JP6287665B2 - Method and apparatus for predicting ductile brittle fracture characteristics of thin steel plate members, and program and recording medium therefor - Google Patents

Description

  The present invention relates to a method and apparatus for predicting ductile brittle fracture characteristics for a member made of a thin steel plate, a program, and a recording medium.

  Recently, there is an increasing need for weight reduction of the vehicle body in the automobile field and the like, and in order to meet this demand, high tensile strength steel (steel plate) has been made.

JP2011-169745A

In general, it is known that a metal having a body-centered cubic lattice structure represented by iron (Fe) (bcc metal) causes two types of fracture, ductile fracture and brittle fracture.
In a normal mild steel sheet, fracture occurs due to ductile fracture. On the other hand, high tensile strength increases the possibility of breakage due to brittle fracture as well as ductile fracture due to its high deformation resistance.

Brittle fracture is characterized in that the absorbed energy at the time of crack propagation is less than that in the case of ductile fracture. Therefore, it is necessary to evaluate quantitatively about the brittle fracture easiness in a thin steel plate.
However, conventionally, a technology for quantitatively evaluating the brittle fracture characteristics of a member made of a thin steel sheet has not yet been established.

  The present invention has been made in view of the above problems, and it is easy to accurately and quantitatively evaluate the brittle fracture characteristics of a member made of a thin steel plate used for an automobile body or a railway car body. It is an object of the present invention to provide a method and apparatus for predicting ductile brittle fracture characteristics of a highly reliable thin steel plate member, a program, and a recording medium.

The method for predicting ductile brittle fracture characteristics of a thin steel sheet member according to the present invention is a method for predicting brittle fracture characteristics intended for a thin steel sheet member, wherein the evaluation item of brittle fracture in the thin steel sheet member is a fracture displacement, A step of selecting from absorbed energy or a brittle fracture surface ratio, a step of referring to a fracture characteristic curve of a thin steel plate corresponding to the evaluation item, and a step of obtaining a stress triaxiality at the time of fracture of the thin steel plate member Predicting the brittle fracture characteristics of the steel sheet member using the fracture characteristic curve and the stress triaxiality .

The predictive apparatus for ductile brittle fracture characteristics of a thin steel sheet member of the present invention is a predictive apparatus for brittle fracture characteristics intended for a thin steel sheet member, and the evaluation item of brittle fracture in the thin steel sheet member is a fracture displacement, an evaluation item input unit for selecting from the absorption energy or brittle fracture rate, and the evaluation Save and has the storage unit destruction characteristic curve of the thin steel sheet as the data about an item, said stress triaxial at break equivalent time of the thin steel member and stress three Jikudo calculating unit for obtaining a degree, with the fracture characteristic curve and the stress three Jikudo, and a calculator for calculating the brittle fracture properties of the thin steel member.

The program of the present invention is a program for predicting ductile brittle fracture characteristics for thin steel sheet members , and selects an evaluation item of brittle fracture in the thin steel sheet members from fracture displacement, absorbed energy or brittle fracture surface ratio A procedure for referring to a fracture characteristic curve of the thin steel plate corresponding to the evaluation item, a procedure for obtaining a stress triaxiality at the time of the fracture of the thin steel plate member, the fracture characteristic curve and the stress three This is for causing a computer to execute a procedure for predicting the brittle fracture characteristics of the thin steel plate member using the axial degree .

  ADVANTAGE OF THE INVENTION According to this invention, it becomes possible to predict the brittle fracture characteristic easily and correctly about the member made from a thin steel plate used for a motor vehicle body, a railway vehicle body, etc.

It is a flowchart which shows the prediction method of the ductile brittle fracture characteristic by 1st Embodiment in order of a step. It is a flowchart which shows the method of acquiring a fracture characteristic curve in order of the step in the prediction method of a ductile brittle fracture characteristic by 1st Embodiment. It is a schematic diagram which shows a member and an impact 3 point | piece bending test about 1st Embodiment. It is a characteristic view which shows the result of having calculated the relationship between a stress-strain state and an evaluation item about each test piece of a thin steel plate. It is a characteristic view which shows the relationship between the obtained drooping displacement and stress triaxiality. It is a characteristic view which shows the relationship between the falling weight displacement of a member, and a brittle fracture surface rate. It is a schematic plan view which shows the test piece with a notch of a thin steel plate. It is a schematic plan view which shows another test piece with a notch of a thin steel plate. It is a schematic plan view which shows a mode that the simulation of the tension test in a test piece was performed. It is a block diagram which shows the material characteristic acquisition apparatus by 1st Embodiment. It is a schematic diagram which shows the internal structure of a personal user terminal device.

