Fatigue failure mode prediction method for composite-to-metal hybrid joint structure

Abstract

The application discloses a composite material and metal rivet hybrid connection structure fatigue failure mode prediction method, which comprises the following steps: presetting a composite material / metal rivet hybrid connection, including mechanical connection failure, and constructing a material parameter prediction model based on water absorption rate, chemical structure and environmental temperature; calculating the maximum strain of each material in the joint structure; processing the original material epsilon-N curve, that is, the relationship curve between the maximum strain under fatigue loading and the fatigue cycle life, to meet the prediction demand of different environmental influences; and judging the failure corresponding to the preset specific type according to the maximum strain of each material in the joint structure and the equal life curve. The application simplifies the test process, significantly shortens the test period, and can realize rapid calculation based on an ordinary personal notebook computer.

Description

Technical Field

[0001] This invention relates to a technology in the field of composite material processing, specifically a rapid prediction method for fatigue failure modes of composite material and metal adhesive riveting hybrid connection structures that takes into account the influence of environmental factors. Background Technology

[0002] Hybrid (riveting / bonding) connections involve joining advanced composite materials and high-strength alloys to form a complete structure. Unlike traditional joints with relatively singular failure types, hybrid connections exhibit a variety of failure modes. Furthermore, due to the combined effects of damp heat aging and varying fatigue loading during service, the failure modes of hybrid connections are difficult to predict. For fatigue failure issues in service environments of composite material and metal-bonded hybrid connection structures, analyzing the influence of damp heat aging and predicting the relative strength of different parts of the joint under different fatigue loading conditions are crucial for achieving rapid prediction of fatigue failure modes. Summary of the Invention

[0003] This invention addresses the shortcomings of existing technologies that fail to consider complex connection structures containing adhesive layers and cannot calculate the fatigue life of connections under the influence of damp heat aging during actual service. It proposes a fatigue failure mode prediction method for hybrid connections of composite materials and metal adhesive rivets. Based on prior knowledge and pre-defined failure categories, the proposed internal grouping competition fatigue mechanism of the hybrid joint is used to compare the relative advantages of materials under specific fatigue loading and environmental influence conditions. The results are matched with the pre-defined failure categories, thereby achieving rapid prediction of fatigue failure types.

[0004] This invention is achieved through the following technical solution:

[0005] This invention relates to a method for predicting fatigue failure modes in a composite material and metal adhesive riveting hybrid connection structure, comprising:

[0006] The first step is to pre-determine fatigue failure modes for composite material / metal adhesive riveting hybrid connections, including mechanical connection failure (Type A failure) and adhesive failure (Type B failure).

[0007] The fatigue failure mode refers to the fact that under the combined effects of different humid and hot environments and fatigue load conditions, the materials constituting the hybrid connection have different relative competitive advantages, which may lead to multiple failure modes. Among them, the coexistence of mechanical connection failure, i.e., Type A failure, and adhesive failure, i.e., Type B failure, will lead to complete failure of the connection.

[0008] The mechanical connection failures include fastener failures, i.e., type A1 failures, and substrate failures, i.e., type A2 failures.

[0009] The adhesive failures mentioned include composite material delamination, i.e., type B1 failure, and adhesive layer failure, i.e., type B2 failure.

[0010] In the fatigue failure modes described, when any form of mechanical connection failure occurs, the hybrid joint degenerates into a bonded joint. Therefore, type A1 and type A2 failures cannot occur on the same joint during the failure process. Similarly, type B1 and type B2 failures cannot occur simultaneously during the failure process. Due to the different competitive advantages of various materials under different conditions, the order of their failure modes will also change. Therefore, for the same hybrid connection, under different humid and hot conditions and fatigue load conditions, the specific failure modes include: type A1_B1 failure, type A1_B2 failure, type A2_B1 failure, type A2_B2 failure, type B1_A1 failure, type B1_A2 failure, type B2_A1 failure, and type B2_A2 failure, where "_" indicates the sequential order of the failure types, i.e., the first stage and the second stage. The preset failure modes already include all possible failures for this type of connection.

[0011] The fatigue failure mode refers to the specific form in which the connection structure completely breaks after fatigue loading, or partially fails upon reaching the fatigue limit.

[0012] The aforementioned hot and humid environment refers to a temperature range of -40 to 80°C and a humidity range of 0% to 100% relative humidity.

[0013] The competitive advantage refers to the superior performance of a material or structure compared to other materials or structures under specific humid and hot conditions and fatigue loading conditions.

[0014] The aforementioned mechanical connection failure refers to the release of the constraint effect of the rivet on the composite material substrate and the metal substrate.

[0015] The adhesive failure refers to the release of the binding effect of the adhesive on the composite substrate and the metal substrate.

