Testing device for sandwich structure of composite material and I-type fracture toughness testing method

Abstract

The invention discloses a testing device for a sandwich structure of a composite material and an I-type fracture toughness testing method. The testing device comprises a guide slide rail; a test connection structure fixed relative to the guide slide rail; the guide sliding block is in sliding connection with the guide sliding rail; the sample connecting structure is detachably fixed with the sliding block; loading the connecting structure; provided is a crack detection device. The I-type fracture toughness testing method is suitable for any one of the testing devices for the sandwich structure of the composite material, and comprises the following steps: preparing a sample of the sandwich structure of the composite material; the composite material sandwich structure sample is installed on a testing device, and the composite material sandwich structure testing device is installed on a universal testing machine; adjusting a crack detection device; carrying out initial loading; carrying out secondary loading; recording a load value, a displacement value and an effective crack propagation value; after the test is finished, the chuck of the universal testing machine is loosened, the computer calculates the GIC value of the composite sandwich structure sample, a test result is derived, and a load displacement curve and an R curve are made.

Description

Technical Field

[0001] The invention relates to the technical field of composite sandwich structure detection, in particular to a testing device for composite sandwich structures and a method for testing mode I fracture toughness. Background Art

[0002] Mode I fracture toughness G of composite sandwich structures IC It is one of the indicators used to characterize the interlayer interface bonding ability of composite sandwich structures and is an important parameter for the delamination and extension analysis of composite sandwich structures. The currently popular test method for the mode I fracture toughness of polymer-based composite sandwich structures is the double cantilever beam test (DCB method). However, when the traditional DCB test method is used to test the polymer-based composite sandwich structure of the sandwich structure, due to the asymmetric loading load and the different material properties of the face sheet and the core material, the normal deformation and shear deformation are coupled during the loading process, resulting in a bending moment in the core area. The load direction will gradually deviate from the normal direction of the face-core structure, and shear stress will be generated in the core material. The component of the type II shear fracture energy in the result accounts for too high a proportion. At the same time, the shear stress at the crack tip is likely to cause the crack to extend from the face-core to the inside of the core material, and the value of the mode I fracture toughness of the sandwich structure composite sandwich structure obtained is inaccurate. Therefore, the DCB test is not suitable for the test of the mode I fracture toughness of the face-core interface of the composite sandwich structure.

[0003] In summary, there is currently a lack of effective equipment and methods for testing the mode I fracture toughness of composite sandwich structures. Summary of the invention

[0004] The technical problem to be solved by the present invention is to provide a testing device for a composite sandwich structure and a method for testing the type I fracture toughness, so as to solve one or more technical problems existing in the prior art and at least provide a beneficial choice or create conditions.

[0005] The solution of the present invention to solve its technical problem is:

[0006] A guide rail, wherein the guide rail is provided with a test connection structure;

[0007] A guide slider, wherein the guide slider is slidably connected to the guide rail;

[0008] A sample connection structure, the sample connection structure is used to fix the composite sandwich structure sample, and the sample connection structure is detachably fixed to the guide slider;

[0009] A loading connection structure, wherein the loading connection structure is used to connect a composite sandwich structure specimen and a universal testing machine;

[0010] A crack detection device is used to detect cracks and record the effective length of the cracks.

[0011] Through the above technical scheme, during the test, the prefabricated crack end of the composite sandwich structure specimen is adhered to the loading connection structure, and the non-cracked end is fixed on the specimen connection structure. Since the composite sandwich structure specimen is completely fixed to the specimen connection structure, during the process of applying load to the crack end of the composite sandwich structure specimen, the guide slider will move relative to the guide rail, thereby offsetting a part of the shear force, which can ensure that the relative position of the crack tip and the loading axis of the universal testing machine always remains constant, greatly reducing the shear stress concentration inside the core material, reducing the tendency of the crack to expand into the core material, and increasing the proportion of mode I fracture toughness in the obtained results, thereby improving the accuracy of the value of mode I fracture toughness of the composite sandwich structure.

[0012] As a further improvement of the above technical solution, the loading connection structure is a hinge structure.

[0013] Through the above technical solution, the hinge structure can keep the loading direction perpendicular to the face plate of the composite sandwich structure specimen.

[0014] As a further improvement of the above technical solution, the sample connection structure and the guide sliding block are detachably fixed by screws.

