Bionic gradient lap joint composite material finger-shaped joint and design and manufacturing method

Abstract

The invention relates to the technical field of composite materials, and particularly discloses a bionic gradient lap joint composite material finger-shaped connector and a design and manufacturing method. The composite material finger-shaped connector comprises a plurality of fiber lap joint units stacked in the center position in the thickness direction. The fiber lap joint unit is formed by stacking carbon fiber prepregs layer by layer, and a fiber layer laid each time and a fiber layer laid last time form a lap joint area in the center position, so that the joint effect is achieved; the different lap joint areas have different lap joint lengths at the center positions, and finger-shaped joint shapes with gradient lap joint are formed in the thickness direction. The comprehensive performance of the composite material joint can be effectively improved, and the problem of toughness mutual exclusion of a traditional uniform lap joint composite material joint is solved.

Description

Technical Field

[0001] The invention relates to the technical field of composite materials, in particular to a bionic gradient overlap composite material finger joint and a design and manufacturing method thereof. Background Art

[0002] In recent years, both academia and industry have been vigorously developing lightweight, high-performance and environmentally friendly materials and structures. Composite materials have become a promising solution due to their light weight, high stiffness and strength, and excellent design flexibility. Composite structures are usually composed of large-sized components that require various parts to be connected and assembled through joint methods. In order to meet the needs of complex structural shapes, as well as considerations of repair, maintenance, weight reduction and strength enhancement, effective joint design is essential.

[0003] Currently, there are three main types of joints commonly used in composite structures: mechanical fastening joints, bonded joints, and hybrid joints that combine mechanical fastening and bonding. The first type of mechanical joints, such as bolts and rivets, can provide certain strength and reliability, but they also have obvious disadvantages. For example, the introduction of holes will reduce the strength and fatigue life of the overall structure, while also increasing its weight and complexity.

[0004] In contrast, the second type of bonded joint has become the preferred alternative to mechanical joints, with advantages including nearly seamless connection, reduced structural weight, reduced manufacturing cost, improved damage tolerance and more uniform stress distribution. The single lap joint (SLJ) design in bonded joints has always been one of the common microstructures in aircraft structures, and its main purpose is to reduce manufacturing complexity and cost. However, this traditional uniform lap joint form has only a single fiber lap unit stacked along the thickness direction, and its failure strength is relatively low. There is a problem of "strong and tough mutual exclusion", so the traditional single lap design is not the best design. It can be seen that effective solutions must be found to improve and balance the strength, toughness and damage tolerance of joints. Summary of the invention

[0005] The purpose of the present invention is to solve the problem of improving the strength and toughness of existing composite joints because the geometric topological structure is too simple. From the perspective of bionics, a bionic gradient lap composite finger joint and a design and manufacturing method are proposed. The performance of the composite joint is improved by a simple method of gradient design of the lap length.

[0006] The present invention provides a bionic gradient overlap composite material finger joint and a design and manufacturing method thereof, which adopts the following technical solutions:

[0007] In the first aspect, the present invention provides a bionic gradient lap composite material finger joint, comprising a plurality of fiber lap units stacked at the center position along the thickness direction; the fiber lap units are stacked layer by layer through carbon fiber prepreg, and each laid fiber layer forms an lap area with the previous fiber layer at the center position, thereby achieving a joint effect; different lap areas have different lap lengths at the center position, forming a finger joint shape with gradient lap along the thickness direction.

[0008] In a second aspect, the present invention provides a method for designing a bionic gradient overlap composite finger joint, characterized in that it comprises the following steps:

[0009] S1: Determine the total thickness T of the laminate and the thickness t of the single-layer prepreg, thereby calculating the total number of plies N, which includes the number of continuous overlap plies and the number of gradient overlap plies;

[0010] S2: Determine the length L of the laminate and set the laminate length l formed by the selected fiber overlap unit i , thus calculating the length L of the fiber overlap unit in the corresponding overlap area i ;

[0011] S3: According to the determined total number of plies N and the ply length l of the fiber overlap unit i Parameter, set the same stack length l to be selected i The number of fiber overlap units N i ;

[0012] S4: The single fiber layer begins to be stacked layer by layer. Each fiber layer laid forms an overlap area with the previous fiber layer at the center. The stacking length is l i Finally, different topological structures of overlapping composite finger joints can be stacked according to different stacking lengths.

