Nested centering adjustment-free loading device and method for testing shear strength of CFRP material

Abstract

The invention discloses a nested centering adjustment-free loading device and method for testing the shear strength of a CFRP material, and belongs to the technical field of material performance tests.The device comprises an upper clamping mechanism, a lower clamping mechanism and a limiting mechanism, the upper clamping mechanism is fixedly connected with a first fixed mounting mechanism, the lower clamping mechanism is fixedly connected with a second fixed mounting mechanism, and the limiting mechanism is fixedly connected with the lower clamping mechanism; the limiting mechanism is fixedly connected with the lower clamping mechanism, and the upper clamping mechanism is connected with the lower clamping mechanism through the limiting mechanism. By adopting the nested centering adjustment-free loading device and method for testing the shear strength of the CFRP material, the sample can be stably and accurately clamped, the shear force can be uniformly applied, and the damage and slippage of the sample can be effectively avoided, so that the shear strength of the sample can be accurately measured, and the urgent requirements of multiple fields such as scientific research and production on the accurate test of the mechanical property of the CFRP material can be met.

Description

Technical Field

[0001] The present invention relates to the technical field of material property testing, and more particularly to a nested centering and non-adjustable loading device and method for testing the shear strength of CFRP materials. Background Art

[0002] The current standardized test method for in-plane shear strength of carbon fiber reinforced resin matrix composites (CFRP materials) is mainly implemented according to ASTM D7078 specification. This standard uses a mechanical clamping type loading mechanism to characterize the in-plane shear performance of V-notch specimens. The core test principle is as follows: The pretreated specimen is fixed to the testing machine through a double-opposed loading track clamping system. When an axial tensile load is applied, based on the frictional coupling effect at the contact interface between the loading track and the specimen, a shear stress field is constructed.

[0003] Specifically, the ASTM D7078 standard requires the specimen to be machined into a V-notch geometric configuration at a specific angle. This design effectively eliminates the stress concentration phenomenon caused by edge effects through stress field reconstruction. During the loading process, two pairs of surface-hardened steel guide rails constrain both ends of the specimen with a preset clamping force. Through the continuous application of a quasi-static tensile load, the shear friction resistance generated at the guide rail-specimen contact interface forms the in-plane shear principal stress, which is applied on the a-b material plane, causing the shear deformation to occur within the plane of the laminate. The test system calculates key mechanical parameters such as the ultimate shear strength and shear modulus of the material by collecting load-displacement data in real time and combining with the effective shear area of the specimen.

[0004] However, as Figure 9 shown, the existing loading devices and methods have the following problems: (1) The upper and lower fixtures lack necessary constraints, which may cause lateral slippage at the fixture-specimen interface during the loading process, resulting in the composite specimen being subjected to non-designed loads; (2) The original design requires the central axes of the upper and lower fixtures and the three points of the specimen notch to be in a straight line, which has strict centering requirements for assembly and is prone to deviating from the preset load path; (3) The original design uses a clamping design. If the clamping force is too small, slippage may occur between the composite specimen and the fixture; if the clamping force is too large, damage may occur to the composite specimen, resulting in the failure part not being in the designed area. Summary of the Invention

[0005] The purpose of the present invention is to provide a nested centering and non-adjustable loading device and method for testing the shear strength of CFRP materials, which can stably and accurately clamp the specimen, uniformly apply shear force, effectively avoid specimen breakage and slippage, and thus accurately measure its shear strength, meeting the urgent needs of precise testing of the mechanical properties of CFRP materials in various fields such as scientific research and production.

[0006] To achieve the above object, the present invention provides a nested centering and non-adjustable loading device for testing the shear strength of CFRP materials, including an upper clamping mechanism, a lower clamping mechanism, and a limiting mechanism. The upper clamping mechanism is fixedly connected to a first fixed mounting mechanism, the lower clamping mechanism is fixedly connected to a second fixed mounting mechanism, the limiting mechanism is fixedly connected to the lower clamping mechanism, and the upper clamping mechanism is connected to the lower clamping mechanism through the limiting mechanism.

[0007] Preferably, the upper clamping mechanism includes a first pressing piece, an upper fixture, a first positioning gasket, and a second positioning gasket. The upper fixture is in an inverted L shape and is fixedly connected to the first pressing piece by a first bolt. A first specimen mounting groove is provided on the upper fixture, and the first pressing piece, the first positioning gasket, and the second positioning gasket are placed in the first specimen mounting groove. A first universal joint mounting hole is provided in the upper part of the upper fixture, and limiting guide rail mating holes are symmetrically provided on both sides of the first universal joint mounting hole.

[0008] Preferably, the lower clamping mechanism includes a lower fixture, a second pressing piece, a third positioning gasket, and a fourth positioning gasket. The lower fixture is in a regular L shape and is fixedly connected to the second pressing piece by a second bolt. A second specimen mounting groove is provided on the lower fixture, and the second pressing piece, the third positioning gasket, and the fourth positioning gasket are installed in the second specimen mounting groove.