[0015] As a further improvement of the above technical solution, the crack detection device is an extensometer.

[0016] As a further improvement of the above technical solution, the extensometer is a video extensometer.

[0017] Through the above technical solution, the crack detection device is set as a video extensometer, which can greatly improve the detection accuracy of cracks.

[0018] The method for testing the mode I fracture toughness of a composite sandwich structure is applicable to a composite sandwich structure testing device as described in any one of the above, comprising the following steps:

[0019] A composite sandwich structure specimen is prepared, and a crack is prefabricated at the interface between the core material and the face sheet at one end of the composite sandwich structure specimen;

[0020] Installing a composite sandwich structure specimen on a testing device, and installing the composite sandwich structure testing device on a universal testing machine;

[0021] Adjust the crack detection device to ensure that the detection range of the crack detection device is within the effective range of the composite sandwich structure specimen;

[0022] Initial loading: the chuck of the universal testing machine loads the sample at a first loading rate, and when the initial split layer line grows to a first set value, the universal testing machine is unloaded until the displacement reading is zero;

[0023] Secondary loading: the chuck of the universal testing machine loads the sample at a second loading rate. When the effective crack length expands to a second set value, the universal testing machine is unloaded until the displacement reading is zero. The universal testing machine automatically records the load value P, the displacement value δ, and the expansion value a of each effective crack on the composite sandwich structure specimen.

[0024] After the test, the universal testing machine chuck is released, the sample is removed or replaced to continue the test, and the computer calculates the G of the composite sandwich structure sample. IC value, export the test results, and make load-displacement curve and R curve.

[0025] Through the above technical solution, the composite sandwich structure specimen is fixed to the specimen connection structure. When a load is applied to one side of the crack of the composite sandwich structure specimen, the guide slider will move relative to the guide rail, thereby offsetting a part of the shear force, ensuring that the relative position of the crack tip and the loading axis of the testing machine always remains constant, reducing the tendency of the crack to extend into the core material.

[0026] As a further improvement of the above technical solution, in the step of installing the composite sandwich structure specimen on the testing device, the following steps are included:

[0027] Fixing the composite sandwich structure specimen to the specimen connection structure;

[0028] The specimen connection structure is fixedly mounted on the guide slide block;

[0029] The loading connection structure is arranged between the prefabricated crack end of the composite sandwich structure specimen and the chuck of the universal testing machine, so that the chuck of the universal testing machine drives the prefabricated crack end of the composite sandwich structure specimen to move through the loading connection structure, so that the loading direction of the prefabricated crack end of the composite sandwich structure specimen is always perpendicular to the panel;

[0030] Fix the guide rail to the universal testing machine.

[0031] As a further improvement of the above technical solution, the step of preparing the composite sandwich structure specimen further includes the following steps: spraying white paint evenly on the side surface of the composite sandwich structure specimen along the crack extension surface.

[0032] The above technical solution can facilitate the projection of the electronic probe of the CCD video extensometer.

[0033] As a further improvement of the above technical solution, in the step of preparing the composite sandwich structure specimen, the dimensions of the prepared composite sandwich structure specimen are as follows: 260 mm in length, 30.67 mm in width, and 13.22 mm in thickness; the length of the prefabricated crack at the interface between the core material and the panel at one end of the composite sandwich structure specimen is a0=60 mm, and only one prefabricated crack is made on each composite sandwich structure specimen.

[0034] As a further improvement of the above technical solution, the first loading rate and the second loading rate are both 1-5 mm / min, the first set value is 3-5 mm, and the second set value is not less than 30 mm.

[0035] The beneficial effect of the present invention is to improve the accuracy of the value of the mode I fracture toughness of the composite sandwich structure.

[0036] The invention is used in the technical field of composite sandwich structure detection. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] In order to more clearly illustrate the technical solutions in the embodiments of the present invention, the following briefly describes the drawings required for use in the description of the embodiments. Obviously, the drawings described are only part of the embodiments of the present invention, not all of the embodiments, and those skilled in the art can also obtain other design solutions and drawings based on these drawings without creative work.

