JP4672616B2 - Evaluation method of stress corrosion crack growth rate - Google Patents

Description

  The present invention relates to a method for evaluating a stress corrosion crack growth rate, which is evaluated using an appropriate stress intensity factor when evaluating the stress corrosion crack growth characteristics of an anisotropic material.

  Conventionally, when evaluating the development characteristics of stress corrosion cracking (hereinafter referred to as SCC) of a material, how the crack of the material develops when a test piece having a crack in advance is exposed to a corrosive environment and a load is applied. The data is obtained by observing the behavior.

  In addition, when evaluating the crack growth characteristics based on the obtained data, the correlation between the stress intensity factor (hereinafter referred to as the K value) and the corrosion data such as the corrosion potential and conductivity is evaluated, and under various environmental conditions. A reference diagram (master curve) of the crack growth rate of the material is created.

  In particular, when evaluating the SCC progress rate of materials used in boiling water reactors (BWRs), the test is mainly performed under a constant load using compact tensile (CT) specimens. Data collected from the SCC progress test were used.

  In addition, the above-mentioned reference diagram of the crack growth rate is often constructed by test results obtained under a load with a certain load applied, that is, under the K value increasing condition.

  Such reasons include the ease of grasping the test conditions in the load control type test. For example, in order to simulate the reactor water of a nuclear reactor and evaluate the SCC progress rate, it is necessary to store a test piece in high-temperature and high-pressure pure water. In this case, the test piece is housed in a heat-resistant / pressure-resistant pressure vessel such as an autoclave, which is provided with a connecting shaft take-out portion for load transmission.

  In such a test apparatus, it is difficult to confirm the amount of opening displacement of the test piece from outside the container using a test piece with a crack.

  On the other hand, regarding the load condition, the conventional test apparatus is suitable for controlling the load because the load cell provided on the connecting shaft for load transmission has a relatively high accuracy.

  By the way, when assuming defects in the reactor internal structure to be applied as a non-pressure-resistant member in a nuclear reactor such as a core shroud or shroud support that are actual structures, the residual stress due to welding becomes dominant on the stress surface. It has been pointed out that the K value increases as the SCC progresses or decreases as the SCC progresses depending on the residual stress distribution.

  In the recent SCC test, a WOL (Wedge Operating Lord) type test piece is used. In this test piece, a constant displacement is given to the crack opening in advance, and the K value can be reduced as the crack progresses.

  That is, FIG. 4 is a conceptual diagram showing a WOL type test apparatus.

  This WOL type test apparatus includes a load bolt 3 for adjusting the opening displacement of the crack 2 formed in the test piece 1, an opening displacement measuring sensor (clip gauge) 4 for electrically detecting the opening displacement value of the crack 2, A voltage monitor 5 that converts an electrical signal detected by the opening displacement measuring sensor 4 into opening displacement (size value) is provided.

  When measuring the opening displacement dimension value of the crack 2 for calculating the predicted K value based on the data measured from the WOL type test apparatus having such a configuration, as shown in FIG. After cutting into the formed shape and forming the crack 2, the load bolt 3 is attached (step 1).

Next, WOL type test apparatus, a load bolt 3, for example, tightening in the direction of rotation indicated by the arrow AR _1, with is widened toward the tip of the crack 2 in the arrow AR ₂ direction, the crack 2 opening When the opening displacement V at the position of the entrance Q or the load line PL is measured by the opening displacement measuring sensor 4 and the voltage monitor 5 (step 2), and the measured opening displacement V is obtained as a predicted K value shown below, an equation ( Substituting into 1), the initial value of the predicted K value is set (step 3).

  When the initial value of the predicted K value is set, the WOL type test method is immersed in a solution in an environment that causes SCC cracking, and the SCC crack is reduced under the decrease of the predicted K value while maintaining the load of the load bolt 3. The progress of the length is observed, and the SCC crack is evaluated (step 4).

On the other hand, when calculating the predicted K value from the test piece, it is obtained by FEM analysis or the like,
[Equation 1]
K = V × E / √a × f ₁ (a · W) / f ₂ (a · W) (1)
It is expressed as

  Here, V is the displacement of the load line of the test piece, E is the longitudinal elastic modulus of the test material, a is the crack length, and W is the thickness of the test piece.

  To summarize the above equation (1), the K value at the time of the test is expressed by the opening displacement amount, the longitudinal elastic modulus, and the crack length. Of these, assuming that the longitudinal elastic modulus takes a constant value depending on the material and the opening displacement is not relaxed from the time of setting, the K value is a function of the crack length a after all.

