A design method for improving the effectiveness of biaxial tensile specimens

Abstract

The application provides a design method for improving the effectiveness of a biaxial tensile sample, comprising the following steps: S1, establishing a finite element model and performing a simulation test; S2, judging whether the biaxial tensile sample needs to be adjusted according to the simulation result of the biaxial tensile test condition; if the deformation of the biaxial tensile sample mainly occurs in a non-central region, S3 is entered; S3 comprises the following steps: (1) increasing the length of four tensile arms of the biaxial tensile sample and the width of the widest part by 5-20% of the side length of the central region; (2) increasing 1-3 grooves on the four tensile arms respectively; (3) adjusting the width of the grooves on the tensile arms to be not more than 0.2 mm; (4) increasing the length of the grooves by 10-50% of the width of the widest part of the tensile arms; and S4, verifying the effectiveness of the biaxial tensile sample after each round of adjustment. The application solves the problems of the existing sample design method, such as high complexity in processing, easy deformation and fracture of the sample in a non-central region, poor stability in the test process and the like.

Description

Technical Field

[0001] This invention relates to the field of formability analysis technology for sheet metal materials, and more particularly to a design method for improving the effectiveness of biaxial tensile specimens. Background Technology

[0002] In the process of formability analysis of thin metal sheets, biaxial tensile tests are usually performed on the specimens under different loading states to obtain the mechanical property curves of the material under different stress states. At present, there are various shapes and sizes of biaxial tensile specimens in the industry. The specimen shape is usually designed to be approximately cross-shaped, including tensile arms in four directions, to meet the equipment requirements of various biaxial tensile testing machines.

[0003] However, although different types of cross-shaped biaxial tensile specimens can all undergo tensile testing under the action of a biaxial tensile testing machine, the deformation of biaxial tensile specimens of different shapes is not necessarily generated under biaxial loading. If the deformation of the biaxial tensile specimen is mainly concentrated in the non-central region, the specimen will inevitably fracture in the non-central region. The fracture of the specimen in the non-central region is generally a uniaxial tensile stress state. The test results collected under this stress state will inevitably not match the actual deformation characteristics of the material, and cannot meet the application requirements of engineering practice and scientific research. In the existing technology, some biaxial tensile specimens have undergone thickness reduction treatment in the central region, but due to the thickness reduction... Milling or grinding machines are generally used. This processing method has a significant impact on material properties. After milling, the material will undergo work hardening, resulting in a higher yield strength after milling and a lower elongation after milling. Changes in material properties will affect the collected test results and cause the test results to be inconsistent with the actual properties of the material. In addition, some biaxial tensile specimens have holes on the tensile arms or lack grooves aligned with the center line of the specimen. Such structures will affect the force transmission stability of the specimen, resulting in uneven biaxial stress on the biaxial specimen during the test. Moreover, it will increase the complexity of specimen processing and increase the test cost.

[0004] In summary, existing technologies, in order to prevent specimen deformation from occurring in non-central regions and to ensure that the central region of the specimen is under biaxial tensile stress as required by test requirements, will reduce the thickness of the specimen center during specimen design. However, the design of the thickness reduction will affect the key performance indicators such as the strength and elongation of the test material, leading to serious distortion of performance test results. Moreover, the existing specimen design methods also have problems such as complex structure, poor uniformity of specimen stress, and poor stability of deformation state. Therefore, it is very important to develop a design method to improve the effectiveness of biaxial tensile specimens. Summary of the Invention

[0005] To address the problems mentioned above, such as the impact of existing specimen design methods on the performance of test materials leading to severe distortion of performance test results, high specimen processing complexity, poor stress uniformity and deformation stability during the test, and inability to meet the requirements of biaxial tensile testing, this invention provides a design method to improve the effectiveness of biaxial tensile specimens. This method ensures that the deformation state of the specimen always meets the loading characteristics of biaxial tensile testing, overcomes the technical difficulties of performance distortion in existing specimen test results, poor stress uniformity and deformation stability during the test, and significantly improves the effectiveness and stability of biaxial tensile specimens. It is particularly suitable for the study and analysis of the formability of thin metal sheet materials and widely meets the application requirements of engineering practice and scientific research.

