Insert for enhancing hole edge strength through composite material and using method of insert

Abstract

The invention discloses a composite insert for enhancing hole edge strength and a using method thereof, and belongs to the technical field of composite structure connection. The hole edge insert has rotational symmetry or spiral geometric characteristics and comprises a limiting structure, an insert inner ring, a self-rotating structure and an insert outer ring, wherein the insert inner ring, the self-rotating structure and the insert outer ring are used in combination with the limiting structure and are integrated. The method comprises the steps that firstly, a hole edge rigidity adjusting insert with a self-rotating structure is designed, and a bolt hole is subjected to chambering treatment; secondly, the insert is embedded into the hole in an interference fit mode and connected with the unthreaded section of the bolt; thirdly, the composite material laminated board and the metal plate are assembled to form a multi-bolt lap joint structure; in the service process of the structure, the spinning structure in the insert generates controllable deformation under the action of external load, the hole edge rigidity is dynamically adjusted, and balanced transmission of loads among multiple bolts is achieved. By introducing the rigidity-adjustable structural unit, local stress concentration is effectively reduced, the damage tolerance and mechanical stability of the connecting structure are improved, and the connecting structure has good engineering applicability and popularization prospects.

Description

Technical Field

[0001] This invention belongs to the field of composite material structure connection technology, and relates to an insert for reinforcing the strength of holes in composite materials and its application method. In particular, it relates to a method for adjusting the stiffness of holes in carbon fiber reinforced thermoplastic (CFRTP) and metal multi-bolt connection structures and its special insert, which falls within the technical scope of load distribution optimization and reinforcement design of multi-bolt connection structures. Background Technology

[0002] Continuous fiber reinforced materials (CFRPs) have been widely used in aerospace, automotive, wind power, and sports and leisure equipment due to their excellent properties such as lightweight, high specific modulus, high specific strength, and corrosion resistance. In such structures, the multi-bolt mechanical connection between metal and CFRTP is widely adopted due to its detachability and engineering versatility.

[0003] However, due to the inherent lack of plastic buffering capacity in composite materials, CFRTP exhibits significantly enhanced local load sensitivity in multi-bolted connection structures. When the connection structure is subjected to external loads, the load often concentrates on a portion of the bolts, leading to premature overload of local bolts and causing early damage such as hole edge peeling, delamination, or matrix cracking, which seriously affects the reliability and service life of the structure.

[0004] Existing improvement methods mainly focus on bolt arrangement optimization, structural stiffness configuration, composite material layup design, and load-bearing capacity prediction. Although these methods improve overall performance to some extent, they lack systematic research on the mechanism of dynamic and coordinated load transfer among multiple bolts during the connection process. In addition, existing structures typically cannot actively or passively adjust local stiffness during service, making it difficult to achieve real-time mitigation and control of stress concentration areas.

[0005] Therefore, there is an urgent need to propose a new load control strategy that is simple in structure, easy to integrate, and can adjust the stiffness of the hole edge during the connection process, so as to improve the load balancing capability and damage tolerance of CFRTP / metal multi-bolt connection structure. Summary of the Invention

[0006] To address the problems existing in the prior art, this invention provides an insert for reinforcing the strength of holes in composite materials and its application method. This invention can overcome the problems of local bolt overload and early damage caused by uneven load distribution in existing composite material / metal multi-bolt connection structures. This invention achieves load balanced transmission through a control method and a dedicated insert structure by adjusting the local stiffness of the hole edge, thereby improving the overall stability and damage tolerance of the connection system.

[0007] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0008] An insert for reinforcing hole edge strength using composite materials is a hole edge stiffness adjustment insert 1 with rotational symmetry or helical geometry. The insert includes an inner ring 11, a spin structure 12, a limiting structure 13, and an outer ring 14. The inner ring 11, spin structure 12, and outer ring 14 are an integrated structure used in conjunction with the limiting structure 13. The spin structure 12 can generate controllable rotational deformation under external loads, thereby achieving local stiffness adjustment. Details are as follows:

[0009] The integrated structure is a circular ring structure with openings at the top and bottom. Its inner ring is an inner insert ring 11, and its outer ring is an outer insert ring 14. A spin structure 12 is located between the inner insert ring 11 and the outer insert ring 14, forming a hollow structure and connecting the inner and outer rings. The inner insert ring 11 is used to mate with bolts and achieve load transfer via an inner connection; the outer insert ring 14 is used to mate with the edge of a hole in a metal plate, achieving interference fit and boundary support.