[0038] Figure 1 is a schematic diagram of the overall structure of an embodiment of the present invention;

[0039] Figure 2 is a schematic cross-sectional structural diagram of an embodiment of the present invention;

[0040] Figure 3 is a detection flow chart of an embodiment of the present invention;

[0041] Figure 4 It is a flow chart of installing a composite sandwich structure specimen and a composite sandwich structure testing device according to an embodiment of the present invention;

[0042] Figure 5 is a load displacement curve diagram of an embodiment of the present invention;

[0043] Figure 6 is an R curve diagram of an embodiment of the present invention.

[0044] In the figure, 100, loading connection structure; 200, sample connection structure; 300, guide slider; 400, guide rail; 500, fixture base; 600, locking ring; 700, crack detection device; 800, composite sandwich structure sample; 801, core material; 802, panel; 810, prefabricated crack; 900, rail assembly. DETAILED DESCRIPTION

[0045] The following will clearly and completely describe the concept, specific structure and technical effects of the present invention in combination with the embodiments and drawings, so as to fully understand the purpose, characteristics and effects of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of them. Based on the embodiments of the present invention, other embodiments obtained by technicians in this field without creative work are all within the scope of protection of the present invention. In addition, all the connection / connection relationships mentioned in the text do not refer to the direct connection of components, but refer to the formation of a better connection structure by adding or reducing connection accessories according to the specific implementation situation. The various technical features in the invention can be combined interchangeably without conflicting with each other.

[0046] Conventional polymer composite sandwich structures can be accurately measured using the double cantilever beam test in ASTM D5528 to obtain the Mode I fracture toughness G. IC . However, when using the traditional DCB method to test the polymer-based composite sandwich structure of the sandwich structure, due to the asymmetric loading load and the different material properties of the face sheet 802 and the core material 801, the normal deformation and shear deformation are coupled during the loading process, resulting in a bending moment in the core area. The load direction will gradually deviate from the normal direction of the face-core structure, and shear stress will be generated in the core material 801. As a result, the component of the type II shear fracture energy accounts for too high a proportion. At the same time, the shear stress at the crack tip is likely to cause the crack to extend from the face-core to the inside of the core material 801. In addition, the traditional DCB method uses a manual visual point-taking method to read the crack length, which is inefficient and produces large errors in reading. The above problems result in the final obtained type I fracture toughness G of the sandwich structure composite sandwich structure. IC The value is inaccurate.

[0047] In view of the above problems, the present invention provides a set of testing equipment suitable for mode I fracture toughness of the face-core interface of a composite sandwich structure and a testing method for mode I fracture toughness of a composite sandwich structure.

[0048] Reference Figure 1 and Figure 2 The composite sandwich structure testing device includes: a loading connection structure 100, a composite sandwich structure specimen 800 connection structure 200, a slide rail assembly 900 (the slide rail assembly 900 includes a guide slider 300 and a guide rail 400), a test connection structure, a crack detection device 700 and other components.

[0049] Specifically, the test connection structure is fixedly installed at the lower end of the guide rail 400, and the test connection structure includes a fixture base 500 and a locking ring 600. The fixture base 500 is used to connect with the sleeve base of the universal testing machine, and the fixture base 500 and the sleeve base of the universal testing machine are locked by a latch and a locking ring 600 to ensure that the guide rail 400 and other components of the test device will not shake. Specifically, in this embodiment, the guide rail 400 and the fixture base 500 are connected by screws.

[0050] The guide slider 300 is installed on the guide rail 400 , and the guide slider 300 can slide relative to the guide rail 400 along the extension direction of the guide rail 400 .

[0051] The basic dynamic rated load of the guide slider 300 and the guide rail 400 should be greater than the maximum breaking load of the test material itself.

[0052] The connection structure 200 of the composite sandwich structure specimen 800 is a sheet-like component, and the connection structure 200 of the composite sandwich structure specimen 800 is detachably connected to the guide slider 300 by screws. By setting the connection structure 200 of the composite sandwich structure specimen 800 as a detachable connection structure with the guide slider 300, the composite sandwich structure specimen 800 can be quickly installed on the guide slider 300 or removed from the guide slider 300, thereby realizing the rapid installation and removal of the composite sandwich structure specimen 800, and then realizing the rapid replacement of the composite sandwich structure specimen 800, which is conducive to the rapid conduct of the test.

[0053] Specifically, the loading connection structure 100 is configured as a hinge structure, which is a hinge-like structure, and one end of the hinge structure is adhesively connected to the end surface of the composite sandwich structure specimen 800 to achieve fixation of the hinge structure and the end surface of the composite sandwich structure specimen 800.

