A fatigue life prediction method based on notch type division correction field strength method

Abstract

The present application relates to the technical field of engineering metal, and particularly relates to a kind of fatigue life prediction method of modified field intensity method based on notch type division, comprising the following steps: S1: the notch component to be analyzed is carried out elastic-plastic finite element analysis, obtains stress-strain distribution, extracts the stress distribution on the most dangerous path of component, and the corresponding relative stress gradient is calculated out;S2: according to the obtained relative stress gradient, the notch type is divided;S3: according to the difference of stagnation point position, the field path of corresponding notch is defined;S4: according to the field path divided, it is brought into the simplified one-dimensional field intensity method model, and the equivalent stress corresponding to the field path divided is calculated;S5: in combination with the S-N curve of smooth material, the fatigue life of notch component is predicted according to the calculated equivalent stress.The present application will distinguish the correction of peak stress according to notch type, and based on this, more accurate life prediction can be obtained.

Description

Technical Field

[0001] This invention relates to the field of engineering metals technology, and in particular to a fatigue life prediction method based on a modified field strength method according to notch type classification. Background Technology

[0002] Notch fatigue research on metallic materials is an important engineering and scientific problem involving multiple fields such as aerospace, automotive, construction, and machinery manufacturing. With the progress of modern society and the innovation of cutting-edge technology, more and more high-precision equipment and transportation tools are being developed and manufactured to meet people's aspirations for a better life. This places higher demands on the mechanical performance of key components. To meet the diverse functional requirements of components, their structures are often designed to be quite complex, with a large number of irregular and geometrically discontinuous structures. These geometrically discontinuous features can lead to local stress concentration, which inevitably introduces the influence of the notch effect. The parts with local stress concentration are often the most prone to fatigue failure, which brings new challenges to the reliability design and integrity assessment of structures.

[0003] To address the problem of multiaxial fatigue life assessment of components under notch effect, the traditional field strength method model involves a huge amount of computation for predicting notched components and has difficulty in defining the fatigue failure zone. Summary of the Invention

[0004] To achieve the above objectives, this invention provides a fatigue life prediction method based on a modified field strength method with notch type classification.

[0005] A fatigue life prediction method based on a modified field strength method with notch type classification includes the following steps:

[0006] S1: Perform elastoplastic finite element analysis on the notched component to be analyzed, obtain the stress-strain distribution, extract the stress distribution on the most dangerous path of the component, and calculate the corresponding relative stress gradient;

[0007] S2: Based on the obtained relative stress gradient, classify the notch type;

[0008] S3: Define the field diameter of the corresponding gap according to the different locations of the stagnation points;

[0009] S4: Based on the divided field path, substitute it into the simplified one-dimensional field strength method model, and calculate the corresponding equivalent stress according to the divided field path;

[0010] S5: Based on the SN curve of smooth materials, predict the fatigue life of notched components according to the calculated equivalent stress.

[0011] Furthermore, the stress selection on the most dangerous path takes into account various influencing factors, including the notch shape and geometry, and the relative stress gradient takes into account the supporting effect brought by the notch morphology.

[0012] Furthermore, the formula for calculating the relative stress gradient is as follows:

[0013]

[0014] Where, χ * This refers to the relative stress gradient, σ yy The stress distribution along the selected path is given by r, which is the distance from the root of the notch.

[0015] Furthermore, the types of gaps specifically include:

[0016] Passivation notch: There is no stagnation point in the relative stress gradient;

[0017] Sharp notch: A stationary point exists after a given nominal stress;

[0018] Moderate notch: Stagnation points exist before a given nominal stress.

[0019] Furthermore, the definition of the field diameter corresponding to the notch includes defining the field diameter of a sharp notch, defining the field diameter of a moderate notch, and defining the field diameter of a passivated notch, wherein;

[0020] The field diameter of the sharp notch is defined as the distance from the notch root to the stagnation point;

[0021] The field diameter of a moderate notch is defined as the distance from the notch root to the stagnation point.

[0022] The field diameter of the passivation notch is defined as the distance from the notch root to the point where the stress change is stable.

