CN117725774A - Fatigue life prediction method for mounting seat structure with round hole by considering two-dimensional stress gradient line - Google Patents
Fatigue life prediction method for mounting seat structure with round hole by considering two-dimensional stress gradient line Download PDFInfo
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Abstract
The invention provides a fatigue life prediction method for a mounting seat structure with a round hole, which considers a two-dimensional stress gradient line. The basic idea is as follows: the invention is improved on the basis of a traditional critical distance method (Theory ofCritical Distance, TCD) model, and because the thickness of the hole edge structure of the mounting seat is larger, the hole edge stress gradient line is not simply deflected in one-dimensional along the direction of the extension line of the hole edge, but gradually deflects towards the thickness direction, so that the invention is based on the actual two-dimensional stress gradient line on the dangerous section of the hole edge of the mounting seat, and the critical distance is determined by combining SWT parameters on the curve, thereby establishing a TCD-SWT fatigue life prediction model considering the multidimensional stress gradient line. The life prediction model is suitable for fatigue life prediction of the mounting seat, which is a notch structure with a large two-dimensional stress gradient, and the prediction results are all in a 2-time dispersion band, so that the prediction accuracy is high, and the method has good engineering application value.
Description
Technical Field
The invention belongs to the technical field of fatigue life prediction of a mounting seat structure with a round hole, and particularly relates to a fatigue life prediction method of a mounting seat structure with a round hole, which takes a two-dimensional stress gradient line into consideration.
Background
Aeroengines are the "heart" of an aircraft and are important power take-off components. The casing is one of important components of the aeroengine, and plays a role in supporting a rotor, fixing stator blades and accessories, and forms an air flow passage of the engine together with other components to transmit thrust generated by the engine to an aircraft.
In a typical construction of a combustor casing, various types of circular hole mounts are commonly included due to structural design requirements. The combustion chamber casing bears various complex load actions, such as pressure difference between the inside and outside of the casing, axial force of gas, torque brought by front and rear mounting edges of the casing, joint loads of various mounting seats on the outer wall of the casing and the like, and under the action of multiaxial load, the mounting seat structures can generate stress concentration on the hole edges. The mounting seat hole Bian Buwei is high in stress level and stress gradient, cracks are easy to initiate, damage occurs further, and the mounting seat hole Bian Buwei is a dangerous part of the combustion chamber casing, so that low-cycle fatigue damage of the edge part of the mounting seat hole needs to be focused when the fatigue life of the combustion chamber casing is evaluated. In addition, the working temperature of the modern high-performance engine combustion chamber casing is up to 450-650 ℃, and the whole casing is subjected to high-temperature gas pressure, so that the influence of temperature is also required to be considered when the service life of the combustion chamber casing is estimated. In this context, the present invention proposes a new method for predicting fatigue life of a round hole mount structure.
Disclosure of Invention
The invention aims to establish a TCD-SWT fatigue life prediction model considering a multidimensional stress gradient line by determining a critical distance by combining SWT parameters on a dangerous section of a hole edge of a mounting seat on the basis of an actual two-dimensional stress gradient line, and provides a fatigue life prediction method considering the two-dimensional stress gradient line for a mounting seat structure with a round hole;
in order to achieve the above purpose, the present invention adopts the following technical scheme: a fatigue life prediction method for a round hole-carrying mounting seat structure considering a two-dimensional stress gradient line comprises the following steps:
step 1: performing finite element simulation calculation on the dangerous section of the hole edge structure of the mounting seat by adopting a numerical simulation method to obtain the dangerous section stress distribution of the hole edge structure of the mounting seat, and then performing calculation on the two-dimensional stress gradient of the dangerous section stress distribution of the hole edge structure of the mounting seat by adopting a gradient descent method to obtain an actual two-dimensional stress gradient curve on the dangerous section of the hole edge structure of the mounting seat;
step 2: determining SWT parameter distribution lines on the two-dimensional stress gradient curves according to the actual two-dimensional stress gradient curves on the dangerous section of the hole edge structure of the mounting seat;
step 3: determining an SWT model and related parameters thereof through a material smooth piece test, and combining with a fatigue test of the test piece of the mounting seat hole edge structure to obtain an effective SWT parameter (sigma) of the test piece of the mounting seat hole edge structure max ε a ) eff ；
Step 4: fitting SWT parameter distribution lines on a two-dimensional stress gradient curve, and integrating to obtain an average value in a critical distance section, so that the average value in the critical distance section and an effective SWT parameter (sigma max ε a ) eff Equality, establishing an equation, and solving the equation to obtain a critical distance;
step 5: fitting an SWT parameter distribution line on a two-dimensional stress gradient line, and integrating to obtain an average value in a critical distance section on the SWT parameter distribution line, so that the average value in the critical distance section on the SWT parameter distribution line is equal to an SWT model of the material smooth piece, and a fatigue life prediction model of the hole edge structure of the mounting seat is obtained.
