Construction method of composite fatigue degradation model considering stress coupling effect

Abstract

The method for constructing a composite material fatigue degradation model considering stress coupling effect belongs to the technical field of composite material fatigue research. The method is as follows: the stress size of a stress unit in each direction is calculated; the equivalent strength of the stress unit in 1, 2 and 3 directions is calculated; the equivalent fatigue life of the stress unit in each stress direction is calculated; the equivalent fatigue stress values of 1, 2 and 3 stress directions are calculated, and the fatigue peak load in the degradation model is corrected according to the equivalent fatigue stress values; a composite material uniaxial fatigue stiffness degradation model and a uniaxial fatigue strength degradation model are established, and the related parameters in the models are determined; and the fatigue stiffness model and the fatigue strength degradation model considering the stress coupling effect are obtained. The method optimizes the problem that the traditional degradation model only considers single stress, which leads to insufficient prediction accuracy of the model. The method provides a strong reference for the reliability and safety design of the composite material, and improves the prediction accuracy of the residual stiffness and the residual strength in the fatigue process.

Description

TECHNICAL FIELD

[0001] The application relates to a method for constructing a composite material fatigue degradation model considering stress coupling effects, and belongs to the technical field of composite material fatigue research. BACKGROUND

[0002] In the field of aerospace, composite materials are widely used in the manufacture of aircraft, rockets, satellites and other aircraft due to their light weight, high strength, corrosion resistance, high temperature resistance and other characteristics. However, these aircraft will be subjected to complex alternating loads such as aerodynamic force, vibration, temperature change and the like during flight, which will cause fatigue damage to the composite materials. Therefore, fatigue testing and performance evaluation of the composite materials are of great significance to ensure the structural safety and service life of the aircraft.

[0003] The composite material fatigue degradation model is currently commonly used to define the damage variable of the material damage in the composite material fatigue process, and mainly includes a stiffness degradation model and a strength degradation model. The traditional stiffness degradation model and the strength degradation model are both for a single direction stress, and in the case that the element is subjected to multi-stress coupling, the model precision will be seriously reduced. Therefore, it is particularly important to propose a reasonable and effective composite material fatigue degradation model considering stress coupling effects, and to provide important model support for the numerical simulation of the composite material fatigue. SUMMARY

[0004] To solve the problems in the background art, the application provides a method for constructing a composite material fatigue degradation model considering stress coupling effects.

[0005] To achieve the above-mentioned purpose, the application adopts the following technical scheme: a method for constructing a composite material fatigue degradation model considering stress coupling effects, the method comprising the following steps:

[0006] S1: according to the composite material fatigue test load spectrum, determining the fatigue peak load σ max and the fatigue stress ratio R f , and calculating the stress size of the stressed element in each direction;

[0007] S2: considering the multi-stress coupling effect, calculating the equivalent strength of the stressed element in 1, 2 and 3 directions based on the hashin criterion;

[0008] S201: for 1-direction tensile failure, calculating the equivalent tensile strength of 1 direction

[0009]

[0010] In formula (1):

[0011] S1t uniaxial tensile strength in direction 1;

[0012] a represents a shear correction coefficient in the Hashin criterion;

[0013] σ 12 shear stress in direction 12;

[0014] σ 13 shear stress in direction 13;

[0015] S 12 shear strength in direction 12;

[0016] S 13 shear strength in direction 13;

[0017] S202: for 1-direction compression failure, calculate the equivalent compression strength in direction 1

[0018]

[0019] In formula (2):

[0020] S 1c uniaxial compression strength in direction 1;

[0021] S203: for 2-direction tensile failure, calculate the equivalent tensile strength in direction 2

[0022]

[0023] In formula (3):

[0024] S 2t uniaxial tensile strength in direction 2;

[0025] σ 33 tensile stress in direction 3;

[0026] σ 23 shear stress in direction 23;

[0027] S 23 shear strength in direction 23;

[0028] S204: for 2-direction compression failure, calculate the equivalent compression strength in direction 2

[0029]

[0030] In formula (4):

[0031] S 2cuniaxial compressive strength in 2 direction;

[0032] S205: Calculate the equivalent tensile strength in 3 direction for 3 direction tensile failure

[0033]

[0034] In formula (5):

[0035] S 3t uniaxial tensile strength in 3 direction;

[0036] σ 22 tensile stress in 2 direction;

[0037] S206: Calculate the equivalent compressive strength in 3 direction for 3 direction compressive failure

[0038]

[0039] In formula (6):

[0040] S 3c uniaxial compressive strength in 3 direction.

