Method for estimating formation property

Abstract

To provide a method for estimating a formation property which can easily estimate the formation property of a test body.SOLUTION: The method for estimating the formation property includes: an imaging step S1 of taking an image of a tensile test on a test body formed of a steel material; a distribution acquisition step S2 of acquiring a strain distribution in a longer direction of the test body by using an image correlation technique on the basis of the image taken of the test body; a calculation step S3 of calculating the maximum value and the minimum value of the strain between predetermined gauge marks in the strain distribution of a longer direction of the acquired test body; and a formation property estimation step S4 of determining the difference between the maximum value and the minimum value of the calculated strain and estimating the formation property of the test body on the basis of the determined difference.SELECTED DRAWING: Figure 2

Description

Technical Field

[0001] The present invention relates to a method for estimating formability, and particularly to a method for estimating the formability of a specimen using the image correlation method.

Background Art

[0002] The image correlation method is known as one of the techniques for analyzing the deformation behavior of the material surface. As a technique using the image correlation method, for example, as described in Patent Document 1 below, images of a specimen before and after a tensile test are taken, and the strain amount and / or deformation amount of the specimen are measured from the taken images using the image correlation method.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] When a tensile test is performed using a metal specimen having parallel portions, based on the obtained stress-strain (so-called SS curve), the mechanical properties in a macroscopic region such as between gauge marks can be confirmed, but there is a problem that the formability of the specimen cannot be confirmed from a microscopic viewpoint. For example, in the case of a high-strength steel sheet, as a trade-off for increasing the strength, the formability of the material decreases, so the superiority or inferiority of the material cannot be strictly judged only from a macroscopic viewpoint. For this reason, a method for estimating the formability of a material from a microscopic viewpoint has been demanded.

[0005] The present invention has been made to solve such technical problems, and an object thereof is to provide a formability estimation method capable of easily estimating the formability of a specimen.

Means for Solving the Problems

[0006] The formability estimation method according to the present invention is a method for estimating the formability of a test piece formed of a steel material, and includes a photographing step of photographing an image of a tensile test of the test piece, and based on the photographed image of the test piece, using an image correlation method, a distribution acquisition step of acquiring a strain distribution in the longitudinal direction of the test piece, a calculation step of calculating, for the acquired strain distribution in the longitudinal direction of the test piece, the maximum value and the minimum value of the strain between predetermined reference points, respectively, and a formability estimation step of obtaining the difference between the calculated maximum value and minimum value of the strain and estimating the formability of the test piece based on the obtained difference.

[0007] In the formability estimation method according to the present invention, based on an image of a test piece using an image correlation method, a strain distribution in the longitudinal direction of the test piece is acquired. For the acquired strain distribution in the longitudinal direction of the test piece, the maximum value and the minimum value of the strain between predetermined reference points are calculated, respectively. The difference between the calculated maximum value and minimum value of the strain is obtained, and the formability of the test piece is estimated based on the obtained difference. In this way, by focusing on the difference between the maximum value and the minimum value of the strain between predetermined reference points and using the difference between the maximum value and the minimum value of the strain as an index for estimating the formability of the test piece, it becomes possible to estimate the formability of the test piece from a microscopic perspective. As a result, the formability of the test piece can be easily estimated.

[0008] Further, in the formability estimation method according to the present invention, in the formability estimation step, it is preferable to compare the obtained difference with a preset threshold value and estimate that the formability of the test piece is good when the obtained difference is less than or equal to the threshold value. In this way, it is possible to easily estimate whether the formability of the test piece is good.

[0009] Furthermore, in the formability estimation method according to the present invention, there are a plurality of the test pieces, and in the formability estimation step, it is preferable to compare the differences between the maximum value and the minimum value of the strain of each obtained test piece, and estimate that the formability of the test piece is better as the difference is smaller. In this way, it is possible to easily select a test piece with good formability among a plurality of test pieces.

Advantages of the Invention

[0010] According to the present invention, the formability of a specimen can be easily estimated.

