CN111860993B - Weld joint fatigue life prediction method considering residual stress evolution - Google Patents

Weld joint fatigue life prediction method considering residual stress evolution Download PDF

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CN111860993B
CN111860993B CN202010672199.8A CN202010672199A CN111860993B CN 111860993 B CN111860993 B CN 111860993B CN 202010672199 A CN202010672199 A CN 202010672199A CN 111860993 B CN111860993 B CN 111860993B
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蒋文春
解学方
王天娇
张显程
涂善东
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China University of Petroleum East China
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Abstract

The invention discloses a method for predicting fatigue life of a welded joint by considering residual stress evolution, which comprises the following steps: performing a cyclic load fatigue test on the base material; substituting the parameters into a parent metal fatigue prediction model considering the average stress, and fitting to obtain material parameters; performing thermal simulation on the welded joint weld joint by adopting a sequential coupling method, and determining a welding state residual stress field in the welding process; based on stress-strain data of the base material, establishing a cyclic constitutive model; simulating the redistribution behavior of the residual stress under the cyclic load, and simulating the maximum steady-state tensile residual stress value sigma re‑max The method comprises the steps of carrying out a first treatment on the surface of the Will have a maximum stress value sigma max Corrected to the maximum stress value sigma without considering the residual stress max0 And a maximum steady-state tensile residual stress value sigma re‑max And establishing a fatigue life prediction model of the welding joint taking the residual stress evolution into consideration. The invention combines the fatigue test with the finite element calculationA fatigue life prediction model of the welding joint considering the residual stress evolution is established so as to realize scientific prediction of the fatigue life of the welding joint.

Description

Weld joint fatigue life prediction method considering residual stress evolution
Technical Field
The invention belongs to the technical field of fatigue life prediction of welded joints, and particularly relates to a method for predicting the fatigue life of a welded joint by considering residual stress evolution.
Background
Modern industrial equipment tends to develop in the directions of high yield, high parameters and high efficiency, the production process is more and more complex, and the production conditions are more and more severe, so that the service environment of the equipment structure is more and more severe, and the equipment is often threatened by cyclic load caused by factors such as frequent start and stop of the equipment, vibration or fluctuation of working pressure. At the geometric discontinuous part of the structure or at the position with initial defects, fatigue cracks are extremely easy to generate, so that serious safety accidents such as leakage, explosion and the like are caused, and the long-term stable and reliable operation of equipment is seriously influenced.
The welding technology has the advantages of cost saving, convenient operation condition, easy realization of automation and mechanization, and the like, and is widely applied in the manufacturing process of large and complicated engineering structure. However, the welded joint is inevitably subjected to large residual welding stresses due to the influence of the locally short-time highly concentrated heat input and the rapid cooling process during welding. Under the combined action of welding residual stress and external cyclic load, the fatigue failure of the welding joint gradually becomes a more common failure mode in engineering. Therefore, the evolution of the welding residual stress under the cyclic load and the influence of the welding residual stress on the fatigue life are clear, a scientific and accurate welding joint fatigue life prediction method is established, and the method has important significance for the reliability design and the structural integrity evaluation of industrial equipment.
Aiming at the problem of fatigue life prediction of welded joints, a plurality of life prediction models and methods exist at present, and the models and the methods are mainly obtained by establishing the relation between parameters such as stress, strain, shaping strain energy, fatigue damage and the like of materials and the fatigue life. The main fatigue life prediction methods are as follows: (1) The S-N curve method comprises the steps of firstly calculating the variation process of the nominal stress or the node stress of a welded joint, determining the equivalent stress amplitude S, and then calculating the fatigue life of the joint by combining the S-N curves determined by a large number of fatigue tests of the welded joint; (2) Fracture mechanics considers that defects exist in the material inevitably, and the defects are regarded as cracks, and the residual life of the material is predicted according to the crack propagation property of the material under the action of a using load; (3) The damage mechanics method introduces the concept of damage, uses a continuous damage field to describe the internal defects of vacancies, dislocation, microcracks and the like of the metal material and the influence of the internal defects on the constitutive relation of the material, and establishes the constitutive relation of the damage mechanics based on the irreversible thermodynamic principle so as to predict the fatigue life.
