Composite Aerospace Structures Questions and Answers

Answer: C

Correct answer: 4 times

A quasi-isotropic layup uses 0, plus 45, minus 45 and 90 degrees in equal proportion, so in 16 plies each angle appears \( 16/4 = 4 \) times. The plies are then stacked symmetrically about the midplane to eliminate the B matrix, and the plus and minus 45 plies are paired to keep the laminate balanced.

Answer: D

Correct answer: 40 per cent

The reduction is expressed as a fraction of the undamaged strength: \[ \frac{450 - 270}{450} = \frac{180}{450} = 0.40 = 40\ \text{per cent.} \] A knock-down of this order from damage that may be invisible on the surface is exactly why compression after impact is the sizing case for so many composite panels.

Answer: A

Multiplying, \( 0.80 \times 500 = 400 \) MPa. The knock-down reflects moisture absorbed over years of service combined with the highest temperature the structure will see, and it applies chiefly to matrix-dominated properties.

Answer: B

The change is essentially \( (0.62-0.55) \times (230 - 3.5) = 0.07 \times 226.5 = 15.9 \) GPa. Control of fibre volume fraction therefore matters a great deal, which is one of the arguments for prepreg over wet lay-up.

Answer: C

Correct answer: Symmetric and balanced

Reading from one face: four zeros, eight nineties, four zeros. The sequence is its own mirror image about the midplane, so it is symmetric and the B matrix vanishes. It is also balanced, since every off-axis ply is matched by one at the opposite angle - and 0 and 90 degree plies are their own opposites, so a cross-ply laminate is balanced automatically.

Answer: D

Correct answer: 6 MPa

The bonded area is the overlap length times the width: \[ \tau = \frac{30\,000}{50 \times 100} = \frac{30\,000}{5000} = 6\ \mathrm{MPa}. \] This is an average only. The real distribution peaks sharply at both ends of the overlap and falls almost to nothing in the middle, so the peak stress may be several times this figure - which is why lengthening an overlap gives much less than proportional benefit.

Answer: A

Comparing mass per unit area, \( 2.0 \times 1.56 = 3.12 \) against \( 3.0 \times 2.70 = 8.10 \), a saving of \( 1 - 3.12/8.10 = 0.615 \). Real structures achieve rather less, since joints, lightning protection and damage allowables all erode the theoretical gain.

Answer: B

Balance requires each off-axis ply to be paired with one at the opposite angle, which cancels the extension-shear coupling. Symmetry is a separate requirement concerning position through the thickness rather than angle.

Answer: C

Numbers fall and complexity rises as the pyramid is climbed, so that confidence built cheaply at coupon level supports the few expensive tests at the top. Composite certification relies on it far more heavily than metallic certification does.

Answer: D

Correct answer: 4.8 kg

\[ 400 \times 0.012 = 4.8\ \text{kg.} \] It is a small figure in absolute terms but not a negligible one: the same absorbed water that adds this mass softens the matrix and lowers every matrix-dominated property, which is why design allowables are quoted for the hot and wet condition rather than for the as-manufactured state.