Sheet metal forming limit evaluation method and crack prediction method

Abstract

To evaluate bending cracks within a plate surface with high accuracy on the basis of a time point at which local necking in a plate thickness direction occurs.SOLUTION: A plurality of types of tests are performed to: apply at least one of bending and tension to a plate-shaped test piece of the same composition so that it is in a plane strain state and a strain gradient in a thickness direction is different; acquire a strain gradient in the plate thickness direction; and measure molding limit strain (S1). The relation between the strain gradient in the plate thickness direction and the molding limit strain is acquired from the molding limit strain and the strain gradient in the plate thickness direction obtained from the plurality of types of tests (S2). The measured molding limit strain is the maximum principal strain on a surface of the test piece at the time when local necking in the plate thickness direction is detected in the test piece.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a forming limit evaluation method and a crack prediction method for a sheet material. [Background technology]

[0002] In order to predict the press formability of blanks such as steel sheets, a method for evaluating the forming limit of sheet materials, more specifically, a method for evaluating the bending crack limit, is known.

[0003] In the evaluation method disclosed in Patent Document 1, the critical strain on the plate surface when bending cracks occur at the end face of a test piece is evaluated using the plate thickness strain gradient (the gradient of strain in the plate thickness direction in the bending direction) and the in-plane strain gradient. Specifically, a V-bend test and a HAT test are performed to measure the strain when bending cracks occur at the end face of the test piece, and the plate thickness strain gradient and the in-plane strain gradient are calculated by numerical analysis.

[0004] In the evaluation method disclosed in Non-Patent Document 1, a tensile bending test is performed using a special jig, and the critical strain for bending cracks is evaluated based on the stress distribution in the plate thickness direction. Both the critical strain and the stress distribution in the plate thickness direction are calculated by numerical analysis under the maximum load in the tensile bending test. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Patent No. 6547920 [Non-patent literature]

[0006] [Non-Patent Document 1] Ryo Yonebayashi and Masahiro Nakata, "Effects of Tool Size and Initial Tension on Tensile Bending Workability of High-Tensile Steel Sheets," Proceedings of the 62nd Japan Joint Conference on the Technology of Plasticity, 2011, pp. 367-368 Summary of the Invention [Problem to be solved by the invention]

[0007] The evaluation method in Patent Document 1 determines the location of bending cracks at the end surface of the test piece, so it is not suitable for evaluating bending cracks within the plate plane (not the end surface of the blank but the inside of the blank) to predict cracks within the product during forming. Furthermore, because the plate thickness direction strain gradient and the in-plane direction strain gradient are calculated by numerical analysis, the evaluation method in Patent Document 1 has problems in terms of accuracy.

[0008] The evaluation method in Non-Patent Document 1 also has a problem in terms of accuracy because the critical strain for bending cracks and the stress distribution in the plate thickness direction are calculated by numerical analysis.

[0009] In the evaluation method of Non-Patent Document 1, the occurrence of bending cracks is judged to be when the maximum load is reached. However, in order to predict the occurrence of cracks during press forming of a blank with higher accuracy, it is necessary to judge the occurrence of bending cracks as when local necking occurs in the thickness direction of the test piece.

[0010] The present invention aims to provide a method for evaluating the forming limits of sheet metal that enables highly accurate evaluation of bending cracks within the sheet surface based on the time point at which localized necking occurs in the sheet thickness direction, and a crack prediction method based on the same. [Means for solving the problem]

[0011] One aspect of the present invention provides a method for evaluating the forming limit of a sheet material, which comprises applying at least one of bending and tension to sheet-shaped test pieces of the same composition so that they are in a plane strain state and have different strain gradients in the sheet thickness direction, obtaining the strain gradients in the sheet thickness direction, and performing multiple types of tests to measure the forming limit strain, determining the relationship between the strain gradient in the sheet thickness direction and the forming limit strain from the forming limit strain and the strain gradient in the sheet thickness direction obtained in the multiple types of tests, and the measured forming limit strain is the maximum principal strain on the surface of the test piece at the time when local necking in the sheet thickness direction is detected in the test piece.

[0012] In each of the multiple types of tests, at least one of bending and tension is applied to a plate-shaped test specimen so that it is in a plane strain state and the strain gradient through the plate thickness differs among the multiple types of tests. Then, for each of the multiple types of tests, the forming limit strain, which is the maximum principal strain on the surface of the test specimen at the time when local necking in the plate thickness direction is detected, is measured, and the relationship with the strain gradient through the plate thickness is determined. As a result, it is possible to evaluate in-plane bending cracking of the plate with high accuracy, based on the time when local necking in the plate thickness direction occurs.

