A composite material arc-shaped part bidirectional compression mechanical property testing device and testing method

Abstract

The present application belongs to the field of composite material testing method, and discloses a kind of composite material arc-shaped workpiece bidirectional compression mechanical property testing device and testing method.Composite material arc-shaped workpiece bidirectional compression mechanical property testing device includes loading system and measuring system, loading system includes axial loading system and vertical loading system, and measuring system includes axial measuring system and circumferential measuring system.The present application provides a kind of composite material arc-shaped workpiece under bidirectional compression state workpiece mechanical property rapid testing method and testing device, so that the precise evaluation of the mechanical property of composite material arc-shaped workpiece under bidirectional compression state becomes possible.

Description

Technical Field

[0001] This invention provides a testing device and method for bidirectional compressive mechanical properties of composite arc-shaped parts, belonging to the field of composite material testing methods. Background Technology

[0002] Composite materials possess advantages such as light weight, high strength, corrosion resistance, and high temperature resistance, leading to their increasingly widespread application in aerospace, shipbuilding, and marine fields. Evaluating the mechanical properties of composite components through experimental and theoretical methods is fundamental to structural design. Various national standards, ASTM standards, aviation standards, and enterprise standards have been established both domestically and internationally for testing the basic mechanical properties of composite materials. However, these standards primarily focus on uniaxial loading conditions. Examples include GB / T 3354-1999, ASTM D3039 / D3039M-08, and HB6740-1993 for testing the tensile properties of composite materials; ASTM D695-2015 for testing the uniaxial compressive properties of composite materials; and GB / T 1449-2005 and ASTM D5023-07 for testing the flexural properties of composite materials. Research on the biaxial loading state of composite materials focuses on both theoretical studies and performance testing of composite flat plates under biaxial tensile loads. For example, the national invention patent CN201610527437X, "A Test Method and Procedure for Biaxial Tensile Properties of Composite Laminates," designs a biaxial tensile test specimen scheme for composite laminates and its biaxial tensile performance testing method. Composite pressure-resistant structures used in deep-sea applications are typically cylindrical, and under hydrostatic pressure, the materials usually experience biaxial compressive loads. However, research on the biaxial compression state of curved composite components is scarce, and experimental studies are not reported. This is mainly because there is no readily available experimental platform for biaxial compression testing of curved composite components, and there is a lack of corresponding experimental standards for reference, making implementation very difficult. However, for composite pressure-resistant structures, the load-bearing failure process is extremely complex. Existing design methods typically utilize the uniaxial compressive mechanical properties of flat plate specimens for mechanical modeling and failure analysis. For example, the literature X. Zhang, Z. Li, P. Wang, et al. Experimental and numerical analyses on buckling and strength failure of composite cylindrical shells under hydrostatic pressure. Ocean Engineering, 2022, 249: 110871 refers to test standards such as ASTM D3039, ASTM 6641, and ASTM D3518 to conduct tensile, compression, and shear tests on composite flat plate specimens to obtain the basic mechanical properties of the specimens for modeling and simulation calculations.Because the mechanical responses of flat and curved components under compressive loads differ, and the effects of uniaxial and biaxial compressive loads on structural failure vary significantly, relying solely on the uniaxial compressive performance analysis of flat components cannot accurately assess the mechanical response of composite pressure-resistant structures under biaxial compressive loads. Therefore, a reliable biaxial compression testing device and method are urgently needed to provide technical support for the safe design of composite pressure-resistant structures. This invention provides a testing device and method for the biaxial compressive mechanical properties of composite curved components. The testing process is simple and effective, and can test the axial and circumferential stiffness and strength of composite curved components. Summary of the Invention

[0003] The purpose of this invention is to provide a testing device and method for testing the biaxial compressive mechanical properties of composite curved parts, which achieves accurate measurement of the biaxial compressive mechanical properties of composite curved parts through a precise loading system and measurement system.

