A test method for determining effective sample to measure hole expansion rate

Abstract

The present invention discloses a test method for determining an effective sample to determine the hole expansion rate, and the steps include: preparing a hole expansion sample, setting an initial fixed loading stroke, determining whether the sample has a crack state of "presence" or "absence", and determining an effective sample. Aiming at the problem of crack judgment standards existing in the current hole expansion test standards, the present invention has formulated corresponding technical solutions, solved the technical deficiencies in traditional testing methods, improved the measurement accuracy of the material hole expansion rate, and laid a solid foundation for the engineering application of the hole expansion rate in the stamping simulation model, which will promote the solution to the edge cracking problem of advanced high-strength steel. The present invention has the technical advantages of practicality, low cost, high precision and standardization, and will have important engineering practice significance and broad engineering application prospects.

Description

Technical Field

[0001] The invention belongs to the field of metal sheet forming, and in particular relates to a test method for measuring the hole expansion rate of metal materials. Background Art

[0002] Advanced high-strength steel is prone to edge cracking during part stamping, which is a common engineering problem faced by OEMs and steel mills. "How to prevent edge cracking?" and "How to predict edge cracking?" are the current research focuses on edge cracking in academia and engineering.

[0003] Regarding "how to prevent edge cracking?", OEMs and steel mills have reached a broad consensus that improving the hole expansion rate of materials can effectively reduce the risk of edge cracking of parts. Regarding "how to predict edge cracking?", some experts and scholars have realized the feasibility and importance of hole expansion rate as an evaluation indicator of edge cracking. For example, the United States Steel Corporation has fully demonstrated this in its public document, SAE 2007-01-1693; the inventor of this application submitted "An Experimental and Calculation Method for Determining the Equivalent Plastic Strain Forming Limit Diagram (Patent Application No. 2021101850376)", which successfully introduced the hole expansion rate into the forming limit diagram for predicting edge cracking. Therefore, how to accurately measure and monitor the hole expansion rate of materials is particularly important to solve the problem of edge cracking of parts.

[0004] The current hole expansion test standard ISO 16630 has been widely criticized for its high uncertainty and volatility in test results. ArcelorMittal’s early research showed that the “human judgment factor” will significantly affect the hole expansion rate measurement results. The so-called “human judgment factor” refers to the very large uncertainty in judging the end time of crack penetration, which leads to high volatility in measurement results.

[0005] On the issue of "How to prevent edge cracking?", the Swedish Steel Company (SSAB) has fully studied and realized the importance of controlling and improving the hole expansion rate of advanced high-strength steel to solve the edge cracking problem; on the issue of "How to predict edge cracking?", due to the "uncertainty and volatility of test results", it abandoned the use of hole expansion rate as an evaluation indicator and proposed a "three-dimensional edge forming limit diagram" based on a large number of experimental measurements.

[0006] At the 2008 IDDRG (International Deep Drawing Research Group) International Conference, USS introduced a new measurement system for hole expansion test, which consists of two parts, including the Digital Recording and Measurement System (DRMS) and the Tinus-Olsen forming machine. Based on the photos recorded by DRMS, the photos with critical through-hole or through-thickness cracks are selected. DRMS ​​can calculate the inner diameter of the hole in this photo and use it to calculate the hole expansion rate. Since the new measurement system can determine the state of critical through-hole cracks, the influence of "human judgment factors" is avoided.

[0007] The ISO 16630 standard determines crack termination as "when a crack that penetrates the thickness of the specimen appears". During the test, the edge of the hole should be under visual surveillance at all times. When a microcrack first appears, the advancement rate of the hole expansion punch should be immediately slowed down to minimize the increase in the loading stroke from the appearance of the microcrack to the cessation of the punch movement. According to the ISO 16630 standard, the determination of crack termination varies greatly from person to person. On the other hand, even if the same person performs hole expansion tests on different specimens of the same material, the determination of crack termination is uncertain, resulting in large differences in the results. Therefore, the ISO 16630 standard hole expansion test results are difficult to be repeatable.

[0008] The standard for crack compliance in ISO 16630:2017 is "the maximum width of the inner opening of a through-crack is 0.1 mm, and if it exceeds 0.1 mm, the test result is invalid". According to ISO 16630, on the one hand, it is very difficult to control the maximum width of the inner opening of a through-crack to be less than 0.1 mm by visual inspection; on the other hand, there are certain difficulties in the measurement method to measure the maximum width of the inner opening less than 0.1 mm. Therefore, if the ISO 16630 standard is strictly followed, the efficiency of the hole expansion test is a serious problem.

