Which of the following is a type of environmental cracking?

• A. Mechanical fatigue cracking
• B. Fretting corrosion
• C. Erosion cracking
• D. Stress corrosion cracking (SCC)

Answer: (D)

Environmental cracking happens when a corrosive environment and tensile stress work together to cause a crack to form and grow in a material. The best example here is stress corrosion cracking, which is a crack that advances under the influence of a specific corrosive environment even at stresses that might not cause ordinary fracture. The environment affects the metal at the crack tip, facilitating brittle-like fracture as the crack propagates, often along grain boundaries or through the crystal structure, and it can occur at relatively modest stresses and sometimes at room temperature in certain alloys.

The other mechanisms don’t fit the environmental-cracking concept as closely. Mechanical fatigue cracking is driven by cyclic mechanical loading without requiring a corrosive environment. Fretting corrosion involves wear and corrosion at small oscillatory motions between contacting surfaces, not a classic stress-assisted crack in a single material under a sustained environment. Erosion cracking mainly results from high-velocity fluid impacts causing wear and crack initiation, with erosion being the dominant driver rather than a chemical environment-induced crack growth.