Fatigue life prediction method of thin-wall circular tube test piece in elastic stability state

Abstract

The invention relates to the technical field of fatigue life prediction of metal materials, and provides a method for predicting the fatigue life of a thin-wall circular tube test piece in an elastic stability state, which comprises the following steps: S1, calculating the stress state at a dangerous point O in the thin-wall circular tube test piece according to the external load of the thin-wall circular tube test piece, and judging whether the thin-wall circular tube test piece is in the elastic stability state; s2, when it is judged that the thin-wall circular tube test piece is in an elastic stable state, extracting a stress state, searching a critical surface and an azimuth angle delta cr of the critical surface, and obtaining a maximum shear stress amplitude Ca, max and a shear average stress Cm, a on the critical surface; s3, constructing a calculation formula of a damage variable Q; and S4, calculating parameters in the fatigue life prediction model, and substituting the parameters, the maximum shear stress amplitude Ca, max and the shear average stress Cm, a of the critical plane into the fatigue life prediction model to calculate the fatigue life. According to the invention, the application space of the thin-wall circular tube test piece in more loading ranges is expanded, and more choices are provided for the field of metal fatigue research.

Description

Technical Field

[0001] This invention relates to the field of fatigue life prediction technology for metallic materials, and specifically to a method for predicting the fatigue life of a thin-walled circular tube specimen under elastic stability. Background Technology

[0002] Stability is the study of the plastic behavior of structures under complex loading conditions, with the aim of determining the stable loads of various engineering structures. With the rapid development of modern industrial technology and engineering construction, in many fields such as chemical engineering, aerospace engineering, and civil engineering, stability not only requires that the loads on the structure sometimes exceed the elastic limit load, but also requires determining the cumulative plastic strain within the structure to ensure that the structure does not produce excessive plastic deformation leading to failure under complex loading. On the other hand, the structural bearing capacity calculation results given by traditional linear elastic mechanics analysis tend to be conservative. Stability analysis can better reflect the bearing limit of the structure, fully utilize the material's bearing capacity, significantly reduce engineering costs, and improve economic efficiency. Determining whether a structure is in an elastically stable state plays a crucial role in stability analysis. In an elastically stable state, most areas of the structural components are in an elastic state, only entering a plastic state at stress concentration points. Furthermore, after a finite number of cycles, the structural components no longer produce new plastic deformations as the cycle continues.

[0003] The goal of stability analysis is to determine the elastic stability range of a structural component. Thin-walled circular tube specimens are frequently used in high-cycle or low-cycle fatigue tests because their unique structural characteristics create a one-to-one correspondence between the external loads they experience and the stresses in their test section. Therefore, this structure can be used to reflect the fatigue performance of the material itself under different external loads. However, a thin-walled circular tube specimen is still a structure, and thus it also possesses its own elastic stability space. After stability analysis, it was found that when the external load is a combined load of symmetrical tension / compression and symmetrical torsion, the equivalent stress synthesized at the critical point inside the thin-walled circular tube specimen is also symmetrical; if the external load is asymmetrical, the equivalent stress synthesized at the critical point is also asymmetrical. If the stress state at the critical point is... If the range is within a certain range, the thin-walled circular tube specimen can be determined to be in an elastically stable state. This method of judgment is very simple because the external load in the test is often given in the form of stress. Only the magnitude of the external load is needed to obtain the elastically stable range of the thin-walled circular tube specimen, avoiding the complex finite element calculations in the traditional stability analysis process and reducing the calculation time.

[0004] The fatigue life of structural components in an elastically stable state is close to that of high-cycle fatigue life. The simple and practical critical surface method is frequently used to predict high-cycle fatigue life. The critical surface method posits that fatigue damage to a structure originates most probably from a plane at a specific angle within the stress state at the critical point, and then expands and intensifies from this plane. Because of its ease of calculation and accurate prediction, it has been extensively studied and experimentally validated, and its application is more widespread than other methods. This method selects different damage variables depending on the type of critical surface. It first searches within a certain range (usually [0°, 180°)) and selects the plane with the largest damage variable as the critical surface; the corresponding angle is the critical surface azimuth. For high-cycle fatigue life prediction models, many studies have used the plane of maximum shear stress amplitude as the critical surface, but accurate prediction is still not possible. Summary of the Invention

[0005] To address the shortcomings of the prior art, the present invention aims to provide a method for predicting the fatigue life of thin-walled circular tube specimens under elastic stability. This method can conveniently solve the problem of predicting the fatigue life of thin-walled circular tube specimens under elastic stability, and overcomes the limitation of previous fatigue life prediction models that could only predict the fatigue life of thin-walled circular tube specimens in the elastic segment. The accuracy of fatigue life prediction is ensured by accurately selecting the critical surface.

