A stress–strain curve records how a material deforms as stress is increased. It connects the internal loading of a specimen with its measured deformation.

Elastic behaviour

In the elastic region, removing the load allows the material to recover approximately its original shape. Many materials are nearly linear over part of this region.

Yielding and plastic deformation

Beyond the elastic range, some materials begin to deform plastically. After unloading, a permanent strain remains.

The stress at which significant plastic deformation begins is characterized by a yield strength.

Ultimate strength and fracture

During a tensile test, engineering stress may rise to an ultimate tensile strength and later fall before fracture as deformation localizes into a neck.

Different materials produce very different curves: brittle materials may fracture after little plastic strain, while ductile materials can deform substantially first.