A composite biaxial compression testing device and method

Abstract

This invention discloses a biaxial compression testing apparatus and method for composite materials. The apparatus includes an X-direction compression section and a Y-direction compression section. The X-direction compression section includes a first upper pressure plate, a first lower pressure plate, a first anti-bending cover plate, and a first fastening bolt. The first anti-bending cover plate presses against the first upper and lower pressure plates and has a gap with the first upper pressure plate. The X-direction compression section employs a staggered asymmetrical structure to cover the entire compression working section of the test specimen in the X direction. The Y-direction compression section includes a second upper pressure plate, a second lower pressure plate, a second anti-bending cover plate, a third anti-bending cover plate, and a second fastening bolt. The second and third anti-bending cover plates respectively press against the first upper and lower pressure plates of the X-direction compression section, providing out-of-plane support for the test specimen. This invention solves the problems of easy buckling and insufficient constraint in the working section of the specimen in existing biaxial compression tests, achieving uniform introduction of biaxial compression load and reliable anti-instability, thus improving the test success rate and data accuracy.

Description

Technical Field

[0001] This invention relates to the field of composite material mechanical property testing technology, specifically to a biaxial compression testing device and method for composite materials. Background Technology

[0002] Composite materials, with their high specific strength, high specific modulus, designability, and excellent fatigue and corrosion resistance, have become core structural materials in aerospace, high-end equipment, and other fields. In actual engineering structures, composite material components commonly bear complex stress states such as biaxial in-plane loads, shear, and bending. For example, fuselage frames, beams, and panels are frequently subjected to simultaneous compression in two in-plane directions under the combined action of aerodynamic loads, inertial loads, and pressurization loads. This biaxial compressive stress state significantly alters the damage initiation, propagation modes, and final failure modes of composite materials, showing a marked difference from the mechanical response under uniaxial loading. Therefore, relying solely on traditional uniaxial mechanical property data cannot accurately reflect the load-bearing characteristics and failure mechanisms of composite materials in actual service.

[0003] Currently, there is no unified international standard for biaxial compression testing of composite materials (ASTM / GB), leading to issues such as inconsistent specimen size and configuration design, and inconsistent buckling prevention methods. Existing technologies mostly apply biaxial compressive loads through large action cylinders, with designs biased towards load delivery rather than material testing itself, making them unsuitable for characterizing biaxial mechanical properties. Japanese Patent JP2019219235A clamps and constrains the specimen near its center, adding devices around the perimeter for compression. However, the constraint in the main compression section is insufficient (constraint only exists in the R-zone), leaving a free compression zone of several centimeters in the central compression section. This zone is highly susceptible to buckling under load, making it impossible to obtain effective failure modes and accurate test results. Furthermore, this device has a complex structure with dozens of parts, requiring numerous screws for assembly. Parallelism and alignment are difficult to guarantee through machining, and the device is prone to wear during use, resulting in high testing costs, low efficiency, and significant specimen waste.

[0004] Therefore, there is an urgent need for a biaxial compression test scheme for composite materials that can uniformly introduce biaxial compressive loads, provide sufficient out-of-plane constraints, prevent premature instability of the specimen, and has a simple and reliable structure. Summary of the Invention

[0005] The purpose of this invention is to address the problems of insufficient constraint of the working section of the specimen, easy buckling, complex structure, and unreliable test results in existing biaxial compression testing devices for composite materials, and to provide a biaxial compression testing device and method for composite materials that can achieve uniform compression in the X and Y directions, effectively prevent instability, and avoid excessive constraint.

