T/CSAE 49-2016Test Method of Forming Limit Diagram of Automotive Sheets (English PDF)

Foreword

This standard draws on the characteristics of existing automotive sheet forming performance test methods at home and abroad, and has accumulated a large number of tests through special research.

On the basis of data and experience, in accordance with GB/T 1.1-2000 "Standardization Work Guidelines Part I. Standard Structure and Writing Rules" Request for development. In order to meet the test requirements, this standard has the following modifications compared with the current national standard.

1) Optimize the sample size according to the forming limit diagram characteristics;

2) According to the material thickness and strength level, a detailed test lubrication scheme is recommended;

3) Propose the sampling direction requirement of the sample and give a quick determination method;

4) According to the development of strain test technology and the need to further improve the test accuracy, a new test data acquisition and processing method is proposed.

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This technical standard is one of the technical standards for steel materials of the China Automotive Engineering Society.

This technical standard was proposed by the Materials Branch of the China Automotive Engineering Society.

This technical standard is approved by the China Automotive Engineering Society.

This technical standard is under the jurisdiction of the China Automotive Engineering Society.

This technical standard was drafted. China Automotive Engineering Research Institute Co., Ltd., Maanshan Iron and Steel Co., Ltd., Wuhan Iron and Steel (set Group) company.

1 Scope

This standard specifies the laboratory measurement method for the forming limit diagram (FLD) of automotive sheets at room temperature.

This standard is applicable to automotive sheets with a thickness of 0.3mm to 3.0mm.

2 Normative references

The terms in the following documents become the terms of this standard by reference to this standard.

GB/T 15825.2-2008 Sheet metal forming performance and test methods - Part 2. General test procedures Sheet metal sheets -- Determination of forming properties and test methods - Part 8 . Guide to forming limit diagrams ( FLD ) GB/T 24171.2-2009 Determination of sheet metal and sheet strip forming limit curves - Part 2 Determination

3 Symbols, names and units

The symbols, names and units used in this standard are shown in Table 1.

Table 1 Symbols, names and units

Symbolic name unit

FLD forming limit diagram -FLC forming limit curve -Dd die inner diameter mm Dp punch diameter mm Db pad middle hole diameter mm Rd die radius phi Rp punch ball nose radius mm E1, epsilon2 engineering main strain, secondary strain -E1, e2 true principal strain, secondary strain -D0 strain analysis graphic initial size mm D1 strain analysis after deformation, large deformation direction dimension mm D2 strain analysis after deformation, small deformation direction dimension mm

4 Test principle and method

4.1 The forming limit diagram characterizes the degree of ultimate deformation that a sheet can achieve under different near linear paths during deformation. By printing a certain map the sample of the case is bulged to obtain the in-plane strain distribution (ie, the primary strain and the secondary strain) at the limit point before the necking failure, to the lower end of the different strain paths.

The curve drawn by the in-plane strain of the limit point is the forming limit curve. Hemispherical rigid punches can be used when testing the forming limit curve the bulging test is carried out by the bulging method or the cylindrical rigid male bulging method. The forming limit curves obtained by the two bulging modes cannot be used for comparison.

of. Under laboratory conditions, it is recommended to use the hemispherical rigid male bulging method preferentially.

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a) hemispherical rigid male bulging method b) cylindrical rigid male bulging method

Figure 1 Forming limit curve bulging method

4.2 When the hemispherical rigid male bulge test is carried out, a sample with a pattern printed on one side of the plate is placed between the concave die and the bead ring, and the test is pressed with a bead ring.

kind. The specimen is bulged by the punch to form a convex hull, and the pattern on the sample is also changed, and the neck is locally deformed on the convex hull.

Or when rupturing, stop the test, determine the limit point according to the neck or the rupture site (or near these parts), and then according to the pattern printed on the sample the change condition measures and calculates the in-plane strain of the limit point, which is the ultimate strain of the limit point in the bulging state, the ultimate strain Also known as in-plane ultimate strain.

Note. The in-plane ultimate strain can be expressed by the engineering principal strain (epsilon1, epsilon2) and can also be expressed by the true principal strain (e1, e2).

Fig. 2 Internal ultimate strain under different strain paths in the forming limit curve

4.3 The complete forming limit curve generally consists of five strain paths. uniaxial tension, general tensile, plane strain, general bulging and equal bidirectional bulging.

The in-plane ultimate strain of the lower limit point is obtained by using the following method to obtain the in-plane ultimate strain at different strain paths.

a) Use different width specimens. In general, the larger the difference in the width of the specimen, the greater the difference in the in-plane ultimate strain of the obtained limit points.

Selecting more width specifications is beneficial to obtain the ultimate strain under various strain paths. If the sample width is appropriate, the approximation can also be obtained separately the ultimate strain point of a uniaxial tensile stress state or an approximate plane strain state;

b) Change the lubrication or contact conditions between the sample and the punch. Generally, the difference in the lubrication effect of different lubrication conditions is greater.

The difference in the numerical value of the ultimate strain is greater. In order to test a sufficient ultimate strain point, a certain amount of lubrication conditions should be appropriately selected. If necessary If necessary, a rubber (or rubber) thin pad of a suitable thickness may be placed between the sample and the punch to change the contact condition between the mold and the sample.

