1. Introduction: choice of materials in aircraft design

The current trends in commercial aircraft operations are showing an increasing demand for lower operational and maintenance costs. The maintenance costs, directly incurred by the airlines' operation, are an important measure of the economic benefits associated with reducing direct operating cost [1]. Practically, most aircraft structures are being designed for longer design lifetime with extension of inspection intervals. For this purpose, the fatigue and

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and eproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

damage tolerance (F&DT) properties have been received considerable attention for further use of lightweight materials on next generation aircrafts [2]. Therefore, there is a strong need for the application of more durable and damage tolerant materials to minimize the total maintenance costs of commercial aircraft. Reducing the structural weight can lead to better fuel efficiency, reduced CO2 emissions and lower maintenance costs. Nowadays, two competing materials, such as modern aluminum alloys and composites, have the potential to improve the cost effectiveness, but they still have limitations that restrict their widespread use, for example corrosion-fatigue resistance for aluminum alloys and blunt notch strength and impact resistance for carbon fiber reinforced plastics (CFRP) [3]. The basic characteristics of materials for aircraft structures are given in Table 1.

A chronological history of materials for aircraft structures is illustrated in Figure 1. New multilayered hybrid materials, FMLs consist of thin metal sheets bonded into a laminate with intermediate thin fiber reinforced composite layers, and combines the benefits of both material classes [5]. Recently, the use of FMLs leads to subsequent benefits for primary aircraft structures, for example upper fuselage skin panels as shown in Figure 2 [6–8]. This figure also presents typical load cases for dimensioning criteria in the design of fuselage structures. To date, the representative commercially available FML is glass reinforced aluminum laminate (GLARE), which combines thin aluminum sheets with unidirectional glass fiber reinforced epoxy layers [9, 10]. It has been produced for the upper fuselage skin panels of Airbus A380 (Toulouse, France) at GKN Aerospace's Fokker Technologies (Papendrecht, The Netherlands) in collaboration with AkzoNobel (Amsterdam, The Netherlands) and Alcoa (New York, US) [3, 4, 11]. The FMLs are also being considered for thin-walled structures for single aisle fuselage shells. In addition, their superior F&DT properties which are addressed as essential design principles in JAR/FAR 25.571 (Damage-tolerance and fatigue evaluation of structure) make them the ideal candidate for military aircrafts that such applications are not only subject to high fatigue stresses, but also high-velocity impact damages (e.g. battle damages) [12]. Concurrently, other types of commercially available FMLs are aramid aluminum laminate (ARALL) based on aramid fibers and carbon reinforced aluminum laminate (CARALL) based on carbon fibers, respectively [11].


The first generation FML, the ARALL, was introduced at 1978 in Faculty of Aerospace Engineering at TU Delft (Delft University of Technology, The Netherlands) [14]. The ARALL consists of alternating thin aluminum alloy layers (0.2–0.4 mm) and uniaxial or biaxial aramid fibers. The GLARE which is the second generation of FML presents the excellent fatigue resistance with high blunt notch strength than either 2024-T3 or ARALL. This new hybrid material also offers the actual weight reduction when it is applied to the fuselage skin panels [15, 16]. Finally, a much stiffer FML which is made by carbon fiber instead of aramid and glass fibers, the CARALL, had been also investigated in TU Delft [17]. The use of high modulus of carbon fiber (in typical, ranging from 230 to 294 GPa) exhibits more efficient crack bridging at the preliminary stage of fatigue crack propagation within composite layers [18]. However, the residual strength of notched CARLL is significantly lower than the monolithic aluminum alloys due to the limited failure strain of carbon fiber (in typical, 2.0%) [11]. Furthermore, it is more susceptible to galvanic corrosion when aluminum alloys are electrically connected to

The Guidelines of Material Design and Process Control on Hybrid Fiber Metal Laminate for Aircraft Structures

http://dx.doi.org/10.5772/intechopen.78217

71

, 27 panels (reproduced

Figure 1. Chronological history of materials for aircraft structures (reproduced from Fontain [3]).

Figure 2. GLARE application on Airbus A380 fuselage section-13/18: Total GLARE area is 469 m<sup>2</sup>

from Beumler [4]) and typical load cases on GLARE sections (reproduced from Assler and Telgkamp [13]).

carbon fiber reinforced composites [19–21].

Table 1. Strength and weakness of materials for aircraft structures [4].

