Riveted joint fatigue life prediction method based on dissipated energy and considering stress concentration coefficient

Abstract

The invention discloses a riveted joint fatigue life prediction method based on dissipated energy and considering a stress concentration coefficient, and belongs to the field of riveted joint fatigue life evaluation. The objective of the invention is to solve the problem of accurate prediction of the fatigue life of a riveted joint under the condition that a material plastically deforms after a rivet pierces a connected piece. The principle of the method comprises the following steps: firstly, determining a stress concentration coefficient by carrying out numerical simulation under the tensile load effect of the riveted joint; secondly, riveting joint infrared thermography variable amplitude and constant amplitude fatigue experiments are carried out, and dissipative energy damage model material parameters are obtained; and finally, coupling the stress concentration coefficient to construct a fatigue life prediction model, and programming to calculate the fatigue life value of the riveted joint under the action of different cyclic loads. According to the method, calculation errors caused by equivalent nominal stress are corrected through the stress concentration coefficient, fatigue damage of the riveted joint is represented through dissipated energy reflecting fatigue failure essence, and the prediction precision is remarkably improved; and meanwhile, the calculation efficiency is also greatly improved in a mode of combining numerical simulation and a fatigue experiment.

Description

Technical Field

[0003] This invention relates to a method for predicting the fatigue life of riveted joints based on the stress concentration factor and considering the dissipation energy, and belongs to the field of fatigue life prediction for riveted joints. Background Technology

[0005] Riveting joints are a common connection method between sheet metal parts. Due to their high forming efficiency, good connection performance, and low production cost, they are widely used in equipment fields such as automobiles, rail transportation, and aerospace. However, the connected parts in riveting joints are often pierced by the rivets, causing irreversible plastic deformation. Compared to the base material and the rivet material itself, the fatigue performance of riveting joints is significantly weakened. Therefore, accurately calculating the fatigue life of riveting joints is crucial to ensuring the safe and reliable operation of riveted structures.

[0006] With the increasing demands of users for the quality of high-end equipment in automobiles, rail transportation, aerospace, and other fields, many researchers have focused on the problem of fatigue life prediction for riveted joints. Northwestern Polytechnical University has published an electromagnetic riveted joint fatigue life prediction method (Publication No.: CN103455671B). This method uses finite element analysis to simulate the residual stress during the riveted joint forming process and couples it into a fatigue life analysis model. Based on the strain fatigue life curve, it predicts the fatigue life of the riveted joint. This method can effectively calculate the fatigue life of the riveted joint before crack propagation, but it fails to consider the fatigue life during the crack propagation stage. Furthermore, the damage parameter is vector strain, making it difficult to determine the maximum shear strain within the critical plane for riveted joints under multi-axis and multi-physics field conditions. Tsinghua University has published a fatigue prediction method, device, equipment, and storage medium for riveted structures (Publication No.: CN111611654B). This method predicts the fatigue life of riveted structures based on the Paris formula. It obtains the response signal of the riveted structure through an excitation signal, determines fatigue characteristic parameters by combining a neural network model, and uses a large amount of data to drive a physical mechanism model, enabling accurate calculation of the fatigue life of the riveted structure. However, this method requires extensive experimental observation of fatigue crack length, mechanical response, and other data, which obviously increases the time cost of predicting the fatigue life of riveted structures. Shanghai Jiao Tong University has published a method for predicting the fatigue failure mode of composite material and metal-plastic-riveted hybrid joint structures (publication number: CN116504334A). This method constructs a material parameter prediction model based on water absorption rate, chemical structure, and ambient temperature, obtains the maximum strain of the material, and predicts the service life of the composite material and metal-plastic-riveted hybrid joint structure by combining the material strain fatigue curve after considering environmental conditions. However, this method addresses the fatigue life problem under complex boundary conditions of composite structures and is highly dependent on accurate material strain fatigue curves. Furthermore, the ability to quickly and efficiently calculate the maximum strain of the material is limited by factors such as structural shape and boundary conditions. Therefore, further in-depth and systematic research is needed to accurately and efficiently characterize the fatigue damage of riveted joints. Summary of the Invention

[0008] To address the problems of low calculation accuracy and efficiency in existing methods for predicting the fatigue life of riveted joints, and to overcome the shortcomings described in the background art, this invention provides a method for predicting the fatigue life of riveted joints based on dissipated energy and considering the stress concentration factor. The method of this invention includes the following steps:

[0009] (1) Establish a finite element model of the riveted joint, define the contact conditions between the rivet and the connected parts and between the connected parts, apply constraints and tensile loads, output the maximum stress result of the riveted joint, and determine the stress concentration factor.