[0016] The failure mode sequence refers to the fact that, since hybrid connections are divided into two sub-parts—adhesive bonding and riveting—the failure of one part does not cause structural breakage. This means that the complete failure of an adhesive-riveted hybrid connection structure is accompanied by the complete failure of both the adhesive bonding and riveting, and a failure sequence is necessarily present. Specifically, A1 and A2 cannot coexist in the failure process, nor can B1 and B2 coexist.

[0017] To determine which of the aforementioned preset failure modes a composite material-metal adhesive riveting joint will match under specific humid and hot conditions and fatigue loading, it is first necessary to analyze the stress on the joint structure under those conditions. This analysis is based on finite element theory, and its core is to consider the prediction of material parameters under environmental conditions.

[0018] The second step is to construct a material parameter prediction model based on water absorption rate, chemical structure, and ambient temperature, specifically as follows: ,in: These are predicted values ​​of material mechanical properties, representing mechanical performance indicators such as stiffness and strength. ; Aging item Where m is the moisture absorption value, k is the number of aging-sensitive functional groups obtained from infrared spectroscopy analysis, and λ is the functional group spectral absorption intensity as a function of moisture absorption. a, b, c, and λ are fitting parameters obtained by fitting experimental data on moisture absorption, infrared spectroscopy, and the material's room temperature mechanical properties. The ambient temperature term... The ambient temperature term is used to account for the coupling effect of aging and temperature, and to correct for the influence of temperature on material properties; where T is the actual ambient temperature value. The room temperature is taken as 20℃; α and β represent the fitting coefficients of each material to the temperature term, and i represents the order of the fitting curve, which is determined according to the law of material properties changing with temperature, and is generally taken as 2.

[0019] The material parameters mentioned refer to the elastic modulus, Poisson's ratio, and strength of the material used in the finite element calculation.

[0020] The aforementioned functional group spectral absorption intensity refers to the spectrum obtained by analyzing the aged polymer material using an infrared spectrometer, where the absorption peak value corresponding to the functional group is specified.

[0021] The aging-sensitive functional groups mentioned refer to those whose spectral absorption peaks change significantly after aging. It is recommended to select the two functional groups with the most significant changes as variables based on the actual situation.

[0022] The third step is to calculate the maximum strain of each material in the joint structure: After assigning the material parameter prediction model obtained in the second step to the corresponding material properties of the connection, mesh generation, displacement boundary condition setting and contact setting are performed, and the load peak value in the fatigue cycle to be predicted, that is, the force value corresponding to the peak in the fatigue loading cycle curve, is extracted as the force boundary condition; then, the finite element calculation result file containing the strain field of each part of the joint material is obtained through finite element calculation; then, the maximum in-plane shear strain of composite material, the maximum interlaminar strain of composite material, the maximum principal strain of adhesive layer, the maximum principal strain of metal plate and the maximum principal strain of rivet are extracted and recorded.

[0023] The aforementioned mesh generation refers to: establishing a numerical model based on the actual structural geometric information and then dividing it into meshes.

[0024] The aforementioned displacement boundary condition setting refers to setting displacement constraints on the connecting parts based on the actual external displacement constraints.

[0025] The contact setting refers to the contact between the fastener and the substrate being set to a rigid contact type.

[0026] The peak load in the fatigue cycle to be predicted is extracted by a force sensor.

[0027] The fourth step is to process the raw materials. The curve, i.e., the relationship between maximum strain under fatigue loading and fatigue cycle life, is processed to meet the prediction needs of different environmental impacts: An improved Kawai method is used to construct isolife curves (CLDs) on the tension-tension and compression-compression SN curves to describe the fatigue life of materials under different stress ratios of tension / compression and shear, specifically including:

[0028] 4.1) Using the Kawai model, SN curve data prediction fatigue life, and then utilize The separator line constructs the right half of the CLD;

[0029] 4.2) Tension-tensile fatigue prediction will be used. The fatigue life was established using the Kawai model. The dividing line constructs the left half of the CLD. Since the Kawai model is constructed based on tensile fatigue tests, the compressive fatigue prediction results differ significantly from the experimental results.

[0030] 4.3) Tension-Tension Fatigue Prediction Construction of fatigue life and compressive strength points The separator line improves the linear CLD on the left half. The Kawai model CLD constructed in section 4.2 is proportionally combined with the linear CLD to form the left half of the CLD;

