CN113239477B - Application of cyclic hardening model based on dislocation entanglement of weld joint in fatigue life prediction of welded joint - Google Patents

Application of cyclic hardening model based on dislocation entanglement of weld joint in fatigue life prediction of welded joint Download PDF

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CN113239477B
CN113239477B CN202110356503.2A CN202110356503A CN113239477B CN 113239477 B CN113239477 B CN 113239477B CN 202110356503 A CN202110356503 A CN 202110356503A CN 113239477 B CN113239477 B CN 113239477B
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李海舟
刘永杰
王清远
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Sichuan University
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Abstract

The invention provides an application of a cyclic hardening model based on dislocation entanglement of weld joints in fatigue life prediction of the weld joints, wherein the cyclic hardening model is as follows:
Figure DDA0003003414110000011
in the method, in the process of the invention,
Figure DDA0003003414110000012
the invention is based on dislocation entanglement of the weld joint, considers the influence of flow stress caused by dislocation entanglement of the weld joint and back stress generated by the blocking effect of the dislocation of the weld joint on the cyclic yield strength, establishes a cyclic hardening model of the weld joint on the basis, well interprets the relation with the maximum stress, the cyclic times and the plastic strain, and interprets the microscopic mechanism based on fatigue failure of the dislocation entanglement of the weld joint, thereby being capable of evaluating the cyclic deformation behavior of the weld joint more rapidly and accurately, and providing a better theoretical basis for predicting the fatigue life of the weld joint.

