JP2023005230A - Damage behavior evaluation method of micro-structure of metallic material - Google Patents

Abstract

To provide an evaluation method of a damage behavior of a micro-structure of a metallic material in conformity to a real condition targeting the micro-structure of the metallic material to be evaluated without using a complicated numerical analysis model.SOLUTION: A method for evaluating a damage behavior of a micro-structure of a metallic material using a computer executes the steps of: causing a computer to acquire a micro-structure image of the metallic material; creating an FEM analysis model simulating the micro-structure image; acquiring a material property as a parameter; setting a history of the stress or strain applied to each element of the FEM analysis model as a boundary condition; outputting equivalent plastic strain and stress tri-axial degree of each element by executing the FEM analysis; obtaining the distribution of the output equivalent plastic strain and stress tri-axial degree; and evaluating the damage behavior of the micro-structure from the distribution.SELECTED DRAWING: Figure 1

Description

The present invention relates to a method for evaluating damage behavior of microstructures of metallic materials.

Many metal materials can receive a large load, for example, during plastic working in the manufacturing process or when using a structure formed of the metal material. When a large strain exceeding the breaking strain amount of the metal material is generated by receiving a large load, the metal material breaks, resulting in a manufacturing defect in the manufacturing process and loss of its function when used as a structure. Therefore, it is very important to know the rupture properties of metallic materials in advance, for example, to evaluate whether the material will rupture under the occurrence of a certain strain.

Metallic materials exhibiting ductile fracture generate microvoids in the microstructure during deformation due to the above strain, which eventually lead to fracture due to their growth and coalescence. In this specification, the generation (including growth) of microvoids is sometimes referred to as "damage to metal materials." Therefore, if microvoid generation/growth behavior can be evaluated, the rupture properties of the material, such as elongation at break and local ductility, can be evaluated. It is known that the generation of microvoids is related to the plastic strain and hydrostatic stress generated in the material. A GISSMO model is used that defines the relationship of axiality (=hydrostatic stress/equivalent plastic strain) and expresses the fracture of the material on simulations such as the finite element method (FEM).

By the way, in the development of the above metal materials, metal materials with various types of microstructures, structure shapes, phase fractions of each structure, etc. have been developed by adjusting the alloy components and devising heat treatments so as to obtain the desired mechanical properties. manufactured. However, with the techniques of Non-Patent Document 1 and Non-Patent Document 2, it was difficult to clearly grasp the generation and growth behavior of microvoids according to the various microstructures.

On the other hand, a method for directly evaluating the microvoid generation behavior of materials has been proposed. For example, Non-Patent Document 3 proposes direct evaluation of the microvoid generation behavior of a material by stress analysis using a microstructure analysis model to which continuum damage mechanics is applied.

F. Neukamm et al. , 7th European LS-DYNA Conference, 2009 Shota Chinzei et al., R&D Kobe Steel Technical Report, Vol. 66, No. 2 (2017), pages76-81 Shigeru Yonemura et al., Nippon Steel & Sumitomo Metal Technical Report, No. 410 (2018), page 47-56

Conventionally, evaluation of microstructure damage behavior required the use of a complex and computationally intensive computational model such as the continuum damage mechanics model of Non-Patent Document 3. However, the continuum damage mechanics model contains many difficult-to-determine material parameters. Therefore, for example, in the case of dual-phase steel, it is difficult to determine all the material parameters for each constituent phase, and the above material parameters have to be set based on empirical assumptions, making it difficult to evaluate the rupture properties in line with the actual situation. rice field.

The present invention has been made in view of the above circumstances, and does not use a complicated numerical analysis model, and targets the microstructure of the metal material to be evaluated. is to provide an evaluation method for

Aspect 1 of the present invention is
A computer-aided method for evaluating microstructural damage behavior of a metallic material, the computer comprising:
a step 1 of obtaining a microstructure image of the metallic material;
step 2 of dividing the microstructure image into a plurality of elements and creating an FEM analysis model simulating the microstructure image by a finite element method (FEM);
Step 3 of obtaining material properties of the metallic material as parameters of the FEM analysis model;
Step 4 of setting, as boundary conditions, the stress or strain that occurs until the metal material fractures, or the history of the stress or strain that is applied to each element of the FEM analysis model;
Step 5 of performing an FEM analysis under the boundary conditions and outputting the equivalent plastic strain and stress triaxiality of each element;
Step 6 for obtaining the distribution of the equivalent plastic strain and the stress triaxiality of each of the output elements for each stress or strain applied to the FEM analysis model in the FEM analysis;
A method for evaluating damage behavior of a microstructure of a metal material, comprising: performing step 7 of evaluating damage behavior of a microstructure from the distribution of the equivalent plastic strain and the stress triaxiality.

Aspect 2 of the present invention is
The step 4 sets the stress or the strain, or the history of the stress or the strain, to be applied to the integration point of each element,
The step 5 is the damage behavior evaluation method of the microstructure of the metal material according to the aspect 1, wherein the equivalent plastic strain and the stress triaxiality at the integration point of each element are output.

Aspect 3 of the present invention is
The metal material has two or more constituent phases,
3. A method for evaluating damage behavior of a microstructure of a metal material according to aspect 1 or 2, wherein steps 5 to 7 are performed for each constituent phase.

Aspect 4 of the present invention is
A method for evaluating damage behavior of a microstructure of a metallic material according to aspect 3, comprising obtaining the stress-strain curves of each of the two or more constituent phases prepared in advance as the material properties in the step 3. is.

Aspect 5 of the present invention is
5. The metal material according to any one of aspects 1 to 4, wherein in step 7, a damage criterion is determined according to a predetermined procedure, and the presence or absence of microstructure damage is determined using the determined damage criterion. It is a damage behavior evaluation method of microstructure.

Aspect 6 of the present invention is
determining the damage criteria,
A step 11 of preparing a test piece made of the metal material and acquiring a plurality of microstructure images of the same field of view of the test piece while continuously applying strain to the test piece;
Step 12 of determining the microvoid generation position and the amount of plastic strain at the time of microvoid generation from the plurality of microstructure images;
step 13 of creating an FEM analysis model simulating the microstructure image by dividing the microstructure image into a plurality of elements;
a step 14 of obtaining material properties of the metallic material as parameters of the FEM analysis model;
Step 15 of setting a strain amount to be applied to each element of the FEM analysis model as a boundary condition;
A step 16 of performing FEM analysis under the boundary conditions and outputting the equivalent plastic strain and stress triaxiality of each element at each plastic strain amount at the time of microvoid generation;
a step 17 of extracting the data of the equivalent plastic strain and the stress triaxiality at the microvoid generation position from the output data of the equivalent plastic strain and the stress triaxiality of each element;
a step 18 of obtaining distributions of equivalent plastic strain and stress triaxiality at each extracted position;
A method for evaluating the damage behavior of a microstructure of a metallic material according to aspect 5, which is performed in the step 19 of determining the damage criteria from the relationship between the equivalent plastic strain at which microvoids are generated and the stress triaxiality in the distribution. is.