[0041] S3: According to the tensile strength value of the composite material, perform uniaxial fatigue test at four stress levels of 90%, 85%, 80% and 75% under the condition of fatigue stress ratio R f = 0.1, obtain the corresponding fatigue data, correct the equivalent strength input after interpolating the related parameters of the traditional fatigue equivalent life model, obtain the fatigue equivalent life model considering the stress coupling effect, and then calculate the equivalent fatigue life of the force unit in each stress direction;

[0042] The fatigue equivalent life model considering the stress coupling effect in S3 is as follows

[0043]

[0044] In formula (7):

[0045]

[0046] wherein σ min represents the fatigue valley load, represents the uniaxial equivalent compressive strength, represents the uniaxial equivalent tensile strength;

[0047] f represents a constant value;

[0048] represents the equivalent fatigue life of the force unit in the current stress direction;

[0049] A and B are constants related to the material.

[0050] S4: Based on the Hashin criterion, the final degradation strength and degradation stiffness of the material are corrected, and the equivalent fatigue stress values of 1, 2, and 3 stress directions are calculated, and the fatigue peak load in the degradation model is corrected accordingly;

[0051] The calculation of the equivalent fatigue stress values of 1, 2, and 3 stress directions in S4 includes the following steps:

[0052] S401: When the 1-direction stress value is greater than 0, the 1-direction equivalent fatigue stress is the tensile equivalent fatigue stress, and the calculation formula is as follows:

[0053]

[0054] In formula (8):

[0055] σ 11 represents the tensile stress in the 1-direction;

[0056] S402: When the 1-direction stress value is less than 0, the 1-direction equivalent fatigue stress is the compressive equivalent fatigue stress, and the calculation formula is as follows:

[0057]

[0058] S403: When the 2-direction stress value is greater than 0, the 2-direction equivalent fatigue stress is the tensile equivalent fatigue stress, and the calculation formula is as follows:

[0059]

[0060] S404: When the 2-direction stress value is less than 0, the 2-direction equivalent fatigue stress is the compressive equivalent fatigue stress, and the calculation formula is as follows:

[0061]

[0062] S405: When the 3-direction stress value is greater than 0, the 3-direction equivalent fatigue stress is the tensile equivalent fatigue stress, and the calculation formula is as follows:

[0063]

[0064] S406: When the 3-direction stress value is less than 0, the 3-direction equivalent fatigue stress is the compressive equivalent fatigue stress, and the calculation formula is as follows:

[0065]

[0066] S5: Based on the real-time stress and strain data of the uniaxial fatigue test in 1, 2, 3 stress directions, the uniaxial fatigue stiffness degradation model and the uniaxial fatigue strength degradation model of the composite material are established, and the related parameters in the model are determined;

[0067] The uniaxial fatigue stiffness degradation model in S5 is as follows:

[0068]

[0069] In formula (14):

[0070] E0represents the initial fatigue stiffness of the material;

[0071] E r (n) represents the remaining stiffness of the material fatigue;

[0072] ε f =S0 / E0is a fixed value, wherein: S0represents the initial fatigue strength of the material;

[0073] x=n / N f represents the fatigue damage of the material, and the damage size increases with the increase of the fatigue cycle number, wherein: n represents the current fatigue cycle number, N f represents the total fatigue life;

[0074] p and q represent constants related to the material.

[0075] The uniaxial fatigue strength degradation model in S5 is as follows:

[0076]

[0077] In formula (15):

[0078] S0represents the initial fatigue strength of the material;

[0079] S r (n) represents the remaining strength of the material fatigue;

[0080] α and β represent constants related to the material.

