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Part-66 Module 6 study materials
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Part-66 Module 6 practice questions

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Question 1Aircraft Materials - Ferrous

In plain carbon steel, increasing the carbon content up to about 0.8% generally results in:

  1. Lower hardness but noticeably better weldability
  2. Improved ductility and impact toughness together with lower tensile strength
  3. Higher hardness and tensile strength, lower ductility Correct answer.
  4. No significant change in the mechanical properties

Explanation

Carbon is the principal hardening element in plain carbon steel; as the carbon content rises towards the eutectoid composition of about 0.8%, hardness and tensile strength increase. The same rise in carbon restricts slip, so ductility and impact toughness fall. Above roughly 0.8% the steel becomes progressively more brittle without a useful gain in strength.

Question 2Aircraft Materials - Ferrous

When eutectoid plain carbon steel (about 0.8% carbon) is cooled slowly from the austenitic range, the resulting structure consists mainly of:

  1. Pearlite Correct answer.
  2. Austenite and cementite
  3. Ferrite only
  4. Martensite

Explanation

Slow cooling allows carbon to diffuse, so the austenite transforms into pearlite, an alternating lamellar mixture of ferrite and cementite. Ferrite alone would require a very low carbon content, and martensite only forms on rapid quenching. Austenite is not retained at room temperature after slow cooling.

Question 3Aircraft Materials - Ferrous

Cementite, or iron carbide (Fe3C), in steel is best described as:

  1. Having the same properties as ferrite
  2. A solid solution of carbon in gamma iron
  3. Soft and highly ductile
  4. Hard and brittle Correct answer.

Explanation

Cementite is an intermetallic compound of iron and carbon that is very hard and very brittle. It contributes wear resistance but, in large amounts or coarse form, reduces toughness. A solid solution of carbon in gamma iron is austenite, not cementite.

Question 4Aircraft Materials - Ferrous

After a steel component has been hardened by quenching, tempering is carried out mainly to:

  1. Soften the component completely so that it can be machined
  2. Increase the carbon content of the surface layer
  3. Refine the grain size before the hardening operation
  4. Relieve internal stresses and reduce brittleness Correct answer.

Explanation

As-quenched martensite is very hard but brittle and contains high internal stresses. Tempering, a controlled reheating below the critical range, allows some carbon to precipitate and relieves those stresses while retaining useful hardness. The amount of tempering determines the final balance of hardness and toughness.

Question 5Aircraft Materials - Ferrous

Martensite forms when austenite is quenched rapidly because:

  1. The steel recrystallises directly into fine pearlite colonies
  2. Carbon is trapped in a supersaturated body-centred tetragonal lattice Correct answer.
  3. Carbon diffuses out of solution to form coarse carbide plates at the grain boundaries
  4. Ferrite and cementite separate into alternate lamellae

Explanation

Rapid cooling suppresses the diffusion of carbon, so the face-centred cubic austenite transforms by a shear mechanism into a body-centred tetragonal structure supersaturated with carbon. This distorted lattice is what makes martensite hard and brittle. Any process in which carbon can diffuse would give pearlite or other softer products instead.

Question 6Aircraft Materials - Ferrous

Case carburising is normally applied to:

  1. Low carbon steel in order to give a hard wear-resistant surface Correct answer.
  2. Austenitic stainless steel in order to improve corrosion resistance
  3. High carbon tool steel in order to toughen the core
  4. Grey cast iron in order to reduce machining costs

Explanation

Carburising diffuses carbon into the surface of a low carbon steel, which is then quenched so that the enriched surface becomes hard martensite while the unaffected core stays tough. A high carbon steel would not need extra carbon and would be too brittle throughout. The process is a surface treatment and does not change corrosion behaviour appreciably.

Question 7Aircraft Materials - Ferrous

Compared with carburising, the nitriding process:

  1. Can only be applied to high carbon steels
  2. Produces a deeper case in a much shorter time
  3. Uses a lower temperature and requires no quenching Correct answer.
  4. Requires a rapid water quench immediately after treatment

Explanation

Nitriding introduces nitrogen into the surface, usually from ammonia gas, at around 500 °C, which is below the critical range. Because no austenite is formed, no quench is needed and distortion is very small. The case produced is thinner but extremely hard and wear resistant.

