A method for testing room temperature interlaminar tensile strength of a laminated ceramic matrix composite

Abstract

This invention discloses a room-temperature interlaminar tensile strength testing method for layered ceramic matrix composites, belonging to the technical field of composite material mechanical property testing. The invention bonds the adhesive medium and the sample between upper and lower pull-out clamps, then uses vacuum bag pressure for sample preparation. The vacuum negative pressure allows the adhesive film to flow and wet the layered ceramic matrix composite, achieving rapid curing. This optimizes the adhesive sample preparation process in conventional room-temperature testing, avoiding premature adhesive failure and improving the test success rate. In room-temperature testing, the inherent porous structure of the ceramic matrix composite enhances the mechanical anchoring effect of the adhesive interface, improving test stability.

Description

Technical Field

[0001] This invention relates to the field of mechanical property testing technology for composite materials, and in particular to a method for testing the interlaminar tensile strength of a layered ceramic matrix composite material at room temperature. Background Technology

[0002] Traditional ceramic materials possess high strength, high modulus, low coefficient of thermal expansion, and excellent corrosion resistance, making them ideal for applications in extreme environments such as aerospace, nuclear energy, and power. However, their inherent brittleness and the resulting lack of reliability pose a risk of unpredictable sudden failures, which to some extent limits their further application. Therefore, ceramic matrix composites (CMCs) prepared by introducing continuous ceramic fibers to reinforce the ceramic matrix have become an effective technical approach to improve the toughness and reliability of traditional ceramic materials. Traditional layup CMCs exhibit excellent in-plane mechanical properties, but their load-bearing capacity is relatively weak when subjected to loads perpendicular to the layup direction (out-of-plane loads), making them prone to early failure phenomena such as delamination. To meet the structural design requirements for CMCs with sufficient strength, it is essential to ensure excellent structural performance in the interlaminar direction, and the main indicator for measuring interlaminar mechanical properties is interlaminar tensile strength.

[0003] Currently, the testing procedures for most conventional properties of CMCs (Chemical Molecular Weights) have been standardized by relevant standards both domestically and internationally, including tensile, compressive, bending, in-plane shear, and interlaminar shear tests. However, compared to other properties, systematic research on interlaminar tensile properties is relatively limited, and the few existing relevant standards and research methods generally have certain limitations. For example, ISO 20975-1:2023 specifies a method for directly testing interlaminar tensile strength by preparing samples along the thickness direction. While this method is direct and relatively accurate, it has specific thickness requirements for the specimen (thickness above 20 mm), which limits its applicability to some extent. In addition, some scholars use bending tests such as three-point bending (CN120521996 A) or four-point bending (CN120489801A) to indirectly measure the interlaminar tensile strength of materials. Since the specimen actually bears a bending load rather than direct tension in the test, it is necessary to convert the measured bending load into the corresponding interlaminar tensile strength using bending beam theory formulas. The feasibility and accuracy of the calculation results of this indirect method still need further research and verification. Summary of the Invention

[0004] To address the technical problems existing in the prior art, this invention provides a method for testing the interlaminar tensile strength of layered ceramic matrix composites at room temperature. This invention utilizes composite material vacuum bag sealing technology, relying on vacuum negative pressure to allow the adhesive film to flow and wet the layered ceramic matrix composite, achieving rapid curing. This optimizes the adhesive sample preparation process in room temperature testing, preventing premature adhesive failure and improving the test success rate. Furthermore, the inherent porous structure of the ceramic matrix composite enhances the mechanical anchoring effect of the adhesive interface during room temperature testing, improving test stability.

[0005] To solve the above-mentioned technical problems, the present invention provides the following technical solution:

[0006] This invention innovatively draws upon the prepreg vacuum curing technology commonly used in resin-based composite material molding, namely, composite vacuum bag sealing (OOA) technology. Unlike traditional autoclave molding processes that rely on high pressure, OOA technology typically uses a curing pressure of no more than one atmosphere, achieving rapid curing primarily through lower negative pressure. This characteristic gives it potential advantages in cost control and mass production.