[0010] (2) Conduct variable amplitude infrared thermal imaging fatigue tests and constant amplitude infrared thermal imaging fatigue tests on the base material specimens. Record the temperature change curves during the fatigue damage evolution process of the base material using an infrared thermal imager to determine the thermophysical properties of the base material. Combine the stress concentration factor of the riveted joint to determine the material parameters of the energy dissipation fatigue damage model.

[0011] (3) Establish the correlation between stress concentration factor, dissipated energy, and fatigue life, and construct a fatigue life prediction model for riveted joints, as shown in the following formula:

[0012] (1)

[0013] In the formula The density of the material; The specific heat capacity of the material; The frequency at which the test load is applied; Stress ratio; It is a time constant; The coefficient of thermal conductivity; It is the thermal constant; Energy tolerance for material fatigue failure; The stress concentration factor; The stress is the equivalent nominal stress of the specimen; once the applied cyclic load is determined, the stress ratio is calculated. Stress concentration factor Equivalent nominal stress of the specimen This allows for the calculation of the fatigue life of the riveted joint under applied cyclic load.

[0014] Furthermore, the stress concentration factor mentioned above Calculate according to the following formula:

[0015] (2)

[0016] The maximum stress of the riveted joint mentioned in the formula The equivalent Von Mises stress maximum value obtained from finite element simulation calculation of the riveted joint under tensile load; the equivalent nominal stress of the specimen. It is the ratio of the tensile load to the cross-sectional area of ​​the rivet connection part of the riveted joint.

[0017] Furthermore, both the variable amplitude infrared thermal imaging fatigue test of the parent material specimen and the constant amplitude infrared thermal imaging fatigue test are performed with reference specimens. The material of the reference specimens is the same as that of the parent material, and the size of the reference specimens is half the size of the parent material specimens.

[0018] Furthermore, in the amplitude-modulated infrared thermography fatigue test of the parent material specimen, the initial loading stress level is equal to 0.7 times the yield strength of the parent material, and then the loading is applied in increments of 10 MPa to 100 MPa until the riveted joint completely fails.

[0019] Furthermore, after the loading stress level is cycled 5000 times, the specimen is allowed to cool to room temperature before being moved to the next loading stress level.

[0020] Furthermore, in the constant-amplitude infrared thermographic fatigue test of the parent material specimen, the initial loading stress level is equal to 0.7 times the yield strength of the parent material specimen, and cyclic loading is continuously applied until the specimen fails or the number of cycles exceeds 10. 6 Then, the specimen was replaced and the loading was applied in stages, with the loading increment ranging from 10MPa to 100MPa, and the maximum loading stress level not exceeding the tensile strength of the riveted joint.

[0021] Furthermore, the material parameters of the dissipated energy fatigue damage model include the density of the material. The specific heat capacity of the material The frequency of the test loading The time constant mentioned above The aforementioned thermal coefficient The thermal constant mentioned above The energy tolerance of material fatigue failure .

[0022] Furthermore, the aforementioned thermal coefficient and the aforementioned thermal constant Determined by the following formula:

[0023] (3)

[0024] In the formula The asymptotic temperature rise is the difference between the fatigue specimen temperature and the reference specimen temperature corresponding to the first inflection point of the temperature change curve of the base material specimen under a certain loading stress level in the amplitude infrared thermography fatigue test. The stress amplitude; the asymptotic temperature rise can be obtained through the amplitude-dependent infrared thermographic fatigue test of the base material specimen. and stress amplitude The thermal coefficient can be determined by fitting the data using the least squares method. and the aforementioned thermal constant .

