CN111860993A - Welding joint fatigue life prediction method considering residual stress evolution - Google Patents

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

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CN111860993A
CN111860993A CN202010672199.8A CN202010672199A CN111860993A CN 111860993 A CN111860993 A CN 111860993A CN 202010672199 A CN202010672199 A CN 202010672199A CN 111860993 A CN111860993 A CN 111860993A
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蒋文春
解学方
王天娇
张显程
涂善东
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Abstract

The invention discloses a method for predicting the fatigue life of a welding joint by considering residual stress evolution, which comprises the following steps of: carrying out cyclic load fatigue test on the parent metal; substituting the average stress into a parent metal fatigue prediction model considering the average stress, and fitting to obtain material parameters; performing thermal simulation on the welding seam of the welding joint by adopting a sequential coupling method, and determining a welding state residual stress field in the welding process; establishing a cyclic constitutive model based on the stress-strain data of the parent metal; simulation of residual stressSimulating the redistribution behavior of the force under the cyclic load to obtain the maximum steady-state tensile residual stress value sigmare‑max(ii) a Maximum stress value sigmamaxCorrected to the maximum stress value σ without taking into account residual stressmax0And maximum steady state tensile residual stress value sigmare‑maxAnd establishing a welding joint fatigue life prediction model considering residual stress evolution. The method combines fatigue test and finite element calculation, and establishes a welding joint fatigue life prediction model considering residual stress evolution so as to realize scientific prediction of the welding joint fatigue life.

Description

Welding joint fatigue life prediction method considering residual stress evolution
Technical Field
The invention belongs to the technical field of fatigue life prediction of welding joints, and particularly relates to a method for predicting the fatigue life of a welding 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 increasingly complex, and the production conditions are increasingly harsh, so that the service environment of an equipment structure is increasingly severe, and the equipment structure is often threatened by cyclic load caused by factors such as frequent start and stop of the equipment, vibration or working pressure fluctuation. Fatigue cracks are easily generated at the position of a structural geometric discontinuity or the position with an initial defect, so that major 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 to the manufacturing process with large and complicated engineering structure. However, the welded joint inevitably generates large welding residual stress due to the influence of local short-time highly concentrated heat input and rapid cooling process in the welding process. Under the combined action of welding residual stress and external cyclic load, the fatigue failure of the welding joint gradually becomes a 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 determined, the scientific and accurate prediction method of the fatigue life of the welding joint is established, and the method has important significance on the reliability design and structural integrity evaluation of industrial equipment.
Aiming at the problem of fatigue life prediction of a welding joint, at present, a plurality of life prediction models and methods are provided, and the models and methods are mainly obtained by establishing a relation between parameters of materials such as stress, strain, plastic strain energy, fatigue damage and the like and the fatigue life. The main fatigue life prediction methods include the following methods: (1) the S-N curve method comprises the steps of firstly calculating the change history of nominal stress or node stress of a welding joint, determining the equivalent stress amplitude S, and then calculating the fatigue life of the joint by combining an S-N curve determined by a large number of welding joint fatigue tests; (2) the fracture mechanics method considers that the 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 the using load; (3) the damage mechanics method introduces the concept of damage, describes the internal defects of metal materials such as vacancies, dislocations and microcracks and the influence thereof on the constitutive relation of the materials by using a continuous damage field, and establishes the constitutive relation of the damage mechanics to predict the fatigue life based on the irreversible thermodynamic principle.
On the one hand, based on an S-N curve method, necessary theoretical support is lacked, an over-conservative safety coefficient is adopted, resource waste is easily caused, on the other hand, a fatigue test result of a welding joint is often high in contingency, the determination of the S-N curve of the welding joint depends on a large number of fatigue tests, and the cost is high; the fatigue life prediction method based on fracture mechanics is mainly developed from the angle of fatigue crack propagation, and cannot cover the initiation behavior of the fatigue crack, and the initiation of the fatigue crack is not allowed in most practical engineering requirements; although the method based on continuous damage mechanics considers the initiation behavior of fatigue cracks, the implementation process is complex, the model parameters are numerous, an operator needs to have a strong mechanical foundation, and the damage is accumulated cycle by analysis method, so that the calculation process is long and the method is difficult to popularize and apply in engineering.
