Answer
Fatigue failure typically manifests as a clean, brittle-appearing fracture originating from a wave crest or trough, where the bending stress is maximum. Scanning Electron Microscopy (SEM) will reveal 'striations'—microscopic ridges each representing one load cycle. In the case of a Crest-to-Crest spring, the crack usually starts at the inner diameter (ID) of the crest because that is where the highest tensile stress occurs during compression. If the fracture surface shows 'beach marks' (macroscopic ridges), it indicates periods of varying load intensity. To prevent recurrence, the stress range $Δ\sigma$ must be reduced, or the surface finish must be improved (e.g., by vibratory finishing) to remove micro-scratches that act as stress risers.
