Method for testing normal shear strength of T-shaped joint based on ceramic matrix composite material

Abstract

The invention discloses a method for testing normal shear strength based on a T-shaped joint of a ceramic matrix composite material. The method comprises the following steps: acquiring the geometric dimension of a T-shaped sample of the ceramic matrix composite material to be tested; the T-shaped sample is placed in a loading clamp, and the mounting position of the T-shaped sample is adjusted according to the structure of the T-shaped sample; by arranging the gasket and tightening the puller screw, the sample is effectively fixed; adjusting the position of the loading pressure head, enabling the loading pressure head to act on the web region of the T-shaped sample, implementing a normal shear test, recording the maximum load value in the whole loading process, and calculating the normal shear strength of the T-shaped joint of the ceramic matrix composite material by utilizing a formula according to the recorded maximum load value and the sectional dimension of the T-shaped sample. The problems that the sample is difficult to fix and the test result is inaccurate are solved, and the accuracy of the normal shear strength test of the T-shaped joint made of the ceramic matrix composite material is improved.

Description

Technical Field

[0001] The present invention relates to a method for testing the normal shear strength of a T-joint, in particular to a method for testing the normal shear strength of a T-joint based on ceramic-based composite materials, and belongs to the technical field of new aerospace materials. Background Art

[0002] Ceramic Matrix Composites (CMCs) are a class of high-performance composite materials with a ceramic matrix and typically fiber reinforcement. They offer significant advantages such as high high-temperature strength, excellent thermal shock resistance, corrosion resistance, and low density. They are widely used in aerospace, energy, metallurgy, and high-temperature structural applications. In gas turbine engines, thermal protection systems, and high-temperature combustion components, CMCs are considered an ideal alternative to metals and traditional ceramics due to their excellent thermomechanical properties.

[0003] Due to the inherent brittleness of ceramic-based composites, their overall structural performance depends largely on the connection method and strength between components. In complex service environments, to meet structural connection requirements, various composite components are usually connected and assembled in T-shaped, L-shaped, and other ways. Among them, T-joints are a common connection form, widely used in load-bearing structures, and are particularly suitable for structural systems with vertical connection requirements between components. T-joints not only serve as a bridge for component force transmission, but also directly affect the stiffness, strength, and reliability of the overall structure. They are a key part that determines the service life and safety of ceramic-based composite components.

[0004] However, ceramic matrix composite (CMC) T-joints often face a variety of complex loads in practice, particularly in the joint region, which is prone to stress concentration and multiaxial stress states. Under these conditions, normal shear loading is a common and important loading mode, often encountered in situations such as uneven thermal expansion, vertical load transfer, or bending of the joint. Due to the laminated structure and fiber-matrix interface characteristics of CMCs, they are sensitive to normal shear loading, prone to localized microcracks and even overall failure.

[0005] In the prior art, mechanical property evaluation of ceramic matrix composite (CMC) T-joints primarily focuses on tensile, compression, or bending tests, while systematic testing methods for normal shear conditions are still immature. For example, a method for testing the shear strength of a CMC pin, disclosed in Publication No. CN107014701A, includes the following steps: preparing a CMC pin specimen, placing the pin specimen in a test mold, and then placing the test mold with the specimen on a testing machine for testing; controlling the upper chuck of the compression testing machine to move downward, subjecting the specimen to a small shear stress; and testing the shear performance of the specimen using a displacement-controlled loading method. The data from the force and displacement sensors on the testing machine are processed to ultimately obtain the load-displacement curve and shear performance of the pin composite. However, the existing device is designed for pin specimens. T-shaped specimens, such as CMC T-joints, are often subjected to a variety of complex loads in actual operation, particularly in the joint area, which is prone to stress concentration and multiaxial stress states. Under these conditions, normal shear loading is a common and important loading method, often encountered in situations such as uneven thermal expansion, vertical load transfer, or bending of the connector. Due to the laminated structure and fiber-matrix interface characteristics of ceramic-based composites, they are more sensitive to normal shear loads and are prone to microcracks in local areas or even overall failure. On the one hand, due to the structural asymmetry of the T-shaped structure during loading, traditional testing equipment is difficult to achieve effective normal shear force application; on the other hand, unreasonable fixation of the specimen during loading can easily lead to slippage or abnormal stress distribution, which seriously affects the accuracy and repeatability of the test results. Summary of the Invention

[0006] The purpose of the present invention is to provide a normal shear strength testing method for T-joints made of ceramic matrix composite materials in order to solve at least one of the above technical problems.

