Fatigue simulation method for local stress ratio change caused by nonlinear deformation

Abstract

The invention discloses a fatigue simulation method for local stress ratio change caused by nonlinear deformation, and belongs to the technical field of composite material fatigue performance research. The method comprises the following steps: determining a fatigue load peak-valley value and calculating a fatigue stress ratio; the fatigue loading process is divided into two stages; calculating the equivalent fatigue life under the current fatigue cyclic loading; calculating a stress ratio under current fatigue cyclic loading; calculating the equivalent fatigue life of the current unit; establishing a uniaxial fatigue rigidity degradation model and a uniaxial fatigue strength degradation model of the composite material, and calculating the overall fatigue life of the models. By considering the influence of local stress ratio change on the fatigue life of the composite material in the fatigue process and based on the equal-life model, the rigidity degradation model, the strength degradation model and the fatigue failure criterion, a more accurate composite material fatigue simulation analysis method is established, the prediction precision of a traditional model is optimized, and the prediction efficiency is improved. And the blank in the field of composite material fatigue simulation calculation is filled.

Description

Technical Field

[0001] The invention relates to a fatigue simulation method for local stress ratio change caused by nonlinear deformation, and belongs to the technical field of composite material fatigue performance research. Background Art

[0002] Composite material fatigue is a process in which the internal structure of a material is gradually damaged under cyclic stress or strain, resulting in a degradation of its performance and eventual destruction. Fatigue failure usually does not have obvious macroscopic plastic deformation, and there is no obvious sign before fracture, but it is a sudden failure. The fatigue performance of composite materials is crucial to the safety and reliability of engineering structures. Fatigue failure is the most common form of failure in engineering, and usually occurs when the stress level is lower than the strength of the material. Therefore, understanding the fatigue performance of materials is of great significance for preventing structural failure and ensuring the structural safety and use of aircraft.

[0003] Traditional composite fatigue simulation methods mainly perform fatigue simulation calculations based on fatigue stress peak values, fatigue equal life models, stiffness degradation and strength degradation, and fatigue failure criteria. During the calculation process, the stress ratio of the peak-to-valley value of the fatigue load on the unit remains unchanged by default. However, in the real fatigue process, due to the failure of some units first, the local stress is redistributed, causing the stress ratio of the fatigue load on the unit to change. At this time, if the stress ratio change is not considered, the accuracy of the fatigue simulation calculation will be seriously reduced. Therefore, it is particularly important to propose a fatigue simulation method that changes the local stress ratio due to nonlinear deformation, which can solve the problem of decreased calculation accuracy due to the change of the unit stress ratio and make the fatigue simulation calculation results more accurate. Summary of the invention

[0004] In order to solve the problems existing in the background technology, the present invention provides a fatigue simulation method for local stress ratio change caused by nonlinear deformation.

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

[0006] S1: Determination of fatigue load peak σ based on load input from composite 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 loaded from 0 to the fatigue load peak 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 of the equivalent fatigue life under the current fatigue cycle loading described in S3 is as follows:

[0010] ΔN i =Dn×Δt i / Δt scale (1)

[0011] In formula (1):

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

[0013] Δt scale represents the dimensionless time scale parameter;

[0014] i represents the current fatigue cycle.

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

[0016] S401: numerically compare the absolute values ​​of stress of the elements under a fatigue cycle loading;

[0017] S402: Filter and save the current fatigue load peak stress point 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 element;

[0021]

[0022] In formula (3):

[0023]

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

[0025] f represents a constant value;

[0026] N fIndicates the equivalent fatigue life of the unit 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 the traditional uniaxial fatigue test, the uniaxial fatigue stiffness degradation model and uniaxial fatigue strength degradation model of the composite material are established;

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

[0030]

[0031] In formula (4):

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

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

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

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

[0036]

[0037] In formula (5):

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

[0039] α, β are the fitting parameters related to the material in the model.

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

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

[0042]

[0043] Compared with the prior art, the present invention has the following beneficial effects:

[0044] The present invention considers the influence of local stress ratio changes on the fatigue life of composite materials during the fatigue process, and establishes a more accurate composite material fatigue simulation analysis method based on the equal life model, stiffness degradation model, strength degradation model and fatigue failure criterion, optimizes the prediction accuracy of the traditional model, 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. BRIEF DESCRIPTION OF THE DRAWINGS

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

[0046] Figure 2 It is a schematic diagram of the load fatigue loading curve considering the change of the local stress ratio of the unit due to nonlinear deformation;

[0047] Figure 3 It is a flow chart of the present invention. DETAILED DESCRIPTION

[0048] The technical solution of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the invention, rather than all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of the present invention.

