A method for evaluating the constraint degree of structure creep based on creep strain volume

Abstract

This invention belongs to the field of service structure safety evaluation technology, specifically a method for evaluating the degree of structural creep restraint based on creep strain volume. The method includes: Step 1, determining the structure to be evaluated and the reference sample; Step 2, obtaining the mechanical and creep performance parameters of the material; Step 3, constructing a finite element model and performing creep finite element simulation; Step 4, extracting the characteristic length and creep strain volume; and Step 5, constructing the creep strain volume restraint parameter Vc. This method for evaluating the degree of structural creep restraint introduces creep strain volume as a characterizing physical quantity, overcoming the shortcomings of traditional parameters (such as Ac) which suffer from large calculation errors, low efficiency, and inability to accurately characterize the overall restraint degree of the structure under complex deformation. This method is computationally simple, clearly defined, and the calculation results are consistent with C. * It is horizontally independent and can more accurately characterize the overall restraint level of the structure, providing important support for predicting the creep crack propagation life of high-temperature components.

Description

Technical Field

[0001] This invention relates to the field of service structure safety evaluation technology, and in particular to a method for evaluating the degree of structural creep restraint based on creep strain volume. Background Technology

[0002] For pressure vessels and pipelines operating in high-temperature environments for extended periods, creep damage and fracture are the dominant modes of structural failure. Therefore, accurately predicting the long-term damage evolution of high-temperature components and scientifically evaluating their service condition has become a critical issue that urgently needs to be addressed in the engineering field.

[0003] Currently, engineering assessments commonly employ high-constraint standard compact tensile (CT) specimens to obtain the creep rupture properties of materials, and life transfer is based on the conservative assumption that "high-constraint conditions cover actual harsh conditions." However, the essence of the constraint effect is the physical constraint of the crack tip material on the plastic deformation of the crack tip: thick-walled members are subject to strong out-of-plane constraints, and the crack tip is in a state of high triaxial stress, which easily induces brittle failure; while shallow cracks in actual structures are mostly at a low-constraint level, allowing plastic deformation to be released. If the difference in constraint effect is not considered and life prediction is made directly, it will not only lead to a serious deviation between the evaluation results and actual service conditions, but also make it difficult to achieve a balance between structural safety and economy.

[0004] Existing technologies still have significant limitations in quantifying creep restraint levels. For example, the patent technologies with publication numbers CN109932242B and CN109933815B introduce a restraint parameter Q. * To predict creep life, but Q * The parameters can only quantify in-plane restraint and cannot fully characterize the unified effect of in-plane and out-of-plane restraint, resulting in limited prediction accuracy under complex stress states. To address this issue, although some studies have proposed Ac parameters based on the equivalent creep strain area, in practical applications, their solution heavily relies on manual statistical cells or graphics processing software. When dealing with large deformations or complex geometric surfaces in engineering structures, the computational efficiency is low and human error is difficult to avoid. More importantly, current methods often characterize the overall restraint by using the average value of Ac at representative locations or points at the crack tip. This simplified treatment of "transforming the volume into a surface" cannot truly capture the mechanical state of the three-dimensional damage zone at the crack tip and fails to form a complete evaluation system that can take into account both local restraint characteristics and macroscopic structural response.

[0005] In summary, existing technologies lack a parameter that can uniformly characterize in-plane / out-of-plane restraint while also balancing computational efficiency and accuracy. Therefore, this invention proposes a method using the equivalent creep strain volume at the crack front as a restraint parameter, aiming to achieve efficient and accurate assessment of the creep restraint degree of cracked structures through the strain accumulation characteristics in three-dimensional space. Summary of the Invention

[0006] Based on existing technical problems, this invention proposes a method for evaluating the degree of structural creep restraint based on creep strain volume.

[0007] The present invention proposes a method for evaluating the degree of structural creep restraint based on creep strain volume, including step one: determining the structure to be evaluated and the reference specimen;

[0008] Step 2: Obtain the mechanical and creep performance parameters of the material;

[0009] Step 3: Construct a finite element model and perform creep finite element simulation;

[0010] Step 4: Extract feature length and creep strain volume;

[0011] Step 5: Construct the creep strain volumetric restraint parameter Vc.

