Fatigue cracks usually initiate on the surface at notches or __________ under cyclic loading.

Mechanical fatigue is a localized and progressive structural damage mechanism that occurs when component materials are subjected to cyclic fluctuating stresses over time. Under repeated stress cycles, microscopic cracks form and propagate through the material until final failure occurs. API RP 571 identifies stress concentrations as critical initiation sites because localized geometry variations, such as sharp corners, keyways, small radii, or notches, intensify the local magnitude of applied cyclic stresses well above the nominal stress level. Consequently, fatigue cracks predominantly initiate on the outer surface at these stress risers, where local stress amplitude exceeds the fatigue endurance limit of the material.

A. Branches: Incorrect because branching is a characteristic morphology of stress corrosion cracking propagation rather than a primary surface initiation feature for fatigue.
B. Laterals: Incorrect because "laterals" refers to piping configuration geometry rather than stress-raising microstructural or mechanical surface discontinuities.
D. Grinding marks: Incorrect because while surface roughness contributes to fatigue initiation, API RP 571 explicitly defines the primary initiation sites as notches or stress concentrations.

1. American Petroleum Institute, API Recommended Practice 571: Damage Mechanisms Affecting Fixed Equipment in the Refining Industry, 3rd Edition, 2020, Section 3.42 "Mechanical Fatigue," Paragraph 3.42.1.