Fatigue life prediction method for defective sample

Abstract

The invention discloses a fatigue life prediction method for a defective sample, and relates to the technical field of life prediction, and the method comprises the following steps: S1, obtaining basic attribute data and fatigue life experiment data of a complete test piece, and obtaining fatigue life experiment data of a notched test piece; s2, performing finite element modeling on the notched test piece by using the basic attribute data of the complete test piece to obtain a finite element model; s3, calculating the equivalent stress of the notched test piece; and S4, constructing a life prediction equation to obtain the predicted life of the notched test piece. According to the method, the low-cycle fatigue life of the test piece with a single-side large-size square notch is predicted through finite element numerical simulation, the stress field is comprehensively considered on the total residual thickness in a layered and path-divided integration mode for the residual thickness area, the prediction accuracy is high, and the prediction precision is high. And a new method is provided for predicting the fatigue life of the large-size notched component.

Description

Technical Field

[0001] The invention relates to the technical field of life prediction, and in particular to a method for predicting fatigue life of a defective sample. Background Art

[0002] Engineering structures are often subjected to cyclic loads in actual service environments, and the study of low-cycle fatigue life of materials is an important branch of this research. In order to meet the actual needs of engineering, when designing engineering components, it is often considered to introduce notches of various geometric shapes and sizes into the structure. The sharp change in geometric continuity near the notch will inevitably lead to a stress concentration effect, and the local stress-strain level will increase significantly, resulting in a decrease in the fatigue resistance of the component. How to predict the fatigue life of notched specimens has received widespread attention, which is of great significance for designing engineering structures and preventing unexpected accidents.

[0003] Most of the existing prediction methods for notched specimens are aimed at arc-shaped or V-shaped notches. Such geometric shapes often have only one "hot spot" at the root of the notch, while prediction methods for square notches, especially those with a certain thickness in the axial direction, are very scarce. In addition, most studies focus on small-sized notched specimens with structural symmetry, and the research on single-sided large-sized notched specimens is still very limited. Summary of the invention

[0004] In order to solve the problem of predicting the low cycle fatigue life of a cylindrical specimen with a large square notch on one side, the present invention proposes a fatigue life prediction method for a defective specimen.

[0005] The technical solution of the present invention is: a method for predicting fatigue life of a defective specimen comprises the following steps:

[0006] S1. Obtain basic property data and fatigue life test data of complete specimens, and obtain fatigue life test data of notched specimens;

[0007] S2. Using the basic property data of the complete specimen, finite element modeling is performed on the notched specimen to obtain a finite element model;

[0008] S3. Calculate the equivalent stress of the notched specimen;

[0009] S4. Based on the finite element model and equivalent stress of the notched specimen, the fatigue life experimental data of the complete specimen and the fatigue life experimental data of the notched specimen are used to construct a life prediction equation to obtain the predicted life of the notched specimen.

[0010] Furthermore, in S1, a uniaxial tensile test is performed on the complete specimen to obtain basic property data of the complete specimen, specifically including elastic modulus, Poisson's ratio and plastic stress-strain curve.

[0011] Furthermore, in S1, low cycle fatigue tests are performed on complete specimens and notched specimens of the same size to obtain corresponding fatigue life experimental data.

[0012] Furthermore, S3 includes the following sub-steps:

[0013] S31, perform layered processing on the finite element model of the notched specimen and calculate the equivalent stress of each integral path;

[0014] S32. Calculate the equivalent stress of each layer according to the equivalent stress of each integral path;

[0015] S33. Calculate the contribution stress of each layer according to the equivalent stress of each layer;

[0016] S34. Calculate the equivalent stress of the notched specimen based on the contributing stress of each layer.

[0017] Furthermore, in S31, the equivalent stress σ of the integral path eff The calculation formula is:

[0018]

[0019] Where, L RT represents the length of the residual thickness, and σ(x) represents the stress field function on each integration path.

[0020] Furthermore, in S32, the equivalent stress of each layer The calculation formula is:

[0021]

[0022] In the formula, j represents the number of integral paths on each layer, represents the equivalent stress of the jth integration path.

[0023] Furthermore, in S33, the contribution stress of each layer is The calculation formula is:

[0024]

[0025] In the formula, q i represents the allocation factor of each layer, σ(z) i represents the stress of each layer in the local area of ​​the notch along the axis of the specimen, q(z) i represents the distribution factor of each layer along the axis of the specimen, L ZRepresents the geometric length of the notch along the axis of the specimen.

[0026] Furthermore, in S3, the expression of the life prediction equation is:

[0027] lnσ=-0.04684lnN f +6.32629;

[0028] Where σ is the maximum stress amplitude of the complete specimen during the cyclic fatigue test, N f is the low cycle fatigue life of the notched specimen.