Here, it is known that f ₁ (a · W) / f ₂ (a · W) shows a decrease in the K value as the crack length a increases. You can see that it can be set.

For example, Patent Document 1 discloses a technique that can set a K value reduction condition using a WOL type test piece.
JP 2004-340571 A

  By the way, when the K value is set by using the above-mentioned opening displacement type SCC test piece, as shown in the above equation (1), only the longitudinal elastic modulus E is used for the deformation characteristics of the material. It is a parameter (factor) that represents.

  On the other hand, for materials with anisotropy in the metal structure, including cold rolling with weld metal, the load-displacement line (stress-strain) of the specimen depends on the specimen sampling direction and notch setting direction. Line) may be different.

  However, as confirmed from the above equation (1), the K value evaluation formula in the conventional WOL test does not consider the influence of the anisotropic material, and the prediction K of the material having the anisotropic property The error of value setting became large, and even when the SCC test was conducted, it was often far from the actual situation.

  The present invention has been made in consideration of such circumstances, and stress corrosion cracking that can evaluate the SCC crack growth rate of a material using an appropriate K value even if the material has anisotropy. The purpose is to provide a method for evaluating the progress rate.

In order to achieve the above-mentioned purpose, the stress corrosion cracking crack growth rate evaluation method according to the present invention gives a crack to an anisotropic test piece, applies a load from a loading means to the crack, and corrosive environment. In the method for evaluating the stress corrosion crack growth rate, which is housed underneath and evaluates the degree of cracking by determining the crack growth rate of the anisotropic test piece, the test load is 100% load on the test piece. load from crack length of the displacement to the load that low rate to a predetermined than - while obtaining a displacement line, load from said predetermined load to 100% load of the load - displacement line in extrapolation calculated, the load is determined by extrapolation - calculates the corresponding displacement of 100% load of the test load displacement line, the stress intensity after applying the calculated the displacement on the test piece 100% load of the test load Calculate coefficient A stress corrosion cracking evaluation method of crack growth rate, wherein Rukoto.

  The stress corrosion crack crack growth rate evaluation method according to the present invention can evaluate a stress corrosion crack crack growth rate with high accuracy even for a material having deformation anisotropy.

  Hereinafter, an embodiment of a method for evaluating a stress corrosion crack growth rate according to the present invention will be described with reference to the drawings and reference numerals attached to the drawings.

  The SCC tester applied to the stress corrosion cracking crack growth rate evaluation method according to the present invention shown in FIG. 1 includes a load bolt 12 for adjusting the opening displacement of the crack 11 formed in the test piece 10, and the opening of the crack 11. An opening displacement measuring sensor (clip gauge) 13 that electrically detects a displacement value and a voltage monitor 14 that converts an electric signal detected by the opening displacement measuring sensor 13 into opening displacement are provided.

  Next, the evaluation method of the stress corrosion crack growth rate according to the present invention will be described with reference to FIG. 2 using an SCC testing machine having such a configuration.

  FIG. 2 is a procedure block diagram showing a method for evaluating the stress corrosion crack growth rate according to the present invention.

  The evaluation method of the stress corrosion crack growth rate according to the present embodiment is as follows. First, the test piece 10 is processed and cut into a predetermined shape, the crack 11 is forcibly provided, and then the load bolt 12 is attached. (Step 11).

Next, the present embodiment, the load bolts 12, for example, tightening in the direction of rotation indicated by the arrow AR _1, thereby expanding towards the tip of the crack 11 a load line PL in the base point in the direction of arrow AR ₂ At the same time, the opening displacement V of the crack 11 is measured by the opening displacement measuring sensor 13 and the voltage monitor 14.

  Thereafter, the test piece 10 is loaded with a load bolt 12 and immersed in a solution simulated in reactor water.

  On the other hand, in the present embodiment, as shown in FIG. 3, the load-displacement line (stress-strain line) PV of the test piece 10 is taken using, for example, a fatigue tester (tensile tester), and a diagram is obtained. (Step 12).

In this case, the load-displacement line PV affects the material, but if the test load P1: (0.8 to 0.9) × P0 is exceeded, an error appears in the predicted K value shown below. Therefore, the load-displacement line P1V1 indicated by the broken line from the test load P1 to the 100% load P0 of the test load is obtained by extrapolation and created as an experimental approximate expression (step 13). Here, the test load P1 is determined in advance in a range that may be greatly separated by a test or the like, and is set to 0.8 to 0.9 of a 100 % load P0 of the test load in a normal material.

