JP2010236930A - Method of evaluating brittle crack propagation stop property - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method of evaluating brittle crack propagation stop properties capable of evaluating brittle crack propagation stop properties of materials under an extremely low-temperature environment while reducing test costs. <P>SOLUTION: In the method of evaluating brittle crack propagation stop properties of materials by generating cracks in a test piece by executing a tensile test to the test piece, the test piece comprises a sample material made of a material to be evaluated, a brittle material that is welded to one end side of the sample material in a direction orthogonal to a tension direction in the tensile test and has toughness smaller than that of the sample material, and an auxiliary material welded to the other end side of the sample material in the orthogonal direction. The cracks occur forcibly in the brittle material, and load stress in the tensile test is set such that the forcibly generated cracks are stopped at the sample material. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a method for evaluating brittle crack propagation stopping characteristics of a steel material or the like.

  Steel materials used in cryogenic storage tanks that store liquefied natural gas (LNG) and the like are required to have excellent fracture toughness in terms of ensuring safety. Here, fracture toughness includes resistance to brittle fracture that occurs based on a predetermined initial defect and resistance to crack propagation when brittle fracture occurs. These two tolerances are fundamentally independent, although related to each other. For this reason, just because one of the required resistances is satisfied, the other resistance is not necessarily satisfied, and each resistance needs to be evaluated.

  Among the above two resistances, in the evaluation of resistance to crack propagation (hereinafter referred to as brittle crack propagation stop characteristic), it is necessary to indicate numerically that the steel material can stop the predetermined crack propagation. . Usually, this brittle crack propagation stop characteristic is expressed by a toughness value Kca using a stress intensity factor (K value) as a parameter.

  The toughness value Kca is generally evaluated by a temperature gradient type ESSO test, a temperature gradient type double tensile test, or the like (see, for example, Non-Patent Document 1). Hereinafter, a temperature gradient type ESSO test will be briefly described as an example of an evaluation method.

  FIG. 1 is a diagram illustrating an example of a temperature gradient type ESSO test. As shown in FIG. 1, in the temperature gradient type ESSO test, a jig 11 is attached to both sides of a specimen 10 (for example, a large specimen having a width of about 500 mm), and a notch 10a is formed at the upper end of the specimen 10. Is formed. Moreover, a predetermined temperature gradient is given to the test material 10 by cooling the upper part of the test material 10 and heating the lower part. In this state, a predetermined load stress is applied to the specimen 10 by pulling the jig 11, and a brittle crack is forcibly generated in the specimen 10 by driving the wedge 12 into the notch 10a.

  Here, the toughness value Kca (crack propagation stop characteristic) of the material has a large temperature dependence, and becomes higher as the temperature of the material becomes higher and lower as the temperature of the material becomes lower. The temperature gradient type ESSO test is a test utilizing the characteristics of such a material. That is, in the above temperature gradient type ESSO test, the toughness value Kca is low on the upper side of the specimen 10, but the toughness value Kca is high on the lower side. Therefore, the crack does not stop on the upper side of the specimen 10 because the crack propagation driving force (stress intensity factor) is larger than the toughness value Kca (crack propagation stopping characteristic) of the specimen 10, but the crack does not stop. As the crack propagates to the lower side of the specimen 10, the difference between the toughness value Kca and the crack propagation driving force gradually decreases. When the toughness value Kca and the crack propagation driving force coincide with each other, the crack stops. Using such a relationship, in the temperature gradient type ESSO test, the toughness value Kca is obtained based on the crack stop position, the temperature of the specimen 10 at that position, the tensile stress of the specimen 10, and the like. be able to.

  By the way, since the temperature of LNG stored in the LNG storage tank is extremely low as about −165 ° C., the LNG storage tank has a material having high toughness even in a cryogenic environment such as 9% Ni steel. ("%" Means "mass%" unless otherwise specified). When a material having such a high toughness is evaluated by a temperature gradient type ESSO test, the temperature of the material must be sufficiently lowered in order to generate a brittle crack. However, in a tough material such as 9% Ni steel, a brittle crack is generated even if the temperature is lowered to about -196 ° C. (a temperature that can be lowered using liquid nitrogen). It's not easy. Moreover, in order to further lower the temperature of the material, liquid hydrogen or the like must be used, which is problematic in terms of safety. Such a problem also occurs in the temperature gradient type double tensile test.

  Therefore, a hybrid ESSO test has been proposed as a test for evaluating the toughness value Kca of such a high toughness material. FIG. 2 is a diagram showing an example of a mixed molding ESSO test.

