frailty vs toughness

frailty

noun
  • The condition quality of being frail, physically, mentally, or morally; weakness of resolution; liability to be deceived. 

  • A fault proceeding from weakness; foible; sin of infirmity. 

toughness

noun
  • A formidable difficulty. 

  • Resistance to fracture when stressed, or the degree of such resistance. 

  • Fortitude; hardiness; mettle. 

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