Sheet Metal Formability Test Method
Metal sheet, as a type of sheet material, has a wide range of applications and is used in various fields. The formability test of metal sheets is an important indicator to measure their adaptability to the forming process. It is widely used in industries such as stamping, automotive, light industry, and even aerospace. Mastering this test and combining it with conventional metal material performance parameters can provide detailed and accurate data support for simulation, modeling, and forming processes.
The formability of metal sheets is not an inherent property but rather a state related to material variables, process variables, and product design variables. This state reflects the difficulty of forming the workpiece and depends on the static quality of the metal sheet, the stress-strain conditions it undergoes during dynamic stamping, and the geometric characteristics of the sheet metal component. Based on the basic stamping forming methods, it can be divided into several categories, including bulging performance, deep drawing performance, hole expansion performance, compound forming performance, and bending performance. Bending performance refers to the ability of the metal sheet to resist fracture caused by tensile stress on the outer layer during bending forming. Understanding the test method for the bending formability of metal sheets is essential as it serves as one of the acceptance criteria for commercial metal sheet supply and one of the routine inspection methods for the working accuracy of metal sheet bending machines. Below, we will specifically introduce the test method for the bending formability of metal sheets according to GB/T15825.5.
1. Test Scope
This method is applicable to determining the bending formability of metal sheets with a thickness ranging from 0.30mm to 4.00mm, using the minimum relative bending radius as the indicator.
2. Test Principle
This test uses a series of punch dies with different bottom radii of curvature (or different thicknesses of cushion dies). After the sample is formed according to the specified bending angle, the outer surface of the deformation zone is inspected to determine the minimum relative bending radius at which no cracks or significant depressions occur. This value is used as the indicator of the bending formability of the metal sheet.
3. Sample Preparation and Mold Selection
1. This test uses strip-shaped samples, with a width greater than 10 times the basic thickness of the sheet but not less than 20mm. The length should be sufficient to allow the sample to be bent into a "V" or "U" shape.
2. It is recommended to use strip-shaped samples with a width of 50mm ± 0.5mm and a length of 150mm ± 2.0mm.
3. The length direction of the sample should be perpendicular to the rolling direction (sampling angle θ = 90°). If there are special requirements, other values for the sampling angle are allowed, but they should be indicated in the test report.
4. Prepare the samples and molds according to the provisions in GB/T15825.2. It is particularly important to measure the thickness of each sample and number them in groups according to the front and back sides of the sample. Record the actual measured thickness of the samples.
5. The width of the punch die, cushion die, and die should all be greater than the width of the sample.
6. The radius of curvature rd at the mouth of the die should be 10mm. (Note: The mouth of the die can also be designed as a rotatable cylinder with a radius of rd.)
7. The bottom radius of curvature Rp of the punch die should ideally be manufactured with a tolerance range of ±0.01mm for each 0.1mm interval. To reduce the workload of the test, it is recommended to use the listed punch die specifications.
8. The thickness of the cushion die used for 180° bending should ideally be manufactured with a tolerance range of ±0.01mm for each 0.2mm interval. To reduce the workload of the test, it is recommended to double the listed data as the cushion die thickness, i.e., tp = 2Rp.
IV. Test Apparatus and Testing Machine
1. Test Apparatus
Prepare the test apparatus according to GB/T15825.2 and ensure the following technical conditions are met:
- The test apparatus should be equipped with various molds of different specifications that meet the testing requirements and allow for convenient and quick replacement of punch dies.
- Within the working stroke, the punch die and the die should be aligned, with a deviation not exceeding 0.1mm.
- The test apparatus should provide accurate positioning of the sample.
- The test apparatus should be able to adjust the opening of the die (L) and lock it after adjustment to ensure that L does not change during the test.
- The test apparatus should not tilt, shift, or vibrate during operation.
V. Test Procedure
1. The test is typically conducted in a temperature environment of 10°C to 35°C. If necessary, the temperature environment can be set to 23°C ± 5°C.
2. Before the test, clean, inspect, and lubricate the molds, test apparatus, and testing machine according to the provisions in GB/T15825.2.
3. Prior to each test, lubricate the rounded corner area of the die mouth with oil using a fully consumed oiling system.
4. Adjust the opening of the die and lock it after adjustment. When adjusting the die opening (L), calculate its value according to Equation (1) and Equation (2), rounding the calculation result to two decimal places.
5. Following the specified bending angle, select the punch die or cushion die specifications in descending order and perform bending tests on the samples one by one until cracks or significant depressions appear on the outer surface of the deformation zone under a 5x magnifying glass. The initial punch die or cushion die specifications used in the test can be determined based on experience.
6. When using the compression bending test method, if the smallest punch die or cushion die specifications still do not cause cracks or significant depressions on the outer surface of the deformation zone, the bending angle of the sample can be changed or the sample can be subjected to 180° folding bending. However, this should be indicated in the test report.
7. If the stiffness of the punch die allows, the compression bending test method can be used directly for 180° bending of the sample, with the die opening determined according to Equation (2).
8. If the compression bending test method cannot be used for 180° bending, the sample should be initially bent to a certain angle and then moved between the upper and lower pressure plates for 180° bending.
9. Repeat the test at least three times on both the front and back sides of the same material.
10. The test is considered invalid if there are no cracks on the outer surface of the deformation zone but cracks appear on the longitudinal side. To avoid this phenomenon, it may be necessary to remove burrs by grinding.
VI. Determination of Minimum Bending Radius and Calculation of Test Results
1. For compression bending tests or 180° bending tests with cushion dies, determine the minimum bending radius (Rmin) according to Equation (3).
