Testing method for limiting strain at edge of metal sheet

Abstract

The invention discloses a method for testing the edge limit strain of a metal sheet. A long metal sheet is selected as a sample. One end of the sample is narrow, the other end is wide, and the middle is an isosceles trapezoid of transition. The two long sides of the sample are vertically bent to form a U-shaped groove. An outer bending core is used to press against the outer side of the bottom wall of the U-shaped groove and limit the outer side of the two side walls. An inner bending core is used to press against the inner side of the bottom wall of the U-shaped groove and limit the inner side of the two side walls, so that the U-shaped groove is bent around the outer bending core at a certain angle. A stretching chuck is used to clamp the short end of the side wall of the U-shaped groove, and the outer bending core and the inner bending core are clamped to fix the U-shaped groove away from the tangent point position of the stretching chuck. Then the U-shaped groove is continuously stretched by the stretching chuck. The side wall of the bent part is observed during the stretching process. When the initial crack appears on the side wall, the side wall height at the cracking position is measured. The critical strain is calculated. The method can accurately and quickly obtain the limit strain at which the material begins to crack at the edge, and can be directly used as a criterion for edge cracking in part forming simulation analysis.

Description

Technical Field

[0001] The invention belongs to the field of material mechanical property detection, and in particular relates to a method for testing the edge limit strain of a metal sheet. Background Art

[0002] There is currently no mature and reliable test method for the criterion of edge cracking of advanced high-strength steel:

[0003] For uniaxial tensile tests, it has been shown that the original gauge length and non-uniform necking deformation are not suitable for the measurement of the ultimate strain at break.

[0004] For the hole expansion test - the principle of the hole expansion test is as follows Figure 1 As shown in the figure, a cone punch is used to expand and flanging a standard diameter circular hole metal sheet specimen. When a through crack appears on the edge, the test is stopped, the hole diameter after expansion is measured, and the hole expansion and flanging performance of the material is measured by the change in the hole diameter. Since the diameter and edge quality of the circular hole of the specimen, the material thickness, the hole edge compressive stress, and the judgment time of the through crack will affect the measurement result of the hole expansion rate, and the actual flanging performance of the material often cannot reach the hole expansion performance, the hole expansion test is no problem for evaluating the transverse performance of the material, but the measurement result of the hole expansion rate is difficult to be directly used for mold design or material selection. Generally, the hole expansion rate of ordinary mild steel is above 60%, while that of high-strength steel rarely exceeds 50%. The hole expansion rate of many high-strength and ultra-high-strength steels is between 15-30%. Since the limit hole expansion rate of high-strength steel is small, the test process is greatly disturbed by the test conditions, material performance fluctuations and human factors, and the fluctuation of the test data is also large, which can reach more than 5%. Therefore, it is not appropriate to use the hole expansion rate as the limit strain of edge cracking of high-strength steel. In addition, the actual structure of the parts changes greatly after flanging deformation, which is inconsistent with the hole expansion flange structure of the material, so it is difficult to directly use the hole expansion rate in the design of parts and the selection of materials. Summary of the invention

[0005] The purpose of the present invention is to provide a method for testing the limit strain of the edge of a metal sheet. The method can accurately and quickly obtain the limit strain at which the material begins to crack at the edge, and can be directly used as a criterion for edge cracking in part forming simulation analysis, which is more practical.

[0006] The technical solution adopted by the present invention is:

[0007] A method for testing the edge limit strain of a metal sheet comprises the following steps: selecting a long metal sheet as a sample, wherein one end of the sample is narrow, the other end is wide, and the middle is an isosceles trapezoid; the two long sides of the sample are vertically bent to form a U-shaped groove, and the bending lines of the two sides are parallel and symmetrical about a center line; then an outer bending core is used to abut against the outer side of the bottom wall of the U-shaped groove and limit the outer side of the two side walls of the U-shaped groove, and an inner bending core is used to abut against the inner side of the bottom wall of the U-shaped groove and limit the inner side of the two side walls of the U-shaped groove; the U-shaped groove is bent by the inner bending core so that the U-shaped groove is bent around the outer bending core at a certain angle; then a stretching chuck is used to clamp the short end of the side wall of the U-shaped groove, and the outer bending core and the inner bending core are clamped to fix the U-shaped groove away from the tangent point of the stretching chuck; then the U-shaped groove is continuously stretched by the stretching chuck; the side wall of the bent part is observed during the stretching process; when the initial crack appears on the side wall, the side wall is stopped, and the height of the side wall at the cracking position is measured; the critical strain, i.e., the limit strain at which the edge cracking of the material begins to occur, is calculated by the side wall height at the cracking position, the sample thickness, and the radius of the outer bending core.

[0008] Furthermore, the size of the specimen is set according to the expected edge cracking limit strain range of the material to ensure that the initial crack can be obtained.

[0009] Furthermore, when the two long sides of the sample are vertically bent to form a U-shaped groove, the bending radius r = (0.1-0.5) t, the width of the bottom wall of the U-shaped groove b = (30-70) t, and t is the thickness of the sample.

