FAA Aviation Mechanic Airframe (AMA)Airframe StructuresMedium
A technician inspecting a wing skin panel notices a small, sharp-cornered rectangular access hole cut into the structure without any reinforcement or radius at the corners. Why is this a structural concern?
- AThe sharp corners reduce the panel's natural vibration frequency
- BThe sharp corners reduce the overall weight of the structure excessively
- CThe sharp corners create stress risers that promote fatigue crack initiation
- DThe sharp corners increase the panel's resistance to galvanic corrosion
Show answer & explanationAnswer & explanation
Correct answer: C. The sharp corners create stress risers that promote fatigue crack initiation
Sharp internal corners concentrate stress (stress risers), significantly increasing the local stress level compared to the surrounding material. Under cyclic loading, this concentrated stress promotes fatigue crack initiation, which is why cutouts should always have rounded corners.
Why the other options are wrong
- A. Vibration frequency change is not the primary structural concern with sharp cutout corners.
- B. Weight reduction is not a structural safety concern related to sharp corners.
- D. Corrosion resistance is unrelated to the corner geometry described.
Stress Riser
A geometric discontinuity, such as a sharp corner, notch, or scratch, that concentrates stress locally far above the nominal stress level, making it a likely site for fatigue crack initiation.
- Sharp corners, notches, and scratches all act as stress risers.
- Fatigue cracks typically start at stress risers under cyclic loading.
- Rounding corners (adding a fillet radius) reduces stress concentration.
Memory trick: Sharp corner = a lightning rod for stress — cracks strike there first