During what process do materials change permanently in shape or size under stress?

The process during which materials change permanently in shape or size under stress is known as plastic deformation. This phenomenon occurs when a material is subjected to a stress level that exceeds its yield strength, leading to a permanent change in its structure.

In plastic deformation, the atomic or molecular structure of the material is altered in such a way that once the stress is removed, the material does not return to its original shape. This is a crucial concept in materials science and engineering because it affects how materials behave under various conditions and influences their applications in manufacturing.

Understanding plastic deformation is essential for engineers and manufacturers because it helps in selecting appropriate materials for specific applications, predicting how they will behave under load, and ensuring the integrity and durability of the components being produced.