A method for testing the fracture toughness of fiber-reinforced ceramic matrix composites

Abstract

This invention relates to the field of fracture toughness testing technology, specifically to a method for testing the fracture toughness of fiber-reinforced ceramic matrix composites. This invention utilizes in-situ mechanical testing technology to achieve real-world observation of the entire crack initiation and propagation process of ceramic matrix composites under external loads, providing microscopic experimental data for fracture toughness testing. Then, it combines in-situ testing with finite element simulation, fully considering the influence of microstructure and interlaminar transverse cracking behavior on the overall fracture toughness of ceramic matrix composites, significantly improving the effectiveness and rationality of fracture toughness prediction.

Description

Technical Field

[0001] This invention relates to the field of material fracture toughness testing technology, and specifically to a method for testing the fracture toughness of fiber-reinforced ceramic matrix composites. Background Technology

[0002] Ceramic matrix composites are composite materials with continuous fibers as reinforcement and ceramic materials as the matrix. Ceramic matrix composites possess excellent properties such as high specific strength, high temperature resistance, oxidation resistance, and corrosion resistance. They are widely used in high-temperature and extreme environments such as aerospace hot-end components and hypersonic vehicles. The mechanical properties and cracking behavior of fiber-reinforced ceramic matrix composites under external loads are closely related to their microstructure. The fiber volume ratio and distribution, the interfacial bonding strength between the fiber and the matrix, the porosity of the preform, and residual stress all affect the crack propagation path of ceramic matrix composites, thus increasing the difficulty of fracture toughness testing. Unlike the propagation of the main crack in general ceramic materials along the direction perpendicular to the load, the vertical main crack propagation of two-dimensional braided fiber-reinforced ceramic matrix composites is prone to deflection under external loads due to the weaker interfacial bonding between the fiber and the matrix. Upon reaching the fiber-matrix interface, the crack will propagate laterally along the interface. For three-dimensional puncture-reinforced fiber-reinforced ceramic matrix composites, when the main crack deflects and propagates along the fiber-matrix interface under external load, the longitudinal fibers hinder further propagation of the interface crack, causing the crack to deflect again and propagate along the longitudinal fiber-matrix interface. Therefore, the deflection phenomenon of the main crack poses a technical challenge to predicting the overall fracture toughness of two-dimensional and three-dimensional fiber-reinforced ceramic matrix composites. Conventional methods for testing the fracture toughness of ceramic matrix composites, including the double cantilever beam method, three-point bending method, single-sided notched beam method, and indentation method, are more suitable for ceramics or metals with isotropic microstructures, but cannot fully consider the influence of microstructure and interlaminar transverse cracking behavior on the overall fracture toughness of ceramic matrix composites, resulting in low accuracy in fracture toughness prediction. Currently, there are no effective experimental methods or mathematical models that can reasonably consider the influence of crack deflection on the overall fracture toughness of ceramic matrix composites. Summary of the Invention

[0003] To address the problems existing in the prior art, the present invention aims to provide a method for testing the fracture toughness of fiber-reinforced ceramic matrix composites. This method takes into account the influence of crack deflection on the overall fracture toughness of the ceramic matrix composites, making the test results more reasonable and accurate.

[0004] Based on the above objectives, the technical solution adopted by the present invention is as follows:

[0005] A method for testing the fracture toughness of fiber-reinforced ceramic matrix composites, characterized in that the method comprises the following steps:

[0006] Referring to GB / T 23806-2009 "Test Method for Fracture Toughness of Fine Ceramics", a three-point bending test specimen of a single-sided notched beam was prepared and designated as the first specimen. An in-situ three-point bending test was carried out until the first specimen fractured. During the test, a fixed displacement was used to control the loading, and the load-indenter displacement curve was recorded in real time. The pre-crack tip and its surrounding area were observed to obtain the load and indenter displacement P corresponding to the initiation of the transverse crack, the indenter displacement Q corresponding to the maximum load, and the functional relationship between the indenter displacement and the transverse crack length or the length of the transverse crack at different indenter displacements. The material of the first specimen is a two-dimensional braided fiber reinforced ceramic matrix composite material.

