Construction method of composite material fatigue degradation model considering stress coupling effect

Abstract

The invention discloses a method for constructing a composite material fatigue degradation model considering a stress coupling effect, and belongs to the technical field of composite material fatigue research. The method comprises the following steps: calculating the stress of a stress unit in each direction; calculating the equivalent strength of the stress unit in the directions 1, 2 and 3; calculating the equivalent fatigue life of the stress unit in each stress direction; calculating equivalent fatigue stress values in the stress directions 1, 2 and 3, and correcting a fatigue peak load in the degradation model according to the equivalent fatigue stress values; establishing a uniaxial fatigue rigidity degradation model and a uniaxial fatigue strength degradation model of the composite material, and determining related parameters in the models; and obtaining a fatigue rigidity model and a fatigue strength degradation model considering the stress coupling effect. According to the method, the problem of insufficient model prediction precision caused by the fact that a traditional degradation model only considers single stress is optimized. Powerful reference is provided for reliability and safety design of the composite material, and the prediction accuracy of the residual rigidity and the residual strength in the fatigue process is improved.

Description

Technical Field

[0001] The invention relates to a method for constructing a composite material fatigue degradation model taking stress coupling effect into consideration, and belongs to the technical field of composite material fatigue research. Background Art

[0002] In the aerospace field, 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 during flight, such as aerodynamics, vibration, temperature changes, etc. These loads can cause fatigue damage to composite materials. Therefore, fatigue testing and performance evaluation of composite materials are of great significance to ensure the structural safety and service life of aircraft.

[0003] Composite material fatigue degradation models are currently commonly used to define damage variables that occur in composite material fatigue processes, mainly including stiffness degradation models and strength degradation models. Traditional stiffness degradation models and strength degradation models are both based on single-direction stress, which will cause a serious decrease in model accuracy when the unit is coupled with multiple stress loads. Therefore, it is particularly important to propose a reasonable and effective composite material fatigue degradation model that takes into account the stress coupling effect, and at the same time provide important model support for the numerical simulation of composite material fatigue. Summary of the invention

[0004] In order to solve the problems existing in the background technology, the present invention provides a method for constructing a composite material fatigue degradation model taking into account the stress coupling effect.

[0005] To achieve the above object, the present invention adopts the following technical solution: a method for constructing a composite material fatigue degradation model considering stress coupling effect, the method comprising the following steps:

[0006] S1: Determine the fatigue peak load σ according to the composite material fatigue test load spectrum max and fatigue stress ratio R f , and calculate the stress magnitude of the force-bearing unit in each direction;

[0007] 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;

[0008] S201: Calculate the equivalent tensile strength in one direction for tensile failure in one direction

[0009]

[0010] In formula (1):

[0011] S1t It represents the uniaxial tensile strength in 1 direction;

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

[0013] σ 12 represents the shear stress in the 12-direction;

[0014] σ 13 represents the shear stress in the 13 direction;

[0015] S 12 Indicates the shear strength in 12 directions;

[0016] S 13 It represents the shear strength in the 13 direction;

[0017] S202: Calculate the equivalent compressive strength in one direction for compression failure in one direction

[0018]

[0019] In formula (2):

[0020] S 1c It represents the uniaxial compressive strength in the 1 direction;

[0021] S203: Calculate the equivalent tensile strength in two directions for tensile failure in two directions

[0022]

[0023] In formula (3):

[0024] S 2t Indicates the uniaxial tensile strength in two directions;

[0025] σ 33 Indicates the tensile stress in 3 directions;

[0026] σ 23 represents the shear stress in the 23 direction;

[0027] S 23 It indicates the shear strength in the 23 direction;

[0028] S204: Calculate the equivalent compressive strength in two directions for compression failure in two directions

[0029]

[0030] In formula (4):

[0031] S 2cIt indicates the uniaxial compressive strength in two directions;

[0032] S205: Calculate the equivalent tensile strength in three directions for three-direction tensile failure

[0033]

[0034] In formula (5):

[0035] S 3t It indicates the uniaxial tensile strength in three directions;

[0036] σ 22 Indicates the tensile stress in two directions;

[0037] S206: Calculate the equivalent compressive strength in three directions for three-way compression failure

[0038]

[0039] In formula (6):

[0040] S 3c Indicates the uniaxial compressive strength in three directions.

