Tensile bending test method

Abstract

To achieve a tensile bending test having a constant bending angle and capable of measuring strain during a test by a digital image correlation method with a simple testing machine configuration.SOLUTION: A longitudinal end section of a test piece 2 is held by a holder 3 and a die 4, and a punch 5 is pressed to be tensile bending deformed. The punch 5 has a punch body 11 and a projection unit 12A. The punch body having a constant cross-sectional shape in a lateral direction of the test piece 2 at a tip has first convex curved surfaces 14e, 14f provided at both ends in a longitudinal direction of the test piece 2 at the tip. The projection unit is provided so as to protrude to a test piece side at the tip of the punch body 11, has a second convex curved surface 12d in which a cross-sectional shape in the lateral direction of the test piece 2 is constant, and has a second breadth WP narrower than a first breadth WM that is a separation distance in the longitudinal direction of the test piece 2 between the first convex curved surfaces 14e and 14f of the punch body 11.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a tensile bending test method.

Background Art

[0002] There is known a test (tensile bending test) for evaluating the cracking of a test piece by bending with tension (tensile bending) for predicting the press formability of a blank such as a steel sheet.

[0003] In the tensile bending test disclosed in Non-Patent Document 1, both sides of the test piece are chucked so that material flow does not occur, and the test piece is tensile bent and deformed using punches with different radii until cracking occurs.

[0004] In the tensile bending test disclosed in Non-Patent Document 2, after allowing the test piece to conform to the punch with a low tension at the initial stage of forming, the displacement of the punch is stopped, and tension is applied to the test piece until cracking occurs.

Prior Art Documents

Non-Patent Documents

[0005]

Non-Patent Document 1

Non-Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0006] In the tensile bending test described in Non-Patent Document 1, both the bending angle and tension change with the displacement of the punch. As a result, the strain gradient in the thickness direction (the gradient of strain in the bending direction in the thickness direction) also changes with the displacement of the punch.

[0007] In the tensile bending test described in Non-Patent Document 2, the tension can be controlled by keeping the bending angle constant, making it possible to conduct the test under conditions where the strain gradient in the thickness direction of the plate is constant. However, performing this tensile bending test requires a complex testing machine and special fixtures. Furthermore, this testing machine is unsuitable for measuring strain during the test using the Digital Image Correlation (DIC) method because it is difficult to secure a field of view for photographing the test specimen with a camera.

[0008] The present invention aims to realize a tensile bending test with a constant bending angle and the ability to measure strain during the test using the digital image correlation method, using a simple testing machine configuration. [Means for solving the problem]

[0009] One aspect of the present invention provides a tensile bending test method comprising: a holder and a die that clamp the longitudinal end of a plate-shaped test piece; a punch that presses against the test piece to cause tensile bending deformation; the punch comprising: a punch body having a constant cross-sectional shape in the short direction of the test piece at its tip and first convex curved surfaces at both ends of the tip in the longitudinal direction of the test piece; and a projection provided at the tip of the punch body so as to protrude toward the test piece, having a second convex curved surface having a constant cross-sectional shape in the short direction of the test piece, and having a second width which is the longitudinal dimension of the test piece and is narrower than a first width which is the distance between the first convex curved surfaces of the punch body in the longitudinal direction of the test piece.

[0010] When the punch is pressed against the test specimen, initially only the second convex surface of the projection contacts the specimen. As the punch displaces, in addition to the second surface, the first convex surfaces at both ends of the tip of the punch body also come into contact with the specimen. In this state, that is, when the specimen has settled into the punch, the area between the part of the specimen that is in contact with the second convex surface of the projection and the area between the pair of first convex surfaces of the punch body become straight lines. In other words, when the specimen has settled into the punch, a bending angle is created between the straight line between the part of the specimen that is in contact with the second convex surface of the projection and one of the first convex surfaces of the punch body, and the straight line between the part of the specimen that is in contact with the second convex surface of the projection and the other first convex surface of the punch body.

[0011] After the specimen has settled into the punch, even if the punch is further displaced, the orientation of these linear sections is maintained, so the bending angle of the specimen in the region between the pair of first convex surfaces remains constant. In other words, once the specimen has settled into the punch, even if the punch is further displaced, the bending angle in the region between the first convex surfaces of the specimen does not change, and only the tension increases.

[0012] As described above, in the region between the first convex curved surfaces of the specimen, the tension acting on the specimen can be increased while maintaining a constant bending angle. Therefore, the specimen can be subjected to tensile bending deformation while maintaining a constant strain gradient in the thickness direction.

[0013] Furthermore, there is nothing obstructing the camera's field of view on the surface of the test specimen opposite to the side against which the punch is pressed, ensuring a clear field of view. Therefore, strain measurement during testing is possible using the digital image correlation method.