[0007] This invention utilizes OOA (Out-of-Area) technology to prepare test specimens specifically for testing the interlaminar tensile strength of ceramic matrix composites (CMCs). For resin-based composites, porosity is a crucial process quality indicator, typically requiring strict control below 2%; exceeding this limit is considered detrimental to performance. However, CMCs inherently possess relatively high density and porosity. Therefore, compared to resin-based composites, CMCs exhibit a higher tolerance for porosity. Although vacuum bag technology cannot provide a high-pressure environment to promote efficient resin flow, the adhesives used in the test, under vacuum bag conditions, can still penetrate the inherent pores on the CMC surface due to their own fluidity. This not only does not impair the bonding effect but also enhances interfacial connectivity through mechanical anchoring, thereby improving the success rate and reliability of the pull-out test.

[0008] The adhesive medium has poor adhesion at room temperature and cannot be directly bonded to the fixture, nor can it be firmly bonded to the upper and lower pull-out fixtures. Typically, a temperature-controlled resistance furnace is used to heat the adhesive film to make it adhesive. After preheating, the adhesive medium allows the sample to bond relatively firmly to the fixture, preventing easy detachment, thus completing the bonding process. This invention uses SY-65 adhesive film or HY-JM-05-40 high-strength epoxy adhesive film as the adhesive medium. Before use, it needs to be preheated to 40℃-60℃ to enhance adhesion and facilitate pre-assembly. Before bonding to the pull-out fixture, the bonding surface needs to be sanded with fine sandpaper and cleaned with anhydrous ethanol. The adhesive film is cut to a size matching the bonding surface of the fixture, and two pieces are taken and bonded to the bonding surfaces of the upper and lower fixtures respectively. The sample is placed between the upper and lower fixtures, ensuring alignment and that the loading axis is consistent with the direction between the sample layers. Cut a suitable size of high-temperature resistant vacuum bag film and seal the bag opening with high-temperature resistant sealing tape (generally four sections of tape, overlapping at the edges). After sealing, press firmly with a special roller to ensure vacuum sealing. Wrap the pre-assembled sample with an anti-stick film and place it in the vacuum bag. Install the vacuum exhaust valve in one corner of the vacuum bag and tighten it. Connect the vacuum pump through a vacuum hose and evacuate to -0.1 MPa to -0.3 MPa, holding the pressure for 1 minute to check the seal integrity. Place the sealed sample along with the vacuum bag in a high-temperature furnace and cure at 130 ℃ for 1.5 hours. Control the heating and cooling rates at 2 ℃ / min to avoid adhesive layer cracking. After curing, wait for the furnace to cool to room temperature, remove the sample and remove the vacuum bag. Then, install and fix the cured sample in the tensile fixture of the universal testing machine and perform a tensile test until the upper and lower pull-out fixtures separate to obtain the "load-displacement" curve. Determine the peak load and then calculate the interlaminar tensile strength according to existing formulas. For the same ceramic matrix composite material, multiple samples can be cut simultaneously, and multiple parallel tests can be conducted. The average value of the interlaminar tensile strength obtained is the interlaminar tensile strength of the material.

[0009] Ceramic matrix composites are typically two-dimensional layup structures with a thickness usually exceeding 3 mm. It is particularly important to note that the applicability of the method of this invention to three-dimensional ceramic matrix composites (such as three-dimensional woven or three-dimensional needle-punched structures) may be limited. Because these materials often contain continuous reinforcing fibers in the tensile direction, the influencing factors and failure mechanisms of their interlaminar tensile strength are more complex, and the method of this invention may not be effective when testing such materials.

[0010] Compared with the prior art, the present invention has the following beneficial effects:

[0011] This invention addresses the high porosity of ceramic matrix composites by utilizing the fluidity of the adhesive to penetrate the pores on the CMC surface under vacuum conditions. The pore structure of the CMC enhances the adhesive anchoring, forming a mechanically interlocking structure, thus improving bonding reliability and test success rate. This invention employs a low-temperature, slow-curing process (heating / cooling at 2℃ / min) to prevent adhesive layer cracking; it has been successfully applied to the testing of interlaminar tensile strength in 2D silicon carbide and alumina layered composites. Attached Figure Description

[0012] Figure 1 The load-displacement curve for Example 1;

[0013] Figure 2 This is a flowchart of the preparation of interlaminar tensile test specimens using the vacuum bag curing adhesive method in Example 1;

[0014] Figure 3 This is a diagram showing the state of the sample after interlaminar damage in Example 1;

[0015] Figure 4 This is a diagram showing the state of the sample after interlaminar damage in Example 2;

[0016] Figure 5 This is a diagram showing the state of the sample after interlaminar damage in Example 3;

[0017] Figure 6 The image shows the film damage caused by the traditional mechanical loading curing method in Comparative Example 1. Detailed Implementation

[0018] To make the technical problems, technical solutions and advantages of the present invention clearer, a detailed description will be given below in conjunction with specific embodiments.