[0031] 4.4) Based on With both left and right CLDs obtained from the dividing line, and the complete material CLD constructed, the improved Kawai model is obtained as follows: Among them, composite materials are greatly affected by environmental factors, and the original curve expression is: In the case of environmental influence The curve needs to be degenerated to ; This refers to the number of loading cycles required to cause fatigue fracture of a composite material without considering environmental impact. This refers to the maximum strain corresponding to the peak fatigue loading of the composite material without considering environmental impact; This refers to the number of loading cycles required to cause fatigue fracture of a composite material when environmental impacts are taken into account. This refers to the maximum strain corresponding to the peak fatigue loading of composite materials when environmental impacts are taken into account. and These refer to strength and modulus unaffected by the environment, respectively. and These refer to the strength and modulus affected by the environment, respectively, predicted by the aforementioned prediction equation P(m,T); the adhesive layer is also easily affected by environmental factors, but its stiffness and strength often change proportionally, leading to its The curve is almost unaffected by the environment, so the original adhesive layer is used. The curve; the metal substrate and rivets are made of metallic materials, and their fatigue properties do not change significantly within the environmental variation range involved in this invention. Therefore, the original method of using metallic materials is applicable. curve.

[0032] The aforementioned The curves are obtained using, but are not limited to, fatigue testing machines.

[0033] The fifth step involves determining the specific failure type corresponding to the preset type in the first step based on the maximum strain of each material in the joint structure obtained in the third step and the isochronous life curve obtained in the fourth step. This includes:

[0034] 5.1) Substitute the predicted maximum strain of each component material under specific environmental influences into the value of each component material under specific environmental influences. The curves were used to calculate the fatigue life of each component material. ,in: =1,2,……5, corresponding to in-plane composite material, interlayer composite material, adhesive layer, metal plate, and rivet, respectively, where: The smallest (denoted as) The material corresponding to the first failure point is identified to determine the first stage of failure type.

[0035] 5.2) Considering the differences in fatigue competitive advantage among different materials in the joint under different fatigue loading and environmental factors, different failure types may be predicted when the environmental conditions of the same joint are changed. A fatigue competition mechanism diagram is drawn according to the qN curves of the corresponding parts for different failure types. A fatigue type partitioning diagram is drawn by extracting the intersection points of the qN curves under different environments. This diagram can be used to determine the influence of different fatigue loading and environmental factors on the first-stage failure types.

[0036] The fatigue competitive advantage mentioned above refers to the fact that the relative superiority or inferiority of the fatigue properties of various materials in the joint will change under different conditions.

[0037] The qN curve refers to the relationship between the ratio of peak fatigue load to joint strength (q) and the structural fatigue failure life (N).

[0038] 5.3) Considering the failure sequence, the structure is not completely disconnected after the first stage of failure. Returning to step three, the mesh of the numerical model corresponding to the failed parts is deleted, and the adhesive-riveted hybrid connection structure degenerates into a simple adhesive or riveted form, and the maximum strain is recalculated. Considering that some fatigue damage has already occurred in each part under cyclic loading during the first stage of failure, this damage needs to be calculated using the damage accumulation criterion and superimposed on the fatigue damage of the second failure stage to predict the fatigue type of the second stage. Specifically, this involves determining the accumulated damage amount in the first stage. When the damage reaches 100%, fatigue failure is considered to have occurred, marking the start of the second stage. The remaining fatigue life percentage of the unfractured parts is... After mesh deletion, the maximum strain is recalculated, and then based on... The curve yields the second stage of pseudo-fatigue life. Furthermore, the actual fracture life of the remaining second stage is obtained as follows: ,in: The material corresponding to the smallest failure point is the part that fails first in the second stage, and the failure type in the second stage is determined based on the failure mode preset in step one.

[0039] 5.4) Similarly, the second-stage failure type is also affected by the material fatigue competition mechanism. A fatigue competition mechanism diagram can be drawn based on the qN curves of the corresponding parts for different failure types. Based on the fatigue type zoning diagram of the first stage, the intersection points of the qN curves under different environments are extracted to draw the second-stage fatigue type zoning diagram. This diagram can be used to determine the influence of different fatigue loading and environmental factors on the second-stage failure type. Thus, the entire process of predicting the fatigue failure type of hybrid connections affected by the environment is realized.

[0040] The fatigue damage mentioned refers to the initiation and propagation of microcracks within the material during fatigue loading, even if fracture does not occur. Therefore, areas that do not fail in the first stage also accumulate a certain amount of damage.

[0041] The aforementioned damage accumulation criterion refers to the Miner fatigue damage linear accumulation criterion. Using this linear accumulation criterion satisfies the computational power requirements for rapid prediction while ensuring prediction accuracy.

[0042] The pseudo-fatigue life refers to the material fatigue life calculated assuming that no initial fatigue damage occurs at the start of the second stage.