[0009] In summary, the ISO 16630 standard has major deficiencies and controversies in both "crack termination determination" and "crack compliance determination", which seriously limits the engineering application of the hole expansion test standard. Summary of the invention

[0010] 1. Technical Problems Solved by the Invention

[0011] The technical problems to be solved by the present invention are: in view of the problem of "crack termination judgment" in the ISO 16630:2017 standard, the traditional test method of determining the result (calculating the hole expansion rate) by the condition (subjectively judging whether the crack is penetrating during the test) is changed to determining the condition (objectively judging whether the crack is penetrating according to the sample after the test) by the result (estimated hole expansion rate); in view of the problem of "crack compliance judgment" in the ISO 16630 standard, by limiting the relationship between the fixed loading strokes of the crack states "presence" and "absence", the average value of the two is set to carry out the hole expansion test with fixed stroke loading, and the effective sample obtained can be directly used to determine the hole expansion rate without judging its crack state, thereby converting the "uncertainty and lack of operability of the crack judgment standard" into the "relative technical provisions of fixed stroke loading".

[0012] 2. Technical solution of the present invention

[0013] In order to achieve the purpose of the technical problem to be solved by the present invention, the present invention provides a test method for determining an effective sample to measure the hole expansion rate, the steps of which include:

[0014] Step 1: Prepare the hole expansion specimens: Prepare several hole expansion specimens of the material to be tested with an initial diameter of D0 according to the specimen size specified in the ISO 16630:2017 hole expansion standard;

[0015] Step 2, setting the initial fixed loading stroke: take out a sample from the prepared hole expansion sample, place the sample with the burr facing upward on the test device, and set the initial fixed loading stroke d0 to perform the hole expansion test on the sample. The fixed loading stroke refers to the distance the punch moves from the initial position to the movement stop position;

[0016] Step 3: Determine whether the specimen has cracks: Take out the expanded specimen after the hole expansion test and observe the crack state at the edge of the hole. There are two situations:

[0017] Case 1: If the crack state of the sample after hole expansion is "none", and the crack state "none" means "no cracks appear on the inner diameter that penetrate the thickness of the sample", then the fixed loading stroke is increased in increments relative to the initial fixed loading stroke d0, and the hole expansion test with the burr facing upward fixed stroke loading is repeated until the crack state of the sample after hole expansion is "yes", and the crack state "yes" means "cracks appear on the inner diameter that penetrate the thickness of the sample", then the fixed loading strokes d1 corresponding to the adjacent crack states "none" and "yes" are recorded as S0 and S1 respectively, and the absolute value of the difference between the two ΔS is required to be less than or equal to Δs, and the value range of Δs is greater than zero;

[0018] Case 2: If the crack state of the specimen after hole expansion is "yes", then the fixed loading stroke is reduced by a decreasing increment relative to the initial fixed loading stroke d0, and the hole expansion test with the fixed stroke loading with the burr facing upward is repeated until the crack state of the specimen after hole expansion is "no", then the fixed loading strokes d1 corresponding to the adjacent crack states "yes" and "no" are recorded as S1 and S0 respectively, and the absolute value of the difference between the two ΔS is required to be less than or equal to Δs;

[0019] Step 4. Determine the effective sample: Take a sample, set the fixed loading stroke d2 equal to (S0+S1) / 2, and perform a hole expansion test on the sample with a fixed stroke loading with the burr facing upward. The sample after hole expansion is the effective sample for determining the hole expansion rate.

[0020] The best test method for determining the effective sample to determine the hole expansion rate is characterized in that: in step one, according to the punching regulations of the ISO 16630:2017 hole expansion standard, a number of hole expansion samples with an initial diameter D0 equal to 10 mm are prepared; in step three, Δs is equal to 0.5 mm.

[0021] The best, described test method for determining effective samples to determine the hole expansion rate is characterized in that: in step 2, the method for determining the initial fixed loading stroke d0 is to use a conventional hole expansion test equivalent to an initial fixed stroke loading hole expansion test, and the specific method is: take a sample from the prepared hole expansion sample, place the sample with the burr facing up on the hole expansion testing machine, and stop the hole expansion test when it is determined by naked eyes that "cracks that penetrate the thickness of the sample appear"; the maximum loading stroke when the hole expansion test is terminated is read from the hole expansion load-loading stroke curve as the initial fixed loading stroke d0.