Aspect 7 of the present invention is
The metal material has two or more constituent phases,
7. A method for evaluating damage behavior of a microstructure of a metallic material according to claim 6, wherein steps 17 to 19 in determining the damage criteria are performed for each constituent phase.

According to the present invention, it is possible to provide a method for evaluating the microstructure damage behavior of a metal material that is in line with the actual situation, without using a complicated numerical analysis model, and targeting the microstructure of the metal material to be evaluated.

1 is a block diagram of an evaluation system according to this embodiment; FIG. FIG. 2 is a flow chart showing step by step a method for evaluating damage behavior of a microstructure according to the present embodiment. It is a figure which shows an example of the procedure of creating the analysis model which simulated the micro structure which concerns on this embodiment. FIG. 2 is a flow diagram showing the steps of a method for determining microstructural damage criteria according to the present embodiment. 1 is a micrograph of materials used in Examples. 4 is a graph of equivalent plastic strain and stress triaxiality in Examples. It is another graph of equivalent plastic strain and stress triaxiality in the example. It is another graph of equivalent plastic strain and stress triaxiality in the example. It is another graph of equivalent plastic strain and stress triaxiality in the example. It is a figure which shows the shape of the tensile test piece used in the Example. It is a side view of a quasi-continuous microstructure observation-tensile test apparatus used in Examples. It is a figure which shows the observation position in the tensile test piece used in the Example. It is a figure which shows the microscope observation time during a tensile test in an Example. It is a microscope photograph which shows a part of the microscopic observation result in an Example. It is a microscope photograph which shows another part of the microscopic observation result in an Example. It is a microscope photograph which shows another part of the microscopic observation result in an Example. It is a microscope photograph which shows another part of the microscopic observation result in an Example. It is a microscope photograph which shows another part of the microscopic observation result in an Example. It is a microscope photograph which shows another part of the microscopic observation result in an Example. It is a microscope photograph which shows another part of the microscopic observation result in an Example. 4 is a graph showing the relationship between the equivalent plastic strain of the ferrite phase and the stress triaxiality and the damage criteria in Examples. 4 is a graph showing the relationship between the equivalent plastic strain of the martensite phase and the stress triaxiality and the damage criteria in Examples.

The present inventors have conducted intensive research to realize a method that can evaluate the damage behavior of the microstructure to be evaluated in accordance with the actual situation without using a complicated calculation model as in the conventional technology. did As a result, the equivalent plastic strain and stress triaxiality obtained by numerical analysis of the metal structure should be applied to the evaluation of the damage behavior of the microstructure. , a structural calculation is performed by setting the stress or strain that occurs until the fracture of the metal material or their history as boundary conditions, and the distribution of the equivalent plastic strain (εeq ^P ) and stress triaxiality (η) obtained from the microscopic It was found that the method can be applied to microvoid generation behavior such as microvoid generation position, generation timing, generation amount, etc. in the tissue.

An evaluation system 1, which is an example of executing a method for evaluating microstructure damage behavior of a metal material according to the present embodiment, will be described. FIG. 1 is a block diagram of an evaluation system 1 according to this embodiment. The evaluation system 1 is a computer system and includes a testing section 10 , a control section 20 , a storage section 30 , a display section 40 and an input section 50 . At least the control unit 20 and the storage unit 30 constitute a computer.

The testing section 10 has a stress/strain imparting section 11 and an imaging section 12 . The stress/strain applying unit 11 is an experimental apparatus capable of holding a metal material and applying a predetermined stress or strain (see FIG. 11 described later for details). The stress/strain imparting unit 11 includes an imaging device for measuring elongation of a metal piece (for example, a video camera equipped with an imaging device such as a CCD, CMOS, etc.), and measures deformation of the metal material during the application of stress or strain. be able to. Details of the metal material will be described later. The stress/strain imparting unit 11 starts photographing so that the entire metal material is sufficiently within the angle of view, and continuously photographs the deformation of the metal material over time. The stress/strain imparting unit 11 can measure deformation by measuring the distance between the ends of the metal material in the left-right direction or the up-down direction. Further, as will be described later, the deformation may be measured by attaching a mark (for example, a sticker) to the metal material and measuring the distance between the marks.

The imaging unit 12 is, for example, a scanning electron microscope (SEM) or the like, and partially captures at least a portion of the metal material before stress or strain is applied and to which stress or strain is applied in the stress/strain applying unit. A magnified microstructure image can be taken. The photographing unit 12 transmits data of the photographed microstructure image to the control unit 20 . As will be described later, the control unit 20 stores the received microstructure image data in the storage unit 30 .

The control unit 20 executes model creation processing 21 , parameter setting processing 22 , analysis processing 23 and distribution creation processing 24 . The control unit 20 includes a general-purpose processor such as a CPU or MPU that implements predetermined functions by executing programs. The control unit 20 realizes various types of processing in the evaluation system 1 by calling and executing a calculation program or the like stored in the storage unit 30, for example. The control unit 20 can implement a model creation process 21 , a parameter setting process 22 , an analysis process 23 and a distribution creation process 24 by calling up and executing a calculation program or the like stored in the storage unit 30 . The control unit 20 is not limited to one that realizes a predetermined function through the cooperation of hardware and software, and may be a hardware circuit designed exclusively for realizing a predetermined function. That is, the control unit 20 can be realized by various processors such as GPU, FPGA, DSP, ASIC, etc., in addition to CPU and MPU. Such a control unit 20 can be configured by, for example, a signal processing circuit that is a semiconductor integrated circuit.

The control unit 20 creates an FEM analysis model for use in FEM analysis from the microstructure image of the metal material captured by the image capturing unit 12 by model creation processing 21 . Programs, parameters, and the like for creating the FEM analysis model are stored, for example, in the storage unit 30, and are read by the control unit 20 as necessary and used for model creation. As will be described later, the control unit 20 can create an FEM analysis model by image-based modeling, for example. When performing image-based modeling, the control unit 20 applies any image processing filter to at least a partial region of the microstructure image to form shape data based on the microstructure shape of the region of the metal material. . Then, the control unit 20 simulates the region based on each parameter of the FEM analysis model (for example, the type of element of the model to be created, the number of elements, the number of nodes, etc.) set by the parameter setting process 22 described later. It is possible to create a FEM analysis model with The created FEM analysis model is output to the storage unit 30 and stored.

The control unit 20 sets various parameters by the parameter setting process 22 . The parameters may include, for example, the stress or strain applied by the stress/strain applying unit 11, the type of elements of the FEM analysis model created by the control unit 20 by the model creation process 21, the number of elements, and the number of nodes. . In addition, the parameters include the characteristics of the metal material to be analyzed by FEM analysis (Young's modulus, yield stress and yield strength, work hardening index and crystal orientation of the metal material, etc.), boundary conditions when performing FEM analysis by analysis processing 23, etc. may be included. The parameters are output to and stored in the storage unit 30 . Then, each component reads the parameter from the storage unit 30 and uses it as necessary.