[0007] The present invention achieves the above-mentioned object through the following technical solution: a normal shear strength testing method for a T-joint based on a ceramic matrix composite material, the normal shear strength testing method comprising the following steps: S1. Obtain the geometric dimensions of the T-shaped specimen of the ceramic matrix composite material to be tested to provide basic data for subsequent loading and strength calculations; S2. Place the T-type specimen in a specially designed loading fixture and accurately adjust its installation position according to the structure of the T-type specimen; S3. By setting the gasket and tightening the top screw, the T-type specimen can be effectively fixed to avoid slippage or deviation during loading. S4. Adjust the position of the loading head so that it acts on the web area of ​​the T-type specimen, perform the normal shear test, and record the maximum load value during the entire loading process; S5. Calculate the normal shear strength of the ceramic matrix composite T-joint using the prescribed formula based on the recorded maximum load value and the cross-sectional dimensions of the T-specimen.

[0008] As a further solution of the present invention: the T-type specimen has a "T"-shaped structure, consisting of a web and a flange, and the web and the flange are connected by two circular arc segments distributed at 90 degrees.

[0009] As a further solution of the present invention: the geometric dimensions of the T-type specimen include the overall thickness of the "T"-shaped structure, the length and width of the web, the length and width of the flange, and the radius of the arc segment.

[0010] As a further solution of the present invention: the T-shaped specimen is installed in a loading fixture designed specifically for the T-shaped structure, and its installation position is precisely adjusted according to the geometric dimensions of the T-shaped specimen. The loading fixture covers the flange portion of the T-shaped specimen, while the web portion of the T-shaped specimen protrudes from the central area of ​​the loading fixture.

[0011] As a further solution of the present invention: when fixing the T-type specimen, a gasket made of a high-strength and moderately rigid material is set on the side of the flange of the T-type specimen. Then, by adjusting and tightening the tightening screw, the T-type specimen is reliably pressed in the fixture.

[0012] As a further solution of the present invention: the gasket includes but is not limited to a flexible graphite composite gasket, a metal spiral wound gasket, a stainless steel gasket and a carbon steel gasket.

[0013] As a further solution of the present invention: the loading head is a loading component with a cylindrical end, and the loading point is located inward from the free end of the web of the T-shaped specimen, that is, in the range of 1 / 5 to 1 / 4 of the total length of the web. During the loading process, the loading head moves downward in the vertical direction to apply a normal shear load to the web. At the same time, the load-displacement curve is recorded by the testing machine, and the maximum load value generated during the loading process is obtained based on the load-displacement curve.

[0014] As a further solution of the present invention: by recording the maximum load value obtained during the loading process and combining it with the effective load-bearing cross-sectional dimensions of the web of the T-type specimen, the stress calculation formula is used to evaluate the specimen's destructive strength. This destructive strength is the shear strength of the ceramic matrix composite T-type joint under normal shear load, and the calculation formula is as follows:

[0015] in, is the normal shear strength, is the maximum load during the test, is the effective shear area of ​​the web, is the width of the web, is the thickness of the T-type specimen of ceramic matrix composites.

[0016] A loading device includes a loading fixture and a loading pressure head for testing; a T-shaped specimen is clamped in the loading fixture, a tightening screw is threadedly connected to the loading fixture, a gasket is provided between the tightening screw and the clamping surface of the T-shaped specimen, and the loading pressure head is located directly above the loading point of the T-shaped specimen.

