Which mechanical property refers to a material's capacity to undergo permanent shape change without breaking when subjected to stress?

- Plasticity refers to a material’s ability to undergo permanent deformation or shape change without breaking when subjected to stress.
- Unlike elastic deformation, which is reversible, plastic deformation means the material retains its new shape even after the applied stress is removed.
- This property is crucial in processes like metal forming, shaping, and manufacturing.

To clarify the other options:
- Elasticity is the ability of a material to return to its original shape after the removal of stress (reversible deformation).
- Ductility specifically refers to a material’s ability to undergo significant plastic deformation under tensile stress, often leading to stretching into a wire.
- Toughness is a measure of how much energy a material can absorb before fracturing; it signifies resistance to impact and fracture but does not directly describe permanent shape change.

Therefore, plasticity best describes the capacity for permanent shape change without breaking under stress.

Reference: Materials Science and Engineering, Volume 1, Chapter 3, pp. 45-50