[0012] Preferably, the reference specimen selected in step one is generally a high-constraint standard CT specimen.

[0013] Preferably, in step two, the mechanical property parameters of the material obtained by conducting uniaxial tensile tests include elastic modulus E, Poisson's ratio μ, and yield strength, and the creep property parameters of the material obtained by conducting uniaxial tensile creep tests include power-law creep parameter A and creep exponent n.

[0014] Preferably, in step three, creep finite element simulations are performed on the reference specimen and the structure to be evaluated to obtain the creep fracture parameters C of the specimen and structure under steady-state creep. * .

[0015] Preferably, in step four, the characteristic length L of the cracks in the reference specimen and the specimen to be evaluated is extracted from the finite element secondary development program in step three. Ref L CEEQ In C * The creep strain volume V obtained under horizontal and identical creep strain isolines Ref V CEEQ value.

[0016] Preferably, in step five, the feature length L extracted in step four is combined with... Ref L CEEQ and equivalent creep strain volume V Ref V CEEQ Substitute the value into the expression to obtain the constraint parameter Vc:

[0017] .

[0018] The beneficial effects of this invention are as follows:

[0019] This method of using the equivalent creep strain volume as a constraint characterization parameter, while considering the characteristics of other constraint parameters, and based on the characteristics of structural creep constraint and combined with the secondary development of finite element software, achieves a more accurate, convenient, and effective assessment of structural creep constraint. It has the advantages of clear definition and simple calculation. Attached Figure Description

[0020] Figure 1 This is a flowchart of a structural creep restraint evaluation method based on creep strain volume proposed in this invention;

[0021] Figure 2 This is a finite element model diagram of a CT specimen used to characterize different degrees of restraint in a structural creep restraint evaluation method based on creep strain volume proposed in this invention.

[0022] Figure 3 This is a planar view of a CT specimen used to characterize different degrees of restraint in a method for evaluating the degree of structural creep restraint based on creep strain volume proposed in this invention.

[0023] Figure 4 This invention proposes a method for evaluating the degree of structural creep restraint based on creep strain volume, which applies different thicknesses and different creep crack tip fracture parameters C. * A comparison of restraint parameters Ac and Vc on a CT specimen. Detailed Implementation

[0024] The technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings of the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments.

[0025] Reference Figures 1-4 A method for evaluating the degree of structural creep restraint based on creep strain volume, such as Figure 1 As shown, the process includes step one: determining the structure to be evaluated and the reference specimen; the reference specimen selected in step one is generally a high-constraint standard CT specimen.

[0026] In this embodiment, a standard CT specimen with a width W=25.4mm and a thickness B / W=1 is selected as a high-constraint reference specimen; CT specimens or defective pipes with different thicknesses (such as B / W=0.5 or B / W=10.25) are selected as the structures to be evaluated.

[0027] Step 2: Obtain the mechanical and creep properties of the material; in Step 2, uniaxial tensile tests are conducted to obtain the mechanical properties of the material, including elastic modulus E, Poisson's ratio μ, and yield strength σ. 0.2(0.2 represents the stress corresponding to 0.2% plastic strain in the material, which is considered the yield stress.) Uniaxial tensile creep tests are conducted to obtain the creep performance parameters of the material, including the power-law creep parameter A and the creep exponent n. The steady-state creep strain rate is... σ represents stress.

[0028] In this step, a uniaxial tensile test and a uniaxial creep test are performed on a martensitic steel at 650℃ to obtain the specific elastic modulus E, Poisson's ratio μ, yield strength, power-law creep parameter A, and creep index n of the material.

[0029] Step 3: Construct a finite element model and perform creep finite element simulation to obtain the crack tip fracture parameter C under steady-state creep. * .

[0030] Specifically: Creep finite element simulations were performed on the reference specimen and the structure to be evaluated to obtain the creep fracture parameters C of the specimen and structure under steady-state creep. * .