[0029] The beneficial effects of the present invention are as follows: the present invention realizes the prediction of the low-cycle fatigue life of the specimen with a single-sided large-sized square notch by using finite element numerical simulation, and realizes the comprehensive consideration of the stress field over the entire residual thickness by integrating the residual thickness area in layers and paths, and the prediction accuracy is high, which provides a new method for predicting the fatigue life of large-sized notch components. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 This is a flow chart of the fatigue life prediction method for defective specimens;

[0031] Figure 2 This is a schematic diagram of the dimensions of a low-cycle fatigue notch specimen;

[0032] Figure 3 It is the stress cloud diagram of the notched part;

[0033] Figure 4 It is a schematic diagram of the layers in the remaining thickness area;

[0034] Figure 5 This is the low cycle fatigue life prediction result diagram. DETAILED DESCRIPTION

[0035] The embodiments of the present invention will be further described below in conjunction with the accompanying drawings.

[0036] like Figure 1 As shown, the present invention provides a method for predicting fatigue life of a defective specimen, comprising the following steps:

[0037] S1. Obtain basic property data and fatigue life test data of complete specimens, and obtain fatigue life test data of notched specimens;

[0038] S2. Using the basic property data of the complete specimen, finite element modeling is performed on the notched specimen to obtain a finite element model;

[0039] S3. Calculate the equivalent stress of the notched specimen;

[0040] S4. Based on the finite element model and equivalent stress of the notched specimen, the fatigue life experimental data of the complete specimen and the fatigue life experimental data of the notched specimen are used to construct a life prediction equation to obtain the predicted life of the notched specimen.

[0041] In the embodiment of the present invention, in S1, a uniaxial tensile test is performed on the complete specimen to obtain basic property data of the complete specimen, specifically including elastic modulus, Poisson's ratio and plastic stress-strain curve.

[0042] The same size of the complete and notched specimens were subjected to strain-controlled low-cycle fatigue tests to obtain multiple sets of fatigue life experimental data under strain amplitude. Figure 2 shown.

[0043] In the embodiment of the present invention, in S1, low cycle fatigue tests are performed on complete specimens and notched specimens of the same size to obtain corresponding fatigue life experimental data.

[0044] In the embodiment of the present invention, in S2, the notched component is modeled in Abaqus, wherein the region with the notch in the middle needs to be corrected in geometric structure according to the actual electron microscope image of the fracture of the notched specimen. This is because it is impossible to ensure that the notched component is correctly modeled in the actual component processing process. Figure 2 The theoretical geometries shown often introduce fillets at right angles, which should also be taken into account during finite element modeling.

[0045] After the modeling is completed, the material properties are assigned according to the test data in S1, and then the entire model is meshed. Since the stress calculation accuracy of the notch part is very high, the mesh needs to be refined in the notch area. The calculation analysis step is set to displacement loading according to triangular waveform loading, and the first few cycles of the low-cycle fatigue test are taken. The boundary condition is set to completely fix one clamping end and generate axial displacement at the other end.

[0046] In this embodiment of the present invention, S3 includes the following sub-steps:

[0047] S31, perform layered processing on the finite element model of the notched specimen and calculate the equivalent stress of each integral path;

[0048] S32. Calculate the equivalent stress of each layer according to the equivalent stress of each integral path;

[0049] S33. Calculate the contribution stress of each layer according to the equivalent stress of each layer;

[0050] S34. Calculate the equivalent stress of the notched specimen based on the contributing stress of each layer.

[0051] In the embodiment of the present invention, in S31, during the post-processing of the finite element calculation, the corresponding integral path is selected according to the grid node and the maximum principal stress value on the path is output to form a stress field function curve on each integral path. eff The calculation formula is:

[0052]

[0053] Where, L RT represents the length of the residual thickness, and σ(x) represents the stress field function on each integration path.

[0054] In the embodiment of the present invention, in S32, the equivalent stress of each layer The calculation formula is:

[0055]

[0056] In the formula, j represents the number of integral paths on each layer, represents the equivalent stress of the jth integration path.

[0057] In the embodiment of the present invention, in S33, the contribution stress of each layer The calculation formula is:

[0058]

[0059] In the formula, q i represents the allocation factor of each layer, σ(z) i represents the stress of each layer in the local area of ​​the notch along the axis of the specimen, q(z) i represents the distribution factor of each layer along the axis of the specimen, L Z Represents the geometric length of the notch along the axis of the specimen.

[0060] In the embodiment of the present invention, after the finite element calculation is completed, only the thickness of the test piece with the notch is taken for analysis. Figure 3 The stress cloud diagram of the notched part is shown in Figure 2. Different from the traditional notch geometry, since it has thickness in the axial direction, it is necessary to fully consider the stress state in the entire residual thickness area. To this end, the residual thickness area is divided into Figure 4 The schematic diagram shown is layered, and multiple integration paths are selected at equal distances on each layer.

