Answer
Torsional fatigue in wave springs usually occurs when the spring is not only compressed axially but also subjected to a twisting moment. This happens if the mating components rotate at different speeds or if there is significant vibration. The stress state becomes complex: $σ_{total} = σ_{axial} + τ_{torsional}$. Failure manifests as cracks initiating at the inner or outer edges of the waves, where stress concentrations are highest. Using SEM (Scanning Electron Microscopy), one would observe 'striations' characteristic of fatigue. To prevent this, the spring should be 'keyed' or the mating surfaces must have enough friction to prevent relative rotation. Using a material with a higher fatigue limit, like Inconel 718, and ensuring a smooth edge finish (deburring) are critical mitigations.
