Applications Of Composite Honeycomb Sandwich Structures in Civil Aircrafts

Abstract: Composite honeycomb sandwich structures are used in a large number of applications in the field of civil aircraft, which are characterized by high strength, high specific stiffness, low density, good compressive and flexural properties, etc. The application parts include fuselage, engine cowl, fairing, hatch, antenna cover, leading edge and so on. The large number of applications of honeycomb sandwich structure significantly reduces the weight of aircraft structure. With the progress of molding process and the improvement of material properties, honeycomb sandwich structure is gradually applied to the floor and other main bearing structures. This paper firstly analyzes the research progress and development process of honeycomb sandwich structures. Secondly, it sort out the application of honeycomb sandwich structures on typical aircraft components. Finally, it introduces the current status of the application of honeycomb sandwich structure on the floor of civil aircraft.

001 Introduction

With the socio-economic development, there are two tasks facing the field of transportation, especially air transportation: the overall weight reduction of structures and the issue of benefits directly related to weight reduction - the reduction of fuel consumption. Structural weight and fuel consumption are particularly important in the aviation field. Currently, honeycomb sandwich structures are widely used in fuselages, hatches, wings, tails, floors, fairings and antenna cover in the aviation industry. Honeycomb sandwich structures offer a good strength-to-weight ratio. Compared to composite skins, they perform better under lateral and bending loads due to their structural form, which consists of a thick, lightweight core surrounded by two thin, rigid layers.

The large number of applications of honeycomb sandwich structures has significantly reduced the weight of aircraft structures, thanks to their good compressive and flexural properties, so much so that honeycomb sandwich structures are gradually moving towards main bearing structures such as floors. The content of this paper is mainly divided into three parts: firstly, it analyzes the research progress and development process of honeycomb sandwich structure; secondly, it combines the application of honeycomb sandwich structure on typical aircraft components with the currently used models; finally, it introduces the current status of the application of honeycomb sandwich structure on the floors of civil aircraft.

002 Honeycomb Sandwich Structure

The honeycomb sandwich structure, which first originated in bionics, is characterized by high specific strength, high specific stiffness, and light weight compared with the same type of solid material, and thus has been widely used in aviation , transportation, and other fields. The composite honeycomb sandwich structure can effectively improve the bending stiffness and enhance the ability to withstand bending moments and pressures with a very small weight increase, making it a more ideal lightweight structure for aviation.

Composite honeycomb sandwich structures consist of three components: panels, honeycomb core and adhesive.

(1) Panel. The panel is the main load-bearing part of the sandwich structure. Compared with the core material, the panel material is characterized by high density, high modulus and high strength. Composite panel material is usually aluminum alloy, titanium alloy, glass fiber reinforced plastic and other materials. Most of the unidirectional tape or fabric composites of carbon fiber or glass fiber are currently used in aerospace structures.

(2) Honeycomb Core. The performance of honeycomb sandwich structure and the geometry of honeycomb are related to the material of honeycomb core. In general, honeycomb cores are available as aluminum honeycomb core, aramid honeycomb core, fiberglass honeycomb core, etc. according to the material. Aramid honeycomb is divided into meta-aramid honeycomb and para-aramid honeycomb. Meso-aramid honeycomb is a meta-aramid obtained by interfacial polycondensation or low-temperature solution polycondensation of m-toluene dicarbonyl chloride (MDCL) and m-phenylenediamine (MPA). The most common on the market is the meta-aramid first invented and put into use by DuPont in the 1960s. Para-aramid honeycomb is p-phenylenediamine and p-phenylenedicarbonyl chloride polycondensation to get para-aramid, trade name Kevlar. The existing aramid honeycomb core is mainly meta-aramid, and the research of para-aramid is very little. The main application models are shown in Table 1. Aramid paper honeycomb (NOMEX honeycomb) is made of aramid paper moistened with phenolic resin. Compared with aluminum honeycomb, aramid honeycomb is much more resistant to local instability because the honeycomb cell of aramid honeycomb is thicker than that of aluminum honeycomb.The preparation of Nomex honeycomb consists of nine processes: gluing of aramid paper, laminating, pressing, cutting, stretching, sizing, dipping, curing, and slicing. AVIC Composites Co., Ltd. has improved the dip gluing process of aramid honeycomb, concluded the process parameters such as cell and paper thickness, and deduced the empirical formula for density control. Suzhou Fanglei Honeycomb Composites Co., Ltd. uses domestic aramid paper to prepare honeycomb, compressive and shear properties have been able to meet the requirements of HB5435-89 and BMS8-124, dielectric and flame retardant properties also meet the standard requirements. Meishi, a subsidiary of China's Yantai Spandex Group Co., Ltd. has prepared different types of honeycomb cores from its meta-aramid paper through a series of mature processes. The in-plane compression resistance and shear resistance of its core and sandwich structure at room temperature are able to reach the index values in the Boeing BMS8-124 standard, and can meet the use requirements of mechanical properties.

