EASA Module 6: Materials and Hardware Encyclopedic Study Guide

Properties of Metals and Materials

A fusible material refers to the ability of two dissimilar metals to melt together. In the context of steel manufacturing, the electric arc process provides the advantage of producing steel where no gas fumes or impurities are generated. Ferrous metals are defined by their iron content, and all ferrous metals are magnetic. Medium carbon steels typically contain carbon in a range of $0.3\,\% \times 0.5\,\%$. At normal temperatures, high carbon (HC) steel is harder due to the specific percentage of carbon present in the granules.

Titanium alloys are characterized by a high strength-to-weight ratio, while nickel chromium is utilized in many exhaust systems due to its high corrosion resistance and low expansion coefficient. Pure aluminium is highly resistant to corrosion on its own; however, it is often alloyed for strength. When aluminium is exposed to air, it oxidizes and becomes electrically insulated.

Metallurgy and Heat Treatment

Normalizing of steel is performed to remove residual stresses resulting from the manufacturing process. During normalizing, the material is left to cool at room temperature. Annealing is a process that relieves internal stress suffered after engineering processes and allows the material to withstand greater stress per unit area. Tempering steel involves heating the metal under the Upper Critical Point (UCP) followed by slow cooling, which provides relief of internal stress after hardening and reduces brittleness. The formation of steel is dependent upon the formation of pearlite into austenite.

Aluminium alloys are used in aircraft because they are stronger than pure aluminium. Clad aluminium alloys (Alclad) are preferred because they are significantly less subject to corrosion than uncoated alloys. Alclad features a pure aluminium coating that is typically $5\,\%$ of the alloy thickness. Non-heat-treatable aluminium alloys cannot be hardened by heat treatment but can be hardened through strain hardening or cold working. In identified alloys like $2024-TH6$, the first digit ($2$) indicates the primary alloying agent.

Solution treatment of aluminium alloys can be carried out a maximum of $3$ times (the original manufacturer treatment plus $2$ subsequent times). Following solution treatment, the metal may be placed into service after $5\,\text{days}$ to allow for natural age hardening. Precipitation treatment (artificial aging) makes the metal harder and stronger but less ductile. For heat treatment quenching, the time between removal from the furnace and quenching must not exceed $10\,\text{seconds}$. If aluminium alloy is not quenched correctly, it will be subject to corrosion. Heat treatment on aluminium is identified by a combination of letter and number codes.

Aircraft Hardware: Bolts, Screws, and Studs

An American National (AN) steel bolt is identified by an "$\times$" on the head, while a corrosion-resistant steel (CRS) bolt is identified by a raised dash. British standard bolts use specific marking systems: a BSF (British Standard Fine) bolt with a raised disk on the head face indicates a close tolerance bolt, whereas a lowered ring under the hexagon indicates the head marking for a close tolerance BSF bolt. A British close tolerance bolt may also be identified by a raised ring on the head.

Bolt sizing and identification vary by system. A (British) $\frac{3}{8}\,\text{inch}$ diameter UNF (Unified National Fine) bolt uses the diameter code letter "$J$". Capped nuts are used to prevent leaks from pipe ends, and nyloc (non-metallic) locknuts must never be used in environments exceeding $120\,^\circ\text{C}$. Studs with a larger thread at one end are called stepped studs. A unified thread is identified by $2$ or $3$ rings on the head.

Nominal lengths are defined differently by component. For a UNF bolt, the nominal length is the distance from under the head to the end of the threads. For a screw, the nominal length refers to the overall length of a countersunk screw (including the head) or the non-threaded portion of a bolt. Multi-start threads are designed to increase the lead of the screw without increasing the pitch.

Rivets and Fastening Systems

Rivets are designed primarily to resist shear loads. Common alloy rivets, such as those made from $2017$ and $2024$, must be used immediately after heat treatment or refrigerated to retard age hardening. D and DD rivets are refrigerated to retard age hardening indefinitely until used. When a solid rivet is formed, it initially holds $75\,\%$ of its shear strength; the remaining $25\,\%$ is gained as the assembly age hardens at room temperature for four hours.

American rivets use head markings for identification: a cross on the head indicates $2117$ aluminium alloy, and a rivet with $5\,\%$ magnesium is identified as anodic green with an "$\times$". British rivets use color coding: a violet anodic finish identifies certain British aluminium alloy rivets, while those made from $L36$ (aluminium) are natural anodic (green anodic). The coding on a British rivet typically indicates the head type and the material.

Specialized fasteners include Rivnuts, which were originally used for securing rubber deicing boots to the airframe. The "HUCK" rivet is similar in design to a cherry rivet. On a structural repair where the backside is inaccessible, blind rivets are used. A Jo-bolt is a blind fastener described as having an external sleeve and being internally threaded. Dzus fasteners are used as quick-release fasteners for inspection panels. When calculating rivet allowance, the formula used is $1.5 \times d + \text{thickness of material}$. The minimum space between rivets is determined by the diameter of the rivets ($d$).

Control Cables and Transmission Systems

Control cables are preferred because they are strong and light. Cable sizing, such as a $9 \times 16$ cable, refers to $9$ cables each containing $16$ strands. British aircraft cables are classified by their breaking load in hundredweights. Cable tension is maintained by a cable tension regulator. Turnbuckles are used to adjust minor tension on the cable. A turnbuckle is considered in safety if the witness hole (inspection hole) is covered or the amount of threads showing is within Maintenance Manual limits. On British turnbuckles, safety is checked by attempting to pass a hardened pin probe through the inspection hole.

