In-plane bending test apparatus and test method, and method for evaluating the fracture limit of a plate.

Abstract

This invention provides an in-plane bending test technology that enables high-precision measurement of the forming limit of the edge face of a metal plate using a simple device. [Solution] The method comprises a pair of support parts positioned symmetrically on either side of the longitudinal center of the test piece, which restrain translational movement and fix the test piece so that it can rotate around a pair of support axes that are perpendicular to the flat surface of the test piece and parallel to each other; a punch that presses one end face in the width direction at the longitudinal center of the test piece in a direction perpendicular to the plane on which the pair of support axes are located, thereby applying in-plane bending deformation to the longitudinal center of the test piece and tensile stress to the other end face in the width direction; and means for recording the stroke and load of the punch. Preferably, the method also includes a buckling prevention mechanism to prevent out-of-plane deformation of the test piece, an imaging means for photographing the surface of the test piece displaying a strain analysis pattern, an imaging means for photographing the end face, a storage means for accumulating the images taken by the imaging means, and a calculation means for analyzing the images and calculating the strain distribution.

Description

【Technical Field】 【0001】 The present invention relates to an in-plane bending test apparatus and a test method for testing stretch flangeability, and more particularly, to an apparatus and a method capable of evaluating the fracture limit of a metal plate or the like used for automobile crashworthy components. 【Background Art】 【0002】 One of the required performances of an automobile body is crashworthiness. That is, it is required to reduce damage to the vehicle body during a collision and protect the occupants. In addition, due to the need for vehicle body weight reduction, the application of high-strength steel sheets that are advantageous for crashworthiness and weight reduction is progressing. With the increase in the strength of materials, the ductility of the materials decreases, increasing the risk of fracture and crack propagation during a collision. 【0003】 In particular, the end faces of components are often processed by shearing with a die or a laser beam. Therefore, compared with the base material, strain accumulates in the edge portion of the end face due to processing, and as a result, there are cases where it is extremely easier to break than the ductility of the base material. For example, as shown in FIG. 5(a), in the deformation of the vehicle body during a collision, when the automobile frame component 6 is greatly deformed, tensile deformation or bending deformation occurs locally in the edge portion of the end face of the component. Therefore, the amount of plastic strain generated in the material at that location becomes very high, and fracture often occurs exceeding the ductility limit of the material. In particular, in high-strength steel sheets having a tensile strength exceeding 980 MPa, catastrophic fracture that divides the base material may occur, and the energy absorption performance during a collision may decrease. Therefore, it has become a major issue in the application of high-strength steel sheets to vehicle bodies. Therefore, as a material, a characteristic having ductility that can withstand such deformation is required, and various high-strength steel sheets with enhanced ductility have been developed. 【0004】 Generally, the ductility of metal sheets is evaluated by indicators such as uniform elongation and local elongation obtained from tensile tests. For edge fracture, the elongation flange test is known as an evaluation method for press formability. However, there is no method to accurately evaluate edge fracture due to impact deformation, and in order to develop and select appropriate materials, various material testing methods have been proposed to determine whether or not edge fracture occurs during impact deformation. 【0005】 As a material testing method, hole expansion tests using cylinders and cones are performed. In addition, Patent Document 1 discloses a side-bend test technique that applies bending deformation and tensile force to the end face of a test piece using a dedicated jig. [Prior art documents] [Patent Documents] 【0006】 [Patent Document 1] Japanese Patent Publication No. 2009-145138 [Overview of the project] [Problems that the invention aims to solve] 【0007】 Incidentally, material testing methods require simple testing techniques that can reproduce the fracture phenomenon occurring during impact, and that are highly accurate with minimal variability. While simplified methods have been proposed for conventional testing, they have had the problem of not being able to adequately reproduce the actual phenomenon. 【0008】 For example, elongation flange tests have traditionally been performed using hole expansion tests. In cylindrical hole expansion tests, fracture often occurs at a point where the material enters the interior from the edge. Therefore, it is difficult to accurately measure the fracture limit. In conical hole expansion tests, deformation occurs in the out-of-plane direction. Therefore, the strain becomes non-uniform in the thickness direction, making it difficult to quantitatively evaluate the fracture strain. 