  Hereinafter, embodiments of a method and apparatus for predicting ductile brittle fracture characteristics, a program, and a recording medium for thin steel plate members will be described in detail with reference to the drawings.

(First embodiment)
FIG. 1 is a flowchart showing a method for predicting ductile brittle fracture characteristics according to the first embodiment in order of steps. FIG. 2 is a flowchart showing a method of acquiring a fracture characteristic curve in the order of steps in the method for predicting ductile brittle fracture characteristics according to the first embodiment.
FIG. 3 is a schematic view showing a thin steel plate member used in the first embodiment. Here, a thin steel plate having a tensile strength of 1180 MPa class and a thickness of 1.2 mm was used. As shown in FIG. 2A, the member is a hat-shaped member having a size of 900 mm × 125.3 mm.

A method for predicting ductile brittle fracture characteristics according to the first embodiment will be described with reference to FIG.
First, a brittle fracture surface ratio is selected as an evaluation item for a thin steel plate member (step S1). The definition of the brittle fracture surface ratio will be described later.

Subsequently, the fracture characteristic curve of the thin steel sheet to be used is referred from the database (step S2).
The database referred to here is, for example, a database storing data that can be obtained from results of testing under various test piece shapes or test temperature conditions by a tensile test shown in Step S11 and Step S12 described later. Here, as shown in FIG. 4, the breaking characteristic curve is a characteristic curve showing the relationship between the evaluation item and the stress triaxiality which is a representative value of the stress strain state. Here, the relationship between the brittle fracture surface ratio and the stress triaxiality is referred to.

Subsequently, the stress strain state of the assumed fracture portion by the collision analysis of the thin steel plate member is calculated (step S3).
Here, a simulation of an impact three-point bending test is performed. As shown in FIG. 3 (b), the simulation conditions were as follows: a weight of 400 kg having a cylindrical shape with a radius of curvature of 60 mm (R60) was used on the collision surface with the specimen, and the specimen was tested at an initial speed of 4.4 m / s. Drop against material. When a bending displacement is given to the member by simulation, the stress strain state of the assumed fracture portion, specifically, the stress triaxiality is calculated, and the relationship between the bending displacement and the stress triaxiality is obtained. The relationship obtained is shown in FIG.

Subsequently, the ductile brittle fracture characteristics of the thin steel plate member are predicted (step S4).
Using the fracture characteristic curve of the thin steel sheet, as shown in FIG. 6, the ductile brittle fracture characteristic of the member, specifically, the relationship between the bending displacement and the brittle fracture surface ratio is predicted. By using the relationship between the bending displacement and the brittle fracture surface ratio, assuming the bending displacement that causes the thin steel plate member to break, the brittle fracture surface ratio at that portion can be predicted.

  When the impact three-point bending test of the member using the above-described thin steel plate was performed, the bending displacement causing the fracture was about 160 mm, and the brittle fracture surface ratio at the fractured portion was 50%. According to FIG. 6, the brittle fracture surface ratio when the bending displacement is 160 mm is also about 50%, confirming the effectiveness of the present invention.

  Hereinafter, a method for obtaining a fracture characteristic curve used in the ductile brittle fracture property prediction method according to the first embodiment will be described with reference to FIG.

The fracture characteristic curve is acquired according to the following procedure. First, a tensile test is performed using each test piece (step S1). Here, a plurality of test pieces as exemplified in FIG. 7 are used.
7A to 7C show U-notch test pieces 11 to 13, and FIG. 7D shows a V-notch test piece 14. Each of the test pieces 11 to 14 is formed in a strip shape, and is formed of a cold-rolled material having a tensile strength of 590 N / mm ² or more (and 3000 N / mm ² or less, for example, in consideration of manufacturing technology restrictions). Thin steel plate. The test pieces 11 to 14 have notches having a predetermined curvature on both sides (both ends) in the longitudinal direction, and the distances between the notches are the same.
In addition, it can apply also to the hot stamped steel plate after hot stamping instead of the thin steel plate formed with the said cold-rolled material.

