Fracture toughness value prediction method, prediction device, program, and method of obtaining local breaking stress

Abstract

To improve versatility and simplicity of fracture toughness value perdition.SOLUTION: A fracture toughness value prediction method is provided, involving acquiring a fracture toughness parameter at the time when a locally generated stress on a test piece evaluation part in a fracture toughness test as obtained through a first numerical analysis reaches a local breaking stress at a fracture initiation point obtained based on a brittle fracture test as a fracture toughness value of a target material.SELECTED DRAWING: Figure 2

Description

Technical Field

[0001] The present invention relates to a method for predicting fracture toughness values, a prediction device, a program, and a method for obtaining a local fracture limit stress used therein.

Background Art

[0002] In order to evaluate the fracture safety of a structure, there are brittle fracture tests such as the Charpy impact test, which is a simple industrial test. In this specification, the brittle fracture test is a general term for tests conducted to evaluate the ductility of a member, and is a higher concept including the Charpy impact test and the fracture toughness test. The test format of the brittle fracture test assumes that the member is brittlely fractured, and the test piece has a notch (referring to a change part of the geometric cross-sectional shape in the member. There are various types such as holes, screw parts, key grooves, stepped parts, cracks, scratches, defects, etc., but usually they are generically called notches.), is in a stress triaxial state, has a large deformation speed of the member such as an impact load, or has a low test temperature, or a combination of these conditions. Among the brittle fracture tests, there is a fracture toughness test for obtaining a fracture toughness value, which is an index necessary for the design and fracture management of a structure considering defects in the member. The fracture toughness value is the resistance value of the material against fracture from a crack in a situation where a load in a certain direction is generated on a member having a crack, and there are limit CTOD (Crack Tip Opening Displacement at which brittle fracture occurs), K1C, J1C, etc.

[0003] In the fracture toughness test, it is necessary to generate a crack (referring to a defect with a sharp shape at the tip of the defect. Ideally, it refers to a defect with a curvature radius as small as about one atom at the tip of the defect. Strictly speaking, it refers to a fracture state in which atomic plane separation occurs in a local region at the tip of the defect.) in the test piece in advance by a fatigue test, and the test cost (cost and time) is large. In addition, it is necessary to analyze and judge the fracture surface after the test to determine whether there was a problem in the prior fatigue test, etc., and high expertise is required to judge the validity of the test results.

[0004] On the other hand, a method for predicting the fracture toughness value has been proposed without actually conducting a fracture toughness test.

[0005] In Non-Patent Document 1, a method for predicting the limit CTOD has been proposed for the cleavage fracture of ferrite-cementite steel. It is assumed that cleavage fracture occurs when the local stress generated during the CTOD test calculated by FEM analysis (the vertical stress acting on the cleavage plane of the microstructure) exceeds the local fracture stress calculated in consideration of the microstructure information and the three stages of cleavage fracture generation. At that time, the quasi-CTOD is obtained. Here, the stages of cleavage fracture generation are divided into the nucleation of cracks due to cementite cracking (Stage I), the formation of cleavage cracks due to the propagation of cementite cracks into the ferrite matrix (Stage II), and the breakthrough of cleavage cracks through ferrite grain boundaries (Stage III). In Stage I, the cracking ratio of cementite is calculated, in Stage II, the local limit stress based on Petch's formula is calculated, and in Stage III, the local limit stress based on Griffith's condition is calculated.

[0006] In Patent Document 1, a method for predicting the toughness (e.g., limit CTOD) of steel materials in the grain boundary fracture mode based on the method of Non-Patent Document has been proposed. It is determined that unstable fracture occurs when the local stress generated in the toughness test system including the CTOD test calculated by FEM analysis (the vertical stress acting on the plane corresponding to the <100> plane of the microstructure) exceeds the local limit stress. For example, in the case of the CTOD test system, the CTOD at the time of unstable fracture occurrence is predicted as the limit CTOD.

[0007] In Patent Document 1, an empirical correlation formula with the microstructure information (the amount of P at grain boundaries and the particle size distribution of prior austenite) contributing to the grain boundary fracture phenomenon is created in advance, and the grain boundary fracture stress is obtained by inputting the microstructure information of the target material into this formula.

[0008] In Patent Document 2, a method combining Non-Patent Document 1 and Patent Document 1 proposes a prediction method for ductility (CTOD) considering both fracture modes of cleavage fracture and intergranular fracture for steel materials in which martensite or bainite is the matrix phase and contains a cementite structure. Based on the microstructural information, the fracture stresses of cleavage fracture and intergranular fracture are respectively obtained, and the smaller stress is determined as the fracture stress. In the same way as Patent Document 1, the local generated stress is calculated by FEM analysis. When this stress exceeds the fracture stress, it is judged that unstable fracture occurs. For example, in the case of a CTOD test system, the CTOD at the time of fracture occurrence is obtained.

[0009] In Patent Document 2, the method for deriving the fracture stress in cleavage fracture is generally an extension and partial simplification of the method in Non-Patent Document 1 to steel having martensite or bainite, and the fracture stress in intergranular fracture is derived by the same method as in Patent Document 1.

Prior Art Documents

Non-Patent Documents

[0010]

Non-Patent Document 1

Patent Documents

[0011]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0012] For fracture toughness value prediction methods using empirical correlation equations, such as in Patent Document 1 and Patent Document 2 (grain boundary fracture), the applicable range of the target material is limited, lacking versatility, and cannot be predicted for materials with new characteristics without prior experience. Also, regarding the acquisition of the amount of P at grain boundaries among microstructural information, for example, it is necessary to measure the amount of P at grain boundaries on the grain boundary fracture surface sample of the specimen by Auger electron spectroscopy using a scanning electron microscope (SEM (Scanning Electron Microscope)) equipped with an Auger electron spectrometer. For the acquisition of austenite grain size, it is necessary to perform a series of processes including subjecting the prior γ grain boundaries of the specimen to corrosion revelation treatment with a saturated aqueous solution of picric acid, obtaining a microstructural photograph, and further quantifying the distribution of prior γ grain boundaries by image processing of the acquired image. As described above, microstructural information needs to be obtained through multiple processes using advanced equipment and analysis or analytical techniques, lacking simplicity.

