Fiber Structures: A Driver of Innovation in Aerospace

 

Abstract

For structural reinforcement in aerospace applications, the reinforcing fibers need to have very high properties in terms of strength, stiffness, thermal or chemical resistance. While textile structural composites are materials containing rigid fabrics designed for structural or load-bearing applications, specific combinations of flexible fiber materials (fibers, yarns and fabrics) are called textile composite preforms. Textile preforms vary greatly in terms of fiber orientation, entanglement and geometry.

 

The structures and properties of various high-performance fibers: toughness, modulus, thermal resistance, chemical resistance, etc. must be transferred to yarn and fabric structures to produce preforms with the required properties. In the case of mechanical properties, the efficiency of this transfer will largely depend on the degree of orientation of the fibers in the yarn and fabric structures. Over the past decades, the driving force in the field of aerospace composites has been to reduce costs, improve component performance and reduce component weight. The use of textile structural composites in aerospace is increasing due to their excellent specific strength, specific stiffness, and their potential for overall design.

 

Composite structures account for approximately 17%, 25% and 50% of the Airbus A340, A380 and A350 XWB respectively, and are growing rapidly. The in-plane mechanical properties of textile composites prepared by liquid molding (vacuum assisted resin transfer molding VARTM, resin transfer molding (RTM) or resin film infusion (RFI)) are almost equal to those of composites prepared by prepreg. However, the out-of-plane impact resistance of textile structural composites manufactured by liquid molding is lower than that of fabric structural composites developed by prepreg.

 

It is also important to note that the use of glass fiber and aramid fiber in aerospace is relatively limited due to the higher density and lower stiffness of glass fiber and the high hygroscopicity of aramid fiber. Therefore, most composite parts used in the aerospace industry are produced using carbon fiber fabrics pre-impregnated with thermosetting matrices. Woven, knitted, oriented structures (DOS), braided and other fabric structures have been used to develop composite parts for aerospace applications, with the most commonly used fabric structure being DOS.

 

2 Woven Structures

An example of the use of woven structures in the aerospace field was achieved by Kawasaki Heavy Industries, Japan, which used Teijin Tenax HTS 5631 12K fiber to manufacture a 380 g/m2 2×2 twill carbon fiber fabric through a special weaving technology. The prepreg developed using this fabric has been used in a variety of applications, including the Embraer ERJ 170 inboard flap, the ERJ190 outboard flap and wingtip, and the Boeing 737-300 winglet (Figure 1).

 

Figure 1 Boeing 737-300 Winglet

 

3 Knitted Structures

Daimler-Benz develops aerospace composite parts with knitted structures. Figure 2 shows a stiffened panel made of warp-knitted skin and 3D-woven reinforcement sewn together to form a complex preform. The fiber structure is then impregnated by DLR Braunschweig using the RTM process.

 

Figure 2 Reinforced Panel Consisting of a Woven Profile and a Warp Knitted Skin

 

4 Directed Structure

Directed Structure (DOS) using 12K carbon fiber is the most commonly used structure in aerospace applications. There are two main typical application examples: the Airbus A380 rear pressure bulkhead (RPB) and the Airbus A400M cargo door. The A380 RPB is produced at the Airbus plant in Stade near Hamburg using a preform made of multiaxial carbon fiber DOS supplied by Saertex. This preform is covered on the positive mold and then laminated using the RFI process (Figure 3). After the initial curing of the 3 mm thick basic laminate and the connection of the stringers, the part is finally cured in an autoclave. The finished bulkhead weighs about 240 kg and measures 6.2 meters by 5.5 meters.

 

Figure 3 DOS Fabric in the Production of the Rear Pressure Bulkhead of the Airbus A380

 

The cargo door of the Airbus A400M is mainly composed of multiaxial carbon fiber DOS and additional uniaxial DOS fabric for local reinforcement and skinning. The A400M cargo door is processed using the EADS/Premium Aerotec patented vacuum assisted process (VAP) infusion technology (Figure 4). The A400M cabin door is manufactured at the Premium Aerotecs plant in Augsburg.

Figure 4 Airbus A400M cargo door manufactured by Premium Aerotec In addition to the above large structures, DOS has some additional aerospace applications at the substructure level, such as Airbus A380 flap track partitions, side shells and belts.

 

5 Braided Structures

Large braiding machines open up interesting applications of braided structures in the aerospace industry. A&P developed a Vectron sock with a diameter of 2 meters and a length of 3 meters using an 800 carrier braiding machine.

 

Figure 5 shows a braided sock produced by A&P mainly used in a prototype airlock developed by NASA.

Figure 5 Braided Airbrake Developed by NASA

 

6 Future Applications

 

Currently, the application of textile structures in the aerospace industry is mainly based on 2D structures using prepreg technology, because 2D textile reinforcements are stronger in-plane than 3D textile reinforcements. Out-of-plane impact resistance is another important load case consideration in the development of aerospace composite parts. 3D fabric structures with oriented yarns in the thickness direction can give composites very strong out-of-plane properties. Therefore, the use of 3D fabric structures is conducive to avoiding delamination and fracture of aerospace composite parts.

 

However, in parts such as stiffeners and stringers, not all loads are in-plane, which makes prepreg laminates less suitable. It can be foreseen that in the near future, three-dimensional braided, knitted and woven fiber structures will attract great attention in the aerospace industry. (Source: Carbon fiber and its composite material technology)

 

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