[0006] Specifically, on the one hand, the present invention provides a method for predicting the fatigue life of a thin-walled circular tube specimen under elastic stability, which includes the following steps: S1. Perform a stability analysis on the thin-walled circular tube specimen, calculate the stress state at the critical point O, and determine whether the thin-walled circular tube specimen is in an elastically stable state based on the stress state at the critical point O. If the thin-walled circular tube specimen is in an elastically stable state, proceed to step S2. S2. Extract the stress state of the thin-walled circular tube specimen at the critical point O, and calculate the critical surface azimuth angle θ. cr And based on the critical surface azimuth angle θ cr Obtain the maximum shear stress amplitude C on the critical surface a,max and shear mean stress C m,a ; S3. The formula for calculating the damage variable Q is as follows: ; Where Q is the damage variable; C a,max It is the maximum shear stress amplitude on the critical surface; C m,a It is the average shear stress on the critical surface; ω C It is the Walker shearing factor, which takes values ​​between 0 and 1; σ -1 and τ -1These are the symmetrical tensile-compressive fatigue strength and symmetrical torsional fatigue strength of the material at a stress ratio R = -1; α is a model coefficient, a dimensionless constant; σ H It is hydrostatic stress; S4. Combine the formula for calculating the damage variable Q with the Basquin formula to construct a fatigue life prediction model for thin-walled circular tube specimens under elastic stability: Q=F(N f ), calculate the parameters in the fatigue life prediction model and the maximum shear stress amplitude C on the critical surface calculated in step S2. a,max and shear mean stress C m,a We input these values ​​into the fatigue life prediction model and solve for the fatigue life N. f The value of .

[0007] Preferably, the calculation of the critical surface azimuth angle in step S2 includes the following sub-steps: S21. Within one loading cycle, σ is increased by the time increment step Δt. x and τ xy Discretize, σ x and τ xy These represent tensile stress and shear stress, respectively. S22. At different times, calculate the δ of the i-th inclined plane using the formulas for normal stress and shear stress respectively. i Normal stress and shear stress on the surface; S23. Calculate the shear stress amplitude C on the i-th inclined plane δi respectively. a(δi) Shear mean stress C m(δi) and maximum normal stress C n,max,δi : ; in, Normal stress; Shear stress; S24. Within the range [0°, 180°), use the bisection method based on the calculation formula from step S23 to continuously calculate the shear stress amplitude C on different inclined planes. a(δi) and maximum normal stress C n,max,δi First, find the maximum shear stress amplitude C. a(δi) At the corresponding azimuth angle, two shear stress amplitudes C will be generated. a(δi) When the azimuth angles are equal but complementary, compare the maximum normal stress C corresponding to the two azimuth angles. n,max,δi The plane with the largest maximum normal stress and the largest maximum shear stress amplitude is the critical surface. The azimuth angle δ of the critical surface is determined based on the critical surface. cr Furthermore, the maximum shear stress amplitude C on the critical surface was calculated. a,max and shear mean stress C m,a .

[0008] Preferably, the normal stress in step S22 and shear stress The calculation formula is as follows: ; Where j is the number of discrete substeps in a loading cycle, σ a τ represents the amplitude of tensile stress. a σ represents the shear stress amplitude. m τ represents the average tensile stress. m This represents the average shear stress.

[0009] Preferably, step S4 specifically includes the following steps: S41. Construct a fatigue life prediction model for thin-walled circular tube specimens under elastic stability: Q=F(N f The formula is as follows: ; in, It is the shear fatigue strength coefficient under pure torsional loading. It is the shear fatigue strength index under pure torsional loading; S42. Calculate the symmetrical tensile-compressive fatigue strength σ of the material when the stress ratio R = -1. -1 and symmetrical torsional fatigue strength τ -1 , shuffling Walker factor ω C Shear fatigue strength coefficient under pure torsional loading Shear fatigue strength index under pure torsional loading ; S43. Substitute the material parameters from step S42 into the formula from step S41 to calculate the fatigue life N. f The value of .