[0009] Preferably, the limiting mechanism includes limiting guide rails, a first shear gasket, and a second shear gasket. The first shear gasket and the second shear gasket are respectively placed in the first specimen mounting groove and the second specimen mounting groove. A first limiting groove is provided in the middle of the first shear gasket, and the first pressing piece passes through the first limiting groove. A second limiting groove is provided in the middle of the second shear gasket, and the second pressing piece passes through the second limiting groove. The number of limiting guide rails is two, and the two limiting guide rails are respectively fixedly installed on two platforms of the lower fixture, and the two limiting guide rails are placed in the two limiting guide rail mating holes and are slidably connected to the upper fixture.

[0010] Preferably, the first fixed mounting mechanism includes a first fixed mounting part, and the first fixed mounting part is fixedly connected to an upper universal joint by an upper universal joint fixing screw, and the upper universal joint is fixedly connected to the upper fixture by a bolt;

[0011] The second fixed mounting mechanism includes a second fixed mounting part, and the second fixed mounting part is fixedly connected to a lower universal joint by a lower universal joint fixing screw, and the lower universal joint is fixedly connected to the lower fixture by a bolt.

[0012] Preferably, two first mounting holes are provided at the bottom of the upper fixture, two second mounting holes and two through holes are provided at the bottom of the lower fixture, two first ejector rods and two second ejector rods penetrate through the lower fixture, the first ejector rod penetrates through the first mounting hole and the through hole and contacts the first shear gasket, and the second ejector rod penetrates through the second mounting hole and contacts the second shear gasket.

[0013] The present invention also provides a nested centering and non-adjustable loading method for testing the shear strength of CFRP materials. Using the above-mentioned nested centering and non-adjustable loading device for testing the shear strength of CFRP materials, it includes the following steps:

[0014] S1. Prepare the specimen;

[0015] S2. Install the specimen into the nested centering and non-adjustable loading device;

[0016] S3. Test the shear strength of the specimen and record and process the data;

[0017] S4. Error analysis.

[0018] Preferably, the specific operation of S1 is:

[0019] S11. Select a representative CFRP material sample. The length of the specimen is 75.5 - 76.5 mm, the thickness is 2 - 5 mm, and the middle width is 55.5 - 56.5 mm. And number the specimen;

[0020] S12. Check each component of the test device to detect whether there are any structural integrity, damage, or looseness;

[0021] S13. Check whether the surface of the double slide rail composed of two limit guide rails is smooth, whether there is any foreign object attachment or scratch. Apply an appropriate amount of high-precision grease on the double slide rail. Check whether the installation position of the limit guide rail is accurate, whether the parallelism and perpendicularity between the double slide rails meet the design requirements, and whether there is any obvious shaking or deviation during the movement process.

[0022] Preferably, the specific operation of S2 is:

[0023] S21. First, insert the limit guide rail on the lower fixture into the two limit guide rail mating holes of the upper fixture. The upper fixture and the lower fixture can only slide up and down on the two limit guide rails, further restricting the relative displacement in the left - right and in - out - of - plane directions, and keeping the upper fixture, the lower fixture, and the specimen vertical;

[0024] S22. Firmly install the lower fixture on the base of the universal testing machine through bolts. The clamping surface of the lower fixture and the surface of the base maintain strict parallelism;

[0025] S23, placing the first positioning gasket and the third positioning gasket in the first sample installation groove and the second sample installation groove respectively, then placing the sample above the first positioning gasket and the third positioning gasket, and placing the second positioning gasket and the fourth positioning gasket in the first sample installation groove and the second sample installation groove respectively, after adjusting the positions of the first positioning gasket, the second positioning gasket, the third positioning gasket and the fourth positioning gasket, placing the first clamping plate and the second clamping plate in the first sample installation groove and the second sample installation groove respectively, and pushing the first push rod and the second push rod upward to drive the first shear gasket and the second shear gasket to move upward until the sample is fixed, and then fixing the first clamping plate and the second clamping plate with the first bolt and the second bolt respectively;

[0026] S24. Check the installation position and clamping status of the sample again. After confirming that they are correct, use a vernier caliper or micrometer to measure the actual clamping length and width of the sample in the fixture and record them. The measurement accuracy should reach ±0.01mm.

[0027] Preferably, the specific operation of S3 is:

[0028] S31, start the universal testing machine and select the displacement control mode, open the data acquisition software, set the acquisition parameters, including acquisition frequency and data storage path, and record the displacement and force data in the test process in real time and accurately;

[0029] S32, start the loading system, and apply shear force to the sample at a slow and stable displacement loading rate according to the preset loading program. The loading rate is set according to the material and size factors of the sample and is controlled at 1-2 mm / min to make the sample evenly stressed during the test;

[0030] S33. During the loading process, closely observe the deformation of the specimen and the operating status of the test device, monitor the displacement changes in real time through the displacement sensor, and at the same time, pay attention to check whether the loading system is working normally, whether the pressure of the hydraulic pump is stable, and whether the loading head moves smoothly. If there is any abnormality, stop loading immediately, check and eliminate the fault;

[0031] S34. As the loading force gradually increases, the sample breaks. At this time, the loading system will automatically detect the sudden drop in force and immediately trigger the loading stop program. At the same time, the data acquisition software records the maximum shear force and displacement data at the moment of sample fracture;

[0032] S35. Export the force-displacement data recorded during the test from the data acquisition software, organize and analyze the data, and draw a curve of shear displacement change.

