CN111610109A - A creep strain calculation method of a material under small punch test and its application - Google Patents

Abstract

The invention establishes a creep strain calculation method and application of materials under the small punch test based on the small punch technology. The relationship between strain and flexural deformation at the center of the sample and the simplification of the formula, the prediction of the creep strain performance of the material. The invention solves the limitation of the traditional uniaxial creep test in the small punching creep test, obtains important material parameters that cannot be directly obtained in the small punching creep test, and has high calculation accuracy for the material creep strain.

Description

A creep strain calculation method of a material under small punch test and its application

technical field

The invention relates to the technical field of material performance evaluation in high temperature service, in particular to a method for calculating creep strain of a material under a small punch test and its application.

Background technique

In the fields of nuclear power, petrochemical and other fields, harsh service conditions such as high temperature and high pressure will cause deterioration and damage to materials with the increase of service time, and gradually reduce the mechanical properties of materials, eventually leading to sudden damage to equipment and instruments, resulting in Irreversible heavy losses and even casualties. Therefore, how to accurately evaluate the remaining life of service materials during the creep process has far-reaching practical significance.

At present, the commonly used method is to obtain a certain amount of material on the service part by using the small punching creep test, and then perform the creep test after preparing the sample. The researchers compared the central flexural deformation-time curve obtained under the small punching creep test condition with the uniaxial creep test curve, and used the similarity between the two to conduct performance analysis to predict the remaining life of the service material. Most of the research is to use the load in the small punching test to calculate the equivalent stress under the uniaxial creep test by using the linear transformation relationship, and then use the parameters such as creep strain or rupture time obtained under the uniaxial stress to carry out further creep. Analysis and research of variable performance. However, there are certain differences between the small punching creep test and the uniaxial creep test. For example, in the small punching test, the sample is in a state of multiaxial stress, and the load on the sample is affected by the deformation of the sample. Therefore, the direct conversion of the load of the small punching creep test into the equivalent stress of the uniaxial test has certain limitations. The small punching creep test process can only obtain the center deflection deformation of the sample during the creep process, and cannot directly obtain the equivalent creep strain during the sample deformation process.

It can be seen that there is an urgent need to study a calculation method for obtaining creep strain under the condition of small punching creep test, so as to meet the requirements of accurate analysis of material creep performance and life prediction.

SUMMARY OF THE INVENTION

The purpose of the present invention is to provide a method for calculating the creep strain of materials under the small punching creep test, which can effectively solve the problems of the material creep strain that cannot be obtained under the existing small punching creep test, and is It provides a new method for evaluating the creep performance of important components under high temperature and high pressure conditions.

Another aspect of the present invention is to provide an application of the creep strain calculation method in evaluating the remaining life of a service material in a creep process, which can simplify the evaluation steps and have a high evaluation accuracy.

The technical scheme adopted for realizing the purpose of the present invention is:

A method for calculating creep strain of a material under a small punching creep test, comprising the following steps:

Step 1, analyze the deformation law of different materials in the small punch creep process, and divide the small punch sample into three deformation areas, namely spherical area a, unequal wall thickness cone area b and equal wall thickness area c , and establish the theoretical deformation model relations (1)-(3) based on the experimental conditions.

V _C =π[R ² -(RA) ² ]h ₀ (3)

In formulas (1), (2) and (3): VA is the volume of spherical area _a , mm ³ , _VB is the volume of cone area b with unequal wall thickness, mm ³ , _VC is the area of equal wall thickness The volume of c, mm ³ , r is the radius of the punching ball, mm; h _min is the minimum sample thickness, mm; α is the angle between the center line of the sample and the lower surface of the sample, °; h0 is the initial sample thickness, mm; A is the width of the equal wall thickness area c, mm; R is the radius of the sample, mm;

Step 2: According to the principle of constant volume during the deformation of the sample, formula (4) is used to establish the center deflection deformation of the small punching creep test - the angle between the center of the sample and the deformation area, the thickness of the sample - the center deflection deformation The relationship between (5) and (6). Using equations (5) and (6) and the test data, the corresponding relationship between the thickness of the sample and the deflection of the center of the sample is established, that is, h _min =f(L).

V _A +V _B +V _C =πR ² h ₀ (4)

In the formula: L is the deflection deformation of the sample center collected during the test, mm;

Step 3, use the relationship between the equivalent creep strain and the thickness of the sample in the small punching creep test proposed by Chakrabarty (7):

ε=ln(h ₀ /h _min ) (7)

where ε is the equivalent creep strain of the small punch sample, a dimensionless parameter.