[0043] This invention relates to a system for implementing the above-described method, comprising: a failure mode predefinition module, a material stiffness and strength performance prediction module considering the influence of environmental factors, a maximum strain extraction module for each material in the joint structure, and a module for processing the original material. The system includes a curve module and a failure mode discrimination module. Specifically: a failure mode predefinition module summarizes and analyzes the fracture mechanism of the connection structure to obtain all theoretically possible failure modes; a material stiffness and strength performance prediction module considering environmental factors constructs a material parameter prediction model based on water absorption rate, chemical structure, and ambient temperature using experimental data on moisture absorption, infrared spectroscopy, and material mechanical properties to obtain the stiffness and strength performance of the material under different humid and hot conditions; a maximum strain extraction module for each material in the joint structure establishes a numerical model based on the actual geometry of the connector and its service loading conditions, performs finite element simulation calculations, and obtains the maximum strain of each material in the connection through post-processing of the finite element calculation results; and processes the original material... Based on material fatigue loading test data, the curve module constructs equal life curves (CLD) on two SN curves, tension-tension and compression-compression, using an improved Kawai method to describe the fatigue life of composite materials under different stress ratios of tension, compression, and shear. The failure mode discrimination module determines the failure type of the hybrid connection in two steps based on the maximum strain and the material equal life curve, realizing a complete prediction of the fatigue failure type of the hybrid connection affected by the environment.

[0044] Technical effect

[0045] This invention constructs a parameter prediction model for hybrid bonding materials based on water absorption rate, chemical structure, and ambient temperature for fatigue simulation; it improves the Kawai method by constructing isolife curves (CLD) on two SN curves (tension-tension and compression-compression) to describe the fatigue life of composite materials under different stress ratios; it summarizes the fatigue failure mode of hybrid bonding into two stages and designs a staged fatigue failure type discrimination process, thus considering the influence of humid heat aging in the fatigue prediction process of glued hybrid bonding structures; the improved accuracy of the isolife curves ensures the accuracy of structural fatigue failure prediction; and it achieves complete prediction of the two-stage process of fatigue failure of hybrid bonding. Attached Figure Description

[0046] Figure 1 This is a flowchart of the present invention;

[0047] Figure 2 This is a geometric shape diagram of the object to be predicted in this invention;

[0048] Figure 3 This is a schematic diagram of the preset failure modes in this invention;

[0049] Figure 4 shows the variation of mechanical property parameters of the composite material in this invention;

[0050] In the figure: a~g represent: in-plane tensile modulus of composite material, in-plane compressive modulus of composite material, in-plane shear modulus of composite material, in-plane tensile strength of composite material, in-plane compressive strength of composite material, in-plane shear strength of composite material, and interlaminar shear strength of composite material, respectively.

[0051] Figure 5 shows the variation of mechanical property parameters of the polyurethane adhesive in this invention;

[0052] In the diagram: ab represents the modulus and strength of the polyurethane adhesive, respectively.

[0053] Figure 6 shows the infrared spectrum analysis of the composite material in this invention;

[0054] In the figure: a, b, and c represent the 500~4000 wavenumber infrared spectrum, the 2800~3000 wavenumber infrared spectrum, and the 3000~3700 wavenumber infrared spectrum, respectively.

[0055] Figure 7 is an infrared spectrum analysis diagram of the polyurethane adhesive in this invention;

[0056] In the figure: a, b, and c represent the 500~4000 wavenumber infrared spectrum, the 1650~1750 wavenumber infrared spectrum, and the 3100~3400 wavenumber infrared spectrum, respectively.

[0057] Figure 8 This is a schematic diagram of finite element mesh generation in this invention;

[0058] Figure 9 This is a schematic diagram of the simulated strain prediction results in this invention;

[0059] Figure 10 shows the materials used in this invention. Line graph;

[0060] In the figure: (a) shows the longitudinal direction of the composite material. Curve (b) represents the shear strength of the composite material. Curve (c) represents the adhesive material. Curve (d) represents 304 stainless steel. Curve (e) represents 7075 aluminum alloy. curve;

[0061] Figure 11 This is a diagram of the fatigue competition mechanism in this invention;

[0062] Figure 12a This is the first stage of failure prediction in the embodiment; Figure 12b For the second stage of failure prediction;

[0063] Figure 13 These are the actual test results of the fatigue types in this invention. Detailed Implementation

[0064] like Figure 1 As shown, this embodiment uses a hybrid adhesive-riveted connection of a 12k plain weave composite laminate of T700 carbon fiber and a 7075 aluminum alloy plate as an example for illustration. The fatigue failure mode prediction method for this connection structure specifically includes:

[0065] Step 1: Determine the object of prediction:

[0066] 1.1) The materials used to predict the joint composition are determined to be: plain woven carbon fiber composite laminate, 7075 aluminum alloy substrate, 304 stainless steel rivets, and polyurethane adhesive layer. Each layer of the plain woven carbon fiber composite laminate is 0.4 mm thick, with a total of 5 layers, and the fiber direction is consistent at 0°.

