KR100927738B1 - Low cycle environmental fatigue test method using small fatigue specimen - Google Patents

Abstract

The present invention relates to a low cycle environmental fatigue test method using a small fatigue specimen to accurately measure the displacement and strain of a low cycle fatigue specimen using a small fatigue specimen, in particular through a low cycle environmental fatigue experiment applying a formula The present invention relates to a low cycle environmental fatigue test method using small fatigue specimens to check the strain hardening properties and the reliability of the test.

Description

Method for low-cycle environment fatigue test using small sized fatigue specimen

The present invention relates to a low cycle environmental fatigue test method using a small fatigue specimen to accurately measure the displacement and strain of a low cycle fatigue specimen using a small fatigue specimen, in particular through a low cycle environmental fatigue experiment applying a formula The present invention relates to a low cycle environmental fatigue test method using small fatigue specimens to check the strain hardening properties and the reliability of the test.

The life curve for fatigue design in Section III Appendix I of the American Society of Mechanical Engineers (ASME), which designs the fatigue integrity of current pressure boundary equipment in nuclear power plants, is based on the results of low cycle fatigue tests at room temperature. It is applied conservatively by giving margin of twice of strain and 10 times of life.

However, even if the pressure boundary equipment is conservatively designed in terms of fatigue, corrosion may occur on the surface of the material under high temperature and high pressure operating conditions under actual operating conditions. Therefore, the service life should be evaluated using fatigue test results under actual operating conditions. It is claimed.

In addition, fatigue design for safety-grade devices in the current nuclear power plant is available in ASME B & PV Sec. Evaluation of the growth behavior of cracks, carried out by III and found by nondestructive testing, was carried out by ASME B & PV Sec. It follows the procedure of XI.

However, the fatigue life assessment curve and fatigue crack growth assessment curve currently used by ASME are not taken into account by the influence of the hydrochemical environment during operation, and some hydrochemical conditions may not be sufficiently conservative.

Therefore, the characteristics of the cyclic strain hardening effect appearing in the process of developing the environmental fatigue life assessment curve for the fatigue life evaluation of CF8M cast stainless steel, the pressure boundary piping material, should be studied. Low cycle environmental fatigue tests should be performed by developing experiments and calibration methods to precisely measure the displacement and strain of rod-shaped miniature fatigue specimens in a small autoclave to simulate operating conditions. In addition, the cyclic strain hardening characteristics and the reliability of the experiments should be verified by performing separate experiments using the calibration method.

The purpose of the present invention is to apply the development of additional environmental fatigue life assessment curves to secure various strain rates of small fatigue specimens and environmental fatigue data for small fatigue specimens through low-cycle environmental fatigue experiments using formulas. It is to provide a low cycle environmental fatigue test method using small fatigue specimens that can contribute to the establishment of the technical basis for the production of fatigue property values required for the environmental fatigue health evaluation of major nuclear power plants.

Low cycle environmental fatigue test method using a small fatigue specimen according to the present invention for achieving the above object, the first step of measuring the strain of the telescopic system and LVDT through a high temperature test in air; And a second step of grasping the displacement behavior of the LVDT inside the autoglove in the PWR experimental environment based on the strain values of the telescopic system and the LVDT measured after the first step.

In the low cycle environmental fatigue test method using the small fatigue specimen according to the present invention, the additional life fatigue life assessment for securing the various strain rates of the small fatigue specimen and the environmental fatigue data for the small fatigue specimen through the low cycle environmental fatigue experiment applying the equation It can be used for curve development research and contribute to the establishment of technology base for the production of fatigue property values required for the environmental fatigue health evaluation of major domestic nuclear power plants.

The present invention will be described in more detail based on the accompanying drawings as follows.

In the following description of the present invention, detailed descriptions of well-known functions or configurations will be omitted if it is determined that the detailed description of the present invention may unnecessarily obscure the subject matter of the present invention. And the following terms are terms defined in consideration of functions in the present invention, which may vary according to the intention or custom of the user, operator, etc. Accordingly, the meaning of each term should be interpreted based on the contents throughout the present specification. will be.

First, the small fatigue test specimen used in the present invention was manufactured according to the test specimen standard set forth in the American Society of Testing Materials (ASTM) E 606-92.

