Research and application of the hottest composite

Research and application of composite patch bonding repair (Part I)

1 Introduction

composite materials have high specific strength and specific modulus, strong designability, good fatigue resistance, corrosion resistance and dimensional stability, and are widely used in the aerospace field. It is inevitable that the aircraft will be damaged during service, and the service life of the appropriate products will also be extended. Since the 1970s, the Royal Australian Air Force Aeronautical Research Laboratory and the U.S. Naval Research Laboratory have developed a new structural repair technology, that is, using composite patches to repair the damaged metal structures of aircraft, which has been successfully applied to the metal and composite structures of some military and civil aircraft

2 advantages of composite patch bonding repair

as a new structural repair technology, compared with the traditional mechanical repair methods (riveting, welding, bolting), composite patch bonding repair has obvious advantages. (1) The composite has high specific strength and modulus, and its patch thickness is about 1/3 ~ 1/2 of that of aluminum alloy patch, which can achieve the same repair effect. Therefore, the weight gain of the structure after bonding repair is small. (2) The composite material has strong designability, and the ply design of the material can be carried out according to the use requirements and stress conditions. (3) Composite materials have a variety of forming processes, which are convenient for large-area integral forming, and can be made into large structural parts and parts with complex surface shape, which can be repaired in situ by bonding and co curing process. For complex curved surfaces, composite patch repair is easier to implement than traditional mechanical repair, and after repair, the patch is closely bonded with the matrix, basically maintaining the original structural shape, which is easy to meet the complex aerodynamic requirements. (4) Composite patch bonding repair can improve the stiffness and static strength of the damaged area and reduce the stress intensity factor at the crack tip. Patch bonding repair does not need to open holes in the original structure and will not form new stress concentration sources, which is conducive to improving the damage tolerance and fatigue resistance of the structure [1]. (5) The repair time is short, and the fatigue life test steps of the automobile front axle stand: This is low. (6) The equipment required for field repair is simple, mainly including repair kit, repair instrument and portable NDT equipment

3 key technology of composite patch bonding repair

for composite patch bonding repair, the selection of adhesive, the matching of patch material and repaired metal components, the surface treatment of repaired structure, repair curing process and construction process guarantee are key technologies

3.1 selection of adhesive

adhesive is the intermediate medium to realize the crack arrest effect of patch, so the selection of adhesive is very important. Adhesives with good fatigue resistance, high shear and peel strength, good medium resistance and moisture and heat aging resistance should be selected according to the actual bearing level and service environment of the repaired structure. There are two main types of adhesives used in composite bonding repair: one is two-component adhesives, which are mainly used for room temperature curing repair of composite structures (a few two-component adhesives can be used for thermal bonding curing repair); The other is film adhesive (adhesive film), which is used for hot bonding, curing and repairing. The adhesives used in composite patch bonding repair abroad mostly belong to epoxy system, such as fm-73, FM ○ r300-2, af-126, etc. This kind of adhesive has high toughness, shear strength and high peel strength. It is generally cured at 100 ℃ ~ 120 ℃, belonging to medium temperature curing system. Similar adhesives in China include j-88, j-47 and j-159

3.2 matching between the composite patch and the repaired metal components

the composite patch should be cured at the lowest temperature possible and can be consistent with the curing temperature of the adhesive; The thermal expansion coefficient of the patch should match that of the parent material. Boron/epoxy composite patches are mostly used to repair damaged metal structures in foreign countries. Because boron/epoxy composites have high strength and elastic modulus, relatively high coefficient of thermal expansion, good thermal matching performance with metal materials, which is conducive to reducing the residual thermal stress in the bonding repair structure, low conductivity, easy to use eddy current nondestructive testing, and the electrochemical corrosion performance is better than that of carbon/epoxy composites after contacting with metal. However, the cost of boron fiber is high and processing is difficult. Carbon/epoxy patch and glass/epoxy patch are mostly used in China, and the price is relatively cheap. Carbon/epoxy composites are widely used in the aviation industry and are easy to operate. They can be made into parts with large radius of curvature, which are suitable for the repair of complex structures, but their strength and stiffness are not as good as those of boron/epoxy composites, which are prone to electrochemical corrosion when in contact with metal