[0007] A finite element model containing a pre-existing crack was established in the finite element software. The constructed specimen was designated as the second specimen, and the following conditions were set:

[0008] The second sample is made of a purely elastic, isotropic, homogeneous material. The input values ​​are the elastic modulus and Poisson's ratio.

[0009] A rigid indenter is placed above the second specimen, and displacement boundary conditions are used to control the pressing behavior. Two rigid rollers are placed below to support the specimen, fixing the movement of the second specimen in all directions. There is no friction between the second specimen and the rollers and indenter, and all face-to-face contact conditions are hard contact.

[0010] The length, width, height, chamfer, pre-crack depth and notch width, lower span, roller diameter, indenter loading rate, and test temperature of the second specimen were all consistent with those of the in-situ three-point bending test.

[0011] When the indenter displacement reaches the value of the indenter displacement P, the vertical crack deflects into a transverse crack. Based on the in-situ three-point bending test, the functional relationship between the indenter displacement and the transverse crack length is set, or the transverse crack length is set for different indenter displacements. Loading is stopped when the indenter depth increases linearly to depth D.

[0012] A ring mesh of 5-10 layers is drawn at the crack tip, with the ring closest to the crack tip set as a singular element mesh. The J integral of each layer of mesh is solved by the perimeter integral method to obtain the energy release rate and stress intensity factor. The stress intensity factor-displacement load curve is plotted. The stress intensity factor corresponding to the indenter displacement Q in the curve is the fracture toughness.

[0013] The depth D is greater than the indenter displacement Q.

[0014] In this invention, the energy release rate = the sum of the J integrals of each loop mesh layer / the total number of loop mesh layers.

[0015] The method described above enables the in-situ mechanical testing technique to realize the full-process observation of crack initiation and propagation in ceramic matrix composites under external load, providing microscopic experimental data for fracture toughness testing. Based on the combination of in-situ testing and finite element simulation, the method fully considers the influence of microstructure and interlaminar transverse cracking behavior on the overall fracture toughness of ceramic matrix composites, significantly improving the effectiveness and rationality of fracture toughness prediction.

[0016] Preferably, 0.1mm ≤ the depth D - the pressure head displacement Q ≤ 0.3mm.

[0017] Preferably, the stress intensity factor is calculated according to equation (1):

[0018]

[0019] in, Stress intensity factor, in MPa·m 1 / 2 ;

[0020] This refers to the elastic modulus, expressed in MPa.

[0021] Poisson's ratio;

[0022] G is the energy release factor, measured in J / m³. 2 It should be noted that, since the interface crack propagation is a mixed propagation mode, the stress intensity factor here is the equivalent stress intensity factor. Equation (1) mainly considers the influence of type II cracking.

[0023] Preferably, when establishing a finite element model containing pre-existing cracks, the length of the transverse crack is set for different indenter displacements, and the set indenter displacements include at least one indenter displacement greater than the indenter displacement Q.

[0024] Preferably, the first sample is 18 mm long, 3 mm wide, and 4 mm high, with a chamfer of 0.12 mm and a roughness of 0.2 μm; a wire saw is used to pre-crack the sample, with the crack located in the middle of the length direction of the first sample, the lower surface being a tensile surface, the pre-crack depth being 2 mm, the cut width being 0.1 mm, the lower span being 16 mm, and the roller diameter being 4 mm.

[0025] Preferably, in the in-situ three-point bending test, a 10N preload is applied before loading for sample positioning and fixation; displacement-controlled loading is used with a loading rate of 0.002mm / s; and the test temperature is 25~1300℃.

[0026] Preferably, the two-dimensional braided fiber reinforced ceramic matrix composite material includes at least one of carbon fiber reinforced ceramic matrix composite material or silicon carbide fiber reinforced ceramic matrix composite material.

[0027] Preferably, the material of the first sample is a three-dimensional puncture fiber reinforced ceramic matrix composite material;

[0028] The in-situ three-point bending test also yielded the indenter displacement L corresponding to the obstruction of the transverse crack.

[0029] In the process of establishing the finite element model containing the pre-existing crack, it is also set that the transverse crack is hindered when the indenter displacement reaches the value of the indenter displacement L.