[0023] Furthermore, the simplified model of the field strength method is expressed as follows:

[0024] σ FI =σ max -ζ(Ω);

[0025] σ FI =σ max +ξ(Ω);

[0026]

[0027] Where, σ FI For the equivalent stress, σ max The peak stress at the root of the notch is ξ(Ω), which represents the correction parameter for the main body acting on the target field strength and is related to the stress distribution in the fatigue failure zone. rLet θ represent the stress distribution along a one-dimensional path, and R be the field diameter defined for the corresponding notch. For symmetrical structures, θ is 0.

[0028] Furthermore, the passivation notch and the moderate notch adopt σ FI =σ max -ζ(Ω) is used to correct the peak stress, and the sharp notch is achieved using σ. FI =σ max +ξ(Ω) is used to correct for peak stress.

[0029] Furthermore, the prediction of the fatigue life of the notched component specifically includes:

[0030] Obtain the SN curve of the corresponding smooth material;

[0031] The calculated equivalent stress is used as the input parameter for the SN curve;

[0032] Find the point on the SN curve corresponding to the equivalent stress and read the corresponding fatigue life.

[0033] Determine the expected fatigue life of the notched component under actual working conditions.

[0034] Furthermore, S1 also includes performing finite element analysis of the notched component based on the external load environment and the shape parameters of the loaded component, and extracting the stress distribution on the path at the root of the notch.

[0035] The beneficial effects of this invention are:

[0036] This invention classifies three notch types by the location of stagnation points in the relative stress gradient and defines the field diameter of the three notch types by the stagnation points. This enables the classification of any notch component and distinguishes the correction of peak stress according to the notch type, thereby obtaining a more accurate life prediction.

[0037] This invention, by mastering the SN curve of the material, can obtain the predicted life without conducting notched component tests, but only by performing finite element analysis incorporating elasticity and plasticity. It is simple to operate, convenient to calculate, and widely applicable. Attached Figure Description

[0038] To more clearly illustrate the technical solutions in this invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only for this invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0039] Figure 1This is a flowchart illustrating the fatigue life prediction method for notched structures based on notch type classification according to an embodiment of the present invention.

[0040] Figure 2 This is a schematic diagram of a passivation notch subjected to a relative nominal stress of 270 MPa as defined in an embodiment of the present invention;

[0041] Figure 3 This is a schematic diagram of a moderate notch with a relative nominal stress of 270 MPa as defined in an embodiment of the present invention;

[0042] Figure 4 This is a schematic diagram of a sharp notch subjected to a relative nominal stress of 270 MPa as defined in an embodiment of the present invention;

[0043] Figure 5 This is a schematic diagram illustrating fatigue life prediction results as a specific example of an embodiment of the present invention. Detailed Implementation

[0044] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to specific embodiments.

[0045] It should be noted that, unless otherwise defined, the technical or scientific terms used in this invention should have the ordinary meaning understood by one of ordinary skill in the art to which this invention pertains. The terms "first," "second," and similar terms used in this invention do not indicate any order, quantity, or importance, but are merely used to distinguish different components. Terms such as "comprising" or "including" mean that the element or object preceding the word encompasses the elements or objects listed following the word and their equivalents, without excluding other elements or objects. Terms such as "connected" or "linked" are not limited to physical or mechanical connections, but can include electrical connections, whether direct or indirect. Terms such as "upper," "lower," "left," and "right" are used only to indicate relative positional relationships; when the absolute position of the described object changes, the relative positional relationship may also change accordingly.

[0046] like Figure 1-5 As shown, a fatigue life prediction method based on notch type classification using a modified field strength method includes the following steps:

[0047] Step S1: First, perform finite element analysis on the notched component based on the external load environment and the shape parameters of the loaded component. Then, perform elastoplastic finite element analysis on the analyzed notched component to obtain its stress-strain distribution, extract the stress distribution on the most dangerous path of the component, and calculate the corresponding relative stress gradient. The selection of the stress on the most dangerous path can take into account the influence of factors such as notch shape and geometric dimensions, and the relative stress gradient takes into account the supporting effect brought by the notch morphology. The specific calculation method of the relative stress gradient is as follows:

[0048]

[0049] Where, χ * This refers to the relative stress gradient, σ yy The stress distribution along the selected path is given by r, which is the distance from the root of the notch.

[0050] Proceed to step S2.

