CN108458930A - The method for determining material creep parameter with clamped straight-bar small sample creep test - Google Patents

Abstract

The present invention relates to a kind of methods determining material creep parameter with clamped straight-bar small sample creep test, include the following steps：Step (1) obtains the clamped straight-bar small sample creeping displacement time graph of material；Step (2), determines sample small deformation and the critical displacement of large deformation；Creeping displacement time graph is carried out deformation stage division, determines the deformation stage residing for steady state creep by step (3)；Step (4), using beam deflection theory analysis small deformation stage creeping displacement curve, obtains the deformation of creep parameter of material when steady state creep appears in the small deformation stage；Step (5) analyzes large deformation stage creeping displacement curve using global Deformation Theory, obtains the deformation of creep parameter of material when steady state creep appears in the large deformation stage.The present invention can more accurately obtain the true creep parameters of material by analyzing clamped straight-bar small sample creeping displacement time graph by stages.

Description

Method for Determination of Material Creep Parameters Using Creep Test of Small Specimen with Fastened Straight Rod

technical field

The invention relates to material creep, in particular to a method for determining material creep parameters by using a fixed-supported straight bar small sample creep test.

Background technique

Creep, also called creep, is the tendency of a solid material to slowly and permanently move or deform under the influence of stress. It occurs as a result of prolonged stress below the yield strength of the material. Creep will be more severe when the material is heated for a long time or near the melting point. Creep often increases with increasing temperature. The rate of this deformation is related to material properties, loading time, loading temperature and loading structural stress.

In the chemical process and power generation industries, in order to obtain high energy utilization, the working temperature and working pressure of equipment and components are constantly increasing, and the working environment is becoming more and more harsh. In order to ensure the efficient and safe operation of equipment and components, it is essential to detect the creep performance of in-service equipment and components.

Creep test is a kind of material mechanical performance test to determine the slow plastic deformation of metal materials under the action of constant temperature and constant stress for a long time. The higher the temperature or the greater the stress, the more significant the creep phenomenon. However, the volume of the traditional uniaxial creep specimen is large, and sampling will cause great damage to the equipment. Some equipment and components cannot even provide the materials required for the test due to their small volume. These promote the development of the small specimen creep method. develop.

The creep test of small specimens supported by straight rods is a test method that uses small specimens supported by straight rods to test the creep properties of materials. Relatively large research value.

Regarding the determination of material creep parameters by the creep test of small specimens supported by fixed straight rods, the current research is mostly focused on the beam bending theory based on the assumption of small deformation. However, the assumption of small deformation is inconsistent with the phenomenon of large deformation that often occurs in actual creep. Therefore, Using the beam bending theory based on the assumption of small deformation to determine the creep parameters of materials in the stage of large deformation has insufficient theoretical assumptions. A method for determining the creep parameters of materials by creep tests on small straight rod specimens.

Contents of the invention

The purpose of the present invention is to provide a method for determining the creep parameters of a material by using the creep test of a small sample of a fixed straight rod. The present invention can accurately determine the creep of the material by using different creep deformation theories for different deformation stages of the material. The variable parameters are used to solve the problem that the creep parameters of the material in the large deformation stage are determined by the beam bending theory based on the small deformation assumption, which makes the determination of the material creep parameters not accurate enough.

In order to achieve the above object, the solution of the present invention is: a method for determining the creep parameters of a material by using the creep test of a small sample of a fixed straight rod, and the steps of the method are as follows:

Step (1), making a small straight rod sample with a fixed support for the material whose creep parameters are to be determined, performing a creep test on the small straight rod sample with a fixed support, and obtaining the creep displacement-time curve of the small straight rod sample;

Step (2), according to the error function and the equivalent gauge length of the small straight-bar sample, determine the critical displacement of the small deformation of the straight-bar small sample, and determine the straight-bar small test according to the equivalent gauge length of the straight-bar small sample The critical displacement of large deformation occurs;

Step (3), divide the obtained creep displacement-time curve into a small deformation stage and a large deformation stage according to the critical displacement of small deformation and large deformation of the small straight rod sample;

Step (4), according to step (3), determine the deformation stage when the steady-state creep of the small straight rod sample occurs;

Step (5), when the steady-state creep occurs in the small deformation stage, the beam bending theoretical mechanical model is used to analyze the creep displacement-time curve in the small deformation stage, and determine the creep of the small straight rod sample in the small deformation stage. The creep parameter in the small deformation stage is the real creep parameter of the material;

Step (6), when the steady-state creep occurs in the large deformation stage, use the global deformation theoretical mechanical model to analyze the creep displacement-time curve in the large deformation stage, and determine the creep of the small straight rod sample in the large deformation stage The creep parameter of the large deformation stage is the real creep parameter of the material.

