FAA Aviation Mechanic Airframe (AMA)Airframe InspectionHard
A technician finds cracking in a high-strength steel landing gear component that had been under sustained tensile stress from an improperly torqued fitting and was exposed to a corrosive environment over time. The cracks propagate perpendicular to the applied stress with little surface deformation. What is the most likely cause?
- AExfoliation corrosion from layered grain structure in an extrusion
- BStress corrosion cracking from sustained tensile stress combined with a corrosive environment
- CFatigue cracking caused solely by repeated cyclic loading
- DGalvanic corrosion from contact with a dissimilar metal
Show answer & explanationAnswer & explanation
Correct answer: B. Stress corrosion cracking from sustained tensile stress combined with a corrosive environment
Stress corrosion cracking (SCC) results from the combined effects of sustained tensile stress and a corrosive environment on susceptible high-strength alloys, producing cracks that propagate perpendicular to the stress with minimal ductile deformation, unlike fatigue cracks which require cyclic loading.
Why the other options are wrong
- A. Exfoliation corrosion is a layered flaking pattern in extrusions, not perpendicular cracking from sustained stress.
- C. Fatigue cracking requires repeated cyclic loading, not sustained static stress, and typically shows beach marks.
- D. Galvanic corrosion requires dissimilar metals in contact, not mentioned here.
Stress Corrosion Cracking (SCC)
Cracking caused by the combined action of sustained tensile stress and a corrosive environment on a susceptible metal, propagating perpendicular to the applied stress.
- Requires sustained (static) tensile stress, not cyclic loading
- Common in high-strength steel and aluminum alloys under residual or applied stress
- Cracks propagate transgranularly or intergranularly, often with little visible deformation
Memory trick: 'Stress + corrosion = a silent crack sneaking through.'