A method for testing the fracture toughness of ceramic materials based on the constant load endurance method

Abstract

This invention discloses a method for testing the fracture toughness of ceramic materials based on the constant load endurance method, comprising the following steps: (1) processing ceramic sheets to obtain ceramic specimens; (2) processing ceramic notches; (3) preliminary test: performing a three-point bending test on the ceramic specimen with the notch, referring to the Type II specimen crushing test in GB / T23806-2009, and obtaining the maximum load F1; (4) constant load endurance test: loading to F2, holding the load for a certain time and then unloading; if the specimen fails during the holding period, record F2; if it does not fail, gradually increase the load based on F2, repeat step (4) until the specimen breaks, and record F3; (5) measuring the crack size; (6) calculation. The test method of this invention is simple to operate, avoids the disadvantage of low success rate of pre-crack in GB / T 23806, and also eliminates the influence of blunt tip of the notch. The test has a high success rate and good reproducibility, and is easy to widely promote and apply.

Description

Technical Field

[0001] This invention relates to a method for testing the fracture toughness of brittle ceramic materials, and particularly to a method for testing the fracture toughness of ceramic materials based on the constant load endurance method. Background Technology

[0002] Ceramic materials are among the stiffest and hardest engineering materials, characterized by high compressive strength, low tensile strength, and poor plasticity and toughness. Specialty ceramics, as an advanced functional material, are used in critical components of tanks, armored vehicles, aircraft, ships, vehicles, and nuclear power plants due to their high hardness, high elastic modulus, wear resistance, high temperature resistance, and low density.

[0003] The excellent physical properties of ceramic materials determine their broad application prospects, but the brittleness of ceramic materials seriously restricts their application. Therefore, research on ceramic toughening technology has been carried out extensively at home and abroad for many years.

[0004] Ceramic material fracture toughness K IC As an important mechanical property indicator for measuring the brittleness or toughness of ceramics, its accurate measurement has always been a core and hot topic of widespread concern for researchers both at home and abroad. Currently, the main methods for testing the fracture toughness of ceramic materials include: indentation method (IM), single-sided pre-cracked beam method (SEPB), single-sided notched beam method (SENB), mountain-shaped notched beam method (CN), and single-sided V-notched beam method (SEVNB).

[0005] The indentation method is the simplest to operate, but its mechanical calculation theory is incomplete. JIS R1607, ISO21618 and many other scholars have proposed calculation formulas. Different formulas often yield different or even significantly different evaluation results for the same material. It is also affected by the resolution of the equipment.

[0006] The single-sided pre-cracked beam method (JIS R1607, GB / T 23806, ASTM C1421) is the most typical fracture toughness test model. However, its sample preparation success rate is extremely low and its operability is poor. This method is rarely used for fracture toughness testing both domestically and internationally, making it difficult to promote its application. Most domestic testing institutions use wire cutting for direct testing, which results in severe notch tip blunting, causing the test results to be artificially inflated by half or even double. In response to this situation, scholars at home and abroad have turned to femtosecond lasers to obtain sharp notch sizes for testing. However, the influence of laser heat input and notch effect on the results is unknown.

[0007] The notched beam method (ISO 23146) is simple to prepare and is currently the most widely used method for testing the fracture toughness of ceramics by various testing institutions. However, the blunting effect at the notch tip is significant, leading to overestimation of the measured results. Some testing institutions even use wire cutting to obtain notch tips larger than 100 μm for testing, resulting in abnormally large results. In view of this, scholars at home and abroad have turned to new methods such as femtosecond lasers to obtain ultra-sharp V-notches, and have initially obtained relatively reliable results for the fracture toughness of ceramics. However, the reliability of the results is still questionable due to the thermal effect of the notch tip on the laser. Summary of the Invention

[0008] To address the shortcomings of the existing technology, this invention provides a method for testing the fracture toughness of ceramic materials based on the constant load endurance method.