[0033] Therefore, the present invention adopts the above-mentioned nested centering and load-adjustment-free loading device and method for testing the shear strength of CFRP materials, and has the following beneficial effects:

[0034] (1) The loading device uses a double-rail design to further optimize the stability and accuracy during testing, ensuring the stable fixation and accurate loading of the CFRP material during the test, and effectively avoiding problems such as jamming and specimen offset during testing caused by uneven spacing between the upper fixture and the lower fixture and unevenness of the double rails.

[0035] (2) A "built-in" loading scheme is designed, and the specimen is directly "embedded" in the test part in a highly concentrated and evenly distributed manner, effectively avoiding the problems of force dispersion and uneven force transmission that may occur in the traditional loading process, ensuring that the shearing area of the specimen can accurately bear the preset loading force, thereby greatly improving the acting accuracy and stability of the force during the test.

[0036] (3) The clamping surfaces of the upper fixture and the lower fixture match the shape of the CFRP material, and a plurality of silicone pad protrusions are evenly distributed on the clamping surfaces. These silicone pad protrusions can further enhance the friction force on the specimen and avoid damaging the surface of the specimen; in addition, the specimens of this test method can be installed repeatedly and without damage, avoiding damage to the specimen in the form of clamping and loading.

[0037] (4) The present invention sets a first shear gasket and a second shear gasket in the embedded area, and its characteristics can realize fine adjustment and positioning within the range of ±1 mm of the specimen; a first positioning gasket and a second positioning gasket are also set to ensure automatic centering and positioning of the specimen.

[0038] (5) Four ejector rods are designed at the bottom of the lower fixture, which can further reinforce the specimen, further enhance the friction force on the specimen, and at the same time can adaptively compensate for the tolerance of the specimen width.

[0039] The technical solution of the present invention will be further described in detail below with reference to the drawings and embodiments. Description of the Drawings

[0040] Figure 1 is a schematic structural diagram of the upper clamping mechanism, the lower clamping mechanism and the limiting mechanism of the embodiment of the nested centering and load-adjustment-free loading device and method for testing the shear strength of CFRP materials of the present invention;

[0041] Figure 2 is a schematic structural diagram of the upper clamping mechanism of the embodiment of the nested centering and load-adjustment-free loading device and method for testing the shear strength of CFRP materials of the present invention;

[0042] Figure 3It is a schematic structural diagram of the lower clamping mechanism and the limiting mechanism (the first shear gasket is not drawn) in the embodiment of the nested centering and non-adjustable loading device and method for testing the shear strength of CFRP materials of the present invention;

[0043] Figure 4 It is a partial structural schematic diagram during the test in the embodiment of the nested centering and non-adjustable loading device and method for testing the shear strength of CFRP materials of the present invention;

[0044] Figure 5 It is a bottom view of the embodiment of the nested centering and non-adjustable loading device and method for testing the shear strength of CFRP materials of the present invention;

[0045] Figure 6 It is a schematic structural diagram of the specimen, the first positioning gasket, the second positioning gasket, the third positioning gasket, and the fourth positioning gasket in the embodiment of the nested centering and non-adjustable loading device and method for testing the shear strength of CFRP materials of the present invention;

[0046] Figure 7 It is a shape diagram of the specimen in Embodiment 1 of the nested centering and non-adjustable loading device and method for testing the shear strength of CFRP materials of the present invention;

[0047] Figure 8 It is the CFRP material coordinate system in Embodiment 1 of the nested centering and non-adjustable loading device and method for testing the shear strength of CFRP materials of the present invention;

[0048] Figure 9 It is a schematic structural diagram of the existing loading device in the nested centering and non-adjustable loading device and method for testing the shear strength of CFRP materials of the present invention.

[0049] Reference numerals

[0050] 1. Upper clamp; 2. First pressing piece; 3. First specimen installation groove; 4. First universal joint installation hole; 5. Limiting guide rail mating hole; 6. Lower clamp; 7. Second pressing piece; 8. Second specimen installation groove; 9. Limiting guide rail; 10. First fixed installation part; 11. Upper universal joint; 12. Upper universal joint fixing screw; 13. Second fixed installation part; 14. Lower universal joint; 15. Lower universal joint fixing screw; 16. Specimen; 17. First positioning gasket; 18. Second positioning gasket; 19. Third positioning gasket; 20. Fourth positioning gasket; 21. First shear gasket; 22. Second shear gasket; 23. First installation hole; 24. Second installation hole; 25. Through hole. Detailed implementation manners

[0051] The technical solutions of the present invention will be further described below with reference to the drawings and embodiments.

