CN109932241B - Creep induction period prediction method for coupling residual stress and constraint effect under plastic condition - Google Patents

Creep induction period prediction method for coupling residual stress and constraint effect under plastic condition Download PDF

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CN109932241B
CN109932241B CN201711352974.6A CN201711352974A CN109932241B CN 109932241 B CN109932241 B CN 109932241B CN 201711352974 A CN201711352974 A CN 201711352974A CN 109932241 B CN109932241 B CN 109932241B
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creep
crack
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residual stress
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徐连勇
邬栋权
荆洪阳
韩永典
赵雷
吕小青
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Tianjin University
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Abstract

The invention discloses a creep induction period prediction method for coupling residual stress and constraint effect under a plastic condition, and provides a creep induction period prediction model for coupling residual stress and constraint effect on the basis of Davies work. And (3) utilizing a toughness dissipation damage model, and introducing a load-independent constraint parameter Q to calculate a creep induction period considering a constraint effect. The invention uses a compact tension sample (CT) to apply main load to carry out a creep simulation experiment, and has the following beneficial effects: the creep induction period under the plastic condition can be simply and effectively predicted in the structure.

Description

Creep induction period prediction method for coupling residual stress and constraint effect under plastic condition
Technical Field
The invention relates to a creep induction period engineering critical evaluation of a high-temperature structure coupling residual stress and constraint effect under a plastic condition, namely, the creep crack initiation life of the high-temperature structure is evaluated when a surface crack exists in the structure and the structure is under the plastic stress condition.
Background
The energy structure mainly based on coal burning is one of the main causes of haze weather in China, and coal burning power generation is the most main power generation mode in China at present, and the trend exists for a long time. Therefore, besides changing the energy structure, the development of a high-efficiency clean Ultra Supercritical (USC) unit is one of the important ways of energy conservation and emission reduction. However, the service environment of the key high-temperature pipeline of the unit is very severe due to the improvement of parameters such as steam temperature, steam pressure and the like, and particularly, various defects such as cracks, incomplete penetration, welding pores, slag inclusion and the like exist in the pipeline, so that the safe operation of the unit is seriously threatened, and scientific and accurate service life evaluation needs to be carried out on the unit.
For decades, various high temperature creep life assessment criteria and methods have been developed abroad for crack-containing components at high temperatures. The creep induction period is the longest period in the creep process, and the accurate prediction of the induction period has great significance for predicting the creep life of a high-temperature structure; an incubation period prediction model provided by Davies et al based on a toughness dissipation model considers the integrity of stress change in a creep process, but the influence of structural residual stress and restraint effect on the incubation period is not researched; residual stresses, confinement effects, are widely present in machined high temperature components and have a significant impact on the service life of the component. A large number of studies have also been widely conducted on the residual stress and restraint effect in the case of high-temperature creep. Therefore, a creep induction period prediction model coupling the residual stress and the constraint effect is established, and the creep induction period of the composite loading structure can be more accurately and completely evaluated.
Disclosure of Invention
The invention provides a creep induction period prediction model coupling residual stress and constraint effect on the basis of Davies work. And (3) calculating a creep induction period considering the residual stress by introducing an elastic following factor Z by utilizing a reference legislation. Creep experiments were performed using compact tensile specimens (CT) to generate residual stress by precompression and applying a main load.
The technical scheme adopted for realizing the purpose of the invention is as follows:
the creep induction period prediction method for the high-temperature structure coupling the residual stress and the constraint effect under the plastic condition comprises the following steps of:
s1: and establishing a prediction model, wherein the prediction model comprises a CT sample body, the front end of the middle part of the CT sample body is provided with a groove, the rear part of the groove is provided with a notch, the groove and the notch are on the same plane, the CT sample body is also provided with an upper main load pin hole and a lower main load pin hole, and the upper main load pin hole and the lower main load pin hole are arranged in an up-and-down symmetrical manner and are respectively arranged at the upper end and the lower end of the groove.
