crumble (away) vs wear down

crumble (away)

wear down

verb
  • To cause (someone) physical or mental fatigue. 

  • To eventually persuade or defeat (someone) through persistent effort. 

  • Used other than figuratively or idiomatically: see wear, down. 

How often have the words crumble (away) and wear down occurred in a corpus of books? (source: Google Ngram Viewer )