The invention divides the creep displacement-time curve into different deformation stages, and by judging the deformation stage where the steady-state creep of the material occurs, different theoretical models are used to analyze the different deformation stages to determine the creep parameters of the material , the result is more realistic, accurate and reliable.

Further, according to the method of determining the creep parameters of the material by using the creep test of the fixed straight rod small sample according to the present invention, it is characterized in that: the error function described in the step (2) is:

Among them, Φ is the error function, x is the length variable in the axial direction of the sample, d is the creep displacement of the sample center, and θ is the section rotation angle of the sample.

Further, according to the method of determining the material creep parameters by using the creep test of the fixed straight rod small sample according to the present invention, in the described step (2), the critical displacement of the small deformation is:

Among them, d _cs is the critical displacement of small deformation, l is the overall length between the lower fulcrums of the beam, and θ _max is the critical rotation angle when the error function reaches 0.1.

Further, according to the method of determining material creep parameters by using the creep test of a small sample of a fixed straight rod according to the present invention, in the described step (2), the critical displacement of large deformation is:

_dcl = 0.2·l

Among them, d _cl is the critical displacement of large deformation, and l is the overall length between the lower fulcrums of the beam.

Further, according to the method of determining the material creep parameters by using the creep test of the fixed straight bar small sample according to the present invention, the beam bending theoretical mechanical model described in step (5) is:

in, is the creep displacement rate at the center of the sample, b is the width of the sample, 2h is the original thickness of the sample, P is the creep load of the sample, l is the overall length between the lower fulcrums of the beam, n is the index parameter of the material, B is the material constant, σ _eq ' is the equivalent stress in the stage of small deformation, is the equivalent strain rate in the small deformation stage.

The present invention modifies the existing theoretical mechanical model of beam bending, and the modified theoretical mechanical model of beam bending takes into account the overall deformation length of the sample, and has better correlation with uniaxial creep.

Further, according to the method of determining the material creep parameters by using the creep test of the fixed straight rod small sample according to the present invention, the global deformation theoretical mechanical model described in step (6) is:

Among them, d is the creep displacement of the sample center, R is the radius of the test indenter, _θ0 is the maximum contact angle between the indenter and the sample, l is the overall length between the lower fulcrums of the beam, ε _eq is the overall length of the large deformation stage, etc. effect strain, σ _eq is the equivalent stress in the stage of large deformation, P is the creep load of the specimen, The overall equivalent strain rate for the large deformation stage.

Further, according to the method of determining the creep parameters of the material by using the creep test of the fixed straight rod small sample according to the present invention, in the step (5), the described creep displacement-time curve is differentially processed to obtain creep displacement rate-time curve, and determine the steady-state creep displacement rate of the material according to the creep displacement rate-time curve, and substitute the steady-state creep displacement rate and load into the beam bending theoretical mechanical model to obtain the straight rod small The equivalent stress and equivalent strain rate of the sample in the small deformation stage, and draw the steady-state strain rate-equivalent stress relationship curve of the sample under the double-logarithmic coordinates, and through the steady-state strain rate-equivalent stress relationship curve Linear fitting to obtain the creep parameters of the material.

Further, according to the method for determining the material creep parameters by using the creep test of the fixed straight rod small sample according to the present invention, in the step (6), the steady-state creep is determined according to the creep displacement-time curve Displacement, and the steady-state creep displacement and load are substituted into the global deformation theoretical mechanical model to obtain the equivalent strain and equivalent stress in the large deformation stage, and the equivalent strain is differentiated with respect to time to obtain a straight rod small sample The equivalent strain rate in the large deformation stage, and draw the steady-state strain rate-equivalent stress relationship curve of the sample in log-logarithmic coordinates, and through linear fitting of the steady-state strain rate-equivalent stress relationship curve, the material creep parameters.

The beneficial effects that the present invention reaches: (1) the present invention divides the creep displacement-time curve into a large deformation stage and a small deformation stage by calculating the critical displacement of small deformation and large deformation of a small sample of fixed straight rod, different Different theoretical analysis methods are used for creep deformation in the creep deformation stage, so that the obtained creep parameters are more accurate.

