A method for reinforcing the interface of a composite end frame structure

Abstract

The present application aims at the problem of weak interlaminar interface of composite materials and the mechanism of tensile failure of end frame under concentrated load, and proposes an interface strengthening method for composite end frame structure, which improves the failure threshold of edge cracking of the composite end frame under tensile working condition. The project belongs to the field of structural composite material manufacturing technology.

Description

Technical Field

[0001] This project addresses the problem of weak interlayer interfaces in composite materials and the mechanism of tensile failure of end frames under concentrated loads. It proposes an interface strengthening method to suppress edge cracking of composite end frames, thereby increasing the failure threshold of edge cracking of composite end frames under tensile conditions. The project belongs to the field of structural composite material manufacturing technology. Background Technology

[0002] The weak interlaminar properties, tendency to delaminate, and low impact damage tolerance of composite laminates have always been barriers to their application. As the plylay design of missile section structures has not fully considered the impact of stress concentration on structural load-bearing capacity under multi-directional load transmission, the in-plane strength of the composite end frame structure cannot be fully utilized when subjected to large out-of-plane tensile loads at a single point. Due to insufficient strength, the end frame experiences delamination damage, leading to a reduction in stiffness and an inability to directly transfer the load to the root of the end frame ribs, ultimately resulting in failure. Summary of the Invention

[0003] The technical problem solved by this invention is to address the issue of weak interlayer interfaces in composite materials and the mechanism of tensile failure of end frames under concentrated loads. A method for strengthening the interface of composite end frames is proposed, which improves the failure threshold of edge cracking of composite end frames under tensile conditions.

[0004] To further address the load-bearing capacity of the end frame, overcome internal delamination, and achieve effective load transfer, this invention employs a locally optimized and reinforced design for the end frame, resulting in a composite material end frame structure suitable for concentrated tensile loads.

[0005] The technical solution of this invention is: a method for strengthening the interface of a composite material end frame structure, comprising: first, selecting an end frame molding mold; cutting the prepreg; inspecting and cleaning the molding mold, and applying a release agent to the mold surface; completing the layup molding according to the layup sequence; and continuing the following processing after layup molding:

[0006] The preformed parts are pre-compacted inside the autoclave.

[0007] An edge reinforcement layer is laid at the top edge of the rib plate. The edge reinforcement layer needs to be tightly laid and spread out in a star-shaped pattern on the end face of the end frame.

[0008] An internal edge reinforcement layer is laid on the top and sides of the rib plate, and an edge reinforcement layer is laid on the top edge of the rib plate to form a cover, and the top edge reinforcement layer is connected to the side of the rib plate body.

[0009] An edge sealing layer is added to the outside of the end frame ply. The edge sealing layer covers the outer skin ply and the overlapping ply of the two side ribs in reverse.

[0010] The composite material end frame structure is obtained after overall co-curing.

[0011] The thickness of the edge sealing layer shall not exceed 5 mm. The optimal thickness is 1-3 mm.

[0012] Preferably, the edge reinforcement layer at the top edge of the rib is a unidirectional prepreg layer with a thickness not exceeding 3 mm, and is flattened by a 60° cone angle star-shaped pattern on the upper surface of the end frame; if the thickness of the edge reinforcement layer at the top edge of the rib is ≥2 mm, it is permissible to alternate between laying edge reinforcement layers and internal edge reinforcement layers at the top edge of the rib, with a minimum thickness of 0.5 mm for the edge reinforcement layer at the top edge of the rib.

[0013] Preferably, the side ply of the rib is a thin-walled ply with a thickness not exceeding 1 mm.

[0014] Preferably, the pre-compaction treatment is a zero-adhesion pre-compaction treatment, in which a release material, a porous plastic film, and an air-conducting layer are sequentially laid on the outside of the prepreg, and finally the mold is sealed with a vacuum bag, followed by vacuuming, heating, and removal of volatile components; the temperature of the zero-adhesion pre-compaction treatment is 60-140℃; the heating rate is 10-50℃ / h; the pressure is 0.2-0.8MPa; and the time is 30-90min.