[0025] Furthermore, the energy tolerance for material fatigue failure... Calculate according to the following formula:

[0026] (4)

[0027] In the formula The fatigue life of the fatigue specimen under a certain loading stress level in the constant amplitude infrared thermography fatigue test of the parent material specimen; The temperature difference between the fatigue specimen temperature and the reference specimen temperature corresponds to the first inflection point of the temperature change curve under the corresponding loading stress level in the constant amplitude infrared thermography fatigue test of the parent material specimen.

[0028] The beneficial effects of this method are: First, it corrects the calculation error caused by the equivalent nominal stress by using the stress concentration factor, and uses dissipated energy, which reflects the essence of fatigue failure, to characterize the fatigue damage of the riveted joint, thus significantly improving the accuracy of fatigue life prediction for the riveted joint. Second, the combination of numerical simulation and fatigue experimentation in predicting the fatigue life of the riveted joint also greatly improves the efficiency of fatigue life calculation. Attached Figure Description

[0030] Figure 1 A flowchart of a method for predicting the fatigue life of riveted joints based on dissipated energy and stress concentration factor.

[0031] Figure 2 Dimensional drawing of the self-piercing riveting joint specimen;

[0032] Figure 3 This is a photograph of a self-piercing riveting joint specimen.

[0033] Figure 4 Comparison of the cross-section of the self-piercing riveting joint and the cross-section of the finite element mesh;

[0034] Figure 5 Finite element mesh image of a self-piercing riveting joint;

[0035] Figure 6 Stress cloud diagram for finite element simulation of self-piercing riveting joint;

[0036] Figure 7 This is a dimensional drawing of the base material specimen;

[0037] Figure 8 This describes the setup method for the infrared thermal imager and the installation method for the reference specimen;

[0038] Figure 9 Thermal coefficient of the base material and thermal constant The fitting results;

[0039] Figure 10 Energy tolerance for fatigue failure of the base material The fitting results;

[0040] Figure 11 To compare the calculation results of the prediction model with the actual fatigue life prediction. Detailed Implementation

[0042] The present invention will now be described in further detail with reference to the accompanying drawings and specific embodiments.

[0043] The following is an example of fatigue life calculation for a certain riveted joint, but the scope of protection of this invention is not limited to the following implementation example.

[0044] Step 1: Establish the finite element model of the riveted joint. Based on the cross-sectional shape and overall dimensions of the self-piercing riveted joint, perform 3D modeling and finite element mesh generation. See the specimen photograph as follows: Figure 2 As shown, the specimen dimensions are as follows: Figure 3 As shown in the photograph, the specimen cross-section is compared with the mesh cross-section. Figure 4 As shown, the self-piercing riveting mesh is as follows Figure 5 As shown, the mesh consists of four parts: an upper plate, a lower plate, an upper plate circle, and rivets. Five contact pairs are set: upper plate to lower plate, upper plate to rivet, lower plate to rivet, upper plate circle to lower plate, and upper plate circle to rivet. Universal contact is selected, and the friction coefficient is set to 0.2. The mechanical property parameters of the materials for the upper plate, lower plate, and upper plate circle are input, including the elastic modulus. Poisson's ratio Material density Input the mechanical property parameters of the rivet material, including the elastic modulus. Poisson's ratio Material density The upper plate is fixed at one end, and a tensile load of 6000N is applied to the other end. The output request domain is set to the upper and lower plates, and the stress field is finally obtained as follows: Figure 6 As shown, the stress concentration factor of the joint is calculated to be 2.755.