[0010] The main body of the limiting structure 13 is a bottom-sealed, top-opening circular ring structure. Its inner and outer rings are arranged along the axial direction of the circular ring structure, namely an inner ring 134 and an outer ring 133. Between the inner ring 134 and the outer ring 134, there are multiple outer plates 131 and inner plates 132 with the same structure, which are used to insert into the spin structure 12. The multiple inner plates 132 are arranged circumferentially at intervals, and the multiple outer plates 131 are arranged circumferentially at intervals. The structure formed by the inner plates 132 and the outer plates 131 is coaxial with the circular ring structure.

[0011] Furthermore, the spin structure 12 has a rotationally symmetric or helical geometry, which can undergo controllable rotational deformation under axial load, thereby adjusting the local stiffness of the hole edge.

[0012] Furthermore, the limiting structure 13 is arranged around the outer periphery of the spin structure 12, that is, the outer plate 131 and inner plate 132 of the limiting structure 13 are inserted into the hollow part of the spin structure 12 to constrain the maximum deformation of the spin structure 12 and prevent structural instability or damage due to excessive deformation.

[0013] Furthermore, the top of the inner ring 11 of the insert is machined with notches at intervals for fitting and installation with the internal protrusions of the limiting structure 13; the top of the outer ring 14 of the insert is machined with notches at intervals for fitting and installation with the external protrusions of the limiting structure 13.

[0014] A method of using a composite material insert for reinforcing hole edge strength includes the following steps:

[0015] The first step is to prepare the hole edge stiffness adjustment insert 1;

[0016] Laser powder bed fusion additive manufacturing technology was used to prepare hole edge inserts using titanium alloy (TC4) powder, and heat treatment was performed to improve their mechanical properties.

[0017] The second step is to machine connection holes on the metal connecting plate;

[0018] The original bolt holes on the metal connecting plate are enlarged to match the outer diameter of the outer ring 14 of the hole edge stiffness adjustment insert 1, forming an interference fit interface, that is, forming a cavity structure for interference fit installation.

[0019] The third step is to install the hole edge stiffness adjustment insert 1;

[0020] The hole edge stiffness adjustment insert 1 is embedded in the hole of the metal plate with an interference fit, so that its spin structure 12 is around the bolt axis, and the inner ring 11 of the insert fits with the unthreaded section of the bolt to avoid thread interference with the hole wall of the composite laminate, and the outer ring 14 of the insert contacts the hole wall of the metal plate connection hole to achieve assembly stability and establish a load transfer path.

[0021] The fourth step is structural assembly and connection;

[0022] The composite laminate, the metal plate with the hole edge stiffness adjustment insert 1, and the bolts are connected by a multi-bolt lap connection to obtain a multi-bolt lap connection structure. The composite laminate has holes, and the threaded section of the bolt does not contact the hole wall of the composite laminate, but only passes through the hole edge stiffness adjustment insert 1 and the metal plate to achieve mechanical protection of the hole wall.

[0023] Step 5: Load regulation and response;

[0024] During the stress process of the multi-bolt lap joint structure, the spin structure 12 undergoes controllable deformation under axial load. By adjusting the local stiffness of the hole edge, the load between the multiple bolts is coordinated and controlled, thereby reducing stress concentration and suppressing local early failure.

[0025] Compared with the prior art, the present invention has the following beneficial effects:

[0026] (1) The present invention achieves dynamic adjustment of hole edge stiffness during connection by using the structural response characteristics of the insert, thereby improving the problem of uneven bolt load distribution;

[0027] (2) The spin structure of the present invention has good energy absorption and deformation control capabilities, which can effectively alleviate stress concentration at the hole edge and improve the mechanical stability and damage resistance of the joint. During the stress process of the multi-bolt lap connection structure, the spin structure of the insert undergoes controllable deformation, dynamically adjusts the stiffness at the hole edge, realizes the balanced coordination of loads between bolts, reduces local stress concentration, and inhibits early damage evolution.

[0028] (3) The insert structure designed in this invention is connected to the metal plate hole wall by an interference fit, which makes the insertion simple and has good assemblability and engineering adaptability.

[0029] (4) The present invention, combined with the thermoplastic formability of CFRTP material, can further expand the application potential of this control strategy in smart connection structures.

[0030] In summary, this invention establishes a synergistically optimized insert control mechanism in terms of structural design, manufacturing process, and service response, which has significant engineering practical value and promising prospects for promotion. Attached Figure Description

[0031] Figure 1 This is a schematic diagram of the hole edge stiffness adjustment insert.

[0032] Figure 2 This is a schematic diagram of the hole edge stiffness adjustment insert.