S2: firstly, the upper round pin and the lower round pin are utilized to carry out compression loading with a preset size on the upper end and the lower end of the CT sample body, and then the upper round pin and the lower round pin are released, so that residual stress distribution can be generated near a gap of the CT sample body;
s3: inserting a prefabricated crack at the notch containing the residual stress to perform a creep test;
s4: applying main loads to the upper main load pin hole and the lower main load pin hole by using the pins, and performing a high-temperature creep test;
s5: necessary parameters required for calculating the incubation period of the CT sample containing residual stress and constraint parameters can be obtained through creep finite element simulation. Under plastic conditions, the calculation of the induction period mainly comprises the following steps:
(1) firstly, calculating a stress intensity factor under composite loading, wherein the calculation formula is as follows:
Figure BDA0001510611570000021
in (I):
Figure BDA0001510611570000022
Figure BDA0001510611570000023
wherein:
Figure BDA0001510611570000028
is a stress intensity factor under the simulation calculation and only contains residual stress, and the unit is MPa (m)1/2)；
Figure BDA0001510611570000027
Is the main load stress intensity factor with the unit of MPa (m)1/2) (ii) a P is the primary load in N; b is the thickness of the specimen in mm, BnIs the net thickness of the sample, in mm; a/W is the length ratio of the prefabricated crack, a is the length of the prefabricated crack, and the horizontal straight line distance from the center of the upper main load pin hole to the rear end of the prefabricated crack is adopted, and the unit is mm; w is the nominal sample width, and the horizontal linear distance from the circle center of the upper main load pin hole to the rear end of the CT sample body is adopted, and the unit is mm; f (a/W) is the geometric coefficient of the CT sample and is only related to a/W; v is a dimensionless plastic related term calculated as follows:
Figure BDA0001510611570000024
in (II) V0Is a non-dimensional parameter, and the parameter is,
Figure BDA0001510611570000025
wherein:
Figure BDA0001510611570000029
is a plastic residual stress intensity factor with the unit of MPa (m)1/2)；
Figure BDA00015106115700000210
Is an elastic residual stress intensity factor with the unit of MPa (m)1/2)，
Figure BDA00015106115700000211
Using JSCalculation, JSIs a fracture parameter under a residual stress field, and the unit is MPa.m;
Figure BDA0001510611570000026
wherein: e' is the effective modulus of elasticity: e ═ E/(1-v)2) E is elastic modulus, ν is poisson's ratio, E, ν is described in literature: (Zhao L, sting H, Xu L, Han Y, Xiu J. evaluation of contract effects on crop grow growth by experiment evolution and numerical simulation. Engng frame Mech 2012; 96: 251-66.),
Figure BDA00015106115700000311
and JSExtracting by using finite element simulation results;
in (II) LrIs a dimensionless parameter describing the main load amplitude:
Figure BDA0001510611570000031
wherein: sigmayIs yield strength, related to material properties, in MPa, see literature: (ZHao L, sting H, Xu L, Han Y, Xiu J. evaluation of contract effects on crop grow growth by experimental initiation and numerical simulation. Engng frame Mech 2012; 96: 251-66.); sigmaref PIs the main load reference stress in MPa, calculated by the following formula:
Figure BDA0001510611570000032
wherein: n isLFor dimensionless crack aspect ratio parameters, the following is calculated:
Figure BDA0001510611570000033
constant number
Figure BDA0001510611570000034
(II):
Figure BDA0001510611570000035
Figure BDA00015106115700000312
is the stress intensity factor of the elastic main load,
Figure BDA00015106115700000313
is a plastic main load stress intensity factor with the unit of MPa (m)1/2)；
Figure BDA00015106115700000314
Calculating by using a finite element simulation result:
in the (II), beta describes the amplitude of the residual stress and is a dimensionless parameter;
Figure BDA0001510611570000036
σref Sis the secondary load reference stress, utilizes finite element simulation calculation,
in the step (II), Z is a dimensionless elastic following factor, a stress-strain relation is extracted from a finite element simulation result, and equivalent creep is takenDelta of strain
Figure BDA0001510611570000037
Equivalent elastic strain increment
Figure BDA0001510611570000038
The ratio of (A) to (B):
Figure BDA0001510611570000039
(2) calculating the J integral value under the plastic composite stress field, wherein the calculation formula is as follows:
Figure BDA00015106115700000310
wherein: kIIs the composite stress intensity factor, E' is the effective modulus of elasticity: e ═ E/(1-v)2) Where ν is poisson's ratio, E is elastic modulus, ν is poisson's ratio, and both E and ν are described in the literature: (ZHao L, sting H, Xu L, Han Y, Xiu J. evaluation of contract effects on crop grow growth by experimental initiation and numerical simulation. Engng frame Mech 2012; 96: 251-66.);
(3) then calculating constraint parameter Q under the plastic conditionHRRThe calculation formula is as follows:
Figure BDA0001510611570000041
Figure BDA0001510611570000042
the opening stress value at the front edge of the crack is calculated by using a finite element, and the unit is MPa. SigmaP0Is the normalized stress in units of MPa,. epsilonP0Is the normalized strain in units of 1, alpha is the strain hardening coefficient, N is the strain hardening index, sigmaP0，εP0α and N are described in literature: (Zhao L, Xu L, Han Y, sting H.two-parameter transformation of constrained effect induced by specimen size on creep crack growth.Engng Fract Mech 2012；96:251–66.)。INIs a dimensionless function related to N, and the specific value can be obtained by looking up the literature: shih, C.F. 1983.Tables of Hutchinson-Rice-Rosenggren Single Field Technical Report, MRL E-147.L is a scalar distance, taken to be 1 mm;
(III) in (III): sigma22The opening stress value of the crack front is calculated by using the HRR stress field, the unit is MPa,
Figure BDA0001510611570000043
wherein: r is the distance in mm from the crack trailing tip to the crack front investigation point. Theta is the crack tip angle.
