Composite material grating and glue injection method and preparation method thereof
By injecting micro-immersion resin at low pressure and using capillary action to achieve self-immersion in the composite grille structure, the bubbles and holes at the connection between longitudinal and transverse grille are solved, and the fatigue resistance and overall strength of composite grille are improved performance.
Patent Information
- Application Number
- CN202411425426.1
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- Filing Date
- 2024-10-12
- Publication Date
- 2024-12-31
AI Technical Summary
The composite grille structure has bubbles and holes at the connection between the longitudinal rib strips and the transverse rib strips, resulting in a reduced fatigue resistance. There are serious internal layers and loose problems in the filling area of the twist strips, which are difficult to effectively solve through the existing RTM process.
Use low-pressure injection of micro-immersion resin to achieve self-immersion by capillary action, ensuring that the resin micro-flow front always leads in the filling area of the twist strip, avoids the formation of bubbles and holes, and further inhibits layering through circulating boosting and bubble discharge treatment, thereby enhancing resin flow Uniformity.
It effectively avoids the looseness and layering problems of the connection areas of the composite grating structure, and significantly improves its fatigue resistance and overall performance.
Smart Images
Figure CN119217751A_ABST
Abstract
Description
Technical Field
[0001] The invention relates to the technical field of composite material resin transfer molding, in particular to a composite material grid, a glue injection method and a preparation method thereof. Background Art
[0002] Composite materials have the advantages of high specific strength and large specific modulus, so they are widely used in the aviation field, which has strict requirements on material performance. As a typical representative of new materials, composite grid structures have received great attention and development due to their better damage tolerance, superior rigidity and structural stability. Composite grid structures for aviation must meet aerodynamic surface requirements, but their cavity surfaces are complex and require high precision, and are very difficult to mold. Traditional autoclaves and molding processes are unable to manufacture such complex curved multi-cavity structure products. In order to realize this product, integrated RTM (resin transfer molding) molding technology is required. At present, the molding technology of this type of product is almost monopolized by Japan's Nikkiso company internationally.
[0003] The composite grid structure is composed of a combination of longitudinal ribs and transverse ribs, and a certain amount of twist strips need to be filled in the joints to fill the gaps formed by the connection between the longitudinal ribs and the transverse ribs. Among them, the longitudinal ribs and the transverse ribs are made of bidirectional continuous dry fiber cloths, and the twist strips are made of unidirectional dry carbon fiber yarns. Then the resin is injected and cured through RTM to finally form the required composite material. However, even with the use of RTM, there are still holes and bubbles in the composite grid structure, especially in the twist strip filling area, there are serious internal stratification and looseness problems, which makes the internal quality problems at the joints between the longitudinal ribs and the transverse ribs cause the fatigue resistance of the composite grid structure to be greatly reduced.
[0004] In view of this, the present invention is proposed. Summary of the invention
[0005] The purpose of the present invention is to provide a composite material grid, a glue injection method thereof and a preparation method thereof. The new glue injection method for the composite material grid provided in the embodiment of the present invention can avoid the formation of bubbles and / or holes at the connection between the longitudinal ribs and the transverse ribs, and then can solve the internal looseness and stratification problems in the connection area of the composite material grid structure product, so as to improve the fatigue resistance of the composite material grid structure.
[0006] The present invention is achieved in that:
[0007] In a first aspect, the present invention provides a method for injecting a composite material grid, comprising: injecting a microscopic impregnation resin into a grid embryo forming mold at an injection pressure not higher than 0.5 bar;
[0008] Then, the injection is stopped and no injection pressure is provided, and the microscopic infiltration resin is self-infiltrated in the grid embryo until the microscopic infiltration resin flows to the edge of the twist strip of the grid embryo.
[0009] It should be noted that the resin used can be a liquid resin available on the market.
[0010] In an optional embodiment, the mass of the microscopic impregnation resin accounts for 25-35% of the total mass of the resin forming the composite grid.
[0011] In an optional embodiment, the self-infiltration time is simulated by resin injection process simulation software.
[0012] In an optional embodiment, the method includes: cyclically performing pressure increase and bubble removal after the completion of self-infiltration; and then performing pressure maintenance and curing.