The control unit 20 can use the created FEM analysis model by the analysis processing 23 to perform FEM analysis for simulating shape deformation when stress or strain is applied to the metal material. The FEM analysis model used for the analysis, the program for performing the simulation, the boundary conditions, etc. are stored, for example, in the storage unit 30, and are read by the control unit 20 as necessary and used for the analysis. The control unit 20 of the evaluation system 1 according to the present embodiment calculates the stress triaxiality and the equivalent plastic strain at the integration point of each element of the FEM analysis model for each stress or strain applied to the FEM analysis model by the analysis processing 23. can be output. The stress triaxiality and the equivalent plastic strain may be output to the display unit 40 and displayed on the display unit 40, may be output to the storage unit 30 and stored, or may be output to the output unit 60 described later. and may be transmitted to another control device.

The control unit 20, for example, by distribution creation processing 24 such as graph creation processing, for each applied stress or strain, distribution of stress triaxiality and equivalent plastic strain at each integration point of the FEM analysis model calculated and output by analysis processing 23 can be asked for. Specifically, for example, they can be plotted on a graph with these as axes. Alternatively, the user can select an integration point using the input unit 50, for example, and plot the stress triaxiality and equivalent plastic strain of the selected integration point on the graph. Programs, parameters, and the like for creating the graph are stored, for example, in the storage unit 30, and are read by the control unit 20 as necessary and used for creating the graph.

The storage unit 30 is a recording medium capable of recording various information. The storage unit 30 is realized by, for example, a memory such as a DRAM, an SRAM, a flash memory, an HDD, an SSD, other storage devices, or an appropriate combination thereof. The storage unit 30 can store data such as the image captured by the imaging unit 12 and the FEM analysis model created by the model creating process 21 . In addition, the storage unit 30 can store parameters such as material properties and boundary conditions used for analysis set in the parameter setting process 22, data of analysis results calculated and output in the analysis process 23, and the like. . Furthermore, the storage unit 30 can transmit the stored information to each component such as the control unit 20 and the display unit 40 as necessary. Each component may transmit and receive information directly between the components without going through the storage unit 30 .

The display unit 40 is a display that can display arbitrary data, such as microstructure images of metal materials, FEM analysis models, parameters such as material properties and boundary conditions used for analysis, calculated analysis results, created graphs, and the like. , can display arbitrary information. The user operates the input unit 50 based on the information displayed on the display unit 40 to set the parameters by the parameter setting processing 22 of the control unit 20, or the microstructure image captured by the imaging unit 12 and the control The analysis result calculated by the analysis processing 23 by the unit 20 can be compared.

The input unit 50 is, for example, a general computer input device such as a keyboard and a mouse, and inputs parameters and the like to the parameter setting process 22 of the control unit 20, and analyzes the control unit 20 displayed on the display unit 40. A position can be indicated with respect to the analysis result by the processing 23 . Further, when a touch panel is used for the display section 40, the display section 40 may also have a function corresponding to the input section 50, such as a mechanism for detecting a position touched by the user.

The evaluation system 1 may also include an output unit 60 or the like capable of transmitting acquired information, calculated information, and the like to other devices. The output unit 60 is, for example, a USB terminal or an Ethernet terminal. Moreover, it may be a communication device for performing wireless communication. As such a communication device, a standard conforming to the Wi-Fi (registered trademark) standard, which performs wireless communication using frequencies such as 2.4 GHz/5.2 GHz/5.3 GHz/5.6 GHz, is adopted. obtain.

Note that the evaluation system 1 only needs to have at least the control unit 20 and the storage unit 30 . For example, the control section 20 has a receiving terminal for receiving an image from the testing section 10 and a receiving terminal for receiving a user's instruction from the input section 50, and outputs a signal to the display section 40 and/or the output section 60. It is only necessary to have an output terminal for At this time, some or all of the testing section 10, the display section 40, the input section 50, and the output section 60 may not be included in the evaluation system 1.

A method for evaluating the microstructure damage behavior of a metal material according to the present embodiment will be described with reference to the microstructure damage behavior evaluation flowchart of FIG. 2 as an example.

[Step 1: Acquisition of microstructure image of metal material]
First, as in step 1 (S1) of FIG. 2, a microstructure image of the metal material is acquired.

The microstructure image is, for example, an image observed with a scanning electron microscope (SEM), an optical microscope (OM), electron beam backscatter diffraction (EBSD), computed tomography (CT), ultrasonic inspection (UT), or the like. You can use it. Pretreatment such as etching may be performed in advance depending on the type of constituent phases that constitute the microstructure. The size, magnification, etc. of the microstructure image may be appropriately set according to the evaluation region, evaluation position, and the like.

The steel type, strength level, etc. of the target metal material do not matter. The metal material is not limited to steel, and includes aluminum, titanium, copper, and magnesium pure metals or alloys. When the metal material is steel, the type of microstructure, structure shape, and phase fraction are not limited. Constituent phases constituting the microstructure include, for example, one or more of ferrite, martensite, bainite, pearlite, retained austenite, and MA, and may also include precipitates such as cementite. The structure shape refers to the shape of crystals, and examples thereof include branch-like, granular, needle-like, and the like. The chemical composition of the steel is also not limited. The method for producing the metal material is not limited, either, and production conditions such as hot rolling conditions and heat treatment conditions can be appropriately set so as to obtain a desired microstructure. When evaluating a plurality of metal materials, for example, when comparing a plurality of metal materials, it is possible to prepare a plurality of metal materials that differ in one or more of, for example, the type of constituent phase of the microstructure, the shape of the structure, and the phase fraction. mentioned. For the sake of simplicity, in the evaluation method described below, the metallic material is a duplex steel composed of ferrite as the soft phase and martensite as the hard phase, and the microstructural damage behavior of the metallic material is evaluated. .

[Step 2: Creation of FEM analysis model]
As shown in step 2 (S2) of FIG. 2, the control unit 20 creates a mesh model as an FEM analysis model to be used for FEM analysis in the model creation process 21. FIG. The analysis model can be created, for example, by image-based modeling using acquired microstructure images of the metal material. An example of the model creation method is described in FIG. FIG. 3A shows the acquired microstructure image. The control unit 20 sets a predetermined threshold value for the density or brightness of pixels in such a microstructure image, and sets a value for a given pixel position depending on whether the density or the like at an arbitrary pixel position exceeds the threshold value. A binary image is created by performing binarization processing. FIG. 3B is a binary image created from the microstructure image of FIG. 3A. This allows the martensite phase to be extracted and separated from the other phases.

Next, the control unit 20 acquires the contour of the region corresponding to the martensite phase from the binary image. The contour can be acquired, for example, by using an image processing filter (eg, Sobel filter, Laplacian filter, etc.) that detects edges. FIG. 3C represents the contour of the region corresponding to the martensite phase (hard phase) obtained from the binary image.