[0041] S3: According to the tensile strength value of the composite material, the fatigue stress ratio R f =0.1, uniaxial fatigue tests were carried out at four stress levels of 90%, 85%, 80% and 75%, and the corresponding fatigue data were obtained. The relevant parameters of the traditional fatigue equal life model were interpolated and the equivalent strength input was corrected to obtain the fatigue equal life model considering the stress coupling effect, and then the equivalent fatigue life of the load-bearing unit in each stress direction was calculated;

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

[0043]

[0044] In formula (7):

[0045]

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

[0047] f represents a constant value;

[0048] Indicates the equivalent fatigue life of the load-bearing unit under the current stress direction;

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

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

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

[0052] 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:

[0053]

[0054] In formula (8):

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

[0056] 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:

[0057]

[0058] 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:

[0059]

[0060] 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:

[0061]

[0062] S405: When the stress value in the three directions is greater than 0, the equivalent fatigue stress in the three directions is the tensile equivalent fatigue stress, and the calculation formula is as follows:

[0063]

[0064] S406: When the stress value in the three directions is less than 0, the equivalent fatigue stress in the three directions is the compression equivalent fatigue stress, and the calculation formula is as follows:

[0065]

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

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

[0068]

[0069] In formula (14):

[0070] E0 represents the initial fatigue stiffness of the material;

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

[0072] ε f =S0 / E0 is a fixed value, where: S0 represents the initial fatigue strength of the material;

[0073] 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 of fatigue;

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

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

[0076]

[0077] In formula (15):

[0078] S0 represents the initial fatigue strength of the material;

[0079] S r (n) represents the residual strength of 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 fatigue strength degradation model considering the stress coupling effect are obtained.

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

[0083]

[0084] In formula (16):

[0085] represents the equivalent fatigue stress;

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

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

[0088]

[0089] Compared with the prior art, the present invention has the following beneficial effects:

[0090] The present invention is based on the life, stiffness and strength degradation models of composite material fatigue, and takes into account the influence of stress coupling effect in fatigue, and constructs a composite material degradation model that more accurately considers stress coupling effect, which optimizes the problem that the traditional degradation model only considers a single stress and leads to insufficient model prediction accuracy. It not only fills the gap in the research field of composite material fatigue performance, but also provides a powerful reference for the reliability and safety design of composite materials, and significantly improves the prediction accuracy of residual stiffness and residual strength during fatigue. At the same time, the present invention provides key model inputs for the numerical simulation of composite materials under composite fatigue loads, which greatly promotes the development of fatigue simulation calculation technology. BRIEF DESCRIPTION OF THE DRAWINGS

[0091] Figure 1 is a flow chart of the present invention;

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

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

[0094] 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.

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

[0096] S1: Determine the fatigue peak load σ according to the composite material fatigue test load spectrum max and fatigue stress ratio R f, and rely on abaqus finite element software to calculate the stress magnitude of the force unit in all directions;

[0097] 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 to reflect the difference between the failure strength of the main direction of the load-bearing unit under multi-stress coupling conditions and the failure strength under uniaxial load;

[0098] S201: Calculate the equivalent tensile strength in direction 1 (fiber direction) for tensile failure in direction 1

[0099]

[0100] In formula (1):

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

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

[0103] σ 12 represents the shear stress in the 12-direction;

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

[0105] S 12 Indicates the shear strength in 12 directions;

[0106] S 13 It represents the shear strength in the 13 direction;

[0107] S202: Calculate the equivalent compressive strength in direction 1 (fiber direction) for compression failure in direction 1

[0108]

[0109] In formula (2):

[0110] S 1c It represents the uniaxial compressive strength in the 1 direction;

[0111] S203: Calculate the equivalent tensile strength in two directions for tensile failure in two directions (transverse direction of the fiber)

[0112]

[0113] In formula (3):

[0114] S 2t Indicates the uniaxial tensile strength in two directions;

[0115] σ 33 Indicates the tensile stress in 3 directions;

[0116] σ 23 represents the shear stress in the 23 direction;

[0117] S 23 It indicates the shear strength in the 23 direction;

[0118] S204: Calculate the equivalent compressive strength in two directions for compression failure in two directions (transverse direction of the fiber)

[0119]

[0120] In formula (4):

[0121] S 2c It indicates the uniaxial compressive strength in two directions;

[0122] S205: Calculate the equivalent tensile strength in three directions for tensile failure in three directions (transverse direction of the fiber)

[0123]

[0124] In formula (5):

[0125] S 3t It indicates the uniaxial tensile strength in three directions;

[0126] σ 22 Indicates the tensile stress in two directions;

[0127] S206: Calculate the equivalent compressive strength in three directions for compression failure in three directions (transverse direction of the fiber)

[0128]

[0129] In formula (6):

[0130] S 3c Indicates the uniaxial compressive strength in three directions.

[0131] S3: According to the tensile strength value of the composite material, the fatigue stress ratio R f =0.1, uniaxial fatigue tests were carried out at four stress levels of 90%, 85%, 80% and 75%, and the corresponding fatigue data were obtained. The relevant parameters of the traditional fatigue equal life model were interpolated and the equivalent strength input was corrected to obtain the fatigue equal life model considering the stress coupling effect, and then the equivalent fatigue life of the load-bearing unit in each stress direction was calculated;

[0132] The fatigue life model considering stress coupling effect described in S3 is as follows:

[0133]

[0134] In formula (7):

[0135]

[0136] where σ min represents the fatigue valley load, represents the uniaxial equivalent compressive strength, It represents the uniaxial equivalent tensile strength;

[0137] f represents a constant value, which is usually 1.06;

[0138] Indicates the equivalent fatigue life of the load-bearing unit under the current stress direction;

[0139] A and B are constants related to the material and are calculated by fitting the test data.

