CN109933817B - Creep induction period prediction method considering constraint parameters irrelevant to load under elastic transient creep condition - Google Patents
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Abstract
The invention discloses a creep induction period prediction method considering constraint parameters irrelevant to load under an elastic transient creep condition, and provides a creep induction period prediction model considering constraint effect on the basis of Davies work. A toughness dissipation damage model is utilized, and a load-independent restraint parameter Q is introduced to calculate a creep induction period considering a restraint 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 elastic transient creep condition can be simply and effectively predicted in the structure.
Description
Technical Field
The invention relates to a creep induction period engineering critical evaluation of a high-temperature structure under an elastic transient creep condition by considering a constraint parameter irrelevant to load, which is to evaluate the creep crack initiation life of the high-temperature structure when a surface crack exists in the structure and the structure is under the elastic transient creep 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 efficient and clean Ultra Supercritical (USC) units 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 a constraint effect of a structure on an incubation period is not researched; in recent years, researchers have conducted extensive studies on the influence of the restraining effect on creep crack growth. The confinement effect is widely present in machined and manufactured high temperature components and has a significant impact on the service life of the component. A number of studies have also been extensively conducted on the constraining effect in the case of high temperature creep. Therefore, a creep induction period prediction model considering 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
On the basis of Davies work, the invention provides a creep induction period prediction model considering constraint effect. By using a toughness dissipation damage model, a creep induction period considering Q-x calculation and constraint effect is introduced. Creep simulation experiments were performed using compact tensile test specimens (CT) to apply the primary load.
The technical scheme adopted for realizing the purpose of the invention is as follows:
the creep induction period prediction method considering the constraint parameters irrelevant to the load under the elastic transient creep condition comprises the following steps of:
s1: establishing a model: the model comprises a CT sample body, wherein a groove is formed in the front end of the middle of the CT sample body, a notch is formed in the rear part of the groove, an upper main load pin hole and a lower main load pin hole are further formed in the CT sample body, and the upper main load pin hole and the lower main load pin hole are arranged in a vertically corresponding mode and are respectively arranged at the upper end and the lower end of the groove;
s2: firstly, inserting a prefabricated crack at the rear part of the gap, enabling the groove, the gap and the prefabricated crack to be on the same plane, applying main loads to an upper main load pin hole and a lower main load pin hole by using pins, and performing a high-temperature creep test;
s3: necessary parameters required for calculating the incubation period of the CT sample containing the restraint effect can be obtained through creep finite element simulation, and the method mainly comprises the following steps of:
(1) Firstly, calculating constraint parameters under transient creep conditions
The calculation formula is as follows:
in (I): c is a high-temperature fracture parameter calculated by using finite elements and has a unit of MPa, mm (h) -1 ，
The opening stress value at the front edge of the crack is calculated by utilizing a finite element, the unit is Mpa, L is a scalar distance, and 1mm is taken.
In (I): sigma 22 The opening stress value of the crack front is calculated by using the HRR stress field, the unit is MPa,
in (I) and (II): sigma 0 Is the yield strength of the material, in MPa, see literature: (Zhao L, xu L, han Y, sting H.two-parameter characterization of constrained effect by specific size on street crack growth. Engng Fract Mech 2012 96),
is the creep strain rate of change, in units of h -1 Associated with the high temperature creep properties of the material, I n Is a dimensionless function related to n, (ii) wherein: r is the distance from the crack trailing tip to the crack front investigation point in mm, θ is the crack tip angle, n is the dimensionless creep stress hardening index, n and
see the literature: (Zhao L, sting H, xu L, han Y, xiu J. Evaluation of constraint effects on creep crack growth by experimental investigation and numerical simulation.Engng Fract Mech 2012；96:251–66.)，
Is a dimensionless function related to theta and n, I n 、
Specific values can be found by consulting the literature: (Shih, C.F. 1983.Tables of Hutchinson-Rice-Rosenggren Single Field Technical Report, MRL E-147.);
(II): the C (t) integral is a time-dependent high-temperature fracture parameter in MPa mm (h) -1 And calculating a formula:
wherein: t is time in units of h, C is a high temperature fracture parameter calculated by finite elements in units of MPa mm (h) -1 And E' is the effective modulus of elasticity: e' = E/(1-v) 2 ) E is the modulus of elasticity, v is the poisson's ratio, both E and v are described in the literature: ( Zhao L, jin H, xu L, han Y, xiu J. Evaluation of contract effects on crop crack growth by experimental improvement and numerical simulation. Engng frame Mech 2012;96, 251-66.), K is a stress intensity factor in MPa (m) ) 0.5 And calculating a formula:
(III) in (III):
in (III): p is the main load in N; b is the 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 related to only a/W.