[0013] In each of the plurality of types of tests, the surface of the test piece may be photographed, and local necking in the thickness direction of the test piece may be detected by a time-dependent method or a position-dependent method.

[0014] The forming limit strain for the thickness direction strain gradient that exceeds the maximum thickness direction strain gradient obtained in the multiple types of tests may be estimated to be the same as the forming limit strain for the maximum thickness direction strain gradient.

[0015] The multiple types of tests may include a plane strain tensile test, a tensile bend test, and a crush bend test.

[0016] Another aspect of the present invention provides a crack prediction method, which involves performing a plurality of types of tests in which a plate-shaped test piece of the same composition is placed in a plane strain state and subjected to at least one of bending and tension so as to produce different strain gradients in the thickness direction, obtaining the strain gradient in the thickness direction, and measuring the forming limit strain. The relationship between the strain gradient in the thickness direction and the forming limit strain is determined from the forming limit strain and the strain gradient in the thickness direction obtained in the plurality of types of tests. The measured forming limit strain is the maximum principal strain on the surface of the test piece at the time when local necking in the thickness direction is detected in the test piece. For specific processing of a blank of the same composition as the test piece, if the maximum principal strain on the surface of the blank relative to the expected maximum strain gradient in the thickness direction is lower than the forming limit strain, it is determined that bending cracks will not occur in the blank due to the processing. [Effects of the Invention]

[0017] According to the present invention, it is possible to evaluate the forming limit of sheet material, which enables highly accurate evaluation of bending cracks within the sheet surface based on the time point at which localized necking occurs in the sheet thickness direction, and to predict bending cracks based on this. [Brief explanation of the drawings]

[0018] [Figure 1] 1 is a flowchart showing an outline of a forming limit evaluation method for a sheet material according to an embodiment of the present invention, and a crack prediction method based on the same. [Figure 2A] FIG. 1 is a schematic front view showing a testing machine for a plane strain tensile test. [Figure 2B] 2A is a schematic plan view of the testing machine of FIG. 2A. [Figure 3] FIG. 1 is a schematic front view showing a testing machine for a tensile bending test. [Figure 4] Schematic plan view of the testing machine of Figure 3. [Figure 5] Schematic diagram for explaining U-shaped bending. [Figure 6] FIG. 1 is a schematic front view showing a testing machine for a crush bending test. [Figure 7] 1 is a conceptual diagram for explaining the strain gradient through the plate thickness. [Figure 8] FIG. 1 is a schematic side view for explaining a strain gradient in the plate thickness direction. [Figure 9] 1 is a graph showing an example of the relationship between the strain gradient in the thickness direction and the forming limit strain. DETAILED DESCRIPTION OF THE INVENTION

[0019] Next, an embodiment of the present invention will be described with reference to the accompanying drawings.

[0020] Referring to Figure 1, the relationship between the strain gradient through the thickness direction and the forming limit strain obtained by the sheet material forming limit evaluation method (steps S1 and S2) is used to predict (crack prediction method) by numerical analysis (e.g., finite element method) whether bending cracks will occur in the blank when a specific processing (e.g., press forming) is performed on the blank (step S3).

[0021] In this embodiment, three types of tests are performed: a plane strain tensile test (FIGS. 2A and 2B), a tensile bending test (FIGS. 3 and 4), and a crush bending test (FIGS. 5 and 6). Plate-shaped test pieces 1, 2, and 3 used in each of these three types of tests are made of a metal material (e.g., steel plate) of the same composition. Test pieces 1 to 3 are all elongated rectangular shapes with longitudinal and transverse directions, but other specific shapes of test pieces 1 to 3 may be different.

[0022] In the plane strain tensile test (Fig. 2A, B), tensile bending test (Fig. 3, 4), and crush bending test (Fig. 5, 6), specimens 1 to 3 are deformed to a plane strain state. In all three types of tests, the strain gradient through the thickness is obtained and the forming limit strain is measured.

[0023] Referring to FIG. 7, the thickness direction strain gradient SB is the gradient in the thickness direction of the strain in the bending direction of Test Specimens 1 to 3 (the same direction as the longitudinal direction of Test Specimens 1 to 3) indicated by the arrow BD in the figure.