Next, the control unit 20 creates shape data of the region corresponding to the martensite phase and shape data of the region corresponding to the ferrite phase from the contours. FIG. 3D shows shape data for creating a region M corresponding to the martensite phase and a region F corresponding to the ferrite phase. After that, the control unit 20 creates a two-dimensional mesh model by dividing the shape data into meshes set based on predetermined conditions. FIG. 3E is an example of a mesh region created in this way. The mesh model according to this embodiment is divided into meshes of quadrilateral elements. FIG. 3E shows an example of the shape of the mesh by partially enlarging a portion of the upper right region. A quadrilateral element need not be rectangular. Moreover, it is not essential to be a quadrilateral, and it is sufficient if the polygon is a triangle or more.

As the FEM analysis model in this embodiment, the above mesh model is created as a 2D model with a total number of elements of 40,279 and a total number of nodes of 40,507 by setting 4-node isoparametric elements. Then, the 2D model is expanded by one layer in the depth direction of the image, and a 3D model with a total number of elements of 40,279 and a total number of nodes of 81,014 is created by 8-node isoparametric elements, and the 3D model is used. FEM analysis is performed by The conditions of the model are not limited to those described above, and for example, elements with different numbers of constituent nodes may be used with respect to the types of elements. In addition, FEM analysis may be performed by creating 3D models with different total number of elements and total number of nodes. Also, instead of the 3D model, a 2D model may be created and used as the FEM analysis model.

[Step 3: Acquisition of material properties]
As shown in step 3 (S3) of FIG. 2, the control unit 20 acquires the properties of each material related to each phase that constitutes the duplex steel and sets them as parameters by the parameter setting process 22 in order to perform the FEM analysis. Here, the material properties to be obtained are, for example, Young's modulus, yield stress and yield strength, work hardening index and crystal orientation of each of the ferrite phase and the martensite phase. In addition, the control unit 20 acquires the stress-strain curve of each material constituting the metal material (in this embodiment, the stress-strain curve of ferrite and the stress-strain curve of martensite) as material characteristics. As a result, when the control unit 20 performs the FEM analysis described later in the analysis processing 23, each A stress or strain applied to the integration point is calculated. A stress-strain curve for each material that constitutes a metal material can be obtained, for example, by performing a tensile test on a test piece that is composed of the material.

[Step 4: Setting Boundary Conditions]
As in step 4 (S4) of FIG. 2, the control unit 20 sets boundary conditions to be given to the FEM analysis model by the parameter setting process 22. FIG. The boundary conditions provide the stress or strain history that is actually considered to occur in the material until the structure that constitutes the metal material fails. The history of stress or strain can be estimated in advance by structural analysis of the structure. Moreover, it may be measured by actually using, for example, a strain gauge from the structure, or arbitrary conditions may be set. Boundary conditions are not limited to stress or strain histories and may, for example, impose stress or strain. In this case, the control unit 20 sets the parameters for performing the FEM analysis by the parameter setting process 22 with a predetermined increase width based on the boundary conditions.

[Step 5: Execution of FEM analysis (output of equivalent plastic strain and stress triaxiality)]
As shown in step 5 (S5) of FIG. 2, the control unit 20 executes the FEM analysis using the created FEM analysis model by the analysis processing 23. FIG. In FEM analysis, by giving the history of stress or strain set as boundary conditions to the created FEM analysis model, it is possible to analyze by simulation how the metal material deforms when stress or strain is applied. can be done. Through the analysis, the "stress triaxiality" and "equivalent plastic strain" in each element of the FEM analysis model can be calculated and output for each stress or strain applied to each element of the FEM analysis model. Each element of the FEM analysis model has preset integration points for performing integration during analysis. "Stress triaxiality" and "equivalent plastic strain" are calculated for each stress or strain applied to the integration points. In addition to the integration point of each element of the FEM analysis model, for example, the element solution or nodal solution calculated from the solution calculated for the integration point is used to calculate the "stress triaxiality" and "equivalent plastic strain". good too.

[Step 6: Creation of distribution of equivalent plastic strain and stress triaxiality]
As shown in step 6 (S6) in FIG. 2, the control unit 20 can obtain the distribution of the equivalent plastic strain and the stress triaxiality by the distribution creating process 24 such as the graph creating process. Specifically, for example, a graph of equivalent plastic strain and stress triaxiality is created. Specifically, for each stress or strain set as the boundary condition, the output equivalent plastic strain and stress triaxiality at each position are plotted on a graph with these as axes. In addition, the control unit 20 plots the calculated equivalent plastic strain and stress triaxiality on the graph for each predetermined time interval (for example, for each time interval advanced per analysis in FEM analysis). good. In the graph, for example, the vertical axis can be the equivalent plastic strain, and the horizontal axis can be the stress triaxiality. In the examples below, the equivalent plastic strain and stress triaxiality of all the integration points are plotted, but not limited to this, the equivalent plastic strain and stress of some integration points in the model as the position Triaxiality may be plotted.

[Step 7: Evaluation of microstructural damage behavior]
As shown in step 7 (S7) of FIG. 2, the damage behavior of the microstructure is evaluated from the distribution of the equivalent plastic strain and the stress triaxiality. An example of the evaluation method is to observe the distribution form of the plot points for each stress or strain set as the boundary condition. Alternatively, by comparing two or more distributions of plot points with different stresses or strains set as the boundary conditions, observe the transition of the plot points accompanying the deformation process, that is, the change in the amount of plastic strain (εmicro), and microvoids It can be used to evaluate developmental behavior. According to this embodiment, it is not necessary to use a complicated continuum damage model as in the prior art, and it is possible to easily evaluate the microvoid generation behavior of a metal material.

As described above, the evaluation method according to this embodiment does not particularly limit the target metal material. The metal material may have one type of constituent phase, or may have two or more types of constituent phases. When the metal material has two or more constituent phases, steps 5 to 7 can be performed for each constituent phase. In this case, in step 7, the susceptibility to microvoid generation may be evaluated by comparing the plot point distributions of the equivalent plastic strain and the stress triaxiality shown for each constituent phase.

In the above step 7, damage criteria prepared separately may be used to determine whether or not the microstructure is damaged. As the damage criteria, experimentally determined damage criteria are preferably used as described below. The damage criteria are first obtained by a simple experiment to determine the location of microvoid generation, which is damage to the microstructure, and the amount of plastic strain at the time of microvoid generation. It is obtained by relating the equivalent plastic strain and the stress triaxiality at the point of origin. By using the damage criteria, it is possible to evaluate the damage behavior of microstructures in line with actual conditions. In this embodiment, a plurality of plot points are displayed on the graph to derive the damage criteria, but it is not essential to display a plurality of plot points. Instead of displaying a plurality of plot points, the derived damage criteria may be displayed.

The method for experimentally determining the damage criteria will be described below with reference to the microstructure damage criteria determination flow diagram of FIG. 4 as an example.