[0010] Preferably, in step S42, σ -1 and τ -1 The calculation method is as follows: Based on symmetrical tensile fatigue tests and symmetrical torsional fatigue tests with different amplitudes of stress ratio R=-1, obtain the SN curves for symmetrical tension / compression and symmetrical torsion, and then let the fatigue life N... f =10^7, then σ can be calculated. -1 and τ -1 The value.

[0011] Preferably, the shear fatigue strength coefficient and shear fatigue strength index The calculation method is as follows: by setting the SN curves at different amplitudes, the Basquin formula is used for linear fitting to obtain the corresponding values.

[0012] Preferably, the specific process for calculating the stress state at the critical point O in step S1 is as follows: Assuming the thin-walled circular tube specimen is simultaneously subjected to cyclic loads of axial tension / compression and circumferential torsion, with both loading waveforms being sinusoidal, it exhibits... The body form is: ; Due to the stress characteristics of the thin-walled circular tube specimen, the stress state at the critical point O is as follows: ; According to the stability analysis, if the cyclic load is in If the stress is in an elastically stable state, then due to the characteristic that the stress generated by the external load on the thin-walled circular tube specimen corresponds one-to-one with the stress at its critical point, then if the stress state at the critical point O is in a state of elastic stability... If the values ​​are within a certain range, the thin-walled circular tube specimen is determined to be in an elastically stable state. Where, σ x and τ xy These represent tensile stress and shear stress, respectively; the subscripts a and m represent stress amplitude and stress mean, respectively; δ is the phase angle, representing the phase difference between the two loading waveforms; ω is the loading frequency. and These are the maximum and minimum values ​​of the stability ultimate load, respectively; and These represent the maximum and minimum stress values ​​at the critical point of the smooth cylinder corresponding to the peak / valley values ​​of the stability limit load, respectively.

[0013] Preferably, in step S3, α is obtained from the symmetrical tensile and compressive fatigue strength and the symmetrical torsional fatigue strength of the material.

[0014] Preferably, the wall thickness of the thin-walled circular tube specimen is 1-2 mm.

[0015] Preferably, in step S24, i varies in steps of 0.05°.

[0016] Compared with the prior art, the beneficial effects of the present invention are as follows: (1) This invention performs a stability analysis on thin-walled circular tube specimens. Due to the special structural characteristics of thin-walled circular tube specimens, the tensile stress generated by external loads will only be generated in the thin-walled circular tube test section, and the shear stress generated by external loads will only be generated in the thin-walled circular tube test section. Based on this characteristic, it is only necessary to know whether the equivalent stress at the danger point caused by external loads is within the elastically stable range to determine whether the thin-walled circular tube specimen is in an elastically stable state, which is very convenient for calculation.

[0017] (2) This invention uses the critical surface method to predict fatigue life, and employs the bisection method to quickly shorten the calculation range during the calculation of the critical surface, which can conveniently and quickly obtain the critical surface azimuth angle and reduce a large amount of calculation time. The critical surface method only requires knowing the damage variable value on the critical surface and combining it with the shear form of the Basquin equation to quickly calculate the predicted fatigue life; the shear form of the Basquin equation only requires the shear fatigue strength coefficient. and shear fatigue strength index It has fewer parameters and is easy to calculate.

[0018] (3) The present invention uses the improved Dang-Van model as the damage variable in the fatigue life prediction model. It only needs to know the symmetrical tensile-compressive fatigue strength and symmetrical torsional fatigue strength σ of the material at a stress ratio R=-1. -1 and τ -1 It does not require solving for too many parameters, and the damage variables include the shear mean stress C. m,a It can reflect the impact of asymmetric loads on fatigue life to a certain extent.

