API 579 Fitness for Service Practice Exam

Thermal fatigue in high-temperature, oxidizing environments produces cracks that grow through grains (transgranular) and often fill with oxide as the metal oxidizes in the crack faces. The cycling of temperature and stresses drives crack propagation in a way that yields slender, dagger‑shaped shapes with a sharp tip, and the crack surfaces become filled with oxide from the surrounding oxide scale. This combination—dagger-like geometry, transgranular path, and oxide filling—is characteristic of thermal fatigue rather than brittle fracture, erosion, or mechanical fatigue, which do not typically show this oxide-filled dagger morphology.