[0013] For methods that determine the fracture stress using the microstructural information and theoretical equations of materials, such as in Non-Patent Document 1 and Patent Document 2 (cleavage fracture), and predict the fracture toughness value through this, advanced observation or analysis equipment is required for the acquisition of microstructural information, and prediction cannot be made without this information. In addition, the applicable range of the material type (microstructure of steel) or fracture mode of the prediction target is limited, lacking versatility, and cannot be predicted for other materials. Also, regarding the prior phase grain size distribution among microstructural information, it is necessary to obtain it using crystal orientation analysis such as the EBSD (Electron BackScatter Diffraction) method. For the cementite grain size distribution, it is necessary to electrolytically polish the specimen to expose cementite on the specimen surface, obtain a cementite microstructure photograph using SEM or the like, and quantify the grain size distribution of cementite by image processing of the acquired image. As described above, microstructural information needs to be obtained through multiple processes using advanced equipment and analysis or analytical techniques, lacking simplicity.

[0014] As described above, in the conventional fracture toughness value prediction methods, there are application ranges depending on the target material (steel type) and the mode of fracture (transgranular fracture or intergranular fracture), lacking versatility. Alternatively, it is necessary to obtain microstructural information using sophisticated analysis and observation equipment or conduct fracture toughness tests, lacking simplicity.

[0015] An object of the present invention is to improve the versatility and simplicity of predicting fracture toughness values.

Means for Solving the Problems

[0016] The gist of the present invention is as follows. The present invention calculates the local generated stress during a fracture toughness test and predicts the value of the fracture toughness parameter (such as stress intensity factor or CTOD) when the value exceeds a predetermined local fracture limit stress as the fracture toughness value. There is a finding that the local fracture limit stress at the time of ductile fracture occurrence is the same for the same material regardless of the ductile fracture test system. For example, there is a mention of this finding in "Yasuto Takashima, Mitsuru Ohata, & Juzo Minami. (2008). Proposal of a method for predicting the transition temperature difference between Charpy impact test and CTOD fracture toughness test by fracture mechanics consideration. Transactions of the Japan Society of Naval Architects and Ocean Engineers, 7, 271-282." The inventors of the present invention focused on this finding in predicting fracture toughness values and obtained the idea of experimentally obtaining the local fracture limit stress in a ductile fracture test (such as Charpy impact test) with simple test implementation and using the local fracture limit stress to predict the fracture toughness value.

[0017] A first aspect of the present invention provides a method for predicting a fracture toughness value, which acquires, as the fracture toughness value of a target material, a fracture toughness parameter when the local generated stress in a test piece evaluation part in a fracture toughness test obtained by a first numerical analysis becomes the local fracture limit stress at the fracture initiation position determined based on a ductile fracture test.

[0018] By determining the fracture toughness value based on the unique local fracture limit stress for each target material, the fracture toughness value can be predicted even for materials without empirical rules, and the versatility is high. In addition, by determining the local fracture limit stress based on the brittle fracture test, it becomes unnecessary to obtain microstructural information that requires sophisticated equipment and technology, and the simplicity is high. By obtaining the fracture toughness parameter through numerical analysis, it becomes unnecessary to prepare a fracture toughness test piece that is difficult to fabricate because a crack must be introduced, and the simplicity is improved.

[0019] The local fracture limit stress may be obtained by determining the fracture occurrence time and the fracture initiation position based on the brittle fracture test, and obtaining the local generated stress at the fracture occurrence time and the fracture initiation position when the brittle fracture test is performed by a second numerical analysis.

[0020] The local fracture limit stress can be obtained without using the microstructural information of the target material.

[0021] The target material may be a steel material.

[0022] The brittle fracture test may be a Charpy impact test.

[0023] When the brittle fracture test is a Charpy impact test, in the second numerical analysis, a dynamic stress-strain curve is used, and considering the temperature and strain rate at each position of the test piece, the load-time history is obtained, and based on the load-time history at the fracture initiation position, the fracture occurrence time and the local generated stress at the fracture initiation position may be obtained.

[0024] When the brittle fracture test is a Charpy impact test, the coefficient used in the second numerical analysis may be adjusted based on the comparison between the load-time history obtained from the actual Charpy impact test and the load-time history obtained from the second numerical analysis.

[0025] The fracture toughness parameter is CTOD, and the fracture toughness value may be the critical CTOD.

[0026] A second aspect of the present invention provides a fracture toughness value prediction device that acquires, as the fracture toughness value of a target material, a fracture toughness parameter when the local generated stress in the test piece evaluation part in a fracture toughness test obtained by numerical analysis becomes the local fracture limit stress at the fracture initiation position determined based on a brittle fracture test.

[0027] A third aspect of the present invention provides a program for predicting a fracture toughness value that acquires, as the fracture toughness value of a target material, a fracture toughness parameter when the local generated stress in the test piece evaluation part in a fracture toughness test obtained by numerical analysis becomes the local fracture limit stress at the fracture initiation position determined based on a brittle fracture test.

[0028] A fourth aspect of the present invention is a method for obtaining a local fracture limit stress, which obtains, as the local fracture limit stress, the local generated stress at the fracture initiation position at the unstable fracture occurrence time based on a brittle fracture test.

[0029] In order to determine the local generated stress at the fracture initiation position at the unstable fracture occurrence time of a brittle fracture test as the local fracture limit stress, there is no need for techniques for acquiring and obtaining microstructure information that requires advanced observation devices and analysis devices.

[0030] The unstable fracture occurrence time and the fracture initiation position are determined based on a brittle fracture test, and the local generated stress may be determined based on numerical analysis.

[0031] The above local fracture limit stress may be used for predicting the fracture toughness value by numerical analysis.

Advantages of the Invention

[0032] According to the present invention, the versatility and simplicity of predicting the fracture toughness value can be improved.