[0024] In all three types of tests, localized necking occurs after diffusional necking as deformation of specimens 1 to 3 progresses. The forming limit is the point at which composition instability, i.e., localized necking in the thickness direction, is detected, and the maximum principal strain on the surface of specimens 1 to 3 at the time the forming limit is reached is the forming limit strain. As will be described later, in each test, localized necking in the thickness direction of specimens 1 to 3 is detected using the time-dependent method or the position-dependent method.

[0025] Below, the plane strain tensile test (Fig. 2A, B), tensile bending test (Fig. 3, 4), and crush bending test (Fig. 5, 6) will be explained in order.

[0026] 2A and 2B show a testing machine 11 used for a plane strain tensile test.

[0027] The testing machine 11 includes a holder 12 and a die 13 for holding a plate-shaped test specimen 1. The testing machine 11 also includes a punch 14 below the test specimen 1 held by the holder 12 and die 13 for deforming the test specimen 1. The testing machine 11 also includes two cameras 15A and 15B arranged on the opposite side of the punch 14 from the test specimen 1 held by the holder 12 and die 13, i.e., above the test specimen 1 held by the holder 12 and die 13. In FIG. 2, the symbol B indicates the force with which the die 13 presses the test specimen 1 against the holder 12, i.e., the holding force of the test specimen 1, and the symbol M indicates the direction in which the punch moves (upward in the testing machine 11).

[0028] This testing machine 11 is for carrying out a test method known as the Nakajima method, which is specified by ISO, and the tip of the punch 14 is hemispherical with a diameter (for example, 100 mm) large enough relative to the thickness of the test piece 1. Therefore, substantially only tension acts on the test piece 1. The test piece 1 may also be deformed by the Marchiak method, which is specified by ISO, so that substantially only tension acts on it. In this case, a punch 14 with a flat tip is used.

[0029] The punch 14 rises until the test piece 1 reaches the forming limit. During this time, the surface (upper surface) 1a of the test piece 1 is photographed by cameras 15A and 15B.

[0030] In this plane strain tensile test, only tension acts on the test piece 1, so there is no strain distribution in the thickness direction, and the strain gradient in the thickness direction is almost zero.

[0031] Images of the surface 1a of the test piece 1, continuously captured by the cameras 15A and 15B, are input to the calculation unit 16. The calculation unit 16 determines that the test piece 1 has reached the forming limit when it detects, by a time-dependent method or a position-dependent method, that a predetermined amount of local necking has occurred in the thickness direction of the test piece 1. The maximum principal strain of the surface 1a of the test piece 1 at the time when the calculation unit 16 determines that the test piece 1 has reached the forming limit is the forming limit strain. The forming limit strain may be determined by the calculation unit 16 using digital image correlation (DIC) based on the images input from the cameras 15A and 15B, or by other methods such as the scribed circle method. 2B, in order to make the strain gradient through the thickness of the portion of the surface 1a of the test piece 1 photographed by cameras 15A and 15B nearly zero, the portion of the test piece 1 where the punch 14 presses is defined as the band-shaped portion 1b, and the portion of the band-shaped portion 1b that is sandwiched between the holder 12 and the die 13 at both ends is defined as the wide portion 1c. If the width W of the wide portion 1c is narrow, uniaxial tension occurs, and if it is wide, equibiaxial tension occurs. In this embodiment, the width W of the wide portion 1c is set to a width that causes plane strain tensile deformation between the narrow width that causes uniaxial tension and the wide width that causes equibiaxial tension, specifically, 70 mm or more and 100 mm or less.

[0032] 3 and 4 show a testing machine 21 used for the tensile bending test.

[0033] The testing machine 21 includes a holder 22 and a die 23 for holding the test specimen 2. The testing machine 21 also includes a punch 24 below the test specimen 2 held by the holder 22 and the die 23 for subjecting the test specimen 2 to tensile bending deformation. The testing machine 21 also includes two cameras 25A and 26B arranged on the opposite side of the punch 24 from the test specimen 2 held by the holder 22 and the die 23, i.e., above the test specimen 2 held by the holder 22 and the die 23.

[0034] The punch 24 has a punch body 27 and a protrusion 28, and when the punch 24 rises as shown by the arrow M in the figure, the protrusion 28 and the punch body 27 are pressed against the test piece 2, thereby causing the test piece 2 to undergo tensile bending deformation.

