FAA Aviation Mechanic General (AMG)Materials, Hardware and ProcessesHard
During a repair, a technician repeatedly bends and straightens a section of 6061-T6 aluminum sheet while trial-fitting a patch. The metal becomes progressively stiffer and more brittle, eventually cracking before the final bend is completed. Which condition describes this behavior?
- AStress corrosion cracking
- BHydrogen embrittlement
- CWork hardening
- DIntergranular corrosion
Show answer & explanationAnswer & explanation
Correct answer: C. Work hardening
Repeated cold working (bending back and forth) increases the internal dislocation density in the metal's grain structure, making it progressively harder and less ductile — a condition called work hardening (strain hardening). Continued working eventually causes the material to crack because it can no longer deform plastically.
Why the other options are wrong
- A. Stress corrosion cracking requires sustained tensile stress combined with a corrosive environment, not mechanical flexing alone.
- B. Hydrogen embrittlement results from hydrogen absorption during plating processes, not mechanical bending.
- D. Intergranular corrosion is an electrochemical attack along grain boundaries, not a mechanical hardening effect.
Work Hardening
An increase in hardness and decrease in ductility of a metal caused by repeated plastic deformation (cold working), which can lead to cracking if working continues.
- Also called strain hardening or cold working effect
- Repeated bending/straightening is a common cause
- Annealing (heat treatment) can restore ductility
Memory trick: Bend a paperclip enough times and it work-hardens until it snaps