Brief Description of the Drawings

[0033]

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Mode for Carrying Out the Invention

[0034] Next, embodiments of the present invention will be described with reference to the accompanying drawings.

[0035] In this embodiment, a test piece made of a target material is produced, a Charpy impact test is performed as a brittle fracture test, and the local fracture limit stress of the target material is determined based on the test results. Then, based on this local fracture limit stress, the limit CTOD is predicted as the fracture toughness value of the target material. Note that the limit CTOD (fracture toughness value) is determined as the value of the CTOD (fracture toughness parameter) when unstable fracture occurs in the CTOD test (fracture toughness test). In this embodiment, it is assumed that fracture occurs when the local stress generated at the stress concentration part of the test piece in the CTOD test (fracture toughness test) reaches the local fracture limit stress of the target material obtained based on the Charpy impact test (brittle fracture test), and the value of the CTOD (fracture toughness parameter) at that time is determined by static elasto-plastic FEM analysis (first numerical analysis).

[0036] Referring to FIG. 1, a limit CTOD value prediction device (fracture toughness value prediction device) 1 according to an embodiment of the present invention includes a fracture limit stress acquisition unit 2, a local stress acquisition unit 3, and a limit CTOD acquisition unit 4. The limit CTOD value prediction device 1 can be constructed by a computer including a storage device such as a ROM and a RAM, an input / output device, an arithmetic device such as an MPU, and software installed thereon. The limit CTOD value prediction device 1 does not necessarily have to be constituted by a single computer, and may be constituted by a plurality of computers.

[0037] The fracture limit stress acquisition unit 2, in cooperation with the instrumented Charpy impact test apparatus (brittle fracture test apparatus) 5, executes the acquisition of the fracture limit stress of the target material (for example, steel material) (step A in FIG. 2 described later). The fracture limit stress acquisition unit 2 includes an FEM execution unit 2a, a requirement setting unit 2b, and a prediction unit 2c. The FEM execution unit 2a and the requirement setting unit 2b execute step A4 in FIG. 3, which is part of step A in FIG. 2. The prediction unit 2c executes step A5 in FIG. 3, which is part of step A in FIG. 2. Steps A2 and A3 in FIG. 3 of step A are executed using the instrumented Charpy impact test apparatus 5.

[0038] The local stress acquisition unit 3 executes the acquisition of the local stress in the CTOD test (steps B in FIG. 2, steps B1 and B2 in FIG. 4). The local stress acquisition unit 3 includes an FEM execution unit 3a and a requirement setting unit 3b.

[0039] The limit CTOD acquisition unit 4, in cooperation with the fracture limit stress acquisition unit 2 and the local stress acquisition unit 3, acquires the predicted value of the limit CTOD.

[0040] Hereinafter, with reference to the flowcharts of FIGS. 2 to 4, as an example of a method for predicting the fracture toughness value of a metal material, a method for predicting the limit CTOD executed by the limit CTOD prediction device 1 according to the embodiment will be described.

[0041] Referring to FIG. 2, this prediction method generally includes the acquisition of the local fracture limit stress σ cr in step A, the acquisition of the local generated stress σ CTOD,local during the CTOD test in step B, and the acquisition of the limit CTOD in step C.

[0042] In step A, by dynamic elastoplastic FEM analysis (second numerical analysis), the local generated stress at the fracture initiation position when performing the Charpy impact test (brittle fracture test) is defined as the local fracture limit stress σ crIt is obtained as follows. Step A includes, before the dynamic elastoplastic FEM analysis, a test process of performing a Charpy impact test (ductile fracture test) to obtain the fracture initiation position in a test piece made of the target material. As a preferred embodiment, in the present embodiment, in the above step A, when obtaining the local fracture limit stress σ cr , the microstructural information of the target material is not used as in Non-Patent Document 1 and Patent Documents 1 and 2. In step B, the relationship between the local stress generated at the stress concentration part of the test piece and the CTOD (fracture toughness parameter) when performing a CTOD test (fracture toughness test) is obtained by static elastoplastic FEM analysis (first numerical analysis). In step C, based on the above relationship obtained in step B, the value of CTOD (fracture toughness parameter) when the local stress becomes the local fracture limit stress σ cr obtained in step A is obtained as the limit CTOD (fracture toughness value). The execution order of steps A and B is not particularly limited.

[0043] Regarding the acquisition of the local fracture limit stress σ cr in step A, referring to FIG. 3, after setting the test temperature in step A1, the instrumented Charpy test (an example of a ductile fracture test using a notched test piece, which is a different ductile fracture test from the CTOD test (fracture toughness test) in step B described later) in step A2, the fracture surface observation of the test piece after the instrumented Charpy test in step A3, and the local stress σ Charpy,local generated during the instrumented Charpy test using dynamic elastoplastic FEM in step A4 are obtained, and finally the local fracture limit stress σ cr in step A5 is obtained. At this time, the execution order of steps A2, A3 and step A4 is not particularly limited.

[0044] In step A1, the test temperature of the instrumented Charpy impact test is determined. At this time, in the metal material to be predicted, any temperature at which ductile fracture surely occurs may be used. It may be determined with reference to conventional knowledge such as documents, or if there is no prior information, the ductile-brittle transition temperature may be obtained experimentally and set with reference to this. Alternatively, for a metal material that surely undergoes brittle fracture, it may be set to the temperature of the CTOD test to be predicted.

[0045] In step A2, a test piece made of the target material is prepared, and an instrumented Charpy impact test is actually carried out to obtain the fracture occurrence time (unstable fracture occurrence time). Referring to FIG. 5, for example, the time when the load rapidly decreases can be defined as the fracture occurrence time. The test piece shape may be, for example, a 2-mm V-notch standard test piece of JIS Z 2242. Alternatively, the notch shape may be a U-notch shape, and the test dimensions may be large or small. The number of tests is usually three at the same temperature for a normal Charpy test, but more is preferable. This is because by performing the same test multiple times, the reproducibility of the load time history response can be confirmed, and it is possible to regard the point at which the reproducibility of the load-time response is lost as the occurrence of fracture, making it easier to specify the fracture occurrence time.