[0009] The technical solution of the present invention to solve the above-mentioned technical problems is as follows:

[0010] A method for testing the fracture toughness of ceramic materials based on the constant load endurance method includes the following steps:

[0011] (1) Ceramic strips are prepared by processing ceramic sheets;

[0012] (2) Cut a ceramic strip to obtain a notched specimen, wherein the notch tip curvature diameter is less than 20 μm;

[0013] (3) Preliminary test: The sample obtained in step (2) is subjected to a three-point bending test. The notch is facing down, and a deflection meter is placed under the notch. The sample of type II in GB / T23806-2009 is crushed at a certain loading speed to obtain the maximum load F1.

[0014] (4) Fixed load endurance test: Take the sample obtained in step (2) and load it to F2 at the same loading speed as in step (3). F2 is not greater than F1. After holding the load for a certain period of time, unload it. If the notched sample fails during the holding load process, record the load F2. If it does not fail, increase the load gradually on the basis of F2 and repeat step (4) until the notched sample breaks. Record the load as F3.

[0015] (5) Measure the crack size: Use a scanning electron microscope to observe the size of the crack propagation area in step (4), and take the notch size obtained by cutting in step (2) and the crack propagation area size in step (4) as the total notch size;

[0016] (6) Calculation: Fracture toughness was calculated based on the Type II specimen of GB / T23806-2009 and formula (1):

[0017]

[0018] Wherein, Pt represents the maximum load N at which the specimen fractures;

[0019] d - Three-point bending span (mm);

[0020] t - Sample thickness (mm);

[0021] w - Sample width (mm);

[0022] l - Precrack length (mm);

[0023] Y - shape factor, which can be obtained from a table.

[0024] In step (3), the deflectometer (LVDT) is used to measure the deformation of the sample. During the initial test, the sample can be crushed to break in one go. When using the constant load endurance method, the equipment software is set to load 0.5 mm / min to 90% of F1. After reaching the load, it is held for 40-60 seconds and then unloaded (if the ceramic material is sensitive, it will break within a few seconds to tens of seconds. If it is not sensitive, it will not break within one minute. It is recommended to hold the load for no more than 60 seconds). If the sample fails during the holding period, the maximum load F2 is recorded. If it does not fail, the load is increased by 2N or other loads as a gradient on the basis of F2. The above operation is repeated until the sample breaks and the load F3 is recorded.

[0025] The final coarse cut, fine cut, and constant-load creep propagation crack size are taken as the overall notch size. Extensive experiments have shown that the constant-load creep propagation crack size is approximately 30-70 μm, which is relatively small compared to the overall notch size.

[0026] Y(l)

[0027] The shape factor w is obtained from a standard table. 50μm is about 0.01 relative to 4mm w. The difference in shape factor is not significant. If possible, it can be tested using a scanning electron microscope. If not, the notch size of coarse and fine cuts can be used directly, or 50μm can be added for calculation.

[0028] Furthermore, in step (3), the loading speed is 0.5 mm / min.

[0029] Furthermore, in step (4), F2 is 90% of F1, and the holding time does not exceed 60s.

[0030] Furthermore, the holding time is 40-60 seconds.

[0031] Further, in step (1), the size of the ceramic sample is 3mm×4mm×40mm. The ceramic sheet is cut into a sample using a ceramic cutting machine, and then the ceramic is finely ground into a sample of 3mm×4mm×40mm using a ceramic grinding machine. In step (2), the cutting of the ceramic sample includes ceramic coarse cutting and ceramic fine cutting. The total size of the notch is 1.4mm to 2mm, and the same parameters are used for samples in the same group to obtain notch sizes that are as close as possible.

[0032] Furthermore, the ceramic rough cutting is performed by using a 0.25mm or 0.12mm wire cutter to cut a rough kerf on the 3mm side of the ceramic template. The depth of the rough kerf is 1.2mm to 1.4mm, which is 30% to 35% of the thickness in the 4mm direction.

[0033] Furthermore, the ceramic fine cutting is achieved by laser cutting or cutting with an ultra-thin razor blade dipped in polishing paste to obtain a fine ceramic kerf. The depth of the fine kerf is 0.2mm to 0.4mm, ultimately ensuring that the diameter of the kerf tip curvature is no greater than 20μm. Optionally, the laser cutting is selected from laser engraving machines, femtosecond lasers, or other fine-beam laser cutting methods.