[0054] Specifically, the crack detection device 700 is configured as a CCD video extensometer, and the crack detection device 700 is fixed to the side of the universal testing machine using a separate tripod, and the detection end of the crack detection device 700 faces the composite sandwich structure specimen 800. The crack detection device 700 projects an electronic probe to the side of the composite sandwich structure specimen 800 through software, and can automatically detect the effective crack extension length, thereby improving test efficiency and reducing errors.

[0055] During the test, the prefabricated crack end of the composite sandwich structure specimen 800 is pasted to the loading connection structure 100, and the non-cracked end is fixed to the composite sandwich structure specimen 800 connection structure 200. Since the composite sandwich structure specimen 800 is completely fixed to the composite sandwich structure specimen 800 connection structure 200, during the process of applying load to the crack end of the composite sandwich structure specimen 800, the guide slider 300 will move relative to the guide rail 400, thereby offsetting a part of the shear force, which can ensure that the relative position of the crack tip and the loading axis of the universal testing machine is always constant, greatly reducing the shear stress concentration inside the core material 801, and reducing the tendency of the crack to expand inside the core material 801. The proportion of type I fracture toughness in the obtained results increases, up to 95%. After the test, the type I fracture toughness of the composite sandwich structure face-core interface can be calculated by the calculation formula in the ASTMD5528 standard.

[0056] Reference Figure 3 The test method for mode I fracture toughness of composite sandwich structures includes the following steps:

[0057] S100: preparing a composite sandwich structure specimen 800, and prefabricating a crack 810 at an interface between a core material 801 and a face plate 802 at one end of the composite sandwich structure specimen 800;

[0058] S200: installing the composite sandwich structure specimen 800 on a testing device, and installing the composite sandwich structure testing device on a universal testing machine;

[0059] S300: adjusting the crack detection device 700 to ensure that the detection range of the crack detection device 700 is within the effective range of the composite sandwich structure specimen 800;

[0060] S400: Initial loading, the chuck of the universal testing machine loads the sample at a first loading rate. When the initial split layer line grows to a first set value, the universal testing machine is unloaded until the displacement reading is zero;

[0061] S500: Secondary loading, the chuck of the universal testing machine loads the sample at a second loading rate, and when the effective crack length expands to a second set value, the universal testing machine is unloaded until the displacement reading is zero; the universal testing machine automatically records the load value P and the displacement value δ, and the crack detection device 700 automatically records the expansion value a of each effective crack on the composite sandwich structure specimen 800;

[0062] S600: After the test, the universal testing machine chuck is released, the sample is removed or replaced to continue the test, and the computer calculates the G of the composite sandwich structure sample 800 IC value, export the test results, and make load-displacement curve and R curve.

[0063] Reference Figure 3 and Figure 4 In step S200, the following steps are included:

[0064] S210: fixing the composite sandwich structure specimen 800 to the composite sandwich structure specimen 800 connection structure 200;

[0065] S220: The composite sandwich structure sample 800 is connected to the structure 200 and fixedly mounted on the guide slider 300;

[0066] S230: The loading connection structure 100 is disposed between the prefabricated crack end of the composite sandwich structure specimen 800 and the chuck of the universal testing machine, so that the chuck of the universal testing machine drives the prefabricated crack end of the composite sandwich structure specimen 800 to move through the loading connection structure 100, so that the loading direction of the prefabricated crack end of the composite sandwich structure specimen 800 is always perpendicular to the panel 802;

[0067] S240: Fix the guide rail 400 to the universal testing machine.

[0068] Specifically, the detailed steps of the test method for mode I fracture toughness of composite sandwich structures are:

[0069] The sample is prepared according to the size of the composite sandwich structure specimen 800 recommended in the ASTM D5528 standard. A crack 810 is prefabricated at the interface between the core material 801 and the panel 802 at one end of the composite sandwich structure specimen 800. The length of the prefabricated crack 810 is a0=60 mm. Only one prefabricated crack 810 is made on each composite sandwich structure specimen 800. The average width B of each composite sandwich structure specimen 800 is measured and recorded. White water-based paint is evenly sprayed on the side of the composite sandwich structure specimen 800 along the crack extension surface to facilitate the projection of the electronic probe by the crack detection device 700.

