Composite materials have been widely used in the structure of civil aircraft. The biggest advantages of composite materials are corrosion resistance and insensitivity to fatigue, and can effectively reduce the weight of aircraft. Therefore, research on aircraft composite material maintenance has high practical engineering significance.
1. Aircraft composite materials
1.1 Types of application
Aircraft composite structures are often referred to as "fiber-reinforced plastics." This is because it uses high-strength fiber-reinforced materials, embedded in a resin matrix, stacked in the form of layers or sheets to form a laminate. A precisely controlled pressure heating process is then used to cure the laminate into a very strong and rigid structure.
The components that make up the aircraft composite material include fiber reinforced material, matrix and interface layer.
The fiber-reinforced material body is a load-bearing component, which is evenly distributed in the matrix and strengthens (toughens) the matrix;
The matrix is used to connect the fiber reinforced materials, make the composite material obtain a certain shape, and protect the fiber reinforced materials;
The interface layer is a coating wrapped around the reinforcement, its function is to transmit force, at the same time prevent the matrix from damaging the fiber reinforced material, and adjust the physical and chemical bonding between the matrix and the fiber reinforced material to ensure the role of the fiber reinforced material Play.
Through the composite effect produced by the interface layer, the composite material can surpass the performance of the original components and achieve the purpose of maximally improving the strength or toughness. Aircraft composite materials are not only multi-component materials, but also the mechanical and physical properties of the materials change with the direction, and are also anisotropic materials.
2. Damage to composite materials
2.1 Crack damage of composite matrix resin
When the composite laminate is under tensile load or alternating load, we can first observe the matrix crack in the off-axis layer. The earliest cracks are often 90 ° layers, followed by other off-axis layers. Generally speaking, the smaller the angle with respect to the axial load direction, the less likely it is that a matrix resin crack will be formed. The matrix resin crack in the off-axis layer is the main form of damage in the off-axis layer. The onset of matrix resin cracking depends on the stress level in this layer. As long as the stress level in the layer reaches the breaking strength of the matrix resin material, or although the stress level is lower than the breaking strength of the matrix resin, the matrix resin crack will appear in the off-axis layer after sufficient load cycling. The matrix resin crack damage in the off-axis layer is related to the layering sequence.
For example, [0/90/45] s laminates have more cracks in 90 ° layup than [0/45/90] s laminates in 90 ° layup. All the cracks of the off-axis layer are added together, and 60% to 90% of the cracks are generated before the 20% fatigue life. However, the appearance of a large number of matrix resin cracks does not affect the safety of the components during application. A large number of static tests and fatigue tests have proved that the composite materials have unique "damage-safety" characteristics.
2.2 Impact damage of composite materials
The impact resistance of composite materials is poor. It is often damaged by the impact of foreign objects. When the impact energy is below a certain level, although the damage cannot be visually perceived, such damage may cause a significant reduction in strength. In the process of using the composite material structure, various damages may occur due to the impact, which can be divided into hard object impact damage and soft object impact damage.
The impact of hard objects is often caused by local damage of the composite material, which may cause the composite material to significantly decrease in strength, and even fatigue damage may occur during a short period of fatigue. The stones on the runway when the aircraft takes off and landing and the hail during flight in the air may cause impact damage to the composite materials. In addition, during the manufacturing and maintenance process, incorrect maintenance behaviors, such as the impact of falling tools It can also cause impact damage to composite components.
The impact of soft objects mainly refers to the impact of birds. Sometimes such impacts directly cause structural damage, and sometimes only cause local damage. Mainly depends on the mass, material, impact speed, geometry and impact angle of the impactor.
2.3 Lamination damage of composite materials
Under the in-plane axial load, interlayer stress or compressive stress (perpendicular to the plane direction of the laminate) will be generated along the edge of the composite material. If the interlayer stress caused by external load (static load) is tensile stress and exceeds the interlayer strength of the material, delamination will occur at the free edge. It should be noted that when the alternating stress level is lower than the static stress level at which delamination begins, delamination may also occur early in the fatigue life.