[0004] The technical solution of the present invention:

[0005] A biaxial compressive mechanical property testing device for composite arc-shaped parts, comprising a loading system and a measurement system, wherein the loading system includes an axial loading system and a vertical loading system, and the measurement system includes an axial measurement system and a circumferential measurement system; specifically comprising a base 1, a first axial support 2, a support roller 3, a limiting frame 4, a limiting head 5, a second axial support 6, a first distance sensor 7, a second distance sensor 8, a first axial loading plate 9, an axial guide rail 10, a first axial slider 11, a loading head 12, a loading frame 13, a first vertical slider 14, a first vertical guide rail 15, a vertical support 16, a second vertical guide rail 17, a second vertical slider 18, a vertical screw 19, a third distance sensor 20, a fourth distance sensor 21, a second axial slider 22, an axial screw 23, and a second axial loading plate 24;

[0006] The base 1 is mainly composed of two semi-circular, oppositely arranged grooved plates. The support rollers 3 are fixed at both ends along the semi-circular edges of the grooved plates of the base 1 via bearings. The limiting head 5 is bolted to the limiting frame 4, which is bolted to the upper surface of the two grooved plates on one side of the base 1. The first axial support 2 and the second axial support 6 are bolted to the outer side of the grooved plates of the base 1. The axial guide rail 10 is bolted to the first axial support 2 and the second axial support 6 at both ends. The first axial slider 11 and the second axial slider 22 are connected to the axial guide rail 10 via dovetail grooves. The first axial loading plate 9 and the second axial loading plate 24 are bolted to the first axial slider 11 and the second axial slider 22, respectively. The first axial loading plate 9 and the second axial loading plate 24 are connected to the axial screw 23 via internally threaded copper sleeves. Axial screw 23 is connected to the first axial support 2 and the second axial support 6 via an inner bearing; the first ranging sensor 7, the second ranging sensor 8, the third ranging sensor 20 and the fourth ranging sensor 21 are connected to the axial guide rail 10 by magnetic attraction; the vertical support 16 is bolted between two grooved plates on the other side of the base 1; the first vertical guide rail 15 and the second vertical guide rail 17 are bolted to the inner sides of the two grooved plates of the base 1; the loading head 12 is bolted to the loading frame 13; the two ends of the loading frame 13 are bolted to the first vertical slider 14 and the second vertical slider 18 respectively; the first vertical slider 14 and the second vertical slider 18 are connected to the first vertical guide rail 15 and the second vertical guide rail 17 via dovetail grooves; and the vertical screw 19 is connected to the loading frame 13 via an internal threaded copper sleeve through the vertical support 16.

[0007] The axial loading system mainly consists of a base 1, a first axial support 2, a second axial support 6, an axial guide rail 10, a first axial slider 11, a second axial slider 22, a first axial loading plate 9, a second axial loading plate 24, and an axial screw 23. The axial screw 23 adopts a positive and negative thread design. When the axial screw 23 rotates, it drives the first axial loading plate 9 and the second axial loading plate 24 to move towards each other through the thread. At the same time, due to the restriction of the first axial slider 11 and the second axial slider 22, the first axial loading plate 9 and the second axial loading plate 24 can only translate along the axial guide rail 10. That is, the first axial loading plate 9 and the second axial loading plate 24 do not rotate during axial movement, thereby realizing the compression loading of the sample 27 without changing the area of ​​action.

[0008] The vertical loading system mainly consists of a base 1, a vertical support 16, a vertical screw 19, a first vertical guide rail 15, a second vertical guide rail 17, a first vertical slider 14, a second vertical slider 18, a loading frame 13, and a loading head 12. When the vertical screw 19 rotates, it drives the loading frame 13 to move along the first vertical guide rail 15 and the second vertical guide rail 17 via the first vertical slider 14 and the second vertical slider 18, thereby realizing the compression loading of the sample 27 by the loading head 12.

[0009] The axial measurement system mainly consists of a first ranging sensor 7 and a second ranging sensor 8. It indirectly measures the axial deformation of the sample 27 by directly measuring the position changes of the first axial loading plate 9 and the second axial loading plate 24.

[0010] The circumferential measurement system mainly consists of a third distance sensor 20, a fourth distance sensor 21, a first positioning ruler 25, and a second positioning ruler 26. The positioning ruler is attached to the sample 27, and the indirect measurement of the circumferential displacement of the sample 27 is achieved through the cooperation of the distance sensor and the positioning ruler.

