When a material is deformed beyond its elastic limit, it undergoes:

When a material is deformed beyond its elastic limit, it undergoes plastic deformation. This means that the material will not return to its original shape when the deforming forces are removed. In the context of materials science, the elastic limit is the point at which the material transitions from elastic behavior, where it can fully recover its original dimensions, to plastic behavior, where permanent deformation occurs.

Plastic deformation is characterized by a rearrangement of the internal structure of the material, often resulting in changes such as dislocation movements in crystalline materials. This change is irreversible, meaning that even after the load is removed, the material will retain some deformation. Understanding this concept is crucial for millwrights when selecting materials and designing components that will endure specific loads and stresses in mechanical systems.

The other options relate to different types of material behavior but do not correctly represent what occurs beyond the elastic limit. Elastic deformation refers to the reversible change that occurs within the elastic limit. Fracture involves breaking apart, often due to excessive stress, while buckling is a stability issue associated with slender structures under compressive stress. Each has its own criteria and conditions, but they do not describe the specific process when a material crosses the threshold of elastic behavior into plastic behavior.