[0019] (4) The fatigue life prediction method for thin-walled circular tube specimens under elastic stability proposed in this invention expands the application space of thin-walled circular tube specimens in more loading ranges and provides more options for the field of metal fatigue research. Attached Figure Description

[0020] Figure 1 This is a flowchart of the fatigue life prediction method for thin-walled circular tube specimens under elastic stability conditions according to the present invention. Figure 2 This is a diagram showing the stress conditions of the thin-walled circular tube specimen of the present invention; Figure 3 This is a stress state diagram of the critical point unit of the thin-walled circular tube of the present invention when it is in an elastically stable state; Figure 4 This is a schematic diagram illustrating the loading conditions of the three materials in this invention; Figure 5 This is a schematic diagram of the fatigue life prediction results of the improved Dang-Van model of the present invention. Detailed Implementation

[0021] Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings.

[0022] This invention provides a method for predicting the fatigue life of thin-walled circular tube specimens under elastic stability conditions, wherein the wall thickness of the thin-walled circular tube specimens is 1-2 mm. Elastic stability is a core concept in materials mechanics and structural engineering, directly related to the safety of structures under repeated loading. Its core meaning is that under repeated alternating loads, although local plastic deformation may occur in a structure or component, the overall structure remains stable and will not accumulate deformation leading to permanent deformation or failure.

[0023] The fatigue life prediction method for thin-walled circular tube specimens under elastic stability conditions of the present invention, as shown in the figure, includes the following steps: S1. Perform stability analysis on the thin-walled circular tube specimen, calculate the stress state at the critical point O, and determine whether the thin-walled circular tube specimen is in an elastically stable state based on the stress state at the critical point O. If the thin-walled circular tube specimen is in an elastically stable state, proceed to step S2.

[0024] First, the thin-walled circular tube specimen is subjected to stability testing. It is assumed that the specimen is simultaneously subjected to cyclic loads of axial tension / compression and circumferential torsion, with both loading waveforms being sinusoidal. Specifically, the form is as follows: ; Where, σ x and τ xy These represent tensile stress and shear stress, respectively; the subscripts a and m represent stress amplitude and stress mean, respectively; δ is the phase angle, representing the phase difference between the two loading waveforms; ω is the loading frequency.

[0025] Figure 2 The diagram shows the stress state of the thin-walled circular tube specimen. Figure 3 The stress state diagram of the critical point element of a thin-walled circular tube under elastic stability is shown. Due to the stress characteristics of the thin-walled circular tube specimen, the tensile stress and shear stress on the entire test section correspond one-to-one with the tensile, compressive, and torsional loads. Meanwhile, when a smooth specimen is subjected to tensile-torsional or bending-torsional loading, the specimen surface can be considered as a plane stress state. In this case, point O is selected as the critical point for analysis, and the stress state at critical point O is as follows: .

[0026] When performing stability analysis, stress and strain need to be integrated and converted into equivalent stress and equivalent strain. The formula is as follows: ; in, and These are the equivalent stress amplitude and the equivalent variable amplitude, respectively. and These are the tensile stress amplitude and the shear stress amplitude, respectively. and These are the equivalent elastic strain amplitude and the equivalent plastic strain amplitude, respectively. and These are the amplitudes of axial elastic strain and plastic strain, respectively. and These are the amplitudes of shear elastic strain and plastic strain, respectively. It is the elastic Poisson's ratio.

[0027] Based on relevant studies on stability analysis, it is believed that the structure under cyclic loading... The amplitude of the structural stability load under action The stress cannot exceed twice the structural elastic limit load. For thin-walled circular tube specimens, the stress generated by the external load corresponds one-to-one with the stress at the critical point O within the specimen; that is, the tensile stress of the external load will only produce tensile stress at the critical point, and the shear stress of the external load will only produce shear stress at the critical point. If the external load is a coupled symmetrical cyclic tension-compression and symmetrical cyclic torsion loading, the resulting equivalent stress should also be symmetrical, and the peak and valley values ​​of the stability limit load are: ; If the equivalent stress obtained by synthesis is asymmetric, then the peak and valley values ​​of the stability ultimate load are: ; in, and These represent the peak and trough values ​​of the structural stability ultimate load under cyclic loading. and The elastic limit load and the plastic limit load corresponding to the initial yield strength of the structural material.

