JP2018173356A - Fracture toughness test method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method of fracture toughness test capable of accurately evaluating the fracture toughness using small size test pieces and a small number of samples.SOLUTION: Disclosed is a method of fracture toughness test for evaluating the fracture toughness of a material using a test piece. In the method, the displacement-load characteristics are acquired of a test piece using an instrumented Charpy impact tester, and based on the displacement-load characteristics, a dynamic fracture toughness value is calculated. After calculating dynamic reference temperature by applying the dynamic fracture toughness value to a dynamic fracture toughness master curve, and the same is converted into static reference temperature. On the basis of the converted static reference temperature, a static fracture toughness master curve is calculated, and a static fracture toughness value is obtained.SELECTED DRAWING: Figure 5

Description

  The present disclosure relates to a fracture toughness test method for evaluating the fracture toughness of a structural material in a large structure such as a reactor pressure vessel.

  In general, it is known that as the plate thickness increases, the toughness decreases, and in structural materials in large structures such as reactor pressure vessels, plastic deformation due to load loading exceeding the yield strength is not only ductile fracture, but also yield. It is required to have sufficient resistance against brittle fracture that may occur even at a load lower than the strength. A fracture toughness test is known as a material test for such a structural material.

  In the fracture toughness test, when a monotonously increasing load is applied using a test piece having a notch, the relationship of the opening displacement of the notch with respect to the load is evaluated. The parameters for evaluating fracture toughness include the load at the time of fracture and the opening displacement.However, when evaluating a structure, these parameters themselves cannot be used because they depend on the specifications of the specimen, and cracks are not possible. A stress intensity factor indicating the strength of the stress / strain field near the tip is used as the fracture toughness value.

  The fracture toughness test includes a static test and a dynamic test, but the static test has a large demand in practice. Static tests are classified into Klc test, CTOD test, and Jlc test according to the structural material specifications (refer to the standards and explanations of each test method for details of these static tests) and suitable for each. Specimens are used. In Patent Document 1, as an example of this type of fracture toughness test, there is a method for measuring the fracture toughness value of a metal material, in which a micro flat plate is cut out from a metal material to be measured to form a test piece, and the load on the test piece and It is disclosed that the fracture toughness value is obtained by applying to the master curve from the strain energy from the displacement of the test piece to the occurrence of unstable fracture.

JP 2007-155540 A

  In the fracture toughness test, since there are generally variations in test results, it is necessary to prepare many test pieces and to statistically evaluate the test results. In recent years, by adopting the master curve method as in Patent Document 1, it has become possible to reduce the number of tests compared to before, but in order to obtain sufficient accuracy, still a certain amount of tests It is necessary to secure the number.

  As an application field of this type of fracture toughness test, for example, there is a large structure such as a reactor pressure vessel. In the reactor pressure vessel, neutrons irradiated from the core contained in the reactor cause embrittlement of the vessel wall surface. In order to monitor the progress of such embrittlement, a test piece is placed inside the reactor pressure vessel. Enclose a test specimen capsule containing the sample, take it out during regular inspections, etc., and conduct a Charpy impact test, fracture toughness test, etc. (collectively referred to as a monitoring test).

  However, the core structure is tightly accommodated inside the reactor pressure vessel, and there is a severe restriction on the space in which the specimen capsule that accommodates the specimen can be installed. Therefore, it is difficult to secure a sufficiently large number of test pieces to be accommodated in the test piece capsule. In particular, the test pieces (CT test pieces and bending test pieces) required in the Klc test, the CTOD test, and the Jlc test, which are static tests, are relatively large and difficult to secure a sufficient number. Many Charpy impact specimens with relatively small sizes are stored.