Meanwhile, when the welding residual stress is superposed with the applied cyclic load, the component is subjected to obvious plastic deformation, and the redistribution of the residual stress is caused. Therefore, ignoring the welding residual stress and its redistribution evolution is inaccurate when making a fatigue strength safety assessment of a welded joint.
In view of the defects of the existing fatigue life prediction method for the welding joint and the influence of the welding residual stress on the fatigue life, a method for predicting the fatigue life of the welding joint by considering the evolution of the residual stress is urgently needed.
Disclosure of Invention
The invention aims to overcome the defects of the prior art and provide a method for predicting the fatigue life of a welding joint by considering the evolution of residual stress.
In order to achieve the purpose, the invention adopts the following technical scheme:
a method for predicting the fatigue life of a welding joint by considering the evolution of residual stress comprises the following steps:
step 1: carrying out a plurality of groups of cyclic load fatigue tests with different average stresses on the parent metal, and recording stress-strain data of the parent metal under the cyclic load and fatigue lives of the parent metal under the different average stress loads;
Step 2: substituting the fatigue life of the parent metal under different average stress loads recorded in the step 1 into a parent metal fatigue prediction model considering the average stress, and fitting to obtain material parameters;
and step 3: performing thermal simulation and mechanical simulation on the welding seam of the welding 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;
and 4, step 4: establishing a mother material circulation constitutive model based on the stress-strain data of the mother material recorded in the step 1;
and 5: taking the welding state residual stress field obtained by simulation in the step 3 as an initial stress strain field of fatigue analysis, carrying out fatigue analysis through the circulation constitutive model in the step 4, simulating redistribution behavior of the residual stress under the circulation load, determining the steady state welding residual stress field, and obtaining the maximum steady state tensile residual stress value sigmare-max；
Step 6: on the basis of a parent metal fatigue life prediction model considering the influence of average stress, the maximum stress value sigma is calculatedmaxCorrected to the maximum stress value σ without taking into account residual stressmax0And maximum steady state tensile residual stress value sigmare-maxAnd establishing a welding joint fatigue life prediction model considering 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)
Figure BDA0002582705570000031
In the formula (2), N is the fatigue life of the base material; sigmaaIs the stress amplitude; sigmamaxIs the maximum stress value; e is the modulus of elasticity;
Figure BDA0002582705570000032
for fatigue limit, tensile strength σ of the passing materialbThe calculation is carried out to obtain the result,
Figure BDA0002582705570000033
the parameter t is a weighting factor of ratchet wheel damage;
a and b are the material parameters 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;
the strain decomposition equation: under small deformation conditions, principal straintInvolving elastic straineAnd viscoplastic strainvpTwo parts, namely:
t＝e+vp(3)
in the formula (3), elastic straineAnd the stress tensor sigma, the elastic equation is satisfied, namely:
e＝D-1:σ (4)
in the formula (4), D is a fourth-order Hook elasticity tensor which is determined by an elastic modulus E and a Poisson ratio v;
yield function equation: yield letterNumber FyThe classical Von-Mises yield criterion was adopted, namely:
Figure BDA0002582705570000041
in the formula (5), s is a bias stress tensor; α is the back stress tensor; r is a scalar representing effective stress; the evolution rule of alpha is determined by a follow-up hardening equation;
flow equation: the power-law rate-dependent flow criterion is used, as follows:
Figure BDA0002582705570000042
in the formula (6), the reaction mixture is,
Figure BDA0002582705570000043
represents the viscoplastic strain rate;
and < > is Macauley operator, and the meaning is as follows: when x is less than or equal to 0, the ratio of < x > -0; when x >0, < x > -x;
K and n are material constants related to the material rate; and | s- α | represents the modulo operation of the s- α vector.