Further, the step 1 includes:
step 1.1: performing finite element simulation calculation on the dangerous section of the hole edge structure of the mounting seat by adopting a numerical simulation method to obtain the stress of each node on the dangerous section of the hole edge structure of the mounting seat, extracting the stress of each node on the dangerous section of the hole edge structure of the mounting seat and forming a matrix, drawing matrix data into a two-dimensional plan, and finally obtaining the stress distribution of the dangerous section of the hole edge structure of the mounting seat according to the two-dimensional plan;
step 1.2: the point of maximum stress is set as the initial point (x 1 ,y 1 ) And (3) determining the position of a second point according to a formula of a gradient descent method, and so on, determining the positions of all the remaining points, and drawing the obtained points on a two-dimensional plan in the step (1.1) to obtain an actual two-dimensional stress gradient curve on the dangerous section of the hole edge structure of the mounting seat.
Further, the gradient descent method has the formula:
wherein x and y are coordinates; λ is the step size; f is a function of x, y and σ is the stress in the stress matrix.
Further, the step 2 specifically includes:
taking the starting point of an actual two-dimensional stress gradient curve on a dangerous section of a hole edge structure of the mounting seat as a circle center, making a plurality of circular arcs with different radiuses, and setting the intersection point of the circular arcs and the stress gradient line as P 1 、P 2 、P 3 Etc., extract intersection point P 1 、P 2 、P 3 Coordinates at … … (x i ,y i ) And then extracting the stress and strain of the intersection points, substituting the stress and strain into a formula of SWT parameters to obtain the SWT parameters of the intersection points, and then plotting by taking the SWT parameters as ordinate and taking l/r as abscissa to obtain the SWT parameter distribution line on the two-dimensional stress gradient curve.
Further, the formula of the SWT parameter is:
wherein sigma max Epsilon is the maximum stress of the hole edge of the mounting seat structure max And epsilon min Respectively maximum strain and minimum strain of hole edge epsilon a Is the strain amplitude.
Further, the step 3 includes:
step 3.1: fitting according to a fatigue test of a mounting seat hole edge structure test piece to obtain a nominal stress-life curve, wherein the nominal stress-life curve is as follows:
lgσ nor ＝A+BlgN f
wherein A and B are material parameters, sigma nor Is the nominal stress, N f Is the fatigue test life.
