heart of oak
A trait said to belong to a very emotionally and/or mentally strong person. I don't know how a soldier does it—they must have a heart of oak. My brother has a heart of oak and has remained totally calm while coping with his wife's sudden illness.
heart of oak
a courageous nature. Literally, the heart is the solid central part of the oak tree traditionally used for timber for ships. The phrase was popularized by the words of an 18th-century song: ‘Heart of oak are our ships, Heart of oak are our men’.