[0014] Furthermore, constant-angle tensile bending is achieved by a two-stage punch with a narrow projection at the tip of the punch body. Therefore, there is no need to manufacture special jigs to achieve constant-angle tensile bending, nor is a complex mechanism or control required for the testing machine.

[0015] The projection may be detachably fixed to the tip of the punch body.

[0016] By replacing the protrusions with different shapes, widths, and heights (the amount of protrusion from the tip of the punch body) of the second convex curved surface, it becomes possible to perform tensile bending tests with a constant bending angle under conditions where the radius of the part of the test piece that bends along the second convex curved surface and the bending angle differ, enabling evaluation at various strain gradients in the thickness direction.

[0017] In the first state, when only the second convex curved surface of the projection is in contact with the test piece, the test piece holding force that presses the die against the holder to hold the test piece is set to the first holding force. In the second state, when both the second convex curved surface of the projection and the first convex curved surface of the punch body are in contact with the test piece, the test piece holding force is set to the second holding force that prevents material from flowing in from the portion of the test piece held between the holder and the die, and the first holding force may be smaller than the second holding force.

[0018] By controlling the specimen holding force in this way, it is possible to prevent cracking of the specimen before it conforms to the punch, that is, before it is subjected to constant tensile bending at a constant bending angle, even when the total elongation of the material constituting the specimen is low (for example, about 35% or less).

[0019] A portion of the test piece may be bent in the longitudinal direction and provided between the portion of the test piece held between the holder and the die and the portion pressed by the punch.

[0020] Before the specimen conforms to the punch, that is, before it contacts the first convex curved surface of the punch body, the excess material stretches, relieving the tension acting on the specimen. Therefore, even if the total elongation of the material constituting the specimen is low (for example, about 35% or less), it is possible to prevent cracking of the specimen before it conforms to the punch, that is, before it undergoes constant tensile bending at a constant bending angle, without controlling the specimen holding force.

[0021] Notches are provided at both ends in the short-side direction of the portion of the test piece where the protrusion of the punch is pressed. The shape of the notch may be such that the width gradually decreases linearly or curvilinearly toward the tip of the protrusion of the punch, and the width at the outermost tip may be the narrowest.

[0022] By providing the notches, the deformation of the test piece is concentrated in a narrow region where the dimension in the longitudinal direction is sufficiently smaller than the dimension in the width direction. As a result, the strain of the test piece can be made into a plane strain state.

[0023] The test piece during tensile bending deformation due to the pressing of the punch may be photographed by a camera disposed on the side opposite to the punch with respect to the test piece.

[0024] The camera can photograph the surface of the test piece on the side opposite to the punch, that is, the surface of the test piece on the bending outer side. Therefore, by the digital image correlation method, the change in the strain of the test piece during the test can be measured, and the strain of the forming limit due to cracking can be calculated.

[0025] The protrusion length, which is the dimension in the short-side direction of the test piece of the protrusion, is shorter than the test piece width, which is the dimension in the short-side direction of the test piece. When the punch is pressed against the test piece to cause tensile bending deformation, both end portions in the short-side direction of the test piece may protrude in the short-side direction with respect to the protrusion.

[0026] Both end portions in the short-side direction of the test piece bend toward the end face of the protrusion, and it is possible to suppress the test piece from shrinking in the short-side direction at the portion pressed by the protrusion. Therefore, an ideal plane strain state can be created at the portion pressed by this protrusion.

[0027] The protrusion length may be longer than half of the test piece width.

[0028] The dimension of the portion pressed by the protrusion in the short-side direction of the test piece becomes longer. The region in the plane strain state becomes wider.

[0029] Auxiliary protrusions are provided at both ends of the projection in the short-side direction of the test piece, and are provided at the tip of the punch body so as to protrude toward the test piece. The first height, which is the amount of the projection protruding from the punch body, may be greater than the second height, which is the amount of the auxiliary protrusion protruding from the punch body.

[0030] The aforementioned projection and the auxiliary projection may be continuous via a curved surface.

[0031] This makes it easier to conform the ends of the short side of the test specimen to the end faces of the protrusions. [Effects of the Invention]

[0032] According to the tensile bending test of the present invention, a tensile bending test in which the bending angle is constant and strain measurement during the test is possible using the digital image correlation method can be realized with a simple testing machine configuration. [Brief explanation of the drawing]