Preferably, in the yield function equation, the evolution of α is determined by the follow-up hardening equation as:
Figure BDA0002582705570000044
Figure BDA0002582705570000045
wherein the critical state is reflected by critical surfaces, namely:
Figure BDA0002582705570000046
Figure BDA0002582705570000051
in the above formula, M represents the number of back stresses;
αkis the kth back stress;
Figure BDA0002582705570000052
is the evolution rate of the kth back stress;
Figure BDA0002582705570000053
is the equivalent plastic strain rate;
H(fk) When f denotesk<0, H (f)k) 0, otherwise H (f)k)＝1；
||αkI represents alphakPerforming modular operation on the vector;
ζkand gammakMu is the ratchet coefficient for the material parameter.
Preferably, in step 6, the fatigue life prediction model of the welding joint considering the residual stress evolution is as follows:
Njoint＝a′×[exp(b′×FPCorrection)](11)
Figure BDA0002582705570000054
σmax-Correction ═ σmax0+σre-max(13)
Wherein N isJointFatigue life of the welded joint to account for residual stress evolution; sigmaaIs the stress amplitude; sigmamax0Maximum stress without considering residual stress; sigmare-maxThe maximum steady state tensile residual stress value is obtained; e is the modulus of elasticity;
Figure BDA0002582705570000055
for fatigue limit, tensile strength σ of the passing materialbThe calculation is carried out to obtain the result,
Figure BDA0002582705570000056
the parameter t is a weighting factor of ratchet wheel damage;
a 'and b' are the material parameters found by the step 2 fitting.
The invention has the beneficial effects that:
the fatigue life prediction method of the welding joint considering the residual stress evolution combines the fatigue test with the finite element calculation, starts from the life prediction model of the base metal, avoids the high cost of the fatigue test of the welding joint, simultaneously, only needs to carry out fatigue calculation for tens of cycles through finite element simulation, can accurately obtain the steady state residual stress field of the joint, and corrects the tensile residual stress value with the maximum harm to the fatigue life into the life prediction model of the base metal, rapidly and accurately calculates the fatigue life of the welding joint from the angle of crack initiation, realizes the scientific and accurate guarantee, and is convenient for engineering application; meanwhile, the evolution redistribution rule of the welding residual stress under the action of the cyclic load can be reasonably calculated, the influence of the residual stress on the fatigue life is reasonably considered, and a welding joint fatigue life prediction model considering the evolution of the residual stress is established so as to realize scientific prediction of the fatigue life of the welding joint.
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The accompanying drawings, which 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 are not intended to limit the application.
FIG. 1 is a flow chart of a method for predicting fatigue life of a weld joint in consideration of residual stress evolution according to the present invention;
FIG. 2 is an evolution rule of the maximum strain of the parent metal period along with the cycle number;
FIG. 3 shows fatigue life distributions of the base material and the joint at different average stress levels;
FIG. 4 is a cloud of distribution of residual stress in a welded state (unit: Pa);
FIG. 5 is a graph of the transverse residual stress distribution for different cycle periods under a 150 + -200 MPa cyclic load;
FIG. 6 is a graph of weld toe position residual stress distribution along the weld bead direction at different cycles under a 150 + -200 MPa cyclic load;
FIG. 7 is a graph showing the distribution of steady-state transverse residual stress at different mean stresses along the weld bead direction toe;
FIG. 8 is a comparison of the residual stress before and after redistribution versus fatigue fracture morphology of the joint;
FIG. 9 is a comparison of the load history of a point on the weld toe side with or without residual stress;
fig. 10 is a comparison of the life prediction results with the test values.
Detailed Description
It should be noted that the following detailed description is exemplary and is intended to provide further explanation of the disclosure. 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 according to the present application. As used herein, the singular forms "a", "an" and "the" are intended to include the plural forms as well, and it should be understood that when the terms "comprises" and/or "comprising" are used in this specification, they specify the presence of stated features, steps, operations, devices, components, and/or combinations thereof, unless the context clearly indicates otherwise.
The invention is further illustrated with reference to the following figures and examples.
