GB/T 15825.8-1995 Formability and test methods of sheet metal Forming limit diagram (FLD) test
Some standard content:
National Standard of the People's Republic of China
Sheet metal formability and test methods-Forming limit diagram (FLD) test
Sheet metal formability and test methods-Forming limit diagram (FLD) test Subject content and scope of application
GB/T15825.8-1995
This standard specifies the laboratory determination method of the forming limit diagram (FLD) of metal sheet. This standard is applicable to metal sheets with a thickness of 0.2~3.0mm. 2 Reference standards
GB/T15825.2 General test procedures for the formability and test methods of metal sheets 3 Symbols, names and units
The symbols, names and units used in this standard are shown in Table 1. Table 1
E1, E2
Forming limit diagram
Forming limit curvebzxZ.net
Inner diameter of return die
Radius of punch ball head
Punch diameter
Radius of return die corner
Punch force
Blank holding force
Surface engineering (limit) principal strain
Surface true (limit) principal strain
Initial diameter of grid circle
Major axis size of distorted grid circle
Minor axis size of distorted grid circle
Strain hardening exponent
Plastic strain ratio
Test principle
4.1 When determining the forming limit diagram under laboratory conditions, the method of bulging the specimen with a rigid punch is usually used, which can be supplemented by tensile test and hydraulic bulging test when necessary.
4.2 During the rigid punch bulging test, a sample with a grid circle on one side is placed between the die and the blank holder. The blank holder force is used to press and pull the sample material outside the deep ribs. The middle part of the sample is bulged and deformed under the punch force to form a convex hull (see Figure 1). The grid circle on its surface is distorted. When a certain part of the convex hull is necked or ruptured, the test is stopped and the major and minor axis dimensions of the grid circle near the necking area (or near the necking area) or the rupture area are measured. The local surface limit principal strain (e1, ez) or (e1, e2) allowed for the metal sheet is calculated. Note: Surface strain refers to the two-dimensional strain parallel to the plane of the sheet. In this standard, (ele2) represents the surface engineering limit principal strain, and (ej, e2) represents the surface true limit principal strain.
Neck shrinkage or rupture
Upper barrier
Draw rib
Figure 1 Rigid punch bulging test
4.3 The following two methods can be used to obtain the surface limit principal strain under different strain paths. 4.3.1 Changing the lubrication conditions between the specimen and the punch contact surface is mainly used to determine the right half of the forming limit diagram (double tension deformation zone, that is, e,>0, e2≥0 or ε>0, 2≥0). If a rubber (or rubber) pad of appropriate thickness is added between the specimen and the punch, the surface limit strain under the state close to equal double tension strain (e=e2 or s1=ε2) can be obtained more conveniently. Generally, the more different lubrication conditions are selected, the more reliable the forming limit diagram determined by the test. 4.3.2 The use of specimens of different widths
is mainly used to determine the left half of the forming limit diagram (tension-compression deformation zone, i.e., e>0, e2<0 or e>0, 2≤0). If the specimen width is selected appropriately, the surface limit strain close to the uniaxial tensile strain state (ei=-2e2 or =-2e2) and plane strain state (e2=0 or ε2=0) can be obtained. Generally, the more width specifications of the specimen, the more reliable the forming limit diagram determined by the test. Note: When the length and width of the specimen are close, the limit strain may also be located in the right half of the forming limit diagram, the double tension deformation zone. 5. Specimens
5.1 Determine the size, shape and number of specimens according to the characteristics of the test device and the test principle. If the punch size recommended in 7.1 of this standard is used, it is recommended to use a square specimen with a side length of 180 mm (or a regular polygon with an inscribed circle diameter of 180 mm, or a circle with a diameter of 180 mm) and a rectangular specimen with widths of 160, 140, 120, 100, 80, 60, 40 and 20 mm (the length can be determined by the test device). 5.2 Prepare the specimen according to the provisions of Chapter 3 of GB/T15825.2 and record the measured thickness of the specimen. 5.3 In order to prevent the narrow rectangular specimen from cracking at the drawing rib, it is allowed to change its shape to a stepped shape with a slightly narrower middle and slightly wider ends, imitating the sheet metal tensile test specimen.