[0013] Furthermore, in step S4, when the stacking lengths are the same along the thickness direction, a uniformly overlapped composite material joint is formed.

[0014] Furthermore, in step S4, when there are two stacking lengths, a two-phase overlap composite material joint is formed.

[0015] Furthermore, in step S4, when the stacking length forms a continuous trend in the thickness direction, a gradient lap joint of the composite material is formed.

[0016] Furthermore, in step S4, when the composite material joint is not a symmetrical structure, a single gradient lap composite material joint is formed.

[0017] Furthermore, in step S4, when the composite material joint is a symmetrical structure, a double-gradient lap-jointed composite material joint is formed.

[0018] Furthermore, in step S2, the length L of the fiber splicing unit is i Should satisfy L i =(L+l i ) / 2.

[0019] Furthermore, in step S3, the total number of plies N should satisfy N=N / 2+N1+N2+N3+...+N i .

[0020] Furthermore, preferably, the composite laminate is a carbon fiber composite material or a glass fiber composite material.

[0021] In a third aspect, the present invention provides a method for manufacturing a bionic gradient lap joint composite material finger joint, comprising the following steps:

[0022] S1: cutting the carbon fiber prepreg according to the design requirements of the overlap length of each fiber overlap unit to obtain fiber sheets of appropriate size and number;

[0023] S2: Laying the fiber sheets layer by layer on a flat mold according to the optimized laying sequence of gradient overlap length;

[0024] S3: performing high temperature and high pressure molding of the prepregs stacked according to step S2 according to a hot-press molding process, and finally taking the finished product out of the mold to obtain a bionic gradient lap composite material finger joint.

[0025] The design principle of the present invention is: bionics and materials science are combined, infiltrated and influenced with each other, and nature provides us with many structural design inspirations for coordinated enhancement and toughening. The rib components of the tortoise shell form a solid exterior, and the sutures connecting these ribs provide the necessary flexibility in activities such as walking, breathing, and eating. This is a biological structure optimization design strategy, which has attracted much attention due to its excellent "rigidity and flexibility" performance. When the tortoise shell is subjected to external impact loads, the overlapping parts around the sutures can adapt to significant deformation and dissipate dynamic energy. Inspired by this, the optimization design of the bionic joint can start from the ply design of the directional adjustment of the overlap area, and by introducing fiber overlap units with different overlap lengths in the laminated area, the damage behavior of the structure is regulated to achieve the improvement of the strength and toughness of the composite joint. Therefore, this bionic gradient overlap composite finger joint, as a multi-layer joint structure inspired by the biological suture structure, can avoid the difficult preparation process and heavy weight problems brought by similar mechanical joints on the one hand, and on the other hand, by regulating the overlap area of ​​the composite joint, break through and improve the performance of the traditional uniform overlap joint, and achieve a good balance between the mechanical properties of the carbon fiber reinforced composite (CFRP) finger joint.

[0026] Inspired by the suture structure found in the ribs of biological structures such as tortoise shells, we proposed a bionic gradient lap composite finger joint, which fully utilized the excellent design space of ultra-thin prepregs, started from the topological structure of the joint, changed the stacking length, and proposed a gradient structure to improve the strength and damage tolerance problems in the traditional joint structure. The dual continuous gradient joint (DGJ) we designed can obtain structural strength comparable to that of laminates with continuous fibers, and enhance toughness and significantly improve ductility.

[0027] The advantages of this structural design are: (1) From the perspective of mechanical stress analysis, the gradient change of the overlap length along the thickness direction is exactly consistent with the stress distribution in the failed joint (FJ) under three-point bending. (2) From the perspective of damage form change, this gradient joint design can induce matrix crack deflection and limit the occurrence of large destructive delamination, thereby effectively improving the damage tolerance. In addition, the short overlap length in the structural design acts as a weak interface, which changes the path of crack propagation, forms a gradual failure process, and avoids catastrophic failure.

