Why is section loss susceptible to fatigue?

Why is section loss susceptible to fatigue?

• A. Higher loads alone cause acceleration
• B. Fatigue is unaffected by section size
• C. Section loss improves fatigue life
• D. Less section means higher stresses, higher stresses increase stress range and therefore lower fatigue life

Answer: (D)

Section loss makes fatigue life worse because it reduces the member’s cross‑section, which raises the stresses that occur under the same loads. In bending, for example, the stress is inversely related to the section modulus (sigma = M/Z). When the section is thinned, Z decreases, so the same bending moment produces higher peak stresses. With cyclic loading, the difference between the maximum and minimum stress—the stress range—becomes larger. Higher stress ranges drive faster initiation and growth of cracks, so the component accumulates damage more quickly and fatigues sooner. The reduced section also concentrates stresses at the thinned area, compounding the effect. That’s why section loss is particularly susceptible to fatigue.

Section loss makes fatigue life worse because it reduces the member’s cross‑section, which raises the stresses that occur under the same loads. In bending, for example, the stress is inversely related to the section modulus (sigma = M/Z). When the section is thinned, Z decreases, so the same bending moment produces higher peak stresses. With cyclic loading, the difference between the maximum and minimum stress—the stress range—becomes larger. Higher stress ranges drive faster initiation and growth of cracks, so the component accumulates damage more quickly and fatigues sooner. The reduced section also concentrates stresses at the thinned area, compounding the effect. That’s why section loss is particularly susceptible to fatigue.