Step 3.2: describing the relation between the strain and the service life by adopting a Manson-Coffin equation, and obtaining related material parameters through a material smooth piece strain test to obtain the relation between the strain and the service life;
based on symmetrical cycle parameters, while considering the strain amplitude ε a Maximum stress sigma max Action on low cycle fatigue and based on the strain versus life, the strain amplitude ε a And maximum stress sigma max Fitting to establish a SWT model of the material smooth piece, wherein the SWT model of the material smooth piece is as follows:
wherein σ' f For fatigue strength coefficient, ε' f B is a fatigue strength index, c is a fatigue ductility index, and E is an elastic modulus;
step 3.3: nominal stress sigma for each load stage nor The theoretical service life corresponding to each load level can be obtained by bringing the nominal stress-service life curveTheoretical lifetime->Substituting into SWT model of smooth piece of material to obtainEffective SWT parameter (σ) of mount hole edge structure test piece max ε a ) eff 。
Further, the equation in the step 4 is:
wherein D is LM Is critical distance, sigma max ε a Is the SWT parameter (σ) on the stress gradient line max ε a ) eff The effective SWT parameter of the test piece of the hole edge structure of the mounting seat is that l is the distance from the hole bottom angle on the dangerous section stress gradient line.
Further, the fatigue life prediction model of the hole edge structure of the mounting seat is as follows:
wherein D is LM Is critical distance, sigma max ε a For SWT parameter on stress gradient line, σ' f For fatigue strength coefficient, ε' f For the fatigue ductility coefficient, b is the fatigue strength index, c is the fatigue ductility index, and E is the elastic modulus.
The beneficial effects are that: compared with the prior art, the fatigue performance prediction method for the mounting seat structure with the round hole has the following advantages: when the traditional critical distance method is used for solving the critical distance, the hole edge extension line in the vertical loading direction is used as a stress gradient line, the method has good adaptability to the sheet with the notch, but the thickness of the hole edge of the mounting seat with the round hole is larger, and the hole edge stress gradient line does not extend along the hole edge extension line direction but gradually deflects towards the thickness direction, so the method is based on the two-dimensional stress gradient line on the dangerous section of the hole edge of the mounting seat with the round hole, and combines SWT parameters on the curve to determine the critical distance, the prediction result is in a 2-time dispersion band, the prediction precision is higher, and the method has better engineering application value.
Drawings
FIG. 1 is a flow chart of a fatigue life prediction method of the present invention.
Fig. 2 is a stress gradient calculation flow.
FIG. 3 is a schematic diagram of a conventional TCD method for a test piece with a hole edge structure of a mounting seat according to the present invention.
FIG. 4 is a geometric diagram of a test piece of the hole edge structure of the mounting seat in the invention.
FIG. 5 is a schematic diagram of an improved critical distance determination method.
FIG. 6 is a schematic diagram of a stress gradient line of a dangerous section of a test piece of a GH4169 mount hole edge structure.
FIG. 7 is a TCD-SWT model GH4169 fatigue life prediction result of the invention considering two-dimensional stress gradients.
Detailed Description
The invention is further explained below with reference to the drawings.
As shown in FIG. 1, the invention provides a fatigue life prediction method for a round hole-equipped mounting seat structure taking two-dimensional stress gradient lines into consideration, which comprises the following steps:
step 1: and carrying out finite element simulation calculation on the dangerous section of the hole side structure of the mounting seat by adopting a numerical simulation method to obtain the dangerous section stress distribution of the hole side structure of the mounting seat, and then calculating the two-dimensional stress gradient of the dangerous section stress distribution of the hole side structure of the mounting seat by adopting a gradient descent method to obtain an actual two-dimensional stress gradient curve on the dangerous section of the hole side structure of the mounting seat.
In step 1, firstly, on the basis of the finite element simulation calculation of a casing, stress of a selected node is led out by ANSYS software, then the stress of each node on a dangerous section is extracted to form a matrix, matrix data are drawn into a two-dimensional plan, and then the dangerous section stress distribution of the hole edge structure of the mounting seat is obtained according to the two-dimensional plan.
After the dangerous section stress distribution of the hole edge structure of the mounting seat is obtained, the point with the maximum stress is taken as an initial point (x 1 ,y 1 ) The point gradient (dx 1 ,dy 1 )＝(dx 11 ,dy 11 ) Determining the position of the second point according to the formula of the gradient descent method, and so on, determining all the remaining pointsThe specific process is shown in the stress gradient calculation flow of figure 2, the obtained points are drawn in a two-dimensional plan in step 1.1, and an actual two-dimensional stress gradient curve on a dangerous section of the hole edge structure of the mounting seat is obtained, wherein the process can be realized by adopting a gradient descent method to automatically develop a calculation program by Matlab software, and the formula of the gradient descent method is as follows:
wherein x and y are coordinates; λ is the step size; f is a function of x, y and σ is the stress in the stress matrix.