[0081] S6: Combining the equivalent fatigue stress and the traditional fatigue stiffness degradation model, the fatigue stiffness model and the fatigue strength degradation model considering the stress coupling effect are obtained.

[0082] The fatigue stiffness degradation model considering the stress coupling effect in S6 is as follows:

[0083]

[0084] In formula (16):

[0085] represents equivalent fatigue stress;

[0086] represents the corrected fatigue damage of the material, represents the corrected fatigue life.

[0087] The fatigue strength degradation model considering the stress coupling effect in S6 is as follows:

[0088]

[0089] Compared with the prior art, the beneficial effects of the present application are:

[0090] The present application is based on the composite material fatigue equivalent life, stiffness and strength degradation model, and considers the influence of stress coupling effect in fatigue, and constructs a more precise composite material degradation model considering the stress coupling effect, which optimizes the problem of insufficient model prediction accuracy caused by the traditional degradation model only considering single stress. Not only fills the gap in the field of composite material fatigue performance research, but also provides a strong reference for the reliability and safety design of composite materials, significantly improves the prediction accuracy of residual stiffness and residual strength in the fatigue process. At the same time, the present application provides a key model input for numerical simulation of composite materials under composite fatigue load, greatly promoting the development of fatigue simulation calculation technology. BRIEF DESCRIPTION OF DRAWINGS

[0091] Figure 1 is a flowchart of the present application;

[0092] Figure 2 is a comparison chart of fatigue stiffness degradation considering stress coupling effect and traditional model;

[0093] Figure 3 is a comparison chart of fatigue strength degradation considering stress coupling effect and traditional model. DETAILED DESCRIPTION

[0094] The technical solutions in the present application will be described clearly and completely below in combination with the drawings in the embodiments of the present application. Obviously, the described embodiments are only a part of the embodiments of the present application, but not all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative labor fall within the scope of protection of the present application.

[0095] A method for constructing a composite material fatigue degradation model considering stress coupling effect, the method comprising the following steps:

[0096] S1: According to the composite material fatigue test load spectrum, the fatigue peak load σ max and the fatigue stress ratio R fand stress size of stress unit in each direction is calculated by relying on abaqus finite element software;

[0097] S2: considering multi-stress coupling effect, the equivalent strength of stress unit in 1, 2 and 3 directions is calculated respectively based on hashin criterion, so as to reflect the difference between the main direction failure strength of stress unit under multi-stress coupling condition and the failure strength under uniaxial load;

[0098] S201: for 1 direction (fiber direction) tensile failure, the equivalent tensile strength of 1 direction is calculated

[0099]

[0100] In formula (1):

[0101] S 1t represents the uniaxial tensile strength of 1 direction;

[0102] α represents the shear correction coefficient in hashin criterion;

[0103] σ 12 represents the shear stress of 12 direction;

[0104] σ 13 represents the shear stress of 13 direction;

[0105] S 12 represents the shear strength of 12 direction;

[0106] S 13 represents the shear strength of 13 direction;

[0107] S202: for 1 direction (fiber direction) compression failure, the equivalent compression strength of 1 direction is calculated

[0108]

[0109] In formula (2):

[0110] S 1c represents the uniaxial compression strength of 1 direction;

[0111] S203: for 2 direction (fiber transverse direction) tensile failure, the equivalent tensile strength of 2 direction is calculated

[0112] In formula (3):

[0113] S 2t represents the uniaxial tensile strength of 2 direction;

[0114] σ 33Indicates the tensile stress in three directions;

[0115] σ 23 represents the shear stress in the 23 direction;

[0116] S 23 Indicates the shear strength in the 23 direction;

[0117] S204: Calculate the equivalent compressive strength in two directions for compression failure (transverse direction of the fiber)

[0118] In formula (4):

[0119] S 2c Indicates the uniaxial compressive strength in two directions;

[0120] S205: Calculate the equivalent tensile strength in three directions (transverse direction of the fiber) for tensile failure

[0121] In formula (5):

[0122] S 3t Indicates the uniaxial tensile strength in three directions;

[0123] σ 22 Indicates the tensile stress in two directions;

[0124] S206: Calculate the equivalent compressive strength in three directions for compression failure (transverse direction of the fiber)

[0125] In formula (6):

[0126] S 3c Indicates the uniaxial compressive strength in three directions.

