This is similar in principle to X-ray testing but uses gamma rays, which have far greater penetration power. Great care must be taken due to the harm they can have on humans.

Also known as the magnetic dust method, this process is used to find cracks in a part. The piece of metal is placed across two magnetic poles, or a magnetic field is induced in it, and it is then sprinkled with a magnetic powder. The excess powder is removed and cracks are revealed by magnetic powder sticking to the area each side of the crack. This occurs because each side of the crack becomes a magnetic pole.

The major limitation of his method is that it will only work with magnetic materials, which makes it useless with aluminium and titanium alloys, which find extensive use in aircraft.

Titanium alloys are used in airframes, landing gear and jet engines because of the combination of properties they offer. The have moderate density, high strength, long fatigue life, toughness and resistance to corrosion and oxidation. They also maintain their mechanical properties up to 500°C. Using titanium alloys to replace some steels and nickel alloys in jet engine construction saves significant weight in the complete jet engine.

Pure titanium is an allotropic material, having an HCP structure (a-Ti) at room temperature and a BCC structure (B-Ti) above 885°C. However, with the addition of alloying elements it is possible to get both structures to exist at room temperature. Pure titanium, a-Ti alloys and ß-Ti alloys are all used in aviation. It is also possible to have an alloy with a mix of a-Ti and ß-Ti and this is called an a-ß-Ti alloy. The titanium alloy using Ti-6A1-4V is an a-ß-Ti alloy and it is one of the most used titanium alloys in the aviation industry. The yield strength of annealed Ti-6A1-4V is approximately 925 MPa.

Nickel-based superalloys, owing to their superior mechanical and chemical properties, find wide applications in the aerospace, marine, nuclear reactor, and chemical industries. But the machinability of nickel-based superalloys is always a matter of concern, while processing them. Wide areas are being explored with a view toward having improved machining performance of nickel-based superalloys. The use of hybrid machining processes, such as thermal-assisted machining, cryogenically enhanced machining, and media-assisted machining, is becoming favoured as they offer enhanced machining conditions and extended cutting tool life during machining of nickel-based superalloys. Future research endeavors might focus on the development of new grades of cutting tools, coating material, and hybridized machining techniques to ensure their stability and thermomechanical properties at high production rates.

https://www.sciencedirect.com/topics/materials-science/nickel-based-superalloys

Like the nickel superalloys, steel's density means it is only used where is strength is essential. While a variety of alloy steels are used in aviation two alloy steels are of particular note. Maraging steels are very low carbon steels (≤0.03%C) with large amounts of nickel (17-19%) with cobalt (8-12%), molybdenum (3-5%) and small amounts of titanium and aluminium. The large amount of nickel means that after austenitising, the steel can be slow cooled to form a martensitic structure, but the martensite is not like that in a quenched plain carbon steel, it is softer with more ductility and toughness. The maraging steel can then be machined and shaped, but it is then aged at around 500°C which results in precipitates forming that strengthen the steel to a great degree; leading to yield strengths up to 2,000 MPa.

Precipitation hardened stainless steels are also used in some structural applications for aircraft where corrosion resistance and strength are required. Stainless steel were used in some early supersonic and hypersonic aircraft to cope with the outer skin heating due to friction, but lighter alloys such as titanium alloys have largely replaced them.

This process takes an unpolymerised preform and compresses it in a mould with heat. The heat and the pressure form the shape and polymerise the polymer. The finished moulding is then ejected. It is used for making plugs, switches and casters.

This is similar to compression moulding but instead of the polymerising happening in the mould, it happens in the adjacent cavity. The molten polymer is transferred via a sprue to the actual mould. It is also used for moulding thermosets.

This is where fibres are placed into a mould and the resin is manually applied onto the fibres. First a gel coat is applied, and then the fibre reinforcement is added in the form of mat or cloth. The resin is poured, brushed or sprayed on, then rollers are used to force the resin to impregnate the fibre mat. In areas where extra thickness sis needed more fibre mat is placed down and the resin rolled into that area again. It is essential that the resin bonds properly to the fibres. An alternative to hand lay-up is the spray-up process where a special gun sprays the resin and continuous strand roving (glass fibre) which is chopped and sprayed at the same time. This chopped roving and resin mix in the mould. It is then rolled to remove air and improve impregnation of the resin into the fibres.

The vacuum bag autoclave process is usually used with epoxy resin based composites. The material used is called prepreg and is uni directional carbon fibres within a partially cured epoxy resin. The material is arranged in years in the mould and then placed in a bag. A vacuum is applied to the bag and this removes any air within the laminates. The bag is then placed in an autoclave at an appropriate temperature and elevated pressure where curing is then completed. For epoxy resins the autoclave will be set to a temperature of around 190°C and at a pressure of around 690 kPa (6.9 bar). This vacuum bag process finds use in forming composite aircraft panels such as wings and other large surfaces

Glass Fibre Reinforced Polymers (GFRP), also known by the trade name Fibreglass, are fine glass fibres embedded in a resin matrix. Polyester, vinyl ester and epoxy resins are all possible resin materials. The glass fibre provides tensile strength while the resin provides toughness. Although not quite as strong as other fibres and also heavier, they are a cost effective option that has also found aeronautical use in Fibre Metal Laminates (FML).