  At least one embodiment of the present invention has been made in view of the above circumstances, and an object thereof is to provide a fracture toughness test method capable of accurately evaluating fracture toughness with a small number of samples using a small-sized test piece. And

(1) In order to solve the above-mentioned problem, the fracture toughness test method according to at least one embodiment of the present invention provides the fracture toughness of a material used for a reactor pressure vessel using a test piece having a notch on one side. A fracture toughness test method for evaluating, using an instrumented Charpy impact tester, obtaining a displacement-load characteristic for the test piece, and a dynamic fracture toughness value based on the displacement-load characteristic Calculating a dynamic reference temperature by applying the dynamic fracture toughness value to a dynamic fracture toughness master curve, converting the dynamic reference temperature to a static reference temperature, and Calculating a static fracture toughness master curve based on a static reference temperature; and determining a static fracture toughness value based on the static fracture toughness master curve.

  According to the above method (1), the displacement-load characteristic of the test piece is obtained in the Charpy impulse test which is a dynamic test, and the dynamic fracture toughness value is obtained. Then, the dynamic reference temperature is calculated by applying the obtained dynamic fracture toughness value to the master curve. The dynamic reference temperature is converted into a static reference temperature and then used to obtain a static fracture toughness curve that defines a static toughness value. A static fracture toughness value is obtained based on the static fracture toughness curve thus obtained. Thus, based on the measurement result by the Charpy impact test which is a dynamic test, the static fracture toughness value conventionally obtained by the static test can be evaluated.

(2) In some embodiments, in the method of (1), the dynamic fracture toughness value includes an elastic component calculated based on a brittle fracture load specified based on the displacement-load characteristics; It is calculated as the sum of the plastic component calculated based on the integral value from the crack initiation load to the load at the time of brittle fracture in the displacement-load characteristic.

  According to the above method (2), the dynamic fracture toughness value to be applied to the master curve can be easily calculated by adding the elastic component and the plastic component calculated based on the displacement-load characteristics.

(3) In some embodiments, in the method of (2) above, the crack generation load is determined as the yield impact load Fgy, the maximum impact load Fm, and the coefficient κ (0) specified based on the displacement-load characteristics. ≦ κ ≦ 1), the following formula Fint = κ × (Fgy + Fm)
Is calculated by

  According to the method (3) above, the crack initiation load used when calculating the plastic component of the dynamic fracture toughness value can be calculated based on the coefficient κ determined according to the specifications of the test piece.

(4) In some embodiments, in any one of the above methods (1) to (3), a characteristic function that defines a relationship between a load speed of the test piece and a reference temperature is obtained based on the dynamic reference temperature. The static reference temperature is calculated by inputting a static test load speed into the characteristic function.

  According to the above method (4), the result of the static test can be estimated based on the result of the dynamic test by converting the dynamic reference temperature into the static reference temperature.

(5) In some embodiments, in any one of the above methods (1) to (4), the test piece is a Charpy impact test piece housed in a capsule disposed in the reactor pressure vessel. is there.

  According to the method of (5) above, by using a relatively small test piece used for the Charpy impact test, it is possible to cope with a severe space in the reactor pressure vessel.

  According to at least one embodiment of the present invention, it is possible to provide a fracture toughness test method capable of accurately evaluating fracture toughness with a small number of samples using a small test piece.

It is sectional drawing which shows roughly the reactor pressure vessel by which the test piece used for the fracture toughness test method which concerns on at least 1 embodiment of this invention is arrange | positioned. It is principal part sectional drawing which shows the state which installed the test piece capsule which accommodated the test piece in the reactor pressure vessel of FIG. It is a schematic diagram which shows the test piece accommodated in the test piece capsule of FIG. It is a fracture surface view of the test piece after implementation of the Charpy impact test. It is a flowchart which shows the fracture toughness test method which concerns on at least 1 embodiment of this invention for every process. It is an example of the displacement-load characteristic acquired by step S2 of FIG. It is a graph which shows a dynamic fracture toughness master curve. It is a graph which shows the relationship between a load speed and reference temperature. It is a graph which shows the example of a measurement of the time history of the load which a test piece receives.