[0010] Furthermore, both the outer bending core and the inner bending core can rotate freely.

[0011] Furthermore, the widths of the outer bending core and the inner bending core are fine-tuned by gaskets to ensure the limitation of the U-shaped groove.

[0012] Furthermore, when the U-shaped groove is bent in the form of an outer bending core cooperating with two inner bending cores, the outer bending core and an inner bending core are clamped to fix the U-shaped groove, and when the other inner bending core bends the U-shaped groove around the outer bending core to stretch the U-shaped groove, the inner bending core at the tangent point away from the stretching clamp is retained, and the inner bending core at the tangent point close to the stretching clamp is removed.

[0013] Furthermore, the critical strain The calculation formula is

[0014]

[0015] Among them, h is the side wall height at the cracking position, t is the specimen thickness, and R is the outer bending core radius.

[0016] Furthermore, the test was performed multiple times, and the average value of the critical strain was taken as the final value.

[0017] The beneficial effects of the present invention are:

[0018] This method can accurately and quickly obtain the limiting strain at which edge cracking of the material begins to occur. This parameter can be directly used as a criterion for edge cracking in part forming simulation analysis. Compared with the limiting hole expansion rate obtained by methods such as hole expansion test, it has more practical use value in judging edge cracking of the material; the limit design of the outer bending core and the inner bending core can prevent the opening and collapse of the U-shaped groove during the tensile bending process, ensuring the accuracy of the test; the isosceles trapezoid in the middle of the specimen can allow the curved part of the U-shaped groove to form a gradually increasing side wall, which can be measured after the initial crack appears, which is convenient and quick. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 It is the principle diagram of hole expansion test.

[0020] Figure 2 3 is a top view of the test piece in the embodiment of the present invention.

[0021] Figure 3 It is a cross-sectional view of one end of the short side wall of the U-shaped groove in an embodiment of the present invention.

[0022] Figure 4 It is a cross-sectional view of one end of the U-shaped groove with a high side wall in an embodiment of the present invention.

[0023] Figure 5 It is a schematic diagram before the U-shaped groove starts to be bent in an embodiment of the present invention.

[0024] Figure 6 It is a schematic diagram before starting to stretch the U-shaped groove in an embodiment of the present invention.

[0025] Figure 7 It is a schematic diagram of stretching the U-shaped groove in an embodiment of the present invention.

[0026] Figure 8 Schematic diagram of the initial crack in the embodiment of the present invention.

[0027] In the figure: 1-specimen; 2-bending line; 3-U-shaped groove; 4-external bending core; 5-inner bending core; 6-tensile chuck; 7-initial crack. DETAILED DESCRIPTION

[0028] The present invention will be further described below in conjunction with the accompanying drawings and embodiments.

[0029] A method for testing the edge limit strain of a metal sheet comprises the following steps:

[0030] 1. Select a long metal sheet as the sample 1. The sample 1 is narrow at one end, wide at the other end, and has a transitional isosceles trapezoid in the middle. The size of the sample 1 is set according to the expected edge cracking limit strain range of the material to ensure that the initial crack 7 can be obtained. In this embodiment, the material is 1 mm thick, the expected edge cracking limit strain is in the range of 15% to 30%, and the size is as follows: Figure 2As shown; the two long sides of the sample 1 are bent vertically to form a U-shaped groove 3, and the bending lines 2 on both sides are parallel and symmetrical about the center line. In this embodiment, when the two long sides of the sample 1 are bent vertically to form the U-shaped groove 3, the bending radius r=(0.1-0.5)t, the width of the bottom wall of the U-shaped groove b=(30-70)t, t is the thickness of the sample 1, and the size of the U-shaped groove 3 is as follows Figure 3 and Figure 4 shown.

[0031] 2. If Figure 5 and Figure 6 As shown, the outer bending core 4 is pressed against the outer side of the bottom wall of the U-shaped groove 3 and limits the outer side of the two side walls of the U-shaped groove 3, and the inner bending core 5 is pressed against the inner side of the bottom wall of the U-shaped groove 3 and limits the inner side of the two side walls of the U-shaped groove 3. The U-shaped groove 3 is bent by the inner bending core 5, so that the U-shaped groove 3 is bent around the outer bending core 4 at a certain angle; in this embodiment, the outer bending core 4 and the inner bending core 5 can rotate freely, effectively reducing the resistance during bending and subsequent stretching; in this embodiment, the width of the outer bending core 4 and the inner bending core 5 are fine-tuned by gaskets to ensure the limitation of the U-shaped groove 3; in this embodiment, the diameter of the outer bending core 4 is 100mm, the width excluding the flanges on both sides is 72mm, the diameter of the inner bending core 5 is 35mm, and the width is 70mm.