[0127] S3: According to the tensile strength value of the composite material, the fatigue stress ratio R f = 0.1, uniaxial fatigue tests were conducted at four stress levels: 90%, 85%, 80%, and 75%. The corresponding fatigue data were obtained. The relevant parameters of the traditional fatigue equivalent life model were interpolated and the equivalent strength input was corrected. The fatigue equivalent life model considering the stress coupling effect was obtained, and the equivalent fatigue life of the load-bearing unit in each stress direction was then calculated.

[0128] The fatigue equivalent life model considering the stress coupling effect described in S3 is as follows:

[0129]

[0130] In formula (7):

[0131]

[0132] where σ min represents the fatigue valley load, represents the uniaxial equivalent compressive strength, It represents the uniaxial equivalent tensile strength;

[0133] f represents a constant value, generally 1.06;

[0134] Indicates the equivalent fatigue life of the load-bearing element under the current stress direction;

[0135] A and B are constants related to the material and are calculated by fitting the test data.

[0136] S4: For the strength degradation and stiffness degradation of composite materials during fatigue, when the load-bearing unit is subjected to multiaxial stress coupling effect and the strength value of the material has not degraded to the fatigue peak load σ max When , the fatigue failure criterion is triggered and the load-bearing unit fails. At this time, the final degradation strength and degradation stiffness of the material are corrected based on the Hashin criterion, and the equivalent fatigue stress values ​​in the 1st, 2nd and 3rd stress directions are calculated respectively. Based on this, the fatigue peak load in the degradation model is corrected.

[0137] The calculation of the equivalent fatigue stress values ​​in stress directions 1, 2, and 3 described in S4 includes the following steps:

[0138] S401: When the stress value in one direction is greater than 0, the equivalent fatigue stress in one direction is the tensile equivalent fatigue stress, and the calculation formula is as follows:

[0139]

[0140] In formula (8):

[0141] σ 11 represents the tensile stress in direction 1;

[0142] S402: When the stress value in one direction is less than 0, the equivalent fatigue stress in one direction is the compression equivalent fatigue stress, and the calculation formula is as follows:

[0143]

[0144] S403: When the stress value in two directions is greater than 0, the equivalent fatigue stress in two directions is the tensile equivalent fatigue stress, and the calculation formula is as follows:

[0145]

[0146] S404: When the 2-direction stress value is less than 0, the 2-direction equivalent fatigue stress For compressive equivalent fatigue stress, the calculation formula is as follows:

[0147]

[0148] S405: When the 3-direction stress value is greater than 0, the 3-direction equivalent fatigue stress For tensile equivalent fatigue stress, the calculation formula is as follows:

[0149]

[0150] S406: When the 3-direction stress value is less than 0, the 3-direction equivalent fatigue stress For compressive equivalent fatigue stress, the calculation formula is as follows:

[0151]

[0152] S5: Based on the real-time stress and strain data of the uniaxial fatigue test of the 1, 2, and 3 stress directions, a composite material uniaxial fatigue stiffness degradation model and a uniaxial fatigue strength degradation model are established, and relevant parameters in the models are determined;

[0153] The uniaxial fatigue stiffness degradation model described in S5 is as follows:

[0154]

[0155] In formula (14):

[0156] E0 represents the initial fatigue stiffness of the material;

[0157] E r (n) represents the remaining stiffness of the material fatigue;

[0158] ε f =S0 / E0 is a fixed value, wherein: S0 represents the initial fatigue strength of the material;

[0159] x=n / N f represents the fatigue damage of the material, and the damage size increases with the increase of the number of fatigue cycles, wherein: n represents the current fatigue cycle number, N f represents the total fatigue life;

[0160] p and q represent constants related to the material, which are obtained by fitting calculation of test data.