[0033] [Figure 1] A schematic front view of the testing machine in the tensile bending test method according to the first embodiment of the present invention. [Figure 2] Schematic plan view of the test machine shown in Figure 1. [Figure 3] A perspective view of a punch. [Figure 4] Front view of the punch. [Figure 5] Disassembled front view of the punch. [Figure 6] Front view of the punch with the protruding part replaced. [Figure 7] Front view of the punch with the protruding part replaced. [Figure 8] A diagram showing the change in blank holding force with respect to punch displacement in the first embodiment. [Figure 9] A schematic front view of the test machine with the punch in its initial position. [Figure 10] A schematic front view of the test machine when the punch is in a higher position than the position shown in Figure 9. [Figure 11]A schematic front view of the test machine when the punch is in a higher position than the position shown in Figure 10. [Figure 12] A schematic front view of the test machine when the punch is in a higher position than the position shown in Figure 11. [Figure 13] A conceptual diagram illustrating the strain gradient in the thickness direction of the plate. [Figure 14] A schematic front view of the testing machine in the tensile testing method according to the second embodiment of the present invention. [Figure 15] Schematic plan view of the test machine shown in Figure 14. [Figure 16] A diagram showing the change in blank holding force with respect to punch displacement in the second embodiment. [Figure 17] A perspective view of the punch of the testing machine in the tensile testing method according to the third embodiment. [Figure 18] Schematic plan view of the test machine shown in Figure 17. [Figure 19] A cross-sectional view along the line XIX-XIX in Figure 18. [Figure 20] A cross-sectional view showing the punch and blank when the punch is in the raised position. [Figure 21] A front view showing the punch and blank when the punch is in the raised position. [Modes for carrying out the invention]

[0034] (First Embodiment) Figures 1 and 2 show the testing machine 1 in the tensile bending test method according to the first embodiment of the present invention.

[0035] The testing machine 1 includes a holder 3 and a die 4 for holding a blank (test piece) 2. The testing machine 1 also includes a punch 5 below the blank 2 held by the holder 3 and die 4 for tensile bending deformation of the blank 2. Furthermore, the testing machine 1 includes two cameras 6A and 6B positioned above the blank 2 held by the holder 3 and die 4, on the opposite side of the punch 5 from the blank 2 held by the holder 3 and die 4.

[0036] In this embodiment, the blank 2 is a long, narrow rectangular plate whose length (Y direction in the figure) is significantly greater than its width (X direction in the figure). The thickness of the blank 2 (Z direction in the figure) is constant. The blank 2 is provided with a pair of notches 2a and 2b at both ends in the short direction, facing each other in the short direction. Notches 2a and 2b will be described in detail later.

[0037] In this embodiment, both the holder 3 and the die 4 are annular in shape. The blank 2 is held at both longitudinal ends between the upper surface of the holder 3 and the lower surface of the die 4, thereby retaining the blank 2 with respect to the holder 3 and the die 4. In detail, the holder 3 is fixed, while the die 4 presses the blank 2 against the holder 3 with a downward force indicated by arrow B in Figure 1, i.e., a blank-holding force. The shapes of the holder 3 and the die 4 are not particularly limited as long as they adequately hold both ends of the blank 2.

[0038] The punch 5 is positioned below the blank 2, which is held by the holder 3 and die 4, in the space surrounded by the holder 3 and die 4, as described above.

[0039] Referring to Figures 3 and 4, the punch 5 comprises a punch body 11 and a projection 12A (which can be replaced with other projections 12B and 12C, as will be described later). In this embodiment, the punch body 11 comprises a cylindrical punch base 13 and an auxiliary member 14 fixed to the tip of the punch base 13. The auxiliary member 14 constitutes the tip of the punch body 11. The shape of the punch base 13 is not particularly limited and may be prismatic (see Figure 17). Also, the punch base 13 and the auxiliary member 14 may be an integrated structure. The punch 5 is movable up and down. As indicated by the arrow UP in the figure, when the punch 5 rises, the projection 12A and the auxiliary member 14 are pressed against the blank 2 in a manner that will be described in detail later, and the blank 2 is deformed by tensile bending as a result.

[0040] Referring to Figures 3 and 4, the auxiliary member 14 is a flattened rectangular parallelepiped overall, and the cross-sectional shape in the short direction (X direction in the figure) of the blank 2 is constant. The bottom surface 14a, front surface 14b, and rear surface 14c of the auxiliary member 14 are flat surfaces. The bottom surface 14a, front surface 14b, and rear surface 14c are not limited to flat surfaces, and may have irregularities in part or all. The top surface 14d of the auxiliary member 14 has convex curved surfaces (first convex curved surfaces) 14e and 14f at both ends in the longitudinal direction (Y direction in the figure) of the blank 2, respectively.

[0041] The shape of the top surface 14d of the auxiliary member 14 in this embodiment will be described further.

