Distortion Induced Fatigue results from what type of forces?

Distortion Induced Fatigue results from what type of forces?

• A. Primary bending forces
• B. Secondary forces
• C. Torsional loads
• D. Thermal expansion

Answer: (B)

Distortion Induced Fatigue comes from forces that are not the main load path, but instead cause the member to distort as it cycles with traffic. These secondary forces—such as restraint, misalignment, fabrication/erection tolerances, residual stresses from welding, and temperature effects—induce small, repeated shape changes. That distortion changes the local stress field at connections and weld details, so the crack growth is driven by these distortion-driven stress reversals rather than just the primary bending moment. Over many cycles, that distortion-induced alternating stress concentrates at weld toes and other detail knots, leading to fatigue cracks. Primary bending forces drive the overall flexing of a member, but distortion-induced fatigue specifically hinges on those secondary forces that distort the member and alter the stress state at connections. Torsional loads and thermal effects can cause distortion as well, but the mechanism of fatigue here is tied to secondary forces causing distortion, not to a pure torsional fatigue mode or to thermal effects alone.

Distortion Induced Fatigue comes from forces that are not the main load path, but instead cause the member to distort as it cycles with traffic. These secondary forces—such as restraint, misalignment, fabrication/erection tolerances, residual stresses from welding, and temperature effects—induce small, repeated shape changes. That distortion changes the local stress field at connections and weld details, so the crack growth is driven by these distortion-driven stress reversals rather than just the primary bending moment. Over many cycles, that distortion-induced alternating stress concentrates at weld toes and other detail knots, leading to fatigue cracks.

Primary bending forces drive the overall flexing of a member, but distortion-induced fatigue specifically hinges on those secondary forces that distort the member and alter the stress state at connections. Torsional loads and thermal effects can cause distortion as well, but the mechanism of fatigue here is tied to secondary forces causing distortion, not to a pure torsional fatigue mode or to thermal effects alone.