Chains must be checked for elongation by laying them flat on a table, applying a tensile load, and measuring against the original length. They are stored oiled and coiled in greaseproof paper. If a single segment of a long chain is defective, the entire chain must be replaced. Stiff links are checked by running the chain over a finger at a $180\,^\circ$ angle.

Bearings and Gears

Bearings are selected based on the load type. Needle bearings resist radial loads only. Taper roller bearings are used to transmit thrust loads in conjunction with radial loads. Spherical roller bearings can resist large radial loads and moderate thrust. Self-aligning bearings are precision bearings, and journal bearings typically involve an interference fit for the stationary race. In high-temperature environments, Group $4$ bearings are utilized. Bearings should be inspected when dismantled from an assembly. A watermark on a bearing indicates intergranular corrosion, while a damaged shield requires the rejection of the bearing.

Gear systems include epicyclic gears, which are used on shafts rotating on the same axis, often found in propeller reduction gearboxes. Spiral gears offer less stress concentration compared to spur gears. A worm drive creates a drive in two planes but only one direction. Correct gear meshing is identified by the pattern (the mark it leaves on a mating gear) and the lash. The large diameter on a bevel gear is called the "heel," and the small diameter is called the "toe."

Corrosion and Surface Protection

Corrosion is the destruction of metal by electrochemical action. Galvanic corrosion occurs when two dissimilar metals are joined; in this reaction, the more noble metal allows the less noble metal to corrode first. The anode is the electrode that loses electrons and suffers the corrosion. Intergranular (or intercrystalline) corrosion attacks the grain boundaries and is often caused by improper heat treatment. Exfoliation corrosion, also known as layer corrosion, appears as the lifting or flaking of metal at the surface due to delamination of grain boundaries.

Fretting corrosion occurs when there is slight movement or vibration between two bolted parts. On steel, it presents as rust, while on aluminium alloy, it appears as a black powder or "cocoa" staining. Stress corrosion is associated with a corrosion pit in a member under continuous tensile stress. Spilled mercury on aluminium causes rapid and severe corrosion upon prolonged contact.

Protective treatments include:

• Anodizing: An electrolytic process that forms a protective oxide film on aluminium alloys, providing a surface for paint to adhere to.

• Chromating: An anti-corrosive treatment applied specifically to magnesium alloys, often resulting in a yellowish surface.

• Cadmium Plating: A common anti-corrosion process for iron (Fe) aircraft parts. These rivets should not be used in temperatures exceeding $250\,^\circ\text{C}$.

• Phosphating: Immersing steel in a solution of phosphoric acid and metal phosphates.

• Alumina: A ceramic oxide of aluminium.

Composites and Adhesives

Composite structures like honeycomb utilize hexagonal cores. Slight waviness in a composite structure may be tolerable if the part is lightly loaded. Bubbles are removed from a wet composite lay-up using a roller. For honeycomb repair, the damaged area is removed using a router, and the largest hole repairable with microballoons is $2.54\,\text{cm}$. Microballoons are used as a filler because they are the lightest material available.

Resin systems involve a pot life that decreases as temperature increases. It is dangerous to mix the accelerator and catalyst of a three-part polyester resin directly together; they must be mixed into the resin separately. Using excessive hardener in polyester resin leads to reduced pot life and a more brittle repair. Stepped curing involves more than two dwell periods at specific temperatures. In an autoclave, a vacuum bag is used to apply pressure, and the vacuum connection must overlap the breather mat.

Thermosetting adhesives give off heat during their curing cycle. The adhesive process involves two parts: wetting and setting. Adhesive joints are tested using peel and shear tests.

Electrical and Fluid Hardware

Electrical cables are primarily made of copper due to its low resistance to current flow. Tersil cable conductors use nickel-plated copper. The temperature limits for cable coatings are: tin-coated copper ($105\,^\circ\text{C}$), silver-plated copper ($200\,^\circ\text{C}$), and nickel-coated copper ($200\,\text{to}\,250\,^\circ\text{C}$). Coaxial cables utilize an electrostatic field around the conductor to reduce electromagnetic fields and interference. Ribbon cables prevent interference by grounding alternate wires. An "$\times$" in a wiring code indicates the control system. Fireproof electrical cables must withstand $1100\,^\circ\text{C}$ for $15\,\text{minutes}$.

Flexible hydraulic hoses are identified by their internal bore size, date of manufacture, and manufacturers' name. Storage life for a flexible hose is up to $5\,\text{years}$. Skydrol-resistant seals are made of Butyl rubber, Ethylene Propylene, or Teflon. When installing a flareless coupling, the fitting should be turned hand tight plus $2\,\text{flats}$. Hydraulic hoses are pressure tested at $1.5 \times \text{working pressure}$. Rigid pipes on gas turbine engines are typically made of stainless steel.

Measurement and Testing

Hardness of steel depends on the iron-austenite grain structure. The Vickers and Brinell hardness tests require a contact time of $15\,\text{seconds}$ between the indentor and the test piece. The Brinell test specifically measures the diameter of the indentation. The Izod impact test measures the toughness of a material, with a striking energy of approximately $163\,\text{J}$.

Sheet metal should be stored on its edge in racks to prevent scratching. When drilling stainless steel, the drill bit should have an included angle of $140\,^\circ$. For carbon fiber reinforced plastic, a tungsten drill is recommended. When using a hacksaw, the teeth of the new blade must face away from the user. For precision marking of steel, copper sulphate or engineers blue is used.