(2) The present invention has revised the mechanical model when the steady-state creep occurs in the small deformation stage. Compared with the effective span method, the corrected beam bending theoretical model has considered the true span of the sample, and is different from the uniaxial creep method. Relevance is better.

(3) The present invention studies the mechanical model of steady-state creep occurring in the large deformation stage, and proposes an analysis method for anchored creep in the large deformation stage based on the global deformation theory, which can obtain the creep deformation of the material under large deformation parameter.

Description of drawings

Fig. 1 is a flow chart of the method for determining the creep parameters of small straight rod specimens.

Figure 2 is a schematic diagram of the installation of the creep test of a small sample on a fixed straight rod.

Figure 3 is a creep test device for a small straight rod sample.

Fig. 4 is the creep displacement time curve of a small sample of a fixed straight rod of A7N01 aluminum alloy at 380°C.

Figure 5 shows the division of the deformation stages of the creep displacement-time curve of a small sample of A7N01 aluminum alloy fixed straight rod.

Fig. 6 is the theoretical mechanical model of beam bending based on the assumption of small deformation.

Fig. 7 is a theoretical mechanical model of global deformation based on the assumption of large deformation.

Figure 8 is the uniaxial creep test curve of A7N01 aluminum alloy at 380°C.

Figure 9 shows the relationship between the equivalent stress and the equivalent strain rate of the A7N01 aluminum alloy under different deformation theories.

In the figure, 1 is the upper mold of the fixture, 2 is the lower mold of the fixture, 3 is the indenter, and 4 is a small straight rod sample.

Detailed ways

The present invention will be described in further detail below in conjunction with the accompanying drawings and specific embodiments.

Method of the present invention is as follows:

(1) The present invention obtains the creep displacement-time curve of the material by using the creep test of a small straight bar sample, and divides the creep displacement-time curve into a small deformation stage, a transition stage and a large deformation stage.

The creep test of the fixed-supported straight bar small sample in the present invention meets the creep test conditions stipulated in "GB/T2039-2012 Metal Material Uniaxial Tensile Test Method".

(2) Analyzing and judging the deformation stage of the steady-state creep of the straight rod small sample through the critical displacement of small deformation and large deformation of the straight rod small sample;

(3) When the steady-state creep occurs in the small deformation stage, the beam bending theoretical mechanical model is used for analysis to obtain the creep parameters in the small deformation stage of the material, and the creep parameters in the small deformation stage are taken as the real creep of the material parameter. Specifically:

Differentiate the creep displacement-time curve obtained in step (1) to obtain the creep displacement rate-time curve. In the creep displacement rate-time curve, when the creep displacement rate changes slowly or hardly changes with time, the The corresponding creep displacement rate is the steady-state creep displacement rate under the corresponding load, and the above-mentioned steady-state creep displacement rate and load are substituted into the beam bending theoretical mechanical model to obtain the equivalent effect under the beam bending theory of the straight rod small sample Stress and steady-state strain rate, draw the steady-state strain rate and equivalent stress on the log-logarithmic coordinates, obtain the steady-state strain rate-equivalent stress relationship curve, and perform linear fitting on the steady-state strain rate-equivalent stress curve Combined, the creep parameters B and n in the small deformation stage under the beam bending theory are obtained, which are the real creep parameters of the material.

The beam bending theoretical mechanics model that the present invention adopts is:

in, is the creep displacement rate, b is the width of the sample, 2h is the original thickness of the sample, P is the creep load of the sample, l is the overall length between the lower fulcrums of the beam, n is the index parameter of the material, and B is the material constant , σ _eq ' is the equivalent stress in the stage of small deformation, is the equivalent strain rate in the small deformation stage.

(4) When the steady-state creep occurs in the large deformation stage, the global deformation theoretical mechanical model is used for analysis to obtain the creep parameters in the large deformation stage of the material, and the creep parameters in the large deformation stage are taken as the real creep of the material parameter. Specifically:

Substitute the load, creep displacement and time obtained in step (1) into the global deformation theoretical mechanical model to obtain the equivalent strain-time curve and equivalent stress-time curve in the large deformation stage, and perform differential processing on the equivalent strain-time curve , obtain the equivalent strain rate-time curve under the global deformation theory, and determine the steady-state strain rate and corresponding equivalent stress of the sample under the global deformation theory, and plot the steady-state strain rate and equivalent stress on the logarithmic coordinate , the steady-state strain rate-equivalent stress relationship curve is obtained, as shown in Figure 9, under the log-logarithmic coordinates, the steady-state strain rate and the equivalent stress present a linear relationship, and the steady-state strain rate-equivalent stress in Figure 9 The force curve is linearly fitted to obtain the creep parameters B and n in the large deformation stage under the global deformation theory, which are the real creep parameters of the material.