[0034] Furthermore, the size of the crack propagation region is 30μm to 70μm.

[0035] Compared with the prior art, the present invention has the following technical effects:

[0036] This invention employs a near-realistic notch stress concentration factor and utilizes a constant load endurance method for testing. If the notch is insensitive to load (the notch stress concentration factor is lower than the material's inherent crack propagation strength factor), the sample will not change after constant load endurance. If the notch is sensitive to load (the notch stress concentration factor is close to or higher than the material's inherent crack propagation strength factor), then the notch tip will induce crack formation under a certain load, and once the crack forms, the ceramic will fracture and fail.

[0037] This invention, based on obtaining a sharp notch by cutting a ceramic sample, employs a constant load persistence method, gradually applying load and maintaining it for a specified time until fracture occurs within a predetermined period. This induces crack propagation and failure, yielding more realistic fracture toughness results. Fracture occurring later in this constant load persistence process inevitably involves crack propagation. Compared to machining or simple laser cutting, this method provides a more accurate crack size, and the crack propagation avoids the influence of the laser. Therefore, the fracture toughness of ceramic materials measured using this method is accurate and reliable. Attached Figure Description

[0038] Figure 1 The diagram shows a coarse cut made using a 0.25mm wire cutter in an embodiment of the present invention.

[0039] Figure 2The diagram shows a coarse cut made using a 0.12mm wire cutter in an embodiment of the present invention.

[0040] Figure 3 This illustrates a fine cut made using a razor blade in an embodiment of the present invention;

[0041] Figure 4 This illustrates a fine slit created by laser cutting in an embodiment of the present invention;

[0042] Figure 5 The fracture toughness curves of different cuts on ceramic samples from different embodiments and comparative examples of the present invention are shown.

[0043] Figure 6 The curves showing the fracture of the ceramic sample during the loading process according to an embodiment of the present invention are shown.

[0044] Figure 7 The load curve of the ceramic sample according to an embodiment of the present invention is shown. Detailed Implementation

[0045] The following specific embodiments illustrate the implementation of the present invention. Those skilled in the art can easily understand other advantages and effects of the present invention from the content disclosed in this specification. Although the description of the present invention is presented in conjunction with preferred embodiments, this does not mean that the features of the invention are limited to these embodiments. On the contrary, the purpose of describing the invention in conjunction with embodiments is to cover other options or modifications that may be derived based on the claims of the present invention. To provide a deep understanding of the invention, many specific details will be included in the following description. The invention may also be implemented without using these details. Furthermore, to avoid confusion or obscuring the focus of the invention, some specific details will be omitted in the description. It should be noted that, unless otherwise specified, the embodiments and features in the embodiments of the present invention can be combined with each other.

[0046] Example

[0047] (1) Processing ceramic templates

[0048] Three types of ceramic samples were selected: hot-pressed boron carbide sample, pressureless sintered silicon carbide sample, and gas-pressure sintered silicon nitride sample. The ceramic sheets were cut into strips using a ceramic cutting machine, and then the ceramics were finely ground into strips of 3mm×4mm×40mm using a ceramic grinding machine.

[0049] (2) Ceramic rough cutting

[0050] A U-shaped coarse cut was made on the 3mm side of the ceramic sample using a 0.25mm wire cutter and a 0.12mm wire cutter, respectively, to obtain a coarse cut depth of approximately 1.3mm. The measured diameter of the curvature at the tip of the coarse cut was 0.28mm. Figure 1 ) and about 0.18mm ( Figure 2 );

[0051] (3) Fine cutting of ceramics

[0052] Using a YLP-M30 laser marking machine and a razor blade, the bottom of the U-shaped coarse cut groove of the ceramic, which was cut with 0.25mm abrasive wire in step (2), was cut to obtain a V-shaped fine cut at the tip, ultimately yielding samples with different notch tip sizes. Figure 3 and Figure 4 After measurement, the curvature diameter of the fine notch tip is within about 20μm, of which the size of the laser-cut fine notch is within 5μm, and the diameter of the razor blade cutting fine kerf is about 20μm.