[0028] In summary, the present invention includes at least one of the following beneficial technical effects:

[0029] 1. The embodiment of the present invention is based on the structural characteristics of biological sutures, draws on the design strategy of biological multi-layer gradient structures, integrates it with the ply design of carbon fiber composite laminates, and uses the ply staggered method to manufacture a multi-lap gradient joint structure. Design optimization is performed in view of the requirements of light weight, high strength and high toughness of joint structure materials in existing engineering technology, and a ply design method is provided to improve the strength and toughness of the joint and the damage tolerance.

[0030] 2. The embodiments of the present invention avoid the difficult preparation process and heavy weight problems brought about by similar mechanical joints, and the present invention can effectively improve the comprehensive performance of composite joints, breaking through the problem of mutual exclusion of strength and toughness of traditional uniformly overlapped composite joints. The bionic gradient overlap design method provides new design ideas and technical support for the research and application of composite joints. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 Schematic diagram of the cross-sectional structure of the uniformly overlapped and two-phase overlapped composite joints provided by the embodiments of the present invention: (a) UJ-10, i.e., a short overlap composite joint with the same fiber overlap unit (L6=45mm); (b) UC-80, i.e., a composite laminate without a joint design; (c) BHJ-10_20, i.e., a composite joint with two fiber overlap units (L6=45mm, L4=50mm); (d) BHJ-10_80, i.e., a composite joint with two fiber overlap units (L6=45mm, L1=80mm);

[0032] Figure 2 For the present invention Figure 1 (a) Load-displacement curves and (b) strength-toughness comparison diagrams of uniform and two-phase overlap composite joints;

[0033] Figure 3 Schematic diagram of the cross-sectional structure of a multi-lap composite joint provided by an embodiment of the present invention: (a) SGJ-5 / 20, i.e., a composite joint having four fiber lap units (L7=42.5mm, L6=45mm, L5=47.5mm, L4=50mm); (b) SGJ-5 / 80, i.e., a composite joint having six fiber lap units (L7=42.5mm, L6=45mm, L4=50mm, L3=60mm, L2=70mm, L1=80mm); (c) DGJ-80 / 5 / 80, i.e., a symmetrically stacked composite joint having six fiber lap units (L7=42.5mm, L6=45mm, L4=50mm, L3=60mm, L2=70mm, L1=80mm);

[0034] Figure 4 For the present invention Figure 3 (a) Load-displacement curves and (b) strength-toughness comparison diagrams of gradient lap joints of four-phase and six-phase composite materials.

[0035] Figure 5 For the present invention Figure 1 and Figure 3 Radar plots of various properties of uniform and multi-lap composite joints, including peak load, dissipated energy, stiffness, and ultimate failure displacement. DETAILED DESCRIPTION

[0036] The present invention is further described and illustrated below in conjunction with the accompanying drawings and specific embodiments. The technical features of each embodiment of the present invention can be combined accordingly without conflicting with each other.

[0037] The bionic composite material joint of the present invention comprises a plurality of fiber overlap units stacked and overlapped in the center along the thickness direction. Each fiber overlap unit is stacked layer by layer through carbon fiber prepreg, and each laid fiber layer forms an overlap area with the previous fiber layer at the center position. Different fiber overlap units have different overlap areas, and different overlap areas have different overlap lengths at the center position, forming a finger-shaped joint shape with gradient overlap along the thickness direction.

[0038] In actual use, facing the three-point bending load condition, the bionic composite material joint of the present invention is in different stress distribution states along the thickness direction. The maximum normal stress is located on the upper and lower surfaces, resulting in premature fiber breakage. The highest shear stress exists near the neutral layer, making the joint prone to delamination. The evolution of these initial damages significantly affects the mechanical response of the structure.

[0039] According to the present invention, the design method of bionic gradient overlap composite finger joints is not limited to the design of total number of plies, uniform thickness and stacking length, and the appropriate total number of plies, thickness and stacking length can be selected according to actual needs.

[0040] According to the present invention, the composite material laminate is not particularly limited, and is preferably a carbon fiber composite material or a glass fiber composite material.

[0041] The present invention will be further described in detail below in conjunction with specific implementation examples, taking the design of unidirectional carbon fiber prepreg and the preparation of bionic gradient lap composite finger joints as an example to illustrate the present invention.