[0006] The technical means employed in this invention are as follows:

[0007] A design method for improving the effectiveness of biaxial tensile specimens, wherein the biaxial tensile specimen is a cross-shaped specimen comprising four tensile arms, and a square central region is defined at the center of the cross-shaped specimen where the four tensile arms intersect; the design method specifically includes the following steps:

[0008] S1. Establish a finite element model based on the shape and size of the biaxial tensile specimen, and conduct simulation tests according to the biaxial tensile test conditions.

[0009] S2. Determine whether the biaxial tensile specimen needs adjustment based on the simulation results of the biaxial tensile test conditions:

[0010] If the deformation of the biaxial tensile specimen mainly occurs in the central region, it indicates that the biaxial tensile specimen is effective and can meet the test requirements.

[0011] If the deformation of the biaxial tensile specimen mainly occurs in the non-central region, it indicates that the biaxial tensile specimen cannot meet the test requirements, and proceed to S3.

[0012] S3. Adjust the shape and size of the finite element model of the biaxial tensile specimen:

[0013] (1) Increase the length of the four tensile arms of the biaxial tensile specimen and the width of the widest part by 5-20% of the side length of the central region; ensure that the length of the tensile arm is not less than 150% of the side length of the central region and the width of the widest part of the tensile arm is not less than the side length of the central region.

[0014] (2) Add 1 to 3 grooves on each of the four tensile arms, and ensure that the number of grooves on each tensile arm is the same and that the number of grooves is odd. The groove located in the middle of the tensile arm is coaxial with the center line of the biaxial tensile specimen, and the other grooves are symmetrically arranged on both sides of the groove located in the middle of the tensile arm.

[0015] (3) Adjust the width of the grooves on each tension arm to not exceed 0.5mm; the width of the grooves on each tension arm should be the same.

[0016] (4) Increase the length of the groove by 10-50% of the width of the widest part of the tension arm; the length of the groove on each tension arm is the same.

[0017] S4. Verify the effectiveness of the biaxial tensile specimen after each round of adjustments:

[0018] Repeat steps S1 and S2 for the biaxial tensile specimen after adjustment S3. If it is determined by S2 that the biaxial tensile specimen does not meet the test requirements, repeat S3 for the next round of adjustment. Repeat the above process until it is determined by S2 that the biaxial tensile specimen is valid and meets the test requirements.

[0019] Furthermore, in S2, the main areas where deformation of the biaxial tensile specimen occurs are determined by finite element analysis cloud diagrams.

[0020] Furthermore, in (2), the grooves are opened along the length direction of the stretching arm, and the spacing between adjacent grooves is equal.

[0021] Furthermore, in (2), ensure that the number of grooves on each tension arm is not less than 3.

[0022] Furthermore, in (2), if the number of existing grooves on the stretch arm is odd, then 2 more grooves are added; if the number of existing grooves on the stretch arm is 0, then 3 more grooves are added; if the number of existing grooves on the stretch arm is an even number other than 0, then 1 more groove is added.

[0023] Furthermore, in (3), if there is already a groove on the stretching arm, the groove width is reduced in (3) by a ratio of 10 to 25% of the existing groove width.

[0024] Furthermore, in (3), if the width of the existing groove on the stretch arm exceeds 0.2 mm, the existing groove width is gradually adjusted by reducing it by 10 to 25% each time until the existing groove width does not exceed 0.2 mm.

[0025] Furthermore, in (4), if the number of existing grooves on the stretch arm is 0, the length of the newly opened groove is controlled to be 10 to 50% of the width of the stretch arm at its widest point.

[0026] Furthermore, in (4), the length of the groove is guaranteed to be no more than 90% of the length of the extension arm.

[0027] Compared with the prior art, the present invention has the following advantages:

[0028] The design method for improving the effectiveness of biaxial tensile specimens provided by this invention does not alter the structure of the central region of the specimen, ensuring that the material properties of the specimen are not affected. This invention utilizes the finite element method to determine the loading history of the specimen under complex deformation and progressively adjusts the shape and size of the specimen. The progressive adjustment method adopted by this invention ensures that the deformation of the final specimen occurs in the central region, and simplifies the specimen processing as much as possible while ensuring the specimen's usage requirements, reducing the specimen preparation cost. At the same time, it ensures that the deformation state of the final specimen always meets the loading characteristics of the biaxial tensile test, improving the stress uniformity, effectiveness, and test stability of the biaxial tensile specimen. It overcomes the technical problems of performance distortion in existing specimen test results, poor stress uniformity and deformation stability during the test, and reduces unnecessary specimen manufacturing costs.