On the one hand, the S-N curve method lacks necessary theoretical support, and adopts too conservative safety coefficient, so that resource waste is easy to cause, on the other hand, the fatigue test result of the welding joint often has larger contingency, the determination of the S-N curve of the welding joint is required to depend on a large number of fatigue tests, and the cost is higher; the fatigue life prediction method based on fracture mechanics is mainly developed from the angle of fatigue crack growth, and cannot cover the initiation behavior of the fatigue crack, but in most practical engineering requirements, the initiation of the fatigue crack is not allowed; the method based on continuous damage mechanics considers the initiation behavior of fatigue cracks, but the implementation process is complex, model parameters are numerous, an operator needs to have a strong mechanical foundation, and the analysis method for the accumulation of the damage week by week makes the calculation process lengthy, and is difficult to popularize and apply in engineering.
Meanwhile, when the welding residual stress is overlapped with the externally applied cyclic load, obvious plastic deformation can be generated on the component, and the redistribution of the residual stress is caused. Therefore, when fatigue strength safety evaluation is performed on welded joints, it is inaccurate to ignore the welding residual stress and its redistribution evolution.
In view of the defects of the existing fatigue life prediction methods of welded joints and the influence of welding residual stress on the fatigue life, a method for predicting the fatigue life of the welded joints by considering the evolution of the residual stress is needed.
Disclosure of Invention
The invention aims to overcome the defects of the prior art and provide a fatigue life prediction method for a welding joint, which considers the evolution of residual stress.
In order to achieve the above purpose, the invention adopts the following technical scheme:
a fatigue life prediction method of a welded joint considering residual stress evolution comprises the following steps:
step 1: carrying out a plurality of groups of cyclic load fatigue tests with different average stress on the base material, and recording stress-strain data of the base material under cyclic load and fatigue life under different average stress load;
step 2: substituting the fatigue life of the base metal under different average stress loads recorded in the step 1 into a base metal fatigue prediction model considering average stress, and fitting to obtain material parameters;
step 3: performing thermal simulation and mechanical simulation on the welding joint weld joint by using finite element analysis software and adopting a sequential coupling method to determine a welding state residual stress field of the welding joint;
step 4: establishing a base material circulation constitutive model based on the stress-strain data of the base material recorded in the step 1;
step 5: the welding state residual stress field obtained through simulation in the step 3 is used as an initial stress strain field of fatigue analysis, the fatigue analysis is carried out through the cyclic constitutive model in the step 4, the redistribution behavior of the residual stress under cyclic load is simulated, the steady-state welding residual stress field is determined, and the maximum steady-state tensile residual stress value sigma is obtained re-max ；
Step 6: the maximum stress value sigma is calculated based on the fatigue life prediction model of the base material considering the influence of the average stress max Corrected to the maximum stress value sigma without considering the residual stress max0 And a maximum steady-state tensile residual stress value sigma re-max The sum is taken into accountAnd a welded joint fatigue life prediction model for residual stress evolution.
Preferably, in step 2, the base material fatigue prediction model considering the average stress is as follows:
N＝a×[exp(b×FP)] (1)
in the formula (2), N is the fatigue life of the base material; sigma (sigma) a Is the stress amplitude; sigma (sigma) max Is the maximum stress value; e is the elastic modulus;
for the fatigue limit, the tensile strength sigma of the passing material b Calculated out->
The parameter t is the weight factor of the ratchet injury;
a and b are the material parameters that need to be fitted.
Preferably, the cyclic constitutive model in the step 4 comprises a strain decomposition equation, an elasticity equation, a yield function equation and a flow equation;
strain decomposition equation: under small deformation conditions, principal strain ε t Comprising elastic strain epsilon e Viscoplastic strain ε vp Two parts, namely:
ε t ＝ε e +ε vp (3)
in the formula (3), elastic strain ε e And the stress tensor sigma satisfies an elasticity equation, namely:
ε e ＝D -1 :σ (4)
in the formula (4), D is a fourth-order Hook elastic tensor, and is determined by an elastic modulus E and a Poisson ratio v;
yield function equation: yield function F y Using classical Von-Mises yield criterion, namely:
in the formula (5), s is a bias stress tensor; alpha is the back stress tensor; r is a scalar quantity representing effective stress; the evolution law of alpha is determined by a follow-up hardening equation;
flow equation: the power law type rate dependent flow criteria are employed as follows:
in the formula (6), the amino acid sequence of the compound,represents the viscoplastic strain rate;
the meaning of Macauley operator: when x is less than or equal to 0, < x > =0; when x >0, < x > =x;
k and n are material constants related to material rate; the s-alpha represents the modulo operation of the s-alpha vector.