  As shown in FIG. 2, in the hybrid ESSO test, a hybrid ESSO test specimen 100 (hereinafter simply referred to as “test specimen 100”) in which a low toughness embrittlement material 13 is welded to the upper side of the specimen 10. .) Is used. In the test piece 100, a notch 13 a is formed at the upper end portion of the embrittlement material 13. The temperature of the test material 10 and the embrittlement material 13 is kept constant. In this state, a predetermined load stress is applied to the test piece 100 by pulling the jig 11, and a brittle crack is forcibly generated in the embrittled material 13 by driving the wedge 12 into the notch 13a.

  Here, the propagation driving force of the crack increases as the crack length increases. As described above, in the mixed molding ESSO test, a crack is first generated in the embrittled material 13, so that the crack propagation driving force must be sufficiently increased before the crack reaches the specimen 10. Can do. Thereby, even when the specimen 10 is a high toughness material, a crack generated in the embrittlement material 13 can be propagated to the specimen 10.

"Fracture Mechanics and Material Strength Lecture 8 Brittle Fracture 2-Fracture Toughness Test"

  However, the above-described mixed molding ESSO test also has the following problems. That is, in the mixed molding ESSO test, since the temperature of the specimen 10 is kept constant, the toughness value Kca (crack propagation stop characteristic) of the specimen 10 is constant. Here, the propagation of a crack from the brittle material 13 to the specimen 10 means that the crack propagation driving force is larger than the toughness value Kca of the specimen 10. In this case, since the toughness value Kca of the specimen 10 is constant, the crack cannot be stopped in the specimen 10. The crack propagation driving force can be adjusted by adjusting the load stress (tensile stress) acting on the specimen 10, but the crack propagation driving force is higher than the toughness value Kca of the specimen 10. When it becomes smaller, the crack does not propagate from the brittle material 13 to the test material 10. Therefore, in order to evaluate the toughness value Kca in detail by the hybrid ESSO test, a large number of specimens 10 are prepared, and the test is performed by changing the load stress (tensile stress). When the propagation force of the crack when propagating to the specimen 10 (Go), the boundary point between the propagation driving force of the crack when the crack does not propagate to the specimen 10 (No-Go) must be found. In this case, since the test must be performed many times, the test cost becomes high.

  The present invention has been made in view of the above problems, and is capable of evaluating brittle crack propagation stopping characteristics of a material in a cryogenic environment while reducing the test cost. The purpose is to provide an evaluation method.

  The present inventors have made various studies in order to provide an evaluation method that makes it possible to evaluate the brittle crack propagation stopping property of a high toughness material even in a cryogenic environment. As a result, the following findings (a) to (d) were obtained.

  (A) In order to generate a crack in a specimen made of a high toughness material at an extremely low temperature, a brittle material having a lower toughness than the specimen is welded to the specimen, and the crack is caused in the brittle material. May be forcibly generated.

  (B) In order to reduce the test cost in the evaluation of the brittle crack propagation stop property of the test material, that is, in order to evaluate the brittle crack propagation stop property with a small number of tests, crack propagation in the test material. As long as it can be stopped. For this purpose, it is necessary to reduce the crack propagation driving force in the specimen. In order to reduce the crack propagation driving force in the specimen, it is necessary to generate a residual stress (compressive stress) in the compression direction in the specimen. In order to generate a sufficient compressive stress in the test material, it is necessary to weld the embrittlement material and the auxiliary material so as to sandwich the test material.

  (C) It can be considered that the toughness value of the specimen is equal to the crack propagation driving force at the position where the crack stops. Therefore, based on the residual stress of the specimen and the load stress of the tensile test, the propagation driving force of the crack at an arbitrary position of the specimen is calculated, and the crack at the position where the crack stops is calculated from the calculation result. The propagation driving force can be obtained, and the obtained propagation driving force can be used as the toughness value of the specimen.

  (D) In order to reliably reduce cracks in the specimen, it is necessary to appropriately adjust the load stress in the tensile test. Further, in order to reliably propagate the crack from the embrittled material to the test material, it is necessary to appropriately select a welding material for joining the embrittled material and the test material. Further, in order to generate an appropriate compressive stress in the test material, it is necessary to appropriately adjust the welding position between the embrittlement material and the test material and the welding position between the test material and the auxiliary material.

  This invention is made | formed based on said knowledge, The summary exists in the brittle crack propagation stop evaluation method of the following (1)-(6).