(Note: Rpf is the bottom radius of curvature of the punch die or half the thickness of the cushion die used when cracks or significant depressions appear on the outer surface of the deformation zone.)
2. For 180° bending tests without cushion dies, determine the minimum bending radius based on the following principles:
- If cracks or significant depressions appear on the outer surface of the deformation zone under a 5x magnifying glass, the minimum bending radius (Rmin) is 0.1mm.
- If there are no cracks or significant depressions on the outer surface of the deformation zone under a 5x magnifying glass, the minimum bending radius (Rmin) is 0.
3. Calculate the minimum relative bending radius (Rmin/t), rounding the calculation result to one decimal place.
4. Calculate the average minimum relative bending radius (Rmin/t), rounding the calculation result to one decimal place.
Test Method for Bending Formability of Metal Sheets
The bending formability of metal sheets refers to their ability to resist deformation during the bending forming process. The test method for evaluating the bending formability of metal sheets is as follows:
1. Test Scope
This method is applicable to determining the bending formability of metal sheets with a thickness ranging from 0.30mm to 4.00mm, using the minimum relative bending radius as the indicator.
2. Test Principle
The test involves using a series of punch dies with different bottom radii of curvature or cushion dies of varying thicknesses. The sample is bent according to the specified bending angle, and the outer surface of the deformation zone is examined. The minimum relative bending radius at which no cracks or significant depressions occur is measured and used as the indicator of the bending formability of the metal sheet.
3. Sample Preparation and Mold Selection
- Strip-shaped samples are used in the test, with a width greater than 10 times the basic thickness of the sheet (but not less than 20mm). The length should be sufficient to allow the sample to be bent into a "V" or "U" shape.
- Recommended dimensions for the strip-shaped samples are a width of 50mm ± 0.5mm and a length of 150mm ± 2.0mm.
- The samples should be oriented perpendicular to the rolling direction (sampling angle θ = 90°). If there are specific requirements, other values for the sampling angle can be used, but they should be clearly stated in the test report.
- Prepare the samples and molds according to the specifications in GB/T15825.2. It is important to measure the thickness of each sample and label them accordingly. The width of the punch die, cushion die, and die should all be greater than the width of the sample.
- The radius of curvature at the mouth of the die should be 10mm. (Note: The mouth of the die can also be designed as a rotatable cylinder with a radius of 10mm.)
- The punch die should have a bottom radius of curvature (Rp) manufactured with a tolerance range of ±0.01mm for each 0.1mm interval. The thickness of the cushion die used for 180° bending should have a tolerance range of ±0.01mm for each 0.2mm interval. To reduce the workload, it is recommended to use the listed specifications for the punch dies and cushion dies.
4. Test Apparatus and Testing Machine
- Prepare the test apparatus according to GB/T15825.2, ensuring that it can accommodate various mold specifications required for the test and facilitate the quick and easy replacement of punch dies.
- The punch die and the die should be aligned within the working stroke, with a deviation not exceeding 0.1mm.
- The test apparatus should provide accurate positioning of the sample.
- The test apparatus should allow for the adjustment of the die opening (L), which should be securely locked in place to ensure it does not change during the test.
- The test apparatus should not tilt, shift, or vibrate during operation.
5. Test Procedure
- The test is typically conducted in a temperature environment of 10°C to 35°C. If necessary, the temperature can be set to 23°C ± 5°C.
- Prior to the test, clean, inspect, and lubricate the molds, test apparatus, and testing machine according to the specifications in GB/T15825.2.
- Lubricate the rounded corner area of the die mouth with oil using a fully consumed oiling system before each test.
- Adjust the die opening (L) and securely lock it in place. The value of L should be calculated according to the provided equations and rounded to two decimal places.
- Perform the bending test on the samples, starting with the largest punch die or cushion die specifications and gradually decreasing in size, following the specified bending angle. Continue the test until cracks or significant depressions appear on the outer surface of the deformation zone under a 5x magnifying glass. The initial punch die or cushion die specifications used in the test can be determined based on experience.
- If the smallest punch die or cushion die specifications still do not cause cracks or significant depressions on the outer surface of the deformation zone, the bending angle of the sample can be adjusted or the sample can be subjected to 180° folding bending. Any changes made should be clearly stated in the test report.
- If the punch die stiffness allows, perform a direct 180° bending of the sample using the compression bending test method, with the die opening determined according to the provided equation.
- If the compression bending test method cannot be used for 180° bending, initially bend the sample to a certain angle and then place it between the upper and lower pressure plates for 180° bending.
- Repeat the test at least three times on both the front and back sides of the same material.
- The test is considered invalid if there are no cracks on the outer surface of the deformation zone but cracks appear on the longitudinal side. To avoid this phenomenon, it may be necessary to remove burrs by grinding.
6. Determination of Minimum Bending Radius and Calculation of Test Results
- For compression bending tests or 180° bending tests with cushion dies, determine the minimum bending radius (Rmin) using the provided equation. Rpf represents the bottom radius of curvature of the punch die or half the thickness of the cushion die used when cracks or significant depressions appear on the outer surface of the deformation zone.
- For 180° bending tests without cushion dies, determine the minimum bending radius based on the provided principles. If cracks or significant depressions appear on the outer surface of the deformation zone, the minimum bending radius (Rmin) is 0.1mm. If there are no cracks or significant depressions, the minimum bending radius (Rmin) is 0.
- Calculate the minimum relative bending radius (Rmin/t) and round the result to one decimal place.
- Calculate the average minimum relative bending radius (Rmin/t) and round the result to one decimal place.