[0140] 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 degraded strength and degraded 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, and the fatigue peak load in the degradation model is corrected accordingly;

[0141] The calculation of the equivalent fatigue stress values ​​in the stress directions 1, 2, and 3 described in S4 includes the following steps:

[0142] 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:

[0143]

[0144] In formula (8):

[0145] σ 11 represents the tensile stress in direction 1;

[0146] 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:

[0147]

[0148] 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:

[0149]

[0150] 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:

[0151]

[0152] S405: When the stress value in the three directions is greater than 0, the equivalent fatigue stress in the three directions is the tensile equivalent fatigue stress, and the calculation formula is as follows:

[0153]

[0154] S406: When the stress value in the three directions is less than 0, the equivalent fatigue stress in the three directions is the compression equivalent fatigue stress, and the calculation formula is as follows:

[0155]

[0156] S5: Based on the real-time stress-strain data of uniaxial fatigue tests in stress directions 1, 2, and 3, a uniaxial fatigue stiffness degradation model and a uniaxial fatigue strength degradation model of composite materials are established, and relevant parameters in the model are determined;

[0157] The uniaxial fatigue stiffness degradation model described in S5 is as follows:

[0158]

[0159] In formula (14):

[0160] E0 represents the initial fatigue stiffness of the material;

[0161] E r (n) represents the residual stiffness of the material after fatigue;

[0162] ε f =S0 / E0 is a fixed value, where: S0 represents the initial fatigue strength of the material;

[0163] 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 of fatigue;

[0164] p and q represent constants related to the material and are calculated by fitting the test data.

[0165] The uniaxial fatigue strength degradation model described in S5 is as follows:

[0166]

[0167] In formula (15):

[0168] S0 represents the initial fatigue strength of the material;

[0169] S r (n) represents the residual strength of material fatigue;

[0170] α and β are constants related to the material and are calculated by fitting the test data.

[0171] 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.

[0172] The fatigue stiffness degradation model considering the stress coupling effect described in S6 is as follows:

[0173]

[0174] In formula (16):

[0175] represents the equivalent fatigue stress;

[0176] represents the fatigue damage of the modified material, represents the modified fatigue life.

[0177] The fatigue strength degradation model considering the stress coupling effect described in S6 is as follows:

[0178]

[0179] Comparison of the fatigue stiffness degradation model and strength degradation model under multi-stress coupling with the traditional uniaxial degradation model under the same uniaxial fatigue load, as shown in the following figure. Figure 2 and Figure 3 shown.

[0180] 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.

[0181] 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 method for constructing a composite material fatigue degradation model considering stress coupling effect, characterized in that: The method comprises the following steps: S1: Determine the fatigue peak load σ according to the composite material fatigue test load spectrum max and fatigue stress ratio R f , and calculate the stress magnitude of the force-bearing unit 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 carried out at four stress levels of 90%, 85%, 80% and 75%, and the corresponding fatigue data were obtained. The relevant parameters of the traditional fatigue equal life model were interpolated and the equivalent strength input was corrected to obtain the fatigue equal life model considering the stress coupling effect, and then the equivalent fatigue life of the load-bearing unit in each stress direction was calculated; S4: Based on the Hashin criterion, the ultimate degraded strength and degraded stiffness of the material are corrected, and the equivalent fatigue stress values ​​in the 1st, 2nd and 3rd stress directions are calculated respectively, and 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, a uniaxial fatigue stiffness degradation model and a uniaxial fatigue strength degradation model of composite materials are established, and relevant parameters in the model are determined; 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 effect 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 It represents the uniaxial tensile strength in 1 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 It represents 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 It represents the uniaxial compressive strength in the 1 direction; S203: Calculate the equivalent tensile strength in two directions for tensile failure in two directions In formula (3): S 2t Indicates the uniaxial tensile strength in two directions; σ 33 Indicates the tensile stress in 3 directions; σ 23 represents the shear stress in the 23 direction; S 23 It 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 It 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-way 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 effect according to claim 2, characterized in that: The fatigue life model considering 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 unit under the current stress direction; A and B are constants related to the material.

4. The method for constructing a composite material fatigue degradation model considering stress coupling effect according to claim 3, characterized in that: The calculation of the equivalent fatigue stress values ​​in the 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 the three directions is greater than 0, the equivalent fatigue stress in the three directions is the tensile equivalent fatigue stress, and the calculation formula is as follows: S406: When the stress value in the three directions is less than 0, the equivalent fatigue stress in the three directions is the compression equivalent fatigue stress, and the calculation formula is as follows:

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 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 of fatigue; p and q represent constants related to the material.

6. The method for constructing a composite material fatigue degradation model considering stress coupling effect according to claim 5, 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 material fatigue; α and β represent constants related to the material.

7. The method for constructing a composite material fatigue degradation model considering stress coupling effect according to claim 6, 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 fatigue damage of the modified material, represents the modified fatigue life.

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

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