[0028] Due to the unique stress distribution in thin-walled circular tube specimens, the stability of the specimen can be determined by the stress range at the critical point O, i.e., when the stress at the critical point O is symmetrical. ; When the stress at the critical point is asymmetric: ; in, and These represent the maximum and minimum stress values ​​at the critical points of the smooth cylinder corresponding to the peak and trough values ​​of the stability limit load, respectively. , and These represent the yield strength, tensile strength, and converted average stress of the material in the thin-walled circular tube specimen.

[0029] If the stress state at the danger point O is... If the values ​​are within a certain range, the thin-walled circular tube specimen can be determined to be in an elastically stable state.

[0030] S2. Extract the stress state of the thin-walled circular tube specimen at the critical point O, and calculate the critical surface azimuth angle θ. cr And based on the critical surface azimuth angle θ cr Obtain the maximum shear stress amplitude C on the critical surface a,max and shear mean stress C m,a .

[0031] Step S2, calculating the critical surface azimuth angle, includes the following sub-steps: S21. Within one loading cycle, σ is increased by the time increment step Δt. x and τ xy Discretize.

[0032] S22. At different times, calculate the δ of the i-th inclined plane using the formulas for normal stress and shear stress respectively. i The normal stress and shear stress on the surface are calculated using the following formulas: ; Where j is the number of discrete substeps in a loading cycle.

[0033] S23. Calculate the shear stress amplitude C on the i-th inclined plane δi respectively. a(δi) Shear mean stress C m(δi) and maximum normal stress C n,max,δi .

[0034] .

[0035] S24. Within the range [0°, 180°), i varies in steps of 0.05°. Using the calculation formula from step S23, the shear stress amplitude C on different inclined planes is continuously calculated using the bisection method. a(δi) and maximum normal stress C n,max,δi First, find the maximum shear stress amplitude C. a(δi) At the corresponding azimuth angle, two shear stress amplitudes C will be generated. a(δi) When the azimuth angles are equal but complementary, compare the maximum normal stress C corresponding to the two azimuth angles. n,max,δi The plane with the largest maximum normal stress and the largest maximum shear stress amplitude is the critical surface. The azimuth angle δ of the critical surface is determined based on the critical surface. cr Furthermore, the maximum shear stress amplitude C on the critical surface was calculated. a,max and shear mean stress C m,a .

[0036] S3. The formula for calculating the damage variable Q is as follows: ; Where Q is the damage variable; C a,max It is the maximum shear stress amplitude on the critical surface; C m,a ω is the shear mean stress on the critical surface. The absolute value is used to reflect the effect of the shear mean stress on fatigue life because, under the same equivalent stress, regardless of whether it is positive or negative, the equivalent stress on the critical surface will reduce fatigue life; C This is the shear Walker factor, which reflects the material's sensitivity to mean shear stress; its value ranges between 0 and 1. -1 and τ -1 These are the symmetrical tensile-compressive fatigue strength and symmetrical torsional fatigue strength of the material at a stress ratio R = -1, respectively; α is a Dang-Van model coefficient, a dimensionless constant obtained from the symmetrical tensile-compressive fatigue strength and symmetrical torsional fatigue strength of the material; σ H It is hydrostatic stress.

[0037] S4. Combine the formula for calculating the damage variable Q with the Basquin formula to construct a fatigue life prediction model for thin-walled circular tube specimens under elastic stability: Q=F(N f ), calculate the parameters in the fatigue life prediction model and the maximum shear stress amplitude C on the critical surface calculated in step S2. a,max and shear mean stress C m,a We input these values ​​into the fatigue life prediction model and solve for the fatigue life N. f The value of .

[0038] S4 specifically includes the following steps: S41. Construct a fatigue life prediction model for thin-walled circular tube specimens under elastic stability: Q=F(N f The formula is as follows: ; in, It is the shear fatigue strength coefficient under pure torsional loading. It is the shear fatigue strength index under pure torsional loading.

[0039] S42. Calculate the symmetrical tensile-compressive fatigue strength σ of the material when the stress ratio R = -1. -1 and symmetrical torsional fatigue strength τ -1 , shuffling Walker factor ω C Shear fatigue strength coefficient under pure torsional loading Shear fatigue strength index under pure torsional loading .

[0040] S43. Substitute the material parameters from step S42 into the formula from step S41 to calculate the fatigue life N. f The value of .