[0019] Unless otherwise specified, all reagents and materials used in this invention are commercially available. The bonding medium is either SY-65 adhesive film produced by a research institute in Beijing, or commercially available HY-JM-05-40 epoxy adhesive film.

[0020] This invention provides a method for testing the interlaminar tensile strength of layered ceramic matrix composites at room temperature, and specific embodiments are as follows.

[0021] Example 1

[0022] A method for testing the room temperature interlaminar tensile strength of a layered ceramic matrix composite material, comprising:

[0023] Step 1: Polish the bonding surfaces of the upper and lower pull-out clamps with fine sandpaper and clean them with anhydrous ethanol. The upper and lower pull-out clamps are integral stainless steel structures with identical structures, including a cylindrical base and a clamping structure on top of the cylindrical base. The bottom surface of the cylindrical base is the bonding surface, which is a flat, 20mm diameter circle. The clamping structure is flat with a through hole in the middle for easy pull-out testing.

[0024] Step 2: Trim the SY-65 adhesive film into two circles with a diameter of 20mm, and the thickness of the adhesive medium is 0.5mm;

[0025] Step 3: Preheat the two SY-65 adhesive films at 40℃ for 3 minutes, and then stick them to the bonding surfaces of the upper and lower pull-out clamps respectively;

[0026] Step 4: Prepare samples of two-dimensional SiC laminated ceramic matrix composites. f / SiC composite material, the diameter of the sample is consistent with the bonding surface size of the upper and lower pull-out fixtures, and the sample thickness is 3 mm; then the sample is placed between the bonding surfaces of the upper and lower pull-out fixtures in step 3, ensuring alignment, so that the loading axis is consistent with the interlayer direction of the sample.

[0027] Step 5: Select a high-temperature resistant vacuum bag and seal the opening of the vacuum bag around the perimeter with high-temperature resistant sealing tape. After sealing, press it firmly with a special roller to ensure vacuum sealing. Then, wrap the pre-assembled sample with a special anti-stick cloth for vacuum bag curing and place it in the high-temperature resistant vacuum bag. Install the vacuum exhaust valve in one corner of the vacuum bag and tighten it. Connect the vacuum pump through a vacuum hose, evacuate to -0.1 MPa and hold the pressure for 1 minute. After reaching the set pressure, close the exhaust valve and check whether the vacuum bag is sealed properly. After confirming that there is no leakage, the subsequent high-temperature curing treatment can be carried out.

[0028] Step 6: Then place the vacuum bag in a high-temperature furnace, control the heating rate to be 2℃ / min, reach 130℃, keep it at the temperature for 1.5 hours to cure, and then cool it down to room temperature at a rate of 2℃ / min to complete the curing.

[0029] Step 7: Remove the vacuum bag, then install and fix the cured sample in the tensile fixture of the universal testing machine, and perform a tensile test until the upper and lower pull-out fixtures separate to obtain the load-displacement curve, as shown in Figure 1.

[0030] Step 8: Based on the peak load and the area of ​​the delaminated region measured in the test, the interlaminar tensile strength of the material can be calculated. The specific calculation formula is as follows.

[0031]

[0032] Where, σ iP is the interlaminar tensile strength, MPa; P is the maximum compressive load, N; A is the area of ​​the delaminated region, mm. 2 .

[0033] Five specimens were tested in this example. These five specimens were cut from the same composite material sheet. The tests were conducted according to the method described above, and the average interlaminar tensile strength of the material was calculated. The results are shown in Table 1. The flowchart for preparing specimens for the vacuum bag curing adhesive method for interlaminar tensile testing is as follows: Figure 2 As shown; images of interlayer damage in the sample are as follows. Figure 3 As shown.