[0045] Step 2: Conduct infrared thermographic fatigue testing on the base material specimen. The dimensions of the base material specimen are as follows: Figure 7 As shown, during the experiment, a reference specimen was clamped on one side of the original specimen. Infrared thermal imagers were used to collect the surface temperatures of both the original specimen and the reference specimen during the experiment. The setup of the infrared thermal imager and the installation method of the reference specimen are as follows: Figure 8 As shown; for the variable amplitude infrared thermography fatigue test of the base material joint, the initial loading stress level is equal to 0.7 times the yield strength of the base material, and then the loading is increased in increments of 10 MPa to 100 MPa until the riveted joint completely fails; for the constant amplitude infrared thermography fatigue test of the base material specimen, the initial loading stress level is equal to 0.7 times the yield strength of the base material specimen, and cyclic loading is continuously applied until the specimen fails or the number of cycles exceeds 10. 6 Then, the specimens were replaced and gradually loaded in increments ranging from 10 MPa to 100 MPa, with the maximum loading stress level not exceeding the tensile strength of the riveted joint. The results of the infrared thermal fatigue test are shown in Table 1.

[0046] Table 1 Results of infrared thermal fatigue test on base material specimens

[0047]

[0048] To calculate the thermal parameters of the base material, the thermal coefficient is mentioned first. and the aforementioned thermal constant Determined by the following formula:

[0049] (1)

[0050] In formula (1) The asymptotic temperature rise is the difference between the fatigue specimen temperature and the reference specimen temperature corresponding to the first inflection point of the temperature change curve of the base material specimen under a certain loading stress level in the amplitude infrared thermography fatigue test. The stress amplitude; the asymptotic temperature rise can be obtained through the amplitude-dependent infrared thermographic fatigue test of the base material specimen. and stress amplitude The thermal coefficient can be determined by fitting the data using the least squares method. and the aforementioned thermal constant .

[0051] Then, the fatigue failure energy tolerance of the base material is calculated. Calculate according to the following formula:

[0052] (2)

[0053] In the formula The fatigue life of the fatigue specimen under a certain loading stress level in the constant amplitude infrared thermography fatigue test of the parent material specimen; The temperature difference between the fatigue specimen temperature and the reference specimen temperature corresponds to the first inflection point of the temperature change curve under the corresponding loading stress level in the constant amplitude infrared thermography fatigue test of the parent material specimen.

[0054] Base material thermal coefficient and thermal constant The fitting results are as follows Figure 9 As shown, the fatigue failure energy tolerance of the base material like Figure 10 As shown in Table 2, based on the above experimental data and mathematical fitting relationship, the parameters of Equation (1) and Equation (2) are as follows.

[0055] Table 2 Thermal parameters of the base material

[0056]

[0057] At this point, all thermal parameters and stress concentration factors of the base material have been determined.

[0058] Step 3: Establish the correlation between stress concentration factor, dissipated energy, and fatigue life. The fatigue life prediction model for riveted joints is shown in the following formula:

[0059]

[0060] In the formula The density of the material; The specific heat capacity of the material; The frequency at which the test load is applied; Stress ratio; It is a time constant; The coefficient of thermal conductivity; It is the thermal constant; Energy tolerance for material fatigue failure; The stress concentration factor; The stress is the equivalent nominal stress of the specimen; once the applied cyclic load is determined, the stress ratio is calculated. Stress concentration factor Equivalent nominal stress of the specimen This allows for the calculation of the fatigue life of the riveted joint under applied cyclic load. The calculated fatigue life can then be compared with the actual fatigue life of the riveted joint. Figure 11 As shown.

Claims

1. A method for predicting the fatigue life of riveted joints based on dissipated energy and considering stress concentration factor, characterized in that... The method includes the following steps: Step 1: Establish a finite element model of the riveted joint, define the contact conditions between the rivet and the connected parts, apply constraint and tensile loads, output the maximum stress result of the riveted joint, and determine the stress concentration factor; Step 2: Conduct variable amplitude infrared thermal imaging fatigue tests and constant amplitude infrared thermal imaging fatigue tests on the base material specimens, record the temperature change curves during the fatigue damage evolution process of the base material using an infrared thermal imager, determine the thermophysical properties of the base material, and determine the material parameters of the energy dissipation fatigue damage model by combining the stress concentration factor of the riveted joint; Step 3: Establish the correlation between the stress concentration factor, energy dissipation, and fatigue life, and construct a fatigue life prediction model for the riveted joint, as shown in the following formula: (1) In the formula The density of the material; The specific heat capacity of the material; The frequency at which the test load is applied; Stress ratio; It is a time constant; The coefficient of thermal conductivity; It is the thermal constant; Energy tolerance for material fatigue failure; The stress concentration factor; The stress is the equivalent nominal stress of the specimen; once the applied cyclic load is determined, the stress ratio is calculated. Stress concentration factor Equivalent nominal stress of the specimen This allows for the calculation of the fatigue life of the riveted joint under applied cyclic load.