The distance between the gauges was 19.05mm, the diameter of the central part of the specimen was 9.63mm, and the shape of the end joint of the specimen was made according to the design shape of the specimen holding device.

The test specimen was made of CF8M cast austenitic stainless steel (CASS) ingot that is identical to the pipe in operation by investigating the chemical composition of CF8M, which is the material of the PWR primary coolant piping. After heat treatment and quenching.

Table 1 below is a table showing the chemical composition of the cast stainless steel.

[Table 1] Chemical composition table of cast stainless steel

ingredient C Mn Si Cr Ni Mo S P CF8M (1) 0.059 0.916 1.167 19.88 8.21 2.329 0.007 0.043 CF8M (2) 0.058 0.918 1.116 19.35 8.39 2.351 0.006 0.043 Average 0.058 0.917 1.141 19.62 8.30 2.340 0.006 0.043

In addition, in order to perform the experiment in an environment similar to that of a pressurized water reactor, the maximum temperature of the experimental solution in the pressure vessel was set to 315 ° C., the maximum pressure of 15 MPa, and the dissolved oxygen of 5 ppb or less. The load ratio is a sawtooth wave in which tension and compression are repeated at R = -1 and the strain rate is 0.04% / s, and the maximum strain amplitude is 0.4%, 0.6%, 0.8% of the specimen gauge distance. In the case of the experiment was performed. Table 2 below is a table showing the experimental conditions and water quality measurement values.

[Table 2] Experimental conditions and measured values

Load rate (R) -1 (tensile / compress) Strain rate 0.04% / s Maximum Strain (ε _α ) 0.4%, 0.6%, 0.8% Temperature 315 ℃ pressure 15 MPa Dissolved oxygen <5 ppb

All experiments were performed by the method of strain control and the fatigue life was determined by the number of cycles (N ₂₅ ) when the experimental load decreased by 25% of the initial tensile load value.

1 is a view showing a small fatigue specimen used in a low cycle environmental fatigue test method using a small fatigue specimen according to the present invention.

The low cycle environmental fatigue experiment is carried out in a small autoclave in order to simulate the operating conditions as an experiment, and as shown in FIG. 1, the gage distance (Real gage: RG) of the small fatigue specimen 1 is shown. (4) Linear displacement differential transformer (LVDT) (2) Displacement sensor (not shown) is used to measure displacement and strain.

The gauge distance 4 is used to measure or control the deformation of the small fatigue specimen 1 by an extensometer 3, and the small fatigue specimen 1 in which the LVDT 2 built in the equipment is installed. The position of is defined as the actual measuring distance (SG) (5).

In addition, the cyclic strain hardening occurring within the gauge distance 4 appears in the form of strain or displacement even at the actual measuring distance 5, and the relationship between the fatigue cycle and strain since the measuring position in the small fatigue specimen 1 is different. There is a difference.

Prior to describing the present invention, a low-cycle environmental fatigue test method using a finite element analysis method (FEM) will be described first, and then the present invention will be described to explain the suitability of the present invention.

2 is a view showing the results of the analysis of the axial length elongation by performing a nonlinear stress analysis in a low cycle environmental fatigue test method using a conventional FEM.

First, the low-cycle environmental fatigue test is controlled based on the deformation amount of the gage distance, and since the gage distance and the actual measurement distance are different, the correlation between the actual measured deformation amount and the gage distance deformation amount should be obtained and corrected.

Using the results of the high temperature tensile test with the same material and the experimental conditions as input data, the difference in deformation between the gauge distance and the measurement interval was calculated by the finite element analysis method as shown in FIG. 2 and the following [Table 3].

[Table 3] Difference in deformation between the gauge distance and the measurement interval calculated by the finite element method

Maximum Strain (ε _α ) Corrected Maximum Strain (ε _α ) 0.4 0.580 0.6 0.800 0.8 1.014

The miniature fatigue specimen (1) is modeled using the general finite element analysis program ABAQUS, and the distributed load is applied in the axial direction with one end of the small fatigue specimen (1) as the fixed end and the other side as the load end.

Then, the axial displacement at the fixed end is fixed, and the distribution load at the load end is gradually applied under non-linear conditions, and the change in the gage distance displacement and the distance between the grips are calculated.