Preferably, in the yield function equation, the evolution of α is determined by the following hardening equation:
wherein the critical state is reflected by a critical plane, namely:
in the above formula, M represents the number of back stress;
α k is the kth back stress;
is the evolution rate of the kth back stress;
is equivalent plastic strain rate;
H(f k ) Expressed as f k <At 0, H (f) k ) =0, otherwise H (f k )＝1；
||α k I represents alpha k Performing modular operation on vectors;
ζ k and gamma k Is a material parameter, μ is a ratchet coefficient.
Preferably, in the step 6, a fatigue life prediction model of the welded joint considering the residual stress evolution is as follows:
N joint ＝a′×[exp(b′×FP Correction )] (11)
σ max- Correction = sigma max0 +σ re-max (13)
Wherein N is Joint The fatigue life of the welded joint to account for residual stress evolution; sigma (sigma) a Is the stress amplitude; sigma (sigma) max0 Maximum stress when the residual stress is not considered; sigma (sigma) re-max Is the maximum steady-state tensile residual stress value; e is the elastic modulus;
for the fatigue limit, the tensile strength sigma of the passing material b Calculation ofFind out->
The parameter t is the weight factor of the ratchet injury;
a 'and b' are the material parameters obtained by fitting in step 2.
The beneficial effects of the invention are as follows:
according to the fatigue life prediction method of the welded joint considering the residual stress evolution, a fatigue test is combined with finite element calculation, the high cost of performing the fatigue test of the welded joint is avoided from a life prediction model of a base metal, meanwhile, through finite element simulation, only tens of cycles of fatigue calculation are needed, a steady-state residual stress field of the joint can be accurately obtained, and a tensile residual stress value with the greatest harm to the fatigue life is corrected into the life prediction model of the base metal; meanwhile, the method and the device can reasonably calculate the evolution redistribution rule of the welding residual stress under the action of the cyclic load, reasonably consider the influence of the residual stress on the fatigue life, and establish a welding joint fatigue life prediction model considering the evolution of the residual stress so as to realize scientific prediction of the fatigue life of the welding joint.
Drawings
The accompanying drawings, which are included to provide a further understanding of the application and are incorporated in and constitute a part of this application, illustrate embodiments of the application and together with the description serve to explain the application and do not constitute an undue limitation to the application.
FIG. 1 is a flow chart of a weld joint fatigue life prediction method that accounts for residual stress evolution in accordance with the present invention;
FIG. 2 is a graph showing the evolution rule of the maximum strain of the parent metal period with the cycle number;
FIG. 3 is a graph of the fatigue life distribution of the base material and the joint at different average stress levels;
FIG. 4 is a cloud chart of the as-welded residual stress distribution (unit: pa);
FIG. 5 is a graph showing the transverse residual stress distribution for different cycle periods under a 150+ -200 MPa cyclic load;
FIG. 6 is a plot of weld toe position residual stress distribution along the weld bead direction at different cycles under a 150+ -200 MPa cyclic load;
FIG. 7 shows the toe position distribution of steady-state transverse residual stress along the weld bead direction for different average stresses;
FIG. 8 is a graph comparing residual stress redistribution before and after joint fatigue fracture morphology;
FIG. 9 is a graph showing a comparison of load history for a point on the toe side with or without residual stress;
fig. 10 is a comparison of life prediction results with test values.
Detailed Description
It should be noted that the following detailed description is illustrative and is intended to provide further explanation of the present application. Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs.