[0013] In an optional embodiment, the process includes: after the infiltration is completed, the pressure is increased to 1-1.2 bar, and the resin is continuously introduced until the flow rate no longer changes, and then the bubbles are discharged; then the pressure is increased to 2-2.2 bar, and the resin is continuously introduced until the flow rate no longer changes, and then the bubbles are discharged; then the pressure is increased to 4-4.2 bar, and the resin is continuously introduced until the flow rate no longer changes, and then the bubbles are discharged; then the pressure is increased to 5.8-6.2, and the resin is continuously introduced until the flow rate no longer changes, and then the bubbles are discharged; then the pressure is maintained and the resin is cured by heating.
[0014] In an optional embodiment, the process includes: after the infiltration is completed, the pressure is increased to 2-2.2 bar, and the resin is continuously introduced until the flow rate no longer changes, and then the bubbles are discharged; then the pressure is increased to 4-4.2 bar, and the resin is continuously introduced until the flow rate no longer changes, and then the bubbles are discharged; then the pressure is increased to 5.8-6.2, and the resin is continuously introduced until the flow rate no longer changes, and then the bubbles are discharged; then the pressure is maintained and the resin is cured by heating.
[0015] In an optional embodiment, the total amount of resin entering the grid embryo forming mold through the cyclic pressurization and degassing treatment accounts for 65%-75% of the total resin mass forming the composite grid.
[0016] In an optional embodiment, microscopic infiltration resin is injected into the grid embryo forming mold at a glue injection pressure of 0.4-0.5 bar.
[0017] In a second aspect, the present invention provides a method for preparing a composite material grid, which includes the glue injection method for the composite material grid described in any one of the aforementioned embodiments.
[0018] In a third aspect, the present invention provides a composite material grid, which is prepared by the glue injection method for a composite material grid described in any one of the aforementioned embodiments or the preparation method for a composite material grid described in any of the aforementioned embodiments.
[0019] The present invention has the following beneficial effects: the embodiment of the present invention uses low-pressure injection of part of the resin before conventional RTM glue injection, and then allows the resin to self-infiltrate, and then reasonably controls the resin micro-flow front and the resin macro-flow front, ensuring that the resin flow front in the twist strip filling area is always in a leading state, thereby avoiding the formation of bubbles and holes in this area, and can well solve the internal looseness and stratification problems in the connection area of the composite grid structure product, and improve the fatigue resistance of the composite grid structure product. BRIEF DESCRIPTION OF THE DRAWINGS
[0020] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings required for use in the embodiments are briefly introduced below. It should be understood that the following drawings only show certain embodiments of the present invention and therefore should not be regarded as limiting the scope. For ordinary technicians in this field, other related drawings can be obtained based on these drawings without creative work.
[0021] Figure 1 A schematic diagram of micro-holes at the connection between the longitudinal ribs and the transverse ribs of the composite material grid in the prior art provided by the embodiment of the present invention;
[0022] Figure 2 A schematic diagram of a composite material grid in the prior art provided by an embodiment of the present invention showing that the connection between the longitudinal ribs and the transverse ribs is delaminated;
[0023] Figure 3 A schematic diagram of a composite material grid with holes at the connection between the longitudinal ribs and the transverse ribs in the prior art provided by an embodiment of the present invention;
[0024] Figure 4 A schematic diagram of the structure of a composite material grid provided by an embodiment of the present invention;
[0025] Figure 5 A schematic diagram of the structure of the composite material grid after self-infiltration provided by an embodiment of the present invention;
[0026] Figure 6 A schematic diagram showing that the resin microscopic flow front of the composite material grid provided by an embodiment of the present invention takes precedence over the resin macroscopic flow front;
[0027] Figure 7 A schematic diagram of the connection between the longitudinal ribs and the transverse ribs of the composite material grid provided by an embodiment of the present invention;
[0028] Figure 8 A waveform diagram of the connection between the longitudinal ribs and the transverse ribs of the composite material grid provided by an embodiment of the present invention;
[0029] Fig. 9A schematic diagram of the connection between the longitudinal ribs and the transverse ribs of the composite material grid provided as a comparative example of the present invention;
[0030] Fig.10 The waveform diagram of the connection between the longitudinal ribs and the transverse ribs of the composite material grid provided as a comparative example of the present invention. DETAILED DESCRIPTION
[0031] In order to make the purpose, technical scheme and advantages of the embodiments of the present invention clearer, the technical scheme in the embodiments of the present invention will be described clearly and completely below. If the specific conditions are not specified in the embodiments, they are carried out according to conventional conditions or conditions recommended by the manufacturer. If the manufacturer of the reagents or instruments used is not specified, they are all conventional products that can be purchased commercially.
