strength vs toughness

strength

noun
  • The quality or degree of being strong. 

  • The intensity of a force or power; potency. 

  • The strongest part of something; that on which confidence or reliance is based. 

  • A positive attribute. 

toughness

noun
  • Fortitude; hardiness; mettle. 

  • A formidable difficulty. 

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

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