As shown in fig. 1, a method for predicting fatigue life of a welded joint considering residual stress evolution includes the following steps:
step 1: carrying out a plurality of groups of cyclic load fatigue tests with different average stresses on the parent metal, and recording stress-strain data of the parent metal under the cyclic load and fatigue lives of the parent metal under the different average stress loads;
step 2: substituting the fatigue life of the parent metal under different average stress loads recorded in the step 1 into a parent metal fatigue prediction model considering the average stress, and fitting to obtain material parameters a and b; specifically, the 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)
Figure BDA0002582705570000071
in the formula (2), N is the fatigue life of the base material; sigmaaIs the stress amplitude; sigmamaxIs the maximum stress value; e is the modulus of elasticity;
Figure BDA0002582705570000072
for fatigue limit, tensile strength σ of the passing materialbThe calculation is carried out to obtain the result,
Figure BDA0002582705570000073
the parameter t is a weight factor of ratchet damage, 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 to be fitted.
In the step 2, fatigue life values under different average stress loads are substituted into the formula (1) and the formula (2), and values of the material parameters a and b are fitted.
In order to verify the effectiveness of the service life prediction method, a cyclic load fatigue test is performed on the welded joint when the cyclic load fatigue test is performed on the base metal.
In order to improve the reliability of the test, each group of fatigue tests is provided with a parallel test.
And step 3: utilizing finite element analysis software, adopting a sequential coupling method, namely neglecting the influence of a stress field on a temperature field, firstly carrying out thermal simulation and mechanical simulation on a welding seam of a welding joint, and determining a welding state residual stress field of the welding joint; specifically, a welding seam temperature field calculation result in the welding process is used as an input condition, and a final welding state residual stress field is determined; wherein, the finite element analysis software can be ABAQUS or ANSYS software;
And 4, step 4: establishing a mother material circulation constitutive model based on the stress-strain data of the mother 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;
the strain decomposition equation: under small deformation conditions, principal straintInvolving elastic straineAnd viscoplastic strainvpTwo parts, namely:
t＝e+vp(3)
in the formula (3), elastic straineAnd the stress tensor sigma, the elastic equation is satisfied, namely:
e＝D-1:σ (4)
in the formula (4), D is a fourth-order Hook elasticity tensor which is determined by an elastic modulus E and a Poisson ratio v;
yield function equation: yield function FyThe classical Von-Mises yield criterion was adopted, namely:
Figure BDA0002582705570000081
in the formula (5), s is a bias stress tensor; α is the back stress tensor; r is a scalar representing effective stress; the evolution rule of alpha is determined by a follow-up hardening equation;
flow equation: the power-law rate-dependent flow criterion is used, as follows:
Figure BDA0002582705570000091
in the formula (6), the reaction mixture is,
Figure BDA0002582705570000092
represents the viscoplastic strain rate;
and < > is Macauley operator, and the meaning is as follows: when x is less than or equal to 0, the ratio of < x > -0; when x >0, < x > -x;
k and n are material constants related to the material rate; and | s- α | represents the modulo operation of the s- α vector.
Specifically, in the yield function equation, the evolution of α is determined by the follow-up hardening equation as:
Figure BDA0002582705570000093
Figure BDA0002582705570000094
Wherein the critical state is reflected by critical surfaces, namely:
Figure BDA0002582705570000095
Figure BDA0002582705570000096
in the above formula, M represents the number of back stresses;
αkis the kth back stress;
Figure BDA0002582705570000097
is the evolution rate of the kth back stress;
Figure BDA0002582705570000098
is the equivalent plastic strain rate;
H(fk) When f denotesk<0, H (f)k) 0, otherwise H (f)k)＝1；
||αkI represents alphakPerforming modular operation on the vector;
ζkand gammakAs material parameters, μ is the ratchet coefficient, ζk、γkAnd mu is determined by the stress-strain data of the parent material under the cyclic load based on the least square method.
Wherein the evolution of α uses the well-known Ohno-Karim model. The model considers the ratchet phenomenon under the condition of non-proportional loading and avoids the occurrence of an overhigh nonlinear order in a back stress evolution equation. The total back stress is divided into a plurality of components, and the 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 each dynamic recovery item, which can be reflected by a critical surface. When the back stress component is within the critical plane, the dynamic recovery term does not work; when the back stress component is on the critical plane, the dynamic recovery term is fully activated, thereby suppressing the generation of an excessive ratchet behavior prediction result by reducing the effect of the dynamic recovery term.
