Plasticity is the property where an object loses its elasticity and undergoes irreversible deformation, even after the deformation forces are eliminated. If a material deforms irreversibly without increasing stress or load, then this is called ideal plasticity. For example, when a force is applied to an aluminum rod, it changes its shape, but it does not return to its original shape once the force is removed. Plastic deformation or ductility is thus a permanent deformation or change in the shape of a solid caused by continuous force.

Once a material goes beyond its elasticity limit and experiences plastic deformation, it remains plastically deformed until the stress reaches the fracture point (breaking point). The value of stress at the fracture point is called breaking stress (or ultimate stress). Materials with similar properties, such as two metals, can have very different breaking stresses. For example, the ultimate stress for aluminum is far lower than that of steel. Beyond the fracture point, the body is fractured into pieces.

Table 1. Approximate breaking stresses of various materials

This text is adapted from Openstax, University Physics Volume 1, Section 12.4: Elasticity and Plasticity.