[0042] This embodiment provides a method for designing finger joints of a bionic gradient lap joint composite material, the steps of the method are as follows:

[0043] (1) It is preliminarily determined that the total number of plies N is 96, the number of continuous overlap plies is 48, the number of gradient overlap plies is 48, the ultra-thin carbon fiber prepreg model is USN 02000, the thickness of the cured layer is 0.02 mm, the plies have a certain gap, and the thickness of the composite material plate after curing is 2 mm.

[0044] (2) According to the standard test method for bending properties of polymer matrix composite materials (ASTM D7264), the overall size of the bending sample is designed to be 80mm×13mm, that is, the length L of the laminate is 80mm and the width is 13mm. The width of the present invention is not designed, and the width of the fiber overlap unit is consistent with the width of the laminate. Therefore, the longest laminate length l1 that can be set is 80mm and it belongs to continuous laminate fiber, which belongs to jointless design. In addition, the laminate lengths that can be selected are l2=60mm, l3=40mm, l4=20mm, l5=15mm, l6=10mm and l7=5mm. According to L i =(L+l i ) / 2, so the lengths of the fiber splicing units are set to be L1=80mm, L2=70mm, L3=60mm, L4=50mm, L5=47.5mm, L6=45mm, and L7=42.5mm.

[0045] The choice of stacking length depends on the cutting process of the prepreg and the size of the sample. When the cutting process of the prepreg is accurate enough, a smaller length change of the stacking length along the thickness direction can be achieved, which means that the design of structures such as ten gradients and twenty gradients can be achieved. The sample size limits the maximum value of the stacking length, and the stacking length should be less than or equal to the sample size. The embodiments of this article only show a certain situation of a specific example of the design strategy.

[0046] (3) According to the total number of gradient overlap layers is 48, different stacking lengths l i The number of fiber overlap units N i Designed with the same number of layers, continuous overlap plies are evenly distributed in the gradient overlap plies.

[0047] This embodiment provides a method for manufacturing a bionic gradient overlap composite material finger joint, comprising the following steps:

[0048] S1: cutting the carbon fiber prepreg according to the design requirements of the overlap length of each fiber overlap unit to obtain fiber sheets of appropriate size and number;

[0049] S2: Laying the fiber sheets layer by layer on a flat mold according to the optimized laying sequence of gradient overlap length;

[0050] S3: performing high temperature and high pressure molding of the prepregs stacked according to step S2 according to a hot-press molding process, and finally taking the finished product out of the mold to obtain a bionic gradient lap composite material finger joint.

[0051] Example 1

[0052] In this embodiment, the lamination length is the same along the thickness direction to form a uniformly overlapped composite joint. The designed lamination length is l6 = 10 mm. i =(L+l i ) / 2, so the length of the fiber overlap unit is L6=45mm. 48 fiber overlap units with a stacking length of l6=10mm are stacked in the center along the thickness direction (i.e. N6=48), and 48 fiber overlap units with a stacking length of l1=80mm are stacked in the center along the thickness direction (i.e. the number of continuous overlap layers is 48), N=N / 2+N6=48+48=96. Its structural schematic diagram is shown as follows Figure 1 (a)UJ-10.

[0053] Comparative Example 1

[0054] The design of the comparative example has a stacking length of l1 = 80 mm, that is, a composite laminate without a joint design, that is, 96 fiber overlap units with a stacking length of l1 = 80 mm are stacked in the center along the thickness direction. The structural schematic diagram is shown as follows Figure 1(b) UC-80.

[0055] Example 2

[0056] This embodiment has two stacking lengths to form a two-phase lap composite joint. The two stacking lengths are designed to be l6 = 10 mm and l4 = 20 mm respectively. i =(L+l i ) / 2, so the lengths of the fiber overlap units are L6=45mm and L4=50mm respectively. 24 fiber overlap units with a stacking length of l6=10mm (i.e. N6=24) and 24 fiber overlap units with a stacking length of l4=20mm (i.e. N4=24) are stacked in the center along the thickness direction, and 48 fiber overlap units with a stacking length of l1=80mm are stacked in the center along the thickness direction (i.e. the number of continuous overlap layers is 48 layers), N=N / 2+N6+N4=48+24+24=96. Its structural schematic diagram is shown as follows Figure 1 (c) As shown in BHJ-10_20.

