Strength, ductility and toughness are three very important, closely related material properties. The yield and ultimate strengths tell us how much stress a material can withstand, and are often used to define failure. Ductility tells us how much plastic deformation a material undergoes before fracture. Brittle materials fracture at very small strains, and can fail catastrophically. Toughness tells us how much energy a material can absorb before fracture. It is closely linked to both strength and ductility.