[0031] In this embodiment: the plan view and finite element diagram of the standard CT specimen finite element model are as follows: Figure 2 and Figure 3 As shown, creep finite element simulation was performed to obtain the creep parameter C of the CT specimen under steady-state creep. * The value was used to calculate the degree of restraint of CT specimens with different thicknesses, taking the CT specimen with B / W=1 as the reference specimen.

[0032] Step 4: Extract characteristic lengths and creep strain volumes; In step 4, combined with a finite element secondary development program, a Python program can be used to automatically extract the characteristic length L of the cracks in the reference specimen and the specimen to be evaluated from the finite element results of step 3. Ref L CEEQ and the equivalent creep strain volume V under the same creep strain isoline Ref V CEEQ Value. Specifically:

[0033] Run a pre-written Python post-processing script to extract the characteristic length L of the cracks in the reference specimen and the specimen to be evaluated from the finite element results of step three. Ref L CEEQ For this example, the characteristic length value of the CT specimen is taken as the crack length value at the corresponding thickness, and the equivalent creep strain volume V under the same creep strain isopleth line. Ref V CEEQ value.

[0034] Step 5: Construct the creep strain volumetric constraint parameter Vc; in step 5, the characteristic length L extracted in step 4 is combined. Ref LCEEQ and equivalent creep strain volume V Ref V CEEQ Substitute the value into the expression to obtain the constraint parameter Vc:

[0035] .

[0036] In this embodiment, the Ac and Vc values ​​of CT specimens at different thicknesses, representing the degree of restraint, are compared as follows: Figure 4 As shown, the trends characterized by the two parameters are basically consistent, and the value of Vc is obtained in relation to the creep parameter C. * Regardless of skill level, the advantages are obvious.

[0037] This invention employs a combination of numerical simulation and automated data processing, successfully constructing a full-scale creep restraint evaluation method from the "sample level" to the "structural level" by introducing an equivalent creep strain volume parameter Vc. This method for evaluating the degree of creep restraint in structures, by introducing a volume parameter, achieves accurate quantification of the creep restraint degree in cracked structures, thereby improving the accuracy of predicting the creep crack propagation rate of high-temperature components. The method is computationally simple, clearly defined, and the calculation results are consistent with C... * It is horizontally independent and can more accurately characterize the overall restraint level of the structure, providing important support for the prediction of creep crack propagation life of high-temperature components. It has the advantages of simple form and higher accuracy.

[0038] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. A method for evaluating the degree of structural creep restraint based on creep strain volume, characterized in that: This includes step one: determining the structure to be evaluated and the reference specimen; Step 2: Obtain the mechanical and creep performance parameters of the material; Step 3: Construct a finite element model and perform creep finite element simulation; Step 4: Extract feature length and creep strain volume; Step 5: Construct the creep strain volumetric constraint parameter Vc.

2. The method for evaluating the degree of structural creep restraint based on creep strain volume according to claim 1, characterized in that: The reference specimen selected in step one is generally a high-constraint standard CT specimen.

3. The method for evaluating the degree of structural creep restraint based on creep strain volume according to claim 1, characterized in that: In step two, the mechanical property parameters of the material obtained by conducting uniaxial tensile tests include elastic modulus E, Poisson's ratio μ, and yield strength. The creep property parameters of the material obtained by conducting uniaxial tensile creep tests include power-law creep parameter A and creep exponent n.

4. The method for evaluating the degree of structural creep restraint based on creep strain volume according to claim 1, characterized in that: In step three, creep finite element simulations are performed on the reference specimen and the structure to be evaluated to obtain the creep fracture parameters C of the specimen and structure under steady-state creep. * .

5. The method for evaluating the degree of structural creep restraint based on creep strain volume according to claim 1, characterized in that: In step four, the characteristic length L of the cracks in the reference specimen and the specimen to be evaluated is extracted from the finite element secondary development program in step three. Ref L CEEQ In the same C * Creep strain volume V is obtained under the same horizontal and equivalent creep strain isobars. Ref V CEEQ value.

6. The method for evaluating the degree of structural creep restraint based on creep strain volume according to claim 5, characterized in that: In step five, the feature length L extracted in step four is combined with... Ref L CEEQ and creep strain volume V Ref V CEEQ Substitute the value into the expression to obtain the constraint parameter Vc: 。

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