[0052] Unless otherwise defined, the technical terms or scientific terms used in the present invention shall have the ordinary meanings understood by those of ordinary skill in the field to which the present invention pertains. The "first", "second" and similar terms used in the present invention do not denote any order, quantity or importance, but are only used to distinguish different components. Words such as "comprising" or "including" mean that the elements or objects appearing before this word cover the elements or objects listed after this word and their equivalents, without excluding other elements or objects. Words such as "connected" or "coupled" are not limited to physical or mechanical connections, but may include electrical connections, whether direct or indirect. "Upper", "lower", "left", "right", etc. are only used to indicate relative positional relationships, and when the absolute position of the object being described changes, the relative positional relationship may also change accordingly.

[0053] As Figure 1 , Figure 4 shown, the present invention provides a nested centering and non-adjustable loading device for testing the shear strength of CFRP materials, including an upper clamping mechanism, a lower clamping mechanism and a limiting mechanism. The upper clamping mechanism is fixedly connected to a first fixed mounting mechanism, the lower clamping mechanism is fixedly connected to a second fixed mounting mechanism, the limiting mechanism is fixedly connected to the lower clamping mechanism, and the upper clamping mechanism is connected to the lower clamping mechanism through the limiting mechanism.

[0054] The upper clamping mechanism and the lower clamping mechanism are used to fix the specimen 16, the limiting mechanism is used to limit the upper clamping mechanism and the lower clamping mechanism, and the first fixed mounting mechanism and the second fixed mounting mechanism are used to provide a loading force to the specimen 16.

[0055] As Figure 2 , Figure 6 shown, the upper clamping mechanism includes a first pressing piece 2, an upper clamp 1, a first positioning gasket 17 and a second positioning gasket 18. The upper clamp 1 is in an inverted L shape. The upper clamp 1 is fixedly connected to the first pressing piece 2 through a first bolt. The first pressing piece 2 can firmly hold the specimen 16 on the upper clamp 1, prevent the specimen 16 from falling off during the test, and ensure the smooth progress of the test. A first specimen mounting groove 3 is formed on the upper clamp 1. The first specimen mounting groove 3 is used to place the specimen 16. A first universal joint mounting hole 4 is formed in the upper part of the upper clamp 1. The first universal joint mounting hole 4 is used to mount the upper universal joint 11. The upper universal joint 11 can enable the upper clamp 1 to flexibly adjust the angle within a certain range, compensate for the possible small angular deviation during the loading process, ensure that the loading force can be applied to the specimen 16 more accurately along the preset direction, and improve the test accuracy. The first pressing piece 2, the first positioning gasket 17 and the second positioning gasket 18 are placed in the first specimen mounting groove 3. The first positioning gasket 17 and the second positioning gasket 18 are used to cooperate with the first pressing piece 2 to clamp and fix the specimen 16.

[0056] On both sides of the first universal joint mounting hole 4, limiting guide rail mating holes 5 are symmetrically provided. The limiting guide rail mating holes 5 cooperate with the limiting guide rails 9 in the limiting mechanism to limit the relative displacement between the upper clamp 1 and the lower clamp 6, ensuring that the upper clamp 1 and the lower clamp 6 can only slide in a limited direction, improving the stability and accuracy of the clamp during the test, and preventing the specimen 16 from shifting.

[0057] As Figure 3 , Figure 6 shown, the lower clamping mechanism includes a lower clamp 6, a second pressing piece 7, a third positioning gasket 19, and a fourth positioning gasket 20. The lower clamp 6 is in a positive L shape. A second universal joint mounting hole is provided at the bottom of the lower clamp 6. The lower clamp 6 is fixedly connected to the second pressing piece 7 by bolts. The second pressing piece 7 is used to provide a stable support base for the specimen 16. A second specimen mounting groove 8 is provided on the lower clamp 6. The second specimen mounting groove 8 is used to place the specimen 16. A plurality of silica gel pad protrusions are evenly distributed on the clamping surfaces of the first specimen mounting groove 3 and the second specimen mounting groove 8, which can enhance the friction force on the first positioning gasket 17 and the third positioning gasket 19. The first positioning gasket 17, the second positioning gasket 18, the third positioning gasket 19, and the fourth positioning gasket 20 are all square structures.