Step 2: and determining the SWT parameter distribution line on the two-dimensional stress gradient curve according to the actual two-dimensional stress gradient curve on the dangerous section of the hole edge structure of the mounting seat.
As shown in fig. 3, when θ=0° is adopted in the conventional TCD method, that is, the average stress of units at a certain distance along the bottom edge of the hole is used as the effective stress to predict the fatigue strength, the formula is as follows:
wherein Δσ eff As effective stress, Δσ y For stress at a point at a certain distance from the root of the notch, θ is the angle between the line connecting the point to the point of maximum stress at the edge of the hole and the bottom edge of the hole, r is the radius of the notch, L M (N f ) Is the critical distance.
In the step 2, the invention takes the starting point of the actual two-dimensional stress gradient curve on the dangerous section of the hole edge structure of the mounting seat as the circle center, makes a plurality of circular arcs with different radiuses, and sets the intersection point of the circular arc and the stress gradient line as P 1 、P 2 、P 3 Etc., extract intersection point P 1 、P 2 、P 3 Coordinates at … … (x i ,y i ) Then extracting the stress and strain of the intersection points, substituting the stress and strain into a formula of SWT parameters to obtain the SWT parameters of the intersection points, and then plotting the SWT parameters with the SWT parameters as ordinate and with l/r as abscissa to obtain a SWT parameter distribution line on a two-dimensional stress gradient curve, wherein the distance between the intersection points and the center of the circular arc, namely the radius of the circular arc, is l 1 、l 2 、l 3 And the like, carrying out normalization processing on the I by taking the radius r of the circular hole of the mounting seat as a reference, wherein the formula of SWT parameters is as follows:
wherein sigma max Epsilon is the maximum stress of the hole edge of the mounting seat structure max And epsilon min Respectively maximum strain and minimum strain of hole edge epsilon a Is the strain amplitude.
Step 3: determining an SWT model and related parameters thereof through a material smooth piece test, and combining with a fatigue test of the test piece of the mounting seat hole edge structure to obtain an effective SWT parameter (sigma) of the test piece of the mounting seat hole edge structure max ε a ) eff 。
In step 3, firstly, fitting according to a fatigue test of a mounting seat hole edge structure test piece to obtain a nominal stress-life curve, wherein the nominal stress-life curve is as follows:
lgσ nor ＝A+BlgN f
wherein A and B are material parameters, and are obtained by fitting fatigue test data of a mounting seat hole edge structure test piece, and sigma nor Is the nominal stress.
Then, describing the relation between the strain and the service life by adopting a Manson-Coffin equation, and obtaining relevant material parameters through a material smooth piece strain test to obtain the relation between the strain and the service life;
based on symmetrical cycle parameters, while considering the strain amplitude ε a Most preferablyLarge stress sigma max Action on low cycle fatigue and based on the strain versus life, the strain amplitude ε a And maximum stress sigma max Fitting to establish a SWT model of the material smooth piece, wherein the SWT model of the material smooth piece is as follows:
wherein σ' f For fatigue strength coefficient, ε' f And b is a fatigue strength index, c is a fatigue ductility index, E is an elastic modulus, and the fatigue ductility index is obtained through fitting strain test data of a material smooth piece.
Finally, nominal stress sigma of each load level nor The theoretical service life corresponding to each load level can be obtained by bringing the nominal stress-service life curveTheoretical lifetime->Substituting the SWT parameter into SWT model of the material smooth piece, and reversely pushing to obtain effective SWT parameter (sigma of the test piece of the hole edge structure of the mounting seat max ε a ) eff 。
Step 4: fitting SWT parameter distribution lines on a two-dimensional stress gradient curve, and integrating to obtain an average value in a critical distance section, so that the average value in the critical distance section and an effective SWT parameter (sigma max ε a ) eff Equality, an equation is established, and the equation is solved to obtain the critical distance.