[0033] Figure 3 This is a schematic diagram of the hole edge stiffness adjustment insert assembly.

[0034] Figure 4 This is a schematic diagram of a multi-bolt composite material joint.

[0035] In the diagram: 1. Hole edge stiffness adjustment insert; 2. Fiber reinforced composite material layer; 3. Metal plate; 4. Bolt; 5. Nut;

[0036] 11 Inner ring of insert; 12 Spin structure; 13 Limiting structure; 14 Outer ring of insert;

[0037] 131 Outer plate; 132 Inner plate; 133 Outer ring; 134 Inner ring. Detailed Implementation

[0038] It should be noted that, unless otherwise specified, the embodiments and features described in the present invention can be combined with each other. The present invention will now be described in detail with reference to the accompanying drawings and embodiments.

[0039] Example 1

[0040] (1) Connection structure and load conditions;

[0041] The upper plate is made of carbon fiber reinforced thermoplastic composite material (CFRTP, matrix PAEK, layup pattern [0 / 90 / ±45]s, symmetrical layup, thickness 3.0 mm), and the lower plate is made of TC4 titanium alloy metal plate (thickness 4.0 mm). The connection method is a single lap four-bolt connection, with bolt nominal diameter M6, pitch 24 mm, edge distance 18 mm, and shear load is static tensile shear load, with a loading rate of 2 mm / min.

[0042] (2) Design of hole edge stiffness adjustment insert;

[0043] According to the method of the present invention, the insert is designed using a combination of rotational symmetry and helical rib geometry:

[0044] Inner ring: Cylindrical aperture, inner diameter 6.1 mm, clearance fit (0.05 mm) with the unthreaded section of the bolt.

[0045] Spin structure: It consists of 4 45° spiral ribs, with a rib thickness of 0.5 mm and a height of 2.0 mm. It can undergo controllable torsional deformation under external load.

[0046] Limiting structure: A radial boss located between the inner and outer rings controls the spin angle to not exceed 5°.

[0047] Outer ring: outer diameter 9.0 mm, forming an interference fit of 0.03 mm with the diameter of the enlarged hole in the metal plate.

[0048] (3) Manufacturing inserts;

[0049] Laser-assisted powder bed fusion (LPBF) additive manufacturing was employed, using spherical TC4 alloy powder (particle size 15–45 μm) as the raw material. An EOS M290 metal 3D printer was used, with a print layer thickness of 30 μm, laser power of 200 W, and a scanning speed of 1200 mm / s. After printing, vacuum annealing (700 °C, 1 h) was performed to eliminate residual stress and improve toughness.

[0050] (4) Hole enlargement and embedding of metal plates;

[0051] The bolt holes of the TC4 metal connecting plate were enlarged from 6.0 mm to 9.0 mm, with the surface roughness Ra controlled at 1.6 μm. The insert was then cooled with liquid nitrogen (–196 ℃) to shrink and quickly inserted into the enlarged hole in the metal plate. After returning to room temperature, a stable interference fit was formed. The inner ring of the insert matched the unthreaded section of the bolt to avoid thread interference with the CFRTP plate hole wall.

[0052] (5) Structural assembly;

[0053] Assemble the CFRTP laminate and the TC4 metal plate with inserts in a single overlap configuration, install four M6 titanium alloy bolts with a preload of 8 kN, and use steel washers to prevent in-situ damage to the composite material.

[0054] (6) Loading and effects;

[0055] Tensile and shear loads were applied to the limit on a universal testing machine, and the load distribution and damage evolution of the bolts were recorded. The results show that:

[0056] In the specimen with inserts, the load distribution difference among the four bolts decreased from the original 25% to 7%. The strain distribution at the hole edge was more uniform, and the maximum strain at the hole wall decreased by 18%. The failure mode changed from early peeling at the hole edge to overall shear failure of multiple bolts, and the damage tolerance was significantly improved.

[0057] This embodiment demonstrates that the hole edge stiffness adjustment insert of the present invention can effectively improve load balance and delay damage without changing the overall connection arrangement, and is suitable for multi-bolt connections of various CFRTP / metal hybrid structures.

[0058] The above-described embodiments are merely illustrative of the implementation methods of the present invention, but should not be construed as limiting the scope of the present invention. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these modifications and improvements all fall within the protection scope of the present invention.

