Aerospace Composites & Plastics

Composite and Non-Metallic Aircraft Materials

• A composite is a material made from two or more constituent parts. This term applies to a wide range of engineering materials, including metals, brick, concrete, and glass.
• In the aerospace industry, "composite" refers to materials combining fibrous and synthetic resin materials.
• Composites offer excellent strength-to-weight ratios.

Plastics

• Plastics are a generic name for materials based on long-chain molecules (polymers) of carbon.
• Plasticity is an ability to retain a deformation after load is removed.
• Plastics are useful for applications with relatively low stress levels where lightness and low electrical/thermal conductivity are desired.
• Plastics can be cast, extruded, or molded into various shapes, or drawn into filaments for use as fibers.
• Two major groups of plastics are thermoplastics and thermosetting.

Thermoplastics

• Thermoplastic materials are hard but become soft and pliable when heated (thermo-heat).
• They can be reshaped and retain their new shape when cooled.
• Thermoplastics can be reheated multiple times without damage.
• Cellulose acetate and acrylic are transparent thermoplastics used for aircraft windshields and side windows in older aircraft. Cellulose acetate shrinks and discolors over time, and fades to yellowish when exposed to light over time, producing black smoke when burned.
• Acrylic is more rigid than cellulose acetate, more transparent, and mostly colorless. Acrylic also produces white marks when exposed to acetone and burns with a clear flame, producing a pleasant odour.
• Common uses of thermoplastics involve tool handles, electrical goods, flexible tubing, cable insulation, and packaging. Poly-vinyl-chloride (PVC) can be rigid and molded, or flexible piping/cable insulation. Polystyrene is typically used in expanded form, as thermal insulation, buoyancy, and shock-resistant packaging. Acrylic is useful in applications needing light transmission, like windows.

Thermoplastic Uses

• Acetate: widely used in tool handles and electrical objects.
• Poly-ethylene: used for flexible tubing, cable insulation, and packaging.
• Poly-propylene: stronger, harder, and more rigid than polyethylene
• Poly-vinyl-chloride (PVC): commonly used for rigid and flexible applications.
• Polystyrene: in expanded form, used for thermal insulation.
• Acrylic (Perspex/Plexiglass): excellent light transmission and resistance to shattering. Laminated forms offer further shatter resistance. Enhanced properties result from stretching, increasing strength and stiffness. Care should be taken not to scratch acrylic surfaces, and avoid the use of solvents which can cause crazing of the material.

Thermosetting Materials

• Thermosetting materials (thermosets) initially soften but will harden if heated further.
• The hardening process is called "curing" and is achievable through chemical reactions.
• Materials are chosen where there are high temperatures because they can endure temperatures exceeding 250 degrees Celcius before beginning to char. Thermosets are generally stronger and have lower ductility/impact properties than thermoplastics.

Resins

• Natural resins are obtained from trees and other plants. They include clear, translucent, yellow (amber), brown, or solid, or semi-solid materials. Uses for resins include inks, lacquers, linoleum, varnishes, and plastics.
• Resins are usually mixed with additives to assist during the molding process. Further, they can be used to increase finished product properties.
• Resins can be thickened and given body by adding inert fillers, such as microballoons, cotton flock, or aerosil (fumed silica). Additional reinforcing agents, plasticizers, stabilizers, colorants, flame retardants, smoke suppressants, and processing aides, among other additives, can be added to resins.

Polyester Resins

• Polyesters cure by chemical action rather than by solvent evaporation.
• Monostyrrene is added to improve workability of the polyester resin mixture.
• Inhibitors are added to improve shelf life of the polyester and styrene mixture by slowing or delaying curing.
• A catalyst is used to initiate the curing process. The curing time can be expedited by adding an accelerator. The chemical reaction between catalyst and accelerator produces heat accelerating curing of the resin, with thicker materials curing more rapidly than thinner materials.
• Thixotropic agents are added to thicken the resin and prevent running, particularly on vertical surfaces.

Epoxy Resin

• Epoxy resin is another type of resin that can be used as a substitute to polyester resins in laminated structures.
• Epoxy resin has low shrinkage, high strength, and adheres to a wide range of materials.
• Epoxy resins use a hardener or curing agent and require no heat to cure.
• Mixing ratio for polyester resins is 64:1 (resin to catalyst), while the epoxy ratio is 4:1 (resin to hardener).