[Step 11: Acquisition of multiple microstructure images during plastic deformation]
As in step 11 (S11) of FIG. 4, the imaging unit 12 acquires a plurality of microstructure images during plastic deformation. Specifically, a test piece made of a metal material is prepared, and a plurality of microstructure images of the same field of view of the test piece are acquired while continuously applying strain to the test piece.
The method of applying the strain is not particularly limited as long as the strain can be applied continuously. The method includes, for example, a tensile test, for example, performing a uniaxial tensile test. When performing the above tensile test, as described in the examples described later, the test piece is pulled until a predetermined amount of plastic strain is reached, the test piece is removed from the tensile tester and observed under a microscope, and the test piece is placed in the tensile tester again. A quasi-continuous microstructure observation-tensile test, in which the specimen is placed in a slab and repeatedly pulled, can be mentioned. However, the present invention is not limited to this, as long as it is possible to acquire a plurality of microstructure images when applying various amounts of plastic strain. For example, the method of microstructure observation and tensile test as shown in Non-Patent Document 3 may be adopted.

In the tensile test piece used in the tensile test, the surface for obtaining the microstructure image may be mirror-polished and, if necessary, subjected to pretreatment such as etching depending on the type of microstructure. . For the microstructure image, an image observed with a scanning electron microscope (SEM) or the like can be used, as in step 1 above. The size, magnification, etc. of the microstructure image may be appropriately set according to the evaluation region, evaluation position, and the like.

The nominal strain amount of the tensile test piece in the tensile test is obtained by photographing the deformation of the test piece with a mark (sticker) attached to the test piece for measuring elongation, as described in the examples described later. It can be obtained from the position of the mark. In addition, by comparing the structure shape of the metal material displayed in the microstructure photographs before and after deformation of the test piece, the same region was identified, and the photograph was taken by estimating the deformation amount of the region. The amount of plastic strain (ε _micro ) in a localized region of the microstructure at the point in time can be determined. Note that the nominal strain amount and the plastic strain amount are expressed, for example, by degrees of displacement (%) when the shape of the test piece before deformation is taken as 100. As an example, we find that at nominal strain ε _nominal =13.3% and nominal strain ε _nominal =18.2%, plastic strain ε _micro =17.1% and plastic strain ε _micro =35.2%. 5% microstructural images were acquired.

[Step 12: Grasping the microvoid generation position from the microstructure image and the plastic strain at the time of microvoid generation]
As shown in step 12 (S12) of FIG. 4, microvoid generation positions and plastic strain amounts (ε _micro ) at the time of microvoid generation are grasped from the plurality of microstructure images. The positions where microvoids are generated can be confirmed visually or by image analysis in the microstructure image. In the case where there are two or more types of constituent phases constituting the metal material, the location of occurrence of microvoids is determined in which constituent phase. For example, a microstructure image may be used to record the location of microvoid generation and the amount of plastic strain (ε _micro ) imparted to the metal material when microvoids were generated.

[Step 13: Creation of FEM analysis model]
As shown in step 13 (S13) of FIG. 4, the control unit 20 creates a mesh model as an FEM analysis model to be used for FEM analysis by the model creating process 21. FIG. The FEM analysis model can be created in a manner similar to step 2 above.

[Step 14: Acquisition of material properties]
As shown in step 14 (S14) of FIG. 4, the control section 20 acquires the properties of each material constituting the metal material tested in step 11 by the parameter setting process 22. FIG. Here, the material properties to be acquired are, for example, the Young's modulus, yield stress and yield strength, work hardening index, crystal orientation, and stress-strain curve of each constituent material, as described in step 3. .

[Step 15: Setting Boundary Conditions]
As shown in step 15 (S15) of FIG. 4, the control unit 20 sets boundary conditions to be given to the FEM analysis model by the parameter setting process 22. FIG. Similar to step 4 above, the boundary condition gives the history of the amount of stress or strain that is actually considered to occur in the material until the structure that constitutes the metal material is destroyed.

[Step 16: Execution of FEM analysis (output of equivalent plastic strain and stress triaxiality)]
As shown in step 16 (S16) of FIG. 4, the control unit 20 performs the FEM analysis using the created FEM analysis model by the analysis processing 23. FIG. As in step 5 above, by giving the stress or strain history set as a boundary condition to the created FEM analysis model, we can simulate how the metal material will deform when stress or strain is applied. can be analyzed. By the analysis, the stress triaxiality and the equivalent plastic strain at each position of the FEM analysis model can be calculated and output for each stress or strain applied to the FEM analysis model.

[Step 17: Extraction of equivalent plastic strain and stress triaxiality at microvoid occurrence position]
As shown in step 17 (S17) in FIG. 4, the control unit 20 selects the equivalent plastic strain at the microvoid occurrence position from the data of the equivalent plastic strain and the stress triaxiality output at each position by the analysis processing 23. and extract the stress triaxiality data. The extracted data is a value calculated by applying the plastic strain ε _micro when the microvoid is generated recorded in step 12 to the FEM analysis model and analyzing how it is deformed. Therefore, it is possible to extract the data of the equivalent plastic strain and the stress triaxiality corresponding to the plastic strain ε _micro when microvoids are generated.

[Step 18: Creation of distribution of equivalent plastic strain and stress triaxiality at the extracted microvoid occurrence position]
As shown in step 18 (S18) of FIG. 4, the control unit 20 obtains the distribution of the equivalent plastic strain and the stress triaxiality at each of the extracted positions by the distribution creation process 24 such as the graph creation process. Specifically, for example, they are plotted on a graph with these as axes. Obtaining the distribution clarifies the relationship between the equivalent plastic strain and the stress triaxiality (combined value of the equivalent plastic strain and the stress triaxiality) that causes microvoids in the target constituent phase. When creating a graph as a distribution, the graph can have, for example, the equivalent plastic strain (εeq ^P ) on the vertical axis and the stress triaxiality (η) on the horizontal axis.

[Step 19: Determine Damage Criteria in Distribution]
As shown in step 19 (S19) of FIG. 4, the damage criteria are determined from the relationship between the equivalent plastic strain at which microvoids are generated and the stress triaxiality in the distributions such as the graphs. According to one example of the distribution of this determination method, the combined values of equivalent plastic strain and stress triaxiality at which microvoids occur are plotted on the graph. In addition, as described above, the greater the equivalent plastic strain and stress triaxiality, the more likely damage is to occur. In other words, microvoids are less likely to occur in regions where the equivalent plastic strain and stress triaxiality are small. Therefore, the damage criterion is indicated as a boundary line between the region where microvoids are unlikely to occur and the clustered region of the plot points where microvoids occur. As the number of plotted points increases, the accuracy increases. Therefore, it is preferable to indicate, for example, 3 points or more, more preferably 5 points or more. The control unit 20 may display the damage criteria on a graph from the results plotted by the distribution creation process 24 . Alternatively, the control unit 20 uses the distribution creation processing 24 to graph the damage criteria from the output equivalent plastic strain and stress triaxiality of each element for each stress or strain applied to the FEM analysis model in the FEM analysis. may be displayed above.

In the examples described later, an inverse proportional approximation formula is shown as a criterion for the relationship between the equivalent plastic strain and the stress triaxiality, but the present invention is not limited to this. The coefficients of the approximation formula may also change depending on the data. The criterion formula may be a linear approximation line or the like depending on the obtained data.