Question 8Aircraft Materials - Ferrous

Which statement about austenitic stainless steels is correct?

  1. They are non-magnetic and not hardened by heat treatment Correct answer.
  2. They are strongly magnetic and can be hardened by quenching
  3. They are hardened mainly by precipitation of carbides
  4. They contain less than 10.5% chromium

Explanation

Austenitic stainless steels, of the 18/8 family, keep a face-centred cubic structure at room temperature and are therefore essentially non-magnetic. Because no martensite can form on quenching, they cannot be hardened by heat treatment and are strengthened only by cold working. Their chromium content is well above the 10.5% minimum needed for stainless behaviour.

Question 9Aircraft Materials - Ferrous

The designation 18/8 stainless steel indicates a content of:

  1. 18% nickel and 8% chromium
  2. 18% chromium and 8% nickel Correct answer.
  3. 18% chromium and 8% carbon
  4. 18% carbon and 8% chromium

Explanation

In the traditional 18/8 notation the first figure is the chromium percentage and the second is the nickel percentage. Chromium provides the passive surface film and nickel stabilises the austenitic structure. Such a steel therefore contains about 18% chromium and 8% nickel with a carbon content of only a few hundredths of a percent.

Question 10Aircraft Materials - Ferrous

Martensitic stainless steels can be hardened by heat treatment because they:

  1. Are fully austenitic at room temperature
  2. Contain nickel as the main alloying element
  3. Depend on nitrogen for their strength
  4. Have a higher carbon content together with 12-18% chromium Correct answer.

Explanation

Martensitic grades contain enough carbon, typically 0.1% to about 1%, for austenite to transform to martensite on quenching, while 12-18% chromium gives moderate corrosion resistance. They are magnetic and are used where hardness and wear resistance matter more than corrosion resistance. Nickel-rich grades, by contrast, stay austenitic and cannot be hardened this way.

Question 11Aircraft Materials - Ferrous

An austenitic stainless steel that has been held in the 450-850 °C range may later suffer intergranular corrosion because:

  1. The passive oxide layer becomes thicker and then cracks
  2. The structure transforms locally into martensite at the grain boundaries
  3. Nickel diffuses to the surface and oxidises rapidly
  4. Chromium carbides precipitate at the grain boundaries Correct answer.

Explanation

In this temperature range carbon diffuses to the grain boundaries and precipitates as chromium carbides, which removes chromium from the adjacent metal. The depleted zones then fall below the level needed to maintain a passive film and corrode preferentially. This effect is called sensitisation and is avoided by using low carbon or stabilised grades.

Question 12Aircraft Materials - Ferrous

In grey cast iron, the carbon is mainly present as:

  1. Spheroidal nodules of graphite
  2. Graphite flakes Correct answer.
  3. Carbon dissolved in solid solution in ferrite
  4. Combined carbon in the form of cementite

Explanation

Slow cooling of grey cast iron allows the carbon to separate as graphite flakes, which give the characteristic grey fracture and provide good damping and machinability. White cast iron, by contrast, retains its carbon combined as cementite because it cools too quickly for graphite to form. Nodular graphite is found in spheroidal graphite iron, not in ordinary grey iron.

Question 13Aircraft Materials - Ferrous

Malleable cast iron is produced by:

  1. Quenching grey cast iron from above its critical temperature
  2. Adding magnesium to the molten iron just before casting
  3. Heat treating white cast iron so that the cementite decomposes Correct answer.
  4. Rolling and forging grey cast iron at high temperature

Explanation

Malleable iron is made by casting white iron, in which all the carbon is combined as cementite, and then giving it a long annealing treatment. This breaks the cementite down into temper carbon, giving a ductile material that can be bent without fracturing. Adding magnesium instead produces spheroidal graphite iron, a different material.

Question 14Aircraft Materials - Ferrous

Which property is typical of wrought iron?