Elastomers

• Elastomers are derived from the Greek word elastos—elastic.
• Elastomers, both natural and synthetic, are materials with significant elastic properties and may be molded into shapes that they retain, making them a type of plastic.
• BuNa 'N' (or nitrile) is a synthetic rubber polymer made from butadiene and sodium. It has excellent resistance to fuels and oils.
• Other elastomers, such as BuNa 'S', are generally cheaper and offer performance comparable to natural rubber. Lower resistance to fuels and oils makes them unsuitable for seals.
• Other elastomers, such as fluoro-elastomers, have exceptional high-temperature properties.
• Neoprenes have excellent tensile properties, excellent recovery qualities, and good resistance to solvents. They're used in hydraulic seals and gaskets.
• Poly-sulfide rubber (PRC or Thiokol) is widely used on fuel tank linings/seals for its high resistance to fuels and oils.
• Silicone rubber is often used in seals due to its exceptional high and low temperature properties.

Glass Fiber Reinforced Plastic (GFRP)

• Glass fibres can be spun into cloth, then used in fire-proof curtains or as transmission media (transmitting light).
• GFRP offers high tensile strength for the weight, though some damage can reduce it significantly.
• GFRP is stiffer than steel, but not as stiff or strong as aluminium.
• GFRP is used for structural applications and when considerable strength is required. In applications like aircraft structures, it is often combined with unidirectional glass cloth, laid at 90° to each other, resembling plywood construction. GFRP can be provided as pre-preg (pre-impregnated with resin).

Carbon Fiber Reinforced Plastic (CFRP)

• CFRP (graphite) is a composite material developed to enhance strength/weight ratios of GFRP.
• CFRP filaments are typically less than 0.01mm in diameter and are manufactured by subjecting fine threads of a nylon-type plastic to high temperatures that decompose the polymer, leaving only the carbon. The threads are then stretched/heated to develop strength (complex and costly process).
• CFRP is used in a variety of applications where weight savings are an advantage, particularly where stiffness or strength is important. One such application is the manufacture of helicopter rotor blades.

Ceramic Fiber

• Ceramic fibers are made by firing clay or other non-metallic materials—a form of glass fiber suitable for high-temperature applications, typically up to 1650°C.
• Ceramic fibers are heavier and more expensive than other materials and are only used where no other material suitable materials are available.

Laminated Plastics

• Laminated plastics are composed of layers of synthetic resin-impregnated fibers (or other fillers) bonded together by heat and pressure to form a single laminate/sheet.
• Laminates improve the appearance (color, porosity, smoothness), increase strength/rigidity, and enhance other properties such as corrosion resistance, and ease of keeping the surface clean.
• They can enhance the surface by forming various structures. Sandwich structures offer a lightweight and strong composite by using a lightweight interior core material like low density balsa wood (or other materials) sandwiched by resin-reinforced layers. Further, this structure is used as floor and wall panels, or for aircraft skins.

Composite Defects Detection

• Composites, resistant to corrosion and cracking, may exhibit characteristics that require regular inspection for defects.
• Areas like airframes and propellers should be inspected for such defects.
• Inspection methods and structural repair procedures should be available.
• Repairs for unexpected areas or damage need specific repair diagrams from manufacturers.

Causes of Damage

• Sharp objects can cause cracking/shattering in thermosetting plastics, which will travel in a straight line.
• Ground handling (e.g. dropped tools) or equipment damage can also damage composite structures.
• Bird strikes, erosion (rain/hail/dust), overloads (heavy landings/turbulent flight/excessive loading), lightning strikes/static discharge, and chafing on internal fittings can further cause damage.

Types of Damage

• Cracks.
• Delamination (separation of fiberglass layers).
• Debonding (separation of honeycomb core from skin layer).
• Blisters (breakdown in the bond within outer laminations).
• Holes (caused by moisture penetration, poor initial bonding, lightning/static discharge or impacts).

Inspection Methods

• Visual inspection.
• Ring/percussion tests.
• Ultrasonic inspection.
• Radiography.

Discusses composite materials and plastics in aerospace, focusing on specific applications, characteristics like plasticity, major plastic groups, and the advantages/disadvantages of thermoplastics such as cellulose acetate in aircraft components. Also, the benefits of using composite materials in the aerospace industry.