[0017] The beneficial effects of the present invention are: 1) A specially designed loading fixture is used to effectively position and stably fix the T-shaped specimen, ensuring that the load can be accurately applied along the normal shear path during loading; 2) This test method solves existing problems such as specimen slippage, uneven force distribution, or loading interference, significantly improving the controllability and repeatability of the test process. Furthermore, by combining the maximum load value obtained in the test with the specimen geometric parameters, the normal shear strength of the joint can be accurately calculated, assessing its load-bearing capacity under actual working conditions. 3) The testing method of the present invention has a simple structure and is easy to operate. It can overcome the problems of difficult specimen fixation and inaccurate test results, effectively improving the accuracy and reliability of the normal shear strength test of T-joints of ceramic-based composite materials. It has high engineering application value and promotion prospects. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 This is a flow chart of the testing method of the present invention; Figure 2 It is a structural schematic diagram of the sample of the present invention; Figure 3 A schematic diagram of the clamp and the pressing head of the present invention; Figure 4 This is a schematic diagram of sample loading in the present invention; Figure 5 The load-displacement response image of the sample during the loading process recorded by the testing machine of the present invention; In the figure, 1. Loading fixture; 2. Gasket; 3. Tightening screw; 4. Specimen; 5. Loading pressure head. DETAILED DESCRIPTION

[0019] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0020] Example 1, as Figure 1As shown, this embodiment provides a normal shear strength test method for a T-joint based on a ceramic matrix composite material. The normal shear strength test method specifically includes the following steps: First: obtain the geometrical dimension parameters of the T-shaped specimen 4 of the ceramic matrix composite material to be tested, and provide basic data for subsequent loading and strength calculation.

[0021] like Figure 2 As shown, the T-type specimen 4 is a "T"-shaped structure, which consists of a web and a flange, and the web and the flange are connected by two arc segments distributed at 90 degrees.

[0022] Second: Place the T-shaped specimen 4 in a specially designed loading fixture 1 and accurately adjust its installation position according to the structure of the T-shaped specimen 4 .

[0023] The T-specimen 4 is installed in a loading fixture 1 designed specifically for "T"-shaped structures, and its installation position is precisely adjusted according to the geometric dimensions of the T-specimen 4. The loading fixture 1 covers the flange part of the T-specimen 4 to achieve its stable fixation, while allowing the web part of the T-specimen 4 to protrude from the middle area of ​​the loading fixture 1 for subsequent loading.

[0024] It should be noted that during the installation process, it is necessary to ensure that there is no geometric interference between the web and the loading fixture 1 on the loading path, so as to avoid contact conflict or mechanical anomaly during the loading process and ensure the accuracy and reliability of the test.

[0025] Third: By setting the gasket 2 and tightening the tightening screw 3, the T-shaped specimen 4 is effectively fixed to avoid slippage or deviation during loading.

[0026] In order to prevent the T-shaped specimen 4 from being damaged due to direct force during the fixing process, when fixing the T-shaped specimen 4, a gasket 2 made of a high-strength and moderately rigid material is set on the side of the flange of the T-shaped specimen 4 to play a buffering and protective role, so as to effectively disperse the local stress applied by the screw and avoid damage to the specimen structure. Subsequently, by adjusting and tightening the tightening screw 3, the T-shaped specimen 4 is reliably pressed in the fixture to ensure that it maintains a stable position during the subsequent loading process to prevent slippage, rotation or offset, thereby improving the accuracy of the loading path and the repeatability of the test results.

[0027] The gasket 2 includes but is not limited to a flexible graphite composite gasket, a metal spiral wound gasket, a stainless steel gasket and a carbon steel gasket.

[0028] Fourth: adjust the position of the loading head 5 so that it acts on the web area of ​​the T-shaped specimen 4, perform a normal shear test, and record the maximum load value during the entire loading process.

[0029] The loading ram 5 used is a loading component with a cylindrical end, which can achieve relatively uniform force transmission. By adjusting the position of the loading ram 5, it can accurately act on the specified position of the web area of ​​the T-type specimen 4. The loading point is located inward from the free end of the web of the T-type specimen, that is, in the range of 1 / 5 to 1 / 4 of the total length of the web. During the loading process, the loading ram 5 moves downward in the vertical direction to apply a normal shear load to the web. At the same time, the load-displacement curve is recorded by the testing machine, and the maximum load value generated during the loading process is obtained according to the load-displacement curve, providing an accurate basis for the subsequent calculation of the normal shear strength of the specimen.