It is noted that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of example embodiments in accordance with the present application. As used herein, the singular is also intended to include the plural unless the context clearly indicates otherwise, and furthermore, it is to be understood that the terms "comprises" and/or "comprising" when used in this specification are taken to specify the presence of stated features, steps, operations, devices, components, and/or combinations thereof.
The invention will be further described with reference to the drawings and examples.
As shown in fig. 1, a method for predicting fatigue life of a welded joint in consideration of residual stress evolution includes the steps of:
step 1: carrying out a plurality of groups of cyclic load fatigue tests with different average stress on the base material, and recording stress-strain data of the base material under cyclic load and fatigue life under different average stress load;
step 2: substituting the fatigue life of the base metal under different average stress loads recorded in the step 1 into a base metal fatigue prediction model considering average stress, and fitting to obtain material parameters a and b; specifically, material parameters a and b are fitted by a least square method;
specifically, in step 2, the base material fatigue prediction model considering the average stress is as follows:
N＝a×[exp(b×FP)] (1)
in the formula (2), N is the fatigue life of the base material; sigma (sigma) a Is the stress amplitude; sigma (sigma) max Is the maximum stress value; e is the elastic modulus;
for the fatigue limit, the tensile strength sigma of the passing material b Calculated out->
The parameter t is a weight factor of ratchet damage and is used for describing the sensitivity of the fatigue life of the material to ratchet deformation and depends on the cyclic hardening/softening characteristics;
a and b are the material parameters that need to be fitted.
In step 2, fatigue life values under different average stress loads are substituted into the formula (1) and the formula (2), and values of material parameters a and b are fitted.
In order to verify the validity of the life prediction method, the cyclic load fatigue test is performed on the welded joint at the same time when the cyclic load fatigue test is performed on the base material.
To improve the reliability of the test, parallel tests were set up for each group of fatigue tests.
Step 3: by utilizing finite element analysis software, adopting a sequential coupling method, namely neglecting the influence of a stress field on a temperature field, firstly performing thermal simulation and mechanical simulation on a welding joint welding seam to determine a welding state residual stress field of the welding joint; specifically, a welding line temperature field calculation result in the welding process is used as an input condition to determine a final welding state residual stress field; the finite element analysis software can be software such as ABAQUS or ANSYS;
step 4: establishing a base material circulation constitutive model based on the stress-strain data of the base material recorded in the step 1;
specifically, the cyclic constitutive model in the step 4 comprises a strain decomposition equation, an elasticity equation, a yield function equation and a flow equation;
strain decomposition equation: under small deformation conditions, principal strain ε t Comprising elastic strain epsilon e Viscoplastic strain ε vp Two parts, namely:
ε t ＝ε e +ε vp (3)
in the formula (3), elastic strain ε e And the stress tensor sigma satisfies an elasticity equation, namely:
ε e ＝D -1 :σ (4)
in the formula (4), D is a fourth-order Hook elastic tensor, and is determined by an elastic modulus E and a Poisson ratio v;
yield function equation: yield function F y The classical Von-Mises yield criterion is adopted, namely:
in the formula (5), s is a bias stress tensor; alpha is the back stress tensor; r is a scalar quantity representing effective stress; the evolution law of alpha is determined by a follow-up hardening equation;
flow equation: the power law type rate dependent flow criteria are employed as follows:
in the formula (6), the amino acid sequence of the compound,represents the viscoplastic strain rate;
the meaning of Macauley operator: when x is less than or equal to 0, < x > =0; when x >0, < x > =x;
k and n are material constants related to material rate; the s-alpha represents the modulo operation of the s-alpha vector.
Specifically, in the yield function equation, the evolution of α is determined by the following hardening equation:
wherein the critical state is reflected by a critical plane, namely:
in the above formula, M represents the number of back stress;
α k is the kth back stress;
is the evolution rate of the kth back stress;
is equivalent plastic strain rate;
H(f k ) Expressed as f k <At 0, H (f) k ) =0, otherwise H (f k )＝1；
||α k I represents alpha k Performing modular operation on vectors;
ζ k and gamma k Is the material parameter, mu is the ratchet coefficient, ζ k 、γ k μ is determined by the stress-strain data of the base material under cyclic load based on the least squares method.