(2) Calculating transient creep equivalent stress
The calculation formula is as follows:
in the (IV):
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.
(3) Calculating the conversion time t by using MATALAB software K-RR : at this point in time:
elastic stage damage cumulative value:
(V) in: elastic equivalent stress
The calculation formula is as follows:
wherein:
is an allowance related to the crack tip angle theta and Poisson's ratio vClass functions, available as a table lookup (Webster, G.A.,1994. Fracturemanechanicherplan. Journal of StrainNalyssi for engineering Design29, 215-223.);
(V) in: MSF K The multiaxial stress factor under elastic conditions is calculated according to the relationship of Cocks and Ashby:
wherein: n is a dimensionless creep stress hardening index, sinh is a hyperbolic sine function, h k Three degrees of elastic stress, in the elastic stress state:
wherein: θ is the crack tip angle and ν is the poisson's ratio;
(4) Then MATALAB software is used for calculating the incubation period time t under the transient creep stress field i The calculation formula is as follows:
(VI) wherein: d is the distance extended by the creep damage before the crack tip reaches 1 when the creep initiation occurs, and the unit is mm, namely the critical distance of the creep initiation;
(VI) wherein: MSF RR Calculating a multiaxial stress factor under a plastic condition according to a relational expression of Cocks and Ashby:
sinh is a hyperbolic sine function, h RR Three degrees of transient creep stress, in the plastic stress state:
wherein: mean stress
The unit is MPa, and the calculation formula is as follows:
wherein: sigma 11 And σ 33 The stress value of the crack front is calculated by utilizing the RRss stress field, the unit is MPa,
wherein:
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.。
preferably, d is taken as the grain size of the material under investigation.
Preferably, the
Finite element simulation of C is computationally simulated using ABAQUS6.14,
c, the extraction process comprises the following steps:
(1) Firstly, establishing a finite element model of a CT sample subjected to main load tensile loading, 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 value, rupture parameter C value set up tensile load in the load module to and restrain the condition: comprises a symmetrical condition and a fixed condition;
(2) Submitting task calculation in the operation module to obtain calculation results containing creep-stretch experiment, obtaining fracture parameters C from historical variables in result files, and obtaining stress values from field variables
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 with the restraint effect, so that the simplified prediction method of the creep induction period under the elastic transient creep condition is provided, and the creep induction period under the elastic transient creep condition can be effectively predicted in the structure.
Drawings
FIG. 1 is a schematic drawing of a compact tensile specimen (CT) stretch;
wherein: 1-CT sample body, 2-upper main load pin hole, 3-groove, 4-notch, 5-prefabricated crack and 6-lower main load pin hole.
FIG. 2 is a schematic representation of critical conditions for creep crack initiation;
fig. 3 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 do not limit the invention.
