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Presentation of an Aeronautical Unidirectional Composite

1.1. Introduction

As it stands, the two main materials in use in aircraft structures are aluminum and a carbon fiber-based material. These two materials make up approximately 70% of the mass of the structure of a typical commercial plane such as the Boeing 787 (Figure 1.1).

Figure 1.1. Material breakdown of the Boeing 787 (according to http://www.boeing.fr). For a color version of this figure, see www.iste.co.uk/bouvet/aeronautical2.zip

The remaining structural materials are glass fiber-based composites, sandwich structures (a honeycomb core covered with two composite sheets), titanium, steel, etc.

Keep in mind that the weight ratio of composite materials is of over 50% of the overall mass for the Boeing 787 or the Airbus A350, but more standard commercial aircrafts such as the Airbus A320 or A380 are primarily composed of aluminum alloy, which makes up over 60% of its mass.

1.2. Carbon/epoxy composite T300/914

We will now look at a carbon/epoxy composite that is widely used in aircraft structures, called T300/914. The T300 portion of the name refers to a carbon fiber produced by Toray® [TOR 16], while 914 is a reference to an epoxy resin produced by Hexcel® [HEX 16]. T300/914 is a first generation carbon/epoxy composite that is 50% (in volume) carbon fiber and 50% epoxy resin. It takes the form of a thin fabric (less than a millimeter thick) that can subsequently be cut and draped to obtain a desired thickness (Figures 1.2 and 1.3).

Figure 1.2. Unidirectional carbon/epoxy laminate

Figure 1.3. Unidirectional and quasi-isotropic laminate

Performing a test along the direction of the fibers, also called the longitudinal direction, we can observe a brittle elastic behavior comparable to the fibers. The elastic limit is obviously lower than that of the fibers, since approximately 50% of the resin has been added, which has a relatively low elastic limit (Figure 1.4). This resin is necessary in order to obtain a less brittle material and to shape it. The carbon fibers are indeed thoroughly interesting elements from the aspect of their mechanical characteristics but cannot be used to adopt a desired geometry. Furthermore, when a crack appears in the material and propagates perpendicularly to the fibers, it will cause a lot of fiber failures and fiber debonding, thus requiring an elevated dissipation of energy; the material will therefore be less brittle (Figure 1.5). In practice, a crack would travel parallel to the fibers if it could, which is why there are plies in other directions, so as to reinforce the material in different orientations of loading (in practice, we can demonstrate that four directions 0°, +45°, -45° and 90° are enough). It is then a case of a composite laminate, as opposed to a composite with fibers facing only one direction, which is called a unidirectional composite (Figure 1.3).

Figure 1.4. Tension along the longitudinal direction of a composite: behavior of fiber, resin and composite. For a color version of this figure, see www.iste.co.uk/bouvet/aeronautical2.zip

Figure 1.5. Tension along the longitudinal direction: damaged area

Comparing Young's modulus and the strength of the main structural materials according to density, we note that composite materials are very well positioned compared to the metals (Figures 1.6 and 1.7). Ceramic materials are also very interesting but often too brittle for any structural use.

Figure 1.6. Young's modulus according to density [ASH 00a] CFRP: carbon fiber reinforced plastic; GFRP: glass fiber reinforced plastic. For a color version of this figure, see www.iste.co.uk/bouvet/aeronautical2.zip

Figure 1.7. Strength according to density [ASH 00a] CFRP: carbon fiber reinforced plastic; GFRP: glass fiber reinforced plastic. For a color version of this figure, see www.iste.co.uk/bouvet/aeronautical2.zip

The way to better understand a composite material is to take a close look at its composition and microstructure, in particular for epoxy resin.

1.3. Polymers

The epoxy matrix is part of the polymer family commonly referred to as plastics. The word plastic comes from the mechanical behavior of polymers that present plastic strains, i.e. the deformation does not return to its original point when loading is released.

Polymers are composed, as indicated by the name, of chains of monomers linked together with covalent bonds. In this particular work, we will limit ourselves to organic polymers. Keep in mind that organic matter is created by living creatures (plants, mushrooms, animals, microorganisms), particularly by their decomposition. In contrast, inorganic or mineral matter is composed of metals, glass, ceramics, rocks, etc.

Organic polymers are therefore based on chains of monomers linked together by carbon atoms. The carbon-carbon covalent bond is strong and will provide elevated mechanical properties. These carbon-carbon bonds serve as a basis for macromolecules that make up the skeleton of the polymer material. On top of these strong bonds, these macromolecules are linked together via the intermediary of weak bonds (hydrogen bonds, Van der Waals bonds, etc.). It is the deformation of these weak bonds which will induce important plastic strain on behalf of the polymers.

Take for instance, the case of polyethylene, one of the simplest and cheapest polymers. It is made up of the polymerization of ethylene monomers (CH2=CH2) which leads to the creation of long chains. These chains are simply linked together via weak bonds. The mechanical characteristics obtained are therefore relatively weak and depend on temperature.

Figure 1.8. Structure of polyethylene

Figure 1.9. Structure of polymer in monomer chains

If we trace, for example, the Young's modulus according to temperature, we then obtain a characteristic three-part curve that demonstrates the existence of two distinct temperatures points within the studied material; its glass transition temperature Tg (g for glass; this glass appellation will be refined further on) and its melting temperature Tm. Below Tg, the behavior of a material is typical of a solid material; beyond Tm, it takes on a practically fluid (more or less viscous) behavior and in between the two, we observe a rubbery behavior characterized by very low rigidity and a high capacity for deformation. Typically, polymers cannot be used as structure materials, in particular as a resin for a composite material, for a temperature beyond Tg (there are, nonetheless, exceptions which we will see further along).

Figure 1.10. Rigidity of a polymer depending on temperature. For a color version of this figure, see www.iste.co.uk/bouvet/aeronautical2.zip

This rubber-like behavior is made possible because the molecular chains present only weak bonds (hydrogen bonds, Van der Waals bonds, etc.) linking them. They can therefore reorient and align when loaded. This type of polymer is called thermoplastic, because its plastic behavior depends on temperature.

If we want to increase the mechanical characteristics of a polymer, we need to lock the relative movement of molecules by creating covalent bonds between molecule chains; this phenomenon is called cross-linking. We can then describe the polymer as thermoset if the level of cross-linking is significant, while it is referred to as thermoplastic in the absence of these cross-links. We note that the term thermoset stems from the fact that the chemical reaction that allows this cross-linking is activated by temperature; in other words, the material hardens with temperature. This is true during manufacturing, but is no longer the case once the polymer is already cross-linked.

Figure 1.11. Secondary links between monomer chains: cross-linking. For a color version of this figure, see www.iste.co.uk/bouvet/aeronautical2.zip

There is a third class of polymers, elastomers, that present an intermediary behavior somewhere between thermoset and thermoplastic. Elastomers generally present a low level of cross-linking and these cross-links are realized from sulfur-based covalent bonds (a process referred to as vulcanization). These sulfur-based secondary bonds grant the material a high level of elasticity (nonlinear in general).

Going back to Young's modulus depending on temperature, we distinguish a thermosetting polymer from a thermoplastic polymer via a less pronounced - if not non-existent - glass transition, and the absence of a melting temperature. The glass transition is a result of the dissociation of weak bonds by thermal agitation. Nonetheless, because thermosetting materials present high levels of cross-linking, this network of cross-links will last beyond Tg and grant the thermosetting material a decent mechanical behavior after Tg. Increasing the temperature further, we reach the material's decomposition pyrolysis, i.e. decomposition into different forms of gas and residue (CO, CO2, H2,...