In addition, the results of 315 ° C. high temperature tensile test of the same material are input and nonlinear stress analysis is performed to analyze the axial length extension of the small fatigue specimen 1, and the results are shown in FIG. 2.

Based on the analysis results of the axial length elongation of the small fatigue specimen (1) shows the correlation between the displacement of the gage distance (3) parallel part and the measurement part displacement, and if the regression (regression) of this, ] Can be obtained.

y = -8.0102x ³ + 3.7336x ² + 0.365519x + 0.00018653 [Equation 1]

here,

x is the displacement of the actual experimental section,

y is the displacement at the specimen mark distance parallel.

By using the above Equation 1, the calculated maximum strain amplitude can be obtained by obtaining the displacement value of the gage distance parallel part from the displacement value of the measurement part applied in the actual experiment.

[Table 4] below is a table showing the results of low cycle environmental fatigue experiments by applying the corrected maximum strain according to the above analysis results.

[Table 4] Low cycle environmental fatigue test by applying the corrected maximum strain

Strain (%) ε _α = 0.4 ε _α = 0.6 ε _α = 0.8 Strain rate (% / s) 0.04 0.04 0.04 Frequency (Hz) 0.025 0.0167 0.0125 Fatigue cycles 8,300 3,040 1,660

3 is a graph showing the change in the maximum tensile load value of each cycle of the fatigue load measured by applying the corrected maximum strain obtained through the low cycle environmental fatigue test method using a conventional FEM.

As shown in Figure 3, it can be seen that the maximum fatigue load at a certain strain increases due to the effect of cyclic strain hardening in the initial fatigue cycle number interval. Periodic strain hardening has a great effect on the initial cycle number of cycles, and proceeds stably in the section after 20% of fatigue life.

In addition, it can be seen that the hardening phenomenon decreases at the same number of repetition cycles as the strain size decreases to 0.8%, 0.6%, and 0.4%.

Figure 4 is a flow chart showing a low cycle environmental fatigue test method using a small fatigue specimen according to the present invention.

As shown in FIG. 4, the low-cycle environmental fatigue test method using the small fatigue test specimen according to the present invention includes a first step (ST1) of measuring the strain of the telescopic system 3 and the LVDT 2 through a high temperature test in air. ); And a second step (ST2) of grasping the displacement behavior of the LVDT (2) inside the autoglove in the PWR experimental environment through the strain values of the telescopic system (3) and the LVDT (2) measured after the first step. Characterized in that made.

FIG. 5 is a graph showing the strain correlation between the LVDT and the LVDT measured during the high temperature air experiment of FIG. 4, and FIG. This is a graph.

First, Table 5 below summarizes the results of high-temperature experiments in air performed in a low cycle environmental fatigue test method using a small fatigue specimen according to the present invention.

[Table 5] High temperature test result in air

Strain (%) ε _α = 0.4 ε _α = 0.6 ε _α = 0.8 Strain rate (% / s) 0.04 0.04 0.04 Frequency (Hz) 0.025 0.0167 0.0125 Fatigue cycles 10,230 5,214 2,654

As shown in FIG. 5, it shows a correlation between the strain system ST1 of the stretch system 3 and the LVDT 2 measured through a high temperature experiment in air, and shows a linear correlation between the experiments. Can be.

In addition, as shown in FIG. 6, the displacement behavior of the LVDT 2 measured in the high-temperature experiment in the air controlled by the telescopic system 3 is shown. Determine the displacement behavior of) (ST2).

In addition, the equations reflected in the experiment through the general control program were divided into [Equation 2] and [Equation 3]. This is because, differently, one waveform cannot adequately simulate the waveform.

First, [Equation 2] is an initial fatigue cycle number interval and extrapolated initial partial data having a large influence on the cyclic strain hardening, and is represented by a third order polynomial. will be.

The equation derived to take into account the cyclic strain hardening is the case of the maximum strain 0.6% experiment and are as follows.

^{y = 0.00000000000005x 3 - 0.00000000203561x 2 +} 0.00002694498772x [ Formula 2]

y = 0.000000000000000002x ³ -0.000000000001043333x ² + 0.000000172683793925x + 0.114354586 [Equation 3]

here,

x is a cycle,

y is the displacement of LVDT.

[Equation 2] and [Equation 3] can be applied to low-cycle environmental fatigue experiments to simulate the actual behavior of the LVDT displacement sensor (not shown).

