Which statement is true about fatigue and stress range?

Which statement is true about fatigue and stress range?

• A. The primary controlling factor is stress range
• B. Fatigue life is determined by maximum stress only
• C. Stress range is irrelevant
• D. Weld details are not related to fatigue

Answer: (A)

Fatigue life in bridges is governed mainly by the fluctuating component of the load, the stress range—the difference between the maximum and minimum stresses in each cycle. This stress range drives crack initiation and growth, so the number of cycles to failure (fatigue life) drops as the range increases. Even if the peak stress is high, a small stress range can allow many more cycles to occur before failure, whereas a large stress range can lead to rapid fatigue failure. Weld details matter here because welds create stress concentrations and notch effects that amplify the local stress range at the detail, accelerating fatigue. That’s why the statement that stress range is the primary controlling factor is correct. The other choices contradict how fatigue typically behaves: maximum stress alone doesn’t determine fatigue life, stress range isn’t irrelevant, and weld details do relate to fatigue.

Fatigue life in bridges is governed mainly by the fluctuating component of the load, the stress range—the difference between the maximum and minimum stresses in each cycle. This stress range drives crack initiation and growth, so the number of cycles to failure (fatigue life) drops as the range increases. Even if the peak stress is high, a small stress range can allow many more cycles to occur before failure, whereas a large stress range can lead to rapid fatigue failure. Weld details matter here because welds create stress concentrations and notch effects that amplify the local stress range at the detail, accelerating fatigue. That’s why the statement that stress range is the primary controlling factor is correct. The other choices contradict how fatigue typically behaves: maximum stress alone doesn’t determine fatigue life, stress range isn’t irrelevant, and weld details do relate to fatigue.