Answer
Fretting fatigue is a failure mechanism that occurs in nested wave springs where the parallel layers of wire rub against each other during cyclic loading. This micro-motion (fretting) removes the protective oxide layer of the metal and creates small pits that act as stress concentrators. Eventually, these pits initiate fatigue cracks that propagate through the wire thickness. This is particularly common in high-frequency applications like vibration isolators. To prevent fretting, engineers specify dry-film lubricants (like MoS2 or PTFE) or ensure the spring is submerged in oil. Failure analysis typically reveals 'red rust' (cocoa) in carbon steel or shiny worn patches in stainless steel at the contact points between turns, accompanied by crack initiation sites.