  (1) An evaluation method for evaluating a brittle crack propagation stopping property of a material by generating a crack in the test piece by performing a tensile test on the test piece, and the test piece is made of an evaluation target material. The test material, an embrittled material that is welded to one end side of the test material in a direction orthogonal to the tensile direction in the tensile test and has a lower toughness than the test material, and the test material in the orthogonal direction An auxiliary material welded to the other end side, and the crack is forcibly generated in the embrittled material, and the load stress in the tensile test is the force generated by the crack. A brittle crack propagation stopping property evaluation method characterized by being set so as to stop at a point.

  (2) The crack propagation driving force at an arbitrary position of the test piece is calculated by numerical calculation based on the residual stress and the load stress of the test piece, and the crack at the position where the crack stops is calculated from the calculation result. The brittle crack propagation stop property evaluation method according to (1) above, wherein a crack propagation driving force is obtained and the obtained propagation driving force is used as a toughness value of the specimen.

  (3) The embrittlement material and the auxiliary material are welded to the test material so that the crack propagation driving force decreases as the crack grows in the test material (1) ) Or (2), the brittle crack propagation stopping property evaluation method.

  (4) The embrittlement material and the test material are welded by a Ni-based welding material containing 1.5% to 9% Ni by mass%. 3) The brittle crack propagation stop property evaluation method according to any one of 3).

  (5) The brittle crack propagation stop property evaluation method according to (1) to (4) above, wherein the load stress in the tensile test is set so as to satisfy the following formula (1).

σ _g <(0.24σ _y −6.1) / (0.000053σ _y +0.295) (1)
In the above formula (1), σ _g represents the load stress, and σ _y represents the yield stress at room temperature of the welding material joining the brittle material and the test material.

  (6) The width of the test piece is 400 mm to 600 mm, and the joint between the embrittlement material and the test material is located within a range of 130 mm to 170 mm from the position where the crack is forcibly generated. In any one of the above (1) to (5), the joint between the test material and the auxiliary material is located in a range of 300 mm to 350 mm from a position where the crack is forcibly generated. The brittle crack propagation stop property evaluation method according to crab.

  According to the brittle crack propagation stop property evaluation method according to the present invention, it is possible to evaluate the brittle crack propagation stop property of a material in a cryogenic environment while reducing the test cost.

It is a figure which shows an example of a temperature gradient type | mold ESSO test. It is a figure which shows an example of a mixed molding ESSO test. It is a figure which shows the test piece used in the brittle crack propagation stop characteristic evaluation method. It is a figure which shows the tension test in the brittle crack propagation stop characteristic evaluation method. It is a graph which shows an example of the residual stress of the test piece shown in FIG. It is a figure which shows the propagation driving force of the crack calculated by FEM analysis. It is a graph which shows an example of the residual stress of the test piece shown in FIG. It is a figure which shows the propagation driving force of the crack calculated by FEM analysis. It is the graph which showed the relationship between the load stress in a tensile test, and the propagation driving force of the crack in a test material. It is the graph which showed the relationship between the load stress in a tensile test, and the propagation driving force of the crack in a test material. It is a figure which shows an example of the test piece used when implementing a double tension test. 3 is a graph showing a crack propagation driving force in the test piece of Example 1. FIG. 6 is a graph showing a crack propagation driving force in the test piece of Example 2.

  Hereinafter, a brittle crack propagation stop property evaluation method according to an embodiment of the present invention will be described with reference to the drawings.

1. Outline of Brittle Crack Propagation Stop Property Evaluation Method First, an outline of a brittle crack propagation stop property evaluation method will be described. FIG. 3 is a diagram showing a test piece used in the brittle crack propagation stop property evaluation method according to the present embodiment. FIG. 4 is a diagram showing a tensile test in the brittle crack propagation stop property evaluation method according to the present embodiment.

  As shown in FIG. 3, the test piece 20 used in the present embodiment has a configuration in which an embrittlement material 13 is welded to the upper side of the test material 10 and an auxiliary material 14 is welded to the lower side of the test material 10. Have A notch 13 a is formed at the center of the upper end portion of the embrittlement material 13.

  As the embrittlement material 13, a material having lower toughness than the test material 10 is used. As the embrittlement material 13, for example, a material that is furnace-cooled after reheating 9% Ni steel to 700 ° C. can be used. Any material can be used as the auxiliary material 14, and the same material as the sample material 10 may be used. As the welding material 15 for joining the test material 10 and the embrittlement material 13, it is preferable to use a material having a high yield stress. For example, a 3.5% Ni-based welding material can be used. When a Ni-based welding material is used as the welding material 15, the Ni content is preferably about 1.5% to 9%. As the welding material 16 for joining the test material 10 and the auxiliary material 14, any welding material can be used, and the same welding material as the welding material 15 may be used. As the welding materials 15 and 16, a material having a larger thermal expansion coefficient than the embrittlement material 13, the test material 10, and the auxiliary material 14 is used.