Tab. 1 Main application models of honeycomb type

(3) Adhesive. The adhesive used for aviation honeycomb sandwich structures is usually a structural adhesive. Structural adhesives are those that can withstand considerable force in a predetermined period of time, in the environment in which they are used, and have a strength and durable service life that match that of the object to which they are adhered. According to the resin matrix is generally divided into three major categories. The first epoxy, epoxy resin has the advantages of excellent workmanship, long application period, and the highest temperature resistance of up to 232 ℃; The second category is the bismaleimide, which can reach temperatures of more than 232 ℃, and is mainly used in military aircraft with higher temperatures; The third is the type of cyanoacid ester, which has excellent resistance to temperature, and dielectricity, and resistance to heat and humidity, thus it is mainly used in components with Electrical performance requirements of the components. Aviation commonly used epoxy resin structural adhesive as shown in Table 2. The United States Hexcel, Cytec and other companies have developed a variety of uses of the material system, and in recent years the domestic AVIC Composite Company has also developed a medium and high temperature resin system, such as BA9913, BA9916, SY-24C-300 and so on.

Tab. 2 Domestic and foreign aviation structural adhesives

003 Honeycomb sandwich structures on typical aircraft components

As far as large civil aircraft are concerned, the Boeing 747 (first flown on February 9, 1969) was designed with a large percentage of sandwich construction (Figure 1). It has about half of the wing surface, including the leading and trailing edges, made of fiberglass and Nomex honeycomb. In the Boeing 747, the fuselage cylindrical shell is mainly Nomex honeycomb, and the floor, side panels, overhead garbage cans, and ceiling are also made of honeycomb. Most of the flaps are made of the same honeycomb structure, but aluminum honeycomb and skin are also used. Honeycomb sandwich structure applications on the B787 include the rudder, elevator, wingtips, and engine nacelles. Among them, the engine nacelles and thrust reversers are made of HexWeb honeycomb and HexPly8552/AS4 prepreg. With the advantages of light weight and high stiffness, as well as acoustic absorption, HexWeb honeycomb has been utilized by all major engine manufacturers in the world. For the Boeing 707, only 8% is honeycomb, and for the newer Boeing 757/46, 46% is honeycomb.

Fig. 1 Illustration of a portion of the B747 sandwich structure application (blue area)

Honeycomb sandwich structure in the application of large civil aircraft is the earliest in the Airbus A310 aircraft rudder, and then used in the A320, A340 rudder. The largest honeycomb sandwich structure is the A340 rudder, with an area of 15.3 m² of honeycomb sandwich structure.The largest single-aircraft dosage for the A380 aircraft, the amount of its honeycomb sandwich structure amounted to 4,000 m², mainly used for large-size structural components, including the belly fairing, flooring, etc.. And the flooring is mainly made of Gillfab4909 sandwich panels of M.C. GillCorp. Kevlar honeycomb. The floor structure on the Airbus A380 is made of a large number of Nomex honeycomb sandwich composites, which are excellent materials that enable the A380 to reach an operating life of 20,000 hours, ensuring that the aircraft can be in service for more than 10 years. Since then, the use of composites has increased significantly, especially with the ATR72, which was the first civil aircraft to be certified with a carbon basic structure (wing box) (see Figure 2). As shown in Figure 3, the proportion of sandwich materials in secondary structures has been decreasing. For the A380, Boeing 787 or Airbus A350, only the belly fairing, the nacelles and the front landing gear doors, some ailerons and rudders are still made of sandwich construction, the rest being self-reinforcing monolithic structures.

Fig. 2 Application of ATR72 composites

(a) Sandwich structure of A380 (b) Sandwich structure of A350 and B787 set of composite structure

Fig. 3 Diagram of aircraft sandwich structure

Compared with civil aircraft, the application of honeycomb sandwich structure in general aircraft is mainly manifested in the use of the fuselage, such as HawkerBeechcraf's PremierI, Hawker 4000 and "Prime Minister" IA aircraft, Bombardier Aerospace Learjet 85 aircraft and so on. PremierI is the first FAA-certified all-composite fuselage jet business jet, the entire fuselage is made of carbon panels of honeycomb sandwich structure, the overall thickness of 20.6mm, the fuselage is free of trusses and frames, than the traditional aluminum structure to reduce the weight of 25%, and increased by 13% of the cabin space. The fuselage structure of the Hawker 4000 aircraft currently in production consists of three barrel structures connected at the wings, the barrels are honeycomb sandwich structures, and the skin is laid using MAG Cincinnati's Viper automated wire-laying machine. Nomex honeycomb sandwich structure composite components are used in a large number of applications in Russia for aircraft components such as flaps, ailerons, fuselage wall panels, engines, cockpits, cargo compartments, horizontal tails, wing leading edges, helicopter rotor tail beams and other aircraft components.