[0011] Axial loading is achieved by rotating the axial screw 23, which causes the first axial loading plate 9 and the second axial loading plate 24 to drive the first axial slider 11 and the second axial slider 22 to make axial linear displacement along the axial guide rail 10. The vertical loading system consists of a base 1, a vertical support 16, a vertical screw 17, a first vertical guide rail 15, a second vertical guide rail 17, a first vertical slider 14, a second vertical slider 18, a loading frame 13, and a loading head 12. Vertical loading is achieved by rotating the vertical screw 17, which causes the loading seat 13 to drive the loading head 12 to make vertical linear movement along the first vertical guide rail 15 and the second vertical guide rail 17 via the first vertical slider 14 and the second vertical slider 18. The measurement system measures the axial compression deformation of the composite material arc-shaped part by cooperating with the first distance sensor 7 and the second distance sensor 8, and measures the circumferential compression deformation of the composite material arc-shaped part by cooperating with the third distance sensor 20 and the fourth distance sensor 21.

[0012] The beneficial effects of this invention are as follows: 1. Using a freely rotatable support roller as a support platform reduces the friction between the sample and the support platform during compression deformation, thus reducing the impact of additional loads on the test results; 2. Using a freely rotatable loading head and limiting head for load application and sample positioning reduces the friction between the loading head and limiting head and the sample during bidirectional loading, improving the reliability of the experimental results; 3. Through the rational design of the sample geometry and its close cooperation with the test device, vertical loading is transformed into circumferential compressive load on the sample; 4. Through the use of a distance measuring sensor and theoretical analysis, accurate measurement of the circumferential deformation of the sample is achieved; 5. This invention can achieve separate or simultaneous axial and circumferential loading on arc-shaped samples, making it possible to accurately evaluate the mechanical properties of arc-shaped samples under bidirectional compression. Attached Figure Description

[0013] Figure 1 This is a schematic diagram of a bidirectional compression testing device for composite material arc-shaped parts.

[0014] Figure 2 This is a schematic diagram of the circumferential displacement measurement of a composite material sample.

[0015] Figure 3 This is a schematic diagram of the geometric shape of a composite material arc-shaped sample.

[0016] Figure 4 Schematic diagram of circumferential stiffness of composite material arc component.

[0017] In the diagram: 1. Base; 2. First axial support; 3. Support roller; 4. Limiting frame; 5. Limiting head; 6. Second axial support; 7. First distance sensor; 8. Second distance sensor; 9. First axial loading plate; 10. Axial guide rail; 11. First axial slider; 12. Loading head; 13. Loading frame; 14. First vertical slider; 15. First vertical guide rail; 16. Vertical support; 17. Second vertical guide rail; 18. Second vertical slider; 19. Vertical screw; 20. Third distance sensor; 21. Fourth distance sensor; 22. Second axial slider; 23. Axial screw; 24. Second axial loading plate; 25. First positioning ruler; 26. Second positioning ruler; 27. Sample; P represents the uniformly distributed pressure on the sample; R0 represents the initial radius of the sample; R represents the radius of the sample after loading; a and b represent any two points on the arc-shaped sample before loading; a' and b' represent the actual positions of points a and b after loading; L0 represents the length of chord ab; L represents the length of chord a'b'. Detailed Implementation

[0018] The specific embodiments of the present invention are described in detail below with reference to the technical solutions and accompanying drawings.

[0019] like Figure 1As shown, before testing the compressive properties of the composite curved part, the loading capacity of the axial screw 23 and the vertical screw 19 needs to be calibrated, that is, the relationship between the torque M applied to the screw and the load F applied to the sample 27 needs to be given. The calibration process of the axial screw 23 is illustrated as an example. A force sensor is placed between the first axial loading plate 8 and the second axial loading plate 24. The axial screw 23 is manually adjusted to clamp the force sensor between the first axial loading plate 8 and the second axial loading plate 24. Afterwards, the torque wrench is adjusted to a certain value M. i Then, a fixed torque M is applied using a torque wrench through the countersunk head hole at the top of the axial screw 23. i And read the value F from the ranging sensor. i Then, the torque M is continuously increased and the corresponding F value is read. This allows us to obtain the relationship between the applied torque M and the axial load F, i.e., F = f1(M), where f1 is a fitting function. The calibration needs to be repeated three times, and the average value is taken as the basis for load application. The same method is used to calibrate the vertical screw 19 to obtain the relationship between the applied torque M and the load F, F = f2(M), where f2 is a fitting function.

