fall back vs turn back

fall back

verb
  • Used other than figuratively or idiomatically: see fall, back. 

  • To retreat. 

  • To fail to fulfill a promise or purpose. 

turn back

verb
  • To fold something back; to fold down. 

  • To cause to reverse direction and retrace one's steps. 

  • To return to a previous state of being. 

  • To reverse direction and retrace one's steps. 

  • To prevent or refuse to allow passage or progress. 

  • To adjust to a previous setting. 

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