Hereinafter, some embodiments of the present invention will be described with reference to the accompanying drawings. However, the dimensions, materials, shapes, relative arrangements, etc. of the components described in the embodiments or shown in the drawings are not intended to limit the scope of the present invention, but are merely illustrative examples. Absent.
For example, expressions expressing relative or absolute arrangements such as “in a certain direction”, “along a certain direction”, “parallel”, “orthogonal”, “center”, “concentric” or “coaxial” are strictly In addition to such an arrangement, it is also possible to represent a state of relative displacement with an angle or a distance such that tolerance or the same function can be obtained.
In addition, for example, expressions representing shapes such as quadrangular shapes and cylindrical shapes not only represent shapes such as quadrangular shapes and cylindrical shapes in a strict geometric sense, but also within the range where the same effect can be obtained. A shape including a chamfered portion or the like is also expressed.
On the other hand, the expressions “comprising”, “comprising”, “comprising”, “including”, or “having” one constituent element are not exclusive expressions for excluding the existence of the other constituent elements.

  FIG. 1 is a cross-sectional view schematically showing a reactor pressure vessel in which a test piece used in a fracture toughness test method according to at least one embodiment of the present invention is arranged, and FIG. 2 is a view inside the reactor pressure vessel of FIG. It is principal part sectional drawing which shows the state which installed the test piece capsule which accommodated the test piece in.

  As shown in FIG. 1, the reactor pressure vessel 1 is a structure including a vessel main body 2 formed in a bottomed cylindrical shape and a lid 3 that is detachably attached to an upper opening of the vessel main body 2. is there. A coolant inlet nozzle 4 and a coolant outlet nozzle 5 are provided on the upper side wall of the container body 2. A cylindrical core tank 6 is supported inside the container body 2 from a shelf near the upper opening, and a downcomer 7 is defined between the core tank 6 and the container body 2. .

A lower core support plate 8 and a lower core plate 9 are horizontally provided below the core tank 6. The core tank 6 is supported in the horizontal direction from the container body 2 at the position of the lower core support plate 8. A number of fuel assemblies (not shown) are disposed adjacent to each other above the lower core plate 9 to form a core 10. The upper end of the fuel assembly is pressed by the upper core plate 11, and an upper plenum 12 is formed above it. The upper plenum 12 communicates with the coolant outlet nozzle 5 through the nozzle flange of the core 6.
The bottom of the container body 2 is formed as a hemispherical end plate 13, and a lower plenum 14 is formed between the end plate 13 and the lower core support plate 8.

  The flow of the coolant C in the reactor pressure vessel 1 having such a structure will be described. First, the coolant C flows through a pipe by a coolant pump (not shown), passes through the coolant inlet nozzle 4 and the internal downcomer. Flows into 7. Then, the coolant C flows down in the downcomer 7 as indicated by an arrow, reverses in the lower plenum 14, flows through the lower core support plate 8 and the lower core plate 9, and enters the core 10. In the core 10, while the coolant C flows upward along the outside of the fuel rod of the fuel assembly, it absorbs the nuclear reaction heat and rises in temperature. Thereafter, the coolant C rises in the reactor core 10 and reaches the upper plenum 12, and then the coolant C turns in the horizontal direction and flows out of the coolant outlet nozzle 5 toward the steam generator.

  As shown in FIG. 2, a sealed long test piece capsule 15 is arranged in the reactor pressure vessel 1. In the test piece capsule 15, the test pieces 16 used for monitoring the deterioration state of the material used for the reactor pressure vessel 1 by neutrons over time and evaluating the embrittlement amount are enclosed in a plurality of rows. Yes. As a result, the test piece 16 is placed in the same environment as the reactor pressure vessel 1 (strictly speaking, since the test piece 16 is arranged inside the inner wall of the reactor pressure vessel 1, It is placed in a more severe environment than the furnace pressure vessel 1).