This is one of the most important composites in the aircraft industry. Carbon fibre reinforced composites are a composite of carbon fires embedded in a resin matrix, which in aeronautical applications is normally epoxy resin. The combination of the two components produces the desirable strength and toughness. Typically, carbon fire reinforced polymers consist of 62% by volume carbon fires. The overall properties are lightweight, very high specific strength and high modulus of elasticity. Particularly important is the resistance to cyclic stress that carbon fibre exhibits. It is these properties that make it so desirable for aircraft. Not only are fighters such as the Saab Gripen and Eurofighter Typhoon using CFRP, but CFRP is also used in the Boeing 737 and Airbus control surfaces and wingtips. The Boeing 787 Dreamliner has its fuselage predominately made from CFRP One disadvantage of CFRP is that fails like a brittle material; with no plastic failure first. This contrasts with a metal like steel, which progressively fails and usually shows warnings such as cracks or bending. Damage detection and repair are also much more complex.

Aramid fire reinforced composites are very similar to carbon fibre composites. The difference is that Aramid fibres are used in lieu of the carbon fibres in an epoxy resin matrix. Aramid, better known under the trade name Kevlar, offers similar properties to carbon fibre, but is more resistant to impacts and abrasion. This is important in some battlefield conditions where shrapnel or debris could shorten the life of a carbon fibre component by producing surface defects.

These are laminated structures of metal and fibre reinforced composites., the most common one being GLARE (Glass Laminate Aluminium Reinforced Epoxy) which is used in the Airbus A380. Because they are laminated with metals they perform more like metal sheeting than fibre reinforced composite sheets, but they offer enhanced properties to a non-laminated metal sheet. Advantages include, improved impact resistance, better fatigue and corrosion resistance, and weight advantages. They can also have their strength properties directional for specific locations in the fuselage unlike a standard metal sheet.

A lot of research has been done into metal matrix composites, with an intention to use them in advanced high-speed jets. MMCs are similar to the two above except that the matrix is not a polymer but a metal. An example would be boron fibre aluminium, a composite using an aluminum matrix with boron fibres to reinforce it. This improves the tensile strength of the aluminium alloy by a huge factor. For example aluminium alloy 6061 has a tensile strength of 310 MPa. With a boron/6061 composite with 51% boron, the tensile strength is increased to 1417 MPa. MC's exist in three forms, continuous fibre reinforced MMCs, discontinuous fibre reinforced MMCs and particulate reinforced MMCs. The continuous fibre type gives far greater tensile strength but is also more difficult to make. The discontinuous fibre type is manufactured using powder metallurgy methods (see Chapter 2). MMCs are capable of withstanding higher temperatures, as the metal matrix can withstand elevated temperatures better than the epoxy matrix. They are, however, heavier than polymer matrix composites.

Epoxy adhesives are used to join composite aircraft surfaces to the base structure. Aircraft such as the Eurofighter Typhoon utilise epoxy adhesives in this way.

Corrosion is a great problem in aircraft design. The frames and skin of aircraft are already highly stressed, so weakening via corrosion is a major concern. Although composites offer resistance to electrochemical corrosion, UV light and the weather may degrade them. A large majority of aircraft still use metal airframes and skins, so corrosion is still a problem. Corrosion mechanisms are outlined in Chapter 1 and the student should refer to this section.

This is a concentration cell that occurs because of different oxygen levels at the top and bottom of a crevice. The example in Chapter 1 of a car door is similar to crevice corrosion, but crevice corrosion usually involves a very fine gap that traps a fluid at the base. This type of corrosion is a concern, as metal aircraft are often riveted with fine gaps where the skins join. Moisture forms in these gaps each time the aircraft crosses the "dew point". The dew point is a particular combination of pressure, temperature and moisture content in the air, where water precipitates onto a surface (e.g. misting of a car windscreen). Aircraft always have moisture on them because of: condensation on the airframe from temperature changes in the atmosphere [20°C at sea level, -53°C at 10,671 m (35,000 ft)] airborne moisture accumulating rain

This is a particularly dangerous form of corrosion, where the action of tensile stress and corrosive environment conspire to bring about crack formation. The stresses that cause SCC can be residual stresses or applied. On their own neither would cause failure but with the two-combined failure occurs. Different alloys are effected by different environments; stainless steels are susceptible to SCC in chloride environments but safe in ammonia-based environments, while brasses are the reverse. Some aluminum alloys may suffer SCC in seawater or in the presence of air and water vapour.

In SCC the combined action involves the tensile stress causing pits or cracks to form and then the corrosion occurring with the tip of the crack becoming anodic. It has been shown that applying cathodic protection will stop SC occurring. But once the protection is removed the SC continues. SC can be reduced or prevented though lowering stress levels, removing the corrosive environment if possible, changing alloys to one not effected by the environment present or applying cathodic protection.