[0058] The limiting mechanism includes a limiting guide rail 9, a first shear gasket 21, and a second shear gasket 22. The first shear gasket 21 and the second shear gasket 22 are respectively placed in the first specimen mounting groove 3 and the second specimen mounting groove 8. A first limiting groove is provided in the middle of the first shear gasket 21. The first pressing piece 2 passes through the first limiting groove. A second limiting groove is provided in the middle of the second shear gasket 22. The second pressing piece passes through the second limiting groove. The first shear gasket 21 and the second shear gasket 22 are used to jack up the specimen 16 upward to fix the specimen 16. The widths of the openings of the first limiting groove and the second limiting groove are slightly larger than the widths of the first pressing piece 2 and the second pressing piece 7, enabling them to slide normally. When the first pressing piece presses the first positioning gasket 17, the specimen 16, and the third positioning gasket 19, the first shear gasket 21 is also pressed and cannot move anymore. The same applies to the second shear gasket 22.

[0059] As Figure 5 shown, two first mounting holes 23 are provided at the bottom of the upper clamp 1. Two second mounting holes 24 and two through holes 2524 are provided at the bottom of the lower clamp 6. Two first ejector rods and two second ejector rods penetrate through the lower clamp 6. The first ejector rod penetrates through the first mounting hole 23 and the through hole 2524 and contacts the first shear gasket 21. The second ejector rod penetrates through the second mounting hole 24 and contacts the second shear gasket 22.

[0060] There are two limiting guide rails 9, and the two limiting guide rails 9 are respectively fixedly installed on the two platforms of the lower fixture 6, and the two limiting guide rails 9 are placed in the two limiting guide rail mating holes 5 and are slidably connected with the upper fixture 1. The limiting guide rails 9 are used to limit the left - right and in - out - of - plane relative displacements of the upper fixture 1 and the lower fixture 6, so that the upper fixture 1 and the lower fixture 6 are kept vertical with respect to the specimen 16. Ensure that during the loading process, the upper fixture 1 and the lower fixture 6 always maintain a relatively stable positional relationship, avoiding problems such as jamming and specimen 16 offset caused by uneven spacing between the upper fixture 1 and the lower fixture 6 or uneven double slide rails, and improving the stability and accuracy of the test.

[0061] The first fixed - installation mechanism includes a first fixed - installation part 10. The first fixed - installation part 10 serves as the basic component for connecting the upper clamping mechanism to external equipment, playing the role of fixing and supporting the upper clamping mechanism, and ensuring the stability and reliability of the entire upper clamping mechanism during the test process. The first fixed - installation part 10 is fixedly connected to the upper universal joint 11 through the upper universal - joint fixing screw 12, and the upper universal joint 11 is fixedly connected to the upper fixture 1 through bolts. The upper universal joint 11 enables the upper fixture 1 to rotate within a certain range, can better adapt to different loading conditions, reduce the influence of installation errors or loading - direction deviations on the test results, and ensure that the loading force is accurately applied to the specimen 16.

[0062] The second fixed - installation mechanism includes a second fixed - installation part 13. The second fixed - installation part 13 is the key component for connecting the lower clamping mechanism to external equipment, providing a stable installation foundation for the lower clamping mechanism, and ensuring the fixed position of the lower clamping mechanism during the test process. The second fixed - installation part 13 is fixedly connected to the lower universal joint 14 through the lower universal - joint fixing screw 15, and the lower universal joint 14 is fixedly connected to the lower fixture 6 through bolts. The lower universal joint 14 can adjust the angle of the lower fixture 6 to a certain extent, and cooperate with the upper universal joint 11 to further ensure that the loading force can be accurately applied to the specimen 16, improving the accuracy of the test.

[0063] The present invention also provides a nested centering and non - adjustable loading method for testing the shear strength of CFRP materials. Using the above - mentioned nested centering and non - adjustable loading device for testing the shear strength of CFRP materials, it includes the following steps:

[0064] S1. Prepare the specimen 16, and the specific operation is as follows:

[0065] S11. Select a representative CFRP material sample. The length of the specimen 16 is 75.5 - 76.5 mm, the thickness is 2 - 5 mm, and the middle width is 55.5 - 56.5 mm, and number the specimen 16;

[0066] S12. Check each component of the test device, including the base, fixed bracket, loading system, specimen fixture, measuring system, etc. on the universal testing machine, and detect whether there are any problems such as complete structure, no damage, and looseness;

[0067] S13. Check whether the surfaces of the double slide rails 9 formed by the two limit guide rails are smooth, whether there is any foreign object attachment or scratches, apply an appropriate amount of high-precision lubricating grease on the slide rails, check whether the installation position of the limit guide rail 9 is accurate, whether the parallelism and perpendicularity between the double slide rails meet the design requirements, and whether there is any obvious shaking or deviation during the movement process.

[0068] S2. Install the specimen 16 into the loading device. The specific operation is as follows:

[0069] S21. First, insert the limit guide rail 9 on the lower fixture 6 into the two limit guide rail mating holes 5 of the upper fixture 1. The upper fixture 1 and the lower fixture 6 can only slide up and down on the two limit guide rails 9, further restricting the relative displacement in the left-right and in-plane and out-of-plane directions, and keeping the upper fixture 1, the lower fixture 6 and the specimen 16 vertical.

