I. Substrate materials
1.1 Resin
1.1.1 Thermoset Resin
Resin is the generic term for polymers. The resin and its chemical composition and physical properties fundamentally affect the processing, manufacturing and final properties of the composite material. Thermoset resin is the most diverse and widely used of all man-made materials. It is easy to cast or form into any shape, is compatible with most other materials, and cures easily (by heat or catalysts) to an insoluble solid. Thermoset resin is also an excellent adhesive and bonding agent.
1.1.2 Polyester Resin
Polyester resin is a relatively inexpensive and easy to process resin that is often used in low cost applications. Low smoke polyester resin is used for interior parts of aircraft. Fiber-reinforced polyester can be processed in a variety of ways. Common processing methods include matched metal mold forming, wet lay-up lamination (vacuum bagging) forming, injection molding, fiber winding, pultrusion, and high pressure steam.
1.1.3 Vinyl Ester Resin
Vinyl ester resin has the same appearance, handling properties, and curing characteristics of conventional resins as polyester resins. However, the corrosion resistance and mechanical properties of vinyl ester composites are much improved over the standard polyester resin composites.
1.1.4 Phenolic Resin
Phenolic resin was first used commercially in the market in the early 20th century. Urea formaldehyde and melamine formaldehyde emerged in the 1920s and 1930s as lower cost alternatives for low temperature use. Phenolic resin is used for interior components because of its low smoke and low flammability characteristics.
1.1.5 Epoxy Resin
Epoxy resin is a polymerizable thermosetting resin that has a wide range of viscosities from liquid to solid. With many different types of epoxy resin, the technician should use a maintenance manual to select the correct type for a particular repair. Epoxy resin is widely used in prepregs and structural adhesives. The advantages of epoxies are high strength and modulus, low volatile content, good adhesion, low shrinkage, good chemical resistance, and ease of processing. Its major disadvantages are fragility and degradation of properties in the presence of moisture. Epoxy resin is slower to process or cure than polyester resin. Processing techniques include autoclave molding, fiber winding, molding, vacuum bagging, resin transfer molding and pultrusion molding. Curing temperatures range from room temperature to approximately 350°F (180°C). The most common cure temperature range is between 250° and 350°F (120-180°C). As shown in Figure 10.
Figure 10: Both wet epoxy dispenser layup systems with pumps
1.1.6 Polyimide Resin
Polyimide resin excels in high-temperature environments, where its heat resistance, oxidative stability, low coefficient of thermal expansion, and solvent resistance facilitate design. Its main uses are circuit boards, heat engines and airframe structures. Polyimide resin can be a thermoset resin or thermoplastic. The polyimide resin requires high curing temperatures, usually in excess of 550°F (290°C). As a result, common epoxy composite bagging materials are not available, and steel tooling becomes a necessity. It is very important to use polyimide bagging and release films such as Kapton®. upilex® instead of lower cost nylon sleeving and polytetrafluoroethylene (PTFE) release films is a common procedure for epoxy composite processing.
Fiberglass topping due to the low melting point of polyester fibers has to be replaced with a dischargeable breathable material as a bedding material.
1.1.7 Polybenzimidazole Resin (PBI)
PBI is used in high temperature resistant materials because of its extreme high temperature resistance. The resin are used as adhesives and fibers.
1.1.8 Bismaleimide Resin (BMI)
BMI has higher temperature resistance and higher toughness than epoxy resins and offer excellent performance in both ambient and elevated temperatures. The BMI is processed similarly to epoxy resins. BMI is used in aero-engines and high-temperature components. bMIs are suitable for standard hot-press can processing, injection molding, resin cast molding, and molded composite molding (SMC), among others.
1.1.9 Thermoplastic resin
Thermoplastic materials can be softened repeatedly by increasing temperature and hardened repeatedly by decreasing temperature. Processing speed is the main advantage of thermoplastic materials. No chemical curing occurs during processing, and materials can be molded or extruded when soft.
1.1.10 Semi-crystalline Thermoplastics
Semi-crystalline thermoplastics have fixed flame retardant properties, superior toughness, good high temperature and post-impact mechanical properties, and low moisture absorption. They are used in secondary and primary aircraft structures. In combination with reinforcing fibers, they can be used for injection molding compounds, compression molded random sheets, unidirectional molds, prepregs made from prepreg tows (prepregs), and fabric prepregs. Fibers impregnated in semicrystalline thermoplastics include carbon fibers, nickel-plated carbon, aramid, glass fibers, quartz, and others.
1.1.11 Amorphous Thermoplastics
Amorphous thermoplastics are available in a variety of physical forms, including films, filaments, and powders. In combination with reinforcing fibers, they are also used in injection molded composites, compression moldable random sheets, unidirectional rubber molds, woven prepregs, and others. The fibers used are mainly carbon, aramid and glass fibers. The particular advantages of amorphous thermoplastics depend on the polymer. Typically, the resins are known for their ease of processing, speed, high temperature capability, good mechanical properties, excellent toughness and impact strength, and chemical stability. The stability results in an unlimited storage life span, eliminating the requirement for cold storage of thermoset prepregs.
1.1.12 Polyetheretherketone (PEEK)
PEEK is a high-temperature thermoplastic. This aromatic ketone material has excellent high heat and combustion characteristics and is resistant to a wide range of solvents and proprietary soluble fluids. PEEK can also be reinforced with glass and carbon fibers.
1.2 Curing Stages of Resins
Thermosetting resin is cured using a chemical reaction. There are three stages of curing, called A, B and C.
-Stage A:The resin components (substrate and hardener) have been mixed, but the chemical reaction has not yet begun. During wet lay-up, the resin is in stage A.
