What are the three primary factors of fatigue life?

• A. Stress range, Number of cycles and Detail type
• B. Temperature, Pressure, Humidity
• C. Crack length, Growth rate, Material type
• D. Amplitude, Frequency, Surface finish

Answer: (A)

Fatigue life in a bridge member is controlled by three things: how large the fluctuating stress is (the stress range), how many load cycles the member experiences (the number of cycles), and the detail type, meaning the geometric features like welds, holes, and notches that concentrate stress. The stress range sets the amount of damage produced per cycle, the number of cycles determines how much total loading is accumulated over time, and the detail type governs where and how easily a crack can initiate and how quickly it can grow under the repeating load. While factors such as environmental conditions or material differences can influence fatigue in some situations, they don’t define the basic life expectation as directly as these three. For example, a sharp notch or hole creates a higher stress concentration, leading to earlier crack initiation and faster growth than a smoother detail, even under the same stress range and cycle count.