  As a welding method of the test piece 20, any welding method can be used. For example, a covered arc welding method (SMAW) or a submerged arc welding method (SAW) can be used. The amount of heat input during welding is preferably determined according to the toughness of the welding materials 15 and 16, but if the amount of heat input becomes too large, the deterioration of toughness becomes large, so normally 0.6 to 3.0 kJ / mm. It is preferable to perform welding in the range of.

  The width W of the test piece 20 is set to 400 mm to 600 mm, for example. When the width W of the test piece 20 is set to 400 mm to 600 mm and the length D in the tensile direction is set to 500 mm, the length d1 between the upper end of the embrittlement material 13 and the welding material 15 is For example, the length d2 between the upper end of the embrittlement material 13 and the welding material 16 is set to 300 mm to 350 mm, for example.

  As shown in FIG. 4, in the brittle crack propagation stop property evaluation method according to the present embodiment, a hybrid ESSO test is performed using the test piece 20. Specifically, the jig 11 is attached to both sides of the test piece 20, and a predetermined load stress (tensile stress) is applied to the test piece 20 by pulling the jig 11, and embrittlement is performed by driving the wedge 12 into the notch 13a. A brittle crack is forcibly generated in the material 13. Note that the temperature of the test piece 20 is maintained at a constant value (for example, −196 ° C.).

  Here, in the present embodiment, since the embrittlement material 13 and the auxiliary material 14 are welded to the upper side and the lower side of the test material 10 as described in FIG. Crack propagation can be stopped in the specimen 10. Hereinafter, the reason will be described in detail.

  FIG. 5 is a graph showing an example of the residual stress of the test piece 20 (FIG. 3). In FIG. 5, the stress in the direction indicated by the arrow X in FIG. 3 is shown, the tensile stress is shown as a positive value, and the compressive stress is shown as a negative value. The values shown in FIG. 5 are 500 mm for width W (see FIG. 3) and length D (see FIG. 3), 150 mm for length d1 (see FIG. 3), and 330 mm for length d2 (see FIG. 3). This is an actual measurement value of a residual stress of a certain test piece 20.

  As shown in FIG. 5, the residual stress of the test piece 20 becomes tensile stress at a portion where the distance from the upper end of the test piece 20 is about 150 mm and about 330 mm, and becomes compressive stress at the other portions. Yes. As a result, tensile stress was generated in the welding material 15 (FIG. 3) and the welding material 16 (FIG. 3), and the embrittlement material 13 (FIG. 3), the test material 10 (FIG. 3), and the auxiliary material 14 (FIG. 3). ) Indicates that compressive stress is generated.

  The reason why the residual stress of the test piece 20 becomes a value as shown in FIG. 5 is as follows. That is, since the thermal expansion coefficients of the welding materials 15 and 16 are larger than those of the embrittlement material 13, the test material 10, and the auxiliary material 14, the embrittlement material 13, the test material 10 and the auxiliary material 14 are welded. When doing so, the welding materials 15 and 16 expand | swell greatly by the high heat at the time of welding. However, when the test piece 20 is cooled after the welding is completed, the welding materials 15 and 16 contract more greatly than the embrittlement material 13, the test material 10, and the auxiliary material 14. Thereby, the embrittlement material 13, the test material 10 and the auxiliary material 14 are pulled by the welding materials 15 and 16, and compressive stress is generated in the embrittlement material 13, the test material 10 and the auxiliary material 14. On the other hand, since the welding materials 15 and 16 are pulled by the embrittlement material 13, the test material 10 and the auxiliary material 14, tensile stress is generated in the welding materials 15 and 16. In addition, since the specimen 10 is pulled by the welding material 15 and the welding material 16, the compressive stress generated in the specimen 10 becomes larger than the compressive stress generated in the embrittlement material 13 and the auxiliary material 14.