[0053] (4) Preliminary test

[0054] The specimens were subjected to a three-point bending test using an electronic universal testing machine. A three-point bending fixture was used with the notch facing down, and an LVDT (deflectometer for measuring deformation) was placed under the notch of the specimen. The two notch sizes (razor blade cut and laser cut) of the above-mentioned Type II specimens according to GB / T23806-2009 were crushed once at a speed of 0.5 mm / min to obtain the maximum load F1 of the two notch sizes of the three ceramic specimens.

[0055] (5) Constant load endurance test

[0056] Another sample was selected after laser engraving and razor blade cutting in step (3). The loading rate of the equipment was set to 0.5 mm / min through the equipment software. The load was loaded to 90% of F1. After reaching the load, it was held for 40-60 seconds and then unloaded. If the sample failed during the holding period, the maximum load F2 was recorded. If it did not fail, the load was increased by 2N or other loads as a gradient based on F2. The above operation was repeated until the sample broke and the maximum load F3 was recorded.

[0057] (6) Measure the crack size

[0058] Scanning electron microscopy was used to observe the size of the crack propagation region (crack propagation caused by constant load and endurance process) of the sample. Finally, the overall notch size was obtained by coarse cutting + fine cutting + constant load and endurance propagation crack.

[0059] (7) Calculation

[0060] Fracture toughness was calculated based on the Type II specimen and formula in GB / T23806-2009, and the results are as follows. Figure 5 As shown.

[0061] Comparative Example

[0062] (1) Processing ceramic templates

[0063] Three types of ceramic samples were selected: hot-pressed boron carbide sample, pressureless sintered silicon carbide sample, and gas-pressure sintered silicon nitride sample. The ceramic sheets were cut into strips using a ceramic cutting machine, and then the ceramics were finely ground into strips of 3mm×4mm×40mm using a ceramic grinding machine.

[0064] (3) Ceramic rough cutting

[0065] A U-shaped coarse cut was made on the 3mm side of the ceramic sample using a 0.25mm wire cutter and a 0.12mm wire cutter, respectively, to obtain a coarse cut depth of approximately 1.3mm. The measured diameter of the curvature at the tip of the coarse cut was 0.28mm. Figure 1 ) and about 0.18mm ( Figure 2 );

[0066] (3) Fine cutting of ceramics

[0067] Using a YLP-M30 laser marking machine and a razor blade, the bottom of the U-shaped coarse cut groove of the ceramic, which was cut with 0.25mm abrasive wire in step (2), was cut to obtain a V-shaped fine cut at the tip, ultimately yielding samples with different notch tip sizes. Figure 3 and Figure 4 After measurement, the curvature diameter of the fine notch tip is within about 20μm, of which the size of the laser-cut fine notch is within 5μm, and the diameter of the razor blade cutting fine kerf is about 20μm.

[0068] (4) Fracture toughness test

[0069] The specimens were subjected to a three-point bending test using an electronic universal testing machine with the notch facing downwards. An LVDT (deflectometer for measuring deformation) was placed under the notch of the specimen. Four notch sizes (0.25mm wire cut, 0.12mm wire cut, razor blade cut, and laser cut) were subjected to compression testing according to the Type II standard of GB / T23806-2009 at a speed of 0.5mm / min. The maximum loads for the four notch sizes of the three ceramic specimens were obtained. The fracture toughness was calculated using the formula, and the relationship was obtained. Figure 5 .

[0070] pass Figure 5 It can be seen that the notch size has a significant impact on the fracture toughness results. When the notch size is reduced to below 20 μm, the difference in fracture toughness values ​​is not significant. However, there is still a problem in the comparative example: the notch tip is not a real crack.