[0067] 1.2) The structure of the joint to be predicted is determined to be a single-lap connection with a combination of adhesive and riveting, and its structure is as follows: Figure 2 As shown. This structure conforms to the predicted range of this embodiment and satisfies the preset failure mode for complete fracture, that is, under different humid heat conditions and fatigue load conditions, there are a total of the following... Figure 3 The eight possible failure modes shown are: A1_B1 type failure, A1_B2 type failure, A2_B1 type failure, A2_B2 type failure, B1_A1 type failure, B1_A2 type failure, B2_A1 type failure, and B2_A2 type failure.

[0068] Step 2: Construct a material parameter prediction model that considers the influence of environmental factors using experimental data, specifically including:

[0069] 2.1) The mechanical properties, infrared spectral information, and moisture absorption information of composite materials and adhesive materials under the influence of the test environment are shown in Figures 4-7 and Table 1, respectively. The mechanical properties of metallic materials under the influence of the environment are shown in Table 2.

[0070] Table 1 Material moisture absorption rate

[0071] Table 2 Mechanical Properties of Metallic Materials

[0072] 2.2) Based on the proposed prediction formula: Based on the above data, a material performance prediction model for different environments is constructed, where: for composite materials, k is set to 2, i.e., [the model is then selected]. and This is used as the band for predicting infrared spectral analysis. For polyurethane adhesive, k is set to 2, i.e., selected... and As a band for predicting infrared spectral analysis.

[0073] Step 3: Based on the fatigue loading peak value, calculate the maximum strain of each material in the joint structure using the finite element method, specifically:

[0074] 3.1) Establish a numerical model and mesh it based on the actual structural geometry information. The meshing is as follows: Figure 8 As shown.

[0075] 3.2) Under aging conditions (unaged and aging in environments with 30%RH, 60%RH, and 90%RH), fatigue load peak values ​​were distributed at 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, and 0.9 times the tensile strength of the joint (the ratio of peak fatigue load to tensile strength is represented by q). The peak fatigue load was applied evenly to the nodes at the joint end (the position with the largest x-coordinate), and displacement constraints were applied to these nodes in the y and z directions. Displacement constraints in the x, y, and z directions were applied to the nodes at the other end (the position with the smallest x-coordinate). Simultaneously, surface-to-surface contact was established between the rivets and the composite material substrate and the metal substrate.

[0076] 3.3) Strain information of each part of the joint material is extracted through simulation calculation, such as... Figure 9 As shown in the figure, the simulation process under different environmental influences demonstrates that the proposed method has the ability to predict the performance of hybrid joints under different environmental influences. After simple post-processing, the maximum strain of each material can be extracted.

[0077] Step 4: The raw materials obtained from the experiment must be analyzed. The curve is processed as follows:

[0078] 4.1) Carbon fiber composite materials without considering the impact of environmental factors The curve is shown in Figure 10. The curve expression at this time is: After being affected by the environment The curve needs to be degenerated to The required strength and stiffness information has been predicted in step two.

[0079] 4.2) This embodiment specifies the adhesive and metal materials The curve does not change with the environment; its curve is as follows: Figure 11 As shown in the figure. The tests under several working conditions affecting the adhesive due to environmental influences are all illustrated in the figure, demonstrating the preset parameters of this embodiment. The accuracy of the curve remains unchanged regardless of the environment.

[0080] Step 5: Determine the failure type, specifically:

[0081] 5.1) Substitute the predicted maximum strain of each component material under specific environmental influences into the maximum strain of each component material under specific environmental influences. The curves were used to calculate the fatigue life of each component material. ,in: =1,2,……5, corresponding to in-plane composite material, interlayer composite material, adhesive layer, metal plate, and rivet, respectively, where: The smallest (denoted as) The material corresponding to the first failure point is identified to determine the first stage of failure type.

[0082] 5.2) Under the combined effects of different fatigue loading and environmental influences, the same connection may exhibit changes in failure mode, i.e., competitive fatigue behavior of the material. When the peak fatigue load changes, the ratio q of the peak fatigue load to the static strength of the joint is calculated. When this value changes with environmental factors, the preset failure mode is compared. Figure 3 Two possible first-stage failure modes, A1 and B2, were identified. The qN curves representing A1 and B2 type failures caused by variations in q-values ​​under different environments were plotted as a fatigue competition mechanism diagram. Figure 11 As shown in Figure 12, the intersection points of the qN curves for the two predicted failure modes are extracted, and a fatigue type partitioning diagram for the first stage is plotted. In the first stage of failure, comprehensive prediction of failure types is achieved for environmental conditions with relative humidity from 0% to 90% and fatigue loading from 0 to the peak joint load.