Figure BDA0001510611570000044
Is a dimensionless function related to theta and N, and the specific value can be obtained by looking up the literature: shih,C.F..1983.Tables of Hutchinson-Rice-Rosengren Singular Field Quantities.Brown University Technical Report,MRL E-147.
(4) Calculating the equivalent stress
Figure BDA0001510611570000045
The unit is MPa, and the calculation formula is as follows:
Figure BDA0001510611570000046
wherein:
Figure BDA0001510611570000047
is a dimensionless function related to theta and N, and the specific value can be obtained by looking up the literature: shih, C.F. 1983.Tables of Hutchinson-Rice-Rosenggren Single Field Technical Report, MRL E-147;
(5) then calculating the incubation period time t under the plastic stress fieldi HRRThe calculation formula is as follows:
Figure BDA0001510611570000048
(V) in: n is the dimensionless creep stress hardening index, εcritIs uniaxial creep toughness, which is related to material properties and has a unit of 1,
Figure BDA0001510611570000049
is the creep strain rate of change in units of h-1N, epsilon depending on the high temperature creep properties of the materialcritAnd
Figure BDA00015106115700000410
see literature: (Zhao L, sting H, Xu L, Han Y, Xiu J. evaluation of contract effects on crop grow growth by experiment evolution and numerical simulation. Engng frame Mech 2012; 96: 251-66.),
(V) in: MSFHRRThe multiaxial stress factor under plastic condition is calculated according to the relationship of Cocks and Ashby:
Figure BDA0001510611570000051
sinh is a hyperbolic sine function, hHRRThree degrees of plastic stress, in the plastic stress state:
Figure BDA0001510611570000052
wherein the mean stress
Figure BDA0001510611570000053
The unit is MPa, and the calculation formula is as follows:
Figure BDA0001510611570000054
wherein: sigma11,σ22And σ33The stress value of the crack front is calculated by using the HRR stress field, the unit is MPa,
Figure BDA0001510611570000055
Figure BDA0001510611570000056
Figure BDA0001510611570000057
wherein:
Figure BDA0001510611570000058
is a dimensionless function related to theta and N, and the specific value can be obtained by looking up the literature: shih, C.F. 1983.Tables of Hutchinson-Rice-Rosenggren Single Field Technical Report, MRL E-147.
Preferably, d is taken as the distance r from the tip of the rear part of the crack to the research point of the front edge of the crack, and d is the distance extending to 1 of creep damage before the crack is judged when creep initiation occurs, namely the critical distance of the creep initiation.
Preferably, d is the grain size of the material under investigation.