[0022] The most optimal test method for determining effective samples to measure hole expansion rate is characterized in that: in step three, an electron microscope is used to assist in judging the "presence" or "absence" of the crack state.

[0023] Furthermore, the test method for determining effective samples to measure the hole expansion rate is characterized in that all samples are placed with the burrs facing downward for the hole expansion test.

[0024] A test method for determining an effective sample to determine a hole expansion rate comprises the following steps:

[0025] Step 1: Prepare the hole expansion specimens: Prepare several hole expansion specimens of the material to be tested with an initial diameter of D0 according to the specimen size specified in the ISO 16630:2017 hole expansion standard;

[0026] Step 2, setting the initial fixed loading stroke: take out a sample from the prepared hole expansion sample, place the sample with the burr facing upward on the test device, and set the initial fixed loading stroke d0 to perform the hole expansion test on the sample. The fixed loading stroke refers to the distance the punch moves from the initial position to the movement stop position;

[0027] Step 3: Determine whether the specimen has cracks: Take out the expanded specimen after the hole expansion test and observe the crack state at the edge of the hole. There are two situations:

[0028] Case 1: If the crack state of the sample after hole expansion is "none", and the crack state "none" means "no initial crack appears on the outer diameter edge", then the fixed loading stroke is increased in increments relative to the initial fixed loading stroke d0, and the hole expansion test with fixed stroke loading with the burr facing upward is repeated until the crack state of the sample after hole expansion is "yes", and the crack state "yes" means "initial crack appears on the outer diameter edge", then the fixed loading strokes d1 corresponding to the adjacent crack states "none" and "yes" are recorded as S0 and S1 respectively, and the absolute value of the difference between the two ΔS is required to be less than or equal to Δs, and the value range of Δs is greater than zero;

[0029] Case 2: If the crack state of the specimen after hole expansion is "yes", then the fixed loading stroke is reduced by a decreasing increment relative to the initial fixed loading stroke d0, and the hole expansion test with the fixed stroke loading with the burr facing upward is repeated until the crack state of the specimen after hole expansion is "no", then the fixed loading strokes d1 corresponding to the adjacent crack states "yes" and "no" are recorded as S1 and S0 respectively, and the absolute value of the difference between the two ΔS is required to be less than or equal to Δs;

[0030] Step 4. Determine the effective sample: Take a sample, set the fixed loading stroke d2 equal to (S0+S1) / 2, and perform a hole expansion test on the sample with a fixed stroke loading with the burr facing upward. The sample after hole expansion is the effective sample for determining the hole expansion rate.

[0031] The best test method for determining the effective sample to determine the hole expansion rate is characterized in that: in step one, according to the punching regulations of the ISO 16630:2017 hole expansion standard, a number of hole expansion samples with an initial diameter D0 equal to 10 mm are prepared; in step three, Δs is equal to 0.5 mm.

[0032] The best, described test method for determining effective samples to determine the hole expansion rate is characterized in that: in step 2, the method for determining the initial fixed loading stroke d0 is to use a conventional hole expansion test equivalent to an initial fixed stroke loading hole expansion test, and the specific method is: take a sample from the prepared hole expansion sample, place the sample with the burr facing up on the hole expansion testing machine, and stop the hole expansion test when it is determined by naked eyes that "initial cracks appear on the outer diameter edge"; the maximum loading stroke when the hole expansion test is terminated is read from the hole expansion load-loading stroke curve as the initial fixed loading stroke d0.

[0033] The most optimal test method for determining effective samples to measure hole expansion rate is characterized in that: in step three, an electron microscope is used to assist in judging the state of "presence" and "absence" of cracks.

[0034] Furthermore, the test method for determining effective samples to measure the hole expansion rate is characterized in that all samples are placed with the burrs facing downward for the hole expansion test.

[0035] 3. Beneficial effects of the present invention

[0036] The present invention provides a test method for determining an effective sample to determine the hole expansion rate, and the beneficial effects produced by the method are mainly reflected in the following three aspects:

[0037] First, accurate measurement of hole expansion rate is achieved. Since the present invention adopts fixed stroke loading, the intermediate state between the two is directly used as the effective sample for measuring hole expansion rate by judging the presence or absence of crack state, thus avoiding the measurement error caused by "artificial judgment of cracks" during the test process, thus ensuring the repeatability of the test results and significantly improving the test efficiency.