[0015] After the above-mentioned zero-absorption pre-compaction treatment is completed, the vacuum bag, air-conducting layer, porous plastic film, and release material are removed in sequence.

[0016] Preferably, the overall co-curing temperature is 140-230℃; the heating rate is 10-30℃ / h; the curing pressure is 0.6-0.8MPa; and the curing time is 15-20h.

[0017] Preferably, the prepreg is a hot-melt prepreg, which is one or more of carbon fiber prepreg, aramid prepreg, and glass fiber prepreg, and the matrix resin system is epoxy, bismaleimide, phenolic, cyano resin, or a combination of different materials.

[0018] Preferably, an edge sealing layer is added to the connection hole position on the end frame, or a metal part is added to the connection hole position and pre-compacted with countersunk bolts.

[0019] A concentrated load composite material end frame structure is obtained based on the interface strengthening method of the composite material end frame structure. An edge sealing layer is added to the outside of the end frame ply. The edge sealing layer covers the outer skin ply and the overlapping ply of the two side ribs in reverse, increasing the anti-delamination ability of the inner side of the end frame. An edge reinforcement layer is added to the top of each side rib, and the edge reinforcement layer is connected to the rib body through the side ply of the rib. The edge reinforcement layer at the top of the rib overlaps with the outer skin in the end frame area, and the upper surface is covered and reinforced with the end frame edge sealing reinforcement ply.

[0020] The advantages of this invention compared to the prior art are:

[0021] (1) This invention proposes a layup design method for suppressing edge cracking of composite end frames, which improves the failure threshold of edge cracking of composite end frames under tensile conditions and increases the edge strength of end frames by more than 100%.

[0022] (2) In view of the failure mechanism of composite end frames under concentrated tensile load, the present invention proposes a plywood structure for all composite end frames, which increases the tensile strength of the end frames from 2-3t to more than 14t. Attached Figure Description

[0023] Figure 1 Schematic diagram of the upper frame ply sealing;

[0024] Figure 2 This is a schematic diagram of the edge reinforcement of the upper frame rib plate;

[0025] Figure 3 This is a partial plywood diagram of the upper frame in the embodiment;

[0026] Figure 4 The loading method for the test of the cap-shaped structure of the end frame composite material. Detailed Implementation

[0027] A method for strengthening the interface of a composite material end frame structure includes the following steps:

[0028] (1) Select the end frame forming mold;

[0029] (2) Take the prepreg and feed it using an automatic feeder; the hot melt prepreg can be one or more of carbon fiber prepreg, aramid prepreg, and glass fiber prepreg, and the matrix resin system can be epoxy, bismaleimide, phenolic, cyano resin, or a combination of different materials.

[0030] (3) Inspect and clean the molding mold, and apply release agent to the surface of the mold;

[0031] (4) Complete the layering and forming process according to the layering sequence. Multiple layers need to be alternated during the forming process.

[0032] (5) Pre-compact the preformed parts in the autoclave;

[0033] In the zero-adhesion pre-compaction process, the pre-compaction process involves sequentially laying a release material, a porous plastic film, and an air-conducting layer on the outside of the prepreg. Finally, the mold is sealed with a vacuum bag, then vacuumed, heated, and volatile components are removed. The zero-adhesion pre-compaction temperature is 60-140℃; the heating rate is 10-50℃ / h; the pressure is 0.2-0.8MPa; and the time is 30-90min. After the zero-adhesion pre-compaction process is completed, the vacuum bag, air-conducting layer, porous plastic film, and release material are sequentially removed.

[0034] (6) Figure 1 As shown, an edge reinforcement layer is laid at the top edge of the rib. The edge reinforcement layer needs to be tightly laid and spread out in a star-shaped pattern on the end face of the end frame; as shown... Figure 2 As shown, an internal edge reinforcement layer is laid on the top and sides of the rib plate, and an edge reinforcement layer is laid on the top edge of the rib plate to form a covering. The top edge reinforcement layer is connected to the side of the rib plate body by a thin-walled layer (thickness not exceeding 1mm) on the side of the rib plate. The star-shaped arrangement here refers to the prepreg being laid flat (thinned) at different angles from the folding point when folding from the top of the inner plate to the upper surface of the end frame. Generally, the angles are symmetrically arranged.

