At present, the whole forming co curing and co bonding technology is widely used in the manufacturing of advanced resin matrix composite materials, which greatly reduces the number of aircraft mechanical connectors. However, due to the limitation of the current co curing and co bonding technology level, As well as the inevitable connection or opening of composite materials with metal, there are still a lot of processing connection problems in the composite structure from the needs of use, installation and maintenance. Compared with the metal structure, the composite joint is the weak link of the structure. According to statistics, more than 70% of the damage of composite materials occurs in the connection part. Therefore, it is of great significance to solve the problem of composite structure processing connection for reducing the structure weight, improving the performance of aircraft and promoting the application of composite materials.
Although advanced resin matrix composite has the advantages that general composite does not have, it also has some weaknesses, such as relatively brittle (elongation at break is only 1% - 3%), low interlaminar strength and poor impact resistance. During the processing and connection of composite, problems such as installation damage, easy to be pulled off, inconsistency of installation load and high concentration of stress around the hole must be solved, Corrosion and other problems should be considered in the assembly of composite materials. Electromagnetic compatibility, stealth and other structures are widely used in advanced aircraft composite structures. These structures put forward higher requirements for the processing and connection, as well as the hole making quality, fasteners used in the assembly, and connection and assembly methods. Therefore, the processing and connection of composite materials will directly affect the idea of composite structure design, manufacturing quality, production efficiency, and even limit the application level of composite materials.
After more than 20 years of development, China's advanced composite materials have made great progress in connection, hole making, fastener, assembly process, etc.; with the increase of the amount of composite materials for aircraft, the work of processing connection has increased greatly. In addition, to meet the increasing requirements of new materials, new structures and new functions of aircraft, therefore, the improvement of composite technology application level requires that the connection technology should have a greater development in order to keep up with the development speed of composite materials.
Mechanical connection
The resin matrix composite includes three levels of connection: the connection of fiber and matrix in micro mechanics level, the connection between layers in macro level and the connection of composite parts in structural level.
Resin matrix composites can be divided into two basic types: thermoplastic matrix composites and thermosetting matrix composites. For thermosetting matrix composites, the long chains of the cured polymer are cross-linked through the main chemical bond. Therefore, thermosetting resin matrix composite materials can not be softened or melted by heating, nor can they be welded. Therefore, thermosetting composite materials can only be mechanically connected or glued.
For thermoplastic materials, the long chains of polymers are mainly connected by subchemical bonds. When heated, the bonds break and the chains can move relative to each other. Therefore, thermoplastic composite materials can be softened or melted by heating, and can also be welded. Thermoplastic composites can also be mechanically or glued.
Thermoplastic composites can be further divided into two categories: amorphous or semi crystalline. The chains of amorphous thermoplastic materials are arranged randomly; the chains of semi crystalline thermoplastic materials are arranged randomly in some intervals, while the chains in other intervals are arranged orderly. For amorphous polymers, the critical temperature for softening is the glass transition temperature; for semi crystalline polymers, the critical temperature is the glass transition temperature and melting temperature.
The joining methods of polymer matrix composites mainly include three types: mechanical joining, bonding and hybrid joining (mechanical fastening and bonding). The hybrid bonding method is rarely used except for strengthening the weak bonding area or eliminating the type I peeling caused by the bonding end. Therefore, the following discussion focuses on the previous two connection methods. The bonding connection can be divided into ordinary bonding and welding. The adhesive layer of ordinary bonding forms an independent phase, while the welding is based on the matrix polymer material (typically thermoplastic plastic) to form the connection interface.
The choice of specific joining technology depends on the application requirements, such as load strength, geometry, operation environment, reliability weight and the cost of the selected polymer matrix system. As a general rule, mechanical connection is mainly used for high load, high reliability and key connection. Gluing and welding are used for medium load connections that give priority to weight and cost. For similar geometry and loading configuration, the bonding joint has better rigidity.
Mechanical connection has the following advantages: no need of surface treatment; no negative impact of heating cycle and high humidity environment; easy to detect and measure.
Mechanical connections include drilling, proper assembly techniques, and generally more stringent tolerances. These will increase the cost of composite processing. In addition, due to the introduction of holes, the corresponding stress concentration is caused, and then the strength is limited. Increasing metal fasteners will also increase the number and weight of parts.
Pore making technology of resin matrix composite
With the complex and large-scale structure of composite materials and the improvement of machining accuracy, a large number of robot drilling, CNC drilling machine drilling, CNC machining center drilling are developed to ensure the requirements of advanced composite material drilling.
Mechanical connection plays an important role in the connection of composite components. Therefore, in the assembly of composite components, thousands of fastener holes need to be processed. Fastener holes are not only large in quantity, high in quality, but also difficult, which is one of the most difficult processing procedures in composite processing.