【0009】 The technology described in Patent Document 1 requires specialized jigs and testing equipment, resulting in a significant financial burden. 【0010】 This invention has been made in view of these circumstances, and aims to provide an in-plane bending test apparatus and test method, as well as a method for evaluating the fracture limit of a metal sheet, that can measure the forming limit of the edge face of a metal sheet with high precision without introducing special and complex equipment. [Means for solving the problem] 【0011】 The present invention provides an in-plane bending test apparatus that advantageously solves the above problems, comprising: a pair of support parts arranged symmetrically on either side of the center of the longitudinal direction of a flat plate-shaped test piece, which restrain the translational movement of the test piece and fix the test piece so as to be rotatable around a pair of support axes perpendicular to the flat surface of the test piece and parallel to each other; a punch that presses one end face in the width direction at the center of the longitudinal direction of the test piece in a direction perpendicular to the plane on which the pair of support axes are located, thereby applying in-plane bending deformation to the center of the longitudinal direction of the test piece and tensile stress to the other end face in the width direction; and means for recording the stroke and load of the punch. 【0012】 Furthermore, the in-plane bending test apparatus according to the present invention is (a) The test specimen is equipped with a buckling prevention mechanism to prevent out-of-plane deformation. (b) comprising: a first imaging means for photographing the surface of a test specimen having an arbitrary grid or strain analysis pattern displayed on one or both sides; a second imaging means for photographing the longitudinal center of the test specimen and the other end face in the width direction; a storage means for storing the images captured by the first imaging means and the second imaging means, respectively; and a calculation means for analyzing the images and calculating the strain distribution. These could be more preferable solutions. 【0013】 The present invention provides an in-plane bending test method that advantageously solves the above problems, characterized in that, using the in-plane bending test apparatus, the punch presses against one end face in the width direction and at the longitudinal center of a test piece fixed by the pair of support parts so as to be rotatable around the pair of support axes, thereby applying tensile and bending deformation so as to spread the other end face of the test piece, and from the relationship between the punch stroke and load obtained, the punch stroke and load when a crack penetrates the end face of the test piece in the thickness direction is calculated. 【0014】 Furthermore, a more preferable solution to the in-plane bending test method (c) according to the present invention may be to use an in-plane bending test apparatus equipped with (b), fix a test piece on one or both sides so as to be rotatable around a pair of support axes by the pair of support parts, then press the center of the longitudinal direction and one end face in the width direction of the test piece with the punch, apply tensile and bending deformation so as to expand the other end face in the width direction of the test piece, and calculate the strain value when a crack penetrates the end face of the test piece in the thickness direction based on the grid or strain analysis pattern. 【0015】 The present invention provides a method for evaluating the fracture limit of a plate that advantageously solves the above problems, characterized by using the above-described in-plane bending test apparatus to perform an in-plane bending test on test pieces with different material properties or processing methods at the longitudinal center of the test piece, according to the above-described in-plane bending test method, and evaluating the fracture limit characteristics of the test piece from the punch stroke and load when a crack penetrates the end face at the longitudinal center of the test piece. 【0016】 Furthermore, a more preferable solution for evaluating the fracture limit of a plate according to the present invention may be to use an in-plane bending test apparatus equipped with (b) to perform an in-plane bending test on test pieces with different material properties or processing methods at the longitudinal center of the test piece, according to the in-plane bending test method (c), and evaluate the fracture limit characteristics of the test piece from the strain value when a crack penetrates the end face at the longitudinal center of the test piece. [Effects of the Invention] 【0017】 According to the present invention, without introducing a special and complex device, it becomes possible to measure with high precision the fracture limit of the end face of a metal plate that simulates a collision deformation. 