The test piece 11 has a pair of notches 11a and 11b having a radius of curvature of 30 mm as U-notches on each side in the longitudinal direction, and the distance between the notches 11a and 11b is 12.5 mm. is there.
The test piece 12 is formed with a pair of notches 12a and 12b having a curvature radius of 5 mm as U-notches on each side in the longitudinal direction, and the distance between the notches 12a and 12b is 12.5 mm. is there.
The test piece 13 is formed with a pair of notches 13a and 13b having a radius of curvature of 0.5 mm as U-notches on each side in the longitudinal direction, and the distance between the notches 13a and 13b is 12.5 mm. Is.
The test piece 14 is formed with a pair of first notches 14a1 and 14b1 on each side in the longitudinal direction, and a pair of second notches having a curvature radius of 0.25 mm as a V-notch in the central portion of the first notches 14a1 and 14b1. Notches 14a2 and 14b2 are formed, and the distance between the second notches 14a2 and 14b2 is 12.5 mm.

In addition, as a comparative example of this embodiment, a test piece in which the distance between the test piece and the notch in each drawing of FIG. 8A and 8B show U-notch test pieces 21 and 22, respectively.
The test piece 21 relates to Comparative Example 1, and a pair of notches 21a and 21b having a curvature radius of 5 mm are formed as U-notches on each side in the longitudinal direction, and the distance between the notches 21a and 21b. Is 15 mm.
The test piece 22 relates to Comparative Example 2, and a pair of notches 22a and 22b having a curvature radius of 5 mm are formed as U-notches on each side in the longitudinal direction, and the distance between the notches 22a and 22b. Is 10 mm.

  In step S1, each test piece is subjected to a tensile test using an intron type tensile tester. For example, each condition of the tensile test is set as follows for each of the test pieces 11 to 14 and 21 to 22. The crosshead speed is set to a speed within a range defined by JIS, for example, 3 mm / min. The ambient temperature is about −120 ° C. to about 200 ° C., preferably about −60 ° C. to about 80 ° C., for example, room temperature (25 ° C.), which is the temperature in the usage environment of the target thin steel sheet.

Subsequently, an evaluation item of brittle fracture in each test piece is measured (step S2).
Evaluation items for brittle fracture in the present embodiment are fracture displacement, absorbed energy, and brittle fracture surface rate. The breaking displacement is a displacement amount (mm) of a break occurrence place in the test piece. The absorbed energy is a characteristic value indicating the toughness (toughness) of the material, and is the energy (Nm) absorbed by the test piece when the test piece breaks. The brittle fracture surface ratio is a value indicating the percentage of the area of the brittle fracture surface in the fracture surface due to the fracture occurrence of the specimen.

  Specifically, the brittle fracture surface ratio is determined when the fracture surface is generated in the specimen when the impact three-point bending test or the tensile test described later is performed at a predetermined temperature (when fracture occurs). The specimen is disassembled to observe the fracture surface. Observe the entire fracture surface of the specimen with a stereomicroscope, and recognize the fracture surface that is shining as a result of cleavage or cleavage at the grain boundary as a brittle fracture surface, and occupy the entire fracture surface area in the observed image. The ratio of the brittle fracture surface was determined by image analysis at 100 minutes. Note that the ductile fracture surface on the fracture surface of the test piece generates a large number of dimples sufficiently smaller than the crystal grain size on the surface. For this reason, there is almost no gloss when observed with a stereoscopic microscope, and it can be distinguished from the brittle fracture surface. When the fracture surface did not occur in the test piece (when fracture did not occur), the brittle fracture surface ratio was 0%.

In step S2, the fracture displacement, the absorbed energy, and the brittle fracture surface ratio are measured for each test piece based on the result of the tensile test performed in step S1.
An example of specific measurement results of fracture displacement, absorbed energy, and brittle fracture surface ratio is shown in Table 1 below. Table 1 exemplifies the measurement results of test pieces 11 to 14 and 21 to 22 having a tensile strength of 980 MPa and a plate thickness of 1.6 mm (referred to as A material).