[0035] The punch body 27 has a constant cross-sectional shape at its tip in the width direction of the test piece 2 (X direction in the figure). The top surface 27a of the punch body 27 has convex curved surfaces 27b and 27c at both ends in the longitudinal direction of the test piece 2 (Y direction in the figure). In this embodiment, the convex curved surfaces 27b and 27c have cross-sectional shapes in the width direction that are partial arcs obtained by dividing a perfect circle into four equal parts. In other words, the convex curved surfaces 27b and 27c are curved surfaces obtained by dividing a cylindrical surface extending in the width direction of the test piece 2 into four equal parts. The radius RM of the convex curved surfaces 27b and 27c and the distance between the outermost ends of the convex curved surfaces 27b and 27c in the longitudinal direction of the test piece 2, i.e., the width WM of the tip of the punch body 27 in the longitudinal direction of the test piece 2, are factors that determine the bending angle θ after the test piece 2 has fitted to the punch 24, as described below. The cross-sectional shapes of the convex curved surfaces 27b and 27c are not limited to partial arcs. For example, the cross-sectional shape of the convex curved surfaces 27b and 27c may be a partial elliptical arc, a curved surface that is a combination of a partial circular arc and a partial elliptical arc, or a cross-sectional shape that is partially linear.

[0036] The protrusion 28 has a constant cross-sectional shape in the width direction of the test piece 2 (X direction in the figure) and has a convex curved surface 28a, which is its top surface. The cross-sectional shape of the convex curved surface 28a in the width direction is a partial arc obtained by dividing a perfect circle into two equal parts, i.e., a semicircular arc. In other words, the convex curved surface 28a at the tip of the protrusion 28 is a semicylindrical surface obtained by dividing a cylindrical surface extending in the width direction of the test piece 2 into two equal parts. The radius RP of the convex curved surface 28a and the distance from the top surface 27a of the punch body 27 to the apex of the convex curved surface 28a, i.e., the height HP of the protrusion 28, are factors that determine the bending angle θ after the test piece 2 has fitted into the punch 24, as described below. The width WP of the protrusion 28, which is the longitudinal dimension of the test piece 2, is set narrower than the width WM of the tip of the punch body 27.

[0037] The punch 24 rises as shown by the arrow UP from an initial state in which the tip of the convex curved surface 28a of the protrusion 28 of the punch 24 contacts the underside 2 of the test piece 2. In the initial state, the convex curved surfaces 27b and 27c of the punch body 27 are positioned away from the underside 2 of the test piece 2 and are not in contact with it.

[0038] As the punch 24 rises from its initial state, not only is the convex curved surface 28a of the protrusion 28 brought into contact with and pressed against the underside of the test piece 2, but the convex curved surfaces 27b and 27c of the punch body 27 also come into contact with it. In this state, that is, when the test piece 2 has fitted to the punch 24, the portion of the test piece 2 in contact with the convex curved surface 28a of the protrusion 28 and the portion in contact with the convex curved surfaces 27b and 27c of the punch body 27 are linear. In other words, when the test piece 2 has fitted to the punch 24, a bending angle θ is formed between the linear portion between the portion in contact with the convex curved surface 28a of the protrusion 28 and one convex curved surface 27b of the punch body 27 and the linear portion between the portion in contact with the convex curved surface 28a of the protrusion 28 and the other convex curved surface 27c of the punch body 27.

[0039] Even if the punch 24 is further raised from the height position where the test piece 2 has fitted to the punch 24, the bending angle θ is maintained constant in the region between the pair of convex curved surfaces 27b, 27c of the punch body 27. In other words, once the test piece 2 has fitted to the punch 24, even if the punch 24 is further raised, the bending angle θ does not change in the region between the convex curved surfaces 27b, 27c of the test piece 2, and only the tension increases.

[0040] As in the plane strain tensile test, the calculation unit 26 determines the forming limit by a time-dependent method or a position-dependent method based on images of the surface 2a of the test piece 2 input from the cameras 25A and 25B. Also, as in the plane strain tensile test, the calculation unit 36 ​​can determine the forming limit strain by DIC or another method (for example, the scribed circle method) based on the images input from the cameras 25A and 25B.

[0041] Referring to FIG. 8, the strain gradient through the thickness direction at the forming limit can be determined by linear approximation through numerical analysis (e.g., finite element method) using the forming limit strain determined by the calculation unit 36, i.e., the longitudinal strain (maximum principal strain) ε1 of the surface 2a of the test piece 2 at the forming limit.

[0042] Referring to FIG. 4, a pair of notches 2b, 2c that are provided at both ends of the blank 2 in the width direction and that face each other in the width direction will be described.