Description

Application of cyclic hardening model based on dislocation entanglement of weld joint in fatigue life prediction of welded joint
Technical Field
The invention relates to the technical field of member fatigue life prediction, in particular to application of a cyclic hardening model based on dislocation entanglement of weld joints in fatigue life prediction of welded joints.
Background
Fatigue failure is widely present in engineering components, so that it is highly necessary to effectively predict the fatigue life of the component, and weak links (such as welded joints) in engineering components are the places where fatigue failure is most likely to occur. Therefore, in order to ensure safe operation of the engineering components, the mechanism of cyclic plastic deformation of the welded joint needs to be studied. The fatigue properties of a welded joint depend on the chemical composition of the welding material and the welding process. In order to design the welding material reasonably, shorten the development time of the welding material, and properly select the welding process, the relationship between the microstructure of the weld and the fatigue properties of the welded joint needs to be determined, and a corresponding cyclic hardening model needs to be established.
In recent years, a plurality of cyclic hardening models are established at home and abroad, but the cyclic hardening models are all based on macroscopic mechanical parameters, and cannot explain microscopic mechanisms in fatigue failure, and dislocation morphology is an important mechanism for determining cyclic plastic deformation in the fatigue process. And when the welded joint of the steel works under the working condition of fatigue failure, fatigue fracture often occurs at the center of the welding line. Therefore, under the condition, the cyclic hardening model of the welding joint based on dislocation entanglement of the welding joint is established, so that the cyclic deformation behavior of the welding joint can be estimated more accurately, and a theoretical basis is provided for predicting the fatigue life of the welding joint.
Disclosure of Invention
The invention aims to establish a weld joint cyclic hardening model based on weld dislocation entanglement, considers the influence of flow stress caused by the weld dislocation entanglement and back stress generated by the blocking effect of weld grain boundaries on dislocation on cyclic yield strength, explains the microscopic mechanism of fatigue failure based on the weld dislocation entanglement, can rapidly and accurately evaluate the cyclic deformation behavior of the weld joint, and further provides a better theoretical basis for the prediction of the fatigue life of the weld joint.
The technical scheme adopted by the invention is as follows:
the application of a cyclic hardening model based on dislocation entanglement of weld joints in fatigue life prediction of the welded joints is that:
Figure BDA0003003414090000011
wherein sigma max Is the maximum stress; n is the number of times of circulation; e is the elastic modulus; delta epsilon t Is the total plastic strain range; alpha is the Taylor hardening coefficient, dimensionless; m is a Taylor factor, dimensionless; g is shear modulus, GPa, b is Bose vector, nm; ρ is the dislocation density, m -2 ；λ G Is the average distance between the slip lines, nm; d is the weld grain size; delta epsilon p Is an attribute strain range, is dimensionless, and has the following relation:
Figure BDA0003003414090000021
/>
Figure BDA0003003414090000022
Figure BDA0003003414090000023
wherein e and C are integral constants; k (k) 1 And k 2 The values of the parameters related to the dislocation formation rate and the annihilation rate are constant and dimensionless.
Further, lambda G And D is obtained by photo measurement of the microstructure, and
Figure BDA0003003414090000024
in sigma G Back stress generated by the blocking effect of the weld grain boundary on dislocation.
The design idea of the invention is as follows:
1. flow stress sigma based on dislocation entanglement of weld joints Taylor ：
Figure BDA0003003414090000025
In the formula (1), the values of α, M, G and b are available by referring to the literature, and for austenitic steels, typically α=0.2, m=3.06, g=55.69 gpa, b=0.255 nm; the value of ρ can be calculated from Electron Back Scattering Diffraction (EBSD) experimental data.
2. Deriving back stress sigma generated by barrier effect of weld grain boundaries on dislocations G ：
Figure BDA0003003414090000026
In the formula (2), lambda G And the value of D can be obtained by photo measurement of the microstructure; deltaε p Can be obtained from fatigue process experimental data (e.g., room temperature low cycle fatigue test, room temperature high cycle fatigue test, high temperature low cycle fatigue test, and creep-fatigue test).
1. 2, the Nano Measurer is adopted as the measuring software.
3. Deriving maximum stress sigma max Theoretical relationship with cycle number N:
adding the two forces in equations 1 and 2, respectively, yields the maximum stress:
Figure BDA0003003414090000027
differentiating the dislocation density on both sides of the formula (3) can be obtained:
Figure BDA0003003414090000031
the relationship between the maximum stress and the plastic strain amplitude is known from the hysteresis loop as follows:
Figure BDA0003003414090000032
equation (5) vs. plastic strain range Δε p Differentiation, can be obtained:
Figure BDA0003003414090000033
equation (4) and equation (6) are equal, and can be obtained:
Figure BDA0003003414090000034
in addition, the relationship between dislocation density and the number of cycles can be expressed as:
Figure BDA0003003414090000035
bringing equation (8) into equation (7) yields:
Figure BDA0003003414090000036
from the formula (3) and the formula (5), it is possible to obtain:
Figure BDA0003003414090000037
bringing equation (10) into equation (9) yields:
(Z″′+Y″′Δε p )Δε p dN+dΔε p ＝0 (11)
wherein:
Figure BDA0003003414090000038
Figure BDA0003003414090000039
then, the integral of formula (11) yields:
Figure BDA00030034140900000310
finally, bringing equation (14) into equation (5) yields the relationship between maximum stress and cycle number:
Figure BDA0003003414090000041
thus, based on the fitting of experimental data, a relationship curve of maximum stress and cycle number (the software used for fitting is Origin) can be obtained.
Compared with the prior art, the invention has the following beneficial effects:
(1) The invention is based on weld dislocation entanglement, considers the influence of flow stress caused by weld dislocation entanglement and back stress generated by the blocking effect of weld grain boundary on dislocation on cyclic yield strength, and establishes a cyclic hardening model of a welding joint on the basis:
Figure BDA0003003414090000042
(i.e.: A. I. Is:>
Figure BDA0003003414090000043
) The model well explains the relation between the maximum stress, the cycle number and the plastic strain, and the verification result shows that the calculated value of the model of the maximum stress increased along with the cycle number is compared with the actual value, and the calculated value and the actual value are very close, and the deviation is not more than 10%. This demonstrates that the cyclic hardening model based on dislocation entanglement of the weld seam designed in the present invention is effective.