Subsequently, the stress strain state at the time of fracture is calculated for each test piece (step S3).
The stress strain state in the present embodiment is a triaxial stress. A stress gradient or a strain gradient may be used as the stress strain state instead of the stress triaxiality or together with the stress triaxiality.
The stress triaxiality is a parameter obtained by dividing the triaxial average stress (hydrostatic pressure stress) by the equivalent stress, that is, is defined as follows.

The hydrostatic stress and Mises equivalent stress can be expressed as follows using principal stresses σ ₁ , σ ₂ , and σ ₃ , respectively.

  From this, the stress triaxiality can be expressed as follows.

In step S3, a simulation of a tensile test in each test piece is performed using FEM analysis. For example, as shown in FIG. 9, a central cross section indicated by a broken line in the drawing of the test piece at the time when fracture displacement occurs in the test piece. The stress triaxiality distribution is calculated. The maximum stress triaxiality in the central section is derived as the representative stress triaxiality in the specimen.
An example of a specific calculation result of the stress triaxiality is shown in Table 2 below.

Subsequently, the relationship between the stress strain state and the evaluation item is calculated for each test piece (step S4).
As a specific example, the result calculated about the test pieces 11-14 and 21-22 is shown in FIG. In FIG. 4, (a) illustrates the correlation between the stress triaxiality and the absorbed energy, and (b) illustrates the correlation between the stress triaxiality and the brittle fracture surface ratio.

  As shown in FIGS. 4A to 4B, by plotting each test piece result, a destructive characteristic curve is uniquely determined as each of the above correlations. Thereby, the material characteristic in the test piece of a some shape can be evaluated uniformly.

FIG. 4 shows the calculated results for the test piece 21 as the comparative example 1 and the test piece 22 as the comparative example 2 of the present embodiment. In Comparative Examples 1 and 2, in any of FIGS. 4A to 4B, values deviating from the material characteristic curves defined by the test pieces 11 to 14 and 21 to 22 are shown. This suggests that the distance between the pair of notches in each test piece needs to be the same in order to accurately obtain one material characteristic curve.
By the above procedure, the fracture characteristic curve of the thin steel sheet can be obtained.

FIG. 10 is a block diagram showing a predicting apparatus for ductile brittle fracture characteristics according to the first embodiment.
The ductile brittle fracture property prediction apparatus according to the present embodiment includes an evaluation item input unit 31 for a thin steel plate member, a fracture property curve storage unit 32 for a thin steel plate, a stress strain state calculation unit 33, and a brittle fracture property calculation unit 34. And a control unit 35.

The evaluation item input part 31 of a thin steel plate member inputs the evaluation item in the part which a thin steel plate member destroys. In other words, the evaluation item input unit inputs either the brittle fracture surface ratio or the absorbed energy at the portion where the member breaks.
The thin steel sheet fracture characteristic curve storage unit 32 shows, for each thin steel sheet, a fracture characteristic curve that is a relationship between the brittle fracture surface ratio or absorbed energy, which is an evaluation item, and the stress triaxiality, which is a representative value of the stress-strain state. I remember it.

  The stress strain state calculation unit 33 calculates a stress strain state when a load acts on the member. For example, using FEM analysis, a structural analysis simulation of each member is executed to calculate the stress triaxiality distribution of the fracture assumed portion. The stress strain state is calculated as a stress triaxiality under a specific condition (load or displacement) or a relationship between the load or displacement and the stress triaxiality.

  The brittle fracture characteristic calculator 34 calculates the brittle fracture characteristic using the fracture characteristic curve and the stress strain state of the member. Specifically, the value of the evaluation item under the specific condition of the member, or the relationship between the load or displacement of the member and the value of the evaluation item is calculated.

  The control unit 35 is an operation of each component of the apparatus including the evaluation item input unit 31 of the thin steel plate member, the fracture characteristic curve storage unit 32 of the thin steel plate, the stress strain state calculation unit 33, and the brittle fracture characteristic calculation unit 34. Is to control. The control unit 35 includes a central processing unit (CPU) for operating a control program and an auxiliary storage device for storing data.