And 5: taking the welding state residual stress field obtained by simulation in the step 3 as an initial stress strain field of fatigue analysis, carrying out fatigue analysis through the circulation constitutive model in the step 4, simulating redistribution behavior of the residual stress under the circulation load, determining the steady state welding residual stress field, and obtaining the maximum steady state tensile residual stress value sigmare-max；
Step 6: on the basis of a parent metal fatigue life prediction model considering the influence of average stress, the maximum stress value sigma is calculatedmaxCorrected to the maximum stress value σ without taking into account residual stressmax0And maximum steady state tensile residual stress value sigmare-maxAnd (3) establishing a welding joint fatigue life prediction model considering residual stress evolution, which comprises the following steps:
Ajoint＝a′×[exp(b′×FPCorrection)](11)
Figure BDA0002582705570000101
σmax-correction＝σmax0+σre-max(13)
Wherein N isJointFatigue life of the welded joint to account for residual stress evolution; sigmaaIs the stress amplitude; sigmamax0Maximum stress without considering residual stress; sigmare-maxThe maximum steady state tensile residual stress value is obtained; e is the modulus of elasticity;
Figure BDA0002582705570000111
to the fatigue limit, byTensile strength σ of the MaterialbThe calculation is carried out to obtain the result,
Figure BDA0002582705570000112
the parameter t is a weight factor of ratchet damage, 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 found by the step 2 fitting.
Example (b):
the fatigue life prediction is carried out by taking a 316L stainless steel welding 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 material and a welding joint, and recording stress-strain data of the base material under cyclic load and fatigue lives of the base material and the welding joint under different average stress loads;
in the test process, a stress control mode is adopted for both a base material and a welding joint fatigue 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 a triangular wave, the loading rate is constant at 100MPa/s (f is 0.125Hz), and the test is carried out until the sample breaks. In order to improve the reliability of the test result, each group of fatigue tests is provided with parallel tests.
The following results are obtained through experiments: the evolution law of the maximum strain of the period of the base metal along with the cycle number shown in fig. 2 and the distribution of the fatigue life of the base metal and the welded joint under different average stresses shown in fig. 3.
Obtained by analysis: under the condition of asymmetric stress load, the base metal and the welding joint of the 316L stainless steel show obvious ratchet behavior; meanwhile, the average stress level has a great influence on the fatigue life of the base metal and the welded joint, namely the fatigue life is reduced along with the increase of the average stress level.
Step 2: substituting the fatigue life of the parent metal under different average stress loads in the step 1 into a parent metal fatigue prediction model considering the average stress, and fitting by a least square method to obtain material parameters a and b; wherein, according to the formula (1) and the formula (2), the material parameter a of 316L stainless steel is 3.01e6, and b is-1.45 e-4;
and step 3: using finite element analysis software, adopting a sequential coupling method, namely neglecting the influence of a stress field on a temperature field, firstly carrying out thermal simulation, determining the evolution law of the temperature field in the welding process, and determining a residual stress field in a welding state by taking the evolution law as an input condition, as shown in figure 4;
it can be seen from fig. 4 that the transverse stress in the as-welded residual stress of the 316L stainless steel welded joint is much larger than the longitudinal and normal stresses, the maximum value is larger than the yield strength of the material, and the distribution rule of the "tension and compression" is presented along the direction of the weld joint. And since the fatigue load loading direction is lateral, the lateral residual stress is mainly considered.
And 4, step 4: establishing a cyclic constitutive model according to a formula (3) to a formula (10) based on the stress-strain data of the base metal recorded in the step 1, and determining cyclic material parameters of the 316L stainless steel;
E＝197000MPa,ν＝0.3,R＝105Mpa；
K＝290MPa，n＝4.5；
ζ1＝6948,γ1＝61.2MPa,ζ2＝5,γ2＝67.6MPa,μ＝0.1。
and 5: through the steps 3 and 4, based on ABAQUS finite element software, by means of a restart analysis technology, the welding state residual stress in the step 3 is used as an initial stress strain field, and a redistribution rule of the welding state residual stress is obtained by means of the circulating constitutive model in the step 4.
