Ductility describes how much a material can deform plastically (elongation A and reduction of area Z from the tensile test). Toughness describes how much energy it can absorb before fracturing – measured for example as impact energy KV in the Charpy test. The two are related but not identical: a material can be ductile at room temperature yet fail in a brittle manner at low temperatures (ductile-to-brittle transition). That is why both values are often tested together.
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