Why is the fatigue strength of a rod subjected to cyclic axial force less than that of a rotating beam of the same dimensions?

• A. Axial stiffness is less than bending stiffness
• B. Absence of centrifugal effects in the rod
• C. The number of discontinuities vulnerable to fatigue is more in the rod
• D. At a particular time, the rod experiences only one type of stress whereas the beam experiences both tensile and compressive stresses

Answer: (D)

The fatigue strength of a rod subjected to cyclic axial force being less than that of a rotating beam of the same dimensions can be attributed primarily to the nature of the stress experienced by each component.

When a rod is subjected to a cyclic axial load, it experiences a single type of stress—tensile or compressive—depending on the direction of the load. This uniaxial stress can lead to simpler loading conditions but does not effectively dissipate the accumulated strain energy. In contrast, a rotating beam not only experiences bending stresses due to the dynamic nature of its rotation but also undergoes variations in tensile and compressive stresses throughout its cycle. This complex loading scenario allows for better distribution of stress and can lead to a more favorable fatigue response, as the maximum stress is balanced by alternating stresses, reducing the likelihood of crack initiation and propagation.

Additionally, the nature of how materials resist fatigue plays a role; components experiencing bending stresses can better absorb and distribute stress concentrations compared to those under uniaxial loads. Therefore, the combination of multiple stress states in the rotating beam contributes to its higher fatigue strength compared to the rod under cyclic axial loading.