  1. It is used mainly for casting gearbox housings
  2. Very high hardness combined with poor ductility
  3. A carbon content of about 2%
  4. A low carbon content with fibrous slag inclusions Correct answer.

Explanation

Wrought iron is a very low carbon ferrous material containing characteristic threads of slag rolled into the metal. These inclusions give it good ductility, toughness and resistance to corrosion, and it can be forged and welded readily. It is not a casting material and does not contain the high carbon levels found in cast iron.

Question 15Aircraft Materials - Ferrous

In the AISI/SAE designation 4130, the figures '41' indicate:

  1. A chromium-molybdenum steel Correct answer.
  2. A carbon content of 0.41%
  3. A chromium content of 4.1%
  4. A nickel-chromium steel

Explanation

In the four-digit system the first two digits identify the alloy type, with 41 denoting a chromium-molybdenum (chrome-moly) steel. The last two digits give the carbon content in hundredths of a percent, so 4130 contains about 0.30% carbon. Such steels are widely used for welded tubular structures such as aircraft engine mounts.

Question 16Aircraft Materials - Ferrous

Normalising of steel consists of:

  1. Heating above the critical range and cooling in still air Correct answer.
  2. Cold working followed by a low temperature soak
  3. Heating below the critical range and quenching in oil
  4. Heating above the critical range and cooling slowly in the furnace

Explanation

Normalising heats the steel above its upper critical temperature to form austenite and then cools it in still air. The moderate cooling rate gives a finer, more uniform grain structure than annealing, with slightly higher strength. Furnace cooling is annealing and produces a softer structure.

Question 17Aircraft Materials - Ferrous

The main purpose of fully annealing a steel component is to:

  1. Introduce compressive stresses at the surface
  2. Produce a hard martensitic structure throughout
  3. Increase its surface hardness for wear resistance
  4. Soften it and relieve internal stresses Correct answer.

Explanation

Full annealing heats the steel above the critical range and cools it very slowly, usually in the furnace, producing a soft, stress-free structure with good machinability. The slow cooling gives coarse pearlite rather than martensite. Hardening treatments require rapid cooling instead.

Question 18Aircraft Materials - Ferrous

Spheroidising heat treatment is used mainly to:

  1. Increase the corrosion resistance of the surface
  2. Improve the machinability of high carbon steels Correct answer.
  3. Give the steel maximum hardness for cutting tools
  4. Produce a very fine grain size before forging

Explanation

Spheroidising holds the steel just below the critical range for a long time so that the lamellar carbide turns into small rounded globules. This globular structure is soft and machines much more readily, particularly in high carbon and tool steels. It is a softening treatment, not a hardening one.

Question 19Aircraft Materials - Ferrous

The Brinell hardness test uses:

  1. A diamond cone under a 150 kg load
  2. A hardened ball under a heavy load Correct answer.
  3. A swinging pendulum dropped onto the surface
  4. A sharp pyramidal diamond under a light load

Explanation

The Brinell test presses a hardened steel or tungsten carbide ball of standard diameter into the surface under a heavy load, typically 500 to 3000 kg. The diameter of the resulting impression is measured and converted to a hardness number. A diamond cone is used in the Rockwell C test and a pyramid in the Vickers test.

Question 20Aircraft Materials - Ferrous

Decarburisation of the surface of a steel part during heat treatment will:

  1. Reduce surface hardness and fatigue strength Correct answer.
  2. Improve the corrosion resistance of the part
  3. Have no measurable effect on the mechanical properties
  4. Increase surface hardness and wear resistance

Explanation

Decarburisation occurs when carbon is lost from the surface by reaction with the surrounding atmosphere at high temperature. The depleted surface layer is softer and weaker, and it lowers fatigue resistance because cracks initiate easily there. It is controlled by using a protective atmosphere or by removing the affected layer afterwards.

Question 21Aircraft Materials - Ferrous

Stainless steel resists corrosion because chromium:

  1. Prevents any oxygen from reaching the surface
  2. Makes the steel fully austenitic at all temperatures
  3. Forms a thin self-repairing passive oxide film Correct answer.
  4. Raises the carbon content of the surface layer

Explanation

Chromium in excess of about 10.5% allows a very thin, adherent chromium-rich oxide film to form on the surface. This passive film is stable and reforms quickly if damaged, isolating the metal from the environment. It is the film, not the bulk metal, that provides the protection.