[0020] like Figure 3 As shown, a composite material arc-shaped test specimen is prepared. The specific dimensions of specimen 27 can be adjusted according to experimental needs. Its basic geometry consists of a semi-cylindrical surface and a flat plate section tangent to the cylindrical surface. By adjusting... Figure 1 The dimensions of the support roller 3 are such that the curved surface formed by the support roller 3 is concentric with the outer surface of the semi-cylindrical surface of the sample 27. The flat plate portion of the sample 27 is used to apply vertical loads. Figure 3 The middle section is on the left.

[0021] like Figure 2 As shown, the sample 27 is placed on the test apparatus, with its outer cylindrical surface naturally fitting against the arc surface formed by the support roller 3. The vertical screw 19 is adjusted so that the loading head 12 contacts the left flat plate of the sample 27. Subsequently, the axial screw 23 or the vertical screw 19 is adjusted as needed to apply axial and circumferential compressive loads to the sample 27. The deformation of the sample 27 is measured by the first, second, third, and fourth distance sensors, and the relevant mechanical property parameters of the sample 27 are calculated.

[0022] Axial load mechanical property test of sample 27: A small torque M is applied to the axial screw 23 using a torque wrench, so that the first axial loading plate 9 and the second axial loading plate 24 gently clamp sample 27, such as M = 1 Nm. At this time, the measurement results of the first ranging sensor 7 and the second ranging sensor 8 are recorded, which are Y and Y respectively. 10 and Y 20 Increase the torque M and record the measurement results of the first ranging sensor 7 and the second ranging sensor 8, which are Y respectively.1n and Y 2n n = 1, 2, 3, ... When M increases to M... a When sample 27 experiences failure modes such as crushing, fracture, and buckling, f(M) a This refers to the axial failure load of sample 27. The calculation of the compressive area A of sample 27 is as follows: a Then, the axial compressive strength σ of sample 27 can be calculated. a =f1(M a ) / A a Based on previously recorded test data, the axial compressive stiffness K of sample 27 can be calculated. a =f1(M n ) / (Y 10 -Y 1n +Y 20 -Y 2n M n This represents the torque applied during the nth test.

[0023] Circumferential load-bearing mechanical performance test of sample 27: A small torque M is applied to the vertical screw 19 using a torque wrench, so that the loading head 12 gently presses against the upper surface of the flat plate part of sample 27, while ensuring that the other side of sample 27 abuts against the limiting head 5, for example, M = 1 Nm. The first positioning ruler 25 and the second positioning ruler 26 are attached to the surface of sample 27, ensuring that the positioning rulers are flat and vertical. The values ​​measured by the third distance sensor 20 and the fourth distance sensor 21 are recorded as X. 10 and X 20 Increase the torque M and record the measurement results of the third ranging sensor 20 and the fourth ranging sensor 21, which are X and X, respectively. 1n and X 2n n = 1, 2, 3, ... When M increases to M... c When sample 27 experiences failure modes such as crushing, fracture, and buckling, σ c =f(M c ) / A c This refers to the circumferential compressive strength of sample 27. Circumferential compressive strength is defined as K... c =F c / (R0-R), where F c This indicates the load applied to the sample in the 27th circumferential direction; other parameters are as follows: Figure 4 As shown. Considering sinθ=L / 2R=L0 / 2R0, therefore R0-R=R0(1-L / L0), thus K c =f2(M c ) / [R0(1-L / L0)], where L and L0 can be obtained from the measurement results of the third ranging sensor 20 and the fourth ranging sensor 21, and R0 is the design parameter of the sample 27. Therefore, the circumferential compressive stiffness K of the sample 27 is... c =f2(Mn ) / (X 10 -X 1n +X 20 -X 2n M n This represents the torque applied during the nth test.

[0024] Bidirectional load mechanical performance test of sample 27: A small torque M is applied to the vertical screw 19 using a torque wrench, so that the loading head 12 gently presses against the upper surface of the flat plate part of sample 27, while ensuring that the other side of sample 27 abuts against the limiting head 5, for example, M = 1 Nm. The first positioning ruler 25 and the second positioning ruler 26 are attached to the upper surface of the test sample 27, ensuring that the positioning rulers remain vertical. The measurement results of the third distance sensor 20 and the fourth distance sensor 21 are recorded, respectively X 10 and X 20 A small torque M is applied to the axial screw 23 using a torque wrench, causing the first axial loading plate 9 and the second axial loading plate 24 to gently clamp the sample 27, such as M = 1 Nm. At this time, the measurement results of the first ranging sensor 7 and the second ranging sensor 8 are recorded, which are Y and Y respectively. 10 and Y 20 Subsequently, the torque applied to the vertical screw 19 and the axial screw 23 is increased sequentially by a certain increment, such as 0.5 Nm, and the measurement results of the first ranging sensor 7, the second ranging sensor 8, the third ranging sensor 20, and the fourth ranging sensor 21 are recorded until the sample 27 experiences failure modes such as crushing, crushing, and buckling. At this point, the compressive strength of the sample 27 under bidirectional compression conditions can be obtained. By changing the load application conditions, the compressive strength of the sample 27 under different compressive load conditions can be obtained.