[0029] Based on the above reasons, this invention can be widely applied in the field of formability analysis of sheet metal materials. Attached Figure Description

[0030] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0031] Figure 1 This is the initial state of the biaxial tensile specimen in Example 1 of the present invention.

[0032] Figure 2 This is a schematic diagram of the simulation test results of the biaxial tensile specimen in Embodiment 1 of the present invention under the biaxial tensile test conditions.

[0033] Figure 3 This is a schematic diagram of the simulation test results of the biaxial tensile specimen in Embodiment 1 of the present invention after the shape and size were adjusted and tested under the biaxial tensile test conditions.

[0034] Figure 4 This is the initial state of the biaxial tensile specimen in Example 2 of the present invention.

[0035] Figure 5 This is a schematic diagram of the simulation test results of the biaxial tensile specimen in Embodiment 2 of the present invention after the shape and size were adjusted and tested under the biaxial tensile test conditions. Detailed Implementation

[0036] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. The following description of at least one exemplary embodiment is merely illustrative and is in no way intended to limit the present invention or its application or use. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0037] like Figure 1-3 As shown, the present invention provides a design method to improve the effectiveness of biaxial tensile specimens. The biaxial tensile specimen is a cross-shaped specimen including four tensile arms, and a square central region is divided in the center of the cross-shaped specimen where the four tensile arms intersect.

[0038] The design method specifically includes the following steps:

[0039] S1. Establish a finite element model based on the shape and size of the biaxial tensile specimen, and conduct simulation tests according to the biaxial tensile test conditions.

[0040] S2. Determine whether the biaxial tensile specimen needs adjustment based on the simulation results of the biaxial tensile test conditions:

[0041] If the deformation of the biaxial tensile specimen mainly occurs in the central region, it indicates that the biaxial tensile specimen is effective and can meet the test requirements.

[0042] If the deformation of the biaxial tensile specimen mainly occurs in the non-central region, it indicates that the biaxial tensile specimen cannot meet the test requirements, and proceed to S3.

[0043] S3. Adjust the shape and size of the finite element model of the biaxial tensile specimen:

[0044] (1) Increase the length of the four tension arms of the biaxial tensile specimen and the width of the widest part by 5-20% of the side length of the central region; ensure that the length of the tension arm is not less than 150% of the side length of the central region and the width of the widest part of the tension arm is not less than the side length of the central region; such specimen size can prevent insufficient force transmission space between the tension arm and the central region of the specimen due to the tension arm size being too small, which would make it difficult for the specimen to deform in the central region. Moreover, if the length of the tension arm is too short, the area of ​​the specimen clamping area (the clamping area is the entire tension arm minus the groove area) will be too small, and the specimen will easily slip during the test; however, the size of the tension arm should not be too large, as an excessively large size will easily lead to waste of the test plate material. The specific size can be determined by the method of this invention through each round of adjustment and verification.

[0045] (2) Add 1 to 3 grooves on each of the four tension arms, and ensure that the number of grooves on each tension arm is the same and that the number of grooves is odd. The groove located in the middle of the tension arm is coaxial with the center line of the biaxial tensile specimen, and the other grooves are symmetrically arranged on both sides of the groove located in the middle of the tension arm. Since it is necessary to collect the change process of two pairs of gauge lengths with the center line of the specimen as the reference during the biaxial tensile test, the center line of one groove on the tension arm can be aligned with the center line of the specimen. The other grooves are symmetrically arranged on both sides to ensure the orthogonality of the strain state of the two pairs of gauge lengths and avoid shear force during the test due to the skew position of the groove, which would affect the test results.

[0046] (3) Adjust the width of the grooves opened on each tension arm to not exceed 0.5 mm; the width of the grooves on each tension arm is the same; making the groove width not exceed 0.5 mm can reduce the influence of the groove on the stiffness of the specimen. Moreover, if the groove is too wide, stress concentration is likely to occur at the groove of the tension arm during the test, making it difficult for the specimen to deform at the center of the specimen.

