CN108733862B - Creep induction period prediction method considering restraint effect under steady-state creep condition - Google Patents

Creep induction period prediction method considering restraint effect under steady-state creep condition Download PDF

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CN108733862B
CN108733862B CN201710271728.1A CN201710271728A CN108733862B CN 108733862 B CN108733862 B CN 108733862B CN 201710271728 A CN201710271728 A CN 201710271728A CN 108733862 B CN108733862 B CN 108733862B
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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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Abstract

The invention discloses a creep induction period prediction method considering restraint effect under a steady-state creep condition, provides a corrected creep induction period prediction model under the steady-state creep condition, introduces a restraint parameter Q by utilizing a toughness dissipation damage model, and calculates the creep induction period considering the restraint effect. A main load is applied by using a compact tensile sample (CT) to carry out a creep simulation experiment, so that the creep induction period under a plastic condition can be simply and effectively predicted in the structure. The invention has the beneficial effects that: a simplified creep induction period prediction method under a steady-state creep condition is provided, so that the creep induction period under a plastic condition can be simply and effectively predicted in a structure.

Description

Creep induction period prediction method considering restraint effect under steady-state creep condition
Technical Field
The invention relates to a creep induction period engineering critical evaluation of a high-temperature structure considering a constraint effect under a steady-state creep 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 steady-state 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 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 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 the machined high temperature components and has a significant impact on the service life of the components. 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 under a steady-state creep condition. A toughness dissipation damage model is utilized, and a constraint parameter Q is introduced to calculate a creep induction period considering a constraint effect. 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 invention discloses a creep induction period prediction method considering restraint effect under steady-state creep condition, which comprises the following steps:
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: and inserting the prefabricated cracks at the rear parts of the gaps, wherein the grooves, the gaps and the prefabricated cracks are on the same plane. 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;
s3: necessary parameters required for calculating the induction period of the CT sample containing the restraint effect can be obtained through creep finite element simulation. Under steady state creep conditions, the calculation of the induction period mainly comprises the following steps:
(1) first, a constraint parameter Q under a steady-state creep condition is calculatedRRssThe calculation formula is as follows:
Figure GDA0002230099690000021
(I) the method comprises the following steps:
Figure GDA0002230099690000022
the opening stress value at the front of the crack is calculated by using finite elements, the unit is MPa, the sigma 0 is the yield strength of the material, and the unit is MPa, see the literature: (Zhao L, Xu L, Han Y, sting H.two-parameter transformation of constrained effect by specific size on street crack growth. Engng Fract Mech 2012; 96: 251-66.);
(I) the method comprises the following steps: sigma22The opening stress value of the crack front is calculated by utilizing the steady-state creep stress field, the unit is MPa,
Figure GDA0002230099690000023
wherein: c*Is a high-temperature fracture parameter calculated by utilizing finite elements, and the unit is MPa.mm (h)-1R is the distance from the tip of the rear part of the crack to the research point of the front edge of the crack, the unit is mm, r is d, d is mm, the distance extending from the creep damage before the crack tip to 1 when the creep initiation occurs is judged, namely the critical distance of the creep initiation, theta is the angle of the crack tip,
Figure GDA0002230099690000024
is the creep strain rate of change in units of h-1Related to the high temperature creep properties of the material, n is the dimensionless creep stress hardening index, n and
Figure GDA0002230099690000025
see literature: (Zhao L, king H, Xu L, Han Y, Xiu J. evaluation of constraint effect street peel growth by experimental initiation and numerical analysis. Engng frame Mech 2012; 96: 251-66.), InIs a non-dimensional function related to n,
Figure GDA0002230099690000026
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 simple Field technical Report, MRL E-147.);
(2) calculating the equivalent stress
Figure GDA0002230099690000027
The calculation formula is as follows:
Figure GDA0002230099690000028
(II): sigma11The unit is MPa, the stress value of the crack front is calculated by utilizing a high-temperature creep stress field,
Figure GDA0002230099690000029
wherein:
Figure GDA00022300996900000210
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 Single Field Technical Report, MRL E-147.);
(3) the steady state creep stress is then calculatedTime t of inoculation period under fieldi RRssThe calculation formula is as follows:
Figure GDA00022300996900000211
(III) in (III): epsiloncritIs uniaxial creep toughness, related to material properties, in units of 1, see literature: (Zhao L, sting H, Xu L, Han Y, Xiu J. evaluation of relationship effects on crop blackgrowth by experimental initiation and numerical simulation. Engng FractMech 2012; 96: 251-66.),
(III) in (III): MSFRRssThe multiaxial stress factor under steady state creep conditions was calculated according to the relationship of Cocks and Ashby:
Figure GDA0002230099690000031
wherein: n is a dimensionless creep stress hardening index, sinh is a hyperbolic sine function, hRRssThree degrees for steady state creep stress, at steady state creep state:
Figure GDA0002230099690000032
wherein: theta is the crack tip angle, n is the dimensionless creep stress hardening index,
Figure GDA0002230099690000033
and
Figure GDA0002230099690000034
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 simple Field technical Report, MRL E-147.).
Preferably, d is the grain size of the material under investigation.
Preferably, the
Figure GDA0002230099690000035
C*Finite element simulations were computationally simulated using ABAQUS6.14,
Figure GDA0002230099690000036
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, fracture parameter C*The values set tensile load in the load module, and the restraint conditions: including symmetric conditions and fixed conditions;
