Aircraft Structures
How the structure carries load
The exam tests whether you can relate load paths, materials and inspection findings. A semi-monocoque fuselage uses skin, stringers, frames and bulkheads: the skin carries shear and hoop tension, stringers carry bending, frames keep the shape and stabilise the skin, and bulkheads collect concentrated loads. Wings are torsion boxes: spars take bending and shear, skins take torsion, ribs maintain aerofoil shape and support the skin. Failures usually start at stress raisers - rivet holes, cutouts, corrosion pits, sharp repairs.
Fatigue, damage tolerance and composites
Fatigue life depends on stress range and cycles, not just peak stress. Damage-tolerant design assumes cracks exist and uses inspection to find them before critical length. Composites carry load in fibres, so damage may be delamination or matrix cracking with little external sign; tap testing and ultrasound are common.
Worked example
A 0.8 mm 2024-T3 skin panel carries 12 kN per 100 mm of width. Area per 100 mm = 0.8 mm × 100 mm = 80 mm². Stress = 12,000 N / 80 mm² = 150 N/mm² (MPa). If the material yield is about 345 MPa, the yield margin is 345/150 − 1 = 1.3. That is a static check only; the joint at a rivet hole may still be fatigue-critical because the local stress is higher.
Traps
Do not call a semi-monocoque structure 'monocoque'; do not assume a visible crack is the only damage; do not compare stress in MPa with a yield in ksi without converting; and do not ignore fretting or galvanic corrosion at fasteners.