[0061] In the present invention, considering that there is also a stress gradient phenomenon along the z-axis direction, it is necessary to distribute the contribution of each layer of equivalent stress to the final stress value, and define the stress distribution factor q, which is calculated as follows:

[0062]

[0063] In the formula, σ(z) represents the stress field function along the z-axis, z represents the axial direction of the specimen, and σ(z) max It represents the maximum stress in the z direction obtained by the z-axis stress field function.

[0064] In an embodiment of the present invention, in S3, the present invention predicts the fatigue life of the notched specimen by the fatigue life of the smooth specimen, and the two are associated with equivalent stress. According to the low-cycle fatigue test data, the maximum stress amplitude data and the corresponding fatigue life generated by the smooth specimen in the fatigue life process at each strain amplitude are obtained, and the two satisfy a logarithmic relationship. The corresponding equation is obtained by fitting using the data analysis and processing software Origin. The theory of the present invention is that when the equivalent stress in the notched specimen reaches the maximum stress amplitude level in the smooth specimen, it is considered to have a corresponding life under the corresponding strain amplitude loading. The expression of the life prediction equation is:

[0065] lnσ=-0.04684lnN f +6.32629;

[0066] Where σ is the maximum stress amplitude of the complete specimen during the cyclic fatigue test, N f is the low cycle fatigue life of the notched specimen.

[0067] In the embodiment of the present invention, the lifespan calculated under different strain amplitudes is compared with the lifespan obtained by experimental test to determine the accuracy of the lifespan prediction. The accuracy of the lifespan prediction is evaluated by the lifespan prediction factor, which is expressed as the ratio of the predicted value to the experimental value. The specific prediction results are as follows: Figure 5 As shown, it can be seen that most of the life prediction results fall within the error band of ±1.5, showing satisfactory prediction accuracy.

[0068] Those skilled in the art will appreciate that the embodiments described herein are intended to help readers understand the principles of the present invention, and should be understood that the protection scope of the present invention is not limited to such specific statements and embodiments. Those skilled in the art can make various other specific variations and combinations that do not deviate from the essence of the present invention based on the technical revelations disclosed by the present invention, and these variations and combinations are still within the protection scope of the present invention.

Claims

1. A method for predicting fatigue life of defective specimens, characterized in that: The following steps are involved: S1. Obtain basic property data and fatigue life test data of complete specimens, and obtain fatigue life test data of notched specimens; S2. Using the basic property data of the complete specimen, finite element modeling is performed on the notched specimen to obtain a finite element model; S3. Calculate the equivalent stress of the notched specimen; S4. Based on the finite element model and equivalent stress of the notched specimen, the fatigue life experimental data of the complete specimen and the fatigue life experimental data of the notched specimen are used to construct a life prediction equation to obtain the predicted life of the notched specimen.

2. The method for predicting fatigue life of defective specimens according to claim 1, characterized in that: In S1, a uniaxial tensile test is performed on the complete specimen to obtain basic property data of the complete specimen, specifically including elastic modulus, Poisson's ratio and plastic stress-strain curve.

3. The method for predicting fatigue life of defective specimens according to claim 1, characterized in that: In S1, low cycle fatigue tests are performed on complete specimens and notched specimens of the same size to obtain corresponding fatigue life experimental data.

4. The method for predicting fatigue life of defective specimens according to claim 1, characterized in that: The S3 comprises the following sub-steps: S31, perform layered processing on the finite element model of the notched specimen and calculate the equivalent stress of each integral path; S32. Calculate the equivalent stress of each layer according to the equivalent stress of each integral path; S33. Calculate the contribution stress of each layer according to the equivalent stress of each layer; S34. Calculate the equivalent stress of the notched specimen based on the contributing stress of each layer.

5. The method for predicting fatigue life of defective specimens according to claim 4, characterized in that: In S31, the equivalent stress σ of the integral path eff The calculation formula is: Where, L RT represents the length of the residual thickness, and σ(x) represents the stress field function on each integration path.

6. The method for predicting fatigue life of defective specimens according to claim 4, characterized in that: In the S32, the equivalent stress of each layer The calculation formula is: In the formula, j represents the number of integral paths on each layer, represents the equivalent stress of the jth integration path.

7. The method for predicting fatigue life of defective specimens according to claim 4, characterized in that: In S33, the contribution stress of each layer The calculation formula is: In the formula, q i represents the allocation factor of each layer, σ(z) i represents the stress of each layer in the local area of ​​the notch along the axis of the specimen, q(z) i represents the distribution factor of each layer along the axis of the specimen, L Z Represents the geometric length of the notch along the axis of the specimen.

8. The method for predicting fatigue life of defective specimens according to claim 1, characterized in that: In S3, the expression of the life prediction equation is: lnσ=-0.04684lnN f +6.32629; Where σ is the maximum stress amplitude of the complete specimen during the cyclic fatigue test, N f is the low cycle fatigue life of the notched specimen.

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