If the metal material is a dual phase steel and there are two or more constituent phases, the damage criteria are determined for each constituent phase. Therefore, in the case of dual-phase steel, the damage criteria are determined by performing steps 17 to 19 above for each constituent phase. For example, when the constituent phases are a ferrite phase and a martensite phase, the damage criteria may be determined for each of these ferrite phase and martensite phase as illustrated in the examples below.

As described above, when using the conventional continuum damage model, there are multiple parameters that are difficult to determine when directly evaluating the microvoid generation behavior. Since the damage criteria are determined by comparing the actual state of void generation with the equivalent plastic strain and the stress triaxiality, it is possible to determine the threshold value easily and in line with the actual state, and using this damage criterion, it is possible to quickly determine the actual state. damage assessment can be performed.

EXAMPLES Hereinafter, the present invention will be described more specifically with reference to Examples. The present invention is not limited by the following examples, and can be implemented with appropriate modifications within the scope that can match the spirit described above and below. subsumed in That is, although the examples using the duplex steel composed of the two phases of the ferrite phase as the soft phase and the martensite phase as the hard phase are shown below, the present invention is not limited to this. In addition, although the graph is created below as an example of the distribution of the stress triaxiality and the equivalent plastic strain, the present invention is not limited to this.

In this embodiment, two types of steel sheets, which are a duplex steel composed of two phases of a soft layer and a hard phase, have the same type of phases that constitute the duplex steel, but have different structural shapes and phase fractions. The microvoid generation behavior of these two types of steel sheets was evaluated as follows.

1. Preparation of metal material As material 1, steel containing 0.063% by mass of C, 0.50% by mass of Si, and 1.46% by mass of Mn (actual result) was melted, obtained an ingot, and then forged. , heat-treated to prepare a dual-phase steel of martensite and ferrite. As material 2, a dual-phase steel of martensite and ferrite was prepared in which the area fraction of martensite, which is a hard phase, was different from that of material 1. Using the above materials 1 and 2, microstructure damage behavior was evaluated according to the following procedure.

2. Acquisition of microstructure image of metal material Each of the above materials is cut into a size of about 10 mm x 10 mm for microscopic observation, etched with nital, and then photographed with a scanning electron microscope at a magnification of 1000. Microstructural images were acquired. A photomicrograph of material 1 is shown in FIG. 5A and a photomicrograph of material 2 is shown in FIG. 5B. In FIG. 5A, the white portion indicates the hard phase Q, which is the martensite phase, and the gray portion indicates the soft phase R, which is the ferrite phase. In FIG. 5B, the light gray portion indicates the martensite phase, which is the hard phase Q, and the dark gray portion indicates the ferrite phase, which is the soft phase R. 5A and 5B, image analysis of the micrographs revealed that the area fraction of martensite, which is the hard phase Q, was about 15% for material 1 and about 50% for material 2.

3. Creation of FEM analysis model Next, the control unit 20 of the evaluation system 1 is caused to execute the model creation process 21, and the mesh imitating the image of the microstructure of each of the materials 1 and 2 is used as the FEM analysis model to be used for the FEM analysis. created the model. A FEM analysis model can be created in a manner similar to step 2 above. In this embodiment, similar to the FEM analysis model described in step 2, the mesh model is created as a 2D model with a total number of elements of 40,279 and a total number of nodes of 40,507 by setting 4-node isoparametric elements. It is By causing the control unit 20 to execute the model creation process 21, the 2D model is expanded by one layer in the depth direction of the image, and the total number of elements is 40,279 and the total number of nodes is 40,279 by the 8-node isoparametric element. Create a 3D model of 81,014. After that, the controller 20 was caused to execute the analysis processing 23 to perform FEM analysis using the 3D model.

The created 2D mesh model is created so that the area fraction of the region corresponding to the hard phase martensite is about 15% for material 1 and about 50% for material 2. . Similarly, the 3D mesh model is made such that the volume fraction of the region corresponding to the hard phase martensite is about 15% for material 1 and about 50% for material 2. .

4. Acquisition of material properties The properties of ferrite and martensite, which are materials constituting the metal material, are acquired and input as parameter setting processing 22 of the control unit 20, and the regions corresponding to ferrite and martensite in the FEM analysis model was set as a parameter of Here, the acquired material properties are, for example, Young's modulus, yield stress and yield strength, work hardening index and crystal orientation of ferrite and martensite, respectively, as described in step 3. In addition, stress-strain curves for ferrite and martensite, which constitute the metal material, were obtained and set in the control unit 20 as material properties. These stress-strain curves were obtained by performing tensile tests on the single-phase steel composed of ferrite and the single-phase steel composed of martensite, respectively.

5. Setting of Boundary Conditions Next, the boundary conditions given to each FEM analysis model were set in the controller 20 . Similar to step 4 above, the boundary conditions provided a history of the amount of strain that would actually occur in the material until the structure constituting the metal material was destroyed. In this example, strain is generated in a tensile test piece by a uniaxial tensile test, as will be described later. Therefore, as boundary conditions given to the FEM analysis model, the direction corresponding to the tensile direction in the tensile test is the x axis (horizontal direction in FIGS. 5A and 5B), and the direction orthogonal to the x axis is the y and z axes (FIGS. 5A and 5B, respectively). 5B), the strain increment was given so that ε _x =−νε _y =−νε _z . where ε _x is the strain increment along the x-axis, ε _y is the strain increment along the y-axis, and ε _z is the strain increment along the z-axis. Also, ν is Poisson's ratio. In this embodiment, ν is set to 0.5. Therefore, the strain increment in the direction of each axis was set to x-axis direction:y-axis direction:z-axis direction=1:-0.5:-0.5.

6. Execution of FEM analysis (output of equivalent plastic strain and stress triaxiality)
Next, the controller 20 is caused to execute the analysis processing 23 to execute the FEM analysis using the created FEM analysis model. In the FEM analysis, when the plastic strain set by the boundary conditions as described above is applied to the FEM analysis model, it is possible to analyze by simulation how the analysis model deforms. Thereby, the displacement at each position in the analysis model can be calculated, and the stress triaxiality and the equivalent plastic strain can be calculated and output. In this embodiment, the values at the integration points of the elements of the FEM analysis model are used as the respective positions. By changing the plastic strain set as a boundary condition, the above FEM analysis can analyze any plastic strain.

7. Graph creation of equivalent plastic strain and stress triaxiality The control unit 20 is caused to execute the distribution creation process 24, and each position in the microstructure obtained from the above calculation result, that is, the stress triaxiality and the equivalent The values of plastic strain were plotted on a graph with equivalent plastic strain (εeq ^P ) on the vertical axis and stress triaxiality (η) on the horizontal axis for each strain value given as a boundary condition. As the graphs plotted above, the graphs of the soft phases of material 1 and material 2 when the plastic strain in the x-axis direction is 5% are shown in FIGS. 6A and 6B, respectively. 7A and 7B are graphs of the hard phase of material 2 and material 2, respectively, and FIGS. Biaxial graphs of the hard phases of Material 1 and Material 2 at a plastic strain of 20% are shown in FIGS. 9A and 9B, respectively.