The Guidelines of Material Design and Process Control on Hybrid Fiber Metal Laminate for Aircraft Structures http://dx.doi.org/10.5772/intechopen.78217 71

Figure 1. Chronological history of materials for aircraft structures (reproduced from Fontain [3]).

damage tolerance (F&DT) properties have been received considerable attention for further use of lightweight materials on next generation aircrafts [2]. Therefore, there is a strong need for the application of more durable and damage tolerant materials to minimize the total maintenance costs of commercial aircraft. Reducing the structural weight can lead to better fuel efficiency, reduced CO2 emissions and lower maintenance costs. Nowadays, two competing materials, such as modern aluminum alloys and composites, have the potential to improve the cost effectiveness, but they still have limitations that restrict their widespread use, for example corrosion-fatigue resistance for aluminum alloys and blunt notch strength and impact resistance for carbon fiber reinforced plastics (CFRP) [3]. The basic characteristics of materials for

A chronological history of materials for aircraft structures is illustrated in Figure 1. New multilayered hybrid materials, FMLs consist of thin metal sheets bonded into a laminate with intermediate thin fiber reinforced composite layers, and combines the benefits of both material classes [5]. Recently, the use of FMLs leads to subsequent benefits for primary aircraft structures, for example upper fuselage skin panels as shown in Figure 2 [6–8]. This figure also presents typical load cases for dimensioning criteria in the design of fuselage structures. To date, the representative commercially available FML is glass reinforced aluminum laminate (GLARE), which combines thin aluminum sheets with unidirectional glass fiber reinforced epoxy layers [9, 10]. It has been produced for the upper fuselage skin panels of Airbus A380 (Toulouse, France) at GKN Aerospace's Fokker Technologies (Papendrecht, The Netherlands) in collaboration with AkzoNobel (Amsterdam, The Netherlands) and Alcoa (New York, US) [3, 4, 11]. The FMLs are also being considered for thin-walled structures for single aisle fuselage shells. In addition, their superior F&DT properties which are addressed as essential design principles in JAR/FAR 25.571 (Damage-tolerance and fatigue evaluation of structure) make them the ideal candidate for military aircrafts that such applications are not only subject to high fatigue stresses, but also high-velocity impact damages (e.g. battle damages) [12]. Concurrently, other types of commercially available FMLs are aramid aluminum laminate (ARALL) based on aramid fibers and carbon reinforced aluminum laminate (CARALL) based on carbon fibers, respectively [11].

> • Fatigue behaviors • Low density (1.54 g/cm<sup>3</sup>

• Poor impact behaviors • No plasticity • Reparability • Recycling

• No corrosion • Best suited for smart structures

)

• Improved fatigue • Better tailoring • Higher fire resistance • Less corrosion

• Lower stiffness

• Higher density (2.52 g/cm<sup>3</sup>

• Less industrialized process (compared to CFRP)

)

Materials Aluminum alloys Composites (CFRP) FML

)

Table 1. Strength and weakness of materials for aircraft structures [4].

aircraft structures are given in Table 1.

70 Optimum Composite Structures

Strength • Broad experience • Repairability • Static behaviors • Improvement potential

Weakness • High density (2.78 g/cm<sup>3</sup>

• Fatigue behaviors • Corrosion behaviors • High costs of new alloys

Figure 2. GLARE application on Airbus A380 fuselage section-13/18: Total GLARE area is 469 m<sup>2</sup> , 27 panels (reproduced from Beumler [4]) and typical load cases on GLARE sections (reproduced from Assler and Telgkamp [13]).

The first generation FML, the ARALL, was introduced at 1978 in Faculty of Aerospace Engineering at TU Delft (Delft University of Technology, The Netherlands) [14]. The ARALL consists of alternating thin aluminum alloy layers (0.2–0.4 mm) and uniaxial or biaxial aramid fibers. The GLARE which is the second generation of FML presents the excellent fatigue resistance with high blunt notch strength than either 2024-T3 or ARALL. This new hybrid material also offers the actual weight reduction when it is applied to the fuselage skin panels [15, 16]. Finally, a much stiffer FML which is made by carbon fiber instead of aramid and glass fibers, the CARALL, had been also investigated in TU Delft [17]. The use of high modulus of carbon fiber (in typical, ranging from 230 to 294 GPa) exhibits more efficient crack bridging at the preliminary stage of fatigue crack propagation within composite layers [18]. However, the residual strength of notched CARLL is significantly lower than the monolithic aluminum alloys due to the limited failure strain of carbon fiber (in typical, 2.0%) [11]. Furthermore, it is more susceptible to galvanic corrosion when aluminum alloys are electrically connected to carbon fiber reinforced composites [19–21].