[0030] Fifth: Calculate the normal shear strength of the ceramic matrix composite T-joint based on the maximum load value and the cross-sectional dimensions of the T-specimen 4.

[0031] By recording the maximum load value obtained during the loading process and combining it with the effective load-bearing cross-sectional dimensions of the web of T-shaped specimen 4, the stress calculation formula is used to evaluate the destructive strength of T-shaped specimen 4. This destructive strength is the shear strength of the ceramic matrix composite T-joint under normal shear load, and the calculation formula is as follows:

[0032] in, is the normal shear strength, is the maximum load during the test, is the effective shear area of ​​the web, is the width of the web, is the thickness of the T-type specimen of the ceramic matrix composite. This formula can be used to quantitatively evaluate the normal shear bearing capacity of the specimen, providing a reliable experimental basis for the design and optimization of the structural connection performance of ceramic matrix composites.

[0033] Example 2, as Figure 3 and Figure 4 As shown, this embodiment provides a loading device, which is used in the normal shear strength test method in Example 1. The loading device includes a loading fixture 1 and a loading pressure head 5 for testing; a T-shaped specimen 4 is clamped in the loading fixture 1, and the loading fixture 1 is threadedly connected with a tightening screw 3, and a gasket 2 is provided between the tightening screw 3 and the clamping surface of the T-shaped specimen 4, and the loading pressure head 5 is located directly above the loading point of the T-shaped specimen 4.

[0034] Example 3: Based on Example 1 and Example 2, this example provides a method for testing the normal shear strength of a T-joint of a ceramic matrix composite material. Taking a T-shaped specimen of a ceramic matrix composite material as an example, the normal shear strength test is performed, specifically including: First, a batch of T-shaped specimens 4 of ceramic matrix composite materials are obtained (i.e., the plane is formed by connecting the web and the flange through two circular arc segments distributed at 90°, and the T-shaped specimen 4 has a certain thickness in the direction perpendicular to the structural plane), such as Figure 2 As shown, the specific geometric parameters of the T-shaped specimen 4 include the overall thickness of the T-shaped specimen 4. , the length of the web , the width of the web , flange length , flange width , the radius of the arc connecting segment , the specific parameters of T-shaped specimen 4 are shown in Table 1.

[0035]

[0036] Then, install the sample to be tested in the loading fixture designed for T-type specimen 4, and adjust its installation position precisely according to the geometric characteristics of the specimen. By setting gaskets and tightening the tightening screws, the specimen can be effectively fixed to avoid slippage or deviation during loading. Adjust the position of the loading head so that it acts on the web area of ​​the T-type specimen and implement normal shear load, such as Figure 5 As shown, record acquisition payload -Displacement The maximum load value generated during the test is extracted from the curve .

[0037] Finally, the normal shear strength of the test sample is calculated using the formula:

[0038] Working process: Obtain the geometric dimensions of the T-type specimen of the ceramic-based composite material to be tested to provide basic data for subsequent loading and strength calculations; Place the T-type specimen in a specially designed loading fixture, and accurately adjust its installation position according to the structure of the T-type specimen; Effectively fix the specimen by setting gaskets and tightening the tightening screws to avoid slippage or displacement during loading; Adjust the position of the loading head so that it acts on the web area of ​​the T-type specimen, implement a normal shear test, and record the maximum load value during the entire loading process; Based on the recorded maximum load value and the cross-sectional dimensions of the specimen, use the formula to calculate the normal shear strength of the ceramic-based composite material T-joint.

[0039] It will be apparent to those skilled in the art that the present invention is not limited to the details of the exemplary embodiments described above and that the invention can be embodied in other specific forms without departing from the spirit or essential characteristics of the invention. Therefore, the embodiments should be considered in all respects as illustrative and non-restrictive, and the scope of the invention is defined by the appended claims, not the foregoing description, and all variations within the meaning and range of equivalents of the claims are intended to be included therein. Any reference sign in a claim should not be construed as limiting the claim to which it relates.