なお、上記式（２）において、σは引張応力を示し、ａはき裂長さを示し、Ｗは試験片の幅を示す。 FIG. 6 is a diagram showing a crack propagation driving force calculated by FEM analysis (finite element method) in consideration of the residual stress of the test piece 20 shown in FIG. The propagation driving force shown in FIG. 6 is obtained by setting the load stress (nominal stress) in the hybrid ESSO test to 100 MPa. For comparison, FIG. 6 shows the crack propagation driving force when a tensile stress of 100 MPa is generated in a normal specimen (normal ESSO test) by a solid line. The crack propagation driving force in this case is calculated based on the following equation (2).

In the above formula (2), σ indicates tensile stress, a indicates the crack length, and W indicates the width of the test piece.

  As shown in FIG. 6, in the ESSO test of a normal specimen, since no residual stress is generated in the specimen, the crack propagation driving force increases in proportion to the square root of the crack length. . Therefore, when the toughness value Kca of the specimen is constant, the crack does not stop in the specimen.

On the other hand, in the test piece 20 (FIG. 3) according to the present embodiment, since the residual stress due to welding is generated in the test piece 20, the propagation driving force of the crack is greatly affected by the residual stress. Specifically, in the welding materials 15 and 16, since the tensile stress is generated, the crack propagation driving force greatly increases as the crack length increases. However, since the compressive stress is generated in the embrittlement material 13 and the test material 10, the crack propagation driving force does not increase greatly even if the crack length is increased. In particular, since a compressive stress larger than that of the embrittlement material 13 is generated in the test material 10, a predetermined region of the test material 10 (in FIG. 6, about 150 mm to 280 mm from the upper end portion of the test piece 20. In the region), the crack propagation driving force decreases as the crack length increases. Therefore, in the hybrid ESSO test according to the present embodiment, when the propagation driving force of the crack is reduced to a value smaller than the toughness value Kca of the test material 10 in the predetermined region of the test material 10, Crack propagation stops. For example, in the example of FIG. 6, when the toughness value Kca of the specimen 10 is 80 MPa · m ^1/2 , when the crack propagates from the upper end of the test piece 20 to a position of about 200 mm (the propagation driving force is When the pressure drops to about 80 MPa · m ^1/2 ), crack propagation stops.

  Based on the above theory, in the present embodiment, the crack propagation driving force is calculated based on the FEM analysis in consideration of the residual stress generated when the test piece 20 is produced, and the position where the crack stops is calculated from the calculation result. The propagation driving force of the crack at is determined, and the obtained propagation driving force is used as the toughness value Kca of the specimen 10. In this case, as in the case of a conventional hybrid ESSO test, when the crack propagates to the specimen 10 (Go), the crack propagation driving force and when the crack does not propagate to the specimen 10 (No-Go) Since it is not necessary to find a boundary point with the crack propagation driving force, it is not necessary to perform a test using a large number of test pieces. Thereby, the test cost can be reduced. Moreover, since the crack can be stopped in the specimen 10, the brittle crack propagation stopping characteristic of the specimen 10 can be evaluated more accurately. Moreover, since the low toughness embrittlement material 13 is provided, a crack can be easily generated in the embrittlement material 13 even in a cryogenic environment.

  In the test piece 100 (see FIG. 2) used in the conventional hybrid ESSO test, residual stress due to welding is generated. However, in the conventional test piece 100, the specimen 10 is cracked. Can not be stopped. The reason will be described below.

  FIG. 7 is a graph showing an example of the residual stress of the conventional test piece 100 (FIG. 2). FIG. 8 is a diagram showing a crack propagation driving force calculated by FEM analysis based on the residual stress of the test piece 100 shown in FIG. In addition, the dimension of the test piece 100 shall be 500 mm x 500 mm, and the embrittlement material 13 (FIG. 2) and the test material 10 (FIG. 2) shall be welded in the position of 150 mm from the upper end of the test piece 100. did. Further, the propagation driving force shown in FIG. 8 is calculated assuming that the load stress (nominal stress) in the hybrid ESSO test is 100 MPa, as in FIG.

  As shown in FIG. 7, even in the conventional test piece 100, residual stress (compressive stress) is generated in the embrittlement material 13 and the test material 10. However, compared with the example of the present invention shown in FIG. 5, the value of the residual stress is particularly small in the region exceeding 150 mm from the upper end of the test piece 100 (that is, the specimen 10). This is because in the test piece 100, the lower side of the specimen 10 is not constrained by the welding material, so that sufficient residual stress cannot be generated in the specimen 10. In this case, as shown in FIG. 8, the crack propagation driving force does not decrease in the test material 10, so that the crack propagation cannot be stopped in the test material 10.