[0006] To achieve the above objectives, the present invention adopts the following technical solution:

[0007] A biaxial compression testing device for composite materials includes an X-direction compression section and a Y-direction compression section; The X-direction compression section includes: a first upper pressure plate, a first lower pressure plate, a first anti-bending cover plate, and a first fastening bolt; the first upper pressure plate and the first lower pressure plate are for direct contact with the test piece, and the first anti-bending cover plate is pressed against the first upper pressure plate and the first lower pressure plate by the first fastening bolt, and a gap is provided between the first anti-bending cover plate and the first upper pressure plate; the X-direction compression section adopts a staggered asymmetrical structure to cover the entire compression working section of the test piece in the X direction; The Y-direction compression section includes: a second upper pressure plate, a second lower pressure plate, a second anti-bending cover plate, a third anti-bending cover plate, and a second fastening bolt; the second upper pressure plate and the second lower pressure plate of the Y-direction compression section are used to directly contact the test piece, and the second anti-bending cover plate and the third anti-bending cover plate are respectively pressed against the first upper pressure plate and the first lower pressure plate of the X-direction compression section by the second fastening bolt, providing out-of-plane support for the test piece.

[0008] Furthermore, the inner surfaces of the first upper pressure plate, the first lower pressure plate, the second upper pressure plate, and the second lower pressure plate that contact the test piece are provided with a mesh pattern.

[0009] Furthermore, the gap between the first anti-bending cover plate and the first upper pressure plate is less than 0.05 mm.

[0010] Furthermore, the first and second fastening bolts are M10 bolts, which are tightened in multiple stages using a torque wrench, with a tightening torque of 7 N·m to 15 N·m.

[0011] Furthermore, the test device reserves a 4mm compression allowance in the compression direction of the test piece.

[0012] Furthermore, the parallelism of the first upper pressure plate, the first lower pressure plate, and the first anti-bending cover plate of the X-direction compression section after assembly is no greater than 0.05 mm.

[0013] Furthermore, after installation, the second and third anti-bending cover plates of the Y-direction compression section form a combined loading surface with the first upper pressure plate and the first lower pressure plate of the X-direction compression section.

[0014] The present invention also provides a method for biaxial compression testing of composite materials, which is implemented using the above-mentioned apparatus and includes the following steps: S1: Install the X-direction compression clamp, assemble the first upper pressure plate, the first lower pressure plate, and the first anti-bending cover plate in the X direction, and press the first anti-bending cover plate onto the first upper pressure plate and the first lower pressure plate using the first fastening bolt. Adjust the gap between the first anti-bending cover plate and the first upper pressure plate to less than 0.05mm, and adjust the parallelism between the first upper pressure plate and the first lower pressure plate to not exceed 0.05mm. Insert the test piece into the clamping area on one side of the first upper pressure plate and the first lower pressure plate, and pre-tighten the first fastening bolt. After rotating 180°, insert it into the clamping area on the other side, and pre-tighten the first fastening bolt. Use a torque wrench to tighten the first fastening bolt evenly in multiple stages, with a tightening torque of 7N·m to 15N·m. S2: Install the Y-direction compression clamp, and place the assembly of the already installed X-direction compression clamp between the second upper pressure plate and the second lower pressure plate; use the second fastening bolts to press the second anti-bending cover plate and the third anti-bending cover plate onto the first upper pressure plate and the first lower pressure plate of the X-direction compression part respectively, and pre-tighten the second fastening bolts; after rotating 180°, install the second anti-bending cover plate and the third anti-bending cover plate on the other side, and pre-tighten the second fastening bolts; use a torque wrench to tighten the second fastening bolts in multiple stages, with a tightening torque of 7 N·m to 15 N·m; S3: Place the entire fixture on the compression platform and adjust the alignment of the vertical and horizontal axes of the entire fixture. The alignment error should not exceed 0.1mm. S4: Apply compressive load to the test piece at a preset rate and continuously collect load and displacement data; stop the test when the load drops by 30% from the maximum peak value and record the maximum failure load and failure mode.

[0015] Furthermore, the preset speed is 1 mm / min to 2 mm / min.

[0016] Furthermore, in step S1, the bolts are tightened 3 to 5 times, with the same increment each time.

[0017] Compared with the prior art, the present invention has at least the following beneficial effects: 1. The test device of the present invention can adjust the clamping force according to the sample thickness, is applicable to different layup schemes and thickness ranges, allows certain machining tolerances, and has strong versatility.