  FIG. 3 is a schematic view showing the test piece 16 accommodated in the test piece capsule 15 of FIG. The test piece 16 is a Charpy impact test piece, and is a notch formed by a 45-degree V-shaped groove (V notch) having a depth t at the center of one side surface of a B × W square bar-shaped test piece main body 16a having a length L. 23 is provided. (Note that the dimensions of the test piece used in the monitoring test are 10 mm × 10 mm rods with a length of 53.7 mm and a notch depth of 2 mm.)

The test piece 16 is attached to the anvil part of an instrumented Charpy impact tester (not shown), and a hammer (not shown) is attached to the held test piece 16 from the back side where the notch part 23 is not provided. The test piece 16 is broken by the collision, and the Charpy impact test is performed. The instrumented Charpy tester can measure mechanical parameters such as load, displacement, and time along with the absorbed energy value during testing, and these measurement results can be output to an external computer as an electrical signal. ing.
The details of the test carried out by the instrumented Charpy impact tester are based on “JIS B7755 (Charpy pendulum impact test for metal—instrumentation device)”, and the detailed description thereof is omitted here.

  FIG. 4 is a broken sectional view of the test piece 16 after the Charpy impact test. The fracture surface of the test piece 16 includes a ductile fracture region 18 in which ductile fracture with large absorbed energy and plastic deformation occurred, and a brittle fracture region 20 in which brittle fracture with small absorbed energy and small contribution to fracture of plastic deformation occurred. including. The ratio between the ductile fracture region 18 and the brittle fracture region 20 on the fracture surface depends on the test temperature. Generally, the ductile fracture region 18 mainly occurs at the transition temperature DBTT or higher, and mainly the brittle fracture at the transition temperature DBTT or lower. Region 20 occurs.

  In the fracture toughness test method according to at least one embodiment of the present invention, as described below, a static fracture is performed using the measurement result of dynamic characteristics using such an instrumented Charpy impulse tester. Toughness can be accurately evaluated. FIG. 5 is a flowchart showing a fracture toughness test method according to at least one embodiment of the present invention for each step.

  First, a Charpy impact test is performed on the test piece 16 using an instrumented Charpy tester as described above (Step S1), and displacement-load characteristics are acquired (Step S2). 6 is an example of the displacement-load characteristic acquired in step S2 of FIG. 5. From the time when the hammer of the instrumented Charpy tester starts to contact the test piece 16, the test piece 16 with respect to the displacement of the hammer. It shows the transition of the load that is received. When the displacement of the hammer increases, first, in the elastic deformation region (F <gy), the load changes substantially linearly with respect to the displacement. When the hammer displacement further increases and the plastic deformation region (Fgy ≦ F <Fui) is reached, the load changes nonlinearly with respect to the displacement, and plastic deformation occurs. When the hammer is further displaced and the load reaches the load Fui at the time of occurrence of brittle fracture, the test piece 16 breaks brittlely and breaks.

Subsequently, a dynamic fracture toughness value KJd is calculated based on the displacement-load characteristic acquired in step S2 (step S3). Here, the dynamic fracture toughness value KJd is calculated using the following equation KJd = {E (Je + Jp) / (1-v ² )} ^0.5 using the J-integral elastic component Je and the plastic component Jp ^0.5 (1)
Is required. Here, E is Young's modulus and v is Poisson's ratio. That is, the dynamic fracture toughness value is calculated as the sum of the elastic component Je of the J integral and the plastic component Jp.

The elastic component Je is calculated based on the load Fui at the time of occurrence of brittle fracture specified from the displacement-load characteristics obtained in step S2 (for details, refer to “JEAC4216 (determining the fracture toughness reference temperature T0 of the electrical engineering stipulated nuclear steel). (See Test methods for
Ke = [Fui · S / (B × W ^1.5 )] f (a / W) (2)
Je = (1-v ² ) Ke ² / E (3)
(The load Fui at the time of occurrence of brittle fracture is obtained as a load causing brittle fracture as described above with reference to FIG. 4, and the length a is measured on the fracture surface of the test piece 16 shown in FIG. Can be obtained). B is the thickness in the direction perpendicular to the crack propagation direction of the test piece, and W is the thickness of the test piece in the direction of crack propagation.