[0042] In this embodiment, the convex curved surfaces 14e and 14f at both ends of the top surface 14d are partial arcs formed by dividing a perfect circle into four equal parts in the cross-sectional shape of the blank 2 in the short direction. In other words, the convex curved surfaces 14e and 14f are curved surfaces formed by dividing a cylindrical surface extending in the short direction of the blank 2 into four equal parts. The radius RM of the convex curved surfaces 14e and 14f, and the distance between the outermost ends of the convex curved surfaces 14e and 14f in the longitudinal direction of the blank 2, i.e., the width (first width) WM of the auxiliary member 14 in the longitudinal direction of the blank 2, are factors that determine the bending angle θ after the blank 2 has settled into the punch 5, as will be described later. The cross-sectional shape of the convex curved surfaces 14e and 14f is not limited to partial arcs. For example, the cross-sectional shape of the convex curved surfaces 14e and 14f may be a partial elliptical arc, a curved surface that is a combination of a partial arc and a partial elliptical arc, or part of the cross-sectional shape may be a straight line.

[0043] In this embodiment, the longitudinal region of the blank 2 sandwiched between the convex curved surfaces 14e and 14f of the top surface 14d is a flat surface 14g. This region is not limited to a flat surface and may have irregularities in part or all of it.

[0044] Next, the shape of the projection 12A will be described.

[0045] Referring to Figures 3 and 4, the projection 12A has a constant cross-sectional shape in the short direction (X direction in the figures) of the blank 2. In this embodiment, the bottom surface 12a, front surface 12b, and rear surface 12c of the projection 12A are flat surfaces. The bottom surface 12a, front surface 12b, and rear surface 12c are not limited to flat surfaces and may have irregularities in part or all. The projection 12A has a convex curved surface (second convex curved surface) 12d which is its top surface.

[0046] The convex curved surface 12d of the projection 12A is a semicircular arc, which is a partial arc obtained by bisecting a perfect circle, in the cross-sectional shape of the blank 2 in the short direction. In other words, the convex curved surface 12d is a semi-cylindrical surface obtained by bisecting the cylindrical surface extending in the short direction of the blank 2. The radius RP of the convex curved surface 12d and the distance from the top surface 14d of the auxiliary member 14 to the vertex of the convex curved surface 12d, i.e., the height HP of the projection 12A, are factors that determine the bending angle θ after the blank 2 has settled into the punch 5, as will be described later. The width (second width) WP of the projection 12A, which is the longitudinal dimension of the blank 2, is set to be narrower than the width WM of the top surface 14d of the auxiliary member 14.

[0047] Referring to Figure 5, the auxiliary member 14 is equipped with a pair of pins 14h protruding from its bottom surface 14a. By inserting these pins 14h into a pair of holes 13b provided in the top surface 13a of the punch base 13, the auxiliary member 14 is held in place on the punch base 13. With the auxiliary member 14 removed from the punch base 13, the projection 12A can be removed from the auxiliary member 14 and replaced with the other projections 12B, 12C (Figures 6 and 7). The bottom surface 12a of the projections 12A to 12C is provided with a screw hole 12e. On the other hand, the auxiliary member 14 is provided with a stepped through hole 14i. By inserting a screw 15 through the stepped through hole 14i and screwing it into the screw hole 12e, the projections 12A to 12C are fixed to the auxiliary member 14. Therefore, by removing the auxiliary member 14 from the punch base 13 and loosening and removing the screw 15, the protrusions 12A to 12C can be removed from the auxiliary member 14 and replaced with other protrusions 12A to 12C.

[0048] In Figure 6, the projection 12B has a smaller radius RP of the convex curved surface 12d than projection 12A, and the height HP is the same as projection 12A. In Figure 7, the projection 12C has a smaller radius RP of the convex curved surface 12d than projection 12B, and the height HP is the same as projections 12A and 12B.

[0049] Next, the tensile bending test method of the first embodiment will be described with reference to Figures 8 to 12.

[0050] Figure 8 shows the change in blank holding force B with respect to the displacement of punch 5. The control of this blank holding force B will be described in detail later.

[0051] As shown in Figure 9, the initial state in which the leading edge of the convex curved surface 12d of the projection 12A of the punch 5 is in contact with the lower surface 2c of the test piece 2, the punch 5 rises as indicated by the arrow UP. In the initial state, the pair of convex curved surfaces 14e and 14f of the auxiliary member 14 of the punch body 11 are located apart from the lower surface 2c of the test piece 2 and are not in contact with it.

[0052] Subsequently, the punch 5 rises until a crack occurs in the test specimen 2, during which time the upper surface 2d of the test specimen is photographed by cameras 6A and 6B.

[0053] Referring to Figure 10, when the punch 5 rises slightly from its initial state (Figure 9), the curved surface 12d of the projection 12A is pressed against the lower surface 2c of the test piece 2, causing the test piece 2 to bend in the longitudinal direction (the bending angle is indicated by the sign θ). In the state shown in Figure 10, the pair of convex curved surfaces 14e and 14f of the punch body 11 are separated from the lower surface 2c of the test piece 2, and the distance from the part of the test piece 2 that is in contact with the curved surface 12d of the projection 12A to both ends sandwiched between the holder 3 and the die 4 is generally straight.