[0041] Preferably, in step S42, σ -1 and τ -1 The calculation method is as follows: Based on symmetrical tensile fatigue tests and symmetrical torsional fatigue tests with different amplitudes of stress ratio R=-1, obtain the SN curves for symmetrical tension / compression and symmetrical torsion, and then let the fatigue life N... f =10^7, then σ can be calculated. -1 and τ -1 The value.

[0042] This invention first performs a stability analysis on a thin-walled circular tube specimen. Due to the structural characteristics of the thin-walled circular tube specimen, the tensile stress generated by the external load will only be generated in the thin-walled circular tube test section, and the shear stress generated by the external load will only be generated in the thin-walled circular tube test section. Based on this characteristic, it is only necessary to know whether the equivalent stress at the danger point caused by the external load is within the elastically stable range to determine whether the thin-walled circular tube specimen is in an elastically stable state, making the calculation very convenient.

[0043] This invention employs an improved Dang-Van model as the fatigue life prediction model, using three materials: 2017A-T4, 6082-T6, and LY12CZ. Their performance data are shown in Table 1, and the loading conditions are as follows. Figure 4 As shown, the comparisons between all loading paths and the corresponding elastic stability ranges of the specimens are all... Figure 5 As shown in Table 2, all loading conditions are within the elastic stability range of each material.

[0044] Table 1 Table 2 The fatigue life prediction method for thin-walled circular tube specimens under elastically stable conditions of the present invention is applicable to specimens with a fatigue life of 0.5 < τ. -1 / σ -1 Metallic materials in the range of <0.8.

[0045] The embodiments described above are merely preferred embodiments of the present invention and are not intended to limit the scope of the present invention. Various modifications and improvements made by those skilled in the art to the technical solutions of the present invention without departing from the spirit of the present invention should fall within the protection scope defined by the claims of the present invention.

Claims

1. A method for predicting the fatigue life of a thin-walled circular tube test specimen in an elastic stable state, characterized by: It comprises the following steps: S1, stability analysis is performed on the thin-walled pipe specimen, stress state at the dangerous point O is calculated, and whether the thin-walled pipe specimen is in an elastic stability state is judged according to the stress state at the dangerous point O, if the thin-walled pipe specimen is in the elastic stability state, step S2 is entered; S2, extract the stress state of the thin-walled tube test piece at the dangerous point O, calculate the critical interface azimuth angle θ cr , and obtain the maximum shear stress amplitude C on the critical interface based on the critical interface azimuth angle θ cr a,max , and the shear average stress C m,a ;​ S3, the calculation formula of the damage variable Q is as follows: ； where Q is a damage variable; C a,max is the maximum shear stress amplitude at the critical plane; C m,a is the shear mean stress at the critical plane; ω C is the shear Walker factor, which takes values between 0 and 1; σ -1 and τ -1 are the symmetric tension-compression fatigue strength and the symmetric torsional fatigue strength of the material at the stress ratio R = -1, respectively; a is a model coefficient, which is a dimensionless constant; σ H is the hydrostatic stress; S4, combine the calculation formula of damage variable Q with Basquin formula to construct fatigue life prediction model of thin-walled circular tube specimen in elastic stable state Q=F(N f ), calculate the parameters in fatigue life prediction model and the maximum shear stress amplitude C a,max and shear average stress C m,a on the critical surface calculated in step S2, and bring them into the fatigue life prediction model to solve, to obtain the value of fatigue life N f .

2. The method of predicting the fatigue life of a thin-walled circular tube test specimen in the elastic regime according to claim 1, characterized in that: The critical surface azimuth angle in step S2 comprises the following sub-steps: S21, within a loading cycle, σ x and τ xy are discretized using a time increment step Δt, σ x and τ xy denote respectively the tensile and shear stresses; S22, at different time, using the normal stress and shear stress formula respectively to calculate the normal stress and shear stress on the i-th slope δ i i S23, calculate the shear stress amplitude C on the i-th slope δi a(δi) , the shear average stress C m(δi) , and the maximum normal stress C n,max,δi : ； wherein is the normal stress; is the shear stress; S24, in [0°, 180°), the shear stress amplitude C on different inclined planes is calculated continuously using the calculation formula of step S23 by dichotomy a(δi) and the maximum normal stress C n,max,δi , the maximum shear stress amplitude C a(δi) corresponding to the maximum normal stress C a(δi) is found first, at this time two shear stress amplitude C n,max,δi equal but with complementary azimuth δ are produced, the maximum normal stress C cr corresponding to the two azimuths is compared, the maximum shear stress amplitude plane with the maximum normal stress is the critical plane, the azimuth δ of the critical plane is determined according to the critical plane, and the maximum shear stress amplitude C a,max and the shear average stress C m,a on the critical plane are further calculated.