The model comprises a CT sample body 1, wherein a groove 3 is formed in the front end of the middle of the CT sample body 1, a notch 4 is formed in the rear portion of the groove 3, an upper main load pin hole 2 and a lower main load pin hole 6 are further formed in the CT sample body 1, and the upper main load pin hole 2 and the lower main load pin hole 6 are vertically and correspondingly arranged and are respectively arranged at the upper end and the lower end of the groove 3;
a CT sample of P92 high-temperature heat-resistant steel, B =10mm, W =20mm, a/W =0.5, was selected as a study object, and a main load P =1200N was selected as a study load. The main material properties are given in the following table:
the creep induction period prediction method of the high-temperature structure containing the restraint effect under the elastic transient creep condition comprises the following steps of:
s1: the prefabricated crack 5 is inserted into the gap, and the groove 3, the gap 4 and the prefabricated crack 5 are on the same plane. Applying main loads to the upper main load pin hole 2 and the lower main load pin hole 6 by using pins, and performing a high-temperature creep test;
s2: applying main loads to an upper main load pin hole and a lower main load pin hole on a CT sample containing a prefabricated crack by using pins, and performing a high-temperature creep test;
s3: the creep finite element simulation can obtain necessary parameters required for calculating the incubation period of the CT sample. Under plastic conditions, the calculation of the induction period mainly comprises the following steps:
(1) Constraint parameter Q under transient creep condition RR :
The following data were extracted from the finite element results:
firstly, establishing a finite element model of a CT sample subjected to main load tensile loading, 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 value, rupture parameter C integral value set up tensile load in the load module to and restrain the condition: including symmetric conditions and fixed conditions;
submitting task calculation in the operation module to obtain calculation results containing creep-stretch experiments, and acquiring fracture parameters C =0.000666564MPa mm h from historical variables in a result file -1 Where a presence variable may acquire an answerForce value of
(a) And (6) looking up a table to obtain:
I n material parameter ε of P92 steel =4.99 crit =0.2; n =5.23, and when calculating creep stress and constraint, we take the distance r = d =0.05mm before the crack tip.
(b)E'＝E/(1-ν 2 )＝137362MPa
(c)
(d)
(e) And (4) looking up a table to obtain:
elastic equivalent stress
(f)
Opening stress at crack front:
(2) And (4) looking up a table to obtain:
transient creep equivalent stress
(3) Transition time t K-RR : by using
And MATALAB calculated as: t is t K-RR ＝0h
Elastic stage damage integrated value:
(4) The germination occurring under the transient creep stress field is then calculated:
and (6) looking up a table to obtain:
average stress:
stress triaxial degree:
multiaxial stress factor:
d (mm) is the distance extending to 1 from the creep damage before the crack tip when the creep initiation occurs, i.e. the critical distance for the creep initiation, and the grain size of the material under study is generally taken as shown in FIG. 2.
Incubation period under transient creep conditions:
using MATALAB to solve the integral to obtain: t is t i K-RR ＝1255h。
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 (3)
1. The creep induction period prediction method considering the constraint parameters irrelevant to the load under the elastic transient creep condition is characterized by comprising the following steps of:
s1: establishing a model: the model comprises a CT sample body, wherein a groove is formed in the front end of the middle of the CT sample body, a notch is formed in the rear part of the groove, an upper main load pin hole and a lower main load pin hole are further formed in the CT sample body, and the upper main load pin hole and the lower main load pin hole are arranged up and down correspondingly and are respectively arranged at the upper end and the lower end of the groove;
s2: firstly, inserting a prefabricated crack at the rear part of the gap, enabling the groove, the gap and the prefabricated crack to be on the same plane, applying main loads to an upper main load pin hole and a lower main load pin hole by using pins, and performing a high-temperature creep test;
s3: necessary parameters required for calculating the incubation period of the CT sample containing the restraint effect can be obtained through creep finite element simulation, and the method mainly comprises the following steps of:
(1) Firstly, calculating constraint parameter Q under transient creep condition * RR The calculation formula is as follows:
in (I): c is a high-temperature fracture parameter calculated by using finite elements and has a unit of MPa, mm (h) -1 ，