(2) submitting task calculation in the operation module to obtain calculation result containing creep-stretch experiment, and obtaining fracture parameter C from historical variables in result file*Obtaining stress values in field variations
Figure GDA0002230099690000037
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 steady-state creep condition is provided, and the creep induction period under the plastic condition can be simply and 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;
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.
In this example, a CT sample of P92 high temperature heat resistant steel, B10 mm, W20 mm, and a/W0.5, was selected as an object of study, and a main load P1200N was selected as an investigation load. The main material properties are given in the following table:
Figure GDA0002230099690000041
the invention discloses a creep induction period prediction method considering restraint effect under steady-state creep condition, which comprises the following steps:
s1: a model as shown in fig. 1 was established: 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, the upper main load pin hole 2 and the lower main load pin hole 6 are arranged in a vertically corresponding mode and are respectively arranged at the upper end and the lower end of the groove 3;
s2: 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;
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) first, each parameter is calculated:
(a) constraint parameter Q under plastic conditionsHRR:
The following data were extracted from the finite element results:
i. 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 of 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 moduleQuantity: stress value, fracture parameter C*Integral value, setting tensile load in the load module, and constraint condition: including symmetric conditions and fixed conditions;
submitting task calculation in the operation module to obtain calculation result containing creep-stretch experiment, and obtaining fracture parameter C from historical variables in result file*＝0.000666564MPa mm h-1Obtaining stress values in field variations
Figure GDA0002230099690000042
(b) And (6) looking up a table to obtain:
Figure GDA0002230099690000043
In4.99, P92 steelcrit0.2; when calculating creep stress and constraint, the distance r to the crack tip is 0.05mm, and d (mm) is the distance extending from the creep damage to the crack tip to 1 when creep initiation occurs, i.e., the critical distance for creep initiation, and as shown in fig. 2, the grain size of the material under study is generally taken.
Opening stress of crack front:
Figure GDA0002230099690000051
Figure GDA0002230099690000052
(2) and (6) looking up a table to obtain:
Figure GDA0002230099690000053
Figure GDA0002230099690000054
equivalent stress
Figure GDA0002230099690000055
(3) The germination occurring under the steady state creep stress field is then calculated:
and (6) looking up a table to obtain:
Figure GDA0002230099690000056
In4.99, P92 steel has a material parameter n of 5.23, ∈crit＝0.2；
Stress triaxial degree:
Figure GDA0002230099690000057
multiaxial stress factor:
Figure GDA0002230099690000058
incubation period under steady state creep conditions:
Figure GDA0002230099690000059
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 effect under the steady-state creep condition is characterized by comprising the following steps of: the method comprises the following steps:
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: inserting the prefabricated cracks at the rear part of the gap, enabling the groove, the gap and the prefabricated cracks to be on the same plane, 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;
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 under the condition of steady creep, the method for calculating the incubation period mainly comprises the following steps:
(1) first, a constraint parameter Q under a steady-state creep condition is calculatedRRssThe calculation formula is as follows:
Figure FDA0002230099680000011
(I) the method comprises the following steps:
Figure FDA0002230099680000012
the expansion stress value at the front edge of the crack is calculated by utilizing finite elements, and the unit is MPa and sigma0Is the yield strength of the material, in MPa,
(I) the method comprises the following steps: sigma22The opening stress value of the crack front is calculated by utilizing the steady-state creep stress field, the unit is MPa,
Figure FDA0002230099680000013
wherein: c is a high-temperature fracture parameter calculated by using finite elements and has a unit of MPa, mm (h)-1R is the distance from the tip of the rear part of the crack to the research point of the front edge of the crack, the unit is mm, r is d, d is mm, the distance extending from the creep damage before the crack tip to 1 when the creep initiation occurs is judged, namely the critical distance of the creep initiation, theta is the angle of the crack tip,
Figure FDA0002230099680000014
is the creep strain rate of change in units of h-1Related to the high temperature creep properties of the material, n is the dimensionless creep stress hardening index, InIs a non-dimensional function related to n,
Figure FDA0002230099680000015
is a dimensionless function related to θ and n;
(2) calculating the equivalent stress
Figure FDA0002230099680000016
The calculation formula is as follows:
Figure FDA0002230099680000017
(II): sigma11The unit is MPa, the stress value of the crack front is calculated by utilizing a high-temperature creep stress field,
Figure FDA0002230099680000018
wherein:
Figure FDA0002230099680000019
is a dimensionless function related to θ and n;
(3) then calculating the incubation period time t under the steady-state creep stress fieldi RRssThe calculation formula is as follows:
Figure FDA0002230099680000021
(III) in (III): epsiloncritIs uniaxial creep toughness, which is related to material properties and has a unit of 1,
(III) in (III): MSFRRssThe multiaxial stress factor under steady state creep conditions was calculated according to the relationship of Cocks and Ashby:
Figure FDA0002230099680000022
wherein: n is a dimensionless creep stress hardening index, sinh is a hyperbolic sine function, hRRssThree degrees for steady state creep stress, at steady state creep state:
Figure FDA0002230099680000023
wherein: theta is the crack tip angle and n is dimensionlessThe creep stress-hardening index of the steel,
Figure FDA0002230099680000024
and
Figure FDA0002230099680000025
is a dimensionless function with respect to theta and n.
2. The method of claim 1, wherein the creep induction period is predicted by considering constraint effect under steady state creep condition, and the method comprises: d takes the grain size of the material under study.
3. The method of claim 1, wherein the creep induction period is predicted by considering constraint effect under steady state creep condition, and the method comprises: the above-mentioned
Figure FDA0002230099680000026
C-finite element simulation a computational simulation was performed using ABAQUS6.14,
Figure FDA0002230099680000027
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
Figure FDA0002230099680000028
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