[0030] Preferably, the three-dimensional puncture fiber reinforced ceramic matrix composite material includes silicon carbide fiber reinforced ceramic matrix composite material.

[0031] In this invention, during the in-situ three-point bending test, for two-dimensional braided fiber reinforced ceramic matrix composites, the focus is on the process of the main crack being laterally deflected and the initiation and propagation of the transverse crack along the fiber matrix interface; for three-dimensional puncture fiber reinforced ceramic matrix composites, in addition to focusing on the process of the main crack being laterally deflected and the initiation and propagation of the transverse crack along the fiber matrix interface, the focus is also on the process of the longitudinal fibers causing the transverse crack to be deflected a second time and changing the crack propagation path.

[0032] Preferably, the finite element software is ABAQUS or ANASYS.

[0033] Preferably, the in-situ three-point bending test is carried out on an in-situ mechanical testing system equipped with a scanning electron microscope or a high-powered microscope.

[0034] Compared with the prior art, the beneficial effects of the present invention are as follows: The present invention realizes the factual observation of the entire process of crack initiation and propagation in ceramic matrix composites under external load through in-situ mechanical testing technology, providing microscopic experimental data for fracture toughness testing; based on the method of combining in-situ testing and finite element simulation, it fully considers the influence of microstructure and interlaminar transverse cracking behavior on the overall fracture toughness of ceramic matrix composites, and greatly improves the effectiveness and rationality of fracture toughness prediction. Attached Figure Description

[0035] Figure 1 This is a flowchart of the method for testing the fracture toughness of fiber-reinforced ceramic matrix composites according to the present invention;

[0036] Figure 2 For example a, the finite element model is (a) a three-point bending finite element model with longitudinal pre-existing cracks and no interface cracking, and (b) finite element mesh generation.

[0037] Figure 3For example b and c, the finite element models are: (a) a three-point bending finite element geometric model with a transverse deflection interface crack, where the crack propagates transversely along the interface; and (b) a three-point bending finite element geometric model where the transverse deflection interface crack is blocked, where the crack stops propagating transversely at the longitudinal fiber.

[0038] Figure 4 Finite element geometric model and mesh generation for 2D-SiCf / SiC composite materials containing interface cracks of different lengths;

[0039] Figure 5 The curves showing the functional relationship between stress intensity factor and displacement load in three finite element models are shown, corresponding to examples a-1, b-1, and c-1, respectively.

[0040] Figure 6 The results show the predicted fracture toughness of ceramic matrix composites with different preform structures at room temperature (25℃) and high temperature (1300℃). Detailed Implementation

[0041] To better illustrate the objectives, technical solutions, and advantages of this invention, the invention will be further described below with reference to specific embodiments. Those skilled in the art should understand that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention.

[0042] Unless otherwise specified, the experimental methods used in the examples are conventional methods; the materials and reagents used are commercially available unless otherwise specified. The same raw materials were used in each parallel experiment.

[0043] Calculation example a-1

[0044] Step 1: Prepare a single-sided notched beam three-point bending test specimen, designated as the first specimen, according to GB / T 23806-2009 "Test Method for Fracture Toughness of Fine Ceramics". The first specimen has dimensions of 18 mm in length, 3 mm in width, and 4 mm in height, with a chamfer of 0.12 mm and a surface roughness of 0.2 μm. A pre-crack is created using a wire saw, located at the exact center of the specimen's length, with the lower surface being the tensile surface. The pre-crack depth is 2 mm, the notch width is 0.1 mm, and the span is 16 mm. The roller diameter is 4 mm. The material of the first specimen is 2D-SiC. f / SiC.

[0045] Step 2: Conduct an in-situ three-point bending test at room temperature (25℃, the same below) on an in-situ mechanical testing system equipped with a scanning electron microscope or high-powered microscope. Before loading, apply a 10N preload to position and fix the first specimen. During loading, use fixed displacement control to control the loading rate at 0.002mm / s. During the test, record the load-indenter displacement curve in real time. Simultaneously with loading, use a scanning electron microscope or high-powered microscope to observe the pre-existing crack tip and its surrounding area in situ, recording the entire process of crack initiation and propagation, and obtaining the indenter displacement Q corresponding to the maximum load. The test ends when the specimen fractures, where the indenter displacement Q = 0.24mm.