  On the other hand, the plastic component Jp is calculated by evaluating the area of the hatching region R shown in the displacement-load characteristic of FIG. The hatched area R is defined by boundaries R1 and R2. The boundary R1 is defined as a line segment that is parallel to the inclination in the elastic deformation region (in the elastic deformation region, the load-displacement characteristic behaves substantially linearly as described above) and passes through the crack generation load Fint. . The boundary R2 is defined as a line segment that is parallel to the slope in the elastic deformation region (in the elastic deformation region, the load-displacement characteristic behaves substantially linearly as described above) and passes through the load Fui at the time of occurrence of brittle fracture. The

The crack generation load Fint is a load at which a crack starts to occur in the test piece 16, and the following formula Fint = κ × (Fgy + Fm) (4)
It is prescribed by. Here, Fgy is a yielding impact load, Fm is a maximum impact load, and κ is an arbitrary coefficient (0 ≦ κ ≦ 1). FIG. 6 shows a case where κ = ½ as an example.

  When the elastic component Je and the plastic component Jp of the J integral are calculated in this way, the dynamic fracture toughness value KJd is obtained by substituting into the above equation (1).

Subsequently, the dynamic reference temperature T0d is calculated by applying the dynamic fracture toughness value KJd to the dynamic fracture toughness master curve (step S4). The dynamic fracture toughness master curve is a characteristic curve that defines the temperature dependence of the median value of dynamic fracture toughness, and its shape is invariant regardless of the type of structural material. Specifically, the dynamic fracture toughness master curve is a function having the dynamic fracture toughness value KJd and the measurement temperature T as variables, and using the dynamic reference temperature T0d, the following equation Kjd = 30 + 70exp [0.019 (T + T0d) ] (5)
It is represented by FIG. 7 is a graph showing a dynamic fracture toughness master curve, in which the measurement temperature T of the test piece 16 and the measurement points corresponding to the dynamic fracture toughness value KJd calculated in step S3 are plotted. In step S4, the processing according to the Weibull statistics is performed on the equation (5) based on the measured temperature T of the test piece 16 and the dynamic fracture toughness value KJd calculated in step S3 (for details, see JEAC4216 and the like). ), The dynamic reference temperature T0d is obtained.

Subsequently, the dynamic reference temperature T0d calculated in step S4 is converted into a static reference temperature T0s (step S5). Here, according to the knowledge of the present inventor, the relationship between the load speed dK / dt and the reference temperature T0 is expressed by the following equation T0 = β · dK / dt + γ using coefficients β and γ (6)
It is represented by The coefficient β does not change depending on the type of the structural material, and is a fixed value obtained from an empirical rule.

  FIG. 8 is a graph showing the relationship between the load speed dK / dt and the reference temperature T0, and the results of the Charpy impact test performed in step S1 are plotted as measurement points. The measurement point corresponds to the load speed dK / dt actually generated on the test piece 16 in the Charpy impact test. The load speed dK / dt at the measurement point and the dynamic reference temperature T0d calculated in step S5 are (6). By substituting into the equation, the coefficient γ is determined. When the relationship between the load speed dK / dt and the reference temperature T0 is obtained in this way, as shown in FIG. 8, a static reference temperature T0s corresponding to a relatively small load speed corresponding to the static characteristics is obtained. .

  The load speed dK / dt at the measurement point may be obtained by directly measuring with a sensor or the like. However, if measurement is difficult, for example, the initial velocity calculated from the time history of the load on the test piece 16 may be used. It may be calculated using a load speed dF / dt. FIG. 9 is a graph showing a measurement example of the time history of the load received by the test piece 16. As shown in FIG. 9, the load F received by the test piece 16 exhibits a behavior that rises substantially linearly at the initial timing from the start of the test until the load F reaches the impact load Fgy at yield. The initial load speed dF / dt indicates a linear load change inclination until the yield impact load Fgy is reached in the time history of such a load.