[0035] (7) Add an edge sealing layer to the outside of the end frame ply, and the edge sealing layer will wrap the outer skin ply and the overlapping ply of the two side ribs in reverse.

[0036] Metal parts can also be used to replace the sealing layer at the connection hole position on the end frame, and are used for pre-tightening of countersunk bolts to improve the ability to resist the propagation of delamination cracks.

[0037] (8) After curing, a composite material end frame structure is obtained. The overall co-curing temperature is 140-230℃; the heating rate is 10-30℃ / h; the curing pressure is 0.6-0.8MPa; and the curing time is 15-20h.

[0038] A concentrated load composite end frame structure includes the following: an edge sealing layer is added to the outside of the end frame ply, the edge sealing layer reverses the overlapping ply of the outer skin and the two side ribs to increase the anti-delamination ability of the inner side of the end frame; an edge reinforcement layer is added to the top of the rib, and is connected to the rib body through the thin-walled ply of the rib side and the edge reinforcement layer to ensure the continuity and effectiveness of the rib load-bearing capacity, and the load is directly transferred to the rib; the inner edge reinforcement layer of the rib overlaps with the outer skin in the end frame area, and the upper surface is covered and reinforced by the end frame edge sealing reinforcement ply.

[0039] The method proposed in this project can be directly applied to end frame compartment structures, and this technology can be applied to the research and production of aerospace composite material components.

[0040] Example 1: Composite material end frame structure;

[0041] A schematic diagram of the preparation of the end frame structure of the composite material to be molded is shown below. Figure 3 As shown, the component dimensions are 159mm high × 91mm wide × 170mm long, with a rib width of 11mm, a top cover thickness of 26.5mm, an inner rib spacing of 88mm, and a back plate thickness of 8.2mm. It is made of bismuth prepreg.

[0042] The specific steps are as follows:

[0043] (1) Select the end frame forming mold;

[0044] (2) Collect the prepreg and feed it using an automatic feeder;

[0045] (3) Inspect and clean the molding mold, and apply release agent to the surface of the mold;

[0046] (4) Complete the layering and forming process according to the layering sequence. Multiple layers need to be alternated during the forming process.

[0047] (5) Pre-compact the preformed part in an autoclave. During the process, after isolating the release material and the porous plastic film, and laying an air-guiding layer on the outside of the porous plastic film, seal the mold with a vacuum bag, evacuate, heat up and absorb glue for pre-compacting to remove volatile components. The glue absorption temperature is controlled at 130-135℃; the heating rate is controlled within the range of 30-35℃ / h; the glue absorption pressure is controlled at 0.60-0.65MPa; and the glue absorption time is controlled at 35min.

[0048] (6) Lay an edge reinforcement layer at the top edge of the rib plate. The edge reinforcement layer needs to be tight and laid flat on the end face of the end frame.

[0049] An internal edge reinforcement layer is laid on the top and sides of the rib plate, and an edge reinforcement layer is laid on the top edge of the rib plate to form a cover, and the top edge reinforcement layer is connected to the side of the rib plate body.

[0050] (7) Add an edge sealing layer to the outside of the end frame ply, and the edge sealing layer will wrap the outer skin ply and the overlapping ply of the two side ribs in reverse.

[0051] (8) Cover the metal mold and cure it in a hot autoclave. Cover the mold, draw a vacuum, and heat and pressurize it in the hot autoclave to cure it as a whole. The curing temperature is controlled within the range of 210 to 220℃; the heating rate is controlled between 30 and 35℃ / h; the curing pressure is controlled at 0.6MPa; and the curing time is controlled within 15h.

[0052] (11) After being removed from the can, the parts are demolded, polished, and the flash is removed to obtain the tensile end frame structure.

[0053] The composite material end frame structure can be obtained through the above steps. Tensile tests were then conducted on the end frame structure specimens.