[0071] Extensive testing has confirmed that if the ceramic material is sensitive, it will fracture within seconds to tens of seconds (e.g., Figure 6 As shown), if it is not sensitive, it will not break within a minute (as shown). Figure 7(As shown), therefore, considering the efficiency of the test, it is recommended that the load hold not exceed 60 seconds. Extensive testing has shown that the size of the sustained-delay crack under constant load is approximately 30μm to 70μm, which is relatively small compared to the overall notch size. The shape factor is obtained from the standard table. 50μm accounts for about 0.01 of 4mm w. The difference in shape factor is not significant. If possible, it can be tested with a scanning electron microscope. If not, the notch size of the rough and fine cut ceramic can be used directly, or 50μm can be added for calculation.

[0072] This invention involves cutting and grinding ceramic specimens into 3mm×4mm×40mm samples, then sequentially cutting a 1.2mm–1.4mm deep notch on the 3mm side using abrasive wire, laser engraving, or cutting a 0.2mm–0.4mm V-groove at the base of the notch using an ultra-thin razor blade. The sample is then subjected to a constant load test using an electronic universal testing machine to determine the load at which fracture occurs within 60 seconds. Finally, the fracture toughness is calculated using this load and the notch crack size. This invention's testing method is simple to operate and particularly suitable for fracture toughness testing of brittle ceramic materials such as boron carbide and silicon carbide. It avoids the low success rate of pre-crack testing in GB / T 23806 and eliminates the influence of blunt cut tips. The test has a high success rate and good reproducibility, making it easy to widely promote and apply.

[0073] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for testing the fracture toughness of a ceramic material based on the constant strain rate method, characterized in that, The method comprises the following steps: (1) processing ceramic sheets to obtain ceramic samples; (2) cutting the ceramic samples to obtain notched samples, wherein the curvature diameter of the notched tip is less than 20 μm; (3) preliminary testing: performing three-point bending test on the samples obtained in step (2), wherein the notched tip faces downward, and a deflection meter is placed under the notched tip, and a type II sample according to GB / T23806-2009 is pressed to break at a certain loading speed to obtain the maximum load F1; (4) constant load endurance test: loading the samples obtained in step (2) at the same loading speed as in step (3) to F2, wherein F2 is not greater than F1, and the load is unloaded after a certain holding time; if the notched sample fails during the holding process, the load F2 is recorded, and if it does not fail, the load is increased on the basis of F2, and step (4) is repeated until the notched sample breaks, and the load is recorded as F3; (5) measuring the crack size: observing the crack propagation area size in step (4) using a scanning electron microscope, and taking the notched size obtained in step (2) and the crack propagation area size in step (4) as the total notched size; (6) calculation: calculating the fracture toughness according to the type II sample of GB / T23806-2009 and the formula; In step (4), F2 is 90% of F1, and the holding time is not more than 60 s.

2. The method for testing the fracture toughness of a ceramic material based on the constant strain rate method according to claim 1, characterized in that, In step (3), the loading speed is 0.5 mm / min.

3. The method of testing the fracture toughness of a ceramic material based on the constant strain rate method according to claim 1, characterized in that, The holding time is 40-60 s.

4. The method of testing the fracture toughness of a ceramic material based on the constant strain rate method according to claim 1, characterized in that, In step (1), the size of the ceramic sample is 3 mm x 4 mm x 40 mm; in step (2), the ceramic sample is cut into a ceramic rough cut and a ceramic fine cut, and the total size of the notched tip is 1.4 mm-2 mm.

5. The method of testing the fracture toughness of a ceramic material based on the constant strain rate method according to claim 4, characterized in that, The ceramic rough cut is cut into a ceramic rough cut using a 0.25 mm or 0.12 mm sand line cutting machine on the 3 mm side of the ceramic sample, and the rough cut depth is 1.2 mm-1.4 mm.

6. The method of testing the fracture toughness of a ceramic material based on the constant strain rate method according to claim 4, wherein The ceramic fine cut is cut into a ceramic fine cut using a laser cutting or an ultra-thin razor blade dipped in grinding paste, and the fine cut depth is 0.2 mm-0.4 mm.

7. The method of testing the fracture toughness of a ceramic material based on the constant strain rate method according to claim 1, wherein The crack propagation area size is 30 µm-70 μm by inducing crack propagation by constant load endurance method.

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