FIG. 5 illustrates an example of a cloud chart of residual stress distribution of the welded state residual stress under a cyclic load of 150 + -200 MPa after different cyclic periods, which shows an evolution law of the welded state residual stress under the cyclic load. It can be seen that: the welding residual stress can generate obvious stress relaxation and redistribution under the action of cyclic load, and mainly occurs in the first cyclic period, and after the residual stress is released, the residual stress is mainly concentrated near the welding toe;
fig. 6 further shows the evolution law of the residual stress along the toe direction under the cyclic load of 150 ± 200MPa, and it can be seen that after 10 cycles, the residual stress is substantially stable, and the residual maximum steady-state tensile residual stress is 27.1 MPa.
The distribution of the steady state residual stress along the weld toe direction at different mean stress levels is shown in fig. 7, and it can be seen that: with the increase of the average stress level, the redistribution of the residual stress is more obvious, the stress at the middle position of the welding 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 distributions with the actual fracture morphology of the joint, with respect to the failure position, both as-welded residual stresses being compressive stresses on both sides of the weld toe of the welded joint specimen and tensile stresses in the middle of the weld. Generally, the existence of compressive stress hinders the crack from opening, so that the fatigue life can be extended, and therefore, under cyclic load, a welded joint fatigue specimen should initiate a fatigue crack from the middle of a weld and then propagate to both sides. However, the test results were in contrast, and fatigue cracks were initiated on one side of the weld toe and propagated to the other side in the width direction, in accordance with the situation after redistribution of residual stress occurred. Therefore, the distribution rule of the residual stress of the welding joint after redistribution under the cyclic load can be proved to truly reflect the fatigue fracture behavior of the welding joint.
Fig. 9 shows the load history of a point on the weld toe side with or without residual stress, and it can be seen that: the effect of the welding residual stress on the real load course of the welded structure is mainly achieved by improving the average stress without changing the fatigue amplitude. This also further demonstrates the accuracy of the proposed model for predicting fatigue life of the base material by using the steady-state residual stress as the average stress.
According to the welding joint fatigue life prediction model considering the residual stress evolution obtained in the step 5 in the application, 316L stainless steel is a cycle hardening material insensitive to ratchet wheel deformation, and t is 0.1. The fatigue life of the 316L stainless steel welded joint calculated was compared with the test values, as shown in fig. 10: in the method, the fatigue life prediction result considering the evolution of the welding residual stress is consistent with the test value, namely the fatigue life prediction method of the welding joint considering the evolution of the residual stress can be used for accurately predicting the service life of the welding joint, and the prediction precision is high.
Although the embodiments of the present invention have been described with reference to the accompanying drawings, it is not intended to limit the present invention, and it should be understood by those skilled in the art that various modifications and changes may be made without inventive efforts based on the technical solutions of the present invention.

Claims (5)

1. A method for predicting the fatigue life of a welding joint by considering the evolution of residual stress is characterized by comprising the following steps:
step 1: carrying out a plurality of groups of cyclic load fatigue tests with different average stresses on the parent metal, and recording stress-strain data of the parent metal under the cyclic load and fatigue lives of the parent metal under the different average stress loads;
step 2: substituting the fatigue life of the parent metal under different average stress loads recorded in the step 1 into a parent metal fatigue prediction model considering the average stress, and fitting to obtain material parameters;
and step 3: performing thermal simulation and mechanical simulation on the welding seam of the welding 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;
and 4, step 4: establishing a mother material circulation constitutive model based on the stress-strain data of the mother material recorded in the step 1;
and 5: taking the welding state residual stress field obtained by simulation in the step 3 as an initial stress strain field of fatigue analysis, carrying out fatigue analysis through the circulation constitutive model in the step 4, simulating redistribution behavior of the residual stress under the circulation load, determining the steady state welding residual stress field, and obtaining the maximum steady state tensile residual stress value sigma re-max；
Step 6: fatigue of parent material in consideration of influence of average stressBased on the life prediction model, the maximum stress value sigma is calculatedmaxCorrected to the maximum stress value σ without taking into account residual stressmax0And maximum steady state tensile residual stress value sigmare-maxAnd establishing a welding joint fatigue life prediction model considering residual stress evolution.