The global deformation theory mechanics model that the present invention adopts is:

Among them, d is the creep displacement of the sample center, R is the radius of the test indenter, _θ0 is the maximum contact angle between the indenter and the sample, l is the overall length between the lower fulcrums of the beam, ε _eq is the overall length of the large deformation stage, etc. effect strain, σ _eq is the equivalent stress in the stage of large deformation, P is the creep load of the specimen, The overall equivalent strain rate for the large deformation stage.

Embodiment: The material is A7N01 aluminum alloy, and the test temperature of the small straight rod sample fixedly supported is 380°C. The process of using the method of the present invention to determine the creep parameters of A7N01 aluminum alloy is as follows:

1. Take materials from A7N01 aluminum alloy, and make a small straight rod sample of A7N01 aluminum alloy. In the middle area of small sample 4, creep tests were carried out on A7N01 aluminum alloy with different creep loads at 380°C, and after the test, the creep displacement-time curve of the straight rod sample was obtained and recorded, as shown in Figure 4 shown.

2. Through the error function, combined with the size of the test device, it is determined that the critical displacement of the sample for small deformation and large deformation is 1.003mm and 2.40mm respectively.

The error function of this embodiment is:

Among them, Φ is the error function, x is the axial direction of the sample, d is the creep displacement of the sample center, and θ is the section rotation angle of the sample.

The critical displacement calculation method for small deformation is: The critical displacement calculation method for large deformation is: d _cl =0.2·l. Among them, l is the overall length between the lower fulcrums of the beam, that is, the span between the lower dies of the fixture of the test device, and θ _max is the critical rotation angle when the error function reaches 0.1.

3. According to the critical displacement of small deformation and large deformation of the sample, the obtained creep displacement-time curve is divided into small deformation stage, transition stage and large deformation stage, as shown in Figure 5. It can be determined from Figure 5 that this In the example, the steady-state creep of the sample occurs in the stage of large deformation.

4. Since the steady-state creep occurs in the large deformation stage, the global deformation theoretical mechanical model is used to analyze the creep displacement-time curve in the large deformation stage in Fig. 5, and the large deformation stage is determined according to the above method (4). Creep parameters, the creep parameters obtained in this embodiment are B=2.381E-9, n=5.80.

5. Use the material creep parameters obtained from the global deformation theory as the real creep parameters of the material.

In this embodiment, the creep parameter determined by the method of the present invention is compared with the creep parameter obtained by the uniaxial creep test, as shown in Figure 8, it is the curve of the uniaxial creep test, and the creep parameter obtained by the uniaxial creep test The variable parameters are B=1.988E-9, n=5.75. It can be seen that the creep parameters determined by the method of the present invention are very close to the creep parameters obtained by uniaxial creep, and the creep parameters of materials determined by the method of the present invention are relatively accurate. The relationship between equivalent stress and equivalent strain rate of A7N01 aluminum alloy under rod creep test conditions.

The present invention divides the creep displacement-time curve into a large deformation stage and a small deformation stage by calculating the critical displacement of small deformation and large deformation of a small straight rod sample, and different theories are adopted for the creep deformation of different creep deformation stages Analysis method: when the steady-state creep occurs in the small deformation stage, the beam bending theory is used to obtain the creep parameters of the material; when the steady-state creep occurs in the large deformation stage, the global deformation theory is used for analysis to obtain the material's Creep parameter. Different theoretical analysis methods are adopted in different deformation stages, so that the obtained creep parameters are more accurate and real.