[0051] Step S2: Based on the obtained relative stress gradient, classify the notch into three different types, such as... Figure 2-4 Defined as follows: Passivation notch: No stationary point exists relative to the stress gradient; Sharp notch: Stationary point exists after a given nominal stress; Moderate notch: Stationary point exists before a given nominal stress. The three different notch types, classified according to the location of the stationary point in the relative stress gradient, mean that the stress gradient distribution before the stationary point is relatively aggressive, while the stress gradient change will change after the stationary point. For a passivation notch: No stationary point exists relative to the stress gradient, indicating that the entire stress gradient change is relatively gradual. For a sharp notch: Stationary point exists after a given nominal stress, indicating that the stress gradient before that point is in a relatively aggressive change. Finally, for a moderate notch: Stationary point exists before a given nominal stress, indicating that only the stress change before that distance needs to be considered.

[0052] Proceed to step S3.

[0053] Step S3: Based on the location of the stagnation point, the field diameter of sharp and moderate notches is defined as the distance from the notch root to the stagnation point, and the field diameter of passivated notches is defined as the distance from the notch root to the point where stress changes smoothly. See details... Figure 2-4 .

[0054] Proceed to step S4.

[0055] Step S4: Substitute the field diameter defined in Step S3 into the simplified one-dimensional field strength method model. The definition of the field diameter R is as described in Step S2. The simplified model of the field strength method is as follows:

[0056] σ FI =σ max -ζ(Ω) (2)

[0057] σ FI =σ max +ξ(Ω) (3)

[0058]

[0059] Where, σ FI For the equivalent stress, σ max σ represents the peak stress at the root of the notch, and ξ(Ω) represents the correction parameter for the main body acting on the target field strength, which is related to the stress distribution within the fatigue failure zone. r Let R be the stress distribution along a one-dimensional path, and R be the field diameter defined according to step S3; for symmetrical structures, θ is 0. Among them, for passivated notches and moderate notches, the correction of peak stress satisfies equation (2), and for sharp notches, the correction of peak stress satisfies equation (3).

[0060] Proceed to step S5.

[0061] Step S5: Based on the SN curve of the smooth material, the life corresponding to the equivalent stress is calculated, which is the fatigue life of the notched component.

[0062] The specific steps for predicting the fatigue life of notched components are as follows:

[0063] Obtain the SN curve of the corresponding smooth material;

[0064] The calculated equivalent stress is used as the input parameter for the SN curve;

[0065] Find the point on the SN curve corresponding to the equivalent stress and read the corresponding fatigue life.

[0066] Determine the expected fatigue life of the notched component under actual working conditions.

[0067] Specific examples are as follows:

[0068] The fatigue life prediction method for notched structures based on notch type classification of the present invention was used to predict the cycle life of arbitrary notched components under stress-controlled fatigue loads under different conditions and with different materials.

[0069] The selected data consisted of 21 sets of stress-controlled fatigue tests of 316H stainless steel at 600℃. The 21 sets of data were composed of notched specimens with four different notch types and three different stress concentration factors. The applied stress was the nominal stress, and the stress ratio was -1. The test methods all complied with the national standard GBT3075—2020 "Method for controlling axial force in fatigue testing of metallic materials".

[0070] Low-cycle fatigue tests were conducted on six groups of 316H stainless steel center-hole plate specimens, with a stress concentration factor of 2.71 and stress amplitudes of 220 MPa, 230 MPa, 240 MPa, 270 MPa, and 300 MPa. Low-cycle fatigue tests were also conducted on six groups of 316H stainless steel double-notched specimens, with a stress concentration factor of 2.2 and stress amplitudes of 230 MPa, 240 MPa, 270 MPa, 300 MPa, and 310 MPa. Low-cycle fatigue tests were also conducted on six groups of 316H stainless steel double-notched specimens, with a stress concentration factor of 1.77 and stress amplitudes of 240 MPa, 270 MPa, 300 MPa, 320 MPa, and 340 MPa. Finally, low-cycle fatigue tests were conducted on three groups of 316H stainless steel ring specimens, with a stress concentration factor of 1.77 and stress amplitudes of 240 MPa and 270 MPa. The test methods are as described in the instruction manual.