The opening stress value at the front edge of the crack is calculated by utilizing a finite element, the unit is Mpa, L is a scalar distance, and the unit is 1mm;
in (I): sigma 22 The opening stress value of the crack front is calculated by using the HRR stress field, the unit is MPa,
in (I) and (II): sigma 0 Is the yield strength of the material, in MPa,
is the creep strain rate of change in units of h -1 Associated with the high temperature creep properties of the material, I n Is a dimensionless function related to n, (II): r is the distance from the crack rear tip to the crack front investigation point in mm, theta is the crack tip angle, n is the dimensionless creep stress hardening index,
is a dimensionless function related to θ and n;
(II): the C (t) integral is a time-dependent high-temperature fracture parameter in MPa mm (h) -1 And calculating the formula:
wherein: t is time in units of h, C is a high temperature fracture parameter calculated by finite elements in units of MPa mm (h) -1 And E' is the effective modulus of elasticity: e' = E/(1-v) 2 ) E is the modulus of elasticity and v is PoissonRatio, K is the stress intensity factor in MPa (m) 0.5 And calculating the formula:
(III) in (III):
in (III): p is the main load in N; b is the sample thickness 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;
(2) Calculating transient creep equivalent stress
The calculation formula is as follows:
(IV) in the following steps:
is a dimensionless function related to theta and n,
(3) Calculating the conversion time t by using MATALAB software K-RR : at this moment:
elastic stage damage integrated value:
(V) in: elastic equivalent stress
The calculation formula is as follows:
wherein:
is a dimensionless function related to the crack tip angle θ and the poisson ratio v;
(V) in: MSF K The multiaxial stress factor under elastic conditions is calculated according to the relationship of Cocks and Ashby:
wherein: n is a dimensionless creep stress hardening index, sinh is a hyperbolic sine function, h k Three degrees of elastic stress, in the elastic stress state:
wherein: θ is the crack tip angle, ν is the poisson's ratio;
(4) Then MATALAB software is used for calculating the incubation period time t under the transient creep stress field i The calculation formula is as follows:
(VI): d is the distance extended by the creep damage before the crack tip reaches 1 when the creep initiation occurs, and the unit is mm, namely the critical distance of the creep initiation;
(VI) wherein: MSF RR Is a multiaxial stress factor under plastic condition according to the relational expression of Cocks and AshbyAnd (3) calculating:
sinh is a hyperbolic sine function, h RR Three degrees of transient creep stress, in the plastic stress state:
wherein: mean stress
The unit is MPa, and the calculation formula is as follows:
wherein: sigma 11 And σ 33 The stress value of the crack front is calculated by utilizing the RRss stress field, the unit is MPa,
wherein:
is a dimensionless function with respect to theta and n.
2. The method of claim 1, wherein d is the grain size of the material under consideration.
3. The method of claim 1, wherein the method of predicting creep induction period under elastic transient creep conditions taking into account load-independent constraints is characterized in that
Finite element simulation of C is computationally simulated using ABAQUS6.14,
c, the extraction process comprises the following steps:
(1) Firstly, establishing a finite element model of a CT sample subjected to main load tensile loading, 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 value, rupture parameter C value set up tensile load in the load module to and restrain the condition: including symmetric conditions and fixed conditions;
(2) Submitting task calculation in the operation module to obtain calculation results containing creep-stretch experiment, obtaining fracture parameters C from historical variables in result files, and obtaining stress values from field variables
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| CN106529017A (en) * | 2016-11-08 | 2017-03-22 | 华南理工大学 | High-temperature creep deformation prediction and creep damage analysis method for high-chrome steel component |
| CN106644784A (en) * | 2016-12-31 | 2017-05-10 | 北京航空航天大学 | Turbine disc damage tolerance assessment method considering multiple parts and multiple failure modes |
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| CN105934797A (en) * | 2014-01-27 | 2016-09-07 | 泰拉能源公司 | Modeling for fuel element deformation |
| WO2016045024A1 (en) * | 2014-09-25 | 2016-03-31 | 华东理工大学 | Method for measuring and determining fracture toughness of structural material in high-temperature environment |
| CN106529017A (en) * | 2016-11-08 | 2017-03-22 | 华南理工大学 | High-temperature creep deformation prediction and creep damage analysis method for high-chrome steel component |
| CN106644784A (en) * | 2016-12-31 | 2017-05-10 | 北京航空航天大学 | Turbine disc damage tolerance assessment method considering multiple parts and multiple failure modes |
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