[0046] Step 3: Based on the in-situ test results, establish a finite element model in the finite element software ABAQUS containing pre-existing cracks and without interfacial transverse cracking (e.g., Figure 2 (As shown). The constructed specimen is designated as the second specimen, and the following conditions are set:

[0047] The second sample was made of a purely elastic, isotropic, and homogeneous material, with an input elastic modulus of 100 GPa and a Poisson's ratio of 0.1.

[0048] A rigid indenter is placed above the second specimen, and displacement boundary conditions are used to control the pressing behavior. Two rigid rollers are placed below to support the specimen, fixing the movement of the second specimen in all directions. There is no friction between the second specimen and the rollers and indenter, and all face-to-face contact conditions are hard contact.

[0049] The length, width, height, chamfer, pre-crack depth and notch width, lower span, roller diameter, indenter loading rate, and test temperature of the second specimen were all consistent with those of the in-situ three-point bending test.

[0050] Loading stops when the pressure head presses down linearly to a depth D, where depth D is 0.4 mm.

[0051] Using the software described above, an 8-layer ring mesh is automatically drawn at the crack tip, with the ring closest to the crack tip set as a singular element mesh. The J-integral of each layer of mesh is automatically solved using the perimeter integral method. The average of the 8 J-integrals obtained under the same displacement load is the energy release rate under that displacement load. Then, the stress intensity factor under that displacement load is automatically calculated according to equations (1) and (2).

[0052]

[0053] in, Stress intensity factor, in MPa·m 1 / 2 ,

[0054] This refers to the elastic modulus, with units of MPa.

[0055] Poisson's ratio,

[0056] G is the energy release factor, measured in J / m³. 2 ,

[0057] Automatically plot the stress intensity factor-displacement load curve.

[0058] Step 4: The stress intensity factor corresponding to the indenter displacement Q in the stress intensity factor-displacement load curve is the fracture toughness.

[0059] Example b-1

[0060] Step 1: Prepare a three-point bending test specimen with a single-sided notched beam according to GB / T 23806-2009 "Test Method for Fracture Toughness of Fine Ceramics". This specimen is designated as the first specimen. The first specimen has dimensions of 18 mm in length, 3 mm in width, and 4 mm in height, with a chamfer of 0.12 mm and a surface roughness of 0.2 μm. A pre-crack is created using a wire saw, located at the exact center of the specimen's length. The lower surface is the tensile surface. The pre-crack depth is 2 mm, the notch width is 0.1 mm, and the span is 16 mm. The roller diameter is 4 mm. The material of the first specimen is 2D-SiC. f / SiC.

[0061] Step 2: Conduct a room temperature in-situ three-point bending test on an in-situ mechanical testing system equipped with a scanning electron microscope or high-powered microscope. Before loading, apply a 10N preload to position and fix the first specimen. During loading, use a fixed displacement control method with a loading rate of 0.002 mm / s. During the test, record the load-indenter displacement curve in real time. Simultaneously with loading, use a scanning electron microscope or high-powered microscope to observe the pre-existing crack tip and its surrounding area in situ, recording the entire process of crack initiation and propagation. Obtain the indenter displacement P corresponding to the initiation of the transverse crack, the indenter displacement Q corresponding to the maximum load, and the length of the transverse crack at different indenter displacements. The test ends when the specimen fractures, where the indenter displacement P = 0.074 mm and the indenter displacement Q = 0.24 mm.

[0062] Step 3: Based on the in-situ test results, establish a finite element model containing transverse interface cracks in the finite element software ABAQUS (e.g., Figure 3 (As shown). The constructed specimen is designated as the second specimen, and the following conditions are set:

[0063] The second sample was made of a purely elastic, isotropic, and homogeneous material, with an input elastic modulus of 100 GPa and a Poisson's ratio of 0.1.

