FAA Aviation Mechanic Airframe (AMA)Airframe StructuresMedium
A technician is forming a 90-degree bend in a piece of 2024-T3 aluminum sheet using a hand brake. To reduce the risk of cracking along the bend, the bend line should be oriented relative to the sheet's grain direction how?
- AAcross (perpendicular to) the grain direction whenever possible
- BGrain direction has no effect on aluminum sheet forming
- CParallel to the grain direction whenever possible
- DAt exactly 45 degrees to the grain regardless of alloy
Show answer & explanationAnswer & explanation
Correct answer: A. Across (perpendicular to) the grain direction whenever possible
Aluminum sheet has directional grain from the rolling process, and it is stronger and more resistant to cracking when bent across the grain rather than parallel to it. Bending parallel to the grain concentrates stress along weaker fiber lines and increases cracking risk.
Why the other options are wrong
- B. Grain direction significantly affects bend radius requirements and cracking tendency.
- C. Bending parallel to the grain increases the likelihood of cracking.
- D. A 45-degree orientation is sometimes used as a compromise but is not the preferred orientation when avoidable.
Grain Direction in Sheet Metal Forming
The directional alignment of metal fibers from the rolling process, which affects bend strength and cracking tendency.
- Bends across the grain resist cracking better than bends parallel to grain
- Minimum bend radius charts assume bending across the grain
- Bending parallel to grain requires a larger bend radius to avoid cracking
Memory trick: Bend across the grain like breaking a log across its rings, not along them