A fatigue simulation method for local stress ratio changes caused by nonlinear deformation

Abstract

A fatigue simulation method based on local stress ratio changes caused by nonlinear deformation belongs to the field of composite material fatigue performance research technology. The method is as follows: determine the peak and valley values ​​of fatigue load and calculate the fatigue stress ratio; divide the fatigue loading process into two stages; calculate the equivalent fatigue life under the current fatigue cycle loading; calculate the stress ratio under the current fatigue cycle loading; calculate the equivalent fatigue life of the current element; establish a uniaxial fatigue stiffness degradation model and a uniaxial fatigue strength degradation model for composite materials, and calculate the overall fatigue life of the model. This invention considers the influence of local stress ratio changes on the fatigue life of composite materials during fatigue, and based on the equal life model, stiffness degradation model, strength degradation model, and fatigue failure criteria, establishes a more accurate composite material fatigue simulation analysis method, optimizing the prediction accuracy of traditional models and filling the gap in the field of composite material fatigue simulation calculation.

Description

Technical Field

[0001] This invention relates to a fatigue simulation method for local stress ratio changes caused by nonlinear deformation, belonging to the technical field of fatigue performance research of composite materials. Background Technology

[0002] Composite material fatigue is the process by which the internal structure of a material gradually deteriorates under cyclic stress or strain, leading to performance degradation and eventual failure. Fatigue failure typically involves no obvious macroscopic plastic deformation and occurs suddenly without warning. The fatigue properties of composite materials are crucial for the safety and reliability of engineering structures. Fatigue failure is one of the most common failure modes in engineering, usually occurring when the stress level is below the material's strength. Therefore, understanding the fatigue properties of materials is of great significance for preventing structural failures and ensuring the structural safety and operational safety of aircraft.

[0003] Traditional composite material fatigue simulation methods primarily rely on peak fatigue stress, employing fatigue life models, stiffness and strength degradation, and fatigue failure criteria for calculation. During these calculations, the stress ratio between the peak and trough values ​​of the fatigue load on each element is assumed to remain constant. However, in real fatigue processes, the failure of some elements leads to local stress redistribution, causing changes in the stress ratio of the fatigue load on each element. Ignoring these stress ratio changes results in a significant decrease in the accuracy of the fatigue simulation. Therefore, proposing a fatigue simulation method that addresses the local stress ratio changes caused by nonlinear deformation is crucial. This method can resolve the accuracy degradation due to variations in element stress ratios, leading to more accurate fatigue simulation results. Summary of the Invention

[0004] To address the problems existing in the background technology, the present invention provides a fatigue simulation method for local stress ratio changes caused by nonlinear deformation.

[0005] To achieve the above objectives, the present invention adopts the following technical solution: a fatigue simulation method for local stress ratio changes caused by nonlinear deformation, the method comprising the following steps:

[0006] S1: Determining the peak fatigue load σ based on the load input from composite material fatigue tests max and fatigue load valley value σ min And calculate the fatigue stress ratio R = σ min / σ max ;

[0007] S2: The entire fatigue loading process is divided into two stages. The first stage is the quasi-static loading stage, that is, the load is applied from 0 to the peak fatigue load σ. max The second stage is the stage of continuous load application.

[0008] S3: Incremental time Δt based on the current simulation i Calculate the equivalent fatigue life under the current fatigue cycle loading;

[0009] The calculation process for the equivalent fatigue life under the current fatigue cyclic loading described in S3 is as follows:

[0010] ΔN i =Dn×Δt i / Δt scale (1)

[0011] In formula (1):

[0012] Dn represents the cyclic parameter of fatigue life, which is a fixed constant;

[0013] Δt scale Represents a dimensionless time scale parameter;

[0014] i represents the current fatigue week.

[0015] S4: Calculate the stress ratio R under the current fatigue cycle loading. i ;

[0016] S401: Numerical comparison of the absolute stress values ​​of the elements under one fatigue cycle loading;

[0017] S402: Filter and save the current peak stress points under fatigue load. and the current fatigue load valley stress point

[0018] S403: Calculate the stress ratio R under the current fatigue cycle loading. i :

[0019]

[0020] S5: Calculate the equivalent fatigue life of the current unit;

[0021]

[0022] In formula (3):

[0023]

[0024] c = S c / S t , where: S c S represents compressive strength. t Indicates tensile strength;

[0025] f represents a constant value;

[0026] N fThis represents the equivalent fatigue life of the element under the current stress direction;

[0027] A and B represent constants related to the material.

[0028] S6: Based on the equivalent fatigue life of the current unit and the real-time stress-strain data of traditional uniaxial fatigue tests, establish a uniaxial fatigue stiffness degradation model and a uniaxial fatigue strength degradation model for composite materials.

