FAA Aviation Mechanic Airframe (AMA)Airframe StructuresMedium
A technician examines a fractured steel bolt from a landing gear attachment under magnification and observes a series of smooth, curved concentric lines radiating outward from an origin point near the surface, transitioning into a rough, granular final rupture zone. This fracture surface pattern is most characteristic of which type of failure?
- AOverload (tensile) failure
- BFatigue failure
- CCreep failure
- DStress corrosion cracking
Show answer & explanationAnswer & explanation
Correct answer: B. Fatigue failure
Fatigue failure produces characteristic 'beach marks' or 'clam shell' marks — smooth concentric lines that show progressive crack growth from a stress concentration point, ending in a rough final overload fracture area when the remaining material can no longer support the load.
Why the other options are wrong
- A. Overload failure typically shows a single rough, granular fracture without progressive beach marks.
- C. Creep failure results from sustained load at high temperature and shows elongation/necking, not beach marks.
- D. Stress corrosion cracking shows branching intergranular cracks, not smooth concentric beach marks.
Fatigue Failure
A progressive fracture caused by repeated cyclic loading, identifiable by smooth concentric 'beach marks' radiating from an origin, ending in a rough final fracture zone.
- Begins at a stress concentration point (stress riser)
- Beach marks show progressive crack growth over cycles
- Final rupture area is rough/granular from sudden overload
- Common cause of in-service structural failures
Memory trick: Beach marks = waves on a beach, showing repeated cycles.