[0083] 5.3) For the first-stage failure mode predicted under a specific working condition, return to step three and delete the mesh of the numerical model corresponding to the material of the failed part, and recalculate the maximum strain; then calculate and superimpose the second-stage fatigue using the damage accumulation criterion to achieve the prediction of the second-stage fatigue type, specifically: calculate the accumulated damage amount of the first stage. Fatigue failure is considered to have occurred when the damage reaches 100%, marking the start of the second stage. The remaining fatigue life percentage of the unfractured parts is... After deleting the mesh of the material in the failed area, the maximum strain was recalculated, and then... The curve yields the second stage of pseudo-fatigue life. Accordingly, the actual fracture life of the remaining second stage is obtained as follows: ,in: The material corresponding to the smallest failure point is the part that fails first in the second stage, and the failure type in the second stage is determined based on the failure mode preset in step one.

[0084] The results show that for the first-stage failure mode A1, only the second-stage failure mode B2 is possible; for the first-stage failure mode B2, both A1 and A2 failure modes are possible in the second stage. Similar to the prediction method for the first stage, a fatigue type partitioning diagram can be drawn to show the impact of changes in environmental and fatigue loading conditions on the second-stage failure, as shown in Figure 12.

[0085] The effectiveness of this method was verified through specific practical experiments. The fatigue performance test considering damp heat aging includes two steps: The first step is to obtain the aged hybrid joint specimens. The damp heat aging test is based on ASTM D5229 / D5229M. Before conducting the joint fatigue performance test, the specimens are adjusted to a state of damp heat equilibrium. The specimens are subjected to damp heat aging in an environmental test chamber. This study introduces three aging states: (1) 30% RH, 70℃, 30 days; (2) 60% RH, 70℃, 30 days; (3) 90% RH, 70℃, 30 days. After aging, the specimens are exposed to ambient temperature for 10 minutes before the fatigue performance test is conducted. The second step is to conduct fatigue performance testing. For hybrid joints, referring to ASTM D3166, the Shimadzu fatigue testing machine was used to conduct tensile fatigue tests on the glued hybrid joint specimens under room temperature conditions. The cyclic load peak was set at three force levels: q=0.2, q=0.6, and q=0.3, and applied under a sinusoidal cyclic load with a frequency of 2 Hz (cyclic peak load ≥ 5 kN) or 10 Hz (cyclic peak load < 5 kN). If 10 Hz is reached... 6 If the specimen does not fracture during the second cycle, the fatigue test is terminated; this is defined as the fatigue limit of the joint. The test results are as follows: Figure 13 As shown. The experimental results Figure 13 Comparing the prediction results with those in Figure 12, it can be found that for the glue-rivet hybrid connection aged in a 30% RH environment, the simulation-predicted failure modes are consistent with experimental observations: when q=0.2, only the rivet fractures; when q=0.6, both the glue layer and the rivet fracture; and when q=0.9, the CFRP fails after fatigue cracking of the glue layer. This demonstrates the effectiveness of the failure mode prediction method.

[0086] Compared with existing technologies, this invention is based on the finite element method, pre-determines failure categories based on prior knowledge, and utilizes the proposed internal grouping competition fatigue mechanism of the hybrid joint to compare the relative advantages of materials under specific working conditions. The results are then matched with the pre-determined failure categories, thereby achieving rapid prediction of fatigue failure types. Based on this technology, the main effects are threefold:

[0087] First, the method can consider the effects of damp heat aging in the fatigue prediction of adhesive-riveted hybrid joint structures. The proposed method for predicting material elasticity and strength parameters considering damp heat conditions correlates the water absorption rate, chemical structure changes, and mechanical parameter decay caused by environmental aging with the material's performance changes at different temperatures.

[0088] Secondly, the improved accuracy of the isolife curves ensures the accuracy of structural fatigue failure prediction. By improving the Kawai method, isolife curves are constructed on the tension-tension and compression-compression SN curves to describe the fatigue life of composite materials under different stress ratios.

[0089] Finally, a complete prediction of the two-stage process of fatigue failure in hybrid joints was achieved. The fatigue failure modes of hybrid joints were summarized into two stages, and a staged fatigue failure type discrimination process was designed. The finite element calculations performed in this invention only require quasi-static loading of the joint based on the fatigue peak load and extraction of the maximum strain. After simple calculations, the fatigue failure modes can be quickly identified according to a pre-set failure mode table.

[0090] Due to the above advantages, this method greatly simplifies the experimental process and significantly shortens the experimental cycle compared to traditional methods. Furthermore, the prediction requires only relatively low computing power; a typical personal laptop can perform the calculations quickly.

[0091] The above-described specific implementations can be partially adjusted by those skilled in the art in different ways without departing from the principles and purpose of the present invention. The scope of protection of the present invention is defined by the claims and is not limited to the above-described specific implementations. All implementation schemes within the scope of the claims are bound by the present invention.