Brief Description of the Drawings

[0011]

Figure 1

Figure 2

Figure 3

Embodiments for Carrying Out the Invention

[0012] Hereinafter, embodiments of a formability estimation method according to the present invention will be described with reference to the drawings.

[0013] FIG. 1 is a flowchart showing a formability estimation method according to an embodiment. As shown in FIG. 1, the formability estimation method of the present embodiment includes a photographing step S1, a distribution acquisition step S2, a calculation step S3, and a formability estimation step S4. Hereinafter, each will be described in detail.

[0014] In the photographing step S1, an image of a tensile test of a specimen is taken. The specimen of the present embodiment is a tensile test piece formed of a steel material, and has a parallel portion having a predetermined length, a pair of shoulders disposed at both ends of the parallel portion, and a pair of gripping portions integrated with the shoulders and disposed at both ends of the specimen, similar to the test piece described in, for example, JIS Z2241 Metallic Materials - Tensile Test Method. The specimen may be plate-shaped, rod-shaped, tubular, arc-shaped, or linear, but a plate-shaped specimen is used in the present embodiment.

[0015] For a test piece having such a structure, first, a random pattern is applied to its surface by spraying or the like, and then, for example, a uniaxial tensile test is performed in accordance with the JIS Z2241 Metallic Materials - Tensile Testing Method, and at that time, an image of the test piece is taken using a camera or the like. That is, using a camera or the like, the random pattern applied to the surface of the test piece before and after the tensile test is photographed.

[0016] In the distribution acquisition step S2 following the photographing step S1, based on the photographed image of the test piece, the strain distribution in the longitudinal direction of the test piece is acquired using the image correlation method. Specifically, the displacement is measured from the amount of change (also referred to as the amount of movement) of the random pattern before and after the tensile test, and the strain distribution of the test piece is acquired based on the result of the measured displacement. The result of the acquired strain distribution is, for example, as shown in FIG. 2.

[0017] In FIG. 2, (a) is a schematic diagram showing the strain distribution of the acquired test piece, and (b) is a diagram showing the relationship between the position (mm) and the maximum principal strain (%). As shown in FIG. 2(b), in this embodiment, for example, the strain distribution in a section 50 mm away from the center in the longitudinal direction of the test piece on each of the left and right sides (a section with a total length of 100 mm) is acquired.

[0018] In the calculation step S3 following the distribution acquisition step S2, for the strain distribution in the longitudinal direction of the acquired test piece, the maximum value and the minimum value of the strain between predetermined reference points are calculated respectively. Specifically, as shown in FIG. 2(b), for example, reference points 25 mm away from the center in the longitudinal direction of the test piece on each of the left and right sides (the distance between the reference points is 50 mm) are picked up, and the maximum value and the minimum value of the strain between these reference points are calculated respectively.

[0019] In the formability estimation step S4 following the calculation step S3, the difference between the calculated maximum value and minimum value of the strain is obtained, and the formability of the test piece is estimated based on the obtained difference. Hereinafter, the estimation of the formability of the test piece will be described in detail with reference to FIG. 3.

[0020] FIG. 3 is a schematic diagram for explaining the formability estimation method. In FIG. 3, the horizontal axis represents the strain (%) between punctuation marks, and the vertical axis represents the difference between the maximum and minimum values of the strain (εmax - εmin) (%). Also, X, Y, and Z in FIG. 3 represent specimens having the same dimensions and shapes respectively made of steel materials of Company X, Company Y, and Company Z.

[0021] The line graph showing the X, Y, and Z specimens in FIG. 3 is obtained based on the result of the above calculation step S3. As shown in FIG. 3, the εmax - εmin of each of the X, Y, and Z specimens changes sharply in slope with Ts (tensile strength) as the boundary.