[0046] In step A3, the fracture surface of the test piece after the instrumented Charpy impact test is observed to obtain the fracture initiation position on the test piece. Referring to FIG. 6 showing the test piece 10 near the fracture surface, the distance x from the bottom of the notch 11 and the distance z from the center of the thickness are grasped as the position coordinates of the fracture initiation point. The fracture initiation position is identified, for example, by observing the fracture surface of the test piece and from the chevron pattern (the mountain-shaped pattern of the brittle fracture surface). At this time, since the accuracy of the position information may be on the sub-millimeter order, the observation method may be any of visual inspection, a stereomicroscope, SEM, etc.

[0047] In step A4, a dynamic elastoplastic FEM analysis of the Charpy impact test is carried out to obtain the time change of the local generated stress σ Charpy,local in the vicinity of the notch bottom (the region including the fracture initiation position) of the test piece during the test (see FIG. 8). When performing the analysis, for example, Abaqus / Explicit 2017 of Dassault Systems, which is general-purpose analysis software, is used, but other software may be used as long as dynamic elastoplastic FEM analysis is possible. When carrying out this analysis, the following requirements are set. The dynamic elastoplastic FEM analysis is executed by the FEM execution unit 2a, and the requirement setting is executed by the requirement setting unit 2b (both are shown in FIG. 1).

[0048] Requirement A4-1: Element Division of Analytical Models for Impact Striker, Charpy Test Specimen, and Anvil Requirement A4-2: Inertia Effect during Impact of Impact Striker Requirement A4-3: Impact Velocity of Impact Striker Requirement A4-4: Initial Temperature (Test Temperature) of Charpy Test Specimen Requirement A4-5: Heat Generation Conditions of Charpy Test Specimen due to Plastic Work Requirement A4-6: Thermophysical Properties of Charpy Test Specimen Requirement A4-7: True Stress-True Strain Relationship (SS Curve) according to Test Temperature and Strain Rate of Charpy Test Specimen Requirement A4-8: Mechanical Properties in Elastic Region of Charpy Test Specimen For the contact condition analysis software of the impact striker, Charpy test specimen, and anvil, other commercially available general-purpose analysis software may be used, or self-developed analysis software may be used.

[0049] In order to reproduce the test systems of Requirements A4-1 and A4-2 on a computer, mesh division is performed according to the actual dimensions for the analytical models of the Charpy test specimen, anvil (the stand for installing the Charpy test specimen), and the cutting edge of the hammer (one of the test machine components that imparts an impact to the Charpy test specimen) (hereinafter referred to as the impact striker). The shapes of the impact striker and anvil comply with JIS B 7722, and the element type may be, for example, 3D solid voxel elements or modeled with 3D rigid shell elements. For mesh division, general-purpose mesher software may be used, or mesh division may be performed independently on the computer. Figure 7 shows the analytical model after mesh division, where reference numeral 11 is the test specimen, reference numeral 12 is the cutting edge of the hammer, and reference numeral 13 is the anvil. This analytical model is a 1 / 4 symmetric model targeting 1 / 2 of the longitudinal direction and 1 / 2 of the width direction of the test specimen 11.

[0050] The load acting on the test piece of Requirement A4-3 vibrates violently due to inertia because the impact striker collides at high speed. When improving the analysis accuracy, it is advisable to consider this inertial effect in the analysis. In that case, for example, dynamic stress analysis is used. For the analysis, for example, the Dynamic analysis function in Abaqus / Explicit, which is general-purpose analysis software, is utilized. Other commercially available general-purpose analysis software functions may be used for the analysis software, or self-developed analysis software may be used. Alternatively, if a decrease in analysis accuracy is acceptable, it is not necessary to implement this item. Regarding the numerical analysis method, in addition to FEM (Finite Element Method), BEM (Boundary Element Method), FDM (Finite Difference Method), FVM (Finite Volume Method), etc. may also be used.

[0051] Regarding the impact speed of the impact striker of Requirement A4-3, since the mass of the Charpy test piece is extremely small compared to the hammer, the change in momentum during the collision between the impact striker and the Charpy test piece can be ignored, and the impact speed may be set to a constant value. In that case, it is preferable to select from the range of 5.0 ± 0.5 m / s assumed in a general-purpose Charpy impact testing machine. Alternatively, the measured value of the speed of the impact striker of the actually used testing machine may be adopted.

[0052] The initial temperature (test temperature) of the Charpy test piece of Requirement A4-4 shall be the value set in Step A1.

[0053] Regarding Requirement A4-5, immediately after the striker impacts the test piece, heat generation due to plastic deformation occurs near the notch bottom of the Charpy test piece. This heat generation may be set, for example, by referring to conventional knowledge, as the value at which plastic work is converted into thermal energy.

[0054] The temperature rise of the Charpy test piece due to heat generation in Requirement A4-6 is set, for example, by referring to the thermal conductivity physical property values of the metal material to be predicted. Using this value, the thermal conductivity is calculated by coupling heat conduction analysis and stress analysis. This coupled analysis may be executed using the functions of the general-purpose software used, or may be executed by a self-developed coupled analysis algorithm.

[0055] Regarding Requirement A4-7, in the vicinity of the notch bottom of the Charpy test specimen, in addition to the temperature change associated with the heat generation described above, it is under a very high strain rate. Therefore, inside the test specimen after impact, the temperature and strain rate are not constant, and accordingly, the mechanical properties of the material after the yield region, that is, the SS curve (stress-strain relationship) in the plastic region, changes. Assuming the range of temperature and strain rate changes immediately after impact, for example, the range from the initial temperature of the test to +200 °C is set at several levels in increments of 20 to 50 °C, and the strain rate is 10 -4 ~10 3 / s in increments of ×10 / s at several levels, and a tensile test is carried out under the conditions of combining the temperature and strain rate at each level to obtain the true stress-true strain relationship. For the conditions of test temperature or strain rate without data, for example, it is complemented using the functions of general-purpose software to be used.