[0070] S22. Securely install the lower fixture 6 on the base with bolts. The clamping surface of the lower fixture 6 maintains strict parallelism with the base surface.

[0071] S23. Place the first positioning gasket 17 and the third positioning gasket 19 into the first specimen mounting groove 3 and the second specimen mounting groove 8 respectively. Then place the specimen 16 above the first positioning gasket 17 and the third positioning gasket 19, and place the second positioning gasket 18 and the fourth positioning gasket 20 into the first specimen mounting groove 3 and the second specimen mounting groove 8 respectively. After adjusting the positions of the first positioning gasket 17, the second positioning gasket 18, the third positioning gasket 19 and the fourth positioning gasket 20, place the first pressing piece 2 and the second pressing piece 7 into the first specimen mounting groove 3 and the second specimen mounting groove 8 respectively, and push the first ejector rod and the second ejector rod upward to drive the first shear gasket 21 and the second shear gasket 22 to move upward until the specimen 16 is fixed. Then fix the first pressing piece 2 and the second pressing piece 7 with the first bolt and the second bolt respectively.

[0072] S24. Check the installation position and clamping state of the specimen 16 again. After confirming that it is correct, use a vernier caliper or a micrometer to measure the actual clamping length and width of the specimen 16 in the fixture and record them. The measurement accuracy should reach ±0.01 mm.

[0073] S3. Test the shear strength of the specimen 16 and record and process the data. The specific operation is as follows:

[0074] S31. Start the universal testing machine and select the displacement control mode. Open the data acquisition software and set the acquisition parameters, including the acquisition frequency and the data storage path, and record the displacement and force data during the test in real time and accurately.

[0075] S32. Start the loading system and apply a shear force to specimen 16 at a slow and stable displacement loading rate according to a preset loading program. The loading rate is set according to the material and size factors of specimen 16, controlled at 1 - 2 mm / min, and ensure that specimen 16 is uniformly stressed during the test;

[0076] S33. During the loading process, closely observe the deformation of specimen 16 and the operating state of the test device. Real-time monitor the displacement change through a displacement sensor. At the same time, pay attention to checking whether the loading system is working properly, such as whether the pressure of the hydraulic pump is stable and whether the loading head moves smoothly. If any abnormal situation occurs, immediately stop the loading and check and eliminate the faults;

[0077] S34. As the loading force gradually increases, specimen 16 fractures. At this time, the loading system will automatically detect the sudden drop in force and immediately trigger the stop loading program. Meanwhile, the data acquisition software records the maximum shear force and displacement data at the moment when specimen 16 fractures;

[0078] S35. Export the force-displacement data recorded during the test from the data acquisition software, organize and analyze the data, and plot the curve of shear displacement change, that is, the F-ΔL curve. Through the curve, the mechanical behavior and deformation characteristics of specimen 16 during shearing can be visually observed.

[0079] Determine the fracture shear force F of specimen 16 b , that is, the maximum shear force borne by specimen 16 at the moment of fracture. The accuracy of reading the data should reach ±1 N. At the same time, according to the original dimensions of specimen 16 (width w and thickness t, in mm) and the clamping length L (in mm), use the following formula to calculate the shear strength σ of the CFRP material t :

[0080] σ t = F b / (w × t);

[0081] In the formula, the unit of the shear strength σ t is MPa (megapascals).

[0082] Conduct repeated tests on multiple specimens 16 of the same batch (usually not less than 5), and calculate the average shear strength of each specimen 16 respectively.

[0083] S4. Error analysis: The "embedded" loading device is adopted to solve the errors caused by the misalignment between the fixture and the testing machine and the errors caused by the slippage of the specimen 16 during clamping due to excessive or insufficient clamping force. The test needs to be carried out in a dry environment at room temperature to prevent the influence of temperature, humidity, etc. in the test environment on the material properties. At least five groups of data should be measured during the test to eliminate random errors. When processing the data, statistical quantities such as the average value, standard deviation, and coefficient of variation should be calculated for each group of tests, and the discreteness and reliability of the test results should be analyzed to reduce errors.

[0084] Cleaning and maintenance after the test:

[0085] (1) After the test is completed, carefully remove the fragments of the fractured specimen 16 and place them in the designated container to avoid contaminating and damaging the test environment.

[0086] (2) Dip a clean dust-free cloth in an appropriate amount of anhydrous ethanol and clean the specimen fixture, loading head, and other parts of the test device that come into contact with the specimen 16 to remove the residual debris of the specimen 16 to ensure the surface is clean and tidy for the next test.

[0087] (3) Check whether each part of the test device is damaged or worn, especially the surface wear of the double slide rails and the limit guide rail 9. If there is slight wear or scratches, it can be polished and repaired with sandpaper; if the wear is serious, the corresponding parts should be replaced in time to ensure that the performance and accuracy of the test device are not affected.

[0088] (4) Carry out overall protection for the test device, such as covering it with a dust cover to prevent impurities such as dust and moisture from entering the inside of the device and affecting its service life and performance.