And, for the cyclic hardening material, since the maximum stress can reflect the degree of cyclic hardening of the material, it is an important factor for determining fatigue damage, and in the present invention, since k 1 And k 2 For parameters related to dislocation formation rate and annihilation rate, values of Z '", Y'" are fitted through macroscopic experimental data, and then the formula is utilized
Figure BDA0003003414090000044
Figure BDA0003003414090000045
Can calculate k 1 And k 2 To explain the microscopic mechanism of fatigue failure based on dislocation entanglement of the weld. Therefore, when the related research is carried out later, the corresponding maximum stress and k are obtained by combining the model calculation according to the results of the plastic strain and the cycle number 1 And k 2 The cyclic deformation behavior of the welded joint can be evaluated quickly and accurately.
(2) The invention has reasonable design, convenient operation and reliable result, and provides a better theoretical basis for predicting the fatigue life of the welding joint.
Drawings
FIG. 1 is a graph showing the results of fitting the relationship between the number of cycles and the plastic strain range in example 1 of the present invention.
FIG. 2 is a graph showing the results of fitting the relationship between the number of cycles and the maximum stress amplitude in example 1 of the present invention.
FIG. 3 is a graph showing the results of fitting the relationship between the number of cycles and the plastic strain range in example 2 of the present invention.
FIG. 4 is a graph showing the results of fitting the relationship between the number of cycles and the maximum stress amplitude in example 2 of the present invention.
Detailed Description
The invention is further illustrated by the following description and examples, including but not limited to the following examples.
Example 1
And (3) selecting a novel heat-resistant steel Sanmicro 25 steel pipe as a base material, and butting the Sanmicro 25 steel pipe by utilizing manual argon tungsten-arc welding. A low cycle fatigue test with a total strain amplitude of 0.4% was performed at 700 ℃ for the welded joint. Specific experimental procedures can be carried out by reference to the descriptions in the literature Low cycle fatigue behavior and microstructure evolution of a novel Cr-3W-3Co tempered martensitic steel at 650 ℃ (Jing H, luo Z, xu L, et al materials Science & Engineering A,2018,731.).
Then, according to the obtained experimental data, obtaining the plastic strain range delta epsilon under different cycle times N p Is a numerical value of (2). Then fitting N and Δε using equation (14) above p The result of the fitting is shown in figure 1.
Thus, equation (14) may be determined as:
Figure BDA0003003414090000051
from the experimental parameters, Δε can be seen t =0.008 and e=154.6 GPa. Equation (S1), Δε t And E is brought into the above formula (15), the maximum stress can beExpressed as:
Figure BDA0003003414090000052
comparing the calculated value of the model of the maximum stress increased with the cycle number with the experimental obtained true value, as shown in fig. 2, the result shows that: the calculated and actual values are very close. For example, when the number of cycles is 191, the theoretical value of the maximum stress amplitude calculated according to the formula (S2) is 325.56MPa, the true value is 330.36MPa, and the calculated deviation is 1.45%.
Example 2
And selecting the 316H austenitic stainless steel welding joint as a verification object. At 550 ℃ and a strain rate of 1 x 10 for a welded joint -3 s -1 A low cycle fatigue test was performed with a total strain amplitude of 0.5%. For specific experimental procedures, reference is made to the document Low cycle fatigue behavior and microstructure evolution of a novel Cr-3W-3Co tempered martensitic steel at 650 ℃ (Jing H, luo Z, xu L, et al materials Science&Engineering a,2018,731).
Then, according to the obtained experimental data, obtaining the plastic strain range delta epsilon under different cycle times N p Is a numerical value of (2). Then fitting N and Δε using equation (14) above p The result of the fitting is shown in figure 3.
Thus, equation (14) may be determined as:
Figure BDA0003003414090000061
from the experimental parameters, Δε can be seen t =0.01 and e= 163.33GPa. Equation (S3), Δε t And E brings the value into equation (15) above, the maximum stress can be expressed as:
Figure BDA0003003414090000062
comparing the calculated value of the model of the maximum stress increased with the cycle number with the experimental obtained true value, as shown in fig. 4, the result shows that: the calculated and actual values are very close. For example, when the number of cycles is 423, the theoretical value of the maximum stress amplitude calculated according to the formula (S4) is 337.67MPa, the true value is 341.13MPa, and the calculated deviation is 1.01%.
From the results of examples 1 and 2, it can be seen that the model calculation values and the true values are very close to each other by verifying the welded joint using the heat-resistant steel Sanmicro 25 steel tube and 316H austenitic stainless steel as the base materials, and the deviation ranges from 0.01% to 7%, while the deviation is not more than 10% by macroscopic verification using other common austenitic heat-resistant steels (such as 316 and 316L). This demonstrates that the cyclic hardening model based on dislocation entanglement of the weld seam designed in the present invention is effective. At the same time, k can be calculated according to the model of the invention 1 And k 2 And then to explain the microscopic mechanism at fatigue failure based on dislocation entanglement of the weld. Therefore, the method can be well applied to the prediction of the fatigue life of the welding joint, and provides a theoretical basis for a prediction means.
The above embodiments are only preferred embodiments of the present invention, and should not be used to limit the scope of the present invention, and all the modifications or color changes that are not significant in the spirit and scope of the main body design of the present invention are still consistent with the present invention.

Claims (2)

1. The application of a cyclic hardening model based on dislocation entanglement of weld joints in fatigue life prediction of the welded joints is that:
Figure FDA0003994636760000011
wherein sigma max Is the maximum stress; n is the number of times of circulation; e is the elastic modulus; delta epsilon t Is the total plastic strain range; alpha is the Taylor hardening coefficient, dimensionless; m is a Taylor factor, dimensionless; g is shear modulus, GPa, b is cypressVector, nm; ρ is the dislocation density, m -2 ；λ G Is the average distance between the slip lines, nm; d is the weld grain size; delta epsilon p Is an attribute strain range, is dimensionless, and has the following relation:
Figure FDA0003994636760000012
Figure FDA0003994636760000013
Figure FDA0003994636760000014
wherein e and C are integral constants; k (k) 1 And k 2 The value of the parameter related to dislocation formation rate and annihilation rate is constant and dimensionless; lambda (lambda) G And D is obtained by photo measurement of the microstructure.
2. Use of a cyclic hardening model based on dislocation entanglement of weld joints in the prediction of fatigue life of welded joints according to claim 1,
Figure FDA0003994636760000015
in sigma G Back stress generated by the blocking effect of the weld grain boundary on dislocation. />
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