Basquin formula parameter fitting results

[0044]

[0045] Further, according to the following formulas (2) and (3), the environmental corrosion influence weight coefficient λ is calculated: i The results are shown in Table 5. In the formula, σ a,i represents the stress amplitude corresponding to the i-th level fatigue loading under corrosive environment, σ aeq,i Indicates σ under corrosive environment a,i When the fatigue life of the load acting alone is equal, the stress amplitude in the air environment is as shown in formula (3). i Indicates the fatigue stress amplitude σ in a non-corrosive environment when the fatigue life is the same aeq,i and fatigue stress amplitude σ in corrosive environment a,i ratio;

[0046] λ i =σ aeq,i / σ a,i (2)

[0047]

[0048] Table 5. Calculation results of environmental corrosion influence weight coefficient

[0049]

[0050] Loading order influence factor μ i Calculation of μ i The calculation formula is shown in (4):

[0051]

[0052] in, It indicates the maximum axial stress amplitude when the fatigue load is applied to level i. For example, for the "HML" loading sequence, μ1=121.5 / 121.5=1, μ1=121.5 / 112.5=1.080, μ1=121.5 / 103.5=1.174.

[0053] By formula Calculate the cumulative damage value of uniaxial fatigue under multi-stage loading in a corrosive environment, where M represents the loading level of the fatigue test, n i Indicates the number of cycles of the i-th level fatigue load, N f,i represents the life when only the i-th level fatigue load acts; therefore, for the two-level and three-level loading described in this embodiment, the nonlinear damage accumulation model formulas are shown in (6) and (7), respectively:

[0054]

[0055] The model results are shown in the figure:

[0056] This embodiment selects three commonly used damage accumulation models, including Miner linear damage accumulation model, Manson model and Morrow plastic work model, for comparison with the model constructed by the present invention. For each damage accumulation model, the critical damage D CR The closer the model calculation result is to 1, the better the prediction effect is.

[0057] As a classic linear cumulative damage model, the Miner model can be used to predict life under variable amplitude fatigue loads. For multi-level load spectra, the total damage can be calculated according to formula (8):

[0058]

[0059] Manson proposed a model based on the damage curve method to calculate damage accumulation. The total damage under multi-level fatigue load spectrum is calculated by formula (9):

[0060]

[0061] Morrow proposed a plastic work interaction model by introducing the equivalent stress amplitude. The total damage under multi-level load spectrum can be calculated according to formula (10):

[0062]

[0063] In formula (10), σ a,i represents the vonMises equivalent stress during each level of loading, σ eq,maxIt is the maximum equivalent stress in the entire loading process, and d is the Morrow plastic work interaction index. According to the literature, for aluminum alloys, its value is -0.45.

[0064] In the above three commonly used models, n i and N f,i The meaning of is the same as described above. For secondary loading, M = 2, and for tertiary loading, M = 3. Table 6 lists the calculation results of three commonly used damage accumulation models and the model proposed in this paper under the multi-level load spectrum of this experiment.

[0065] Table 6. Damage accumulation calculation results of each model

[0066]

[0067] It can be seen from Table 6 that among the four damage accumulation models, the model established by the present invention has achieved the best calculation results, with the smallest average damage accumulation value, which is closest to the critical damage value of 1 among the four models. In particular, for the damage assessment under the salt spray environment, since the model introduces the environmental corrosion influence weight coefficient, the range of variation is the smallest compared with the other three models, and the mean value of the accumulated damage under the corrosive environment is also the smallest at 1.130.

[0068] The principles and implementation methods of the present invention are described in this article using specific examples. The description of the above embodiments is only used to help understand the method and core idea of ​​the present invention. At the same time, for those skilled in the art, according to the idea of ​​the present invention, there will be changes in the specific implementation methods and application scope. In summary, the content of this specification should not be understood as limiting the present invention.

Claims

1. A method for evaluating uniaxial fatigue cumulative damage under multi-stage loading in a corrosive environment, characterized in that: The method comprises the following steps: Step S1: Design fatigue pattern according to standards or requirements; Step S2: performing a multi-stage loading fatigue test under a corrosive environment and a multi-stage loading fatigue test under a non-corrosive environment on the fatigue specimen; Step S3: By formula The single tensile fatigue strength coefficient σ of the material is obtained by fitting the parameters obtained from the multi-stage loading fatigue test under corrosive environment fc , material single tension fatigue strength index b c ; Through the formula The single tensile fatigue strength coefficient σ of the material is obtained by fitting the parameters obtained from the multi-stage loading fatigue test under non-corrosive environment f , material single tension fatigue strength index b; Step S4: By formula λ i =σ aeq,i / σ a,i (2) and formula Calculate the environmental corrosion influence weight coefficient λ i ; Step S5: According to the formula Calculate the loading order influence factor μ i The value of Step S6: By formula Calculate the accumulated damage value of uniaxial fatigue under multi-level loading in a corrosive environment.

2. The method for evaluating uniaxial fatigue cumulative damage under multi-stage loading in a corrosive environment according to claim 1 is characterized in that: The M represents the loading level of the fatigue test, n i Indicates the number of cycles of the i-th level fatigue load, N f,i Represents the life when only the i-th level fatigue load acts.

3. The method for evaluating uniaxial fatigue cumulative damage under multi-stage loading in a corrosive environment according to claim 1 is characterized in that: The lambda i is the weight coefficient of corrosion influence of the i-th level loading environment, which is used to characterize the influence of corrosion on fatigue life.

4. The method for evaluating uniaxial fatigue cumulative damage under multi-stage loading in a corrosive environment according to claim 3 is characterized in that: The formula λ i =σ aeq,i / σ a,i (2), where σ a,i represents the stress amplitude corresponding to the i-th level fatigue loading under corrosive environment, σ aeq,i Indicates σ under corrosive environment a,i Stress amplitude in air environment when fatigue life is equal for each load acting alone.

5. The method for evaluating uniaxial fatigue cumulative damage under multi-stage loading in a corrosive environment according to claim 1 is characterized in that: The μ i Defined as the loading sequence influence factor, it is used to consider the effect of loading sequence and load level on fatigue life.

6. The method for evaluating uniaxial fatigue cumulative damage under multi-stage loading in a corrosive environment according to claim 5 is characterized in that: The formula It represents the maximum axial stress amplitude when the fatigue load is loaded to the i-th level.

7. The method for evaluating uniaxial fatigue cumulative damage under multi-stage loading in a corrosive environment according to claim 1 is characterized in that: The corrosive environment is a liquid corrosive environment or a salt spray corrosive environment.

8. The method for evaluating uniaxial fatigue cumulative damage under multi-stage loading in a corrosive environment according to claim 1 is characterized in that: The formula The σ fc Indicates the single tensile fatigue strength coefficient of the material under corrosive environment, b c Represents the single tensile fatigue strength index of the material in a corrosive environment and satisfies 9. The method for evaluating uniaxial fatigue cumulative damage under multi-stage loading in a corrosive environment according to claim 1, characterized in that: The formula In n is the uniaxial fatigue stress range, σ a represents the uniaxial fatigue stress amplitude, σ f is the material single tension fatigue strength coefficient, and b represents the material single tension fatigue strength index.

10. The method for evaluating uniaxial fatigue cumulative damage under multi-stage loading in a corrosive environment according to claim 1, characterized in that: The multi-stage loading fatigue test includes two-stage and three-stage loading, and the nonlinear damage accumulation model formulas are:

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