Composite Aerospace Structures Questions and Answers

Answer: A

Supplying the fibre pre-impregnated gives close control of the resin content and so of the fibre volume fraction. The trade is a limited out-time at room temperature and a requirement for cold storage, which complicates the supply chain.

Answer: B

The combination of elevated temperature and several atmospheres of pressure produces low void content and consistent thickness. The cost of the vessel and the cycle time are why out-of-autoclave processes are pursued for large parts.

Answer: C

Machine placement gives the consistency and throughput that hand lay-up cannot achieve on a wing skin or a fuselage barrel. It also allows steered fibre paths, though those bring their own wrinkling and gap-and-overlap concerns.

Answer: D

Tapering the repair through the thickness transfers load ply by ply and avoids the bending introduced by an external doubler. It demands accurate machining and careful process control, so it is reserved for structurally significant repairs.

Answer: A

Delamination and disbonds leave little surface evidence, so methods that see into the thickness are needed. Magnetic and eddy current techniques rely on ferromagnetic or conductive behaviour and do not transfer usefully to a laminate.

Answer: B

Correct answer: 139 GPa

Along the fibres the two constituents strain together, so the moduli add in proportion to volume: \[ E_1 = V_fE_f + V_mE_m = 0.6(230) + 0.4(3.5) \] \[ = 138 + 1.4 = 139\ \text{GPa.} \] The matrix term contributes barely one per cent, which is why the longitudinal modulus is called fibre-dominated and why fibre volume fraction is the number that matters most in manufacture.

Answer: C

Correct answer: 8.6 GPa

Across the fibres the two constituents carry the same stress and their strains add, so the compliances add: \[ \frac{1}{E_2} = \frac{V_f}{E_f} + \frac{V_m}{E_m} = \frac{0.6}{230} + \frac{0.4}{3.5} \] \[ = 0.00261 + 0.11429 = 0.1169, \quad E_2 = 8.6\ \text{GPa.} \] The soft matrix dominates the sum, so the transverse modulus is about a sixteenth of the longitudinal - the anisotropy the whole design method exists to exploit.

Answer: D

Correct answer: 1.56 g/cm^3

Density is a simple volume-weighted average, since mass and volume both add: \[ \rho = V_f\rho_f + V_m\rho_m = 0.6(1.8) + 0.4(1.2) \] \[ = 1.08 + 0.48 = 1.56\ \mathrm{g/cm^3}. \] At roughly 1.6 against aluminium’s 2.7, this is where much of the weight saving originates, before any account is taken of the higher stiffness.

Answer: A

Specific stiffness is \( 139/1.56 = 89.1 \) against \( 70/2.70 = 25.9 \), a ratio of about 3.4. That figure, along the fibre direction, is the headline argument for composites, though a real laminate with off-axis plies does far less well.

Answer: B

Correct answer: 2.0 mm

\[ 16 \times 0.125 = 2.0\ \text{mm.} \] The nominal cured ply thickness is a design constant for a given material and process, which is what makes laminate thickness a discrete quantity: it can only be changed a ply at a time. Adding or dropping plies in symmetric pairs is therefore the usual way of tailoring thickness across a part.