CN108732031B - Creep induction period prediction method considering restraint effect under elastic condition - Google Patents

Creep induction period prediction method considering restraint effect under elastic condition Download PDF

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CN108732031B
CN108732031B CN201710271302.6A CN201710271302A CN108732031B CN 108732031 B CN108732031 B CN 108732031B CN 201710271302 A CN201710271302 A CN 201710271302A CN 108732031 B CN108732031 B CN 108732031B
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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 a restraint effect under an elastic condition, and provides a creep induction period prediction model considering the restraint effect, introduces a restraint parameter Q by utilizing a toughness dissipation damage model, calculates the creep induction period considering the restraint effect, and applies a main load by using a compact tension sample (CT) to perform a creep simulation experiment. The invention has the beneficial effects that: a simplified creep induction period prediction method under an elastic condition is provided, and the creep induction period considering the constraint effect under the plastic condition can be simply and effectively predicted in the structure.

Description

Creep induction period prediction method considering restraint effect under elastic condition
Technical Field
The invention relates to a creep induction period engineering critical evaluation of a high-temperature structure considering a constraint effect under an elastic condition, namely determining that 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 elastic 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 an elastic 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 creep induction period prediction method considering the constraint effect under the elastic condition comprises the following steps:
s1: establishing a model: the model comprises a CT sample body, wherein 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 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 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 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:
(2) firstly, calculating a constraint parameter Q under an elastic conditionKThe calculation formula is as follows:
Figure GDA0002230099760000021
in (I):
Figure GDA0002230099760000022
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, see literature: (Zhao L, Xu L, Han Y, sting H. two-parameter characterization of constrained effect by specific size on street crack growth. Engng frame Mech 2012; 96: 251-66.).
In (I): sigma22The opening stress value of the crack front is calculated by using the elastic stress field, the unit is MPa,
Figure GDA0002230099760000023
wherein: r 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, d is the distance extending from the creep damage before the crack tip to 1 when the creep initiation occurs, the unit is mm, namely the critical distance of the creep initiation, and herein, r is taken as d; theta is the crack tip angle, f22(θ) is a dimensionless function related to θ, and specific values can be found by looking up the literature: (Webster, G.A.,1994. frame mechanisms in the hierarchy range. journal of Strainayanlysis for Engineering Design 29,215-0.5And calculating a formula:
Figure GDA0002230099760000024
Figure GDA0002230099760000025
wherein: p is the primary 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 the equivalent stress
Figure GDA0002230099760000026
The unit is MPa, and the calculation formula is as follows:
Figure GDA0002230099760000027
(II): sigma11The stress value of the crack front is calculated by using HRR stress field (plastic crack tip stress field), the unit is MPa,
Figure GDA0002230099760000028
wherein: f. of11(θ) is a dimensionless function related to θ, and specific values can be found by looking up the literature: (Webster, G.A.,1994. frame mechanisms in the hierarchy range. journal of train Analysis for engineering Design 29, 215-223.);
(3) then calculating the incubation period time t under the elastic stress fieldi KThe calculation formula is as follows:
Figure GDA0002230099760000031
(III) in (III): 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 GDA0002230099760000032
is the creep strain rate of change in units of h-1N, ε relating to the high temperature creep properties of the materialcritAnd
Figure GDA0002230099760000033
see all documents: (ZHao L, lacing H, Xu L, Han Y, Xiu J. evaluation of constraint effects on crop grid growth by experimental initiation and numerical simulation. EngngFract Mech 2012; 96: 251-66.);
(III) in (III): MSFKThe multiaxial stress factor under elastic conditions is calculated according to the relationship of Cocks and Ashby:
Figure GDA0002230099760000034
wherein: n is a dimensionless creep stress hardening index, sinh is a hyperbolic sine function, hkThree degrees of elastic stress, in the elastic stress state:
Figure GDA0002230099760000035
wherein: θ is the crack tip angle, v is the poisson ratio, v is found in literature: (Zhao L, sting H, Xu L, Han Y, Xiu J. evaluation of contract effects on crop grow growth by experiment simulation and numerical simulation. Engng frame Mech 2012; 96: 251-66.).
Preferably, d is the grain size of the material under investigation.
Preferably, the finite element simulation is a computational simulation using ABAQUS6.14,
Figure GDA0002230099760000036
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 J integral 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 result containing creep-tension experiment, and obtaining stress value from field variable in result file
Figure GDA0002230099760000037