As one of the main characteristics of composite laminates is the low interlaminar shear strength, which makes the axial force in the borehole easy to produce interlaminar delamination and the delamination at the outlet. If not prevented, it will lead to the scrapping of expensive composite materials. According to foreign statistics, in the assembly of aircraft composite materials, the scrapping caused by hole making defects accounts for more than 60% of all scrapped parts. Another main problem of composite hole making is the high hardness of carbon fiber composite (62 ~ 65hrc), which is equivalent to the hardness of high-speed steel. Therefore, the tool wear is particularly serious, and the tool durability is very low. For example, when the high-speed steel drill bit is used to drill carbon fiber composite materials, only 3-5 holes can be drilled for each edge grinding, so it is impossible to carry out industrial production. There is also a problem that must be paid attention to in the process of composite material hole making: the pollution of fiber dust will endanger human health, and its conductivity will make electrical equipment and power grid short-circuit, so safety measures must be taken in the construction. The drilling technology of resin matrix composite mainly includes: the selection of cemented carbide cutting tools, the selection of bit geometry parameters, the selection of drilling technology parameters, reaming technology, and the technological measures to prevent delamination.
The introduction of an opening in a composite (or any material) will result in a stress concentration at the edge of the hole, so a mechanical connection that requires drilling in the component will result in a stress concentration. For isotropic elastic materials, the stress distribution around the hole of infinite plate under tensile load is given by Timoshenko and Goodier.
It can be seen from equation (3-50) that the circumferential stress δ θ θ is the maximum, and the maximum value occurs at the same hole edge position as the loading direction (θ = 490); the stress concentration (coefficient) at the maximum value point is equal to 3. Similarly, for infinite orthotropic plates under tensile load, the maximum stress is also the hoop stress at θ = ± 90 °. Lekhnitski gives the hoop stress of infinite orthotropic plates under tensile load.
Equation (3-51) shows that the stress concentration coefficient in the positive direction is much higher than that in isotropy (4 for glass fiber / epoxy, 6 for boron / epoxy and 9 for carbon epoxy). Greszczuk has given the circumferential stress at the hole edge of isotropic and anisotropic materials.
The stress of orthotropic material is larger and more complex than that of isotropic material. For isotropic materials, the bearing capacity of perforated plates can be estimated by the stress concentration factor plus the yield or failure criterion (maximum stress criterion). Composite materials have many failure mechanisms, so a series of failure prediction should be adopted.
The failure of laminated plates is more complex. The classical theory of laminated plates is used to estimate the stress of each layer, and the composite stress criterion is used to predict the failure of each layer. In fact, the influence of holes in laminates is more complicated due to edge effect. At the edge of the hole, there is the interlaminar shear stress area which is caused by the loading bow in the direction of the lamina.
In the exact solution of the plate with holes, the interlaminar stress and edge effect should be included in the calculation of the failure point. This kind of complexity will increase the cost of analysis and test verification, thus increasing the cost of products. However, in recent years, the practice foundation is increasing, and many useful guidelines can be used for reference. In the process of composite drilling, the damage of laminate and fiber should be minimized. The insertion of fasteners is particularly important for thick composite materials. If the fasteners are inserted at a certain angle or forced, the laminates will be extruded and damaged prematurely. The single lap joint and double lap joint will have similar failure due to the bending of fasteners. When locking fasteners, it is generally considered that the preload is beneficial. The preload can reduce the influence of laying sequence and free edge, and make the load distribution more uniform.
In the mechanical connection design, the composite material should be considered as brittle material, so the stress concentration around the fastening hole will not lead to plastic deformation and reduce, but the local matrix damage and delamination stress concentration will be relieved. The size design of the hole is very important. If more than one fastener is used, the hole alignment is also very important. Otherwise, the load between the nails cannot be evenly distributed, leading to early failure. This problem will slow down the thermoplastic materials to a certain extent, mainly because the thermoplastic materials can withstand greater plastic deformation. In the design process, the environment of the connector is also an aspect to be considered in the design. Mismatching of thermal properties of laminates and fasteners may result in damage or loosening of fasteners. In the same way, the wet expansion of laminate caused by moisture absorption (or absorption of other solvents) will lead to loosening of fasteners. Resin matrix composite fastener and manufacturing I process. Composite fastener is an ideal method to solve the problems of weight, strength, corrosion, lightning stroke, etc. this kind of fastener is only suitable for light-duty members, but it still needs metal fasteners for heavy-duty members. The research scope of composite fastener includes installation and manufacturing. Composite fasteners need fiber and resin to have proper matching, so that the fastener head can bear enough tensile stress, and the nail bar part should have enough shear strength.
The fasteners used in composite structures must solve four major problems: potential corrosion, easy to be "stuck", installation, damage and low pull off strength. In all materials, only stainless steel and composite have the lowest potential difference, but their specific strength is the lowest. In other materials, only titanium alloy has both high specific strength and low potential difference. Therefore, titanium alloy has become the best choice for the structural connection of composite structural parts.
The fasteners of resin matrix composite materials mainly include bimetal rivets, half hollow titanium rivets, high lock bolts, bolts, 100 ° countersunk pull riveted titanium ring grooved nails, large foot thread pull nails, rivets, composite bolts and interference fit titanium alloy ring grooved nails. Table 3-10 lists the characteristics of composite special fasteners.
The composite fastener has the following advantages: avoiding arc light in oil tank when lightning strikes; reducing weight; reducing different materials