The domestic production by AVIC Harbin Aircraft Industry Group Co., Ltd. and other units of a large number of resin matrix composite materials (such as drogue, paddle, flat tail and rotor, etc.) and Nomex honeycomb sandwich structure composite materials (fuselage panels, floors, tail beams, etc.), which significantly reduces the weight of the overall fuselage, improves the fatigue strength of the components, and improves aerodynamic characteristics so as to enhance the flight quality of the helicopter. It is estimated that, in addition to the engine and drive train, nearly 300 aircraft parts are made of Nomex honeycomb sandwich composites. All parts use Nomex honeycomb core materials of 3 densities and 12 specifications. The amount of Nomex honeycomb core materials in the whole fuselage is more than 200m2, and its coverage area accounts for about 80% of the whole fuselage, which is one of the aircraft types that use Nomex honeycomb sandwich structural composites the most in China at present.

004 Application of honeycomb sandwich structure on civil aircraft flooring

4.1 Research Status of Aircraft Passenger and Cargo Compartment Flooring

Conventional aircraft floors passenger and cargo floors are constructed of metal. Each floor has hundreds of rivets and screws, but also need to use elastic pads and fuselage structure isolation, and therefore expensive. And the use of composite materials can greatly improve the floor structure of the strength, stiffness, fatigue and corrosion resistance, can reduce the connectors by an order of magnitude, and can significantly reduce the weight of the floor structure.

The panel of the composite sandwich structure of the aircraft floor is usually made of glass fiber prepreg, carbon fiber and Kevlar prepreg, and the matrix is selected from epoxy resin or phenolic resin. Honeycomb cores are commonly used with Nomex honeycomb.

Fig. 4 Aircraft passenger and cargo compartment flooring

4.2 Application of sandwich structures to floors

The main cabin floor of Boeing's Model 234 civil Chinook helicopter measures 2.5m x 2.2m each and consists of four panels. The panels are fiberglass/Kevlar49 composites with Nomex honeycomb cores, and the sandwich floor has four layers of upper and lower panel layups each, symmetrical to the honeycomb core in the middle. The first and second layers are 0° glass fiber prepreg, and the third and fourth layers are 0° direction Kevlar49 fiber prepreg. The Nomex core has a height of 38mm, cell length of 2mm, and a density of 48kg/m3. The panels and the core are glued together with EA9 adhesive film. Composite sandwich structure is also chosen for the cabin floor of Boeing 777, Boeing 787 Dreamliner and other airplane models.

M.C. GillCorp. production of Gillfab4223 sandwich structure panel selection of fiberglass fabric, matrix for the phenolic resin, core for the GillcoreHD meta-aramid honeycomb, adhesive film selection of epoxy system. The thickness of the whole sandwich structure is 12.6mm, the thickness of the upper panel is 1.27mm, and the thickness of the lower panel is 0.508mm. Gillfab4223 sandwich structure is used in Airbus A318/ A319/ A320/ A321/ A330/ A340/ A300/ A310/ A300-600. M.C. GillCorp. production of Gillfab4505 sandwich structure panel selection of carbon fiber unidirectional belt (the surface layer for the glass fiber prepreg), the matrix for the phenolic resin, the core for the GillcoreHD meta-aramid honeycomb, adhesive film selection of epoxy system. The thickness of the whole sandwich structure is 9.5mm, and the thickness of the upper and lower panels is 0.5mm. Gillfab4505 sandwich structure is used in Airbus A318/A319/A320/A321/A330/A340.

The Fokker 100's cargo floor is a relatively simple structure, which is a composite sandwich structure with an external skin of sheets of high-performance thermoplastic composites and a core of Nomex honeycomb. The skin and the core are glued together with an epoxy film.

The cargo hold floor of a certain model of Nomex honeycomb sandwich structure was prepared by hot press molding using self-made glass fiber reinforced phenolic resin prepreg as the raw material of the panel by Niu Fangxu et al. of Beijing Glass Steel Institute Composites Ltd. The mechanical properties and flame retardant properties of the floor were systematically evaluated, and the effects of prepreg type, curing temperature, and molding process on the mechanical properties of the honeycomb panels were analyzed, and the influencing factors of the flame retardant properties of the honeycomb panels were studied. The results show that the mechanical properties of the cargo compartment floor prepared with self-made prepreg are excellent, with the ultimate load of long beam bending up to 4348N, the deflection of only 6.79mm at a load of 445N, the impact performance up to 17.1J, and the roller peeling strength up to 66.4N-mm/mm, and at the same time, meeting the strict requirements for flame retardancy, which can realize the localized substitution of the flooring materials for the bulk cargo area of the aircraft.

005 Conclusion

Honeycomb sandwich structure with high strength, high specific stiffness and low density, benefiting from its good compressive and flexural properties, has been used in a large number of applications in the field of civil aircraft. This paper analyzes the research progress and development status of honeycomb sandwich structure, further combs its application on typical aircraft structures, and concludes that with the development and production of domestic aircraft C919/C929, composite sandwich structure for aircraft floors has a wide range of application prospects.(Source: Fiber Composites)