Plastic deformation is:

• A. A dimensional change that disappears when the initiating stress is removed.
• B. The three-dimensional array of atoms in a metal.
• C. A dimensional change that does not disappear when the initiating stress is removed; bonds are broken, and the material will not return to its original shape.
• D. The measure of the acidity of a solution.

Answer: (C)

Plastic deformation is a permanent, non-recoverable change in shape that remains after the applied load is removed. When a metal is stressed beyond its elastic limit, its crystal structure yields and dislocations move so that planes slip past one another. The lattice rearranges into a new configuration, so the material does not snap back to its original shape. Bonds aren’t simply broken and rebuilt to restore the original form; instead, the arrangement shifts to a new geometry through dislocation motion, leaving a permanent deformation.

This is different from an elastic deformation, which disappears when the load is released, and from simply describing the crystal lattice or chemical acidity, which relate to other concepts entirely.