6 Preparation of grid circles
6.1' In order to determine the surface strain of the specimen, a certain number of grid circles should be prepared on the surface of one side of the specimen. The number and arrangement pattern of the grid circles can be designed by yourself (some necessary symbols can be added). The pattern shown in Figure 2 is for reference. 44
GB/T15825.8-1995
Figure 2 Grid circle pattern
6.2 The grid circle on the surface of the sample can be made by photolithography, photolithography, electrochemical corrosion or other methods. 6.3 The size of the initial diameter d of the grid circle affects the measurement and calculation results of the test. The selection principle is: when using a large-sized mold, the value of d can be larger, and when using a small-sized mold, it can be smaller. 6.4 If the punch size recommended in Article 7.1 of this standard is used, it is recommended to use a grid circle with d = 1.5 to 2.5 mm. 6.5 The deviation of the diameter of the grid circle shall not exceed 2% of its value. 7 Mold
7.1 There is no specific provision for the geometric dimensions of the test mold (including the location, shape and size of the drawing ribs, etc.). It is only recommended to use a cylindrical ball head punch with a diameter of 100 mm.
7.2 Prepare the mold according to the provisions of Article 4.1 of GB/T15825.2. 8 Test conditions
8.1 Lubrication
8.1.1 When using specimens of different widths, refer to the provisions of Chapter 6 of GB/T15825.2. It is recommended to use lubricant 1 to lubricate the surface of the specimen without the grid circle pattern. 8.1.2 When the lubrication conditions between the contact surface of the specimen and the punch are changed for testing, the lubricant or the combination of lubricants can be selected by yourself, but the surface limit strain of each specimen should be evenly distributed in the coordinate system as much as possible. It is recommended to use lubricant 1 or 2# specified in Chapter 6 of GB/T15825.2 as a liquid lubricant. For solid lubricants, it is recommended to use polyethylene (or polyvinyl chloride, polytetrafluoroethylene) films of different thicknesses, rubber (or rubber) pads of appropriate thickness, and other applicable lubricating materials. 8.1.3 When the test is conducted by changing the lubrication conditions between the contact surface of the specimen and the punch, refer to the provisions of Chapter 6 of GB/T15825.2, and only lubricate the surface of the specimen without the grid circle pattern. It is allowed to use lubricating oil to stick the solid lubricating film on the surface of the specimen to be lubricated. 8.2 Blank holding force
8.2.1 The blank holding force should press the specimen material other than the drawing ribs to ensure that they do not deform and flow. 8.2.2 When repeating the test on specimens of the same size or the same lubrication method, the blank holding force deviation shall not exceed 5%. 8.3 Test speed
There is no specific provision for the test speed (punch movement speed), but it is not allowed to generate large inertial movement when the test is stopped, so as to capture the moment when the specimen convex hull shrinks or breaks in a timely and accurate manner. 9 Test device and testing machine
9.1 Prepare the test device according to Article 5.1 of GB/T15825.2. If the punch size recommended in Article 7.1 of this standard is used, the following technical conditions must be met:
a. Within the working stroke, the center lines of the punch and the die should coincide with each other, and the deviation should not exceed 0.15mm. 45
CB/T15825.8—1995
The test device should be able to position the specimen, and the deviation between the center of the specimen and the center line of the punch should not exceed 0.5mm. 9.2 Prepare the testing machine according to Article 5.2 of GB/T15825.2. 10 Measurement and calculation
10.1 The grid circle used to measure and calculate the surface limit strain is called the critical grid circle. 10.2 When determining the surface limit strain of a point on the specimen, in principle, the calculation should be performed by measuring the diameter change of the critical mesh circle in the necking area. However, from the perspective of engineering application, it is also allowed to select a critical mesh circle near the necking area or the rupture area for measurement to approximately calculate the surface limit strain of a point on the specimen.
10.3 From the perspective of engineering application, it is recommended to select the critical mesh circle using the following method: a.
The mesh circle located in the necking area but not broken is taken as the critical mesh circle; the mesh circle close to the necking or crack is taken as the critical mesh circle; the mesh circle adjacent to the mesh circle through which the necking or crack runs is taken as the critical mesh circle. 10.4 When selecting the critical mesh circle, the following matters should be noted: the number of critical mesh circles should not be too many (usually three are acceptable), and they should be as adjacent or close as possible, and the corresponding a.
measurement difference between them should not be greater than 10%;
b. In order to maintain the consistency of the test results, the same critical mesh circle selection method must be used for measurement, calculation and plotting of the forming limit diagram.
