Toughness

A material's resistance to the propagation of a crack from an existing flaw. It underpins damage-tolerant design, and in laminates the related interlaminar fracture toughness describes resistance to delamination growth.

The capacity of a material to undergo significant permanent deformation before fracture, as displayed by most metals and many thermoplastics. Ductile behaviour gives visible warning of overload and allows local stresses to redistribute.

The tendency of a material to fracture with little or no plastic deformation, as seen in glass and many cured thermosets. Brittle materials are sensitive to flaws, notches and impact, and fail suddenly rather than yielding.