[0018] 2. The test device of the present invention adopts a staggered asymmetric design in the X direction to achieve full coverage of the compression working section, effectively preventing sample instability and ensuring uniform compression.

[0019] 3. The test device of the present invention uses the second and third anti-bending cover plates in the Y direction to constrain the pressure plate in the X direction, thereby indirectly providing out-of-plane support for the sample and solving the problem that the symmetrical structure in the Y direction cannot directly cover the working section.

[0020] 4. The present invention reserves a 4mm compression allowance. Extensive testing has verified that this ensures that the sample is fully damaged while avoiding stress concentration and unreasonable damage due to excessive allowance.

[0021] 5. The out-of-plane support of the test device of the present invention is moderate, which will not excessively restrict the effective deformation of the sample and ensure the accuracy of the test results; moreover, the device has a simple structure, controllable installation accuracy, high testing efficiency, and reduces sample waste. Attached Figure Description

[0022] To more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0023] Figure 1 This is a schematic diagram of the compression portion in the X direction in an embodiment of the present invention; Figure 2 This is a schematic diagram of the X-direction compression section after the anti-bending cover plate has been removed in an embodiment of the present invention; Figure 3 This is a front view of the X-direction compression portion in an embodiment of the present invention; Figure 4 This is a schematic diagram of the compression portion in the Y direction in an embodiment of the present invention; Figure 5 This is a schematic diagram of the Y-direction compression portion after the anti-bending cover plate has been removed in an embodiment of the present invention; Figure 6 This is a front view of the Y-direction compression portion in an embodiment of the present invention; Figure 7 This is a front view of the biaxial compression testing apparatus for composite materials according to an embodiment of the present invention (with the cover plate removed). Figure 8 This is a front view (including the cover plate) of the composite material biaxial compression testing device according to an embodiment of the present invention.

[0024] In the figure: 1-First upper pressure plate; 2-First lower pressure plate; 3-First anti-bending cover plate; 4-First fastening bolt; 5-Test piece; 6-Second upper pressure plate; 7-Second lower pressure plate; 8-Second anti-bending cover plate; 9-Third anti-bending cover plate; 10-Second fastening bolt. Detailed Implementation

[0025] The embodiments of the present invention will now be described in detail with reference to the accompanying drawings.

[0026] The following specific examples illustrate the implementation of the present invention. Those skilled in the art can easily understand other advantages and effects of the present invention from the content disclosed in this specification. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of them. The present invention can also be implemented or applied through other different specific embodiments, and the details in this specification can also be modified or changed based on different viewpoints and applications without departing from the spirit of the present invention. It should be noted that, in the absence of conflict, the following embodiments and features in the embodiments can be combined with each other. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are within the scope of protection of the present invention.

[0027] It should be noted that, unless otherwise defined, the technical or scientific terms used in this invention should have the ordinary meaning understood by one of ordinary skill in the art to which this invention pertains. The terms "first," "second," and similar words used in this invention do not indicate any order, quantity, or importance, but are merely used to distinguish different components. Words such as "comprising" or "including" mean that the element or object preceding the word encompasses the elements or objects listed following the word and their equivalents, without excluding other elements or objects.

[0028] Example 1 like Figures 1 to 8 As shown, this embodiment provides a biaxial compression testing device for composite materials, including a compression section in the X direction ( Figures 1-3 ) and the compression part in the Y direction ( Figures 4-6 ).