2. Method for Determining Appropriate Load Stress in Tensile Test (ESSO Test) As described above, the present invention utilizes the fact that the crack propagation driving force decreases in the specimen 10 (FIG. 3). And in this invention, the evaluation possible range of the brittle crack propagation stop characteristic of the specimen 10 is determined according to the fall amount of the crack propagation driving force. In the example of FIG. 6, the propagation driving force of the crack in the test specimen 10 is dropped from about 120 MPa ^{· m 1/2} to about 60 MPa ^{· m 1/2,} toughness Kca test materials 10 to about When the value is between 120 MPa · m ^1/2 and about 60 MPa · m ^1/2 , the brittle crack propagation stop property evaluation method according to the present invention can be applied. That is, in the example of FIG. 6, evaluation range of the brittle crack propagation stop characteristics in the range between 120 MPa ^{· m 1/2} to about 60 MPa ^{· m 1/2.}

  Here, the amount of decrease in the crack propagation driving force in the specimen 10 is determined by the load stress in the tensile test (ESSO test) and the yield stress of the welding material 15. Hereinafter, it demonstrates using drawing.

  FIG. 9 and FIG. 10 are graphs showing the relationship between the load stress in the tensile test and the crack propagation driving force in the specimen 10. 9 and 10 show the maximum value of the crack propagation driving force in the specimen 10 as the evaluable upper limit value, and the minimum value of the crack propagation driving force in the specimen 10 is the lower limit value that can be evaluated. Is shown as The evaluable range of the brittle crack propagation stop property is a range between the evaluable upper limit value and the evaluable lower limit value. 9 shows the relationship when 50 kg class steel (yield stress at room temperature: 300 MPa) is used as the welding material 15 (FIG. 3) in the test piece 20 (FIG. 3), and FIG. The relationship is shown when a grade steel (yield stress at normal temperature: 600 MPa) is used as the welding material 15. FIG. 9 shows values obtained by FEM analysis.

  As shown in FIGS. 9 and 10, the evaluation range of the brittle crack propagation stop characteristic decreases as the load stress increases. This is because the influence of the residual stress on the driving force decreases as the load stress increases. Then, when the value of the load stress becomes a predetermined value (about 200 MPa in FIG. 9), the evaluable upper limit value and the evaluable lower limit value become equal, and the evaluable range of the brittle crack propagation stop characteristic becomes zero. Become. When the evaluable range becomes 0, the crack propagation driving force does not decrease in the specimen 10, so that the crack propagation cannot be stopped in the specimen 10. Therefore, in order to effectively use the present invention, it is necessary to perform a tensile test with a load stress at which the evaluable range is greater than zero.

Here, as shown in FIGS. 9 and 10, the load stress when the evaluable range becomes 0 (hereinafter referred to as “evaluable maximum load stress”) varies depending on the yield stress of the welding material 15. Specifically, when the yield stress of the welding material 15 is large, the maximum load stress that can be evaluated also increases. Therefore, the present inventors derived the following equation (1) from the relationship between FIGS. 9 and 10 in order to determine an appropriate load stress in the tensile test based on the yield stress of the welding material 15. In the following formula (1), σ _g represents the load stress in the tensile test, and σ _y represents the yield stress of the welding material 15 at room temperature.

σ _g <(0.24σ _y −6.1) / (0.000053σ _y +0.295) (1)
By using the above formula (1), the tester can easily determine an appropriate load stress in the tensile test. Specifically, the tester may determine the load stress of the tensile test according to the yield stress of the welding material 15 so as to satisfy the above formula (1).

  The above formula (1) is derived by the present inventors in consideration of the convenience of the tester, and in the tensile test if the crack propagation driving force in the specimen 10 is reduced. The load stress may not satisfy the above formula (1). Further, an appropriate load stress may be determined in consideration of the yield stress of the welding material according to the test environment.

  In addition, although the evaluation range of the brittle crack propagation stop characteristic can be widened by using the welding material 15 having a high yield stress, the welding material 15 should be appropriately determined according to the environment in which the tensile test is performed. Is preferred. That is, when the welding material 15 is extremely susceptible to brittle fracture in the environment where the tensile test is performed (for example, a cryogenic environment), the longitudinal direction of the welding material 15 when the crack enters the welding material 15. The crack may split in the direction indicated by the arrow X in FIG. In this case, the crack cannot be propagated to the specimen 10 and the brittle crack propagation stop characteristic cannot be evaluated. On the other hand, if the toughness of the welding material 15 is too high, the crack may stop at the welding material 15. In this case as well, the crack cannot be propagated to the specimen 10 and the brittle crack propagation stop characteristic cannot be evaluated. Therefore, it is preferable to select a material having appropriate toughness as the welding material 15 according to the environment in which the tensile test is performed. For example, when the specimen 10 is 9% Ni steel, it is preferable to use a 3.5% Ni-based welding material 15.