Wherein, the evolution of alpha selects a well-known Ohno-Karim model. The model considers the ratchet phenomenon under non-proportional loading and avoids the occurrence of excessive nonlinear order in a back stress evolution equation. The total back stress is divided into a plurality of components, and an evolution equation of each back stress component comprises a linear strengthening term and a dynamic recovery term. Wherein the model has a critical state for dynamic recovery terms, which can be reflected by critical planes. When the back stress component is within the critical plane, the dynamic recovery term is not active; when the back stress component is at the critical plane, the dynamic recovery term is fully activated, thereby suppressing the generation of the excessively large ratchet behavior prediction by reducing the effect of the dynamic recovery term.
Step 5: the welding state residual stress field obtained through simulation in the step 3 is used as an initial stress strain field of fatigue analysis, the fatigue analysis is carried out through the cyclic constitutive model in the step 4, the redistribution behavior of the residual stress under cyclic load is simulated, the steady-state welding residual stress field is determined, and the maximum steady-state tensile residual stress value sigma is obtained re-max ；
Step 6: the maximum stress value sigma is calculated based on the fatigue life prediction model of the base material considering the influence of the average stress max Corrected to the maximum stress value sigma without considering the residual stress max0 And a maximum steady-state tensile residual stress value sigma re-max And establishing a fatigue life prediction model of the welded joint considering residual stress evolution, wherein the fatigue life prediction model is as follows:
A joint ＝a′×[exp(b′×FP Correction )] (11)
σ max correction ＝σ max0 +σ re-max (13)
Wherein N is Joint The fatigue life of the welded joint to account for residual stress evolution; sigma (sigma) a Is the stress amplitude; sigma (sigma) max0 Maximum stress when the residual stress is not considered; sigma (sigma) re-max Is the maximum steady-state tensile residual stress value; e is the elastic modulus;
for the fatigue limit, the tensile strength sigma of the passing material b Calculated out->
The parameter t is a weight factor of ratchet damage and is used for describing the sensitivity of the fatigue life of the material to ratchet deformation and depends on the cyclic hardening/softening characteristics;
a 'and b' are the material parameters obtained by fitting in step 2.
Examples:
the fatigue life prediction is performed by taking a 316L stainless steel welded joint as a research object and considering the evolution of residual stress, and the specific implementation mode is as follows.
Step 1: respectively carrying out fatigue tests with different average stresses on a 316L stainless steel base metal and a welded joint, and recording stress-strain data of the base metal under cyclic load and fatigue life of the base metal and the welded joint under different average stress loads;
in the test process, a stress control mode is adopted for the base metal and the welded joint fatigue test sample, four groups of fatigue tests under different stress levels are respectively carried out, the axial stress in the four groups of tests is 125+/-200 MPa, 150+/-200 MPa, 175+/-200 MPa and 200+/-200 MPa in sequence, the cyclic load waveform is triangular wave, the loading rate is constant to 100MPa/s (f=0.125 Hz), and the test is carried out until the test sample breaks. To improve the reliability of the test results, parallel tests were set for each group of fatigue tests.
The test results show that: the evolution rule of the maximum strain of the parent metal period along with the cycle number shown in fig. 2, and the distribution of the fatigue life of the parent metal and the welded joint under different average stresses shown in fig. 3.
The method comprises the following steps of: under the asymmetric stress load condition, the 316L stainless steel base material and the welding joint show obvious ratchet behavior; at the same time, the average stress level has a great influence on the fatigue life of both the base metal and the welded joint, i.e. the fatigue life decreases with increasing average stress level.
Step 2: substituting the fatigue life of the base metal under different average stress loads in the step 1 into a base metal fatigue prediction model considering average stress, and obtaining material parameters a and b through least square fitting; wherein, according to the formula (1) and the formula (2), the material parameters a=3.01e6 and b= -1.45e-4 of the 316L stainless steel are obtained;
step 3: by utilizing finite element analysis software, adopting a sequential coupling method, namely neglecting the influence of a stress field on a temperature field, firstly performing thermal simulation, determining the evolution rule of the temperature field in the welding process, and determining a welding state residual stress field by taking the evolution rule as an input condition, wherein the welding state residual stress field is shown in fig. 4;
as can be seen from fig. 4, the transverse stress is much greater than the longitudinal and normal stresses in the residual stress in the welded state of the 316L stainless steel welded joint, the maximum value is greater than the yield strength of the material, and the distribution rule of "compressive, tensile and compressive" is shown along the welding line direction. And since the fatigue load loading direction is transverse, the transverse residual stress is mainly considered.