[0032] Composite grilles are a commonly used high-performance composite material. In the prior art, many methods are used to improve the performance of composite materials, such as optimizing the structure, type or ratio of the resin, or optimizing the production process, such as using RTM. However, the inventors found that even if RTM is used to prepare composite materials, the performance of composite grilles is still poor. The inventors conducted in-depth research and found that even if RTM is used to prepare composite materials, there are still bubbles or holes in the composite grilles, especially bubbles and / or holes at the connection between the longitudinal ribs and the transverse ribs (see Figure 1 and Figure 2 ), the twist strip filling area has serious internal stratification and looseness problems (see Figure 2 and Figure 3 ). These cannot be improved by optimizing the structure, type or ratio of the resin, or by the existing optimized RTM.
[0033] In response to this problem, the inventors conducted an in-depth study of the RTM process and found that there are significant differences in resin flow during the RTM process. Specifically, the inventors found that the resin infiltration of the rib area formed by the longitudinal ribs and transverse ribs in the composite grid is mainly based on the macroscopic flow of resin, and the thrust of the macroscopic flow of resin is provided by the RTM injection machine; the resin infiltration of the twist bar filling area is mainly based on the microscopic flow of resin, and the microscopic flow of resin relies on capillary action. The difference between the macroscopic flow rate of resin and the microscopic flow rate of resin is huge. The conventional RTM process only considers the macroscopic flow of resin, while ignoring the microscopic flow of resin in the twist bar filling area. That is to say, when the conventional RTM process is used for molding, the macroscopic flow front of the resin in the rib area far exceeds the resin flow front of the twist bar filling area, resulting in the formation of resin wrapping in the twist bar area, and then the formation of holes and bubble aggregation in this area, resulting in serious internal stratification and looseness in the twist bar filling area. The internal quality problems at the joints of the composite grid will cause the fatigue resistance of the entire structure to be greatly reduced. This technical problem has seriously restricted the application of composite grid structural parts in the aviation field.
[0034] Therefore, in response to the above-mentioned problems discovered by the inventors, the inventors improved the RTM process to avoid the formation of bubbles and / or holes at the connection between the longitudinal ribs and the transverse ribs, thereby solving the internal looseness and stratification problems in the connection area of the composite grid structure product, thereby improving the fatigue resistance of the composite grid structure.
[0035] First, a grid embryo, that is, a grid structure preform or a grid embryo forming mold is assembled. For example, its structure is shown in Figure 4 The assembly process of the grille embryo is prior art and will not be described in detail in the embodiment of the present invention.
[0036] Then, the glue is injected into the grille blank forming mold. Specifically, the pressure of the RTM glue injection machine is set to no higher than 0.5 bar, for example, 0.4-0.5 bar. This pressure is only a contact pressure, which can inject the resin into the grille blank mold cavity without generating obvious macroscopic flow. In other words, if the glue injection pressure is higher than this pressure, the injected resin may form a macroscopic flow, which in turn affects the resin microscopic flow. In the twist strip filling area, the resin flow front cannot be formed first, which in turn causes bubbles, holes and / or delamination in the twist strip filling area.
[0037] Open the injection port, wait for a certain amount of resin to flow in, and then close the injection port. The resin injected at this time is the microscopic impregnation resin. The amount of microscopic impregnation resin injected at this time needs to be converted according to the volume of the twist bar filling area and the small amount of macroscopic flow resin generated in the first stage. The specific calculation method is M (microscopic impregnation resin mass) = V1 (twist bar filling area volume) * ρ (resin density) * 0.4 (resin mass fraction) + (0.2-0.3) * V2 (longitudinal and transverse reinforcement filling area volume) * ρ (resin density) * 0.4 (resin mass fraction). Among them, 0.2-0.3 means that after the twist bar is infiltrated in the microscopic infiltration stage, the longitudinal and transverse reinforcement area is just infiltrated with resin at a height of 0.2-0.3. This data will have slight changes for different longitudinal and transverse reinforcement layup structures, and needs to be obtained through basic experiments or PAM-RTM simulation according to different longitudinal and transverse reinforcement layup structures. Generally, the mass of the microscopic impregnation resin accounts for 25-35% of the total resin mass of the composite grid. If too much microscopic impregnation resin is injected at this time, there may be rib areas where resin flows, affecting the performance of the composite grid. If too little microscopic impregnation resin is injected at this time, the resin may not flow to the edge of the twist strip, also affecting the performance of the composite grid.