In step 4, the equation is:
wherein D is LM Is critical distance, sigma max ε a Is the SWT parameter (σ) on the stress gradient line max ε a ) eff The effective SWT parameter of the test piece of the hole edge structure of the mounting seat is that l is the distance from the hole bottom angle on the dangerous section stress gradient line.
Step 5: fitting an SWT parameter distribution line on a two-dimensional stress gradient line, and integrating to obtain an average value in a critical distance section on the SWT parameter distribution line, so that the average value in the critical distance section on the SWT parameter distribution line is equal to an SWT model of the material smooth piece, and a fatigue life prediction model of the hole edge structure of the mounting seat is obtained.
In step 5, the fatigue life prediction model of the hole edge structure of the mounting seat is as follows:
wherein D is LM Is critical distance, sigma max ε a For SWT parameter on stress gradient line, σ' f For fatigue strength coefficient ε f ' is the fatigue ductility factor, b is the fatigue strength index, c is the fatigue ductility index, and E is the elastic modulus.
The GH4169 mounting seat hole edge structure test piece is measured according to the method, and the specific implementation steps are as follows:
the structure of the GH4169 mounting seat hole edge structure test piece is shown in fig. 4, a stress fatigue test (600 ℃) is carried out on the GH4169 mounting seat hole edge structure test piece, and then a nominal stress-life curve is obtained by fitting, wherein the nominal stress-life curve is specifically as follows:
lgσ＝3.616-0.142lgN f 。
strain fatigue test is carried out on GH4169 material smooth standard parts, and nominal stress sigma of each load level is carried out nor The theoretical service life corresponding to each load level can be obtained by bringing the nominal stress-service life curveTheoretical lifetime->Is carried into SWT model of smooth piece of material, and can be reversely pushed to obtain each loadThe SWT model of the material smooth piece is specifically as follows:
σ max ·Δε/2＝11.185(2N f ) -0.1518 +8174.1(2N f ) -1.1492
at this time ε a ＝Δε/2。
And 5-6, calculating a stress gradient line on a dangerous section of the test piece by using Matlab software through a gradient descent method according to the finite element calculation result of the test piece of the hole edge structure of the mounting seat. Taking the starting point of the stress gradient line as the center of a circle, making a plurality of circular arcs with different radiuses, and setting the intersection point of the circular arc and the stress gradient line as P 1 、P 2 、P 3 Etc., extract intersection point P 1 、P 2 、P 3 Coordinates at … … (x i ,y i ). The stresses and strains at these intersections are then extracted, and the SWT parameters (σ) at these points can be found max ε a ). The distance between the intersection point and the center of the arc, namely the radius of the arc is l 1 、l 2 、l 3 And the like, taking the radius r of the hole as a reference, and carrying out normalization treatment on the l. And then, drawing by taking the SWT parameter as an ordinate and taking l/r as an abscissa, so as to obtain a SWT parameter distribution map on the stress gradient line, wherein r is the radius of a circular hole of the mounting seat. Finally, fitting the SWT parameter distribution line on the two-dimensional stress gradient curve, and integrating to obtain the average value in the critical distance section, so that the average value in the critical distance section and the effective SWT parameter (sigma) of the mounting seat hole edge structure test piece max ε a ) eff Equality, an equation is established, and the equation is solved to obtain the critical distance. The equation is:
based on the determined critical distance D LM =0.716 r, and a SWT parameter distribution diagram on a stress gradient line obtained through a finite element calculation result of the mount hole edge structure test piece, solving a predicted SWT parameter, and taking the predicted SWT parameter into a corresponding mount hole edge structure fatigue life prediction model to obtain the following formula:
solving the formula to obtain the predicted life of the GH4169 smooth standard component, wherein sigma max ε a The SWT parameter on the stress gradient line can be regarded as a function of l, and the calculation formula of the SWT parameter is as follows:
comparison analysis with the fatigue test results under the same conditions, as shown in fig. 7, shows that the error is within a tolerance zone of 2 times, indicating that the prediction method is good.
