Answer
Yielding is characterized by 'permanent set', where the spring no longer returns to its original free height $H_{free}$ after being unloaded, but the material remains in one piece. Visually, the waves will appear flattened. This indicates that the applied load $P$ exceeded the elastic limit. Fatigue, however, results in a complete fracture. Under SEM (Scanning Electron Microscopy), a fatigue failure will show 'striations' indicating progressive crack growth and a 'beach mark' pattern, followed by a final fast-fracture zone. If the fracture is brittle and occurs shortly after installation, it suggests Hydrogen Embrittlement or a material defect; if it occurs after millions of cycles, it is typical mechanical fatigue near the endurance limit.
