FAA Aviation Mechanic Airframe (AMA)Airframe StructuresHard
A composite structures technician is laying up a repair patch that must resist loads from multiple directions equally. Which ply orientation scheme should the technician use?
- ARandom fiber orientation with no set pattern
- BA quasi-isotropic layup using 0, +45, -45, and 90 degree plies
- CAll plies oriented at 0 degrees only
- DAll plies oriented at 90 degrees only
Show answer & explanationAnswer & explanation
Correct answer: B. A quasi-isotropic layup using 0, +45, -45, and 90 degree plies
A quasi-isotropic layup, alternating plies at 0, +45, -45, and 90 degrees, distributes strength and stiffness evenly in multiple directions, approximating the load-carrying behavior of an isotropic (metal-like) material. This is required when the repair must handle multidirectional loading rather than loads along a single known axis.
Why the other options are wrong
- A. Random orientation is not an engineered layup and would not meet approved repair data requirements.
- C. 0-degree-only plies give strength in one direction only, leaving the patch weak to loads in other directions.
- D. 90-degree-only plies also give strength in only one direction, perpendicular to the 0-degree case.
Quasi-Isotropic Layup
A composite ply stacking sequence (typically 0°, +45°, -45°, 90°) designed to give approximately equal strength and stiffness in all in-plane directions.
- Mimics isotropic behavior of metals
- Common in structural repair patches needing multidirectional strength
- Must follow approved repair data for ply count, order, and orientation
Memory trick: '0-45-Negative45-90, spread strength all around' — quasi-isotropic covers every angle.