Preferably, Bn＝B。
Preferably, the finite element simulation is a computational simulation using ABAQUS6.14, σref S、
Figure BDA00015106115700000510
JS、
Figure BDA0001510611570000059
The extraction process comprises the following steps:
(5) firstly, establishing a finite element model of a pre-compressed loaded CT sample, setting elastic-plastic parameters in a material property module, setting compression load in a load module, and setting a constraint condition: including symmetric conditions and fixed conditions. The contact module is internally provided with a compression round pin which is in rigid contact with the upper surface and the lower surface of the sample, and the analysis step module is internally provided with output parameters: stress value, dividing grids in the grid module;
(6) the task calculation is submitted in the operation module to obtain the calculation result of the residual stress, and the secondary load reference stress sigma can be directly extracted from the field variable in the result fileref S；
(7) Establishing a sample model with the same size, carrying out a main load tensile test, setting elastic-plastic creep parameters at high temperature in a material property module, dividing grids in a grid module, setting rigid contact between a tensile pin and a pin hole in a contact module, inserting a prefabricated crack in the model, and setting output parameters in an analysis step module: stress strain value, stress intensity factor K value set up tensile load in the load module to and the constraint condition: the method comprises the steps of (1) introducing the calculated residual stress into a preloading stress field under a symmetrical condition and a fixed condition;
(8) submitting task calculation in the operation module to obtain the calculation result of the creep-tensile experiment containing residual stress, and obtaining the stress value in the field variable at the moment that tensile load is not applied after the crack is inserted in the result file
Figure BDA0001510611570000061
The elastic residual stress intensity factor can be obtained from historical variables
Figure BDA0001510611570000065
And residual stress rupture parameter JSAt the initial moment of applying the tensile load, the plastic main load strength factor can be obtained
Figure BDA0001510611570000066
Obtaining the change curve of the equivalent stress along with the total strain increment from the historical variables, obtaining the equivalent creep strain increment from the curve,
Figure BDA0001510611570000062
equivalent elasticityIncrement of strain
Figure BDA0001510611570000063
And further obtaining an elastic following factor Z calculation method.
Compared with the prior art, the invention has the beneficial effects that:
compared with the existing model, the design method can expand the original prediction model into the model containing the residual stress, so that the creep induction period prediction method under the plastic condition is provided, and the creep induction period under the plastic condition can be effectively predicted in the structure.
Drawings
FIG. 1 is a schematic illustration of a compact tensile specimen (CT) precompression;
wherein: 1-upper round pin, 2-CT sample body, 3-upper main load pin hole, 4-groove, 5-notch, 6-prefabricated crack, 7-lower main load pin hole and 8-lower round pin.
FIG. 2 is a schematic representation of critical conditions for creep crack initiation.
Fig. 3 shows a method for calculating the elastic tracking factor Z.
Fig. 4 is a schematic diagram of stress conversion.
Detailed Description
The invention is described in further detail below with reference to the figures and specific examples. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention.
The high-temperature heat-resistant steel P92 was selected, and a CT sample with B20 mm, W40 mm, and a/W0.5 was used as an object to be studied, and a preload of 12000N and a main load P12000N were used as the study loads. The main material properties are given in the following table:
Figure BDA0001510611570000064
Figure BDA0001510611570000071
the creep induction period prediction method for coupling the residual stress and the constraint effect under the plastic condition comprises the following steps of:
s1: establishing a prediction model: as shown in fig. 1, the prediction model includes a CT sample body 2, a groove 4 is provided at the front end of the middle part of the CT sample body, a gap 5 is provided at the rear part of the groove, the groove 4 and the gap 5 are on the same plane, an upper main load pin hole 3 and a lower main load pin hole 7 are further provided on the CT sample body, and the upper main load pin hole and the lower main load pin hole are arranged in an up-and-down symmetrical manner and are respectively provided at the upper end and the lower end of the groove.
