iron skull

例句：

1. 1.The iron skull of the old man was a constant reminder of his past mistakes.
   老人的铁石心肠总是提醒着他过去的错误。
2. 2. The scientist's iron skull made him unyielding in his pursuit of knowledge.
   科学家的铁石心肠使他在追求知识时变得坚定不移。
3. 3. The iron skull of the leader kept everyone on their toes, ensuring order and discipline.
   领导者的铁石心肠让每个人都保持警惕，确保秩序和纪律。