Question 22Aircraft Materials - Ferrous

Manganese is added to steel mainly to:

  1. Give the steel a non-magnetic structure
  2. Improve hardenability and act as a deoxidiser Correct answer.
  3. Raise the resistance to seawater corrosion
  4. Increase the carbon content of the melt

Explanation

Manganese combines with sulphur to prevent hot shortness and acts as a deoxidiser during melting, and in amounts around 1-2% it also improves hardenability and strength. It is present in almost all steels for these reasons. It does not raise the carbon content or provide corrosion resistance.

Question 23Aircraft Materials - Non-Ferrous

In an aluminium alloy designation, what does the temper letter 'T6' indicate?

  1. Annealed to its softest possible condition
  2. As fabricated, with no further treatment
  3. Solution heat treated, then artificially aged Correct answer.
  4. Strain hardened, then partially annealed

Explanation

The 'T' prefix denotes a temper produced by thermal treatment, and the digit 6 specifically means the metal was solution heat treated and then artificially aged, sometimes after a small amount of cold work. This is the temper normally used for high-strength alloys such as 7075 and 2014. Other prefixes cover entirely different conditions: O for annealed, H for strain hardened and F for as fabricated.

Question 24Aircraft Materials - Non-Ferrous

What is the purpose of the thin pure aluminium surface layer on Alclad sheet?

  1. To protect the core by sacrificial action Correct answer.
  2. To raise the tensile strength of the sheet
  3. To lower the weight of the finished panel
  4. To allow the sheet to be welded more easily

Explanation

The cladding is anodic to the alloy core, so it corrodes preferentially and protects the core from attack, including at cut edges where it is smeared over the exposed core. It also provides a good key for paint. It contributes no strength and slightly increases weight.

Question 25Aircraft Materials - Non-Ferrous

Which element is the principal alloying addition in aluminium alloy 2024?

  1. Zinc
  2. Silicon
  3. Copper Correct answer.
  4. Magnesium

Explanation

2024 is an aluminium-copper alloy, containing roughly 4.4 per cent copper together with smaller amounts of magnesium and manganese, and it is heat treatable to high strength. Its copper content also makes it less corrosion resistant than pure aluminium, which is why it is often supplied clad.

Question 26Aircraft Materials - Non-Ferrous

Which property of magnesium alloys demands particular care during repair and overhaul?

  1. They are strongly magnetic at high temperature
  2. They have a very high melting point
  3. They are highly resistant to all common chemicals
  4. They are flammable and burn with intense heat Correct answer.

Explanation

Magnesium in thin sections, and especially as machining swarf or dust, ignites readily and burns with a very hot, bright flame. Water and carbon dioxide react with burning magnesium, so special dry powders are used to extinguish it. Repair areas are also kept free of contamination from other metals to limit galvanic corrosion.

Question 27Aircraft Materials - Non-Ferrous

The very high strength of aluminium alloy 7075-T6 derives mainly from the presence of:

  1. Zinc Correct answer.
  2. Copper
  3. Manganese
  4. Titanium

Explanation

7075 belongs to the Al-Zn-Mg-Cu family, in which zinc is the dominant addition and provides the greatest response to precipitation hardening. It is one of the strongest aluminium alloys used in aircraft structures but has poor corrosion resistance unless protected.

Question 28Aircraft Materials - Non-Ferrous

Solution heat treatment of a wrought aluminium alloy such as 2024 is normally carried out at approximately:

  1. 650 °C
  2. 350 °C
  3. 500 °C Correct answer.
  4. 250 °C

Explanation

The alloying elements must be taken into solid solution, which for 2024 requires roughly 495 to 505 °C, close to but safely below the temperature at which the grain-boundary eutectic would melt. Temperature control is critical because overheating causes incipient melting and irreparable damage. Quenching follows immediately after soaking.