[0029] The uniaxial fatigue stiffness degradation model described in S6 is as follows:

[0030]

[0031] In equation (4):

[0032] E f This represents the initial fatigue stiffness of the element;

[0033] ε f Indicates the failure strain of the element;

[0034] λ, γ represent the fitting parameters related to the material.

[0035] The uniaxial fatigue strength degradation model described in S6 is as follows:

[0036]

[0037] In equation (5):

[0038] S f Indicates the initial fatigue strength of the element;

[0039] α and β are the material-related fitting parameters in the model.

[0040] S7: During fatigue simulation calculation, the stiffness and strength of the elements degrade with the number of fatigue cycles. At the same time, the stress ratio of the elements needs to be recalculated in each cycle. When the current stress of the element triggers the fatigue failure criterion, the element fails, and the stiffness and strength no longer degrade with the number of fatigue cycles. When all elements in the model fail, the fatigue simulation ends. The equivalent life under each fatigue cycle is accumulated to calculate the overall fatigue life of the model.

[0041] The fatigue life calculation formula for the overall model described in S7 is as follows:

[0042]

[0043] Compared with the prior art, the beneficial effects of the present invention are:

[0044] This invention considers the impact of local stress ratio changes on the fatigue life of composite materials during fatigue and establishes a more accurate fatigue simulation analysis method for composite materials based on the equal life model, stiffness degradation model, strength degradation model, and fatigue failure criteria. This method optimizes the prediction accuracy of traditional models, obtains more accurate fatigue simulation calculation results, fills the gap in the field of composite material fatigue simulation calculation, provides an important reference for the reliability and safety design of composite material structures, and improves the calculation efficiency and accuracy of fatigue life. Attached Figure Description

[0045] Figure 1 This is a schematic diagram of the overall load fatigue loading curve considering the local stress ratio variation;

[0046] Figure 2 This is a schematic diagram of the load fatigue loading curve considering the change in the local stress ratio of the element due to nonlinear deformation.

[0047] Figure 3 This is a flowchart of the present invention. Detailed Implementation

[0048] The technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings of the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the invention, not all embodiments. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are within the scope of protection of the present invention.

[0049] A fatigue simulation method for local stress ratio changes caused by nonlinear deformation, the method comprising the following steps:

[0050] S1: Determining the peak fatigue load σ based on the load input from composite material fatigue tests max and fatigue load valley value σ min And calculate the fatigue stress ratio R = σ min / σ max ;

[0051] S2: In finite element software, the entire fatigue loading process is divided into two stages, such as... Figure 1 and Figure 2 As shown, the first stage is the quasi-static loading stage, that is, the load is applied from 0 to the peak fatigue load σ. max To accurately describe the change in stress ratio of an element during fatigue, the process of increasing and decreasing load once is considered as one fatigue cycle, and the second stage is the stage of continuous load application.

[0052] S3: Incremental time Δt based on the current simulation i Calculate the equivalent fatigue life under the current fatigue cycle loading;

[0053] The calculation process for the equivalent fatigue life under the current fatigue cyclic loading described in S3 is as follows:

[0054] ΔN i =Dn×Δt i / Δt scale (1)

[0055] In formula (1):

[0056] Dn represents the cyclic parameter of fatigue life, which is a fixed constant;

[0057] Δt scale Represents a dimensionless time scale parameter;

[0058] Together, they control the equivalent fatigue life under a single fatigue cycle loading.

[0059] i represents the current fatigue week.

[0060] S4: Calculate the stress ratio R under the current fatigue cycle loading. i ;

[0061] S401: Numerical comparison of the absolute stress values ​​of the elements under one fatigue cycle loading;

[0062] S402: Filter and save the current peak stress points under fatigue load. and the current fatigue load valley stress point

[0063] S403: Calculate the stress ratio R under the current fatigue cycle loading. i :

[0064]

[0065] S5: Calculate the equivalent fatigue life of the current unit;

[0066]

[0067] In formula (3):

[0068]

[0069] c = S c / S t , where: S c S represents compressive strength. t Indicates tensile strength;

[0070] f represents a constant value, typically 1.06;

[0071] N fThis represents the equivalent fatigue life of the element under the current stress direction;

[0072] A and B represent material-related constants, which are obtained by fitting experimental data.

[0073] S6: Based on the equivalent fatigue life of the current unit and the real-time stress-strain data of traditional uniaxial fatigue tests, establish a composite material uniaxial fatigue stiffness degradation model and a uniaxial fatigue strength degradation model to determine the relevant parameters in the model;

[0074] The uniaxial fatigue stiffness degradation model described in S6 is as follows:

[0075]

[0076] In equation (4):

[0077] E f This represents the initial fatigue stiffness of the element;

[0078] ε f Indicates the failure strain of the element;

[0079] λ and γ represent the fitting parameters related to the material, which are obtained by fitting experimental data.