[0057] Example 3

[0058] This embodiment has two stacking lengths to form a two-phase lap composite joint. The two stacking lengths are designed to be l6 = 10mm and l1 = 80mm respectively. i =(L+l i ) / 2, so the lengths of the fiber overlap units are L6=45mm and L1=80mm respectively. 24 fiber overlap units with a stacking length of l6=10mm (i.e. N6=24) and 24 fiber overlap units with a stacking length of l1=80mm (i.e. N1=24) are stacked in the center along the thickness direction, and 48 fiber overlap units with a stacking length of l1=80mm are stacked in the center along the thickness direction (i.e. the number of continuous overlap layers is 48 layers), N=N / 2+N6+N4=48+24+24=96. Its structural schematic diagram is shown as follows Figure 1 (d) Shown by BHJ-10_80.

[0059] Example 4

[0060] In this embodiment, the lamination length forms a continuous trend in the thickness direction, and the composite material joint is not a symmetrical structure, forming a single gradient lap composite material joint. The four lamination lengths are designed to be l7 = 5mm, l6 = 10mm, l5 = 15mm, and l4 = 20mm. i =(L+l i) / 2, so the lengths of the fiber overlap units are set respectively L7=42.5mm, L6=45mm, L5=47.5mm, L4=50mm. Along the center of the thickness direction, 12 fiber overlap units with a stacking length of l7=5mm (i.e. N7=12), 12 fiber overlap units with a stacking length of l6=10mm (i.e. N6=12), 12 fiber overlap units with a stacking length of l5=15mm (i.e. N5=12) and 12 fiber overlap units with a stacking length of l4=50mm (i.e. N4=12) are stacked, and 48 fiber overlap units with a stacking length of l1=80mm are stacked along the center of the thickness direction (i.e. the number of continuous overlap layers is 48 layers), N=N / 2+N7+N6+N5+N4=48+12+12+12+12=96. Its structural schematic diagram is shown as follows Figure 3 (a) SGJ-5 / 20.

[0061] Example 5

[0062] In this embodiment, the lamination length forms a continuous trend in the thickness direction, and the composite material joint is not a symmetrical structure, forming a single gradient lap composite material joint. Six lamination lengths are designed, namely, l7 = 5mm, l6 = 10mm, l4 = 20mm, l3 = 40mm, l2 = 60mm, l1 = 80mm. i =(L+l i ) / 2, so the lengths of the fiber splicing units are set respectively L7=42.5mm, L6=45mm, L4=50mm, L3=60mm, L2=70mm, L1=80mm. Along the center of the thickness direction, 8 fiber overlap units with a stacking length of l7=5mm (i.e., N7=8), 8 fiber overlap units with a stacking length of l6=10mm (i.e., N6=8), 8 fiber overlap units with a stacking length of l4=20mm (i.e., N4=8), 8 fiber overlap units with a stacking length of l3=40mm (i.e., N3=8), 8 fiber overlap units with a stacking length of l2=60mm (i.e., N2=8), 8 fiber overlap units with a stacking length of l1=80mm (i.e., N1=8) are stacked, and 48 fiber overlap units with a stacking length of l1=80mm are stacked along the center of the thickness direction (i.e., the number of continuous overlapped layers is 48 layers), N=N / 2+N7+N6+N4+N3+N2+N1=48+8+8+8+8+8+8=96. Its structural schematic diagram is shown as follows Figure 3 (b) SGJ-5 / 80.