Since h _min = f(L), the relationship between the equivalent creep strain based on the ideal deformation model and the deflection deformation at the center of the sample is established (8);

ε=ln(h ₀ /f(L)) (8)

In step 4, the relational expressions (5), (6) and (8) obtained in steps 2 and 3 are coupled into the central flexural deformation data collected in the small punching creep test to obtain the small punching creep test creep. Correspondence between variable strain and flexural deformation at the center of the specimen. In order to facilitate the practical application of engineering, the nonlinear fitting is carried out by formula (9), and the important physical parameters a, b, c and d in the relationship between the creep strain and the central flexural deformation in the small punching creep test are obtained. The expression of the relationship between the creep strain and the center deflection deformation of the punching creep test;

ε=a(L/h ₀ ) ⁴ +b(L/h ₀ ) ³ +c(L/h ₀ ) ² +d(L/h ₀ ) (9)

In step 5, formula (9) is coupled into the central flexural deformation-time curve data directly obtained by the small punching creep test, and finally the relationship between the creep strain and the test time of the small punching creep test is established.

Another aspect of the present invention further includes the application of the method for calculating the creep strain of the material under test conditions in evaluating the remaining life of the service material in the creep process.

In the above technical solution, the service material is Sanicro25 austenitic stainless steel.

In the above technical solution, the service temperature of the service material is 500-100 degrees Celsius.

Compared with the prior art, the beneficial effects of the present invention are:

1. The present invention establishes a creep strain calculation method for materials under test conditions based on the small punch technology, solves the limitations of the traditional uniaxial creep test in the small punch creep test, and obtains the small punch creep. Important material parameters that cannot be directly obtained in the test have high calculation accuracy for material creep strain.

2. The establishment of the theoretical deformation model in the calculation process is based on experiments, with high accuracy;

3. The obtained empirical formula has a simple form, which is convenient for popularization and application;

4. The calculation efficiency is high, and the calculation accuracy of the material creep strain is high;

5. It can replace the uniaxial creep test, and the required data can be obtained by the small punching creep test to reduce the cost.

Description of drawings

Fig. 1 is a schematic diagram of the research route of the creep strain calculation method under the small and medium punching creep test of the present invention.

Figure 2 is the small punch creep test curve.

Figure 3 is a microscopic metallographic analysis diagram of the cross-section of the small punch sample.

Figure 4 is a schematic diagram of the deformation of the small punch sample during the test.

Figure 5 is a graph of the thickness-center deflection curve of the small punch creep test specimen.

Figure 6 is a graph of creep strain-(L/h ₀ ) in a typical small punch creep test.

FIG. 7 is a graph showing the comparison between the creep strain calculation method proposed by the present invention and the finite element calculation results.

Detailed ways

The present invention will be further described below with reference to specific embodiments and accompanying drawings.

Referring to Fig. 1, the present invention provides a method for calculating creep strain of materials under a small punching creep test, including: establishing a theoretical deformation model and obtaining a correlation, establishing an equivalent creep strain and a sample The relationship between the central deflection and the simplification of the formula, the prediction of the creep strain performance of the material. Taking Sanicro25 austenitic stainless steel as an example, the central flexural deformation-time curve obtained directly from the small punching creep test under different load conditions is shown in Figure 2. In order to obtain the creep strain of the sample during the test, the specific steps as follows:

The first step is to analyze the deformation law of the sample in the test by using the microscopic metallographic diagram of the cross-section of the small punch sample, as shown in Figure 3. The deformation of the sample during the test can be mainly divided into three parts, (1) the spherical area A in contact with the punching ball, the thickness is uniform, and is the minimum value h _min ; (2) the unequal thickness cone area B, the thickness is from the spherical area. The h _min of the region increases linearly to the initial thickness h ₀ of the sample; (3) the equal thickness cone region C, with uniform thickness, is the initial sample thickness h ₀ . According to the deformation law of the sample and the principle of constant volume, the theoretical deformation model of the small punching creep test as shown in Figure 4 is established. In this example, the aperture R of the lower die is 0.2 mm, the width A of the c region is 0.02 mm, the initial thickness h ₀ of the sample is 0.5 mm, and the radius r of the punching ball is 1.25 mm. According to the formula (5), the relationship between the thickness of the sample and the angle α can be established (10):

According to Figure 4, the relationship between the thickness of the sample and the central deflection (11) can be obtained:

According to equations (10) and (11) and the test data, the relationship between the thickness of the sample and the deflection of the center of the sample is drawn, as shown in Figure 5 when the test condition is 425N.