[0064] A rigid indenter is placed above the second specimen, and displacement boundary conditions are used to control the pressing behavior. Two rigid rollers are placed below to support the specimen, fixing the movement of the second specimen in all directions. There is no friction between the second specimen and the rollers and indenter, and all face-to-face contact conditions are hard contact.

[0065] The length, width, height, chamfer, pre-crack depth and notch width, lower span, roller diameter, indenter loading rate, and test temperature of the second specimen were all consistent with those of the in-situ three-point bending test.

[0066] When the indenter displacement reaches the aforementioned indenter displacement P, the vertical crack deflects into a transverse crack (i.e., transverse crack initiation, the same below). Based on the results of the aforementioned in-situ three-point bending test, the length of the transverse crack at different indenter displacements is set as follows:

[0067] When the indenter's downward pressing distance (i.e., indenter displacement, the same below) is 0.08 mm, the transverse crack length is approximately 0.25 mm;

[0068] When the indenter's downward pressing distance is 0.11 mm, the transverse crack length is approximately 0.55 mm;

[0069] When the indenter's downward pressing distance is 0.18 mm, the transverse crack length is approximately 0.75 mm;

[0070] When the indenter's downward pressing distance is 0.23 mm, the transverse crack length is approximately 1.0 mm;

[0071] When the pressure head presses down a distance of 0.3 mm, the transverse crack length is approximately 1.5 mm.

[0072] The system is set to stop loading when the pressure head presses down linearly to a depth D, where depth D is 0.4 mm.

[0073] Using the software described above, an 8-layer ring mesh is automatically drawn at the crack tip, with the ring closest to the crack tip set as a singular element mesh. The J-integral of each layer of mesh is automatically solved using the perimeter integral method. The average of the 8 J-integrals obtained under the same displacement load is the energy release rate under that displacement load. Then, the stress intensity factor under that displacement load is automatically calculated according to equations (1) and (2).

[0074]

[0075] in, Stress intensity factor, in MPa·m 1 / 2 ,

[0076] This refers to the elastic modulus, with units of MPa.

[0077] Poisson's ratio,

[0078] G is the energy release factor, measured in J / m³. 2 ,

[0079] Automatically plot the stress intensity factor-displacement load curve.

[0080] Step 4: The stress intensity factor corresponding to the indenter displacement Q in the stress intensity factor-displacement load curve is the fracture toughness.

[0081] Example c-1

[0082] Step 1: Prepare a single-sided notched beam three-point bending test specimen, designated as the first specimen, according to GB / T 23806-2009 "Test Method for Fracture Toughness of Fine Ceramics". The first specimen has dimensions of 18 mm in length, 3 mm in width, and 4 mm in height, with a chamfer of 0.12 mm and a surface roughness of 0.2 μm. A pre-crack is created using a wire saw, located at the exact center of the specimen's length, with the lower surface being the tensile surface. The pre-crack depth is 2 mm, the notch width is 0.1 mm, and the span is 16 mm. The roller diameter is 4 mm. The material of the first specimen is 3D-SiC. f / SiC.

[0083] Step 2: Conduct a room temperature in-situ three-point bending test on an in-situ mechanical testing system equipped with a scanning electron microscope or high-powered microscope. Before loading, apply a 10N preload to position and fix the first specimen. During loading, use a fixed displacement control method with a loading rate of 0.002mm / s. During the test, record the load-indenter displacement curve in real time. Simultaneously with loading, use a scanning electron microscope or high-powered microscope to observe the pre-existing crack tip and its surrounding area in situ, recording the entire process of crack initiation and propagation. Obtain the indenter displacement P corresponding to transverse crack initiation, the indenter displacement L corresponding to transverse crack obstruction, the indenter displacement Q corresponding to the maximum load, and the length of the transverse crack at different indenter displacements. The test ends when the specimen fractures, where indenter displacement P = 0.074mm, indenter displacement L = 0.110mm, and indenter displacement Q = 0.195mm.

[0084] Step 3: Based on the in-situ test results, establish a finite element model of the interface with hindered transverse cracking in the finite element software ABAQUS (e.g., Figure 4 (As shown). The constructed specimen is designated as the second specimen, and the following conditions are set:

[0085] The second sample was made of a purely elastic, isotropic, and homogeneous material, with an input elastic modulus of 100 GPa and a Poisson's ratio of 0.1.