[0038] Close the injection port, stop injection, and do not provide injection pressure. The microscopic impregnation resin in the composite grid achieves self-infiltration through the capillary action between the fiber and the resin. After the microscopic impregnation resin is self-infiltrated, it enters the flow waiting time to achieve full infiltration. This stage time needs to be simulated by basic experiments or resin injection process simulation software, such as PAM-RTM simulation software.
[0039] Self-infiltration allows the microscopic infiltration resin to flow to the edges of the twisted strips of the grid embryo, for example Figure 5 In the embodiment of the present invention, the resin micro-flow generated by capillary action preferentially forms a resin flow front in the twist bar filling area, and the twist bar filling area enters the flow waiting time after the entire infiltration. That is to say, the present invention reasonably controls the resin micro-flow front and the resin macro-flow front to ensure that the resin flow front in the twist bar filling area is always in a leading state (for example, Figure 6 ), thereby avoiding the formation of bubbles and holes in this area, and can effectively solve the internal looseness and stratification problems in the connection area of the grid structure product.
[0040] Next, after the infiltration is completed, the pressure is increased and the bubbles are discharged in a cycle; and then the pressure is maintained and solidified. The embodiment of the present invention relies on the macroscopic flow of the resin to infiltrate the longitudinal ribs and the transverse ribs, and after the macroscopic flow resin front and the microscopic flow resin front meet, the pressure is further increased and the bubbles are discharged, so as to further suppress the formation of bubbles, holes or stratification.
[0041] Specifically, the above cycle is for pressure increase and bubble removal, followed by pressure maintenance and curing, and the existing normal RTM injection steps can be adopted. The following cycle of pressure increase and bubble removal followed by pressure maintenance and curing is only an example of the embodiment of the present invention.
[0042] For example, after the infiltration is completed, the pressure is increased to 1-1.2 bar and the resin is continued to be introduced until the flow rate no longer changes, and then the bubbles are removed; then the pressure is increased to 2-2.2 bar and the resin is continued to be introduced until the flow rate no longer changes, and then the bubbles are removed; then the pressure is increased to 4-4.2 bar and the resin is continued to be introduced until the flow rate no longer changes, and then the bubbles are removed; then the pressure is increased to 5.8-6.2 bar and the resin is continued to be introduced until the flow rate no longer changes, and then the bubbles are removed; then the pressure is maintained and the resin is heated to cure.
[0043] Or for example, after the impregnation is completed, the pressure is increased to 2-2.2 bar and the resin is continued to be introduced until the flow rate no longer changes, and then the bubbles are removed; then the pressure is increased to 4-4.2 bar and the resin is continued to be introduced until the flow rate no longer changes, and then the bubbles are removed; then the pressure is increased to 5.8-6.2 bar and the resin is continued to be introduced until the flow rate no longer changes, and then the bubbles are removed; then the pressure is maintained and the resin is heated to cure.
[0044] Among them, during the entire pressure-increasing operation in the above-mentioned RTM injection process, the longitudinal ribs and the transverse ribs are macroscopically infiltrated with resin, and after the macroscopic resin flow front and the microscopic resin flow front meet (i.e., overflow occurs at the glue outlet), the bubbles are removed.
[0045] The total amount of resin entering the grid embryo forming mold after cyclic pressurization and degassing treatment accounts for 65%-75% of the total resin mass forming the composite grid, that is, the total resin forming the composite grid except the microscopic impregnation resin is all injected during the normal RTM injection process.
[0046] The resin subjected to debubbling treatment includes the lost part of the resin and the resin entering into the grille embryo forming mold, while the resin subjected to debubbling treatment defined above refers to the resin entering into the grille embryo forming mold and does not include the lost part of the resin due to debubbling.
