Composite Aerospace Structures Questions and Answers

Answer: A

The fibres carry the load along their length while the matrix binds them, transfers load between them and protects them. The consequence is a material far stronger along the fibres than across them, which is the defining feature of composite design.

Answer: B

The matrix is much the weaker constituent, but without it the fibres could not share load or resist compression. Matrix-dominated properties, such as transverse and shear strength, are consequently far lower than the fibre-dominated ones.

Answer: C

Distinct properties along the fibres, transverse to them and through the thickness require far more elastic constants than an isotropic metal. Four independent constants describe a lamina in plane stress against two for a metal.

Answer: D

Along the fibres both constituents strain equally, so their contributions add in proportion to volume. Because the fibre modulus so far exceeds the matrix's, the result is dominated by the fibres and by their volume fraction.

Answer: A

Across the fibres the constituents carry the same stress rather than the same strain, so their compliances add. The result sits close to the matrix value, which is why transverse properties are so much poorer than longitudinal ones.

Answer: B

Symmetry about the mid-plane removes the coupling between in-plane loading and bending, so the laminate does not warp when loaded or when cooled from cure. Almost all practical laminates are made symmetric for this reason.

Answer: C

Pairing each plus-angle ply with a minus-angle one removes the coupling between extension and in-plane shear. A laminate that is both symmetric and balanced behaves in-plane much like an orthotropic sheet, which greatly simplifies design.

Answer: D

Correct answer: The B matrix

Classical laminate theory writes the laminate stiffness as three matrices. A relates in-plane forces to in-plane strains, D relates moments to curvatures, and B couples the two - so a non-zero B means stretching the laminate also bends it. B vanishes for a symmetric stacking sequence, which is why symmetry is imposed on almost every structural laminate.

Answer: A

Layups such as zero, plus and minus forty-five and ninety in equal proportion give in-plane stiffness the same in every direction. It is a convenient starting point, though it wastes much of the advantage of tailoring the layup to the load.

Answer: B

Correct answer: High specific stiffness, tailorable by layup

Two things are being claimed at once. Carbon composite gives more stiffness per unit mass than aluminium along the fibres, and because the designer chooses the ply angles and their proportions, that stiffness can be placed where the loads are rather than being the same in every direction. Repair, inspection and material cost all go the other way, which is the price paid.