[0161] The uniaxial fatigue strength degradation model described in S5 is as follows:

[0162]

[0163] In formula (15):

[0164] S0 represents the initial fatigue strength of the material;

[0165] S r (n) represents the residual strength of the material fatigue;

[0166] α and β represent constants related to the material, which are calculated by fitting the experimental data.

[0167] S6: combining the equivalent fatigue stress and the traditional fatigue stiffness degradation model, the fatigue stiffness model and the fatigue strength degradation model considering the stress coupling effect are obtained.

[0168] The fatigue stiffness degradation model considering the stress coupling effect described in S6 is as follows:

[0169]

[0170] In formula (16):

[0171] represents the equivalent fatigue stress;

[0172] represents the modified fatigue damage of the material, represents the modified fatigue life.

[0173] The fatigue strength degradation model considering the stress coupling effect described in S6 is as follows:

[0174]

[0175] Comparing the fatigue stiffness degradation model and the strength degradation model considering the multi-stress coupling effect under the same uniaxial fatigue load with the traditional uniaxial degradation model, as shown in Figure 2 and Figure 3 .

[0176] It is apparent to those skilled in the art that the present application is not limited to the details of the foregoing exemplary embodiments, but can be implemented in other forms without departing from the spirit or essential characteristics of the present application. Therefore, the embodiments should be considered in all respects as illustrative and not restrictive, the scope of the application being defined by the appended claims rather than by the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein. Any reference signs in the claims should not be construed as limiting the claims to the figures in which the reference signs are used.

[0177] Furthermore, it should be understood that although the specification is described in terms of embodiments, not every embodiment includes every feature or implementation described herein. The specification can include implicit combinations of explicitly mentioned features and / or implicit combinations of implicitly mentioned features. Such combinations are also expressly included within the scope of the specification and an embodiment.

Claims

1. A method for constructing a composite material fatigue degradation model considering stress coupling effects, characterized by: The method comprises the following steps: S1: Determine the fatigue peak load σ based on the composite material fatigue test load spectrum max and fatigue stress ratio R f , and calculate the stress magnitude of the force element in each direction; S2: Considering the multi-stress coupling effect, the equivalent strength of the load-bearing unit in directions 1, 2, and 3 is calculated based on the Hashin criterion; S3: According to the tensile strength value of the composite material, the fatigue stress ratio R f = 0.1, uniaxial fatigue tests were conducted at four stress levels: 90%, 85%, 80%, and 75%. The corresponding fatigue data were obtained. The relevant parameters of the traditional fatigue equivalent life model were interpolated and the equivalent strength input was corrected. The fatigue equivalent life model considering the stress coupling effect was obtained, and the equivalent fatigue life of the load-bearing unit in each stress direction was then calculated. The fatigue equivalent life model considering the stress coupling effect described in S3 is as follows: In formula (7): where σ min represents the fatigue valley load, represents the uniaxial equivalent compressive strength, It represents the uniaxial equivalent tensile strength; f represents a constant value; Indicates the equivalent fatigue life of the load-bearing element under the current stress direction; A and B are constants related to the material; S4: Based on the Hashin criterion, the ultimate degradation strength and degradation stiffness of the material are corrected, and the equivalent fatigue stress values ​​in stress directions 1, 2, and 3 are calculated respectively. The fatigue peak load in the degradation model is corrected accordingly. S5: Based on the real-time stress-strain data of uniaxial fatigue tests in stress directions 1, 2, and 3, establish a uniaxial fatigue stiffness degradation model and a uniaxial fatigue strength degradation model for composite materials and determine the relevant parameters in the model; S6: Combining the equivalent fatigue stress and the traditional fatigue stiffness degradation model, the fatigue stiffness model and fatigue strength degradation model considering the stress coupling effect are obtained.