According to the invention, a numerical simulation method is adopted to perform finite element simulation calculation on the dangerous section of the hole edge structure of the mounting seat, a Matlab software is utilized to obtain an actual two-dimensional stress gradient curve on the dangerous section by adopting a gradient descent method, and a test result of a simulation piece of the mounting seat is combined to obtain a critical distance, so that a fatigue life prediction model of the hole edge structure of the mounting seat is established. The life prediction method is suitable for fatigue life prediction of the mounting seat, namely the notch structure with the large two-dimensional stress gradient, the prediction results are in a 2-time dispersion band, the prediction precision is higher than that of the traditional method, and the method has better engineering application value.
The foregoing is merely a preferred embodiment of the present invention and it should be noted that modifications and adaptations to those skilled in the art may be made without departing from the principles of the present invention, which are intended to be comprehended within the scope of the present invention.
Claims (8)
1. A fatigue life prediction method for a mounting seat structure with a round hole taking a two-dimensional stress gradient line into consideration is characterized by comprising the following steps:
step 1: performing finite element simulation calculation on the dangerous section of the hole edge structure of the mounting seat by adopting a numerical simulation method to obtain the dangerous section stress distribution of the hole edge structure of the mounting seat, and then performing calculation on the two-dimensional stress gradient of the dangerous section stress distribution of the hole edge structure of the mounting seat by adopting a gradient descent method to obtain an actual two-dimensional stress gradient curve on the dangerous section of the hole edge structure of the mounting seat;
step 2: determining SWT parameter distribution lines on the two-dimensional stress gradient curves according to the actual two-dimensional stress gradient curves on the dangerous section of the hole edge structure of the mounting seat;
step 3: determining an SWT model and related parameters thereof through a material smooth piece test, and combining with a fatigue test of the test piece of the mounting seat hole edge structure to obtain an effective SWT parameter (sigma) of the test piece of the mounting seat hole edge structure max ε a ) eff ；
Step 4: fitting SWT parameter distribution lines on a two-dimensional stress gradient curve, and integrating to obtain an average value in a critical distance section, so that the average value in the critical distance section and an effective SWT parameter (sigma max ε a ) eff Equality, establishing an equation, and solving the equation to obtain a critical distance;
step 5: fitting an SWT parameter distribution line on a two-dimensional stress gradient line, and integrating to obtain an average value in a critical distance section on the SWT parameter distribution line, so that the average value in the critical distance section on the SWT parameter distribution line is equal to an SWT model of the material smooth piece, and a fatigue life prediction model of the hole edge structure of the mounting seat is obtained.
2. The method for predicting fatigue life of a round hole-equipped mount structure taking into account two-dimensional stress gradient lines according to claim 1, wherein the step 1 comprises:
step 1.1: performing finite element simulation calculation on the dangerous section of the hole edge structure of the mounting seat by adopting a numerical simulation method to obtain the stress of each node on the dangerous section of the hole edge structure of the mounting seat, extracting the stress of each node on the dangerous section of the hole edge structure of the mounting seat and forming a matrix, drawing matrix data into a two-dimensional plan, and finally obtaining the stress distribution of the dangerous section of the hole edge structure of the mounting seat according to the two-dimensional plan;
step 1.2: the point of maximum stress is set as the initial point (x 1 ,y 1 ) Determining the position of the second point according to the formula of the gradient descent method, and so onAnd (3) determining the positions of all the remaining points, drawing the obtained points on a two-dimensional plan in the step (1.1), and obtaining an actual two-dimensional stress gradient curve on the dangerous section of the hole edge structure of the mounting seat.