[0089] Example 1

[0090] The design of the shear specimen 16 aims at meeting the test accuracy, reducing stress concentration, and improving the test efficiency. The geometric shape of the specimen 16 is carefully designed so that the shear stress is mainly concentrated in the middle area of the notch of the specimen 16, thereby achieving the control of the fracture position and ensuring the reliability of the test results. At the same time, the specimen 16 is easy to process to reduce the processing difficulty.

[0091] Use a parametric modeling tool to design the specimen 16, define parameters such as the notch shape and size, and the parameters are shown in Table 1. The shape of the specimen 16 is as Figure 7 shown.

[0092] Figure 8 is the CFRP material coordinate system, which is established using the Cartesian coordinate system and describes the main material coordinate system using a, b, and c as axes. The a, b, and c axes correspond to the fiber direction, transverse direction, and lamination direction respectively, σ aa ,σbb and σ cc are the principal stresses along the a, b, and c axes; in addition, the load transfer mechanism of the present invention strictly follows the constitutive relationship of the composite material and adopts an in-plane shear loading mode:

[0093] (1) Ensure that more than 85% of the shear stress components act on the main plane of the laminate a-b material;

[0094] (2) Through decoupling in the orthotropic coordinate system, decouple the shear stress component τ ab effectively control it within the range of 60%-75% of the interlaminar shear strength (ILSS);

[0095] (3) The innovative in-plane shear restraint device reduces the interlaminar shear stress τ ac and τ bc components to <5% ILSS, thus avoiding the delamination failure mode.

[0096] Table 1 Basic dimensions of Specimen 16

[0097] Length Width Thickness 76mm 56mm 4.8mm

[0098] Therefore, by adopting the nested centering and non-adjusting loading device and method for testing the shear strength of the above CFRP material, the present invention can stably and accurately clamp the specimen, uniformly apply the shear force, effectively avoid specimen breakage and slippage, and thus accurately measure its shear strength, meeting the urgent needs of accurate testing of the mechanical properties of CFRP materials in various fields such as scientific research and production.

[0099] 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 preferred embodiments, those of ordinary skill in the art should understand that they can still modify or equivalently replace the technical solutions of the present invention, and these modifications or equivalent replacements cannot make the modified technical solutions deviate from the spirit and scope of the technical solutions of the present invention.

Claims

1. A nested centering-free adjustment loading device for CFRP material shear strength testing, characterized in that: It includes an upper clamping mechanism, a lower clamping mechanism and a limiting mechanism. The upper clamping mechanism is fixedly connected to the first fixed mounting mechanism, the lower clamping mechanism is fixedly connected to the second fixed mounting mechanism, the limiting mechanism is fixedly connected to the lower clamping mechanism, and the upper clamping mechanism is connected to the lower clamping mechanism through the limiting mechanism.

2. The nested pair non-adjustable loading device for testing the shear strength of CFRP materials according to claim 1, wherein: The upper clamping mechanism includes a first pressing piece, an upper fixture, a first positioning gasket and a second positioning gasket. The upper fixture is in an inverted L shape. The upper fixture and the first pressing piece are fixedly connected by a first bolt. A first specimen mounting groove is provided on the upper fixture. The first pressing piece, the first positioning gasket and the second positioning gasket are placed in the first specimen mounting groove. A first universal joint mounting hole is provided in the upper part of the upper fixture, and limiting guide rail matching holes are symmetrically provided on both sides of the first universal joint mounting hole.

3. The nested pair non-adjustable loading device for testing the shear strength of CFRP materials according to claim 2, characterized in that: The lower clamping mechanism includes a lower fixture, a second pressing piece, a third positioning gasket and a fourth positioning gasket. The lower fixture is in a regular L shape. The lower fixture and the second pressing piece are fixedly connected by a second bolt. A second specimen mounting groove is provided on the lower fixture. The second pressing piece, the third positioning gasket and the fourth positioning gasket are installed in the second specimen mounting groove.

4. The nested pair non-adjustable loading device for testing the shear strength of CFRP materials according to claim 3, wherein: The limiting mechanism includes limiting guide rails, a first shear gasket and a second shear gasket. The first shear gasket and the second shear gasket are respectively placed in the first specimen mounting groove and the second specimen mounting groove. A first limiting groove is provided in the middle of the first shear gasket, and the first pressing piece passes through the first limiting groove. A second limiting groove is provided in the middle of the second shear gasket, and the second pressing piece passes through the second limiting groove. The number of limiting guide rails is two. The two limiting guide rails are respectively fixedly installed on two platforms of the lower fixture, and the two limiting guide rails are placed in the two limiting guide rail matching holes and are slidably connected to the upper fixture.

