The initiation of a fatigue crack could occur at:

The initiation of a fatigue crack could occur at:

• A. All of the above
• B. Weld toes
• C. Notches
• D. Section changes

Answer: (A)

Fatigue crack initiation happens at places where the cyclic stress is concentrated or where the material’s surface or microstructure is weakened, allowing tiny cracks to form under repeated loading. Weld toes are classic initiation sites because the weld introduces abrupt geometry and residual tensile stresses that elevate local stress. Notches create deliberate or incidental geometric sharpness that concentrates stress, especially under bending or fluctuating loads. Section changes, such as abrupt reductions or enlargements in cross-section, also cause stress intensification at the transition, making initiation more likely there. Since any of these features can serve as a starting point for fatigue cracks, all of the listed locations are valid initiation sites.

Fatigue crack initiation happens at places where the cyclic stress is concentrated or where the material’s surface or microstructure is weakened, allowing tiny cracks to form under repeated loading. Weld toes are classic initiation sites because the weld introduces abrupt geometry and residual tensile stresses that elevate local stress. Notches create deliberate or incidental geometric sharpness that concentrates stress, especially under bending or fluctuating loads. Section changes, such as abrupt reductions or enlargements in cross-section, also cause stress intensification at the transition, making initiation more likely there. Since any of these features can serve as a starting point for fatigue cracks, all of the listed locations are valid initiation sites.