Question 29Aircraft Materials - Non-Ferrous

Which of these aluminium alloys normally needs artificial ageing, rather than natural ageing, to reach full strength?

  1. 7075 aluminium-zinc-magnesium-copper alloy Correct answer.
  2. 1050A commercially pure aluminium
  3. 3003 aluminium-manganese alloy
  4. 5052 aluminium-magnesium alloy

Explanation

7075 does not harden sufficiently at room temperature and must be given a controlled furnace ageing treatment at elevated temperature to precipitate the strengthening phases. Alloys such as 2024 harden largely by natural ageing at room temperature over several days. Alloys of the 1xxx, 3xxx and 5xxx series are not heat treatable at all and gain strength only by cold work.

Question 30Aircraft Materials - Non-Ferrous

The oxide film produced on aluminium by sulphuric acid anodising is best described as:

  1. Metallic and soluble in water
  2. Electrically conductive and soft
  3. Porous and an electrical insulator Correct answer.
  4. Magnetic and used for crack detection

Explanation

Anodising grows a hard oxide layer that is porous at the surface, so it can be dyed and then sealed, and it provides an excellent paint base. Because it is a ceramic oxide it is an electrical insulator, which is why anodised parts need special treatment where electrical bonding is required.

Question 31Aircraft Materials - Non-Ferrous

Rivets marked 'AD' (2117-T4) can be driven straight from the stores because:

  1. They must be solution treated just before driving
  2. They must be heated to a red heat before driving
  3. They must be refrigerated until they are driven
  4. They need no further heat treatment before driving Correct answer.

Explanation

2117 is a low-copper alloy supplied already solution treated and naturally aged, soft enough to be upset cold yet strong enough in service, so no further treatment is needed. This makes it the standard 'field rivet' for general structural repair. Rivets that must be kept in the quenched condition are stored in a refrigerator instead.

Question 32Aircraft Materials - Non-Ferrous

Titanium alloys are extensively used in aero-engine compressors because they:

  1. Combine high strength with good heat resistance Correct answer.
  2. Are the cheapest of the structural metals
  3. Have the highest thermal conductivity of any metal
  4. Can be easily soft soldered to aluminium

Explanation

Titanium offers an excellent strength-to-weight ratio and retains useful strength at temperatures where aluminium would soften, together with good resistance to corrosion and oxidation. That combination makes it ideal for compressor blades and discs and for hot airframe areas near engines. It is, however, expensive and difficult to machine.

Question 33Aircraft Materials - Non-Ferrous

Monel is an alloy of:

  1. Nickel and copper Correct answer.
  2. Copper and tin
  3. Aluminium and silicon
  4. Copper and zinc

Explanation

Monel contains roughly two-thirds nickel with the balance mainly copper, giving very good resistance to corrosion and to moderately high temperatures. It is used for firewalls, exhaust components and marine fittings. It is considerably stronger and harder than the copper alloys it is often compared with.

Question 34Aircraft Materials - Non-Ferrous

Brass is an alloy of copper with:

  1. Zinc Correct answer.
  2. Tin
  3. Aluminium
  4. Nickel

Explanation

Brass is copper alloyed principally with zinc; varying the zinc content changes colour, strength and ductility. It is used for fittings, bushes and pipework where good corrosion resistance and easy machining are needed. Adding further elements such as aluminium or tin produces the special brasses.

Question 35Aircraft Materials - Non-Ferrous

Bronze is traditionally an alloy of copper with:

  1. Lead
  2. Magnesium
  3. Tin Correct answer.
  4. Zinc

Explanation

The classic bronze is copper alloyed with tin, producing a hard bearing material with good corrosion resistance. Modern bronzes may use aluminium, silicon or beryllium instead of tin to obtain particular properties such as higher strength or non-sparking behaviour.

Question 36Aircraft Materials - Non-Ferrous

Compared with aluminium alloys, magnesium alloys have:

  1. An identical density but better weldability
  2. A higher melting point and better ductility
  3. A higher density but lower strength
  4. A lower density but poorer corrosion resistance Correct answer.