[0043] Generally, when the width dimension of the test piece 2 is sufficiently long, deformation in the longitudinal direction (which corresponds to the bending direction indicated by the arrow BD in Figure 7) of the test piece 2 is suppressed, resulting in a plane strain state. By providing a pair of notches 2b and 2c facing each other in the width direction, deformation of the test piece 2 is concentrated in a narrow region A connecting the vertices of the notches 2b and 2c. This region A has an elongated shape in the width direction (a shape with a narrow longitudinal width), and the width dimension is sufficiently longer than the longitudinal dimension. Therefore, this region A is in a plane strain state. In this way, by providing the notches 2b and 2c, deformation of the test piece 2 is concentrated in a narrow region whose longitudinal dimension is sufficiently smaller than its width dimension, creating a plane strain state.

[0044] In order to concentrate deformation in the narrow region A and create a plane strain state, the shapes of the notches 2b and 2c in this embodiment are generally semi-elliptical, with the width gradually decreasing in a curved manner toward the tip of the protrusion 28 of the punch 24 and being narrowest at the tip. The notches 2b and 2c may also have a shape in which the width gradually decreases linearly toward the tip of the protrusion 28 of the punch 24 and is narrowest at the tip. Furthermore, the shapes of the notches 2b and 2c may include both a portion where the width gradually decreases linearly and a portion where the width gradually decreases curvedly, as long as the width gradually decreases toward the tip of the protrusion 28 of the punch 24 and is narrowest at the tip.

[0045] 6 shows a testing machine 31 used for the crush bending test. The testing machine 31 includes a die 33 and a punch 34, as well as two cameras 35A and 35B arranged on either side of the die 33 and punch 34.

[0046] The test piece 3 for the crush bending test is first subjected to U-bending as shown in Figure 5. In this U-bending, a punch 38 is lowered, as indicated by the symbol M, toward the test piece 3 supported on a pair of rotatable rollers 37A, 37B, so that the test piece 3 is bent without tension. The tip of the punch 38 crosses the test piece 3 in the width direction. In this example, the cross-sectional shape of the test piece 3 in the width direction at the tip of the punch 38 is semicircular.

[0047] After being bent to a certain extent in the U-bend shown in Fig. 5, the test piece 3 is set in the testing machine 31 shown in Fig. 6. Specifically, the test piece 3 is placed on its side with one of the straight portions at both ends of the U-bent portion resting on the die 33. As shown by the symbol M, the punch 34 moves, i.e., descends, toward the die 33 while contacting the other straight portion at both ends of the test piece 3, and the test piece 3 is further bent without any tension being applied.

[0048] As in the plane strain tensile test and the tensile bending test, the calculation unit 36 ​​determines the forming limit by a time-dependent method or a position-dependent method based on images of the surface 3a of the test piece 3 input from the cameras 35A and 35B. Also, as in the plane strain tensile test and the tensile bending test, the calculation unit 36 ​​can determine the forming limit strain by DIC or another method (for example, the scribed circle method) based on the images input from the cameras 35A and 35B.

[0049] Referring to FIG. 8, in the crush bending test, no tension acts on the test piece 3, so the strain gradient in the thickness direction at the forming limit can be calculated by the following formula.

[0050]

number

[0051] Figure 9 is a graph summarizing the results of the three types of tests mentioned above, namely, the plane strain tensile test, the tensile bending test, and the crush bending test, conducted on two different materials (Material A and Material B). In this graph, the vertical axis is the forming limit strain, and the horizontal axis is the strain distribution through the thickness direction corresponding to the forming limit strain.

[0052] In the plane strain tensile test, the strain gradient through the thickness is zero, so the forming limit strain for both Material A and Material B is distributed along the vertical axis. In the tensile bending test and the crush bending test, bending is applied to the test piece, so both have a strain gradient through the thickness. In the tensile bending test, the bending angle θ (see Figure 3) is constant, whereas in the crush bending test, the bending of the test piece increases until it reaches the forming limit. Therefore, for both Material A and Material B, the forming limit strain is distributed in a region where the strain gradient through the thickness is larger in the crush bending test than in the tensile bending test.

[0053] As shown by lines La and Lb, the relationship between the thickness strain gradient and the forming limit strain was obtained from the data obtained from the three types of tests for both Material A and Material B. For Material A, this means that the processing conditions in the region below line La do not reach the forming limit, while the processing conditions in the region on line La and above reach the forming limit. The same is true for Line Lb for Material B.