2. The method for predicting fatigue life of a welded joint considering residual stress evolution according to claim 1, wherein in the step 2, the fatigue prediction model of the base material considering the average stress is as follows:
N＝a×[exp(b×FP)](1)
Figure FDA0002582705560000011
in the formula (2), N is the fatigue life of the base material; sigmaaIs the stress amplitude; sigmamaxIs the maximum stress value; e is the modulus of elasticity;
Figure FDA0002582705560000021
for fatigue limit, tensile strength σ of the passing materialbThe calculation is carried out to obtain the result,
Figure FDA0002582705560000022
the parameter t is a weighting factor of ratchet wheel damage;
a and b are the material parameters to be fitted.
3. The method for predicting the fatigue life of the welded joint by considering the evolution of the residual stress as claimed in claim 2, wherein the cyclic constitutive model in the step 4 comprises a strain decomposition equation, an elasticity equation, a yield function equation and a flow equation;
the strain decomposition equation: under small deformation conditions, principal straintInvolving elastic straineAnd viscoplastic strainvpTwo parts, namely:
t＝e+vp(3)
In the formula (3), elastic straineAnd the stress tensor sigma, the elastic equation is satisfied, namely:
e＝D-1:σ (4)
in the formula (4), D is a fourth-order Hook elasticity tensor which is determined by an elastic modulus E and a Poisson ratio v;
yield function equation: yield function FyThe classical Von-Mises yield criterion was adopted, namely:
Figure FDA0002582705560000023
in the formula (5), s is a bias stress tensor; α is the back stress tensor; r is a scalar representing effective stress; the evolution rule of alpha is determined by a follow-up hardening equation;
flow equation: the power-law rate-dependent flow criterion is used, as follows:
Figure FDA0002582705560000024
in the formula (6), the reaction mixture is,
Figure FDA0002582705560000025
represents the viscoplastic strain rate;
and < > is Macauley operator, and the meaning is as follows: when x is less than or equal to 0, the ratio of < x > -0; when x >0, < x > -x;
k and n are material constants related to the material rate; and | s- α | represents the modulo operation of the s- α vector.
4. The method of predicting fatigue life of a welded joint considering the evolution of residual stress as set forth in claim 3, wherein the evolution of α in the yield function equation is determined by the following hardening equation as:
Figure FDA0002582705560000031
Figure FDA0002582705560000032
wherein the critical state is reflected by critical surfaces, namely:
Figure FDA0002582705560000033
Figure FDA0002582705560000034
in the above formula, M represents the number of back stresses;
αkis the kth back stress;
Figure FDA0002582705560000035
is the evolution rate of the kth back stress;
Figure FDA0002582705560000036
is the equivalent plastic strain rate;
H(fk) When f denotesk<0, H (f) k) 0, otherwise H (f)k)＝1；
||αkI represents alphakPerforming modular operation on the vector;
ζkand gammakMu is the ratchet coefficient for the material parameter.
5. The method for predicting fatigue life of a welded joint considering residual stress evolution according to claim 4, wherein in the step 6, the model for predicting fatigue life of a welded joint considering residual stress evolution is as follows:
Njoint＝a′×[exp[(b′×FPCorrection)](11)
Figure FDA0002582705560000037
σmax-correction＝σmax0+σre-max(13)
Wherein N isJointFatigue life of the welded joint to account for residual stress evolution; sigmaaIs the stress amplitude; sigmamax0Maximum stress without considering residual stress; sigmare-maxThe maximum steady state tensile residual stress value is obtained; e is the modulus of elasticity;
Figure FDA0002582705560000041
for fatigue limit, tensile strength σ of the passing materialbThe calculation is carried out to obtain the result,
Figure FDA0002582705560000042
the parameter t is a weighting factor of ratchet wheel damage;
a 'and b' are the material parameters found by the step 2 fitting.
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