FAA Aviation Mechanic Airframe (AMA)Airframe StructuresHard
A technician inspecting a high-strength steel landing gear trunnion finds fine intergranular cracks that developed while the part was under sustained tensile stress in a corrosive environment, even though the part was never subjected to a fatigue-cycling load. What type of failure mechanism does this describe?
- AFatigue cracking
- BExfoliation corrosion
- CStress corrosion cracking
- DHydrogen embrittlement from improper plating alone
Show answer & explanationAnswer & explanation
Correct answer: C. Stress corrosion cracking
Stress corrosion cracking (SCC) occurs when a susceptible high-strength metal (often heat-treated steel or aluminum) is subjected to sustained tensile stress in a corrosive environment, producing intergranular cracks without the need for cyclic fatigue loading. High-strength steel landing gear components are particularly susceptible.
Why the other options are wrong
- A. Fatigue cracking requires cyclic (repeated) loading, not sustained static stress, and this scenario specifies no fatigue cycling.
- B. Exfoliation corrosion is layer separation from corrosion product buildup, not a stress-driven intergranular crack process, and is more typical of aluminum extrusions.
- D. Hydrogen embrittlement is a related but distinct mechanism typically caused by plating processes introducing hydrogen, not simply by stress plus corrosive exposure.
Stress Corrosion Cracking (SCC)
A failure mechanism in which sustained tensile stress combined with a corrosive environment causes intergranular cracking in susceptible metals, notably high-strength steels and some aluminum alloys, without requiring cyclic fatigue loading.
- Requires three factors: susceptible material, tensile stress, corrosive environment.
- Common in high-strength steel landing gear components and heat-treated aluminum.
- Prevention includes proper heat treatment, stress relief, and corrosion protection coatings.
Memory trick: SCC = Stress + Corrosion = Cracks, even without cycling