[0038] Second, it saves the cost of updating the test equipment. Based on the existing test equipment and test conditions, the present invention obtains effective samples for measuring the hole expansion rate without increasing any cost and affecting the test process. Compared with the new test system introduced by USS, although the basic idea of ​​the two methods is to determine whether the "conditions" are met by the "results", and both are to solve the problem of hole expansion rate measurement accuracy, the present invention saves the purchase cost and management cost of the new system. At the same time, the present invention is based on the actual judgment, while the latter is based on the picture. Obviously, the present invention is more intuitive and more reliable.

[0039] Third, the application space of the hole expansion test standard is enhanced. The hole expansion rate of the material is a key factor affecting the edge cracking of parts. The biggest obstacle to using the hole expansion rate as an evaluation indicator of edge cracking is that the traditional method cannot accurately measure the hole expansion rate of the material, which makes the engineering value of the hole expansion test seriously underestimated. With the efficient acquisition of effective samples for measuring the hole expansion rate and strong operability of the present invention, a solid foundation has been laid for the engineering application of the hole expansion rate in the stamping simulation model, which will enhance the attention of OEMs and steel mills to the hole expansion test, thereby promoting the solution to the edge cracking problem of advanced high-strength steel.

[0040] In summary, through the implementation of the present invention, the technical deficiencies in the traditional testing method are solved and the application space of the hole expansion test is improved. Therefore, the present invention has important engineering practice significance and broad engineering application prospects. BRIEF DESCRIPTION OF THE DRAWINGS

[0041] The present invention will be further described in detail below in conjunction with the accompanying drawings.

[0042] Figure 1 It is a schematic diagram of the new hole expansion test process of the present invention;

[0043] Figure 2 is a schematic diagram of a hole-expanding sample after punching of the present invention;

[0044] Figure 3 is a schematic diagram of a sample after hole expansion judged by naked eyes in Example 1 of the present invention;

[0045] Figure 4 Schematic diagram of the specimen and crack state after hole expansion with 7.5 mm fixed stroke loading in Example 1 of the present invention;

[0046] Figure 5 Schematic diagram of the specimen and crack state after hole expansion with 7.3 mm fixed stroke loading in Example 1 of the present invention;

[0047] Figure 6 It is a schematic diagram of the effective sample and crack state after hole expansion with 7.4 mm fixed stroke loading in Example 1 of the present invention;

[0048] Figure 7 is a schematic diagram of a sample after hole expansion judged by naked eyes in Example 2 of the present invention;

[0049] Figure 8 Schematic diagram of the specimen and crack state after hole expansion with 5.8 mm fixed stroke loading in Example 2 of the present invention;

[0050] Fig. 9 Schematic diagram of the specimen and crack state after hole expansion with 5.6 mm fixed stroke loading in Example 2 of the present invention;

[0051] Fig.10It is a schematic diagram of the effective sample and crack state after hole expansion with 5.7 mm fixed stroke loading in Example 2 of the present invention. DETAILED DESCRIPTION

[0052] The technical solution of the present invention will be clearly and completely described below in conjunction with the accompanying drawings of the present invention.

[0053] Example 1

[0054] This embodiment takes material DP780 / 1.5mm as an example, and uses an electron microscope to assist in observation and judgment of the crack state. The flow chart is as follows Figure 1 The new hole expansion test flow chart shown includes the following steps:

[0055] Step 1: Prepare the hole-expanding specimens: According to the specimen size requirements and punching regulations specified in GB / T 24524 or ISO 16630, prepare a number of hole-expanding specimens with an initial diameter D0 equal to 10 mm.

