Which two factors control the number of cycles in fatigue life?

• A. Temperature and humidity
• B. ADTT and n
• C. Material grade and weld size
• D. Cross-section area and yield strength

Answer: (D)

Fatigue life is set by how much cyclic stress the member endures at the critical section and how well the material can resist that stress over time. The cross-sectional area at the weakest section determines the stress produced by a given load (stress = load divided by area). A larger area lowers the stress, so more cycles are needed before cracks grow to a critical size; a smaller area raises stress and shortens the cycle count to failure. The material’s yield strength indicates the stress level the material can withstand before it deforms plastically, which accelerates crack initiation and growth if exceeded. A higher yield strength means the component can tolerate higher cyclic stresses before damage progresses, increasing the number of cycles before failure. These two factors—the stress incurred at the section and the material’s resistance to that stress—primarily govern the fatigue life in this context. In real-world scenarios other factors like environment, surface finish, and weld details influence fatigue, but the combination of area and yield strength directly sets the capacity against cyclic loading.