[0086] A rigid indenter is placed above the second specimen, and displacement boundary conditions are used to control the pressing behavior. Two rigid rollers are placed below to support the specimen, fixing the movement of the second specimen in all directions. There is no friction between the second specimen and the rollers and indenter, and all face-to-face contact conditions are hard contact.

[0087] The length, width, height, chamfer, pre-crack depth and notch width, lower span, roller diameter, indenter loading rate, and test temperature of the second specimen were all consistent with those of the in-situ three-point bending test.

[0088] When the indenter displacement reaches the aforementioned indenter displacement P, the vertical crack deflects into a transverse crack. Based on the results of the aforementioned in-situ three-point bending test, the length of the transverse crack is set for different indenter displacements, specifically as follows:

[0089] When the indenter's downward pressing distance is 0.08 mm, the transverse crack length is approximately 0.25 mm;

[0090] When the indenter's downward pressing distance is 0.11 mm, the transverse crack length is approximately 0.55 mm;

[0091] When the indenter's downward pressing distance is 0.18 mm, the transverse crack length is approximately 0.55 mm;

[0092] When the indenter's downward pressure distance is 0.23 mm, the transverse crack length is approximately 0.55 mm;

[0093] When the pressure head presses down a distance of 0.3 mm, the transverse crack length is approximately 0.55 mm.

[0094] The system is set to allow crack coalescence (i.e., transverse cracks are hindered) when the indenter displacement reaches the value of the aforementioned indenter displacement L. Loading is stopped when the indenter depth increases linearly to depth D, where depth D is 0.4 mm.

[0095] Using the software described above, an 8-layer ring mesh is automatically drawn at the crack tip, with the ring closest to the crack tip set as a singular element mesh. The J-integral of each layer of mesh is automatically solved using the perimeter integral method. The average of the 8 J-integrals obtained under the same displacement load is the energy release rate under that displacement load. Then, the stress intensity factor under that displacement load is automatically calculated according to equations (1) and (2).

[0096]

[0097] in, Stress intensity factor, in MPa·m 1 / 2 ,

[0098] This refers to the elastic modulus, with units of MPa.

[0099] Poisson's ratio,

[0100] G is the energy release factor, measured in J / m³. 2 ,

[0101] Automatically plot the stress intensity factor-displacement load curve.

[0102] Step 4: The stress intensity factor corresponding to the indenter displacement Q in the stress intensity factor-displacement load curve is the fracture toughness.

[0103] Calculation example a-2

[0104] The difference from example a-1 is that the in-situ three-point bending test temperature is 1300℃ and the indenter displacement Q=0.265mm; when establishing the finite element model in the finite element software ABAQUS, the input elastic modulus is 70GPa.

[0105] Example b-2

[0106] The difference from Example b-1 is that the in-situ three-point bending test was conducted at a temperature of 1300℃, with indenter displacements P=0.062mm and Q=0.265mm. When establishing the finite element model in the ABAQUS software, the input elastic modulus was 70GPa. The specific lengths of the transverse cracks under different indenter displacements are as follows:

[0107] When the indenter's downward pressing distance is 0.070 mm, the transverse crack length is approximately 0.21 mm;

[0108] When the indenter's downward pressing distance is 0.130 mm, the transverse crack length is approximately 0.43 mm;

[0109] When the indenter's downward pressing distance is 0.176 mm, the transverse crack length is approximately 0.50 mm;

[0110] When the indenter's downward pressing distance is 0.19 mm, the transverse crack length is approximately 0.85 mm;

[0111] When the pressure head presses down a distance of 0.30 mm, the transverse crack length is approximately 1.60 mm.