S2: firstly, the upper round pin 1 and the lower round pin 8 are used for carrying out compression loading with a preset size on the upper end and the lower end of the CT sample body 2, and then the upper round pin and the lower round pin are released, so that residual stress distribution can be generated near a gap 5 of the CT sample body;
s3: inserting a prefabricated crack 6 into the notch 5 containing residual stress to perform a creep test;
s4: applying main loads to the upper main load pin hole 3 and the lower main load pin hole 7 by using the pins, and performing a high-temperature creep test;
s5: necessary parameters required for calculating the incubation period of the CT sample containing residual stress can be obtained through creep finite element simulation, and the method mainly comprises the following steps of:
(1) first, each parameter is calculated:
(a) elastic main load strength factor:
Figure BDA0001510611570000072
Figure BDA0001510611570000073
the following data were extracted from the finite element results:
i) first a finite element model of the pre-compressed loaded CT specimen is built by size. And setting elastic-plastic parameters in the material property module. Set up compression load in the load module to and restrain the condition, restrain the condition and include symmetry condition and fixed condition, set up the rigid contact of compression round pin and sample upper and lower surface in the contact module, set up output parameter in the analysis step module: stress value, dividing grids in the grid module;
and ii) submitting task calculation in the operation module to obtain a calculation result of the residual stress. In the result file, the secondary load reference stress can be directly extracted from the field variables
Figure BDA0001510611570000074
Iii) establishing a sample model of the same dimensions and performing a main load tensile test, see FIG. 1. Elastic-plastic creep parameters under high temperature are set in the material property module, grids are divided in the grid module, rigid contact between a stretching pin and a pin hole is set in the contact module, a prefabricated crack is inserted into the model, and output parameters are set in the analysis step module: stress value, stress intensity factor K value, rupture parameter J integral value set up tensile load in the load module to and the constraint condition: the method comprises the steps of (1) introducing the calculated residual stress into a preloading stress field under a symmetrical condition and a fixed condition;
iv) submitting task calculation in the operation module to obtain a calculation result of a creep and tension experiment containing residual stress, wherein in a result file, when no tensile load is applied after the crack is inserted, an elastic residual stress intensity factor can be obtained from historical variables
Figure BDA00015106115700000814
And residual stress rupture parameter JSThe plastic residual stress intensity factor can be calculated as 0.013MPa · m:
Figure BDA0001510611570000081
at the initial moment of applying the tensile load, the plastic main load strength factor can be obtained
Figure BDA0001510611570000082
From historical variablesTaking the change curve of the equivalent stress along with the total strain increment, as shown in FIG. 3, obtaining the equivalent creep strain increment from the curve
Figure BDA0001510611570000083
Increment of equivalent elastic strain
Figure BDA0001510611570000084
And further obtaining an elastic following factor Z calculation method.
(b) Main load reference stress:
Figure BDA0001510611570000085
Figure BDA0001510611570000086
(c) amplitude of main load:
Figure BDA0001510611570000087
Figure BDA0001510611570000088
(d) residual stress reference stress:
Figure BDA00015106115700000815
magnitude of residual stress:
Figure BDA0001510611570000089
(e) elastic following factor:
Figure BDA00015106115700000810
Figure BDA00015106115700000811
Figure BDA00015106115700000812
(f) plastic related terms:
Figure BDA00015106115700000813
Figure BDA0001510611570000091
(2) therefore, the stress intensity factor under composite loading
Figure BDA0001510611570000092
J integral value under composite loading:
Figure BDA0001510611570000093
(3) stress values can be obtained in the presence of a variable
Figure BDA0001510611570000094
(a) And (6) looking up a table to obtain:
Figure BDA0001510611570000095
IN4.49, P92 steelcrit0.2; when calculating HRR stress and constraint, we take the distance r-d-0.05 mm before the crack tip.
Opening stress of crack front:
Figure BDA0001510611570000096
Figure BDA0001510611570000097
(4) and (6) looking up a table to obtain:
Figure BDA0001510611570000098
equivalent stress
Figure BDA0001510611570000099
(5) The germination that occurred under the plastic stress field was then calculated:
(a) and (6) looking up a table to obtain:
Figure BDA00015106115700000910
Figure BDA00015106115700000911
Figure BDA00015106115700000912
average stress:
Figure BDA00015106115700000913
stress triaxial degree:
Figure BDA00015106115700000914
multiaxial stress factor:
Figure BDA00015106115700000915
d (mm) is the distance extending to 1 from the creep damage before the crack tip when creep initiation occurs, i.e. the critical distance for creep initiation, and the grain size of the material under study is generally taken, as shown in fig. 2.
Incubation period under plastic conditions:
Figure BDA0001510611570000101
the foregoing is only a preferred embodiment of the present invention, and it should be noted that, for those skilled in the art, various modifications and decorations can be made without departing from the principle of the present invention, and these modifications and decorations should also be regarded as the protection scope of the present invention.