Claims

1. A testing device for the bidirectional compressive mechanical properties of composite arc-shaped parts, characterized in that, The biaxial compression mechanical property testing device for composite arc-shaped parts includes a loading system and a measurement system. The loading system includes an axial loading system and a vertical loading system, and the measurement system includes an axial measurement system and a circumferential measurement system. The axial loading system is mainly composed of a base (1), a first axial support (2), a second axial support (6), an axial guide rail (10), a first axial slider (11), a second axial slider (22), a first axial loading plate (9), a second axial loading plate (24), and an axial screw (23). The vertical loading system is mainly composed of a base (1), a vertical support (16), a vertical screw (19), a first vertical guide rail (15), a second vertical guide rail (17), a first vertical slider (14), a second vertical slider (18), a loading frame (13), and a loading head (12). The axial measurement system is mainly composed of a first distance sensor (7) and a second distance sensor (8). The circumferential measurement system is mainly composed of a third distance sensor (20), a fourth distance sensor (21), a first positioning ruler (25), and a second positioning ruler (26). The base (1) is mainly composed of two semi-circular, oppositely arranged grooved plates. The two ends of the support roller (3) are fixed along the inner edge of the semi-circular grooved plate of the base (1) by bearings. The limiting head (5) is installed on the limiting frame (4) by bolts. The limiting frame (4) is installed on the upper surface of the two grooved plates on one side of the base (1) by bolts. The first axial bracket (2) and the second axial bracket (6) are installed on the outer side of the grooved plate of the base (1) at the position of the semi-circular groove by bolts. The two ends of the axial guide rail (10) are installed on the first axial bracket (2) and the second axial bracket (6) by bolts respectively. The first axial slider (11) and the second axial slider (22) are connected to the axial guide rail (10) by dovetail groove. The first axial loading plate (9) and the second axial loading plate (24) are connected to the first axial slider (11) and the second axial slider (22) by bolts respectively. The first axial loading plate (9) and the second axial loading plate (24) are connected to the axial screw (23) by internal threaded copper sleeve. The axial screw (23) is connected to the first axial bracket (2) and the second axial bracket (6) through the inner bearing; the first distance sensor (7), the second distance sensor (8), the third distance sensor (20) and the fourth distance sensor (21) are connected to the axial guide rail (10) by magnetic attraction; the vertical bracket (16) is connected to the two groove plates on the other side of the base (1) by bolts; the first vertical guide rail (15) and the second vertical guide rail (17) are connected to the inner side of the two groove plates of the base (1) by bolts; the loading head (12) is connected to the loading frame (13) by bolts; the two ends of the loading frame (13) are connected to the first vertical slider (14) and the second vertical slider (18) by bolts respectively; the first vertical slider (14) and the second vertical slider (18) are connected to the first vertical guide rail (15) and the second vertical guide rail (17) through the dovetail groove; the vertical screw (19) is connected to the loading frame (13) through the vertical bracket (16) through the internal thread copper sleeve. The axial screw (23) adopts a positive and negative thread design. When the axial screw (23) rotates, it drives the first axial loading plate (9) and the second axial loading plate (24) to move towards each other through the thread. At the same time, due to the restriction of the first axial slider (11) and the second axial slider (22), the first axial loading plate (9) and the second axial loading plate (24) can only translate along the axial guide rail (10). That is, the first axial loading plate (9) and the second axial loading plate (24) do not rotate during axial movement, thereby realizing the extrusion loading of the sample without changing the action area. When the vertical screw (19) rotates, the loading frame (13) is driven by the screw to move along the first vertical guide rail (15) and the second vertical guide rail (17) through the first vertical slider (14) and the second vertical slider (18), so as to realize the compression loading of the loading head (12) on the sample (27); Indirect measurement of the axial deformation of the sample is achieved by directly measuring the positional changes of the first axial loading plate (9) and the second axial loading plate (24); The positioning ruler is pasted on the sample (27), and the circumferential displacement of the sample is indirectly measured by the cooperation of the distance sensor and the positioning ruler. The sample (27) consists of a semi-cylindrical surface and a plate section tangent to the cylindrical surface. The plate section is used to apply vertical loads. Under the combined action of the sample geometry and the test device, the applied vertical load will be converted into a compressive load along the circumference of the sample.