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

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

[0051] S2: In the 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 loaded from 0 to the fatigue load peak σ max ; In order to accurately describe the change of stress ratio of the unit during fatigue process, the process of load rise and fall is regarded as a 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 of the equivalent fatigue life under the current fatigue cycle loading described in S3 is as follows:

[0054] ΔN i =Dn×Δt i / Δt scale (1)

[0055] In formula (1):

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

[0057] Δt scale represents the dimensionless time scale parameter;

[0058] The two together control the equivalent fatigue life under one fatigue cycle loading;

[0059] i represents the current fatigue cycle.

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

[0061] S401: numerically compare the absolute values ​​of stress of the elements under a fatigue cycle loading;

[0062] S402: Filter and save the current fatigue load peak stress point 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 element;

[0066]

[0067] In formula (3):

[0068]

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

[0070] f represents a constant value, which is usually 1.06;

[0071] N fIndicates the equivalent fatigue life of the unit under the current stress direction;

[0072] A and B represent constants related to the material, which are calculated by fitting the test data.

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

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

[0075]

[0076] In formula (4):

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

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

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

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

[0081]

[0082] In formula (5):

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

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

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

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

[0087]

[0088] It will be apparent to those skilled in the art that the 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 the spirit or essential features of the invention. 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, and it is intended that all variations within the meaning and range of equivalents of the claims be included in the invention. Any reference numeral in a claim should not be considered as limiting the claim to which it relates.

[0089] In addition, it should be understood that although the present specification is described according to implementation modes, not every implementation mode contains only one independent technical solution. This description of the specification is only for the sake of clarity. Those skilled in the art should regard the specification as a whole. The technical solutions in each embodiment may also be appropriately combined to form other implementation modes that can be understood by those skilled in the art.

Claims

1. A fatigue simulation method for local stress ratio change caused by nonlinear deformation, characterized in that: The method comprises the following steps: S1: Determination of fatigue load peak σ based on load input from composite 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 loaded from 0 to the fatigue load peak 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; S4: Calculate the stress ratio R under the current fatigue cycle loading i ; S5: Calculate the equivalent fatigue life of the current element; S6: Based on the equivalent fatigue life of the current unit and the real-time stress-strain data of the traditional uniaxial fatigue test, the uniaxial fatigue stiffness degradation model and uniaxial fatigue strength degradation model of the composite material are established; S7: During the fatigue simulation calculation process, the stiffness and strength of the unit degrade with the number of fatigue cycles. At the same time, the stress ratio of the unit needs to be recalculated in each cycle. When the current stress of the unit triggers the fatigue failure criterion, the unit fails, and the stiffness and strength no longer degrade with the number of fatigue cycles. When all the units in the model fail, the fatigue simulation ends, and the equivalent life under each fatigue cycle is accumulated to calculate the fatigue life of the entire model.

2. A fatigue simulation method for local stress ratio change caused by nonlinear deformation according to claim 1, characterized in that: The calculation process of the equivalent fatigue life under the current fatigue cycle loading described in S3 is as follows: ΔN i =Dn×Δt i / Δt scale (1) In formula (1): Dn represents the cycle parameter of fatigue life, which is a fixed constant; Δt scale represents the dimensionless time scale parameter; i represents the current fatigue cycle.

3. A fatigue simulation method for local stress ratio change caused by nonlinear deformation according to claim 2, characterized in that: The S4 comprises the following steps: S401: numerically compare the absolute values ​​of stress of the elements under a fatigue cycle loading; S402: Filter and save the current fatigue load peak stress point and the current fatigue load valley stress point S403: Calculate the stress ratio R under the current fatigue cycle loading i :

4. A fatigue simulation method for local stress ratio change caused by nonlinear deformation according to claim 3, characterized in that: The S5 comprises the following steps: In formula (3): c=S c / S t , where: S c Indicates the compressive strength, S t Indicates tensile strength; f represents a constant value; N f Indicates the equivalent fatigue life of the element under the current stress direction; A and B represent constants related to the material.

5. A fatigue simulation method for local stress ratio change caused by nonlinear deformation according to claim 4, characterized in that: The uniaxial fatigue stiffness degradation model described in S6 is as follows: In formula (4): E f represents the initial fatigue stiffness of the element; ε f represents the failure strain of the element; λ,γ are the fitting parameters related to the material.

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

7. A fatigue simulation method for local stress ratio change caused by nonlinear deformation according to claim 6, characterized in that: The fatigue life calculation formula of the whole model described in S7 is as follows:

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