[0032] 3. Such as Figures 6 to 8 As shown, a stretching chuck 6 is used to clamp the shorter end of the side wall of the U-shaped groove 3, and the outer bending core 4 and the inner bending core 5 are clamped to fix the U-shaped groove 3 away from the tangent point of the stretching chuck 6, and then the U-shaped groove 3 is continuously stretched by the stretching chuck 6. During the stretching process, the side wall of the bent part is observed, and the stretching is stopped when an initial crack 7 appears on the side wall, and the height of the side wall at the crack position is measured; in this embodiment, when the U-shaped groove is bent in the form of an outer bending core 4 and two inner bending cores 5, the outer bending core 4 and an inner bending core 5 are clamped to fix the U-shaped groove 3, and when the other inner bending core 5 bends the U-shaped groove 3 around the outer bending core 4 to stretch the U-shaped groove 3, the inner bending core 5 at the tangent point away from the stretching chuck 6 is retained, and the inner bending core 5 at the tangent point close to the stretching chuck 6 is removed.

[0033] 4. Calculate the critical strain by the side wall height at the cracking position, the thickness of the specimen 1 and the radius of the outer bending core 4, that is, the limit strain at which the material begins to crack at the edge. The calculation formula is

[0034]

[0035] Wherein, h is the side wall height at the cracking position, t is the thickness of the specimen 1, and R is the radius of the outer bending core 4.

[0036] 5. Test multiple times and take the average critical strain value as the final value; the results of the test of 5 samples 1 in this embodiment are as follows

[0037]

[0038] According to the test results in the above table, the ultimate strain of edge cracking of the material is 0.136 and the engineering ultimate strain is 14.6%.

[0039] This method can accurately and quickly obtain the limiting strain at which edge cracking of the material begins to occur. This parameter can be directly used as a criterion for edge cracking in part forming simulation analysis. Compared with the limiting hole expansion rate obtained by methods such as hole expansion test, it has more practical use value in judging edge cracking of the material; the limit design of the outer bending core 4 and the inner bending core 5 can prevent the opening and collapse of the U-shaped groove 3 during the tensile bending process, ensuring the accuracy of the test; the isosceles trapezoid in the middle of the sample 1 can allow the curved part of the U-shaped groove 3 to form a gradually increasing side wall, which can be measured after the initial crack 7 appears, which is convenient and quick.

[0040] It should be understood that those skilled in the art can make improvements or changes based on the above description, and all these improvements and changes should fall within the scope of protection of the appended claims of the present invention.

Claims

1. A method for testing the limit strain of a metal sheet edge, characterized in that: A long metal sheet is selected as a specimen, one end of the specimen is narrow, the other end is wide, and the middle is an isosceles trapezoid. The two long sides of the specimen are vertically bent to form a U-shaped groove, and the bending lines on both sides are parallel and symmetrical about the center line; then an outer bending core is used to press against the outer side of the bottom wall of the U-shaped groove and limit the outer side of the two side walls of the U-shaped groove, and an inner bending core is used to press against the inner side of the bottom wall of the U-shaped groove and limit the inner side of the two side walls of the U-shaped groove, and the U-shaped groove is bent by the inner bending core to bend around the outer bending core at a certain angle; then a stretching chuck is used to clamp the short end of the side wall of the U-shaped groove, and the outer bending core and the inner bending core are clamped to fix the U-shaped groove away from the tangent point of the stretching chuck, and then the U-shaped groove is continuously stretched by the stretching chuck, and the side wall of the bent part is observed during the stretching process. When the initial crack appears on the side wall, the side wall is stopped, and the height of the side wall at the cracking position is measured; the critical strain, that is, the limit strain at which the material begins to crack at the edge, is calculated by the side wall height at the cracking position, the specimen thickness and the radius of the outer bending core; Critical strain The calculation formula is Among them, h is the side wall height at the cracking position, t is the specimen thickness, and R is the outer bending core radius.

2. The metal sheet edge limit strain testing method according to claim 1, characterized in that: The size of the specimen is set according to the expected edge cracking limit strain range of the material to ensure that the initial crack can be obtained.

3. The metal sheet edge limit strain testing method according to claim 1, characterized in that: When the two long sides of the sample are bent vertically to form a U-shaped groove, the bending radius r = (0.1-0.5) t, the width of the bottom wall of the U-shaped groove b = (30-70) t, and t is the thickness of the sample.

4. The metal sheet edge limit strain testing method according to claim 1, characterized in that: Both the outer and inner bending cores can rotate freely.

5. The metal sheet edge limit strain testing method according to claim 1, characterized in that: The widths of the outer bending core and the inner bending core are fine-tuned by gaskets to ensure the limitation of the U-shaped groove.

6. The metal sheet edge limit strain testing method according to claim 1, characterized in that: When the U-shaped groove is bent by using an outer bending core and two inner bending cores, the outer bending core and an inner bending core are clamped to fix the U-shaped groove, and the other inner bending core bends the U-shaped groove around the outer bending core to stretch the U-shaped groove, retaining the inner bending core at the tangent point away from the stretching chuck, and removing the inner bending core at the tangent point close to the stretching chuck.

7. The metal sheet edge limit strain testing method according to claim 1, characterized in that: The test was performed multiple times, and the average critical strain value was taken as the final value.

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