5. The nested pair centering-free adjustment loading device for CFRP material shear strength testing according to claim 2, characterized in that: The first fixed mounting mechanism includes a first fixed mounting part. The first fixed mounting part is fixedly connected to an upper universal joint through an upper universal joint fixing screw, and the upper universal joint is fixedly connected to the upper fixture by a bolt; The second fixed mounting mechanism includes a second fixed mounting part. The second fixed mounting part is fixedly connected to a lower universal joint through a lower universal joint fixing screw, and the lower universal joint is fixedly connected to the lower fixture by a bolt.

6. The nested pair non-adjustable loading device for testing the shear strength of CFRP materials according to claim 4, characterized in that: Two first mounting holes are provided at the bottom of the upper fixture. Two second mounting holes and two through holes are provided at the bottom of the lower fixture. Two first ejector rods and two second ejector rods penetrate through the lower fixture. The first ejector rod penetrates through the first mounting hole and the through hole and contacts the first shear gasket, and the second ejector rod penetrates through the second mounting hole and contacts the second shear gasket.

7. A nested alignment-free adjustment loading method for testing the shear strength of CFRP materials, characterized in that: Adopting the nested centering non-adjustable loading device for testing the shear strength of CFRP materials described in any one of claims 1-6, it is characterized in that it includes the following steps: S1. Prepare specimens; S2. Install the specimens into the nested centering non-adjustable loading device; S3. Test the shear strength of the specimens and record and process the data; S4. Error analysis.

8. The nested pair centering and load adjustment-free loading method for testing the shear strength of CFRP materials according to claim 7, characterized in that: The specific operation of S1 is: S11. Select representative CFRP material samples. The length of the specimens is 75.5 - 76.5 mm, the thickness is 2 - 5 mm, and the middle width is 55.5 - 56.5 mm, and number the specimens; S12. Check each component of the test device to detect whether there are any problems such as complete structure, no damage and looseness; S13. Check whether the surface of the double slide rail composed of two limit guide rails is smooth, whether there is any foreign object attachment or scratch. Apply an appropriate amount of high-precision grease on the double slide rail. Check whether the installation position of the limit guide rail is accurate, whether the parallelism and perpendicularity between the double slide rails meet the design requirements, and whether there is any obvious shaking or deviation during the movement process.

9. The nested pair centering-free adjustment loading method for CFRP material shear strength testing according to claim 7, characterized in that: The specific operation of S2 is as follows: S21. First, insert the limit guide rail on the lower fixture into the two limit guide rail mating holes of the upper fixture. The upper fixture and the lower fixture can only slide up and down along the two limit guide rails, further restricting the relative displacement in the left-right and front-back directions, so that the upper fixture, the lower fixture and the specimen are kept vertical. S22. Firmly install the lower fixture on the base of the universal testing machine through bolts. The clamping surface of the lower fixture and the surface of the base should maintain strict parallelism. S23. Place the first positioning gasket and the third positioning gasket in the first specimen mounting groove and the second specimen mounting groove respectively. Then place the specimen above the first positioning gasket and the third positioning gasket, and place the second positioning gasket and the fourth positioning gasket in the first specimen mounting groove and the second specimen mounting groove respectively. After adjusting the positions of the first positioning gasket, the second positioning gasket, the third positioning gasket and the fourth positioning gasket, place the first pressing piece and the second pressing piece in the first specimen mounting groove and the second specimen mounting groove respectively, and push the first ejector rod and the second ejector rod upward to drive the first shear gasket and the second shear gasket to move upward until the specimen is fixed. Then fix the first pressing piece and the second pressing piece with the first bolt and the second bolt respectively. S24. Check the installation position and clamping state of the specimen again. After confirmation, use a vernier caliper or a micrometer to measure the actual clamping length and width of the specimen in the fixture and record them. The measurement accuracy should reach ±0.01 mm.

10. The nested pair centering-free adjustment loading method for CFRP material shear strength testing according to claim 7, characterized in that: The specific operation of S3 is as follows: S31. Start the universal testing machine and select the displacement control mode. Open the data acquisition software and set the acquisition parameters, including the acquisition frequency and the data storage path, to record the displacement and force data during the test in real time and accurately. S32. Start the loading system. According to the preset loading program, apply a shear force to the specimen at a slow and stable displacement loading rate. The loading rate is set according to the material and size factors of the specimen, and is controlled at 1 - 2 mm / min to make the specimen evenly stressed during the test. S33. During the loading process, closely observe the deformation of the specimen and the operation state of the test device. Real-time monitor the displacement change through the displacement sensor. At the same time, pay attention to checking whether the loading system is working properly, whether the pressure of the hydraulic pump is stable, and whether the loading head moves smoothly. If there is any abnormal situation, immediately stop loading and check and eliminate the fault. S34. As the loading force gradually increases, the specimen breaks. At this time, the loading system will automatically detect the sudden drop in force and immediately trigger the stop loading program. At the same time, the data acquisition software records the maximum shear force and displacement data at the moment when the specimen breaks. S35. Export the force-displacement data recorded during the test from the data acquisition software, sort out and analyze the data, and draw a curve of the shear displacement change.

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