[0029] The X-direction compression section includes: a first upper pressure plate 1, a first lower pressure plate 2, a first anti-bending cover plate 3, and a first fastening bolt 4. The first upper pressure plate 1 and the first lower pressure plate 2 are for direct contact with the test specimen 5. Their contact surfaces are textured, for example, with a texture specification of m0.4 (GB / T 6403.3-2008) to increase friction and prevent slippage of the test specimen during compression. The first anti-bending cover plate 3 presses against the first upper pressure plate 1 and the first lower pressure plate 2 via the first fastening bolt 4. A gap of less than 0.05 mm is provided between the first anti-bending cover plate 3 and the first upper pressure plate 1. This gap is checked using a feeler gauge during actual installation; if it is not satisfied, a metal shim is added under the first anti-bending cover plate 3 for adjustment. This gap setting ensures that the first anti-bending cover plate 3 can effectively limit the out-of-plane bending of the first upper pressure plate 1, while avoiding additional friction caused by direct contact between the cover plate and the pressure plate, thus preventing excessive constraint on the axial deformation of the specimen. The first fastening bolt 4 is an M10 bolt, which is tightened evenly in multiple (preferably 3 to 5 times) using a torque wrench. The tightening torque is preferably 7 N·m to 15 N·m. This process ensures the accuracy and repeatability of the clamping force.

[0030] The X-direction compression section adopts a staggered asymmetric structure, that is, the clamping end faces of the first upper pressure plate 1 and the first lower pressure plate 2 are staggered in the compression direction (vertical direction), so that the clamping areas of the two overlap on the horizontal projection plane and jointly cover the entire compression working section of the test piece in the X direction, avoiding the appearance of free compression area, effectively preventing out-of-plane buckling of the test piece during compression, ensuring the uniform introduction of biaxial compression load and stable failure of the specimen.

[0031] The Y-direction compression section includes: a second upper pressure plate 6, a second lower pressure plate 7, a second anti-bending cover plate 8, a third anti-bending cover plate 9, and a second fastening bolt 10. The second upper pressure plate 6 and the second lower pressure plate 7 are also used for direct contact with the test piece 5, and their inner surfaces are also textured. Unlike the X-direction, the Y-direction, due to its symmetrical structure, cannot directly constrain the working section using a staggered covering method. Therefore, this embodiment of the invention employs a unique indirect constraint scheme for the Y-direction compression section: the second anti-bending cover plate 8 and the third anti-bending cover plate 9 respectively press the first upper pressure plate 1 and the first lower pressure plate 2 of the X-direction compression section using the second fastening bolt 10. After pressing, the second anti-bending cover plate 8 and the third anti-bending cover plate 9 form a combined loading surface with the first upper pressure plate 1 and the first lower pressure plate 2. When a compressive load is applied in the Y-direction, this combined loading surface can uniformly transfer the load to the test piece, while providing effective out-of-plane support for the test piece, preventing lateral bending of the test piece under Y-direction compression.

[0032] After the above installation, the parallelism of the first upper pressure plate 1, the first lower pressure plate 2, and the first anti-bending cover plate 3 in the X-direction compression section of the entire device is no greater than 0.05 mm. This high precision requirement ensures that the load is introduced perpendicularly along the sample axis, avoiding premature failure caused by off-center loading.

[0033] Furthermore, this test apparatus includes a 4mm compression allowance in the compression direction (i.e., axial direction) of the test specimen. This allowance was determined based on extensive experience from biaxial compression tests of composite laminates: 4mm is sufficient to cause complete compressive failure (fiber crushing, delamination, etc.) of the specimen, while avoiding unreasonable localized failure due to stress concentration in unsupported areas caused by excessive allowance. This design balances the contradiction between sufficient failure and effective constraint.

[0034] Example 2 This embodiment provides a method for biaxial compression testing of composite materials, implemented using the apparatus described in Embodiment 1. The specific steps are as follows: S1: Install the X-direction compression clamp Assemble the first upper pressure plate 1, the first lower pressure plate 2, and the first anti-bending cover plate 3 in the X direction, and press the first anti-bending cover plate 3 onto the first upper pressure plate 1 and the first lower pressure plate 2 using the first fastening bolt 4. First, adjust the gap between the first anti-bending cover plate 3 and the first upper pressure plate 1 to less than 0.05mm (in actual installation, use a feeler gauge to check; if the gap is too large, add a metal shim under the cover plate); at the same time, adjust the parallelism between the first upper pressure plate 1 and the first lower pressure plate 2 to no more than 0.05mm. Then, insert the test piece 5 into the clamping area on one side of the first upper pressure plate 1 and the first lower pressure plate 2, and manually pre-tighten the first fastening bolt 4. Next, rotate the entire assembly 180°, and gently insert the other side of the test piece into the clamping area on the other side of the first upper pressure plate 1 and the first lower pressure plate 2, and manually pre-tighten the first fastening bolt 4 again. Finally, the first fastening bolt 4 was tightened evenly in multiple stages (3 to 5 times) using a torque wrench, with the same increment each time, and the final tightening torque was controlled between 7 N·m and 15 N·m. This staged tightening process ensured a uniform distribution of clamping force and avoided local crushing of the sample or deformation of the clamp due to excessive tightening at one time.