[0063] Example 6

[0064] In this embodiment, the lamination length forms a continuous trend in the thickness direction, and the composite material joint is a symmetrical structure, forming a double-gradient lap joint composite material joint. The six lamination lengths are designed as l7=5mm, l6=10mm, l4=20mm, l3=40mm, l2=60mm, l1=80mm. i =(L+l i ) / 2, so the lengths of the fiber overlap units are set to be L7 = 42.5mm, L6 = 45mm, L4 = 50mm, L3 = 60mm, L2 = 70mm, and L1 = 80mm, and the six fiber overlap units are symmetrically stacked. This double-gradient overlap composite joint is divided into two parts, the upper and lower parts are symmetrically structured, and the upper part has a stacking length of l i Lay according to the gradient from large to small, the lower part of the stack length l i Lay in a gradient from small to large. In the center of the upper part along the thickness direction, there are 4 fiber overlap units with a stacking length of l1=80mm (i.e. N1=4), 4 fiber overlap units with a stacking length of l2=60mm (i.e. N2=4), 4 fiber overlap units with a stacking length of l3=40mm (i.e. N3=4), 4 fiber overlap units with a stacking length of l4=20mm (i.e. N4=4), 4 fiber overlap units with a stacking length of l6=10mm (i.e. N6=4), 4 fiber overlap units with a stacking length of l7=5mm (i.e. N7=4), and the lower part has a stacking length of l i According to the gradient laying from small to large, 48 fiber overlap units with a stacking length of l1=80mm are stacked in the center along the thickness direction (that is, the number of continuous overlap layers is 48 layers), and the continuous overlap layers are evenly distributed in the gradient overlap layers. N=N / 2+2(N7+N6+N4+N3+N2+N1)=48+2*(4+4+4+4+4+4)=96. Its structural schematic diagram is as follows Figure 3 (c)DGJ-80 / 5 / 80.

[0065] like Figure 2 As shown in the figure, for UJ-10 with a shorter overlap length, although its strength and stiffness are relatively low, its advantage is that it improves ductility without sudden ultimate failure. UC-80 shows better strength and stiffness, but from the load-displacement curve, it can be seen that there is a sudden drop after the peak load, and the overall failure process is small; BHJ-10_20 still has lower strength and stiffness, but from the load-displacement curve, it can be seen that the structure will not suffer catastrophic failure and has the characteristics of progressive failure. BHJ-10_80 has lower strength and stiffness, but the final failure displacement is higher, and a load plateau period may occur before the final failure occurs, thereby prolonging the entire failure process.

[0066] like Figure 2 and Figure 4As shown in the figure, compared with the two-phase hybrid structure and the single gradient topology, BHJ-10_80 and SGJ-5 / 80 have higher load-bearing capacity, while BHJ-10_20 and SGJ-5 / 20 show excellent toughness and delayed failure properties. Both BHJ-10_20 and SGJ-5 / 20 show significantly larger final failure displacements, which are 112.4% and 83.5% higher than UC-80, respectively. Even after reaching the peak and experiencing a large drop in load-bearing capacity, BHJ-10_20 and SGJ-5 / 20 can still withstand gradually stable loads, showing pseudo-ductile responses similar to yielding and strain hardening observed in metals. The dual gradient design DGJ-80 / 5 / 80 stands out among these bionic designs, with a 35.8% increase in strength and a 17.2% increase in toughness compared to UJ-10. On the other hand, DGJ-80 / 5 / 80 has high stiffness and strength close to the upper limit of UC, and even better toughness and ductility than UC.

[0067] like Figure 5 As shown, compared with other structures, our proposed gradient overlap design SGJ-5 / 80 and DGJ-80 / 5 / 80 show a good balance between the four properties. It shows high strength comparable to that of continuous laminates and even exceeds toughness. Although the stiffness is reduced, it can effectively delay the arrival of final failure. In addition, SGJ-5 / 20 still shows excellent performance at short overlap lengths. Compared with UC-10, it shows a superior combination of strength, toughness and delayed failure ability. It can be seen that by changing the design of the joint, different topologies and overlap lengths are used to offset the poor strength and damage tolerance.

[0068] from Figure 4 It can be seen that compared with the uniform design, the bionic gradient design has higher structural performance and achieves a balance of mechanical properties, including peak load, final failure displacement, energy dissipation, and damage tolerance. In particular, the gradient change in the overlap length of the double continuous gradient joint ply is well matched to the stress distribution in the FJ subjected to three-point bending, which can achieve comparable structural strength and better toughness to laminates with continuous fibers, and significantly delay catastrophic failure.

[0069] All the above composite joints were verified by three-point bending test. Figure 5 It is shown that under bending load, the designed bionic gradient lap composite finger joints exhibit higher structural performance, including maximum load, ultimate displacement, energy dissipation and damage tolerance. In particular, the DGJ design can achieve structural strength comparable to that of laminates with continuous fibers, while significantly improving toughness and delaying catastrophic failure. The gradient variation of the laminate overlap length matches well with the stress distribution in the finger joints subjected to three-point bending, and has the mechanical response characteristics of different fiber lap units.

