inflexibility vs iron fist

inflexibility

noun
  • The quality or state of being inflexible, or not capable of being bent or changed; unyielding stiffness 

iron fist

noun
  • Ruthless and usually unrestricted control. 

  • Symbol of an army's tank or cavalry branch. 

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