By substituting dF / dt obtained from the load time history as Fui in equation (2), dK / dt is expressed by the following equation: dK / dt = [(dF / dt) · S / (B × W ^1.5 ] F (a / W) (7)
Is obtained.

Subsequently, the following expression Kjc = 30 + 70exp [0.019 (T + T0s)] obtained by substituting the static reference temperature T0s obtained in step S5 into the expression (5) (8)
Thus, a static fracture toughness master curve is obtained, and the static fracture toughness is estimated (step S6).

  As described above, according to the above-described embodiment, the dynamic fracture toughness value is obtained from the displacement-load characteristic obtained using the Charpy specimen used in the Charpy impulse test which is a dynamic test. A static fracture toughness curve can be estimated by converting a dynamic reference temperature obtained by the master curve method from a dynamic fracture toughness value to a static reference temperature. Thus, based on the measurement result by the Charpy impact test which is a dynamic test, the static fracture toughness value conventionally obtained by the static test can be evaluated.

  The Charpy test piece is smaller in size than the test pieces (CT test piece and bending test piece) required for the Klc test, CTOD test and Jlc test, which are static tests. It can be secured. In particular, the Charpy test piece does not need to form a pre-crack in the test piece main body unlike the CT test piece, so that the work load and cost can be reduced.

  Furthermore, as described above, in this method, the static fracture toughness characteristic can be estimated based on the measurement result of at least one Charpy test, so that good accuracy can be obtained with a small number of samples. In this way, it is possible to provide a fracture toughness test method capable of accurately evaluating fracture toughness with a small number of samples using a small-sized test piece.

  At least one embodiment of the present invention can be used in a fracture toughness test method for evaluating the fracture toughness of a structural material in a large structure such as a reactor pressure vessel.

DESCRIPTION OF SYMBOLS 1 Reactor pressure vessel 2 Vessel body 3 Lid 4 Coolant inlet nozzle 5 Coolant outlet nozzle 6 Core tank 7 Downcomer 8 Lower core support plate 9 Lower core plate 10 Core 11 Upper core plate 12 Upper plenum 13 End plate 14 Lower plenum 15 Test piece capsule 16 Test piece 18 Ductile fracture region 20 Brittle fracture region 23 Notch

Claims (5)

A fracture toughness test method for evaluating the fracture toughness of a material used for a reactor pressure vessel using a test piece having a notch on one side,
Using an instrumented Charpy impact tester to obtain displacement-load characteristics for the specimen;
Calculating a dynamic fracture toughness value based on the displacement-load characteristics;
Calculating a dynamic reference temperature by fitting the dynamic fracture toughness value to a dynamic fracture toughness master curve;
Converting the dynamic reference temperature to a static reference temperature;
Calculating a static fracture toughness master curve based on the static reference temperature;
Obtaining a static fracture toughness value based on the static fracture toughness master curve;
A fracture toughness test method comprising:   The dynamic fracture toughness value includes an elastic component calculated based on a load at brittle fracture specified based on the displacement-load characteristic, and a load at the time of brittle fracture from a crack generation load of the displacement-load characteristic. The fracture toughness test method according to claim 1, wherein the fracture toughness test method is calculated as the sum of the plastic component calculated based on the integrated value up to. The crack generation load is calculated by using the following expression Fint = κ × (Fgy + Fm) using the yield impact load Fgy, the maximum impact load Fm, and the coefficient κ (0 ≦ κ ≦ 1) specified based on the displacement-load characteristics. )
The fracture toughness test method according to claim 2 calculated by: Based on the dynamic reference temperature, calculate a characteristic function that defines the relationship between the load speed of the test piece and the reference temperature,
The fracture toughness test method according to any one of claims 1 to 3, wherein the static reference temperature is calculated by inputting a load speed of a static test into the characteristic function. The fracture toughness test method according to any one of claims 1 to 4, wherein the test piece is a Charpy impact test piece accommodated in a capsule disposed in the reactor pressure vessel.

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