[0070] In this embodiment, the number of units of the gradient stack can be reasonably adjusted according to the ply length and the single-layer thickness of the fiber sheet.

[0071] The present invention regulates the three-point bending damage characteristics and damage distribution of the composite material laminate through the synergistic effect of fiber overlap units with different stacking lengths, thereby improving the strength, toughness and delayed failure capacity of the composite material joint.

[0072] The above-described embodiment is only one solution of the present invention, but it is not intended to limit the present invention. A person skilled in the art can make various changes and modifications without departing from the spirit and scope of the present invention.

[0073] The above are all preferred embodiments of the present invention, and are not intended to limit the protection scope of the present invention. Therefore, any equivalent changes made based on the structure, shape, and principle of the present invention should be included in the protection scope of the present invention.

Claims

1. A bionic gradient lap joint composite material finger joint, characterized in that: It includes a plurality of fiber overlap units stacked at the center position in the thickness direction; the fiber overlap units are stacked layer by layer through carbon fiber prepreg, and each laid fiber layer forms an overlap area with the previous fiber layer at the center position, thereby achieving a joint effect; different overlap areas have different overlap lengths at the center position, forming a finger-like joint shape with gradient overlap along the thickness direction.

2. A design method for a bionic gradient lap joint composite material finger joint, characterized in that: The following steps are involved: S1: Determine the total thickness T of the laminate and the thickness t of the single-layer prepreg, thereby calculating the total number of plies N, which includes the number of continuous overlap plies and the number of gradient overlap plies; S2: Determine the length L of the laminate and set the laminate length l formed by the selected fiber overlap unit i , thus calculating the length L of the fiber overlap unit in the corresponding overlap area i ; S3: According to the determined total number of plies N and the ply length l of the fiber overlap unit i Parameter, set the same stack length l to be selected i The number of fiber overlap units N i ; S4: The single fiber layer begins to be stacked layer by layer. Each fiber layer laid forms an overlap area with the previous fiber layer at the center. The stacking length is l i Finally, different topological structures of overlapping composite finger joints can be stacked according to different stacking lengths.

3. The design method of a bionic gradient overlap composite material finger joint according to claim 2, characterized in that: In step S4, when the stacking lengths are all the same along the thickness direction, a uniformly overlapped composite material joint is formed.

4. The design method of a bionic gradient overlap composite material finger joint according to claim 2, characterized in that: In step S4, when there are two stacking lengths, a two-phase overlap composite material joint is formed.

5. The design method of a bionic gradient overlap composite material finger joint according to claim 2, characterized in that: In the step S4, when the length of the laminate forms a continuous trend in the thickness direction, a gradient overlap composite material joint is formed.

6. The design method of a bionic gradient overlap composite material finger joint according to claim 5, characterized in that: In step S4, when the composite material joint is not a symmetrical structure, a single gradient lap composite material joint is formed.

7. The design method of a bionic gradient overlap composite material finger joint according to claim 5, characterized in that: In step S4, when the composite material joint is a symmetrical structure, a double-gradient overlap composite material joint is formed.

8. The design method of a bionic gradient overlap composite material finger joint according to claim 2, characterized in that: In step S2, the length L of the fiber splicing unit i Should satisfy L i =(L+l i ) / 2.

9. The design method of a bionic gradient overlap composite material finger joint according to claim 2, characterized in that: In step S3, the total number of plies N should satisfy N=N / 2+N1+N2+N3+...+N i .

10. A method for manufacturing a bionic gradient lap joint composite material finger joint, characterized in that: The following steps are involved: S1: cutting the carbon fiber prepreg according to the design requirements of the overlap length of each fiber overlap unit to obtain fiber sheets of appropriate size and number; S2: Laying the fiber sheets layer by layer on a flat mold according to the optimized laying sequence of gradient overlap length; S3: performing high temperature and high pressure molding of the prepregs stacked according to step S2 according to a hot-press molding process, and finally taking the finished product out of the mold to obtain a bionic gradient lap composite material finger joint.

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