The stress at fracture is called the fracture strength . On unloading from the plastic range a permanent strain remains in the material. On reloading the stress-strain behavior is elastic until a new yield stress is reached which is equal to the stress at which unloading started. After reaching the UTS, deformation becomes localized in a necking region of the specimen. While the material continues to strain harden, the load necessary to cause continuing deformation decreases since the specimen cross section area decreases quickly enough to overcome the strain hardening effect.
Deformation continues up to fracture .
Some material characteristics include strain in their definition. The amount of deformation that the material undergoes before fracture is often called ductility . The energy needed to cause deformation depends on the force and displacement or stress and strain. Toughness is the energy required to cause fracture. The energy absorbed in the elastic deformation range, and so recovered on unloading, is the resilience of the material.