[0056] Regarding the mechanical properties in the elastic region of the Charpy test specimen of Requirement A4-8, that is, the Young's modulus and Poisson's ratio, they are set at the representative values of the metal material to be predicted. Alternatively, the values measured by tensile tests or the like may be used.

[0057] Regarding Requirement A4-9, in this analysis model, the collision between the impact striker and the Charpy test specimen and the contact between the Charpy test specimen and the anvil are handled. Therefore, when improving the analysis accuracy, for example, the interaction (contact stiffness) of two objects through the contact surface is set as the contact condition. The contact stiffness between the test specimen and the impact striker may be set with reference to the Hertz contact theory formula as the relationship between the indentation amount due to impact and the load acting on the impact part considering the indentation at the contact surface.

[0058] Regarding the contact between the test specimen and the anvil, it may be set as a perfect rigid contact where no indentation occurs at the contact surface, assuming that the influence on the local generated stress σ Charpy,local at the evaluation part is small. Alternatively, when allowing a decrease in analysis accuracy, it is not necessary to implement this item.

[0059] In Step A5, as conceptually shown in Fig. 9, the local generated stress σ near the notch bottom obtained in Step A4 Charpy,localIn the time variation, the local generated stress σ corresponding to the fracture occurrence time and the fracture initiation position obtained in the instrumented Charpy impact test Charpy,local is obtained as the local fracture limit stress σ cr of the target material.

[0060] In the acquisition of the local generated stress σ CTOD,local during the CTOD test in Step B, referring to FIG. 4, after setting the fracture initiation position (specimen evaluation part) of the CTOD test in Step B1, a static elasto-plastic FEM analysis of the CTOD test in Step B2 is performed. The execution order of Steps B1 and B2 is not particularly limited, but considering the efficiency of the prediction procedure, it is preferable to execute Step B2 after Step B1.

[0061] The reference position of the local generated stress σ CTOD,local used for the prediction in Step B1, that is, the fracture occurrence position, cannot use the actual fracture initiation position because it is determined without performing the fracture toughness test to be predicted. Therefore, as a method of determining the reference position based on empirical knowledge, for example, using CTOD, it is determined as a position at an arbitrary distance between 2 and 5 times CTOD from the crack tip. Also, for example, if the target material is steel, it may be set to about 0.1 mm from the crack tip.

[0062] In Step B2, a static elasto-plastic FEM analysis of the CTOD test system is performed, and the relationship between the local generated stress σ CTOD,local at the reference position corresponding to the location set in Step B1 of the specimen during the test, that is, the fracture occurrence position, and CTOD (fracture toughness parameter) is obtained (see FIG. 11). In performing the analysis, for example, Abaqus / Standard of Dassault Systems, a general-purpose analysis software, is used. When performing this analysis, the following requirements are set.

[0063] Requirement B2-1: Element division of the analysis model of the CTOD test specimen Requirement B2-2: Initial temperature (test temperature) of the CTOD test specimen Requirement B2-3: True stress-true strain relationship (SS curve) corresponding to the initial temperature of the CTOD test specimen Requirement B2-4: Mechanical properties in the elastic range of the CTOD test piece

[0064] For the analysis software, it is acceptable to utilize the functions of other commercially available general-purpose analysis software, or to use self-developed analysis software. Alternatively, regarding the numerical analysis method, in addition to FEM (finite element method), it may also be BEM (boundary element method), FDM (finite difference method), FVM (finite volume method), etc.

[0065] Regarding Requirement B2-1, in order to reproduce the CTOD test system on a computer, for example, in accordance with WES1108 and in accordance with the actual dimensions, mesh division of the analysis model of the CTOD test piece is performed. The element type of the analysis model is, for example, a three-dimensional solid voxel element. The shapes of the punch and the fixed support pin are, for example, in accordance with WES1108, and the element type is modeled by, for example, three-dimensional rigid shell elements. For mesh division, general-purpose meshing software may be used, or mesh division may be performed independently on the computer. Regarding the validity verification of the analysis model such as the mesh division size, for example, elastic analysis may be performed to confirm whether the stress field near the crack corresponds to the theoretical solution. Fig. 10 shows the analysis model after mesh division, where reference numeral 21 is the test piece, reference numeral 22 is the punch, and reference numeral 23 is the support pin. This analysis model is a 1 / 4 symmetric model targeting 1 / 2 in the longitudinal direction and 1 / 2 in the width direction of the test piece 21.

[0066] The initial temperature (test temperature) of the CTOD test piece in Requirement B2-2 is set to the test temperature to be predicted.

[0067] In Requirement B2-3, the static SS curve (true stress - true strain relationship) at the test temperature set in Requirement B2-2 is set. At this time, among the dynamic SS curves set in Requirement A4-7, the SS curve under the corresponding temperature and strain rate conditions may be directly transferred and set. Alternatively, it may be set from the SS curve obtained by performing a tensile test under predetermined test conditions. Alternatively, the true stress - true strain relationship under arbitrary temperature and strain rate conditions may be set using the empirical correlation formula of the temperature and strain rate dependence of the strength in the target material.

[0068] For the mechanical properties in the elastic range of the CTOD test piece of Requirement B2-4, that is, the Young's modulus and Poisson's ratio, they are set to the representative values of the metal material to be predicted. Alternatively, the values measured by tensile tests or the like may be used.

[0069] In Step C, as conceptually shown in FIG. 12, the local fracture limit stress σ cr obtained in Step A5 and the value of the local generated stress of the CTOD test piece obtained in Step B2 corresponding to it, σ CTOD,local are referred to, and the CTOD at that time is obtained as the limit CTOD. That is, in the relationship between the local generated stress σ CTOD,local obtained in Step B2 and the CTOD (fracture toughness parameter), the local fracture limit stress σ cr obtained in Step A5 and the local generated stress σ CTOD,local equal to it, the CTOD (fracture toughness parameter) corresponding to it is obtained as the limit CTOD (fracture toughness value).