-Stage B: The resin components have been mixed and the chemical reaction has started. The material becomes thick and sticky. The resin of the prepreg is in stage B. To prevent further curing, the resin is placed in the freezer at 0°F. In the frozen state, the resin of the prepreg stays in the B stage. Curing begins when the material is removed from the refrigerator and heated again.
-Stage C:The resin is fully cured. Some resins cure at room temperature, others require high temperature curing cycles to fully and adequately cure.
1.3 Prepregs
Prepreg consists of a combination of matrix and reinforcing fibers. It is available in unidirectional form (one reinforcement direction) and in fabric laminated form (several reinforcement directions). All five main matrix resin families can be used to impregnate various fiber forms. The resins are then no longer in the low viscosity stage, but have been advanced to a class B cure level for better handling characteristics. The following products are available in prepreg form: unidirectional rubber molds, woven fiber articles, continuous tows and chopped cut mats. Prepregs must be stored in a refrigerator below 0°F to retard the curing process. Prepregs are cured at elevated temperatures. Many prepregs used in aerospace are impregnated with epoxy resins that cure at 250°F or 350°F. Prepregs are cured in autoclaves, ovens or hot blankets. They are usually purchased and stored in a sealed plastic bag roll to avoid moisture contamination. As shown in Figure 11.
Figure 11: Adhesive Film and Fabric Prepreg
1.4 Dry fiber materials
Dry fiber materials, such as carbon fiber, glass fiber and kevlar®, are used in many aircraft repair procedures. The dry fabric is impregnated with resin before the repair work begins. This process is often referred to as wet lamination. The main advantage of using the wet lay-up process is that the fibers and resin can be stored at room temperature for long periods of time. The composite can be cured at room temperature or cured at high temperatures to speed up the curing process and increase strength. The disadvantages are that the process is messy and the properties of the reinforced material are lower than those of the prepreg. As shown in Figure 12.
Figure 12: Dry fabric materials (from top to bottom: aluminum lightning protection material, kevlar®, glass fiber and carbon fiber)
1.5 Auxiliaries (thixotropic agents)
Auxiliaries (thixotropic agents) are in the form of a gel when at rest and become liquid when stirred. These materials have high static shear strength and low dynamic shear strength while losing viscosity under stress.
II. Adhesives
2.1 Film adhesives
Structural adhesives for aerospace applications are typically supplied in film form, supported on release paper, and stored under refrigerated conditions (-18°C, or 0°F). Film adhesives can use high temperature aromatic amines or catalyzed curing agents with a wide range of flexibilizers and tougheners. Rubber toughened epoxy film adhesives are widely used in the aerospace industry. The upper temperature limit of 121-177°C (250-350°F) is usually dependent on the degree of toughening required and the overall selection of resin and hardener. In general, toughened resins result in lower service temperatures. The film material is usually supported by fibers to improve handling of the film prior to curing, to control adhesive flow during the bonding process, and to assist in controlling the thickness of the bondline. The fibers can be made into randomly oriented staple mats or into woven fabrics. Common fibers are polyester, polyamide (nylon) and glass fibers. Adhesives containing woven fabrics may have a slight environmental degradation due to water being absorbed by the fibers. Random matting is not as effective as woven fabrics in controlling film thickness because the unrestricted fibers move during the bonding process. Spunlace nonwoven fabrics do not move and are therefore widely used. As shown in Figures 13 and 14.
Figure 13: Use of Film Adhesives, Kevlar®, Glass Fiber and Carbon Fiber
Figure 14: Adhesive film
2.2 Adhesives
Adhesives are used as a substitute for film adhesives. These are often used for secondary bonding to repair damaged parts of patches and in areas where film adhesives are difficult to apply. In the case of epoxy resins, a paste is mainly used to adhere to the structural binder. One-part and two-part systems are available. The advantage of paste adhesives is that they can be stored at room temperature and have a long shelf life. The disadvantage is that the thickness of the bondline is difficult to control, which affects the strength of the bond.
When the adhesive is applied, it is possible to keep the fabric glued during the bonding process. As shown in Figure 15.
Figure 15: Adhesives
2.3 Foam Adhesives
Most foam adhesives are 0.025-inch to 0.10-inch thick Class B epoxy resins. Foam adhesives cure at 250°F (121°C) or 350°F (176°C). During the cure cycle, the foam adhesive unfolds. Foam adhesives need to be stored in the refrigerator and, like prepregs, they have a limited storage life. In the pre-repair, the foam adhesive is used to splice onto the honeycomb in the sandwich structure and repair in the existing core. As shown in Figure 16.
Figure 16: Using Foam Adhesive
III. Description of sandwich structure (description of sandwich structure)
Theoretically, sandwich construction is a structural panel concept that consists of two relatively thin, parallel facings separated by a relatively thick or light core. The core supports the facing against bending and against self-planar shear loads. The core must have high shear strength and compressive stiffness. Composite sandwich structures are usually manufactured by autoclave curing, press curing or vacuum bag curing. Skin laminations can be pre-cured and then combined in a co-curing operation, or a combination of both methods. Examples of honeycomb structures are: wing spoilers, talc, ailerons, flaps, nacelles, floors, and rudders. As shown in Figure 17.
Figure 17: Honeycomb sandwich structure
IV. Performance
In a comparison of aluminum and composite laminate structures, the bending stiffness of sandwich structures is very high. Most honeycombs are anisotropic, i.e., the properties are oriented. The advantages of using honeycomb structures are illustrated in Figure 18. Increasing the core thickness greatly increases the stiffness of the honeycomb structure with minimal weight increase. Due to the high stiffness of the honeycomb structure, there is no need to use external hardboards, as in the case of a beam frame.
Figure 18: Strength and stiffness of honeycomb sandwich materials compared to solid lamination values