3. The method for predicting fatigue life of a round hole-equipped mount structure taking into account two-dimensional stress gradient lines according to claim 2, wherein the formula of the gradient descent method is as follows:
wherein x and y are coordinates; λ is the step size; f is a function of x, y and σ is the stress in the stress matrix.
4. The method for predicting the fatigue life of the round hole-equipped mounting seat structure taking two-dimensional stress gradient lines into consideration according to claim 1, wherein the step 2 is specifically:
taking the starting point of an actual two-dimensional stress gradient curve on a dangerous section of a hole edge structure of the mounting seat as a circle center, making a plurality of circular arcs with different radiuses, and setting the intersection point of the circular arcs and the stress gradient line as P 1 、P 2 、P 3 Etc., extract intersection point P 1 、P 2 、P 3 Coordinates at … … (x i ,y i ) And then extracting the stress and strain of the intersection points, substituting the stress and strain into a formula of SWT parameters to obtain the SWT parameters of the intersection points, and then plotting by taking the SWT parameters as ordinate and taking l/r as abscissa to obtain the SWT parameter distribution line on the two-dimensional stress gradient curve.
5. The method for predicting fatigue life of a round hole-equipped mount structure taking into account two-dimensional stress gradient lines according to claim 4, wherein the formula of the SWT parameter is:
wherein sigma max Epsilon is the maximum stress of the hole edge of the mounting seat structure max And epsilon min Respectively maximum strain and minimum strain of hole edge epsilon a Is the strain amplitude.
6. The method for predicting fatigue life of a round hole-equipped mount structure taking into account two-dimensional stress gradient lines according to claim 1, wherein the step 3 comprises:
step 3.1: fitting according to a fatigue test of a mounting seat hole edge structure test piece to obtain a nominal stress-life curve, wherein the nominal stress-life curve is as follows:
lgσ nor ＝A+BlgN f
wherein A and B are material parameters, sigma nor Is the nominal stress, N f Is the fatigue test life.
Step 3.2: describing the relation between the strain and the service life by adopting a Manson-Coffin equation, and obtaining related material parameters through a material smooth piece strain test to obtain the relation between the strain and the service life;
based on symmetrical cycle parameters, while considering the strain amplitude ε a Maximum stress sigma max Action on low cycle fatigue and based on the strain versus life, the strain amplitude ε a And maximum stress sigma max Fitting to establish a SWT model of the material smooth piece, wherein the SWT model of the material smooth piece is as follows:
wherein σ' f Is strong in fatigueCoefficient of degree, ε' f B is a fatigue strength index, c is a fatigue ductility index, and E is an elastic modulus;
step 3.3: nominal stress sigma for each load stage nor The theoretical service life corresponding to each load level can be obtained by bringing the nominal stress-service life curveTheoretical lifetime->Substituting the SWT parameter into SWT model of the material smooth piece, and reversely pushing to obtain effective SWT parameter (sigma of the test piece of the hole edge structure of the mounting seat max ε a ) eff 。
7. The method for predicting fatigue life of a round hole-equipped mount structure taking into account two-dimensional stress gradient lines according to claim 1, wherein the equation in step 4 is:
wherein D is LM Is critical distance, sigma max ε a Is the SWT parameter (σ) on the stress gradient line max ε a ) eff The effective SWT parameter of the test piece of the hole edge structure of the mounting seat is that l is the distance from the hole bottom angle on the dangerous section stress gradient line.
8. The method for predicting the fatigue life of the circular hole-carrying mounting seat structure taking two-dimensional stress gradient lines into consideration according to claim 1, wherein the fatigue life prediction model of the mounting seat hole edge structure is as follows:
wherein D is LM Is critical distance, sigma max ε a For SWT parameter on stress gradient line, σ' f For fatigue strength coefficient, ε' f For the fatigue ductility coefficient, b is the fatigue strength index, c is the fatigue ductility index, and E is the elastic modulus.
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