[0034] Table 1

[0035]

[0036] As shown in Table 1, the tensile strength tested by the method of the present invention exhibits good uniformity, and the strength achievable by the selected adhesive film and method is sufficiently high; Figure 3 It can be seen that the sample underwent obvious interlaminar failure, and the fracture surface was relatively flat and uniform, with no obvious local abnormal damage.

[0037] Example 2

[0038] In this embodiment, two-dimensional SiC f / SiC composite material replaced with two-dimensional Al2O 3f The Al2O3 composite material was tested under the same conditions as in Example 1. The interlaminar tensile strengths of the five samples are shown in Table 2, and images of the samples after interlaminar failure are shown below. Figure 4 .

[0039] Table 2

[0040]

[0041] As shown in Table 2, due to the inherent properties of the material, the interlaminar strength of alumina composites is not as high as that of silicon carbide, but the tensile strength uniformity is better. Figure 4 It can be seen that the sample experienced significant interlaminar failure, but no obvious local abnormal damage was observed.

[0042] Example 3

[0043] In this embodiment, the sample used is a two-dimensional SiC with different geometric dimensions than that in Example 1. f The / SiC composite material had a diameter of 10 mm and a thickness of 10 mm, and the remaining test conditions were consistent with those in Example 1. The test results are shown in Table 3. Figure 5 .

[0044] Table 3

[0045]

[0046] Depend on Figure 5 It can be seen that the method of the present invention has good applicability to specimens with different geometric dimensions. When the specimen diameter decreases and the thickness increases, the specimen can still undergo normal interlaminar tensile failure during the test, indicating that the method has a certain degree of adaptability to changes in specimen size.

[0047] Example 4

[0048] In this embodiment, the sample used is the same two-dimensional SiC as in Example 1. f The test used a SiC composite material, but the bonding medium was HY-JM-05-40 epoxy film, with a thickness of 0.5 mm, consistent with Example 1. All other test conditions remained the same as in Example 1. The test results are shown in Table 4.

[0049] Table 4

[0050]

[0051] Interlaminar tensile strength can also be successfully determined using different films, and the test results are not significantly discrete when compared with those in Tables 1 and 4, indicating their reliability.

[0052] As can be seen from the above embodiments, the success rate of interlaminar tensile strength testing achieved by preparing specimens according to the method of the present invention reaches 100%, and there is no problem of interlaminar failure. Furthermore, the interlaminar tensile strength obtained using the method of the present invention has high accuracy.

[0053] To further illustrate the beneficial effects of the present invention, the following comparative examples were constructed.

[0054] Comparative Example 1

[0055] While maintaining the same adhesive film, steps 1 to 4 of Example 1 were repeated to prepare a pre-cured sample. However, during the curing process, a vacuum bag process was not used; instead, a special fixture was used for mechanical pressure curing via bolt fastening. The specific fixture structure is shown in [link to fixture description]. Figure 6 As shown.

[0056] This specially designed fixture features a split upper and lower structure. The sample can be fixed within the slots created in both the upper and lower sections. During assembly, the sample is first placed in the slot of the lower section, then the slot on the upper cover is aligned with the sample. The applied pressure is controlled by adjusting the tightness of the bolts. The tightened fixture, along with the sample, is then placed into a high-temperature furnace, and the curing procedure in step 6 of Example 1 is repeated for curing.

[0057] Experimental results show that this method failed to achieve the expected results. Due to the uneven load applied by the pressure fixture during the curing process, the thickness of the cured adhesive layer was inconsistent. Under these circumstances, the adhesive layer often cracked preferentially before the interlaminar layers during tensile testing, making it impossible to accurately measure the true interlaminar tensile strength of the material. Specific failure morphologies are shown in [reference needed]. Figure 6 As shown.

[0058] Furthermore, when the inventors used the method of Comparative Example 1 to prepare specimens for interlaminar tensile strength testing, the adhesive layer cracked preferentially before the material interlaminars during the tensile process, making accurate testing impossible.

[0059] Comparative Example 2

[0060] In step 6 of this comparative example, the heating rate and cooling rate were both adjusted to 10℃ / min, and the remaining curing conditions were the same as in Example 1. The specific tensile data are shown in Table 5.