2. The method for constructing a composite material fatigue degradation model considering stress coupling effects according to claim 1, characterized in that: The S2 comprises the following steps: S201: Calculate the equivalent tensile strength in one direction for tensile failure in one direction In formula (1): S 1t Indicates the uniaxial tensile strength in one direction; α represents the shear correction coefficient in the Hashin criterion; σ 12 represents the shear stress in the 12-direction; σ 13 represents the shear stress in the 13 direction; S 12 Indicates the shear strength in 12 directions; S 13 Indicates the shear strength in the 13 direction; S202: Calculate the equivalent compressive strength in one direction for compression failure in one direction In formula (2): S 1c Indicates the uniaxial compressive strength in direction 1; S203: Calculate the equivalent tensile strength in two directions for two-way tensile failure In formula (3): S 2t Indicates the uniaxial tensile strength in two directions; σ 33 Indicates the tensile stress in three directions; σ 23 represents the shear stress in the 23 direction; S 23 Indicates the shear strength in the 23 direction; S204: Calculate the equivalent compressive strength in two directions for compression failure in two directions In formula (4): S 2c Indicates the uniaxial compressive strength in two directions; S205: Calculate the equivalent tensile strength in three directions for three-direction tensile failure In formula (5): S 3t Indicates the uniaxial tensile strength in three directions; σ 22 Indicates the tensile stress in two directions; S206: Calculate the equivalent compressive strength in three directions for three-direction compression failure In formula (6): S 3c Indicates the uniaxial compressive strength in three directions.

3. The method for constructing a composite material fatigue degradation model considering stress coupling effects according to claim 2, characterized in that: The calculation of the equivalent fatigue stress values ​​in stress directions 1, 2, and 3 described in S4 includes the following steps: S401: When the stress value in one direction is greater than 0, the equivalent fatigue stress in one direction is the tensile equivalent fatigue stress, and the calculation formula is as follows: In formula (8): σ 11 represents the tensile stress in direction 1; S402: When the stress value in one direction is less than 0, the equivalent fatigue stress in one direction is the compression equivalent fatigue stress, and the calculation formula is as follows: S403: When the stress value in two directions is greater than 0, the equivalent fatigue stress in two directions is the tensile equivalent fatigue stress, and the calculation formula is as follows: S404: When the stress value in two directions is less than 0, the equivalent fatigue stress in two directions is the compression equivalent fatigue stress, and the calculation formula is as follows: S405: When the stress value in three directions is greater than 0, the equivalent fatigue stress in three directions is the tensile equivalent fatigue stress, and the calculation formula is as follows: S406: When the stress value in three directions is less than 0, the equivalent fatigue stress in three directions is the compression equivalent fatigue stress, and the calculation formula is as follows:

4. The method for constructing a composite material fatigue degradation model considering stress coupling effects according to claim 3, characterized in that: The uniaxial fatigue stiffness degradation model described in S5 is as follows: In formula (14): E0 represents the initial fatigue stiffness of the material; E r (n) represents the residual stiffness of the material after fatigue; ε f =S0 / E0 is a fixed value, where: S0 represents the initial fatigue strength of the material; x=n / N f Indicates the fatigue damage of the material. The damage increases with the number of fatigue cycles, where n represents the current fatigue cycle, N f represents the total life span from fatigue; p and q represent material-related constants.

5. The method for constructing a composite material fatigue degradation model considering stress coupling effect according to claim 4, characterized in that: The uniaxial fatigue strength degradation model described in S5 is as follows: In formula (15): S0 represents the initial fatigue strength of the material; S r (n) represents the residual strength of the material due to fatigue; α and β represent constants related to the material.

6. The method for constructing a composite material fatigue degradation model considering stress coupling effects according to claim 5, characterized in that: The fatigue stiffness degradation model considering the stress coupling effect described in S6 is as follows: In formula (16): represents the equivalent fatigue stress; represents the corrected fatigue damage of the material, represents the modified fatigue life.

7. The method for constructing a composite material fatigue degradation model considering stress coupling effects according to claim 6, characterized in that: The fatigue strength degradation model considering the stress coupling effect described in S6 is as follows:

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