【Brief Description of the Drawings】 【0018】 [Figure 1] It is a schematic perspective view showing the configuration of an in-plane bending test device according to an embodiment of the present invention, where (a) shows the overall configuration, (b) is a view for explaining a bearing device, and (c) is a view for explaining a buckling prevention jig. [Figure 2] It is a schematic front view showing an example of the dimensions of a punch and a test piece of the in-plane bending test device according to the above embodiment. [Figure 3] It is a schematic front view for explaining the in-plane bending test method according to the above embodiment. [Figure 4] It is the result of an in-plane bending test performed using the in-plane bending test device according to the above embodiment, where (a) is a front view showing the state where the end face of the test piece is broken, and (b) is a graph showing the relationship between the stroke of the punch and the load. [Figure 5] (a) is a schematic diagram of an image showing an example of a collision fracture phenomenon of an actual part, and (b) is a schematic perspective view showing the state where the end face of the test piece is broken by the in-plane bending test method according to an embodiment of the present invention. [Figure 6] It explains the state of implementation of the in-plane bending test according to the above embodiment, where (a1) to (c1) are observation image diagrams of the end face during the test, and (a2) to (c2) are graphs showing the relationship between the corresponding punch stroke and load. [Figure 7] (a) is a front image diagram showing an example of a test piece whose end face is broken by the in-plane bending test according to the above embodiment, and (b) is an enlarged image diagram of the vicinity of the broken part. [Figure 8] (a) and (b) are graphs showing the relationship between the punch stroke and the load obtained as a result of the in-plane bending test for test pieces with different end face treatments. [Figure 9](a) to (c) are graphs showing the relationship between punch stroke and load obtained from the in-plane bending test according to the above embodiment for test pieces of different materials. [Figure 10] This is a schematic perspective view showing the configuration of an in-plane bending test apparatus according to another example of the above embodiment. [Modes for carrying out the invention] 【0019】 The embodiments of the present invention will be described in detail below. Note that the drawings are schematic and may differ from actual examples. Furthermore, the following embodiments are illustrative examples of methods for realizing the technical concept of the present invention and do not limit the configuration to those described below. In other words, the technical concept of the present invention can be modified in various ways within the technical scope described in the claims. 【0020】 Figure 1 is a schematic perspective view of an in-plane bending test apparatus according to an embodiment of the present invention. Figure 2 is a schematic front view illustrating the shape of the test piece and bending punch according to this embodiment. Figure 1(a) shows the overall configuration of the in-plane bending test apparatus 100. The test piece 1 is a metal plate having a processed end face 11. The test piece 1 has holes 12 at longitudinally symmetrical positions on either side of the longitudinal center. The end faces opposite each other in the width direction of the processed end face 11 have notched surfaces 13 that the bending punch 3 contacts. The in-plane bending test apparatus 100 comprises a pair of shafts 2 that pass through and support the holes 12 of the test piece 1, a bearing device 20 having two sets of bearings 21 that rotatably support both ends of the shafts 2 and restrain movement in the interaxial direction, and a bending punch 3. With the pair of shafts fixed to the bearing device 20, the in-plane bending test apparatus 100 is moved in the direction of the arrow indicated by the symbol FD, for example, using a tensile testing machine, as shown in Figure 3. In this process, the notched surface of the test piece is pressed in by the bending punch, causing in-plane bending deformation at the longitudinal center of the test piece. Then, tensile stress is applied to the processed end face 11 of the test piece, causing elongation flange deformation. 【0021】 In this embodiment, the system is equipped with means for recording the bending load and axial displacement, i.e., the stroke of the bending punch, during the process of elongation flange deformation of the processed end face 11. An in-plane bending test is then performed until the processed end face 11 breaks, and the breaking limit is evaluated using a load-stroke curve. 【0022】 In this embodiment, the test piece 1 is a flat metal plate. In the example shown in the figure, the processed end face is an arc shape with radius R1, with one end open. A straight processed end face may also be tested, but an arc shape is preferable from the viewpoint of stress concentration. In actual parts, fracture often occurs from the arc-shaped processed end face, as shown in Figure 5(a). Figure 5(a) is a schematic diagram of an image showing an example of the collision fracture phenomenon of an automobile frame part 6. The bottom of the part notch 6A is arc-shaped, and in-plane deformation 7 occurs due to the collision, and a crack propagation direction 8 toward the part hole 6B is observed as the part fractures from a part of the arc. Examples of processing methods for the end face include punching, machine cutting, and laser cutting. As shown in Figure 5(b), the in-plane deformation 7 according to this embodiment causes in-plane tensile deformation in the processed end face 11, and stress can be concentrated at the edge of the end in the thickness direction. Then, after fracture of the end, a crack propagation direction 8 similar to that of an actual part can be observed. 