The rst airplanes were constructed mainly from wood and later from metal alloys (Federal Aviation Administration, 2012b; Konieczny, 2013). In the mid-20th century, signi cant ad-vancement in aircraft construction technology occurred with the application of composite materials. Composites have become competitive alternatives to traditional metallic (alu-minium, steel and titanium) materials due to their numerous advantages. A combination of two or more dierent materials, based on the de nition of composites, enables the creation of a superior and unique material. The most commonly adapted composites in the aircraft industry, among the Polymer Matrix Composites (PMCs), are based on Fibre Reinforced Polymers (FRPs). They have found a wide application in military, general aviation as well as in commercial aircraft. They have also been used in the production of helicopter rotor blades, sailplanes and gliders. This is due to the signi cant weight savings, greater strength and stiness, aerodynamic smoothness, and resistance to corrosion and fatigue that FRPs ensure over traditional metallic structures. Moreover, a possibility of the shape forming is much more advanced than in the metal processing technologies. Lightness is of key importance in the aircraft industry since reducing structural weight of an aircraft results in reduced fuel consumption and increased payload capacity (Taylor, 2008).
2.1.1 Fibre Reinforced Polymer (FRP) composites
An FRP is a composite made of a polymeric matrix reinforced with bres. Plies of the matrix are layered together to form a composite laminate. Generally, bres are responsi-ble for transferring loads, whereas a binder material is responsiresponsi-ble for holding the bres in a desired location and orientation, and for lling voids between them (Taylor, 2008).
Structural properties of a composite laminate, such as stiness and strength, depend on the stacking sequence of the plies, i.e. the distribution of ply orientations through the com-posite thickness. Proper selection of ply orientation is necessary to provide a structurally e cient design. Orientations of the plies at 0◦, ±45◦, and 90◦ ensure reaction to axial,
2.1. Composite materials and their application in the aircraft industry 6
shear, and side loads, respectively. Because a composite material is strong and sti only in the direction of the bres, successive plies of the matrix are layered together at various angles to achieve the required strength characteristics. This enables the transfer of loads in dierent directions and the use of such a composite as a structural material. There are two basic arrangements of bre reinforcement: unidirectional and bidirectional, where the former provides the strength and stiness only in one direction and the latter ensures strength in two directions (not necessarily of the same strength) typically oriented at 90◦ to each other (Federal Aviation Administration, 2012a). An example of unidirectional ply orientation is a pre-impregnated (prepreg) tape (Fig. 2.1 on the left side), which is an FRP reinforcement that is pre-impregnated with a thermoplastic resin. An example of bidirectional ply orientation is a plain weave fabric (Fig. 2.1 on the right side), which oers more exibility for a lay-up of complex shapes when compared to straight unidi-rectional tapes. Prepregs are the standard in the aerospace industry (Federal Aviation Administration, 2012a). It should be noted that there are also other architectures of bre reinforcement which can be multidirectional or even three-dimensional (Mallick, 2007).
Fig. 2.1: A scheme of basic prepreg products (Federal Aviation Administration, 2012a)
Many aerospace composite structures are made of quasi-isotropic materials, which sim-ulate the properties of an isotropic material, ensuring the same properties in all directions.
The plies of the quasi-isotropic lay-up are stacked in the following sequence: 0◦, 90◦, +45◦, and -45◦ (Fig. 2.2), or 0◦, -60◦, and +60◦ (Federal Aviation Administration, 2012a).
2.1.2 Manufacturing of aircraft structural elements
There are numerous methods of manufacturing of composite structures such as hand lam-inating (or wet lay-up), autoclave processing, lament winding, pultrusion, resin transfer
2.1. Composite materials and their application in the aircraft industry 7
Fig. 2.2: An example of quasi-isotropic material lay-up (0/90/±45)s (Federal Aviation Administration, 2012a)
moulding, etc. (Mallick, 2007; Hoa, 2009; Masuelli, 2013; Divya et al., 2016). In the case of manufacturing of high quality polymer composite aerospace/aircraft structural compo-nents, autoclaves (heated pressure vessels) have become irreplaceable tools. Autoclaves are now being used to produce very large aircraft components such as wings and fuselages.
Autoclaves can process a wide variety of materials with varying contours and complex shapes (Upadhya et al., 2011). Manufacturing with use of autoclaves is a very common process in the aerospace industry because it aords precise control over the moulding process due to a long and slow curing cycle. This precise control creates the exact lam-inate geometric forms needed to ensure strength and safety in the aerospace industry (Masuelli, 2013). The typical vacuum bagging scheme adopted in the autoclave moulding technique for a composite component is shown in Fig. 2.3.
Fig. 2.3: Typical autoclave moulding bagging scheme (Aero Consultants AG, n.a.)
2.1. Composite materials and their application in the aircraft industry 8
The consecutive steps of the manufacturing process are as follow. Individual sheets of a prepreg material (previously stored in a fridge) are laid-up and placed in an open mould.
It has to be mentioned that detailed QA/QC (quality assurance and quality control) of the date of validity of the material for the use has to be veri ed. Moreover, those materials have to be certi ed in accordance with relevant standards used in the aerospace industry.
Before this ply, a release agent can be inserted as an option, which enables more resin to remain on the laminate surface. Then, the laminate is covered with a peel ply, a release
lm, a breather/bleeder material and a vacuum bagging lm (Masuelli, 2013). The peel ply provides an easy release barrier between the laminate surface and the breather and bleeder. The breather and bleeder traps and holds the excess resin from the laminate. The release lms and the breather/bleeder can be held in place by use of pressure sensitive tapes (Aero Consultants AG, n.a.). The bagging lm is an air-tight seal placed over the sealant tape and it applies vacuum pressure over the entire laminate. A vacuum is pulled on the component and the entire mould is placed into an autoclave. A vacuum valve connects the bagging lm to a vacuum tubing. The component is cured with an elevated temperature and a continuous vacuum to extract entrapped gasses from the laminate (Masuelli, 2013) and to create chemically irreversible bonds between the resin and the
bre plies (Kjelgaard, 2012).
2.1.3 Polymer composites in the aircraft industry
In aviation, a composite aircraft usually contains one or a combination of the following components (Taylor, 2008):
carbon bre/epoxy resin (CFRP) used as a primary structural and skin material,
aramid (Kevlar®) bre/epoxy resin mostly used in military applications, in pri-mary structures and armour plating,
glass bre used as a structural and skin material (on general aviation aircraft),
glass bre/phenolic resin (GFRP Glass Fibre Reinforced Polymer) used in inte-rior ttings, furnishings and structures,
boron bre/epoxy resin used in composite repair patches, or on older composite structures,
honeycomb (e.g. DuPontTM Nomex®) core/face sheets made of aluminium, carbon
bre, glass bre, or Kevlar® used in oor boards, interior walls, storage bins, wing spoilers, fairings, ailerons, aps, engine nacelles, and rudders (Federal Aviation Administration, 2012a; DuPont, n.a.).