[0047] In summary, the embodiment of the present invention first forms a resin flow front in the twist strip filling area by means of the resin micro-flow generated by capillary action. After the twist strip filling area is fully infiltrated, the flow waiting time is entered to ensure that the resin flow front in the twist strip filling area is always in a leading state, thereby avoiding the formation of bubbles and holes in this area. Then, the longitudinal ribs and transverse ribs are infiltrated by the resin macro-flow, and after the macro-flow resin front and the micro-flow resin front meet, the pressure is further increased to remove bubbles.
[0048] It should be noted that the resin described above may be a resin conventionally used for RTM.
[0049] In a second aspect, the present invention provides a method for preparing a composite material grid, which includes the glue injection method for the composite material grid described in any one of the aforementioned embodiments.
[0050] In a third aspect, the present invention provides a composite material grid, which is prepared by the glue injection method for a composite material grid described in any one of the aforementioned embodiments or the preparation method for a composite material grid described in any of the aforementioned embodiments.
[0051] The features and performance of the present invention are further described in detail below in conjunction with the embodiments.
[0052] Example
[0053] An embodiment of the present invention provides a method for preparing a composite material grid, comprising:
[0054] (1) After the grid structure prefabricated body is prepared and assembled, it is connected to the glue injection pipeline and sent into the curing furnace to be heated to the glue injection temperature.
[0055] (2) RTM injection begins. Specifically, the injection pressure of the RTM injection machine is set to 0.4-0.5 bar, and after a certain amount of resin has flowed in, the injection port is closed. The amount of resin injected at this stage is 1 / 3 of the mass of the resin injected into the entire grid structure. Among them, the above-mentioned resin Hengshen Company developed a special RTM resin system, brand EH301, and other similar liquid molding special resins can also be implemented in this way.
[0056] (3) The closed injection port does not provide injection pressure, and the self-infiltration effect is achieved by relying on the capillary action between the fiber and the resin. Specifically, the self-infiltration time is obtained through basic experiments or resin injection process simulation software, such as PAM-RTM simulation software. The simulation results also need to be verified by experiments, that is, the resin flow process in the self-infiltration stage is simulated through board-level experiments to find the time when the resin flows to the edge of the twist strip.
[0057] (4) Then open the injection valve and adjust the injection pressure to 1 bar to continue to introduce resin until the flow rate no longer changes, then open the discharge port to remove bubbles, and close the discharge port; continue to increase the pressure to 2 bar and continue to introduce resin until the flow rate no longer changes, open the discharge port to remove bubbles, and close the discharge port; then increase the pressure to 4 bar and continue to introduce resin until the flow rate no longer changes, open the discharge port to remove bubbles, and close the discharge port; then increase the pressure to 6 bar and continue to introduce resin until the flow rate no longer changes, open the discharge port to remove bubbles, and close the discharge port; then the injection machine maintains a pressure of 6 bar and heats the resin to the curing temperature, and cures it at a constant temperature. The curing system for EH301 resin is 140°C for three hours + 180°C for two hours.
[0058] Comparative Example
[0059] An embodiment of the present invention provides a method for preparing a composite material grid, including: opening a glue injection valve and adjusting the glue injection pressure to 1 bar to continue to introduce resin until the flow rate no longer changes, then opening a glue discharge port to remove bubbles, and closing the glue discharge port; continuing to increase the pressure to 2 bar to continue to introduce resin until the flow rate no longer changes, opening a glue discharge port to remove bubbles, and closing the glue discharge port; then increasing the pressure to 4 bar to continue to introduce resin until the flow rate no longer changes, opening a glue discharge port to remove bubbles, and closing the glue discharge port; then increasing the pressure to 6 bar to continue to introduce resin until the flow rate no longer changes; then maintaining the pressure and heating to 140°C for three hours + 180°C for two hours to solidify the resin.
[0060] The microscopic metallographic image and waveform diagram of the connection between the longitudinal ribs and the transverse ribs in the composite material grid prepared in Example 1 were tested by electron microscope and ultrasonic nondestructive flaw detector. The results are shown in Figure 7 and Figure 8 .
[0061] The microscopic image and waveform diagram of the connection between the longitudinal ribs and the transverse ribs in the composite grid prepared by the comparative example were tested by electron microscope and ultrasonic nondestructive flaw detector. The results are shown in Fig. 9 and Fig.10 .