【0023】 The notched surface 13 of the test piece 1 pressed by the bending punch 3 has a radius of curvature R of the punch head. P It is preferable to have an arc shape with a larger radius R2. By making the notch surface 13 arc-shaped, it can smoothly contact the punch head, disperse stress concentration, and prevent the test piece 1 from compressing and buckling during in-plane bending deformation. Radius of curvature R of the punch head P The ratio of the radius R2 of the arc of the notched surface to R2 is R2 / R P It is preferable that the value is greater than 1 and less than 4. R2 / R P When R2 / R is 4 or greater, there is a possibility that the punch head will contact the notched surface at a single point on its top. P If the ratio is 1 or less, the top of the punch head may not touch the notched surface, but rather the two points on either side of the punch head. Ultimately, R2 / R P If it is 4 or more, or R2 / R PIn any case where the value is 1 or less, it may not be possible to make smooth contact between the punch head and the notched surface, making it impossible to apply ideal in-plane bending deformation. 【0024】 If out-of-plane deformation occurs in test specimen 1, the accuracy of the in-plane bending test will decrease. Therefore, it is preferable to first control the difference between the diameter d of the hole 12 in test specimen 1 and the diameter D of the shaft 2. Specifically, it is preferable to control dD to be greater than 0 and 0.1 mm or less. By doing so, test specimen 1 can be smoothly inserted into the shaft 2, making it easier to perform an in-plane bending test without any rattle. 【0025】 Next, as shown in Figure 1(b), it is preferable to construct the bearing device with a highly rigid frame. This prevents the shaft from twisting or the distance between the shaft centers from changing due to the load. 【0026】 Furthermore, as shown in Figure 1(c), especially when the plate thickness is thin, it is preferable to provide a buckling prevention mechanism that suppresses out-of-plane deformation by being positioned to sandwich the flat portion of the test piece 1 from both sides. As an example, the buckling prevention jig 4 shown in Figure 1(c) can be used. Alternatively, a slit can be provided in the punch head, the central portion of the test piece 1 can be inserted into the slit, and the flat portion of the test piece 1 can be sandwiched from both sides. 【0027】 In the above embodiment, a shaft 2 that penetrates the test specimen was used as a pair of support parts that constrain the translational movement of the test specimen and fix the test specimen so that it can rotate around a pair of support axes that are perpendicular to the flat surface of the test specimen and parallel to each other. The support parts are not limited to shafts that penetrate the specimen, but can be of any form as long as they constrain the translational movement of the test specimen and are rotatable around the support axes. For example, as shown in Figure 10, two sets of fixing devices 22, in which the shaft 2 and the rotation axis are common and rotatable, may be used to clamp and fix the test specimen 1. If the contact surface of the fixing device 22 with the test specimen 1 is made large, it will also function as a buckling prevention mechanism. 【0028】 Figure 4(a) is a front view showing the results of an in-plane bending test performed using the in-plane bending test apparatus according to this embodiment, where the end face of the test piece has fractured. Figure 4(b) is a graph showing the relationship between the punch stroke and the load in the in-plane bending test. Here, the punch stroke refers to the relative distance traveled between the punch 3 and the shaft 2 as the pair of shafts 2 move. As the punch stroke increases, that is, as the punch 3 presses into the notched surface 13 of the test piece 1, the load increases uniformly, and when the processed end face 11 fractures, the load decreases sharply. 【0029】 The in-plane bending test apparatus according to this embodiment preferably includes a camera that images the processed end face and the test surface near the fracture of the test piece 1. Figures 6(a1) to (c1) are images of the processed end face 11 captured by the camera during the in-plane bending test of the test piece. Figures (a2) to (c2) are graphs showing the position of each image on the load-stroke curve. Figures 6(a1) and (a2) show the initial state of the test piece during the in-plane bending test. Although the plate thickness has decreased due to tensile stress caused by surface deformation at the processed end face, there are no cracks at the edges at both ends in the thickness direction. 【0030】 Figures 6(b1) and (b2) show how a crack D1 originates and propagates from one edge of the machined end face of the test specimen. In this case, the load-stroke curve increases uniformly with increasing stroke. 【0031】 Figures 6(c1) and (c2) show the state after a crack has penetrated the machined end face of the test specimen from one edge to the other in the thickness direction, resulting in fracture DP. At this time, the load-stroke curve shows a maximum load at the time of fracture, after which the load decreases sharply. Here, "fracture" refers to the point when a crack that originates at one edge of the machined end face 11 penetrates in the thickness direction and reaches the other edge. 