Step 4: based on the stress-strain data of the parent metal recorded in the step 1, establishing a circulating constitutive model according to a formula (3) -a formula (10), and determining circulating material parameters of the 316L stainless steel;
E＝197000MPa,ν＝0.3,R＝105Mpa；
K＝290MPa，n＝4.5；
ζ 1 ＝6948,γ 1 ＝61.2MPa,ζ 2 ＝5,γ 2 ＝67.6MPa,μ＝0.1。
step 5: and (3) through the step (3) and the step (4), based on ABAQUS finite element software, using the welding state residual stress in the step (3) as an initial stress strain field by means of a restarting analysis technology, and obtaining a redistribution rule of the welding state residual stress by means of a circulating constitutive model in the step (4).
Fig. 5 illustrates an evolution rule of the welding state residual stress under the cyclic load by taking a cloud chart of the distribution of the residual stress after different cyclic periods under the cyclic load of 150±200MPa as an example. It can be seen that: the welding residual stress can be obviously relaxed and redistributed under the action of cyclic load, and mainly occurs in the first cycle, and after the residual stress is released, the residual stress is mainly concentrated near the weld toe;
fig. 6 further shows the evolution law of the residual stress along the toe direction under a cyclic load of 150±200MPa, and it can be seen that the residual stress is substantially stable after 10 cycles, and the residual maximum steady-state tensile residual stress is 27.1MPa.
The distribution of steady state residual stresses along the toe direction at different average stress levels is shown in fig. 7, as can be seen: with the increase of the average stress level, the redistribution of the residual stress is more obvious, the stress in the middle of the weld toe is further released, and the stress level at the two ends is increased. The maximum tensile steady-state residual stress values at different average stresses can also be determined from fig. 7.
Fig. 8 compares the as-welded and steady-state residual stress distribution with the actual fracture profile of the joint, with compressive stress on both sides of the weld toe of the welded joint sample and tensile stress in the middle of the weld from the failure location. In general, the presence of compressive stress prevents crack propagation and thus increases fatigue life, and therefore, from the viewpoint of residual stress in the welded state, a fatigue crack should be initiated from the center of the weld and then propagated to both sides in the fatigue test specimen of the welded joint under cyclic load. However, the test results were in contrary to each other, and fatigue cracks were initiated on one side of the weld toe and propagated to the other side in the width direction, consistent with the situation after the residual stress was redistributed. Therefore, the distribution rule of the residual stress of the welded joint after the residual stress is redistributed under the cyclic load can truly reflect the fatigue fracture behavior of the welded joint.
Fig. 9 shows the load history of a point on the toe side with or without residual stress, as can be seen: the effect of the welding residual stress on the actual load history of the welding structure is mainly through the improvement of the average stress, but the fatigue amplitude of the welding residual stress is not changed. This further demonstrates the correctness of the proposed model for predicting fatigue life of the base metal with steady-state residual stress as the average stress correction.
According to the welding joint fatigue life prediction model taking the residual stress evolution into consideration, which is obtained in step 5 in the application, wherein the 316L stainless steel is a cyclic hardening material insensitive to ratchet deformation, and t=0.1 is taken. The fatigue life of the calculated 316L stainless steel welded joint was compared with the test value as shown in fig. 10: the fatigue life prediction result considering the weld residual stress evolution in the method is consistent with the test value, namely, the fatigue life prediction method considering the residual stress evolution can accurately predict the life of the welding joint, and the prediction precision is high.
While the foregoing is directed to embodiments of the present invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.