[0035] S2: Install the Y-direction compression clamp The assembly with the X-direction clamp installed is placed between the second upper pressure plate 6 and the second lower pressure plate 7 in the Y direction. Then, the second anti-bending cover plate 8 and the third anti-bending cover plate 9 are pressed onto the first upper pressure plate 1 and the first lower pressure plate 2 of the X-direction compression section using the second fastening bolt 10, and the second fastening bolt 10 is manually pre-tightened. After rotating 180°, the second anti-bending cover plate 8 and the third anti-bending cover plate 9 on the other side are installed in the same way, and the second fastening bolt 10 is pre-tightened. Finally, the second fastening bolt 10 is tightened evenly in multiple passes (3-5 times) using a torque wrench, with a tightening torque of 7 N·m to 15 N·m. In this step, after the second anti-bending cover plate 8 and the third anti-bending cover plate 9 press against the first upper pressure plate 1 and the first lower pressure plate 2, a combined loading surface is formed, providing out-of-plane support in the Y direction for the sample and preventing lateral bending of the sample during Y-direction compression.

[0036] S3: Central Adjustment Place the entire fixture set on the compression platform of the universal testing machine. Adjust the vertical and horizontal axes of the fixture set to ensure they are precisely aligned with the loading direction of the testing machine, with an alignment error not exceeding 0.1 mm. Precise alignment ensures that the compressive load is transmitted along the centerline of the specimen, avoiding the introduction of bending moments, thereby ensuring that the test results reflect pure compressive performance.

[0037] S4: Loading and Data Acquisition A compressive load is applied to the test specimen at a preset rate, typically a constant rate of 1 mm / min to 2 mm / min. Load and displacement data are continuously acquired from the start of loading. As the load increases, the specimen gradually fails; the specimen is loaded until the maximum load is reached, and the test is stopped when the load decreases by 30% from its peak value to prevent damage to the test fixture. The maximum failure load (i.e., peak load) and failure mode (such as fiber crushing, delamination, buckling, etc., determined by observation after specimen failure) are recorded. The continuously acquired load-displacement data can be used to plot load-displacement curves and calculate mechanical properties such as compressive strength and instability critical point.

[0038] By following the steps above, the ultimate strength and failure mode of the composite laminate under biaxial compression can be accurately obtained.

[0039] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. For those skilled in the art, various modifications and variations can be made to the embodiments of the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A biaxial compression testing device for composite materials, characterized in that, Includes compression portions in the X direction and compression portions in the Y direction; The X-direction compression section includes: a first upper pressure plate (1), a first lower pressure plate (2), a first anti-bending cover plate (3), and a first fastening bolt (4); the first upper pressure plate (1) and the first lower pressure plate (2) are used to directly contact the test piece (5), and the first anti-bending cover plate (3) presses the first upper pressure plate (1) and the first lower pressure plate (2) together with the first fastening bolt (4), and there is a gap between the first anti-bending cover plate (3) and the first upper pressure plate (1); the X-direction compression section adopts a staggered asymmetrical structure to cover the entire compression working section of the test piece in the X direction; The Y-direction compression section includes: a second upper pressure plate (6), a second lower pressure plate (7), a second anti-bending cover plate (8), a third anti-bending cover plate (9), and a second fastening bolt (10); the second upper pressure plate (6) and the second lower pressure plate (7) of the Y-direction compression section are used to directly contact the test piece (5), and the second anti-bending cover plate (8) and the third anti-bending cover plate (9) are respectively pressed by the second fastening bolt (10) to the first upper pressure plate (1) and the first lower pressure plate (2) of the X-direction compression section to provide out-of-plane support for the test piece.