8. Evaluation of Microstructure Damage Behavior Material 1 (microstructure with less hard phase) and material 2 (microstructure with more hard phase) were compared for each strain amount and each constituent phase. As a result, in both the soft and hard phases of Material 1 and Material 2, when the same strain is applied, Material 2 tends to show higher values for stress triaxiality and equivalent plastic strain. I understand. Moreover, it can be seen that this tendency is more pronounced at a strain of 20% than at a strain of 5%. As mentioned above, the greater the equivalent plastic strain and stress triaxiality, the more likely damage is to occur. From the comparison of these graphs, it can be judged that microscopic damage in the microstructure tends to occur more quickly.

9. Determination of damage criteria Damage criteria were determined according to the following procedure.

(1) Acquisition of multiple microstructure images during plastic deformation (quasi-continuous microstructure observation-tensile test)
Microstructural observations were performed at multiple stages during the tensile test to photograph the stress/strain during the tensile test and the microstructure deformed under the stress/strain.
Specifically, a tensile test piece having the shape shown in FIG. 10 was prepared. The tensile test piece was made of a material having a martensite area ratio of 13.7%. Then, one plane of the tensile test piece is mirror-polished (with nital etching) for microscopic observation, and a video extensometer seal is attached to the other plane, and an example of the stress/strain imparting unit 11 As shown in the side view of a quasi-continuous microstructure observation-tensile test apparatus 70 in FIG. 11, a (tensile) test piece 71 is fixed with a chuck 72, and a video camera 74 was placed. A tensile test was performed while filming with a video camera. The test conditions for the tensile test were a crosshead displacement rate of 1 mm/min under a room temperature environment. The surface of the (tensile) test piece 71 opposite to the surface to which the seal 73 is applied has a polished surface 75 for microstructure observation.

In the test, SEM observation was first performed to obtain a microstructure photograph of the state before strain was applied. Then, the tensile test piece 71 is placed in a tensile tester and pulled to apply strain, stop pulling when a predetermined amount of elongation is reached, remove the tensile test piece 71 from the tensile tester, and remove the tensile test piece 71. The polished surface 75 for microstructure observation was observed with an SEM, and then the tensile test piece 71 was placed again in the tensile tester and pulled repeatedly until a predetermined amount of elongation was reached. By repeating this process, it is possible to grasp the microstructure at a certain timing (amount of strain). The observation position was the position of the arrow in the tensile test piece as shown in FIG. Observation of the microstructure was performed using a scanning electron microscope. The microstructure was observed in the same visual field at a plurality of times from the start of measurement (no strain) to immediately before fracture including before and after occurrence of constriction, as shown in FIG. 13 . In each of the above SEM observations, when each stress was applied to the tensile test piece, the tensile test piece was temporarily removed from the tensile tester, and the polished surface of the test piece was microscopically observed.

(2) Ascertainment of Microvoid Occurrence Location and Plastic Strain at Microvoid Occurrence from Microstructure Images Some of the microscopic observation results are shown in FIGS. 14A to 14C. 14A is a microstructure photograph at the time of no strain, that is, at the time of the black circle in [1] of FIG. 13, and FIG. 14B is the time of the black circle in [2] in FIG. Plastic strain in the same axial direction (that is, x-axis direction) (a predetermined area displayed in the microstructure photograph, which is estimated from the amount of deformation in the field of view based on the structure shape of the metal material displayed in the microstructure photograph Strain value) ε _micro is 17.1% (nominal strain ε _nominal estimated from the amount of displacement of the seal for the video extensometer is 13.3%). ] and a microstructure photograph when the plastic strain ε _micro is 35.5% (nominal strain ε _nominal is 18.2%). 14A to 14C, the area surrounded by broken lines represents the predetermined area, and it can be seen that it extends in the horizontal direction (x-axis direction) and shrinks in the vertical direction (y-axis direction). In this quasi-continuous microstructure observation-tensile test, the amount of plastic strain in the x-axis direction (ε _micro ), which is the amount of macro strain, the position where microvoids are generated at the amount of plastic strain, and the type of constituent phase that is the origin of microvoids was used as an evaluation item.

Observation of quasi-continuous microstructure—The occurrence of microvoids in the tensile test was confirmed visually in the above microstructure photographs, such as voids indicated by arrows in FIG. 15A or circles in FIG. 15B. An example of the confirmation result is shown in FIGS. 16 to 19. FIG. 16 and 17 are photographs of locations where microvoids are generated in the ferrite phase, and FIGS. 18 and 19 are photographs of locations where microvoids are generated in the martensite phase. FIGS. 16A to 16C are photographs of microvoid generation in the ferrite phase before constriction occurs in the tensile test piece, and FIGS. , is a photograph of microvoid generation in the ferrite phase. In addition, FIG. 18A is a photograph of microvoid generation in the martensite phase before constriction occurs in the tensile test piece, and FIGS. 18B to 18D, FIGS. It is a photograph of generation of microvoids in the martensite phase afterward. 16 to 19, the photograph on the left including the locations where microvoids are generated indicated by arrows and the enlarged photograph on the right where the locations where microvoids are generated are indicated by circles.

As an example of evaluation, FIG. 20 shows the results of evaluating the items shown in FIGS. 16 to 19 in the microstructure photograph of FIG. 14C. In FIG. 20, the portions surrounded by solid lines or dotted lines indicate the locations where voids are generated, the solid line surroundings indicate voids generated in the ferrite phase, and the dotted line surroundings indicate voids generated in the martensite phase. indicates that there is FIG. 20 shows the results when the _nominal strain ε _nominal is 18.2%. Similar to this FIG. , and the evaluation results shown in FIG. 20 were obtained.

(3) Output of Analysis Results at Strains Corresponding to the Tensile Test In order to perform FEM analysis, the controller 20 was caused to execute the model creation process 21 to create an FEM analysis model in the same manner as described above. The model was created by image-based modeling using the area enclosed by the dotted line in FIG. 14A. Since the area fraction of the martensite phase in the test piece is 13.7%, an FEM analysis model was created in which the volume fraction of the region corresponding to the martensite phase is 13.7%. Also, the characteristics of ferrite and martensite, which are materials constituting the metal material, are set in the control unit 20 as parameters.

Next, the boundary conditions given to each FEM analysis model were set in the controller 20 as parameters. Similar to the above, the boundary condition gives the history of the amount of strain that is actually considered to occur in the material until the structure that constitutes the metal material is destroyed. In addition, the strain increment is given so that ε _x = -νε _y = -νε _z , and ν is set to 0.5, and the strain increment in the direction of each axis is x-axis direction: y-axis direction: z It was set so that the axial direction=1:-0.5:-0.5.