Claims (1)

1. A method for predicting fatigue life of a welded joint by considering residual stress evolution is characterized by comprising the following steps:
step 1: carrying out a plurality of groups of cyclic load fatigue tests with different average stress on the base material, and recording stress-strain data of the base material under cyclic load and fatigue life under different average stress load;
step 2: substituting the fatigue life of the base metal under different average stress loads recorded in the step 1 into a base metal fatigue prediction model considering average stress, and fitting to obtain material parameters;
step 3: performing thermal simulation and mechanical simulation on the welding joint weld joint by using finite element analysis software and adopting a sequential coupling method to determine a welding state residual stress field of the welding joint;
step 4: establishing a base material circulation constitutive model based on the stress-strain data of the base material recorded in the step 1;
step 5: the welding state residual stress field obtained through simulation in the step 3 is used as an initial stress strain field of fatigue analysis, the fatigue analysis is carried out through the cyclic constitutive model in the step 4, the redistribution behavior of the residual stress under cyclic load is simulated, the steady-state welding residual stress field is determined, and the maximum steady-state tensile residual stress value sigma is obtained re-max ；
Step 6: the maximum stress value sigma is calculated based on the fatigue life prediction model of the base material considering the influence of the average stress max Corrected to the maximum stress value sigma without considering the residual stress max0 And a maximum steady-state tensile residual stress value sigma re-max The sum of the above is used for establishing a fatigue life prediction model of the welding joint considering residual stress evolution;
in step 2, the base material fatigue prediction model considering the average stress is as follows:
N＝a×[exp(b×FP)] (1)
in the formula, N is the fatigue life of the base metal; sigma (sigma) a Is the stress amplitude; sigma (sigma) max Is the maximum stress value; e is the elastic modulus;
for the fatigue limit, the tensile strength sigma of the passing material b Calculated out->
The parameter t is the weight factor of the ratchet injury;
a and b are material parameters to be fitted;
the cyclic constitutive model in the step 4 comprises a strain decomposition equation, an elasticity equation, a yield function equation and a flow equation;
strain decomposition equation: under small deformation conditions, principal strain ε t Comprising elastic strain epsilon e And a viscoplastic strain epsivp, namely:
ε t ＝ε e +ε vp (3)
in the formula (3), elastic strain ε e And the stress tensor sigma satisfies an elasticity equation, namely:
ε e ＝D -1 :σ (4)
in the formula (4), D is a fourth-order Hook elastic tensor, and is determined by an elastic modulus E and a Poisson ratio v;
yield function equation: yield function F y The classical Von-Mises yield criterion is adopted, namely:
in the formula (5), s is a bias stress tensor; alpha is the back stress tensor; r is a scalar quantity representing effective stress; the evolution law of alpha is determined by a follow-up hardening equation;
flow equation: the power law type rate dependent flow criteria are employed as follows:
in the formula (6), the amino acid sequence of the compound,represents the viscoplastic strain rate;
the meaning of Macauley operator: when x is less than or equal to 0, < x > =0; when x >0, < x > =x;
k and n are material constants related to material rate; the s-alpha represents the modulo operation of the s-alpha vector;
in the yield function equation, the evolution of α is determined by the following hardening equation:
wherein the critical state is reflected by a critical plane, namely:
in the above formula, M represents the number of back stress;
α k is the kth back stress;
is the evolution rate of the kth back stress;
is equivalent plastic strain rate;
H(f k ) Expressed as f k <At 0, H (f) k ) =0, otherwise H (f k )＝1；
||α k I represents alpha k Performing modular operation on vectors;
ζ k and gamma k As material parameters, μ is a ratchet coefficient;
in the step 6, a fatigue life prediction model of the welded joint considering the residual stress evolution is as follows:
N joint ＝a′×[exp(b′×Fp Correction )] (11)
σ max correction ＝σ max0 +σ re-max (13)
Wherein N is Joint The fatigue life of the welded joint to account for residual stress evolution; sigma (sigma) a Is the stress amplitude; sigma (sigma) max0 Maximum stress when the residual stress is not considered; sigma (sigma) re-max Is the maximum steady-state tensile residual stress value; e is the elastic modulus;
for the fatigue limit, the tensile strength sigma of the passing material b Calculated out->The parameter t is the weight factor of the ratchet injury;
a 'and b' are the material parameters obtained by fitting in step 2.
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