[0056] Step 2: Set the initial fixed stroke loading: The method for determining the initial fixed loading stroke d0 is to use a conventional hole expansion test equivalent to an initial fixed stroke loading hole expansion test. The specific method is: First, take a sample from the prepared hole expansion sample, such as Figure 2 For the punched specimen shown in the figure, place the specimen with the burr facing upward on the hole expansion test machine. When the "cracks that penetrate the thickness of the specimen appear on the inner diameter" are determined by naked eyes, the hole expansion test is stopped. During the test, the edge of the hole should be under visual observation all the time. The maximum loading stroke at the end of the hole expansion test is equal to 8.0 mm as read from the hole expansion load-loading stroke curve. Figure 3 The specimen and crack state after hole enlargement are judged by naked eyes. The inner diameter is 12.36mm. Figure 3 -c indicates the presence of "cracks that penetrate the thickness of the specimen" visible to the naked eye, and the crack state is "yes", which is equivalent to the hole expansion test with an initial fixed loading stroke d0 equal to 8.0 mm.

[0057] Step 3: Determine whether the crack state of the hole expansion specimen is "presence" or "absence": Take a specimen, take the initial fixed loading stroke d0 as the reference value, set the fixed loading stroke d1, which is smaller than d0, to 7.5 mm, and perform a hole expansion test with the burr facing upwards on the specimen. Figure 4 -The inner diameter of the expanded specimen with a fixed stroke of 7.5 mm shown in a is 12.01 mm. Figure 4 -c indicates that the crack state of the sample after hole expansion is "yes", that is, there is still a "crack that penetrates the thickness of the sample". Figure 3-c is closer to the critical penetration state. Then, the fixed loading stroke is reduced to 7.3 mm with a decreasing increment of 0.2 mm, and the hole expansion test with the burr facing upward is repeated. Figure 5 -The inner diameter of the expanded specimen with a fixed stroke of 7.3 mm shown in a is 11.91 mm. Figure 5 -c indicates that the crack state of the sample after hole expansion is "none", that is, there is no "crack that penetrates the thickness of the sample". Therefore, the adjacent crack states "yes" and "no" correspond to fixed loading strokes S1 equal to 7.5mm and S0 equal to 7.3mm respectively. The absolute value of the difference between the two ΔS is equal to 0.2mm, which meets the technical requirement of less than or equal to 0.5mm.

[0058] Step 4: Determine the effective sample: Take a sample, set the fixed loading stroke d2 equal to (S0+S1) / 2=7.4mm, and perform a hole expansion test on the sample with the burr facing upward. The sample after hole expansion is as follows: Figure 6 -The effective sample for measuring the hole expansion rate shown in a is measured to have an inner diameter of 11.96 mm. Figure 6 -c indicates that the crack state of the sample after hole expansion is "present". The measurement error of this result: the relative accuracy described by fixed stroke is 0.1mm; the absolute error directly described by hole expansion rate is 0.5%, thus cleverly solving the problem of determining whether the crack meets the standard.

[0059] Example 2

[0060] This embodiment takes material DP780 / 2.1mm as an example, and uses an electron microscope to assist in observation and judgment of the crack state. The flow chart is as follows Figure 1 The new hole expansion test flow chart shown includes the following steps:

[0061] Step 1: Prepare the hole-expanding specimens: According to the specimen size requirements and punching regulations specified in GB / T 24524 or ISO 16630, prepare a number of hole-expanding specimens with an initial diameter D0 equal to 10 mm.

[0062] Step 2: Set the initial fixed stroke loading: The method for determining the initial fixed loading stroke d0 is to use a conventional hole expansion test equivalent to the hole expansion test with initial fixed stroke loading. The specific method is: take a sample from the prepared hole expansion sample, such as Figure 2 For the punched specimen shown in the figure, place the specimen with the burr facing upward on the hole expansion test machine. When the "initial crack appears on the outer diameter edge" is determined by naked eyes, the hole expansion test is stopped. It is required that the edge of the hole should be under naked eye monitoring during the test. The maximum loading stroke at the end of the hole expansion test is equal to 6.2mm as read from the hole expansion load-loading stroke curve. Figure 7The specimen and crack state after hole enlargement are judged by naked eyes. The outer diameter is 13.23mm. Figure 7 -c indicates that there is "initial crack on the outer diameter edge" visible to the naked eye, and the crack state is "yes", which is equivalent to the hole expansion test with an initial fixed loading stroke d0 equal to 6.2 mm.

