During the plastic stage of the stress–strain curve, deformation is...

• A. Elastic deformation
• B. Creep
• C. Fatigue
• D. Permanent deformation

Answer: (D)

The key idea is that once the material yields, deformation becomes permanent. Up to the yield point, returning the load to zero removes the strain—this is elastic behavior. Beyond yield, dislocations move and the material undergoes plastic deformation, causing a real, irreversible change in shape that remains after removing the load. Creep describes time-dependent deformation under a constant load (often at high temperature), and fatigue is failure from cyclic loading; neither describes the immediate permanent shape change that defines the plastic stage. So, during the plastic stage, the deformation is permanent.