[0112] Example c-2

[0113] The difference from example c-1 is that the in-situ three-point bending test was conducted at a temperature of 1300℃, with indenter displacements P=0.062mm, L=0.176mm, and Q=0.205mm. When establishing the finite element model in the ABAQUS software, the input elastic modulus was 70GPa. The specific lengths of the transverse cracks under different indenter displacements are as follows:

[0114] When the indenter's downward pressing distance is 0.070 mm, the transverse crack length is approximately 0.21 mm;

[0115] When the indenter's downward pressing distance is 0.130 mm, the transverse crack length is approximately 0.43 mm;

[0116] When the indenter's downward pressing distance is 0.176 mm, the transverse crack length is approximately 0.50 mm;

[0117] When the indenter's downward pressing distance is 0.19 mm, the transverse crack length is approximately 0.50 mm;

[0118] When the pressure head presses down a distance of 0.30 mm, the transverse crack length is approximately 0.50 mm.

[0119] Depend on Figure 5 It can be seen that when interface cracking is not considered (as in example a-1), the stress intensity factor increases linearly with the increase of displacement load. When interface cracking is considered (as in example b-1), the energy release rate decreases significantly, and therefore the stress intensity factor decreases compared to the case without interface cracking. With the indentation of the indenter and the propagation of the interface crack, the growth of the stress intensity factor follows a quadratic function shape until it reaches a critical value and the crack becomes unstable. When the interface crack encounters resistance and stops propagating (as in example c-1), strain energy continues to accumulate, and the stress intensity factor will be higher than the growth rate of the continuously propagating interface crack, until the critical fracture toughness specimen fails.

[0120] The stress intensity factor-displacement load curves obtained from the finite element model in the above examples are as follows: Figure 6As shown in the figure, RT represents the room temperature environment, HT represents the 1300℃ high temperature environment, solid lines of different colors represent the numerical simulation values ​​of stress intensity factors of different composite materials, dashed lines represent the numerical simulation values ​​of stress intensity factors under ideal conditions without considering interface cracking, and lines of the same color represent the same set of parallel tests. The stress intensity factor corresponding to the indenter displacement when the load of each material reaches its maximum is the predicted fracture toughness value. The dotted line represents the experimentally measured values ​​of fracture toughness of each composite material (calculated by the three-point bending fracture toughness formula provided by the national standard GB / T 23806-2009). As can be seen from the figure, the corresponding indenter displacements for the initiation of interface cracks are different for different materials and temperatures, and the inflection points of the decrease in stress intensity factor appear at different locations. For 2D-SiCf / SiC and 3D-SiCf / SiC composite materials, interface cracking releases some strain energy, causing the stress intensity factor to decrease compared to the case without interface cracking. Furthermore, due to the decrease in elastic modulus caused by high temperature, the stress intensity factors of both composite materials are slightly lower than those at room temperature, while the indenter displacement corresponding to the maximum load is slightly greater than that at room temperature. This results in the high-temperature fracture toughness of both composite materials being slightly lower than their room-temperature fracture toughness, consistent with experimental results. Compared to the 2D-SiCf / SiC composite material, at both room temperature and high temperature, the interfacial cracks in the 3D-SiCf / SiC composite material are unable to propagate laterally due to the obstruction of the longitudinal fiber bundles, leading to strain energy accumulation and a downward propagation tendency. Therefore, the 3D-SiCf / SiC composite material has a higher stress intensity factor, is more prone to crack instability, and exhibits lower fracture toughness, consistent with experimental results. In addition, due to... Figure 6 As shown, the abscissas of the intersection points of the stress intensity factors and the experimentally measured fracture toughness values ​​of the two composite materials at room temperature and high temperature are concentrated in the range of 0.18 mm to 0.27 mm. This indicates that the predicted crack instability time for all three composite materials occurs within the period of 0.18 mm < indenter displacement < 0.27 mm, which is consistent with the in-situ test results. In summary, the fracture toughness prediction model for ceramic matrix composites based on the finite element model agrees well with the experimental results, demonstrating good effectiveness and reliability.

[0121] 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 the scope of protection of the present invention. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the essence and scope of the technical solutions of the present invention.

