crack up vs titter

crack up

verb
  • To cause to laugh heartily. 

  • To become insane; to suffer a mental breakdown. 

  • To crash an aircraft or automobile. 

  • To laugh heartily. 

  • To cry up; to extol. 

adj
  • Funny; hilarious 

titter

verb
  • To laugh or giggle in a somewhat subdued or restrained way, as from nervousness or poorly-suppressed amusement. 

noun
  • A nervous or somewhat repressed giggle. 

  • A woman's breast. 

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