[0054] Referring to Figure 11, as the punch 5 rises further from the height position shown in Figure 10, not only does the convex curved surface 12d of the projection 12A contact and press against the lower surface 2c of the test piece 2, but the convex curved surfaces 14e and 14f of the punch body 11 also come into contact. In this state, that is, when the test piece 2 has settled into the punch 5, the space between the portion of the test piece 2 that is in contact with the convex curved surface 12d of the projection 12A and the space between the pair of convex curved surfaces 14e and 14f of the punch body 11 becomes straight. In other words, when the test piece 2 has settled into the punch 5, a bending angle θ is created in the test piece 2 between the straight space between the portion of the test piece 2 that is in contact with the convex curved surface 12d of the projection 12A and one of the convex curved surfaces 14e of the punch body 11, and between the straight space between the portion of the test piece 2 that is in contact with the convex curved surface 12d of the projection 12A and the other convex curved surface 14f of the punch body 11.

[0055] Referring to Figure 12, even if the punch 5 rises further from the height position where the test piece 2 shown in Figure 11 is fitted to the punch 5, the bending angle θ in the region between the pair of convex curved surfaces 14e and 14f of the punch body 11 remains constant. In other words, once the test piece 2 is fitted to the punch 5, even if the punch 5 rises further, the bending angle θ in the region between the first convex curved surfaces 14e and 14f of the test piece 2 does not change, and only the tension increases. The punch 5 rises until a crack occurs in the test piece.

[0056] As described above, in the region between the convex curved surfaces 14e and 14f of specimen 2, the tension can be increased while maintaining a constant bending angle θ of specimen 2. As a result, specimen 2 can be subjected to tensile bending deformation while maintaining a constant strain gradient in the thickness direction.

[0057] Referring to Figure 13, the plate thickness direction strain gradient SB is the gradient in the plate thickness direction of the strain in the bending direction of test piece 2 (the same direction as the longitudinal direction of test piece 2), indicated by arrow BD in the same figure.

[0058] The punch 5 is positioned on the underside of the test piece 2 and pressed against the lower surface 2c of the test piece 2. However, on the opposite side, i.e., the upper side of the test piece 2, there is nothing obstructing the field of view of cameras 6A and 6B. During the test, the field of view of cameras 6A and 6B for the upper surface 2d of the test piece 2 is maintained between the pair of convex curved surfaces 14e and 14f. Therefore, cameras 6A and 6B can continuously photograph the portion of the upper surface 2d of the test piece 2 between the pair of convex curved surfaces 14e and 14f, enabling strain measurement of the upper surface 2d (bent outer surface) of the test piece 2 during the test using the digital image correlation method, and allowing calculation of the strain at the forming limit due to cracking.

[0059] Tension bending with a constant bending angle θ is achieved by a two-stage punch 5, which has a narrow projection 12A on an auxiliary member 14 that is the tip of the punch body 11. Therefore, there is no need to manufacture a special jig to achieve tensile bending with a constant bending angle θ, nor is a complex mechanism or control required for the testing machine 1.

[0060] By replacing the projection 12A with projections 12B and 12C, which have different shapes, widths WP and HPa of the convex curved surface 12d, it becomes possible to perform tensile bending tests with a constant bending angle under conditions where the radius of the portion of the test piece 2 that bends along the convex curved surface 12d and the bending angle θ are different, enabling evaluation at various strain gradients in the plate thickness direction.

[0061] Referring to Figure 8, in this embodiment, the blank holding force B is changed in response to the displacement of the punch 5. Specifically, from the initial state shown in Figure 9 until the test piece 2 settles into the punch 5 as shown in Figure 11, that is, when only the convex curved surface 12d of the projection 12A is in contact with the lower surface 2c of the test piece 2 (first state), the blank holding force B is set to a relatively small holding force (first holding force) B1. As shown in Figure 11, when the test piece 2 settles into the punch 5 and the punch 5 rises further, that is, when both the convex curved surface 12d of the projection 12A and the convex curved surfaces 14e and 14f of the punch body 11 are in contact with the lower surface 2c of the test piece 2 (second state), the blank holding force B is set to a relatively large holding force (second holding force) B2. The holding force B2 is set to a size that prevents material from flowing in from both ends in the longitudinal direction of the test piece 2, which is held between the holder 3 and the die 4. The holding force B1 is set to be smaller than the holding force B2.

[0062] By controlling the blank holding force B in this way, even when the total elongation of the material constituting the test piece 2 is low (for example, about 35% or less), it is possible to prevent cracking of the test piece 2 before it conforms to the punch 5, that is, before it becomes a constant tensile bend at a constant bending angle θ.