2. The composite material biaxial compression testing apparatus according to claim 1, characterized in that, The inner surfaces of the first upper pressure plate (1), the first lower pressure plate (2), the second upper pressure plate (6), and the second lower pressure plate (7) that are in contact with the test piece are provided with a mesh pattern.

3. The composite material biaxial compression testing apparatus according to claim 1, characterized in that, The gap between the first anti-bending cover plate (3) and the first upper pressure plate (1) is less than 0.05 mm.

4. The composite material biaxial compression testing apparatus according to claim 1, characterized in that, The first fastening bolt (4) and the second fastening bolt (10) are M10 bolts, which are tightened in multiple steps using a torque wrench, with a tightening torque of 7 N·m to 15 N·m.

5. The composite material biaxial compression testing apparatus according to claim 1, characterized in that, The test device has a 4mm compression allowance in the compression direction of the test piece.

6. The composite material biaxial compression testing apparatus according to claim 1, characterized in that, The parallelism of the first upper pressure plate (1), the first lower pressure plate (2), and the first anti-bending cover plate (3) of the X-direction compression section after assembly is no greater than 0.05 mm.

7. The composite material biaxial compression testing apparatus according to claim 1, characterized in that, The second anti-bending cover plate (8) and the third anti-bending cover plate (9) of the Y-direction compression section form a combined loading surface with the first upper pressure plate (1) and the first lower pressure plate (2) of the X-direction compression section after installation.

8. A method for biaxial compression testing of composite materials, implemented using the apparatus described in any one of claims 1 to 7, characterized in that, Includes the following steps: S1: Install the X-direction compression clamp, combine the first upper pressure plate (1), the first lower pressure plate (2) and the first anti-bending cover plate (3) in the X direction, and press the first anti-bending cover plate (3) onto the first upper pressure plate (1) and the first lower pressure plate (2) with the first fastening bolt (4), adjust the gap between the first anti-bending cover plate (3) and the first upper pressure plate (1) to less than 0.05mm, and adjust the parallelism between the first upper pressure plate (1) and the first lower pressure plate (2) to not greater than 0.05mm; insert the test piece (5) into the clamping area on one side of the first upper pressure plate (1) and the first lower pressure plate (2), and pre-tighten the first fastening bolt (4); after rotating 180°, insert it into the clamping area on the other side, and pre-tighten the first fastening bolt (4); use a torque wrench to tighten the first fastening bolt (4) evenly in multiple times, with a tightening torque of 7N·m to 15N·m; S2: Install the Y-direction compression clamp, place the assembly of the installed X-direction compression clamp between the second upper pressure plate (6) and the second lower pressure plate (7); press the second anti-bending cover plate (8) and the third anti-bending cover plate (9) onto the first upper pressure plate (1) and the first lower pressure plate (2) of the X-direction compression part respectively by the second fastening bolt (10), and pre-tighten the second fastening bolt (10); after rotating 180°, install the second anti-bending cover plate (8) and the third anti-bending cover plate (9) on the other side, and pre-tighten the second fastening bolt (10); tighten the second fastening bolt (10) in multiple stages with a torque wrench, with a tightening torque of 7 N·m to 15 N·m; S3: Place the entire fixture on the compression platform and adjust the alignment of the vertical and horizontal axes of the entire fixture. The alignment error should not exceed 0.1mm. S4: Apply compressive load to the test piece at a preset rate and continuously collect load and displacement data; stop the test when the load drops by 30% from the maximum peak value and record the maximum failure load and failure mode.

9. The method according to claim 8, characterized in that, The preset speed is 1 mm / min to 2 mm / min.

10. The method according to claim 8, characterized in that, In step S1, the bolts are tightened 3 to 5 times, with the same increment each time.

Images