10.5 The distorted shape of the mesh circle on the specimen surface is shown in Figure 3. The major axis of the distorted mesh circle is denoted as d, and the minor axis is denoted as d2, and d, and d, are approximately regarded as the two principal strain directions at a point in the specimen plane. d,>de, d,d.、d=d.
Figure 3 Grid circle distortion
dd>do.d>d,
10.6 When measuring the major and minor axes d and d2 of the critical grid circle, a reading microscope, a measuring microscope, a projector or a specially designed measuring tool, a detection device, etc. [such as an engineering strain scale, see Appendix A (reference)] may be used. 10.7 Based on the measurement results, calculate the surface limit strain of the specimen according to formulas (1) and (2). e,
d=de × 100%
d二de × 100%
（1）
11 Test procedure and operation method
GB/T15825.8-1995
e = Ind
= In(1 +e)
11.1 Prepare the specimen in accordance with the provisions of Chapter 5 and Chapter 6 of this standard. ln(1+e2）
11.2 Clean, inspect and lubricate the mold, test device and test machine according to the provisions of 4.2, 5.1.2, 5.2.2 and 5.2.3 of GB/T15825.2.
11.3 Carry out preliminary test.
11.4 When placing the sample before the formal test, the side of the sample with the grid circle should be placed against the die. During the test, the sample should be pressed tightly until local necking or rupture occurs on the sample. 11.5 For samples of the same size and the same lubrication method, more than 3 effective repeated tests are carried out. 11.6 The test is invalid if any of the following situations occurs: a.
Necking or rupture of the sample occurs near the die orifice; when using samples of different widths, the side of the sample is torn; the sample ruptures near the drawing rib;
Unable to select a suitable critical grid circle.
11.7 Measure the major and minor axis dimensions of the critical mesh circle and calculate the surface limit strain. 12 Plot the forming limit diagram
12.1 Use the surface strain e(or) as the abscissa and the surface strain e;(or e,) as the ordinate to establish a surface strain coordinate system. In the e-e2 coordinate system, the division ratio of e2 and ei is usually taken as 2:1 (Figure 4b), while in the E2-E, coordinate system, the division ratio of the two is generally the same. 12.2 Plot the surface limit strain (e1, e2) or (s1, 5) measured by the test in the surface strain coordinate system (see Figure 4). e.%
Figure 4 Forming Limit Diagram (FLD) Plot
Note:,
Surface limit strain
12.3 According to the distribution characteristics of the surface limit strain in the coordinate system, connect them into an appropriate curve (Figure 4a) or form a strip-shaped area (Figure 4b), namely the forming limit curve (FLC). 47
13 Test report
The test report format is designed by yourself.
GB/T15825.8-1995
The test report should include the following main contents. a.
Specifications, grades and states of test materials; actual thickness of test specimens;
Testing methods: in accordance with GB/T15825.8;
Molds: including punch diameter, die inner diameter, drawing rib size, materials and hardness of punch, die and blank holder; size and specifications of test specimens;
Lubrication conditions between the contact surface of test specimen and punch; number of valid test specimens with the same size and specifications and the same lubrication conditions; initial diameter of mesh circle;
selection method of critical mesh circle;
measurement method;
testing machine;
measurement and calculation results of the test: including di, d2, (erve2) or (srE2); test date.
Note: The strain hardening exponent n value and plastic strain ratio of metal sheets have a certain influence on the strain value level and shape of the forming limit diagram. If necessary, these two performance indicators can be tested and included in the test report. 48
GB/T15825.8—1995
Appendix A
Engineering strain scale
(reference)
A1 When using a microscope or projector to measure the length and minor axis of the distorted grid circle, the measured value is affected by the curvature of the convex hull of the specimen. For this reason, the soft strain scale shown in Figure A1 can be used for measurement. This scale can be made of soft film. Figure A1
Engineering strain scale
Additional instructions:
This standard is proposed by the Ministry of Machinery Industry of the People's Republic of China. This standard is under the jurisdiction of the National Forging Standardization Technical Committee. This standard is drafted by Wuhan Institute of Technology. The main drafters of this standard are Cao Hongchen and Jiang Kuihua. From the date of implementation of this series of standards, the original Ministry Standard JB4409--88 "Formability and Test Methods of Thin Steel Plates" will be invalid.
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