Then, the control unit 20 is caused to perform the analysis processing 23 to perform FEM analysis, and a plastic strain equivalent to each plastic strain ε _micro estimated from the microstructure photograph is applied to the FEM analysis model in the tensile direction. We analyzed the deformation of the FEM analysis model in the case. That is, in this example, each plastic strain when 17.1%, 35.5% and other estimated plastic strains ε _micro in the x-axis direction are given to each integration point of the FEM analysis model We analyzed the deformation of the FEM analysis model at ε _micro . Then, the displacement at each position in the analysis model was calculated, the stress triaxiality and the equivalent plastic strain were calculated, and output to the storage unit 30 . That is, when the plastic strain ε _micro occurs in a predetermined region of the microstructure photograph of the test piece, the stress triaxiality and the equivalent plastic strain at an arbitrary position within the predetermined region are output.

(4) Extraction of the equivalent plastic strain and stress triaxiality at the microvoid occurrence position from the analysis results, and creation of a graph of the extracted equivalent plastic strain and stress triaxiality. Then, the data of the equivalent plastic strain and the stress triaxiality at the microvoid occurrence position were extracted from the output data of the equivalent plastic strain and the stress triaxiality at each position. The extracted data is a value calculated by applying the plastic strain ε _micro when the microvoid is generated to the FEM analysis model. Then, for each constituent phase corresponding to each position, the equivalent plastic strain and stress triaxiality of each extracted position were set to the equivalent plastic strain (εeq ^P ) on the vertical axis and the stress triaxiality (η) on the horizontal axis. Plotted on a graph, the relationship between the equivalent plastic strain at which microvoids occur and the stress triaxiality was obtained for each constituent phase. The results are shown in FIG. 21 for the ferrite phase and FIG. 22 for the martensite phase. From FIG. 21 and FIG. 22 above, the values of the equivalent plastic strain and the stress triaxiality become higher early with an increase in strain, that is, the earlier the microvoids are generated, the earlier it shifts to the upper right of the graph. I can say

(5) Determination of damage criteria in graphs In each of FIGS. 21 and 22, as a boundary line between the area of the plot points and the area in the lower left direction of the graph where the equivalent plastic strain and stress triaxiality decrease, An inversely proportional approximation curve was calculated. The approximation curve is a threshold curve at which damage occurs in each constituent phase, and indicates the area above the curve where damage occurs.

10. Evaluation of damage behavior of microstructure using damage criteria Damage behavior can be determined more clearly. More specifically, in all of FIGS. 6, 7, 8 and 9, there were more plotted points in the upper right corner of the graph for material 2 than for material 1. FIG. From this, it is considered that microvoids are generated earlier in material 2, and it is inferred that material 2 is inferior in rupture properties.

In recent years, as the component composition and manufacturing process of metallic materials have become more complex, on the scale of the microstructure of metallic materials, the distribution (heterogeneity) of the structure and strength tends to unavoidably appear, and the structure etc. The inhomogeneity of steel greatly affects the macroscopic strength properties. Furthermore, the strength characteristics required for structures formed from the above metal materials have become more and more severe in recent years. However, according to the method according to the present embodiment, it is possible to accurately evaluate the macro-strength characteristics of a practical-scale structure (member) by accurately considering the influence of the inhomogeneity of the structure and the like.

The damage behavior evaluation method of the microstructure of the metallic material according to the claims of the present disclosure is realized by cooperation of hardware resources, such as processors, memories, and software resources (computer programs).

The method according to the present embodiment can be effectively utilized in the development of metal materials that constitute structures such as buildings, ships, offshore structures, bridges, tanks, etc., which require stricter strength characteristics.

1 evaluation system 10 test unit 11 stress/strain applying unit 12 imaging unit 20 control unit 21 model creation processing 22 parameter setting processing 23 analysis processing 24 distribution creation processing 30 storage unit 40 display unit 50 input unit 60 output unit 70 quasi-continuous microstructure Observation-Tensile Testing Apparatus 71 Tensile Specimen 72 Clamping Chuck 73 Seal for Video Extensometer 74 Video Camera 75 Polished Surface for Microstructural Observation

Claims (7)

A computer-aided method for evaluating microstructural damage behavior of a metallic material, the computer comprising:
a step 1 of obtaining a microstructure image of the metallic material;
step 2 of dividing the microstructure image into a plurality of elements and creating an FEM analysis model simulating the microstructure image by a finite element method (FEM);
Step 3 of obtaining material properties of the metallic material as parameters of the FEM analysis model;
Step 4 of setting, as boundary conditions, the stress or strain that occurs until the metal material fractures, or the history of the stress or strain that is applied to each element of the FEM analysis model;
Step 5 of performing an FEM analysis under the boundary conditions and outputting the equivalent plastic strain and stress triaxiality of each element;
Step 6 for obtaining the distribution of the equivalent plastic strain and the stress triaxiality of each of the output elements for each stress or strain applied to the FEM analysis model in the FEM analysis;
A method for evaluating damage behavior of a microstructure of a metal material, comprising: step 7 of evaluating damage behavior of a microstructure from the distribution of the equivalent plastic strain and the stress triaxiality. The step 4 sets the stress or the strain, or the history of the stress or the strain, to be applied to the integration point of each element,
2. The method for evaluating damage behavior of a microstructure of a metallic material according to claim 1, wherein said step 5 outputs said equivalent plastic strain and stress triaxiality at integration points of each element. The metal material has two or more constituent phases,
3. The method for evaluating damage behavior of a microstructure of a metallic material according to claim 1, wherein steps 5 to 7 are performed for each constituent phase. Damage behavior evaluation of the microstructure of the metallic material according to claim 3, comprising obtaining the stress-strain curves of each of the two or more constituent phases prepared in advance as the material properties in the step 3. Method. 5. The metal material according to any one of claims 1 to 4, wherein in step 7, a damage criterion is determined according to a predetermined procedure, and the presence or absence of microstructure damage is determined using the determined damage criterion. damage behavior evaluation method of the microstructure of. determining the damage criteria,
A step 11 of preparing a test piece made of the metal material and acquiring a plurality of microstructure images of the same field of view of the test piece while continuously applying strain to the test piece;
Step 12 of determining the microvoid generation position and the amount of plastic strain at the time of microvoid generation from the plurality of microstructure images;
step 13 of creating an FEM analysis model simulating the microstructure image by dividing the microstructure image into a plurality of elements;
a step 14 of obtaining material properties of the metallic material as parameters of the FEM analysis model;
Step 15 of setting a strain amount to be applied to each element of the FEM analysis model as a boundary condition;
A step 16 of performing FEM analysis under the boundary conditions and outputting the equivalent plastic strain and stress triaxiality of each element at each plastic strain amount at the time of microvoid generation;
a step 17 of extracting the data of the equivalent plastic strain and the stress triaxiality at the microvoid generation position from the output data of the equivalent plastic strain and the stress triaxiality of each element;
a step 18 of obtaining distributions of equivalent plastic strain and stress triaxiality at each extracted position;
6. Damage behavior evaluation of the microstructure of the metal material according to claim 5, which is performed in the process including step 19 of determining the damage criteria from the relationship between the equivalent plastic strain at which microvoids are generated and the stress triaxiality in the distribution. Method. The metal material has two or more constituent phases,
7. The method for evaluating damage behavior of a microstructure of a metal material according to claim 6, wherein steps 17 to 19 in determining the damage criteria are performed for each constituent phase.

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