Compared with the prior art, the invention has the beneficial effects that:
the invention provides a corrected creep induction period prediction model under the elastic condition containing the constraint effect, and therefore, a simplified creep induction period prediction method under the elastic 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) tensile;
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 diagram 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.
The test piece of P92 high temperature heat resistant steel was selected as the test object, and the CT sample of B10 mm, W20 mm, and a/W0.5 was selected as the test load, and the main load P1200N was selected as the test load. The main material properties are given in the following table:
Figure GDA0002230099760000041
the creep induction period prediction method considering the constraint effect under the elastic condition 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. Considering restraint under elastic conditions, calculating the incubation period mainly comprises the following steps:
(1) first, each parameter is calculated:
(a) restraint parameter Q under elastic conditionsK:
(b) Elastic main load strength factor:
Figure GDA0002230099760000042
Figure GDA0002230099760000043
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 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: the stress value sets up tensile load in the load module to and the restraint condition: including symmetric conditions and fixed conditions;
ii, submitting task calculation in the operation module to obtain calculation results containing creep-stretch experiments, and obtaining stress values from field variables in result files
Figure GDA0002230099760000051
(c) And (6) looking up a table to obtain: f. of22Material parameter epsilon of 1, P92 steelcrit0.2; when calculating the elastic stress and the constraint, the distance r between the crack tips is 0.05 mm.
Opening stress of crack front:
Figure GDA0002230099760000052
Figure GDA0002230099760000053
(2) and (6) looking up a table to obtain: f. of11(θ)＝1
Figure GDA0002230099760000054
Equivalent stress:
Figure GDA0002230099760000055
(3) the initiation occurring under the elastic stress field was then calculated:
stress triaxial degree:
Figure GDA0002230099760000056
multiaxial stress factor:
Figure GDA0002230099760000057
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 GDA0002230099760000058
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 restraint effect under the elastic 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 the method mainly comprises the following steps of:
(1) firstly, calculating a constraint parameter Q under an elastic conditionKThe calculation formula is as follows:
Figure FDA0002230099750000011
in (I):
Figure FDA0002230099750000012
is a crack front calculated by finite elementThe value of the opening stress of (A) is in MPa, sigma0Is the yield strength of the material in MPa;
in (I): sigma22The opening stress value of the crack front is calculated by using the elastic stress field, the unit is MPa,
Figure FDA0002230099750000013
wherein: r 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, d is the distance extending from the creep damage before the crack tip to 1 when the creep initiation occurs, the unit is mm, namely the critical distance of the creep initiation, and herein, r is taken as d; theta is the crack tip angle, f22(theta) is a dimensionless function related to theta, and K is a stress intensity factor in units of MPa (m)0.5And calculating a formula:
Figure FDA0002230099750000014
Figure FDA0002230099750000015
wherein: p is the primary 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 the equivalent stress
Figure FDA0002230099750000016
The unit is MPa, and the calculation formula is as follows:
Figure FDA0002230099750000021
(II): sigma11The stress value of the crack front is calculated by using the HRR stress field, the unit is MPa,
Figure FDA0002230099750000022
wherein: f. of11(θ) is a dimensionless function related to θ;
(3) then calculating the incubation period time t under the elastic stress fieldi KThe calculation formula is as follows:
Figure FDA0002230099750000023
(III) in (III): 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 FDA0002230099750000024
is the creep strain rate of change in units of h-1Related to the high temperature creep properties of the material;
(III) in (III): MSFKThe multiaxial stress factor under elastic conditions is calculated according to the relationship of Cocks and Ashby:
Figure FDA0002230099750000025
wherein: n is a dimensionless creep stress hardening index, sinh is a hyperbolic sine function, hkThree degrees of elastic stress, in the elastic stress state:
Figure FDA0002230099750000026
wherein: theta is the crack tip angle and ν is the poisson's ratio.
2. The method of claim 1, wherein the creep induction period prediction method is based on the consideration of constraint effect under elastic conditions, and comprises: d takes the grain size of the material under study.
3. The method of claim 1, wherein the creep induction period prediction method is based on the consideration of constraint effect under elastic conditions, and comprises: the above-mentioned
Figure FDA0002230099750000027
The finite element simulation of (a) was computationally simulated using ABAQUS6.14,
Figure FDA0002230099750000028
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 J integral 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 result containing creep-tension experiment, and obtaining stress value from field variable in result file
Figure FDA0002230099750000029
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