[0070] In the method for predicting the limit CTOD of the present embodiment, by obtaining the limit CTOD based on the unique local fracture limit stress for each target material, the fracture toughness value can be predicted even for materials without empirical rules, and the versatility is high. Also, by obtaining the local generated stress at the fracture initiation position as the local fracture limit stress based on the instrumented Charpy impact test, it becomes unnecessary to obtain microstructural information that requires advanced equipment and technology, and the simplicity is high. Furthermore, by obtaining the limit CTOD through numerical analysis, it becomes unnecessary to prepare a fracture toughness test piece that is difficult to fabricate because a crack must be introduced, and the simplicity is improved.

[0071] Hereinafter, modified examples of the embodiment will be described.

[0072] (Modified Example 1) In the present embodiment, the local fracture limit stress σ crThe case where an instrumented Charpy impact tester is used to obtain the occurrence position and time of unstable fracture, which are necessary for the determination, has been described as an example. However, a general non-instrumented Charpy impact tester may also be used. In that case, the occurrence time of unstable fracture may be determined based on the swing angle or the like that can be confirmed by the Charpy impact tester. Alternatively, if it is a test system capable of identifying the occurrence time of unstable fracture, a brittle fracture test performed to evaluate the ductility of a metallic material, such as a Charpy impact test or a fracture toughness test, may be used.

[0073] (Modification Example 2) In the present embodiment, the case where the local stress σ Charpy,local acting on the Charpy impact test piece is derived using dynamic elastoplastic FEM has been described as an example. However, it is not always necessary to carry out in this way as long as the numerical solution or analytical solution of the local stress σ Charpy,local at each position of the test piece when a load is applied to the test piece so that the notch bottom of the test piece spreads in the longitudinal direction of the test piece is derived. Also, regarding the numerical analysis method, BEM (Boundary Element Method), FDM (Finite Difference Method), FVM (Finite Volume Method), etc. may be used instead of FEM (Finite Element Method). Alternatively, instead of analyzing every time a prediction is made, a method of obtaining in advance and referring to the relationship between the swing angle and the local stress σ Charpy,local may be used. Alternatively, a method of performing static elastoplastic FEM analysis and simply correcting the influence of the impact load may be used.

[0074] (Modification Example 3) In the present embodiment, when performing dynamic elastoplastic FEM analysis, the temperature and strain rate dependence of the material strength was obtained by conducting tensile tests at a plurality of temperature and strain rate levels and set in the analysis software. However, the true stress-true strain relationship under arbitrary temperature and strain rate conditions may be set using an empirical correlation formula for the temperature and strain rate dependence of the strength in the target material, or it may be obtained using a technique capable of predicting the temperature and strain rate dependence of the material from simple tensile test results.

[0075] (Modification Example 4) In the present embodiment, the local stress σCTOD,local Taking as an example the case of deriving it using static elasto-plastic FEM, it was explained. However, when a load is applied to the test piece so that the crack tip of the test piece spreads in the longitudinal direction of the test piece, the local stress σ CTOD,local at each position of the test piece when the numerical solution or analytical solution of is derived does not necessarily have to be carried out in this way. Also, regarding the numerical analysis method, it may be BEM (Boundary Element Method), FDM (Finite Difference Method), FVM (Finite Volume Method), etc. instead of FEM (Finite Element Method). Alternatively, instead of analyzing every time a prediction is made, a method of obtaining in advance and referring to the relationship between the swing angle and the locally generated stress may be used.

[0076] (Modification 5) In this embodiment, taking as an example the case of predicting the limit CTOD among the fracture toughness values was explained. However, it may be the case of predicting the fracture toughness value for other parameters such as K1C and J1C. In that case, the shape of the analysis model of the static elasto-plastic FEM may be changed according to the fracture toughness test system to be targeted.

[0077] Next, examples will be described. In the following description, when referring to steps, they mean each step of the flowcharts of FIGS. 2 to 4 referred to in the description of the embodiment.

[0078] As the target steel grade, 590 MPa grade steel for construction (material code: 590B) was selected.

[0079] The local fracture limit stress σ cr in step A was obtained as follows.

[0080] In step A1, the test temperature of the instrumented Charpy impact test was set to -90°C.

[0081] In step A2, the fracture occurrence time was obtained by the instrumented Charpy impact test. At this time, the test piece shape was a 2 mm V-notch standard test piece of JIS Z 2242, and the number of tests was 7. An example of the results of the instrumented Charpy impact test is shown in FIG. 13. The fracture occurrence time was defined as the time when the load rapidly decreased.

[0082] In step A3, SEM fracture surface observation was performed on each of the seven test pieces after the instrumented Charpy impact test, the position where brittle fracture originated from the chevron pattern was identified, and the distance x from the notch bottom and the distance z from the center of the thickness were grasped as the position coordinates of the fracture origin (see Fig. 6).

[0083] In step A4, dynamic elastoplastic FEM analysis of the Charpy impact test was performed to obtain the time variation of the local stress σ Charpy,local near the notch bottom of the test piece during the test. When performing the analysis, Abaqus / Explicit 2017 of Dassault Systems, a general-purpose analysis software, was used. When performing this analysis, the following requirements were set.

[0084] Requirement A4-1: Element division of the analytical models of the impact striker, Charpy test piece, and anvil Requirement A4-2: Inertia effect during impact striker impact Requirement A4-3: Impact velocity of the impact striker Requirement A4-4: Initial temperature (test temperature) of the Charpy test piece Requirement A4-5: Heat generation condition of the Charpy test piece due to plastic work Requirement A4-6: Thermophysical properties of the Charpy test piece Requirement A4-7: True stress-true strain relationship (SS curve) according to the test temperature and strain rate of the Charpy test piece Requirement A4-8: Mechanical properties in the elastic region of the Charpy test piece Requirement A4-9 Contact conditions of the impact striker, Charpy test piece, and anvil

[0085] Regarding Requirement A4-1, mesh division was performed on the Charpy test specimen, anvil, and analytical model of the impact striker according to the actual dimensions (see Figure 7). For the Charpy test specimen, a 1 / 4 portion was modeled based on the symmetry of the test system to reduce the analysis cost. The element type of the test specimen model was 3D solid voxel elements, and the minimum element size was set to 0.05 mm. The setting area of the minimum element size was 1 mm in the depth direction from the notch bottom, 0.5 mm in the longitudinal direction, and the full width in the width direction. The area 1 mm outside the periphery of the setting area of the minimum element size was divided into elements with a size of 0.1 - 0.2 mm. The element size of the contact area between the test specimen and the impact striker, and the contact area between the test specimen and the anvil was set to 0.1 mm. In the thickness direction, the element size was finely divided on the surface side and coarsely divided on the symmetry plane side into 7 layers. The shapes of the impact striker and the anvil conformed to JIS B 7722 and were modeled with 3D rigid shell elements.