【0032】 Figure 7(a) is a front view showing an example of a test specimen whose end face fractured in an in-plane bending test according to this embodiment. Figure 7(b) is a magnified photograph of the vicinity of the fracture in the same image. An arbitrary grid or strain analysis pattern can be displayed on one or both test surfaces of the test specimen, and an in-plane bending test can be performed while imaging with the camera 5 of the in-plane bending testing apparatus. It is preferable to further include a calculation means for analyzing the obtained images and calculating the strain distribution. 【0033】 The imaging means for the test specimen is preferably configured to acquire high-resolution images using, for example, a high-speed camera. Examples of arbitrary grids include grid lines, grid points, a Kagome grid, and scribed circles. In the example in Figure 7, a dot pattern is used. 【0034】 The calculation means for calculating the strain distribution can be a strain measuring device. The strain measuring device may include an image analysis unit, a strain measuring unit, and a strain data storage unit. The strain measuring device may also include a control unit, a storage unit, an operation unit, a display unit, a communication unit, etc., and each unit may be connected by a bus. 【0035】 The control unit is a computer composed of a central processing unit (CPU), random access memory (RAM), etc. The CPU of the control unit, in response to operations on the operation unit, reads system programs and various processing programs stored in the memory unit, for example, in the memory area that stores programs within the memory unit, and expands them into the RAM's working area. It then executes various processes, described later, according to the expanded programs, thereby realizing the various functions of the strain measuring device. In addition, the CPU receives signals and data from other components via the bus, sends control signals and commands, and receives images from the imaging means and transmits and receives strain data via the communication unit. The communication unit is configured to communicate with other devices via wired or wireless connections. 【0036】 The storage unit consists of non-volatile semiconductor memory such as an SSD (Solid State Drive) or a hard disk drive (HDD). The storage unit may also include removable flash memory. The storage unit stores various programs, including programs for executing various processes in the control unit, parameters necessary for executing the processes by the programs, or data such as processing results. The various programs stored in the storage unit are stored in the form of program code that can be read by a computer, and the control unit sequentially executes operations according to the program code. 【0037】 The display device can be configured as the display unit of a strain measuring device, and examples include monitors such as liquid crystal displays (LCDs), CRTs, and organic light-emitting diodes (LEDs). 【0038】 The communication unit includes a LAN (Local Area Network) adapter, modem, wireless communication device, etc., and controls transmission and reception with each device connected to the communication network. The communication unit may also include a communication interface such as a network card. The communication unit is capable of sending and receiving various types of data with external devices; for example, image data captured by the imaging means 5 is input to the strain measuring device via this communication unit. The operation unit includes a keyboard with cursor keys, numeric input keys, and various function keys, a mouse, a touch panel, or other pointing device, and outputs instruction signals input by key operations, mouse operations, etc., to the control unit. 【0039】 The in-plane bending test apparatus of this embodiment can be used to evaluate the fracture limit characteristics of metal plates with different material properties and test pieces with different processing methods for the processed end faces by in-plane bending tests. As fracture limit characteristics, for example, the maximum load at fracture and the fracture stroke can be compared using the in-plane bending test of test pieces of the same shape and thickness according to this embodiment. It is also possible to compare the limit strain at fracture. From the evaluation results of the fracture limit by in-plane bending tests, it is possible to optimize the processing method of the end face or select material properties that have excellent impact characteristics. [Examples] 【0040】 Based on this embodiment, an in-plane bending test was performed on a 980MPa-class high-strength steel plate with a thickness of t=1.4mm. The dimensions of the test specimen, as shown in Figure 2, were: specimen length W0=60mm, specimen width A=22.5mm. The hole diameter d=10mm, shaft diameter D=9.9mm, and distance between shaft centers W1=33mm. The arc radius R1=12.5mm of the machined end face, the arc radius R2=12.5mm of the notched face, and the radius of curvature R of the punch head. P The minimum distance H between the machined end face and the notched surface was set to 4 mm. 【0041】 Figure 8 shows the load-stroke curves from in-plane bending tests of specimens with different end-face processing methods. For the same material, Steel A, Figure 8(a) shows the load-stroke curve of a specimen with an end-face processed by mechanical cutting, and Figure 8(b) shows the load-stroke curve of a specimen with an end-face processed by punching. From the results in Figure 8, since the material is the same, the initial load-stroke curves overlap. Machining requires a longer punch stroke until the end-face breaks, and the critical breaking load is also higher than that of punching. 