3. The method of predicting the fatigue life of a thin-walled circular tube test specimen in the elastic regime according to claim 1, characterized in that: The formula for calculating the normal stress in step S22 and the shear stress is as follows: ； where j is the number of discrete sub-steps in a loading cycle, σ a represents the tensile stress amplitude, τ a represents the shear stress amplitude, σ m represents the tensile stress mean, τ m represents the shear stress mean.

4. The method of predicting the fatigue life of a thin-walled circular tube test specimen in the elastic regime according to claim 1, wherein: Step S4 Specifically comprises the following steps: S41, construct the elastic stability state of thin-walled tube fatigue life prediction model Q = F (N f ) formula as follows: ； wherein is the shear fatigue strength coefficient in the pure torsion loading state, is the shear fatigue strength exponent in the pure torsion loading state. S42, the symmetrical tension-compression fatigue strength of the material at stress ratio R = -1 is calculated -1 and the symmetrical torsional fatigue strength τ -1 , the shear Walker factor ω C , the shear fatigue strength coefficient under pure torsional loading state and the shear fatigue strength exponent under pure torsional loading state ; S43, substituting the material parameters in step S42 into the formula in step S41 to calculate the fatigue life N f .

5. The method of predicting the fatigue life of a thin-walled circular tube test specimen in the elastic regime according to claim 4, characterized in that: The calculation method of σ -1 and τ -1 is as follows: according to the symmetric tension-compression fatigue test and the symmetric torsion fatigue test of different amplitude of stress ratio R=-1, the S-N curves of symmetric tension-compression and symmetric torsion are obtained, and then the fatigue life N f =10^7, the values of σ -1 and τ -1 can be calculated.

6. The method of predicting the fatigue life of a thin-walled circular tube test specimen in the elastic regime according to claim 5, characterized in that: Shear fatigue strength coefficient And shear fatigue strength index The calculation method is: by setting the S-N curve under different amplitude, linear fitting is carried out by using Basquin formula, and the corresponding value is obtained.

7. The fatigue life prediction method for the thin-walled pipe specimen in the elastic stability state according to claim 1, characterized in that: The specific process of calculating the stress state at the dangerous point O in step S1 is as follows: Suppose that the thin-walled pipe specimen is subjected to axial tension and compression and circumferential torsion cyclic load at the same time, the loading waveform is a sine wave, and the load has The specific form is as follows: ； Because of the stress characteristics of the thin-walled pipe specimen, the stress state at the dangerous point O is as follows: ； According to the stability analysis, the cyclic load is in the elastic stability state if it is in , and because of the characteristic that the stress generated by the external load of the thin-walled pipe specimen corresponds to the stress at the dangerous point, if the stress state at the dangerous point O is between , it is determined that the thin-walled pipe specimen is in the elastic stability state. where σ x and τ xy represent tensile and shear stresses, respectively; subscripts a and m represent stress amplitude and stress mean, respectively; δ is the phase angle, representing the phase difference between the two loading waveforms; and ω is the loading frequency; and are the maximum and minimum values of the shakedown limit load, respectively; and are the maximum and minimum values of the stress at the critical point of the smooth cylinder corresponding to the peak / trough of the shakedown limit load, respectively.

8. The method for predicting the fatigue life of a thin-walled circular tube test specimen in the elastic regime according to Claim 1, characterized by: In step S3, α is obtained from the symmetric tension and compression fatigue strength and the symmetric torsion fatigue strength of the material.

9. The method for predicting the fatigue life of a thin-walled circular tube test specimen in the elastic regime according to Claim 1, characterized in that: The wall thickness of the thin-walled pipe specimen is 1-2 mm.

10. The method of predicting the fatigue life of a thin-walled circular tube test specimen in the elastic regime according to claim 1, wherein: In step S24, i changes with a step length of 0.05°.

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