[0040] In addition, it should be understood that although this specification is described in terms of implementation methods, not every implementation method contains only one independent technical solution. This narrative method of the specification is only for the sake of clarity. Those skilled in the art should regard the specification as a whole. The technical solutions in each embodiment can also be appropriately combined to form other implementation methods that can be understood by those skilled in the art.

Claims

1. A method for testing the normal shear strength of a T-joint of a ceramic matrix composite material, characterized in that: The normal shear strength testing method comprises the following steps: S1. Obtaining the geometric dimensions of the T-shaped specimen (4) of the ceramic matrix composite material to be tested; S2, placing the T-shaped specimen (4) in the loading fixture (1), and adjusting its installation position according to the structure of the T-shaped specimen (4); S3. Fix the T-type specimen (4) by setting the gasket (2) and tightening the top screw (3); S4, adjusting the position of the loading head (5) so that it acts on the web area of ​​the T-type specimen (4), performing a normal shear test, and recording the maximum load value during the entire loading process; S5. Calculate the normal shear strength of the ceramic matrix composite T-joint according to the maximum load value and the cross-sectional dimensions of the T-specimen (4).

2. The normal shear strength testing method according to claim 1, wherein: In the S1, the T-type specimen (4) has a "T"-shaped structure, consisting of a web and a flange, and the web and the flange are connected by two circular arc segments distributed at 90 degrees.

3. The normal shear strength testing method according to claim 1, wherein: In said S1, the geometrical dimensions of the T-type specimen (4) include the overall thickness of the "T"-shaped structure, the length and width of the web, the length and width of the flange, and the radius of the arc segment.

4. The normal shear strength testing method according to claim 2, wherein: In the above-mentioned S2, the T-type specimen (4) is installed in the loading fixture (1), and its installation position is adjusted according to the geometric dimensions of the T-type specimen (4), wherein the loading fixture (1) covers the flange portion of the T-type specimen (4) and simultaneously makes the web portion of the T-type specimen protrude from the middle area of ​​the loading fixture (1).

5. The normal shear strength testing method according to claim 2, wherein: In the above-mentioned S3, when fixing the T-type specimen (4), a gasket (2) is set on the side of the flange of the T-type specimen (4), and then the T-type specimen (4) is pressed into the loading fixture (1) by adjusting and tightening the tightening screw (3).

6. The normal shear strength testing method according to claim 5, wherein: The types of gaskets (2) include but are not limited to flexible graphite composite gaskets, metal spiral wound gaskets, stainless steel gaskets and carbon steel gaskets.

7. The normal shear strength testing method according to claim 1, wherein: In the S4, the loading head (5) is a loading member with a cylindrical end, and the loading point is located inward of the free end of the web of the T-shaped specimen. During the loading process, the loading head (5) moves downward in the vertical direction to apply a normal shear load to the web, and at the same time, a load-displacement curve is recorded by a testing machine, and the maximum load value generated during the loading process is obtained based on the load-displacement curve.

8. The normal shear strength testing method according to claim 1, wherein: In said S5, by recording the maximum load value obtained during the entire loading process and combining it with the effective load-bearing cross-sectional dimensions of the web of the T-type specimen (4), the stress calculation formula is used to evaluate the destructive strength of the T-type specimen (4). The destructive strength is the shear strength of the ceramic matrix composite T-type joint under the normal shear load, and the calculation formula is as follows: ； in, is the normal shear strength, is the maximum load during the test, is the effective shear area of ​​the web, is the width of the web, is the thickness of the T-type specimen of ceramic matrix composites.

9. A loading device used in the normal shear strength testing method according to any one of claims 1 to 8, characterized in that: The loading device comprises a loading fixture (1) and a loading pressure head (5) for testing; a T-shaped specimen (4) is clamped in the loading fixture (1); a tightening screw (3) is threadedly connected to the loading fixture (1); a gasket (2) is provided between the tightening screw (3) and the clamping surface of the T-shaped specimen (4); and the loading pressure head (5) is located directly above the loading point of the T-shaped specimen (4).

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