[0063] Step 3: Determine whether the crack state of the hole expansion specimen is "presence" or "absence": Take a specimen, take the initial fixed loading stroke d0 as the reference value, set the fixed loading stroke d1, which is smaller than d0, to be equal to 5.8 mm, and perform a hole expansion test on the specimen with the burr facing upward with fixed stroke loading. Figure 8 -The outer diameter of the expanded specimen with a 5.8 mm fixed stroke loading as shown in a is 12.47 mm. Figure 8 -c indicates that the crack state of the sample after hole expansion is "yes", that is, "initial crack on the outer diameter edge" still exists. Then, the set loading stroke is reduced to 5.6mm with a decreasing increment of 0.2mm, and the hole expansion test with fixed stroke loading and burr facing upward is carried out again. Fig. 9 -The outer diameter of the expanded specimen with a 5.6 mm fixed stroke loading as shown in a is 12.20 mm. Fig. 9 -c indicates that the crack state of the sample after hole expansion is "none", that is, there is no "initial crack on the outer diameter edge". Therefore, the adjacent crack states "yes" and "no" correspond to fixed loading strokes S1 equal to 5.8mm and S0 equal to 5.6mm respectively. The absolute value of the difference between the two ΔS is equal to 0.2mm, which meets the technical requirement of less than or equal to 0.5mm.

[0064] Step 4: Determine the effective sample: Take a sample, set the fixed loading stroke d2 equal to (S0+S1) / 2=5.7mm, and perform a hole expansion test on the sample with the burr facing upward. The sample after hole expansion is as follows: Fig.10 The effective sample for measuring the hole expansion rate shown in -a is measured to have an outer diameter of 12.37 mm. Fig.10 -c indicates that the crack state of the sample after hole expansion is "present". The measurement error of this result: the relative accuracy described by fixed stroke is 0.1mm; the absolute error directly described by hole expansion rate is 1.35%, thus cleverly solving the problem of determining whether the crack meets the standard.

[0065] The above embodiments are only exemplary embodiments of the present invention and are not intended to limit the present invention. The protection scope of the present invention is defined by the claims. Various modifications or equivalent substitutions made by those skilled in the art to the present invention within the substantial protection scope of the present invention also fall within the protection scope of the present invention.

Claims

1. A test method for determining an effective sample to determine a hole expansion rate, the steps comprising: Step 1: Prepare the hole expansion specimens: Prepare several hole expansion specimens of the material to be tested with an initial diameter of D0 according to the specimen size specified in the ISO 16630:2017 hole expansion standard; Step 2, setting the initial fixed loading stroke: take out a sample from the prepared hole expansion sample, place the sample with the burr facing upward on the test device, and set the initial fixed loading stroke d0 to perform the hole expansion test on the sample. The fixed loading stroke refers to the distance the punch moves from the initial position to the movement stop position; Step 3: Determine whether the specimen has cracks: Take out the expanded specimen after the hole expansion test and observe the crack state at the edge of the hole. There are two situations: Case 1: If the crack state of the sample after hole expansion is "none", the crack state "none" means "no cracks appear on the inner diameter that penetrate the thickness of the sample", then the fixed loading stroke is increased in increments relative to the initial fixed loading stroke d0, and the hole expansion test with the burr facing upward fixed stroke loading is repeated until the crack state of the sample after hole expansion is "yes", and the crack state "yes" means "cracks appear on the inner diameter that penetrate the thickness of the sample", then the fixed loading strokes d1 corresponding to the adjacent crack states "none" and "yes" are recorded as S0 and S1 respectively, and the absolute value of the difference between the two ΔS is required to be less than or equal to Δs, and the value range of Δs is greater than zero; Case 2: If the crack state of the specimen after hole expansion is "yes", then the fixed loading stroke is reduced by a decreasing increment relative to the initial fixed loading stroke d0, and the hole expansion test with the fixed stroke loading with the burr facing upward is repeated until the crack state of the specimen after hole expansion is "no", then the fixed loading strokes d1 corresponding to the adjacent crack states "yes" and "no" are recorded as S1 and S0 respectively, and the absolute value of the difference between the two ΔS is required to be less than or equal to Δs; Step 4. Determine the effective sample: Take a sample, set the fixed loading stroke d2 equal to (S0+S1) / 2, and perform a hole expansion test on the sample with a fixed stroke loading with the burr facing upward. The sample after hole expansion is the effective sample for determining the hole expansion rate.

2. The test method for determining effective samples to measure hole expansion rate according to claim 1, characterized in that: In step one, according to the punching regulations of ISO 16630:2017 hole expansion standard, several hole expansion specimens with an initial diameter D0 equal to 10 mm are prepared; in step three, Δs is equal to 0.5 mm.