[0071] First, according to step S1 of this invention, the tensile stress-strain distribution under nominal stress is obtained based on elastoplastic finite element analysis. The most dangerous path is selected, its stress distribution is extracted, and the relative stress gradient distribution on the path is calculated. Then, according to step S2, three types of notches are classified. Next, according to step S3, the field diameter of different notched components is determined. According to step S4, the equivalent stress of the simplified one-dimensional field strength method model is calculated. Finally, according to step S5, combined with the SN curve of the smooth material, the life corresponding to the equivalent stress is the fatigue life of the notched component.

[0072] For different notch types and various stress concentration factors, the lifetimes predicted using this invention are all within a 2x error band. Therefore, the lifetime prediction model of this invention is universally applicable to different test materials, different test loads, different notch types, and different stress concentration factors.

[0073] Those skilled in the art should understand that the discussion of any of the above embodiments is merely exemplary and is not intended to imply that the scope of the invention is limited to these examples; within the framework of the invention, the technical features of the above embodiments or different embodiments can also be combined, the steps can be implemented in any order, and there are many other variations of the different aspects of the invention as described above, which are not provided in detail for the sake of brevity.

[0074] This invention is intended to cover all such substitutions, modifications, and variations falling within the broad scope of the claims. Therefore, any omissions, modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of this invention should be included within the scope of protection of this invention.

Claims

1. A fatigue life prediction method based on a modified field strength method using notch type classification, characterized in that, Includes the following steps: S1: Perform elastoplastic finite element analysis on the notched component to be analyzed, obtain the stress-strain distribution, extract the stress distribution on the most dangerous path of the component, and calculate the corresponding relative stress gradient; S2: Based on the obtained relative stress gradient, classify the notch type; The specific types of gaps include: Passivation notch: There is no stagnation point in the relative stress gradient; Sharp notch: A stationary point exists after a given nominal stress; Moderate notch: The stagnation point exists before a given nominal stress; S3: Define the field diameter of the corresponding gap according to the different locations of the stagnation points; The field diameter defined for the corresponding notch includes the field diameter for a sharp notch, the field diameter for a moderate notch, and the field diameter for a passivated notch, wherein; The field diameter of the sharp notch is defined as the distance from the notch root to the stagnation point; The field diameter of a moderate notch is defined as the distance from the notch root to the stagnation point. The field diameter of the passivation notch is defined as the distance from the notch root to the point where the stress change is stable. S4: Based on the divided field path, substitute it into the simplified one-dimensional field strength method model, and calculate the corresponding equivalent stress according to the divided field path; The simplified model of the field strength method is expressed as follows: in, For equivalent stress, This represents the peak stress at the root of the notch. This represents the correction parameter for the main body acting on the target field strength, and is related to the stress distribution within the fatigue failure zone. The stress distribution along a one-dimensional path, R To define the field diameter corresponding to the notch, for a symmetrical structure, Take 0, This is the distance from the root of the gap; S5: Based on the SN curve of smooth materials, predict the fatigue life of notched components according to the calculated equivalent stress.

2. The fatigue life prediction method based on notch type classification according to claim 1, characterized in that, The stress selection on the most dangerous path takes into account a variety of influencing factors, including the shape and geometry of the notch, and the relative stress gradient takes into account the supporting effect brought by the notch morphology.

3. The fatigue life prediction method based on notch type classification using a modified field strength method according to claim 2, characterized in that, The formula for calculating the relative stress gradient is: in, This refers to the relative stress gradient. The stress distribution along the selected path.

4. The fatigue life prediction method based on notch type classification according to claim 3, characterized in that, The passivation notch and the moderate notch are adopted To correct for peak stress, the sharp notch is used. Correction is performed for peak stress.

5. The fatigue life prediction method based on notch type classification using a modified field strength method according to claim 4, characterized in that, The predicted fatigue life of the notched component specifically includes: Obtain the SN curve of the corresponding smooth material; The calculated equivalent stress is used as the input parameter for the SN curve; Find the point on the SN curve corresponding to the equivalent stress and read the corresponding fatigue life. Determine the expected fatigue life of the notched component under actual working conditions.

6. The fatigue life prediction method based on notch type classification according to claim 1, characterized in that, S1 further includes performing finite element analysis of the notched component based on the external load environment and the shape parameters of the loaded component, and extracting the stress distribution along the path at the root of the notch.

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