Claims (5)

1. The creep induction period prediction method for coupling the residual stress and the constraint effect under the plastic condition is characterized by comprising the following steps of:
s1: establishing a prediction model, wherein the prediction model comprises a CT sample body, the front end of the middle part of the CT sample body is provided with a groove, the rear part of the groove is provided with a notch, the groove and the notch are on the same plane, the CT sample body is also provided with an upper main load pin hole and a lower main load pin hole, the upper main load pin hole and the lower main load pin hole are arranged in an up-and-down symmetrical manner and are respectively arranged at the upper end and the lower end of the groove,
s2: firstly, the upper round pin and the lower round pin are utilized to carry out compression loading with a preset size on the upper end and the lower end of the CT sample body, and then the upper round pin and the lower round pin are released, so that residual stress distribution can be generated near a gap of the CT sample body;
s3: inserting a prefabricated crack at the notch containing the residual stress to perform a creep test;
s4: applying main loads to the upper main load pin hole and the lower main load pin hole by using the pins, and performing a high-temperature creep test;
s5: necessary parameters required for calculating the incubation period of the CT sample containing residual stress and constraint parameters can be obtained through creep finite element simulation, and the calculation of the incubation period mainly comprises the following steps under the plastic condition:
(1) firstly, calculating a stress intensity factor under composite loading, wherein the calculation formula is as follows:
Figure FDA0002964569060000011
in (I):
Figure FDA0002964569060000012
Figure FDA0002964569060000013
wherein:
Figure FDA0002964569060000014
is a stress intensity factor under the simulation calculation and only contains residual stress, and the unit is MPa (m)1/2)；
Figure FDA0002964569060000015
Is the main load stress intensity factor with the unit of MPa (m)1/2) (ii) a P is the primary load in N; b is the thickness of the specimen in mm, BnIs the net thickness of the sample, in mm; a/W is the length ratio of the prefabricated crack, a is the length of the prefabricated crack, and the horizontal straight line distance from the center of the upper main load pin hole to the rear end of the prefabricated crack is adopted, and the unit is mm; w is the nominal sample width, and the horizontal linear distance from the circle center of the upper main load pin hole to the rear end of the CT sample body is adopted, and the unit is mm; f (a/W) is the geometric coefficient of the CT sample and is only related to a/W; v is a dimensionless plastic related term calculated as follows:
Figure FDA0002964569060000016
in (II) V0Is a non-dimensional parameter, and the parameter is,
Figure FDA0002964569060000017
wherein:
Figure FDA0002964569060000018
is a plastic residual stress intensity factor with the unit of MPa (m)1/2)；
Figure FDA0002964569060000019
Is calculated by simulation and only contains residual stressStress intensity factor in MPa (m)1/2)，
Figure FDA00029645690600000110
Using JSCalculation, JSIs a fracture parameter under a residual stress field, and the unit is MPa.m;
Figure FDA0002964569060000021
wherein: e' is the effective modulus of elasticity: e ═ E/(1-v)2) E is the modulus of elasticity, v is the Poisson's ratio,
Figure FDA0002964569060000022
and JSExtracting by using finite element simulation results;
in (II) LrIs a dimensionless parameter describing the main load amplitude:
Figure FDA0002964569060000023
wherein: sigmayIs yield strength, related to material properties, in MPa;
Figure FDA0002964569060000024
is the main load reference stress in MPa, calculated by the following formula:
Figure FDA0002964569060000025
wherein: n isLFor dimensionless crack aspect ratio parameters, the following is calculated:
Figure FDA0002964569060000026
constant number
Figure FDA0002964569060000027
(II):
Figure FDA0002964569060000028
Figure FDA0002964569060000029
is the stress intensity factor of the main load,
Figure FDA00029645690600000210
is a plastic main load stress intensity factor with the unit of MPa (m)1/2)；
Figure FDA00029645690600000211
Calculating by using a finite element simulation result:
in the (II), beta describes the amplitude of the residual stress and is a dimensionless parameter;
Figure FDA00029645690600000212
Figure FDA00029645690600000213
is the secondary load reference stress, utilizes finite element simulation calculation,
in the step (II), Z is a dimensionless elastic following factor, a stress-strain relation is extracted from a finite element simulation result, and an equivalent creep strain increment is taken
Figure FDA00029645690600000214
Equivalent elastic strain increment
Figure FDA00029645690600000215
The ratio of (A) to (B):
Figure FDA00029645690600000216
(2) calculating the J integral value under the plastic composite stress field, wherein the calculation formula is as follows:
Figure FDA00029645690600000217
wherein: kIIs the composite stress intensity factor, E' is the effective modulus of elasticity: e ═ E/(1-v)2) V is the poisson's ratio, E is the elastic modulus, and v is the poisson's ratio;
(3) then calculating the constraint parameters under the plastic condition
Figure FDA0002964569060000031
The calculation formula is as follows:
Figure FDA0002964569060000032
Figure FDA0002964569060000033
the opening stress value at the front edge of the crack is calculated by finite elements, and the unit is Mpa and sigmaP0Is the normalized stress in units of MPa,. epsilonP0Is the normalized strain with the unit of 1, alpha is the strain hardening coefficient, N is the strain hardening index, INIs a dimensionless function related to N, L is a scalar distance, taken to be 1 mm;
(III) in (III): sigma22The opening stress value of the crack front is calculated by using the HRR stress field, the unit is MPa,
Figure FDA0002964569060000034
wherein: r is the distance from the crack trailing tip to the crack front investigation point,in mm, theta is the crack tip angle,
Figure FDA0002964569060000035
is a dimensionless function related to theta and N,
(4) calculating the equivalent stress
Figure FDA0002964569060000036
The unit is MPa, and the calculation formula is as follows:
Figure FDA0002964569060000037
wherein:
Figure FDA0002964569060000038
is a dimensionless function related to θ and N;
(5) then calculating the incubation period time t under the plastic stress fieldi HRRThe calculation formula is as follows:
Figure FDA0002964569060000039
(V) wherein: n is the dimensionless creep stress hardening index, εcritIs uniaxial creep toughness, which is related to material properties and has a unit of 1,
Figure FDA00029645690600000310
is the creep strain rate of change in units of h-1Which is related to the high temperature creep properties of the material,
(V) wherein: MSFHRRThe multiaxial stress factor under plastic condition is calculated according to the relationship of Cocks and Ashby:
Figure FDA00029645690600000311
sinh is a hyperbolic sine function, hHRRThree degrees of plastic stress, in the plastic stress state:
Figure FDA00029645690600000312
wherein the mean stress
Figure FDA0002964569060000041
The unit is MPa, and the calculation formula is as follows:
Figure FDA0002964569060000042
wherein: sigma11,σ22And σ33The stress value of the crack front is calculated by using the HRR stress field, the unit is MPa,
Figure FDA0002964569060000043
Figure FDA0002964569060000044
Figure FDA0002964569060000045
wherein:
Figure FDA0002964569060000046
is a dimensionless function related to theta and N.
2. The method of claim 1, wherein d is the distance r from the rear tip of the crack to the research point of the front edge of the crack, and d is the distance extending to 1 from the creep damage before the crack tip when the creep initiation occurs, i.e. the critical distance of the creep initiation.
3. The method of claim 2, wherein d is the grain size of the material under investigation.
4. The method of claim 1, wherein B is B, the method of predicting creep induction period coupled with residual stress and constraint effect under plastic conditionsn＝B。
5. The method of claim 1, wherein the finite element modeling is computational modeling using ABAQUS6.14, σref S、
Figure FDA0002964569060000047
JS、
Figure FDA0002964569060000048
The extraction process comprises the following steps:
(1) firstly, establishing a finite element model of a pre-compressed loaded CT sample, setting elastic-plastic parameters in a material property module, setting compression load in a load module, and setting a constraint condition: including symmetry condition and fixed condition, set up the rigid contact of compression round pin and sample upper and lower surface in the contact module, set up output parameter in the analysis step module: stress value, dividing grids in the grid module;
(2) the task calculation is submitted in the operation module to obtain the calculation result of the residual stress, and the secondary load reference stress can be directly extracted from the field variables in the result file
Figure FDA0002964569060000049
(3) Establishing a sample model with the same size, carrying out a main load tensile test, setting elastic-plastic creep parameters at high temperature in a material property module, dividing grids in a grid module, setting rigid contact between a tensile pin and a pin hole in a contact module, inserting a prefabricated crack in the model, and setting output parameters in an analysis step module: stress strain value, stress intensity factor K value set up tensile load in the load module to and the constraint condition: the method comprises the steps of (1) introducing the calculated residual stress into a preloading stress field under a symmetrical condition and a fixed condition;
(4) submitting task calculation in the operation module to obtain the calculation result of the creep-tensile experiment containing residual stress, and obtaining the stress value in the field variable at the moment that tensile load is not applied after the crack is inserted in the result file
Figure FDA0002964569060000051
The elastic residual stress intensity factor can be obtained from historical variables
Figure FDA0002964569060000052
And residual stress rupture parameter JSAt the initial moment of applying the tensile load, the plastic main load strength factor can be obtained
Figure FDA0002964569060000053
Obtaining the change curve of the equivalent stress along with the total strain increment from the historical variables, obtaining the equivalent creep strain increment from the curve,
Figure FDA0002964569060000054
increment of equivalent elastic strain
Figure FDA0002964569060000055
And further obtaining an elastic following factor Z calculation method.
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