2. The method for predicting the fatigue life of riveted joints based on dissipated energy and considering stress concentration factor as described in claim 1, characterized in that... In step one, the stress concentration factor Calculate according to the following formula: (2) The maximum stress of the riveted joint mentioned in the formula The equivalent Von Mises stress maximum value obtained from finite element simulation calculation of the riveted joint under tensile load; the equivalent nominal stress of the specimen. It is the ratio of the tensile load to the cross-sectional area of ​​the rivet connection part of the riveted joint.

3. The method for predicting the fatigue life of riveted joints based on dissipated energy and considering stress concentration factor as described in claim 1, characterized in that... In step two, both the variable amplitude infrared thermal imaging fatigue test of the parent material specimen and the constant amplitude infrared thermal imaging fatigue test are performed with reference specimens. The material of the reference specimens is the same as that of the parent material, and the size of the reference specimens is half the size of the parent material specimens.

4. The method for predicting the fatigue life of riveted joints based on dissipated energy and considering stress concentration factor as described in claim 1, characterized in that... In step two, the initial loading stress level of the variable amplitude infrared thermography fatigue test of the base material specimen is equal to 0.7 times the yield strength of the base material, and then the loading is applied in increments of 10 MPa to 100 MPa until the riveted joint completely fails.

5. The method for predicting the fatigue life of riveted joints based on dissipated energy and stress concentration factor as described in claim 4, wherein after 5000 cycles of the applied stress level, the specimen is allowed to cool to room temperature before being replaced with the next applied stress level.

6. The method for predicting the fatigue life of riveted joints based on dissipated energy and considering stress concentration factor as described in claim 1, characterized in that... In step two, the initial loading stress level of the constant amplitude infrared thermography fatigue test on the parent material specimen is equal to 0.7 times the yield strength of the parent material specimen, and cyclic loading is continuously applied until the specimen fails or the number of cycles exceeds 10. 6 Then, the specimen was replaced and the loading was applied in stages, with the loading increment ranging from 10MPa to 100MPa, and the maximum loading stress level not exceeding the tensile strength of the riveted joint.

7. The method for predicting the fatigue life of riveted joints based on dissipated energy and considering stress concentration factor as described in claim 1, characterized in that... In step two, the material parameters of the dissipated energy fatigue damage model include the density of the material. The specific heat capacity of the material The frequency of the test loading The time constant mentioned above The aforementioned thermal coefficient The thermal constant mentioned above The energy tolerance of material fatigue failure .

8. The method for predicting the fatigue life of a riveted joint based on dissipated energy and considering the stress concentration factor as described in claim 7, characterized in that... The thermal coefficient and the aforementioned thermal constant Determined by the following formula: (3) In the formula The asymptotic temperature rise is the difference between the fatigue specimen temperature and the reference specimen temperature corresponding to the first inflection point of the temperature change curve of the base material specimen under a certain loading stress level in the amplitude infrared thermography fatigue test. The stress amplitude; the asymptotic temperature rise can be obtained through the amplitude-dependent infrared thermographic fatigue test of the base material specimen. and stress amplitude The thermal coefficient can be determined by fitting the data using the least squares method. and the aforementioned thermal constant .

9. The method for predicting the fatigue life of riveted joints based on dissipated energy and considering stress concentration factor as described in claim 7, wherein the material fatigue failure energy tolerance... Calculate according to the following formula: (4) In the formula The fatigue life of the fatigue specimen under a certain loading stress level in the constant amplitude infrared thermography fatigue test of the parent material specimen; The temperature difference between the fatigue specimen temperature and the reference specimen temperature corresponds to the first inflection point of the temperature change curve under the corresponding loading stress level in the constant amplitude infrared thermography fatigue test of the parent material specimen.

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