Physical metallurgy

La tenacidad a la fractura importa cuando ya hay una grieta: fracture toughness matters once a crack is already there. Toughness — not tenacity, and the false friend is the whole point. English owns both words and gave them different jobs: « toughness » is the property of the material, how much energy it absorbs before it breaks, and « tenacity » is a quality of people, the refusal to let go. Spanish uses la tenacidad for both, so la tenacidad del acero and la tenacidad de un negociador are the same noun. No metallurgist writing English would say « the tenacity of the steel », and that is the sentence a Spanish speaker writes first.