Explanation

Magnesium has a density of about 1.74 g/cm³ against 2.7 g/cm³ for aluminium, making it the lightest structural metal in common aircraft use. Its drawback is chemical activity: it corrodes readily and needs thorough surface protection and careful isolation from other metals.

Question 37Aircraft Materials - Non-Ferrous

Intergranular corrosion in a 2024 component is most likely to be caused by:

  1. An anodic film that is thinner than specified
  2. Mechanical polishing of the external surface
  3. Long exposure to ultraviolet light in service
  4. Improper heat treatment at the grain boundaries Correct answer.

Explanation

Incorrect solution treatment or quenching can leave a continuous network of anodic precipitate along the grain boundaries, which then corrodes preferentially and eats the metal away along narrow paths. The correct heat treatment keeps the precipitate finely distributed within the grains instead. Cladding and good surface protection further reduce the risk.

Question 38Aircraft Materials - Non-Ferrous

Exfoliation corrosion of aluminium alloy plate is characterised by:

  1. Small pits of uniform depth over the whole surface
  2. A white powder deposit on the upper surfaces only
  3. Cracking that follows the direction of applied stress
  4. Flaking of the metal in layers parallel to the surface Correct answer.

Explanation

Exfoliation follows the elongated grain structure of rolled or extruded material, so the corrosion products lift successive layers and the metal appears to flake or swell. It is a severe form of intergranular attack and causes a significant loss of section thickness. It is often found at edges, fastener holes and faying surfaces.

Question 39Aircraft Materials - Non-Ferrous

The cladding on Alclad sheet consists of:

  1. A low-alloy or pure aluminium layer bonded to the core Correct answer.
  2. An electroplated cadmium layer applied after rolling
  3. A paint film applied at the rolling mill
  4. A sprayed zinc coating applied after forming

Explanation

Alclad is produced by roll-bonding a thin layer of commercially pure aluminium or a low-alloy aluminium onto one or both faces of the alloy core during manufacture. The layer is metallurgically bonded, not applied later, and it protects the core galvanically. Care is needed in service so that deep scratches do not expose the core.

Question 40Aircraft Materials - Non-Ferrous

Why must a solution-treated aluminium alloy be quenched rapidly?

  1. To increase the grain size of the material
  2. To keep the alloying elements in supersaturated solution Correct answer.
  3. To allow coarse particles of alloying elements to form
  4. To relieve the internal stresses from rolling

Explanation

Rapid cooling suppresses the precipitation of alloying elements, so they remain trapped in a supersaturated solid solution at room temperature. That unstable condition is what allows the subsequent ageing treatment to produce fine, uniformly distributed precipitates and hence high strength. Slow cooling would instead produce coarse particles and a weaker, corrosion-prone structure.

Question 41Aircraft Materials - Non-Ferrous

Compared with conventional aluminium alloys, aluminium-lithium alloys offer:

  1. Better electrical conductivity and lower cost
  2. Lower density and higher stiffness Correct answer.
  3. Improved corrosion resistance and easier welding
  4. Higher density and improved fatigue strength

Explanation

Lithium is the lightest metallic element, so each per cent added reduces the density of aluminium appreciably while also raising the elastic modulus. This makes Al-Li alloys attractive for weight-critical structure. Their disadvantages include higher cost and greater sensitivity to damage and to certain joining processes.

Question 42Aircraft Materials - Non-Ferrous

Which non-ferrous metal is most widely used for aircraft electrical cables?

  1. Copper Correct answer.
  2. Titanium
  3. Magnesium
  4. Aluminium

Explanation

Copper combines low electrical resistance with good ductility and reliable crimped terminations, which is why it is the standard conductor material. Aluminium is lighter but has higher resistance and poorer termination behaviour and is used mainly for large power feeders where weight saving justifies it.

Question 43Aircraft Materials - Non-Ferrous

Nickel-based superalloys such as the Nimonic series are selected for turbine blades because they:

  1. Retain strength at high temperature and resist creep Correct answer.
  2. Have a very high coefficient of thermal expansion
  3. Have the lowest density of the heat-resistant metals
  4. Are easily formed cold into complex shapes

Explanation

Nickel superalloys keep useful strength and creep resistance at temperatures where aluminium and even steel would fail, which is essential in the turbine section. They are usually vacuum cast or machined because they are difficult to form. Their relatively low, controlled thermal expansion is a further advantage in close-tolerance assemblies.

