Plastic deformation is the permanent deformation that occurs when the stress exceeds the yield strength of the material. It generally occurs in two different ways:
1. Macroscopically, the contact load between the rolling elements and the raceway causes yielding to occur over most of the contact trajectory.
2. Microscopically, the foreign object is rolled between the rolling elements and the raceway, and yielding occurs only in a small part of the contact trajectory.