  The load-displacement line PV may be obtained by an interpolation method when obtaining between loads or between displacements.

As shown in FIG. 3, when the approximate equation line P ₁ V ₁ and the load-displacement line PV from the fatigue tester are created, the displacement V obtained from the SCC tester shown in FIG. 14) The load P shown in FIG. 3 is calculated from the set displacement V.

For example, when the measured value of displacement from the SCC testing machine is V ₀ , the 100% load of the test load is P ₀ .

As described above, when the load P is obtained from the load-displacement line PV shown in FIG. 3, in this embodiment, the K value is calculated from the following experimental formula (2) (step 15).
[Equation 2]
K = P × [√W × {F ₁ (a / W)}] / B (2)
Where P is the load, B is the thickness of the test piece, W is the length of the test piece, a is the length of the crack, and F ₁ (a / W) is a function of a / W depending on the shape of the test piece. is there.

  In addition, since the above formula (2) is irrelevant to the longitudinal elastic modulus E, it can be sufficiently applied to anisotropic materials.

As described above, in the present embodiment, the test load P ₁ equal to or lower than the load P _{0 of the} test piece having the forcibly applied crack 11 is defined as (0.8 to 0.9) P ₀ as P ₁ , for example. load takes the data of the load-displacement PV value of the up test load _{P 1} - as well as the diagram as a displacement line PV (step 11, 12), test load _P 1 _{((0.8~0.9) P 0} ) To the load P ₀ , the load-displacement line P ₁ V ₁ is obtained by extrapolation (step 13), and the load-displacement obtained from the load-displacement line P ₁ V ₁ obtained by the extrapolation method and experimental data is obtained. and a line PV, the displacement _{V 0} determined corresponding to a test load of 100% (step 14), obtains a load _{P 0} by applying the displacement _{V 0} to the test piece, a load _{P 0} determined by formula (2) Since the K value is calculated by giving (Step 5), use an appropriate K value even for materials with anisotropic properties. Evaluation of even higher SCC crack growth rate of accuracy Te can be performed.

The conceptual diagram which shows the stress corrosion cracking testing machine applied to the evaluation method of the stress corrosion cracking crack growth rate which concerns on this invention. The block diagram which shows the procedure of the evaluation method of the stress corrosion crack crack growth rate which concerns on this invention. The load-displacement diagram applied to the evaluation method of the stress corrosion crack growth rate according to the present invention. The conceptual diagram which shows the stress corrosion cracking testing machine applied to the evaluation method of the conventional stress corrosion cracking crack growth rate. The block diagram which shows the procedure of the evaluation method of the conventional stress corrosion cracking crack growth rate.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Test piece 2 Crack 3 Load bolt 4 Opening displacement measurement sensor 5 Voltage monitor 10 Test piece 11 Crack 12 Load bolt 13 Opening displacement measurement sensor 14 Voltage monitor

Claims (4)

A crack is given to an anisotropic specimen, and the load from the loading means is applied to the crack, and the specimen is accommodated in a corrosive environment, and the crack progress rate of the anisotropic specimen is obtained to evaluate the crack degree. In the stress corrosion cracking crack growth rate evaluation method, the load-displacement line is determined from the displacement of the crack length up to a predetermined load lower than the 100% load with the test load as 100% load in the test piece. Meanwhile, the load from said predetermined load to 100% load of the load - calculated displacement line in extrapolation, the load was determined by the extrapolation - corresponds to 100% load of the test load displacement line It calculates the displacement, calculated stress corrosion cracking evaluation method of crack growth rate, wherein the calculating the stress intensity factor of 100% load of displaced by applying to the test piece the test load. 2. The stress corrosion crack growth rate evaluation method according to claim 1, wherein the load-displacement line based on the predetermined load and crack length is obtained from a fatigue tester. 2. The stress corrosion crack growth rate evaluation according to claim 1, wherein the test piece placed in a corrosive environment is immersed in a solution simulated in reactor water while being loaded by means for loading. Method. 2. The stress corrosion crack crack growth rate evaluation method according to claim 1, wherein the stress intensity factor K is calculated from the following equation.
[Equation 1]
K = P × [√W × {F ₁ (a / W)}] / B
Here, P is the load, B is the thickness of the test piece, W is the length of the test piece, a is the length of the crack, and F ₁ (a / W) is a function of a / W depending on the shape of the test piece. is there.

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