[0054] The tensile loading was performed using a universal testing machine with a capacity of 30 tons or more. The upper and lower loading axes of the tensile test specimen were kept as aligned as possible, with a deviation of ≤1mm. The loading test structure is as follows: Figure 4 As shown, the upper end uses an M18 screw, which is locked in place by a flat washer and an M18 high-strength nut; the lower end boundary has two M16 screw holes with a thread depth of more than 20mm, and the final breaking load is 145.35kN.

[0055] The parts of this invention not described in detail are common knowledge to those skilled in the art.

Claims

1. A method for strengthening the interface of a composite material end frame structure, comprising: firstly, selecting an end frame molding mold; cutting prepreg; inspecting and cleaning the molding mold; applying a release agent to the mold surface; and completing the layup molding according to the layup sequence; characterized in that... After the layers are laid up, the following processing is continued: The preformed parts are pre-compacted inside the autoclave. An edge reinforcement layer is laid at the top edge of the rib plate. The edge reinforcement layer needs to be tightly laid and spread out in a star-shaped pattern on the end face of the end frame. An internal edge reinforcement layer is laid on the top and sides of the rib, and an edge reinforcement layer is laid on the top edge of the rib to form a cover, and the top edge reinforcement layer is connected to the side of the rib body. An edge sealing layer is added to the outside of the end frame ply. The edge sealing layer covers the outer skin ply and the overlapping ply of the two side ribs in reverse. The composite material end frame structure is obtained after overall co-curing.

2. The method according to claim 1, characterized in that: The thickness of the edge sealing layer shall not exceed 5mm.

3. The method according to claim 2, characterized in that: The thickness of the edge sealing layer is 1-3mm.

4. The method according to claim 1, characterized in that: The top edge reinforcement layer of the rib is a unidirectional prepreg layer with a thickness not exceeding 3mm, and is flattened by a 60° cone angle star-shaped pattern on the upper surface of the end frame; if the thickness of the top edge reinforcement layer of the rib is ≥2mm, the edge reinforcement layer and the inner edge reinforcement layer are allowed to be laid alternately at the top edge of the rib, and the minimum thickness of the top edge reinforcement layer of the rib is 0.5mm.

5. The method according to claim 1, characterized in that: The side ply of the rib is a thin-walled ply with a thickness not exceeding 1 mm.

6. The method according to claim 1, characterized in that: The pre-compaction treatment is a zero-adhesion pre-compaction treatment. A release material, a porous plastic film, and an air-conducting layer are sequentially laid outside the prepreg. Finally, the mold is sealed with a vacuum bag, then a vacuum is drawn, and the temperature is raised to remove volatile components. The temperature of the zero-adhesion pre-compaction treatment is 60-140℃; the heating rate is 10-50℃ / h; the pressure is 0.2-0.8MPa; and the time is 30-90min. After the above-mentioned zero-absorption pre-compaction treatment is completed, the vacuum bag, air-conducting layer, porous plastic film, and release material are removed in sequence.

7. The method according to claim 1, characterized in that: The overall co-curing temperature is 140-230℃; the heating rate is 10-30℃ / h; the curing pressure is 0.6-0.8MPa; and the curing time is 15-20 h.

8. The method according to claim 1, characterized in that: The prepreg is a hot-melt prepreg, which is one or more of carbon fiber prepreg, aramid prepreg, and glass fiber prepreg, and the matrix resin system is epoxy, bismaleimide, phenolic, cyano resin or a combination of different materials.

9. The method according to claim 1 or 2, characterized in that: Add an edge sealing layer to the connection hole position on the end frame, or add a metal part to the connection hole position and pre-compact it with countersunk bolts.

10. A concentrated load composite material end frame structure, obtained based on the interface strengthening method for a composite material end frame structure according to claim 1, characterized in that: An edge sealing layer is added to the outside of the end frame ply. The edge sealing layer covers the outer skin ply and the overlapping ply of the two side ribs in reverse, increasing the anti-delamination ability of the inner side of the end frame. An edge reinforcement layer is added to the top of each side rib, and the edge reinforcement layer is connected to the rib body through the side ply of the rib. The edge reinforcement layer at the top of the rib overlaps with the outer skin in the end frame area, and the upper surface is covered and reinforced with the end frame edge sealing reinforcement ply.

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