[0070] The loading connection structure 100 is pasted on the upper end of the prefabricated crack end of the composite sandwich structure specimen 800, and the lower end is pasted on the connection structure 200 of the composite sandwich structure specimen 800. The connection structure 200 of the composite sandwich structure specimen 800 is fixed on the guide slider 300 by a cross screw. The guide slider 300 is moved so that the chuck of the universal testing machine clamps the loading connection structure 100, and the loading direction is perpendicular to the panel 802 of the composite sandwich structure specimen 800.

[0071] Install the fixture base 500 to the sleeve base of the universal testing machine, insert the latch, and tighten the locking ring 600 to fix the fixture base 500 and the sleeve base of the universal testing machine relatively to ensure that the entire fixture does not shake;

[0072] Adjust the position of the crack detection device 700 to ensure that the projected electronic probe is on the crack propagation path, the head and tail of the electronic probe are within the effective range of the composite sandwich structure specimen 800, and set the probe sensitivity;

[0073] Initial loading: Set the universal testing machine in displacement control mode, and load the sample with the chuck at a constant rate of 1-5 mm / min. When the initial delamination crack grows to between 3 and 5 mm, the universal testing machine stops loading and unloads at a rate higher than 25 mm / min until the displacement reading is zero. The purpose of this step is to eliminate the resin-rich area at the tip of the prefabricated crack 810.

[0074] Secondary loading: Set the universal testing machine in displacement control mode, and load the sample with the chuck at a constant rate of 1-5mm / min. When the effective crack length expands to at least 30mm, the universal testing machine stops loading and unloads at a rate higher than 25mm / min until the displacement reading is zero. The computer of the universal testing machine automatically records the chuck load value P, displacement value δ, and the extension value a of each effective crack on the composite sandwich structure specimen 800;

[0075] After the test, release the universal testing machine's chuck, remove or replace the sample and continue the test. IC The calculation formula for calculating the G of composite sandwich structure specimen 800 is IC value, export the test results, and make load-displacement curve and R curve.

[0076] The present invention has the following advantages:

[0077] The traditional DCB method uses manual visual point selection, which is inefficient and produces large errors in reading. The present invention fixes the CCD video extensometer through a tripod and projects an electronic probe to the side of the composite sandwich structure sample 800 through software, which can automatically read the effective crack extension length and automatically calculate G IC , reducing human errors.

[0078] At the same time, the connecting structure 200 of the composite sandwich structure specimen 800 and the guide slider 300, the guide rail 400 and the fixture base 500 are all connected by screws. The guide slider 300, the guide rail 400 and the connecting structure 200 of the composite sandwich structure specimen 800 can be quickly replaced according to actual conditions to adapt to composite sandwich structure specimens 800 of various sizes, thereby improving the test efficiency.

[0079] The device and method of the present invention can effectively prevent the test angle of the composite sandwich structure specimen 800 from changing during the test process.

[0080] Due to the asymmetric loading load and the different material properties of the face sheet 802 and the core material 801, the normal deformation and shear deformation are coupled during the loading process, resulting in a change in the test angle of the composite sandwich structure specimen 800, and the load direction gradually deviates from the normal direction of the face-core structure, causing the crack to expand into the core material 801, and the result is inaccurate. In the present invention, since the composite sandwich structure specimen 800 is fixed to the composite sandwich structure specimen 800 connection structure, when a load is applied to one side of the crack of the composite sandwich structure specimen 800, the guide slider 300 will move relative to the guide rail 400, thereby offsetting a part of the shear force, ensuring that the relative position of the crack tip and the loading axis of the test machine is always kept constant, reducing the tendency of the crack to expand into the core material 801.

[0081] The device and method of the present invention can significantly increase the proportion of mode I fracture toughness in the results.

[0082] In the traditional DCB method, the double material interface of the face-core contact will lead to the coupling of normal deformation and shear deformation, and shear stress will be generated in the core material 801. As a result, the component of the type II shear fracture energy is too high. At the same time, the shear stress at the crack tip is likely to cause the crack to extend from the face-core to the inside of the core material 801. The obtained type I fracture toughness G of the sandwich structure composite sandwich structure is IC The slide rail assembly in the present invention moves to offset a portion of the shear force, greatly reducing the shear stress concentration inside the core material 801 and the tendency of cracks to extend inside the core material 801. The obtained results show that the proportion of type I fracture toughness increases to as high as 95%.