【0042】 Figure 9 shows the load-stroke curves of in-plane bending tests conducted on 980 MPa-class high-strength steel plates: Steel A to C, each with different material properties, prepared as test specimens as described above. Figures 9(a) to 9(c) correspond to Steel A to C, respectively. The punch stroke until the processed end face fractures increases in the order of Steel A to C, and the critical fracture load also increases. 【0043】 As described above, using the in-plane bending test apparatus and test method of this embodiment, the forming limit of the edge face of a metal sheet can be measured with high accuracy without introducing special and complex equipment. Furthermore, fracture limit characteristics can be evaluated using test pieces with different edge processing methods and material properties. [Explanation of symbols] 【0044】 100 In-plane bending test apparatus 1. Test specimen (metal plate) 10 Test surface 11 Processed end face 12 holes 13 Notched surface 2 axes 20 Bearing device 21 Bearings 22 Fixtures 3 (bending) punch 4. Buckling prevention jig 5. Camera (imaging means) 6. Automotive frame parts 6A Component notch 6B Parts hole 7. In-plane deformation 8. Crack propagation direction FD (direction of movement of the test specimen) D1 Crack DP fracture

Claims

[Claim 1] A pair of support parts are positioned symmetrically on either side of the longitudinal center of a flat test specimen, constraining the translational movement of the specimen and fixing the specimen so that it can rotate around a pair of support axes that are perpendicular to the flat surface of the specimen and parallel to each other. A punch that presses the longitudinal center of the test specimen and one end face in the width direction in a direction perpendicular to the plane on which the pair of support shafts are located, thereby applying in-plane bending deformation to the longitudinal center of the test specimen and tensile stress to the other end face in the width direction, Means for recording the stroke and load of the punch, An in-plane bending test apparatus equipped with the following features. [Claim 2] The in-plane bending test apparatus according to claim 1, comprising a buckling prevention mechanism for preventing out-of-plane deformation of the test specimen. [Claim 3] A first imaging means for photographing the surface of a test specimen on which an arbitrary grid or strain analysis pattern is displayed on one or both sides, A second imaging means for photographing the longitudinal center of the test specimen and the other end face in the width direction, A storage means for storing images captured by the first imaging means and the second imaging means, A calculation means for analyzing the aforementioned image and calculating the strain distribution, An in-plane bending test apparatus according to claim 1 or 2, comprising the following: [Claim 4] Using the in-plane bending test apparatus described in claim 1 or 2, The punch presses against the longitudinal center and one end face in the width direction of the test piece, which is fixed by the pair of support parts so as to be rotatable around the pair of support shafts, Tensile and bending deformations are applied to the other end face of the aforementioned test piece so that it is widened. An in-plane bending test method for calculating the punch stroke and load when a crack penetrates the end face of the test piece in the thickness direction, based on the obtained relationship between the punch stroke and load. [Claim 5] Using the in-plane bending test apparatus described in claim 3, A test specimen, on which an arbitrary grid or strain analysis pattern is displayed on one or both sides, is fixed by the pair of support parts so as to be rotatable around the pair of support axes, The center of the longitudinal direction and one end face in the width direction of the test piece are pressed with the punch. Tensile and bending deformations are applied to the other end face in the width direction of the test specimen so that it is widened. Based on the aforementioned grid or strain analysis pattern, An in-plane bending test method for calculating the strain value when a crack penetrates the end face of the aforementioned test specimen in the thickness direction. [Claim 6] Using the in-plane bending test apparatus described in claim 1 or 2, A method for evaluating the fracture limit of a plate, comprising performing an in-plane bending test on test specimens with different material properties or processing methods at the longitudinal center of the test specimen, according to the in-plane bending test method described in claim 4, and evaluating the fracture limit characteristics of the test specimen from the punch stroke and load when a crack penetrates the end face at the longitudinal center of the test specimen. [Claim 7] Using the in-plane bending test apparatus described in claim 3, A method for evaluating the fracture limit of a plate, comprising performing an in-plane bending test on test specimens with different material properties or processing methods at the longitudinal center of the test specimen, according to the in-plane bending test method described in claim 5, and evaluating the fracture limit characteristics of the test specimen from the strain value when a crack penetrates the end face at the longitudinal center of the test specimen.

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