The layup sequence of the composite laminate will determine whether the normal stress between layers at the free edge is tensile or compressive. For example, [30/90] s laminates under tensile load, the interlayer normal stress generated at the free edge is tensile stress, and the compressive stress generated under compressive load, so ] s Laminated plates will produce extensive delamination damage under the action of tensile load, but will not produce such severe delamination damage under the action of compression load. The reason is that under normal compressive load, the normal stress between layers at the free edge is compressive stress. [90/30] s Laminated plates produce interlaminar tensile stress at the free edge under compressive load, so it produces delamination damage under compressive load. In addition, it should also be pointed out that the two 90 ° plies are pasted together easily to cause delamination damage at the free edge. The method of covering the edges with sewing or woven cloth and improving the interlayer strength of the base material can improve the resistance of the interlayer to delamination.
3. Repair of composite material damage
3.1 Composite repair equipment
In the process of repairing and curing the composite material structure, it is necessary to heat the repaired parts; during the repair process, the commonly used heating equipment includes an oven and a heating blanket, or you can choose to use a hot-pressed tank. The hot press tank uses positive pressure to compact the material layer, while using a hot mixed gas of nitrogen and air to cure the material through high-speed circulation. The oven uses the negative pressure in the vacuum bag to compact the material layer, while using high-speed circulating air to solidify the material. The heating lamp is used for curing and low temperature repair. The distance between the heating lamp and the repair surface will determine the temperature of the repair site, and an adjustable bracket is needed to change the distance to the repair site. The heating lamp must not touch or be close to the repair parts or components, otherwise it will cause damage to the repair area or components. At the same time, a thermocouple can be used to measure the surface temperature. If the temperature under the direct beam of the heating lamp is relatively high, the temperature of the heating lamp can be controlled by using a thermal patch controller. The electric blanket is composed of two layers of silica gel sandwiched with a layer of metal resistance heating elements. Composite repairs use electric blankets that output 5 watts per square inch. To ensure that the edge of the repaired area is fully cured, an electric blanket 4 inches larger than the repaired area should be used.
During the repair, a thermocouple indicating the highest temperature can be set to control the repair and curing cycle. The thermal repair instrument can monitor the vacuum level in the vacuum bag during the repair and curing cycle. If the vacuum bag is abnormal, the thermal repair instrument will issue an alarm.
3.2 Composite materials repair auxiliary materials
The auxiliary material is not part of the repaired parts after repair. Auxiliary materials refer to materials that are used during the repair process to assist the curing process or to help cure to the correct fiber-resin ratio.
Auxiliary materials are: separation film / woven fabric, suction material and breathable cotton, vacuum bag film, sealing tape for vacuum bag, etc. These materials are available in different compositions, thicknesses and temperature ranges.
Separation membranes / woven fabrics are used in situations where resin flow needs to be controlled, or in contact with resins / adhesives.
The non-porous separation membrane / woven fabric acts as a kind of separator and plays a role of isolation.
The porous separation membrane / woven fabric allows resin and air to pass through and can be easily removed from the part after curing. Suction material and breathable cotton can be the same material, but have different applications. This material is highly absorbent and porous, and is usually made of polyester. Suction materials are used to absorb excess resin on the parts, and provide channels for chemical volatiles and air when the resin is cured, so that they can escape during the curing process. Breathable cotton is usually used when it is not in contact with the resin. It is only used between the vacuum bag film and other vacuum bag materials to provide a path for air to escape from the layer. The sealing tape is used between the parts and the vacuum bag film to play the role of air sealing and generate the vacuum pressure required for repair.
3.3 Composite repair materials
1. Resin material: resin is used to impregnate fiber fabrics, the resin is a two-component epoxy resin series. Before the two components are mixed and used, each component can be stored at room temperature.
2. Fiber fabric: Fiber fabric (fiber fabric and fiber unidirectional belt) is the pavement material for wet ply repair. Wet layup repairs are performed by users using fiber fabric dip coating resins.