[0083] The preferred embodiments of the present invention are specifically described above, but the invention is not limited to the embodiments. Those skilled in the art may make various equivalent modifications or substitutions without violating the spirit of the invention. These equivalent modifications or substitutions are all included in the scope defined by the claims of this application.

Claims

1. Composite sandwich structure testing device, characterized by: include: A guide rail, wherein the guide rail is provided with a test connection structure; A guide slider, wherein the guide slider is slidably connected to the guide rail; A sample connection structure, the sample connection structure is used to fix the composite sandwich structure sample, and the sample connection structure is detachably fixed to the guide slider; A loading connection structure, wherein the loading connection structure is used to connect a composite sandwich structure specimen and a universal testing machine; A crack detection device is used to detect cracks and record the effective length of the cracks.

2. The composite sandwich structure testing device according to claim 1, characterized in that: The loading connection structure is a hinge structure.

3. The composite sandwich structure testing device according to claim 1, characterized in that: The sample connection structure and the guide slide block are detachably fixed by screws.

4. The composite sandwich structure testing device according to claim 1, characterized in that: The crack detection device is an extensometer.

5. The composite sandwich structure testing device according to claim 4, characterized in that: The extensometer is a video extensometer.

6. A method for testing the mode I fracture toughness of a composite sandwich structure, applicable to a composite sandwich structure testing device as claimed in any one of claims 1 to 5, characterized in that: The following steps are involved: A composite sandwich structure specimen is prepared, and a crack is prefabricated at the interface between the core material and the face sheet at one end of the composite sandwich structure specimen; Installing a composite sandwich structure specimen on a testing device, and installing the composite sandwich structure testing device on a universal testing machine; Adjust the crack detection device to ensure that the detection range of the crack detection device is within the effective range of the composite sandwich structure specimen; Initial loading: the chuck of the universal testing machine loads the sample at a first loading rate, and when the initial split layer line grows to a first set value, the universal testing machine is unloaded until the displacement reading is zero; Secondary loading: the chuck of the universal testing machine loads the sample at a second loading rate. When the effective crack length expands to a second set value, the universal testing machine is unloaded until the displacement reading is zero. The universal testing machine automatically records the load value P, the displacement value δ, and the expansion value a of each effective crack on the composite sandwich structure specimen. After the test, the universal testing machine chuck is released, the sample is removed or replaced to continue the test, and the computer calculates the G of the composite sandwich structure sample. IC value, export the test results, and make load-displacement curve and R curve.

7. The method for testing mode I fracture toughness of a composite sandwich structure according to claim 6, characterized in that: The step of installing the composite sandwich structure specimen on the testing device includes the following steps: Fixing the composite sandwich structure specimen to the specimen connection structure; The specimen connection structure is fixedly mounted on the guide slide block; The loading connection structure is arranged between the prefabricated crack end of the composite sandwich structure specimen and the chuck of the universal testing machine, so that the chuck of the universal testing machine drives the prefabricated crack end of the composite sandwich structure specimen to move through the loading connection structure, so that the loading direction of the prefabricated crack end of the composite sandwich structure specimen is always perpendicular to the panel; Fix the guide rail to the universal testing machine.

8. The method for testing mode I fracture toughness of composite sandwich structures according to claim 6, characterized in that: The step of preparing the composite sandwich structure sample also includes the following steps: spraying white paint evenly on the side surface of the composite sandwich structure sample along the crack extension surface.

9. The method for testing mode I fracture toughness of composite sandwich structures according to claim 6, characterized in that: In the step of preparing the composite sandwich structure specimen, the dimensions of the prepared composite sandwich structure specimen are as follows: 260 mm in length, 30.67 mm in width, and 13.22 mm in thickness; the length a0 of the prefabricated crack at the interface between the core material and the panel at one end of the composite sandwich structure specimen is 60 mm, and only one prefabricated crack is made on each composite sandwich structure specimen.

10. The method for testing mode I fracture toughness of composite sandwich structures according to claim 6, characterized in that: The first loading rate and the second loading rate are both 1-5 mm / min, the first setting value is 3-5 mm, and the second setting value is not less than 30 mm.

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