[0080] The uniaxial fatigue strength degradation model described in S6 is as follows:

[0081]

[0082] In equation (5):

[0083] S f Indicates the initial fatigue strength of the element;

[0084] α and β are the material-related fitting parameters in the model, which are obtained by fitting experimental data.

[0085] S7: During fatigue simulation calculation, the stiffness and strength of the elements degrade with the number of fatigue cycles. At the same time, the stress ratio of the elements needs to be recalculated in each cycle. When the current stress of the element triggers the fatigue failure criterion, the element fails, and the stiffness and strength no longer degrade with the number of fatigue cycles. When all elements in the model fail, the fatigue simulation ends. The equivalent life under each fatigue cycle is accumulated to calculate the overall fatigue life of the model.

[0086] The fatigue life calculation formula for the overall model described in S7 is as follows:

[0087]

[0088] It will be apparent to those skilled in the art that the present invention is not limited to the details of the exemplary embodiments described above, and that the invention can be implemented in other forms without departing from its spirit or essential characteristics. Therefore, the embodiments should be considered in all respects as exemplary and non-limiting, and the scope of the invention is defined by the appended claims rather than the foregoing description. Thus, all variations falling within the meaning and scope of the equivalents of the claims are intended to be included within the present invention. No reference numerals in the claims should be construed as limiting the scope of the claims.

[0089] Furthermore, it should be understood that although this specification describes embodiments, not every embodiment contains only one independent technical solution. This narrative style is merely for clarity. Those skilled in the art should consider the specification as a whole, and the technical solutions in each embodiment can also be appropriately combined to form other embodiments that can be understood by those skilled in the art.

Claims

1. A fatigue simulation method for local stress ratio changes caused by nonlinear deformation, characterized in that: The method includes the following steps: S1: Determining the peak fatigue load σ based on the load input from composite material fatigue tests max and fatigue load valley value σ min And calculate the fatigue stress ratio R = σ min / σ max ; S2: The entire fatigue loading process is divided into two stages. The first stage is the quasi-static loading stage, that is, the load is applied from 0 to the peak fatigue load σ. max ; The second stage is the stage of continuous load application; S3: Incremental time Δt based on the current simulation i Calculate the equivalent fatigue life under the current fatigue cycle loading; The calculation process for the equivalent fatigue life under the current fatigue cyclic loading described in S3 is as follows: ΔN i =Dn×Δt i / Δt scale (1) In formula (1): Dn represents the cyclic parameter of fatigue life, which is a fixed constant; Δt scale Represents a dimensionless time scale parameter; i represents the current fatigue cycle; S4: Calculate the stress ratio R under the current fatigue cycle loading. i ; S5: Calculate the equivalent fatigue life of the current unit; S5 includes the following steps: In formula (3): c = S c / S t , where: S c S represents compressive strength. t Indicates tensile strength. This indicates the current peak stress point under fatigue load; f represents a constant value; N f This represents the equivalent fatigue life of the element under the current stress direction; A and B represent material-related constants; S6: Based on the equivalent fatigue life of the current unit and the real-time stress-strain data of traditional uniaxial fatigue tests, establish a uniaxial fatigue stiffness degradation model and a uniaxial fatigue strength degradation model for composite materials. S7: During fatigue simulation calculation, the stiffness and strength of the elements degrade with the number of fatigue cycles. At the same time, the stress ratio of the elements needs to be recalculated in each cycle. When the current stress of the element triggers the fatigue failure criterion, the element fails, and the stiffness and strength no longer degrade with the number of fatigue cycles. When all elements in the model fail, the fatigue simulation ends. The equivalent life under each fatigue cycle is accumulated to calculate the overall fatigue life of the model.

2. The fatigue simulation method for local stress ratio changes caused by nonlinear deformation according to claim 1, characterized in that: S4 includes the following steps: S401: Numerical comparison of the absolute stress values ​​of the elements under one fatigue cycle loading; S402: Filter and save the current peak stress points under fatigue load. and the current fatigue load valley stress point S403: Calculate the stress ratio R under the current fatigue cycle loading. i :

3. The fatigue simulation method for local stress ratio changes caused by nonlinear deformation according to claim 2, characterized in that: The uniaxial fatigue stiffness degradation model described in S6 is as follows: In equation (4): E f This represents the initial fatigue stiffness of the element; ε f Indicates the failure strain of the element; λ and γ represent the fitting parameters related to the material.

4. The fatigue simulation method for local stress ratio changes caused by nonlinear deformation according to claim 3, characterized in that: The uniaxial fatigue strength degradation model described in S6 is as follows: In equation (5): S f Indicates the initial fatigue strength of the element; α and β are the material-related fitting parameters in the model.

5. The fatigue simulation method for local stress ratio changes caused by nonlinear deformation according to claim 4, characterized in that: The fatigue life calculation formula for the overall model described in S7 is as follows:

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