[0047] (4) Increase the length of the groove by 10 to 50% of the width of the widest part of the tension arm. Increasing the length of the groove can improve the force transmission stability during the tensile process of the specimen, thereby ensuring that the specimen deformation is more likely to occur in the central area; the length of the groove on each tension arm is the same.

[0048] S4. Verify the effectiveness of the biaxial tensile specimen after each round of adjustments:

[0049] Repeat steps S1 and S2 for the biaxial tensile specimen after adjustment S3. If it is determined by S2 that the biaxial tensile specimen does not meet the test requirements, repeat S3 for the next round of adjustment. Repeat the above process until it is determined by S2 that the biaxial tensile specimen is valid and meets the test requirements.

[0050] Furthermore, in S2, the main areas where deformation of the biaxial tensile specimen occurs are determined by finite element analysis cloud diagrams.

[0051] Furthermore, in (2), the grooves are opened along the length direction of the stretching arm, and the spacing between adjacent grooves is equal.

[0052] Furthermore, in (2), ensure that the number of grooves on each tension arm is not less than 3.

[0053] Furthermore, in (2), if the number of existing grooves on the stretch arm is odd, then 2 more grooves are added; if the number of existing grooves on the stretch arm is 0, then 3 more grooves are added; if the number of existing grooves on the stretch arm is an even number other than 0, then 1 more groove is added.

[0054] Furthermore, in (3), if there is already a groove on the stretching arm, the groove width is reduced in (3) by a ratio of 10 to 25% of the existing groove width.

[0055] Furthermore, in (3), if the width of the existing groove on the stretch arm exceeds 0.5 mm, the existing groove width is gradually adjusted by reducing it by 10 to 25% each time until the existing groove width does not exceed 0.5 mm.

[0056] Preferably, in (3), adjusting the width of the grooves opened on each tension arm to not exceed 0.2 mm will achieve better results and stress concentration will be more likely to occur at the grooves; if the width of the existing grooves on the tension arm exceeds 0.2 mm, the existing groove width will be gradually adjusted by reducing the existing groove width by 10 to 25% each time until the existing groove width does not exceed 0.2 mm.

[0057] Furthermore, in (4), if the number of existing grooves on the stretch arm is 0, the length of the newly opened groove is controlled to be 10 to 50% of the width of the stretch arm at its widest point; if the number of existing grooves on the stretch arm is not 0, the length of the groove is increased by 10 to 50% of the width of the stretch arm at its widest point.

[0058] Furthermore, in (4), the length of the groove is guaranteed to be no more than 90% of the length of the extension arm.

[0059] The design method for improving the effectiveness of biaxial tensile specimens provided by this invention eliminates the need for thickness reduction in the central region, ensuring the true material properties of the specimen. Based on the biaxial tensile deformation mechanism of materials, this invention correlates the loading characteristics of the biaxial tensile testing machine with the bidirectional deformation characteristics of the part. It utilizes the finite element method to determine the loading history of the specimen under complex deformation, progressively adjusting the specimen shape and size. This progressive adjustment method ensures that the deformation of the final specimen occurs in the central region, simplifying the specimen processing as much as possible while ensuring the specimen meets usage requirements, reducing processing difficulty and preparation costs. Simultaneously, this invention overcomes the technical difficulties of distorted test results, poor stress uniformity, and poor deformation stability in existing specimens. It significantly improves the stress uniformity, effectiveness, and stability of biaxial tensile specimens, while ensuring the specimen remains under biaxial loading, broadly meeting practical engineering needs.

[0060] Example 1

[0061] The following describes the design method for improving the effectiveness of biaxial tensile specimens according to the present invention, with reference to specific embodiments, and specifically includes the following steps:

[0062] S1, such as Figure 1As shown, a finite element model was established based on the shape and size of the biaxial tensile specimen, and a simulation test was conducted according to the biaxial tensile test conditions.

[0063] Simulation results for S2, biaxial tensile test condition are as follows: Figure 2 As shown, it can be seen that the deformation of the initial biaxial tensile specimen in this embodiment mainly occurs in the non-central region, such as... Figure 2 As shown by the dashed line, it is necessary to enter S3 to adjust the shape and size of the finite element model of the biaxial tensile specimen;

[0064] S3. Adjust the shape and size of the finite element model of the biaxial tensile specimen:

[0065] (1) Increase the length of the four tensile arms and the width of the widest part of the biaxial tensile specimen by 5 to 20% of the side length of the central region, so that the length of the tensile arm = 2 × the side length of the central region and the width of the widest part of the tensile arm = the side length of the central region.