3. Appropriate Dimensions of Test Specimens As described above, the present invention generates an appropriate residual stress (compressive stress) in the specimen 10 (FIG. 3), thereby reducing the crack propagation driving force in the specimen 10. It is characterized by making it. Therefore, each dimension of the test piece 20 (FIG. 3) is preferably determined so that an appropriate residual stress is generated in the specimen 10. For example, when the width W (FIG. 3) of the test piece 20 is 400 mm to 600 mm, the length d1 (FIG. 3) is set to 130 mm to 170 mm, and the length d2 (FIG. 3) is set to 300 mm to 350 mm. I like it. Moreover, when the width W (FIG. 3) of the test piece 20 is 400 mm to 600 mm, the distance between the welding material 15 and the welding material 16 is preferably set to 130 mm to 220 mm.

  In addition, the dimension of the test piece 20 is not limited to said example, It is possible to change suitably according to the test objective etc.

4). Other Embodiments In the above embodiment, the crack propagation driving force is calculated based on FEM analysis, but the crack propagation driving force is calculated by other numerical calculations (for example, boundary element method, etc.). May be.

  Moreover, in the said embodiment, although the residual stress of the test piece 20 is measured, you may calculate the residual stress of the test piece 20 by numerical calculation (for example, a finite element method, a boundary element method, etc.).

  Moreover, in the said embodiment, although the case where a hybrid ESSO test was implemented as a tensile test was demonstrated, a tensile test is not limited to a hybrid ESSO test, You may implement another tensile test. Hereinafter, a case where a double tensile test is performed as an example of another tensile test will be briefly described.

  FIG. 11 is a diagram illustrating an example of a test piece used when a double tensile test is performed. The test piece 21 used in the double tensile test differs from the specimen 10 shown in FIG. 3 in the following points. That is, in the test piece 21, the portion to be pulled 17 is formed integrally with the embrittlement material 13 on the upper portion of the embrittlement material 13. A notch hole 17 a is formed in the portion to be pulled 17.

  In the double tensile test, as in FIG. 4, the jig 11 is attached to the embrittlement material 13, the test material 10, and the auxiliary material 14, and another tensile jig is also attached to the portion 17 to be pulled. And when the to-be-tensioned part 17 is pulled by the tension jig, a crack will generate | occur | produce from the notch 17a, and a crack will grow when the test piece 21 is pulled by the jig 11 (FIG. 4). Also in this case, crack propagation can be stopped in the specimen 10 as in the test piece 20 described above.

  The effects of the present invention will be described below based on examples.

In Examples, a steel sheet having a chemical composition shown in Table 1 below was rolled to a plate thickness of 35 mm, and then subjected to a quenching and tempering treatment to produce a steel sheet. From the steel sheet, a No. 4 test piece specified in JISZ2201 and a V-notch test piece specified in JISZ2202 were collected, subjected to a tensile test at room temperature and a Charpy impact test at -196 ° C., and a tensile strength (TS: MPa), yield strength (YS: MPa) and absorbed energy (vE-196: J, average value of three). The results are shown in Table 1. In addition, the No. 4 test piece was extract | collected in the rolling direction in the position of the plate thickness 1/4 of a steel plate, and the V notch test piece was extract | collected in the rolling orthogonal direction in the position of the plate thickness 1/4 of a steel plate.

  Using the steel plate as the test material 10 (FIG. 3), test pieces of Example 1 and Example 2 were prepared so that the rolling direction of the steel plate and the tensile direction in the tensile test coincided. In addition, the test piece of Example 1, 2 has the structure similar to the specimen 10 demonstrated in FIG. 3, respectively, and the width D (FIG. 3) and the length D (FIG. 3) in a tension direction are 500 mm. Yes, the length d1 (FIG. 3) is 150 mm, and the length d2 (FIG. 3) is 330 mm. Further, as the embrittlement material 13 (FIG. 3), a material obtained by reheating 9% Ni steel to 700 ° C. and furnace-cooling is used, the steel plate is used as the auxiliary material 14, and the welding materials 15 and 16 are as follows: A 3.5% Ni-based welding material was used.