[0061] Table 5

[0062]

[0063] As shown in Table 5, an excessively rapid heating rate can lead to insufficient flow of the adhesive film, thus affecting the uniformity of the adhesive layer and the bonding quality, ultimately reducing the curing success rate. At this heating rate, the curing success rate decreased, and some samples exhibited debonding. Even successfully cured samples may experience adhesive film failure in subsequent tensile tests, resulting in a significant decrease in tensile strength that fails to reflect the true interlaminar tensile strength of the material.

[0064] The adhesive medium of this invention has a thickness of 0.1mm-1mm. For ceramic matrix composites with a thickness of 3-12mm, the inventors found that when the adhesive medium is too thin (less than 0.1mm), a strong mechanical interlocking structure cannot be formed during the curing process at 130℃, resulting in cracking of the adhesive layer; when the adhesive medium is too thick (greater than 1mm), it will not be able to completely penetrate into the pores of the CMC surface in a short time, which will greatly reduce the success rate of the test.

[0065] In summary, this invention relies on vacuum negative pressure to allow the adhesive film to flow and wet the laid-up ceramic matrix composite material, and achieve rapid curing, thereby avoiding premature failure of the adhesive layer and improving the success rate and stability of testing.

[0066] The above description is a preferred embodiment of the present invention. For those skilled in the art, any improvements and modifications made without departing from the principles of the present invention should also be considered within the scope of protection of the present invention.

Claims

1. A method for testing the interlaminar tensile strength of a layered ceramic matrix composite at room temperature, characterized in that, include: Step 1: Pre-treat the bonding surfaces of the upper and lower pull-out clamps respectively; Step 2: Cut the adhesive medium to a thickness of 0.1 mm-1 mm; Step 3: Place the two adhesive media on the bonding surfaces of the upper and lower pull-out clamps, and then preheat them to adhere the adhesive media to the bonding surfaces of the upper and lower pull-out clamps; Step 4: Prepare the sample of the layered ceramic matrix composite material. Place the sample between the bonding surfaces of the upper and lower pull-out clamps in Step 3, ensuring alignment and that the loading axis is consistent with the direction between the sample layers. Step 5: After wrapping the pre-assembled sample with an anti-stick film, place it in a high-temperature resistant vacuum bag, evacuate to -0.1 MPa to -0.3 MPa and hold the pressure for 1 minute; Step 6: Then, the vacuum bag is kept at 130 ℃ for 1.5 hours to cure, and then cooled to room temperature in the oven; Step 7: Remove the vacuum bag, then install and fix the cured sample in the tensile fixture of the universal testing machine, and perform a tensile test until the upper and lower pull-out fixtures separate to obtain the "load-displacement" curve; Step 8: Determine the peak load and then calculate the interlaminar tensile strength.

2. The method according to claim 1, characterized in that, The pretreatment in step 1 is as follows: the bonding surfaces of the upper and lower pull-out clamps are sanded with fine sandpaper and cleaned with anhydrous ethanol.

3. The method according to claim 1, characterized in that, In step 2, the adhesive medium has poor adhesion at room temperature and cannot be firmly adhered to the upper and lower pull-out clamps. After preheating, the adhesive medium can make the sample and the clamps bond relatively firmly and not easily fall off.

4. The method according to claim 3, characterized in that, In step 3, the preheating temperature is 40℃-60℃ and the preheating time is 2 min-3 min.

5. The method according to claim 4, characterized in that, The bonding medium is SY-65 adhesive film or HY-JM-05-40 epoxy adhesive film.

6. The method according to claim 1, characterized in that, In step 4, the sample size is less than or equal to the bonding surface size; The dimensions of the adhesive medium are the same as the dimensions of the sample.

7. The method according to claim 6, characterized in that, The layered ceramic matrix composite material has a two-dimensional layered structure with a thickness of 3-12 mm.

8. The method according to claim 1, characterized in that, In step 5, the high-temperature resistant vacuum bag is first sealed around the opening of the vacuum bag with high-temperature resistant sealing tape. After sealing, it is pressed with a special roller to ensure vacuum sealing. Before evacuating, first install the vacuum exhaust valve in one corner of the vacuum bag and tighten it. Then connect it to the vacuum pump through the vacuum hose to evacuate.

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