  As described above, according to the present embodiment, it is possible to easily and accurately perform quantitative evaluation of material properties related to brittle fracture of thin steel plates used for automobile bodies, railway bodies, and the like.

(Second Embodiment)
Each component of the apparatus for predicting brittle fracture characteristics of the brittle fracture member according to the first embodiment described above (evaluation item input unit 31 of the thin steel plate member in FIG. 10, fracture characteristic curve storage unit 32 of the thin steel plate, stress strain state) The functions of the calculation unit 33, the brittle fracture characteristic calculation unit 34, the control unit 35, etc.) can be realized by operating a program stored in a RAM, a ROM, or the like of the computer. Similarly, each step according to the first embodiment can be realized by operating a program stored in a RAM or ROM of a computer. This program and a computer-readable recording medium on which the program is recorded are included in this embodiment.

  Specifically, the above program is recorded on a recording medium such as a CD-ROM, or provided to a computer via various transmission media. As a recording medium for recording the program, a flexible disk, a hard disk, a magnetic tape, a magneto-optical disk, a nonvolatile memory card, and the like can be used in addition to the CD-ROM. On the other hand, a communication medium in a computer network system for propagating and supplying program information as a carrier wave can be used as the program transmission medium. Here, the computer network is a WAN such as a LAN or the Internet, a wireless communication network, or the like, and the communication medium is a wired line such as an optical fiber or a wireless line.

  Further, the program included in the present embodiment is not limited to the one in which the function of the first embodiment is realized by the computer executing the supplied program. For example, when the function of the first embodiment is realized in cooperation with an OS (operating system) running on a computer or other application software, the program is included in this embodiment. . Further, when all or part of the processing of the supplied program is performed by the function expansion board or function expansion unit of the computer and the functions of the first embodiment are realized, such a program is included in this embodiment. It is

  For example, FIG. 11 is a schematic diagram illustrating an internal configuration of a personal user terminal device. In FIG. 13, reference numeral 1200 denotes a personal computer (PC) having a CPU 1201. The PC 1200 executes device control software stored in the ROM 1202 or the hard disk (HD) 1211 or supplied from the flexible disk drive (FD) 1212. The PC 1200 generally controls each device connected to the system bus 1204.

  By the program stored in the CPU 1201, the ROM 1202, or the hard disk (HD) 1211 of the PC 1200, the procedure of steps S1 to S4 in FIG. 1 of the first embodiment is realized.

  Reference numeral 1203 denotes a RAM which functions as a main memory, work area, and the like for the CPU 1201. A keyboard controller (KBC) 1205 controls instruction input from a keyboard (KB) 1209, a device (not shown), or the like.

  Reference numeral 1206 denotes a CRT controller (CRTC), which controls display on a CRT display (CRT) 1210. Reference numeral 1207 denotes a disk controller (DKC). The DKC 1207 controls access to a hard disk (HD) 1211 and a flexible disk (FD) 1212 that store a boot program, a plurality of applications, an edit file, a user file, a network management program, and the like. Here, the boot program is a startup program that starts execution (operation) of hardware and software of a personal computer.

Reference numeral 1208 denotes a network interface card (NIC) which performs bidirectional data exchange with a network printer, another network device, or another PC via the LAN 1220.
Instead of using a personal user terminal device, a predetermined computer specialized for the material property acquisition device may be used.

11-14, 21-22 Test piece 11a, 11b, 12a, 12b, 13a, 13b, 21a, 21b, 22a, 22b Notch part 14a1, 14b1 First notch part 14a2, 14b2 Second notch part 31 Evaluation item input part 32 Fracture characteristic curve storage unit 33 Stress strain state calculation unit 34 Brittle fracture characteristic calculation unit 35 Control unit

Claims (16)