[0062] according to Figure 7 , Figure 8 and Fig. 9 , Fig.10 It can be seen that the preparation method provided by the embodiment of the present invention can effectively solve the serious internal stratification and looseness problems in the twist strip filling area, and improve the overall static strength and fatigue performance of the composite material grid structure.
[0063] The composite grid structure manufactured in this embodiment is subjected to a low cycle fatigue strength test and assessment using loads under aircraft S19-1, aborted takeoff, and emergency landing conditions, and aerodynamic loads are provided separately.
[0064] Experimental results: No damage occurred after completing 30,000 cycles of S19 conditions, 150 cycles of rejected takeoff conditions, and 30 cycles of emergency landing. Ultrasonic nondestructive testing of the samples revealed no damage.
[0065] The load under the critical landing state is selected to carry out static strength test assessment on the composite material grid structure manufactured in this embodiment, and the aerodynamic load is provided separately.
[0066] After completing the 1.15 times limit load and the extreme load conditions, the specimen did not show any harmful deformation. Ultrasonic testing was performed on the test, and no extension of defect damage was found. Under the extreme load condition, the specimen did not break after being loaded to the extreme load for 60 seconds. During the load test for the bottom load of the bearing capacity, the specimen did not break after being loaded to 36588N (4 times the limit load for the critical landing condition).
[0067] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. For those skilled in the art, the present invention may have various modifications and variations. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.
Claims
1. A method for injecting glue into a composite material grid, characterized in that: include: Inject microscopic impregnation resin into the grille embryo molding mold at an injection pressure not higher than 0.5 bar. Then, the injection is stopped and no injection pressure is provided, and the microscopic infiltration resin is self-infiltrated in the grid embryo until the microscopic infiltration resin flows to the edge of the twist strip of the grid embryo.
2. The method for injecting glue into a composite material grid according to claim 1, characterized in that: The mass of the microscopic impregnation resin accounts for 25-35% of the total mass of the resin forming the composite material grid.
3. The method for injecting glue into a composite material grid according to claim 1, characterized in that: The self-infiltration time is simulated by the resin injection process simulation software.
4. The method for injecting glue into a composite material grid according to any one of claims 1 to 3, characterized in that: include: After the infiltration is completed, the pressure is increased and the bubbles are discharged in a cycle; Then the pressure is maintained and the curing is carried out.
5. The method for injecting glue into a composite material grid according to claim 4, characterized in that: include: After the impregnation is completed, the pressure is increased to 1-1.2 bar, and the resin is continued to be introduced until the flow rate no longer changes, and then the bubbles are removed; then the pressure is increased to 2-2.2 bar, and the resin is continued to be introduced until the flow rate no longer changes, and then the bubbles are removed; then the pressure is increased to 4-4.2 bar, and the resin is continued to be introduced until the flow rate no longer changes, and then the bubbles are removed; then the pressure is increased to 5.8-6.2 bar, and the resin is continued to be introduced until the flow rate no longer changes, and then the bubbles are removed; then the pressure is maintained and the resin is heated to cure.
6. The method for injecting glue into a composite material grid according to claim 5, characterized in that: include: After the impregnation is completed, the pressure is increased to 2-2.2 bar and the resin is continued to be introduced until the flow rate no longer changes, and then the bubbles are removed; then the pressure is increased to 4-4.2 bar and the resin is continued to be introduced until the flow rate no longer changes, and then the bubbles are removed; then the pressure is increased to 5.8-6.2 bar and the resin is continued to be introduced until the flow rate no longer changes, and then the bubbles are removed; then the pressure is maintained and the resin is heated to cure.
7. The method for injecting glue into a composite material grid according to claim 4, characterized in that: The total amount of resin entering the grid embryo forming mold after the cyclic pressurization and degassing treatment accounts for 65%-75% of the total resin mass of the composite grid.
8. The method for injecting glue into a composite material grid according to claim 1, characterized in that: Microscopic infiltration resin is injected into the grille embryo forming mold at an injection pressure of 0.4-0.5 bar.
9. A method for preparing a composite material grid, characterized in that: The invention comprises the glue injection method of the composite material grid as claimed in any one of claims 1 to 8.
10. A composite material grid, characterized in that: The composite material grid is prepared by the glue injection method of any one of claims 1 to 8 or the preparation method of the composite material grid of claim 9.