[0054] In Figure 9, for both Material A and Material B, the data obtained from the crush bending test have the maximum thickness direction strain gradient. The forming limit strain for a thickness direction strain gradient that exceeds the maximum thickness direction strain gradient obtained from the crush bending test is assumed to be the same as the forming limit strain at the maximum thickness direction strain gradient obtained from the crush bending test. In other words, for both lines La and Lb, in the region where the thickness direction strain gradient is larger than the data obtained from the crush bending test, they are parallel to the horizontal axis (constant forming limit strain).

[0055] In this embodiment, the strain gradient through the thickness is obtained for each of three types of tests: a plane strain tensile test (tension only), a tensile bending test (tension bending with a constant bending angle), and a crush bending test (bending only). The forming limit strain, which is the maximum principal strain on the surface of the test piece at the time when local necking in the thickness direction of the test piece is detected, is also measured. As a result, bending cracks within the surface of the test piece can be evaluated with high accuracy based on the time when necking occurs.

[0056] Using the relationship between lines La and Lb in Figure 9, a numerical analysis (e.g., finite element method) is performed on a specific processing of a blank with the same composition as the test piece.If the maximum principal strain on the surface of the blank for the maximum strain gradient through the plate thickness estimated from the results of the numerical analysis is below the forming limit strain shown by lines La and Lb, it can be determined that bending cracks will not occur in the blank due to processing.

[0057] When used for evaluation in numerical analysis, the lines La and Lb may be expressed as a function or a table. [Explanation of symbols]

[0058] 1, 2, 3 Test pieces 1a,2a,3a surface 1b Belt 1c Wide part 2b, 2c notch 11 Testing machine (plane strain tensile test) 12 Holder 13 Die 14 Punch 15A, 15B Camera 16 Arithmetic section 21 Testing machine (tensile test) 22 Holder 23 Die 24 Punch 25A, 25B Camera 26 Arithmetic section 27 Punch body 27a Top surface 27b,27c Convex curved surface 28 Protrusion 28a Convex curved surface 31 Testing machine (tensile test) 32 Holder 34 Punch 35A, 35B Camera 36 Arithmetic section 37A, 37B Roller 38 Punch Area A B Holding force BD bending direction HP protrusion 28 height M Direction in which the punch moves RM Convex curved surface 27b, 27c radius RP Convex curved surface 28a radius SB Strain gradient through the thickness WM punch body 27 tip width WP Width of protrusion 28

Claims

1. A plurality of types of tests are performed to plate-shaped test pieces of the same composition, in which the test pieces are placed in a plane strain state and subjected to at least one of bending and tension so as to have different strain gradients in the thickness direction, and the strain gradients in the thickness direction are obtained to measure the forming limit strain; The relationship between the strain gradient in the thickness direction and the forming limit strain is determined from the forming limit strain and the strain gradient in the thickness direction obtained from the plurality of types of tests; The method for evaluating the forming limit of a sheet material, wherein the measured forming limit strain is the maximum principal strain on the surface of the test piece at the time when local necking in the sheet thickness direction is detected in the test piece.

2. 2. The method for evaluating the forming limit of a sheet material according to claim 1, wherein a surface of the test specimen is photographed in each of the plurality of types of tests, and local necking in the thickness direction of the test specimen is detected by a time-dependent method or a position-dependent method.

3. The forming limit strain for the thickness direction strain gradient that exceeds the maximum thickness direction strain gradient obtained in the plurality of types of tests is estimated to be the same as the forming limit strain for the maximum thickness direction strain gradient.

4. The forming limit evaluation method for a sheet material according to claim 1 , wherein the plurality of types of tests include a plane strain tensile test, a tensile bending test, and a crush bending test.

5. A plurality of types of tests are carried out on plate-shaped test pieces of the same composition, in which the test pieces are placed in a plane strain state and subjected to at least one of bending and tension so as to produce different strain gradients in the thickness direction, and the strain gradients in the thickness direction are obtained to measure the forming limit strain; From the forming limit strain and the thickness direction strain gradient obtained from the plurality of types of tests, a relationship between the thickness direction strain gradient and the forming limit strain is determined; The measured forming limit strain is a maximum principal strain on the surface of the test piece at a time when local necking in the thickness direction of the test piece is detected, A crack prediction method that determines that bending cracks will not occur in the blank due to specific processing on a blank of the same composition as the test piece if the maximum principal strain on the surface of the blank relative to the expected maximum strain gradient in the thickness direction is below the forming limit strain.

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