In other words, the correlation between the gage distance (4) and the actual measurement distance (5) through high temperature experiments in the air under the same conditions except for the PWR environment is derived, and a formula considering the strain hardening phenomenon is derived.

Using this equation, the displacement of the LVDT 2 mounted on the actual measurement distance 5 is controlled according to a cycle.

The high temperature and high pressure test under the PWR condition is performed using the formula derived through the high temperature test. The reason is that the thermal expansion and fatigue properties are the same in the high temperature state in the air and in the hydrochemical state at the same temperature. This is because the fatigue life is assumed to be different according to the environment.

Table 6 below shows the results of the low cycle environmental fatigue test method using a small fatigue specimen according to the present invention.

[Table 6] Low cycle environmental fatigue test results

Strain (%) ε _α = 0.4 ε _α = 0.6 ε _α = 0.8 Strain rate (% / s) 0.04 0.04 0.04 Frequency (Hz) 0.025 0.0167 0.0125 Fatigue cycles 6,818 3,610 1,473

In the application of the induction formula, the experiment was paused after the section where [Equation 2] was applied, replaced with [Equation 3], and the experiment was resumed, and the load and deformation amount were maintained without significant change.

7 is a graph showing a change in the maximum tensile load value of each cycle of the fatigue load measured by applying the equation obtained through FIG.

As shown in FIG. 7, it can be seen that the maximum fatigue load at a certain strain is increased by the influence of cyclic strain hardening caused by plastic deformation in the initial fatigue cycle number interval.

In addition, the cyclic strain hardening effect greatly affects the initial cycle number of fatigue cycles and proceeds stably in the range of 20% ~ 40% of fatigue life, and the values of strain size, which are experimental conditions, are 0.8%, 0.6%, and 0.4%. It can be seen that the hardening phenomenon decreases at the same number of repetition cycles.

8 is a graph showing the displacement behavior of the LVDT measured after performing a low cycle environmental fatigue experiment with the maximum strain of 0.6% and applying the equation.

The formula for applying the cyclic strain hardening phenomenon in the second step of the low cycle environmental fatigue test method using the small fatigue specimen according to the present invention was derived individually for all experimental conditions. Displacement behavior of the LVDT measured after the low cycle environmental fatigue experiment using is shown in FIG. 8.

Therefore, it can be seen that the displacement behavior of the LVDT 2 measured in the high temperature experiment in air was successfully simulated.

Figure 9 is a graph showing the fatigue life data of CF8M material obtained through a low cycle environmental fatigue test method using a small fatigue specimen according to the present invention.

As shown in FIG. 9, fatigue life data (CF8M, PWR (CSH), 0.04) and conventional finite element analysis methods of CF8M material obtained through a low cycle environmental fatigue test method using a small fatigue specimen according to the present invention The fatigue life data used (CF8M, PWR (FEM), 0.04).

It can be seen that the low cycle environmental fatigue test method using the small fatigue specimen according to the present invention is similar to the results of the low cycle environmental fatigue test method using the conventional FEM.

Figure 10 shows a comparison of the low cycle environmental fatigue test method using a small fatigue specimen according to the present invention and the experimental results of the prior art.

As shown in Figure 10, the low cycle environmental fatigue test method using the FEM of the present invention and 2004 KEPRI (KEPRI) is almost the same experimental results.

That is, the low-cycle environmental fatigue test method using the present invention and the conventional FEM defines the correlation between the gauge distance and the actual measurement distance and compared with the results of experiments in the PWR environment, the low-cycle environmental fatigue test method using the conventional FEM It can be seen that slightly conservative than the fatigue life according to the present invention, but shows a similar fatigue life tendency.

From the above results, it can be confirmed that the conventional low-cycle environmental fatigue test method through FEM is an effective method, and also the low-cycle environmental fatigue test method using the small fatigue specimen of the present invention also has reliability.

Although Japan's data are close to the ASME fatigue design curve, the results of this experiment are close to the average air curve and the fatigue life is longer than that of Japan's data.

This is because the strain rate and the deterioration state are similar but the hydrochemical conditions are different. The Japanese data is maintained at 5ppb or less while the dissolved oxygen is 8ppb. Hydrochemical conditions are also different.