[0086] Regarding Requirement A4-2, since the load acting on the test specimen vibrates violently due to inertia because the impact striker collides at high speed, this inertial effect was considered in the analysis. For the analysis, the Dynamic analysis function in the general-purpose analysis software Abaqus / Explicit was used.

[0087] Regarding Requirement A4-3, the impact speed of the impact striker was set to 5.0 m / s.

[0088] Regarding Requirement A4-4, the initial temperature (test temperature) of the Charpy test specimen was set to -90°C.

[0089] Regarding Requirement A4-5, immediately after the striker impacts the test specimen, heat generation due to plastic deformation occurs near the notch bottom of the Charpy test specimen. This heat generation was set with reference to the report by Taylor and Quinney, assuming that 90% of the plastic work is converted into thermal energy.

[0090] Regarding Requirement A4-6, the temperature rise of the Charpy test piece due to heat generation was set as shown in Table 1 below with reference to the thermal conductivity physical property values of the steel material. Using this value, the thermal conductivity was calculated by coupling thermal conduction analysis and stress analysis. This coupled analysis was executed using the functions of the general-purpose software utilized.

[0091]

Table 1

[0092] Regarding Requirement A4-7, the true stress-true strain relationship corresponding to the test temperature and strain rate was obtained experimentally for the strength corresponding to the temperature, and for the strength corresponding to the strain rate, it was obtained by fitting the results of the instrumented Charpy test and the analysis. A low-temperature static tensile test was carried out with the test temperature at five levels of room temperature (about 20°C), -60°C, -90°C, -120°C, and -150°C, one condition each. As a result, the yield stress (0.2% proof stress) and tensile strength at each test temperature were obtained. Regarding the determination of the strain rate dependence of the material strength, the parameter R expressing the equivalence of the strain rate-temperature parameter shown in Equation (1) was used.

[0093]

Equation

[0094] Based on the static tensile test, the strain rate was set to 10 -4 [s -1 . The value of A was given as 10 8 [s -1 with respect to the yield stress and 10 9 [s -1 with respect to the tensile strength as provisional values. By regressing the relationship between the yield stress or tensile strength obtained from the low-temperature tensile test results and this parameter R, the yield stress or tensile strength at an arbitrary temperature or strain rate was tentatively set. Using the obtained value and the uniform elongation at room temperature, the true stress-true plastic strain relationship was obtained as conforming to the Swift law shown in Equation (2).

[0095]

Equation

[0096] The processing index n and the material constant α were obtained by solving the following simultaneous equations (3).

[0097]

Number

[0098] Assuming that the uniform elongation does not depend on temperature and strain rate, it was set as the value obtained from the room temperature tensile test.

[0099] After giving the settings of Requirements A4-8 and A4-9 described later, the dynamic elastoplasticity of the Charpy impact test was executed to obtain a load-time curve. At this time, when compared with the load-time curve of the instrumented Charpy test obtained in Step A2, since the time change of the load deviated from the experimental value, the value of the above coefficient A and the true stress-true plastic strain relationship corresponding to the value of coefficient A were adjusted so that this deviation became the smallest. As a result of the adjustment, the value of coefficient A was 10 11 [s -1 with respect to the yield stress and 10 12 [s -1 with respect to the tensile strength. The relationship between the yield stress or tensile strength and R (A = 10 11 at the yield stress and A = 10 12 at the tensile strength) is shown in Fig. 14.

[0100] Regarding Requirement A4-8, the mechanical properties in the elastic region of the Charpy test specimen, that is, the Young's modulus and Poisson's ratio, were set to 200 GPa and 0.3, which are representative values of steel materials, respectively.

[0101] Regarding Requirement A4-9, the contact stiffness between the test specimen and the impact striker can be defined by Hertz's contact theory as the relationship between the amount of indentation due to impact and the load per unit time acting on the impact part, and the following equation (4) for the contact of cylinders parallel to each other was used as an approximate formula for setting.

[0102]

Number

[0103] Here, r 2 was approximated to ∞, and e was set to 1. Regarding the contact between the test piece and the anvil, it was set as a completely rigid contact where no indentation occurred on the contact surface.

[0104] Examples of the load-time curves of the instrumented Charpy impact test and the dynamic elastoplastic FEM analysis are shown in Fig. 15. In the analysis, the experimental results can be generally reproduced.

[0105] In step A5, the local generated stress σ Charpy,local corresponding to the fracture occurrence time and position in each of the seven test pieces obtained from the instrumented Charpy impact test was cr obtained as the local fracture limit stress σ Charpy,local . At this time, in the analysis, since the mesh division is divided into a finite number, the output point of the local generated stress corresponding to the fracture occurrence position obtained in the test may not match. In this case, the local generated stress σ Charpy,local at the position closest to the fracture starting point position obtained in the test was referred to. Examples of the local generated stress σ Charpy,local and the fracture occurrence time at the fracture starting point position are shown in Fig. 16. The local generated stress σ cr at this fracture occurrence time was obtained as the fracture limit stress σ cr . Although only a representative example is shown in Fig. 16, actually, for each test piece for which the Charpy impact test was carried out as necessary, that is, here, the fracture limit stress σ

[0106] The acquisition of the local generated stress σ CTOD,local during the CTOD test in step B was performed as follows.

[0107] In step B1, the reference position and the fracture starting point position of the local generated stress σ CTOD,local used for prediction were set as the positions 0.1 mm from the crack tip.