[0066] (2) Grooves are added to the four tension arms respectively. In this embodiment, the number of existing grooves on the tension arms is 0. Three grooves are added during the first round of adjustment, and two more grooves are added during each subsequent adjustment. The number of grooves on each tension arm is the same. The grooves are opened along the length of the tension arm. The spacing between adjacent grooves is equal. The groove located in the middle of the tension arm is coaxial with the center line of the biaxial tensile specimen. Other grooves are symmetrically arranged on both sides of the groove located in the middle of the tension arm.

[0067] (3) Adjust the width of the grooves on each tension arm to ensure that the width of the grooves does not exceed 0.2 mm, so as to reduce the impact of the grooves on the stiffness of the specimen; the width of the grooves on each tension arm is the same.

[0068] (4) Set the groove length to be equal to the width of the widest part of the tension arm to ensure the stability of the force transmission of the sample; the groove length on each tension arm is the same.

[0069] S4. Verify the effectiveness of the biaxial tensile specimen after each round of adjustments:

[0070] Repeat steps S1 and S2 for the biaxial tensile specimen after adjustment S3. If it is determined by S2 that the biaxial tensile specimen does not meet the test requirements, repeat S3 for the next round of adjustment. Repeat the above process until it is determined by S2 that the biaxial tensile specimen is valid and meets the test requirements.

[0071] In this embodiment, the final effective biaxial tensile specimen has 7 grooves on the tensile arm, each groove being 0.1 mm wide and its length equal to the width of the tensile arm. After simulation testing according to the biaxial tensile test conditions, the deformation mainly occurs in the central region. Figure 3As shown, this indicates that the sample meets the test requirements.

[0072] Example 2

[0073] The following describes the design method for improving the effectiveness of biaxial tensile specimens according to the present invention, with reference to specific embodiments, and specifically includes the following steps:

[0074] S1, such as Figure 4 As shown, a finite element model was established based on the shape and size of the biaxial tensile specimen, and a simulation test was conducted according to the biaxial tensile test conditions.

[0075] S2. In this embodiment, the deformation of the initial biaxial tensile specimen mainly occurs in the non-central region. Therefore, it is necessary to proceed to S3 to adjust the shape and size of the finite element model of the biaxial tensile specimen.

[0076] S3. Adjust the shape and size of the finite element model of the biaxial tensile specimen:

[0077] (1) Increase the length of the four tensile arms of the biaxial tensile specimen and the width of the widest part by 10% of the side length of the central region, so that the length of the tensile arm = 2.5 × the side length of the central region and the width of the widest part of the tensile arm = the side length of the central region.

[0078] (2) Grooves are added to each of the four tension arms. In this embodiment, the tension arms already have two grooves. Figure 4 As shown, one groove is added during the first round of adjustment, and two more grooves are added during each subsequent adjustment, ensuring that the number of grooves on each tension arm is the same. The grooves are opened along the length of the tension arm, the spacing between adjacent grooves is equal, and the groove located in the middle of the tension arm is coaxial with the center line of the biaxial tensile specimen. Other grooves are symmetrically arranged on both sides of the groove located in the middle of the tension arm.

[0079] (3) Adjust the width of the grooves opened on each tension arm. In this embodiment, the existing groove width is 1.5 mm. The existing groove width is gradually adjusted by reducing it by 25% each time until the existing groove width does not exceed 0.2 mm, so as to reduce the impact of the grooves on the stiffness of the sample. The groove widths on each tension arm are the same.

[0080] (4) Increase the length of the groove by 50% of the width of the widest part of the tension arm to ensure the stability of the force transmission of the sample; the length of the groove on each tension arm is the same.

[0081] S4. Verify the effectiveness of the biaxial tensile specimen after each round of adjustments:

[0082] Repeat steps S1 and S2 for the biaxial tensile specimen after adjustment S3. If it is determined by S2 that the biaxial tensile specimen does not meet the test requirements, repeat S3 for the next round of adjustment. Repeat the above process until it is determined by S2 that the biaxial tensile specimen is valid and meets the test requirements.

