Question 1 of 8: Aluminium alloy versus triaxially braided composite for a fan casing

Question 1: Aluminium alloy versus triaxially braided composite for a fan casing (20 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

The duty has to be stated before the two candidates can be compared, because it is the duty and not the datasheet that decides this selection. An inlet fan casing is a large-diameter, thin-walled annular structure that reacts fan blade aerodynamic and gyroscopic loads, carries the acoustic liner, forms the outer flowpath wall, and — the requirement that governs everything else — must contain a released fan blade. Certification in Canada follows Transport Canada Civil Aviation airworthiness standards for aircraft engines, harmonised with FAR 33.94, which demands a full-scale blade-off rig test in which the casing retains the liberated blade and the debris that follows it. The casing is also struck by birds, ice shed from the spinner, hail and runway debris, and it is wetted by jet fuel, hydraulic and de-icing fluids. Its thermal environment, however, is benign: inlet air at the fan face runs from about −55 ℃ at altitude to roughly 60–120 ℃ on a hot day at take-off power, nowhere near the 250 ℃ capability quoted for the composite.

(a) Comparison against the seven criteria.

Strength. A 7050-T7451 or 2219 aerospace aluminium offers roughly 450–500 MPa yield and 70 GPa modulus at a density of 2.80 g/cm³, isotropically and in every direction equally. The triaxial braid, at a fibre volume fraction near 55 %, gives 700–900 MPa in the axial tow direction and an in-plane modulus of 50–60 GPa at 1.58 g/cm³. On specific strength the composite wins by a factor of about three and on specific stiffness by about a third, which is the entire commercial motivation. The qualification is that the composite is strong only where the tows run: through-thickness and interlaminar strengths are an order of magnitude lower, so joints, cut-outs and flange bending are designed against matrix-dominated allowables, not fibre ones.

Impact resistance. The two materials contain a blade by completely different physics. Aluminium absorbs energy by gross plastic stretching: the casing bulges, thins and petals, and the work done is the area under the true stress–strain curve times the volume deformed, so a high strain-to-failure alloy in the T7 temper is preferred over a stronger but less ductile T6. Damage is visible, and the residual strength of a dented case can be judged by inspection. The braided composite absorbs energy by progressive fibre failure, tow pull-out, matrix cracking, delamination and friction over a large area; the triaxial architecture is important here, because the interlocked bias tows resist the delamination that would otherwise unzip a tape laminate. Braided cases are routinely wrapped with an additional aramid belt for the residual energy. The composite’s weakness is low-energy impact: a dropped tool or hail strike can produce barely visible impact damage that halves compressive strength, so the design must be damage-tolerant to a detectable damage threshold and the case must be inspected ultrasonically rather than visually.

Manufacturing methods. The aluminium casing is machined from a rolled ring forging or spun from plate, with integral flanges machined in and stiffeners either machined or riveted on; heat treatment, stress relief and anodising follow. The process is mature, the tooling cost modest, the scrap rate predictable, and buy-to-fly ratios of five or more are accepted. The composite casing is braided as a dry preform over a mandrel on a triaxial braiding machine — a continuous, automated, highly repeatable textile operation — then infused by resin transfer moulding and cured out of the autoclave. It arrives near net shape with metallic flanges co-bonded or bolted on, so material utilisation is far better, but the mandrel and injection tooling are expensive, cycle development is long, and every part needs full-field non-destructive inspection.

Chemical resistance. Cured epoxy is essentially inert to jet fuel, engine oil, phosphate-ester hydraulic fluid and glycol de-icers, and it does not corrode. Its real vulnerability is water: an epoxy laminate absorbs 1–2 wt % moisture in service, which plasticises the matrix and depresses the glass transition temperature, so matrix-dominated allowables must be taken hot and wet. Aluminium is the opposite case. It resists organic fluids but corrodes, and in this installation it would sit next to carbon-fibre hardware, steel fasteners and a salt-laden airfield atmosphere. Every aluminium-to-carbon interface needs a glass-fibre isolation ply or a sealant barrier, because carbon is strongly cathodic to aluminium and the resulting galvanic couple attacks the aluminium preferentially — exactly the mechanism examined in Question 4.

Heat resistance. The aluminium alloy is limited by over-ageing rather than by melting: sustained service above about 120–150 ℃ coarsens the strengthening precipitates and the alloy loses strength irreversibly. The braided composite is quoted at 250 ℃, which implies a bismaleimide or a high-temperature toughened epoxy with a dry glass transition well above that. Against the fan-face environment both are adequate, so heat resistance does not discriminate between them — unless a local hot spot such as an anti-ice bleed duct or a fire zone raises the requirement, in which case the composite’s margin is the larger. Fire is a separate matter: aluminium does not burn but softens and melts near 500 ℃, while an epoxy laminate chars, delaminates and releases smoke, so a nacelle fire zone imposes additional protection on the composite.

Cost. On raw material and process the aluminium casing is far cheaper, perhaps by a factor of five to ten per kilogram of finished part. The composite carries expensive intermediate-modulus carbon, high-temperature resin, a large steel mandrel, RTM tooling, a long qualification programme and a heavier inspection burden. That comparison inverts once life-cycle cost is counted: on a large turbofan a braided composite fan case saves of the order of 150 kg per engine, and each kilogram removed from an airframe is worth thousands of dollars of fuel over the aircraft’s life, quite apart from the secondary saving in mounts, pylon and structure sized by the engine mass.

Overall suitability. The table below collects the comparison.

(b) Selection. The triaxially braided fibre-reinforced polymer is selected. The reasoning is that the fan casing is a large, mass-dominant structure whose governing requirement — blade containment — is an energy-absorption problem rather than a strength problem, and the braided composite absorbs that energy at roughly half the mass. Its thermal capability of 250 ℃ exceeds the fan-face requirement by a wide margin, so the criterion on which a polymer would ordinarily be rejected in an engine simply does not bite at this station. It cannot corrode, which removes a lifetime maintenance burden from a component washed by rain, salt and de-icing fluid, and the braiding plus RTM route makes the part in one automated operation with far less material waste than machining a ring forging. The mass saved on the casing propagates outward into lighter mounts and pylon, so the life-cycle fuel saving repays the higher acquisition cost. This is the judgement the industry has already made in service: braided carbon fan cases are flying on current large and single-aisle turbofans.

The selection is made subject to four conditions, and an answer that omits them is incomplete. First, the case must be qualified by a full-scale blade-off test, not by analysis alone, because progressive composite failure is not predictable to certification confidence. Second, every aluminium and steel fitting must be galvanically isolated from the carbon by a glass-fibre ply or a sealant barrier. Third, a lightning protection layer — typically an expanded copper foil — is required, since carbon conducts too poorly to carry a strike and too well to be treated as an insulator. Fourth, allowables must be set hot and wet and the structure designed to carry limit load with barely visible impact damage present, with ultrasonic rather than visual inspection at scheduled checks.