Claims

1. A method for predicting fatigue failure modes in a composite material and metal adhesive riveting hybrid connection structure, characterized in that, include: The first step is to pre-determine fatigue failure modes for composite material / metal adhesive riveting hybrid connections, including mechanical connection failure (Type A failure) and adhesive failure (Type B failure). The second step is to construct a material parameter prediction model based on water absorption rate, chemical structure, and ambient temperature, specifically as follows: ,in: These are predicted values ​​for the mechanical properties of the material. ; Aging item m is the moisture absorption value, k is the number of aging-sensitive functional groups obtained from infrared spectroscopy analysis, a, b, c, and λ are fitting parameters, and the ambient temperature term is also included. , T represents the actual ambient temperature. The value is room temperature; α and β represent the fitting coefficients of each material to the temperature term, and i represents the order of the fitting curve; The third step is to calculate the maximum strain of each material in the joint structure: After assigning the material parameter prediction model obtained in the second step to the corresponding material properties of the connection, mesh generation, displacement boundary condition setting and contact setting are performed, and the load peak value in the fatigue cycle to be predicted, that is, the force value corresponding to the peak in the fatigue loading cycle curve, is extracted as the force boundary condition; then, the finite element calculation result file containing the strain field of each part of the joint material is obtained through finite element calculation; then, the maximum in-plane shear strain of composite material, the maximum interlaminar strain of composite material, the maximum principal strain of adhesive layer, the maximum principal strain of metal plate and the maximum principal strain of rivet are extracted and recorded from it; The fourth step is to process the raw materials. The curve, namely the relationship between the maximum strain under fatigue loading and the fatigue cycle life, is processed to meet the prediction needs of different environmental impacts: by improving the Kawai method, isolife curves are constructed on the tension-tension and compression-compression SN curves to describe the fatigue life of materials under different stress ratios of tension, compression, and shear. The fifth step is to determine the specific type of failure corresponding to the preset failure type in the first step based on the maximum strain of each material in the joint structure obtained in the third step and the equal life curve obtained in the fourth step.

2. The fatigue failure mode prediction method for composite material and metal adhesive riveting hybrid connection structures according to claim 1, characterized in that, The fatigue failure mode refers to the fact that under the combined action of different humid and hot environments and fatigue load conditions, the materials constituting the hybrid connection have different relative competitive advantages, which may lead to multiple failure modes. Among them, the coexistence of mechanical connection failure, i.e., Type A failure, and adhesive failure, i.e., Type B failure, will lead to complete connection failure. The mechanical connection failures include fastener failures, i.e., type A1 failures, and substrate failures, i.e., type A2 failures. The adhesive failures mentioned include composite material delamination, i.e., type B1 failure, and adhesive layer failure, i.e., type B2 failure; In the fatigue failure modes described, when any form of mechanical connection failure occurs, the hybrid joint degenerates into a bonded joint. Therefore, type A1 and type A2 failures cannot occur on the same joint during the failure process; similarly, type B1 and type B2 failures cannot occur simultaneously during the failure process. Due to the different competitive advantages of various materials under different conditions, the order of their failure modes will also change. Therefore, for the same hybrid connection, under different humid and hot conditions and fatigue load conditions, the specific failure modes include: type A1_B1 failure, type A1_B2 failure, type A2_B1 failure, type A2_B2 failure, type B1_A1 failure, type B1_A2 failure, type B2_A1 failure, and type B2_A2 failure, where "_" indicates the order in which the failure types appear, i.e., the first stage and the second stage; the preset failure mode already includes all possible failures of this type of connection. The fatigue failure mode refers to the specific form in which the connection structure completely breaks after fatigue loading, or partially fails when the fatigue limit is reached. The failure mode sequence refers to the following: Since the hybrid connection is divided into two sub-parts, adhesive bonding and riveting, the failure of one part does not cause the structure to break; this means that the complete failure of the adhesive-riveting hybrid connection structure is accompanied by the complete failure of both adhesive bonding and riveting, and there must be a failure sequence; in the failure, A1 and A2 cannot coexist, and B1 and B2 cannot coexist either.

3. The fatigue failure mode prediction method for composite material and metal adhesive riveting hybrid connection structures according to claim 1, characterized in that, The material parameters mentioned refer to the elastic modulus, Poisson's ratio, and strength of the material used in the finite element calculation. The spectrum of the aged polymer material was obtained by analyzing it with an infrared spectrometer, where the absorption peaks corresponding to the functional groups are shown. The aging-sensitive functional groups mentioned refer to those whose spectral absorption peaks change significantly after aging; it is recommended to select the two functional groups with the most significant changes as variables based on the actual situation.

4. The fatigue failure mode prediction method for composite material and metal adhesive riveting hybrid connection structures according to claim 1, characterized in that, The aforementioned mesh generation refers to: establishing a numerical model based on the actual structural geometric information and then dividing it into meshes; The aforementioned displacement boundary condition setting refers to setting displacement constraints on the connectors based on the actual external displacement constraints. The contact setting refers to the contact between the fastener and the substrate being set to a rigid contact type.