3.3 surface treatment of repaired structures

surface treatment of repaired structures refers to the removal of oxide layers on the surface of structures that affect the bonding force by mechanical methods before bonding Increase the mechanical interface of pollutants; Or through chemical methods, a layer of uniform special oxide is deposited on the structural surface or chemical bonds are formed on the bonding surface, so that the treated structural surface has high surface energy, so as to obtain high bonding strength and durability. The commonly used methods to purify the surface include solvent cleaning, steam degreasing, etc. Common chemical treatment methods include grooveless chemical oxidation and silane coupling agent coating. The grooveless chemical oxidation method will be limited by cracks, cracks and other structural forms when implemented in the field, which is easy to produce secondary pollution, so it is not suitable for use in the field. At present, the commonly used simple surface treatment method is to apply silane coupling agent after mechanical grinding

3.4 repair and curing process and construction process the main control factors to ensure repair and curing are pressure, temperature and time. Because the repaired part and the surrounding body structure form a complex thermal conductor, and the structural forms are diverse, it is necessary to provide continuous temperature and pressure protection for the repair area through special repair equipment (such as heat repair instrument), supporting materials and special tools

4 design analysis of composite patch bonding repair

4.1 durability principle of bonding repair

durability principle should be considered in the design of bonding repair. (1) Control the design stress level. The geometric parameters of the patch are selected through the stress calculation of strength design, so as to at least ensure that the stress level of the patch in the bonding structure is consistent with the damaged part of the original structure, and the load shared by the patch does not exceed the bearing capacity of the adhesive. (2) Avoid or reduce eccentricity. Single side bonding is easy to make the structure repaired by bonding eccentric. Under the action of external force, tensile stress perpendicular to the bonding surface is generated in the adhesive layer, making the peak stress exceed the nominal stress and reducing the bearing capacity of the repaired structure. (3) Reduce stress concentration. The stiffness change at the edge of the patch should be gentle, so as to avoid stress concentration in the adhesive layer caused by sudden change in the stiffness of the edge structure, resulting in the early destruction of the adhesive layer at this position. (4) Arrange patches reasonably. In order to ensure the repair efficiency, unidirectional fiber laminates can be used in the repair, and 90 ° and ± 45 ° layers can be appropriately added in the complex loaded parts as required. (5) Take appropriate protective measures after repair. Because damp heat, corrosive media, ultraviolet radiation and other environments will accelerate the aging of adhesives and patches and reduce the adhesive capacity. (6) Control the bonding quality. The bonding repair of damaged structures with composite patches should strictly follow the relevant process. Mauser has installed recycling centers in the United States, Brazil and Europe

4.2 main parameters of bonding repair

through the design of patch size, ply, shape and other details, the repaired structure has good service function and long service life under the comprehensive action of load, environment and other factors. From the design point of view, the parameters of bonding repair mainly include the shape, size and ply of the patch

4.2.1 shape of patch

when determining the shape of patch, consider the specific characteristics of damaged structure, and note that the shape of patch should not be too special. When repairing the metal plate with a crack in the center, the best shape of the patch is designed to be skewed (symmetrical four pointed leaf shape or waist drum shape), and pay attention to ensure that the internal stress of the plate does not exceed the allowable stress; Rectangular patch takes the second place, but it is more effective than elliptical, circular and square patches. The optimal length perpendicular to the crack direction is equal to the crack length, and the length parallel to the crack direction is equal to the plate length; The patch of ellipse is inferior to that of skew and rectangle. The ellipse whose long axis is parallel to the crack is more effective than that perpendicular to the crack. The optimal length of the long axis is equal to the width of the motherboard, and the optimal length of the short axis (perpendicular to the crack direction) is equal to the length of the crack. For patches with the same volume, increasing the thickness can reduce the stress intensity factor by 18% more than increasing the area [6]. The patch edge should be designed as a wedge with a certain taper. When the taper ratio is 30:1 to 20:1, the stress concentration in the adhesive layer caused by the sudden change of edge structural stiffness can be avoided. And Mary a Mahler[7] believes that the uniform dispersion estimation: when the taper ratio is 16:1, the bonding strength is higher; Stress analysis shows that in some areas, when the taper ratio is 10 ∶ 1, it can also provide enough high bonding strength. Even if there is a slightly higher stress/strain concentration in the area with small bearing capacity of the repaired structure, it is still within the safe range. In order to avoid shear stress, especially in the 0 ° direction of the patch, cutting its end into a serrated shape can reduce the shear stress and greatly improve the bonding strength of the patch