[0022] Also, the B-pillar threshold shown in FIG. 3 means the threshold of εmax - εmin of the steel material suitable for forming the B-pillar. This B-pillar threshold is set based on the results of confirmation such as the presence or absence of necking and cracking and the reduction rate of the plate thickness for the formed trial products when performing the forming trial of the B-pillar using the steel material. On the other hand, the rocker reinforcement threshold shown in FIG. 3 means the threshold of εmax - εmin of the steel material suitable for forming the rocker reinforcement. This rocker reinforcement threshold is set based on the results of confirmation such as the presence or absence of necking and cracking and the reduction rate of the plate thickness for the formed trial products when performing the forming trial of the rocker reinforcement using the steel material. Note that the B-pillar is an example of a part with relatively high forming difficulty, and the rocker reinforcement is an example of a part with relatively low forming difficulty.

[0023] As shown in FIG. 3, for example, in the forming trial of the B-pillar, when necking and cracking are not observed in the Y specimen but are observed in the X and Z specimens, when the εmax - εmin of the Y specimen at the time of reaching Ts is below the B-pillar threshold, it can be estimated that the formability of the Y specimen is good. In other words, among the X, Y, and Z specimens, it can be said that the Y specimen deforms more uniformly. As a result, it can be determined that the steel material used for the Y specimen can be applied to the forming of the B-pillar, but the steel materials used for the X and Z specimens cannot be applied to the forming of the B-pillar.

[0024] Similarly, for example, in the forming trials of the rocker reinforcement, when no necking or cracking is observed in the X test specimen and the Y test specimen, but necking or cracking is observed in the Z test specimen, when the εmax - εmin of the X test specimen and the Y test specimen at the time of Ts arrival are each below the rocker reinforcement threshold value, it can be estimated that the formability of the X test specimen and the Y test specimen is good. In other words, among the X test specimen, the Y test specimen, and the Z test specimen, it can be said that the X test specimen and the Y test specimen deform more uniformly. As a result, it can be determined that the steel materials used for the X test specimen and the Y test specimen are each applicable to the forming of the rocker reinforcement, but the steel material used for the Z test specimen is not applicable to the forming of the rocker reinforcement.

[0025] In addition, as shown in FIG. 3, when there are multiple test specimens, the difference between the maximum value and the minimum value of the strain of each obtained test specimen may be compared, and it may be estimated that the better the formability of the test specimen, the smaller the difference. Specifically, among the εmax - εmin of the X test specimen, the Y test specimen, and the Z test specimen shown in FIG. 3, since the εmax - εmin of the Y test specimen is the smallest, it can be estimated that the formability of the Y test specimen is good. In this way, it is possible to easily select a test specimen with good formability from multiple test specimens.

[0026] In the formability estimation method according to this embodiment, the strain distribution in the longitudinal direction of the test specimen is obtained based on the image of the test specimen using the image correlation method. For the obtained strain distribution in the longitudinal direction of the test specimen, the maximum value and the minimum value of the strain between predetermined reference points are calculated respectively, the difference between the calculated maximum value and minimum value of the strain is obtained, and the formability of the test specimen is estimated based on the obtained difference. In this way, by focusing on the difference between the maximum value and the minimum value of the strain between predetermined reference points and using the difference between the maximum value and the minimum value of the strain as an index for estimating the formability of the test specimen, it becomes possible to estimate the formability of the test specimen from a microscopic perspective. As a result, the formability of the test specimen can be easily estimated.

[0027] As described above in detail, the embodiments of the present invention are not limited to the above-described embodiments, and various design changes can be made without departing from the spirit of the present invention described in the claims.

Explanation of Reference Numerals

[0028] S1: Photographing step, S2: Distribution acquisition step, S3: Calculation step, S4: Formability estimation step

Claims

[Claim 1] A method for estimating formability of a test specimen formed from a steel material, comprising: A photographing step of photographing an image of the tensile test of the test specimen; A distribution acquisition step of acquiring a strain distribution in the longitudinal direction of the test specimen using an image correlation method based on the captured image of the test specimen; A calculation step of calculating maximum and minimum values ​​of strain between predetermined reference points for the acquired strain distribution in the longitudinal direction of the test specimen; a formability estimation step of calculating a difference between a maximum value and a minimum value of the calculated strain and estimating the formability of the test specimen based on the calculated difference; A formability estimation method comprising:

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