  Using the test pieces of Examples 1 and 2 having the above-described configuration, the mixed ESSO test described with reference to FIG. 4 was performed in an environment of −165 ° C. In the mixed ESSO test of Example 1, the load stress was 130 MPa, and in the mixed ESSO test of Example 2, the load stress was 160 MPa. As a result, in Example 1, the crack propagation stopped at a position of 205.6 mm from the upper end of the test piece, and in Example 2, the crack propagation stopped at a position of 249.2 mm from the upper end of the test piece. That is, crack propagation stopped at the specimen 10 in both test pieces.

  Further, in this example, the residual stress of the test pieces of Examples 1 and 2 before the mixed molding ESSO test was measured, and the crack propagation driving force was calculated by FEM analysis based on the residual stress. FIG. 12 and FIG. 13 show the calculated crack propagation driving force. FIG. 12 shows the propagation driving force of the test piece of Example 1, and FIG. 13 shows the propagation driving force of the test piece of Example 2.

As described above, in the hybrid ESSO test of Example 1, crack propagation stopped at a position of 205.6 mm from the upper end position of the test piece. From the result of FIG. It can be seen that the propagation driving force was 107 MPa · m ^1/2 . Therefore, it can be estimated that the toughness value Kca of the test piece of Example 1 is 107 MPa · m ^1/2 .

Further, in the hybrid ESSO test of Example 2, crack propagation stopped at a position of 249.2 mm from the upper end position of the test piece. Therefore, from the result of FIG. 13, the crack propagation driving force at the crack propagation stop position. It can be seen that was 126 MPa · m ^1/2 . Therefore, it can be estimated that the toughness value Kca of the test piece of Example 2 is 126 MPa · m ^1/2 .

  The experimental results are shown in Table 2 below. From the above, according to the present invention, crack propagation can be stopped in the specimen 10, and the crack propagation driving force when the crack propagation is stopped is calculated based on FEM analysis. Thus, it was found that the toughness value Kca of the specimen 10 can be evaluated.

  According to the brittle crack propagation stop property evaluation method according to the present invention, it is possible to evaluate the brittle crack propagation stop property of a material in a cryogenic environment while reducing the test cost.

DESCRIPTION OF SYMBOLS 10 Test material 11 Jig 12 Wedge 13 Brittle material 13a Notch 14 Auxiliary material 15,16 Welding material 17 Tensile part 17a Notch hole 20,21 Test piece 100 Test piece

Claims (6)

An evaluation method for evaluating a brittle crack propagation stopping property of a material by generating a crack in the test piece by carrying out a tensile test on the test piece,
The test piece is made of an evaluation target material, and an embrittlement material that is welded to one end side of the test material in a direction orthogonal to the tensile direction in the tensile test and has lower toughness than the test material. It consists of an auxiliary material welded to the other end side of the test material in the orthogonal direction,
The crack is forcibly generated in the brittle material,
The load stress in the tensile test is set so that the forcibly generated crack stops in the specimen, The brittle crack propagation stop property evaluation method characterized by the above-mentioned. Based on the residual stress and the load stress of the test piece, the propagation driving force of the crack at an arbitrary position of the test piece is calculated by numerical calculation, and the propagation of the crack at the position where the crack stops is calculated from the calculation result. The brittle crack propagation stop property evaluation method according to claim 1, wherein a driving force is obtained and the obtained propagation driving force is used as a toughness value of the specimen. The said embrittlement material and the said auxiliary material are welded to the said test material so that the propagation drive force of a crack may fall with the growth of a crack in the said test material. Evaluation method for brittle crack propagation stopping properties of steel. The said embrittlement material and the said test material are welded by the Ni-type welding material containing 1.5%-9% Ni by the mass%, Either of Claims 1-3 characterized by the above-mentioned. The method for evaluating brittle crack propagation stop characteristics as described. The load stress in the said tensile test is set so that the following formula (1) may be satisfied, The brittle crack propagation stop characteristic evaluation method in any one of Claims 1-4 characterized by the above-mentioned.
σ _g <(0.24σ _y −6.1) / (0.000053σ _y +0.295) (1)
In the above formula (1), σ _g represents the load stress, and σ _y represents the yield stress at room temperature of the welding material joining the brittle material and the test material. The width of the test piece is 400 mm to 600 mm, and the joint between the embrittlement material and the test material is located within a range of 130 mm to 170 mm from the position where the crack is forcibly generated. The brittle crack according to any one of claims 1 to 5, wherein a joint portion between the sample and the auxiliary material is located in a range of 300 mm to 350 mm from a position where the crack is forcibly generated. Crack propagation stop property evaluation method.

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