Claims

1. An insert for reinforcing the edge strength of a composite material hole, characterized by, The insert is a hole edge stiffness adjusting insert (1) with rotational symmetry or spiral geometric characteristics, comprising an insert inner ring (11), a self-rotation structure (12), a limiting structure (13) and an insert outer ring (14), wherein the insert inner ring (11), the self-rotation structure (12) and the insert outer ring (14) are integrated structures, which are used in combination with the limiting structure (13); specifically as follows: The integrated structure body is an upper and lower opening circular ring structure, the inner ring is the insert inner ring (11), the outer ring is the insert outer ring (14), the self-rotation structure (12) is arranged between the insert inner ring (11) and the insert outer ring (14), forming a hollow structure and connecting the inner and outer rings.

2. An insert for reinforcing the edge strength of a hole in a composite material according to claim 1, characterized in that The main body of the limiting structure (13) is a circular ring structure with a closed bottom and an open top, and the inner and outer rings are provided with circular rings along the axial direction of the circular ring structure, which are respectively the inner ring circular ring (134) and the outer ring circular ring (133), a plurality of structure same outer plates (131) and inner plates (132) are arranged between the inner ring circular ring (134) and the outer ring circular ring (133), which are used for inserting into the self-rotation structure (12); wherein a plurality of inner plates (132) are arranged along the circumferential direction at intervals, a plurality of outer plates (131) are arranged along the circumferential direction at intervals, and the structure formed by the inner plate (132) and the outer plate (131) is coaxial with the circular ring structure.

3. An insert for reinforcing the edge strength of a hole in a composite material according to claim 2, wherein The limiting structure (13) is arranged around the outer periphery of the self-rotation structure (12), that is, the outer plate (131) and the inner plate (132) of the limiting structure (13) are inserted into the hollow part of the self-rotation structure (12), which is used to constrain the maximum deformation degree of the self-rotation structure (12).

4. An insert for reinforcing the edge strength of a hole in a composite material according to claim 3, wherein The insert inner ring (11) is provided with notches at the top at intervals, which are used for cooperating with the internal protrusions of the limiting structure (13) for installation.

5. A hole edge insert for a multi-bolt composite joint according to claim 4, wherein, The insert outer ring (14) is provided with notches at the top at intervals, which are used for cooperating with the external protrusions of the limiting structure (13) for installation.

6. An insert for reinforcing the edge strength of a hole in a composite material according to claim 5, wherein The insert inner ring (11) is used for inner side connection with the bolt to realize load transmission; the insert outer ring (14) is used for cooperation with the metal plate hole edge to realize interference fixation and boundary support; the self-rotation structure (12) can produce controllable rotational deformation under external load to realize local stiffness adjustment.

7. An insert for reinforcing the edge strength of a hole in a composite material according to claim 6, wherein The self-rotation structure (12) has rotational symmetry or spiral geometric shape, which can produce controllable rotational deformation under axial load to adjust the local stiffness of the hole edge.

8. A method of using the insert of any one of claims 1-7 to reinforce the edge strength of a composite material. The method comprises the following steps: First step, preparing the hole edge stiffness adjusting insert (1); Second step, processing the connecting hole on the metal connecting plate; The bolt hole on the metal connecting plate is treated by hole expansion, so that the hole diameter matches the outer diameter size of the insert outer ring (14) of the hole edge stiffness adjusting insert (1), forming an interference fit interface, that is, a hole cavity structure for interference fit installation; Third step, embedding the hole edge stiffness adjusting insert (1); The hole edge stiffness adjusting insert (1) is embedded in the metal plate hole in an interference fit manner, so that the self-rotation structure (12) is around the bolt axis, the insert inner ring (11) cooperates with the unthreaded section of the bolt, and the insert outer ring (14) contacts the hole wall of the metal plate connecting hole; Fourth step, structure assembly and connection; The composite laminate, the metal plate provided with the hole edge rigidity adjusting insert (1) and the bolt are subjected to multi-bolt lap joint connection to obtain a multi-bolt lap joint structure. Fifth step, load regulation and response; In the stress process of the multi-bolt lap joint structure, the self-spinning structure (12) is subjected to controllable deformation under the action of the axial load, and the load among the multi-bolts is cooperatively regulated and controlled by adjusting the local rigidity of the hole edge.

9. The method of using an insert for reinforcing the edge strength of a hole in a composite material according to claim 8, wherein, The first step is specifically: using a laser powder bed fusion additive manufacturing technology, titanium alloy TC4 powder is used to prepare a hole edge insert, and heat treatment is performed.

10. The method of using an insert for reinforcing the edge strength of a hole in a composite material according to claim 9, wherein, In the fourth step, the composite laminate is provided with a hole, and the threaded section of the bolt does not contact the hole wall of the composite laminate, but only passes through the hole edge rigidity adjusting insert (1) and the metal plate.

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