[0108] In Step B2, a static elastoplastic FEM analysis of the CTOD test was performed to obtain the relationship between the local stress σ CTOD,local and CTOD at the location set in Step B1 of the test piece during the test. When performing the analysis, Abaqus / Standard of Dassault Systems, a general-purpose analysis software, was used. When performing this analysis, the following requirements were set.

[0109] Requirement B2-1: Element division of the analysis model of the CTOD test piece Requirement B2-2: Initial temperature (test temperature) of the CTOD test piece Requirement B2-3: True stress-true strain relationship (SS curve) corresponding to the initial temperature of the CTOD test piece Requirement B2-4: Mechanical properties in the elastic region of the CTOD test piece

[0110] Regarding Requirement B2-1, in order to reproduce the CTOD test system on the computer, the mesh division of the analysis model of the CTOD test piece was performed according to the actual dimensions (see Figure 10). The analysis model was modeled with a 1 / 4 part from the symmetry of the test system to reduce the analysis cost, and the element type was 3D solid voxel elements. The element types of the punch and fixed support pins were modeled with 3D rigid shell elements.

[0111] Regarding Requirement B2-2, the initial temperature (test temperature) of the CTOD test piece was set to the test temperature - 90°C to be predicted.

[0112] Regarding Requirement B2-3, from the dynamic SS curves set in Requirement A4-7, the SS curve under the corresponding temperature and strain rate conditions was diverted.

[0113] Regarding Requirement B2-4, the mechanical properties in the elastic region of the CTOD test piece, that is, the Young's modulus and Poisson's ratio, were set to 200 GPa and 0.3, which are representative values of steel materials, respectively.

[0114] In the prediction of the limit CTOD in Step C, for each Charpy impact test, the local fracture limit stress σ crThe value σ of the local stress of the CTOD test piece obtained in step B2 corresponding thereto CTOD,local was referred to, and the CTOD at that time was obtained as the limiting CTOD. An example of predicting the limiting CTOD is shown in FIG. 17. Although FIG. 17 shows an example, actually, for each test piece for which the Charpy impact test was carried out as necessary, that is, here, the limiting CTODs of 7 points were obtained.

[0115] The validity of the prediction method of the example was examined. Specifically, three CTOD tests were carried out at -90 °C, the same temperature as the analysis for the same material, to experimentally obtain the limiting CTOD, and it was compared with the limiting CTOD obtained by the prediction method of the example. Comparing the prediction results and experimental results of the limiting CTOD as shown in Table 2, it can be seen that the experimental results can be well reproduced including the variation in the limiting CTOD.

[0116]

Table 2

Explanation of symbols

[0117] 1 Limiting CTOD value prediction device (fracture toughness value prediction device) 2 Fracture limit stress acquisition unit 2a FEM execution unit 2b Requirement setting unit 2c Prediction unit 3 Local stress acquisition unit 4 Limiting CTOD acquisition unit 5 Instrumented Charpy impact test device (brittle fracture test device) 10 Test piece 11 Test piece 12 Tooth tip of hammer 13 Anvil 21 Test piece 22 Push punch 23 Support pin

Claims

1. A method for predicting a fracture toughness value, which obtains, as a fracture toughness value of a target material, a fracture toughness parameter when the local stress generated in a test piece evaluation part in a fracture toughness test obtained by a first numerical analysis becomes the local fracture limit stress at the fracture initiation position determined based on a brittle fracture test.

2. The method for predicting a fracture toughness value according to Claim 1, wherein the local fracture limit stress is obtained by determining a fracture occurrence time and a fracture initiation position based on the brittle fracture test, and obtaining, by a second numerical analysis, the local stress generated at the fracture occurrence time and the fracture initiation position when the brittle fracture test is performed.

3. The method for predicting a fracture toughness value according to Claim 1 or 2, wherein the local fracture limit stress is obtained without using microstructural information of the target material.

4. The method for predicting a fracture toughness value according to Claim 1 or 2, wherein the target material is a steel material.

5. The method for predicting a fracture toughness value according to Claim 1 or 2, wherein the brittle fracture test is a Charpy impact test.

6. The brittle fracture test is a Charpy impact test, In the second numerical analysis, a dynamic stress-strain curve is used, a load-time history is obtained for each position of the test piece in consideration of temperature and strain rate, and the local stress generated at the fracture occurrence time and the fracture initiation position is obtained based on the load-time history at the fracture initiation position. The method for predicting a fracture toughness value according to Claim 2.

7. The brittle fracture test is a Charpy impact test, Based on a comparison between the load-time history obtained from an actual Charpy impact test and the load-time history obtained from the second numerical analysis, the coefficient used in the second numerical analysis is adjusted. The method for predicting a fracture toughness value according to Claim 2.

8. The method for predicting a fracture toughness value according to Claim 1 or 2, wherein the fracture toughness parameter is CTOD and the fracture toughness value is the critical CTOD.

9. A fracture toughness value prediction device that obtains, as a fracture toughness value of a target material, a fracture toughness parameter when the local stress generated in a test piece evaluation part in a fracture toughness test obtained by numerical analysis becomes the local fracture limit stress at the fracture initiation position determined based on a brittle fracture test.

10. A program for predicting fracture toughness values that obtains, as the fracture toughness value of the target material, the fracture toughness parameter when the local stress generated at the test piece evaluation part in the fracture toughness test obtained by numerical analysis becomes the local fracture limit stress at the fracture initiation position determined based on the brittle fracture test.

11. A method for obtaining a local fracture limit stress, which obtains, as the local fracture limit stress, the local stress generated at the fracture initiation position at the unstable fracture occurrence time based on the brittle fracture test.

12. The unstable fracture occurrence time and the fracture initiation position are determined based on the brittle fracture test, The method for obtaining a local fracture limit stress according to claim 8, wherein the local stress generated is determined based on numerical analysis.

13. The method for obtaining a local fracture limit stress according to claim 11 or 12, wherein the local fracture limit stress is used for predicting the fracture toughness value by numerical analysis.

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