A method for predicting brittle fracture characteristics for thin steel sheet members,
Selecting an evaluation item of brittle fracture in the thin steel plate member from fracture displacement, absorbed energy or brittle fracture surface ratio ;
Referring to the fracture characteristic curve of the thin steel sheet corresponding to the evaluation item;
Obtaining a stress triaxiality at the time of fracture of the steel sheet member;
Predicting the brittle fracture characteristics of the sheet steel member using the fracture characteristic curve and the stress triaxiality . The method for predicting ductile brittle fracture characteristics of a sheet steel member. The fracture characteristic curve of the thin steel sheet is a relationship between stress triaxiality of a test piece in which a notch having a predetermined curvature is formed at both ends of a strip shape, and an evaluation item of brittle fracture. 2. A method for predicting ductile brittle fracture characteristics of a thin steel plate member according to 1. The fracture characteristic curve of the thin steel sheet is a relationship between stress triaxiality of a plurality of the test pieces in which the distance between the notches in the test piece is the same, and an evaluation item of brittle fracture. A method for predicting ductile brittle fracture characteristics of a thin steel plate member according to claim 2.   The said evaluation items are absorption energy and a brittle fracture surface rate, The prediction method of the ductile brittle fracture characteristic of the member made from a thin steel plate of any one of Claims 1-3 characterized by the above-mentioned. The method for predicting ductile brittle fracture characteristics of a thin steel plate member according to any one of claims 1 to 4 , wherein the thin steel plate is a cold-rolled material having a tensile strength of 590 N / mm ² or more. . A device for predicting brittle fracture characteristics for thin steel plate members,
An evaluation item input unit for selecting an evaluation item of brittle fracture in the thin steel plate member from fracture displacement, absorbed energy or brittle fracture surface ratio ,
A storage unit that stores a fracture characteristic curve of the thin steel sheet related to the evaluation item as data,
A stress triaxiality calculation unit for obtaining a stress triaxiality at the time of fracture of the steel sheet member;
A calculation unit for calculating the brittle fracture characteristics of the thin steel plate member using the fracture characteristic curve and the stress triaxiality, and a predicting apparatus for ductile brittle fracture characteristics of the thin steel plate member. The fracture characteristic curve data is data indicating the relationship between the stress triaxiality of a plurality of test pieces in which notches having a predetermined curvature are formed at both ends of a strip shape and the evaluation items of the brittle fracture. The apparatus for predicting ductile brittle fracture characteristics of a thin steel plate member according to claim 6 . The fracture characteristic curve data of the thin steel sheet is data indicating the relationship between the stress triaxiality of the plurality of test pieces in which the distance between the notches in the test piece is the same, and the evaluation items of brittle fracture The predictive device for ductile brittle fracture characteristics of a thin steel plate member according to claim 7 . The evaluation items are predicting device ductile brittle fracture characteristics of the thin steel member according to any one of claims 6-8, characterized in that the absorbed energy and brittle fracture rate. The steel sheet has a tensile strength predictor ductile brittle fracture characteristics of the thin steel member according to any one of claims 6-9, characterized in that a 590N / mm ² or more cold-rolled material . A program for predicting ductile brittle fracture characteristics for thin steel sheet members,
A procedure for selecting an evaluation item of brittle fracture in the thin steel plate member from fracture displacement, absorbed energy or brittle fracture surface ratio ,
A procedure for referring to a fracture characteristic curve of a thin steel plate corresponding to the evaluation item,
A procedure for obtaining the stress triaxiality at the time of the fracture of the steel sheet member,
A program for causing a computer to execute a procedure for predicting the brittle fracture characteristics of the thin steel plate member using the fracture characteristic curve and the stress triaxiality . The fracture characteristic curve is data indicating a relationship between stress triaxiality of a plurality of test pieces in which notches having a predetermined curvature are formed at both ends of a strip shape and evaluation items of brittle fracture. The program according to claim 11 . The fracture characteristic curve of the thin steel sheet is data indicating the relationship between the stress triaxiality of a plurality of the test pieces and the evaluation items of brittle fracture in which the distance between the notches in the test piece is the same. The program according to claim 12 . The program according to any one of claims 11 to 13 , wherein the evaluation items are absorbed energy and a brittle fracture surface ratio. The program according to any one of claims 11 to 14 , wherein the thin steel plate is a cold-rolled material having a tensile strength of 590 N / mm ² or more. A computer-readable recording medium storing a program according to any one of claims 11-15.

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