Obtaining the ultimate strain of the double tensile or approximate double tensile strain state;

c) supplemented by other tests. in-plane in the uniaxial tension, plane strain and equal bidirectional bulging strain paths for accurate determination of the forming limit diagram the ultimate strain can be supplemented by other test methods such as uniaxial stretching, hydraulic bulging and cylindrical convex bulging.

When the lubrication condition is not good, it is easy to produce a phenomenon in which the double necking and the rupture position are far from the dome. To ensure the accuracy and avoidance of test results Multi-peak phenomenon of strain distribution when using the fitting method to calculate the ultimate strain is usually obtained by testing a series of samples with different widths.

Forming limit curve.

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5 Sample

6 Strain analysis pattern

6.1 In order to obtain the ultimate strain of the bulging specimen, a strain analysis pattern shall be prepared on one side of the specimen, and the pattern may have a certain dimension Grids, speckles, or other identifiable patterns of inch and shape features.

6.2 If the grid is made on the surface of the specimen, the diameter or side length of the grid is chosen to be 1 to 2 times the thickness of the specimen. When using a diameter of 100 mm when the punch is used, the maximum grid size printed is generally no more than 2.5 mm. Grid preparation can be photographic, lithographic, electrochemically etched or in other methods, the mesh size deviation obtained is not more than 2% of the value, the edge of the mesh is clear, and the test process does not fall off.

6.3 If the surface of the sample is sprayed with speckle or other patterns, the sprayed pattern shall have good adhesion properties and shall not fall off before and after the test.

The selected spray pattern should ensure that the accuracy of the strain obtained is not less than 0.1%.

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7 Mold

7.1 When rigid bulge is used, it is recommended to use a hemispherical rigid punch with a diameter of (100 ± 2) mm under laboratory conditions.

It is recommended to use 108mm for the diameter. The fillet radius of the die is a minimum of 5mm or 2 times the thickness of the material.

7.2 Test molds of different constructions are permitted under conditions that guarantee compliance with the test principle and obtain ultimate strain. Specific mold shape and for the dimensions, refer to 4.3.3.2 or 4.3.4.2 of GB/T 24141.2.

7.3 Prepare the mold according to the provisions of 4.1 of GB/T 15825.2-2008.

8 Test conditions

9 Test procedure

9.1 The test is usually carried out at a temperature of 10 ° C to 35 ° C. If required, the temperature environment can also be set to 23 ° C ± 5 ° C.

9.2 Prepare the specimen in accordance with the provisions of Chapter 5 of this standard and prepare a strain analysis pattern on the specimen as specified in Chapter 6.

9.3 Inspect and clean the test equipment.

9.4 Perform pre-testing in order to select a reasonable blanking force. For sample materials with uncertain sampling direction, a small amount of pre-test may be passed first.

Determine the sampling direction.

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9.5 Conduct formal test. Lubricate one side of the sample without the strain analysis pattern and place it on the side facing the punch before the test.

During the stroke, ensure that the center line of the die and the punch overlap, the deviation is not more than 0.15mm, and the deviation between the center of the sample and the center line of the die is not more than 0.5mm.

The sample is positioned by the test positioning device. During the test, the crimping ring is pressed to firmly press the sample material until the sample is partially necked or broken.

9.6 Perform an effective repeat test of 3 or more samples of the same size and lubrication method.

9.7 In the event of any of the following, the test is considered invalid.

a) the necking or rupture of the specimen occurs near the orifice of the die;

b) tearing occurs on the sides of the specimen when using specimens of different widths;

c) the specimen ruptures near the drawbead;

9.8 Analyze the strain analysis pattern and calculate the in-plane ultimate strain.

10 Strain measurement and calculation

10.1 In the case of a hemispherical rigid male bulge test, the strain can be measured either in-line or off-line. Online measurement means installing the camera directly in on the shape tester, by recording and analyzing the deformation pattern on the sample from the start of the test to the time the sample is broken, the sample is obtained throughout the deformation process.

In-plane strain distribution; off-line measurement refers to obtaining the in-plane strain distribution after deformation of the sample by comparing the patterns on the sample before and after forming.

10.2 The ultimate strain in the surface of the specimen can be calculated according to the following formulas (1) to (4) or directly according to the corresponding graphical changes.

E1=(d1-d0)/d0.(1)

E2=(d2-d0)/d0.(2)

E1=ln(d1/d0)=ln(1 epsilon1) (3) E2=ln(d2/d0)=ln(1 epsilon2) (4)

11 Data processing

11.1 In-plane ultimate strain the forming limit curve is composed of the in-plane ultimate strain of the limit point under different strain paths, and the limit point refers to the instantaneous critical position before the necking occurs.

The corresponding point, the in-plane strain corresponding to the point is the in-plane ultimate strain. There are two ways to obtain the limit point and the in-plane ultimate strain.

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12 Test report

The test report should include the following information.

a) test method;

b) mold shape and size;

c) the grade, specification and condition of the test material;

d) ambient temperature;

e) the shape, size, number and sampling direction of the sample;

f) test equipment;

g) lubrication method;

h) strain analysis pattern and measurement method;

j) in-plane ultimate strain and average;

k) forming limit diagram;

l) Test date.