Claims (8)

1. a method for determining material creep parameters using a fixed straight bar small sample creep test, is characterized in that: the method steps are as follows: Step (1), making a small straight rod sample with a fixed support for the material whose creep parameters are to be determined, performing a creep test on the small straight rod sample with a fixed support, and obtaining the creep displacement-time curve of the small straight rod sample; Step (2), according to the error function and the size of the small straight bar sample test device, respectively determine the critical displacement of small deformation and large deformation of the small straight bar sample; Step (3), divide the obtained creep displacement-time curve into a small deformation stage and a large deformation stage according to the critical displacement of small deformation and large deformation of the small straight rod sample; Step (4), according to step (3), determine the deformation stage when the steady-state creep of the small straight rod sample occurs; Step (5), when the steady-state creep occurs in the small deformation stage, the beam bending theoretical mechanical model is used to analyze the creep displacement-time curve in the small deformation stage, and determine the creep of the small straight rod sample in the small deformation stage. The creep parameter in the small deformation stage is the real creep parameter of the material; Step (6), when the steady-state creep occurs in the large deformation stage, use the global deformation theoretical mechanical model to analyze the creep displacement-time curve in the large deformation stage, and determine the creep of the small straight rod sample in the large deformation stage The creep parameter of the large deformation stage is the real creep parameter of the material. 2. according to claim 1, the creep test of the small specimen of the fixed straight bar is used to determine the material creep parameter method, it is characterized in that: the error function described in the step (2) is: Among them, Φ is the error function, x is the length variable in the axial direction of the sample, d is the creep displacement of the sample center, and θ is the section rotation angle of the sample. 3. according to claim 2, the method for determining the material creep parameter by using the creep test of the small specimen of the fixed straight bar is characterized in that: in the described step (2), the critical displacement of the small deformation is: Among them, d _cs is the critical displacement of small deformation, l is the overall length between the lower fulcrums of the beam, and θ _max is the critical rotation angle when the error function reaches 0.1. 4. according to claim 1, the method for determining the material creep parameter by using the creep test of the small specimen of fixed straight bar is characterized in that: in the described step (2), the critical displacement of large deformation is: _dcl = 0.2·l Among them, d _cl is the critical displacement of large deformation, and l is the overall length between the lower fulcrums of the beam. 5. according to claim 1, the method for determining the material creep parameter by using the creep test of the small specimen of the straight rod fixedly supported is characterized in that: the theoretical mechanical model of beam bending described in the step (5) is: in, is the creep displacement rate, b is the width of the sample, 2h is the original thickness of the sample, l is the overall length between the lower fulcrums of the beam, σ _eq ' is the equivalent stress in the stage of small deformation, is the equivalent strain rate in the small deformation stage, P is the creep load of the sample, n is the exponent parameter of the material, and B is the material constant. 6. the method for determining the material creep parameters by using the creep test of the small specimen of fixed straight bar according to claim 1, is characterized in that: the global deformation theory mechanical model described in the step (6) is: Among them, d is the creep displacement of the sample center, R is the radius of the test indenter, _θ0 is the maximum contact angle between the indenter and the sample, P is the creep load of the sample, l is the overall length between the lower fulcrums of the beam, ε _eq is the overall equivalent strain in the large deformation stage, σ _eq is the equivalent stress in the large deformation stage, The overall equivalent strain rate for the large deformation stage. 7. according to claim 5, the creep test of the small specimen of the fixed straight bar is used to determine the material creep parameter method, it is characterized in that, in the described step (5), the described creep displacement-time curve is carried out Differential processing to obtain the creep displacement rate-time curve, and determine the steady-state creep displacement rate of the material according to the creep displacement rate-time curve, and substitute the steady-state creep displacement rate and load into the beam bending theoretical mechanical model, Obtain the equivalent stress and equivalent strain rate of the small straight bar sample in the small deformation stage, and draw the steady-state strain rate-equivalent stress relationship curve of the sample under the log-logarithmic coordinates, and through the steady-state strain rate-equivalent effect The force relationship curve is linearly fitted to obtain the creep parameters of the material. 8. according to claim 6, utilize fixed straight bar small sample creep test to determine material creep parameter method, it is characterized in that, in described step (6), determine according to described creep displacement-time curve Steady-state creep displacement, and the above-mentioned steady-state creep displacement and load are substituted into the global deformation theoretical mechanical model to obtain the equivalent strain and equivalent stress in the large deformation stage, and the equivalent strain is differentiated with respect to time to obtain a direct The equivalent strain rate of the small rod sample in the large deformation stage, and draw the steady-state strain rate-equivalent stress relationship curve of the sample in the log-logarithmic coordinates, and linearly simulate the steady-state strain rate-equivalent stress relationship curve Combined, the creep parameters of the material are obtained.