Fracture mechanics and fatigue
Which statement correctly describes the initiation of fatigue cracks when a stress amplitude below the yield stress is repeatedly applied to a metal with a smooth surface?
- AThe surface is protected by its oxide film, so fatigue cracks almost always start inside the material near its center
- BBelow the yield stress, dislocations do not move at all, so no cracks form and the material lasts indefinitely
- CLocalized slip repeats in grains near the surface, and surface intrusions and extrusions become the starting points
Show answer and explanation
Answer: C. Localized slip repeats in grains near the surface, and surface intrusions and extrusions become the starting points
Even at a stress amplitude below the yield stress, localized slip occurs in favorably oriented grains near the surface, and with repetition persistent slip bands form. Slip does not fully reverse even when it goes back and forth, so tiny irregularities called intrusions and extrusions appear on the surface, and these act as notches where cracks start. The surface is less constrained, carries scratches and corrosion, and in bending or torsion bears the highest stress, so most fatigue cracks start at the surface. Initiation from the interior is limited to cases such as inclusions acting as origins in high-strength steels in the very-high-cycle regime. Dislocations do not stay completely still, so fatigue failure occurs even below the yield stress.