5. The fatigue failure mode prediction method for composite material and metal adhesive riveting hybrid connection structures according to claim 1, characterized in that, The fifth step specifically includes: 5.1) Substitute the predicted maximum strain of each component material under specific environmental influences into the value of each component material under specific environmental influences. The curves were used to calculate the fatigue life of each component material. ,in: =1, 2, ..., 5, corresponding to in-plane composite material, interlayer composite material, adhesive layer, metal plate, and rivet, respectively, with the smallest being... ,Right now The corresponding material is the part that first fails, thus identifying the first stage of failure type; 5.2) Considering the differences in fatigue competitive advantage of different materials in the joint under different fatigue loading and environmental factors, different failure types may be predicted when the environmental conditions of the same joint are changed. A fatigue competition mechanism diagram is drawn according to the qN curves of the corresponding parts of different failure types. By extracting the intersection points of the qN curves under different environments, a fatigue type partition diagram is drawn. The influence of different fatigue loading and environmental factors on the first stage failure type is determined by this diagram. 5.3) Considering the failure sequence, the structure is not completely disconnected after the first stage of failure. Returning to step three, the mesh of the numerical model corresponding to the failed part is deleted, and the glue-riveting hybrid connection structure degenerates into a simple glue or riveting form, and the maximum strain is recalculated. Considering that some fatigue damage has already occurred in each part under cyclic loading during the first stage of failure, this damage needs to be calculated using the damage accumulation criterion and superimposed on the fatigue of the second failure stage to achieve the fatigue type prediction for the second stage. Specifically, this involves determining the accumulated damage amount in the first stage. When the damage reaches 100%, fatigue failure is considered to have occurred, marking the start of the second stage. The remaining fatigue life percentage of the unfractured parts is... After mesh deletion, the maximum strain is recalculated, and then based on... The curve yields the second stage of pseudo-fatigue life. Furthermore, the actual fracture life of the remaining second stage is obtained as follows: ,in: The smallest corresponding material is the part that fails first in the second stage, and the failure type in the second stage is determined based on the failure mode preset in step one. 5.4) Similarly, the second-stage failure type is also affected by the material fatigue competition mechanism. A fatigue competition mechanism diagram is drawn according to the qN curves of the corresponding parts of different failure types. Based on the fatigue type partition diagram of the first stage, the intersection points of the qN curves under different environments are extracted to draw the second-stage fatigue type partition diagram. The influence of different fatigue loading and environmental factors on the second-stage failure type is determined through this diagram. Thus, the fatigue failure type prediction of hybrid connection affected by the environment is realized.

6. The fatigue failure mode prediction method for composite material and metal adhesive riveting hybrid connection structures according to claim 5, characterized in that, The fatigue damage mentioned above refers to the fact that even if fracture does not occur during the fatigue loading process, microcracks still initiate and propagate inside the material, so the parts that do not fail in the first stage also accumulate a certain amount of damage. The aforementioned damage accumulation criterion refers to the Miner fatigue damage linear accumulation criterion; the linear accumulation criterion is used to meet the computing power requirements for rapid prediction while ensuring prediction accuracy. The pseudo-fatigue life refers to the material fatigue life calculated assuming that no initial fatigue damage occurs at the start of the second stage.

7. A system for predicting fatigue failure modes of composite material and metal adhesive riveting hybrid connection structures as described in any one of claims 1-6, characterized in that, include: Failure mode predefinition module, material stiffness and strength performance prediction module considering environmental factors, maximum strain extraction module for each material in joint structure, and raw material processing module. The system includes a curve module and a failure mode discrimination module. Specifically: a failure mode predefinition module summarizes and analyzes the fracture mechanism of the connection structure to obtain all theoretically possible failure modes; a material stiffness and strength performance prediction module considering environmental factors constructs a material parameter prediction model based on water absorption rate, chemical structure, and ambient temperature using experimental data on moisture absorption, infrared spectroscopy, and material mechanical properties to obtain the stiffness and strength performance of the material under different humid and hot conditions; a maximum strain extraction module for each material in the joint structure establishes a numerical model based on the actual geometry of the connector and its service loading conditions, performs finite element simulation calculations, and obtains the maximum strain of each material in the connection through post-processing of the finite element calculation results; and processes the original material... The curve module uses material fatigue loading test data and an improved Kawai method to construct isolife curves on two SN curves, tension-tension and compression-compression, to describe the fatigue life of composite materials under different stress ratios of tension, compression, and shear. The failure mode discrimination module determines the failure type of the hybrid connection in two steps based on the maximum strain and the material isolife curve, realizing a complete prediction of the fatigue failure type of the hybrid connection affected by the environment.

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