[0063] Referring to Figure 2, a pair of notches 2a and 2b, which are located at both ends of the blank 2 in the short direction and face each other in the short direction, will be described.

[0064] Generally, when the dimension of the specimen 2 in the short direction is sufficiently long relative to the deformation in the longitudinal direction of the specimen 2 (which coincides with the bending direction indicated by arrow BD in Figure 13), the deformation in the short direction is suppressed, resulting in a plane strain state. By providing a pair of opposing notches 2a and 2b in the short direction, the deformation of the specimen 2 is concentrated in a narrow region A connecting the vertices of notches 2a and 2b. This region A has an elongated shape in the short direction (a shape with a narrow width in the longitudinal direction), and the dimension in the short direction is sufficiently long relative to the dimension in the longitudinal direction. Therefore, a plane strain state is achieved in this region A. In this way, by providing notches 2a and 2b, the deformation of the specimen 2 can be concentrated in a narrow region where the dimension in the longitudinal direction is sufficiently smaller than the dimension in the width direction, creating a plane strain state. In order to concentrate the deformation in a narrow region A and create a plane strain state, the shapes of the notches 2a and 2b in this embodiment are generally semi-elliptical, with the width gradually decreasing curvilinearly toward the tip of the projection 12A of the punch 5, and the width being narrowest at the very tip. The notches 2a and 2b may also have a shape in which the width gradually decreases linearly toward the tip of the projection 12A of the punch 5, and the width being narrowest at the very tip. Furthermore, the shapes of the notches 2a and 2b may include both a portion where the width gradually decreases linearly and a portion where the width gradually decreases curvilinearly, as long as the width gradually decreases toward the tip of the projection 12A of the punch 5, and the width is narrowest at the very tip.

[0065] (Second Embodiment) A tensile bending test method according to a second embodiment of the present invention will be described with reference to Figures 14 to 16. Except for matters mentioned in the following description, the second embodiment is the same as the first embodiment, and the same elements as in the first embodiment are denoted by the same reference numerals in Figures 14 to 16.

[0066] Referring to Figures 14 and 15, in this embodiment, the configuration of the testing machine 1 is the same as in the first embodiment, but the shape of the test piece 2 is different from that of the first embodiment.

[0067] The test piece 2 has excess portions 2e that are bent in the longitudinal direction between the longitudinal ends that are clamped by the holder 3 and the die 4 and the portion that is pressed by the punch 5.

[0068] Referring to Figure 16, in this embodiment, the blank holding force B is set to a holding force B2 such that no material flows in from both ends in the longitudinal direction of the test piece 2, which is held between the holder 3 and the die 4, from the initial state (see Figure 9) until a crack occurs in the test piece 2.

[0069] Before the test piece 2 conforms to the punch 5, that is, before the test piece 2 comes into contact with the convex curved surfaces 14e and 14f of the punch body 11, the excess portion 2e stretches, relieving the tension acting on the test piece 2. Therefore, even if the total elongation of the material constituting the test piece 2 is low (for example, about 35% or less), it is possible to prevent cracking of the test piece 2 before it conforms to the punch 5, that is, before it becomes a constant tensile bend at a bending angle θ, without controlling the blank holding force B (increasing it stepwise from holding force B1 to holding force B2 as in the first embodiment).

[0070] (Third embodiment) A tensile bending test method according to a third embodiment of the present invention will be described with reference to Figures 17 to 21. Except for matters mentioned in the following description, the third embodiment is the same as the first embodiment, and the same elements as in the first or second embodiment are denoted by the same reference numerals in Figures 17 to 21. In this embodiment, the structure and operation of the holder 3 and die 4 (not shown in Figures 17 to 21) of the testing machine 1 are the same as in the first embodiment described with reference to Figures 1 and 9 to 12.

[0071] Referring to Figure 17, the punch base 13 is prismatic in shape. The auxiliary member 14 is formed in the same manner as in the first embodiment and has a top surface 14d that includes convex curved surfaces 14e, 14f and a flat surface 14g, and is fixed to the tip of the punch base 13, forming the tip of the punch body 11.

[0072] The projection 12A is provided on the tip of the punch body 11 so as to protrude upward, similar to the first embodiment. The projection 12A has a convex curved surface 12d with a constant cross-sectional shape in the width direction of the blank. The width WP of the projection 12A is narrower than the width WM of the punch body 11.

[0073] Referring to FIG. 18, the blank 2 in the present embodiment does not have cutouts 2a and 2b. The blank 2 is a steel plate. The blank 2 is in the shape of an elongated rectangular plate with its dimension in the longitudinal direction (Y direction in the figure), that is, the length, being sufficiently larger than its dimension in the short transverse direction (X direction in the figure), that is, the width. The thickness of the blank 2 (dimension in the Z direction in the figure) is constant.