Claims

1. A method for testing the fracture toughness of fiber-reinforced ceramic matrix composites, characterized in that, The method includes the following steps: Referring to GB / T 23806-2009 "Test Method for Fracture Toughness of Fine Ceramics", a three-point bending test specimen of a single-sided notched beam was prepared and designated as the first specimen. An in-situ three-point bending test was carried out until the first specimen fractured. During the test, a fixed displacement was used to control the loading, and the load-indenter displacement curve was recorded in real time. The pre-crack tip and its surrounding area were observed to obtain the load and indenter displacement P corresponding to the initiation of the transverse crack, the indenter displacement Q corresponding to the maximum load, and the functional relationship between the indenter displacement and the transverse crack length or the length of the transverse crack at different indenter displacements. The material of the first specimen is a two-dimensional braided fiber reinforced ceramic matrix composite material. A finite element model containing a pre-existing crack was established in the finite element software. The constructed specimen was designated as the second specimen, and the following conditions were set: The second sample is made of a purely elastic, isotropic, homogeneous material. The input values ​​are the elastic modulus and Poisson's ratio. A rigid indenter is placed above the second specimen, and displacement boundary conditions are used to control the pressing behavior. Two rigid rollers are placed below to support the specimen, fixing the movement of the second specimen in all directions. There is no friction between the second specimen and the rollers and indenter, and all face-to-face contact conditions are hard contact. The length, width, height, chamfer, pre-crack depth and notch width, lower span, roller diameter, indenter loading rate, and test temperature of the second specimen were all consistent with those of the in-situ three-point bending test. When the indenter displacement reaches the value of the indenter displacement P, a transverse crack initiates. Based on the in-situ three-point bending test, a functional relationship between the indenter displacement and the transverse crack length is set, or the transverse crack length is set for different indenter displacements. Loading is stopped when the indenter depth increases linearly to depth D. A ring mesh of 5-10 layers is drawn at the crack tip, with the ring closest to the crack tip set as a singular element mesh. The J integral of each layer of mesh is solved by the perimeter integral method to obtain the energy release rate and stress intensity factor. The stress intensity factor-displacement load curve is plotted. The stress intensity factor corresponding to the indenter displacement Q in the curve is the fracture toughness. The depth D is greater than the indenter displacement Q.

2. The method as described in claim 1, characterized in that, 0.1mm ≤ Depth D - Indenter Displacement Q ≤ 0.3mm.

3. The method as described in claim 1, characterized in that, The stress intensity factor is calculated according to equation (1): in, Stress intensity factor, in MPa·m 1 / 2 ; This refers to the elastic modulus, expressed in MPa. Poisson's ratio; G is the energy release factor, measured in J / m³. 2 .

4. The method as described in claim 1, characterized in that, In the process of establishing a finite element model containing pre-existing cracks, when setting the length of the transverse crack for different indenter displacements, the set indenter displacement must include at least one indenter displacement greater than the indenter displacement Q.

5. The method as described in claim 1, characterized in that, The first sample is 18 mm long, 3 mm wide, and 4 mm high, with a chamfer of 0.12 mm and a roughness of 0.2 μm. A wire saw is used to pre-crack the sample, which is located in the middle of the length direction of the first sample. The lower surface is a tensile surface, the pre-crack depth is 2 mm, the cut width is 0.1 mm, the lower span is 16 mm, and the roller diameter is 4 mm.

6. The method as described in claim 1, characterized in that, In the in-situ three-point bending test, a 10N preload is applied before loading for sample positioning and fixation; displacement-controlled loading is used with a loading rate of 0.002mm / s; and the test temperature is 25~1300℃.

7. The method as described in claim 1, characterized in that, The two-dimensional braided fiber reinforced ceramic matrix composite material includes at least one of carbon fiber reinforced ceramic matrix composite material or silicon carbide fiber reinforced ceramic matrix composite material.

8. The method as described in claim 1, characterized in that, The material of the first sample is a three-dimensional puncture fiber reinforced ceramic matrix composite material; The in-situ three-point bending test also yielded the indenter displacement L corresponding to the obstruction of the transverse crack. In the process of establishing the finite element model containing the pre-existing crack, it is also set that the transverse crack is hindered when the indenter displacement reaches the value of the indenter displacement L.

9. The method as described in claim 8, characterized in that, The three-dimensional puncture fiber-reinforced ceramic matrix composite material includes silicon carbide fiber-reinforced ceramic matrix composite material.

10. The method as described in claim 1, characterized in that, The finite element software is ABAQUS or ANASYS.

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