3. The test method for determining effective samples to determine the hole expansion rate according to claim 2, characterized in that: In step 2, the method for determining the initial fixed loading stroke d0 is to use a conventional hole expansion test equivalent to a hole expansion test with initial fixed stroke loading. The specific method is: take a sample from the prepared hole expansion sample, place the sample with the burr facing upward on the hole expansion testing machine, and stop the hole expansion test when it is determined by naked eyes that "cracks that penetrate the thickness of the sample appear"; the maximum loading stroke when the hole expansion test is terminated is read from the hole expansion load-loading stroke curve as the initial fixed loading stroke d0.

4. The test method for determining effective samples to measure hole expansion rate according to claim 3, characterized in that: In step three, an electron microscope is used to assist in determining whether a crack is present or absent.

5. The test method for determining effective samples to measure hole expansion rate according to claim 4, characterized in that: All specimens were placed with the burrs facing downwards for hole expansion tests.

6. A test method for determining an effective sample to determine the hole expansion rate, the steps comprising: Step 1: Prepare the hole expansion specimens: Prepare several hole expansion specimens of the material to be tested with an initial diameter of D0 according to the specimen size specified in the ISO 16630:2017 hole expansion standard; Step 2, setting the initial fixed loading stroke: take out a sample from the prepared hole expansion sample, place the sample with the burr facing upward on the test device, and set the initial fixed loading stroke d0 to perform the hole expansion test on the sample. The fixed loading stroke refers to the distance the punch moves from the initial position to the movement stop position; Step 3: Determine whether the specimen has cracks: Take out the expanded specimen after the hole expansion test and observe the crack state at the edge of the hole. There are two situations: Case 1: If the crack state of the sample after hole expansion is "none", and the crack state "none" means "no initial crack appears on the outer diameter edge", then the fixed loading stroke is increased in increments relative to the initial fixed loading stroke d0, and the hole expansion test with fixed stroke loading with the burr facing upward is repeated until the crack state of the sample after hole expansion is "yes", and the crack state "yes" means "initial crack appears on the outer diameter edge", then the fixed loading strokes d1 corresponding to the adjacent crack states "none" and "yes" are recorded as S0 and S1 respectively, and the absolute value of the difference between the two ΔS is required to be less than or equal to Δs, and the value range of Δs is greater than zero; Case 2: If the crack state of the specimen after hole expansion is "yes", then the fixed loading stroke is reduced by a decreasing increment relative to the initial fixed loading stroke d0, and the hole expansion test with the fixed stroke loading with the burr facing upward is repeated until the crack state of the specimen after hole expansion is "no", then the fixed loading strokes d1 corresponding to the adjacent crack states "yes" and "no" are recorded as S1 and S0 respectively, and the absolute value of the difference between the two ΔS is required to be less than or equal to Δs; Step 4. Determine the effective sample: Take a sample, set the fixed loading stroke d2 equal to (S0+S1) / 2, and perform a hole expansion test on the sample with a fixed stroke loading with the burr facing upward. The sample after hole expansion is the effective sample for determining the hole expansion rate.

7. The test method for determining effective samples to measure hole expansion rate according to claim 6, characterized in that: In step one, according to the punching regulations of ISO 16630:2017 hole expansion standard, several hole expansion specimens with an initial diameter D0 equal to 10 mm are prepared; in step three, Δs is equal to 0.5 mm.

8. The test method for determining effective samples to measure hole expansion rate according to claim 7, characterized in that: In step 2, the method for determining the initial fixed loading stroke d0 is to use a conventional hole expansion test equivalent to a hole expansion test with initial fixed stroke loading. The specific method is: take a sample from the prepared hole expansion sample, place the sample with the burr facing upward on the hole expansion testing machine, and stop the hole expansion test when it is determined by naked eyes that "initial cracks appear on the outer diameter edge"; read the maximum loading stroke when the hole expansion test is terminated from the hole expansion load-loading stroke curve as the initial fixed loading stroke d0.

9. The test method for determining effective samples to measure hole expansion rate according to claim 8, characterized in that: In step three, an electron microscope is used to assist in determining whether a crack is present or absent.

10. The test method for determining effective samples to measure hole expansion rate according to claim 9, characterized in that: All specimens were placed with the burrs facing downwards for hole expansion tests.

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