[0083] In this embodiment, the final effective biaxial tensile specimen has nine grooves on the tensile arm, each groove being 0.15 mm wide and twice the widest dimension of the tensile arm. After simulation testing under biaxial tensile test conditions, deformation mainly occurs in the central region. Figure 5 As shown, this indicates that the sample meets the test requirements.

[0084] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and are not intended to limit them. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features therein. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method of designing a biaxially-stretched specimen for improving the effectiveness of the biaxially-stretched specimen, the biaxially-stretched specimen being a cross-shaped specimen including four stretching arms, a square central region being divided at the center of the cross-shaped specimen where the four stretching arms meet; characterized in that, The design method specifically comprises the following steps: S1, a finite element model is established according to the shape and size of the biaxial tensile sample, and a simulation test is carried out according to the biaxial tensile test condition; S2, whether the biaxial tensile sample needs to be adjusted is judged according to the simulation result of the biaxial tensile test condition: If the deformation of the biaxial tensile sample mainly occurs in the central region, it indicates that the biaxial tensile sample is effective and can meet the test requirements; If the deformation of the biaxial tensile sample mainly occurs in the non-central region, it indicates that the biaxial tensile sample cannot meet the test requirements, and S3 is entered; S3, the shape and size of the finite element model of the biaxial tensile sample are adjusted: (1) the length of the four stretching arms of the biaxial tensile sample and the width of the widest part are increased by 5-20% of the side length of the central region; the length of the stretching arm is not less than 150% of the side length of the central region, and the width of the widest part of the stretching arm is not less than the side length of the central region; (2) 1-3 grooves are respectively increased on the four stretching arms, and the number of grooves on each stretching arm is the same and is an odd number, and the groove located at the middle position of the stretching arm is coaxial with the center line of the biaxial tensile sample, and the other grooves are symmetrically arranged on both sides of the groove located at the middle position of the stretching arm; (3) the width of the groove is adjusted to be not more than 0.5 mm; the width of the groove on each stretching arm is the same; (4) the length of the groove is increased by 10-50% of the width of the widest part of the stretching arm; the length of the groove on each stretching arm is the same; S4, the effectiveness of the biaxial tensile sample after each round of adjustment is verified: The biaxial tensile sample after S3 is adjusted is repeated S1 and S2, if the biaxial tensile sample cannot meet the test requirements after S2, the next round of adjustment is repeated S3; the above process is repeated until the biaxial tensile sample is effective and can meet the test requirements after S2.

2. The design method of improving the effectiveness of biaxially-stretched test specimens according to claim 1, characterized in that, In S2, the deformation of the biaxial tensile sample is mainly determined by the finite element analysis cloud map.

3. The design method of improving the effectiveness of biaxially-stretched test specimens according to claim 1, characterized in that, In (2), the grooves are opened along the length direction of the stretching arm, and the distance between adjacent grooves is equal.

4. The design method of improving the effectiveness of biaxially-stretched test specimens according to claim 1, characterized by, In (2), the number of grooves on each stretching arm is not less than 3.

5. The design method of improving the effectiveness of biaxially-stretched test specimens according to claim 1, characterized by, In (2), if the number of existing grooves on the stretching arm is odd, 2 more grooves are added; If the number of existing grooves on the stretching arm is 0, 3 more grooves are added; if the number of existing grooves on the stretching arm is other even number except 0, 1 more groove is added.

6. The design method of improving biaxially-stretched sample effectiveness according to claim 1, characterized by, In (3), if there is already a groove on the stretching arm, the width of the groove is reduced by 10-25% of the width of the existing groove in (3).

7. The design method of improving biaxially-stretched sample effectiveness according to claim 1, characterized by, In (3), if the width of the existing groove on the stretching arm is more than 0.5 mm, the width of the existing groove is gradually adjusted by reducing the width of the existing groove by 10-25% each time until the width of the existing groove is not more than 0.5 mm.

8. The design method of improving biaxially-stretched sample effectiveness according to claim 1, characterized by, In (4), if the number of existing grooves on the stretching arm is 0, the length of the groove to be increased is controlled to be 10-50% of the width of the widest part of the stretching arm.

9. The design method of improving biaxially-stretched sample effectiveness according to claim 1, characterized by, In (4), the length of the groove is not more than 90% of the length of the stretching arm.

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