In the second step, according to the relationship between the equivalent creep strain and the thickness of the specimen in the small punching creep test proposed by Chakrabarty, the relationship between the equivalent creep strain based on the ideal deformation model and the deflection deformation at the center of the specimen is established. Relation (12)

ε=ln(f(L)/0.5)=ln(2f(L)) (12)

The equations (10)-(12) are used to draw the relationship diagram of the equivalent creep strain-(L/h ₀ ) in the small punch creep test, as shown in Fig. 6 .

In order to facilitate practical engineering applications, equation (13) is used for fitting, and the creep test coefficients a, b, c, and d of small punches with different materials are obtained, as shown in Table 1.

ε=a(L/h ₀ ) ⁴ +b(L/h ₀ ) ³ +c(L/h ₀ ) ² +d(L/h ₀ ) (13)

Table 1 Small punch creep test parameter values

In the third step, use the relationship (13) of the previous step to couple into the small punching creep test data shown in Figure 2 (coupling is performed by the substitution method, one data corresponds to one result, after fitting the curve, and then from The data read in this curve), the creep strain of Sanicro25 steel under different load conditions at 700 °C is obtained, and the creep strain obtained by the finite element analysis is compared, as shown in Figure 7. It can be found that the invention can simply and accurately calculate the creep strain in the small punching creep test, has clear physical meaning, and has stronger operability and persuasion.

Claims (4)

1. A creep strain calculation method of a material under a small punch creep test is characterized by comprising the following steps:

step 1, analyzing deformation rules of different materials in a small punch creep process, dividing a small punch sample into three deformation regions, namely a spherical region a, a conical region b with unequal wall thickness and a region c with equal wall thickness, and establishing theoretical deformation model relational expressions (1) - (3) based on test conditions;

V_C＝π[R²-(R-A)²]h₀(3)

in formulae (1), (2), and (3): v_AVolume of the spherical region a, mm³，V_BVolume of conical region b of unequal wall thickness, mm³，V_CIs the volume of the region c with equal wall thickness, mm³R is the punching radius, mm; h is_minIs the minimum thickness of the sample in mm, α is the included angle between the central line of the sample and the lower surface of the sample in DEG, h₀Is the initial specimen thickness, mm; a is the width of the equal wall thickness area c, mm; r is the radius of the sample, mm;

step 2, establishing relational expressions (5) and (6) between small punching creep test center deflection deformation-included angle between sample center and deformation area and sample thickness-center deflection deformation by using the expression (4) according to the volume invariance principle in the sample deformation process; the corresponding relation between the thickness of the sample and the central deflection deformation of the sample, namely h, is established by using the formulas (5) and (6) and test data_min＝f(L)；

V_A+V_B+V_C＝πR²h₀(4)

In the formula: l is the deflection deformation of the center of the sample collected in the test process, and is mm;

step 3, utilizing a relation (7) between equivalent creep strain and sample thickness in a small punch creep test proposed by Chakrabarty:

＝ln(h₀/h_min) (7)

in the formula: equivalent creep strain of a small punch test sample without dimensional parameters;

due to h_minEstablishing a relation (8) between equivalent creep strain and sample center flexural deformation based on an ideal deformation model;

＝ln(h₀/f(L)) (8)

step 4, coupling the relational expressions (5), (6) and (8) obtained in the steps 2 and 3 into central deflection deformation data collected in a small punch creep test to obtain a corresponding relation between creep strain of the small punch creep test and sample central deflection deformation; in order to facilitate practical engineering application, the formula (9) is adopted to carry out nonlinear fitting to obtain important physical parameters a, b, c and d in a creep strain and center deflection deformation relational expression in a small punching creep test, and a creep strain and center deflection deformation relational expression in the small punching creep test is established;

＝a(L/h₀)⁴+b(L/h₀)³+c(L/h₀)²+d(L/h₀) (9)

and 5, coupling the formula (9) into central deflection deformation-time curve data directly obtained by the small punch creep test, and finally establishing a relation graph of creep strain and test time of the small punch creep test.

2. Use of the material of claim 1 in creep strain calculation under small punch creep test to evaluate the remaining life of a service material during creep.

3. Use according to claim 2, wherein the service material is Sanicro25 austenitic stainless steel.

4. The application of claim 2, wherein the service temperature of the service material is 500-100 ℃.

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