Question 44Aircraft Materials - Non-Ferrous

In the alloy Ti-6Al-4V, the role of vanadium is to:

  1. Act only as a grain refiner in the melt
  2. Improve machinability by forming graphite
  3. Stabilise the beta phase and aid heat treatment Correct answer.
  4. Raise the density to that of steel

Explanation

Aluminium is an alpha-phase stabiliser while vanadium stabilises the beta phase, so the two together give a balanced alpha-beta alloy that responds well to heat treatment and has good strength. Ti-6Al-4V is the most widely used titanium alloy in airframes and engines. Its density remains about 4.4 g/cm³, well below that of steel.

Question 45Aircraft Materials - Composite and Non-Metallic

Which statement best defines a structural composite material as used in aircraft construction?

  1. A metal alloy containing more than one element
  2. Two or more distinct materials combined with a recognisable interface Correct answer.
  3. A single material with a homogeneous internal structure
  4. A plastic that has been given a thin metallic coating

Explanation

A composite consists of two or more distinct constituents, normally a reinforcement such as fibres and a matrix such as resin, which remain separately identifiable at their interface. The combination produces properties that neither constituent could provide alone.

Question 46Aircraft Materials - Composite and Non-Metallic

Why can a cured thermosetting resin not be softened and re-formed by reheating?

  1. Its polymer chains are permanently cross-linked Correct answer.
  2. Its fibres prevent any movement of the resin
  3. It contains no polymer chains at all
  4. Its chains are held only by weak secondary forces

Explanation

Thermosetting resins cure by a chemical reaction that builds a three-dimensional network of covalent cross-links between the polymer chains. Once formed, this network cannot be broken by heat, so the material chars or degrades rather than softening.

Question 47Aircraft Materials - Composite and Non-Metallic

Which property is characteristic of a thermoplastic matrix composite?

  1. It needs a hardener added before use
  2. It cures by an irreversible chemical reaction
  3. It softens on heating and hardens again on cooling Correct answer.
  4. It cannot be re-formed after shaping

Explanation

Thermoplastic polymers are not cross-linked, so raising the temperature allows the chains to move and the material to soften; cooling restores rigidity. The process can be repeated, which allows re-forming and welding.

Question 48Aircraft Materials - Composite and Non-Metallic

Which type of glass fibre is the general-purpose reinforcement most widely used in aircraft composite laminates?

  1. E-glass, used for general structural and electrical-grade laminates Correct answer.
  2. S-glass, used where cost is secondary to strength
  3. C-glass, used mainly for chemical resistance
  4. A-glass, used where alkali resistance is required

Explanation

E-glass is the standard general-purpose grade, combining adequate strength and stiffness with moderate cost and good electrical insulation properties. The other grades are selected for specific characteristics and are produced in much smaller quantities.

Question 49Aircraft Materials - Composite and Non-Metallic

What property does aramid fibre reinforcement give to a composite?

  1. High stiffness combined with high density
  2. Very high compressive strength
  3. High tensile strength with good impact resistance Correct answer.
  4. Very high electrical conductivity

Explanation

Aramid fibres have high specific tensile strength and excellent energy absorption, which is why they are used for impact-prone and ballistic-resistant parts. They are notably weak in compression and are electrically insulating.

Question 50Aircraft Materials - Composite and Non-Metallic

Why must carbon fibre composite be electrically isolated from aluminium structure?

  1. Aluminium expands more than carbon fibre when heated
  2. Carbon fibre melts at a lower temperature than aluminium
  3. Carbon and aluminium form a galvanic cell when wet Correct answer.
  4. Carbon fibre releases corrosive acids when wetted

Explanation

Carbon is cathodic relative to aluminium, so when moisture provides an electrolyte path the aluminium becomes the anode and corrodes rapidly. Isolation with sealant, glass cloth or protective coatings prevents the couple from forming.