[0074] Unlike the above embodiment, the dimension of the protruding portion 12A in the short transverse direction of the blank 2, that is, the protruding length LP, is shorter than the width WB of the blank 2. On the other hand, the protruding length LP is larger than half of the width WB of the blank 2. That is, the protruding length LP and the width WB of the blank 2 satisfy the following formula: WB / 2 < LP < WB.

[0075] The protruding portion 12A is arranged at the center of the tip of the punch body 11 in the short transverse direction of the blank 2 and is arranged at the center of the tip of the punch body 11 in the longitudinal direction of the blank 2.

[0076] The punch 5 includes a pair of auxiliary protruding portions 22 provided at both ends of the protruding portion 12A in the short transverse direction of the blank 2. The auxiliary protruding portions 22 also protrude upward at the tip of the punch body 11.

[0077] The cross-sectional shape of the auxiliary protruding portion 22 in the short transverse direction of the blank 2 is constant. The auxiliary protruding portion 22 has a convex curved surface 22d which is the top surface. The convex curved surface 22d of the auxiliary protruding portion 22 is in a semi-arc shape when viewed in the short transverse direction of the blank 2. That is, the convex curved surface 22d is a semi-cylindrical surface obtained by bisecting a cylindrical surface extending in the short transverse direction of the blank 2 in the same manner as the convex curved surface 12d of the protruding portion 12A.

[0078] The auxiliary protruding portion 22 is low-profile and narrow-width with respect to the protruding portion 12A. The amount of upward protrusion of the protruding portion 12A from the punch body 11, that is, the first protrusion height HP, is higher than the amount of upward protrusion of the auxiliary protruding portion 22 from the punch body 11, that is, the second protrusion height HP2. The width WP of the protruding portion 12A is wider than the dimension of the auxiliary protruding portion 22 in the longitudinal direction of the blank 2, that is, the second protruding width WP2.

[0079] The longitudinal center of the auxiliary projection 22 on the blank 2 coincides with the longitudinal center of the projection 12A on the blank 2. In other words, when viewed in the short direction of the blank 2, the semicircular arc formed by the convex curved surface 12d of projection 12A and the semicircular arc formed by the convex curved surface 22d of auxiliary projection 22 are concentric with each other.

[0080] The projection 12A and the auxiliary projection 22 are continuous via a curved surface 23. The convex curved surface 12d of the projection 12A is continuous with the convex curved surface 22d of the auxiliary projection 22 via the curved surface 23. A gentle step is formed between the projection 12A and the auxiliary projection 22. The curved surface 23 is a cross section that extends in the short direction of the blank 2 and in any direction perpendicular to the short direction, and has a uniform shape in the cross section that passes through the center of the projection 12A and the auxiliary projection 22 in the longitudinal direction of the blank 2.

[0081] The curved surface 23 has a first curved surface 23a extending from the convex curved surface 12d of the projection 12A, and a second curved surface 23b that is continuous with the first curved surface 23a and connects to the convex curved surface 22d of the auxiliary projection 22. The first curved surface 23a is an upwardly convex surface, and is curved downward as it moves outward in the short direction from the edge of the convex curved surface 12d, which is its apex. The second curved surface 23b is a downwardly convex surface, and is curved upward as it moves towards the center in the short direction from the edge of the convex curved surface 22d, which is its apex. The first curved surface 23a and the second curved surface 23b are smoothly continuous.

[0082] Referring to Figures 18 and 19, when the blank 2 is pressed with the punch 5 to deform it by tensile bending, both ends of the blank 2 in the longitudinal direction are held between the upper surface of the holder 3 and the lower surface of the die 4, in the same manner as in the first embodiment described with reference to Figure 1. Both ends of the blank 2 in the short direction protrude in the short direction relative to the projection 12A, and the lower surface of the blank 2 contacts the apex of the projection 12A. In this state, the punch 5 is raised in the same manner as in the first embodiment described with reference to Figures 9 to 12.

[0083] Referring to Figure 20, as the punch 5 continues to rise, the blank 2 is pulled in the longitudinal direction. As a result, the blank 2 tends to contract in the transverse direction. On the other hand, the portion of the blank 2 that is in contact with the apex of the convex curved surface 12d of the projection 12A receives greater surface pressure from the punch 5 than the portion adjacent to the contact portion in the transverse direction that is floating away from the punch 5. As a result, this floating portion deforms so as to tilt downward relative to the portion in contact with the apex, and comes into contact with the transverse outer portion of the projection 12A, i.e., the curved surface 23. The greater the displacement of the punch 5, the more both ends of the blank 2 in the transverse direction conform to the curved surface 23.