In general, under laboratory conditions, the flow rate is closer to a stagnant compared to the operating state of the power plant, and accordingly, influences such as conductivity and pH may be increased, and thus the difference in the experimental results may be large. Experimental conditions in the present invention are set based on the operating environment of domestic operating nuclear power plants.

The low-cycle environmental fatigue test method using the small fatigue specimen of the present invention is formulated by dividing the displacement behavior of the LVDT measuring the actual measurement distance of the small fatigue specimen into two stages according to the cycle and inputting it into the experimental device control program. The cyclic strain hardening characteristics that occur at the gauge lengths of small fatigue specimens can be summarized in the low cycle environmental fatigue test.

As a result of comparing the fatigue life characteristics using the correction method using the equation, the results obtained by the low-cycle environmental fatigue test method using the conventional FEM are similar to those of the existing data or lower than the ASME Mean Curve RT air curve.

In addition, although the reliability of the low cycle environmental fatigue test method using the conventional indirect FEM has been verified and verified through the present invention, it is realistic to apply the direct phenomenon to the experiment in consideration of the limitations of the indirect analysis method. I think it can be simulated.

The present invention will be used to develop additional environmental fatigue life assessment curves to secure data on various strain rates and deterioration specimens in the future, and to establish a technology base for the production of fatigue property values necessary for the environmental fatigue assessment of major domestic nuclear power plants. Will contribute.

1 is a view showing a small fatigue specimen used in a low cycle environmental fatigue test method using a small fatigue specimen according to the present invention.

2 is a view showing the results of the analysis of the axial length elongation by performing a nonlinear stress analysis in a low cycle environmental fatigue test method using a conventional FEM.

3 is a graph showing the change in the maximum tensile load value of each cycle of the fatigue load measured by applying the corrected maximum strain obtained through the low cycle environmental fatigue test method using a conventional FEM.

Figure 4 is a flow chart showing a low cycle environmental fatigue experiment method using a small fatigue specimen according to the present invention.

FIG. 5 is a graph showing the strain correlation between the telescopic system and the LVDT measured during the high temperature air experiment of FIG. 4.

FIG. 6 is a graph showing the displacement behavior of the LVDT measured in the air experiment controlled by the telescopic system in FIG. 4.

7 is a graph showing a change in the maximum tensile load value of each cycle of the fatigue load measured by applying the equation obtained through FIG.

8 is a graph showing the displacement behavior of the LVDT measured after performing a low cycle environmental fatigue experiment with the maximum strain of 0.6% and applying the equation.

Figure 9 is a graph showing the fatigue life data of CF8M material obtained through a low cycle environmental fatigue test method using a small fatigue specimen according to the present invention.

Figure 10 shows a comparison of the low cycle environmental fatigue test method using a small fatigue specimen according to the present invention and the experimental results of the prior art.

<Description of the symbols for the main parts of the drawings>

1 ---- Small Fatigue Specimen 2 ---- Linear Displacement Differential Transformer (LVDT)

3 ---- Telescopic System 4 ---- Real Gage

5 ---- Actual Gage

Claims (3)

A first step (ST1) of measuring the strain of the telescopic system 3 and the LVDT 2 through a high temperature experiment in air, and The second step (ST2) is to determine the displacement behavior of the LVDT (2) in the auto glave of the PWR experimental environment through the strain values of the telescopic system (3) and LVDT (2) measured after the first step, By using the data obtained by measuring the strains of the telescopic system 3 and the LVDT 2 in the first step ST21, the initial cycle number period in which the periodic strain hardening phenomenon is severe is simulated and the third polynomial as shown in [Equation 2] Low cycle environmental fatigue test method using a small fatigue specimen, characterized in that shown. ^{y = 0.00000000000005x 3 - 0.00000000203561x 2 +} 0.00002694498772x [ Formula 2] Where x is a cycle and y is the displacement of LVDT. delete The method of claim 1, Low cycle environmental fatigue experiment method using a small fatigue specimen, characterized in that to calculate the [Equation 3] by arranging the area in which the displacement behavior of the LVDT (2) of the second step is stabilized. y = 0.000000000000000002x ³ -0.000000000001043333x ² + 0.000000172683793925x + 0.114354586 [Equation 3] Where x is a cycle and y is the displacement of LVDT.

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