imperturbability vs sangfroid

imperturbability

noun
  • A state of calm, unruffled self-assurance; aplomb, composure. 

sangfroid

noun
  • Composure, self-possession or imperturbability especially when in a dangerous situation. 

How often have the words imperturbability and sangfroid occurred in a corpus of books? (source: Google Ngram Viewer )