[0084] As a result, when the blank 2 is pressed by the punch 5, both ends in the shorter direction catch on the ends of the projection 12A, making it difficult for it to shrink in the shorter direction even when pulled in the longitudinal direction. Therefore, an ideal plane strain state can be created in the part pressed by the projection 12A.

[0085] The projection length LP is shorter than the width WB of the blank 2. Therefore, this constraint on the deformation (shrinkage) of the blank 2 in the short direction can be achieved, and an ideal plane strain state can be created. The projection length LP is longer than half the width WB of the blank 2. Therefore, the plane strain region becomes wider.

[0086] While embodiments have been described above, the above configuration can be modified, added to, or deleted as appropriate within the scope of the present invention. [Explanation of Symbols]

[0087] 1. Test machine 2 Blank 2a, 2b Notches 2c Bottom surface 2d top surface 2e Surplus 3 holders 4 Dies 5 punches 6A, 6B Camera 11 Punch body 12A, 12B, 12C protrusion 12a Bottom 12b Front 12c rear 12d convex surface (second convex surface) 12e threaded hole 13 Punch base 13a Top surface 13b hole 14 Auxiliary members 14a Base 14b Front 14c rear 14d top surface 14e,14f Convex curved surface (first convex curved surface) 14g flat surface 14h pin 14i stepped through hole 15 screws 22 Auxiliary protrusion 22d convex surface 23 Curved surface 23a First curved surface 23b Second curved surface Area A B Blank retention force UP Punch 5 is rising in the direction of upward movement. RM Convex surface 14e,14 radius Width of the top surface 14d of the WM auxiliary member 14 (first width) RP convex surface 12d radius LP protrusion length HP projection height 12A~C (first projection height) HP2 Second protrusion height WP projection width 12A~C (second width) Width of auxiliary projection 22 of WP2 Width of WB test specimen 2 SB strain gradient BD Test specimen 2 bending direction

Claims

1. The holder and die clamp the longitudinal end of the plate-shaped test piece, The aforementioned test piece is pressed with a punch to cause tensile bending deformation. The aforementioned punch, The punch body has a constant cross-sectional shape in the short direction of the test piece at the tip, and a first convex curved surface at each end of the test piece in the long direction at the tip, A projection is provided at the tip of the punch body so as to protrude toward the test piece, the test piece having a second convex curved surface with a constant cross-sectional shape in the short direction, and having a second width which is the longitudinal dimension of the test piece, and is narrower than the first width which is the distance between the first convex curved surfaces of the punch body in the longitudinal direction of the test piece. A tensile bending test method comprising the following components.

2. The tensile bending test method according to claim 1, wherein the projection is detachably fixed to the tip of the punch body.

3. In the first state, where only the second convex curved surface of the projection is in contact with the test piece, the test piece holding force applied by the die to the holder to hold the test piece is set to the first holding force. In the second state in which both the second convex curved surface of the projection and the first convex curved surface of the punch body are in contact with the test piece, the test piece holding force is set to a second holding force that prevents material from flowing in from the portion of the test piece held between the holder and the die. The tensile bending test method according to claim 1 or claim 2, wherein the first holding force is smaller than the second holding force.

4. The tensile bending test method according to claim 1 or 2, wherein an excess portion is provided between the portion of the test piece held between the holder and the die and the portion pressed by the punch, the test piece being bent back in the longitudinal direction.

5. The tensile bending test method according to claim 1 or 2, wherein notches are provided at both ends in the short direction of the portion of the test piece to which the projection of the punch is pressed, and the shape of the notches is such that the width gradually decreases linearly or curved toward the tip of the projection of the punch, and the width is narrowest at the very tip.

6. The projection length, which is the dimension of the projection in the short direction of the test piece, is shorter than the width, which is the dimension of the test piece in the short direction. When the punch is pressed against the test piece to cause tensile bending deformation, both ends of the test piece in the shorter direction are made to protrude in the shorter direction relative to the projection. The tensile bending test method according to claim 1 or 2.

7. The length of the projection is longer than half the width of the test piece. The tensile bending test method according to claim 6.

8. Auxiliary protrusions are provided at both ends of the projection in the short direction of the test piece, and at the tip of the punch body so as to protrude toward the test piece. The height of the first projection, which is the amount of the projection protruding from the punch body, is greater than the height of the second projection, which is the amount of the auxiliary projection protruding from the punch body. The tensile bending test method according to claim 6.

9. The aforementioned projection and the auxiliary projection are continuous via a curved surface. The tensile bending test method according to claim 8.

10. The tensile bending test method according to claim 1 or 2, wherein a camera positioned on the opposite side of the test piece from the punch is used to photograph the test piece as it undergoes tensile bending deformation due to the pressure of the punch.

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