22-Mec-B8 Engineering Materials · May 2013
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
Paper format. National Exams, May 2013 — 07-Mec-B8 Engineering Materials. Three hours, open book; any non-communicating calculator permitted. Eight questions, all of equal value; any FIVE constitute a complete paper, so each question is worth 20 marks. Candidates are urged to state any assumptions made. All eight questions are solved below, because the set as a whole is the study resource.
Reference texts (22-Mec-B8 Engineering Materials).
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.
Given. A 100–150 seat jet whose competitive claims are (i) substantially greater range at the same cabin capacity and (ii) market-leading environmental performance, with three candidate families for the semi-monocoque body: advanced carbon fibre-reinforced polymer (CFRP), conventional aircraft-grade aluminium (2xxx/7xxx), and third-generation aluminium–lithium (Al–Li) alloys.
Find. Five selection criteria genuinely decisive for a pressurised fuselage in this size class, a reasoned evaluation of each family against them, and a defended recommendation.
Range at fixed cabin capacity is bought almost entirely with structural mass: every kilogram removed from the body is a kilogram of fuel that can be carried instead, and the Breguet range equation rewards a lower empty weight fraction twice over, once through the mass ratio and once through the reduced induced drag of a lighter aircraft. The environmental claim is the same physics seen from the other end — fuel burn per seat-kilometre — with a life-cycle tail concerning manufacture and disposal. The selection criteria below are therefore ordered by how directly they act on those two claims, and only then by cost and producibility, which decide whether the programme is viable at all.
Criterion 1 — specific stiffness and specific strength. A semi-monocoque body is a stiffened thin-walled tube: the skin carries shear and pressure hoop load, the stringers and frames carry bending and stabilise the skin against buckling. The governing material indices are therefore E/ρ and σy/ρ for the stiffened panel, not raw strength. Criterion 2 — damage tolerance, fatigue and crack growth. A pressurised fuselage is a cyclically loaded pressure vessel; certification demands demonstrated slow crack growth and residual strength with a two-bay crack. A short-haul 100–150 seat aircraft flies far more pressurisation cycles per year than a long-haul widebody, so cyclic life dominates. Criterion 3 — corrosion and in-service durability. Galvanic attack at fastened joints, exfoliation in high-strength aluminium, and moisture ingress into honeycomb all drive inspection intervals and therefore direct operating cost. Criterion 4 — manufacturability and production rate. A narrow-body programme lives or dies on rate: hundreds of aircraft per year, each demanding repeatable, inspectable joints. Criterion 5 — life-cycle environmental footprint. Embodied energy of the raw material, energy consumed in the autoclave or the heat-treat line, in-service fuel burn, and end-of-life recyclability.
| Criterion | CFRP (advanced composite) | Aircraft-grade Al (2024/7075) | 3rd-generation Al–Li |
|---|---|---|---|
| Specific stiffness / strength | Best. Laminate can be tailored ply-by-ply to the load path; 15–20 % body mass saving is realistic | Baseline. Isotropic, well characterised, but every kilogram is carried whether the load path needs it or not | Strong. Each 1 wt % Li lowers density about 3 % and raises modulus about 6 %; roughly 8–12 % lighter than 2024 at equal capability |
| Damage tolerance / fatigue | Excellent in fatigue, but poor in through-thickness impact: barely visible impact damage causes delamination that must be assumed present at design allowables | Best understood. Ductile, visible cracking, mature crack-growth data and repair practice | Good; modern 3rd-generation grades have corrected the anisotropy and low short-transverse toughness that plagued the 2nd generation |
| Corrosion / durability | Immune to electrochemical attack, but drives galvanic corrosion of any aluminium it touches; needs isolation plies at fittings | Weakest. Requires cladding, anodising, sealants and a lifetime of inspection | Better than 2024/7075; still a metal, still needs protection |
| Manufacturability / rate | Weakest at this size. Autoclave cycle time, capital cost and inspection burden scale badly to hundreds of small barrels per year | Best. Rolling, stretch forming, machining and riveting are fast, cheap and certified | Nearly as good as conventional aluminium — the same mills, forms and joining methods, at a raw-material premium |
| Life-cycle footprint | Lowest fuel burn, but high embodied energy and no established recycling route — structure is largely landfilled or pyrolysed | Highest fuel burn of the three; fully recyclable with a mature closed-loop scrap market | Best balance: most of the fuel-burn benefit, and the aluminium recycling stream still applies |
Recommendation. For the body of a 100–150 seat aircraft I recommend a third-generation aluminium–lithium semi-monocoque fuselage, with CFRP reserved for the wing, empennage and control surfaces. The reasoning follows the criteria in order of leverage. Criterion 1 is nearly a draw once Al–Li is on the table: the composite barrel wins on paper, but a large part of that advantage is given back at the many cut-outs, floor attachments and system penetrations that a narrow-body fuselage is full of, where the laminate must be locally thickened and metallic fittings reintroduced. Criterion 2 favours the metal outright for this mission — a short-sector aircraft accumulates pressurisation cycles roughly three times as fast as a long-haul type, and metallic damage tolerance is visible, inspectable and repairable at line stations. Criterion 4 is decisive: at narrow-body production rates the autoclave is the bottleneck, and a rate-limited programme cannot meet its delivery commitments however light the aeroplane is. Criteria 3 and 5 split, and Al–Li takes the middle ground on both.
This is not a theoretical position. It is the choice Bombardier made for the CSeries (now the Airbus A220), an aircraft in exactly this seat class that pairs an aluminium–lithium fuselage with a resin-transfer-moulded composite wing, and it delivers the range and fuel-burn improvements the question describes. The wing is where the composite genuinely pays: it is a high-aspect-ratio bending structure with few cut-outs, so the laminate can be run continuously along the load path and the aeroelastic tailoring is worth real drag.
Check: the recommendation assumes a short-to-medium-haul mission (high cycles, moderate sector length) and a production rate typical of the narrow-body market. If the same 100–150 seat cabin were instead specified for very long thin routes — few cycles, very long sectors, modest rate — the weight leverage on range grows and the cycle-life and rate penalties shrink, and a full CFRP barrel becomes the better answer.