2. The test apparatus for bidirectional compressive mechanical properties of composite arc-shaped parts according to claim 1, characterized in that, A freely rotatable support roller (3) is used as the support platform for the sample (27). The friction between the sample and the support platform under circumferential loading is reduced by the low-friction rotation of the support roller (3).

3. The composite material arc-shaped part bidirectional compressive mechanical property testing device according to claim 1, characterized in that, The sample (27) is limited and loaded by a freely rotatable limiting head (5) and loading head (12), ensuring that the friction between the sample (27) and the limiting head (5) and loading head (12) is minimized when the sample (27) is subjected to axial compressive load.

4. A method for testing the circumferential load-bearing mechanical properties of composite arc-shaped parts using the biaxial compressive mechanical property testing device of claim 1, characterized in that, Using a torque wrench, apply a torque M to the vertical screw (19) to press the loading head (12) against the upper surface of the sample (27), while ensuring that the other side of the sample (27) abuts against the limiting head (5); attach the first positioning ruler (25) and the second positioning ruler (26) to the surface of the sample (27) and ensure that the positioning rulers remain vertical, and record the measured values ​​of the third distance sensor (20) and the fourth distance sensor (21), which are X and X, respectively. 10 and X 20 The torque M was increased multiple times, and the measurement results of the third ranging sensor (20) and the fourth ranging sensor (21) were recorded, respectively X 1n and X 2n n=1,2,3,…; when M increases to M c When sample (27) fails, σ c =f(M c ) / A c This refers to the circumferential compressive strength of the sample (27); the circumferential compressive strength is defined as K. c =F c / (R0-R), where F c Let R0 represent the circumferential load applied to the sample (27), R0 represent the initial radius of the sample (27), and R represent the radius of the sample (27) after loading. Considering sinθ=L / 2R=L0 / 2R0, R0-R=R0(1-L / L0), therefore, K c =f2(M c ) / [ R0(1-L / L0)], where L and L0 are obtained by the measurement results of the third ranging sensor (20) and the fourth ranging sensor (21), respectively, and are the lengths of chord ab and chord a'b'. a and b represent any two points on the arc-shaped sample (27) before loading, and a' and b' represent the actual positions of points a and b after loading; the circumferential compressive stiffness K of the sample (27) c =f2(M n ) / (X 10 -X 1n +X 20 -X 2n M n This represents the torque applied during the nth test.

5. A method for testing the biaxial compressive mechanical properties of composite material samples under load using the biaxial compressive mechanical properties testing apparatus for composite material arc-shaped parts as described in claim 1, characterized in that, Using a torque wrench, apply a torque M to the vertical screw (19) to press the loading head (12) against the upper surface of the sample (27), while ensuring that the other side of the sample (27) abuts against the limiting head (5); attach the first positioning ruler (25) and the second positioning ruler (26) to the upper surface of the sample (27) and ensure that the positioning rulers remain vertical, and record the measurement results of the third distance sensor (20) and the fourth distance sensor (21), which are X respectively. 10 and X 20 Using a torque wrench, apply a torque M to the axial screw (23) to clamp the sample (27) with the first axial loading plate (9) and the second axial loading plate (24). At this time, record the measurement results of the first ranging sensor (7) and the second ranging sensor (8), which are Y and Y respectively. 10 and Y 20 The torque applied to the vertical screw (19) and the axial screw (23) is increased sequentially, and the measurement results of the first distance sensor (7), the second distance sensor (8), the third distance sensor (20) and the fourth distance sensor (21) are recorded until the sample (27) fails. At this time, the compressive strength of the sample under bidirectional compression is obtained. The compressive strength of the sample under different compressive load conditions can be obtained by changing the load application conditions.

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