Load Induced Fatigue occurs due to fatigue from which type of stresses?

• A. Fatigue due to in-plane stresses
• B. Fatigue due to out-of-plane stresses
• C. Fatigue due to thermal gradients
• D. Fatigue due to shear at bolt holes

Answer: (A)

Load-induced fatigue comes from the stress cycles that act within the plane of the member’s cross-section. As traffic loads vary, bending and axial stresses alternate inside that plane, causing tension on one side and compression on the other with each load cycle. Over many cycles, these in-plane stress reversals drive crack initiation and growth at critical details such as weld toes or bolt holes.

Thermal gradients can cause a different type of fatigue (thermal fatigue), which isn’t driven by mechanical loads. Stresses out of the plane, or shear at bolt holes, are not the primary mechanism for load-induced fatigue in typical bridge members, where the dominant damaging cycles are the in-plane bending and axial stresses.